Improved Cycling Performance and Safety Characteristics of NMC811/G-Si Battery Cells with an Optimised Electrolyte Formulation

Abstract

An increase in the practical energy density of anodes can be achieved when using 10%–25% silicon in graphite composite anodes. This is sufficient as the practical capacity of lithium-ion batteries (LIBs) is limited by the cathode capacity and applicable current densities (<4 mA cm−2). Using an NMC811 cathode and a graphite–15% silicon (G-Si15%) composite anode, the performance of more than 20 electrolyte formulations have been evaluated. Of these formulations, EC/EMC+1.2 M LiPF6 containing FEC/VC or LiFSI/VC combinations were shown to have superior performance over the parent electrolyte. Safer to operate ionic liquid-based electrolytes (ILELs) were found to be compatible with these electrodes at moderate cycling rates, with discharge capacities 15% lower compared to the conventional electrolytes. DSC data for both electrodes, cycled and uncycled, in contact with three different electrolytes were recorded. For the cathode in contact with the ILEL, temperatures of exothermic events were higher compared to those of carbonate-based electrolyte counterparts, whereas they were significantly lower for the G-Si15% anodes. In both cases when in contact with ILEL, the heat generated is considerably higher not only compared to samples containing carbonate-based electrolytes but also with respect to data reported for common graphite electrodes with the same ILEL.