阅读说明：本技术 非水电解质二次电池用负极以及非水电解质二次电池
(Negative electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery
) 是由 四宫拓也 于 2018-05-24 设计创作，主要内容包括：本公开的目的在于提供一种放电负荷特性以及长期循环特性优异的非水电解质二次电池。本公开的实施方式的一例的非水电解质二次电池具备正极、负极(30)、隔板以及非水电解质。负极(30)具有负极集电体(31)和形成于负极集电体(31)上的负极合剂层(32)。负极合剂层(32)包括以碳被覆石墨(35)为主要成分的第1合剂层(33)和以石墨(36)为主要成分的第2合剂层(34),第1合剂层(33)配置在负极合剂层(32)的表面侧,第2合剂层(34)配置在负极集电体(31侧)。(The purpose of the present disclosure is to provide a nonaqueous electrolyte secondary battery having excellent discharge load characteristics and long-term cycle characteristics. A nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode (30), a separator, and a nonaqueous electrolyte. The negative electrode (30) has a negative electrode current collector (31) and a negative electrode mixture layer (32) formed on the negative electrode current collector (31). The negative electrode mixture layer (32) comprises a 1 st mixture layer (33) mainly composed of carbon-coated graphite (35) and a 2 nd mixture layer (34) mainly composed of graphite (36), the 1 st mixture layer (33) is disposed on the surface side of the negative electrode mixture layer (32), and the 2 nd mixture layer (34) is disposed on the negative electrode current collector (31) side.)
1. A negative electrode for a nonaqueous electrolyte secondary battery, comprising a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector,
the negative electrode material mixture layer includes a 1 st material mixture layer containing graphite coated with amorphous carbon as a main component and a 2 nd material mixture layer containing graphite not coated with amorphous carbon as a main component,
the 1 st mixture layer is disposed on the surface side of the negative electrode mixture layer, and the 2 nd mixture layer is disposed on the negative electrode current collector side.
2. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1,
the ratio of the thicknesses of the 1 st mixture layer and the 2 nd mixture layer is 30: 70-70: 30.
3. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1 or 2,
the 1 st and 2 nd mixture layers comprise SiOxWherein 0.5. ltoreq. x.ltoreq.1.6.
4. A nonaqueous electrolyte secondary battery comprising the negative electrode according to any one of claims 1 to 3, a positive electrode, and a nonaqueous electrolyte.
Technical Field
The present disclosure relates to a negative electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery.
Background
In a nonaqueous electrolyte secondary battery such as a lithium ion battery, it is widely known to use graphite as a negative electrode active material. For example, patent document 1 discloses a nonaqueous electrolyte secondary battery including, as a negative electrode active material, a mixture of coated graphite particles having particle surfaces coated with amorphous carbon and uncoated graphite particles having particle surfaces not coated with amorphous carbon, for the purpose of improving safety and cycle characteristics at the time of rapid charge.
Prior art documents
Patent document
Patent document 1: japanese laid-open patent publication No. 2005-294011
Disclosure of Invention
Problems to be solved by the invention
However, in the nonaqueous electrolyte secondary battery, improvement of discharge load characteristics and long-term cycle characteristics is an important issue. In the nonaqueous electrolyte secondary battery disclosed in patent document 1, there is room for improvement in discharge load characteristics and long-term cycle characteristics.
Means for solving the problems
A negative electrode for a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure is a negative electrode for a nonaqueous electrolyte secondary battery including a negative electrode collector and a negative electrode mixture layer formed on the negative electrode collector, the negative electrode mixture layer including a 1 st mixture layer containing graphite mainly coated with amorphous carbon and a 2 nd mixture layer containing graphite mainly uncoated with amorphous carbon, the 1 st mixture layer being disposed on a surface side of the negative electrode mixture layer, and the 2 nd mixture layer being disposed on a side of the negative electrode collector.
A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure is characterized by including the negative electrode, the positive electrode, and a nonaqueous electrolyte.
Effects of the invention
According to the negative electrode for a nonaqueous electrolyte secondary battery of one embodiment of the present disclosure, a nonaqueous electrolyte secondary battery excellent in discharge load characteristics and long-term cycle characteristics can be provided.
Drawings
Fig. 1 is a perspective view of a nonaqueous electrolyte secondary battery according to an example of the embodiment.
