Reasons for depth analysis of lithium ion batteries

　　Author ：Iflowpower – Portable Power Station Supplier

Due to its high life, lithium-ion battery is widely used, with the extension of the use time, the problem of bulging, safety performance is not ideal and circulating attenuation is also more serious, causing the analysis and suppression of lithium battery depth Research. According to experimental research and development experience, the author divides the causes of lithium batteries into two categories, one is bulging caused by the thickness of the battery (second, due to the bulging of the electrolytic liquid oxidation). In different battery systems, the dominant factors of the battery thickness are different.

For example, in the lithium titanate negative electrode battery, the main factors of the bulging are the drum; in the graphite negative electrode system, the thickness of the pole thickness and the bulging of the gas supply Act. First, the thickness of the electrode pole is changed in the use of lithium batteries, and the thickness of the electrode pole having a thickness change, especially the graphite negative electrode. According to the existing data, the lithium battery has passed high temperature storage and circulation, which is prone to drumming, with a thickness growth rate of about 6% to 20%, wherein the positive polar expansion ratio is only 4%, and the negative expansion ratio is 20%.

The root cause of the bulging of the thickness of lithium battery changes is affected by the essence of graphite. The negative electrode graphite forms LICX (LIC24, LiC12 and LIC6, etc.), and the linear spacing changes, resulting in the formation of microscopic internal stress, resulting in a negative electrode Expand.

The figure below is a schematic structural chart of the structure of the graphite negative electrode plate in place and charge and discharge. The expansion of the graphite negative electrode is mainly caused by ineffective expansion. This part of the expansion is mainly related to the structure of particle size, adhesive agent and pole sheet.

The expansion of the negative electrode causes the core to deform, and the electrode is formed between the diaphragm, and the negative electrode particles form a microcrack, the solid electrolyte phase interface (SEI) film is broken and recombinant, consuming electrolyte, and detergeting the circulating performance. There are many factors affecting the negative electrode poles, and the nature of the adhesive and the structural parameters of the polar sheet are two most important. The adhesive commonly used in graphite negative electrode is SBR, different adhesive elastic modulus, different mechanical strength, and different effects on the thickness of the plate.

The rolling force after the finish coating is also affected by the thickness of the negative electrode plate in the battery. Under the same stress, the larger the elastic modulus of the adhesive, the smaller the polarity physical shelving, when charging, due to Li + embedding, the graphite lattice expansion; at the same time, due to the deformation of the negative electrode particles and SBR, internal stress is completely released , Make the negative expansion rate rise sharply, SBR is in the stage of plastic deformation. This part of the expansion ratio is related to the elastic modulus of the SBR, which leads to the larger the elastic modulus and the strength of the SBR, and the smaller the expansion of irreversible expansion.

When the amount of SBR is inconsistent, the pressure is different when the polar roller is pressed, and the pressure difference causes the residual stress produced by the pole, the greater the residual stress, leading to the pre-physical shelving expansion, full electricity and Empty power expansion ratio; the less SBR content, the smaller the pressure of the rolling, the less physical shelves, the expansion ratio of the pre-electricity and the empty electrocositis, the smaller the negative expansion causes the core to deform, affect the negative The degree of lithium is lithium and Li + diffusion rate, thereby generating a serious impact on battery cycle performance. Second, the internal gas intake of the bulk battery caused by battery gas is another important reason that causes battery bulging, whether it is battery temperature cycle, high temperature cycle, high temperature shelving, it produces different degrees of bulging gas. According to current research results, the essence of the electrical core swelling is caused by the decomposition of electrolyte.

There are two cases of the electrolyte decomposition, one is an impurity of the electrolyte, such as moisture and metal impurities to decompose the electrolytic fluid, and the other is too low of the electrolytic fluid, which causes the decomposition during charging, and in electrolyte Solvents such as EC, DEC are generated after obtaining electrons, and the direct consequences of free radical reactions are hydrocarbons, esters, ethers and CO2, etc. After the lithium battery assembly is completed, a small amount of gas is generated during the predetermined process, and these gases are inevitable, and the so-called electrical core irreversible capacity loss source. During the first charge and discharge process, the electrons reaches the electrolytic solution with the electrolytic solution of the negative electrode after the external circuit, forming a gas.

In this process, SEI is formed on the surface of the graphite negative electrode, with the thickness of the SEI increase, electrons cannot penetrate the continuous oxidation of the electrolyte. During the battery life, the internal gas volume will gradually increase, because of the cause of impurities or moisture in the electrolyte or in the electrolyte. The presence of the electrolyte requires serious exclude, and moisture control is not strict.

The electrolytic solution itself is not strict, and the battery pack is not strictly introduced into water, the angular dispensing is caused, and the overtilization of the battery will also accelerate the gas production of the battery. Speed, causing battery failure. In different systems, the amount of battery production is different.

In the graphite negative electrode battery, the cause of gas production is mainly due to the SEI film formation, the moisture in the battery is exceeded, and the chemical flow is abnormal, the package is poor, and the battery florescent ratio in the lithium titanate The NCM battery system should be more serious. In addition to the impurities, moisture and processes in the electrolyte, another difference from the graphite negative electrode is that lithium titanate cannot be like a graphite negative electrode battery, forming a SEI film on its surface, inhibiting its Electrolyte reaction. The electrolyte is always directly in contact with the surface of Li4Ti5O12 during charge and discharge, resulting in a continuous reduction of the surface of Li4Ti5O12 material, which may be the root cause of Li4Ti5o12 battery flatulence.

The main components of the gas are H2, CO2, CO, CH4, C2H6, C2H4, C3H8, etc. When the lithium titanate is separately immersed in the electrolyte, only CO2 is produced, and after preparing a battery with an NCM material, the generated gases include H2, CO2, CO, and a small amount of gaseous hydrocarbons, and after the battery, only in the cycle When charging and discharging, H2 is generated, and in the gas generated, the H2 content exceeds 50%. This indicates that H2 and CO gas will be generated during charge and discharge.

LIPF6 exists in the electrolyte: PF5 is a very strong acid, which is easy to cause decomposition of carbonate, and increase the amount of PF5 with temperature increase. PF5 contributes to electrolyte decomposition, producing CO2, CO and CXHY gas. According to relevant research, the production of H2 is derived from trace water in the electrolyte, but the water content in the general electrolyte is about 20 ¡Á 10-6, which is very low for the Yield of H2.

Shanghai Jiaotong University Wu Kai's experiment was used as a battery for graphite / NCM111. The conclusion concluded that the source of H2 is the decomposition of carbonate under high voltage. At present, there are three solutions for suppression of lithium titanate batteries.

, Solvent system; third, improve battery process technology.