JP2019192325A - Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Abstract

To provide a positive electrode active material for a non-aqueous electrolyte secondary battery capable of improving the initial charge/discharge efficiency while including a nickel-containing lithium transition metal oxide having a high Ni content.SOLUTION: A positive electrode active material for a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes Ni-containing lithium transition metal oxide with layered structure (Ni≥80 mol%), and boron is present in the particles of the Ni-containing lithium transition metal oxide, and the ratio b of the boron to the total molar amount of the metal elements excluding lithium in the Ni-containing lithium transition metal oxide is 0 mol%<B≤0.5 mol%, and the discharge capacity ratio d of 3.5 V or less in the discharge capacity of 2.5 V to 4.3 V (vs.Li/Li) is in the range of 6%<d.SELECTED DRAWING: None

Description

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

The nickel-containing lithium transition metal oxide (eg, LiNiO ₂ ), which is one of the positive electrode active materials of the lithium ion secondary battery, has a higher capacity than the cobalt-containing lithium transition metal oxide (eg, LiCoO ₂ ), Nickel is expected to be a next-generation positive electrode material because it has advantages such as being cheaper than cobalt and being stably available.

  In Patent Document 1, during the synthesis of lithium nickelate, by adding a firing aid to the fired material, crystal growth is promoted at a temperature lower than the firing temperature necessary for lithium nickelate to obtain the desired crystal growth, It is disclosed that substitution of elements contributing to structural stability into the crystal is promoted, and that distortion of the crystal and oxygen deficiency during synthesis are suppressed, and a lithium ion secondary battery having excellent cycle characteristics can be provided. ing.

International Publication No. 2011-111377

  By the way, when nickel-containing lithium transition metal oxide is used as the positive electrode active material, the charge / discharge capacity increases as the nickel content increases, but the initial charge / discharge efficiency decreases. The initial charge / discharge efficiency is the ratio of the initial discharge capacity to the initial charge capacity.

  Therefore, the present disclosure provides a positive electrode active material for a non-aqueous electrolyte secondary battery that can improve initial charge and discharge efficiency while containing a nickel-containing lithium transition metal oxide having a high Ni content. Objective.

A positive electrode active material for a non-aqueous electrolyte secondary battery that is one embodiment of the present disclosure includes a layered Ni-containing lithium transition metal oxide (Ni ≧ 80 mol%), and the Ni-containing lithium transition metal oxide particles Boron is present therein, and the ratio b of the boron to the total molar amount of the metal element excluding lithium in the Ni-containing lithium transition metal oxide is in the range of 0 mol% <b ≦ 0.5 mol%. The ratio d of the discharge capacity of 3.5 V or less to the discharge capacity of 2.5 V to 4.3 V (vs. Li / Li ⁺ ) is in the range of 6% <d.

  According to the positive electrode active material for a non-aqueous electrolyte secondary battery that is one embodiment of the present disclosure, it is possible to improve the initial charge / discharge efficiency while containing a nickel-containing lithium transition metal oxide having a high Ni content. Become.

The curve which carried out the secondary differential process of the discharge capacity of test cell A1-A4 and test cell B1 with the electric potential is shown.

As described above, the initial charge / discharge efficiency of the Ni-containing lithium transition metal oxide decreases as the nickel content increases. This is considered to be because when the Ni content is high (particularly Ni ≧ 80%), Li is less likely to be reinserted at the time of discharge due to expansion and contraction of the crystal lattice and structural change accompanying charging. Here, the presence of a predetermined amount of boron in the particles of the Ni-containing lithium transition metal oxide suppresses the structural change and the like, enables Li reinsertion, and around 3.4 to 3.5 V during discharge. (For example, 3.35 to 3.55 V), a new potential flat portion is generated due to a change in the valence of Ni, and the discharge capacity is increased. Thereby, it is possible to improve the initial charge / discharge efficiency. However, in the Ni-containing lithium transition metal oxide in which the ratio d of the discharge capacity of 3.5 V or less to the discharge capacity of 2.5 V to 4.3 V (vs. Li / Li ⁺ ) is 6% or less, It is considered that a part of the existing boron is deposited on the particle surface, and a new potential flat part due to Ni valence change is unlikely to be generated in the vicinity of 3.4 to 3.5 V during discharge. Conceivable.

