There are many reasons for the bulk lithium ion battery bulge.. According to the experimental research and development related experience, the author divides the causes of lithium-ion battery bulge into three categories.
First, the thickness of the battery cells caused by the expansion during the cycle; the second is due to the bulging of the electrolytic liquid oxidation. Third, the battery pack is not strictly introduced into the bulging defect caused by water, angular damage..
Different in different battery systems, the leading factors of battery thickness variations, such as in the lithium titanate negative electrode battery cells, the important factors of bulging are gas drums; in the graphite negative electrode system, the thickness of the pole thickness and the bulk of the battery To promote use. I. Electrode pole thickness variational graphite negative expansion factors and mechanism discussion that the lithium-ion battery has increased in the charging process to the negative expansion, the positive electrode expansion ratio is only 2 ~ 4%, and the negative electrode is usually from graphite, sticky.
Connecting, conductive carbon composition, wherein the expansion ratio of the graphite material itself reaches ~ 10%, which causes an important influencing factor in the change in graphite negative expansion ratio, including: SEI film formation, tipping state, SOC), process parameters, and other influencing factors. (1) SEI film forms a lithium ion battery for the first charge and discharge process, the electrolyte reacts in graphite particles in the solid-liquid phase interface, forming a passivation layer (SEI film) covering the surface of the electrode material, SEI film The anode thickness has increased significantly, and since the SEI film occurs, the thickness of the battery is increased by about 4%..
From the long-term circulation process, according to the physical structure and specific surface of different graphuses, the circulatory process will occur and the dynamic process of the new SEI production, such as the sheet graphite compared with a larger expansion ratio.. (2) In the circulation process, the graphite anode bulk expansion and the battery SOC have a good periodic functional relationship, that is, as lithium ions are constantly embedded in graphite (the increase in battery SOC) volume.
Gradually expand, when the lithium ion is discharged from the graphite anode, the electrical core SOC gradually decreases, and the corresponding graphite anode is gradually reduced.. (3) Process parameters From the aspect of the process parameters, the compaction density has a large influence on the anode of the graphite.
During the cold pressure of the pole, there is a larger pressure stress in the graphite anode film layer, which is a high-temperature baking of the pole. It is difficult to completely release. When the cell is circulated, the electrolyte is commonly used due to the embedding and extraction of lithium ions, and the electrolytic solution is commonly used, and the diaphragm stress is released in the circulation process, and the expansion ratio is increased.
. On the other hand, the compact density size determines the size of the void capacity of the anode film, and the pore capacity in the film layer can effectively absorb the volume of the pole expansion, the void capacity is small, when the pole is expanded, there is no sufficient spatial absorption expansion The volume, at this time, the expansion can only be expanded outside the membrane layer, and the volume expansion of the anode sheet can be expanded..
(4) Other factor adhesive bond strength (adhesive, graphite particles, conductive carbon, and bonding strength of the interface between each other), charge and discharge ratio, adhesive, and electrolyte swelling, graphite particles The shape and its bulk density, as well as the electrode expansion of the electrode expansion, etc.. The expansion ratio is calculated: the expansion ratio calculation is measured by the secondary element X, Y direction size, the mean measurement Z direction thickness, and measured after the blanket and the battery.
Figure 1 The influence of the anode sheet measurement and the effect of the coating quality on the negative polar expansion is used as factor of compaction density and coating quality, and each of the three different levels are taken, and the whole factor is orthogonal experimental design (as shown in Table 1) Other conditions of each group. Figure 2 (a), (b) can be seen that after the battery is full, the expansion ratio of the anode sheet in the X / Y / Z direction increases as the compact density increases..
When the compaction density increases from 1.5 g / cm3 to 1.7 g / cm3, the X / Y direction expansion ratio increases from 0.
7% to 1.3%, and the Z-direction expansion ratio increases from 13% to 18%..
As can be seen from Figure 2 (a), at different compaction densities, the X-direction expansion ratio is greater than the Y direction, and the reason for this phenomenon is important is caused by the pole refrigeration step, during the cold pressure process, the pole passes through the pressure When the roller, according to the minimum resistance, the material is flowed in an external force, and the material point will flow in a direction in which the resistance is minimal. When the expansion rate of the anode is cold in different directions, the minimum direction of resistance is MD direction (pole) The Y direction, as shown in FIG. 3), the stress is more easily released in the MD direction, and the resistance of the TD direction (the X direction of the plate) is large, the stress of the roller press is not easy to release, the TD direction should be larger than the MD direction.
