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 (Li6.4Al0.2La3Zr2O12), 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. •Composite solid electrolytes were prepared via PEO, Al-LLZO, SCN and Li salts.•The CSE sheet with SCN exhibits a ionic conductivity of 2.5 × 10−4 S cm−1 at 40 °C.•SCN provides an additional lithium ion conduction path and suppresses PEO oxidation.•Bipolar cell exhibits stable cycle performance without short circuit.