Tailoring Practically Accessible Polymer/Inorganic Composite Electrolytes for All-Solid-State Lithium Metal Batteries: A Review
为全固态锂金属电池定制切实可行的聚合物/无机复合电解质：综述

AbstractSection Highlights  亮点

• The current issues and recent advances in polymer/inorganic composite electrolytes are reviewed.
  综述了聚合物/无机复合电解质的当前问题和最新进展。

• The molecular interaction between different components in the composite environment is highlighted for designing high-performance polymer/inorganic composite electrolytes.
  在设计高性能聚合物/无机复合电解质时，重点考虑了复合环境中不同成分之间的分子相互作用。

• Inorganic filler properties that affect polymer/inorganic composite electrolyte performance are pointed out.
  指出了影响聚合物/无机复合电解质性能的无机填料特性。

• Future research directions for polymer/inorganic composite electrolytes compatible with high-voltage lithium metal batteries are outlined.
  概述了与高压锂金属电池兼容的聚合物/无机复合电解质的未来研究方向。

AbstractSection Abstract  摘要

Solid-state electrolytes (SSEs) are widely considered the essential components for upcoming rechargeable lithium-ion batteries owing to the potential for great safety and energy density. Among them, polymer solid-state electrolytes (PSEs) are competitive candidates for replacing commercial liquid electrolytes due to their flexibility, shape versatility and easy machinability. Despite the rapid development of PSEs, their practical application still faces obstacles including poor ionic conductivity, narrow electrochemical stable window and inferior mechanical strength. Polymer/inorganic composite electrolytes (PIEs) formed by adding ceramic fillers in PSEs merge the benefits of PSEs and inorganic solid-state electrolytes (ISEs), exhibiting appreciable comprehensive properties due to the abundant interfaces with unique characteristics. Some PIEs are highly compatible with high-voltage cathode and lithium metal anode, which offer desirable access to obtaining lithium metal batteries with high energy density. This review elucidates the current issues and recent advances in PIEs. The performance of PIEs was remarkably influenced by the characteristics of the fillers including type, content, morphology, arrangement and surface groups. We focus on the molecular interaction between different components in the composite environment for designing high-performance PIEs. Finally, the obstacles and opportunities for creating high-performance PIEs are outlined. This review aims to provide some theoretical guidance and direction for the development of PIEs.
固态电解质（SSE）具有极高的安全性和能量密度，因此被广泛认为是未来可充电锂离子电池的重要组成部分。其中，聚合物固态电解质（PSE）因其灵活性、形状多样性和易加工性，成为替代商用液态电解质的竞争性候选材料。尽管 PSE 发展迅速，但其实际应用仍面临着离子导电性差、电化学稳定窗口窄和机械强度低等障碍。在 PSE 中添加陶瓷填料形成的聚合物/无机复合电解质（PIEs）融合了 PSE 和无机固态电解质（ISEs）的优点，由于界面丰富且各具特色，因而表现出令人赞赏的综合性能。一些 PIE 与高压阴极和锂金属阳极高度兼容，为获得高能量密度的锂金属电池提供了理想的途径。本综述阐明了 PIEs 的当前问题和最新进展。填料的特性（包括类型、含量、形态、排列和表面基团）对 PIEs 的性能有显著影响。我们重点研究了设计高性能 PIEs 的复合环境中不同成分之间的分子相互作用。最后，概述了创造高性能 PIE 的障碍和机遇。本综述旨在为开发 PIE 提供一些理论指导和方向。

1 Introduction
1 简介

The ever-increasing energy consumption sparks widespread interest in energy-efficient storage and flexible conversion. Lithium-ion batteries (LIBs) have been heavily marketed in consumer electronics and traffic electrification owing to their eco-friendliness, high energy density and working voltage [1,2,3]. Currently, the energy density of LIBs has approached 260 Wh kg⁻¹ and is challenging to break through [4, 5]. Meanwhile, LIBs have repeatedly experienced catastrophic failure in recent years, resulting in severe property damage and raising public concern. Developing LIBs with high energy density and safety has become unremitting pursuit. Organic liquid electrolyte frequently employed in commercial LIBs is blamed for thermal runaway [6]. It has volatility and flammability, posing safety issues about leakage and fire. The constituent solvents such as ethylene carbonate have strong reactivity with lithium metal anodes (LMAs) known as “holy grail” anodes, causing dendrite growth and continual side reactions [7,8,9]. Solid-state electrolytes (SSEs) can effectively enhance safety by eliminating the flammable liquid electrolyte. They can inhibit the dissolution of transition metal ions of the cathode materials and block the by-product cross talk between the electrodes [10]. SSEs can also limit the shuttle effect of polysulfide in lithium–sulfur batteries and reduce the cross talk of O₂ and H₂O as well as the nucleophilic attack of reduced oxygen in lithium-oxygen batteries [11, 12]. Some SSEs exhibit thermodynamic/electrochemical compatibility on the interfaces of LMAs, which further broaden the electrochemical window and enhance the energy density [13].
日益增长的能源消耗引发了人们对高能效存储和灵活转换的广泛兴趣。锂离子电池（LIB）因其环保、高能量密度和工作电压而在消费类电子产品和交通电气化领域大受欢迎 [1,2,3]。目前，锂离子电池的能量密度已接近 260 Wh kg^-1，要想突破这一极限还很困难[4、5]。同时，近年来锂电池屡次发生灾难性故障，造成严重的财产损失，引起了公众的关注。开发具有高能量密度和安全性的 LIB 已成为人们不懈的追求。商用锂离子电池中经常使用的有机液态电解质被指会导致热失控[6]。有机液态电解质具有挥发性和易燃性，会引发泄漏和火灾等安全问题。碳酸乙烯酯等组成溶剂与被称为 "圣杯 "阳极的锂金属阳极（LMA）有很强的反应性，会导致枝晶生长和持续的副反应[7,8,9]。固态电解质（SSE）可消除易燃液体电解质，从而有效提高安全性。固态电解质可抑制阴极材料中过渡金属离子的溶解，并阻断电极之间的副产物串扰[10]。 SSE 还可以限制锂硫电池中多硫化物的穿梭效应，减少锂氧电池中 O₂ 和 H₂O 的交叉反应以及还原氧的亲核攻击 [11, 12] 。一些 SSE 在 LMA 的界面上表现出热力学/电化学兼容性，从而进一步拓宽了电化学窗口并提高了能量密度 [13]。

SSEs can be categorized into two groups: polymer solid-state electrolytes (PSEs) and inorganic solid-state electrolytes (ISEs). Single PSEs and ISEs are challenging to fulfill general requirements, such as adequate ionic conductivity (> 10^–4 S cm⁻¹), high operating voltage (up to 4–5 V vs. Li/Li⁺), appropriate mechanical strength (> 6 GPa) and excellent interfacial contact (Fig. 1) [14, 15]. PSEs exhibit good elasticity and adaptability to volume variations, which are widely used for flexible batteries. However, the polymers crystallize easily at ambient temperature, resulting in limited ionic conductivity [16]. The thermodynamic instability of the interface restricts their compatibility with high-voltage cathode materials and the inferior mechanical properties cannot suppress dendrite growth [17, 18]. ISEs own acceptable ionic conductivity, extensive electrochemical window and satisfactory mechanical strength, while their brittleness and fragility cause poor machinability and large contact resistance. Recently, researchers have been committed to integrating inorganic fillers into PSEs to form polymer/inorganic composite electrolytes (PIEs) and realize the synergistic effect of different materials. Inorganic fillers not only increase the mechanical strength of the polymer matrix but also act as plasticizers, preventing polymer crystallization and boosting the ionic conductivity of the electrolyte [19,20,21]. The interaction of the fillers with the polymer increases the redox stability of the electrolyte, hence extending the electrochemical window [22]. PIEs with sufficient ionic conductivity, electrochemical stability and outstanding mechanical strength represent tremendous potential for the next generation of LIBs.
固态电解质可分为两类：聚合物固态电解质（PSE）和无机固态电解质（ISE）。单一的 PSE 和 ISE 很难满足一般要求，例如足够的离子电导率（> 10^-4 S cm^-1）、高工作电压（高达 4-5 V vs. Li/Li^-4 S cm^-1）、无机固态电解质（ISE）和聚合物固态电解质。Li/Li⁺）、适当的机械强度（> 6 GPa）和出色的界面接触（图1）[14、15]。PSE 具有良好的弹性和对体积变化的适应性，可广泛用于柔性电池。然而，聚合物在环境温度下容易结晶，导致离子传导性有限[16]。界面的热力学不稳定性限制了它们与高电压阴极材料的兼容性，而较差的机械性能也无法抑制枝晶的生长[17, 18] 。ISE 具有可接受的离子导电性、广泛的电化学窗口和令人满意的机械强度，而其脆性和易碎性则导致加工性能差和接触电阻大。近年来，研究人员致力于将无机填料融入 PSE 中，形成聚合物/无机复合电解质（PIE），实现不同材料的协同效应。无机填料不仅能增加聚合物基体的机械强度，还能起到增塑剂的作用，防止聚合物结晶，提高电解质的离子导电性[19,20,21] 。 填料与聚合物的相互作用提高了电解质的氧化还原稳定性，从而延长了电化学窗口期[22]。PIE 具有足够的离子导电性、电化学稳定性和出色的机械强度，是下一代 LIB 的巨大潜力所在。

Fig. 1 图 1

Performance comparison of different electrolytes
不同电解质的性能比较

Numerous inorganic fillers have emerged to enhance the performance of PIEs, including metal oxides, ceramic Li⁺ conductors and novel porous materials like metal–organic frameworks (MOFs). Despite extensive researches asserting that certain fillers have the potential to dramatically enhance PIE performance, the mechanism underlying these improvements lacks in-depth understanding and sortation. This review provides a comprehensive summary of the existing challenges and current advancements in PIEs. The properties of the PIEs are profoundly influenced by the nature of ceramic fillers including content, morphology, arrangement and surface groups. The molecular interaction in different phases and interface regions are highlighted to understand the improvement. The major purpose of this review is to propose alternative solutions to overcome the defects of PIEs and inspire the engaged contributors and new entrants to explore scalable strategies for the industrialization of PIEs.
为了提高 PIE 的性能，出现了许多无机填料，包括金属氧化物、陶瓷锂⁺ 导体和新型多孔材料（如金属有机框架 (MOF)）。尽管大量研究表明，某些填料具有显著提高 PIE 性能的潜力，但对这些改进的内在机理还缺乏深入的了解和分类。本综述全面总结了 PIE 的现有挑战和当前进展。PIE 的性能深受陶瓷填料性质（包括含量、形态、排列和表面基团）的影响。本综述重点介绍了不同相和界面区域的分子相互作用，以帮助读者了解相关改进。本综述的主要目的是提出克服 PIEs 缺陷的替代解决方案，并激励参与研究的人员和新加入者探索 PIEs 工业化的可扩展战略。

2 Key Issues in the Development of PIEs
2开发项目执行实体的关键问题

The key issues in the development of PIEs are illustrated in Fig. 2. PIEs must be engineered to be thin (thickness < 30 μm) and have fast Li⁺ transport capability to compete with the available commercial liquid LIBs [23]. Besides, PIEs should match the electrodes with high loading, specific capacity and working voltage to gain advantages in energy density and power capability. Combining SSEs with 4 V-class cathode and LMAs can increase the energy density, which also poses a significant challenge since it may cause performance deterioration due to high reactivity between electrolyte and charged electrodes [24]. The poor point-to-point contact generates a substantial interface resistance and uneven distribution of local current density, driving dendritic growth. Periodic volume changes of the electrode lead to the formation and accumulation of structural stress, which will deteriorate the ion transport on the electrode/electrolyte interface [25]. Improving the voltage window and developing a stable interface of PIEs are crucial challenges for achieving high-performance all-solid-state batteries (ASSBs).
图2说明了开发 PIE 的关键问题。PIE 必须设计得很薄（厚度 < 30 μm），并具有快速的锂⁺ 传输能力，才能与现有的商用液态 LIB [23]竞争。此外，PIE 应与高负载、高比容量和高工作电压的电极相匹配，以获得能量密度和功率能力方面的优势。将 SSE 与 4 V 级阴极和 LMA 结合使用可提高能量密度，但这也是一项重大挑战，因为电解液和带电电极之间的高反应性可能会导致性能下降 [24]。点对点接触不良会产生巨大的界面电阻和不均匀的局部电流密度分布，从而导致树枝状生长。电极的周期性体积变化会导致结构应力的形成和积累，从而恶化电极/电解质界面上的离子传输[25]。要实现高性能全固态电池（ASSB），改善 PIE 的电压窗口和开发稳定的界面是至关重要的挑战。

