Carbon coating stability on high-voltage cathode materials in H₂O-free and H₂O-containing electrolyte

Carbon coatings on cathode materials with low electrical conductivity like phospho-olivines LiMPO₄ (M = 3d-transition metal) are known to improve their performance in Li-ion batteries. However, at high potentials and in the presence of water, the stability of carbon coatings on high-voltage materials (e.g., LiCoPO₄) may be limited due to the anodic oxidation of carbon. In this work, we describe the synthesis of LiFePO₄ (LFP) with an isotopically labeled ¹³C carbon coating (characterized by Raman spectroscopy, electrical conductivity, and charge/discharge rate capability tests) as a model compound to study the anodic stability of carbon coated cathode materials in ethylene carbonate-based electrolytes. We characterize the degradation of the ¹³C carbon coating by On-line Electrochemical Mass Spectrometry (OEMS) through the ¹³CO₂ and ¹³CO signals in order to differentiate the anodic oxidation of the coating (¹³C) from the oxidation of electrolyte, conductive carbon, and binder (all ¹²C) in the electrode. The oxidation of the carbon coating takes place at potentials ≥4.75 V for electrolyte without H₂O (< 20 ppm) and ≥ 4.5 V for electrolyte with 4000 ppm H₂O, and it is strongly enhanced for H₂O-containing electrolyte. The extent of carbon coating oxidation over 100 h at 4.8 and 5.0 V vs. Li/Li⁺ (25°C) is projected on the basis of our OEMS data, suggesting that carbon coatings have insufficient stability at such high cathodic potentials. Furthermore, our results prove the in situ formation of H₂O during the anodic decomposition of ethylene carbonate-containing electrolyte. The H₂O formation is monitored via the detection of gaseous POF3, which is formed from the reaction of LiPF₆ with H₂O.