JP2016012500A - Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte secondary battery which has a high capacity and excellent thermal stability.SOLUTION: A cathode active material for a non-aqueous electrolyte secondary battery mainly contains lithium composite oxide in which the rate of nickel (Ni) to the total mole number of metal elements excluding lithium (Li) is more than 30 mol%. The lithium composite oxide contains particles each of which is constructed by aggregation of primary particles having an average particle diameter of 0.5 μm or more, and at least one of boron (B) and phosphor (P) is fixed on the surface of the particles.

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

Patent Document 1 discloses that when the battery temperature rises due to an internal short circuit or the like, boron, phosphorus, or the like is formed on the surface of the lithium composite oxide particles constituting the positive electrode active material in order to suppress the temperature rise caused by the decomposition reaction of the electrolytic solution. A nonaqueous electrolyte secondary battery in which a layer containing nitrogen is formed is disclosed. Here, the lithium composite oxide has a general formula Li _x Ni _y Co _z O _a (0 <x <1.3, y + z = 1, y> z, 1.8 ≦ a ≦ 2.2).

JP-A-7-192720

  By the way, when boron or phosphorus is added to the lithium composite oxide, the thermal stability of the non-aqueous electrolyte secondary battery is improved. However, when the amount of addition increases, the capacity per unit weight of the positive electrode active material decreases. There is a problem. Therefore, it is required to improve the thermal stability of the nonaqueous electrolyte secondary battery while suppressing the addition amount of boron and phosphorus.

  FIG. 4 is an electron microscope image of a conventional positive electrode active material (Comparative Example 1) disclosed in Patent Document 1. FIG. 4 shows that the conventional positive electrode active material is formed by agglomerating primary particles having a small particle diameter, and the particle surface is rough. The present inventor uses a lithium composite oxide constituted by agglomerating primary particles having a large particle diameter, so that even if the amount of boron or phosphorus added is small compared to the conventional, better thermal stability It was found that can be obtained.

  The positive electrode active material for a nonaqueous electrolyte secondary battery according to the present invention is mainly composed of a lithium composite oxide in which the ratio of nickel (Ni) to the total number of moles of metal elements excluding lithium (Li) is more than 30 mol%. The lithium composite oxide is a particle formed by agglomerating primary particles having an average particle diameter of 0.5 μm or more, and at least one of boron (B) and phosphorus (P) is fixed to the surface of the particle. It is characterized by.

  The nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode including the positive electrode active material, a negative electrode, and a nonaqueous electrolyte.

  The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention can provide a non-aqueous electrolyte secondary battery having a high capacity and good thermal stability. In the nonaqueous electrolyte secondary battery according to the present invention, excellent thermal stability can be obtained even when the amount of boron or phosphorus added is small, for example, as compared with the case where a conventional positive electrode active material is used.

It is a figure which shows typically the positive electrode active material which is an example of embodiment of this invention. It is an electron microscope image of the positive electrode active material (Example 1) which is an example of embodiment of this invention. It is a figure for demonstrating the measuring method of the average surface roughness of a positive electrode active material. It is an electron microscope image of the conventional positive electrode active material (Comparative Example 1).

Hereinafter, an example of an embodiment of the present invention will be described in detail.
A nonaqueous electrolyte secondary battery which is an example of an embodiment of the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. A separator is preferably provided between the positive electrode and the negative electrode. The nonaqueous electrolyte secondary battery has a structure in which, for example, a wound electrode body in which a positive electrode and a negative electrode are wound via a separator, and a nonaqueous electrolyte are housed in an exterior body. Alternatively, instead of the wound electrode body, other types of electrode bodies such as a stacked electrode body in which a positive electrode and a negative electrode are stacked via a separator may be applied. In addition, the form of the nonaqueous electrolyte secondary battery is not particularly limited, and examples thereof include a cylindrical shape, a square shape, a coin shape, a button shape, and a laminate shape.

[Positive electrode]
The positive electrode includes a positive electrode current collector such as a metal foil and a positive electrode active material layer formed on the positive electrode current collector. As the positive electrode current collector, a metal foil that is stable in the potential range of the positive electrode such as aluminum, a film in which the metal is disposed on the surface layer, or the like can be used. The positive electrode active material layer preferably includes a conductive material and a binder in addition to the positive electrode active material. The positive electrode active material 10 described later is used as the positive electrode active material.

  The conductive material is used to increase the electrical conductivity of the positive electrode active material layer. Examples of the conductive material include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. These may be used alone or in combination of two or more.

  The binder is used to maintain a good contact state between the positive electrode active material and the conductive material and to enhance the binding property of the positive electrode active material and the like to the surface of the positive electrode current collector. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and modified products thereof. The binder may be used in combination with a thickener such as carboxymethyl cellulose (CMC) or polyethylene oxide (PEO). These may be used alone or in combination of two or more.

  Hereinafter, the positive electrode active material 10 which is an example of an embodiment of the present invention will be described in detail with reference to FIGS.

FIG. 1 is a diagram schematically showing the positive electrode active material 10.
The positive electrode active material 10 contains, as a main component, a lithium composite oxide in which the ratio of nickel (Ni) to the total number of moles of metal elements excluding lithium (Li) is more than 30 mol%. The lithium composite oxide is a particle formed by agglomerating primary particles 11 having an average particle diameter of 0.5 μm or more, and at least one of boron (B) and phosphorus (P) (hereinafter, referred to as “surface”). These are collectively referred to as “element A”). Hereinafter, the lithium composite oxide constituting the positive electrode active material 10 is referred to as “composite oxide Z”.

