Structural design strategies of polymer binders for Silicon-based anodes in lithium-ion batteries

Interaction mechanisms between polymer binders and electrode constituents

Polymer binders used in battery electrodes are applied in small quantities, typically below 3 wt% of the total electrode composition. Despite their limited proportion, significant research has been devoted to enhancing high-energy battery performance by engineering the chemical properties of polymeric binders. A comprehensive understanding of both the functional roles of binders in LIB electrodes and the operational mechanisms of Si anodes is essential for developing advanced binder systems that improve electrochemical performance. Various binder materials have been designed through structural tailoring and the incorporation of functional moieties to achieve high elasticity, strong interfacial adhesion, and mechanical flexibility. The following sections explore these roles in greater detail within the context of Si-based anodes. Although the adhesive mechanisms of binders are inherently complex and cannot be fully described by a single model, the most widely accepted binding mechanisms include mechanical interlocking, chemical bonding, van der Waals interactions, and electrostatic forces^[52]. When binders interact with active material and conductive additive particles, the system typically forms three distinct interfacial layers^[53,54]. First, upon dispersion in a solvent, the binder spreads uniformly across particle surfaces, forming a primary adhesion layer. Surrounding this, a secondary immobilized layer is established, where molecular-level interactions anchor adjacent polymer chains, reinforcing structural cohesion. Finally, residual free polymer chains give rise to an outer surplus layer, which contributes to the overall mechanical and chemical properties of the binder system. The specific configuration and properties of these interfacial layers are largely governed by the inherent characteristics of the polymer and the surface chemistry of the contacting materials. Variations in polymer composition, molecular weight, and functional groups can significantly influence how the binder interacts with different particle surfaces, thereby affecting interfacial stability and electrode performance [Figure 2A]^[52]. Beyond this framework, several complementary theories such as adsorptive interactions, wetting dynamics, and acid-base interactions have also been proposed to further explain the complexity of binder adhesion mechanisms^[52]. In the case of Si anodes, this topic becomes even more critical. Si can expand by up to 300% during cycling, leading to severe mechanical disruption of the electrode and rapid capacity degradation. Understanding the precise bonding mechanisms at play is therefore essential for developing binders capable of accommodating these extreme volume fluctuations without compromising electrode integrity.

Figure 2. Schematic illustration of the functional roles of polymer binders in electrodes (A) Binding action; (B) Mitigation of volume expansion; (C) Dispersion of active materials; (D) Facilitation of ionic conductivity; (E) Enhancement of electronic conductivity; (F) Contribution to SEI formation. The original figure was created by the authors.

Maintaining structural integrity of Si electrodes with binders

The mechanical properties imparted by polymer binders play a critical role in accommodating the substantial volumetric fluctuations experienced by Si electrodes during cycling [Figure 2B]. To effectively mitigate the mechanical stress induced by expansion and contraction, binders must exhibit key characteristics, such as tensile strength, adhesive strength, flexibility, and elasticity^[55]. When the interaction between the binder and Si particles is sufficiently strong, the resulting interfacial adhesion can exceed the stress generated by volume expansion, thereby stabilizing the electrode interface and maintaining structural integrity over repeated cycles. In addition to mechanical reinforcement, another essential function of polymer binders lies in their ability to disperse electrode components uniformly throughout the slurry. The primary role of a binder is to uniformly bind the active material and conductive agents via dispersion in solvent and to facilitate the formation of a homogeneous electrode slurry that coats the current collector evenly during the drying process [Figure 2C]. One of the critical challenges during slurry preparation is the propensity of Si particles to aggregate with conductive additives, which typically exhibit high specific surface area and fine particle size. This non-uniform dispersion can disrupt the uniformity of the electrode architecture and impair electrochemical performance. Therefore, polymer binders should be designed to promote homogeneous distribution of all electrode components throughout the mixing process.

In particular, polymers containing aromatic moieties are highly effective in dispersing carbon-based conductive agents via π-π stacking interactions between the aromatic rings of the binder and the graphitic surfaces of the additives. These noncovalent interactions suppress aggregation and promote uniform dispersion, thereby enhancing the formation of a continuous electronic network. As a result, both electronic and ionic transport within the electrode are improved [Figure 2D and E]. Given the importance of these functions, a wide range of polymeric materials has been employed as binders in LIB electrodes, each offering distinct structural and chemical properties.

Moreover, the solid electrolyte interphase (SEI) formed on the surface of active material critically influences the electrochemical behavior of LIBs [Figure 2F]. In Si anodes, continuous volume changes expose new surfaces, leading to ongoing SEI formation. The composition of the SEI layer can vary depending on the electrolyte used, and the resulting surface chemistry can affect the adhesion of the binder. The addition of binders plays a key role in promoting uniform dispersion of electrode components, which contributes not only to a more homogeneous SEI but also to a more stable interfacial structure. A uniformly distributed SEI is beneficial in reducing surface resistance during electrochemical reactions. In Si anodes, the SEI is primarily composed of reduction products such as Li₂CO₃, LiF, and Li₂O^[54]. Among these, LiF is particularly important for maintaining the mechanical and chemical stability of the SEI^[56]. Therefore, the formation of a stable SEI of appropriate thickness on the active material surface is critical for maintaining electrode capacity and prolonging cycle life. In addition to mechanical stability and SEI formation, the chemical stability of polymer binders toward liquid electrolytes is a critical factor affecting the long-term performance of Si anodes. During extended cycling, binders may undergo swelling, partial dissolution, or degradation due to reactions with electrolyte components, such as LiPF₆ salt and carbonate solvents. Such degradation can weaken interfacial adhesion and disrupt the electronic network within the electrode. Functional groups in the binder, including -OH, -COOH, and amide moieties, can interact with electrolyte additives, such as fluoroethylene carbonate (FEC), influencing SEI composition and stability. As reported in a recent study by An et al.,^[57] optimizing the chemical compatibility between the binder and electrolyte is essential to suppress undesirable side reactions and maintain the mechanical integrity of the electrode. Therefore, designing binders with enhanced chemical resistance, reduced solubility in electrolyte, and the capability to maintain robust adhesion under electrochemical stress is critical for improving both cycling stability and coulombic efficiency of Si-based electrodes. Polymer binders aid in this process by providing mechanical support, preventing interfacial collapse during repeated volume changes of Si^[58]. Following the discussion on the mechanical properties and dispersibility of polymer binders, it is also essential to examine how molecular-level interactions, particularly those involving functional groups, govern the performance of Si anodes.

