In recent years, accidents caused by battery safety issues are common, and the consequences of many problems are shocking, such as the lithium battery fire incident of the Boeing 787 "Dream" airliner that shocked the industry, and the large-scale battery fire and explosion incident of Samsung mobile phones, which has given rise to The safety issues of lithium-ion batteries have once again sounded the alarm. In the field of energy storage, the safety of energy storage batteries is also an important indicator. This article will introduce issues related to lithium battery safety and testing.

　　1. Composition and working principle of lithium-ion battery

　　Lithium-ion batteries are mainly composed of positive electrodes, negative electrodes, electrolytes, separators, and external connections and packaging components. Among them, the positive and negative electrodes include active electrode materials, conductive agents, binders, etc., and are evenly coated on copper foil and aluminum foil current collectors.

　　The positive electrode of lithium-ion batteries has a high potential and is often a lithium-embedded transition metal oxide, or a polyanion compound, such as lithium cobalt oxide, lithium manganate, ternary, lithium iron phosphate, etc.; the negative electrode material of lithium-ion batteries is usually carbon material , such as graphite and non-graphitized carbon; the lithium-ion battery electrolyte is mainly a non-aqueous solution, consisting of an organic mixed solvent and a lithium salt. The solvent is mostly an organic solvent such as carbonic acid, and the lithium salt is mostly a monovalent polyanion lithium salt, such as Lithium hexafluorophosphate, etc.; lithium-ion battery separators are mostly polyethylene and polypropylene microporous membranes, which serve to isolate positive and negative electrode materials, prevent electrons from passing through and causing short circuits, and at the same time allow ions in the electrolyte to pass through.

　　During the charging process, inside the battery, lithium is released from the positive electrode in the form of ions, transported through the separator by the electrolyte, and embedded in the negative electrode; outside the battery, electrons migrate from the external circuit to the negative electrode. During the discharge process: lithium ions inside the battery escape from the negative electrode, pass through the separator, and are embedded in the positive electrode; outside the battery, electrons migrate from the external circuit to the positive electrode. With charging and discharging, "lithium ions" migrate between batteries instead of elemental "lithium", so the battery is called a "lithium-ion battery."

　　2. Safety hazards of lithium-ion batteries

　　Generally speaking, safety problems in lithium-ion batteries occur in the form of combustion or even explosion. The root cause of these problems lies in thermal runaway inside the battery. In addition, some external factors, such as overcharging, fire source, extrusion, puncture, and short circuit Issues such as this can also lead to security issues. Lithium-ion batteries will generate heat during charging and discharging. If the heat generated exceeds the heat dissipation capacity of the battery, the lithium-ion battery will overheat, and the battery material will decompose the SEI film, the electrolyte, the positive electrode, and the negative electrode. Destructive side reactions such as the reaction of the electrolyte and the reaction of the negative electrode with the binder.

　　1. Safety hazards of cathode materials

　　When a lithium-ion battery is used improperly, the internal temperature of the battery will rise, causing the positive electrode material to decompose the active material and the electrolyte to oxidize. At the same time, these two reactions can generate a large amount of heat, causing the battery temperature to further rise. Different delithiation states have very different effects on the lattice transformation of the active material, decomposition temperature and thermal stability of the battery.

　　2. Safety hazards of negative electrode materials

　　The negative electrode material used in the early days was metallic lithium. The assembled battery is prone to produce lithium dendrites after repeated charging and discharging, which can pierce the separator, causing the battery to short circuit, leak, or even explode. Lithium-embedded compounds can effectively avoid the generation of lithium dendrites and greatly improve the safety of lithium-ion batteries. As the temperature increases, the carbon negative electrode in the lithium-embedded state first undergoes an exothermic reaction with the electrolyte. Under the same charge and discharge conditions, the heat release rate of the reaction between the electrolyte and lithium-embedded artificial graphite is much greater than the heat release rate of the reaction with lithium-embedded mesophase carbon microspheres, carbon fibers, coke, etc.

　　3. Potential safety hazards of diaphragm and electrolyte

　　The electrolyte of lithium-ion batteries is a mixed solution of lithium salt and organic solvent. The commercial lithium salt is lithium hexafluorophosphate. This material is prone to thermal decomposition at high temperatures and undergoes thermochemical reactions with trace amounts of water and organic solvents, reducing the Thermal stability of the electrolyte. The organic solvent of the electrolyte is carbonate. This type of solvent has a low boiling point and flash point. It easily reacts with lithium salt to release PF5 at high temperatures and is easily oxidized.

　　4. Safety hazards in the manufacturing process

　　During the manufacturing process of lithium-ion batteries, processes such as electrode manufacturing and battery assembly will have an impact on the safety of the battery. The quality control of various processes such as mixing the positive and negative electrodes, coating, rolling, cutting or punching, assembly, adding the amount of electrolyte, sealing, formation, etc., all affect the performance and safety of the battery. The uniformity of the slurry determines the uniformity of the distribution of active materials on the electrode, thereby affecting the safety of the battery. If the slurry fineness is too small, the negative electrode material will undergo large expansion and contraction changes during charging and discharging, and metal lithium may precipitate; if the slurry fineness is too small, the internal resistance of the battery will be too large. If the coating heating temperature is too low or the drying time is insufficient, the solvent will remain and the binder will partially dissolve, causing some active materials to be easily peeled off; if the temperature is too high, the binder will be carbonized and the active material will fall off, causing an internal short circuit in the battery.

