Lithium battery safety, detection and solution popularization

With the popularity of mobile phones, digital products and electric vehicles, lithium-ion batteries play an increasingly important role in people's lives. The use of low energy density, limited cycle life and other problems are often criticized, but compared with these issues, battery safety issues are the focus of attention.

In recent years, accidents caused by battery safety problems are everywhere, and the consequences of many problems are shocking, such as the shocking Boeing 787 "Dream" passenger aircraft lithium battery fire incident, and the large-scale battery fire explosion of SamsungGalaxyNote7, giving lithium ions The battery safety issue is again ringing the alarm.

First, the composition and working principle of lithium ion battery

The lithium ion battery is mainly composed of a positive electrode, a negative electrode, an electrolyte, a separator, and an external connection and packaging member. Among them, the positive electrode and the negative electrode include an active electrode material, a conductive agent, a binder, and the like, and are uniformly applied to a copper foil and an aluminum foil current collector.

The lithium ion battery has a high positive electrode potential, often a lithium intercalation transition metal oxide, or a polyanionic compound such as lithium cobaltate, lithium manganate, ternary, lithium iron phosphate, etc.; the lithium ion battery anode material is usually a carbon material. Such as graphite and non-graphitizable carbon; lithium ion battery electrolyte is mainly non-aqueous solution, composed of organic mixed solvent and lithium salt, wherein the solvent is mostly organic solvent such as carbonic acid, and the lithium salt is mostly monovalent polyanion lithium salt, such as Lithium hexafluorophosphate and the like; lithium ion battery separators are mostly polyethylene and polypropylene microporous membranes, which function to isolate positive and negative materials, prevent electrons from passing through short circuits, and allow ions in the electrolyte to pass.

During the charging process, inside the battery, lithium is extracted 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 are taken out from the negative electrode, passed through the separator, and embedded in the positive electrode; outside the battery, electrons migrate from the external circuit to the positive electrode. With charging and discharging, it is "lithium ion" that migrates between the batteries, rather than the simple substance "lithium", so the battery is called "lithium ion battery".

Second, the safety hazard of lithium-ion batteries

In general, lithium-ion batteries have safety problems that appear to be burning or even exploding. The root cause of these problems lies in the thermal runaway inside the battery. In addition, some external factors such as overcharge, fire, extrusion, puncture, and short circuit. Other issues can also lead to security issues. Lithium-ion batteries will generate heat during charging and discharging. If the heat generated exceeds the dissipation capacity of the battery, the lithium ion battery will overheat and the battery material will have destructive side reactions such as SEI film decomposition, electrolyte decomposition and positive electrode decomposition, negative electrode reaction with electrolyte and negative electrode reaction with adhesive.

1. Safety hazards of cathode materials

When the lithium ion battery is used improperly, the internal temperature of the battery is increased, so that the decomposition of the active material and the oxidation of the electrolyte occur in the positive electrode material. At the same time, both reactions can generate a large amount of heat, causing a further increase in battery temperature. Different delithiation states have a great influence on the lattice transition of the active material, the decomposition temperature and the thermal stability of the battery.

2. The safety hazard of the anode material

The anode material used in the early stage is metallic lithium. The assembled battery is prone to lithium dendrite after repeated charge and discharge, and then pierces the diaphragm, causing short circuit, liquid leakage and even explosion of the battery. The lithium intercalation compound can effectively avoid the generation of lithium dendrites and greatly improve the safety of the lithium ion battery. As the temperature increases, the carbon negative electrode in the lithium intercalation state first undergoes an exothermic reaction with the electrolyte. Under the same charge and discharge conditions, the heat release rate of the electrolyte reacted with lithium-incorporated artificial graphite is much higher than that of the intercalated carbon microspheres, carbon fibers, coke, etc. with lithium intercalation.

3. The safety hazard of the diaphragm and electrolyte

The electrolyte of the lithium ion battery is a mixed solution of a lithium salt and an organic solvent, wherein the commercial lithium salt is lithium hexafluorophosphate, the material is susceptible to thermal decomposition at a high temperature, and is subjected to a thermo chemical reaction with a trace amount of water and an organic solvent to reduce Thermal stability of the electrolyte. The organic solvent of the electrolyte is a carbonate. These solvents have a low boiling point and a low flash point, and are easily reacted with a lithium salt to release PF5 at a high temperature, and are easily oxidized.

