Graphite and pregraphitic carbons capable of reversibly reacting with lithium ions are hosts commonly used in Li-ion cells. As a continuation of previous work, we have used chemical vapour deposition of benzene and silicon-containing precursors to prepare carbons containing nanodispersed silicon. The silicon resides within the unorganized regions in the pregraphitic carbons. These materials reversibly react with lithium in electrochemical cells and the reversible specific capacity has been known to increase from -300 mAhg−1, in the absence of silicon, to near 500 mAhg−1 as silicon is added. We also report on Si-O-C materials which have been shown to reversibly react with Li in electrochemical cells with reversible specific capacities as high as 770mAhg−1. These materials have been made by thermal pyrolysis of siloxane polymers and epoxy-silane composites prepared from hardened mixtures of epoxy novolac resin and epoxy-functional silane. These materials all show promise for use as anode materials in advanced rechargeable lithium batteries.

Found inherent in the submicron-diameter vapor-grown carbon filament fabrication process was a tarry residue, which comprised polyaromatic hydrocarbons. Cyclic voltammetry conducted using carbon working electrodes and an iron cyanide electrolyte showed that the residue harmed the electrochemical performance. Removal of the residue from the filaments using a solvent resulted in increases in the electron transfer rate (to values as high as 0.2 cm/s) and reversibility of the iron cyanide redox species, increase in the packing density and decrease in the filament-filament contact electrical resistivity.

High-capacity carbon electrodes for rechageable lithium-ion batteries were prepared by carbonization of thermosetting resins such as phenol-formaldehyde at temperatures between 500°C and 600°C. Their structures were characterized by high resolution transmission electron microscopy, in-situ transmission electron microscopy and x-ray diffraction analysis. These studies suggest that the carbons consist predominantly of disorganized (amorphous) phase. However evidence was found in carbon containing nickel cobalt oxide for the presence of organized graphite-like regions of parallel and curved layer planes. These graphitized structure usually appear as agglomerate particles which are composed of many smaller (100-nm diameter) particles. The high degree of graphitization is attributed to catalytic graphitization that occurs in the presence of the metal oxide.

Carbons derived from the phase separation of polyacrylonitrile/solvent mixtures were investigated as lithium intercalation anodes for rechargeable lithium-ion batteries. The carbon electrodes have a bulk density of 0.35-0.5 g/cm3, relatively low surface areas (< 10 m2/g), and micron-size cells. Pyrolysis temperature influences the reversible lithium intercalation and the irreversible capacity (associated with the formation of the passivating layer). Carbon electrodes pyrolyzed at 600°C have first-cycle capacity as high as 550 mAh/g as well as large irreversible capacity, 440 mAh/g. Electrodes prepared at 1050°C have reversible capacities around 270 mAh/g with relatively lower capacity losses (120 mAh/g). Doping the organic precursors with phosphoric acid, prior to pyrolysis at 1050 C, leads to carbon electrodes with reversible capacities as high as 450 mAh/g. The capacity of doped carbon increased with increasing phosphorus concentration in the samples. The doped carbon anodes exhibited good cycleability and excellent coulombic efficiency. The electrochemical performance is related to morphology, chemical composition, and local structural order.

Electrochemically polymerized polymers from pyrrole, fluoranthene and pyrene were heat-treated at temperatures between 600 to 3000 °C. The electrochemical properties of these heat-treated samples were examined by chronoamperometry using an electrolyte solution of IM LiClO4 in propylene carbonate. Lithium doping capacities of carbons from fluoranthene and pyrrole polymer heat-treated at 3000 °C were 1.5 and 1.2 times that of the theoretical capacity of graphite as LiC6, respectively.

