Influence of Carbon Coating on Intercalation Kinetics and Transport Properties of LiFePO4

We report on a comparative study of Li+ intercalation kinetics for LiFePO4 materials with polydopamine-derived and glucose-derived carbon coatings. We demonstrate that the uniform polydopamine-derived carbon coating affects the charge transfer rates and transport properties (electronic conductivity) of composite electrodes only slightly (compared to the inhomogeneous glucose-derived coatings), whereas much more pronounced effects are observed for the apparent diffusion coefficients and nucleation barriers. Thick and uniform carbon surface layers cause a sharp decrease in the ionic diffusion coefficients, which results in the loss of capacity at higher charge/discharge rates for LiFePO4 composite electrodes. The new phase nucleation rates are also demonstrated to decrease significantly for the samples with thicker coatings inducing higher potential differences between charge and discharge curves. The analysis of the rate determining factors for the LiFePO4 electrodes points to the rate limitations induced by the slow nucleation and slow ionic diffusion, whereas the formation of a polydopamine-derived uniform carbon layer tends to hinder the (de)intercalation reaction and does not provide additional benefits compared to glucose-derived carbon coatings comprising conductive yet inhomogeneous layers.