Understanding Anode Capacity Fade in Symmetric Cells

Symmetric cells have been developed to evaluate electrode degradation directly associated with solid electrolyte interphase (SEI) growth. This limits their applications to some certain anodes, such as graphite and Li 4 Ti 5 O 12 . 1 In this work, symmetric cells have been applied to understand the degradation of high energy density anodes, such as Si-alloys, which undergoes multiple degradation mechanisms, including mechanical failure, SEI growth, and excess capacity. The excess capacity is related to the reversible capacity changes, caused by the electrode upper endpoint potential shift. A Li inventory model is developed to interpret anode capacity fade in symmetric cells. The Li inventory model attempts to establish a mathematical relationship between multiple anode degradation processes and measured experimental data, such as cycling charge/discharge capacity. This model allows for the measurement of coulombic efficiencies of individual components of blended electrodes (e.g. alloy + graphite). Figure 1 illustrates the main reactions and side reactions during symmetric cell discharge at i th cycle. Except the main reactions in working electrodes about lithiation and delithiation, the side reactions, as mentioned above, have been included. In this work, a Si-alloy, Si 80 W 20 , 2 was used as an example. One major benefit of symmetric cells is to measure the coulombic efficiency (CE) of electrodes with standard chargers. 3 After deconvolution, the total irreversible capacity (IC) per cycle due to surface reactions on Si 80 W 20 can be obtained from graphite and Si-alloy/graphite blended electrodes. The temperature effects on IC can also be obtained, as summarized in Figure 2 . Reference J. C. Burns et al., J. Electrochem. Soc. , 158 , A1417–A1422 (2011). Y. Liu, B. Scott, and M. N. Obrovac, J. Electrochem. Soc. , 166 , A1170–A1175 (2019) 10.1149/2.0851906jes. Z. Yan and M. N. Obrovac, J. Electrochem. Soc. , 164 , A2977–A2986 (2017). Figure 1 (a) An example of potential curves of electrode A, electrode B, and the symmetric cell during symmetric cell discharge. Illustrations of main and side reactions occurred at electrode A and electrode B during symmetric cell discharge at i th cycle are illustrated in (b) and (c), respectively. The capacity for each reaction during a single symmetric cell discharge/charge process are labelled in the following brackets. Li-Eld , Eld , and Elyt stand for lithiated electrode, delithiated electrode, and electrolyte, respectively. Figure 2 A summary of the average IC of graphite (MAG-E, 20 μm average size, Hitachi) and on Si 80 W 20 per cycle normalized with respect to reversible capacity at different temperatures, as derived from symmetric cells with graphite and alloy/graphite blended electrodes. Figure 1