Therefore, a predetermined amount of boron is present in the particles of Ni-containing lithium transition metal oxide (Ni ≧ 80 mol%) as in the positive electrode active material for a non-aqueous electrolyte secondary battery that is one embodiment of the present disclosure, and 2.5 V By setting the ratio d of the discharge capacity of 3.5 V or less to the discharge capacity of ˜4.3 V (vs. Li / Li ⁺ ) in the range of 6% <d, the discharge capacity is reduced by 3.5 V or more. While suppressing, it becomes possible to increase the discharge capacity of 3.5 V or less, and the initial charge / discharge efficiency can be improved.

  Hereinafter, the configuration of the positive electrode active material for a nonaqueous electrolyte secondary battery will be described in detail.

  The positive electrode active material for a non-aqueous electrolyte secondary battery which is one embodiment of the present disclosure includes a Ni-containing lithium transition metal oxide (Ni ≧ 80 mol%) having a layered structure and boron present in the particles. And the ratio b of the boron with respect to the total molar amount of the metal element except lithium in Ni containing lithium transition metal oxide (Ni> = 80 mol%) is the range of 0 mol% <b <= 0.5 mol%. The presence of boron in the particles means, for example, a form in which boron is uniformly dispersed in the crystal structure of a Ni-containing lithium transition metal oxide, or a form in which boron is present in a part of the crystal structure. Indicates. Further, the Ni-containing lithium transition metal oxide (Ni ≧ 80 mol%) means that the ratio of Ni is 80 mol% or more with respect to the total molar amount of metal elements excluding lithium in the transition metal oxide. is doing. Hereinafter, the Ni-containing lithium transition metal oxide (Ni ≧ 80 mol%) in which boron is present in the particles may be simply referred to as a high Ni-containing lithium transition metal oxide.

  Examples of the layered structure of the high Ni-containing lithium transition metal oxide include a layered structure belonging to the space group R-3m, a layered structure belonging to the space group C2 / m, and the like. Among these, a layered structure belonging to the space group R-3m is preferable in terms of increasing the capacity, stability of the crystal structure, and the like.

The high Ni-containing lithium transition metal oxide is not particularly limited as long as the proportion of Ni is 80 mol% or more as described above and a predetermined amount of boron is present in the particles. The Ni-containing lithium transition metal oxide represented by the composition formula is preferable.
Li _x Ni _y Co _α M _β B _z O _2-γ (1)
In the formula, x, y, α, β, z and γ are 0.95 <x <1.05, 0.80 ≦ y <1, 0 <α <0.15, 0 <β <0.05, respectively. Y + α + β = 1, 0 <z ≦ 0.005, and 0 ≦ γ <0.05. In the formula, M is an additive metal element other than Ni and Co present in the crystal structure. For example, Al, Mg, Si, Ge, Sn, Mg, Cr, Ti, W, Nb, Zr, Mn and One or more elements selected from Fe are listed. In the composition formula (1), the molar abundance ratio of each element is shown by assuming that the total amount of Ni, Co and M is 1 mol, that is, y + α + β = 1.

  X in the composition formula (1) indicates the content (molar ratio) of lithium (Li) with respect to the total amount of Ni, Co, and M. In view of improving the charge / discharge capacity of the nonaqueous electrolyte secondary battery, the lithium content is preferably in the range of 0.95 <x <1.05, and 0.98 <x ≦ 1. A range is more preferable.

  Y in the composition formula (1) represents the content (molar ratio) of nickel (Ni) with respect to the total amount of Ni, Co, and M. In view of improving the charge / discharge capacity of the nonaqueous electrolyte secondary battery, the nickel content is preferably in the range of 0.80 ≦ y <1, and 0.85 <y <1. Is more preferable.

  Α in the composition formula (1) indicates the content (molar ratio) of cobalt (Co) with respect to the total amount of Ni, Co, and M. By setting the cobalt content in the range of 0 <α, the durability of the nonaqueous electrolyte secondary battery can be improved. Moreover, the charge / discharge capacity of the nonaqueous electrolyte secondary battery can be improved by setting the cobalt content in the range of α <0.15. A more preferable cobalt content is in the range of 0.03 <α <0.12.

Β in the composition formula (1) indicates the content (molar ratio) of M with respect to the total amount of Ni, Co, and M.
By setting the content of M in the range of 0 <β, the durability of the nonaqueous electrolyte secondary battery can be improved. Moreover, the charging / discharging capacity | capacitance of a nonaqueous electrolyte secondary battery can be improved by making content of M into the range of (beta) <0.05. A more preferable content of M is in the range of 0.005 <β <0.05.

