Lithium-ion secondary batteries have become the primary energy storage devices due to their high energy density, long cycle life, and low cost, and are widely applied in fields such as electric vehicles. However, the use of organic electrolytes in traditional lithium-ion batteries poses safety risks such as leakage and combustion^[1-3]. Additionally, the adoption of graphite anodes results in low energy density. There is an increasing demand for higher safety and energy density in lithium batteries. Replacing commercial electrolytes and separators with solid-state electrolytes improves the spatial utilization within batteries, thereby enabling compatibility with lithium metal anodes. This approach can effectively enhance battery safety while further increasing energy density^[4-5]. Therefore, the development of solid-state electrolytes has become one of the research hotspots in the field of energy storage both domestically and internationally.

Common solid-state electrolytes can be classified into polymer solid-state electrolytes, inorganic solid-state electrolytes, and organic-inorganic composite solid-state electrolytes (Composite Solid-State Electrolyte, CSSE)^[6-8]. Among them, polymer solid-state electrolytes possess excellent flexibility and processability, but exhibit low ionic conductivity at room temperature. Inorganic solid-state electrolytes can be divided into oxides^[9], halides^[10], and sulfides^[11] according to their composition. Oxide-based solid-state electrolytes have high air stability and a wide electrochemical window; however, they suffer from large grain boundary impedance and usually require high-temperature heat treatment to reduce the grain boundary impedance and improve ionic conductivity. Halide-based solid-state electrolytes possess high ionic conductivity and compatibility with oxide cathodes, but show poor compatibility with lithium metal and higher preparation costs. Sulfide electrolytes exhibit the highest ionic conductivity (Figure 1)^[12] and also possess softness and ease of cold pressing, making them promising candidates for all-solid-state batteries. However, sulfides are unstable toward lithium metal anodes and demonstrate poor electrochemical stability. Organic-inorganic composite solid-state electrolytes combine the advantages of inorganic and polymer electrolytes, achieving both high ionic conductivity and electrochemical stability, and have become one of the current research hotspots.

According to the composition of raw materials, sulfide-based solid electrolytes can be divided into two major categories: binary sulfides and ternary sulfides. Binary sulfide electrolytes mainly include three types of systems: Li₂S-P₂S₅^[13-14], Li₂S-SiS₂^[15-16], and Li₂S-GeS₂^[17-18]. Ternary sulfide electrolytes mainly include two categories: Li₁₀MP₂S₁₂ (M = Si^[19], Ge^[20], Sn^[21]) and Li₆PS₅X^[22] (X = Cl, Br, I). Ternary sulfide electrolytes are materials obtained by high-energy ball milling and doping modification based on binary sulfides and usually exhibit higher ionic conductivity. Among them, the most representative are Li₁₀GeP₂S₁₂ (LGPS) and Li₆PS₅Cl (LPSC). The room-temperature ionic conductivity of LGPS-type sulfides can reach 1.2×10^-2 S·cm^-1, but the high cost of Ge restricts its further development. In contrast, LPSC-type sulfides have attracted more attention due to their relatively lower production cost, simple synthesis process, and high room-temperature ionic conductivity (1×10^-2 S·cm^-1). However, pure LPSC sulfides also face issues such as instability in air, moisture absorption, and the generation of toxic gases. These issues can be addressed through composite modification with polymers by preparing composite solid-state electrolytes (CSSE) to improve their practicality. This paper elaborates on the classification and ion transport mechanisms of LPSC-CSSE, discusses their composition, composite methods, and modification strategies, and finally summarizes and prospects the future development direction of LPSC-CSSE.

Currently, there are typically three transport pathways for lithium ions in CSSE: the polymer organic phase pathway, the interfacial space charge region pathway formed between the polymer and inorganic electrolyte, and the inorganic phase pathway of the electrolyte^[23] (Fig. 2).

