Patent Description:
As negative electrode materials for non-aqueous electrolyte batteries, carbonaceous materials obtained by firing graphite or organic materials have high charge/discharge efficiency and excellent cycle life characteristics, and thus have been commercialized widely. However, as portable electronic communication devices or power tools have been downsized, weight-lightened and provided with high quality and a need for high capacity for commercialization of electric vehicles has been increased, there has been a need for a negative electrode material substituting for such carbonaceous materials having low battery capacity due to structural characteristics.

Recently, active studies about metals, such as Si and Sn, have been conducted despite several problems of metallic negative electrodes. Particularly, since a silicon-based negative electrode has a theoretical capacity of <NUM> mAh/g corresponding to about <NUM> times or more of the theoretical capacity <NUM> mAh/g of a carbonaceous negative electrode, studies about silicon-based negative electrodes have been conducted intensively.

Silicon as a negative electrode material undergoes volumetric swelling to <NUM>% or more through the reaction with lithium during charge and then is shrunk during discharge. Due to such a rapid swelling and shrinking phenomenon, the negative electrode material particles cause cracking or separation of the electrode undesirably, resulting in a serious problem of rapid degradation of cycle life.

A silicon oxide-based negative electrode suggested to solve the above-mentioned problem provides a low capacity as compared to silicon, but shows higher capacity as compared to the capacity of a carbonaceous negative electrode and a lower volumetric swelling ratio as compared to metals, and thus has been spotlighted as an advanced material having excellent cycle life characteristics. When using such silicon oxide as a negative electrode material for a secondary battery, it is possible to obtain a capacity approximately <NUM> times higher than the capacity of a carbonaceous negative electrode active material but corresponding to approximately a half of the capacity of a silicon negative electrode active material. However, when using silicon oxide as a negative electrode material for a secondary battery, there still is a problem in that it shows a larger change in volume during charge/discharge as compared to a carbonaceous negative electrode material.

In addition, when using silicon oxide as a negative electrode material for a secondary battery, silicon oxide undergoes reaction with lithium upon the initial charge to produce lithium silicide and lithium oxide (lithium oxide and lithium silicate). Particularly, lithium oxide cannot participate in the subsequent electrochemical reaction and a part of lithium transported to a negative electrode upon the initial charge cannot be returned to a positive electrode, and thus irreversible reaction occurs, resulting in depletion of lithium undesirably.

In the case of silicon oxide, it shows high irreversible capacity as compared to the other silicon-based negative electrodes and provides a significantly low initial charge efficiency (ICE, ratio of initial discharge capacity to charge capacity) of <NUM>-<NUM>%. Such low initial efficiency requires excessive capacity of a positive electrode, when manufacturing a secondary battery, thereby causing a setoff of the capacity per unit weight of a negative electrode.

To solve the above-mentioned problems, magnesium-doped silicon-silicon oxide composites have been prepared in the art. However, there still has been a need for improvement in terms of cycle characteristics. <CIT>, <CIT> and <CIT> disclose silicon oxide materials for lithium secondary batteries.

The present disclosure is designed to solve the problems of the related art, and therefore the present disclosure is directed to providing a negative electrode active material capable of improving the initial efficiency, capacity and cycle characteristics of a lithium secondary battery. The negative electrode active material for a lithium secondary battery includes a silicon-silicon oxide-magnesium silicate (Si-SiOx-Mg silicate) composite, wherein two types of magnesium silicates comprised in the composite have a predetermined composition and content.

The present disclosure is also directed to providing a method for manufacturing the negative electrode active material for a lithium secondary battery.

According to the first embodiment of the present disclosure, there is provided a negative electrode active material for a lithium secondary battery which includes a silicon-silicon oxide-magnesium silicate composite comprising a silicon oxide (SiOx, <NUM> < x ≤ <NUM>) matrix; and silicon (Si) crystal grains, MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains present in the silicon oxide matrix, wherein the content ratio of MgSiO<NUM> crystal grains to Mg<NUM>SiO<NUM> crystal grains is <NUM>:<NUM>-<NUM>:<NUM> on the weight basis.

According to the second embodiment of the present disclosure, there is provided the negative electrode active material for a lithium secondary battery as defined in the first embodiment, wherein the content ratio of MgSiO<NUM> crystal grains to Mg<NUM>SiO<NUM> crystal grains is <NUM>:<NUM>-<NUM>:<NUM> on the weight basis.

According to the third embodiment of the present disclosure, there is provided the negative electrode active material for a lithium secondary battery as defined in the first or the second embodiment, wherein the MgSiO<NUM> crystal grains in the silicon-silicon oxide-magnesium silicate composite have a crystal size of <NUM>-<NUM>.

