Calendar aging of lithium-ion batteries is mainly driven by cell voltage and temperature. The aging of the cell is attributed to mechanisms such as SEI formation, electrolyte dissolution on the cathode or even reversible shuttle reaction that lead to a voltage decay. To keep the cell at a desired voltage and avoid the strong influence of anode overhang, the cells are held potentiostatically for more than 30 days at 30°C. This steady-state forms the basis for the comparison of the two methods: open circuit voltage measurement OCV vs. potentiostatic hold measuring the float currents. In previous publications we found a strong correlation of the float current to capacity loss.
In the following we will discuss the pros and cons of both methods and if they deliver the same results. OCV and float current can be transferred into each other by the local dU/dQ value at the float voltage by the formula:
dU/dt = I x dU/dQ
In our experiment we investigated different cell types and cell voltages. To further evaluate the temperature dependence of dU/dQ, we additionally applied temperature ramps from 5 to 50°C with different temperature velocities to the steady-state float current and OCV measurements as it is shown in the attached graph on the left. With the formula and the measured OCV and float currents, we can then calculate the corresponding dU/dQ curves for the different temperature ramps which are illustrated in the graph on the right. We observe a clear temperature dependence of the dU/dQ curves, and it generally fits well to the measured dU/dQ values with a low current measurement at 5°C, 25°C and 50°C (red stars in the graph).
As a result, we found a significant temperature dependence of the dU/dQ at given voltage that generally fits best to the local dU/dQ value measured at low currents while the standard differential voltage curve at C/10 shows strong deviations. However, the local dU/dQ exhibits still some deviations and the measurement for various temperatures is quite an effort. Deviations will result from differences in the voltage decay and the float current method regarding entropy-induced SOC-change and lowering SOC during voltage decay during aging. The impact of the ramp speed itself is not relevant for lower velocities and for higher velocities the transient parts at the begin of the ramps become significant distorting the measurement.
As a conclusion, measuring float currents is more effort regarding the quality of the measurement equipment compared to the simple OCV measurement. However, the evaluation of OCV is more complicated as the temperature dependency of the local dU/dQ values must be measured before. This causes errors transferring OCV to float currents. In general, we emphasize that both methods come to the same results within the here measured voltages and temperatures, and we can exclude any additional triggering of aging mechanisms even at high cell voltages caused by potentiostatic hold. Regarding entropy effect, one must have a closer look on the respective experiment as they are are differently affected by entropy effect, anode overhang effect or aging.