Li-containing silicon oxide powder and production method thereof

Abstract

There is produced a Li-containing silicon oxide powder containing a crystallized lithium silicate that is mostly water-insoluble Li.sub.2Si.sub.2O.sub.5 and containing little crystalline Si. This object is attained through the mixing of a lower silicon oxide powder represented by a compositional formula SiO.sub.x (0.5<x<1.5) with a powdered lithium source that involves grinding of the powdered lithium source; controlling a median diameter D1 of the lower silicon oxide powder and a median diameter D2 of the powdered lithium source so as to fulfill 0.05D2/D12; and calcining the mixed powder at 300 C. or higher and 800 C. or lower.

Claims

1. A method for producing a Li-containing silicon oxide powder for use in a negative electrode material of a lithium ion secondary battery, the method comprising: a step of mixing a lower silicon oxide powder represented by a compositional formula SiO.sub.x (0.5<x<1.5) with a powdered lithium source, and a step of calcining the mixed powder at 300 C. or higher and 800 C. or lower, wherein a median diameter D1 of the lower silicon oxide powder and a median diameter D2 of the powdered lithium source fulfill Requirement (2): 0.05D2/D12.

2. The method for producing a Li-containing silicon oxide powder according to claim 1, comprising grinding the powdered lithium source to be mixed with the lower silicon oxide powder for the fulfillment of Requirement (2).

3. The method for producing a Li-containing silicon oxide powder according to claim 1, wherein a mixing ratio of the powdered lithium source to the lower silicon oxide powder at the mixing step is 0.2Li/O0.6, when the mixing ratio is expressed in terms of an elemental ratio Li in the powdered lithium source relative to 0 in the lower silicon oxide powder.

4. The method for producing a Li-containing silicon oxide powder according to claim 1, comprising allowing the silicon oxide powder to be subjected to the mixing step to undergo a carbon coating treatment that gives a conductive carbon film.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 schematically shows powder particles, shown in (a), produced according to conventional methods, which are compared with powder particles, shown in (b), produced according to the method of the present invention for producing a Li-containing silicon oxide powder.

(2) FIG. 2 shows an X-ray diffraction chart of a Li-containing silicon oxide powder produced according to the present invention.

(3) FIG. 3 shows an X-ray diffraction chart of a conventional Li-containing silicon oxide powder.

(4) FIG. 4 shows an X-ray diffraction chart of silicon oxide powder not doped with Li.

DESCRIPTION OF EMBODIMENTS

(5) Embodiments of the present invention will be described hereinafter. The method according to embodiments of the present invention for producing a Li-containing silicon oxide powder begins with the preparation of a silicon oxide powder serving as a raw material used in the production method, and of a powdered lithium source to be mixed therewith.

(6) The silicon oxide powder serving as a raw material is a lower silicon oxide powder represented by a compositional formula SiO.sub.x (0.5<x<1.5). Used here is an amorphous SiO produced by precipitation method, i.e., the one represented by SiO.sub.x (x=1). The silicon oxide powder has a particle diameter, in terms of median diameter, of 0.5 to 30 m.

(7) Examples of the powdered lithium source include lithium hydride (LiH), lithium oxide (Li.sub.2O), lithium hydroxide (LiOH) and lithium carbonate (Li.sub.2CO.sub.3). Used here is lithium hydride (LiH).

(8) The silicon oxide powder serving as the raw material is subjected to C-coating that gives a carbon film. The C-coating is performed by a thermal CVD method using a hydrocarbon gas, for example, under an atmosphere of a mixed gas of propane and argon, at 850 C. The C-coating amount when expressed in terms of a weight proportion of carbon with respect to a mass of the whole of the silicon oxide powder is 0.5 to 20 wt %.

(9) The powdered lithium source is subjected to grinding treatment. The grinding treatment is done, for example by the use of a mortar, so as to give a controlled particle size after grinding such that the powdered lithium source and the silicon oxide powder provide the median diameter ratio (D2/D1) ranging from 0.05 to 2, preferably from 0.05 to 1.

(10) Subsequently, the C-coated silicon oxide powder is mixed with the grinding-treated powdered lithium source. The mixing ratio is controlled to give a Li/O molar ratio ranging from 0.2 to 0.6 considering, from an equilibrium standpoint, the promotion of the formation of Li.sub.2Si.sub.2O.sub.5 together with the inhibition of the formation of Li.sub.2SiO.sub.3.