  Z in the composition formula (1) indicates the content (molar ratio) of boron (B) with respect to the total amount of Ni, Co, and M. By containing boron in the range of 0 <z ≦ 0.005, the layered oxide increases the discharge capacity of 3.5 V or less, and suppresses the decrease in the initial charge / discharge efficiency of the nonaqueous electrolyte secondary battery. it can. In addition, when there is too much content of boron, depending on the case, lithium borate may produce | generate on particle | grains and charge / discharge capacity may fall.

  “2-γ” in the composition formula (1) indicates the content (molar ratio) of oxygen atoms (O) with respect to the total amount of Ni, Co, and M. Here, γ indicates the amount of oxygen deficiency, and when the value of γ increases, the amount of divalent Ni increases accordingly, so-called “rock chloride” in which the layered structure becomes a rock salt structure occurs, and the charge / discharge capacity Is expected to decrease. For this reason, from the viewpoint of improving the charge / discharge capacity, it is preferable to set γ, which is an acceleration factor for rock chloride, in the range of 0 ≦ γ <0.05.

  The Ni-containing lithium transition metal oxide may contain a metal element other than Li, Ni, Co, and M as long as the object of the present disclosure is not impaired.

  The content of the element constituting the positive electrode active material can be measured by an inductively coupled plasma optical emission spectrometer (ICP-AES), an electron beam microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like. .

In the high Ni-containing lithium transition metal oxide, the ratio d of the discharge capacity of 3.5 V or less to the discharge capacity of 2.5 V to 4.3 V (vs. Li / Li ⁺ ) should be in the range of 6% <d. The range is preferably 7% <d. The upper limit of the ratio d of the discharge capacity of 3.5 V or less is preferably 20% or less, and more preferably 15% or less, from the viewpoints of initial charge / discharge efficiency, crystal structure stability, and the like.

Here, the ratio d of the discharge capacity of 3.5 V or less is a value measured as follows. Using a cell (cell capacity 30 mAh) comprising an electrode made of a high Ni-containing lithium transition metal oxide as a positive electrode active material and a lithium metal as a counter electrode, the voltage is 4.3 V under a temperature condition of 25 ° C. Then, constant current charging is performed at a current value of 6 mA, and then constant voltage charging is performed at 4.3 V until the current value reaches 1.5 mA. Thereafter, constant current discharge is performed at 6 mA until the voltage reaches 2.5V. In such charge and discharge, the ratio of the discharge capacity of 2.5 to 3.5 V occupying the discharge capacity of 2.5 V to 4.3 V (vs. Li / Li ⁺ ) is the ratio d of the discharge capacity of 3.5 V or less. And The discharge capacity is a discharge capacity (mAh / g) per unit weight of the positive electrode active material.

  The ratio of the high Ni-containing lithium transition metal oxide in which boron is present in the particles to the total amount of the positive electrode active material for a nonaqueous electrolyte secondary battery is preferably 90% by mass or more, and more preferably 99% by mass. That's it. When the ratio of the high Ni-containing lithium transition metal oxide is less than 90% by mass, the discharge capacity is reduced.

  Further, the positive electrode active material for a non-aqueous electrolyte secondary battery of the present embodiment may contain other lithium transition metal oxides in addition to the high Ni-containing lithium transition metal oxide in which boron is present in the particles. good. Examples of other lithium transition metal oxides include lithium transition metal oxides having a Ni content of 0 mol% to less than 80 mol%, and boron-containing oxides in which boron is contained in the lithium transition metal oxide. It is done.

  In the high Ni-containing lithium transition metal oxide, a part of boron present in the particles may be deposited on the particle surface within a range not impairing the object of the present invention.

  The ratio d of the discharge capacity of 3.5 V or less can be set to 6% <d by the method for synthesizing the Ni-containing lithium transition metal oxide described below.

The synthesis method of the Ni-containing lithium transition metal oxide in which boron is present in the particles includes, for example, a Li-containing compound (Li raw material), a metal-containing compound containing Ni (metal raw material containing Ni), and a B-containing compound (B raw material). ) In a mixing ratio based on the target Ni-containing lithium transition metal oxide, and firing the mixture. Here, after synthesizing a Ni-containing lithium transition metal oxide with a Li raw material and a metal raw material containing Ni, a method of adding and mixing the B raw material and firing the mixture is also conceivable. It is difficult to synthesize a Ni-containing lithium transition metal oxide in which a new potential flat portion appears in the vicinity of .4 to 3.5 V. The ratio of the discharge capacity of 3.5 V or less to the discharge capacity of 2.5 V to 4.3 V (vs. Li / Li ⁺ ) by the method of adding the B raw material in the synthesis stage of the Ni-containing lithium transition metal oxide is described above. It becomes easy to obtain a Ni-containing lithium transition metal oxide within the range.

