The Character of Dislocations in LiCoO2

Dislocations in LiCoO2 were observed by transmission electron microscopy, and their Burgers vectors were determined by analysis of diffraction contrast in tilting experiments. The configuration of all dislocations indicates that they are glissile, and dislocation configurations were found that are indicative of active slip planes. Perfect dislocations of a/3 type Burgers vectors were observed on {0001} habit planes. These perfect dislocations sometimes dissociate into Shockley partial dislocations with a/3 type Burgers vectors. Glide of these partial dislocations can account for the sequence of crystal structures O3, H1-3, O1 that occur with the delithiation of LiCoO2. The presence of glissile dislocations also suggests possible damage mechanisms during cycling

Thermodynamics of Lithium Intercalation into Graphites and Disordered Carbons

The temperature dependence of the open-circuit potential of lithium half-cells was measured for electrodes of carbon materials having different amounts of structural disorder. The entropy of lithium intercalation, DeltaS, and enthalpy of intercalation, DeltaH, were determined over a broad range of lithium concentrations. For the disordered carbons, DeltaS is small. For graphite, an initially large DeltaS decreases with lithium concentration, becomes negative, and then shows two plateaus associated with the formation of intercalation compounds. For all carbons DeltaH is negative, and decreases in magnitude with increased lithium concentration. For lithium concentrations less than x = 0.5 in LixC6, for the disordered carbons the magnitude of DeltaH is significantly more negative than for graphite (i.e., intercalation is more exothermic). The measurements of DeltaH provide an energy spectrum of chemical environments for lithium. This spectrum can be used to understand some of the concentration dependence of configurational entropy, but the negative values of DeltaS require another contribution to entropy, perhaps vibrational in origin

Phase Diagram of Li_xFePO_4

The phase diagram for LixFePO4 has been determined for different lithium concentrations and temperatures. The two low-temperature phases, heterosite and triphylite, have previously been shown to transform to a disordered solid solution at elevated temperatures. This disordered phase allows for a continuous transition between the heterosite and triphylite phases and is stable at relatively low temperatures. At intermediate temperatures the proposed phase diagram resembles a eutectoid system, with eutectoid point at around x=0.6 and 200°C. Kinetics of mixing and unmixing transformations are reported, including the hysteresis between heating and cooling. The enthalpy of this transition is at least 700 J/mol

Highly Reversible Lithium Storage in Nanostructured Silicon

Anode materials of nanostructured silicon have been prepared by physical vapor deposition and characterized using electrochemical methods. The electrodes were prepared in thin-film form as nanocrystalline particles (12 nm mean diameter) and as continuous amorphous thin films (100 nm thick). The nanocrystalline silicon exhibited specific capacities of around 1100 mAh/g with a 50% capacity retention after 50 cycles. The amorphous thin-film electrodes exhibited initial capacities of 3500 mAh/g with a stable capacity of 2000 mAh/g over 50 cycles. We suggest that the nanoscale dimensions of the silicon circumvents conventional mechanisms of mechanical deterioration, permitting good cycle life

Nanocrystalline and Thin Film Germanium Electrodes with High Lithium Capacity and High Rate Capabilities

Germanium nanocrystals (12 nm mean diam) and amorphous thin films (60-250 nm thick) were prepared as anodes for lithium secondary cells. Amorphous thin film electrodes prepared on planar nickel substrates showed stable capacities of 1700 mAh/g over 60 cycles. Germanium nanocrystals showed reversible gravimetric capacities of up to 1400 mAh/g with 60% capacity retention after 50 cycles. Both electrodes were found to be crystalline in the fully lithiated state. The enhanced capacity, rate capability (1000C), and cycle life of nanophase germanium over bulk crystalline germanium is attributed to the high surface area and short diffusion lengths of the active material and the absence of defects in nanophase materials

Electron Diffraction Studies of LiNi_(1/3)Mn_(1/3)Co_(1/3)O_2 After Charge–Discharge Cycling

The performance of LiNi_(1/3)Mn_(1/3)Co_(1/3)O_2 depends largely on the distribution of transition-metal TM ions over the available lattice sites within the layered rocksalt structure. Despite predictions that the TM ions take on an ordered arrangement described by a √3 X √3R30° supercell, little experimental evidence is available to confirm this model. However, the observed cation distribution likely depends on synthesis conditions. Here, we present a study of commercially produced LiNi_(1/3)Mn_(1/3)Co_(1/3)O_2 synthesized at 1000°C before and after charge–discharge cycling. Electron diffraction shows that in-plane ordering is observed to small extents. After charge–discharge cycling, a transformation toward the cubic spinel structure and significant morphology changes are observed

TiO₂ Nanoparticles Prepared by Sol-Gel Method for Anode Application in Lithium-Ion Batteries

TiO 2 nanoparticles are prepared via sol-gel method using titanium tetraisopropoxide (TTIP) as a precursor under refluxing and controlled pH. It is found that pure anatase phase is obtained with pH 10. Further characterization studies are carried out on pure nanoparticle anatase phase by XRD, SEM, and transmission electron microscope (TEM). Their electrochemical performances as anode material in lithium-ion batteries are investigated by cyclic voltammetry, galvanostatic cycling, and rate capability measurements. A high discharge capacity of 321 mAh/g (vs. 335 mAh/g theoretical) is achieved at 1C rate. After the first galvanostatic charge/discharge cycle, voltage profiles show plateaus at 1.75 and 1.95 V versus Li at discharge and charge, respectively. High Coulombic efficiency (>99%) is maintained after 300 cycles, which makes anatase TiO 2 nanoparticles prepared by sol-gel method, a very promising material for anode application in lithium rechargeable batteries