Voltage Fade in Lmr-NMC Oxides Cycled below the Activation Plateau

A common feature of lithium-excess layered oxides, nominally of composition xLi₂MnO₃•(1-x)LiMO₂ (M = transition metal) is 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 Li₂MnO₃ component, 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 is commonly referred to as voltage fade.

In this talk we show evidence of voltage fade well below the activation plateau.  We argue that this 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 LiMO₂ component into the lithium planes during electrochemical cycling.