Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolytic solution secondary battery includes a positive electrode having a positive electrode mixture layer, a negative electrode, and a non-aqueous electrolytic solution. The positive electrode mixture layer contains a positive electrode active material and inert particles. The positive electrode active material contains a lithium-containing complex oxide. The average particle diameter D1 of the positive electrode active material and the average particle diameter D2 of the inert particles satisfy D1>D2. The non-aqueous electrolytic solution has a viscosity of less than 2 mPa·s at 30ºC.

Description

Technical Field

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

Background Art

[0002] A non-aqueous electrolyte secondary battery typified by a lithium-ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode contains a positive electrode active material.

[0003] Patent Document 1 discloses a non-aqueous electrolyte secondary battery including a positive electrode plate having a positive electrode mixture layer containing a positive electrode active material, a negative electrode plate, and a non-aqueous electrolyte containing an electrolyte salt in a non-aqueous solvent. The positive electrode active material is Li x Ni 1-y M y O z (0.9 < x ≤ 1.2, 0 < y ≤ 0.7, 1.9 < z ≤ 2.1, M is an element containing at least one of Al and Co), which is a lithium nickel composite oxide. Ceramic particles are attached to the surface of the particles of the positive electrode active material, and a copolymer of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene is contained in the positive electrode mixture layer.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] Patent Document 1 aims to provide a non-aqueous electrolyte secondary battery in which gas generation due to the reaction between the positive electrode and the non-aqueous electrolyte during high-temperature charge storage is suppressed when a lithium nickel composite oxide is used as the positive electrode active material.

[0006] On the other hand, in a non-aqueous electrolyte secondary battery, it is required to increase the mobility of ions in the electrode and improve the load characteristics.

[0007] One aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode having a positive electrode mixture layer, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode mixture layer includes a positive electrode active material and inactive particles, the positive electrode active material includes a lithium-containing composite oxide, an average particle diameter D1 of the positive electrode active material and an average particle diameter D2 of the inactive particles satisfy D1>D2, and the viscosity of the non-aqueous electrolyte at 30°C is less than 2 mPa s.

[0008] According to the present disclosure, it is possible to improve the load characteristics while suppressing side reactions in a nonaqueous electrolyte secondary battery. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a partially cutaway plan view that illustrates a schematic structure of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure. [Diagram 2] FIG. 2 is a cross-sectional view taken along line XX' of the nonaqueous secondary battery shown in FIG. [Diagram 3] FIG. 3 is a graph showing the relationship between the viscosity of the non-aqueous electrolyte and the capacity obtained by high-rate discharge. [Figure 4] FIG. 4 is an enlarged view of a portion of the graph of FIG. [Diagram 5] FIG. 5 is a graph showing the Log differential pore size distribution of the positive electrode mixture layer of evaluation cell A1 and cell B1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] A nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode having a positive electrode mixture layer, a negative electrode, and an electrolyte. The positive electrode mixture layer includes a positive electrode active material and inactive particles. The positive electrode active material includes a lithium-containing composite oxide. An average particle diameter D1 of the positive electrode active material and an average particle diameter D2 of the inactive particles satisfy D1>D2. The viscosity of the electrolyte at 30°C is less than 2 mPa·s.

[0011] The positive electrode active material has high hardness, and even when densely packed in the positive electrode mixture layer, voids of various sizes may be formed between the particles of the positive electrode active material. Among them, the lithium-containing composite oxide often forms substantially spherical secondary particles, and voids are easily formed in the positive electrode mixture layer.

[0012] On the other hand, when the positive electrode mixture layer contains a positive electrode active material and inactive particles, and the average particle diameter D1 of the positive electrode active material and the average particle diameter D2 of the inactive particles satisfy D1>D2, the inactive particles fill the relatively large gaps between the particles of the positive electrode active material, and the size of the gaps is made uniform. This increases the number of fine paths through which lithium ions can move, and reduces the movement distance of the lithium ions that contribute to the reaction in the positive electrode mixture layer. As a result, the load characteristics of the nonaqueous electrolyte secondary battery are improved. For example, when high-rate discharge is performed, the discharge capacity is improved.

