Dow Corning Toray Co Ltd exhibited a silicon-containing carbon material for negative electrode of lithium-ion secondary batteries at Automotive Engineering Exposition 2009.
It is a Si-O-C composite material produced by burning a Si-containing polymer material. Unlike the existing method to mix Si alloy with a carbon material, which has been studied thus far, the new production method is suitable to finely disperse Si particles, the company said.
The prototyped material has a current capacity of 650-800mAh/g, which is more than double that of graphite, a commonly-used material. In addition, it has a stable charge/discharge cycle performance because its expansion/contraction caused by charging/discharging is as small as that of graphite. For those characteristics, Dow Corning Toray expects that the material will be used for next-generation automotive Li-ion secondary batteries.
However, the material is faced with a problem that its discharge capacity in the first cycle is rather small with respect to the charge capacity. This means that lithium stored in the negative electrode during the first charging process is not adequately released to the positive electrode during discharging.
The ratio of discharge capacity to charge capacity is called "coulombic efficiency." The initial coulombic efficiency of the prototype is only in the 70-80% range, according to the company. When the coulombic efficiency is not high enough, a positive electrode material containing lithium must be excessively packed during the cell production, or lithium ions must be doped.
In order to improve the initial coulombic efficiency, Dow Corning Toray intends to promote research on the polymer composition, burning conditions, etc in the future.
The company delivered a lecture on the new negative-electrode material under the title of "Battery Characteristics of Si-O-C Composite Material for Lithium-ion Secondary Batteries" at the 49th Battery Symposium in Japan in November 2008.