Ion-Doping-Site-Variation-Induced Composite Cathode Adjustment: A Case Study of Layer-Tunnel Na_0.6MnO₂ with Mg²⁺ Doping at Na/Mn Site

Composite cathodes have attracted great attention due to the integrated advantages of each pure structure. Also, the component ratio deserves a careful modulation to further improve the corresponding electrochemical performance. Mn-based layer-tunnel hybrid composite became a focus in sodium-ion batteries due to the superiority in terms of high performance, low cost, and nontoxicity. In the previous reports, the structure modulation was carried out via changing the synthesis condition, varying the transition-metal-element ratio, and different ion doping. Also, it is still challenging to explore a more feasible method to simplify the adjustment process. Herein, we introduced Mg²⁺ into Na sites or transition-metal sites in Na_0.6MnO₂ and first discovered the doping-site-variation-induced structural adjustment phenomenon. Specifically, Mg doping in transition-metal sites could be beneficial for the growth of the P2-type structure, while layer/tunnel component ratio decreased when locating Mg²⁺ in Na sites. The P2-O2 phase transformations could be effectively suppressed by locating Mg²⁺ in both sites in high-voltage regions and thus improve the cycling performance. The designed material, Na_0.6Mn_0.99Mg_0.01O₂, could attain a decent capacity of 100 mA h g^-1 at 1000 mA g^-1 and a satisfied retention of 76.6% after 500 cycles. Additionally, ex situ X-ray diffraction analysis experiments verify the excellent structural stability of Mg-substituted samples during charge-discharge processes. Moreover, the Na_0.6Mn_0.99Mg_0.01O₂ also displays superior sodium-ion full-cell properties when merged with hard carbon anode. Thus, this research may indicate a proper novel thread for designing high-performance composite electrodes.