CN118318324A

CN118318324A - Negative electrode active material, negative electrode including the same, secondary battery including the same, and method of preparing the negative electrode active material

Info

Publication number: CN118318324A
Application number: CN202380015035.XA
Authority: CN
Inventors: 崔静贤; 朴世美; 朴熙娟; 申善英; 吴一根; 李龙珠
Original assignee: Lg Energy Solution
Current assignee: Lg Energy Solution
Priority date: 2022-09-07
Filing date: 2023-09-06
Publication date: 2024-07-09

Abstract

The present invention relates to a negative electrode active material containing silicon-based particles containing SiO _x (0 < x < 2) and a Li compound, wherein the Li compound contains at least one crystalline lithium silicate selected from the group consisting of crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, and crystalline Li ₂Si₂O₅, the content of the crystalline Li ₂SiO₃ is higher than the sum of the content of the crystalline Li ₂Si₂O₅ and the content of the crystalline Li ₄SiO₄ based on 100 parts by weight of the total crystalline lithium silicate, the content of the crystalline Li ₂SiO₃ is 60 parts by weight or more based on 100 parts by weight of the total crystalline lithium silicate, and the total content of crystal phases present in the silicon-based particles is higher than the total content of amorphous phases.

Description

Negative electrode active material, negative electrode including the same, secondary battery including the same, and method of preparing the negative electrode active material

Technical Field

The present application claims priority and rights of korean patent application No. 10-2022-0110245 filed on 7 th 9 of 2022 and korean patent application No. 10-2023-0117434 filed on 5 th 9 of 2023 to the korean intellectual property office.

The present invention relates to a negative electrode active material, a negative electrode including the same, a secondary battery including the same, and a method of preparing the negative electrode active material.

Background

Recently, with the rapid spread of electronic devices such as mobile phones, notebook computers, and electric vehicles using batteries, the demand for small and lightweight secondary batteries having a relatively high capacity has rapidly increased. In particular, lithium secondary batteries are lightweight and have high energy density, and thus are attracting attention as driving power sources for mobile devices. Accordingly, research and development efforts for improving the performance of lithium secondary batteries have been actively conducted.

In general, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, an organic solvent, and the like. Further, for the positive electrode and the negative electrode, active material layers each including a positive electrode active material and a negative electrode active material may be formed on the current collector. In general, lithium-containing metal oxides such as LiCoO ₂ and LiMn ₂O₄ have been used as positive electrode active materials for positive electrodes, and a carbon-based active material or a silicon-based active material that does not contain lithium has been used as a negative electrode active material for negative electrodes.

Among the negative electrode active materials, silicon-based active materials are attracting attention because they have high capacity and excellent high-rate charging characteristics as compared with carbon-based active materials. However, the silicon-based active material has disadvantages in that initial efficiency is low because the degree of volume expansion/contraction due to charge/discharge is large and irreversible capacity is large.

On the other hand, among silicon-based active materials, silicon-based oxides, specifically, silicon-based oxides represented by SiO _x (0 < x < 2), have an advantage that the degree of volume expansion/contraction due to charge/discharge is low compared to other silicon-based active materials such as silicon (Si). However, the silicon-based oxide still has a disadvantage in that the initial efficiency is lowered according to the presence of the irreversible capacity.

In this regard, studies have been continuously conducted to reduce the irreversible capacity and improve the initial efficiency by doping or embedding metals such as Li, al and Mg in silicon-based oxides. However, in the case of an anode slurry containing a metal-doped silicon-based oxide as an anode active material, there is a problem in that a metal oxide formed by doping a metal reacts with moisture, the pH of the anode slurry is increased and the viscosity thereof is changed, and thus there is a problem in that the state of the prepared anode is deteriorated and the charge/discharge efficiency of the anode is lowered.

Accordingly, there is a need to develop an anode active material capable of improving the phase stability of an anode slurry containing a silicon-based oxide and improving the charge/discharge efficiency of an anode prepared therefrom.

Korean patent No. 10-0794192 relates to a method for preparing a carbon-coated silicon-graphite composite anode material for a lithium secondary battery and a method for preparing a secondary battery including the same, but has limitations in solving the above-described problems.

[ Related art literature ]

[ Patent literature ]

(Patent document 1) Korean patent No. 10-0794192

Disclosure of Invention

Technical problem

The present invention is directed to providing a negative electrode active material, a negative electrode including the same, a secondary battery including the same, and a method of preparing the negative electrode active material.

Technical proposal

An exemplary embodiment of the present invention provides a negative electrode active material containing silicon-based particles including SiO _x (0 < x < 2) and a Li compound, wherein the Li compound includes at least one crystalline lithium silicate selected from the group consisting of crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, and crystalline Li ₂Si₂O₅, the content of the crystalline Li ₂SiO₃ is higher than the sum of the content of the crystalline Li ₂Si₂O₅ and the content of the crystalline Li ₄SiO₄, the content of the crystalline Li ₂SiO₃ is 60 parts by weight or more based on 100 parts by weight of the total of the crystalline lithium silicate, and the content of crystalline phases present in the silicon-based particles is higher than the total content of amorphous phases.

Another exemplary embodiment of the present invention provides a method of preparing a negative active material according to one exemplary embodiment of the present invention, the method including:

forming particles comprising a silicon-based oxide represented by SiO _x (0 < x < 2); and

Particles comprising the silicon-based oxide are mixed with a lithium precursor, and the resulting mixture is then heat treated.

Still another exemplary embodiment of the present invention provides a negative electrode including the negative electrode active material according to one exemplary embodiment of the present invention.

Still another exemplary embodiment of the present invention provides a secondary battery including the negative electrode according to one exemplary embodiment of the present invention.

Technical effects

Since the content of crystalline Li ₂SiO₃ is dominant in the crystalline lithium silicate, the discharge capacity loss per unit weight depending on the increase in the content of Li in the anode active material of the present invention is small, and SiO ₂, which serves as an irreversible capacity during Li doping, and Li react in a large amount to form a lithium silicate structure of electrochemically stable Li ₂SiO₃, so that the anode active material of the present invention can significantly increase the initial efficiency. In addition, since crystalline Li ₂SiO₃ is structurally stable during charge and discharge, it has excellent life characteristics.

With the anode active material of the present invention, the total content of the crystalline phase is higher than the total content of the amorphous phase, and therefore, the content of lithium oxide and lithium silicate that react with moisture is low, so that gas generation and viscosity change of the anode slurry can be prevented, and the phase stability of the slurry containing the anode active material is improved. Therefore, there is an effect of improving the quality of the anode including the anode active material and the secondary battery including the anode and improving the charge/discharge capacity, initial efficiency, and/or service life characteristics thereof.

Detailed Description

Hereinafter, the present specification will be described in more detail.

In this specification, when a part "includes" one constituent element, unless specifically described otherwise, this is not meant to exclude another constituent element, but means that another constituent element may be further included.

In this specification, when one component is arranged "on" another component, this includes not only the case where the one component is in contact with the other component but also the case where the other component exists between the two components.

The terms and words used in the present specification should not be construed as limited to typical or dictionary meanings, but should be construed based on the principle that the inventor can properly define the concept of terms in order to describe his/her own invention in the best manner to correspond to the meaning and concept of the technical idea of the present invention.

As used in this specification, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise.

In this specification, crystallinity of a structure included in the anode active material can be confirmed by X-ray diffraction analysis which can be performed using an X-ray diffraction (XRD) analyzer (trade name: D4-endavor, manufacturer: bruker), and an apparatus used in the art may be appropriately employed in addition to the apparatus.

In this specification, the presence or absence of an element in the anode active material and the content thereof can be confirmed by ICP analysis, and the ICP analysis can be performed using an inductively coupled plasma atomic emission spectrometer (ICPAES, perkin Elmer 7300).

In the present specification, the average particle diameter (D ₅₀) may be defined as a particle diameter of 50% of the cumulative volume in the particle size distribution curve (curve of the particle size distribution diagram) of the particles. The average particle diameter (D ₅₀) can be measured using, for example, a laser diffraction method. Laser diffraction methods can generally measure particle diameters from the submicron region to about several millimeters, and can obtain results with high reproducibility and high resolution.

Hereinafter, preferred exemplary embodiments of the present invention will be described in detail. However, the exemplary embodiments of the present invention can be modified in various different forms, and the scope of the present invention is not limited to the exemplary embodiments to be described below.

< Cathode active Material >)

Hereinafter, the anode active material will be described in detail.

The present invention relates to a negative electrode active material, and more particularly, to a negative electrode active material for a lithium secondary battery.

In the related art, in the anode active material including the silicon-based oxide, research has been conducted to remove the irreversible capacity of the silicon-based oxide or increase the initial efficiency by doping or distributing lithium or the like into the anode active material. However, since the content of an amorphous phase is high in such a negative electrode active material, there are problems in that during preparation of a negative electrode slurry, particularly an aqueous negative electrode slurry, the reaction of water with lithium oxide and/or lithium silicate increases gas generation, increases pH of the negative electrode slurry, and decreases phase stability, and thus there are problems in that quality of the prepared negative electrode is poor and charge/discharge efficiency is lowered.

In order to solve this problem, the anode active material according to one exemplary embodiment of the present invention is characterized in that the silicon-based particles including SiO _x (0 < x < 2) and a Li compound include a Li compound in the form of at least one crystalline lithium silicate selected from the group consisting of crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, and crystalline Li ₂Si₂O₅ or in the form of an amorphous lithium silicate, the content of crystalline Li ₂SiO₃ is higher than the sum of the content of crystalline Li ₂Si₂O₅ and the content of crystalline Li ₄SiO₄, and the total content of crystalline phases present in the silicon-based particles is higher than the total content of amorphous phases.

According to the anode active material of the present invention, since the content of crystalline Li ₂SiO₃ is dominant in the crystalline lithium silicate, the loss of discharge capacity per unit weight depending on the increase of the content of Li in the anode active material is small, and SiO ₂ and Li, which serve as irreversible capacities during Li doping, react in a large amount to form a lithium silicate structure of electrochemically stable Li ₂SiO₃, so that the anode active material of the present invention can significantly increase initial efficiency. In addition, since crystalline Li ₂SiO₃ is structurally stable during charge and discharge, it has excellent life characteristics.

According to the anode active material of the present invention, the total content of the crystalline phase is higher than the total content of the amorphous phase, and therefore, the content of lithium oxide and lithium silicate reacting with moisture is low, so that gas generation and viscosity change of the anode slurry can be prevented, and the phase stability of the slurry containing the anode active material is improved. Therefore, there is an effect of improving the quality of the anode including the anode active material and the secondary battery including the anode and improving the charge/discharge capacity, initial efficiency, and/or service life characteristics thereof.

The anode active material according to an exemplary embodiment of the present invention includes silicon-based particles. The silicon-based particles contain SiO _x (0 < x < 2) and a Li compound.

