NANOPARTICULATE SILICON CARBIDE AND ELECTRODE COMPRISING NANOPARTICULATE SILICON CARBIDE

Abstract

The present invention relates to nanoparticulate stoichiometric doped or non-doped silicon carbide SiC in the form of secondary particles, which consist of agglomerates of SiC primary particles, wherein the primary particles have a particle size in the range of 40-100 nm and the secondary particles have an average size of 1-10 μm. Furthermore, the present invention relates to an anode of a secondary lithium-ion battery containing the SiC according to the invention and a secondary lithium-ion battery having this anode.

Claims

1. A nanoparticulate silicon carbide SiC in the form of secondary particles, which consist of agglomerates of SiC primary particles, wherein the primary particles have a particle size in the range of 5-100 nm and the secondary particles have an average size of 1-10 μm.

2. The nanoparticulate silicon carbide SiC as claimed in claim 1, wherein the agglomerates have a bulk density of 1200-1600 g/l.

3. The nanoparticulate silicon carbide SiC as claimed in claim 1, wherein the agglomerates have a compressed density of 1500-3000 g/l (1.5-3 g/cm.sup.3).

4. The nanoparticulate silicon carbide SiC as claimed in claim 1, the powder resistance of which is >28 Ω/cm.

5. The nanoparticulate silicon carbide SiC as claimed in claim 1, wherein the SiC is doped using an element selected from us Mg, Nb, Zr, B, Cr, V, Sc, Y, Al, N, P, La, Er, and Ga and mixtures thereof.

6. The nanoparticulate silicon carbide SiC as claimed in claim 5, wherein the element is B and/or Al or N and/or P.

7. The nanoparticulate silicon carbide SiC as claimed in claim 1, wherein the SiC is present in the 3C crystal structure.

8. A use of a nanoparticulate silicon carbide as claimed in claim 1 as an electrode for a secondary lithium-ion battery.

9. An electrode for a secondary lithium-ion battery containing a silicon carbide as the active material as claimed in claim 1.

10. The electrode as claimed in claim 9, wherein the density of the active material of the electrode is 1.5 bis 3 g/cm.sup.3.

11. The electrode as claimed in claim 9, wherein the electrode is an anode.

12. The electrode as claimed in claim 11, wherein the SiC is doped using N or Al.

13. The electrode as claimed in claim 12, wherein the electrode doped using Al has a plateau at 0.4 V+/−0.05 V vs. Li.

14. A secondary lithium-ion battery comprising an anode as claimed in claim 12.

15. The electrode as claimed in claim 10, wherein the electrode is an anode.

16. A secondary lithium-ion battery comprising an anode as claimed in claim 13.

Description

[0027] Furthermore, in the figures

[0028] FIG. 1: shows a SEM picture of SiC doped using aluminum: 3C—SiC:Al

[0029] FIG. 2: shows the XRD spectrum of 3C—SiC:Al

[0030] FIG. 3: shows a SEM picture of SiC doped using nitrogen: 3C—SiC:N

[0031] FIG. 4: shows a voltage diagram of an electrode according to the invention with 3C—SiC:Al vs. Li

EXEMPLARY EMBODIMENTS

Measurement Methods

[0032] The determination of the BET surface area was carried out according to DIN 66131 (DIN-ISO 9277). Micromeritics Gemini V or Micromeritics Gemini VII were used for this purpose as measuring devices.

[0033] The measurement of the x-ray powder diffractogram (XRD) was carried out using a Siemens XPERTSYSTEM PW3040/00 and the software DY784.

[0034] The SEM pictures were carried out using a LEO 1530 VP microscope, which was connected to a Gemini TFE column, at an acceleration voltage of 4 kV.

[0035] The determination of the compressed density and the powder resistance were carried out simultaneously on a Mitsubishi MCP-PD51 tablet press device with Loresta-GP MCP-T610 resistance measuring device, which were installed in a glovebox to which nitrogen was applied to preclude potential interfering effects of oxygen and moisture. The hydraulic actuation of the tablet press took place via a manual hydraulic press Enerpac PN80-APJ (max. 10,000 psi/700 bar).

[0036] The measurements were carried out at the following settings

TABLE-US-00001 Sample quantity 4 g Applied pressure 7.5 kN Resistance meter Loresta GP Loresta GP Measurement sensor setting ESP ESP Poles linear linear Pole interval 3 mm 3 mm Pole size 1.4 mm 1.4 mm Sample shape round round Size of the sample diameter = 20 mm 20 mm Thickness of the sample sample dependent 5 mm Measurement position X: 10 mm; Y: 10 mm (10, 10) RCF automatic calculation 2.758

[0037] The powder resistance was subsequently calculated according to the following equation:

Powder resistance [Ω/cm]=resistance [Ω]×thickness [cm]×RCF

[0038] The compressed density was calculated according to the following formula:

Compressed density (g/cm.sup.3)=mass of the sample (g)

Π×r.sup.2 (cm.sup.2)×thickness of the sample (in cm)

[0039] Typical manufacturing tolerances are at most 3%.

