JP2019050098A - Cathode for nonaqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery - Google Patents

Abstract

To provide a cathode capable of realizing a non-aqueous electrolyte secondary battery which is capable of securing sufficient battery capacity, reduces internal resistance and has an excellent high-rate property.SOLUTION: A cathode for non-aqueous electrolyte secondary battery comprises a cathode mixture layer containing a layered lithium metal composite oxide and a conductive material. In the cathode mixture layer, a total of a specific surface area of the conductive material contained per unit volume is equal to or more than 20 m/cm.SELECTED DRAWING: Figure 2

Description

  The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

A nonaqueous electrolyte secondary battery using a layered lithium metal composite oxide is widely known as a positive electrode active material. For example, Patent Document 1 discloses a surface enhanced Raman spectrum, 800 cm ^-1 or more, the lithium metal composite oxide is non-aqueous electrolyte secondary battery that the cathode active material is disclosed having a peak at 1000 cm ^-1 or less . Patent Document 1 discloses that carbon black having a nitrogen adsorption specific surface area of 70 m ² / g or more and 300 m ² / g or less and an average particle diameter of 10 nm or more and 35 nm or less is used as a conductive material of the positive electrode. Has been.

JP 2012-38724 A

  By the way, in a non-aqueous electrolyte secondary battery using a layered lithium metal composite oxide, if the packing density of the mixture layer of the positive electrode is low, the space in the mixture layer is sufficiently impregnated with the electrolyte solution, ensuring the capacity. However, the distance between the particles of the active material becomes long, and the resistance during discharge tends to increase. On the other hand, when the packing density of the mixture layer is high, the resistance decreases, but the amount of the electrolyte in the mixture layer is insufficient, and the capacity may be reduced.

  The objective of this indication is providing the positive electrode which can implement | achieve the nonaqueous electrolyte secondary battery which can fully ensure battery capacity, has low internal resistance, and has a favorable high-rate characteristic.

The positive electrode for a non-aqueous electrolyte secondary battery according to the present disclosure has a positive electrode mixture layer including a layered lithium metal composite oxide and a conductive material, and the conductive material included per unit volume of the positive electrode mixture layer. The total specific surface area of the material is 20 m ² / cm ³ or more, and the packing density of the positive electrode mixture layer is 2.2 g / cm ³ or more.

  A nonaqueous electrolyte secondary battery according to the present disclosure includes an electrode body including the positive electrode, a negative electrode having a negative electrode mixture layer, a separator interposed between the positive electrode and the negative electrode, a nonaqueous electrolyte, It is characterized by providing.

  According to the positive electrode for a nonaqueous electrolyte secondary battery according to the present disclosure, even when the packing density of the mixture layer is low, an energization path can be secured and the resistance can be suppressed low. For this reason, a nonaqueous electrolyte secondary battery having a high battery capacity and a low internal resistance can be provided. Further, even when the packing density of the mixture layer is high, more electrolytic solution can be held on the surface of the conductive material. Therefore, the electrolytic solution in the electrode plate can be preferentially guided to the surface of the active material through the conductive material. Therefore, the shortage of the electrolytic solution in the vicinity of the active material can be suppressed, and a decrease in capacity can be suppressed. A non-aqueous electrolyte secondary battery having both high capacity and low resistance can be obtained.

It is a figure which shows the nonaqueous electrolyte secondary battery which is an example of embodiment. It is sectional drawing of the positive electrode which is an example of embodiment.

  As described above, in the conventional positive electrode for a nonaqueous electrolyte secondary battery, when the packing density of the mixture layer is low, the space between the active material particles is wide, so that the liquid retention in the mixture layer is good. Therefore, a sufficient battery capacity can be secured. However, the resistance is high because the distance between the particles of the active material is long and there are few energization paths. On the other hand, when the packing density of the mixture layer is high, an energization path can be secured, but the amount of the electrolyte in the mixture layer is small, so that a sufficient capacity cannot be secured. That is, in both cases, it is difficult to ensure both capacitance and low resistance.

