WO2015146864A1

WO2015146864A1 - Negative electrode active material for lithium ion secondary battery, and method for producing same

Info

Publication number: WO2015146864A1
Application number: PCT/JP2015/058623
Authority: WO
Inventors: 三崎日出彦; 阿部昌則; 津吉徹; 向後雅則; 高東修二; 荒川太地; 岩永宏平
Original assignee: 東ソー株式会社
Priority date: 2014-03-25
Filing date: 2015-03-20
Publication date: 2015-10-01

Abstract

Provided are: a negative electrode active material for a lithium ion secondary battery with which high initial efficiency and battery capacity can be maintained and excellent cycling characteristics achieved; and a method for producing said active material. This negative electrode active material for a lithium ion secondary battery, said active material comprising an Si compound, and a carbon substance or a carbon substance and graphite, is obtained by means of: a step of mixing an Si compound, a carbon precursor, and, in accordance with requirements, a graphite powder; a step of granulation/compaction; a step of pulverizing the mixture to form composite particles; a step of firing the composite particles in an inert gas atmosphere; and a step of subjecting the pulverized and spheroidized composite powder or the fired powder to air classification.

Description

Negative electrode active material for lithium ion secondary battery and method for producing the same

The present invention relates to a negative electrode active material for a lithium ion secondary battery and a method for producing the same.

As mobile devices such as smartphones and tablet terminals become more sophisticated and vehicles equipped with lithium ion secondary batteries such as EVs and PHEVs are increasingly required, there is an increasing demand for higher capacity lithium ion secondary batteries. At present, graphite is mainly used as the negative electrode material of lithium ion secondary batteries. However, for further increase in capacity, the theoretical capacity is high, and elements such as silicon and tin that can absorb and release lithium ions are used. Development of negative electrode materials using metals or alloys with other elements has been activated.

On the other hand, it is known that an active material made of a metal material capable of inserting and extracting lithium ions remarkably expands when alloyed with lithium by charging. Therefore, the active material is cracked and refined, and the structure of the negative electrode using these is also broken and the conductivity is cut. Therefore, the negative electrode using these metal materials has a problem that the capacity is remarkably lowered with the passage of cycles.

In response to this problem, a method has been proposed in which these metal materials are made into fine particles and combined with carbonaceous material or graphite. Such composite particles have significantly higher cycle characteristics than the use of these materials alone as a negative electrode material, because these metal materials are alloyed with lithium and conductivity is ensured by carbonaceous materials and graphite even when they are miniaturized. It is known to improve. For example, in Patent Document 1, the active material of the negative electrode includes fine particles having a carbonaceous material layer formed on the surface, and the fine particles are composed of at least one element selected from Mg, Al, Si, Ca, Sn, and Pb. Further, it is disclosed that the average particle diameter is 1 to 500 nm, and the atomic ratio of fine particles in the active material is 15% by weight or more.

Patent Document 2 discloses metal-carbon composite particles in which metal particles are embedded in a plurality of phases of carbon, and the carbon contains graphite and amorphous carbon. The metal particles include Mg, Al, It is described that any one of Si, Zn, Ge, Bi, In, Pd, and Pt is used, and the average particle size is preferably 0.1 to 20 μm. Patent Document 3 discloses a so-called core shell in which the negative electrode active material includes graphite core particles, a carbon coating (shell) that covers the graphite core particles, and metal particles that are dispersed and positioned inside the carbon coating. The graphite core particle has an average particle size of 1 to 20 μm, the carbon coating has a coating thickness of 1 to 4 μm, and the metals alloyed with lithium include Cr, Sn, Si, Al, Mn, Ni , Zn, Co, In, Cd, Bi, Pb, and V, and at least one substance selected from the group consisting of V and an average particle size of 0.01 to 1.0 μm is preferable.

Furthermore, Patent Document 4 discloses a mixing step of obtaining a mixture by mixing expanded graphite or flaky graphite having a BET specific surface area of 30 m ² / g or more and a battery active material that can be combined with lithium ions, and adding a spherical shape to the mixture. A composite active material for a lithium secondary battery having a spheroidizing step of manufacturing a substantially spherical composite active material for a lithium secondary battery containing a battery active material that can be combined with graphite and lithium ions A method is disclosed, and the battery active material that can be combined with lithium ions contains at least one element selected from Si, Sn, Al, Sb, and In, and the average particle diameter is preferably 1 μm or less. Yes.

By using these fine metal materials, there is less expansion per particle due to lithium insertion during charging, and the cycle life is improved by making it difficult to crack, but it has not yet reached the required characteristics, and further There was a need for improved cycle life.

In the method using the composite particles, the denser the composite particles are filled in the negative electrode thin film, the higher the energy density of the negative electrode and the better the performance as a battery. In addition, by uniformly and as isotropically filling the composite particles, lithium can be desorbed and inserted uniformly, local deterioration of the negative electrode can be avoided, and cycle life is improved. For example, Patent Document 5 discloses a negative electrode material for a lithium secondary battery including spherical graphite particles derived from scaly natural graphite particles, and describes that the circularity is preferably 0.85 or more. .

Japanese Laid-Open Patent Publication No. 10-3920 Japanese Unexamined Patent Publication No. 2000-272911 Japanese Unexamined Patent Publication No. 2010-129545 Japanese Patent No. 5227483 Japanese Unexamined Patent Publication No. 2012-221951

The present invention relates to a negative electrode active material for a lithium ion secondary battery comprising Si or Si alloy (hereinafter collectively referred to as “Si compound”) and a carbonaceous material or a carbonaceous material and graphite in combination. Another object of the present invention is to provide a negative electrode active material capable of providing a lithium ion secondary battery having a large energy density, cycle life or discharge capacity, and a long cycle life, and a method for producing the same.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the composite material is a negative electrode active material for a lithium ion secondary battery comprising a Si compound and a carbonaceous material or a carbonaceous material and graphite. The negative electrode active material (active material A) and the average particle size (D50) that give a lithium ion secondary battery having high energy density and cycle life by controlling the particle size and shape of the particles are 0.01 to 0.6 μm , D90 is 0.01 ~ 1.0 .mu.m, a Si compound and 10 to 80 wt% is a BET specific surface area of 40 ~ 300m ² / g as measured by the BET method, of 90 to 5 wt% of carbonaceous material Lithium ion secondary battery negative electrode active material containing graphite in an amount of 0 to 80% by weight and having an average circularity of 0.7 to 1.0 has a large discharge capacity and a long cycle life. It found that the negative electrode active material (active material B) is obtained to give on secondary battery, and completed the present invention.

That is, the present invention has the following gist.
(1) In a negative electrode active material for a lithium ion secondary battery comprising Si or an Si alloy and a carbonaceous material or a carbonaceous material and graphite, the negative electrode active material has an average particle diameter (D50) of 1 to 40 μm. And a negative active material for a lithium ion secondary battery, wherein the negative active material is a substantially spherical composite particle having an average circularity of 0.7 to 1.0.
(2) Composite particles containing 1 to 80% by weight of flat fine particles having an average particle diameter (D50) of 1 to 10 μm and a minor axis length of less than 1 μm measured by SEM image observation. The negative electrode active material for a lithium ion secondary battery as described in (1) above, wherein
(3) The average particle diameter (D50) of the Si or Si alloy is 0.01 to 5 μm, and the carbonaceous material covers at least the surface of the active material, (1) or (2), Negative electrode active material for lithium ion secondary battery.
(4) The average particle diameter (D50) of the Si or Si alloy is 0.01 to 1 μm, and the carbonaceous material covers at least the active material surface, as described in (1) or (2) above Negative electrode active material for lithium ion secondary battery.
(5) A structure in which the Si or Si alloy is sandwiched between carbonaceous materials and a thin graphite layer having a thickness of 0.2 μm or less, and the structure spreads in a laminated and / or network shape. Any one of (1) to (4) above, wherein the active material particles are curved in the vicinity of the surface of the active material particles to cover the active material particles, and the carbonaceous material covers the surface of the outermost layer. Negative electrode active material for lithium ion secondary battery.
(6) The graphite is composed of 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, by ICP emission spectroscopy. The purity obtained from the semi-quantitative value of impurities of Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U) is 99.9% by weight or more, or the impurity amount is 1000 ppm or less. IC) The lithium according to any one of the above (1) to (5), wherein the S amount by the measurement method is 0.3% by weight or less and / or the BET specific surface area is 40 m ² / g or less. Negative electrode active material for ion secondary battery.
(7) Any one of (1) to (6) above, wherein the content of the Si or Si alloy is 10 to 80% by weight and the content of the carbonaceous material is 90 to 20% by weight. The negative electrode active material for lithium ion secondary batteries as described in 2.
(8) The content of the Si or Si alloy is 10 to 60% by weight, the content of the carbonaceous material is 5 to 40% by weight, and the content of the graphite is 20 to 80% by weight. The negative electrode active material for a lithium ion secondary battery according to any one of (1) to (6).
(9) The negative electrode active material for a lithium ion secondary battery as described in any one of (1) to (8) above, wherein the BET specific surface area is 0.5 to 80 m ² / g.
(10) a step of mixing Si or an Si alloy, a carbon precursor, and graphite, a step of granulating and compacting, a step of pulverizing and spheroidizing the mixture to form substantially spherical composite particles, The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of the above (1) to (9), comprising a step of firing the composite particles in an inert gas atmosphere.
(11) The method for producing a negative electrode active material for a lithium ion secondary battery as described in (10) above, wherein, in the spheronization treatment, the pulverized particles are rebound to form substantially spherical composite particles. .
(12) a step of mixing Si or an Si alloy, a carbon precursor, and graphite, a step of granulating and compacting, a step of pulverizing and spheronizing the mixture to form substantially spherical composite particles, Any one of the above (1) to (9), comprising a step of firing the composite particles in an inert gas atmosphere, and a step of air-classifying the pulverized and spherically processed powder or the fired powder. The manufacturing method of the negative electrode active material for lithium ion secondary batteries as described in any one of.
(13) In the spheronization treatment, the pulverized particles are rebound to form substantially spherical composite particles and flat fine particles, or the substantially spherical composite particles and flat fine particles are mixed, stirred, and classified. The manufacturing method of the negative electrode active material for lithium ion secondary batteries as described in said (12).
(14) The negative electrode for a lithium ion secondary battery as described in any one of (10) to (13) above, wherein the carbon precursor is a carbon-based compound having a weight average molecular weight (Mw) of 1000 or less. A method for producing an active material.
(15) The method for producing a negative electrode active material for a lithium ion secondary battery as described in any one of (10) to (14) above, wherein the graphite is expanded graphite or flaky graphite.
(16) The lithium ion secondary battery as described in any one of (10) to (15) above, wherein the temperature of the step of firing the composite particles in an inert atmosphere is 600 to 1200 ° C. For producing a negative electrode active material.
(17) Si or Si alloy having an average particle size (D50) of 0.01 to 0.6 μm, D90 of 0.01 to 1.0 μm, and a BET specific surface area of 40 to 300 m ² / g by BET method. Lithium ion secondary battery comprising 10 to 80% by weight, carbonaceous material 90 to 5% by weight, graphite 0 to 80% by weight, and having a substantially spherical shape with an average circularity of 0.7 to 1.0 Negative electrode active material.
(18) The Si or Si alloy has an average particle size (D50) of 0.01 to 0.3 μm, a D90 of 0.01 to 0.5 μm, and a BET specific surface area of 70 to 300 m ² / g by the BET method. The negative electrode active material for a lithium ion secondary battery as described in (17) above, wherein
(19) The above (17), wherein the average particle diameter (D50) is 1 to 40 μm, the BET specific surface area by the BET method is 5 to 120 m ² / g, and the active material surface is covered with a carbonaceous material. Or the negative electrode active material for lithium ion secondary batteries as described in (18).
(20) The graphite is composed of 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, ICP emission spectroscopic analysis. Ion chromatography (IC) by oxygen flask combustion method with purity of 99.9 wt% or more (1000 ppm or less) determined from impurity semi-quantitative values of Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U) The lithium ion secondary according to any one of (17) to (19) above, wherein the S amount by the measurement method is 0.3% by weight or less and / or the BET specific surface area is 40 m ² / g or less. Negative electrode active material for batteries.
(21) Si or Si alloy is sandwiched between carbonaceous materials and a graphite thin layer having a thickness of 0.2 μm or less, and the structure spreads in a laminated and / or network form. The negative electrode active material for a lithium ion secondary battery according to any one of the above (17) to (20), wherein the active material particle is curved in the vicinity of the surface of the active material particle to cover the active material particle.
(22) Si or an Si alloy having an average particle diameter (D50) of 0.01 to 0.6 μm, D90 of 0.01 to 1.0 μm, and a BET specific surface area of 40 to 300 m ² / g by the BET method; A step of mixing a carbon precursor, and optionally graphite, a step of granulating and compacting, a step of forming composite particles by pulverization and spheronization, and the composite particles in an inert gas atmosphere The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of the above (17) to (21), comprising a step of firing.
(23) The method for producing a negative electrode active material for a lithium ion secondary battery as described in (22) above, wherein the temperature of the step of firing the composite particles in an inert atmosphere is 600 to 1000 ° C.

