Review on comprehending and enhancing the initial Coulombic efficiency of anode materials in lithium-ion/sodium-ion batteries
锂离子/钠离子电池负极材料初始库仑效率的理解与提升研究进展

Lithium-ion battery 锂离子电池

Sodium-ion battery 钠离子电池

Initial Coulombic efficiency
初始库仑效率

Energy density 能量密度

Anode materials 阳极材料

Due to the rapid consumption of fossil fuels and the accompanying serious environmental issues, the utilization of new sustainable and environmentally friendly energy, such as wind, solar, geothermal, water, and tidal energy, has attracted great attentions in the past decades [1-6]. However, the characteristics of unpredictability, capacity instability and intermittence impede the exploitation of these energy sources [7-9]. Thus, the exploration of high-performance energy storage devices is urgently required in order to most efficiently utilize renewable energy sources. Among the various available energy storage systems, rechargeable LIBs with their high energy conversion efficiency, stable cyclability, simple maintenance, and adaptable power/energy features have become one of the most widely used candidates [10-16]. LIBs have been successfully applied in many fields ranging from portable electronics and electric vehicles to large-scale smart grids [17-22]. Nevertheless, with the increasing need for energy storage, it is more and more necessary to develop LIBs with substantially better electrochemical
由于化石燃料的快速消耗以及随之而来的严重环境问题，风能、太阳能、地热能、水能和潮汐能等新型可持续环保能源的利用在过去几十年中引起了人们的极大关注[1-6]。然而，不可预测性、容量不稳定性和间歇性等特点阻碍了这些能源的开发[7-9]。因此，为了最有效地利用可再生能源，迫切需要探索高性能储能设备。在各种可用的储能系统中，可充电锂离子电池具有能量转换效率高、可循环性稳定、维护简单、电力/能量适应性强等特点，成为应用最广泛的候选系统之一[10-16]。锂离子电池已成功应用于从便携式电子产品和电动汽车到大规模智能电网等多个领域[17-22]。然而，随着对储能需求的增加，越来越有必要开发具有明显更好的电化学性质的锂离子电池

Scheme 1. The relationship between causes of low ICE, ICE and energy density.
方案 1.低ICE，ICE和能量密度的原因之间的关系。

performance, including higher power/energy density, longer lifespan, and faster charge/discharge rates, especially for transportation applications [23-29]. In addition to LIBs, SIBs are being considered as potential alternatives for large-scale stationary energy storage applications, and have also attracted worldwide interest over the past few years due to the rich reserves and low cost of sodium resources [29-37]. However, the practical application of SIBs is still undesirable owing to their unsatisfactory electrochemical performance [38-40].
性能，包括更高的功率/能量密度、更长的使用寿命和更快的充放电速率，特别是对于运输应用[23-29]。除锂离子电池外，系统重要性银行也被认为是大规模固定式储能应用的潜在替代品，并且由于钠资源储量丰富且成本低廉，在过去几年中也引起了全世界的兴趣[29-37]。然而，由于SIBs的电化学性能不理想，其实际应用仍然不理想[38-40]。

As an essential part of rechargeable batteries, anode materials play an important role in electrochemical performance for both LIBs and SIBs. Currently, the most widely used commercial anode material for LIBs is graphite, which has a relative limited theoretical capacity of 372 and practical capacity of [41-44]. However, commercial graphite has limitations for the storage of sodium-ions due to the large atomic radius of sodium-ions [45-48]. Hence, in order to successfully meet the increasingly higher demands for LIBs equivalent or better energy storage, and to resolve the practical application for SIBs, the creation of anode materials that have commercial electrochemical
负极材料作为可充电电池的重要组成部分，对锂离子电池和系统实科电池的电化学性能都起着重要作用。目前，锂离子电池使用最广泛的商业负极材料是石墨，其理论容量相对有限，为372 ，实际容量为 [41-44]。然而，由于钠离子的原子半径较大，商业石墨在钠离子的储存方面存在局限性[45-48]。因此，为了成功满足对锂离子电池等效或更好储能的日益增长的要求，并解决SIB的实际应用，创造了具有商业电化学的阳极材料