[0013] When D1≦D2, the inactive particles cannot be expected to have the effect of reducing the relatively large voids between the particles of the positive electrode active material and making the size of the voids uniform.

[0014] Inactive particles filling the gaps between particles of the positive electrode active material usually do not contribute to the charge / discharge reaction, and are not involved in the side reactions of non-aqueous electrolyte secondary batteries. Therefore, excessive film formation due to the progression of side reactions is unlikely to occur, and the fine paths through which lithium ions move are unlikely to be blocked. In addition, by suppressing side reactions, gas generation accompanying charge / discharge cycles is also suppressed.

[0015] However, the improved discharge performance during high-rate discharge (hereinafter referred to as high-rate discharge performance) is an effect specific to the case where the viscosity of the non-aqueous electrolyte is low. Specifically, the viscosity of the non-aqueous electrolyte at 30°C must be less than 2 mPa·s. If the viscosity of the non-aqueous electrolyte at 30°C is 2 mPa·s or higher, the discharge capacity during high-rate discharge drops drastically. This is thought to be because, when the viscosity of the non-aqueous electrolyte increases to a certain level, the non-aqueous electrolyte's ability to circulate around the migration paths of fine lithium ions drops.

[0016] The lower the viscosity of the nonaqueous electrolyte at 30°C, the more desirable it is, and for example, when the viscosity is 1.9 mPa s or less, the effect of improving the high-rate discharge performance becomes significant. Furthermore, when the viscosity of the nonaqueous electrolyte at 30°C is 1.5 mPa s or less, or even 1.3 mPa s or less, the effect of improving the high-rate discharge performance becomes even more significant.

[0017] (Viscosity measurement of non-aqueous electrolyte) The viscosity of the non-aqueous electrolyte at 30° C. can be determined, for example, by a microchip differential pressure type viscometer (for example, Viscometer-Rheometer-on-a-Chip (m-VROC) manufactured by RheoSense Inc.).

[0018] The positive electrode active material (especially the lithium-containing composite oxide) usually has a form of secondary particles formed by aggregation of primary particles. The average particle diameter D1 of the positive electrode active material may be, for example, 2 μm or more and 20 μm or less, or 4 μm or more and 15 μm or less.

[0019] The average particle diameter D2 of the inactive particles depends on the average particle diameter D1 of the mixed positive electrode active material, but may be, for example, 0.1 μm or more and 10 μm or less, or 0.5 μm or more and 5 μm or less. Here, the average particle diameter refers to the median diameter at which the cumulative volume in the volume-based particle size distribution is 50%. The volume-based particle size distribution can be measured by a laser diffraction type particle size distribution measuring device. By setting the average particle diameter D2 of the inactive particles to 0.1 μm or more, the dispersibility of the inactive particles when mixed with the positive electrode active material is improved, and by setting it to 10 μm or less, the inactive particles are easily filled into the relatively large gaps between the particles of the positive electrode active material.

[0020] The ratio D1 / D2 of the average particle diameter D1 to the average particle diameter D2 may be, for example, 2 to 50, or 5 to 30. When D1 / D2 is within the above range, the relatively large gaps between the particles of the positive electrode active material are easily filled with the inactive particles, and the size of the gaps is easily made more uniform.

[0021] In the positive electrode mixture layer, the amount of inactive particles in the total of the positive electrode active material and the inactive particles may be, for example, 0.1% by mass or more and 15% by mass or less, 0.5% by mass or more and 10% by mass or less, or 0.5% by mass or more and 5% by mass or less. In such a range, the inactive particles are generally more likely to be preferentially filled in the space in the positive electrode mixture layer that is not filled with the positive electrode active material (i.e., the space that does not contribute to the capacity), and the space that should be occupied by the positive electrode active material is less likely to be eroded by the inactive particles. Therefore, since the space that does not contribute to the capacity can be effectively used, the positive electrode capacity is sufficiently secured even when the positive electrode mixture layer contains the inactive particles.

