1. Variation of electrode thickness
Discussion on Influencing Factors and Mechanisms of Graphite Anode Expansion
The increase in the thickness of the cell during the charging process of lithium-ion batteries is mainly due to the expansion of the negative electrode. The positive electrode expansion rate is only 2~4%. The negative electrode is usually composed of graphite, binder, and conductive carbon. The expansion rate of the graphite material itself reaches ~10%, the important factors that cause the change of the expansion rate of graphite anode include: SEI film formation, state of charge (SOC), process parameters and other influencing factors.
(1) SEI film formation During the first charge and discharge process of lithium-ion batteries, the electrolyte undergoes a reduction reaction at the solid-liquid interface of the graphite particles, forming a passivation layer (SEI film) covering the surface of the electrode material. The appearance of the SEI film The anode thickness is significantly increased, and the thickness of the cell is increased by about 4% due to the appearance of the SEI film. From the perspective of the long-term cycle process, according to the physical structure and specific surface of different graphites, the dissolution of SEI and the dynamic process of new SEI production will occur during the cycle. For example, flake graphite has a larger expansion rate than spherical graphite.
(2) During the cycle of the state of charge cell, the volume expansion of the graphite anode and the SOC of the cell have a good periodic functional relationship, that is, with the continuous insertion of lithium ions in the graphite (the increase of the cell SOC) volume Gradually expand, when lithium ions are extracted from the graphite anode, the SOC of the cell gradually decreases, and the corresponding graphite anode volume gradually decreases.
(3) Process parameters From the perspective of process parameters, the compaction density has a great influence on the graphite anode. During the cold pressing of the pole piece, a large compressive stress appears in the graphite anode film layer, and this stress is caused by the subsequent high temperature baking of the pole piece. It is difficult to completely release the process. When the battery is cyclically charged and discharged, due to the combined use of multiple factors such as the insertion and extraction of lithium ions and the swelling of the electrolyte to the adhesive, the diaphragm stress is released during the cycle and the expansion rate increases. On the other hand, the compaction density determines the void capacity of the anode film layer. The large void capacity in the film layer can effectively absorb the expanded volume of the pole piece, and the void capacity is small. When the pole piece expands, there is not enough space to absorb the expansion The volume that appears, at this time, the expansion can only expand to the outside of the membrane layer, which is manifested as the volume expansion of the anode sheet.
(4) Other factors Adhesive strength of the adhesive (the adhesive strength of the adhesive, graphite particles, conductive carbon and the interface between the current collectors), charge and discharge rate, swellability of the adhesive and electrolyte, graphite particles The shape and bulk density of the anode, as well as the increase in the volume of the pole piece caused by the failure of the binder during the cycle, all have a certain degree of influence on the anode expansion.
Expansion rate calculation:
For the calculation of expansion ratio, the dimensions of the anode sheet in X and Y directions were measured by the quadratic element, and the thickness in the Z direction was measured with a micrometer.
Effects of compaction density and coating quality on anode expansion
Taking the compaction density and coating quality as factors, and taking three different levels for each, a full factorial orthogonal experimental design was carried out (as shown in Table 1), and other conditions were the same for each group.
After the cell is fully charged, the expansion rate of the anode sheet in the X/Y/Z direction increases with the increase of the compaction density. When the compaction density increased from 1.5g/cm3 to 1.7g/cm3, the expansion rate in X/Y direction increased from 0.7% to 1.3%, and the expansion rate in Z direction increased from 13% to 18%. It can be seen from Figure 2(a) that under different compaction densities, the expansion rate in the X direction is greater than that in the Y direction. The reason for this phenomenon is mainly caused by the cold pressing process of the pole piece. When rolling, according to the law of least resistance, when the material is subjected to external force, the material particles will flow in the direction of least resistance.
When the negative plate is cold pressed, the direction with the least resistance is the MD direction (the Y direction of the pole piece, as shown in Figure 3), and the stress is easier to release in the MD direction, while the TD direction (the X direction of the pole piece) has greater resistance, and the roller The stress in the compressive process is not easy to release, and the stress in the TD direction is larger than that in the MD direction. Therefore, after the electrode sheet is fully charged, the expansion rate in the X direction is greater than the expansion rate in the Y direction. On the other hand, the compaction density increases, and the pore capacity of the electrode sheet decreases (as shown in Figure 4). When charging, there is no internal anode film layer. There is enough space to absorb the expanded volume of graphite, and the external manifestation is that the whole pole piece expands in the three directions of X, Y and Z. It can be seen from Figure 2(c) and (d) that the coating quality increased from 0.140g/1, 540.25mm2 to 0.190g/1, 540.25mm2, the expansion rate in the X direction increased from 0.84% to 1.15%, Y The directional expansion rate increased from 0.89% to 1.05%, and the trend of the Z-direction expansion rate was opposite to that of the X/Y direction, showing a downward trend, decreasing from 16.02% to 13.77%. It shows that the graphite anode expansion shows one after another in the three directions of X, Y and Z, and the change of coating quality is mainly reflected in the significant change of the film thickness. The above variation rules of the negative electrode are consistent with the literature results, that is, the smaller the ratio of the current collector thickness to the film thickness, the greater the stress in the current collector.
