Surface aging at olivine LiFePO₄: A direct visual observation of iron dissolution and the protection role of nano-carbon coating

LiFePO₄ has attracted much attention as a potential cathode material for advanced lithium-ion batteries due to its superior thermal stability. In spite of this, LiFePO₄ still suffers from fast capacity fading at high temperature and/or moisture-contaminated electrolyte. The influence of moisture and the detailed corrosion mechanism is still not clear. Here, for the first time, we present a direct visual observation of the surface corrosion process at olivine LiFePO₄ stored in moisture-contaminated electrolyte, and found the direct relationship between iron dissolution and LiFePO₄ corrosion. By using the LiFePO₄ ingot sample with a flat surface as model materials, iron dissolution and surface chemistry change can be clearly observed and identified by field-emission scanning electron microscopy (SEM), time-of-flight-secondary ion mass spectroscopy (TOF-SIMS), and X-ray absorption near-edge structure (XANES). These iron dissolutions at some corrosion sites evoked the overall LiFePO₄ surface corrosion. A significant improvement of the surface stability of LiFePO₄ was obtained by nano-carbon coating, and the carbon surface layer protects LiFePO₄ from direct contact with corrosive medium, effectively restraining the surface corrosion and preserving the initial surface chemistry of LiFePO₄.