KR20190113541A - Lithium ion rechargeable battery - Google Patents

Abstract

Provided is a lithium ion secondary battery which comprises: a negative electrode mixture containing a graphite active material; and a negative electrode comprising a solid electrolyte interphase (SEI) film. The ration C (004) / C (110) of an X-ray diffraction peak intensity of the negative electrode is 20 or less. The porosity in the particle of the graphite active material is 5% or more. The SEI film contains sulfur atoms forming a SO_4 bond.

Description

Lithium Ion Secondary Battery {LITHIUM ION RECHARGEABLE BATTERY}

The present invention relates to a lithium ion secondary battery.

A nonaqueous electrolyte secondary battery such as a lithium ion secondary battery is a notebook computer (note PC). BACKGROUND ART It is widely used as a power source for portable devices such as mobile phones, and has attracted attention as a power source suitable for xEVs such as electric vehicles or hybrid vehicles. In response to this need, the demand for lithium ion secondary batteries is rapidly increasing.

The lithium ion secondary battery suitable for the xEV is required to ensure the same performance as a vehicle using a conventional gasoline engine (gasoline engine), high capacity and long life characteristics are required. In addition, a lithium ion secondary battery suitable for xEV also requires fast charging characteristics in order to complete charging within a time equivalent to the fueling time of a gasoline engine car.

On the other hand, the development of high capacity or long life of lithium ion secondary batteries has been actively progressed, but the development of rapid charge characteristics (high rate charge / discharge characteristics) is poor.

On the other hand, in the secondary batteries other than the lithium ion secondary battery, a study on the charging characteristics in various aspects has been carried out, for example, in the following Patent Documents 1 to 3 to charge a specific additive in the electrolyte by charging the lithium ion secondary battery A technique for improving discharge characteristics is disclosed.

Patent Document 1: Patent Publication No. 1998-189042

Patent Document 2: Patent Publication No. 2013-101968

Patent Document 3: Patent Publication No. 2013-229307

In recent years, as the capacity of a lithium ion secondary battery becomes higher, the packing density of the negative electrode containing a graphite active material is also increasing. In addition, control of the graphite active material crystal structure included in the negative electrode has been studied as one of methods for improving the reduction of rapid charge and discharge characteristics (ie, high rate charge and discharge characteristics) according to an increase in the packing density.

However, there is no detailed study on the interaction between the graphite active material whose crystal structure is controlled as described above and the solid electrolyte interface (SEI) film formed by the additives described in Patent Documents 1 to 3 on the negative electrode surface. . Therefore, it is expected that new results can be obtained by examining the action of the surface of the negative electrode and the crystal structure of the graphite active material contained in the negative electrode in detail.

Accordingly, an object of the present invention is to provide a lithium ion secondary battery with improved charge and discharge characteristics at high rates.

According to one embodiment, the negative electrode mixture comprising a graphite active material; And a cathode including a solid electrolyte interphase (SEI) film, wherein the ratio C (004) / C (110) of the X-ray diffraction peak intensity of the cathode is 20 or less, and the porosity in the particles of the graphite active material is 5% or more. The SEI film provides a lithium ion secondary battery containing sulfur atoms constituting a SO ₄ bond. Thereby, the charge / discharge characteristic at the high rate of a lithium ion secondary battery can be improved.

The negative electrode mixture including the graphite active material may be 50 μm or more. Thereby, high capacity | capacitance of a lithium ion secondary battery can be realized.

The negative electrode mixture including the graphite active material may be 60 μm or more. Thereby, high capacity | capacitance of a lithium ion secondary battery can be realized.

The ratio C (004) / C (110) of the X-ray diffraction peak intensity of the cathode may be 8 or less. Thereby, high capacity | capacitance of a lithium ion secondary battery can be realized.

The porosity in the particles of the graphite active material may be 8% or more. Thereby, high capacity | capacitance of a lithium ion secondary battery can be realized.

The sulfur atom present on the surface of the negative electrode may be 1 atomic% or more based on the total number of atoms present on the surface of the negative electrode. Thereby, high capacity | capacitance of a lithium ion secondary battery can be realized.

The packing density of the negative electrode mixture including the graphite active material may be 1.5 g / cm ^{3 or} more. Thereby, high capacity | capacitance of a lithium ion secondary battery can be realized.

The lithium ion secondary battery may further include an electrolyte solution containing 1,3,2-dioxathiolane 2,2-dioxide. Thereby, high capacity | capacitance of a lithium ion secondary battery can be realized.

It is possible to provide a lithium ion secondary battery with improved high rate charge and discharge characteristics.

1 is a side cross-sectional view schematically showing the configuration of a lithium ion secondary battery.

EMBODIMENT OF THE INVENTION Preferred embodiment of this invention is described in detail, referring an accompanying drawing below. In addition, in this specification and drawing, the same code | symbol is attached | subjected about the component which has a substantially same functional structure, and it abbreviate | omits duplication description.

