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What improvements have been made to the safety performance of lithium-ion batteries in the past decade? Aug 15, 2018

Lithium-ion battery is a rechargeable battery. Lithium-ion batteries usually use an embedded lithium compound as the electrode material, and mainly rely on the movement of lithium ions between the positive and negative electrodes - lithium-ion batteries have specific energy compared to conventional aqueous secondary batteries. High battery life, long cycle life and environmentally friendly advantages are a promising battery system that has been widely used in notebook computers. Lithium-ion batteries are the most commonly used in portable electronic devices. One of the sub-battery types, but also in the military field, pure electric vehicles and aerospace.

However, the characteristics of high specific energy not only bring advantages to lithium-ion batteries, but also bring high-risk risks. Because of the non-aqueous electrolyte system, lithium-ion batteries are prone to irreversible oxidative decomposition. Or chemical reactions with highly active cathode materials, these are very likely to cause the battery to explode and burn, causing dangerous accidents - thus, safety issues have always been an important issue in lithium-ion battery applications.

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Factors affecting the safety of lithium-ion batteries

In general, in addition to meeting the requirements of capacity, internal resistance and high and low temperature charge and discharge performance, lithium ion batteries also need to pass various safety performance tests. However, due to manufacturing technology and operation methods, it still has certain risks - the cause of lithium-ion battery explosion, often overcharge, short circuit, thermal shock and mechanical shock.

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Impact of acupuncture, short circuit and extrusion on battery safety

When the battery is externally short-circuited or pinched and squeezed, it often causes the internal diaphragm of the battery to rupture, causing a large amount of current to flow from the inside of the battery in a very short time, and directly causing a sharp rise in the internal temperature of the battery. High, which triggers a series of violent reactions in a short period of time - for batteries with poor safety, the process can be completed in a matter of seconds and end in a "blow". Due to the high specific energy of lithium-ion batteries, mobile phone batteries like 2200mAh can only reach about 30 KJ. Once a short circuit occurs, the aluminum explosion-proof valve at the battery seal will be melted, resulting in user property damage.

The impact of therm

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al shock on battery safety

In addition, when the battery is subjected to thermal shock, if the temperature of the thermal shock is high, the risk of "thermal runaway" is also caused. This condition usually causes the SEI film on the negative electrode surface of the battery to decompose, and causes the lithium-plated negative electrode material to react exotherm with the electrolyte. If the heat dissipation rate of the battery is slow, it is easy to cause the temperature inside the battery to rise further. , melting the diaphragm, causing a short circuit - and eventually causing a dangerous accident such as an explosion.

 Overcharge affects battery safety

In daily life, the battery explosion that we may be exposed to is caused by over-charging of the battery. When the battery is overcharged, the electrical energy generated by the internal chemical reaction of the battery is easily converted into heat energy, which causes a rapid rise in the temperature of the battery. As with the above two cases, if the battery has better heat dissipation capacity when overcharged, it is not easy to cause danger. The battery has poor heat dissipation capability, and high rate charging is often the culprit of such a situation. In this case, the transition metal oxide Due to the significant de-lithiation and chemical activity increase of the positive electrode material, it often leads to a more serious safety problem than the simple thermal shock.

Safety measures that manufacturers will take

In general, for manufacturers, safety issues are naturally best at the design stage. Therefore, for the above factors that may cause battery explosion, there are naturally corresponding, design-level Eliminating the method, after that, through the safe monitoring of qualified batteries, will only enter the market - because lithium-ion batteries have more safety hazards, the battery must be designed to take some special measures to ensure safety, currently More structural design uses a heat-sealable diaphragm, a series of positive temperature coefficient resistors, and an explosion-proof cap.

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Heat-closed diaphragm

When the battery causes a large current to flow through the battery due to needle sticking or squeezing, the porous diaphragm inside the battery will soften rapidly. Because the battery core is tight, the compressed diaphragm will be damaged. The porous structures adhere to each other to form an almost completely closed structure, thereby stopping the transport of ions and terminating the chemical reaction, thereby achieving the purpose of rapidly cutting off the current - the battery with safe performance is often blocked at this point. Further rises and makes the battery no longer dangerous.

Products that are not up to standard may be at this stage because the diaphragm does not form a good closed structure at the temperature rise, or shrinkage deformation caused by uneven force, which causes the internal of the battery to be dangerous due to a short circuit.

Positive temperature coefficient resistor (PTC)

In addition, the insurance measures designed by the principle of increasing the resistance of the resistors are also very effective. When the battery is overcharged or externally short-circuited, a large amount of current will heat up the positive temperature coefficient of the series connected to the battery, resulting in The resistance heats up, raises the resistance, and thus reduces the current flowing through the battery to ensure the safety of the battery - its design idea is similar to the fuse concept in the circuit, and it is also more reliable.


Safety valve

In addition to the above two conventional solutions, there is also a security measure that is open to the brain and is also used by today's manufacturers. After the battery is overcharged, the vaporization of the electrolyte and the decomposition of the electrolyte to generate gas after heating may cause a sudden increase in the internal pressure of the battery, resulting in the explosion of the battery - the explosion-proof valve or the explosion-proof membrane is designed for this purpose, they will Timely rupture, release the internal pressure of the battery, thus protecting the user's property and personal safety (in addition, the explosion-proof valve in the positive electrode cap of the cylindrical battery may also cut off the connection with the electrode piece while preventing deformation, thereby preventing the battery from being dangerous).

Chemical methods to improve battery safety

In addition, chemical methods such as polymerizable additives, voltage-sensitive separators, redox couples, and flame retardant additives are also the safety measures often used by manufacturers.

Of course, in the past decade, due to the improvement of the thermal stability of battery materials, the understanding of battery safety performance and new safety-enhancing devices and measures have been continuously developed, and the safety performance of lithium-ion batteries has indeed been greatly improved. Improvements - however, these methods are often exchanged for sacrificing capacity - so improving lithium-ion battery manufacturing processes and optimizing lithium-ion battery safety design will remain the main efforts of future lithium-ion battery manufacturers. direction.

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