Introduction to failure analysis of lithium batteries
The national standard GB3187-82 defines: “Failure (failure) – the product loses its specified function. For repairable products, it is usually called a failure.” The failure of lithium batteries refers to the deterioration of battery performance due to some specific essential reasons. Or use performance exceptions. The failures of lithium batteries are mainly divided into two categories: one is performance failure, and the other is safety failure, as shown in Figure 1. Performance failure refers to when the performance of lithium batteries does not meet the requirements and related indicators, mainly including capacity attenuation or diving, short cycle life, poor rate performance, poor consistency, easy self-discharge, high and low-temperature performance degradation, etc.; safety failure refers to The main problem is that due to improper use or abuse of lithium batteries, failures with certain safety risks occur, mainly including thermal runaway, flatulence, liquid leakage, lithium precipitation, short circuit, expansion deformation, etc.
Classification of common lithium battery failures
The birth of failure analysis is accompanied by failure phenomena, and its purpose is to determine and prevent its occurrence. Failure analysis is a technical and management activity for judging product failure modes, analyzing failure causes, and predicting or preventing failure phenomena. People have put forward higher requirements for the performance indicators of lithium batteries, especially in terms of volume/mass-energy density, power density, cycle life, cost, safety performance, etc. For example, energy-based lithium batteries are mentioned in “Made in China 2025” The specific energy of the battery is greater than 300 W·h/kg, and the specific power of the power lithium battery is greater than 4000 W/kg. Figure 2 shows the development roadmap of lithium-ion battery energy density from 1990 to 2025. In order to meet the needs of the market, improve the performance and safety of batteries, and shorten the development cycle of new systems, it is necessary to carry out a failure analysis of lithium batteries.
Development route of lithium-ion battery energy density from 1990 to 2025
Although the birth of a product is accompanied by failure, the failure is recognized by people as starting from the failure phenomenon, so the failure analysis work should start with the failure phenomenon. First of all, we should start with the failure phenomenon of lithium batteries. The failure phenomenon of lithium batteries is the first step in the failure analysis of lithium batteries, and it is one of the most direct and important failure information. If the failure information of a lithium battery is not fully grasped and analyzed, the root cause of the failure of a lithium battery cannot be accurately obtained, so it is not only impossible to provide constructive suggestions or reliability assessment. The failure phenomenon is divided into two parts: dominant and recessive. Dominant refers to directly observable manifestations and characteristics, such as fracture and deformation of the surface structure that occur at the failure site and can be observed by macroscopic analysis, including fire burning, heating, bulging (gas production), deformation, leakage, encapsulation Material damage and distortion, packaging material burrs, virtual welding or missing welding, melting and deformation of plastic materials, etc. Recessive refers to the performance and characteristics that cannot be directly observed but need to be obtained after disassembly, analysis or simulated experiments, such as microscopic failures detected by laboratory disassembly, and electrical information in simulated batteries, etc. In the process of lithium battery failure, there are often hidden failure phenomena such as short circuits in the positive and negative electrodes, lithium precipitation, powder drop of the pole piece, diaphragm aging, diaphragm blockage, diaphragm puncture, electrolyte drying, electrolyte denaturation failure, negative electrode dissolution, transition metal Precipitation (including copper precipitation), pole piece burr, abnormal winding (or lamination), capacity diving, abnormal voltage, high resistance, abnormal cycle life, abnormal high/low-temperature performance, etc. The scope of the failure phenomenon often intersects with the scope of the failure mode, and the failure phenomenon is more inclined to the direct description of the phenomenon, which belongs to the information collection and description of the failure process; the failure mode is generally understood as the nature and type of failure, which is the attribute of failure Classes and divisions. The lithium battery failure phenomenon is a large cluster of battery failure performance, and it is necessary to define and classify it.
Failure is the final manifestation of the failure cause, and it is also the result of the superimposed failure phenomenon of the failure caused within a certain period of time. One of the important tasks of failure analysis is to accurately determine the cause of failure. Common causes of lithium battery failure include structural changes of active materials, phase transitions of active materials, cracks or breakage of active particles, dissolution of transition metals, volume expansion, solid electrolyte interface (SEI) overgrowth, SEI decomposition, lithium dendrite growth, electrolysis Liquid decomposition or failure, insufficient electrolyte, mismatch of electrolyte additives, corrosion or dissolution of the current collector, failure of a conductive agent, failure of binder, failure of diaphragm aging, blocking of diaphragm pores, segregation of pole pieces, material agglomeration, abnormal cell design, abnormal cell aging process, etc.
