Structure of H2Ti3O7 and its evolution during sodium insertion as anode for Na ion batteries

Abstract

H₂Ti₃O₇ was prepared as a single phase material by ionic exchange from Na₂Ti₃O₇. The complete ionic exchange was confirmed by ¹H and ²³Na solid state Nuclear Magnetic Resonance (NMR). The atomic positions of H₂Ti₃O₇ were obtained from the Rietveld refinement of powder X-ray diffraction (PXRD) and neutron diffraction experimental data, the latter collected at two different wavelengths to precisely determine the hydrogen atomic positions in the structure. All H⁺ cations are hydrogen bonded to two adjacent [Ti₃O₇]²⁻ layers leading to the gliding of the layers and lattice centring with respect to the parent Na₂Ti₃O₇. In contrast with a previous report where protons were located in two different positions of H₂Ti₃O₇, 3 types of proton positions were found. Two of the three types of proton are bonded to the only oxygen linked to a single titanium atom forming an H–O–H angle close to that of the water molecule. H₂Ti₃O₇ is able to electrochemically insert Na⁺. The electrochemical insertion of sodium into H₂Ti₃O₇ starts with a solid solution regime of the C-centred phase. Then, between 0.6 and 1.2 inserted Na⁺ the reaction proceeds through a two phase reaction and a plateau at 1.3 V vs. Na⁺/Na is observed in the voltage–composition curve. The second phase resembles the primitive Na₂Ti₃O₇ cell as detected by in situ PXRD. Upon oxidation, from 0.9 to 2.2 V, the PXRD pattern remains mostly unchanged probably due to H⁺ removal instead of Na⁺, with the capacity quickly fading upon cycling. Conditioning H₂Ti₃O₇ for two cycles at 0.9–2.2 V before cycling in the 0.05–1.6 V range yields similar specific capacity and better retention than the original Na₂Ti₃O₇ in the same voltage range.