[0022] Here, the inactive particles are particles of a material that is substantially electrochemically inactive, specifically, particles of a material with a theoretical capacity per unit mass of 10 mAh / g or less. As the inactive particles, it is desirable to use ceramic particles that are stable and inexpensively available in the battery. In addition, ceramic particles are advantageous over carbon materials such as carbon black used as a conductive material because they retain their shape and tend to maintain voids in the positive electrode mixture layer even when rolling is performed to increase the density of the positive electrode mixture layer.

[0023] Examples of electrochemically inactive ceramics include silica, alumina, titania, magnesia, zirconia, etc. Among them, at least one selected from the group consisting of silica, alumina, and titania is preferable because of easy availability.

[0024] The effect of improving the high-rate discharge performance becomes more prominent as the thickness of the positive electrode mixture layer increases. In other words, the greater the thickness of the positive electrode mixture layer, the greater the absolute distance traveled by lithium ions, and therefore shortening the distance traveled is important for improving the load characteristics of the nonaqueous electrolyte secondary battery. Specifically, when the thickness of the positive electrode mixture layer is 100 μm or more (even 110 μm or more or 120 μm or more), the degree of improvement in the high-rate discharge characteristics due to the synergistic effect of the use of inactive particles that satisfy D1>D2 and the use of a low-viscosity nonaqueous electrolyte with a viscosity of 2 mPa·s or less at 30°C tends to be particularly prominent. However, from the viewpoint of suppressing the decrease in the liquid circulation and realizing the above synergistic effect, it is desirable to set the thickness of the positive electrode mixture layer to 300 μm or less.

[0025] In order to make the size of the voids uniform, it is necessary that the inactive particles present in only a small amount in the positive electrode mixture layer are efficiently filled into the voids. Therefore, unlike the proposal of Patent Document 1 mentioned above, it is not necessary to attach the inactive particles to the surface of the positive electrode active material. The coverage Rc of the positive electrode active material by the inactive particles may be 30% or less.

[0026] The coverage rate Rc is obtained from the image data of elemental mapping of the cross section of the positive electrode mixture layer. In the image data, inactive particles present at a position more distant from the particle surface of the positive electrode active material than a distance d corresponding to 3% of the average particle diameter D1 of the positive electrode active material cannot be said to be attached to the surface of the positive electrode active material. Therefore, when a curve distanced by d from the particle surface of the positive electrode active material is drawn along the particle surface of the positive electrode active material in the image data, the inactive particles present in the region A between the curve and the particle surface of the positive electrode active material are regarded as inactive particles attached to the positive electrode active material. At this time, the coverage rate Rc is the ratio of the area corresponding to the inactive particles present in the region A to the total area corresponding to the inactive particles in the image data. At this time, image data in which five or more particles of the positive electrode active material with a maximum diameter of the average particle diameter D1 ± 20% can be confirmed and at least two of the particles can be confirmed in their entirety is used.

[0027] Although it is difficult to pack the lithium-containing composite oxide into the positive electrode mixture layer at a high density, it is desirable to increase the density of the positive electrode mixture layer as much as possible in order to increase the capacity. Generally, the density of the positive electrode mixture layer is, for example, 2 g / cm 3 More than 4g / cm 3 or less, and 3 g / cm for higher density. 3 More than 4g / cm 3 The density (d) of the positive electrode mixture layer is set in the following range: For example, a positive electrode piece of a predetermined size is cut out from a positive electrode, the thickness (t) and area (S) of the positive electrode mixture layer of the positive electrode piece are measured, and the mass (M) of the positive electrode mixture layer of the positive electrode piece is measured, and the density (d) of the positive electrode mixture layer is calculated from the formula: d = M / (t x S).

[0028] The porosity of the positive electrode mixture layer is, for example, 15 volume % or more and 30 volume % or less. The porosity of the positive electrode mixture layer is calculated from the apparent volume of the positive electrode mixture layer, the composition of the positive electrode mixture layer, and the true specific gravity of the material contained in the positive electrode mixture layer.

