Delineating the intricacies of niobium-modified high-nickel layered cathodes with a single-step synthesis

Abstract

High-nickel layered oxides suffer from shortened cycle life due to high surface reactivity with the electrolyte. Modifications with Nb, whether doping or coating, result in improved electrochemical stability. This improvement is often at the expense of initial capacity. This study identifies the origins of this commonly reported decrease in initial capacity and the “activation” region of increasing capacity. Through the identification of the mechanisms behind the initial capacity penalty, a modified cycling schedule is employed that improves both the initial capacity output and stability by compensating for the polarization loss induced by the presence of lithium niobate (Li_xNbO_y) phases with an increase in the cutoff charge voltage. This results in a 30% increase in initial capacity for a 2% Nb-modified sample in full cells with graphite anodes by adjusting the cycling parameters, as well as a 27% longer cycle life when half cells were cycled to 180 mA h g⁻¹ instead of 4.4 V. Electrochemical impedance spectroscopy (EIS) identifies a decrease in cell impedance for Nb-modified samples at higher voltages (>4.4 V vs. Li/Li⁺) compared to those cycled to the standard 4.4 V (vs. Li/Li⁺) cutoff. These findings allow realization of improved electrochemical performance with Nb-modified samples synthesized with single-step calcinations. By elucidating the mechanisms behind why the lithium niobate/cathode interface results in higher impedance at 4.4 V cutoff, we suggest new cycling parameters that can improve the performance of high nickel cathode materials modified with lithium niobate phases.