Factors affecting the safety of power lithium-ion batteries

With the continuous updating and development of lithium-ion battery technology, the advantages of light weight, high capacity and long life have gradually been favored by consumers. Its market has been extended from mobile phones to cameras, DVDs, model airplanes, toys and other fields. In recent years, lithium-ion power batteries have received increasing attention in large-scale equipment such as electric hardware, tools, backup power supplies, voltage regulators, electric bicycles, and light electric vehicles. These devices require batteries with large capacity and high power conditions. Whether lithium-ion battery can meet the basic requirements of high safety, high power and long life is an important topic for many experts and scholars.

The safety of power lithium-ion batteries generally refers to accidents caused by improper use of power batteries, malfunctions, accidents and abuse, battery bulging, temperature pressures exceeding the standards of use, and even explosions and fires, which threaten the safety of users' lives and property. . Explosions and fires are the most serious safety incidents. The internal causes of lithium-ion battery abuse causing an explosion are complicated, but most of them are caused by excessive temperature and pressure, and most of the causes of the two can be attributed to the thermal runaway reaction of the lithium-ion battery, that is, the exothermic reaction. These exothermic reactions generally include decomposition of a battery electrode SEI film, reaction of intercalating lithium with a solvent, decomposition reaction of a positive electrode material, and the like.

The positive electrode material is the key lithium manganate

In the safety of power lithium-ion batteries, the positive electrode material in the electrode material is the key, and it is also the main cause of the safety hazard of lithium-ion battery. The search for electrode materials with high energy density, high safety, environmental protection and low price is the key to the development of power batteries. The cathode materials currently in common use are lithium cobaltate, lithium manganate, lithium nickel cobalt manganese oxide, and lithium iron phosphate. According to the data obtained at home and abroad, lithium cobalt oxide is a safety hazard for power batteries. It is generally not used as a positive electrode material for power batteries; the use of lithium manganate is common; nickel-cobalt-manganate is less used in China. Some manufacturers in Taiwan and Japan use it.

Due to the rich manganese resources (especially in China, the third highest reserves in the world), low cost, friendly to environment, and lithium manganese acid with high discharge voltage, mature technology, good safety, with other layered structure can't be matched by the anode material advantages such as high rate charge and discharge ability, thus in the promotion of lithium electricity power battery, lithium manganese acid has a big advantage. Although the specific capacity is relatively low, the large size of the power battery itself does not constitute an obvious weakness. Lithium - ion batteries with lithium manganate as anode material have good safety and thermal stability. However, lithium manganate also has the disadvantages of fast capacity attenuation (especially at high temperature) and short cycle life, which hinder the actual use process.

The advantage of lithium nickel cobalt manganese oxide material is high specific capacity and excellent cycle performance. The fabricated lithium ion power battery has a very high specific energy and is suitable for use in fields where weight and volume are strictly limited but high energy is required. Due to the use of metals such as cobalt and nickel, the price of lithium nickel cobalt manganese oxide is second only to lithium cobalt oxide. However, at present, lithium nickel cobalt manganese oxide has yet to be evaluated in the application of power batteries. Domestically applied materials are usually modified by doping to improve their safety performance, but no large-scale applications have been observed.

lithium iron phosphate has the advantages of abundant raw materials, low price, high specific capacity, excellent high temperature cycle performance and high safety performance, and is a promising power battery cathode material. The use of lithium iron phosphate as a new positive electrode material for power batteries has been heating up in recent years. Due to the limitations of patent issues, there are not many manufacturers of large-scale lithium iron phosphate batteries in Europe and North America, and the market is basically monopolized by a few manufacturers. Although many lithium ion manufacturers in China have invested in research and development of lithium iron phosphate power batteries , the quality of products varies greatly due to differences in technology and process levels.

Additives and electrolytes ensure safety

The design of the power lithium-ion battery also affects the safety of the battery.

Only through the careful calculation and control of the positive and negative ratios, the type of bonding agent, the assembly method of the diaphragm, the type of the ear, the welding method and the packaging form, can the current distribution of the pole piece be reasonable, the battery polarization is small, and the heat dissipation is good and efficient and safe. For safety reasons, power batteries are often designed to take into account whether the rate at which the battery system transfers heat to the environment is greater than the rate at which the system generates heat when anomalies occur, as this determines the probability of a thermal runaway reaction.

In order to further enhance the safety of the power battery, many manufacturers generally add functional additives to the electrolyte of the battery. These additives have anti-overcharge, anti-flatulence, low temperature and flame retardant effects. It is worth mentioning that the addition of flame retardant in the electrolyte is also an important direction to improve the safety of the power battery. The purpose of adding a flame retardant is to increase the oxygen index of the electrolyte combustion. It is well known that when the material's oxygen index exceeds 27, it is a flame retardant substance, and the currently developed flame retardant has approached or reached this index. For power battery electrolytes, the development of additives for individual functional indicators is not difficult. The difficulty lies in ensuring the safety of the electrolyte while taking into account other conventional properties. In order to balance the functions of the electrolyte, the research of multi-functional groups and composite additives is also the research and development direction.

Protection board is essential

In order to enhance the safety of power lithium-ion batteries, the protection board is a must-have device for the battery pack. In most cases, the lithium protection board should have the function of controlling the working conditions of the lithium ion battery, such as voltage, current, temperature and so on. Due to the special use of lithium-ion batteries, power lithium-ion batteries must be used together with the protection board to ensure the safety and reliability of the entire system. In the process of paired assembly and long-term use, there are inevitably individual differences in the battery packs. The protection board (or battery management system) is responsible for monitoring the voltage, current, temperature and even the capacity of the entire battery. To be accurate and correct, it can play its due role. At the same time, due to the difference in individual performance of the battery pack, the voltage and available capacity of each battery will be different under the same charge and discharge current. As the number of cycles increases, the performance difference of each battery will become more and more significant, which will make The performance of the entire battery pack is greatly reduced, and the life span is greatly shortened. In order for the performance of the battery pack to reach or approach the performance level of the single battery, it is necessary to equalize the respective batteries to minimize the deterioration of the charge and discharge performance of each monomer in the battery pack. At present, most manufacturers use a balanced charging protection board to maintain the lithium-ion battery pack.

Therefore, in the safety factor of the power lithium-ion battery, the electrode material is a crucial factor, and the material system that can guarantee the industry test standard is only lithium manganate and lithium iron phosphate. The safety performance additives and compatible electrolytes commonly used by manufacturers ensure the better use of power lithium-ion batteries. In addition, the lithium ion protection board provides various functions to protect the lithium battery, which can ensure the safe and reliable use of the power battery. The practical use of the charge equalization function ensures that the power lithium ion battery can work with high safety and long life. From the perspective of future trends, the joint development of power lithium-ion batteries, special protection boards and equipment is a scientific development path.

The page contains the contents of the machine translation.