by Etienne Kipping, LEITAT Managing Technologies
Among all the characterizations carried out in the CoFBAT project, PITT may not be the most common one, but this is an elegant technique, adapted to the specific batteries developed in CoFBAT, used to evaluate the lithium-ion diffusion into the battery electrode.
PITT stands for Potentiostatic Intermittent Titration Technique. In other words, this is a technique to determine concentrations by applying a step signal of constant voltages, more precisely to study the diffusion coefficient of solutes (e.g. Li+) in electrode materials.
Originally proposed by Huggins et al. for studying Li diffusion in Li–Al alloy (C.J. Wen, B.A. Boukamp, R.A. Huggins, W. Weppner, Journal of the Electrochemical Society 126 (1979) 2258), but used and declined since then in different variants to evaluate the Li-anion diffusion coefficient in Li-ion battery electrodes and to clarify the intrinsic limiting factors of electrochemical performances, especially rate capability. This Li-diffusion coefficient represents how fast the species move from one electrode to another in the battery during charging/discharging. The higher it is, the faster they can move, the faster the battery can be charged.
There are many other techniques to evaluate the diffusion of species, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) or Galvanostatic Intermittent Titration Techniques (GITT). All have their pros and cons, but it is the kind of material in which the species diffuse that determines which technique is most suitable. In a liquid solution, a simple CV could be enough to assess the diffusion coefficient as we would be testing in classic conditions where the common Cottrell Equation can be applied. However, in CoFBAT we tried to determine the diffusion of Li-anion in a solid electrode, where the species are slower than in a liquid. Then the conditions are not the same and different equations that describe better the species behavior should be applied. Other considerations like the homogeneity of the material in terms of particle size distribution and porosity are also to be taken into account. This is why other technique(s) could be complementary.
In the CoFBAT project, this characterization is mainly employed on testing and characterization of battery cells, to gain insights in aging phenomena and for optimization purposes. It is used to study how the repeated charge and discharge of the battery are affecting the battery cells. It was also a way to gather useful data for the development of the battery model.
For the moment, TNO, LNMO and Si/C electrodes were characterized at 10, 25 and 40ºC. The temperature is, indeed, another parameter that can influence the battery behavior. A good reproducibility of the results has been obtained as well as comparable values with the one in the literature.