Surface Fluorination of Commercial Lto in Order to Overcome the Low Electrochemical Performances of Li2TiO3 Set Onto Lto
Authors: Guerin Katia , Avignant Daniel , El-GHOZZI Malika 
Published: February 2020

Spinel Li4Ti5O12 is known as a “zero-strain” anode material for rechargeable Li-ion batteries due to the minute structural changes upon Li insertion/extraction. Li4Ti5O12 possesses a flat potential of about 1.55 V (vs Li+/Li) with a theoretical capacity of 175 mAh/g. Compared with the currently most used anode material, graphite, Li4Ti5O12 has lower energy density, but it is safer has longer cycle life  and an excellent power density resulting from its 3D Li-ion mobility in the spinel structure. This implies that Li4Ti5O12 has huge potential  as anode material in high power density and long cycle life applications, such as electric vehicles (EV) and stationary power plants. Spinel Li4Ti5O12can be synthesized by many different techniques: solid-state reaction sol−gel methods microwave-assisted synthesis and spray pyrolysis and in hydrothermal batch reactors .

For practical applications, the solid-state method is the most widely used method since it is easy to scale-up, and the precursors are cheap and abundant. Anatase TiO2 and Li2CO3 are the most commonly used starting material for the solid state synthesis of Li4Ti5O12 . However, it has proven difficult to achieve pure phase Li4Ti5O12, and rutile TiO2 and Li2TiO3 are usually observed as impurity phases in the synthesis product . The lithium rich Li2TiO3 phase is known to be located on the Li4Ti5O12 surface . There are few reports considering the impact of Li2TiO3 on the electrochemical performance of Li4Ti5O12 LTO.

In this work, first of all, we have been interested in preparing monoclinic Li2TiO3 by solid-state method (binary Li2O-TiO2).  When used as cathode in primary lithium battery, the performances appear as low (theoretical capacity for 1e- process of 244 mAh/g) but at the same potential than the one of a LTO.

By accurate phase determination of a commercial LTO containing as impurities Li2TiO3  we have calculated the true efficiency of the LTO phase. As for most of commercial products, the 150 mAh/g discharge capacity registered appears as due to the proportion of low electrochemically active Li2TiO3 phase. Then in order to come over this Li2TiO3phase, we have chosen to make surface fluorination on this commercial LTO.

Indeed, surface fluorination has already been successfully performed on for example LiMn2O4 by using NF3 gas . Recently, a thin and homogeneous Li2TiF6 coating has been introduced onto an over-lithiated layered oxide (OLO, namely Li1.17Ni0.17Co0.1Mn0.56O2) surface via simple co-precipitation at ambient temperature by using Li2CO3 and H2TiF6 aqueous solutions . The cycle performance and rate capability of the coated samples were superior to that of pristine OLO by reducing polarizations owing to the surface lithium conductive phase. Furthermore, the Li2TiF6coating, which had strong fluoride bonding, hindered thermal side reactions at elevated temperature and the associated drawbacks by passivating the OLO surface.

So we have decided to manage fluorination in order to try to convert Li2TiO3 into Li2TiF6. Two fluorination ways were made: flash fluorination by a quick exposure of LTO with pure fluorine gas flow at 100°C and fluorination by XeF2 decomposition at 120°C. In the first case, the fluorinating agent is F2 whereas in the second, it is mainly F°. Both fluorination ways succeeded in disappearing LiTi2O3.Only flash fluorination allows to get Li2TiF6 onto LTO whereas after fluorination by XeF2, Li2TiF6 is generated together with LiF and TiOF2 as shown by XRD  and XPS.

The electrochemical performances of commercial LTO fluorinated either by flash fluorination or XeF2 decomposition have been used as cathode in secondary lithium battery. For both fluorinations, the charge-discharge capacities at the first cycle are clearly enhanced by comparison with those of commercial LTO. Fluorinated parts give an oxidation potential at around 3.0V for LTO fluorinated by  XeF2 and this oxidation phenomenon remains upon cycle. It can be attributed to redox properties of TiOF2 . For LTO-F2, no clear new redox potential is visible, Li2TiF6 seems to act only on the SEI formation and as a consequence on the first cycle. The own electrochemical properties of Li2TiF6 could be interesting to perform latter.