Lithium doped silicon oxide-based negative electrode material and method of manufacturing the same

Abstract

[Problem] Provided is a silicon oxide-based negative electrode material capable of avoiding, as much as possible, decreased battery performance resulting from a heterogeneous distribution of a Li concentration. [Solution] Provided is a powder having an average composition of SiLi.sub.xO.sub.y wherein 0.05<x<y<1.2 and a mean particle size of 1 μm or more. Further, 10 particles randomly selected from particles of the powder each satisfy 0.8<L1/L2<1.2 with the standard deviation of L2 being 0.1 or less, L1 being a Li concentration at a depth of 50 nm from an outermost surface of each of the 10 particles, and L2 being a Li concentration at a depth of 400 nm from the outermost surface.

Claims

1. A silicon oxide-based negative electrode material, comprising a powder having an average composition of SiLi.sub.xO.sub.y wherein 0.05<x<y<1.2, wherein 10 particles randomly selected from particles of the powder each satisfy 0.8<L1/L2<1.2 with the standard deviation of L2 being 0.1 or less, L1 being a Li concentration at a depth of 50 nm from an outermost surface of each of the 10 particles, and L2 being a Li concentration at a depth of 400 nm from the outermost surface.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The FIGURE shows an image of a cross-section of a particle of a powder observed under a TEM, the powder pertaining to the silicon oxide-based negative electrode material according to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENT

(2) Below, the embodiments of the present invention will be described. The silicon oxide-based negative electrode material according to an embodiment of the present invention may typically be manufactured by the following methods.

(3) First, a Si powder is mixed with, for example, a Li.sub.2Si.sub.2O.sub.5 powder as a lithium-silicate powder to prepare a raw material containing Si, O, and Li. A SiO.sub.2 powder is mixed to adjust the O content if necessary. The mixing ratio of the powders is adjusted so as to obtain the desired element ratio (Li:Si:O) of Li, Si, and O (for example, 1:0.4:1) within a range where the average composition SiLi.sub.xO.sub.y of the powder mixture satisfies 0.05<x<y<1.2.

(4) Next, the above powder mixture as a raw material is changed into a reaction vessel, and heated under reduced pressure to generate a gas from, in particular, lithium silicate contained in the mixed raw material. In the above gas-generating reaction, a SiO gas and a Li gas are generated simultaneously. With reference to chemical formulae, the reaction may be expressed generally by formula (1), or by formula (2) when lithium silicate is Li.sub.2Si.sub.2O.sub.5. As described above, lithium silicate may be represented by the general formula of .sub.xLi.sub.2O.Math..sub.ySiO.sub.2.
[Formulae 1]
(x+y)Si+(xLi.sub.2O.Math.ySiO.sub.2).fwdarw.(x+2y)SiO↑+2xLi↑  (1)
3Si+Li.sub.2Si.sub.2O.sub.5.fwdarw.5SiO↑+2Li↑  (2)

(5) As shown in formula (1) and formula (2), a SiO gas and a Li gas are generated simultaneously from lithium silicate by heating in the co-presence of elemental Si. This reaction is believed to be a reductive reaction by Si.

(6) While allowing the gases to be generated from the raw material in the reaction vessel, the generated gases are cooled and deposited on a surface of a vapor deposition platform arranged at an upper portion within the reaction vessel. After the end of the reaction, a deposit is collected from the surface of the vapor deposition platform. The collected deposit, which is a Li-containing silicon oxide material, is pulverized to obtain a powder for a negative electrode material having a predetermined particle size.

(7) The SiO gas and the Li gas are simultaneously generated from the raw material within the reaction vessel. This allows for the production of a gas mixture of the two having their concentrations homogeneously distributed. Therefore, a deposit obtained by cooling the above gas mixture on the same surface of a vapor deposition platform will also have homogeneously distributed concentrations. Therefore, a powder obtained by pulverizing the above deposit will have both a homogeneous distribution of a Li concentration among particles of the powder and a homogeneous distribution of a Li concentration in individual particles of the powder. When the above powder is used as a powder for a negative electrode material, development of a Li-rich phase can be prevented, leading to decreased reactivity and improved battery performance.