For Si anodes, polymers have been actively engineered not only to improve electrolyte wettability and ionic/electronic conductivity, but also to reinforce mechanical robustness and electrochemical stability, both of which are crucial for managing drastic volume changes during cycling. Therefore, the molecular architecture of the binder polymer is a key determinant of electrode performance.

Natural polymer binders

In this subsection, we focus on naturally derived polysaccharide binders, which offer environmental benefits, aqueous processability, and abundant functional groups for Si adhesion. We discuss their advantages, performance limitations at high Si loading, and industrial considerations for large-scale implementation.

Naturally derived polysaccharides (e.g., CMC, chitosan, alginate, carrageenan) provide dense networks of -OH/-COOH^[63] groups that enable multi-point adsorption on hydroxylated Si surfaces via hydrogen bonding and, under appropriate processing, esterification. Compared with synthetic binders, their advantages are low cost, aqueous processability, and benign rheology for large‐area coating^[65]. However, their molecular-weight distributions and batch variability can lead to dispersion inconsistency and viscosity drift at scale. Across the literature, three common behaviors emerge: (i) cellulose- or alginate-type binders improve early-cycle cohesion and facilitate more uniform SEI formation; (ii) performance gains diminish at higher Si fractions unless elasticity and interparticle bridging are reinforced; and (iii) slurry stability is sensitive to the degree of substitution/oxidation and to the presence of co-binders or salts. These trends indicate that polysaccharides are highly effective as industrial baselines for low-to-moderate Si loading, while high-loading electrodes require either compositional hybridization or architectural modification to sustain mechanical compliance and stable interfaces over long cycling.

From a manufacturing viewpoint, we find that controlling substitution/oxidation levels and ionic strength is as important as choosing the polysaccharide family itself, because these parameters dictate slurry stability and coating uniformity at line scale. Representative examples include Carboxymethyl cellulose (CMC) [Figure 4A]^[16,65,66], Chitosan (CS) [Figure 4B]^[67,68], Sodium alginate (SA) [Figure 4C]^[69], and Lambda (λ)-carrageenan (L-CGN) [Figure 4D]^[70,71]. Among them, CS is one of the most abundant natural polysaccharides, similar to CMC. To overcome its poor water solubility, Lee et al. developed a network binder by incorporating epoxidized natural rubber (ENR) into chitosan. When applied to Si anodes, this binder maintained a high specific capacity of approximately 2,310 mAh g^-1 even after 500 cycles [Figure 4E]^[72]. This performance was attributed to the synergistic effect of strong adhesion and elastic buffering in the polymer network. Polysaccharide-based binders can form not only hydrogen bonds but also covalent ester bonds through condensation reactions between the carboxyl groups in the binder and the hydroxyl groups on the Si surface. CMC, a representative linear binder, is widely used in commercial applications, typically in combination with styrene-butadiene rubber (SBR)^[16]. Owing to its excellent thickening and dispersing properties, low cost, safety, and eco-friendliness, CMC/SBR binders have been commonly adopted. Ko et al. demonstrated that a graphite/a-Si nanolayer composite electrode using 4 wt% CMC/SBR maintained a reversible capacity of 450-500 mAh g^-1 for more than 100 cycles^[73]. This performance was ascribed to the high elongation of the binder, strong adhesion, and formation of a stable SEI layer. CMC offers reliable thickening and dispersion in water, strong polar interactions with Si, and compatibility with SBR in commercial lines. Its main limitation at higher Si contents is insufficient elastic recovery under large volume change; this can be mitigated by modest PAA blending or by tuning the degree of substitution to balance viscosity and film ductility. In recent years, the development of water-soluble cellulose-derived binders, such as CMC, has further enhanced the stability of Si anodes by reinforcing interparticle cohesion and facilitating the formation of a stable SEI layer^[31]. These improvements are primarily attributed to their strong hydrogen-bonding capability and effective bridging effects between active material particles. However, while the CMC/SBR binder may be suitable for Si contents below 5 wt%, higher Si loadings demand binders with enhanced interfacial adhesion, better dispersibility, and improved mechanical elasticity. In this context, another natural polysaccharide, SA, exhibits excellent elasticity and contains abundant carboxyl groups, contributing to strong adhesion with Si surfaces. Kovalenko et al. were the first to demonstrate the effectiveness of SA as a binder for Si anodes, showing its ability to mitigate large volume expansion of Si through superior mechanical support^[69]. The resulting electrode retained 85% of its initial capacity after 100 cycles [Figure 4F]. To further enhance its performance, mussel-inspired catechol groups with strong binding capability were grafted onto the alginate backbone. A catechol-grafted PAA binder improved cycling performance and slurry rheology^[64,74], and a catechol-grafted alginate binder (Alg-C) maintained > 2,000 mAh g^-1 over 400 cycles [Figure 4G]. Overall, alginate offers high elasticity and abundant carboxylates for strong adhesion, but batch variability and slurry sensitivity to multivalent cations can complicate scale-up. Partial functionalization (e.g., catechol grafts) effectively improves adhesion and crack suppression, although it adds synthesis complexity and cost.

Figure 4. Chemical structures of polymer binders containing hydrophilic functional groups and their effects on Si anode performance. (A) CMC; (B) SA; (C) CS; (D) L-CGN; (E) Design of the binder material (left) and cycling stability (right) of a Si anode using a natural rubber/chitosan network binder. Adapted with permission from ref.^[72]; (F) Chemical structures of the mannuronic and guluronic acid repeat units (left); cycling performance of nano-Si anodes with alginate (SA), CMC, and PVDF binders (right; 0.01-1.0 V vs. Li/Li⁺). Adapted from ref.^[64]; (G) Catechol-conjugated polymer binders and cycling performance of Si electrodes using Alg-C, Alg, and PVDF binders. Adapted from ref.^[64]; (H) Cycling performance of Si electrodes using different natural polymer binders (CGN, PAA, SA, CMC). The Si@CGN electrode demonstrated the highest capacity retention over 100 cycles, underscoring the superior binding and structural stabilizing effect of lambda carrageenan. Adapted with permission from ref.^[71]; (I) Schematic illustration of the synthesis of Ox-L-CGN, comparison of its solubility in distilled water with that of L-CGN, and cycling performance of Si electrodes using different polymer binders. The optimized Ox L-CGN 5 binder demonstrated improved solubility and enhanced electrochemical performance, attributed to its optimized viscosity and formation of a stable polymer network. Adapted with permission from ref.^[70].