A nongraphitizable carbon prepared from the cross-linked petroleum pitch and carbonized at 1473 K was found to have a unique structure and a charge capacity of more than 600 Ah/kg. A main peak of the 7Li Nuclear Magnetic Resonance spectra of the charged carbon shifted downfield with an increase of charge capacity. A Knight shift of lithium in the carbon charged to 600 Ah/kg reached 110 ppm when LiCl was used as the reference of 0 ppm. This shift was clearly distinguished from that of the lithium state in the first stage of the graphite intercalation compound, because the latter was observed at 45 ppm. A modified extended Hükel molecular orbital calculation showed that the average net electron density on lithium atoms drastically increased with increasing concentration of lithium atoms if the aromatic molecular planes are more than 0.5 nm apart. Both the experimental and theoretical results suggest that lithium atoms form clusters in this nongraphitizable carbon.

Carbon fibers prepared at 1000 °C from mesophase pitch were treated with trimethyl silazane for modifying surface -OH group into trimethyl silyl group. The irreversible reduction current on the first cathodic sweep was diminished. Low-temperature carbon fibers prepared at 700°C and active carbon fibers were evaluated for electrochemical lithium doping and undoping reaction in propylenecarbonate(PC)/ethylene carbonate or PC/dimethyl carbonate mixtures(l:l) containing 1 M LiClO4 at room temperature.

Pristine samples prepared at 700 °C gave very poor reversibility and capacity on cyclic voltammograms(cv), but a constant current charge/discharge with a very low current gave a capacity over 400 mAh/g. After prolonged heating at 700°C, the cv characteristics were gradually improved. Partial oxidation in a mild oxidizing atmosphere at 700 °C for a few minutes was found to be quite effective in improving the cv characteristics remarkably. The charge/discharge capacity, however, was very similar to that obtained for the pristine sample at a very low consatant current. This indicates that the heat treatments effectively increase the electrochemical reaction rate of lithium doping and undoping, but have no effect on the intrinsic capacity.

Active carbon fibers were found to have considerable capacity after repeated oxidation/reduction cycles in solution. Partial oxidation treatment was also effective in increasing the charge/discharge current. The capacity after the treatment was over 400mAh/g, which is far larger than that of the double layer capacity of the sample.

The improvement due to the heat treatment is elucidated based on the removal of a surface inhibiting layer through the treatment.

Mesophase pitch-based Carbon Fibers(MPCF) have been investigated as anode materials for lithium secondary cells by examining their physical and electrochemical properties. Discharge capacity and initial charge-discharge efficiency of the materials were studied in relation to the heat treatment temperatures of MPCF. Carbon fiber which was heat treated at about 3,000’C gave the highest discharge capacity(over 300mAh/g), good efficiency (92%) and superior current capability (600mA/g). Carbon fiber heat treated at less than 1,000·C, also has superior discharge capacity(over 500mAh/g) at the first cycle, however efficiency was relatively low. Some of the relationships between structure of MPCF and electrochemical properties are discussed below.

Submicron carbon filaments used in place of carbon black as porous reduction electrodes in carbon limited lithium batteries in plate and jellyroll configurations with the BCX (bromine chloride in thionyl chloride) catholyte gave a specific capacity (at 2 V cut-off) of up to 8700 mAh/g carbon, compared to a value of up to 2900 mAh/g carbon for carbon black. The high specific capacity per g carbon (demonstrating superior carbon efficiency) for the filament electrode is partly due to the filaments' processability into sheets as thin as 0.2 mm with good porosity and without a binder, and partly due to the high catholyte absorptivity and high rate of catholyte absorption of the filament electrode.