According to the present invention, by making the composite particles into substantially spherical particles having a high bulk density, a negative electrode active material suitable for forming a negative electrode having a high energy density and excellent cycle characteristics can be obtained. Moreover, excellent cycle characteristics and high initial efficiency can be obtained by suppressing the reaction between the electrolytic solution and silicon by reducing the expansion volume per particle by the fine silicon particles and by combining the carbonaceous material.

Further, according to the present invention, silicon particles having an average particle diameter (D50) of 0.01 to 0.6 μm, D90 of 0.01 to 1.0 μm, and a BET specific surface area of 40 to 300 m ² / g by the BET method. By reducing the expansion volume per unit, excellent cycle characteristics by suppressing the reaction between the electrolytic solution and silicon and high initial efficiency can be obtained. In addition, a high bulk density negative electrode active material suitable for forming a high density negative electrode can be obtained by the production method of the present invention.

2 is a secondary electron image of the negative electrode active material particles obtained in Example 1 by SEM. 2 is a secondary electron image obtained by FE-SEM of a cross section of a negative electrode active material particle obtained in Example 1. FIG. 2 is a secondary electron image of the negative electrode active material particles obtained in Example 2 by SEM. 3 is a secondary electron image by SEM of negative electrode active material particles obtained in Example 3. FIG. 4 is a secondary electron image obtained by FE-SEM of a cross section of a negative electrode active material particle obtained in Example 3. FIG. 4 is a secondary electron image of the negative electrode active material particles obtained in Example 4 by SEM. 7 is a secondary electron image of the negative electrode active material particles obtained in Example 5 by SEM. 4 is a secondary electron image of the negative electrode active material particles obtained in Example 6 by SEM. 2 is a secondary electron image of the negative electrode active material particles obtained in Comparative Example 1 by SEM. 3 is a secondary electron image of the negative electrode active material particles obtained in Comparative Example 2 by SEM. It is a secondary electron image by SEM of the negative electrode active material particles obtained in Comparative Example 3. 7 is a secondary electron image obtained by FE-SEM of a cross section of a negative electrode active material particle obtained in Example 7. 7 is a secondary electron image of the negative electrode active material particles obtained in Example 8 by SEM.

First, the active material A of the present invention will be described in detail.

The active material A is a negative electrode active material for a lithium ion secondary battery comprising Si or a Si alloy, and a carbonaceous material or a carbonaceous material and graphite, and the average particle size (D50) of the negative electrode active material is 1 to 40 μm. And a negative active material for a lithium ion secondary battery, which is a substantially spherical composite particle having an average circularity of 0.7 to 1.0.

Si in the active material A is a general grade metal silicon having a purity of about 98% by weight, a chemical grade metal silicon having a purity of 2 to 4N, a polysilicon having a purity higher than 4N purified by chlorination and distillation, a single crystal Ultrahigh-purity single crystal silicon that has undergone a deposition process by a growth method, or those that are doped with elements of

Group

13 or 15 of the periodic table to be p-type or n-type, wafers generated in the semiconductor manufacturing process, There is no particular limitation as long as it has a purity equal to or higher than that of general-purpose grade metal silicon, such as cutting scraps and discarded wafers that have become defective in the process.

The Si alloy referred to as the active material A is an alloy containing Si as a main component. In the Si alloy, the element contained other than Si is preferably one or more of elements of Groups 2 to 15 of the periodic table, and the selection and / or addition amount of the element having a melting point of the phase contained in the alloy of 900 ° C. or more. Is preferred.

In the active material A, the average particle diameter (D50) of the Si compound is preferably 0.01 to 5 μm, more preferably 0.01 to 1 μm, and particularly preferably 0.05 to 0.6 μm. If it is smaller than 0.01 μm, the capacity and initial efficiency due to surface oxidation are drastically reduced, and if it is larger than 5 μm, cracking is severely caused by expansion due to lithium insertion, and cycle deterioration tends to be severe. The average particle size (D50) is a volume average particle size measured with a laser particle size distribution meter.

The content of the Si compound is preferably 10 to 80% by weight, particularly preferably 15 to 50% by weight. When the content of the Si compound is less than 10% by weight, a sufficiently large capacity cannot be obtained as compared with the conventional graphite, and when it is more than 80% by weight, the cycle deterioration tends to become severe.

The carbonaceous material referred to as the active material A is an amorphous or microcrystalline carbon material, easily graphitized carbon (soft carbon) that is graphitized by a heat treatment exceeding 2000 ° C., and hardly graphitized carbon (hard). Carbon).

When the active material A contains a carbonaceous material, the content of the carbonaceous material is preferably 90 to 20% by weight, particularly preferably 40 to 20% by weight. When the content of carbonaceous material is less than 20% by weight, the carbonaceous material cannot cover the Si compound, and the conductive path becomes insufficient, and capacity deterioration is likely to occur severely. I can't get it.

Graphite as the active material A is a crystal whose graphene layer is parallel to the c-axis, natural graphite obtained by refining ore, artificial graphite obtained by graphitizing the pitch of oil or coal, etc. , Oval or spherical, cylindrical or fiber. In addition, these graphites are subjected to acid treatment, oxidation treatment, and then expanded by heat treatment. Part of the graphite layer is exfoliated to form an accordion shape, or a pulverized product of expanded graphite, or an ultrasonic layer or the like. Exfoliated graphene or the like can also be used. The particle size of the graphite contained in the active material A of the present invention is not particularly limited as long as it is smaller than the size of the negative electrode active material particles, and the thickness of the graphite particles is 1/5 or less of the average particle diameter (D50) of the active material. It is preferable. Addition of graphite increases the conductivity and strength of the active material particles, and improves charge / discharge rate characteristics and cycle characteristics. The (002) plane spacing d002 measured by X-ray diffraction of graphite particles is preferably 0.338 nm or less, which means highly graphitized graphite. When d002 exceeds this value, the effect of improving conductivity by graphite becomes small.

In addition, graphite as active material A has a purity of 99.9% by weight or more, or an impurity amount of 1000 ppm or less, an S amount of 0.3% by weight or less, and / or a BET specific surface area of 40 m ² / g or less. It is preferable. If the purity is less than 99.9% by weight or the amount of impurities is more than 1000 ppm, the irreversible capacity due to the formation of SEI derived from impurities increases, so the initial charge / discharge efficiency, which is the discharge capacity with respect to the initial charge capacity, decreases. Tend. Moreover, since the irreversible capacity | capacitance similarly becomes high when S amount becomes higher than 0.3 weight%, initial charge / discharge efficiency becomes low. More preferably, the amount of S is preferably 0.1% by weight or less. If the BET specific surface area of graphite is higher than 40 m ² / g, the area that reacts with the electrolytic solution increases, so the initial charge / discharge efficiency is likely to be low.

Impurities were analyzed by ICP emission spectroscopic analysis with the following 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co). , Cu, Mo, Pb, Sb, Se, Th, Tl, U). In addition, the amount of S is measured by ion chromatography (IC) measurement after filtering and filtering with an oxygen flask combustion method.

In the active material A, when carbonaceous material and graphite are contained, the content of each is preferably 5 to 40% by weight and 20 to 80% by weight, particularly 8 to 30% by weight and 40 to 70% by weight. preferable. When the content of the carbonaceous material is less than 5% by weight, the carbonaceous material cannot cover the Si compound and graphite, adhesion between the Si compound and graphite becomes insufficient, and formation of active material particles tends to be difficult. Moreover, when larger than 40 weight%, the effect of the graphite whose electroconductivity is higher than a carbonaceous material is not fully drawn out. On the other hand, when the graphite content is less than 20% by weight, the effect of graphite having higher conductivity than the carbonaceous material is not sufficient, and when it is more than 80% by weight, a sufficiently large capacity can be obtained compared to conventional graphite. Absent.

The active material A is a substantially spherical composite particle having an average particle diameter (D50) of 1 to 40 μm, preferably 2 to 30 μm, particularly preferably 2 to 20 μm. When the average particle diameter (D50) is less than 1 μm, it becomes bulky and it becomes difficult to produce a high-density electrode, and when it exceeds 40 μm, the unevenness of the applied electrode becomes intense and it becomes difficult to produce a uniform electrode. Moreover, it is preferable that the average particle diameter of the Si compound is 1/5 or less of the average particle diameter of the negative electrode active material, and the carbonaceous material covers at least the active material surface.

The substantially spherical composite particles are those in which the corners of particles generated by pulverization, etc. are removed, spherical or spheroid shapes, discs or oval shapes with rounded corners, or deformed Including those having rounded corners, the circularity is 0.7 to 1.0. The circularity was measured by image analysis of a particle image taken with a scanning electron microscope. That is, when the projected area (A) and the perimeter (PM) of a particle are measured from a photograph and the area of a perfect circle having the same perimeter (PM) is (B), the circularity is defined as A / B. The When the radius of the true circle is r, PM = 2πr and B = πr ² are established, and from this, the circularity A / B = A × 4π / (PM) ² is calculated. Thereby, the average value of the substantially spherical particles excluding the flat fine particles having a minor axis length of less than 1 μm among the arbitrary 100 or more composite particles was defined as the average circularity of the composite particles. In addition, flat fine particles are those in which the corners of particles generated by pulverization, etc. are rounded, those having a round shape with a disc or oval shape, or those in which they are deformed and rounded in corners, etc. The short axis length of the particle image taken with a scanning electron microscope was less than 1 μm. The content of the flat fine particles was defined as the total projected area of the flat fine particles divided by the total projected area of all the particles. When the shape is rounded, the bulk density of the composite particles is increased, and the packing density when the negative electrode is formed is increased. Further, since the carbonaceous material covers at least the active material surface, lithium ions solvated in the electrolytic solution during the charge / discharge process are separated from the solvent on the surface of the carbonaceous material, and only lithium ions are Si compounds. Since it reacts with graphite and / or graphite, it becomes difficult to produce a decomposition product of the solvent, and the efficiency of charging and discharging is increased.

When the average circularity of the substantially spherical composite particles decreases, the bulk density decreases, the packing density when the negative electrode is made, and the contact points and regions between the composite particles decrease. Due to the volume expansion / contraction, the probability that the electrical continuity is interrupted increases, and the cycle capacity retention rate tends to decrease. When the composite particles are composed of substantially spherical particles and flat composite fine particles, the flat fine particles fill the gaps between the substantially spherical particles as the content of the flat fine particles increases. Electrical conduction is maintained even in volume expansion and contraction. When the average particle diameter (D50) is 1 to 10 μm and the flat particles having a minor axis length of less than 1 μm measured by SEM image observation are contained in an amount of 1 wt% to 80 wt%, the negative electrode active material has an excellent cycle capacity. Indicates the maintenance rate. When the content of the flat fine particles is less than 1% by weight and / or when the circularity of the substantially spherical particles is less than 0.7, the effect of improving the cycle capacity retention rate is not recognized.

In the active material A, the Si compound is sandwiched between thin carbon layers having a thickness of 0.2 μm or less together with the carbonaceous material, and the structure spreads in a laminated and / or network shape. It is preferable that the thin layer bends in the vicinity of the surface of the active material particles to cover the active material particles, and the carbonaceous material covers the surface of the outermost layer.

The thin graphite layer referred to as the active material A is an expanded accordion or expanded graphite in which the graphite described above is subjected to acid treatment and oxidation treatment and then expanded by heat treatment, and part of the graphite layer is peeled off to form an accordion shape. Graphite pulverized material, graphene delaminated by ultrasonic waves, or the like, or composed of one layer of graphene (thickness 0.0003 μm) to several hundred layers (thickness 0.2 μm) generated by receiving compression force It is a graphite thin layer. When the thickness of the graphite thin layer is thinner, the Si compound sandwiched between the graphite thin layers and the carbonaceous material layer become thinner, and the transmission of electrons to the Si compound is improved, and the thickness exceeds 0.2 μm. The electron transfer effect of the graphite thin layer is diminished. When the graphite thin layer is linear when viewed in cross section, its length is preferably at least half the size of the negative electrode active material particles for electron transfer, and more preferably about the same as the size of the negative electrode active material particles. When the graphite thin layer is network-like, it is preferable for electron transfer that the graphite thin layer network is connected to more than half of the size of the negative electrode active material particles, and it may be about the same size as the negative electrode active material particles. Further preferred.

In the active material A, it is preferable that the graphite thin layer bends near the surface of the active material particles to cover the active material particles. By adopting such a shape, there is a risk that the electrolyte enters from the end face of the graphite thin layer, the Si compound or the end face of the graphite thin layer is in direct contact with the electrolyte, and a reaction product is formed during charge and discharge, resulting in reduced efficiency. Is reduced.

