Insights of potassium hexafluorophosphate additive in solid polymer electrolyte for realizing high performance all-solid-state lithium metal batteries

High energy density all-solid-state lithium metal batteries (ASSLMBs) with flexible, low-cost solid polymer electrolytes (SPEs) have received extensive research attention, but the low ionic conductivity of SPEs and the rapid growth of lithium dendrite seriously limit their performance. Here, polyethylene oxide (PEO) based solid polymer electrolytes (SPEs) containing potassium hexafluorophosphate (KPF6) salt additive are developed for lithium dendrite-free ASSLMBs. The results show that with the additive of KPF6, the dissociation of lithium salt is promoted, providing more free Li+ ions in SPEs and enhancing the ionic conductivity. KPF6 additive also makes SPE more flexible and promotes the formation of Li2S and LiF-rich solid electrolyte interphase (SEI). Besides, the presence of K+ is also expected to work as a shield to inhibit the lithium dendrite growth. With the synergistic effect of above advantages, lithium dendrite growth and interfacial side-reactions in ASSLMBs are prevented. Li/Li symmetric cell with SPE can achieve a stable cycling performance over 1500 h with low polarization potential. The all-solid-state LiFePO4/Li cell releases an initial capacity of 142.1 mAh g−1 with a capacity retention of 91.3% after 200 cycles at 0.5 C and 60 °C. This study provides a facile scheme for designing high performance ASSLMBs.