JP6524610B2 - Positive electrode active material for non-aqueous secondary battery and method for producing the same - Google Patents

Description

  The present invention relates to a positive electrode active material for non-aqueous secondary batteries and a method for producing the same.

  A non-aqueous secondary battery typified by a lithium ion secondary battery removes a substance capable of desorbing and inserting an alkali metal ion into a positive electrode active material and removes an alkali metal simple substance such as metallic lithium or an alkali metal ion into a negative electrode active material. It is a battery that uses a non-aqueous electrolyte as an alkali metal ion conduction medium, and exchanges alkali metal ions between the positive and negative electrodes through the alkali metal ion conduction medium to exchange electric power with the outside. In lithium ion secondary batteries, lithium transition metal complex oxides such as lithium cobaltate are typically used as a positive electrode active material.

  By the way, there exists a technique which coat | covers the surface of a positive electrode active material with a specific substance in the technique of modify | reforming the interface of a positive electrode active material.

In Patent Document 1, for the purpose of suppressing an increase in impedance due to high temperature storage etc., firing is carried out after niobium oxide or the like is present on the surface of a lithium nickel composite oxide such as LiNi _0.82 Co _0.15 Al _0.03 O ₂ Technology has been proposed.

In Patent Document 2, for the purpose of increasing the capacity and improving the charge and discharge efficiency, complex oxide particles such as Li _1.03 Ni _0.77 Co _0.20 Al _0.03 O _{2 and the} like, and borates such as ammonium pentaborate There has been proposed a technique in which an acid compound or the like is deposited and then fired in an oxidizing atmosphere.

In Patent Document 3, lithium such as Li _1.03 (Ni _0.8 Co _0.2 ) _0.9 Al _0.1 O ₂ or the like is used for the purpose of improving the thermal stability without significantly deteriorating the initial charge and discharge capacity. There has been proposed a technique for forming a surface layer containing W and the like and Li on the surface of a composite oxide powder. Specific examples was mixed with _{Li 1.03 (Ni 0.8 Co 0.2)} 0.9 Al 0.1 O 2 and _Li 4 WO _5, discloses that heat-treated at 752 ° C..

Unexamined-Japanese-Patent No. 2004-253305 JP, 2009-146739, A Japanese Patent Application Laid-Open No. 2002-75367

  With the progress of improvement of non-aqueous secondary batteries up to now, the field of application for larger-scale and long-term use, such as power source of large equipment such as electric car and storage battery for electric power leveling, is examined as its application field It is done. With the expansion of such application fields, non-aqueous secondary batteries are required to have higher performance in charge / discharge capacity, output characteristics, thermal stability, life characteristics (storage characteristics, cycle characteristics, etc.), etc. .

  In the case of an all-solid secondary battery employing a solid electrolyte as the alkali metal ion conductive medium, the thermal and chemical stability is extremely improved as compared to the case of a non-aqueous electrolyte secondary battery employing a non-aqueous electrolyte. However, since the alkali metal ion conductivity in the solid electrolyte is lower than that of the non-aqueous electrolyte, the discharge capacity of the all-solid secondary battery is lower than that of the non-aqueous electrolyte secondary battery when the extraction current is the same. It gets lower.

  In the case of non-aqueous electrolyte secondary batteries, there is still room for improvement given the recent demand for rapid charging. This is related to the fact that the crystal structure of the lithium transition metal complex oxide is easily broken when the current at the time of charge and discharge becomes high, and the reaction with the electrolyte in the non-aqueous electrolyte is promoted.

  As described above, there are points to be overcome in the application to the field which has been studied in recent years, whether it is the all solid secondary battery or the non-aqueous electrolyte secondary battery. An object of the present invention is to provide a positive electrode active material that can realize a non-aqueous secondary battery that can withstand use in fields intended for larger scale and long-term use.

  In order to achieve the above object, the present inventors have intensively studied and completed the present invention. The inventor of the present invention can increase the discharge capacity in an all solid secondary battery by forming a covering layer containing a plurality of specific elements on the surface of a core particle made of a lithium transition metal composite oxide, and charging with high current. It has been found that even if the discharge is repeated, the reaction between the electrolyte in the non-aqueous electrolyte and the positive electrode active material is suppressed.

