Failure Analysis and Failure Mechanism of Lithium Ion Battery Electrode Materials

First, the negative active material

The analysis of the failure mechanism of the anode material is mainly based on commercial carbon-based materials. Although, new anode materials, such as silicon, tin and some oxides, are currently being extensively studied and have made great progress in scientific research. However, due to the large volume expansion of these materials during the lithium ion deintercalation cycle, the electrochemical performance is seriously affected. Therefore, it has not been widely used in commercial batteries.

Formation and growth of 1SEI film

In commercial lithium-ion battery systems, the loss of capacity of the battery is partly due to the side reaction between graphite and the organic electrolyte. Graphite is easily electrochemically reacted with the lithium ion organic electrolyte, especially the solvent is ethylene carbonate ( EC) and dimethyl carbonate (DMC). When the lithium battery is in the first charging process (chemical stage), the graphite of the negative electrode reacts side by side with the lithium ion electrolyte and forms a solid electrolyte interface (SEI) film on the graphite surface, which causes a part of the irreversible capacity to be generated. The SEI membrane is capable of permeating Li+, ensuring the transport of ions, while protecting the active material, preventing further occurrence of side reactions, and maintaining the stability of the active material of the battery. However, during the subsequent cycle of the battery, new active sites are exposed due to the continuous expansion and contraction of the electrode material, which causes a continuous loss failure mechanism, that is, the battery capacity is continuously degraded. This failure mechanism can be attributed to the electrochemical reduction process of the electrode surface, which is manifested by the increasing thickness of the SEI film. Therefore, the study of the chemical composition and morphology of SEI membranes can provide a deeper understanding of the reasons for the decline in capacity and power of lithium-ion batteries.

In recent years, researchers have attempted to study the properties of SEI membranes by disassembling experiments on small battery systems. The battery disassembly process needs to be carried out in an anhydrous, oxygen-free inert gas glove box (<5 ppm). After the battery is disassembled, it can be analyzed byspecial magnetic resonance (NMR), time-of-flight secondary ion mass spectrometry (TOF-SIMS), scanning electron microscopy (SEM) transmission electron microscopy (TEM) atomic force microscopy (AFM), X-ray absorption spectroscopy. (XAF) as well as infrared (FTIR) and Raman (Raman) spectroscopy methods to study the thickness, morphology, composition, growth process and mechanism of SEI film, although many test methods have been used to characterize SEI films, there is still a pressing need to utilize a more advanced and straightforward way to characterize the actual model of SEI film growth in batteries. The difficulty is that the SEI film is a composite of organic and inorganic materials. The composition is complex, very fragile and easy to react with the environment. If it is not properly protected, it is difficult to obtain the true information of the SEI film.

Schematic diagram of SEI membrane composition

SEI film thickening is a typical electrochemical parasitic side reaction, which is closely related to the reaction kinetics, mass transfer process and structural geometry of the battery. However, the change of the SEI film does not directly lead to catastrophic failure, and its decomposition will only cause an increase in the internal temperature of the battery, which may lead to decomposition of gas production, which may cause thermal runaway. In FMMEA, the formation and growth of the SEI film are regarded as a loss mechanism, resulting in a decrease in the capacity of the battery and an increase in internal impedance.

2. Lithium dendrite formation

If the battery is rapidly charged at a current density higher than its rated current or charged at a low temperature, the surface of the negative electrode is liable to form metal lithium dendrites. Such dendrites easily pierce the membrane and cause a short circuit inside the battery. This condition can cause battery rupture failure, and dendrites are difficult to detect before a battery short circuit occurs.

