The main function of SBR as an adhesive is to bind graphite particles together to form a strong anode material. As one of the auxiliary materials for lithium-ion batteries, SBR is used in very small quantities (only for screeding and coating of graphite anode materials), but it is an indispensable component. During the coating process, SBR interacts with the graphite particles, enabling them to be evenly distributed on the electrode and maintaining good adhesion. At the same time, SBR also provides a certain degree of elasticity and flexibility, enabling the battery to better withstand stress and deformation during the charging and discharging process. However, due to drying speed and solvent evaporation, SBR migrates during the coating process. This migration leads to changes in the distribution state of SBR on the electrodes, which in turn affects the microstructure of the formed slurry and pole piece. If the migration of SBR is uneven or unstable, it may lead to the aggregation or uneven dispersion of graphite particles, thus affecting the compactness and conductivity of the electrode.
Unreasonable use of SBR will cause differences in the microstructure of the electrode sheet, affecting the bonding performance of the graphite negative electrode, which is prone to sticking rollers during rolling; affecting the bonding performance between the graphite negative electrode and the copper foil, and the electrode sheet will be easy to be polarized during the battery charging and discharging process, which will cause the negative electrode to drop out and reduce the service life of the battery. Therefore, the correct understanding of SBR, analyze the impact of SBR on the performance of lithium batteries, the rational use of SBR on lithium-ion batteries is of great significance.
1.SBR Connection Mechanism
First of all, understand how SBR in the slurry can play the role of adhesive. Only graphite and carbon black particles are uniformly dispersed in the slurry and pole piece, lithium ion battery can show better performance. Graphite and carbon black particles because both are surface hydrophobic, non-polar, without additives its aggregation in water can not be dispersed. Graphite anode and carbon black dispersion with anionic dispersant as the main, supplemented by non-ionic dispersant, you can get a stable dispersion system, the general anode graphite anode selection of SBR and CMC both synergistic as a binder, CMC is known as a thickener, SBR is known as a binder.
The reasons for choosing SBR and CMC as binder: 1) Although SBR has strong adhesion, it can not be stirred at high speed for a long time. If homogenization when adding SBR and then a long time of mixing, SBR is easy to break the emulsion, due to the destruction of its structure and reduce the adhesion, generally SBR choose to join the low-speed mixing in the mixing of the late, the slurry is equipped with if you can not be coated, low-speed mixing instead of static. In addition SBR dispersion effect is not good, too much SBR will produce a large dissolution, so do not use SBR alone as a binder.
2) CMC can play a good role in the dispersion of anode graphite, CMC will decompose in aqueous solution, and its decomposition products will be adsorbed on the graphite surface, and after adsorption, the graphite particles will repel each other due to electrostatic force, so as to achieve good dispersion effect. When the proportion of CMC is very high, the extra CMC is not adsorbed on the surface of graphite particles, the combination of these CMC leads to the gravitational force between each other is greater than the repulsive force between the graphite particles after adsorption, the formation of graphite particles agglomerated.CMC is brittle, if the binder is only CMC equipped with graphite anode slurry, in the post-processing process, the graphite anode will be collapsed during the roll pressure, and there is a serious powder fall when slitting.
3), The reasonable proportion of CMC and SBR mixed in the homogenization process can make up for each other's defects, so that the graphite negative electrode slurry has good coating performance.The proportion between CMC and SBR and graphite, carbon black needs to be chosen through a series of test data, and then choose the optimized proportioning scheme. In addition, the way of mixing CMC and SBR and the mixing process also affect the performance of the slurry, which need time to find out the stable process through the experimental data, in which SBR mainly plays the role of bonding and CMC plays the role of thickening. Different CMC/SBR/graphite/carbon black need to optimize the process to get the best slurry performance.
From the composition of the battery negative electrode, graphite is about 96 parts, SBR is about 1.5-2.3 parts, but the specific surface area of graphite is the smallest, the SBR film covers the surface of the graphite particles, as well as exists in the middle of the graphite particles, and the connecting network is formed between the SBR to play the role of a bridge. At the same time, SBR particles are only about 150nm, there is no connecting force of individual SBR particles, in the slurry, only a lot of SBR combined together to form a SBR film, in order to form a connecting force on the graphite negative electrode particles to play the role of adhesion, SBR more connection is point-to-point connection, linking the graphite with the middle of the graphite, graphite and carbon black, graphite and copper foil together.
