Safety hidden danger and safety design of lithium-ion battery

Lithium-ion batteries-safety hazards:

The safety of lithium-ion batteries is not only related to the properties of the pool material, but also to the technology of battery preparation and use. The frequent explosion of mobile phone batteries is due to the failure of the protection circuit, but more importantly, there is no fundamental solution to the problem in terms of materials.

Lithium cobalt acid positive cathode active material is a very mature system in terms of small cores, but after being fully charged, there are still a large number of lithium ions left at the positive pole. When overcharged, lithium ions remaining at the positive pole will flock to the negative pole. The formation of dendrites on the negative electrode is an inevitable result of overcharging of batteries using lithium cobalt acid materials. Even during the normal charging and discharging process, there may be excess lithium ions free to the negative electrode to form dendrites. The theoretical than energy of lithium cobalt acid material is more than 270 mA per gram, but in order to ensure its recycling performance, the actual capacity of use is only half of the theoretical capacity. During use, due to some reason(such as damage to the management system), the battery charging voltage is too high, and the remaining part of the lithium in the positive electrode will be removed and deposited as metal lithium via the electrolyte to the negative electrode surface to form dendrites. The dendritic perforates the diaphragm, forming an internal short circuit.

The main component of the electrolyte is carbonate. The flash point is very low and the boiling point is also low. It will burn or even explode under certain conditions. If the battery is overheated, it will cause the carbonate in the electrolyte to be oxidized and reduced, resulting in a large amount of gas and more heat. For example, if the safety valve is lacking or the gas is too late to be released through the safety valve, the internal pressure of the battery will rise sharply and cause an explosion.

Polymer electrolyte lithium-ion batteries do not fundamentally solve the safety problem. Lithium cobalt acid and organic electrolytes are also used, and the electrolytes are colloidal and are not prone to leakage. More violent combustion will occur. Combustion is the safety of polymer batteries. The biggest problem.

There are also some problems in use. An external short circuit or an internal short circuit of the battery will generate hundreds of amps of excessive current. When the battery is short-circuited, the battery instantly discharges a large amount of current, which consumes a lot of energy and produces a lot of heat. The internal short circuit forms a large current, and the temperature rises, causing the diaphragm to melt and the short circuit area to expand, thus forming a vicious circle.

In order to achieve a high operating voltage of 3 to 4.2 V of a single core, a lithium-ion battery must adopt an organic electrolyte with a decomposition voltage greater than 2 V, and an organic electrolyte will be electrolyzed under high current and high temperature conditions to electrolyze to produce gas. Causes the internal pressure to increase and will seriously break through the shell.

Overcharge may precipitate metallic lithium. In the case of rupture of the shell, direct contact with the air leads to combustion, while igniting the electrolyte, a strong flame occurs, and the gas expands rapidly and explodes.

In addition, for mobile phone lithium-ion batteries, due to improper use, such as extrusion, impact and water intake, resulting in battery expansion, deformation and cracking, etc., these will lead to short circuit batteries, during the discharge or charging process heat explosion.

Lithium-ion battery-safety design:

Lithium-ion batteries have a triple protection mechanism in a single lithium ion battery in order to avoid overdischarge or overcharging of the battery due to improper use. One is the use of switching components. When the temperature in the battery rises, its resistance rises. When the temperature is too high, the power supply will automatically stop. The second is to select an appropriate partition material. When the temperature rises to a certain value, the micron micro-holes on the partition will automatically dissolve, so that lithium ions can not pass and the internal reaction of the battery will stop; The third is to set up a safety valve(that is, the air release hole at the top of the battery). When the internal pressure of the battery rises to a certain value, the safety valve automatically opens to ensure the safety of the battery.

Sometimes, although the battery itself has safety control measures, due to certain reasons, the control failure, the lack of a safety valve, or the gas can not be released through the safety valve, the internal pressure of the battery will rise sharply and cause an explosion.

Under normal circumstances, the total energy stored in lithium-ion batteries is inversely proportional to its safety. With the increase in battery capacity, the battery volume is also increasing, and its heat dissipation performance is deteriorating, and the possibility of an accident will increase significantly. For lithium ion batteries for mobile phones, the basic requirement is that the probability of a safety accident is less than one millionth, which is the lowest standard that the public can accept. For large-capacity lithium-ion batteries, especially large capacity lithium-ion batteries such as automobiles, it is particularly important to use forced heat dissipation.

Choosing a safer electrode material and selecting lithium manganate material ensure that the lithium ion of the positive electrode is completely embedded in the carbon hole of the negative electrode in the molecular structure, which fundamentally avoids the generation of dendrites. At the same time, the stable structure of lithium manganate makes its oxidation performance far lower than that of lithium cobaltate. The decomposition temperature exceeds 100 °C of lithium cobaltate. Even if internal short circuit (acupuncture) due to external force, external short circuit, overcharge, it is fully capable. The danger of burning and explosion due to precipitation of metallic lithium is avoided.

In addition, the use of lithium manganese acid materials can also significantly reduce costs.

To improve the performance of the existing safety control technology, it is necessary to improve the safety performance of the lithium ion battery core, which is particularly important for large-capacity batteries. Select a diaphragm with good thermal closure performance. The diaphragm's role is to separate the positive and negative poles of the battery while allowing the passage of lithium ions. When the temperature rises, it is closed before the diaphragm melts, so that the internal resistance rises to 2000oumu and the internal reaction stops.

When the internal pressure or temperature reaches the preset standard, the blast Valve will open and begin to discharge to prevent the accumulation of too much internal gas and deformation, which will eventually cause the shell to burst.

Improved control sensitivity, more sensitive selection of control parameters and joint control with multiple parameters (this is particularly important for large-capacity batteries). For large-capacity lithium ion batteries, which are composed of series/parallel multiple cores, for example, the voltage of a notebook computer is more than 10V, and the capacity is large. Generally, 3 to 4 single cells can be used in series to meet the voltage requirements. Then two to three series batteries are connected in parallel to ensure a larger capacity.

The large-capacity battery itself must be equipped with a relatively complete protection function. Two circuit substrate modules should also be considered: the Protection Framework PCB module and the SmartBatteryGaugeBoard module. The battery protection design of the entire set includes: Level 1 protection IC (to prevent battery overcharge, overdischarge, short circuit), Level 2 protection IC(to prevent second overpressure), Fuse, LED indication, temperature adjustment and other components.

Under the multi-level protection mechanism, even in the case of abnormal power chargers and laptops, the laptop battery can only be switched to the automatic protection state. If the situation is not serious, it will still work normally after re-plugging. An explosion will occur.

At present, the underlying technology used for lithium ion batteries used in laptops and mobile phones is unsafe and needs to consider safer structures.

In short, the future of lithium-ion batteries will become safer with the Advancement of material technology and the increasing understanding of the requirements for the design, manufacture, testing and use of lithium-ion batteries.

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