First, a configuration of a lithium ion secondary battery according to one embodiment will be described with reference to FIG. 1.

As shown in FIG. 1, the lithium ion secondary battery 10 includes a positive electrode 20, a negative electrode 30, a separator 40, and an electrolyte solution. On the other hand, the form of the lithium ion secondary battery 10 is not particularly limited, for example, any type such as cylindrical, square, laminate type, button type can be used without limitation.

The positive electrode 20 includes a positive electrode current collector 21 and a positive electrode active material layer 22.

The positive electrode current collector 21 is not particularly limited as long as it is used as a conductor, and may be, for example, aluminum, stainless steel, nickel coated steel, or the like.

The cathode active material layer 22 may include a cathode active material and a conductive material, and may further include a binder. In addition, content of a positive electrode active material, a electrically conductive material, and a binder is not specifically limited, Content in a general lithium ion secondary battery can be used.

The positive electrode active material is, for example, a solid solution oxide containing lithium. However, the positive electrode active material is not particularly limited as long as it is a material capable of electrochemically storing and releasing lithium ions. For example, the solid solution oxide may be lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, or a combination thereof. In addition, the solid solution oxide is Li _a Mn _x Co _y Ni _z M _1-xyz O ₂ (M is at least one selected from Ni, Co, Mn, Al, Cr, Ti or Mg, 0.8≤a≤1.45, 0≤x≤ 0.6, 0≤y≤0.3, 0.2≤z≤0.98), LiMn _x Co _y Ni _z O ₂ (0.3≤x≤0.85, 0.1≤y≤0.3, 0.1≤z≤0.3), LiMn _1.5 Ni _0.5 O _4, etc. Can be.

The conductive material may be, for example, carbon black such as ketjen black or acetylene black, natural graphite, artificial graphite, carbon nanotubes, graphene or carbon nano Fibrous carbon such as fibers (carbon nanofibers), a composite of the fibrous carbon and carbon black, and the like can be used. However, the conductive material is not particularly limited as long as it is for enhancing the conductivity of the positive electrode.

The binder is, for example, polyvinylidene fluoride, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber ), Fluoro rubber, polyvinyl acetate, polymethylmethacrylate, polyethylene, cellulose nitrate, etc. can be used. If it can bind | conclude on the collector 21, it will not specifically limit.

The negative electrode 30 includes a negative electrode current collector 31 and a negative electrode active material layer 32. The negative electrode current collector 31 can be used as long as it is a conductor.

The negative electrode current collector 31 may be, for example, copper, copper alloy, aluminum, stainless steel, nickel plated steel, or the like.

The negative electrode active material layer 32 may include at least a negative electrode active material, and may further include a conductive material and a binder. In addition, the ratio of content of a negative electrode active material, a electrically conductive material, and a binder is not restrict | limited, The ratio of content which can be used with a general lithium ion secondary battery can be used.

The negative electrode active material includes at least a graphite active material, and may be, for example, artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, and the like. In addition, the negative electrode active material may further include a silicon (Si) or tin (Sn) -based active material, a titanium oxide (TiO _x ) -based active material, etc., in addition to the graphite active material. As the silicon or tin-based active material, fine particles of silicon or tin, fine particles of oxides of silicon or tin, alloys of silicon or tin, and the like may be used. The titanium oxide-based active material may be Li ₄ Ti ₅ O ₁₂ or the like.

The conductive material may be the same as the conductive material used for the positive electrode active material layer 22.

As the binder, for example, styrene-butadiene rubber (SBR) may be used. However, the binder is not particularly limited as long as the binder can bind the negative electrode active material and the conductive material onto the negative electrode current collector 31.

The negative electrode 30 according to the embodiment includes a graphite active material as a negative electrode active material, and the ratio C (X-ray diffraction peak intensity of the (004) plane of graphite and the X-ray diffraction peak intensity of the (110) plane of graphite) 004) / C 110 may be 20 or less, preferably 8 or less. The ratio of the X-ray diffraction peak intensities represented by C (004) / C (110) indicates the degree of orientation of the graphite crystals.

On the other hand, the larger the ratio of the X-ray diffraction peak intensities represented by C (004) / C (110), the higher the orientation of the graphite layer, which means that the graphite layer is more parallel to the measurement plane.

When the orientation of the graphite layer is high, the graphite layers are stacked more regularly, and it is difficult for lithium ions to be inserted or released into the crystal structure of the graphite because the gap between the stacked graphite layers is reduced. Accordingly, the higher the ratio of the X-ray diffraction peak intensities represented by C (004) / C (110), the lower the high rate charge / discharge characteristics of the lithium ion secondary battery may be. Therefore, the lithium ion secondary battery 10 according to an embodiment limits the ratio C (004) / C110 of the X-ray diffraction peak intensity of the negative electrode 30 to 20 or less, for example, 8 or less.

