Consequences of Electrolyte Degradation for the Electrochemical Performance of Li1+x(NiaCobMn1-a-b)1-xO₂

In lithium-ion batteries, the stability of the electrolyte during cycling is among the key parameters that govern cycle life and specific-charge retention. For the case of carbonate-based electrolytes, one of the main reduction products is the alkoxide anion. Here, we report a study of the parameters influencing the formation of these anions during cycling and of the effects that this process has on cycling performance. To this end, conventional and overlithiated lithium-nickel-cobalt-manganese-oxide cathodes were studied in combination with an electrolyte containing ethyl methyl carbonate. The extent of trans-esterification reaction within the electrolyte, serving as an indicator for the alkoxide-anion formation, was estimated based on gas-chromatography results. An analysis of the electrolyte extracted from cells after testing them at different cycling rates revealed a crucial role of anode passivation in the process of alkoxide-anion formation. The upper cutoff potential was found to be inversely proportional to the extent of trans-esterification. Further analysis of the electrolyte extracted from the full-cells containing Li 1.05 (Ni 0.33 Co 0.33 Mn 0.33 ) 0.95 O 2 suggested that electrolyte instability toward the anode can cause a surface layer buildup on the cathode side, which in turn deteriorates the overall cell performance.