(a)
- Graphite/NCA
    O Binder
Electrolyte
    :8 Conductive carbon
Separator
Current collector

(c) （三）
(d) （四）

Fig. 1. The influence of initial capacity loss on energy density. (a) Schematic depiction of an 18650-type cell. (b) The bar chart demonstrates the specific energies and energy density for the calculations. (Step 1 indicates the theoretical capacity and Step 2 indicates the practical capacity in consideration of the anode and cathode balancing due to the effect of initial capacity loss). Adapted with permission from Refs. [41], Richard Schmuch et al., Adv. Energy Mater., Copyright 2018, John Wiley and Sons. (c) The typical schematic diagram of the sodium-ion full cell. Reprinted from Nano Energy, 28, Wenhao Ren et al., Cathodic polarization suppressed sodium-ion full cell with a 3.3V high-voltage, 8, Copyright (2016), with permission from Elsevier. (d) Sodium-ion storage properties including energy density and power density of full cells reported. Adapted from Ref. [61] with permission from The Royal Society of Chemistry.
图 1.初始容量损失对能量密度的影响。（a） 18650型细胞的示意图。（b） 条形图显示了计算的比能量和能量密度。（步骤 1 表示理论容量，步骤 2 表示考虑由于初始容量损失影响而导致的阳极和阴极平衡的实际容量）。经 Refs 许可改编。[41]，Richard Schmuch等人，Adv. Energy Mater.，版权所有 2018，John Wiley and Sons。（c） 钠离子全电池的典型示意图。转载自Nano Energy，28，Wenhao 任等人，阴极偏振抑制钠离子全电池与3.3V高压，8，版权所有（2016），经爱思唯尔许可。（d） 报告的钠离子储存特性，包括能量密度和全电池的功率密度。改编自参考文献[61]，经英国皇家化学学会许可。

Table 1 表1

Summary of ICE for various anode materials in LIBs/SIBs and the reasons for initial capacity loss.
锂离子电池/SIB中各种负极材料的ICE总结以及初始容量损失的原因。

performance offers a great opportunity.
性能提供了一个很好的机会。

Key variables that must be taken into consideration in order to obtain a satisfactory electrochemical performance for anode materials include ICE, cycling performance, rate capability, and average charge voltage. The methods and mechanisms to modify these parameters have attracted a great deal of attention during the past decades, and some excellent reviews have summarized the desired realization of stability and the enhancement of the rate capabilities of various anodes for LIBs/SIBs. This has provided vital and precious values for further modification of anode materials for LIBs/SIBs [49-54]. However, to the best of our knowledge, there is no review focusing in detail on ICE as a key variable for practical applications of various anode materials for LIBs/SIBs. Here we provide a comprehensive overview of ICE for different kinds of anode materials in LIBs/SIBs. We focus on (i) the association between ICE and energy density, (ii) the cause of low ICE for various types of anode materials, and (iii) various methods and related fundamental mechanisms being used to enhance ICE with specific reasons for various anode materials. We end this review with a perspective on the future challenges and potential research directions for the anode materials that might lead to further improvements of ICE and the fabrication of high-energy-density batteries.
为了获得令人满意的负极材料电化学性能，必须考虑的关键变量包括内燃机、循环性能、倍率能力和平均充电电压。在过去的几十年中，修改这些参数的方法和机制引起了人们的广泛关注，一些优秀的综述总结了LIB/SIBs各种阳极的稳定性和速率能力的增强。这为锂离子电池/SIB的阳极材料的进一步改性提供了重要而宝贵的价值[49-54]。然而，据我们所知，目前还没有综述将ICE作为LIBs/SIBs各种负极材料实际应用的关键变量进行详细讨论。在这里，我们全面概述了 LIB/SIB 中不同类型负极材料的 ICE。我们专注于（i）ICE与能量密度之间的关联，（ii）各种类型负极材料低ICE的原因，以及（iii）用于增强ICE的各种方法和相关基本机制，以及各种负极材料的特定原因。最后，我们展望了负极材料的未来挑战和潜在研究方向，这些挑战和潜在研究方向可能导致内燃机的进一步改进和高能量密度电池的制造。

