WO2021059706A1 - リチウムイオン二次電池用負極及びリチウムイオン二次電池 - Google Patents
リチウムイオン二次電池用負極及びリチウムイオン二次電池 Download PDFInfo
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E60/10—Energy storage using batteries
Definitions
- the present disclosure relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery.
- Patent Document 1 discloses a negative electrode for a lithium ion secondary battery including a negative electrode mixture layer containing a Si-containing compound and graphite as a negative electrode active material.
- An object of the present disclosure is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery capable of improving the charge / discharge cycle characteristics of the lithium ion secondary battery even when the alloying material is used as the negative electrode active material. To provide.
- the negative electrode for a lithium ion secondary battery which is one aspect of the present disclosure, includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector, and the negative electrode mixture layer has voids inside particles.
- the negative electrode mixture having a negative electrode active material containing graphite particles A having a ratio of 10% or less, graphite particles B having a particle internal void ratio of more than 10%, and an alloying material alloying with lithium.
- the layer includes a first layer formed on the negative electrode current collector and a second layer formed on the first layer, and the graphite particles A and the alloying material are the first layer.
- the graphite particles B are contained more in the first layer than in the second layer, and the content of the alloying material is the negative electrode activity in the negative electrode mixture layer. It is 15% by mass or less with respect to the total amount of the substance.
- the lithium ion secondary battery according to one aspect of the present disclosure includes the negative electrode for the lithium ion secondary battery.
- the alloying material is used as the negative electrode active material, it is possible to improve the charge / discharge cycle characteristics of the lithium ion secondary battery.
- the negative electrode for a lithium ion secondary battery which is one aspect of the present disclosure, includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector, and the negative electrode mixture layer has voids inside particles.
- the negative electrode mixture having a negative electrode active material containing graphite particles A having a ratio of 10% or less, graphite particles B having a particle internal void ratio of more than 10%, and an alloying material alloying with lithium.
- the layer includes a first layer formed on the negative electrode current collector and a second layer formed on the first layer, and the graphite particles A and the alloying material are the first layer.
- the graphite particles B are contained more in the first layer than in the second layer, and the content of the alloying material is the negative electrode activity in the negative electrode mixture layer. It is 15% by mass or less with respect to the total amount of the substance.
- the alloying material is used as the negative electrode active material, it is possible to improve the charge / discharge cycle characteristics of the lithium ion secondary battery. The mechanism that exerts this effect is not sufficiently clear, but the following can be inferred, for example.
- the layer containing the graphite particles A and the alloying material is a layer having few interparticle voids in the negative electrode active material.
- the negative electrode containing the alloying material expands due to a large volume change of the alloying material due to charging and discharging, but on the other hand, the battery case containing the expanding negative electrode tries to maintain its shape, so that the negative electrode Is applied with pressure in the thickness direction. Therefore, by arranging more graphite particles A having a particle internal porosity of 10% or less and an alloying material in the second layer on the surface side of the negative electrode than in the first layer, the above-mentioned pressure is effectively utilized.
- the surface side of the negative electrode is a surface facing the separator and the positive electrode. Further, since the graphite particles A having a particle internal void ratio of 10% or less are not easily crushed during the negative electrode production, the binding property between the negative electrode current collector and the graphite particles A is low, but the particle internal void ratio is 10%. Since the natural graphite particles B are easily crushed during the production of the negative electrode, the adhesiveness between the negative electrode current collector and the graphite particles B is high.
- the alloying material undergoes a large volume change with charging and discharging, the adhesiveness between the negative electrode current collector and the alloying material tends to decrease. Therefore, in the first layer arranged on the negative electrode current collector, the graphite particles A having a particle internal void ratio of 10% or less and the graphite particles B having a small amount of alloying material and having a particle internal void ratio of more than 10% are used. By using a large number of layers, it is possible to prevent particles of the negative electrode active material from peeling off from the negative electrode current collector.
- the negative electrode of the present disclosure even if the lithium ion secondary battery is repeatedly charged and discharged, the formation of interparticle voids in the negative electrode active material and the peeling of the negative electrode active material from the negative electrode current collector are suppressed. Therefore, the increase of particles of the negative electrode active material isolated from the conductive path in the negative electrode mixture layer is suppressed, so that the charge / discharge cycle characteristics of the lithium ion secondary battery are improved.
- this effect is effective when the content of the alloying material is 15% by mass or less with respect to the total amount of the negative electrode active material in the negative electrode mixture layer.
- number (1) to numerical value (2) means a numerical value (1) or more and a numerical value (2) or less.
- FIG. 1 is a cross-sectional view of a lithium ion secondary battery which is an example of the embodiment.
- a winding type electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound via a separator 13, an electrolyte, and above and below the electrode body 14, are arranged, respectively.
- the insulating plates 18 and 19 and a battery case 15 for accommodating the above members are provided.
- the battery case 15 is composed of a bottomed cylindrical case body 16 and a sealing body 17 that closes an opening of the case body 16.
- the winding type electrode body 14 instead of the winding type electrode body 14, another form of an electrode body such as a laminated type electrode body in which positive electrodes and negative electrodes are alternately laminated via a separator may be applied.
- the battery case 15 include a metal case such as a cylinder, a square, a coin, and a button, and a resin case (laminated battery) formed by laminating a resin sheet.
- the case body 16 is, for example, a bottomed cylindrical metal container.
- a gasket 28 is provided between the case body 16 and the sealing body 17 to ensure the airtightness inside the battery.
- the case body 16 has, for example, an overhanging portion 22 that supports the sealing body 17 with a part of the side surface overhanging inward.
- the overhanging portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and the sealing body 17 is supported on the upper surface thereof.
- the sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in this order from the electrode body 14 side.
- Each member constituting the sealing body 17 has, for example, a disk shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other.
