Towards better estimability of electrode-specific state of health: Decoding the cell expansion

Li-ion batteries are prone to adverse physical and chemical mechanisms that can degrade their performance over time. For this reason, identifying each electrode's capacity and utilization window is important for the safe operation of the battery, unfortunately the standard capacity estimation method cannot provide this. In this work, we introduce electrode-specific State of Health (eSOH) related parameters, including individual electrode capacity and utilization window. We explore the identifiability of the parameters using terminal voltage alone and voltage plus cell expansion measurements. The analysis here is based on the constrained Cramer-Rao Bound (CRB) formulation, which provides the error bounds for the parameters. The model utilizes the voltage/expansion functions of lithium stoichiometry for the individual electrodes based on the underlying physics of phase transitions. It is shown that slope changes in voltage and expansion that correspond to phase transitions in the electrodes enhance the estimation. As a result, with the addition of the expansion, the parameters are estimable without the need to discharge the battery to a high Depth of Discharge (>70%), which rarely happens in automotive applications. This makes eSOH estimation feasible for a wider range of real-world driving scenarios.