The boron secret from the 2000 patent:
"It is possible, when the content of boron is confined to the aforementioned range, to enhance the lithium ion absorption and desorption potential of the negative electrode, so that it is now possible to inhibit lithium metal from being precipitated on the carbonaceous material at the occasion of quick charging or at the occasion of charging/discharging under a low temperature (0.degree. C. or less) environment. Further, since the crystallinity of graphite can be improved, it is possible to improve the rate of lithium ion absorption and desorption in the negative electrode, and hence to improve the discharge capacity of the negative electrode. If the content of boron is less than 1% by weight, it would become difficult to sufficiently enhance not only the lithium ion absorption and desorp! tion potential but also the crystallinity of graphite. On the other hand, if the content of boron exceeds over 10% by weight, a B.sub.4 C compound would be generated in the carbonaceous material. Since this B.sub.4 C compound is incapable of taking part in the absorption and desorption of lithium ion, the capacity of the negative electrode would be deteriorated. As the content of boron in the carbonaceous material becomes larger, the content of boron compound tends to become larger. The content of boron should preferably be in the range of from 1 to 7% by weight, more preferably from 1 to 5% by weight, still more preferably from 1 to 3% by weight, most preferably from 1.5 to 2.5% by weight. "
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