Self-Healing Binders for Silicon-Based Electrodes

Silicon is a promising active material for anodes in lithium-ion batteries owing to its high theoretical capacity (3579 mAh/g)¹. However, it is well known that the compound contends with large volume changes during cycling, creating cracks in the material and therefore limiting the lifetime and capacity of the cell. To cope with these volume changes, this project aims to develop self-healing binders to improve the cycling stability of lithium-ion batteries.<br/> <br/>Both hydrogen bonds and dynamic covalent bonds have been previously used to modify binder systems to get self-healing properties through reversible cross-linking with the binder in the electrode. While the hydrogen bonds have eminent reversibility, the dynamic covalent bonds provide high mechanical stability^2–4. Both of which is desirable properties to implement in the polymeric binder system.<br/> <br/>Specifically, the focus of this work has been on synthesizing borate ester bonds⁵, as well as using other commercially available borate groups and coupling them with the polymer binder poly(vinyl alcohol) (PVA). Furthermore, in order to gain more knowledge of the different self-heling groups, other binder systems containing hydrogen bonds have been used for comparison.<br/> <br/>The electrochemical performance of the cells with and without the self-healing groups has been investigated, showing an increased capacity with the latter. Besides the performance, understanding the self-healing mechanism of these binders and possible degradation reactions from these functionalities is key to further improve the system and cycle life. Therefore, the effect of the self-heling functional groups on the formation of the solid electrolyte interphase layer has been investigated with X-ray photoelectron spectroscopy. Furthermore, the electrochemical stability of the functional groups was investigated in order to compare the impact of the hydrogen bonding and dynamic covalent bonding, especially at the low operating voltages of silicon.<br/> <br/>1. Nguyen, C. C <i>et al.,</i> Systematic Investigation of Binders for Silicon Anodes: Interactions of Binder with Silicon Particles and Electrolytes and Effects of Binders on Solid Electrolyte Interphase Formation. <i>ACS Appl Mater Interfaces</i> <b>8</b>, 12211–12220 (2016).<br/>2. Chen, G <i>et al.,</i> Design of Robust Self-Healing Silicone Elastomers Based on Multiple H-Bonding and Dynamic Covalent Bond. <i>Langmuir</i> (2021).<br/>3. Dahlke, J <i>et al.,</i> How to Design a Self-Healing Polymer: General Concepts of Dynamic Covalent Bonds and Their Application for Intrinsic Healable Materials. <i>Adv Mater Interfaces</i> <b>5</b>, 1800051 (2018).<br/>4. Xu, J. H. <i>et al.,</i> Intrinsic self-healing polymers for advanced lithium-based batteries: Advances and strategies. <i>Appl Phys Rev</i> <b>7</b>, 031304 (2020).<br/>5. Cromwell, O. R <i>et al.,</i> Malleable and Self-Healing Covalent Polymer Networks through Tunable Dynamic Boronic Ester Bonds. <i>J Am Chem Soc</i> <b>137</b>, 6492–6495 (2015).