JP2004362781A - Cathode active substance and nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cathode active substance capable of improving discharge load characteristics of a nonaqueous electrolyte secondary battery and with excellent thermal stability. <P>SOLUTION: Of the cathode active substance containing secondary particles which are coagulations of primary particles of lithium-containing metal complex oxide, an average maximum particle size L<SB>1</SB>max of the primary particles is within the range of 0.1×L<SB>2</SB>max≤L<SB>1</SB>max<0.5×L<SB>2</SB>max, when an average maximum particle size of the secondary particles is L<SB>2</SB>max, and an average value of a shape factor SF2 calculated by a formula (1): SF2=R<SB>1</SB>/R<SB>2</SB>, is more than 1 and less than 2, (provided, R<SB>2</SB>denotes an area of any two-dimensional image of the secondary particle, and R1 denotes an area of a region surrounded by a shortest envelope connecting tip points of protruded parts of the two-dimensional image). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０１３５】
表１から明らかなように、二次粒子の形状係数ＳＦ２の平均値が１を超え、２未満の範囲にあり、かつ一次粒子の平均最大粒径Ｌ_１ｍａｘが０．１×Ｌ_２ｍａｘ≦Ｌ_１ｍａｘ＜０．５×Ｌ_２ｍａｘの範囲にある正極活物質を備えた実施例１〜７の二次電池は、放電レートを高くした際の容量低下幅が小さく、優れた放電負荷特性を有していた。また、実施例１〜７の二次電池は、充電状態で高温貯蔵した際の膨れが比較例１〜４に比較して小さかった。
【０１３６】
これに対して、二次粒子の形状係数ＳＦ２の平均値が１である正極活物質を備えた比較例１の二次電池と、二次粒子の形状係数ＳＦ２の平均値が２以上である正極活物質を備えた比較例２の二次電池は、実施例１〜７の二次電池に比較して放電レートを高くした際の容量低下が著しく、そのうえ充電状態で高温貯蔵時の電池膨れが大きかった。
【０１３７】
一方、二次粒子の形状係数ＳＦ２の平均値は１を超え、２未満の範囲にあるものの、一次粒子の平均最大粒径Ｌ_１ｍａｘが０．１×Ｌ_２ｍａｘ未満である正極活物質と、ＥＣ、ＭＥＣ及びＶＣからなる非水溶媒を用いた液状非水電解質とを備えた比較例３の二次電池は、実施例１〜７の二次電池に比較して放電レートを高くした際の容量低下が大きく、充電高温貯蔵時の電池膨れも大きかった。
【０１３８】
二次粒子の形状係数ＳＦ２の平均値は１を超え、２未満の範囲にあるものの、一次粒子のＬ_１ｍａｘが０．５×Ｌ_２ｍａｘ以上である正極活物質と、ＥＣ、ＭＥＣ及びＶＣからなる非水溶媒を用いた液状非水電解質とを備えた比較例４の二次電池は、実施例１〜７の二次電池に比較して放電レートを高くした際の容量低下が著しく、そのうえ充電高温貯蔵時の電池膨れが大きかった。
【０１３９】
二次凝集していない単粒子を正極活物質として備えた比較例５の二次電池は、実施例１〜７の二次電池に比較して放電レートを高くした際の容量低下が著しかった。これは、単粒子であると、同じ粒径を有する二次粒子に比較してリチウムイオンの移動距離が長くなり拡散性が低下すると共に、表面が滑らかな曲面で構成されているために非水電解質に対する濡れ性が低いからである。
【０１４０】
【発明の効果】
以上詳述したように、本発明によれば、非水電解質二次電池の放電負荷特性を向上させることが可能で、かつ、熱安定性に優れる正極活物質を提供することができる。また、本発明によれば、優れた放電負荷特性を有するとともに、高温貯蔵時の電池膨れが抑制された非水電解質二次電池を提供することができる。従って、本発明に係る正極活物質と非水電解質二次電池は、近年急成長している携帯電話、ノート型パソコン等の携帯端末機器、電気自動車、電力貯蔵用の電源等として工業的及び商業的に産業界に大きく貢献することが可能である。
【図面の簡単な説明】
【図１】Ｒ_１及びＲ_２の定義を説明するために使用するリチウム含有金属複合酸化物の模式図。
【図２】本発明に係る非水電解質二次電池の一例である薄型非水電解質二次電池を示す断面図。
【図３】図２のＡ部を示す拡大断面図。
【図４】本発明に係る非水電解質二次電池の一例である角型非水電解質二次電池を示す部分切欠斜視図。
【図５】実施例１の正極の断面についての第１の視野の走査型電子顕微鏡写真（倍率２０００倍）。
【図６】実施例１の正極の断面についての第２の視野の走査型電子顕微鏡写真（倍率２０００倍）。
【図７】実施例１の正極の断面についての第３の視野の走査型電子顕微鏡写真（倍率２０００倍）。
【符号の説明】
１…容器、２…電極群、３，２５…セパレータ、４…正極活物質含有層、５…正極集電体、６，２６…正極、７…負極層、８…負極集電体、９，２４…負極、１０…正極端子、１１，３０…負極端子、２１…外装缶、２２…絶縁体、２３…渦巻電極体、２７…スペーサ、２８…蓋体、２９…取出穴、３１…絶縁材、３２…負極リード、３３…絶縁封口板、３４…外装チューブ。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a positive electrode active material and a non-aqueous electrolyte secondary battery.
[0002]
[Prior art]
Currently, mobile terminals such as mobile phones and notebook computers, which are rapidly growing in demand, are highly evaluated for their features of high energy density, small size and light weight. A non-aqueous electrolyte secondary battery provided as a battery is often used. The use of non-aqueous electrolyte secondary batteries as power sources for electric vehicles and electric power storage as well as portable terminal devices is also being studied. Demands for improving the performance of non-aqueous electrolyte secondary batteries have been increasing, and various reports have been issued by many research institutions.
[0003]
Examples of the Li-containing transition metal oxide capable of providing a practical discharge capacity include LiCoO. ₂ , LiNiO ₂ , LiMn ₂ O ₄ Etc. have been proposed. Among them, LiCoO is easy to manufacture and has stable characteristics. ₂ Is dominant.
[0004]
Patent Document 1 discloses a compound having a layered structure and a general formula A _x B _y C _z D _w O ₂ A non-aqueous secondary battery using a composite oxide represented by the formula (1) as a positive electrode is disclosed. In the general formula, A is at least one selected from alkali metals, B is a transition metal, C is at least one selected from the group consisting of Al, In, and Sn, and D is ( a) an alkali metal other than A, (b) a transition metal other than B, (c) a group IIa element, (d) a group IIIb, group IVb, group Vb, or VIb excluding Al, In, Sn, carbon, nitrogen and oxygen. And at least one element selected from the group consisting of elements of the second to sixth periods of the group. Also, x, y, z and w are respectively 0.05 ≦ x ≦ 1.10, 0.85 ≦ y ≦ 1.00, 0.001 ≦ z ≦ 0.10, 0.001 ≦ w ≦ 0.10. Represents the number of
[0005]
As described in the upper right column of page 3, lines 8 to 13 and the upper right column of page 5, line 20 to lower left column, line 6, A, B, C, D And then firing at 600 to 950 ° C. in the air or in an oxygen atmosphere.
