JP5464717B2 - Method for producing positive electrode active material - Google Patents

Description

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

  As a positive electrode active material for a lithium ion secondary battery, a lithium manganese composite oxide (hereinafter referred to as a Li-Mn composite oxide) that is excellent in safety and has abundant resources has attracted attention. However, the Li—Mn composite oxide has a lower capacity per active material than a lithium cobalt composite oxide (abbreviated as Li—Co composite oxide), and has many voids in the secondary particles. Accordingly, the mass of the active material charged in the battery whose size is limited because the secondary particles are light and the oil absorption is large is reduced. As a result, there is a problem that the electric capacity per unit battery is small.

In recent years, tap density (vibrating the container under certain conditions) has been improved by pressing and molding a mixture of a manganese compound and a lithium compound at a pressure of 500 kg / cm ² or higher, followed by heat treatment and crushing. Have been proposed to obtain a Li—Mn composite oxide having an apparent density of 1.7 g / ml or more (US Pat. No. 5,807,646: Patent Document 1, JP-A-9-86933: Patent Document). 2). However, the specific tap density disclosed is only 1.9 g / ml at most, which is not a satisfactory level.

  Further, the above publication discloses the average particle diameter of secondary particles in which primary particles of Li—Mn-based composite oxide are aggregated, but secondary particles use the interaction between primary particles. Even if the filling property is improved, the aggregation does not occur at the stage of forming a paint (electrode paste) during the preparation process of the electrode material, which is not an essential improvement measure.

  In addition, as a method for producing a Li-Mn composite oxide having a spinel structure, a method of producing a mixture of a manganese compound and a lithium compound by firing at a high temperature (for example, at a temperature of 250 ° C. to 850 ° C.) -86933 gazette: Patent Document 2) or Li-Mn-B in which a manganese compound and a lithium compound are further mixed with an oxide of a boron element that can be substituted with manganese, fired at high temperature, and Mn is partially substituted with B A method for producing a positive electrode active material of a system oxide (Japanese Patent Laid-Open No. 4-237970: Patent Document 3) is disclosed.

  However, when these raw materials are fired at high temperature in the atmosphere or in an oxygen gas flow, the secondary particles after pulverization have a high average porosity (15% or more) and a low tap density (1.9 g). For this reason, the mass of the positive electrode active material charged into the electrode cannot be increased to increase the capacity.

  JP-A-4-14752 (Patent Document 4) discloses the use of a manganese-based oxide obtained by blending and sintering titanium oxide in a spinel type lithium manganese oxide for a positive electrode active material. If titanium oxide is not 950 ° C. to 1000 ° C. or higher, lithium and manganese will not react to form a melt, and if titanium oxide is not added in an amount of 10% by mass, the tap density is only 1.60 g / ml. There was a problem.

US Pat. No. 5,807,646 JP-A-9-86933 JP-A-4-237970 JP-A-4-14752

  An object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that has excellent filling properties, a high initial capacity, and a small decrease in capacity when charging and discharging are repeated (a high capacity retention rate), a method for producing the same An object of the present invention is to provide a lithium ion secondary battery using a positive electrode active material.

As a result of intensive studies, the present inventors have crushed a fired product of Li-Mn composite oxide having a spinel structure, and then added a sintering accelerator to these pulverized particles, and granulated and fired. As a result, the above-mentioned problems were solved by succeeding in densifying the particles.
That is, the present invention provides the following positive electrode active material for a lithium ion secondary battery, a production method thereof, an electrode paste containing the positive electrode active material, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

