WO2016002158A1 - Positive electrode active material for non-aqueous electrolyte secondary cell and non-aqueous electrolyte secondary cell using same - Google Patents

Abstract

　The purpose of the present invention is to provide a non-aqueous electrolyte secondary cell with which it is possible to obtain exceptional cycle characteristics. Using a positive electrode active material for non-aqueous electrolyte secondary cells comprising secondary particles in which a plurality of primary particles are coagulated, the positive electrode active material indicating multi-modality in a distribution curve that represents the frequency of appearance of the aspect ratio of the primary particles, makes it possible to minimize the occurrence of cracking in a grain boundary face between the primary particles attributable to an orientation with a high aspect ratio at the time of expansion or contraction during charging or discharging, and to provide a non-aqueous electrolyte secondary cell having improved cycle characteristics.

Description

Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery using the same

The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery with high capacity and high durability, and a non-aqueous electrolyte secondary battery using the same.

In recent years, non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, have high energy density and high capacity, so mobile phones, laptop computers, smartphones, etc. that are required to be small and light are moved. Widely used as a driving power source for information terminals.

In addition, not only mobile information terminals, but also attracting attention as power source applications for power tools, electric vehicles (EV), hybrid electric vehicles (HEV, PHEV), and the like.

Furthermore, in recent years, it is also expected as an emergency power source for base stations and a power source for stationary power storage such as load fluctuation adjustment.

Such a lithium ion secondary battery includes a positive electrode, a negative electrode, and a separator interposed therebetween, and as a positive electrode active material, a lithium cobalt oxide (for example, LiCoO ₂₎ having a high potential with respect to lithium and being easily synthesized. ) Is used. In recent years, a layered active material mainly composed of nickel or a layered compound composed of three components of nickel cobalt manganese has been used as a positive electrode active material for the purpose of increasing the capacity.

Here, the positive electrode active material of the lithium ion secondary battery (the positive electrode active material including the secondary particles in which the primary particles are aggregated) is the primary particles due to insertion and desorption of lithium ions when the charge / discharge cycle is repeated for a long time. Expansion and contraction occur, cracks occur at the crystal grain interface between the primary particles, and the cycle characteristics deteriorate.

That is, at the initial stage of the charge / discharge cycle, the primary particles in the secondary particles are joined at the crystal grain interface, but the volume change accompanying the expansion / contraction causes distortion at the grain interface between the primary particles, and the charge / discharge cycle As the process progresses, cracks occur from the grain interface with weak bonding strength. As a result, the conductive path between the primary particles is interrupted, the primary particles are isolated as the active material, the discharge capacity is reduced, and a problem occurs in the cycle characteristics.

On the other hand, in Patent Document 1, by using positive electrode active material particles including secondary particles in which primary particles having an aspect ratio of 1.5 or more are aggregated, the contact area between adjacent primary particles is increased, and the primary particles have a larger area. It is disclosed that the bonding strength is increased, cracks at the crystal grain interface between primary particles are suppressed, and cycle characteristics are improved.

JP 2009-120734 A

However, even when the positive electrode active material disclosed in Patent Document 1 is used, sufficient improvement in cycle characteristics may not be obtained.

Since primary particles having a high aspect ratio used in Patent Document 1 have ab surfaces of crystallites aligned in the major axis direction, they tend to preferentially expand and contract in the minor axis direction during charge and discharge.

Such secondary particles composed only of particles having a high aspect ratio form an aggregate in which the particles are aligned in the same direction as shown in FIG. 1, and the aggregate is randomly arranged to form secondary particles. Form.

And when the crystal orientation of primary particles with a high aspect ratio is aligned, the expansion and shrinkage of the primary particles in the minor axis direction increases, and cracks are generated due to delamination between the particles at the primary particle interface. Conductivity is lost.

In particular, the presence of an aggregate of primary particles having a high aspect ratio at the center of the secondary particles causes a large distortion inside the secondary particles during charge / discharge, thereby causing more significant collapse of the active material particles.

The problem to be solved by the present invention is to provide a non-aqueous electrolyte secondary battery capable of obtaining excellent cycle characteristics.

The present invention obtains excellent cycle characteristics by using a positive electrode active material composed of secondary particles in which a plurality of primary particles are aggregated and the distribution curve representing the appearance frequency of the primary particle aspect ratio is multimodal. Provided is a nonaqueous electrolyte secondary battery.

By using the positive electrode active material of the present invention, it is possible to suppress particle cracks due to the orientation of primary particles having a high aspect ratio in the secondary particles, and it is possible to relieve the stress in the particles during charging and discharging. Since cracks are suppressed, a non-aqueous electrolyte secondary battery with improved cycle characteristics can be provided.

The schematic diagram which shows the secondary particle of the positive electrode active material which concerns on one experiment example of this invention The schematic diagram which shows the primary particle of the positive electrode active material which concerns on one Embodiment of this invention. (A) A diagram showing a unimodal distribution curve representing the appearance frequency of the primary particle aspect ratio, (b) a diagram showing a multimodal (bimodal) distribution curve representing the appearance frequency of the primary particle aspect ratio. (A) Schematic sectional view showing secondary particles of the positive electrode active material according to one embodiment of the present invention, (b) Schematic sectional view showing secondary particles of the positive electrode active material according to one embodiment of the present invention. The schematic diagram which shows the secondary particle of the positive electrode active material which concerns on one experiment example of this invention Sectional drawing of the cylindrical nonaqueous electrolyte secondary battery which concerns on one Embodiment of this invention.

Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the embodiment described below is exemplified to embody the technical idea of the present invention, and is not intended to limit the present invention to this embodiment. The present invention can be equally applied to various modifications without departing from the technical idea shown in the scope.

The present invention consists of secondary particles in which a plurality of primary particles are aggregated, and by using a positive electrode active material in which the distribution curve representing the appearance frequency of the aspect ratio of the primary particles exhibits multimodality, It becomes possible to mix primary particles having a low aspect ratio in the same secondary particles, and to suppress generation of cracks at the grain interface between the primary particles due to expansion of the primary particles in the coaxial direction.

As a result, it is possible to provide a non-aqueous electrolyte secondary battery in which stress in the particles at the time of charge / discharge is relaxed, cracks between the particles are suppressed, and cycle characteristics are improved.

Here, the aspect ratio of the primary particles of the present invention is obtained by dividing “the length x of the longest diameter of the particle image” shown in FIG. 2 by “the length y of the maximum diameter perpendicular to x”. This is an index that represents the shape of the secondary particle, and is evaluated using a cross-sectional image of secondary particles photographed by a scanning ion microscope (SIM).

加工 A processing device using an ion beam and a shielding plate is used to adjust the secondary particle cross-sectional analysis sample. For evaluation of the particle state, a particle cross section obtained by cutting the vicinity of the center of the secondary particle is selected.

Then, in order to evaluate the aspect ratio of the primary particles in the secondary particles, a plurality of primary particles are selected at random, and the aspect ratio is calculated for the ratio (x / y) of the primary particles.

Further, a measurement result of the area occupied by each primary particle obtained from the cross-sectional image of the secondary particle and a curve indicating the distribution of appearance frequency (area standard) for each aspect ratio of each primary particle are obtained.

In addition, in the distribution curve showing the distribution of the appearance frequency of the aspect ratio of the primary particles, a structure composed of only one peak as shown in FIG. 3A is called a unimodal distribution, and FIG. A distribution in which the appearance frequency is composed of a plurality of peaks is called a multimodal (bimodal) distribution.

The present invention also includes spherical primary particles having an aspect ratio appearance frequency peak of 1 or more and 2 or less, and elongated primary particles having an aspect ratio appearance frequency peak of greater than 2 and 10 or less. As a result, the orientation of the primary particles is suppressed, a sufficient amount of the bonding area between the primary particles can be ensured, and even better cycle characteristics can be obtained.

In addition, when the appearance frequency of the aspect ratio of the elongated primary particles is larger than the appearance ratio of the aspect ratio of the spherical primary particles, the characteristics of the elongated primary particles having a high aspect ratio appear. As a result, the elongated primary particles can increase the contact area with adjacent particles, and it is easy to suppress the occurrence of cracks at the grain interface. Moreover, since the distance of the average ab surface in the primary particle which is a lithium diffusion surface is long, it is advantageous for cycle characteristics at a low load.

On the other hand, if the appearance frequency of the aspect ratio of the spherical primary particles is larger than the appearance frequency of the aspect ratio of the elongated primary particles, the characteristics of the spherical primary particles with a low aspect ratio appear. As a result, the spherical primary particles have a shorter distance of the average ab plane in the particles, which is the surface of lithium diffusion, and therefore are less affected by defects in the crystallite plane, which is advantageous for high-rate cycle characteristics. .

In any case, the orientation of the primary particles is suppressed, and a sufficient amount of the bonding area between the primary particles can be ensured, and good cycle characteristics can be obtained.

In addition, the peak of the appearance frequency of the aspect ratio of the elongated primary particles is more preferably 4 or more and 6 or less.

Further, when the primary particles aggregated in the central part of the secondary particles are spherical primary particles and the primary particles aggregated in the outer peripheral part of the secondary particles are elongated primary particles, even better cycle characteristics Can be obtained.

In other words, the primary particles with a small aspect ratio are placed in the center of the particles where stress is likely to concentrate in the secondary particles, reducing the stress inside the particles during charge and discharge, while the aspect ratio on the secondary particle surface where stress is difficult to concentrate. It is considered that by disposing primary particles having large and excellent adhesion, it is possible to suppress particle peeling and to improve cycle characteristics.

More preferably, excellent cycle characteristics can be obtained by arranging the primary particles formed on the outer peripheral portion of the secondary particles so that the major axis direction is radial with respect to the center of the secondary particles. By arranging in this way, it is considered that stress stress vectors accompanying expansion and contraction are arranged in the circumferential direction and can cancel each other, and the stress is easily relaxed.

In addition, the region that reaches half the length from the center of the secondary particle to the outermost circumference (50% of the radius of the secondary particle) is the central part of the secondary particle, and the region outside it is the secondary particle. The outer periphery of the. In addition, regarding the particles present on the boundary line between the central portion and the outer peripheral portion, the particles are regarded as belonging to the higher presence ratio of the particles.

In addition, the distribution of the appearance frequency of the aspect ratio of the elongated primary particles and the aspect ratio of the spherical primary particles (area standard) is calculated, and at the center of the secondary particles and the outer periphery of the secondary particles, If the values match, the primary particle distribution within the secondary particles is considered uniform.

Also, it is desirable that each primary particle is composed of the same transition metal composition. When the molar ratio of each transition metal element to the total amount of the transition metal element is determined, even when the composition analysis of any primary particle constituting the secondary particle is performed by EDX (energy dispersive X-ray spectroscopy), When the difference in composition is 3 mol% or less, it can be considered that the composition is the same.

