Elucidating the Electrochemical Behavior of a P3-type High-Na-Content Cathode

Layered oxide materials are often regarded as prospective positive electrodes for Na-ion batteries owing to their superior electrochemical properties and facile synthesis. In this work, a high-Na-content P3-type cathode (NaMn0.6Ni0.3Cu0.1O2; P3-NMNC) was prepared by the sol–gel technique. These materials exhibited excellent rate performance and specific capacity (discharge specific capacity at 3C being 77% of that at 0.1C). Even at 10C, the cells retained ∼45 mAh g–1. The P3-NMNC half-cells were cycled between two voltage ranges, 2.0–4.0 V and 2.0–4.2 V, among which the former exhibited an 83% capacity retention after 200 cycles, which was vastly superior to the latter, where the degradation in the capacity dropped below 80% in just 75 cycles. The dQ/dV vs V plots revealed an irreversible peak above 4.0 V during the first desodiation process, which is attributed to an irreversible anionic redox leading to poor cyclability. Operando synchrotron X-ray diffraction studies revealed a reversible P3 ↔ P3′ ↔ O3 transformation in NMNC during sodiation–desodiation. The repeated P3′ ↔ O3 transformations resulted in strain due to changes in lattice parameters causing capacity degradation. The DNa+ was determined using the galvanostatic intermittent titration technique in the order of 10–12 to 10–10 cm2 s–1. These results underscore the importance of the scarcely explored high-Na-content P3-type layered oxide cathodes toward the advancement of Na-ion batteries.