Interaction of Boron-Based Cross-Linkers with Polymer Binders for Silicon Anodes in Lithium-Ion Batteries

Intrinsic self-healing chemistries based on dynamic bonds have been shown to solve many issues in advanced applications, not the least batteries. Herein, we investigate the interactions within a polymer system containing dynamic covalent bonds as cross-linkers and their role when they are used as binders in silicon electrodes. We introduce 1,4-benzenediboronic acid (BDBA) as a cross-linking agent forming boronic ester groups and sodium tetraborate (borax) forming borate ester bonds with poly(vinyl alcohol) (PVA). Silicon electrodes with the cross-linked binders show improved electrochemical performance with a capacity of 1500 mA h g–1 after 200 cycles compared to PVA alone featuring 1000 mA h g–1. In contrast, another hydroxyl-containing polymer, carboxymethyl cellulose, able to form the same cross-linking functionalities, showed poorer performance with the addition of BDBA. The choice of polymer and cross-linker not only impacted the cell performance but also the electrode fabrication and morphology. The presence of cross-linkers decreased the electrode cracking of the pristine and cycled electrodes while having little effect on the solid electrolyte interphase composition. Hence, the properties of the polymeric binder system as a whole and the electrode manufacturing process have a significant impact on the cell performance. Solid-state NMR is shown to be a powerful technique to investigate the interaction between the different components and to confirm the formation of boronic ester bonds via the hydroxyl groups of PVA. Furthermore, the presence of silicon particles changes the chemical environment of the boron in BDBA favoring the formation of borate species, which are not present with PVA only, indicating interactions between BDBA and the silicon particles. Hence, PVA in combination with the boron-based cross-linkers, BDBA and borax, provide higher capacity and cycling stability than PVA alone, showing a promising approach to overcome the challenges of silicon anodes using binders with dynamic covalent bonds.