[0006]
However, a compound of a metal A such as lithium carbonate, a compound of a metal B such as cobalt oxide, a compound of a metal C such as stannic oxide, and a compound of a metal D such as scandium oxide are simultaneously formed. When mixed and fired at 600 to 950 ° C. in the air or in an oxygen atmosphere, crystal growth proceeds by a melting reaction of the mixture, and flat-plate-like particles are obtained. When the flat particles are used as a positive electrode active material, there arises a problem that excellent discharge load characteristics cannot be obtained.
[0007]
On the other hand, Patent Document 2 discloses a general formula: Li _x M _y Sn _z O ₂ It is disclosed that a Li-containing transition metal composite oxide having a composition represented by the following formula is used as a positive electrode active material. In the general formula, M represents at least one element selected from transition metals, and x, y, and z are 0.9 ≦ x ≦ 1.15, 0.85 ≦ y ≦ 1.00, 0, respectively. <Z <0.001 is satisfied.
[0008]
This Li-containing transition metal composite oxide is a starting material for each metal element (for example, cobalt oxide and lithium carbonate, as described on page 11, lines 11 to 15 and page 11, lines 18 to 20). (Tin oxide) is mixed at a predetermined ratio, and the mixture is baked in air at 900 ° C. for 5 hours to be synthesized.
[0009]
However, when cobalt oxide, lithium carbonate, and tin oxide are mixed at a time and fired at a high temperature such as 900 ° C., the crystal growth proceeds due to the melting reaction of the mixture, and flat plate-like particles are obtained. There is a problem that load characteristics cannot be obtained.
[0010]
By the way, Patent Document 3 discloses that at least one selected from the group consisting of cobalt hydroxide in which cobalt valence is trivalent and cobalt oxyhydroxide, and B, Mg, Si, Cu, Ce, A compound consisting of at least one element selected from the group consisting of Y, Ti, V, Mn, Fe, Ni, Sn, Zr, Sb, Nb, Ru, Pb, Hf, Ta, La, Pr and Nd; A method for producing lithium-cobalt composite oxide particles, which is subjected to a heat treatment at 60 to 500 ° C. after being dispersed in an aqueous lithium hydroxide solution, is disclosed.
[0011]
Also, in paragraph [0038], according to this production method, unlike the solid-state reaction at a high temperature conventionally used for producing lithium-cobalt composite oxide particles, the primary particles are not fused. Further, it is described that lithium-cobalt composite oxide particles having a submicron size and uniform size can be obtained.
[0012]
However, the positive electrode active material composed of the primary particles has poor wettability to the non-aqueous electrolyte, which causes a decrease in the discharge load characteristics of the secondary battery.
[0013]
[Patent Document 1]
JP-A-63-112258
[0014]
[Patent Document 2]
International Publication No. WO00 / 77869A1
[0015]
[Patent Document 3]
JP 10-1316 A
[0016]
[Problems to be solved by the invention]
An object of the present invention is to provide a positive electrode active material capable of improving the discharge load characteristics of a nonaqueous electrolyte secondary battery and having excellent thermal stability.
[0017]
Another object of the present invention is to provide a non-aqueous electrolyte secondary battery having excellent discharge load characteristics and suppressed gas generation during high-temperature storage.
[0018]
[Means for Solving the Problems]
The positive electrode active material of the present invention is a positive electrode active material including secondary particles in which primary particles of a lithium-containing metal composite oxide are aggregated,
The average maximum particle size of the secondary particles is L ₂ The average maximum particle size L of the primary particles when max ₁ max is 0.1 × L ₂ max ≦ L ₁ max <0.5 × L ₂ It is characterized by being in the range of max and the average value of the shape factor SF2 calculated by the following equation (1) is more than 1 and less than 2.
[0019]
SF2 = R ₁ / R ₂ … (1)
Where R ₂ Indicates the area of an arbitrary two-dimensional image of the secondary particles, ₁ Indicates the area of a region surrounded by the shortest envelope connecting the vertices of the convex portions of the two-dimensional image.
[0020]
A first nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery including a positive electrode including secondary particles in which primary particles of a lithium-containing metal composite oxide are aggregated, a negative electrode, and a nonaqueous electrolyte. And
The average maximum particle size of the secondary particles is L ₂ The average maximum particle size L of the primary particles when max ₁ max is 0.1 × L ₂ max ≦ L ₁ max <0.5 × L ₂ max, and the average value of the shape factor SF2 calculated by the formula (1) is more than 1 and less than 2.
[0021]
The second non-aqueous electrolyte secondary battery according to the present invention is one in which the primary particles of the lithium-containing metal composite oxide are aggregated, and the average value of the shape factor SF2 calculated by the formula (1) exceeds 1, A positive electrode comprising secondary particles that is less than 2;
A negative electrode,
A non-aqueous electrolyte containing at least one of cyclohexylbenzene and 2,4-difluoroanisole, and vinylene carbonate;
It is characterized by having.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the positive electrode active material according to the present invention will be described.
[0023]
The positive electrode active material of the present invention is a positive electrode active material including secondary particles in which primary particles of a lithium-containing metal composite oxide are aggregated,
The average maximum particle size of the secondary particles is L ₂ The average maximum particle size L of the primary particles when max ₁ max is 0.1 × L ₂ max ≦ L ₁ max <0.5 × L ₂ max, and the average value of the shape factor SF2 calculated by the following equation (1) exceeds 1 and is less than 2.
[0024]
SF2 = R ₁ / R ₂ … (1)
Where R ₂ Indicates the area of an arbitrary two-dimensional image of the secondary particles, ₁ Indicates the area of a region surrounded by the shortest envelope connecting the vertices of the convex portions of the two-dimensional image.
[0025]
First, the R ₁ And the R ₂ Will be described with reference to FIG. FIG. ₁ And R ₂ FIG. 2 is a schematic diagram of secondary particles used for explaining the following.
[0026]
In FIG. 1, the area indicated by oblique lines is the area R of an arbitrary two-dimensional image of the secondary particles. ₂ It is. On the other hand, a region surrounded by the shortest envelope A connecting the vertices of the convex portion of an arbitrary two-dimensional image of the secondary particle has an area R ₁ It is. Since there are a plurality of methods for connecting the vertices of the convex portion of the two-dimensional image with a curve, a plurality of types of envelopes can be drawn for one two-dimensional image. Among these envelopes, the shortest envelope A is one in which the area surrounded by the tangent X contacting the two adjacent protrusions and the envelope is the smallest. That is, the shortest envelope A is an envelope that draws the shortest trajectory among the group of envelopes connecting the vertices of the convex portions of an arbitrary two-dimensional image of the secondary particles.
[0027]
R thus defined ₁ , R ₂ The average value of the shape factor SF2 calculated from the above is defined in the above range for the reason described below.
