Sensitivity and Reliability of Global Electrochemical Lithium Detection Signatures
Authors: Chinnam Parameswara , Tanim Tanvir R. , Dufek Eric J. , Li Meng , Dickerson Charles C 
Published: July 2021

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.

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