  The positive electrode active material of the present invention comprises a core particle composed of a lithium transition metal complex oxide and a covering layer present on the surface of the core particle, wherein the covering layer is at least selected from the group consisting of boron and tungsten. It is characterized in that it is a covering layer containing one kind of element M and niobium.

  Since the positive electrode active material of the present invention has the above features, the discharge capacity in the all solid secondary battery is increased. In addition, since the positive electrode active material of the present invention has the above-described features, reaction with the electrolyte in the non-aqueous electrolytic solution is suppressed even if charge and discharge are repeated at high current. Therefore, high current cycle characteristics are improved in the non-aqueous electrolyte secondary battery.

FIG. 1 is a conceptual view of an example of a preferred embodiment for producing the positive electrode active material of the present invention.

  Hereinafter, the positive electrode active material of the present invention and the method for producing the same will be described in detail using embodiments and examples.

  The positive electrode active material of the present invention includes core particles made of a lithium transition metal composite oxide, and a covering layer present on the surface of the core particles. Hereinafter, these will be mainly described.

[Core particle]
The core particle may be a known lithium transition metal complex oxide. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate with spinel structure (LiMn ₂ O ₄ ), lithium nickel cobalt manganate (Li (Ni, Co, Mn) O ₂ ), olivine structure Lithium iron phosphate (LiFePO ₄ ) and the like.

A lithium transition metal complex oxide having a layered structure such as lithium cobaltate is preferable because it is easy to obtain a non-aqueous secondary battery having a good balance of charge and discharge capacity, energy density and the like. In particular, lithium transition metal complex oxides containing nickel, cobalt and manganese as transition metals are preferred. The content of nickel in the above three elements is about 30 mol% to about 70 mol% in terms of balance between charge and discharge capacity and easiness of synthesis, and the content of cobalt is about 10 mol% to about 35 mol% in terms of cost and output characteristics. The amount is preferably about 20 mol% to about 35 mol% in view of thermal stability and charge and discharge capacity. As transition metals other than nickel, cobalt and manganese, tungsten, zirconium, molybdenum, niobium and the like may be contained up to about 2 mol% of all transition metals according to the purpose. When expressed as a composition of the general formula _{Li a Ni 1-x-y} Co x Mn y L z O 2 (0.95 ≦ a ≦ 1.2,0.10 ≦ x ≦ 0.35,0.20 ≦ y ≦ Particularly preferred is a lithium transition metal complex oxide represented by 0.35, 0 ≦ z ≦ 0.02, L is at least one element selected from the group consisting of W, Zr, Mo and Nb.

[Covering layer]
The covering layer contains at least one element M selected from the group consisting of boron and tungsten, and niobium. When the positive electrode active material has a coating layer containing these specific at least two elements, the interfacial resistance between the solid electrolyte and the positive electrode active material is dramatically reduced, and as a result, the discharge capacity of the all solid secondary battery is increased. To increase. Moreover, the crystal structure of the whole positive electrode active material is stabilized, and even when charging and discharging with high current, the reaction between the electrolyte in the non-aqueous electrolyte and the positive electrode active material is suppressed.

The element M in the covering layer mainly contributes to the stabilization of the crystal structure of the whole positive electrode active material. If the amount of the element M is too small, the above-mentioned effects become insufficient, and if it is too large, the capacity decreases. If the element M is 0.1 mol% or more and 5.0 mol% or less with respect to the core particle, the above effect can be sufficiently exhibited by coexisting with niobium, which is preferable.

The element M in the covering layer mainly contributes to the stabilization of the crystal structure of the whole positive electrode active material. If the amount of the element M is too small, the above-mentioned effects become insufficient, and if it is too large, the capacity decreases. If the element M is 0.1 mol% or more and 7.0 mol% or less with respect to the core particle, the above effect can be sufficiently exhibited by coexisting with niobium, which is preferable.

Appropriate adjustment of the ratio of the element M to niobium in the covering layer is preferable because the effects of the element M and niobium are amplified. The preferable range of the ratio is 0.01 mol% or more of the element M to niobium . It is a range which becomes 0 mol% or less.

  Next, the method for producing the positive electrode active material of the present invention will be described. In the positive electrode active material of the present invention, a coating layer may be formed on core particles obtained by a known method by a known method.