Figure 3 shows the internal failure of a lithium battery. The research content on the failure reasons of lithium batteries, it can be divided into external causes and internal causes. Among them, external factors include impact, acupuncture, corrosion, high-temperature combustion, man-made damage, and other external factors; while internal factors mainly refer to the nature of physical and chemical changes in failure, the research scale can be traced back to the atomic and molecular scale, and the thermodynamics and dynamics of the failure process are studied Changes in science. The failure of lithium batteries comes down to the failure of materials. The failure of materials mainly refers to the abnormal occurrence of material structure, properties, and morphology and the mismatch between materials. For example, the particle crushing of the positive electrode material due to the uneven stress of the material due to the inconsistent rate of local Li+ DE intercalation, the crushing and pulverization of the silicon negative electrode material due to the volume expansion and contraction during the charging and discharging process, the electrolyte is subjected to humidity and temperature. It affects the decomposition or deterioration, the solvent co-intercalation problem of the graphite anode and the additive propylene carbonate (PC) in the electrolyte, and the lithium precipitation caused by N/P (the ratio of the capacity of the negative electrode to the capacity of the positive electrode) that is too small.
The failure causes of lithium batteries do not always correspond to failures one-to-one, and there are “one-to-many”, “many-to-one” and “many-to-many” relationships. A certain failure cause may have different manifestations in the time span. For example, the abnormal charging and discharging system leads to high current charging and discharging, which may show a large polarization at the beginning, and an internal short circuit due to the precipitation of lithium dendrites in the intermediate stage. With the decomposition and regeneration of Li dendrites, thermal runaway may eventually occur. A variety of completely different failures may occur for a certain failure cause, such as the precipitation of local transition metals, which may generate gas, resulting in a bulging failure performance, but it may also cause local heating due to internal short circuits, which will cause the diaphragm to shrink and cause a large area. Of thermal runaway. A certain failure phenomenon may correspond to a variety of failure reasons, such as the failure mechanism of capacity fading, such as material structure change, microstructure damage, contact failure between materials, electrolyte failure or decomposition, conductive additive failure, and so on.
Figure 3- Internal failure of lithium battery
Failure analysis is divided into two directions: one is the diagnosis analysis based on the failure of lithium batteries, which starts from the failure and traces the failure mechanism of the battery material to achieve the purpose of analyzing the failure cause; the other mechanism exploration and analysis is based on the failure point of the design material and explores various factors affecting the failure process of lithium batteries, in order to achieve the purpose of prevention. The diagnosis and analysis of the lithium battery start from the failure of the lithium battery. According to the failure performance of the battery, the battery appearance inspection, the battery non-destructive inspection, the battery damage inspection, and the comprehensive analysis are carried out. In the face of actual cases, it is necessary to adjust and optimize the analysis process and test items according to different situations. Taking the failure analysis of capacity-fading batteries as an example (as shown in Figure 4), the failure behavior is refined based on the failure performance and service conditions, and the corresponding analysis focus is provided. Such as normal cycle decay, the later analysis focuses on material structure changes, SEI overgrowth, lithium precipitation, and other factors. Through the visual inspection of the failed battery, it is determined whether there are external structural changes or leakage of electrolytes and other factors. Non-destructive testing mainly includes micron X-ray cross-sectional scanning (XCT) and full-cell electrochemical testing. Through the conclusion of the non-destructive testing analysis, further, confirm the internal structure changes, quantify the failure behavior, select test items, and adjust the analysis process. For example, comparing the charge-discharge curve analysis of a certain LiFe PO 4 /C failed battery and a fresh battery in Figure 5 shows that the discharge capacity has decreased by 21%, and further processing the charge-discharge curve to obtain a capacity increment (IC) curve, according to the overall peak position of the curve. The shift to high potential indicates that there is an increase in the difficulty of lithium DE intercalation due to the change in material structure. Combined with the more obvious peak intensity changes at 3.27 V and 3.32 