Solid-state lithium-ion batteries (SSLBs) are one of the most promising next-generation batteries due to their excellent safety, higher energy density and longer cycle life. Solid-state electrolytes (SSEs) with a high ionic conductivity, a wide electrochemical window, and good mechanical properties are the importance of the high-performance SSLBs. The organic polymer SSEs have some advantages of flexibility and good mechanical property, that makes them almost have no serious interfacial problems. However, the low ionic conductivity is a major drawback for most polymer SSEs. Inorganic SSEs show a high ionic conductivity, a good electrochemical stability, and a high thermal stability, but their main disadvantages are a weak mechanical property (i.e., brittle and fragile), a poor air stability and an inferior compatibility with metal lithium anode. Compared to inorganic and organic polymer SSEs, composite solid-state electrolytes (CSSEs) integrating the merits of organic polymers and inorganic electrolytes exhibit a good electrochemical performance and an excellent interface compatibility, which have attracted extensive attention in the SSEs research. This review detailly summarized the research progress on CSSEs in lithium batteries.

The CSSEs are usually composed of inorganic fillers, organic polymers and lithium salts. The inorganic fillers are classified into inert and active fillers. The inert fillers, such as SiO₂, Al₂O₃ and TiO₂, do not transport Li⁺ themselves, but can promote the ionic properties of polymer matrices. The active fillers conduct Li⁺ themselves including oxides (i.e., perovskite-type, garnet-type, NASICON-type), sulfides, and halides, etc.. Lithium salts used for the investigation of CSSEs include LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂(LiTFSI), etc..

The first CSSE with an inert filler was prepared via introducing an Al₂O₃ into a matrix, thus improving the mechanical property of LiClO₄/PEO CSSEs at 100 ℃. SiO₂ and TiO₂ were a commonly used inert ceramic filler for the fabrication of CSSEs. The content, particle size and shape of metal oxide inert fillers affect the performance of CSSEs. The layered nano-sized claylike montmorillonite (MMT) as a passive filler was doped into polyvinylidene fluoride (PVDF)-hexafluoropropylene (HFP) polymer to prepare a ultraviolet (UV)-crosslinked MMT/PVDF-HFP membrane, having a room-temperature ionic conductivity of 1.6×10^-3 S/cm. Carbon materials were also served as inert fillers to prepare CSSEs.

The CSSEs with active fillers were synthesized via adding active fillers to polymers. Li_0.5La_0.5TiO₃ (LLTO) is a typical perovskite-type active filler. The reduction of Ti ions in LLTO with lithium was prevented as combining LLZO with polymers, and the LLZO CSSEs exhibited a wide electrochemical window and a good stability. For instance, a PVDF polymer CSSEs with LLTO nanowires delivers a room temperature ion conductivity of 5.8×10^-4 S/cm, an electrochemical window of 5.2 V, and a mechanical strength of 10 MPa. The cell of Li|PVDF/LLTO-15%/Li⁺|LiFePO₄ (LFP) has a superior Coulomb efficiency of approaching 100% after 200 cycles. The garnet-type active filler Li₇La₃Zr₂O₁₂ (LLZO) and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) were divided into zero-dimensional nanoparticles, one-dimensional nanowires and nanotubes, two-dimensional nanosheets, and three-dimensional structures according to the geometric structures. For instance, a LLZTO/PEO CSSEs was prepared by dispersing a zero-dimensional LLZTO powder into acetonitrile solution of PEO/LiTFSI. The room-temperature ion conductivity and electrochemical stability window of LLZTO/PEO CSSEs are 4.76×10^-4 S/cm and 4.75 V, respectively. The multi-mixing CSSE membrane consisting of LLZTO, PEO, PVDF-HFP, and LiTFSI has a high ionic conductivity of 1.05×10^-4 S/cm at 35 ℃, an electrochemical window of 5.2 V, and a high Li⁺ transference number of 0.52 at 60 ℃. A three-dimensional PVDF/LLZO/LiClO₄ CSSEs was obtained by adding 3D coral-like LLZO nanofiller into PVDF/LiClO₄. This CSSE membrane structured 3D interconnected framework has an enhanced ionic conductivity of 1.51×10^-4 S/cm at room temperature. The assembled battery delivers a capacity retention of 95.2% after 200 cycles at 1 C. The NASICON-type Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) and Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) ceramics as active fillers were added into PEO and PVDF-HFP polymers. A PVDF-HFP/LiTFSI CSSE containing 50% (in mass fraction) LAGP displays the maximum ionic conductivity of 9.2×10^-4 to 9.6×10^-4 S/cm at room temperature. After 50 cycles, the cell with PVDF-HFP/50% LAGP/LiTFSI CSSE remains 141.3 mA·h/g and a capacity retention rate of 89.5%.

Besides, sulfide fillers were investigated. However, sulfides are instability in air because it intrinsically tends to react with moisture to generate toxic H₂S gas. The CSSEs prepared with sulfide fillers improve asulfide stability and offer a high ion conductivity. For instance, adding sulfide active filler Li₁₀GeP₂S₁₂ (LGPS) to PEO polymer can prepare a PEO/LGPS/LiTFSI CSSE membrane with a stability in air. The CSSE has an ionic conductivity of 1.18×10^-4 S/cm at room temperature. After 150 cycles at 0.5 C, the cells using PEO/LGPS/LiTFSI as a membrane demonstrates superior capacity retention and rate performance.

To meet high-voltage cathodes and lithium metal, CSSEs with bi-layered or multi-layered structures maximize the synergistic effect of each layer without or withless sacrificing their properties, particularly for adjustable interphase. A bi-layer structure used Li_6.4La₃Zr₂Al_0.2O₁₂/PEO/LiTFSI and I₂/PEO/LiTFSI CSSE layers to contact with cathode and anode, respectively. After 500 cycles at 0.2 C, the LFP||Li cell maintains a specific capacity of 146.20 mA·h/g. A sandwich structure CSSE with PAN/PVDF-80% (in mass) LLTO/PEO was prepared by a casting method, having an ionic conductivity of 2.81×10^-4 S/cm and an electrochemical window of 4.92 V. The assembled battery delivers the Coulomb efficiency of 99.2% and a capacity retention rate of 88% at 0.5 C after 500 cycles.

The mechanism of ion transport in CSSEs has not yet clarified due to itscomplexity. The current well-known ion transport pathways in CSSEs include polymer matrix, inorganic active fillers and interfacial regions between polymer matrix and active fillers. Among them, the ion-transport pathway at the interfaces is relatively complex, and more efforts are needed to reveal the mechanism.