WO2014102219A1

WO2014102219A1 - Silicon nanosheet and preparing method of the same

Info

Publication number: WO2014102219A1
Application number: PCT/EP2013/077834
Authority: WO
Inventors: Seong-Ju Hwang; Seung Mi Oh; Kanyaporn ADPAKPANG; Sharad Bandu PATIL
Original assignee: Solvay Sa
Priority date: 2012-12-24
Filing date: 2013-12-20
Publication date: 2014-07-03
Also published as: KR101924925B1; KR20140082571A

Abstract

The present disclosure relates to a silicon nanosheet comprising a layered structure substantially similar to a clay, a preparing method of the same, an anode for lithium- ion battery including the silicon nanosheet, and a lithium-ion battery including the anode for lithium-ion battery.

Description

Silicon nanosheet and preparing method of the same

This application claims priority to Korean Patent application

No. 10-2012-0152344 filed on December 24^th, 2012, the whole content of this application being incorporated herein by reference for all purposes.

FIELD OF THE INVENTION

The present disclosure relates to a silicon nanosheet comprising a layered structure substantially similar to a clay, a preparing method of the same, an electrode for battery including the silicon nanosheet, and a battery including the electrode.

BACKGROUND OF THE INVENTION

Lithium- ion batteries have been widely used as energy storage devices for driving electronic equipment. In lithium-ion secondary batteries, graphite- containing materials have been widely used as an anode active material. When a graphite-containing material releases lithium, an average potential is about 0.2 V (based on Li/Li+) and its potential changes in a relatively uniform manner during discharge. Therefore, the voltage of a battery is constantly high. However, a graphite material has a low electrical capacity per unit mass of about 372 mAh/g, but a capacity of a graphite material is currently increased to be close to such a theoretical capacity. Therefore, it is difficult to further increase its capacity.

In order to further increase a capacity of a lithium- ion battery, various anode active materials have been studied. A material forming an intermetallic compound with lithium, such as silicon and tin, has been expected to be promising as an anode active material having a high capacity. In particular, silicon is an alloy-type anode material having a theoretical capacity

(about 4,200 mAh/g) about ten times higher than that of graphite and has been drawing attention as an anode active material of a lithium-ion battery at present. Further, silicon is the second most abundant element in the Earth's crust

(about 28 % by mass), and thus, its mass production can be achieved at a relatively low cost. By way of example, Korean Patent Laid-open Publication No. 2012-0061941 describes a silicon oxide and an anode material for lithium- ion secondary battery.

However, silicon undergoes a great change in volume (-about 300 %) during charge and discharge of a battery, and thus, a physical contact between elements is cut off and an ion conductivity, an electrical conductivity, and the like of silicon are sharply decreased. Therefore, an actual capacity tends to be sharply decreased. Accordingly, when silicon having a high theoretical capacity is applied to a lithium- ion battery, development of a technology for minimizing a volume change during charge and discharge is demanded.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, the present disclosure provides a silicon nanosheet including a layered structure substantially similar to a clay, a preparing method of the same, an electrode for battery including the silicon nanosheet, and a battery including the electrode.

However, the problems to be solved by the present disclosure are not limited to the above description and other problems can be clearly understood by those skilled in the art from the following description.

In accordance with a first aspect of the present disclosure, there is provided a preparing method of a silicon nanosheet including heating a mixture including a clay and an alkaline earth metal to reduce a silica included in the clay to a silicon nanosheet.

In accordance with a second aspect of the present disclosure, there is provided a silicon nanosheet including a layered structure substantially similar to a clay.

In accordance with a third aspect of the present disclosure, there is provided an electrode for battery including the silicon nanosheet of the second aspect of the present disclosure.

In accordance with a fourth aspect of the present disclosure, there is provided a battery including the electrode for battery of the third aspect of the present disclosure as an anode, a cathode, and an electrolyte.

In a method for preparing a silicon nanosheet in accordance with the present disclosure, a silicon nanosheet including a layered structure substantially similar to a clay can be prepared. The silicon nanosheet can be prepared from an easily available clay, and thus, it is economical and can be readily prepared. Further, the silicon nanosheet has a structure of a two-dimensional crystalline form, and thus, it is resistant to a volume change during charge and discharge. Therefore, when the silicon nanosheet is used as an anode active material of a battery, a volume change during charge and discharge of the battery can be minimized and a high electrical conductivity can be maintained. Furthermore, since silicon has a high electrical capacity, the silicon nanosheet also has a high electrical capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments will be described in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be intended to limit its scope, the disclosure will be described with specificity and detail through use of the accompanying drawings, in which :

Fig. 1 provides a result of a powder X-ray diffraction pattern analysis of silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 2 provides scanning electron microscope (SEM) images of a silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 3 provides a result of a powder X-ray diffraction pattern analysis of silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 4 provides scanning electron microscope (SEM) images of a silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 5 provides a result of a powder X-ray diffraction pattern analysis of silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 6 provides a result of a powder X-ray diffraction pattern analysis of silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 7 provides a result of a powder X-ray diffraction pattern analysis of silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 8 provides an image of high resolution transmission electron microscopy (HR-TEM) of the silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 9 provides a graph and a table showing B.E.T. surface areas of silicon nanosheet prepared from talc in accordance with an example of the present disclosure; Fig. 10 provides a graph showing a pore diameter distribution of silicon nanosheet prepared from talc in accordance with an example of the present disclosure;

Fig. 11 provides images of exfoliated laponite prepared in accordance with an example of the present disclosure;

Fig. 12 provides images of exfoliated bentonite prepared in accordance with an example of the present disclosure;

Fig. 13 provides TEM image (a) and SAED pattern (b) of exfoliated laponite prepared in accordance with an example of the present disclosure;

Fig. 14 provides TEM image (a) and SAED pattern (b) of exfoliated bentonite prepared in accordance with an example of the present disclosure;

Fig. 15 provides a result of a powder X-ray diffraction pattern analysis of exfoliated laponite (a) and exfoliated bentonite (b) prepared in accordance with an example of the present disclosure;

Fig. 16 provides a result of a powder X-ray diffraction pattern analysis of silicon nanosheet prepared from exfoliated laponite (a) and silicon nanosheet prepared from exfoliated bentonite (b) in accordance with an example of the present disclosure;

Fig. 17 provides a result of a galvanostatic test of a silicon nanosheet prepared in accordance with an example of the present disclosure;

Fig. 18 provides a result of a galvanostatic test of a silicon nanosheet prepared in accordance with an example of the present disclosure; and

Fig. 19 provides a result of a galvanostatic test of a silicon nanosheet prepared in accordance with an example of the present disclosure.

Figs. 20a to 20c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 21a to 21c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 22a to 22c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 23a to 23c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure; Figs. 24a to 24b provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Fig. 25a provides a result of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure, and Fig. 25b provides a capacity retention analysis graph of a silicon

nanomaterial prepared in accordance with an example of the present disclosure;

Fig. 26 provides a photo of colloid including a silica-containing material and a powderized silica-containing material prepared in accordance with an example of the present disclosure;

Figs. 27a to 27c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 28a to 28c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 29a to 29c provide an image of transmission electron

microscopy (Fig. 29a), an image of high resolution transmission electron microscopy (Fig. 29b), and a selected area electron diffraction pattern (Fig. 29c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 30a to 30c provide an image of transmission electron

microscopy (Fig. 30a), an image of high resolution transmission electron microscopy (Fig. 30b), and a selected area electron diffraction pattern (Fig. 30c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 3 la to 3 lc provide an image of transmission electron

microscopy (Fig. 31a), an image of high resolution transmission electron microscopy (Fig. 31b), and a selected area electron diffraction pattern (Fig. 31c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 32a to 32c provide results of an N₂ adsorption/desorption isotherm analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 33a to 33c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure; Figs. 34a to 34c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 35a to 35c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 36a to 36c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 37a to 37c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 38a to 38c provide an image of transmission electron

microscopy (Fig. 38a), an image of high resolution transmission electron microscopy (Fig. 38b), and a selected area electron diffraction pattern (Fig. 38c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 39a to 39c provide an image of transmission electron

microscopy (Fig. 39a), an image of high resolution transmission electron microscopy (Fig. 39b), and a selected area electron diffraction pattern (Fig. 39c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 40a to 40c provide an image of transmission electron

microscopy (Fig. 40a), an image of high resolution transmission electron microscopy (Fig. 40b), and a selected area electron diffraction pattern (Fig. 40c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 41a to 41c provide results of an N₂ adsorption/desorption isotherm analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 42a to 42c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 43a to 43c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure; Figs. 44a to 44c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 45a to 45c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 46a to 46c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 47a to 47c provide an image of transmission electron

microscopy (Fig. 47a), an image of high resolution transmission electron microscopy (Fig. 47b), and a selected area electron diffraction pattern (Fig. 47c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 48a to 48c provide an image of transmission electron

microscopy (Fig. 48a), an image of high resolution transmission electron microscopy (Fig. 48b), and a selected area electron diffraction pattern (Fig. 48c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 49a to 49c provide an image of transmission electron

microscopy (Fig. 49a), an image of high resolution transmission electron microscopy (Fig. 49b), and a selected area electron diffraction pattern (Fig. 49c) of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 50a to 50c provide results of an N₂ adsorption/desorption isotherm analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 51a to 51b provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 52a to 52c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 53a to 53c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure;

Figs. 54a to 54c provide results of an X-ray diffraction analysis of a silicon nanomaterial prepared in accordance with an example of the present disclosure; Figs. 55a to 55c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure; and

Figs. 56a to 56c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial prepared in accordance with an example of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, examples of the present disclosure will be described in detail with reference to the accompanying drawings so that the present disclosure may be readily implemented by those skilled in the art. However, it is to be noted that the present disclosure is not limited to the examples but can be embodied in various other ways. In drawings, parts irrelevant to the description are omitted for simplicity of explanation, and like reference numerals denote like parts through the whole document.

Through the whole document, the term "comprises or includes" and/or

"comprising or including" used in the document means that one or more other components, steps, operation and/or existence or addition of elements are not excluded in addition to the described components, steps, operation and/or elements unless context dictates otherwise.

The term "about or approximately" or "substantially" are intended to have meanings close to numerical values or ranges specified with an allowable error and intended to prevent accurate or absolute numerical values disclosed for understanding of the present disclosure from being illegally or unfairly used by any unconscionable third party. Through the whole document, the term "step of does not mean "step for".

Through the whole document, the term "on" that is used to designate a position of one element with respect to another element includes both a case that the one element is adjacent to the another element and a case that any other element exists between these two elements.

Through the whole document, the term "combination of included in

Markush type description means mixture or combination of one or more components, steps, operations and/or elements selected from a group consisting of components, steps, operation and/or elements described in Markush type and thereby means that the disclosure includes one or more components, steps, operations and/or elements selected from the Markush group. Through the whole document, the term "A and/or B" means "A or B, or A and B".