In contrast, a non-aqueous electrolyte secondary battery using a positive electrode for a non-aqueous electrolyte secondary battery according to the present disclosure (hereinafter also referred to as a positive electrode) can sufficiently ensure battery capacity and has low internal resistance. Therefore, it has a good high rate characteristic. In the positive electrode according to the present disclosure, since the total specific surface area of the conductive material per unit volume in the positive electrode mixture layer is as high as 20 m ² / cm ³ or more, the contact area between the conductive materials or between the conductive material and the active material particles Tends to be large. For this reason, even if the packing density of the mixture layer is low, it is considered that the energization path can be secured and the resistance can be reduced. Moreover, even when the packing density of the mixture layer is high, the electrolyte solution can be guided to the particle surface of the active material by setting the total specific surface area of the conductive material in the positive electrode mixture layer to 20 m ² / cm ³ or more. It is possible to suppress the decrease in capacity. The total specific surface area (m ² / cm ³ ) of the conductive material is the specific surface area (m ² / g) of the conductive material, the packing density (g / cm ³ ) of the mixture layer, and the mixture layer. It is calculated from the added amount (% by mass) of the conductive material. In addition, when the addition amount of the conductive material is calculated from the mass ratio (a mass%) of the conductive material to the active material, the addition amount (b mass%) of the binder (binder) to the active material is used. The compounding ratio (100: a: b) of the substance, the conductive material and the binder can be calculated and calculated as a / (100 + a + b).

In addition, when the packing density of the mixture layer of the positive electrode is increased, wrinkles occur due to the difference in elongation at rolling between the coated portion and the non-coated portion of the mixture, and the wrinkles are uneven on the surface of the current collector. There is a risk of becoming. And this wrinkle may reduce the current collecting property between the mixture layer and the current collector. The degree of influence varies depending on the diameter of the rolling roll and the rolling technique. For this reason, the upper limit value of the packing density varies depending on the rolling method and the like. An example of the upper limit of the packing density is 3.8 g / cm ³ . In addition, when the packing density of the positive electrode mixture layer is lower than 2.2 g / cm ³ , the resistance is particularly increased, and the effect of the conductive material in the present disclosure is hardly obtained. Further, when the packing density is lower than 2.2 g / cm ³ , the resistance variation with respect to the change in the packing density becomes large. Therefore, even if the same battery is continuously manufactured, each battery has a small packing density. There is a possibility that the characteristics of the battery may fluctuate due to the difference. From the above, in the positive electrode of the present disclosure, it is preferably 2.2 g / cm ³ or more of the mixture layer.

  In the nonaqueous electrolyte secondary battery according to this embodiment, the negative electrode active material is not particularly limited, but a suitable example is a lithium titanium composite oxide typified by lithium titanate. When a lithium-titanium composite oxide is used for the negative electrode, a surface coating (SEI coating) that can be an obstacle when lithium ions enter and exit is not formed, so that the high-rate characteristics of the battery are improved. Moreover, since the lithium titanium complex oxide has a small volume change accompanying charging / discharging, the cycle characteristics of the battery are improved. Therefore, deterioration of battery performance such as increased resistance is easily affected by deterioration of the positive electrode material. Since the positive electrode according to the present disclosure is excellent in conductivity and liquid retention, the positive electrode material is not easily deteriorated. That is, the combination of the positive electrode according to the present disclosure and the negative electrode using the lithium-titanium composite oxide greatly contributes to improvement of battery capacity, high rate characteristics, and durability.

  FIG. 1 is a diagram illustrating a nonaqueous electrolyte secondary battery 10 which is an example of an embodiment. FIG. 1 shows a non-aqueous electrolyte secondary battery 10 that is a square battery including a square battery case 11. However, the non-aqueous electrolyte secondary battery according to the present disclosure is not limited to a rectangular battery, and may be a cylindrical battery, a coin-shaped battery, or the like provided with a metal case such as a cylindrical shape or a coin shape. A so-called laminate battery provided with a resin case constituted by: In FIG. 1, the electrode body exemplifies a wound electrode body 14 in which a strip-shaped positive electrode and a strip-shaped negative electrode are spirally wound via a separator, but a plurality of positive electrodes and a plurality of negative electrodes are combined. A laminated electrode body laminated via a separator may also be used.

  As illustrated in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a battery case 11 and an electrode body 14 accommodated in the case. Further, the battery case 11 is filled with a non-aqueous electrolyte (not shown). The battery case 11 is a rectangular metal case including a bottomed cylindrical case main body 12 and a sealing plate 13 that closes an opening of the main body. The electrode body 14 includes a positive electrode 15, a negative electrode 16, and a separator 17 interposed between the positive electrode 15 and the negative electrode 16. The electrode body 14 illustrated in FIG. 1 has a stacked structure in which strip-shaped positive electrodes 15 and strip-shaped negative electrodes 16 are alternately stacked via separators 17.

  The nonaqueous electrolyte secondary battery 10 includes a positive electrode terminal 18 that is electrically connected to the positive electrode 15 and a negative electrode terminal 19 that is electrically connected to the negative electrode 16. In the example shown in FIG. 1, the positive electrode terminal 18 is provided on one end side in the longitudinal direction of the upper surface portion of the battery case 11 constituted by the sealing plate 13, and the negative electrode terminal 19 is provided on the other end side in the longitudinal direction of the upper surface portion. Yes. The nonaqueous electrolyte secondary battery 10 may be provided with a conductive member that connects the electrode and the terminal.

