High-entropy P2/O3 biphasic cathode materials for wide-temperature rechargeable sodium-ion batteries

Layered sodium manganese-based oxides are highly attractive cathode materials for sodium-ion batteries but suffer from limited initial coulombic efficiency (ICE) and poor structural stability. Herein, a high-entropy biphasic Na0.7Mn0.4Ni0.3Cu0.1Fe0.1Ti0.1O1.95F0.1 cathode material is reported to exhibit remarkable ICE, rate capability and cyclability. In-situ structural analysis during the preparation of cathode reveals tunable P2/O3 ratios by changing the sintering temperature. The synthesized high-entropy oxide with a P2/O3 ratio of 23:77 (wt%) delivers a high ICE of 97.6%, a considerable discharge capacity of 86.7 mAh g−1 at current density of 800 mA g−1, and respectable capacity retention in a wide temperature range from -40 to 50 °C. Additionally, full cell coupling Na0.7Mn0.4Ni0.3Cu0.1Fe0.1Ti0.1O1.95F0.1 and hard carbon exhibits an energy density of 268.3 Wh kg−1 at power density of 1172 W Kg−1 based on the mass of cathode. Combined experimental and computational investigations suggest that the as-prepared Na0.7Mn0.4Ni0.3Cu0.1Fe0.1Ti0.1O1.95F0.1 cathode favors reversible structural evolution, fast Na+ diffusion kinetics, and low energy barriers due to the unique P2/O3 biphasic structure and high-entropy effect. This study brings an in-depth insight into the design and preparation of high-entropy P2/O3 biphasic cathode to build advanced sodium-ion batteries.