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 LiMPO 4 (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., LiCoPO 4 ) may be limited due to the anodic oxidation of carbon. In this work, we describe the synthesis of LiFePO 4 (LFP) with an isotopically labeled 13 C 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 13 C carbon coating by On-line Electrochemical Mass Spectrometry (OEMS) through the 13 CO 2 and 13 CO signals in order to differentiate the anodic oxidation of the coating ( 13 C) from the oxidation of electrolyte, conductive carbon, and binder (all 12 C) in the electrode. The oxidation of the carbon coating takes place at potentials ≥ 4.75 V for electrolyte without H 2 O (< 20 ppm) and ≥ 4.5 V for electrolyte with 4000 ppm H 2 O, and it is strongly enhanced for H 2 O-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 H 2 O during the anodic decomposition of ethylene carbonate-containing electrolyte. The H 2 O formation is monitored via the detection of gaseous POF 3 , which is formed from the reaction of LiPF 6 with H 2 O.