Electrochemical performances of a new solid composite polymer electrolyte based on hyperbranched star polymer and ionic liquid for lithium-ion batteries

Abstract

A new composite polymer electrolyte membrane composed of hyperbranched star polymers (HBPS-(PMMA-b-PPEGMA)30 (the hyperbranched star polymer with hyperbranched polystyrene as core and polymethyl methacrylate block poly(ethylene glycol) methyl ether methacrylate) as arms)), ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) or 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6)), and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) has been fabricated by solution casting method. This type of solid polymer electrolyte membrane shows an excellent flexibility, transparency, and free-standing properties. Room temperature ionic conductivity of composite electrolytes with 40 wt% BMImBF4 or 40 wt% BMImPF6 can reach 2.5 × 10−4 and 4.1 × 10−5 S cm−1, respectively. The HBPS-(PMMA-b-PPEGMA)30/BMImPF6/LiTFSI (the composite polymer electrolyte with 1-butyl-3-methylimidazolium hexafluorophosphate) composite polymer electrolyte displays better performances including a good thermal stability with decomposition temperature of 350 °C, a wide electrochemical window with oxidation potential of 4.3 V, and the excellent interfacial compatibility with lithium electrode. Moreover, the Li/LiFePO4 batteries based on HBPS-(PMMA-b-PPEGMA)30/BMImPF6/LiTFSI electrolyte retain about 96% of their highest discharge capacity (120.5 mAh g−1) after 100 cycles under a current density of 0.1 C at 60 °C, exhibiting the excellent reversible cyclability.