In addition to polysaccharides, various other bio-based polymers have been explored as sustainable binder materials. Jang et al. designed a nature-inspired L-CGN binder derived from sulfated polysaccharides of red algae^[71]. This binder contains abundant -OH and -SO₃^- functional groups, which enable strong hydrogen bonding with the -OH groups on the Si surface. As a result, even after 50 cycles, the CGN binder demonstrated superior interfacial adhesion to Si particles and internal cohesion within the electrode, as confirmed by SEM analysis, compared to conventional binders such as SA, CMC, and PAA. This case highlights the effective applicability of naturally derived functional groups in Si anode binder systems [Figure 4H]^[71]. In addition, Kim et al. addressed the high viscosity and solubility issues of natural polysaccharide binders by introducing oxidized λ-carrageenan (Ox-L-CGN)^[70]. Through partial oxidation using sodium periodate, aldehyde functional groups were introduced, leading to a reduction in molecular weight and an improvement in rheological properties. The optimized binder (Ox-L-CGN-5), with its appropriate solubility, achieved a high initial Coulombic efficiency of 78.6% and a reversible capacity of 2,534 mAh g^-1 after 100 cycles, while also exhibiting excellent performance under high-mass-loading and nanoscale Si conditions.

Notably, excessive oxidation (Ox-L-CGN-10) hindered effective network formation, resulting in increased electrode pulverization and unstable SEI regeneration, which significantly degraded electrochemical performance. This degradation is presumed to result from reduced solubility, which compromised the structural integrity and stability of the water-processable binder system [Figure 4I]^[70]. Sulfated polysaccharides can enhance adhesion and internal cohesion via hydrogen bonding and electrostatic interactions; controlled oxidation lowers viscosity and improves coatability, yet over-oxidation weakens network formation and accelerates pulverization at high strain. Despite their appeal in terms of sustainability and processing advantages, natural polymer binders pose challenges, such as irregular molecular weights and complex chemical structures, which can lead to inconsistent performance. Nevertheless, the environmental and functional benefits of naturally derived materials continue to drive intensive research in this area. While polysaccharides, such as guar gum (GG) and gum arabic (GA), have been employed as components in certain binder systems, they are not considered key materials for Si anode applications and are thus excluded from the scope of this study.

Synthetic polymer binders

This subsection examines synthetic binders, highlighting their tunable architectures and ability to integrate functional groups not easily accessible in natural polymers. We outline how molecular design impacts mechanical resilience, adhesion to Si, and compatibility with high-solid-loading slurry processes.

Synthetic binders offer tunable architectures and functionalities that are difficult to achieve with natural polymers. For Si electrodes, the central design problem is to combine strong interfacial bonding with elastic stress dissipation and electrolyte compatibility while maintaining slurry processability at high solid loading. Here we synthesize cross-study evidence into three takeaways: linear fluorinated baselines are robust but adhesion-limited on Si; acrylic chemistries (e.g., PAA) add dense -COOH sites and stiffness for higher Si content; and architectural modifications (branched/cross-linked/dynamic) are required when areal loading and volumetric strain exceed the elastic window of linear binders. In the following discussion, we focus on representative synthetic binders and their performance-structure relationships, beginning with the widely used fluorinated baseline PVDF and progressing to non-fluorinated acrylic and nitrile-based systems. Notably, synthetic polymers, like polyvinylidene fluoride (PVDF) [Figure 5A]^[75,76], a fluoropolymer, as well as non-fluorinated options like polyacrylic acid (PAA) [Figure 5B]^{[11,75,77-80]}, and Polyacrylonitrile (PAN) [Figure 5C]^[81], are being actively researched. PVDF, a fluorocarbon-based binder, has a chemical structure of -(CH₂-CF₂)n- and possesses excellent thermal and electrochemical stability, making it the most widely used binder in commercial LIBs. From an industrial standpoint, PVDF remains a commercial baseline due to thermal/chemical stability and well-established processing. However, its non-functional linear backbone interacts weakly with Si, and NMP-based processing plus gel-like swelling in certain electrolytes can impair coating uniformity and interparticle contact under large strain. Heat-treatment or copolymer strategies partially enhance mechanical integrity, but for Si-rich electrodes, the adhesion/elasticity deficit typically necessitates alternative or hybrid binders. The presence of fluorine (F) atoms in fluoropolymer materials reacts with lithium ions to form LiF in the SEI layer, which is electronically insulating but mechanically and chemically stabilizing; this contributes to interfacial stability rather than providing electronic conductivity^[56,63]. However, PVDF binders have several issues when used as binders for Si anode materials. The first issue with PVDF arises from its non-functional linear chain structure, which can lead to weak interactions with the Si surface, potentially causing a decline in cycle performance. PVDF also lacks flexibility and mechanical properties, which limits its ability to accommodate the volume changes occurring in Si anode electrodes. This can result in the breakage of adhesive bonds between particles, further impacting battery performance. Various copolymers containing the PVDF group have been applied and studied as electrode binders; however, several limitations have been reported. Despite its high polarity and good interaction with polar organic electrolytes, this fluoropolymer tends to expand and become gel-like under high-temperature conditions, reducing its physical performance. When PVDF dissolves in a solvent, it forms a high-viscosity solution, which can significantly hinder the uniform dispersion of active materials and conductive additives. This can lead to uneven electrode coatings, ultimately blocking ion channels and negatively impacting performance. To address the challenges associated with fluoropolymer materials, several approaches have been suggested. A key method involves enhancing performance through thermal treatment. For example, after 50 cycles, PVDF-based binders treated at 120 °C showed a 20.1% retention in capacity (< 250 mAh g^-1), while heat treatment at 220 °C effectively reduced the expansion of the Si electrode [Figure 5D]. This resulted in a significant increase in capacity retention, reaching 75.4% (> 1,000 mAh g^-1), while also improving the stability of the electrode^[82,83]. While fluorinated polymers such as PVDF have been widely employed due to their excellent electrolyte compatibility and wettability, they present several limitations. Most notably, the use of flammable and potentially explosive organic solvents raises secondary safety and environmental concerns. In response, fluorine-free polymer binders are emerging as cost-effective, safer, and more environmentally friendly alternatives, garnering increasing attention and research interest. Among these alternatives, acrylic-based polymers represent a typical class synthesized via controlled polymerization techniques. Their physical and chemical properties vary depending on the functional groups introduced into the monomers. One representative example is PAA, polymerized from acrylic acid monomers bearing -COOH, which has demonstrated promising performance as a binder for Si anodes^[84]. Because each monomer unit of PAA contains a -COOH, the polymer provides a high density of acidic sites for strong interactions with Si. Moreover, PAA exhibits robust mechanical properties that help maintain electrode integrity during the severe volume changes associated with lithiation and delithiation. The use of PAA in Si anodes was first reported in 2010, where a Si/PAA electrode achieved stable cycling performance, retaining approximately 75% of its initial capacity over 100 cycles at 0.5 C^[62]. Subsequent studies found that the addition of alkaline agents, such as NaOH, could significantly enhance the electrochemical performance of PAA-based binders, increasing their capacity from 1,300 to 2,000 mAh g^-1. Furthermore, PAA has emerged as a viable alternative to increase the Si content in graphite/Si composite electrodes. PAA not only exhibits strong adhesion to Si particles but also offers water-processability, eliminating the need for toxic organic solvents such as NMP, which is a key advantage for eco-friendly and scalable electrode fabrication^[85]. PAA provides dense carboxylates for strong interfacial bonding and higher matrix stiffness, supporting increased Si fractions in graphite/Si composites. Industrial interest stems from aqueous processing, formulation flexibility with CMC/SBR/CNT, and tunability via molecular weight/viscosity (≈ 20-40 kcps) to balance dispersion and coating quality. The main trade-offs are brittleness at very high stiffness and potential electrolyte interactions; recent practice emphasizes MW control and limited co-binders to preserve elastic recovery while maintaining line stability.

