Overview of Failure Analysis and Fault Mechanism of Lithium Ion Battery

　　Author ：Iflowpower – Portable Power Station Supplier

The generation and growth of the 1SEI film is in a commercial lithium-ion battery system, and the battery capacity loss portion is from the side effect between graphite and organic electrolyte, and the graphite is easily electrochemically reacted with lithium ion organic electrolyte, especially Solvent is vinyl carbonate (EC) and dimethyl carbonate (DMC). When the lithium ion battery is during the first charging (stage), the negative electrolyte and the lithium ion electrolyte occurred and the lithium ion electrolyte occurred and a layer of solid electrolyte interface (SEI) film is formed in the graphite surface, which can cause a portion of the irreversible capacity. The SEI film ensures the transmission of ions while protecting the reactive substance, and prevents the stability of the active material operation of the battery active material while preventing the active substance.

However, during the subsequent cycle of the battery, since the constant expansion and contraction of the electrode material cause a new active site to expose, this can cause a continuous loss failure mechanism, that is, the capacity of the battery is continuously lowered. This failure mechanism can be attributed to the electrochemical reduction process of the surface of the electrode, which is expressed as the continuous increase in the thickness of the SEI film. Therefore, the study of SEI film chemical components and morphology can be more in-depth, the cause of lithium-ion battery capacity and power decline.

SEI film formation process In recent years, researchers have tried to study the nature of SEI membranes through dismantling experiments of small battery systems. The disassembly process of the battery is carried out in an aerosolic inert gas glove box (<5 ppm). After the battery is disassembled, it can pass a nuclear magnetic resonance technology (NMR), a flight time secondary ion mass spectrometry (TEMS), a scanning electron microscope (SEM), a transmission electron microscope (TEM), an atomic force microscope (AFM), X-ray absorption spectrum (XAF), and Infrared (FTIR) and Raman Spectroscopy and other test methods study the thickness, morphology, composition, growth process and mechanism of SEI membranes.

Although many test methods have been used to characterize the SEI film, the actual model of the SEI film grows in the battery is used to characterize more advanced and direct ways. The difficulty is that the SEI film is complicated with a variety of substances such as organic and inorganic, and the ingredient is complicated, and it is very fragile and easy to respond to the environment. If it is improper, it is difficult to obtain the true information of the SEI film.

The thickening of the SEI film is a typical electrochemical parasitic side reaction, which has a close relationship with the reaction kinetics, mass transfer process, and structural geometry of the battery. However, the change of the SEI film does not directly lead to the failure of destructive failure, and its decomposition will only cause an increase in the internal temperature of the battery, which in turn can cause the decomposition gas, and severe heat will cause thermal out of control. In FMMEA, the formation and growth of the SEI film is considered a loss mechanism, which can cause the battery to reduce capacity and increase internal impedance.

2 Lithium dendrites generate, if the battery is quickly charged at a current density higher than its rated current, and the negative surface is easily formed to form a metal lithium dendride. This dendritic crystal is easy to pierce the diaphragm, causing a short circuit inside the battery. This situation may result in failure of battery destruction, and it is difficult to detect before the battery is short-circuited.

In recent years, the researchers have studied the growth rate of lithium dendride and the relationship between the growth rate of lithium dendrites and the lithium ion diffusion capacity of lithium dendrites. Experiments show that the growth of lithium delegra is difficult to detect or observe in a complete battery system, and the current model is limited to the growth of lithium dendrites under a single system. In the experimental system, the transparent battery constructed by quartz glass can observe the growth process of lithium dendrites in situ.

The Zhang Yuegono researcher in Suzhou Nanotechnology and Nano Bionic Research Institute in my country has revealed the formation process of lithium dendrites (as shown by video) technology in the scanning electron microscope (SEM) technology. However, in the commercial lithium-ion battery system, it is difficult to achieve the original observation of lithium branches. The universal situation is to observe its lithium branch crystals by dismantling the battery.