- the lower valve body 24 and the upper valve body 26 are connected to each other at the central portion thereof, and an insulating member 25 is interposed between the peripheral portions thereof.
- the lower valve body 24 When the internal pressure of the lithium ion secondary battery 10 rises due to heat generated by an internal short circuit or the like, for example, the lower valve body 24 is deformed and broken so as to push the upper valve body 26 toward the cap 27 side, and the lower valve body 24 and the upper valve body are broken. The current path between 26 is cut off. When the internal pressure further rises, the upper valve body 26 breaks and gas is discharged from the opening of the cap 27.
- the positive electrode lead 20 attached to the positive electrode 11 extends to the sealing body 17 side through the through hole of the insulating plate 18, and the negative electrode lead 21 attached to the negative electrode 12 is the insulating plate. It extends to the bottom side of the case body 16 through the outside of 19.
- the positive electrode lead 20 is connected to the lower surface of the filter 23, which is the bottom plate of the sealing body 17, by welding or the like, and the cap 27, which is the top plate of the sealing body 17 electrically connected to the filter 23, serves as the positive electrode terminal.
- the negative electrode lead 21 is connected to the inner surface of the bottom of the case body 16 by welding or the like, and the case body 16 serves as a negative electrode terminal.
- the positive electrode 11, the negative electrode 12, the separator 13, and the electrolyte constituting the lithium ion secondary battery 10 will be described in detail.
- the positive electrode 11 includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector.
- a metal foil stable in the potential range of the positive electrode such as aluminum or an aluminum alloy, a film in which the metal is arranged on the surface layer, or the like can be used.
- the positive electrode mixture layer includes, for example, a positive electrode active material, a binder, and a conductive material.
- the positive electrode mixture layer is preferably formed on both sides of the positive electrode current collector.
- a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive material, etc. is applied onto the positive electrode current collector, the coating film is dried and rolled, and the positive electrode mixture layer is formed on the positive electrode current collector. It can be manufactured by forming it on both sides.
- the positive electrode active material is composed mainly of a lithium-containing metal composite oxide.
- Metal elements contained in the lithium-containing metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn. , Ta, W, Ca, Sb, Pb, Bi, Ge and the like can be exemplified.
- An example of a suitable lithium-containing metal composite oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.
- Examples of the conductive material contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, graphene, carbon nanotubes, and graphite.
- Examples of the binder contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefins. These resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO) and the like.
- the negative electrode 12 includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector.
- a negative electrode current collector for example, a foil of a metal stable in the potential range of the negative electrode such as copper or a copper alloy, a film in which the metal is arranged on the surface layer, or the like can be used.
- the negative electrode mixture layer is composed of graphite particles A having a particle internal porosity of 10% or less, graphite particles B having a particle internal porosity of more than 10%, and an alloying material alloying with lithium. Will be done. Each of the above materials will be described in detail below.
- FIG. 2 is a schematic view showing a cross section of graphite particles.
- the graphite particle 30 has a closed void 34 which is not connected to the particle surface from the inside of the particle and a void 36 which is connected to the particle surface from the inside of the particle in a cross-sectional view of the graphite particle 30. ..
- the void 34 is hereinafter referred to as an internal void 34.
- the void 36 is hereinafter referred to as an external void 36.
- the internal porosity of the graphite particles is a two-dimensional value obtained from the ratio of the area of the internal voids of the graphite particles to the cross-sectional area of the graphite particles.
- the internal porosity of the graphite particles A may be 10% or less, preferably 1% to 5%, and more preferably 3% to 5% in terms of improving charge / discharge cycle characteristics. ..
- the internal porosity of the graphite particles B may be more than 10%, preferably 12% to 25%, more preferably 12% to 23%, in terms of being appropriately crushed by the compression step in the negative electrode production. Is.
- the internal porosity of the graphite particles is determined by the following procedure.
- ⁇ Measurement method of internal porosity> The cross section of the negative electrode active material is exposed.
- Examples of the method of exposing the cross section include a method of cutting out a part of the negative electrode and processing it with an ion milling device (for example, IM4000PLUS manufactured by Hitachi High-Tech) to expose the cross section of the negative electrode mixture layer.
- an ion milling device for example, IM4000PLUS manufactured by Hitachi High-Tech
- the cross-sectional image obtained as described above is taken into a computer and binarized using image analysis software (for example, ImageJ manufactured by the American National Institutes of Health) to make the cross-sectional image of the particles in the cross-sectional image black.
- image analysis software for example, ImageJ manufactured by the American National Institutes of Health
- a binarized image obtained by converting the voids existing in the cross section of the particle into white is obtained.
- the area of the graphite particle cross section and the area of the internal voids existing in the graphite particle cross section are calculated.
- the area of the cross section of the graphite particles refers to the area of the region surrounded by the outer periphery of the graphite particles, that is, the area of the entire cross section of the graphite particles.
- Graphite particles A and B are produced, for example, as follows.
- ⁇ Graphite particles A having an internal porosity of 10% or less> For example, coke (precursor), which is the main raw material, is crushed to a predetermined size, and the coke (precursor), which is the main raw material, is agglomerated with a binder, fired at a temperature of 2600 ° C. or higher, graphitized, and then sieved. A graphite particle A having a desired size is obtained.
- the internal porosity can be adjusted to 10% or less depending on the particle size of the precursor after pulverization, the particle size of the precursor in an agglomerated state, and the like.
- the average particle size (median diameter D50) of the precursor after pulverization is preferably in the range of 12 ⁇ m to 20 ⁇ m.
- coke precursor
- a binder aggregated with a binder
- the graphitized block-shaped molded product is pulverized and sieved to obtain graphite particles B having a desired size.
- the internal porosity can be adjusted to more than 10% depending on the amount of volatile components added to the block-shaped molded product.
