Silicon Anodes For Lithium-Ion Batteries

Silicon anodes are promising for high energy density lithium-ion batteries because of their high theoretical capacity (3579 mAh/g) and low potential of ~0.2 V vs. Li/Li+. However, silicon undergoes >300% volume changes during cycling. This causes delamination from the current collector and unstable solid electrolyte interphase (SEI) build-up, leading to rapid capacity fade during battery cycling. To address these issues, binder and conducting carbon, are added to the silicon anode. In this dissertation, we explored a new binder, conductive additive, and anode architecture, and also identified the interactions between the anode components that led to improved cycling performance. Binders improve cohesion between anode components and adhesion to the current collector. We demonstrated the use of tannic acid, a natural polyphenol, as a binder for silicon anodes. Tannic acid was explored as a small molecule binder with abundant hydroxyl (−OH) groups (14.8 mmol of OH/g of tannic acid). This allowed for the specific evaluation of hydrogen-bonding interactions without the consideration of particle bridging that occurs otherwise with high molecular weight long-chain polymers. The resultant silicon anodes demonstrated a capacity of ~850 mAh/g at 0.5 C-rate. Along with huge volume expansion, silicon has poor conductivity which requires the addition of hydrophobic carbon, thus effectively diluting the active silicon material. To address this issue, we used minimal amount of MXene nanosheets (4 wt% in the entire anode) to maximize total silicon anode capacity. We made silicon anodes using a composite binder of sodium alginate and MXenes that demonstrated capacities of ~900 mAh/g at 0.5 C-rate. The stable anode performance even with a minimal MXene content is attributed to homogenous electrode formation with improved interactions due to high conductivity, hydrophilicity, and large lateral size of MXene nanosheets. To stabilize SEI build-up, the contact between silicon and electrolyte should be minimized. Thus, we made a yolk-shell type structure by crumpling MXene nanosheets around silicon particles via a spray-dryer. Our electrodes made using crumpled MX/Si capsules demonstrated decent cycling capacities, while minimizing the electrode’s through-plane expansion. An in-house comparison of crumpled with uncrumpled anode showed that crumpling does improve cycling stability due to stable SEI formation