Influence of Silicone Foam on Expansion Force of Rechargeable Battery Module

Influence of Silicone Foam on Expansion Force of Rechargeable Battery Module

 

Silicone foam materials have higher tear and tensile strengths than foam materials with softer density grades. Such materials can be used for more durable high closure force gaskets. Silicone foam is ideal for sealing and protecting outdoor communications, lighting and electronics enclosures from fine dust particles, wind, rain and fire. Silicone foam provides enhanced durability and sealing performance in a variety of environmental conditions and applications. Silicone foam is available in coils of different thicknesses and is easy to process to the required size. Let's introduce the influence of silicone foam on the expansion force of the rechargeable battery module.

 

Lithium iron phosphate battery has expansion problems in the whole process of application. On the one hand, it is transformed into SEI film in the whole process, resulting in vapor, and the standard air pressure inside the battery is increased, and with the development of the circulation system, the thickness of the SEI film expands, and then Promote the deformation of the shell; on the other hand, during charging and discharging, Li slippage and the insertion of lamellar raw materials cause the change in the thickness of the negative pole piece, the change caused by the lattice constant of the positive pole piece is small, and the harm is not great .

 

The expansion force of the battery is not prominent in the short term, but appears slowly with the increase of the charging and discharging frequency. Some studies have shown that the external diffusion characteristic impedance of Li increases with the increase of the working pressure, and a moderate working pressure is beneficial to reduce the specificity. The damage of lithium can alleviate the decay coefficient speed of the battery volume. With the increase of the working pressure of battery assembly, the service life of the battery circulation system will continue to increase, but after a certain working pressure is exceeded, the working pressure is increased again, and the service life of the battery circulation system will decrease.

In addition, the expansion force also involves stability and safety factor. Whether the structure of the battery pack can bear the expansion force of the battery will immediately endanger the application safety of the battery; Substances can reasonably reduce the expansion force of the battery. At this stage, there are relatively few scientific researches on the physical characteristics of the buffer substance itself to alleviate the damage of the expansion force. In this paper, the buffer silicone foam is selected as the research object, and the damage of the thickness and strength to the expansion force of the battery is scientifically studied.

 

01 test

 

The test uses a square shell lithium battery, the positive electrode is LiNixCoyMn1-x-yO2 ternary material, the negative is graphite material, the lithium battery specification is 100.7*148*39.7mm (aspect ratio * total width * thickness), and the tolerance volume is 81Ah , the maximum working voltage is 4.35V; the two lithium batteries are lined with silicone foam, the width of the foam is 142 mm and 90 mm respectively, the two batteries are connected in series with an aluminum row, and the method of laser welding is used to fix it. The physical model of the control module is shown in the figure. shown in 1.

 

Test 1 selects the same thickness and different strength of silicone foam, its thickness is 2.0mm, and the Shore A strength values ​​are 12 and 31 respectively. Adjust the spacing between the straight hair splints in the fixture tooling to release the preload force, and both sets of tests release 160kgf force to simulate the preload force in the entire process of the specific combination. In test 2, silicone foam with a strength value of Shao A of 18 was selected, and the thickness was 1.5 mm and 2.5 mm, respectively. Different preloads were released. The foam was reduced by 0.2 mm, and the shrinkage of the foam was the same, and then the charge-discharge cycle system was carried out. Testing, testing procedures are shown in Table 1, all testing equipment is shown in Figure 2, the working pressure is recorded according to the sensor and storage equipment, and the recording equipment records the standard value every 1 minute.

 

02Analysis of test results

 

In test 1, silicone foams with the same thickness and different strengths were selected. The Shore A strength values ​​of the foams were 12 and 31 respectively. The stress-strain curves of the two types of silicone foams are shown in Figure 3(a). It can be seen that the smaller the strength value of the silicone foam, the lighter the stress-strain curve, that is, the smaller the in-situ stress required to reduce the same strain force, and the larger the strength. When the foam is compressed to the middle and late stages, the in-situ stress has an increase. , the transformation is obvious, and the actual stress-strain curve is shown in Table 2.

 

Put the two parallel battery control modules into the fixture tool, the original volume of the control module is 30% SOC, the preload force at the start and end release is 160kgf, carry out the charge and discharge test, cycle the system for 100 weeks, and select the whole process of each charge and discharge. The results are shown in Figure 3(b), the minimum value of expansion force is at the end of discharge, and the maximum value is at the end of battery charging. The distance between the minimum expansion forces of the two modes is not large, and both start and end. The 115kgf was reduced to 108kgf and remained stable.

 

In the whole process of battery charging in the first week, because the original volume is 30% SOC, when the battery is fully charged, the expansion forces of Shao A of 12 and 31 are 165.3 and 174.9 kgf respectively. In the 7-week cycle system from start to finish , the larger expansion force gradually decreases, which may be caused by a stable whole process of the preloading force. The overalls have a certain bending stiffness. During the whole process of clamping the control module by the tooling fixture, the control module also aligns the hair straightening splint. There is a certain recoil force, and the equilibrium relationship between each other must take a certain period of time, reducing to 159.6 and 173.5 kgf respectively, and then with the development of the charge-discharge cycle system, the larger expansion force of the battery at the end of charging gradually expands, tending to For linear transformation, 100-cycle circulatory system, the maximum expansion force is 178.7 and 204 kgf respectively, and the transformation speed is 0.205 and 0.328 kgf/week respectively.