The positive electrode active material 10 may contain a component other than the composite oxide Z, for example, a lithium composite oxide other than the composite oxide Z as long as the object of the present invention is not impaired. However, the composite oxide Z is preferably contained in an amount of 50% by weight or more with respect to the total weight of the positive electrode active material 10, and is 100% by weight (when the element A is dissolved in the composite oxide Z). There may be. On the particle surface of the positive electrode active material 10, fine particles of an inorganic compound, for example, an oxide such as aluminum oxide (Al ₂ O ₃ ) or a compound containing a lanthanoid element may be present.

  The element A plays a role of suppressing the decomposition reaction of the electrolytic solution and improving the thermal stability of the battery when the battery temperature rises due to an internal short circuit or the like. The decomposition reaction of the electrolytic solution easily occurs on the surface of the positive electrode active material 10. For this reason, in order to suppress the reaction, it is effective that the element A is present on the particle surface of the positive electrode active material 10 (the particle surface of the composite oxide Z). The element A may be contained inside the composite oxide Z particles, but is preferably present at least on the surface of the particles. As element A, either B or P may be used, but B and P may be used in combination.

In addition, the element A sticking to the particle surface of the composite oxide Z includes a state in which the element A is dissolved in the composite oxide Z. Preferably, after adding the element A to the composite oxide Z, firing is performed at a high temperature to dissolve the element A into a solid solution. When the element A is not dissolved in the composite oxide Z, it is preferable that the element A is fixed to the particle surface with such a strength that it does not fall off when the positive electrode active material 10 is washed under the following conditions. is there.
Washing conditions: 500 g of the positive electrode active material 10 is dropped into 1 L of water and stirred for 30 minutes. Thereafter, the suspension is filtered to separate a solid substance, and cleaning is performed using pure water until the cleaning liquid becomes neutral.

  The addition amount of the element A is preferably 0.01 to 1% by weight, preferably 0.02 to 0.7% by weight, and 0.03 to 0.5% with respect to the weight (100% by weight) of the composite oxide Z. Weight percent is particularly preferred. In terms of moles, 0.03 to 3 mole% is preferable, 0.06 to 2 mole% is more preferable, and 0.09 to 1.5 mole% with respect to the mole number (100 mole%) of composite oxide Z. Is particularly preferred. In the case where B and P are used in combination, it is preferable that the total addition amount is within the range. If the addition amount of the element A is within the range, it is possible to efficiently achieve both high battery capacity and good thermal stability. The content of element A in the positive electrode active material 10 is measured by an inductively coupled plasma (ICP) emission spectroscopic analyzer (ICP-AES), an electron beam microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like. be able to.

The element A is preferably dissolved in the complex oxide Z as described above. The complex oxide Z in which the element A is solid-solved has the general formula Li _x Ni _y M _(1-yz) A _z O ₂ {0.1 ≦ x ≦ 1.2, 0.3 <y <1, 0.001 It is preferable that <z <0.05, M ^* is at least one element A, and M is at least one metal element other than Li and Ni}. The Ni content y is preferably greater than 0.3 from the viewpoints of cost reduction and capacity increase. The complex oxide Z has a layered rock salt type crystal structure.

As the metal element M other than Li and Ni contained in the composite oxide Z, Co, Mn, Mg, Zr, Al, Cr, V, Ce, Ti, Fe, K, Ca, Ga, In, Bi, Ba, Sr etc. are mentioned. Among these, it is preferable to include at least one of Co and Mn. Suitable examples of the composite oxide Z include LiNi _0.82 Co _0.15 B _0.03 O _{2 and the} like. The composite oxide Z may be used alone or in combination of two or more.

The composite oxide Z can also be synthesized from a lithium raw material in the same manner as a conventionally known lithium composite transition metal oxide (LiCoO ₂ , LiNi _0.33 Mn _0.33 Co _0.33 O _2, etc.). However, in the same synthesis method as in the prior art, in order to obtain a layered rock salt phase as a stable phase, it is necessary to make the amount of Li excessive to some extent and to set the firing temperature to 700 to 900 ° C. If the firing temperature is less than 700 ° C, crystal growth becomes insufficient. If it exceeds 900 ° C, Ni ions enter the Li site and site exchange (cation mixing) of Ni ions and Li ions occurs, resulting in distortion of the crystal structure and battery characteristics. May be reduced. Thus, it is difficult to synthesize the composite oxide Z while controlling the firing temperature as compared with the case where a conventionally known lithium composite transition metal oxide is similarly produced from a lithium raw material.

  A preferred method for synthesizing the composite oxide Z is a method in which Na of the sodium-nickel composite oxide synthesized by mixing and baking a sodium raw material and a nickel raw material is ion-exchanged with Li. Then, the element A is added to the composite oxide after the ion exchange and re-baked. Compared with the method of synthesizing lithium-nickel composite oxide from a lithium raw material, this method can obtain a layered rock-salt phase even if the firing temperature and the amount of Na of the sodium-nickel composite oxide are greatly changed. Yes, the physical properties and crystal size of the composite can be controlled. Ni-containing composite oxides tend to have primary particles with small particle sizes and large surface roughness, but this method allows crystal growth without distortion or collapse of the crystal structure during firing. The particle shape can be controlled.

The method for synthesizing the sodium-nickel composite oxide is as follows.
As the sodium raw material, at least one selected from metallic sodium and sodium compounds is used. Any sodium compound can be used without particular limitation as long as it contains Na. Suitable sodium raw materials include oxides such as Na ₂ O and Na ₂ O ₂ , salts such as Na ₂ CO ₃ and NaNO ₃ , and hydroxides such as NaOH. Of these, NaNO ₃ is particularly preferable.