In the active material A, the Si compound content is preferably 10 to 80% by weight, and the carbonaceous material content is preferably 90 to 20% by weight.

In the active material A, the Si compound content is preferably 10 to 60% by weight, the carbonaceous material content is 5 to 40% by weight, and the graphite content is preferably 20 to 80% by weight. .

In the active material A of the present invention, the BET specific surface area is preferably 0.5 to 80 m ² / g.

In the active material A of the present invention, the carbonaceous material is a carbon material formed by carbonizing a carbon precursor described later inside the negative electrode active material. Therefore, the lithium ions solvated in the electrolyte during charging and discharging have a structure that is difficult to directly contact the Si compound and / or graphite, and the BET specific surface area is 0.5 to 60 m ² / g. Moreover, since the reaction between the carbonaceous material and the electrolytic solution on the surface is kept small, the charge / discharge efficiency is further increased.

Next, a method for producing the active material A of the present invention will be described.

The manufacturing method of the active material A of the present invention includes a step of mixing a Si compound, a carbon precursor, and graphite as necessary, a step of granulating and densifying, a step of pulverizing to form composite particles, And a step of firing the composite particles in an inert atmosphere.

The raw material Si compound is preferably a powder having an average particle diameter (D50) of 0.01 to 5 μm. In order to obtain a Si compound having a predetermined particle diameter, the above-described Si compound raw material (ingot, wafer, powder, etc.) is pulverized by a pulverizer, and in some cases, a classifier is used. In the case of a lump such as an ingot or a wafer, it can be first pulverized using a coarse pulverizer such as a jaw crusher. After that, for example, a ball or bead is used to move the grinding medium, and the impact force, frictional force, or compression force of the kinetic energy is used to grind the material to be crushed, the media agitation mill, or the compression force of the roller. Rotation of a roller mill that pulverizes, a jet mill that collides crushed objects with the lining material or collides with each other at high speed, and pulverizes by the impact force of the impact, and a rotor with a fixed hammer, blade, pin, etc. It can be finely pulverized by using a hammer mill, pin mill, disk mill that pulverizes the material to be crushed using the impact force of the colloid, a colloid mill that uses shear force, or a high-pressure wet-on-front collision disperser "Ultimizer". .

Grinding can be used for both wet and dry processes. For further fine pulverization, very fine particles can be obtained, for example, by using a wet bead mill and gradually reducing the diameter of the beads. In order to adjust the particle size distribution after pulverization, dry classification, wet classification, or sieving classification can be used. In the dry classification, the process of dispersion, separation (separation of fine particles and coarse particles), collection (separation of solid and gas), and discharge are performed sequentially or simultaneously, mainly using air flow. Pre-classification (adjustment of moisture, dispersibility, humidity, etc.) before classification, or the moisture in the airflow used so that the classification efficiency is not lowered due to the influence of shape, air flow disturbance, velocity distribution, static electricity, etc. Adjust the oxygen concentration. In a dry type in which a classifier is integrated, pulverization and classification are performed at a time, and a desired particle size distribution can be obtained.

As another method for obtaining a Si compound having a predetermined particle size, a method in which the Si compound is heated and evaporated by plasma or laser and solidified in an inert gas, or a CVD or plasma CVD using a gas raw material is used. These methods are suitable for obtaining ultrafine particles of 0.1 μm or less.

The carbon precursor of the raw material is not particularly limited as long as it is a carbon-based compound mainly composed of carbon and becomes a carbonaceous material by heat treatment in an inert gas atmosphere. Petroleum pitch, coal pitch, synthetic pitch , Tars, cellulose, sucrose, polyvinyl chloride, polyvinyl alcohol, phenol resin, furan resin, furfuryl alcohol, polystyrene, epoxy resin, polyacrylonitrile, melamine resin, acrylic resin, polyamideimide resin, polyamide resin, polyimide resin, etc. Can be used. Furthermore, in the step of forming composite particles described later, it is preferable to use a carbon precursor having a strong binding force in that the pulverized particles can be rebound to form substantially spherical composite particles. In particular, when the carbon precursor is a carbon-based compound and the weight average molecular weight (Mw) is 1000 or less, a strong binding force is expressed, which is preferable.

As the raw material graphite, natural graphite, artificial graphite obtained by graphitizing the pitch of petroleum or coal, etc. can be used, and scale-like, oval or spherical, cylindrical or fiber-like are used. In addition, these graphites are subjected to acid treatment, oxidation treatment, and then expanded by heat treatment, and a portion of the graphite layer is exfoliated to form an accordion shape, or a pulverized product of expanded graphite, or an ultrasonic wave, etc. Exfoliated graphene or the like can also be used. Expanded graphite or a pulverized product of expanded graphite is superior in flexibility to other graphites, and in the process of forming composite particles, which will be described later, the pulverized particles can be rebound to easily form substantially spherical composite particles. Can be formed. In view of the above, it is preferable to use expanded graphite or a pulverized product of expanded graphite. The raw material graphite is preliminarily adjusted to a size that can be used in the mixing process, and the particle size before mixing is 1 to 100 μm for natural graphite or artificial graphite, expanded graphite or a pulverized expanded graphite, and 5 μm to 5 mm for graphene. Degree.

Mixing with these Si compounds, carbon precursors, and, if necessary, graphite, when the carbon precursors are softened or liquefied by heating, the Si compounds, carbon precursors, and further necessary under heating Accordingly, it can be performed by kneading graphite. When the carbon precursor is dissolved in a solvent, the Si compound, the carbon precursor, and, if necessary, graphite are added to the solvent, and the Si compound and the carbon precursor are dissolved in the solution in which the carbon precursor is dissolved. Body, and if necessary, graphite can be dispersed and mixed, and then the solvent can be removed. The solvent to be used can be used without particular limitation as long as it can dissolve the carbon precursor. For example, when pitch or tar is used as the carbon precursor, quinoline, pyridine, toluene, benzene, tetrahydrofuran, creosote oil or the like can be used, and when polyvinyl chloride is used, tetrahydrofuran, cyclohexanone, nitrobenzene or the like can be used. When phenol resin or furan resin is used, ethanol, methanol or the like can be used.

As a mixing method, when the carbon precursor is heat-softened, a kneader (kneader) can be used. In the case of using a solvent, in addition to the above-described kneader, a Nauter mixer, a Roedige mixer, a Henschel mixer, a high speed mixer, a homomixer, or the like can be used. Further, the jacket is heated with these apparatuses, and then the solvent is removed with a vibration dryer, a paddle dryer or the like.

With these devices, the carbon precursor is solidified, or stirring in the process of solvent removal is continued for a certain amount of time, so that the Si compound, the carbon precursor, and, if necessary, the mixture with graphite are granulated and consolidated. The Further, the carbon precursor is solidified or the mixture after removing the solvent is compressed by a compressor such as a roller compactor and coarsely pulverized by a crusher, whereby granulation and consolidation can be achieved. The size of the granulated / consolidated product is preferably 0.1 to 5 mm in view of ease of handling in the subsequent pulverization step.

The granulated / consolidated material is pulverized by ball mill, medium agitation mill, roller mill for pulverizing using the compressive force of the roller, or lining material to be crushed at high speed. A jet mill that collides with each other or collides with each other and crushes by the impact force of the impact, a hammer mill that crushes the material to be crushed using the impact force of the rotation of a rotor with a fixed hammer, blade, pin, etc. A dry pulverization method such as a pin mill or a disk mill is preferred. In order to adjust the particle size distribution after pulverization, dry classification such as air classification and sieving is used. In the type in which the pulverizer and the classifier are integrated, pulverization and classification are performed at a time, and a desired particle size distribution can be obtained.

The composite particles obtained by pulverization are fired in an argon gas or nitrogen gas stream or in an inert atmosphere such as a vacuum. The firing temperature is preferably 600 to 1200 ° C. When the firing temperature is less than 600 ° C., the irreversible capacity of the amorphous carbon derived from the carbon precursor is large, and the cycle characteristics are poor, so that the battery characteristics tend to deteriorate. On the other hand, when the firing temperature exceeds 1200 ° C., there is a strong possibility that a reaction between the Si compound and the amorphous carbon or graphite derived from the carbon precursor occurs, and the discharge capacity tends to decrease.

The production method of the active material A of the present invention includes a step of mixing and dispersing an Si compound, a carbon precursor, and, if necessary, a graphite in a solvent in which the carbon precursor is dissolved, a step of granulating and densifying, It is preferable to include a step of forming composite particles having a round shape by pulverization and spheronization treatment, and a step of firing the composite particles in an inert atmosphere.

As a method of pulverizing the granulated / consolidated product and subjecting it to spheronization, it is pulverized by the above-mentioned pulverization method to adjust the particle size, and then passed through a dedicated spheronization device, and the above-mentioned jet mill or rotor rotation. There is a method of spheroidizing by repeating the method of pulverizing the material to be crushed by using the impact force of or by extending the processing time. Specialized spheroidizing devices include Hosokawa Micron's Faculty (registered trademark), Nobilta (registered trademark), Mechano-Fusion (registered trademark), Nippon Coke Industries' COMPOSI, Nara Machinery Co., Ltd. hybridization system, Earth Technica's Examples include kryptron orb and kryptron eddy.

In addition, the method for producing the active material A of the present invention comprises a step of mixing and dispersing a Si compound, a carbon precursor, expanded graphite or flaky graphite in a solvent in which the carbon precursor is dissolved, and granulating and densifying the mixture. It is preferable to include a step, a step of pulverizing and spheroidizing to form substantially spherical composite particles, and a step of firing the composite particles in an inert atmosphere.

Expanded graphite and flaky graphite are made from acid-treated graphite obtained by acid-treating and oxidizing natural graphite and artificial graphite. Expanded graphite is an acid-treated graphite that is expanded by heat treatment, and part of the graphite layer is peeled off to form an accordion. In addition, exfoliated graphite is pulverized, or graphene delaminated with ultrasonic waves or the like is flaky graphite. In expanded graphite, it can be expanded greatly by sufficiently performing acid treatment and increasing the temperature gradient of heat treatment, and the thickness of the graphite thin layer of the negative electrode active material obtained by sufficiently mixing and dispersing can be increased. Since it can be made thin, good electrical conductivity and cycle characteristics can be obtained.

The active material A of the present invention thus obtained can be used as a negative electrode material for a lithium secondary battery.

The active material A of the present invention is, for example, kneaded with an organic binder, a conductive additive and a solvent, and formed into a sheet shape, a pellet shape or the like, or applied to a current collector, and the current collector The negative electrode for a lithium secondary battery is integrated with the body.

As the organic binder, for example, polyethylene, polypropylene, ethylene propylene polymer, butadiene rubber, styrene butadiene rubber, butyl rubber, and a polymer compound having high ion conductivity can be used. Polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide and the like can be used as the polymer compound having a high ionic conductivity. The content of the organic binder is preferably 3 to 20% by weight based on the whole negative electrode material. In addition to the organic binder, carboxymethyl cellulose, polysodium acrylate, other acrylic polymers, or fatty acid esters may be added as a viscosity modifier.

The type of the conductive auxiliary agent is not particularly limited, and may be any electron conductive material that does not cause decomposition or alteration in the constructed battery. Specifically, Al, Ti, Fe, Ni, Cu, Zn, Ag , Sn, Si and other metal powders and metal fibers, natural graphite, artificial graphite, various coke powders, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, various resin fired bodies, etc. Graphite or the like can be used. The content of the conductive assistant is 0 to 20% by weight, more preferably 1 to 10% by weight, based on the whole negative electrode material. When the amount of the conductive auxiliary agent is small, the negative electrode material may have poor conductivity, and the initial resistance tends to be high. On the other hand, an increase in the amount of conductive aid may lead to a decrease in battery capacity.

The solvent is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone, dimethylformamide, isopropanol, pure water, and the amount thereof is not particularly limited. As the current collector, for example, a foil such as nickel or copper, a mesh, or the like can be used. The integration can be performed by a molding method such as a roll or a press.

The negative electrode thus obtained has a cycle characteristic compared to a lithium secondary battery using conventional silicon as a negative electrode material by placing the positive electrode opposite to each other with a separator interposed therebetween and injecting an electrolytic solution. In addition, a lithium secondary battery having excellent characteristics such as high capacity and high initial efficiency can be manufactured.

The material used for the positive electrode, for example _{_{_{LiNiO 2, LiCoO 2, LiMn 2}}} O 4, LiNi x Mn y Co 1-x-y O 2, LiFePO 4, Li 0.5 Ni 0.5 Mn 1.5 O 4 Li ₂ MnO ₃ —LiMO ₂ (M═Co, Ni, Mn) or the like can be used alone or in combination.