  The preferable formation method of a coating layer is demonstrated using FIG. FIG. 1 is for explaining the concept, and each element in the figure is exaggerated, omitted, and the like.

[First coating process]
First, the first covering material 2 made of a compound containing the element M is attached to the surface of the core particle 1 ((a) in FIG. 1) to obtain the first covering particle 3 ((b) in FIG. 1) . In (b) of FIG. 1, although the 1st coating raw material 2 coat | covers the whole surface of the core particle 1 uniformly, such a form is only an example, It is not necessary to be such a form necessarily. .

  The first coating material may be any compound as long as it contains the element M. An oxide, a halide, an oxo acid salt, a hydroxide and the like can be taken. Further, in the present specification, the “compound containing the element M” also includes a single element of the element M. In addition, when a plurality of elements M are selected, an alloy of a plurality of elements M is also included.

  There is no particular limitation on the method of attaching the first coating material to the surface of the core particle. For example, the core particles and the first coating material are stirred at high speed and attached to the mechanochemical, the core particles and the first coating material are dispersed in a dispersion medium, mixed and then dried and physically attached, A method may be adopted such as mixing an aqueous solution as a precursor of one coating material and core particles and depositing a first coating material on the surface of the core particles.

[First heat treatment process]
The resulting first coated particles may be further subjected to heat treatment. The heat treatment causes the first coating material and a part of the core particles to react, and more areas of the surface of the core particles are coated with the first coating material and the like. As a result, the discharge capacity is further increased in the all solid secondary battery. Therefore, the first heat treatment step is particularly preferable in the case of obtaining a positive electrode active material for an all solid secondary battery.

It should be noted that if the heat treatment temperature in the first heat treatment step is too high, the reaction between the first coating material and the core particles may proceed, which may impair the inherent properties of the core particles. Not usually a problem if the heat treatment temperature is 5 00 ℃ or less. The effect of the heat treatment temperature is about 150 ° C. Therefore, the heat treatment temperature is preferably 150 ° C. or more and 500 ° C. or less.

[Second coating step]
The second coated raw material 4 consisting of a compound containing niobium is attached to the surface of the first coated particle 3 obtained ((c) in FIG. 1), to obtain a second coated particle 5 (see FIG. d) The positive electrode active material is used. In (d) of FIG. 1, although the 2nd coating raw material 2 coat | covers the whole surface of the 1st coated particle 3 uniformly, such a form is only an example and is not necessarily such a form. There is no need. Further, in FIG. 1D, the second coated particle 5 has a two-layered coating layer, but such a form is merely an example, and it is not necessary to be such a form. . In addition, the coating layer does not have to be clearly distinguished from the area derived from the first coating material and the area derived from the second coating material.

  About the method which adheres the compound which can be taken as a 2nd coating raw material and a 2nd coating raw material to the surface of 1st coating particle, it applies to them of a 1st coating process.

[Second heat treatment step]
The resulting second coated particles may be further subjected to heat treatment. The heat treatment is preferable because the coating becomes strong.

It should be noted that if the heat treatment temperature in the second heat treatment step is too high, the reaction between the second coating material and the first coated particles may proceed to damage the original characteristics of the coating layer. There is usually no problem if the heat treatment temperature is 500 ° C. or less. The effect of the heat treatment temperature is about 250 ° C. Therefore, the heat treatment temperature is preferably 5 00 ° C. less than 250 ° C..

  Hereinafter, the present invention will be described more specifically using examples and the like.

Lithium transition metal complex oxide represented by general formula Li _1.12 Ni _0.33 Co _0.33 Mn _0.33 W _0.005 O ₂ is used as a core particle, and 0.5 mol% as tungsten with respect to the core particle The tungsten oxide (VI) and core particles were mixed in a high speed shear mixer to obtain a first mixture. After mixing, the first mixture was heat treated in air at 400 ° C. for 10 hours to obtain first coated particles.

  A commercially available niobium oxide sol (containing 5% as niobium) is dropped to an amount of 2.0 mol% as niobium with respect to the core particles while the first coated particles are stirred with a high-speed shear mixer, and the second mixture is added. Obtained. After dropping, the second mixture was heat-treated at 350 ° C. in the atmosphere for 5 hours to obtain the intended positive electrode composition.

