Sandwich-Structured Composite Polymer Electrolyte Based on PVDF-HFP/PPC/Al-Doped LLZO for High-Voltage Solid-State Lithium Batteries

High-performance solid-state lithium-metal batteries (SSLMBs) require solid electrolytes displaying outstanding electrochemical stability, excellent ionic conductivity, and high Li+ ion transference number. On top of these, it should also be compatible with the electrodes applied and functionable under room temperature. To achieve these, a solution-casting technique is proposed herein to prepare a flexible composite polymer electrolyte (CPE), which is equipped with a high ionic conductivity and Li+ ion transference number, concurrently applicable in the construction of high-voltage solid-state Li batteries. The proposed CPE, which is made up of poly(vinylidene difluoride-co-hexafluoropropylene) (PVDF-HFP)/polypropylene carbonate (PPC) blend with an Al-doped Li7La3Zr2O12 (Al-LLZO) filler, was sandwiched between PVDF-HFP/PPC–lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) skin layers with SN plasticizer added. This formulation of PVDF-HFP/PPC/Al-LLZO/LiTFSI/SN was abbreviated as sandwich-PPA in our study. Such configuration permits notable resistance reduction at the electrode–electrolyte interface while suppressing Li dendrite growth throughout the robust charging–discharging process. This can be attributed to the excellent performance of the sandwich composite electrolyte membrane, which promises high ionic conductivity (ca. 4.04 × 10–4 S cm–1) and a high Li+ ion transference number (ca. 0.583) at room temperature. A CR2032 coin cell, which is assembled with Al2O3-C@NCA/Sandwich-PPA/Li, delivered a high specific capacity (186.20 mAh g–1 at 0.1C at room temperature), along with its excellent rate performance and cycle stability (discharge capacity of 126.23 mAh g–1; capacity retention of 80.03% after 100 cycles at a rate of 0.5C at room temperature). This verified the potential of our novelty-formulated solid-state electrolyte to secure excellent performance of SSLMBs.