JP2018073481A - Positive electrode active material for nonaqueous electrolyte secondary battery, method for manufacturing the same, positive electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a positive electrode active material for a nonaqueous electrolyte secondary battery, which has both of a high discharge capacity and a high charge/discharge cycle performance; and a method of manufacturing the positive electrode active material for a nonaqueous electrolyte secondary battery.SOLUTION: A positive electrode active material for a nonaqueous electrolyte secondary battery comprises a lithium transition metal composite oxide. The lithium transition metal composite oxide has an α-NaFeOstructure, of which the transition metal element (Me) includes Mn and at least one of Co and Ni; the mole ratio (Mn/Me) of Mn to Me is given by 0.5<Mn/Me, and the mole ratio (Li/Me) of Li to Me is given by 1<Li/Me<1.5. The positive electrode active material further comprises boron. The surface of each particle of the lithium transition metal composite oxide is coated with the born; and the boron is included at 0.5 to 5 mol% to a quantity of Me.SELECTED DRAWING: Figure 1

Description

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

Non-aqueous electrolyte secondary batteries represented by lithium secondary batteries have been increasingly used in recent years, and development of higher-capacity cathode materials has been demanded.
Conventionally, lithium transition metal composite oxides having an α-NaFeO ₂ type crystal structure have been studied as positive electrode active materials for non-aqueous electrolyte secondary batteries, and non-aqueous electrolyte secondary batteries using LiCoO ₂ have been widely put into practical use. It was. However, the discharge capacity of LiCoO ₂ was about 120 to 130 mAh / g.

As the transition metal (Me) constituting the lithium transition metal composite oxide, abundant Mn is used as an earth resource, and the molar ratio Li / Me of Li to the transition metal constituting the lithium transition metal composite oxide is approximately 1. In addition, a so-called “LiMeO ₂ type” active material having a Mn molar ratio Mn / Me in the transition metal of 0.5 or less has been put into practical use. For example, a positive electrode active material containing LiNi _1/2 Mn _1/2 O ₂ or LiNi _1/3 Co _1/3 Mn _1/3 O ₂ has a discharge capacity of 150 to 180 mAh / g.
The molar ratio of Mn in the transition metal (Me) to the transition metal constituting the lithium transition metal composite oxide exceeds 0.5, and the molar ratio of Li to the transition metal (Me) Li Since the so-called “lithium-excess type” active material with / Me exceeding 1 has a higher discharge capacity than the “LiMeO ₂ type” active material, studies have been conducted for its practical use.

Patent Document 1 states that “general formula Li _{1 + x} Mn _{1- xy My} O ₂ (where x and y are in the range of 0 <x <0.33, 0 <y <0.66, M represents at least one transition metal other than Mn.) And is a lithium-containing transition metal oxide having a layered structure, and a boron oxide layer is formed on the surface thereof. "Positive electrode active material for lithium secondary battery" (Claim 1).
For the example, “<Experiment 1> [Preparation of lithium-containing transition metal oxide] Lithium hydroxide (LiOH) and a coprecipitated hydroxide of Mn, Co and Ni were used as starting materials. The mixed powder was formed into pellets, and the pellets were baked at 900 ° C. for 24 hours to obtain Li _1.2 Mn _0.54 Co _0.13 Ni _0.13. A lithium-containing transition metal oxide having a composition of O ₂ was obtained. The average particle diameter of the obtained lithium-containing transition metal oxide was 11 μm ”(paragraph [0048]),“ [positive electrode active material Preparation] On the surface of the obtained lithium-containing transition metal oxide, a layer of boron oxide was formed as described in the following examples to obtain a positive electrode active material ”(paragraph [0049]),“ (implementation). Examples 1 to 5) A layer of boron oxide was formed on the surface of the lithium-containing transition metal oxide obtained as described above as follows. "(Paragraph [0056])," lithium-containing transition metal oxide 2 parts by weight of H ₃ BO ₃ and 50 parts by weight of water were prepared for 100 parts by weight of the product, and this aqueous solution was mixed with a lithium-containing transition metal oxide. Next, the dried powder was heat-treated in air at a predetermined temperature for 5 hours at 200 ° C. (Example 1), 300 ° C. (Example 2), 400 ° C. (Example 3). , 500 ° C. (Example 4), and 600 ° C. (Example 5) ”(paragraph [0057]).
Moreover, as an evaluation result of the test cells using the positive electrode active materials of Examples and Comparative Examples, “As shown in Table 1, in Examples 2 to 4 in which a boron oxide layer was formed on the surface according to the present invention, The discharge capacity at the first cycle is higher than those of Comparative Examples 1 to 3 having the same heat treatment temperature where no boron oxide layer is formed on the surface ”(paragraph [0062]).

Patent Document 2 states that “formula Li _{1 + x} M _1−x O _2−z F _z (M is a non-lithium metal element or a combination thereof, 0.01 ≦ x ≦ 0.3, 0 ≦ z Lithium ion battery cathode material comprising a lithium metal oxide approximately represented by ≦ 0.2 and coated with about 0.1 to about 0.75 weight percent metal / metalloid oxide. (Claim 1), “The metal / metalloid oxide is aluminum oxide (Al ₂ O ₃ ), bismuth oxide (Bi ₂ O ₃ ), boron oxide (B ₂ O ₃ ), zirconium oxide (ZrO ₂ ). , Magnesium oxide (MgO), chromium oxide (Cr ₂ O ₃ ), magnesium aluminate (MgAl ₂ O ₄ ), gallium oxide (Ga ₂ O ₃ ), silicon oxide (SiO ₂ ), tin oxide (SnO ₂ ), oxidation Mosquito Siumu _(CaO 2), strontium oxide (SrO), barium oxide (BaO), titanium oxide _(TiO 2), iron oxide _{(Fe 2 O} 3), molybdenum oxide (MoO ₃ and _MoO 2), cerium oxide _(CeO 2) , Lanthanum oxide (La ₂ O ₃ ), zinc oxide (ZnO), lithium aluminum oxide (LiAlO ₂ ), or a combination thereof. Yes.
Then, for the examples, the paragraphs [0068] - [0103], according to Table 3 - _{"xLi 2 MnO 3 (1-x} ) x in LiMO _2" 0.5,0.3,0 a composition that is 1 to 8.1, aluminum oxide _{(Al 2 O} 3), is coin battery using bismuth oxide _(Bi 2 _{O 3)} or the positive electrode material coated with magnesium oxide (MgO) is described, Al ₂ In the case of the O ₃ coating, “cells formed from samples with a 0.2 weight percent coating showed surprisingly good performance up to 55 cycles at C / 3 discharge rate” (paragraph [0086]). “Samples with a 0.2 weight percent coating showed higher battery capacity than other coated samples at all rates.” (Paragraph [008 7]) and in the case of Bi ₂ O ₃ coating, “battery made from lithium metal oxide powder with 0.1 weight percent Bi ₂ O ₃ coating and 0.5 weight percent Bi ₂ O ₃ coating Had a higher specific discharge capacity at any rate than batteries made from uncoated samples ”(paragraph [0096]), and in the case of MgO coating,“ the coated sample is charged and discharged. It showed an increase in specific capacity and a corresponding comparable or increased first cycle irreversible capacity loss (IRCL) ”(paragraph“ 0101 ”).