  Instead of tungsten oxide (VI), 0.5 mol% of boric acid as boron with respect to the core particles and the core particles were mixed by a high speed shear mixer to obtain a first mixture. After mixing, the first mixture was heat treated in air at 250 ° C. for 10 hours to obtain first coated particles. The subsequent steps were performed in the same manner as in Example 1 to obtain a target positive electrode composition.

  The lithium transition metal complex oxide in Example 1 is used as a core particle, 0.5 mol% of tungsten (VI) oxide as tungsten with respect to the core particle, and 0.5 mol% of boric acid and core particles with respect to the core particle. The mixture was mixed with a high speed shear mixer to obtain a first mixture. The subsequent steps were performed in the same manner as in Example 1 to obtain a target positive electrode composition.

  A target positive electrode composition was obtained in the same manner as in Example 2 except that the amount of boric acid mixed was 4.0 mol% as boron based on the core particles.

Comparative Example 1
The core particles in Example 1 were used for comparison.

Lithium transition metal complex oxide represented by the general formula Li _1.05 Ni _0.6 Co _0.2 Mn _0.2 Zr _0.005 O ₂ is used as a core particle, and 0.5 mol% as boron based on the core particle The boric acid was mixed with a high speed shear mixer to obtain a first mixture.

  Commercially available niobium oxide sol (containing 5% as niobium) is added dropwise to the core particles in an amount of 0.5 mol% as niobium while stirring the first coated particles with a high speed shear mixer, and the second mixture is added. Obtained. After dropping, the second mixture was heat-treated in air at 450 ° C. for 10 hours to obtain a target positive electrode composition.

Comparative Example 2
The core particles in Example 5 were used for comparison.

Comparative Example 3
A lithium transition metal complex oxide similar to that of Example 5 is used as a core particle, and a commercially available niobium oxide sol (containing 5% as niobium) is mixed with a core particle in an amount of 0.5 mol% as niobium with stirring using a high speed shear mixer. The following amount was dropped to obtain a second mixture. After dropping, the second mixture was heat-treated in air at 450 ° C. for 10 hours to obtain a target positive electrode composition.

<3. Evaluation of all solid secondary battery>
All solid secondary batteries were produced using the positive electrode active materials of Examples 1 to 4 and Comparative Example 1, and the battery characteristics were evaluated.

[3-1. Preparation of solid electrolyte]
Under an argon atmosphere, lithium sulfide and phosphorus pentasulfide were weighed so that their mass ratio would be 7: 3. The weighed material was ground and mixed in an agate mortar to obtain a sulfide glass. This was used as a solid electrolyte.

[3-2. Preparation of positive electrode]
60 parts by weight of a positive electrode active material, 36 parts by weight of a solid electrolyte, and 4 parts by weight of VGCF (gas phase grown carbon fiber) were mixed to obtain a positive electrode mixture.

[3-3. Fabrication of negative electrode]
An indium foil having a thickness of 0.05 mm was cut out into a circular shape having a diameter of 11.00 mm and used as a negative electrode.

[3-4. Assembly of evaluation battery]
A cylindrical lower die having an outer diameter of 11.00 mm was inserted into a cylindrical outer die having an inner diameter of 11.00 mm from the lower portion of the outer die. The upper end of the lower mold was fixed at a position in the middle of the outer mold. In this state, 80 mg of the solid electrolyte was charged from the top of the outer mold to the top of the lower mold. After the introduction, a cylindrical upper mold having an outer diameter of 11.00 mm was inserted from the top of the outer mold. After insertion, a pressure of 90 MPa was applied from above the upper mold to form a solid electrolyte to obtain a solid electrolyte layer. After molding, the upper mold was pulled out from the upper part of the outer mold, and 20 mg of the positive electrode mixture was charged from the upper part of the outer mold to the upper part of the solid electrolyte layer. After charging, the upper die was inserted again, and this time, a pressure of 360 MPa was applied to shape the positive electrode mixture, to form a positive electrode layer. After molding, the upper mold was fixed, the lower mold was released from fixation, and the lower mold was pulled out from the lower part of the lower mold, and the negative electrode was introduced from the lower part of the lower mold to the lower part of the solid electrolyte layer. After charging, the lower mold was inserted again, and a pressure of 150 MPa was applied from below the lower mold to mold the negative electrode, thereby forming a negative electrode layer. The lower mold was fixed in a state where pressure was applied, and a positive electrode terminal was attached to the upper mold and a negative electrode terminal was attached to the lower mold to obtain an all solid secondary battery.