  Hereinafter, each component of the nonaqueous electrolyte secondary battery 10 will be described in detail.

[Positive electrode]
FIG. 2 is a diagram illustrating a part of the cross section of the positive electrode 11, where (a) shows a case where the packing density of the positive electrode mixture layer 21 is low, and (b) shows a case where the packing density of the positive electrode mixture layer 21 is high. Respectively. As illustrated in FIG. 2, the positive electrode 11 includes a strip-shaped positive electrode current collector 20 and a positive electrode mixture layer 21 formed on the current collector. For the positive electrode current collector 20, a metal foil that is stable in the potential range of the positive electrode 11 such as aluminum, a film in which the metal is disposed on the surface layer, or the like can be used.

  The positive electrode mixture layer 21 includes a lithium metal composite oxide 22 and a conductive material 23, and is formed on both surfaces of the positive electrode current collector 20. The positive electrode mixture layer 21 includes a binder that binds the lithium metal composite oxide 22 and the conductive material 23 to form a layer. The lithium metal composite oxide 22 functions as a positive electrode active material. In the positive electrode 11, a positive electrode mixture slurry containing a positive electrode active material, a conductive material 23, a binder, and the like is applied onto the positive electrode current collector 20, the coating film is dried, and then rolled to form a positive electrode mixture layer 21. It can be produced by forming on both surfaces of the positive electrode current collector 20.

The lithium metal composite oxide 22 is, for example, a general formula Li _{1 + x} M _a O _{2 + b} (where, x + a = 1, −0.2 <x ≦ 0.2, −0.1 ≦ b ≦ 0.1, M is a composite oxide represented by (including at least one selected from Ni, Co, Mn, and Al). The positive electrode active material may contain a small amount of other lithium metal composite oxides and the like, but it is preferable that the main component is lithium metal composite oxide 22 represented by the above general formula.

  The lithium metal composite oxide 22 may contain elements other than Ni, Co, Mn, and Al. Examples of other elements include alkali metal elements other than Li, transition metal elements other than Ni, Co, and Mn, alkaline earth metal elements, Group 12 elements, Group 13 elements other than Al, and Group 14 elements. Can be mentioned. Specific examples include Zr, B, Mg, Ti, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Si, and the like.

The particle size of the lithium metal composite oxide 22 is not particularly limited, but for example, the average particle size is preferably 2 μm or more and less than 30 μm. When the average particle size is less than 2 μm, there is a case where resistance increases due to obstruction of energization by the conductive material 23 in the positive electrode mixture layer 21. On the other hand, when the average particle size is 30 μm or more, the load characteristics may be reduced due to a reduction in the reaction area. The average particle diameter is a volume average particle diameter measured by a laser diffraction method, and means a median diameter at which the volume integrated value is 50% in the particle diameter distribution. The average particle diameter can be measured using, for example, a laser diffraction / scattering particle size distribution measuring apparatus (manufactured by Horiba, Ltd.). The specific surface area of the positive electrode active material is preferably 0.1 m ² / g or more and 7 m ² / g or less. When the specific surface area is lower than 0.1 m ² / g, the resistance is high, and when the specific surface area exceeds 7 m ² / g, the fluidity of the slurry is lowered.

  As the conductive material 23, for example, a carbon material such as carbon black, acetylene black, ketjen black, or graphite can be used. These may be used alone or in combination of two or more. The electrical conductivity of the conductive material 23 is preferably 0.1 Ω · cm or more and 0.3 Ω · cm or less. When the electrical conductivity of the conductive material 23 is lower than 0.1 Ω · cm, the resistance in the positive electrode mixture layer cannot be sufficiently reduced, and when the electrical conductivity of the conductive material 23 exceeds 0.3 Ω · cm, This is because the resistance reduction effect in the mixture layer is saturated.

A suitable example of the conductive material 23 is a carbon material having a specific surface area of 30 to 140 m ² / g. When the specific surface area of the conductive material 23 is lower than 30 m ² / g, it is necessary to increase the amount of the conductive material 23 added, and the dispersibility in the positive electrode mixture slurry is lowered. In this case, reaction spots are generated in the positive electrode mixture layer 21, and the battery performance may be deteriorated. On the other hand, when the specific surface area exceeds 140 m ² / g, the viscosity of the positive electrode mixture slurry is increased, the coating property is lowered, and it becomes difficult to form the homogeneous positive electrode mixture layer 21. Moreover, the average primary particle diameter of the electrically conductive material 23 is 10 nm or more and 50 nm or less, for example.

