Interfacial borate engineering enables strain-regulated and gas-stable O3-type layered cathodes for durable sodium-ion batteries

The practical deployment of O3-type layered oxide cathodes for sodium-ion batteries is hindered by structural degradation, interfacial side reactions, and high-voltage gas evolution. Herein, we report an interfacial borate engineering strategy that constructs a conformal borate layer on Na_0.98Ca_0.02Ni_1/3Fe_1/3Mn_1/3O₂, inducing surface densification and Ca²⁺ enrichment to form a gradient-stabilized structure. This architecture strengthens the structural pinning effect, synergistically regulating lattice strain and passivating reactive sites. Consequently, the modified NFM@B cathode suppresses c-axis expansion and intragranular cracking, while nearly eliminating O₂ release and markedly reducing CO₂ evolution at 4.5 V. Electrochemically, NFM@B delivers 139.3 mAh g⁻¹ at 0.2C, maintains a higher average voltage of 3.08 V at 1C, and achieves 44.6% greater energy retention after 350 cycles. The borate interface also enhances moisture tolerance. Practical viability is validated in 1 Ah pouch cells and all-solid-state batteries. This work highlights interfacial strain regulation and surface passivation as an effective pathway for durable high-voltage sodium-ion batteries.