In accordance with a first aspect of the present disclosure, there may be provided a silicon nanosheet preparing method including heating a mixture including a clay and an alkaline earth metal to reduce a silica included in the clay to a silicon nanosheet.

As used in the present disclosure, the term "silicon nanosheet" is understood to denote, in particular, a planar structure or a substantially planar structure in which silicon atoms having two-dimensional crystalline form or tetrahedral crystalline form are arranged two-dimensionally and periodically.

Further, silicon nanosheet having layered structure may include one layer of the planar structure or may include at least two layers of the planar structure in which they are layered.

By way of example, a silicon nanosheet prepared by the silicon nanosheet preparing method of the present disclosure may have a layered structure, wherein one or more than two of silicon nanosheets are layered to form layered structure. However, the present disclosure may not be limited thereto.

In accordance with an illustrative embodiment of the present disclosure, the clay may include a clay having a layered structure such as a layered clay, and thus, the silicon nanosheet prepared by reducing silica contained in the clay may also have, but may not be limited to, a crystalline form and/or a structure corresponding to or substantially corresponding to that of the clay.

A two-dimensional planar structure of the silicon nanosheet has a high specific surface area and a short moving route of a lithium ion and also has stability due to its structural characteristics. Therefore, the silicon nanosheet having the two-dimensional planar structure has maximized properties as an electrode material and can be used as, but may not be limited to, an anode active material for battery, in particular lithium-ion battery. If the silicon nanosheet is used as an anode active material for lithium-ion battery, it is possible to minimize a shock caused by a volume change of the silicon nanosheet during insertion and deinsertion of a lithium ion due to the structure of the silicon nanosheet. Further, the silicon nanosheet according to the present disclosure can be a desirable material having a high electrical capacity due to natural properties of silicon. In accordance with the silicon nanosheet preparing method, a silicon material having a planar structure may be readily prepared from, for example, but may not be limited to, an easily available clay. The clay having the layered structure may have a structure in which a sheet containing silica (Si0₂) and a sheet containing a metal element are layered alternately. If the clay is reduced, the silica (Si0₂) is reduced to silicon (Si) and silica nanosheet is formed. By etching and removing impurities, for example, a metallic compound or a metallic ion, from the reduced clay except the silicon, the silicon nanosheet can be separately obtained. By way of example, the silicon nanosheet may have, but may not be limited, a layered structure.

By way of example, if the clay includes a silicate mineral, silicon tetrahedrons are bonded in their own ways and form an intrinsic crystalline form. The silicon nanosheet obtained from the clay may have, but may not be limited to, the intrinsic crystalline form of the used clay For example, if the layered clay contains a layered silicate mineral, a silicon nanosheet produced from the layered silicate mineral having an intrinsic bonding in a layered manner, may include a layered structure originated from the crystalline form of the layered silicate mineral. However, the present disclosure may not be limited thereto.

In accordance with an illustrative embodiment of the present disclosure, the step of reducing the silica contained in the clay may be carried out by means of, but may not be limited to, a magnesiothermic reaction. The magnesiothermic reaction includes a reaction for reducing a material to be reduced by heating a mixture of an alkaline earth metal, for example, magnesium, and the material to be reduced in a reduction gas atmosphere.

In accordance with an illustrative embodiment of the present disclosure, the silicon nanosheet preparing method may further comprise, but may not be limited to, etching the heated mixture by using an acidic solution to separately obtain the silicon nanosheet.

By heating a mixture including a clay and an alkaline earth metal to reduce a silica included in the clay, the silica is reduced while the alkaline earth metal may be oxidized. An alkaline earth metal oxide formed from the oxidation of the alkaline earth metal may be dissolved in an acidic solution and washed away during etching the heated mixture by using an acidic solution, but it may not be limited thereto. By way of example, during etching the heated mixture by using the acidic solution, the alkaline earth metal oxide and other impurities are washed away and only the reduced and layered silica mainly remains, but the present illustrative embodiment may not be limited thereto.

In accordance with an illustrative embodiment of the present disclosure, the clay may include, but may not be limited to, a layered clay. In accordance with an illustrative embodiment of the present disclosure, the layered clay may include, but may not be limited to, a layered silicate mineral. By way of example, the layered silicate mineral may include a natural layered silicate or a synthetic layered silicate. The layered silicate mineral may include, but may not be limited to, a three-layered silicate mineral of a pyrophyllite group, a di- or tri-octahedral three-layered silicate mineral of a mica group such as muscovite, paragonite, phlogopite or biotite, a four-layered silicate mineral of a chlorite group, and clay minerals such as kaolinite, montmorillonite, and illite.

In accordance with an illustrative embodiment of the present disclosure, the layered silicate mineral may include, but may not be limited to, a

phyllosilicate.

In accordance with an illustrative embodiment of the present disclosure, the phyllosilicate may be represented by, but may not be limited to, a

formula (I) :

[Formula (I)]

[M_n/Valenz]^lnter [M^M^f [M^]^ X₁₀Y₂

wherein

M is a metal cation having oxidation state of from 1 to 3,

M¹ is a metal cation having oxidation state of 2 or 3,

Mⁿ is a metal cation having oxidation state of 1 or 2,

Mⁱⁿ is an atom having oxidation state of 3 or 4,

X is a dianion,

Y is a monoanion,

M≤ about 2.0 for a metal atom M¹ having the oxidation state 3, and m≤ about 3.0 for a metal atom M¹ having the oxidation state of 2,

≤ about 1.0, and

a layer charge n is from about 0.2 to about 0.8.

In the formula (I), the term "Valenz" means the number of covalent bonds that can be shared by the relevant element, the term "inter" means presence of the relevant elements between phyllosilicate layers, the term "oct(octahedral)" means that the relevant elements may have an octahedral structure, and the term "tet(tetrahedral)" means that the relevant elements may have a tetrahedral structure, but the present illustrative embodiment may not be limited thereto.

In accordance with an illustrative embodiment of the present disclosure, the phyllosilicate may include a material selected from, but may not be limited to, the group consisting of talc, mica, smectite clays, magadiite, kenyaite, stevensite, halloysite, aluminate oxide, hydrotalcite, and their combinations.

In accordance with an illustrative embodiment of the present disclosure, the smectite clays may include a material selected from the group, but may not be limited to, consisting of montmorillonite, nontronite, beidellite, bentonite, hectorite, laponite, saponite, sauconite, vermiculite, and their combinations.

In accordance with an illustrative embodiment of the present disclosure, the alkaline earth metal may include a metal selected from the group consisting of Be, Mg, Ca, Sr, Ba, and their combinations. Preference of the alkaline earth metal in the present invention is given to Mg.

In accordance with an illustrative embodiment of the present disclosure, the alkaline earth metal may be in a form of, but may not be limited to, powder. By way of example, if the alkaline earth metal is in a form of powder, it can be easily mixed with the clay when the mixture is prepared, but the present illustrative embodiment may not be limited thereto.

In accordance with an illustrative embodiment of the present disclosure, heating the mixture may be performed at a temperature of, but may not be limited to, from about 300°C to about 1000°C. By way of example, heating the mixture may be performed at a temperature of, but may not be limited to, from about 300°C to about 1000°C, from about 400°C to about 1000°C, from about 500°C to about 1000°C, from about 600°C to about 1000°C, from about 700°C to about 1000°C, from about 800°C to about 1000°C, from about 900°C to about 1000°C, from about 300°C to about 900°C, from about 300°C to about 800°C, from about 300°C to about 700°C, from about 300°C to about 600°C, from about 300°C to about 500°C or from about 300°C to about 400°C.

In accordance with an illustrative embodiment of the present disclosure, heating the mixture may be performed at a temperature of, but may not be limited to, from about 400°C to about 800°C.

In accordance with an illustrative embodiment of the present disclosure, heating the mixture may be performed at a temperature of, but may not be limited to, from about 550°C to about 750°C. By way of example, heating the mixture may be performed at a temperature of, but may not be limited to, from about 600°C to about 650°C, from about 600°C to about 700°C, from about 600°C to about 750°C, from about 550°C to about 700°C, from about 550°C to about 650°C or from about 550°C to about 600°C. In accordance with an illustrative embodiment of the present disclosure, the acidic solution may have a pH of, but may not be limited to, about 6 or less. By way of example, the acidic solution may have a pH of, but may not be limited to, about 6 or less, about 5 or less, about 4 or less, about 3 or less, about 2 or less or about 1 or less.

In accordance with an illustrative embodiment of the present disclosure, the acidic solution may have a pH of, but may not be limited to, about 3 or less.

In accordance with an illustrative embodiment of the present disclosure, the acidic solution may include, but may not be limited to, an inorganic acid.

In accordance with an illustrative embodiment of the present disclosure, the inorganic acid may include an acid selected from the group, but may not be limited to, consisting of a hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, hydrofluoric acid, perchloric acid, chloric acid, chlorous acid, hypochlorous acid, iodic acid, and their combinations.

In accordance with an illustrative embodiment of the present disclosure, a mole ratio of a silicon included in the clay to an alkaline earth metal included in the mixture may be, but may not be limited to, from about 1 : 0.1 to about 1 : 10. By way of example, a mole ratio of a silicon included in the clay to an alkaline earth metal included in the mixture may be an alkaline earth metal, but may not be limited to, from about 1 : 0.1 to about 1 : 10, from about 1 : 0.1 to about 1 : 9, from about 1 : 0.1 to about 1 : 8, from about 1 : 0.1 to about 1 : 7, from about 1 : 0.1 to about 1 : 6, from about 1 : 0.1 to about 1 : 5, from about 1 : 0.1 to about 1 : 4, from about 1 : 0.1 to about 1 : 3, from about 1 : 0.1 to about 1 : 2, from about 1 : 0.1 to about 1 : 1, from about 1 : 0.1 to about 1 : 0.5, from about 1 : 0.5 to about 1 : 10, from about 1 : 1 to about 1 : 10, from about 1 : 2 to about 1 : 10, from about 1 : 3 to about 1 : 10, from about 1 : 4 to about 1 : 10, from about 1 : 5 to about 1 : 10, from about 1 : 6 to about 1 : 10, from about 1 : 7 to about 1 : 10, from about 1 : 8 to about 1 : 10, or from about 1 : 9 to

about 1 : 10.

In accordance with an illustrative embodiment of the present disclosure, a mole ratio of a silicon included in the clay to an alkaline earth metal included in the mixture may be, but may not be limited to, from about 1 : 0.5 to about 1 : 5.

In accordance with an illustrative embodiment of the present disclosure, a mole ratio of a silicon included in the clay to an alkaline earth metal included in the mixture may be, but may not be limited to, from about 1 : 0.5 to about 1 : 3. By way of example, the clay may be, but may not be limited to, an oxide having a layered structure including aluminum and silicon. The clay is cheap and easily available and includes layered silica (Si0₂), and thus, it is possible to easily and economically prepare a silicon nanosheet by reducing the silica.

