Research on 6LR61 battery electrode binders

　　 Lithium-ion battery is a new type of high-performance rechargeable battery. As an important component of battery positive and negative electrode materials, binders have a great impact on battery performance. The current research status of lithium-ion battery adhesives and their applications in different electrode materials are introduced. In particular, some new electrode materials and more complex application environments have put forward higher requirements for lithium-ion battery adhesives.

　　 Due to their high energy density, lithium-ion batteries play an increasingly important role in new energy sources. Lithium-ion batteries have an energy density of over 150 Wh kg-1, which is the highest energy density of almost any known secondary battery. To further improve the performance of lithium-ion batteries, researchers are working hard to find new electrode materials, electrolytes and additives. However, the efficiency of lithium-ion batteries depends largely on the optimization of electrode preparation conditions [1-2]. One of the most important aspects is finding the best adhesive for the electrode being used. Binder is an important component of the positive and negative electrode materials of lithium batteries. It can closely combine the active material, conductive agent and current collector in the electrode material, enhance the electronic contact between the active material and conductive agent and between the active material and the current collector, and better stabilize the structure of the pole piece.

　　In lithium-ion batteries, non-aqueous carbonates such as propylene carbonate and ethylene carbonate are commonly used. Therefore, the adhesive is required to have the following characteristics [3]: It can maintain stability when heated to 130~180°C during drying and dehydration. Can be wetted by organic electrolytes; has good processing properties; is not easy to burn; is stable to lithium chloride, lithium fluoride, etc. Electrolyte and by-products LiOH, Li2CO3, etc. It has high electronic ion conductivity; small dosage and low price. In addition, the binder is also required to have good electrochemical stability and not react under the working voltage of the electrode. The adhesive is required to be insoluble in the polar electrolyte and expand less to ensure that the electrode material does not fall off and the powder does not fall off. In some electrode materials whose volume changes greatly during charging and discharging, the binder is required to play a certain buffering role in the volume change. At present, commercial lithium-ion batteries generally use polyvinylidene fluoride (polyvinylidene fluoride) as the binder of lithium-ion batteries because polyvinylidene fluoride has good electrochemical stability and resistance to electrode materials and current collectors. High adhesion. However, as further demands are placed on the performance of lithium batteries, some other types of binders have emerged.

　　 This article mainly introduces a series of different binders, including polyvinylidene fluoride binders, water-soluble binders, conductive binders, ionic polymer binders, etc. This paper expounds the current research progress on binders for lithium-ion battery electrode materials at home and abroad from the aspects of binder requirements for electrode materials, binder characteristics and bonding mechanisms.

　　1PVDF adhesive Organic fluoropolymer adhesive is a commonly used adhesive, mainly polyvinylidene fluoride (polyvinylidene fluoride), including homopolymers, copolymers and other vinylidene fluoride Modification[4]. The fluorine content of polyvinylidene fluoride reaches 59.3%. Thermoplastic polyvinylidene fluoride has excellent mechanical and processing properties compared to perfluoropolytetrafluoroethylene (PTEF). When polyvinylidene fluoride is used as a binder for batteries, N-methylpyrrolidone (NMP) is often used as a solvent. This process is relatively mature and widely used in electrode materials.

　　Huang [5-6] et al. used lithium iron phosphate as the active material, carbon black as the conductive agent, and polyvinylidene fluoride as the binder to prepare lithium cathode materials. The resulting battery reaches 90% of the theoretical capacity at C2 and has high chemical stability. In addition, the author also prepared Li3V2(PO4)3 cathode material and used polyvinylidene fluoride as the binder. The battery capacity almost reached the theoretical capacity, and the prepared electrode material has good chemical stability. This indicates that polyvinylidene fluoride can be well applied to this electrode material.

　　Chen [7] et al. studied the properties of polyvinylidene fluoride-polytetrafluoroethylene-polypropylene triblock copolymer. Compared with polyvinylidene fluoride, the study found that polyvinylidene fluoride-polytetrafluoroethylene-polypropylene The component mixture has an elongation at break of 100%, while polyvinylidene fluoride has an elongation at break of less than 10%. The author believes that the binder of amorphous alloy cathode materials must have high elasticity and be able to withstand large volume changes during charge and discharge to maintain the aggregation state of electrode materials and ensure electron transfer between active materials and current collectors. Therefore, polyvinylidene fluoride-tetrafluoroethylene-polyvinylidene fluoride can replace polyvinylidene fluoride as a binder for large volume changes in electrode materials.

　　Zhou [8] and others studied the effect of polyvinylidene fluoride binder content on the performance of cathode materials. The study found that as the mass fraction of polyvinylidene fluoride increases, the first charge and discharge efficiency of lithium batteries increases, but the capacity decreases. Comprehensive consideration, when the polyvinylidene fluoride content is 4%, the battery performance is optimal. The first charge and discharge efficiency of the battery is 91%, and the specific capacity is 190mah/g. The author believes that if the polyvinylidene fluoride content is too high, the effective contact between the active material and the conductive agent will be reduced, resulting in a decrease in capacity. When the polyvinylidene fluoride content is too low, the adhesion between the active material and the conductive agent is reduced, which also reduces the performance of the battery.

