Unveiling the potential of P3 phase in enhancing the electrochemical performance of a layered oxide cathode

Abstract

Controlling the phase structure of layered oxide cathodes is considered an effective strategy to realize high-performance Na-ion batteries. Herein, we unveil the synthesis and electrochemical behavior of a hitherto unreported monophasic P3 structure in Na0.75Mn0.75Ni0.25O2 (NNM) calcined at 750 °C. The conversion of P3 to a P2-type single phase (at 850 °C) upon increasing the calcination temperature, with P2/P3 biphasic structures at intermediate temperatures, was confirmed by the Rietveld refinement of the x-ray diffraction (XRD) data. The monophasic P3 cathode showed a specific capacity of 190 mAh/g at 0.1 C and retained 90% of its initial capacity after 200 cycles at 1 C in the 1.5–4.2 V range. The galvanostatic charge-discharge (GCD) curves also showed a decrease in specific capacity with an increase in the P2 phase. The P3-dominant biphasic cathodes displayed superior rate performance, exhibiting capacities up to 97 mAh/g at 6 C, attributed to their smaller particle size and optimal P2/P3 phase fraction. Operando Synchrotron XRD data confirmed the suppression of P3→O′3 + O3 transformation in the biphasic cathode that was present in the monophasic P3 structure during charge-discharge cycling. The superior electrochemical performance and excellent structural stability of biphasic cathodes make them ideal for stationary and mobile storage applications. © 2023 Elsevier Ltd