[0040] Determination of the Density of the Active Material in an Electrode

[0041] To determine the material density of the active material, electrodes (thickness approximately 60 μm) having a composition 50% active material, 30 wt. % Super-P carbon, and 20 wt. % binder (NMP, N-methyl-2-pyrrolidone) were produced.

[0042] For this purpose, the appropriate quantities were weighed in a 50 ml screwtop jar and mixed for 5 minutes at 600 RPM on a magnetic stirrer using a crossbar stirring element, dispersed for 1 minute using an ultrasonic finger Hielscher UP200S, and subsequently after addition of 20 glass beads of the diameter 4 mm and closing of the glass, rotated at a speed of 10 RPM on a rolling table for at least 15 hours. For the electrode coating, the homogeneous suspension thus obtained was applied using a doctor blade laboratory squeegee having 200 μm gap width and a feed speed of 20 mm/sec on an aluminum carrier foil.

[0043] After drying at 80° C. in the vacuum drying cabinet, electrodes of 13 mm diameter were punched out of the film and mechanically post-compressed at room temperature on a uniaxial hydraulic laboratory press from Specac at a load of 10 tons for 60 seconds. For the density determination, the net electrode weight was determined from the gross weight and the known weight per unit of area of the carrier foil and the net electrode thickness was determined using a micrometer screw minus the known thickness of the carrier foil.

[0044] The active mass density in g/cm.sup.3 in the electrode is calculated therefrom via

(Active material proportion in electrode formula (50%)*electrode net weight in g/(π((0.65 cm).sup.2*net electrode thickness in cm)

As the value for the active material density in the electrode, 1.7 g/cm.sup.3 was found for the material according to the invention.

[0045] Determination of the Particle Size Distribution:

[0046] The particle size distributions for the mixtures or suspensions and the produced material are determined on the basis of the light scattering method using commercially available devices. This method is known per se to a person skilled in the art, wherein reference is also made in particular to the disclosure in JP 2002-151082 and WO 02/083555. In the present case, the particle size distributions were determined according to DIN 66133 with the aid of a laser diffraction meter (Mastersizer S, Firma Malvern Instruments GmbH, Herrenberg, DE) and the software of the producer (Version 2.19) using a Malvern Small Volume Sample Dispersion Unit, DIF 2002 as the measuring unit. The following measurement conditions were selected: Compressed range; active beam length 2.4 mm; measurement range: 300 RF; 0.05 bis 900 μm. The sample preparation and measurement were carried out according to the producer specifications.

[0047] The D.sub.90 value specifies the value at which 90% of the particles in the measured sample have a smaller or equal particle diameter. Correspondingly, the D.sub.50 value and the D.sub.10 value specify the value at which 50% or 10%, respectively, of the particles in the measured sample have a smaller or equal particle diameter.

[0048] According to one particularly preferred embodiment according to the invention, the values mentioned in the preceding description apply for the D.sub.10 values, D.sub.50 values, the D.sub.90 values and the difference of the D.sub.90 and the D.sub.10 values in relation to the volume proportion of the respective particle in the total volume. Accordingly, the D.sub.10, D.sub.50, and D.sub.90 values mentioned here according to this embodiment according to the invention specified the values at which 10 vol. % or 50 vol. % or 90 vol. % of the particles in the measured sample have a smaller or equal particle diameter. If these values are maintained, particularly advantageous materials are provided according to the invention and negative influences of relatively coarse particles (having comparatively larger volume component) on the processing ability and the electrochemical product properties are avoided. The values mentioned in the present description for the D.sub.10 values, the D.sub.50 values, the D.sub.90 values and the difference of the D.sub.90 and the D.sub.10 values particularly preferably apply both with respect to percent and also volume-percent of the particles.

