Yasuyuki Kiya

Batteries and electrochemical capacitors

Present and future applications of electrical energy storage devices are stimulating research into innovative new materials and novel architectures.

CO2 and O2 Evolution at High Voltage Cathode Materials of Li-Ion Batteries: A Differential Electrochemical Mass Spectrometry Study

A three-electrode differential electrochemical mass spectrometry (DEMS) cell has been developed to study the oxidative decomposition of electrolytes at high voltage cathode materials of Li-ion batteries. In this DEMS cell, the working electrode used was the same as the cathode electrode in real Li-ion batteries, i.e., a lithium metal oxide deposited on a porous aluminum foil current collector. A charged LiCoO2 or LiMn2O4 was used as the reference electrode, because of their insensitivity to air, when compared to lithium. A lithium sheet was used as the counter electrode. This DEMS cell closely approaches real Li-ion battery conditions, and thus the results obtained can be readily correlated with reactions occurring in real Li-ion batteries. Using DEMS, the oxidative stability of three electrolytes (1 M LiPF6 in EC/DEC, EC/DMC, and PC) at three cathode materials including LiCoO2, LiMn2O4, and LiNi(0.5)Mn(1.5)O4 were studied. We found that 1 M LiPF6 + EC/DMC electrolyte is quite stable up to 5.0 V, when LiNi(0.5)Mn(1.5)O4 is used as the cathode material. The EC/DMC solvent mixture was found to be the most stable for the three cathode materials, while EC/DEC was the least stable. The oxidative decomposition of the EC/DEC mixture solvent could be readily observed under operating conditions in our cell even at potentials as low as 4.4 V in 1 M LiPF6 + EC/DEC electrolyte on a LiCoO2 cathode, as indicated by CO2 and O2 evolution. The features of this DEMS cell to unveil solvent and electrolyte decomposition pathways are also described.

Dramatic Acceleration of Organosulfur Redox Behavior by Poly(3,4-ethylenedioxythiophene)

We report unprecedentedly high lectrocatalytic activity of electropolymerized films of Poly(3,4-ethylenedioxythiophene) (PEDOT) toward the redox reactions of 2,5-dimercapto-1,3,4-thiadiazoie (DMcT) which is a promising cathode material for high-energy Li-ion batteries. The irreversible redox reaction of DMcT monomer at an unmodified glassy carbon electrode was dramatically accelerated at a PEDOT-film-coated electrode to give a reversible response. PEDOT also showed high electrocatalytic activity toward the redox reactions of DMcT dimer and oligomers. Such acceleration of the DMcT redox reactions, which is essential to practical application of organosulfur compounds in batteries, was ascribed to the electronic properties of PEDOT. Chronopotentiometric results indicated that at PEDOT film-coated electrodes the redox reactions of DMcT proceed fast enough to satisfy an electrolysis current as large as 1.0 mA/cm 2 .

4-Amino- 4H-1,2,4-triazole-3,5-dithiol A Modifiable Organosulfur Compound as a High-Energy Cathode for Lithium-Ion Rechargeable Batteries

We have studied the electrochemistry of the organosulfur compound, 4-amino-4H-1,2,4-triazole-3,5-dithiol (ATAD), as a potential cathode electroactive material for lithium-ion rechargeable batteries. The redox behavior was investigated via cyclic voltammetry, and the charge-transfer kinetics of ATAD were compared to those of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) and thiophene-2,5-bis(thiolate) (TBT). The redox reactions of ATAD were ascribed to the thiol groups at the 2 and 5 positions, forming a disulfide polymer during oxidation and cleaving the disulfide bonds during reduction. In addition to its chemical tunablity via the amine group, an important feature that DMcT does not possess, ATAD exhibited comparable charge-transfer kinetics to DMcT. Moreover, the charge-transfer kinetics of ATAD were significantly greater than those of TBT, which possesses chemically tunable points. These results point to the importance of heteroatoms adjacent to the thiolate groups to obtain fast charge-transfer kinetics. Furthermore, it was revealed that the redox reactions of ATAD could be accelerated by the conducting polymer poly(3,4-ethylenedioxythiophene). The chemical tunability of ATAD and the fast charge-transfer kinetics, as well as the high positive potential of the redox reactions, could enable the practical use of this organosulfur compound as a charge-storage material in lithium-ion rechargeable batteries.

Synthesis, computational and electrochemical characterization of a family of functionalized dimercaptothiophenes for potential use as high-energy cathode materials for lithium/lithium-ion batteries

We present a family of a novel class of organosulfur compounds based on dimercaptothiophene and its derivatives, with a variety of functional groups (electron-donating or electron-withdrawing groups) and regiochemistries, designed as potential high-energy cathode materials with sufficient charge/discharge cyclability for lithium/lithium-ion rechargeable batteries. This study uses as a point of departure the electrochemical and computational understanding of the electrocatalytic effect of poly(3,4-ethylenedioxythiophene) (PEDOT) towards the redox reactions of 2,5-dimercapto-1,3,4-thiadiazole (DMcT). The effective redox potentials of these materials exhibited good correlation with the highest-occupied molecular orbital (HOMO) levels predicted via computational modeling. Furthermore, the redox reactions of all the compounds studied were electrocatalytically accelerated at PEDOT film-coated glassy carbon electrodes (GCEs), although some materials exhibited higher energy output than others. By using this approach we have identified several compounds that exhibit clear promise as potential cathode materials and have characterized the molecular interactions between the organosulfur compounds and PEDOT film surfaces involved in the electrocatalytic reactions.