Patent Document 3 states that “a positive electrode active material particle powder made of a compound having at least a crystal system belonging to space group R-3m and a crystal system belonging to space group C2 / m, wherein the compound includes at least Li and Mn. Intensity of maximum diffraction peak at 2θ = 20.8 ± 1 ° in a powder X-ray diffraction diagram using a Cu—Kα ray of a positive electrode active material particle powder, which is a composite oxide containing boron and Co and / or Ni. The positive electrode active material particle powder in which the relative intensity ratio (a) / (b) between (a) and the intensity (b) of the maximum diffraction peak at 2θ = 18.6 ± 1 ° is 0.02 to 0.5. The positive electrode active material particle powder is characterized in that the positive electrode active material particle powder has a molar ratio of Mn / (Ni + Co + Mn) of 0.55 or more and contains boron in an amount of 0.001 to 3 wt%. (Claim 1) is described.
And about the Example, "Example 1 14L of water was put into the sealed reaction tank, and it maintained at 50 degreeC, distribute | circulating nitrogen gas. Furthermore, it stirred so that it might become pH = 8.2 (± 0.2). While adding 1.5M Ni, Co, Mn mixed sulfate aqueous solution, 0.8M sodium carbonate aqueous solution and 2M ammonia aqueous solution continuously, only the filtrate was discharged out of the system by the concentrator during the reaction. After the reaction for 20 hours, the slurry of the coprecipitation product was collected, and the collected slurry was filtered, washed with water, and dried at 105 ° C. overnight to obtain a coprecipitation precursor powder. The obtained coprecipitation precursor, lithium carbonate powder and boric acid were weighed and mixed well, and this was fired at 800 ° C. for 5 hours under an air flow using an electric furnace to obtain a positive electrode active material particle powder. As a result of X-ray diffraction measurement The positive electrode active material particle powder includes a crystal system belonging to the space group R-3m and a crystal system belonging to the space group C2 / m, and the peak intensity ratio (a) / (b) was 0.11. As a result of ICP composition analysis, the molar ratios are Li / (Ni + Co + Mn) = 1.33, Ni: Co: Mn = 21.6: 12.4: 66 (Mn / (Ni + Co + Mn) = 0.66), B = 0.05 wt% BET specific surface area by nitrogen adsorption method was 3.5 m ² / g, and the result of observation of particles of the positive electrode active material particles by a scanning electron microscope (SEM) It was observed that primary particles having an average primary particle diameter of 0.07 μm aggregated to form secondary particles having an average secondary particle diameter of 12.1 μm ”(paragraph [0063]). ing. Examples 2 to 13 also describe a positive electrode active material containing 0.003 to 1.600 wt% of B.

Patent Document 4 states that “a positive electrode active material that expresses a potential of 4.5 V or more on the basis of metallic lithium, a conductive agent, a positive electrode having a positive electrode mixture having a binder, a negative electrode, and a lithium salt. A lithium ion secondary battery having a non-aqueous electrolyte dissolved in a non-aqueous solvent, having a positive electrode coating layer containing boron on at least a part of the surface of the positive electrode mixture, and boron in the positive electrode coating layer The amount of the lithium ion secondary battery is 0.0001 wt% or more and 0.005 wt% or less with respect to the weight of the positive electrode mixture. ”(Claim 1) is described.
And about the Example, "The battery of this embodiment, the battery A, the battery B, and the battery C were produced as follows. As a positive electrode active material which expresses the electric potential of 4.5V or more on a metal lithium basis, LiMn _1.52 Ni _0.48 O ₄ was prepared as a raw material, manganese dioxide (MnO ₂ ) and nickel oxide (NiO) were weighed to a predetermined composition ratio and wet-mixed with pure water. The calcined product was calcined in an air atmosphere at a temperature increase of 3 ° C./min and a temperature decrease of 2 ° C./min for 12 hours at 1000 ° C. Lithium carbonate (Li ₂ CO ₃ ) was pulverized and weighed so as to have a predetermined composition ratio with this The mixture was wet mixed in the same manner, and dried, and then fired in an air atmosphere at 800 ° C. for 20 hours at a temperature increase of 3 ° C./min and a temperature decrease of 2 ° C./min. Paragraphs [0065]-[0067]), “ Aqueous electrolyte was prepared as follows ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate at a volume ratio 2:. 4: in a non-aqueous solvent mixture of 4, lithium 1 mol / dm ³ dissolved phosphate hexafluoride as lithium salt To this, boron ethoxide (B (OC ₂ H ₅ ) ₃ ) was added in an amount of 0.2% by weight (Battery A), 1% by weight (Battery B), and 4% by weight (Battery C), respectively. Used "(paragraphs [0071], [0072]).

JP 2011-171113 A Special table 2013-503449 gazette JP 2011-96650 A JP 2012-94459 A

A so-called “lithium-excess” active material having a Mn / Me ratio of more than 0.5 and a Li molar ratio of Li / Me of more than 1 has a more noble charge upper limit potential than a so-called “LiMeO ₂ type” active material. Since the potential can be set, a high discharge capacity can be taken out. However, there is a problem that the charge / discharge cycle performance is not sufficient. As a method for producing a “lithium-rich” active material, a method using a hydroxide precursor and a method using a carbonate precursor are known. By adopting a method using a carbonate precursor, an active material having a large specific surface area can be obtained, so that the diffusion distance from the interface between the active material particles and the nonaqueous electrolyte to the inside of the particles can be shortened. Therefore, a nonaqueous electrolyte battery excellent in terms of initial efficiency and discharge capacity can be obtained. However, an active material having a large specific surface area has a problem in charge / discharge cycle performance because there is a tendency for side reactions and elution of transition metals to be promoted with a large contact area with the electrolyte.
In Patent Document 1, when an active material provided with a boron oxide surface coating on a lithium-excess type active material prepared using a hydroxide precursor is used, the initial discharge capacity is improved and the irreversible capacity is reduced. Have been described. However, since the active material derived from the hydroxide precursor usually has a specific surface area as small as 3 m ² / g or less, the above-mentioned problem during the charge / discharge cycle cannot be obtained from the description in Patent Document 1.
Patent Document 2 describes that a lithium / rich active material is coated with a metal / metalloid oxide such as aluminum oxide, thereby improving charge / discharge cycle performance and rate characteristics of a battery using the same. Yes. However, no specific example of coating the positive electrode active material with boron oxide is described.
Patent Document 3 discloses a non-aqueous electrolyte secondary battery using a lithium-excess active material prepared by mixing and baking a transition metal carbonate precursor, a lithium compound, and boric acid, and has a high capacity and cycle characteristics. Is described as improving. And the BET specific surface area of the active material which concerns on Example 1, ² , 8-13 containing B 0.050 wt% or more and baking at 800-950 degreeC in air is as small as 3.5 m < ² > / g or less. This is presumably because boric acid acts as a sintering aid that promotes sintering by firing in air. Moreover, in the manufacturing method described in the Example of patent document 3, the boron is taken in in the particle | grains rather than coat | covering the active material particle surface.
Patent Document 4 describes a nonaqueous electrolyte secondary battery including a positive electrode having a coating layer containing boron on the surface of a positive electrode mixture, which has a high discharge capacity and coulomb efficiency, and an excellent cycle life. ing. However, what is specifically shown as the active material used for the positive electrode is a spinel type, and no lithium-excess type active material is shown.

  The present invention relates to a positive electrode active material for a nonaqueous electrolyte secondary battery having high discharge capacity and high charge / discharge cycle performance, a method for producing the positive electrode active material, and a positive electrode for a nonaqueous electrolyte secondary battery containing the positive electrode active material. And a non-aqueous electrolyte secondary battery including the positive electrode.