Therefore, after the electrode film is full, the X direction expansion rate is greater than the Y direction expansion rate. On the other hand, the compact density is increased, the polar pore capacity is lowered (as shown in Figure 4), when charging, there is no inside the anode film layer Sufficient space absorbs the volume of graphite expansion, and the external manifestation is expanded in three directions of the maximum sheet to X, Y, Z..
As can be seen from Figures 2 (c), (d), the coating mass increases from 0.140 g / 1, 540.25 mm2 to 0.
190 g / 1, 540.25mm2, and the X direction expansion ratio increases from 0.84% to 1.
15%, Y The direction of expansion is increased from 0.89% to 1.05%, the Z-direction expansion ratio and the X / Y direction change trend are reversed, which is decreased, from 16.
02% to 13.77%..
Explanation of the ink anode expansion in the three directions of X, Y, Z, and the change of the coating quality changes are important to significantly changes in the thickness of the film layer.. The above negative electrode variations and the results of the literature are consistent, that is, the smaller the thickness of the aggregate fluid and the thickness of the film layer, the greater the stress in the collector.
. Figure 3 Anode cold pressure process schematic diagram Figure 4 Changes of voids at different compaction densities The thickness of the copper foil on the negative expansion is selected from the thickness of the copper foil and the coating quality, the thickness of the copper foil is taken from 6 and 8 μm, and the anode, respectively. The coating mass was 0.
140 g / 1, 540.25 mm2 and 0.190 g / 1, 540.
25 mm2, and the compaction density was 1.6 g / cm3, and the other groups of experiments were the same, and the experimental results are shown in Figure 5..
As can be seen from Figures 5 (a), (c), under the quality of the two different coating, the anode flake of the copper foil in the X / Y direction is less than 6 μm, indicating that the thickness of the copper foil is added, due to its elastic modulus Added (see Figure 6), that is, the anti-deformation ability is enhanced, and the anode expansion constraint use is enhanced, and the expansion ratio is reduced.. According to the literature, under the same coating quality, the thickness of the copper foil increases, the concentrated thickness and the thickness ratio of the film layer have increased, and the stress in the current collector is small, and the polar expansion ratio is small.
. In the Z direction, the expansion rate change trend is completely opposite, and it can be seen from Figure 5 (b), the thickness of the copper foil increases, and the expansion ratio is added; from Fig. 5 (b), (d) can be seen, when coated When the quality is increased from 0.
140g / 1,540.25mm2 to 0.190g / 1,540.
25mm2, the thickness of the copper foil has increased, and the expansion rate is reduced.. The thickness of the copper foil has increased, although it is conducive to reducing its own stress (strength), the stress in the new increase in the film, resulting in a new Z direction expansion rate, as shown in Figure 5 (b); as the coating quality is new Increased, thicker copper foil has increased the application of the membrane stress, but at the same time, the constraint ability of the film layer is also enhanced, and the binding force is more obvious, the Z direction expansion rate is reduced.
. Figure 5 Copper foil thickness and coating mass change the membrane expansion ratio of the anode Figure 6 Different thickness copper foil stress-strain curve graphite type on negative expansion using five different types of graphite (see Table 2), The coating mass is 0.165 g / 1, 540.
25mm2, the compact density is 1.6 g / cm3, the copper foil thickness is 8 μm, the other conditions are the same, and the experimental results are shown in Figure 7..
As can be seen from Figure 7 (a), different graphuses differ in X / Y direction, minimum 0.27%, maximum 1.14%, Z-direction expansion ratio, maximum 17.
47%, X / Y direction expansion In the z direction, the results of the same analysis are consistent. Among them, the electrical core of the A-1 graphite is severely deformed, the deformation ratio is 20%, and the other set of cells do not deform, indicating that the X / Y expansion ratio has a significant impact on the electrical cell deformation..
Figure 7 Different graphite expansion rate conclusions (1) increase the compaction density, the anode sheet increases in the X / Y, Z three directions in the full charge process, and the expansion ratio of the X direction is greater than the expansion ratio in the Y direction. (The X direction is the roller axis direction during the cold pressure process, the Y direction is the direction of the machine). (2) The quality of the new coating, the expansion ratio in the X / Y direction has increased, the Z direction expansion rate is reduced; the quality of the new coating can cause increased stretch stress in the current collector.
. (3) Improve the intensity of the collector, and suppress expansion of the anode sheet in the X / Y direction. (4) Different types of graphite, the difference in expansion ratio in X / Y, Z is large, wherein the expansion magnitude in the X / Y direction is large to the electrical cell deformation.