Figure 5. Chemical structures of synthetic polymer-based binders and their effects on Si anode performance. (A) PVDF; (B) PAA; (C) PVA. PVDF-based binder strategies; (D) Cross-sectional SEM images of Si-graphite/PVDF composite electrodes before and after the first cycle at various temperatures, along with the corresponding cycling performance of the Si anode. Adapted with permission from ref.^[82]; (E) Introduction of Si-functional groups via an azide-nitrile click reaction on a PAN-based polymer and the resulting electrochemical performance. Adapted with permission from ref.^[87]; (F) Schematic illustration of ion transport pathways and structural evolution in PAA@Si and PPD PAA@Si electrodes during cycling. The long-term cycling performance of Si electrodes using PAA, CMC SBR, and PPD PAA binders at a current density of 1.2 A g^-1 is also presented. Adapted with permission from ref.^[39].

In our view, PVDF is an excellent baseline for graphite but structurally mismatched to high-strain Si. PAA-centered formulations currently offer the most pragmatic route to scale, provided viscosity and elasticity are co-optimized to avoid cracking at high Si loading. In addition to PAA, PAN has also been investigated as a binder, owing to the polar nature of its nitrile groups (-CN)^[81]. PAN’s nitrile (-C≡N) groups confer high polarity, improving interfacial adhesion to Si and enhancing slurry rheology and electrolyte wettability rather than creating intrinsic ion-conducting pathways within the binder phase. High-molecular-weight PAN forms entangled chains that wrap and bridge Si particles, increasing mechanical integrity and adhesion; however, its intrinsic electronic conductivity remains low, and neat PAN binders may exhibit brittleness under large strains. In practice, PAN is less widely adopted than PVDF or PAA owing to higher cost and more complex processing, although targeted functionalization (e.g., silane coupling, click chemistry) has yielded hybrid organic-inorganic networks with improved durability. These modifications, while effective, must be balanced against synthesis complexity and solvent compatibility to ensure feasibility for large-scale manufacturing. The strong polarity of the nitrogen atom in the nitrile group improves both ionic conductivity and interfacial adhesion with the Si surface. One key advantage of high molecular weight PAN is its ability to form entangled polymer networks around Si particles, thereby improving mechanical integrity and adhesion. This structural advantage results in a notable improvement in capacity retention -37.95% for high-molecular-weight PAN compared with 17.5% for PVDF^[86]. While the nitrile group imparts beneficial characteristics, the intrinsic conductivity of PAN remains limited. Consequently, further improvements can be achieved by compositing PAN with conductive additives or applying doping strategies. For example, a study utilizing an azide nitrile click reaction to enhance adhesion of PAN to Si showed a significant improvement in capacity retention, achieving 522 mAh g^-1 after 50 cycles compared to just 152 mAh g^-1 for unmodified PAN [Figure 5E]^[87]. This work highlights a promising approach wherein silane-treated PAN (Si-PAN) was integrated into Si alloy electrodes. The Si-PAN binder forms a hybrid organic-inorganic structure through silane functionalization, enhancing both electrode durability and electrochemical performance. Notably, Si alloy electrodes fabricated with the optimized Si-PAN binder and thermally annealed at 450 °C delivered an initial discharge capacity of approximately 760 mAh g^-1 and maintained stable performance over 50 cycles^[87]. From an industrial standpoint, PAN’s strong polarity and film-forming ability make it attractive for niche applications requiring high adhesion, but its relatively high cost and limited stretchability under large Si expansion mean it is more viable as a component in hybrid systems rather than as a sole binder in high-loading Si electrodes. More recently, Sun et al. developed a neural-network-inspired binder system by integrating an ion-conductive polymer synthesized from poly(ethylene glycol) diglycidyl ether (PEGDGE) and polyethylenimine (PEI) with a PAA backbone through noncovalent interactions [Figure 5F]^[39]. This hybrid binder (PPD-PAA) forms a flexible physical network that avoids chemical cross-linking, while enabling efficient Li-ion transport and effectively buffering the large volume fluctuations of Si via hydrogen bonding and electrostatic interactions. Impressively, the resulting electrode demonstrated superior rate capability and high capacity retention over 200 cycles, outperforming both CMC-SBR@Si and conventional PAA@Si systems^[39].

