Effects of Overcharge Voltage on Capacity Degradation Behaviors in Graphite/Ncm Full Cells
Authors: Lim Chang Hun , Ryu Ji Heon 
Published: December 2024

As the electric vehicle (EV) market continues to expand in efforts to reduce CO2 emissions, so does the market for lithium-ion batteries. However, instances of overcharging can occur due to battery management system (BMS) inaccuracies or voltage discrepancies among cells. Overcharging can result in structural collapse and transition metal elution due to excessive delithiation from the positive electrode. Additionally, issues such as lithium plating at the negative electrode and electrolyte decomposition due to side parasitic reactions can arise. These problems collectively compromise the cycle life of lithium-ion batteries (LIBs), emphasizing the importance of understanding the effects of overcharging on the internal components of the cell.

It's widely understood that overcharging diminishes battery performance. This study aims to analyze the capacity degradation mechanism of cells when subjected to overcharging using coin-type full cells with LiNi0.6Co0.2Mn0.2O2 (NCM622) positive electrode and graphite negative electrode. The cells were cycled at a current of 0.1 C (170 mA/g for NCM622), with normal charging conditions (4.2 V), and subjected to overcharging at three voltage thresholds (4.35 V, 4.5 V, and 4.65 V). The analysis aimed to identify the causes of capacity degradation when the capacity dropped to 80% of its initial value. Three degradation models were employed for analysis: (i) loss of active material (LAM), involving the NCM material loss from structural collapse and elution; (ii) loss of lithium inventory (LLI), attributed to electrolyte decomposition from the irreversible reaction of lithium plating and solid electrolyte interphase (SEI) reformation; and (iii) polarization increase (PI), arising from factors such as film thickening or inadequate contact.

The cell degradation mechanism was analyzed in two ways. The first method involved calculating the predicted full cell data using half cell data. Using the voltage profile at the beginning-of-life (BoL) of NCM622 and graphite half cells, the contributions of LAM, LLI, and PI were applied differently to each to simulate voltage profiles and derivative capacity plot (dQ/dV) profiles of a full cell with 20% capacity degradation at the end-of-life (EoL) to be similar. Through this simulated data, it was possible to quantitatively calculate the contribution of the degradation model according to each overcharged voltage. The second method involves using the dV/dQ and dQ/dV profiles of the full cell. The alteration in the shape of these profiles was interpreted as the capacity degradation. A comparison was made for each degree of degradation of the full cell regarding the size and shifted voltage of the peaks in the dV/dQ and dQ/dV profiles. To validate the degradation contributions derived from the failure model, degraded full cells were disassembled, and positive and negative electrodes were fabricated and tested independently as half cells with Li metal counter electrode. The results obtained were compared against those derived from the model, confirming the accuracy of the degradation model calculations. This study provided deeper insights into the capacity degradation modes based on the extent of overcharging.