Fig. 2 图 2

Key issues in the development of PIEs
开发 PIE 的关键问题

2.1 Lithium-Ionic Conductivity
2.1锂离子电导率

Ionic conductivity is a critical metric for accessing the migratory ability of Li⁺ in electrolytes. It is proportional to carrier concentration and transference number (t₊). The ionic conductivity of commercial liquid organic electrolytes can approach 10^–3–10^–2 S cm⁻¹, while the ionic conductivity of PSEs is less than 10^–4 S cm⁻¹ at room temperature [26]. PSEs with conductivity less than 5 × 10^–4 S cm⁻¹ are incapable of meeting the operational requirements of thick electrodes (thickness > 70 μm) [27], and gain no advantage in terms of energy density. According to the free-volume model, polymer matrix transfers Li⁺ ions through the polar sites and local segmental motions in amorphous regions [28, 29]. The ion diffusion kinetics in crystalline region is negligible. Reducing the crystallinity of polymer matrix at normal temperature is a crucial method for enhancing conductivity. However, lowering crystallinity reduces polymer strength, causing the polymer behaves as a viscous liquid and incapable of forming a self-supporting membrane. The balance between conductivity and mechanical strength raises concerns regarding polymers as hosts. Given that the reported ionic conductivity of certain ISEs has reached 10^–3 S cm⁻¹ and they feature great mechanical strength, the development of polymer/ceramic composite electrolytes should be a viable solution to enhance ion conductivity to some extent (Fig. 3a) [30, 31]. Ionic conductivity of PIEs is primarily influenced by interactions between Li⁺, anions, polymers and fillers [32]. The ion–dipole interaction between ions and the polymer matrix impacts the concentration of free Li⁺. The Lewis acid–base interaction generated by inorganic fillers influences polymer segment motion, lithium salt solubility and Li⁺ ion diffusion behavior. Making full use of the interaction between different components to optimize ionic conductivity has emerged as a primary focus for developing PIE.
离子电导率是了解锂⁺ 在电解质中迁移能力的关键指标。它与载流子浓度和转移数（t₊）成正比。商用液态有机电解质的离子电导率可接近 10^-3-10^-2 S cm^-1 、而 PSE 在室温下的离子电导率小于 10^-4 S cm^-1 [26]。电导率小于 5 × 10^-4 S cm^-1 的 PSE 无法满足厚电极（厚度大于 70 μm）的工作要求 [27]，在能量密度方面也没有优势。根据自由体积模型，聚合物基质通过无定形区域的极性位点和局部分段运动转移 Li⁺ 离子[28, 29] 。结晶区域的离子扩散动力学可忽略不计。在常温下降低聚合物基体的结晶度是提高导电性的重要方法。然而，降低结晶度会降低聚合物强度，使聚合物表现为粘性液体，无法形成自支撑膜。导电性和机械强度之间的平衡引起了人们对聚合物作为宿主的关注。 鉴于某些 ISE 的离子电导率已达到 10^-3 S cm^-1 且具有很高的机械强度，开发聚合物/陶瓷复合电解质应该是在一定程度上提高离子电导率的可行解决方案（图 3）。3a) [30, 31] 。PIE 的离子导电性主要受 Li⁺、阴离子、聚合物和填料之间相互作用的影响[32]。离子与聚合物基质之间的离子-偶极相互作用会影响游离锂的浓度⁺。无机填料产生的路易斯酸碱相互作用会影响聚合物段的运动、锂盐溶解度和锂⁺ 离子扩散行为。充分利用不同成分之间的相互作用来优化离子传导性已成为开发 PIE 的主要重点。

Fig. 3 图 3

a Ionic conductivity and electrochemical window of different SSEs [33]. Copyright 2020, American Chemical Society. b HOMO and LUMO values of different polymers and lithium salts [32]. Copyright 2019, John Wiley and Sons Publisher
a 不同 SSE 的离子电导率和电化学窗口 [33].美国化学学会版权所有，2020 年。b 不同聚合物和锂盐的 HOMO 值和 LUMO 值 [32].Copyright 2019, John Wiley and Sons Publisher

The t₊ quantifies the contribution of Li⁺ to the transport charge. Since anions do not engage in reversible electrochemical reactions, their migration cannot transfer effective charges [34]. Nevertheless, for anions with a large volume and mass, their transfer number is always bigger than that of Li⁺; hence, the t₊ of PSEs is always less than 0.5 [35]. t₊ is determined by the ability of polymer to dissociate lithium salt, and thus polymer with a high dielectric constant and lithium salt with a low lattice energy can achieve high t₊ [36, 37]. Besides, the transport of Li⁺ strongly depends on the segmental movement of the amorphous region in the polymer matrix. Polymer with a low glass transition temperature (T_g) can facilitate the segment movement and enhance t₊. Adding fillers in polymer can change the local environment of Li⁺ ions. Especially, the strong interaction between fillers and anions results in the dissociation of lithium salts and an increase in t₊. Fixing anions with fillers to increase t₊ can reduce the concentration polarization on the electrode and inhibits fractal dendrites caused by the depletion of Li⁺ on the anode.
t₊ 量化了 Li⁺ 对迁移电荷的贡献。由于阴离子不进行可逆的电化学反应，因此它们的迁移不能转移有效电荷 [34]。然而，对于体积和质量较大的阴离子，其转移数总是大于 Li⁺ 的转移数，因此 PSE 的 t₊ 总是小于 0.5 [35]。t₊ 由聚合物离解锂盐的能力决定，因此高介电常数的聚合物和低晶格能的锂盐可以实现高 t₊ [36, 37] 。此外，锂⁺ 的传输在很大程度上取决于聚合物基体中无定形区域的分段移动。玻璃化转变温度（T_g）较低的聚合物可促进锂段的移动，并提高t₊。在聚合物中添加填料可以改变锂⁺ 离子的局部环境。特别是，填料与阴离子之间的强相互作用会导致锂盐解离和 t₊ 的增加。用填料固定阴离子以增加 t₊ 可以降低电极上的浓度极化，并抑制阳极上因锂⁺ 耗尽而产生的分形树枝状。

2.2 Electrochemical Stability
2.2 电化学稳定性

The electrolyte decomposes when the working potential of the battery exceeds its redox potential window [38, 39]. To achieve high-voltage stability, PIEs require every component has a HOMO energy level less than the Fermi energy of cathode. The HOMO values of most polymers are greater than those of lithium salts, indicating that the polymers preferentially undergo interfacial side reactions (Fig. 3b) [32]. Furthermore, adding lithium salt reduces the oxidation stability of polymers because the anions shield positive charges on the chains [40]. Electrolytes based on PEO are typically utilized for 3 V-grade cathode materials due to the labile lone pairs on the ether-oxygen atoms in the PEO chains [41]. Yu et al. found that the C-H bonds became weak after partially oxidizing the ether-oxygen atoms, causing the H protons to be carried away by the TFSI⁻ and generate hydrogenated HTFSI. As a potent acid, it can impair interface and produce H₂ on the anode [42]. In addition, the cathodes such as LiNiO₂, LiCoO₂ and LiNi_xCo_yMn_1−x−yO₂ possess large specific surface areas and show strong catalytic ability, due to the transition metal ions or conductive carbon, hence accelerating electrolyte degradation [43, 44]. The molecule interaction of the components changes the chemical environment of the polymers, which consequently affects their HOMO value. Incorporating inorganic fillers can improve the oxidative stability of polymers via Lewis acid–base interaction, hydrogen-bonding or dipolar interactions between the lone pairs of polymers and the surface groups of fillers [32, 45]. Cui et al. reported that the ether-oxygen segments in the polymer matrix can interact with the P atoms in the Li₆PS₅Cl fillers, thereby reducing the HOMO energy level of the polymer and widening the electrochemical window [46]. Chen et al. found that the strong Lewis acid–base interaction between anions and the surface groups of Li₇La₃Zr₂O₁₂ fillers can decrease the oxidation of anions [47]. Meanwhile, combining anions with the fillers can diminish the shielding effect of anions on the positive charges of polymers and effectively stabilize the polymers at high voltage. Furthermore, rational design of polymer and inorganic Li⁺ conductors can inhibit the direct contact of unstable interface and improve the compatibility with Li and high-voltage cathode [48]. Specifically, most polymers are stable at the Li anode but poor at the high-voltage cathode, whereas certain inorganic oxides and sulfides are the exact reverse. Properly designing PIEs with two or more layers of vertical heterostructure provides a viable option for concurrently meeting cathode and anode requirements, exploring a new pathway for high-voltage ASSBs.
当电池的工作电位超过其氧化还原电位窗口时，电解质就会分解[38，39]。为了实现高压稳定性，PIE 要求每个组件的 HOMO 能级都小于阴极的费米能。大多数聚合物的 HOMO 值都大于锂盐的 HOMO 值，这表明聚合物会优先发生界面侧反应（图 3b) [32]。此外，添加锂盐会降低聚合物的氧化稳定性，因为阴离子会屏蔽链上的正电荷[40]。基于 PEO 的电解质通常用于 3 V 级阴极材料，这是因为 PEO 链中的醚氧原子上存在易变孤对[41]。Yu 等人发现，醚氧原子部分氧化后，C-H 键变弱，导致 H 质子被 TFSI^- 带走，生成氢化 HTFSI。作为一种强酸，它会损害界面并在阳极上产生 H₂ [42]。此外，阴极如 LiNiO₂、LiCoO₂ 和 LiNi_xCo_yMn_1-x-yO₂ 具有较大的比表面积，并显示出较强的催化能力、43, 44] 。 各成分的分子相互作用会改变聚合物的化学环境，从而影响其 HOMO 值。加入无机填料可通过路易斯酸碱作用、氢键作用或聚合物孤对与填料表面基团之间的偶极作用提高聚合物的氧化稳定性[32、45]。Cui 等人报告说，聚合物基体中的醚氧段可以与 Li₆PS₅Cl 填料中的 P 原子相互作用，从而降低聚合物的 HOMO 能级，拓宽电化学窗口[46]。Chen 等人发现，阴离子与 Li₇La₃Zr₂O₁₂ 填料表面基团之间的强路易斯酸碱相互作用可降低阴离子的氧化性[47]。同时，阴离子与填料的结合可以减弱阴离子对聚合物正电荷的屏蔽作用，有效稳定高压下的聚合物。此外，合理设计聚合物和无机锂⁺导体可抑制不稳定界面的直接接触，提高锂与高压阴极的相容性[48]。具体来说，大多数聚合物在锂阳极上都很稳定，但在高压阴极上却很差，而某些无机氧化物和硫化物则正好相反。 正确设计具有两层或多层垂直异质结构的 PIE，为同时满足阴极和阳极的要求提供了可行的选择，为高压 ASSB 探索了一条新的途径。

2.3 Dendrite Inhibition
2.3 树突抑制

LMA possesses unique superiority in energy density because it owns the lowest molar mass and reduction potential among metallic elements [49, 50]. However, notorious dendrite propagation gives rise to large volume expansion, low reversibility and potential safety hazards [51]. In polymer electrolytes, heterogeneous interface, limited ion transport and low mechanical strength are the primary reasons driving dendrite growth [52]. Firstly, solid electrolyte interface (SEI) realizes the dynamic passivation of the electrode, which expands the electrochemical window of LMBs to a certain extent [53]. However, the heterogeneous SEI induces uneven Li⁺ flux on the anode, triggering the propagation of mossy and whiskery dendrites (Fig. 4a) [7, 54]. Isotropic inert interface with uniform ionic conductivity can effectively homogenize lithium flux; thus, dendrite growth can be effectively alleviated by constructing a stable electrolyte layer on the anode. Furthermore, the limited transport results in local ion depletion on the interface, creating a space charge layer (SCL) [55]. The large electric field in the SCL leads to electric convection and rapid growth of fractal dendrites. Fixing anions to enhance t₊ and prevent SCL formation is regarded as an effective method for inhibiting dendrites [56]. Additionally, enhancing mechanical strength can regulate Li nucleation and growth by altering the surface energy at the Li top surface [57]. According to the theoretical model proposed by Monroe and Newman, lithium dendrites can be eliminated when the surface shear modulus is at least 2–3 times that of metallic lithium (4.5 GPa) [58]. The polymer electrolytes have a low shear modulus (typically < 0.1 GPa) and are incapable of inhibiting Li dendrites (Fig. 4b). Viswanathan et al. further established a universal criterion for stable electroplating using the shear modulus ratio of SSEs and lithium anode ($G_{\text{SSE}}$/$G_{\text{Li}}$) and the molar volume ratio of Li⁺ ions and lithium anode ($V_{{\text{Li}}^{+}}/V_{\text{Li}}$) [59]. They concluded stable electroplating necessitated the use of SSEs with a high (low) Li molar volume and high (low) shear modulus. PSEs have soft texture and low shear modulus, while the formation of Li⁺ solvated “cages” leads to high volume expansion and high $V_{{\text{Li}}^{+}}$, causing they cannot inhibit dendrites. To verify the feasibility of this criterion, Helms et al. prepared nano-LiF@polymer PIEs by in situ cation metathesis [60]. The modified PIEs had minimally reconfigurable, ceramic-like, ion-conducting domains contained in a soft, polymer-like matrix with a low shear modulus, which can inhibit the growth of dendrites.
LMA 具有独特的能量密度优势，因为它在金属元素中具有最低的摩尔质量和还原潜力 [49, 50] 。然而，臭名昭著的枝晶扩展会导致体积膨胀大、可逆性低和潜在的安全隐患[51]。在聚合物电解质中，异质界面、有限的离子传输和较低的机械强度是导致枝晶生长的主要原因[52]。首先，固体电解质界面（SEI）实现了电极的动态钝化，在一定程度上扩大了 LMB 的电化学窗口[53]。然而，异质 SEI 会导致阳极上的锂⁺ 通量不均匀，引发苔藓状和须状树枝状突起的传播（图 4a ）[7, 54] 。各向同性的惰性界面具有均匀的离子传导性，可有效均化锂通量；因此，通过在阳极上构建稳定的电解质层，可有效缓解枝晶的生长。此外，有限的传输会导致界面上的局部离子耗竭，形成空间电荷层（SCL）[55]。SCL 中的大电场导致电对流和分形树突的快速增长。固定阴离子以增强t₊和防止SCL形成被认为是抑制树突的有效方法[56]。此外，提高机械强度可通过改变锂顶表面的表面能来调节锂的成核和生长[57]。 根据 Monroe 和 Newman 提出的理论模型，当表面剪切模量至少是金属锂的 2-3 倍（4.5 GPa）时，锂枝晶就可以被消除[58]。聚合物电解质的剪切模量较低（通常< 0.1 GPa），无法抑制锂枝晶（图4b）。Viswanathan 等人使用 SSE 和锂阳极的剪切模量比（G_{{text\{SSE}}}}$\text{G\_{{text{SSE}}}}}$/$G_{textLi}$) 和 Li⁺ 离子与锂阳极的摩尔体积比（$V_{{{text{Li}}^{ + }} /V_{{{text{Li}}}}$)[59]。他们认为，稳定的电镀需要使用具有高（低）锂摩尔体积和高（低）剪切模量的 SSE。PSE 具有柔软的质地和较低的剪切模量，而 Li⁺ 溶剂 "笼 "的形成会导致高体积膨胀和高 (V_{{\text{Li}}^{ + }}) ，从而导致它们无法抑制树枝状突起。为了验证这一标准的可行性，Helms 等人通过原位阳离子偏析法制备了纳米 LiF@ 聚合物 PIEs [60]。改性后的 PIEs 具有最小可重构、类似陶瓷的离子导电域，包含在具有低剪切模量的软聚合物基质中，可抑制树枝状突起的生长。

