Lithium- and Manganese-Rich Nickel-Manganese-Cobalt Oxides (LMR-NMC), nominally of composition xLi₂MnO₃•(1-x)LiMO₂ (with M a combination of Ni, Mn, and/or Co), present a high-voltage plateau (~4.5 V vs. Li/Li⁺) in their capacity-voltage profile during the first delithiation cycle.  This plateau is believed to result from activation of the Li₂MnO₃component, which makes additional lithium available for electrochemical cycling.  Once activated (i.e., cycled beyond the activation plateau) the voltage v. capacity characteristic of LMR-NMC oxides is known to decrease continuously with cycling, resulting in a progressive loss of energy density.  This phenomenon, commonly referred to as voltage fade, results in a gradual loss of energy density with cycling and constitutes one of the main barriers toward broad application of LMR-NMC as cathode materials for automotive applications.

In this work we show evidence of voltage fade well below the activation plateau and its relation to structural changes during LMR-NMC cycling.  We argue that voltage fade results from a gradual accumulation of spinel environments in the crystal structure.  Some of these spinel sites result from lithium deficiencies during oxide synthesis and are likely to be at the particle surfaces; other sites result from the migration of transition metal atoms in the partially-delithiated LiMO₂component into the lithium planes during electrochemical cycling.  We also present evidence that these structural changes are related to the oxygen-to-metal ratio and could be driving oxygen evolution from LMR-NMC materials which, as a potential trigger of thermal runaway, is a process of fundamental concern for safety. 

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