Field of the Invention

The present invention generally relates to the field of lithium ion batteries. Particularly, the present invention relates to a method for preparing hollow silicon spheres, the obtained hollow silicon spheres and its application as anode materials for lithium ion batteries.

Prior Art

The capacity of graphite anode material in current commercial lithium ion battery is approaching its theoretical value (372 mAh/g), which will limit the application of lithium ion battery in automobile, energy storage and smart grid field. Silicon is considered to be a promising alternative to graphite because of its high capacity (-4200 mAh/g, more than 10 times as high as graphite) and rich reserves.

In spite of its high capacity, silicon suffers from fast capacity fading, which is mainly caused by the huge volume change during the lithium insertion and extraction. Recent study shows that the cycling performance of silicon-based material could be effectively improved by designing a hollow structure which provides reserved volume to accommodate the expansion of the materials during cycling (Nemo Lett. 2011, 11, 2949 - 2954, Angew. Chem. Int. Ed. 2012, 51, 2409 - 2413). In the reference (Ref. 1, Nano Lett. 2011, 11, 2949 - 2954), interconnected silicon hollow nanospheres were prepared by Si CVD on nano silica templates which were drop casted on stainless steel substrates. In another reference (Ref. 2, Angew. Chem. Int. Ed. 2012, 51, 2409 - 2413), hollow porous Si0 ₂ nanoparticles were synthesized using cetyltrimethylammonium bromide (CTAB) as surfactant to produce pores and polystyrene (PS) nanoparticles as templates to generate hollow structures; the resulting hollow porous Si0 ₂ nanoparticles were reduced to porous hollow Si by magnesiothermic reduction, followed by Ag coating to reach certain conductivity.

Problem to be Solved

Although the prepared hollow silicon spheres in above previous work exhibited enhanced cycling performance, there are still serious restrictions for industrialization.

In Ref. 1, the vital process drop-casting is quite complex and low efficient. In order to form a uniform coating layer, the nano Si0 ₂ templates need to be dispersed into ethanol solvent and the substrates need to be 0 ₂ plasma-treated before coating; during drop-casting, the operation need to be strictly controlled and the evaporation of solvent is not environment friendly. Even though the drop-casting process is repeated several times, the final silicon mass loading is only 0.1-0.2 mg/cm , which is too low for commercial application.

In Ref. 2, the high costs of CTAB surfactant and Ag modification, and the complexity of the process make it difficult to realize in industry scale.

Considering the high costs and complexity of the processes mentioned above, a facile and environmental friendly synthetic method with low cost is needed to produce hollow silicon spheres.

Summary of the Invention

In the present invention, hollow silicon spheres (named HSS) as a novel anode material with high capacity and good stability for Li-ion battery is synthesized by a simple and low cost method.

According to one aspect of the present invention, there is provided a simple and low cost method to prepare hollow silicon spheres. Particularly, the method for preparing hollow silicon spheres of the present invention can be summarized as below:

using nano particles as a template, a silicon coating layer is formed on the nano particles using a silicon source by means of chemical vapor deposition, followed by template removal and purification.

According to another aspect of the present invention, there is provided hollow silicon spheres obtained by the process according to the present invention.

According to a further aspect of the present invention, there is provided an anode material comprising the hollow silicon spheres of the present invention.

According to a further aspect of the present invention, there is provided a negative electrode for a lithium ion battery comprising the anode material.

According to a further aspect of the present invention, there is provided a lithium ion battery comprising the negative electrode.

Brief Description of the Drawings

Figure 1 shows the typical synthesis process of the inventive hollow silicon spheres and the final product HSS. Figure 2 shows the TEM image of commercially available nano-calcium carbonate particles (20-100 nm).

Figures 3a, 3b show TEM images of HSS-1 obtained in Example 1. Figure 3c shows SAED pattern of HSS-1.

Figure 4 shows SEM images and EDS pattern of HSS-1.

Figure 5 shows nitrogen adsorption/desorption isotherms and pore size distribution of HSS-1.

Figure 6 shows an XRD pattern of HSS-1.

Figure 7 (a) shows an XPS pattern of HSS-1, and Figure 7 (b) shows Si 2p spectra of HSS-1.

Figure 8 shows TEM images of HSS-2.

Figure 9 shows SEM images of HSS-2.

Figure 10 shows an XRD pattern of HSS-2.

Figure 11 shows the particle size distribution of HSS-2.

Figure 12 shows the cycling curve of HSS-1 at 400 mA/g between 0.02 and 1.5 V (100 m A/g in first 3 cycles).

Figure 13 shows the cycling curve of HSS-2 at 400 mA/g between 0.02 and 1.5 V (100 m A/g in first 3 cycles).