[0028]
In the case where the average value of the shape factor SF2 is 1, the fusion reaction between the primary particles progresses and the surface is smoother, so that the contact angle of the positive electrode active material with respect to the nonaqueous electrolyte increases, and the wettability decreases. In addition, high discharge load characteristics cannot be obtained in a non-aqueous electrolyte secondary battery. In addition, it takes a long time to produce secondary particles having an average value of the shape factor SF2 of 1, so that the production cost increases. On the other hand, when the average value of the shape factor SF2 is 2 or more, the fusion reaction between the primary particles has not progressed, and the unevenness of the particle surface has increased, so that the conductive material is easily attached locally, Therefore, high discharge load characteristics cannot be obtained in the non-aqueous electrolyte secondary battery. The more preferable range of the average value of the shape factor SF2 is 1.05 to 1.5, and the more preferable range is 1.1 to 1.3. By setting the average value of SF2 in the range of 1.1 to 1.3, the discharge load characteristics of the secondary battery can be further improved.
[0029]
The lithium-containing metal composite oxide primary particles may be spherical crystals or plate-like crystals. Further, it may have another shape.
[0030]
Average maximum particle size L of primary particles ₁ The reason why max is defined in the above range is as follows.
[0031]
Average maximum particle size L of primary particles ₁ Max is 0.1 × L ₂ If it is less than max, the thermal stability of the positive electrode active material decreases. On the other hand, the average maximum particle size L of the primary particles ₁ max is 0.5 × L ₂ If it is greater than max, the movement distance of lithium ions becomes longer and the lithium ion diffusion rate of the positive electrode active material decreases, so that a high discharge load characteristic cannot be obtained in the nonaqueous electrolyte secondary battery. Average maximum particle size L of primary particles ₁ A more preferred range of max is 0.15 × L ₂ max ≦ L ₁ max ≦ 0.45 × L ₂ max, a more preferred range is 0.2 × L ₂ max ≦ L ₁ max ≦ 0.45 × L ₂ max.
[0032]
Specific surface area of secondary particles by BET method is 0.1 to 10 m ² / G is preferable. This is for the reason described below. 0.1m specific surface area ² If it is less than / g, the reactivity of the positive electrode active material is reduced, so that sufficient discharge load characteristics may not be obtained in the nonaqueous electrolyte secondary battery. On the other hand, the specific surface area is 10m ² If it exceeds / g, the thermal stability of the positive electrode active material decreases, so that the safety of the nonaqueous electrolyte secondary battery at the time of overcharging may be impaired or the amount of gas generated during high-temperature storage may increase. . A more preferable range of the specific surface area is 0.2 to 5 m. ² / G.
[0033]
The average particle size of the secondary particles is preferably in the range of 4 to 20 μm. This is for the reason described below. When the average particle diameter of the secondary particles is less than 4 μm, the secondary particles become closer to the form of a single particle in which the primary particles exist alone, so that the contact angle of the positive electrode active material with the nonaqueous electrolyte increases, and the wettability increases. Tends to decrease. Therefore, it may be difficult to improve the discharge load characteristics of the non-aqueous electrolyte secondary battery. On the other hand, if the average particle size of the secondary particles exceeds 20 μm, the moving distance of lithium ions is increased and the diffusivity is reduced, so that it may be difficult to improve the discharge load characteristics of the nonaqueous electrolyte secondary battery. is there. A more preferable range of the average particle size is 5 to 15 μm.
[0034]
The composition of the lithium-containing metal composite oxide is Li _x CoO ₂ (However, the molar ratio x preferably represents 0.95 ≦ x ≦ 1.15).
[0035]
When the molar ratio x is less than 0.95, Li required for the reaction is insufficient, so that the crystallinity is reduced and the discharge load characteristics may be reduced. If the molar ratio x exceeds 1.15, Li remaining in the active material surface without being completely reacted due to an excessive amount of Li may increase, and the surface resistance may increase, resulting in a decrease in discharge load characteristics.
[0036]
The positive electrode active material according to the present invention is produced, for example, by the method described below. That is, by adding an aqueous alkali solution to an aqueous solution containing a cobalt salt, the cobalt hydroxide is precipitated to obtain an aggregate of cobalt hydroxide crystals. This is filtered to collect a precipitate, which is repeatedly washed with water and dried when the pH is stabilized, thereby obtaining secondary particles in which primary particles of cobalt hydroxide are aggregated. Next, the secondary particles and the lithium compound are mixed at a predetermined ratio, and the mixture is fired in an air atmosphere or an oxygen atmosphere at a temperature of 700 to 1000 ° C. for 5 to 9 hours to obtain a positive electrode active material.
[0037]
Examples of the cobalt salt include cobalt sulfate. Examples of the alkaline aqueous solution include an aqueous solution containing sodium hydroxide, potassium hydroxide, or lithium hydroxide. As the lithium compound, dry powder fine particles or wet sol fine particles can be used.
[0038]
Shape factor SF2 and average maximum particle size L ₁ The adjustment of max can be performed, for example, by changing the firing temperature, the firing time, or the mixing ratio between the cobalt hydroxide secondary particles and the lithium compound. For example, when the firing time is shortened, the fusion reaction between the primary particles is suppressed, and the irregularities on the surface of the secondary particles can be increased, so that the shape factor SF2 can be increased. When single particles of cobalt oxide are used instead of the aggregates of cobalt hydroxide crystals, a positive electrode active material consisting of single particles, not secondary particles, is obtained.
[0039]
The positive electrode active material according to the present invention described above includes secondary particles in which primary particles of a lithium-containing metal composite oxide are aggregated, and has an average maximum particle size of L ₂ The average maximum particle size L of the primary particles when max ₁ max is 0.1 × L ₂ max ≦ L ₁ max <0.5 × L ₂ max and the average value of the shape factor SF2 calculated by the formula (1) is more than 1 and less than 2, so that the lithium ion diffusion rate can be increased without impairing the thermal stability. It is possible to improve the wettability to the non-aqueous electrolyte and to make the distribution of the conductive agent to the positive electrode active material uniform. As a result, the discharge load characteristics of the non-aqueous electrolyte secondary battery can be improved without impairing safety and high-temperature storage characteristics.
[0040]
Next, a nonaqueous electrolyte secondary battery including the positive electrode active material according to the present invention (hereinafter, referred to as a first nonaqueous electrolyte secondary battery) will be described.
[0041]
The first non-aqueous electrolyte secondary battery includes a positive electrode including the positive electrode active material according to the present invention, a negative electrode, and a non-aqueous electrolyte.
[0042]
Hereinafter, the positive electrode, the negative electrode, and the nonaqueous electrolyte will be described in detail.
[0043]
(I) Positive electrode
The positive electrode includes a current collector and a positive electrode active material-containing layer formed on one or both surfaces of the current collector and containing the positive electrode active material according to the present invention.
[0044]
This positive electrode is produced, for example, by suspending the above-described positive electrode active material, conductive agent and binder in an appropriate solvent, applying the obtained suspension to the surface of the current collector, drying and pressing. Is done.
