The energy density of conventional lithium-ion batteries with graphite anodes is getting close to its theoretical maximum. One of the most promising approaches to significantly increase the energy density of lithium-ion batteries is to swap out the typical graphite anodes with high-capacity silicon-based anodes. The practical use of silicon anodes is, however, severely constrained by the inherent enormous volume expansion of silicon-based materials after lithiation and the ensuing series of unsolvable issues, such as unstable solid electrolyte interphase layer, polymer binders electrode cracking, and particularly, the rapid capacity degradation of cells. 

Reducing the active particle size to the nanometre range results in higher reactivity due to the higher percentage of surface atoms compared to bulk silicon particles. Nanosized particles can accommodate greater stress without cracking, and encasement, coatings, or other methods that limit surface-electrolyte contact can further mitigate this. Many studies have also introduced carbon coating of silicon nanoparticles to improve battery integrity. However, even by shrinking the size of the silicon particles and integrating some level of structural rigidity, some of the inherent flaws of adopting silicon are still present.

Several studies over the past ten years have shown that polymer binders are essential for reducing volume expansion, maintaining the integrity of silicon-based anodes, and ensuring stable cycling. Manufacturers can produce polymer binders from natural or synthetic sources, each of which offers benefits and shortfalls. For instance, synthetic polymer binders have several benefits compared to natural polymer binders, such as a structure-optimised design and functionality-tuned capabilities for generating high-performance silicon electrodes. Furthermore, researchers can tune the obtained conductive and self-healing synthetic polymer binders to enhance both the mechanical strength and electrical integrity of silicon electrodes.

Innovations in Polymer Binders for Silicon Nanoparticle Batteries

To strengthen the electrode structure during lithiation and increase the electronic conductivity of silicon nanoparticles, scientists at North Carolina State University in the US developed a hydrogel binder. This binder is composed of carbon black and guar, chemically cross-linked using glutaraldehyde. The researchers observed the cross-linking reaction using dynamic rheological measurements, demonstrating the significance of rheology in binder performance. In the presence of carbon black, the cross-linking reaction proceeds more quickly and results in stronger networks, as shown by the greater gel elastic modulus in guar-plus-carbon black gels than in guar gels alone.

Silicon nanoparticle electrodes that use binders with low cross-link densities (trxn < 2 days) demonstrate discharge capacities of approximately 1200mAhg–1 and Coulombic efficiencies >99.8% after 300 cycles. Low cross-link densities probably increase the capacity of silicon nanoparticle anodes due to interactions between the binder and silicon that allow for volume expansions. Additionally, polymer binders the cross-linked binder showcases the potential for self-healing. The enhanced elastic modulus observed after mechanically fragmenting the gel provides evidence of its ability to help preserve the electrode microstructure during lithiation and elevate capacity retention.

Researchers from Dresden University in Germany have investigated the suitability of Polyvinyl Butyral (PVB) polymers as a binder component for lithium-ion batteries based on silicon nanoparticles. Commercial PVB and Polyacrylic Acid (PAA) polymers’ distinctive binder characteristics were contrasted. The research focused on PVB and PAA polymer blends for an enhanced binder composition that combines their respective benefits. Various polymer ratios were carefully investigated to comprehend the impact of specific polymer chains, functional groups, and mass fractions on the electrochemical performance. Some combinations demonstrated high performance, strong adhesion to copper foil, and good binder-particle interaction. A PAA/PVB-based electrode with a silicon loading of approximately 1mg/cm2 tested between 0.01V and 1.2V vs. Li/Li+ demonstrated specific capacities as high as 2170mAhg-1 after 100 cycles.

