3.7V Lithium Polymer Battery

Introduction to 3.7V Lithium Polymer Battery safety, testing and solutions

In recent years, accidents caused by battery safety issues are everywhere, and the consequences of many problems are shocking, such as the 3.7V Lithium Polymer Battery fire incident of the Boeing 787 "Dreamliner" that shocked the industry, and the large-scale battery fire and explosion incident of Samsung Galaxy Note 7, which once again sounded the alarm for the safety of lithium-ion batteries.

1. Composition and working principle of lithium-ion batteries

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

The positive electrode potential of lithium-ion batteries is relatively high, and is often a lithium-embedded transition metal oxide, or a polyanion compound, such as lithium cobalt oxide, lithium manganese oxide, ternary, lithium iron phosphate, etc.; the negative electrode material of lithium-ion batteries is usually a carbon material, such as graphite and non-graphitized carbon, etc.; the electrolyte of lithium-ion batteries is mainly a non-aqueous solution, composed of an organic mixed solvent and a lithium salt, in which 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.; the lithium-ion battery separator is mostly a polyethylene or polypropylene microporous membrane, which serves to isolate the positive and negative electrode materials, prevent electrons from passing through and causing a short circuit, and 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, transmitted by the electrolyte through the separator, 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 released 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. As the battery is charged and discharged, it is the "lithium ions" that migrate between the batteries, not the single substance "lithium", so the battery is called a "lithium-ion battery".

2. Safety hazards of lithium-ion batteries

Generally speaking, safety problems of lithium-ion batteries manifest as combustion or even explosion. The root cause of these problems is thermal runaway inside the battery. In addition, some external factors, such as overcharging, fire, extrusion, puncture, short circuit, etc., can also cause safety problems. Lithium-ion batteries 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 undergo destructive side reactions such as SEI film decomposition, electrolyte decomposition, positive electrode decomposition, reaction between the negative electrode and the electrolyte, and reaction between the negative electrode and the adhesive.

1. Safety hazards of positive electrode materials

When lithium-ion batteries are used improperly, the internal temperature of the battery will rise, causing the positive electrode material to decompose the active material and oxidize the electrolyte. At the same time, these two reactions can generate a lot of heat, which will cause the battery temperature to rise further. Different delithiation states have very different effects on the lattice transformation of the active material, the decomposition temperature, and the 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 was prone to lithium dendrites after multiple charge and discharge, which then punctured the diaphragm, causing battery short circuit, leakage and even explosion. Lithium-intercalated compounds can effectively avoid the generation of lithium dendrites and greatly improve the safety of lithium-ion batteries. As the temperature rises, the carbon negative electrode in the lithium-intercalated state first reacts with the electrolyte to release heat. Under the same charge and discharge conditions, the heat release rate of the electrolyte reaction with lithium-intercalated artificial graphite is much greater than the heat release rate of the reaction with lithium-intercalated intermediate carbon microspheres, carbon fibers, coke, etc.

3. Safety hazards of diaphragms and electrolytes

The electrolyte of lithium-ion batteries is a mixed solution of lithium salts and organic solvents. 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 a carbonate ester. This type of solvent has a low boiling point and flash point. It is easy to react with lithium salts 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 affect the safety of the battery. For example, the quality control of the positive and negative electrode mixing, coating, rolling, cutting or punching, assembly, electrolyte filling, sealing, formation and other processes will 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, which affects the safety of the battery. If the slurry fineness is too large, the expansion and contraction of the negative electrode material will change greatly during the battery charging and discharging, and the precipitation of metallic lithium may occur; 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 may be carbonized, and the active material may fall off, causing an internal short circuit in the battery.

5. Safety hazards during battery use

Lithium-ion batteries should be used to minimize overcharging or over-discharging, especially for batteries with high single-cell capacity, because thermal disturbances may trigger a series of exothermic side reactions, leading to safety problems.

III. Lithium-ion battery safety test indicators

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

1. Extrusion test: Place a fully charged battery on a plane, apply an extrusion force of 13±1KN by a hydraulic cylinder, and squeeze the battery with a steel rod with a diameter of 32mm. Once the extrusion pressure reaches the maximum, stop squeezing. The battery does not catch fire or explode.

2. Impact test: After the battery is fully charged, place it on a plane, place a steel column with a diameter of 15.8mm vertically in the center of the battery, and let a weight of 9.1kg fall freely from a height of 610mm onto the steel column above the battery. The battery does not 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 it approaches a certain time, the battery voltage rises rapidly. When it rises to a certain limit, the battery cap is pulled off, and the voltage drops to 0V. The battery does not catch fire or explode.

4. Short circuit test: After the battery is fully charged, the positive and negative poles of the battery are short-circuited with a wire with a resistance of no more than 50mΩ to test the surface temperature change of the battery. The highest surface temperature of the battery is 140℃. The battery cap is pulled open, and the battery does not catch fire or explode.

5. Needle puncture test: Place the fully charged battery on a plane and pierce the battery radially with a steel needle with a diameter of 3mm. Test that the battery does not catch fire or explode.

6. Temperature cycle test: The temperature cycle test of lithium-ion batteries is used to simulate the safety of lithium-ion batteries repeatedly exposed to low and high temperature environments during transportation or storage. The test is carried out using rapid and extreme temperature changes. After the test, the sample should not catch fire, explode or leak.

IV. Lithium-ion battery safety solution

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

1. Improving 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, improving the safety of the electrolyte is one of the more effective methods. The 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 the additives, they can be mainly divided into the following types: safety protection additives, film-forming additives, positive electrode protection additives, stable lithium salt additives, lithium precipitation-promoting additives, current collector anticorrosive additives, and wettability enhancement additives.

In order to improve the performance of commercial lithium salts, researchers have carried out atomic substitution and obtained many derivatives. Among them, the compounds obtained by replacing atoms with perfluoroalkyl have many advantages such as high flash point, similar conductivity, and enhanced water resistance. It is a class of lithium salt compounds with great application prospects. In addition, the anionic lithium salt obtained by chelating the boron atom with the oxygen ligand as the central atom has high thermal stability.

For 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 class of highly safe electrolytes, but compared with the commonly used carbonate electrolytes, the viscosity of ionic liquids is several orders of magnitude higher, and the conductivity and ion self-diffusion coefficient are lower, so there is still a lot of work to be done before practical application.

2. Improving the safety of electrode materials

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

For negative electrode materials, since their surface is often the part that is most susceptible to thermochemical decomposition and heat release in lithium-ion batteries, improving the thermal stability of the SEI film is a key method to improve the safety of negative electrode materials. The thermal stability of negative electrode materials can be improved by weak oxidation, metal and metal oxide deposition, polymer or carbon coating.

3. Improve the safety protection design of batteries

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

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