[0045]
Examples of the conductive agent include acetylene black, carbon black, graphite, and the like.
[0046]
Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluorine-based rubber.
[0047]
The mixing ratio of the positive electrode active material, the conductive agent and the binder is preferably in the range of 80 to 95% by weight of the positive electrode active material, 3 to 18% by weight of the conductive agent, and 2 to 7% by weight of the binder.
[0048]
The current collector can be used without particular limitation as long as it is a conductive material.In particular, as the current collector for the positive electrode, it is preferable to use a material that is not easily oxidized during a battery reaction, for example, aluminum, stainless steel, Titanium or the like can be used.
[0049]
(Ii) negative electrode
The negative electrode includes a current collector and a negative electrode layer formed on one or both surfaces of the current collector.
[0050]
This negative electrode is produced, for example, by kneading a powder of a negative electrode material and a binder in the presence of an organic solvent, applying the obtained suspension to a current collector, drying and pressing.
[0051]
Examples of the negative electrode material include carbonaceous materials that occlude and release lithium ions, metals such as aluminum, magnesium, tin, and silicon, metal oxides, metal sulfides, metal nitrides, and lithium alloys.
[0052]
Examples of the carbonaceous material include graphite, coke, carbon fiber, graphite material or carbonaceous material such as spherical carbon, thermosetting resin, isotropic pitch, mesophase pitch, mesophase pitch-based carbon fiber, mesophase spheroid and the like (particularly, And mesophase pitch-based carbon fibers are preferable because they have an increased capacity and charge / discharge cycle characteristics) and are subjected to a heat treatment at 500 to 3000 ° C. Above all, it can be obtained by setting the temperature of the heat treatment to 2000 ° C. or more, and the plane distance d ₀₀₂ It is preferable to use a graphitic material having a graphite crystal having a particle size of 0.34 nm or less. A nonaqueous electrolyte secondary battery provided with a negative electrode containing such a graphite material as a carbonaceous material can significantly improve the battery capacity and large current discharge characteristics. The surface distance d ₀₀₂ Is more preferably 0.336 nm or less.
[0053]
Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC). Can be used.
[0054]
The compounding ratio of the negative electrode material and the binder is preferably in the range of 90 to 98% by weight of the negative electrode material and 2 to 10% by weight of the binder.
[0055]
As the current collector, any conductive material can be used without particular limitation. Among them, foil, mesh, punched metal, lath metal, and the like made of copper, stainless steel, or nickel can be used.
[0056]
(Iii) Non-aqueous electrolyte
As the non-aqueous electrolyte, those having a liquid or gel form can be used.
[0057]
The liquid non-aqueous electrolyte is prepared by dissolving the electrolyte in a non-aqueous solvent.
[0058]
As the non-aqueous solvent, for example, a non-aqueous solvent mainly composed of a cyclic carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) or a mixed solvent of these cyclic carbonate and a non-aqueous solvent having a lower viscosity than the cyclic carbonate is used. Can be used.
[0059]
Examples of the low-viscosity nonaqueous solvent include linear carbonates (eg, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, etc.), γ-butyrolactone, acetonitrile, methyl propionate, ethyl propionate, cyclic ethers (eg, tetrahydrofuran) , 2-methyltetrahydrofuran, etc.) and chain ethers (eg, dimethoxyethane, diethoxyethane, etc.).
[0060]
As the electrolyte, a lithium salt is used. Specifically, lithium hexafluorophosphate (LiPF) ₆ ), Lithium tetrafluoroborate (LiBF ₄ ), Lithium arsenic hexafluoride (LiAsF) ₆ ), Lithium perchlorate (LiClO) ₄ ), Lithium trifluorometasulfonate (LiCF ₃ SO ₃ ). In particular, lithium hexafluorophosphate (LiPF ₆ ), Lithium tetrafluoroborate (LiBF ₄ ) Is a preferred example.
[0061]
The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2 mol / L.
[0062]
The gel non-aqueous electrolyte is obtained by, for example, compounding the above-mentioned liquid non-aqueous electrolyte with a polymer material. Examples of the polymer material include a polymer of a monomer such as polyacrylonitrile, polyacrylate, polyvinylidene fluoride (PVdF), and polyethylene oxide (PECO), or a copolymer with another monomer.
[0063]
Next, the second nonaqueous electrolyte secondary battery according to the present invention will be described.
[0064]
This second non-aqueous electrolyte secondary battery is one in which the primary particles of the lithium-containing metal composite oxide are aggregated, and the average value of the shape factor SF2 calculated by the above-mentioned formula (1) is more than 1 and less than 2 A positive electrode comprising secondary particles,
A negative electrode,
A non-aqueous electrolyte containing at least one of cyclohexylbenzene (CHB) and 2,4-difluoroanisole (DFA) and vinylene carbonate (VC);
Is provided.
[0065]
The negative electrode may be the same as that described in the first nonaqueous electrolyte secondary battery, and the positive electrode and the nonaqueous electrolyte will be described below.
[0066]
1) Positive electrode
The positive electrode includes a current collector, and a positive electrode active material-containing layer formed on one or both surfaces of the current collector and containing the secondary particles as a positive electrode active material.
[0067]
When a non-aqueous electrolyte containing CHB is used, by setting the average value of the shape factor SF2 of the secondary particles to the above range, the organic film formed by decomposition of CHB due to overcharge or high-temperature storage in a charged state is used. Since the surface of the secondary particles can be uniformly coated, the reaction between the positive electrode and the nonaqueous electrolyte can be suppressed, and heat generation and gas generation during overcharge and during high-temperature storage in a charged state can be suppressed. . However, when the average value of SF2 is 2 or more, the unevenness of the surface of the secondary particles becomes large, so that it is difficult to uniformly cover the surface of the secondary particles with the organic film, and the high-temperature storage when the battery is charged and stored at a high temperature is difficult. Battery swelling increases. When the average value of SF2 is smaller, an organic film is more likely to be uniformly formed on the surface of the secondary particles, but when the average value of SF2 is 1, high discharge load characteristics cannot be obtained.
[0068]
When a non-aqueous electrolyte containing DFA is used, the average value of the shape factor SF2 of the secondary particles is set in the above range. When the average value of SF2 is 1 or 2 or more, battery swelling during high-temperature storage increases. This is because the charge / discharge cycle characteristics deteriorate.
[0069]
Further, among the secondary particles, by using the above-described positive electrode active material according to the present invention, it is possible to further improve the discharge load characteristics of the secondary battery and further suppress battery swelling during high-temperature storage. Can be.
[0070]
As a method for manufacturing the positive electrode, the same method as described in the first nonaqueous electrolyte secondary battery can be used.
[0071]
2) Non-aqueous electrolyte
The non-aqueous electrolyte includes a non-aqueous solvent containing at least one of cyclohexylbenzene and 2,4-difluoroanisole, and vinylene carbonate, and an electrolyte dissolved in the non-aqueous solvent.
