Preparation and electrochemical properties of polymer electrolyte containing lithium difluoro(oxalato)borate or lithium bis(oxalate)borate for Li-ion polymer batteries

摘要

A lithium difluoro(oxalato)borate (LiODFB) and lithium bis(oxalate)borate (LiBOB) salts in combination with an ionic liquid (N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulphonyl)imide - MePrPyrNTf(2) or 1-ethyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide - EtMeImNTf(2)), sulfolane (TMS) and poly(vinylidene fluoride) (PVdF) were tested as polymer electrolytes (PEs) for Li-ion polymer batteries. Quaternary lithium salt (LiBOB or LiODFB) + ionic liquid (EtMeImNTf(2) or MePrPyrNTf(2)) + PVdF + TMS polymer electrolytes were prepared by the casting technique. All the membranes are free-standing, flexible and transparent. Obtained PE films were investigated by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and the flammability test. Conductivity of the polymer electrolytes ranged from 0.52 to 3.21 mS cm(-1) with the activation energy of 34.24 and 19.58 kJ mol(-1), respectively. Decomposition of PE does not result in flammable products. The presence of large pores in these membranes ensures better lithium ion transport processes. The polymer electrolytes were used as electrolytes in Li vertical bar LiFePO4 and Li broken vertical bar Li cells which were tested applying EIS, cyclic voltammetry and the galvanostatic method. The electrochemical formation of SEI protects the Li vertical bar polymer electrolyte system against its aging (the total impedance evolution with time is much smaller). The LiFePO4 cathode with the membrane (24.7 wt% PVdF, 2.3 wt% LiODFB, 51.1 wt% EtMeImNTf(2) and 21.9 wt% TMS) exhibited a good reversible capacity of 138 mAh g(-1) and 120 mAh g(-1) at high current densities (the C/2 and 1C rates).