On the other hand, the lower limit of the ratio C (004) / C (110) of the X-ray diffraction peak intensity of the cathode 30 is not particularly limited, but may be, for example, 7 or more on the crystal structure of graphite.

On the other hand, the ratio C (004) / C (110) of the X-ray diffraction peak intensity of the negative electrode 30 described above is a measured value at the time of full charge after the initial charge and discharge of the lithium ion secondary battery 10, the negative electrode 30 ) Is not a measured value at the time of manufacture. In the graphite active material included in the negative electrode 30, the crystal structure of the graphite active material may be changed by occluding or releasing lithium ions. Accordingly, it should be noted that the value of C (004) / C110 measured at full charge after the initial charge and discharge cannot be directly compared with the value measured at the time of fabrication of the negative electrode 30.

The porosity in the particles of the graphite active material contained in the negative electrode 30 may be 5% or more, preferably 8% or more. As described above, the negative electrode 30 according to the embodiment may form voids to facilitate the insertion and desorption of lithium ions in the graphite active material particles by controlling the orientation of the graphite active material contained in the negative electrode. In particular, since the negative electrode 30 according to the embodiment forms more voids in the particles of the graphite active material rather than the voids between the particles of the graphite active material, the lithium ion secondary battery 10 according to this repeats high rate charge and discharge. Even in this case, a decrease in capacity can be suppressed. This is because the voids between the particles of the graphite active material tend to be distorted or deformed due to the slippage between particles due to the expansion or contraction of the graphite active material accompanying charge and discharge, whereas the voids in the particles are difficult to be distorted or deformed. It is thought that the same effect difference occurs. Therefore, when the intraparticle porosity of the graphite active material is less than 5%, high rate charge and discharge characteristics may be lowered, which is not preferable. On the other hand, the upper limit of the intraparticle porosity of the graphite active material is not particularly limited, but may be, for example, 20% or less on the crystal structure of the graphite active material.

In the particle porosity of the graphite active material, for example, after initial charging, the fully charged lithium ion secondary battery 10 is disassembled to separate the negative electrode 30, and the cross section of the negative electrode mixture of the negative electrode 30 is SEM (Scanning Electron). It can be calculated by observing with a microscope.

Specifically, the intraparticle porosity of the graphite active material can be calculated by dividing the cross sectional area of the empty region in the particles of the graphite active material observed in the SEM image by the entire cross sectional area of the particle.

The negative electrode mixture may have a thickness of 50 μm or more, for example, 60 μm or more. When the thickness of the negative electrode mixture is in the above-described range, high capacity of the lithium ion secondary battery 10 can be realized.

However, when the thickness of the negative electrode mixture increases, the capacity of the lithium ion secondary battery 10 may be increased, whereas the positive electrode 20, the negative electrode 30, and the separator 40 are stacked and wound to form an electrode assembly. It can be difficult to do. Therefore, the upper limit of the thickness of the negative electrode mixture may be 150 μm, for example. In the lithium ion secondary battery 10 according to one embodiment, while maintaining the thickness of the negative electrode mixture to the extent that it is not difficult to form the electrode assembly, by controlling the orientation of the graphite active material and the porosity in the particles in the above range, high capacity and high rate charge and discharge Characteristics can be achieved.

On the other hand, the thickness of the negative electrode mixture described above is the thickness of the cross-section of the negative electrode (30). In other words, when the negative electrode active material layer 32 is disposed on both surfaces of the current collector 31, the thickness of the negative electrode mixture described above is the total value of the negative electrode active material layers 32 on both sides, and the cross section of the current collector 31 is used. When the negative electrode active material layer 32 is disposed only, the value is only the negative electrode active material layer 32 in the cross section.

The packing density of the negative electrode mixture in the negative electrode 30 is preferably 1.5g / cm ³ or more. When the packing density of the negative electrode mixture containing the graphite active material falls within the above-described range, high capacity of the lithium ion secondary battery 10 can be achieved. However, when the charge density of the negative electrode mixture is increased, the capacity of the lithium ion secondary battery 10 may be increased while the charge and discharge characteristics of the lithium ion secondary battery 10 may be reduced. However, in the lithium ion secondary battery 10 according to the embodiment, since the high rate charge / discharge characteristics may be improved by controlling the orientation and porosity of the graphite active material, the packing density of the negative electrode mixture may be increased. Accordingly, in the lithium ion secondary battery 10, high capacity and high rate charge / discharge characteristics may be compatible. The upper limit of the packing density of the negative electrode mixture of the graphite active material is not particularly limited, but may be, for example, 1.8 g / cm ³ .

