A reversible phase transition for sodium insertion in anatase TiO₂

Journal Article · 07 February 2017 · Chemistry of Materials

DOI: https://doi.org/10.1021/acs.chemmater.7b00098 · OSTI ID:1372078

Li, Wei; Fukunishi, Mika; Morgan, Benjamin J.; Borkiewicz, Olaf J.; Chapman, Karena W.; Pralong, Valerie; Maignan, Antoine; Lebedev, Oleg I.; Ma, Jiwei; Groult, Henri; Komaba, Shinichi; Dambournet, Damien

Anatase TiO₂ is a potential negative electrode for sodium-ion batteries. The sodium storage mechanism is, however, still under debate, yet its comprehension is required to optimize the electrochemical properties. To clarify the sodium storage mechanism occurring in anatase, we have used both electrochemical and chemical routes from which we obtained similar trends. During the first discharge, an irreversible plateau region is observed which corresponds to the insertion of Na+ within the interstitial sites of anatase and is accompanied by a drastic loss of the long-range order as revealed by X-ray diffraction, high resolution of high angle annular dark-field scanning transmission electron microscope (HAADF-STEM), and pair distribution function (PDF) analysis. Further structural analysis of the total scattering data indicates that the sodiated phase displays a layered-like rhombohedral R3m structure built from the stacking of Ti and Na slabs. Because of the initial 3D network of anatase, the reduced phase shows strong disorder due to cationic intermixing between the Ti and Na slabs and the refined chemical formula is (Na_0.43Ti_0.57)_{3a 0.22}Na_0.39Ti_0.39)_3bO₂, where refers to vacancy. The presence of high valence Ti ions in the Na layers induces a contraction of the c-parameter as compared to the ordered phase. Upon desodiation, the structure further amorphized and the local structure probed by PDF is shown to be similar to the anatase TiO₂, suggesting that the 3D network is recovered. The reversible sodium insertion/deinsertion is thus attributed to the rhombohedral active phase formed during the first discharge, and an oxidized phase featuring the local structure of anatase. Due to the amorphous nature of the two phases, the potential-composition curves are characterized by a sloping curve. Lastly, a comparison between the intercalation of lithium and sodium into anatase TiO₂ performed by DFT calculations confirmed that, for the sodiated phase, the rhombohedral structure is more stable than the tetragonal phase observed during the lithiation of nanoparticles.

• Research Organization: Argonne National Laboratory (ANL)
• Sponsoring Organization: European Commission - Community Research and Development Information Service (CORDIS) - Seventh Framework Programme (FP7); Engineering and Physical Sciences Research Council (EPSRC); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22), Scientific User Facilities Division
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 1372078
• Journal Information: Chemistry of Materials, Journal Name: Chemistry of Materials Journal Issue: 4 Vol. 29; ISSN 0897-4756
• Publisher: American Chemical Society (ACS)Copyright Statement
• Country of Publication: United States
• Language: English