  The Li raw material is preferably added so that the molar ratio L of Li to the metal excluding Li is in the range of 1.03 <L <1.18. More preferably, 1.04 ≦ L ≦ 1.15. When the molar ratio L of Li is 1.03 or less or 1.18 or more, in the obtained Ni-containing lithium transition metal oxide, a new potential flat portion appears in the vicinity of 3.4 to 3.5 V during discharge. It becomes difficult.

The firing temperature of the raw material mixture is in the range of 650 ° C. to 750 ° C., and the firing time is in the range of 1 hour to 20 hours. The firing of the raw material mixture is preferably performed in an oxygen stream. By satisfying the above conditions, a new potential flat portion appears in the vicinity of 3.4 to 3.5 V, and it is 3.5 V or less that occupies a discharge capacity of 2.5 V to 4.3 V (vs. Li / Li ⁺ ). The ratio of the discharge capacity can be within the above range.

  Further, by the above synthesis method, other parameters such as the lattice constant and the amount of transition metal described below can be controlled within the target range.

[X-ray diffraction pattern]
The X-ray diffraction pattern of the high Ni-containing lithium transition metal oxide is obtained by using a powder X-ray diffractometer (trade name “RINT-TTR”, source Cu-Kα, manufactured by Rigaku Corporation) and powder X under the following conditions: Obtained by analysis based on the line diffraction method.
Measurement range: 15-120 °
Scanning speed: 4 ° / min
Rietveld analysis; PDXL2 (Rigaku Corporation) is used.
Analysis range: 30-120 °
Background; B-spline profile function; split pseudo-Voigt function constraint; Li (3a) + Ni (3a) = 1
Ni (3a) + Ni (3b) = y
ICSD No .; 98-009-4814

  The high Ni-containing lithium transition metal oxide has a lattice constant a indicating the a-axis length of the crystal structure obtained by the analysis result of the X-ray diffraction pattern obtained by the X-ray diffraction of 2.867Å <a <2.873Å. The lattice constant c indicating the c-axis length is preferably in the range of 14.17Å <c <14.19Å. If the lattice constant a is 2.867 Å or less, the interatomic distance becomes narrow and unstable, and the cycle characteristics may be deteriorated, and if it is 2.873 ８７ or more, the load characteristics may be deteriorated. Further, if the lattice constant c is 14.17 mm or less, the interatomic distance is narrow and unstable, and the cycle characteristics may be deteriorated. If it is 14.19 mm or more, the interatomic distance is wide and unstable. May lead to deterioration of cycle characteristics.

  The high Ni-containing lithium transition metal oxide is the amount of transition metal m present at the 3a site (lithium site) in the crystal structure obtained from the Rietveld analysis result of the X-ray diffraction pattern obtained by the X-ray diffraction. Is preferably included in the range of 0 mol ≦ m <2 mol% with respect to the total molar amount of the transition metal. If the amount of transition metal m present at the 3a site (lithium site) in the crystal structure exceeds 2 mol%, the crystal structure may be distorted to reduce the diffusibility of lithium ions, leading to deterioration of battery characteristics.

  The high Ni-containing lithium transition metal oxide has a crystallite size s calculated by Scherrer equation from the half-value width of the diffraction peak of the (104) plane in the X-ray diffraction pattern obtained by the X-ray diffraction. 300Å ≦ s ≦ 700Å, preferably 350Å ≦ s ≦ 550 好 ま し い. Scherrer's formula is expressed by the following formula (2).

D = Kλ / Bcos θ (2)
In equation (2), D is the crystallite size, λ is the X-ray wavelength, B is the full width at half maximum of the diffraction peak on the (104) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this embodiment, K is 0.9.

  When the crystallite size s of the high Ni-containing lithium transition metal oxide is smaller than 300 mm, the crystallinity is lowered and the durability may be lowered. Further, when the crystallite size s of the high Ni-containing lithium transition metal oxide exceeds 700%, the rate characteristics may be deteriorated.

[Compressive fracture strength]
The compressive fracture strength K of the secondary particles of the high Ni-containing lithium transition metal oxide is preferably in the range of 50 MPa <K <150 MPa, and more preferably in the range of 70 MPa <K <120 MPa. Secondary particles are a form in which primary particles are aggregated. If the compressive fracture strength K is 50 MPa or less, the particles may collapse with the charge / discharge cycle and the cycle characteristics may be deteriorated. If it is 150 MPa or more, the filling property is lowered and the volume capacity density of the battery is reduced. May lead to a connection.