[0049] In the case of compositions (for example electrode materials) which contain further components in addition to the silicon carbide according to the invention, in particular in the case of carbonaceous compositions, the above light scattering method can lead to misleading results, since the silicon carbide particles can be bonded to form larger agglomerates by the additional (for example carbonaceous) material. The particle size distribution of the material according to the invention in such compositions can be determined, however, on the basis of SEM recordings as follows: A small quantity of the powder sample is suspended in acetone and dispersed for 10 minutes using ultrasound Immediately thereafter, several drops of the suspension are dripped on a sample plate of a scanning electron microscope (SEM). The solid concentration of the sample and the number of the drops are dimensioned so that a substantially single-ply layer made up of powder particles forms on the carrier to prevent mutual concealment of the powder particles. The dripping has to take place rapidly before the particles can separate according to size by sedimentation. After drying in air, the sample is transferred into the measurement chamber of the SEM. In the present example, it is a device of the type LEO 1530, which is operated using a field emission electrode at 1.5 kV excitation voltage and a sample distance of 4 mm. At least 20 randomly placed detail enlargements having an enlargement factor of 20,000 are recorded of the sample. These are each printed on a DIN A4 sheet together with the overlaid magnification scale. If possible, at least 10 freely visible particles of the material according to the invention, from which the powder particles are constructed, are randomly selected on each of the at least 20 sheets, wherein the boundaries of the particles of the material according to the invention are defined by the absence of fixed, direct adhesion bridges. In contrast, bridges due to possibly present carbon material are included in the particle boundary. The longest and shortest axis in the projection are each measured using a ruler for each of the selected particles and converted to the real particle dimensions on the basis of the scale ratio. For each measured SiC particle, the arithmetic mean value of the longest and the shortest axis is defined as the particle diameter. Subsequently, the measured SiC particles are classified into size classes similarly to the light scattering measurement. If one plots the number of the respective associated particles over the size class, a differential particle size distribution with respect to the number of particles is obtained. If the particle numbers are summed progressively from the small to the large particle classes, the cumulative particle size distribution is obtained, from which D.sub.10, D.sub.50, and D.sub.90 can be read directly on the size axis.

[0050] The described method is also applied to battery electrodes containing the material according to the invention. In this case, however, instead of a powder sample, a fresh cut surface or fracture surface of the electrode is fastened on the sample carrier and studied in SEM.

Exemplary Embodiments

[0051] The SiC according to the invention was produced by means of a modified sol-gel method as was similarly described in broad strokes, for example, by Yajima et al. Chem. Lett. 1975, 931 or by B. Friedel, Dissertation Paderborn, 2007, B. Kettner et al. In Adv. Eng. Mater. 2018, 1701067.

Example 1

[0052] Production of Nanoparticulate Silicon Carbide (3C—SiC)

[0053] 1.1 Production of the Sol-Gel Si—C Precursor:

[0054] 135 g tetraethyl orthosilicate (TEOS) was dissolved in 170 ml ethanol. Furthermore, a solution of 60 g sucrose was produced at 60° C. in 75 mL distilled water, to which 37.15 ml HCl (1M) was added drop by drop as a catalyst to form invert sugar. Subsequently, both solutions were mixed with one another with stirring and permitted to cool. A ratio of 1/6.5/0.3/0.06 has proven to be advantageous for the molar ratios TEOS/water/sucrose/HCl used. Variations of these ratios (individual or all) in the range of +/−10% are also usable in the scope of the present invention without changes occurring in the final product. Alternatively, instead of the sucrose solution, liquid sugar (invert sugar, 122 g 70%) can be used directly. Water is then not added and only very little HCl (5.2 mL 1M), since it is only still required to start the gelling process.

[0055] The resulting sol was dried for 48 hours and 60° C. and subsequently for 24 hours at 100-160° C., preferably at 150° C. The dry black coarse-grained granulate thus obtained (“Xerogel”) was subsequently sintered under argon at 1100° C. over 15 hours and optionally ground. The molar ratio of C/Si in the granulate was 3.6. This ratio is particularly preferred, however, pure-phase 3C—SiC according to the invention is also obtained in a range of 3.2 to 4.0 C/Si. Outside this range, various foreign phases are found in the final product, such as molten SiO.sub.2 and carbon residues and other SiC. Graphene residues can also form on the surface of the material according to the invention. Kettner et al. (op. cit.) uses different quantities of starting materials in relation to the method described here and also modified reaction conditions, whereby mixed phases are obtained (op. cit. chapter 3.1 and 3.2).

[0056] 1.2. Production of SiC from the Precursor

[0057] Subsequently, the granulate was sintered at 1800° C. for 5 hours, wherein the heating rate from 1000° C. to 1800° C. took place at a temperature gradient of 100° C./min. It was subsequently cooled to room temperature (25° C.) within 30 minutes. The particle size of the nanoparticulate pure-phase and stoichiometric 3C—SiC thus obtained was 40-100 nm, with a D.sub.90 value of 63 nm (+/−1 nm) for the primary particles and 1-10 μm for the secondary particles with a D.sub.90 value of 8 μm. The size of the primary particles may advantageously be controlled via the heating speed (heating rate) and the duration of the temperature treatment at 1800° C. Particularly large primary crystallites between 80 and 100 μm in size are obtained upon slower heating of the granulate, for example, at 10° C./minute and 8 hours sintering. It has fundamentally been found that with a faster heating rate and shorter duration of the temperature treatment at 1800° C., smaller primary crystallites are obtained.