SiO _x (0 < x < 2) may correspond to the matrix in the silicon-based particles. SiO _x (0 < x < 2) may be in a form comprising Si and/or SiO ₂, and Si may also form a phase. For example, siO _x (0 < x < 2) may be a composite material comprising amorphous SiO ₂ and Si crystals. That is, x corresponds to the number ratio of O contained in SiO _x (0 < x < 2) to Si. When the silicon-based particles contain SiO _x (0 < x < 2), the discharge capacity of the secondary battery can be improved. Specifically, in terms of structural stability of the active material, siO _x (0 < x < 2) may be a compound represented by SiO _x (0.5+.x+.1.5).

The Li compound may correspond to a matrix in the silicon-based composite particles. The Li compound may be present in the silicon-based particles in the form of at least one of a lithium atom, a lithium silicate, a lithium silicide, and a lithium oxide. When the silicon-based particles contain Li compounds, there is an effect of improving the initial efficiency.

The Li compound is in a form in which the silicon-based particles are doped with the compound, and may be distributed on the surface and/or inside the silicon-based particles. The Li compound is distributed on the surface and/or inside of the silicon-based particles, and thus the volume expansion/contraction of the silicon-based particles can be controlled to an appropriate level, and can be used to prevent damage to the active material. Further, in terms of reducing the ratio of the irreversible phase (e.g., siO ₂) of the silicon-based oxide particles to increase the efficiency of the active material, a Li compound may be contained.

In particular, lithium may be distributed on, in, or on and in the surface of the silicon-based particles. In addition, the silicon-based particles may be doped with lithium.

In one exemplary embodiment of the present invention, the content of Li element may be more than 7 parts by weight and 10 parts by weight or less based on 100 parts by weight total of the anode active material.

Preferably, the content of the Li element may be 7.2 parts by weight or more and 9.8 parts by weight or less, 7.5 parts by weight or more and 9.7 parts by weight or less, 8 parts by weight or more and 9.5 parts by weight or less, 8.2 parts by weight or more and 9.5 parts by weight or less, or 8.5 parts by weight or more and 9.2 parts by weight or less, based on 100 parts by weight of the total negative electrode active material. The lithium content is preferably within the above range because the characteristics of the initial efficiency and the charge/discharge efficiency of the anode active material can be improved. In contrast, when the content of Li is less than the above content, there is a problem in that the content of Li in the silicon-based particles is insufficient, resulting in significant deterioration of discharge capacity and efficiency, and when the content of Li exceeds the above content, there is a problem in that the silicon-based particles and unreacted Li remain as byproducts, and thus the aqueous slurry becomes strongly alkaline, which may adversely affect the slurry processability.

The upper limit of the Li content may be 10 parts by weight, 9.8 parts by weight, 9.7 parts by weight, 9.6 parts by weight, 9.5 parts by weight, 9.4 parts by weight, 9.3 parts by weight, 9.2 parts by weight, 9.1 parts by weight, or 9 parts by weight, and the lower limit thereof may be 7.2 parts by weight, 7.5 parts by weight, 7.8 parts by weight, 8 parts by weight, 8.2 parts by weight, or 8.5 parts by weight, based on 100 parts by weight of the total negative electrode active material.

The content of Li element can be confirmed by ICP analysis. Specifically, after a predetermined amount (about 0.01 g) of the anode active material was aliquoted, the anode active material was completely decomposed on a hot plate by transferring the aliquoted sample to a platinum crucible and adding nitric acid, hydrofluoric acid or sulfuric acid thereto. Thereafter, a reference calibration curve was prepared by measuring the intensity of a standard solution prepared using a standard solution (5 mg/kg) at the inherent wavelength of the element to be analyzed using an inductively coupled plasma atomic emission spectrometer (ICPAES, perkin-Elmer 7300). Thereafter, the pretreated sample solution and the blank sample are each introduced into the apparatus, the actual strength is calculated by measuring each strength, the concentration of each component is calculated with respect to the prepared calibration curve, and then the elemental content of the prepared anode active material can be analyzed by converting the sum into a theoretical value.

In one exemplary embodiment of the present invention, the Li compound may be distributed in the form of lithium silicate in the silicon-based particles, and thus the Li compound may be used to improve the initial efficiency and charge/discharge efficiency of the anode active material by removing the irreversible capacity of the particles. In this case, silicate means a compound containing silicon, oxygen and one or more metals.

In particular, the Li compound may be distributed on, inside, or both on and inside the surface of the silicon-based particles in the form of lithium silicate, and the lithium silicate may correspond to the matrix in the particles. Lithium silicate is represented by Li _aSi_bO_c (2.ltoreq.a.ltoreq.4, 0.ltoreq.b.ltoreq.2, 2.ltoreq.c.ltoreq.5), and can be classified into crystalline lithium silicate and amorphous lithium silicate. The crystalline lithium silicate may be present in the silicon-based particles in the form of at least one lithium silicate selected from the group consisting of Li ₂SiO₃、Li₄SiO₄ and Li ₂Si₂O₅, and the amorphous lithium silicate may be composed of a complex structure in the form of Li _aSi_bO_c (2.ltoreq.a.ltoreq.4, 0.ltoreq.b.ltoreq.2, 2.ltoreq.c.ltoreq.5), and the lithium silicate is not limited to the form.

In one exemplary embodiment of the present invention, the Li compound may include at least one crystalline lithium silicate selected from the group consisting of crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, and crystalline Li ₂Si₂O₅. In addition, the Li compound may further contain amorphous lithium silicate.

Specifically, the Li compound contained in the silicon-based particles contains crystalline Li ₂SiO₃ and crystalline Li ₂Si₂O₅, and optionally, may further contain crystalline Li ₄SiO₄ or amorphous lithium silicate.

When the anode active material contains a large amount of crystalline Li ₂SiO₃, the loss of discharge capacity per unit weight of the active material can be reduced, and crystalline Li ₂SiO₃ has an electrochemically stable structure, and can significantly increase initial efficiency, and since the structure is stable even during charge and discharge, service life characteristics can be effectively improved.

Crystalline Li ₂Si₂O₅ is stable in the anode active material and has particularly few side reactions with moisture in the anode slurry, particularly in aqueous anode slurry. However, when the anode active material contains a large amount of crystalline Li ₂Si₂O₅, there are problems in that the loss of discharge capacity per unit weight of the active material is large, and crystalline Li ₂Si₂O₅ has an unstable structure during charge and discharge, resulting in deterioration of service life characteristics.

In the case of crystalline Li ₄SiO₄, there is a problem in that side reactions with moisture in the anode slurry occur, which makes gas generation serious, and there may occur a problem in that by-products formed by side reactions with moisture, such as Li ₂ O, increase the pH of the anode slurry, destabilize the phase of the slurry, and change the viscosity.

In this regard, since the content of crystalline Li ₂SiO₃ is higher than the content of crystalline Li ₂Si₂O₅ and the content of crystalline Li ₄SiO₄ in the anode active material of the present invention, by smoothly removing the irreversible capacity of the anode active material, the initial efficiency and the charge/discharge efficiency can be improved, and by improving the phase stability of the anode slurry containing the anode active material and preventing the problem of viscosity reduction, the quality of the anode can be improved, the charge/discharge capacity can be made to exhibit an excellent level, and the charge/discharge efficiency can be improved. Further, as described below, since the anode active material of the present invention reduces the total content of amorphous phase while increasing the content of crystalline Li ₂SiO₃, it is possible to improve the phase stability of the anode slurry described above, prevent anode malfunction, and significantly improve charge/discharge capacity and efficiency.

The anode active material of the present invention may contain Li element in an amount of more than 7 parts by weight and 10 parts by weight or less, and when the content of crystalline Li ₂SiO₃ is higher than the content of crystalline Li ₂Si₂O₅ in this range, the anode active material has a small discharge capacity loss per unit weight (depending on the increase in the Li content) and has an electrochemically stable lithium silicate structure, so that it has an effect that a high discharge capacity can be achieved when the same initial efficiency is achieved, and is excellent in service life characteristics, because of a stable structure even during charge and discharge. In addition, as described below, by reducing the total content of the amorphous phase in the anode active material, there is an effect that side reactions with moisture can be prevented.

In contrast, when the content of crystalline Li ₂Si₂O₅ is higher than that of crystalline Li ₂SiO₃, side reactions with moisture may occur less, but there may occur problems in that the discharge capacity loss per unit weight depending on the increase in the Li content in the anode active material is large, the discharge capacity is low due to an unstable structure during charge and discharge, and the service life characteristics are deteriorated when the same initial efficiency is achieved.

In one exemplary embodiment of the present invention, the content of crystalline Li ₂SiO₃ is higher than the sum of the content of crystalline Li ₂Si₂O₅ and the content of crystalline Li ₄SiO₄. In another exemplary embodiment, the content of crystalline Li ₂SiO₃ may be higher than the content of crystalline Li ₂Si₂O₅.

In one exemplary embodiment of the present invention, the content of crystalline Li ₂SiO₃ is 60 parts by weight or more based on 100 parts by weight of the total of the crystalline lithium silicate. In another exemplary embodiment, the content of the crystalline Li ₂SiO₃ may be 65 parts by weight or more, 70 parts by weight or more, 74 parts by weight or more.

In one exemplary embodiment of the present invention, the content of the crystalline Li ₂SiO₃ may be 15 parts by weight or more and 50 parts by weight or less based on 100 parts by weight of the total silicon-based particles. In another exemplary embodiment, the content of the crystalline Li ₂SiO₃ may be 17 parts by weight or more and 48 parts by weight or less, 20 parts by weight or more and 45 parts by weight or less, 25 parts by weight or more and 45 parts by weight or less, 30 parts by weight or more and 45 parts by weight or less, or 35 parts by weight or more and 42 parts by weight or less. When the content of crystalline Li ₂SiO₃ satisfies the above range, it is preferable in terms of realization of capacity/efficiency and excellent service life characteristics, because it is advantageous to realize capacity per gram, and Li ₂SiO₃ is stable during charge and discharge. When the content exceeds the above range, there are problems in that the processability of the aqueous slurry is adversely affected due to the high reactivity of Li ₂SiO₃ with water, and when the content of crystalline Li ₂SiO₃ is below the above range, there are problems in that the realization of capacity/efficiency and the service life characteristics are deteriorated because it is unfavorable for realizing capacity per gram, and Li ₂SiO₃ stable during charge and discharge is insufficient.

The upper limit of the content of crystalline Li ₂SiO₃ may be 50 parts by weight, 48 parts by weight, 45 parts by weight, 42 parts by weight, or 40 parts by weight, and the lower limit thereof may be 15 parts by weight, 17 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 33 parts by weight, 35 parts by weight, or 38 parts by weight, based on 100 parts by weight of the total silicon-based particles.