In addition, in the cathode 30 according to the exemplary embodiment, an SEI film may be formed on the surface of the cathode, and the SEI film may include sulfur atoms constituting the SO ₄ coupling unit. The SEI film is an electrode thin film formed by decomposing an electrolyte solution on the surface of the cathode 30 during initial charge and discharge. Specifically, the SEI film is an electrode thin film formed of an electrolyte solution impregnated with the cathode 30 and a decomposition product of an additive of the electrolyte solution. The SEI film can suppress further electrolyte solution decomposition reaction of the negative electrode 30 by assisting insertion or removal of lithium ions into the negative electrode 30.

In the lithium ion secondary battery 10 according to the exemplary embodiment, as the sulfur atom constituting the SO ₄ coupling unit is included in the SEI film of the negative electrode 30, the negative electrode may be formed through electrostatic interaction between SO ₄ and lithium ions. 30) It is possible to increase the concentration of lithium ions on the surface. Accordingly, it is easy to insert or detach lithium ions from the negative electrode 30 to improve the high rate charge / discharge characteristics of the lithium ion secondary battery 10.

However, when the orientation represented by C (004) / C (110) of the graphite active material is high, the effect of containing sulfur atoms constituting the SO ₄ bonding unit in the SEI film may be reduced. This is considered to be because when the pores into which lithium ions can be inserted into the graphite active material decrease, even if the lithium ion concentration on the surface of the negative electrode 30 increases, the insertion of lithium ions into the graphite active material cannot be promoted. On the other hand, when the orientation represented by C (004) / C (110) of the graphite active material is low, such as the lithium ion secondary battery 10 according to the embodiment, sulfur atoms constituting the SO ₄ bonding unit are formed in the SEI film. By containing, higher effect can be achieved.

On the other hand, the content ratio of the SEI film on the sulfur atom of the SO ₄ bonding units is preferably not less than 1 atom%. In this case, since the SEI film on the surface of the negative electrode may include SO ₄ that draws lithium ions close to each other, high rate charge and discharge characteristics of the lithium ion secondary battery 10 may be improved. The upper limit of the content ratio of sulfur atoms in the SEI film is not particularly limited, but may be, for example, 10 atomic%.

The separator 40 can be used without particular limitation as long as it is used as a separator of a lithium ion secondary battery. For example, a porous membrane, a nonwoven fabric, or the like having excellent high rate discharge performance may be used alone or in combination. The resin constituting the separator is, for example, a polyolefin resin such as polyethylene or polypropylene, polyester terephthalate, polyester such as polybutylene terephthalate, or the like. (polyester) resin, polyvinylidene fluoride (PVdF), vinylidene fluoride (VdF) -hexafluoropropylene (HFP) copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride Tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone aerial Copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride propylene copolymer, vinylidene fluoride Trifluoro propylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene air Coalescing and the like.

The electrolyte can be used without particular limitation as long as it can be used as an electrolyte for a lithium ion secondary battery. The electrolyte may have a composition including an electrolyte salt in the nonaqueous solvent.

The nonaqueous solvent is, for example, a cyclic carbonate such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, and vinylene carbonate. ); cyclic esters such as γ-butyrolactone and γ-valerolactone; Chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; Chain esters such as methyl formate, methyl acetate and methyl butyrate; Tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyl diglyme, etc. Ethers; Nitriles such as acetonitrile and benzonitrile; Dioxolane or derivatives thereof; Ethylene sulfide, sulfolane, sultone, or derivatives thereof may be used alone or in combination.

The electrolyte salt can be used without particular limitation as used in conventional lithium ion batteries. Electrolyte salts include, for example, LiN (SO ₂ F) ₂ , LiFSA (lithium bis fluoro sulfonyl amide), LiFSI (lithium bis fluoro sulfonyl imide), LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiPF _6-x (CnF _{2n + 1} ) _x (1 <x <6, n = 1 or 2), LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr , Inorganic ion salt containing one of lithium (Li), sodium (Na) or potassium (K) such as KClO ₄ , KSCN, LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , (CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (nC ₄ H ₉ ) ₄ NClO ₄ , (nC ₄ H ₉ ) ₄ NI, (C ₂ H ₅ ) ₄ N-maleate, (C ₂ H ₅ ) ₄ N-benzoate, (C ₂ H ₅ ) ₄ N-phthalate, lithium ste Lithium stearyl sulfonate, lithium octyl sulfonate, lithium dodecyl benzenesulfonate organic ionic salts such as esulfonate) and the like, and the electrolyte salts may be used alone or in combination. The electrolyte salt may comprise a lithium salt. Meanwhile, the concentration of the electrolyte salt may be a concentration (0.8 mol / L to 2.0 mol / L) used in a general lithium ion secondary battery.

In addition, various additives can be added to electrolyte solution. As the additive, for example, cathodic additives, anodizing additives, ester additives, carbonate ester additives, sulfuric acid ester additives, phosphate ester additives, boric acid ester additives, acid anhydride additives, electrolyte additives, etc. may be used. The additives may be added alone or in mixture of two or more kinds thereof.

