Elucidating of moisture-induced degradation and rehealing of alluaudite Na2+2xFe2-x(SO4)3 cathode for Sodium-Ion batteries

Abstract

Polyanion-type iron-based sulfates are promising candidates for cathode in sodium-ion batteries due to the cost-effectiveness. However, the material exhibits a high sensitivity to humidity, which significantly increases the costs associated with cell fabrication. Therefore, it’s critical for the industrial application to understand the humidity-induced degradation mechanism and develop regeneration strategies. In this study, Na_2.67Fe_1.67(SO₄)₃ (NFS) exhibiting an outstanding capacity retention of 77.8 % after 6000 cycles, is used as the model to systematically investigate the intrinsic causes of degradation upon exposure to moisture. Specifically, water ingress and the strong Coulombic repulsion between Fe-Fe induce the decoupling of the [Fe₂O₁₀] dimer, causing the transformation into bloedite-type Na₂Fe(SO₄)₂·4H₂O. The π-contributing orbital interaction between O of water and Fe, along with the hydrogen-bonding network formed between H in water and the lattice O, confers additional structural stability to the hydrate. Theoretical calculations and enthalpy measurements indicate that the hydration reaction is thermodynamically spontaneous, with the Gibbs free energy change of −0.91 eV and the enthalpy change of –22.083 kJ/mol. After two days of exposure to 50 % humidity, the capacity degrades to approximately 83.5 % and a secondary heating strategy is developed to restore the fully degraded NFS to its original crystal structure and recover up to 94 % of the initial capacity. This study provides comprehensive insights into the causes of air instability in NFS and proposes an effective strategy for capacity regeneration.