Lithium-ion batteries (LIBs) with nickel-rich cathodes and silicon-graphite (SiC) anodes are expected to be deployed in the next generation electric vehicles (EV) due to their high specific energy density and price advantages. However, recent data from companies working at developing Si-rich anodes suggests that high-energy Si cells (>300 Wh kg−1) tend to present particularly low calendar lifetimes, suggesting that this is, besides volume expansion, a crucial technical barrier preventing near-term commercialization of LIBs with high Si content1.
Float current analysis is a method to measure the calendar aging by precisely measuring the currents to keep the voltage constant. The float currents are linked to the capacity loss rate after anode overhang effect is concluded. To quantify this relationship, a scaling factor for the anode and cathode is necessary which is calculated by the ratio of the slopes of the anode and cathode to the full cell. Multiplying the aging currents like SEI current with the scaling factor, one obtains the measured float current. This method is already validated with graphite containing cells and will be applied for silicon-graphite composites in the following2, 3.
Si-containing anodes are susceptible to rapid degradation due to severe volume changes, as it undergoes significant phase changes during the lithiation and delithiation processes. These transformations involve moving between different lithium-silicon phases with differing electrochemical potentials (like amorphous LixSi and crystalline Li15Si4) 4. Consequently, the voltage profiles during charge and discharge forming a strong voltage hysteresis. Thus, approaching a constant voltage from charge and discharge direction, leads to deviating anode potentials and with this to deviating cathode potentials.
This study investigates the impact of hysteresis on calendar aging in 3-electrode Li-ion cells with Si-rich anodes using float current and OCV analysis. Results show higher float currents when approaching target voltage from the charge direction due to strong voltage hysteresis effects. OCV tests confirm greater capacity loss for charged cells, highlighting the complex degradation mechanisms of Si anodes. While the float current method remains a useful tool, quantifying aging currents in Si-graphite cells is more complex due to hysteresis, requiring further optimization for accurate evaluation.

References
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2. M. Lewerenz, S. Käbitz, M. Knips, J. Münnix, J. Schmalstieg, A. Warnecke and D. U. Sauer, Journal of Power Sources, 353, 144–151 (2017).
3. M. Azzam, C. Endisch and M. Lewerenz, Batteries, 10(1), 3 (2024).