The electrolyte of a lithium ion secondary battery according to one embodiment may contain 1,3,2-dioxathiolane 2,2-dioxide (DTD) additive. DTD is a cyclic sulfuric acid ester, and decomposed during initial charge and discharge of the lithium ion secondary battery 10 to form an SEI film on the negative electrode 30. Since the DTD is mainly decomposed into SO ₄ bonding units, the decomposition products of the DTD may be included as SO ₄ bonding units in the SEI film on the surface of the cathode 30. Therefore, it is possible to contain the sulfur atom of the SO ₄ in the coupling unit described above SEI film on the surface of the incorporation of a DTD in the electrolyte, the negative electrode 30. In particular, since the DTD is likely to be decomposed into SO ₄ bonding units, the DTD may more efficiently include sulfur atoms constituting the SO ₄ bonds in the SEI film on the surface of the cathode 30.

Meanwhile, the additive decomposed into the SO ₄ bonding unit is 4-methyl-1,3,2-dioxathiolane 2,2-dioxide, 4-ethyl-1,3,2-dioxathiolane 2,2 in addition to DTD. -Dioxide, 4-fluoro-1,3,2-dioxathiolan 2,2-dioxide, 4-chloro-1,3,2-dioxathiolan 2,2-dioxide, 4-phenyl-1,3, 2-dioxathiolane 2,2-dioxide and the like.

The manufacturing method of the said lithium ion secondary battery 10 is demonstrated.

The positive electrode 20 is produced by the following method. First, the positive electrode active material, the conductive material, and the binder are mixed at a predetermined ratio, and then dispersed in a solvent (for example, N-methyl-2-pyrrolidone) to form a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector 21 and dried to form the positive electrode active material layer 22. On the other hand, the coating method is not particularly limited, for example, a knife coater (grave coater) method, a gravure coater (gravure coater) method and the like can be used. Thereafter, the positive electrode active material layer 22 is compressed to a predetermined density by using a press to produce a positive electrode 20.

The cathode 30 is also manufactured in the same manner as the anode 20. First, the negative electrode active material and the binder are mixed in a predetermined ratio and dispersed in a solvent (for example, water) to form a negative electrode slurry, which is applied onto the negative electrode current collector 31 and It is dried to form the negative electrode active material layer 32. Thereafter, the negative electrode active material layer 32 is compressed to a predetermined density by using a press to produce a negative electrode 30.

An electrode assembly is manufactured through the separator 40 between the anode 20 and the cathode 30 manufactured by the above-described method. The electrode assembly is processed into a desired shape (eg, cylindrical, square, laminated, button, etc.) and inserted into a processed container. Thereafter, the electrolyte is injected into the container to impregnate the electrolyte in each of the pores of the separator 40. Thereby, the lithium ion secondary battery 10 is produced.

Hereinafter, the manufacturing method of the lithium ion secondary battery and lithium ion secondary battery which concerns on this embodiment is demonstrated concretely, referring an Example and a comparative example. On the other hand, the following embodiment is only one example, the method of manufacturing a lithium ion secondary battery and a lithium ion secondary battery according to the present invention is not limited to the following examples.

Example

Example 1 Fabrication of a Lithium Ion Secondary Battery

As the positive electrode active material, the conductive material, and the binder, lithium nickel cobalt oxide, carbon powder, and polyvinylidene fluoride represented by LiNi _0.88 Co _0.1 Al _0.02 O ₂ were used, respectively. The positive electrode active material, the conductive material, and the binder were mixed in a weight ratio of 94: 4: 2, and N-methyl-2-pyrrolidone was mixed thereto to prepare a positive electrode slurry (positive electrode mixture).

The positive electrode slurry was applied on a current collector made of aluminum foil having a thickness of 12 μm, a length of 238 mm, and a width of 29 mm, with a length of 222 mm and a width of 29 mm on one side of the current collector, and a length of 172 mm and a width on the other surface facing one side of the current collector. 29 mm was applied. After drying the collector which apply | coated the positive electrode slurry, it rolled and produced the positive electrode plate. On the other hand, the thickness of the positive electrode was 125 µm on both sides, the amount of the positive electrode mixture on the current collector was 42.5 mg / cm ² , and the packing density of the positive electrode mixture was 3.75 g / cm ³ .

Thereafter, a current collecting tab composed of an aluminum flat plate having a thickness of 70 μm, a length of 40 mm, and a width of 4 mm was attached to the portion of the positive electrode plate where the positive electrode mixture was not applied.

