Enhanced Performance by Zn-Substitution in Biphasic P2/P3-Na_0.75Mn_0.68Ni_0.25Zn_0.07O₂

Zn-substituted Na_0.75Mn_0.68Ni_0.25Zn_0.07O₂ has been synthesized in P3, P2, and composite P2/P3 structures and compared with unsubstituted Na_0.7Mn_0.75Ni_0.25O₂ analogues as positive electrodes for sodium-ion batteries. The synthesis temperature was shown to provide a means of controlling the phase ratio of P2 and P3 phases. Powder diffraction measurements, high-resolution transmission electron microscopy (TEM), and selected area electron diffraction (SAED) revealed that Zn substitution enhanced the ordering of the transition metal (TM) layers. Electrochemical studies combined with XAS measurements showed that after Zn substitution, Ni activity was enhanced, while the irreversible activity of O and Mn was suppressed. Structural transformations were suppressed, and the reversibility of Zn-substituted samples on cycling was improved. Among the Zn-substituted samples, Zn-P2/P3 delivers the best electrochemical performance with an initial capacity of 121 mAh g^-1 at a rate of 25 mA g^-1 and 90% capacity retention after 100 cycles in half-cells. This work reveals the intrinsic correlation among cation doping, synthesis conditions, and crystal phase compositions but also provides a reliable strategy for designing high-stability composite layered cathode materials for sodium-ion batteries.