SiOx/Si/C composite material and process of producing thereof, and anode for lithium ion battery comprising said composite material

Abstract

An SiOx/Si/C composite material, includes SiOx/Si composite particles and a carbon coating layer coated on the SiOx/Si composite particles. The SiOx/Si composite particles include nano-silicon crystallites embedded in an SiOx (0<x2) amorphous matrix phase. The SiOx/Si composite particles have an Si:O molar ratio of 5:1-1.1:1, preferably 2:1-1.2:1. A process for producing an SiOx/Si/C composite material, includes a) milling SiO powder together with a metal reductant in a molar ratio of 125:1-10:1, preferably 2:11-5:1, b) totally removing the oxidation product of the metal reductant to obtain SiOx/Si composite particles, and c) coating the SiOx/Si composite particles with carbon to obtain the SiOx/Si/C composite material.

Claims

1. A process for producing a SiOx/Si/C composite material, wherein 0<x<2, said process comprising: milling SiO powder together with a metal reductant in a molar ratio of 1.25:1-10:1; totally removing the oxidation product of said metal reductant to obtain SiOx/Si composite particles; and coating said SiOx/Si composite particles with carbon to obtain said SiOx/Si/C composite material.

2. The process of claim 1, further comprising: selecting said metal reductant from the group consisting of Mg, Al, Zn, Li, and the combination thereof.

3. The process of claim 1, wherein during said milling said SiO powder is milled by a high energy ball milling process in advance for 0-15 h, and further milled together with said metal reductant for 3-20 h.

4. The process of claim 1, wherein said coating is carried out by chemical vapor deposition or pyrolysis to achieve a carbon coating layer having a thickness of 2-15 nm.

5. The process of claim 4, wherein said chemical vapor deposition or pyrolysis is conducted by using a precursor or carbon source selected from toluene, acetylene, polyvinyl chloridepolyvinylidene fluoride, citric acid, glucose, pitch, and the combination thereof.

6. The process of claim 1, wherein an SiOx/SiC composite material is produced.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the schematic structure of the SiOx/Si/C composite according to the invention.

(2) FIG. 2 shows the SEM image of the SiOx/Si/C composite according to an embodiment of the invention.

(3) FIGS. 3A and 3B show TEM images of the SiOx/Si composite material according to an embodiment of the invention.

(4) FIG. 4 shows the particle size distribution of the SiOx/Si composite according to an embodiment of the invention.

(5) FIG. 5 shows the XRD profile of the silicon monoxide ball milled for 25 h (bottom) and SiOx/Si composite material according to an embodiment of the invention (upper).

(6) FIGS. 6 (A) and (B) show (a) the N.sub.2 adsorption/desorption isotherm of the SiOx/Si composite according to an embodiment of the invention and (b) the pore size distribution plot calculated by the BJH formula in the desorption branch isotherm of the SiOx/Si composite, respectively.

(7) FIG. 7 shows the first two discharge/charge curves of SiOx/Si/C composite material according to an embodiment of the invention.

(8) FIG. 8 shows the cycling performance of the pristine SiO/C, SiO/C ball milled for 25 h and SiOx/Si/C composite according to an embodiment of the invention.

(9) FIG. 9 shows the XPS spectra of the pristine SiO milled 10 h (m-SiO) and the SiOx/Si/C composite material according to an embodiment of the invention (m-SiOx/Si).

(10) FIG. 10 shows the HR-TEM image of the SiOx/Si/C composite according to an embodiment of the invention.

DETAILED DESCRIPTION

(11) Referring to the following description, examples and accompanying claims, other features, aspects and advantages of the present invention will be apparent.

(12) Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, will control.

(13) When an amount, concentration, or other value or parameter is given as either a range, a preferred range or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

(14) When the term about is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to.

(15) Unless stated otherwise, all percentages, parts, ratios, etc., are by weight.

(16) The SiOx/Si/C Composite Material

(17) On one aspect, the invention provides a SiOx/Si/C composite material, comprising SiOx/Si composite particles and a carbon coating layer coated on said SiOx/Si composite particles, wherein 0<x2.

