Composite solid electrolyte with improved ionic conductivity and high lithium transference number through reduced PVDF-HFP crystallinity

Solid-state electrolytes (SSEs) with excellent stability and flexibility have been used to overcome the safety and stability issues associated with the liquid electrolyte used in lithium-ion batteries (LIBs). In this study, thin composite solid electrolytes (CSEs) based on poly(vinylidenefluoride-cohexafluoropropylene) (PVDF-HFP) with high mechanical strength were fabricated. Garnet-type Li6.5Ga0.2La2.95Rb0.05Zr2O12 (Ga-Rb LLZO) with high ionic conductivity was added to investigate the improvements in mechanical strength and ionic conductivity. The CSEs with Ga-Rb LLZO added to PVDF-HFP were referred to as PVDF-HFP + LLZO (PHL). Next, the interfacial resistance of the CSEs was reduced by adding a low-molecular-weight poly(ethylene glycol) (PEG) polymer, and the improvement in ionic conductivity and Li transfer was investigated. The CSEs with Ga-Rb LLZO and PEG added to the PVDF-HFP were referred to as PVDF-HFP + LLZO + PEG (PHLP). The ionic conductivity of the fabricated CSEs was 6.58 × 10−10 S cm−1 for the PVDF-HFP sheet, which increased to 1.10 × 10−5 S cm−1 and 2.33 × 10−4 S cm−1 for PHL and PHLP, respectively, at 70 °C. PHLP also had a very high lithium transference number of 0.789, and exhibited a discharge capacity of 123.16 mAh g−1 and a capacity retention rate of 88.29% after 200 cycles at 0.33C and 70 °C.