Unexpected Voltage Fade in LMR-NMC Oxides Cycled below the “Activation” Plateau

Abstract

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. However, oxides cycled beyond this activation plateau are known to display voltage fade which is a continuous reduction in their equilibrium potential. In this article we show that these oxides display gradual voltage fade even on electrochemical cycling in voltage ranges well below the activation plateau. The average fade is ∼0.08 mV-cycle−1 for Li1.2Ni0.15Mn0.55Co0.1O2 vs. Li cells after 20 cycles in the 2–4.1 V range at 55°C; a ∼54 mV voltage hysteresis, expressed as the difference in average cell voltage between charge and discharge cycles, is also observed. The voltage fade results from a gradual accumulation of local 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. The observed rate of voltage fade depends on a combination of factors that includes the phase equilibrium between the layered and spinel components and the kinetics of transition metal migration.