In secondary batteries, the anode/electrolyte interphase plays a key role in the electrochemical performances. As the liquid organic electrolyte undergoes degradation in the electrochemical potential window of a cycling battery, a Solid Electrolyte Interphase (SEI) is formed upon cycling. This interphase layer leads to a double-edged problematic: the formation of the SEI lowers the coulombic efficiency and causes irreversible capacity loss, but it also passivates the electrode from the electrolyte and prevents further aging processes. Knowing this, any modification of the SEI should be performed with parsimony as it could break the balance between the positive and negative aspect of the SEI. By synthetizing a chemisorbed thin fluorinated layer upon anode material, we managed to improve the passivating power of the SEI leading to enhanced electrochemical performances. We also determine that very low quantities of fluorine on the active electrode material surface leads to several beneficial effects.

The chemical nature of the surface layer was describe by the mean of the XPS (figure 1), as well as the fluorine distribution on the surface with both AES and SAM. The fluorine has been quantified around 10 at. % of the extreme surface of the Li₄Ti₅O₁₂ (LTO) material, without diffusion in particles bulks. The bulk and sub-surface properties of fluorinated LTO (LTO-F) were also investigated by coupling XRD, Raman Spectroscopy and NMR ¹⁹F, showing no modifications of the crystallographic structure. The influence of the surface fluorination on the electrochemical performance was investigate by galvanostatic cycling and by coupling XPS and SAM on cycled electrodes. We had a specific attention to the impact of the fluorination on the SEI thickness and stability in charge and discharge. Indeed, LTO-F exhibit a new reactivity toward the electrolyte, leading to a thinner and stabilized SEI. Finally, the gas generation of the LTO-F electrodes has been investigate by Gas Chromatography – Mass Spectrometry (GC-MS), as gassing is known to be a roadblock to the commercialization of LTO^1,2. We demonstrate that the CO₂ outgassing is reduced by the surface fluorination. The strategy implemented in this work, from synthesis to thorough characterization, allow to propose new solutions to improve SEI for Lithium ion batteries.

(1) He, Y.-B.; Li, B.; Liu, M.; Zhang, C.; Lv, W.; Yang, C.; Li, J.; Du, H.; Zhang, B.; Yang, Q.-H.; et al. Gassing in Li(4)Ti(5)O(12)-Based Batteries and Its Remedy. Sci. Rep. 2012, 2, 913.

(2) Zhang, L.; Zhang, S.; Zhou, Q.; Snyder, K.; Miller, T. Electrolytic Solvent Effects on the Gassing Behavior in LCO||LTO Batteries. Electrochimica Acta 2018, 274, 170–176.