Fig. 4 图 4

a 3D reconstructed volumes of the dendrite located on the subsurface below the polymer/electrode interface [54]. Copyright 2014, Springer Nature. b Stress–strain curves of different SSEs [23]. Copyright 2021, Springer Nature. c PIEs can improve the contact stability with electrode [23]. Copyright 2021, Springer Nature
a 位于聚合物/电极界面下方亚表面的树突的三维重建体积 [54]。Copyright 2014, Springer Nature.b 不同 SSE 的应力-应变曲线 [23].版权归 Springer Nature 2021 所有。c PIE 可提高与电极接触的稳定性 [23].2021 年《施普林格-自然》杂志版权所有

2.4 Contact Stability
2.4 接触稳定性

During reciprocating charge and discharge, the electrode materials may undergo lattice and phase changes, causing volume fluctuation and particle pulverization [61, 62]. Inadequate contact between the electrode and PIEs leads to high contact resistance even complete loss of electric disconnection (Fig. 4c). Besides, the uneven plating/stripping behavior of metallic lithium reduces the effective contact area, hence exacerbating the inhomogeneous distribution of Li⁺ flux. A decent electrolyte design requires a compromise between the appropriate modulus and surface adhesion [63].
在往复充放电过程中，电极材料可能发生晶格和相变，导致体积波动和颗粒粉碎[61，62]。电极与 PIE 之间接触不充分会导致接触电阻过高，甚至完全断电（图4c）。此外，金属锂不均匀的电镀/剥离行为减少了有效接触面积，从而加剧了锂⁺ 通量的不均匀分布。合理的电解质设计需要在适当的模量和表面附着力之间做出折衷[63]。

In addition to the typical issues listed above, PIEs face additional challenges with some specific energy storage systems. Lithium–sulfur batteries have an overwhelming advantage in energy density (500–600 Wh kg⁻¹), which partly attributes to the reduction of S to Li₂S yields a high specific capacity of 1675 mAh g⁻¹. However, the shuttle effect of soluble polysulfide intermediates (Li₂S_n, 3 ≤ n ≤ 8) results in the rapid attenuation of capacity and low coulombic efficiency [64]. Some polymers such as PEO have a strong solvation effect on polysulfides at high temperatures, causing polysulfides to dissolve in polymers and trigger side reactions [65]. The polysulfides on the anode induce uneven plating/stripping behavior of lithium and further deterioration of interface contact [66]. As physical barriers, inorganic fillers can reduce the contact with polymers [67]. Meanwhile, they can adsorb polysulfides and mitigate the shuttle effect by forming chemical bonds with polysulfides [68]. Li–O₂ (air) batteries also have a much higher energy density (~ 950 Wh kg⁻¹) than the existing graphite||layered ternary cathode system. Polymers with non-toxic, non-combustible and nonvolatile characteristics provide feasible solutions to solve safety problems. However, most routinely used polymers, including PAN, PVDF, PVDF-HFP and PEO, are reactive to reduction products such as Li₂O₂ [69, 70]. Fortunately, Lewis acid base interaction between inorganic fillers and polymers can improve the electrochemical stability of the polymers [71, 72]. The charge transfer of Li–O₂ batteries using liquid electrolyte occurs at the solid–liquid-gas interface, while that of Li–O₂ batteries using PIEs occurs at the solid–gas interface. Due to the increased contact resistance, the reaction rate would be drastically slowed down. It is essential to develop catalysts to accelerate the kinetics of conversion reaction.
除上述典型问题外，PIE 还面临一些特定储能系统的额外挑战。锂硫电池在能量密度方面具有压倒性优势（500-600 Wh kg^-1），部分原因是将 S 还原为 Li₂S 可产生 1675 mAh g^-1 的高比容量。然而，可溶性多硫化物中间体（Li₂S_n, 3 ≤ n ≤ 8）的穿梭效应导致容量迅速衰减，库仑效率降低[64]。某些聚合物（如 PEO）在高温下对多硫化物有很强的溶解作用，导致多硫化物溶解在聚合物中并引发副反应[65]。阳极上的多硫化物会导致锂的不均匀镀层/剥离行为，并进一步恶化界面接触[66]。作为物理屏障，无机填料可减少与聚合物的接触[67]。同时，它们还能吸附多硫化物，并通过与多硫化物形成化学键来减轻穿梭效应[68]。锂-氧化₂（空气）电池的能量密度（~ 950 Wh kg^-1）也远高于现有的石墨||层状三元阴极系统。具有无毒、不可燃和不挥发特性的聚合物为解决安全问题提供了可行的解决方案。 然而，包括 PAN、PVDF、PVDF-HFP 和 PEO 在内的大多数常规聚合物对 Li₂O₂ 等还原产物具有反应性 [69, 70] 。幸运的是，无机填料和聚合物之间的路易斯酸碱相互作用可以提高聚合物的电化学稳定性[71, 72] 。使用液态电解质的 Li-O₂ 电池的电荷转移发生在固-液-气界面，而使用 PIE 的 Li-O₂ 电池的电荷转移发生在固-气界面。由于接触电阻增大，反应速度会大大降低。因此，开发催化剂以加速转化反应的动力学过程至关重要。

3 Fillers of PIEs
3 PIE 的填充物

3.1 Components of PIEs
3.1项目执行实体的组成部分

PIEs are made up of polymer matrix, lithium salt and ceramic filler. Wright et al. proposed that alkali metal salts mixed with polyethylene oxide (PEO) could conduct ions in 1973 [73]. And then Armand used the composite as electrolyte in batteries [74]. Subsequently, a broad array of polymers, including polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), emerged as matrices [73, 75]. It is challenging for a single polymer to satisfy all the requirements as electrolyte material (Table 1). By combining the benefits of several hosts, polymer/polymer cooperation offers the chance to create superior polymer matrices. Copolymerization, cross-linking, interpenetration and blending are the most explored techniques in this field [22]. These polymer segments typically include polar groups to dissolve lithium salts and transfer Li⁺ ions, such as C=O, –O–, –N–, C=N and –P– [37]. Li⁺ ions coordinate with polar groups on the polymer chains at certain places and generate free volume by local segment movement of the polymer chains, allowing Li⁺ to be transmitted within and between the chains [26] (Fig. 5a). Table 1 lists the fundamental characteristics of typical polymer matrices in terms of T_g and melting point (T_m). These two parameters govern the conductivity of Li⁺ ions. Specifically, T_g is crucial for the phase transition of polymer electrolytes since most studies hold that Li⁺ ion transport only takes place in the amorphous zone above T_g.
PIE 由聚合物基体、锂盐和陶瓷填料组成。1973 年，Wright 等人提出碱金属盐与聚氧化乙烯（PEO）混合后可以传导离子[73]。随后，Armand 将这种复合材料用作电池的电解质[74]。随后，包括聚偏二氟乙烯 (PVDF)、聚甲基丙烯酸甲酯 (PMMA)、聚丙烯腈 (PAN) 和聚偏二氟乙烯-共六氟丙烯 (PVDF-HFP) 在内的多种聚合物成为基质[73、75]。单一聚合物要满足作为电解质材料的所有要求具有挑战性（表 1）。通过结合几种宿主的优点，聚合物/聚合物合作提供了创造优质聚合物基质的机会。共聚、交联、互渗和混合是这一领域最常用的技术[22]。这些聚合物段通常包括极性基团，如 C=O、-O-、-N-、C=N 和 -P- [37]，以溶解锂盐并转移锂⁺ 离子。Li⁺ 离子在某些地方与聚合物链上的极性基团配位，并通过聚合物链的局部分段移动产生自由体积，从而使 Li⁺ 在链内和链之间传输[26]（图5a）。表 1 列出了典型聚合物基质的基本特性（T_g 和熔点 T_m ）。 这两个参数决定了锂⁺ 离子的电导率。具体来说，T_g 对聚合物电解质的相变至关重要，因为大多数研究认为，锂⁺ 离子的传输只发生在 T_g 以上的无定形区。

Table 1 Main properties of polymer matrix [76,77,78,79]
表 1 聚合物基质的主要特性 [76,77,78,79].

Fig. 5 图 5

a Li⁺ ions diffuse through the polar groups and segment movement of the polymer chains [26]. Copyright 2018, Royal Society of Chemistry. b Common lithium salts used in PIEs [26]. Copyright 2018, Royal Society of Chemistry
a Li⁺ 离子通过极性基团扩散并分段移动聚合物链 [26]。版权 2018 年，英国皇家化学会。b PIE 中使用的常见锂盐[26] 。版权 2018 年，英国皇家化学学会

Ordinary lithium salts usually contain the characteristics of large anionic radius and delocalization charge, such as LiPF₆, LiFSI, LiTFSI and LiClO₄, which have high solubility in polymers and easily generate stable SEI [26, 80] (Fig. 5b). Ceramic fillers can be classified as either inert or active fillers depending on whether they can conduct Li⁺ ions. The inert fillers include SiO₂, ZrO₂, Al₂O₃, Y₂O₃, LiAlO₂, and the active fillers include garnet, NASICON, perovskite, sulfide, Li₃N, etc. [71, 81,82,83,84]. Both inert and active fillers can be utilized as plasticizers to diminish the crystallization, hence facilitating the movement of Li⁺ ions. As fast ion conductors, active fillers can also promote Li⁺ diffuse through the defects or vacancies in the crystal structure, such as Schottky defects and Flenker defects, thus enhancing the ionic conductivity. If the active fillers are highly concentrated, Li⁺ ions can diffuse through the permeation network provided by continuous filler particles [85]. In this case, the polymer matrix only acts as a flexible host and is not responsible for Li⁺ ion diffusion. Therefore, high ion conductivity and t₊ can also be achieved without lithium salts [86,87,88].
普通锂盐通常具有阴离子半径大、电荷分散的特点，如LiPF₆、LiFSI、LiTFSI和LiClO₄、LiTFSI 和 LiClO₄，它们在聚合物中的溶解度高，容易生成稳定的 SEI [26, 80] （图 2）。5b）。根据陶瓷填料是否能传导锂⁺ 离子，可将其分为惰性填料和活性填料。惰性填料包括 SiO₂, ZrO₂, Al₂O₃ 、Y₂O₃, LiAlO₂, 活性填料包括石榴石、NASICON、透辉石、硫化物、Li₃N 等。[71, 81, 82, 83, 84] 。惰性和活性填料都可用作增塑剂来减少结晶，从而促进锂⁺ 离子的移动。作为快速离子导体，活性填料还能促进锂⁺ 通过晶体结构中的缺陷或空位（如肖特基缺陷和弗伦克缺陷）扩散，从而增强离子导电性。如果活性填料高度浓缩，锂⁺ 离子可通过连续填料颗粒提供的渗透网络进行扩散[85]。在这种情况下，聚合物基体只起到柔性宿主的作用，并不负责锂⁺ 离子的扩散。 因此，在不使用锂盐的情况下，也能实现高离子传导性和 t₊ [86,87,88] 。

3.2 Inert Fillers
3.2 惰性填充物

The thermal and mechanical strength of the polymer matrix can be improved by inert fillers. Moreover, fillers dispersed in the polymer matrix typically have tiny particle sizes and large specific surface areas, creating abundant interface with massive defects and high reactivity, which easily interact with other components [89]. Interaction between components affects the ionic conductivity and electrochemical stable window (ESW). Precisely regulating the intermolecular force is essential for achieving PIEs with high performance [89].
惰性填料可提高聚合物基体的热强度和机械强度。此外，分散在聚合物基体中的填料通常具有极小的粒径和较大的比表面积，从而形成具有大量缺陷和高反应性的丰富界面，容易与其他成分相互作用[89]。成分之间的相互作用会影响离子电导率和电化学稳定窗口（ESW）。精确调节分子间作用力对于获得高性能的 PIE 至关重要 [89]。

Inert fillers can weaken the interaction among the chains and increase free volume in the polymer matrix, which speeds up segmentation dynamics and delays polymer crystallization. Furthermore, fillers with Lewis acidic surface can interact with the anions [90]. As a result, the newly established hydrogen bonds make the fillers become cross-linking centers between polymer and anions, further disrupting the crystallinity (Fig. 6a) [32, 91, 92]. Instead, the fillers with Lewis basic surface can interact with Li⁺, causing the decrease of t₊. Neutral fillers interact weakly with lithium salts and polymer, hence having a negligible effect on the transport characteristics. Therefore, fillers with Lewis acidic surface are more favorable to Li⁺ ion diffusion. The inert fillers can also facilitate salt dissociation and increase Li⁺ ion concentration. Fixing anions on fillers can prevent anion–polymer interaction to increase the oxidation stability of PIEs [45, 93]. Meanwhile, most inorganic fillers are stable at high voltages. Well-designed PIEs can broaden ESW by inhibiting the direct contact of thermodynamically unstable components to realize compatibility with LMAs and high-voltage cathode. The recent research on PIEs with inert fillers and their properties is presented in Table 2.
惰性填料会减弱链之间的相互作用，增加聚合物基体中的自由体积，从而加快分段动力学，延迟聚合物结晶。此外，具有路易斯酸性表面的填料可与阴离子相互作用 [90]。因此，新建立的氢键使填料成为聚合物和阴离子之间的交联中心，进一步破坏结晶性（图 6a) [32, 91, 92] 。相反，具有路易斯碱性表面的填料可以与 Li⁺ 发生相互作用，导致 t₊ 下降。中性填料与锂盐和聚合物的相互作用较弱，因此对传输特性的影响可以忽略不计。因此，表面呈路易斯酸性的填料更有利于锂⁺ 离子的扩散。惰性填料还能促进盐解离，增加锂⁺ 离子浓度。将阴离子固定在填料上可防止阴离子与聚合物之间的相互作用，从而提高 PIE 的氧化稳定性[45、93]。同时，大多数无机填料在高电压下是稳定的。设计良好的 PIE 可抑制热力学不稳定成分的直接接触，实现与 LMA 和高压阴极的兼容，从而拓宽 ESW 的应用范围。表2介绍了近期有关惰性填料 PIE 及其特性的研究。

