1 Introduction 1 引言

Rapidly depleting fossil fuel resources coupled with increasing environmental pollution has accelerated the pace of developing environmentally sustainable and high-energy–density renewable energy [1,2,3]. Lithium-ion batteries (LIBs), as a new clean energy source, have become important energy storage candidates in the electronic and communication equipment market [4]. However, their restricted energy density (150–240 Wh kg−1) and lack of memory retention render them unsuitable for deployment in grid and hybrid/electric vehicle. Recently, lithium–sulfur (Li–S) batteries, as rechargeable batteries incorporating multi-electron chemistry, have garnered intensive attention [5,6,7]. Their theoretical capacity (1675 mAh g−1) is much higher than that of LIBs (e.g., 274 mAh g−1 for lithium cobalt oxide (LiCoO2)), and even surpasses those of selenium and tellurium-based batteries (678 and 419 mAh g−1, respectively). Moreover, sulfur is abundant and environmentally friendly, making Li–S batteries competitive for widespread deployments [8]. Even though Li–S batteries possess appealing advantages, several challenges still limit their practicality: (i) the intrinsic electrical insulation (5 × 10–30 S cm−1) and volumetric expansion of sulfur; (ii) the reaction between the Li anode and the electrolyte resulting in unstable solid electrolyte interphase formation (SEI) and dendrite formation due to non-homogeneous nucleation at anode; (iii) the shuttling effect initiated by the polysulfides dissolution [9].
化石燃料资源的迅速枯竭加上环境污染的加剧,加快了开发环境可持续和高能量密度可再生能源的步伐[1,2,3]。锂离子电池(LIB)作为一种新型清洁能源,已成为电子和通信设备市场中重要的储能候选者[4]。然而,它们的能量密度有限(150-240 Wh kg −1 )和缺乏内存保留,使其不适合部署在电网和混合动力/电动汽车中。近年来,锂硫(Li-S)电池作为结合多电子化学的可充电电池,引起了人们的广泛关注[5,6,7]。它们的理论容量(1675 mAh g −1 )远高于锂离子电池(例如,钴酸锂(LiCoO 2 )为274 mAh g −1 ),甚至超过了硒和碲基电池(分别为678和419 mAh g −1 )。此外,硫含量丰富且环保,使锂硫电池在广泛部署方面具有竞争力[8]。尽管锂硫电池具有吸引人的优势,但仍有一些挑战限制了它们的实用性:(i)固有电绝缘(5×10 –30 S cm −1 )和硫的体积膨胀;(ii)锂阳极与电解质之间的反应,由于阳极不均匀成核导致不稳定的固体电解质界面形成(SEI)和枝晶形成;(iii)多硫化物溶解引发的穿梭效应[9]。

To solve the aforementioned problems, improvements have been made to different components of the battery [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. Specifically, to solve the intrinsic insulating properties of sulfur, carbon materials (e.g., active carbon, carbon nanotubes (CNT) [26] (108 S m−1), graphene [27], and their composites [28,29,30]) have been designed and prepared to improve the overall conductivity of the electrodes [30,31,32]. Specifically, an impressive capacity of 1006 mAh g−1 at 2 C was observed for the super-aligned CNT/S [33]. To reduce lithium dendrites and undesired side reactions, in situ or artificial SEI membranes are a one-two-punch approach to both insulate the electrolyte and avoid polysulfide attack on the lithium metal [31]. These will motivate additional efforts including electrode structure, electrolyte solvent, and electrolyte additive screening to customize the composition/structural characteristics of the SEI layer and the associated ion transport at the interface. Another strategy for the anode is to substitute Li metal with Li-free anodes (e.g., Li2S) for a new Li–S battery configuration, which has been identified as one of prospective directions to achieve ideal anodes [33]. Naturally, Li anode is accommodated in the 3D current collector with a submicron skeleton [32] may well be an ideal candidate to take on this task [34].
为了解决上述问题,对电池的不同组件进行了改进[10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]。具体而言,为了解决硫的固有绝缘性能,设计并制备了碳材料(例如活性炭、碳纳米管(CNT)[26](108 S m −1 )、石墨烯[27]及其复合材料[28,29,30]),以提高电极的整体导电性[30,31,32]。具体而言,在2 C时,超对准CNT/S的容量为1006 mAh g −1 [ 33]。为了减少锂枝晶和不良副反应,原位或人工SEI膜是一种一双冲头的方法,既可以绝缘电解质,又可以避免多硫化物对锂金属的侵蚀[31]。这些将激励额外的努力,包括电极结构、电解质溶剂和电解质添加剂筛选,以定制 SEI 层的组成/结构特性以及界面处的相关离子传输。阳极的另一种策略是用无锂阳极(例如Li 2 S)代替Li金属,以替代新的Li-S电池配置,这已被确定为实现理想阳极的潜在方向之一[33]。当然,锂阳极被安置在具有亚微米骨架的3D集流体中[32]很可能是承担这项任务的理想人选[34]。

Although the above-mentioned strategies can solve the first two issues, the shuttle effect in liquid electrolyte still significantly challenges the efficient sulfur reduction reaction (SRR) within Li–S batteries, which heavily damages the cathode, electrolyte, and lithium metal [35]. Soluble polysulfides diffuse toward the anode side, where they react with lithium metal to produce insoluble and insulating Li2S2/Li2S [36, 37]. This reaction leads to the loss of sulfur material and reduces Coulombic efficiency. So far, many strategies to curb the shuttle effect have been developed and effectively improved the utilization rate of sulfur in the cathode, and thus boosted the electrochemical performance of Li–S cells [38]. For instance, nonpolar carbon-based materials not only act as a conductive carrier of sulfur but also physically confine LPS [39, 40]. In recent times, a synergistic approach that involves a combination of multiple functional materials (e.g., heterogeneous atoms [41,42,43], catalysts with double defects [44,45,46], and heterostructure materials [47, 48]) has been devised, which possesses appropriate adsorption and catalytic capabilities to effectively execute SRR and enhances the performances of Li–S batteries [49]. The enhanced performances are ascribed to the promoted bidirectional conversion of LPS as a redox accelerator and regulated uniform Li plating/stripping to slow the growth of Li dendrites [50]. Moreover, tailoring electrolyte systems to construct an anode SEI layer [51] and a cathode solid electrolyte interphase (CEI) layer form on both electrodes by molecular regulation of electrolytes using optimal solvents/co-solvents [51,52,53,54,55], highly concentrated electrolytes (HCE) [56,57,58,59,60,61], and electrolyte additives [62, 63], etc. with various numbers of anchoring sites which significantly improved the stability of the Li anode interface, controlled the kinetics of sulfur redox, and suppressed side reactions toward polysulfides. Consequently, there is an improvement in the retention rate of capacity.
尽管上述策略可以解决前两个问题,但液体电解质中的穿梭效应仍然对锂硫电池内的高效硫还原反应(SRR)提出了重大挑战,严重损害了正极、电解质和锂金属[35]。可溶性多硫化物向阳极扩散,在那里它们与锂金属反应生成不溶性和绝缘的Li 2 S 2 /Li 2 S[36,37]。该反应导致硫材料的损失并降低库仑效率。迄今为止,已经开发出许多抑制穿梭效应的策略,并有效地提高了硫在阴极中的利用率,从而提高了Li-S电池的电化学性能[38]。例如,非极性碳基材料不仅充当硫的导电载体,而且还在物理上限制LPS [39,40]。近年来,人们设计了一种协同方法,该方法涉及多种功能材料(例如,非均相原子[41,42,43],具有双重缺陷的催化剂[44,45,46]和异质结构材料[47,48])的组合,该方法具有适当的吸附和催化能力,可以有效地执行SRR并提高Li-S电池的性能[49]。LPS作为氧化还原促进剂的双向转化和调节均匀的Li镀层/剥离以减缓Li枝晶的生长[50]。 此外,通过对电解质进行分子调控,使用最佳溶剂/助溶剂[51,52,53,54,55]、高浓度电解质(HCE)[56、57、58、59、60、61]和电解质添加剂[62、63]等,定制电解质系统,在两个电极上形成阳极SEI层[51]和阴极固体电解质界面(CEI)层,显著提高了锂阳极界面的稳定性, 控制硫氧化还原的动力学,抑制对多硫化物的副反应。因此,容量的保留率有所提高。

Extensive research has gone into the development of efficient strategies to inhibit the shuttling effect and achieve excellent performance in Li–S batteries [64,65,66]. Several reviews have summarized the design of cathode materials, high-sulfur loading, the inhibition of shuttle effect on the anode or electrolyte [67], etc. However, a comprehensive and systematical review regarding the strategies for suppressing the shuttling effect for all components of Li–S batteries is lacking and desired, especially for their practical application in future commercialization. In this review, we center on the shuttle effect issues and suppressing strategies in Li–S batteries (Fig. 1). We will first discuss the electrochemical principles and shuttle effect of Li–S batteries to give an overview of the mechanism and original of the shuttle effect. The designed principles for prohibiting LPS shuttle will be elaborated, including boosting the sulfur conversion rate of sulfur, confining sulfur or LPS within cathode host, confining LPS in the shield layer, and preventing LPS from contacting the anode, which offers guidance for further design novel materials of Li–S batteries. Then, we summarize the inhibition of shuttle effect from all components in Li–S batteries (cathode, electrolyte, separator and anode) with the above-designed principles. Finally, the prospects for inhibition of the shuttle effect and future development directions in Li–S batteries will be elucidated.

Fig. 1 图1
figure 1

Schematic illustrations of the strategies and operation mechanisms for inhibiting LPS shuttling starting

2 Electrochemical Principles and Shuttle Effect of Li–S Batteries
2 锂硫电池的电化学原理和穿梭效应

A typical half Li–S cell comprises sulfur cathode, separator, electrolyte, and lithium metal anode. And a conversion-type working mechanism is inherited during charge/discharge process. Specifically, each sulfur atom undergoes a complete two-electron redox reaction:

S8+16Li++16e8 Li2S

As shown in Fig. 2a, Li–S batteries typically show two plateaus during discharging, wherein S electrochemically reduces to Li2S via soluble intermediate polysulfides, i.e., Sk−2 (4 ≤ k ≤ 8), relating to a “solid–liquid-solid” process. Specifically, the discharge voltage plateaus at ~ 2.35 V contribute to 25% of the total theoretical specific capacity (419 mAh g−1). The electrochemical reduction of this part goes through two stages. Initially, solid S8 transforms into soluble higher-order Li2S8 upon reaction with migrating Li-ions and electrons, relating to the reaction of converting between solid–liquid [68]. Subsequently, the highly soluble higher-order Li2S8 tends to be disproportionate in the aprotic electrolytes and lower-order polysulfide anions (Sk−2, k > 4) produced by single-phase liquid–liquid reactions. The low discharge voltage plateaus (< 2.1 V) represent a further reduction of these lower-order intermediate polysulfides to solid state products (Li2S2/Li2S), which contributes to the 75% of the total specific capacity (~ 1256 mAh g−1). This part undergoes a two-stage electrochemical reduction as well. In the first stage, soluble lower-order polysulfides are reduced to insoluble Li2S2 or Li2S, and this process is a slow two-phase reaction involving liquid–solid phases [69]. In the last ramp stage, Li2S2 dissociates to Li2S eventually, two low-conductivity solid-phase transformation processes that is always subject to large polarization and slow kinetics. Reversible solid–liquid-solid reactions also occur in the oxidation process (charging) whereby the Li2S is converted to elemental S through dissolved intermediate polysulfides [70].
如图2a所示,Li-S电池在放电过程中通常呈现出两个平台,其中S通过可溶性中间多硫化物(即S k −2 (4≤k≤8)电化学还原为Li 2 S,与“固-液-固”过程有关。具体而言,~ 2.35 V 的放电电压平台占总理论比容量 (419 mAh g −1 ) 的 25%。该部分的电化学还原经历两个阶段。最初,固体 S 8 在与迁移的锂离子和电子反应时转化为可溶性高阶 Li 2 S 8 ,这与固液转化反应有关 [ 68]。随后,高溶度高阶Li 2 S 8 在单相液-液反应产生的非质子电解质和低阶多硫化物阴离子(S,k k −2 >4)中趋于不成比例。低放电电压平台(< 2.1 V)代表这些低阶中间多硫化物进一步还原为固态产物(Li 2 S 2 / Li 2 S),占总比容量的75%(~1256 mAh g −1 )。该部件也经历了两阶段电化学还原。在第一阶段,可溶性低级多硫化物被还原为不溶性Li 2 S 2 或Li 2 S,该过程是涉及液固相的缓慢两相反应[69]。在最后的斜坡阶段,Li 2 S 2 最终解离到 Li 2 S,这是两个低电导率的固相相变过程,总是受到大极化和缓慢动力学的影响。 可逆的固-液-固反应也发生在氧化过程(充电)中,其中Li 2 S通过溶解的中间多硫化物转化为元素S[70]。

Fig. 2 图2
figure 2

a Schematic diagram of Li–S battery containing differnt components. b Response mechanisms of “solid–solid,” “quasi-solid,” “solid–liquid-solid,” and corresponding discharge profiles
包含不同组件的锂硫电池示意图。b “固-固”、“准-固”、“固-液-固”的响应机理及相应的放电曲线

The reaction about sulfur in Li–S batteries often leads to an internal shuttle effect, particularly during the “solid–liquid-solid” process. This issue can be solved by transitioning to quasi-solid or solid–solid transformation, which will be explained in Sect. 3. The soluble intermediate polysulfides move toward the anode driven by the concentration gradient and then combine with the Li anode to form lower-order polysulfides. As short-chain polysulfides experience greater electric field force than concentration gradient force, they move back toward the cathode and cause the production of higher-order polysulfide. The ongoing process of shuttle effect may result in significant self-discharge of the anode due to corrosion, and poor Coulombic efficiency (CE).
锂-硫电池中硫的反应通常会导致内部穿梭效应,特别是在“固-液-固”过程中。这个问题可以通过过渡到准固体或固体-固体转变来解决,这将在第 3 节中解释。可溶性中间多硫化物在浓度梯度的驱动下向阳极移动,然后与锂阳极结合形成低级多硫化物。由于短链多硫化物承受的电场力大于浓度梯度力,它们会向阴极移动并导致产生更高阶的多硫化物。持续的穿梭效应过程可能导致阳极因腐蚀而发生显著的自放电,库仑效率(CE)较差。

In particular, the soluble polysulfides would become disconnected from the current collector, which would separate them from engaging in future electrochemical reactions. This would result in significant sulfur loss, leading to a subsequent decrease in the CE. Furthermore, the non-dissolvable layer of Li2S2/Li2S that collects on the Li anode surface would not turn into long-chain LPS and S8 again due to their deficiency in electrical conductivity. As a result, the active materials will be permanently lost, and the diffusion and transfer of Li+ ions will be delayed, resulting in a rapid decline in capacity and a short cycle life. Moreover, when the electrolyte viscosity increases due to the dissolution of LPS, the resistance to charge transfer in Li–S batteries also increases. This occurs because the solid Li2S2/Li2S during oxidation encounters a nucleation barrier, and Li2S2/Li2S decomposes and requires extra activation energy during discharge. Moreover, when the electrolyte viscosity increases resulting from the dissolution of LPS, the resistance to charge transfer also increases in Li–S batteries. The observed phenomenon may result from the nucleation barrier encountered during the oxidation of solid Li2S2/Li2S, as well as from the decomposition of Li2S2/Li2S which requires overcoming additional activation energy during reduction.
特别是,可溶性多硫化物将与集流体断开连接,这将使它们无法参与未来的电化学反应。这将导致大量的硫损失,导致随后的CE下降。此外,由于锂阳极表面的不溶 2 解层Li S 2 /Li 2 S,由于导电性不足,它们不会再次变成长链LPS和S 8 。结果,活性材料将永久丢失,锂 + 离子的扩散和转移将延迟,导致容量迅速下降和循环寿命短。此外,当电解质粘度因LPS的溶解而增加时,Li-S电池中的电荷转移阻力也会增加。这是因为固体 Li 2 S 2 /Li 2 S 在氧化过程中遇到成核屏障,而 Li 2 S 2 /Li 2 S 分解并在放电过程中需要额外的活化能。此外,当LPS溶解导致电解质粘度增加时,Li-S电池的电荷转移电阻也会增加。观察到的现象可能是由于固体 Li 2 S 2 /Li 2 S 在氧化过程中遇到的成核势垒,以及 Li 2 S 2 /Li 2 S 的分解,这需要克服还原过程中的额外活化能。

