Graphite electronic conductivity

Graphite has an electron conductivity of about 200 to 700 d cm is relatively cheap, and forms gaseous anodic reaction products. The material is, however, mechanically weak and can only be loaded by low current densities for economical material consumption. Material consumption for graphite anodes initially decreases with increased loading [4, 5] and in soil amounts to about 1 to 1.5 kg A a at current densities of 20 A m (see Fig. 7-1). The consumption of graphite is less in seawater than in fresh water or brackish water because in this case the graphite carbon does not react with oxygen as in Eq. (7-1),... 

Perhaps the first practical application of carbonaceous materials in batteries was demonstrated in 1868 by Georges Le-clanche in cells that bear his name [20]. Coarsely ground MnO, was mixed with an equal volume of retort carbon to form the positive electrode. Carbonaceous powdered materials such as acetylene black and graphite are commonly used to enhance the conductivity of electrodes in alkaline batteries. The particle morphology plays a significant role, particularly when carbon blacks are used in batteries as an electrode additive to enhance the electronic conductivity. One of the most common carbon blacks which is used as an additive to enhance the electronic conductivity of electrodes that contain metal oxides is acetylene black. A detailed discussion on the desirable properties of acetylene black in Leclanche cells is provided by Bregazzi [21], A suitable carbon for this application should have characteristics that include (i) low resistivity in the presence of the electrolyte and active electrode material, (ii) absorption and retention of a significant... 

Nonmetal electrodes are most often fabricated by pressing or rolling of the solid in the form of fine powder. For mechanical integrity of the electrodes, binders are added to the active mass. For higher electronic conductivity of the electrode and a better current distribution, conducting fillers are added (carbon black, graphite, metal powders). Electrodes of this type are porous and have a relatively high specific surface area. The porosity facilitates access of dissolved reactants (H+ or OH ions and others) to the inner electrode layers. 

Carbon. The electronically conductive carbons are derived from the hexagonal crystalline modification—graphite. 

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

The idea of electronic conductivity in the crystals of this cluster is stimulated by the metallic reflectance of the crystals. A potential conductivity is expected to be anisotropic because of the anisotropic order of the clusters inside the crystal. As a consequence, the electric resistance is expected to be smaller in the direction of the tubes than in the vertical direction where there is no graphite-like bridging between the clusters. 

The future remains bright for the use of carbon materials in batteries. In the past several years, several new carbon materials have appeared mesophase pitch fibers, expanded graphite and carbon nanotubes. New electrolyte additives for Li-Ion permit the use of low cost PC based electrolytes with natural graphite anodes. Carbon nanotubes are attractive new materials and it appears that they will be available in quantity in the near future. They have a high ratio of the base plane to edge plain found in HOPG. The ultracapacitor application to deposit an electronically conductive polymer on the surface of a carbon nanotube may be the wave of the future. 

Graphite has a layer structure and each layer can be regarded as a network of fused benzene rings. The delocalised electrons extend over the whole layer and allow graphite to conduct electricity. The benzene molecule also contains delocalised electrons and this would imply that individual molecules would conduct electricity. However, a collection of benzene molecules, such as in a beaker of benzene liquid, does not conduct. This is because the delocalised electrons are confined to the individual benzene molecules and cannot jump from one molecule to another. 

The potassium donates an electron to the graphite (forming and the conductivity of the graphite increases. Graphite electron-acceptor intercalation compounds have also been made with NO3T CrOs, Br2, FeCls, and ASF5. Some of these compounds have electrical conductivity approaching that of aluminum (see Chapter 6). 

The cathode mix is a compressed mixture of electrolytic Mn02 (EMD) and synthetic graphite or acetylene black to provide electronic conductivity, in a ratio of 4—5 1, wetted with electrolyte. The cathode current collector is generally the external steel can, which may be nickel-plated or coated with conductive carbon. Reduction of Mn02 in alkaline conditions is a complex process and follows a number of steps which can be written formally as... 

The active material used in pocket plate cells consists of Ni(OH)2 together with up to 5% of Co(OH)2, Ba(OH)2, etc. to improve cell capacity and cycle life, and 20% of graphite in various forms to increase the electronic conductivity. The nickel hydroxide is precipitated from nickel sulphate in a controlled manner to produce fine particles of large surface area. As in the case of the cadmium electrode, the nickel hydroxide powder may be formed into pellets before insertion into the pockets. 

In pocket plate cells, the active materials are a mixture of finely powdered metallic iron and Fe304. The preparation of this mixture varies from manufacturer to manufacturer, but generally involves a final process in which controlled air oxidation of iron powder or reduction of Fe304 with hydrogen is used to form the appropriate composition. Additives such as cadmium, cadmium oxide or graphite are commonly included to improve the capacity retention and electronic conductance. The performance of the electrode is improved by the addition of up to 0.5% of FeS the mechanism of the sulphide involvement is not well understood. If sulphide is lost by oxidation after prolonged use, small amounts of soluble sulphide may be added to the electrolyte,... 

Optimum behaviour was obtained with a cathode mixture comprising the intercalation host with electrolyte and graphite which, respectively, reduced contact polarization and enhanced the electronic conductance. 

Because the bisulfate anions occupy spaces between the planes of the graphite layers, they will separate the layers more than they are separated in pure graphite. The conduction between layers will be reduced. The conduction within the layers, however is enhanced because the partial oxidation produces some empty spaces in the delocalized molecular orbitals, which extend over the graphite sheet. The presence of these empty spaces makes it easier for the electrons to migrate through the solid along the planar carbon networks. 

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