Silicon oxide containing active silicon and its evaluation

#This invention relates to silicon oxide containing active silicon and a method for evaluating the same.

#Silicon oxide is well known in the art and generally used as a raw material for the manufacture of ceramics, a raw material for the manufacture of organosilicon compounds, and a negative electrode active material in lithium ion secondary batteries. By taking advantage of its chemical activity, silicon oxide is used in the synthesis of industrially useful alkylhalosilanes (Gary N. Bokerman et al., U.S. Pat. No. 5,051,247) and the direct synthesis of siloxanes (Peter L. Timms and William N. Rowlands, EPA 0406000A2). It is reported in E. Fuglein and U. Schubert, Chem. Mater., 1999, 11, 865-866 that silicon oxide is reacted with magnesium at low temperature to produce magnesium silicide. JP-A 9-7638 discloses that using SiOx as a negative electrode active material in lithium ion secondary batteries facilitates occlusion and release of lithium ions. The use of silicon oxide as the lithium ion secondary battery negative electrode active material is expected to expand in the near future.

#A need exists for silicon oxide finding effective use in these applications.

#FIG. 1 is a diagram showing solid state NMR spectra of silicon oxide according to the invention and a SiO2/Si mixture in a comparative example.

#FIG. 2 is a diagram showing solid state NMR spectra of Examples 1 and 2 and Comparative Example 1.

#FIG. 3 is a diagram showing solid state NMR spectra of Example 1 and Comparative Examples 2 to 4.

#On analysis by solid state NMR (29Si DD/MAS) of the silicon oxide containing active silicon according to the invention, the measured spectrum has two definitely separate peaks, a broad peak (A1) centered at −70 ppm and a broad peak (A2) centered at −110 ppm. The area ratio A1/A2 should fall in the range of 0.1≦A1/A2≦1.0, preferably 0.15≦A1/A2≦0.9, and more preferably 0.2≦A1/A2≦0.8. In general, on analysis of the relevant inorganic compound by solid state NMR, a long relaxation time must be set. The NMR analysis herein prefers to set a relaxation time of at least 100 seconds, and more preferably at least 500 seconds. The upper limit of relaxation time is not critical although it is usually 3,600 seconds or less. A typical spectrum of active silicon-containing silicon oxide as measured under such conditions is shown in FIG. 1 together with a spectrum of a comparative mixture of amorphous silicon dioxide and crystalline metal silicon in a molar ratio of 1/1. It is appreciated from the chemical shift of each peak and the measured oxygen content that the broad peak centered at −110 ppm is assigned to tetravalent silicon (SiO2) and the broad peak centered at −70 ppm is assigned to atomic or zero-valent silicon. When zero-valent silicon is not fully dispersed in the atomic state, but exists in blocks, a broad peak appears in proximity to −85 ppm. As crystallization proceeds, a sharp spectral axis appears in proximity to −84 ppm. On x-ray diffraction analysis of such a sample, the diffraction lines of crystalline silicon appear.

#It is noted that the area of the broad peak centered at −70 ppm is approximately the area between −50 ppm and −80 ppm, and the area of the broad peak centered at −110 ppm is approximately the area between −90 ppm and −130 ppm.

#The active silicon-containing silicon oxide according to the invention is represented by the general formula: SiOx wherein x is from 0.8 to 1.9. If x is less than 0.8, undesirably metallic silicon becomes substantially excessive so that the silicon oxide becomes crystalline and/or block-like and lacks active silicon. If x is more than 1.9, undesirably the silicon oxide becomes substantially silicon dioxide and lacks active silicon. Preferably x is from 0.8 to 1.6, and more preferably from 0.9 to 1.3.