According to the fourth embodiment of the present disclosure, there is provided the negative electrode active material for a lithium secondary battery as defined in any one of the first to the third embodiments, wherein the Mg<NUM>SiO<NUM> crystal grains in the silicon-silicon oxide-magnesium silicate composite have a crystal size of <NUM>-<NUM>.

According to the fifth embodiment of the present disclosure, there is provided the negative electrode active material for a lithium secondary battery as defined in any one of the first to the fourth embodiments, wherein the Mg<NUM>SiO<NUM> crystal grains have a crystal size corresponding to <NUM>-<NUM> times of the crystal size of the MgSiO<NUM> crystal grains.

According to the sixth embodiment of the present disclosure, there is provided the negative electrode active material for a lithium secondary battery as defined in any one of the first to the fifth embodiments, wherein the total content of Mg metal is <NUM>-<NUM> wt% based on the weight of the silicon-silicon oxide-magnesium silicate composite particles.

According to the seventh embodiment of the present disclosure, there is provided the negative electrode active material for a lithium secondary battery as defined in any one of the first to the sixth embodiments, wherein the content ratio of MgSiO<NUM> crystal grains to Mg<NUM>SiO<NUM> crystal grains is <NUM>:<NUM>-<NUM>:<NUM> on the weight basis.

According to the eighth embodiment of the present disclosure, there is provided a negative electrode for a lithium secondary battery including the negative electrode active material for a lithium secondary battery as defined in any one of the first to the seventh embodiments.

According to the ninth embodiment of the present disclosure, there is provided a lithium secondary battery including the negative electrode for a lithium secondary battery as defined in the eighth embodiment.

According to the tenth embodiment of the present disclosure, there is provided a method for preparing the silicon-silicon oxide-magnesium silicate composite as defined in the first embodiment, including the steps of: (S1) introducing a mixture comprising silicon powder, silicon dioxide (SiO<NUM>) powder and Mg powder at a molar ratio of silicon powder to silicon dioxide (SiO<NUM>) powder of <NUM>:<NUM>-<NUM>:<NUM> and a molar ratio of silicon powder to Mg powder of <NUM>:<NUM>-<NUM>:<NUM> to a reactor; (S2) heating the mixture comprising silicon powder, silicon dioxide (SiO<NUM>) powder and Mg powder to <NUM>,<NUM>-<NUM>,<NUM> under vacuum so that silicon oxide vapor resulting from a mixed powder of silicon powder and silicon dioxide (SiO<NUM>) powder and magnesium vapor may be generated at the same time to carry out vapor-phase reaction, and then cooling the reaction mixture to precipitate a silicon-silicon oxide-magnesium silicate composite at <NUM>-<NUM>; and (S3) pulverizing the precipitated silicon-silicon oxide-magnesium silicate composite to an average particle diameter of <NUM>-<NUM>.

The non-aqueous secondary battery including a Si-SiOx-Mg silicate composite as a negative electrode active material according to the present disclosure can satisfy high battery capacity, excellent cycle characteristics and high charge/discharge efficiency at the same time.

The effects of the present disclosure are not limited to the above-mentioned effect and it should be understood that the effects of the present disclosure include any effects inferable from the constitution of the present disclosure as described in the following detailed description and claims.

Hereinafter, preferred embodiments of the present disclosure will be described in detail.

It will be further understood that the terms "comprises" and/or "comprising", or "includes" and/or "including" when used in this specification, specify the presence of stated features, numbers, steps, elements or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements or combinations thereof.

As used herein. 'D<NUM> average particle diameter' means a value measured as weight average D<NUM> (particle diameter when accumulated weight reaches <NUM>% of the total weight, or median diameter) in particle size distribution analysis through laser light diffraction.

In one aspect of the present disclosure, there is provided a negative electrode active material for a lithium secondary battery which includes a silicon-silicon oxide-magnesium silicate composite comprising a silicon oxide (SiOx, <NUM> < x ≤ <NUM>) matrix; and silicon (Si) crystal grains, MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains present in the silicon oxide matrix, wherein the content ratio of MgSiO<NUM> crystal grains to Mg<NUM>SiO<NUM> crystal grains is <NUM>:<NUM>-<NUM>:<NUM> on the weight basis.

As used herein, 'crystal size' is determined by analyzing the ratio of diffraction peak intensities through X-ray diffractometry, and the particular analysis method is referred to the following Test Example <NUM>.

Preferably, the silicon crystal grains, MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains are distributed homogeneously in the silicon-silicon oxide-magnesium silicate composite. More preferably, the silicon crystal grains, MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains are dispersed and distributed homogeneously in the silicon oxide matrix and embedded/buried therein.

