Binder, Conductive Additive, Active Material, Current Collector: Who Controls the Critical Factors of Electrode Flexibility?

1. Introduction

Lithium-ion batteries (LIBs), as efficient and environmentally friendly energy storage devices, are widely used in electric vehicles, consumer electronics, and other fields. The electrode, one of the core components of a battery, directly impacts its energy density, cycle life, and safety performance. Electrode flexibility refers to the ability of the electrode to deform under external force without fracturing and is a crucial indicator for evaluating the mechanical properties of the electrode.

Poor electrode flexibility can lead to cracking and powdering of the electrode during charge/discharge cycles, resulting in deteriorated contact between the active material and the current collector, increased internal resistance, accelerated capacity decay, and ultimately shortened battery lifespan. Furthermore, poor electrode flexibility increases the risk of internal short circuits under mechanical impact, threatening battery safety. Therefore, investigating the factors influencing electrode flexibility and developing highly flexible electrode materials are of significant importance. The electrode is primarily composed of active material, conductive additive, binder, and current collector (Figure 1). These components synergistically regulate its flexibility. The roles and influencing mechanisms of each component are analyzed individually below.

Figure 1. Composition of electrode components

2. Active Material: The “Dominant Player” in Volume Change and Stress Distribution

The active material is the primary component in the electrode responsible for storing and releasing lithium ions. Its type and morphology influence electrode flexibility.

• Type: Different active materials exhibit varying degrees of volume change during charge/discharge. For instance, silicon-carbon anodes can experience volume expansion rates as high as 300%, while graphite anode materials typically expand around 10%. Volume changes induce stress; localized stress can cause internal fracture of anode particles or delamination of the Solid Electrolyte Interphase (SEI) layer (Figure 2). Both particle fracture and SEI layer delamination expose fresh electrode surfaces, leading to continuous SEI formation and accelerated electrode aging.

• Morphology: Spherical or near-spherical active material particles are beneficial for enhancing electrode flexibility. This is attributed to their higher packing density and more uniform stress distribution, which effectively buffer the stress generated by volume changes. Conversely, plate-like or needle-like particles tend to create stress concentration points, leading to localized cracking within the electrode.

Figure 2. Schematic diagram of material cracking

3. Conductive Additive: The “Double-Edged Sword” of Conductive Network and Mechanical Strength

The primary role of the conductive additive is to enhance the electronic conductivity of the electrode. Its type, content, and dispersion also affect electrode flexibility.

• Type: One-dimensional (1D, e.g., carbon nanotubes) or two-dimensional (2D, e.g., graphene) conductive additives can form a three-dimensional (3D) conductive network, connecting active material particles and the current collector, thereby improving both the mechanical strength and flexibility of the electrode.

• Content:

  • Optimal Content: Forms a continuous conductive network, enhancing electrode conductivity and mechanical strength.

  • Excessive Content: Occupies space intended for active material, reducing electrode energy density, and potentially increasing brittleness.

• Dispersion: Uniform distribution of the conductive additive effectively buffers stress and improves electrode flexibility. Agglomeration of the conductive additive leads to localized stress concentration, reducing electrode flexibility.

4. Binder: The “Adhesive” for Electrode Components

The binder is a vital component of the electrode. Its main function is to bond the active material, conductive additive, and current collector, imparting mechanical strength to the electrode. The type, content, and molecular weight of the binder influence electrode flexibility.

• Type: Different binders possess varying mechanical properties and adhesion strengths. For example, polyvinylidene fluoride (PVDF) binders offer high mechanical strength but relatively poor flexibility, making them prone to cracking during cycling. Conversely, styrene-butadiene rubber (SBR), with its high elastic modulus, can effectively buffer stresses induced by volume changes, enhancing electrode flexibility.

• Content:

  • Insufficient Content: Results in inadequate mechanical strength, making the electrode prone to cracking.

  • Excessive Content: Reduces the proportion of active material in the electrode, negatively impacting battery energy density.

• Molecular Weight: High molecular weight binders form stronger adhesive networks, increasing the mechanical strength of the electrode, but may reduce its flexibility.

5. Current Collector: The “Supporting Skeleton” of the Electrode

The current collector serves as the carrier for electron transport within the electrode. Its material, thickness, and surface treatment affect electrode flexibility.

• Material: Metals like copper foil and aluminum foil possess inherent ductility and flexibility, providing fundamental mechanical support to the electrode and contributing to its overall flexibility. However, if the current collector itself is excessively thick, has low purity, or contains numerous impurities, its flexibility may be compromised, adversely affecting the electrode’s flexibility.

• Thickness: Increasing current collector thickness enhances the overall strength of the electrode but may reduce flexibility. Thicker current collectors require greater force to bend and are more susceptible to crack formation.

• Surface Treatment: Roughening the surface of the current collector increases the contact area with the active material, enhancing adhesion and creating a more robust bond. This generally improves electrode flexibility. However, excessive roughness can lead to non-uniform distribution of the active material, potentially decreasing flexibility.

6. Innovative Testing Methods for Electrode Flexibility

Electrode flexibility is a key factor influencing the cycle life and safety performance of lithium-ion batteries. By optimizing electrode composition and manufacturing processes, highly flexible electrodes can be developed, thereby improving battery cycle life and safety, and advancing lithium battery technology. To accurately assess electrode flexibility, IEST Instrument has developed a testing instrument based on stress-strain curve analysis (Figure 3). This method secures the electrode, applies displacement, and measures the force and displacement (force-displacement curve) in real-time during deformation, enabling quantitative analysis of flexibility. This technology provides crucial data support for optimizing electrode composition and processing, facilitating the development of high-flexibility electrodes.

Figure 3. IEST Battery Electrode Flexibility Testing System (BEF1000) Equipment

• Compare the shape of the curves: Higher flexibility electrodes may exhibit smoother curves with no distinct abrupt changes or inflection points.
• Compare the slopes: Electrodes with better flexibility typically show a smaller initial slope in their curve, indicating they undergo larger deformation under the same applied force.
• Analyze the fracture point: Generally, electrodes exhibiting a larger compression displacement at the fracture point possess superior flexibility.
• Observe the first derivative: Electrodes with poorer flexibility may exhibit larger peaks or sharp changes in their first derivative curve.

7. Summary

Electrode flexibility is a core indicator for battery performance and safety. By synergistically optimizing the selection and ratios of active material, conductive additive, binder, and current collector, and leveraging advanced testing methodologies, the comprehensive performance of electrodes can be significantly enhanced. This lays a solid foundation for the development of next-generation lithium-ion batteries with high energy density and long cycle life.