JP6989430B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are widely used as a power source for portable devices such as notebook personal computers (note PCs) and mobile phones. In addition to these applications, lithium-ion secondary batteries are also attracting attention as a power source for xEVs such as electric vehicles and hybrid vehicles. Therefore, in recent years, the demand for lithium-ion secondary batteries is expected to grow even more actively.

Here, the lithium ion secondary battery for xEV is required to have a high capacity and a long life in order to secure the same performance as a conventional gasoline engine (gasoline engine) vehicle. Further, in the lithium ion secondary battery for xEV, there is a strong demand for quick charging characteristics (that is, charging / discharging characteristics at a high rate) for completing charging within a time equivalent to the refueling time of a gasoline engine vehicle. ..

Conventionally, as disclosed in Patent Documents 1 to 3 below, there is known a technique for improving the charge / discharge characteristics of a lithium ion secondary battery by containing a specific additive in an electrolytic solution.

JP-A-1998-189042 Japanese Unexamined Patent Publication No. 2013-101968 Japanese Unexamined Patent Publication No. 2013-229307

Here, as the capacity of the lithium ion secondary battery has increased in recent years, the packing density of the negative electrode containing the graphite active material has become higher. Further, in order to improve the rapid charge / discharge characteristics (that is, the charge / discharge characteristics at a high rate) that tend to decrease by increasing the filling density, the crystal structure of the graphite active material contained in the negative electrode should be controlled more strictly. Is being considered.

However, the interaction between the graphite active material whose crystal structure is strictly controlled in this way and the SEI (Solid Electrolyte Interface) film formed on the surface of the negative electrode by the additives as described in Patent Documents 1 to 3 above. Has not been examined in detail in the past. Therefore, new findings may be obtained by examining the crystal structure of the graphite active material contained in the negative electrode and the state of the surface of the negative electrode in more detail.

Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is a new and improved lithium ion secondary battery capable of improving charge / discharge characteristics at a high rate. Is to provide.

In order to solve the above problems, according to a certain viewpoint of the present invention, a negative electrode containing a graphite active material is provided, and the orientation of the negative electrode represented by C (004) / C (110) is 20 or less, and the intraparticle porosity of the graphite active material is 5% or more, wherein the negative electrode surface, the presence of sulfur atom constituting the SO ₄ binding is provided a lithium ion secondary battery.

According to this viewpoint, the charge / discharge characteristics at a high rate can be improved in the lithium ion secondary battery.

The thickness of the negative electrode mixture containing the graphite active material may be 50 μm or more.

According to this viewpoint, the capacity of the lithium ion secondary battery can be increased.

The thickness of the negative electrode mixture containing the graphite active material may be 60 μm or more.

According to this viewpoint, the capacity of the lithium ion secondary battery can be increased.

The orientation of the negative electrode represented by C (004) / C (110) may be 8 or less.

According to this viewpoint, in the lithium ion secondary battery, the charge / discharge characteristics at a high rate can be further improved.

The void ratio in the particles of the graphite active material may be 8% or more.

According to this viewpoint, in the lithium ion secondary battery, the charge / discharge characteristics at a high rate can be further improved.

The abundance ratio of the sulfur atom on the surface of the negative electrode may be 1 atom% or more.

According to this viewpoint, in the lithium ion secondary battery, the charge / discharge characteristics at a high rate can be further improved.

The packing density of the negative electrode mixture containing the graphite active material ^{may be 1.5 g / cm 3} or more.

According to this viewpoint, the capacity of the lithium ion secondary battery can be increased.

The lithium ion secondary battery may further include an electrolytic solution containing 1,3,2-dioxathiolane 2,2-dioxide.

According to this viewpoint, in the lithium ion secondary battery, the charge / discharge characteristics at a high rate can be further improved.

As described above, according to the present invention, it is possible to provide a lithium ion secondary battery having improved charge / discharge characteristics at a high rate.

It is a side sectional view schematically showing the structure of a lithium ion secondary battery.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the present specification and the drawings, components having substantially the same functional configuration are designated by the same reference numerals, and duplicate description will be omitted.

<1. Lithium-ion secondary battery configuration>
First, with reference to FIG. 1, the configuration of the lithium ion secondary battery 10 according to the embodiment of the present invention will be described. FIG. 1 is a side sectional view schematically showing the configuration of a lithium ion secondary battery.