This is because the chemical composition between the primary particles is made uniform, and the degree of expansion and contraction associated with charging and discharging of each primary particle is made constant, so that cracks at the grain interface between the primary particles due to the difference in expansion and contraction occur. This can be prevented and higher cycle characteristics can be obtained.

The positive electrode active material of the present invention is a lithium-containing transition metal oxide having a layered structure, and has a general formula Li _{1 + x} Ni _a Mn _b Co _c O _{2 + d} (wherein x, a, b, c, d are x + a + b + c = 1, 0 <x ≦ 0.2, 0 ≦ c / (a + b) <0.6, 1 ≦ a / b ≦ 3, −0.1 ≦ d ≦ 0.1. It is preferable to use one.

The lithium transition metal oxide represented by the above general formula is used in which the cobalt composition ratio c, the nickel composition ratio a, and the manganese composition ratio b satisfy the condition of 0 ≦ c / (a + b) <0.6. This reduces the material ratio of the positive electrode active material by reducing the proportion of cobalt, and when the proportion of cobalt is small, the diffusion of lithium in the solid phase is slow, so that the primary particles are made small. On the other hand, in order to improve the filling property of the positive electrode active material, it is necessary to agglomerate primary particles to form secondary particles. Considering this, it is more desirable that 0 ≦ c / (a + b) <0.4, and it is even more desirable that 0 ≦ c / (a + b) <0.3.

Also, the nickel composition ratio a and the manganese composition ratio b satisfying the condition of a / b ≦ 3.0 are used because the change in the c-axis direction is suppressed even when the particle aspect ratio is relatively large. Therefore, even better cycle characteristics can be obtained. On the other hand, when 1> a / b, it is difficult to obtain a lithium transition metal oxide having a uniform composition, and the degree of expansion / contraction associated with charging / discharging of each particle is difficult to be constant, resulting in deterioration of cycle characteristics. It is.

Furthermore, when the lithium composition ratio (1 + x) satisfies the condition of 0 <x ≦ 0.2, the cycle characteristic is improved when 0 <x. On the other hand, when x> 0.2, there are many alkali components on the surface of the lithium-containing transition metal oxide, so that side reactions increase and cycle characteristics deteriorate.

In addition, d in the oxygen composition ratio (2 + d) satisfies the condition of −0.1 ≦ d ≦ 0.1 because the lithium-containing transition metal oxide is in an oxygen deficient state or an oxygen excess state. This is because the crystal structure is damaged, the active material is easily cracked, and the cycle characteristics are deteriorated.

In the positive electrode active material of the present invention, the average particle diameter (volume basis) of secondary particles is preferably in the range of 5 μm to 30 μm, and more preferably in the range of 10 μm to 20 μm.

Here, when the average particle diameter of the secondary particles is larger than 30 μm, sufficient diffusibility of lithium in the particles cannot be ensured, and the cycle characteristics deteriorate. On the other hand, when the average particle diameter of the secondary particles is 3 μm or less, the specific surface area is increased and the side reaction is increased, whereby the cycle characteristics are deteriorated.

Further, in the positive electrode active material of the present invention, the average particle diameter of the spherical primary particles is preferably 0.3 μm or more and 4 μm or less, and the average particle diameter of the elongated primary particles is preferably 1.5 μm or more and 13 μm or less.

In the spherical primary particles, if the average particle diameter is less than 2 μm, the bonding characteristics between the primary particles cannot be sufficiently obtained, and the cycle characteristics are deteriorated.

On the other hand, when the average particle diameter is larger than 14 μm, the strain due to the stress due to expansion / contraction during charging / discharging is not easily reduced at the center of the secondary particles, so that the cycle characteristics are deteriorated.

Further, in the elongated primary particles, when the average particle diameter of the primary particles is less than 1 μm, the number of constituent particles of the secondary particles increases, so that the interfacial resistance increases, the electron conductivity decreases, and the cycle characteristics are reduced. descend. On the other hand, when the average particle diameter of the primary particles is larger than 26 μm, the strain due to the stress due to expansion / contraction during charging / discharging is not easily reduced in the central part of the secondary particles, so that the cycle characteristics are deteriorated.

In the positive electrode active material of the present invention, the long axis length of the elongated primary particles in the secondary particles is preferably 25% or more and 60% or less of the major axis of the secondary particles.

If the major axis length of the primary particles is 60% or more of the major axis of the secondary particles, the strain caused by the stress due to expansion and contraction during charge / discharge is not easily reduced at the center of the secondary particles. Characteristics are degraded. On the other hand, if it is less than 25%, a sufficient fixing force between the primary particles cannot be obtained, so that the cycle characteristics deteriorate.

As the positive electrode active material of the nonaqueous electrolyte secondary battery of the present invention, boron (B), fluorine (F), magnesium (Mg), aluminum (Al), chromium (Cr), vanadium (V), iron (Fe) , Copper (Cu), zinc (Zn), molybdenum (Mo), zirconium (Zr), tin (Sn), tungsten (W), titanium (Ti), niobium (Nb), tantalum (Ta), sodium (Na) , Potassium (K), or at least one selected from the group consisting of rare earths may be included.

These addition amounts are preferably 0.1 mol% or more and 5.0 mol% or less with respect to the transition metal in the lithium-containing transition metal composite oxide, and particularly 0.1 mol% or more and 3.0 mol% or less. Is more preferable. This is because when the amount added exceeds 5.0 mol%, the capacity is lowered and the energy density is lowered. On the other hand, when the added amount is less than 0.1 mol%, the effect on the crystal growth by the added element is reduced.