[1] In a positive electrode active material for a lithium ion secondary battery mainly composed of Li-Mn composite oxide particles having a spinel structure, the following formula
Porosity (%) = (A / B) × 100 (1)
(A is the total cross-sectional area of the pores included in the cross section of one secondary particle, and B is the cross-sectional area of one secondary particle.) The average value of the porosity of the particles shown in FIG. A positive electrode active material for a lithium ion secondary battery.
[2] The positive electrode active material for a lithium ion secondary battery according to [1], wherein the average porosity is 10% or less, and the average particle diameter of primary particles is 0.2 to 3 μm.
[3] The positive electrode active material for a lithium ion secondary battery according to [1], wherein the positive electrode active material has a tap density of 1.9 g / ml or more.
[4] The positive electrode active material for a lithium ion secondary battery according to [3], wherein the positive electrode active material has a tap density of 2.2 g / ml or more.
[5] The positive electrode active material for a lithium ion secondary battery according to the above [1], wherein the crystallite size of the positive electrode active material is 400 to 960 Å.
[6] The positive electrode active material for a lithium ion secondary battery according to [1], wherein the positive electrode active material has a lattice constant of 8.240 angstroms or less.
[7] The positive electrode active material is mainly composed of a Li-Mn composite oxide having a spinel structure, and the oxide contains an element or an element that can be an oxide or oxide that melts at a temperature of 550 ° C to 900 ° C. In the above [1], which is a compound, or an active material that is granulated and sintered, consisting of a compound, or an element that can be dissolved or reacted with lithium or manganese, or an oxide that can be melted by reaction or an oxide The positive electrode active material for lithium ion secondary batteries as described.
[8] An element or a compound containing an element that can be an oxide or oxide that melts at a temperature of 550 ° C. to 900 ° C., or an oxide or oxide that melts or reacts with lithium or manganese. An element or a compound containing an element is at least one element selected from the group consisting of Bi, B, W, Mo, and Pb, or a compound that combines B ₂ O ₃ and LiF, or MnF ₂ and LiF The positive electrode active material for a lithium ion secondary battery according to [7], which is a compound in which
[9] In a method for producing a positive electrode active material for a lithium ion secondary battery mainly composed of a Li-Mn composite oxide having a spinel structure, the pulverized product of the Li-Mn composite oxide having a spinel structure is 550 ° C. Contains an element or compound that can be an oxide or oxide that melts at a temperature of ˜900 ° C., or an element or element that can be dissolved or reacts with lithium or manganese to become an oxide or oxide that melts The manufacturing method of the positive electrode active material for lithium ion secondary batteries characterized by having the process of adding a compound, mixing, and granulating.
[10] The method for producing a positive electrode active material for a lithium ion secondary battery according to [9], which includes a step of sintering the granulated product in addition to the granulating step.
[11] In addition to the granulation step, the granulated product is heated at a rate of at least 100 ° C./min from the sintering shrinkage start temperature to a temperature higher by at least 100 ° C. and held at that temperature for 1 to 10 minutes. The method for producing a positive electrode active material for a lithium ion secondary battery according to the above [9], comprising a step of lowering the temperature to the sintering start temperature at a rate of at least 100 ° C./min and sintering.
[12] The method for producing a positive electrode active material for a lithium ion secondary battery according to [11], wherein sintering is performed using a rotary kiln.
[13] The sintering step includes at least one element selected from the group consisting of Bi, B, W, Mo, and Pb on the surface of the Li—Mn composite oxide particles, or a compound containing the element, or B ₂ O _[3 ] The method for producing a positive electrode active material for a lithium ion secondary battery according to [10], wherein the method is performed by melting and sintering a compound combining ₃ and LiF or a compound combining MnF ₂ and LiF.
[14] The method for producing a positive electrode active material for a lithium ion secondary battery according to [9], wherein the average particle size of the pulverized product of the Li-Mn composite oxide having a spinel structure is 5 μm or less.
[15] The method for producing a positive electrode active material for a lithium ion secondary battery according to [9], wherein the average particle size of the pulverized product of the Li—Mn composite oxide having a spinel structure is 3 μm or less.
[16] The method for producing a positive electrode active material for a lithium ion secondary battery according to [9], wherein the granulation step is performed by a spray granulation method, a stirring granulation method, a compression granulation method or a fluidized granulation method. .
[17] In the granulation step, an acrylic resin, a copolymer of isobutylene and maleic anhydride, polyvinyl alcohol, polyethylene glycol, polyvinyl pyrrolidene, hydroxypropyl cellulose, methyl cellulose, corn starch, gelatin, The method for producing a positive electrode active material for a lithium ion secondary battery according to the above [9], wherein at least one organic compound selected from the group consisting of lignin is used.
[18] The method for producing a positive electrode active material for a lithium ion secondary battery according to the above [17], which has a degreasing step at a temperature of 300 ° C. to 550 ° C. in a gas flow atmosphere containing air or oxygen.
[19] A positive electrode active material for a lithium ion secondary battery obtained by the method according to any one of [9] to [18].
[20] An electrode paste containing the positive electrode active material for a lithium ion secondary battery according to any one of [1] to [8].
[21] A positive electrode for a lithium ion secondary battery comprising the positive electrode active material for a lithium ion secondary battery according to any one of [1] to [8] or [19].
[22] A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to [21].
[23] The lithium ion secondary battery according to [22], wherein the lithium ion secondary battery is a coin battery, a wound battery, a cylindrical battery, a prismatic battery, or a stacked battery.

It is a scanning electron micrograph (x15,000 times) of an example (Example 14) of the positive electrode active material granulated, fired, and sized according to the present invention. It is a particle size distribution of an example (Example 14) of the positive electrode active material granulated, fired, and sized according to the present invention.

Hereinafter, the present invention will be specifically described.
The present invention relates to a Li—Mn based composite oxide positive electrode active material having a spinel structure in which the porosity of secondary particles is greatly reduced to 15% or less compared to conventional products. In addition, the present invention relates to a Li—Mn composite oxide having a spinel structure in which the average porosity of secondary particles is 10% or less and the cycle characteristics are particularly excellent compared to conventional products.

That is, the positive electrode active material of a lithium-manganese (Li-Mn) based composite oxide having a spinel structure in the present invention has chemical formulas LiMn ₂ O ₄ , Li _{1 + x} Mn _2−x O ₄ (where x is 0 < x <0.2) or a chemical formula in which Mn is substituted with at least one element selected from the group consisting of chromium, cobalt, aluminum, nickel, iron, and magnesium (abbreviated as M in the chemical formula). _{li 1 + x Mn 2-xy} M y O 4 ( wherein x is in the range of 0 <x <0.2, y is 0 <y <0.4.) a general term of a compound represented by It is.

In the present invention, the positive electrode active material for a lithium ion secondary battery is mainly composed of the Li-Mn composite oxide having the spinel structure, and the porosity of one secondary particle is represented by the following formula (1 )
Porosity (%) = (A / B) × 100 (1)
(A is the total cross-sectional area of pores included in the cross section of one secondary particle, and B is the cross-sectional area of one secondary particle), and the average porosity is 15% or less Is used.