As an electrolytic solution, a lithium salt such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF _{3 is used as} a non-aqueous solvent such as ethylene carbonate, diethyl carbonate, dimethoxyethane, dimethyl carbonate, tetrahydrofuran, and propylene carbonate. A so-called dissolved organic electrolyte solution can be used. Furthermore, ionic liquids using imidazolium, ammonium, and pyridinium type cations can be used. The counter anion is not particularly limited, and examples thereof include BF ₄ ⁻ , PF ₆ ⁻ , (CF ₃ SO ₂ ) ₂ N ^{− and the} like. The ionic liquid can be used by mixing with the organic electrolyte solvent described above. An SEI (solid electrolyte interface layer) forming agent such as vinylene carbonate or fluoroethylene carbonate can also be added to the electrolytic solution.

In addition, a solid electrolyte obtained by mixing the above salts with polyethylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, or the like, derivatives, mixtures, composites, or the like can also be used. In this case, the solid electrolyte can also serve as a separator, and the separator becomes unnecessary. As the separator, for example, a nonwoven fabric mainly composed of polyolefin such as polyethylene or polypropylene, cloth, microporous film, or a combination thereof is used. can do.

Next, the active material B of the present invention will be described in detail.

The active material B has an average particle diameter (D50) of 0.01 to 0.6 μm, D90 of 0.01 to 1.0 μm, and a BET method BET specific surface area of 40 to 300 m ² / g Si or Si A negative active material for a lithium ion secondary battery comprising 10 to 80% by weight of an alloy, 90 to 5% by weight of a carbonaceous material, 0 to 80% by weight of graphite, and having an average sphericity of 0.7 to 1.0. It is a substance.

Si in the active material B is a general grade metal silicon having a purity of about 98% by weight, a chemical grade metal silicon having a purity of 2 to 4N, a polysilicon having a purity higher than 4N purified by chlorination and distillation, a single crystal Ultrahigh-purity single crystal silicon that has undergone a deposition process by a growth method, or those that are doped with elements of

Group

The Si alloy referred to as the active material B is an alloy containing Si as a main component. In the Si alloy, the element contained other than Si is preferably one or more of elements of Groups 2 to 15 of the periodic table, and the selection and / or addition amount of the element having a melting point of the phase contained in the alloy of 900 ° C. or more. Is preferred.

In the active material B of the present invention, the average particle diameter (D50) of the Si compound is 0.01 to 0.6 μm. If it is smaller than 0.01 μm, the capacity and initial efficiency due to surface oxidation are drastically reduced, and if it is larger than 0.6 μm, cracks are severely caused by expansion due to lithium insertion, and cycle deterioration tends to become severe. 0.3 μm. The average particle diameter (D50) is a volume average particle diameter measured by a laser diffraction method or a dynamic light scattering method.

D90 is 0.01 to 1.0 μm. If it is larger than 1.0 μm, it is preferable because large particles expand more greatly, not only the Si compound, but also cracks due to expansion stress occur between the Si compound and the carbonaceous material, and the cycle deterioration tends to become severe. Is 0.01 to 0.6 μm. D90 is a particle diameter corresponding to a cumulative value of 90% from the minimum particle diameter value measured by the laser diffraction method or the dynamic light scattering method.

Further, the BET specific surface area measured by the BET method is 40 to 300 m ² / g. When the BET specific surface area is less than 40 m ² / g, the particles are large, and cracks occur violently due to expansion due to lithium insertion, and cycle deterioration tends to become severe. When the BET specific surface area is greater than 300 m ² / g, the reactivity with the electrolyte increases. As cycle deterioration easily occurs and oxygen on the Si surface increases, the irreversible capacity increases and the initial charge / discharge efficiency decreases. Preferably, it is 70 to 300 m ² / g.

The content of the Si compound is 10 to 80% by weight, preferably 15 to 50% by weight. When the content of the Si compound is less than 10% by weight, a sufficiently large capacity cannot be obtained as compared with the conventional graphite, and when it is more than 80% by weight, the cycle deterioration tends to become severe.

The carbonaceous material referred to as the active material B is an amorphous or microcrystalline carbon material, and easily graphitized carbon (soft carbon) that is graphitized by heat treatment exceeding 2000 ° C. and hard-graphitizable carbon (hard) Carbon).

In the active material B of the present invention, the carbonaceous material content is 90 to 5% by weight, preferably 40 to 8% by weight. When the content of carbonaceous material is less than 5% by weight, the carbonaceous material cannot cover the Si compound, the conductive path becomes insufficient, and the capacity deterioration easily occurs. When the content is larger than 90% by weight, the capacity is sufficient. It is not obtained.

Graphite as the active material B is a crystal whose graphene layer is parallel to the c-axis, natural graphite obtained by refining ore, artificial graphite obtained by graphitizing the pitch of oil or coal, etc. , Oval or spherical, cylindrical or fiber. In addition, these graphites are subjected to acid treatment, oxidation treatment, and then expanded by heat treatment. Part of the graphite layer is exfoliated to form an accordion shape, or a pulverized product of expanded graphite, or an ultrasonic layer or the like. Exfoliated graphene or the like can also be used. The particle size of the graphite contained in the negative electrode active material of the active material B is not particularly limited as long as it is smaller than the size of the negative electrode active material particles, and the thickness of the graphite particles is 1/5 or less of the average particle diameter (D50) of the active material. Preferably there is. Addition of graphite increases the conductivity and strength of the active material particles, and improves charge / discharge rate characteristics and cycle characteristics. The (002) plane spacing d002 measured by X-ray diffraction of graphite particles is preferably 0.338 nm or less, which means highly graphitized graphite. When d002 exceeds this value, the effect of improving conductivity by graphite becomes small.

Further, the graphite referred to as the active material B has a purity of 99.9% by weight or more, or an impurity amount of 1000 ppm or less, an S amount of 0.3% by weight or less, and / or a BET specific surface area of 40 m ² / g or less. It is preferable. If the purity is less than 99.9% by weight or the amount of impurities is more than 1000 ppm, the irreversible capacity due to the formation of SEI derived from impurities increases, so the initial charge / discharge efficiency, which is the discharge capacity with respect to the initial charge capacity, decreases. Tend. Moreover, since the irreversible capacity | capacitance similarly becomes high when S amount becomes higher than 0.3 weight%, initial charge / discharge efficiency becomes low. More preferably, the amount of S is preferably 0.1% by weight or less. If the BET specific surface area of graphite is higher than 40 m ² / g, the area where it reacts with the electrolytic solution increases, so the initial charge / discharge efficiency decreases.

Impurity is measured by ICP emission spectroscopic analysis using the following 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd. , Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U). Further, the amount of S is measured by ion chromatography (IC) measurement after filtering and filtering by an oxygen flask combustion method.

When the carbonaceous material and graphite are contained in the active material B of the present invention, the content of each is preferably 5 to 40% by weight and 20 to 80% by weight, and 8 to 30% by weight and 40 to 70% by weight. The ratio is more preferable. When the content of the carbonaceous material is less than 5% by weight, the carbonaceous material cannot cover the Si compound and graphite, adhesion between the Si compound and graphite becomes insufficient, and formation of active material particles tends to be difficult. Moreover, when larger than 40 weight%, the effect of the graphite whose electroconductivity is higher than a carbonaceous material is not fully drawn out. On the other hand, when the graphite content is less than 20% by weight, the effect of graphite having higher conductivity than the carbonaceous material is not sufficient, and when it is more than 80% by weight, a sufficiently large capacity can be obtained compared to conventional graphite. Absent.

The active material B of the present invention is a substantially spherical composite particle having an average particle diameter (D50) of 1 to 40 μm, preferably 2 to 30 μm. When D50 is less than 1 μm, it becomes bulky and it is difficult to produce a high-density electrode, and when it exceeds 40 μm, the unevenness of the applied electrode becomes intense and it is difficult to produce a uniform electrode. Moreover, it is preferable that the average particle diameter of the Si compound is 1/5 or less of the average particle diameter of the negative electrode active material, and the carbonaceous material covers at least the active material surface.

The substantially spherical composite particles are those in which the corners of particles generated by pulverization, etc. are removed, spherical or spheroid shapes, discs or oval shapes with rounded corners, or deformed The roundness is 0.7 to 1.0, and preferably 0.7 to 0.8. The circularity was measured by image analysis of a particle image taken with a scanning electron microscope. That is, when the projected area (A) and the perimeter (PM) of a particle are measured from a photograph and the area of a perfect circle having the same perimeter (PM) is (B), the circularity is defined as A / B. The When the radius of the true circle is r, PM = 2πr and B = πr ² are established, and from this, the circularity A / B = A × 4π / (PM) ² is calculated. Thus, the sphericity of any 100 or more composite particles was obtained, and the average value was defined as the average circularity of the composite particles. At this time, the average value of the substantially spherical particles excluding the flat fine particles having a minor axis length of less than 1 μm can be used as the average circularity of the composite particles. When the shape is rounded, the bulk density of the composite particles is increased, and the packing density when the negative electrode is formed is increased. Further, since the carbonaceous material covers at least the active material surface, lithium ions solvated in the electrolytic solution during the charge / discharge process are separated from the solvent on the surface of the carbonaceous material, and only lithium ions are Si compounds. Since it reacts with graphite and / or graphite, it becomes difficult to produce a decomposition product of the solvent, and the efficiency of charging and discharging is increased.

In the active material B of the present invention, the Si compound is sandwiched between carbonaceous materials and a graphite thin layer having a thickness of 0.2 μm or less, and the structure spreads in a laminated and / or network form, It is preferable that the graphite thin layer bends in the vicinity of the surface of the active material particles and covers the active material particles.

The thin graphite layer referred to as the active material B is an expanded accordion or expanded graphite in which the above-mentioned graphite is subjected to acid treatment and oxidation treatment and then expanded by heat treatment, and a part of the graphite layer is peeled off to form an accordion shape. Graphite pulverized material, graphene delaminated by ultrasonic waves, or the like, or composed of one layer of graphene (thickness 0.0003 μm) to several hundred layers (thickness 0.2 μm) generated by receiving compression force It is a graphite thin layer. When the thickness of the graphite thin layer is thinner, the Si compound sandwiched between the graphite thin layers and the carbonaceous material layer become thinner, and the transmission of electrons to the Si compound is improved, and the thickness exceeds 0.2 μm. The electron transfer effect of the graphite thin layer is diminished. When the graphite thin layer is linear when viewed in cross section, its length is preferably at least half the size of the negative electrode active material particles for electron transfer, and more preferably about the same as the size of the negative electrode active material particles. When the graphite thin layer is network-like, it is preferable for electron transfer that the graphite thin layer network is connected to more than half of the size of the negative electrode active material particles, and it may be about the same size as the negative electrode active material particles. Further preferred.

In the active material B, it is preferable that the graphite thin layer bends near the surface of the active material particles to cover the active material particles. By adopting such a shape, there is a risk that the electrolyte enters from the end face of the graphite thin layer, the Si compound or the end face of the graphite thin layer is in direct contact with the electrolyte, and a reaction product is formed during charge and discharge, resulting in reduced efficiency. Is reduced.

In the active material B of the present invention, when no graphite is contained, the Si compound content is preferably 10 to 80% by weight, and the carbonaceous material content is preferably 90 to 20% by weight.

More preferably, the active material B of the present invention has a BET specific surface area of 5 to 120 m ² / g as measured by the BET method.

Next, a method for producing the active material B of the present invention will be described.

The manufacturing method of the active material B of the present invention includes a step of mixing a Si compound, a carbon precursor, and, if necessary, graphite, a step of granulating and densifying, a pulverization and spheronization treatment, and A step of forming, and a step of firing the composite particles in an inert atmosphere.

The Si compound as a raw material has an average particle diameter (D50) of 0.01 to 0.6 μm, D90 of 0.01 to 1.0 μm, and a BET specific surface area measured by the BET method of 40 to 300 m ² / g. Of powder. In order to obtain a Si compound having a predetermined particle diameter, the above-described Si compound raw material (ingot, wafer, powder, etc.) is pulverized by a pulverizer, and in some cases, a classifier is used. In the case of a lump such as an ingot or a wafer, it can be first pulverized using a coarse pulverizer such as a jaw crusher. After that, for example, a ball or bead is used to move the grinding medium, and the impact force, frictional force, or compression force of the kinetic energy is used to grind the material to be crushed, the media agitation mill, or the compression force of the roller. Rotation of a roller mill that pulverizes, a jet mill that collides crushed objects with the lining material or collides with each other at high speed, and pulverizes by the impact force of the impact, and a rotor with a fixed hammer, blade, pin, etc. After pulverizing using a hammer mill, pin mill, disk mill that pulverizes the material to be crushed using the impact force of, colloid mill using shear force or high-pressure wet opposed collision disperser `` Ultimizer '', etc., By performing pulverization, a Si compound having a predetermined particle size is obtained.

