The impact of SBR in lithium-ion batteries!

SBR, as one of the auxiliary materials for lithium-ion batteries,

 although used in very small quantities (only for homogenization and coating of graphite negative electrode materials), is an indispensable component.

 During the electrode coating process, the evaporation of solvents at different drying speeds affects the migration of SBR, resulting in different distribution states of SBR.

 The resulting slurry and electrode microstructure have significant differences, which directly affect the performance of the battery.

Improper use of SBR can cause differences in the microstructure of the electrode sheets,

 affecting the bonding performance of graphite negative electrodes, and making it easy to stick to the roller during rolling;

 The bonding performance between graphite negative electrode and copper foil is affected,

 and the electrode sheet is prone to polarization during battery charging and discharging, causing negative electrode material to fall off and reducing the service life of the battery.

 Therefore, a correct understanding of SBR, analysis of its impact on the performance of lithium-ion batteries, and rational use of SBR are of great significance.

1 SBR connection mechanism

Firstly, let's understand how SBR can act as a bonding agent in the slurry. 

Only when graphite and carbon black particles are uniformly dispersed in the slurry and electrode sheets, can lithium-ion batteries exhibit good performance.

 Graphite and carbon black particles, due to their surface hydrophobicity and non-polar nature, cannot disperse in water without additives. 

When dispersing graphite negative electrodes with carbon black, anionic dispersants are mainly used, supplemented by non-ionic dispersants, to achieve a stable dispersion system.

 Generally, SBR and CMC are used as binders in combination for graphite negative electrodes, with CMC being called thickener and SBR being called binder.

The reason for choosing the combination of SBR and CMC as binders is that although SBR has strong adhesion, it cannot be stirred at high speed for a long time.

 If SBR is added to the homogenate and stirred for a long time, it is easy for SBR to break down and reduce its adhesion due to structural damage. 

Generally, SBR is added with low-speed stirring in the later stage of stirring. If the slurry cannot be coated after preparation, low-speed stirring is needed instead of standing.

 In addition, the dispersion effect of SBR is not good, and too much SBR will cause significant swelling, so SBR should not be used alone as a binder.

2) CMC can play a good role in dispersing negative graphite. 

CMC will decompose in aqueous solution, and its decomposition products will adsorb on the surface of graphite. 

After adsorption, graphite particles will repel each other due to static electricity, achieving good dispersion effect.

 When the proportion of CMC is high, the excess CMC does not adsorb onto the surface of graphite particles. 

The combination of these CMCs causes the attraction between them to be greater than the repulsion between the adsorbed graphite particles, resulting in the aggregation of graphite particles. 

CMC is brittle.

 If only CMC is used as a bonding agent with graphite negative electrode slurry, during the subsequent film making process, 

the graphite negative electrode will collapse during rolling and severe powder loss will occur during slitting.

3) Reasonable mixing of CMC and SBR in the homogenization process can compensate for each other's defects, thus graphite negative electrode slurry has good coating performance. 

The ratio of CMC and SBR to graphite and carbon black needs to be determined through a series of experimental data, and then an optimized ratio scheme should be selected. In addition, 

the mixing method and stirring process of CMC and SBR also have an impact on the performance of the slurry, which requires time to explore stable processes through experimental data. 

SBR mainly plays a bonding role, while CMC plays a thickening role. Different CMC/SBR/graphite/carbon black require process optimization to achieve optimal slurry performance.

From the composition of the negative electrode of the battery, graphite accounts for about 96 parts and SBR accounts for about 1.5-2.3 parts.

 However, the specific surface area of graphite is the smallest.

 SBR film covers the surface of graphite particles and exists in the middle with them, forming a connecting network between SBR and serving as a bridge. 

At the same time, SBR particles with a diameter of only about 150nm have no bonding force. 

Only when many SBR particles are combined together to form an SBR film in the slurry can a bonding force be formed to bond the graphite negative electrode particles. 

SBR is more of a point-to-point connection, connecting graphite between graphite, graphite and carbon black, and graphite and copper foil together.