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 electrolytic solution. The form of the lithium ion secondary battery 10 is not particularly limited. That is, the lithium ion secondary battery 10 may be in any form such as a cylindrical shape, a square shape, a laminate type, or a button type.

(Positive electrode 20)
The positive electrode 20 includes a current collector 21 and a positive electrode active material layer 22. The current collector 21 may be any conductor as long as it is a conductor. The current collector 21 may be made of, for example, aluminum, stainless steel, nickel-coated steel, or the like.

The positive electrode active material layer 22 contains at least the positive electrode active material, and may further contain a conductive agent and a binder. The ratio of the contents of the positive electrode active material, the conductive agent and the binder is not particularly limited, and the ratio of the contents used 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 substance that can electrochemically occlude and release lithium ions. Solid solution oxides, for _{example, Li a Mn x Co y Ni} z O 2 (1.150 ≦ a ≦ 1.430,0.45 ≦ x ≦ 0.6,0.10 ≦ y ≦ 0.15,0. _{20 ≦ z ≦ 0.28), LiMn} x Co y Ni z O 2 (0.3 ≦ x ≦ 0.85,0.10 ≦ y ≦ 0.3,0.10 ≦ z ≦ 0.3), or It _{may be LiMn 1.5} Ni _0.5 O ₄ or the like.

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

The binder is, for example, polyvinylidene fluoride, ethylene propylene diene (ethylene-polyethylene-diene) ternary copolymer, styrene butadiene rubber (Stylene-butadiene rubber), acrylonitrile butadiene rubber (acrylloburi). (Fluororubber), polyvinyl acetate, polymethylmethacrylate, polyethylene, nitrocellulose, and the like. However, the binder is not particularly limited as long as it can bind the positive electrode active material and the conductive agent onto the current collector 21.

(Negative electrode 30)
The negative electrode 30 includes a current collector 31 and a negative electrode active material layer 32. The current collector 31 may be any conductor. The current collector 31 may be, for example, copper, a copper alloy, aluminum, stainless steel, nickel-plated steel, or the like.

The negative electrode active material layer 32 contains at least the negative electrode active material, and may further contain a conductive agent and a binder. The ratio of the contents of the negative electrode active material, the conductive agent and the binder is not particularly limited, and the ratio of the contents used in a general lithium ion secondary battery can be used.

The negative electrode active material contains at least a graphite active material. The graphite active material may be, for example, artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, or natural graphite coated with artificial graphite. Further, the negative electrode active material may contain a silicon (Si) or tin (Sn) -based active material, a titanium oxide (TiO _x ) -based active material, or the like, in addition to the graphite active material. The silicon or tin-based active material is, for example, fine particles of silicon or tin, fine particles of an oxide of silicon or tin, or an alloy of silicon or tin, and the titanium oxide-based active material is, for example, Li ₄ Ti ₅ O _12. And so on.

As the conductive agent, the same conductive agent as that used in the positive electrode active material layer 22 can be used.

As the binder, for example, styrene-butadiene rubber (Stylene-Butagene Rubber: SBR) or the like can be used. However, the binder is not particularly limited as long as it can bind the negative electrode active material and the conductive agent onto the current collector 31.

The negative electrode 30 according to the present embodiment contains a graphite active material as the negative electrode active material, and the ratio C (ratio) of the X-ray diffraction peak intensity of the (004) plane of graphite to the X-ray diffraction peak intensity of the (110) plane of graphite ( 004) / C (110) is 20 or less, preferably 8 or less. Here, the ratio of the X-ray diffraction peak intensities represented by C (004) / C (110) indicates the degree of orientation of the graphite crystals. The larger the value of C (004) / C (110), the more parallel the graphite layer is to the measurement plane.

When the graphite layer is highly oriented, the graphite layers are more regularly and more tightly laminated, making it difficult for lithium ions to be inserted or removed from inside the graphite crystal structure. Therefore, the larger the ratio of the X-ray diffraction peak intensities represented by C (004) / C (110), the lower the charge / discharge characteristics, especially at high rates. Therefore, in the lithium ion secondary battery 10 according to the present embodiment, the ratio C (004) / C (110) of the X-ray diffraction peak intensity of the negative electrode 30 is limited to 20 or less, preferably 8 or less. The lower limit of the ratio C (004) / C (110) of the X-ray diffraction peak intensity of the negative electrode 30 is not particularly limited, but the lower limit may be, for example, 7 due to the crystal structure of graphite.

