Sensitivity and Reliability of Global Electrochemical Lithium Detection Signatures

Abstract

Electric vehicles’ (EVs’) more-limited driving range and longer charging times—compared with gasoline vehicle refueling—cause range anxiety that inhibits mass adoption of EVs. Extreme fast charging (XFC) is the ability to charge batteries in a length of time (i.e., ~10 minutes or less) comparable to that required to refuel gasoline vehicles [1]. The cell performance and aging mechanisms under XFC conditions are quite distinct from those seen under slow charging conditions. Among the various XFC challenges, Li-plating on the anode is the cause of poor cell performance, with severe capacity fade, dendrites growth and internal shorts that could lead to catastrophic safety issues [2].Literature reports on electrochemical (EC) Li detection, mostly at sub-zero temperatures and moderate-to-high charging rates, show Li plating displaying higher-favorability, reliable plating-driven mixed potential dynamics [3-5]. Those EC signatures have not been evaluated extensively for fast-charging conditions. Our study focuses on the sensitivity and reliability of EC Li-plating signatures at varying XFC conditions with long-term cycling and aging. The global EC signatures investigated were coulombic efficiency (CE), incremental capacity (dQ/dV), differential voltage (dV/dt), and end of relaxed charge (EOC) voltage on moderate loading Li/graphite and graphite/NMC532 coin cells and graphite/NMC532 single-layer pouch cells under XFC conditions at 25°C [6]. The sensitivity and reliability of investigated signatures vary with cell-to-cell and cycling conditions. For example, the sensitivity and reliability of dQ/dV is good with moderate C-rate lithiation and reasonable de-lithiation (C/5) conditions. With increasing C-rate (e.g., 6C), dQ/dV sensitivity decreases even under the same discharge (C/5) conditions. The dV/dt signatures at various states-of-charge with 6C-lithiation conditions varies with cell-to-cell and cycling conditions. In all cases, the dV/dt signature appears during initial cycling at different relaxation times, intensities, but within few cycles, the signal strength decreases and/or disappears even though CE and EOC EC signatures confirm continuous Li plating on the graphite electrode. The EC Li-detection signatures in half-cells, full cells and full pouch cells align with each other within the experimental error. Long-term and continuous reliability of Li-plating detection with EC signatures is still an issue for XFC conditions. Ahmed, S., et al., Enabling fast charging – A battery technology gap assessment. Journal of Power Sources, 2017. 367: p. 250-262.Santhanagopalan, S., P. Ramadass, and J. Zhang, Analysis of internal short-circuit in a lithium ion cell. Journal of Power Sources, 2009. 194(1): p. 550-557.Campbell, I.D., et al., How Observable Is Lithium Plating? Differential Voltage Analysis to Identify and Quantify Lithium Plating Following Fast Charging of Cold Lithium-Ion Batteries. Journal of The Electrochemical Society, 2019. 166(4): p. A725-A739.Petzl, M. and M.A. Danzer, Nondestructive detection, characterization, and quantification of lithium plating in commercial lithium-ion batteries. Journal of Power Sources, 2014. 254: p. 80-87.Schindler, S., et al., Voltage relaxation and impedance spectroscopy as in-operando methods for the detection of lithium plating on graphitic anodes in commercial lithium-ion cells. Journal of Power Sources, 2016. 304: p. 170-180.Tanim, T.R., et al., Electrochemical Quantification of Lithium Plating: Challenges and Considerations. Journal of The Electrochemical Society, 2019. 166(12): p. A2689-A2696.