The talc is represented by a chemical formula of Mg3Si30io(OH)₂. In a crystalline form of the talc, a sheet (Mgi₂0i₂H4) containing magnesium is interposed between two silica (Si0₂) sheets. These three sheets are bonded to form a single layer. The single layer is electrically neutral. Respective layers are bonded to each other by a weak van der Waals force.

The laponite also has a structure very similar to the talc. In the laponite, a sheet containing magnesium is interposed between two silica sheets. However, unlike the talc, a part of magnesium in the magnesium sheet of the laponite is substituted with lithium, and thus, respective layers are negatively charged. Therefore, in order to maintain an electrically neutral state overall, sodium (Na) cations are included between the respective layers and the respective layers are bonded to each other by an electrical interaction.

In accordance with an illustrative embodiment of the present disclosure, heating a mixture including a clay and an alkaline earth metal to reduce a silica included in the clay to a silicon nanosheet may be performed, but may not be limited to, in a reducing environment.

In accordance with an illustrative embodiment of the present disclosure, heating a mixture including a clay and an alkaline earth metal to reduce a silica included in the clay to a silicon nanosheet may be performed, but may not be limited to, in an environment including an inert gas. By way of example, heating a mixture including a clay and an alkaline earth metal to reduce a silica included in the clay to a silicon nanosheet may be performed, but may not be limited to, in an environment including a hydrogen gas and an inert gas.

By way of example, the inert gas may include, but may not be limited to, a gas selected from the group, but may not be limited to, consisting of a nitrogen gas, an argon gas, a neon gas, a helium gas, and their combinations.

In accordance with an illustrative embodiment of the present disclosure, the silicon nanosheet preparing method may further comprise, but may not be limited to, stirring and/or washing the heated mixture after the reduction of the alkaline earth metal. In accordance with an illustrative embodiment of the present disclosure, stirring and/or washing may be performed using, but may not be limited to, a polar solvent.

In accordance with an illustrative embodiment of the present disclosure, the polar solvent may include, but may not be limited to, water, alcohol, an organic polar solvent, or their combinations. By way of example, the water may include, but may not be limited to, primary distilled water, secondary distilled water or tertiary distilled water.

In accordance with an illustrative embodiment of the present disclosure, the clay may be exfoliated into layers, but may not be limited to.

In accordance with an illustrative embodiment of the present disclosure, before heating a mixture including a clay and an alkaline earth metal, restacking the clay by adding an alkaline earth metal cation to the clay or freeze-drying the clay may be included, but the illustrative embodiment may not be limited thereto. The alkaline earth metal cation which is intended for this purpose may include at least one metal cation selected from the group consisting of Be, Mg, Ca, Sr, Ba, and their combinations. Preference of the alkaline earth metal cation in the present invention is given to magnesium cation.

By way of example, the specific surface area of the silicon nanosheet may be increased by, but may not be limited to, the restacking or freeze-drying. If the specific surface area of the silicon nanosheet is increased, there is an increase in area where an interaction with an electrolyte occurs. Therefore, performance of a battery, in particular lithium- ion battery, comprising the silicon nanosheet can be improved, but the illustrative embodiment may not be limited thereto.

In accordance with an illustrative embodiment of the present disclosure, the mixture including the clay and the alkaline earth metal may further include, but may not be limited to, a graphene oxide. In a preferred embodiment of the present invention, the graphene oxide has a form of nanosheet.

In accordance with an illustrative embodiment of the present disclosure, by heating the mixture including the clay and the alkaline earth metal and further including the graphene oxide to reduce the silica contained in the clay and the graphene oxide to a silicon nanosheet and graphene, respectively, a composite containing silicon and graphene may be formed, but the illustrative embodiment may not be limited thereto. In a preferred embodiment of the present invention, a composite containing a silicon nanosheet and a graphene nanosheet can be obtained by reducing the silica contained in the layered clay and the graphene oxide nanosheet.

By way of example, while the silica contained in the clay and the graphene oxide are reduced to a silicon nanosheet and graphene, respectively, silicon and graphene may be hybridized with each other, but the illustrative embodiment may not be limited thereto. By way of example, a silicon nanosheet formed by the hybridizing of silicon and graphene may contain, but may not be limited to, silicon and graphene.

In accordance with a second aspect of the present disclosure, there may be provided a silicon nanosheet including a layered structure substantially similar to a clay.

By way of example, the clay may include a layered clay, and may have a planar crystalline form and/or a layered structure, and thus, the silicon nanosheet prepared by reducing silica contained in the clay may also have, but may not be limited to, a crystalline form and/or a structure corresponding to or substantially corresponding to that of the clay. By way of example, the silicon nanosheet may have a structure of a two-dimensional plate-crystalline form or a tetrahedral crystalline form.

In accordance with an illustrative embodiment of the present disclosure, the silicon nanosheet may be produced, but may not be limited to, by a method of the first aspect of the present disclosure.

In accordance with an illustrative embodiment of the present disclosure, the silicon nanosheet may have a B.E.T. surface area of, but may not be limited to, from about 20 m²/g to about 300 m²/g. By way of example, the silicon nanosheet may have a B.E.T. surface area of, but may not be limited to, from about 20 m²/g to about 300 m²/g, from about 40 m²/g to about 300 m²/g, from about 60 m²/g to about 300 m²/g, from about 80 m²/g to about 300 m²/g, from about 100 m²/g to about 300 m²/g, from about 150 m²/g to about 300 m²/g, from about 200 m²/g to about 300 m²/g, from about 250 m²/g to about 300 m²/g, from about 20 m²/g to about 250 m²/g, from about 20 m²/g to about 200 m²/g, from about 20 m²/g to about 150 m²/g, from about 20 m²/g to about 100 m²/g, from about 20 m²/g to about 80 m²/g, from about 20 m²/g to about 60 m²/g or from about 20 m²/g to about 40 m²/g.

In accordance with an illustrative embodiment of the present disclosure, the silicon nanosheet may have a size of, but may not be limited to, from about 10 nm to about 300 nm and an aspect ratio of, but may not be limited to, from about 1 : 1 to about 1 : 10. By way of example, the silicon nanosheet may have a size of, but may not be limited to, from about 10 nm to about 300 nm, from about 10 nm to about 250 nm, from about 10 nm to about 200 nm, from about 10 nm to about 150 nm, from about 10 nm to about 100 nm, from about 10 nm to about 50 nm, from about 10 nm to about 30 nm, from

about 10 nm to about 20 nm, from about 20 nm to about 300 nm, from about 30 nm to about 300 nm, from about 50 nm to about 300 nm, from about 100 nm to about 300 nm, from about 150 nm to about 300 nm, from about 200 nm to about 300 nm or from about 250 nm to about 300 nm. By way of example, the silicon nanosheet may have an aspect ratio of, but may not be limited to, from about 1 : 1 to about 1 : 10, from about 1 : 2 to about 1 : 10, from about 1 : 3 to about 1 : 10, from about 1 : 4 to about 1 : 10, from about 1 : 5 to about 1 : 10, from about 1 : 6 to about 1 : 10, from about 1 : 7 to about 1 : 10, from about 1 : 8 to about 1 : 10, from about 1 : 9 to about 1 : 10, from about 1 : 1 to about 1 : 9, from about 1 : 1 to about 1 : 8, from about 1 : 1 to about 1 : 7, from about 1 : 1 to about 1 : 6, from about 1 : 1 to about 1 : 5, from about 1 : 1 to about 1 : 4, from about 1 : 1 to about 1 : 3 or from about 1 : 1 to about 1 : 2.

In accordance with an illustrative embodiment of the present disclosure, the silicon nanosheet may have an average particle size of, but may not be limited to, from about 10 nm to about 400 nm. By way of example, the silicon nanosheet may have an average particle size of, but may not be limited to, from about 10 nm to about 400 nm, from about 30 nm to about 400 nm, from about 50 nm to about 400 nm, from about 80 nm to about 400 nm, from about 100 nm to about 400 nm, from about 150 nm to about 400 nm, from about 200 nm to about 400 nm, from about 250 nm to about 400 nm, from about 300 nm to about 400 nm, from about 350 nm to about 400 nm, from about 10 nm to about 350 nm, from about 10 nm to about 300 nm, from about 10 nm to about 250 nm, from about 10 nm to about 200 nm, from about 10 nm to about 150 nm, from about 10 nm to about 100 nm, from about 10 nm to about 80 nm, from about 10 nm to about 50 nm, from

about 10 nm to about 30 nm, from about 100 nm to about 300 nm, from about 100 nm to about 200 nm or from about 100 nm to about 150 nm.

In accordance with an illustrative embodiment of the present disclosure, the silicon nanosheet may be hybridized with at least one conductive carbon compound. For instance, the conductive carbon compounds may be selected from the group consisting of carbon black, acetylene black, active carbon, carbon nanotube (CNT), graphite, graphene, and any combinations thereof, but the illustrative embodiment may not be limited thereto. In a particular embodiment of the present invention, the silicon nanosheet may be hybridized with graphene, and thus, may exhibit further improved performance of an electrode.

In accordance with a third aspect of the present disclosure, there may be provided an electrode for battery including a silicon nanosheet of the second aspect of the present disclosure. By way of example, the electrode can be intended for lithium batteries, such as lithium-ion battery, lithium air battery, and lithium sulfur battery, or sodium batteries, such as sodium ion battery, and sodium sulfur battery, but the present invention is not limited thereto. By way of example, the silicon nanosheet may be included as, but may not be limited to, an anode active material of a lithium- ion battery.

In accordance with a fourth aspect of the present disclosure, there may be provided a battery comprising the electrode of the third aspect of the present disclosure as an anode, a cathode, and an electrolyte. In a particular embodiment of the present invention, provided is a lithium-ion battery comprising the electrode of the third aspect of the present disclosure as an anode, a cathode, and an electrolyte.

In accordance with an illustrative embodiment of the present disclosure, the cathode may include one selected from, but may not be limited to, the group consisting of a lithium-containing oxide, a lithium- containing sulfide, a lithium- containing selenide, a lithium-containing halide, and their combinations.

In accordance with an illustrative embodiment of the present disclosure, the lithium-containing oxide may include one selected from, but may not be limited to, the group consisting of Li_xCo0₂ (0.5<x<l .3), Li_xNi0₂ (0.5<x<l .3), Li_xMn0₂ (0.5<x<l .3), Li_xMn₂0₄ (0.5<x<l .3), Li_x(NiaCo_bMn_c)0₂ (0.5<x<l .3, 0<a<l, 0<b<l, 0<c<l, a+b+c=l), Li_xNii__yCo_yO₂ (0.5<x<1.3, 0<y<l),

Li_xCoi__yMn_y0₂ (0.5<x<1.3, 0<y<l), Li_xNii__yMn_y0₂ (0.5<x<1.3, 0<y<l), Li_x(Ni_aCo_bMn_c)0₄ (0.5<x<1.3, 0<a<2, 0<b<2, 0<c<2, a+b+c=2), Li_xMn₂__zNi_z0₄ (0.5<x<l .3, 0<z<2), Li_xMn₂__zCo_z0₄ (0.5<x<l .3, 0<z<2), Li_xCoP0₄ (0.5<x<l .3), Li_xFeP0₄ (0.5<x<1.3), and their combinations.