As the negative electrode active material, artificial graphite and silicon-containing carbon were used, and as the binder, carboxymethyl cellulose and styrene butadiene rubber were used. Artificial graphite, silicon-containing carbon, carboxymethylcellulose and styrene butadiene rubber were mixed at a weight ratio of 92.2: 5.3: 1.0: 1.5, and water was added and mixed thereto to prepare a negative electrode slurry (cathode mixture).

Thereafter, the negative electrode slurry was applied on a current collector made of aluminum foil having a thickness of 8 μm, a length of 271 mm, and a width of 30 mm on one side of the current collector with a length of 235 mm and a width of 30 mm, and the length was on the other surface facing one side of the current collector. 178 mm and width 30 mm were applied. The collector after drying of the negative electrode slurry was dried and then rolled to produce a negative electrode plate. On the other hand, the thickness of the negative electrode was 148 μm on both sides, the amount of the negative electrode mixture on the current collector was 23.0 mg / cm ² , and the packing density of the negative electrode mixture was 1.65 g / cm ³ . On the other hand, rolling is performed in multiple steps, and suppresses the increase in the value of the orientation represented by C (004) / C (110) of a cathode. In the negative electrode plate, a current collector tab of a nickel flat plate having a thickness of 70 μm, a length of 40 mm, and a width of 4 mm was attached to a portion where the negative electrode mixture was not applied.

Ethylene carbonate, methylethyl carbonate and dimethyl carbonate were mixed in a volume ratio of 20:40:40 to prepare a nonaqueous solvent. Electrolyte salt LiPF ₆ was dissolved in the nonaqueous solvent so as to have a concentration of 1.15 mol / L to prepare an electrolyte solution. Vinylene carbonate was added to the electrolyte at 1.5 wt% based on the total weight of the electrolyte, and 1,3,2-dioxathiolane 2,2-dioxide (DTD) was added at 1.0 wt% based on the total weight of the electrolyte. The lithium secondary battery was produced using the positive electrode, negative electrode, and electrolyte solution produced above. Specifically, the positive electrode and the negative electrode were disposed to face each other with a separator therebetween, folded at a predetermined position, wound, and pressed to produce a flat electrode assembly. On the other hand, the separator used two separators containing the polyethylene porous body of length 350mm and width 32mm.

The electrode assembly was housed in a battery container composed of aluminum laminate, and the electrolyte solution was injected into the battery container. At this time, it arrange | positioned so that the collector tab of a positive electrode and a negative electrode may be taken out outside. On the other hand, it arrange | positioned so that the collector tab of a positive electrode and a negative electrode may be taken out. The designed capacity of the prepared lithium ion secondary battery was 480 mAh.

(Evaluation of Lithium Ion Secondary Battery)

Subsequently, initial charge and discharge of the lithium ion secondary battery produced above was performed. Specifically, the lithium ion secondary battery was charged at 25 ° C. until it became 4.3 V at a constant current of 48 mA and further charged at a constant voltage of 4.3 V until the current value became 24 mA, followed by 2.8 at a constant current of 48 mA. The discharge was performed until it reached V. The discharge capacity at this time was made into initial stage Q1.

Subsequently, the lithium secondary battery subjected to initial charge and discharge as described above was charged at 25 ° C. and 240 mA constant current until it became 4.3 V, and then charged at a constant voltage of 4.3 V until the current value became 24 mA. . Thereafter, the charged lithium ion secondary battery was dismantled, and the negative electrode plate was separated. For the separated negative electrode plate, a wide angle X-ray diffraction profile was measured. On the other hand, the X-ray source was CuKα, and the output was 45 kV and 200 mA. About the obtained X-ray diffraction profile, the peak intensities of the (004) plane of graphite observed around 2θ = 48 ° to 56 ° and the (110) plane of graphite observed near 2θ = 76 ° to 79 ° were measured. . And the peak intensity ratio C (004) / C (110) of the (004) surface of graphite and the (110) surface was computed. The results are shown in Table 1 below.

On the other hand, the higher the peak intensity ratio C (004) / C (110) means that the graphite layer is oriented more parallel to the measurement plane.

In addition, the lithium secondary battery subjected to the initial charge and discharge as described above was charged at 25 ° C. until it became 4.3 V at a constant current of 240 mA, and then again charged at a constant voltage of 4.3 V until the current value became 24 mA. did.

Thereafter, the charged lithium ion secondary battery was dismantled, and the negative electrode plate was separated. About the separated negative electrode plate, the cross section of the negative electrode mixture was produced by the Ar ion milling (cross section polisher) method, and the SEM image of the produced cross section was obtained. About the acquired SEM image, the particle | grains of a graphite active material and the intraparticle voids were discriminated by image processing, each cross-sectional area ratio was computed, and the intraparticle porosity of the graphite active material was computed. The results are shown in Table 1 below.