(11) Subsequently, the mixed powder of the silicon oxide powder and the powdered lithium source is subjected to its calcining under an inert gas atmosphere. The calcining temperature ranges from 300 to 800 C., and more specifically such a temperature region as to avoid involving the disproportionation of the silicon oxide powder. The calcining allows the silicon oxide powder to undergo the Li-doping, and consequently gives the Li-containing silicon oxide powder.

(12) Here, the mixing ratio of the powdered lithium source to the silicon oxide powder restricted to be in the lower range considered from an equilibrium standpoint to contribute to the inhibition of the formation of Li.sub.2SiO.sub.3, and the powdered lithium source finely ground so that both the powders give the median diameter ratio (D2/D1) ranging from 0.05 to 2, particularly from 0.05 to 1, lead to the inhibition of the local reaction on the particles constituting the silicon oxide powder thereby inhibiting the concentration of lithium, with a result that the lithium silicate phase of the Li-containing silicon oxide is mostly composed of Li.sub.2Si.sub.2O.sub.5, and concurrently the formation of the crystalline Si in the Li-containing silicon oxide is inhibited.

(13) Specifically, the produced Li-containing silicon oxide powder, when observed by X-ray diffractometry using CuK ray, exhibits peaks that include a height of peak attributed to Li.sub.2Si.sub.2O.sub.5, P1, exhibited at a diffraction angle 2 ranging from 24.4 to 25.0; a height of peak attributed to Li.sub.2SiO.sub.3, P2, exhibited at a diffraction angle 2 ranging from 18.6 to 19.2; and a height of peak attributed to crystalline Si, P3, exhibited at a diffraction angle 2 ranging from 27.4 to 29.4, fulfill P2/P1<1 and P3/P1<0.5.

(14) The produced Li-containing silicon oxide powder is used as a negative electrode material of a lithium ion secondary battery. Specifically, the Li-containing silicon oxide powder is mixed with an aqueous binder to give a slurry, and the slurry is applied on a collector composed of e.g., a copper foil, followed by drying, to provide a thin film working electrode. The lithium silicate phase in the Li-containing silicon oxide powder is mostly composed of water-insoluble Li.sub.2Si.sub.2O.sub.5 and hardly contains water-soluble Li.sub.2SiO.sub.3. This configuration prevents the elution of lithium from the silicon oxide, and improves the initial efficiency included in battery performances as intended. Also, the configuration inhibits the silicon oxide from having the crystalline Si, and accordingly inhibits the reduction of the cycle properties included in battery performances. Improvement benefits attained by the inhibition of the formation of Li.sub.2SiO.sub.3, which is Li-rich, are seen not just in the use of an aqueous binder but also in the use of a solvent-based binder composed of an organic solvent as a solvent, since most of such polymer components as polyimides is reactive with lithium.

EXAMPLES

Example 1

(15) A silicon oxide powder serving as a raw material prepared for producing a Li-containing silicon oxide powder was an amorphous SiO powder produced by precipitation method. The SiO powder raw material had a median diameter of 8.0 m. For the SiO powder raw material to undergo the C-coating through a thermal treatment, the SiO powder was thermally-treated at 850 C. for 30 minutes in a furnace into which a carbonizing gas was supplied at a flow rate of 1 liter per minute, the carbonizing gas being given by mixing argon and propane at a weight ratio of 1:1.

(16) Subjecting the C-coated SiO powder to combustion-infrared absorption method revealed that the particles constituting the Si powder had a conductive carbon film formed at a weight ratio of 1.00%. The C-coated SiO powder had a median diameter D1 of 8.2 m.

(17) A powdered lithium source selected for mixing with the raw material SiO was LiH powder, which originally had a median diameter of 20.8 m, considerably larger than the median diameter of the C-coated SiO powder. The LiH powder was thus finely ground with a mortar in a glove box under an argon atmosphere, and the ground powder was classified by using a test sieve with an opening of 16 m.

(18) The finely-ground LiH powder was subjected to a dry-particle size distribution measurement using a laser diffraction-type particle size distribution measuring equipment, HELOS, manufactured by Sympatec GmbH. The finely-ground LiH powder was found to have a median diameter D2 of 5.1 m, which was smaller than the median diameter D1 of the C-coated SiO powder (8.2 m), which gave a median diameter ratio D2/D1 of 0.62.

