JP2016031852A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

A non-aqueous electrolyte secondary battery including a positive electrode active material exhibiting a high charge / discharge voltage and having improved cycle deterioration and life characteristics is provided.
A non-aqueous electrolyte secondary battery having a positive electrode including positive electrode active material particles composed of a transition metal oxide exhibiting a charge / discharge potential of 4.5 V or more as a metal lithium counter electrode potential, the positive electrode active material but on its surface, with a coating layer comprising LiTi ₂ O ₄ having a spinel crystal structure, a non-aqueous electrolyte secondary battery.
[Selection figure] None

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery including positive electrode active material particles composed of a transition metal oxide.

As a positive electrode material for a lithium ion secondary battery, a positive electrode material exhibiting a high charge / discharge voltage of 4.5 V or more has attracted attention. As a positive electrode material exhibiting such a high charge / discharge voltage, for example, LiNi _0.5 Mn _1.5 O ₄ described in Patent Documents 1 and 2 is known. However, in a battery including a positive electrode material exhibiting such a high charge / discharge voltage, a current and a part of the electrolytic solution are consumed due to the electrolytic oxidation decomposition of the electrolytic solution at a high potential, and a by-product of the electrolytic oxidation decomposition is generated. Since the discharge capacity is reduced by adversely affecting the electrodes, the cycle deterioration is large, and there is a problem in terms of battery life characteristics.

JP 2014-096326 A JP 2003-142101 A

  In view of the above problems, the present invention is a non-aqueous electrolyte secondary battery including a positive electrode active material exhibiting a high charge / discharge voltage, which has improved cycle deterioration and thus improved life characteristics. An object is to provide a secondary battery.

The present inventors can suppress electrolytic oxidative decomposition of the electrolytic solution by forming a coating layer containing LiTi ₂ O ₄ on the surface of the positive electrode active material particles composed of the high potential transition metal oxide. I found. Since electrolytic oxidative decomposition of the electrolytic solution can be suppressed, deterioration due to the charge / discharge cycle of the secondary battery is suppressed, and the life of the secondary battery can be extended.

In one embodiment, the present invention is a non-aqueous electrolyte secondary battery having a positive electrode including positive electrode active material particles composed of a transition metal oxide exhibiting a charge / discharge potential of 4.5 V or more at a metal lithium counter electrode potential. Then, the positive electrode active material particle is a non-aqueous electrolyte secondary battery having a coating layer containing LiTi ₂ O ₄ having a spinel crystal structure on the surface thereof.

FIG. 1 shows the discharge capacities after the secondary batteries of Examples 1 to 3 and the comparative example were stored at 60 ° C. for 7 days. FIG. 2 shows initial discharge curves of Examples 1 and 2 and a comparative example.

In the non-aqueous electrolyte secondary battery of the present invention, the positive electrode active material is composed of a transition metal oxide that exhibits a charge / discharge potential of 4.5 V or more as a metal lithium counter electrode potential. Specific examples of the positive electrode active material exhibiting such a high charge / discharge potential include positive electrode active materials having a spinel crystal structure such as LiNi _0.5 Mn _1.5 O ₄ , LiFe _0.5 Mn _1.5 O ₄ , LiCo _0.5 Mn _1.5 O ₄ and LiCr _0.5. Examples include Mn _1.5 O ₄ and positive electrode active materials having a non-spinel crystal structure, such as LiCoPO ₄ and LiNiPO ₄ . The positive electrode active material particles preferably have an average particle size of 0.1 to 5 μm. Here, the average particle diameter of the positive electrode active material means a particle diameter d50 corresponding to a 50% cumulative value on the basis of the number in the particle size distribution of the area equivalent circle diameter obtained by image analysis of a scanning electron microscope (SEM). . The positive electrode active material composed of the transition metal oxide can be produced by a method known in the technical field such as a solid phase method, a sol-gel method, a hydrothermal synthesis method, and a coprecipitation method, and is not particularly limited. For example, when the positive electrode active material is produced by the sol-gel method, a solution containing the raw material in a predetermined ratio is prepared, the raw material is hydrolyzed and polymerized in the solution to form a sol, and further subjected to hydrolysis and polymerization reaction by heating. The positive electrode active material can be obtained by proceeding to gelation, preliminarily firing and pulverizing the obtained precursor, followed by main firing. The main firing temperature is desirably 500 to 900 ° C, and more desirably 600 to 800 ° C. The firing time is desirably 0.5 to 5 hours, and more desirably 1 to 3 hours. The higher the firing temperature and the longer the firing time, the more the particles grow.