3 Design Principles for Prohibiting Shuttle Effect of Li–S Batteries
3 禁止锂硫电池穿梭效应的设计原则

A complex reaction kinetics, multi-step phase transition, accompanied by the shuttle effect, is generally involved in the operation of Li–S batteries. Shuttle effect represents a significant challenge for achieving efficient SRR within Li–S batteries. The phenomenon can cause considerable damage to all battery components. At the sulfur cathode, sulfur has a significant reaction barrier with Li-ions during the electrochemical cycle, as shown by the overpotential. This leads to sluggish reaction kinetics, prolonged retention time of polysulfide intermediates, an exacerbated shuttle effect, as well as a reduction in both the electrochemical stability and lifetime of the cell. This means that Li–S batteries can achieve stable cycling and high energy density if the shuttle effect is effectively suppressed. In this section, we will discuss the design strategies to prevent the shuttle effect through boosting the sulfur conversion rate, confining sulfur or LPS within cathode host, confining LPS in the shield layer, and preventing LPS from contacting the anode.
Li-S电池的运行通常涉及复杂的反应动力学、多步相变,并伴有穿梭效应。穿梭效应是实现锂硫电池高效SRR的重大挑战。这种现象会对所有电池组件造成相当大的损坏。在硫阴极,硫在电化学循环中与锂离子具有显着的反应势垒,如过电位所示。这导致反应动力学缓慢,多硫化物中间体的保留时间延长,穿梭效应加剧,以及电池的电化学稳定性和寿命降低。这意味着,如果有效抑制穿梭效应,锂硫电池可以实现稳定的循环和高能量密度。在本节中,我们将讨论通过提高硫转化率、将硫或 LPS 限制在阴极主体内、将 LPS 限制在屏蔽层以及防止 LPS 接触阳极来防止穿梭效应的设计策略。

3.1 Boosting the Sulfur Conversion Rate
3.1 提高硫转化率

The shuttle effect is inevitable in the traditional solid–liquid-solid conversion process, but if the conversion process of the sulfur-lithium reaction process can be changed to avoid the formation of soluble polysulfide or reduce the existence time of polysulfide, this will be a fundamental way for eliminating the shuttle effect. Researchers have developed two alternative conversion mechanisms for Li–S batteries. One is a quasi-solid conversion mechanism whereby the activated sulfur can be directly reduced into the short-chain substance Li2Sx (where x is less than or equal to 4); this substance is then reduced to form solid Li2S [71]. Through this reaction pathway, the formation of higher-order LPS intermediates is minimized; in turn, this prevents the detrimental parasitic shuttling that afflicts Li–S batteries [72]. A typical quasi-solid conversion pathway behaves a single-platform discharge process, with a capacity-voltage curve that is usually characterized by a weakened high plateau (≈2.3 V) and an extended discharge plateau (< 2.1 V) in first cycle (Fig. 2b). The other is solid–solid pathway which only involves the conversion between S and Li2S, avoiding the production of intermediates. So, it is always a single slope platform curve at 1.70 V [73]. Therefore, achieving the quasi-solid/solid–solid transition mechanism is main approach to boosting the sulfur conversion rate.
在传统的固-液-固转化过程中,穿梭效应是不可避免的,但如果能够改变硫锂反应过程的转化过程,避免可溶性多硫化物的形成或减少多硫化物的存在时间,这将是消除穿梭效应的根本途径。研究人员已经为锂硫电池开发了两种替代转换机制。一种是准固转化机理,将活性硫直接还原为短链物质Li 2 S x (其中x小于等于4);然后将该物质还原成固体Li 2 S[71]。通过这种反应途径,高阶LPS中间体的形成被最小化;反过来,这又防止了困扰锂硫电池的有害寄生穿梭[72]。典型的准固体转换路径表现为单平台放电过程,其容量-电压曲线通常以第一周期的高平台期(≈2.3 V)减弱和放电平台期延长(<2.1 V)为特征(图2b)。 另一种是固-固途径,仅涉及S和Li 2 S之间的转换,避免了中间体的产生。因此,它始终是1.70 V的单斜率平台曲线[ 73]。因此,实现准固/固-固转变机制是提高硫转化率的主要途径。

3.1.1 Ultra-microporous Carbon Confined Small Molecules
3.1.1 超微孔碳约束小分子

A practical way of reducing the dissolution of active material and thus the shuttle effect, is to confine sulfur with ultra-microporous carbon (UMC) whose pore size ≤ 0.7 nm. The sulfur confined in such micropore structures can be existed as small molecules due to steric constraints, such as S2−4 [74, 75]. At the same time, UMC materials prevent solvent molecules from entering the pores because they have a smaller size than the solvated ion (with EC and DMC having calculated diameters of 5.74 and 7.96 Å, respectively) (Fig. 3a) [76]. On the other hand, the solvent molecules become highly distorted, which would lead to the solvation energy turns lower than surface energy of the sub-nanopore, which forces the Li+ to desolvate and then moves through the carbon to react with sulfur [74]. Therefore, regardless of the electrolyte in use (whether ether or carbonate based), the concentration of solvent molecules inside the pore is almost negligible, resulting in a quasi-solid reaction mechanism [77]. For instance, a highly ordered microporous carbon FDU (0.46 nm) is used as the confinement matrix for S2−4 composites (FDU/S-40) in different electrolytes, exhibiting comparable electrochemical behavior: single discharge plateau at approximately 1.8 V, initial reversible capacity above 1000 mAh g−1, and stable cycling (Fig. 3b).
减少活性材料溶解从而减少穿梭效应的实用方法是用孔径≤ 0.7 nm的超微孔碳(UMC)限制硫。由于空间约束,限制在这种微孔结构中的硫可以以小分子的形式存在,例如S 2−4 [74,75]。同时,UMC材料可以防止溶剂分子进入孔隙,因为它们的尺寸比溶剂化离子小(EC和DMC的计算直径分别为5.74和7.96 Å)(图3a)[76]。另一方面,溶剂分子变得高度扭曲,这将导致溶剂化能转低于亚纳米孔的表面能,这迫使Li + 去溶剂化,然后穿过碳与硫反应[74]。因此,无论使用何种电解质(无论是醚基还是碳酸盐基),孔内溶剂分子的浓度几乎可以忽略不计,从而产生准固态反应机理[77]。例如,高度有序的微孔碳FDU(0.46 nm)被用作不同电解质中S 2−4 复合材料(FDU/S-40)的约束基质,表现出相当的电化学行为:约1.8 V的单放电平台,1000 mAh g −1 以上的初始可逆容量和稳定的循环(图3b)。

Fig. 3 图3
figure 3

Copyright 2014, Wiley–VCH. c Overall reaction of Li/SPAN cell [78]. Copyright 2014, MDPI. d The lithiation process of SPAN from ex situ electron paramagnetic resonance (EPR) spectra [79]. Copyright 2018, American Chemical Society
版权所有 2014,Wiley–VCH。c Li/SPAN电池的整体反应[ 78].版权所有 2014, MDPI。d 从非原位电子顺磁共振(EPR)光谱中SPAN的锂化过程[79]。版权所有 2018,美国化学学会

a Schematics of the lithiation process for UMC/S and mesoporous carbon/S cathode in carbonate-based electrolyte. b Electrochemical curves of FDU/S-40 and FDU/S-60 [74].
a 碳酸盐基电解质中UMC/S和介孔碳/S阴极的锂化过程示意图。b FDU/S-40和FDU/S-60的电化学曲线[ 74].

3.1.2 Sulfur-conjugated Organic Skeleton (Organic Sulfides/Sulfur-containing Polymers)
3.1.2 硫共轭有机骨架(有机硫化物/含硫聚合物)

Sulfur-containing polymers have exhibited positive attributes in Li–S batteries with solid–solid conversion owing to chemically bonded short-chain sulfur, in which element S existed as short -S2- and -S3- chains through reversible C–S/S–S bonds and transformed exclusively to Li2S to facilitate the solid-to-solid process. Li–S batteries achieve superior cycling stability compared to conventional cyclo-S8 cathodes, which generate lithium polysulfide (LPS) during cycling, due to the fundamental elimination of the shuttle effect [80]. Specifically, covalent attachment of Sn (n = 2–4) species to the PAN backbone was achieved through a one-step pyrolysis of sulfur and commercial polyacrylonitrile (PAN). Figure 3c displays the formula of lithiated SPAN, where sulfur was covalently bonded to the π-conjugated carbon skeleton through C–S bonds [78]. Furthermore, SPAN is optimally matched with the carbonate electrolyte, a typical SPAN discharge/charge curve (Fig. 3d) shows that no LPS is generated. During the first discharge, sulfhydryl radicals are produced as the S–S bond in the pristine SPAN is broken, as shown in Fig. 3d. Encapsulating these smaller sulfur molecules in the cathode, while covalently bonding and physically constraining, completely eliminates LPS dissolution and shuttling between the anode and cathode [79].
由于化学键合的短链硫,含硫聚合物在具有固-固转化的 Li-S 电池中表现出积极的属性,其中元素 S 通过可逆的 C-S/S-S 键以短 -S 2 和 -S 3 -链的形式存在,并专门转化为 Li 2 S 以促进固-固过程。与传统的环硫正极相比,锂硫电池具有优异的循环稳定性,传统的环硫 8 正极在循环过程中产生多硫化锂(LPS),这从根本上消除了穿梭效应[80]。具体来说,S n (n = 2–4) 物种与 PAN 主链的共价连接是通过硫和商业聚丙烯腈 (PAN) 的一步热解实现的。图3c显示了锂化SPAN的公式,其中硫通过C-S键与π共轭碳骨架共价键合[78]。此外,SPAN与碳酸盐电解质最佳匹配,典型的SPAN放电/充电曲线(Fig. 3d)显示不会产生LPS。在第一次放电期间,当原始SPAN中的S-S键被破坏时,会产生巯基自由基,如Fig. 3d所示。将这些较小的硫分子封装在阴极中,同时共价键合和物理约束,完全消除了LPS的溶解和阳极和阴极之间的穿梭[79]。

Moreover, porous organic polymers with much larger specific surface areas and more accessible pores can efficiently eliminate the volume expansion of sulfur cathode and enable higher sulfur loading (> 50%). An illustration of this can be with the preparation of the graphdiyne (GDY) type of the porous organic framework (GPOF). The pyrene nodes' π–π interaction resulted in interconnected channels in planes, providing an accommodation for more sulfur species in GPOF. Moreover, the acetylenic groups enriched with electrons are highly reactive, facilitating the combination with sulfur molecules in the form of C–S–S–C in the nanochannel and by solid-phase conversion to inhibiting the shuttle effect [81]. Therefore, GPOF-S composite enabled 56.8 wt% sulfur loading and average discharge capacities of 925 mA h−1 at 0.2 C, accompanied by a negligible reduction in capacity after 250 cycles.
此外,具有更大比表面积和更易接近孔隙的多孔有机聚合物可以有效地消除硫阴极的体积膨胀,并实现更高的硫含量(>50%)。可以通过制备多孔有机框架(GPOF)的石墨炔(GDY)类型来说明这一点。芘节点的π-π相互作用导致平面上的互连通道,为GPOF中更多的硫物种提供了便利。此外,富含电子的乙炔基团具有高反应性,有利于在纳米通道中以C-S-S-C的形式与硫分子结合,并通过固相转化来抑制穿梭效应[81]。因此,GPOF-S复合材料在0.2 C下实现了56.8 wt%的硫负荷和925 mA h −1 的平均放电容量,同时在250次循环后容量的降低可以忽略不计。

3.1.3 Sulfur-rich Compounds/Sulfur-Containing Inorganic Compounds
3.1.3 富硫化合物/含硫无机化合物

Instead of directly using elemental sulfur, the sulfur-containing compounds with incorporating the transition metals (e.g., Mo, Fe, Ti, Nb) have exhibited sulfur-like electrochemical behaviors [82]. Sulfur-rich compounds operate on a single voltage profile platform through the reversible breaking and formation of S–S bonds, lithium-ion insertion and extraction mechanisms. For instance, MoS3 has been used as cathode material replacing the pure element sulfur, which has a chain-like structure composed of Mo ions bridged by sulfide and disulfide ligands (as shown in Fig. 4a), enabling highly efficient active storage and rapid ion transfer [83]. Additionally, the amorphous chain-like structure of MoS3 is mostly maintained during lithiation/delithiation process with no Mo–S bond broken, and also no LPS intermediates generation has been observed in the electrochemical reaction process revealed by operando X-ray absorption spectroscopy (Fig. 4b).
掺入过渡金属(如Mo、Fe、Ti、Nb)的含硫化合物没有直接使用元素硫,而是表现出类似硫的电化学行为[82]。富硫化合物通过可逆断裂和形成 S-S 键、锂离子插入和提取机制在单一电压曲线平台上运行。例如,MoS 3 已被用作替代纯元素硫的正极材料,纯元素硫具有由硫化物和二硫键配体桥接的钼离子组成的链状结构(如图4a所示),可实现高效的活性储存和快速离子转移[83]。此外,MoS 3 的无定形链状结构在锂化/脱锂过程中大多保持,没有Mo-S键断裂,并且在操作X射线吸收光谱显示的电化学反应过程中也没有观察到LPS中间体的产生(图4b)。

Fig. 4 图4
figure 4

Copyright 2017, Natl Acad Sciences. c A schematic diagram of the sulfur (yellow) confined in CMK-3. And d galvanostatic electrochemical curves of the first cycles with CMK-3/S [84]. Copyright 2009, Nature Portfolio. e TEM images and elemental mapping of DHCS-S. f Electrochemical performance of DHCS-S and carbon black-sulfur (CB-S) [85]. Copyright 2012, Wiley–VCH. (Color figure online)
版权所有 2017,Natl Acad Sciences。c CMK-3中限制的硫(黄色)示意图。CMK-3/S第一循环的静电流电化学曲线[ 84] 。版权所有 2009,Nature Portfolio。e DHCS-S 的 TEM 图像和元素图谱。f DHCS-S与炭黑硫(CB-S)的电化学性能[ 85].版权所有 2012,Wiley–VCH。(彩色图在线)

a XRD pattern and b Fourier-transformed Mo K-edge EXAFS spectrum of 1D chain-like MoS3 [82].
a XRD 图谱和 b 傅里叶变换的 Mo K-edge EXAFS 光谱,一维链状 MoS 3 [ 82]。

Quasi-solid and solid–solid reactions are effective in addressing major challenges of Li–S batteries e.g., the shuttle effect caused by polysulfides and high dependency on electrolyte consumption. Nonetheless, the practical energy density of Li–S cells is significantly restricted due to the low sulfur content and inert redox kinetics of such cathodes [86].