Table 1 summarizes representative linear binders used for Si anodes, including their structural features and functional groups. These binders offer good processability and enable polar interactions with Si particles; however, their limited mechanical reversibility often restricts long-term cycling stability.

Linear binders, such as PVDF, PAA, and PAN, offer well-defined molecular structures, high processability, and strong polar interactions with Si surfaces, making them compatible with scalable electrode fabrication. However, their limited mechanical reversibility and susceptibility to structural degradation during repeated cycling remain critical challenges, often restricting long-term stability. In particular, PAA, while not delivering the highest electrochemical performance, has attracted significant attention in the domestic cell industry due to its low cost and compatibility with aqueous processing.

Current R&D efforts focus on optimizing slurry viscosity and improving adhesion to enhance electrode integrity without compromising manufacturability. Future developments are expected to focus on controlling molecular weight, diversifying functional groups, and developing hybridization strategies to balance mechanical resilience with ionic/electronic conductivity in next-generation Si-based anodes. Recent studies have also explored advanced linear synthetic binder architectures, including copolymer systems with zwitterionic functional groups and supramolecular host-guest assemblies, which have demonstrated improved Li⁺ transport, enhanced interfacial adhesion, and superior cycling stability under high-Si-loading and industrially relevant conditions^[88]

Branched or grafted polymer binders

Here, we examine branched and grafted binder architectures, which introduce multiple interaction sites for enhanced adhesion and stress dispersion. We assess their performance benefits, synthesis challenges, and potential as an intermediate strategy between linear and fully cross-linked systems.

Branched or grafted architectures increase the density and spatial distribution of interaction sites relative to linear analogs, improving multi-point anchoring and stress dispersion around Si particles. Across reported systems, elasticity and capacity retention benefit from side-chain mediated hydrogen bonding/ionic interactions, yet synthesis complexity and viscosity management remain practical bottlenecks. We therefore view branched motifs as effective intermediates especially when used to tune chain mobility and solvent uptake between simple linear baselines and fully cross-linked networks^[32,89,90]. Recent studies have demonstrated that compared to linear binders, branched architectures or grafted systems composed of functional moieties can better withstand the mechanical stress associated with Si expansion. Among these approaches, grafting functional groups onto a polymer backbone to generate side chains has been shown to improve both mechanical strength and interfacial adhesion in binder systems. For instance, He et al. developed a branched-type water-soluble binder, poly(vinyl alcohol)-grafted poly(acrylic acid) (PVA-g-PAA), synthesized via esterification between the hydroxyl groups of PVA and the carboxyl groups of PAA [Figure 6A]^[89]. Although the synthesis involves covalent ester bonding, it does not result in a cross-linked network but rather introduces functional branches onto the PVA backbone. This architecture offers abundant -COOH and -OH groups that form hydrogen bonds with Si particles, thereby improving interfacial adhesion and structural elasticity. While branched structures offer enhanced solubility and functional tunability, they often lack the robust network cohesion required to fully suppress Si volume fluctuation. Nevertheless, PVA-g-PAA exhibited promising electrochemical performance, retaining a capacity of approximately 1,000 mAh g^-1 after 1,000 cycles at a current density of 2.0 A g^-1, significantly outperforming conventional PAA binders^[89]. Building on this strategy, Shen et al. introduced a multidimensional branched composite binder (CA-g-PAA) by covalently grafting PAA onto a hyperbranched polyamide framework [Figure 6B]^[32]. The resulting three-dimensional network provided both mechanical resilience and strong interfacial adhesion with Si particles. Electrochemical tests revealed a high reversible capacity of ~2,500 mAh g^-1 over 200 cycles at 0.5 A g^-1, markedly superior to traditional binders^[32]. Furthermore, SEM analysis confirmed that CA-g-PAA maintained a dense, crack-free electrode morphology after prolonged cycling, unlike the heavily degraded control samples. This improved performance is attributed to the multidimensional grafted structure, which effectively buffers volumetric stress and maintains electrode integrity. Similarly, He et al. developed a GA-grafted poly(acrylic acid) (GA-g-PAA) binder by grafting GA onto PAA via a free radical polymerization process. The resulting binder exhibited enhanced cycle stability, higher Coulombic efficiency, and improved rate capability compared to pure PAA^[91]. In another notable example, Jolley et al. proposed a branched-type binder by grafting PAA onto a SBR backbone, forming PAA-g-SBR [Figure 6C]^[90]. This design combines the strong adhesion of PAA with the elasticity of SBR, resulting in improved cycling stability of Si/graphite composite anodes when tested at a charge and discharge rate of 1 C. The improved performance is primarily due to the branched configuration, in which grafted PAA chains establish hydrogen bonds and acid-base interactions with Si surfaces. Unlike physically blended PAA/SBR systems, this covalently grafted network offers superior homogeneity and interfacial bonding strength^[90]. Despite their promising attributes, branched-type polymer binders are still relatively underexplored for Si anodes. Their limited mechanical durability and weak internal cohesion often hinder their ability to fully accommodate the extreme volume fluctuations of Si. Furthermore, the loosely packed architecture can result in poor slurry dispersibility and restricted ionic conductivity. Consequently, recent research has increasingly focused on multifunctional hybrid systems that combine covalent bonding, hydrogen bonding, and ion-conductive components to enhance both mechanical stability and electrochemical performance. Recent developments have also reported highly elastic hyperbranched polymer binders that provide exceptional mechanical compliance and strong interfacial adhesion, effectively accommodating large Si volume changes and maintaining electrode integrity over extended cycling^[92].

Figure 6. Electrochemical performance and structural stability of Si anodes employing polymeric branched binders. (A) Cycling performance of a Si electrode using the Si@PVA-g-10PAA binder over 1,000 cycles at a current density of 400 mA g^-1, accompanied by rate performance comparisons with various binders including PVA, PVA-g-10PAA, PVA-g-15PAA, PAA, and CMC. Adapted with permission from ref.^[89]; (B) Schematic illustration of the interaction between Si particles and the CA-g-PAA binder. Cross-sectional SEM images of Si electrodes with PAA and CA-g-PAA binders before cycling and after 5 and 50 cycles, along with corresponding cycling performance using different binders. Adapted with permission from ref.^[32]; (C) Schematic illustration and long-term cycling performance of Si/graphite composite anodes using Na-PAA/SBR and Na-PAA-g-SBR binders, demonstrating the superior stability and capacity retention imparted by the grafted binder system. Adapted with permission from ref.^[90].