The influence of SBR on graphite dispersion

1) When there is only a low content of CMC in the slurry without SBR, graphite particles agglomerate during the homogenization process and cannot be well dispersed.

2) When the ratio of CMC to graphite is moderate, adding 1.0% to 4.5% SBR to the slurry will cause SBR to adsorb onto the graphite surface, 

dispersing the graphite particles and reducing the viscosity and modulus of the slurry.

3) When the CMC is between 0.7% and 1.0%, the slurry exhibits viscoelasticity, and continuous addition of SBR does not change the rheological properties of the slurry. 

A comparison was made between the two mixing methods of adding SBR and CMC simultaneously and adding CMC first followed by SBR.

 The results showed that CMC played a dominant role in the dispersion of graphite in the slurry, and CMC preferentially adsorbed on the surface of graphite particles.

In summary, when the amount of CMC added is very low, the addition of SBR will adsorb onto the surface of graphite particles, 

which has a certain impact on the dispersion of graphite; 

As the amount of CMC added increases, the adsorption capacity on the graphite surface also increases, 

and SBR cannot adsorb on the graphite surface, thus having no effect on the dispersion of graphite;

 After a certain amount of CMC is reached, the excess CMC that cannot be adsorbed on the graphite surface combines, resulting in attraction greater than repulsion,

 which leads to agglomeration between graphite particles. Therefore, CMC plays a crucial role in the dispersion of graphite negative electrode slurry.

3. Adhesive rollers related to SBR

1) During the coating process, if the temperature of the pole plate oven is set too high, the negative electrode plate will be baked relatively quickly. 

Due to the rapid evaporation of the solvent, most of the SBR migration will be carried to the surface of the pole plate,

 resulting in a significant increase in surface SBR concentration and the formation of a microstructure of the pole plate with surface viscosity greater than that between the copper foil and the negative electrode material. 

This can easily lead to the formation of sticky rollers in the roller press, causing particles that fall off due to the sticky rollers to fall onto the pole plate. 

We can adjust the drying and exhaust frequency settings for coating to better control the operation of the coating machine, suppress SBR migration, and optimize the coating baking drying curve.

2) The SBR connection force is insufficient, and the SBR content in the slurry is too low, resulting in insufficient bonding force between the active substances and insufficient bonding force with the foil. 

When rolled (in contact with other substances), 

there is a tendency to immediately detach and stick to other objects. For water-based negative electrode slurry, the ratio of CMC to SBR can be considered. 

If it is too small, it will definitely not stick well. The storage film amount and viscoelasticity of SBR can be adjusted and controlled to improve the adhesive roller performance.

3) When SBR floats and floats during pulp making, the concentration distribution of SBR will be uneven after coating, 

and the adhesion between the active substance and the foil will deteriorate, making it easy to stick to the roll during rolling.

 Main measures: Reduce the settling time after pulping, or use low-speed stirring instead of settling; 

Adjust the mixing method and ratio of graphite CMC SBR through different processes, 

and select the matching graphite CMC SBR process scheme based on experimental data; 

Special modified SBR can also be chosen to enhance the interaction between its surface functional groups and CMC, reducing the phenomenon of SBR bleaching.

The influence of drying temperature of lithium batteries on SBR

Strictly controlling moisture during the manufacturing process of lithium-ion batteries and increasing the drying temperature of the battery cells are the main ways to reduce moisture.

 During the baking and drying process of battery cells, the adhesive will be heated at high temperatures, and adhesives with different properties may cause cross-linking of crosslinkable groups,

 thereby affecting electrode performance. Therefore, it is also very important to study the influence of battery cell drying on the performance of adhesives.

There is an experimental analysis of the thermal properties of water-based adhesives LA132 and SBR. 

When the temperature is too high, LA132 will undergo intermolecular crosslinking, which will damage the adhesion between the active substance and the current collector. 

The cycling performance of the battery will deteriorate. The drying temperature should not be higher than 120 ℃.

 However, the performance of SBR electrodes is almost unaffected by the drying temperature. SBR does not undergo crosslinking when heated, and the peel strength is maintained at around 3.5N/mm.

