Li-ion battery is a mature technology widely applied as a power source for consumer electronic devices, and nowadays, their use is expanded towards electric vehicles and stationary applications [1,2]. However, improvement of current battery systems is needed to meet the requirements of the transport sector in terms of energy density, safety, cycle life, and costs [2,3]. Among the different strategies to increase battery autonomy, one is focusing on electrode loading to increase the ratio of active materials (negative and positive electrode thickness) to inactive components (separator, current collector...,) [4,5]. However, by doing so, the output power density becomes strongly limited by the charge transport within the composite electrodes [5]. Thus, our objective is to optimize the battery design to find the best compromise between energy and power density.

In this work, we studied Li-ion battery capacity as a function of the current density with respect to electrode loading, formulation, porosity, and aging. For this purpose, (LiFePO₄) LFP based electrodes were formulated at different loadings (0.4, 0.7, 1.6, and 3.2 mAh.cm^-2), compositions (LFP%, C65%, PVDF %), and calendered to reach different porosities (50%, 45% and, 35%). The microstructure of electrodes is investigated using SEM and BET to determine their specific area. Subsequently, the power performance is fully captured and analyzed using a time-saving methodology [6,7]. The limiting C-rate (resp. current density, J_lim) is then obtained through capacity vs. discharge current curves, which allows us to determine the effective diffusion coefficient of the limiting transport process (D_eff) above J_lim thanks to the Sand equation [7]. Figure 1 shows an example of D_eff obtained by varying the electrode composition for loading of 0.7 mAh.cm^-2 and a porosity of 35%.

Finally, D_eff is compared to the diffusion coefficient obtained using conventional Galvanostatic Intermittent Titration Technique (GITT) to verify whether the limiting parameter is the Li⁺ diffusion within the solid phase or a different process such as the Li⁺ diffusion in liquid phase through electrode porosity. The coupling of the various processes according to the studied parameters is discussed.

References:

1. Nakayama, M., et al, Energy & Environmental Science, 2010. 3 (12): p. 1995-2002.
2. Armand, M. et al. Power Sources, 2020. 479: 228708.
3. Du, Z., et al. Journal of Applied Electrochemistry, 2017. 47(3): p. 405-415.
4. Heubner, C., et al., Journal of Power Sources, 2019. 419: p. 119-126.
5. Heubner, C., et al., Journal of Power Sources, 2018. 380: p. 83-91.
6. Doyle, M., J. Newman, and J. Reimers. Journal of Power Sources, 1994. 52(2): p. 211-216.
7. Devaux, D., et al, Frontiers in Energy Research, 2020. 7, (168).