The ICE of anode materials in half cells can be calculated from the ratio of initial charge capacity to initial discharge capacity, which defines the ability of anode materials to prevent the occurrence of irreversible reactions and the loss of irreversible capacity. This ability is of great importance because it is associated with the practical application for anode materials in a battery in terms of available energy density. In general, a low ICE for the anode material can be attributed to various causes, as shown in Scheme 1 and will lead to a low energy density for a battery.
半电池中负极材料的ICE可以从初始充容量与初始放电容量的比值计算出来，它定义了负极材料防止不可逆反应发生和不可逆容量损失的能力。这种能力非常重要，因为它与电池中阳极材料的实际应用有关，就可用能量密度而言。一般来说，阳极材料的低ICE可以归因于各种原因，如方案1所示，并导致电池的能量密度低。

For example, the low ICE of anode materials is compensated for in current commercial LIBs by additional loading of cathode materials [55]. Nevertheless, as common lithium-metal oxide cathodes have lower specific capacity ( ) than that of anodes, excessive amount of cathode materials ( for a graphite anode material) must be
例如，在当前的商业锂离子电池中，负极材料的低ICE可以通过增加阴极材料的负载来补偿[55]。然而，由于普通锂金属氧化物阴极的比容量（ ）低于阳极，因此过量的阴极材料（ 对于石墨负极材料）必须
(a) （一）

(b) （二）

(c) （三）

Fig. 2. Formation of SEI layer. (a) Schematic open-circuit energy diagram of an electrolyte. (b) Computed reduction potential for several common solvents, additives, and salts. Data compiled from Delp et al. Reprinted from Electrochim. Acta, 209, Samuel A. Delp et al., Importance of reduction and oxidation stability of high voltage electrolytes and additives, 13, Copyright (2016), with permission from Elsevier. (c) Schematic diagram of SEI on graphite particle. Reprinted from J. Power Sources, 153, Kristina Edström et al., A new look at the solid electrolyte interphase on graphite anodes in Li-ion batteries, 5, Copyright (2006), with permission from Elsevier.
图 2.SEI层的形成。（a） 电解质的开路能量示意图。（b） 几种常见溶剂、添加剂和盐的计算还原潜力。数据汇编自 Delp 等人。转载自 Electrochim。Acta， 209， Samuel A. Delp et al.， Importance of reduction and oxidation stability of high voltage electrolytes and additives， 13， 版权所有 （2016），经爱思唯尔许可。（c） 石墨颗粒SEI示意图。转载自 J. Power Sources， 153， Kristina Edström et al.， A new look at the solid electrolyte interphase on graphite anodes in Li-ion batteries， 5， Copyright （2006）， with permission from Elsevier.

added to overcome the insufficient ICE of anode materials, which leads to an appreciable reduction of energy density [56]. Specifically, the reduction is about of total available capacity in commercial graphite electrodes and it can even reach to as high as for next-generation high-capacity anode materials (such as Si) [57-59]. Betz et al. compared the difference of energy densities in LIBs by using calculations based on theoretical and practical situations [41]. As shown in Fig. 1, they used (NCA) as the cathode material and graphite as the anode material in 18650-type cell for a LIB system. The theoretical energy content of the full cell at the active material level amounted to and , respectively. However, when taking the ICE effect of graphite and balancing the amount of anode and cathode material into consideration, the useable energy content at the active material level of a graphite/NCA battery cell decreases to and , respectively. As for the practical application of SIBs, the ICE of anode materials has similar effect on energy density with that of LIBs [60]. Consequently, to develop LIBs/SIBs with higher energy density, it is necessary and urgent to put more effort into the enhancement of ICE for anode materials.
添加以克服阳极材料的ICE不足，这导致能量密度明显降低[56]。具体而言，这种减少大约 是商用石墨电极的总可用容量，甚至可以达到与下一代高容量负极材料（如Si）一样高 的降低[57-59]。Betz等人通过基于理论和实际情况的计算比较了锂离子电池中能量密度的差异[41]。如图1所示，他们在LIB系统的18650型电池中使用 （NCA）作为阴极材料，石墨作为阳极材料。在活性物质水平上，全电池的理论能量含量分别为 和 。然而，当考虑到石墨的ICE效应并平衡阳极和阴极材料的数量时，石墨/NCA电池的活性材料水平上的可用能量含量分别降低到 和 。在SIB的实际应用中，负极材料的ICE对能量密度的影响与LIB相似[60]。因此，为了开发具有更高能量密度的LIB/SIBs，有必要且迫切需要加大对阳极材料ICE的增强力度。

