A Discussion on the Unconventional Electrochemistry of Silicon Anodes
Authors: Rodrigues Marco-Tulio F 
Published: December 2024

Replacing graphite anodes with silicon can potentially increase cell energy by >20%. Performance of high-energy cells based on silicon was historically limited by mechanics, as Si particles would experience extensive fracturing that led to capacity fade. These concerns appear to have been mitigated, as data recently disclosed by several US-based manufacturers of Si-containing cells display superb capacity and energy retention over extended cycling.[1] Rather, this newly gained durability has brought to the fore issues with calendar aging,[1] which currently limits the wider adoption of high-energy Si cells. The Silicon Consortium Project has been tasked by the U.S. Department of Energy to help advance the science needed to enable Si anodes.

During our work as part of this Consortium, our team uncovered many ways in which the behavior of Si anodes stands apart from that of graphite. These differences are often very subtle and can affect aging trends and measurement output, causing common methods used for characterizing graphite cells to become unreliable when applied to silicon. This presentation will highlight some of these behaviors and discuss how changing the anode material can have broader consequences to how we study Li-ion cells.

For example, two cases among the topics of the presentation relate to properties of Si anodes at low states of lithiation (SOL). As cells lose capacity, the potential experienced by the anode at the end of discharge (EOD) will increase (Figure 1a), causing the anode properties at low SOL to increasingly contribute to measurable cell behavior. Two such properties of interest are: A) Si exhibits higher impedance at SOL (Figure 1b); and B) the voltage profile of Si has a lower slope than that of graphite at low SOL (Figure 1c). A consequence of property A is that Si cells can appear to present elevated impedance rise, when in reality the higher impedances are simply coming from the increased contributions of Si at low SOL, rather than being connected to any physical phenomena, as we demonstrate using 3-electrode experiments.[2]

The consequences of property B are even more subtle. An increase in anode potential implies that the anode is being delithiated by a larger extent. In other words, cell capacity loss will progressively expose the anode to lower SOLs, by removing extra Li+ from the anode and transferring them to the cathode. As a result, the more the cell loses capacity due to SEI growth, the more extra capacity is provided by the anode to offset some of these losses. Graphite electrodes have a nearly vertical voltage profile at typical EOD conditions, and so an increase in anode potential will release negligible amounts of extra Li+. On the other hand, Si anodes can hold a substantial amount of capacity at these high-potential regions of their voltage profile, causing a much larger offset in capacity. Due to this mechanism, capacity measurements in Si full-cells can yield artificially high values, masking as much as 60% of the Li+ loss to the SEI.[3] Accounting for the behaviors we discuss in this presentation is essential in the progress towards commercial high-energy Si cells.

Figure 1. a) Anode potentials measured at the end of discharge for cells containing Si electrodes. Terminal potentials increase as the cell loses capacity. Measurements performed using a 3-electrode cell. (Data extracted from Luo et al., ACS Appl. Energy Mater. 5, 5513 [2022].) b) Impedance of a Si anode in a full-cell as measured using high-current pulses in a 3-electrode cell. Impedance increases drastically at lower states of lithiation (higher anode potentials). c) Assumed voltage profiles for anodes during the discharge of hypothetical full-cells. At the discharge cutoff voltage, all anodes will initially experience a typical value of ~0.6 V vs. Li/Li+ (indicated by the intersection point). Regions to the right of that point indicate capacity that remains stored at the anode even when the full-cell is nominally fully discharged.

References

[1] McBrayer et al., Nat. Energy 6, 866 (2021)

[2] Klett et al., J. Electrochem Soc. 163, A875 (2016)

[3] Rodrigues, J. Electrochem Soc. 169, 080524 (2022)

Acknowledgments: This research was supported by the U.S. Department of Energy’s Vehicle Technologies Office under the Silicon Consortium Project, directed by Brian Cunningham, Thomas Do, Nicolas Eidson and Carine Steinway, and managed by Anthony Burrell.

Figure 1