Enhancing the Cycling and Rate Performance of NaNi_1/3Fe_1/3Mn_1/3O₂ Cathodes by La/Al Codoping

O3-type layered oxides hold significant promise as the material for cathodes in sodium-ion batteries for their favorable electrochemical properties, while irreversible structural degradation and harmful phase transitions during cyclic operation limit the practical application of these materials. In this work, we proposed a La³⁺/Al³⁺ codoping strategy in O3-Na(Ni_1/3Mn_1/3Fe_1/3)O₂ cathode materials and found that batteries with the Na (Ni_1/3Mn_1/3Fe_1/3)_0.998La_0.001Al_0.001O₂ (NFM-La/Al) cathodes exhibited not only promoted capacity from 135.80 to 170.42 mAh g^-1 at 0.2 C but also significantly enhanced cycling stability, with a 10% improvement in capacity retention compared with NFM cathodes after 300 cycles. Particularly, their rate performance was significantly improved as well. XRD and XPS tests indicated that La could expand the <i>c</i>-axis of NFM due to its larger ionic radius and thus significantly increased Na⁺ ion diffusion efficiency, and in addition, Al doping could effectively increase the content of Ni²⁺ and Mn⁴⁺ and thus greatly alleviated the negative Jahn-Teller effect caused by Mn³⁺. Moreover, consistent with XRD analyses, DFT calculations further substantiated the effectiveness of the La/Al codoping strategy by demonstrating the detailed atom substitution mechanism in the NFM crystal lattice. The boosted structure stability and Na⁺ diffusion kinetics may enhance the potential for practical applications of O3-type oxide cathodes.