(18) As used herein, 0<x2 means that the Si atoms of SiOx exist in a valency of +4, +3, +2, +1 and the combination thereof.

(19) The structure of the SiOx/Si/C composite material can be schematically illustrated as in FIG. 1. As shown in FIG. 1, the SiOx/Si composite particles comprise nano-silicon crystallites embedded in a SiOx amorphous matrix phase, and the nano-silicon crystallites are uniformly distributed in the SiOx amorphous matrix phase. In addition, the SiOx/Si particles are surrounded by a carbon coating layer.

(20) In an embodiment of the invention, the SiOx/Si composite particles have a Si:O molar ratio of about 3:1-1.1:1, preferably about 2:1-1.2:1.

(21) In another embodiment of the invention, the nano-silicon crystallites have a particle size of about 1-about 50 nm, preferably about 4-about 20 nm.

(22) In another embodiment of the invention, the SiOx/Si/C composite material has a carbon coating layer with a thickness of about 2-about 15 nm, preferably about 4-about 12 nm, more preferably about 6-about 10 nm.

(23) In still a further embodiment of the invention, the SiOx/Si composite particles may have a D50 particle size within the range of no more than 10.0 m, preferably no more than 5.0 m, more preferably no more than 2.0 m, still more preferably no more than 1.1 m. In addition, the D90 particle size of the SiOx/Si composite particles may be within the range of no more than 20.0 m, preferably no more than 11.1 m, and the D10 particle size may be within the range of no less than 0.1 m, preferably no less than 0.3 m.

(24) As used herein, D50, D90 or D10 particle size refers to the particle diameter in case the cumulative distribution percentage reaches the specified value. For example, if D90=5 m, there are 10% particles larger than 5 m, and 90% smaller than 5 m. The specified value, e.g. 50, 90 and 10 used herein may be by mass, weight, length, etc, depending on the test equipment and method used. Particularly, the D50, D90 and D10 particle sizes used herein are by volume, and they can be obtained by the commonly used volume averaged particle size distribution curve. In an embodiment of the invention, the SiOx/Si composite particles may have a particle distribution ranging from about 0.1-about 1000 m.

(25) The Process for Producing a SiOx/Si/C Composite Material

(26) On another aspect, in order to arrive at the SiOx/Si/C composite material, the invention provides a process for producing a SiOx/Si/C composite material of the invention, wherein 0<x2, comprising: a) milling SiO powder together with a metal reductant in a molar ratio of about 1.25:1-about 10:1, preferably about 2:1-about 5:1, b) totally removing the oxidation product of said metal reductant to obtain SiOx/Si composite particles, c) coating said SiOx/Si composite particles with carbon to obtain said SiOx/Si/C composite material.

(27) In an embodiment of the invention, the metal reductant is selected from the group consisting of Mg, Al, Zn, Li and the combination thereof.

(28) In another embodiment of the invention, during step a) the SiO powder is milled by a high energy ball milling process in advance for 0-about 15 h, and further milled together with said metal reductant for about 3-about 20 h.

(29) In still a further embodiment of the invention, step C) is carried out by chemical vapor deposition or pyrolysis to achieve a carbon coating layer having a thickness of about 2-about 15 nm, preferably about 4-about 12 nm, more preferably about 6-about 10 nm.

(30) Specifically, in an embodiment of the invention, the starting SiO powder in step a) may be commercially available or prepared from Si and SiO.sub.2 at high temperatures according to the processes known in the prior art. The particle size of the starting SiO powder includes, but not limited to about 200 mesh to about 500 mesh, preferably about 200 mesh to about 400 mesh. Representative commercially available SiO powders can be commercially purchased from Aldrich (325 mesh) and Aladdin (200 mesh).

