Editors' Choice—Growth of Ambient Induced Surface Impurity Species on Layered Positive Electrode Materials and Impact on Electrochemical Performance

Abstract

Surface impurity species, most notably Li<sub>2</sub>CO<sub>3</sub>, that develop on layered oxide positive electrode materials with atmospheric aging have been reported to be highly detrimental to the subsequent electrochemical performance. LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA) was used as a model layered oxide compound to evaluate the growth and subsequent electrochemical impact of H<sub>2</sub>O, LiHCO<sub>3</sub>, LiOH and Li<sub>2</sub>CO<sub>3</sub>. Methodical high temperature annealing enabled the systematic removal of each impurity specie, thus permitting the determination of each specie's individual effect on the host material's electrochemical performance. Extensive cycling of exposed and annealed materials emphasized the cycle life degradation and capacity loss induced by each impurity, while rate capability measurements correlated the electrode impedance to the impurity species present. Based on these characterization results, this work attempts to clarify decades of ambiguity over the growth mechanisms and the electrochemical impact of the specific surface impurity species formed during powder storage in various environments.