The compressive fracture strength can be obtained by measuring under the following measurement conditions using a micro compression tester (manufactured by Shimadzu Corporation “MCT-W201”). Specifically, the deformation amount and the load of the sample particle when a load is applied to the sample particle at the following load speed are 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) ) Is substituted into the following equation (1) to calculate the compressive fracture strength.
Compressive fracture strength (MPa) = 2.8 × load (N) / {π × (particle diameter (mm)) 2} (1)

<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

[Porosity and primary particle size]
The porosity h of the secondary particles of the high Ni-containing lithium transition metal oxide is preferably in the range of 2% <h <6%. When the porosity is 2% or less, the permeability of the electrolytic solution into the secondary particles may be poor and the diffusion of lithium may be reduced. When the porosity is 6% or more, electron conduction is caused by the problem of contact between the primary particles. May decrease.

  The primary particle diameter R of the high Ni-containing lithium transition metal oxide is preferably in the range of 400 nm <R <1300 nm. When the primary particle diameter R is 400 nm or less, the surface area is increased and the reactivity with the electrolytic solution is increased. Therefore, decomposition products of the electrolytic solution are likely to be deposited, which may affect the increase in DC resistance. . When the primary particle diameter is 1300 nm or more, the surface area becomes small and the lithium ion desorption / insertion area becomes small, which may affect the increase of the DC resistance value.

  The measuring method of the porosity and the primary particle diameter is as follows. After mixing a high Ni-containing lithium transition metal oxide and a thermosetting resin, curing the resin, embedding the high Ni-containing lithium transition metal oxide in the resin, and using a mechanical polishing together to produce a rough cross-section, A finished cross section is processed by a cross section polisher (CP) method, and the polished surface is observed with a SIM (Scanning Ion Microscope) at a magnification of 1,000 to 10,000 times. From the obtained image, the measurement of the primary particle diameter and the porosity of the high Ni-containing lithium transition metal oxide are 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 is determined from the area ratio.

  Below, the nonaqueous electrolyte secondary battery to which the positive electrode active material for nonaqueous electrolyte secondary batteries of this embodiment is applied will be described.

  A nonaqueous electrolyte secondary battery which is an example of an embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. A separator is preferably provided between the positive electrode and the negative electrode. Specifically, it has a structure in which a wound electrode body in which a positive electrode and a negative electrode are wound through 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 includes the positive electrode active material for a non-aqueous electrolyte secondary battery described above. The description of the positive electrode active material is as described above, and the description is omitted. The positive electrode active material layer preferably includes a conductive material and a binder in addition to the positive electrode active material.

  Examples of the conductive material include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. One of these may be used, or two or more may be used in combination. The content of the conductive material is preferably 0.1 to 30% by mass, more preferably 0.1 to 20% by mass, and particularly preferably 0.1 to 10% by mass with respect to the total mass of the positive electrode active material layer.

  As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, polyvinyl acetate, polymethacrylate, polyacrylate, polyacrylonitrile, polyvinyl alcohol, or a mixture of two or more thereof may be used. The binder may be used in combination with a thickener such as carboxymethyl cellulose (CMC) or polyethylene oxide (PEO). One of these may be used, or two or more may be used in combination. The content of the binder is preferably 0.1 to 30% by mass, more preferably 0.1 to 20% by mass, and particularly preferably 0.1 to 10% by mass with respect to the total mass of the positive electrode active material layer.

[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 surface of 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 in which lithium is previously occluded, silicon, 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.

[Nonaqueous 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.

  Examples of 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, acetic acid Examples thereof include carboxylic acid esters such as methyl, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone.

  Examples of 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 non-aqueous solvent preferably contains a halogen-substituted product obtained by substituting hydrogen of the above various solvents with a halogen atom such as fluorine. In particular, a fluorinated cyclic carbonate and a fluorinated chain carbonate are preferable, and it is more preferable to use a mixture of both. Thereby, a good protective film is formed not only in the negative electrode but also in the positive electrode, and the cycle characteristics are improved. As preferable examples of the fluorinated cyclic carbonate, 4-fluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4,4-difluoroethylene carbonate, 4,4,5-trifluoroethylene carbonate, 4,4,5 , 5-tetrafluoroethylene carbonate and the like. Preferable examples of the fluorinated chain ester include ethyl 2,2,2-trifluoroacetate, methyl 3,3,3-trifluoropropionate, methyl pentafluoropropionate and the like.

