JP6685640B2 - Non-aqueous electrolyte Positive electrode active material for secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery.

  In recent years, small electronic devices have become more sophisticated, and secondary batteries used in these electronic devices are required to have higher energy density. A lithium-ion secondary battery is expected as a secondary battery that can meet such requirements, and has been put to practical use as a drive power source for mobile phones, laptop computers, and the like. As a positive electrode active material for a lithium ion secondary battery, lithium cobalt oxide is typically put into practical use.

  In addition to lithium cobalt oxide, a lithium transition metal composite oxide has been proposed as a positive electrode active material. Furthermore, there is also a technique of adding a trace amount of boron in addition to the main metal element.

  For example, in Patent Document 1, in a secondary battery using a lithium-transition metal composite oxide containing at least Ni and Mn as a positive electrode active material, an object is to improve thermal stability at a high charging potential and in a state where an electrolytic solution coexists. As such, a technique has been proposed in which the positive electrode active material further contains boron. The method of containing boron specifically disclosed is a method of mixing a lithium compound, a boron compound, and a composite hydroxide of nickel, manganese, and cobalt and then firing the mixture.

  On the other hand, there is also a technique in which boron is present on the surface of the lithium-transition metal composite oxide particles.

  For example, in Patent Document 2, for the purpose of increasing the capacity of the secondary battery and improving the charging / discharging efficiency of the secondary battery, ammonium borate, boron or boron is formed on the surface of the lithium-transition metal composite oxide particles in which nickel or cobalt is essential. A technique has been proposed in which a boric acid compound such as lithium oxide is adhered and heat treatment is performed in an oxidizing atmosphere. The lithium-transition metal composite oxide specifically disclosed is lithium nickelate in which a part of nickel is replaced with cobalt and aluminum.

JP 2004-281158 A JP, 2009-146739, A

  Along with the improvement in various characteristics of lithium-ion secondary batteries, there is a movement to apply non-aqueous electrolyte secondary batteries as driving power sources for larger devices such as electric vehicles. An important characteristic required for a driving power source for such a large device is the energy density stored in the secondary battery. Energy density can be increased by higher charging capacity and higher charging voltage.

  Another important characteristic required for a driving power source for large equipment is output characteristic. One of the methods for improving the output characteristics of the positive electrode active material is to increase the specific surface area of the positive electrode active material and increase the area of the positive electrode active material in contact with the non-aqueous electrolyte.

  According to the research by the present inventor, it has been found that the viscosity of a positive electrode slurry using a lithium-transition metal composite oxide containing boron as a positive electrode active material as in Patent Document 1 is likely to increase with time. It has been found that this tendency is stronger as the specific surface area of the positive electrode active material is larger, the ratio of lithium to the transition metal element in the lithium transition metal composite oxide is larger, or the ratio of nickel in the transition metal is higher. The fact that the positive electrode slurry changes with time means that the characteristics of the obtained positive electrode vary. Further, the cycle characteristics also need to be improved in applications such as electric vehicles.

  The present invention has been made in view of these circumstances. An object of the present invention is to provide a positive electrode active material that has high cycle characteristics (high voltage cycle characteristics) and output characteristics under a use condition at a high charging voltage (about 4.3 V or higher) and that can efficiently manufacture a positive electrode. It is to provide the substance.

  In order to achieve the above-mentioned object, the present inventor has conducted extensive studies and completed the present invention. The present inventor has provided a coating layer containing boron and oxygen on a core particle composed of a lithium-transition metal composite oxide to increase the viscosity of the positive electrode slurry even if the core particle contains boron and the specific surface area of the core particle is large. It was found that the above was suppressed and the high voltage cycle characteristics were improved.

The positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention has a general formula of Li _a Ni _1-x-y Co _x M ¹ _y M ² _z B _w O ₂ (1.00 ≦ a ≦ 1.50,0. 0.00 <x <0.50, 0 <y <0.50, 0.000 <z <0.020, 0.002 <w <0.020, 0.00 <x + y <0.70, M ¹ is At least one selected from the group consisting of Mn and Al, M ² is at least one selected from the group consisting of Zr, Ti, Mg, W and V), and its specific surface area is 1.2 m ² / g or more. And a coating layer that is present on the surface of the core particle and that contains boron and oxygen.

