Carbon-containing negative electrodes

A renewed interest in noncarbonaceous lithium alloy electrodes arose recently as the result of the announcement by Fuji Photo Film Co. of the development of a new generation of lithium batteries based upon the use of an amorphous tin-based composite oxide in the negative electrode [13]. It is claimed that these electrodes have a volumetric capacity of 3200, AhL" which is four times that commonly achieved with carbonaceous negative electrodes, and a specific capacity of 800, mAhg" twice that generally found in carbon-containing negative electrodes. 

The first zinc-carbon cell made in 1876 by the French engineer G.-L. Leclanchd was a glass jar containing an aqueous solution of ammonium chloride into which were immersed an amalgamated zinc rod (the negative electrode) and a porous... 

For manufacturing of negative electrodes, suspensions containing 45wt% powder of the carbon material being examined, 5wt% PVDF, and 50wt% of the solvent. Copper foil with thickness of 0,02 mm was applied as substrate. 

An electrochemical cell generally consists of two half-cells, each containing an electrode in contact with an electrolyte. The electrode is an electronic conductor (such as a metal or carbon) or a semiconductor. Current flows through the electrodes via the movement of electrons. An electrolyte is a phase in which charge is carried by ions. For example, a solution of table salt (sodium chloride, NaCl) in water is an electrolyte containing sodium cations (Na+) and chloride anions (CE). When an electric field is applied across this solution, the ions move Na+ toward the negative side of the field and CE toward the positive side. 

Owens and Iqbal [146] succeeded in an electrochemical hydrogenation of open-ended SWCNTs synthesized by CVD. Sheets of SWCNT bucky paper were used as the negative electrode in an electrochemical cell containing aqueous KOH solution as electrolyte. The authors claimed to have incorporated up to 6 wt. % of hydrogen into the tubes, determined by laser Raman IR spectroscopy and hydrogen release by thermolysis at 135 °C under TGA conditions [146], However, the stability of exohydrogenated carbon nanotubes and the low temperature of hydrogen release at 135 °C [146] is contradictory with the 400-500 °C reported elsewhere [79a, 145],... 

FIGURE 7.8 SEM pictures of a LiCo02 positive electrode (left) and of a surface-treated graphite negative electrode (right) both containing TIMREX KS6 graphite and SUPER P Li carbon black conductive additives. 

Materials obtained by pyrolysis of pitch-polysilane blends have been extensively studied as carbon materials containing Si [157-161], For some of these materials, ca. 600mAh/g of Crev for Li insertion, as well as small irreversible capacities and small hysteresis effects, were reported. It has been shown that the materials contain nanodispersion of Si-O-C and Si-O-S-C instead of nanodispersed Si particles [162-165], Furthermore, the oxygen and sulfur contents are proved to be correlated to the irreversible capacity. There is a report about the fabrication of porous Si negative electrodes with 1-D channels, where the usefulness of the fabricated negative electrodes for rechargeable microbatteries is also suggested [166],... 

Nonaqueous electrolyte solutions can be reduced at negative electrodes, because of an extremely low electrode potential of lithium intercalated carbon material. The reduction products have been identified with various kinds of analytical methods. Table 3 shows several products that detected by in situ or ex situ spectroscopic analyses [16-29]. Most of products are organic compounds derived from solvents used for nonaqueous electrolytes. In some cases, LiF is observed as a reduction product. It is produced from a direct reduction of anions or chemical reactions of HF on anode materials. Here, HF is sometimes present as a contaminant in nonaqueous solutions containing nonmetal fluorides. Such HF would be produced due to instability of anions. A direct reduction of anions with anode materials is a possible scheme for formation of LiF, but anode materials are usually covered with a surface film that prevents a direct contact of anode materials with nonaqueous electrolytes. Therefore, LiF formation is due to chemical reactions with HF [19]. Where does HF come from Originally, there is no HF in nonaqueous electrolyte solutions. HF can be produced by decomposition of fluorides. For example, HF can be formed in nonaqueous electrolyte solutions by decomposition of PF6 ions through the reactions with H20 [19,30]. 

The Electrolysis of Molten Sodium Chloride. Molten sodium chloride (the salt melts at 801° C) conducts an electric current, as do other molten salts. During the process of conducting the current a chemical reaction occurs—the salt is decomposed. If two electrodes (carbon rods) are dipped into a crucible containing molten sodium chloride and an electric potential (from a battery or generator) is applied, metallic sodium is produced at the negative electrode—the cathode—and chlorine gas at the positive electrode—the anode. Such electrical decomposition of a substance is called electrolysis. 

F yrolysis of gaseous hydrocarbons at 1000-1700 °C is a common route (cf. Nos. 6 and 7 in Table 9, where two examples involving benzene are considered [441, 442]). The substrate was nickel, and dense black layers were obtained to serve as a host lattice for the lithium negative electrode. The pyrolytic carbon from benzene at 1000 °C gave a lithium GIC (CeLi) and could be cycled at 99% current efficiency [407]. Pyrolysis of epoxy Novolac resin and epoxy-functionalized silane gave a material containing silicon with a capacity of 770 mAh/g for the lithiated form [443]. 

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