Titanium Oxyfluoride as a Material for Negative Electrodes of Lithium-Ion Batteries

Abstract

A study of the electrochemical characteristics of titanium oxyfluoride obtained with the direct interaction of titanium with hydrofluoric acid is reported. Two materials T1 and T2 synthesized under different conditions in which some TiF3 is formed in T1 are compared. Both materials exhibit conversion-type anode properties. Based on the analysis of the charge–discharge curves of the half-cell, a model is proposed according to which the first electrochemical introduction of lithium occurs in two stages: the first stage is the irreversible reaction resulting in a reduction in Ti4+/3+, and the second stage is the reversible reaction with a change in the charge state Ti3+/1.5+. The difference in material behavior is quantitative: T1 has a higher reversible capacity but lower cycling stability and a slightly higher operating voltage. The Li diffusion coefficient determined from the CVA data for both materials averages 1.2–3.0 × 10−14 cm2/s. A distinctive feature of titanium oxyfluoride anodes is the asymmetry in kinetic characteristics that revealed themselves during lithium embedding and extraction. In the long cycling regime, the excess of Coulomb efficiency over 100% was found in the present study.

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

“…The charge and discharge plateaus are located in the region of 1.5 and 2 V, respectively, which indicates that the direct and reverse parts of reaction (see Equation ( 2)) proceed at different potential values. According to the study of Astrova et al, [5] it can be divided into two independent reactions (see Equation (4a) and (4b))…”

“…where R dch is the resistance of the electrode in the delithiated state, R dch I dch is the voltage drop when the discharge current is turned off, and R dch I ch is the voltage drop when the charge reflex (rutile). [5] current is turned on. Similarly, the potential at point F is greater than 0.01 V by the magnitude of the voltage drops across the electrode resistance in the lithiated state R ch during charging and discharging currents (see Equation ( 6))…”

“…where applied current density i = I/S [A cm À2 ], I = j m [A], j [A g À1 ] current density per unit mass m of the active substance in the electrode, S is the true surface area of the electrode under study S = S s m, where, S s = 1.96 m 2 g À1 [5] is the specific surface area, whence i = j/S s = 1.53 μA cm À2 ; D is diffusion coefficient of lithium ions [cm…”

“…[1] Until now, only a small number of studies [2][3][4] have been devoted to the electrochemical properties of TiOF 2 anodes, and there was no generally accepted concept of the lithiation mechanism. In our previous work on TiOF 2 , [5] we proposed a model according to which the first Li introduction is a two-stage process consisting of an irreversible (see Equation (1)) and a reversible process (see Equation ( 2))…”

“…During the total reaction of the first lithiation, the conversion reaction, one TiOF 2 molecule absorbs 2.5 lithium atoms, and during further cycling the insertion/extraction of 1. 5 Li atoms occurs as a result of the reaction (see Equation ( 2)), which corresponds to a specific capacity of 395 mAh g À1 .…”