Detailed Description of the Invention

In the specification of the present invention, unless expressly indicated otherwise, all the percentages (%) shown are percentages by weight. Any interval of values denoted by the expression "between a and b" and "from a to b" means the range of values extending from a up to b (that is to say, including the strict limits a and b).

The invention describes a novel anode material of hollowed silicon spheres with good stability and capacity for lithium ion battery and its preparation method.

Compared with traditional silicon powders, our HSS material shows high capacity and good stability due to the designed hollow structure. The hollow structure leaves enough space for volume expansion during the lithium intercalation and the interconnection of silicon spheres provide a conductive network for the transference of electrons .

Compared with the reported work in Ref. 1 as mentioned previously, the present method is simple and easy to control. The template nano particles are directly used for Si CVD without drop-casting onto a particular substrate. More importantly, compared with low active substance loading (0.1-0.2 mg/cm ) in Ref.l, an active substance loading between 0.2 to 1.5 2

mg/cm is achieved easily in the present method and the active substance loading can be further increased up to 3 mg/cm by controlling the coating process.

Compared with ef. 2 as mentioned previously, the method of the present invention is low cost and involves no expensive reactant (surfactant, Ag). The template nano particles in present method are commercially available with a low cost. Therefore, the present method has great potential for practical application in industry.

Preparation of HSS material

The nano particles used as a template in the present invention are selected from carbonates and oxides. The carbonate is preferably selected from the group consisting of calcium carbonate, magnesium carbonate, strontium carbonate, barium carbonate. The oxides are preferably selected from the group consisting of AI2O3, MgO, ZnO and Si0 ₂ .

The size of the nano particles used as a template in the present invention is in a range of lO nm to 100 nm.

The silicon source used in the present invention is selected from the group consisting of high purity silane, chlorosilane and the like. Various kinds of chlorosilane can be used in the present invention, preferably trichlorosilane.

The chemical vapor deposition (CVD) used in the present invention is a conventional technique. CVD can be simply described as that a carrier gas is used to carry one or more source vapor gases into the reaction chamber, chemical reactions occur on the substrate surface, and the desired solid material is deposited onto the substrate. As to the present invention, during the CVD process, nano particles as the substrate are placed in a horizontal tube furnace or a fluidized-bed furnace, and silicon is deposited onto the substrate.

The particle size of the nano particles used in the present invention lies in a range of 10 nm to 100 nm, preferably in a range of 40 nm to 80 nm.

In a preferred embodiment, a mixture of 5% H ₂ and 95% inert gas such as Ar with a flow rate of 120 seem is introduced into the furnace before the temperature reaches 400-500 °C so as to form a reductive atmosphere. Then, high-purity silane with a purity of 99.999% carried by an inert gas such as Ar is introduced to the furnace which is at a temperature of 400-500°C at a flow rate of 80-120 seem for 1 to 3 hours. The mixing weight ratio of silane and Ar is in a range of 10:80- 2:98, preferably 5:95.

During the chemical vapor deposition process, at the same temperature and flow rate, silicon coating layers with different thicknesses can be obtained by controlling the time for deposition. For example, in the case that the temperature is 450 °C, the flow rate is 100 seem, when the time for deposition is 1.5 h, a silicon coating layer of about 10 nm is obtained; when the time for deposition is 2 h, a silicon coating layer of about 16 nm was obtained.

After reaction, in the cooling process, the gas mixture of 5% H ₂ and 95% Ar is reintroduced into the furnace.

In the subsequent process, acid with a concentration of for example 2 wt% is used to remove the nano particles, so as to obtain a hollow structure material. A person skilled in the art can choose suitable acids for this treatment, as long as the acid can react with the nano particles template to form a soluble salt or gas and does not impair the properties of silicon. For example, based on the common knowledge in the art, the acid can be suitably selected by a person skilled in the art from the group consisting of hydrochloric acid, sulfuric acid and hydrofluoric acid. A person skilled in the art knows that these acids may not be suitable for any kind of the template mentioned above. For example, hydrofluoric acid is suitable when Si0 ₂ is used as the template, but may not be suitable for other templates.

Further, considering that silicon atoms may be oxidized in the deposition process and acid treatment, hydrofluoric acid with a concentration of for example 10 wt% is used to purify the product, so as to obtain the final product hollow silicon spheres.

The hollow part size of the hollow silicon spheres is in a range of 10 nm to 90 nm, the primary particle size is in a range of 80 nm to 100 nm, and the secondary particle size is in a range of 1 to 30 μιτι, the thickness of the silicon wall is about 9-17 nm.

In the present invention, the term "primary particle" refers to the original particles of the hollow silicon spheres, and the term "secondary particle" refers to the particles agglomerated by the original particles of the hollow silicon spheres.

