<p>Sodium-ion batteries have garnered extensive attention as potential alternatives to lithium-ion batteries due to their advantages of abundant sodium resources and low production costs. However, the larger ionic radius of sodium restricts the charge storage capacity of electrode materials and hinders ion transport efficiency, resulting in inferior energy density and power characteristics that severely impede practical applications. This study investigates the influence mechanism of sodium content adjustment on the energy storage properties of layered oxides and employs MgO surface coating to enhance structural stability while suppressing interfacial side reactions. A series of Na_xMn_0.7Ni_0.2Co_0.1O₂ (x = 0.65, 0.70, 0.75, denoted as Na_xMNCO) samples were synthesized via a sol-gel method. Experimental results demonstrate optimal electrochemical performance at x = 0.70: the material exhibits a specific capacity of 156.75 mAh·g^− 1 during initial charge-discharge cycles within 2.0–4.2&#xa0;V voltage window at 0.1&#xa0;C rate (1&#xa0;C = 200 mA·g^− 1). After 200 charge-discharge cycles, 84.6% of the initial capacity remains, indicating excellent cyclic stability. Building on these findings, liquid-phase chemical deposition was used to create adjustable MgO protective layers on the positive electrode material. Optimal electrochemical performance was achieved with 3 wt% MgO coating. The modified sample delivered an initial discharge specific capacity of 159.9 mAh·g^− 1 and maintained 89.4% capacity retention after 200 cycles, demonstrating significantly improved cyclic stability compared to unmodified materials. This systematic investigation of sodium content optimization and MgO coating engineering in layered Ni/Mn-based sodium-ion battery cathode materials may represent an important step toward the commercial viability of sodium-ion battery technology.</p>