Effects of Ball Milling on the Electrochemical Capacity and Interfacial Stability of Li 2MnO3 Cathode Materials

The cycling mechanism of Li2MnO3 cathode materials synthesized by conventional solid-state methods at high temperatures (800–900 °C) has been intensively investigated. Previous studies showed that CO₂ and O₂ gas evolution accounts for most of the charge capacity, followed by some Mn reduction during discharge. In this work, we analyze the effects of ball milling on the structure, surface contaminant, and electrochemical capacity of Li₂MnO₃ cathode material, with or without a graphitic fluoride (C–F) additive. At the same time, C–F is added to form a protective coating layer that reduces unwanted reactions with the electrolyte during later electrochemical cycling. We find that the C–F ball-milled material shows Li₂MnO₃/LiMnO₂ composite phases, while the purely ball-milled material shows a single Li₂MnO₃ phase. Furthermore, we characterize surface species and gas evolution during the first cycle, which reveals the decomposition of Li₂CO₃ and the carbonate electrolyte during the first charge, especially during the high potential region (>4.4 V), and the electrochemical reduction of only a small fraction of the evolved gas on the first discharge (<2.75 V). The appearance further demonstrates the repetitive nature of this process during charge and disappearance during discharge of Mn 2p₃/₂ X-ray photoelectron spectroscopy (XPS) spectra signals during the first two cycles. These processes result in first discharge specific capacities of only 155 and 170 mAh/g after first charge specific capacities of 210 and 320 mAh/g for the pure ball-milled and ball-milled with C–F materials, respectively. These studies demonstrate the interfacial instability introduced by ball milling. However, the electrochemical capacity is significantly increased, necessitating further investigation to determine whether ball milling can activate Mn-containing cathode materials.