  In addition, it is preferable that the mixture ratio of the electrically conductive material 23 is 3 mass% or more and 15 mass% or less with respect to the mass of an active material. This is because when the blending ratio of the conductive material 23 is less than 3% by mass, the resistance in the positive electrode mixture layer increases, and when the blending ratio of the conductive material exceeds 15% by mass, the slurry properties are not stable.

In the positive electrode 11, the total specific surface area of the conductive material 23 contained per unit volume of the positive electrode mixture layer 21 is 20 m ² / cm ³ or more, and the packing density of the positive electrode mixture layer 21 is 2.2 m ² / cm. ³ or more. By satisfying this condition, it is possible to obtain the nonaqueous electrolyte secondary battery 10 having a sufficient battery capacity, low internal resistance, and good high rate characteristics. Here, the total specific surface area of the conductive material 23 contained per unit volume of the positive electrode mixture layer 21 can be calculated from the specific surface area of the conductive material 23 and the content of the conductive material 23. The specific surface area of the conductive material 23 can be measured by the BET method using a BET specific surface area measuring device by nitrogen adsorption / desorption. The packing density of the positive electrode mixture layer 21 is estimated from the measured thickness of the electrode mixture layer, the thickness of the current collector, and their mass.

The upper limit of the packing density of the positive electrode mixture layer 21 is not particularly limited, but a suitable example is 3.8 g / cm ³ . Packing density of the positive electrode mixture layer 21, for example 2.2 g / cm ³ or more, is set in the range of 3.8 g / cm ^3. As described above, when the packing density of the positive electrode mixture layer 21 is higher than 3.8 g / cm ³ , wrinkles are likely to occur due to the difference in elongation at rolling between the coated portion and the non-coated portion of the positive electrode mixture. Become. The wrinkles may become uneven on the surface of the current collector. And this wrinkle may reduce the current collecting property between the mixture layer and the current collector. On the other hand, when the charge density of the positive electrode mixture layer 21 is lower than 2.2 g / cm ^{3, the} influence due to the fact that the distance between the particles of the positive electrode active material is too long and a sufficient conductive path cannot be secured increases. The effect of conductive material is small. Therefore, the resistance of the positive electrode mixture layer having a packing density lower than 2.2 g / cm 3 is remarkably increased.

As illustrated in FIG. 2A, the conductive material per unit volume of the positive electrode mixture layer 21 even when the packing density of the positive electrode mixture layer 21 is low (however, 2.2 g / cm ³ or more). By setting the total of the specific surface areas of 23 to 20 m ² / cm ³ or more, a current-carrying path can be sufficiently secured. Thereby, the resistance of the positive electrode 11 can be kept low. On the other hand, as illustrated in FIG. 2B, even if the packing density of the positive electrode mixture layer 21 is high, the total specific surface area of the conductive material 23 per unit volume of the positive electrode mixture layer 21 is 20 m ^2. By setting it to / cm ³ or more, more electrolytic solution can be held in or near the conductive material 21. Therefore, the electrolytic solution can be guided to the particle surface of the positive electrode active material (lithium metal composite oxide 22) through the conductive material 21, and the capacity reduction can be suppressed.

Although the upper limit of the total specific surface area of the conductive material 23 included per unit volume of the positive electrode mixture layer 21 is not particularly limited, a suitable example is 40 m ² / cm ³ . The total specific surface area of the conductive material 23 in the positive electrode mixture layer 21 is set to 20 to 40 m ² / cm ³ , for example. In addition, the total of the specific surface area of the electrically conductive material 23 should just be in the said range, and the specific surface area of the electrically conductive material 23 to be used is not specifically limited as mentioned above. The total specific surface area of the conductive material 23 is prepared mainly by the specific surface area of the conductive material 23 to be used and its addition amount.

  Examples of the binder contained in the positive electrode mixture layer 21 include fluorine resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like. These may be used alone or in combination of two or more.

  In addition, it is preferable that the mixture ratio of the binder contained in the positive mix layer 21 is 1 mass% or more and 5 mass% or less with respect to the mass of a positive electrode active material. When this blending ratio is lower than 1% by mass, the active material easily falls off from the electrode plate. When this compounding ratio exceeds 5 mass%, there exists a possibility that the resistance in a positive mix layer or the resistance between a positive mix layer and a collector may increase.

[Negative electrode]
The negative electrode 16 has a strip-shaped negative electrode current collector and a negative electrode mixture layer formed on the current collector. As the negative electrode current collector, a metal foil that is stable in the potential range of a negative electrode such as copper, a film in which the metal is disposed on the surface layer, or the like can be used. The negative electrode mixture layer includes a negative electrode active material and a binder, and is formed on both surfaces of the negative electrode current collector. The negative electrode is prepared by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc. on the negative electrode current collector, drying the coating film, and rolling the negative electrode mixture layer on both sides of the negative electrode current collector. It can be manufactured by forming.