Further, as the Li—Mn based composite oxide, those having an average porosity of 10% or less and an average primary particle diameter of 0.2 to 3 μm are preferably used. .
That is, in order for the tap density to exceed 1.9 g / ml as the positive electrode active material, the average porosity of secondary particles needs to be 15% or less. The average porosity of the secondary particles is preferably 13% or less, more preferably 10% or less.

  In general, in the composite oxide manufacturing method, if the sintering temperature is increased and the sintering time is increased to cause the shrinkage of the secondary particles to reduce the average porosity of the secondary particles as much as possible, the primary particles cause the sintering shrinkage. Accompanying this, the grains grow and become large, and when this material is used as a battery positive electrode active material, the capacity retention rate decreases. As a result, battery characteristics after battery assembly are deteriorated.

  As a result of diligent research on a method for suppressing grain growth and sintering shrinkage, the present inventors have achieved a temperature at least 100 ° C. higher than the sintering shrinkage initiation temperature measured by a thermo-mechanical analysis. After raising the temperature at a rate of at least 100 ° C./min, holding for 1 minute to 10 minutes, and then lowering the temperature to the sintering shrinkage start temperature at a rate of at least 100 ° C./min to suppress grain growth and sinter It was found that it can shrink.

  Here, the sintering shrinkage start temperature refers to the shrinkage start temperature obtained with a thermomechanical tester. The holding temperature needs to be at least 100 ° C. higher than the sintering shrinkage start temperature. When the holding temperature is made lower than 100 ° C. below the sintering start temperature, the sintering shrinkage rate is slow, so that a longer sintering time is required. As a result, the grains grow and the primary particle diameter becomes larger than 0.5 μm. End up.

  Further, in the sintering step, the primary particle diameter is 0.2 μm or more and 0.5 μm or less at a temperature at least 100 ° C. higher than the sintering shrinkage start temperature, and excellent battery characteristics are obtained. The holding time is at least 1 minute and within 10 minutes. If the holding time is less than 1 minute, the heat transfer time is too short, the primary particle size is as small as less than 0.2 μm, crystallization is insufficient, and the initial capacity is reduced. When the holding time exceeds 10 minutes, grain growth proceeds even after sintering shrinkage, so that the primary particles become large and the capacity retention rate decreases.

In the present invention, the holding time is preferably 2 to 8 minutes, more preferably 2 to 5 minutes.
In the temperature range from the sintering start temperature to the holding temperature, the rate of temperature increase and the rate of temperature decrease was limited to at least 100 ° C./minute because the holding time in the temperature range where grain growth proceeds is made as short as possible. This is because the grain growth is suppressed only by proceeding.
Further, in order for the tap density of the positive electrode active material to exceed 2.2 g / ml, the average porosity of the secondary particles needs to be 10% or less, preferably 7% or less, more preferably 5%. It is as follows.

  The crystallite size of the positive electrode active material in the present invention is preferably 400 to 960 Å. When the crystallite size is less than 400 angstroms, the initial capacity of the battery is low because of insufficient crystallinity, and the capacity retention rate is low. On the other hand, when the crystallite size exceeds 960 angstroms, the capacity retention rate is drastically reduced. More specifically, the preferred crystallite size is 500 to 920 angstroms, and more desirably 700 to 920 angstroms.

  In addition, the positive electrode active material of the Li—Mn composite oxide having a spinel structure of the present invention preferably has a lattice constant of 8.240 angstroms or less. When the lattice constant exceeds 8.240 angstroms, the capacity retention rate of the battery is drastically reduced. Therefore, the preferable range of the lattice constant is 8.235 angstroms or less, and more preferably 8.233 angstroms or less.

  The positive electrode active material of the present invention mainly composed of a Li-Mn composite oxide having a spinel structure is obtained by crushing a fired product of a Li-Mn composite oxide having a spinel structure and then obtaining pulverized particles (this is 1 Dense particles obtained by adding and mixing a sintering acceleration aid (granulation accelerator) to secondary particles in which secondary particles are aggregated, preferably having an average particle size of 0.5 μm or less. Particles are used. Here, the dense granulated particles mean that there are no or few voids between the primary particles of the oxide. The positive electrode active material of the present invention is the above-mentioned dense granulated particles, and is formed using a sintering acceleration aid described later.

Hereinafter, the manufacturing method of the positive electrode active material of this invention is demonstrated.
In a method for producing a Li-Mn composite oxide having a spinel structure, a mixture of a manganese compound and a lithium compound, or a mixture to which a compound containing a different element that can be substituted with manganese is further added in the atmosphere or in an oxygen gas flow. What is necessary is just to bak at least 1 hour or more at the temperature of 300-850 degreeC.

  There is no restriction | limiting in particular about the crystallinity of Li-Mn type complex oxide which has a spinel structure, The unreacted lithium compound and manganese oxide may remain | survive. When using a Li—Mn composite oxide having a spinel structure with high crystallinity, there is no particular limitation on the lattice constant, but when used for a positive electrode active material having a lattice constant of 8.240 angstroms or less, the capacity retention rate is reduced. Reduction is suppressed.