As a method of pulverization, for example, a wet bead mill is used, and very fine particles can be obtained by gradually reducing the diameter of the beads. Zirconia, which has high strength, is preferable as the medium used for the bead mill. The diameter of the beads is changed depending on the particle size of the Si compound to be crushed. For example, if the average particle size (D50) of the Si compound is 10 to 40 μm, 0.5 to 1 mm beads are used, and the average particle size of the Si compound is When the diameter (D50) is 0.5 to 10 μm, beads having a diameter of 0.1 to 0.5 mm are used, and when the average particle diameter (D50) of the Si compound is 0.1 to 0.5 μm, 0.03 to 0.003. It is preferred to use 1 mm beads. When using beads smaller than 0.1 mm, it is preferable to use a centrifugal separation system for separating the beads and the slurry.

When a dispersant is used at the time of pulverization, the dispersant is preferably an alcohol such as methanol, ethanol or isopropyl alcohol, or a hydrocarbon solvent such as hexane or toluene. Water is not suitable because of the intense oxidation of Si. Moreover, you may add the anionic, cationic, and nonionic dispersing agent for lowering | hanging a slurry viscosity as needed. The concentration of the slurry is not particularly limited, and is preferably 5 to 25% by weight, particularly preferably 5 to 20% by weight as a concentration for performing efficient pulverization, preventing aggregation during pulverization and lowering the slurry viscosity. %. If the concentration is lower than 5% by weight, the pulverization efficiency is lowered. If the concentration is higher than 25% by weight, the slurry viscosity increases, and pulverization may not be possible due to a decrease in pulverization efficiency or clogging.

In order to adjust the particle size distribution after pulverization, dry classification, wet classification or sieving classification can be used. In the dry classification, the process of dispersion, separation (separation of fine particles and coarse particles), collection (separation of solid and gas), and discharge are performed sequentially or simultaneously, mainly using air flow. Pre-classification (adjustment of moisture, dispersibility, humidity, etc.) before classification, or the moisture in the airflow used so that the classification efficiency is not lowered due to the influence of shape, air flow disturbance, velocity distribution, static electricity, etc. It is done by adjusting the oxygen concentration. In a dry type in which a classifier is integrated, pulverization and classification are performed at a time, and a desired particle size distribution can be obtained.

The carbon precursor of the raw material is not particularly limited as long as it is a polymer mainly composed of carbon and becomes a carbonaceous material by heat treatment in an inert gas atmosphere, petroleum pitch, coal pitch, synthetic pitch, Tar, cellulose, sucrose, polyvinyl chloride, polyvinyl alcohol, phenol resin, furan resin, furfuryl alcohol, polystyrene, epoxy resin, polyacrylonitrile, melamine resin, acrylic resin, polyamideimide resin, polyamide resin, polyimide resin, etc. it can.

As the raw material graphite, natural graphite, artificial graphite obtained by graphitizing the pitch of petroleum or coal, etc. can be used, and scale-like, oval or spherical, cylindrical or fiber-like are used. In addition, these graphites are subjected to acid treatment, oxidation treatment, and then expanded by heat treatment, and a portion of the graphite layer is exfoliated to form an accordion shape, or a pulverized product of expanded graphite, or an ultrasonic wave, etc. Exfoliated graphene or the like can also be used. The raw material graphite is preliminarily adjusted to a size that can be used in the mixing process, and the particle size before mixing is 1 to 100 μm for natural graphite or artificial graphite, expanded graphite or a pulverized expanded graphite, and 5 μm to 5 mm for graphene. Degree.

With these devices, the carbon precursor is solidified, or stirring in the process of solvent removal is continued for a certain amount of time, so that the Si compound, carbon precursor, lithium compound, and, if necessary, a mixture with graphite are granulated. Consolidated. Further, the carbon precursor is solidified or the mixture after removing the solvent is compressed by a compressor such as a roller compactor and coarsely pulverized by a crusher, whereby granulation and consolidation can be achieved. The size of the granulated / consolidated product is preferably 0.1 to 5 mm in view of ease of handling in the subsequent pulverization step.

The composite particles obtained by pulverization are fired in an argon gas or nitrogen gas stream or in an inert atmosphere such as a vacuum. The firing temperature is preferably 600 to 1000 ° C. When the firing temperature is less than 600 ° C., the irreversible capacity of the amorphous carbon derived from the carbon precursor is large, and the cycle characteristics are poor, so that the battery characteristics tend to deteriorate.

The production method of the active material B of the present invention includes a step of mixing and dispersing an Si compound, a carbon precursor, and, if necessary, a graphite in a solvent in which the carbon precursor is dissolved, and a step of granulating and densifying. It is preferable to include a step of forming composite particles having a round shape by pulverization and spheronization treatment, and a step of firing the composite particles in an inert atmosphere.

In addition, the method for producing the active material B of the present invention comprises a step of mixing and dispersing a Si compound, a carbon precursor, expanded graphite or flaky graphite in a solvent in which the carbon precursor is dissolved, and granulating and densifying the mixture. It is preferable to include a step, a step of forming a rounded composite particle by pulverization and spheronization, and a step of firing the composite particle in an inert atmosphere.

Expanded graphite and flaky graphite are made from acid-treated graphite obtained by acid-treating and oxidizing natural graphite and artificial graphite. Expanded graphite is an acid-treated graphite that is expanded by heat treatment, and part of the graphite layer is peeled off to form an accordion. In addition, exfoliated graphite is pulverized, or graphene delaminated with ultrasonic waves or the like is flaky graphite. In expanded graphite, it can be expanded greatly by sufficiently performing acid treatment and increasing the temperature gradient of the heat treatment, and the thickness of the graphite thin layer of the negative electrode active material obtained by sufficiently mixing and dispersing can be increased. Since it can be made thin, good electrical conductivity and cycle characteristics can be obtained.

The active material B of the present invention thus obtained can be used as a negative electrode material for a lithium secondary battery.

The active material B of the present invention may contain, for example, an organic binder and / or a conductive additive (negative electrode active material mixture), kneaded with the negative electrode active material mixture and a solvent, It shape | molds in shapes, such as a pellet form, or it apply | coats to a collector and is united with this collector and it is set as the negative electrode for lithium secondary batteries.

As the organic binder, for example, polyethylene, polypropylene, ethylene propylene polymer, butadiene rubber, styrene butadiene rubber, butyl rubber, and a polymer compound having high ion conductivity can be used. Polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide and the like can be used as the polymer compound having a high ionic conductivity. The content of the organic binder is preferably 3 to 20% by weight with respect to the negative electrode active material mixture. In addition to the organic binder, the active material B of the present invention may contain carboxymethyl cellulose, sodium polyacrylate, other acrylic polymer, fatty acid ester, or the like as a viscosity modifier.

The type of the conductive auxiliary agent is not particularly limited, and may be any electron conductive material that does not cause decomposition or alteration in the constructed battery. Specifically, Al, Ti, Fe, Ni, Cu, Zn, Ag , Sn, Si and other metal powders and metal fibers, natural graphite, artificial graphite, various coke powders, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, various resin fired bodies, etc. Graphite or the like can be used. The content of the conductive assistant is preferably 0 to 20% by weight, particularly 1 to 10% by weight, based on the negative electrode active material mixture. When the amount of the conductive assistant is small, the conductivity of the negative electrode may be poor and the initial resistance tends to be high. On the other hand, an increase in the amount of conductive aid may lead to a decrease in battery capacity.

The negative electrode thus obtained has a cycle characteristic compared to a lithium secondary battery using conventional silicon as a negative electrode material by placing the positive electrode opposite to each other through a separator and injecting an electrolytic solution. In addition, a lithium secondary battery having excellent characteristics such as high capacity and high initial efficiency can be manufactured.

The material used for the positive electrode, for example _{_{_{LiNiO 2, LiCoO 2, LiMn 2}}} O 4, LiNi x Mn y Co 1-x-y O 2, LiFePO 4, Li 0.5 Ni 0.5 Mn 1.5 O 4 Li ₂ MnO ₃ —LiMO ₂ (M═Co, Ni, Mn), Li foil, etc. can be used alone or in combination.

Hereinafter, the present invention will be specifically described by way of examples and comparative examples, but the present invention is not limited to these examples.

Example 1
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol at 25% by weight, and a finely pulverized wet bead mill using zirconia beads having a diameter of 0.3 mm was performed for 6 hours. An ultrafine Si slurry having a D50) of 0.3 μm and a dry BET specific surface area of 60 m ² / g was obtained.

Natural graphite having a particle diameter of 0.5 mm (width in the (200) plane direction) and a thickness of 0.02 mm is immersed in a solution obtained by adding 1 wt% sodium nitrate and 7 wt% potassium permanganate to concentrated sulfuric acid for 24 hours. Thereafter, it was washed with water and dried to obtain acid-treated graphite. This acid-treated graphite was placed in a vibrating powder feeder, placed on nitrogen gas at a flow rate of 10 L / min, passed through a mullite tube having a length of 1 m and an inner diameter of 11 mm heated to 850 ° C. with an electric heater, and released from the end face to the atmosphere. A gas such as sulfurous acid was exhausted at the top, and expanded graphite was collected at the bottom in a stainless steel container. The expanded graphite had a (200) plane width of 0.5 mm and maintained the original graphite value, but the thickness expanded to 4 mm and 200 times, the appearance was coiled, and the graphite layer was peeled off by SEM observation. The accordion was confirmed.

24 g of the ultrafine Si slurry, 12 g of the expanded graphite, 5 g of a resol type phenolic resin (weight average molecular weight (Mw) = 460), 1.6 L of ethanol were placed in a stirring vessel, and after ultrasonic treatment for 15 minutes The mixture was stirred with a homomixer for 30 minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 65 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a fume hood and dried for 2 hours while evacuating, passed through a mesh with a mesh opening of 2 mm, and further dried for 2 days to obtain 20 g of a mixed dried product (light bulk density 67 g / L).

The mixed dried product was passed through a three-roll mill twice and granulated and consolidated to a particle size of 2 mm and a light bulk density of 385 g / L.

Next, this granulated / consolidated product was put in a new power mill and ground with water at 21000 rpm for 900 seconds while being cooled with water, and at the same time, it was spheroidized to obtain a spheroidized powder having a light bulk density of 650 g / L. The obtained powder was put into an alumina boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace. Thereafter, a negative active material having an average particle diameter (D50) of 19 μm and a light bulk density of 761 g / L was obtained through a mesh having an opening of 45 μm.

FIG. 1 shows an SEM image of the obtained negative electrode active material particles. The negative electrode active material particle graphite thin layer (12) is curved and has a substantially spherical shape covering the active material particle, the average circularity is 0.74, and the content of flat fine particles is 0% by weight. there were.

FIG. 2 shows a secondary electron image by FE-SEM of a cross section obtained by cutting the obtained negative electrode active material particles with an ion beam. The negative electrode active material particles are substantially spherical, and the inside of the negative electrode active material particles is 0.05 to 0.2 μm long Si fine particles together with the carbonaceous material and a 0.02 to 0.2 μm thick graphite thin layer ( 11) (13) (gap is 0.05 to 1 μm), the structure sandwiched between the layers spreads and is laminated. The carbonaceous material was in close contact with and covered the Si fine particles. Further, near the surface of the active material particles, the graphite thin layer (12) was curved to cover the active material particles.

The BET specific surface area by the BET method using nitrogen gas was 50 m ² / g. In powder X-ray diffraction, a diffraction line corresponding to the (002) plane of graphite was observed, and d002 was 0.336 nm. In addition, a very broad diffraction line derived from amorphous carbonization of the carbonaceous material was observed in the vicinity thereof. A diffraction line corresponding to the (100) plane of Si was observed, and d002 was 0.314 nm.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.2% by weight (content in the total solid content, the same applies hereinafter), acetylene black 0.6% by weight as a conductive auxiliary, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.6 wt%, styrene butadiene rubber (SBR) 2.6 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 3.5 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 29 μm. A negative electrode for a secondary battery was obtained.

"Production of evaluation cells"
In the glove box, the evaluation cell was prepared by dipping the negative electrode, a 24 mmφ polypropylene separator, a 21 mmφ glass filter, a 18 mmφ 0.2 mm thick metal lithium and a stainless steel foil of the base material into the electrolyte solution in the glove box. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolytic solution used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1. The evaluation cell was further placed in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

"Evaluation conditions"
The evaluation cell was cycle tested in a constant temperature room at 25 ° C. Charging was performed after charging to 0.01 V at a constant current of 2.2 mA until the current value reached 0.2 mA at a constant voltage of 0.01 V. The discharge was performed at a constant current of 2.2 mA up to a voltage value of 1.5V. The initial discharge capacity and initial charge / discharge efficiency were the results of the initial charge / discharge test.

In addition, the cycle characteristics were evaluated as the capacity retention rate by comparing the discharge capacity after 30 charge / discharge tests under the charge / discharge conditions with the initial discharge capacity.