In accordance with another illustrative embodiment of the present disclosure, the cathode may include at least one cathode active material which can insert and de-insert sodium, particularly sodium-containing compound, such as sodium-metal mixed oxides. Examples of the cathode active material in the present invention include NaFe0₂, NaCo0₂, NaCr0₂, NaMn0₂, NaNi0₂, NaNii/2Tii/₂02, NaNii/₂Mni/₂02, Na₂/3Fei/3Mn₂/302, NaNii/3Coi/3Mni/₃0₂, NaMn₂0₄, NaNii/₂Mn_3/20₂, NaFeP0₄, NaMnP0₄, NaCoP0₄, Na₂FeP0₄F, Na₂MnP0₄F, Na₂CoP0₄F, and any combination thereof, but the present invention is not limited thereto.

In accordance with an illustrative embodiment of the present disclosure, the electrolyte may include, but may not be limited to, a typical lithium salt and a solvent. By way of example, the lithium-ion battery may include, but may not be limited to, an electrolyte containing a silane-based compound represented by the following chemical formula as an additive :

In the above chemical formula, R is an alkyl group or a vinyl group, R' is an alkyl group or an alkyl group substituted for an alkoxy group, and y is an integer selected from 1 to 3.

In the above chemical formula, the alkyl group may be, but may not be limited to, a C1-C30 alkyl group, and the vinyl group may be, but may not be limited to, C₂-C₂o vinyl group. The silane-based compound may be represented by Si-(R)i(OR')₃ and may include, but may not be limited to,

trimethoxy(methyl)silane (SiCFL^OCFL;^) or tris(2-methoxyethoxy)vinylsilane (CH₂=CHSi(OCH₂CH₂OCH₂)3).

The electrolyte containing the above-described silane-based compound may include a silane-based compound of, but may not be limited to, from about 2 wt % to about 10 wt %. In this case, an inherent function of the electrolyte can be maintained and formation of a protection layer on a surface of a silicon oxide may be induced efficiently, which may be more advantageous. However, the present disclosure may not be limited thereto.

By way of example, the electrolyte may include, but may not be limited to, an electrolyte including a lithium salt selected from the group consisting of LiTFSi, LiPF₆, LiFSi, and their combinations and a non-aqueous carbonate- based solvent or a lithium salt selected from the group consisting of LiTFSi, LiPF₆, LiFSi, and their combinations and a room temperature ionic liquid solvent selected from the group consisting of an imidazolium-based solvent, a pyrrolidinium-based solvent, and a piperidinium-based solvent. In a particular embodiment of the present disclosure, the electrolyte may comprise LiP0₂F₂ optionally in combination with one or more above-described lithium salts.

In accordance with an illustrative embodiment of the present disclosure, the electrolyte may comprise at least one solvent additives, in particular fluorinated organic compounds, for example, fluorinated carbonic esters which are selected from the group of fluorosubstituted ethylene carbonates,

polyfluorosubstituted dimethyl carbonates, fluorosubstituted ethyl methyl carbonates, and fluorosubstituted diethyl carbonates. Preferred fluorosubstituted carbonates are monofluoroethylene carbonate (F1EC), 4,4-difluoro ethylene carbonate, 4,5-difluoro ethylene carbonate, 4-fluoro-4-methyl ethylene carbonate, 4,5-difluoro-4-methyl ethylene carbonate, 4-fluoro-5-methyl ethylene carbonate, 4,4-difluoro-5-methyl ethylene carbonate, 4-(fluoromethyl)-ethylene carbonate, 4-(difluoromethyl)-ethylene carbonate, 4-(trifluoromethyl)-ethylene carbonate, 4-(fluoromethyl)-4-fluoro ethylene carbonate, 4-(fluoromethyl)-5- fluoro ethylene carbonate, 4-fluoro-4,5-dimethyl ethylene carbonate,

4,5-difluoro-4,5-dimethyl ethylene carbonate, and 4,4-difluoro-5,5-dimethyl ethylene carbonate ; dimethyl carbonate derivatives including fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, trifluoromethyl methyl carbonate, bis(difluoro)methyl carbonate, and bis(trifluoro)methyl carbonate ; ethyl methyl carbonate derivatives including 2-fluoroethyl methyl carbonate, ethyl fluoromethyl carbonate, 2,2-difluoroethyl methyl carbonate, 2-fluoroethyl fluoromethyl carbonate, ethyl difluoromethyl carbonate, 2,2,2-trifluoroethyl methyl carbonate, 2,2-difluoroethyl fluoromethyl carbonate, 2-fluoroethyl difluoromethyl carbonate, and ethyl trifluoromethyl carbonate ; and diethyl carbonate derivatives including ethyl (2-fluoroethyl) carbonate, ethyl

(2,2-difluoroethyl) carbonate, bis(2-fluoroethyl) carbonate, ethyl

(2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl 2'-fluoroethyl carbonate, bis(2,2-difluoroethyl) carbonate, 2,2,2-trifluoroethyl 2'-fluoroethyl carbonate, 2,2,2-trifluoroethyl 2',2'-difluoroethyl carbonate, and bis(2,2,2-trifluoroethyl) carbonate, 4-fluoro-4-vinylethylene carbonate, 4-fluoro-5-vinylethylene carbonate, 4,4-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4-vinylethylene carbonate, 4-fluoro-4,5-divinylethylene carbonate, 4,5-difluoro-4,5- divinylethylene carbonate, 4-fluoro-4-phenylethylene carbonate, 4-fluoro-5- phenylethylene carbonate, 4,4-difluoro-5-phenylethylene carbonate, 4,5-dif uoro- 4-phenylethylene carbonate and 4,5-difluoro-4,5-diphenylethylene carbonate, fluoromethyl phenyl carbonate, 2-fluoroethyl phenyl carbonate, 2,2-difluoroethyl phenyl carbonate and 2,2,2-trifluoroethyl phenyl carbonate, fluoromethyl vinyl carbonate, 2-fluoroethyl vinyl carbonate, 2,2-difluoroethyl vinyl carbonate and 2,2,2-trifluoroethyl vinyl carbonate, fluoromethyl allyl carbonate, 2-fluoroethyl allyl carbonate, 2,2-difluoroethyl allyl carbonate and 2,2,2-trifluoroethyl allyl carbonate.

The electrolyte may be, but may not be limited to, a room temperature ionic liquid solvent that does not include a lithium salt of LiPF₆. If a room temperature ionic liquid solvent does not include a lithium salt of LiPF₆, there is no interfacial reaction between a silicon oxide included in an anode and a LiPF₆ derivative. Further, the room temperature ionic liquid solvent forms a stable SEI (solid electrolyte interphase) layer on a thin film surface of a silicon oxide during an initial charge/discharge process and suppresses a future interfacial reaction with the electrolyte. Thus, a charge/discharge cycling performance can be stable.

The solvent may include a non-aqueous carbonate-based solvent in addition to the room temperature ionic liquid solvent as the solvent. If the electrolyte includes the non-aqueous carbonate-based solvent, the electrolyte may include, but may not be limited to, about 5 to 70 parts by weight of the non- aqueous carbonate-based solvent on the basis of 100 parts by weight of the room temperature ionic liquid solvent. In this case, flame retardancy of the ionic liquid solvent can be maintained or ignition can be suppressed, which may be advantageous. However, the present disclosure may not be limited thereto.

The non-aqueous carbonate-based solvent may include, but may not be limited to, dimethyl carbonate (DMC), diethyl carbonate (DEC) or ethylmethyl carbonate (EMC).

The lithium- ion battery includes a silicon nanosheet prepared by the silicon nanosheet preparing method of the present disclosure as an anode material. The silicon nanosheet may undergo little change in volume during charge and discharge and may have a high ion conductivity, a high electrical conductivity, and a high capacity.

Hereinafter, examples of the present disclosure will be explained in detail, but the present disclosure may not be limited thereto.

[Examples]

In the present example, magnesium powder (SAMCHUN) was mixed with talc (Aldrich) at various ratios (a ratio of Si : Mg was about 1 : 1.56,

about 1 : 2.34, about 1 : 3.12, and about 1 : 3.9) and the mixture was put into a tube-shaped electric furnace manufactured in a laboratory and heated in an atmosphere of H₂ (5 %)/Ar (95 %) at about 650°C for about 3 hours. Then, a powder X-ray diffraction pattern of the heated mixture was observed by using a powder X-ray diffraction analyzer (Rigaku) (Fig. 1). As depicted in Fig. 1, peaks of MgO, Si, and Mg₂Si were observed. Therefore, it was confirmed that silica contained in the talc was reduced and the magnesium was oxidized and MgO, Si, and Mg₂Si were produced accordingly. When the mixture was prepared at a ratio of Si : Mg of about 1 : 1.56, a peak of the talc was shown. Therefore, it could be seen that the mixture needed to be prepared at a higher ratio of the magnesium in order to completely reduce the silica.

Fig. 2 provides scanning electron microscope (SEM) images of the heated mixture. Fig. 2a is a SEM image showing a case where a ratio of Si : Mg was about 1 : 3.12 and Fig. 2b is a SEM image showing a case where a ratio of Si : Mg was about 1 : 2.34.

Then, hydrochloric acid (SAMCHUN) of from about 35 wt % to about 37 wt % was mixed with distilled water to prepare a 1 M hydrochloric acid solution. The heated mixture was mixed at a ratio of about 1 g per about 500 ml of the 1 M hydrochloric acid solution and stirred and washed for about a day. Thereafter, centrifugation of the mixture was carried out at about 3500 rpm for about 30 minutes by a centrifuge (Combi 514R, Hanil) and the mixture was washed with distilled water until a pH of the mixture reached neutrality. The mixture was dried in a vacuum oven at about 100°C. Then, an X-ray diffraction pattern of the mixture from which a magnesium oxide dissolved therein was removed during the washing process was observed (Fig. 3). As depicted in Fig. 3, as a result of a washing process with the 1 M hydrochloric acid solution, peaks of silicon only were shown. Therefore, it could be seen that most elements except the silicon were washed and removed from the mixture.

Fig. 4 provides scanning electron microscope (SEM) images of the heated mixture after being washed with the HC1 solution. Fig. 4a is a SEM image showing a case where a ratio of Si : Mg was about 1 : 3.12 and Fig. 4b is a SEM image showing a case where a ratio of Si : Mg was about 1 : 2.34. As can be seen from Fig. 4b, plate-shaped silicon nanosheets were layered.

