Understanding the degradation mechanisms of lithium-ion batteries is crucial for improving their performance and safety. Such studies could be accelerated by exploring the impact of different stress-inducing scenarios on battery performance and degradation mechanisms. In research conducted by Kemeny, Ondrejka, & Mikolasek [1], a comprehensive analysis of NCA Li-Ion batteries' degradation was carried out using various electrochemical characterization methods.
Exploring Individual Scenarios:
1. The Reference Scenario:
In the Reference Scenario, the battery exhibited a slow, linear decrease in capacity over 126 cycles. The degradation was primarily induced by cycling between 0% & 100% State of Charge (SOC). The stability of the Solid Electrolyte Interface (SEI) layer suggested stable battery performance. However, increasing values of RCHT (charge transfer resistance) pointed to microstructural and/or phase changes, supported by decreasing Li+ diffusion. The degradation of the cathode active material was found to be the main contributor.
2. The Under-Charge Scenario:
Under-charging the battery resulted in a similar capacity decrease trend compared to the Reference Scenario. Higher degradation was observed due to over-discharging, which induced additional stress on the graphite electrodes. The analysis highlighted the dominance of LAMliPE (lithium-ion absorption mode with limited power efficiency) & LLI (loss of lithium inventory) as the primary degradation modes.
3. The Over-Charge Scenario:
Unique to the Over-Charge Scenario, the battery experienced a two-stage capacity fade, with a rapid decline leading to battery failure. Conductivity losses, indicated by increasing ROHM (ohmic resistance), were observed due to the decomposition & deposition of transition metals on the SEI interface. The degradation of the cathode active material played a significant role in this scenario, with LLI & LAMliPE as the dominant degradation modes.
4. The High-Current Charging Scenario:
This scenario exhibited the most severe degradation, particularly on the cathode side of the battery. High-current charging induced CO2 & O2 gas generation, resulting in increased internal pressure, electrolyte depletion & conductivity loss. The anode showed only insignificant degradation. The analysis revealed structural changes & microcracking in the cathode material.
Cycling between extreme SOC levels, over-discharging, & over-charging were identified as significant contributors to battery degradation. The findings emphasize the importance of proper charge management & SOC control to mitigate degradation.
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