- the binder When a part of the binder added to coke (precursor) volatilizes during firing, the binder can be used as a volatile component. Pitch is exemplified as such a binder.
- the graphite particles A and B used in the present embodiment are not particularly limited to natural graphite, artificial graphite and the like, but artificial graphite is preferable in terms of ease of adjusting the internal porosity and the like.
- the surface spacing (d 002 ) of the (002) planes of the graphite particles A and B used in the present embodiment by the X-ray wide-angle diffraction method is preferably, for example, 0.3354 nm or more, and is 0.3357 nm or more. Is more preferable, and it is preferably less than 0.340 nm, and more preferably 0.338 nm or less.
- the crystallite size (Lc (002)) of the graphite particles A and B used in the present embodiment determined by the X-ray diffraction method is, for example, preferably 5 nm or more, and more preferably 10 nm or more. Further, it is preferably 300 nm or less, and more preferably 200 nm or less.
- the average particle size of the graphite particles A and B is not particularly limited, but is, for example, 1 ⁇ m to 30 ⁇ m.
- the average particle size means a volume average particle size (Dv50) at which the integrated volume value is 50% in the particle size distribution measured by the laser diffraction / scattering method.
- the alloying material is composed of an element that alloys with lithium, a compound that contains an element that alloys lithium, or both.
- the element that alloys with lithium that can be applied to the negative electrode active material include Al, Ga, In, Si, Ge, Sn, Pb, As, Sb, and Bi. Among them, Si and Sn are preferable, and Si is particularly preferable, from the viewpoint of increasing the capacity.
- Si-containing compound examples include a silicon oxide phase, a Si-containing compound dispersed in the silicon oxide phase, a lithium silicate phase, and a Si-containing compound dispersed in the lithium silicate phase.
- the silicon oxide phase and the compound containing Si dispersed in the silicon oxide phase are hereinafter referred to as “SiO”.
- the lithium silicate phase and the compound containing Si dispersed in the lithium silicate phase are hereinafter referred to as “LSX”.
- a conductive layer made of a highly conductive material may be formed on the surface of the SiO and LSX particles.
- An example of a suitable conductive layer is a carbon coating made of a carbon material.
- the carbon film is composed of, for example, carbon black, acetylene black, ketjen black, graphite, and a mixture of two or more of these.
- Examples of the method of carbon-coating the particle surfaces of SiO and LSX include a CVD method using acetylene, methane, etc., a method of mixing coal pitch, petroleum pitch, phenol resin, etc. with SiO, LSX particles, and performing heat treatment. it can.
- a carbon film may be formed by fixing carbon powder such as carbon black to the particle surface using a binder.
- a suitable SiO has a sea-island structure in which fine Si particles are substantially uniformly dispersed in an amorphous silicon oxide phase, and is represented by the general formula SiO x (0.5 ⁇ x ⁇ 1.6).
- the content of Si particles is preferably 35 to 75% by mass with respect to the total mass of SiO, from the viewpoint of achieving both battery capacity and cycle characteristics.
- the average particle size of the Si particles dispersed in the silicon oxide phase is generally 500 nm or less, preferably 200 nm or less, and more preferably 50 nm or less before charging / discharging. After charging and discharging, 400 nm or less is preferable, and 100 nm or less is more preferable. By making the Si particles finer, the volume change during charging and discharging becomes smaller and the cycle characteristics are improved.
- the average particle size of the Si particles is measured by observing the cross section of SiO using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and specifically, the longest diameter of 100 Si particles. Obtained as an average value.
- the silicon oxide phase is composed of, for example, a set of particles finer than Si particles.
- a suitable LSX has a sea-island structure in which fine Si particles are substantially uniformly dispersed in a lithium silicate phase represented by the general formula Li 2z SiO (2 + z) (0 ⁇ z ⁇ 2).
- the content of Si particles is preferably 35 to 75% by mass with respect to the total mass of LSX, as in the case of SiO.
- the average particle size of the Si particles is generally 500 nm or less, preferably 200 nm or less, and more preferably 50 nm or less before charging / discharging.
- the lithium silicate phase is composed of, for example, an aggregate of particles finer than Si particles.
- SiO can be produced by the following steps. (1) Si and silicon oxide are mixed at a weight ratio of, for example, 20:80 to 95: 5 to prepare a mixture. (2) At least before or after the preparation of the above mixture, Si and silicon oxide are pulverized into fine particles by, for example, a ball mill. (3) The pulverized mixture is heat-treated at 600 to 1000 ° C., for example, in an inert atmosphere.
- LSX can be produced by using lithium silicate instead of silicon oxide.
- FIG. 3 is a cross-sectional view of a negative electrode which is an example of the embodiment.
- the negative electrode mixture layer 42 formed on the negative electrode current collector 40 includes the first layer 44 and the second layer 46.
- the first layer 44 is arranged on the negative electrode current collector 40
- the second layer 46 is arranged on the first layer 44.
- the negative electrode mixture layer 42 is preferably formed on both surfaces of the negative electrode current collector 40.
- the second layer 46 when the second layer 46 is "arranged" on the first layer 44 "above", the second layer 46 may be arranged directly on the first layer 44, or the second layer 46 and the second layer 46 and the second layer 46 may be arranged directly.
- An intermediate layer may be interposed between the layer 44 and the layer 44.
- the first layer 44 contains graphite particles B as a negative electrode active material.
- the graphite particles B contained in the first layer 44 may be larger than the graphite particles B contained in the second layer 46 in terms of improving the charge / discharge cycle characteristics, but the total amount of the graphite particles B in the negative electrode mixture layer 42. It is preferably in the range of 60% by mass to 100% by mass.