(8) In another embodiment, a Si powder is mixed with a LiOH powder. A SiO.sub.2 powder is mixed to adjust the O content if necessary. The resulting powder mixture as a primary raw material is changed into a reaction vessel and heated and calcined under an Ar atmosphere. With reference to chemical formulae, a reaction in which LiOH is heated in the co-presence of elemental Si may be expressed by the first part of formula 3).
[Formulae 2]
4Si+4LiOH.fwdarw.3Si+Li.sub.4SiO.sub.4+2H.sub.2↑3Si+Li.sub.4SiO.sub.4.fwdarw.4SiO↑+4Li↑  (3)
4Si+2Li.sub.2CO.sub.3.fwdarw.3Si+Li.sub.4SiO.sub.4+2CO↑3Si+Li.sub.4SiO.sub.4.fwdarw.4SiO↑+4Li↑  (4)

(9) As shown in the first part of formula (3), heating and calcining LiOH in the co-presence of elemental Si generates lithium silicate (Li.sub.4SiO.sub.4) while an undesired element H is removed as a gas component. The resulting calcined material is a mixture of lithium silicate (Li.sub.4SiO.sub.4) and residual element Si. This corresponds to the raw material containing Si, O, and Li used in the aforementioned embodiment.

(10) Then, heating of the resulting calcined material as a secondary raw material is continued under reduced pressure. Heating of lithium silicate (Li.sub.4SiO.sub.4) contained in the secondary raw material in the co-presence of elemental Si then generates a Si gas and a Li gas simultaneously from that lithium silicate (Li.sub.4SiO.sub.4) as shown in the latter part of formula (3). Here, the generated gases can be cooled on and collected from the same surface to obtain a powder for a negative electrode material having a homogeneous distribution of a Li concentration as in the aforementioned embodiment. Instead of continuously heating the secondary raw material, the secondary raw material may be subsequently reheated.

(11) As described above, a raw material including elemental Si and lithium silicate (a Si.Math.lithium silicate-containing raw material) can be obtained by heating and calcining a primary raw material including elemental Si and LiOH. The resulting raw material obtained as a secondary raw material can be heated to generate a SiO gas and a Li gas simultaneously.

(12) Li.sub.2CO.sub.3 may also be used instead of LiOH. That is, a Si powder is mixed with a Li.sub.2CO.sub.3 powder. A SiO.sub.2 powder is mixed to adjust the O content if necessary. The resulting powder mixture as a primary raw material is changed into a reaction vessel and heated and calcined under an Ar atmosphere. With reference to chemical formulae, a reaction in which Li.sub.2CO.sub.3 is heated in the co-presence of elemental Si may be expressed by the first part of formula (4).

(13) As shown in the first part of formula (4), heating and calcining Li.sub.2CO.sub.3 in the co-presence of elemental Si generates lithium silicate (Li.sub.4SiO.sub.4) while an undesired element C is removed as a gas component. The resulting calcined material is a mixture of lithium silicate (Li.sub.4SiO.sub.4) and residual element Si. This corresponds to the raw material containing Si, O, and Li used in the aforementioned embodiment.

(14) Then, heating of the resulting calcined material as a secondary raw material is continued under reduced pressure. Heating of lithium silicate (Li.sub.4SiO.sub.4) contained in the secondary raw material in the co-presence of elemental Si then generates a Si gas and a Li gas simultaneously from that lithium silicate (Li.sub.4SiO.sub.4) as shown in the latter part of formula (4). Here, the generated gases can be cooled on and collected from the same surface to obtain a powder for a negative electrode material having a homogeneous distribution of a Li concentration as in the aforementioned embodiment. Instead of continuously heating the secondary raw material, the secondary raw material may be subsequently reheated.

(15) As described above, a raw material including elemental Si and lithium silicate (a Si.Math.lithium silicate-containing raw material) can be obtained by heating and calcining a primary raw material including elemental Si and Li.sub.2CO.sub.3. The resulting raw material as a secondary raw material can be heated to generate a SiO gas and a Li gas simultaneously. LiOH and Li.sub.2CO.sub.3 may also be used instead of using LiOH or Li.sub.2CO.sub.3.

(16) It is noted that the chemical reactions in the embodiments are represented by chemical formulae (1) to (4), but these merely represent putative reactions in model cases in which these phenomena are simplified. The actual reactions may likely be more complicated due to the addition of SiO.sub.2 for adjusting the O content.

Example 1

(17) A Si powder, a SiO.sub.2 powder, and a Li.sub.2Si.sub.205 powder were mixed in a molar ratio of 21:15:2. The element ratio of the powder mixture is Si:Li:O=1:0.1:1. This powder as a raw material was charged into a reaction vessel and heated to 1400° C. under reduced pressure. Generated gases were cooled on and collected from a vapor deposition platform arranged at an upper portion within the reaction vessel. Then, the collected material (deposit) was pulverized into a powder with a ball mill using a zirconia container and balls. The mean particle size of the powder was 5.2 μm as determined by the laser diffraction particle size distribution measurement.