[0029] The nonaqueous electrolyte secondary battery according to the present disclosure will be described in further detail below. The nonaqueous electrolyte secondary battery includes, for example, a positive electrode, a negative electrode, a nonaqueous electrolyte, and a separator as described below.

[0030] [Positive electrode] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer of the above-mentioned configuration formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed, for example, by applying a positive electrode slurry in which a positive electrode mixture containing a positive electrode active material, inactive particles, a binder, etc. is dispersed in a dispersion medium to the surface of the positive electrode current collector and drying it. The coating film after drying may be rolled as necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0031] The positive electrode mixture layer contains a positive electrode active material as an essential component, and contains optional components such as a binder, a conductive material, a thickener, etc. As the binder, the conductive material, the thickener, etc., known materials can be used.

[0032] The positive electrode active material contains a lithium-containing composite oxide. The lithium-containing composite oxide is not particularly limited, but those having a layered rock salt-type crystal structure containing lithium and a transition metal are promising. Specifically, the lithium-containing composite oxide may be, for example, Li a Ni 1-x-y Co x M y O 2 (where 0 < a ≤ 1.2, 0 ≤ x ≤ 0.1, 0 ≤ y ≤ 0.1, 0 < x + y ≤ 0.1, and M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Cu, Zn, Al, Cr, Pb, Sb, and B). From the viewpoint of the stability of the crystal structure, M may contain Al. The value of a indicating the molar ratio of lithium increases or decreases during charge and discharge. As a specific example, LiNi 0.9 Co 0.05 Al 0.05 O 2 , LiNi 0.91 Co 0.06 Al 0.03 O 2 etc. may be mentioned.

[0033] For the positive electrode current collector, for example, a metal sheet or a metal foil is used. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc.

[0034] [Negative electrode] The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer can be formed, for example, by applying a negative electrode mixture containing a negative electrode active material, a binder, etc. dispersed in a dispersion medium to the surface of the negative electrode current collector and drying it. The dried coating film may be rolled if necessary. That is, the negative electrode active material may be a negative electrode mixture layer. The negative electrode active material layer may be formed on one surface of the negative electrode current collector or on both surfaces.

[0035] Also, the negative electrode active material layer may be a lithium metal foil or a lithium alloy foil. In this case, the negative electrode current collector is not essential.

[0036] The negative electrode mixture layer contains a negative electrode active material as an essential component, and contains optional components such as a binder, a conductive material, a thickener, etc. As the binder, the conductive material, the thickener, etc., known materials can be used.

[0037] The negative electrode active material includes a material that electrochemically absorbs and releases lithium ions, lithium metal, lithium alloy, etc. As the material that electrochemically absorbs and releases lithium ions, a carbon material, an alloy-based material, etc. are used. As the carbon material, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc. can be exemplified. Among them, graphite is preferable because of its excellent charge / discharge stability and small irreversible capacity.

[0038] An alloy material is a material that contains an element that can form an alloy with lithium. Examples of elements that can form an alloy with lithium include silicon and tin, with silicon (Si) being particularly promising.

[0039] The material containing silicon may be a silicon alloy, a silicon compound, or a composite material. Among them, a composite material containing a lithium ion conductive phase and silicon particles dispersed in the lithium ion conductive phase is promising. As the lithium ion conductive phase, for example, a silicon oxide phase, a silicate phase, a carbon phase, or the like can be used. The silicon oxide phase is a material with a relatively large irreversible capacity. On the other hand, the silicate phase is preferable in that it has a small irreversible capacity.

[0040] The main component of the silicon oxide phase (for example, 95 to 100 mass%) may be silicon dioxide. The composition of the composite material containing the silicon oxide phase and silicon particles dispersed therein is, as a whole, SiO x It can be expressed as SiO x is a material in which silicon particles are amorphous SiO 2 The oxygen content ratio x relative to silicon is, for example, 0.5≦x<2.0, and more preferably 0.8≦x≦1.5.

