First Principles Investigation of Sodium Intercalation Mechanisms into Corrugated Titanate Structures for Sodium-Ion Battery Anodes
Authors: Markus Isaac Manny , Asta Mark , Doeff Marca , Shirpour Mona , Engelke Simon 
Published: February 2020

Sodium titanates have emerged as attractive anode materials for sodium ion batteries due to their ability to reversibly intercalate sodium ions at low voltages, while exhibiting a rich compositional range with varying crystal structures. In this work we investigate two types of titanates that have been recently synthesized and tested exhibiting similar sloping voltage profiles. The first set of materials is based on Na1+xTi3O6(OH)×2H2O, also known as sodium nonatitanate1,2 (NNT), and the second is based on a family of lepidocrocite structures such as Na0.8Ti1.73Li0.27O4 and Na0.8Ti1.4Mg0.6O4
3. Lepidocrocite titanates consist of corrugated metal oxide layers between which large cations such as potassium or sodium and, optionally, water are inserted. The layers can be stacked in-phase (P-type) or anti-phase (C-type) with one another. NNT shares some structural features with the lepidocrocite type titanates, having a stepped layered structure consisting of edge-sharing Ti6O14 units linked by corners to form steps, with sodium ions and water between the layers. A dehydration process irreversibly removes the water while maintaining the gross structural features of NNT.
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.

References

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Seshadri D., Shirpour M., Doeff M. “Electrochemical Properties of Electrodes Derived from NaTi3O6OH·2H2O in Sodium and Lithium Cells” J. Electrochem. Soc. 2015Vol 162, issue 1, A52-A59.

Shirpour M., Cabana J., Doeff M. “Lepidocrocite-type Layered Titanate Structures: New Lithium and Sodium Ion Intercalation Anode Materials.” Chemistry of Materials 2014 26 (8), 2502-2512.