Experimental study on detecting and quantifying lithium-deposition with differential voltage analysis and coulomb-counting

Summary:

High currents during fast charging of lithium-ion cells at elevated states of charge (SOC) induce undesired lithium deposition (LD), which leads to rapid loss of cyclable lithium and therewith an accelerated capacity fade. Furthermore, dendritic LD may penetrate the separator and can cause safety-critical conditions. Much research effort has been spent on detection methods of LD, to prevent this severe degradation mechanism [1–3]. The aim of this study is to demonstrate opportunities and limits of differential voltage analysis as a state-of-the-art detection technique for LD. Quantification, reversibility and reproducibility of LD and sensitivity of the detection method is discussed.
In our approach, commercial lithium-ion cells with a graphite anode and a LiFePO4 cathode are charged under critical conditions to provoke LD on purpose. The differential voltage analysis and coulomb-counting method [4, 5] are used to detect and quantify reversible and irreversible LD during the following discharge process. Before and after the test series, a capacity test at 25°C is conducted to determine the actual capacity loss. The intention is to analyse if the sum of detected irreversibly plated lithium during cycling equals the capacity fade determined by the capacity test.
The cells are fast-charged from a SOC of 65 % to 80 % with varying C-rates by the constant current charging strategy. Different extends of reversible and irreversible LD are quantified to identify the minimum detection limits for both methods and the initiation point of the degradation process. Furthermore, the analysis of reversible and irreversible effects of LD shows a functional dependency between fast detectable anomalies and the charge loss due to parasitic reactions.
In a secondary series of measurement, the cells are charged under identical critical circumstances with an additional relaxation phase between charging and discharging to investigate the influence of cell treatment after charging on the reversibility of LD.
The results of this study provide a comprehensive overview of opportunities and limits of detection methods for LD. The knowledge of the amount and reversibility of LD at certain circumstances facilitates the decision, whether a small capacity fade is acceptable to decrease charging duration.

References
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