Synergistic effect of P2, O3 phase on biphasic layered oxide with enhanced electrochemical performance for sodium storage.

Abstract

Oxides with a layered structure are regarded as prospective candidates for use as cathodes in the next generation of sodium ion batteries. These materials, which exhibit a P2 structure and O3 structure, possess distinctive advantages that give rise to disparate electrochemical performance. Herein, a thermodynamically and kinetically stable P2/O3 biphasic layered oxide with a chemical formula of K_0.05Na_0.8Ni_0.5Mn_0.5O₂ is synthesized using a simplistic high temperature solid-state method. P2 phase enhances the structural stability of the material, while O3 phase provides additional sodium storage sites, thereby increasing the material's capacity. The joint action of the two phases results in an improvement in the electrochemical characteristics. Moreover, the Ni-Mn-based layered oxide is enhanced by Fe-substitution, which effectively mitigates the Jahn-Teller effect caused by Mn³⁺, thereby improving the comprehensive electrochemical performance of the cathode. The Fe-doped specimen offers 102.8 mAh g^-1 as the initial reversible discharge capacity under a high current density of 200 mA g^-1, and a high capacity retention amounting to 83.17 % is attained after 200 cycles. In addition, the structural development of the P2/O3 biphasic sample during Na⁺ extraction/insertion was elucidated by in situ XRD. This paper employs the advantages of P2 and O3 phases to augment material electrochemical characteristics and verifies the possibility of the P2/O3 biphasic structure by density functional theory (DFT) calculations, providing new ideas for biphasic sodium ion battery cathode materials.