V, it shows that the capacity decline of the battery is mainly due to the loss of active lithium source and the destruction of active material structure. And further corroborates the focus of the analysis. The so-called battery destructive testing refers to determining the role of positive and negative electrodes, active materials, and separators in battery failure through battery disassembly, electrode observation, and material testing and analysis. The testing and analysis of materials are based on physical and chemical properties and electrical For example, the scanning electron microscope (SEM) morphology test results of the above-mentioned LiFe PO 4 /C failed battery pole piece show that the cathode material has obvious structural damage. The X-ray diffraction (XRD) structure spectrum shows 18.5 and the increase in 31 peak intensity reveals that Fe the increase of x (PO y) phase means that there is a phase transition phenomenon in the cathode material (as shown in Figure 6). The X-ray photoelectron spectroscopy (XPS) analysis of the pole piece surface and the half-cell test of the pole piece can qualitatively and quantitatively analyze the SEI and capacity loss of the pole piece surface. Finally, the qualitative or quantitative failure reasons are concluded and an analysis report is provided. The research on the failure mechanism of lithium batteries is to accurately simulate and analyze the complex physical and chemical reaction processes inside the battery through a large number of basic scientific research, as well as the construction of reasonable models and verification experiments. The battery mechanism analysis may be carried out from different angles, including the design material angle and the design failure angle.
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The failure analysis process of a certain battery capacity decay
charge-discharge curve and (b) IC curve of the corresponding discharge curve of a certain LiFePO4/C failed battery and a fresh battery.
SEM image of a certain LiFePO4/C failed battery and fresh battery; (b) XRD pattern
Taking the material system as the starting point, different variables are designed to study the failure mechanism of the battery or material respectively (as shown in Figure 7). Among them, the mechanism analysis work based on the material system is often carried out in the form of basic scientific research, and this kind of work is mostly in scientific research institutions. The purpose of the experiment should be clarified, such as “comparative research on the capacity fading mechanism of high-rate charge-discharge of a certain material system at room temperature”, “research on the effect of a certain electrolyte additive on the high-temperature cycle performance of the battery”, etc. Design the experimental process, and by preparing the battery, simulate the battery use environment or use conditions to achieve the purpose of expected failure. The reverse analysis of the failed battery was carried out, and the failure mechanism of the battery was analyzed in combination with the material system.
Schematic diagram of the research flow of the failure mechanism of lithium batteries
In addition to the design of the failure analysis process, the main steps of lithium battery failure analysis also include failure information collection, failure mechanism research, testing and analysis methods, and so on. Collect failure information of lithium batteries, including direct failure phenomenon, use environment, use conditions, etc. Although the content of failure analysis mainly includes clarifying analysis objects, collecting failure information, determining failure modes, studying failure mechanisms, determining failure causes, and proposing preventive measures. However, failure analysis should not be limited to the purpose of finding out the essential causes of product failure, and should It leads to thinking about technical management methods, standardized specifications, and deep-level mechanisms of failure phenomena, as well as integrating new thinking such as big data and simulation. The ultimate goal of failure analysis is to determine the exact failure mode Clarify the failure mechanism, accumulate the failure analysis database, and complete the complete data chain of “failure phenomenon-failure mode-failure cause-improvement measures-simulation experiment” and the full life of “original materials-preparation process-use environment- gradient utilization and dismantling and recycling” Periodic failure studies. At this stage, a “Lithium Battery Failure Database” is being constructed. In the future, the failure analysis of lithium batteries will be electronic and intelligent. By collecting failure phenomena and combining them with the “lithium battery failure database”, a preliminary prediction of the failure mechanism and a reasonable and efficient test and analysis process are given. In this process, it is still necessary to solve there are many difficulties, such as: optimizing the failure analysis process, providing test analysis technology, overcoming the difficulties of test technology, standardizing test analysis methods, etc.
