Lithium battery binders such as PVDF, CMC, PAA, etc

The electrodes of Lithium-ion Batteries (LIBs) are primarily composed of electrochemically active electrode materials, conductive additives, binders, current collectors, and other components. Among these, binders serve as a critical component of LIBs electrodes. Binders can firmly adhere active materials and conductive materials to the current collector, forming a complete electrode structure. They prevent the detachment or exfoliation of active materials during charging and discharging processes, while also uniformly dispersing active materials and conductive agents. This enables the formation of a favorable electron and ion transport network, thereby facilitating efficient transport of electrons and lithium ions.

Currently, substances used as electrode binders include poly(vinylidene fluoride) (PVDF), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), poly(vinyl pyrrolidone) (PVP), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), poly(acrylic acid) (PAA), poly(vinyl alcohol) (PVA), sodium alginate (Alg), β-cyclodextrin polymer (β-CDp), polypropylene emulsion (LA132), poly(tetrafluoroethylene) (PTFE), and so on, as well as functionalized derivatives of the above-mentioned polymers or copolymers formed by monomers.

In lithium-ion battery (LIB) electrodes, the ideal binder performance should include:

(1) chemical and electrochemical stability in a given electrode/electrolyte system, resistance to electrolyte corrosion, and no occurrence of redox reactions within the operating voltage range;

(2) It should exhibit good solubility, with fast dissolution rate and high solubility in solvents, and the required solvents should be safe, environmentally friendly, and non-toxic, with water-based solvents being preferred;

(3) It should have moderate viscosity to facilitate slurry mixing and maintain slurry stability, while also possessing strong adhesion, resulting in electrodes with high peeling strength, excellent mechanical properties, and low binder usage;

(4) It should demonstrate good flexibility to tolerate bending during electrode handling and volume changes of active material particles during the charge-discharge cycles of LIBs;

(5) It should be capable of forming an ideal conductive network with conductive agents, leading to electrodes with good electrical conductivity and lithium ion conduction capability;

(6) It should be widely available and low-cost.

This paper summarizes recent research achievements related to LIB electrode binders, with a focus on introducing the adhesion mechanisms of binders in electrodes and the commonly used oil-based and water-based binders in current LIB electrodes.

1 Adhesion Mechanism of Binders in Lithium-Ion Battery Electrodes

The production process of LIB electrodes typically involves four steps: mixing various materials (including electrode active materials) in a solvent to form a battery slurry, coating the slurry onto a current collector, drying, and rolling. It is generally believed that LIB electrodes consist of three components: active material particles (AM) serving as sources of ions and electrons, electrolyte-filled pore space for ion conduction, and carbon-binder domains (CBD) that provide conductivity.

CBD is typically composed of carbon nanoparticles connected by a polymeric binder (Fig. 1), whereas the precursor slurry required for electrode preparation consists of micrometer-sized active material (AM) particles suspended within the CBD. The CBD directly influences the transport efficiency of ions and electrons in the electrode, as well as the quality of the passivation layers (e.g., solid electrolyte interphase [SEI] and cathode electrolyte interphase [CEI] films) formed on the surface of active materials in contact with the electrolyte. Therefore, the CBD plays a critical role in the electrode manufacturing process: insufficient CBD leads to poor electrode connectivity, resulting in inadequate electron transport and insufficient electrode mechanical strength; excessive CBD increases the self-weight and volume of the battery, and may even slow down ion transport.

Zielke et al. employed a novel approach combining X-ray computed tomography (CT) and virtual design to compare the influences of two carbon-binder domain (CBD) models on the surface area, tortuosity, and electrical conductivity of the solid electrolyte interphase (SEI) film on electrodes in lithium-ion batteries (LIBs) during charging and discharging states. The results demonstrated that the CBD content significantly impacts the transport parameters of LIBs under both charging and discharging conditions, whereas the morphology of the CBD only exerts a critical effect on the discharge state.

Prasher's group proposed a micro-rheological model that incorporates interparticle colloidal interactions and hydrodynamic interactions, which was used to predict the viscosity of suspensions of conductive carbon nanoparticles and polymer binders, as well as the entire anode slurry. The findings revealed that the interactions between carbon nanoparticle particles depend largely on the ratio of particles to polymer binder and the molecular weight of the polymer binder. Furthermore, changes in interparticle interactions are clearly reflected in the self-assembled structure of the particles, which in turn manifests in the viscosity of the slurry.

Srivastava et al. elucidated the influence of the adhesion between active material (AM) and carbon-binder domain (CBD) and the cohesion within the CBD on the electrode microstructure and key properties related to electrochemical transport (such as ion transport tortuosity, electronic conductivity, and available active material (AM)-electrolyte interfacial area) through particle dynamics and colloid dynamics simulations.