Fig. 6 图 6

a Lewis acid–base interaction between different components in PIEs [32]. Copyright 2019, John Wiley and Sons Publisher. b Synthetic process of the PEO-LiClO₄@SiO₂ PIEs [94]. Copyright 2020, American Chemical Society. c Schematics of the PEO-LiTFSI@Li₂SO₄ modified SiO₂ PIEs [95]. Copyright 2019, John Wiley and Sons. d Structure and ionic conductivity of PAN-LiClO₄@Y₂O₃-doped ZrO₂ PIEs [96]. Copyright 2016, American Chemical Society
a PIEs 中不同成分之间的路易斯酸碱相互作用 [32].版权归 John Wiley and Sons Publisher 2019 所有。b PEO-LiClO₄@SiO₂ PIEs 的合成过程 [94].Copyright 2020, American Chemical Society.c PEO-LiTFSI@Li₂SO₄ 改性 SiO₂ PIE 的示意图 [95].Copyright 2019, John Wiley and Sons. 版权所有。d PAN-LiClO₄@Y₂O₃ 掺杂 ZrO₂ PIEs [96].美国化学学会 2016 年版权所有

Table 2 PIEs filled with passive fillers and their properties
表 2 填充了被动填料的 PIE 及其特性

3.2.1 Oxide Materials
3.2.1氧化物材料

Al₂O₃ is inexpensive and widely available with robust thermal stability and is one of the earliest materials used as filler [126,127,128]. Pereira et al. reported that the addition of Al₂O₃ did not change the T_g of PEO-LiClO₄, but increased amorphous regions, thereby promoting the segment mobility and the transport of Li⁺ ions [129]. Wieczorek and Chen used Al₂O₃ with two distinct properties as fillers to demonstrate the validity of Lewis acid–base theory in elucidating the modification of ionic conductivity [130, 131]. Fourier transform infrared spectroscopy (FTIR) showed that Al₂O₃ with acidic groups enhanced the interaction with ClO₄⁻, thus promoting the dissolution of LiClO₄. Therefore, O atoms on Al₂O₃ with basic groups can interact with Li⁺, which increased free anions and diminished the t₊.
Al₂O₃ 价格低廉，来源广泛，热稳定性强，是最早用作填料的材料之一 [126,127,128] 。Pereira 等人报告称，添加 Al₂O₃ 并未改变 PEO-LiClOT_g 的T_g 、但增加了非晶区，从而促进了段流动性和锂⁺ 离子的传输[129]。Wieczorek 和 Chen 使用具有两种不同性质的 Al₂O₃ 作为填料，证明了路易斯酸碱理论在阐明离子导电性的改变方面的有效性 [130, 131] 。傅立叶变换红外光谱（FTIR）显示，带有酸性基团的 Al₂O₃ 增强了与 ClO₄^- 的相互作用，从而促进了 LiClO₄ 的溶解。因此，带有碱性基团的 Al₂O₃ 上的 O 原子可以与 Li⁺ 相互作用，从而增加了游离阴离子，减小了 t₊ 。

SiO₂ is easily accessible and rich in reserves and is commonly utilized as filler material [132,133,134]. Zhang et al. constructed a three-dimensional network of PEO-LiClO₄@SiO₂ by in situ hydrolysis reaction (Fig. 6b) [94]. SiO₂ promoted the segmental motion by the synergistic effect of Lewis acid–base and hydrogen bond. In addition, the enhanced interfacial stability allowed for an ESW of up to 4.8 V at 90 °C. Lu et al. created Li₂SO₄-modified SiO₂ nanofibers through electrospinning and calcination (Fig. 6c) [95]. The doping of Li₂SO₄ enhanced the ionic conductivity of SiO₂ and the wettability to the polymer. Meanwhile, the created mesopores encouraged anion absorption. After integrating PEO-LiTFSI matrix, the nanofiber networks can produce rapid and continuous Li⁺ diffusion routes. The sturdy 3D network served as a solid skeleton, reinforcing the entire membrane and inhibiting dendrite growth.
SiO₂ 易于获得且储量丰富，通常用作填充材料 [132,133,134] 。Zhang 等人通过原位水解反应构建了 PEO-LiClO₄@SiO₂ 的三维网络（图6b）[94]。SiO₂ 在路易斯酸碱和氢键的协同作用下促进了分段运动。此外，界面稳定性的增强使得在 90 °C 时的 ESW 电压可达 4.8 V。Lu 等人通过电纺丝和煅烧制造出了 Li₂SO₄ 改性 SiO₂ 纳米纤维（图6c）[95]。Li₂SO₄ 的掺杂增强了 SiO₂ 的离子导电性和对聚合物的润湿性。同时，产生的介孔促进了阴离子的吸收。在整合了 PEO-LiTFSI 基质后，纳米纤维网络可以产生快速、连续的锂⁺ 扩散途径。坚固的三维网络可作为坚实的骨架，加固整个膜并抑制树枝状突起的生长。

TiO₂ has a high dielectric constant (ε > 180) and strong Lewis acid–base action, making it a popular choice as a filler for PIEs. Ghosh et al. explored the impact of TiO₂ nanoparticles on the characteristics of the PMMA-LiClO₄ [135]. 1 wt% addition of TiO₂ raised the ionic conductivity of the PIEs to 3 × 10^–4 S cm⁻¹ at room temperature. It contributed to that the strong interaction between TiO₂ nanoparticles and ClO₄⁻ inhibited ion pair formation and increased free carriers. Lithium-ion poly (ethyl citrate) embedded with TiO₂ nanoparticles was in situ produced by thermal-initiated polymerization [136]. Polymer esterification catalyzed in situ hydrolysis of titanium alkoxide, leading to the production of nano-TiO₂. As the increase in TiO₂ concentration, polymerization of PIEs decreased and the thermal stability improved marginally. The addition of 20 wt% TiO₂ to the PIEs increased ionic conductivity by two orders of magnitude (1.74 × 10^–4 S cm⁻¹).
TiO₂ 具有较高的介电常数（ε 180）和较强的路易斯酸碱作用，因此成为 PIE 填料的热门选择。Ghosh 等人探讨了 TiO₂ 纳米粒子对 PMMA-LiClO₄ 特性的影响 [135]。在室温下，添加 1 wt% 的 TiO₂ 可将 PIEs 的离子电导率提高到 3 × 10^-4 S cm^-1 。这是因为 TiO₂ 纳米粒子与 ClO₄^- 之间的强相互作用抑制了离子对的形成，增加了自由载流子。嵌入 TiO₂ 纳米粒子的锂离子聚（柠檬酸乙酯）是通过热引发聚合在原位生成的[136]。聚合物酯化催化了烷氧基钛的原位水解，从而产生了纳米 TiO₂ 。随着 TiO₂ 浓度的增加，PIE 的聚合度降低，热稳定性略有提高。在 PIEs 中添加 20 wt% 的 TiO₂ 可将离子传导性提高两个数量级（1.74 × 10^-4 S cm^-1 ）。

ZrO₂ has good chemical and thermal stability. In addition, ZrO₂ nanoparticles have Lewis acidity, which can attract anions and encourage lithium salt dissociation [137]. Jing et al. fabricated polypropylene oxide (PPO)-based PIEs by combining the bis[3-(methyldimethoxysilyl)]-terminated PPO (BSPPO) oligomers with ZrO₂ nanofillers, succinonitrile (SN) plasticizer and cellulose membrane (CM) framework. LiBOB was used to trigger the cross-linking of BSPPO oligomers. ZrO₂ nanofillers decreased the T_g of the polymer and promoted the dissociation of LiTFSI. The ionic conductivity was further increased by the SN, which was an efficient ionizer. The prepared PPO-LiTFSI@ ZrO₂ had good flexibility, high ionic conductivity (9.62 × 10^–4 S cm⁻¹), excellent thermal and electrochemical stability (5 V) [138]. Cui et al. employed Y₂O₃-doped ZrO₂ to tailor the PAN-LiClO₄ (Fig. 6d). High concentration of oxygen vacancies in ZrO₂ can be created by doping with Y³⁺ with a low oxidation state. The positively charged oxygen vacancies as Lewis acid sites can combine with ClO₄⁻ to liberate additional Li⁺ ions, which increased the conductivity to 1.07 × 10^–5 S cm⁻¹ and the t₊ rose to 0.56 [96].
ZrO₂ 具有良好的化学稳定性和热稳定性。此外，ZrO₂ 纳米粒子具有路易斯酸性，可以吸引阴离子并促进锂盐解离[137]。Jing 等人通过将双[3-（甲基二甲氧基硅基）]端 PPO（BSPPO）低聚物与 ZrO₂ 纳米填料、琥珀腈（SN）增塑剂和纤维素膜（CM）框架相结合，制备了基于聚丙烯氧化物（PPO）的 PIE。LiBOB 用于触发 BSPPO 低聚物的交联。ZrO₂ 纳米填料降低了聚合物的 T_g 值，并促进了 LiTFSI 的解离。作为高效离子发生器的 SN 进一步提高了离子导电率。制备的 PPO-LiTFSI@ ZrO₂ 具有良好的柔韧性、高离子电导率（9.62 × 10^-4 S cm^-1 ）、优异的热稳定性和电化学稳定性（5 V）[138]。Cui 等人采用 Y₂O₃ 掺杂 ZrO₂ 来定制 PAN-LiClO₄ （图6d）。通过掺杂低氧化态的 Y³⁺ 可以在 ZrO₂ 中产生高浓度的氧空位。作为路易斯酸位点的带正电的氧空位可与 ClO₄^- 结合，释放出更多的 Li⁺ 离子，从而将电导率提高到 1。07 × 10^-5 S cm^-1 而 t₊ 则上升到 0.56 [96] 。

3.2.2 Ferroelectric Materials
3.2.2铁电材料

Ferroelectric materials with permanent dipoles have strong Lewis acid–base characteristics, which are also employed as fillers [97, 139]. Due to their unique crystal structure, ferroelectric materials exhibit spontaneous polarization. They can effectively alleviate the generation of SCL formed by Li⁺ depletion due to the large chemical potential difference at the electrode/electrolyte interface [140, 141]. Sohn et al. incorporated BaTiO₃, PbTiO₃ and LiNbO₃ into the PEO polymer [98]. The fillers decreased the contact resistance between the LMAs and the electrolyte, increasing the mechanical strength and Li⁺ ionic conductivity.
具有永久偶极子的铁电材料具有很强的路易斯酸碱特性，也可用作填料 [97, 139] 。由于其独特的晶体结构，铁电材料表现出自发极化。由于电极/电解质界面存在较大的化学电位差，铁电材料可有效缓解因 Li⁺ 耗尽而产生的 SCL [140, 141] 。Sohn 等人在 PEO 聚合物中加入了 BaTiO₃、PbTiO₃ 和 LiNbO₃ [98]。填料降低了 LMA 与电解质之间的接触电阻，提高了机械强度和 Li⁺ 离子电导率。

3.2.3 Porous Materials
3.2.3 多孔材料

Porous materials, such as zeolite and MOFs, provide an adaptable pore structure and large specific surface area, hence generating abundant contact sites [142]. In addition, the channel structure has a nanoscale effect that allows for the effective regulation of charged particle adsorption. Moreover, these materials possess excellent thermal stability and mechanical properties which have been extensively explored as inorganic fillers.
多孔材料（如沸石和 MOFs）具有适应性强的孔隙结构和较大的比表面积，因此可产生丰富的接触点 [142]。此外，通道结构具有纳米级效应，可有效调节带电粒子的吸附。此外，这些材料还具有优异的热稳定性和机械性能，已被广泛用作无机填料。

Zeolites are widely available from nature and possess ultra-high structural stability. Kim et al. treated the surface of aluminosilicate zeolite (SSZ-13) with polyacrylic acid (Fig. 7a) [143]. SSZ-13 with a hydrophobic surface enhanced the dispersion of LiTFSI in PEO and provided continuous channels for Li⁺ diffusion. It increased dissociation of LiTFSI and liberation of Li⁺ ions. The conductivity of PEO-LiTFSI@SSZ-13 was increased to 5.34 × 10^–2 cm S⁻¹ (70 °C) with a t₊ of 0.85. The ASSBs assembled with Li and LiFePO₄ delivered capacity retention of 94.1% after 80 cycles at 60 °C. Additionally, they employed YNa zeolite as a ceramic filler and combined it with PEO-LiFSI to create PIEs (PEO-LiFSI@YNa) (Fig. 7b) [99]. The ionic conductivity was elevated to 1.66 × 10^–2 S cm⁻¹ and t₊ was significantly increased to 0.84. Li||Li symmetric cells maintained a stable overpotential of ~ 60 mV for 1500 h, revealing the PIEs can inhibit dendrite growth.
沸石在自然界中广泛存在，具有超高的结构稳定性。Kim 等人用聚丙烯酸处理了铝硅酸盐沸石（SSZ-13）的表面（图7a）[143]。具有疏水表面的 SSZ-13 增强了 LiTFSI 在 PEO 中的分散，并为 Li⁺ 扩散提供了连续通道。它增加了 LiTFSI 的解离和 Li⁺ 离子的释放。PEO-LiTFSI@SSZ-13 的电导率提高到 5.34 × 10^-2 cm S^-1 (70 °C)，t₊ 为 0.85。使用锂和 LiFePO₄ 组装的 ASSB 在 60 °C 下循环 80 次后，容量保持率达到 94.1%。此外，他们还采用 YNa 沸石作为陶瓷填料，并将其与 PEO-LiFSI 结合，制成了 PIE（PEO-LiFSI@YNa）（图7b）[99]。离子导电率提高到 1.66 × 10^-2 S cm^-1 ，t₊ 显著提高到 0.84。锂||锂对称电池在1500小时内保持了约60 mV的稳定过电位，这表明PIEs能抑制树突的生长。