Difficulties in failure analysis
The failure cause and failure of lithium batteries is not a simple “one-to-one “model, but also multi-dimensional relationships such as “one-to-many”, “many-to-one”, and “many-to-many”. In addition, the reasons for the failure of lithium batteries are divided into internal and external factors, which can be from changes in the structure and physicochemical properties of the constituent materials themselves, or from complex factors such as design and manufacture, use environment, and time span. Therefore, the failure causes of lithium batteries and the structure-activity relationship between failures are very complex (as shown in Figure 8). For example, the structural change or destruction of positive and negative electrode materials will cause problems such as capacity attenuation, rate performance decline, and internal resistance increase; diaphragm aging and puncture are important factors for short circuits in the battery; battery design, electrode coating The processes of cloth, rolling, winding, etc. are directly related to the performance of battery capacity and rate performance; the high-temperature environment will lead to the decomposition and deterioration of battery electrolyte, as well as capacity attenuation, internal resistance increase, gas production, and other problems. Therefore, it is not correct to describe and analyze the failure with a single failure cause, and it is necessary to quantitatively analyze the influence weight and primary-secondary relationship of multiple failures causes at a certain stage, in order to accurately evaluate the failed battery and target it to propose reasonable measures.
The relationship between the use conditions, failure causes, and failure phenomena of lithium batteries.
The lithium battery itself is a gray box (gray system) in modern cybernetics, that is, it does not fully understand its internal physical and chemical change mechanism and thermodynamic and kinetic processes. As we all know, lithium batteries are mainly composed of positive electrode materials, negative electrode materials, separators, electrolytes, solvents, conductive agents, binders, current collectors, tabs, etc. The battery preparation process includes three parts: the front, middle, and final stages, including beating, coating, drying, rolling, slitting, sheeting, die-cutting or winding, shell insertion, tab welding, liquid injection, sealing welding, chemical composition, and other steps. Figure 9 shows the common preparation process of lithium batteries. The factors that affect the performance of the battery in each production process are described in. However, each key material does not exist independently, and each preparation step does not exist independently. They are interrelated and affect each other, and will change greatly due to changes in the application field. Figure 10 shows the relationship between the properties and performance of battery materials. At present, the common cathode materials for lithium batteries are LiCoO 2, LiFePO 4, LiMn2O4, Li2MnO3-LiMO2, LiNix Co y Al 1−x−y O2, LiNix Co y Mn 1−x−y O2, LiNi 0.5 Mn 1.5 O4, etc. Common anode materials for lithium batteries include natural graphite, artificial graphite, miso carbon microspheres MCMB, Li4Ti 5O 12, soft carbon, hard carbon, silicon anode, SiOx -C anode, metal lithium, composite metal lithium, etc. According to different use environments and requirements, different positive and negative electrode systems are selected, and together with appropriate electrolyte systems and other auxiliary materials, under the appropriate preparation process, various forms of lithium batteries that meet the needs of users are made. Qualified lithium batteries will be used in all walks of life, especially in electric vehicles, ships, aerospace, and other fields. The process from material preparation to product use is full of variability and complexity. Therefore, the failure analysis of lithium batteries should not be limited to the failure of key battery materials, but also to the material structure, synthesis and processing, performance design, manufacturing process, and service conditions. , failure performance, etc. shall be comprehensively considered.
Influencing factors of common preparation process design of lithium batteries
Sample transfer cassettes for common testing and analysis equipment
In addition to the above difficulties, there are also some technical difficulties, including the need to use sample collection/screening technology, sample transfer technology, and reasonable and accurate characterization and analysis technology to analyze the failure of lithium battery materials. Before collecting and screening samples, it is very important to dismantle cells of different specifications reasonably and effectively. At present, it is mostly manual or semi-automatic disassembly, and there are hidden dangers such as short circuits and damage to key materials during the disassembly process. There are still some difficulties in the collection of gas production and electrolyte in the battery, especially in the process of gas production collection, it is easy to introduce impurity gas, and the remaining electrolyte is too small, which makes it difficult to collect and test. Most lithium battery materials are sensitive to air, especially Moisture and oxygen in the air. This also puts forward.