[0072]
As the non-aqueous electrolyte, those having a liquid or gel form can be used. The liquid non-aqueous electrolyte and the gel non-aqueous electrolyte are prepared in the same manner as described in the first non-aqueous electrolyte secondary battery.
[0073]
As the electrolyte, those similar to those described in the first nonaqueous electrolyte secondary battery described above can be used, and the nonaqueous solvent will be described below.
[0074]
Since vinylene carbonate (VC) can form a protective film on the surface of the negative electrode at the time of initial charge, the reaction between the negative electrode and the nonaqueous electrolyte can be suppressed, and the deterioration of the negative electrode can be suppressed. For this reason, the charge / discharge cycle life of the nonaqueous electrolyte secondary battery can be improved.
[0075]
The ratio of VC in the non-aqueous solvent is desirably in the range of 10% by volume or less. This is because, when the volume ratio of VC exceeds 10% by volume, when the nonaqueous electrolyte secondary battery is stored in a charged state in a high-temperature environment, the VC reacts with the positive electrode, so that the interface resistance of the positive electrode greatly increases. Or the generation of gas due to the decomposition of VC. A more preferable range of the volume ratio of VC is 0.01 to 5% by volume.
[0076]
When CHB is added, the ratio of CHB in the non-aqueous solvent is desirably 7% by volume or less. If the volume ratio of CHB exceeds 7% by volume, the cycle characteristics and the low-temperature discharge characteristics of the nonaqueous electrolyte secondary battery may decrease due to an increase in the internal impedance. In order to sufficiently suppress battery swelling during high-temperature storage, it is desirable to secure a volume ratio of CHB of at least 0.01% by volume.
[0077]
When DFA is added, the ratio of DFA in the non-aqueous solvent is desirably in the range of 0.01 to 5% by volume. If the volume ratio of DFA is less than 0.01% by volume, the reaction between the positive electrode and VC may not be suppressed. When the positive electrode reacts with VC, the battery swells when stored in a charged state under a high-temperature environment, and the charge / discharge cycle life is shortened. On the other hand, if the volume ratio of DFA exceeds 5% by volume, the internal impedance may increase due to the resistive film generated by the decomposition of DFA, and the charge / discharge cycle life may decrease.
[0078]
The non-aqueous solvent may contain a solvent other than VC, CHB and DFA. Other solvents include, for example, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), chain carbonates (eg, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, etc.), γ-butyrolactone, acetonitrile, propionic acid Examples thereof include methyl, ethyl propionate, cyclic ethers (eg, tetrahydrofuran, 2-methyltetrahydrofuran), and chain ethers (eg, dimethoxyethane, diethoxyethane, etc.). The type of the other solvent to be contained in the non-aqueous solvent can be one or more.
[0079]
As described above, the second nonaqueous electrolyte secondary battery according to the present invention is formed by agglomerated primary particles of a lithium-containing metal composite oxide, and has an average shape factor SF2 calculated by the above-described equation (1). Since it has a positive electrode including secondary particles having a value of more than 1 and less than 2 and a non-aqueous electrolyte containing at least one of CHB and DFA and VC, the wettability with respect to the non-aqueous electrolyte In addition to improving the properties and the distribution of the conductive agent with respect to the positive electrode active material, the reaction between the positive electrode and the nonaqueous electrolyte during high-temperature storage can be suppressed. As a result, it is possible to realize a non-aqueous electrolyte secondary battery in which gas generation during high-temperature storage is suppressed and discharge load characteristics are improved.
[0080]
A thin non-aqueous electrolyte secondary battery, which is an example of the non-aqueous electrolyte secondary battery according to the present invention, will be described in detail with reference to FIGS.
[0081]
FIG. 2 is a sectional view showing a thin non-aqueous electrolyte secondary battery which is an example of the non-aqueous electrolyte secondary battery according to the present invention, and FIG. 3 is an enlarged sectional view showing a portion A in FIG.
[0082]
As shown in FIG. 2, an electrode group 2 is housed in a container 1 made of, for example, a laminate film. The electrode group 2 has a structure in which a laminate comprising a positive electrode, a separator made of a porous material such as a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, and the like, and a negative electrode are wound into a flat shape. Having. As shown in FIG. 3, the laminate includes a separator 3 (from the lower side of the figure), a positive electrode 6 including a positive electrode active material containing layer 4, a positive electrode current collector 5, and a positive electrode active material containing layer 4, a separator 3 A negative electrode 9 having a negative electrode layer 7, a negative electrode current collector 8 and a negative electrode layer 7, a separator 3, a positive electrode 6 having a positive electrode active material containing layer 4, a positive electrode current collector 5 and a positive electrode active material containing layer 4, The negative electrode 9 including the separator 3, the negative electrode layer 7, and the negative electrode current collector 8 is laminated in this order. In the electrode group 2, the negative electrode current collector 8 is located in the outermost layer. One end of the strip-shaped positive electrode lead 10 is connected to the positive electrode current collector 5 of the electrode group 2, and the other end is extended from the container 1. On the other hand, one end of the strip-shaped negative electrode lead 11 is connected to the negative electrode current collector 8 of the electrode group 2, and the other end is extended from the container 1.
[0083]
A prismatic nonaqueous electrolyte secondary battery, which is another example of the nonaqueous electrolyte secondary battery according to the present invention, will be described with reference to FIG.
[0084]
FIG. 4 is a partially cutaway perspective view showing a rectangular nonaqueous electrolyte secondary battery which is an example of the nonaqueous electrolyte secondary battery according to the present invention.
[0085]
As shown in FIG. 4, a bottomed rectangular cylindrical outer can 21 made of metal, for example, aluminum, also serves as a positive electrode terminal, for example, and an insulator 22 is disposed on the inner surface of the bottom. A spiral electrode body 23 is housed in the outer can 21. The spiral electrode body 23 is formed by spirally winding the negative electrode 24, the separator 25, and the positive electrode 26 such that the positive electrode 26 is located at the outermost periphery, and then press-molding the electrode group into a flat shape. It is. A positive electrode lead (not shown) is connected to the current collector of the positive electrode 26, and the other end of the positive electrode lead is connected to the outer can 21.
[0086]
A spacer 27 made of, for example, a synthetic resin and having a lead extraction hole near the center is disposed on the spiral electrode body 23 in the outer can 21. The metal lid 28 is hermetically joined to the upper end opening of the outer can 21 by, for example, laser welding. Near the center of the lid 28, an extraction hole 29 for the negative electrode terminal 30 is opened. The lid 28 is provided with a safety valve mechanism (not shown) that breaks and releases the internal pressure when the internal pressure in the outer can 21 rises extremely. The negative electrode terminal 30 is hermetically sealed in the extraction hole 29 of the lid 28 via an insulating material 31 made of glass or resin. A negative electrode lead 32 is connected to a lower end surface of the negative electrode terminal 30, and the other end of the negative electrode lead 32 is connected to a current collector of the negative electrode 24 of the spiral electrode body 23. The insulating sealing plate 33 is arranged on the upper surface of the lid 28. The insulating outer tube 34 covers the side and bottom edges of the outer can 21 and the outer edge of the insulating sealing plate 33.