Branched-like polymer architectures offer enhanced multi-point anchoring and network connectivity compared to their linear counterparts, enabling improved adhesion to Si surfaces and better stress distribution during cycling. The incorporation of multiple functional side chains can facilitate both mechanical stability and Li-ion transport pathways, thereby enhancing long-term capacity retention. However, the synthesis of branched polymers often requires more complex monomer feed ratios and reaction controls, which can lead to batch-to-batch variability. Additionally, higher viscosity in aqueous processing may limit slurry uniformity at a large scale. To advance industrial applicability, future work should prioritize scalable synthetic strategies, optimized branching density, and cost-efficient monomers that balance mechanical robustness with manufacturability.

Cross-linked polymer binders

In this section, the term “cross-linked” encompasses both covalent and physical network formations. While covalent cross-links (e.g., esterification, amidation) provide permanent structural reinforcement, physical cross-links (e.g., hydrogen bonding, ionic interactions, supramolecular assemblies) can also deliver significant mechanical and interfacial benefits. Both types are considered here due to their relevance in suppressing Si anode pulverization and delamination. To overcome the limitations of conventional linear polymer binders, cross-linked polymer systems have been extensively explored. These systems form strong covalent bonds between polymer chains, thereby achieving high mechanical strength, an essential property for preserving the structural integrity of Si anodes during repeated cycling. On a molecular level, covalent bonding between the binder and hydroxyl-terminated Si surfaces (Si-OH) helps mitigate stress from volumetric expansion. Covalently cross-linked binders form robust three-dimensional networks that effectively suppress active material pulverization and electrode delamination, but their structural irreversibility can limit long-term adaptability under extreme cycling. Hybrid designs incorporating partially dynamic cross-links have recently been explored to mitigate this trade-off. However, since these covalent bonds are typically irreversible, they cannot reform once broken, which may lead to a gradual deterioration in adhesion between the binder and Si particles. Despite this limitation, chemically cross-linked binders built on well-chosen polymer backbones have demonstrated excellent long-term cycling performance. Their interconnected polymer networks effectively accommodate the repeated volume changes of Si while maintaining mechanical cohesion and interfacial adhesion over extended cycles^[93,94]. A notable challenge is that highly cross-linked binders are often insoluble in common solvents, necessitating thermal activation or specialized processing to form stable networks. A representative example is the thermally cross-linked PAA-PVA binder system, where esterification occurs between the hydroxyl groups of PVA and the carboxyl groups of PAA under elevated temperatures^[69,89]. The resulting interpenetrating polymer network (IPN) significantly enhances mechanical robustness and provides abundant functional groups for strong interactions with Si surfaces. This architecture effectively stabilizes the electrode against Si expansion/contraction during lithiation and delithiation, while outperforming linear alternatives in both adhesion and elasticity. Notably, the PAA-PVA binder delivered a stable capacity of ~2,283 mAh g^-1 over 100 cycles, surpassing PVDF and NaCMC systems [Figure 7A]^[95]. Cross-linked binders also excel in suppressing electrode thickness changes during cycling. For instance, electrodes using linear binders such as SA experienced irreversible thickness growth of ~16.5% after just five cycles. In contrast, PAA-PVA cross-linked binders limited this change to ~4.5%, owing to the rigidity of their IPN structure^[95]. In another study, a cross-linked binder synthesized via esterification between PAA and CMC retained over 60% of its initial capacity after 100 cycles at 0.5 C, even under high mechanical stress^[84], highlighting the structural resilience of the covalent network.

Figure 7. (A) Schematic illustration of the interaction between cross-linked PAA-PVA and the Si anode, and the cycling performance of Si electrodes using PAA-PVA, NaCMC, and PAA binders. Adapted from ref.^[64]; (B) Schematic illustration of polymer interaction among PAA, NaCMC, and functionalized SCNT, enhancing mechanical integrity, and peeling test results and cycle performance comparison among μSi electrodes using PAA-NaCMC 1, PAA, and NaCMC binders. Adapted with permission from ref.^[34]; (C) Schematic illustration of Si/C composite anodes incorporating the cross-linked binder P(SH-BA). Rate capability comparison of Si/C₄₅₀ electrodes with various binder systems. Long-term cycling performance and Coulombic efficiency of coin-type Si/C₆₀₀//Li half cells employing P(SH-BA3%) and PSH binders (tested at 0.2 C). Adapted with permission from ref.^[13]; (D) Schematic representations of LiPAA, TA, and SS, followed by a depiction of the physicochemically cross-linked LPTS composite polymer binder and its functional integration into Si-based anodes during lithiation/delithiation. Initial galvanostatic charge-discharge profiles of Si/PTS, Si/LPT, Si/LPS, and Si/LPTS electrodes at 0.1 C are shown, along with their cycling stability at 0.5 C, highlighting the improved electrochemical performance enabled by the LPTS binder system. Adapted with permission from ref.^[38]; (E) Schematic illustration of the Si/CPU-PAA anode architecture and hierarchical stress-dissipating polymeric network. Cycling performance of Si anodes in half cells at 1 A g^-1 with a high Si mass loading of 1.5 mg cm^-2. Morphological comparison between Si/PAA and Si/CPU-PAA anodes before and after 50 cycles in Si//Li half cells, demonstrating the structural reinforcement provided by the CPU-PAA binder. Adapted with permission from ref.^[22].

Zhang et al. also introduced a chemically cross-linked binder system comprising PAA, CMC, and CNT-NH₂ to improve mechanical and electrical properties [Figure 7B]^[34]. Spontaneous esterification and amidation reactions, which involve the formation of ester bonds between -COOH and -OH groups and amide bonds between -COOH and -NH₂ groups, occurred during electrode fabrication and resulted in a stable three-dimensional polymer network. This structure buffered Si expansion while preserving electronic pathways, resulting in improved cycling stability and overall performance. Recent studies have further demonstrated the potential of multifunctional cross-linked networks for Si anodes^[96]. For example, a CA@CMC binder system formed via esterification between the -COOH groups of citric acid and the -OH groups of CMC created a mechanically robust, three-dimensional covalent network that not only mitigated Si pulverization but also in situ regulated the SEI composition, enriching LiF content and enhancing interfacial stability over 500 cycles. In another approach, Sun et al. synthesized a covalently cross-linked 3D binder, P(SH-BA), via RAFT polymerization of hydroxyethyl acrylamide, sodium acrylate, and phenylboronic acid^[13]. Boronic acid moieties form dynamic covalent B-O and B-N linkages with -OH and -CONH₂ groups, constructing a resilient network [Figure 7C]. Remarkably, at a binder dosage of just 3 wt%, the optimized P(SH-BA3%) retained 83.56% of its capacity after 600 cycles at 0.5 C, with high Coulombic efficiency. This confirms that boron-mediated dynamic linkages offer an effective balance between mechanical strength and ion transport, making them suitable for high-energy LIBs.