Electrochemical capacitors (ultracapacitors) are one approach to meeting the high power requirements for the energy storage system in an electric vehicle. Energy is stored in an electrochemical capacitor by charge separation in the double layer formed in the micropores of a very high surface area electrode material, which does not undergo chemical change as in a battery. Consequently, the material requirements for capacitors are very different from those of batteries. In the last several years, a number of promising material technologies have been identified for use in electrochemical capacitors. These include activated carbon fibers, foams, and composites, doped conducting polymers, and mixed metal oxides. The most important material property is its specific capacitance (F/gm or F/cm3). Carbon materials with specific capacitances of 100 to 300 F/gm have been developed. Doped polymer materials having specific capacitances of 300 to 400 F/gm are also being studied. In addition to high specific capacitance, the electrode material must also have a low electronic resistivity ( < 0.1 Ω-cm) in order that charge can be distributed with minimum voltage drop in the electrode. Electrochemical capacitor cells have been fabricated using the various material technologies with both aqueous and organic electrolytes. Tests of the cells have shown near ideal charge/discharge characteristics — that is, the voltage versus time curves are nearly linear for constant current tests. The energy densities of 1 V cells, using aqueous electrolytes, are 1 to 1.5 W-h/kg and those of 3 V cells, using organic electrolytes are 7 to 10 W-h/kg. Most of the cells have high power densities of 1 to 3 kW/kg. Numerous new materials for electrochemical capacitors have been identified, processed, and tested in electrodes and cells in recent years and progress is rapid in this relatively new field of research.

The ability to tailor the structure and properties of porous carbons has led to their increased use as electrodes in energy storage devices. Our research focuses on the synthesis and characterization of carbon aerogels for use in electrochemical double layer capacitors. Carbon aerogels are formed from the sol-gel polymerization of (1) resorcinol-formaldehyde or (2) phenolic-furfural, followed by supercritical drying from carbon dioxide, and subsequent pyrolysis in an inert atmosphere. These materials can be produced as monoliths, composites, thin films, powders, or microspheres. In all cases, the aerogels have an open-cell structure with an ultrafine pore size (<100 nm), high surface area (400-1100 m2/g), and a solid matrix composed of interconnected particles, fibers, or platelets with characteristic dimensions of 10 nm. This paper examines the effects of the carbon precursor and processing conditions on electrochemical performance in aqueous and organic electrolytes.

Several different mechanisms can be used to provide short term high power pulses from electrochemical systems. Their fundamental characteristics and applicability to the different types of transient output requirements will be discussed. When significant amounts of energy must be delivered, the most favorable materials operate by means of insertion reactions. The methods that can be employed to quantitatively evaluate the critical materials parameters will be described. Spreadsheet techniques are especially useful for modelling the transient transport behavior of components containing such materials under various boundary conditions. LaPlace transform methods can then be used to convert information about the parameters of individual components into the dynamic response of the total system.

NiO/Ni composite thin films consisting of nano-sized particles have been found to perform as good electrodes in electrochemical capacitor applications. These films can provide a specific capacitance of 25-40 F/g. The low cost of raw materials and easy manufacturing process of this system should allow one to produce low-cost electrochemical capacitors.

Ruthenium oxide thin films of an amorphous phase were successfully prepared on a titanium (Ti) substrate at temperatures below 160 °C. The sol-gel process using metal alkoxide precursor in nonaqueous solvents was used to prepare these films. The preliminary results showed that a specific capacitance of 430 F/g can be achieved for amorphous ruthenium oxide electrode in sulfuric acid. Films prepared by this method are compared with the films prepared by the thermal decomposition of the aqueous ruthenium chloride solution at temperatures above 300 °C. The specific capacitance, the crystalline structure, and the surface morphology of these films as a function of the preparation temperature were also discussed.

The hydrous ruthenium oxide (RuO2.xH2O) powders with different contents of H20 have been formed by a sol-gel process and have been annealed at different temperatures. It was found that the specific capacitance of RuO2.xH2O is strongly dependent on the crystalline structure. The specific capacitance of over 720 F/g was obtained in an amorphous RuO2.xH2O. This value is at least two times greater than ever reported of this type of material.The specific capacitance dropped rapidly as the amorphous phase transforms into the crystalline phase at high temperatures. In contrast to the crystalline phase the specific capacitance of an amorphous RuO2.xH2O was not dependent on the specific surface area. The charge storage mechanism in amorphous RuO2.xH2O is different from that in the crystalline RuO2. The current in the cyclic voltammetric curve of RuO2.xH2O is nearly constant and is symmetrical with the potential in a range of 1 volts. The resistivity of pellets made from these powders is in the order of 10−3 Ω-cm.