The positive electrode active material particles described above, on the surface thereof, having a coating layer containing the LiTi ₂ O ₄ having a spinel crystal structure. When the positive electrode active material particles are composed of a transition metal oxide having a spinel crystal structure, both the transition metal oxide and LiTi ₂ O ₄ constituting the positive electrode active material particles have a spinel crystal structure. at the interface between the coating layer comprising a surface and LiTi ₂ O ₄ of the material particles, with good bonding properties to the surface of the positive electrode active material particle LiTi ₂ O ₄ can be joined, as a result, Li ion conductivity Can be increased. The coating layer containing LiTi ₂ O ₄ having a spinel crystal structure preferably has a thickness of 0.1 to 50 nm. Here, the positive electrode active material having a coating layer containing LiTi ₂ O ₄ having a spinel crystal structure on the surface, for example, is obtained by dispersing positive electrode active material particles in an alcohol solution containing titanium tetraisopropoxide, Propoxide can be hydrolyzed, dried, and fired at 500 ° C. for 1 hour.

In the present invention, the coating layer containing LiTi ₂ O ₄ preferably further contains a conductive agent selected from TiN, TiC, or a combination thereof. When the coating layer contains a conductive agent selected from TiN, TiC, or a combination thereof, electrolytic oxidation decomposition of the electrolytic solution can be suppressed, and conductivity can be improved. TiN and TiC, which are conductive agents, can have a spherical or fibrous form. When the conductive agent is spherical, it preferably has an average particle size of 0.1 to 100 nm. Here, the average particle diameter of the conductive agent means a particle diameter d50 corresponding to a 50% cumulative value on the basis of the number in the particle size distribution of the equivalent area circle diameter obtained by image analysis of a scanning electron microscope (SEM). When the coating layer containing LiTi ₂ O ₄ contains a conductive agent selected from TiN, TiC, or a combination thereof, the amount of the conductive agent is preferably 0.5 to 10 with respect to 100 parts by mass of the positive electrode active material. Part by mass.

  In addition to the positive electrode active material, the positive electrode mixture may include optional components known in the art, for example, a conductive agent other than TiN and TiC, a binder, a solid electrolyte, and the like. Examples of conductive agents other than TiN and TiC include particulate or fibrous metals, particulate or fibrous conductive oxides, and the like. The binder is not particularly limited as long as it can impart flexibility to the positive electrode. Examples of binders include, for example, fluorine-based binders such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE), rubber-based binders such as styrene butadiene rubber (SBR) and butadiene rubber (BR), and carboxymethyl cellulose. , Polyacrylic acid, polyimide, polyamic acid, polyamideimide and the like. The ratio of each component constituting the positive electrode and the thickness of the positive electrode mixture layer are not particularly limited.

  The positive electrode can be obtained by forming a layer of the positive electrode mixture on at least one surface of the positive electrode current collector by any method known in the art, and the method for forming the positive electrode is not particularly limited. The positive electrode current collector is not particularly limited as long as it is known in the art as being capable of functioning as a current collector. Examples of the material constituting the current collector include aluminum, stainless steel (SUS), nickel, iron, copper, and the like. The thickness of the positive electrode current collector can be changed according to their intended use and is not particularly limited, but is typically 5 μm to 30 μm. The positive electrode mixture may be any method known in the art using an appropriate solvent as necessary, for example, electrostatic coating, dip coating, spray coating, roll coating, doctor blade method. It can be applied to the surface of the positive electrode current collector by a gravure coating method, a screen method or the like. For example, a positive electrode mixture paste or slurry containing a positive electrode active material and, if necessary, optional components such as a conductive material and a binder and a solvent is prepared, and the obtained positive electrode mixture paste or slurry is used as a positive electrode current collector. After being applied to the surface of the substrate, it is dried and pressed to obtain a positive electrode having a positive electrode mixture layer on the positive electrode current collector. The density of the positive electrode mixture layer on the positive electrode current collector is preferably 1.3 to 2.7 g / cc. If the density of the positive electrode mixture layer is too low, it is difficult to ensure an electron conduction path. The higher the density of the positive electrode mixture layer, the smaller the amount of electrolyte solution that can be present in the positive electrode mixture layer, and hence the smaller the amount of Li ions that move between the electrolyte solution and the positive electrode active material particles. Therefore, the density of the positive electrode mixture layer determines the Li ion conduction in the electrode.

