Electrochemical Characteristics of Li-Ion Battery Anodes Based on Titanium Oxyfluoride

The properties of titanium oxyfluoride-based anodes are studied by galvanostatic cycling and galvanostatic intermittent titration technique in a TiOF2-Li half-cells. This anode material is shown to have a high degradation resistance and a specific gravimetric capacity comparable to that of carbon and Li4Ti5O12. The chemical diffusion coefficient of Li+ in TiOF2 varies by two orders of magnitude in the range from 2.3 x 10(-13) to 2.5 x 10(-15) cm(2) s(-1) depending on the degree of lithiation and the direction of charge or discharge process. The electrode electrical resistance changes with cycling and its value is always lower in the lithiated state than in the delithiated state. The phenomenon of superreversibility inherent in this material has been studied. This phenomenon consists in the excess of the discharge capacity (Li extraction) over the charge capacity (Li insertion), that is, the Coulomb efficiency (CE) exceeds 100%. In the first few tens of cycles, the phenomenon is due to the extraction of excess lithium, which was previously stored in the electrode during the first cycle. On subsequent cycles, the CE stabilizes at a level above or below 100%, depending on the ratio between the charge and discharge currents, and is not related to the accumulation or spending of Li.