(31) In an embodiment of the invention, the method for conducting milling includes, but not limited to ball milling. In a particular embodiment of the invention, the milling is conducted by high energy ball-milling (high energy mechanical milling) using a commercially available planetary balling mill including zirconica vial and hardened stainless vial. The operation parameters including the duration time, the rotation speed, the number and size of the zirconica balls or stainless steel balls of the ball mill may be adopted according to the instruction of the manufacturer, and a person skilled in the art can easily adjust these operation parameters of milling. In a particular embodiment of the invention, the SiO and reductant powder is milled in a planetary balling mill with 10 zirconia balls (=10 mm) at a rotation speed of about 300-about 500 rpm, preferably about 400-about 500 rpm for about 3-about 20 h, preferably about 5-about 15 h. In another embodiment of the invention, the SiO powder is pre-milled in a planetary balling mill at a rotation speed of about 400 rpm for about 0-about 15 h, preferably about 5-about 15 h in an alternative pretreatment step before step a).

(32) The term a reductant used herein means a substance capable of reducing the higher oxidation states of silicon, e.g. Si.sup.4+ to a lower state, and not negatively affecting in the milling process. In step a), the reductant used herein includes, but not limited to metals, such as Mg, Al, Zn, Li and the combination thereof. The molar ratio of SiO to the reductant may be in the range of about 1.25:1-about 10:1, preferably about 2:1-about 5:1. In a particular embodiment of the invention, the reductant is Mg, and thus the reaction between the reductant and the starting SiO powder can be assumed as follows:
SiO+Mg=MgO+Si.

(33) In the case that Mg is used as the reductant, it is preferred that Mg is used in the form of powder having a particle size from about 50 to about 200 mesh, preferably from about 100 to about 200 mesh. The commercial available Mg powders can be commercially available from Sinopharm Chemical Reagent Co., Ltd (100-200 mesh) and from Aladdin (100-200 mesh). In a particular embodiment of the invention, the molar ratio of SiO to Mg was about 5:1.

(34) In an embodiment of the invention, the treatment of totally removing the oxidation product of the reductant in step a) is conducted by immersing the oxidation product in an acid solution. The term an acid solution used herein means those dissolving the oxidized product of the reductant, but inert to SiO. Accordingly, the acid solution is selected from the solution of hydrochloric acid, perchloric acid, nitric acid, sulfuric acid, phosphoric acid and the combination thereof. Hydrofluoric acid will be avoided to use in the invention due to its possible reactivity in material containing Si. The acid solution may be in the form of diluted or concentrated solution, as long as it can dissolve the oxidized product of the reductant, e.g. MgO, Al.sub.2O.sub.3, ZnO and/or Li.sub.2O. The concentration of the acid solution includes, but not limited to about 0.5 M-about 5 M, preferably about 1 M-about 2 M. In a particular embodiment of the invention, the acid solution will be a 2 M hydrochloric acid solution if the oxidized product of the reductant is MgO. The term totally removing used herein means that at least about 90%, preferably about 95%, more preferably about 99% of the oxidized product of the reductant is substantially dissolved and removed. The acid immersing period is not limited as long as such an amount of oxidized product can be removed. Preferably, the acid immersing period holds about 1-about 12 h, more preferably, about 4-about 6 h.

(35) In an embodiment of the invention, after step b), the resulting mixture may be optionally washed by a washing reagent. The selection of the washing reagent is not limited, and it may include ethanol, deionized water or combinations thereof. Taking account of the ease to handle and cost-efficiency, in a particular embodiment of the invention, deionized water is used to wash off the dissolved salt produced in step b) from the SiOx/Si material.

(36) In an embodiment of the invention, after the washing step, the washed SiOx/Si material may be optionally dried in order to obtain the final product. In the present invention, the method for conducting drying includes, but not limited to vacuum drying. In addition, the drying parameters, such as temperature and duration time can be easily adjusted by a person skilled in the art. Specifically, the drying temperature includes but not limited to about 50 C. -about 100 C. The drying duration may hold for about 4-about 12 h depending on the temperature used in the drying process. In a particular embodiment of the invention, the washed SiOx/Si material is vacuum dried at about 60 C. for about 10 h.

(37) The carbon coating of an anode material for lithium ion battery has been demonstrated in the art to have many advantages, such as electrical conductivity, chemical and electrochemical stability, unique physical properties and low cost.