Until now, broadly three types of reaction mechanisms have been reported for anode materials in LIBs and SIBs. In this part, we will present the current research status of ICE (as shown in Table 1) and analyze the underlying causes of low ICE for various types of anode materials (anode materials are sorted by reaction mechanisms) to give an exhaustive understanding of why and how the irreversible capacity loss happens in the initial cycle.
到目前为止，锂离子电池和SIB中的负极材料大致有三种类型的反应机理。在这一部分中，我们将介绍ICE的现状（如表1所示），并分析各类负极材料（负极材料按反应机理排序）ICE低的根本原因，以详尽地了解初始循环中不可逆容量损失发生的原因和方式。

Intercalation-type anode materials mainly include carbon materials (such as commercial graphite and hard carbon) and Ti-based materials (such as , and ). The mechanism of intercalation-type anode materials is based on the reversible intercalation and extraction of lithium-ions/sodium-ions in the lattice of a host anode material with layered structures (e. g., 6C (graphite) ). For intercalation-type anode materials, for both LIBs and SIBs, there are mainly two causes of irreversible capacity loss during the initial cycling process: (i) the formation of a solid-electrolyte interface (SEI) layer and (ii) the irreversible absorption of lithium-ions or sodium-
插层型负极材料主要包括碳材料（如商业石墨和硬碳）和钛基材料（如 、和 ）。插层型负极材料的机理是基于具有层状结构（例如6C（石墨） ）的主体负极材料晶格中锂离子/钠离子的可逆插层和萃取。对于插层型负极材料，无论是锂离子电池还是SIBs，在初始循环过程中造成不可逆容量损失的原因主要有两个：（i）固体电解质界面（SEI）层的形成和（ii）锂离子或钠的不可逆吸收。

Fig. 3. Some causes of initial capacity loss for conversion-type anode materials. (a) FTIR spectra of before cycling, after first discharge, and after first cycle showing that both the first discharged specimen and the first charged specimen have , and . (b) Charge-discharge curves of confirming the occurrence of irreversible electrochemical reaction. Reprinted with permission from Refs. [80], J. Electrochem. Soc., 150, A1539 (2003). Copyright 2003, The Electrochemical Society. (c) Schematic illustration of the sodiation and desodiation process for the two electrodes with different binders toward different results. Reproduced from Ref. [98] with permission from The Royal Society of Chemistry.
图 3.转换型负极材料初始容量损失的一些原因。（a） 循环 前、首次放电后和第一次循环后的 FTIR 光谱显示，第一放电试样和第一带电试样均具有 和 。（b）确认不可逆电化学反应发生的充放电曲线 。经 Refs 许可转载。[80]，J.电化学。Soc.， 150， A1539 （2003）。版权所有 2003，电化学学会。（c） 两种电极的钠化和脱钠过程示意图，其粘合剂不同，结果不同。经英国皇家化学学会许可，转载自参考文献[98]。

ions within the host materials. Extensive research has been done in order to understand the mechanism of formation of SEI layers in both lithium ion and sodium ion batteries [62-67].
主体材料中的离子。为了了解锂离子和钠离子电池中SEI层的形成机制，已经进行了广泛的研究[62-67]。