Fig. 7 图 7

a Schematic of the PIEs with M-SSZ-13 zeolite as the filler [143]. Copyright 2021, Royal Society of Chemistry. b Structure of the YNa zeolite and schematic of the synthesis process of PEO-LiFSI@YNa [99]. Copyright 2021, Royal Society of Chemistry. c Synthetic process and ion transport channels of 3D PAN/PEO-LiTFSI@UIO-66 PIEs [100]. Copyright 2022, Elsevier
a 以 M-SSZ-13 沸石为填料的 PIEs 示意图 [143]。版权 2021 年，英国皇家化学学会。b YNa 沸石的结构和 PEO-LiFSI@YNa 的合成过程示意图 [99]。版权 2021 年，英国皇家化学会。c 三维 PAN/PEO-LiTFSI@UIO-66 PIEs 的合成过程和离子传输通道 [100]。版权 2022 年，爱思唯尔

MOFs are comprised of inorganic clusters containing center metal ions and organic ligands [144]. In addition to sharing some characteristics with zeolites, including great thermal stability, large specific surface area and Lewis acidic surface, MOFs also contain their own distinct organic functional groups, which allow for the flexible control of surface properties [145, 146]. Unsaturated metal sites in MOFs can interact with anions to facilitate Li⁺ ion transport, hence enhancing ionic conductivity [147]. The periodic crystal structure and organized channels in MOFs provide uniform Li⁺ flux, ensuring uniform Li⁺ plating behavior and inhibiting dendrite growth. Stephan et al. enhanced the ionic conductivity of PEO-LiTFSI by two orders of magnitude with aluminum benzenetricarboxylate (Al-BTC) as filler [117]. The obtained PIEs exhibited excellent thermal stability and cycle stability to LMAs. They also reported that the insertion of aluminum terephthalate (Al-TPA) can reduce the migration of polysulfides in lithium–sulfur batteries and realize a stable cycle performance [118]. Zheng et al. constructed a 3D MOF network (Zirconium benzenedicarboxylate MOF, UIO-66) by electrospinning and then filled it with PAN/PEO-LiTFSI to obtain PIEs@UIO-66 (Fig. 7c) [100]. Density functional theory (DFT) demonstrated that UIO-66 had strong adsorption to Li⁺ ions. The interconnected particles offered continuous pathways for the rapid transport of Li⁺ ions, efficiently enhancing the ionic conductivity (2.89 × 10^–4 S cm⁻¹) and promoting the homogeneous distribution of Li⁺ flux. The PIEs@UIO-66 had high t₊ (0.52), wide ESW (4.7 V), remarkable ability to suppress lithium dendrites and high mechanical strength. Guo et al. produced a novel cationic MOF (CMOF) by grafting pyridine onto UiO-66 and dispersed it in PEO-LiTFSI to form PIEs (Fig. 8a) [101]. CMOF fixed anions through electrostatic interaction and its large specific surface area further enhanced the adsorption of anions, making its t₊ reach 0.72. Moreover, CMOF grafted with -NH₂ groups protected the ether-oxygen on the polymer chains by hydrogen bonding, extending the electrochemical window to 4.97 V. After 300 cycles at 1C, the ASSBs combined with LMAs and LiFePO₄ retained 85.4% of their initial capacity. Zhang et al. grafted polyethylene glycol diacrylate chain (PEGDA) onto vinyl-functionalized MOF nanoparticles (UIO66-NH₂) through UV photopolymerization and formed PIEs with LiTFSI (Fig. 8b) [148]. The PIEs have a fivefold increase in ionic conductivity over PEGDA-LiTFSI, reaching 10^–5 S cm⁻¹.
MOFs由含有中心金属离子和有机配体的无机团簇组成[144]。MOFs 除了与沸石具有相同的一些特性（包括热稳定性强、比表面积大和表面呈路易斯酸性）外，还含有自己独特的有机官能团，可灵活控制表面性质[145、146]。MOF 中的不饱和金属位点可与阴离子相互作用，促进 Li⁺ 离子的传输，从而增强离子导电性[147]。MOFs 中的周期性晶体结构和有组织的通道提供了均匀的锂⁺ 通量，确保了均匀的锂⁺ 电镀行为，并抑制了树枝状晶粒的生长。Stephan 等人使用苯三羧酸铝（Al-BTC）作为填料，将 PEO-LiTFSI 的离子电导率提高了两个数量级[117]。获得的 PIE 与 LMA 相比，具有出色的热稳定性和循环稳定性。他们还报道了插入对苯二甲酸铝（Al-TPA）可减少锂硫电池中多硫化物的迁移，实现稳定的循环性能[118]。Zheng 等人通过电纺丝构建了三维 MOF 网络（苯二甲酸锆 MOF，UIO-66），然后填充 PAN/PEO-LiTFSI 得到 PIEs@UIO-66（图7c）[100]。密度泛函理论（DFT）表明，UIO-66 对 Li⁺ 离子有很强的吸附作用。 相互连接的颗粒为锂⁺离子的快速传输提供了连续的通道，有效地提高了离子电导率（2.89 × 10^-4 S cm^-1 ），促进了锂⁺ 通量的均匀分布。PIEs@UIO-66 具有高 t₊ (0.52)、宽 ESW (4.7 V)、显著的抑制锂枝晶能力和高机械强度。Guo 等人通过在 UiO-66 上接枝吡啶并将其分散在 PEO-LiTFSI 中形成 PIE，制备了一种新型阳离子 MOF（CMOF）（图8a）[101]。CMOF 通过静电作用固定阴离子，其巨大的比表面积进一步增强了对阴离子的吸附，使其 t₊ 达到 0.72。此外，接枝了 -NH₂ 基团的 CMOF 通过氢键保护了聚合物链上的醚氧，将电化学窗口延长至 4.97 V。在 1C 下循环 300 次后，与 LMA 和 LiFePO₄ 结合的 ASSB 保留了 85.4% 的初始容量。Zhang 等人通过紫外光聚合将聚乙二醇二丙烯酸酯链（PEGDA）接枝到乙烯基官能化 MOF 纳米粒子（UIO66-NH₂ ）上，并与 LiTFSI 形成 PIEs（图 8b ）[148] 。PIE 的离子电导率比 PEGDA-LiTFSI 提高了五倍，达到 10^-5 S cm^-1 。

Fig. 8 图 8

a Li plating behavior with PEO-LiTFSI and PEO-LiTFSI@CMOF [101]. Copyright 2019, Elsevier. b Synthesis process of the PEGDA-LiTFSI@UIO66-NH₂ [148]. Copyright 2018, Royal Society of Chemistry
a 使用 PEO-LiTFSI 和 PEO-LiTFSI@CMOF 的锂电镀行为 [101]。版权 2019 年，爱思唯尔。b PEGDA-LiTFSI@UIO66-NH₂ 的合成过程 [148].版权 2018 年，英国皇家化学学会

3.2.4 Other Inert Fillers
3.2.4 其他惰性填料

Other inert inorganic materials, such as mixed metal oxides, phosphates, layered clay materials, are also widely used as fillers. Stephan et al. incorporated MgAl₂O₄ into PEO-LiPF₆ to create PIEs by hot press [149]. The addition of MgAl₂O₄ improved the T_g and ionic conductivity of the polymer, which attributed to Lewis acid properties of MgAl₂O₄, can compete with Li⁺ ions and form complexes with PEO chains, thus decreasing polymer crystallization. Nanosized Ca₃(PO₄)₂ was reported to produce a similar effect on the performance of PEO-LiTFSI and PEO-LiClO₄ [150]. Nano-layered clays, such as montmorillonite and kaolinite, were utilized as inorganic fillers due to their high dielectric characteristics and specific surface area, which were conducive to the dissociation of lithium salts [151,152,153].
其他惰性无机材料，如混合金属氧化物、磷酸盐、层状粘土材料，也被广泛用作填料。Stephan 等人将 MgAl₂O₄ 加入 PEO-LiPF₆ 中，通过热压制造出 PIE [149]。添加 MgAl₂O₄ 提高了聚合物的 T_g 和离子导电性、由于 MgAl₂O₄ 的路易斯酸特性，它可以与 Li⁺ 离子竞争，并与 PEO 链形成络合物，从而降低聚合物的结晶度。据报道，纳米级 Ca₃(PO₄)₂ 对 PEO-LiTFSI 和 PEO-LiClO₄ 的性能产生类似的影响 [150]。蒙脱石和高岭石等纳米层状粘土具有高介电特性和比表面积，有利于锂盐的解离，因此被用作无机填料[151、152、153]。

In summary, whereas inert fillers are unable to transport Li⁺ ions, numerous surface groups can interact with polymers and lithium salts to prevent polymer crystallization and promote lithium salt dissociation. Additionally, inert fillers can improve the mechanical and thermal stability of polymers. The interaction between inorganic fillers and anions can inhibit the continuous oxidative decomposition of anions and widen the ESW of the PIEs.
总之，虽然惰性填料无法传输锂⁺ 离子，但许多表面基团可以与聚合物和锂盐相互作用，从而防止聚合物结晶并促进锂盐解离。此外，惰性填料还能提高聚合物的机械稳定性和热稳定性。无机填料与阴离子之间的相互作用可抑制阴离子的持续氧化分解，并拓宽 PIE 的 ESW。

3.3 Active Fillers
3.3 活性填充物

Active fillers allow efficient conduction of Li⁺ ions. Li⁺ ions exhibit different migration patterns in different regions of the PIEs with active fillers: (1) segment movement within the polymer, (2) vacancy or interstitial migration in the active fillers (Fig. 9a) and (3) interfacial migration between the fillers and polymer (Fig. 9b) [28, 154, 155]. Debates still exist regarding the migration paths of Li⁺ ions in PIEs containing active fillers, which will be described in depth in the following section. Based on the type of solid-state electrolyte used as fillers, they can be classified as garnet-type, NASICON-type, perovskite-type and sulfide-type PIEs (Fig. 9c) [156].
活性填料可实现锂⁺ 离子的高效传导。锂⁺离子在带有活性填料的 PIE 的不同区域表现出不同的迁移模式：(1) 聚合物内部的段移动，(2) 活性填料中的空位或间隙迁移（图 9a）。9a) 和 (3) 填料与聚合物之间的界面迁移（图 9b) [28, 154, 155] 。关于 Li⁺ 离子在含有活性填料的 PIE 中的迁移路径仍存在争议，下文将对此进行深入介绍。根据用作填料的固态电解质类型，可将它们分为石榴石型、NASICON 型、透辉石型和硫化物型 PIE（图 9c) [156]。

Fig. 9 图 9

a Diffusion modes of Li⁺ ion in ISEs [28]. Copyright 2018, Royal Society of Chemistry. b Diffusion modes of Li⁺ ion in PIEs with active fillers [155]. Copyright 2018, Elsevier. c Crystal structure, conductivity and activation energy of different active fillers [156]. Copyright 2016, American Chemical Society. d Schematic diagram of Li⁺ ion diffusion routes in PEO-LiTFSI with different contents of LLZO fillers [157]. Copyright 2018, American Chemical Society. e Schematic of different Li⁺ diffusion pathways in the PIEs and ⁶Li NMR spectra of the PAN-LiClO₄@LLZO, LLZO nanowires, PAN-LiClO₄ and cycled PIEs [158]. Copyright 2017, American Chemical Society. f Schematic of the rapid ion diffusion route in space charge regions and the TEM image of space charge regions [159]. Copyright 2019, American Chemical Society
a Li⁺ 离子在 ISE 中的扩散模式 [28].版权归英国皇家化学学会 2018 年所有。b Li⁺ 离子在含有活性填料的 PIEs 中的扩散模式 [155].版权 2018 年，爱思唯尔。c 不同活性填料的晶体结构、电导率和活化能 [156].美国化学学会 2016 年版权所有。d 不同 LLZO 填料含量的 PEO-LiTFSI 中 Li⁺ 离子扩散路线示意图 [157].Copyright 2018, American Chemical Society.e PIEs 中不同 Li⁺ 扩散途径示意图和 PAN-LiClO₄@LLZO 的 ⁶Li NMR 光谱、LLZO 纳米线、PAN-LiClO₄ 和循环 PIE [158]。美国化学学会 2017 年版权所有。f 空间电荷区离子快速扩散路线示意图和空间电荷区的 TEM 图像 [159]。美国化学学会 2019 年版权所有

3.3.1 Garnet-Type PIEs
3.3.1石榴石类型 PIE

Thangadurai et al. reported the garnet-type Li₅La₃M₂O₁₂ (M = Nb, Ta) with an ionic conductivity of 10^–6 S cm⁻¹ at room temperature for the first time in 2003 [160]. By inserting more lithium atoms into the framework, a series of SSEs with garnet structure were created. Li_6.4La₃Zr_1.4Ta_0.6O₁₂ has the highest bulk ionic conductivity of 10^–3 S cm⁻¹ at 25 °C among the known Li-rich garnets [161]. Garnet-type SSEs have the advantages of excellent ionic conductivity (~ 10^–4–10^–3 S cm⁻¹), oxidation resistance under high voltage, stability to lithium metal and superior mechanical strength. Nevertheless, they also have the issue of significant interfacial resistance brought by inadequate contact with the rough interface [162]. Composited with polymer can accomplish robustness and flexibility, minimize interfacial contact impedance and overcome the poor processability of powder ceramics. The recent research on PIEs filled with garnet-type fillers and their properties is summarized in Table 3.
Thangadurai 等人报告了石榴石型 Li₅La₃M₂O₁₂ （M = Nb、Ta）的离子电导率为 10^-6 S cm^-1 [160]。通过在框架中加入更多的锂原子，一系列具有石榴石结构的 SSE 诞生了。Li_6.4La₃Zr_1.4Ta_0.6O₁₂ 在已知的富锂辉石[^{-3S cm-1 25 °C]中具有最高的体离子电导率[161]。石榴石型 SSE 具有出色的离子导电性（约 10-4-10-3 S cm-1 ）、高电压下的抗氧化性、对锂金属的稳定性和卓越的机械强度等优点。不过，它们也存在因与粗糙界面接触不充分而导致界面电阻较大的问题 [162]。与聚合物复合可实现坚固性和灵活性，最大限度地减少界面接触阻抗，并克服粉末陶瓷加工性差的问题。表3概述了最近关于填充石榴石型填料的 PIE 及其性能的研究。}