[0087]
In addition, in FIG. 2 and FIG. 4 described above, an example in which the positive electrode, the separator, and the negative electrode are used in a flatly wound electrode group has been described, but instead, a laminate of the positive electrode, the separator, and the negative electrode is used. An electrode group having a structure in which a laminate of a positive electrode, a separator, and a negative electrode is bent at least once is also applicable.
[0088]
FIG. 2 illustrates an example applied to a thin non-aqueous electrolyte secondary battery, and FIG. 4 illustrates an example applied to a rectangular non-aqueous electrolyte secondary battery. The present invention can be similarly applied to a water electrolyte secondary battery and the like.
[0089]
Note that the present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the components without departing from the scope of the invention at the stage of implementation. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiments. For example, some components may be deleted from all the components shown in the embodiment. Further, components of different embodiments may be appropriately combined.
[0090]
【Example】
Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings.
[0091]
<Preparation of positive electrode active material>
(Example 1)
A saturated aqueous solution containing cobalt sulfate as a cobalt salt was prepared. By gradually adding an aqueous sodium hydroxide solution as an aqueous alkaline solution to the saturated aqueous solution with stirring, a cobalt hydroxide was precipitated, and an aggregate of cobalt hydroxide crystals was obtained. This was filtered to collect a precipitate, which was repeatedly washed with water and dried when the pH was stabilized, whereby secondary particles in which primary particles of cobalt hydroxide were aggregated were obtained.
[0092]
Subsequently, lithium carbonate powder as a lithium compound was mixed with the cobalt hydroxide at a ratio of Li: Co = 1.00: 1, and the mixture was calcined at 920 ° C. for 6 hours in an oxygen atmosphere to obtain Li. _1.00 CoO ₂ Was obtained.
[0093]
(Example 2)
Performed in the same manner as in Example 1 except that secondary particles of cobalt hydroxide and lithium carbonate powder were mixed at a ratio of Li: Co = 1.00: 1 and fired in an oxygen atmosphere at 900 ° C. for 6 hours. Li _1.00 CoO ₂ Was obtained.
[0094]
(Example 3)
Performed in the same manner as in Example 1, except that the secondary particles of cobalt hydroxide and lithium carbonate powder were mixed at a ratio of Li: Co = 1.03: 1 and calcined at 920 ° C. for 6 hours in an oxygen atmosphere. Li _1.03 CoO ₂ Was obtained.
[0095]
(Example 4)
Performed in the same manner as in Example 1 except that the secondary particles of cobalt hydroxide and lithium carbonate powder were mixed at a ratio of Li: Co = 1.03: 1 and calcined at 920 ° C. for 5 hours in an oxygen atmosphere. Li _1.03 CoO ₂ Was obtained.
[0096]
(Example 5)
Performed in the same manner as in Example 1, except that secondary particles of cobalt hydroxide and lithium carbonate powder were mixed at a ratio of Li: Co = 1.00: 1 and calcined at 920 ° C. for 5 hours in an oxygen atmosphere. Li _1.00 CoO ₂ Was obtained.
[0097]
(Comparative Example 1)
Performed in the same manner as in Example 1 except that secondary particles of cobalt hydroxide and lithium carbonate powder were mixed at a ratio of Li: Co = 1.05: 1 and calcined at 920 ° C. for 6 hours in an oxygen atmosphere. Li _1.05 CoO ₂ Was obtained.
[0098]
(Comparative Example 2)
Performed in the same manner as in Example 1 except that secondary particles of cobalt hydroxide and lithium carbonate powder were mixed at a ratio of Li: Co = 0.99: 1 and calcined at 920 ° C. for 6 hours in an oxygen atmosphere. Li _0.99 CoO ₂ Was obtained.
[0099]
(Comparative Example 3)
Performed in the same manner as in Example 1 except that secondary particles of cobalt hydroxide and lithium carbonate powder were mixed at a ratio of Li: Co = 0.99: 1 and calcined at 900 ° C. for 6 hours in an oxygen atmosphere. Li _0.99 CoO ₂ Was obtained.
[0100]
(Comparative Example 4)
Performed in the same manner as in Example 1 except that secondary particles of cobalt hydroxide and lithium carbonate powder were mixed at a ratio of Li: Co = 1.03: 1 and calcined at 930 ° C. for 6 hours in an oxygen atmosphere. Li _1.03 CoO ₂ Was obtained.
[0101]
(Comparative Example 5)
The cobalt oxide powder and the lithium carbonate powder are mixed at a ratio of Li: Co = 1.03: 1, and calcined at 920 ° C. for 5 hours in an oxygen atmosphere to obtain Li. _1.03 CoO ₂ Was obtained.
[0102]
<Measurement of average particle size>
The particle size distribution of the obtained positive electrode active materials of Examples 1 to 5 and Comparative Examples 1 to 5 was measured by the method described below.
[0103]
After stirring 0.5 g of the sample in 100 mL water, ultrasonic dispersion was performed under the conditions of 100 W for 3 min, and the average particle size at D50 was measured using MICROTRACII PARTICLE-ANALYZER TYPE7997-10 manufactured by LEEDS & NORTHRUP. . The results are shown in Table 1 below.
[0104]
<Measurement of specific surface area by BET method>
The specific surface areas of the obtained positive electrode active materials of Examples 1 to 5 and Comparative Examples 1 to 5 were measured by the BET method by the method described below.
[0105]
The specific surface area was measured using QUANTASOBE manufactured by QANTACHROME. ₂ Measurement was performed by gas adsorption and BET one-point method. The results are shown in Table 1 below.
[0106]
<Preparation of positive electrode>
Positive electrodes were manufactured using the positive electrode active materials of Examples 1 to 5 and Comparative Examples 1 to 5 by the method described below.
[0107]
A mixture of 90% by weight of the obtained positive electrode active material, 6% by weight of a conductive agent, and 4% by weight of a binder is dispersed in a solvent to form a slurry, which is applied to a current collector made of aluminum foil. Then, after drying, it was compression-molded by a roller press and cut into a predetermined size to obtain a positive electrode.
[0108]
<Scanning electron microscope observation of positive electrode>
With respect to the arbitrary cross sections of the obtained positive electrodes of Examples 1 to 5 and Comparative Examples 1 to 5, scanning electron micrographs were photographed in three visual fields at a magnification of 2000 times. Scanning electron micrographs of Example 1 are shown in FIGS. From the scanning electron micrographs of FIGS. 5 to 7, the positive electrode active material of Example 1 was obtained by aggregating secondary particles of lithium-cobalt composite oxide primary particles having various shapes such as a sphere, a plate, and a flat shape. It was confirmed that it included. In addition, JSM-5800LV manufactured by JEOL Datum Ltd. was used for the scanning electron microscope. The accelerating voltage was set to 20 kV, and observation was performed with Au deposition being 100 °.