[0041] The silicate phase may contain, for example, at least one selected from the group consisting of Group 1 elements and Group 2 elements of the long-period periodic table. Examples of the Group 1 element and Group 2 element of the long-period periodic table include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. Other elements may include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), titanium (Ti), etc. Among them, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferable because it has a small irreversible capacity and high initial charge-discharge efficiency.

[0042] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may also contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li 2z SiO 2+z (0 < z < 2). z preferably satisfies the relationship of 0 < z < 1, and more preferably z = 1 / 2. Examples of elements other than Li, Si, and O that may be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), etc.

[0043] The carbon phase may be composed of, for example, low-crystalline amorphous carbon (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or others.

[0044] For the negative electrode current collector, for example, a metal sheet or a metal foil is used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy, etc.

[0045] Examples of the conductive material used in the positive electrode mixture layer and the negative electrode mixture layer include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNT), graphite, etc. These may be used alone or in combination of two or more.

[0046] Examples of the binder used in the positive electrode mixture layer and the negative electrode mixture layer include fluororesins (polytetrafluoroethylene, polyvinylidene fluoride, etc.), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, etc. These may be used alone or in combination of two or more.

[0047] [Electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent and a solute dissolved in the non-aqueous solvent. Here, the solute means an electrolyte salt that ionizes in the non-aqueous solvent, and includes a lithium salt. The components of the non-aqueous electrolyte other than the non-aqueous solvent and the solute are additives. The electrolyte may contain various additives.

[0048] Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic carboxylates, and chain carboxylates. Examples of the cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of the chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of the cyclic carboxylates include γ-butyrolactone (GBL), and γ-valerolactone (GVL). Examples of the chain carboxylates include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). The non-aqueous solvents may be used alone or in combination of two or more.

[0049] Among them, the chain carboxylate ester is suitable for preparing a non-aqueous electrolyte solution with low viscosity. Therefore, the non-aqueous electrolyte solution may contain 90 mass% or less of the chain carboxylate ester. Among the chain carboxylate esters, methyl acetate has a particularly low viscosity. Therefore, 90 mass% or more of the chain carboxylate ester may be methyl acetate.

[0050] Other examples of the non-aqueous solvent include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0051] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ethers, and the like.

[0052] Examples of chain ethers include 1,2-dimethoxyethane, dimethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0053] These solvents may be fluorinated solvents in which some of the hydrogen atoms are replaced with fluorine atoms. As the fluorinated solvent, fluoroethylene carbonate (FEC) may be used.

[0054] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO 4 , LiAlCl 4 , LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF 6 , LiPF 2 O 2 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiCF 3 SO 3 , LiCF 3 CO 2 etc.), lithium salts of fluorine-containing acid imides (LiN(FSO 2 ) 2 , LiN(CF 3 SO 2 ) 2 , LiN(CF 3 SO 2 )(C 4 F 9 SO 2 ), LiN(C 2 F 5 SO 2 ) 2 Lithium salts that can be used include lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salts may be used alone or in combination of two or more.

[0055] The concentration of the lithium salt in the non-aqueous electrolyte may be 1 mol / L or more and 2 mol / L or less, or may be 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ion conductivity and low viscosity can be obtained.

[0056] The additives include 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, and the like.

[0057] [Separator] A separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and has appropriate mechanical strength and insulation. As the separator, a microporous thin film, a woven fabric, a nonwoven fabric, etc. can be used. As the material of the separator, polyolefin such as polypropylene and polyethylene is preferable.

[0058] An example of the structure of the secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a non-aqueous electrolyte are housed in an exterior body. Alternatively, instead of a wound type electrode group, other types of electrode groups may be applied, such as a stacked type electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The non-aqueous electrolyte secondary battery may be in any form, such as a cylindrical type, a square type, a coin type, a button type, or a laminate type.

[0059] Hereinafter, a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure will be described with reference to Fig. 1 and Fig. 2. Fig. 1 is a partially cutaway plan view showing a schematic example of a structure of a nonaqueous electrolyte secondary battery. Fig. 2 is a cross-sectional view taken along line XX' in Fig. 1.

