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Gel polymer electrolytes (GPEs) have attracted ever-increasing attention in Li-ion batteries, due to their great thermal stability and excellent electrochemical performance. Here, a flexible poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-based GPE doped with an appropriate proportion of the PEO and SiO2 is developed through a universal immersion precipitation method. This porous PVDF-HFP-PEO-SiO2 GPE with high ionic conductivity and lithium-ion transference number (tLi+) can enhance the electrochemical performance of LiFePO4 cells, leading to superior rate capability and excellent cycling stability. Moreover, the PVDF-HFP-PEO-SiO2 GPE effectively inhibits the lithium dendrite growth, thereby improving the safety of Li-ion batteries. In view of the simplicity in using the gel polymer electrolyte, it is believed that this novel GPE can be used as a potential candidate for high-performance Li-ion batteries.

The cyclability of lithium-ion batteries (LIBs) is often affected by the components of the solid electrolyte interphase (SEI) layer which is generated from electrochemical decomposition of electrolyte. Here, lithium difluorophosphate (LiPO2F2) is studied in this work. When 1.6 wt% LiPO2F2 additive is incorporated into the reference electrolyte, the capacity retention of graphite/Li half-cell is increased from 82.53% to 98.04% and the capacity retention of LiCoO2/Li half-cell is increased from 89.60% to 97.53% after 160 cycles. Electrochemical impedance spectroscopy (EIS) indicates that the SEI layer containing LiPO2F2 can decrease the surface impedance of cells in the last stage cycle. In situ atomic force microscopy (AFM), DFT calculations and X-ray photoelectron spectroscopy (XPS) results show that LiPO2F2 is deposited on the surface of both LiCoO2 and graphite electrodes, which effectively protects the graphite anode and suppresses the degradation of the cathode during the long-term cycling of LIBs.

Electrochemical prelithiation is a facile, effective and extensively used method to improve the initial coulombic efficiency of SiO. However, much less research attention has been devoted to prelithiation effect on initial several cycles. Here, we introduce a new perspective to evaluate the prelithiation behaviors, which could understand in depth the electrochemical prelithiation behaviors and their effects on the following two cycles. Then X-ray photoelectron spectroscopy was further performed to pinpoint the reaction products. It has been found that the quantity of irreversible Li4SiO4, Li2O and SEI (Li2CO3 and LiF) and reversible LixSi are increasing as the prelithiation time extending. When the prelithiation time extending over 20 min, only alloy reaction of Si has been revealed. The regeneration of SEI discloses at least 7% capacity loss in the first cycle which depends on the prelithiated time. However, the reformation of SEI in the second cycles reveals 3% capacity loss. Because the coulombic efficiencies are independent on prelithiated time in the second cycle which indicates only one discharge/charge cycle is enough to form integrated SEI and adequate irreversible Li4SiO4 and Li2O except part of unreactive SiOx.