According to an embodiment of the present disclosure, in the MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains, elements in each phase are interfused so that the boundary surfaces of each phase may be bound with one another. In other words, each phase is bound at an atomic level. As a result, even when lithium ions are intercalated/deintercalated, a change in volume of the silicon-silicon oxide-magnesium silicate is small, and particle cracking hardly occurs even after repeating charge/discharge.

According to an embodiment of the present disclosure, the MgSiO<NUM> crystal grains have a crystal size of <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. When the MgSiO<NUM> crystal grains have the above-defined range of crystal size, it is possible to obtain an effect of increasing initial efficiency and improving life characteristics. The crystal size may be determined according to the Scherrer equation (see, the following Mathematical Formula <NUM>) based on the full width at half maximum (FWHM) of the diffraction peak of MgSiO<NUM> (<NUM>) in X-ray diffractometry, and is based on a silicon-silicon oxide-magnesium silicate composite obtained at a precipitation temperature of <NUM> during the preparation process. X-ray diffractometry may be carried out according to the conventional XRD method. According to an embodiment of the present disclosure, XRD may be carried out in a 2θ range of <NUM>-<NUM>° by using CuKα rays (e.g. wavelength of light source: <NUM>. Herein, scanning speed may be selected suitably. <MAT> wherein K = <NUM>, λ = <NUM>, B = full width at half maximum (FWHM, rad), and θ = peak position (angle).

According to an embodiment of the present disclosure, the Mg<NUM>SiO<NUM> crystal grains have a crystal size of <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. When the Mg<NUM>SiO<NUM> crystal grains have the above-defined range of D<NUM> average particle diameter, it is possible to obtain an effect of increasing initial efficiency and improving life characteristics. The crystal size may be determined according to the Scherrer equation (see, the following Mathematical Formula <NUM>) based on the full width at half maximum (FWHM) of the diffraction peak of the Mg<NUM>SiO<NUM> (<NUM>) in X-ray diffractometry, and is based on a silicon-silicon oxide-magnesium silicate composite obtained at a precipitation temperature of <NUM> during the preparation process. <MAT> wherein K = <NUM>, λ = <NUM>, B = full width at half maximum (FWHM, rad), and θ = peak position (angle).

According to an embodiment of the present disclosure, each of the MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains has the above-defined range of D<NUM> average particle diameter, while the Mg<NUM>SiO<NUM> crystal grains have a crystal size corresponding to <NUM> time or more, or <NUM>-<NUM> times of the crystal size of the MgSiO<NUM> crystal grains. Even though each of the MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains has the above-defined range of crystal size, it is possible to obtain an effect of improving electrical conductivity and an effect of improving life characteristics through the improvement of physical strength only when the Mg<NUM>SiO<NUM> crystal grains have a D<NUM> average particle diameter corresponding to <NUM> time or more of the crystal size of the MgSiO<NUM> crystal grains.

According to the present disclosure, the MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains are distributed in the silicon oxide matrix. As used herein, the term 'silicon oxide' refers to general compounds represented by the formula of SiOx (<NUM> < x ≤ <NUM>). According to an embodiment of the present disclosure, the silicon oxide matrix includes silicon oxide SiOx (<NUM> < x ≤ <NUM>), and is an ingredient inactive to Li so that Li intercalation/deintercalation may not occur during the charge of a battery.

According to the present disclosure, in addition to the MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains, Si crystal grains are dispersed and distributed in the silicon oxide matrix. The Si crystal grains preferably have a crystal size of <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. When the Si crystal size is larger than the above-defined range, life characteristics may be degraded. The Si crystal grain size may be determined according to the Scherrer equation (see, the following Mathematical Formula <NUM>) based on the full width at half maximum (FWHM) of the diffraction peak of Si (<NUM>) in X-ray diffractometry, and is based on a silicon-silicon oxide-magnesium silicate composite obtained at a precipitation temperature of <NUM> during the preparation process. <MAT> wherein K = <NUM>, λ = <NUM>, B = full width at half maximum (FWHM, rad), and θ = peak position (angle).

According to an embodiment of the present disclosure, total content of Mg metal is <NUM>-<NUM> wt%, <NUM>-<NUM> wt%, or <NUM>-<NUM> wt%, based on the weight of the silicon-silicon oxide-magnesium silicate composite. When the content of Mg metal satisfies the above-defined range, it is possible to provide an effect of improving the efficiency of a lithium secondary battery using the silicon-silicon oxide-magnesium silicate composite as a negative electrode active material, while minimizing a decrease in capacity of the battery.