Xiao et al. developed a multifunctional composite binder composed of LiPAA, tannic acid (TA), and silk sericin (SS), forming a physically cross-linked network via hydrogen bonding [Figure 7D]^[38]. TA acts as a molecular bridge between the carboxyl groups of LiPAA and the amino/hydroxyl groups of SS, forming an interpenetrating supramolecular network. This structure provided superior elasticity, cohesion, and adhesion. The Si/LPTS electrode showed excellent rate capability, delivering over 1,500 mAh g^-1 even at 2 C, and nearly full capacity recovery at 0.1 C. The use of silk sericin, a biodegradable protein, enhances structural stability and environmental compatibility, offering a practical and sustainable solution for Si anodes. He et al. developed a dual cross-linked binder system composed of cross-linked polyurethane (CPU) and PAA, where covalent and hydrogen bonds synergistically construct a physicochemical network [Figure 7E]^[22]. The resulting CPU-PAA binder exhibited high toughness, superior tensile strength, and hierarchical stress dissipation, thereby maintaining the structural integrity of the Si anode and reducing thickness swelling. Finite element analysis revealed that this binder effectively mitigates internal stress during cycling. As a result, the Si/CPU-PAA anode achieved a capacity retention of 82.3% after 150 cycles at a high current density of 5 A g^-1[22]. This physicochemical dual-network(Si/CPU-PAA) preserved electrode integrity during prolonged cycling, as confirmed by scanning electron microscopy (SEM) images showing dense, crack-free structures compared to Si/PAA after 50 cycles.

Covalently cross-linked binders provide robust, permanent networks that effectively suppress Si pulverization and electrode delamination, delivering high mechanical strength and stable cycling even under high areal loadings. Their dense network structure minimizes irreversible deformation, maintaining electrical and ionic pathways over prolonged operation. Nonetheless, the irreversible nature of covalent bonds can limit adaptability to extreme volumetric changes, and the multi-step synthesis or thermal curing requirements may increase production cost and energy consumption. Future research should focus on hybrid cross-linking approaches that integrate partial reversibility, reduce curing temperatures, and employ eco-friendly, water-based processing routes to bridge the gap between laboratory performance and industrial-scale electrode fabrication.

Dynamically cross-linked polymer binders

Here, we review binders that employ reversible cross-links to enable autonomous healing and stress relaxation. We analyze their potential to combine mechanical adaptability with ionic conductivity, while addressing the challenges of bond lifetime control, rheology management, and scalability.

Self-healing polymers refer to materials in which physically separated chains can recombine through the formation of new bonds at reactive sites on adjacent chains via dynamic cross-linking mechanisms^[97]. Unlike conventional cross-linking, in which broken bonds are irreversible, dynamic cross-linking enables the formation of new interactions at alternative active sites even after bond dissociation. This mechanism is driven by a multitude of noncovalent interactions, such as hydrogen bonding, ionic interactions, and π-π stacking, acting collectively like microscopic Velcro. These reversible bonds allow the material to adapt to mechanical deformation by temporarily dissociating and reassembling, thereby enabling flexible and self-healing behavior. In the context of Si anodes, the use of self-healing binders helps mitigate the mechanical stress induced by large volume expansion. Upon expansion, the cross-linked polymer network stretches to dissipate mechanical stress, while upon contraction, it reverts to its original configuration by re-establishing bonds and healing microcracks. One representative example is the dynamic host-guest system utilizing cyclodextrin (β-cyclodextrin polymer, β-CD) and adamantane groups by Kwon et al. [Figure 8A]^[97]. The hydrophobic interior and hydrophilic exterior of β-CD enable specific supramolecular binding with adamantane, and when combined with a multifunctional adamantine (AD) linker, the resulting network achieved ~90% capacity retention after 150 cycles. Another notable approach is the hydrogen-bond-based dynamic binder developed by Chen et al., which involves the reaction of TA and PAA^[79]. The branched TA structure, rich in hydrogen bonding sites, forms a dynamic network that accommodates Si volume fluctuations and improves electrode integrity. Additionally, a PAA-free strategy combining CMC and cationic polyacrylamide (CPAM) was introduced to form an ionically cross-linked network through electrostatic interactions^[98]. This dynamic yet physical network showed excellent adaptability to volume changes without relying on covalent bonding, leading to improved structural resilience. In a more recent study, Kang et al. proposed a dynamic covalently cross-linked network (xPAA-B-DA), in which boronic ester linkages undergo reversible exchange reactions [Figure 8B]^[28]. This imparts self-healing functionality, preserves ionic transport pathways, and alleviates structural disintegration under cycling stress. Importantly, the residual dopamine moieties enhance adhesion via catechol metal and hydrogen bonding, stabilizing SEI formation and reducing charge transfer resistance over time. In another advanced system, Kang et al. designed a multifunctional binder integrating multiple dynamic interactions - boronic ester (B-N) bonds, Al³⁺ carboxylate coordination, and hydrogen bonding - within a semi-interpenetrating polymer network of PAA and CMC [Figure 8C]^[99]. This design imparted robust mechanical strength, elasticity, and rapid autonomous healing under ambient conditions. The inclusion of Al³⁺ ions enhanced compressive resistance and interfacial adhesion. A particularly notable feature was the visible healing of separated hydrogel segments within 3 h without external stimuli. The binder (CMC-AAc-3) retained over 1,000 mAh g^-1 after 300 cycles, exhibited a high initial Coulombic efficiency of 84.7%, suppressed swelling in electrolyte, and facilitated stable SEI formation. These results underscore the versatility of dynamic hydrogel binders for Si anode applications. Building on such multifunctional dynamic architectures, recent work has reported a supramolecular polymer binder featuring hard/soft phase interactions^[100]. In this design, the hard domain integrates reversible disulfide exchange (dynamic covalent bonds), cooperative hydrogen bonding, and π-π interactions, imparting self-healing capability and robust mechanical strength. Meanwhile, the soft polyether domain establishes Li⁺ ion-hopping pathways, enhancing stretchability and ionic conductivity. When applied to Si electrodes, this binder effectively suppressed volumetric expansion, promoted the formation of a dense and homogeneous SEI, and achieved a high initial coulombic efficiency of 85.2%, along with superior rate capability and cycling stability in both half-cell and NCM523||Si/C full-cell configurations. This synergy between dynamic covalent bonding and supramolecular interactions exemplifies a promising pathway for enhancing both mechanical adaptability and electrochemical performance in next-generation Si anode binders. Lastly, Xu et al. developed a fast self-healing and highly adhesive aqueous binder (PAA-LS) by incorporating lignosulfonate into PAA, utilizing dynamic Fe³⁺ catechol coordination^[29]. This system enables efficient healing of microcracks and stress redistribution during lithiation/delithiation, while providing mechanical robustness and strong interfacial adhesion. The SiO_x@PAA-LS electrode demonstrated an impressive capacity of 997.3 mAh g^-1 after 450 cycles at 0.5 A g^-1, outperforming conventional binders such as PAA and CMC [Figure 8D]. In full-cell tests (SiO_x||NCM622), the binder achieved 82.72% capacity retention after 100 cycles. The superior performance stems from the synergy of dynamic coordination bonding, polar functional groups enhancing Li-ion transport, and the optimized LS content (0.5 wt%) ensuring network integrity. This work validates the practical potential of dynamic cross-linked binders for next-generation high-capacity Si oxide anodes^[29].