2 Common Electrode Binders for Lithium-Ion Batteries
2.1 Poly(vinylidene fluoride) (Oil-Based)

Poly(vinylidene fluoride) (PVDF) is one of the earliest binders used. It exhibits high mechanical strength and a wide electrochemical stability window, making it widely employed as a binder for battery electrodes in various systems, including lithium-ion batteries (LIBs). In the large-scale production of the lithium-ion battery industry, strongly polar organic compounds such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylformamide (DMF) are commonly used as solvents. PVDF is first dissolved in these solvents to form an oil-soluble solution, which is then used as a lithium battery binder.

Zhong et al. investigated the bonding mechanism between active material (AM) and poly(vinylidene fluoride) (PVDF) in lithium-ion batteries (LIBs) through density functional theory (DFT) simulations and analysis of the bonding interface between AM particles and the binder in LIB electrodes (Fig. 2). Results from process simulations and theoretical calculations indicated that in LiFePO₄ (LFP) batteries, the binding interaction between LFP and PVDF was significantly stronger than that between PVDF and aluminum (Al), whereas in Ni-Co-Mn (NCM) batteries, the binding interaction between NCM and PVDF was weaker than that between PVDF and Al. Scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) analyses revealed that in LFP batteries, PVDF was primarily distributed on the surface of LFP, indicating poor adhesive performance of PVDF in LFP batteries. In contrast, in NCM batteries, PVDF was uniformly distributed on the surfaces of active materials and Al, indicating good adhesive performance of PVDF in NCM batteries. These research findings suggest that the development of new PVDF-based binders for LIBs should prioritize enhancing the binding interaction between the binder and Al. Additionally, they confirmed that the interactions between AM, Al, and PVDF in LIBs are primarily physical rather than chemical.

2.2 Carboxymethyl Cellulose and Styrene-Butadiene Rubber (Water-Based)
Carboxymethyl cellulose (CMC) is a linear polymer derivative of cellulose, formed by the differential substitution of natural cellulose with carboxymethyl groups. As a polyprotic weak acid, CMC can dissociate to form carboxylate anionic functional groups. Additionally, the presence of carboxymethyl groups makes CMC more soluble in water compared to ethyl cellulose (EC), methyl cellulose (MC), and hydroxyethyl cellulose (HEC). This water solubility enables CMC to be used in water-based electrode production, offering advantages over PVDF in terms of low cost, non-toxicity, and environmentally friendly manufacturing. The free carboxylic acid groups in CMC can interact with hydroxyl groups on the surfaces of materials such as silicon/carbon, facilitating the formation of an ideal carbon-binder domain (CBD) network in electrodes. Furthermore, CMC is characterized by low cost, good thermal stability, and environmental friendliness, making it a potential binder for anodes in lithium-ion batteries (LIBs).

Studies by Lee et al. have shown that graphite slurries using carboxymethyl cellulose (CMC) with a lower degree of substitution as a binder exhibit better suspension stability. This is because CMC with a lower degree of substitution has stronger hydrophobicity, which enhances its interaction with the graphite surface in the aqueous medium. Drofenik et al. demonstrated that a graphite anode using a small amount of CMC (with a mass fraction of 2%) can achieve the same effect as a large amount of poly(vinylidene fluoride) (PVDF) (10%) binder. Moreover, this does not affect the normal intercalation/deintercalation of lithium ions in the graphite electrode or the formation of the solid electrolyte interphase (SEI) film. These findings indicate that the use of CMC reduces the required binder amount, which is beneficial for improving the energy density of lithium-ion battery (LIB) electrodes, making CMC an excellent anode binder for LIBs.

However, the aqueous carboxymethyl cellulose (CMC) binder exhibits strong rigidity and brittleness. After vacuum drying, cracks are visibly present on the surface of electrodes using CMC as the binder, which may even lead to gaps between the active material coating and the current collector, causing "material shedding" in the electrode. To address this issue, Liu et al. used styrene-butadiene rubber (SBR) as a flexible additive for the CMC binder. They compared the effects of SBR-CMC composite binders with traditional PVDF binders on the cycling stability of silicon (Si) anodes and investigated the mechanical properties and swelling behavior of SBR-CMC composite binders in electrolyte solutions. The results showed that the addition of SBR effectively reduced the brittleness of the electrode. Compared with PVDF binders, Si anodes using SBR-CMC composite binders exhibited a smaller Young’s modulus, greater maximum elongation, and stronger adhesion strength to the current collector.

Research by Dahn's group has shown that silicon (Si) electrodes fabricated with SBR-CMC composite binders exhibit better capacity retention compared to those made using only CMC binders. Meanwhile, their study revealed that due to CMC being a highly rigid and brittle polymer, the aqueous CMC binder performs well as a binder in electrodes with a high volume change rate of active material particles. However, the aqueous CMC binder absorbs less organic carbonate electrolytes than PVDF, which may affect the rate capability of electrodes using CMC as the binder.