The positive electrode active material used in the nonaqueous electrolyte secondary battery of the present invention does not need to be composed of only the positive electrode active material described above, and has a layered structure capable of reversibly inserting and extracting lithium. If it is, it will not specifically limit. Examples of the lithium-containing transition metal composite oxide include lithium cobaltate, lithium composite oxide of Ni—Mn—Al, lithium composite oxide of Ni—Co—Al, lithium composite oxide of Co—Mn, iron, manganese, and the like. Examples include transition metal oxides. In addition, the active material, a compound having a spinel structure, a phosphoric acid compound, a boric acid compound, and a silicic acid compound (the active material is at least one selected from the group consisting of Li, Ni, Mn, Co, Fe, and rare earths) May be used in combination.

The packing density of the positive electrode used in the nonaqueous electrolyte secondary battery of the present invention is preferably 2.0 g / cm ³ or more and 4.0 g / cm ³ or less, particularly 2.8 g / cm ³ or more and 3.7 g. / Cm ³ or less is more preferable. When the packing density of the positive electrode exceeds 4.0 g / cm ³ , the amount of the electrolytic solution in the positive electrode is reduced, and the cycle characteristics are deteriorated due to a heterogeneous reaction. On the other hand, when the packing density of the positive electrode is less than 2.0 g / cm ³ , not only the energy density is decreased, but also the electron conductivity in the positive electrode is decreased, resulting in a decrease in capacity and cycle characteristics due to a heterogeneous reaction. Because.

Examples of the negative electrode active material for the non-aqueous electrolyte secondary battery of the present invention include carbon materials such as various natural graphites, cokes, graphitized carbon, carbon fibers, spherical carbon, various artificial graphites, amorphous carbon, and metals, Two or more kinds of metal fibers, oxides, nitrides, tin compounds, silicon compounds, various alloy materials and the like can be used in combination. As a material used together with the carbon material, a simple substance such as silicon (Si) or tin (Sn), or a silicon compound or tin compound such as an alloy, a compound, or a solid solution is preferable from the viewpoint of a large capacity density. For example, as the silicon compound, SiO _x (0.05 <x <1.95), or any one of these may be B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, An alloy, a compound, a solid solution, or the like in which a part of Si is substituted with at least one element selected from the group consisting of Ta, V, W, Zn, C, N, and Sn can be used. More preferably, the silicon oxide has a ratio of oxygen atom to silicon atom (O / Si) of 0.5 to 1.5. As the tin compound, Ni ₂ Sn ₄ , Mg ₂ Sn, SnO _x (0 <x <2), SnO ₂ , SnSiO ₃ or the like can be applied. In addition to the above, although the energy density is lowered, a material having a higher charge / discharge potential with respect to lithium metal such as lithium titanate than a carbon material can be used.

Examples of the positive electrode or negative electrode binder include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polyacrylic acid methyl ester, and polyacrylic. Acid ethyl ester, polyacrylic acid hexyl ester, polymethacrylic acid, polymethacrylic acid methyl ester, polymethacrylic acid ethyl ester, polymethacrylic acid hexyl ester, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene Styrene butadiene rubber, carboxymethyl cellulose, etc. can be used. Two types selected from tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene A copolymer of the above materials may be used. Two or more selected from these may be mixed and used.

Examples of the conductive agent included in the electrode include natural graphite and artificial graphite graphite, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon nanotubes, and other carbon blacks, gas phase Conductive fibers such as carbon fibers and metal fibers such as growth carbon fiber (VGCF), metal powders such as carbon fluoride and aluminum, conductive whiskers such as zinc oxide and potassium titanate, and conductivity such as titanium oxide Organic conductive materials such as metal oxides and phenylene derivatives can be used.

The mixing ratio of the positive electrode active material, the conductive agent, and the binder is within the range of 80 to 99% by mass of the positive electrode active material, 0.5 to 20% by mass of the conductive agent, and 0.5 to 20% by mass of the binder, respectively. It is preferable. This is because when the positive electrode active material is less than 80% by mass, the energy density decreases, and when it exceeds 99% by mass, the electron conductivity in the positive electrode decreases, resulting in a decrease in capacity and cycle characteristics due to heterogeneous reactions. . The blending ratio of the negative electrode active material and the binder is preferably in the range of 93 to 99% by mass of the negative electrode active material and 1 to 10% by mass of the binder, respectively. This is because if the negative electrode active material is less than 93% by mass, the energy density decreases, and if it exceeds 99% by mass, the binder is insufficient and the active material collapses.

As the current collector, a long porous conductive substrate or a non-porous conductive substrate is used. As a material used for the conductive substrate, as the positive electrode current collector, for example, stainless steel, aluminum, titanium, or the like is used. As the negative electrode current collector, for example, stainless steel, nickel, copper, or the like is used. The thickness of these current collectors is not particularly limited, but is preferably 1 to 500 μm, and more preferably 5 to 20 μm. By setting the thickness of the current collector within the above range, it is possible to reduce the weight while maintaining the strength of the electrode plate.

As the separator interposed between the positive electrode and the negative electrode, a microporous thin film, a woven fabric, a non-woven fabric or the like having a large ion permeability and having a predetermined mechanical strength and an insulating property is used. As a material of the separator, for example, polyolefin such as polypropylene and polyethylene is preferable from the viewpoint of safety of the nonaqueous electrolyte secondary battery because it has excellent durability and has a shutdown function. The thickness of the separator is generally 6 to 300 μm, preferably 40 μm or less. Further, the range of 10 to 30 μm is more preferable, and the more preferable range of the separator thickness is 10 to 25 μm. Furthermore, the microporous film may be a single layer film made of one kind of material, or a composite film or a multilayer film made of one kind or two or more kinds of materials. The porosity of the separator is preferably in the range of 30 to 70%. Here, the porosity indicates the volume ratio of the pores to the separator volume. A more preferable range of the porosity of the separator is 35 to 60%.