Fig. 2 is a sectional view of an electrode body according to an example of the embodiment.
Fig. 3 is a cross-sectional view of a negative electrode according to an example of the embodiment.
Detailed Description
As a means for improving the discharge load characteristics (output characteristics) of the nonaqueous electrolyte secondary battery, it is considered to apply graphite coated with amorphous carbon to the negative electrode active material. However, when carbon-coated graphite is used, the adhesion between the negative electrode mixture layer and the negative electrode current collector tends to be weak, and there is a problem that the conductivity is lowered. This decrease in conductivity causes an increase in resistance during a long-term cycle, and also affects output characteristics. On the other hand, in the case of using soft particles such as natural graphite, although good adhesion between the negative electrode mixture layer and the negative electrode current collector can be obtained, there is a problem that the lithium ion diffusibility in the negative electrode mixture layer is reduced and the output characteristics are greatly reduced because the particles are crushed and oriented during rolling in the electrode production.
As a result of intensive studies to solve the above problems, the inventors of the present invention succeeded in satisfying both excellent discharge load characteristics and long-term cycle characteristics by using a negative electrode mixture layer including a 1 st mixture layer mainly composed of carbon-coated graphite and a 2 nd mixture layer mainly composed of graphite not coated with amorphous carbon, the 1 st mixture layer being disposed on the surface side of the negative electrode mixture layer, and the 2 nd mixture layer being disposed on the negative electrode current collector side. According to the negative electrode having the negative electrode mixture layer, a nonaqueous electrolyte secondary battery having excellent discharge load characteristics and long-term cycle characteristics can be provided.
Hereinafter, the nonaqueous electrolyte secondary battery 10, which is a laminated battery including an outer package formed of a laminate sheet including a resin sheet and a metal layer, is exemplified as an example of the embodiment, but the nonaqueous electrolyte secondary battery of the present disclosure is not limited thereto. The nonaqueous electrolyte secondary battery of the present disclosure may be, for example, a cylindrical battery having a cylindrical metal case, a prismatic battery having a prismatic metal case, or the like.
Fig. 1 is a perspective view of a nonaqueous electrolyte secondary battery 10 as an example of the embodiment, and fig. 2 is a sectional view of an electrode body 12 constituting the nonaqueous electrolyte secondary battery 10. As illustrated in fig. 1 and 2, the nonaqueous electrolyte secondary battery 10 includes a package 11 and a power generating element housed in the package 11. A preferred example of the nonaqueous electrolyte secondary battery 10 is a lithium ion battery. The power generating element is composed of an electrode body 12 and a nonaqueous electrolyte. As illustrated in fig. 2, the electrode body 12 has a positive electrode 20, a negative electrode 30, and a separator 40, and has a structure in which the positive electrode 20 and the negative electrode 30 are wound in a spiral shape with the separator 40 interposed therebetween. The electrode body may have a structure in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with separators interposed therebetween.
The non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the water-insoluble medium include cyclic carbonates such as Ethylene Carbonate (EC) and Propylene Carbonate (PC), chain carbonates such as dimethyl carbonate (DMC) and Ethyl Methyl Carbonate (EMC), cyclic ethers, chain ethers, carboxylic acid esters, nitriles, amides, and mixed solvents of 2 or more of these. The nonaqueous solvent may contain a halogen-substituted compound (for example, 4-fluoroethylene carbonate) in which at least a part of the hydrogen atoms in the solvent is substituted with a halogen atom such as fluorine. The electrolyte salt is preferably LiBF4、LiPF6And the like lithium salts.
Outer package 11 is formed by laminating two laminated sheets. Each laminate sheet preferably has a laminated structure in which at least one resin sheet (resin layer) is laminated on each of both surfaces of the metal layer, and the resin layers in contact with each other in each sheet are made of a heat-weldable resin. The metal layer is a thin film layer mainly composed of aluminum, for example, and has a function of preventing permeation of moisture and the like.
The exterior body 11 includes a housing portion 13 for housing the power generating element and a seal portion 14 formed around the housing portion 13. One of the laminated sheets constituting the exterior body 11 is formed into a cup shape, and a flat substantially rectangular parallelepiped housing portion 13 is formed in the sheet. The housing portion 13 is formed by, for example, drawing at least one of the laminated films, and projects toward the opposite side of the other laminated sheet disposed to face the other laminated sheet. The sealing portion 14 is formed by thermally welding the end portions of the laminated films to each other, and seals the internal space of the housing portion 13 in which the power generating element is housed.