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 / discharge of the lithium ion secondary battery 10, and the negative electrode 30 is manufactured. Not a measured value at the time. The crystal structure of the graphite active material contained in the negative electrode 30 may change due to occlusion or release of lithium ions. Therefore, it should be noted that the value of C (004) / C (110) measured at the time of full charge after the initial charge / discharge cannot be directly compared with the value measured at the time of manufacturing the negative electrode 30, for example.

In the negative electrode 30 according to the present embodiment, the void ratio in the particles of the graphite active material contained is 5% or more, preferably 8% or more. As described above, the negative electrode 30 according to the present embodiment is provided with voids in which lithium ions can be easily inserted inside the graphite active material by controlling the orientation of the graphite active material contained therein to be low. In particular, the negative electrode 30 according to the present embodiment is provided with more voids in the particles of the graphite active material instead of voids between the particles of the graphite active material, so that lithium ions are formed even when charging and discharging at a high rate are repeated. It is possible to suppress a decrease in the capacity of the secondary battery 10. It is considered that this is because the voids between the particles of the graphite active material are easily crushed by the slip between the particles due to the expansion or contraction of the graphite active material due to charge and discharge, while the voids in the particles are not easily crushed. Be done. Therefore, when the void ratio in the particles of the graphite active material is less than 5%, the charge / discharge characteristics at a high rate are deteriorated, which is not preferable. The upper limit of the void ratio in the particles of the graphite active material is not particularly limited, but the upper limit may be, for example, 20% due to the crystal structure of the graphite active material.

For the void ratio in the particles of the graphite active material, for example, the lithium ion secondary battery 10 fully charged after the initial charge is disassembled, the negative electrode 30 is taken out, and the cross section of the negative electrode mixture of the taken out negative electrode 30 is SEM (Scanning Electron Microscope). It can be calculated by analyzing the image observed in the above. Specifically, the in-particle void ratio of the graphite active material is obtained by dividing the cross-sectional area of the hollow region in the particles of the graphite active material observed in the SEM image by the cross-sectional area of the solid region in the particles. , Can be calculated.

The thickness of the negative electrode mixture on the negative electrode 30 is preferably 50 μm or more, more preferably 60 μm or more. By setting the thickness of the negative electrode mixture within the above range, the lithium ion secondary battery 10 can realize a higher capacity.

However, when the thickness of the negative electrode mixture is increased, the capacity of the lithium ion secondary battery 10 can be increased, but on the other hand, the positive electrode 20, the negative electrode 30, and the separator 40 are laminated and wound to form an electrode structure. It can be difficult to form. Therefore, the upper limit of the thickness of the negative electrode mixture on the negative electrode 30 may be, for example, 150 μm. In the lithium ion secondary battery 10 according to the present embodiment, the orientation of the graphite active material and the void ratio in the particles are controlled within the above ranges while keeping the thickness of the negative electrode mixture to such that the formation of the electrode structure is not difficult. It is possible to achieve both high capacity and charge / discharge characteristics at a high rate. The thickness of the negative electrode mixture described above is the thickness per one side of the negative electrode 30. That is, the thickness of the above-mentioned negative electrode mixture is the total value of the negative electrode active material layers 32 on both sides when the negative electrode active material layers 32 are provided on both sides of the current collector 31, and is only on one side of the current collector 31. When the negative electrode active material layer 32 is provided, the value is only for the negative electrode active material layer 32 on one side.

The filling density of the negative electrode mixture on the negative electrode 30 ^{is preferably 1.5 g / cm 3} or more. By setting the packing density of the negative electrode mixture containing the graphite active material within the above range, the lithium ion secondary battery 10 can realize a higher capacity. However, when the filling density of the negative electrode mixture is increased, the capacity of the lithium ion secondary battery 10 can be increased, but on the other hand, the charge / discharge characteristics of the lithium ion secondary battery 10 may deteriorate. However, in the lithium ion secondary battery 10 according to the present embodiment, the charge / discharge characteristics at a high rate can be ensured by controlling the orientation of the graphite active material and the void ratio in the particles. It is possible to increase the filling density. According to this, the lithium ion secondary battery 10 can achieve both high capacity and charge / discharge characteristics at a high rate. 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.}

Furthermore, the negative electrode 30 according to this embodiment, the SEI layer is formed on the surface, the SEI film, include sulfur atoms constituting the SO ₄ binding units. The SEI film is an ultrathin film formed by decomposing the electrolytic solution on the surface of the negative electrode 30 at the time of initial charge / discharge. Specifically, the SEI film is an ultrathin film formed by an electrolytic solution impregnating the negative electrode 30 and a decomposition product of an additive of the electrolytic solution. The SEI film can suppress further decomposition reaction of the electrolytic solution on the negative electrode 30 while playing a role of supporting the insertion or desorption of lithium ions into the negative electrode 30.

