Although extensive studies have been made, there is still a great demand for organic materials that allow for fast charging and discharging with high cyclability for the storage of electrical energy in practical use. We initiated our project on organic cathode materials for Li-ion batteries based on our experience in organic electrochemistry and organolithium chemistry.

Versatility of chemical structures is a benefit of functional materials based on organic molecules. Although there are various types of redox-active functional groups, it is preferable to choose those consisting of atoms in the second row of the periodic table because of their low atomic weights, availability, and sustainability. Thus, we focus on molecules that consist of hydrogen, carbon, nitrogen, and oxygen, and we found that the polymer (polymethacrylate) bearing pyrene-4,5,9,10-tetraone (PYT) as a redox-active core exhibited remarkable charge−discharge properties as a cathode material in a Li-ion battery.

—Nokami et al.

They designed core structures of Li-ion organic cathode materials based on density functional theory (DFT) calculations, which indicated that six-membered cyclic 1,2-diketones serve as excellent core structures because of the high redox energy change resulting from favorable coordination of the oxygen atoms to Li and the aromaticity of the reduced form.

They chose pyrene-4,5,9,10-tetraone (PYT), which contains two six-membered-ring 1,2-diketone units, a redox core structure because of its favorable capacity.

To prepare the cathodes, they mixed polymer-bound PYT (PPYT) with acetylene black and poly(vinylidene fluoride (PVDF) as a binder. These materials were mixed with (N-methyl-2-pyrrolidone) NMP as a solvent, and the thus-obtained paste was coated on aluminum sheet using a coater. Next, NMP was removed under vacuum at 85 °C for 1 h.

Hermetically sealed two-electrode cells were used for electrochemical experiments. The cathode was separated from the lithium anode by a polyethylene porous film (Celgard) imbued with an equimolar LiNTf₂/G4 salt. The three layers were pressed between two current collectors, one in contact with the cathodic material and the other in contact with a lithium disk.

Among their findings:

• The cell was successively discharged or charged at increasing rates (1 C, 3 C, 5 C, 10 C, 20 C, and 30 C). Even at 30 C, which corresponds to a time of 2 min to fully discharge, the capacity was about 90% of that at the 1 C rate, implying that the battery is suitable for high-power applications.

• The physical flexibility and affinity to Li ions of methacrylate polymer backbone seem to be responsible for fast charge−discharge ability, although the details are not clear at present.

• Even after 500 charge−discharge cycles at the rate of 1 C (0.2 C every 20 times), 83% (first cycle, 231 mAh/g; 500th cycle, 193 mAh/g) of the capacity of the material was retained

• The average Coulombic efficiency between the first and the 500^th cycle is 99.96%, which is significantly higher than that of PYT (95.21% between the first and 20^th cycle).

High-capacity, fast charge−discharge ability, and excellent cyclability speak well for the high potential of organic materials for Li-ion batteries, and open a new aspect of energy storage. Further work is in progress to explore the detailed mechanism and to develop practical batteries for high-capacity high-power applications.

—Nokami et al.

Resources

• Toshiki Nokami, Takahiro Matsuo, Yuu Inatomi, Nobuhiko Hojo, Takafumi Tsukagoshi, Hiroshi Yoshizawa, Akihiro Shimizu, Hiroki Kuramoto, Kazutomo Komae, Hiroaki Tsuyama, and Jun-ichi Yoshida (2012) Polymer-Bound Pyrene-4,5,9,10-tetraone for Fast-Charge and -Discharge Lithium-Ion Batteries with High Capacity. Journal of the American Chemical Society doi: 10.1021/ja306663g