The raw material of the Li-Mn composite oxide having a spinel structure is not particularly limited, but is preferably a known manganese compound such as manganese dioxide, manganese sesquioxide, manganese trioxide, hydrated manganese oxide, manganese carbonate. Manganese nitrate can be used, and lithium hydroxide, lithium carbonate, lithium nitrate and the like can be used as the lithium compound.
Desirably, the manganese compound is preferably manganese carbonate that easily reacts at a low temperature with a lithium compound having excellent battery characteristics when applied to a positive electrode active material.

The preparation of manganese substituted for _{Li 1 + x Mn Li-Mn} -M ( different element) indicated by the _2-xy M _y O ₄ based complex oxide, chromium together with the raw material of the manganese compound and the lithium compound, a cobalt At least one element selected from the group consisting of aluminum, nickel, iron and magnesium is used. The compound containing the different element M may be any compound that can form the oxide by a heating reaction, and may be added together with a lithium compound or a manganese compound in the heating reaction.

  The method for crushing and crushing secondary particles of the Li—Mn composite oxide having the spinel structure is not particularly limited, and known crushers and crushers can be used. Examples thereof include a medium agitating pulverizer, a ball mill, a paint shaker, a jet mill, and a roller mill. The crushing and pulverizing method may be dry or wet. There is no restriction | limiting in particular also about the solvent which can be used in the case of wet, For example, water, alcohol, etc. are used.

From the viewpoint of promoting sintering shrinkage, the particle size of the Li—Mn based composite oxide having a spinel structure after pulverization and pulverization is important.
The particle size is preferably 5 μm or less in average particle size as measured with a laser particle size distribution analyzer. More preferably, the average particle diameter is 2 μm or less without including coarse particles of 5 μm or more. More preferably, it does not contain coarse particles exceeding 3 μm and has an average particle size of 1.5 μm or less, more preferably 0.5 μm or less, still more preferably 0.3 μm or less, and particularly preferably 0.2 μm or less. .

There are no particular limitations on the method of mixing the pulverized and pulverized Li-Mn composite oxide particles and the sintering accelerating aid. For example, the above-mentioned medium agitating pulverizer, ball mill, paint shaker, mixing mixer, etc. can be used. . The mixing method may be either dry or wet. When pulverizing and pulverizing the Li—Mn-based composite oxide, a sintering promoting aid may be added and mixed at the same time.
The sintering promoting auxiliary agent only needs to be capable of sintering the pulverized / pulverized particles of Li-Mn composite oxide particles for granulation, more preferably a compound that melts at a temperature of 900 ° C. or lower, For example, a precursor capable of becoming an oxide or oxide that can be melted at a temperature of 550 ° C. to 900 ° C., or a compound that can be dissolved or reacted with lithium or manganese to become an oxide or oxide that melts by reaction. Good.

Examples of the sintering acceleration aid include compounds containing elements such as Bi, B, W, Mo, and Pb, and these compounds may be used in any combination. A compound in which B ₂ O ₃ and LiF are combined or a compound in which MnF ₂ and LiF are combined is also used. Among these, compounds containing Bi, B, and W elements are preferable because they have a large sintering shrinkage effect.

  Examples of the Bi compound include bismuth trioxide, bismuth nitrate, bismuth benzoate, bismuth oxyacetate, bismuth oxycarbonate, bismuth citrate, and bismuth hydroxide. Examples of the B compound include boron trioxide, boron carbide, boron nitride, and boric acid. Examples of the W compound include tungsten dioxide and tungsten trioxide.

  The addition amount of the sintering accelerating aid is preferably in the range of 0.0001 to 0.05 mol with respect to 1 mol of Mn in the Li-Mn composite oxide in terms of added metal element. If the added amount in terms of added metal element is less than 0.0001 mol, the sintering shrinkage effect cannot be obtained, and if it exceeds 0.05 mol, the initial capacity of the active material becomes small. A preferable addition amount is 0.005 to 0.03 mol.

  The sintering aid may be used in a powder state or in a liquid state dissolved in a solvent. When added in a powder state, the average particle size of the sintering accelerator aid is preferably 50 μm or less, more preferably 10 μm or less, and even more preferably 3 μm or less. The sintering promoting aid is preferably added before granulation / sintering, but the granulated product may be impregnated and sintered at a temperature at which the sintering promoting aid can be melted after granulation.

  In many cases, the sintering accelerator aid remains in the positive electrode material used in the battery after firing. For example, the sintering accelerator assistant used in the production method of the present invention remains in the cathode active material. It is detected by analysis.

Next, the granulation method will be described.
Examples of the granulation method include spray granulation method, fluidized granulation method, compression granulation method, stirring granulation method and the like using the above-mentioned sintering acceleration aid, and also include medium fluidized drying and medium vibration drying. You may use together.

  In the present invention, it is sufficient that dense secondary particles (including granulated particles) can be formed, and there is no particular limitation on the granulation formation method. Agitation granulation and compression granulation are particularly preferred because the density of secondary particles is high, and spray granulation is particularly preferred because the granulated particle shape is a true sphere. Examples of the agitating granulator include a vertical granulator manufactured by Paulek Co., Ltd. and a Spartan Luther manufactured by Fuji Paudal Co., Ltd. Examples of the compression granulator include Kurimoto Tekko Co., Ltd. Examples thereof include a roller compactor MRCP-200 type. As an example of a spray granulator, Ashizawairo atomizer Co., Ltd. Mobile minor type spray dryer etc. are mentioned.

