Layered transition metal oxides with regulated phase transition and low interlayer spacing variation for sodium-ion batteries

Abstract

Ni/Mn-based layered transition metal oxides (LTMOs) without the involvement of expensive Co are regarded as promising cathode materials for sodium-ion batteries. Severe phase transitions at high charge voltages would result in significant variation in interlayer spacing, deteriorating cycling performance. Designing LTMOs with low interlayer spacing variation through the regulated phase transition remains a significant challenge. Herein, Na_0.67Ni_0.20Zn_0.05Mn_0.55Ti_0.20O₂ (NZMT) materials are synthesized through the co-substitution of Zn and Ti at the transition metal sites, Ni and Mn, respectively, of Na_0.67Ni_0.25Mn_0.75O₂ (NM). In situ X-ray diffraction analyses confirm that the resultant NZMT undergoes the regulated P2-OP4 phase transition, rather than the conventional P2-O2 phase transition or the multi-step phase transformation observed in NM. The interlayer spacing is marginally decreased from 5.6 Å in the P2 phase to 5.4 Å in the OP4 phase for NZMT. A discharge capacity of 99.9 mAh g⁻¹ is delivered by NZMT at 1 C, with a capacity retention of 79.4 % after 200 cycles, significantly higher than those of NM (78.2 mAh g⁻¹, 29.3 % retention). A high energy density of 274.3 Wh kg⁻¹ is achieved for NZMT-based full cell at a power density of 78.3 W kg⁻¹. This work paves a new avenue for the development of LTMOs.