(18) From the resulting powder, 10 particles were randomly selected for the cross-sectional TEM observation of each particle. EELS measurements were performed at a depth of 50 nm from the outmost surface of a particle for a region of 20 nm in the longitudinal direction and 400 nm in the transverse direction to obtain a Si spectral intensity and a Li spectral intensity. The ratio of the Li spectral intensity to the Si spectral intensity was taken as L1, i.e., a Li concentration at the surface of the particle. A similar procedure was performed at a depth of 400 nm from the outmost surface of the particle to obtain the ratio of a Li spectral intensity to a Si spectral intensity, which was taken as L2, i.e., a Li concentration in the inside of the particle. L1/L2 was determined for each of the 10 particles, and the standard deviation of L2 was then determined.

(19) For the measurements of a particle, the particle was cut to expose a cross-section under an inert atmosphere by the FIB process using an FB-2000A (Hitachi). The TEM observation was performed under an atomic resolution-analytical electron microscope JEM-ARM 200F (JEOL), and then EELS measurements were performed with a GATAN GIF Quantum energy filter. The TEM measurements were performed under the following conditions: the acceleration voltage was 200 kV; the diameter of a beam was 0.2 nmφ; and the energy resolution was 0.5 eV FWHM.

(20) Further, the Li content (Li/Si) and the O content (O/Si) of the powder obtained were measured by the ICP emission analysis method and the infrared absorption method.

Example 2

(21) A Si powder and a Li.sub.2Si.sub.2O.sub.5 powder were mixed in a molar ratio of 3:1. The element ratio of the powder mixture is Si:Li:O=1:0.4:1. Other procedures were the same as Example 1, and a powder with an average particle size of 5.4 μm was produced. Then, L1/L2 and the standard deviation of L2 of the resulting powder were determined as well as the Li content and the O content. The FIGURE shows a cross-sectional TEM image of a particle of the powder which was used for measuring L1 and L2 of that particle.

Example 3

(22) A Si powder and a Li.sub.2SiO.sub.3 powder were mixed in a molar ratio of 2:1. The element ratio of the powder mixture is Si:Li:O=1:0.67:1. Other procedures were the same as Example 1, and a powder with an average particle size of 5.1 μm was produced. Then, L1/L2 and the standard deviation of L2 of the resulting powder were determined as well as the Li content and the O content.

Example 4

(23) A Si powder, a SiO.sub.2 powder, and a LiOH powder were mixed in a molar ratio of 4:1:3. The resulting powder as a primary raw material was charged into a reactional vessel and heated and calcined to 1400° C. under an Ar atmosphere at the atmospheric pressure. A portion of the calcined material was collected and analyzed. The element ratio was found to be Si:Li:O=1:0.6:1, and no residual H component was found.

(24) Then, the calcined material was used as a secondary raw material, and allowed to be continuously heated at 1400° C. in the same reaction vessel under reduced pressure. Generated gases were then cooled on and collected from a vapor deposition platform arranged at an upper portion within the reaction vessel. Subsequently, the collected material (deposit) was subjected to powderization as in Example 1, and then L1/L2 and the standard deviation of L2 of the powder were determined as well as the Li content and the O content. The mean particle size of the resulting powder was 5.6 μm.

Comparative Example 1

(25) A Si powder and a SiO.sub.2 powder were mixed in a molar ratio of 1:1. The element ratio of the powder mixture is Si:Li:O=1:0:1. Other procedures were the same as Example 1, and a powder with an average particle size of 5.1 μm was produced. The resulting powder did not contain Li, and thus only the O content was measured.

Comparative Example 2

(26) A powder of lithium hydride (LiH) was added to the powder produced in Comparative Example 1, i.e., a SiO powder so that Li was present at 0.4 mol relative to Si and O, and then the resulting powder was heated and calcined to 850° C. under an Ar atmosphere to obtain a powder with a mean particle size of 5.2 μm. Then, L1/L2 and the standard deviation of L2 of the resulting powder were determined as well as the Li content and the O content.