[0109]
Next, the shape factor SF2 and the average maximum particle size L are calculated by the method described below. ₂ max was calculated. For each of the three field-of-view scanning electron micrographs, ten secondary particle images (two-dimensional images of secondary particles) from which the entire contour can be observed are randomly selected, and the R of each secondary particle image is selected. ₁ And R ₂ Was measured and the shape factor SF2 was calculated to obtain a total of 30 shape factors SF2 of the secondary particle images. The average value of the obtained SF2 is also shown in Table 1 below.
[0110]
Further, the maximum particle size is measured for each of a total of 30 selected secondary particles, and the average value thereof is obtained to obtain the average maximum particle size L ₂ max was calculated. In addition, the maximum particle size of the primary particles whose entire contour is observable for each secondary particle is measured, and the average value thereof is determined to obtain the average maximum particle size L. ₁ max was calculated. The results are shown in Table 1 below. In addition, the obtained L ₁ max to L ₂ The results are shown in Table 1 below.
[0111]
In the case of the positive electrode active material of Comparative Example 5, since no secondary aggregation was observed, the area of the two-dimensional image of the primary particles was calculated as R ₂ And the area of a region surrounded by the shortest envelope A connecting the vertices of the convex portion of this two-dimensional image is R ₁ And the shape factor SF2 was calculated.
[0112]
Using the positive electrodes of Examples 1 to 5 and Comparative Examples 1, 2, and 5, prismatic nonaqueous electrolyte secondary batteries were assembled by the method described below.
[0113]
<Preparation of negative electrode>
N-methyl-2-pyrrolidone in which 6% by weight of polyvinylidene fluoride was dissolved was added to 94% by weight of mesophase pitch-based carbon fibers heat-treated at 3000 ° C. to prepare a mixture slurry. This mixture slurry was applied to both sides of a current collector made of copper foil, dried, and then heated and roll-pressed to produce a negative electrode.
[0114]
<Preparation of liquid non-aqueous electrolyte>
Ethylene carbonate (EC), methyl ethyl carbonate (MEC), cyclohexylbenzene (CHB), 2,4-difluoroanisole (DFA), and vinylene carbonate (VC) have a volume ratio (EC: MEC: CHB: DFA: VC) of 32. : 64: 1: 1: 2 in a non-aqueous solvent mixed with lithium hexafluorophosphate (LiPF ₆ Was dissolved at a concentration of 1 mol / L to obtain a liquid non-aqueous electrolyte.
[0115]
<Assembly of secondary battery>
The positive electrode current collector of the positive electrode was ultrasonically welded with a positive electrode lead made of a strip-shaped aluminum foil (thickness of 100 μm), and the negative electrode current collector was ultrasonically welded with a negative electrode lead made of a strip of nickel foil (thickness of 100 μm). The positive electrode and the negative electrode were spirally wound therebetween with the separator interposed therebetween, and then formed into a flat shape to produce a spiral electrode group.
[0116]
An aluminum sheet having a thickness of 300 μm was formed into a rectangular parallelepiped can having a thickness of 5 mm, a width of 30 mm, and a height of 48 mm, and the spiral electrode group was housed in the obtained container.
[0117]
Next, moisture adsorbed on the spiral electrode body and the aluminum can was removed by subjecting the spiral electrode group in the container to vacuum drying at 80 ° C. for 12 hours.
[0118]
The liquid non-aqueous electrolyte was injected into the spiral electrode group in the container such that the amount per battery capacity of 1 Ah was 3.4 g, and after sealing, the insulating sealing plate was placed on the lid and covered with an outer tube. As a result, a rectangular non-aqueous electrolyte secondary battery having the structure shown in FIG. 4 described above and having a thickness of 5 mm, a width of 30 mm, and a height of 48 mm was assembled.
[0119]
(Example 6)
The volume ratio (EC: MEC: CHB: VC) of ethylene carbonate (EC), methyl ethyl carbonate (MEC), cyclohexylbenzene (CHB) and vinylene carbonate (VC) is 33: 66: 0.5: 0.5. Hexafluorophosphate (LiPF) ₆ Was dissolved at a concentration of 1 mol / L to obtain a liquid non-aqueous electrolyte.
[0120]
A rectangular non-aqueous electrolyte secondary battery having the same configuration as that described in Example 1 was assembled except that the obtained liquid non-aqueous electrolyte was used.
[0121]
(Example 7)
The volume ratio (EC: MEC: DFA: VC) of ethylene carbonate (EC), methyl ethyl carbonate (MEC), 2,4-difluoroanisole (DFA), and vinylene carbonate (VC) is 33: 66: 0.5: 0. Lithium hexafluorophosphate (LiPF) ₆ Was dissolved at a concentration of 1 mol / L to obtain a liquid non-aqueous electrolyte.
[0122]
A rectangular non-aqueous electrolyte secondary battery having the same configuration as that described in Example 1 was assembled except that the obtained liquid non-aqueous electrolyte was used.
[0123]
(Comparative Examples 3 and 4)
Except for using the positive electrodes of Comparative Examples 3 and 4 and a liquid non-aqueous electrolyte described below, a prismatic non-aqueous electrolyte secondary battery having the same configuration as that described in Example 1 was assembled.
[0124]
Lithium hexafluorophosphate in a non-aqueous solvent obtained by mixing ethylene carbonate (EC), methyl ethyl carbonate (MEC) and vinylene carbonate (VC) so that the volume ratio (EC: MEC: VC) becomes 30:60:10. (LiPF ₆ Was dissolved at a concentration of 1 mol / L to obtain a liquid non-aqueous electrolyte.
[0125]
<Discharge load test>
For the secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 5, capacity confirmation and discharge load test were performed by the methods described below.
[0126]
After charging to 4.2 V with a constant current of 0.3 C, charging and discharging were repeated to discharge to 3 V with a constant current of 0.3 C, and a stable discharge capacity at the fifth cycle was measured. The capacity is also shown in Table 1 below.
[0127]
Next, after charging to 4.2 V with a constant current of 0.3 C, the discharge capacity when discharging to 3 V at a constant current of 1 C was measured, and 1 C discharge when the discharge capacity at 0.3 C was 100%. The capacity ratio was calculated, and the results are shown in Table 1 below.
[0128]
Further, after the battery was charged to 4.2 V at a constant current of 0.3 C, the discharge capacity at the time of discharging to 3 V at a constant current of 2.7 C was measured, and the discharge capacity at 0.3 C was taken as 100%. The 2.7 C discharge capacity ratio was calculated, and the results are also shown in Table 1 below.
[0129]
Here, the discharge load characteristics are output characteristics when a discharge test is performed at various current values. In the case of rechargeable batteries, since the size of the capacity varies, it is easy to know what the current value of each battery is substantially, even if the current value itself is displayed. is not. Therefore, a common measure of time rate (h) is used instead of the current value. As shown in the following equation (1), a value obtained by dividing a nominal capacity (rated capacity) X by a predetermined time rate H is defined as a reference discharge current value Y. If the discharge is performed at this current value Y, the value is equivalent to the nominal capacity X. That is, the required capacity can be obtained.
[0130]
X (Ah) / H (h) = Y (A) ... {1}
In the formula (1), X indicates a nominal capacity (Ah), H indicates a predetermined time rate (h), and Y indicates a reference discharge current (A).