In the present example, magnesium powder (SAMCHUN) was mixed with laponite (RDS grade, ROCKWOOD) at a ratio of Mg : Si of about 3.9 : 1 and the mixture was heated in an atmosphere of H₂ (5 %)/Ar (95 %) at about 650°C for about 3 hours. A powder X-ray diffraction pattern (before etching) of the heated mixture and a powder X-ray diffraction pattern (after etching) of the heated mixture washed with a HCl solution were observed (Fig. 5). As depicted in Fig. 5, even when the laponite was used, it could be seen that silicon was prepared by reducing silica in the same manner as the case where the talc was used. Further, when the heated mixture was washed with the HCl solution (after etching), it could be seen that most elements including a magnesium oxide except the silicon were washed and removed from the mixture by comparing peaks before and after etching.

In the present example, magnesium powder (SAMCHUN) was mixed with talc (Aldrich) at various ratios (a ratio of Si : Mg was about 1 : 1, about 1 : 2, and about 1 : 2.3) and the mixture was put into a tube-shaped electric furnace manufactured in a laboratory and heated in an atmosphere of H₂ (5 %)/Ar (95 %) at about 650°C for about 3 hours. Then, a powder X-ray diffraction pattern of the heated mixture was observed by using a powder X-ray diffraction analyzer (Rigaku) (Fig. 6). As depicted in Fig. 6, when the mixture mixed at a molar ratio of Si : Mg of about 1 : 1 and about 1 : 2 was heated, peaks of the remaining talc were observed in addition to peaks of Si and MgO. In case of the mixture mixed at a molar ratio of Si : Mg of about 1 : 2.3, peaks of the talc were not shown but peaks of Mg₂Si Mg₂Si0₄ were observed.

Then, the heated mixture was washed with a 0.5 M hydrochloric acid solution for about a day unlike the example 1 and further washed with a newly substituted 0.5 M hydrochloric acid solution for about a day. Thereafter, an X-ray diffraction pattern of the mixture was analyzed (Fig. 7). As can be seen from Fig. 7, in the mixture mixed at a molar ratio of Si : Mg of about 1 : 1 and about 1 : 2, peaks of the silicon and the talc were shown, and in the mixture mixed at a molar ratio of Si : Mg of about 1 : 3, peaks of the silicon and unidentified elements could be observed.

According to the observation, when magnesium in an amount less than a necessary amount for preparing a silicon nanosheet was used, the talc remained, and when magnesium in an amount more than the necessary amount was used, the magnesium influenced on obtaining a pure silicon.

Fig. 8 is a high-resolution transmission electron microscope (HR-TEM, Jeol JEM-2100F) image acquired after the mixture mixed at a molar ratio of Si : Mg of about 1 : 2.3 was heated and washed with the 0.5 M hydrochloric acid solution. An average aspect ratio of the silicon nanosheet was measured from the image of Fig. 8 in a range of from about 1 : 1 to about 1 : 3.

A silicon nanosheet was prepared from talc and laponite, respectively, in the same manner as the Example 3 except that talc and laponite were

respectively used instead of talc. A surface area of the prepared silicon nanosheet was measured by using a Micromeritics ASAP 2020 instrument by means of a N₂ adsorption-desorption isotherm measurement method. Further, a size of a pore was calculated by applying the BJH (Barrett, Johner and Halenda) equation to a result of the measurement. The pore could serve as an

insertion/deinsertion route of lithium. When the pore size was suitable for insertion and deinsertion of lithium and the pore was large in quantity, a surface area involved in a reaction was increased, and thus, the silicon nanosheet had a higher capacity and a higher performance as an anode material.

Fig. 9 provides a graph showing B.E.T. surface areas of a silicon nanosheet depending on various molar ratios of Si : Mg using talc and a table showing B.E.T. surface areas of a silicon nanosheet derived therefrom. Fig. 10 is a graph showing pore diameters of a silicon nanosheet depending on various molar ratios of Si : Mg using talc. Referring to Figs. 9 and 10, it could be seen that when talc was mixed with magnesium and the mixture was heated for reduction and washed with a hydrochloric acid solution, a B.E.T. surface area and a pore diameter of a silicon nanosheet were remarkably increased as compared with a case of using pristine talc.

<Example 5> Preparation of silicon nanosheet using clay exfoliated into layers>

In the present example, laponite and bentonite were respectively exfoliated and used for preparing silicon nanosheets, and characteristics thereof were analyzed. First, aqueous dispersion solutions of laponite and bentonite of about 0.2 weight % were prepared respectively. The dispersion solutions were vigorously stirred for about 24 hours and ultrasonicated for about an hour. Then, the unltrasonicated dispersion solutions were freeze-dried to obtain exfoliated laponite and bentonite, respectively. The silicon nanosheet was prepared in the same manner as Example 3 except that the exfoliated laponite and bentonite were used respectively instead of talc. Fig. 1 la is a field emission scanning electron microscope (FE-SEM) image of freeze-dried laponite after ultrasonication. Fig. 1 lb is a photo showing a Tyndall effect of an ultrasonication-treated laponite. Fig. 1 lc is a photo of a freeze-dried exfoliated laponite.

Fig. 12a is a FE-SEM image of freeze-dried bentonite after ultrasonication.

Fig. 12b is a photo showing a Tyndall effect of an ultrasonication-treated bentonite. Fig. 12c is a photo of a freeze-dried exfoliated bentonite.

Fig. 13a is a TEM image of an exfoliated laponite. Fig. 13b shows a selected area electron diffraction (SAED, acceleration voltage 200 kV,

FEI-TecnaiG² F20 microscope, FEI company) pattern of an exfoliated laponite. Typically, a diameter of a single laponite particle is known as about 25 nm. Referring to Fig. 13a, laponite particles of about 20 nm and about 40 nm were observed.

Fig. 14a is a TEM image of an exfoliated bentonite. Fig. 14b shows a SAED pattern of an exfoliated bentonite. Referring to Fig. 14a, bentonite particles of about 40 nm or more were observed.

Fig. 15a is a graph showing XRD patterns of exfoliated and freeze-dried laponite and non-exfoliated laponite. Fig. 15b is a graph showing XRD patterns of exfoliated and freeze-dried bentonite and non-exfoliated bentonite. Referring to Figs. 15a and 15b, intensity of the XRD peaks were decreased after exfoliating and freeze-drying. However, it is analyzed that restacking was occurred during freeze-drying process, considering that the peaks were appeared at the same position.

Fig. 16a shows an XRD pattern of a silicon nanosheet prepared in the same manner as the example 3 except that exfoliated laponite was used instead of talc. Fig. 16b shows an XRD pattern of a silicon nanosheet prepared in the same manner as the example 3 except that exfoliated bentonite was used instead of talc. Referring to Figs. 16a and 16b, it was confirmed that silicon was formed after reducing. In case of bentonite, peaks of other materials than silicon were also observed, because of various metal elements included in clay.

In the present example, a mixture of magnesium powder and talc at a molar ratio of Si : Mg of about 1 : 2 was heated and washed with a 0.5 M hydrochloric acid solution in the same manner as described in the above examples. Then, a galvanostatic test was performed thereto by using a charge/discharge

tester (MACCOR SERIES 4000). A silicon nanosheet prepared from talc was used as an active material. The silicon nanosheet, Super P as a conductive agent, and PAA (polyacrylic acid) were mixed in ethanol at a weight ratio of about 50 : 35 : 15. Then, copper foil was coated with the mixture. Thereafter, the coating mixture was dried at about 100°C for about 4 hours and fabricated into a 2016 cell. As a counter electrode of the test cell, lithium metallic foil was used and as a separator, Celgard 2500 (Celgard) was used. As an electrolyte, 1 M LiPF₆ (in EC/DEC = 1/1 (v/v), Soulbrain) was used.

In order to check how a battery capacity was maintained depending on a change in current density, a charge/discharge test was performed with different current densities. A cycle including a discharge step carried out at a current density of about 255 mA/g (0.061 C) and about 68 niA/g (0.016 C) until a voltage reached about 0.01 V, a charge step carried out at a current density of about 255 mA/g until a voltage reached about 1.0 V, and a rest step carried out for about 1 minute was repeated about 20 times (Figs. 17 and 18).

Referring to Figs. 17 and 18 each showing a graph obtained from measurement of a capacity of a battery, a charge/discharge capacity after a first discharge step (a first cycle, a curve indicated by 1) was maintained to be substantially similar to a charge/discharge capacity after a second discharge step (a second cycle, a curve indicated by 2). In particular, referring to Fig. 17, a capacity was maintained stably at about 800 mAh/g even after the second cycle. Thus, it could be seen that a capacity of the silicon nanosheet of the present disclosure was maintained even after the cycle was repeated. Further, it was confirmed that insertion and deinsertion of a lithium ion were occurred at low voltage of 0.5 V or less, in steps of discharge(lithiation) and charge(delithiation) Further, referring to Figs. 17 and 18 each showing a graph of potential, charge curves in the first cycle and the second cycle including various irreversible reactions were similar to each other. Therefore, it was expected that a similar tendency was observed in the following cycles. Generally, as a current density is increased, a capacity of a material is decreased. Therefore, it can be seen how much a capacity is changed as a current density is changed. When the charge step and the discharge step were carried out with a low current density, a charge/discharge capacity was relatively high. Referring to Figs. 17 and 18, even when the charge step and the discharge step were carried out with a high current density, a capacity was not much different from the case with a low current density. Accordingly, it could be seen that the silicon nanosheet of the present disclosure enabled rapid charge and discharge with a high capacity.

Referring to Fig. 19, even when a charge and a discharge were repeated about 20 times, there was a very small change in capacity of the cell. Therefore, such a result showed that the cell including the silicon nanosheet of the present disclosure as an anode active material stably maintained its capacity even when a charge and a discharge were repeated.

<Example 7 : Preparation of silicon nanomaterial from laponite

(Restacking)>

500 mL of a laponite suspension including laponite dispersed in distilled water at a concentration of 2 g/L was stirred for 12 hours to exfoliate the laponite. Then, Mg²⁺ was added to colloid containing the exfoliated laponite to restack the exfoliated laponite. To be specific, since the laponite contained Na⁺ ions between layers, a Mg²⁺ amount having a sufficient electric charge to substitute the total Na⁺ ion was calculated, and then MgCl₂ about twice that amount, i.e. excessive MgCl₂, was added to the colloid. In the present example, 20 mL of 3.5 M MgCl₂ aqueous solution was added to 100 mL of the colloid containing 0.2 g of the laponite and restacking was carried out with stirring for 12 hours, thereby powderizing the colloid.