Influence of copper foil thickness on anode expansion
The two influencing factors of copper foil thickness and coating quality are selected. The thickness of copper foil is 6 and 8 μm, respectively. The quality of anode coating is 0.140g/1, 540.25mm2 and 0.190g/1, 540.25mm2, respectively. The compaction density is 1.6g/cm3, the other conditions of the experiments in each group are the same, and the experimental results are shown in Figure 5. It can be seen from Figure 5(a) and (c) that under the two different coating qualities, the expansion rate of the 8 μm copper foil anode sheet in the X/Y direction is less than 6 μm, indicating that the thickness of the copper foil is increased due to its elastic modulus. Added (see Figure 6), that is, the resistance to deformation is enhanced, the use of anode expansion constraints is enhanced, and the expansion rate is reduced. According to the literature, under the same coating quality, when the thickness of the copper foil is increased, the ratio of the thickness of the current collector to the thickness of the film layer is increased, the stress in the current collector becomes smaller, and the expansion rate of the pole piece becomes smaller. In the Z direction, the change trend of the expansion rate is completely opposite. It can be seen from Figure 5(b) that the thickness of the copper foil increases and the expansion rate increases; from the comparison of Figures 5(b) and (d), it can be seen that when the When the cloth mass is increased from 0.140g/1, 540.25mm2 to 0.190g/1, 540.25mm2, the thickness of the copper foil is increased and the expansion rate is decreased. The increase in the thickness of the copper foil is beneficial to reduce its own stress (high strength), but it will increase the stress in the film layer, resulting in an increase in the expansion rate in the Z direction, as shown in Figure 5(b); Although the thickness of the copper foil can promote the increase of the stress of the film layer, the restraint ability of the film layer is also enhanced. At this time, the restraint force is more obvious and the expansion rate in the Z direction decreases.
Influence of graphite type on anode expansion
Experiments were carried out with 5 different types of graphite (see Table 2), the coating mass was 0.165g/1, 540.25mm2, the compaction density was 1.6g/cm3, the copper foil thickness was 8μm, and other conditions were the same. The experimental results are shown in Figure 7 . It can be seen from Figure 7(a) that the expansion rate of different graphites in the X/Y direction is quite different, the minimum expansion rate is 0.27%, the maximum expansion rate is 1.14%, the Z direction expansion rate is the minimum 15.44%, and the maximum expansion rate is 17.47%. , the expansion in the Z direction is small, which is consistent with the analysis results. Among them, the cells using A-1 graphite were severely deformed, and the deformation ratio was 20%. The other groups of cells did not deform, indicating that the X/Y expansion rate had a significant impact on the deformation of the cells.
(1) Increase the compaction density, the expansion rate of the anode sheet increases along the X/Y and Z directions during the full charging process, and the expansion rate in the X direction is greater than the expansion rate in the Y direction (the X direction is the cooling of the pole piece) The direction of the roller axis during the pressing process, the Y direction is the direction of the machine belt).
(2) With the new coating quality, the expansion rate in the X/Y direction has an increasing trend, and the expansion rate in the Z direction decreases; the new coating quality will lead to an increase in the tensile stress in the current collector.
(3) Improving the strength of the current collector can suppress the expansion of the anode sheet in the X/Y direction.
(4) Different types of graphite have large differences in the expansion rates in the X/Y and Z directions, and the expansion in the X/Y direction has a greater impact on the deformation of the cell.
