Preparation and electrochemical properties of polymer electrolyte containing lithium difluoro(oxalato)borate or lithium bis(oxalate)borate for Li-ion polymer batteries
By Swiderska-Mocek, Agnieszka; Kubis, Aleksandra
Published in Solid State Ionics
2021
Abstract
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 - MePrPyrNTf2 or 1-ethyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide - EtMeImNTf2), 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 (EtMeImNTf2 or MePrPyrNTf2) + 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|LiFePO4 and Li|Li cells which were tested applying EIS, cyclic voltammetry and the galvanostatic method. The electrochemical formation of SEI protects the Li| 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% EtMeImNTf2 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).