LiF and LiNO₃ as synergistic additives for PEO-PVDF/LLZTO-based composite electrolyte towards high-voltage lithium batteries with dual-interfaces stability

Solid electrolytes with desirable properties such as high ionic conductivity, wide electrochemical stable window, and suitable mechanical strength, and stable electrode–electrolyte interfaces on both cathode and anode side are essential for high-voltage all-solid-state lithium batteries (ASSLBs) to achieve excellent cycle stability. In this work, a novel strategy of using LiF and LiNO₃ as synergistic additives to boost the performance of PEO-PVDF/LLZTO-based composite solid electrolytes (CSEs) is developed, which also promotes the assembled high-voltage ASSLBs with dual-interfaces stability characteristic. Specifically, LiF as an inactive additive can increase the electrochemical stability of the CSE under high cut-off voltage, and improve the high-voltage compatibility between cathode and CSE, thus leading to a stable cathode/CSE interface. LiNO₃ as an active additive can lead to an enhanced ionic conductivity of CSE due to the increased free-mobile Li⁺ and ensure a stable CSE/Li interface by forming stable solid electrolyte interphase (SEI) on Li anode surface. Benefiting from the improved performance of CSE and stable dual-interfaces, the assembled NCM622/9[PEO₁₅-LiTFSI]-PVDF-15LLZTO-2LiF-3LiNO₃/Li cell delivers a high rate capacity of 102.1 mAh g⁻¹ at 1.0 C and a high capacity retention of 77.4% after 200 cycles at 0.5 C, which are much higher than those of the ASSLB assembled with additive-free CSE, with only 60.0 mAh g⁻¹ and 52.0%, respectively. Furthermore, novel cycle test modes of resting for 5 h at different charge states after every 5 cycles are designed to investigate the high-voltage compatibility between cathode and CSE, and the results suggest that LiF additive can actually improve the high-voltage compatibility of cathode and CSE. All the obtained results confirm that the strategy of using synergistic additives in CSE is an effective way to achieve high-voltage ASSLBs with dual-interfaces stability.