The nonaqueous electrolyte secondary battery 10 includes a pair of electrode terminals (a positive electrode terminal 15 and a negative electrode terminal 16) drawn out from the exterior body 11. The positive electrode terminal 15 and the negative electrode terminal 16 are drawn out from one end in the longitudinal direction of the package 11. The positive electrode terminal 15 and the negative electrode terminal 16 are both substantially flat plate-like bodies, and are joined to the respective laminate films in the sealing portion 14, and are drawn out to the outside of the package 11 from between the two laminate sheets.
The electrode body 12 preferably has a flat shape so as to be efficiently housed in the housing 13. The flat shape of the electrode body 12 is formed by winding each electrode and the separator 40 into a cylindrical shape, and then flattening the cylindrical shape in the radial direction. Alternatively, the electrode body 12 may be formed by winding the electrodes and the separators 40 in a flat shape. The positive electrode 20 is provided with an exposed portion that exposes the surface of the positive electrode current collector 21, and the positive electrode terminal 15 is connected to this exposed portion. Alternatively, a conductive member may be connected to the exposed portion, and the positive electrode terminal 15 may be connected to the conductive member. Similarly, the negative electrode 30 may have the negative electrode terminal 16 connected to the exposed portion of the negative electrode current collector 31, or the negative electrode terminal 16 may be connected to a conductive member connected to the exposed portion.
Hereinafter, the respective constituent elements (the positive electrode 20, the negative electrode 30, and the separator 40) of the electrode body 12, particularly, the negative electrode 30 will be described in detail with reference to fig. 2 and 3 as appropriate. Fig. 3 is a cross-sectional view of a negative electrode 30 as an example of the embodiment.
[ Positive electrode ]
As illustrated in fig. 2, the positive electrode 20 includes a positive electrode current collector 21 and a positive electrode mixture layer 22 formed on the positive electrode current collector 21. As the positive electrode current collector 21, a metal foil such as aluminum which is stable in the potential range of the positive electrode 20, a film in which the metal is disposed on the surface layer, or the like can be used. The positive electrode mixture layer 22 includes a positive electrode active material, a conductive agent, and a binder. The positive electrode 20 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like onto the positive electrode current collector 21, drying the coating film, rolling the coating film, and forming positive electrode mixture layers 22 on both surfaces of the positive electrode current collector 21.
The positive electrode active material includes a lithium transition metal oxide as a main component. The positive electrode active material may be substantially composed of only the lithium transition metal oxide, or may be formed by fixing inorganic compound particles such as alumina and a compound containing a lanthanum element to the surface of particles of the lithium transition metal oxide. The lithium transition metal oxide may be used in 1 kind, or 2 or more kinds may be used in combination.
Examples of the metal element contained In the lithium transition metal oxide include nickel (Ni), cobalt (Co), manganese (Mn), aluminum (Al), boron (B), magnesium (Mg), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), gallium (Ga), strontium (Sr), zirconium (Zr), niobium (Nb), indium (In), tin (Sn), tantalum (Ta), and tungsten (W). One example of a preferred lithium transition metal oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.
Examples of the conductive agent contained in the positive electrode mixture layer 22 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 22 include fluorine resins such as Polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), Polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins may be used together with cellulose derivatives such as carboxymethyl cellulose (CMC) or salts thereof, polyethylene oxide (PEO), and the like.
[ negative electrode ]
As illustrated in fig. 2 and 3, the negative electrode 30 includes a negative electrode current collector 31 and a negative electrode mixture layer 32 formed on the negative electrode current collector 31. As the negative electrode current collector 31, a metal foil such as copper which is stable in the potential range of the negative electrode 30, a film on which the metal is disposed on the surface layer, or the like can be used. The negative electrode mixture layer 32 includes a negative electrode active material and a binder. The negative electrode 30 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and the like to a negative electrode current collector 31, drying the coating, rolling the coating, and forming negative electrode mixture layers 32 on both surfaces of the negative electrode current collector 31.