Polymers in CSSE primarily serve to connect inorganic particles and provide flexibility. The transport mechanism of lithium ions follows the free volume theory, mainly through the movement of organic chain segments in the amorphous regions of the polymer; mobile functional groups on the chain segments allow lithium ions to migrate via a "hopping" mechanism, thus enabling lithium ion conduction^[24]. In addition, the microporous and nanoporous structures inherent to both the polymer and the inorganic phase can also act as transport channels for lithium ions.

In composite electrolytes composed of polymers and inorganic electrolytes, a space charge region forms at the interface between the two due to significant differences in their physical and chemical properties. Lithium-ion transport is achieved through the synergistic effect of both components within this region^[25]. In certain cases, lithium ions may need to adsorb onto the interface first before passing through it into another material; in other cases, lithium ions may migrate directly across the interface. Due to the complex transport mechanisms of lithium ions at the organic-inorganic interface in CSSE, a systematic understanding is currently lacking.

In CSSE, the inorganic phase of the electrolyte is typically the main channel for lithium ion conduction. Macroscopically, functional groups on the surface of inorganic phase particles can coordinate with lithium ions, thereby reducing the energy barrier for lithium ions during migration^[26]. Microscopically, the transport mechanism of lithium ions mainly follows the direct gap mechanism and vacancy mechanism. Within the inorganic phase of the electrolyte, lithium ions are primarily transported through lattice channels. These channels are determined by the crystal structure of the inorganic electrolyte and are typically interstitial sites or specific crystal planes within the lattice. In addition to lattice channel transport, vacancies can also assist in lithium ion transport. These vacancies can act as "traps" for lithium ions, enabling their migration.

LPSC-CSSE typically consists of LPSC sulfides and organic polymers. Depending on the specific roles of the polymer and sulfide in the CSSE, they can be categorized into two types: polymer-based and sulfide-based. The former uses a polymer network as the supporting framework to construct an efficient percolation network, while the latter employs sulfides as the matrix, with the polymer serving a modifying function.

With the continuous development of the lithium battery industry, research on solid-state batteries has entered a new round of popularity. All-solid-state batteries are widely recognized as one of the preferred solutions for next-generation batteries. Sulfide all-solid-state batteries have attracted significant attention in the industry due to their excellent overall performance. This review summarizes the research work on LPSC-CSSE, briefly introduces the basic properties, characteristics, and development history of sulfide electrolytes, and discusses the composite methods and modification strategies of LPSC sulfides with polymers. LPSC composite solid-state electrolytes possess not only high ionic conductivity, high energy density, and excellent rate performance, but also good solid-solid contact, achieving remarkable results in the field of solid-state lithium batteries. Future research should focus on the following aspects.

(1) Reduce the production cost of electrolytes. Currently, sulfide electrolytes generally face issues such as high raw material costs, complex synthesis processes, and low yields, which are also one of the main reasons why solid-state batteries are difficult to mass produce. Therefore, developing novel low-cost sulfide electrolytes and optimizing the synthesis and preparation processes of electrolytes are crucial for the mass production of sulfide-based solid-state batteries.

(2) Enhancing electrolyte stability. Sulfide electrolytes exhibit sensitivity to air and moisture, and tend to generate toxic hydrogen sulfide gas, which hinders their industrialization to some extent. Currently, researchers have explored strategies involving functional nanolayer coatings on electrolyte particles and the development of phosphorus-free sulfide systems. Studies have shown that these strategies and systems significantly improve the air and chemical stability of the electrolytes. Enhancing the stability of sulfide electrolytes is of great significance for their commercial application.

(3) Interface improvement. In the field of solid-state batteries, the interface has always been a complex and challenging issue. Sulfide electrolytes, due to their soft texture, exhibit relatively good solid-solid contact. However, unstable physicochemical reactions still occur at the interfaces between sulfides and both the anode and cathode, which can lead to a sharp increase in interfacial impedance, thereby affecting Li⁺ migration and conduction. Therefore, for LPSC-CSSE, comprehensive and multidimensional interfacial modification techniques are crucial.