The solute of the non-aqueous electrolyte used in the present invention is not limited, and solutes conventionally used for non-aqueous electrolyte secondary batteries can be used. For example, as such a lithium salt, a lithium salt containing one or more elements among P, B, F, O, S, N, and Cl can be used. Specifically, LiPF ₆ , LiBF ₄ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiPF _6-x (C _n F _2n-1 ) _x (where 1 <x < 6. In addition to n = 1 or 2), LiPO ₂ F _{2 and the} like, a lithium salt having an oxalato complex as an anion can also be used. Examples of the lithium salt having the oxalato complex as an anion include LiBOB [lithium-bisoxalate borate] and a lithium salt having an anion in which C ₂ O ₄ ²⁻ is coordinated to the central atom, such as Li [M (C ₂ O ₄ ) _x R _y ] (wherein M is a transition metal, an element selected from groups IIIb, IVb and Vb of the periodic table, R is selected from a halogen, an alkyl group and a halogen-substituted alkyl group) Group, x is a positive integer, and y is 0 or a positive integer). Specifically, there are Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ], and the like.

The above solutes may be used alone or in combination of two or more. The concentration of the solute is not particularly limited, but is preferably 0.8 to 1.7 mol per liter of the electrolyte.

The nonaqueous electrolyte solvent used in the present invention can be used by mixing the following solvents. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, propionic acid Compounds containing esters such as ethyl and γ-butyrolactone, compounds containing sulfone groups such as propane sultone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane, 1, 4 -Compounds containing ethers such as dioxane and 2-methyltetrahydrofuran, butyronitrile, valeronitrile, n-heptanenitrile, succinonitrile, glutaronitrile, adiponitrile, pimeronite Le, 1,2,3-propanetriol-carbonitrile, 1,3,5-pentanetricarboxylic carbonitrile compounds containing nitrile such as nitrile or can be used compounds comprising an amide such as dimethylformamide. In particular, a solvent in which some or all of these H are substituted with F can be used. Further, these can be used alone or in combination, and a solvent in which a cyclic carbonate and a chain carbonate are combined, and a solvent in which a compound containing a small amount of nitrile or an ether is further combined with these is preferable. .

Further, the non-aqueous electrolyte may contain a known benzene derivative that decomposes during overcharge to form a film on the electrode and inactivate the battery. As the benzene derivative, those having a phenyl group and a cyclic compound group adjacent to the phenyl group are preferable. As the cyclic compound group, a phenyl group, a cyclic ether group, a cyclic ester group, a cycloalkyl group, a phenoxy group, and the like are preferable. Specific examples of the benzene derivative include cyclohexylbenzene, biphenyl, diphenyl ether, and tertiary amylbenzene. These may be used alone or in combination of two or more. However, the content of the benzene derivative is preferably 10% by volume or less of the entire non-aqueous solvent.

A layer made of an inorganic filler that has been conventionally used can be formed at the interface between the positive electrode and the separator or the interface between the negative electrode and the separator. As the filler, it is possible to use oxides or phosphate compounds using titanium, aluminum, silicon, magnesium, etc., which have been used conventionally, or those whose surfaces are treated with hydroxide or the like. .

The filler layer can be formed by directly applying a filler-containing slurry to a positive electrode, a negative electrode, or a separator, or by attaching a sheet formed of a filler to the positive electrode, the negative electrode, or the separator. it can.

As the outer package used in the present invention, a cylindrical battery may be an easily deformable one such as an aluminum laminate and a stainless steel can, as well as an aluminum laminate.

(Experimental example 1)
[Preparation of positive electrode active material]
First, an aqueous solution containing cobalt ions, nickel ions, and manganese ions prepared from cobalt sulfate, nickel sulfate, and manganese sulfate is prepared in a reaction vessel, and a molar ratio of cobalt, nickel, and manganese in the aqueous solution (nickel: Cobalt: manganese) was adjusted to 5: 2: 3. Next, an aqueous sodium hydroxide solution was added dropwise over 1 hour while maintaining the temperature of the aqueous solution at 40 ° C. and the pH at 9. Thereby, after obtaining a precipitate containing cobalt, nickel, and manganese, the precipitate was filtered, washed with water and dried to obtain Ni _0.5 Co _0.2 Mn _0.3 (OH) ₂ . .

Next, Ni _0.5 Co _0.2 Mn _0.3 (OH) ₂ obtained by the coprecipitation method is roasted to obtain an oxide, and then Li ₂ so as to have a molar ratio of 1: 0.54. By mixing CO ₃ and firing these in air at 900 ° C. for 12 hours, Li _1.08 Ni _{0.5 0} Co _0.20 Mn _0.30 O ₂ (lithium-containing transition metal oxide) having a layered structure ) Was produced.

The thus produced Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ has a multimodal distribution showing the appearance frequency of the aspect ratio of the primary particles, and the aspect ratio of the primary particles The appearance frequency peaks were 6 and 1, and as shown in FIG. 4A, the primary particles having different aspect ratios were composed of secondary particles that were uniformly dispersed and aggregated.