Comparative Example 3

(27) Two reactional vessels were provided. A mixture in which a Si powder and a SiO.sub.2 powder were mixed in a molar ratio of 1:1 was charged into one vessel. The element ratio of the powder mixture is Si:Li:O=1:0:1. Further, metal Li was charged into the other vessel under an inert gas atmosphere. Then, the weight ratio and the heating temperature of the raw materials in the two vessels were adjusted so that a SiO gas generated in one vessel and a Li gas generated in the other vessel showed a partial pressure of 1:0.4. The gases generated in both vessels were mixed and cooled on and collected from the common vapor deposition platform.

(28) Subsequently, the collected material (deposit) was subjected to powderization as in Example 1, and L1/L2 and the standard deviation of L2 of the resulting powder were determined as well as the Li content and the O content. The mean particle size of the resulting powder was 5.2 μm.

(29) Battery Evaluation

(30) Battery evaluation was performed according to the following procedure for the powder samples produced in Examples 1 to 4 and Comparative Examples 1 to 3.

(31) A powder sample, a PI binder as a nonaqueous (organic) binder, and a KB as an electrically conductive auxiliary agent were mixed in a weight ratio of 80:15:5 and kneaded with an organic solvent NMP to obtain a slurry. The resulting slurry was applied onto a copper foil and subjected to vacuum heat treatment at 350° C. for 30 minutes to obtain a negative electrode. The resulting negative electrode, a counter electrode (a Li foil), an electrolytic solution (EC:DEC=1:1), an electrolyte (1 mol/L of LiPF.sub.6), and a separator (a polyethylene porous film with a film thickness of 30 μm) were combined to fabricate a coin cell battery.

(32) The resulting coin cell battery was subjected to a charge and discharge test. Charge was performed at a constant current of 0.1 C until the voltage across the two electrodes of the battery reached 0.005 v. After the voltage reached 0.005 v, constant-potential charge was then performed until the electric current reached 0.01 C. Discharge was performed at a constant current of 0.1 C until the voltage across the two electrodes of the battery reached 1.5 V.

(33) The initial charging capacity and initial discharge capacity were measured by this charge and discharge test to determine the initial efficiency. Results are shown in Table 1 along with the main specifications (the Li content, the O content, L1/L2, and the standard deviation of L2) of powder samples.

(34) TABLE-US-00001 TABLE 1 Raw Post- Post- Material rxn rxn L2 Initial Li/Si Li/Si O/Si L1/L2 Std. Dev. efficiency Example 1 0.1 0.08 1.03 0.94-1.03 0.05 74.30% Example 2 0.4 0.41 0.99 0.95-1.06 0.04 79.10% Example 3 0.67 0.95 1.05 0.91-1.05 0.05 82.50% Example 4 0.6 0.93 1.02 0.88-1.15 0.07 82.20% Comp. Exp. 1 0 0 1.04 — — 71.40% Comp. Exp. 2 0 0.4 1.08 1.19-1.57 0.13 68.60% Comp. Exp. 3 0.4 0.34 0.98 0.90-1.11 0.16 29.70%

(35) In Comparative Example 1, Li was not doped to a SiO powder. As compared with this, the initial efficiency was improved for all of Examples 1 to 4, demonstrating that the performance was improved by Li doping. Incidentally, L1/L2 as a ratio of a Li concentration in the inside of a particle to a Li concentration at the surface of the particle falls within a range of 0.8<L1/L2<1.2, and the standard deviation of L2 is also 0.1 or less for all cases.

(36) In contrast, in Comparative Example 2, Li was doped in accordance with the solid phase method (calcination method) as described in Patent Document 1. As often observed in this method, Li in a higher concentration was unevenly distributed at the surface of a particle, and L1/L2 as a ratio of a Li concentration in the inside of a particle to a Li concentration at the surface of the particle showed significant variation toward a value of greater than 1.2. This resulted in deteriorated binder performance and a decreased initial efficiency as compared with even Comparative Example 1.

(37) In contrast, in Comparative Example 3, Li was doped in accordance with the gas phase method (deposition method) as described in Patent Document 2. Use of a gas mixture of a SiO gas and a Li gas allowed for a smaller difference in the Li concentration between the surface of a particle and the inside of the particle, but the Li concentration showed significant variation among particles, resulting in an even lower initial efficiency as compared with Comparative Example 2. This is likely because particles with higher Li concentrations were produced, and they reacted with a binder.

(38) Incidentally, the Li content (x=Li/Si) and the O content (y=O/Si) in a powder satisfies 0.05<x<y<1.2 as defined in the present invention for all samples except for Comparative Example 1 where Li was not doped. This also indicates that L1/L2 and the standard deviation of L2 are effective performance measures for a powder of a negative electrode material.