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Operating temperature range of lithium-ion batteries


Lithium-ion batteries have the advantages of high voltage, high capacity, high energy density and long life, so they are used in smart phone batteries, electric vehicle batteries, household batteries and so on. However, in recent years, the number of fire accidents caused by lithium-ion batteries such as smartphones has increased rapidly, and the safety (danger) of lithium-ion batteries has been recognized, and improving this safety is the subject of popularization of lithium-ion batteries. As the Internet of Things advances, the importance of lithium-ion batteries will increase, so increasing your knowledge of lithium-ion batteries will help you live a more comfortable life.

In particular, it is well known that the performance of lithium-ion batteries tends to deteriorate under conditions that significantly deviate from normal use temperatures (such as high or low temperatures), and they can even operate under harsh conditions (such as high or low temperatures). The basic thing is that design can be done. The temperature at which a lithium-ion battery can be used is called the operating temperature range or simply the temperature range.

(1) What is the upper limit of the operating temperature range of lithium-ion batteries?

The upper limit of the operating temperature range of lithium-ion batteries is mainly determined at high temperatures where the capacity does not deteriorate, the internal resistance does not increase, and the battery does not explode and other dangerous temperatures. As described on this page about the high temperature characteristics of lithium-ion batteries, under normal circumstances, the chemical reaction of the negative electrode of lithium-ion batteries (SEI film growth) is the main cause of capacity degradation, internal resistance increase, etc.

Since this deterioration is a chemical reaction, it follows a formula called Arrhenius's equation, which is characterized by a faster rate of deterioration at high temperatures. In other words, once deterioration occurs in a high temperature environment, it will appear in a form of deterioration that cannot be refreshed (resurrected). Therefore, in order to extend the life of lithium-ion batteries, be sure not to expose them to high temperatures.

In addition, when the temperature exceeds the operating temperature range, the diaphragm, which is an integral part of the lithium-ion battery, begins to shrink, resulting in a short circuit caused by direct contact between the positive and negative electrodes. When a short circuit occurs, an exothermic reaction occurs, leading to further contraction of the diaphragm and the risk of further short circuits. This causes the temperature to rise rapidly, causing various exothermic reactions such as the reaction between the electrolyte and the negative electrode, the decomposition reaction of the electrolyte, and eventually leading to the risk of thermal runaway leading to rupture and fire. Although it depends on the positive electrode material, negative electrode material, electrolyte, etc., which constitutes the lithium-ion battery, the upper limit of the operating temperature range is often around 60 ° C due to the above reaction.

(2) What is the lower limit of the operating temperature range of lithium-ion batteries?

On the other hand, the lower limit of the operating temperature range of the lithium-ion battery is set to ensure that the output will not drop too much at low temperatures and prevent electrical deposition during rapid charging at low temperatures. As explained by the relationship between external temperature, capacity, and internal resistance, the internal resistance of all batteries, including lithium-ion batteries, increases at low temperatures. (In contrast, the ohmic resistance of a metal increases with decreasing temperature.)

If the internal resistance of the battery increases, the output decreases, so if the battery is installed in the product, there is a risk that the product will not operate. In addition, when the lower limit of the operating temperature range is specified, the operating temperature range should be set to prevent this from happening, because rapid charging at low temperatures can lead to the phenomenon of negative electrodeposition, which significantly reduces the battery capacity and output has been set lower limits. Due to these factors, the lower limit of the temperature range for lithium-ion batteries is usually around -20 to 0°C, although this depends on the product.

(3) How to expand the temperature range of lithium-ion batteries

So, can we expand the temperature range (operating temperature range) of lithium-ion batteries by doing this? On the high temperature side, the main cause of degradation is the formation of sediment (SEI) due to the reaction between the negative electrode material and the electrolyte on the negative surface.

In particular, by adding additives such as VC (vinylidene carbonate) and FEC to the electrolyte composition, and optimizing the ratio of other substances such as ethyl carbonate (EC) and diethyl carbonate (DEC), the performance of lithium-ion batteries can be improved. In some cases, degradation at high temperatures is greatly reduced. Therefore, electrolyte optimization is very important for extending the temperature range at high temperatures. (Of course, if the type, composition or electrode structure of the positive and negative materials is not suitable, there are also cases where the material deteriorates first, but in most cases, the main cause is the electrolyte.)

In addition, it is best to use lithium titanate, because lithium titanate is a material that is not easy to cause electrodeposition at low temperatures, otherwise even the use of graphite negative electrode will reduce the available capacity of the negative electrode. One way to lower the lower end of your operating temperature range is to mix a small amount of a substance with a very low melting point (below freezing), such as PC (Propylene carbonate).

However, if the PC component is too high, the SEI will not form well with the graphite negative and the battery performance will deteriorate significantly, so carefully consider the material composition before using this method. Ultimately, materials such as electrolytes, anode materials, and negative materials and their combinations need to be optimized, and various evaluations and considerations are made.

(4) What are the temperature characteristics of lithium-ion batteries?

Previously explained the temperature range of lithium-ion batteries, there is a similar term to the temperature range called temperature characteristics. The temperature characteristic of a lithium-ion battery is a vague term, similar to the term temperature range, which basically refers to "a wide operating temperature range" or "the temperature (high or low) when used in the end product." It is used as an indicator to show whether the required values for yield, capacity, etc., are being met. It can also be used as a general term to describe high temperature characteristics and low temperature characteristics. In addition to the operating temperature range, it is also necessary to understand the term temperature characteristics.

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