Strategy Formulation for Mitigating Capacity Fading of Na-Layered Oxides

The mechanisms underlying capacity fading during cycling in layered oxide cathode materials for sodium-ion batteries remain inadequately understood. It is essential to elucidate the reasons and propose effective strategies. Here, the capacity-fading mechanism of commercial NaFe_1/3Mn_1/3Ni_1/3O₂ is due to the dissolution of iron ions. Additionally, the extraction of sodium ions (after the Fe³⁺/Fe⁴⁺ reaction) lowers the energy level of NaFe₁/₃Mn₁/₃Ni₁/₃O₂ below that of the electrolyte solvent, thereby inducing solvent decomposition. We establish screening criteria for electrolyte additives through theoretical calculations to improve capacity retention. We identified a series of nitrogen-containing Lewis base additives that can kinetically bind efficiently to iron ions in NaFe₁/₃Mn₁/₃Ni₁/₃O₂ and thermodynamically exhibit stronger electron-donating abilities than the solvents. A new compound, sodium bis(trimethylsilyl)amide (which has not been studied as a Na-ion battery additive before), is selected through the Reaxys database (out of 61 molecules) because it is commercially available at a low price and is relatively stable in the electrochemical process. Such an additive is demonstrated to greatly improve the Coulombic efficiency and reduce the dissolution of iron ions of NaFe₁/₃Mn₁/₃Ni₁/₃O₂//hard carbon cells.