  The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention comprises a mixing step of mixing the core particles and a coating layer raw material compound containing boron and oxygen to obtain a raw material mixture, and the mixing step. A heat treatment step of heat treating the obtained raw material mixture.

  Since the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention has the above characteristics, it is possible to suppress an increase in viscosity of the obtained positive electrode slurry even if the core particle contains boron and has a large specific surface area. You can As a result, it becomes possible to efficiently manufacture a non-aqueous electrolyte secondary battery having high output characteristics and high voltage cycle characteristics.

FIG. 1 shows the relationship between the specific surface area and output characteristics of core particles having the composition according to the present embodiment. FIG. 2 shows the relationship between the specific surface area of a plurality of types of core particles and the viscosity change of the positive electrode slurry.

  Embodiments of the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention and the method for producing the same will be described below. However, the present invention is not limited to the following description.

  First, an embodiment of the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention will be described.

  The positive electrode active material for a non-aqueous electrolyte secondary battery of the present embodiment includes core particles made of a specific lithium transition metal composite oxide, and a coating layer formed on the surface of the core particles. Hereinafter, these will be mainly described.

[Core particles]
<Composition>
The core particle is composed of a lithium-transition metal composite oxide containing lithium and nickel as main components, and further boron as an essential component.

  When part of the nickel site is replaced with cobalt, up to 50 mol% of nickel can be replaced. If the amount of cobalt substituting the nickel site is too large, it will lead to an increase in manufacturing cost, and therefore the smaller amount is preferable. Considering the balance with various characteristics, the amount of cobalt substituting the nickel sites is preferably 5 mol% or more and 35 mol% or less.

  When part of the nickel site is replaced with at least one selected from the group consisting of manganese and aluminum, up to 50 mol% of nickel can be replaced. Be aware that if it is too large, it may adversely affect the output characteristics and charge / discharge capacity. If the nickel content of the nickel sites is too small, the charge / discharge capacity tends to decrease, so the total substitution amount of the nickel sites is 70 mol% or less. Considering the balance with various characteristics, the total substitution amount of nickel sites is preferably 20 mol% or more and 60 mol% or less.

  If the amount of lithium in the core particle composition is large, the output characteristics tend to be improved, but if it is too large, it is difficult to synthesize. Further, even if it can be synthesized, it tends to be sintered and it becomes difficult to handle it thereafter. Based on these, the amount of lithium is 100 mol% or more and 150 mol% or less with respect to the nickel site element. Considering balance of characteristics, easiness of synthesis, etc., it is preferably 105 mol% or less and 125 mol% or less.

  The composition of the core particles further contains boron in addition to the above transition metal. When the content of boron is too large, the charge / discharge capacity of the whole positive electrode active material is reduced. Moreover, the viscosity of the positive electrode slurry is increased, and the coating layer described later cannot be suppressed. On the other hand, if the amount is too small, the cycle characteristics under the usage conditions at high charging voltage become insufficient. Based on these, the content of boron is 0.2 mol% or more and 2.0 mol% or less with respect to the core particles. The preferable content of boron is 0.3 mol% or more and 0.75 mol% or less with respect to the core particles.

  Zirconium, titanium, magnesium, tantalum, tungsten, vanadium or the like can be selected as an element to be further contained in the composition of the core particles. When the content of these elements is up to 2 mol%, various purposes can be achieved without hindering the characteristic improvement by other elements. For example, zirconium is suitable for improving storage characteristics, titanium and magnesium are suitable for further improving cycle characteristics, tungsten is suitable for further improving output characteristics, and vanadium is suitable for improving safety.