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 F2 p} + 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 ], LiPO ₂ F _{2 and the} like. One type of these lithium salts may be used, or two or more types may be used in combination.

[Separator]
For the separator, for example, 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.

  Hereinafter, the present disclosure will be further described by way of examples. However, the present disclosure is not limited to the following examples.

<Example 1>
[Preparation of positive electrode active material (high Ni content lithium transition metal oxide)]
A nickel cobalt aluminum composite hydroxide represented by the composition formula of Ni _0.88 Co _0.09 Al _0.03 (OH) ₂ was obtained by coprecipitation and then heat-treated at 500 ° C. to prepare a composite oxide. . Next, LiOH, the composite oxide, and H ₃ BO ₃ are mixed in such an amount that the total amount of Li, transition metals (Ni, Co, and Al) and the molar ratio of B are 1.04: 1: 0.01. did. Thereafter, the mixture was baked in an oxygen stream at 710 ° C. for 3 hours, and then impurities were removed by washing with water. The composition of the resulting high Ni-containing lithium transition metal oxide was measured using an ICP emission spectroscopic analyzer (manufactured by Thermo Fisher Scientific, trade name “iCAP6300”). As a result, it was confirmed that it was a high Ni-containing lithium transition metal oxide represented by the composition formula Li _0.95 Ni _0.88 Co _0.09 Al _0.03 B _0.001 O ₂ . This was used as the positive electrode active material of Example 1.

[Production of positive electrode]
91 parts by mass of the positive electrode active material, 7 parts by mass of acetylene black as a conductive material, and 2 parts by mass of polyvinylidene fluoride as a binder were mixed. The mixture was kneaded using a kneader (TK Hibismix, manufactured by Primics Co., Ltd.) to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to an aluminum foil having a thickness of 15 μm, and the coating film was dried to form a positive electrode active material layer on the aluminum foil. This was used as a positive electrode.

[Preparation of non-aqueous electrolyte]
Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 3: 3: 4. A nonaqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) in the mixed solvent so as to have a concentration of 1.2 mol / liter.

[Production of test cell]
The positive electrode and a negative electrode made of lithium metal foil were laminated so as to face each other with a separator interposed therebetween, and wound to prepare a wound electrode body. Next, the spirally wound electrode body and the nonaqueous electrolyte were inserted into an aluminum exterior body to produce a nonaqueous electrolyte secondary battery (test cell A1).

<Example 2>
In the preparation of the positive electrode active material, LiOH, the above composite oxide, and H ₃ BO ₃ were combined with a total amount of Li, transition metals (Ni, Co, and Al) and a molar ratio of B of 1.1: 1: 0.03. The high Ni-containing lithium transition represented by the composition formula Li _0.96 Ni _0.88 Co _0.09 Al _0.03 B _0.003 O ₂ in the same manner as in Example 1 except that the amount of A metal oxide was obtained. Using this as the positive electrode active material of Example 2, a nonaqueous electrolyte secondary battery (test cell A2) was produced in the same manner as in Example 1.

<Example 3>
In the preparation of the positive electrode active material, LiOH, the above composite oxide, and H ₃ BO ₃ were combined with a total amount of Li, transition metals (Ni, Co, and Al) and a molar ratio of B of 1.15: 1: 0.03. The high Ni-containing lithium transition represented by the composition formula Li _0.97 Ni _0.88 Co _0.09 Al _0.03 B _0.003 O ₂ in the same manner as in Example 1 except that the amount is mixed. A metal oxide was obtained. Using this as the positive electrode active material of Example 3, a nonaqueous electrolyte secondary battery (test cell A3) was produced in the same manner as in Example 1.

<Example 4>
In the preparation of the positive electrode active material, LiOH, the above composite oxide, and H ₃ BO ₃ were mixed with a total amount of Li, transition metals (Ni, Co, and Al) and a molar ratio of B of 1.15: 1: 0.05. The high Ni-containing lithium transition represented by the composition formula Li _0.97 Ni _0.88 Co _0.09 Al _0.03 B _0.005 O ₂ in the same manner as in Example 1 except that the amount is mixed. A metal oxide was obtained. Using this as the positive electrode active material of Example 4, a nonaqueous electrolyte secondary battery (test cell A4) was produced in the same manner as in Example 1.