[0060] As shown in FIGS. 1 and 2, the nonaqueous electrolyte secondary battery 100 is a sheet-type battery, and includes an electrode plate group 4 and an exterior case 5 that houses the electrode plate group 4.

[0061] The electrode plate group 4 has a structure in which a negative electrode 10, a separator 30, and a positive electrode 20 are laminated in this order, with the negative electrode 10 and the positive electrode 20 facing each other via the separator 30. This forms the electrode plate group 4. The electrode plate group 4 is impregnated with a non-aqueous electrolyte.

[0062] The negative electrode 10 includes a negative electrode active material layer 1a and a negative electrode current collector 1b. The negative electrode active material layer 1a is formed on the surface of the negative electrode current collector 1b.

[0063] The positive electrode 20 includes a positive electrode mixture layer 2a and a positive electrode current collector 2b. The positive electrode mixture layer 2a is formed on the surface of the positive electrode current collector 2b.

[0064] A negative electrode tab lead 1c is connected to the negative electrode current collector 1b, and a positive electrode tab lead 2c is connected to the positive electrode current collector 2b. The negative electrode tab lead 1c and the positive electrode tab lead 2c each extend to the outside of the outer case 5.

[0065] The negative electrode tab lead 1c and the exterior case 5, and the positive electrode tab lead 2c and the exterior case 5 are insulated from each other by insulating tab films 6, respectively.

[0066] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0067] Example 1 (1) Preparation of the positive electrode The positive electrode active material, inactive particles, conductive material, and binder were mixed in a mass ratio of 100:1.6:0.75:0.6, and N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to one side of a positive electrode current collector to form a coating film. An aluminum foil was used as the positive electrode current collector. After drying the coating film, the coating film was rolled together with the positive electrode current collector by a rolling roller to form a film with a thickness of 120 to 130 μm and a density of 3.7 g / cm. 3 As a result, a positive electrode having a positive electrode mixture layer with a porosity of 22% was obtained.

[0068] The positive electrode was cut into a predetermined shape to obtain a positive electrode for evaluation. The positive electrode was provided with a 20 mm x 20 mm region to function as a positive electrode and a 5 mm x 5 mm region to connect to the tab lead. The positive electrode mixture layer formed on the connection region was then scraped off to expose the positive electrode current collector. The exposed portion of the positive electrode current collector was then connected to the positive electrode tab lead, and a predetermined region on the periphery of the positive electrode tab lead was covered with an insulating tab film.

[0069] The following materials were used:

[0070] Cathode active material: LiNi 0.9 Co 0.05 Al 0.05 O2 (Average particle diameter D1=11.1μm) Inert particles: Alumina (Al 2 O 3 )(Average particle diameter D2=0.79μm, D1 / D2 ratio=14.1) Conductive material: Acetylene black Binder: Polyvinylidene fluoride (2) Preparation of the negative electrode A lithium metal foil (thickness: 300 μm) was attached to one side of an electrolytic copper foil to prepare a negative electrode.

[0071] The negative electrode was cut into the same shape as the positive electrode to obtain a negative electrode for evaluation. The lithium metal foil formed on the connection area formed in the same manner as the positive electrode was peeled off to expose the negative electrode current collector. Thereafter, the exposed portion of the negative electrode current collector was connected to a negative electrode tab lead in the same manner as the positive electrode, and a predetermined area on the periphery of the negative electrode tab lead was covered with an insulating tab film.

[0072] (3) Preparation of non-aqueous electrolyte The mixture of solvents with the composition (volume ratio) shown in Table 1 was mixed with LiPF 6 The viscosity of the nonaqueous electrolyte at 30°C was measured using a Viscometer-Rheometer-on-a-Chip (m-VROC (registered trademark)) manufactured by RheoSense, Inc., with a channel depth of 50 μm and a shear rate of 4000 to 10000 s -1 The measurements were performed under the following conditions. The average viscosity value in the measurement range where the parameter %-Full-scale was 20% or more was used. The results are shown in Table 1.