Based on these, the core particle has a general formula of Li _a Ni _1-x-y Co _x M ¹ _y M ² _z B _w O ₂ (1.00 ≦ a ≦ 1.50, 0.00 ≦ x ≦ 0.50, 0 ≦ y ≦ 0.50, 0.000 ≦ z ≦ 0.020, 0.002 ≦ w ≦ 0.020, 0.00 ≦ x + y ≦ 0.70, M ¹ is selected from the group consisting of Mn and Al A lithium transition metal composite oxide represented by the following formula (1), M ² is at least one selected from the group consisting of Zr, Ti, Mg, W and V).

<Specific surface area>
In order to increase the contact area between the non-aqueous electrolyte solution and the positive electrode active material and enhance the output characteristics, the specific surface area of the core particles needs to be above a certain level. FIG. 1 shows the relationship between the specific surface area Ss ₁ of the core particles having the composition according to the present embodiment and the output characteristics (details will be described later). Details on the composition of the core particles differ, the inflection point of the curve _is present in ^{Ss 1 = 1.0m 2 /g~1.2m 2 /} g Atari. The specific surface area of the positive electrode active material is 1.2 m ² / g or more so that the positive electrode active material can be suitably used for applications requiring high output such as electric vehicles. As the specific surface area, a value (so-called BET specific surface area) measured by a gas adsorption method using nitrogen gas is used.

  FIG. 2 shows the relationship between the specific surface area of a plurality of types of core particles and the viscosity change of the positive electrode slurry. As can be seen from FIG. 2, when the core particle composition contains boron and its specific surface area is large, the viscosity of the positive electrode slurry is likely to increase with time. The cause is not clear, but it is presumed that if lithium and boron coexist in the core particle, lithium easily elutes from the core particle into the liquid phase. When the liquid phase is the organic solution used for the positive electrode slurry, it is considered that the organic solvent, the binder, etc. in the organic solution change, and the viscosity of the positive electrode slurry increases. If the specific surface area of the core particles is large, the contact area with the liquid phase is large, and this tendency is considered to be stronger. Therefore, a coating layer described below is provided on the surface of the core particles.

[Coating layer]
The coating layer is present on the surface of the core particle and contains boron and oxygen. The coating layer is formed by the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention, which is exemplified in the embodiments described below. The morphology of the coating layer is slightly different depending on the forming process. This difference can be distinguished by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and the like.

  When the amount of boron contained in the coating layer is too small, the effect is not sufficiently exhibited, and when the amount is too large, it means that there are many electrochemically inactive regions in the positive electrode active material. The preferable range of the boron content is 2.0 mol% or less based on the core particles. A more preferable range is 0.2 mol% or more and 1.5 mol% or less.

  Next, an embodiment of the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention will be described.

  The manufacturing method of the positive electrode active material for a non-aqueous electrolyte secondary battery of the present embodiment includes a mixing step and a heat treatment step as main steps. Hereinafter, these steps will be mainly described.

[Mixing process]
The core particles and the coating layer raw material compound containing boron and oxygen are mixed to obtain a raw material mixture. The core particles may be obtained by appropriately using a known method. The coating layer raw material compound may be composed of a single compound containing boron and oxygen, or may be composed of a plurality of compounds. Further, a single or a plurality of compounds containing boron but not oxygen may be combined with a single or a plurality of compounds containing oxygen but not containing boron. Alternatively, for at least two kinds selected from the group consisting of a compound containing boron and oxygen, a compound containing boron but not oxygen and a compound containing oxygen but not boron, a single compound or a plurality of compounds are selected respectively. However, they may be combined. When the coating layer raw material compound is composed of a single compound or a plurality of compounds containing boron and oxygen, it is easy to form a more preferable coating layer.

  Examples of compounds containing boron and oxygen include oxides such as boron oxide, orthoboric acid (so-called ordinary boric acid), boric acids such as metaboric acid, lithium borate, and borate salts such as ammonium borate. To be Boric acid is particularly preferable because it is easy to obtain and handle, and the form of the coating layer is optimized.

