Experimental and Theoretical Investigation of Lib-Active Materials Properties
Authors: Chaouachi Oumaima , Chandesris Marion , Réty Jean-Michel , Bultel Yann , Geniès Sylvie 
Published: May 2020

Li-ion batteries (LIB) are currently used for energy storage in several applications. The design of LIB system must ensure the holding of performance for a given application during its entire life. Nowadays, Newman based models [1] are among the electrochemical models developed to compute the physical phenomena at the electrode scale, while predicting the behaviour of the entire cell. However, they require electrode’s components physical parameters. Finding out these physical parameters and their evolution during their lifetime is thus critical to simulate and predict with a high reliability the electrochemical response of the LIB via these models. Among the physical parameters, the equilibrium potential Eq
, the solid diffusion Ds
, and the exchange current density i0
of positive and negative active materials are affecting significantly this electrochemical response. Determining their expressions according to stoichiometry and their consistency based on thermodynamic approach [2] is an improvement for modelling Li-ion batteries. In this work, Ni0.6Mn0.2Co0.2O2 (NMC 622) is considered as the active material of the positive electrode and graphite for the negative one.

In this paper, a methodology to extract these parameters from experimental data as function of the stoichiometry is presented. Then, an analytical model of their stoichiometry dependence is derived based on thermodynamic considerations. Eq
and Ds
have been measured using the Galvanostatic Intermittent Titration Technique (GITT) [3] on both half cells and a pouch cell with a lithium reference electrode. Results on the first design (half-cell) allow to quantify the active materials available at the positive and negative electrodes of the second design (pouch cell) and determine active materials stoichiometry as function of cell’s state of charge (SOC). i0
is determined from electrochemical impedance spectroscopy measurements (EIS) [4] performed at different cell’s state of charge on a specific experimental design with two references electrodes (Figure 1a). A lithium iron phosphate (LFP) reference electrode has been used to measure simultaneously the impedance spectra of both positive and negative electrodes. Lithium has been implemented as a second reference electrode to follow the LFP reference electrode potential, and the negative electrode potential at the different SOC of the EIS measurements. The exchange current densities are extracted using a Transmission Line Model (TLM) [5] and obtained results are presented (Figure 1b) for the two materials as function of the stoichiometry. A comparison of these results with the classical Butler-Volmer expression (classical BV) used in porous electrode theory shows a gap between them. This difference was highlighted by Bazant [6] where he presented an intercalation kinetics model dedicated to lithium iron phosphate intercalation, going beyond the elementary reaction considered in classical BV. With the classical BV, the molecular intercalations (repulsive or attractive forces) are neglected.

The measured stoichiometry dependence of these three active material’ parameters is further analyzed, based on thermodynamic considerations. For the positive material, a model is proposed for this observed stoichiometry dependence, highlighting the non-ideal molecular behavior of lithium inside the host material.

Finally, to validate the methodology and assess the predictability of Newman based LIB model, a comparative numerical simulation of the impedance spectra for the positive and negative electrodes with Newman electrochemical model with Eq
, Ds
, and i0
has been performed. Results at 20 and 80% SOC are presented Figure 1c for the negative electrode.

References:

1. J. Newman, W. Tiedemann. J.AIChE, 21:25–41 (1975)

2. M.Z. Bazant, Accountes of chemical research, 46, 1144 (2013)

3. W. Weppner, R.A. Huggins, J.Elechtrochem.Soc ,1569 :1578-124 (1977)

4. J. Illig et al. 670 :679-239(2013)

5. L. Nielsen, J.Hjelm, J.Electrochimica Acta,31:45-115(2014)

6. M.Z. Bazant, Faraday Discussions 199, 423-463 (2017)

Figure 1