In addition, the calculation of the distribution of the appearance frequency of the primary particle aspect ratio is performed by first using a cross section polisher of JEOL, creating a cross section, observing using SIM, and then calculating the average particle diameter observed by particle surface observation. Ten secondary particles having a similar cross-sectional diameter were selected and calculated using Image Pro Plus from Roper Industries.

[Production of positive electrode]
Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ as a positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder in a mass ratio of 95: 2 After mixing to a ratio of 5: 2.5, an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode slurry. Next, the positive electrode mixture slurry was applied to both surfaces of a positive electrode current collector made of aluminum foil, dried, and then rolled with a rolling roller, whereby a positive electrode mixture layer was formed on both surfaces of the positive electrode current collector. A positive electrode was produced. The filling density of the positive electrode was 3.5 g / cm ³ .

[Production of negative electrode]
Artificial graphite as a negative electrode active material, CMC (carboxymethylcellulose sodium) as a dispersant, and SBR (styrene-butadiene rubber) as a binder are mixed in an aqueous solution at a mass ratio of 98: 1: 1. A negative electrode mixture slurry was prepared. Next, this negative electrode mixture slurry was uniformly applied to both surfaces of a negative electrode current collector made of copper foil, dried, and further rolled with a rolling roller. This obtained the negative electrode in which the negative mix layer was formed on both surfaces of the negative electrode collector. The packing density of the negative electrode active material in this negative electrode was 1.63 g / cm ³ .

[Adjustment of non-aqueous electrolyte]
Lithium hexafluorophosphate with respect to a mixed solvent in which ethylene carbonate (EC), propylene carbonate (PC) and ethyl methyl carbonate (EMC) dimethyl carbonate (DMC) are mixed at a volume ratio of 10: 10: 50: 30 (LiPF ₆ ) was dissolved at a rate of 1 mol / liter to prepare a non-aqueous electrolyte.

[Production of non-aqueous electrolyte secondary battery]
A 18650 cylindrical nonaqueous electrolyte secondary battery having a nominal capacity of 2300 mAh was fabricated using a separator composed of a positive electrode, a negative electrode, a nonaqueous electrolyte, and a polyethylene microporous membrane. FIG. 6 is a schematic view showing the produced nonaqueous electrolyte secondary battery.

The nonaqueous electrolyte secondary battery shown in FIG. 6 includes a battery case 1 made of stainless steel and an electrode plate group accommodated in the battery case 1. The electrode plate group includes a positive electrode 5, a negative electrode 6, and a polyethylene separator 7, and the positive electrode 5 and the negative electrode 6 are wound in a spiral shape via the separator 7. An upper insulating plate 8a and a lower insulating plate 8b are disposed above and below the electrode plate group. The battery case 1 is sealed by caulking the opening plate 2 with a sealing plate 2 through a gasket 3. One end of an aluminum positive electrode lead 5a is attached to the positive electrode 5, and the other end of the positive electrode lead 5a is connected to a sealing plate 2 that also serves as a positive electrode terminal.

One end of a nickel negative electrode lead 6 a is attached to the negative electrode 6, and the other end of the negative electrode lead 6 a is connected to the battery case 1 that also serves as a negative electrode terminal.

First, an aluminum positive electrode lead 5a and a nickel negative electrode lead 6a were attached to current collectors of a predetermined positive electrode 5 and negative electrode 6, respectively, and then wound through a separator 7 to constitute an electrode plate group. Insulating plates 8a and 8b are arranged on the upper and lower parts of the electrode plate group, the negative electrode lead 6a is welded to the battery case 1, and the positive electrode lead 5a is welded to the sealing plate 2 having an internal pressure actuated safety valve. 1 was stored inside. Thereafter, a non-aqueous electrolyte was injected into the battery case 1 by a reduced pressure method. Finally, the 18650 type nonaqueous electrolyte secondary battery was completed by caulking the opening end of the battery case 1 to the sealing plate 2 via the gasket 3. The battery thus produced was designated as battery A1.

(Experimental example 2)
The temperature of the aqueous solution at the time of coprecipitation is maintained at 40 ° C. and pH is 9 and an aqueous solution of sodium hydroxide is added dropwise for 15 minutes, then the temperature of the aqueous solution is raised to 50 ° C. The positive electrode active material Li _1.08 was obtained in the same manner as in Experimental Example 1, except that the obtained Ni _0.5 Co _0.2 Mn _0.3 (OH) ₂ and Li ₂ CO ₃ were fired at 910 ° C. for 12 hours. Ni _0.50 Co _0.20 Mn _0.30 O ₂ was produced.

The Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ produced in this way has a multimodal distribution of the frequency of appearance of the primary particle aspect ratio, and the primary particle aspect ratio. The appearance frequency peaks were 4 and 2.

Further, as shown in FIG. 4B, the elongated primary particles form the outer periphery of the secondary particles, the long axis is radially from the center, and the short axis is parallel to the tangential direction of the outer periphery. The appearance frequency of the aspect ratio of the elongated primary particles was higher than the appearance frequency of the aspect ratio of the spherical primary particles. A battery produced using this positive electrode active material was designated as battery A2.

(Experimental example 3)
Ni _0.5 Co _0.2 Mn _0.3 (OH) ₂ and Li ₂ CO ₃ obtained by raising the temperature of the aqueous solution during coprecipitation to 40 ° C. and then increasing to 55 ° C. were calcined at 920 ° C. Except for the above, a positive electrode active material Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ was produced in the same manner as in Experimental Example 2.

The Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ produced in this way has a multimodal distribution of the frequency of appearance of the primary particle aspect ratio, and the primary particle aspect ratio. The appearance frequency peaks of 6 and 2 were.

