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

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 (BMImBF 4 ) or 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF 6 )), 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% BMImBF 4 or 40 wt% BMImPF 6 can reach 2.5 × 10 −4 and 4.1 × 10 −5  S cm −1 , respectively. The HBPS-(PMMA- b -PPEGMA) 30 /BMImPF 6 /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/LiFePO 4 batteries based on HBPS-(PMMA- b -PPEGMA) 30 /BMImPF 6 /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. The free-standing composite electrolyte films composed of ionic liquids, hyperbranched star polymers and lithium salts show the high ionic conductivities.