SoH estimation using Differential Capacity Analysis in Li-ion batteries

Image Reference: Kurzweil P, Scheuerpflug W, Frenzel B, Schell C, Schottenbauer J. Differential Capacity as a Tool for SOC and SOH Estimation of Lithium Ion Batteries Using Charge/Discharge Curves, Cyclic Voltammetry, Impedance Spectroscopy, and Heat Events: A Tutorial. Energies. 2022; 15(13):4520.

Presently, evaluating State of Health (SoH) often involves full charge-discharge cycling of lithium-ion batteries under controlled conditions to map out the capacity & internal resistance changes over time. Such methods induce additional stress & degrade battery life.

The quest for less invasive methodologies has led to the development of differential capacity based analysis, as researched by Kurzweil P et. al. The method by which differential capacity, expressed as dQ/dU and understood as capacitance, derived from discharge curves, impedance spectroscopy, and cyclic voltammetry, can be adeptly employed for the immediate diagnosis of li-ion batteries.

This approach facilitates the assessment of battery conditions without the necessity for a complete charge & discharge cycle, offering a more efficient & potentially less degradative means of determining battery vitality.

Differential capacity analysis is a promising, non-destructive approach that monitors subtle changes in a battery's behavior during normal operation. It revolves around measuring the dQ/dU which is the derivative of the charge with respect to the voltage during battery operation. By plotting it against voltage, researchers can identify characteristic peaks that correlate with specific SoC levels, electrochemical reactions, and phase transitions within the battery.

This approach offers a window into the battery's internal chemistry without necessitating full charge-discharge cycles. For instance, the height & position of the peaks in the dQ/dU plot can signal the loss of active material or the onset of undesirable side reactions, both indicative of SoH. Furthermore, monitoring the evolution of these peaks over time can reveal the rate of battery degradation. A peak may correspond to a phase transition within the electrode material or indicate the voltage at which intercalation is most active. Shifts in these peak positions over time can point to changes in the electrode structure or composition, while alterations in peak magnitude may signal a reduction in the electrochemically active surface area or lithium inventory within the battery.

What makes differential capacity analysis particularly appealing is its sensitivity to early signs of degradation. With minimal intervention, this method aligns with the industry's shift towards predictive health monitoring & proactive maintenance strategies.