Defect Engineering of Iron-Rich Orthosilicate Cathode Materials with Enhanced Lithium-Ion Intercalation Capacity and Kinetics

Abstract

Defect engineering via nonstoichiometric composition control can serve as an effective strategy to tune the electronic and crystal structures of intercalation compounds, as has been recently evidenced in Li-rich cathode materials. To extend this strategy in another direction, Fe-richness as opposed to Li-richness is investigated in improving the electrochemical performance of a promising cathode material, Li2FeSiO4 (LFS). Nonstoichiometric LFS compounds in orthorhombic Pmn21 phase with up to 8% Fe-excess are successfully synthesized via controlled hydrothermal synthesis. It is demonstrated that, in addition to the higher electron capacity from the accessible Fe2+/Fe3+ redox couple, the presence of excess Fe enhances the intercalation kinetics vis-à-vis the stoichiometric composition. From combined electrochemical evaluation and first-principles DFT calculations, the enhanced kinetics are rationalized by the introduction of the FeLi• + VLi′ defect pair and newly generated electron conducting states from the creation of local Fe-O-Fe configurations. Moreover, the Fe-rich structure facilitates Fe migration to the Li-site due to a lower energy barrier than that of stoichiometric LFS, hence apparently leading to faster phase transformation from Pmn21 toward the cycled inverse Pmn21 phase. More generally, this study opens alternative defect and compositional engineering approaches in designing next generation intercalation materials with improved electrochemical performance.