Keijiro Sawai

Carbon materials for lithium-ion (shuttlecock) cells

Abstract Carbon materials as possible alternatives to metallic lithium were described with emphasis on the electrochemical characters of carbon materials, such as (natural and artificial) graphite, petroleum coke, pitch-based carbon fibers, high-area carbons, and other carbonaceous materials. Well-defined graphite showed the lowest operating voltage (0-0.3 V versus Li) and the highest volumetric capacity (about 0.6 Ah·cm−3 based on the observed density and rechargeable capacity) in addition to excellent rechargeability. The theoretical capacity of graphite was 372 mAh·g−1 or 850 mAh·cm−3 based on the graphite sample for the reaction to form the first-stage compound (LiC6). Among the materials examined, some of them showed the capacity more than 372 mAh·g−1, while the operating voltage was higher and the density was lower than that of graphite. To understand such an anomalous behavior of carbonaceous materials, we modeled carbon electrodes and explained why some of carbonaceous materials exceed a capacity limit of 372 mAh·g−1. According to our model, high-capacity materials more than 500 mAh·g−1 were possible but traded off volumetric capacity, operating voltage, and consequently energy density. A lithium-ion (shuttlecock) cell consisting of natural graphite and LiNiO2 was also reported and the specific problems in applying carbon materials to lithium-ion cells were discussed.

Electrochemistry and Structural Chemistry of Li [ CrTi ] O 4 ( Fd3̄m ) in Nonaqueous Lithium Cells

Li[CrTi]O 4 (Fd3m; a = 8.32 A) having a spinel-framework structure was prepared and examined in nonaqueous lithium cells. Li/Li[CrTi]O 4 cells showed the flat operating voltages of 1.50 V and rechargeable capacity of 150 mAh/g. The X-ray diffraction examinations in this region indicated that Li[CrTi]O 4 was reduced to Li 2 [CrTi]O 4 in a topotactic manner. Electrochemical oxidation of Li[CrTi]O 4 was also examined. Redox potential of 4.7 V vs. Li was observed while reversibility was poor due to the destruction of crystallite. Differences and similarities between Li[CrTi]O 4 and Li[Li 0.33 Ti 1.67 ]O 4 were discussed with respect to operating voltage, reversibility, and insertion scheme.

Heat-Treated Transition Metal Hexacyanometallates as Electrocatalysts for Oxygen Reduction Insensitive to Methanol

Highly active catalysts for electrochemical reduction of oxygen insensitive to the presence of methanol were prepared from transition metal hexacyanometallate precursors by heat-treatment with carbon black under an inert atmosphere. The catalytic activity for oxygen reduction was examined with the floating electrode technique under an air atmosphere at room temperature. The electrolyte used in most of the measurements was 1 M sodium phosphate buffer solution (pH 7.5), whereas acid and alkaline solutions were also used in addition to the neutral buffer solution to examine the catalytic activity of the prepared catalyst over a wide range of pH. Remarkable enhancements in the catalytic activity were observed for samples heat-treated at temperatures higher than 500°C. Among several 3d-transition metals incorporated in the inorganic precursor, the combination of cobalt and iron incorporated at neighboring sites gave the highest activity, comparable to that of platinum black catalyst (Pt/C). The catalytic activity for oxygen reduction was not affected by the presence of 2.5 M methanol in the electrolyte, while that of Pt/C was severely impaired by the presence of methanol. The catalysts prepared from the inorganic precursors were characterized by X-ray diffraction (XRD), infrared (IR), and X-ray photoelectron spectroscopy (XPS) measurements. The XRD and IR data indicated that the cyanide structure of the inorganic precursor was decomposed when heating beyond 500°C. The XPS data indicated that the oxidation states of cobalt and iron are close to metallic ones and two types of nitrogen forming new bonding are present in the heat-treated samples. The same structural and spectral changes were observed for samples heat-treated without carbon black. From these results, the evolution of the high catalytic activity by heat-treating the inorganic precursors is discussed.

Monitoring of Particle Fracture by Acoustic Emission during Charge and Discharge of Li / MnO2 Cells

Charge and discharge of Li/MnO 2 cells were examined with monitoring of particle fracture of manganese dioxide by acoustic emission. Manganese dioxide used was electrolytic manganese dioxide heat-treated at 400°C for 24 h in air [HEMD(400)]. The acoustic-emission technique worked well to monitor events that occurred inside a cell. During the first discharge to prepare a deep-discharge product, a closely packed series of acoustic events was observed, especially in the latter half of the discharge process, which contained most of the acoustic events. During cycling, acoustic events were concentrated at the end of discharge while no event was observed during charge, indicating that particle fracture took place during lithium-ion insertion into a solid matrix. Rate-capability tests showed that the rate of acoustic events was a function of current drain, i.e., a higher discharging current accelerated particle fracture. From these results we discuss the important role of mechanical properties of materials upon the lithium-insertion scheme. We also discuss the ideal considerations regarding insertion materials for advanced batteries.

