Determination of Diffusion Coefficient of Li+ Ions in Porous Electrodes from Rate Capability Tests

To apply lithium-ion batteries as a power source of an electric vehicle, it requires high input- and output-power capability. Our previous study revealed that the decrease in capacity on rate capability tests is due to the migration process of Li ions in a porous electrode [1]. Specifically, solid state diffusion of Li ions in an active material is much faster than diffusion of Li ions in liquid phase. This indicates that transportation of Li ions in an electrolyte within porous electrode is a rate determining step. In this paper, the diffusion coefficient of Li ions in porous electrode was determined from the results of rate capability tests by means of electrochemical simulation program (Dualfoil 5.1 [2-4]), because the mass transport of lithium ions in the electrolytic solution within porous electrode occurs by the diffusion process. The discharge behavior of Li/LAMOF cells was calculated with changing the electrode thickness of the LAMOF electrode. The results showed that the area-specific capacity (ASC) per geometrical electrode area on discharge was correlated to the discharge current density based on the geometrical electrode area ( J a ) in the high current density region, whereas ASC was constant in the low current density region. The linear relationship between the two can be expected from Sand’s equation described as follows, Jτ ½ C R *-1 = nFAD R ½ π ½ 2 -1 The above equation ca be transformed in the following equation, JτA -1 = (D R n²F²πC R *² 4 -1 ) × (J A -1 ) -1 From this equation, diffusion coefficient of Li ions ( D R ) can be determined from the slope of the linear relation. From the results on rate-capability tests with changing various electrode structure, thickness and porosity, the relationship between the diffusion coefficient and the rate characteristic is discussed. [1] K. Ariyoshi et al., J. Electrochem. Soc., 165, A3965-A3970 (2018). [2] M. Doyle et al., J. Electrochem. Soc., 140, 1526-1533 (1993). [3] T. F. Fuller et al., J. Electrochem. Soc., 141, 1-10 (1994). [4]T. F. Fuller et al., J. Electrochem. Soc., 141, 982-990(1994). Figure 1