(19) For Li-doping treatment, the C-coated SiO powder (median diameter 8.2 m) was mixed with the finely-ground LiH powder so as to give a Li/O molar ratio of 0.5, and thereafter the mixture was calcined in an argon atmosphere in a furnace (1 atm, 600 C.) for 72 hours.

(20) In the X-ray diffractometry using CuK ray of the produced Li-containing SiO powder, a height of peak attributed to Li.sub.2Si.sub.2O.sub.5, P1, exhibited at a diffraction angle 2 ranging from 24.4 to 25.0; a height of peak attributed to Li.sub.2SiO.sub.3, P2, exhibited at a diffraction angle 2 ranging from 18.6 to 19.2; and a height of peak attributed to crystalline Si, P3, exhibited at a diffraction angle 2 ranging from 27.4 to 29.4 were examined to calculate P2/P1 and P3/P1.

(21) The SiO powder undergoing the C-coating and the Li-doping was used to fabricate a negative electrode of a lithium ion secondary battery. Specifically, the SiO powder, ketjen black, and a polyimide precursor serving as a non-aqueous solvent-based binder were mixed together at a mass ratio of 85:5:10. To the mixture, NMP (n-methylpyrrolidone) was added, followed by kneading, to give a slurry. The slurry was applied on a copper foil with a thickness of 40 m, and pre-dried at 80 C. for 15 minutes. This was followed by punching so as to give a diameter of 11 mm, and further by an imidizing treatment. In this way, a negative electrode was provided.

(22) The fabricated negative electrode was used to produce a lithium ion secondary battery. A counter electrode used was a lithium foil. An electrolyte used was a solution prepared by dissolving LiPF.sub.6 (lithium hexafluorophosphate) at 1 mol/L in a solution that had been given by mixing ethylene carbonate with diethyl carbonate at a volume ratio of 1:1. A separator used was a polyethylene porous film with a thickness of 30 m. In this way, a coin cell was fabricated.

(23) The fabricated lithium ion secondary battery was subjected to charging-discharging tests with the use of a secondary battery charging-discharging tester (manufactured by NAGANO Co., Ltd.). Conditions for the charging-discharging operations are shown in Table 1. Through the charging-discharging tests, initial charging capacity, initial discharging capacity, ratio of the initial discharging capacity to the initial charging capacity (hereinafter referred to as initial efficiency), and ratio of discharging capacity given at 50th cycles relative to the initial discharging capacity (hereinafter referred to as the discharging capacity retentivity after 50 cycles) were determined.

(24) TABLE-US-00001 TABLE 1 Charging Discharging 1st CC-CV 0.1 C 5 mV-0.01 C CC 0.1 C 1.5 V 2nd CC-CV 0.3 C 5 mV-0.01 C CC 0.3 C 1.5 V 3rd~50th CC-CV 0.5 C 5 mV-0.01 C CC 0.5 C 1.5 V

Example 2

(25) The same operation as in Example 1 was performed, except that in Example 1, the SiO powder raw material was changed to the one with a median diameter of 5.6 m, which was smaller than in Example 1, and together with this change, the thermal treatment time for the C-coating was shortened from 30 minutes to 27 minutes. The C-coating amount in the C-coated SiO powder was 0.94 wt %. The median diameter D1 was 5.8 m, and the median diameter ratio D2/D1 was 0.88.

Example 3

(26) The same operation as in Example 1 was performed, except that in Example 1, the Li-doping was performed so that the mixing ratio between the C-coated SiO powder and the finely-ground LiH powder (Li/O molar ratio) was 0.2.

Example 4

(27) The same operation as in Example 2 was performed, except that in Example 2, the Li-doping was performed so that the mixing ratio between the C-coated SiO powder and the finely-ground LiH powder (Li/O molar ratio) was 0.2.

Comparative Example 1

(28) The same operation as in Example 2 was performed, except that in Example 2, the Li-doping of the SiO powder that had been C-coated (median diameter 8.2 m) involved the use of a LiH powder that did not undergo the fine grinding and had a median diameter of 20.8 m. The median diameter ratio D2/D1 was 2.54.

Comparative Example 2

(29) The same operation as in Example 4 was performed, except that in Example 4, the Li-doping of the SiO powder that had been C-coated (median diameter 8.2 m) involved the use of a LiH powder that did not undergo the fine grinding and had a median diameter of 20.8 m.