　　5. Potential safety hazards during battery use

　　During use of lithium-ion batteries, overcharging or over-discharging should be minimized. Especially for batteries with high single capacity, thermal disturbance may cause a series of exothermic side reactions, leading to safety issues.

　　3. Lithium-ion battery safety testing indicators

　　After lithium-ion batteries are produced, they need to undergo a series of tests before reaching consumers to ensure the safety of the batteries and reduce safety hazards.

　　1. Extrusion test:

　　Place the fully charged battery on a flat surface, apply a squeezing force of 13±1KN from the hydraulic cylinder, and squeeze the battery flatly with a steel rod with a diameter of 32mm. Once the squeezing pressure reaches the maximum, stop squeezing and the battery will not catch fire. , just don’t explode.

　　2. Impact test:

　　After the battery is fully charged, place it on a flat surface, place a 15.8mm diameter steel column vertically in the center of the battery, and freely drop a 9.1kg weight from a height of 610mm onto the steel column above the battery. Just make sure the battery doesn't catch fire or explode.

　　3. Overcharge test:

　　Fully charge the battery with 1C, and perform an overcharge test according to 3C overcharge 10V. When the battery is overcharged, the voltage rises to a certain voltage and stabilizes for a period of time. When approaching a certain time, the battery voltage rises rapidly. When it rises to a certain limit, the battery high The cap is pulled off, the voltage drops to 0V, and the battery does not catch fire or explode.

　　4. Short circuit test:

　　After the battery is fully charged, short-circuit the positive and negative poles of the battery with a wire with a resistance of no more than 50mΩ, and test the surface temperature change of the battery. The maximum surface temperature of the battery is 140°C. If the battery cap is opened, the battery will not catch fire or explode.

　　5. Acupuncture test:

　　Place the fully charged battery on a flat surface and use a 3mm diameter steel needle to pierce the battery in the radial direction. Just make sure the battery doesn't catch fire or explode.

　　6. Temperature cycle test:

　　The lithium-ion battery temperature cycle test is used to simulate the safety of lithium-ion batteries when they are repeatedly exposed to low and high temperature environments during transportation or storage. The test is conducted using rapid and extreme temperature changes. After the test, the sample should not catch fire, explode, or leak.

　　4. Lithium-ion battery safety solutions

　　In view of the many safety hazards in the materials, manufacturing and use of lithium-ion batteries, how to improve the parts that are prone to safety problems is a problem that lithium-ion battery manufacturers need to solve.

　　1. Improve the safety of electrolyte

　　There is a high reactivity between the electrolyte and the positive and negative electrodes, especially at high temperatures. In order to improve the safety of the battery, improving the safety of the electrolyte is one of the more effective methods. The potential safety hazards of the electrolyte can be effectively solved by adding functional additives, using new lithium salts, and using new solvents.

　　According to the different functions of additives, they can be mainly divided into the following categories: safety protection additives, film-forming additives, cathode protection additives, stabilizing lithium salt additives, lithium precipitation-promoting additives, current collector anti-corrosion additives, enhanced wettability additives, etc.

　　In order to improve the performance of commercial lithium salts, researchers have substituted atoms and obtained many derivatives. Among them, the compounds obtained by substituting atoms with perfluoroalkyl groups have many advantages such as high flash point, similar conductivity, and enhanced water resistance. , is a type of lithium salt compound with great application prospects. In addition, the anionic lithium salt obtained by chelating the boron atom with an oxygen ligand has high thermal stability.

　　Regarding solvents, many researchers have proposed a series of new organic solvents, such as carboxylic acid esters and organic ether organic solvents. In addition, ionic liquids also have a type of electrolyte with high safety. However, compared with the commonly used carbonate electrolytes, the viscosity of ionic liquids is orders of magnitude higher, and the conductivity and ion self-diffusion coefficient are low. There is still a lot of work to be done before practical use. To do.

　　2. Improve the safety of electrode materials

　　Lithium iron phosphate and ternary composite materials are considered to be low-cost, "excellent safety" cathode materials and may be widely used in the electric vehicle industry. For positive electrode materials, a common method to improve their safety is coating modification. For example, surface coating of the positive electrode material with metal oxide can prevent direct contact between the positive electrode material and the electrolyte, inhibit the phase change of the positive electrode material, and improve the safety of the positive electrode material. Its structural stability reduces the disorder of cations in the crystal lattice to reduce the heat generated by side reactions.

　　As for the anode material, since its surface is often the part in the lithium-ion battery that is most prone to thermochemical decomposition and heat release, improving the thermal stability of the SEI film is a key method to improve the safety of the anode material. The thermal stability of negative electrode materials can be improved through weak oxidation, metal and metal oxide deposition, polymer or carbon coating.

　　3. Improve battery safety protection design

　　In addition to improving the safety of battery materials, commercial lithium-ion batteries adopt many safety protection measures, such as setting up battery safety valves, hot-melt fuses, connecting components with positive temperature coefficients in series, using thermally sealed diaphragms, loading special protection circuits, and special battery management systems, etc., are also means to enhance security.

　　5. Lithium-ion battery testing service provider

　　In recent years, the lithium-ion battery performance and safety testing industry has become one of the fastest growing industries in the world, with annual growth of around 20%. my country's testing industry has reached a scale of close to 100 billion yuan, with an average annual growth rate of about 25%. Currently, there are nearly 700 laboratories accredited by CNAS and CMA on the Jiayu Testing Network.

　　6. Lithium-ion battery safety solution provider

　　As the safety issues of lithium-ion batteries have attracted more and more attention, many companies have conducted research and development specifically to address the safety hazards in lithium-ion batteries and proposed effective battery safety solutions.

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