Carbon Materials and the Surface Modification in View of Electrochemical Li Insertion/Extraction

Carbon is an old, and at the same time, a new material. Carbon plays the leading part in the practical design of Li-ion secondary batteries, the most advanced secondary batteries, where as the negative electrode material it takes Li into the internal structure during the charging. This guarantees the reliability and safety by preventing the dangerous deposition of Li metal. In responding to a strong demand of the high power capability, the author has developed several surface modification methods. This article describes the surface modification methods I have developed, together with several new findings in relation to the behavior of Li in the interior and on the surface of carbon. The following items are described and analyzed: (1) electrochemical reaction rate in relation to the battery reaction could be enhanced by surface modification such as (a) a simple heating in vacuum, (b) mild oxidation, and (c) vacuum deposition of a metal film on the carbon surface with such metals as Ag, Au, Bi, Cu, In, Pb, Pd, Sn, and Zn together with the oxidized ones. The cause of the enhancement was studied on the basis of SEI (solid-electrolyte-interphase); (2) use of a single fiber electrode enabled us to find what really occurs on the electrode surface, to determine the most reliable values of the Li diffusion coefficient in carbon; and to prevent decomposition of propylene carbonate on graphite; (3) electrical contact was demonstrated as a key factor to reduce the initial charging irreversibility and to enhance the cycleability; (4) underpotential deposition of Li on the carbon surface was pointed out; and (5) free movement of Li in metal at an ambient temperature was confirmed by the use of a bipolar cell. In the final remarks I offer a new term of “carbon alloys” based on the nano-technology, which will be an attractive material both in science and technology in the new century.