An electrochemical evaluation of state-of-the-art non-flammable liquid electrolytes for high-voltage lithium-ion batteries

The rapid and accelerating adoption of lithium-ion batteries worldwide, especially in the transportation sector, has focused attention on their safety. One area of particular interest is finding alternatives for their most flammable component, the liquid electrolyte. Over the past 20 years, a number of non-flammable liquid electrolytes have been identified and tested. However, because these data are frequently obtained under a wide range of conditions – e.g., different active materials, current densities or voltage cutoffs – it is difficult to compare them. In this work, eight promising non-flammable liquid electrolytes – four phosphate derivatives and four based on fluorinated hydrocarbons – are identified from the literature and tested in commercially relevant highvoltage systems under identical conditions. The electrochemical stabilities of the electrolytes were studied against both inert electrodes and in LiNi0.6Mn0.2Co0.2O2|graphite cells. Each electrolyte was assessed via longterm cycling experiments and rate-testing and the cell resistance during aging was monitored. It was found that the electrolytes containing phosphate and phosphonate-based solvents generally performed very poorly compared to the phosphorus-free fluorinated solvents; the latter resulted, on average, in twice the capacity retention after 500 cycles of the former. A strong correlation was observed between long-term cycling performance, rate capability and the cell resistance.