In the process of upgrading energy storage structures, sodium‐ion batteries (SIBs) are regarded as the most promising candidates for large‐scale grid storage systems. However, the difficulty in further improving their specific capacity and lifespan has become a major obstacle to promoting extensive application. Herein, by optimizing synthesis conditions, a biphasic‐Na2/3Ni1/3Mn2/3O2 cathode that exhibits an ultrahigh capacity of ≈200 mAh g‐1 without the involvement of anion redox reactions is successfully synthesized. Nevertheless, there is significant electrochemical performance degradation because of failure at the cathode‐electrolyte interface as revealed by comprehensive analyses. Further in‐depth research proves that the surface side reactions that occur at high operating voltages and the transition metal dissolution that occurs in low voltage are the root causes of electrode surface failure. Therefore, the metal oxide atomic layer deposition (ALD) protective layer is deliberately chosen to suppress such failures. The coating effectively blocks corrosion of the cathode material by the electrolyte and successfully anchors the transition metal ions on the particle surface. As a result, the cycle stability and rate performance of the electrode are improved considerably. This surface engineering strategy could provide concepts with broad applicability for suppressing the failure of sodium layered cathodes.

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