2.The Effect of SBR on Graphite Dispersion
1) When there is only a low content of CMC in the slurry without SBR, the graphite particles are agglomerated in the homogenization process and cannot be well dispersed.
2) When the proportion of CMC and graphite is moderate, adding 1.0%~4.5% SBR to the slurry, the graphite particles will be dispersed by the adsorption of SBR on the surface of graphite, and the viscosity and modulus of the slurry will be reduced.
3), When the CMC is 0.7% ~ 1.0% of the slurry performance of viscoelasticity, the continuous addition of SBR will not change the rheological properties of the slurry. SBR and CMC added at the same time and add CMC followed by the addition of SBR mixing of two comparisons, the results show that the graphite in the dispersion of slurry in the CMC played a dominant role in the CMC preferentially adsorbed on the surface of the graphite particles.
In short, when the amount of CMC added is very low, SBR added will be adsorbed on the surface of graphite particles, the dispersion of graphite has a certain effect; with the increase in the amount of CMC added to the surface of graphite adsorption also increased, SBR can not be adsorbed on the surface of graphite, and thus the dispersion of graphite can not play a role; when the CMC reaches a certain amount of the excess failed to adsorb on the surface of the graphite of the CMC combined to lead to the gravitational force than the When the CMC reaches a certain amount, the excess CMC not adsorbed on the graphite surface combines to cause the gravitational force to be greater than the repulsive force, which will form the agglomeration between graphite particles. Therefore, CMC plays a key role in the dispersion of graphite anode slurry.
3. Sticky Rolls Related to SBR
1) During the coating process, the temperature of the pole piece oven is set too high, and the negative pole piece is baked relatively fast, which leads to the migration of SBR to the surface of the pole piece due to the fast evaporation of solvents, and the concentration of SBR on the surface increases significantly, forming the microstructure of the pole piece with the viscosity on the surface greater than that between the copper foil and the negative electrode material, which can easily lead to the formation of sticky rolls on the roller press, resulting in the particles falling off the rolls and dropping on the pole piece. We can adjust the coating drying and extraction frequency settings to better control the operation of the coating machine to inhibit SBR migration and optimize the coating baking and drying curve.
2) Insufficient SBR connecting force, the SBR content in the slurry is low, resulting in insufficient bonding force between the active substances, insufficient bonding force with the foil, and the tendency to detach and stick to other objects when rolled (in contact with other substances). If water-based anode slurry, you can consider the ratio of CMC and SBR, too little is definitely not good adhesion, you can adjust and control the amount of SBR storage film and viscous elasticity to improve the viscous roll performance.
3) The situation of SBR bleaching and floating during pulping will make the concentration of SBR unevenly distributed after coating, and the adhesion between the active substance and the foil will be poor, and it will be easy to stick to the roll when rolling. The main measures: after pulping to reduce the resting time, or can be used to replace the low-speed mixing resting; through different processes to adjust the graphite - CMC - SBR mixing and ratios, based on experimental data to select the matching graphite - CMC - SBR process program; can also choose a special modification of the SBR, so that its surface functional groups and the formation of a better interaction with the CMC, to reduce the phenomenon of blue drifting of the SBR.
4. Lithium Battery Drying Temperature Impact on SBR
Lithium-ion battery in the production process of strict control of moisture, improve the drying temperature of the electric core is the main way to reduce moisture. In the core baking and drying process, the binder will be heated at high temperatures, and different properties of the binder may trigger the phenomenon of cross-linking of cross-linkable groups, thus affecting the electrode performance. Therefore, it is also very important to study the effect of core drying on binder performance.
There are tests to analyze the thermal performance of water-based binder LA132 and SBR, the temperature is too high LA132 will occur intermolecular crosslinking, resulting in the active substance with the collector of the adhesion is damaged battery cycling performance deterioration, the drying temperature should not be as high as 120 ° C, and the use of SBR pole piece, the performance of the drying temperature is almost unaffected by the SBR does not crosslinking by the heat, the peeling strength is maintained at about 3.5 N/mm. N/mm or so.