Battery thermal runaway process diagram

In recent years, researchers have studied the relationship between the growth rate of lithium dendrites and the loading current density and lithium ion diffusion ability to prevent the formation of lithium dendrites. Experiments have shown that the growth of lithium dendrites is difficult to detect or observe in a complete battery system. The current model is limited to the study of lithium dendrite growth in a single system. In the experimental system, the growth process of lithium dendrites can be observed in situ by a transparent battery constructed of quartz glass. Researcher Zhang Yuegang and his colleagues at the Institute of Nanotechnology and Nano-Bionics of the Chinese Academy of Sciences, based on scanning electron microscopy (SEM), revealed the formation of lithium dendrites (as shown in the video). However, it is difficult to achieve in situ observation of lithium dendrite growth in commercial lithium ion battery systems. It is common to observe lithium dendrites by disassembling the battery. However, since the activity of lithium dendrites is very high, it is difficult to analyze the details of their formation. Zier et al. proposed to determine the position of dendrite growth by dyeing the electrode structure to map the electrode electron micrograph. If the formation of lithium dendrites causes an internal short circuit inside the battery before the battery is disassembled, then this part of the dendrites may be difficult to observe because the large pulse current of the internal short circuit may melt the lithium dendrites. Partial micropore closure of the membrane may imply a possible growth location for lithium dendrites, but these locations may also be caused by local overheating or the presence of metallic contaminants. Therefore, the failure model is further developed to predict the generation of lithium dendrites. At the same time, it is very meaningful to study the relationship between lifetime and failure under different working conditions.

3. Powdering of active material particles

In the case of rapid charge and discharge or uneven distribution of the electrode active material, the active material is liable to be pulverized or chipped. In general, as the use of the battery is extended, particles of a micron size may be chipped due to internal stress of ion implantation. The initial crack can be observed on the surface of the active material particles by SEM. As the lithium ions are repeatedly embedded and released, the cracks will continuously extend to cause the particles to crack. Cracked particles expose a new active surface, and an SEI film is formed on the new surface. By studying and analyzing the lithium ion embedded stress, the electrode material of the battery can be better designed. Christensen and Newman et al. developed an initial lithium-ion embedded stress model, which was followed by other researchers to extend the material and geometry of the material. The ion-embedded stress model will help researchers design more excellent active substances. However, the loss of capacity and power caused by the fragmentation of active material particles needs further research, and the failure mechanism of particle fragmentation is fully considered to more accurately predict the service life of lithium ion batteries.

The volume change of the electrode material also causes the active material to lose contact with the current collector, so that this portion of the active material cannot be utilized. The lithium intercalation process of the active material is accompanied by ion migration inside the battery and external electron transfer. Since the electrolyte is electronically insulated, it can only provide ion conduction. The conduction of electrons mainly depends on the conductive network formed by the conductive agent on the surface of the electrode. Frequent changes in the volume of the electrode material can cause some of the active material to leave the conductive network and form an isolated system that cannot be utilized. This change in electrode structure can be measured by measuring the porosity or specific surface area. This process can also use the focused ion beam (FIB) to mill the surface of the electrode, using SEM to perform electrode shape observation or X-ray tomography test.

Si anode material cracks and pulverizes and leaves the conductive network

Second, the positive active material

The positive electrode active materials of commercial lithium ion batteries are mostly transition metal oxides such as lithium cobaltate, lithium manganate, etc., or polyanionic lithium salts such as lithium iron phosphate. Most of the positive active materials are embedded reaction mechanisms, and the stress mechanism and the decay mechanism are mostly due to particle fragmentation and active material shedding, similar to the description in the negative electrode section above. The SEI film is also formed on the surface of the positive electrode and is affected by it, but the surface of the positive electrode has a higher potential than the surface of the negative electrode, and the SEI film is very thin and stable. In addition, the positive electrode material is also susceptible to decomposition due to internal heat generation, especially in the case of battery overcharge. When overcharged, the electrolyte becomes unstable under high pressure, which causes a side reaction between the electrolyte and the positive active material, causing a continuous increase in the internal temperature of the battery, and the positive electrode material releases oxygen. Further upgrades result in thermal runaway and can cause catastrophic failure of the battery. The positive electrode material in which the overcharge occurs may be detected by gas chromatography to analyze the gas composition inside the battery or by detecting the structure of the electrode material by X-ray spectroscopy. However, there is currently no failure model that predicts gas spillage caused by overcharging inside the battery.

Summary: The failure mechanism mode of the positive and negative materials of lithium ion batteries mainly focuses on the decomposition of SEI film, the formation of lithium dendrites or copper dendrites, the pulverization and shedding of active material particles, and the thermal decomposition of materials. Among them, the formation of lithium dendrites or copper dendrites, material decomposition and gas production are likely to cause thermal runaway of the cells, causing burning or even explosion of the battery. The failure study of lithium-ion battery is based on the discovery of the failure mode and mechanism to optimize the material and structure of the battery, and improve the environmental adaptability, reliability and safety of the battery. Therefore, it has very important guiding significance for the manufacture and practical application of batteries.

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