　　 Polyvinylidene fluoride binder has good bonding properties, but its electrical conductivity and ionic conductivity are poor, and it does not work well in some electrode materials (such as silicon, tin, etc.). ) has a large volume change during charging and discharging, so it is necessary to find new binders.

　　  2Water-soluble adhesive

　　 The use of organic solvents will cause certain environmental pollution, while water-soluble adhesives use water as a dispersant, which is more environmentally friendly and has excellent performance. Currently, water-soluble binders have been used as cathode materials, such as sodium carboxymethylcellulose (sodium carboxymethylcellulose) and styrene-butadiene rubber (styrene-butadiene rubber) latex, which have been widely used. Research on water-soluble binders has become an important direction. Buqa [9] et al. studied the binders of graphite and nano-silicon anode materials and compared the properties of styrene-butadiene rubber, sodium carboxymethylcellulose and their blends with polyvinylidene fluoride. The study found that the binding properties of the three adhesives were similar to polyvinylidene fluoride, but the first cycle irreversible capacity of sodium carboxymethylcellulose was lower than that of polyvinylidene fluoride. A mixture of styrene-butadiene rubber and sodium carboxymethylcellulose as a graphite or silicon anode binder has good electrochemical stability, only 1% styrene-butadiene rubber and 1% sodium carboxymethylcellulose as a binder exhibit better electrochemical stability Same cycle stability as 10% polyvinylidene fluoride. At the same time, sodium carboxymethyl cellulose is soluble in water, and styrene-butadiene rubber is soluble in acetate. They do not need to use the organic solvent NMP as a binder, which is more environmentally friendly and has lower processing costs. The author also pointed out that the amount of styrene-butadiene rubber and sodium carboxymethylcellulose blend as a binder should not exceed 6% of the electrode material, otherwise it will affect the migration of lithium ions and lead to a decrease in battery performance.

　　Liu [10] et al. pointed out that the silicon anode using elastic styrene-butadiene rubber (styrene-butadiene rubber) and carboxymethyl cellulose (carboxymethyl cellulose) as negative electrode binders has better performance than polyvinylidene fluoride (polyvinylidene fluoride). Ethylene), the difference in cycle stability comes from macroscopic mechanical properties. However, Li et al. found that the capacity retention rate of carboxymethylcellulose alone as a negative silicone binder was higher than that of a blend of carboxymethylcellulose and styrene-butadiene rubber. Since carboxymethylcellulose is a rigid polymer with only a small elongation at break, it differs from elastic adhesives. Therefore, he believes that the reasons why carboxymethyl cellulose and other rigid polymers achieve better results still need to be further explored. Liu·[12] and Guy·[13] analyzed the bonding mechanism based on the above results. They believe that in the electrode slurry, polymer chain segments are adsorbed or adsorbed between different particle components to form a three-dimensional network structure. When the solvent evaporates, its form is retained, so the particulate materials in the electrode material are linked together by polymer chains that act as a bond.

　　Leistriz [14] and others found that although carboxymethylcellulose is not an elastic material, it can be used as a binder to significantly improve the cycle performance of silicon anode batteries. Carboxymethylcellulose can take extended forms in solution and form a network structure during electrode preparation. At the same time, the study also found that the structure of carboxymethylcellulose can be changed by controlling the pH. When the pH is controlled at 3, the polymer chain segments form a three-dimensional network structure through intramolecular and intermolecular hydrogen bonds. In this case, the reversible capacity of the battery obtained is four times that of normal neutral conditions.

　　  3Conductive Adhesive

　　In the design of common electrode materials, active materials and conductive agents (such as acetylene black) are bonded together with non-conductive adhesives (such as polyvinylidene fluoride and carboxymethylcellulose). The volume of the graphite electrode does not change much during the cycle, only 10%. However, in some electrode materials with large volume changes, the conductive agent acetylene black does not have a flexible structure and cannot adapt to the expansion and contraction of the active material. The volume change of the active material causes stress on the conductive agent such as acetylene black during the charge and discharge process, causing the active particles to separate from the conductive network of the pole piece, thereby reducing the capacity retention rate of the electrode. Therefore, it is of great significance to develop a new adhesive to accommodate this active substance. Conductive glue is an adhesive that can conduct electrons. It can increase the conductivity of electrode materials, reduce the use of conductive agents, and at the same time play a bonding role. It has great advantages in preparing electrode materials. Pan[et al. prepared a polyaniline-polyoxyethylene conductive adhesive with a polyaniline content of 50%. The author pointed out that the conductive adhesive is conductive and can reduce the amount of conductive agent or add no additional conductive agent, thereby increasing the capacity of the battery. At the same time, the addition of conductive glue can reduce the contact resistance between the active material and the current collector, thereby improving the electrochemical performance of the battery. Compared with existing adhesives such as polyvinylidene fluoride, the prepared polyaniline-polyethylene oxide adhesive has significantly improved adhesive properties and conductivity, and has no negative impact on the charge and discharge performance of the battery.