In one exemplary embodiment of the present invention, the content of the crystalline Li ₂Si₂O₅ may be 0.5 parts by weight or more and 20 parts by weight or less based on 100 parts by weight of the total silicon-based particles. In another exemplary embodiment, the content of the crystalline Li ₂Si₂O₅ may be 1 part by weight or more and 18 parts by weight or less, 3 parts by weight or more and 15 parts by weight or less, or 3 parts by weight or more and 12 parts by weight or less.

The upper limit of the content of crystalline Li ₂Si₂O₅ may be 20 parts by weight, 18 parts by weight, 15 parts by weight, or 12 parts by weight, and the lower limit thereof may be 0.1 part by weight, 0.5 parts by weight, 1 part by weight, 3 parts by weight, or 5 parts by weight, based on 100 parts by weight of the total silicon-based particles.

When the content of crystalline Li ₂Si₂O₅ exceeds the above range, there is a problem in that since the capacity loss per unit weight excessively increases and the structure of Li ₂Si₂O₅ becomes unstable, the service life characteristics deteriorate. When the content of crystalline Li ₂Si₂O₅ is less than the above range, the amount of Li ₂Si₂O₅ having low reactivity with water may be too small, resulting in deterioration of the processability of the aqueous slurry. Therefore, when the content of crystalline Li ₂Si₂O₅ satisfies the above range, it is preferable to achieve proper charge capacity, initial efficiency, and service life characteristics during battery driving because the capacity loss per unit weight can be minimized, service life characteristics are not excessively deteriorated, and processability of the aqueous slurry is also properly ensured.

In one exemplary embodiment of the present invention, the content of the crystalline Li ₄SiO₄ may be 5 parts by weight or less, specifically 3 parts by weight or less, 1 part by weight or less, or 0.1 part by weight or less, based on 100 parts by weight of the total of the silicon-based particles, and more specifically, the crystalline Li ₄SiO₄ may not be present in the anode active material. When the content of crystalline Li ₄SiO₄ satisfies the above range, it is preferable in terms of the following facts: during the preparation of the anode slurry, particularly, the aqueous anode slurry, the production of byproducts such as Li ₂ O, which are caused by the reaction of the anode active material and moisture, the increase in the pH of the anode slurry, which is caused by the production of the byproducts, and the deterioration of the anode quality are prevented.

In one exemplary embodiment of the present invention, the difference between the content of crystalline Li ₂SiO₃ and the content of crystalline Li ₂Si₂O₅ may be 1 to 50 parts by weight based on 100 parts by weight of the total silicon-based particles. Specifically, the difference between the content of crystalline Li ₂Si₂O₅ and the content of crystalline Li ₂SiO₃ may be 5 to 45 parts by weight, 8 to 45 parts by weight, 10 to 40 parts by weight, 20 to 40 parts by weight, or 20 to 35 parts by weight.

The upper limit of the difference between the content of crystalline Li ₂SiO₃ and the content of crystalline Li ₂Si₂O₅ may be 50 parts by weight, 45 parts by weight, 40 parts by weight, or 38 parts by weight, and the lower limit thereof may be 1 part by weight, 5 parts by weight, 8 parts by weight, 10 parts by weight, 15 parts by weight, or 20 parts by weight, based on 100 parts by weight of the total of the silicon-based particles.

When the difference between the content of crystalline Li ₂Si₂O₅ and the content of crystalline Li ₂SiO₃ satisfies the above range, the phase stability of the above-described anode slurry can be improved, anode malfunction is prevented, and charge/discharge capacity and efficiency are significantly improved.

The confirmation and content measurement of the crystalline lithium silicate of crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, or crystalline Li ₂Si₂O₅ can be performed by analysis by means of an X-ray diffraction distribution by X-ray diffraction analysis or ²⁹ Si-magic angle spinning-nuclear magnetic resonance (²⁹ Si-MAS-NMR).

Here ²⁹ Si-MAS-NMR analysis is a solid phase NMR technique and is an NMR analysis performed by rapidly rotating the rotor containing the sample at a magic angle B _M (e.g., 54.74 setting) with respect to the magnetic field B ₀. Thus, the presence or absence, the content, and the like of the crystalline Li ₂SiO₃, the crystalline Li ₄SiO₄, the crystalline Li ₂Si₂O₅, the crystalline Si, and the crystalline SiO ₂ contained in the anode active material of the present invention can be measured.

In one exemplary embodiment of the present invention, during ²⁹ Si-MAS-NMR analysis of the anode active material, the height of the peak p1 of Li ₂SiO₃ occurring at the chemical shift peak of-70 ppm to-80 ppm may be greater than the height of the peak p2 of Li ₂Si₂O₅ occurring at the chemical shift peak of-90 ppm to-100 ppm.

In one exemplary embodiment of the present invention, during ²⁹ Si-MAS-NMR analysis of the anode active material, the ratio p2/p1 of the height of the peak p2 of Li ₂Si₂O₅ occurring at the chemical shift peak of-90 ppm to-100 ppm to the height of the peak p1 of Li ₂SiO₃ occurring at the chemical shift peak of-70 ppm to-80 ppm may be 1 or less. Specifically, the ratio may be 0.01 to 0.8, 0.05 to 0.7, 0.1 to 0.6. When the ratio satisfies the above range, crystalline Li ₂SiO₃ is sufficiently present in the anode active material, so that the phase stability of the above anode slurry can be improved, anode malfunction is prevented, and charge/discharge capacity and efficiency are significantly improved.

The upper limit of p2/p1 may be 1, 0.8, 0.7, 0.6, 0.5 or 0.4, and the lower limit thereof may be 0.01, 0.05, 0.1, 0.15 or 0.2.

In one exemplary embodiment of the present invention, during ²⁹ Si-MAS-NMR analysis of the anode active material, there may be no peak p3 of Li ₄SiO₄ present at the chemical shift peak of-60 ppm to-69 ppm. In this case, it is preferable in terms of the following facts: the production of by-products such as Li ₂ O caused by the side reaction of moisture with Li ₄SiO₄ in the anode active material, the increase in the pH of the anode slurry caused by the production of by-products, and the deterioration of the anode quality are prevented.

The contents of crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, and crystalline Li ₂Si₂O₅ may be achieved by performing a heat treatment process, adjusting a heat treatment temperature, performing an acid treatment process, and the like in a method for preparing a negative electrode active material, which will be described later, but are not limited thereto.

In one exemplary embodiment of the present invention, the anode active material may include crystalline SiO ₂ in an amount of less than 5 parts by weight, specifically less than 4 parts by weight, and in still another exemplary embodiment, 3 parts by weight or less, based on a total of 100 parts by weight of silicon-based particles. Preferably, the anode active material contains crystalline SiO ₂ in an amount of 1 part by weight or less based on 100 parts by weight of the total silicon-based particles, but may not contain crystalline SiO ₂ at all. When the content of crystalline SiO ₂ satisfies the above range, the anode is easily charged and discharged, so that the charge/discharge capacity and efficiency can be excellently improved.

In one exemplary embodiment of the present invention, the anode active material may include crystalline Si in an amount of 10 to 50 parts by weight, 15 to 40 parts by weight, 15 to 30 parts by weight, 20 to 30 parts by weight, or 20 to 25 parts by weight, based on a total of 100 parts by weight of the silicon-based particles. When the content of crystalline Si satisfies the above range, the anode is easily charged and discharged, so that the charge/discharge capacity and efficiency can be excellently improved.

In one exemplary embodiment of the present invention, the total content of crystalline phases present in the silicon-based particles is higher than the total content of amorphous phases. The total content of the crystalline phases means the total content of all the crystalline phases including crystalline Si, crystalline SiO ₂, crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, crystalline Li ₂Si₂O₅, and the like present in the silicon-based particles, and the total content of the amorphous phase may mean a content other than the total content of the crystalline phases present in the silicon-based particles. That is, the total content of amorphous phase includes amorphous SiO ₂ and the like in addition to amorphous lithium silicate, and means the sum of the contents of all amorphous phases present in the particles.

Since the total content of crystalline phases present in the silicon-based particles is higher than the total content of amorphous phases in the anode active material of the present invention, the content of amorphous lithium silicate or the like highly reactive with moisture is reduced during the preparation of the anode slurry, particularly the aqueous anode slurry, and is thus preferable in terms of the fact that: the production of by-products such as Li ₂ O caused by side reactions with moisture, the increase in pH of the anode slurry caused by the production of by-products, and the deterioration of the anode quality are prevented.

In one exemplary embodiment of the present invention, the total content of the crystalline phases present in the silicon-based particles may be more than 50 parts by weight and 85 parts by weight or less based on 100 parts by weight of the total silicon-based particles. Specifically, the total content of the crystal phase present in the silicon-based particles may be 55 parts by weight or more and 85 parts by weight or less, 55 parts by weight or more and 80 parts by weight or less, 55 parts by weight or more and 75 parts by weight or less, 60 parts by weight or more and 70 parts by weight or less, 62 parts by weight or more and 68 parts by weight or less, 64 parts by weight or more and 68 parts by weight or less or 64 parts by weight or more and 66 parts by weight or less.

The upper limit of the total content of the crystalline phase may be 85 parts by weight, 83 parts by weight, 80 parts by weight, 75 parts by weight, 70 parts by weight, 68 parts by weight, or 66 parts by weight, and the lower limit thereof may be 55 parts by weight, 60 parts by weight, 62 parts by weight, or 64 parts by weight, based on 100 parts by weight of the total silicon-based particles.

In one exemplary embodiment of the present invention, the total content of the amorphous phase present in the silicon-based particles may be 15 parts by weight or more and less than 50 parts by weight based on 100 parts by weight of the total silicon-based particles. Specifically, the total content of the amorphous phase present in the silicon-based particles may be 20 parts by weight or more and less than 50 parts by weight, 25 parts by weight or more and 45 parts by weight or less, 30 parts by weight or more and 40 parts by weight or less, 32 parts by weight or more and 36 parts by weight or less.

The upper limit of the total content of the amorphous phase may be 48 parts by weight, 45 parts by weight, 40 parts by weight, 38 parts by weight, or 36 parts by weight, and the lower limit thereof may be 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 32 parts by weight, or 34 parts by weight, based on 100 parts by weight of the total silicon-based particles.

In one exemplary embodiment of the present invention, the difference between the total content of the crystalline phase present in the silicon-based particles and the total content of the amorphous phase present in the silicon-based particles may be 10 parts by weight or more and 70 parts by weight or less, 20 parts by weight or more and 68 parts by weight or less, 20 parts by weight or more and 50 parts by weight or less, 25 parts by weight or more and 40 parts by weight or less, 28 parts by weight or more and 36 parts by weight or less, based on 100 parts by weight of the total silicon-based particles.

The upper limit of the difference between the total content of the crystalline phase and the total content of the amorphous phase present in the silicon-based particles may be 70 parts by weight, 68 parts by weight, 66 parts by weight, 60 parts by weight, 50 parts by weight, 40 parts by weight, or 36 parts by weight, and the lower limit thereof may be 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 28 parts by weight, or 30 parts by weight, based on 100 parts by weight of the total silicon-based particles.