  Examples of the compound containing boron but not oxygen include metal borides such as aluminum boride and boron nitride. Examples of the compound containing oxygen but not boron include various compounds that are solid at room temperature, such as various oxides, oxo acids, and oxo acid salts.

  The core particles and the coating layer raw material compound are selected according to the purpose, and after adjusting the substance amount ratio, they are appropriately mixed with a known stirring means such as a blade-type stirrer or a V-type mixer. Thus, the raw material mixture is obtained and used in the heat treatment step.

[Heat treatment process]
The obtained raw material mixture is heat-treated to form a coating layer on the surface of core particles. It is speculated that the coating layer is formed by chemically or physically binding at least a part of the elements constituting the coating layer raw material compound and at least a part of the elements constituting the core particles.

  If the heat treatment temperature is too low, the intended coating layer will not be formed. On the other hand, if the content is too high, the proportion of the elements constituting the core particles to be used for forming the coating layer becomes high, and the original characteristics of the core particles may be impaired. Therefore, it is appropriately adjusted according to the purpose. A preferable heat treatment temperature range is 450 ° C. or lower, and a more preferable range is 200 ° C. or higher and 400 ° C. or lower.

  Hereinafter, a more specific description will be given using examples. In addition, the ratio of elements is expressed as a mass ratio unless otherwise specified.

A composite hydroxide of Ni: Co: Mn = 1: 1: 1 was obtained by the coprecipitation method. The obtained composite hydroxide was mixed with lithium carbonate, boric acid and zirconium (IV) oxide so that Li: (Ni + Co + Mn): B: Zr = 1.15: 1: 0.005: 0.005. The materials were mixed to obtain a raw material mixture. The obtained raw material mixture was fired at 910 ° C. for 10 hours in an air atmosphere to obtain a sintered body. The obtained sintered body was pulverized and passed through a dry sieve to obtain a lithium transition metal composite represented by the general formula: Li _1.15 Ni _0.33 Co _0.33 Mn _0.33 B _0.005 Zr _0.005 O _2. Core particles made of oxide were obtained.

  The obtained core particles and boric acid corresponding to 0.5 mol% of boron with respect to the core particles were mixed with a high-speed shear stirrer to obtain mixed particles. The obtained mixed particles were heat-treated at 250 ° C. for 10 hours in the air atmosphere to obtain the intended positive electrode active material.

[Comparative Example 1]
The core particles in the examples were used as a positive electrode active material for comparison.

[Comparative Example 2]
A lithium transition metal composite oxide represented by the general formula Li _1.15 Ni _0.33 Co _0.33 Mn _0.33 Zr _0.005 O ₂ was prepared in the same manner as in Example except that boric acid was not added to the raw material mixture. Core particles were obtained. Thereafter, in the same manner as in Example 1, a desired positive electrode active material was obtained.

[Comparative Example 3]
The core particle in Comparative Example 2 was used as a positive electrode active material for comparison.

[Particle size evaluation]
With respect to the positive electrode active material, a laser diffraction method was used, and the value at which the cumulative value of the volume frequency was 50% was defined as the median particle diameter D ₅₀ .

[Evaluation of specific surface area]
The BET specific surface area Ss ₁ of the core particles and the BET specific surface area Ss ₂ of the positive electrode active material were measured.

[Evaluation of coating layer]
The boron content c _B in the coating layer was determined using a chemical analysis such as inductively coupled plasma (ICP) analysis. When boron was present in the composition of the core particles, c _B was calculated from the difference between the total boron content of the positive electrode active material and the boron content of the core particles.

[Viscosity evaluation]
For Examples and Comparative Examples 1 to 6, the change in viscosity of the positive electrode slurry was measured as follows.

30 g of the positive electrode active material, 1.57 g of PVDF (polyvinylidene fluoride) and 12.48 g of NMP (normal methyl-2-pyrrolidone) were placed in a polyethylene container and kneaded at room temperature (about 25 ° C.) for 5 minutes to obtain a positive electrode slurry. It was The initial viscosity ν ₀ of the obtained positive electrode slurry was immediately measured with an E-type viscometer. The blade of the E-type viscometer was a cone blade, and the rotation speed of the rotor was 5 rpm.

