High-Performance Sodium Storage Enabled by Structural Optimization of Oxidized (NH₄)_0.5V₂O₅

This study reports the design of an oxidation-engineered (NH 4 ) 0.5 V 2 O 5 (oxidation-NVO) cathode via selective ammonium ion extraction using Na 2 S 2 O 8 to modulate the interlayer framework for enhanced sodium-ion storage. The oxidation process partially removes NH 4 + ions while preserving the layered monoclinic structure, leading to expanded interlayer spacing and improved ion accessibility. Structural characterizations confirm the retention of a stable [VO 6 ] n framework supported by residual NH 4 + species acting as structural pillars. Morphological analysis reveals highly crystalline rod-like microstructures, and chemical state analysis confirms the coexistence of V 4+ /V 5+ state, favorable for redox activity. Electrochemical testing shows that the oxidation-NVO electrode delivers a high reversible capacity of 192 mAh g −1 , approximately 20% higher than the pristine NVO counterpart, with excellent cycling stability, low polarization, and full capacity recovery after rate variation. Further transport and structural analyses under dynamic conditions demonstrate improved Na + transport and a reversible breathing-type interlayer response. These findings highlight the effectiveness of interlayer chemistry modulation through oxidation as a simple and scalable strategy for advancing vanadium-based cathode materials in sodium-ion batteries.