To simultaneously address the inherent limitations of the capacity and cycling instability of Na[Ni0.5Mn0.5]O2 cathodes, it is necessary to concurrently stabilize particle surfaces and crystal structure. Encouraged by previously conducted research activities in this field, we have adopted an effective strategy for simultaneous MgO coating and Mg doping to produce substantially improved high voltage stability compared to the previously reported results in the literature on Na[Ni0.5Mn0.5]O2 cathodes. The MgO coating layer effectively suppressed the unfavorable side reactions during cycling while the partial Mg doping into the bulk Ni sites improved the structural stability by moderating the extent of the irreversible multiphase transformation. As a result, the combination of a MgO coating with Mg doping provides enhanced electrochemical performance and structural stability of Na[Ni0.5Mn0.5]O2 within the voltage range of 2.0–4.2 V. The practical acceptability of the simultaneous MgO coating and Mg doping of the Na[Ni0.5Mn0.5]O2 cathode was obviously verified using scaled-up pouch-type full cells with hard carbon anodes. Compared with previously reported similar cathode materials, the proposed MgO-NM55 cathode showed great competitiveness for high-voltage stability in SIBs. Moreover, the use of earth's abundant and inexpensive Mg and Na elements, and a simple practical strategy are highly desirable for developing high-energy and low-cost SIBs. Although the practical use of the MgO-NM55 cathode in SIBs will require further work, the methodology used in this study will be helpful in developing an efficient design of high performance cathode materials for SIBs.

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