Carbon Coating Stability on High-Voltage Cathode Materials in H2O-Free and H2O-Containing Electrolyte

Abstract

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.