Solid-Solution Phase Transition Induced by Surface Electrochemico-Mechanical Interactions for High-Voltage Sodium-Layered Oxide Cathodes.

Abstract

The phase transition behaviors of sodium-layered oxide (SLO) cathodes play an important role in their electrochemical performances at high voltages. Specifically, SLOs experience a phase transition from the P to OP intergrowth phase with Na-deficient O layers, leading to sluggish Na extraction/insertion kinetics, severe strain formation, and high reactive activity with electrolytes. In addition to the normally used phase engineering strategies such as bulk doping, we demonstrate here that a Mn-gradient surface layer can significantly tune the bulk phase transition from the OP intergrowth to O3 solid-solution transition, evidenced by in situ XRD and Cryo-STEM analyses. The Mn-rich surface has asynchronous Na extraction properties compared to the bulk at high voltages, suppressing the nucleation and growth of the OP intergrowth phase as the generated stress cannot be well released while facilitating the formation of a stressless O3 solid-solution phase. Benefiting from the O3 solid-solution phase change behaviors, the SLO with the Mn-rich surface shows much improved electrochemical performances at high voltages up to 4.3 V in terms of energy density, rate performance, energy conversion efficiency, and cycling stability.