  The negative electrode active material is not particularly limited as long as it can reversibly occlude and release lithium ions. For example, carbon materials such as natural graphite and artificial graphite, and Li and alloys such as silicon (Si) and tin (Sn) Or an oxide containing a metal element such as Si or Sn can be used. The negative electrode mixture layer may contain a lithium titanium composite oxide. The lithium titanium composite oxide functions as a negative electrode active material.

The lithium titanium composite oxide is, for example, lithium titanate represented by a general formula Li _{4 + y} Ti ₅ O ₁₂ (wherein y is 0 or more and 1 or less), and has a spinel crystal structure. The lithium titanium composite oxide may contain a metal element such as Mg, Al, Ca, Ba, Bi, Ga, V, Nb, W, Mo, Ta, Cr, Fe, Ni, Co, and Mn. Good. By using the lithium titanium composite oxide as the negative electrode active material, the high rate characteristics and cycle characteristics of the battery are improved as described above. In the nonaqueous electrolyte secondary battery 10, a combination of the positive electrode 11 having the above-described configuration and a negative electrode using a lithium titanium composite oxide is preferable.

  The lithium titanium composite oxide can be synthesized by a method according to the method for synthesizing the layered lithium metal composite oxide (see Example 1 described later). For example, a lithium-containing compound such as lithium hydroxide and a titanium-containing compound such as titanium dioxide and titanium hydroxide are mixed at a target mixing ratio, and the mixture is baked to be expressed by the above general formula. Lithium titanium composite oxide can be synthesized. The lithium titanium composite oxide obtained by this synthesis method is a secondary particle formed by agglomerating primary particles. The mixture is generally fired in the atmosphere (or in an oxygen stream) under conditions of a firing temperature of about 500 to 1100 ° C. and a firing time of about 1 to 30 hours.

The specific surface area of the lithium titanium composite oxide is preferably 3 m ² / g or more and 7 m ² / g or less. When the specific surface area of the lithium-titanium composite oxide is lower than 3 m ² / g, the reaction active sites decrease and the resistance increases, and when the specific surface area of the lithium-titanium composite oxide is higher than 7 m ² / g, the resistance reduction effect is saturated. In addition, the slurry properties are lowered.

When using a lithium titanium composite oxide, the negative electrode mixture layer preferably contains a conductive material. Examples of the conductive material include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. These may be used alone or in combination of two or more. Moreover, when using lithium titanate, it is preferable that the packing density of a mixture layer is 1.6 g / cm < ³ > or more and 2.2 g / cm < ³ > or less. In addition, when the packing density of a negative mix layer is lower than 1.6 g / cm < ³ >, the electroconductivity in the negative electrode between active material particles falls. On the other hand, when the packing density of the negative electrode mixture layer is higher than 2.2 g / cm ³ , wrinkles are caused by the difference in elongation rate of the current collector during rolling between the coated portion and the non-coated portion of the negative electrode mixture. Is likely to occur. And since wrinkles become the unevenness | corrugation of the surface of a collector, the current collection property between a collector and a negative mix layer will fall, and it will cause resistance increase.

  In addition, it is preferable that the mixture ratio of the electrically conductive material contained in a negative mix layer is the range of 3 mass% or more and 15 mass% or less with respect to the mass of a negative electrode active material. When the blending ratio of the conductive material in the negative electrode mixture layer is lower than 3% by mass, the resistance in the negative electrode mixture layer increases. When the blending ratio of the conductive material in the negative electrode mixture layer is greater than 15% by mass, the slurry properties are not stable. In addition, the average primary particle diameter of the electrically conductive material contained in the negative electrode mixture layer is, for example, 10 nm or more and 50 nm or less.

  As the binder contained in the negative electrode mixture layer, a known binder can be used. Similarly to the case of the positive electrode 20, a fluororesin such as PTFE and PVdF, PAN, a polyimide resin, an acrylic resin, In addition, a polyolefin-based resin or the like can be used. Moreover, as a binder used when preparing a negative electrode mixture slurry using an aqueous solvent, CMC or a salt thereof, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol ( PVA) etc. can be illustrated.

  In addition, it is preferable that the mixture ratio of the binder contained in a negative mix layer is 1 mass% or more and 5 mass% or less with respect to the mass of a negative electrode active material. This is because when the blending ratio of the binder in the negative electrode mixture layer is lower than 1% by mass, the active material tends to fall off from the electrode plate, and when the blending ratio of the binder exceeds 5% by mass, This is because the resistance in the agent layer or the resistance between the negative electrode mixture layer and the current collector may increase.