The restacked powder was obtained, a ratio of Si0₂ : Mg was determined, and the restacked powder was mixed with the restacked powder with Mg powder. To be specific, a molar amount of the silicon was calculated and the Mg powder was mixed at a ratio of 1 : 2 (Si0₂ : Mg) with respect to the molar amount of the silicon and then ground and mixed in a mortar. Thereafter, the mixture was put into a self-produced tube furnace and heated at a heating rate of 3.3°C/min to reduce the silica contained in the mixture by a magnesiothermic reaction under an atmosphere of 5 % H₂/95 % Ar. A heating temperature for reduction varied in the range of from about 500°C to about 650°C. After the reduction was completed, the mixture was taken out and etched with a 0.5 M hydrochloric acid solution in an amount of 500 mL per 1 g of the mixture with stirring for 24 hours. Then, the etching solution used was removed by centrifugation, and the mixture was stirred again with 500 mL of a new 0.5 M hydrochloric acid solution for 24 hours. Thereafter, the etching solution used was removed by centrifugation, and then, the mixture was washed with distilled water. Subsequently, the mixture was dried in a vacuum at 200°C for 12 hours. <Example 8 : Preparation of silicon nanomaterial from laponite (Freeze- drying)>

A process for stirring 100 mL of a laponite suspension including laponite at a concentration of 5 g/L for 24 hours and dispersing the laponite suspension by using an ultrasonic homogenizer (5510, Branson) for 1 hour was repeated twice to exfoliate the laponite. Then, colloid containing the exfoliated laponite was freeze-dried for 5 days to be powderized. To be specific, the colloid was quickly cooled with a liquid nitrogen and then freeze-dried at -75°C and 5 mTorr within a freeze dryer (Ilsin Boibase Freeze Dryer FD8508).

The freeze-dried powder was obtained, a molar ratio of Si0₂ : Mg was determined as 1 : 2, and the freeze-dried powder was mixed with Mg powder according to the predetermined ratio. Thereafter, the mixture was put into a self- produced tube furnace and heated at a heating rate of 3.3°C/min to reduce the silica contained in the mixture by a magnesiothermic reaction. A heating temperature for reduction varied in the range of from about 550°C to

about 650°C. After the reduction was completed, the mixture was taken out and etched with a 0.5 M hydrochloric acid solution in an amount of 500 mL per 1 g of the mixture with stirring for 24 hours. Then, the etching solution used was removed by centrifugation, and the mixture was stirred again with 500 mL of a new 0.5 M hydrochloric acid solution for 24 hours. Thereafter, the etching solution used was removed by centrifugation, and then, the mixture was washed with distilled water. Subsequently, the mixture was dried in a vacuum at 200°C for 12 hours.

In the present example, characteristics of a silicon nanomaterial prepared were analyzed by an X-ray diffraction analysis method (a powder X-ray diffraction analyzer D/max 2000vk, Rigaku). Although silicon nano materials were obtained by the methods of Examples 7 and 8, the silicon nanomaterials might contain impurities or crystallinity might be changed depending on a heat treatment condition and a molar ratio of Si : Mg. Therefore, such characteristics were analyzed by the X-ray diffraction analysis method.

Figs. 20a to 20c provide graphs showing results of an X-ray diffraction analysis of a silicon nanomaterial obtained by powderizing colloid including a silica-containing material from laponite by a restacking method, reducing the powderized colloid by a magnesiothermic reaction, and etching the resultant product. Figs. 20a to 20c show diffraction patterns of silicon nanomaterials obtained by performing a heat treatment at 550°C for 6 hours as shown in Fig. 20a, at 600°C for 1 hour as shown in Fig. 20b, and at 650°C for 3 hours as shown in Fig. 20c, at a molar ratio of Si0₂ : Mg of 1 : 2. In Figs. 20a and 20b, a wide and gentle peak regarded as an amorphous silicon oxide was observed between 20° and 30° together with apparent peaks of silicon. In Fig. 20c, small peaks presumed to be impurities were observed together with apparent silicon peaks.

Figs. 21a to 21c provide graphs showing results of an X-ray diffraction analysis of a silicon material obtained by powderizing colloid including a silica- containing material from laponite by a restacking method and reducing the powderized colloid by a magnesiothermic reaction without etching. Figs. 21a to 21c show diffraction patterns of silicon materials obtained by performing a heat treatment at 550°C for 6 hours as shown in Fig. 21a, at 600°C for 1 hour as shown in Fig. 21b, and at 650°C for 3 hours as shown in Fig. 21c, at a molar ratio of Si : Mg of 1 : 2. It was observed from Figs. 21a to 21c that the laponite- restacked material was changed to a material containing a silicon nanomaterial, MgO, Mg₂Si, and some impurities through a magnesiothermic reaction.

From the above experiment results, it was confirmed that most of the impurities in the reduced silicon material were removed by performing the etching process and only the silicon nanomaterials usually remained.

An electrochemical characteristic was analyzed by conducting an experiment with a Maccor 2000 series charger/discharger in a potential range of from 0.01 V to 1 V at 210 mA/g (0.05°C). Herein, 3 vol % of F1EC as an additive for increasing stability of an anode was added to 1 M LiPF₆

(in EC/DEC 1 : 1 volume ratio) as an electrolyte.

Figs. 22a to 22c provide graphs showing results of measuring

electrochemical characteristics of a silicon nanomaterial obtained by

powderizing colloid including a silica-containing material from laponite by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 550°C for 6 hours, and etching the resultant product. Fig. 22a is a graph showing a measurement result of capacity retention, Fig. 22b is a graph showing a measurement result of rate capability, and Fig. 22c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 6 % was observed after 18 cycles. Figs. 23a to 23c provide graphs showing results of measuring

electrochemical characteristics of a silicon nanomaterial obtained by

powderizing colloid including a silica-containing material from laponite by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 600°C for 1 hour, and etching the resultant product. Fig. 23a is a graph showing a measurement result of capacity retention, Fig. 23b is a graph showing a measurement result of rate capability, and Fig. 23c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 4 % was observed after 17 cycles.

Figs. 24a and 24b provide graphs showing results of measuring

electrochemical characteristics of a silicon nanomaterial obtained by

powderizing colloid including a silica-containing material from laponite by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 650°C for 3 hours, and etching the resultant product. Fig. 24a is a graph showing a measurement result of capacity retention and Fig. 24b is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 70 % was observed after 50 cycles.

Fig. 25a provides a result of an X-ray diffraction analysis of a silicon nanomaterial obtained by powderizing colloid including a silica-containing material from laponite by a freeze-drying method, reducing the powderized colloid by a magnesiothermic reaction with different temperatures and times, and etching the resultant product. The upper graph shows a diffraction pattern of a silicon material obtained by performing a heat treatment at 550°C for 3 hours, and reducing and etching the resultant product, the intermediate graph shows a diffraction pattern of a silicon material obtained by performing a heat treatment at 525°C for 6 hours, and reducing and etching the resultant product, and the lower graph shows a diffraction pattern of a silicon material obtained by performing a heat treatment at 525°C for 3 hours, and reducing and etching the resultant product. From the diffraction patterns of the silicon materials reacted at 525°C for 3 hours, and at 525°C for 6 hours, a wide and gentle peak regarded as an amorphous silicon oxide was observed between 20° and 30° together with apparent silicon peaks. From the diffraction pattern of the silicon material reacted at 550°C for 3 hours, apparent silicon peaks were observed.

Fig. 25b provides a capacity retention analysis graph of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite by a freeze-drying method, reducing the powderized colloid by a magnesiothermic reaction at 525°C for 6 hours, and etching the resultant product. A ratio of Si : Mg was 1 : 2, and F1EC was not used. The silicon nanomaterial obtained by the freeze-drying method had a low capacity but exhibited a high stability as compared with the silicon nanomaterial obtained by the restacking method.

500 mL of a laponite suspension including laponite at a concentration of 2 g/L was stirred for 18 hours to exfoliate the laponite. Then, a suspension containing graphene oxide of 0.05 wt % and prepared by a modified Hummer's method was added to colloid containing the exfoliated laponite. An amount of the graphene oxide added to the colloid containing the exfoliated laponite was determined by comparing a weight of silicon contained in the laponite. The laponite contained Si0₂ of 59.5 wt %, and the Si0₂ contained silicon

of 46.7 wt %. Therefore, the laponite contained silicon of 27.8 wt %. In consideration of this, the graphene oxide was added after calculation of a weight to be 10 %, 7.5 %, and 5 % with respect to the silicon contained in a final product (a composite of the graphene oxide and the silicon). In the present example, each of 62 mL, 45 mL, and 29 mL of a suspension containing graphene oxide of 0.05 wt % was added to 500 mL of a suspension containing 1 g of laponite, and 500 mL of a 3.6 mM MgCl₂ solution was added thereto. Then, the laponite and the graphene oxide were restacked together with stirring

for 12 hours and washed with distilled water by using a centrifuge, and then dried in an oven at 50°C to be powderized.

The restacked powder was obtained, a molar ratio of Si0₂ : Mg was determined as 1 : 2, and the restacked powder was mixed with Mg powder. Thereafter, the mixture was put into a self-produced tube furnace and heated at a heating rate of 3.3°C/min to reduce the silica contained in the mixture by a magnesiothermic reaction. A heating temperature for reduction varied in the range of from about 500°C to about 650°C. After the reduction was completed, the mixture was taken out and etched with a 0.5 M hydrochloric acid solution in an amount of 500 mL per 1 g of the mixture with stirring for 24 hours. Then, the etching solution used was removed by centrifugation, and the mixture was stirred again with 500 mL of a new 0.5 M hydrochloric acid solution for 24 hours.

Thereafter, the etching solution used was removed by centrifugation, and then, the mixture was washed with distilled water. Subsequently, the mixture was dried in a vacuum at 200°C for 24 hours.

In Fig. 26, the photo on the left shows a colloid including a silica- containing material and the photo on the right shows a powderized silica- containing material prepared in accordance with the present example.

In the present example, characteristics of a silicon nanomaterial prepared were analyzed by an X-ray diffraction analysis method.

(1) In the case where a graphene was contained in as amount of 5 wt % in a silicon nanomaterial

In the present example, an analysis was conducted on characteristics of a silicon nanomaterial prepared by using laponite and a graphene oxide solution such that a mass ratio of graphene in a final product became 5 % by mass.

Figs. 27a to 27c provide results of an X-ray diffraction analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction, and etching the resultant product. A molar ratio of Si : Mg was determined as 1 : 2. Figs. 27a to 27c show analysis results of silicon nanomaterials obtained by performing an etching process after a heat treatment at 550°C for 6 hours as shown in Fig. 27a, at 600°C for 1 hour as shown in Fig. 27b, and at 650°C for 3 hours as shown in Fig. 27c. In Figs. 27a and 27c, a wide and gentle peak regarded as an amorphous silicon oxide were observed between 20° and 30° together with a diffraction pattern of silicon.