As the nickel raw material, any compound containing Ni can be used without particular limitation. Examples thereof include oxides such as Ni ₃ O ₄ , Ni ₂ O ₃ and NiO ₂ , salts such as NiCO ₃ and NiCl ₂ , hydroxides such as Ni (OH) ₂ , and oxyhydroxides such as NiOOH. Of these, NiO ₂ and Ni (OH) ₂ are particularly preferable.

The mixing ratio of the sodium raw material and the nickel raw material is preferably such a ratio that a layered rock salt type crystal structure is generated. Specifically, in the general formula Na _z NiO ₂ , the sodium amount z is preferably 0.5 to 2, more preferably 0.8 to 1.5, and particularly preferably 1. For example, both raw materials are mixed so that the chemical composition of NaNiO ₂ is obtained. The mixing method is not particularly limited as long as these can be uniformly mixed, and examples thereof include mixing using a known mixer such as a mixer.

  The mixture of the sodium raw material and the nickel raw material is fired in the air or in an oxygen stream. The firing temperature is preferably 600 to 1100 ° C, more preferably 700 to 1000 ° C. The firing time is preferably 1 to 50 hours when the firing temperature is 600 to 1100 ° C. When the firing temperature is 900 to 1000 ° C., it is preferably 1 to 10 hours. The fired product is preferably pulverized by a known method. In this way, a sodium-nickel composite oxide is obtained.

The ion exchange method of the sodium-nickel composite oxide is as follows.
As a suitable method for ion-exchange of Na to Li, for example, a method in which a molten salt bed of lithium salt is added to sodium composite transition metal oxide and heated can be mentioned. As the lithium salt, it is preferable to use at least one selected from lithium nitrate, lithium sulfate, lithium chloride, lithium carbonate, lithium hydroxide, lithium iodide, lithium bromide, and the like. 200-400 degreeC is preferable and the heating temperature at the time of an ion exchange process has more preferable 330-380 degreeC. The treatment time is preferably 2 to 20 hours, more preferably 5 to 15 hours.

  As a method of the ion exchange treatment, a method of immersing a sodium-containing transition metal oxide in a solution containing at least one lithium salt is also suitable. In this case, sodium composite transition metal oxide is put into an organic solvent in which a lithium compound is dissolved, and the treatment is performed at a temperature not higher than the boiling point of the organic solvent. In order to increase the ion exchange rate, it is preferable to perform the ion exchange treatment while refluxing the solvent near the boiling point of the organic solvent. The treatment temperature is preferably from 100 to 200 ° C, more preferably from 140 to 180 ° C. Although processing time changes also with processing temperature, 5 to 50 hours are preferable and 10 to 20 hours are more preferable.

In the lithium-nickel composite oxide produced using the ion exchange, the ion exchange may not proceed completely, and a certain amount of Na may remain. In this case, for example, the composite oxide Z in which the element A is solid-solved by re-firing as described later has the general formula Li _xu Na _{x (1-u)} Ni _y M _(1-yz) A _z O ₂ {0.1 ≦ x ≦ 1.2, 0.3 <y <1, 0.001 <z <0.05, 0.95 <u ≦ 1}. Here, u is an exchange rate when Na is ion-exchanged with Li.

  Next, the element A is added to the composite oxide Z and refired to obtain the composite oxide Z in which the element A is fixed to the surface of the oxide particles, and preferably the element A is in solid solution. When refiring, for example, the crystallite diameter may grow somewhat, but the physical properties such as the crystallite diameter, surface roughness, and density of the positive electrode active material 10 are substantially determined before the fixing step. Yes.

The element A can be added to and mixed with the composite oxide Z as a simple substance of B and P or as a compound of B and P. The compound is not particularly limited, but is preferably an oxide (B ₂ O ₃ , P ₂ O ₅ ). The mixing method of the element A and the composite oxide Z is not particularly limited as long as they can be mixed uniformly, and examples thereof include mixing using a known mixer such as a mixer.

When boron oxide (B ₂ O ₅ ) is used as the compound of element A, the addition amount is preferably, for example, 0.05 to 1.5% by weight with respect to the weight (100% by weight) of the composite oxide Z. 0.07 to 1.2% by weight is more preferable, and 0.1 to 1% by weight is particularly preferable. In terms of the number of moles, for example, 0.1 to 4 mol% is preferable, 0.2 to 3.5 mol% is more preferable, and 0.3 to 3 is preferable with respect to the number of moles (100 mol%) of the composite oxide Z. Mole% is particularly preferred.

The re-baking is performed in the air or in an oxygen stream. The recalcination temperature needs to be lower than the first firing temperature, that is, the firing temperature for the mixture of the sodium raw material and the nickel raw material.
A preferable rebaking temperature is 700 to 1050 ° C., and a preferable rebaking time is 1 to 50 hours. By the refiring, for example, particles of the composite oxide Z in which the element A is dissolved at least on the particle surface can be obtained without greatly changing the crystallite diameter or surface roughness of the composite oxide Z after ion exchange. . The refired product is preferably pulverized by a known method.

  The positive electrode active material 10 (the composite oxide Z to which the element A is fixed) obtained by the above method is a secondary particle obtained by agglomerating primary particles 11 having an average particle diameter of 0.5 μm or more. For this reason, the grain boundary 12 of the primary particle 11 exists in the positive electrode active material 10. Although the positive electrode active materials 10 that are secondary particles may also aggregate, the aggregation of secondary particles can be separated from each other by ultrasonic dispersion. On the other hand, even if the secondary particles are ultrasonically dispersed, the particles are not separated into the primary particles 11.