- the first layer 44 may contain graphite particles A and an alloying material as the negative electrode active material, but the content of the graphite particles A in the first layer 44 is the negative electrode combination in terms of improving the charge / discharge cycle characteristics.
- the second layer 46 contains graphite particles A and an alloying material as the negative electrode active material.
- the graphite particles A contained in the second layer 46 may be larger than the graphite particles A contained in the first layer 44 in terms of improving the charge / discharge cycle characteristics, but the total amount of the graphite particles A in the negative electrode mixture layer 42. It is preferably in the range of 60% by mass to 100% by mass.
- the alloying material contained in the second layer 46 may be larger than the alloying material contained in the first layer 44 in terms of improving the charge / discharge cycle characteristics, but the alloying material in the negative electrode mixture layer 42 may be used. It is preferably in the range of 60% by mass to 100% by mass with respect to the total amount of.
- the content of the alloying material is the total amount of the negative electrode active material in the negative electrode mixture layer 42. It is necessary to make it 15% by mass or less.
- the lower limit of the content of the alloying material is 5% by mass or more with respect to the total amount of the negative electrode active material in the negative electrode mixture layer 42 in terms of increasing the capacity of the lithium ion secondary battery. It is preferably 8% by mass or more, and more preferably 8% by mass or more.
- the second layer 46 may contain graphite particles B as the negative electrode active material, but the content of the graphite particles B in the second layer 46 is in the negative electrode mixture layer 42 in terms of improving charge / discharge cycle characteristics. It is preferably 40% by mass or less with respect to the total amount of graphite particles B in the above.
- the negative electrode mixture layer 42 preferably contains polyacrylic acid (hereinafter, PAA) or a salt thereof.
- PAA polyacrylic acid
- the salt of PAA is, for example, a lithium salt, a sodium salt, a potassium salt, an ammonium salt and the like.
- PAA or a salt thereof may be contained in the first layer 44 and the second layer 46 in the same amount or in a large amount in either of them, but in order to prevent the alloying material from falling off from the negative electrode 12.
- the second layer 46 contains more than the first layer 44.
- the negative electrode mixture layer 42 preferably contains styrene-butadiene rubber.
- styrene-butadiene rubber By containing the styrene-butadiene rubber, the particles of the negative electrode active material are strongly bonded to each other, and the charge / discharge cycle characteristics can be improved.
- the styrene-butadiene rubber may be contained in the first layer 44 and the second layer 46 in the same amount, or may be contained in a large amount in either of them, but suppresses the negative electrode active material from peeling from the negative electrode current collector 40. Therefore, it is preferable that the first layer 44 contains more than the second layer 46.
- the negative electrode mixture layer 42 preferably contains carboxymethyl cellulose (hereinafter, CMC) or a salt thereof.
- CMC carboxymethyl cellulose
- the salt of CMC is, for example, a lithium salt, a sodium salt, a potassium salt, an ammonium salt and the like.
- the CMC or a salt thereof may be contained in the first layer 44 and the second layer 46 in the same amount or in a large amount in either of them, but in order to prevent the alloying material from falling off from the negative electrode 12.
- the second layer 46 contains more than the first layer 44.
- the negative electrode mixture layer 42 may contain a binder other than PAA or a salt thereof, styrene butadiene rubber, CMC or a salt thereof.
- a binder include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimides, acrylic resins, and polyolefins.
- the content of the binder in the negative electrode mixture layer 42 is, for example, preferably 0.5% by mass to 10% by mass, more preferably 1% by mass to 5% by mass, based on the total amount of the negative electrode mixture layer 42.
- the negative electrode mixture layer 42 preferably contains fibrous carbon. Since the fibrous carbon is contained, a good conductive path is formed in the negative electrode mixture layer 42, and the charge / discharge cycle characteristics can be improved.
- the fibrous carbon may be contained in the first layer 44 and the second layer 46 in the same amount or in a larger amount in either of them, but in terms of maintaining a conductive path to the alloying material, the first layer It is preferable that the second layer 46 contains more than the layer 44.
- fibrous carbon examples include carbon nanotubes (CNT) and carbon nanofibers.
- the CNTs may be not only single-walled CNTs, but also double-walled CNTs, multi-walled CNTs, and mixtures thereof. Further, the CNT may be a vapor-grown carbon fiber.
- the fibrous carbon has, for example, a diameter of 2 nm to 20 ⁇ m and a total length of 0.03 ⁇ m to 500 ⁇ m.
- the content of fibrous carbon in the negative electrode mixture layer 42 is, for example, preferably 0.01% by mass to 5% by mass, more preferably 0.02% by mass to 2% by mass, based on the total amount of the negative electrode mixture layer 42. preferable.
- the thickness of the negative electrode mixture layer 42 is, for example, 30 ⁇ m to 100 ⁇ m or 50 ⁇ m to 80 ⁇ m on one side of the negative electrode current collector 40.
- the thicknesses of the first layer 44 and the second layer 46 may be the same or different from each other.
- an intermediate layer may be provided between the first layer 44 and the second layer 46.
- the intermediate layer may contain the above-mentioned graphite particles A, graphite particles B, and an alloying material, or may contain other conventionally known negative electrode active materials and the like.
- the intermediate layer may be designed within a range that does not impair the effects of the present disclosure.
- the negative electrode 12 is manufactured by, for example, the following method.
- a first negative electrode mixture slurry for the first layer 44 containing graphite particles B, a binder, and the like is prepared.
- a second negative electrode mixture slurry for the second layer 46 containing graphite particles A, an alloying material, a binder and the like is prepared.
- the first negative electrode mixture slurry is applied onto the negative electrode current collector 40, and the coating film is dried to form the first layer 44 on the negative electrode current collector 40.