Figs. 28a to 28c provide results of an X-ray diffraction analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method and reducing the powderized colloid by a magnesiothermic reaction without etching. A molar ratio of Si : Mg was determined as 1 : 2. Figs. 28a to 28c show diffraction patterns of silicon materials obtained by performing a heat treatment at 550°C for 6 hours as shown in Fig. 28a, at 600°C for 1 hour as shown in Fig. 28b, and at 650°C for 3 hours as shown in Fig. 28c. According to Figs. 28a and 28b, it was confirmed that a material containing a silicon nanomaterial, MgO, Mg₂Si, and some impurities was produced, and according to Fig. 28c, it was confirmed that a material containing a silicon nanomaterial and MgO was produced. From the above experiment results, it was confirmed that most of the impurities in the reduced silicon material were removed by performing the etching process and only the silicon nanomaterials usually remained.

Figs. 29a to 29c provide an image of transmission electron

microscopy (Fig. 29a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 29b), and a selected area electron

diffraction (SAED) pattern (Fig. 29c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 550°C for 6 hours, and etching the resultant product. From Fig. 29a, it was observed that particles of 50 nm or less were concentrated on particles of about 300 nm. From Fig. 29b, a grid on a (111) silicon plane with an inter-planar distance of 0.30 nm was observed.

From Fig. 29c, a pattern formed by crystals of silicon was observed.

Figs. 30a to 30c provide an image of transmission electron

microscopy (Fig. 30a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 30b), and a selected area electron

diffraction (SAED) pattern (Fig. 30c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 600°C for 1 hour, and etching the resultant product. From Fig. 30a, it was observed that particles of about 100 nm or 50 nm or less were concentrated on large particles of about 500 nm. From Fig. 30b, a grid on a (111) silicon plane with an inter-planar distance of 0.31 nm was observed. From Fig. 30c, a pattern formed by crystals of silicon was observed.

Figs. 31a to 31c provide an image of transmission electron

microscopy (Fig. 31a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 31b), and a selected area electron

diffraction (SAED) pattern (Fig. 31c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 650°C for 3 hours, and etching the resultant product. From Fig. 31a, particles without containing small particles of about 400 nm were observed. From Fig. 3 lb, a grid on a (111) silicon plane with an inter-planar distance of 0.30 nm was observed. From Fig. 3 lc, a pattern formed by crystals of silicon was observed. Figs. 32a to 32c provide results of an N₂ adsorption/desorption isotherm analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction, and etching the resultant product. Figs. 32a to 32c provide analysis results of silicon nanomaterials obtained by reduction at 550°C for 6 hours as shown in Fig. 32a, at 600°C for 1 hour as shown in Fig. 32b, and at 650°C for 3 hours as shown in Fig. 32c. According to the analysis results, pore sizes of the silicon nanomaterials were about 10.8 nm or less as shown in Fig. 32a, about 10.1 nm or less as shown in Fig. 32b, and about 10.7 nm or less as shown in Fig. 32c.

Figs. 33a to 33c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 550°C for 6 hours, and etching the resultant product. Fig. 33a is a graph showing a measurement result of capacity retention, Fig. 33b is a graph showing a measurement result of rate capability, and Fig. 33c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 23 % was observed after 50 cycles.

Figs. 34a to 34c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 600°C for 1 hour, and etching the resultant product. Fig. 34a is a graph showing a measurement result of capacity retention, Fig. 34b is a graph showing a measurement result of rate capability, and Fig. 34c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 17 % was observed after 48 cycles.

Figs. 35a to 35c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 650°C for 3 hours, and etching the resultant product. Fig. 35a is a graph showing a measurement result of capacity retention, Fig. 35b is a graph showing a measurement result of rate capability, and Fig. 35c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 35 % was observed after 40 cycles.

(2) In the case where a graphene was contained in as amount of 7.5 wt % in a silicon nanomaterial.

In the present example, an analysis was conducted on characteristics of a silicon nanomaterial prepared by using laponite and a graphene oxide solution such that a mass ratio of graphene in a final product became 7.5 % by mass.

Figs. 36a to 36c provide results of an X-ray diffraction analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction, and etching the resultant product. A molar ratio of Si : Mg was determined as 1 : 2. Figs. 36a to 36c show analysis results of silicon nanomaterials obtained by performing an etching process after a heat treatment at 550°C for 6 hours as shown in Fig. 36a, at 600°C for 1 hour as shown in Fig. 36b, and at 650°C for 3 hours as shown in Fig. 36c. In Figs. 36a and 36c, a wide and gentle peak regarded as an amorphous silicon oxide were observed between 20° and 30° together with a diffraction pattern of silicon.

Figs. 37a and 37c provide results of an X-ray diffraction analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method and reducing the powderized colloid by a magnesiothermic reaction without etching. A molar ratio of Si : Mg was determined as 1 : 2. Figs. 37a to 37b show analysis results of materials obtained by performing a heat treatment at 550°C for 6 hours as shown in Fig. 37a and at 600°C for 1 hour as shown in Fig. 37b. According to Figs. 37a and 37b, it was confirmed that a material containing a silicon nanomaterial, MgO, Mg₂Si, and some impurities was produced through a magnesiothermic reaction. Fig. 37c shows an analysis result of a material obtained by performing a heat treatment at 650°C for 3 hours. According to Fig. 37c, it was confirmed that a material containing a silicon nanomaterial and MgO was produced through a magnesiothermic reaction.

Figs. 38a to 38c provide an image of transmission electron

microscopy (Fig. 38a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 38b), and a selected area electron

diffraction (SAED) pattern (Fig. 38c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 550°C for 6 hours, and etching the resultant product. From Fig. 38a, particles of about 400 nm without containing small particles were observed. From Fig. 38b, a grid on a (111) silicon plane with an inter-planar distance of 0.31 nm was observed. From Fig. 38c, a pattern formed by crystals of silicon was observed.

Figs. 39a to 39c provide an image of transmission electron

microscopy (Fig. 39a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 39b), and a selected area electron

diffraction (SAED) pattern (Fig. 39c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 600°C for 1 hour, and etching the resultant product. From Fig. 39a, particles of about 400 nm rarely containing small particles were observed. From Fig. 39b, a grid on a (111) silicon plane with an inter-planar distance of 0.30 nm was observed. From Fig. 39c, a pattern formed by crystals of silicon was observed.

Figs. 40a to 40c provide an image of transmission electron

microscopy (Fig. 40a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 40b), and a selected area electron

diffraction (SAED) pattern (Fig. 40c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 650°C for 3 hours, and etching the resultant product. From Fig. 40a, it was observed that the synthesized material contained small particles having various sizes from about 10 nm or less to about 100 nm or less. From Fig. 40b, a grid on a (111) silicon plane with an inter-planar distance of 0.31 nm was observed. From Fig. 40c, a pattern formed by crystals of silicon was observed.

Figs. 41a to 41c provide results of an N₂ adsorption/desorption isotherm analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction, and etching the resultant product. Figs. 41a to 41c provide analysis results of silicon nanomaterials obtained by reduction at 550°C for 6 hours as shown in Fig. 41a, at 600°C for 1 hour as shown in Fig. 41b, and at 650°C for 3 hours as shown in Fig. 41c. According to the analysis results, pore sizes of the silicon nanomaterials were about 8.3 nm or less as shown in Fig. 41a, about 8.7 nm or less as shown in Fig. 41b, and about 12.0 nm or less as shown in Fig. 41c.

Figs. 42a to 42c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 550°C for 6 hours, and etching the resultant product. Fig. 42a is a graph showing a measurement result of capacity retention, Fig. 42b is a graph showing a measurement result of rate capability, and Fig. 42c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 23 % was observed after 50 cycles.

Figs. 43a to 43c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 600°C for 1 hour, and etching the resultant product. Fig. 43a is a graph showing a measurement result of capacity retention, Fig. 43b is a graph showing a measurement result of rate capability, and Fig. 43c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 18 % was observed after 50 cycles.

Figs. 44a to 44c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 650°C for 3 hours, and etching the resultant product. Fig. 44a is a graph showing a measurement result of capacity retention, Fig. 44b is a graph showing a measurement result of rate capability, and Fig. 44c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 39 % was observed after 30 cycles.

(3) In the case where a graphene was contained in as amount of 10 wt % in a silicon nanomaterial. In the present example, an analysis was conducted on characteristics of a silicon nanomaterial prepared by using laponite and a graphene oxide solution such that a mass ratio of graphene in a final product became 10 % by mass.

Figs. 45a to 45c provide results of an X-ray diffraction analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction, and etching the resultant product. A molar ratio of Si : Mg was determined as 1 : 2. Figs. 45a to 45c show analysis results of silicon nanomaterials obtained by performing an etching process after a heat treatment at 550°C for 6 hours as shown in Fig. 45a, at 600°C for 1 hour as shown in Fig. 45b, and at 650°C for 3 hours as shown in Fig. 45c. In Figs. 45a and 45c, a wide and gentle peak regarded as an amorphous silicon oxide were observed between 20° and 30° together with a diffraction pattern of silicon.

Figs. 46a to 46c provide results of an X-ray diffraction analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method and reducing the powderized colloid by a magnesiothermic reaction without etching. A molar ratio of Si : Mg was determined as 1 : 2. Figs. 46a and 46b show analysis results of materials obtained by performing a heat treatment at 550°C for 6 hours as shown in Fig. 46a and at 600°C for 1 hour as shown in Fig. 46b. According to Figs. 46a and 46b, it was confirmed that a material containing a silicon nanomaterial, MgO, Mg₂Si, and some impurities was produced through a magnesiothermic reaction. Fig. 46c shows an analysis result of a material obtained by performing a heat treatment at 650°C for 3 hours. According to Fig. 46c, it was confirmed that a material containing a silicon nanomaterial and MgO was produced.

Figs. 47a to 47c provide an image of transmission electron

microscopy (Fig. 47a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 47b), and a selected area electron

diffraction (SAED) pattern (Fig. 47c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 550°C for 6 hours, and etching the resultant product. From Fig. 47a, graphene and small particles of about 20 nm or less were observed. From Fig. 47b, a grid on a (111) silicon plane with an inter- planar distance of 0.31 nm was observed. From Fig. 47c, a pattern formed by crystals of silicon was observed.

Figs. 48a to 48c provide an image of transmission electron

microscopy (Fig. 48a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 48b), and a selected area electron

diffraction (SAED) pattern (Fig. 48c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 600°C for 1 hour, and etching the resultant product. From Fig. 48a, small particles of about 20 nm or less were observed.

From Fig. 48b, a grid on a (111) silicon plane with an inter-planar distance of 0.31 nm was observed. From Fig. 48c, a pattern formed by crystals of silicon was observed.

Figs. 49a to 49c provide an image of transmission electron

microscopy (Fig. 49a), an image of high resolution transmission electron microscopy (HR-TEM) (Fig. 49b), and a selected area electron

diffraction (SAED) pattern (Fig. 49c) of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 650°C for 3 hours, and etching the resultant product. From Fig. 49a, particles of about 400 nm and small particles of about 20 nm were observed. From Fig. 49b, a grid on a (111) silicon plane with an inter-planar distance of 0.31 nm was observed. From Fig. 49c, a pattern formed by crystals of silicon was observed.