One aspect of the present invention is a positive electrode active material for a non-aqueous electrolyte secondary battery including a lithium transition metal composite oxide, wherein the lithium transition metal composite oxide has an α-NaFeO ₂ structure, and a transition metal ( Me) contains Mn and contains at least one of Co or Ni, the molar ratio of Mn to Me is Mn / Me 0.5 <Mn / Me, and the molar ratio of Li to Me is Li / Me. 1 <Li / Me <1.5, a coating containing boron is present on the surface of the lithium transition metal composite oxide particles, and the boron is contained in an amount of 0.5 mol% to 5 mol% with respect to Me. It is an active material for water electrolyte secondary batteries.

  Another aspect of the present invention is a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising Mn and at least one of Co or Ni, wherein Mn is a transition metal (Mn). A transition metal carbonate precursor having a molar ratio Mn / Me of 0.5 <Mn / Me is prepared, the transition metal carbonate precursor and the lithium compound are mixed and fired, and a Li molar ratio Li / Me to Me Producing lithium transition metal composite oxide particles having a ratio of 1 <Li / Me <1.5, mixing the particles with a boron compound, and heat-treating to form a coating containing boron on the surface of the particles It is a manufacturing method of the active material for nonaqueous electrolyte secondary batteries provided with this.

  Still another aspect of the present invention is a nonaqueous electrolyte secondary battery positive electrode containing the active material and a nonaqueous electrolyte secondary battery including the positive electrode.

  According to the present invention, a positive electrode active material for a nonaqueous electrolyte secondary battery having high charge / discharge capacity and high charge / discharge cycle performance, a method for producing the positive electrode active material, and a nonaqueous electrolyte secondary battery provided with the positive electrode active material And a non-aqueous electrolyte secondary battery having the positive electrode.

Electron micrograph of active material particles according to examples of the present invention Electron micrograph of active material particles according to a comparative example of the present invention The perspective view which shows one Embodiment of the nonaqueous electrolyte secondary battery which concerns on 1 side of this invention Schematic showing a power storage device comprising a plurality of non-aqueous electrolyte secondary batteries according to one aspect of the present invention

  The configuration and operational effects of the present invention will be described with the technical idea. However, the action mechanism includes estimation, and the correctness does not limit the present invention. It should be noted that the present invention can be implemented in various other forms without departing from the spirit or main features thereof. For this reason, the following embodiments or examples are merely examples in all respects, and should not be interpreted in a limited manner. Further, all modifications and changes belonging to the equivalent scope of the claims are within the scope of the present invention.

<Positive electrode active material (lithium transition metal composite oxide)>
The positive electrode active material for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention (hereinafter referred to as “this embodiment”) is a positive electrode active material containing a lithium transition metal composite oxide.
In the lithium transition metal composite oxide, since a high discharge capacity can be obtained, the transition metal (Me) contains Mn and contains at least one of Co or Ni, and Li _{1 + z} MeO _{2 + z} (z > 0), which is a so-called “lithium-excess type”, and the lithium transition metal composite oxide has a coating containing boron on the particle surface.
This positive electrode active material is typically represented by a composition formula Li _{1 + z} MeB _y O _{2 + z} (Me: transition metal including Mn and Ni or Co). In order to obtain a non-aqueous electrolyte secondary battery having a high discharge capacity, the molar ratio of Mn to transition metal element Me, Mn / Me is greater than 0.5, and the molar ratio of Li to transition metal element Me, Li / Me (the above composition formula (1 + z)) is a lithium-excess type active material of 1 <Li / Me <1.5.

The molar ratio Li / Me of Li to the transition metal element Me is preferably 1.15 to 1.45, more preferably 1.2 to 1.45. Within this range, the discharge capacity is particularly improved.
The molar ratio Mn / Me of the Mn with respect to the transition metal element Me is preferably 0.60 to 0.75, more preferably 0.60 to 0.70, and particularly preferably 0.65 to 0.70. Within this range, the energy density is improved.
Co contained in the lithium transition metal composite oxide has an effect of improving the initial efficiency, but is expensive because it is a scarce resource. Therefore, the molar ratio Co / Me of Co to the transition metal element Me is preferably 0.20 or less, and may be 0.
The molar ratio Ni / Me of Ni to the transition metal element Me is preferably 0.15 to 0.35. Within this range, the energy density is improved.

The lithium transition metal composite oxide according to this embodiment has an α-NaFeO ₂ structure. The lithium transition metal composite oxide after synthesis (before charging and discharging) is attributed to the space group P3 ₁ 12 or R3-m. Among these, those belonging to the space group P3 ₁ 12 are superlattice peaks (Li [Li _1/3 Mn _2/3 ] O ₂ near 2θ = 21 ° on the X-ray diffraction diagram using the CuKα tube. The peak observed in the monoclinic type) is confirmed. However, when charging is performed once and Li in the crystal is desorbed, the symmetry of the crystal changes, whereby the superlattice peak disappears and the lithium transition metal composite oxide belongs to the space group R3-m. Will come to be. Here, P3 ₁ 12 is a crystal structure model in which the atomic positions of the 3a, 3b, and 6c sites in R3-m are subdivided, and when ordering is recognized in the atomic arrangement in R3-m, the P3 ₁ 12 model Is adopted. Note that “R3-m” is originally written by adding a bar “-” on “3” of “R3m”.

  Note that the lithium transition metal composite oxide according to the present embodiment is an alkali metal such as Na and K, an alkaline earth metal such as Mg and Ca, and a 3d transition such as Fe and Zn within a range not impairing the effects of the present invention. It does not exclude inclusion of a small amount of other metals such as transition metals typified by metals.

In the lithium transition metal composite oxide according to the present embodiment, a positive electrode active material excellent in charge / discharge cycle performance can be obtained by making the particle surface have a coating containing boron. The reason for this is not clear, but the presence of a coating containing boron on the surface of the positive electrode active material particles can reduce the contact area with the nonaqueous electrolyte and suppress side reactions associated with elution of transition metals. It is guessed. Moreover, since the coating containing boron can easily transport Li ions, the charge / discharge cycle performance can be improved while maintaining the discharge capacity.
If boron is present uniformly from the center to the surface of the secondary particles of the lithium transition metal composite oxide, it cannot be a positive electrode active material having a high discharge capacity. The presence of a coating containing boron on the particle surface of the lithium transition metal composite oxide means that the distribution of boron is observed on the cross section of the particle of the lithium transition metal composite oxide using EPMA (electron beam microanalyzer). It can be confirmed with. The positive electrode active material according to the present invention requires that the boron abundance ratio be higher on the surface than the center of the secondary particles of the lithium transition metal composite oxide, and the concentration gradient from the center of the particles to the surface. It is preferable to be present together, more preferably to be localized near the surface of the particle, and most preferably to be present only on the surface of the particle. In addition, when a plurality of secondary particles are associated, the particles refer to individual secondary particles.

The boron coating amount is such that the molar ratio B / Me of transition metal element (Me) B / Me (y in the above composition formula) is 0.005 ≦ B / Me ≦ 0.05, ie, B is 0 with respect to Me. It is preferable that it is 5-5 mol%.
If Li / Me and Mn / Me are lithium transition metal composite oxides satisfying the above composition range, the molar ratio B / Me is generally about 0.05 to 0.48 mass of boron in the positive electrode active material. % Can be converted.
By containing 0.5 mol% or more of boron with respect to Me, charge / discharge cycle performance can be improved, and by setting it to 5 mol% or less, a high discharge capacity can be maintained.