[Separator]
As the separator, a porous sheet having ion permeability and insulating properties is used. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. As the material of the separator, olefinic resins such as polyethylene and polypropylene, cellulose and the like are suitable. The separator may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. In addition, a multilayer separator including a polyethylene layer and a polypropylene layer may be used, and a separator whose surface is coated with a resin such as an aramid resin or inorganic fine particles such as alumina or titania can also be used.

[Nonaqueous electrolyte]
The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. As the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and a mixed solvent of two or more of these can be used. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of hydrogen in these solvents is substituted with a halogen atom such as fluorine. The non-aqueous electrolyte is not limited to a liquid electrolyte (non-aqueous electrolyte), and may be a solid electrolyte using a gel polymer or the like.

  Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate, dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), and methyl propyl carbonate. Chain carbonates such as ethyl propyl carbonate and methyl isopropyl carbonate, cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL), methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP ) And chain carboxylic acid esters such as ethyl propionate.

  Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4 -Cyclic ethers such as dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether, 1,2-dimethoxyethane, 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, methoxy toluene, 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, tri Examples thereof include chain ethers such as ethylene glycol dimethyl ether and tetraethylene glycol dimethyl.

  As the halogen-substituted product, it is preferable to use a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), a fluorinated chain carbonate, a fluorinated chain carboxylate such as methyl fluoropropionate (FMP), or the like. .

The electrolyte salt is preferably a lithium salt. _Examples of the lithium _{salt, LiBF 4, LiClO 4, LiPF} 6, LiAsF 6, LiSbF 6, LiAlCl 4, LiSCN, LiCF 3 SO 3, LiCF 3 CO 2, Li (P (C 2 O 4) F 4), LiPF _6-x (C _n F _{2n + 1} ) _x (1 <x <6, n is 1 or 2), LiB ₁₀ Cl ₁₀ , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, Li ₂ B _{4 O 7, Li (B (} C 2 O 4) F 2) boric acid salts such _{as, LiN (SO 2 CF 3)} 2, LiN (C l F 2l + 1 SO 2) (C m F 2m + 1 SO 2) {l , M is an integer greater than or equal to 1} and the like. These lithium salts may be used alone or in combination of two or more. Of these, LiPF ₆ is preferably used from the viewpoints of ion conductivity, electrochemical stability, and the like. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per liter of the nonaqueous solvent.

  Hereinafter, although this indication is further explained by an example, this indication is not limited to these examples.

<Example 1>
[Production of positive electrode]
A lithium-containing compound, a composite oxide containing Ni, Co, and Mn, and zirconium oxide are mixed at a target mixing ratio, and the mixture is fired to obtain a composition formula Li _1.050 Ni _0.189. A layered lithium metal composite oxide represented by Co _0.567 Mn _0.189 Zr _0.005 O ₂ was obtained. The lithium metal composite oxide is used as the positive electrode active material. The lithium metal composite oxide is a secondary particle obtained by agglomerating primary particles. The composition of the positive electrode active material and the negative electrode active material described later was measured using an ICP emission spectroscopic analyzer (Thermo Fisher Scientific iCAP6300).

The positive electrode active material, 7% by mass of carbon black with respect to the active material, and 2% by mass of polyvinylidene fluoride (PVdF) with respect to the active material are mixed, and N-methyl-2 is used as a dispersion medium in the mixture. -Pyrrolidone (NMP) was added and stirred using a mixer (Primix Co., Ltd., TK Hibismix) to prepare a positive electrode mixture slurry. Next, after applying the positive electrode mixture slurry on the aluminum foil as the positive electrode current collector and drying the coating film (positive electrode mixture layer), the packing density of the positive electrode mixture layer was 2.2 g / cm ³ . Thus, the positive electrode mixture layer was rolled with a rolling roll to produce a positive electrode in which the positive electrode mixture layer was formed on both surfaces of the aluminum foil.

In the production of the positive electrode, the specific surface area of carbon black used as the conductive material, the amount of addition thereof, and the rolling conditions were adjusted, and the total specific surface area of carbon black contained per unit volume of the positive electrode mixture layer was 20.5 m. ² / cm ³ .

[Production of negative electrode]
A lithium titanium composite oxide represented by the composition formula Li ₄ Ti ₅ O ₁₂ , carbon black, and polyvinylidene fluoride (PVdF) were mixed at a mass ratio of 90: 8: 2. NMP was added to the mixture, and the mixture was stirred using a mixer (manufactured by PRIMIX Corporation, TK Hibismix) to prepare a negative electrode mixture slurry. Next, after applying the negative electrode mixture slurry on the aluminum foil as the negative electrode current collector and drying the coating film, the coating film is rolled with a rolling roller to form a negative electrode mixture layer on both surfaces of the aluminum foil. A negative electrode was prepared.

