H2Ti6O13, a new protonated titanate prepared by Li+/H+ ion exchange: synthesis, crystal structure and electrochemical Li insertion properties

Abstract

The hexatitanate H₂Ti₆O₁₃ is obtained by a simple successive Na⁺/Li⁺/H⁺ ion exchange of Na₂Ti₆O₁₃. The crystal structure of H₂Ti₆O₁₃ was solved from both synchrotron and neutron powder diffraction. H₂Ti₆O₁₃ crystallizes in the monoclinic space group C2/m, with a = 14.6702(3) Å; b = 3.7447(1) Å; c = 9.2594(2) Å; β = 96.941(2)°. The monoclinic symmetry of the [Ti₆O₁₃]²⁻ framework is preserved during the exchange reaction. When compared to the positions of Na and Li in Na₂Ti₆O₁₃ and Li₂Ti₆O₁₃, the position of the proton is shifted towards the O3 atomic position, where it forms a covalent O–H bond. The vicinity of the proton to the O5 atom across the tunnel allows for the formation of a classical (asymmetric) hydrogen bond. H₂Ti₆O₁₃ has been tested as a Li insertion material to assess its use as an electrode in lithium rechargeable batteries. It reacted irreversibly with ca. 6 Li ions per formula unit at an average voltage of 1.5 V vs. Li⁺/Li, with a specific discharge capacity of 315 mA h g⁻¹. However, after first discharge, a reversible specific capacity of 170 mA h g⁻¹ was developed. H₂Ti₆O₁₃ then yielded a higher reversible specific capacity than Na₂Ti₆O₁₃ and comparable to Li₂Ti₆O₁₃. Besides structural details, IR spectroscopy has been used to further assess possible reaction mechanisms pointing to the transformation of H₂Ti₆O₁₃ to Li₂Ti₆O₁₃ when reacting with the very first two lithium ions.