In the lithium ion secondary battery 10 according to the present embodiment, by the inclusion of sulfur atom constituting the SO ₄ binding units SEI film of the negative electrode 30, the electrostatic interaction between the SO ₄ and the lithium ion, the negative electrode 30 The concentration of lithium ions in the vicinity can be increased. This makes it possible to facilitate the insertion or desorption of lithium ions from the negative electrode 30, so that the charge / discharge characteristics of the lithium ion secondary battery 10 at a high rate can be improved.

However, the inclusion of sulfur atom constituting the SO ₄ binding unit in SEI film described above, when the orientation indicated by the graphite active material C (004) / C (110 ) is high, the effect becomes low .. It is considered that this is because if there are few voids in which lithium ions can be inserted into the graphite active material, even if the lithium ion concentration on the surface of the negative electrode 30 is increased, the insertion of lithium ions into the graphite active material is not promoted. .. On the other hand, as the lithium ion secondary battery 10 according to the present embodiment, when the orientation indicated by the graphite active material C (004) / C (110 ) is low, constituting SO ₄ binding units SEI film It is possible to exert a higher effect by containing a sulfur atom.

Incidentally, the existing ratio of the SEI film of the sulfur atom constituting the SO ₄ binding units is preferably 1 atomic% or more. _{In such a case, since a larger amount of SO 4} that attracts lithium ions can be present in the SEI film on the surface of the negative electrode, the charge / discharge characteristics of the lithium ion secondary battery 10 at a high rate can be further improved. The upper limit of the abundance ratio of sulfur atoms in the SEI film is not particularly limited, but may be, for example, 10 atom%.

(Separator 40)
The separator 40 can be used without particular limitation as long as it is used as a separator for a lithium ion secondary battery. As the separator, it is preferable to use a porous membrane or a non-woven fabric showing excellent high rate discharge performance alone or in combination. The resin constituting the separator is, for example, a polyolefin-based resin typified by polyethylene, polymer, or the like, a polyethylene terephthalate, a polybutylene terephthalate, or the like. Polymer) -based resin, polyvinylidene fluoride (PVDF), vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetra Fluoroethylene polymer, vinylidene fluoride-trifluoroethylene polymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetonee copolymer, Vinylidene fluoride-ethylene (ethylene) polymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene- Hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer and the like may be used.

(Electrolytic solution)
The electrolytic solution is not particularly limited as long as it is used as an electrolytic solution for a lithium ion secondary battery. The electrolytic solution has a composition in which an electrolyte salt is contained in a non-aqueous solvent.

Examples of the non-aqueous solvent include propylene carbonate (propylene carbonate), ethylene carbonate (ethylene carbonate), butylene carbonate (ethylene carbononate), chloroethylene carbonate (chloroethylene carbonate), vinylene carbonate (vinylene carbonate), and the like. Kind: Cyclic esters such as γ-butyrolactone and γ-valerolactone; chains of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and the like. Chain esters such as methyl format, methyl acetate, butyric acid methyl; tetrahydrofuran (Tetrahydrofuran) or derivatives thereof; 1,3-dioxane, 1,4- Ethers such as dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyldilyme; acetonitrile (acetylene), benzonitrile (benez). ) Etc. nitriles; dioxolane or its derivatives; ethylene sulfide (estersulfide), sulfolane, sultone (sultone) or its derivatives, etc. may be used alone or in admixture of two or more. can.

The electrolyte salt, for _{example, LiClO 4, LiBF 4, LiAsF} 6, LiPF 6, LiPF 6-x (C n F 2n + 1) x ( where, 1 <x <6, n = 1or2), LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN and other inorganic ion salts containing one of lithium (Li), sodium (Na) or potassium (K), 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, (n-C ₄ H ₉ ) ₄ NClO ₄ , (n-C ₄ H ₉ ) ₄ NI, (C ₂ H ₅ ) ₄ N-malate, (C ₂ H ₅ ) ₄ N-benzoate, (C) ₂ H ₅ ) ₄ N-phthalate, lithium sulphonic acid lithium, lithium sulphonic acid, lithium dodecylbenzene sulphonate, etc., organic salts, etc. May be good. The electrolyte salt can be used alone or in combination of two or more of these ionic salts or ionic compounds. The concentration of the electrolyte salt can be the same as the concentration used in a general lithium ion secondary battery (about 0.8 mol / L to 2.0 mol / L).