The negative electrode can be made of metallic lithium or a lithium alloy such as LiSn alloy, LiAl alloy, LiSi alloy, or the like, or have a negative electrode mixture layer on the surface of the negative electrode current collector. The negative electrode mixture includes a negative electrode active material. As the negative electrode active material, any active material known to function as a negative electrode active material in the technical field can be used. Examples of the negative electrode active material include metal-based active materials such as elemental metals such as In, Al, Si, and Sn and alloys thereof, inorganic oxides such as Li ₄ Ti ₅ O ₁₂ , carbon-based active materials such as Examples include mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. In addition to the negative electrode active material, the negative electrode mixture is an optional component known in the art as a constituent of the negative electrode mixture for a non-aqueous electrolyte secondary battery, for example, a conductive agent, a binder. In addition, a solid electrolyte may be included. Examples of the conductive agent, the binder, and the solid electrolyte include those exemplified above for the positive electrode, but are not limited to those exemplified. The ratio of each component constituting the negative electrode and the thickness of the negative electrode mixture layer are not particularly limited.

  The negative electrode current collector is not particularly limited as long as it is known in the technical field as being capable of functioning as a current collector, and can be composed of the same material as exemplified for the positive electrode current collector. The thickness of the negative electrode current collector can be changed according to their intended use. Examples of the method of applying to the surface of the negative electrode current collector include an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, and a screen method. It is not limited. As in the case of forming the positive electrode, the negative electrode is prepared by preparing a negative electrode mixture paste or slurry containing, for example, a negative electrode active material, optional components such as a conductive material, a binder, and a solvent, and a solvent. Alternatively, the negative electrode layer having the negative electrode mixture layer on the negative electrode current collector can be obtained by applying the slurry to the surface of the negative electrode current collector and then drying the slurry.

In the present invention, the electrolyte constituting the non-aqueous electrolyte is lithium ion conductive and is dissolved in a non-aqueous solvent, such as a lithium salt electrolyte such as lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate ( LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium bis (pentafluoroethanesulfonyl) imide (LiN (C ₂ F ₅ SO ₂ ) ₂ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), bis (trifluoromethanesulfonyl) imidolithium (LiN (CF ₃ SO ₂ ) ₂ ), lithium tris (trifluoromethanesulfonyl) methide (LiC (CF ₃ SO ₂ ) ₃ ), lithium chloride (LiCl) And lithium bromide (LiBr). The non-aqueous solvent constituting the electrolytic solution is preferably an aprotic polar organic solvent. For example, a high-dielectric constant, high-viscosity cyclic carbonate compound such as ethylene carbonate (EC) or propylene carbonate (PC), and dimethyl carbonate ( Low viscosity chain carbonate compounds such as DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) are preferred. The concentration of the lithium salt in the electrolytic solution is usually 0.5 to 2 mol / L.

  A separator will not be specifically limited if it is a porous film | membrane known to function as a separator in the said technical field. Examples of the material constituting the separator include an organic material such as a porous membrane made of polyolefin such as polypropylene (PP) and polyethylene (PE), and an inorganic material such as a porous membrane made of ceramic. There is no restriction | limiting in particular in the thickness of a porous membrane. The porous film may be composed of one or more organic or inorganic materials, and may be a composite film composed of an organic material and an inorganic material. There are no particular restrictions on the number, material, and thickness of the membranes that make up the composite membrane. The separator can have a single layer structure or a multilayer structure. Examples of the separator having a multilayer structure include a PP / PE two-layer structure or a PP / PE / PP or PE / PP / PE three-layer structure separator.