The impact of SBR on low-temperature performance

The impedance RB, RSEI, and RCT of lithium-ion batteries under low temperature conditions all increase with decreasing temperature, but RCT has the largest increase.

 If the RCT under low temperature conditions can be reduced, it is possible to improve the low-temperature performance of the battery. 

The application of SBR can effectively improve the low-temperature characteristics of batteries by reducing the growth rate of RCT under low temperature conditions due to SBR factors.

During the charging process, the SBR membrane covers a certain specific surface area of graphite, 

and the effective way for lithium ions to embed into graphite during transport is to bypass the SBR membrane and reach the graphite surface.

 Electrolyte is the carrier for the transport of lithium ions between the positive and negative electrodes in lithium batteries. 

The better the wetting performance of electrolyte and SBR, the more favorable it is for the conduction of lithium ions between the interfaces. The wetting of different SBRs with the same electrolyte is different.

 The discharge data of low-temperature batteries using different SBR showed that SBR with good wetting performance had a 4% improvement compared to general SBR, 

while the battery DCR at 0 ℃ was 15% lower than general SBR.

 Although the use of SBR with a smaller contact ratio does not significantly improve battery performance compared to other methods, it has a significant impact on battery performance for SBR.

The Effect of SBR on Negative Electrode Expansion

Graphite negative electrode plates often encounter problems such as material dropping and significant thickness rebound. 

The expansion of negative electrode sheets has a significant impact on the cycling performance and internal resistance of batteries, 

so we need to understand the effect of binder SBR on the expansion of negative electrode sheets. 

The rebound of the negative electrode is mainly related to the physical properties of the material, such as elastic modulus, fracture strength, elongation, and so on.

 CMC mainly plays a thickening role in the negative electrode slurry, while SBR plays a strong bonding role.

 It is precisely because of the high elasticity of SBR that the negative electrode sheet will have a significant thickness rebound after the rolling process.

 The higher the elastic modulus and strength of SBR, the lower the negative electrode expansion rate.

The experiment shows that the expansion of the negative electrode is related to the pressure applied during rolling, as well as the elastic modulus and strength of the binder.

 The SBR content is the same, and the pressure applied during rolling is the same. The higher the elastic modulus and strength of SBR, the lower the negative electrode expansion rate; 

The lower the SBR content, the less pressure is applied during rolling, and the lower the expansion rates of physical idle, fully charged, and empty charged states in the early stage;

 The expansion of the negative electrode causes deformation of the battery core, affecting the lithium ion transport channel and thus having a serious impact on the cycling performance of the battery.

The elastic modulus of SBR affects the rebound of the polarizer, and the larger the elastic modulus, the smaller the rebound of the polarizer thickness. 

When selecting battery materials, priority should be given to choosing binders with high elastic modulus and fracture strength.

 During the material ratio adjustment process, SBR should be minimized as much as possible to improve the cycle life of the battery.

7 Summary

In summary, the slurry process in the manufacturing of lithium-ion batteries is optimized through SBR design,

 which improves the microstructure of SBR in the electrode under specific conditions and enhances the energy storage film capacity of SBR during compaction,

 thereby improving and slowing down the sticking caused by SBR. Improving the low-temperature performance of the battery by enhancing the wettability of the electrolyte to SBR. 

The SBR synthesis process uses different means, and different synthetic monomers are used for SBR. 

SBR has different performance through the adjustment of SBR surface, including the adjustment of decoupling, gel and other aspects.

 In this way, different SBRs will show different wettability to electrolyte, which will help to improve the low-temperature performance of lithium batteries.

The role of SBR in lithium-ion batteries is like "pulling a thousand pounds with four or two strokes", 

although the amount of SBR used is small, it plays a key role in overall performance.

 Insufficient use of SBR can easily lead to low adhesion between the electrode sheets, causing material to fall off and sticking to the rollers during the rolling process,

 which is also detrimental to the later performance of the battery.

 In the manufacturing process of lithium-ion batteries, 

people pay more attention to SBR and explore reasonable ratios and processes with CMC and graphite negative electrodes in order to fully play a role in the performance of lithium-ion batteries.