Mapping the sodium intercalation mechanism, electrochemical properties and structural evolution in non-stoichiometric alluaudite NaFe(SO) cathode materials

In the scientific advancement of future cathode materials, alluaudite sodium iron sulfate Na 2+2 δ Fe 2− δ (SO 4 ) 3 (N x F y S) has emerged as one of the most promising candidates for sustainable sodium-ion batteries due to its high Fe 2+/3+ redox potential (3.8 V vs. Na/Na + ), low cost, and high rate capability. Usually, this material occurs in a non-stoichiometric form with partial Na + substitutions on Fe sites, where δ is close to 0.25 (N 2.5 F 1.75 S) depending on the synthesis conditions. While many contemporary works have primarily been directed to study this non-stoichiometric compound, our previous theoretical prediction unveiled the possibility to synthesize stoichiometric alluaudite (N 2 F 2 S), which is expected to deliver higher specific capacity (∼120 mA h g −1 ) as compared to the non-stoichiometric derivatives. This provokes curiosity toward the non-stoichiometric effect on the electrochemical activities and sodium intercalation mechanism in alluaudite materials. In this work, we therefore perform rigorous first-principles calculations to study the structural evolution, electrochemical behavior, and voltage profile of N x F y S with y = 2, 1.75, and 1.5. We reveal the likelihood of two phase transitions after half desodiation process, whereas the probability is reduced with a higher degree of non-stoichiometry, suggesting improvement in the structural reversibility for N 2.5 F 1.75 S and N 3 F 1.5 S. The prediction of the voltage profiles shows the benefit of non-stoichiometry in enhancing the specific capacity and identifies the structural rearrangement of Fe 2 O 10 dimers as the hidden reason behind the irreversible sharp peak experimentally observed in differential galvanostatic profiles. The influence of non-stoichiometry on sodium intercalation mechanism, electrochemical properties and structural evolution, and their correlation in 3.8 V alluaudite sodium iron sulfate Na 2+2 δ Fe 2− δ (SO 4 ) 3 cathode materials where δ = 0, 0.25 and 0.5.