Kazuma Kumai

Gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cell

Abstract To elucidate the gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cells after long cycling, we developed a device which can accurately determine the volume of generated gas in the cell. Experiments on LixC6/Li1−xCoO2 cells using electrolytes such as 1 M LiPF6 in propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are presented and discussed. In the nominal voltage range (4.2–2.5 V), compositional change due mainly to ester exchange reaction occurs, and gaseous products in the cell are little. Generated gas volume and compositional change in the electrolyte are detected largely in overcharged cells, and we discussed that gas generation due to electrolyte decomposition involves different decomposition reactions in overcharged and overdischarged cells.

Electrochemical and calorimetric approach to spinel lithium manganese oxide

Abstract A simultaneous electrochemical and calorimetric measurement was conducted on a spinel lithium manganese oxide (LiMn 2 O 4 )-cathode, carbon-anode prismatic lithium ion cell. From the comparison of thermal behavior at room temperature (25°C) and at an elevated temperature (40°C), a pronounced exothermic thermal behavior was observed during charging in the high voltage region (0.2 x x Mn 2 O 4 ) at elevated temperatures. The degraded cell that was operated at 40°C indicated that the capacity fading occurred mainly in the same high voltage region in advance of lower region. The irreversible thermal behavior observed by calorimetry clearly indicates the degradation of LiMn 2 O 4 at elevated temperatures.

An X-ray photoelectron spectroscopy study on the surface film on carbon black anode in lithium secondary cells

Abstract The property of the surface film formed on the carbon black anode of a lithium cell at the initial reduction stage was investigated. About 90% of the carbon black surface was covered with a smooth film about 10–15 A in thickness after five cyclic voltammetry cycles. The X-ray photoelectron spectroscopy analysis revealed that this film contained oxygen atom bonding directly to the carbon atoms and was formed by decomposition of the solvent. The inhibiting effect of the surface film against further solvent decomposition reaction disappeared after washing with ethanol or heat treatment above 200 °C. The surface film was confirmed to form also by chemical reduction using lithium naphthalide, where lithium insertion in the carbon black took place. The inhibiting effect of a chemically formed surface film against the solvent decomposition was less marked than that of an electrochemically formed film.

Effects of the macroscopic structure of carbon black on its behaviour as the anode in a lithium secondary cell

Abstract The anodic behaviour of various carbon black (CB) samples prepared by the oil furnace method has been investigated in 1 M LiPF6 solution in ethylene carbonate-1,2-dimethoxyethane 50 50 (by volume). These CB samples had different macroscopic structures, though their microscopic parameters such as d002 and Lc remained constant substantially. The irreversible capacity at the initial reduction stage increased almost linearly with the BET surface area. The reversible capacity, on the other hand, showed no correlation with the BET area and was large for a highly aggregated coarse CB sample. The neck position connecting two primary CB particles was considered to be mainly active for the electrochemical insertion/extraction of lithium including effective electrolyte penetration on to the carbon surface.

Degradation mechanism due to decomposition of organic electrolyte in Li/MoS2 cells during long cycling

Abstract The degradation mechanism due to the decomposition of organic electrolytes during charge and discharge cycling of Li/MoS2 cells has been investigated. The gas products of the electrolyte decomposition during long cycling were measured by gas chromatography (GC), and the changes of electrolyte composition was measured by gas chromatography/mass spectrometry (GC/MS). The gases were produced through two types of electrolyte decomposition: the electrochemical reaction, which was dependent on the charge and discharge voltage, and the chemical reaction, which was independent of the discharge voltage. This decomposition of electrolytes occurred under a discharge voltage lower than 1.4 V. The operating conditions with a lower discharge rate and a deeper depth of discharge (D.O.D.) accelerated the electrochemical decomposition. The rate of gas production by chemical decomposition, which is independent of the discharge condition, was estimated to be about 0.1 ml/h.

Series-connected multi-cell operation of lithium-ion cells by floating method

Abstract The resistance to damage during overcharge and overdischarge of a single cell and the possibility of series-connected multi-cell operation have been investigated using a commercialized lithium-ion cell. The single cell showed sufficient cycleability in overcharge up to 4.5 V and small reversible capacity in overdischarge under 2.5 V. An overdischarged cell below 0 V did not generate subsequent electromotive force and behaved like a resistor of an electron conductor. Multi-cell operations including imbalanced cells both in a preshifted state-of-charge between +30 and −5% and in various ambient temperatures were performed for over 1000 cycles of charge/discharge by the floating method.

Investigation on the spectrophotometric determination of microamounts of iron in water

A bathophenanthroline method was examined to determine microamounts of iron in water. Investigations were made with artificial standards of ferrous diammonium sulfate solution. Analytical procedure was similar to the Wilsons method [found in Analyst, 89, 389 (1964)] but pH adjustment was made with a glass-electrode pH-meter. Ferrous-bathophenanthroline complex was stable in isoamyl alcohol for more than twenty hours. Determinations could be available within the errors 0.4 μg/l (95% confidence limits). This value is smaller than that of authorized in JIS B 8224(1961).