Table 3 PIEs filled with garnet-type fillers and their properties
表 3 填充了石榴石型填料的 PIE 及其特性

Gerbaldi et al. added Li₇La₃Zr₂O₁₂ (LLZO) fillers and a photoinitiator to the PEO-tetra (ethylene glycol dimethyl ether) (G4)-LiTFSI and then induced cross-linking under ultraviolet light to generate PIE films. The PIEs had good flexibility and exhibited an ionic conductivity more than 1 × 10^–4 S cm⁻¹ and a t₊ greater than 0.5 at 20 °C. The Li||LiFePO₄ cells with the PIEs demonstrated a remarkable specific capacity for 400 cycles [175]. The relationship between ion mobility, transport pathways and activity concentration in PEO-LiTFSI@LLZO was determined by solid-state nuclear magnetic resonance (NMR) [157]. The results demonstrated that when the LLZO content in the PIEs was less than 20 wt%, Li⁺ ions were mainly conducted through PEO (Fig. 9d). Once the concentration of LLZO reached a threshold level, the particles joined together, forming an infiltration network. Li⁺ ions migrated through the network rather than the PEO matrix. The critical concentration depended on several factors such as particle size, morphology as well as the dispersity of the fillers. The effects of LLZO fillers on ionic conductivity of PIEs are mainly manifested in the following aspects: (1) LLZO fillers reduced the crystallinity of polymer matrix; (2) Li⁺ ion channels in PEO could be blocked by LLZO particles and reduced the mobility of Li⁺ ions; (3) LLZO contributed as an extra source of Li⁺ ions to the conductivity. The trade-off between three competing effects determined whether the fillers increased or decreased ionic conductivity at a given concentration.
Gerbaldi 等人将 Li₇La₃Zr₂O₁₂ (LLZO) 填料和光引发剂添加到 PEO-四（乙二醇二甲醚）（G4）-LiTFSI，然后在紫外线下诱导交联，生成 PIE 薄膜。PIE 具有良好的柔韧性，在 20 °C 时离子电导率超过 1 × 10^-4 S cm^-1 ，t₊ 大于 0.5。带有 PIEs 的 Li||LiFePO₄ 电池在 400 次循环中显示出显著的比容量[175]。固态核磁共振（NMR）[157]测定了 PEO-LiTFSI@LLZO 中离子迁移率、传输路径和活性浓度之间的关系。结果表明，当 PIE 中的 LLZO 含量低于 20 wt% 时，锂⁺ 离子主要通过 PEO 传导（图9d）。一旦 LLZO 的浓度达到临界值，颗粒就会结合在一起，形成渗透网络。锂⁺离子通过网络而不是 PEO 基质迁移。临界浓度取决于多个因素，如颗粒大小、形态以及填料的分散性。 LLZO填料对PIE离子传导性的影响主要表现在以下几个方面：(1) LLZO填料降低了聚合物基体的结晶度；(2) LLZO颗粒可阻塞PEO中的Li⁺离子通道，降低Li⁺离子的迁移率；(3) LLZO作为额外的Li⁺离子源，有助于提高导电率。三种竞争效应之间的权衡决定了填料在给定浓度下是增加还是减少离子传导性。

Chan et al. improved the ionic conductivity of PAN-LiClO₄ by incorporating 5 wt% LLZO nanowires [158]. NMR revealed that LLZO nanowires changed the local environment in the polymer matrix and Li⁺ ion transport preferentially happened at the LLZO/polymer interface (Fig. 9e). The total ionic conductivity of PIEs adding LLZO nanoparticles (1.13 × 10^–5 S cm⁻¹) was much lower than that of PIEs adding LLZO nanowires (1.31 × 10^–4 S cm⁻¹). This indicated that the morphology and continuous conduction pathways provided by fillers were essential for the improvement of ionic conductivity.
Chan 等人通过加入 5 wt% 的 LLZO 纳米线提高了 PAN-LiClO₄ 的离子电导率[158]。核磁共振显示，LLZO 纳米线改变了聚合物基体中的局部环境，Li⁺ 离子的传输优先发生在 LLZO/聚合物界面上（图9e）。添加了 LLZO 纳米粒子的 PIE 的总离子电导率（1.13 × 10^-5 S cm^-1 ）远低于添加了 LLZO 纳米线的 PIE 的总离子电导率（1.31 × 10^-4 S cm^-1 ）。这表明，填料提供的形态和连续传导路径对离子传导性的改善至关重要。

Percolation effect may contribute significantly to the ionic conductivity of PIEs [157]. Wei et al. observed the space charge regions at the interface of PEO/Li_6.25Ga_0.25La₃Zr₂O₁₂ (LLZO-Ga) nanoparticles by transmission electron microscope (TEM) [159]. Phase-field simulation demonstrated the chemical potential difference between LLZO-Ga and PEO drove the Li⁺ to migrate to the surface sites, leading to the enrichment of Li⁺ ions and low concentration of vacancies. As soon as the space charge region and phase distribution satisfied the criteria for establishing the percolation, percolation effect occurred, creating successive rapid transport routes and increasing ionic conductivity dramatically. Meanwhile, the space charge region surrounding isolated fillers barely impacted the ionic conductivity (Fig. 9f). Hu et al. tracked the Li⁺ diffusion paths in PEO-LiClO₄@LLZO combining isotope labeling and Li NMR. By detecting that ⁶Li in the LMAs replaced ⁷Li in the PEO-LiClO₄@LLZO, they found that Li⁺ ions diffused mainly through LLZO particles rather than through the interface or the polymer matrix (Fig. 10a) [176]. The aforementioned results imply that the observed Li⁺ diffusion path is closely related to the prepared PIEs inherently tied to the morphology, content, dispersion and properties of fillers.
渗流效应可能对 PIE 的离子电导率有很大影响[157]。Wei 等人在 PEO/Li_6.25Ga_0.25La₃Zr₂O₁₂ (LLZO-Ga) 纳米粒子的透射电子显微镜 (TEM) [159]。相场模拟表明，LLZO-Ga 和 PEO 之间的化学势差促使 Li⁺ 迁移到表面位点，导致 Li⁺ 离子富集和空位浓度降低。一旦空间电荷区和相分布满足建立渗流的标准，就会产生渗流效应，形成连续的快速传输路线，离子导电率也会显著提高。同时，孤立填料周围的空间电荷区几乎不影响离子导电率（图9f）。Hu 等人结合同位素标记和锂核磁共振技术，跟踪了锂⁺ 在 PEO-LiClO₄@LLZO 中的扩散路径。通过检测 LMA 中 ⁶Li 取代 PEO-LiClO₄@LLZO 中 ⁷Li 的情况，他们发现 Li⁺ 离子主要通过 LLZO 颗粒扩散，而不是通过界面或聚合物基质（图 2）。10a）[176]。 上述结果表明，所观察到的锂⁺ 扩散路径与制备的 PIE 密切相关，与填料的形态、含量、分散性和特性有着内在联系。

Fig. 10 图 10

a NMR spectra and spectral simulation for LLZO, PEO-LiClO₄ and PEO-LiClO₄@LLZO [176]. Copyright 2019, John Wiley and Sons. b Multiple Li⁺ conduction forms in PMMA-LiClO₄@LLZN [165]. Copyright 2019, Elsevier
a LLZO、PEO-LiClO₄ 和 PEO-LiClO₄@LLZO [176] 的 NMR 光谱和光谱模拟。版权归 John Wiley and Sons 2019 所有。b PMMA-LiClO₄@LLZN [165] 中的多种 Li⁺ 传导形式。版权 2019 年，爱思唯尔

Shen et al. suggested that Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) fillers can induce structure changes in PVDF [163]. La atoms of LLZTO can complex with N atoms and C=O groups of N, N-dimethylformamide (DMF) coupled with electron enrichment at the N atoms. The electron-rich N atoms acted as Lewis bases donated electron pairs and caused the partial dehydrofluorination of PVDF. The C=C on the modified PVDF enhanced the acid–base interaction with different components. LLZTO particles as Lewis acid promoted the dissociation of lithium salt and increased the concentration of Li⁺ ions. Partially dehydrofluorinated PVDF enhanced the interaction with LLZTO and further reduced the crystallinity of PVDF, resulting in enhanced comprehensive performance of the PIEs.
Shen 等人认为 Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) 填料可引起 PVDF 的结构变化 [163]。LLZTO 的 La 原子可与 N 原子和 N，N-二甲基甲酰胺（DMF）的 C=O 基团络合，并在 N 原子上富集电子。富电子的 N 原子作为路易斯碱捐献电子对，导致 PVDF 部分脱氢氟化。改性 PVDF 上的 C=C 增强了与不同成分的酸碱相互作用。作为路易斯酸的 LLZTO 粒子促进了锂盐的解离，增加了 Li⁺ 离子的浓度。部分脱氢氟化的 PVDF 增强了与 LLZTO 的相互作用，并进一步降低了 PVDF 的结晶度，从而提高了 PIE 的综合性能。

Wang et al. suggested that Li_6.75La₃Zr_1.75Nb_0.25O₁₂ (LLZN) nanowires can interact with C=O and O=C–N, which were left by the solvent (DMF) [165]. The interaction reinforced the connection between fillers and polymer thus creating abundant amorphous regions and large free volume for segment movement. The surface group of ceramic filler had strong adsorption for ClO₄⁻, hence facilitating the dissociation of the salt. Moreover, the vacancies of the LLZN nanowires provided special conductive channels for ion transportation. The multiple Li⁺ conduction forms significantly increased the ionic conductivity of PIEs (Fig. 10b).
Wang 等人认为 Li_6.75La₃Zr_1.75Nb_0.25O₁₂ (LLZN) 纳米线可以与溶剂（DMF）中残留的 C=O 和 O=C-N 相互作用 [165]。这种相互作用加强了填料与聚合物之间的连接，从而产生了大量的无定形区域和较大的自由体积，便于段的移动。陶瓷填料的表面基团对 ClO₄^- 有很强的吸附作用，从而促进了盐的解离。此外，LLZN 纳米线的空位为离子传输提供了特殊的导电通道。多重 Li⁺ 传导形式显著提高了 PIE 的离子传导性（图10b）。

3.3.2 NASICON-Type PIEs
3.3.2 NASICON 类型 PIE

The primary NASICON-type SSEs are derived from LiGe₂(PO₄)₃ and LiTi₂(PO₄)₃. The ionic conductivity can be further enhanced by partial replacement of tetravalent Ge⁴⁺ and Ti⁴⁺ with trivalent cations such as Ga³⁺, Al³⁺ and Fe³⁺. The ionic conductivity of Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) can reach 10^–3 S cm⁻¹ and satisfy the requirements of SSEs [177]. Moreover, they are resistant to air and water, enabling large-scale synthesis and battery assembly in an air atmosphere, which decreases processing challenge and cost [178]. While they have an issue with instability to lithium since Ti⁴⁺ and Ge⁴⁺ are easily reduced, generating high-impedance interfacial phases [179, 180]. Composited with polymer electrolyte owning electronic insulation and flexibility can improve electrochemical and contact stability on the interface of NASICON-type SSEs. The recent research on PIEs filled with NASICON-type ISEs and their properties is summarized in Table 4.
主要的 NASICON 型 SSE 来自锗锂₂(PO₄)₃ 和 LiTi₂(PO₄)₃ 。用三价阳离子（如 Ga³⁺ ）部分取代四价的 Ge⁴⁺ 和 Ti⁴⁺ 可进一步提高离子导电性、Al³⁺ 和 Fe³⁺ 等三价阳离子。Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) 可达到 10^-3 S cm^-1 并满足 SSE 的要求 [177]。此外，它们还耐空气和水，可在空气环境中进行大规模合成和电池组装，从而降低了加工难度和成本[178]。但由于 Ti⁴⁺ 和 Ge⁴⁺ 很容易被还原，产生高阻抗界面相，因此它们对锂存在不稳定性问题 [179，180]。与拥有电子绝缘性和柔韧性的聚合物电解质复合，可以提高 NASICON 型 SSE 接口的电化学和接触稳定性。表4概述了最近关于填充 NASICON 型 ISE 的 PIE 及其特性的研究。

Table 4 PIEs filled with NASICON-type fillers and their properties
表 4 用 NASICON 型填料填充的 PIE 及其特性