(30) Results of X-ray diffractometry of Li-containing SiO powders produced in Examples 1 to 4 and Comparative Examples 1 and 2 (P2/P1 and P3/P1), and results of charging-discharging tests (initial charging capacity, initial discharging capacity, initial efficiency, and discharging capacity retentivity after 50 cycles), together with mixing ratios between SiO powder and LiH powder (Li/O molar ratio) in Li-doping and median diameter ratios D2/D1, are shown in Table 2.

(31) An X-ray diffraction chart of the Li-containing SiO powder produced in Example 2 is shown in FIG. 2, and an X-ray diffraction chart of the Li-containing SiO powder produced in Comparative Example 1 is shown in FIG. 3. An X-ray diffraction chart of a SiO powder not doped with Li is shown in FIG. 4 for reference purpose.

(32) TABLE-US-00002 TABLE 2 Discharging capacity Initial Initial retentivity charging discharging Initial after 50 Li/O D2/D1 P2/P1 P3/P1 capacity capacity efficiency cycles Example 1 0.5 0.62 0.21 0.07 1853 1568 84.6 86.5 Example 2 0.5 0.88 0.71 0.20 1802 1515 84.1 87.2 Example 3 0.2 0.62 0.05 0.05 2050 1615 78.8 87.3 Example 4 0.2 0.88 0.11 0.10 2203 1679 76.2 88.5 Comparative 0.5 2.54 1.27 0.79 1900 1225 64.5 2.1 Example 1 Comparative 0.2 2.54 1.12 1.78 2281 1710 75.0 53.1 Example 2

(33) SiO powder not doped with Li does not exhibit any crystalline peaks, as is evident from FIG. 4, and is therefore practically amorphous. The X-ray diffraction chart shown in FIG. 4 is of the SiO powder not doped with Li and not C-coated, which SiO powder was found to be the one that even after undergoing the C-coating at C-coating temperature of 850 C. did not exhibit any crystalline peaks. Allowing this SiO powder to undergo the C-coating and thereafter Li-doping produced powders, in Comparative Examples 1 to 2, that in spite of having undergone the Li-doping at a low temperature of 600 C. markedly exhibited not just peaks attributed to Li.sub.2Si.sub.2O.sub.5 but also peaks attributed to Li.sub.2SiO.sub.3 and the crystalline Si, as is evident from FIG. 3.

(34) The above result is contrasted with Examples 1 to 4, where allowing SiO powder to undergo the C-coating and thereafter the Li-doping produced powders that exhibited high peaks attributed to Li.sub.2Si.sub.2O.sub.5 but with low peaks attributed to Li.sub.2SiO.sub.3 and to the crystalline Si, as is evident from FIG. 2. This is due to the fact that the Li-doping was preceded by finely grinding the LiH powder serving as a raw material of the Li-doping to give a median diameter ratio D2/D1 that had been kept lower as compared with the SiO powder, which accordingly inhibited the local reaction from occurring on the SiO powder particles and suppressed the concentration of lithium, resulting in the inhibition of the formation of Li.sub.2SiO.sub.3 and the formation of the crystalline Si.

(35) In fact, as is evident from Table 2, Examples 1 to 4, as compared with Comparatives Examples 1 and 2, the ratio P2/P1 of a height of peak attributed to Li.sub.2SiO.sub.3, P2, relative to a height of peak attributed to Li.sub.2Si.sub.2O.sub.5, P1, is kept to be not higher than 1; and moreover the ratio P3/P1 of a height of peak attributed to crystalline Si, P3, relative to a height of peak attributed to Li.sub.2Si.sub.2O.sub.5, P1, is kept to as low as 0. This resulted in superior battery performances: high initial efficiency values and high discharging capacity retentivity values after 50 cycles.

(36) The comparison between Examples 1 versus Example 3 and the comparison between Example 2 versus Example 4 each show that the former example attained a larger P2/P1 than the latter example, i.e., the former examples having a higher initial efficiency: this is due to a more amount of Li-doping amount (Li/O) in the Li-doping. The comparison between Example 1 versus Example 2 and the comparison between Example 3 versus Example 4 each show that the latter example attained a lager P2/P1 than the former example: this will be due to a larger median diameter ratio D2/D1 of the LiH powder to the SiO powder.