5.SBR Impact on Low Temperature Performance
The impedance RB, RSEI and RCT of lithium-ion batteries under low temperature conditions will rise with the decrease of temperature, but the increase of RCT is the largest. If the RCT under low temperature conditions can be reduced, it is possible to improve the low temperature performance of the battery.The factor of SBR and reduce the growth of the RCT of the battery under low temperature conditions, the application of SBR can effectively improve the low temperature characteristics of the battery.
During the charging process, the membrane of SBR covers a certain specific surface area of graphite, and lithium ions are effectively embedded in graphite during the transmission process by bypassing the SBR membrane to reach the graphite surface. Electrolyte is the carrier of lithium ion transmission between positive and negative electrodes in lithium batteries, the better the wettability of electrolyte and SBR, the more conducive to the conduction of lithium ions between the interfaces. Different SBR and the same electrolyte wetting is different. Selection of different SBR low-temperature battery discharge data show that the wetting performance of good SBR than the general SBR has a 4% improvement, while the battery DCR at 0 ℃ is 15% lower than the general SBR. Although the magnitude of battery performance enhancement by choosing SBR with a small contact ratio is not as large as other routes, the impact on battery performance is a significant improvement for SBR.
6. The Effect of SBR on Negative Electrode Expansion
Graphite negative electrode pole pieces often encounter problems such as dropout and large thickness rebound. Negative pole piece expansion has an important impact on the battery cycle performance, internal resistance, etc., so we need to understand the effect of binder SBR on negative pole piece expansion. The rebound of the negative pole piece is mainly related to the physical properties of the material, such as elastic modulus, breaking strength, elongation, etc. CMC mainly plays the role of thickening in the negative electrode slurry, and SBR plays a strong binder role, and it is because of the high elasticity of SBR that the negative electrode piece will rebound to a greater extent after the rolling process, and the higher the modulus of elasticity and strength of SBR, the lower the negative electrode expansion rate.
Tests show that: the anode expansion is related to the pressure during rolling and the elastic modulus and strength of the binder; the same SBR content, the same pressure during rolling, the higher the elastic modulus and strength of SBR, the lower the anode expansion rate; the lower the SBR content, the lower the pressure during rolling, the smaller the expansion rate of the pre-physical shelving, the full state and the empty state; the expansion of the anode results in the deformation of the battery roll core and affects the lithium-ion transport channel, which in turn affects the battery cycling cycle and the lithium ion transport channel. transport channel, which in turn has a serious impact on the battery cycle performance.
The modulus of elasticity of SBR affects the bounce of the pole piece, and the larger the modulus of elasticity, the smaller the bounce of the pole piece thickness. In the selection of battery materials, we should give priority to the selection of binder with high modulus of elasticity and high breaking strength, and try to reduce the SBR during the adjustment of the material ratio, so as to improve the cycle life of the battery.
7.Summary
In summary, the lithium-ion battery manufacturing process slurry process through the design of SBR optimization, in specific conditions to improve the microstructure of SBR in the pole piece, in the process of compaction to enhance the amount of SBR energy storage film, and through this to enhance the slowing down of SBR-induced sticky rolls. Improve the low-temperature performance of the battery by improving the wettability of the electrolyte to the SBR.The SBR synthesis process adopts different means, adopts different synthesized monomers for the SBR, and adjusts the surface of the SBR to make the SBR have different properties, including decoupling, gel, etc., so that different SBRs will show different wettability to the electrolyte, which is helpful for improving the low-temperature performance of lithium batteries. help.
As an auxiliary material in lithium-ion batteries, SBR plays an important role in the homogenization and coating process of graphite anode materials. However, the migration inhomogeneity of SBR due to the volatilization rate solvent may lead to the negative impact on the microstructure and performance of the battery. To address this issue, optimizing the drying rate and solvent selection, and improving the formulation and preparation process of SBR are potential solutions. It is important to accurately control the migration of SBR during the manufacturing process of lithium-ion batteries. By adjusting the coating process parameters, such as drying temperature, drying time, and solvent type, the migration behavior of SBR can be optimized to achieve uniform distribution on the electrodes and to form a desirable microstructure. This will help improve the performance and cycle life of the batteries and further advance the development of lithium-ion battery technology.