  The nonaqueous electrolyte secondary battery obtained by using the negative electrode mixture of the present invention has a positive electrode current collector, a positive electrode terminal connected to the negative electrode current collector, a negative electrode terminal, and the like in addition to the above-described constituent members. be able to. The structure of the negative electrode and the positive electrode can be a stacked structure in which flat positive electrodes and negative electrodes are alternately stacked, or a strip of positive electrodes and negative electrodes can be stacked as long as the negative electrode and the positive electrode face each other with a separator and an electrolyte interposed therebetween. It may be wound into a roll and formed into a wound type structure. The types and shapes of these constituent members can be appropriately selected according to the application of the nonaqueous electrolyte secondary battery. Examples of the shape of the nonaqueous electrolyte secondary battery include, but are not limited to, a coin shape, a cylindrical shape, and a square shape.

  The present invention will be described in more detail with reference to the following Examples and Comparative Examples, but it goes without saying that the scope of the present invention is not limited by these Examples.

[Comparative example]
<Manufacture of positive electrode active material>
A particulate positive electrode active material was synthesized by the sol-gel method according to the following procedure. Lithium acetate dihydrate, nickel acetate tetrahydrate and manganese acetate tetrahydrate weighed so that the molar ratio of Li, Ni and Mn is 1: 0.5: 1.5 (all Nacalai Tesque Co., Ltd.) was added to 1 L of pure water in a beaker and dissolved while adjusting the pH to 1.5 or less with concentrated nitric acid. Next, 5 times the amount of glycolic acid (manufactured by Nacalai Tesque Co., Ltd.) as much as the molar amount of the target LiNi _0.5 Mn _1.5 O ₄ was dissolved in the obtained solution. This glycolic acid was used as a chelating agent for inhibiting particle growth. A beaker was immersed in an oil bath at 80 ° C., and water was evaporated from the solution while stirring the solution for about 20 hours to obtain a gel-like active material precursor. This precursor was put into a drying oven at 80 ° C. and further dried for 24 hours. Next, the dried precursor was temporarily fired at 200 ° C. in an argon atmosphere. The obtained powder was pulverized in a mortar and then calcined at 600 ° C. for 1 hour in an argon atmosphere to obtain particulate LiNi _0.5 Mn _1.5 O ₄ (average particle size 1 μm). In the comparative example, this particle was used as a positive electrode active material.
<Preparation of positive electrode>
The positive electrode active material obtained as described above, a conductive agent (acetylene black), and a binder (polyvinylidene fluoride (PVdF)) are dispersed in N-methyl-2-pyrrolidone (manufactured by Nacalai Tesque). This gave a slurry. The mass ratio of the positive electrode active material, acetylene black, and PVdF was 75: 20: 5. The obtained slurry was applied onto an aluminum foil (thickness 15 μm) by a doctor blade method, dried at 80 ° C. for 30 minutes, and pressed with a roll press so that the density of the coated layer after drying was 2 g / cc. And vacuum drying at 120 ° C. to obtain a positive electrode.
<Preparation of non-aqueous electrolyte>
By dissolving lithium salt LiPF ₆ as an electrolyte at a ratio of 1 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl carbonate (EMC) at a volume ratio of 3: 3: 4, non-water An electrolyte solution was prepared.
<Production of negative electrode>
Foil-like metal lithium (thickness: 500 μm) was punched into a circle having a diameter of 16 mm to obtain a negative electrode.
<Separator>
A laminated porous film of polypropylene (PP) and polyethylene (PE) (manufactured by Ube Industries) was used as a separator.
<Production of battery>
A coin-type lithium secondary battery was fabricated by incorporating the battery constituent member into a stainless steel 2032 type (diameter 20 mm, thickness 3.2 mm) coin cell.

[Example 1]
Cathode active material LiNi _0.5 Mn _1.5 O ₄ particles (average particle size 1 μm) produced as described in Comparative Examples were dispersed in an anhydrous ethanol solution of titanium tetraisopropoxide. Water was added to the obtained dispersion, and titanium tetraisopropoxide was hydrolyzed while stirring at 80 ° C. The reaction was continued until water was consumed by the hydrolysis reaction and completely disappeared. The amount of titanium tetraisopropoxide was the theoretical amount necessary for the mass ratio of LiNi _0.5 Mn _1.5 O ₄ and LiTi ₂ O ₄ derived from titanium tetraisopropoxide to be 95: 5. After the hydrolysis reaction, the obtained product was dried and heat-treated at 500 ° C. for 1 hour to obtain positive electrode active material particles coated with LiTi ₂ O ₄ . The average thickness of the LiTi ₂ O ₄ coating layer on the surface of the positive electrode active material particles was about 10 nm when determined with a transmission electron microscope. Using the obtained positive electrode active material particles coated with the LiTi ₂ O ₄ coating layer, a secondary battery was produced in the same manner as in the comparative example.