The negative electrode mixture layer 32 includes a 1 st mixture layer 33 mainly composed of carbon-coated graphite 35 having a coating of amorphous carbon and a 2 nd mixture layer 34 mainly composed of graphite 36 whose particle surface is not coated with amorphous carbon. The 1 st material mixture layer 33 is disposed on the surface side of the negative electrode material mixture layer 32, and the 2 nd material mixture layer 34 is disposed on the negative electrode current collector 31 side. The negative electrode mixture layers 32 are formed on both surfaces of the negative electrode current collector 31, and each has a 2-layer structure composed of a 1 st mixture layer 33 and a 2 nd mixture layer 34.
By applying the 2-layer structure to the negative electrode mixture layer 32, good lithium ion diffusibility can be obtained while ensuring good adhesion between the negative electrode mixture layer 32 and the negative electrode current collector 31. This makes it possible to realize the nonaqueous electrolyte secondary battery 10 having excellent discharge load characteristics and long-term cycle characteristics. In other words, by disposing the carbon-coated graphite 35 having a shape that is less likely to be crushed during rolling and is more spherical than the graphite 36 on the surface side of the negative electrode mixture layer 32, it is possible to achieve good diffusion of lithium ions into the inside of the negative electrode mixture layer 32, and by disposing the graphite 36 in a portion in contact with the negative electrode current collector 31, it is possible to ensure good adhesion between the negative electrode mixture layer 32 and the negative electrode current collector 31.
Here, the main component is a component having the largest mass ratio among the components constituting the 1 st material mixture layer 33, when the 1 st material mixture layer 33 is described as an example. The carbon-coated graphite 35 is contained in an amount of preferably 50 mass% or more, more preferably 80 mass% or more, and particularly preferably 90 mass% or more, based on the total mass of the 1 st material mixture layer 33. The graphite 36 is contained in an amount of preferably 50 mass% or more, more preferably 80 mass% or more, and particularly preferably 90 mass% or more, based on the total mass of the 2 nd mixture layer 34.
In the negative electrode mixture layer 32, the 2 nd mixture layer 34 is preferably formed directly on the surface of the negative electrode current collector 31, and the 2 nd mixture layer 34 is preferably interposed between the negative electrode current collector 31 and the 1 st mixture layer 33. The 2 nd mixture layer 34 is formed on substantially the entire surface of the negative electrode current collector 31 except for the exposed portion for electrical connection to the negative electrode terminal 16. The 1 st material mixture layer 33 is directly formed on substantially the entire surface of the 2 nd material mixture layer 34.
The ratio of the thicknesses of the 1 st mixture layer 33 and the 2 nd mixture layer 34 is preferably 10: 90 to 90: 10, and more preferably 30: 70 to 70: 30. The ratio of the thicknesses of the layers may be 40: 60 to 60: 40, or 50: 50. If the ratio of the thicknesses of the respective layers is within this range, it is easy to achieve both the discharge load characteristics and the long-term cycle characteristics of the battery. The thickness of the negative electrode mixture layer 32 (the total thickness of the 1 st mixture layer 33 and the 2 nd mixture layer 34) is, for example, 50 to 150 μm, and preferably 60 to 120 μm on one side of the negative electrode current collector 31. Examples of preferable thicknesses of the 1 st mixture layer 33 and the 2 nd mixture layer 34 are 30 μm to 60 μm, respectively.
The carbon-coated graphite 35 is core-shell particles having graphite 35a and an amorphous carbon film 35b formed on the surface of the graphite 35 a. The amorphous carbon film 35b is a carbon film in a state of amorphous or microcrystalline disordered layer structure in which the graphite crystal structure does not reach, and is composed of, for example, carbon having a d (002) plane spacing of more than 0.340nm by X-ray diffraction. The amorphous carbon film 35b is preferably formed on the entire particle surface of the graphite 35 a. The amorphous carbon film 35b has a function of, for example, reducing decomposition of the electrolyte and increasing the hardness of the carbon-coated graphite 35. As described above, the carbon-coated graphite 35 is harder than the graphite 36 not coated with amorphous carbon and is less likely to be crushed during rolling.
Specific examples of the amorphous carbon film 35b include carbon black such as hard carbon (hard graphitizable carbon), soft carbon (graphitizable carbon), acetylene black, ketjen black, thermal cracking carbon black, and furnace carbon black, carbon fiber, and activated carbon. An example of a preferable range of the thickness of the amorphous carbon film 35b is 10nm to 200 nm. The thickness of the amorphous carbon film 35b can be measured by observing particle sections of the carbon-coated graphite 35 with a Scanning Electron Microscope (SEM).
