Theoretical Insights into Solid Electrolyte Interphase Formation in an Al Anode Dual-Ion Battery

Dual-ion batteries (DIBs) are emerging as a highly attractive class of batteries as they try to address the shortcomings of the widely used lithium ion batteries. Among the various organic electrolytes used in DIBs, ethyl methyl carbonate (EMC) with LiPF6 salt is recently being considered as a better electrolyte in comparison to commercially used ethylene carbonate (EC). In this work, we have carried out a comparative study of EMC and EC solvent systems to address the greater stability of EMC in contact with aluminum (Al) and lithiated Al (LiAl) electrode as well as the effect of salt in the solid electrolyte interphase (SEI) formation process with the help of ab initio molecular dynamics (AIMD) simulations. We find that EMC can decompose via 1e(-) reduction due to limited charge transfer from the Al surface, whereas 2e(-) reduction becomes more favorable with lithiation of the Al anode surface. The limited decomposition observed in EMC compared to EC in contact with the Al electrode surface justifies the enhanced stability of EMC solvent in DIBs with an Al anode. However, the decomposition and SEI formation process can speed up in the presence of LiPF6 salt as it induces more charge transfer (1.11 vertical bar e vertical bar for Al and 2.86 vertical bar e vertical bar for LiAl) from the electrode surface. Nevertheless, the charge transfer is less than in the case of EC solvent (2.54 vertical bar e vertical bar for AI and 5.42 vertical bar e vertical bar for LiAl), further justifying the stability of EMC solvent. We also find that the charge transfer to the salt molecule from the electrode surface depends on the position of the salt rather than the composition of the electrode surface. Overall, our study shows that the EMC solvent-LiPF6 salt combination can serve as an efficient electrolyte for Al anode DIBs.