Example 2
36 g of ultrafine Si slurry produced in the same manner as in Example 1, 18 g of expanded graphite, 7.5 g of resol type phenol resin (weight average molecular weight (Mw) = 490), and 2.4 L of ethanol were placed in a stirring vessel. And sonication for 15 minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 50 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a fume hood and dried for 2 hours while evacuating, passed through a mesh with a mesh opening of 2 mm, and further dried for 2 days to obtain 32 g of a mixed dried product (light bulk density 66 g / L).

The mixed dried product was passed through a three-roll mill twice, and granulated and consolidated to a particle size of 2 mm and a light bulk density of 340 g / L.

Next, this granulated / consolidated product was put in a new power mill and pulverized at 21000 rpm for 900 seconds while being cooled with water. At the same time, it was spheroidized to obtain a spheroidized powder with a light bulk density of 490 g / L.

The obtained powder was put into an alumina boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace. Thereafter, a negative electrode active material having an average particle diameter (D50) of 9 μm and a light bulk density of 567 g / L was obtained through a mesh having an opening of 45 μm.

FIG. 3 shows an SEM image of the obtained negative electrode active material. The negative electrode active material particle graphite thin layer (12) is curved and has a substantially spherical shape covering the active material particle, the average circularity is 0.77, and the content of flat fine particles is 0% by weight. there were.

The BET specific surface area by the BET method using nitrogen gas was 47 m ² / g. In powder X-ray diffraction, a diffraction line corresponding to the (002) plane of graphite was observed, and d002 was 0.336 nm. In addition, a very broad diffraction line derived from amorphous carbonization of the carbonaceous material was observed in the vicinity thereof. A diffraction line corresponding to the (100) plane of Si was observed, and d002 was 0.314 nm.

A lithium ion secondary battery using the obtained negative electrode active material was produced as follows.

“Preparation of negative electrode for lithium ion secondary battery”
With respect to 90.9 wt% of the obtained negative electrode active material (content in the total solid content, the same applies hereinafter), acetylene black 0.4 wt% as a conductive auxiliary and polyvinylidene fluoride (PVDF) as a binder A negative electrode mixture-containing slurry was prepared by mixing 8.7% by weight of NMP.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 1.8 mg / cm ^2, and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 17 μm. A negative electrode for a secondary battery was obtained.

"Production of evaluation cells"
In the glove box, the evaluation cell was prepared by dipping the negative electrode, a 24 mmφ polypropylene separator, a 21 mmφ glass filter, 16 mmφ and 0.2 mm thick metal lithium, and a stainless steel foil of the base material into the electrolyte solution. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolytic solution used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1. The evaluation cell was further placed in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to the charge / discharge device.

"Evaluation conditions"
The evaluation cell was cycle tested in a constant temperature room at 25 ° C. Charging was performed after charging to 0.01 V with a constant current of 1.4 mA until the current value reached 0.2 mA with a constant voltage of 0.01 V. The discharge was performed at a constant current of 1.4 mA up to a voltage value of 1.5V. The initial discharge capacity and initial charge / discharge efficiency were the results of the initial charge / discharge test.

Example 3
Chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol in an amount of 21% by weight and subjected to fine grinding wet bead mill using zirconia beads having a diameter of 0.3 mm for 6 hours. D50) An ultrafine Si slurry having a diameter of 0.3 μm and a dry BET specific surface area of 100 m ² / g was obtained.

Acid-treated natural graphite having a particle diameter of 0.3 mm (width in the (200) plane direction) and a thickness of 10 μm was placed in a vibrating powder feeder, placed on nitrogen gas at a flow rate of 12 L / min, and heated to 850 ° C. with an electric heater. It was passed through a mullite tube having a length of 1 m and an inner diameter of 20 mm, discharged from the end face to the atmosphere, and a gas such as sulfurous acid was exhausted at the top and expanded graphite was collected at the bottom in a stainless steel container. The expanded graphite had a (200) plane width of 0.3 mm and maintained the original graphite value, but the thickness expanded 2.4 times to 2.4 mm, the appearance was coiled, and the graphite layer was observed by SEM observation. Was peeled off and confirmed to be in the form of an accordion.

95.7 g of the ultrafine Si slurry, 37.5 g of the expanded graphite, 23.5 g of a resol type phenolic resin (weight average molecular weight (Mw) = 370), and 5 L of ethanol were placed in a stirring vessel, and 60 minutes with a homomixer. Mix and stir for minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 60 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a fume hood and dried for 2 hours while being evacuated, passed through a mesh with a mesh opening of 2 mm, and further dried for 1 day to obtain 80 g of a mixed dried product (light bulk density 87 g / L).

The mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and consolidated to a light bulk density of 528 g / L.

Next, this granulated / consolidated product is placed in a new power mill, water-cooled and pulverized at 21000 rpm for 900 seconds. At the same time, it is spheroidized, and the resulting bulk density is 633 g / L. While flowing nitrogen gas in a furnace, firing was performed at a maximum temperature of 900 ° C. for 1 hour. Thereafter, a mesh having an opening of 45 μm was passed through to obtain composite particles having an average particle diameter (D50) of 17.5 μm and a light bulk density of 807 g / L.

The composite particles are put into an air classifier (ATP-20 manufactured by Hosokawa Micron), classified at a classifier rotation speed of 60,000 rpm and an air volume of 8 m ³ / m, and fine particles are captured by a dust collecting bag filter, and the average particle diameter A negative electrode active material having a (D50) of 4.8 μm and a light bulk density of 204 g / L was obtained. FIG. 4 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved and covers the active material particles, flat fine particles are contained, the average circularity is 0.75, and the inclusion of flat fine particles The rate was 77.9% by weight. The BET specific surface area by the BET method using nitrogen gas was 56 m ² / g.

FIG. 5 shows a secondary electron image by FE-SEM of a cross section obtained by cutting the obtained negative electrode active material particles with an ion beam. The negative electrode active material particles are composed of substantially spherical particles and flat fine particles, and inside the substantially spherical particles, a structure in which Si fine particles are sandwiched between carbonaceous materials and a graphite thin layer spreads in a network shape and is laminated. The carbonaceous material was in close contact with and covered the Si fine particles. Further, near the surface of the active material particles, the graphite thin layer was curved to cover the active material particles. Although the flat fine particles have a small number of layers, they have a structure similar to that of the substantially spherical particles, and the surface thereof is covered with a thin graphite layer or a carbon material.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.4% by weight (content in the total solid content; the same applies hereinafter), acetylene black 0.5% by weight as a conductive additive, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.6 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 1.5 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 16 μm. A negative electrode for a secondary battery was obtained.

"Production of evaluation cells"
In the glove box, the evaluation cell was prepared by dipping the negative electrode, a 24 mmφ polypropylene separator, a 21 mmφ glass filter, a 18 mmφ 0.2 mm thick metal lithium and a stainless steel foil of the base material into the electrolyte solution in the glove box. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolyte used was a mixture of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 1, FEC (fluoroethylene carbonate), and LiPF ₆ dissolved to a concentration of 1.2 vol / L. The evaluation cell was further placed in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

The evaluation cell was subjected to a cycle test in a constant temperature room at 25 ° C. Charging was performed after charging to 0.01 V at a constant current of 2.2 mA until the current value reached 0.2 mA at a constant voltage of 0.01 V. The discharge was performed at a constant current of 2.2 mA up to a voltage value of 1.5V. The initial discharge capacity and initial charge / discharge efficiency were the results of the initial charge / discharge test.

Example 4
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol in an amount of 24% by weight and subjected to a fine pulverization wet bead mill using zirconia beads having a diameter of 0.3 mm for 6 hours. D50) An ultrafine Si slurry having a diameter of 0.3 μm and a dry BET specific surface area of 100 m ² / g was obtained.

98.8 g of the ultrafine Si slurry, 48.0 g of the expanded graphite, 20.0 g of a resol type phenol resin (weight average molecular weight (Mw) = 370), and 5.9 L of ethanol were placed in a stirring vessel. And stirred for 90 minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 60 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a fume hood and dried for 2 hours while evacuating, passed through a mesh with a mesh opening of 2 mm, and further dried for 1 day to obtain 86 g of a mixed dried product (light bulk density 77 g / L).

The mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and consolidated to a light bulk density of 303 g / L.

Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 900 seconds while being cooled with water. The obtained powder was put into a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace. Thereafter, a mesh having an opening of 45 μm was passed through to obtain composite particles having an average particle diameter (D50) of 16.5 μm and a light bulk density of 573 g / L.

This composite particle is put into an air classifier (ATP-50 manufactured by Hosokawa Micron), and fine powder is captured by a cyclone collector with a classifier rotating speed of 18,000 rpm and an air volume of 1.6 m ³ / min. A negative electrode active material having a diameter (D50) of 5.9 μm and a light bulk density of 293 g / L was obtained. FIG. 6 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved and covers the active material particles, flat fine particles are contained, the average circularity is 0.74, and the inclusion of flat fine particles The rate was 1.8% by weight. The BET specific surface area by the BET method using nitrogen gas was 30 m ² / g.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.6% by weight (content in the total solid content; the same applies hereinafter), acetylene black 0.5% by weight as a conductive auxiliary, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.4 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 2.5 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 21 μm. A negative electrode for a secondary battery was obtained.

"Production of evaluation cells"
In the glove box, the evaluation cell was prepared by dipping the negative electrode, a 24 mmφ polypropylene separator, a 21 mmφ glass filter, a 18 mmφ 0.2 mm thick metal lithium and a stainless steel foil of the base material into the electrolyte solution in the glove box. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolyte used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, FEC (fluoroethylene carbonate) as the additive, and LiPF ₆ dissolved to a concentration of 1.2 vol / L. did. The evaluation cell was further placed in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

Example 5
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol at 25% by weight, and a finely pulverized wet bead mill using zirconia beads having a diameter of 0.3 mm was performed for 6 hours. D50) An ultrafine Si slurry with 0.4 μm and a dry BET specific surface area of 60 m ² / g was obtained.

Vibrating powder feeder using acid-treated high-purity natural graphite having a particle diameter of 0.15 mm (width in the (200) plane direction), a thickness of 10 μm, a purity of 99.9% by weight or more, and an S content of 0.3% by weight or less Put into nitrogen gas at a flow rate of 12 L / min and pass through a mullite tube with a length of 1 m and an inner diameter of 11 mm heated to 850 ° C. with an electric heater, discharged from the end face to the atmosphere, and a gas such as sulfurous acid exhausted to the top, Expanded graphite was collected in a stainless steel container at the bottom. The width of the expanded graphite in the (200) plane direction was 0.15 mm and the original graphite value was maintained, but the thickness expanded to 40 mm, 0.4 mm, the appearance was coiled, and the graphite layer was observed by SEM observation. Was peeled off and confirmed to be in the form of an accordion.

466.4 g of the ultrafine Si slurry, 426.2 g of the expanded graphite, 86.5 g of a resol type phenolic resin (weight average molecular weight (Mw) = 460), and 6.4 L of ethanol are placed in a stirring vessel. And stirred for 26 minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 60 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a fume hood and dried for 2 hours while being evacuated, passed through a mesh with a mesh opening of 2 mm, and further dried for 1 day to obtain 588 g of a mixed dried product (light bulk density 170 g / L).

The mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and consolidated to a lightly packed compact 308 g / L.

Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 900 seconds while being cooled with water. At the same time, it was spheroidized to obtain a spheroidized powder having a light bulk density of 437 g / L.

The obtained powder was put in a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace to obtain composite particles having a light bulk density of 549 g / L. This composite particle is put into an air classifier (ATP-50 manufactured by Hosokawa Micron), and the fine particle is captured by a cyclone collector with a classifier rotating speed of 5000 rpm and an air volume of 1.6 m ³ / min, and the average particle diameter (D50) Was 10.0 μm, and a lightly loaded bulk density of 558 g / L was obtained. FIG. 7 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved to cover the active material particles, flat fine particles are contained, the average circularity is 0.70, and the inclusion of flat fine particles The rate was 1.2% by weight. The BET specific surface area by the BET method using nitrogen gas was 29.0 m ² / g.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 3.0 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 22 μm. A negative electrode for a secondary battery was obtained.

"Production of evaluation cells"
In the glove box, the evaluation cell was prepared by dipping the negative electrode, a 24 mmφ polypropylene separator, a 21 mmφ glass filter, a 18 mmφ 0.2 mm thick metal lithium and a stainless steel foil of the base material into the electrolyte solution in the glove box. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolyte used is a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, FEC (fluoroethylene carbonate) as the additive, and LiPF ₆ dissolved to a concentration of 1.2 mol / L. did. The evaluation cell was further placed in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

Example 6
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol at 25% by weight, and a finely pulverized wet bead mill using zirconia beads having a diameter of 0.3 mm was performed for 6 hours. D50) An ultrafine Si slurry with 0.4 μm and a dry BET specific surface area of 60 m ² / g was obtained.

