Synergistic effect of electrolyte additives on the improvement in interfacial stability between ionic liquid based gel electrolyte and LiFePO4 cathode

Polyethylene (PE)-supported poly (vinylidene fluoride)-hexafluoropropene P(VdF-HFP) based GPE was prepared by using ionic liquid 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) as solvent, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) as lithium salt, vinylene carbonate (VC) and lithium bis(oxalato)borate (LiBOB) as CEI (solid electrolyte interface formed between cathode and electrolyte) film-forming additive. The performance of ionic liquid type GPE with VC and LiBOB additives was understood by electro-chemical impedance spectroscopy, linear sweep voltammetry, charge/discharge test, scanning electron microscope, transmission electron microscope, X-ray diffraction and inductively coupled plasma. It was found that the ionic liquid based GPE with 5 wt% VC and 1.5 wt% LiBOB as additives helped to improve the interfacial stability by forming superior quality of CEI layer on the surface of LiFePO4 cathode. Thinner and more uniform CEI layer built by the synergistic effect of additives, which VC contributed to a soft organic layer, while LiBOB facilitated the formation of a brittle and tough inorganic layer, could inhibit the further dissolution of Fe ion and keep the structural integrity of LiFePO4. Caused by the preferably comprehensive performance of ionic liquid based GPE, in which exhibited good compatibility with lithium anode, and the improved oxidative potential of 5.4 V, the Li/GPE/LiFePO4 coin cell showed excellent cyclic stability, maintaining 96.7% of its initial discharge capacity after 100 cycles at 0.1C rate, compared to that of 72.3% for the GPE without additive.