Humidity stability and recovery mechanism of the equilibrium Na2.6Fe1.7(SO4)3 cathode material for sodium-ion batteries

Sodium iron sulfates are a promising class of cathode materials for sodium-ion batteries. However, their sensitivity to moisture hinders large-scale application. Dynamic vapor sorption was employed to investigate the moisture sorption equilibrium behavior of Na 2.6 Fe 1.7 (SO 4 ) 3 within the 0%–90% relative humidity (RH) range, as well as its electrochemical performance recovery following thermal treatment. The results indicate that the material undergoes physical adsorption at RH ≤ 50% with its structure and morphology remaining basically unchanged, and the specific capacity is slightly reduced; the material undergoes chemical hydration at RH ≥ 60%, accompanied by the formation of Na 2 Fe(SO 4 ) 2 ·4H 2 O and Fe 3 O 4 , along with the separation of the hydrated phase from the CNT network, resulting in a significant decrease in specific capacity. Thermal treatment at 200 °C can largely restore the electrochemical performance of physically adsorbed samples, but fails to fully recover that of chemically hydrated samples, owing to the irreversible formation of Fe 3 O 4 and the irreversibility of the morphological changes.