The positive electrode active material particles obtained according to the present embodiment preferably have a BET specific surface area of 4.0 m ² / g or more, more preferably 5.0 m ² / g or more, and 5.3 m ² / g or more. More preferably. It is preferable that the BET specific surface area is less than 7.0 m ² / g, and more preferably 6.8 m ² / g or less.
In this specification, the specific surface area of the positive electrode active material is measured under the following conditions. Using the positive electrode active material particles as a measurement sample, a specific surface area measurement device (trade name: MONOSORB) manufactured by Yuasa Ionics Co., Ltd. is used to determine the nitrogen adsorption amount [m ² ] on the measurement sample by a one-point method. The input amount of the measurement sample is 0.5 g ± 0.01 g. Preheating is performed at 120 ° C. for 15 minutes. Cooling is performed using liquid nitrogen, and the amount of nitrogen gas adsorbed during the cooling process is measured. A value obtained by dividing the measured adsorption amount (m ² ) by the active material mass (g) is defined as a BET specific surface area.
FIG. 1 is an electron micrograph of positive electrode active material particles in which a coating with boron according to the present embodiment (Example 4 described later) is present, and FIG. 2 illustrates a conventional coating with boron (Comparative Example 1 described later). It is an electron micrograph of the positive electrode active material particle which does not have. Since both are derived from a carbonate precursor, they are spherical particles.
The positive electrode active material particles having no boron coating of Comparative Example 1 shown in FIG. 2 are true spheres having fine irregularities on the particle surface, have a large BET specific surface area, and exceed 7.0 m ² / g.
On the other hand, in the positive electrode active material particles according to this embodiment in which the coating with boron (B is 5 mol% with respect to Me) shown in FIG. 1 is generated, the generation of particles in which a plurality of secondary particles are associated is observed. The irregularities on the particle surface are larger than those in Comparative Example 1, and the BET specific surface area is reduced to about 5.0 m ² / g. In a comparative example to be described later, when the B content exceeds 5 mol% with respect to Me, the BET specific surface area is further reduced to less than 5.0 m ² / g. However, the BET specific surface area depends on other production conditions. When the boron coating amount is 5 mol% or less with respect to Me, the BET specific surface area should be 4.0 m ² / g or more. If it is, the effect of this invention is show | played.

<Method for producing positive electrode active material>
The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present embodiment includes producing a transition metal carbonate precursor, mixing and calcining the precursor and a lithium compound, and lithium-excess lithium transition metal Forming oxide particles, mixing a boron compound with the particles, and heat-treating to form a coating containing boron on the particle surfaces;
The lithium transition metal composite oxide used for the positive electrode active material according to the present embodiment is basically composed of an active material (oxide) for the purpose of metal elements (Li, Ni, Co, Mn) constituting the active material. It can be obtained by preparing a raw material containing the same and firing it. However, with respect to the amount of the Li raw material, it is preferable to add an excess of about 1 to 5% in view of the disappearance of a part of the Li raw material during firing.

  In producing a composite oxide having a desired composition, a so-called “solid phase method” in which salts of Li, Ni, Co, and Mn are mixed and fired, or a raw material aqueous solution containing Ni, Co, and Mn is dropped. Then, a compound containing Ni, Co and Mn is coprecipitated in a solution to prepare a coprecipitation precursor in which Ni, Co and Mn are present in one particle in advance, and a Li salt is mixed with this. A “coprecipitation method” of firing is known. In the synthesis process by the “solid phase method”, since Mn is difficult to be uniformly dissolved in Ni and Co, it is difficult to obtain a sample in which each element is uniformly distributed in one particle. The “method” is easier to obtain a homogeneous phase at the atomic level. Therefore, in the method for producing a lithium transition metal composite oxide according to the present embodiment, a transition metal coprecipitation precursor is prepared.

  Carbonate precursors and hydroxide precursors are known as transition metal coprecipitation precursors. When the carbonate precursor is used, spherical positive electrode active material particles having a specific surface area larger than that of using the hydroxide precursor can be obtained. Since the positive electrode active material having a large specific surface area has a short diffusion distance from the active material / electrolyte interface to the inside of the particle, a positive electrode active material having a high discharge capacity and high initial efficiency can be obtained. Therefore, in this embodiment, the manufacturing method using a carbonate precursor is selected.

In preparing a transition metal coprecipitation precursor, Mn is easily oxidized among Ni, Co, and Mn, and it is easy to prepare a coprecipitation precursor in which Ni, Co, and Mn are uniformly distributed in a divalent state. Therefore, uniform mixing at the atomic level of Ni, Co, and Mn tends to be insufficient. Therefore, it is preferable to remove dissolved oxygen in order to suppress oxidation of Mn present in the coprecipitation precursor. Examples of the method for removing dissolved oxygen include a method of bubbling a gas not containing oxygen. The gas not containing oxygen is not limited, but nitrogen gas, argon gas, carbon dioxide (CO ₂ ), or the like can be used.

  The pH in the step of preparing a precursor by co-precipitation of a compound containing Ni, Co and Mn in a solution is not limited. However, when preparing a carbonate precursor, 7.5 to 11 is used. It can be. By setting the pH to 9.4 or less, the tap density can be set to 1.25 g / cc or more, and the high rate discharge performance can be improved. Furthermore, since the particle growth rate can be accelerated by setting the pH to 8.0 or less, the stirring continuation time after completion of dropping of the raw material aqueous solution can be shortened.

  The raw material of the coprecipitation precursor is nickel hydroxide, nickel carbonate, nickel sulfate, nickel nitrate, nickel acetate or the like as the Ni compound, and cobalt sulfate, cobalt nitrate, cobalt acetate, or the like as the Mn compound as the Co compound. Examples thereof include manganese oxide, manganese carbonate, manganese sulfate, manganese nitrate, manganese acetate and the like.

  The dropping speed of the raw material aqueous solution greatly affects the uniformity of element distribution in one particle of the coprecipitation precursor to be generated. The preferred dropping rate is influenced by the reaction vessel size, stirring conditions, pH, reaction temperature, etc., but is preferably 30 ml / min or less. In order to improve the discharge capacity, the dropping rate is more preferably 10 ml / min or less, and most preferably 5 ml / min or less.

Further, when a complexing agent such as NH ₃ is present in the reaction tank and a certain convection condition is applied, by continuing the stirring after the dropwise addition of the raw material aqueous solution, the rotation of the particles and in the stirring tank are performed. Revolution is promoted, and in this process, particles collide with each other, and the particles grow into concentric spheres step by step. That is, the coprecipitation precursor undergoes a reaction in two stages: a metal complex formation reaction when the raw material aqueous solution is dropped into the reaction tank, and a precipitation formation reaction that occurs while the metal complex is retained in the reaction tank. It is formed. Therefore, a coprecipitation precursor having a target particle size can be obtained by appropriately selecting a time for continuing stirring after the dropping of the raw material aqueous solution.

  The preferable stirring duration after the dropping of the raw material aqueous solution is influenced by the size of the reaction vessel, stirring conditions, pH, reaction temperature, etc., but 0.5 hours or more in order to grow the particles as uniform spherical particles The reaction time is preferably 1 hour or longer. Further, in order to reduce the possibility that the output performance in the low SOC region of the battery is not sufficient due to the particle size becoming too large, it is preferably 30 hours or less, more preferably 25 hours or less, and most preferably 20 hours or less.