Example 2

[0058] Production of Doped Nanoparticulate Silicon Carbide (3C—SiC)

[0059] The production runs similarly to that of the non-doped SiC. However, before the addition of the sucrose, the corresponding compound(s) of the dopant element(s) or the pure element(s) are put into the water heated to 60° C. Otherwise, the method is as in example 1. The sols thus obtained are partially colored depending on the dopant element/compound.

[0060] The amount of dopant compound/element was 5% in relation to 1M Si in each case.

[0061] The doped SiC thus obtained was studied in each case by means of EPR (electron paramagnetic resonance) spectroscopy and XRD.

[0062] In the present case, the following doped 3C—SiC nanoparticles were obtained: 2.1. 3C—SiC:Al by adding aluminum acetyl acetonate (5 at % Al in relation to Si) or elementary aluminum, dark blue 3C—SiC:Al was obtained.

[0063] An SEM recording of 3C—SiC:Al is shown in FIG. 1. The primary particles (primary crystallites) and the agglomerates consisting of them are clearly recognizable.

[0064] FIG. 2 shows an EDX recording of the 3C—SiC:Al according to the invention; the reflections for Si and Al are clearly recognizable.

[0065] 2.2 3C—SiC:P (5 at % P in relation to Si) by addition of potassium dihydrogen phosphate

[0066] 2.3 3C—SiC—N(5 at % Al in relation to Si) by addition of nitric acid, dark blue 3C—SiC:N was obtained.

[0067] An SEM recording of 3C—SiC:N shown in FIG. 3. The primary particles (primary crystallites) and the agglomerates consisting of them are clearly recognizable.

[0068] 2.4.3C—SiC-B by addition of boron acetyl acetonate

[0069] 2.5 3C—SiC-Er by addition of erbium acetyl acetonate

Example 3

[0070] Thin-film electrodes having 3C—SiC:Al and 3C—SiC:N as the active material were produced, as described, for example, in Anderson et al., Electrochem. and Solid State Letters 3 (2) 2000, pages 66-68. The electrode compositions typically consisted of 50 weight-parts active material, 30 weight-parts super P carbon, and 20% polyvinylidene fluoride (Solvay 21216) as the binder. A suspension was produced therefrom in N-methyl-2-pyrrolidone. The solid content of the slurry was 11.5%.

[0071] The electrode suspension was dispensed using a doctor blade (squeegee) at a height of approximately 200 μm and the N-methyl pyrrolidone was evaporated at 105° C. under vacuum. The dried electrodes were rolled multiple times or compressed using suitable pressure until a thickness of 20 to 25 μm was obtained. Subsequently, the electrodes were cut out (13 mm diameter) and compressed in an IR press at a pressure of 5 tons (3.9 tons/cm.sup.2) over 20 seconds at room temperature. The electrodes were then dried overnight at 120° C. under vacuum and installed in an argon-filled glovebox in half cells against lithium metal and measured electrochemically. The electrode charge was 0.7 mg/cm.sup.2 for SiC:N and 4.6 mg/cm.sup.2 for SiC:Al.

[0072] The electrochemical measurements were carried out against lithium metal (counter and reference electrodes made of lithium) and using LP30 (Merck, Darmstadt) as the electrolyte (EC (ethylene carbonate):DMC (dimethyl carbonate)=1:1, 1 M LiPF.sub.6). The test method was carried out in the CC mode, i.e., cycles with a constant current at the C/100 rate between the voltage limits 0.05 V and 2.0 V against Li/Li.sup.+

[0073] In the electrodes having 3C—SiC:Al as the active material, a reversible capacitance between 400 and 500 mAh/g was obtained. Upon delithiation, a so-called plateau was observed at 0.4 V vs. Li (FIG. 4). The electrode contained 2.6 mg 3C—SiC:Al as active material. The total measuring time was 190 hours.

[0074] 2 cycles were measured:

TABLE-US-00002 Lithiation Delithiation Lithiation Delithiation Capacitance/mAh/ Capacitance/mAh/ Cycle Capacitance/mAh Capacitance/mAh g(SiC.Al) g(SiC.Al) Efficiency/% 1 1.920 1.020 738 392 53 2 1.208 0.897 464 345 74 3 0.994 0.788 382 303 79 4 0.844 0.709 324 273 84 5 0.753 0.657 289 253 87