3.2 Confining Sulfur or LPS within Cathode Host
3.2 将硫或LPS限制在阴极主机内

To increase the performance of Li–S batteries with solid–liquid-solid conversion, the strategies focus on inhibiting polysulfide dissolution or catalyzing the rapid conversion of sulfur into low-solubility discharge products [5, 33, 38, 87]. At the same time, so as to achieve the practical energy density, the electrolyte should be minimized, e.g., lean electrolyte with the electrolyte/sulfur ratio (E/S ratio) < 10 μL mg−1. However, most of the achievements in the last decade have been based on the excessive use of electrolytes. In this section, physical confinement, chemical anchoring and electrochemical catalysts strategies to inhibit the shuttle effect will be presented by designing optimized sulfur hosts that provide a high conductivity/ionic conductivity environment and fast redox kinetics. The recent host materials are summarized in Table 1 [88,89,90,91,92,93,94,95,96,97,98,99,100,101,102].
为了提高Li-S电池的固-液-固转化性能,这些策略侧重于抑制多硫化物溶解或催化硫快速转化为低溶解度放电产物[5,33,38,87]。同时,为了达到实际的能量密度,应尽量减少电解质,例如,电解质/硫比(E/S比)<10μL mg的稀薄电解质 −1 。然而,过去十年的大多数成就都是基于过度使用电解质。在本节中,将通过设计优化的硫主体来介绍抑制穿梭效应的物理约束、化学锚定和电化学催化剂策略,这些主体可提供高电导率/离子电导率环境和快速氧化还原动力学。表1总结了最近的寄主材料[88,89,90,91,92,93,94,95,96,97,98,99,100,101,102]。

Table 1 Summary of recent results on “Confining sulfur or LPS within cathode host”
表1 “将硫或LPS限制在阴极主体内”的最新结果摘要

3.2.1 Physical Confinement Method for Sulfur or LPS
3.2.1 硫磺或LPS的物理限制方法

Considerable effort has gone into solving the shuttle effect. The materials have been designed with a complex internal path and porous, absorbent carbon, to physically confine the sulfur or LPS in the cathode side [103], which strongly inhibit the bulky polysulfide anions from diffusing out of the channels into the electrolyte. Nazar and colleagues conducted a pioneering work that employed mesoporous carbon (CMK-3) as a conductive host material to trap or encapsulate S/Li2S-active material and LPS (Fig. 4c) [84]. The resulting composite showed a capacity of 1005 mAh g−1, which was superiority over that reported in the literature for C-S composites (averaging between 300 and 420 mAh g−1) [104] (Fig. 4d). Subsequently, the preparation of sulfur hybrids with macro/meso/microporous carbons [105,106,107] and carbon nanofibers [108, 109], spheres [110], nanotubes (CNTs) [108] was reported using a similar physical confinement method with some of the advantages of large surface area and short Li+ ions diffusion paths [39]. One example is double-shelled hollow carbon spheres (DHCS) with intricate shell architectures, which can further enhance the benefits of hollow nanostructures [111, 112], enabling high levels of sulfur encapsulation, limiting outward diffusion of LPS, and withstanding volume changes during long-term cycling (Fig. 4e). The distribution of carbon and sulfur is mainly concentrated in the area between the two carbon shells. The analogous distribution of these two elements indicates that sulfur has a strong attraction to carbon. In Fig. 4f, a better capacity retention was achieved using DHCS-S as the cathode (initial discharge capacity was 1020 mAh g−1, second cycle remained at 935 mAh g−1) [85].
为了解决穿梭效应,我们付出了相当大的努力。这些材料设计有复杂的内部路径和多孔的吸收性碳,以物理限制阴极侧的硫或LPS[103],这强烈抑制了笨重的多硫化物阴离子从通道扩散到电解质中。Nazar及其同事进行了一项开创性的工作,即使用介孔碳(CMK-3)作为导电主体材料来捕获或封装S/Li 2 S活性材料和LPS(图4c)[84]。所得复合材料的容量为1005 mAh g −1 ,优于文献报道的C-S复合材料(平均在300至420 mAh g −1 之间)[104](图4d)。随后,使用类似的物理约束方法制备了具有大表面积和短锂 + 离子扩散路径的一些优点的硫杂化物[105,106,107]和碳纳米纤维[108,109],球体[110],纳米管(CNTs)[108]。一个例子是具有复杂壳结构的双壳空心碳球(DHCS),它可以进一步增强中空纳米结构的优点[111,112],实现高水平的硫封装,限制LPS的向外扩散,并承受长期循环期间的体积变化(图4e)。碳和硫的分布主要集中在两个碳壳之间的区域。这两种元素的相似分布表明硫对碳有很强的吸引力。在图4f中,使用DHCS-S作为阴极实现了更好的容量保持(初始放电容量为1020 mAh g −1 ,第二次循环保持在935 mAh g −1 )[85]。

3.2.2 Chemical Anchoring Techniques for Effective Sulfur or LPS Confinement
3.2.2 有效硫或LPS限制的化学锚固技术

While the carbon–sulfur composites exhibit exceptional electrochemical behavior during the initial charge/discharge cycle, a pronounced degradation has been observed in subsequent cycles. This decline can account for the fact that mere physical confinement does not sufficiently expedite the kinetic processes involved in LPS transition. Additionally, the composites lack the requisite adsorption capacity to effectively mitigate LPS dissolution. Consequently, chemical anchorage strategies have been implemented to enhance inhibition of polysulfide solvation, including heteroatom-doped carbons, conjugated polymers, transition metal oxides, nitrides, and sulfides. Heteroatom-doping (N, O, S, P, Se, etc.) and polar materials have been studied for their ability to entrap the soluble LPS effectively was accounted for polar-polar interactions. For example, graphene wrapping nitrogen-doped double-shelled hollow carbon spheres (G-NDHCS-S) have been designed (Fig. 5a). The introduction of nitrogen atoms creates reactive sites for rapid charge transfer, allowing the hollow carbon spheres to immobilize more polysulfide ions (Fig. 5b), delivering a high initial discharge capacity of 1360 mAh g−1 at 0.2 C and rate performance of 600 mAh g−1 at 2 C [113]. Another way is to form coordinate bonds based on Lewis acid–base interactions, in which polysulfide anions (Sx2−, 4 ≤ x ≤ 8) as a Lewis base and most representative Lewis acid from metal ions of metal–organic frameworks (MOFs) [114] or MXenes [115]. As shown in Fig. 5c, soluble polysulfide ions are trapped in the MOF scaffold by the Lewis acidic Ni(II) center of the Ni-MOF, as evidenced by the capacity retention of up to 89% after 100 cycles at 0.1 C [116] (Fig. 5d).
虽然碳硫复合材料在初始充放电循环中表现出出色的电化学行为,但在随后的循环中观察到明显的退化。这种下降可以解释这样一个事实,即单纯的物理限制不足以加速LPS转变中涉及的动力学过程。此外,复合材料缺乏有效减轻LPS溶解所需的吸附能力。因此,已经实施了化学锚固策略来增强对多硫化物溶剂化的抑制,包括杂原子掺杂的碳、共轭聚合物、过渡金属氧化物、氮化物和硫化物。杂原子掺杂(N、O、S、P、Se等)和极性材料被研究为极性-极性相互作用的有效捕获可溶性LPS的能力。例如,已经设计了石墨烯包裹氮掺杂双壳空心碳球(G-NDHCS-S)(图5a)。氮原子的引入为快速电荷转移创造了反应位点,使空心碳球能够固定更多的多硫离子(图5b),在0.2 C时提供1360 mAh g −1 的高初始放电容量,在2 C时提供600 mAh g −1 的倍率性能[113]。另一种方法是形成基于路易斯酸-碱相互作用的配位键,其中多硫化物阴离子(S x 2− , 4 ≤ x ≤ 8)作为路易斯碱,最具代表性的路易斯酸来自金属-有机框架(MOFs)[114]或MXenes[115]的金属离子。如图5c所示,可溶性多硫离子被Ni-MOF的路易斯酸性Ni(II)中心捕获在MOF支架中,在0.1°C下循环100次后,容量保持率高达89%[116](图5d)。

Fig. 5 图5
figure 5

Copyright 2015, Wiley–VCH. c Schematic diagram of the interaction between LPS and Ni–MOF. d Electrochemical performance of Ni-MOF/S composite [116]. Copyright 2014, American Chemical Society. e Schematic diagrams, f lithiophilic/sulfiphilic dual binding sites, and g, h SRR catalyst of CNCO as additive in Li–S battery [117]. Copyright 2022, Elsevier
版权所有 2015,Wiley–VCH。c LPS与Ni-MOF相互作用示意图。d Ni-MOF/S复合材料的电化学性能[ 116].版权所有 2014,美国化学学会。e 中贤钧作为锂硫电池添加剂的亲光/亲硫双结合位点和g、h SRR催化剂示意图[117]。版权所有 2022,爱思唯尔

a Preparation process and b N 1s XPS spectra of the G–NDHCS–S [113].
a 制备过程和 b N 1s G–NDHCS–S 的 XPS 谱图 [ 113]。

The inclusion of catalysts is necessary to improve the sluggish conversion kinetics of polysulfides as a means to reduce the excessive accumulation of long-chain LPS and improve the electrochemical performance. Specifically, with the addition of an electrocatalyst to the sulfur electrode, the conversion process of S will only occur in cathodes, which means that the “solid–liquid-solid” transform of LPS takes place inside the cathode material. The transfer of lithium ions is realized by wetting the electrode with electrolyte [118,119,120]. Thus, as shown in Fig. 5e-h, hollow CoxNi1-xO concave (CNCO) as an additive endowed cathode host with strong affinity and efficient SRR catalysts which derived from sites with dual binding affinity to lithophile and chalcophile elements and abundant oxygen vacancies of CNCO. The CNCO/S can provide a high initial specific capacity of 1355 mAh g−1 at 0.1 C with a low nucleation barrier and overpotential of Li2S [117] and under lean electrolyte (E/S = 9 μL mg−1) keep 796 mAh g−1 at 0.5 C.
催化剂的加入对于改善多硫化物的缓慢转化动力学是必要的,以减少长链LPS的过度积累,提高电化学性能。具体来说,随着在硫电极中加入电催化剂,S的转化过程只会发生在阴极中,这意味着LPS的“固-液-固”转变发生在正极材料内部。锂离子的转移是通过用电解质润湿电极来实现的[118,119,120]。因此,如图5e-h所示,空心 x Co Ni 1-x O凹面(CNCO)作为添加剂赋予的阴极主体,具有强亲和力和高效的SRR催化剂,其来源于对亲石剂和亲硫剂元素具有双重结合亲和力以及CNCO丰富的氧空位的位点。CNCO/S在0.1°C时可提供1355 mAh g −1 的高初始比容量,具有低成核势垒和Li 2 S的过电位[117],在贫电解质(E/S = 9 μL mg −1 )下,在0.5°C时保持796 mAh g −1

The direct usage of elementary sulfur when the formation of chemically stable copolymers is another method of chemical containment [121]. The dissolution and diffusion of LPS species from the cathode region are proficiently mitigated through robust chemical interactions established between the carbonaceous framework and sulfur constituents inherent in the C–S copolymers. However, their cycling performance is impeded by their inherent limitation in facilitating efficient electronic conduction. As shown in Fig. 6e, sulfur reacts with the polymer via organic radicals to produce a sulfur-rich polymer with a sulfur content of 90 wt% [122]. The resulting cathode exhibits an initial discharge capacity of 1100 mAh g−1 at 0.1 C, but capacity rapidly decreases to below 400 mAh g−1 at 2 C. Similarly, higher initial capacity (1143 mAh g−1 at 0.1 C) with inferior rate performance (595 mAh g−1 at 1 C) is observed for sulfur-rich polymer materials synthesized by copolymerization of elemental sulfur with 1,3-diethynylbenzene (as shown in Fig. 6f-h) [123].
当形成化学稳定的共聚物时,直接使用原硫是另一种化学控制方法[121]。通过碳质框架和 C-S 共聚物中固有的硫成分之间建立的稳健化学相互作用,可以熟练地缓解 LPS 物质从阴极区域的溶解和扩散。然而,它们的循环性能受到其在促进有效电子传导方面的固有限制的阻碍。如图6e所示,硫通过有机自由基与聚合物反应,生成硫含量为90wt%的富硫聚合物[122]。所得阴极在0.1°C时的初始放电容量为1100 mAh g −1 ,但在2°C时容量迅速降至400 mAh g −1 以下。同样,对于元素硫与1,3-二炔基苯共聚合成的富硫聚合物材料(如图6f-h所示)[123],观察到较高的初始容量(0.1 C时为1143 mAh g −1 )和较差的速率性能(1 C时为595 mAh g −1 )[123]。

Fig. 6 图6
figure 6

Copyright 2021, Wiley–VCH. c Preparation process of the porous CNTs/SnO2 QDs/S microcapsules; d Binding energies of polysulfides adsorbed on SnO2 and carbon [125]. Copyright 2021, Wiley–VCH. e Diagram of thermal ring-opening of S8 to polymeric sulfur diradicals [122]. Copyright 2013, Nature. f Fabrication of C–S copolymer fabrication; g 1H–NMR spectra and h initial three charge–discharge profiles of C–S copolymer composite [123]. Copyright 2014, The Royal Society of Chemistry
版权所有 2021,Wiley–VCH。c 多孔碳纳米管/SnO 2 QDs/S微胶囊的制备工艺;d 吸附在SnO 2 和碳上的多硫化物的结合能[125]。版权所有 2021,Wiley–VCH。e S 8 对聚合硫二自由基的热开环图[122]。版权所有 2013, Nature.f C-S共聚物的制造;C-S共聚物复合材料的h 1 H–NMR谱图和h初始三个充放电曲线[ 123].版权所有 2014,英国皇家化学学会

a In situ UV–VIS spectra of N–C and CNT/MoS2–Co composite S host; b Illustrates the interaction between CNT/MoS2–Co and LPS [124].
a N-C和CNT/MoS-Co 2 复合S主体的原位紫外-可见光谱;b 说明了CNT/MoS 2 –Co和LPS之间的相互作用[124]。

3.2.3 Physical Confinement and Chemical Anchoring Co-existence for Effective Sulfur or LPS Confinement
3.2.3 物理约束和化学锚固共存,以实现有效的硫或LPS约束

The integration of physical confinement with chemical anchoring/catalysts as a sulfur host is a promising approach. An extraordinary nanostructure consisting of tube-in-tube carbon nanotubes (CNTs), sulfur-deficient molybdenum disulfide (MoS2) embedded with cobalt atom clusters has been developed as an effective regulator of LPS in Li–S batteries (Fig. 6b) [124]. Figure 6a illustrates a unique design that integrates physical confinement, chemical adsorption, and the kinetics of catalytic polysulfide redox reactions in a single package. In a similar manner, as shown in Fig. 6c, a cathode made of polysulfide-confined, porous microcapsules integrated with a composite core of carbon nanotubes, titanium dioxide quantum dots, and sulfur (CNTs/QDs/S) achieved a long life of 700 cycles, a high-sulfur loading of 2.03 mg cm−2, and a CE value of up to 99.9% [125]. The observed results were owing to the efficient adsorption of polar SnO2 quantum dots to LPS, which inhibited the shuttle effect. Moreover, the CNTs provided a fast electron transfer pathway, while the porous shell improved sulfur loading and electrolyte permeability. Additionally, the internal voids successfully adapted to the volumetric change of the sulfur during charging and discharging. (Fig. 6d.)
将物理约束与化学锚定/催化剂作为硫宿主的结合是一种很有前途的方法。已经开发出一种由管中碳纳米管(CNTs)、嵌入钴原子团簇的缺硫二硫化钼(MoS 2 )组成的非凡纳米结构,作为锂硫电池中LPS的有效调节剂(图6b)[124]。图6a展示了一种独特的设计,该设计将物理约束、化学吸附和催化多硫化物氧化还原反应的动力学集成在一个封装中。以类似的方式,如图6c所示,由多硫物限制的多孔微胶囊与碳纳米管、二氧化钛量子点和硫(CNTs/QDs/S)的复合芯集成而成的阴极实现了700次循环的长寿命,高硫负载量为2.03 mg cm,CE −2 值高达99.9% [ 125]。观测结果归因于极性SnO 2 量子点对LPS的有效吸附,抑制了穿梭效应。此外,碳纳米管提供了快速的电子转移途径,而多孔壳改善了硫负载和电解质渗透性。此外,内部空隙成功地适应了充放电过程中硫的体积变化。(图 6d。

