Unveil the impact of high-entropy agent in cathode material for sodium ion batteries

Sodium-ion batteries (SIBs) are now viewed as a prospective energy storage system for grid applications. As one of the representative cathode materials, Ni-Fe-Mn-based ternary layered oxides (NFM) have attracted extensive attention due to the extensive commercialization practices. However, the modification of NFM with multiple dopants, especially using the high-entropy structures, usually results in residual impurity phases, which could lead to an obstacle to the production consistency during commercialization. This can be ascribed to the Gibbs free energy with the correlated formation energy of the system with complex and diverse elements. To address this challenge, herein, a pure phase O3-type Na_0.95Li_0.06Ni_0.2Cu_0.1Fe_0.12Al_0.03Mn_0.39Ti_0.1O₂ (denoted as NFMLCTA) is successfully synthesized via a high-entropy strategy with the assistance of 'high-entropy agent' Al element. Theoretical studies discloses that the existence of Al with certain threshold can effectively promote the formation of a pure phase by reducing in formation energy. The synergistic effect of multiple dopants is also unveiled systematically. Electrochemical performance tests show that NFMLCTA delivers a reversible capacity of 178.02 mAh g^-1 at 0.1C within the voltage range of 2.0-4.2 V. At a high rate of 20C, it maintains a capacity of 72.29 mAh g^-1 with a capacity retention of 80.6% after 500 cycles. Structure evolution study using in-situ X-ray diffraction (XRD) as the implementation discloses that high-entropy doping could markedly restrain the detrimental P3 → O3' phase transition at high-voltages with the formation of OP2 phase at the end of charging, which contributes to the cycling stability. This study offers a novel insight for designing pure phase high-entropy SIBs cathodes and opens an innovative avenue for promoting the advancement of high-performance SIBs.