According to an embodiment of the present disclosure, the content ratio of the MgSiO<NUM> crystal grains to Mg<NUM>SiO<NUM> crystal grains in the silicon-silicon oxide-magnesium silicate composite particles may be <NUM>:<NUM>-<NUM>:<NUM>, <NUM>:<NUM>-<NUM>:<NUM>, or <NUM>:<NUM>-<NUM>:<NUM>, on the weight basis. When the ratio of the MgSiO<NUM> crystal grains to Mg<NUM>SiO<NUM> crystal grains satisfies the above-defined range, it is possible to provide an effect of increasing initial efficiency and improving life characteristics.

According to an embodiment of the present disclosure, the silicon-silicon oxide-magnesium silicate composite particles may have an average particle diameter (D<NUM>) of <NUM>-<NUM>, or <NUM>-<NUM>. When the silicon-silicon oxide-magnesium silicate composite has the above-defined range of average particle diameter, it may be distributed homogeneously in slurry for forming a negative electrode during the preparation of the slurry, only a suitable amount of binder is required, and the problem of swelling of silicon crystal grains during charge/discharge and separation of the silicon-silicon oxide-magnesium silicate composite particles from the current collector may be minimized.

According to an embodiment of the present disclosure, the silicon-silicon oxide-magnesium silicate composite particles may have a BET specific surface area of <NUM>-<NUM><NUM>/g as determined by nitrogen adsorption. When the silicon-silicon oxide-magnesium silicate composite particles have the above-defined range of BET specific surface area, it is possible to form a negative electrode having a uniform coating layer of slurry for forming a negative electrode and to minimize side reactions in a lithium secondary battery during the charge/discharge.

According to an embodiment of the present disclosure, the silicon-silicon oxide-magnesium silicate composite particles may further include a coating layer or deposition layer comprising a carbonaceous material on the surfaces thereof. When using a silicon-based material as a negative electrode active material for a lithium secondary battery, electrical resistance may be increased. Thus, it is possible to improve conductivity by coating the surfaces of the particles comprising a silicon-based material with a carbonaceous material.

There is no particular limitation in the carbonaceous material. For example, the carbonaceous material may include graphite, such as natural graphite, artificial graphite or expandable graphite, carbon black, such as acetylene black or Ketjen black, and carbon fibers. In addition, hydrocarbon materials, such as methane, ethane, propane, butane, acetylene, carbon monoxide, benzene, toluene or xylene, may be used. Preferably, the particle surfaces may include a coating layer or deposition layer including a carbonaceous material with a view to improvement of conductivity of the silicon-silicon oxide-magnesium silicate composite particles.

According to an embodiment of the present disclosure, the carbonaceous material may be used in an amount of <NUM>-<NUM> parts by weight, or <NUM>-<NUM> parts by weight, based on <NUM> parts by weight of the silicon-silicon oxide-magnesium silicate composite particles. When the carbonaceous material is used in the above-defined range, it is possible to prevent the problem of an undesired increase in specific surface area of an electrode, while improving conductivity of the electrode and charge/discharge characteristics and cycle characteristics of a battery.

In another aspect of the present disclosure, there is provided a method for preparing the silicon-silicon oxide-magnesium silicate composite particles to be used as a negative electrode active material for a lithium secondary battery. An embodiment of the method includes the steps of: (S1) introducing a mixture comprising silicon powder, silicon dioxide (SiO<NUM>) powder and Mg powder at a molar ratio of silicon powder to silicon dioxide (SiO<NUM>) powder of <NUM>:<NUM>-<NUM>:<NUM> and a molar ratio of silicon powder to Mg powder of <NUM>:<NUM>-<NUM>:<NUM> to a reactor; (S2) heating the mixture comprising silicon powder, silicon dioxide (SiO<NUM>) powder and Mg powder to <NUM>,<NUM>-<NUM>,<NUM> under vacuum so that silicon oxide vapor resulting from a mixed powder of silicon powder and silicon dioxide (SiO<NUM>) powder and magnesium vapor may be generated at the same time to carry out vapor-phase reaction, and then cooling the reaction mixture to precipitate a silicon-silicon oxide-magnesium silicate composite at <NUM>-<NUM>; and (S3) pulverizing the precipitated silicon-silicon oxide-magnesium silicate composite to an average particle diameter of <NUM>-<NUM>.

When the heating temperature of the mixture comprising mixed powder of silicon with silicon dioxide (SiO<NUM>) and Mg is lower than <NUM>,<NUM>, it is difficult to improve initial efficiency through heat treatment. When the heating temperature is higher than <NUM>,<NUM>, silicon crystals are grown excessively to cause cracking in the silicon oxide composite structure due to volumetric swelling and shrinking of the silicon crystals according to lithium ion intercalation, resulting in degradation of cycle characteristics.

Each of the gaseous silicon oxide (SiOx) and gaseous Mg may be formed by a heating process under a reduced pressure atmosphere of -<NUM> to -<NUM> torr.

