Development of AAA NiMH batteries capacitors: high voltage, large capacity, high safety

　　FDK has developed AAA NiMH batteries capacitors with high output power and excellent charge and discharge cycle characteristics. It is now being used in fields such as high-voltage sag compensation devices and load averaging for solar power generation. In addition, its application in automotive fields that require high output, such as hybrid vehicles, is also progressing. In this article, FDK introduces the characteristics of AAA NiMH batteries capacitors and its initiatives for hybrid vehicles, etc.

　　In recent years, various countermeasures have been taken to cope with the depletion of fossil fuels and prevent global warming. In response to the problem of fossil fuels, natural energy sources such as solar power generation and wind power generation have been actively introduced. In order to prevent global warming, we have begun to implement emission reduction measures such as electrification and motor-assisted driving for vehicles with high CO2 emissions.

　　However, these countermeasures have led to the emergence of new issues such as unstable power systems and increased power consumption. To solve these problems, power storage components are indispensable.

　　Previously, the development of power storage components has been centered on AAA NiMH batteries rechargeable batteries (LIB). However, due to different uses, LIB's output characteristics and charge-discharge cycle life (hereinafter referred to as life) have limits. We have developed "EneCapTen", a high-output and long-life lithium ion capacitor (LIC) for applications that are difficult to support with LIB. This article will introduce LIC’s application solutions for the hybrid vehicle market, a market expected to grow in the future.

　　High voltage and large capacity LIC

　　LIC is a capacitor that uses activated carbon as the positive electrode, carbon material as the negative electrode, and lithium ion organic matter as the electrolyte (salt: LipF6, solvent: pCEC). The positive electrode stores electricity through the effect of the electric double layer. The negative electrode, like LIB, stores electricity through the redox reaction of lithium ions.

　　By adding lithium ions, LIC not only increases the voltage to about 4V, but also increases the electrostatic capacity stored in the negative electrode. The overall electrostatic capacity of the unit can be increased to about twice that of the original electric double layer capacitor (EDLC). Therefore, LIC has the advantages of high voltage and large capacity compared with EDLC (Table 1).

　　For example, the energy density per unit volume is 10-50Wh/L, which is much larger than the capacity of EDLC of 2-8Wh/L.

　　Although the energy density is lower than LIB, LIC has high output density and long life. In addition, it has two major characteristics: excellent high-temperature characteristics and smaller self-discharge than EDLC.

　　Different positive electrode, higher safety

　　At present, there are three main requirements for electricity storage: ① safety, ② long life, and ③ low price. Among them, the safety of ① is the most important factor. Electric storage components are used to store energy. If they cannot be stored stably, the components will become very dangerous as the energy density increases.

　　Currently, in order to improve safety, various measures are taken for LIB, such as coating the diaphragm with insulation, but essentially, the safety of the electricity storage principle itself is the most ideal.

　　The difference between LIB and LIC lies in the positive electrode. The positive electrode of LIB uses lithium oxide, while LIC uses activated carbon. Lithium oxide not only contains a large amount of lithium, but also contains oxygen, an important factor that can cause fire.

　　Therefore, if a short circuit occurs inside the unit for some reason, the heat caused by the short circuit will decompose the lithium oxide and can further develop into thermal decomposition of the entire unit, resulting in severe heat generation.

　　The positive electrode of LIC uses activated carbon. Although it will react with the negative electrode when an internal short circuit occurs, the positive electrode and the electrolyte will not react after that, and it can be said to be safe in principle (Figure 1).

　　Even if an internal short circuit occurs in LIC, the positive electrode and electrolyte will not react. The positive electrode of LIB will react with the electrolyte, causing the constituent materials to thermally decompose, resulting in severe heating.

　　Excellent high temperature durability

　　Regarding ② long life, due to the relatively high price of power storage components, the longer they are used, the more they can reduce product life cycle costs. Moreover, if it has a long service life, it can also reduce the frequency of replacement, reduce waste, etc., and have a smaller load on the environment.

　　In order to reduce degradation and achieve long life, LIB narrows the charge and discharge range (charge and discharge depth), but this actually reduces the usable capacity. It was originally hoped that expanding the charging and discharging depth would also achieve long life.

　　The charging and discharging principle of EDLC is simply to adsorb or remove ions in the electrolyte to achieve long life. However, it is difficult to extend the life under actual use conditions based on this alone.

　　The disadvantage of power storage components is that the temperature rises. When repeatedly charging and discharging, the internal resistance will cause the temperature to rise, which will greatly affect its lifespan. Therefore, high temperature durability is a necessary condition.

　　Deterioration caused by high temperature is mainly caused by oxidative decomposition of the positive electrolyte. The higher the potential of the positive electrode or the higher the ambient temperature, the easier it is for oxidation and decomposition to occur. Therefore, when used in places with high ambient temperatures, it is necessary to reduce the potential of the positive electrode. However, if the positive electrode potential of EDLC is lowered, the cell voltage will also decrease, so the capacity cannot be ensured.

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