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An investigation of the causes of lithium-ion battery swelling

Views: 202     Author: Hedy     Publish Time: 2023-06-15      Origin: Site


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An investigation of the causes of lithium-ion battery swelling

Because of its extended life and large capacity, lithium-ion batteries are frequently utilized. However, as usage duration has increased, the problems of bulging, inadequate safety performance, and fast cycle attenuation have become increasingly significant, prompting in-depth investigation and suppression research in the lithium battery sector. The author divides the causes of lithium battery bulging into two categories based on experimental research and development experience: bulging caused by the thickness change of the battery pole piece, and bulging caused by the oxidation and decomposition of the electrolyte to produce gas.The primary variables influencing battery thickness change differ between various systems. The major cause of bulge in lithium titanate negative electrode system batteries, for example, is air bulge. The thickness of the electrode sheet and gas generation in the graphite negative electrode system both contribute to battery swelling.

Alterations in electrode pad thickness

The thickness of the electrode sheet will fluctuate to some amount throughout the usage of lithium batteries, particularly the graphite negative electrode. Existing data show that lithium batteries are prone to swelling following high-temperature storage and cycling, with a thickness growth rate ranging from 6% to 20%.The expansion rate of the positive electrode is just 4%, whereas the expansion rate of the negative electrode is more than 20%. The nature of graphite is the root cause of the bulging induced by the rise in the thickness of the lithium battery pole component. When intercalating lithium, negative electrode graphite creates LiCx (LiC24, LiC12, and LiC6, for example), and the lattice spacing changes, leading in the production of tiny internal tension, which causes the negative electrode to expand.

The expansion of the graphite negative electrode is mostly driven by irreversible expansion following lithium intercalation. This section of the expansion is primarily concerned with particle size, binder, and pole piece construction. The expansion of the negative electrode causes the winding core to deform, voids to form between the electrode and the separator, microcracks to form in the negative electrode particles, the rupture and reorganization of the solid electrolyte interface (SEI) film, electrolyte consumption, and cycle performance to deteriorate. The nature of the binder and the structural features of the pole piece are two of the most critical aspects that influence the thickness of the negative pole piece.SBR is a typical binder for graphite negative electrodes. varied binders have varied elastic modulus and mechanical strength, therefore their impacts on pole piece thickness vary. In battery use, the rolling force after the electrode sheet is coated influences the thickness of the negative electrode sheet. The bigger the elastic modulus of the adhesive under the same tension, the lower the physical rest rebound of the pole piece.When charging, the graphite lattice expands due to Li + intercalation; at the same time, the internal stress is completely released due to the deformation of the negative electrode particles and SBR, so the expansion rate of the negative electrode increases sharply, and the SBR is in the plastic deformation stage.

This portion of the expansion rate is connected to the elastic modulus and breaking strength of SBR, resulting in a decreased irreversible expansion as the elastic modulus and breaking strength of SBR increase.When the amount of SBR injected varies, the pressure on the pole piece varies when it is rolled. The residual tension created by the pole piece varies depending on the pressure. The higher the pressure, the higher the residual stress, which causes physical shelf expansion in the early stage, full charge state, and The expansion rate of the empty state increases as the SBR content decreases; the lower the SBR content, the lower the pressure during rolling, and the lower the early expansion rate of the physical shelf, full state, and empty state.The expansion of the negative electrode deforms the winding core, affecting the negative electrode's lithium intercalation degree and the Li + diffusion rate, which has a major influence on battery cycle performance.

Bulging due to battery gas

Another major cause of battery swelling is gas generation within the battery. The battery will create varying degrees of bulging and gas generation when cycled at normal temperature, high temperature cycled, or left at high temperature. According to recent study findings, the essence of battery flatulence is generated by electrolyte breakdown. Electrolyte breakdown occurs in two ways.One is that the electrolyte contains impurities such as moisture and metal impurities, which cause the electrolyte to breakdown and create gas. The other is that the electrolyte's electrochemical window is too narrow, resulting in breakdown during charging. After accepting electrons, electrolyte solvents such as EC and DEC create free radicals. The free radical reaction directly produces low-boiling hydrocarbons, esters, ethers, and CO2.

During the preform process, a little quantity of gas is produced after the lithium battery is built. These gases are unavoidable, and they are also the source of the battery cell's so-called irreversible capacity loss. After electrons reach the negative electrode from the external circuit during the initial charge and discharge operation, they will undertake redox reactions with the electrolyte on the negative electrode's surface to create gas. During this process, SEI forms on the surface of the graphite anode, and as the thickness of the SEI rises, electrons cannot enter, inhibiting the electrolyte's continual oxidative breakdown.

The internal gas generation of the battery will steadily grow during usage.

The reason for this is because the electrolyte contains contaminants or the moisture content of the battery exceeds the standard. Impurities in the electrolyte must be eliminated with care. The electrolyte itself, bad battery packing, water introduction, and corner corrosion can all contribute to poor moisture control. Furthermore, battery abuse such as overcharging and discharging, internal short circuiting, and so on will increase the battery's gas generation rate and cause the battery to die.The degree of battery swelling varies between systems. The major causes of gas production and swelling in graphite negative electrode system batteries are the aforementioned SEI film development, excessive moisture in the battery cell, an irregular manufacturing process, and inadequate packaging. Battery flatulence is substantially worse in the lithium titanate negative electrode system than in the graphite/NCM battery system.

Aside from contaminants, moisture, and technology in the electrolyte, another reason it differs from graphite anodes is that lithium titanate, unlike graphite anode system batteries, cannot build an SEI coating on the surface to block its interaction with the electrolyte. The electrolyte is always in direct contact with the surface of the Li4Ti5O12 material during the charge and discharge process, resulting in continuous reduction and breakdown of electricity on the surface of the Li4Ti5O12 material, which may be the root cause of Li4Ti5O12 battery swelling.H2, CO2, CO, CH4, C2H6, C2H4, C3H8, and other gases are the primary components. Only CO2 is created when lithium titanate is submerged in the electrolyte alone. The gas created once it is converted into a battery using NCM materials comprises H2, CO2, CO, and a little quantity of gaseous hydrocarbons, and after the battery is constructed, H2 is only produced when the battery is charged and discharged. The concentration of H2 in the gas generated at the same time approaches 50%. This means that throughout the charging and discharging processes, H2 and CO gases will be produced.

PF5 is a powerful acid that may readily trigger the breakdown of carbonates, and the amount of PF5 rises with temperature. PF5 aids the electrolyte's decomposition, resulting in CO2, CO, and CxHy gas. According to relevant research, the synthesis of H2 originates from trace water in the electrolyte, although the water content in the electrolyte is normally about 2010-6, which contributes very little to the formation of H2.In the experiment, Wu Kai of Shanghai Jiaotong University used graphite/NCM111 as the battery and determined that the source of H2 is the breakdown of carbonate under high voltage. There are now three basic solutions to lithium titanate battery flatulence: first, the processing and modification of LTO anode materials, including enhancing the production procedure and surface modification. Second, create an electrolyte with additives and solvent systems that matches the LTO negative electrode. Third, battery technology should be improved.

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