Toray announced on November 25th, 2016 that it will increase the output density of lithium-ion rechargeable batteries by approximately 1 times and increase the energy density by 20% when multi-layer graphene manufactured using proprietary technology is used as a conductive additive for positive electrodes. . The company is considering mass production of this multi-layered graphene in 2019, "the goal is to reach billions of yen in sales around 2020, and several billion yen in a few years" (Toray). According to the company’s introduction, it is also planned to promote in the rapidly expanding China market for pure electric vehicles. The technical details will be announced at the 57th Battery Symposium held in Chiba Prefecture from November 29 to December 1, 2016.
In general, in the lithium ion rechargeable battery, the active material used for the electrode does not have high conductivity. In this case, the electrical conductivity between the current collector and the active material, which conducts current to the outside of the battery, is often ensured by adding a conductive aid. Originally, conductive additives often used cheap carbon materials - carbon black carbonized from acetylene.
This time, Toray used about 10 layers of multilayer graphene as part or all of the conductive additives to verify the characteristics of the battery. As a result, it was found that both the capacity density and the output density of the battery were improved. In particular, even if the output density is high, the discharge capacity does not easily decrease.
Specifically, the discharge capacity at a time of 12 minutes of rapid discharge of a battery using a carbon black as a conductive aid decreases to 1/2 or less at a time of 1 hour of slow discharge. In addition, when the conductive aid is made of Toray's multilayer graphene, the discharge capacity is reduced by only a factor of 10 even after discharge for 12 minutes.
This effect also changes with the ratio of carbon black to multilayer graphene. According to Toray, when the ratio of carbon black to multilayer graphene is 1:1, the output power is increased by about 1 times. When the conductive additives are all multi-layer graphene, the energy density is increased by about 20%.
Adsorption of dopamine, inhibition of agglutination
In the past, many people tried to use graphene as a conductive additive for the positive electrode. In this case, generally, graphite is first oxidized, graphene is peeled off by applying ultrasonic waves, and the like is reused. However, in many cases, the function of the battery has not been improved. The reason is that the graphene thus prepared is very agglomerated and does not function as a conductive additive.
Toray's graphite oxide and stripped multilayer graphenes adsorbed dopamine (formula C8H11NO2), a brain neurotransmitter, and then reduced it to prevent it from agglutinating in the solvent and stably disperse it.
The obtained multilayer graphene is about 10 layers of graphene with a thickness of 3 to 5 nm, and only 1/4 to 1/3 of the original reduced graphene is thin. Toray said that because of its low agglutination and thin graphene, it shows strong conductivity additives.
The reason why dopamine was selected to inhibit aggregation was "because dopamine and graphene have strong adsorptivity and excellent agglutination inhibition effect" (Toray). According to Toray, several other materials with the same structure were also discovered.
Also has the effect of prolonging the charge and discharge cycle life
Toray said that with this technology, it is also expected to have an effect of prolonging the charge and discharge cycle life of the battery. Several studies have used graphene as a conductive additive for Si-type anodes to absorb the effects of expansion and contraction of Si materials. The results of this time show that the positive electrode also has this effect. (Reporter: Nozawa Tetsuo)
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