such as leakage, corrosion, swelling, and flammability. [3,4] Therefore, developing non-flammable solid electrolytes to replace traditional organic electrolytes has become the focus of this field. [1,2,5] Meanwhile, lithium metal, with the lowest potential (−3.04 V vs standard hydrogen electrode) and high theoretical capacity (3860 mA h g −1 ), has the potential as anodes for solid-state batteries and greatly improves the energy density. [6][7][8] Solid electrolytes, regarded as the most important core of solid-state batteries, could be roughly divided into three categories: inorganic solid electrolytes (ISEs), solid polymer electrolytes (SPEs), and composite polymer electrolytes (CPEs). [5,[9][10][11] ISEs exhibit high ionic conductivity and a wide electrochemical stability window, while their rigidity restricts close contact with electrodes, which leads to interface problems between ISEs and electrodes. [12][13][14] SPEs, based on Li + immigration in polymers, have considerable advantages over ISEs, such as low cost, easy preparation, high flexibility, and especially good ability to form closer contact with electrodes. However, the ionic conductivity of SPEs is quite low at room temperature. [11,15] Meanwhile, the low ionic transference number of SPEs (< 0.4) is supposed to be the positive incentive of dendrite growth. [16][17][18] At present, researchers improve the ionic conductivity mostly by increasing the proportion of amorphous regions, such as cross-linking, forming copolymers, and adding plasticizers. [19][20][21][22] As ISEs and SPEs mentioned above have different advantages and disadvantages, CPEs could combine the superior properties of inorganic ceramics and polymer matrixes, and not only improve the mechanical properties and ionic conductivity at room temperature over SPEs, but also enhance the interface contact over ISEs. [23][24][25][26][27][28] Therefore, CPEs have been attracting more research interest.CPEs are solid conductors consisting of inorganic fillers dispersed in solid polymer hosts. Inorganic fillers could be generally divided into two kinds, non-lithium conductors (such as TiO 2 and SiO 2 ) and lithium-ion conductors (such as garnet-type Li 7 La 3 Zr 2 O 12 (LLZO), perovskite-type Li 3x La 2/3-x TiO 3 (LLTO), NASICON-type Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP), and sulfide Li 10 GeP 2 S 12 (LGPS)). [27,[29][30][31][32][33][34][35][36][37] Generally, compared with nonlithium-ion conductors, lithium-ion conductors could provide more lithium-ion transmission channels and thus improve the ionic conductivity. [11,24] Among them, LLZO, LLTO, LATP, and LGPS are four representative well-studied solid electrolytes.