Topotactic Reduction of Alpha‐Manganese (Di)Oxide in Nonaqueous Lithium Cells

Electrochemical reduction of synthetic α-MnO 2 s was examined in nonaqueous lithium cells. Synthetic α-MnO 2 s containing NH 4 + (empirical formula (NH 4 ) 1.4(1) Mn 8 O 16 , MnO 1.908 ), K + (K 1.3(1) Mn 8 O 16 , MnO 1.913 ), and Rb + ions (Rb 1.3(1) Mn 8 O 16 ,MnO 1.924 ) having tetragonal lattices were examined

Factors Affecting Rate Capability of Graphite Electrodes for Lithium-Ion Batteries

Kinetic problems associated with rate capability of graphite electrode were examined by impedance measurement, current-pulse relaxation technique, and voltage-step chronoamperometry. The impedance and current-pulse relaxation data showed a diffusion behavior. The diffusion behavior was examined as to which process is reflected on the data, i.e., lithium-ion transport in a graphite solid matrix or in a liquid electrolyte, using a lithium electrode substituting for the lithiated graphite electrode and using the same electrolyte, separator, and cell. It was shown that the diffusion behavior observed for the lithiated graphite electrode was the same as that of lithium electrode reflecting lithium salt diffusion in the electrolyte. To examine specifically whether or not lithium transport in the solid matrix is responsible for the rate capability, the voltage-step chronoamperometry of lithiated graphite electrodes having different porosity in the electrode mix was carried out. The current per graphite sample weight was highly dependent on the porosity, confirming that the rate capability problem is not due to lithium transport in the solid matrix. From these results, factors affecting the rate capability of graphite negative electrodes for lithium-ion batteries are discussed.

Highly Active Nonplatinum Catalyst for Air Cathodes

Nonplatinum electrocatalysts for air cathodes showing higher activity than the conventional Pt/C catalysts were prepared by heat-treating cobalt hexacyanoferrate precursors dispersed on carbon support under an inert atmosphere. The activity for oxygen reduction was examined by polarization measurements with a gas-diffusion electrode floating on the surface of an electrolyte under an air atmosphere at room temperature. The electrolyte used was a neutral phosphate buffer solution. To investigate the effect of carbon support on the catalytic activity, different types of carbon supports were used with and without pretreatment with nitric acid prior to dispersion of the inorganic precursor. Among several carbon supports examined, the high-area carbon pretreated with nitric acid at higher concentrations than 8 M led to an enhanced performance for oxygen reduction in the gas-diffusion electrode. The effect of Co/Fe ratio in the precursor on the catalytic activity was also investigated. The catalyst prepared by heat-treating the inorganic precursor with a Co/Fe ratio of 1.2 dispersed on the high-area carbon showed the highest activity. Concerning the polarization voltage for oxygen reduction with the floating electrode, the best catalyst prepared in this study exhibited ca. a 200 mV higher potential at 100 mA cm -2 than the carbon-supported Pt-black catalysts. The remarkable enhancement in the performance of the air cathodes seems to be due to high dispersion on the high-area carbon, leading to effective utilization of the catalyst in addition to its intrinsic activity.

A Method of Impedance Spectroscopy for Predicting the Dynamic Behavior of Electrochemical System and Its Application to a High‐Area Carbon Electrode

A method for calculating the time-domain response to a periodic signal imposed on a system from impedance spectroscopic data is described. Since impedance is one of the system functions, a current (or voltage) response to voltage (or current) signal imposed on a system can be calculated from the impedance spectrum by performing a Fourier transform of voltage or current signals combined with an operational form of Ohms law I(ω) = E(ω)/Z(ω) in the frequency domain. To show the applicability of the method to an electrochemical system, a high-area carbon electrode in a nonaqueous electrolyte was selected. A large capacitance characteristic of high-area carbons, 0.52 F per 3 cm 2 of apparent electrode area (140 F per gram based on the carbon sample weight), appeared in the low-frequency region below 1 mHz, while a small capacitance of 25 μF per 3 cm 2 (a typical value for a double layer) was observed for higher frequencies than 100 Hz. In a midfrequency range of 10 -1 to 10 1 Hz, the electrode behaved like a resistor. Cyclic voltammograms at various sweep rates were calculated from the impedance spectrum. A one-to-one correspondence between calculated and directly measured voltammograms was obtained, and a characteristic feature of a high-area carbon electrode is described. The feasibility of applying the proposed method to materials research for advanced batteries, including supercapacitors, is discussed.

Enzymatic reduction-resistant nitroxyl spin probes with spirocyclohexyl rings

To suppress enzymatic reduction of nitroxyl group of spin probes, this study designed two new nitroxyl probes, 4-hydroxy and 4-oxopiperidine-N-oxyls having 4′-hydroxyspirocyclohexyl groups at the 2- and 6-positions of the piperidine ring (hydroxy-DICPO and oxo-DICPO, respectively). The decay of the EPR signal of these probes in mouse liver homogenates was significantly suppressed compared with that of 4-hydroxy- and 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (hydroxy-TEMPO and oxo-TEMPO, respectively), although hydroxy-DICPO and oxo-DICPO showed no difference in the reactivities with ascorbic acid. While both hydroxy- and oxo-DICPO reacted with hydroxyl radicals, only hydoxy-DICPO lost its EPR signal by the reaction with superoxide anion radical in the presence of cysteine. This feature is similar to that observed for hydroxy- and oxo-TEMPO. These results suggest that the introduction of spirocyclohexyl groups to nitroxyl spin probes is effective for protecting the nitroxyl group against enzymatic reduction without changing the characteristics of the reaction with oxygen radicals.

Topotactic Two‐Phase Reaction of Ruthenium Dioxide (Rutile) in Lithium Nonaqueous Cell

Electrochemical and x-ray diffraction studies were carried out for the reduction of RuO 2 having rutile structure in 1M LiClO 4 propylene carbonate/1,2-dimethoxyethane (1:1) solution. RuO 2 was topotactically reduced to LiRuO 2 , drawing an L-shaped voltage curve