Tannic Acid as a Small-Molecule Binder for Silicon Anodes

Increasing demand for portable electronic devices, electric vehicles, and grid scale energy storage has spurred interest in developing high-capacity rechargeable lithium-ion batteries (LIBs). Silicon is an abundantly available anode material that has a theoretical gravimetric capacity of 3579 mAh/g and a low operating potential of 0–1 V vs Li/Li⁺. However, silicon suffers from large volume variation (>300%) during lithiation and delithiation that leads to pulverization, causing delamination from the current collector and battery failure. These issues may be improved by using a binder that hydrogen bonds with the silicon nanoparticle surface. Here, we demonstrate the use of tannic acid, a natural polyphenol, as a binder for silicon anodes in lithium-ion batteries. Whereas the vast majority of silicon anode binders are high molecular weight polymers, tannic acid is explored here as a small molecule binder with abundant hydroxyl (−OH) groups (14.8 mmol of OH/g of tannic acid). This allows for the specific evaluation of hydrogen-bonding interactions toward effective binder performance without the consideration of particle bridging that occurs otherwise with high molecular weight polymers. The resultant silicon electrodes demonstrated a capacity of 850 mAh/g for 200 cycles and a higher capacity when compared to electrodes fabricated by using high molecular weight polymers such as poly­(acrylic acid), sodium alginate, and poly­(vinylidene fluoride). This work demonstrates that a small molecule with high hydrogen-bonding capability can be used a binder and provides insights into the behavior of small molecule binders for silicon anodes.