Current situation and development suggestions of auxiliary materials for lithium-ion batteries

Abstract: The lithium-ion battery material market has overcapacity, while the competition intensity in the auxiliary material market is low.

 New conductive agents, positive electrode binders and other products are highly dependent on imports and require domestic substitution, with huge development potential. 

Summarize the market and technological development status of four major lithium-ion battery auxiliary materials, including adhesives,

 lithium supplements, conductive agents, and current collectors. 

The current market for lithium-ion battery auxiliary materials is in a booming stage of development. 

The development of composite current collectors for negative electrode binders and positive electrode lithium supplements in China is relatively fast. 

However, there is a significant gap between positive electrode binders, carbon nanotube conductive agents, graphene conductive agents, and foreign counterparts.

 Analyze market demand, sort out key technologies, and look forward to the development trend of these auxiliary materials.

 It is recommended to adopt new product research and development, upstream and downstream cooperation, 

and vertical layout of the industrial chain to focus on the development of four types of auxiliary materials:

 high nickel ternary positive electrode binder, silicon-based negative electrode lithium supplement, single-walled carbon nanotube conductive agent,

 and polyethylene terephthalate (PET) based composite fluid.

Keywords: lithium-ion battery; Auxiliary materials; Collector fluid; Conductive agent; Lithium supplements; Adhesive

Although the proportion of key auxiliary materials such as conductive agents, adhesives, current collectors, and lithium supplements is not large, 

they are indispensable in the structure of lithium-ion batteries and can improve battery cycling performance and safety. 

They are a key part of battery production and manufacturing. 

With the gradual industrialization of battery systems such as high nickel ternary positive electrodes and silicon-based negative electrodes, 

traditional auxiliary materials can no longer meet the requirements of battery systems. 

Composite current collectors, positive electrode lithium supplements, graphene conductive agents,

 water-based binders and other auxiliary materials have gradually become research hotspots in the field of lithium-ion batteries.

The penetration rate of new energy vehicles and new energy storage markets has been increasing year by year, bringing huge demand for lithium-ion batteries and key materials. 

According to the latest data from the Ministry of Industry and Information Technology, 

the total production capacity of lithium-ion batteries in China will exceed 940 GW · h in 2023, a year-on-year increase of 25%. 

The production of four key materials, including positive electrode materials, negative electrode materials, separators, and electrolytes,

 will reach 2.3 million tons, 1.65 million tons, 15 billion m2, and 1 million tons, respectively,with growth rates exceeding 15%. 

The rapid expansion of production has led to increasingly fierce competition for key materials in domestic lithium-ion batteries, resulting in a decrease in corporate profit margins. 

The market for new auxiliary materials is far from being monopolized by leading enterprises, with low competition intensity and broad development space.

The author of this article discusses the current development status and trends of four major auxiliary materials for lithium-ion batteries, 

including adhesives, current collectors, conductive agents, and lithium supplements. 

The author analyzes the domestic market and technology status, proposes development suggestions, and aims to promote the rapid and high-quality development of the lithium-ion battery industry.

1 Adhesive

1.1 Positive electrode adhesive

The positive electrode mainly uses oil soluble binders represented by polyvinylidene fluoride (PVDF). 

At present, PVDF is basically monopolized by foreign companies such as Solvay,

 Wu Yu, and Arkema. Domestic products from Sanaifu and Dongyue Chemical are mainly used in low-end fields.

 PVDF has high mechanical strength and a wide electrochemical stability window, but it is prone to defluorination failure under alkaline conditions and therefore requires modification. 

The common modification methods include homopolymerization modification, copolymerization modification, grafting modification, and blending modification,

 with the aim of obtaining PVDF adhesives with low crystallinity, high adhesion, appropriate molecular weight, and uniform distribution. 

Oil based adhesives require the use of organic solvents that can easily cause environmental pollution, and have high preparation costs and complex processes. 

Therefore, in recent years, water-based adhesives with good thermal stability, green environmental protection,

low cost, and simple processing technology have attracted widespread attention in the industry and have the potential to replace PVDF in the field of positive electrode adhesives. 

Water based adhesives can be divided into two categories: natural polymer and synthetic polymer.

 Natural high molecular weight polymers mainly include brown seaweed, gelatin, and arabic gum; 

Artificially synthesized polymers include polyacrylonitrile (PAN), polyvinyl alcohol (PVA), 

polymethyl methacrylate (PMMA), hydrogenated nitrile rubber (HNBR), polytetrafluoroethylene (PTFE), and polyacrylic acid (PAA). 

At present, research on water-based adhesives mainly focuses on screening natural polymer materials, 

designing and developing artificially synthesized polymer materials, optimizing adhesive modification, and preparing various polymer composite multifunctional adhesives.