 

From this, it can be analyzed that the selection of buffer substances with low strength can reduce the large expansion force of the battery during the whole process of battery charging. Because the same battery is in the same detection working condition, the amount of expansion caused by it may not change. , the expansion of the battery has the effect of extrusion molding on the foam. According to the regularity of the mutual influence of the force, the foam also has a recoil force on the battery, and the recoil force has the same extrusion molding effect on the battery to resist the expansion of the battery. Foam with low strength has low recoil force, and the extrusion molding level of the battery is small, so the expansion force is small; the rigid silicone foam has a large ground stress under the same strain force, and the extrusion deformation of the battery is larger, and the expansion force is small. the greater the force.

Test 2 According to the results of Test 1, the silicone foam with low strength can reduce the expansion force of the battery at the end of charging, and the strength value of the silicone foam is 18. The thickness of the foam is 1.5 mm and 2.5 mm, respectively. The preload force of 83.4kgf reduces the foam by 0.2mm, and the cycle system is cycled for 350 cycles according to the same charging and discharging process; From 114.3kgf at start and end to 312.8gf, the force value change in the whole process of discharge is increased from 274.1kgf at start and end to 398.1kgf; the expansion force of 2.5mm thin and thick foam battery at the end of charging is increased from 265.9kgf at start and end to 524.5kgf, the expansion at the end of discharge The force increases from 69.4kgf at the beginning and the end to 215.3kgf, and the change in the force value during the whole discharge process increases from 274.1kgf at the beginning and end to 309.2kgf. It can be seen from Figure 4(b) that the greater the released preload, the more the battery is charged at the end of the charge. The expansion force error at the end of discharge is larger.

 

Scientific research has confirmed that releasing a moderate working pressure of mechanical equipment to the battery can reduce the way of Li transfer, reduce the damage of Li and improve the service life of the battery [10], but when the working pressure exceeds, the layer interval of the battery cathode material decreases, The van der Waals force between the layers expands, which in turn expands the frictional resistance of Li slippage, resulting in a decrease in volume. It can be seen from Figure 4(d) that the starting and ending discharge volume of the control module using 1.5mm foam is 168.7Ah, and the discharge volume is reduced to 156.7Ah after 350 cycles of the system, and the maintenance rate is 92.9%; the starting and ending discharge of the 2.5mm foam control module is used. The volume is 168.2Ah, the discharge volume is reduced to 160.7Ah after 350 cycles of circulation, and the maintenance rate is 95.5%. After 200 cycles of circulation, the expansion force of the control module is accelerated, the volume attenuation coefficient is accelerated, and the control module attenuation coefficient of 1.5mm foam more significantly.

 

Figure 4(c) is a graph of the difference between the error of the expansion force at the end of charging and the end of discharge and the difference between the preload force. It can be seen that the expansion force caused by the pole piece changes during the whole process of the discharge of the two-parallel control module. The two control The change trend of the pure expansion force of the module in the whole process of discharge is the same, and it increases with the increase of the cycle number of the circulation system. For the control module of the foam, after comparison, the shape of the 1.5mm foam battery is selected to change the shape of the control module of the 2.5mm foam. This may be because the thickness of the foam is small, the compressible stroke is small, and the battery is digested The expansion force of the battery is less; the thick and thick foam, the larger the schedule that can be reduced, the greater the expansion force of the digestion and absorption battery. On the other hand, the greater the deformation, the greater the expansion force, and the more serious the extrusion in the middle of the pole piece, resulting in the reduction of the layer interval of the high-purity graphite of the battery cathode material, the expansion of the van der Waals force between the layers, and the slippage of Li. The frictional resistance expands and reduces the volume of the battery, which is consistent with the results presented in the discharge volume transition curve.

 

To sum up, it must be concluded that the expansion force of the battery is related to the released pre-tightening force. The greater the pre-tightening force, the greater the expansion force. In addition, if the foam of the same strength is selected, the thickness of the foam can be appropriately increased according to the size of the indoor space, which can reduce the expansion force. This in turn improves the battery capacity retention rate.

 

03 Results

 

(1) The soft strength of the buffer in the middle of the battery is harmful to the expansion force of the battery control module. The buffer with low strength has less extrusion to the battery, and the greater the expansion force of the absorbed battery is, the battery can be reasonably reduced. The expansion force at the end of charging, so the expansion force is small; the rigid foam has a large ground stress with the same strain force, and the greater the extrusion deformation of the battery, the greater the expansion force is.

 

(2) The size of the pretightening force has an influence on the expansion force of the battery. The expansion force of the battery is the effect of the preload force and the expansion of the pole piece, which are accumulated together. The expansion force and the discharge volume of the battery affect each other, and the expansion speed of the expansion force The faster the battery volume decay coefficient, the faster the battery volume decay coefficient; in the cache of the same strength, moderately expanding the thickness of the cache can reduce the expansion force of the battery, which is beneficial to improve the service life of the battery. Choose a cache with a thickness of 18 and a thickness of 2.5 mm Foam, the expansion force of the control module is smaller, and the volume retention rate is higher.