In one exemplary embodiment of the present invention, the ratio of the total weight of the crystalline phase present in the silicon-based particles to the total weight of the amorphous phase (total weight of crystalline phase: total weight of amorphous phase) may be 55:45 to 85:15, or 60:40 to 80:20, or 60:40 to 75:25.

When the relation between the content of the crystalline phase and the content of the amorphous phase present in the silicon-based particles satisfies the above-described range, the content of the crystalline phase and the amorphous phase present in the anode active material is appropriately adjusted so that the content of amorphous lithium silicate or the like highly reactive with moisture is reduced during the preparation of the anode slurry (particularly, aqueous anode slurry), whereby the production of by-products such as Li ₂ O caused by side reactions with moisture, the increase in the pH of the anode slurry caused by the production of by-products, and the change in the viscosity can be prevented, and it is preferable in terms of the fact that the content of crystalline SiO ₂ that hinders the expression of charge/discharge capacity and efficiency does not excessively increase.

Although the content of crystalline Li ₂SiO₃ is highest among lithium silicates, when the total content of crystalline phases present in the anode active material does not satisfy the above-described range, the crystalline phases are excessively contained in the anode active material, so that there is a problem in that it is difficult to achieve capacity/efficiency and service life characteristics are also deteriorated because the battery is not easily charged and discharged, and when the content of crystalline Li ₂SiO₃ is less than the above-described range, there is a problem in that it is difficult to effectively control side reactions with moisture because the content of crystalline phases is insufficient.

The total content of the crystalline phase and the amorphous phase present in the silicon-based particles can be measured by a quantitative analysis method using X-ray diffraction analysis (XRD).

In one exemplary embodiment of the present invention, the silicon-based particles may contain additional metal atoms. The metal atoms may be present in the silicon-based particles in the form of at least one of metal atoms, metal silicates, metal silicides, and metal oxides. The metal atom may contain at least one selected from the group consisting of Mg, li, al, and Ca. Accordingly, the initial efficiency of the anode active material can be improved.

In one exemplary embodiment of the present invention, the silicon-based particles have a carbon layer disposed on at least a portion of a surface thereof. In this case, the carbon layer may be in the form of coating on at least a part of the surface, i.e., may be partially coated on the particle surface, or may be coated on the entire particle surface. The negative electrode active material is given conductivity through the carbon layer, and the initial efficiency, service life characteristics, and battery capacity characteristics of the secondary battery can be improved.

In one exemplary embodiment of the present invention, the carbon layer comprises amorphous carbon. Furthermore, the carbon layer may additionally contain crystalline carbon.

The crystalline carbon may further improve the conductivity of the anode active material. The crystalline carbon may contain at least one selected from the group consisting of fullerenes, carbon nanotubes, and graphene.

The amorphous carbon can suppress expansion of the silicon-based particles by appropriately maintaining the strength of the carbon layer. The amorphous carbon may be a carbide of at least one selected from the group consisting of tar, pitch, and other organic materials, or may be a carbon-based material formed using hydrocarbon as a source of the chemical vapor deposition method.

The carbide of the other organic material may be a carbide of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose or ketohexose and a carbide of an organic material selected from a combination thereof.

The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon of the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, hexane or the like. Examples of aromatic hydrocarbons of the substituted or unsubstituted aromatic hydrocarbons include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, phenanthrene, and the like.

In one exemplary embodiment of the present invention, the carbon layer may be an amorphous carbon layer.

In one exemplary embodiment of the present invention, the content of the carbon layer may be 0.1 to 50 parts by weight, 0.1 to 30 parts by weight, or 0.1 to 20 parts by weight, based on 100 parts by weight of the total negative electrode active material. More specifically, the content of the carbon layer may be 0.5 to 15 parts by weight, 1 to 10 parts by weight, or 1 to 5 parts by weight. When the above range is satisfied, a decrease in capacity and efficiency of the anode active material can be prevented.

In an exemplary embodiment of the present invention, the carbon layer may have a thickness of 1nm to 500nm, specifically 5nm to 300 nm. When the above range is satisfied, the conductivity of the anode active material is improved, the volume change of the anode active material is easily suppressed, and the side reaction between the electrolyte and the anode active material is suppressed, thereby having the effect of improving the initial efficiency and/or the service life of the battery.

Specifically, the carbon layer may be formed by a Chemical Vapor Deposition (CVD) method using at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene.

In the present invention, the crystallinity of the carbon layer can be confirmed by calculating the D/G band ratio by raman spectroscopy. Specifically, a Renishaw 2000 raman microscopy system and 532nm laser excitation can be used, and to avoid laser thermal effects, the D/G band ratio is measured using a 100x optical lens at low laser power density and 30 seconds exposure time. After measuring a total of 25 points in the 5 μm region of m x μm and fitting using a lorentz function to reduce positional deviation, the D/G band ratio can be calculated by calculating the average of the D band and the G band.

In one exemplary embodiment of the present invention, lithium by-products may be present on the silicon-based particles. In particular, lithium by-products may be present on the surface of the silicon-based particles, or on the surface of the carbon layer. In addition, lithium by-products may exist between a surface layer, which will be described below, and silicon-based particles.

In particular, the lithium by-product may mean a lithium compound remaining on the surface of the silicon-based particles or in the vicinity of the carbon layer after the silicon-based particles are prepared. As described above, even after the acid treatment process, lithium by-products that have not reacted with the acid may remain.

The lithium byproduct may include one or more selected from the group consisting of Li ₂ O, liOH and Li ₂CO₃.

The presence or absence of lithium by-products can be confirmed by X-ray diffraction analysis (XRD) or X-ray photoelectron spectroscopy (XPS).

The content of the lithium by-product may be 5 parts by weight or less based on 100 parts by weight of the total negative electrode active material. Specifically, the content of the lithium by-product may be 0.01 to 5 parts by weight, 0.05 to 2 parts by weight, or 0.1 to 1 part by weight. More specifically, the content of the lithium by-product may be 0.1 to 0.8 parts by weight or 0.1 to 0.5 parts by weight. When the content of the lithium by-product satisfies the above range, side reactions in the slurry can be reduced, and the aqueous processability can be improved by reducing the change in viscosity. In contrast, when the content of the lithium by-product is higher than the above range, there are problems in that the slurry becomes alkaline during the formation of the slurry, which causes side reactions or viscosity changes, and water-based processability problems.

As for the content of the lithium by-product, the amount of the lithium by-product may be calculated after measuring the amount of the HCl solution within a specific interval of pH change during the process of titrating the aqueous solution containing the anode active material with the HCl solution using a titrator.

In one exemplary embodiment of the present invention, lithium byproducts selected from the group consisting of crystalline lithium silicate, li ₂ O, liOH, and Li ₂CO₃ may be hardly present or may not be present on the surface of the anode active material. The lithium by-product increases the pH of the anode slurry, decreases its viscosity, and thus may cause deterioration of the electrode state of the anode. Therefore, by performing an acid treatment process of the anode active material to remove lithium silicate and by-products such as Li ₂ O existing on the surface of the anode active material, an effect of improving the quality and charge/discharge efficiency of the anode can be achieved at a preferable level.

The silicon-based particles according to an exemplary embodiment of the present invention include a surface layer disposed on at least a portion of the silicon-based particles, and the surface layer may include Al, P, and O.

In an exemplary embodiment of the present invention, the surface layer may further contain Li element.

The surface layer may be in the form of a coating on at least a portion of the silicon-based particles having a carbon layer disposed on the surface. That is, the surface layer may be in the form of a partial coating on the surface of the particle or a coating on the entire surface of the particle. Examples of the shape of the surface layer include an island type, a film type, and the like, but the shape of the surface layer is not limited thereto.

The surface layer may be disposed on at least a portion of the outer surface of the carbon layer. That is, the surface layer is adjacently coated on the carbon layer, and thus may be provided in the form of a silicon-based particle-carbon layer-surface layer containing SiO _x (0 < x < 2) and a Li compound. The surface layer may substantially or completely cover the carbon layer or partially cover the carbon layer.

The surface layer may be provided on a region on the surface of the silicon-based particle containing SiO _x (0 < x < 2) and the Li compound where the carbon layer is not provided. That is, the surface layer is adjacently coated on the silicon-based particles containing SiO _x (0 < x < 2) and the Li compound, and thus may be provided in the form of a silicon-based particle-surface layer containing SiO _x (0 < x < 2) and the Li compound.

In one exemplary embodiment of the present invention, the content of Al may be 0.05 to 0.4 parts by weight based on 100 parts by weight of the total negative electrode active material. Specifically, the content of Al may be 0.1 to 0.4 parts by weight, 0.12 to 0.35 parts by weight, or 0.15 to 0.3 parts by weight.

In one exemplary embodiment of the present invention, the content of P may be 0.05 to 2 parts by weight based on 100 parts by weight of the total negative electrode active material. Specifically, the content of P may be 0.1 to 1.5 parts by weight or 0.15 to 1 part by weight.

The surface layer may contain Al _zP_wO_v (0<z. Ltoreq.10, 0< w. Ltoreq.10, and 0<v. Ltoreq.10) phases. The Al _zP_wO_v phase may contain aluminum oxide, phosphorus oxide, aluminum phosphate, etc., and z, y, and v mean the number ratio of each atom. In one example, the Al _zP_wO_v phase may include a mixture or compound formed of AlPO ₄、Al(PO₃)₃ or the like, but is not limited thereto.

The surface layer may contain Li _yAl_zP_wO_v (0 < y.ltoreq.10, 0< z.ltoreq.10, 0< w.ltoreq.10, and 0<v.ltoreq.10) phases. The Li _yAl_zP_wO_v phase may contain aluminum oxide, phosphorus oxide, lithium oxide, aluminum phosphate, lithium salt, lithium phosphate, lithium aluminate, etc., and y, z, w, and v mean the number ratio of each atom. In one example, the Li _yAl_zP_wO_v phase may include a mixture or compound formed of Li ₃PO₄、AlPO₄、Al(PO₃)₃、LiAlO₂ or the like, but is not limited thereto.

Y can satisfy 0< y.ltoreq.3.

Z may satisfy 0<z.ltoreq.1.

W is more than or equal to 0.5 and less than or equal to 3.

V can satisfy 4<v.ltoreq.12.

When the inorganic surface layer containing the above-described phase is provided, a phenomenon in which the Li compound contained in the silicon-based particles reacts with the moisture of the slurry to reduce the viscosity of the slurry can be prevented, and there is an effect of improving the stability of the electrode state and/or the charge/discharge capacity.

In one exemplary embodiment of the present invention, the surface layer may include an amorphous phase. In particular, the surface layer may be amorphous.

When the surface layer contains the aforementioned amorphous phase, there is an effect that the capacity and/or efficiency can be stably achieved while effectively reducing side reactions on the slurry, because the entrance and exit of Li ions are easier than in the case where the surface layer does not contain the amorphous phase.

