A Comprehensive Study of Hydrolyzed Polyacrylamide as a Binder for Silicon Anodes.

Silicon anodes have a high theoretical capacity for lithium storage, but current composite electrode formulations are not sufficiently stable under long-term electrochemical cycling. The choice of polymeric binder has been shown to impact stability and capacity of silicon anodes for electrochemical energy storage. While several promising polymeric binders have been identified, there is a knowledge gap in how various physicochemical properties-including adhesion, mechanical integrity, and ion diffusion-impact electrochemical stability and performance. In this work, we comprehensively investigate the physical properties and performance of a molecular-weight series (3-20 × 106 g/mol) of partially hydrolyzed polyacrylamide (HPAM) in silicon anodes. We quantify the mechanical strength, electrolyte uptake, adhesion to silicon, copper, and carbon, as well as electrochemical performance and stability and find that HPAM satisfies many of the properties generally believed to be favorable, including good adhesion, high strength, and electrochemical stability. HPAM does not show any electrolyte uptake regardless of any molecular weight studied, and thin films of mid- and high-molecular-weight HPAM on silicon surfaces suppress lithiation of silicon. The resulting composite electrodes exhibit an electrochemical storage capacity greater than 3000 mAh/g initially and 1639 mAh/g after 100 cycles. We attribute capacity fade to failure of mechanical properties of the binder or an excess of the solid electrolyte interphase layer being formed at the Si surface. While the highest-molecular-weight sample was expected to perform the best given its stronger adhesion and bulk mechanical properties, we found that HPAM of moderate molecular weight performed the best. We attribute this to a trade-off in mechanical strength and uniformity of the resulting electrode. This work demonstrates promising performance of a low-cost polymer as a binder for Si anodes and provides insight into the physical and chemical properties that influence binder performance

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

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

Mixed Ionic–Electronic Conduction Increases the Rate Capability of Polynaphthalenediimide for Energy Storage

Conjugated polymers offer a number of unique and useful properties for use as battery electrodes, and recent work has reported that conjugated polymers can exhibit excellent rate performance due to electron transport along the polymer backbone. However, the rate performance depends on both ion and electron conduction, and strategies for increasing the intrinsic ionic conductivities of conjugated polymer electrodes are lacking. Here, we investigate a series of conjugated polynapthalene dicarboximide (PNDI) polymers containing oligo(ethylene glycol) (EG) side chains that enhance ion transport. We produced PNDI polymers with varying contents of alkylated and glycolated side chains and investigated the impact on rate performance, specific capacity, cycling stability, and electrochemical properties through a series of charge–discharge, electrochemical impedance spectroscopy, and cyclic voltammetry measurements. We find that the incorporation of glycolated side chains results in electrode materials with exceptional rate performance (up to 500C, 14.4 s per cycle) in thick (up to 20 μm), high-polymer-content (up to 80 wt %) electrodes. Incorporation of EG side chains enhances both ionic and electronic conductivities, and we found that PNDI polymers with at least 90% of NDI units containing EG side chains functioned as carbon-free polymer electrodes. This work demonstrates that polymers with mixed ionic and electronic conduction are excellent candidates for battery electrodes with good cycling stability and capable of ultra-fast rate performance