Sandwich structures serve as host materials, allowing both physical confinement and chemical anchoring by placing sulfur or its composites between two functional films, typically decorated with catalytically active materials. Functionalized films located at the edges of the sandwich structure enable physical confinement and catalytic conversion of the sulfur and its end-products within the sandwich. The film serves as a three-dimensional catalytic current collector and multifunctional interlayers. For example, by sandwiching sulfur and acetylene black (AB) (S/AB) between two 1 T–MoSe2/CC films (1 T–MoSe2/CC@S/AB@1 T–MoSe2/CC), the excellent catalytic activity and metallic properties of 1 T-MoSe2 can be used to lower the Gibbs free energy barriers for polysulfide conversion. While a single 1 T–MoSe2/CC membrane also inhibits the shuttle effect to some extent, sandwich structure is more suitable for the realization of high-sulfur Li–S batteries with a lean electrolyte. Therefore, the Li–S batteries with 1 T–MoSe2/CC@S/AB@1 T–MoSe2/CC allow a high capacity of 5.43 mAh cm−2 and cycle up to 200 times, which was suitable for lean electrolyte (E/S was 7.8 μL mg−1) and high-sulfur loading (5.7 mg cm−2) applications [126]. Using these strategies described above, sluggish kinetics and the shuttle effect can be overcome even under lean electrolyte conditions.
夹层结构用作主体材料,通过将硫或其复合材料放置在两层功能膜之间,通常用催化活性材料装饰,从而实现物理约束和化学锚定。位于夹层结构边缘的功能化薄膜能够对夹层内的硫及其最终产物进行物理限制和催化转化。该薄膜用作三维催化集流体和多功能中间层。例如,通过将硫和乙炔黑 (AB) (S/AB) 夹在两个 1 T-MoSe 2 /CC 薄膜 (1 T-MoSe 2 /CC@S/AB@1 T-MoSe 2 /CC) 之间,1 T-MoSe 2 的优异催化活性和金属性能可用于降低多硫化物转化的吉布斯自由能势垒。虽然单个1 T-MoSe 2 /CC膜也在一定程度上抑制了穿梭效应,但夹层结构更适合实现具有稀薄电解质的高硫Li-S电池。因此,具有 1 T-MoSe 2 /CC@S/AB@1 T-MoSe 2 /CC 的 Li-S 电池可实现 5.43 mAh cm −2 的高容量和高达 200 次的循环,适用于贫电解质(E/S 为 7.8 μL mg −1 )和高硫负载量 (5.7 mg cm −2 ) 应用 [ 126]。使用上述这些策略,即使在贫电解质条件下也可以克服迟钝的动力学和穿梭效应。

A lower E/S is necessary to utilize the high-energy-density potential of Li–S batteries. Nevertheless, the commonly employed sulfur cathode is typically solid–liquid-solid lithiation process, and to achieve full discharge capacity, a large amount of electrolyte is typically required to completely dissolve the long-chain LPS. In contrast, the solid–solid or quasi-solid-phase conversion pathway eliminates the long-chain LPS generation and significantly reduces the battery's dependence on electrolyte consumption. As a result, sulfur cathodes are more likely to achieve low E/S ratios through solid–solid conversion reactions.
为了利用锂硫电池的高能量密度潜力,需要较低的 E/S。然而,通常采用的硫阴极通常是固-液-固锂化过程,为了达到完全放电能力,通常需要大量的电解质才能完全溶解长链LPS。相比之下,固-固或准固相转换途径消除了长链LPS的产生,并显着降低了电池对电解质消耗的依赖性。因此,硫阴极更有可能通过固-固转化反应实现低 E/S 比。

3.3 Confining LPS in the Shield Layer
3.3 将LPS限制在屏蔽层中

The dissolution of LPS is inevitable in the liquid electrolyte. If the strategies for regulating the cathode are unable to fully avoid dissolving and spreading of LPS, It would spread to the interface of the separator, driven by the concentration gradient. This area between the cathode and the separator is called the shield layer. In this area, engineering cathode electrolyte interphase and constructing functional separators to prevent the shuttle of polysulfide are two potential methods.

3.3.1 Interfacial Protection and Engineering Peculiar Cathode Electrolyte Interphase
3.3.1 界面保护与特殊阴极电解质界面的工程设计

In Li–S batteries, during the first few cycles of electrolyte decomposition, both an SEI and a CEI layers form on the electrodes [127]. Ideally, the SEI and CEI layers should fully develop after a few cycles and become passive, resulting in the formation of thin films created from the by-products of the partially reacted electrolyte solvent and decomposed salt fragments [128]. SPAN cathodes can run for more than 500 cycles in a carbonate-based electrolyte, whereas in an ether electrolyte polysulfide dissolution occurs, leading to rapid capacity degradation (70% capacity loss in 100 cycles) (see Fig. 7a, b). Therefore, it is logical to speculate that the CEI (coating electrolyte interface) can effectively reduce polysulfide release in ether electrolytes, which is a critical factor in mitigating interface-related issues. The use of conformal polycarbonate-CEI derived from cyclic carbonate can significantly reduce the fatal shuttle effect. This safeguarding mechanism ensures the solid-phase mechanism of SPAN [129] (Fig. 7d). Inspired by this, a tailored electrolyte also promotes the formation of a bilayer SEI with improved Li+ ions transport and mechanical strength. This mechanism enables the compatibility of an ultra-thin Li anode [130], achieving high capacity SPAN cathodes (4.08 mAh cm−2).
在锂硫电池中,在电解质分解的最初几个循环中,电极上会形成SEI和CEI层[127]。理想情况下,SEI和CEI层应在几个循环后完全发展并变得钝化,从而形成由部分反应的电解质溶剂和分解盐碎片的副产物形成的薄膜[128]。SPAN阴极在碳酸盐基电解质中可以运行超过500次循环,而在醚电解质中发生多硫化物溶解,导致容量快速下降(100次循环中容量损失70%)(见图7a,b)。因此,可以合理推测CEI(涂层电解质界面)可以有效减少醚电解质中多硫化物的释放,这是缓解界面相关问题的关键因素。使用源自环状碳酸酯的保形聚碳酸酯-CEI可以显着降低致命的穿梭效应。这种保护机制确保了SPAN的固相机制[129](图7d)。受此启发,定制的电解质还促进了双层SEI的形成,改善了锂 + 离子传输和机械强度。这种机制使超薄锂阳极[130]兼容,实现了高容量SPAN阴极(4.08 mAh cm −2 )。

Fig. 7 图7
figure 7

Copyright 2021, American Chemical Society. c Different lithiation mechanism of Se–S cathodes in different electrolytes [131]. Copyright 2019, Wiley–VCH. d Schematic of the structure of CEIs in 1 M LiFSI/DME and 1 M LiFSI/DME-EC [129]. Copyright 2021, American Chemical Society. e Contour plot of NMR signal, and f Areas function of Li+ species consistent with the predicted voltage profile [131]. Copyright 2019, Wiley–VCH
版权所有 2021,美国化学学会。c Se-S阴极在不同电解质中的锂化机理不同[131]。版权所有 2019,Wiley–VCH。d 1 M LiFSI/DME 和 1 M LiFSI/DME-EC 中 CEI 的结构示意图 [ 129].版权所有 2021,美国化学学会。e NMR信号的等值线图,以及与预测的电压曲线一致的Li + 物种的f 面积函数[ 131]。版权所有 2019, Wiley–VCH

a Discharge–charge curves and b cyclic voltammograms (CV) of Li–SPAN cells [129].
a Li-SPAN电池的放电曲线和b循环伏安图(CV)[129]。

Therefore, manipulating the interfacial chemistry of SEI by optimizing the electrolyte and designing the cathode rationally can develop high-performance, high-load Li–S batteries. Indeed, the SEI layer formed acts as a physical barrier, isolating sulfur species and carbonate solvent. This leads to the desolvation of Li+ and prevents solvent molecules from attacking S. Consequently, it prompts the solid-phase lithiation process. Conversely, in the absence of SEI formation, solvated Li+ will readily enter the S-active substance, causing LPS formation and severe shuttle effect. For instance, Se doping S22.2Se/Ketjenblack (KB) cathodes in HFE-based electrolyte (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether replaced DME) formed robust SEI on the KB surface, which avoids active materials and the electrolytes contact, inducing the solid–solid lithiation process [131]. However, DME-based electrolytes do not form an SEI, which leads to the formation of LPS and a lithiation process of solid–liquid-solid in Fig. 7c. Thus, the cell composed of S22.2Se/KB and HFE-based electrolyte have the potential to achieve a higher reversible capacity in long-term and high-rate cycling owing to minimal shuttle effects. In Fig. 7e, f, spatially confined selenium–sulfur cathodes were lithiated conversion from solid–liquid-solid to solid–solid, as confirmed by in situ characterization techniques. Under this mechanism, SEI membranes can be extended to large mesoporous or even macroporous materials that have been deemed unsuitable for sulfur host, which broadens the way for achieving high volumetric energy density batteries.
因此,通过优化电解液和合理设计阴极来操纵SEI的界面化学,可以开发高性能、高负载的锂硫电池。事实上,形成的SEI层起到了物理屏障的作用,隔离了硫物质和碳酸盐溶剂。这导致Li的脱溶剂化 + ,并防止溶剂分子攻击S。因此,它促进了固相锂化过程。相反,在没有SEI形成的情况下,溶剂化的Li + 很容易进入S活性物质,引起LPS的形成和严重的穿梭效应。例如,HFE基电解质(1,1,2,2-四氟乙基-2,2,3,3-四氟丙基醚取代DME)中的Se掺杂S 22.2 Se/Ketjenblack(KB)阴极在KB表面形成稳健的SEI,避免了活性材料与电解质的接触,诱导了固-固锂化过程[131]。然而,基于DME的电解质不会形成SEI,这导致LPS的形成和图7c中固-液-固的锂化过程。因此,由S 22.2 Se/KB和HFE基电解质组成的电池具有在长期和高速率循环中实现更高可逆容量的潜力,因为穿梭效应最小。在图7e,f中,空间限制的硒-硫阴极从固-液-固到固-固的锂化转化,通过原位表征技术证实了这一点。在这种机制下,SEI膜可以扩展到被认为不适合硫主体的大型介孔甚至大孔材料,这拓宽了实现高体积能量密度电池的途径。

To mitigate the shuttle effect of LPS, separators function similarly to fences, impeding the transfer of LPS. In this regard, the long-chain LPS are either fixed by physical barrier and chemical adsorption on the coating material of the modified layer or pushed by electrostatic repulsion toward the cathode side.

3.3.2 Implementing Electric Repulsion Strategies for LPS Confinement
3.3.2 实施LPS约束的电排斥策略

Two molecules with the same type of charge will repel each other electrostatically. In concrete terms, decorating the separator with negatively charged ions or groups can prevent negatively charged Sx2− ions of LPS from crossing the separator due to repulsive forces, while still not affecting Li+ transport. Recently, a sulfonate-rich covalent-organic framework (COF) (SCOF-2) has been used to modify the separator, in which both the soluble polysulfide (polysulfide molecules and polysulfide anions) and the designed SCOF-2 possess strong electronegative properties, repelling polysulfide anions through electrostatic interaction and absorbing polysulfide molecules at the same time (Fig. 8a) [132]. Additionally, SCOF-2 has large layer spacing, which promoted the migration of lithium ions and slowed down the formation of lithium dendrites (Fig. 8b). Similarly, a multifunctional graphene–sodium lignosulfonate (SL) composite membrane (rGO@SL/PP) with a large number of negatively charged sulfonic groups has been designed for inhibiting LPS shuttling and achieving uniform transport of lithium ions (Fig. 8c) [133]. The rGO@SL/PP hindered the transfer of electronegative polysulfide ions by charge interaction without affecting the transport of Li+. Thus, the charge repulsion effect of rGO@SL/PP with abundant sulfonate groups strongly suppresses the LPS shuttle, while maintaining uniform transport of Li-ions, which has resulted in highly robust Li–S batteries that exhibit stable cycling performance over 1000 times, even at a high current density of 5 mA cm–2 (Fig. 8d).
具有相同类型电荷的两个分子会以静电方式相互排斥。具体来说,用带负电荷的离子或基团装饰隔膜可以防止LPS带负电荷的S x 2− 离子因排斥力而穿过隔膜,同时仍然不影响Li + 的输运。最近,富磺酸共价有机框架(COF)(SCOF-2)被用于改性隔膜,其中可溶性多硫化物(多硫化物分子和多硫化物阴离子)和设计的SCOF-2都具有很强的电负性,通过静电相互作用排斥多硫化物阴离子,同时吸收多硫化物分子(图8a)[132]。此外,SCOF-2具有较大的层间距,这促进了锂离子的迁移并减缓了锂树突的形成(图8b)。同样,设计了一种具有大量带负电荷磺酸基团的多功能石墨烯-木质素磺酸钠(SL)复合膜(rGO@SL/PP),用于抑制LPS穿梭并实现锂离子的均匀传输(图8c)[133]。rGO@SL/PP通过电荷相互作用阻碍了电负性多硫离子的转移 + ,而不影响Li的输运。因此,具有丰富磺酸盐基团的 rGO@SL/PP 的电荷排斥效应强烈抑制了 LPS 穿梭,同时保持了锂离子的均匀传输,这导致了高度坚固的 Li-S 电池,即使在 5 mA cm –2 的高电流密度下,也能表现出超过 1000 次的稳定循环性能(图 8d)。

Fig. 8 图8
figure 8

Copyright 2018, Cell Press. e Effect of SEI in shielding LPS of lithium anodes with and without BTB additive [134]. Copyright 2020, Wiley–VCH. f, g Illustration of the role and coating process of LiPON-coated Li metal anode; h Optical images of Li metal foil coated and uncoated with LiPON after soaking in 1 M sulfur/DME solution for 7 days [135]. Copyright 2019, Elsevier
版权所有 2018, Cell Press. e SEI在含BTB添加剂和不含BTB添加剂的锂阳极的LPS屏蔽中的影响[ 134].版权所有 2020,Wiley–VCH。f、g LiPON涂层锂金属阳极的作用及涂覆工艺图示;h Li金属箔在1 M硫/DME溶液中浸泡7天后涂覆和未涂覆LiPON的光学图像[135]。版权所有 2019, Elsevier

a Preparation process of the sulfonated COFs; b Graphic comparison of the batteries with different separators [132]; Copyright 2021, Wiley–VCH. c Flexibility test and d Schematic diagram for inhibiting LPS shuttling effect of the rGO@SL/PP [133].
a 磺化COFs的制备工艺;b 不同隔膜电池的图形比较[ 132];版权所有 2021,Wiley–VCH。c柔韧性试验和抑制LPS rGO@SL/PP穿梭效应的示意图[133]。

3.4 Preventing LPS from Contacting the Anode
3.4 防止LPS接触阳极

Avoiding contact of polysulfides with the anode would be a last resort strategy if the shuttling effect cannot be completely prevented by the aforementioned strategies. The shuttled long-chain polysulfide will directly react with lithium metal to form low-order polysulfide, which results in the reduction in active material and deterioration of capacity. Furthermore, the by-products generated by the reaction of polysulfide and lithium are deposited continuously on the surface of the lithium metal anode, which increases the impedance of the Li anode/electrolyte interface and reduces the interface ion transport efficiency. The side reactions among large households, LPS and electrolytes will also produce gas by-products, mainly H2 and CH4, which are easily confined in the porous lithium deposits, resulting in the increased internal pressure of lithium metal. Thus, constructing the last line of defense (SEI layer) on the lithium metal anode side is an effective method for avoiding reaction of polysulfide in direct contact with lithium metal and deposition of product Li2S2/Li2S on the surface of lithium metal.
如果上述策略不能完全防止穿梭效应,则避免多硫化物与阳极接触将是最后的手段。穿梭的长链多硫化物会直接与锂金属反应生成低阶多硫化物,导致活性物质减少,产能变差。此外,多硫化物与锂反应产生的副产物不断沉积在锂金属阳极表面,增加了锂阳极/电解质界面的阻抗,降低了界面离子传输效率。大户的副反应,LPS和电解质也会产生气体副产物,主要是H 2 和CH 4 ,这些副产物很容易被限制在多孔锂沉积物中,导致金属锂的内压增加。因此,在锂金属负极侧构建最后一道防线(SEI层)是避免多硫化物与锂金属直接接触反应和产物Li 2 S 2 /Li 2 S沉积在锂金属表面的有效方法。