Silicon Anode Binder Breakthroughs: Conducting Polymer Hydrogel and Water-Soluble Polyimide:

Stanford University’s Department of Materials Science and Engineering in the US conducted a research study in which they incorporated a conducting polymer hydrogel into silicon-based anodes. The researchers achieved this by performing in-situ polymerization of the hydrogel, resulting in a well-connected three-dimensional network structure of silicon nanoparticles coated with the conducting polymer binders. A continuous electrically conductive polyaniline network bonded to the silicon surface through either the crosslinker’s hydrogen bonding with phytic acid or electrostatic interaction with the positively charged polymer, and porous space for silicon particle volume expansion are all included in this hierarchical hydrogel framework. With this anode, they demonstrated a cycle life of 5,000 cycles with over 90% capacity retention at a current density of 6.0Ag−1. 

Scientists in the Advanced Materials Division, Korea Research Institute of Chemical Technology, Korea, synthesized an eco-friendly water-soluble polyimide (W-PI) precursor named poly(amic acid) salt (W-PAmAS) as a binder for silicon anodes. They accomplished this through a straightforward one-step process that employed water as a solvent. The researchers further utilized the W-PAmAS binder to create a composite silicon electrode through low-temperature processing at 150°C. Adding 3,5-diaminobenzoic acid, which has free carboxylic acid (-COOH) groups in the W-PAmAS backbone, increased the adhesion between the electrode components.

The -COOH on the surface of the W-PI binder chemically bonded with the silicon nanoparticles, forming ester bonds that remain effective even during extreme volume fluctuations. After 200 cycles at 1200mAg1, the silicon anode with W-PI binder showed remarkable electrochemical performance with a high capacity of 2061mAhg-1 and excellent cyclability of 1883mAhg-1. Hence, W-PI can serve as a highly effective polymeric binder in high-capacity lithium-ion batteries based on silicon nanoparticles.

Self-Healing Binder Elevates Silicon Anode Performance:

Using a self-healing composite polymer binder for the silicon particles, researchers from the Japan Advanced Institute of Science and Technology have enhanced the performance of silicon anodes in lithium-ion batteries. The new binder enhances stability while maintaining a thin solid electrolyte interphase layer. This binder comprises a polymer composite consisting of a carboxylate-containing polymer known as poly(acrylic acid) (PAA) and an n-type conducting polymer named poly(bisiminoacenaphthenequinone) (P-BIAN). These two components are linked through hydrogen bonds. The composite polymer structure acts as a net to hold the silicon nanoparticles together and keeps them from disintegrating.

Since the two polymers can reconnect themselves if they separate at any moment, the hydrogen bonds between them allow the structure to self-heal.Researchers tested the binder by using an anodic half-cell consisting of silicon nanoparticles with graphite (Si/C), the binder (P-BIAN/PAA), polymer binders and a conductive additive known as Acetylene Black (AB).We repeatedly conducted charge-discharge cycles on the Si/C/(P-BIAN/PAA)/AB anode. The P-BIAN/PAA binder stabilized the silicon nanoparticle anode and sustained a specific discharge capacity of 2100mAhg-1 for more than 600 cycles. In contrast, the bare silicon nanoparticle anode’s capacity decreased to 600mAhg-1 in just 90 cycles.

The Future of Polymer Binders for Silicon Nanoparticle Batteries

The eighth-most plentiful substance on earth, silicon, will be a potential, eco-friendly replacement for graphite as anode in lithium-ion batteries as the demand for these batteries rises. Despite their infrequent use, polymer binders are essential for maintaining the stability and integrity of the electrodes in silicon-based lithium-ion batteries. Over the past ten years, researchers have developed numerous polymer binders with diverse structures, properties, functionalities, conductivities, and flexibilities, derived from various chemical reactions and sources (natural or synthetic polymers).Researchers have created these binders to enhance the stable cycling and practical application of silicon-based anodes in high-energy-density lithium-ion batteries.

Researchers have proposed 3D interconnected polymer binders for future lithium-ion batteries using silicon nanoparticles. These binders can be categorized into covalently and dynamically crosslinked polymer binders based on the reversibility of the crosslinking bonds. Many believe that this will additionally enhance the binding strength and efficiency in buffering the volume expansion of silicon-based anodes, a critical factor in the utilization of these high-energy-density batteries.