Vibrating powder feeder using acid-treated high-purity natural graphite having a particle diameter of 0.15 mm (width in the (200) plane direction), a thickness of 10 μm, a purity of 99.9% by weight or more, and an S content of 0.3% by weight or less Put into nitrogen gas at a flow rate of 12 L / min and pass through a mullite tube with a length of 1 m and an inner diameter of 11 mm heated to 850 ° C. with an electric heater, discharged from the end face to the atmosphere, and a gas such as sulfurous acid exhausted to the top Expanded graphite was collected in a stainless steel container at the bottom. The width of the expanded graphite in the (200) plane direction was 0.15 mm and the original graphite value was maintained, but the thickness expanded to 40 mm, 0.4 mm, the appearance was coiled, and the graphite layer was observed by SEM observation. Was peeled off and confirmed to be in the form of an accordion.

145.7 g of the ultrafine Si slurry, 133.2 g of the expanded graphite, 27 g of resol type phenolic resin (weight average molecular weight (Mw) = 460), 2 L of ethanol were placed in a stirring vessel, and 8.25 using an in-line mixer. Mix and stir for minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 60 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a fume hood and dried for 2 hours while being evacuated, passed through a mesh with a mesh opening of 2 mm, and further dried for 1 day to obtain 188 g of a mixed dried product (light bulk density 132 g / L).

The mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and consolidated to a lightly packed compact 235 g / L.

Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 900 seconds while being cooled with water. At the same time, it was spheroidized to obtain a spheroidized powder having a light bulk density of 476 g / L.

The obtained powder was put into a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace to obtain composite particles having a light bulk density of 641 g / L. Thereafter, a negative active material having an average particle diameter (D50) of 17.6 μm and a light bulk density of 629 g / L was obtained through a mesh having an opening of 45 μm. FIG. 8 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved and covers the active material particles, flat fine particles are contained, the average circularity is 0.72, and the inclusion of flat fine particles The rate was 1.1% by weight. The BET specific surface area by the BET method using nitrogen gas was 37 m ² / g.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 3.6 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 36 μm. A negative electrode for a secondary battery was obtained.

Comparative Example 1
In the mixing step, 36 g of ultrafine Si slurry, 18 g of expanded graphite, 7.5 g of resol type phenol resin (weight average molecular weight (Mw) = 3.6 × 10 ³ ), and 2.4 L of ethanol are put in a stirring vessel. A spherical powder having an average particle size (D50) of 4.2 μm and a light bulk density of 250 g / L was obtained in the same manner as in Example 2 except that the steps were performed.

From this spheroidized powder, a negative electrode active material, a negative electrode, and an evaluation cell were prepared in the same manner as in Example 2, and the cells were evaluated.

FIG. 9 shows an SEM image of the obtained negative electrode active material. The particles were not substantially spherical, but were fine powder and flat particles, the average circularity was 0.65, and the content of flat particles was 0.3% by weight. The BET specific surface area by the BET method using nitrogen gas was 33 m ² / g.

“Preparation of negative electrode for lithium ion secondary battery”
With respect to 90.8% by weight of the obtained negative electrode active material (content in the total solid content, the same shall apply hereinafter), acetylene black 0.5% by weight as a conductive assistant, PVDF 8.7% by weight as a binder, NMP was mixed to prepare a negative electrode mixture-containing slurry.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 2.2 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 17 μm. A negative electrode for a secondary battery was obtained.

Comparative Example 2
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol in an amount of 23% by weight and subjected to a fine pulverization wet bead mill using zirconia beads having a diameter of 0.3 mm for 6 hours. D50) An ultrafine Si slurry having a diameter of 0.3 μm and a dry BET specific surface area of 100 m ² / g was obtained.

102.6 g of the ultrafine Si slurry, 48.0 g of the expanded graphite, 20.0 g of a resol-type phenol resin (weight average molecular weight (Mw) = 370), and 5.9 L of ethanol were placed in a stirring vessel. And stirred for 90 minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 60 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a fume hood and dried for 2 hours while being evacuated, passed through a 2 mm mesh, and further dried for 1 day to obtain 86 g of a mixed dried product (light bulk density 66 g / L).

This mixed dried product is passed through a three-roll mill twice, passed through a 1 mm sieve, granulated and compacted to a light bulk density of 287 g / L, and then heat-treated at 150 ° C. for 2 hours in the atmosphere. It was.

Next, the granulated / consolidated product subjected to the heat treatment was placed in a new power mill and pulverized at 21000 rpm for 300 seconds while cooling with water. The obtained powder was put into a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace, and then composite particles were obtained through a mesh having an opening of 45 μm.

This composite particle is put into an air classifier (ATP-50 manufactured by Hosokawa Micron), and fine powder is captured by a cyclone collector with a classifier rotating speed of 18,000 rpm and an air volume of 1.6 m ³ / min. A negative electrode active material having a diameter (D50) of 4.3 μm and a light bulk density of 270 g / L was obtained. FIG. 10 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved and covers the active material particles, flat fine particles are contained, the average circularity is 0.56, and the inclusion of flat fine particles The rate was 30.9% by weight. The BET specific surface area by the BET method using nitrogen gas was 47 m ² / g.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.5% by weight (content in the total solid content; the same shall apply hereinafter), acetylene black 0.5% by weight as a conductive auxiliary, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.5 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 3.1 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 28 μm. A negative electrode for a secondary battery was obtained.

Comparative Example 3
When the baked powder obtained in Example 3 was passed through a sieve having an opening of 45 μm, the obtained particles of 45 μm or more were passed through a mesh having an opening of 53 μm, and the average particle diameter (D50) was 54.8 μm. Composite particles having a light bulk density of 935 g / L were obtained. FIG. 11 shows an SEM image of the obtained negative electrode active material particles. The negative electrode active material particle graphite thin layer (12) is curved and has a substantially spherical shape covering the active material particle, the average circularity is 0.73, and the content of flat fine particles is 0% by weight. there were. The BET specific surface area by the BET method using nitrogen gas was 92 m ² / g.

Table 1 shows the results of Examples 1 to 6 and Comparative Examples 1 to 3.

As is clear from Table 1, the lithium ion secondary batteries of Examples 1 and 2 have a high capacity, high initial charge / discharge efficiency, and good charge / discharge cycle characteristics.

The lithium ion secondary battery using the negative electrode active material containing 1 to 80% of the flat particles of Examples 3 to 6 has better charge / discharge cycle characteristics than Examples 1 to 2. In Examples 5 and 6, high-purity graphite is used as a raw material, so that the initial charge / discharge efficiency is a higher value.

On the other hand, the lithium ion secondary battery of Comparative Example 1 has a low average circularity, and therefore its cycle retention rate is inferior to Examples 1-6. Although the lithium ion secondary battery of Comparative Example 2 contains an appropriate amount of flat particles, the average circularity is low, so that it is inferior to Examples 3 to 5 having a specific amount of flat particles having the same charge / discharge cycle characteristics. In Comparative Example 3, the composite particles were too large in size, so that the electrode could not be formed and evaluation was impossible.

Example 7
A fine-grade wet bead mill using a zirconia bead having a mean particle size (D50) of 7 μm and chemical grade metal Si (purity 3.5N) mixed with methanol in an amount of 20 wt. Average particle diameter (D50) measured by a laser diffraction particle size distribution analyzer LA-950 manufactured by HORIBA, Ltd. with a real part of 3.5 and an imaginary part of 0 by performing ultrafine grinding wet bead mill using 03 mm zirconia beads for 5 hours. Of fine particles, having a DET of 0.16 μm, a D90 of 0.29 μm, and a BET specific surface area measuring device Tristar 3000 manufactured by Shimadzu Corporation having a BET specific surface area of 101 m ² / g upon drying.

Natural graphite having a particle diameter of 0.5 mm (width in the (200) plane direction) and a thickness of 0.02 mm was immersed in concentrated sulfuric acid added with 1 wt% sodium nitrate and 7 wt% potassium permanganate for 24 hours, It was washed with water and dried to obtain acid-treated graphite. This acid-treated graphite was placed in a vibrating powder feeder, placed on nitrogen gas at a flow rate of 10 L / min, passed through a mullite tube having a length of 1 m and an inner diameter of 11 mm heated to 850 ° C. with an electric heater, and released from the end face to the atmosphere. A gas such as sulfurous acid was exhausted at the top, and expanded graphite was collected at the bottom in a stainless steel container. The expanded graphite had a (200) plane width of 0.5 mm and maintained the original graphite value, but the thickness expanded to 4 mm and 200 times, the appearance was coiled, and the graphite layer was peeled off by SEM observation. The accordion was confirmed.

60 g of the ultrafine Si slurry, 24 g of the expanded graphite, 10 g of a resol type phenol resin (weight average molecular weight Mw = 3.7 × 10 ² ) and 1 L of ethanol were stirred so that the Si concentration was 30% by weight. It put in the container and mixed and stirred with the homomixer for 1 hour. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 60 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a fume hood and dried for 2 hours while evacuating, passed through a mesh with a mesh opening of 2 mm, and further dried for 12 hours to obtain 40 g of a mixed dried product (light bulk density 80 g / L).

The mixed dried product was passed through a three-roll mill twice, and granulated and consolidated to a particle size of 2 mm and a light bulk density of 440 g / L.

Next, this granulated / consolidated product was placed in a new power mill and pulverized at 22000 rpm for 900 seconds while cooling with water, and at the same time, spheroidized to obtain a spheroidized powder with a light bulk density of 650 g / L. The obtained powder was put into an alumina boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace. Thereafter, a negative electrode active material having an average particle diameter (D50) of 20 μm and a lightly loaded bulk density of 810 g / L was obtained through a mesh having an opening of 45 μm.

FIG. 12 shows a secondary electron image by FE-SEM of a cross section obtained by cutting the obtained negative electrode active material particles with an Ar ion beam. Inside the negative electrode active material particles, 0.05 to 0.2 μm long Si fine particles and a carbonaceous material (0.02) between the graphite thin layers (11) having a thickness of 0.02 to 0.2 μm (12) (the gap is 0. The structure sandwiched between (05-1 μm) spreads in a mesh pattern and is laminated. The carbonaceous material was in close contact with and covered the Si fine particles. Further, in the vicinity of the surface of the active material particles, the graphite thin layer (11) was curved to cover the active material particles. The BET specific surface area by the BET method using nitrogen gas was 53 m ² / g. Measurement with a Rigaku powder X-ray diffractometer RINT2000 showed a diffraction line corresponding to the (002) plane of graphite, and d002 was 0.336 nm. In addition, a very broad diffraction line derived from amorphous carbonization of the carbonaceous material was observed in the vicinity thereof. A diffraction line corresponding to the (100) plane of Si was observed, and d002 was 0.314 nm.

FIG. 13 shows an SEM image of the obtained negative electrode active material. The negative electrode active material particle graphite thin layer was curved to have a substantially spherical shape covering the active material particle, and the average circularity was 0.73.

"Preparation of negative electrode for lithium ion secondary battery"
The obtained negative electrode active material is 95.5% by weight (content in the total solid content; the same shall apply hereinafter), acetylene black 0.5% by weight as a conductive auxiliary, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.5 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 15 μm using an applicator so that the solid content was 3 mg / cm ² and dried at 110 ° C. in a stationary operation dryer for 0.5 hour. After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 30 μm. A negative electrode for a secondary battery was obtained.

"Production of evaluation cells"
In the glove box, the evaluation cell was prepared by dipping the negative electrode, a 24 mmφ polypropylene separator, a 21 mmφ glass filter, a 18 mmφ 0.2 mm thick metal lithium and a stainless steel foil of the base material into the electrolyte solution in the glove box. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolyte used was a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, dissolved LiPF ₆ to a concentration of 1.2 mol / L, and added with 2% by volume of fluoroethylene carbonate. did. The evaluation cell was further placed in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

"Evaluation conditions"
The evaluation cell was cycle tested in a constant temperature room at 25 ° C. Charging was performed after charging to 0.01 V at a constant current of 2 mA until the current value reached 0.2 mA at a constant voltage of 0.01 V. The discharge was performed at a constant current of 2 mA up to a voltage value of 1.5 V. The initial discharge capacity and the initial charge / discharge efficiency are the results of the initial charge / discharge test.

In addition, the cycle characteristics were evaluated as the capacity retention rate by comparing the discharge capacity after 50 charge / discharge tests under the charge / discharge conditions with the initial discharge capacity. Moreover, the thickness of the negative electrode before charging / discharging and after 50 times charging / discharging was evaluated as an expansion coefficient.