2. Bulging caused by battery gas production
The internal gas production of the battery is another important reason for the bulging of the battery. Whether the battery is cycled at normal temperature, high temperature cycle, or placed on hold at high temperature, it will bulge and produce gas to varying degrees. During the first charge and discharge of the battery, an SEI (Solid Electrolyte Interface) film will be formed on the electrode surface. The formation of the negative electrode SEI film is mainly due to the reduction and decomposition of EC (EthyleneCarbonate). At the same time as the generation of alkyl lithium and Li2CO3, a large amount of CO and C2H4 will be generated. DMC (DimethylCarbonate) and EMC (EthylMethylCarbonate) in the solvent will also form RLiCO3 and ROLi during the film formation process, along with gases such as CH4, C2H6 and C3H8 and CO gas. In the pC (propylenecarbonate)-based electrolyte, there are relatively more gases, and the most important is the C3H8 gas generated by the reduction of pC. The lithium iron phosphate soft-pack lithium battery has the most serious swelling after charging at 0.1C in the first cycle. It can be seen from the above that the formation of SEI will be accompanied by the appearance of a large amount of gas, which is an inevitable process. The presence of H2O in the impurity will destabilize the p-F bond in LipF6 and generate HF, which will lead to the instability of this battery system with the accompanying gas. The presence of excess H2O consumes Li+ and forms LiOH, LiO2 and H2 resulting in gas evolution. Gas will also appear during storage and long-term charging and discharging. For sealed lithium-ion batteries, the presence of a large amount of gas will cause the battery to swell, thereby affecting the performance of the battery and shortening the service life of the battery. There are two important reasons for the occurrence of gas in the battery during storage: (1) H2O existing in the battery system will lead to the generation of HF, resulting in damage to the SEI. The O2 in the system may cause the oxidation of the electrolyte, resulting in the generation of a large amount of CO2; (2) If the SEI film formed by the first chemical formation is unstable, the SEI film will be damaged during the storage stage, and the re-repair of the SEI film will release hydrocarbons. Class-based gases. During the long-term charge-discharge cycle of the battery, the crystal structure of the positive electrode material changes, and the non-uniform point potential on the electrode surface causes some point potentials to be too high, the stability of the electrolyte on the electrode surface decreases, and the electrode surface film continues to thicken. The interface resistance of the electrode is increased, and the reaction potential is further increased, resulting in the decomposition of the electrolyte on the electrode surface to generate gas, and at the same time, the positive electrode material may also release gas.
In different systems, the degree of battery swelling is different. In the graphite anode system battery, the main reasons for gas production and bulging are the above-mentioned SEI film formation, excessive moisture in the cell, abnormal formation process, poor packaging, etc. In the lithium titanate anode system, the industry generally believes that Li4Ti5O12 The flatulence of the battery is mainly caused by the easy absorption of water by the material itself, but there is no definite evidence to prove this speculation. Xiong et al. of Tianjin Lishen Battery Company pointed out in the abstract of the paper of the 15th International Electrochemical Conference that CO2, CO, alkanes and a small amount of alkenes are included in the gas components, but there is no data to support their specific composition and proportion. And Belharouak et al. used gas chromatography-mass spectrometry to characterize the gas production of the battery. The important component of the gas is H2, as well as CO2, CO, CH4, C2H6, C2H4, C3H8, C3H6, etc.
Generally, the electrolyte system selected for lithium-ion batteries is LipF6/EC:EMC, where LipF6 has the following balance in the electrolyte:
pF5 is a strong acid, which is easy to cause the decomposition of carbonates, and the amount of pF5 increases with the increase of temperature. pF5 contributes to the decomposition of the electrolyte, and CO2, CO and CxHy gases appear. The calculation also showed that CO and CO2 gas appeared in the decomposition of EC. C2H4 and C3H6 are C2H6 and C3H8, which are generated by oxidation-reduction reaction with Ti4+, respectively, and Ti4+ is reduced to Ti3+ at the same time. According to relevant research, the appearance of H2 comes from the trace water in the electrolyte, but the general water content in the electrolyte is about 20×10-6, which is responsible for the gas production of H2. Wu Kai's experiment at Shanghai Jiao Tong University used graphite/NCM111 as the battery, which contributed very little to the battery, and concluded that the source of H2 was the decomposition of carbonate under high voltage.
3. Abnormal process leads to gas-induced expansion
1. Poor packaging, the proportion of flat battery cells caused by poor packaging has been greatly reduced. The reasons for the poor sealing of the three sides of Topsealing, Sidesealing and Degassing have been described above. Poor sealing on any side will lead to battery cells. Topsealing and Degassing are the most common. Topsealing is mainly caused by poor sealing of the Tab position, and Degassing is mainly caused by delamination (including electrolysis. Liquid and gel effects lead to pp and Al detachment). Poor packaging causes moisture in the air to enter the interior of the battery cell, causing the electrolyte to decompose and generate gas.
2. The surface of the pocket is damaged, and the battery cell is abnormally damaged or artificially broken during the flow-pulling process, causing the pocket to be damaged (such as pinholes) and allow moisture to enter the interior of the battery cell.
