Multiple Influences of Nickel Concentration Gradient Structure and Yttrium Element Substitution on the Structural and Electrochemical Performances of the NaNi0.25Mn0.25Fe0.5O2 Cathode Material

The research and development of sodium-ion battery (SIB) cathode materials are crucial for the commercialization of SIBs as important energy storage and conversion devices. This work aims to provide new insights into the electrochemical behavior of the NaNi0.25Mn0.25Fe0.5O2 (NMF) cathode material, especially improving its cycle life and rate capability. Herein, a novel nickel concentration gradient NMF material (NCG-NMF) was designed to utilize the characteristic reaction of dimethylglyoxime with nickel. This approach not only demonstrates the role of high nickel content in the core for the high discharge capacity of materials but also observes excellent cycling stability with the manganese-rich outer layer. To further optimize the cycle stability and rate performance, we select the Y element with a strong Y–O binding energy to replace the transition metal in the NCG-NMF material. The Y dopants can promote the Na-ion transport and inhibit the phase transformation of materials from layered structure to spinel or to the rock-salt phase. The Y-doped NCG-NMF material exhibits excellent capacity retention of 71% after 100 cycles at the current density of 125 mA g–1 under a cutoff voltage of 4.2 V, which is much higher than that of the NMF cathode material (42%). These findings provide a new concept for designing and developing high-performance SIB cathode materials.