[3-5. Initial discharge capacity]
Constant current constant voltage charging was performed at a current density of 0.195 μA / cm ² and a charging voltage of 4.0 V. After charging, the current density 0.195μA / cm ^2, a constant current discharge at a discharge voltage 1.9V, the discharge capacity was measured _{Q d.}

<4. Evaluation of Nonaqueous Electrolyte Secondary Battery>
A non-aqueous electrolyte secondary battery was produced using the positive electrode active materials of Example 5 and Comparative Examples 2 and 3, and the battery characteristics were evaluated.

[4-1. Preparation of positive electrode]
A positive electrode slurry was prepared by dispersing 85 parts by weight of the positive electrode composition, 10 parts by weight of acetylene black, and 5.0 parts by weight of PVDF (polyvinylidene fluoride) in NMP (normal methyl 2-pyrrolidone). The obtained positive electrode slurry was applied to an aluminum foil, dried, compression molded using a roll press, and cut into a predetermined size to obtain a positive electrode.

[4-2. Fabrication of negative electrode]
A negative electrode slurry was prepared by dispersing 97.5 parts by weight of artificial graphite, 1.5 parts by weight of CMC (carboxymethylcellulose), and 1.0 parts by weight of SBR (styrene butadiene rubber) in water. The obtained negative electrode slurry was applied to a copper foil, dried, compression molded using a roll press, and cut into a predetermined size to obtain a negative electrode.

[4-3. Preparation of non-aqueous electrolyte]
EC (ethylene carbonate) and MEC (methyl ethyl carbonate) were mixed at a volume ratio of 3: 7, and used as a solvent. Lithium hexafluorophosphate (LiPF ₆ ) was dissolved in the resulting mixed solvent to a concentration of 1 mol / l to obtain a non-aqueous electrolytic solution.

[4-4. Assembly of evaluation battery]
After lead electrodes were attached to the current collectors of the positive electrode and the negative electrode, respectively, vacuum drying was performed at 120 ° C. Subsequently, a separator made of porous polyethylene was disposed between the positive electrode and the negative electrode, and the separator was housed in a bag-like laminate pack. After storage, it was vacuum dried at 60 ° C. to remove the moisture adsorbed on each member. After vacuum drying, the non-aqueous electrolyte described above was injected into the laminate pack and sealed, to obtain a laminate-type non-aqueous electrolyte secondary battery.

[4-5. High current cycle characteristics]
The non-aqueous electrolyte secondary battery was subjected to aging with a weak current, and the positive electrode and the negative electrode were sufficiently impregnated with the electrolyte. After aging, place the battery in a thermostatic chamber set at 45 ° C., charge at a charge voltage of 4.4 V, charge current of 2.0 C (1 C: current at which discharge ends in 1 hour), discharge voltage of 2.75 V, The discharge at a discharge current of 2.0 C was regarded as one cycle, and charge and discharge were repeated. The value obtained by dividing the discharge capacity at the nth cycle by the discharge capacity at the first cycle is taken as the discharge capacity maintenance rate Qs (n) at the nth cycle. A high Qs (n) means good cycle characteristics.

[4-6. Output characteristics]
The non-aqueous electrolyte secondary battery was aged by passing a weak current, and the positive electrode and the negative electrode were sufficiently impregnated with the electrolyte. After that, discharge at high current and charge at weak current were repeated. The charge capacity at the 10th charge was taken as the total charge capacity of the battery. After the 11th discharge, the battery was charged to 40% of the total charge capacity. After the 11th charge, put the battery in a thermostat set at -25 ° C, set it for 6 hours, and then discharge it with discharge current 0.02A, 0.04A, 0.06A, and measure the voltage at each discharge did. The abscissa represents the current, and the ordinate represents the voltage. The intersection is plotted, and the absolute value of the slope of the straight line connecting the intersections is taken as the DC internal resistance R. Low R means that the output characteristics are good.

