Tuning of voltage and cycle performance of organic-tailored rechargeable battery by organic chemical technique

Organic chemistry plays an important role in the development of high-performance rechargeable batteries exceeding Li-ion batteries using LiCoO2 as a cathode-active material. The authors implemented the high-capacity rechargeable batteries using trioxotriangulene (TOT) stable neutral radicals with four-stage redox ability as cathode-active materials. These batteries showed the capacity of 311 A h kg in the first discharge process and high cycle performance. This result was accomplished by a tailor-made approach to organic rechargeable batteries based on frontier-MO engineering by molecular modification and crystal engineering (Nat. Mater. 2011 , 10, 947). For practical organic rechargeable batteries, it is a crucial issue to establish organic tailor-made approaches by illustrating how output voltages of the batteries link the redox potentials/energy levels of the frontier MOs of cathode-active materials. In the present paper, the authors focused on “closed-shell” organic electron-acceptors and -donors shown in Figure 1a. The redox potentials of TCNQ 1 are easily tuned by substituents (2–4), and the gaps between the first and second redox potentials are reduced with the decrease in on-site Coulombic repulsion by π-extension (5 , 6). Electron-donors 7 and 8 possess higher redox potentials than those of the TCNQ derivatives. Furthermore, 5–8 show low solubility in common organic solvents owing to the rigid π-extended structures, which is an advantage for improving cycle performance. The results of battery experiments have demonstrated tuning of the output voltage (Figure 1b) and the cycle performance of rechargeable batteries in terms of organic chemical technique. Furthermore, the authors have found two obvious relationships between the output voltages of the organic batteries and the redox potentials of the cathode-active materials in solution, and between the output voltages and the energy levels of the frontier MOs of the cathode-active materials. This study enables us to design novel organic electrode-active materials on the molecular level and to tune the battery performance in terms of synthetic organic spin chemistry. Notably, the output voltage is predictable in the design phase of batteries, which makes a sharp contrast with the batteries using inorganic materials as cathode-active materials.