[0073] The following was used as the non-aqueous solvent:

[0074] FEC: Fluoroethylene carbonate DMC: Dimethyl carbonate MA: Methyl acetate (4) Preparation of evaluation cells A cell was prepared using the above-mentioned evaluation positive electrode and negative electrode. First, the positive electrode and the negative electrode were placed opposite each other through a polypropylene separator (thickness 30 μm) so that the positive electrode mixture layer and the negative electrode active material layer (lithium metal foil) were exactly overlapped to obtain an electrode plate group. Next, an Al laminate film (thickness 100 μm) cut into a rectangle of 60 × 90 mm was folded in half, and the end of the 60 mm long side was heat sealed at 230 ° C. to form a 60 × 45 mm cylindrical shape. Thereafter, the prepared electrode plate group was placed in the cylinder, and the end face of the Al laminate film was aligned with the position of the heat-sealed resin of each tab lead and heat-sealed at 230 ° C. Next, nonaqueous electrolyte was poured 0.3 cm from the short side of the Al laminate film that was not heat-sealed. 3 After the injection, the positive electrode mixture layer was left to stand for 5 minutes under a reduced pressure of 0.06 MPa to impregnate the positive electrode mixture layer with the nonaqueous electrolyte. Finally, the end face of the Al laminate film on the injected side was heat sealed at 230°C to obtain evaluation cell A1. The evaluation cell was produced in a dry environment with a dew point of -50°C or less.

[0075] (5) Battery evaluation The evaluation cell was clamped between a pair of 80×80 cm stainless steel clamps (thickness: 2 mm) and pressurized and fixed at 0.2 MPa.

[0076] First, five cycles of charging and discharging were repeated at a constant current of 0.05 C (1 C is the current value at which the designed capacity is discharged in one hour) in a thermostatic bath at 25° C. Charging was terminated at a battery voltage of 4.2 V, and discharging was terminated at a battery voltage of 2.5 V. Between charging and discharging, the battery was left to stand in an open circuit for 20 minutes.

[0077] Next, in a thermostatic bath at 25° C., the battery was charged at a constant current of 0.05 C up to 4.2 V, and then held at a constant voltage of 4.2 V until the current value became less than 1 mA. After that, the battery was left standing in an open circuit for 20 minutes, and then discharged at a constant current of 2 C up to 2.5 V in a thermostatic bath at 25° C., and the 2 C discharge capacity was determined as the high-rate discharge performance.

[0078] The results are shown in Table 1. The 2C discharge capacity in Table 1 is a relative value to cell B3 of Comparative Example 3 described below, and the larger the value, the more excellent the high-rate discharge performance.

[0079] Example 2 In preparing the non-aqueous electrolyte solution, the composition of the mixed solvent was changed as shown in Table 1, and an evaluation cell A2 was produced in the same manner as in Example 1.

[0080] Example 3 In the preparation of the positive electrode, alumina (Al 2 O 3 Except for changing the average particle diameter D2 of ) to 2.85 μm and D1 / D2 ratio = 3.9), evaluation cell A3 was produced in the same manner as in Example 1. The loading amount and porosity of the positive electrode active material contained in the positive electrode mixture layer were controlled to be the same as in Example 1.

[0081] Example 4 In preparing the non-aqueous electrolyte solution, the composition of the mixed solvent was changed as shown in Table 1, and an evaluation cell A4 was produced in the same manner as in Example 3.

[0082] Comparative Example 1 In the preparation of the positive electrode, alumina (Al 2 O 3 Except for not adding the above-mentioned cation, an evaluation cell B1 was produced in the same manner as in Example 1. The loading amount of the positive electrode active material contained in the positive electrode mixture layer and the porosity were controlled to be the same as in Example 1.

[0083] Comparative Example 2 In the preparation of the positive electrode, alumina (Al 2 O 3 Except for not adding the positive electrode active material, evaluation cell B2 was produced in the same manner as in Example 2. The loading amount and porosity of the positive electrode active material contained in the positive electrode mixture layer were controlled to be the same as in Example 1.