(38) Therefore, in an embodiment of the invention, the SiOx/Si material obtained by the above process is further coated with a carbon-containing material in step c). There are various carbon coating methods known in the art, such as chemical vapor deposition (CVD) method, pyrolysis, etc. In an embodiment of the invention, the carbon source includes, but not limited to toluene, acetylene, polyvinyl chloride, polyvinylidene fluoride, citric acide, glucose, pitch and combinations thereof. In a particular embodiment of the invention, the coating process comprises substeps of 1) providing the SiOx/Si composite material obtained above in a reaction chamber, and 2) introducing a precursor gas having toluene into the reaction chamber, and maintaining a temperature at about 200 C.-about 1000 C. During the process, the coating duration includes, but not limited to about 0.3-about 2 h depending on the temperature used in the carbon coating process.

(39) As a result, the process produces a SiOx/Si/C material which having a weight ratio of carbon to the sum of silicon monoxide and silicon is from about 5% -about 20%, and preferably from about 12.5%-about 20%. The carbon coating layer has a thickness of about 2-about 15 nm, preferably about 4-about 12 nm, more preferably about 6-about 10 nm.

(40) The Method for Characterizing of SiOx/Si/C Composite Material

(41) For better understanding the structure and more physical properties of the SiOx/Si/C composite material according to the invention, the inventors have conducted the following instrumental tests to characterize the SiOx/Si/C composite material produced in Examples.

(42) The method for testing the atom ratio of Si/O is conducted by an energy dispersive spectrometer (EDS). In the invention, a transmission electron microscope (TEM, JEM-100CX, manufactured by JOEL) is used to conduct the test and calculate the atom ratio of Si/O.

(43) The SEM image as shown in FIG. 2 is obtained by FESEM, JEOL JSM-7401 F.

(44) The TEM images as shown in FIGS. 3 and 10 are obtained by JOEL JEM-100CX.

(45) The particle size distribution of the SiOx/Si composite particles as shown in FIG. 4 is obtained by MASTERSIZER 2000.

(46) The XRD profiles as shown in FIG. 5 are obtained by Rigaku D/max-2200/PC, Cu.

(47) The pore size distribution and N.sub.2 adsorption/desorption isotherm as shown in FIGS. 6 (A) and (B) are obtained by Brunauer-Emmett-Teller (BET) measurement, Micromeritics Inc ASAP 2010 M+C.

(48) The XPS spectra as shown in FIG. 9 are obtained by Kratos Axis Ultra.sup.DLD).

EXAMPLE

Example 1

(49) The Production of SiOx/Si Composite Material

(50) The SiOx/Si composite material was synthesized by high energy ball-milling process with a Planetary Mono Mill P-6 (Fritsch, Germany) at a rotation speed of 400 rpm for 25 h. 2.0 g of SiO (325 mesh, Aldrich) powder was placed in a 80 ml zirconica vial and ball milled with 10 zirconia balls (=10mm) for 10 h. Then 0.218 g of Mg powder (200 mesh, from Sinopharm Chemical Reagent Co., Ltd), was added and it was further ball milled for 15 h. As a result, the SiOx/Si/C composite material having a D50 particle size of about 1.0 m, a D90 particle size of about 11.0 m, and a D10 particle size of about 0.4 m was obtained (as shown in FIG. 4). The obtained powder (SiOx/Si/MgO) was first immersed by 2 M HCl solution for 12 h to remove MgO, followed by washing with deionized water and finally vacuum dried at 60 C. for 10 h. As a result, the obtained SiOx/Si composite material has a Si:O molar ratio of about 1.25:1.

(51) The Production of SiOx/Si/C Composite Material

(52) The obtained SiOx/Si powder (0.2 g) was loaded in an alundum boat and placed at the center of a quartz tube furnace. Next, the precursor gas (argon and toluene) was introduced into the furnace. Then, the furnace temperature was increased from room temperature to 800 C. at a rate of 10 C. min.sup.1 and kept at 800 C. for 60 min. The furnace was cooled slowly to room temperature. At high temperature, the toluene decomposed quickly and deposited onto the surface SiOx/Si particles. As a result, the SiOx/Si/C composite material with a carbon coating having an average thickness of about 10 nm, which is shown in FIG. 10, was obtained. The coated carbon content in the composite was determined to be 12.5%.

