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Which battery technology will stand out?

The new ionic liquid used in air batteries has a higher potential than all solid-state batteries is the lithium-air battery called the final battery. The positive electrode of the lithium-air battery uses oxygen in the air, so the energy density can be greatly increased. However, some opinions point out that there are problems with the reduction reaction of the air pole.



At this battery seminar, Toyota announced the adoption of the ionic liquid N, N─diethyl─N─methyl─N─Methoxyammonium bistrifluoromethylsulfonamide ( DEME-TFSA) can achieve a capacity equivalent to that of organic solvents (Figure 11) Note 6).



Figure 11: Ionic liquids similar to organic solvents



Toyota has achieved a capacity equivalent to that of organic solvents by using ether-based ionic liquid DEME-TFSA in the electrolyte solvent of lithium-air batteries.



Note 6) Toyota and Toyota Central Research Institute gave a speech on the topic of ether-based ionic liquids as electrolytes for Li-O2 batteries [Lecture Number: 2G04].



Although the mainstream organic solvents for the development of electrolyte solvents for lithium-air batteries are expected to achieve higher capacity, they have large side reactions and are volatile, so they lack stability. Toyota used N-methyl-N-propylpiperidine bistrifluoromethanesulfonamide (PP13-TFSA) ionic liquid before, and it was confirmed that charge and discharge reactions can occur in the same way as in theory, but there has always been a problem of low capacity. Compared with PP13-TFSA, the DEME-TFSA is expected to achieve about 3 times higher capacity.



Organic compounds are highly anticipated



Although there are many topics related to the research of new-generation batteries in 2030, the momentum of research and development aimed at improving the performance of current lithium-ion rechargeable batteries has not diminished in the slightest.



The current lithium ion Rechargeable battery cathode materials use lithium cobalt oxide (LiCoO2), ternary system (LiNiMnCoO2), lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4), etc. (Figure 12). However, the theoretical capacity of these cathode materials is below 200mAh/g. Therefore, the search for new materials with a capacity of 200 mAh/g or more, and the development of adding additives to positive electrode materials in order to increase the maximum performance to the theoretical capacity value are increasingly active.



Figure 12: Diversified cathode materials



At this battery seminar, there were many publications on cathode materials such as organic compounds and solid solution materials.



Maximum capacity increased to 1000mAh/g



Among the candidate technologies to realize lithium-ion rechargeable batteries above 200mAh/g through the use of new materials, organic rechargeable batteries are the most concerned. The theoretical capacity of an organic rechargeable battery using organic compounds as the positive electrode can reach nearly 1000 mAh/g. And no heavy metals are used. Therefore, it has the advantages of light weight and less resource restriction.



However, although the energy density per unit weight of organic rechargeable batteries is high, the energy density per unit volume is relatively low. Moreover, the lithium potential is mostly only 2 to 3.5V. Therefore, in order to achieve the same energy density as current lithium-ion rechargeable batteries, at least an organic compound with a capacity of 400-600mAh/g must be found.



Murata Manufacturing Co., Ltd. plans to use rubic acid in organic compounds, and strives to achieve commercialization around 2020 (the company). If a four-electron reaction occurs in red acid, a theoretical capacity of 890 mAh/g can be achieved. At this battery seminar, as a joint research result of Honda Institute of Technology and Carlit of Japan, the charge and discharge characteristics of a half-cell using rubic acid as the positive electrode material were shown (Figure 13) Note 7). The capacity at the first discharge is 750mAh/g, and after the second time it stabilizes at 650mAh/g. The specific capacity of 430mAh/g was maintained even after 100 times of repeated charging and discharging.



Figure 13: Using organic compounds to achieve high capacity



Compared with current materials, organic compounds can increase the specific capacity of the positive electrode. Murata Manufacturing Co., Ltd. has positioned rubine as an important candidate and has confirmed that it can increase the capacity density to about 650mAh/g.



Note 7) Murata Manufacturing Co., Ltd., Honda Institute of Technology, and Japan Carlit gave a speech on the topic of high-energy-density rechargeable batteries using red acid as the positive electrode active material [Lecture No.: 3E18].



Panasonic is also one of the companies dedicated to the development of organic rechargeable batteries. The company has greatly improved the subject charge-discharge cycle characteristics of organic rechargeable batteries Note 8). The result announced by Panasonic is that a polymer material (TTF polymer) with a tetrathiafulvalene (TTF) structure is used as the positive electrode active material, and the discharge capacity is still maintained at 58% after repeated charging and discharging 30,000 times. By increasing the copolymerization ratio, the structure is stable and the cycle characteristics are improved (the company).



Note 8) Panasonic gave a lecture on the electrochemical properties of polymer positive active materials with tetrathiafulvalene [Lecture No.: 3E16]



Although the discharge capacity of the trial-produced battery is only 114mAh/g, which is relatively low as an organic rechargeable battery, a person related to the battery was surprised to say that (Panasonic's) results have proved that if the dissolution of the electrolyte is suppressed, the organic rechargeable battery can also achieve excellent charging. Discharge cycle life.



In addition, Panasonic has also jointly developed with the Yoshida Laboratory of Kyoto University. After the battery seminar ended on November 19, 2012, an organic rechargeable battery that supports 30C high-speed charging and discharging was announced (Figure 14). A cyclic 1,2-diketone is used to connect two ketones to form a cyclic structure. Ketones are composed of carbon and oxygen, so there is no need to worry about resource shortages and can reduce costs. Stabilization is achieved by forming the ketone into a ring. The capacity of the trial-produced battery is 231mAh/g, and it still maintains 83% of the capacity after 500 times of charging and discharging.



Figure 14: Organic rechargeable battery capable of high-speed charging and discharging



Kyoto University and Panasonic have developed an organic rechargeable battery (a) that uses a cyclic 1,2-diketone in which two ketones are connected to form a cyclic structure as a positive electrode material. Support 30C high-speed charge and discharge (b).

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