Impact of Hysteresis on Calendar Aging in 3 Electrode Li-Ion Cells with Silicon-Rich Anodes: OCV and Float Current Insights

Iqra Kiran, Qing Yu, Christian Endisch, Meinert Lewerenz
Research Group Electromobility and Learning Systems, Technische Hochschule Ingolstadt, D-85049, Germany; els@thi.de

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.
In this talk, we evaluate whether our current theory for graphite containing cells can be transferred to Si containing anodes. Therefore, float current analysis and open circuit experiments were performed. At first, we utilized laboratory scale 3-electrode cells (PAT-EL cells) with an anode composition of 69.7 wt% polycrystalline microscale silicon and 19.9 wt% graphite, while the NCA cathode has an active content of 96 wt%. After formation cycles, one group of cells was charged up to the target voltage, while another was discharged down to the target voltage, allowing us to explicitly examine the lithiation states for both charge and discharge paths. This approach is particularly relevant, given the pronounced hysteresis effects impacting the lithiation/delithiation behavior in each direction.
In the first float experiment, cell voltage was held constant to monitor the decay current over time and measure the anode potential individually as shown in Figure 1. Our results indicate that silicon-based anodes exhibit a significantly higher float current when approaching the target voltage from the charge direction compared to the discharge direction. A key observation is that when we convert the measured voltage-time data into voltage-charge curves, we observe that the subsequent voltage relaxation consistently follows a discharge trajectory, reflecting the lower voltage path typical of hysteresis effects. These findings suggest that the degree of lithiation may be more accurately aligned with discharge curves, offering a promising direction for understanding lithiation states in these anodes.

Figure 11: Float Test at 3.4V a) Float current measurement for cell at target voltage from charge and discharge direction b) anode voltage recorded with time, c) Transfer Voltage Vs. Time to Voltage Vs. Charge by using GITT Discharge curve d) Transfer Voltage Vs. Time to Voltage Vs. Charge by using GITT Charge curve.

In the second experiment, the OCV tests are conducted to measure the voltage decay of the full cell and the cathode as well as the voltage increase of the anode as shown in Figure 2. Consistently with float results, OCV measurements reveals greater capacity loss for cells charged to the target voltage rather than discharge. Furthermore, Galvanostatic Intermittent Titration Technique and checkups are performed on fresh cells and after experiment, to validate our results and voltage-slippery theory for cells with a strong hysteresis. The capacity loss results from checkup closely align with those calculated from OCV data (I_CapLoss= I_SEI- I_(CL )) for cells that reached the target voltage from the discharge direction, cell stored at 4.0V has highest capacity loss (17% ) as well as float current (500 nA).
Finally, the results of the 3-electrode tests are compared to 2-electrode 18650 cells containing a Si-graphite compound on the anode.
In the end, our results demonstrate that our theory is still valid for Si containing cells, however the evaluation is due to the voltage hysteresis effect more complex.

Figure 22: a) Full cell OCV measurement for six cell , one cell for charge and one for discharge at each voltage b)Measured Anode Voltage c) Transferred voltage vs. Time to voltage vs. charge/discharge curve for both anode, cathode curve, symbols with square are the full cell voltage values with time (aging) d) shift in cathode curves to match with x-axis (cell capacity) represents Capacity loss.

References
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