Rational structural design can help ceramic and polymer electrolytes overcome their drawbacks and exploit their full potential. Yang et al. constructed PIEs with vertically aligned Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) and flexible PEO/PEG polymer (Fig. 11a) [181]. The vertical arranged LAGP created successive pathways for rapid ion diffusion and the PEO/PEG matrix made the PIEs flexible. The ionic conductivity of the PIEs reached 1.67 × 10^–4 S cm⁻¹ at 25 °C. After 300 cycles, the ASSBs built with LiFePO₄ and LMAs retained 93.3% of the initial capacity. Jiang et al. adopt Janus interface modification strategy to improve the electrochemical stability at LAGP/electrodes interface. They sandwiched LAGP disks between in situ cross-linked PMMA and poly(cyclic carbonate urethane methacrylate)-based polymer electrolytes (Fig. 11b). Polymer electrolyte coatings not only kept PIEs in contact with the electrode, accelerating the interfacial ion transport kinetics, but also built stable CEI and SEI layers. The PIEs enabled the Li||LiNi_0.8Mn_0.1Co_0.1O₂ cells to have outstanding cycle stability at 4.5 V [196]. Yang et al. developed PIEs with “brick–mortar” microstructures (Fig. 11c) [182]. They prepared multilayer PEO-LiTFSI@LAGP by stacking and sintering at 850 °C. Then, the stack was immersed in polymer electrolyte under vacuum, and compressed at 80 °C to break into thin sheet and enable polymer to plug all gaps. The obtained PIEs exhibited extremely high ultimate bending strength and remarkable toughness. The ASSBs assembled with LiFePO₄ and LMAs can retain 92% of their initial capacity at 0.5C after 300 cycles at 60 °C. Xiong et al. embedded silane functionalized LATP nanoparticles into the PVDF framework by electrospinning to form nanofiber membranes and then carried out thermal initiation polymerization of vinylene carbonate-based precursors in the composite network (Si@LATP/PVDF/PVC) [183]. Silane functionalization increased the affinity of Si@LATP with the PVDF skeleton and fully exposed the Lewis acid sites on LATP. The -NH₃⁺ in poly-siloxane further increased the anion adsorption. The PIEs possessed high electrochemical stability to lithium and the ASSBs coupled with LiNi_0.5Co_0.2Mn_0.3O₂ exhibited excellent cycle performance and rate capability. Fan et al. created porous interconnected LATP networks with NaCl as a sacrificial template and introduced PEO-LiTFSI into the networks (Fig. 11d). The PIEs not only served as rapid transport routes for Li⁺ ions, but also as physical barriers to prevent the growth of Li dendrites [184].
合理的结构设计有助于克服陶瓷和聚合物电解质的缺点，充分发挥其潜力。Yang 等人利用垂直排列的 Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) 和柔性 PEO/PEG 聚合物（图11a）[181]。垂直排列的 LAGP 为离子的快速扩散创造了连续的通道，而 PEO/PEG 基质则使 PIEs 具有柔韧性。PIEs 的离子电导率在 25 °C 时达到 1.67 × 10^-4 S cm^-1 。经过 300 次循环后，使用 LiFePO₄ 和 LMA 制成的 ASSB 保留了 93.3% 的初始容量。Jiang 等人采用 Janus 界面改性策略来提高 LAGP/电极界面的电化学稳定性。他们将 LAGP 盘夹在原位交联的 PMMA 和聚（环碳酸聚氨酯甲基丙烯酸酯）聚合物电解质之间（图11b）。聚合物电解质涂层不仅保持了 PIE 与电极的接触，加速了界面离子传输动力学，还建立了稳定的 CEI 和 SEI 层。PIE 使 Li||LiNi_0.8Mn_0.1Co_0.1O₂ 电池在 4.5 V 下具有出色的循环稳定性[196]。Yang 等人开发了具有 "砖-砂 "微结构的 PIE（图11c）[182]。他们通过在 850 °C 下堆叠和烧结制备了多层 PEO-LiTFSI@LAGP。 然后，在真空条件下将叠层浸入聚合物电解液中，并在 80 °C 下压缩，使其断裂成薄片，让聚合物堵塞所有间隙。获得的 PIE 具有极高的极限弯曲强度和出色的韧性。用 LiFePO₄ 和 LMA 组装的 ASSB 在 60 °C 下循环 300 次后，在 0.5C 下仍能保持 92% 的初始容量。Xiong 等人通过电纺丝将硅烷官能化的 LATP 纳米粒子嵌入 PVDF 框架中形成纳米纤维膜，然后在复合网络（Si@LATP/PVDF/PVC）中对碳酸乙烯酯基前体进行热引发聚合[183]。硅烷官能化提高了 Si@LATP 与 PVDF 骨架的亲和力，并充分暴露了 LATP 上的路易斯酸位点。聚硅氧烷中的 -NH₃⁺ 进一步增加了阴离子的吸附。PIEs 对锂具有很高的电化学稳定性，而与 LiNi_0.5Co_0.2Mn_0.3O₂ 相结合的 ASSBs 则表现出优异的循环性能和速率能力。Fan 等人以 NaCl 为牺牲模板创建了多孔互连 LATP 网络，并将 PEO-LiTFSI 引入网络中（图11d）。PIE 不仅是锂⁺离子的快速传输通道，还是防止锂枝晶生长的物理屏障[184]。

Fig. 11 图 11

a Schematic of constructing a vertically aligned PIEs with LAGP and flexible PEO/PEG polymer [181]. Copyright 2019, Elsevier. b Comparison of interfacial evolution in a liquid electrolyte, ex situ polymer modification and in situ Janus polymer modification [196]. Copyright 2022, Elsevier. c Preparation process of PEO-LiTFSI@LAGP PIEs with “brick–mortar” microstructures [182]. Copyright 2020, John Wiley and Sons. d Illustration for the preparation of porous interconnected LATP networks with NaCl as a sacrificial template and SEM images of the PIEs [184]. Copyright 2019, Elsevier
a 利用 LAGP 和柔性 PEO/PEG 聚合物构建垂直排列 PIEs 的示意图 [181]。版权 2019 年，爱思唯尔。b 液体电解质、原位聚合物改性和原位 Janus 聚合物改性的界面演化比较 [196].版权 2022 年，爱思唯尔。c 具有 "砖-砂 "微结构的 PEO-LiTFSI@LAGP PIE 的制备过程 [182]。Copyright 2020, John Wiley and Sons.d 以 NaCl 为牺牲模板制备多孔互连 LATP 网络的图示以及 PIE 的扫描电镜图像 [184]。版权 2019 年，爱思唯尔

3.3.3 Perovskite-Type PIEs
3.3.3透闪石型 PIEs

Perovskite-type SSEs include Li_3xLa_2/3−xTiO₃ and (Li, Sr)(M, M') O₃ (M = Ti, Hf, Zr, Ga, Sn, etc., M' = Ta, Nb, etc.) [197, 198]. They process high ionic conductivity at room temperature (10^–3 S cm⁻¹) as well as outstanding mechanical strength and electrochemical oxidation potential (> 8 V). But they are vulnerable to reduction by the LMAs (Ti⁴⁺  + Li → Ti³⁺  + Li⁺). It is effective in overcoming defects by compositing with polymers. The recent research on PIEs filled with perovskite-type ISEs and their properties is summarized in Table 5.
Perovskite 型 SSEs 包括 Li_3xLa_2/3-xTiO₃ 和 (Li、Sr)(M, M') O₃ （M = Ti、Hf、Zr、Ga、Sn 等。,M' = Ta、Nb 等）[197, 198] 。它们在室温下具有很高的离子导电性（10^-3 S cm^-1）以及出色的机械强度和电化学氧化电位（> 8 V）。但它们很容易被 LMAs 还原（Ti⁴⁺ + Li → Ti³⁺ + Li⁺ ）。通过与聚合物复合，它能有效克服缺陷。表5概述了最近对填充了包晶型 ISE 的 PIE 及其特性的研究。

Table 5 PIEs filled with perovskite-type fillers and their properties
表 5 填充了包晶型填料的 PIE 及其特性

Hu et al. reported flexible PIEs made of PEO-LiFSI and Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ (PEO-LiFSI@LSTZ). The increased bonding of Ta⁵⁺ to F atoms in anions accelerated the release of Li⁺ ions and improved ionic conductivity (Fig. 12a) [199]. Concurrently, the SEI layer formed on LMAs increased the interfacial stability and inhibited lithium dendrites. The symmetrical Li||Li cells with PEO-LiFSI@LSTZ exhibited long-life stripping/plating behavior over 700 h. ASSBs matched with LiFePO₄ or LiNi_0.8Co_0.1Mn_0.1O₂ exhibited high cycle stability and rate performance.
Hu 等人报道了由 PEO-LiFSI 和 Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ 组成的柔性 PIE（PEO-LiFSI@LSTZ）。阴离子中的 Ta⁵⁺ 与 F 原子的结合增加，加速了 Li⁺ 离子的释放，提高了离子导电性（图 12a ）[199]。同时，在 LMA 上形成的 SEI 层增加了界面稳定性并抑制了锂枝晶的产生。与 LiFePO₄ 或 LiNi_0.8Co_0.1Mn_0.1O₂ 均表现出较高的循环稳定性和速率性能。

Fig. 12 图 12

a Structure of the LSTZ and LiTFSI adsorbed on the surface of the LSTZ [199]. Copyright 2019, Proceedings of the National Academy of Sciences. b Ionic conductivity and percolation model of LLTO networks and LLTO nanoparticles [200]. Copyright 2018, John Wiley and Sons. c Schematic diagram of Li⁺ ion conduction in continuous network and randomly scattered LLTO [201]. Copyright 2020, Elsevier. d Schematic diagram of Li⁺ ion conduction in randomly scattered and well-aligned nanowires [203]. Copyright 2017, Springer Nature
a LSTZ 的结构和吸附在 LSTZ 表面的 LiTFSI [199]。Copyright 2019, Proceedings of the National Academy of Sciences. b LLTO 网络和 LLTO 纳米颗粒的离子电导率和渗流模型 [200]。版权归 John Wiley and Sons 2018 所有。c 连续网络和随机分散 LLTO 中 Li⁺ 离子传导示意图 [201]。Copyright 2020, Elsevier.d 随机散布和排列整齐的纳米线中 Li⁺ 离子传导的示意图 [203]。2017 年《施普林格-自然》杂志版权所有

The alignment of the fillers in the polymer matrix has a significant effect on ionic conductivity and cell performance. Yu et al. compared randomly distributed Li_0.35La_0.55TiO₃ (LLTO) fillers with 3D interconnection network on the performance of PIEs (Fig. 12b) [200]. In the random distribution structure, the agglomeration of nanoparticles generated discontinuous Li⁺ conductive paths which reduced the percolation behavior and ionic conductivity. LLTO with a 3D interconnection structure provided a continuous interface phase for Li⁺ conduction, which can significantly improve the ionic conductivity of PIEs. Zhao et al. fabricated PIEs with a vertically aligned LLTO framework embedded in a PEO-LiTFSI matrix [201]. The vertically aligned structure provided a rapid and continuous network for Li⁺ transport, obtaining ionic conductivity of 0.13 × 10^–3 S cm⁻¹, which was 2.4 times more than that of PIEs with randomly scattered LLTO (Fig. 12c). Cui et al. investigated the impact of Li_0.33La_0.557TiO₃ nanowire orientation on the ionic conductivity of PIEs. Compared to the nanowires that were randomly scattered, well-aligned nanowires had a ten-fold increase in ionic conductivity (6.05 × 10^–5 S cm⁻¹ at 30 °C) [203] (Fig. 12d). Cui et al. compared the effect of Li_0.33La_0.557TiO₃ nanoparticles with nanowires on the performance of PAN-LiClO₄. Nanowires allowed for continuous ion transport channels, which shortened the transport distance and increased ionic conductivity compared to nanoparticle packing, where Li⁺ ions must cross several particle–particle junctions [202]. Therefore, developing continuous conduction paths is crucial for achieving high ionic conductivity of PIEs.
填料在聚合物基质中的排列对离子传导性和电池性能有显著影响。Yu 等人比较了随机分布的 Li_0.35La_0.55TiO₃ (LLTO) 填料与三维互连网络对 PIE 性能的影响（图12b）[200]。在随机分布结构中，纳米粒子的聚集产生了不连续的 Li⁺ 导电路径，从而降低了渗流行为和离子导电率。具有三维互连结构的 LLTO 为 Li⁺ 的传导提供了连续的界面相，可显著提高 PIE 的离子传导性。Zhao 等人在 PEO-LiTFSI 基质中嵌入了垂直排列的 LLTO 框架，制成了 PIE [201]。垂直排列的结构为锂⁺ 的传输提供了一个快速、连续的网络，离子电导率达到 0.13 × 10^-3 S cm^-1 ，是随机分散 LLTO 的 PIE 的 2.4 倍（图12c）。Cui 等人研究了 Li_0.33La_0.557TiO₃ 纳米线取向对 PIE 离子电导率的影响。与随机分散的纳米线相比，排列整齐的纳米线的离子电导率增加了十倍（6.05 × 10^-5 S cm^-1 30 °C）[203]（图12d）。Cui 等人比较了 Li_0.33La_0.557TiO₃ 纳米粒子与纳米线对 PAN-LiClO₄ 性能的影响。与纳米粒子填料相比，纳米线可以形成连续的离子传输通道，从而缩短了传输距离，提高了离子传导性，而在纳米粒子填料中，锂⁺离子必须穿过多个粒子-粒子连接点[202]。因此，开发连续的传导路径对于实现 PIE 的高离子传导性至关重要。

3.3.4 Sulfide-Type PIEs
3.3.4硫化物型 PIE

The ionic conductivity of sulfide-type SSEs can reach ~ 10^–2 S cm⁻¹, while the electrochemical stability and interfacial stability are poor (Fig. 13a) [209]. The sensitivity of sulfides to air necessitates treatment in inert gas environment, which impedes their large-scale utilization. Sulfide-type SSEs are classified as binary or ternary based on their compositions. Binary sulfide SSEs comprise P₂S₅ and Li₂S including Li₇P₃S₁₁ and Li₃PS₄, while ternary sulfide electrolytes comprise P₂S₅, Li₂S and MS₂ (M = Si, Ge, Sn) including Li₁₀GeP₂S₁₂ (LGPS) and Li₆PS₅X (X = Cl, Br, I) [210]. Combining sulfide-type SSEs with polymers can increase interfacial stability and improve processability. The recent research on PIEs filled with sulfide-type ISEs and their properties is summarized in Table 6.
硫化物型 SSE 的离子电导率可达约 10^-2 S cm^-1，但电化学稳定性和界面稳定性较差（图 13a ）[209]。硫化物对空气的敏感性要求在惰性气体环境中进行处理，这阻碍了硫化物的大规模利用。硫化物类 SSE 根据其组成分为二元和三元。二元硫化物 SSE 包括 P₂S₅ 和 Li₂S 包括 Li₇P₃S₁₁ 和 Li₃PS₄ 、而三元硫化物电解质包括 P₂S₅ 、Li₂S 和 MS₂ (M = Si, Ge、Sn），包括Li₁₀GeP₂S₁₂ (LGPS)和Li₆PS₅X (X = Cl、Br、I）[210]。将硫化物型 SSE 与聚合物结合可提高界面稳定性并改善加工性能。表6概述了最近关于填充硫化物型 ISE 的 PIE 及其特性的研究。