- the second negative electrode mixture slurry is applied onto the first layer 44, the coating film is dried to form the second layer 46 on the first layer 44, and then the first layer 44 and the second layer 46 are formed. Compress. In this way, the negative electrode 12 in which the negative electrode mixture layer 42 including the first layer 44 and the second layer 46 is formed on the negative electrode current collector 40 is obtained.
- a porous sheet having ion permeability and insulating property is used as the separator 13.
- the porous sheet include a microporous thin film, a woven fabric, and a non-woven fabric.
- an olefin resin such as a copolymer containing at least one of polyethylene, polypropylene, ethylene and propylene, cellulose and the like are suitable.
- the separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 13.
- the electrolyte contains a solvent and an electrolyte salt.
- the electrolyte is not limited to the liquid electrolyte, and may be a solid electrolyte using a gel polymer or the like.
- the electrolyte salt for example, lithium salts such as LiFSI, LiTFSI, LiBF 4 , LiPF 6 and the like are used.
- the solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl acetate (MA), methyl propionate (MP) and the like.
- the non-aqueous solvent may contain a halogen substituent in which at least a part of hydrogen in these solvents is replaced with a halogen atom such as fluorine.
- halogen substituent examples include a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), a fluorinated chain carbonate, and a fluorinated chain carboxylic acid ester such as methyl fluoropropionate (FMP).
- FEC fluoroethylene carbonate
- FMP fluorinated chain carboxylic acid ester
- LiNiCoAlO 2 as the positive electrode active material is mixed so as to be 98 parts by mass, acetylene black as the conductive material is 1 part by mass, and polyvinylidene fluoride powder as the binder is 1 part by mass, and further N-methyl-2-pyrrolidone is added.
- An appropriate amount of (NMP) was added to prepare a positive electrode mixture slurry. This slurry is applied to both sides of a current collector made of aluminum foil (thickness 15 ⁇ m) by the doctor blade method, the coating film is dried, and then the coating film is compressed by a rolling roller to form positive electrodes on both sides of the positive electrode current collector. A positive electrode on which the active material layer was formed was produced.
- the negative electrode active material Using this mixture as the negative electrode active material, these are mixed so that the mass ratio of the negative electrode active material: CMC: PAA: styrene-butadiene rubber is 100: 0.9: 0.95: 1, and an appropriate amount of water is added.
- the second negative electrode mixture slurry for the second layer was prepared.
- the first negative electrode mixture slurry was applied to both sides of the negative electrode current collector made of copper foil, and the coating film was dried to form the first layer on both sides of the negative electrode current collector.
- the second negative electrode mixture slurry was applied onto the first layer formed on both sides of the negative electrode current collector, and the coating film was dried to form the second layer.
- the coating film was rolled using a roller to prepare a negative electrode in which negative electrode mixture layers including the first layer and the second layer were formed on both sides of the negative electrode current collector.
- the internal porosities of the graphite particles A and B were measured in the prepared negative electrode, they were 1% and 22%, respectively. Since the measurement method is as described above, it will be omitted.
- Electrolytic solution 1% by mass of vinylene carbonate (VC) was added to a mixed solvent in which ethylene carbonate (EC), fluorinated ethylene carbonate (FEC), and diethyl carbonate (DEC) were mixed at a volume ratio of 27: 3: 70.
- An electrolytic solution was prepared by dissolving LiPF 6 at a ratio of 1.2 mol / L.
- Test cell The positive electrode and the negative electrode were laminated so as to face each other via a separator, and wound around the positive electrode to prepare an electrode body. Next, the electrode body and the electrolytic solution were housed in a bottomed cylindrical battery case body, the electrolytic solution was injected, and then the opening of the battery case body was sealed with a gasket and a sealing body to prepare a test cell. ..
- Example 2 No Si compound was used in the preparation of the first negative electrode mixture slurry, and 22 parts by mass of graphite particles A, 65 parts by mass of graphite particles B, and 14 parts by mass of Si compound in the preparation of the second negative electrode mixture slurry.
- a test cell was prepared in the same manner as in Example 1 except that the mixture mixed so as to be used as the negative electrode active material.
- Example 3 In the preparation of the first negative electrode mixture slurry, no Si compound was used, and in the preparation of the second negative electrode mixture slurry, a mixture was mixed so that the graphite particles A were 86 parts by mass and the Si compound was 14 parts by mass. A test cell was prepared in the same manner as in Example 1 except that it was used as the negative electrode active material.
- Example 4 No Si compound was used in the preparation of the first negative electrode mixture slurry, and the mixture was mixed so that the graphite particles A were 86 parts by mass and the Si compound was 14 parts by mass in the preparation of the second negative electrode mixture slurry.
- the mass ratio of the negative electrode active material: CMC: PAA: styrene butadiene rubber: CNT was 100: 0.9: 0.95: 1: 0.
- a test cell was prepared in the same manner as in Example 1 except that these were mixed so as to be 5.
- Capacity retention rate (Discharge capacity in the 50th cycle / Discharge capacity in the 1st cycle) x 100 Table 1 shows the evaluation results (capacity retention rate in 50 cycles) of the test cells of Examples 1 to 4 and Comparative Example 1.
- Examples 1 to 4 and Comparative Example 1 7% by mass of the Si compound is contained in the negative electrode active material, but the test cells of Examples 1 to 4 are more filled than the test cells of Comparative Example 1. The capacity retention rate in the discharge cycle became a high value, and the charge / discharge cycle characteristics were improved.
- the test cell of Example 4 is different from the test cell of Example 3 in that CNT is added to the second layer. From the evaluation result of the capacity retention rate, it can be seen that the capacity retention rate of Example 4 was improved more than the capacity retention rate of Example 3.
- Example 5 No Si compound was used in the preparation of the first negative electrode mixture slurry, and 20 parts by mass of graphite particles A, 60 parts by mass of graphite particles B, and 20 parts by mass of Si compound in the preparation of the second negative electrode mixture slurry.
