DETERMINATION OF THE DIFFUSION COEFFICIENT OF LITHIUM IONS IN GRAPHITE COATED WITH POLYMER-DERIVED SICN CERAMIC

摘要

The diffusion coefficients of lithium ions (Du+) in graphite/SiCN ceramic composites, pyrolyzed at temperatures between 850°C and 1300°C, were determined by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). The diffusion coefficient values calculated by these techniques are of the same order of magnitude. Depending on the potential the values vary from 10"15 to 10"08 cm2/s. Electrochemical measurements show that ZLi+ depends on the material composition, pyrolysis temperature and charge/discharge state. For composites of graphite to ceramic ratio 50:50 the diffusion coefficient is constant and not temperature dependent. The composite materials of 75:25 ratio pyrolyzed at different temperatures exhibit an increase in the Du+ with the highest diffusion coefficient of lithium ions of the order~ 10"10 cm2/s for material pyrolyzed at 1300 °C. Cyclic voltammetry curves demonstrate that lithium ions intercalate into graphite and free carbon segregated from the SiCN phase. The presence of SiCN improves diffusion transport of lithium ions into the graphite and the derived free carbon. These findings strongly indicate that the SiCN ceramic network creates a pathway for lithium ion transfer.