Fig. 13 图 13

a Arrhenius of sulfide electrolytes compared to organic liquid electrolytes [209]. Copyright 2019, John Wiley and Sons. b Preparation process of PIEs with Li₆PS₅Cl (LPSCl) and P(VDF-TrFE) by electrospinning-permeation-hot pressing method [88]. Copyright 2022, John Wiley and Sons. c Preparation process of PEO-LiTFSI@LSPSCl [211]. Copyright 2022, John Wiley and Sons. d Comparison of S||Li batteries operated in liquid electrolytes, ceramic solid-state electrolytes and PIEs [212]. Copyright 2020, John Wiley and Sons
a 硫化物电解质的阿伦尼乌斯与有机液态电解质的比较 [209].版权归 John Wiley and Sons 2019 所有。b 电纺-渗透-热压法制备含 Li₆PS₅Cl (LPSCl) 和 P(VDF-TrFE) 的 PIEs [88].版权归 John Wiley and Sons 2022 所有。c PEO-LiTFSI@LSPSCl 的制备过程 [211].版权 2022 年，John Wiley and Sons。d 在液态电解质、陶瓷固态电解质和 PIE 中运行的 S||Li 电池的比较 [212].Copyright 2020, John Wiley and Sons 版权所有

Table 6 PIEs filled with sulfide-type fillers and their properties
表 6 填充了硫化物类填料的 PIE 及其特性

Nan et al. prepared ultra-thin and flexible PIEs with Li₆PS₅Cl (LPSCl) and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) by electrospinning-permeation-hot pressing method (Fig. 13b) [88]. The TrFE groups allowed P(VDF-TrFE) to exhibit dominant-phase with an all-trans conformation, resulting in a higher dielectric constant and greater flexibility than PVDF. The strong polarity of the polymer promoted the interaction with LSPSCl. The P(VDF-TrFE) network enabled the complete infiltration of LPSCl particles to generate interpenetrating P(VDF-TrFE)@LPSCl films. The PIEs had an ionic conductivity of up to 1.2 × 10^–3 S cm⁻¹ and enabled Li-In||LiNi_0.8Co_0.1Mn_0.1O₂ cells to maintain 71% capacity after 20,000 cycles at 1.0 mA cm⁻². PEO-LiTFSI@LSPSCl PIEs were developed by solution casting method (Fig. 13c) [211]. The Li||S batteries assembled by the PIEs retained 97.8% of their initial capacity at 0.1 Ag⁻¹. Cryo-TEM revealed that LSPSCl facilitated the decomposition of TFSI⁻ and enhanced ionic conductivity. Li₂O, LiF and Li₂S-rich SEI formed by anionic decomposition hindered dendrite growth and enhanced interfacial stability. PEO-LiTFSI@LSPSCl also suppressed the shuttling of phosphorus and sulfur specie. By employing PEO-LiTFSI@Li_3.25Ge_0.25P_0.75S₄, Bieker et al. reduced the interfacial contact impedance and increased ionic conductivity (0.42 × 10^–3 S cm⁻¹) and t₊ (0.87). The cells made of vulcanized polyacrylonitrile and LMAs exhibited outstanding rate performance and cycle stability (Fig. 13d) [212].
Nan 等人采用电纺丝-渗透-热压法制备了含 Li₆PS₅Cl (LPSCl) 和聚偏氟乙烯-共三氟乙烯 (P(VDF-TrFE)) 的超薄柔性 PIE（图）。13b）[88]。TrFE 基团使 P(VDF-TrFE)呈现出全反式构象的主相，从而比 PVDF 具有更高的介电常数和更大的柔韧性。聚合物的强极性促进了与 LSPSCl 的相互作用。P(VDF-TrFE)网络使 LPSCl 颗粒能够完全渗入，生成互穿的 P(VDF-TrFE)@LPSCl 薄膜。PIE 的离子电导率高达 1.2 × 10^-3 S cm^-1 ，并使 Li-In||LiNi_0.8Co_0.1Mn_0.1O₂ 电池在 1.0 mA cm^-2 下循环 20,000 次后仍能保持 71% 的容量。PEO-LiTFSI@LSPSCl PIE 是通过溶液浇铸法研制的（图13c）[211]。在 0.1 Ag^-1条件下，PIEs 组装的锂离子电池保持了 97.8% 的初始容量。Cryo-TEM 显示 LSPSCl 促进了 TFSI^- 的分解，并增强了离子导电性。阴离子分解形成的富含Li₂O、LiF和Li₂S的SEI阻碍了枝晶的生长，增强了界面稳定性。PEO-LiTFSI@LSPSCl 还能抑制磷和硫的穿梭。通过使用 PEO-LiTFSI@Li_3.25Ge_0.25P_0.75S₄, Bieker 等人降低了界面接触阻抗，提高了离子电导率 (0.42 × 10^-3 S cm^-1) 和 t₊ (0.87)。由硫化聚丙烯腈和 LMA 制成的电池具有出色的速率性能和循环稳定性（图13d）[212]。

3.4 Synthesis of PIEs
3.4合成 PIE

The preparation methods of PIEs are mainly based on the synthesis of polymers involved solution casting, phase inversion, electrospinning and in situ polymerization. Solution casting entails dispersing the polymer, lithium salts and fillers in solvents, thoroughly agitating and then casting the mixture onto a flat substrate [224]. After removing the solvents, PIEs are obtained. This procedure is straightforward to implement; however, it cannot precisely regulate the porosity and thickness of PIEs. Phase inversion and solution casting share similar beginning steps; however in the former, the mixture coated on the substrate is soaked in a nonsolvent to replace the solvent. The exchange process causes phase transitions in the polymer. After drying at a high temperature, porous PIEs are created. Electrospinning is commonly used to fabricate one-dimensional nanomaterials and nanofiber-woven 3D networks. It can produce PIEs with adjustable porosity, pore size, thickness and excellent elasticity. Long fibers can offer continuous routes for ion transport [16]. In situ polymerization is the process of solidifying procurers containing curable monomers (e.g., tetrahydrofuran, 1,3-dioxolane, etc.), initiators (e.g., PF₅, BF₃, AlCl₃, etc.), lithium salts and inorganic fillers under specific conditions (e.g., heat, UV radiation) [225]. Inorganic fillers shall be uniformly dispersed in the polymer during the process. Grafting allows fillers to covalently join on the polymer to avoid agglomeration of nanoparticles [226].
PIE 的制备方法主要基于聚合物的合成，包括溶液浇注、相位反转、电纺丝和原位聚合。溶液浇铸法是将聚合物、锂盐和填料分散在溶剂中，充分搅拌后将混合物浇铸到平坦的基底上[224]。除去溶剂后，即可得到 PIE。这种方法简单易行，但无法精确调节 PIE 的孔隙率和厚度。相反转和溶液浇铸的起始步骤相似，但在前者中，涂覆在基底上的混合物被浸泡在非溶剂中以取代溶剂。交换过程会导致聚合物发生相变。在高温下干燥后，多孔 PIE 便形成了。电纺丝通常用于制造一维纳米材料和纳米纤维编织的三维网络。它可以制造出孔隙率、孔径、厚度可调且弹性极佳的 PIE。长纤维可提供连续的离子传输路径[16]。原位聚合是将含有可固化单体（如四氢呋喃、1,3-二氧戊环等）、引发剂（如、PF₅, BF₃, AlCl₃ 等）、锂盐和无机填料[225]。在此过程中，无机填料应均匀地分散在聚合物中。接枝可使填料与聚合物共价结合，避免纳米颗粒聚集[226]。

When solvent treatment is performed, the compatibility between solvents and fillers must be evaluated. In the presence of sulfides, the polarity index of the solvent must be less than 3.1 [227]. Physical and chemical properties of different PIE components, such as reactivity and toxicity with wet air and oxygen, must be thoroughly accounted for. For example, sulfides exhibit strong reactivity in humid air, leading to the creation of hazardous H₂S [228]. PIEs composed of sulfides must be treated in a dry environment or even an inert gas atmosphere [229].
在进行溶剂处理时，必须评估溶剂与填料之间的相容性。在存在硫化物的情况下，溶剂的极性指数必须小于 3.1 [227]。必须全面考虑不同 PIE 成分的物理和化学特性，如与湿空气和氧气的反应性和毒性。例如，硫化物在潮湿空气中会发生强烈反应，产生有害的 H₂S [228]。由硫化物组成的 PIE 必须在干燥环境甚至惰性气体环境中进行处理 [229]。

Generally, active fillers can conduct Li⁺ ions and the interface generated by their contact with polymers can provide transport routes for Li⁺ ions. Establishing continuous conduction routes is critical to improving ionic conductivity. The fillers’ type, particle size, shape, arrangement and interaction with other components will influence performance. Vertical heterostructures possess asymmetrical features, which can enrich the design strategies and show great potential in the practical application of ASSBs.
一般来说，活性填料可以传导锂⁺ 离子，其与聚合物接触产生的界面可以为锂⁺ 离子提供传输路径。建立连续的传导路线对于提高离子传导性至关重要。填料的类型、粒度、形状、排列以及与其他成分的相互作用都会影响性能。垂直异质结构具有不对称的特点，这可以丰富设计策略，并在 ASSB 的实际应用中显示出巨大的潜力。

4 Summary and Perspective
4总结与展望

This review presents recent progress on PIEs with inorganic fillers and focuses on the influence of inert and active fillers on the characteristics of the PIEs (Fig. 14). Especially, composite with active fillers can effectively overcome defects of the single component and improve the comprehensive performance of the electrolyte. The characteristics of PIEs are influenced by the type, content, morphology, arrangement and surface groups of the fillers. Proper design of fillers can significantly improve the ionic conductivity, mechanical strength and interfacial stability of the PIEs. Given their superior integrative performance, PIEs have been extensively investigated in ASSBs assembled with high-energy-density cathode and anode including S, O₂ and LMAs. Even though PIEs have made significant strides, fundamental scientific questions remain and widespread implementation confronts substantial obstacles.
本综述介绍了无机填料 PIE 的最新研究进展，并重点讨论了惰性和活性填料对 PIE 特性的影响（图 14）。尤其是与活性填料的复合，能有效克服单组分的缺陷，提高电解液的综合性能。PIE 的特性受填料的类型、含量、形态、排列和表面基团的影响。合理设计填料可显著提高 PIE 的离子导电性、机械强度和界面稳定性。鉴于 PIEs 优越的综合性能，人们对其在组装有高能量密度阴极和阳极（包括 S、O₂ 和 LMAs）的 ASSB 中的应用进行了广泛研究。尽管 PIE 取得了长足的进步，但基本的科学问题依然存在，广泛应用也面临着巨大的障碍。

• 1. Even though ionic conductivity of PIEs has greatly increased compared to traditional PSEs, it is still much lower than that of conventional liquid electrolytes, which is detrimental to develop LIBs with high energy density and power capability. Precisely regulating the characteristics and arrangement of fillers are expected to break through the ionic conductivity limit of PIEs. Understanding the Li⁺ migration routes and interactions between different components can provide crucial theoretical direction for enhancing ionic conductivity. Furthermore, it is essential to develop advanced in situ characterization techniques and theoretical computation methods to conduct mechanistic investigations in PIEs.
  尽管与传统的 PSE 相比，PIE 的离子电导率已大大提高，但仍远低于传统液态电解质的离子电导率，这不利于开发具有高能量密度和功率能力的 LIB。精确调节填料的特性和排列有望突破 PIEs 的离子电导率极限。了解锂⁺的迁移路径和不同成分之间的相互作用可为提高离子导电性提供重要的理论方向。此外，开发先进的原位表征技术和理论计算方法对于开展 PIE 的机理研究至关重要。

• 2. It is critical for realizing the interfacial stability between PIEs and electrodes. Although the flexible polymer matrix can improve the contact with the rough electrode, the frequent expansion and contraction of the electrodes would degrade the contact during the charging/discharging processes. Especially when matching LMAs, the uneven plating/stripping behavior may cause the loss of electrical connection of active lithium. Regulating the electrochemical behavior of LMAs and adding minuscule ionic liquid or liquid electrolyte can considerably optimize surface contact.
  这对于实现 PIE 与电极之间的界面稳定性至关重要。虽然柔性聚合物基质可以改善与粗糙电极的接触，但在充电/放电过程中，电极的频繁膨胀和收缩会降低接触效果。特别是在匹配 LMA 时，不均匀的镀层/剥离行为可能会导致活性锂失去电气连接。调节 LMA 的电化学行为并添加微量离子液体或液态电解质可大大优化表面接触。

• 3. Increasing the operating voltage is a potential avenue for developing high-energy–density batteries. By crafting the arrangement of the fillers and polymer, we can increase the stability on the high-voltage cathode and reduce dendritic growth and side reactions on the anode. Coatings that can withstand high voltages and reduction are expected to encourage the widespread use of PIEs.
  提高工作电压是开发高能量密度电池的一个潜在途径。通过精心设计填料和聚合物的排列，我们可以提高高压阴极的稳定性，减少阳极上的树枝状生长和副反应。能够承受高压和还原的涂层有望促进 PIE 的广泛应用。

• 4. At present, the thickness of PIEs is still significantly higher than that of commercial polyolefin separators. Developing ultra-thin PIEs with moderate rigidity and flexibility is conducive to improving the energy density of batteries. The uniform dispersion of inorganic fillers in polymer matrix facilitates to construct continuous and uniform Li⁺ transport channels. And the efficient dispersion of fillers is critical in the manufacture of PIEs. Meanwhile, processing compatibility with electrodes or other internal components must be guaranteed throughout synthesis and operation.
  目前，PIE 的厚度仍明显高于商用聚烯烃隔膜。开发刚柔并济的超薄 PIE 有利于提高电池的能量密度。无机填料在聚合物基体中的均匀分散有利于构建连续、均匀的锂⁺ 传输通道。而填料的高效分散是制造 PIE 的关键。同时，在整个合成和运行过程中，必须保证与电极或其他内部元件的加工兼容性。

Fig. 14 图 14

Overview of the topics in this paper
本文主题概述

Generally, solving these problems still requires the joint efforts of multidisciplinary fields. The assessment of advanced research and outlook for future research in this paper is expected to benefit the next generation of all-solid-state lithium metal batteries.
一般来说，解决这些问题仍需要多学科领域的共同努力。本文对先进研究的评估和对未来研究的展望，有望为下一代全固态锂金属电池带来裨益。