Lithium transition metal composite oxide particles can be obtained by firing the coprecipitation precursor obtained as described above and a Li compound such as lithium hydroxide or lithium carbonate.
The firing temperature affects the reversible capacity of the active material.
If the firing temperature is too low, crystallization does not proceed sufficiently and the electrode characteristics tend to deteriorate. In the present embodiment, the firing temperature is preferably 800 ° C. or higher. By setting the temperature to 800 ° C. or higher, positive electrode active material particles having a high degree of sintering can be obtained, and charge / discharge cycle performance can be improved.
On the other hand, if the firing temperature is too high, the structural change occurs from the layered α-NaFeO ₂ structure to the rock salt cubic structure, which is disadvantageous for lithium ion movement in the positive electrode active material during the charge / discharge reaction, and the discharge performance decreases. To do. Further, since the sintering progresses too much, the specific surface area becomes small or the pore volume decreases, so that the discharge capacity and the initial efficiency are lowered. In the present embodiment, the firing temperature is preferably 900 ° C. or lower. By setting the temperature to 900 ° C. or less, the charge / discharge cycle performance can be improved.
Therefore, when producing the positive electrode active material containing the lithium transition metal complex oxide according to the present embodiment, the firing temperature is preferably set to 800 to 900 ° C. in order to improve charge / discharge cycle performance. The atmosphere during firing is preferably an oxidizing gas atmosphere, and more preferably normal air. The firing time is preferably 1 to 30 hours.

When the above-mentioned calcination is performed using a carbonate precursor, the pore diameter is 10 to 100 nm where the differential pore volume in the pore distribution obtained by the BJH method from the adsorption isotherm using the nitrogen gas adsorption method shows the maximum value. Lithium transition metal composite oxide particles having a maximum value of 0.20 mm ³ / (g · nm) or more can be obtained.
The pore volume of the lithium transition metal composite oxide particles is determined by measurement using the following nitrogen gas adsorption method. Based on the assumption that the pores are cylindrical with respect to the adsorption isotherm obtained on the desorption side of the measurement, a cumulative pore volume curve is obtained by applying the BJH method. Next, by differentiating the cumulative pore volume curve linearly, differential pores in which the horizontal axis is the pore diameter (nm) and the vertical axis is the pore volume (mm ³ / (g · nm)) Get the volume curve. In the present specification, the “pore diameter at which the differential pore volume exhibits the maximum value” refers to a value on the horizontal axis corresponding to the point at which the differential pore volume curve exhibits the maximum value. “Peak differential pore volume” refers to the value on the vertical axis corresponding to the point at which the differential pore volume curve shows the maximum value.

In the present embodiment, a step is provided in which a boron compound is mixed with the lithium transition metal composite oxide particles synthesized by the firing step and heat-treated to form a coating containing boron on the particle surface.
The mixing amount of the boron compound is 0.005 ≦ B / Me ≦ 0.05 (0.5 to 5.0 mol with respect to Me) at a molar ratio B / Me to the transition metal element (Me) of the lithium transition metal composite oxide. %).
As the boron compound, boric acid (H ₃ BO ₃ ) or boron oxide (B ₂ O ₃ ) is preferably used. When boric acid is used as the boron compound to be mixed, the above molar ratio B / Me can be obtained by mixing approximately 0.29 to 2.8% by mass of boric acid with respect to the positive electrode active material.
The heat treatment after mixing is preferably performed at 200 ° C. to 600 ° C. for 1 to 10 hours in an air atmosphere. By setting the heat treatment temperature to 600 ° C. or lower, it is possible to suppress the diffusion of boron into the lithium transition metal composite oxide particles.

<Negative electrode material>
The negative electrode material is not limited, and any negative electrode material may be selected as long as it can release or occlude lithium ions. For example, titanium-based materials such as lithium titanate having a spinel crystal structure typified by Li [Li _1/3 Ti _5/3 ] O ₄ , alloy-based materials such as Si, Sb, and Sn-based materials, lithium metal, lithium Alloys (lithium-containing alloys such as lithium-silicon, lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloys), lithium composite oxide (lithium-titanium), oxidation In addition to silicon, alloys that can occlude / release lithium, carbon materials (eg, graphite, hard carbon, low-temperature fired carbon, amorphous carbon, etc.), and the like can be given.

<Positive electrode / Negative electrode>
The positive electrode active material powder and the negative electrode material powder preferably have an average particle size (D50) of 100 μm or less. In particular, the positive electrode active material powder is preferably 50 μm or less for the purpose of improving the high output characteristics of the nonaqueous electrolyte battery, and preferably 3 μm or more in order to maintain charge / discharge cycle performance. In order to obtain the powder in a predetermined shape, a pulverizer or a classifier is used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill or a sieve is used. At the time of pulverization, wet pulverization in the presence of water or an organic solvent such as hexane may be used. There is no particular limitation on the classification method, and a sieve, an air classifier, or the like is used as needed for both dry and wet methods.

  The positive electrode active material and the negative electrode material, which are the main components of the positive electrode and the negative electrode, have been described in detail above. In addition to the main components, the positive electrode and the negative electrode include a conductive agent, a binder, a thickener, and a filler. Etc. may be contained as other constituents.

  The conductive agent is not limited as long as it is an electron conductive material that does not adversely affect the battery performance. Usually, natural graphite (such as scaly graphite, scaly graphite, earthy graphite), artificial graphite, carbon black, acetylene black, Conductive materials such as ketjen black, carbon whisker, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fiber, and conductive ceramic material can be included as one kind or a mixture thereof. .

  Among these, as the conductive agent, acetylene black is desirable from the viewpoints of electron conductivity and coatability. The addition amount of the conductive agent is preferably 0.1% by weight to 50% by weight, and particularly preferably 0.5% by weight to 30% by weight with respect to the total weight of the positive electrode or the negative electrode. In particular, it is desirable to use acetylene black by pulverizing into ultrafine particles of 0.1 to 0.5 μm because the required carbon amount can be reduced. These mixing methods are physical mixing, and the ideal is uniform mixing. Therefore, powder mixers such as V-type mixers, S-type mixers, crackers, ball mills, and planetary ball mills can be mixed dry or wet.

  The binder is usually a thermoplastic resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene. Polymers having rubber elasticity such as rubber (SBR) and fluororubber can be used as one kind or a mixture of two or more kinds. The addition amount of the binder is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight, based on the total weight of the positive electrode or the negative electrode.

  As the filler, any material that does not adversely affect the battery performance may be used. Usually, olefin polymers such as polypropylene and polyethylene, amorphous silica, alumina, zeolite, glass, carbon and the like are used. The addition amount of the filler is preferably 30% by weight or less with respect to the total weight of the positive electrode or the negative electrode.

  The positive electrode and the negative electrode are prepared by mixing the main constituents (positive electrode active material in the positive electrode, negative electrode material in the negative electrode) and other materials into a mixture and mixing with an organic solvent such as N-methylpyrrolidone or toluene or water. After that, the obtained liquid mixture is applied on a current collector described in detail below, or pressed and heat-treated at a temperature of about 50 ° C. to 250 ° C. for about 2 hours. . About the application method, for example, it is desirable to apply to any thickness and any shape using means such as roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc. It is not limited.

  As the current collector, a current collector foil such as an Al foil or a Cu foil can be used. The positive electrode current collector foil is preferably an Al foil, and the negative electrode current collector foil is preferably a Cu foil. The thickness of the current collector foil is preferably 10 to 30 μm. Further, the thickness of the mixture layer is preferably 40 to 150 μm (excluding the current collector foil thickness) after pressing.

