Method of producing silicon oxide, negative electrode active material for lithium ion secondary battery and lithium ion secondary battery using the same

Abstract

A method for producing a silicon oxide including the steps of supplying silicon atoms onto a substrate through an oxygen atmosphere to form a silicon oxide layer on the substrate, and separating the silicon oxide layer from the substrate and pulverizing the separated silicon oxide layer to obtain silicon oxide containing silicon and oxygen in predetermined proportions, and a negative electrode active material obtained by the production method.

Description

FIELD OF THE INVENTION[0001]The present invention relates mainly to a lithium ion secondary battery and, more particularly, to a negative electrode active material for a lithium ion secondary battery and a method of producing the material.BACKGROUND OF THE INVENTION[0002]Attention has been given to lithium ion secondary batteries as a power source for driving electronic equipment. For example, graphite materials have an average potential of about 0.2 V (vs. Li / Li+) during desorption of lithium and, therefore, high-voltage lithium ion secondary batteries can be obtained by using graphite materials as a negative electrode active material. Further, graphite materials have a comparatively flat potential characteristic with respect to time during desorption of lithium. For these reasons, a lithium ion secondary battery containing a graphite material as a negative electrode active material is favorably used as a power source for a device which needs to have a high voltage and a flat volta...

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Problems solved by technology

Graphite materials, however, have a small capacity per unit mass of 372 mAh / g, and a further increase in capacity cannot be expected.

However, the crystal structure of each of such materials is changed when the material absorbs lithium, resulting in a change in volume of the material.

But the molar ratio x of oxygen cannot be reduced to 1 or less.

Japanese Patent Laid-Open No. 6-325765 also discloses a method in which SiO2 is reduced by being mixed with carbon or a predetermined metal to control the oxygen ratio x. However, it is difficult to reduce SiO2 so as to obtain the desired uniformity in oxygen ratio x. Therefore, SiOx cannot be obtained with a constant distribution of the oxygen ratio x. If the oxygen ratio x varies among different electrode plate portions, the amount of absorption of Li and the expansion coefficient when Li is absorbed vary, resulting in nonuniformity of the charge / discharge reaction in the electrode plate and deformation of the electrode plate.

Such an impurity has lower reactivity with lithium in comparison with SiOx and therefore reduces the capacity of the negative electrode.

Therefore, SiOx and an unoxidized Si portion coexist in each particle and it is not possible to form SiOx particles having a uniform oxygen distribution.

Thus, none of the production methods disclosed in Japanese Patent Laid-Open No. 6-325765 makes it possible to produce a high-purity silicon oxide.

The method disclosed in Japanese Patent Laid-Open No. 2002-260651 enables production of SiOx controlled so that the oxygen ratio x is 1.05 to 1.5, but does not enable the oxygen ratio x to be reduced to 1 or less.

For these reasons, the negative electrode described in Japanese Patent Laid-Open No. 2002-260651 is incapable of utilizing the characteristics of high-capacity silicon and obtaining the expected capacity.

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