Fabrication and electrochemical behavior of flexible composite solid electrolyte for bipolar solid-state lithium batteries

To improve the performance of solid-state lithium batteries (SSLB), solid-state succinonitrile (SCN) is incorporated into a composite solid electrolyte (CSE) comprising Al-doped LLZO (Li_6.4Al_0.2La₃Zr₂O₁₂), poly(ethylene oxide) (PEO), and lithium salts, thereby conferring flexibility to the CSE. SCN acts as a solid-state plasticizer that increased the ion conduction and interfacial control between the materials. The flexible CSE sheet exhibits a high ionic conductivity of 2.5 × 10⁻⁴ S cm⁻¹ at 40 °C and a wide potential window of ∼5.2 V vs. Li/Li⁺. The solid SCN provides new synergistic properties such as an additional ion conduction path and suppresses PEO oxidation. When the CSE with 7 wt% SCN is utilized as the cathode layer and solid electrolyte layer in SSLBs, instead of the liquid electrolyte, a discharge capacity of approximately 130 mAh g⁻¹ is realized at 45 °C and 0.1C, whilst a capacity retention rate of 90% and a coulombic efficiency of 99% are obtained over 100 cycles. A 3-stack bipolar cell is fabricated to evaluate the mechanical properties of the flexible CSE sheet, demonstrating stable cycle performance over 50 cycles without any short circuiting or leakage current.