Fig. 2a shows the relative electron energies of the anode, electrolyte, and cathode of a thermodynamically stable redox pair in LIBs. In the figure, and are the electrochemical potentials of the anode and cathode respectively. The stability window of the electrolyte is the difference between the energy of the LUMO and HOMO. This window is shown as . If is above the LUMO energy, then it will reduce the electrolyte on the anode, and, likewise, if is below the HOMO energy, it will oxidize the electrolyte on the cathode [91]. Fig. 2b presents the computed reduction potential for several common solvents, additives, and salts. For graphite, the SEI layer contains an inorganic layer inside, porous organic layer outside and the presence of LiF crystals as presented in Fig. 2c [92,93]. Zhang et al. found that the ICE of graphite is for a graphite cell, and ascribed the initial capacity loss to the formation of a SEI layer [94]. Based on their study, they found that the formation of a SEI layer on the graphite electrode went through two major stages. The first stage underwent at a voltage of greater than 0.25
图2a显示了LIB中热力学稳定的氧化还原对的阳极、电解质和阴极的相对电子能。在图中， 和 分别是阳极和阴极的电化学势。电解质的稳定窗口是 LUMO 和 HOMO 的能量之差。此窗口显示为 。如果 高于LUMO能量，则会减少阳极上的电解质，同样，如果 低于HOMO能量，则会氧化阴极上的电解质[91]。图2b显示了几种常见溶剂、添加剂和盐的计算还原电位。对于石墨，SEI层内部包含无机层，外部包含多孔有机层，并且存在LiF晶体，如图2c所示[92,93]。Zhang等人发现石墨的ICE是 针对 石墨电池的，并将初始容量损失归因于SEI层的形成[94]。根据他们的研究，他们发现石墨电极上SEI层的形成经历了两个主要阶段。第一级在电压大于0.25的情况下进行
V , and of the irreversible capacity took place in this stage. The second stage was between 0.25 V and 0.04 V during the intercalation of lithium-ions into the graphite electrode, which accounts for of the irreversible capacity. Agubra et al. [95] summarized the formation reactions of the SEI layer on the graphite anode for different PC-based/EC-based electrolytes and commonly used carbonate solvents by various characterization methods, in order to determine how the SEI layer is formed and what the SEI layer is composed of under different conditions. For instance, the SEI layer can be composed of , , and LiF . As for the irreversible absorption of lithium-ions or sodium-ions, it is related to the properties of the host material itself, depending on whether the host material can offer enough space or energy for ions to be extracted. For example, in order to identify the other sources of ICE loss besides the formation of a SEI layer, Mitlin and co-workers utilized pseudographitic carbon in LIBs and SIBs [96]. They found by analysis of XRD, XPS, and Raman data that irreversible lithium-ions/sodium-ions adsorption takes place on graphene edges, domain surfaces, or in the amorphous regions between the pseudographitic domains, all of which are expected to lead to the initial capacity
V，并且 在这个阶段发生了不可逆的能力。在锂离子插入石墨电极期间，第二级在0.25 V和0.04 V之间，这是 不可逆容量的原因。Agubra等[95]通过各种表征方法总结了不同PC基/EC基电解质和常用碳酸盐溶剂在石墨阳极上SEI层的形成反应，以确定SEI层是如何形成的，以及SEI层在不同条件下由什么组成。例如，SEI 层可以由 、 和 LiF 组成。至于锂离子或钠离子的不可逆吸收，则与主体材料本身的性质有关，取决于主体材料是否能提供足够的空间或能量来提取离子。例如，为了确定除SEI层形成之外的其他ICE损失来源，Mitlin及其同事在LIB和SIB中使用了假石墨碳[96]。他们通过分析XRD、XPS和拉曼数据发现，不可逆的锂离子/钠离子吸附发生在石墨烯边缘、畴表面或伪石墨畴之间的非晶区域，所有这些都有望导致初始容量
(a) （一）