  There are no particular restrictions on the size of the secondary particles to be granulated. When the average particle size of the granulated secondary particles is too large, the particles may be crushed, pulverized and classified immediately after granulation or after sintering, and the particle size may be adjusted to a desired particle size. In general, secondary particles having an average particle size of 10 to 20 μm are preferred.

In order to increase the granulation efficiency, an organic granulation aid may be added.
Examples of such granulation aids include acrylic resins, copolymers of isobutylene and maleic anhydride, polyvinyl alcohol, polyethylene glycol, polyvinyl pyrrolidene, hydroxypropyl cellulose, methyl cellulose, corn starch, gelatin, and lignin. .

  The method of adding the granulation aid is more efficient than adding it in a powdered state by adding and granulating it by dissolving it in an organic solvent such as water or alcohol. The addition amount of the granulation aid is preferably 5 parts by mass or less, more preferably 2 parts by mass or less, with respect to 100 parts by mass of the Li-Mn composite oxide having a spinel structure and the sintering acceleration aid.

Next, a method for firing the granulated secondary particles will be described.
The method for degreasing the granulated particles is carried out by holding for 10 minutes or more in the temperature range of 300 to 550 ° C. in the air or in a gas flow containing oxygen. The carbon residue in the degreased granulated product is preferably 0.1% or less.

  Firing of the granulated particles after degreasing is performed for 1 minute or more in the temperature range of 550 ° C. to 900 ° C. in the air or in a gas flow containing oxygen in order to suppress grain growth and advance sintering shrinkage. The sintering promoting aid can be held in a molten state on the surface of the Li—Mn composite oxide particles, sintered and shrunk, and the secondary particles can be densified.

  Further, in the present invention, the calcination of the granulated particles after degreasing is the sintering measured by a thermomechanical tester in the atmosphere or in a gas flow containing oxygen in order to suppress the grain growth and advance the sintering shrinkage. After raising the temperature at a rate of at least 100 ° C./min to a temperature that is at least 100 ° C. higher than the shrinkage start temperature and holding for 1 to 10 minutes, the temperature is lowered to the sintering shrinkage start temperature at a rate of at least 100 ° C./min. Sintering shrinks to densify the secondary particles. About the temperature increase rate and temperature decrease rate between normal temperature and sintering shrinkage start temperature, it may be 10 degrees C / min or less as usual.

In addition, the firing of the granulated particles without using the above-mentioned organic-based granulating aid is similarly performed by sintering and shrinking in the air or in a gas flow atmosphere containing oxygen, thereby densifying the secondary particles. be able to.
The positive electrode active material of the present invention and the positive electrode active material obtained from the production method of the present invention are processed into a positive electrode for a lithium ion secondary battery according to the same method as a conventional Li-Mn composite oxide, and evaluation of the battery To be served.

Hereinafter, the example which uses the said positive electrode active material of this invention as a positive electrode material of a non-aqueous secondary battery is demonstrated.
First, as a positive electrode material, a solution (for example, N-methylpyrrolidone) in which a positive electrode active material, a conductivity imparting agent such as carbon black or graphite, and a binder (binding material) such as polyvinylidene fluoride are dissolved is kneaded at a predetermined ratio. The electrode paste is applied to the current collector, and then dried and pressed by a roll press or the like. As the current collector, a known metal current collector such as aluminum, stainless steel or titanium is used.

As the electrolyte salt in the electrolytic solution used in the nonaqueous secondary battery of the present invention, a known lithium salt containing fluorine can be used. For example, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ can be used. The electrolyte solution of the non-aqueous secondary battery is used by dissolving at least one electrolyte of a known lithium salt containing fluorine in a non-aqueous electrolyte solution. As the non-aqueous solvent of the non-aqueous electrolyte solution, a chemically and electrochemically stable aprotic solvent can be used.

  For example, dimethyl carbonate, propylene carbonate, ethylene carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, diethyl carbonate, ethyl propyl carbonate, diisopropyl carbonate, dibutyl carbonate, 1,2-butylene carbonate, ethyl isopropyl carbonate, Examples thereof include carbonates such as ethylbutyl carbonate. Also, oligoethers such as triethylene glycol methyl ether and tetraethylene glycol dimethyl ether, aliphatic esters such as methyl propionate and methyl formate, aromatic nitriles such as benzonitrile and tolunitrile, amides such as dimethylformamide, dimethyl Examples include sulfoxides such as sulfoxide, lactones such as γ-butyrolactone, sulfur compounds such as sulfolane, N-vinylpyrrolidone, N-methylpyrrolidone, and phosphate esters. Of these, carbonic acid esters, aliphatic esters, and ethers are preferable.

  The negative electrode used in the nonaqueous secondary battery of the present invention is not particularly limited as long as it is a material capable of reversibly occluding and releasing lithium ions. For example, lithium metal, lithium alloy, carbon material (including graphite) Metal chalcogen etc. can be used.

Next, a method for evaluating electrode characteristics will be described.
A positive electrode active material, Cabot Vulcan XC-72 as a conductive material, and tetrafluoroethylene resin as a binder are mixed in a mass ratio of 50:34:16, and the mixture is swollen with toluene for 12 hours. The swollen mixture is applied onto a current collector made of aluminum expanded metal, and pressure-molded at ² t / cm ² , and toluene is dried to form a positive electrode. On the other hand, lithium foil is used as the negative electrode.