In addition, before and after the addition of the element A, the average particle diameter of the primary particles 11, the volume average particle diameter of the secondary particles (hereinafter referred to as “D ₅₀ ”), the surface roughness of the secondary particles, and the like are not substantially changed. That is, the composite oxide Z to which the element A is not fixed and the positive electrode active material 10 that is the composite oxide Z to which the element A is fixed have an average primary particle diameter, D ₅₀ , average surface roughness, average porosity, The tap density, compressive fracture strength, average crystallite diameter, and the like are substantially the same.

D ₅₀ of the composite oxide Z (positive electrode active material 10) is preferably 5 to 30 μm, more preferably 7 to 30 μm, and particularly preferably 10 to 30 μm. Within D ₅₀ of that range, for example, it tends to be low surface roughness of the composite oxide Z, improved packing density of the composite oxide Z in the positive electrode is. D ₅₀ of the composite oxide Z can be measured by a light diffraction scattering method. D ₅₀ means a particle diameter when the volume integrated value is 50% in the particle diameter distribution, and is also called a median diameter.

The average particle diameter of the primary particles 11 constituting the composite oxide Z (hereinafter referred to as “average primary particle diameter”) is 0.5 μm or more, preferably 1 μm or more, more preferably 2 μm or more. When the average primary particle diameter is within the range, the surface roughness can be reduced while maintaining D ₅₀ of the composite oxide Z in an appropriate range. The primary particles 11 are composed of a plurality of crystallites.

The average primary particle diameter can be evaluated using a scanning electron microscope (SEM).
Specifically, it is as follows.
(1) Ten particles are randomly selected from a particle image obtained by observing the composite oxide Z with SEM (2000 times).
(2) The grain boundaries 12 and the like are observed for the 10 selected particles, and the primary particles 11 are determined.
(3) The longest diameter of the primary particles 11 is obtained, and the average value for 10 particles is taken as the average primary particle diameter.

  The size of the crystallites of the complex oxide Z is the direction in which the layers are stacked in the layered rock salt type crystal structure (0 0 3), the crystallite diameter in the vector direction, and the direction perpendicular to the (1 1 0) vector direction It can be expressed by the crystallite diameter of Here, the crystallite size is evaluated using the crystallite diameter in the (1 1 0) vector direction. The average crystallite diameter in the (1 1 0) vector direction of the composite oxide Z is preferably 70 to 300 nm, more preferably 100 to 300 nm, and particularly preferably 150 to 300 nm. If the average crystallite diameter is within this range, for example, the ion conductivity of the composite oxide Z can be improved.

  As for the crystallite diameter, a powder X-ray diffraction pattern of the composite oxide Z is obtained using a powder X-ray diffraction measurement apparatus (Bruker AXS, trade name “D8ADVANCE”), and the powder X-ray diffraction pattern is decomposed into a whole powder pattern. It is calculated by analysis by the method (hereinafter referred to as WPPD method).

The measurement conditions of the powder X-ray diffraction pattern are as follows.
X-ray output: 40 kV x 40 mA
Detector: Scintillation counter Goniometer radius: 250mm
Divergent slit: 0.6 °
Scattering slit: 0.6 °
Receiving slit: 0.1mm
Solar slit: 2.5 ° (incident side, light receiving side)
The measurement of the powder X-ray diffraction pattern is performed using a 2θ / θ method (2θ = 15 to 140 ° is measured, step width is 0.01 °) with a sample-type concentrated optical system. The scanning time is set so that the intensity of the main peak ((111) plane) is about 10,000 counts.

The analysis procedure using the WPPD method is as follows.
Procedure 1: Start the software (TOPAS) and read the measurement data.
Step 2: Set the Emission Profile. (Select Cu tube, Bragg Brentano concentrated optical system)
Step 3: Set the background. (The Legendre polynomial is used as the profile function, and the number of terms is set to 8-20.)
Procedure 4: Set Instrument. (Use Fundamental Parameter, input slit condition, filament length, sample length)
Step 5: Set Corrections. (Uses sample displacement. Absorbtion is also used when the sample packing density in the sample holder is low. In this case, Absorption is fixed by the linear absorption coefficient of the measurement sample.)
Procedure 6: Set the crystal structure. (Set to space group R3-m. Use lattice constant, crystallite diameter, and lattice strain. Set spread of profile by crystallite diameter and lattice strain to Lorentz function)
Step 7: Perform calculation. (The background, sample displacement, diffraction intensity, lattice constant, crystallite diameter, and lattice distortion are refined, and the Le-ball equation is used for the calculation)
Procedure 8: If the standard deviation of the crystallite diameter is 6% or less of the refined value, the analysis is completed. If greater than 6%, go to step 9.
Step 9: Set the profile spread due to lattice distortion to a Gaussian function. (The crystallite diameter remains the Lorentz function)
Step 10: Perform calculation. (Precision of background, sample displacement, diffraction intensity, lattice constant, crystallite diameter, and lattice distortion)
Procedure 11: If the standard deviation of the crystallite diameter is 6% or less of the refined value, the analysis is completed. If it is greater than 6%, analysis is not possible.

FIG. 2 is an SEM image of the positive electrode active material 10 (the composite oxide Z to which the element A is fixed).
As shown in FIG. 2, the positive electrode active material 10 (composite oxide Z) has a smooth particle surface and small irregularities compared to the conventional positive electrode active material (see FIG. 4). The degree of smoothness (unevenness) on the particle surface can be evaluated by the surface roughness measured by the method described later. The average surface roughness of the composite oxide Z is preferably small, specifically 4% or less, more preferably 3% or less, and particularly preferably 2.5% or less. If the average double-sided roughness is 4% or less, good thermal stability can be obtained even if the amount of element A added is small. The surface roughness of the composite oxide Z is affected by, for example, the primary particle diameter and the closeness between the primary particles 11.