The amorphous carbon film 35b can be formed by mixing coal tar, pitch, naphthalene, anthracene, phenanthrene, or the like with the graphite 35a and performing a heat treatment at a temperature of 800 to 1200 ℃. The amorphous carbon film 35b is formed in an amount of, for example, 0.5 to 15 mass% with respect to the mass of the carbon-coated graphite 35.
The graphite 35a, 36 may be either natural graphite or artificial graphite. The same graphite may be used for the graphite 35a and 36. Further, artificial graphite may be applied to the graphite 35a, and natural graphite may be applied to the graphite 36. In this case, the difference in hardness between the carbon-coated graphite 35 and the graphite 36 becomes larger, and it becomes easy to achieve both the discharge load characteristic and the long-term cycle characteristic of the battery. The average particle diameter of the graphite particles 35a, 36 may be, for example, 5 μm to 30 μm or 10 μm to 25 μm, or may be substantially equal to each other. The average particle diameter of the graphite particles is a volume average particle diameter measured by a laser diffraction method, and means a median diameter at which a volume accumulation value is 50% in a particle diameter distribution. Since the amorphous carbon film 35b is thin, the particle diameter of the carbon-coated graphite 35 is substantially equal to the particle diameter of the graphite 35 a.
The negative electrode mixture layer 32 may contain a negative electrode active material other than graphite. Examples of the negative electrode active material other than graphite include metals that are alloyed with lithium, such as silicon (Si) and tin (Sn), and oxides containing metal elements such as Si and Sn. Among them, SiO is preferablexSilicon oxide as shown. SiO 2xThe content of the negative electrode active material other than graphite is preferably 10% by mass or less, and more preferably 5% by mass or less, based on the total mass of the negative electrode active material.
The negative electrode mixture layer 32 contains SiOxFor example, in the case of a negative electrode active material other than graphite, the negative electrode active material may be included in only one of the 1 st material mixture layer 33 and the 2 nd material mixture layer 34, but is preferably included in both layers. Further, SiO in each layerxEtc. may be different from each other, but are preferably substantially the same.
From SiOxThe silicon oxide represented has, for example, SiO in an amorphous state2A structure in which fine particles of Si are dispersed in a matrix. One example of a preferred silicon oxide is SiOx(x is more than or equal to 0.5 and less than or equal to 1.6). From SiOxThe silicon oxide represented by may contain Li2ySiO(2+y)(0＜y＜2) The lithium silicate represented may have a structure in which fine particles of Si are dispersed in a lithium silicate phase.
Preferably in the form of SiOxThe surface of the silicon oxide particles shown is formed with a conductive film made of a material having higher conductivity than silicon oxide. The material constituting the conductive coating is preferably at least one selected from the group consisting of carbon materials, metals, and metal compounds. Among them, a carbon material is particularly preferably used, and the carbon material may be amorphous carbon similar to the amorphous carbon film 35 b. Conductive coating film for example with respect to SiOxThe particles are formed in an amount of 0.5 to 10 mass%.
As the binder contained in the negative electrode mixture layer 32, as in the case of the positive electrode, a fluororesin, PAN, a polyimide resin, an acrylic resin, a polyolefin resin, or the like can be used. When the mixture slurry is prepared using an aqueous solvent, it is preferable to use CMC or a salt thereof, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol, or the like.
The 1 st adhesive layer 33 and the 2 nd adhesive layer 34 can use the same adhesive. The content of the binder in the 1 st material mixture layer 33 and the 2 nd material mixture layer 34 is, for example, 0.5 to 5 mass% with respect to the total mass of each layer. The content of the binder may be different from each other in the 1 st material mixture layer 33 and the 2 nd material mixture layer 34, or may be substantially equal to each other.
[ partition board ]
The separator 40 is a porous sheet having ion permeability and insulation properties. Specific examples of the porous sheet include a microporous film, a woven fabric, and a nonwoven fabric. As a material of the separator 40, olefin resin such as polyethylene and polypropylene, cellulose, and the like are preferable. The separator 40 may have a single-layer structure or a stacked structure. A heat-resistant layer may be formed on the surface of the separator 40.