In an exemplary embodiment of the present invention, the surface layer may further include one or more selected from the group consisting of Li ₂ O, liOH and Li ₂CO₃. In general, since a material remaining in a process of doping silicon-based particles with lithium may be exposed to moisture or air to form lithium byproducts such as Li ₂ O, liOH and Li ₂CO₃, the surface layer may be in a form including one or more selected from the group consisting of Li ₂ O, liOH and Li ₂CO₃.

In one exemplary embodiment of the present invention, during the X-ray diffraction analysis of the anode active material, a crystallization peak derived from the surface layer may not occur. Specifically, a crystallization peak derived from the Li _yAl_zP_wO_v (0 < y.ltoreq.10, 0< z.ltoreq.10, 0< w.ltoreq.10, and 0<v.ltoreq.10) phase contained on the surface layer may not be detected. When a crystallization peak derived from the surface layer occurs, there is a problem in that capacity and/or efficiency is deteriorated because the surface layer contains an excessive amount of crystalline material. In one example, the crystallization peak from the surface layer can be identified by the change before and after the surface layer is coated. Specifically, in the case of XRD, a crystallization peak was detected, and it was confirmed that when there was no difference in the XRD pattern of the anode active material before and after the surface layer coating, a crystallization peak derived from the surface layer did not occur, and the surface layer was formed of an amorphous phase.

In one exemplary embodiment of the present invention, the content of the amorphous phase contained in the surface layer may be more than 50 parts by weight based on 100 parts by weight of the total surface layer. Specifically, the content of the amorphous phase may be 60 parts by weight or more, 70 parts by weight or more, 80 parts by weight or more, 90 parts by weight or more, 95 parts by weight or more, or99 parts by weight or more and 100 parts by weight or less than 100 parts by weight based on 100 parts by weight of the total surface layer. By satisfying the above range, there is an effect that side reactions on the slurry can be effectively suppressed and capacity and/or efficiency can be stably achieved.

In one exemplary embodiment of the present invention, the content of the surface layer may be 10 parts by weight or less based on 100 parts by weight of the total negative electrode active material. Specifically, the content of the surface layer may be 8 parts by weight or less, 6 parts by weight or less, or 5 parts by weight or less and 0.1 parts by weight or more, or 0.5 parts by weight or more. More specifically, the content of the surface layer may be 1 part by weight or more and 5 parts by weight or less or 1.5 parts by weight or more and 3 parts by weight or less. There are problems in that when the content of the surface layer is less than the above range, it is difficult to prevent gas generation on the slurry, and when the content is more than the above range, it is difficult to achieve capacity or efficiency.

In an exemplary embodiment of the present invention, the weight ratio of the surface layer to the carbon layer may be 1:0.1 to 1:30. Specifically, the weight ratio may be 1:0.5 to 1:5 or 1:1 to 1:4 or 1:1 to 1:3. By satisfying the above range, the silicon-based composite particles can be effectively coated with the carbon layer and the surface layer to effectively suppress side reactions on the slurry, and the capacity and/or efficiency can be stably achieved. In contrast, there are problems in that when the content of the surface layer is much higher than that of the carbon layer, it is difficult to achieve capacity or efficiency, and when the content of the carbon layer is much higher than that of the surface layer, it is difficult to prevent gas generation on the slurry.

In one exemplary embodiment of the present invention, the content of the surface layer may be 90 parts by weight or less based on 100 parts by weight of the carbon layer. Specifically, the content of the surface layer may be 80 parts by weight or less, 70 parts by weight or less, 60 parts by weight or less, and 50 parts by weight or less based on 100 parts by weight of the carbon layer. Further, the content of the surface layer may be 0.1 parts by weight or more, 1 part by weight or more, 5 parts by weight or more, and 10 parts by weight or more based on 100 parts by weight of the carbon layer. By satisfying the above range, there is an effect that the silicon-based composite particles can be effectively coated with the carbon layer and the surface layer to effectively suppress side reactions on the slurry and stably realize capacity and/or efficiency.

The anode active material may have an average particle diameter (D50) of 0.1 μm to 14 μm, specifically 1 μm to 12 μm, and more specifically 1 μm to 10 μm. When the above range is satisfied, structural stability of the active material during charge and discharge is ensured, and the problem that the volume expansion/contraction level also becomes large with an excessive increase in particle size can be prevented, and the problem that the initial efficiency is lowered due to an excessively small particle size can be prevented.

The anode active material may have a BET specific surface area of 1m ²/g to 100m ²/g, specifically 1m ²/g to 70m ²/g, more specifically 1m ²/g to 50m ²/g, and for example 2m ²/g to 30m ²/g. When the above range is satisfied, side reactions with the electrolyte during charge and discharge of the battery can be reduced, so that the service life characteristics of the battery can be improved.

Preparation method of negative electrode active material

The invention provides a method for preparing a negative electrode active material, in particular to a method for preparing the negative electrode active material.

An exemplary embodiment of the present invention provides a method of preparing a negative active material according to the present invention, the method including:

Particles comprising a silicon-based oxide are mixed with a lithium precursor, and the resulting mixture is then heat treated.

The formation of particles comprising a silicon-based oxide represented by SiO _x (0 < x < 2) may include: : these powders were evaporated by heating the Si powder and SiO ₂ powder under vacuum, and then the evaporated mixed gas was deposited.

Specifically, the mixed powder of Si powder and SiO ₂ powder may be heat-treated under vacuum at a temperature of 1300 ℃ to 1800 ℃, 1400 ℃ to 1800 ℃, or 1400 ℃ to 1600 ℃.

The particles formed by the method are represented by SiO _x (0 < x < 2), and in terms of structural stability of the active material, it is preferable that the silicon-based oxide is a compound represented by SiO _x (0.5.ltoreq.x.ltoreq.1.5).

The silicon-based oxide formed may have a form of SiO.

The method of preparing a negative active material according to an exemplary embodiment of the present invention may further include: before mixing the lithium precursor, a carbon layer is formed on the surface of particles containing a silicon-based oxide represented by SiO _x (0 < x < 2).

The carbon layer is disposed or formed on the particles, and thus can serve as a protective layer capable of appropriately controlling the volume expansion of the anode active material according to charge and discharge and preventing side reactions with the electrolyte. On the other hand, in terms of the fact that the change in the crystalline phase and the amorphous phase of the anode active material is prevented, the step of forming the carbon layer may be performed before the step of mixing the particles with the lithium precursor.

The carbon layer may be formed using Chemical Vapor Deposition (CVD) using a hydrocarbon gas or by a method of carbonizing a material as a carbon source.

Specifically, the carbon layer may be formed by: the formed particles are introduced into a reaction furnace, and then a hydrocarbon gas is subjected to Chemical Vapor Deposition (CVD) at 600 to 1,200 ℃ to prevent the change of crystalline and amorphous phases. The hydrocarbon gas may be at least one hydrocarbon gas selected from the group consisting of methane, ethane, propane, and acetylene, and may be heat-treated at 900 ℃ to 1,000 ℃.

The particle size of the particles containing the silicon-based oxide may be adjusted by a method such as a ball mill, a jet mill, or air classification, and the method is not limited thereto.

The method of preparing a negative active material according to an exemplary embodiment of the present invention may include: particles comprising a silicon-based oxide are mixed with a lithium precursor, and the resulting mixture is then heat treated.

Silicon-based particles comprising SiO _x (0 < x < 2) and a Li compound can be prepared by mixing particles comprising a silicon-based oxide with a lithium precursor and then heat treating the resulting mixture. For silicon-based particles, the content of Li ₂SiO₃ in the particles is higher than the content of crystalline Li ₂Si₂O₅ and the content of crystalline Li ₄SiO₄, and the total content of crystalline phases present in the particles may be higher than the total content of amorphous phases.

The lithium precursor may contain the Li compound in particles containing a silicon-based oxide by a heat treatment step to be described later. In particular, the lithium precursor may contain at least one selected from the group consisting of lithium metal, liOH, liH, and Li ₂CO₃, and in particular, the lithium precursor may contain lithium metal in terms of the fact that additional oxidation is prevented when particles containing silicon-based oxides react with the lithium precursor. The lithium precursor may be in the form of particles, in particular lithium metal powder.

In particular, the lithium precursor may comprise a Stable Lithium Metal Powder (SLMP).

In one exemplary embodiment of the present invention, particles including a silicon-based oxide may be solid-phase mixed with a lithium precursor. Specifically, during mixing, the particles including the silicon-based oxide and the lithium precursor may be in a solid state, in which case, during formation of the anode active material by heat treatment to be described later, the porosity and specific surface area in the anode active material may be controlled to be at appropriate levels, so that the volume expansion of the anode active material according to charge and discharge may be preferably controlled.

In an exemplary embodiment of the present invention, the weight ratio of the particles including the silicon-based oxide to the lithium precursor may be 80:20 to 94:6. In particular, the weight ratio may be 85:15 to 93:7, 88:12 to 93:7 or 90:10 to 92:8.

In one exemplary embodiment of the present invention, the particles including the silicon-based oxide and the lithium precursor may be mixed while performing the heat treatment under an inert gas atmosphere.

The heat treatment temperature of the mixture comprising particles of the silicon-based oxide and the lithium precursor may be 650 ℃ to 950 ℃, in particular 670 ℃ to 900 ℃ or 700 ℃ to 850 ℃. When the particles containing a silicon-based oxide and the lithium precursor can be mixed while performing the heat treatment under the aforementioned conditions, it is preferable in view of the fact that Li ₂SiO₃ crystalline lithium silicate is easily formed.

Through the heat treatment process in the above temperature range, the Li compound may be contained in the prepared silicon-based particles at an appropriate level, and specifically, the Li compound may be distributed on, inside, or both on and inside the surface of the silicon-based particles. The silicon-based particles contain lithium silicate as the above-mentioned Li compound, and may further contain lithium silicide, lithium oxide, or the like.

The negative electrode active material can be prepared by a heat treatment process in the above temperature range. Specifically, lithium may be distributed in the form of lithium silicate in the particles through the heat treatment process within the above temperature range, and thus lithium may be used to improve the initial efficiency and charge/discharge efficiency of the anode active material by removing the irreversible capacity of the particles including silicon-based oxide. Specifically, lithium may be present in the form of at least one crystalline lithium silicate or amorphous lithium silicate selected from the group consisting of crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, and crystalline Li ₂Si₂O₅. In this case, in the anode active material prepared by the method of preparing an anode active material of the present invention, the content of crystalline Li ₂SiO₃ may be higher than the sum of the content of crystalline Li ₂Si₂O₅ and the content of crystalline Li ₄SiO₄, and the content of crystalline Li ₂SiO₃ may be 60 parts by weight or more based on 100 parts by weight of the total of crystalline lithium silicate.