After the measurement, the positive electrode slurry was returned to the polyethylene container and allowed to stand in a constant temperature bath at 60 ° C. for 24 hours. After standing, the positive electrode slurry was kneaded for 2 minutes at room temperature, and the viscosity ν _{1 of the} positive electrode slurry was quickly measured with an E-type viscometer. In this way, the viscosity change Δν (≡ν ₁ −ν ₀ ) of the positive electrode slurry was obtained. In each measurement, a cone blade was used as the blade of the E-type viscometer, and the rotation speed of the rotor was 5 rpm.

[Cycle characteristic evaluation]
The cycle characteristics of Examples and Comparative Examples 1 to 3 were evaluated as follows.

[1. Production of positive electrode]
85 parts by weight of the positive electrode active material, 10 parts by weight of acetylene black, and 5.0 parts by weight of PVDF (polyfuccavinylidene) were dispersed and dissolved in NMP (normal methyl-2-pyrrolidone) to obtain a positive electrode slurry. The obtained positive electrode slurry was applied to an aluminum foil, dried, compression-molded by a roll press machine, and cut into a predetermined size to obtain a positive electrode.

[2. Preparation of negative electrode]
97.5 parts by weight of artificial graphite, 1.5 parts by weight of CMC (carboxymethyl cellulose), and 1.0 part by weight of SBR (styrene butadiene rubber) were dispersed in water to obtain a negative electrode slurry. The obtained negative electrode slurry was applied to a copper foil, dried, compression-molded by a roll press machine, and cut into a predetermined size to obtain a negative electrode.

[3. Preparation of non-aqueous electrolyte]
EC (ethylene carbonate) and MEC (methyl ethyl carbonate) were mixed at a volume ratio of 3: 7 to obtain a solvent. Lithium hexafluorophosphate (LiPF ₆ ) was dissolved in the obtained solvent to a concentration of 1 mol / L to obtain a non-aqueous electrolytic solution.

[4. Assembly of evaluation battery]
A lead electrode was attached to each of the positive electrode and negative electrode current collectors, and vacuum drying was performed at 120 ° C. Next, a separator made of porous polyethylene was placed between the positive electrode and the negative electrode, and they were stored in a bag-shaped laminate pack. After the storage, the entire laminate pack was vacuum dried at 60 ° C. to remove the water adsorbed on each member. After vacuum drying, the above-mentioned non-aqueous electrolyte was injected into the laminate pack and sealed to obtain a laminate-type non-aqueous electrolyte secondary battery for evaluation.

[5. Measurement]
The obtained battery was aged with a weak current, and the electrolyte was sufficiently soaked in the positive electrode and the negative electrode. After aging, the battery was placed in a constant temperature bath set at 45 ° C., and a constant-current constant-voltage charge with a charge potential of 4.4 V and a charge current of 2.0 C (1 C is the current at which discharge ends in 1 hour) and the discharge potential Charging and discharging were repeated with a constant current discharge at 2.75 V and a discharge current of 2.0 C as one cycle, and the discharge capacity Qd (n) at the nth cycle was measured. Qd (n) / Qd (1) was defined as the discharge capacity retention rate Rs (n) at the nth cycle.

[Output characteristic evaluation]
The output characteristics of Examples and Comparative Examples 1 to 3 and 6 were evaluated as follows.

[1. Preparation of secondary battery for evaluation]
A laminate type non-aqueous electrolyte secondary battery for evaluation was obtained in the same manner as the secondary battery for evaluating cycle characteristics.

[2. Measurement]
A weak current was passed through the obtained battery to perform aging, so that the positive electrode and the negative electrode were sufficiently wetted with the electrolyte. After that, discharging at high current and charging at weak current were repeated. The charge capacity in the 10th charge was taken as the total charge capacity of the battery, and after the 10th discharge, the battery was charged to 40% of the total charge capacity. After charging, the battery was placed in a constant temperature bath set at -25 ° C, left for 6 hours, then discharged at 0.02A, 0.04A, 0.06A, and the voltage was measured. The current was plotted on the horizontal axis and the voltage was plotted on the vertical axis, and the intersections were plotted.

