An Ultrathin, Flexible Solid Electrolyte with High Ionic Conductivity Enhanced by a Mutual Promotion Mechanism

The pursuit of strong endurance and nonflammable performances has promoted demand for solid-state batteries (SSBs). Meanwhile, the reduction of electrolytes' thickness is the key to improving battery performance. However, a large-scale feasible method to fabricate an ultrathin solid electrolyte exhibiting high ionic conductivities is still a challenge. Here, we show a large-scale feasible method to prepare a succinonitrile/polyacrylonitrile(SN/PAN)-coated Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) with flexibility and high ionic conductivity by tape-casting. The unique dual polymer-coated garnet electrolytes exhibit structural stability through mutual promotion, constructing soft interparticle contact that provides fast lithium-ion transfer channels. In essence, the mutual promotion mechanism is that SN can improve the Li⁺ conductivity of PAN, while PAN can protect SN from aggregation. Therefore, the flexible SN/PAN-coated LLZTO provides high structural stability and satisfactory electrochemical performance, contributing to a high ionic conductivity of 4 × 10^-4 S cm^-1 at room temperature (RT). In this way, a long lifespan of over 500 cycles and a high discharge capacity (163 mAh g^-1) are achieved based on LiFePO4 (LFP) cathodes at 0.2 C.