Experimental Investigation of Fast-Charging Protocols Derived from Physicochemical and Thermal Simulations Using 5.4 Ah Multilayer Pouch Cells with Silicon-Dominant Anodes over Aging
Authors: Friedrich Sven , Durdel Axel , Mährlein Johannes , Jossen Andreas 
Published: December 2024

Fast-charging capability remains a main concern for consumers buying an electric vehicle as a sustainable transportation solution. For lithium-ion batteries with typical graphite anodes, lithium-plating and temperature limitations due to the electrolyte are the two main limitations for fast-charging. Silicon as next-generation anode active material promises faster charging times due to its higher averaged potential versus 0 V Li+/Li avoiding lithium plating,[1] especially if silicon is only partially lithiated.[2] Further advantages are the lower coating thickness and the lower ionic resistance for silicon-dominant anodes when compared to graphite anodes with the same areal capacity, also leading to better fast-charging capability.[1]

In this study, a validated physicochemical Newman-type p2D model[3] for a 70 wt% microscale silicon||NCA cell is extended by a 3D thermal model to account for thermal limitations at high-currents for a 5.4 Ah multilayer pouch cell (MLP). The thermal model includes the MLP, compression pads, and the cell holder, while the tab temperature was measured for validation. Experimental and theoretical parameterization of the thermal model includes the heat capacity and the thermal conductivity.[4] The coupled thermal-physicochemical model is depicted in Figure 1 and was used to simulatively derive different fast-charging protocols for the following input parameters: different state-of-charge windows (SoC
Start and SoC
End), different electrochemical limitations as security buffers to prevent lithium plating (U
Sec vs. 0 V Li+/Li), and different thermal limits as different maximal cell temperatures T
Cell,max were simulated. The cell’s charging current I
Cell was controlled according to four different charging phases: constant current (CC), constant anode potential (CAP), temperature-limited (TDER), and constant voltage (CV) are switched and the applied cell current is controlled based on the simulated anode potential U
Anode or the simulated maximal cell temperature in the cell core T
Cell,max. The resulting simulated cell voltage U
Cell during fast-charging was then experimentally applied to 5.4 Ah multilayer pouch cells as a voltage trajectory, while the cell current I
Cell followed according to the voltage trajectory and the cell impedance. In total, 12 MLPs were aged until 70% state of health (SoH) for 25% - 75% SoC for USec=140 mV and 170 mV as well as for 10%-70% for USec=140 mV. As fast-charging reference, charging at 2C was tested for 25%-75% SoC and all results are compared to our previous study[6] aging identical MLPs at C/2.

In the electrochemical results, the capacity retention could be significantly improved by utilizing the SoC window from 25%-75% SoC compared to 100% SoC from our previous study[6]. At the beginning of life, fast-charging times of 10.2 (±0.3) min for 25%-75% SoC with USec=140 mV and 11.8 (±0.1) mins for 25%-75% SoC with USec=140 mV were achieved. Fast-charging simulations for 25%-75% SoC with USec=140 mV predicted a comparable fast-charging time of 11.2 mins. Comparing the different fast-charging profiles over aging, a similar capacity retention is experimentally measured hinting towards loss of lithium inventory being the main aging mechanism while no charging profile seems to be particularly harmful to the cells. Regarding fast-charging time over aging, an increase in charging time is measured since the cell resistance increases over aging leading to a decreased overpotential being released during fast-charging according to the voltage trajectory.

The trade-off between protecting the MLPs over lifetime versus the increased charging time could be further investigated in future work by adjusting the voltage trajectory based on the SoH. Moreover, even more aggressive charging protocols could be tested to derive the physicochemical limitation U
Sec, e.g., 0 V Li+/Li.

Acknowledgments:

S.F. gratefully acknowledges the financial support from the BMBF (Federal Ministry of Education and Research, Germany) under the auspices of the ExZellTUM III project (grant number 03XP0255). The authors want to thank the research battery production team of iwb at TUM for the multilayer pouch cell production. We also thank Wacker Chemie AG for providing the microscale silicon material (CLM 00001).

References:

[1] Berg: Comparison of Silicon and Graphite Anodes: Temperature-Dependence of Impedance Characteristics and Rate Performance. 2023. J. Electrochem. Soc. 170 030534

[2] Jantke: Silicon-Dominant Anodes Based on Microscale Silicon Particles under Partial Lithiation with High Capacity and Cycle Stability. 2019. J. Electrochem. Soc. 166, A3881–A3885.

[3] Durdel: Modeling Silicon-Dominant Anodes: Parametrization, Discussion, and Validation of a Newman-Type Model. 2023. Batteries, 9, 558.

[4] Bryden: Methodology to determine the heat capacity of lithium-ion cells. 2018. Journal of Power Sources. 395.

[5] Zörr: Electrochemical polarization-based fast charging of lithium-ion batteries in embedded systems. 2023. Journal of Energy Storage 72 108234

[6] Friedrich: Effect of Mechanical Pressure on Rate Capability, Lifetime, and Expansion in Multilayer Pouch Cells with Silicon-Dominant Anodes – manuscript in preparation.

Figure 1