A Reversible Phase Transition for Sodium Insertion in Anatase TiO₂

Abstract

Anatase TiO 2 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 R3̅ m 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 0.43 Ti 0.57 ) 3a □ 0.22 Na 0.39 Ti 0.39 ) 3b O 2 , 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 TiO 2 , 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. Finally, a comparison between the intercalation of lithium and sodium into anatase TiO 2 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.

Enhanced Ionic/Electronic Transport in Nano‐TiO2/Sheared CNT Composite Electrode for Na+ Insertion‐based Hybrid Ion‐Capacitors

“…However, the sodiation mechanism of TiO 2 is still widely contested; here we assume Na + insertion into the lattice . Other theories involve the reversible formation of a new Na/Ti mixed phase or the irreversible formation of metallic Ti and Na 2 O …”

Pseudocapacitance of TiO2−x/CNT Anodes for High‐Performance Quasi‐Solid‐State Li‐Ion and Na‐Ion Capacitors

“…As shown in Figure b, a wide irreversible current peak in the range of 0.3–1.2 V can be found in the first CV curve of TiO 2− x /CNT anode in SIBs, which may be attributed to the formation SEI films. A couple of broad cathodic/anodic peaks at 0.70/0.90 V can be found, correlating with the reversible reaction of Ti 3+ /Ti 4+ for the sodiation/desodiation processes . The first three charge–discharge curves at 0.1 A g −1 of TiO 2− x /CNT anodes in 1–3 V for LIBs and 0.01–2.5 V for SIBs are presented in Figure c,d, respectively.…”

A High‐Performance Sodium‐Ion Hybrid Capacitor Constructed by Metal–Organic Framework–Derived Anode and Cathode Materials

“…Thus, a certain amount of sodium ions might be trapped in the TiO 2 /C host even after the fully desertion of sodium ions, which might be also responsible for the low ICE (besides the formation of the SEI layer). The reversible sodium‐ion storage was accomplished between amorphous sodium‐rich titanate phase and sodium‐poor titanate phase, characterized by the redox peak at 0.77–0.84 V with a sloping potential curve similar to a solid solution mechanism resulted from the amorphous nature of the two phases (Figure b) …”