Through the heat treatment process in the above temperature range, the total content of the crystalline phase present in the silicon-based particles may be higher than the total content of the amorphous phase, and thus since the content of lithium oxide and lithium silicate reacting with moisture is low, gas generation and viscosity change of the anode slurry may be prevented and phase stability of the slurry including the anode active material may be improved, so that quality of the anode including the anode active material and the secondary battery including the anode may be improved and charge/discharge efficiency thereof may be improved.

When the heat treatment process is performed at a temperature lower than 650 ℃, the content of the amorphous phase of the anode active material prepared by the preparation method increases, which may cause problems in the processability of the aqueous slurry. When the heat treatment process is performed at a temperature higher than 950 ℃, the content of crystalline SiO ₂ increases and crystalline SiO ₂ acts as a resistance during charge and discharge, so that there arises a problem in that charge and discharge are unfavorable and charge/discharge capacity and efficiency are deteriorated, which is not preferable.

The heat treatment may be carried out for 1 hour to 12 hours, specifically 2 hours to 8 hours. When the time is within the above range, the lithium silicate may be uniformly distributed in the silicon-based particles, so that the above charge/discharge efficiency improvement effect may be further improved.

The heat treatment may be performed in an inert atmosphere in terms of the fact that additional oxidation of the particles comprising the silicon-based oxide and the lithium precursor is prevented. Specifically, the heat treatment may be performed in an inert atmosphere by at least one gas selected from the group consisting of nitrogen, argon, and helium.

The method of preparing a negative active material of the present invention may further include performing an acid treatment after heat-treating the resultant mixture.

A lithium compound (lithium by-product) is provided on at least a part of the surface of the silicon-based particles formed after heat-treating the resulting mixture. Specifically, in the step (doping) of incorporating the Li compound into the particles containing SiO _x (0 < x < 2), the lithium compound (i.e., lithium by-product formed from unreacted lithium) remains near the surface of the silicon-based particles. The lithium by-product may be lithium silicate, li ₂ O, or the like, which may increase the pH of the anode slurry including the anode active material and reduce the viscosity, resulting in deterioration of the electrode condition of the anode. Therefore, by performing an acid treatment process after the heat treatment process to remove lithium silicate and byproducts such as Li ₂ O present on the surface of the anode active material, the effect of improving the quality and charge/discharge efficiency of the anode can be achieved at a preferable level.

Specifically, the acid treatment may be performed by treating the heat-treated composition for forming the anode active material with an aqueous acid solution containing at least one acid selected from the group consisting of hydrochloric acid (HCl), sulfuric acid (H ₂SO₄), nitric acid (HNO ₃) and phosphoric acid (H ₃PO₄), specifically at least one acid selected from the group consisting of hydrochloric acid (HCl), sulfuric acid (H ₂SO₄) and nitric acid (HNO ₃), for 0.3 to 6 hours, specifically for 0.5 to 4 hours, and is preferable in view of the fact that byproducts present on the surface of the anode active material can be easily removed by the process.

The pH of the aqueous acid solution at 23 ℃ may be 3 or less, specifically 2 or less, and more specifically 1 or less, in view of the fact that byproducts present on the surface of the anode active material can be easily removed.

The method of preparing a negative active material of the present invention may further include: the surface layer is provided on at least a part of the silicon-based particles formed after heat-treating the resulting mixture. In one example, when the acid treatment step is performed after the heat treatment of the resulting mixture, it may further include providing a surface layer after the acid treatment step.

The surface layer may be provided by mixing silicon-based particles and aluminophosphates.

Specifically, the surface layer may be formed on at least a part of the silicon-based particles by: i) Dry-blending the silicon-based particles with an aluminophosphate and heat-treating the resulting mixture, or ii) mixing the silicon-based particles and the aluminophosphate with a solvent, and then heat-treating the resulting mixture to react the silicon-based particles and the aluminophosphate while evaporating the solvent. When the surface layer is formed by the method, the surface layer can be easily formed by reacting Li by-products formed or remaining during the preparation process of the silicon-based particles with the aluminophosphate.

The aluminophosphate may be in the form of Al _bP_cO_d (0<b. Ltoreq.10, 0< c. Ltoreq.10, and 0<d. Ltoreq.10). Specifically, the aluminophosphate may be Al (PO ₃)₃ or AlPO ₄, and is not limited thereto, and salts used in the art for forming a surface layer may be suitably employed.

Alternatively, the surface layer may be formed on at least a part of the silicon-based particles by: iii) Dry-blending the silicon-based particles, the aluminum precursor and the phosphorus precursor and heat-treating the resulting mixture, or iv) mixing the silicon-based particles, the aluminum precursor and the phosphorus precursor with a solvent, and then heat-treating the resulting mixture to react the silicon-based composite particles, the aluminum precursor and the phosphorus precursor while evaporating the solvent. When the surface layer is formed by the method, the surface layer can be easily formed by reacting lithium by-products formed during the preparation process of the silicon-based composite particles with an aluminum precursor and a phosphorus precursor.

The aluminum precursor may be an aluminum oxide having the form of Al _aO_b (0<a. Ltoreq.10, 0< b. Ltoreq.10), and may specifically be Al ₂O₃.

Alternatively, the aluminum precursor may be aluminum hydroxide, aluminum nitrate, aluminum sulfate, or the like, specifically may be Al (OH) ₃、Al(NO₃)₃·9H₂ 0 or Al ₂(SO₄)₃, and is not limited thereto, and an aluminum precursor used in the art for forming the surface layer may be suitably employed.

The phosphorus precursor may be a phosphorus oxide having the form P _cO_d (0<c.ltoreq.10, 0< d.ltoreq.10).

Alternatively, the phosphorus precursor may be ammonium phosphate, diammonium phosphate, phosphoric acid, or the like, and may be (NH ₄)₃PO₄、(NH₄)₂HPO₄、H₃PO₄ or NH ₄H₂PO₄ in particular, and is not limited thereto, and a phosphorus precursor used in the art for forming a surface layer may be suitably employed.

Alternatively, the surface layer may be formed on at least a part of the silicon-based particles by: v) dry-blending the silicon-based particles and the Li-Al-P-O-based precursor and heat-treating the resulting mixture, or vi) mixing the silicon-based composite particles and the Li-Al-P-O-based precursor with a solvent, and then heat-treating the resulting mixture to react the silicon-based composite particles and the Li-Al-P-O-based precursor while evaporating the solvent. When the surface layer is formed by the method, the surface layer may be formed by directly introducing a Li-Al-P-O-based precursor.

The Li-Al-P-O system precursor may have the form of Li _yAl_zP_wO_v (0 < y.ltoreq.10, 0< z.ltoreq.10, 0< w.ltoreq.10, and 0<v.ltoreq.10). Specifically, the Li-Al-P-O-based precursor may be a mixture or compound formed by compounding Li ₃PO₄、AlPO₄、Al(PO₃)₃、LiAlO₂ or the like, and is not limited thereto, and a configuration for forming a surface layer in the art may be suitably employed.

When the surface layer is provided on at least a part of the silicon-based particles, the heat treatment may be performed at 500 ℃ to 700 ℃, specifically 550 ℃ to 650 ℃. However, the heat treatment temperature is not limited thereto, and may vary depending on the salt, precursor, and the like used.

When the heat treatment temperature satisfies the above range, the salt or precursor reacts well with the Li by-product, so that the surface layer contains Li, thereby having an effect that the discharge rate limiting characteristic (rate performance) becomes excellent, because the durability of the formed anode active material against moisture is improved, entry and exit of Li ions through the surface layer becomes easy, and the Li diffusion resistance of the anode active material surface is reduced.

The solvent may be water or ethanol and is not limited thereto, and solvents used in the art may be suitably employed.

The surface layer formed on the silicon-based composite particles preferably contains a Li _yAl_zP_wO_v (0 < y.ltoreq.10, 0< z.ltoreq.10, 0< w.ltoreq.10, and 0<v.ltoreq.10) phase, and the Li _yAl_zP_wO_v phase may be an amorphous phase.

< Cathode >

The anode according to an exemplary embodiment of the present invention may include the anode active material described above.

Specifically, the anode may include an anode current collector and an anode active material layer disposed on the anode current collector. The anode active material layer may contain an anode active material. In addition, the anode active material layer may further include a binder, a thickener, and/or a conductive material.

The anode active material layer may be formed by applying an anode slurry containing an anode active material, a binder, a thickener, and/or a conductive material to at least one surface of a current collector and drying and calendaring the current collector.

The anode slurry may further contain an additional anode active material.

As the additional anode active material, a compound capable of reversibly intercalating and deintercalating lithium may be used. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers and amorphous carbon; a metal species capable of forming an alloy with lithium, such as Si, al, sn, pb, zn, bi, in, mg, ga, cd, si alloy, sn alloy, or Al alloy; metal oxides capable of undoped lithium and doped lithium, such as SiO _β (0<β<2)、SnO₂, vanadium oxide, lithium titanium oxide and lithium vanadium oxide; or a composite material containing the metal substance and a carbonaceous material, such as a si—c composite material or a sn—c composite material, or the like, and any one or a mixture of two or more thereof may be used. In addition, a metallic lithium thin film may be used as the anode active material. Alternatively, both low crystalline carbon and high crystalline carbon, and the like may be used as the carbon material. Typical examples of low crystalline carbon include soft carbon and hard carbon, and typical examples of high crystalline carbon include natural graphite or artificial graphite in an irregular, planar, flaky, spherical or fibrous form, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase pitch, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived coke.

The additional anode active material layer may be a carbon-based anode active material.

In one exemplary embodiment of the present invention, the weight ratio of the anode active material to the additional anode active material included in the anode slurry may be 10:90 to 90:10, specifically 10:90 to 50:50.

The anode slurry may contain a solvent for forming the anode slurry. Specifically, the solvent used to form the anode slurry may contain at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropanol, specifically distilled water in terms of promoting the dispersion of the components.

The anode slurry including the anode active material according to one exemplary embodiment of the present invention may have a pH of 7 to 11 at 25 ℃. When the pH of the anode slurry satisfies the above range, there is an effect of stabilizing the rheological properties of the slurry. In contrast, when the pH of the negative electrode slurry is less than 7 or more than 11, there is a problem in that carboxymethyl cellulose (CMC) used as a thickener is decomposed, resulting in a decrease in viscosity of the slurry and a deterioration in the degree of dispersion of the active material contained in the slurry.

The anode current collector is sufficient as long as the anode current collector has conductivity without causing chemical changes in the battery, and is not particularly limited. For example, as the negative electrode current collector, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or a material of aluminum or stainless steel in which the surface is surface-treated with carbon, nickel, titanium, silver, or the like may be used. In particular, a transition metal such as copper or nickel, which well adsorbs carbon, may be used as the current collector. Although the current collector may have a thickness of 6 μm to 20 μm, the thickness of the current collector is not limited thereto.

The adhesive may comprise at least one selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene Propylene Diene Monomer (EPDM), sulfonated EPDM, styrene Butadiene Rubber (SBR), fluororubber, polyacrylic acid, and a material in which hydrogen thereof is substituted with Li, na, ca, or the like, and may further contain various copolymers thereof.