[0084] Comparative Example 3 Evaluation cell B3 was produced in the same manner as in Comparative Example 1, except that in the preparation of the nonaqueous electrolyte, the composition of the mixed solvent was changed as shown in Table 1.

[0085] Comparative Example 4 Evaluation cell B4 was produced in the same manner as in Example 1, except that in the preparation of the nonaqueous electrolyte solution, the composition of the mixed solvent was changed as shown in Table 1.

[0086] Comparative Example 5 Evaluation cell B5 was produced in the same manner as in Example 3, except that in the preparation of the nonaqueous electrolyte, the composition of the mixed solvent was changed as shown in Table 1.

[0087] [Table 1]

[0088] FIG. 3 shows the relationship between the viscosity of the non-aqueous electrolyte and the 2C discharge capacity. FIG. 4 shows an enlarged view of the area surrounded by the dashed line in FIG. 3. From FIG. 3, it can be seen that when the positive electrode mixture layer contains inactive particles, the 2C discharge capacity increases significantly when the viscosity of the non-aqueous electrolyte is reduced. On the other hand, when the positive electrode mixture layer does not contain inactive particles, the 2C discharge capacity increases to some extent when the viscosity of the non-aqueous electrolyte is reduced, but the increase is relatively small.

[0089] In Figure 3, the increase in 2C discharge capacity when the viscosity of the nonaqueous electrolyte is 1.22 mPa s is extremely significant, making it difficult to grasp the trend in the area enclosed by the dashed line. In this regard, Figure 4 shows that the 2C discharge capacity increases significantly even when the viscosity of the nonaqueous electrolyte is 1.85 mPa s, compared to when the viscosity is 2.0 mPa s.

[0090] Next, the Log differential pore size distribution (cc / g logμm) of the positive electrode mixture layer of the evaluation cell A1 in Example 1 and the positive electrode mixture layer of the evaluation cell B1 in Comparative Example 1 was measured using a mercury porosimeter (AutoPore V by Micromeritics). The results are shown in FIG. 5. It can be seen from FIG. 5 that the addition of inactive particles shifts the peak of the pore size distribution of the positive electrode mixture layer to the smaller particle size side and increases the amount of finer pores. This indicates that the inactive particles are filled in the relatively large gaps between the particles of the positive electrode active material, the size of the gaps is uniformed, and the number of fine paths through which lithium ions can move is increased. [Industrial Applicability]

[0091] The nonaqueous electrolyte secondary battery according to the present disclosure is suitable for use in fields requiring high-rate discharge performance. [Explanation of symbols]

[0092] 1a Negative electrode active material layer 1b Negative electrode current collector 1c Negative electrode tab lead 2a Positive electrode mixture layer 2b Positive electrode current collector 2c Positive electrode tab lead 4 Plate group 5 Outer case 6 Insulation tab film 10 negative electrode 20 positive electrode 30 Separator 100 Lithium-ion secondary battery

Claims

1. The battery includes a positive electrode having a positive electrode mixture layer, a negative electrode, and a non-aqueous electrolyte solution, The positive electrode mixture layer includes a positive electrode active material and inactive particles, The positive electrode active material includes a lithium-containing composite oxide, The inert particles are ceramics, an average particle diameter D1 of the positive electrode active material and an average particle diameter D2 of the inactive particles satisfy D1>D2, The nonaqueous electrolyte secondary battery has a viscosity of 1.9 mPa·s or less at 30° C.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the average particle diameter D2 is 0.1 μm or more and 10 μm or less.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of the average particle diameter D1 to the average particle diameter D2: D1 / D2 satisfies 2 to 50.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the amount of the inactive particles based on the total amount of the positive electrode active material and the inactive particles is 0.1 mass % or more and 15 mass % or less.

5. 2. The nonaqueous electrolyte secondary battery in accordance with claim 1, wherein the ceramic comprises at least one selected from the group consisting of silica, alumina, and titania.

6. 6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode mixture layer has a thickness of 100 μm or more.

Images