. Second, the internal gas production of the bulk battery caused by battery gas is another important reason for battery bulging, whether it is battery temperature cycle, high temperature cycle, high temperature shelving, it will have varying degrees of bulging gas..
Battery In the first charge and discharge process, the electrode surface forms a Sola (SolideElectrolyte Interface film). The formation of the negative electrode SEI film is important to reduce decomposition of EthyleneCarbonate, while the alkyl lithium and Li2CO3 are generated, there will be a large amount of Co and C2H4. DMC (DMETHYLCARBONATE), EMC (Ethylmethylcarbonate) in the solvent is also rlico3 and roli during film formation, accompanied by gas and CO gas such as CH4, C2H6 and C3H8.
. In the PC (PropyleneCarbonate) base electrolytic solution, the occurrence of gas is relatively, and it is important that the C3H8 gas generated by the PC reduction..
Lithium iron phosphate soft boiled lithium battery is the most severe in the end of 0.1c charging during the first cycle..
As can be seen above, the formation of SEI will accompany the appearance of a large amount of gas, this imperfection process. The presence of H2O in impurities causes the P-F bond in LiPF6 to be unstable, generating HF, HF will result in unstable, accompanying gas, accompanying the gas. Excess H2O presence will consume Li +, generate LiOH, LiO2 and H2 results in a gas.
In the storage and long-term charge and discharge process, there will be a gas in a gas. For the sealed lithium-ion battery, a large number of gas can cause the battery to be influenced, which affects the performance of the battery, and shortens the battery life..
The battery is important for the battery in the storage process. The following two points: (1) H2O existing in the battery system causes HF generation, resulting in damage to SEI. O2 in the system may cause oxidation of the electrolyte, resulting in a large amount of CO2; (2) If the SEI film unstable formed in the first time, the SEI film is unstable, and the resiliency of the SEI film will release the hydrocarbon.
Class-based gas. During the long-term charge and discharge cycle of the battery, the crystalline form of the positive electrode material changes, and the unevenness of the point potential of the electrode has caused some point potential too high, the electrolyte decreases, and the electrode surface film is constantly thickening. The electrode interface resistance increases, further improves the reaction potential, resulting in decomposition of the electrolyte in the surface of the electrode, and the positive electrode material may also release the gas.
In different systems, the amount of battery production is different.. In the graphite negative system battery, the cause of gas intake is important or the SEI film generated as described above, the moisture exceeding the standard, the chemical flow is abnormal, the package is poor, and the industry is generally considered Li4TI5O12 The battery’s flatulence is important that the material itself is easy to absorb water, but there is no exact evidence to prove this speculation.
. Tianjin Life Battery Company Xiong et al., In the abstract of the 15th International Electric Conference Papers, there is no data support for CO2, CO, alkanes, and small amounts of olefins in gas components, and have no data support for specific composition and proportion.
. Belharouak, etc. use a gas chromatography-mass spectrometry to obtain battery air condition.
The important component of the gas is H2, as well as CO2, CO, CH4, C2H6, C2H4, C3H8, C3H6, etc.. Figure 8LI4Ti5O12 / LiMN2O4 Battery at 30, 45, 60 ° C for 5 months of gas ingredients The electrolytic liquid system selected by LiPF6 / Ec: EMC, wherein LiPF6 exists in the electrolyte: PF5 is a kind of Very strong acid, it is easy to cause the decomposition of carbonate, and the amount of PF5 has increased with the increase of temperature.
. PF5 contributes to electrolyte decomposition, CO2, CO and CXHY gases. Calculation also shows that EC decomposition occurs CO, CO2 gas.
C2H4 and C3H6 are C2H6 and C3H8 are generated and the C2H6 + oxidative reactions occurred, respectively, and Ti4 + is reduced to Ti3 +.. According to relevant research H2, it derived from trace water in the electrolyte, but the water content in the general electrolyte is about 20 × 10-6, and the gas gas of H2.
Shanghai Jiaotong University Wu Kai’s experiment was used to make battery quantity of graphite / NCM111 very low. The conclusion concluded that the source of H2 is the decomposition of carbonate under high voltage..
Third, the process is abnormal, causing the occurrence of expansion 1. Poor packaging, the proportion of the flattening battery core caused by the poor packaging has been greatly reduced..
The cause of TopSealing, Sidesealing and Degassing three-side packages have been introduced. Any side packaging can cause the battery core. It performs TopSealing and Degassing.