[Preparation of non-aqueous electrolyte]
Propylene carbonate (PC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 25:35:40. LiPF ₆ was dissolved in the mixed solvent to a concentration of 1.2 mol / L to prepare a nonaqueous electrolyte.

[Production of battery]
The positive electrode and the negative electrode were spirally wound through a separator made of a polypropylene microporous film, and then press-molded to produce an electrode body. The current collector part of the sealing body was welded to each non-coated part of the positive and negative electrodes protruding from the electrode body, the electrode body was accommodated in an aluminum battery case, and the nonaqueous electrolyte was injected into the battery case. . Then, the opening part of the battery case was sealed, and the square nonaqueous electrolyte secondary battery (rated capacity 10Ah) shown in FIG. 1 was produced.

<Example 2>
In the production of the positive electrode, the packing density of the positive electrode mixture layer was 2.4 g / cm ³ , and the total specific surface area of the conductive material contained per unit volume was 22.3 m ² / cm ³ (7% by mass with respect to the active material) A battery was fabricated in the same manner as in Example 1 except that the positive electrode mixture layer was formed so that the carbon black was added.

<Example 3>
In the production of the positive electrode, the packing density of the positive electrode mixture layer is 3.0 g / cm ³ , and the total specific surface area of the conductive material contained per unit volume is 39.9 m ² / cm ³ (10% by mass with respect to the active material) A battery was fabricated in the same manner as in Example 1 except that the positive electrode mixture layer was formed so that the carbon black was added.

<Comparative Example 1>
In the production of the positive electrode, the positive electrode mixture layer has a packing density of 2.2 g / cm ³ and a total specific surface area of the conductive material contained per unit volume of 2.6 m ² / cm ^3. A battery was fabricated in the same manner as in Example 1 except that was formed.

<Comparative example 2>
In the production of the positive electrode, the positive electrode mixture layer has a packing density of the positive electrode mixture layer of 2.4 g / cm ³ and a total specific surface area of the conductive material contained per unit volume of 4.9 m ² / cm ^3. A battery was fabricated in the same manner as in Example 1 except that was formed.

<Comparative Example 3>
In the production of the positive electrode, the positive electrode mixture layer has a packing density of the positive electrode mixture layer of 2.4 g / cm ³ and a total specific surface area of the conductive material contained per unit volume of 8.2 m ² / cm ^3. A battery was fabricated in the same manner as in Example 1 except that was formed.

<Comparative example 4>
In the production of the positive electrode, the positive electrode mixture layer has a packing density of the positive electrode mixture layer of 3.0 g / cm ³ and a total specific surface area of the conductive material contained per unit volume of 14.3 m ² / cm ^3. A battery was fabricated in the same manner as in Example 1 except that was formed.

  About each battery of an Example and a comparative example, the performance evaluation test was implemented with the following method. The evaluation results are shown in Table 1.

[Battery capacity measurement test]
In a temperature atmosphere of 25 ° C., each battery was charged with a constant current at a current value of 10 A until the battery voltage reached 2.65V. After a pause of 15 minutes, constant current discharge is performed until the battery voltage reaches 1.5 V at a current value of 1100 mA, the discharge capacity at this time is defined as the battery capacity, and the relative value with the value of Comparative Example 1 as the reference (100) It is shown in Table 1.

[High rate characteristics test]
In a temperature atmosphere of 25 ° C., each battery was charged to a SOC (charge depth) of 50% with a charging current of 1 C. Thereafter, the charge / discharge current was increased in the order of 5C discharge → 5C charge → 10C discharge → 10C charge → 15C discharge → 15C charge → 20C discharge → 20C charge → 25C discharge → 25C charge. At this time, a pause period of 15 minutes was provided between each step, and charging / discharging was performed in the order of discharge for 30 seconds → pause for 15 minutes → charge for 30 seconds → pause for 15 minutes. The battery voltage at the time when 10 seconds had elapsed from this discharge was plotted against the discharge current, and the current value when the straight line obtained by the least square method reached 1.5 V was calculated as the output value. The resistance values at that time are shown in Table 1 as relative values based on the value of Comparative Example 1 as a reference (100).

  As shown in Table 1, each of the batteries of the examples had a lower discharge resistance than the batteries of Comparative Examples 1 and 2, and had a higher capacity than the batteries of Comparative Examples 3 and 4. In other words, according to the battery of the example, it is possible to achieve both high capacity and low resistance, which are difficult to realize with the battery of the comparative example.