(b) （二）

Fig. 4. Some causes of initial capacity loss for alloying-type anode materials. (a) Ball-stick models of crystal structures of and : lithium, green: silicon, purple: oxygen). Reproduced from Ref. [100] with permission from The Royal Society of Chemistry. (b) The relationship of (left side) and (right side) with particle size (d) of Sn in a mixture. (c) Schematic drawing shows the structure and phase evolution of a -based electrode during initial discharge and charge. (d) Schematic diagram for the structural evolution of hierarchical . Reproduced from Refs. [101] with permission from The Royal Society of Chemistry.
图 4.合金型负极材料初始容量损失的一些原因。（a） 和 ：锂，绿色：硅，紫色：氧的晶体结构的球棒模型）。经英国皇家化学学会许可，转载自参考文献[100]。（b） 混合物中Sn 的（左侧）和 （右侧）与粒径（d）的关系。（c） 示意图显示了基于 电极的初始放电和充电过程中的结构和相演化。（d） 层次结构演化示意图 。转载自参考文献。[101] 经英国皇家化学学会许可。

loss. 损失。

Conversion-type anode materials are usually transition-metal oxides, sulfides, selenides, nitrides, and phosphides that are of relatively low cost and relatively high capacity, such as , and . The conversion-type anode materials can store lithium/sodium through reversible replacement redox reactions between lithiumions/sodium-ions and transition-metal cations (e. g., ). For this class of materials, in both LIBs and SIBs, there are mainly four reasons causing the irreversible capacity loss during the initial cycling process: (i) the formation of SEI layer, (ii) the irreversible decomposition of , (iii) the irreversible absorption of lithium-ions or sodium-ions within the host materials, and (iv) adverse side reactions between inactive components and lithium/sodium metal. The formation of the SEI layer is associated with electrolyte decomposition on the surface of active materials, so that a larger contact area between conversion-type anode and the electrolyte can cause a greater amount of SEI layer formation. For the irreversible decomposition of , it is unavoidable because of the nature of the conversion reaction mechanism, the electrochemically inactivity and the inappropriate reaction dynamics of . For example, Kang et al. considered the use of as an anode material candidate for LIBs [80]. They found that has a crucial problem in view of its ICE, which just reaches . The reasons for such a low value of ICE were summarized in detail. First, the SEI layer is formed on the surface of the anode material during the first discharging from 1.2 V to 0.2 V , and it acts as a lithium-ion trap during this first charging. Second, based on the conversion reaction mechanism, the electrochemical reaction leads to the formation of (as shown in Fig. 3a). However, is known as an electrochemically inactive material with a lack of reversible decomposition of during the first charging
转化型负极材料通常是成本相对较低、容量相对较高的过渡金属氧化物、硫化物、硒化物、氮化物和磷化物，如 、 和 。转化型阳极材料可以通过锂离子/钠离子和过渡金属阳离子（例如） 之间的可逆置换氧化还原反应来储存锂/钠。对于这类材料，无论是在锂离子电池还是SIB中，在初始循环过程中造成不可逆容量损失的原因主要有四个：（i）SEI层的形成，（ii）锂离子或钠离子在主体材料中的不可逆 分解，以及（iv）非活性组分与锂/钠金属之间的不良副反应。SEI层的形成与活性材料表面的电解质分解有关，因此转化型阳极与电解质之间的接触面积越大，SEI层的形成量就越大。对于 的不可逆分解，由于转化反应机理的性质、电化学不活泼和反应 动力学的不适当，是不可避免的。例如，Kang等人考虑使用锂 离子电池作为阳极材料候选[80]。他们发现 ，鉴于其 ICE，它有一个关键问题，它刚刚达到 .详细总结了ICE值如此之低的原因。首先，在从1.2 V到0.2 V的第一次放电过程中，在阳极材料的表面形成SEI层，并在第一次充电期间充当锂离子阱。 其次，根据转化反应机理，电化学反应导致形成（ 如图3a所示）。然而， 它被称为电化学非活性材料， 在第一次充电期间缺乏可逆分解