Looking for the Lost Lithium: Lithium Trapping and Its Effect on Capacity Losses in Li-Ion Batteries

Rechargeable lithium batteries are well-known to exhibit progressive capacity decays upon cycling, leading to loss of cell voltage, power and energy. However, the origins of capacity losses are still not fully understood and are largely attributed to the formation of a solid electrolyte interface layer and volume expansion. Another largely overlook capacity decay mechanism is lithium trapping 1–4. Lithium trapping, caused by incomplete delithiation, is here demonstrated to be the main source of capacity losses in high energy density anodes (e.g. Si) while SEI formation and dissolution affect the accumulated capacity loss due to decreased coulombic efficiency. In the following paper we will present our findings on capacity losses in standard and optimized Si-Li half cells and how these systems affect lithium trapping inside the active electrode material. We will discuss how the cycling protocol (e.g. full capacity cycling vs coulombic limited cycling) can mask signs of Li trapping and temporarily delay capacity decay in half-cells. We will highlight the importance of lithium-trapping induced capacity or voltage decays effect in lithium-based batteries, explain the mechanism for lithium-trapping and establish diffusion-controlled lithium-trapping models. Approaches toward identifying and circumventing lithium-trapping problem will also be discussed as well as challenges and proposed future research directions within this exciting and important research field. Figure 1. Capacity evolution for an optimized nano-Silicon composite electrode in a half-cell configuration during coulombic limited cycling (i.e. 1200 mAh/g). Critical capacity decay is observed when the accumulated trapped lithium has saturated the electrode core and counteracts the deposited Li layer 1. References 1. Lindgren, F. et al. On the Capacity Losses Seen for Optimized Nano‐Si Composite Electrodes in Li‐Metal Half‐Cells. Adv. Energy Mater. 9, 1901608 (2019). 2. Rehnlund, D. et al. Lithium trapping in alloy forming electrodes and current collectors for lithium based batteries. Energy Environ. Sci. 10, 1350–1357 (2017). 3. Rehnlund, D., Pettersson, J., Edström, K. & Nyholm, L. Lithium Trapping in Microbatteries Based on Lithium- and Cu2O-Coated Copper Nanorods. ChemistrySelect 3, 2311–2314 (2018). 4. Zhu, B. et al. Minimized lithium trapping by isovalent isomorphism for high initial Coulombic efficiency of silicon anodes. Sci. Adv. 5, eaax0651 (2019). Figure 1