Figs. 50a to 50c provide results of an N₂ adsorption/desorption isotherm analysis of a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction, and etching the resultant product. Figs. 50a to 50c provide analysis results of silicon nanomaterials obtained by reduction at 550°C for 6 hours as shown in Fig. 50a, at 600°C for 1 hour as shown in Fig. 50b, and at 650°C for 3 hours as shown in Fig. 50c. According to the analysis results, pore sizes of the silicon nanomaterials were about 10.2 nm or less as shown in Fig. 50a, about 11.2 nm or less as shown in Fig. 50b, and about 9.7 nm or less as shown in Fig. 50c.

Figs. 51a to 51b provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 550°C for 6 hours, and etching the resultant product. Fig. 51a is a graph showing a measurement result of rate capability, and Fig. 51b is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, this silicon nanomaterial was not much different in speed characteristic but had a relatively low capacity as compared with a nanomaterial heat-treated at a different temperature.

Figs. 52a to 52c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 600°C for 1 hour, and etching the resultant product. Fig. 52a is a graph showing a measurement result of capacity retention, Fig. 52b is a graph showing a measurement result of rate capability, and Fig. 52c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 32 % was observed after 40 cycles.

Figs. 53a to 53c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a restacking method, reducing the powderized colloid by a magnesiothermic reaction at 650°C for 3 hours, and etching the resultant product. Fig. 53a is a graph showing a measurement result of capacity retention, Fig. 53b is a graph showing a measurement result of rate capability, and Fig. 53c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a capacity loss of about 11 % was observed after 50 cycles. <Example 12 : Preparation of silicon nanomaterial using laponite and graphene oxide (Freeze-drying)>

In the present example, a silicon nanomaterial was prepared by

powderizing colloid including silica-containing material from laponite and graphene oxide by a freeze-drying method and reducing the powderized colloid. 500 mL of a laponite suspension including laponite at a concentration of 5 g/L was stirred for 48 hours to exfoliate the laponite. Then, a suspension containing graphene oxide of 0.05 wt % and prepared by a modified Hummer's method was added to colloid containing the exfoliated laponite. An amount of the graphene oxide was determined by comparing a weight of silicon contained in the laponite. The laponite contained Si0₂ of 59.5 wt %, and the Si0₂ contained silicon of 46.7 wt %. Therefore, the laponite contained silicon of 27.8 wt %. In consideration of this, the graphene oxide was added after calculation of a weight to be 6.5 %, 10 %, and 12.5 % with respect to the silicon contained in a final product (a composite of the graphene oxide and the silicon). In the present example, 170 mL of dispersed laponite was added to 30 mL of colloid containing graphene oxide of 0.05 wt %, 155 mL of dispersed laponite was added to 45 mL of colloid containing graphene oxide of 0.05 wt %, or 146 mL of dispersed laponite was added to 54 mL of colloid containing graphene oxide of 0.05 wt %, and then stirred for 1 hour. Thereafter, the mixed colloid was freeze-dried for 5 days to be powderized. To be specific, the colloid was quickly cooled with a liquid nitrogen and then freeze-dried at -75°C and 5 mTorr within a freeze dryer (Ilsin Boibase Freeze Dryer FD8508).

Then, with various molar ratios of Si0₂ : Mg, the powder was mixed with Mg powder. To be specific, molar ratios of Si0₂ : Mg was 1 : 2, 1 : 2.5,

1 : 3, and 1 : 4. Thereafter, the mixture was put into a self-produced tube furnace and heated at a heating rate of 3.3°C/min to reduce the silica contained in the mixture by a magnesiothermic reaction. A heating temperature for reduction varied in the range of from about 520°C to about 550°C. The reduction was carried out under a gas atmosphere of 5 % H₂/95 % Ar, the temperature was maintained for 3 hours after the temperature was increased. After the reduction was completed, the mixture was taken out and etched with a 0.5 M hydrochloric acid solution in an amount of 500 mL per 1 g of the mixture with stirring for 24 hours. Then, the etching solution used was removed by centrifugation, and the mixture was stirred again with 500 mL of a new 0.5 M hydrochloric acid solution for 24 hours. Thereafter, the etching solution used was removed by centrifugation, and then, the mixture was washed with distilled water.

Subsequently, the mixture was dried in a vacuum at 180°C for 24 hours. <Example 13 : Characteristic analysis of silicon nanomaterial prepared from laponite and graphene oxide>

In the present example, an analysis was conducted on characteristics of silicon nanomaterials prepared with various concentrations of a graphene oxide solution used for preparing silicon nanomaterials and various reduction conditions.

Figs. 54a to 54c provide results of an X-ray diffraction analysis of silicon nanomaterials obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a freeze-drying method, and reducing the powderized colloid. Figs. 54a to 54c show analysis results of silicon nanomaterials containing a 6.5 wt % graphen as shown in Fig. 54a, containing a 10 % graphene as shown in Fig. 54b, and containing a 12.5 % graphene as shown in Fig. 54c.

The upper graph of Fig. 54a shows a result of an X-ray diffraction analysis of a silicon material obtained by reduction at 520°C for 3 hours, and in this case, a molar ratio of Si : Mg was 1 : 2.5. The lower graph of Fig. 54a shows a result of an X-ray diffraction analysis of a silicon material obtained by reduction at 520°C for 1.5 hours, and in this case, a molar ratio of Si : Mg was 1 : 2.

The upper graph of Fig. 54b shows a result of an X-ray diffraction analysis of a silicon material obtained by reduction at 550°C for 3 hours, and in this case, a molar ratio of Si : Mg was 1 : 4. The lower graph of Fig. 54b shows a result of an X-ray diffraction analysis of a silicon material obtained by reduction at 550°C for 3 hours, and in this case, a molar ratio of Si : Mg was 1 : 3.

Fig. 54c shows a result of an X-ray diffraction analysis of a silicon material obtained by reduction at 550°C for 3 hours, and in this case, a molar ratio of Si : Mg was 1 : 2.

Figs. 55a to 55c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a freeze-drying method, reducing the powderized colloid by a magnesiothermic reaction at 550°C for 3 hours. A silicon nanomaterial containing 10 wt % graphene was used, and a molar ratio of Si : Mg was 1 : 4. Fig. 55a is a graph showing a measurement result of capacity retention, Fig. 55b is a graph showing a measurement result of rate capability, and Fig. 55c is a graph showing a measurement result of charge-discharge profile. According to an analysis result from the graphs, a change in capacity of a silicon nanomaterial containing 10 wt % graphene depending on a content of FIEC could be checked. However, this result was not considered to be changed entirely depending on a content of FIEC. When an electrolyte contained 10 % FIEC, a capacity loss of about 12 % was observed after 50 cycles. Figs. 55b and 55c provide graphs showing stable capacity even at a high current density, and stable potential vs. capacity.

Figs. 56a to 56c provide graphs showing electrochemical characteristics measured on a silicon nanomaterial obtained by powderizing colloid including silica-containing material from laponite and graphene oxide by a freeze-drying method, reducing the powderized colloid by a magnesiothermic reaction.

Fig. 56a shows capacity retention of a silicon nanomaterial containing a 10 % graphene at a molar ratio of Si : Mg of 1 : 3. Fig. 56b shows rate capability of a silicon nanomaterial containing a 12.5 % graphene at a molar ratio of Si : Mg of 1 : 2 without containing FIEC. Fig. 56c shows charge-discharge profile of a silicon nanomaterial containing a 12.5 % graphene at a molar ratio of Si : Mg of 1 : 2 without containing FIEC. According to an analysis result from the graphs, in the case of charge/discharge at a current density of 0.05 C, capacity was stable but as low as about 600 mAh/g.

The above description of the present disclosure is provided for the purpose of illustration, and it would be understood by those skilled in the art that various changes and modifications may be made without changing technical conception and essential features of the present disclosure. Thus, it is clear that the above- described embodiments are illustrative in all aspects and do not limit the present disclosure. For example, each component described to be of a single type can be implemented in a distributed manner. Likewise, components described to be distributed can be implemented in a combined manner.

The scope of the present disclosure is defined by the following claims rather than by the detailed description of the embodiment. It shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the present disclosure.

Claims

C L A I M S

1. A method for preparing a silicon nanosheet, the method comprising : heating a mixture including a clay and an alkaline earth metal to reduce a silica included in the clay to a silicon nanosheet.

2. The method of claim 1 , further comprising : etching the heated mixture by using an acidic solution to obtain the silicon nanosheet.

3. The method of claim 1 or 2, wherein the clay comprises a layered clay.

4. The method of any one of claims 1 to 3, wherein the clay comprises smectite clays comprising a material selected from the group consisting of montmorillonite, nontronite, beidellite, bentonite, hectorite, laponite, saponite, sauconite, vermiculite, and their combinations.

5. The method of any one of claims 1 to 4, wherein heating the mixture is performed at a temperature of from 300°C to 1000°C, preferably from 400°C to 800°C, more preferably from 550°C to 750°C.

6. The method of any one of claims 1 to 5, wherein a mole ratio of a silicon included in the clay to an alkaline earth metal included in the mixture is from 1 : 0.1 to 1 : 10, preferably from 1 : 0.5 to 1 : 5, more preferably

from 1 : 0.5 to 1 : 3.

7. The method of any one of claims 1 to 6, further comprising : before heating a mixture including a clay and an alkaline earth metal, restacking the clay by adding an alkaline earth metal cation to the clay, or freeze-drying the clay.

8. The method of any one of claims 1 to 7, wherein the mixture including a clay and an alkaline earth metal further includes a graphene oxide.

9. The method of claim 8, further comprising : forming a composite including silicon and graphene by heating the mixture including a clay and an alkaline earth metal and further including the graphene oxide to reduce the silica included in the clay and the graphene oxide to the silicon nanosheet and graphene, respectively.

10. A silicon nanosheet comprising a layered structure substantially similar to a clay.

11. The silicon nanosheet of claim 10, wherein the silicon nanosheet is produced by a method of any one of claims 1 to 9.

12. The silicon nanosheet of claim 10 or 11, wherein the silicon nanosheet has a B.E.T. surface area of from 20 m²/g to 300 m²/g.

13. The silicon nanosheet of any one of claims 10 to 12, wherein the silicon nanosheet has a size of from 10 nm to 300 nm and an aspect ratio of from 1 : 1 to 1 : 10.

14. The silicon nanosheet of any one of claims 10 to 13, wherein the silicon nanosheet is hybridized with at least one conductive carbon compound selected from the group consisting of carbon black, acetylene black, active carbon, carbon nanotube (CNT), graphite, graphene, and any combinations thereof.

15. An electrode, comprising the silicon nanosheet of any one of claims 10 to 14.

16. A battery comprising : the electrode of claim 15 as an anode, a cathode, and an electrolyte.