Assembly of coin cells

According to another aspect of the present invention, an anode material for lithium ion batteries is provided, comprising hollow silicon spheres material, conductive agents and binders.

Preferably, the anode material includes 50 wt%-80 wt% of hollow silicon spheres, 5 wt%-20 wt% of conductive agents and 5 wt%-30 wt% of binders, based on the total weight of the anode material.

The hollow silicon spheres are the hollow silicon spheres of the present invention.

The conductive agent can be selected by a person skilled in the art so as to improve the conductivity. For example, the conductive agent can be selected from the group consisting of conductive carbon black, carbon nanotubes and graphene. The binder is preferably polyacrylic acid (PAA), and it can also be selected from the group consisting of sodium carboxymethylcellulose (CMC), a mixture of sodium carboxymethylcellulose and styrene-butadiene rubber (SB ), and sodium alginate (SA).

As an example, the anode material includes 60 wt% of the hollow silicon spheres of the present invention, 20 wt% of the conductive carbon black and 20 wt% of polyacrylic acid, based on the total weight of the anode material.

In one embodiment, hollow silicon spheres of the present invention, conductive carbon black and polyacrylic acid in a weight ratio of 60:20:20 are dispersed in deioned water to form an anode material in the form of slurry. For illustrative purpose, the anode material in the form of slurry is poured onto a horizontally placed copper foil, preferably a wet film applicator of 150 μηι is used to coat a film to make an electrode. After coating, the electrode is left to dry. Then, the electrode is subjected to a tabletting process under a pressure of for example 8 MPa. After being tabletted, the electrode is placed preferably in a vacuum oven at a temperature of 80 °C to dry overnight, thus a negative electrode is produced. The negative electrode formed as described above together with lithium foil may form counter electrodes of a lithium ion battery. 1 mol/L of LiPF ₆ /EC: DMC: EMC in the volume ratio of 1 : 1 : 1 with 2 wt% VC as an additive may be used as the electrolyte. EC refers to allyl carbonate, DMC refers to dimethyl carbonate, EMC refers to ethyl methyl carbonate and VC refers to vinylene carbonate. The battery mold is for example a 2025 type coin cell. Coin cells are thus prepared.

The active substance loading is determined by the amount of anode materials coated onto the copper foil.

In order to measure the electrochemical properties of the coin cells, the coin cells are subjected to galvanostatic charge-discharge tests. The current density is 100 mA/g (the first three cycles) and 400 mA/g (subsequent cycles), the voltage range is set to 0.02-1.5 V. The capacity is calculated based on the weight of hollow silicon spheres in the anode.

Galvanostatic charge-discharge tests show that the initial discharge capacity of the coin cells employing the hollow silicon spheres of the present invention is up to 3063 mAh/g, the first charge capacity can reach 2246 mAh/g, initial columbic efficiency is up to 82%. After 100 cycles under 400 mAh/g, the reversible capacity can still reach 1150 mAh/g.

Examples

Example 1 Preparation of hollow silicon spheres HSS-1

EV-E-006 produced by Hefei EV NANO Technology Co., Ltd., China was used as the nano calcium carbonate template, its particle size is 50-80 nm. Chemical vapor deposition was carried out in a horizontal tube furnace (its internal diameter is 60 mm) at a temperature of 450 °C. Before the temperature reached 450 °C, a mixture of 5% ¾ and 95% Ar was introduced into the furnace to form a reductive atmosphere and remove the remaining oxygen. Then, high-purity silane with a purity of 99.999% carried by Ar was introduced to the horizontal tube furnace which was at a temperature of 450°C at a flow rate of 100 seem for 1.5 hours. Silane was decomposed into silicon particles and ¾, and silicon particles finally deposited onto the nano-calcium carbonate. The mixing weight ratio of silane and Ar is 5:95. After the deposition, a mixture of 5% ¾ and 95% Ar was reintroduced into the furnace to accelerate the cooling and prevent oxidization of silicon particles. In the subsequent process, 2 wt% of hydrochloric acid was used to remove the nano-calcium carbonate to obtain hollow structure materials. Considering that silicon atoms may be oxidized in the deposition process, 10 wt% of hydrofluoric acid was used to purify the product, so as to obtain the final product hollow silicon spheres HSS-1.

The typical morphologies of HSS-1 in Figures 3a, 3b show that HSS-1 is consisted of interconnected hollow spheres with a narrow primary particle size distribution. The wall thickness of the hollow silicon spheres is 9-12 nm. The SAED pattern in Figure 3 c shows that HSS-1 is almost an amorphous material. This is also verified by the XRD pattern in Figure 6. Figure 6 does not show obvious characteristic peaks of crystalline silicon.