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The failure analysis team of the Institute of Physics, Chinese Academy of Sciences has been engaged in related research work for many years. The fully automatic battery dismantling instrument developed with great effort is currently in the trial stage. It has developed a gas collection device for different types of batteries and developed an atmosphere protective shell for conventional testing equipment. Or sample transfer box (shown in Figure 11) to achieve inert atmosphere protection during sample transfer and testing. Figure 12 shows the conventional characterization and analysis techniques for various failures in lithium batteries, explaining electrode surface coating, particle surface coating, material pore blockage, material contact failure, particle breakage, and transition metal dissolution from the perspectives of electrodes and materials. Characterization techniques for failures such as migration. However, there are still deficiencies in the more microscopic atomic level material failure characterization and 3D imaging characterization. Therefore, some in-situ experimental techniques, synchrotron radiation techniques, neutron diffraction techniques, reconstruction imaging techniques, Nano CT, spherical aberration electron microscopy, etc. have also been introduced into the failure analysis of lithium batteries, revealing a deeper failure mechanism. However, failure analysis does not use high-end characterization analysis methods as a gimmick, but after a rigorous and complete logical analysis of failure problems, formulate appropriate analysis procedures and use necessary characterization analysis methods.
Characterization analysis technology of common battery internal failure points
Normative test analysis methods
Different analysis groups use the same test analysis technology, the experimental results will be different to some extent, and even the same analysis group in the later repeated experiments, the experimental results obtained will be different. The ultimate purpose of failure analysis is to propose key solutions, and the difference in experimental results will make the solutions vary by a thousand miles. These problems are not limited to failure analysis of lithium batteries but widely exist in failure analysis in other fields such as mechanical engineering, automotive engineering, and aerospace engineering. Therefore, the standardized analysis process has become an inevitable trend. In addition to conventional material physicochemical analysis techniques, the standardization of material preprocessing, transfer environment, and data analysis is necessary to accurately analyze materials and identify failure mechanisms. For example, the pretreatment of the test sample will affect the accuracy of the test results. The atmosphere protection of the sample, the collection environment of the electrolyte/gas, and the separation of the electrode material mixture are all closely related to the test results and analysis conclusions. At this stage, there are certain differences in the material systems, battery types, preparation methods, and processes of different manufacturers, and their electrochemical properties, physicochemical properties, and safety properties are directly affected, which brings more variables and inconsistencies to failure analysis. Certainty. The current lithium-ion battery test standards are mostly aimed at the safety and electrical performance testing of products such as battery cells or battery packs, such as IEC 61960, JIS-C-8711 mainly focus on the electrical performance testing of lithium-ion batteries; IEC 62133, UL2054, UL1642 and JISC- 8714 and other standards mainly focus on the test standards for the safety performance of battery products. Most of the current domestic testing and analysis standards are based on materials and involve the determination of material properties and content, as listed in Table 1. In addition, GB/T 31467 “Lithium-ion power battery packs and systems for electric vehicles” for battery packs and battery packs, and GB/T 18287 “General specification for lithium-ion batteries and battery packs for mobile phones” formulated for single cells Contains some security testing and performance testing items.
Research progress and development direction
Lithium batteries appeared in the 1970s, and their development was seriously lagging behind because their safety performance was not guaranteed. Until 1991, the successful commercialization of lithium-ion batteries stimulated interest in lithium battery energy storage technology in countries around the world. Its use performance and safety performance have always been the focus of people’s attention. In the process of its industrialization development, people are constantly aware of the research and development optimization of lithium battery failure analysis in the early stage of the product, the cell manufacturing and large-scale production in the mid-term, the prediction and evaluation of battery performance and safety failure in the later stage, and even in the later stage. Arbitration failure accidents and other aspects have important practical significance. At this stage, there are not many professional institutions engaged in the failure analysis of lithium batteries, especially the institutions specializing in the diagnosis and analysis of lithium batteries are even less. Among them, the Argonne National Laboratory in the United States established an electrochemical analysis and diagnosis department as early as the end of the 20th century to develop new advanced battery and energy storage technologies, as well as all-around battery research and diagnosis. Japan Toray started the research and development of lithium battery analysis and characterization technology as early as 1979 and then focused on characteristic analysis and degradation analysis technology as the main development direction. Now it can provide standardized commercial services for battery failure analysis. In addition, there are Brookhaven National Laboratory, Japan Toyo (Toyo), Sony (Sony), Yuasa (Yuasa), Panasonic (Panasonic), South Korea’s Samsung SDI (Samsung SDI), LG CChem (LG-CChem), and other institutions also doing similar work. Compared with the long-term accumulation and development abroad, domestic development in the field of failure analysis is still in the exploratory stage. At this stage, some excellent research teams have emerged. Among them, the lithium-ion battery failure analysis team of the Institute of Physics, Chinese Academy of Sciences was the first to systematically the research work on the failure analysis of lithium-ion batteries has been carried out. With the support of the Chinese Academy of Sciences pilot A-type project, relying on the Li-ion battery research accumulated by the Institute of Physics for many years, a comprehensive analysis platform for interconnected inert atmosphere batteries has been built.
