Analysis of the causes of swelling of lithium-ion batteries

Lithium-ion batteries are widely used due to their long life and high capacity. However, with the extension of use time, the problems of swelling, unsatisfactory safety performance and accelerated cycle attenuation have become increasingly serious, which has caused deep analysis and suppression in the lithium battery industry. the study. Based on experimental research and development experience, the author divides the causes of lithium battery swelling into two categories, one is the swelling caused by the change in the thickness of the battery pole piece; the other is the swelling caused by the oxidation and decomposition of electrolyte to produce gas. In different battery systems, the leading factors for the change of battery thickness are different. For example, in the lithium titanate anode system battery, the main factor of swelling is air swelling; in the graphite anode system, the thickness of the pole piece and the gas production affect the swelling of the battery. Play a promotional role.

1. Change in thickness of electrode pole piece

During the use of lithium batteries, the thickness of the electrode pole pieces will change to a certain extent, especially the graphite negative electrode. According to existing data, lithium batteries are prone to swelling after high-temperature storage and circulation, and the thickness growth rate is about 6% to 20%. The expansion rate of the positive electrode is only 4%, and the expansion rate of the negative electrode is more than 20%. The fundamental reason for the swelling caused by the increased thickness of the lithium battery pole piece is the essence of graphite. The negative electrode graphite forms LiCx (LiC24, LiC12 and LiC6, etc.) when lithium is inserted, and the lattice spacing changes, resulting in the formation of microscopic internal stress, causing the negative electrode to produce Swell. The figure below is a schematic diagram of the structural changes of the graphite negative pole piece during placement, charging and discharging.

The expansion of graphite negative electrode is mainly caused by irreversible expansion after lithium insertion. This part of the expansion is mainly related to the particle size, the adhesive and the structure of the pole piece. The expansion of the negative electrode causes the core to deform, forming a cavity between the electrode and the diaphragm, forming micro-cracks in the negative electrode particles, breaking and reorganizing the solid electrolyte interface (SEI) membrane, consuming the electrolyte, and deteriorating the cycle performance. There are many factors that affect the thickness of the negative pole piece. The properties of the adhesive and the structure parameters of the pole piece are the two most important.

The commonly used adhesive for graphite negative electrodes is SBR. Different adhesives have different elastic modulus and mechanical strength, and have different effects on the thickness of the pole piece. The rolling force after the pole piece is coated also affects the thickness of the negative pole piece in battery use. Under the same stress, the greater the elastic modulus of the adhesive, the smaller the rebound of the pole piece from physical placement; during charging, the graphite lattice expands due to the insertion of Li +; at the same time, due to the deformation of the negative electrode particles and SBR, the internal stress is completely Release, make the negative electrode expansion rate rise sharply, SBR is in the plastic deformation stage. This part of the expansion rate is related to the elastic modulus and breaking strength of SBR, resulting in the larger the elastic modulus and breaking strength of SBR, the smaller the irreversible expansion.

When the amount of SBR added is inconsistent, the pressure on the pole piece during rolling will be different. Different pressures cause a certain difference in the residual stress generated by the pole piece. The greater the pressure, the greater the residual stress, which leads to physical storage expansion, full electric state and The expansion rate of the empty electric state increases; the less the SBR content, the lower the pressure during rolling, the smaller the expansion rate of the physical storage, the full electric state and the empty electric state in the early stage; the expansion of the negative electrode causes the core to deform and affects the negative electrode The degree of lithium insertion and Li + diffusion rate have a serious impact on battery cycle performance.

2. Bloating caused by battery gas production

       Gas generated inside the battery is another important cause of battery swelling. Whether the battery is stored at room temperature, high temperature cycle, or high temperature, it will produce different degrees of swelling and gas production. According to the current research results, the essence of the cell bloating is caused by the decomposition of the electrolyte. There are two cases of electrolyte decomposition. One is that there are impurities in the electrolyte, such as moisture and metal impurities, which cause the electrolyte to decompose and produce gas. The other is that the electrochemical window of the electrolyte is too low, which causes decomposition during the charging process. The solvents such as EC, DEC, etc. will generate free radicals after getting electrons. The direct consequence of free radical reactions is the production of low-boiling hydrocarbons, esters, ethers and CO2.

After the lithium battery is assembled, a small amount of gas will be generated during the pre-forming process. These gases are inevitable and are also the source of the so-called irreversible capacity loss of the battery. During the first charge and discharge process, after the electrons reach the negative electrode from the external circuit, they will undergo an oxidation-reduction reaction with the electrolyte on the negative electrode surface to generate gas. During this process, SEI is formed on the surface of the graphite negative electrode. As the thickness of the SEI increases, electrons cannot penetrate and inhibit the continuous oxidation and decomposition of the electrolyte. For the formation of SEI, see the article: Dry Goods | What is SEI? So much impact on lithium batteries! During the use of the battery, the internal gas production will gradually increase. The reason is that there are impurities in the electrolyte or excessive moisture in the battery. Impurities in the electrolyte need to be carefully removed. Inadequate moisture control may be caused by the electrolyte itself, the improper battery packaging, the introduction of moisture, and the breakage of the corners. In addition, the battery's overcharge, overdischarge, abuse, and internal short circuit will also accelerate the battery's gas production Speed, causing battery failure.

In different systems, the degree of battery swelling is different. In the graphite anode system battery, the main causes of gas swelling are the above-mentioned SEI film formation, excessive moisture in the cell, abnormal chemical conversion process, poor packaging, etc., while in the lithium titanate anode system, the battery flatulence is more than graphite/ The NCM battery system is much more serious. In addition to the impurities, moisture and process in the electrolyte, another reason different from the graphite anode is that lithium titanate cannot form an SEI film on its surface like a graphite anode system battery to inhibit its The reaction of the electrolyte. During the charging and discharging process, the electrolyte is always in direct contact with the surface of Li4Ti5O12, resulting in continuous reduction and decomposition of electricity on the surface of the Li4Ti5O12 material, which may be the root cause of the Li4Ti5O12 battery flatulence. The main components of the gas are H2, CO2, CO, CH4, C2H6, C2H4, C3H8 and so on. When lithium titanate is immersed in the electrolyte alone, only CO2 is produced. After it is made into a battery with NCM materials, the gases produced include H2, CO2, CO and a small amount of gaseous hydrocarbons. During charging and discharging, H2 is produced, and the content of H2 in the gas produced at the same time exceeds 50%. This indicates that H2 and CO gas will be generated during the charge and discharge process.

LiPF6 has the following balance in the electrolyte:

PF5 is a strong acid, which easily causes the decomposition of carbonates, and the amount of PF5 increases with the increase of temperature. PF5 helps the electrolyte to decompose to produce CO2, CO and CxHy gas. According to related studies, the generation of H2 comes from trace water in the electrolyte, but the water content in the electrolyte is generally about 20×10-6, which contributes very little to the output of H2. Wu Kai of Shanghai Jiaotong University used graphite/NCM111 as the battery in his experiment and concluded that the source of H2 is the decomposition of carbonate under high voltage. At present, there are mainly three solutions to inhibit the flatulence of lithium titanate batteries. First, the processing and modification of LTO anode materials, including improved preparation methods and surface modification, etc.; second, the development of electrolytes that match LTO anodes, including additives , Solvent system; third, improve battery technology.