However, because the activity of the lithium branch is very high, it is difficult to analyze the details of the generation. Zier et al. Proposed to draw electrode electron micrograms by dyeing the electrode structure to determine the position of the dendrites.

If before battery dismantion, the generation of lithium branch crystal has caused the inside short circuit inside, then this part of the dendritic crystal may be difficult to observe because the huge pulse current of the internal short circuit may cause lithium branch crystallization. The local microporous closing of the diaphragm suggests that the possible growth position of lithium dendrites, but these parts may be partially overheating or caused by metal impurity contaminants. Therefore, further development of failure models to predict the emergence of lithium branches, and at the same time, it is very meaningful to study life and failure relationship under different working conditions.

3 The pollization of the active material particles is uneven in the dispensing of rapid charge and discharge or electrode active substance, the active material is prone to powder or fragmentation. In general, as the battery is extended, the micron-sized particles, the internal stress of ion may be broken. The initial crack can be observed by SEM on the surface of the active material particles.

As the repeated embedding of lithium ions, the cracks are constantly extending, resulting in particles cracking. The cracking particles will expose the new active surface, and the SEI film is generated on the new surface. By research and analysis of lithium ion embedding stress, better design battery electrode materials.

Christensen and Newman et al. Developed the initial lithium-ion embedded stress model, and other researchers have expanded different materials, and the geometric morphology of materials, and materials. Ion embedded stress model will facilitate researchers to design more active substances.

However, the loss of capacity and power of active material particles is further studied, and the failure mechanism of particle fragmentation is comprehensively predicted to predict the life of lithium-ion batteries. The volume change of the electrode material can also cause the active substance to be unloaded with the current collector, so that this part of the active substance is not available. The inconed lithium process of the active material is accompanied by ion migration and external electron migration inside the battery.

Since the electrolyte is electronically insulated, only ions can only be supplied. The conduct of electrons is important to the conductive network constructed by the electrode surface by the conductive agent. Frequent changes in the volume of the electrode material can result in partial active substances from the conductive network to form an isolated system, which is not available.

This change in the electrode structure can be measured by measuring a method such as a porosity or a specific surface area. This process can also be milled by milling the electrode surface using the focal ion beam (FIB), using SEM to perform morphological observation or X-ray tomography test using SEM. Si negative electrode material is cleaned and disengaged from the conductive network.

The positive electrode active substance of the positive electrode active substance is mostly transition metal oxide, such as lithium cobaltate (LiMn2O4), or polyanate Lithium salt, lithium iron phosphate (LifePo4). Most of the positive active substances are embedded reaction mechanisms, and their stress mechanisms and recession mechanisms are mostly due to the fall of granules and the description of the active substances above. The SEI film is also generated and affected by the surface of the positive electrode, but the surface of the positive electrode has a high potential, and its SEI film is very thin and stable.

In addition, the positive electrode material is also susceptible to the influence of internal heat generation, especially when the battery is overchaired. At the time of charge, the electrolyte becomes unstable under high pressure, which results in an electrolyte and the positive electrode active substance, which causes the internal temperature of the battery to continue to rise, and the positive electrode material releases oxygen. Further upgrade, resulting in thermal out-of control, it will cause destruction failure to the battery.

The positive electrode material that occurs during the pre-charge can be analyzed by gas chromatography to analyze or detect the electrode material structure by X-ray spectrum detection electrode material structure. However, there is currently no failure model that can predict the inside of the battery by overcharged gas overflow. Summary: The failure mechanism mode of the lithium-ion battery positive and negative electrode material is important to the decomposition of the SEI membrane, the production of lithium delegated crystal or copperprine crystals, the powder of the active material particles and the heat decomposition gas, etc.

Among them, the generation of lithium derivatives or copper delegths, the material decomposition gas is easily caused by thermal out of control of the cell, causing the combustion of the battery, and even exploding. The failure of lithium-ion batteries is analyzed by the faded mode, and the mechanism is optimized by optimizing the battery's material, structure, and improve the environmental adaptability, reliability and safety of the battery. Therefore, there is a very important guiding significance for the production and practical application of the battery.