　　Xu [16] et al. first used conductive poly(9,9-dioctylfluorene-fluorenone-methyl benzoate) (PFM) as a binder for tin anode to prepare a half-cell, and achieved high capacity during cycling. of pure nanotin electrodes. Compared with adhesives such as polyvinylidene fluoride and carboxymethyl cellulose, it was found that electrodes prepared from PFM conductive adhesives can significantly improve cycle performance, with a reversible capacity reaching 520 mAh/g. In addition, since the conductive polymer binder accounts for only 5% of the electrode material, the electrode material containing 95% active material is higher than ordinary tin electrodes, which is also a factor in improving battery performance. It is pointed out that during the continuous intercalation and extraction process of lithium, the nanotin electrode will gradually lose its crystal structure, resulting in powdering. For conductive polymers, active particles embedded in a conductive matrix, or even recycled matrix fragments, can continuously maintain conductivity, which is a very effective way to solve the performance degradation caused by volume changes in electrode materials.

　　 4 Ionic polymer binder

　　 The charge and discharge performance of lithium batteries is affected by the conductivity of lithium ions in the electrode material. When the conductivity of lithium ions is low, the capacity of lithium batteries will decay rapidly under high-speed charging and discharging. Therefore, lithium ion conductivity is a very important factor in both electrode materials and electrolytes. Using ionic polymers containing lithium ions as binders can effectively increase the lithium ion content and lithium ion transfer rate in electrode materials, while also increasing the mobility of lithium ions and reducing polarization.

　　Li [17] et al. used lithium polyacrylate (Li-PAA) as the binder to prepare a 6LR61 battery with Sn30Co30C40 as the active material, and compared it with sodium carboxymethyl cellulose and polyvinylidene fluoride, and found that Li- The performance of PAA is better than sodium carboxymethylcellulose and polyvinylidene fluoride. The results show that when polyvinylidene fluoride is used as a binder, the capacity retention rate is very low, while lithium polyacrylic acid shows good capacity retention rate. After 100 charge and discharge cycles, the capacity can reach 450 mAh/g. The author believes that the use of lithium-polyacrylic acid binder can improve the conductivity of lithium ions, thus improving the cycle performance of the battery and helping to improve SEI. The formation of the film prevents the capacity from continuing to decrease with battery cycling. Oh[18] et al. used an ionic polymer with a perfluorosulfonic acid structure as a binder for lithium iron phosphate electrode materials and compared it with polyvinylidene fluoride. At low discharge rates (C/5), both adhesives showed similar discharge capacities. However, at high magnification (1C-5C), the ionomer binder showed higher discharge capacity. This is because the ionic polymer containing lithium ions increases the lithium ion content in the electrode assembly, effectively preventing the phenomenon of lithium ion vacancies in the electrode material caused by lithium ion migration, which may occur in ordinary binders. . The results show that the ionic polymer used can not only increase the content of lithium ions in the electrode, but also quickly transfer lithium ions in the electrolyte to the surface of the active material, thereby reducing capacity loss during charge and discharge.

　　Shi [19] and others synthesized an ionic polymer PFSILi with a poly(perfluorosulfonimide) structure and blended it with polyvinylidene fluoride to prepare a lithium-ion battery binder. Research has found that polylithium silicate-polyvinylidene fluoride can form a conductive channel for lithium ions in the electrode material, effectively preventing the vacancy problem of lithium ions during rapid charge and discharge. The prepared battery has higher reversible performance, lower polarizability and internal resistance, and higher energy density at high rates and high temperatures. When charging and discharging at 2C at 60°C, the discharge platform is 0.29 volts higher than that of pure polyvinylidene fluoride binder. At room temperature 4C, the discharge capacity and energy density of the assembled lithium iron phosphate/lithium half-cell reached 50% and 66% respectively, which were 1.5 times and 1.66 times that of the polyvinylidene fluoride binder. Therefore, polysiloxane-polyvinylidene fluoride is a promising adhesive.

　　  5Summary and Outlook

　　In summary, binder is an important component of lithium-ion batteries and has a great impact on the performance of the entire battery. When using adhesives, we should fully understand the properties of the adhesive itself and the electrode material. Based on the structure and bonding mechanism of the binder, different binders are selected for different electrode materials and different usage environments. In addition, with the continuous development of 6LR61 battery technology, the requirements for 6LR61 battery binders are becoming higher and higher. Therefore, it is of great significance to continuously explore new materials and processes to improve the performance of existing adhesives.

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