  The production conditions of Examples 1 to 4 and Comparative Example 1 are shown in Table 1, and the characteristics of the positive electrode active material and the characteristics of the all solid secondary battery using the positive electrode active material are shown in Table 2.

  As can be seen from Tables 1 and 2, the discharge capacity of the all-solid secondary battery using the positive electrode active material of Examples 1 to 4 in which the covering layer contains both the element M and niobium is extremely high. This is considered to be a result of the interface resistance between the positive electrode active material and the solid electrolyte being dramatically reduced by the coexistence of the element M and the niobium of the covering layer.

  Table 3 shows the manufacturing conditions of Example 5 and Comparative Examples 2 and 3 and Table 4 shows the characteristics of the positive electrode active material and the characteristics of the non-aqueous electrolyte secondary battery using the positive electrode active material.

  As can be seen from Tables 3 and 4, both the high current cycle characteristics and the output characteristics of the non-aqueous electrolyte secondary battery using the positive electrode active material of Example 5 in which the covering layer contains both the element M and niobium are good. In particular, the improvement of the high current cycle characteristics is considered to be the result of the presence of the element M and niobium in the coating layer, which results in a stable crystal structure that can withstand rapid charge and discharge.

  By using the positive electrode active material for a non-aqueous secondary battery of the present invention, it is possible to obtain an all-solid secondary battery having a high discharge capacity even when taking out a large current. Alternatively, it is possible to obtain a non-aqueous electrolyte secondary battery which can be used for a long time even if charge and discharge are repeated with a large current. The non-aqueous secondary battery obtained in this manner can be particularly suitably used as a power source for large-sized devices such as electric vehicles.

1 core particle 2 first coated material 3 first coated particle 4 second coated material 5 second coated particle

Claims (9)

Core particles comprising lithium transition metal complex oxide,
A first covering layer present on the surface of the core particle;
Including a second covering layer present on the surface of the first covering layer,
The first covering layer contains at least one element M selected from the group consisting of boron and tungsten,
The second covering layer contains niobium,
The lithium transition metal complex oxide has a layered structure,
Formula _{Li a Ni 1-x-y} Co x Mn y L z O 2 (0.95 ≦ a ≦ 1.2,0.10 ≦ x ≦ 0.35,0.20 ≦ y ≦ 0.35,0 The positive electrode active material for non-aqueous secondary batteries represented by <= z <= 0.02, L is at least 1 type of element selected from the group which consists of W, Zr, Mo, and Nb. The positive electrode active material according to claim 1 , wherein the element M in the first covering layer is 0.1 mol% or more and 5.0 mol% or less with respect to the core particle. Wherein said niobium in the second coating layer is not less than 0.1 mol% 7.0 mol% or less with respect to the core particles, the positive active material of claim 1 or 2. 0.01 mol% or more of the element M in the first and second covering layers with respect to the niobium 50 . The positive electrode active material according to any one of claims 1 to 3 , which is 0 mol%. A non-aqueous system comprising a core particle comprising a lithium transition metal complex oxide, and a covering layer present on the surface of the core particle and containing at least one element M selected from the group consisting of boron and tungsten and niobium. A method of manufacturing a positive electrode active material for a secondary battery,
A first coating step of depositing a first coating material comprising a compound containing an element M on the surface of the core particle to obtain a first coated particle;
A second coating step of depositing a second coating material consisting of a compound containing niobium on the surface of the first coated particle to obtain a second coated particle;
And the lithium transition metal complex oxide has a layered structure,
Formula _{Li a Ni 1-x-y} Co x Mn y L z O 2 (0.95 ≦ a ≦ 1.2,0.10 ≦ x ≦ 0.35,0.20 ≦ y ≦ 0.35,0 A manufacturing method represented by ≦ z ≦ 0.02, L is at least one element selected from the group consisting of W, Zr, Mo and Nb). The method according to claim 5 , further comprising a first heat treatment step of heat treating the first coated particles. The manufacturing method according to claim 5 or 6 , further comprising a second heat treatment step of heat treating the second coated particles. The manufacturing method according to claim 6 whose heat treatment temperature in said 1st heat treatment process is 150 ° C or more and 500 ° C or less. The manufacturing method according to claim 7 whose heat treatment temperature in said 2nd heat treatment process is 250 ° C or more and 500 ° C or less.

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