• Energy Research

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.

Cathode using the olivine particles was subjected to an open-circuit voltage measurement under the relaxation condition of 24 h at each SOC and DOD states. The electrochemical reaction in the LiFePO4 cathode was composed of a large plateau around 3.45 V with sloped regions nearby for both the fully charged and discharged states. It was found that the potential profile in the sloped regions exhibited a hysteresis. Furthermore, both sloped regions became narrower when the operating temperature was raised from 30 to 60°C. Furthermore, it was found that the apparent diffusion coefficient of Li+ ions in the sloped regions was much smaller than that in the plateau region. These facts implied that the obtained profiles were not in an equilibrium state with a quasi-OCV profile than the real one, and that the potential relaxation in the sloped regions took an extremely long time.

The change in entropy of Li x Cr y Mn 2-y O 4 (y = 0, 0.3), AS, was determined by potentiometric and calorimetric approaches. The AS obtained by the two different methods showed similar trends. The peak in AS was explained by the partial Li-ion ordering of the Li(1)(0, 0, 0) and Li(2)(0.25, 0.25, 0.25) sites in the sublattice of the 8a site at x = 0.6. The following complete ordering at x = 0.5 was accompanied with the rearrangement of the partial ordering between Li(1) and Li(2). Although the AS peak was diminished by partial Cr-ion substitution because of the incomplete ordering due to the random dispersion of Cr ions, the peak position (x) of AS remained unchanged. This suggested that the voltage step between 0.6 > x > 0.5 in Li x Cr y Mn 2-y O 4 was not due to the structural change in the host spinel structure but mainly caused by the Li-ion partial ordering at x = 0.6, the rearrangement of the two domains (0.6 > x > 0.5), and the following perfect ordering (x = 0.5).

Abstract not Available.

Olivine compounds Life 1-x Mn x PO 4 (0.0 < x < 0.2) were prepared by solid-state reaction and their properties of Fe 2+ /Fe 3+ electrochemical redox reaction were studied. The Mn 2+ substitution strongly affected the electrochemical properties, such as initial capacity, capacity fading, and polarization. From the relationship between the redox peak current and the sweeping rate in the cyclic voltammetry, the apparent diffusion constants were numerically evaluated. They increased monotonically with the Mn 2+ substitution and were always larger in the charging (delithiation) process than that in the discharging (lithiation) process. Additionally, the open-circuit voltage profiles in the beginning of the charge-discharge process were modified with Mn 2+ substitution; the profile was changed from a flat voltage vs the reaction degree to a sloped profile, and the sloped region became wider as the substitution degree was increased. This implied that one-phase reaction, rather than the expected two-phase reaction, was attained and expanded with the substitution. It was found that there existed a relationship between the apparent diffusion constant and the one-phase reaction width; the apparent diffusion constant was enhanced with an increase in the one-phase reaction width.

The relationship between the electrochemical behavior and the arrangement of lithium/vacancies has been investigated with electrochemical Li removal in Li(x)M(y)Mn(2-y)O4 (x or = approximately 0.5 and (2) approximately 4.2 V at x < or = approximately 0.5. To understand the stepwise behavior, entropy measurement of reaction, DeltaS(obs), was performed by using the electrochemical methods. The changes of the sign in deltaS(obs) from negative to positive at the composition x approximately 0.50 in Li(x)M(y)Mn(2-y)O4 indicated that the ordered arrangement of Li/vacancies was formed with electrochemical Li removal. Moreover, such an ordering was suppressed by the substitution of Co3+ and Cr3+ for Mn3+. To clarify the nature and origin of Li/vacancy ordering, the Monte Carlo simulation was performed in view of Coulombic interaction. The simulation reproduced the formation of a new phase arising from Li/vacancy ordering at x = 0.50 in Li(x)Mn2O4. In addition, the ordered arrangement of Li/vacancy at x = 0.5 was perturbed by the trivalent M3+ replacement in spinel structure due to the local clustering of Li+ around M3+. Consequently, the electrochemical behavior in spinel LiMn2O4 was deeply related to the Coulombic interactions, proved by the fact that experimentally observed changes in entropy agreed well with Monte Carlo simulation based on the Coulombic interaction.

Abstract High-voltage cathode LiMn1.5Ni0.5O4 particles with average size of approximately 2 μm were prepared using a two-step process. Low-temperature magnetic measurements indicated them as ferrimagnetic materials with high Neel temperature (146.5 K) and high magnetization (107 emu/g at 4.5 K) that closely approximates the theoretical value of the stoichiometric LiMn1.5Ni0.5O4 compound. Because of their high-purity and high crystallinity, their redox signals were observed only around 4.7 V. Their low-rate capacity was sufficiently high (about 140 mAh/g at 0.1 C). Their capacity retention after the 100th cycle was higher than 95%. In spite of the low specific surface area of the LiMn1.5Ni0.5O4 particles, they exhibited high reversible capacity even at a high-current rate. Additionally, the in-situ X-ray diffraction was conducted to elucidate the reaction process, which indicated that the electrochemical reaction progresses reversibly with two consecutive two-phase reactions. The lattice parameters of three corresponding phases remained almost unchanged on the reactions (0.8165, 0.8087, and 0.8000 nm). The presence of the intermediate phase might give the relaxation of the structural variation during the electrochemical reaction.

Abstract Differential voltage (dV/dQ) curve is examined to analyze the degradation of 30 Ah commercial lithium-ion batteries consisting of a Mn-based cathode and graphite anode during discharge. It is observed that the dV/dQ peak became sharper after the capacity faded. The corresponding peak change is confirmed by analysis of the electrode potential separation analysis using pseudo reference electrode and attributed to graphite voltage step at LiC12 single phase. In addition, the change in peak shape is not related to the degradation of lithium distribution in the electrode but is strongly related to the intercalation content in the graphite anode. Because the change in peak shape is observed only during discharge, charge voltage is recommended for the non-destructive analysis of lithium-ion batteries with graphite anode.