3.4.1 Construction of SEI Films for Effective Anode Protection and LPS Prevention
3.4.1 构建用于有效阳极保护和LPS预防的SEI薄膜

Tailoring liquid electrolytes to construct a thin, strong and stable SEI film on the surface of the lithium anode is an efficient measure to avoid corrosion of lithium anode by LPS, and thus inhibit the side reaction. For instance, an organosulfur-containing SEI was tailored by employing 3,5-bis(trifluoromethyl)thiophenol (BTB) additive for shielding of Li metal from the soluble LPS corrosion [134]. In Fig. 8e, the lithium metal undergoes a reaction with the active sulfhydryl group present in the BTB additive resulting in the formation of a Ph–S component, which forms an electrostatic repulsion with the polysulfide anion. As a result, the organosulfur-containing SEI can decrease the depletion of fresh lithium and electrolyte by avoiding side reactions between Li metal and LPS. So as to deliver an initial areal capacity of 4.0 mAh cm–2 (950 mAh g–1) and keep 3.0 mAh cm–2 (700 mAh g–1) after 82 cycles at 0.1 C.
定制液体电解质,在锂阳极表面构建薄、强、稳定的SEI膜,是避免LPS腐蚀锂阳极,从而抑制副反应的有效措施。例如,通过采用3,5-双(三氟甲基)硫基苯酚(BTB)添加剂来定制含有机硫的SEI,以保护Li金属免受可溶性LPS腐蚀[134]。在图8e中,锂金属与BTB添加剂中存在的活性巯基发生反应,形成Ph-S组分,该组分与多硫化物阴离子形成静电排斥。因此,含有机硫的SEI可以通过避免锂金属和LPS之间的副反应来减少新鲜锂和电解质的消耗。因此,在0.1°C下循环82次后,初始面容量为4.0 mAh cm –2 (950 mAh g –1 ),并保持3.0 mAh cm –2 (700 mAh g –1 )。

In addition, another successful way involves coating Li metal anode with carbon-based interlayers [136], creating a solid electrolyte protection layer [137] and adjusting the composition of the electrolyte to prevent the dissolution of LPS. The protective layer consisting of a dense and average lithium phosphorus oxynitride (LiPON) has been prepared by nitrogen plasma-assisted electron beam evaporative deposition method (Fig. 8f) [135]. As a protective layer with high ionic conductivity, chemical stability and mechanical strength, LiPON can effectively prevent corrosion reactions between lithium metal and organic electrolytes. It also promotes the average deposition or dissolution of lithium metal (Fig. 8g, h). This results in a stable cycle life of lithium metal symmetrical battery at a current density of 3 mA cm–2 for over 900 cycles without any lithium metal dendrite formation. Moreover, the LiPON-coated lithium metal as the anode can also be used to prepare~300 Wh kg–1 high-performance Li–S pouch cell.
此外,另一种成功的方法是在锂金属阳极上涂上碳基中间膜[136],形成固体电解质保护层[137],并调整电解质的组成以防止LPS的溶解。采用氮等离子体辅助电子束蒸发沉积法制备了由致密且平均氮化磷锂(LiPON)组成的保护层(图8f)[135]。LiPON作为一种具有高离子电导率、化学稳定性和机械强度的保护层,可以有效防止锂金属与有机电解质之间的腐蚀反应。它还促进了锂金属的平均沉积或溶解(图8g,h)。这导致锂金属对称电池在电流密度为 3 mA cm –2 时具有稳定的循环寿命,持续 900 多次循环,而不会形成任何锂金属枝晶。此外,LiPON涂层的锂金属作为阳极也可用于制备~300 Wh kg –1 高性能Li-S软包电池。

4 Concrete Strategies to Inhibit the Shuttle Effect in Li–S Batteries

The infamous shuttle effect and slow kinetics have long hindered the practical application of Li–S batteries. Since Li–S batteries are secondary batteries with multi-step reactions, the shuttle effect and slow kinetics affect all parts of the battery components. Researchers have applied various strategies to different components of the battery including designing carbon matrices at the nanoscale [28, 84, 138], using metal oxides [139,140,141]/chalcogenides [141,142,143]/nitride as interlayers or hosts, among others. And the following strategies are categorized according to Sect. 3 to avoid the shuttle effect at different cell components for Li–S batteries.
臭名昭著的穿梭效应和缓慢的动力学长期以来一直阻碍着锂硫电池的实际应用。由于锂硫电池是具有多步反应的二次电池,因此穿梭效应和慢动力学会影响电池组件的所有部分。研究人员将各种策略应用于电池的不同组件,包括设计纳米级碳基质[28,84,138],使用金属氧化物[139,140,141]/硫族化合物[141,142,143]/氮化物作为中间层或主体等。根据第 3 节对以下策略进行分类,以避免 Li-S 电池不同电池组件的穿梭效应。

4.1 Rational Construction of Sulfur Cathodes
4.1 硫阴极的合理构造

Sulfur cathode is a vital element in Li–S batteries for it performs a key function by releasing capacity, increasing energy density, and improving cycle life. Prevention of diffusion of soluble polysulfides is the primary approach for suppressing the shuttle effect. Various sulfur cathode materials with specific properties have been designed to inhibit the shuttle of LPS. As described in Sect. 3, sulfur cathodes can inhibit the shuttle effect by spatially confining and chemically anchoring LPS, using catalysts to accelerate the reaction kinetics for effectively enhancing battery performance.

4.1.1 Short-chain Sulfur Cathodes
4.1.1 短链硫阴极

To prevent the initiation of the shuttle effect, which involves the production and degradation of LPS, one possible strategy is to avoid the formation of soluble long-chain polysulfides in the cathode. This can be achieved by physical confinement of small sulfur molecules (S2−4) in micro-compartments or by chemical attachment of short-chain sulfur species to the polymer backbone through covalent bonding. Short-chain sulfur polymers can be formed when there are unsaturated bonds or dehydrogenating with sulfur to release hydrogen sulfide (H2S) during the pyrolysis, which is similar to other organosulfur compounds like SPAN, SPANI and sulfurized aminophenol–formaldehyde resin (SAF). However, the low conductivity and ionic conductivity of organosulfur compounds impede the kinetics of the SRR, leading to poor rate performance. Novel sulfated polypyrrole (SPPy) compounds were introduced based on pyrolysis and dehydrogenation behaviors [80], in which short-chain sulfur was successfully added to the backbone of SPPy distinct from the pristine S8 form in conventional sulfided polypyrrole (PPy@S) blends (Fig. 9a). The material has both a solid–solid transition mechanism and superior lithium ion and charge transfer kinetics [144]. Therefore, the samples obtained at 320 °C (SPPy320V) exhibited initial capacity of 803 mAh g−1 at a high rate of 2 C, and the decay rate was 0.022% per cycle during 700 cycles (Fig. 9b).
为了防止涉及LPS产生和降解的穿梭效应的发生,一种可能的策略是避免在阴极中形成可溶性长链多硫化物。这可以通过将小硫分子(S 2−4 )物理限制在微区室中或通过共价键将短链硫物种化学附着到聚合物主链上来实现。当存在不饱和键或在热解过程中与硫脱氢释放硫化氢(H 2 S)时,可以形成短链硫聚合物,这与SPAN、SPANI和硫化氨基苯酚-甲醛树脂(SAF)等其他有机硫化合物相似。然而,有机硫化合物的低电导率和离子电导率阻碍了SRR的动力学,导致速率性能不佳。基于热解和脱氢行为,引入了新型硫酸化聚吡咯(SPPy)化合物[80],其中短链硫被成功地添加到SPPy的骨架中,这与传统硫化聚吡咯(PPy@S)共混物中的原始S 8 形式不同(图9a)。该材料具有固-固转变机制和优异的锂离子和电荷转移动力学[144]。因此,在320 °C(SPPy320V)下获得的样品在2 C的高速率下表现出803 mAh g −1 的初始容量,在700次循环中,衰变率为0.022%/循环(图9b)。

Fig. 9 图9
figure 9

Copyright 2023, Elsevier. c Synthetic process and the reaction mechanism of the S/CFS cathode in the VC-ether co-solvent electrolyte d The electrochemical performance of the S/CFS cathode [145]. Copyright 2023, Spring Link. e The plateau curves and f Nyquist plot of the impedance spectra (sulfur content = 70 wt%). g Schematic illustrations of CEI with 70 wt% and 50 wt% sulfur content. h The electrochemical performance with appropriate sulfur content [146]. Copyright 2022, Wiley–VCH
版权所有 2023,爱思唯尔。c S/C FS 阴极在VC-醚助溶剂电解液中的合成过程及反应机理 d S/C FS 阴极的电化学性能[145]。版权所有 2023,Spring Link。e 阻抗谱的平台曲线和 f 奈奎斯特图(硫含量 = 70 wt%)。g 硫含量为 70 wt% 和 50 wt% 的 CEI 示意图。h 硫含量适宜的电化学性能[146]。版权所有 2022, Wiley–VCH

a Preparation process of sulfurized polypyrrole. b CV curves and discharging/charging curves of rate performance the SPPy320V cathode [144].
a 硫化聚吡咯的制备工艺。b SPPy320V阴极的CV曲线和放电/充电曲线[144]。

4.1.2 Interfacial Interaction in Sulfur Cathode
4.1.2 硫阴极中的界面相互作用

Another strategy is to use the electrode design in conjunction with electrolyte modulation for forming a dense SEI film on the surface of cathode by appropriate nucleophilic reaction of LPS with the electrolyte at the early stage of discharge. This research suggests that using edible fungal sludge-derived porous carbon (CFS), paired with vinyl carbonate (VC) as a co-solvent for the ether-based electrolyte, could be an effective strategy in producing a protective layer onto the surface of the S/CFS composites in situ. This protective layer could isolate the internal sulfur from the external electrolyte, inhibiting any further generation of soluble LPS (Fig. 9c). This enables the system to function in a solid–solid conversion mode, resulting in a high reversible capacity of 1557 mAh g-1 along with 99.9% high cycling efficiency over 500 cycles (Fig. 9d) [145].
另一种策略是将电极设计与电解质调制相结合,通过LPS在放电早期与电解质的适当亲核反应,在阴极表面形成致密的SEI膜。本研究表明,使用可食用真菌污泥衍生的多孔碳(C FS )与碳酸乙烯酯(VC)作为醚基电解质的助溶剂,可能是在S/C FS 复合材料表面原位产生保护层的有效策略。该保护层可以将内部硫与外部电解质隔离开来,从而抑制任何进一步产生的可溶性LPS(图9c)。这使得该系统能够在固-固转换模式下运行,从而产生1557 mAh g -1 的高可逆容量,并在500次循环中具有99.9%的高循环效率(图9d)[145]。

The CEI produced onto the sulfur cathode surface plays a prominent role in the solid-phase conversion in Li–S batteries, which can effectively prevent the dissolution of LPS. Figure 9e shows the charging-discharging curve for a cathode with 70 wt% sulfur. During the first discharge, the newly generated LPS underwent a 2.25 V nucleophilic reaction with the limited carbonate solvent, and the reaction products participated in the formation of CEI (Fig. 9e). The corresponding electrochemical impedance spectroscopy (EIS) curve shows that CEI is present in subsequent operations of the battery (Fig. 9f). However, excess sulfur (70 wt%) can cause the formed CEI to crack due to large volume changes caused by repeated reactions during cycling (Fig. 9g-a), which triggers continued decomposition of the electrolyte and nucleophilic reactions between the LPS and the carbonate solvent [147, 148]. This will result in the formation of thick CEI, thus reducing the cycle life. In contrast, the volume of the reduction product (Li2S) under the appropriate sulfur content does not exceed the host volume (Fig. 9g-b), and the biphasic conversion reaction between solid phases based on the CEI strategy has the advantage of prolonging the cell life [146]. Thus, if the sulfur content is sufficient (50 wt%), the assembled battery (the sulfur host used was CMK-3, which has a single pore-size distribution centered among 2.5–4.3 nm) achieves an initial capacity of 819 at 1 C and after 2000 cycles maintains a capacity of 445 with a attenuation rate of only 0.03% (Fig. 9h). Moreover, this strategy for CEI demonstrates that the battery could operate at lean E/S conditions. At lean electrolyte (E/S = 3 µL mg−1) and sulfur content of 60 wt%, the cell provided a high initial areal capacity of 7.4 mAh cm−2 under sulfur loading of 4.3 mg cm−2.
硫正极表面产生的CEI在锂硫电池的固相转化中起着突出的作用,可以有效防止LPS的溶解。图9e显示了硫含量为70 wt%的阴极的充放电曲线。在第一次放电时,新生成的LPS与有限的碳酸盐溶剂进行了2.25 V的亲核反应,反应产物参与了CEI的形成(图9e)。相应的电化学阻抗谱(EIS)曲线显示,CEI存在于电池的后续操作中(图9f)。然而,过量的硫(70 wt%)会导致形成的CEI开裂,这是由于循环过程中重复反应引起的大量体积变化(图9g-a),从而引发电解质的持续分解和LPS与碳酸盐溶剂之间的亲核反应[147,148]。这将导致厚CEI的形成,从而缩短循环寿命。相反,在适当的硫含量下,还原产物(Li 2 S)的体积不超过主体体积(图9g-b),并且基于CEI策略的固相间双相转化反应具有延长细胞寿命的优势[146]。因此,如果硫含量足够(50 wt%),组装好的电池(使用的硫主体是CMK-3,其孔径分布集中在2.5-4.3 nm之间)在1 C时达到819的初始容量,在2000次循环后保持445的容量,衰减率仅为0.03%(图9h)。此外,CEI的这一策略表明,电池可以在稀薄的E/S条件下运行。 在贫电解质 (E/S = 3 μL mg −1 ) 和硫含量为 60 wt% 时,在 4.3 mg cm −2 的硫负载下,电池提供了 7.4 mAh cm −2 的高初始面容量。

4.1.3 Pysical-chemical Confinement of Sulfur Cathode
4.1.3 硫正极的硫化约束

The integration of multiple multidimensional nanostructured materials as excellent hosts for sulfur is a promising strategy. Recently, a synergistic interface bonding enhancement strategy has been enabled by designing a novel sulfur cathode has been developed in a flexible fiber-shape composite form, where using a simple microfluidic assembly technique, uniformly distributed mono-disperse nanospheres (~ 500 nm) of polypyrrole@sulfur (PPy@S) were implanted into the internal cavities of self-assembled reduced graphene oxide fibers (rGOFs). (Fig. 10a) [149]. Notably, in this flexible core–shell structure, both sulfur nanospheres and LPS are confined to the carbon interface (rGOFs) and the polymer interface (PPy) because of the enhanced interfacial chemical bonding which endows the excellent adsorption ability. Interestingly, by wrapping a controllably prepared GO sheet around the outer layer of hollow mesoporous spheres with sulfur (HMCS/S), the advantages of their respective structures can be integrated. The HMCS/S@GO electrode exhibited an initial discharge capacity of 1054 mAh g−1 at 0.5 C, its capacity retention of 60.2% after 100 cycles is higher than that of the HMCS/S electrode which is 54.7%. The GO layer acts as an additional physical barrier and chemical trap for polysulfide intermediates, which in turn reduces charge/discharge shuttling and improves conversion kinetics [100]. Novel multifunctional LSB cathode hosts were used, which utilized bronze TiO2 nanosheets (TiO2–B) to firmly anchor LPS and promote its rapid redox transformation. TiO2-B has a strong chemical affinity for polysulfides because of its exposed (100) surfaces and Ti3+ ions, which effectively restrict LPS to its surface. The combined cathode has better electronic conductivity. This is due to Ti3+ ions and interfacial coupling with carbon, which enhance redox conversion kinetics. Thus, the TiO2–B/S cathode showed a high capacity of 1165 mAh g−1 at 0.2 C, outstanding rate efficiency of 244 mAh g−1 at 5 C [98].
整合多种多维纳米结构材料作为硫的优良宿主是一种很有前途的策略。最近,通过设计一种新型硫阴极,开发了一种柔性纤维形状的复合材料形式,其中使用简单的微流控组装技术,将均匀分布的单分散纳米球(~500 nm)的polypyrrole@sulfur(PPy@S)植入自组装还原氧化石墨烯纤维(rGOFs)的内腔中,从而实现了协同界面键合增强策略。(图 10a)[ 149]. 值得注意的是,在这种柔性核壳结构中,硫纳米球和LPS都局限于碳界面(rGOFs)和聚合物界面(PPy),因为增强了界面化学键合,赋予了优异的吸附能力。有趣的是,通过用硫(HMCS/S)将可控制备的GO片包裹在空心介孔球的外层上,可以整合它们各自结构的优点。HMCS/S@GO电极在0.5 C时的初始放电容量为1054 mAh g −1 ,循环100次后容量保持率为60.2%,高于HMCS/S电极的54.7%。GO层作为多硫化物中间体的额外物理屏障和化学捕集器,从而减少充放电穿梭并改善转化动力学[100]。采用新型多功能LSB正极主体,利用青铜TiO 2 纳米片(TiO 2 –B)牢固地锚定LPS,促进其快速氧化还原转化。TiO-B 2 对多硫化物具有很强的化学亲和力,因为它暴露在外(100)表面和钛 3+ 离子,有效地限制了LPS在其表面。 组合阴极具有更好的电子导电性。这是由于钛 3+ 离子和与碳的界面耦合,增强了氧化还原转化动力学。因此,TiO 2 –B/S阴极在0.2 C时表现出1165 mAh g −1 的高容量,在5 C时表现出244 mAh g −1 的出色速率效率[ 98]。

