Unique control of bulk reactivity by surface phenomena in a positive electrode of lithium battery

Layered LiNi_1/2Mn_1/2O₂ compound prepared by the classical coprecipitation method delivers a capacity loss upon cycling in a lithium battery, which increases upon the 5 first cycles, and then becomes less visible over the next 40 cycles. The charge–discharge polarization follows the same trend. The formation and the evolution of lithium-containing species on the grain surface of layered LiNi_1/2Mn_1/2O₂ and the interfacial charge transfer resistance have been carefully investigated upon cycling, by coupling ex situ ⁷Li MAS NMR and in situ electrochemical impedance spectroscopy. An important increase in the amount of lithiated surface species is observed during the first electrochemical cycles along with an increase of the charge transfer resistance. After reaching a maximum, both integrated intensity of the NMR signal and charge transfer resistance decrease, indicating a strong correlation between these two different surface characteristics, obtained from ex situ and in situ experiments, respectively. The evolution of surface species, probed by NMR and impedance spectroscopy, follow the same kind of variation as electrochemical parameters, demonstrating a unique control by surface phenomena of the overall electrochemical behavior of an electrode material of lithium battery.