Abstract
In this study, a carbon-coated LiFePO4 (LFP/C) powder was chemically grafted with trifluoromethylphenyl groups in order to increase its hydrophobicity and to protect it from moisture. The modification was carried out by the spontaneous reduction of in situ generated 4-trifluoromethylphenyl ions produced by the diazotization of 4-trifluoromethylaniline. X-ray photoelectron spectroscopy was used to analyze the surface organic species of the modified powder. The hydrophobic properties of the modified powder were investigated by carrying out its water contact angle measurements. The presence of the trifluoromethylphenyl groups on the carbon-coated LiFePO4 powder increased its stability in deionized water and reduced its iron dissolution in the electrolyte used for assembling the battery. The thermogravimetric and inductively coupled plasma atomic emission spectroscopy analyses revealed that 0.2–0.3 wt.% Li was deinserted during grafting and that the loading of the grafted molecules varied from 0.5 to 0.8 wt.% depending on the reaction conditions. Interestingly, the electrochemical performance of the modified LFP/C was not adversely affected by the presence of the trifluoromethylphenyl groups on the carbon surface. The chemical relithiation of the grafted samples was carried out using LiI as the reducing agent and the lithium source in order to obtain fully lithiated grafted powders.
Highlights
LiFePO4 is one of the most promising cathode materials for lithium-ion batteries [1]
The aim of this work was to demonstrate the facile protection of an LFP/C powder from sideside-reactions occurring in contact with water
We demonstrated that the functionalization of LFP/C with trifluoromethylphenyl groups by diazonium chemistry is an efficient functionalization of LFP/C with trifluoromethylphenyl groups by diazonium chemistry is an efficient strategy to control its surface reactivity
Summary
LiFePO4 is one of the most promising cathode materials for lithium-ion batteries [1]. The aging and partial oxidation of LiFePO4 upon exposure to ambient air and humidity deteriorate its electrochemical performance [5,6,7] These problems can be mitigated by storing LiFePO4 in costly special packaging under inert gas. The modification of carbon powder (e.g., Vulcan XC72 or carbon black), typically used for the preparation of Pt/C active layers of proton-exchange membrane fuel cells (PEMFC), is carried out by grafting it with 4-trifluoromethylphenyl groups [9,11]. This modification enables the production of gas diffusion electrodes with a precisely controlled degree of hydrophobicity
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