Further, as shown in FIG. 4 (b), it is composed of secondary particles arranged by primary particles as in the positive electrode active material of Experimental Example 2, and the appearance frequency of the aspect ratio of the elongated primary particles is spherical. It was more than the appearance frequency of the aspect ratio of primary particles. A battery produced using this positive electrode active material was designated as battery A3.

(Experimental example 4)
Ni _0.5 Co _0.2 Mn _0.3 (OH) ₂ and Li ₂ CO ₃ obtained by raising the temperature of the aqueous solution during coprecipitation to 40 ° C. and then increasing to 60 ° C. were calcined at 950 ° C. Except for the above, a positive electrode active material Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ was produced in the same manner as in Experimental Example 2.

The Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ produced in this way has a multimodal distribution of the frequency of appearance of the primary particle aspect ratio, and the primary particle aspect ratio. The appearance frequency peaks were 10 and 2.

Further, as shown in FIG. 4 (b), it is composed of secondary particles arranged by primary particles as in the positive electrode active material of Experimental Example 2, and the appearance frequency of the aspect ratio of the elongated primary particles is spherical. It was more than the appearance frequency of the aspect ratio of primary particles. A battery produced using this positive electrode active material was designated as battery A4.

(Experimental example 5)
The temperature of the aqueous solution at the time of coprecipitation is maintained at 40 ° C. and pH is 9, and after adding sodium hydroxide aqueous solution dropwise for 105 minutes, the temperature of the aqueous solution is raised to 50 ° C., and sodium hydroxide aqueous solution is dropped over 15 minutes. A positive electrode active material Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O _{2 in the same} manner as in Experimental Example 2, except that Ni _0.5 Co _0.2 Mn _0.3 (OH) ₂ was obtained. Was made.

The Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ produced in this way has a multimodal distribution of the frequency of appearance of the primary particle aspect ratio, and the primary particle aspect ratio. The appearance frequency peaks were aspect ratios 2 and 4.

Further, as shown in FIG. 4B, the elongated primary particles form the outer periphery of the secondary particles, the long axis is radially from the center, and the short axis is parallel to the tangential direction of the outer periphery. The appearance ratio of the aspect ratio of the spherical primary particles was higher than the appearance ratio of the aspect ratio of the elongated primary particles. A battery produced using this positive electrode active material was designated as battery A5.

(Experimental example 6)
The positive electrode active material Li _1.08 Ni ₀ .0 was prepared in the same manner as in Experimental Example 1 except that the aqueous sodium hydroxide solution was added dropwise over 2 hours while maintaining the temperature of the aqueous solution at 45 ° C. and pH 9 at the time of coprecipitation _{. 50} Co _0.20 Mn _0.30 O ₂ was produced.

The thus produced Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ has a _monomodal distribution indicating the appearance frequency of the primary particle aspect ratio, and the primary particle aspect ratio. The appearance frequency peak was 2.

Moreover, it consisted of secondary particles arranged with primary particles as shown in FIG. A battery produced using this positive electrode active material was designated as battery Z1.

(Experimental example 7)
A positive electrode active material was produced in the same manner as in Experimental Example 6 except that the temperature of the aqueous solution during coprecipitation was 55 ° C.

The thus produced Li _1.08 Ni _0.50 Co _0.20 Mn _0.30 O ₂ has a _monomodal distribution indicating the appearance frequency of the primary particle aspect ratio, and the primary particle aspect ratio. The appearance frequency peak was 5.

Moreover, it consisted of secondary particles arranged with primary particles as shown in FIG. A battery produced using this positive electrode active material was designated as battery Z2.

[Battery evaluation]
The cycle deterioration rates of the batteries A1 to A4, the battery Z1 and the battery Z2 were determined by the following method, and the results are shown in Table 1.

The battery was charged at a constant current of 1150 mA [0.5 It] until the battery voltage reached 4.10 V, charged at a voltage of 4.10 V until the current value reached 46 mA, paused for 10 minutes, and then 1150 mA [0. 5 It] was discharged to a battery voltage of 3.0 V, and then rested for 20 minutes. In addition, charging / discharging of the battery was performed at 25 degreeC.

Then, charging / discharging was repeated 350 times under the above-mentioned charging / discharging conditions, and the reduction rate of the discharge capacity at the 350th cycle with respect to the discharge capacity at the first cycle was obtained. . In addition, it means that cycling characteristics are so favorable that a cycle deterioration rate is small.

Here, when the battery A1 and the battery Z2 are compared, it can be seen that the battery A1 has a lower battery cycle deterioration rate than the battery Z2, and has improved cycle characteristics.

This is because the battery Z2 uses only the positive electrode active material composed of secondary particles in which the primary particles having an aspect ratio of greater than 2 and 10 or less are aggregated, and the primary particles having a high aspect ratio are long axes of each other in the secondary particles. It becomes easy to align in contact with the surface. In such an oriented structure, the direction of volume change due to the expansion and contraction of the primary particles is aligned.

Therefore, in the secondary particles (especially in the central part), when adjacent oriented structures change in volume in different directions, stress is concentrated at the boundary, and cracks are generated at the primary particle interface.

On the other hand, in the positive electrode active material used in Battery A1, the distribution indicating the appearance frequency of the primary particle aspect ratio in the secondary particles is a multimodal distribution, and the appearance frequency peak of the primary particle aspect ratio is 6 and 1.

This is because the distribution showing the appearance frequency of the primary particle aspect ratio in the secondary particles is a multimodal distribution, and the appearance frequency peak of the primary particle aspect ratio is 1 or more and 2 or less and larger than 2. An aspect ratio of 10 or less is provided.