 Due to technological limitations, water-based adhesives are currently only suitable for battery systems such as lithium iron phosphate and lithium rich manganese based cathodes.

1.2 Negative electrode binder Negative electrode main

Water soluble adhesives represented by carboxymethyl cellulose (CMC)/styrene butadiene rubber (SBR) should be used. 

SBR foreign production enterprises are mainly Japan's Rayon and Japan Synthetic Rubber Company (JSR), with domestic Jingrui Corporation taking a leading position. 

CMC's foreign production enterprises mainly include Nippon Paper Co., Ltd. and Nippon Daihatsu, 

while domestic production enterprises mainly include Xinxiang Jinbang Power Technology Co., Ltd. and Chongqing Leehom Fine Chemical Co., Ltd.

Styrene butadiene rubber lotion (SBR) is polymerized from butadiene and styrene. 

The crosslinking degree and glass transition temperature of the product can be adjusted by adjusting the ratio of the two monomers. CMC, 

as a stabilizer and suspension dispersant, 

can make SBR more evenly dispersed, while utilizing the repulsive effect of space charge to ensure the stability of the entire system. 

SBR polymerization has two methods: lotion polymerization and solution polymerization. 

lotion polymerization is the mainstream process for SBR production because of its cost advantage and excellent product comprehensive performance. 

To meet the higher requirements for binders in high-energy density silicon-based negative electrodes, modification methods such as reducing particle size, 

introducing functional groups such as carboxyl groups, developing core-shell structures, and introducing new functional monomers are usually adopted to enhance the bonding strength.

2 lithium supplements

During the initial charging and discharging process of lithium-ion batteries,

 the electrolyte decomposes on the negative electrode surface to form a solid electrolyte interface (SEI) film,

 resulting in irreversible energy loss and reducing battery capacity and energy density.

 To this end, people have developed pre lithiation technology, 

which adds a small amount of lithium source to the electrode material before the operation of lithium-ion batteries to compensate for the loss of active lithium, improve energy density,

 reduce internal resistance, and extend cycle life. Lithium supplements should meet the following four basic requirements: 

① The voltage during the lithium removal process should be within the working voltage range of the electrode material; 

② Has a high specific capacity;

 ③ Compatible with electrolyte and battery production processes;

④ Has good stability. According to different lithium supplementation methods,

 lithium supplements can be divided into positive electrode lithium supplements and negative electrode lithium supplements. 

Negative electrode lithium supplements are incompatible with conventional solvents, binders, and heat treatment processes, and are expensive and difficult to industrialize. 

The industrialization process of positive electrode lithium supplements is relatively fast, 

and currently LNO (Li2NiO2) and LFO (Li5FeO4) have achieved mass production and installation first.

 LNO is currently the most widely used lithium supplement, and with the gradual implementation of LFO production capacity,

 the market share of LFO will significantly increase in the future.

Due to the instability of most high-capacity lithium supplements in air and their tendency to react with water and carbon dioxide,

 modification methods such as coating and passivation are needed to improve stability. 

The research focuses on forming a stable coating layer and controlling the thickness of the coating layer to avoid crystallization on the negative electrode surface. 

The coating layer can ensure the smooth release of Li+from the lithium replenishment agent, while preventing the inner layer from reacting with water and carbon dioxide in the air.

3 conductive agents

After mixing conductive agents with active substances and binders, coating them on the electrode can improve the electron transfer efficiency in the electrode, 

thereby increasing the specific capacity and rate charging and discharging ability of the battery, 

thereby improving charging efficiency and extending service life. 

Traditional conductive agents are mainly conductive carbon black and conductive graphite. 

Conductive carbon black has outstanding cost advantages, 

especially in the lithium iron phosphate lithium-ion battery system, occupying an important position. 

New conductive agents include gas-phase grown carbon fibers, carbon nanotubes (CNTs), graphene, etc. 

Due to their unique point line and point surface contact networks, which differ from traditional conductive agents,

 they have excellent conductivity and can improve battery energy density. 

The addition amount is small, and for silicon carbon negative electrodes, it can also reduce the volume expansion rate and extend battery cycle life.

3.1 Carbon nanotubes (CNT)

CNT conductive agents have irreplaceable performance advantages in the high nickel ternary and silicon-based negative electrode markets. 

The research and development process is constantly accelerating, 

and Tiannai Technology and Sanshun Nano have successfully achieved mass production of CNT conductive agents, 

showing a trend of replacing traditional carbon black in the high-end power battery market. 

At present, CNT products mainly include single-walled CNT and multi walled CNT, with single-walled CNT having better conductivity but higher technical barriers.

 Overseas OCSiAl company (nanotechnology company) holds an absolute leading position in single-walled CNT production capacity, 

while domestically it mainly produces multi walled CNTs, and there is still room for catching up in product performance.