<Comparative Example 1>
In the preparation of the positive electrode active material, H ₃ BO ₃ was not used, and LiOH and the composite oxide were mixed in an amount such that the total amount of Li and transition metals (Ni, Co, and Al) was 1.03: 1. Except for this, a high Ni-containing lithium transition metal oxide represented by the composition formula Li _0.95 Ni _0.88 Co _0.09 Al _0.03 O ₂ was obtained in the same manner as in Example 1. Using this as the positive electrode active material of Comparative Example 1, a nonaqueous electrolyte secondary battery (test cell B1) was produced.

<Comparative example 2>
In the preparation of the positive electrode active material, H ₃ BO ₃ was not used, and LiOH and the composite oxide were mixed in an amount such that the total amount of Li and transition metals (Ni, Co, and Al) was 1.03: 1. The high Ni-containing lithium transition represented by the composition formula Li _0.96 Ni _0.88 Co _0.09 Al _0.03 O ₂ in the same manner as in Example 1 except that firing was performed at 760 ° C. in an oxygen stream. A metal oxide was obtained. Using this as the positive electrode active material of Comparative Example 2, a nonaqueous electrolyte secondary battery (test cell B2) was produced.

<Comparative Example 3>
In the preparation of the positive electrode active material, LiOH, the above composite oxide, and H ₃ BO ₃ were combined with a total amount of Li, transition metals (Ni, Co, and Al) and a molar ratio of B of 1.1: 1: 0.03. The composition formula Li _0.97 Ni _0.88 Co _0.09 Al _0.03 B _0.003 O ₂ was obtained in the same manner as in Example 1 except that the mixture was calcined at 760 ° C. in an oxygen stream. The high Ni content lithium transition metal oxide represented by these was obtained. Using this as the positive electrode active material of Comparative Example 3, a nonaqueous electrolyte secondary battery (test cell B3) was produced.

<Comparative example 4>
In the preparation of the positive electrode active material, LiOH, the composite oxide, and H ₃ BO ₃ are mixed with a total amount of Li, transition metals (Ni, Co, and Al) and a molar ratio of B of 1.2: 1: 0.03. The high Ni-containing lithium transition represented by the composition formula Li _0.99 Ni _0.88 Co _0.09 Al _0.03 B _0.003 O ₂ in the same manner as in Example 1 except that the amount of A metal oxide was obtained. Using this as the positive electrode active material of Comparative Example 4, a nonaqueous electrolyte secondary battery (test cell B4) was produced.

<Comparative Example 5>
In the preparation of the positive electrode active material, LiOH, the composite oxide, and H ₃ BO ₃ were mixed with a total amount of Li, transition metals (Ni, Co, and Al) and a molar ratio of B of 1.03: 1: 0.05. In the same manner as in Example 1, except that the composition formula Li _0.84 Ni _0.88 Co _0.09 Al _0.03 B _0.005 O ₂ The high Ni content lithium transition metal oxide represented was obtained. Using this as the positive electrode active material of Comparative Example 5, a nonaqueous electrolyte secondary battery (test cell B5) was produced.

  As a result of analyzing the X-ray diffraction patterns of the examples and comparative examples, diffraction lines indicating a layered structure were confirmed.

[Charge / discharge test]
Using the test cells A1 to A4 (Examples 1 to 4) and the test cells B1 to B5 (Comparative Examples 1 to 5) each having a battery capacity of about 30 mAh, the voltage is 4. Constant current charging was performed at a current value of 6 mA until the voltage reached 3 V, and then constant voltage charging was performed at 4.3 V until the current value reached 1.5 mA. Thereafter, constant current discharge was performed at 6 mA until the voltage reached 2.5V. The capacity obtained from the test cell by this charge / discharge measurement was defined as the initial charge capacity and initial discharge capacity (mAh / g) of each test cell, and the percentage of the initial discharge capacity in the initial charge capacity was defined as the initial charge / discharge efficiency. . Moreover, the percentage of the discharge capacity of 2.5 to 3.5 V in the initial discharge capacity of 2.5 V to 4.3 V was calculated as the ratio d of the discharge capacity of 3.5 V or less.

FIG. 1 shows a curve (d ² Q / dV ² curve) obtained by subjecting the discharge capacities of Examples 1 to 4 and Comparative Example 1 to second-order differential processing using potential. The value of the second derivative process (d ² Q / dV ² value) has a peak at a potential at which a potential flat portion starts to appear in the charge / discharge curve (a potential at which the change rate of the slope of the charge / discharge curve increases). The appearance of this peak indicates that a potential flat portion is generated in the charge / discharge curve. The appearance of a peak here means that there is a minimum value (maximum value) in a predetermined potential range.