3. Corner damage, due to the special deformation of the aluminum at the folded corner, the shaking of the air bag will distort the angle and cause the Al to be damaged (the larger the battery cell, the larger the air bag, the easier it is to break), and lose its barrier function to water. Wrinkle glue or hot-melt glue can be added to the corners for relief. And in each process after top sealing, it is forbidden to use the air bag to move the battery cell, and more attention should be paid to the operation method to prevent the swing of the battery cell on the aging board.
4. The water content inside the battery cell exceeds the standard. Once the water content exceeds the standard, the electrolyte will fail and gas will appear after formation or Degassing. The main reasons for the excessive water content in the battery are: the water content of the electrolyte exceeds the standard, the water content of the bare cell after Baking exceeds the standard, and the drying room Excessive humidity. If it is suspected that the water content exceeds the standard and causes flatulence, a retrospective inspection of the process can be carried out.
5. The formation process is abnormal, and the wrong formation process will cause flatulence in the battery cell.
6. The SEI film is unstable, and the battery cell is slightly bloated during the charging and discharging process of the capacity test.
7. Overcharge and overdischarge, due to the abnormality of the process or machine or protection board, the battery cell is overcharged or overdischarged, and the battery cell will be seriously inflated.
8. Short circuit, due to operation errors, the contact between the two tabs of the charged cell will be short-circuited, the battery cell will be gassed and the voltage will drop rapidly, and the tab will be burnt black.
9. Internal short circuit, the short circuit of cathode and anode inside the battery cell causes the cell to rapidly discharge and heat up and at the same time serious gassing. There are many reasons for internal short circuits: design problems; isolation diaphragm shrinkage, curling, breakage; Bi-cell dislocation; burrs piercing isolation diaphragm; excessive clamp pressure; For example, due to insufficient width, the ironing machine over-extruded the cell body, resulting in short-circuiting of the cathode and anode and flatulence.
10. Corrosion, the battery core is corroded, the aluminum layer is consumed by the reaction, the barrier function to water is lost, and flatulence occurs.
11. Abnormal vacuum pumping, abnormal vacuum degree due to system or machine. Degassing pumping is not complete; the heat radiation area of VacuumSealing is too large, so that the Degassing pumping bayonet cannot effectively pierce the pocket bag, resulting in unclean pumping.
Four measures to suppress abnormal gas production
Suppressing abnormal gas production should start from both material design and manufacturing process.
First of all, it is necessary to design and optimize the material and electrolyte system to ensure the formation of a dense and stable SEI film, improve the stability of the cathode material, and inhibit the occurrence of abnormal gas production.
For the treatment of the electrolyte, the method of adding a small amount of film-forming additives is often used to make the SEI film more uniform and dense, so as to reduce the SEI film falling off during the use of the battery and the gas production during the regeneration process, which causes the battery to bulge. Related research has been reported and practical. For example, Cheng Su of Harbin University of Science and Technology reported that the use of film-forming additive VC can reduce the phenomenon of battery inflation. However, most studies focus on single-component additives with limited effect. Cao Changhe et al. of East China University of Science and Technology used VC and pS composite as a new type of electrolyte film-forming additive, and achieved good results. The gas production of the battery was significantly reduced during high temperature storage and cycling. Studies have shown that the components of the SEI film formed by EC and VC are linear alkyl lithium carbonate, and the alkyl lithium carbonate attached to LiC is unstable at high temperature, decomposes to generate gas (such as CO2, etc.), and the battery swells. The SEI film formed by pS is lithium alkyl sulfonate. Although the film has defects, it has a certain two-dimensional structure and is still relatively stable when attached to LiC at high temperature. When VC and pS are used in combination, pS forms a defective two-dimensional structure on the negative electrode surface when the voltage is low, and as the voltage increases, VC forms a linear structure of alkyl lithium carbonate on the negative electrode surface, and the alkyl lithium carbonate fills In the defects of the two-dimensional structure, an SEI film with a network structure stably attached to LiC was formed. The SEI film with this structure greatly improves its stability and can effectively suppress the gas generation caused by the decomposition of the film.
In addition, due to the mutual use of the positive electrode lithium cobalt oxide material and the electrolyte, its decomposition products will catalyze the decomposition of the solvent in the electrolyte, so the surface coating of the positive electrode material can not only increase the structural stability of the material, but also reduce the positive electrode and the electrolyte. The contact of the electrolyte reduces the gas generated by the catalytic decomposition of the active cathode. Therefore, forming a stable and complete coating layer on the surface of cathode material particles is also a major development direction at present.