Fig. 10 图例 10
figure 10

Copyright 2022, Wiley–VCH. b Schematic illustration and c, d electrochemical performance of the CC-ZnO@Li||CC-NC-Co@S full battery [150]. Copyright 2023, American Chemical Society. e Schematic illustration of the preparation of COF-MF; f Structural features and g rate capability of COF-MF and COF-CS [151]. Copyright 2019, Elsevier
版权所有 2022,Wiley–VCH。b示意图和c、dCC-ZnO@Li||CC-NC-Co@S 充满电 [ 150]。版权所有 2023,美国化学学会。e COF-MF制备示意图;f COF-MF和COF-CS的结构特征和重力能力[151]。版权所有 2019, Elsevier

a Interface illustration of PPy@S/rGOFs [149].
a PPy@S/rGOFs的界面图示[ 149]。

4.1.4 Heterojunction Sulfur Cathode
4.1.4 异质结硫阴极

The current mainstream strategy for addressing cathode challenges involves the development of multifunctional cathode hosts utilizing physical confinement, chemical anchoring, and prominent electrocatalytic properties. On carbon cloth (CC), two MOFs based on Zn and Co were synthesized: CC–Co–ZIF–L (as a host for S with Co nanoparticles incorporated within the carbon backbone (CC–NC–Co)) and CC–Zn–ZIF–L (as host for lithium metal with lithiophilic ZnO arrays (CC–ZnO)), respectively (Fig. 10b) [150]. On the cathode side, the presence of C nanosheet skeleton enables the confinement of LPS and the enhanced polarization owing to Co nanoparticles embedding further accelerates the redox kinetics of LPS. Thus, in Fig. 10c, d, Li–S half-batteries with CC–NC–Co@S cathodes delivered outstanding rate capability (746 mAh g−1 at 4 C) and long-term stable circulation (capacity retention of 90.8% after 500 cycles). Moreover, the full cells with CC–NC–Co@S cathode and CC–ZnO@Li anode have exhibited exceptional rate capability (793 mAh g−1 at 4 C) and impressive long-term stabilities (per-cycle capacity degradation 0.02% when at 0.5 C cycling 900 times).
目前应对阴极挑战的主流策略涉及利用物理约束、化学锚定和突出的电催化性能开发多功能阴极主机。在碳布(CC)上,合成了两种基于Zn和Co的MOF:CC-Co-ZIF-L(作为碳主链中掺入Co纳米颗粒的S的宿主(CC-NC-Co))和CC-Zn-ZIF-L(作为具有亲石性ZnO阵列(CC-ZnO)的锂金属的宿主)(图10b)[150]。在阴极侧,C纳米片骨架的存在使LPS受到限制,并且由于Co纳米颗粒嵌入而增强的极化进一步加速了LPS的氧化还原动力学。因此,在图10c,d中,采用CC-NC-Co@S阴极的Li-S半电池具有出色的速率能力(4°C时为746 mAh g −1 )和长期稳定的循环(500次循环后容量保持率为90.8%)。此外,具有 CC-NC-Co@S 阴极和 CC-ZnO@Li 阳极的全电池表现出出色的倍率能力(4 C 时为 793 mAh g −1 )和令人印象深刻的长期稳定性(0.5 C 时每周期容量下降 0.02%,循环 900 次)。

4.1.5 Polymer-based Sulfur Cathode
4.1.5 聚合物基硫阴极

Covalent organic frameworks (COFs) are commonly used as hosts with sulfur-redox confinement to enable varied states during cycling of highly efficient Li–S batteries. 3D hierarchical flower superstructures (COF-MF) containing porphyrin-rich conjugated ultra-thin nanosheets were firstly bottom-up synthesized as a multi-scale engineering solution to fully demonstrate the potential of COF in Li–S batteries (Fig. 10e) [151]. With minimal nanosheet stacking, unique macro–meso–micro porosity, and large accessible specific surface area, COF-MF not only transforms COF from conventional diffusion-dominated redox kinetics to a charge transfer-controlled process, but also fully exposed porphyrin then serves as a unique anchoring site to maximize the chemisorption of polysulfides and improve sulfur utilization (Fig. 10f). Therefore, the COF-MF, as a polymer host, endowed Li–S batteries excellent ultra-stable cycles (0.047% ultralow decay rate over 1000 cycles at 1 C) and appealing areal capacity (4.78 mAh cm−2 at a sulfur loading of 4.1 mg cm−2), much superior to the bulk COF counterpart (attenuation of 0.13% over 400 cycles at 1 C) (Fig. 10g).
共价有机框架 (COF) 通常用作硫氧化还原限制的主体,以在高效锂硫电池的循环过程中实现不同的状态。首先自下而上合成了含有富含卟啉的共轭超薄纳米片的三维多级花超结构(COF-MF),作为多尺度工程解决方案,充分展示了COF在Li-S电池中的潜力(图10e)[151]。COF-MF具有最小的纳米片堆积、独特的宏观-中观-微孔隙率和较大的可及比表面积,不仅将COF从传统的扩散主导的氧化还原动力学转变为电荷转移控制过程,而且完全暴露的卟啉随后作为独特的锚定位点,以最大限度地提高多硫化物的化学吸收并提高硫的利用率(图10f)。因此,COF-MF作为聚合物主体,赋予了Li-S电池出色的超稳定循环(在1°C下1000次循环中衰减率为0.047%)和吸引人的面容量(4.78 mAh cm −2 ,硫含量为4.1 mg cm −2 ),远优于块状COF电池(在1°C下400次循环衰减0.13%)(图10g)。

4.1.6 Catalyst-enhanced Sulfur Cathode
4.1.6 催化剂增强硫阴极

Furthermore, single-atom catalysts (SAC) offer significant potential for catalyzing the polysulfide conversion reaction kinetic owing to their maximum atom utilization efficiency (≈100%) and unique catalytic properties [152]. A sulfur host has been developed in the form of a cobalt single-atom catalyst supported on heteroatom (O, N, S) codoped carbon (SACo@HC) with unique CoN3S-active moiety [153]. The SACo@HC is comprised of sulfiphilic and numerous lithiophilic active sites that form Li–O, Li–N, Li–S, Co–S bonds, which can efficiently facilitate the adsorption of LPS (Fig. 11a). As shown in Fig. 11b, in the cyclic voltammograms (CV), symmetrical cells with HC and SACo@HC exhibit eight redox peaks representing four steps during LPS conversion process (S8 ↔ Li2S8 ↔ Li2S6 ↔ Li2S4 ↔ Li2S) [154]. However, the SACo@HC showed higher peak current density, indicating that during the process of solid–liquid transform, SACo@HC displayed greater catalytic activity. As displayed in Fig. 11c, the SACo@HC exhibited lower reduction potentials for LPS compared to HC, demonstrating that atomically dispersed cobalt centers (CoN3S) can encourage the conversion of LPS with faster kinetics and lower polarization. A high capacity of 1425.1 mAh g−1 at 0.05 C and an excellent rate performance of 745.9 mAh g−1 at 4 C were obtained for the S-SACo@HC composite with 80 wt% sulfur loading. In addition, Ni single atoms supported on N-rich mesoporous carbon (Ni-NC(p)) can also act as sulfur host for Li–S batteries [155]. The unique architecture design, N-atom doping and Ni single-atom catalyst synergistically achieved physical confinement, chemical adsorption and catalytic transformation, which suppressed the shuttle effect and accelerated the redox kinetics of LPS. Therefore, the Ni–NC(p)/S delivered an average discharge capacity of 778.1 mAh g−1 at 1 C.
此外,单原子催化剂(SAC)具有最高的原子利用效率(≈100%)和独特的催化性能,在催化多硫化物转化反应动力学方面具有巨大的潜力[152]。已经开发出一种以钴单原子催化剂的形式存在,该催化剂负载在具有独特CoN 3 S活性部分的杂原子(O,N,S)共掺杂碳(SACo@HC)上[153]。该SACo@HC由硫化和许多亲石活性位点组成,形成Li-O、Li-N、Li-S、Co-S键,可有效促进LPS的吸附(图11a)。如图11b所示,在循环伏安图(CV)中,具有HC和SACo@HC的对称电池在LPS转换过程中表现出8个氧化还原峰,代表4个步骤(S 8 ↔ Li 2 S 8 ↔ Li 2 S Li 2 S 46 ↔ Li S 2 )[154]。然而,该SACo@HC表现出较高的峰值电流密度,表明在固液转化过程中,SACo@HC表现出更大的催化活性。如图11c所示,与HC相比,SACo@HC对LPS表现出更低的还原电位,表明原子分散的钴中心(CoN 3 S)可以促进LPS的转化,具有更快的动力学和更低的极化。硫含量为80 wt%的S-SACo@HC复合材料在0.05 C时具有1425.1 mAh g −1 的高容量,在4 C时具有745.9 mAh g −1 的优异倍率性能。此外,负载在富氮介孔碳(Ni-NC(p))上的Ni单原子也可以作为Li-S电池的硫主体[155]。 独特的结构设计,N原子掺杂和Ni单原子催化剂协同实现了物理约束、化学吸附和催化转化,抑制了LPS的穿梭效应,加速了LPS的氧化还原动力学。因此,Ni-NC(p)/S 在 1 C 时的平均放电容量为 778.1 mAh g −1

Fig. 11 图 11
figure 11

Copyright 2022, Wiley–VCH. d Schematic illustration of the design inspiration of VC@INFeD molecular catalyst [156]. Copyright 2022, Wiley–VCH. e Synthesis and f rate capabilities in PVFH-TOC-PEG electrolyte [157]. Copyright 2021, The Royal Society of Chemistry
版权所有 2022,Wiley–VCH。d VC@INFeD分子催化剂的设计灵感示意图[ 156].版权所有 2022,Wiley–VCH。e PVFH-TOC-PEG电解质的合成和f速率能力[157]。版权所有 2021,英国皇家化学学会

a UV–VIS spectra of Li2S6 solutions after interactions with HC and SACo@HC; b CV curves of the symmetric cells in electrolyte with 0.5 mol L−1 Li2S6 and without Li2S6 at a scanning rate of 1 mV s−1; c Free energy of LPS on HC and SACo@HC substrates [153].
与HC和SACo@HC相互作用后Li 2 S 6 溶液的紫外-可见光谱;b 在扫描速率为 1 mV s 时,具有 0.5 mol L −1 Li 2 S 6 和不含 Li 2 S 6 的电解质中对称电池的 CV 曲线; −1 c LPS在HC和SACo@HC基底上的自由能[153]。

4.1.7 Desolvated Sulfur Cathode
4.1.7 脱溶剂硫阴极

Although various catalytic materials (e.g., heterojunctions, heteroatom (N, O, S)-doped carbon, and single atoms catalysts (SACs)) have been simply combined to overcome their respective weaknesses in stepwise SRR, the lack of interfacial connectivity and charge transfer between compounds are still the limiting factors in improving the kinetics of electrode reactions [77, 158]. Thus, designing an electronic reservoir that can release and accept electrons from sulfur species during discharging and charging can efficiently activate stepwise and reduce the activation energy, thus providing an ideal solution for smooth and sustainable catalyzing SRR for Li–S batteries [159]. As a result, a highly efficient VC@INFeD catalyst incorporated on the sulfur cathode with assistance of multiple H/Li-bonds has been developed at the cathode/electrolyte interface. VC@INFeD is capable of capturing dissolved LPS clusters present at the cathode/electrolyte interface through H-bonds, resulting in a local high-concentration distribution. With the assistance of Li-bonds and Fe2+/Fe3+ activity centers, the captured LPS clusters are rapidly transferred and efficiently converted at the gradient catalytic site. In particular, the two components, INFeD and VC, are catalytic for long-chain and short-chain polysulfides, respectively (Fig. 11d). Therefore, even when subjected to lean electrolyte (approximately 7 µL mg−1) and high-sulfur loading (5.2 mg cm−2), VC@INFeD significantly reduced energy barrier for each step of the redox process, suppressing the shuttle effect, and imparting a high utilization of sulfur and excellent cycling stability (441 mAh g−1) in Li–S cells [156].
尽管各种催化材料(如异质结、杂原子(N、O、S)掺杂碳和单原子催化剂(SACs))已被简单地组合以克服它们在逐步SRR中的弱点,但缺乏界面连接和化合物之间的电荷转移仍然是提高电极反应动力学的限制因素[77,158]。因此,设计一种在放电和充电过程中能够释放和接受硫物质中电子的电子的电子储层,可以有效地逐步激活并降低活化能,从而为Li-S电池平稳、可持续地催化SRR提供了理想的解决方案[159]。因此,在正极/电解质界面上开发了一种在多个H/Li键的帮助下掺入硫阴极的高效VC@INFeD催化剂。VC@INFeD能够通过氢键捕获存在于阴极/电解质界面上的溶解LPS团簇,从而产生局部高浓度分布。在锂键和Fe 2+ /Fe 3+ 活性中心的协助下,捕获的LPS团簇在梯度催化位点快速转移和有效转化。特别是,INFeD 和 VC 这两种组分分别催化长链和短链多硫化物(图 11d)。因此,即使经受稀薄电解质(约7 μL mg −1 )和高硫负荷(5.2 mg cm −2 )的影响,氧化还原过程的每个步骤VC@INFeD显著降低了能量势垒,抑制了穿梭效应,并在Li-S电池中赋予了硫的高利用率和出色的循环稳定性(441 mAh g −1 )[156]。

4.2 Tailoring Electrolyte Systems
4.2 定制电解质系统

Modulating the electrolyte suppresses both shuttle effect and the formation of lithium dendrites for Li–S batteries. Electrolytes play a key role in determining cathode and anode chemistry [160, 161]. Researchers have proven that SEIs formed on both sulfur cathodes and lithium anodes are inadequate for long-term cycling using conventional, organic electrolytes. Electrolyte modulation has become prevalent in the literature for adjusting the surface chemistry of active materials and enabling reversible reaction sites.
调节电解质会抑制 Li-S 电池的穿梭效应和锂树突的形成。电解质在决定阴极和阳极化学成分方面起着关键作用[160,161]。研究人员已经证明,在硫阴极和锂阳极上形成的SEI不足以使用传统的有机电解质进行长期循环。电解质调制在文献中已经很普遍,用于调整活性材料的表面化学性质并实现可逆的反应位点。