The SEM image and EDS pattern in Figure 4 show that except C, Si, and O, no other impurity element exists in the HSS material, this is also verified by the XPS spectrum in Figure 7. Further, Si 2p spectra show that, the symmetry peak with a center of 99.4 eV is regarded as the Si-Si bond, indicating that the main state of the material is elemental silicon.

Figure 5 shows nitrogen adsorption/desorption and pore size distribution of HSS-1. It can be seen that hollow silicon spheres HSS-1 has a specific surface area of 35.7 m /g and a pore volume of 0.274 cm /g. Pore size distribution corresponding to hollow part size of HSS-1 is mainly between 10 nm and 90 nm, which is identical to the size of the nano-calcium carbonate template.

Example 2 Preparation of hollow silicon spheres HSS-2

Example 2 is almost the same with Example 1 , except that the time for CVD deposition is 2 hours. That is, during the chemical vapor deposition, high-purity silane with a purity of 99.999% carried by Ar was introduced to the horizontal tube furnace (with an internal diameter of 60 mm) at a temperature of 450 °C at a flow rate of 100 seem for 2 hours.

The typical morphology of HSS-2 in Figure 8 shows that HSS-2 is consisted of mainly interconnected hollow spheres with a narrow primary particle size distribution. The wall thickness of the hollow spheres is about 16 nm. The X-ray diffraction pattern in Figure 10 verifies that HSS-2 is also amorphous, since no obvious characteristic peaks of crystalline silicon is shown.

The SEM images in Figure 9 show that HSS-2 has a secondary sphere structure. The primary particle size is similar to that of the nano-calcium carbonate, and the secondary particle size is approximately 10 μιτι, which is consistent with the measurement results of the particle size distribution shown in Figure 11. Figure 11 shows that about 95% of the hollow silicon spheres have a size of 1-30 μιη.

Example 3 Assembly of coin cell 1

Hollow silicon spheres HSS-1 prepared by Example 1 was used to prepare a negative electrode. The anode material comprised 60 wt% of hollow silicon spheres, 20 wt% of conductive carbon black and 20 wt% binder, wherein the binder is polyacrylic acid (PAA). The formed negative electrode together with lithium foil constituted counter electrodes of a lithium ion battery. 1 mol/L of LiPF ₆ /EC: DMC: EMC in the volume ratio of 1 : 1 : 1 with 2 wt% VC as an additive was used as the electrolyte. The battery mold was a 2025 type coin cell, a coin cell was thus prepared.

It was determined that the active substance loading was 0.85 mg/cm .

In order to measure the electrochemical properties of the coin cell, the coin cell was subjected to galvanostatic charge-discharge tests. The current density was 100 mA/g (the first three cycles) and 400 mA/g (subsequent cycles), the voltage range was set to 0.02-1.5 V.

Figure 12 shows the charge-discharge cycle performance curves. The test results show that the initial discharge capacity of the coin cell is up to 3063 mAh/g, the first charge capacity can reach 2246 mAh/g. The first columbic efficiency is 73%. After 160 cycles under 400 mAh/g, the reversible capacity still reached 1150 mAh/g. That is, after 160 cycles, the capacity retention ratio is 73%. From the 20 ^th to the 160 ^th cycle, the capacity loss was less than 2%.

Example 4 Assembly of coin cell 2

Hollow silicon spheres HSS-2 prepared by Example 2 was used to prepare a negative electrode. The anode material comprised 60 wt% of hollow silicon spheres material, 20% by weight of conductive carbon black and 20 wt% binder, wherein the binder is polyacrylic acid (PAA). The formed negative electrode together with lithium foil constituted counter electrodes of a lithium ion battery. 1 mol/L of LiPF ₆ /EC: DMC: EMC in the volume ratio of 1 : 1 : 1 with 2 wt% VC as an additive was used as the electrolyte. The battery mold was a 2025 type coin cell, a coin cell was thus prepared. 2

It was determined that the active substance loading was 0.62 mg/cm .

In order to measure the electrochemical properties of the coin cell, the coin cell was subjected to galvanostatic charge-discharge tests. The current density was 100 mA/g (the first three cycles) and 400 mA/g (subsequent cycles), the voltage range was set to 0.02-1.5 V.

Figure 13 shows the charge-discharge cycle performance curves. The test results show that the initial discharge capacity of the coin cell is up to 2547 mAh/g, the first charge capacity can reach 2093 mAh/g. The initial columbic efficiency reaches 82%. After 100 cycles under 400 mAh/g, the reversible capacity still reached 800 mAh/g. The capacity retention ratio is 44%.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.