Example 2

(53) The SiOx/Si/C composite material was produced as in the same method used in Example 1, except that 2.0 g of SiO powder and 0.545 g of Mg powder were loaded in the ball-milling process. As a result, the obtained SiOx/Si composite material has a Si:O molar ratio of about 2:1.

Example 3

(54) The SiOx/Si/C composite material was produced as in the same method used in Example 1, except that 2.0 g of SiO powder and 0.872 g of Mg powder were loaded in the ball-milling process. As a result, the obtained SiOx/Si composite material has a Si:O molar ratio of about 5:1.

Example 4

(55) The SiOx/Si/C composite material was produced as in the same method used in Example 1, except that the furnace temperature was increased from room temperature to 800 C. at a rate of 10 C. min.sup.1 and kept at 800 C. for 90 min. As a result, the SiOx/Si/C composite material with a carbon coating having an average thickness of about 10 nm was obtained. The coated carbon content in the composite was determined to be about 15.2%.

Example 5

(56) The SiOx/Si/C composite material was produced as in the same method used in Example 1, except that the furnace temperature was increased from room temperature to 800 C. at a rate of 10 C. min.sup.1 and kept at 800 C. for 30 min. As a result, the SiOx/Si/C composite material with a carbon coating having an average thickness of about 5 nm was obtained. The coated carbon content in the composite was determined to be about 5.6%.

Comparative Example 1

(57) The SiO/C material was produced by coating the pristine SiO/C material with carbon as in the same method used in Example 1, but without ball milling and Mg powder addition. The as purchased SiO was directly loaded into an alundum boat and placed at the center of a quartz tube furnace for carbon coating.

Comparative Example 2

(58) The SiO/C composite material was produced as in the same method used in Example 1, except that no Mg powder was charged in the ball-milling process. The as purchased SiO was loaded into milling machine and milled for 25 hours under 400 rpm. Then the product was loaded into an alundum boat and placed at the center of a quartz tube furnace for carbon coating.

(59) Cell Assembling and Electrochemical Test:

(60) The electrochemical performances of the as-prepared composites in Examples 1-5 and Comparative Examples 1 and 2 were tested using two-electrode coin-type cells. The working electrodes were prepared by pasting a mixture of active material, Super P conductive carbon black (40 nm, Timical) and styrene butadiene rubber/sodium carboxymethyl cellulose (SBR/SCMC, 3:5 by weight) as binder at a weight ratio of 70:20:10. After coating the mixture onto pure Cu foil, the electrodes were dried, cut to 12 mm sheets, pressed at 3 MPa, and then further dried at 50 C. in vacuum for 4 h. The CR2016 coin cells were assembled in an argon-filled glove box (MB-10 compact, MBraun) using 1M LiPF.sub.6/EC+DMC (1:1 by volume, ethylene carbonate (EC), dimethyl carbonate (DMC)) as electrolyte, including 2wt. % vinylene carbonate (VC), ENTEK ET20-26 as separator, and pure lithium foil as counter electrode.

(61) The cycling performances were evaluated on a LAND battery test system (Wuhan Kingnuo Electronics Co., Ltd., China) at 25 C. with a current density of 100 mA g.sup.1 or 300 mA g.sup.1. The cut-off voltage was 0.01 V versus Li/Li.sup.+ for discharge (Li insertion) and 1.2 V versus Li/Li.sup.+ for charge (Li extraction). The test results are shown in FIGS. 7 and 8.

(62) Evaluation:

(63) As shown in FIG. 2, the particles of SiOx/Si/C composite according to Example 1 are small and distribute uniformly, which demonstrates the schematic structure shown in FIG. 1. In addition, Examples 2-5 also have the similar structures as that of Example 1.