The temperature, where the silicon-silicon oxide-magnesium silicate composite is precipitated by cooling the mixture comprising a mixed powder of silicon with silicon dioxide (SiO<NUM>) and Mg after it is heated, may be <NUM>-<NUM>. According to an embodiment of the present disclosure, the temperature where the silicon-silicon oxide-magnesium silicate composite is precipitated may be controlled to <NUM>-<NUM>. When the cooling and precipitation temperature of the silicon-silicon oxide-magnesium silicate composite is lower than <NUM>, the silicon oxide and Mg may be cooled rapidly after it undergoes vapor-phase reaction to produce nanopowder and to cause a rapid increase in specific surface area of the silicon-silicon oxide-magnesium silicate composite, resulting in degradation of battery characteristics. When the cooling and precipitation temperature of the silicon-silicon oxide-magnesium silicate composite is higher than <NUM>, silicon crystal size may be increased to several tens of nanometers or more, resulting in rapid degradation of the life characteristics of a battery.

There is no particular limitation in the method for forming a coating layer or deposition layer comprising a carbonaceous material on the surface of the silicon-silicon oxide-magnesium silicate composite. For example, an organic polymer coating film may be formed on the surface of the silicon-silicon oxide-magnesium silicate composite and then pyrolyzed under inert atmosphere. Otherwise, a coating layer or deposition layer comprising a carbonaceous material may be formed by heat treating hydrocarbon or organic gas through a chemical vapor deposition process. According to a preferred embodiment of the present disclosure, any additional heat treatment is not carried out after forming the coating layer or deposition layer.

When using the chemical deposition process, in the case of at a temperature of <NUM>-<NUM>, a carbonaceous material may be produced at an adequate rate, while decomposition of raw material gases does not occur overly rapidly.

The carbonaceous raw material gas may include a hydrocarbon-based material, such as benzene, toluene, xylene, methane, ethane, ethylene, propane, butane, acetylene or carbon monoxide. In addition, more inexpensive gas, such as liquefied petroleum gas (LPG) or liquefied natural gas (LNG), may be used.

The carbonaceous material formed on the surfaces of the silicon-silicon oxide-magnesium silicate composite particles may include any one selected from crystalline carbon, amorphous carbon, carbon fibers, carbon whiskers, carbon nanofibers, carbon nanotubes, graphene, oxidized graphene and reduced graphene, or a mixture thereof.

In still another aspect of the present disclosure, there is provided a lithium secondary battery which includes a negative electrode including the silicon-silicon oxide-magnesium silicate composite particles as a negative electrode active material, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

The positive electrode may include a positive electrode current collector and a mixture of a positive electrode active material, a conductive material and a binder, disposed on the positive electrode current collector.

The positive electrode current collector should have high conductivity, allow easy adhesion of the above-mentioned mixture, and show no reactivity in the voltage range of the corresponding battery. Particular examples of the positive electrode current collector include aluminum, nickel, alloys thereof, or the like. The positive electrode current collector may have a thickness of <NUM>-<NUM>.

Particular examples of the positive electrode active material include: lithium cobalt oxides such as LixCoO<NUM> (<NUM> < x < <NUM>); lithium nickel oxides such as LixNiO<NUM> (<NUM> < x < <NUM>); lithium manganese oxides such as Li<NUM>+xMn<NUM>-xO<NUM> (wherein x is <NUM>-<NUM>), LiMnO<NUM>, LiMn<NUM>O<NUM>, LiMn<NUM>O<NUM> or LixMnO<NUM> (<NUM> < x < <NUM>); lithium copper oxides such as Li<NUM>CuO<NUM>; lithium iron oxides such as LiFe<NUM>O<NUM>; lithium nickel cobalt manganese oxides such as Li[NixCOyMnz]O<NUM> (x + y + z = <NUM>, <NUM> < x < <NUM>, <NUM> < y < <NUM>, <NUM> < z < <NUM>); lithium nickel cobalt aluminum oxides such as Li[NixCoyAlz]O<NUM> (x + y + z = <NUM>, <NUM> < x < <NUM>, <NUM> < y < <NUM>, <NUM> < z < <NUM>); lithium vanadium compounds such as LiV<NUM>O<NUM>; Ni-site type lithium nickel oxides represented by the chemical formula of LiNi<NUM>-xMxO<NUM> (wherein M is Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x is <NUM>-<NUM>); lithium manganese composite oxides represented by the chemical formula of LiMn<NUM>-xMxO<NUM> (wherein M = Co, Ni, Fe, Cr, Zn or Ta, and x = <NUM>-<NUM>) or Li<NUM>Mn<NUM>MO<NUM> (wherein M = Fe, Co, Ni, Cu or Zn); lithium manganese composite oxides in which Li is partially substituted with an alkaline earth metal ion; disulfide compounds; vanadium oxides such as V<NUM>O<NUM> or Cu<NUM>V<NUM>O<NUM>; Fe<NUM>(MoO<NUM>)<NUM>; or the like. More particularly, the positive electrode active material may include lithium nickel cobalt manganese oxides such as Li[NixCoyMnz]O<NUM> (x + y + z = <NUM> , and each of x, y and z independently represents a number of <NUM>-<NUM>) or lithium nickel cobalt aluminum oxides such as Li[NixCoyAlz]O<NUM> (x + y + z = <NUM>, and each of x, y and z independently represents a number of <NUM>-<NUM>). Such positive electrode active materials may be used alone or in combination.

