Researchers in Europe have been investigating the potential of the protonated hexatitanate H2Ti6O13 as an anode material for rechargeable lithium batteries.
Results of their work suggests that the properties of H2Ti6O13 seem to surpass those of the parent compound Li2Ti6O13. Its reversible capacity is well maintained upon cycling, even at increasing discharge rates. Furthermore, this reversible capacity is similar to that obtained for other titanium oxides already proposed as anode material for lithium rechargeable batteries. They are thus proposing both these materials as candidates for anode material, once their electrochemical performances are optimized.
|Schematic representation of the crystal structure of a) Li2Ti6O13 and b) H2Ti6O13. Figure courtesy of J. C. Pérez-Flores. Click to enlarge.|
The existence of the hexatitanate was only very recently demonstrated; in a paper published in 2012 in the journal RSC Advances, the team reported that the hexatitanate H2Ti6O13 is obtained by a simple successive Na+/Li+/H+ ion exchange of Na2Ti6O13. The team tested the hexatitanate as a Li insertion material to assess its use as an electrode in lithium rechargeable batteries.
It reacted irreversibly with around 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. After first discharge, a reversible specific capacity of 170 mA h g−1 was developed. H2Ti6O13 then yielded a higher reversible specific capacity comparable to Li2Ti6O13.
The researchers combined synchrotron and neutron powder diffraction with IR spectroscopy data to provide a precise determination of the crystal structure, thus shedding light on the electrochemical properties of the material and the charge and discharge processes.
Powder X-ray diffraction provided a preliminary structural characterization. To characterize the structure in more detail, the team used neutron powder diffraction at the Heinz Maier-Leibnitz neutron source (FRM II, Garching, Germany), thanks to NMI3 funding. Additional synchrotron diffraction experiments were performed at the Helmholtz-Zentrum Dresden-Rossendorf outstation in the European Synchrotron Radiation Facility in Grenoble, France.
They also investigated the lithium insertion/deinsertion processes electrochemically.
While X-ray diffraction using synchrotron radiation provides highly accurate phase analysis, neutron diffraction is indispensable for deeper investigations, as both Li and H atoms can be readily located within the structure using subtle intensity changes in the neutron diffraction patterns, the researchers commented.
Juan Carlos Pérez-Flores, Flaviano García-Alvarado, Markus Hoelzel, Isabel Sobrados, Jesús Sanz and Alois Kuhn (2013) “Insight into the channel ion distribution and influence on the lithium insertion properties of hexatitanates A2Ti6O13 (A = Na, Li, H) as candidates for anode materials in lithium-ion batteries” Dalton Transactions doi: 10.1039/C2DT31665J
J. C. Pérez-Flores, C. Baehtz, M. Hoelzel, A. Kuhna and F. García-Alvaradoa (2012) “H2Ti6O13, a new protonated titanate prepared by Li+/H+ ion exchange: synthesis, crystal structure and electrochemical Li insertion properties” RSC Advances doi: 10.1039/C2RA01134D