Self-organized carbon nanostrips with a new LiC10 structure derived from carbon nanotubes

Single walled carbon nanotubes (SWNTs) were reacted with molten lithium at 220 °C for two weeks. This induced dramatic changes in their structure as shown by x-ray and electron diffractometry and Raman spectroscopy. A significant fraction of the initial SWNTs transformed into flat nanostrips having intercalated lithium in between them. Lithium forms a superlattice commensurate with that of the graphitelike nanostrips with [square root of]7×[square root of]3 in-plane distribution. This new structure corresponds to the LiC10 composition

An XRD Study of Chemical Self-Discharge in Delithiated Cobalt Oxide

Changes in samples of Li1–xCoO2 were measured by X-ray diffractometry (XRD) after thermal aging treatments that cause capacity losses in electrochemical cells. Changes in lattice parameters were used to identify lithium re-intercalation into Li1–xCoO2 when it was aged in the presence of LiClO4, LiPF6, and LiAsF6 in propylene carbonate (PC). Li+ re-intercalation could account for the reversible capacity loss. Thermal aging at 75°C in pure PC or pure argon gas resulted in other changes that are attributed to the formation of spinel phase. The rate of the lithium re-intercalation increases in the following sequence: LiPF6<LiClO4<LiAsF6

Hexagonal to Cubic Spinel Transformation in Lithiated Cobalt Oxide

A transmission electron microscopy (TEM) investigation was performed on LiCoO2 before and after it had been subjected to charge/discharge cycling in electrochemical cells, as well as on delithiated Li(1–x)CoO2 before and after thermal aging. Turbostratic disorder involving small rotations of the O-Co-O slabs was found in as-received material, and in material subjected to a few cycles. In LiCoO2 subjected to extensive charge/discharge cycling, it was found that increasing amounts of the trigonal O3 phase had transformed to H1-3 phase and to the cubic spinel phase. The transformation appears to initiate on the surfaces of trigonal crystals. The orientation relationship between the trigonal and spinel phases was determined from diffraction patterns to be {0001}trigonal parallel {111}cubic and trigonal parallel cubic. The difference in unit cell dimensions leads to transformation stresses when spinel crystals are formed, and spallation of surface layers was observed. The formation of a spinel phase could suppress electrochemical performance of LiCoO2 cathodes in heavily cycled cells. Aging in the charged state also can alter particle surfaces and therefore the performance

Transmission Electron Microscope Studies of LiNi1/3Mn1/3Co1/3O2 before and after Long-Term Aging at 70°C

LiNi1/3Mn1/3Co1/3O2 is a potential cathode material for high-power applications in lithium-ion batteries. While cation ordering on a sqrt(3)×sqrt(3) R30° in-plane superlattice was proposed for the layered structure, the experimental data do not fully support this model. Here, we present a systematic electron diffraction study of LiNi1/3Mn1/3Co1/3O2 in the pristine state and after aging. Our results show that a mixture of different phases in the starting material transforms to the O3-type phase and the cubic spinel phase after aging, accompanied by an increase in the percentage of polycrystals

Entropy of Li intercalation in LixCoO2

The entropy of lithiation of LixCoO2 for 0.5 < x less than or equal to 1.0 was determined from measurements of the temperature dependence of equilibrated voltages of electrochemical cells. Measured changes in the entropy of the lithiation reaction were as large as 9.0 k(B)/atom, and as large as 4.2 k(B)/atom within the "O3" layered hexagonal structure of LixCoO2. Three contributions to the entropy of lithiation for the O3 phase were assessed by experiment and calculation. The phonon entropy of lithiation was determined from measurements of inelastic neutron scattering. Phonon entropy can account for much of the negative entropy of lithiation, but its changes with lithium concentration were found to be small. Electronic structure calculations in the local density approximation gave a small electronic entropy of lithiation of the O3 phase. The configurational entropy from lithium-vacancy disorder was large enough to account for most of the compositional trend in the entropy of lithiation of the O3 phase if ordered structures exist at lithium concentrations of x=1/2 and x=5/6. The electrochemical measurements showed the existence of a two-phase region in the composition range between x=5/6 and 0.95. Electronic structure calculations gave evidence that these phases were metallic and insulating, respectively. Changes of the electronic and configurational entropy were found to be of comparable importance for this metal-insulator transition

High-Energy-Density, Low-Temperature Li/CFx Primary Cells

High-energy-density primary (nonrechargeable) electrochemical cells capable of relatively high discharge currents at temperatures as low as -40 C have been developed through modification of the chemistry of commercial Li/CFx cells and batteries. The commercial Li/CFx units are not suitable for high-current and low-temperature applications because they are current limited and their maximum discharge rates decrease with decreasing temperature. The term "Li/CFx" refers to an anode made of lithium and a cathode made of a fluorinated carbonaceous material (typically graphite). In commercial cells, x typically ranges from 1.05 to 1.1. This cell composition makes it possible to attain specific energies up to 800 Wh/kg, but in order to prevent cell polarization and the consequent large loss of cell capacity, it is typically necessary to keep discharge currents below C/50 (where C is numerically equal to the current that, flowing during a charge or discharge time of one hour, would integrate to the nominal charge or discharge capacity of a cell). This limitation has been attributed to the low electronic conductivity of CFx for x approx. 1. To some extent, the limitation might be overcome by making cathodes thinner, and some battery manufacturers have obtained promising results using thin cathode structures in spiral configurations. The present approach includes not only making cathodes relatively thin [.2 mils (.0.051 mm)] but also using sub-fluorinated CFx cathode materials (x 1. It was known from recent prior research that cells containing sub-fluorinated CFx cathodes (x between 0.33 and 0.66) are capable of retaining substantial portions of their nominal low-current specific energies when discharged at rates as high as 5C at room temperature. However, until experimental cells were fabricated following the present approach and tested, it was not known whether or to what extent low-temperature performance would be improved