The preparation methods of CNT include chemical vapor deposition (CVD), graphite arc method, laser evaporation method, hydrothermal method, etc.

The CVD method has the advantages of easy control of the reaction process, low reaction temperature, high product purity, and high single batch yield.

 It is the mainstream production process, mainly using small molecule compounds such as acetylene, methane, CO, ethylene, propylene, butene, and benzene as carbon sources. 

The core process includes powder preparation, powder purification, and slurry dispersion. 

In the process of powder preparation, the catalyst determines the aspect ratio of CNTs, and the larger the aspect ratio, the better the performance. 

Research mainly focuses on transition metal catalysts such as Fe, Co, Mo, Pt, etc.

 Powder purification mainly removes metal ions from carbon nanotube coarse powder to avoid battery short circuits, 

usually including two steps: high-temperature oxidation and acidification. 

High temperature oxidation refers to the oxidation reaction between CNT coarse powder and high-temperature air, 

which oxidizes the impurities contained in carbon nanotubes into carbon dioxide, removes amorphous carbon and other impurities, 

and acidifies mainly to remove residual catalyst in CNT coarse powder. 

Slurry dispersion refers to the process of dissolving, dispersing, and grinding CNT powder, dispersants, and solvents to obtain CNT slurry. 

CNT is prone to agglomeration due to van der Waals forces, which can seriously affect battery performance. 

The main method to improve the dispersibility of CNT is to add dispersants, such as polyetheramine and epoxy resin composites, ethylene and acrylamide copolymers, etc.

3.2 Graphene

Graphene is a honeycomb like planar thin film formed by the ordered arrangement of adjacent carbon atoms through sp2 hybridization in a two-dimensional plane.

 It has a unique layered structure and can form point to surface contact with electrode particles, increasing the contact area and exhibiting good conductivity, 

reducing the amount of conductive agent and lowering costs. 

China's graphene industry started relatively late, and there are significant differences in the development processes of related materials. 

The main production enterprises of conductive graphene slurry include Qingdao Haoxin New Energy Technology Co., Ltd., 

Zhongke Yueda, Xiamen Kaina Graphene Technology Co., Ltd.,

 Dongguan Hongna New Material Technology Co., Ltd., etc.

At present, foreign countries mainly use the "bottom-up" CVD method and epitaxial growth method to prepare graphene, 

and the products are mostly single-layer graphene with large specific surface area and good quality. 

China mostly uses the "top-down" oxidation-reduction method to prepare graphene, which is low-cost and easy to produce on a large scale. 

However, the products are mostly multi-layer graphene with high lattice defects, and the product performance needs to be improved.

 In addition, graphene has a high theoretical specific surface area, high van der Waals forces, and is highly prone to agglomeration.

 Moreover, graphene has few functional groups on its surface and poor compatibility with polymers. 

Therefore, graphene dispersion methods are currently a research hotspot, which mainly include: 

① chemical modification to graft functional groups onto graphene; 

② In situ processing method, adding coating powder during graphene synthesis process; 

③ Introducing the dispersant method, where the dispersant is mainly an organic solvent, including turpentine alcohol, dimethylformamide, N-methylpyrrolidone, etc 

The composite conductive agent method utilizes two or more conductive agents to produce synergistic effects, such as CNT/graphene, carbon black/CNT, carbon black/CNT/graphene, etc., 

which can balance conductivity, cost, and dispersion to a certain extent.

4 collection fluids

The current collector can not only carry active materials, but also collect and output the current generated by the electrode active materials, 

which is beneficial for reducing the internal resistance of the battery, improving the Coulombic efficiency, cycle stability, and rate performance of the battery. 

Thinning the current collector can improve its ability to carry active substances to a certain extent, directly increasing the energy density of the battery. 

Therefore, the current collector has been developed in an extremely thin direction, but the current technology can only have a minimum thickness of 6-10 μ m. 

Otherwise, the mechanical strength of the current collector will decrease, deformation and fracture will occur, affecting safety. 

The composite current collector adopts a sandwich structure and a polymer matrix. 

Two layers of 1 μ m thick aluminum/copper metal are stacked on top and bottom of the base film through vacuum coating and other processes,

which have the advantages of improving safety, energy density, and reducing costs, providing ideas for cost reduction and efficiency improvement of metal current collectors. 

At present, the large-scale supply of composite current collectors has not been achieved, and major enterprises include Chongqing Jinmei, Anmaite, etc.

Polymer matrix is a key component of composite current collectors and a key breakthrough in the industrialization of composite current collectors.

 Common polymers include polyethylene terephthalate (PET), polypropylene (PP), and polyimide (PI).

 PET has high tensile strength, obvious cost advantages, and the fastest industrialization process. 