(64) In addition, as shown in FIGS. 3 (A) and (B), some crystallites can be found in the SiOx/Si/C composite of Example 1. They are within the sizes of 5-10 nm and were confirmed as silicon phases, assigned a Si (111) d-spacing value of 3.1 . Accordingly, silicon nanocrystallites embedded in an amorphous matrix phase, which further demonstrates the schematic structure shown in FIG. 1. In addition, Examples 2-5 have the similar structures as that of Example 1.

(65) As shown in FIG. 5, the main diffraction peaks of SiO and SiOx/Si composite material in Example 1 are similar. SiO exhibited weak diffraction patterns of silicon phases from the disproportionation of SiO during the ball milling process. For SiOx/Si composite material in Example 1, the diffraction pattern of silicon increased. It is presumed that the SiO partially reacted with the Mg and was reduced to silicon nanocrystallites. In addition, a set of tetragonal ZrO.sub.2 peaks after the ball milling are detected. The oxide contaminant must have come from the grinding medium (vial and ZrO.sub.2 balls).

(66) As shown in FIGS. 6 (A) and (B), the specific surface area of the SiOx/Si composite material in Example 1 is 24.5 m.sup.2 g.sup.1. The pores are mainly obtained from the etching MgO reduction product.

(67) FIG. 7 shows the first two discharge/charge curves of SiOx/Si/C composite material according to Example 1. The coin cell was discharged at 100 mA g.sup.1 between 0.01 and 1.2V vs Li/Li.sup.+. As shown in FIG. 7, the first two discharge/charge curves of SiOx/Si/C composite at current density of 100 mA g.sup.1. The discharge and charge capacity of SiOx/Si/C composite is 2080 mAh g.sup.1 and 1286.8 mAh g.sup.1, respectively, with an initial Coulombic efficiency of 61.9% in the first cycle.

(68) FIG. 8 shows the cycling performance of the SiO/C material according to Comparative Example 1, the SiO/C material according to Comparative Example 2 and the SiOx/Si/C composite material according to Example 1. The coin cell was discharged at 100 mA g.sup.1 for the five cycles and 300 mA g.sup.1 in the following cycles between 0.01 and 1.2 V vs Li/Li.sup.+. As shown in FIG. 8, compared to the SiO/C material according to Comparative Example 1, the cycling performance of the SiO/C material according to Comparative Example 2 is increased. However, its reversible capacity is still relative low, only 900 mAh g.sup.1. Surprisingly, the reversible capacity of the SiOx/Si/C composite according to Example 1 is further improved from about 900 mAh g.sup.1 (Comparative Example 2) to over 1200 mAh g.sup.1 (Example 1).

(69) Therefore, by the reduction reaction with Mg and coating with carbon, SiOx/Si/C anode according to Example 1 delivers better cycle stability and larger reversible capacity.

(70) FIG. 9 shows the XPS spectra of the pristine SiO milled 10 h (m-SiO) and the SiOx/Si/C composite material according to Example 1 (m-SiOx/Si). Table 1 gives the abundance ratios for the Si oxidation states of Si 2p spectra for m-SiO and m-SiOx/Si.

(71) TABLE-US-00001 TABLE 1 Abundance ratios for Si oxidation states of Si 2p spectra with m-SiO and m-SiOx/Si. Sample Si.sup.0(%) Si.sup.+(%) Si.sup.2+(%) Si.sup.3+(%) Si.sup.4+(%) m-SiO 9.0 1.5 2.7 17.8 69.0 m-SiOx/Si 11.2 5.8 3.7 46.3 33.0

(72) As shown in Table 1, compared to the pristine SiO milled 10 h, the abundance ratio of Si.sup.4+ is significantly decreased while lower oxidation states, i.e. Si.sup.0, Si.sup.+, Si.sup.2+ and Si.sup.3+ are increased. Consequently, in the SiOx/Si/C composite material produced by the inventive process, the amount of Si in the higher oxidation state, i.e. Si.sup.4+ has been reduced by the reductant, and the amount of Si in the lower oxidation states is increased, and thus improves the cycle stability and obtains a larger reversible capacity.