Ab Initio Study of Stability of Na2Fe2(SO4)3, a High Potential Na-Ion Battery Cathode Material

• Published in: Journal of physical chemistry. C 2017-09, Vol.121 (37), p.20067-20074
• Main Authors: Shishkin, Maxim, Sato, Hirofumi
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b02479

The free energies of thermal decomposition and hydration of Na2Fe2(SO4)3, used as a high operating potential Na-ion battery cathode material (3.8 V), are calculated with the help of the DFT+U method, augmented by magnetic exchange term. In addition, the energetics of hydration of low potential Na2Fe­(SO4)·2H2O cathode (3.25 V) in humidified atmosphere is also analyzed. We find that DFT+U/magnetic exchange method can provide a reasonable agreement between calculated ground state properties of the studied materials (i.e., energetics and magnetic moments) and experiment. Using the evaluated total energies of solid materials and the free energies of the molecular species, we determined that Na2Fe2(SO4)3 is indeed thermally unstable at temperatures T ∼ 450 °C in agreement with experimental observations. Moreover, our calculations predict that Na2Fe2(SO4)3 can be hydrated in room temperature environment (T = 20 °C) also in a reasonable agreement with experiment. Overall, our work shows that the DFT+U method, augmented by magnetic exchange, can be used for analysis of thermal decomposition and hydration of complex compounds such as Na intercalated iron sulfates.