The conductive material is a material having conductivity while not causing any chemical change in the lithium secondary battery according to the present disclosure. Particular examples of the conductive material include: graphite, such as natural graphite or artificial graphite; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal black; conductive fibers, such as carbon fibers or metallic fibers; carbon fluoride; metal powder, such as aluminum or nickel powder; conductive whisker, such as zinc oxide or potassium titanate; conductive metal oxide, such as titanium oxide; conductive materials, such as polyphenylene derivatives; or the like.

The binder is an ingredient which assists binding between the positive electrode active material and the conductive material and binding to the current collector. Particular examples of the binder include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylenepropylene-diene monomer (EPDM) rubber, hydrogenated nitrile butadiene rubber (HNBR), sulfonated ethylene propylene diene, styrene butadiene rubber (SBR), fluororubber, various copolymers, or the like.

The negative electrode includes a negative electrode current collector and a negative electrode active material disposed on the negative electrode current collector.

The negative electrode current collector should have high conductivity, allow easy adhesion of the negative electrode active material, and show no reactivity in the voltage range of the corresponding battery. Particular examples of the negative electrode current collector include copper, gold, nickel, alloys thereof, or the like.

Description of the negative electrode active material is the same as the above description of the embodiments of the negative electrode active material.

The separator prevents a short-circuit between the positive electrode and the negative electrode and provides a lithium ion transport channel. The separator may include an insulation thin film having high ion permeability and mechanical strength. Particular examples of the separator include polyolefin-based polymer membranes, such as polypropylene and polyethylene, or multilayer films, microporous films, woven fabrics or non-woven fabrics thereof, or the like. When a solid electrolyte, such as the polymer described hereinafter, is used, it may also be used as a separator.

The electrolyte may be a lithium salt-comprising electrolyte. Particular examples of the anion of the lithium salt include F-, Cl-, Br-, I-, NO<NUM>-, N(CN)<NUM>-, BF<NUM>-, ClO<NUM>-, PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF-, (CF<NUM>)<NUM>P-, CF<NUM>SO<NUM>-, CF<NUM>CF<NUM>SO<NUM>-, (CF<NUM>SO<NUM>)<NUM>N-, (FSO<NUM>)<NUM>N-, CF<NUM>CF<NUM>(CF<NUM>)<NUM>CO-, (CF<NUM>SO<NUM>)<NUM>CH-, (SF<NUM>)<NUM>C-, (CF<NUM>SO<NUM>)<NUM>C-, CF<NUM>(CF<NUM>)<NUM>SO<NUM>-, CF<NUM>CO<NUM>-, CH<NUM>CO<NUM>-, SCN-, (CF<NUM>CF<NUM>SO<NUM>)<NUM>N-, or the like. Such anions may be incorporated into the electrolyte alone or in combination.

The lithium secondary battery according to an embodiment of the present disclosure may be provided in the form a cylindrical battery using a can, a prismatic battery, pouch-type battery or a coin-type battery, but its outer shape is not limited thereto.

The lithium secondary battery according to an embodiment of the present disclosure may be applied not only to a battery cell used as a power source for compact type devices but also as a unit cell in a medium- or large-scale battery module including a plurality of battery cells. Particular examples of the medium- or large-scale devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles or power storage system, but are not limited thereto.

A mixture comprising silicon powder, silicon dioxide (SiO<NUM>) powder and Mg powder at a molar ratio of silicon powder to silicon dioxide (SiO<NUM>) powder of <NUM>:<NUM> and a molar ratio of silicon powder to Mg powder of <NUM>:<NUM> was introduced to a reactor. The mixture comprising silicon powder, silicon dioxide (SiO<NUM>) powder and Mg powder was heated to <NUM>,<NUM> under a reduced pressure atmosphere of -<NUM> torr and under vacuum to carry out vapor-phase reaction by generating silicon oxide vapor resulting from a mixed powder of silicon powder with silicon dioxide (SiO<NUM>) powder and magnesium vapor at the same time, and then cooled to precipitate a silicon-silicon oxide-magnesium silicate composite at <NUM>. Then, the silicon-silicon oxide-magnesium silicate composite was pulverized with a jet mill to an average particle diameter (D<NUM>) of <NUM>, thereby recovering powder of silicon-silicon oxide-magnesium silicate composite particles. Herein, the content of Mg was <NUM>% based on the weight of the silicon-silicon oxide-magnesium silicate composite particles.