Subsequently, the lithium secondary battery subjected to initial charge and discharge as described above was charged at 25 ° C and 240mA constant current until it became 4.3V, and at 4.3V constant voltage until the current value became 24mA. Thereafter, the charged lithium ion secondary battery was dismantled, the negative electrode plate was separated, and the SEI film formed on the surface of the separated negative electrode plate was analyzed by X-ray photoelectron spectroscopy. On the other hand, the X-ray source was monochrome AlKα (1486.6 eV), and the analysis region of the negative electrode plate was 700 µm to 300 µm. Spectrum obtained by X-ray photoelectron spectroscopy was analyzed to calculate sulfur in atomic percent. The results are shown in Table 1 below.

In addition, atomic state analysis of sulfur atoms contained in the SEI film was performed in the spectra obtained by X-ray photoelectron spectroscopy. The state analysis of sulfur used F (1s) peak and S (2p) peak.

For example, according to the Journal of the Electrochemical Society, 154 (12) A 1088-A1099 (2007), the peak of F (1s) is observed at 685.1 eV, and the peak of S (2p) is SO. Observed at 168.4 eV to 171.1 eV, and 165.5 eV to 167.5 eV in ₄ and SO _3, respectively. Therefore, the difference between the peak of F (1s) and the peak of S (2p) is 516.7 eV to 514.0 eV in SO ₄ and 519.6 eV to 517.6 eV in SO ₃ .

Here, assuming the value of the peak of F (1s) measured by X-ray photoelectron spectroscopy as LiF, the state of sulfur was determined by the shift amount of S (2p) peak based on the peak of F (1s). .

In other words, in the negative electrode plate of Example 1, the LiF (F (1s)) peak was 681.7 eV, and the S (2p) peak was 166.1 eV. Therefore, the difference in the observation position of F (1s) peak and S (2p) peak was 515.6 eV. According to the above non-patent literature, this corresponds to the difference between the peak of F (1s) and the peak of S (2p) of SO ₄ , and the state of sulfur atoms contained in the SEI film of the negative electrode plate according to Example 1 It can be seen that is SO ₄ .

In addition, the lithium secondary battery subjected to initial charge and discharge as described above was charged to 4.3 V at a constant current of 25 ° C. and 240 mA, and then charged to a current value of 24 mA at a constant voltage of 4.3 V, followed by 2.8 at a current of 240 mA. Discharged to V. 1 cycle was used and 100 cycles of charge and discharge were repeated. Then, from the initial capacity Q1 and the discharge capacity Q [0.5C] of 100 cycles, the capacity maintenance ratio of (0.5C) 100 cycles was calculated | required using following formula (1).

[Equation 1]

(0.5C) Capacity retention rate of 100 cycles (%) = (Q [0.5C] / Q1)

In addition, the lithium secondary battery subjected to initial charge and discharge as described above was charged to 4.3 V at a constant current of 960 mA at 25 ° C., and then charged to a current value of 24 mA at a constant voltage of 4.3 V, and then to 2.8 V at a current of 240 mA. Discharged. As this cycle, 99 cycles of charge and discharge were repeated. Thereafter, the battery was charged at a constant current of 240 mV to 4.3 V at the 100th cycle, charged to a current value of 24 m at a constant voltage of 4.3 V, and then discharged to 2.8 V at a constant current of 240 mA. From the initial capacity Q1 and the discharge capacity Q [2.0 C] of 100 cycles, the capacity retention ratio of (2.0 C) 100 cycles was calculated using the following equation.

[Equation 2]

(2.0C) Capacity retention rate of 100 cycles (%) = (Q [2.0C] / Q1)

Each evaluation result of an Example and a comparative example is shown in Table 1 below.

On the other hand, in Examples 2, 3 and Comparative Example 1, the orientation of the negative electrode mixture is controlled by reducing the number of multi-stage presses during the production of the negative electrode, compared to Example 1. In addition, in Examples 4, 5 and Comparative Example 3, the presence ratio of sulfur atoms on the surface of the negative electrode was controlled by reducing the amount of DTD added or not adding DTD as compared with Example 1. In Comparative Example 2, 1,3-propane sultone (PS) was added in place of DTD, and the bonding state of sulfur atoms on the negative electrode surface was controlled by replacing SO ₄ with SO ₃ . . On the other hand, in the above examples and comparative examples, the content of the positive electrode mixture and the negative electrode mixture on the current collector is the same.

