Fabrication of all solid-state lithium polymer secondary batteries using PEG-borate/aluminate ester as plasticizer for polymer electrolyte

All solid-state lithium polymer secondary battery (LPB) was fabricated by using lithium conducting poly(ethylene oxide) film as solid-state polymer electrolyte (SPE). To enhance the ionic conductivity of lithium, PEG-borate ester or PEG-aluminate ester was added to the SPE as plasticizer. The fabricated LPBs with LiFePO4 cathodes were cycled at 60 °C with and without plasticizer. At lower current density (0.1 C) condition, the cell showed no significant change or slightly poor electrochemical performance despite the addition of plasticizer. On the other hand, increased capacity retention and reducing the polarization were observed by addition of PEG-borate/aluminate ester at higher current density (0.5 C) or reduced temperature condition (40 °C). The impedance analyses suggested two factors affecting the electrochemical performance for LPB, interfacial resistance of Li|SPE and cathode|SPE. Former and latter ones would relate to the initial capacity and to the cycle performance and irreversibility of LPB, respectively. Adding PEG-borate/aluminate ester would decrease the interfacial resistance of Li|SPE interface. However, it also caused undesirable side reaction at the cathode|SPE interface.