The conductive material is not particularly limited as long as the conductive material has conductivity without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite may be used; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes; a fluorocarbon; metal powder, aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives and the like.

The thickener may be carboxymethyl cellulose (CMC), and is not limited thereto, and a thickener used in the art may be suitably employed.

In one exemplary embodiment of the present invention, the weight ratio of the anode active material to the additional anode active material included in the anode slurry may be 1:99 to 30:70, specifically 5:95 to 30:70 or 10:90 to 20:80.

In one exemplary embodiment of the present invention, the content of the entire anode active material in the anode slurry may be 60 to 99 parts by weight, specifically 70 to 98 parts by weight, based on the total of 100 parts by weight of the solids of the anode slurry.

In one exemplary embodiment of the present invention, the content of the binder may be 0.5 to 30 parts by weight, specifically 1 to 20 parts by weight, based on 100 parts by weight total of the solids of the negative electrode slurry.

In one exemplary embodiment of the present invention, the content of the conductive material may be 0.5 to 25 parts by weight, specifically 1 to 20 parts by weight, based on 100 parts by weight total of the solids of the negative electrode slurry.

In one exemplary embodiment of the present invention, the content of the thickener may be 0.5 to 25 parts by weight, specifically 0.5 to 20 parts by weight, more preferably 1 to 20 parts by weight, based on 100 parts by weight total of the solids of the negative electrode slurry.

The anode slurry according to an exemplary embodiment of the present invention may further include a solvent for forming the anode slurry. Specifically, the solvent used to form the anode slurry may contain at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropanol, specifically distilled water in terms of promoting the dispersion of the components.

In one exemplary embodiment of the present invention, the solid weight of the negative electrode slurry may be 20 to 75 parts by weight, specifically 30 to 70 parts by weight, based on 100 parts by weight of the total negative electrode slurry.

< Secondary Battery >

The secondary battery according to an exemplary embodiment of the present invention may include the above-described negative electrode according to an exemplary embodiment. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode is identical to the above-described negative electrode. Since the anode has been previously described, a detailed description thereof will be omitted.

The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including a positive electrode active material.

In the positive electrode, the positive electrode current collector is not particularly limited as long as the positive electrode current collector has conductivity without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or a material in which the surface of aluminum or stainless steel is surface-treated with carbon, nickel, titanium, silver, or the like may be used. Further, the positive electrode current collector may typically have a thickness of 3 to 500 μm, and the adhesion of the positive electrode active material may also be improved by forming fine irregularities on the surface of the current collector. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric body.

The positive electrode active material may be a typically used positive electrode active material. Specifically, the positive electrode active material contains: layered compounds such as lithium cobalt oxide (LiCoO ₂) and lithium nickel oxide (LiNiO ₂) or compounds substituted with one or more transition metals; lithium iron oxides, such as LiFe ₃O₄; lithium manganese oxides such as the chemical formulas Li _1+c1Mn_2-c1O₄ (0≤c1≤0.33)、LiMnO₃、LiMn₂O₃ and LiMnO ₂; lithium copper oxide (Li ₂CuO₂); vanadium oxides, such as LiV ₃O₈、V₂O₅ and Cu ₂V₂O₇; ni-site lithium nickel oxide expressed by a chemical formula LiNi _1-c2M_c2O₂ (here, M is at least one selected from the group consisting of Co, mn, al, cu, fe, mg, B and Ga, and c2 satisfies 0.01.ltoreq.c2.ltoreq.0.5); lithium manganese composite oxide expressed as a chemical formula LiMn _2-c3M_c3O₂ (here, M is at least any one selected from the group consisting of Co, ni, fe, cr, zn and Ta, and c3 satisfies 0.01.ltoreq.c3.ltoreq.0.1) or Li ₂Mn₃MO₈ (here, M is at least any one selected from the group consisting of Fe, co, ni, cu and Zn); liMn ₂O₄, wherein Li in the formula is partially replaced by alkaline earth metal ions; etc., but is not limited thereto. The positive electrode may be Li-metal.

The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder, and the positive electrode active materials described above.

In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and may be used without any particular limitation as long as the positive electrode conductive material has electron conductivity without causing chemical changes in the battery to be constructed. Specific examples thereof include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and any one or a mixture of two or more thereof may be used.

Alternatively, the positive electrode binder serves to improve adhesion between positive electrode active material particles and adhesion between a positive electrode active material and a positive electrode current collector. Specific examples thereof may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, hydroxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene Propylene Diene Monomer (EPDM), sulfonated EPDM, styrene Butadiene Rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof may be used.

The separator separates the anode from the cathode, and provides a passage for movement of lithium ions, and may be used without particular limitation as long as the separator is typically used as a separator in a secondary battery, and in particular, a separator having excellent electrolyte moisture-containing ability and low resistance to movement of ions in an electrolyte is preferable. Specifically, a porous polymer film, for example, a porous polymer film formed of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a typical porous nonwoven fabric such as a nonwoven fabric made of glass fiber, polyethylene terephthalate fiber, or the like having a high melting point may also be used. In addition, a coated separator including a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may be selectively used as a single-layer or multi-layer structure.

Examples of the electrolyte include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc., which may be used to prepare a lithium secondary battery, but are not limited thereto.

In particular, the electrolyte may comprise a non-aqueous organic solvent and a metal salt.

As the nonaqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ -butyrolactone, 1, 2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1, 3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphotriester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1, 3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate can be used.

In particular, among the carbonate-based organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate may be preferably used because the cyclic carbonates have a high dielectric constant as a high-viscosity organic solvent, thus dissociating lithium salts well, and such cyclic carbonates may be more preferably used because they may be mixed with linear carbonates such as dimethyl carbonate and diethyl carbonate of low viscosity and low dielectric constant in an appropriate ratio and used to prepare an electrolyte having high conductivity.

As the metal salt, a lithium salt, which is a material easily soluble in a nonaqueous electrolyte, may be used, and as anions of the lithium salt, for example, one or more types ：F^-、Cl^-、I^-、NO₃ ^-、N(CN)₂ ^-、BF₄ ^-、ClO₄ ^-、PF₆ ^-、(CF₃)₂PF₄ ^-、(CF₃)₃PF₃ ^-、(CF₃)₄PF₂ ^-、(CF₃)₅PF^-、(CF₃)₆P^-、CF₃SO₃ ^-、CF₃CF₂SO₃ ^-、(CF₃SO₂)₂N^-、(FSO₂)₂N^-、CF₃CF₂(CF₃)₂CO^-、(CF₃SO₂)₂CH^-、(SF₅)₃C^-、(CF₃SO₂)₃C^-、CF₃(CF₂)₇SO₃ ^-、CF₃CO₂ ^-、CH₃CO₂ ^-、SCN^- and (CF ₃CF₂SO₂)₂N^-) selected from the group consisting of the following anions may be used.

In the electrolyte, in order to improve the life characteristics of the battery, suppress the decrease in the battery capacity, and improve the discharge capacity of the battery, one or more additives such as halogenated alkylene carbonate-based compounds, for example, difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, (poly) ethyleneglycol dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride may be further contained in addition to the above electrolyte-constituting components.

According to still another exemplary embodiment of the present invention, there are provided a battery module including a secondary battery as a unit cell and a battery pack including the same. The battery module and the battery pack include secondary batteries having high capacity, high rate characteristics, and cycle characteristics, and thus can be used as a power source for medium-to-large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and electric storage systems.

Mode for carrying out the invention

Examples and comparative examples

Example 1

100G of powder in which Si and SiO ₂ were mixed in a molar ratio of 1:1 were heated in a reactor under vacuum at a sublimation temperature of 1,400 ℃. Thereafter, the mixture gas of evaporated Si and SiO ₂ was reacted in a cooling zone under vacuum with a cooling temperature of 800 ℃ and condensed into a solid phase. Thereafter, initial silicon-based particles were prepared by performing heat treatment at a temperature of 800 ℃ in an inert atmosphere. After that, after 15 SUS spherical media were introduced into the initial silicon-based particles, the initial silicon-based particles were crushed by using a ball mill for 3 hours, to prepare silicon-based particles having a size (D ₅₀) =6 μm. Thereafter, while maintaining an inert atmosphere by flowing Ar gas, silicon-based particles were placed in a hot zone of a CVD apparatus, and methane was blown into the hot zone at 900 ℃ using Ar as a carrier gas, and reacted at 10 ^-1 torr for 20 minutes to form a carbon layer on the surface of the silicon-based particles.

A composition for forming a negative electrode active material was prepared by solid-phase mixing silicon-based particles having a carbon layer formed thereon with lithium metal powder as a lithium precursor at a weight ratio of 90:10.

The composition for forming the anode active material was heat-treated at 800 ℃ for 3 hours.

The heat-treated composition for forming a negative electrode active material was acid-treated with an aqueous hydrochloric acid solution having a pH of 1 at 23 ℃ for 1 hour.

A material obtained by acid treatment was used as the anode active material of example 1. The negative electrode active material had a specific surface area of D ₅₀ of 6 μm and 3m ²/g.

Example 2

A negative electrode active material was prepared in the same manner as in example 1, except that a composition for forming a negative electrode active material was prepared by solid-phase mixing silicon-based particles having a carbon layer formed thereon with lithium metal powder as a lithium precursor at a weight ratio of 92:8.

Example 3

A negative electrode active material was prepared in the same manner as in example 1 except that a composition for forming a negative electrode active material was prepared by solid-phase mixing silicon-based particles having a carbon layer formed thereon with lithium metal powder as a lithium precursor at a weight ratio of 90:10 and acid treatment was not performed.

Example 4

A negative electrode active material was prepared in the same manner as in example 1, except that a composition for forming a negative electrode active material was prepared by solid-phase mixing silicon-based particles having a carbon layer formed thereon with lithium metal powder as a lithium precursor at a weight ratio of 90:10 and the composition for forming a negative electrode active material was heat-treated at 850 ℃ for 3 hours.

Example 5

By mixing 1.5g of Al (PO ₃)₃ with 98.5g of the anode active material obtained in example 1), and then heat-treating the resultant mixture at 600 ℃ to form a surface layer containing Li, al, P, and O on the surface, the material formed with the surface layer was used as the anode active material of example 5.

Example 6

A negative electrode active material was prepared in the same manner as in example 1, except that silicon-based particles having a size (D ₅₀) =8 μm were prepared by pulverizing the initial silicon-based particles.

Comparative example 1

A negative electrode active material was prepared in the same manner as in example 1, except that the composition for forming a negative electrode active material was heat-treated at 500 ℃ for 3 hours and was not subjected to acid treatment.

Comparative example 2

A negative electrode active material was prepared in the same manner as in example 1, except that the composition for forming a negative electrode active material was heat-treated at 1000 ℃ for 3 hours and was not subjected to acid treatment.