  90% or more of the composite oxide Z preferably has a surface roughness of 4% or less, and more preferably 95% or more has a surface roughness of 4% or less. In other words, the ratio of the composite oxide Z having a surface roughness of 4% or less with respect to the total number of the composite oxide Z is preferably 90% or more.

The average surface roughness of the composite oxide Z is evaluated by determining the surface roughness for each particle. The surface roughness was determined for 10 particles, and the average was taken as the average surface roughness. The surface roughness (%) is calculated using the calculation formula for the surface roughness described in International Publication No. 2011/125577. The calculation formula is as follows.
(Surface roughness) = (maximum value of change amount of particle radius r every 1 °) / (longest diameter of particle)
The particle radius r was determined as the distance from the center C defined as a point that bisects the longest diameter of the particle in shape measurement described later to each point around the particle. The amount of change of the particle radius every 1 ° is an absolute value, and the maximum value means the maximum amount of the amount of change per 1 ° measured for the entire circumference of the particle.

FIG. 3 shows a method for obtaining the particle radius r based on the SEM image of the composite oxide Z.
In FIG. 3, the distance from the center C to each point P _i around the particle is measured as the particle radius r _i . The center C is a position that bisects the longest diameter of the particle. The position around the particle where the particle radius r is maximum was taken as the reference point P ₀ (θ = 0). The angle formed by the line segment CP ₀ connecting the reference point P ₀ and the center C and the line segment CP _i formed by the other peripheral points P _i and the center C of the particle is defined as θ. And the particle | grain radius r in (theta) for every 1 degree was calculated | required. Using this particle radius r, the surface roughness was calculated from the above formula.

  The composite oxide Z is a secondary particle with few internal voids and closely packed with the primary particles 11. The internal voids of the composite oxide Z are easily formed mainly at the interface between the primary particles 11, but in the case of the composite oxide Z, the closeness between the primary particles 11 is high and the porosity is low. The average porosity of the composite oxide Z is preferably 1% or less, and more preferably 0.5% or less. If the average porosity is 1% or less, the output characteristics of the battery will be better, and the packing density of the composite oxide Z in the positive electrode will be improved. The porosity of the composite oxide Z can be obtained by embedding the positive electrode active material in a resin, processing the cross section of the positive electrode active material particles by a cross section polisher (CP) method, and observing the polished surface with an SEM. it can.

  The tap density of the composite oxide Z is preferably 2.7 g / ml or more, and more preferably 3.0 g / ml or more. The tap density of the composite oxide Z can be measured using a powder reduction degree measuring instrument (TPM-1 manufactured by Tsutsui Rika Kikai Co., Ltd.). Specifically, 50 g of a sample (powder) is put into a 150 ml glass graduated cylinder, and a powder filling density is obtained by tapping 1000 times at a stroke of 30 mm using a powder reduction degree measuring device. Density.

The hardness of the composite oxide Z is determined by the closeness between the primary particles 11 constituting the secondary particles, the closeness between the crystallites constituting the primary particles 11, and the like. The hardness of the composite oxide Z can be evaluated by the compressive strength of one particle. The compressive fracture strength (St) is expressed by the formula St = 2.8 P / πd ² described in “Journal of Japan Mining Association” Vol. 81, No. 932, December 1965, pages 1024 to 1030 (where P: depends on the particle) Load, d: indicates particle diameter). Since the compressive fracture strength (St) is divided by the square of the particle size, the dependence on the particle size is high, and the smaller the particle, the greater the compressive fracture strength (St). Therefore, the compressive fracture strength (St) is preferably defined as the compressive fracture strength (St) at a predetermined particle size.

The compressive fracture strength (St) of one particle of the composite oxide Z having a volume average particle diameter (D ₅₀ ) of 5 to 30 μm is preferably 200 to 500 MPa, and more preferably 300 to 500 MPa. If the compression fracture strength is within the range, good cycle characteristics can be obtained even when the battery voltage is high.

[Negative electrode]
The negative electrode includes, for example, a negative electrode current collector such as a metal foil and a negative electrode active material layer formed on the negative electrode current collector. As the negative electrode current collector, a metal foil that is stable in the potential range of the negative electrode such as aluminum or copper, a film in which the metal is disposed on the surface layer, or the like can be used. The negative electrode active material layer preferably contains a binder in addition to the negative electrode active material capable of inserting and extracting lithium ions. Further, a conductive material may be included as necessary.

  Examples of the negative electrode active material include natural graphite, artificial graphite, lithium, silicon, carbon, tin, germanium, aluminum, lead, indium, gallium, lithium alloy, carbon and silicon in which lithium is previously occluded, and alloys and mixtures thereof. Can be used. As the binder, PTFE or the like can be used as in the case of the positive electrode, but styrene-butadiene copolymer (SBR) or a modified body thereof is preferably used. The binder may be used in combination with a thickener such as CMC.

[Non-aqueous electrolyte]
The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The nonaqueous electrolyte is not limited to a liquid electrolyte (nonaqueous electrolyte solution), and may be a solid electrolyte using a gel polymer or the like. As the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and a mixed solvent of two or more of these can be used. The non-aqueous solvent may contain a halogen-substituted product obtained by substituting hydrogen of these solvents with a halogen atom such as fluorine. The halogen-substituted product is preferably a fluorinated cyclic carbonate or a fluorinated chain carbonate, and more preferably used in combination.

  Examples of the esters include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, Examples thereof include carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone.

  Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4 -Cyclic ethers such as dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether , Dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxy toluene, benzyl ethyl ether, diphenyl ether, dibenzyl Ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, tri Examples thereof include chain ethers such as ethylene glycol dimethyl ether and tetraethylene glycol dimethyl.

The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (FSO ₂ ) ₂ , LiN (C ₁ F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂₎ (l, m is an integer of 1 or _{more), LiC (C P F 2p} + 1 SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) (p, q, r Is an integer of 1 or more), Li [B (C ₂ O ₄ ) ₂ ] (bis (oxalate) lithium borate (LiBOB)), 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. These lithium salts may be used alone or in combination of two or more.

[Separator]
As the separator, a porous sheet having ion permeability and insulating properties is used. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. As the material of the separator, olefinic resins such as polyethylene and polypropylene, cellulose and the like are suitable. The separator may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further, this invention is not limited to these Examples.

<Example 1>
[Preparation of positive electrode active material]
Na _1.03 Ni _0.845 Co _0.155 O ₂ as (charge composition) is obtained, sodium nitrate (NaNO _3), nickel oxide (II) (NiO), was mixed cobalt (II, III) oxide and (Co ₃ O ₄₎ . This mixture was kept at a firing temperature of 900 ° C. for 35 hours to obtain a sodium-nickel composite oxide.

A molten salt bed in which lithium nitrate (LiNO ₃ ) and lithium hydroxide (LiOH) were mixed at a molar ratio of 61:39 was equivalent to 5 times equivalent (5 g) to 5 g of the obtained sodium-nickel composite oxide. )added. Thereafter, this mixture was held at a firing temperature of 200 ° C. for 10 hours to ion-exchange Na of the sodium-nickel composite oxide with Li. Furthermore, the material after ion exchange was washed with water to obtain a lithium-nickel composite oxide.

Next, boron oxide (B ₂ O ₃ ) was mixed with the obtained lithium-nickel composite oxide and fired again to prepare a positive electrode active material which is a composite oxide in which B was dissolved. Classification was performed after re-firing, and a D _{50 of 7} to 30 μm was used as the positive electrode active material. B ₂ O ₃ was added in an amount of 0.1 part by weight based on 100 parts by weight of the lithium-nickel composite oxide. The re-baking conditions were 700 ° C. and 50 hours.

The positive electrode active material was analyzed by a powder X-ray diffraction method using a powder X-ray diffractometer (Bruker AXS, trade name “D8ADVANCE”, source Cu-Kα) to identify the crystal structure. The obtained crystal structure was assigned to the layered rock salt type crystal structure. The composition of the positive electrode active material was measured using an ICP emission spectroscopic analyzer (manufactured by Thermo Fisher Scientific, trade name “iCAP6300”), and as a result, it was Li _1.00 Ni _0.82 Co _0.15 B _0.03 O ₂ .

[Production of positive electrode]
The positive electrode active material was mixed at 92% by weight, carbon powder at 5% by weight, and polyvinylidene fluoride powder at 3% by weight, and this was mixed with an N-methyl-2-pyrrolidone (NMP) solution to prepare a slurry. did. This slurry was applied to both surfaces of an aluminum current collector having a thickness of 15 μm by a doctor blade method to form a positive electrode active material layer. Then, after compressing using a compression roller and cutting out to predetermined size, the positive electrode tab was attached and the positive electrode whose short side length was 30 mm and long side length was 40 mm was obtained.

[Production of negative electrode]
A slurry was prepared by mixing 98% by weight of the negative electrode active material, 1% by weight of the styrene-butadiene copolymer and 1% by weight of carboxymethylcellulose, and mixing with water. As the negative electrode active material, a mixture of natural graphite, artificial graphite, and artificial graphite whose surface was coated with amorphous carbon was used. The slurry was applied to both surfaces of a 10 μm thick copper current collector by a doctor blade method to form a negative electrode active material layer. Then, after compressing using a compression roller and cutting out to predetermined size, the negative electrode tab was attached and the negative electrode whose length of a short side is 32 mm and whose length of a long side is 42 mm was obtained.

[Preparation of non-aqueous electrolyte]
LiPF ₆ was added to an equal volume mixed non-aqueous solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) so as to be 1.6 mol / l to prepare a non-aqueous electrolyte.

[Production of non-aqueous electrolyte secondary battery]
Using the positive electrode, the negative electrode, the non-aqueous electrolyte solution, and the separator, a non-aqueous electrolyte secondary battery was produced by the following procedure.
(1) The positive electrode and the negative electrode were wound through a separator to produce a wound electrode body.
(2) Insulating plates were respectively arranged above and below the wound electrode body, and the wound electrode body was housed in a cylindrical battery outer can having a diameter of 18 mm and a height of 65 mm. The battery outer can is made of steel and also serves as a negative electrode terminal.
(3) The negative electrode current collecting tab was welded to the inner bottom portion of the battery outer can, and the positive electrode current collecting tab was welded to the bottom plate portion of the current interrupting sealing body in which the safety device was incorporated.
(4) A nonaqueous electrolyte was supplied from the opening of the battery outer can, and then the battery outer can was sealed with a current interrupting seal provided with a safety valve and a current interrupting device to obtain a nonaqueous electrolyte secondary battery. . The design capacity of the nonaqueous electrolyte secondary battery is 1500 mAh.

<Example 2>
In the production of the positive electrode active material, a positive electrode active material and a nonaqueous electrolyte secondary battery were produced in the same manner as in Example 1 except that the addition amount of B ₂ O ₃ was changed to 0.5 parts by weight.

<Example 3>
In the production of the positive electrode active material, a positive electrode active material and a nonaqueous electrolyte secondary battery were produced in the same manner as in Example 1 except that the amount of B ₂ O ₃ added was changed to 1 part by weight.