Figure 8. Electrochemical performance and structural stability of Si anodes incorporating polymeric dynamically cross-linked binders. (A) Schematic illustration of the self-healing mechanism in a polymer binder and the cycling performance of Si electrodes using dynamically cross-linked polymer binders based on CD and AD host-guest interactions. Adapted with permission from ref.^[97]; (B) Schematic illustration of the self-healing 3D polymeric binder (xPAA-B-DA), constructed via boronic ester and dopamine-based cross-linking chemistry, demonstrating its integration within Si anodes during lithiation/delithiation. Also shown are the electrochemical impedance spectra of Si@xPAA-B-DA and Si@PAA electrodes measured at the 1st, 50th, and 100th cycles under a current density of 1 C. Adapted with permission from ref.^[28]; (C) Schematic illustration of the self-healing mechanism in the CMC-AAc3 binder, along with extended cycling performance data. The CMC-AAc3 binder retained 67.2% of its initial capacity after 300 cycles at a rate of 0.2 C, demonstrating its enhanced durability under long-term operation. Adapted with permission from ref.^[99]; (D) Schematic representation of the PAA-LS binder architecture, followed by long-term cycling performance of SiO_x anodes using PAA-LS, PAA, and CMC binders at a current density of 0.5 A g^-1, demonstrating the improved capacity retention and enhanced electrode stability provided by the lignosulfonate-modified binder. Adapted with permission from ref.^[29].

Table 2 categorizes various polymeric binders according to their structural architectures and bonding mechanisms, including linear, branched, covalently cross-linked, and dynamically cross-linked systems. These binders employ diverse interactions, such as hydrogen bonding, ionic interactions, covalent ester linkages, and coordination bonding to improve mechanical resilience and interfacial stability in Si anodes. Table 3 provides a concise overview of the representative binder types covered in Sections "Branched or grafted polymer binders"-"Dynamically cross-linked polymer binders", including linear, branched, covalently cross-linked, and dynamically cross-linked systems, highlighting their respective advantages and limitations. This comparative summary offers a basis for the subsequent discussion in Section "MECHANICAL PROPERTIES AND THEIR IMPACT ON SI ANODE PERFORMANCE", which focuses on the mechanical properties of these binder systems.

Dynamic cross-linked architectures combine adaptive mechanical buffering with autonomous healing, making them particularly well-suited for mitigating the extreme volume changes of high-capacity Si-based anodes. Dynamic networks leverage reversible bonds to enable autonomous crack healing and stress relaxation, though industrial deployment requires careful control of bond lifetime, mechanical stability, and slurry rheology under high-solid-loading conditions.

Their versatility in incorporating multiple reversible interactions ranging from hydrogen bonding and π-π stacking to metal-ligand coordination enables simultaneous improvement of structural resilience, interfacial adhesion, and ionic conductivity. Nevertheless, the complexity of multi-component synthesis, the inherently high cost of functionalized precursors, potential variability in dynamic bond lifetimes, and uncertainties in long-term stability under industrial processing conditions remain barriers to commercialization. Future efforts will need to focus on cost-effective precursor selection, large-scale aqueous synthesis routes, and rheological optimization to fully leverage the advantages of dynamic cross-linked binders in mass-produced next-generation LIB electrodes. While these dynamic cross-linked architectures have been primarily investigated in conventional liquid electrolyte systems, similar design principles can be extended to solid-state battery (SSB) configurations. In SSBs, the absence of liquid electrolyte eliminates swelling and dissolution issues, but introduces challenges related to maintaining intimate solid-solid interfacial contact and ensuring sufficient ionic conductivity across the electrode-electrolyte interface^[101-103]. Binders in this context must combine strong adhesion to both the Si anode and solid electrolyte with mechanical compliance to accommodate substantial volume changes during cycling. Incorporating ion-conductive polymer segments, such as polyethylene oxide (PEO) or polycarbonate-based backbones, can facilitate Li⁺ transport through the binder phase and reduce interfacial resistance^[104-106]. Furthermore, hybrid binder systems containing ceramic fillers, such as Li₇La₃Zr₂O₁₂ (LLZO), have been explored to enhance both mechanical robustness and ionic conductivity in SSB environments^[107] (Boaretto et al., 2022^[107]). Developing binders that integrate high interfacial adhesion, appropriate mechanical flexibility, and Li⁺ conduction pathways is therefore essential for achieving stable cycling performance of Si anodes in solid-state battery systems.