Comparative example 3

A negative electrode active material was prepared in the same manner as in example 1, except that the composition for forming a negative electrode active material was heat-treated at 1000 ℃ for 3 hours and acid-treated for 3 hours.

Comparative example 4

A negative electrode active material was prepared in the same manner as in example 1, except that the composition for forming a negative electrode active material was prepared by solid-phase mixing silicon-based particles having a carbon layer formed thereon with lithium metal powder as a lithium precursor at a weight ratio of 94:6, the composition for forming a negative electrode active material was heat-treated at 500 ℃ for 3 hours and no acid treatment was performed.

Comparative example 5

A negative electrode active material was prepared in the same manner as in example 1, except that the composition for forming a negative electrode active material was prepared by solid-phase mixing silicon-based particles having a carbon layer formed thereon with lithium metal powder as a lithium precursor at a weight ratio of 94:6, the composition for forming a negative electrode active material was heat-treated at 1000 ℃ for 3 hours, and acid treatment was performed for 3 hours.

Comparative example 6

A negative electrode active material was prepared in the same manner as in example 1, except that silicon-based particles having a size (D ₅₀) =15 μm were prepared by pulverizing the initial silicon-based particles.

The anode active materials prepared in examples 1 to 6 and comparative examples 1 to 6 were analyzed by the following methods, and the analysis results are shown in table 1.

Measurement of p2/p1

In Table 1, p2/p1 was calculated by ²⁹ Si MAS NMR analysis as follows.

-P2/p1: during ²⁹ Si MAS NMR analysis, the ratio of the height of the peak (p 2) of Li ₂Si₂O₅ to the height of the peak (p 1) of Li ₂SiO₃

Measurement of total content of crystalline phase and total content of amorphous phase comprising crystalline Li ₂Si₂O₅, crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, crystalline SiO ₂ and crystalline Si

The measurement was performed using an X-ray diffraction (XRD) apparatus (trade name: D4-endavor, manufacturer: bruker). For the type and wavelength of the light source, the X-ray wavelength generated by cukα was used, and the wavelength (λ) of the light source was 0.15406nm. After the reference material MgO and the anode active material were mixed at a weight ratio of 20:80, the resultant mixture was placed in a cylindrical jig having a diameter of 2.5 cm and a height of 2.5: 2.5 mm, and subjected to a planarization treatment using a glass slide, so that the height of the sample in the jig was constant, to prepare a sample for XRD analysis. The scanning time is set to 1 hour and 15 minutes, the measurement area is set to an area where 2θ is 10 ° to 90 °, and the step time and step size are set so as to scan 2θ of 0.02 ° per second. The measurement of the X-ray diffraction profile was analyzed by Rietveld refinement using X-ray diffraction pattern analysis software. The total content of crystalline phases and the total content of amorphous phases including crystalline Li ₂Si₂O₅, crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, crystalline SiO ₂, and crystalline Si were measured by the analysis.

D ₅₀ of the negative electrode active material was analyzed by a PSD measurement method using Microtrac equipment.

The specific surface area of the anode active material was measured by degassing the gas at 200℃for 8 hours using a BET measuring apparatus (BEL-SORP-MAX, nippon Bell), and performing N ₂ adsorption/desorption at 77K.

Experimental example: evaluation of discharge capacity, initial efficiency and service life (Capacity Retention Rate) characteristics

The negative electrode active materials in examples and comparative examples were used to prepare a negative electrode and a battery, respectively.

The mixture was prepared by mixing the anode active material, the conductive material carbon black, and the binder polyacrylic acid (PAA) in a weight ratio of 80:10:10. After that, 7.8g of distilled water was added to 5g of the mixture, and the resultant mixture was stirred to prepare a negative electrode slurry. The anode slurry was applied to a copper (Cu) metal thin film having a thickness of 20 μm as an anode current collector and dried. In this case, the temperature of the circulated air was 60 ℃. Subsequently, the anode was prepared by calendaring an anode current collector and drying the anode current collector in a vacuum oven at 130 ℃ for 12 hours.

A lithium (Li) metal thin film obtained by cutting a lithium (Li) metal sheet into a circular shape of 1.7671cm ² was used as the positive electrode. A porous polyethylene separator was interposed between the positive electrode and the negative electrode, ethylene carbonate was dissolved in 0.5 parts by weight in a mixed solution of methyl ethyl carbonate (EMC) and Ethylene Carbonate (EC) in a mixed volume ratio of 7:3, and an electrolyte having LiPF ₆ dissolved therein at a concentration of 1M was injected thereto to prepare a lithium coin half-cell.

The discharge capacity, initial efficiency, and capacity retention rate were evaluated by charging and discharging the prepared battery, and are shown in table 2 below.

For cycles 1 and 2, the battery was charged and discharged at 0.1C, and from cycle 3 to cycle 49, the battery was charged and discharged at 0.5C. The 50 th cycle is completed in the charged state (in which lithium is contained in the negative electrode).

Charging conditions: CC (constant current)/CV (constant voltage) (5 mV/0.005C current cut-off)

Discharge conditions: CC (constant current) Condition 1.5V

The discharge capacity (mAh/g) and initial efficiency (%) were obtained from the results during the first charge/discharge. Specifically, the initial efficiency (%) is calculated as follows.

Initial efficiency (%) = (1 st discharge capacity/1 st charge capacity) ×100

The capacity retention rates were each calculated as follows.

Capacity retention (%) = (49 th discharge capacity/1 st discharge capacity) ×100

In table 2, it was confirmed that in examples 1 to 6 using the anode active material according to the present invention, the content of crystalline Li ₂SiO₃ was high, the loss of discharge capacity per unit weight depending on the increase in the Li content in the anode active material was small, the total content of crystalline phases present in the anode active material was higher than the total content of amorphous phases, and therefore, the phase stability of the slurry was improved, and the initial efficiency and the service life characteristics were significantly increased due to the lithium silicate structure of electrochemically stable Li ₂SiO₃.

In the case of example 5, it was confirmed that the surface layer containing Li, al, P, and O was provided on the surface of the anode active material, so that the Li compound contained in the silicon-based particles could react with the moisture of the slurry to prevent the phenomenon of lowering the viscosity of the slurry, and thus the initial efficiency and the service life characteristics were further improved.

In contrast, in comparative examples 1 to 6, it was confirmed that the capacity loss per unit weight increased or the structure of Li ₂Si₂O₅ was unstable because the content of crystalline Li ₂SiO₃ was less than 60 parts by weight or the total content of crystalline phases was low based on 100 parts by weight of the total of crystalline lithium silicate, and thus the initial efficiency and/or the service life characteristics were poor. In particular, in the case of comparative examples 4 and 5, it was confirmed that the Li content was reduced, and thus the discharge capacity was increased, but the efficiency was reduced by 10% or more, and the service life was reduced by about 50% or more.

Claims

1. A negative electrode active material containing silicon-based particles containing SiO _x and a Li compound, wherein 0< x <2,

Wherein the Li compound comprises at least one crystalline lithium silicate selected from the group consisting of crystalline Li ₂SiO₃, crystalline Li ₄SiO₄, and crystalline Li ₂Si₂O₅,

The content of the crystalline Li ₂SiO₃ is higher than the sum of the content of the crystalline Li ₂Si₂O₅ and the content of the crystalline Li ₄SiO₄,

The content of the crystalline Li ₂SiO₃ is 60 parts by weight or more based on 100 parts by weight of the total crystalline lithium silicate, and

The total content of crystalline phases present in the silicon-based particles is higher than the total content of amorphous phases.

2. The anode active material according to claim 1, wherein the content of Li element is more than 7 parts by weight and 10 parts by weight or less based on 100 parts by weight of the anode active material in total.

3. The anode active material according to claim 1, wherein the Li compound contains crystalline Li ₂SiO₃ and crystalline Li ₂Si₂O₅.

4. The anode active material according to claim 1, wherein a total content of crystal phases present in the silicon-based particles is more than 50 parts by weight and 85 parts by weight or less based on 100 parts by weight total of the silicon-based particles.

5. The anode active material according to claim 1, wherein the content of the crystalline Li ₂SiO₃ is 15 parts by weight or more and 50 parts by weight or less based on 100 parts by weight of the total of the silicon-based particles.

6. The anode active material according to claim 1, wherein a difference between a content of the crystalline Li ₂SiO₃ and a content of the crystalline Li ₂Si₂O₅ is 1 to 50 parts by weight based on 100 parts by weight of the silicon-based particles in total.

7. The anode active material according to claim 1, wherein the crystalline Li ₄SiO₄ is not contained.

8. The anode active material according to claim 1, wherein during ²⁹ Si-MAS-NMR analysis, the height of the peak p1 of Li ₂SiO₃ occurring at the chemical shift peak of-70 ppm to-80 ppm is greater than the height of the peak p2 of Li ₂Si₂O₅ occurring at the chemical shift peak of-90 ppm to-100 ppm.

9. The anode active material according to claim 1, wherein a ratio p2/p1 of a height of a peak p2 of Li ₂Si₂O₅ appearing at a chemical shift peak of-90 ppm to-100 ppm to a height of a peak p1 of Li ₂SiO₃ appearing at a chemical shift peak of-70 ppm to-80 ppm is 1 or less during ²⁹ Si-MAS-NMR analysis.

10. The anode active material according to claim 1, wherein during ²⁹ Si-MAS-NMR analysis, there is no peak p3 of Li ₄SiO₄ present at the chemical shift peak of-60 ppm to-69 ppm.

11. The anode active material according to claim 1, wherein a content of crystalline SiO ₂ is less than 5 parts by weight based on 100 parts by weight of the total of the silicon-based particles.

12. The anode active material according to claim 1, further comprising a carbon layer provided on the silicon-based particles.

13. The anode active material according to claim 1, further comprising a surface layer containing Al, P, and O provided on the silicon-based particles.

14. A method of preparing the anode active material of any one of claims 1 to 13, the method comprising:

Forming particles comprising a silicon-based oxide represented by SiO _x, wherein 0< x <2; and

Particles comprising the silicon-based oxide and a lithium precursor are mixed, and then the resulting mixture is heat-treated.

15. The method of claim 14, further comprising performing an acid treatment after the heat treating the resulting mixture.

16. The method of claim 14, the method further comprising: the surface layer is provided on at least a part of the silicon-based particles formed after the heat treatment of the resulting mixture.

17. The method of claim 14, wherein the heat treatment is performed at 650 ℃ to 950 ℃.

18. The method of claim 14, wherein the heat treatment is performed for 1 hour to 12 hours.

19. A negative electrode, the negative electrode comprising:

A negative electrode current collector; and

A negative electrode active material layer disposed on at least one surface of the negative electrode current collector,

Wherein the anode active material layer contains an anode material containing the anode active material according to any one of claims 1 to 13.

20. A secondary battery, the secondary battery comprising:

The negative electrode of claim 19;

a positive electrode facing the negative electrode;

a separator interposed between the negative electrode and the positive electrode; and

An electrolyte.