Polyoxyethylene (PEO)|PEO-Perovskite|PEO Composite Electrolyte for All-Solid-State Lithium Metal Batteries

Composite solid electrolytes (CSEs) are regarded as one of the most promising candidates for all-solid-state lithium metal batteries (ASSLMBs) due to inherited desirable features from both ceramic and polymer materials. However, poor interfacial contact/compatibility between the electrodes and solid electrolytes remains a critical challenge. In this work, we prepare a flexible CSE composed of polyoxyethylene (PEO)-perovskite composite with a layer of PEO on either side. This PEO|PEO-perovskite|PEO structure prevents direct contact between the perovskite and lithium metal at the anode side, avoiding the undesired reaction between the two materials (Ti⁴⁺ + Li → Ti³⁺ + Li⁺). Moreover, the design incorporating the PEO surface on either side enables superb contact between the electrolyte and the electrodes and buffers the change in electrolyte volume from the cathode and lithium metal during repeated cycling, resulting in low interfacial resistances and excellent cycling stability. Meanwhile, perovskite inorganic electrolyte Li_0.33La_0.557TiO₃ (LLTO) 3D nanofiber networks formed by electrospinning enable the CSE to achieve enhanced mechanical strength and high ionic conductivity of 0.16 mS cm^-1 at 24 °C. As a result, a Li|PEO-LiTFSI-LLTO|Li symmetric cell remains stable after 400 h of operation without short-circuiting. Most notably, a Li|PEO-LiTFSI-LLTO|LiFePO₄ full battery is capable of delivering a high capacity of 135.0 mAh g^-1 even at 2 C with a retention rate of 79.0% after 300 cycles at 60 °C. These results demonstrate that the integrated sandwich structure proposed in this work is effective in developing high-performance composite solid electrolytes for ASSLMBs.