  Table 1 shows the production conditions and Table 2 shows the characteristics of the positive electrode active material and the battery characteristics of Examples and Comparative Examples 1 to 6.

  The following can be seen from Tables 1 and 2.

  If boron is not contained in the composition of the core particles, the high voltage cycle characteristics are insufficient (Comparative Examples 2 and 3). On the other hand, when boron is contained in the composition of the core particles, the viscosity of the positive electrode slurry increases with time as its specific surface area increases (Comparative Examples 1, 4 to 6). When boron is contained in the composition of the core particles and the coating layer is formed, even if the specific surface area of the positive electrode active material is high, the increase in viscosity of the positive electrode slurry is suppressed, and the obtained non-aqueous electrolyte secondary battery is excellent. High voltage cycle characteristics and high output characteristics can both be achieved (Example).

  When the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention is used, both high cycle characteristics and high output characteristics can be achieved under usage conditions at high charging voltage. Moreover, since the charging voltage can be increased, the energy density can be improved. Furthermore, such a non-aqueous electrolyte secondary battery can be efficiently obtained. The non-aqueous electrolyte secondary battery thus obtained can be suitably used as a drive power source for large equipment such as hybrid vehicles and electric vehicles.

Claims (6)

Formula ^{_{Li a Ni 1-x-y}} Co x M 1 y M 2 z B w O 2 (1.00 ≦ a ≦ 1.50,0.00 ≦ x ≦ 0.50,0 ≦ y ≦ 0.50 , 0.000 ≦ z ≦ 0.020, 0.002 ≦ w ≦ 0.020, 0.00 ≦ x + y ≦ 0.70, M ¹ is at least one selected from the group consisting of Mn and Al, and M ² is Zr, Ti, Mg, at least one selected from the group consisting of Mg, W and V), a core particle composed of lithium transition metal composite oxide particles having a specific surface area of 1.2 m ² / g or more,
A coating layer that is present on the surface of the core particle and contains boron and oxygen,
Including,
The positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the content of boron in the coating layer is 2.0 mol% or less based on the core particles. The positive electrode active material according to claim 1 , wherein at least a part of the element containing boron and oxygen forming the coating layer and at least a part of the element forming the core particle are chemically or physically bonded. material. Formula ^{_{Li a Ni 1-x-y}} Co x M 1 y M 2 z B w O 2 (1.00 ≦ a ≦ 1.50,0.00 ≦ x ≦ 0.50,0 ≦ y ≦ 0.50 , 0.000 ≦ z ≦ 0.020, 0.002 ≦ w ≦ 0.020, 0.00 ≦ x + y ≦ 0.70, M ¹ is at least one selected from the group consisting of Mn and Al, and M ² is Zr, Ti, Mg, W and at least one selected from the group consisting of V)), a core particle composed of lithium transition metal composite oxide particles having a specific surface area of 1.2 m ² / g or more, A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which is present on the surface of core particles and comprises a coating layer containing boron and oxygen,
A mixing step of mixing the core particles and a coating layer raw material compound containing boron and oxygen to obtain a raw material mixture;
A heat treatment step of heat treating the raw material mixture obtained in the mixing step,
Including,
In the mixing step, the core particles and the coating layer raw material compound are mixed in a proportion such that the ratio of the boron element in the coating layer raw material compound to the core particles is 2.0 mol% or less. A method for producing a positive electrode active material for a water electrolyte secondary battery.   The manufacturing method according to claim 3, wherein the coating layer raw material compound in the mixing step is at least one selected from the group consisting of boron oxide, boron oxoacid, and boron oxoacid salt.   The manufacturing method according to claim 4, wherein the coating raw material compound in the mixing step is orthoboric acid.   The manufacturing method according to claim 3, wherein a heat treatment temperature in the heat treatment step is 450 ° C. or lower.

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