[Example 2]
The positive electrode active material particles coated with LiTi ₂ O ₄ produced as described in Example 1 were mixed with TiN fine particles (average particle size 50 nm) as a conductive agent at 300 rpm for 20 hours by a ball mill. Next, the obtained powder was fired at 500 ° C. for 1 hour in an argon gas atmosphere to obtain positive electrode active material particles coated with a LiTi ₂ O ₄ coating layer containing TiN fine particles. Using the obtained positive electrode active material particles, a secondary battery was produced in the same manner as in the comparative example.

[Example 3]
A secondary battery was fabricated in the same manner as in Example 2 except that TiC fine particles (average particle size 50 nm) were used instead of TiN fine particles as the conductive agent.

The following characteristics evaluation was performed about the secondary battery produced in each case.
(1) Initial discharge capacity Using a charge / discharge tester, after charging up to 5.0 V with a current value of 0.1 C (1 C is a current value that can be fully charged in 1 hour), up to 3.5 V, 0 Discharge was performed at a current of 1 C.
(2) Life characteristics In order to evaluate the life characteristics, the secondary battery was charged as follows, and then stored at 60 ° C for 7 days, and then the discharge capacity was determined.
The secondary battery was charged in a constant current / constant voltage mode at an upper limit of 4.0 V at 0.1 C with respect to an actual capacity of 150 mAh / g at a temperature of 2.5 ° C. Thereafter, the charged secondary battery was stored at 60 ° C. for 7 days, and then discharged at 0.1 C to 3 V, and the discharge capacity was determined. The results are shown in Table 1 below.

The results of Table 1 are shown as a bar graph in FIG. 1, and the initial discharge capacities obtained for the secondary batteries of Comparative Example and Examples 1 and 2 are shown in FIG.
In the secondary battery of the comparative example, Li self-discharged due to the decomposition of the electrolytic solution, and the discharge capacity after storage at 60 ° C. for 7 days was a low value of 50 mAh / g. In the secondary battery of Example 1, the positive electrode active material particles were coated with LiTi ₂ O _{4, and} as a result, the initial discharge capacity decreased as shown in FIG. Since the surface of the material particles was coated with LiTi ₂ O ₄ , electrolytic oxidation decomposition of the electrolytic solution was suppressed, and the discharge capacity after storage at 60 ° C. for 7 days was a high value of 70 mAh / g. In the secondary battery of Example 2 in which the surface of the positive electrode active material particles was coated with a LiTi ₂ O ₄ coating layer containing TiN fine particles as a conductive agent, the initial discharge capacity and the discharge capacity after storage at 60 ° C. for 7 days were Both improved compared to the comparative example. The secondary battery of Example 3 in which the surface of the positive electrode active material particles was coated with a LiTi ₂ O ₄ coating layer containing TiC fine particles as a conductive agent was also compared with the comparative example in terms of discharge capacity after storage at 60 ° C. for 7 days. Improved.
From these results, it can be seen that the life characteristics can be improved by using a Li ion conductor such as LiTi ₂ O ₄ having a spinel crystal structure as in the case of the positive electrode active material. It can also be seen that the life characteristics can be further improved by combining a Li ion conductor having a spinel crystal structure with a stable conductive agent such as TiN or TiC.

Claims (3)

A non-aqueous electrolyte secondary battery having a positive electrode including positive electrode active material particles composed of a transition metal oxide having a charge / discharge potential of 4.5 V or more at a metal lithium counter electrode potential, wherein the positive electrode active material has a surface thereof A nonaqueous electrolyte secondary battery having a coating layer containing LiTi ₂ O ₄ having a spinel crystal structure. The non-aqueous electrolyte secondary battery according to claim 1, wherein the transition metal oxide exhibiting a charge / discharge potential of 4.5 V or more as a metal lithium counter electrode potential is LiNi _0.5 Mn _1.5 O ₄ . The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the coating layer containing LiTi ₂ O ₄ further contains a conductive agent selected from TiN, TiC, or a combination thereof.

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