In such a case, the primary particles having a low aspect ratio in the secondary particles are mixed with the primary particles having a high aspect ratio, and the orientation of the primary particles is suppressed and the bonding area between the primary particles is sufficient. Can be secured. As a result, it is considered that the battery A1 has improved cycle characteristics as compared with the battery Z2.

Further, comparing the battery A1 with the batteries A2 to A5, it can be seen that the battery A2 to the battery A5 have a cycle deterioration rate lower than that of the battery A1, and the cycle characteristics are improved.

In the batteries A1 to A5, the distribution indicating the appearance frequency of the primary particle aspect ratio in the secondary particles is a multimodal distribution, and the aspect ratio peak of the primary particle aspect ratio is 1 or more and 2 or less. 2 and an aspect ratio of 10 or less.

However, as shown in FIGS. 4A and 4B, in the positive electrode active material of the battery A1, primary particles having different aspect ratios exist uniformly throughout the secondary particles. In the positive electrode active material of A5, primary particles having a low aspect ratio are unevenly distributed in the central portion, and primary particles having a high aspect ratio are unevenly distributed in the outer peripheral portion.

In the positive electrode active material, stress is likely to concentrate at the center in the secondary particle, and therefore it is preferable to arrange an active material having a small aspect ratio, but there is a problem that bonding between particles is weak.

On the other hand, in the positive electrode active materials in the batteries A2 to A5, the active material having a high aspect ratio is disposed on the outer peripheral portion, thereby strengthening the bonding between the primary particles and the active material having the low aspect ratio disposed in the central portion. The substance can be prevented from falling off. At the same time, distortion due to stress concentration caused by the volume change of the crystal at the time of charging / discharging is more easily reduced in the outer peripheral portion than in the central portion, so that primary particles having a high aspect ratio can be arranged.

Further, the primary particles forming the outer peripheral portion can obtain excellent cycle characteristics by arranging the long axis so as to be radial with respect to the center of the secondary particles.

By arranging the primary particles in this way, strain stress vectors associated with expansion and contraction are arranged in the circumferential direction and can be canceled each other, and the stress is easily relaxed. As a result, it can be seen that the batteries A2 to A5 have a cycle deterioration rate lower than that of the battery A1, and the cycle characteristics are improved.

Further, it can be seen that the cycle characteristics of the batteries A1 to A5 are improved because the cycle deterioration rate is lower than those of the batteries Z1 and Z2. This experimental result is considered to be due to the following reasons.

When the battery Z1 and the battery Z2 are compared, it can be seen that the cycle deterioration rate of the battery Z2 is reduced and the cycle characteristics are improved. In both positive electrode active materials, the distribution indicating the appearance frequency of the primary particle aspect ratio in the secondary particles is a unimodal distribution, but in the positive electrode active material used for the battery Z1, the aspect ratio is 1 or more, 2 Since the following primary particles are composed of agglomerated secondary particles, cracks are generated because sufficient bonding force cannot be obtained for expansion and contraction during charging and discharging between the particles of the active material.

On the other hand, in the battery Z2 using the positive electrode active material composed of secondary particles in which the primary particles having an aspect ratio of greater than 2 and 10 or less are aggregated, the aspect ratio is higher than the primary particles contained in the positive electrode active material of the battery Z1. The bonding area between the particles in the primary particles is increased. As a result, since a relatively large bonding force can be obtained between the particles of the active material with respect to expansion and contraction during charging and discharging, cracks are hardly generated. As a result, the battery Z2 is considered to have improved cycle characteristics than the battery Z1.

The nonaqueous electrolyte secondary battery according to one aspect of the present invention can be applied to applications that require a high capacity and a long life, such as a mobile phone, a notebook computer, a smartphone, and a tablet terminal.

DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Gasket 5 Positive electrode 6 Negative electrode 7 Separator 10 Aggregate

Claims (8)

A positive electrode active material for a non-aqueous electrolyte secondary battery, which is composed of secondary particles in which a plurality of primary particles are aggregated, and the distribution curve representing the appearance frequency of the aspect ratio of the primary particles exhibits multimodality. Spherical primary particles having a peak appearance frequency of 1 to 2 in aspect ratio,
2. The positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, having an elongated appearance primary particle having an aspect ratio appearance frequency greater than 2 and 10 or less. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the appearance frequency of the aspect ratio of the elongated primary particles is larger than the appearance frequency of the aspect ratio of the spherical primary particles. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the appearance frequency of the aspect ratio of the spherical primary particles is larger than the appearance frequency of the aspect ratio of the elongated primary particles. The primary particles aggregated at the center of the secondary particles are the spherical primary particles, and the primary particles aggregated at the outer periphery of the secondary particles are the elongated primary particles. The positive electrode active material for nonaqueous electrolyte secondary batteries in any one of. The cathode active material for a non-aqueous electrolyte secondary battery according to claim 5, wherein the elongated primary particles are arranged so that the major axis direction is radial from the center of the secondary particles. A lithium-containing transition metal oxide having a layered structure,
General formula Li _{1 + x} Ni _a Mn _b Co _c O _{2 + d} (where x, a, b, c, d are x + a + b + c = 1, 0 <x ≦ 0.2, 0 ≦ c / (a + b) <0.6, The positive electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 6, which satisfies the following conditions: 1 ≦ a / b ≦ 3 and −0.1 ≦ d ≦ 0.1. A non-aqueous electrolyte secondary battery using the positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 7.

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