Dynamic inconsistency between electrochemical reaction and phase transition in Na-deficient layered cathode materials

Electrochemical characteristics, as revealed by the charge-discharge voltage profiles, are commonly used to identify phase transitions in electrode materials during electrochemical cycling. However, such identifications are only valid under the condition of fast kinetics of phase transitions. Herein, layered Na_2/3Ni_0.25Mn_0.75O₂ with various structures, including a P3-type phase (C2/m), a P2-type structure (P6₃), or a mixture of P2- and P3-type phases, were synthesized through precisely controlling the heating temperature. The structural evolution and charge compensation mechanism of these cathode materials were systematically investigated upon cycling using in situ synchrotron-based characterization techniques. Very complicated phase transitions occur when these compounds are cycled between 1.5 and 4.5 V. Upon careful examination of the experimental results, it becomes evident that the phase transformations in P-type layered cathode materials exhibit significant hysteresis during high-voltage cycling, as observed at the end of charge (4.5 V) at 0.1 C, but remain undetected at a slower current density of 0.05 C, indicative of the sluggish kinetics of these transformations. The experimental results unambiguously confirm that while the shape of the voltage curves of the cathodes is consistent with the cationic-anionic redox processes, it does not match the phase transitions. These findings provide a more fundamental understanding of the hysteresis of the phase transitions in P-type layered cathode materials during high-voltage cycling.