As an electrolytic solution, a solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / liter in a mixed solution in which propylene carbonate and dimethyl carbonate are mixed at a volume ratio of 1: 2 is used. A separator made of polypropylene is used, and silica fiber filter paper (for example, QR-100 manufactured by Advantech Toyo Co., Ltd.) is also used as a reinforcing material for the purpose of preventing micro shorts caused by dendrite formation of the negative electrode. . Using these positive electrode, negative electrode, electrolytic solution, separator and reinforcing material, a 2016 coin battery is prepared and subjected to 500 charge / discharge cycle tests in a thermostat set at 60 ° C. The measurement conditions are constant current constant voltage charge-constant current discharge, charge and discharge rate 1C (charge suspend in 2.5 hours from the start of charge), and scanning voltage 3.1V to 4.3V.

EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated, this invention is not limited at all by the following description.
In addition, the characteristic of the positive electrode active material shown to the following example and Tables 1-3 was measured with the following method.
1) Average particle size and specific surface area Disperse the powder by ultrasonic wave in 0.2% aqueous solution of surfactant (Kao Demol P) using CILAS GRANULOMETER (HR850 type) as a laser particle sizer. Then, the particle size distribution was measured and determined.
2) Tap density Measured after tapping 2000 times with an amplitude of 8 mm using a tapping machine (KRS-409 type) manufactured by Kuramochi Scientific Instruments.

3) Porosity The positive electrode active material and thermosetting resin are mixed and cured to embed the positive electrode active material in the resin, cut with a microtome, mirror polished, and photograph the polished surface with a scanning electron microscope (SEM) did. The cross-sectional area B of one secondary particle in the obtained SEM photograph and the total cross-sectional area A of all the pores contained in the cross-section of the secondary particle are measured with an image analyzer, and one secondary particle is expressed by the following formula. The porosity C (%) was calculated, and the average value of the porosity of 50 randomly selected secondary particles was defined as the average porosity.
C (%) = (A / B) × 100
4) Crystallite size It calculated using the Scherrer formula from the X-ray diffraction peak of the (111) plane measured under the following conditions.

  That is, assuming that the outer shape of the crystallite is a cube and does not have a size distribution, a value obtained by calculating the diffraction line broadening due to the crystallite size from the half width was used. In addition, after pulverizing the single crystal silicon with a sample mill made of tungsten carbide and using a powder obtained by sieving to 44 μm or less as an external standard, an apparatus constant correction curve was prepared.

[Measurement apparatus and method]
Rad type goniometer manufactured by Rigaku Corporation, continuous measurement as measurement mode, and RINT2000 series application software was used as analysis software, and the size of crystallites was analyzed.
The measurement conditions are X-ray (CuKα ray), output 50 kV, 180 mA, slit width (3 places) 1/2 °, 1/2 °, 0.15 mm, scan method 2θ / θ method, scan speed 1 The measurement range (2θ) is 17 to 20 °, and the step is 0.004 °. The accuracy of the crystallite size obtained by this method is ± 30 angstroms.

5) Lattice constant
JB Nelson and DP Riley's method (Proc. Phys. Soc., 57 , 160 (1945)).
6) Specific surface area Measured by the BET method.
7) Shape of granulated particles Photographed granulated product of positive electrode active material with SEM, image analysis, secondary particle circularity (circularity = 4π [area / (perimeter length) ² ]) and needle The shape ratio (needle ratio = needle absolute maximum length / diagonal width) was determined. For each sample, 200 secondary particles were measured, and the average value was obtained.

Example 1
Manganese carbonate (Chuo Electric Industry Co., Ltd., C2-10) having a specific surface area of 22 m ² / g and lithium carbonate (Honjo Chemical Co., Ltd. 3N) so that the Li / Mn atomic ratio is 0.51. ) Was mixed with a ball mill, and the temperature was raised from room temperature to 650 ° C. at a heating rate of 200 ° C./hr in an air atmosphere and kept at that temperature for 4 hours to synthesize a Li—Mn based composite oxide. In the composite, a very small amount of manganese sesquioxide other than the Li—Mn composite oxide was detected by an X-ray analyzer (XRD). The average particle size of the synthesized product measured with a laser particle size distribution analyzer was 10 μm, and the specific surface area was 7.7 m ² / g.

The obtained Li-Mn composite oxide having a spinel structure was dispersed in an ethanol solvent and pulverized with a wet ball mill to make the average particle size 0.5 μm. As a result of the measurement, the pulverized powder did not contain large particles of 3 μm or more, and the specific surface area was 27.8 m ² / g. To this pulverized powder, bismuth oxide having an average particle diameter of 2 μm was added and mixed so that the atomic ratio of Bi / Mn was 0.0026, and the mixture was mixed with Fuji Powder Co., Ltd. Spartan Luzer RMO-6H. Agitation granulation was performed.

  As a granulation aid, 1.5 parts by mass of polyvinyl alcohol was dissolved in an aqueous solution and added to 100 parts by mass of the mixed powder of Li-Mn composite oxide and bismuth oxide, and granulated for 16 minutes. The obtained granulated material was lightly crushed and pulverized with a mixer, and sized with an air classifier to an average particle size of 15 μm. The tap density of the granulated product after the sizing was 1.65 g / ml.

