Semi-Empirical Study on Lithium Diffusion into Electrode Materials for Lithium Batteries

The decrease in power density of lithium batteries at low temperature is one of main obstacles for their widespread use in transportation systems. In spite of many works on low temperature performance, the key factors affecting the temperature-dependent power degradation still remain unclarified in many battery systems. Successful analysis of power performance is dependent critically on how to estimate accurately the resistances of the electrode reactions such as interfacial charge transfer and solid state lithium diffusion.

In particular, lithium diffusion is hard to be analyzed when the electrode materials undergo a phase transition during charge-discharge process. As a matter of fact, many researchers have still analyzed lithium transport phenomena by using typical relations derived on the assumption of atom diffusion into single phase. For instance, they carried out typical potentiostatic intermittent titration technique, cyclic/linear sweep voltammetry, or electrochemical impedance spectroscopy and then estimated the solid state diffusion coefficient from Cottrell, Randle-Sevcik, or Warburg equation, respectively. However, it is quite unlikely that such a method might give a reliable value of diffusion coefficient because the phase transition significantly influences the electrochemical response. Although a few works investigated lithium transport by considering the coexistence of two phases, the analysis of lithium transport kinetics through biphasic materials is still a great challenge.

In this work, a viable way to analyze lithium transport phenomena through the materials experiencing phase transition is suggested. The method includes the experimental determination of the charge amount transferred during the phase transition and the subsequent numerical estimation of apparent lithium diffusion coefficient for the same amount of charge in a hypothetic single phase.

The apparent diffusion coefficients of transition metal oxides undergoing phase transformation during charge-discharge process were determined at different temperatures to obtain the activation energy for lithium diffusion. The estimated values of diffusivity and activation energy are compared with those in the literatures and their validity is critically discussed. Furthermore, from the comparison with the activation energies for interfacial reactions estimated from the electrochemical impedance spectroscopy, main factors affecting power performance, esp. low temperature power decline of transition metal oxides are suggested.