<Nonaqueous electrolyte>
The nonaqueous electrolyte used for the nonaqueous electrolyte secondary battery according to the present embodiment is not limited, and those generally proposed for use in lithium batteries and the like can be used. Nonaqueous solvents used for the nonaqueous electrolyte include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, Chain carbonates such as diethyl carbonate and ethyl methyl carbonate; chain esters such as methyl formate, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxy Ethers such as ethane, 1,4-dibutoxyethane and methyldiglyme; Nitriles such as acetonitrile and benzonitrile; Dioxolane or derivatives thereof; Ethylene sulfide, sulfolane, sultone or derivatives thereof Examples thereof include a conductor alone or a mixture of two or more thereof, but are not limited thereto.

Examples of the electrolyte salt used for the nonaqueous electrolyte include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr. , KClO ₄ , KSCN, and other inorganic ion salts containing one of lithium (Li), sodium (Na), or potassium (K), LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , ( CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (n-C ₄ H ₉ ) ₄ NClO ₄ , (n-C _{4 H 9) 4 NI, (} C 2 H 5) 4 N-mal _{ate, (C 2 H 5)} 4 N-benzoate, (C 2 H 5) 4 N-phthalate, lithium stearyl sulfonate, lithium octyl sulfonate, organic ion salts of lithium dodecyl benzene sulfonate, and the like. These These ionic compounds can be used alone or in admixture of two or more.

Furthermore, by mixing and using LiPF ₆ or LiBF ₄ and a lithium salt having a perfluoroalkyl group such as LiN (C ₂ F ₅ SO ₂ ) ₂ , the viscosity of the electrolyte can be further reduced. The low temperature characteristics can be further improved, and self-discharge can be suppressed, which is more desirable.

  Moreover, you may use normal temperature molten salt and an ionic liquid as a nonaqueous electrolyte.

  The concentration of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol / l to 5 mol / l, more preferably 0.5 mol / l to 2 in order to reliably obtain a non-aqueous electrolyte battery having high battery characteristics. .5 mol / l.

<Separator>
As a separator used for the non-aqueous electrolyte secondary battery according to the present embodiment, it is preferable to use a porous film or a nonwoven fabric exhibiting excellent high rate discharge performance alone or in combination. Examples of the material constituting the separator for a nonaqueous electrolyte battery include polyolefin resins typified by polyethylene, polypropylene, etc., polyester resins typified by polyethylene terephthalate, polybutylene terephthalate, etc., polyvinylidene fluoride, vinylidene fluoride-hexa. Fluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, fluorine Vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride - tetrafluoroethylene - hexafluoropropylene copolymer, vinylidene fluoride - ethylene - can be mentioned tetrafluoroethylene copolymer.

  The porosity of the separator is preferably 98% by volume or less from the viewpoint of strength. Further, the porosity is preferably 20% by volume or more from the viewpoint of charge / discharge characteristics.

  The separator may be a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, polyvinylidene fluoride, and an electrolyte. Use of the non-aqueous electrolyte in the gel state as described above is preferable in that it has an effect of preventing leakage.

  Furthermore, it is desirable that the separator be used in combination with the above-described porous film, non-woven fabric, or the like and a polymer gel because the liquid retention of the electrolyte is improved. That is, by forming a film in which the surface of the polyethylene microporous membrane and the microporous wall are coated with a solvophilic polymer having a thickness of several μm or less, and holding the electrolyte in the micropores of the film, Gels.

  Examples of the solvophilic polymer include polyvinylidene fluoride, an acrylate monomer having an ethylene oxide group or an ester group, an epoxy monomer, a polymer having a monomer having an isocyanate group, and the like crosslinked. The monomer can be subjected to a crosslinking reaction using a radical initiator in combination with heating or ultraviolet rays (UV), or using an actinic ray such as an electron beam (EB).

  Other battery components include a terminal, an insulating plate, a battery case, and the like, but these components may be used as they are.

<Nonaqueous electrolyte secondary battery>
FIG. 5 shows a lithium secondary battery which is an embodiment of the nonaqueous electrolyte secondary battery according to one aspect of the present invention. FIG. 5 is a perspective view of the inside of a rectangular lithium secondary battery seen through. The lithium secondary battery 1 is assembled by injecting a nonaqueous electrolyte (electrolytic solution) into the battery container 3 in which the electrode group 2 is housed. The electrode group 2 is formed by winding a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material via a separator. The positive electrode is electrically connected to the positive electrode terminal 4 via the positive electrode lead 4 ′, and the negative electrode is electrically connected to the negative electrode terminal 5 via the negative electrode lead 5 ′.
The shape of the lithium secondary battery according to this embodiment is not particularly limited, and examples thereof include a cylindrical battery, a square battery (rectangular battery), a flat battery, and the like.

  The present invention can also be realized as a power storage device in which a plurality of the above lithium secondary batteries are assembled as another aspect. An example of the power storage device is illustrated in FIG. In FIG. 6, the power storage device 30 includes a plurality of power storage units 20. Each power storage unit 20 includes a plurality of lithium secondary batteries 1. The power storage device 30 can be mounted as a power source for vehicles such as an electric vehicle (EV), a hybrid vehicle (HEV), and a plug-in hybrid vehicle (PHEV).

Example 1
<Precursor production process>
Weigh 13.8 g of cobalt sulfate heptahydrate, 19.9 g of nickel sulfate hexahydrate and 66.3 g of manganese sulfate pentahydrate, and dissolve all of them in 200 ml of ion-exchanged water. Co: Ni: Mn A 2.0 M aqueous sulfate solution with a molar ratio of 12.3: 18.9: 68.7 was prepared. On the other hand, 750 ml of ion exchange water was poured into a ₂ L reaction tank, and CO ₂ gas was bubbled for 30 minutes to dissolve CO _{2 in the} ion exchange water. The temperature of the reaction vessel was set to 50 ° C. (± 2 ° C.), and the aqueous sulfate solution was stirred while stirring the reaction vessel with a baffle plate at a rotational speed of 1000 rpm using a disc turbine blade equipped with a stirring motor. The solution was dropped at a rate of 3 ml / min. Here, during the period from the start to the end of dropping, an aqueous solution containing 1.0 M sodium carbonate and ammonia is appropriately dropped, so that the pH in the reaction tank is always 8.0 (± 0.05), the ammonia concentration. Was controlled to maintain 0.5 g / L. After completion of the dropwise addition, stirring in the reaction vessel was further continued for 3 hours. After the stirring was stopped, the mixture was allowed to stand for 12 hours or more.
Next, using a suction filtration device, the coprecipitated carbonate particles produced in the reaction vessel are separated, and sodium ions adhering to the particles are washed away using ion-exchanged water, and an electric furnace is used. And dried at 80 ° C. under normal pressure in an air atmosphere. Then, in order to arrange | equalize a particle size, it grind | pulverized for several minutes with the smoked automatic mortar. In this way, a coprecipitated carbonate precursor was produced.

