Accelerated Failure in Li[Ni0.5Mn0.3Co0.2]O2/Graphite Pouch Cells Due to Low LiPF6Concentration and Extended Time at High Voltage

Li[Ni0.5Mn0.3Co0.2]O-2/graphite pouch cells were cycled using protocols that included 24 h spent at high voltage (>= 4.3 V) under constant voltage or open circuit conditions to accelerate failure. Compared to traditional cycling, failure was reached up to 3.5 times faster. When this protocol was applied to cells containing low LiPF(6)concentrations (<= 0.4 M) failure was achieved up to 17.5 times faster than traditional cycling with normal LiPF(6)concentrations. This represents a time improvement on the order of years and therefore can be used as a high-throughput screening method. Failure mechanisms for cells containing a range of LiPF(6)concentrations undergoing these aggressive protocols were investigated using charge-discharge cycling, impedance spectroscopy (including symmetric cell analysis) and isothermal microcalorimetry. Long times at high voltage rapidly increase positive electrode impedance but do not seem to consume lithium inventory. The use of lower LiPF(6)concentrations does not seem to introduce new failure mechanisms but makes cells less tolerant to positive electrode impedance growth. The utility of this method is demonstrated by screening cells with a variety of electrolyte additive combinations. Fewer than 3 months were required to distinguish cells containing 1% lithium difluorophospate as superior to cells with other additive combinations.