4.2.1 Co-solvents Electrolyte Systems
4.2.1 助溶剂电解质系统

In Li–S batteries, the nature and quantity of solvents in the electrolyte play a vital role because they function as the medium for Li+ transport and are extensively involved in lithium salt reactions on electrode surfaces. A well-designed electrolyte should suppress shuttle of LPS and safeguard Li anode, thereby extending the cycling duration of Li–S batteries [162]. Long-chain LPS can be dissolved in conventional ether-based electrolytes and move to the anode, whereby they react with the lithium metal, leading to a decrease in capacity and an increase in resistance. Recently, high concentration electrolytes (HCEs) with dilute solvents have been developed to inhibit the dissociation of LPS [161, 163]. Nevertheless, the extensive use of costly lithium salts in HCE electrolytes has given rise by several challenges such as high cost, low ionic conductivity, poor wettability and high viscosity of LPS [58, 164]. In order to solve these problems while maintaining limited solubility of LPS, the proposal suggests localized high-concentration electrolytes (LHCEs) by supplementing fluorinated solvents having weak donating ability (e.g., fluorinated ether and fluoride benzene) to HCEs. As an illustration, fluorinated ethers (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, TTE) have been used as co-solvents in electrolytes, where LPS are trapped within the cathode due to the formation of fluorine-rich SEIs (e.g., LiF), which improved the cycling performance and suppressed the LPS shuttle [165]. Additionally, TTE was also used to dilute a dual-salts based medium-concentrated electrolyte (MCE) to form diluted medium-concentrated electrolytes (DMCEs). Thanks to the LiF-rich SEI film formed by TTE and the effect of inhibiting side reactions, LSB batteries delivered an initial capacity of 682 mAh g−1 in the DMCE electrolyte (667 mAh g−1 in MCE) with 92% capacity retention and lifespan of 500 cycles [166].
在锂硫电池中,电解质中溶剂的性质和数量起着至关重要的作用,因为它们是锂 + 传输的介质,并广泛参与电极表面的锂盐反应。设计良好的电解质应抑制LPS的穿梭并保护锂阳极,从而延长Li-S电池的循环时间[162]。长链LPS可以溶解在传统的醚基电解质中并移动到阳极,从而与锂金属发生反应,导致容量降低和电阻增加。最近,人们开发了含有稀溶剂的高浓度电解质(HCE)来抑制LPS的解离[161,163]。然而,在HCE电解质中广泛使用昂贵的锂盐带来了一些挑战,如LPS的高成本、低离子电导率、低润湿性和高粘度[58,164]。为了在保持LPS有限溶解度的同时解决这些问题,该提案建议通过补充供体能力较弱的氟化溶剂(例如氟化醚和氟化物苯)来定位高浓度电解质(LHCEs)。例如,氟化醚(1,1,2,2-四氟乙基-2,2,3,3-四氟丙基醚,TTE)已被用作电解质中的助溶剂,其中LPS由于富含氟的SEI(例如LiF)的形成而被困在阴极内,从而改善了循环性能并抑制了LPS穿梭[165]。此外,TTE还用于稀释基于双盐的中浓电解质(MCE),以形成稀释的中浓电解质(DMCE)。 由于TTE形成的富含LiF的SEI膜和抑制副反应的作用,LSB电池在DMCE电解质中的初始容量为682 mAh g −1 (MCE中为667 mAh g −1 ),容量保持率为92%,寿命为500次循环[166]。

4.2.2 Quasi-solid Electrolytes
4.2.2 准固态电解质

Gel polymer electrolytes (GPEs) are a type of quasi-solid electrolytes, which are interfacially compatible with both the cathodes and anodes and exhibit enhanced ionic conductivity. They consist of liquid electrolyte enclosed in a polymer matrix [167,168,169,170]. The electrolyte solvent in GPEs facilitates solid/liquid interfacial sulfur conversion [61, 135, 171, 172]. While GPEs enable the creation of high-sulfur-loading Li–S batteries, plastification of the liquid electrolyte can significantly decrease the GPEs mechanical stability. To reinforce GPEs, it is common practice to introduce inorganic nanoparticles (e.g., TiO2, Al2O3, and SiO2) into the polymer matrix [173, 174]. An example is a GPE reinforced with a titanium-oxo cluster (TOC) that was prepared for constructing low E/S ratio Li–S batteries [157] in Fig. 11e. The designed TOC promotes the behavior of the polymer matrix in forming a film, inhibiting polysulfide shuttling, and leading to uniform Li deposition. In the first cycle at 2 mA cm−2, the discharge capacity of the S|PVFH-TOC-PEG|Li cell was as high as 1103 mAh g−1 calculated based on the mass of sulfur. Even increasing to16 mA cm−2 such high current density, the S|PVFH-TOC-PEG|Li cells maintained 802 mAh g−1 such a high specific capacity (Fig. 11f).
凝胶聚合物电解质(GPE)是一种准固体电解质,它与阴极和阳极表面相容,并表现出增强的离子电导率。它们由包裹在聚合物基质中的液体电解质组成[167,168,169,170]。GPE中的电解质溶剂促进固/液界面硫的转化[61,135,171,172]。虽然 GPE 能够制造高硫负载的 Li-S 电池,但液体电解质的塑化会显着降低 GPE 的机械稳定性。为了增强GPE,通常的做法是将无机纳米颗粒(例如TiO 2 、Al 2 O 3 和SiO 2 )引入聚合物基质中[173,174]。一个例子是用钛羰基簇(TOC)增强的GPE,该簇用于构建图11e中的低E/S比Li-S电池[157]。设计的TOC促进了聚合物基体形成薄膜的行为,抑制了多硫化物的穿梭,并导致均匀的Li沉积。在2 mA cm −2 的第一个循环中,S|PVFH-TOC-PEG|Li电池高达1103 mAh g −1 ,根据硫的质量计算。即使将电流密度增加到 16 mA cm,S −2 |PVFH-TOC-PEG|锂电池保持了802 mAh g −1 如此高的比容量(图11f)。

4.2.3 Dual-phase Electrolyte Approaches
4.2.3 双相电解质方法

The dissolution of LPS is crucial for rapid cathode kinetics, particularly under lean electrolyte conditions, although it jeopardizes anode stability. According to the phenomenon of phase separation between different polar solvents and the mediator-solvating property, tetramethyl sulfone (TMS) and dibutyl ether (DBE) have been selected as dual-phase electrolyte system for Li–S battery. Specifically, the high-density TMS-LiTFSI with high-polarity acted as the cathode electrolyte, whereas the anode electrolyte consisted of DBE and a polymeric ion conductor in GPE. The cathode electrolyte can strongly solvate LPS and propel SRR process [175]; meanwhile, the corrosive species such as LPS and ammonia trifluoroacetate can be effectively discouraged by the DBE on the anode side, allowing the activity and stability of batteries to be significantly increased (Fig. 12a). Consequently, pouch cells assembled with the dual-phase electrolytes have delivered 120 cycles under a low-Li-excess condition (N/P = 3) and lean electrolyte (4 µL mg−1).
LPS的溶解对于快速阴极动力学至关重要,特别是在贫电解质条件下,尽管它会危及阳极稳定性。根据不同极性溶剂之间的相分离现象和介质溶剂化特性,选择四甲基砜(TMS)和二丁基醚(DBE)作为锂硫电池的双相电解质体系。具体而言,具有高极性的高密度TMS-LiTFSI充当阴极电解质,而负极电解质由DBE和GPE中的聚合物离子导体组成。阴极电解质能强溶剂化LPS并推动SRR工艺[175];同时,负极侧的DBE可以有效抑制LPS和三氟乙酸氨等腐蚀性物质,从而显著提高电池的活性和稳定性(图12a)。因此,用双相电解质组装的软包电池在低锂过量条件 (N/P = 3) 和稀薄电解质 (4 μL mg) 下进行了 120 次循环 −1

Fig. 12 图例 12
figure 12

Copyright 2022, Wiley–VCH. b Dual SEIs formed with BTT electrolyte additive [176]. Copyright 2021, Nature. c Schematic illustration of the Se0.06SPAN/MMT@PP separator for Li–S battery [177]. Copyright 2022, Wiley–VCH. d The operating principle in Li–S cells with ZnO–ZnS/rGO heterostructures functionalized separators. e Dimensionless transient curves of 7ZnO–3ZnS/rGO with theoretical models [178]. Copyright 2022, Elsevier. f Schematics for the absorption and conversion of LPS on PP, C@PP, and C-Lepidolite@PP separators [179]. Copyright 2021, Wiley–VCH
版权所有 2022,Wiley–VCH。b 用BTT电解质添加剂形成的双SEI [ 176]。版权所有 2021,Nature。c 锂硫电池用Se 0.06 SPAN/MMT@PP隔膜示意图[177]。版权所有 2022,Wiley–VCH。d 使用ZnO-ZnS/rGO异质结构功能化分离器的Li-S电池的工作原理。e 7ZnO–3ZnS/rGO的无量纲瞬态曲线与理论模型[ 178].版权所有 2022,爱思唯尔。f LPS在PP、C@PP和C-Lepidolite@PP分离器上的吸收和转化示意图[179]。版权所有 2021, Wiley–VCH

a Schematic and photographs for the immiscibility of Li2S6 in TMS solution and DBE [175].
a Li 2 S 6 在TMS溶液和DBE中的不混溶性示意图和照片[175]。

4.2.4 Electrolyte Additives for Shutle Inhibition
4.2.4 抑制舒特尔的电解质添加剂

Electrolyte additives that are appropriate should have the ability to carry out electrochemical conversions and produce functional surface films on both electrodes. 1,3,5-benzenetrithiol (BTT) has been employed in the fabrication of dual SEIs (D-SEIs) as electrolyte additives (Fig. 12b), which forms SEI at the anode by reacting with Li metal and meanwhile self-assembling with sulfur at cathode creates an adaptable monolayer on homogeneous surfaces by forming interfacial layers containing S-Li and S–S bonds [176]. These in situ formed bonds changed the redox pathway of the sulfur cathode and highly regulated Li deposition/stripping behavior, thus achieving improved performance. The Mulliken charge distribution of BTT showed that the charge has been transferred from Li electrode to the S counterpart upon adsorption, resulting in stronger ionic bonding. The discharge capacity of Li–S cell with BTT was as high as 1239 mAh g−1 and long-term stable cycling was over 300 cycles at 1 C.
合适的电解质添加剂应具有进行电化学转化并在两个电极上产生功能性表面膜的能力。1,3,5-苯三硫醇(BTT)作为电解质添加剂被用于制造双SEIs(D-SEIs)(图12b),通过与Li金属反应在阳极形成SEI,同时在阴极与硫自组装,通过形成含有S-Li和S-S键的界面层,在均相表面上形成适应性强的单层[176]。这些原位形成的键改变了硫阴极的氧化还原途径,并高度调控了锂沉积/剥离行为,从而实现了性能的提高。BTT的Mulliken电荷分布表明,电荷在吸附时已从Li电极转移到S电极上,从而产生更强的离子键合。含BTT的Li-S电池放电容量高达1239 mAh g −1 ,在1 C下长期稳定循环超过300次。

4.3 Constructing Functional Separators
4.3 构造功能分隔符

Separators play a pivotal role in preventing the dissolution and diffusion of LPS when cathode and electrolyte conditioning strategies are insufficient. Nevertheless, commercial separators, like the polypropylene porous membranes (PP) or polyethylene porous membranes (PE), have pore sizes as wide as 100 nm, which is much larger than that of the long-chain LPS with an average size of several nanometers, resulting in the easy penetration of LPS to the anode side. Therefore, almost any typical host material is possible to be employed to optimize a separator or architecture an interlayer.

4.3.1 Multifunctional Materials Coating for Separators
4.3.1 分离器多功能材料涂层

Depositing functional materials on the separator to trap soluble intermediates is a common strategy to suppress LPS. However, due to the transformation barrier of insoluble Li2S and Li2S2, the captured LPS tend to exit in the subsequent electrochemical process and accumulate as “dead sulfur” on the separator over time [180, 181]. Furthermore, the accumulated “dead sulfur” significantly obstructs the separator's lithium-ion transport channels, leading to clear degeneration in the performance [182,183,184,185,186]. Some catalytically active modifiers have been discovered to alleviate the “dead sulfur” issue via accelerating the conversion of LPS [187,188,189]. The optimizing modification separators with multifunctional coating layer composed of bicomponent composite comprising montmorillonite (MMT) and selenium-doped sulfurized polyacrylonitrile (Se0.06SPAN) efficiently enabled the catalytic activation of the blocked LPS and prevented the cumulative of “dead sulfur” [177]. The layered structure of MMT serves as an effective anchor for dissolved polysulfides while allowing the migration of lithium ions (Fig. 12c). It also served as a support to minimize volume changes as Se0.06SPAN charged and discharged. Moreover, MMT-loaded Se0.06SPAN accelerated the conversion of anchored polysulfides and activated “dead sulfur” owing to the potential barrier of insoluble Li2S and soluble LPS was reduced. As a result, the Li–S battery with the Se0.06SPAN/MMT@PP delivered a low fading rate of 0.034% during 1000 cycles. Furthermore, under high-sulfur loading (26.75 mg cm−2) and lean electrolyte (4.5 µL mg−1), the battery achieved a super-high areal capacity of 33.07 mAh cm−2.
在隔膜上沉积功能材料以捕获可溶性中间体是抑制LPS的常用策略。然而,由于不溶性Li 2 S和Li 2 S 2 的转化势垒,捕获的LPS往往会在随后的电化学过程中退出,并随着时间的推移在隔膜上积累为“死硫”[180,181]。此外,积累的“死硫”会严重阻碍隔膜的锂离子传输通道,导致性能明显退化[182,183,184,185,186]。已经发现了一些催化活性改性剂,通过加速LPS的转化来缓解“死硫”问题[187,188,189]。由蒙脱石(MMT)和硒掺杂硫化聚丙烯腈(Se 0.06 SPAN)组成的双组分复合材料组成的多功能涂层优化改性隔膜有效地催化活化了封闭的LPS,并防止了“死硫”的积累[177]。MMT的层状结构可作为溶解多硫化物的有效锚固,同时允许锂离子迁移(图12c)。它还起到了支撑作用,以最大限度地减少 Se 0.06 SPAN 充电和放电时的体积变化。此外,MMT负载的Se 0.06 SPAN加速了锚定多硫化物的转化,并由于不溶性Li 2 S和可溶性LPS的潜在势垒而活化了“死硫”。因此,采用 Se 0.06 SPAN/MMT@PP 的 Li-S 电池在 1000 次循环中提供了 0.034% 的低衰落率。此外,在高硫负载(26.75 mg cm −2 )和稀薄电解质(4.5 μL mg)下 −1 ,电池实现了33.07 mAh cm −2 的超高面容量。

In order to diminishing the shuttling effects in Li–S batteries, it has been found helpful to add catalysts to the separator to accelerate the conversion of LPS. However, it is challenging to achieve both high catalytic activity and strong adsorption using a single catalyst. The modified heterostructure of ZnO-ZnS/reduced graphene oxide (rGO) with strong balanced adsorption and high catalytic activity has achieved significantly enhanced polysulfide conversion and Li–S performance (Fig. 12d) [178]. In addition, through a controlled vulcanization, the ZnO-ZnS/rGO heterostructure achieved the optimal overall performance with a ZnO/ZnS ratio of 7:3, suggesting a balance between adsorption and catalytic activity. ZnO-ZnS/rGO heterostructures can significantly improve redox kinetics and inhibit polysulfide shuttling when used as functional coatings on separators. As shown in Fig. 12e, under the synergistic effect of the heterostructure, the Li2S deposition pattern showed a mixed model of 3DP and 2DI [9, 190], demonstrating that the heterostructure realized a bidirectional redox reaction between Li2S and LPS. Thus, the pure sulfur cathode matched that the 7ZnO-3ZnS/rGO-modified separator delivered initial specific capacity as high as 1186 mAh g−1 at 0.5 C and possessed 0.06% fading per cycle over 500 cycles at 1 C.
为了减少锂硫电池的穿梭效应,人们发现在隔膜中添加催化剂有助于加速LPS的转化。然而,使用单一催化剂实现高催化活性和强吸附具有挑战性。ZnO-ZnS/还原氧化石墨烯(rGO)的改性异质结构具有较强的平衡吸附和高催化活性,显著增强了多硫化物的转化率和Li-S性能(图12d)[178]。此外,通过可控硫化,ZnO-ZnS/rGO异质结构以7:3的ZnO/ZnS比值实现了最佳的整体性能,表明吸附和催化活性之间的平衡。ZnO-ZnS/rGO异质结构在用作隔膜上的功能涂层时,可以显著提高氧化还原动力学并抑制多硫化物穿梭。如图12e所示,在异质结构的协同作用下,Li 2 S沉积模式呈现出3DP和2DI的混合模型[9,190],表明异质结构实现了Li 2 S和LPS之间的双向氧化还原反应。因此,纯硫阴极与7ZnO-3ZnS/rGO改性隔膜在0.5 C时提供高达1186 mAh g −1 的初始比容量相匹配,并且在1 C下500次循环中每个周期具有0.06%的衰落。