C (004) / C (110)
Orientation of In particle
Porosity
[%] Sulfur atom
Content ratio
[atom%] Bond state of sulfur atom 0.5C cycle capacity maintenance rate
[%] 2.0C cycle capacity maintenance rate
[%] Example 1 7.4 9.2 1.5 S0 ₄ 92 80 Example 2 10.5 7.7 1.5 S0 ₄ 92 79 Example 3 14.0 6.5 1.5 S0 ₄ 92 75 Example 4 7.4 9.2 0.5 S0 ₄ 92 78 Example 5 7.4 9.2 0.1 S0 ₄ 92 73 Comparative Example 1 20.1 4.1 1.5 S0 ₄ 92 70 Comparative Example 2 7.4 9.2 0.6 S0 ₃ 92 70 Comparative Example 3 7.4 9.2 0 - 92 72

Referring to Table 1, Examples 1 to 5 have a configuration according to one embodiment, it can be seen that the capacity retention rate of the 2.0C cycle compared to Comparative Examples 1 to 3 is improved. On the other hand, in Comparative Example 1, since the orientation and intra-particle porosity represented by C (004) / C (110) is out of the range according to one embodiment, it can be seen that the capacity retention rate of the 2.0C cycle is decreasing. In addition, in Comparative Example 2, since the sulfur atoms on the surface of the negative electrode do not constitute a SO ₄ coupling unit, it can be seen that the capacity retention rate of 2.0 C cycles is reduced. Moreover, since the sulfur atom does not exist in the surface of a negative electrode in the comparative example 3, it turns out that the capacity retention rate of 2.0 C cycles is falling.

In addition, Example 1 has a capacity of 2.0 C cycles, as for Examples 2 and 3, since the orientation and intraparticle porosity represented by C (004) / C (110) are included in a preferred range according to one embodiment. It can be seen that the retention rate is further increased. In addition, in Example 1, since the content ratio of sulfur atoms on the surface of the negative electrode is included in the preferred range according to the embodiment for Examples 4 and 5, it can be seen that the capacity retention rate of the 2.0C cycle further increases. have.

The degree to which the volume of the lithium ion secondary battery after 100 cycles of the lithium ion secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 3 increased. Specifically, the change rate of the thickness of the lithium ion secondary battery after initial charge and discharge and the thickness of the lithium ion secondary battery after (2.0C) 100 cycles was measured. In addition, all the thickness of the lithium ion secondary battery was measured in fully charged state.

The increase rate of the thickness of the lithium ion secondary battery after (2.0C) 100 cycles when the thickness of the lithium ion secondary battery after initial stage charging and discharging is based on it is shown in Table 2 below.

(2.0C)% expansion of battery after 100 cycles Example 1 6.0 Example 2 6.2 Example 3 6.5 Example 4 6.4 Example 5 6.4 Comparative Example 1 9.9 Comparative Example 2 7.5 Comparative Example 3 7.1

Referring to Table 2, Examples 1 to 5 having the configuration according to the embodiment can be seen that the increase in battery volume compared to Comparative Examples 1 to 3. That is, the lithium ion secondary battery according to one embodiment may suppress the increase in battery volume due to high rate charge and discharge. Further, Example 1 includes all of the orientations expressed in C (004) / C (110), the intra-particle porosity, and the content ratio of sulfur atoms on the surface of the negative electrode, compared to Examples 2 to 5, in the preferred range according to one embodiment. Therefore, it can be seen that the increase in battery volume is more effectively suppressed.

As mentioned above, although preferred embodiment of this invention was described in detail, referring an accompanying drawing, this invention is not limited to the above-mentioned example. Those who possess ordinary knowledge in the field of technology to which the present invention pertains should also naturally read the present invention, even if it is clear that various changes or modifications can be made within the scope of the technical idea described in the claims. It can be understood that it belongs to the technical scope of.

10: lithium ion secondary battery
20: anode
21: anode current collector
22: positive electrode active material layer
30: cathode
31: negative electrode current collector
32: negative electrode active material layer
40: separator

Claims (8)

A negative electrode mixture containing a graphite active material; And
A cathode including a solid electrolyte interphase (SEI) film,
The ratio C (004) / C (110) of the X-ray diffraction peak intensity of the cathode is 20 or less,
The porosity in the particles of the graphite active material is 5% or more,
The SEI film comprises a sulfur atom constituting the SO ₄ bond, lithium ion secondary battery. The method of claim 1,
The thickness of the negative electrode mixture containing the graphite active material is 50㎛ or more, lithium ion secondary battery. The method of claim 2,
The thickness of the negative electrode mixture containing the graphite active material is 60㎛ or more, lithium ion secondary battery. The method of claim 1,
Lithium ion secondary battery, the ratio C (004) / C (110) of the X-ray diffraction peak intensity of the negative electrode is 8 or less. The method of claim 1,
The porosity in the particle of the graphite active material is at least 8%, lithium ion secondary battery. The method of claim 1,
The sulfur atom present on the surface of the negative electrode is at least 1 atomic% based on the total number of atoms present on the surface of the negative electrode, lithium ion secondary battery. The method of claim 1,
The charge density of the negative electrode mixture containing the graphite active material is 1.5 g / cm ³ or more, lithium ion secondary battery. The method of claim 1,
A lithium ion secondary battery, further comprising an electrolyte solution containing 1,3,2-dioxathiolane 2,2-dioxide.

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