Figure 13 shows the Chinese Academy of Sciences. CAFES, a comprehensive analysis platform for inert atmosphere interconnected by the Institute of Physics, provides diagnostic analysis services for outstanding battery companies at home and abroad and cooperates with scientific research institutions and material companies to carry out research on the failure mechanism of commercial batteries and advanced batteries, as well as failure analysis methods. Establish and improve the failure fault tree and failure analysis process of lithium-ion batteries, and improve the overall failure analysis system of lithium-ion batteries.
Figure 13. Interconnected Inert Atmosphere Comprehensive Analysis Platform of Institute of Physics, Chinese Academy of Sciences
Figure 14 (a) Application of in-situ transmission X-ray imaging technology, (b) in-situ high-frequency
X-ray tomography combined with thermal imaging technology and (c) in-situ transmission electron microscopy technology in lithium battery testing and analysis of the mechanism analysis of lithium batteries is mainly carried out in universities and research institutes. From the perspective of basic science, it analyzes and studies the failure problems of lithium batteries, and has rich experience in testing and analysis technology. A large number of advanced testing and characterization technologies are applied to lithium batteries. The test analysis, such as neutron diffraction, Nano-CT, spherical aberration electron microscope, and in-situ detection technology, provides support for a more accurate analysis of the failure mechanism at the material level. As shown in Figure 14, Duet al. used in-situ transmission X-ray imaging technology to deeply study the relationship between the morphology and structure failure of LiCoO2 materials in pouch batteries and the distribution of chemical elements and the related failure mechanism; Finnegan et al. Using an in-situ high-frequency X-ray tomography scanner combined with thermal imaging technology, the changes in the internal structure and thermodynamics of two commercial batteries during thermal runaway caused by different conditions were visually studied “in situ”. The key factors of generation and dissipation provide technical support; Gong et al., Institute of Physics, Chinese Academy of Sciences, based on spherical aberration transmission electron microscopy, developed an in-situ technology to observe and analyze the lithium-DE intercalation process of battery materials in real-time from the Nano-level, which is very important for batteries. The research on the failure mechanism of materials provides an important technical guarantee. Battery companies and material companies carry out research on failure analysis of lithium-ion batteries, but they mostly focus on the research and development of battery manufacturing processes and materials, with the aim of improving battery performance and reducing battery costs. In order to achieve the goal, a large number of positive verification experiments are used, and rich experience and methods have been accumulated, but there are still deficiencies in experience and technology in reverse analysis and accurate analysis. From the perspective of efficiency and benefit, relevant enterprises are more hopeful On the basis of the existing conventional testing technology, a failure analysis method with high efficiency, accuracy, and universality is developed, which puts forward higher requirements for the design and testing analysis process.
This Article discusses the definition, failure performance, failure reasons, analysis content, and analysis process of lithium battery failure analysis. Brief description. In the future, failure analysis may be carried out from the following aspects: First, the research work on the basic problems of batteries, which is the basis of failure analysis, requires the use of advanced characterization and analysis techniques to explore the structure, properties and reaction laws of materials and cells. ; The second is to standardize, standardize and modularize the test and analysis technology of batteries with different systems and different failure performances, and establish an efficient, accurate, and universal failure analysis process on this basis. This part is the only way to systematize failure analysis. ; Thirdly, make full use of computer simulation technology to simulate and analyze the multi-factors and multi-links that affect the performance of lithium batteries, so as to shorten the database accumulation period and consider the interaction between multiple factors; finally, the failure analysis methods and ideas are summarized and modularized It can maintain good portability for different systems, such as sodium-ion batteries, all-solid-state batteries, lithium-sulfur batteries, air batteries, etc.
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