[0131]
Also, the reciprocal of the time rate is often used and is often referred to as C (discharge rate) as shown in the following equation (2).
[0132]
1 / time rate = C (discharge rate)… ▲ 2 ▼
For example, 0.2C is a discharge rate for discharging the nominal capacity in 5 hours. Generally, as the discharge rate increases, the capacity decreases, but the smaller the degree of the decrease, the better the output characteristics.
[0133]
<Battery swelling during high temperature storage>
After charging each rechargeable battery to 4.2 V at 1 C, it was placed in an oven at 85 ° C. and left for 24 hours, and the battery swelling (%) was calculated from the change in battery thickness before and after leaving. Also shown in Table 1.
[0134]
[Table 1]

[0135]
As is clear from Table 1, the average value of the shape factor SF2 of the secondary particles is more than 1 and less than 2, and the average maximum particle size L of the primary particles is L. ₁ max is 0.1 × L ₂ max ≦ L ₁ max <0.5 × L ₂ The secondary batteries of Examples 1 to 7 provided with the positive electrode active materials in the range of max had a small capacity decrease width when the discharge rate was increased, and had excellent discharge load characteristics. In addition, the secondary batteries of Examples 1 to 7 had smaller swelling when stored at a high temperature in a charged state than Comparative Examples 1 to 4.
[0136]
On the other hand, the secondary battery of Comparative Example 1 including the positive electrode active material in which the average value of the shape factor SF2 of the secondary particles is 1 and the positive electrode in which the average value of the shape factor SF2 of the secondary particles is 2 or more In the secondary battery of Comparative Example 2 including the active material, the capacity was significantly reduced when the discharge rate was increased as compared with the secondary batteries of Examples 1 to 7, and further, the battery swelled during high temperature storage in the charged state. It was big.
[0137]
On the other hand, although the average value of the shape factor SF2 of the secondary particles is more than 1 and less than 2, the average maximum particle size L of the primary particles is ₁ max is 0.1 × L ₂ The secondary battery of Comparative Example 3 including the positive electrode active material of less than max and a liquid non-aqueous electrolyte using a non-aqueous solvent composed of EC, MEC, and VC was compared with the secondary batteries of Examples 1 to 7. As a result, when the discharge rate was increased, the capacity was significantly reduced, and the battery swelled during storage at high temperature during charging.
[0138]
Although the average value of the shape factor SF2 of the secondary particles is more than 1 and less than 2, the L of the primary particles ₁ max is 0.5 × L ₂ The secondary battery of Comparative Example 4 including a positive electrode active material having a maximum of max or more and a liquid nonaqueous electrolyte using a nonaqueous solvent composed of EC, MEC, and VC is compared with the secondary batteries of Examples 1 to 7. As a result, the capacity was significantly reduced when the discharge rate was increased, and the battery swelled during storage at a high temperature during charging.
[0139]
In the secondary battery of Comparative Example 5 including the single particles without secondary aggregation as the positive electrode active material, the capacity was significantly reduced when the discharge rate was increased as compared with the secondary batteries of Examples 1 to 7. This is because, when a single particle is used, the migration distance of lithium ions is longer than that of a secondary particle having the same particle size, the diffusivity is reduced, and the surface is formed of a smooth curved surface. This is because the wettability to the electrolyte is low.
[0140]
【The invention's effect】
As described in detail above, according to the present invention, it is possible to provide a positive electrode active material that can improve the discharge load characteristics of a nonaqueous electrolyte secondary battery and has excellent thermal stability. Further, according to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery having excellent discharge load characteristics and suppressing battery swelling during high-temperature storage. Therefore, the positive electrode active material and the non-aqueous electrolyte secondary battery according to the present invention are industrially and commercially used as portable terminals such as mobile phones and notebook computers, electric vehicles, power sources for power storage, etc., which are rapidly growing in recent years. It can greatly contribute to the industrial world.
[Brief description of the drawings]
FIG. 1 R ₁ And R ₂ FIG. 2 is a schematic view of a lithium-containing metal composite oxide used for explaining the definition of “(1)”.
FIG. 2 is a sectional view showing a thin non-aqueous electrolyte secondary battery which is an example of the non-aqueous electrolyte secondary battery according to the present invention.
FIG. 3 is an enlarged sectional view showing a portion A in FIG. 2;
FIG. 4 is a partially cutaway perspective view showing a prismatic nonaqueous electrolyte secondary battery as an example of the nonaqueous electrolyte secondary battery according to the present invention.
FIG. 5 is a scanning electron micrograph (magnification: 2000 times) of a first field of view of a cross section of the positive electrode of Example 1.
FIG. 6 is a scanning electron micrograph (magnification: 2000 times) of a second field of view of the cross section of the positive electrode of Example 1.
FIG. 7 is a scanning electron micrograph (magnification: 2000 times) of a third field of view of the cross section of the positive electrode of Example 1.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... container, 2 ... electrode group, 3, 25 ... separator, 4 ... positive electrode active material containing layer, 5 ... positive electrode current collector, 6, 26 ... positive electrode, 7 ... negative electrode layer, 8 ... negative electrode current collector, 9, Reference numeral 24: negative electrode, 10: positive electrode terminal, 11, 30: negative electrode terminal, 21: exterior can, 22: insulator, 23: spiral electrode body, 27: spacer, 28: lid, 29: extraction hole, 31: insulating material , 32: negative electrode lead, 33: insulating sealing plate, 34: outer tube.

Claims (3)

A positive electrode active material including secondary particles in which lithium-containing metal composite oxide primary particles are aggregated,
Range of the average maximum particle diameter _L 1 max is _{0.1 × L 2 max ≦ L 1} max <0.5 × L 2 max of the primary particles an average maximum particle diameter of the secondary particles upon the _L 2 max And a mean value of the shape factor SF2 calculated by the following formula (1) is more than 1 and less than 2.
SF2 = R ₁ / R ₂ (1)
Here, R ₂ indicates an area of an arbitrary two-dimensional image of the secondary particle, and R ₁ indicates an area of a region surrounded by a shortest envelope connecting vertexes of convex portions of the two-dimensional image. Is shown. The positive electrode active material according to claim 1, wherein the secondary particles have a specific surface area in a range of 0.1 to ¹⁰ m2 / g and an average particle size in a range of 4 to 20 m. A positive electrode including secondary particles in which lithium-containing metal composite oxide primary particles are aggregated and an average value of shape factor SF2 calculated by the following formula (1) is more than 1 and less than 2;
A negative electrode,
A non-aqueous electrolyte secondary battery comprising: a non-aqueous electrolyte containing at least one of cyclohexylbenzene and 2,4-difluoroanisole; and vinylene carbonate.
SF2 = R ₁ / R ₂ (1)
Here, R ₂ indicates an area of an arbitrary two-dimensional image of the secondary particle, and R ₁ indicates an area of a region surrounded by a shortest envelope connecting vertexes of convex portions of the two-dimensional image. Is shown.

Images