Example 8
In the pulverization of Si, 5 wt% of finely pulverized wet bead mills using zirconia beads having a diameter of 0.3 mm were mixed with 20 wt% of chemical grade metal Si (purity 3.5N) having an average particle diameter (D50) of 7 μm in methanol. The negative electrode active material, the negative electrode, and the evaluation were performed in the same manner as in Example 6 except that the average particle size (D50) was 0.33 μm, D90 was 0.52 μm, and the BET specific surface area was 60 m ² / g. The cells were prepared in the order of the cells. This negative electrode active material had an average particle diameter (D50) of 19 μm, a BET specific surface area of 50 m ² / g, and an average circularity of 0.74. The negative electrode active material was subjected to cell evaluation.

Example 9
As graphite, expanded graphite prepared by heat-treating acid-treated graphite EXP-80MT manufactured by Nippon Graphite in the same manner as in Example 6 was used. This expanded graphite expanded 40 times, and the appearance was a coil shape. The SEM observation confirmed that the graphite layer was peeled off and an accordion shape.

The amount of impurities subjected to semi-quantitative analysis by ICP of this expanded graphite was Al 23 ppm, Ca 29 ppm, Fe 53 ppm, Mg 21 ppm, Na 25 ppm. Other elements were less than 10 ppm, and the purity was 99.9% by weight. The amount of S by the oxygen flask combustion method was less than 0.1% by weight, and the BET specific surface area was 24 m ² / g.

144g Si slurry similar to Example 8, 133g expanded graphite, 27g resol type phenolic resin similar to Example 7 and 2L ethanol similar to Example 7 so that the Si concentration was 20 wt%. Mixing, drying, granulation / consolidation, pulverization, spheroidization, and firing by the method, passing through a mesh with an opening of 45 μm, average particle diameter (D50) 19 μm, BET specific surface area 50 m ² / g, average circularity 0 70, a negative electrode active material having a lightly loaded bulk density of 630 g / L was obtained. With respect to this negative electrode active material, a negative electrode and an evaluation cell were prepared in the same manner as in Example 7 and evaluated.

Example 10
Implementing 144 g of Si slurry similar to Example 8, 133 g of expanded graphite similar to Example 7, 27 g of resol type phenolic resin similar to Example 7 and 2 L of ethanol so that the Si concentration is 50% by weight Mixing, drying, granulation / consolidation, pulverization / spheronization, and firing were performed in the same manner as in Example 7, passed through a mesh with an opening of 45 μm, an average particle size (D50) of 7 μm, and a BET specific surface area of 86 m ² / g. A negative electrode active material having an average circularity of 0.72 was obtained. With respect to this negative electrode active material, a negative electrode and an evaluation cell were prepared in the same manner as in Example 7 and evaluated.

Comparative Example 4
In the mixing step, the same Si ultrafine particle slurry as in Example 7, 36 g, expanded graphite, 18 g, resol type phenol resin (weight average molecular weight (Mw) = 3.6 ×) so that the Si concentration becomes 30% by weight. Spherical powder having a light bulk density of 250 g / L was obtained in the same manner as in Example 8 except that 7.5 g of 10 ³ ) and 2.4 L of ethanol were placed in a stirring vessel and the process was performed.

From this spheroidized powder, a negative electrode active material, a negative electrode, and an evaluation cell were prepared in the same manner as in Examples 7 and 8, and the cells were evaluated.

The negative electrode active material was not substantially spherical, but was a fine powder and flat particles. D50 was 4 μm, BET specific surface area was 33 m ² / g, and average circularity was 0.65.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 90.8% by weight (content in the total solid content; the same applies hereinafter), 0.5% by weight of acetylene black as a conductive assistant, 8.7% by weight of PVDF as a binder, NMP, Were mixed to prepare a negative electrode mixture-containing slurry.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 2.2 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 17 μm. A negative electrode for a secondary battery was obtained.

Comparative Example 5
In the pulverization of Si, a finely pulverized wet bead mill using 20% by weight of chemical grade metal Si (purity 3.5N) having an average particle diameter (D50) of 7 μm in methanol and zirconia beads having a diameter of 0.3 mm is 1 A Si slurry having an average particle size (D50) of 0.93 μm, D90 of 3.92 μm, and a BET specific surface area of 15 m ² / g was obtained. 180 g of this Si slurry, 133 g of expanded graphite similar to Example 7, 27 g of resol type phenolic resin, and 2 L of ethanol were mixed, dried and prepared in the same manner as in Example 7 so that the Si concentration was 20 wt%. Granulated / consolidated, pulverized / spheroidized, fired, passed through a mesh with an opening of 45 μm, average particle size (D50) 10 μm, BET specific surface area 37 m ² / g, average circularity 0.72, light weight bulk density 588 g / L negative electrode active material was obtained. With respect to this negative electrode active material, a negative electrode and an evaluation cell were prepared in the same manner as in Example 7 and evaluated.

Table 2 shows the results of Examples 7 to 10 and the results of Comparative Examples 4 to 5.

As is apparent from Table 2, the lithium particles of Examples 7 to 10 having a small average particle diameter (D50) or D90 of Si, a large BET specific surface area, and a substantially spherical shape having an average circularity of 0.7 to 1.0. The ion secondary battery has a high initial discharge capacity of 785 to 1447 mAh / g, a high initial charge and discharge efficiency of 77 to 85%, a good cycle capacity maintenance rate of 70 to 90%, and a good expansion rate. is there.

In contrast, the lithium ion secondary battery of Comparative Example 4 having an average circularity of less than 0.7 has an initial discharge capacity of 1188 mAh / g and an initial charge / discharge efficiency of 80%, which are almost the same as those of Examples 7 to 10. However, the cycle capacity retention rate was 12%, which was inferior. Further, Comparative Example 2 in which the BET specific surface area of Si is as small as 15 m ² / g has an initial discharge capacity of 839 mAh / g and an initial charge / discharge efficiency of 86%, which are almost the same as those of Examples 7 to 9, The capacity maintenance rate was 61%, which was inferior.

The negative electrode active material for a lithium ion secondary battery and the production method thereof according to the present invention can be used for a lithium ion secondary battery that requires a high capacity and a long life.

11 Graphite thin layer inside negative electrode active material 12 Graphite thin layer near negative electrode active material surface 13 Si fine particle and carbonaceous material layer 14 Si fine particle inside negative electrode active material Although the present invention has been described in detail with reference to specific embodiments It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit and scope of the invention.

The present application is a Japanese patent application filed on May 7, 2014 (Japanese Patent Application No. 2014-096158), a Japanese patent application filed on December 2, 2014 (Japanese Patent Application No. 2014-243869), and March 2014 This application is based on the Japanese patent application (Japanese Patent Application No. 2014-061376) filed on the 25th and the Japanese patent application (Japanese Patent Application No. 2014-247751) filed on December 8, 2014, which are incorporated by reference in their entirety. Also, all references cited herein are incorporated as a whole.

Claims

In a negative electrode active material for a lithium ion secondary battery comprising Si or a Si alloy, carbonaceous material or carbonaceous material, and graphite, the average particle size (D50) of the negative electrode active material is 1 to 40 μm, and the average A negative electrode active material for a lithium ion secondary battery, wherein the negative electrode active material is a substantially spherical composite particle having a circularity of 0.7 to 1.0.
The negative electrode active material has a mean particle diameter (D50) of 1 to 10 μm, and is a composite particle containing 1 to 80 wt% of flat fine particles having a minor axis length of less than 1 μm measured by SEM image observation. The negative electrode active material for a lithium ion secondary battery according to claim 1.
3. The lithium ion secondary battery according to claim 1, wherein the Si or Si alloy has an average particle size (D50) of 0.01 to 5 μm, and a carbonaceous material covers at least the active material surface. Negative electrode active material.
3. The lithium ion secondary battery according to claim 1, wherein the Si or Si alloy has an average particle diameter (D50) of 0.01 to 1 μm, and a carbonaceous material covers at least the active material surface. Negative electrode active material.
The Si or Si alloy has a structure sandwiched between carbonaceous materials and a thin graphite layer having a thickness of 0.2 μm or less, and the structure spreads in a laminated and / or network shape, and the thin graphite layer is an active material. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the active material particles are curved in the vicinity of the surface of the particles, and a carbonaceous material covers the surface of the outermost layer. Negative electrode active material.
The graphite is composed of 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co, Cu, by ICP emission spectroscopy. , Mo, Pb, Sb, Se, Th, Tl, U) Ion chromatography (IC) measurement by oxygen flask combustion method when the purity determined from the semi-quantitative value of impurities is 99.9% by weight or more, or the impurity amount is 1000 ppm or less The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the S amount by the method is 0.3% by weight or less and / or the BET specific surface area is 40m 2 / g or less. Active material.
The lithium ion 2 according to any one of claims 1 to 6, wherein a content of the Si or Si alloy is 10 to 80% by weight and a content of the carbonaceous material is 90 to 20% by weight. Negative electrode active material for secondary battery.
The content of the Si or Si alloy is 10 to 60% by weight, the content of the carbonaceous material is 5 to 40% by weight, and the content of the graphite is 20 to 80% by weight. 6. The negative electrode active material for a lithium ion secondary battery according to any one of 6 above.
The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the BET specific surface area is 0.5 to 80 m 2 / g.
A step of mixing Si or an Si alloy, a carbon precursor, and graphite, a step of granulating and compacting, a step of grinding and spheroidizing the mixture to form substantially spherical composite particles, and the composite particles The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 9, further comprising a step of firing in an inert gas atmosphere.
The method for producing a negative electrode active material for a lithium ion secondary battery according to claim 10, wherein in the spheronization treatment, the pulverized particles are rebound to form substantially spherical composite particles.
A step of mixing Si or an Si alloy, a carbon precursor, and graphite, a step of granulating and compacting, a step of grinding and spheroidizing the mixture to form substantially spherical composite particles, and the composite particles The lithium ion 2 according to any one of claims 1 to 9, comprising a step of firing in an inert gas atmosphere and a step of air-classifying the pulverized and spherically processed powder or the fired powder. The manufacturing method of the negative electrode active material for secondary batteries.
In the spheronization treatment, the pulverized particles are rebound to form substantially spherical composite particles and flat fine particles, or the substantially spherical composite particles and flat fine particles are mixed, stirred, and classified. Item 13. A method for producing a negative electrode active material for a lithium ion secondary battery according to Item 12.
The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 10 to 13, wherein the carbon precursor is a carbon-based compound having a weight average molecular weight (Mw) of 1000 or less.
The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 10 to 14, wherein the graphite is expanded graphite or flaky graphite.
The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 10 to 15, wherein the temperature of the step of firing the composite particles in an inert atmosphere is 600 to 1200 ° C. Method.
Si or Si alloy having an average particle diameter (D50) of 0.01 to 0.6 μm, D90 of 0.01 to 1.0 μm, and a BET specific surface area of 40 to 300 m 2 / g by BET method is 10 to 80 A negative active for a lithium ion secondary battery, characterized in that it contains approximately 90% by weight, 90% to 5% by weight of a carbonaceous material, 0% to 80% by weight of graphite, and an average sphericity of 0.7 to 1.0. material.
The Si or Si alloy has an average particle size (D50) of 0.01 to 0.3 μm, D90 of 0.01 to 0.5 μm, and a BET specific surface area of 70 to 300 m 2 / g as measured by the BET method. The negative electrode active material for a lithium ion secondary battery according to claim 17.
19. The active material surface is covered with a carbonaceous material, having an average particle diameter (D50) of 1 to 40 μm, a BET specific surface area by BET method of 5 to 120 m 2 / g, Negative electrode active material for lithium ion secondary battery.
The graphite is composed of 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co, Cu, by ICP emission spectroscopy. , Mo, Pb, Sb, Se, Th, Tl, U) The purity determined from the semi-quantitative value of impurities is 99.9 wt% or more (1000 ppm or less), and is measured by ion chromatography (IC) measurement by oxygen flask combustion method. The negative electrode active for a lithium ion secondary battery according to any one of claims 17 to 19, wherein the amount of S is 0.3% by weight or less and / or the BET specific surface area is 40m 2 / g or less. material.
Si or Si alloy is sandwiched between carbonaceous materials and a thin graphite layer having a thickness of 0.2 μm or less, and the structure spreads in a laminated and / or network form, and the thin graphite layer is an active material. The negative electrode active material for a lithium ion secondary battery according to any one of claims 17 to 20, wherein the active material particles are curved in the vicinity of the surface of the particles to cover the active material particles.
Si or Si alloy having a mean particle size (D50) of 0.01 to 0.6 μm, D90 of 0.01 to 1.0 μm, and a BET specific surface area of 40 to 300 m 2 / g and a carbon precursor Further, if necessary, a step of mixing graphite, a step of granulating and compacting, a step of forming composite particles by pulverization and spheronization, and a step of firing the composite particles in an inert gas atmosphere The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 17 to 21, wherein:
The method for producing a negative electrode active material for a lithium ion secondary battery according to claim 22, wherein the temperature of the step of firing the composite particles in an inert atmosphere is 600 to 1000 ° C.