Vehicle manufacturers are increasingly electrifying their vehicles using Lithium-ion batteries (LiBs) as the energy storage unit. With a very high battery cost, the requirements on LiB life and performance are necessarily stringent and much effort goes into understanding how these properties are influenced by the conditions of use. For electric vehicles a large State-of-Charge (SOC) operation window has to be balanced with the lifetime expectancy. Being able to correlate load cycles to a specific type of ageing is crucial to understand how to better utilize the LiBs - improving the accuracy of lifetime predictions and optimizing the performance/lifetime balance in the vehicle.

A physics-based ageing model of a commercial Li-ion pouch cell with a graphite anode and a blended LMO/NMC cathode has been developed. The model is based on the Newman theory [1], as implemented in COMSOL [2-6], and is a direct continuation of the work in [6]. The growth of a solid electrolyte interphase (SEI), resulting in increased resistance and capacity loss, constitutes the main ageing mechanisms of the model, which has been tuned with experimental test data as input. The SEI growth has also been linked to a change in active material porosity and thereby a reduced electrolyte volume in the electrode. The resulting effect on the cell resistance and capacity during various test conditions is determined, and the impact from the different ageing parameters evaluated.

The experimental tests used synthetic load cycles for various C-rates, at different Depth of Discharge (DOD) and at different SOC. The results show an expected increased ageing with C-rate, but more surprising is the ageing patterns seen for small DOD, at various SOC intervals, which do not follow the trend observed for LiBs with LFP cathodes [7]. The LiB here degrade most strongly at high SOC (> 60%), in accordance with previous research, while at SOC < 30% ageing is impressively slow. The slow capacity degradation of the cells in SOC intervals < 30% make their cycle lifetime more than 3x longer compared to the cells run at high SOC. The developed ageing model can successfully capture the difference in capacity loss due to larger SEI growth at high SOC.

References:

[1] M. Doyle, T.F. Fuller, J. Newman, J. Electrochem. Soc. 140 (1993) 1526-1533.

[2] H. Ekström, G. Lindbergh, J. Electrochem. Soc. 162 (2015) A1003-A1007.

[3] P. Ramadass, B. Haran, P.M. Gomadam, R. White, B.N. Popov, J. Electrochem. Soc. 151 (2004) A196-A203.

[4] R. Darling, J. Newman, J. Electrochem. Soc. 145 (1998) 990-998.

[5] S. Santhanagopalan, Q. Guo, P. Ramadass, R. E. White, J. Power Sources 156 (2006) 620-628.

[6] E. Wikner, Lithium ion Battery Aging: Battery Lifetime Testing and Physics-based Modeling for Electric Vehicle Applications, Chalmers University of Technology, Göteborg, 2017.

[7] J. Groot, State-of-Health Estimation of Li-ion Batteries: Ageing Models, Chalmers University of Technology, Göteborg, 2014.