- a test cell was prepared in the same manner as in Example 1 except that the mixture mixed so as to be used as the negative electrode active material.
- ⁇ Comparative example 2> In the preparation of the first negative electrode mixture slurry, a mixture in which graphite particles B were 90 parts by mass and Si compound was 10 parts by mass was used as the negative electrode active material, and in the preparation of the second negative electrode mixture slurry. A test cell was prepared in the same manner as in Example 1 except that a mixture of graphite particles A in an amount of 22 parts by mass, graphite particles B in an amount of 68 parts by mass, and a Si compound in an amount of 10 parts by mass was used as the negative electrode active material. did.
- Example 5 For the test cells of Example 5 and Comparative Example 2, the capacity retention rate in 50 cycles was evaluated under the same conditions as described above. Table 2 shows the evaluation results (capacity retention rate in 50 cycles) of the test cells of Example 5 and Comparative Example 2.
- Example 5 the negative electrode active material contains 10% by mass of the Si compound, but the test cell of Example 5 has a larger capacity in the charge / discharge cycle than the test cell of Comparative Example 2. The maintenance rate became a high value, and the charge / discharge cycle characteristics were improved.
- Example 6 In the preparation of the first negative electrode mixture slurry, a mixture in which graphite particles B were 88 parts by mass and Si compound was 12 parts by mass was used as the negative electrode active material, and in the preparation of the second negative electrode mixture slurry. A test cell was prepared in the same manner as in Example 1 except that a mixture in which graphite particles A were 20 parts by mass, graphite particles B were 62 parts by mass, and Si compounds were 18 parts by mass was used as the negative electrode active material. did.
- Example 6 and Comparative Example 3 the negative electrode active material contains 15% by mass of the Si compound, but the test cell of Example 6 has a larger capacity in the charge / discharge cycle than the test cell of Comparative Example 3. The maintenance rate became a high value, and the charge / discharge cycle characteristics were improved.
- the graphite particles A having the particle internal void ratio of 10% or less and the alloying material are the first.
- the charge / discharge cycle characteristics of the lithium ion secondary battery are due to the fact that graphite particles B, which are contained more in the second layer than in the layer and have a particle internal void ratio of more than 10%, are contained in the first layer more than in the second layer. Can be said to have been improved.
Abstract
Description
正極11は、正極集電体と、正極集電体上に形成された正極合材層とを備える。正極集電体には、アルミニウム、アルミニウム合金などの正極の電位範囲で安定な金属の箔、当該金属を表層に配置したフィルム等を用いることができる。正極合材層は、例えば正極活物質、結着材、導電材を含む。正極合材層は、正極集電体の両面に形成されることが好ましい。正極は、例えば正極活物質、結着材、導電材等を含む正極合材スラリーを正極集電体上に塗布し、塗膜を乾燥、圧延して、正極合材層を正極集電体の両面に形成することにより製造できる。