TopSealing is important that the Tab bit is poorly sealed. Degassing is important to layers (including electrical solution) The effects of liquids and gels cause PP and Al to disengage. Package is poorly causing moisture in the air into the internal cell, causing an electrolytic solution to decompose a gas, etc.
. 2. Pocket surface is damaged, the battery core is abnormally damaged or artificially broken in the process of abnormal damage (such as pinholes) to the interior of the battery core.
. 3. The angular disorders are damaged.
Due to the special deformation of aluminum in the flange, the airbag shake will distort the angular position causes Al breakage (the larger the battery core, the larger the air bag, the more broken), and lose the barrier use of water.. Can be relieved in angular wrinkle or hot melt adhesive.
And in the top-sealing process prohibits taking the airbag mobile cell, but also pay more attention to the operation method to prevent the swing of the battery on the aging plate.. 4.
The internal water content of the battery core exceeds the standard, once the water content exceeds the standard, the electrolyte will fail in the formation or degassing.. The reason why the internal water content exceeding the standard is important: the water content of the electrolytic liquid exceeds the standard, BAKING is exceeded, and the dry room is exceeded.
. If the water content exceeds the standard, the retrospective inspection of the process can be carried out..
5. Extraformed process abnormalities, the wrong formulation process can cause a flatulence of the battery core. 6.
Si membrane is unstable, the battery core is slightly flattened during the capacity test charging process. 7. Overcharge, over, due to the abnormality of the process or machine or the protective plate, make the battery core over charge or excessively discharge, the battery core will seriously discharge.
8. Short circuit, due to the operation error causes short circuit in two tab contact in the charging cell, the battery core will drum rapidly, and Tab will be burned black..
9. Internal short circuit, the interior of the battery core causes the battery to quickly discharge heat while severely. There are many reasons for internal short circuits: design problems; isolation membrane shrinkage, curling, damage; Bi-Cell misalignment; glitch-in-isolation membrane; clamp pressure is too large; hot edge pressure is excessive.
For example, it has been due to lack of width, and the heat of the iron is over extruded the battery.. 10.
Corrosion, the battery core is corroded, the aluminum layer is reacted, and the barrier use of water is lost, and the flatulence occurs.. 11.
Vacuum extraversion abnormality, system or machine causes the vacuum abnormality Degassing is not thorough; Vacuumsealing’s thermal radiation area is too large, causing Degassing pumping bayonets to effectively puncture the Pocket bag and cause the breath that is not clean. Four inhibition of abnormal gas production to inhibit abnormal gas production to start from both material design and manufacturing processes. First, the optimization material and the electrolytic liquid system are required to ensure the formation of a dense stable SEI film, improve the stability of the positive electrode material, and inhibit the occurrence of abnormal gas production.
. Treatment of electrolyte often uses a small amount of film formation additive to make SEI membrane more uniform, dense, reducing the SEI membrane falling and regenerative process gas production during use, and related studies have reported and actually Applications, such as the Chengxi of Harbin University of Technology, using film forming additive VC can reduce battery airflant phenomenon. However, research is concentrated on a single component additive, the effect is limited.
The Cao Changhe, etc. of East China University of Science and Technology, using VC and PS composite as a new electrolyte film forming additive, has achieved good results, and the battery is significantly reduced during high temperature shelving and circulation..
Studies have shown that the SEI film components formed by EC and Vc are linear alkyl carbonate, and the alkyl carbonate attached at a high temperature is unstable, decomposing the generating gas (such as CO2, etc.) and the battery is bulging..
The SEI film formed by PS is lithium alkylsulfonate, although defects, but there is a certain two-dimensional structure, which is still more stable at high temperatures in LiC.. When the VC and PS are used, the PS forms a defective two-dimensional structure in the negative surface when the voltage is low, and the elevated Vc of the voltage also forms a linear structure of alkyl carbonate, alkyl carbonate filled.
In the defect of the two-dimensional structure, a SEI film that stabilizes the network structure has a network structure.. The SEI film of this structure greatly increases its stability, which can effectively suppress the gas gas caused by membrane decomposition.
. Further, since the electrode cobalt-based material and the electrolyte of the electrolyte, the decomposition product can catalyze the solvent decomposition in the electrolyte, so that the surface coating of the positive electrode material can not only add the structural stability of the material, but also a positive electrode and Contact in electrolyte, reducing the gas generated by the active positive electrode catalytic decomposition. Therefore, the surface of the positive electrode material particles form a stable and complete coating layer is also a current direction of development.