Reduction of charging time is desirable in most battery-powered applications and may be a key factor for wider acceptance of battery electric vehicles (BEVs). However, fast charging of lithium-ion cells may result in accelerated aging [1]. In state-of-the-art cells, lithium deposition on the graphite anodes is a significant mechanism of capacity loss, especially with high charging rates and/or low temperatures.
In this work, we show that local cell expansion can serve as an indirect measure of lithium deposition and how this relationship can be used to design charging protocols that provide both beneficial battery cycle life and charging time.
Experimental and Results
A 2-D thickness scan setup [2] is used to measure locally resolved thickness changes of commercial pouch cells during operation. After accounting for thermal expansion, the thickness changes are linked to reversible and irreversible lithium plating. The analysis is underpinned by voltage relaxation [3] and post-mortem examination. It is shown that the locally resolved thickness measurement helps detect lithium deposition more reliably and at an earlier stage than a with single-spot or a surface-averaged thickness measurement.
Finally, a screening of charging conditions and corresponding thickness changes is performed. Fast charging protocols are derived based on maximum allowable local irreversible thickness change. An extended cycling experiment serves as a proof of the cycle life advantage of our fast charging protocols compared to standard constant current – constant voltage charging.
Acknowledgements
This work has received funding from the European Union's Horizon 2020 research and innovation programme under the grant 'Electric Vehicle Enhanced Range, Lifetime And Safety Through INGenious battery management' (EVERLASTING-713771).
References
[1] Sikha G, Ramadass P, Haran BS, White RE, Popov BN. Comparison of the capacity fade of Sony US 18650 cells charged with different protocols. Journal of Power Sources 2003;122(1):67–76.
[2] Rieger B, Schuster SF, Erhard SV, Osswald PJ, Rheinfeld A, Willmann C et al. Multi-directional laser scanning as innovative method to detect local cell damage during fast charging of lithium-ion cells. J. Energy Storage 2016;8:1–5.
[3] Zinth V, Lüders C von, Hofmann M, Hattendorff J, Buchberger I, Erhard S et al. Lithium plating in lithium-ion batteries at sub-ambient temperatures investigated by in situ neutron diffraction. J. Power Sources 2014;271:152–9.