The recovered silicon-silicon oxide-magnesium silicate composite particles were subjected to chemical vapor deposition (CVD) by using a tube-shaped electric furnace at <NUM>,<NUM> for <NUM> hours in the presence of a mixed gas of argon (Ar) with methane (CH<NUM>) to obtain silicon-silicon oxide-magnesium silicate composite particles having a carbon coating layer with a carbon content of <NUM> wt%. The resultant silicon-silicon oxide-magnesium silicate composite particles having a carbon coating layer was not subjected to further heat treatment.

After the powder of silicon-silicon oxide-magnesium silicate composite particles (Sample <NUM>) was analyzed with inductively coupled plasma-atomic emission spectroscopy (ICP-AES), it was shown that magnesium concentration was <NUM> wt%. After carrying out X-ray diffractometry (CuKα), it was shown that Si crystal grains had a size of <NUM>.

The resultant silicon-silicon oxide-magnesium silicate composite includes a silicon oxide (SiOx, <NUM> < x ≤ <NUM>) matrix; and silicon (Si) crystal grains, MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains present in the silicon oxide matrix. Each of the crystal size of MgSiO<NUM> crystal grains and that of Mg<NUM>SiO<NUM> crystal grains in the silicon-silicon oxide-magnesium silicate composite, and the ratio of crystal size of Mg<NUM>SiO<NUM> crystal grains to crystal size of MgSiO<NUM> crystal grains are shown in the following Table <NUM>.

Powder of silicon-silicon oxide-magnesium silicate composite particles were obtained in the same manner as Example <NUM>, except that the mixed powder was heat treated at a temperature of <NUM>,<NUM>.

Powder of silicon-silicon oxide-magnesium silicate composite particles were obtained in the same manner as Example <NUM>, except that the cooling temperature after carrying out vapor-phase reaction by generating silicon oxide vapor and magnesium vapor at the same time was set to <NUM>.

A mixture comprising silicon powder, silicon dioxide (SiO<NUM>) powder and Mg powder at a molar ratio of silicon powder to silicon dioxide (SiO<NUM>) powder of <NUM>:<NUM> and a molar ratio of silicon powder to Mg powder of <NUM>:<NUM> was introduced to a reactor. The mixture comprising silicon powder, silicon dioxide (SiO<NUM>) powder and Mg powder was heated to <NUM>,<NUM> under a reduced pressure atmosphere of -<NUM> torr and under vacuum to carry out vapor-phase reaction by generating silicon oxide vapor resulting from a mixed powder of silicon powder with silicon dioxide (SiO<NUM>) powder and magnesium vapor at the same time, and then cooled to precipitate a silicon-silicon oxide-magnesium silicate composite at <NUM>.

The recovered silicon-silicon oxide-magnesium silicate composite particles were subjected to chemical vapor deposition (CVD) by using a tube-shaped electric furnace at <NUM>,<NUM> for <NUM> hours in the presence of a mixed gas of argon (Ar) with methane (CH<NUM>) to obtain silicon-silicon oxide-magnesium silicate composite particles having a carbon coating layer with a carbon content of <NUM> wt%.

Then, the powder of silicon-silicon oxide-magnesium silicate composite particles having a carbon coating layer was further heat treated at <NUM>,<NUM> for <NUM> hours under argon (Ar) atmosphere to obtain powder of silicon-silicon oxide-magnesium silicate composite particles as a final product.

Powder of silicon-silicon oxide-magnesium silicate composite particles were obtained in the same manner as Comparative Example <NUM>, except that additional heat treatment was further carried out at <NUM>,<NUM> for <NUM> hours under argon (Ar) atmosphere in the final step.

Each of the samples obtained from Examples <NUM> and <NUM> and Comparative Examples <NUM>-<NUM> was used as a negative electrode active material, and mixed with Super-P black as a conductive material and polyacrylic acid (PAA) as a binder at a weight ratio of <NUM>:<NUM>:<NUM> to obtain a slurry-like composition.

The composition was applied to one surface of copper foil having a thickness of <NUM>, followed by drying, to form an active material layer having a thickness of <NUM> on one surface of copper foil, and then punched into a circular shape having a diameter of 14Φ to obtain a test electrode. Lithium foil having a thickness of <NUM> was used as a counter electrode.

