O3-type layered oxides are among the most promising cathode candidates for sodium-ion batteries, yet their practical use is constrained by irreversible oxygen redox at high voltages, parasitic side reactions, and pronounced moisture sensitivity. Here, we propose a one-step H₃BO₃ treatment that introduces an in-situ boride complex, enabling near-surface B doping and the formation of a conformal sodium borate interphase. The dual modification operates synergistically: B incorporation reinforces the layered framework and suppresses oxygen redox above 4.0 V, while the sodium borate layer, endowed with high Na⁺ conductivity, functions as a robust interfacial barrier. These effects collectively suppress transition-metal dissolution, mitigate electrolyte decomposition, and promote rapid Na⁺ transport. Benefiting from this design, the optimized cathode delivers 160.5 mAh g⁻¹ at 0.1 C and retains 85.3 % capacity after 200 cycles at 1 C. Moreover, the sodium borate coating effectively blocks H⁺/H₂O ingress, conferring exceptional air stability. After 150 cycles, 3-days-aged pristine cathode retains 4.7 % of the fresh capacity, whereas optimized cathode maintains nearly pristine cycling stability. Even after seven days of exposure, only trace Na₂CO₃ impurities are detected. This work establishes in situ boride complexes as a viable strategy to achieve electrochemically robust and moisture-tolerant sodium-ion cathodes for grid-scale energy storage.