負極12は、負極集電体と、負極集電体上に形成された負極合材層とを備える。負極集電体には、例えば、銅、銅合金などの負極の電位範囲で安定な金属の箔、当該金属を表層に配置したフィルム等を用いることができる。
(1)負極活物質の断面を露出させる。断面を露出させる方法としては、例えば、負極の一部を切り取り、イオンミリング装置(例えば、日立ハイテク社製、IM4000PLUS)で加工し、負極合材層の断面を露出させる方法が挙げられる。
(2)走査型電子顕微鏡を用いて、上記露出させた負極合材層の断面の反射電子像を撮影する。反射電子像を撮影する際の倍率は、3千倍から5千倍である。
(3)上記により得られた断面像をコンピュータに取り込み、画像解析ソフト(例えば、アメリカ国立衛生研究所製、ImageJ)を用いて二値化処理を行い、断面像内の粒子断面を黒色とし、粒子断面に存在する空隙を白色として変換した二値化処理画像を得る。
(4)二値化処理画像から、黒鉛粒子断面の面積、及び当該黒鉛粒子断面に存在する内部空隙の面積を算出する。ここで、黒鉛粒子断面の面積とは、黒鉛粒子の外周で囲まれた領域の面積、すなわち、黒鉛粒子の断面部分全ての面積を指している。また、黒鉛粒子断面に存在する空隙のうち幅が3μm以下の空隙については、画像解析上、内部空隙か外部空隙かの判別が困難となる場合があるため、幅が3μm以下の空隙は内部空隙としてもよい。そして、算出した黒鉛粒子断面の面積及び黒鉛粒子断面の内部空隙の面積から、黒鉛粒子の内部空隙率(黒鉛粒子の内部空隙率=黒鉛粒子断面の内部空隙の面積×100/黒鉛粒断面の面積)を算出する。黒鉛粒子の内部空隙率は、黒鉛粒子10個の平均値とする。
例えば、主原料となるコークス(前駆体)を所定サイズに粉砕し、それらを結着材で凝集させた状態で、2600℃以上の温度で焼成し、黒鉛化させた後、篩い分けることで、所望のサイズの黒鉛粒子Aを得る。ここで、粉砕後の前駆体の粒径や凝集させた状態の前駆体の粒径等によって、内部空隙率を10%以下に調整することができる。例えば、粉砕後の前駆体の平均粒径(メジアン径D50)は、12μm~20μmの範囲であることが好ましい。
例えば、主原料となるコークス(前駆体)を所定サイズに粉砕し、それらを結着材で凝集した後、さらにブロック状に加圧成形した状態で、2600℃以上の温度で焼成し、黒鉛化させる。黒鉛化後のブロック状の成形体を粉砕し、篩い分けることで、所望のサイズの黒鉛粒子Bを得る。ブロック状の成形体に添加される揮発成分の量によって、内部空隙率を10%超に調整することができる。
合金化材料は、リチウムと合金化する元素、リチウム合金化する元素を含有する化合物、又はその両方を含んで構成される。負極活物質に適用可能なリチウムと合金化する元素としては、Al、Ga、In、Si、Ge、Sn、Pb、As、Sb、Bi等が挙げられる。中でも、高容量化の観点から、Si、Snが好ましく、Siが特に好ましい。
(1)Si及び酸化ケイ素を、例えば20:80~95:5の重量比で混合して混合物を作製する。
(2)少なくとも上記混合物の作製前又は作製後に、例えばボールミルによりSi及び酸化ケイ素を粉砕して微粒子化する。
(3)粉砕された混合物を、例えば不活性雰囲気中、600~1000℃で熱処理する。
セパレータ13は、イオン透過性及び絶縁性を有する多孔性シートが用いられる。多孔性シートの具体例としては、微多孔薄膜、織布、不織布等が挙げられる。セパレータ13の材質としては、ポリエチレン、ポリプロピレン、エチレン及びプロピレンの少なくとも一方を含む共重合体等のオレフィン系樹脂、セルロースなどが好適である。セパレータ13は、単層構造、積層構造のいずれであってもよい。セパレータ13の表面には、耐熱層などが形成されていてもよい。
電解質は、溶媒と、電解質塩とを含む。電解質は、液体電解質に限定されず、ゲル状ポリマー等を用いた固体電解質であってもよい。電解質塩には、例えば、LiFSI、LiTFSI、LiBF4、LiPF6等のリチウム塩が用いられる。溶媒には、例えば、エチレンカーボネート(EC)、プロピレンカーボネート(PC)、ジメチルカーボネート(DMC)、エチルメチルカーボネート(EMC)、ジエチルカーボネート(DEC)、酢酸メチル(MA)、プロピオン酸メチル(MP)等のエステル類、エーテル類、二トリル類、アミド類、及びこれらの2種以上の混合溶媒などが用いられる。非水溶媒は、上記これらの溶媒の水素の少なくとも一部をフッ素等のハロゲン原子で置換したハロゲン置換体を含有していてもよい。
[正極]
正極活物質としてのLiNiCoAlO2が98質量部、導電材としてのアセチレンブラックが1質量部、結着材としてのポリフッ化ビニリデン粉末が1質量部となるよう混合し、さらにN-メチル-2-ピロリドン(NMP)を適量加えて、正極合材スラリーを調製した。このスラリーをアルミニウム箔(厚さ15μm)からなる集電体の両面にドクターブレード法により塗布し、塗膜を乾燥した後、圧延ローラーにより塗膜を圧縮して、正極集電体の両面に正極活物質層が形成された正極を作製した。
コークスを平均粒径(メジアン径D50)が15μmとなるまで粉砕し、粉砕したコークスに結着材としてのピッチを添加し、コークスを平均粒径(メジアン径D50)が17μmとなるまで凝集させた。この凝集物を2800℃の温度で焼成して黒鉛化した後、250メッシュの篩いを用いて、篩い分けを行い、平均粒径(メジアン径D50)が23μmの黒鉛粒子Aを得た。
コークスを平均粒径(メジアン径D50)が15μmとなるまで粉砕し、粉砕したコークスに結着材としてのピッチを添加して凝集させた後、さらに等方的な圧力で1.6g/cm3~1.9g/cm3の密度を有するブロック状の成形体とした。このブロック状の成形体を2800℃の温度で焼成して黒鉛化した後、ブロック状の成形体を粉砕し、250メッシュの篩いを用いて、篩い分けを行い、平均粒径(メジアン径D50)が23μmの黒鉛粒子Bを得た。
黒鉛粒子Bが94.4質量部、Si化合物(SiO)が5.6質量部となるようにこれらを混合した。この混合物を負極活物質として、負極活物質:CMC:PAA:スチレンブタジエンゴムの質量比が、100:0.9:0.95:1となるようにこれらを混合し、水を適量加えて、第1層用の第1負極合材スラリーを調整した。また、黒鉛粒子Aが23質量部、黒鉛粒子Bが69質量部、Si化合物(SiO)が8.4質量部となるように混合した。この混合物を負極活物質として、負極活物質:CMC:PAA:スチレンブタジエンゴムの質量比が、100:0.9:0.95:1となるようにこれらを混合し、水を適量加えて、第2層用の第2負極合材スラリーを調整した。
エチレンカーボネート(EC)と、フッ化エチレンカーボネート(FEC)と、ジエチルカーボネート(DEC)とを、27:3:70の体積比で混合した混合溶媒にビニレンカーボネート(VC)を1質量%添加し、LiPF6を1.2モル/Lの割合で溶解させて電解液を調製した。
正極と、負極とを、セパレータを介して互いに対向するように積層し、これを巻回して、電極体を作製した。次いで、電極体及び上記電解液を有底円筒形状の電池ケース本体に収容し、上記電解液を注入した後、ガスケット及び封口体により電池ケース本体の開口部を封口して、試験セルを作製した。
