First Principles Investigation of Sodium Intercalation Mechanisms into Corrugated Titanate Structures for Sodium-Ion Battery Anodes
Using density functional theory (DFT) we investigated the structural changes occurring during sodium intercalation, and calculated voltage profiles for the different materials. We also calculated changes to the sodium diffusion energy barriers at different sodium concentrations.
Structural results indicate that sodium intercalation is a site-limited process, with the NNT having higher capacity for sodium insertion compared to the lepidocrocite structures. Our results also indicate that the layers in the lepidocrocite Na0.8Ti1.73Li0.27O4 structure in space group Cmmm are able to accommodate more sodium cations by shifting layers, compared to the same composition in space group Pmmm. This is corroborated by the experimental evidence on samples with the different stacking arrangements, prepared by different heating regimes. The ability for layers to shift is also attributed to the presence of the lower valence ions Li or Mg in the metal oxide layers, which have different abilities to transition from the titanium layer to sodium sites, depending on charge density.
The energy landscape for the lepidocrocite structure indicates strong M-M interactions with secondary ions such as Li or Mg preferring to distribute among different titanium layers. The presence of lower valence cations also reduces electrostatic repulsion forces leading to sodium binding energies between 0.2-0.3 eV, which dominate sodium diffusion pathways in the lepidocrocite structure. Sodium diffusion is also found to have preferential orientation perpendicular to the corrugated titanium layers.
Energy barriers are also examined at different sodium concentrations in order to determine changes at different states of charge.
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