  The obtained granulated product is kept in the atmosphere at 500 ° C. for 2 hours and degreased (polyvinyl alcohol is decomposed), then heated in the atmosphere at 200 ° C./hr and kept at 750 ° C. for 20 hours to obtain a positive electrode active material. It was. The positive electrode active material produced here was confirmed to contain the Bi element of the bismuth oxide by an ICP-AES method (inductively coupled plasma emission analysis method) and to contain the composition ratio.

The obtained positive electrode active material had an average porosity of 11.2%. Further, the tap density of the positive electrode active material was 1.96 g / ml, the crystallite size was 880 angstroms, and the lattice constant was 8.233 angstroms.
Using the above positive electrode active material, a coin-type battery was produced as follows. A positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone are kneaded at a mass ratio of 80:10:10, applied onto an aluminum foil, and pressed. A positive electrode was obtained. A lithium foil having a predetermined thickness was used as the negative electrode. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / liter in a mixed solution in which propylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 2 was used. Using these positive and negative electrodes, a polypropylene separator, an electrolytic solution and a glass filter, a 2016 type coin-type battery was produced.

  A charge / discharge cycle test at 60 ° C. of the battery produced by the above method was performed with a charge / discharge rate of 1C (charge stopped in 2.5 hours from the start of charge) and a charge / discharge cycle of 100 cycles under the condition of a voltage range of 3.0 to 4.2V Was repeated. Table 1 shows the initial discharge capacity and the capacity retention rate (%) after 100 cycles, together with other measurement results.

Example 2
The same procedure as in Example 1 was carried out except that the manganese raw material in the Li-Mn composite oxide synthesis conditions was electrolytic manganese dioxide, the porosity of the secondary particles, the tap density, the crystallite size, the lattice constant, the electrode The characteristics were evaluated. The results are shown in Table 1.

Example 3
Manganese carbonate, lithium carbonate and aluminum hydroxide were mixed with a ball mill so that the atomic ratio of Li / Mn / Al was 1.02: 1.967: 0.013, and the heating rate in the atmosphere was 200 ° C. / The temperature was raised from room temperature to 650 ° C. with hr and held at 650 ° C. for 4 hours to synthesize a Li—Mn-based composite oxide. In the synthesized product, a very small amount of manganese sesquioxide other than the Li—Mn composite oxide was detected by XRD. The average particle size of the synthesized product measured with a laser particle size distribution analyzer was 10 μm.
The obtained Li—Mn composite oxide was pulverized to an average particle size of 0.5 μm, and boron oxide was added and granulated so that the atomic ratio of B / Mn was 0.0208. Next, the same procedure as in Example 1 was performed except that the granulated product after degreasing was fired at 750 ° C. for 0.5 hr. The results are shown in Table 1.

Example 4
The same procedure as in Example 3 was performed except that the B / Mn atomic ratio was 0.009 and the degreased granulated product was calcined at 760 ° C. for 0.5 hr. The results are shown in Table 1.

Example 5
The same procedure as in Example 3 was performed except that the B / Mn atomic ratio was 0.006 and the degreased granulated product was calcined at 770 ° C. for 0.5 hr. The results are shown in Table 1.

Example 6
It was carried out in the same manner as in Example 1 except that the granulated product after degreasing was fired at 760 ° C. for 20 hours. The results are shown in Table 1.

Example 7
Except for changing the bismuth oxide to tungsten trioxide, adding tungsten trioxide at an atomic ratio of W / Mn of 0.0208, and firing the degreased granulated product at 750 ° C. for 20 hours, The same operation as in Example 1 was performed. The results are shown in Table 1.

Example 8
The Li—Mn-based composite oxide synthesized in Example 1 was further crystallized by raising the temperature from room temperature to 750 ° C. at a heating rate in the atmosphere of 200 ° C./hr and holding at 750 ° C. for 20 hours. Thereafter, the Li-Mn composite oxide crystallized in Example 1 was used, bismuth oxide was changed to boron oxide, and boron oxide was added at a B / Mn atomic ratio of 0.0208. This was carried out in the same manner as in Example 1 except that the granulated product after degreasing was calcined at 750 ° C. for 0.5 hr. The results are shown in Table 1.

Example 9
The same procedure as in Example 3 was performed except that the Li—Mn composite oxide before granulation had an average particle size of 3.5 μm and a specific surface area of 10 m ² / g. The results are shown in Table 1.

Example 10
Li-Mn based composite oxidation synthesized by mixing manganese carbonate, lithium carbonate and aluminum hydroxide with a ball mill so that the atomic ratio of Li / Mn / Al is 1.03: 1.967: 0.013. The same procedure as in Example 3 was performed except that the product was used. The results are shown in Table 1.

Example 11
It was carried out in the same manner as in Example 1 except that the granulated product after degreasing was fired at 830 ° C. for 20 hours. The results are shown in Table 2.

Example 12
Li-Mn based composite oxidation synthesized by mixing manganese carbonate, lithium carbonate and aluminum hydroxide with a ball mill so that the atomic ratio of Li / Mn / Al is 0.99: 1.967: 0.013. The same procedure as in Example 3 was performed except that the product was used. The results are shown in Table 2.

Example 13
The same procedure as in Example 3 was performed except that the average particle size after granulation was adjusted to 65 μm. The results are shown in Table 2.