4. Safety hazards in the manufacturing process

Lithium-ion batteries in the manufacturing process, electrode manufacturing, battery assembly and other processes will have an impact on the safety of the battery. Such as the positive and negative mixture, coating, rolling, cutting or punching, assembly, filling of the amount of electrolyte, sealing, chemical and other processes of quality control, all affect the performance and safety of the battery. The uniformity of the slurry determines the uniformity of the distribution of the active material on the electrode, thereby affecting the safety of the battery. The fineness of the slurry is too large. When the battery is charged and discharged, there will be a large change in the expansion and contraction of the negative electrode material and precipitation of metallic lithium may occur; if the fineness of the slurry is too small, the internal resistance of the battery may 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 be partially dissolved, causing some active materials to be easily peeled off; if the temperature is too high, the binder may be charred, and the active material may fall off and cause internal short circuit of the battery.

5. The safety hazard during the use of the battery

Lithium-ion batteries should be used to minimize over-charging or over-discharging during use. Especially for batteries with high monomer capacity, thermal disturbances may cause a series of exothermic side reactions, leading to safety problems.

Third, lithium ion battery safety testing indicators

After the lithium-ion battery is produced, it needs to carry out a series of tests before reaching the consumer to ensure the safety of the battery and reduce the safety hazard.

1. Extrusion test: Place the fully charged battery on a flat surface, apply a 13±1KN pressing force by a hydraulic cylinder, and squeeze the battery from a 32mm diameter steel rod. Once the extrusion pressure reaches the maximum stop squeeze, the battery does not ignite, it does not explode.

2. Impact test: After the battery is fully charged, it is placed on a flat surface. The steel column with a diameter of 15.8 mm is placed vertically in the center of the battery, and the weight of 9.1 kg is freely dropped from the height of 610 mm onto the steel column above the battery. The battery can't be fired or exploded.

3. Overcharge test: the battery is fully charged with 1C, according to 3C overcharge 10V overcharge test, when the battery overcharges the voltage rises to a certain voltage for a certain period of time, the battery voltage rises rapidly when it reaches a certain time, when it rises to When the limit is high, the battery high cap is broken, the voltage drops to 0V, and the battery does not catch fire or explode.

4. Short-circuit test: After fully charging the battery, short-circuit the positive and negative terminals of the battery with a wire with a resistance of not more than 50mΩ, and test the surface temperature of the battery. The maximum temperature of the battery surface is 140°C. The battery cap is opened and the battery does not ignite or explode.

5. Acupuncture test: Place the fully charged battery on a flat surface and pierce the battery radially with a 3 mm diameter steel needle. The test battery can't be fired or exploded.

6. Temperature cycling test: Lithium-ion battery temperature cycling test is used to simulate the safety of lithium-ion batteries during repeated transportation to low-temperature and high-temperature environments during lithium ion battery transportation. The test is to use rapid and extreme temperature. The change is going on. After the test, the sample should not ignite, explode, or leak.

Fourth, lithium ion battery safety solutions

In view of the many safety hazards in the material, 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 the 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, it is one of the more effective methods to improve the safety of the electrolyte. The safety hazard 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, it can be divided into the following types: safety protection additives, film-forming additives, protection of positive electrode additives, stable lithium salt additives, lithium-precipitating additives, current-carrying anti-corrosion additives, and enhanced infiltration additives.

In order to improve the performance of commercial lithium salts, the researchers have atomically substituted them and obtained many derivatives. Among them, compounds obtained by using perfluoroalkyl substituted atoms have many advantages such as high flash point, approximate conductivity, and enhanced water resistance. , is a kind of promising lithium salt compound. Further, an anionic lithium salt obtained by sequestering an oxygen ligand with a boron atom as a central atom has high thermal stability.

For solvents, many researchers have proposed a series of new organic solvents, such as carboxylates and organic ethers. In addition, ionic liquids also have a kind of high-safety electrolyte, but the relatively common use of carbonate-based electrolytes, the viscosity of ionic liquids is orders of magnitude higher, the conductivity and ion self-diffusion coefficient are lower, and there is still much work away from practical use.

2. Improve the safety of electrode materials

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

For the negative electrode material, since the surface is often the most prone to thermal chemical decomposition and exotherm in the lithium ion battery, improving the thermal stability of the SEI film is a key method to improve the safety of the negative electrode material. The thermal stability of the negative electrode material can be improved by weak oxidation, metal and metal oxide deposition, polymer or carbon coating.

3. Improve the safety protection design of the battery

In addition to improving the safety of battery materials, many safety protection measures for commercial lithium-ion batteries, such as setting battery safety valves, hot-melt fuses, connecting components with positive temperature coefficient in series, using heat-sealed diaphragms, loading special protection circuits and dedicated battery management Systems, etc., are also  means of enhancing security.

The page contains the contents of the machine translation.