Structural stabilities and electrochemistry of Na2FeSiO4 polymorphs: first-principles calculations.

Searching for new cathode materials is very important in the field of Na-ion batteries. Transition metal sodium orthosilicates have attracted much attention due to the significant progress made in its analog of lithium orthosilicates. In this work, density functional theory (DFT) calculations are performed to study the structural stabilities, electronic and electrochemical properties of Na2FeSiO4 polymorphs, and their corresponding desodiated products NaxFeSiO4. Our results show that formation energies of all the calculated structures differ substantially upon removal of Na, indicating that phase transformations should happen during the desodiation for different polymorphs. NaFeSiO4 is the only energetically stable intermediate phase among the cases, leading to two theoretically voltage plateaus, i.e., ~ 2.0 V (2 ≥ x ≥ 1) and ~ 4.5 V (1 ≥ x ≥ 0) in NaxFeSiO4, respectively. Moreover, the oxidation of Fe2+ to Fe3+ ions is observed during the removal of the first Na ion, while both the oxidation of O and Fe ions occur during the further desodiation. [ABSTRACT FROM AUTHOR]