Forming covalent bonding between S and the atoms from the separators is also an efficient strategy to prohibit the shutting effect. For instance, the designed lepidolite-modified polypropylene (C-Lepidolite@PP) separator can induce the electron transfer from S atoms to the 3p antibonding orbitals of Si atoms, which weakened the S–S bonds and formed strong Si-S bonds, thus effectively confining polysulfides (Fig. 12f) [179]. The lithium-ion diffusion barrier in lepidolite is extremely low (0.081 eV), it permits free migration of lithium ions and this, which in turn improves the conversion of polysulfide from liquid Li2S8 to solid Li2S and fast LPS redox for high-rate current operation. Therefore, Li–S batteries with the C-Lepidolite@PP delivered an excellent areal capacity of 7.53 mAh cm−2 under 6.5 mg cm−2 sulfur loading and a superior rate performance of 703 mAh g−1 at 7 C.
在 S 和来自隔膜的原子之间形成共价键也是抑制闭合效应的有效策略。例如,设计的锂云母改性聚丙烯(C-Lepidolite@PP)隔膜可以诱导电子从S原子转移到Si原子的3p反键轨道,从而削弱了S-S键并形成强Si-S键,从而有效地限制了多硫化物(图12f)[179]。锂云母中的锂离子扩散势垒极低(0.081 eV),它允许锂离子自由迁移,这反过来又提高了多硫化物从液态锂 2 8 硫到固体锂 2 硫的转化率,并提高了高速率电流操作的快速LPS氧化还原。因此,采用C-Lepidolite@PP的锂硫电池在6.5 mg cm −2 硫负载下具有7.53 mAh cm −2 的出色面容量,在7 C时具有703 mAh g −1 的优异倍率性能。

4.4 Modified Lithium Metal Anode
4.4 改性锂金属负极

If the approach to eliminate LPS shuttling by the use of optimized sulfur cathode, functional separators, and customized electrolyte compositions is still inadequate, the residue long-chain soluble LPS will inevitably diffuse to Li anode side through a concentration gradient. Once these soluble LPS diffused to the anode surface and reacted with Li metal, it will result in the formation of the insoluble and inert Li2S and Li2S2, resulting in loss of active material, irreversible depletion of Li and reduction in coulombic efficiency. Therefore, introduction of a passivation layer at the interface to inhibit the diffusion of LPS, facilitated fast transport for Li+ ions, and prevent the formation of irregular Li dendrites is effective in inhibiting active S species loss and prolong the cycle life of Li–S batteries. It has been described above that the SEI layer is formed by tailoring the electrolyte to prevent the direct reaction of the polysulfide with the lithium metal. This section will show artificial protective layers and other strategies to avoid shuttle effects on negative electrode materials.
如果通过使用优化的硫阴极、功能性隔膜和定制电解质组合物来消除LPS穿梭的方法仍然不够充分,残留的长链可溶性LPS将不可避免地通过浓度梯度扩散到锂阳极侧。一旦这些可溶性LPS扩散到阳极表面并与Li金属反应,将导致不溶性和惰性Li 2 S和Li 2 S 2 的形成,导致活性物质的损失,Li的不可逆消耗和库仑效率的降低。因此,在界面处引入钝化层以抑制LPS的扩散,促进锂 + 离子的快速传输,并防止不规则Li枝晶的形成,可有效抑制活性S物种的损失并延长Li-S电池的循环寿命。如上所述,SEI层是通过定制电解质来形成的,以防止多硫化物与锂金属的直接反应。本节将介绍人工保护层和其他策略,以避免对负极材料产生穿梭效应。

4.4.1 Artificial Solid Electrolyte Interphase (SEI)
4.4.1 人工固体电解质界面(SEI)

Numerous organic and inorganic materials have been utilized to fabricate artificial SEIs for the purpose of enhancing the performance of lithium anodes [191, 192]. For example, a stable UiO-66-ClO4/PDMS (PDUO-Cl) biomimetic protective layer has been designed to modify Li anode by a drip coating method (Fig. 13a) [193]. When bare Li and PDUO-Cl@Li were immersed into a solution containing Li2S6, it was observed that the bare Li-immersed solution became almost colorless after 36 h, owing to the reaction of Li metal with LPS. Conversely, the discoloration of the PDUO-Cl@Li-immersed solution was markedly reduced, demonstrating that the LPS in the solution were not exhausted (Fig. 13b). This result suggested that the PDUO-Cl could resist the aggression of LPS. Therefore, the symmetric cells of the PDUO-Cl@Li delivered a stable long-term cycle over than 1400 h at 0.5 mA cm−2. The half cells with a PDUO-Cl@Li also showed a relatively high capacity retention of 69% after 100 cycles at 0.1 C.
许多有机和无机材料已被用于制造人工SEI,以提高锂阳极的性能[191,192]。例如,设计了一种稳定的UiO-66-ClO 4 /PDMS(PDUO-Cl)仿生保护层,通过滴涂法对锂阳极进行改性(图13a)[193]。当裸锂和PDUO-Cl@Li浸入含有Li 2 S 6 的溶液中时,由于金属锂与LPS的反应,观察到裸锂浸没的溶液在36小时后变得几乎无色。相反,PDUO浸Cl@Li溶液的变色明显减少,表明溶液中的LPS没有耗尽(图13b)。这一结果表明,PDUO-Cl能够抵抗LPS的侵袭。因此,PDUO-Cl@Li的对称细胞在0.5 mA cm −2 下提供了超过1400小时的稳定长期循环。在0.1°C下循环100次后,具有PDUO-Cl@Li的半电池也显示出相对较高的容量保留率,为69%。

Fig. 13 图 13
figure 13

Copyright 2022, Elsevier. c The LPS in c1) ordered selective permeable polymer interphase and c2) disordered polymer interphase. d The visualized test of Li2S6 electrolyte with pristine Li, unselective-permeable Li, and selective permeable Li. e The shuttle currents in LiNO3-free ether electrolyte [194]. Copyright 2021, Wiley–VCH
版权所有 2022,爱思唯尔。c c1)有序选择性渗透聚合物中间相和c2)无序聚合物中间相中的LPS。d Li S 6 电解质与纯锂、非选择性渗透性 Li 和选择性渗透性 Li 的可视化测试 2 。 e 无 LiNO 3 醚电解质中的穿梭电流 [ 194].版权所有 2021, Wiley–VCH

a The manufacturing steps of PDUO-Cl protective layer for Li anode. b the visualized test of Li2S6 electrolyte for bare Li and PDUO-Cl@Li [193].
a 锂阳极用PDUO-Cl保护层的制造步骤。b Li 2 S 6 电解质裸Li和PDUO-Cl@Li的可视化试验[193]。

4.4.2 In Situ Ion-selective Interphase Engineering
4.4.2 原位离子选择性相间工程

Except for developing a protective layer for the lithium anode, it is essential to enhance the transportation of Li+ ion through the SEI layer as it directly affected the plating/stripping behavior of lithium [181]. In Fig. 13c, parasitic reactions generate disordered Li+ channels on the lithium surface, which obstruct ion transportation and promote electrode corrosion [195]. Thus, it is essential to construct ion-selective, ordered channels which permitted Li+ to diffuse rapidly across the working interface and prevent large amounts of corrosive anions from passing through [196, 197]. For instance, a direct reaction between aminopropyl-terminated polydimethylsiloxane (AT-PDMS) and lithium metal has been used to create selectively permeable interphase for a lithium anode [194]. The entropic ordered organization of the polymer chain reduces the free volume of the polymer, selectively blocking bigger polysulfides which have stronger spatial barriers, while allowing the passage of Li+ ions (Fig. 13d). Without adding a lithium nitrate passivator to the electrolyte, the shuttle current of Li–S batteries reduced by 90% and the coulombic efficiency improved from 82 to 91% (Fig. 13e). Table 2 summarized the recent advancements in Li–S batteries based on the inhibition of the shuttle effect for various components.
除了为锂阳极开发保护层外,还必须增强锂 + 离子通过SEI层的传输,因为它直接影响锂的电镀/剥离行为[181]。在图13c中,寄生反应在锂表面产生无序的Li + 通道,阻碍离子传输并促进电极腐蚀[195]。因此,必须构建离子选择性、有序通道,使Li + 能够快速扩散到工作界面上,并防止大量腐蚀性阴离子通过[196,197]。例如,氨丙基封端聚二甲基硅氧烷(AT-PDMS)与锂金属之间的直接反应已被用于为锂阳极产生选择性渗透的中间相[194]。聚合物链的熵有序组织减少了聚合物的自由体积,选择性地阻挡了具有更强空间势垒的较大多硫化物,同时允许锂 + 离子通过(图13d)。在不向电解液中添加硝酸锂钝化剂的情况下,Li-S电池的穿梭电流降低了90%,库仑效率从82%提高到91%(图13e)。表2总结了Li-S电池在抑制各种组件的穿梭效应方面的最新进展。

Table 2 The recent advancements in Li–S batteries based on the inhibition of the shuttle effect for various components of the devices
表2 基于抑制器件各部件穿梭效应的锂硫电池的最新进展

5 Conclusion and Outlooks
第5章 结论与展望

If LPS exhibits shuttling behavior in Li–S batteries, it may be challenging to meet practical demands. Consequently, it is urgent and meaningful to gain a more comprehensive understanding of the shuttling process of LPS, which can also be used as guidance for future shuttle effect inhibition design for Li–S battery applications. This review focuses on the shuttle path of LPS and suppressing strategies in Li–S batteries. The designed principles for prohibiting LPS shuttle, including boosting the sulfur conversion rate, confining sulfur or LPS within cathode host, confining LPS in the shield layer, and preventing LPS from contacting the anode have been discussed and summarized. The summarized recent advances of inhibition of shuttle effect in the sulfur host, electrolyte system, separator, and anode protection demonstrated that the designed principles for prohibiting LPS shuttle are highly correlated to the activity and stability for Li–S batteries.

Currently, tremendous advancements have been acquired with respect to Li–S batteries with many breakthroughs for high-energy and long-stability. However, it is still challenging to eliminate all the side reactions, shuttle effects and finally commercialization. Therefore, it is still urgent to develop efficient strategies for Li–S batteries to realize the practical application. Achieving more advanced characterization techniques may be beneficial for exploring of reaction mechanisms, analyzing of interface engineering in-depth and expanding of interdisciplinary research. A more microscopic perspective can provide a complete and thorough comprehension of Li–S batteries, which can be effectively used to eliminate the shuttle effect.

1. Upgrading of characterization technologies. Advanced characterization methods, especially more direct in situ characterization methods, should be developed for Li–S batteries. Currently, traditional EIS and Raman can only understand the macro mechanism of these strategies on polysulfides. More recently, some ex situ characterization techniques have been used to explore the changes in the physicochemical properties before and after the cycles, which help us to get a better understanding of the Li–S battery process. However, the interfacial and structure–activity relationships between polysulfides, catalysts and electrolytes are still unclear. Therefore, more advanced characterization techniques are highly desired, especially the in situ characterization method. For example, the in situ synchrotron radiation technique can be used to investigate the near-free evolution kinetic behavior of monatomic catalysts in electrocatalytic reduction reactions [202], and catalytic sites and reaction processes can be detected through the use of in situ X-ray absorption spectroscopy (XAS) and surface enhanced infrared absorption (SEIRA) spectroelectrochemistry. The electronic state of each metal site can be observed in real time during the voltage change process, which can help to determine the initial reaction conditions and the reaction intermediate process [203].

2. Exploration of reaction mechanism. However, much enhanced performance of Li–S batteries has been achieved through the aforementioned strategies, significantly suppressing the shuttle effect. But awareness of mechanism about inhibition for polysulfide is still backward in terms of theory. In addition, experimental characterization methods can reflect the integrated performance of Li–S batteries, while theoretical simulation can provide profound mechanism insights from the atomic scale, and has become an indispensable tool in the study for Li–S batteries. Density functional theory (DFT) calculations can determine structural stability, calculate the free energy of the reaction, analyze the electronic structure, and simulate the ionization/molecular diffusion or adsorption kinetics [204]. At the same time, high-throughput screening and machine learning have broad application prospects on the research on Li–S batteries, which can inspire and guide the further development in this field [205]. Further exploration of the reaction mechanism will play a guiding role in the material design and theoretical calculation, which can provide a strong basis for cathodes and anodes design strategy, catalyst selection, electrolyte customization, and SEI component construction. At the same time, reaction mechanism will also provide the direction for Li–S batteries to achieve high energy density under high loading and lean electrolyte conditions. This will provide a solid foundation for the commercialization of Li–S batteries in the future.

3. In-depth analysis of interface engineering. Whether it is the three processes of the Li–S reaction: “quasi-solid,” “solid–liquid-solid,” “solid–solid” process, or the construction of CEI on the cathode or SEI film on the anode, it involves the interphase transition process. In-depth analyzing the phase transition of interface engineering is conducive to the targeted design of materials or structures to achieve the theoretical capacity of Li–S batteries. Simultaneously, interface engineering can also improve the shuttle energy barrier of polysulfide, inhibit the loss of active substances, and avoid the growth of lithium dendrites. The morphological changes of the anode surface during operation can be monitored by in situ optical microscope. Based on the finite element method, COMSOL Multiphysics simulation can be carried out to simulate the local current density and the overall electric field distribution to reveal the evolution and failure mechanism of interface engineering [206, 207]. It lays a foundation for the realization of safe and stable Li–S batteries.
3、界面工程的深入分析。无论是Li-S反应的三个过程:“准固体”、“固-液-固”、“固-固”过程,还是在阴极上构建CEI或在阳极上构建SEI膜,都涉及相间转变过程。深入分析界面工程的相变,有利于有针对性地设计材料或结构,实现锂硫电池的理论容量。同时,界面工程还可以改善多硫化物的穿梭能垒,抑制活性物质的流失,避免锂枝晶的生长。在操作过程中阳极表面的形貌变化可以通过原位光学显微镜进行监测。基于有限元方法,可进行COMSOL Multiphysics仿真,对局部电流密度和整体电场分布进行仿真,揭示界面工程的演化和失效机理[206,207]。为实现安全稳定的锂硫电池奠定了基础。

4. Expansion of interdisciplinary research. Similar to sulfur cathodes in Li–S battery, many areas of electrochemical energy storage process face challenges in terms of electrodeposition behavior. For example, the alkali metal–chalcogen system, working in a similar way to Li–S battery, namely Li/Na/K/Mg-S/Se/Te, suffers from slow cathode deposition kinetics during discharge [208,209,210,211]. In the case of lithium metal anodes, lithium with high rigidity needs to be deposited flat without forming dendrites to prevent battery short circuits [191]. Although the objects of study for metal anodes and sulfur cathodes are usually different, their related strategies, methods, and materials are of high value for achieving ideal electrodeposition. In addition, electrodeposition has long been recognized as an important technology for material synthesis or device manufacturing in many frontier fields such as solar photovoltaic, thermoelectric and sensors [212]. Therefore, implementing interdisciplinary research to inspire more insightful work far beyond the field of energy storage should be very attractive.
4. 扩大跨学科研究。与锂硫电池中的硫阴极类似,电化学储能过程的许多领域在电沉积行为方面都面临挑战。例如,碱金属-硫族系统的工作方式与Li-S电池类似,即Li/Na/K/Mg-S/Se/Te,在放电过程中阴极沉积动力学缓慢[208,209,210,211]。在锂金属阳极的情况下,高刚性的锂需要平整沉积而不形成枝晶,以防止电池短路[191]。尽管金属阳极和硫阴极的研究对象通常不同,但它们的相关策略、方法和材料对于实现理想的电沉积具有很高的价值。此外,电沉积在太阳能光伏、热电、传感器等许多前沿领域,长期以来一直被认为是材料合成或器件制造的重要技术[212]。因此,实施跨学科研究以激发远远超出储能领域的更有见地的工作应该非常有吸引力。