<Example 4>
In the production of the positive electrode active material, a positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 3 except that the firing temperature during the production of the sodium-nickel composite oxide was changed to 850 ° C.

<Example 5>
In the production of the positive electrode active material, a positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 3 except that the firing temperature during the production of the sodium-nickel composite oxide was changed to 800 ° C.

<Example 6>
In the production of the positive electrode active material, a positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 3 except that the firing temperature during the production of the sodium-nickel composite oxide was changed to 750 ° C.

<Example 7>
In the production of the positive electrode active material, a positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 3 except that the firing temperature during the production of the sodium-nickel composite oxide was changed to 700 ° C.

<Example 8>
A positive electrode active material and a nonaqueous electrolyte secondary battery were prepared in the same manner as in Example 2 except that diphosphorus pentoxide (P ₂ O ₅ ) was used instead of B ₂ O ₃ in the production of the positive electrode active material. did.

<Example 9>
In the production of the positive electrode active material, a positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 8 except that the amount of P ₂ O ₅ added was changed to 1 part by weight.

<Comparative Example 1>
In preparing the positive electrode active _{material, Li 1.03 Ni 0.845 Co 0.155 O} 2 as to _{obtain, LiNO 3, NiO 2, Co} 3 O 4 were mixed, 10 hours at a firing temperature 750 ° C. The mixture at an oxygen atmosphere Examples were obtained except that lithium-nickel composite oxide was obtained by holding, B ₂ O ₃ was mixed with the lithium-nickel composite oxide, and refired at 700 ° C. for 5 hours. A positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in No. 3.

<Comparative Example 2>
In the production of the positive electrode active material, a positive electrode active material and a nonaqueous electrolyte secondary battery were produced in the same manner as in Comparative Example 1 except that the amount of B ₂ O ₃ added was changed to 2 parts by weight.

<Comparative Example 3>
A positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in Comparative Example 1 except that P ₂ O ₅ was used instead of B ₂ O ₃ in the production of the positive electrode active material.

<Comparative Example 4>
In the production of the positive electrode active material, a positive electrode active material and a nonaqueous electrolyte secondary battery were produced in the same manner as in Comparative Example 3 except that the amount of P ₂ O ₅ added was changed to 2 parts by weight.

For positive electrode active material prepared in Examples 1 to 9 and Comparative Examples 1 to 4, an average primary particle diameter, D _50, the average surface roughness, average porosity, tap density, compressive fracture strength, and the average evaluation of the crystallite size Went. The evaluation results are shown in Tables 1-3.

[Evaluation of average primary particle size]
The procedure for measuring the average primary particle size is as follows.
Ten particles are randomly selected from an image of the positive electrode active material obtained by observation with SEM (2000 times). Next, a grain boundary etc. are observed about 10 selected particles, and each primary particle is determined. The longest diameter of the primary particles was obtained, and the average value for 10 particles was taken as the average primary particle diameter.

[Evaluation of D _50]
D ₅₀ of the positive electrode active material (secondary particles) is a laser diffraction scattering particle size obtained by dispersing 1 ml of a surfactant (manufactured by MERCK, trade name “Extran MA02 Neutral”) in 100 ml of ion-exchanged water. Evaluation was performed using a distribution measuring device (trade name “LA-920” manufactured by HORIBA). The measurement conditions are an ultrasonic dispersion for 1 minute, an ultrasonic intensity of 1, a circulation speed of 2, and a relative refractive index of 1.60 to 0.25.

[Evaluation of average surface roughness]
The surface roughness was determined for 10 particles, and the average was taken as the average surface roughness. The surface roughness (%) was calculated using the following calculation formula.
(Surface roughness) = (maximum value of change amount of particle radius r every 1 °) / (longest diameter of particle)
The particle radius r was obtained as the distance from the center C defined as a point that bisects the longest diameter of the particle to the respective points around the particle in the shape measurement described with reference to FIG. The amount of change of the particle radius every 1 ° is an absolute value, and the maximum value means the maximum amount of the amount of change per 1 ° measured for the entire circumference of the particle.

[Evaluation of average porosity]
Mix the positive electrode active material and thermosetting resin, cure the resin, embed the positive electrode active material in the resin, and make a rough cross section by using mechanical polishing etc., then finish cross section by cross section polisher (CP) method The polished surface was processed and observed with a SEM at a magnification of 1,000 to 10,000 times. From the obtained image, the porosity of the positive electrode active material was calculated using Image-Pro Plus analysis software. Specifically, the color of the hole part of the secondary particle and the color of the part other than the hole part of the secondary particle are divided into white and black or black and white part, and the respective areas are obtained, The porosity (area ratio) was determined from the area ratio. The cross-sectional area B of one secondary particle and the total cross-sectional area A of all the pores contained in the cross-section of the secondary particle are randomly selected by calculating the porosity C (%) of one secondary particle using the following formula. The average value of the porosity of 50 secondary particles was defined as the average porosity.
C (%) = (A / B) × 100

[Evaluation of tap density]
50 g of the positive electrode active material was placed in a 150 ml glass graduated cylinder, and the tap density was measured using a powder reduction degree measuring instrument (TPM-1 manufactured by Tsutsui Rika Kikai Co., Ltd.). The tap density is calculated by dividing the sample weight by the sample volume when tapped 1000 times with a stroke of 30 mm.

