Miho Fujita

Capacity Fading of Graphite Electrodes Due to the Deposition of Manganese Ions on Them in Li-Ion Batteries

The capacity fading of lithiated and delithiated graphite electrodes has been studied using lithium manganese oxide as a counter electrode. Higher storage temperatures and longer storage periods give larger capacity losses and larger amounts of manganese (Mn) deposition on the graphite. The capacity losses appear to be related to the amount of deposited Mn in storage experiments in dry electrolyte solutions containing various concentrations of Mn 2+ ions, but definite evidence has not been found that the Mn deposit is a simple and direct cause of the capacity fading. The capacity losses estimated from quantities of Mn deposition on the graphite, assuming a Mn 2+ -Li exchange process, appear negligible when compared with the experimental findings. Scanning electron microscope and transmission electron microscope observations show the presence of some amounts of fluoride compounds accompanying some Mn contents on the graphite. The capacities of the graphites are recovered to some extent during the cycles.

Electrochemical and Structural Properties of a 4.7 V-Class LiNi0.5Mn1.5 O 4 Positive Electrode Material Prepared with a Self-Reaction Method

LiNi 0.5 Mn 1.5 O 4 as 4.7 V-class cathode materials were prepared with our self-reaction method at 650, 700, and 750°C calcination temperatures respectively, and their electrochemical properties and crystal structures were studied. The sample at 700°C was assigned to the space group P4 3 32 in the X-ray diffraction analysis. On the contrary, the space group of the sample calcined at 650 or 750°C fell into Fd3m because diffraction peaks from P4 3 32 were missing. The sample at 700°C showed a one-step profile in the charge/discharge curve and this contrasted with the other samples showing two-step ones. In spite of their different structures, these three samples had the same initial capacities of about 123 mAh/g. They gave similarly good cycle performance as well, but the sample at 650 or 750°C maintained a little higher capacity after 50 cycles of charge/discharge than the sample at 700°C. From the analysis of transformation of the crystal structure of LiNi 0.5 Mn 1.5 O 4 during charging, it was found that LiNi 0.5 Mn 1.5 O 4 underwent the electrochemical reactions at three stages unlike LiMn 2 O 4 with two stages.

Cycle Performance in Each State-of-Charge in LiMn2 O 4

Cycle performances at 55 C were examined for a cell using a spinet-type of lithium manganese oxide as a positive electrode in each of five regions of the state-of-charge. The results demonstrate that larger capacity losses are found in two of the lowest voltage regions of 0-20% and 20-40% states-of-charge and a smaller capacity loss is observed in a higher voltage region of 60-80% state-of-charge. By the addition of the conductive material to the positive electrode after the cycle, the capacities recover in any regions. The results of our present work imply that the capacity losses can he divided into two major parts; one caused by deterioration of the active material itself and another by poor conduction between the active material and collector. The ratio of the former to the latter is estimated to he ahout 1:2. A distinct correlation is found between the capacity loss in the storage experiment at each constant potential and that arising from the ective material itself, and only a weaker correlation is admitted between the capacity loss in the storage and that resulting from the poor conduction. The poor conduction in the positive electrode would he derived from insulating substances formed by the reaction of a protonated lithium manganese oxide with Li and Mn ions, at found in the storage experiment. However, there is no evidence connecting a cause of these capacity losses with lattice expansion of the lithium manganese oxide.

Chromatographic separation of the diastereomers of the bis(2,4-pentanedionato)[2-(methylseleno)ethylamine]-cobalt(III) ion on a column of sulphoethyl-toyopearl and determination of the rate of inversion at the selenium atom

Abstract A high-performance liquid chromatographic (HPLC) method was successfully applied to kinetic studies of the inversion at the selenium atom of [Co(acac) 2 -(CH 3 SeCH 2 CH 2 NH 2 )] + (acac — 2,4-pentanedionate ion). The complex was separated into two racemic pairs, Δ ( R ), Λ ( S ) and Δ ( S ), Λ ( R ), by column chromatography on sulphoethyl (SE)-Toyopearl. The racemate, Δ ( R ), Λ ( S ) or Δ ( S ), Λ ( R ), was epimerized in 0.05 mol dm -3 Na 2 SO 4 , and then the products, Δ ( R ), Λ ( S ) and Δ ( S ), Λ ( R ), were separated on SE-Toyopearl packed in an HPLC column. The activation enthalpy obtained, ΔH * — 113 kJ mol -1 , is the highest reported so far for inversion of coordinated selenide.

