Comprehensive Analysis of Float Current Behavior and Calendar Aging Mechanisms in Lithium‐Ion Batteries

This study presents a framework quantifying electrode-specific degradation in lithium-ion batteries using steady-state float currents, capacity-loss rates, and scaling factors from galvanostatic intermittent titration data. Two graphite-based 18650 cells with nickel-cobalt-aluminum and lithium iron phosphate cathodes were tested. Float current partitions into solid electrolyte interphase growth and cathode lithiation, both showing Arrhenius behavior.

Aiming to quantify degradation currents from solid electrolyte interphase formation (ISEIgrowth$\boldsymbol{I}_{\mathbf{SEI}~ \textbf{growth}}$) and gain of active lithium due to cathode lithiation (ICL$\boldsymbol{I}_{\mathbf{CL}}$), resulting from electrolyte decomposition, the float current behavior of lithium-ion batteries is investigated with different cathode materials. The float current, IFloat$\boldsymbol{I}_{\textbf{Float}}$, represents the recharge current required to maintain the cell at a fixed potential during calendar aging. This current arises as lithium is irreversibly consumed at the anode or inserted into the cathode, shifting the electrode potentials. To account for the asymmetric response of the electrodes, a voltage-dependent scaling factor, SF$\mathbf{SF}$, is introduced, derived from the slopes of the electrode-specific voltage curves. Using this factor in combination with measured float currents and capacity loss rates from check-up tests, ISEIgrowth$\boldsymbol{I}_{\mathbf{SEI}~ \textbf{growth}}$ and ICL$\boldsymbol{I}_{\mathbf{CL}}~$ is quantified at 30 °C across various float voltages. Although the SF$\mathbf{SF}$ and capacity data are limited to 30 °C, the model is extended to a range of 5–50 °C using only float current measurements. The results show that using capacity loss rates alone underestimate ISEIgrowth$\boldsymbol{I}_{\mathbf{SEI}~ \textbf{growth}}$ and that ICL$\boldsymbol{I}_{\mathbf{CL}}~$, contributes significantly to the observed float current at elevated voltages, indicating that cathode lithiation plays an increasingly important role in high-voltage calendar aging.

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