In Examples 1 to 4, a peak appears in the d ² Q / dV ² value near 3.4 to 3.5 V (shaded portion), whereas in Comparative Example 1, no peak is confirmed. In Examples 1 to 4, during discharge, the valence change of Ni in the high Ni-containing lithium transition metal oxide occurs in the vicinity of 3.4 to 3.5 V, and a potential flat portion that does not appear in Comparative Example 1 occurs. As a result, it can be said that the capacity increased as compared with Comparative Example 1.

Table 1 shows the B content (molar ratio), initial charge and discharge efficiency, capacity ratio of 3.5 V or less, d ² Q / dV ² curve around 3.4 to 3.5 V in each Example and Comparative Example. The presence or absence of peaks was summarized.

  In Examples 1 to 4, the initial charge and discharge efficiency was higher than that in Comparative Examples 1 to 5. From this result, boron is present in the particles of the high Ni-containing lithium transition metal oxide, and the ratio b of boron to the total molar amount of the metal elements excluding lithium in the high Ni-containing lithium transition metal oxide is 0 mol% <b. It can be said that the initial charge and discharge efficiency can be improved by setting ≦ 0.5 mol% and setting the ratio d of the discharge capacity of 3.5 V or less to 6% <d. In Comparative Examples 3 to 5, since the firing temperature at the time of synthesis of the positive electrode active material is higher than that of the example or the molar ratio of the Li raw material is 1.03 or less or 1.18 or more, the discharge capacity of 3.5 V or less A high Ni-containing lithium transition metal oxide having a ratio d of 6% or less was synthesized, and the initial charge / discharge efficiency was lower than in the examples.

  Table 2 summarizes the measured values of the lattice constant, 3a-site transition metal amount, crystallite size, and compressive fracture strength of each Example and Comparative Example. The measurement method for each measurement value is as described above.

  The discharge capacity of test cell B5 (Comparative Example 5) was greatly reduced as compared to other test cells. This is because the amount m of the transition metal existing at the 3a site (lithium site) in the crystal structure is 2 mol% or more with respect to the total molar amount of the transition metal, resulting in distortion of the crystal structure and diffusion of lithium ions. It is considered that the discharge capacity has decreased due to the decrease in performance.

Claims (6)

Ni-containing lithium transition metal oxide (Ni ≧ 80 mol%) having a layered structure,
Boron is present in the particles of the Ni-containing lithium transition metal oxide,
The ratio b of the boron to the total molar amount of the metal elements excluding lithium in the Ni-containing lithium transition metal oxide is in the range of 0 mol% <b ≦ 0.5 mol%,
The ratio d of the discharge capacity of 3.5 V or less to the discharge capacity of 2.5 V to 4.3 V (vs. Li / Li ⁺ ) is in the range of 6% <d. The positive electrode active for a non-aqueous electrolyte secondary battery material.   The Ni-containing lithium transition metal oxide has a lattice constant a indicating the a-axis length and a lattice constant c indicating the c-axis length of the crystal structure obtained from the analysis result of the X-ray diffraction pattern obtained by X-ray diffraction. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the ranges are 2.867 Å <a <2.873 Å, 14.17 ＜< c <14.19 範 囲.   In the Ni-containing lithium transition metal oxide, the transition metal amount m present at the 3a site in the crystal structure obtained from the Rietveld analysis result of the X-ray diffraction pattern obtained by X-ray diffraction is such that the total moles of transition metals are The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the range is 0 mol% ≤ m <2 mol% with respect to the amount.   The Ni-containing lithium transition metal oxide has a crystallite size s calculated from Scherrer's equation from the half-value width of the diffraction peak of the (104) plane of the X-ray diffraction pattern obtained by X-ray diffraction, and 350Å ≦ s ≦ The positive electrode active material for nonaqueous electrolyte secondary batteries according to any one of claims 1 to 3, wherein the positive electrode active material is in a range of 550mm.   5. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the compressive fracture strength K of the secondary particles of the Ni-containing lithium transition metal oxide is in the range of 50 MPa <K <100 MPa. Active material.   It is a nonaqueous electrolyte secondary battery provided with a positive electrode and a negative electrode, Comprising: The said positive electrode is a nonaqueous electrolyte secondary containing the positive electrode active material for nonaqueous electrolyte secondary batteries of any one of Claims 1-5. battery.