Example 14
The same operation as in Example 1 was carried out except that the Bi / Mn atomic ratio was 0.0019. The results are shown in Table 2. As a result of observing the granulated, fired and sized positive electrode active material obtained here with a scanning electron microscope (× 15,000 times), it was found that the particles were round particles as shown in FIG. The particle size distribution of the particles is shown in FIG.

Comparative Example 1
The same operation as in Example 1 was performed except that the average particle diameter of the Li—Mn composite oxide before granulation was 6.0 μm. The results are shown in Table 2.

Comparative Example 2
Electrolytic manganese dioxide having an Li / Mn atomic ratio of 0.51 and an average particle size of 20 μm was mixed with lithium carbonate by a ball mill, and the temperature was raised to 760 ° C. at a heating rate of 100 ° C./hr in the atmosphere. The positive electrode active material was synthesized by maintaining at 24 ° C. for 24 hours. The obtained positive electrode active material was evaluated in the same manner as in Example 1. The results are shown in Table 2.

Comparative Example 3
The same procedure as in Example 1 was performed except that granulation was performed without adding a sintering accelerator. The results are shown in Table 2.

Comparative Example 4
It was carried out in the same manner as in Example 3 except that the granulated material was fired at 750 ° C. for 20 hours. The results are shown in Table 2.

Analysis of Shape Measurement Results of Granulated Particles Circularity of the secondary particles produced in Examples 1 to 14 and Comparative Examples 1 to 4 shown in Tables 1 to 2 (circularity = 4π [area / (perimeter length) ^2] ] And the acicular ratio (needle ratio = needle absolute maximum length / diagonal width), the positive electrode active material produced in the example has a circularity of 0.7 or more and an acicular ratio of 1 It can be seen that there are features below .35.

  The positive electrode active material of the present invention is essentially different in that it is granulated and sintered as compared with the conventionally known secondary particles utilizing cohesive force, compared with the positive electrode active material obtained by the conventional method. The particles are dense and spherical, have excellent filling properties to the electrode, and have an effect of increasing the initial capacity and capacity retention rate even in a high temperature environment as a secondary battery.

  According to the method for producing a positive electrode active material of the present invention, by adding a sintering acceleration aid that generates a melt in a high temperature range to the Li-Mn composite oxide, the secondary particles are densified, In the conventional method, excellent battery performance can be obtained even when grown to a crystallite size that deteriorates initial capacity and cycle characteristics. In the conventional method, when the secondary particles are densified, the primary particle size grows larger than 0.5 μm and the initial capacity and cycle characteristics are deteriorated. This is solved by the method of the present invention in which an accelerator aid is added to the Li—Mn composite oxide, and a positive electrode active material having high filling properties and excellent battery performance is obtained.

  Since the lithium ion secondary battery of the present invention uses a positive electrode active material excellent in filling properties, it is excellent in initial capacity and capacity retention at a high temperature.

Claims (12)

In the positive electrode active material for a lithium ion secondary battery mainly composed of Li-Mn composite oxide particles having a spinel structure, the following formula
Porosity (%) = (A / B) × 100 (1)
(A is the total cross-sectional area of the pores included in the cross section of one secondary particle, and B is the cross-sectional area of one secondary particle.) The average value of the porosity of the particles shown in FIG. , and the crystallite size of the positive electrode active material Ri der less 960 angstroms, BET specific surface area of the positive electrode active material is not more than 2.0 m ² / g, der lattice constant 8.243 Å of the positive electrode active material A positive electrode active material for a lithium ion secondary battery.   2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the value of the average porosity is 10% or less and the average particle diameter of primary particles is 0.2 to 3 μm.   The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode active material has a tap density of 1.9 g / ml or more.   The positive electrode active material for a lithium ion secondary battery according to claim 3, wherein the tap density of the positive electrode active material is 2.2 g / ml or more.   The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the crystallite size of the positive electrode active material is 400 to 960 angstroms.   The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the positive electrode active material has a lattice constant of 8.240 angstroms or less.   The particles are formed by granulating and sintering a mixture containing a Li-Mn composite oxide having a spinel structure and a sintering acceleration aid, and the sintering acceleration aid is (1) 550 An oxide that melts at a temperature of 950 ° C. to 900 ° C., or a precursor that can become the same oxide, or (2) an oxide that melts by reacting with lithium or manganese at a temperature of 550 ° C. to 900 ° C., or the same oxidation The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material is a precursor that can become a product. The sintering accelerating aid is a compound containing at least one element selected from the group consisting of Bi, B, W, Mo, Pb, a compound containing B ₂ O ₃ and LiF, or MnF ₂ and LiF. The positive electrode active material for a lithium ion secondary battery according to claim 7, which is a compound in which   The paste for electrodes containing the positive electrode active material for lithium ion secondary batteries of any one of Claims 1 thru | or 8.   The positive electrode for lithium ion secondary batteries containing the positive electrode active material for lithium ion secondary batteries in any one of Claims 1 thru | or 8.   The lithium ion secondary battery provided with the positive electrode for lithium ion secondary batteries of Claim 10.   The lithium ion secondary battery according to claim 11, wherein the lithium ion secondary battery is a coin-type battery, a wound battery, a cylindrical battery, a prismatic battery, or a stacked battery.

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