<Baking process>
1.40 g of lithium carbonate is added to 3.00 g of the coprecipitated carbonate precursor, and mixed well using a smoked automatic mortar. The molar ratio of Li: (Co, Ni, Mn) is 1.42: 1. A mixed powder was prepared. The mixed powder was placed in an alumina crucible and placed in a box-type electric furnace (model number: AMF20). The temperature was raised from ambient temperature to normal temperature to 870 ° C. over 10 hours in an air atmosphere, and the temperature was increased at 870 ° C. for 4 hours. Baked for hours. The box-type electric furnace has internal dimensions of 10 cm in length, 20 cm in width, and 30 cm in depth, and heating wires are inserted at intervals of 20 cm in the width direction. After firing, the heater was turned off and allowed to cool naturally with the alumina crucible placed in the furnace. As a result, the temperature of the furnace decreases to about 200 ° C. after 5 hours, but the subsequent temperature decrease rate is somewhat moderate. After the passage of day and night, it was confirmed that the furnace temperature was 100 ° C. or lower, and then the pellets were taken out and pulverized for several minutes in a smoked automatic mortar in order to make the particle diameter uniform. Thus, the lithium transition metal complex oxide Li _1.42 Ni _{0.189 Co 0.123} Mn _0.687 O _{2 + z according} to Comparative Example 1 was synthesized. Here, the value of z calculated stoichiometrically from the composition formula is 0.42, but the value of z is not necessarily in the stoichiometric ratio as long as it has an α-NaFeO ₂ type crystal structure. It is not necessary. The same applies to the following examples and comparative examples.

<Surface treatment process>
0.0087 g of boric acid is added to 3.0 g of the synthesized lithium transition metal composite oxide and mixed well using a smoked automatic mortar, and the molar ratio of Li: (Co, Ni, Mn): B is 1.42. A mixed powder of 1: 0.005 was prepared. The mixed powder is placed in an alumina crucible and placed in a box-type electric furnace (model number: AMF20), and the temperature is increased from normal temperature to 400 ° C at a temperature increase rate of 1.5 ° C / min in an air atmosphere. Warmed and heat treated at 400 ° C. for 5 hours. The box-type electric furnace has internal dimensions of 10 cm in length, 20 cm in width, and 30 cm in depth, and heating wires are inserted at intervals of 20 cm in the width direction. After the heat treatment, the heater was turned off, and the alumina crucible was left in the furnace and allowed to cool naturally. As a result, the temperature drop rate is slightly moderate. After the passage of day and night, it was confirmed that the furnace temperature was 100 ° C. or lower, and then the pellets were taken out and pulverized for several minutes in a smoked automatic mortar in order to make the particle diameter uniform. Thus, to prepare a lithium transition metal composite oxide _{Li 1.42 Ni 0.189 Co 0.123 Mn 0.687} B 0.005 O 2 + z according to the first embodiment. Here, the value of z calculated stoichiometrically from the composition formula is 0.42, but the value of z is not necessarily in the stoichiometric ratio as long as it has an α-NaFeO ₂ type crystal structure. It is not necessary.

(Comparative Example 1)
Lithium transition metal composite oxide that was not subjected to the surface treatment step on Li _1.42 Ni _0.189 Co _0.123 Mn _0.687 O _{2 + z} synthesized in Example 1 was defined as Comparative Example 1 (see FIG. 2). .

(Examples 2 to 4)
With respect to Li _1.42 Ni _0.189 Co _0.123 Mn _0.687 O _{2 + z} synthesized in Example 1, the molar ratio of boron to transition metal element was 2 mol%, 3 mol%, and 5 mol%, respectively. The lithium transition metal composite oxides Li _1.42 Ni _0.189 Co _{0.123 according to} Examples 2 to 4 were the same as Example 1 except that boric acid was added and the surface treatment step was performed. Mn _0.687 B _0.02 O _{2 + z} , Li _1.42 Ni _0.189 Co _0.123 Mn _0.687 B _0.03 O _{2 + z} and Li _1.42 Ni _0.189 Co _0.123 Mn _0.687 B _0.05 O _{2 + z} was produced (see FIG. 1 for Example 4).

(Comparative Examples 2 and 3)
With respect to Li _1.42 Ni _0.189 Co _0.123 Mn _0.687 O _{2 + z} synthesized in Example 1, the molar ratio of boron to transition metal element is 7 mol% and 10 mol%, respectively. The lithium transition metal composite oxide Li _1.42 Ni _0.189 Co _0.123 Mn _{0.687 according to} Comparative Examples 2 and 3 was obtained in the same manner as in Example 1 except that boric acid was added and the surface treatment step was performed. B _0.07 O _{2 + z} and Li _1.42 Ni _0.189 Co _0.123 Mn _0.687 B _0.10 O _{2 + z} were prepared.

(Comparative Example 4)
Except that cobalt sulfate heptahydrate, nickel sulfate hexahydrate and manganese sulfate pentahydrate were weighed so that the molar ratio of Co: Ni: Mn was 1: 1: 1, the same as in Example 1 was performed. Thus, a coprecipitated carbonate precursor was prepared.
A mixing step and a firing step similar to Example 1 except that lithium carbonate was added to the coprecipitated carbonate precursor so that the molar ratio of Li: (Co, Ni, Mn) was 1.13: 1. The lithium transition metal composite oxide Li _1.13 Ni _0.33 Co _0.33 Mn _0.33 O _{2 + z according} to Comparative Example 4 was synthesized without performing the surface treatment step. The value of z calculated stoichiometrically from the composition formula is 0.13, but the value of z does not necessarily have to be in the stoichiometric ratio as long as it has an α-NaFeO ₂ type crystal structure. .

(Comparative Examples 5 to 8)
With respect to the lithium transition metal composite oxide Li _1.13 Ni _0.33 Co _0.33 Mn _0.33 O _{2 + z} synthesized in Comparative Example 4, the molar ratio of boron to transition metal element was 0.5, Lithium transition metal composite oxides Li _1.13 Ni _{0 according to} Comparative Examples 5 to 8 were the same as Comparative Example 4 except that boric acid was added so as to be 2, 3 and 5 mol% and the surface treatment step was performed. _.33 Co _0.33 Mn _0.33 B _0.005 O _{2 + z} , Li _1.13 Ni _0.33 Co _0.33 Mn _0.33 B _0.02 O _{2 + z} , Li _1.13 Ni _{0 .33 Co 0.33 Mn 0.33 B 0.03 O} 2 + z, and to prepare a _{Li 1.13 Ni 0.33 Co0.33Mn 0.33 B 0.05} O 2 + z.

[Confirmation of crystal structure]
Powder X-ray diffraction measurement was performed on the lithium transition metal composite oxides according to Examples 1 to 4 and Comparative Examples 1 to 8 using an X-ray diffractometer (manufactured by Rigaku, model name: MiniFlex II).
As a result, it was confirmed that the lithium transition metal composite oxides prepared in all Examples and Comparative Examples had an α-NaFeO ₂ structure.

[Specific surface area measurement]
By adopting the measurement conditions described above, the BET specific surface area was measured for the lithium transition metal composite oxides according to Examples 1 to 4 and Comparative Examples 1 to 3. In the measurement, gas adsorption by cooling with liquid nitrogen was performed. In addition, preheating at 120 ° C. for 15 minutes was performed before cooling. The input amount of the measurement sample was 0.5 g ± 0.01 g.

<Preparation of positive electrode for nonaqueous electrolyte secondary battery>
Using the lithium transition metal composite oxides according to Examples 1 to 4 and Comparative Examples 1 to 8 as a positive electrode active material for a nonaqueous electrolyte secondary battery, a positive electrode for a nonaqueous electrolyte secondary battery was produced by the following procedure. did. Using N-methylpyrrolidone as a dispersion medium, a coating paste in which a positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVdF) were kneaded and dispersed at a mass ratio of 90: 5: 5 was prepared. The coating paste was applied to one side of an aluminum foil current collector having a thickness of 20 μm to produce a positive electrode plate. In addition, the mass and coating thickness of the active material applied per fixed area were standardized so that the test conditions were the same among the nonaqueous electrolyte secondary batteries according to all Examples and Comparative Examples.

