Two multielectron redox organic compounds, hexaazatrinaphthylene (3Q) and hexaazatriphenylenehexacarbonitrile (6CN) were evaluated as high energy density cathode materials of quasi-solid cells. The cathode dissolution, which has been an issue for organic cathodes, was suppressed and the cell cyclability drastically improved. 6CN, which exhibited low capacity and cyclability in conventional liquid electrolyte, was measured to possess the specific capacity of ∼420 mAh/g. The energy density of the cathode body was ∼720 Wh/kg. It was also discovered that implementation of depth-of-discharge management further improved cyclability. This study is a demonstrative case that cathode compounds deemed unusable in liquid electrolyte may turn usable in solid-state lithium cells.

No abstract

The molecular design of organic energy-storage devices relies on correlations between the electrochemical properties of organic materials and their molecular structures. Here we report a systematic study of the fundamental electronic states of the quinone family of redox-active materials. Poly(ethylene oxide) coatings, as elution inhibitors, facilitated the evaluation of the electrochemical properties of single quinone molecules. Moreover, we confirmed experimentally how LUMO energies and their corresponding redox potentials depend on molecular structure, including the number of aromatic rings, the positions of functional groups, and coordination structures; this was achieved by elemental and chemical-stateselective X-ray absorption spectroscopy, and DFT calculations. We introduce an energy diagram depicting a segmentalized reduction process; this diagram considers the intermediate states during redox reactions to discuss processes that dominate changes in electrochemical properties as molecular structures are altered. Our results and analysis strategy are widely applicable to the material design of future organic molecular-based devices.

Quinone-based organic materials have reversal multielectron redox reactions. Cations in electrolytes are withdrawn or released by reduced quinone-based materials. Lithium ions being adopted, quinone-based materials are applied as an active material for lithium secondary battery. Due to their multielectron redox reactions, organic materials have an energy density even equal to inorganic materials. Inorganic materials must keep their crystal structure during cell operation, relating to reversibility of charge-discharge capacity, and the lithium conducting path in their crystal structure limit the mobility of lithium. On the other hand, organic materials do not need to keep their crystal structure owing to redox reaction within a molecule. Their electrochemical performances are not controlled by the lithium mobility in the lithium conducting path as in the case of inorganic materials. In addition, organic materials are relatively low-cost due to no metal elements and can be easily designed to fabricate their organic structure. However, their high solubility to electrolytes gives a large capacity loss, and their insulating properties lead directly to poor electron conduction during their redox reaction. These properties limit the rate performance and utilization ratio of organic materials. In this study, several organic materials, anthraquinone (AQ), 9,10-phenanthrenequinone (PQ), 1,10-phenanthroline-5,6-dione (N2PQ), and so on, were supported as much as possible on the surface of activated carbon (Maxsorb®) with nano-sized pores as a current collector for compensating their insulating properties. The organic materials-supported activated carbon was covered by a lithium conductive polymer (polyethyleneoxide, PEO) in order to restrict a dissolution of organic materials to electrolytes. The effect of organic structures on the electrochemical performance as positive electrodes was discussed. Discharge curves of positive electrodes using AQ, PQ, and N2PQ at 0.2 C show that two clear plateaus are detected for PQ (2.7 and 2.3 V) and N2PQ (2.9 and 2.5 V), respectively, and one plateau is detected for AQ (2.1 V). When two ketone groups in PQ and N2PQ are adjacent, radical anions reduced from neutral molecules through one electron redox reaction could exist stably by coordinating a lithium ion for a bridge formation. The coexistence regions of neutral molecules-radical anions and of radical anions-divalent anions result in two plateaus in the charge-discharge test of PQ and N2PQ. Redox potentials of PQ are higher than those of AQ because 1,2-diketone compound (PQ) receives a repulsion of two dipoles from each ketone, leading to a destabilization of neutral molecules and to easily reduction at the higher potential than 1,4-diketone compound (AQ). Heterocyclic compound (N2PQ) offers higher redox potential than PQ. In the field of complex chemistry, N2PQ behaves as a ligand for chelation. A resonance form of reduced N2PQ gives a negative charge to nitrogen atoms, and lithium ions are possibly withdrawn not only at oxygen atoms but also at nitrogen atoms. This coordinating ability of nitrogen atoms stabilizes the reduced form of N2PQ more than that of non-heterocyclic compound (PQ), reflecting higher redox potentials of N2PQ. Low polarization and high redox potentials in the charge-discharge test for N2PQ gives a high energy density of about 600 mWh g-1 when an electric double layer capacitance obtained from the activated carbon was removed. This value is comparable to inorganic active materials. A charge-discharge curve for N2PQ at 1.0 C shows two plateaus at 2.8 and 2.4 V. Discharge capacity in the 5th cycle at 0.2 and 1.0 C was 226 and 194 mAh g-1, respectively, and their utilization ratios of N2PQ (theoretical discharge capacity: 255 mAh g-1) were 89 and 76 %. The redox reaction of N2PQ through stable radical anion occurs smoothly to offer highly discharge capacity and rate performance.