A porous polyethylene sheet having a thickness of <NUM> was used as a separator. As an electrolyte, LiPF<NUM> as a lithium salt dissolved in a mixed solvent of ethylene carbonate (EC) with diethyl carbonate (DEC) (volume ratio <NUM>:<NUM>) to a concentration of about <NUM> mol/L was used. The above-mentioned elements were received in a casing made of stainless steel to obtain a coin cell with a conventional shape for evaluation having a thickness of <NUM> and a diameter of <NUM> (so called a <NUM> type coin cell).

Each of the samples obtained from Examples <NUM> and <NUM> and Comparative Examples <NUM>-<NUM> was analyzed by X-ray diffractometry. The results are shown in the following Table <NUM>.

The size of MgSiO<NUM> crystal grains may be determined according to the Scherrer equation (see, the following Mathematical Formula <NUM>) based on the full width at half maximum (FWHM) of the diffraction peak of the MgSiO<NUM> (<NUM>) in X-ray diffractometry, and is based on a silicon-silicon oxide-magnesium silicate composite obtained at a precipitation temperature of <NUM> during the preparation process. <MAT> wherein K = <NUM>, λ = <NUM>, B = full width at half maximum (FWHM, rad), and θ = peak position (angle).

In addition, the size of Mg<NUM>SiO<NUM> crystal grains may be determined according to the Scherrer equation (see, the following Mathematical Formula <NUM>) based on the full width at half maximum (FWHM) of the diffraction peak of the Mg<NUM>SiO<NUM> (<NUM>) in X-ray diffractometry, and is based on a silicon-silicon oxide-magnesium silicate composite obtained at a precipitation temperature of <NUM> during the preparation process. <MAT> wherein K = <NUM>, λ = <NUM>, B = full width at half maximum (FWHM, rad), and θ = peak position (angle).

The size of Si crystal grains may be determined according to the Scherrer equation (see, the following Mathematical Formula <NUM>) based on the full width at half maximum (FWHM) of the diffraction peak of Si (<NUM>) in X-ray diffractometry, and is based on a silicon-silicon oxide-magnesium silicate composite obtained at a precipitation temperature of <NUM> during the preparation process. <MAT> wherein K = <NUM>, λ= <NUM>, B = full width at half maximum (FWHM, rad), and θ = peak position (angle).

The content of each of Mg, MgSiO<NUM> and Mg<NUM>SiO<NUM> was determined through inductively coupled plasma (ICP).

The coin cell obtained by using each of the powder of silicon-silicon oxide-magnesium silicate composite particles obtained from Examples <NUM> and <NUM> and Comparative Examples <NUM>-<NUM> was charged to a voltage of <NUM>. 01V at a constant current of <NUM>. 05C and discharged to a voltage of <NUM>. 5V at a constant current of <NUM>. 05C to evaluate discharge capacity and initial efficiency.

In addition, the coin cell obtained according to the above Manufacture Example by using each sample was charged to a voltage of <NUM>. 01V at a constant current of <NUM>. 2C and discharged to a voltage of <NUM>. 5V at a constant current of <NUM>. 2C to evaluate cycle characteristics for <NUM> cycles after the first charge/discharge.

The discharge capacity, initial efficiency and cycle characteristics were obtained as follows. The results are shown in the following Table <NUM>.

As shown in Table <NUM>, the case of the coin cell batteries using the silicon-silicon oxide-magnesium silicate composite particles according to Examples <NUM> and <NUM> of the present disclosure as negative electrode active materials provide improved initial charge/discharge efficiency and excellent capacity and cycle characteristics, as compared to the coin cell batteries using the silicon-silicon oxide-magnesium silicate composite particles according to Comparative Examples <NUM>-<NUM>.

Claim 1:
A negative electrode active material for a lithium secondary battery which comprises:
a silicon-silicon oxide-magnesium silicate composite comprising:
a silicon oxide (SiOx, <NUM> < x ≤ <NUM>) matrix; and
silicon (Si) crystal grains, MgSiO<NUM> crystal grains and Mg<NUM>SiO<NUM> crystal grains present in the silicon oxide matrix;
wherein the MgSiO<NUM> crystal grains have a crystal size of <NUM>-<NUM> and the Mg<NUM>SiO<NUM> crystal grains have a crystal size of <NUM>-<NUM> in the silicon-silicon oxide-magnesium silicate composite, the crystal size being determined as described in the specification, and
the content ratio of MgSiO<NUM> crystal grains to Mg<NUM>SiO<NUM> crystal grains is <NUM>:<NUM>-<NUM>:<NUM> on the weight basis.