Lithium ion batteries are the latest and fastest commercialized high-performance batteries. Lithium ion batteries are currently widely used as new energy sources in various fields due to their unique advantages. Lithium ion batteries have the characteristics of high voltage, high specific energy, and good cycling performance, and are increasingly being used in the 3 C market, electric vehicle (E V) and hybrid electric vehicle (HEV) market, automotive applications, and space technology. Although the safety of lithium-ion secondary batteries has been greatly improved compared to metal lithium secondary batteries, there are still many hidden dangers, such as high specific energy of batteries and mostly organic flammable electrolytes. When the heat generation rate of batteries exceeds the heat dissipation rate, safety issues may occur.

According to research, lithium-ion batteries may generate high temperatures (>700 ℃) that melt the aluminum collector under abusive conditions, leading to smoking, ignition, explosion, and even personal injury in the battery. Therefore, for the development and production of lithium-ion batteries, the safety of batteries not only refers to the absence of smoke, fire, explosion and other phenomena under various testing conditions, but also the most important thing is to ensure that personnel are not harmed under battery abuse conditions. This article discusses various factors that affect the safety of lithium-ion power batteries from the aspects of lithium-ion battery design, materials, manufacturing, and usage conditions, and proposes specific measures to solve safety problems.

1 The impact of battery design on safety

The safety of lithium-ion batteries is determined by their own characteristics:

(1) The energy density of the battery is very high, and if there is a thermal runaway reaction, releasing a high amount of heat can easily lead to unsafe behavior

(2) Lithium ion batteries, due to the use of organic electrolyte systems and the organic solvents being hydrocarbons, are prone to oxidation at around 4.6V, and the solvents are flammable. If leakage occurs, it can cause the battery to catch fire, even burn and explode

(3) The overcharge reaction of lithium-ion batteries can cause a change in the structure of the positive electrode material, resulting in strong oxidation of the material, causing strong oxidation of the solvent in the electrolyte. This effect is irreversible, and if the heat generated by the reaction accumulates, there is a risk of thermal runaway.

1.1 Principle of timeliness

Lithium ion power batteries have a large capacity, and the risk increases exponentially with the increase of capacity. Therefore, it is necessary to consider the compatibility of active substances in the later stage of battery design. As the cycle progresses, the battery capacity gradually decreases and the internal resistance increases, resulting in significant structural changes in the positive electrode compared to the negative electrode; At the same time, the SEI film on the negative electrode surface thickens, and at the end of the cycle, lithium and lithium compounds deposit. It is these changes that cause the conventional performance of the battery to deteriorate and the appearance to change as the cycle progresses.

As the cycle progresses, the extraction and insertion of lithium will cause a volume change in the particles, resulting in lattice stress and deteriorating safety. Often, new batteries can pass safety tests, but batteries in the middle and later stages of use may not necessarily pass safety tests again, because the active substances such as positive and negative electrodes do not match during use, and metal lithium will precipitate in the later stages of use. Metal lithium is exceptionally active and easily reacts with many inorganic and organic substances. Therefore, in electrochemical cycling, the uneven surface of lithium can easily cause uneven deposition of metal lithium, stroke lithium dendrites, and cause safety issues. To obtain reliable and safe lithium-ion power batteries, the design must consider timeliness, especially the safety of the battery in the later stages of use

1.2 Reliability principle

The usage environment of batteries varies greatly, and different batteries have different usage requirements. Even the same battery usage environment can be vastly different. What is more important is how to ensure the safety of batteries under misuse or abuse conditions. The heat resistance and abuse resistance of lithium-ion batteries with long-term cycling deteriorate. To avoid excessive heat generated by physical or chemical reactions of the constituent substances due to specific energy inputs in the battery during abuse, targeted design should be adopted for batteries with different structures.

For cylindrical batteries, PTC is often used as an over-current protection component. Due to the presence of a current limiting device (PTC) placed between the positive terminal and the electrode coil inside the battery, when the electrolyte decomposes and the battery temperature rapidly rises during overcharging, the device begins to operate and cut off the current.

For square aluminum shell batteries, there is no current limiting device inside, and due to the softness and easy deformation of aluminum, safety can only be ensured by external devices of the battery; The lithium-ion battery made of aluminum-plastic packaging film, although there is no current limiting device inside the battery, the careful design combined with external safety devices makes the battery safer, especially for the use of cellular phones. This structure has been widely adopted by polymer battery manufacturers.

For lithium-ion batteries with cylindrical and square steel shell structures, the top vent valve structure with safety design activates the safety mechanism when a large amount of gas is generated inside the battery. In addition to this function, it can also reduce the temperature of the battery to eliminate thermal runaway. For aluminum-plastic packaging film batteries, due to the soft aluminum-plastic film on the outer packaging, there is no protective device inside the battery, so the design requirements for the battery are strict. However, compared with cylindrical steel shell batteries, when misuse and abuse occur, causing the gas generated by chemical reactions to gradually increase, the packaging film will bulge or the aluminum film welding sealing position will bulge and release pressure, thereby ensuring the safety of the battery.