As the main binder of water-based negative electrode materials, CMC products are widely used by domestic and foreign battery manufacturers. The optimal amount of binder can obtain relatively large battery capacity, long cycle life and relatively low internal resistance.

Binder is one of the important auxiliary functional materials in lithium-ion batteries. It is the main source of the mechanical properties of the entire electrode and has an important impact on the production process of the electrode and the electrochemical performance of the battery. The binder itself has no capacity and occupies a very small proportion in the battery.

In addition to the adhesive properties of general binders, lithium-ion battery electrode binder materials also need to be able to withstand the swelling and corrosion of the electrolyte, as well as withstand the electrochemical corrosion during charge and discharge. It remains stable in the working voltage range, so there are not many polymer materials that can be used as electrode binders for lithium-ion batteries.

There are three main types of lithium-ion battery binders that are widely used at present: polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR) emulsion and carboxymethyl cellulose (CMC). In addition, polyacrylic acid (PAA), Water-based binders with polyacrylonitrile (PAN) and polyacrylate as the main components also occupy a certain market.

Four characteristics of battery-level CMC

Due to the poor water solubility of the acid structure of carboxymethyl cellulose, in order to better apply it, CMC is a very widely used material in battery production.

As the main binder of water-based negative electrode materials, CMC products are widely used by domestic and foreign battery manufacturers. The optimal amount of binder can obtain relatively large battery capacity, long cycle life and relatively low internal resistance.

The four characteristics of CMC are:

First, CMC can make the product hydrophilic and soluble, completely soluble in water, without free fibers and impurities.

Second, the degree of substitution is uniform and the viscosity is stable, which can provide stable viscosity and adhesion.

Third, produce high-purity products with low metal ion content.

Fourth, the product has good compatibility with SBR latex and other materials.

The CMC sodium carboxymethyl cellulose used in the battery has qualitatively improved its use effect, and at the same time provides it with good use performance, with the current use effect.

The role of CMC in batteries

CMC is a carboxymethylated derivative of cellulose, which is usually prepared by reacting natural cellulose with caustic alkali and monochloroacetic acid, and its molecular weight ranges from thousands to millions.

CMC is a white to light yellow powder, granular or fibrous substance, which has strong hygroscopicity and is easily soluble in water. When it is neutral or alkaline, the solution is a high-viscosity liquid. If it is heated above 80℃ for a long time, the viscosity will decrease and it will be insoluble in water. It turns brown when heated to 190-205°C, and carbonizes when heated to 235-248°C.

Because CMC has the functions of thickening, bonding, water retention, emulsification and suspension in aqueous solution, it is widely used in the fields of ceramics, food, cosmetics, printing and dyeing, papermaking, textiles, coatings, adhesives and medicine, high-end ceramics and lithium batteries The field accounts for about 7%, commonly known as “industrial monosodium glutamate”.

Specifically in the battery, the functions of CMC are: dispersing the negative electrode active material and conductive agent; thickening and anti-sedimentation effect on the negative electrode slurry; assisting bonding; stabilizing the processing performance of the electrode and helping to improve the battery cycle Performance; improve the peel strength of the pole piece, etc.

CMC performance and selection

Adding CMC when making the electrode slurry can increase the viscosity of the slurry and prevent the slurry from settling. CMC will decompose sodium ions and anions in aqueous solution, and the viscosity of CMC glue will decrease with the increase of temperature, which is easy to absorb moisture and has poor elasticity.

CMC can play a very good role in the dispersion of negative electrode graphite. As the amount of CMC increases, its decomposition products will adhere to the surface of graphite particles, and the graphite particles will repel each other due to electrostatic force, achieving a good dispersion effect.

The obvious disadvantage of CMC is that it is relatively brittle. If all CMC is used as the binder, the graphite negative electrode will collapse during the pressing and cutting process of the pole piece, which will cause serious powder loss. At the same time, CMC is greatly affected by the ratio of electrode materials and pH value, and the electrode sheet may crack during charging and discharging, which directly affects the safety of the battery.

Initially, the binder used for negative electrode stirring was PVDF and other oil-based binders, but considering environmental protection and other factors, it has become mainstream to use water-based binders for negative electrodes.

The perfect binder does not exist, try to choose a binder that meets the physical processing and electrochemical requirements. With the development of lithium battery technology, as well as cost and environmental protection issues, water-based binders will eventually replace oil-based binders.

CMC two major manufacturing processes

According to different etherification media, the industrial production of CMC can be divided into two categories: water-based method and solvent-based method. The method using water as the reaction medium is called the water medium method, which is used to produce alkaline medium and low-grade CMC. The method of using organic solvent as the reaction medium is called the solvent method, which is suitable for the production of medium and high-grade CMC. These two reactions are carried out in a kneader, which belongs to the kneading process and is currently the main method for producing CMC.

Water medium method: an earlier industrial production process, the method is to react alkali cellulose and etherification agent under the conditions of free alkali and water, which is used to prepare medium and low-grade CMC products, such as detergents and textile sizing agents Wait. The advantage of the water medium method is that the equipment requirements are relatively simple and the cost is low; the disadvantage is that due to the lack of a large amount of liquid medium, the heat generated by the reaction increases the temperature and accelerates the speed of side reactions, resulting in low etherification efficiency and poor product quality.

Solvent method; also known as organic solvent method, it is divided into kneading method and slurry method according to the amount of reaction diluent. Its main feature is that the alkalization and etherification reactions are carried out under the condition of an organic solvent as the reaction medium (diluent) of. Like the reaction process of the water method, the solvent method also consists of two stages of alkalization and etherification, but the reaction medium of these two stages is different. The advantage of the solvent method is that it omits the processes of alkali soaking, pressing, crushing, and aging inherent in the water method, and the alkalization and etherification are all carried out in the kneader; the disadvantage is that the temperature controllability is relatively poor, and the space requirements are relatively poor. ,higher cost.