<Preparation of nonaqueous electrolyte secondary battery>
As the negative electrode of the nonaqueous electrolyte secondary battery, a metal lithium electrode in which metal lithium having a capacity sufficiently larger than the theoretical capacity of the positive electrode was attached to a nickel current collector was used.

  Using each of the positive electrodes for lithium secondary batteries using the lithium transition metal composite oxides according to Examples 1 to 4 and Comparative Examples 1 to 9, non-aqueous electrolyte secondary batteries using the above negative electrodes were produced (hereinafter referred to as the following). And Example batteries 1 to 4 and Comparative example batteries 1 to 9, respectively).

As a non-aqueous electrolyte, LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) in a volume ratio of 6: 7: 7 so that the concentration would be 1 mol / l. Solution was used. As the separator, a polypropylene microporous film whose surface was modified with polyacrylate was used. A metal resin composite film made of polyethylene terephthalate (15 μm) / aluminum foil (50 μm) / metal-adhesive polypropylene film (50 μm) is used for the exterior body, and the electrodes are exposed so that the open ends of the positive electrode terminal and the negative electrode terminal are exposed to the outside. The metal resin composite film was hermetically sealed with the fusion allowance where the inner surfaces of the metal resin composite films faced each other except for the portion serving as a liquid injection hole, and the liquid injection hole was sealed after the nonaqueous electrolyte was injected.

[Initial charge / discharge process]
The batteries of Examples 1 to 4 and Comparative Examples 1 to 3 were subjected to two cycles of initial charge / discharge at 25 ° C. The charging of these batteries was constant current and constant voltage charging with a current of 0.1 CmA and a voltage of 4.7 V, and the charge termination condition was when the current value attenuated to 1/6. The discharge was a constant current discharge with a current of 0.1 CmA and a final voltage of 2.0 V. Here, a pause process of 10 minutes was provided after charging and discharging, respectively, and the initial discharge capacity was confirmed.
For the batteries of Comparative Examples 4 to 8, initial charge and discharge were performed under the same conditions as those of Examples 1 to 4 and Comparative Examples 1 to 3 except that the charge end voltage was 4.45 V and the discharge end voltage was 2.75 V. The initial discharge capacity was confirmed.

[Charge / discharge cycle test]
After confirming the initial discharge capacity, a 25-cycle charge / discharge cycle test was conducted. Regarding the batteries of Examples 1 to 4 and Comparative Examples 1 to 3, charging was performed at a constant current and a constant voltage at a current of 1 CmA and a voltage of 4.7 V, and the charge termination condition was set at the time when the current value was attenuated to 1/6. The discharge was a constant current discharge with a current of 1 CmA and a final voltage of 2.0V. Here, a pause process of 10 minutes was provided after charging and discharging, respectively. The batteries of Comparative Examples 4 to 8 were charged and discharged under the same conditions as the batteries of Examples 1 to 4 and Comparative Examples 1 to 3 except that the charge end voltage was 4.45 V and the discharge end voltage was 2.75 V. .
The percentage of the discharge capacity at the 25th cycle with respect to the discharge capacity at the 1st cycle in the charge / discharge cycle test was calculated to be “capacity maintenance ratio (%)”.

  The results are shown in Table 1.

From the results of Examples 1 to 4 and Comparative Examples 1 to 3 in Table 1, the molar ratio of Mn in transition metal (Me) Mn / Me exceeds 0.5, and the molar ratio of lithium to transition metal (Me) Li In a lithium-excess type active material with / Me exceeding 1 and adding 0.5 mol% or more of boron to Me and forming a coating containing boron on the active material surface, it can be seen that the capacity retention rate is improved. . However, in Comparative Examples 2 and 3 in which the amount of boron added exceeds 5 mol%, the initial discharge capacity is reduced by 5% or more from Comparative Example 1 (non-boron added product), and the discharge capacity after 25 cycles of charge and discharge is also compared. Below example 1. Moreover, when the boron addition amount increases, the BET specific surface area decreases.
Therefore, it is preferable that the boron addition amount for forming a coating containing boron having both a high discharge capacity and an improvement in capacity retention rate after the charge / discharge cycle is 0.5 to 5 mol%.

Comparative Examples 4 to 8 use a LiMeO _{2 type} active material having a Mn molar ratio Mn / Me in the transition metal of 0.5 or less and a Ni: Co: Mn ratio of 1: 1: 1. It is the example which investigated the influence of the coating containing boron by.
Comparing Comparative Example 1 and Comparative Example 4 in which no boron is added, the LiMeO ₂ type active material has a high capacity retention rate and excellent charge / discharge cycle performance, but the initial discharge capacity is much higher than the lithium excess type active material. It can be seen that it is small. When boron is added to the LiMeO _{2 type} active material, there is no effect in improving the capacity retention rate with a small addition amount (Comparative Examples 5 and 6), and the discharge capacity decreases as the addition amount increases (Comparative Example). 7, 8).
Therefore, by forming a coating containing boron on the surface of the active material particles, the effect of achieving both a high discharge capacity and a high capacity retention rate after the charge / discharge cycle is unique to the lithium-rich active material. I understand.

  By using the positive electrode active material containing the lithium transition metal composite oxide according to the present invention, a non-aqueous electrolyte secondary battery having a high discharge capacity and a high charge / discharge cycle performance can be provided. The electrolyte secondary battery is useful as a non-aqueous electrolyte secondary battery for hybrid vehicles and electric vehicles.

1 Nonaqueous electrolyte secondary battery (lithium secondary battery)
2 Electrode group 3 Battery container 4 Positive electrode terminal 4 ′ Positive electrode lead 5 Negative electrode terminal 5 ′ Negative electrode lead 20 Power storage unit 30 Power storage device

Claims (5)

A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide,
The lithium transition metal composite oxide is
having an α-NaFeO ₂ structure,
The transition metal element (Me) contains Mn and contains at least one of Co or Ni;
The molar ratio of Mn to Me, Mn / Me is 0.5 <Mn / Me,
The molar ratio of Li to Me Li / Me is 1 <Li / Me <1.5,
A coating containing boron is present on the particle surface of the lithium transition metal composite oxide,
Containing 0.5 mol% or more and 5 mol% or less of the boron with respect to Me;
Positive electrode active material for 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 transition metal composite oxide has a BET specific surface area of 4.0 m ² / g or more and 7.0 m ² / g or less. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2,
A transition metal carbonate precursor comprising Mn and at least one of Co or Ni, wherein a molar ratio of Mn to transition metal element (Me) Mn / Me is 0.5 <Mn / Me;
The transition metal carbonate precursor and the lithium compound are mixed and fired to produce lithium transition metal composite oxide particles in which the Li molar ratio Li / Me to Me is 1 <Li / Me <1.5,
A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: mixing a boron compound with the particles, and performing heat treatment to form a coating containing boron on the surface of the particles.   The positive electrode for nonaqueous electrolyte secondary batteries containing the positive electrode active material of Claim 1 or 2. A nonaqueous electrolyte secondary battery comprising the positive electrode according to claim 4, a negative electrode, and a nonaqueous electrolyte.