Foreign production companies include Japan's Toray, Teijin, and the United States' 3M,

 while domestic PET substrate manufacturers include Double Star New Materials, Kanghui New Materials, and Dongma Technology.

 Among them, Double Star New Materials has achieved mass supply and gradually formed an integrated production capacity for PET copper foil. 

At present, the main problem with PET is that it is not resistant to strong acids and alkalis. 

When used as a negative electrode current collector, it is easily corroded at high electrolyte temperatures, leading to a decrease in battery cycling performance. 

PP is resistant to strong acids and alkalis, but has low tensile strength. It is prone to breakage at high coating speeds, resulting in low production yield. 

Moreover, PP is a non-polar molecule with a low melting point and weak adhesion.

 During the long roll processing, it is prone to forming pores and has poor adhesion to the metal layer. 

PI is currently the best comprehensive performance thin film insulation material, 

with excellent mechanical properties, electrical properties, chemical stability, high radiation resistance, high temperature and low temperature resistance. 

However, its cost is high, and its market space is relatively small in the context of cost reduction and efficiency improvement in various aspects of batteries.

5 Conclusion and Suggestions

At present, the four key material technologies for lithium-ion batteries,

 including positive electrode materials, negative electrode materials, electrolytes, and separators, are relatively mature. 

The products have shown a situation of overcapacity, and the price war is becoming increasingly fierce. 

The overall profit margin of enterprises is being compressed. 

The localization level of the four major auxiliary materials, namely adhesives, conductive agents, lithium supplements, and current collectors, is relatively low,

 and the market competition intensity is small. Some products still rely heavily on imports, resulting in a significant market gap. 

At the same time, with the gradual industrialization of battery systems such as high nickel ternary positive electrodes and new silicon-based negative electrodes, 

traditional auxiliary materials can no longer meet the requirements, and supporting auxiliary materials need to be developed. 

The summary and suggestions for various materials are as follows: adhesives.

Adhesive manufacturers should design different types of high-performance adhesives for high nickel ternary positive electrode and silicon-based negative electrode materials, 

broaden the types of adhesives, enrich product grades, and provide industrial support for high-performance electrode materials to improve the various properties of lithium-ion batteries. 

In addition, we should increase the basic research efforts on adhesives, 

starting from the molecular structure of adhesives, further understanding the mechanism of action, improving the bonding ability and thermal stability of adhesives,

 and optimizing the performance of lithium-ion batteries.

Lithium supplements. At present, positive electrode lithium supplements have formed a certain industrial scale, 

and leading enterprises have certain advantages in production capacity and customer stickiness.

 However, silicon-based negative electrode lithium supplements are still in the process of industry and incubation, and have not yet formed an advantageous technological path and leading enterprises,

 with huge market development space. 

Silicon based negative electrode production enterprises should combine their own characteristics of silicon-based negative electrode materials to develop negative electrode lithium supplements, 

solve industrial pain points such as high cost, poor safety, 

and poor adaptability to negative electrodes, 

develop silicon-based negative electrode and pre lithiation production processes, study the optimal ratio of lithium supplements to negative electrode materials, 

reduce the production cost of lithium supplements, solve the key problem of poor cycle efficiency of silicon-based negative electrodes, 

and lay a good foundation for promoting them to the market as soon as possible.

Conductive agent. The intrinsic conductivity of silicon-based negative electrodes is poor, and traditional conductive agents are difficult to meet the demand. 

The research and development of new conductive agents has become a common focus of attention for negative electrode production enterprises. 

CNT raw materials include propylene, N-methylpyrrolidone, and liquid nitrogen.

 Enterprises can adopt an integrated upstream and downstream development strategy to achieve independent production of raw materials and control CNT production costs. 

At present, only low-end CNT products can be produced domestically. 

Enterprises can cooperate with universities to form a single-walled CNT production process with independent intellectual property rights,

 breaking the monopoly of foreign countries on high-end CNTs; 

In terms of graphene conductive agents, 

graphene/CNT composite technology can be developed to reduce costs, accelerate the promotion of related products, and quickly occupy the market.

Collecting fluid. The composite current collector market has a large capacity and low industry concentration, making it a "newcomer" in the field of lithium-ion battery materials investment. 

Multiple companies have entered the current current current collector market, and the industry is about to enter the first year of mass production.

 At present, the three major routes of PET, PP, and PI each have their own advantages and disadvantages: the industrialization process of PET is relatively fast,

 but the problem of acid and alkali resistance has not been solved, and a clear technical route has not yet been formed; 

The technical uncertainty of the other two routes is relatively high, and there are opportunities for overtaking on bends, 

which may form an overall pattern of different substrates for different battery materials. 

It is recommended to develop composite fluid collection production technology based on downstream application market needs and seize the fluid collection track.