第1負極合材スラリーの調製において、Si化合物を用いなかったこと、第2負極合材スラリーの調製において、黒鉛粒子Aが22質量部、黒鉛粒子Bが65質量部、Si化合物が14質量部となるように混合した混合物を負極活物質として用いたこと以外は、実施例1と同様に試験セルを作製した。
第1負極合材スラリーの調製において、Si化合物を用いなかったこと、第2負極合材スラリーの調製において、黒鉛粒子Aが86質量部、Si化合物が14質量部となるように混合した混合物を負極活物質として用いたこと以外は、実施例1と同様に試験セルを作製した。
第1負極合材スラリーの調製において、Si化合物を用いなかったこと、第2負極合材スラリーの調製において、黒鉛粒子Aが86質量部、Si化合物が14質量部となるように混合した混合物を負極活物質として用いたこと、第2負極合材スラリーの調製において、負極活物質:CMC:PAA:スチレンブタジエンゴム:CNTの質量比が、100:0.9:0.95:1:0.5となるようにこれらを混合したこと以外は、実施例1と同様に試験セルを作製した。
第1負極合材スラリーの調製において、黒鉛粒子Bが93質量部、Si化合物が7質量部となるように混合した混合物を負極活物質として用いたこと、第2負極合材スラリーの調製において、黒鉛粒子Bが93質量部、Si化合物が7質量部となるように混合した混合物を負極活物質として用いたこと以外は、実施例1と同様に試験セルを作製した。
試験セルを、25℃の温度環境下、0.3Cの定電流で電池電圧が4.2Vになるまで定電流で充電した後、4.2Vで電流値が1/50Cになるまで定電圧で充電した。その後、1.0Cの定電流で電池電圧が2.5Vになるまで定電流放電を行った。また、この充放電を50サイクル行い、下記の式に基づいて、充放電サイクルにおける容量維持率を求めた。
表1に、実施例1~4及び比較例1の試験セルについての評価結果(50サイクルにおける容量維持率)を示す。
第1負極合材スラリーの調製において、Si化合物を用いなかったこと、第2負極合材スラリーの調製において、黒鉛粒子Aが20質量部、黒鉛粒子Bが60質量部、Si化合物が20質量部となるように混合した混合物を負極活物質として用いたこと以外は、実施例1と同様に試験セルを作製した。
第1負極合材スラリーの調製において、黒鉛粒子Bが90質量部、Si化合物が10質量部となるように混合した混合物を負極活物質として用いたこと、第2負極合材スラリーの調製において、黒鉛粒子Aが22質量部、黒鉛粒子Bが68質量部、Si化合物が10質量部となるように混合した混合物を負極活物質として用いたこと以外は、実施例1と同様に試験セルを作製した。
第1負極合材スラリーの調製において、黒鉛粒子Bが88質量部、Si化合物が12質量部となるように混合した混合物を負極活物質として用いたこと、第2負極合材スラリーの調製において、黒鉛粒子Aが20質量部、黒鉛粒子Bが62質量部、Si化合物が18質量部となるように混合した混合物を負極活物質として用いたこと以外は、実施例1と同様に試験セルを作製した。
第1負極合材スラリーの調製において、黒鉛粒子Bが85質量部、Si化合物が15質量部となるように混合した混合物を負極活物質として用いたこと、第2負極合材スラリーの調製において、黒鉛粒子Aが21質量部、黒鉛粒子Bが64質量部、Si化合物が15質量部となるように混合した混合物を負極活物質として用いたこと以外は、実施例1と同様に試験セルを作製した。
11 正極
12 負極
13 セパレータ
14 電極体
15 電池ケース
16 ケース本体
17 封口体
18,19 絶縁板
20 正極リード
21 負極リード
22 張り出し部
23 フィルタ
24 下弁体
25 絶縁部材
26 上弁体
27 キャップ
28 ガスケット
30 黒鉛粒子
34 内部空隙
36 外部空隙
40 負極集電体
42 負極合材層
44 第1層
46 第2層
Claims (7)
- 負極集電体と、前記負極集電体上に形成された負極合材層とを備え、
前記負極合材層は、粒子内部空隙率が10%以下である黒鉛粒子Aと、粒子内部空隙率が10%超である黒鉛粒子Bと、リチウムと合金化する合金化材料と、を含む負極活物質を有し、
前記負極合材層は、前記負極集電体上に形成された第1層と、前記第1層上に形成された第2層と、を含み、
前記黒鉛粒子A及び前記合金化材料は、前記第1層より前記第2層に多く含まれ、
前記黒鉛粒子Bは、前記第2層より、前記第1層に多く含まれ、
前記合金化材料の含有量は、前記負極合材層内の前記負極活物質の総量に対して15質量%以下である、リチウムイオン二次電池用負極。 - 前記負極合材層は、繊維状炭素を有し、
前記繊維状炭素は、前記第1層より、前記第2層に多く含まれる、請求項1に記載のリチウムイオン二次電池用負極。 - 前記負極合材層は、ポリアクリル酸又はその塩を含み、
前記ポリアクリル酸又はその塩は、前記第1層より、前記第2層に多く含まれる、請求項1又は2に記載のリチウムイオン二次電池用負極。 - 前記負極合材層は、スチレンブタジエンゴムを含み、
前記スチレンブタジエンゴムは、前記第2層より、前記第1層に多く含まれる、請求項1~3のいずれか1項に記載のリチウムイオン二次電池用負極。 - 前記スチレンブタジエンゴムは、前記負極集電体側半分の領域に、前記負極合材層に含まれるすべての前記スチレンブタジエンゴムの90質量%以上100質量%以下が含まれている、請求項4に記載のリチウムイオン二次電池用負極。
- 前記負極合材層は、カルボキシメチルセルロース又はその塩を含み、
前記カルボキシメチルセルロース又はその塩は、前記第1層より、前記第2層に多く含まれる、請求項1~5のいずれか1項に記載のリチウムイオン二次電池用負極。 - 請求項1~6のいずれか1項に記載のリチウムイオン二次電池用負極を備える、リチウムイオン二次電池。
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WO2023054308A1 (ja) * | 2021-09-30 | 2023-04-06 | パナソニックIpマネジメント株式会社 | 非水電解質二次電池 |
EP4187632A1 (en) * | 2021-11-25 | 2023-05-31 | SK On Co., Ltd. | Anode for lithium secondary battery and lithium secondary battery including the same |
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WO2024048733A1 (ja) * | 2022-08-31 | 2024-03-07 | パナソニックエナジー株式会社 | 二次電池用負極、二次電池、および二次電池用負極の製造方法 |
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