15N NMR studies of cobalt(III) complexes: [Co(NH3)5(15NH3)]3+ and [Co(15NH2CH2CH215NH2)3]3+

Abstract 15 N Fourier-transform nuclear magnetic resonance spectroscopy enabled the determination of spin-spin coupling constants 1 J ( 59 Co 15 N) and spin-lattice relaxation times T 1 ( 59 Co) for hexaamminecobalt(III) complex [Co(NH 3 ) 5 ( 15 NH 3 )] 3+ and tris-(ethylenediamine[ 15 N, 15 N])cobalt(III) complex. The obtained values are as follows: 1 J ( 59 Co 15 N) = 62.5 (± 1.0)Hz, T 1 ( 59 Co) = 38 msec for the hexaamminecobalt complex, and 1 J ( 59 Co 15 N) = 63.8 (± 1.0)Hz, T 1 ( 59 Co) = 13 msec for the tris(ethylenediamine)cobalt(III) complex. These T 1 ( 39 Co) values are well consistent with the values obtained from multi-pulse spin-echo measurement.

Improved LiMn2O4/Graphite Li-Ion Cells at 55°C

Sodium phosphate additives and mixed electrolytes of Li(C 2 F S SO 2 ) 2 N (LiBETI) and LiPF 6 were examined to improve cycle performances of Li 1.04 Mn 1.96 O 4 /graphite cells at high temperatures. The cycle performance of those cells exceeded 80% at 100 cycles at 55°C in an optimum combination of a 0.975 M LiBETI and 0.025 M LiPF 6 /ethylene carbonate (EC):dimethylcarbonate (DMC) (1:2) electrolyte solution containing 5% of Na 4 P 2 O 7 or 5% of Na 5 P 3 O 10 additive. Such a cycle performance is considered to fulfill the requirement for practical use.

Geometrical and optical isomers of diethylenetriaminemonoacetato(ethylene-diamine)cobalt(III) ion and their isomerism

Abstract Two isomers of the complex ion in the title were obtained and each isomer was resolved chromatographically into its antipodes. The two isomers with their isomer proportion of 27.9 and 72.1% in the equilibrium mixture were assigned to α and β( mer -N) isomers, respectively, of three possible geometrical isomers, from the measurements of their absorption, circular dichroism, and NMR spectra. Preference of the β( mer -N) to the isomer and very poor yield of an expected β( fac -N) isomer were confirmed by conformational analyses carried out for each structure of the isomers of Λ configuration, with possible configurations around nitrogen atoms and conformations of chelate rings. They gave minimized total strain energies of 43.13, 44.24, and 52.63 kJ/mol for the Λ- R , R (en:λ) structure of a β( mer -N) isomer, the Λ- S , R (δ,δ) structure of an α isomer, and a Λ- R , S (λ,λ) structure of a β( fac -N) isomer, respectively. From the results, configurations and conformations of the enantiomers of the resolved β( mer -N) and its isomers were deduced. An unfound isomer, β( fac -N) isomer, is thought to be very unstable; it would exist as less than 2% of the amount of β( mer - N) isomer, even if it were present in the reaction mixture.

The preparation of trichlorotris(hydrogen-salicylato)rhodate(III) and tetra-μ-(hydrogen-salicylato)diaquadirhodium(II)

The preparation of a new example of a salicylato divalent rhodium complex, namely, tetra-μ-(Hsal)diaquadirhodium(II), and a new trivalent complex, the trichlorotris(Hsal)rhodium(III) ion are reported. The complexes were isolated and characterized by spectrochemical methods including FT-Raman and n.m.r. spectroscopies. The trivalent complex has a monodentate Hsal ligand bonded through a carboxyl oxygen with a facial configuration.

Enantiomers of the tris(trimethylenediamine)cobalt(III) ion of 100% optical purity: preparation and circular dichroism study

Reaction of optically pure [Co(tn)3]3+ isomers (tn = H2NCH2CH2CH2NH2)(in aqueous solutions), obtained by a new chromatographic technique, with L-tartrate ions resulted in stereoselective circular dichroism changes similar to those for [Co(en)3]3+, (en = H2NCH2-CH2NH2), which implies the tris-lel-twist conformation†; of the complex at least in the ion-pair.