Additionally, CMC is also used as an additive to improve the cycling stability of lithium-ion battery (LIB) anodes (e.g., Si and Sn alloys), which exhibit significant volume changes during battery cycling. The mechanism for the improvement in cycling performance is considered to be: (1) bridging between Si particles and carbonaceous conductive additive particles via CMC chains; (2) the formation of stable covalent bonds (Fig. 3) or self-healing hydrogen bonds on the surface of Si particles by CMC.

2.3 Poly(acrylic Acid)-Based Binders (Water-Based)
Poly(acrylic acid) (PAA) is a water-soluble polymer formed by the polymerization of acrylic acid monomers. Owing to the presence of a large number of carboxylic acid groups in its structure (Fig. 4), PAA can form strong interactions with active materials and aluminum foil, thereby exhibiting excellent binding properties. It serves as a promising high-performance binder for lithium-ion battery (LIB) electrodes. Furthermore, during the cycling of LIBs, PAA facilitates the formation of a stable cathode electrolyte interphase (CEI), which enhances the cycling stability of LIBs.

Su et al. first used poly(allyl lithium) (PAALi) as a binder for Li₃V₂(PO₄)₃ (LVP) and investigated the Li⁺ transport behavior in PAALi and its influence on the electrochemical performance of lithium-ion batteries (LIBs). The results showed that the novel PAALi binder exhibited excellent stability in organic electrolytes and good adhesion to all electrode components, thereby forming a continuous conductive network in the electrodes. The LVP battery using the new PAALi binder maintained a capacity retention rate of 91% after 1400 cycles at 10 C. Through testing and analysis such as Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), they demonstrated that the PAALi binder promotes Li⁺ transport at the electrode interface via a reversible H⁺/Li⁺ exchange reaction between –COOH and –COOLi groups (Fig. 5). This highly synergistic transport of electrons and Li⁺ in the PAALi-LVP battery enhances the electrode’s kinetic performance and provides a fast capacitive redox process, enabling it to achieve an excellent rate capability of 107 mAh/g at 70 C.

Chong et al. investigated the electrochemical performance of half-cells and full-cells in graphite/LiFePO₄ battery systems using PAA with added SBR as a binder versus PVDF as a binder. The results showed that PAAX (where X = H, Li, Na, or K) could effectively improve the initial Coulombic efficiency, reversible capacity, and cycling stability of graphite/LiFePO₄ batteries compared to PVDF. The addition of a small amount of SBR (0.5%–3.0%) prevented the formation of brittle cracks on the electrode surface after drying. Among the PAAX series binders, PAALi and PAANa exhibited better battery performance, which was attributed to their ability to form more favorable polymer conformations in the electrode composites (related to CEI). Additionally, water-based PAAX series binders can reduce the manufacturing cost of graphite/LiFePO₄ batteries and minimize environmental harm.

3 Summary and Outlook

Although binders in lithium-ion battery (LIB) electrodes are electrochemically inactive materials, they can co-form a carbon-binder domain (CBD) structure with conductive carbon nanoparticles. When the adhesion between the binder and the current collector is favorable, adjusting the cohesion of the CBD and the adhesion between active material (AM) and the CBD can form a high-quality CBD conductive network. This not only endows the electrode with strong mechanical strength and peeling resistance but also establishes a conductive network within the electrode that facilitates electron conduction, thereby enhancing electron transport efficiency. Additionally, it helps increase the AM-electrolyte interfacial area for ion conduction, reduces ion transport tortuosity within the electrode, and improves the quality of passivation layers (e.g., solid electrolyte interphase [SEI] and cathode electrolyte interphase [CEI] films) formed on the AM surface in contact with the electrolyte. Collectively, these effects exert a significant influence on the electrode’s electrochemical performance.

Currently, the commonly used binders for LIB electrodes primarily include oil-based binders represented by PVDF and water-based binders represented by CMC, SBR, and PAA as discussed in this paper. PVDF exhibits good adhesion to current collectors, and its molecular weight can be adjusted by modifying the degree of polymerization of vinylidene fluoride (VDF), thereby regulating its binding properties. It is currently widely used in the production of electrodes for various battery systems. Compared to the oil-soluble binder PVDF, water-based binders such as CMC, SBR, and PAA do not require the use of organic solvents during practical application, thus avoiding environmental pollution and harm to the health of operators caused by high-temperature organic solvent vapors. Among water-based binders, CMC, as a cellulose derivative, is characterized by its wide availability and low cost, meeting the requirement of low-cost LIBs. It can also serve as an additive for improving the cycling stability of silicon anodes, showing broad application prospects. The PAALi binder exhibits good binding performance and can replenish the consumption of active lithium during the cycling of LIBs, demonstrating significant development potential. It is expected to pave a new path for the development of high-performance binders for LIBs.

References: Fu Tiantian, Tao Fuxing, Li Chaowei, Zhang Yang, Wang Jiuzhou. Research Progress on Binders for Lithium-Ion Batteries[J]. Power Technology, 2023, 47(5): 570-574.