Insight into Na and Sn substitutions on structural, electronic and thermodynamic responses of Li2FeSiO4 for next generation Li+-ion based batteries: A First principles study

In this study, Li2FeSiO4 has emerged as a promising alternative cathode material for next-generation Li-ion-based batteries due to its high theoretical capacity. We used spin density functional to investigate the structural, electronic, and thermodynamic responses of the pure and doped Li16Fe8Si8O32 system. In the Li-site is substituted by Li61Na3Fe32Si32O128, Li59Na5Fe32Si32O128, Li56Na8Fe32Si32O128, and in the Si-site, Li64Fe32Si31Sn1O128, Li64 Fe32Si30Sn2O128, Li64Sn4Fe32Si28O128 atoms, employing the Perdew-Burke-Ernzerhof (PBE) with generalized gradient approximation (GGA). The introduction of dopants induces significant modifications in the lattice parameters and corresponding volumes, thereby affecting the overall structural characteristics. The computed band gaps of both systems experience a decrease in the presence of dopants, indicating an enhancement in electronic conductive responses. The calculated results show that the band gap of Na-doped can be changed from 2.66 eV to 1.94 eV, and Sn-doped can be changed from 2.66 eV to 0.89 eV, respectively. The element, partial, and total density of states investigations reveal impurity states created by the modified atoms extending to the conduction band maximum. Intriguingly, Na and Sn concentrations in the pure system contribute to modified electronic conductivity attributed to volumetric expansion, a narrowed band gap, and reduced ionic bonding. In this analysis, we have recognized four new thermodynamically stable systems within the Li-Si-Fe-O system, both pure and dopant systems complementing each other. The computational investigations presented here are expected to contribute to a deeper understanding of the doping effects on Li2FeSiO4, facilitating further advancements in cathode material design for Li-ion batteries.