  In the battery of Example 1, although the packing density of the positive electrode mixture layer is low, the high-rate discharge resistance is low and the battery capacity and the high-rate characteristics are compatible. This is presumably because an energization path between the active material particles was formed due to a significant increase in the specific surface area of the conductive material in the mixture layer. In the battery of Example 2, although the packing density of the positive electrode mixture layer is higher than that of Example 1, the battery capacity is not reduced, and the battery capacity and the high rate characteristics are compatible. This is presumably because the energization path between the active material particles was formed due to a large increase in the specific surface area of the conductive material in the mixture layer, and the electrolyte was led near the surface of the active material particles. In the battery of Example 3, as in the case of Example 2, the battery capacity and the high rate characteristics are compatible. On the other hand, the reason why the battery characteristics of Example 3 are not improved as compared with Example 2 is because the formation effect of the energization path due to the specific surface area of the conductive material and the formation effect of the introduction path of the electrolyte solution are saturated. it is conceivable that.

In addition, when the relationship between the change in the packing density of the positive electrode mixture layer and the change in resistance is observed using Examples 1 to 3, the batteries of Examples 1 to 3 each have a small difference in resistance value and are stable. You can see that Then, as compared filling density of the positive electrode mixture layer and the battery of Example 1 is 2.2 g / cm ^3, the packing density is the 2.4 g / cm ³ or more, the resistance value of the battery of Example 2 and 3 Although the resistance is low, the physical properties are almost the same. Therefore, it can be said that the variation of the resistance value between the batteries of Examples 2 and 3 is extremely small. Therefore, among the mixture layers having a packing density of 2.2 g / cm ³ or more, the positive electrode mixture layer having a packing density of 2.4 g / cm ³ or more has a slight difference in packing density between batteries. Even if it occurs, it can be seen that particularly excellent output characteristics can be stably exhibited as a mixture layer used in the positive electrode of the present disclosure.

Note that the batteries of Comparative Examples 1 and 2 have high resistance during high-rate discharge. This is because the packing density of the positive electrode mixture layer is as low as 2.2 and 2.4 g / cm ³ and the specific surface area of the conductive material is as low as 2.6 and 4.9 m ² / cm ^3. This is thought to be because the energization path cannot be secured due to separation. The batteries of Comparative Examples 3 and 4 having positive electrodes with a packing density of 2.4 and 3.0 g / cm ³ and a specific surface area of the conductive material of 8.2 and 14.3 m ² / cm ³ are Comparative Examples 1 and 2, respectively. As the conductive material specific surface area per unit volume is increased as compared with the above, a decrease in resistance during high-rate discharge is slightly confirmed, but on the other hand, the battery capacity is reduced. This is presumably because the amount of the electrolyte introduced into the mixture layer was reduced by increasing the packing density of the positive electrode mixture layer.

  DESCRIPTION OF SYMBOLS 10 Nonaqueous electrolyte secondary battery, 11 Battery case, 12 Case main body, 13 Sealing plate, 14 Electrode body, 15 Positive electrode, 16 Negative electrode, 17 Separator, 18 Positive electrode terminal, 19 Negative electrode terminal, 20 Positive electrode collector, 21 Positive electrode combination Agent layer, 22 lithium metal composite oxide, 23 conductive material

Claims (7)

It has a positive electrode mixture layer containing a layered lithium metal composite oxide and a conductive material,
The total specific surface area of the conductive material contained per unit volume of the positive electrode mixture layer is 20 m ² / cm ³ or more,
Positive electrode for non-aqueous electrolyte secondary battery. The packing density of the positive electrode mixture layer is 2.2 g / cm ³ or more,
The positive electrode for nonaqueous electrolyte secondary batteries according to claim 1. The lithium metal composite oxide has a general formula Li _{1 + x} M _a O _{2 + b} (wherein x + a = 1, −0.2 <x ≦ 0.2, −0.1 ≦ b ≦ 0.1, and M is And at least one selected from Ni, Co, Mn, and Al).
The positive electrode for nonaqueous electrolyte secondary batteries according to claim 1. The specific surface area of the conductive material is 30 m ² / g or more and 140 m ² / g or less.
The positive electrode for nonaqueous electrolyte secondary batteries of Claim 1 or 2. The electrical conductivity of the conductive material is 0.1Ω · cm or more and 0.3Ω · cm or less,
The positive electrode for nonaqueous electrolyte secondary batteries as described in any one of Claims 1-3. It is comprised with the positive electrode for nonaqueous electrolyte secondary batteries as described in any one of Claims 1-4, the negative electrode which has a negative mix layer, and the separator interposed between the said positive electrode and the said negative electrode. An electrode body;
A non-aqueous electrolyte,
With
Non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery according to claim 5, wherein the negative electrode mixture layer includes a lithium titanium composite oxide.