Possibility of determination of transport coefficients D and k from relaxation experiments for sphere-shaped powder samples

Precise determination of chemical diffusion coefficient D and surface exchange reaction coefficient k is of importance for characterization of materials exhibiting ionic or mixed ionic-electronic type conductivity. Commonly performed as relaxation-type experiments, such studies require well-defined geometry of the considered samples. Also, additional constrains regarding magnitude of changes of concentration of the mobile species, as well as values of dimensionless parameter L, in the case of simultaneous determination of D and k, have to be fulfilled. In this work, the application of relaxation-type experiments for evaluation of transport coefficients in the case of studies of powders with different grain size is discussed. Numerical diffusion solutions for the application of particle powders are proposed, which allow to determine the transport diffusion coefficients D and k for powdered materials. The case study of diffusion coefficients D and k with the proposed numerical diffusion solutions is performed for LiCoO2 powder-shaped material in lithium-ion batteries by the modified-PITT method. In addition, a systematic discussion of the mathematical basis of the relaxation methods, with three typical cases (diffusion-controlled, surface exchange reaction-controlled and mixed process) is given, with particular attention directed to the role of the dimensionless parameter L.