Comprehensive Solution for Negative Electrode Binders for Lithium Batteries                                                                                                                                                                                            

Overview of Lithium-ion Battery Binders

In the production of lithium-ion batteries, binders are often referred to as "auxiliary materials" together with conductive agents, additives, etc., but they are an indispensable key material. Their main function is to adhere active substances and conductive agents to the current collector to ensure electrical contact between powder material particles in the electrode and between the powder material and the current collector. Binders have a low mass ratio in the electrode and do not participate in electrochemical reactions. Their main function is to adhere active substances and conductive agents to the current collector to keep the electrode intact. Binders affect the formation of the solid electrolyte interface (SEI), charge transfer inside the electrode and between the electrode-electrolyte interface, the wetting behavior of the electrode, and the cycle performance and cost of the battery. Therefore, an ideal binder can ensure the stability of the electrode structure with the least possible usage.

Basic characteristics and mechanism of binders

1: Working mechanism

The essence of adhesive bonding is the interaction between material molecules (van der Waals force, surface tension, etc.), chemical bond force (hydrogen bond, covalent bond, coordination bond, etc.) and interfacial electrostatic attraction. The following are three theories of adhesive bonding, as shown in the schematic diagram:

(1) Diffusion theory: The polymer binder penetrates the surface of the electrode material and diffuses into the complex gaps inside the active material through the Brownian motion of the molecules. This diffusion is carried out at the interface between the electrode material surface and the binder , thus achieving a strong bonding effect.

(2) Electrostatic interaction theory: When the adhesive and adherend system is an electron acceptor/donor combination, a double electric layer can be formed at the interface between the two, thereby generating electrostatic attraction.

(3) Adsorption theory: After the binder diffuses to the electrode surface, adsorption force is generated between the two molecules of the binder and the adherend . Non-reactive binders mainly rely on intermolecular interactions (van der Waals forces), while reactive binders mainly rely on hydrogen bonds, covalent bonds, coordination bonds, etc. to form interfacial forces.

Figure: Schematic diagram of adhesive

2: Failure mechanism

Binder structure damage or failure will lead to ion and electron path destruction and active material loss, resulting in capacity attenuation and safety hazards. The damage mechanism can usually be attributed to three reasons:

(1) Contact interface damage: The adhesive strength is insufficient and the adhesive cannot effectively bond with the adherend , resulting in electrode detachment.

(2) Binder rupture: The binder will be affected by strain and stress changes during the battery cycle. When the stress exceeds the yield strength, the deformation of the polymer is plastic deformation. After the stress is removed, the strain cannot be fully recovered. When the stress exceeds the ultimate strength, the polymer breaks and fails.

(3) Adhesion fracture: The adhesion and mechanical strength of the binder meet the requirements, but during the battery cycle, the electrode material will crack and fall off, which will also cause the battery capacity to decrease.

In recent years, industry practitioners have made great progress in exploring the working mechanism of binders and their failure mechanism, but the structure-activity relationship of binders is very complex, and it is still challenging to establish an effective correspondence between binder structure and performance. In addition, since binders account for a small proportion of the electrode and are difficult to characterize, their failure mechanism in practical applications is still unclear, and more sophisticated characterization techniques (such as in-situ characterization, etc.) and theoretical simulation calculations are needed to further explore the failure mechanism of binders .

3: Performance requirements

Generally speaking, an ideal adhesive should have six characteristics:

(1) Stable thermal properties to maintain strong bonding over a wide operating temperature range

(2) Good mechanical properties, including tensile strength, elasticity, flexibility, hardness and bonding strength, to withstand the huge volume or strain changes in some special systems such as silicon (Si) negative electrode batteries

(3) Good electrical conductivity and ionic conductivity to ensure excellent electrochemical performance

(4) Excellent dispersibility in solvents to cover and connect the various components of the electrode and prevent uneven aggregation of the slurry

(5) Excellent chemical and electrochemical stability to meet the application requirements of different chemical solvents and different voltage windows

(6) Low cost, environmentally friendly, and easy to mass produce

Current status and research practice of lithium battery negative electrode binder

In lithium-ion batteries, the adhesives used for the negative electrode are similar to those used for the positive electrode, mainly including oil-based PVDF, water-based CMC, PAMAC, polyvinyl alcohol (PVA), sodium alginate, etc.

1. Adhesive for graphite negative electrode

CMC-based adhesives have currently become the main adhesives for graphite negative electrode materials. Jernei D et al. studied the bonding mechanism of CMC-based adhesives and the failure mechanism of graphite negative electrodes at different adhesive contents. Four CMC-based adhesives were used, namely methyl cellulose (MC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC) and methyl cellulose (CMC).

When the mass fractions of the four adhesives in the electrode were 1% and 2% respectively, the test results showed that, compared with the other three adhesives, CMC had the smallest irreversible capacity at mass fractions of 1% and 2%, and the critical mass fraction was 2%; when the mass fraction of CMC was 0.25%, when the negative electrode was fully charged, brown spots appeared on the surface of the golden negative electrode (see Figure 3). The experimental results showed that the electrode material at the brown spots was not in a fully charged state (there was a LiC12 diffraction peak in the X-ray diffraction spectrum). The reason was that during the electrode charging process, the graphite material on the electrode surface was charged first. Due to the low content of adhesive in the electrode, the volume of the graphite negative electrode expanded during the charging process, causing part of the material on the electrode surface to separate from the electrode body, resulting in a decrease in the reversible capacity of the electrode. The mechanism is that the adhesive itself forms a network structure and is distributed between the electrode material particles. The adhesive plays a spatial barrier role. If you want to achieve a bonding effect, you need a sufficient amount of adhesive to achieve the bonding effect.

By using gelatin (Fluka. No. 48722) and CMC as adhesives, respectively, and comparing the same graphite materials, it was found that in order to achieve an acceptable capacity value for the electrode material specific capacity, the critical mass fractions of the two adhesives are only 0.25% for gelatin and 2.00% to 5.00% for CMC .

2. Adhesive for silicon-based negative electrode

Silicon anode materials are one of the current research focuses of lithium-ion battery materials. Silicon has a theoretical capacity of up to 4200mA·h/g, a low lithium insertion and extraction potential (0-0.4V to lithium potential), and a lower first irreversible capacity than other metal and alloy anode materials. It is also abundant in nature and the raw material price is low, making it a very promising lithium insertion anode material.

However, during the lithium insertion and extraction process, the pure silicon material has a high expansion-contraction ratio (expansion rate as high as 400%), which leads to structural crushing and poor contact between the active material and the current collector. After several cycles, the electrode capacity has been greatly attenuated.

At present, in order to improve the electrochemical properties of silicon-based materials, the following methods are mainly adopted:

(1) Control electrode porosity

(2) Effective distribution of silicon material in electrodes

(3) Development of functional binders for high-energy-density batteries

At present, the lithium battery negative electrode binder industry is facing a series of technological innovation challenges, such as how to improve adhesion, reduce production costs, and improve battery reliability and cycle performance. With the continuous optimization and iteration of technology, the industrialization development of fluorine-free binders, the difficulty, cost, and environmental protection of PAA and PTFE modification processes are also important considerations and bottlenecks to be broken through in future research.

-End-

Canrd Brief Introduce

Canrd use high battery R&D technology(core members are from CATL) and strong Chinese supply chain to help many foreign companies with fast R&D. We provide lab materials,electrodes, custom dry cells, material evaluation, perfomance and test, coin/pouch/cylindrical cell equipment line, and other R&D services.

Email:janice@canrd.com    

Phone/Wechat/WhatsApp/Skype:+86 18928276992

Website : www.canrud.com