[Evaluation of compressive fracture strength]
The compressive fracture strength was measured under the following measurement conditions using a micro-compression tester (“MCT-W201” manufactured by Shimadzu Corporation). Specifically, the deformation amount and the load of the resin particles when a load was applied to one sample particle at the following load speed were measured. Then, the load (N) when the sample particle is deformed and reaches its breaking point (a point at which the displacement starts to increase suddenly), and the particle diameter of the sample particle before deformation (particle diameter measured by the CCD camera) ) Was substituted into the following equation to calculate the compressive fracture strength.
<Calculation formula of compressive fracture strength>
Compressive fracture strength (MPa) = 2.8 × load (N) / {π × (particle diameter (mm)) ² }
<Measurement conditions of compressive strength>
Test temperature: Normal temperature (25 ° C)
Upper pressure indenter: Flat indenter with a diameter of 50 μm (material: diamond)
Lower pressure plate: SKS flat plate Measurement mode: Compression test Test load: Minimum 10 mN, Maximum 50 mN
Load speed: Min. 0.178 mN / sec, Min. 0.221 mN / sec Displacement full scale: 10 μm

[Evaluation of average crystallite size]
By measuring the powder X-ray diffraction pattern of the positive electrode active material using a powder X-ray diffraction measurement device (Bruker AXS, trade name “D8ADVANCE”, source Cu-Kα), and analyzing by the WPPD method, the average crystal The diameter was obtained. Detailed analysis procedures and the like are as described above.

  The positive electrode active materials and non-aqueous electrolyte secondary batteries produced in Examples 1 to 9 and Comparative Examples 1 to 4 were evaluated for battery capacity and heat generation start temperature. The evaluation results are shown in Tables 1-3.

[Measurement of battery capacity]
About each said nonaqueous electrolyte secondary battery (25 degreeC), it charges until a battery voltage will be 4.3V with a constant current of 1It = 2400mA, and after a battery voltage reaches | attains 4.3V, 1It = 2400mA constant. The battery was discharged with current until the battery voltage reached 2.5V. The discharge capacity at this time was divided by the weight of the positive electrode active material to determine the capacity per unit weight (mAh / g) of the positive electrode active material.

[Measurement of heat generation start temperature]
About each said nonaqueous electrolyte secondary battery (25 degreeC), it charged until the battery voltage became 4.3V with the constant current of 1It = 2400mA. Thereafter, the battery was disassembled, the positive electrode was taken out, washed with a nonaqueous solvent, and the electrolyte solution was removed. Then, 1 mg of the positive electrode active material layer was scraped off and sealed in a pressure-resistant sealed container together with 1 μL of the electrolyte solution to obtain a measurement sample. About this measurement sample, it heated up from 25 degreeC to 550 degreeC with the temperature increase rate of 10 degree-C / min using the differential scanning calorimeter (DSC: Differential Scanning Calorimetry), and the heat_generation | fever start temperature was measured. Higher heat generation start temperature means higher heat stability.

As shown in Tables 1 to 3, when the amount of addition of the element A is the same, the batteries of the examples had higher heat generation start temperatures and excellent thermal stability than the batteries of the comparative examples (implementation). Examples 3 to 7, 9 and Comparative Examples 1 and 3). In Example 3, even when the amount of B ₂ O ₃ added to the composite oxide was 1% by weight, the heat generation start temperature could be raised to 245 ° C. In addition, the battery of the example shows a thermal stability (heat generation start temperature) equal to or higher than that of the battery of the comparative example even when the amount of addition of the element A is small, and has a high battery capacity. (See Examples 1, 2, 8 and Comparative Examples 1 and 3). In the battery of the comparative example, when trying to ensure the same thermal stability as the battery of the example, it is necessary to add a larger amount of the element A than in the case of the example, and the battery capacity is greatly reduced. (See Comparative Examples 2 and 4).

  In other words, by using lithium composite oxide particles with a smooth surface composed of agglomerated primary particles with a large particle size, excellent thermal stability can be achieved even when the amount of element A added is small compared to the prior art. Sex can be obtained. In addition, by suppressing the amount of element A added, a high battery capacity can be maintained. According to the battery of the example, it is possible to achieve both high battery capacity and good thermal stability. On the other hand, in the battery of the comparative example, it was difficult to ensure good thermal stability while maintaining a high capacity.

  In the examples, experimental data is shown for the case where B or P is used as the element A, but it is considered that the same effect can be obtained when B and P are used in combination.

  10 cathode active material, 11 primary particles, 12 grain boundaries

Claims (8)

The main component is a lithium composite oxide in which the ratio of nickel (Ni) to the total number of moles of metal elements excluding lithium (Li) is more than 30 mol%,
The lithium composite oxide is a particle formed by agglomerating primary particles having an average particle diameter of 0.5 μm or more, and at least one of boron (B) and phosphorus (P) is fixed to the surface of the particle. A positive electrode active material for a non-aqueous electrolyte secondary battery.   2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium composite oxide has a volume average particle diameter of 5 to 30 μm and an average surface roughness of 4% or less. The lithium composite oxide has a general formula Li _x Ni _y M _(1-yz) A _z O ₂ {0.1 ≦ x ≦ 1.2, 0.3 <y <1, 0.001 <z <0. The positive electrode active for a nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein 05, A is represented by one or more metal elements including at least one of B and P, and M is at least one of Co and Mn. material.   The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium composite oxide has an average porosity of 1% or less.   5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium composite oxide has a tap density of 2.7 g / ml or more.   6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium composite oxide has a compressive fracture strength of 200 to 500 MPa.   The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the lithium composite oxide has an average crystallite size in the (1 1 0) vector direction of 70 to 300 nm. A positive electrode comprising the positive electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 7,
A negative electrode,
A non-aqueous electrolyte,
A non-aqueous electrolyte secondary battery.