Various additives may be added to the electrolytic solution. Examples of such additives include negative-negative action additives, positive-side action additives, ester-based additives, carbonic acid ester-based additives, sulfate ester-based additives, phosphate ester-based additives, borate ester-based additives, and acids. Examples thereof include an anhydride-based additive and an electrolyte-based additive. One or more of these additives may be added to the electrolytic solution.

In the lithium ion secondary battery according to the present embodiment, the electrolytic solution preferably contains 1,3,2-dioxathiolane 2,2-dioxide (DTD) as an additive. The DTD is a cyclic sulfate ester, which is decomposed during the initial charge / discharge of the lithium ion secondary battery 10 to form an SEI film on the negative electrode 30. In this case, the DTD, primarily because it is degraded in SO ₄ linking unit, degradation of the DTD will be contained as SO ₄ binding units SEI film on the surface of the negative electrode 30. Thus, by incorporating a DTD in the electrolytic solution, the SEI film on the surface of the negative electrode 30, may contain a sulfur atom constituting a SO ₄ binding units described above. In particular, DTD has a high possibility of being decomposed by SO ₄ binding units, may contain a sulfur atom constituting a SO ₄ coupled to SEI film on the surface of the negative electrode 30 more efficiently.

As the additives to be degraded in such a predominantly SO ₄ binding units to other DTD, 4-methyl-1,3,2-dioxathiolane 2,2-dioxide, 4-ethyl-1,3, 2-Dioxathiolane 2,2-dioxide, 4-fluoro-1,3,2-dioxathiolane 2,2-dioxide, 4-chloro-1,3,2-dioxathiolane 2,2-dioxide, and 4-phenyl-1, Examples thereof include 3,2-dioxathiolane 2,2-dioxide and the like.

<2. Manufacturing method of lithium-ion secondary battery ＞
Next, a method for manufacturing the lithium ion secondary battery 10 will be described.

The positive electrode 20 is manufactured by the following method. First, a slurry is formed by dispersing a mixture of a positive electrode active material, a conductive agent and a binder in a predetermined ratio in a solvent (for example, N-methyl-2-pyrrolidone). Next, the slurry is applied onto the current collector 21 and dried to form the positive electrode active material layer 22. The method of application is not particularly limited. As a method of coating, for example, a knife coater method, a gravure coater method, or the like can be used. Subsequently, the positive electrode active material layer 22 is compressed to a predetermined density by a press machine. As a result, the positive electrode 20 is manufactured.

The negative electrode 30 is also manufactured by the same method as that of the positive electrode 20. First, a slurry is formed by dispersing a mixture of a negative electrode active material and a binder in a solvent (for example, water). Next, the slurry is applied onto the current collector 31 and dried to form the negative electrode active material layer 32. Subsequently, the negative electrode active material layer 32 is compressed to a predetermined density by a press machine. As a result, the negative electrode 30 is manufactured.

Next, an electrode structure is manufactured by sandwiching the separator 40 between the positive electrode 20 and the negative electrode 30. Subsequently, the prepared electrode structure is processed into a desired shape (for example, cylindrical shape, square shape, laminated shape, button shape, etc.) and inserted into the container having the desired shape. Then, by injecting the electrolytic solution into the container, each pore in the separator 40 is impregnated with the electrolytic solution. As a result, the lithium ion secondary battery 10 is manufactured.

Hereinafter, the lithium ion secondary battery and the method for manufacturing the lithium ion secondary battery according to the present embodiment will be specifically described with reference to Examples and Comparative Examples. The examples shown below are merely examples, and the method for manufacturing the lithium ion secondary battery and the lithium ion secondary battery according to the present embodiment is not limited to the following examples.

(Manufacturing of Lithium Ion Secondary Battery According to Example 1)
_{LiNi Nickel cobalt oxide represented by LiNi 0.88} Co _0.1 Al _0.02 O ₂ was used as the positive electrode active material, carbon powder was used as the conductive agent, and polyvinylidene fluoride was used as the binder. The positive electrode slurry (positive electrode mixture) was prepared by mixing the positive electrode active material, the conductive agent, and the binder in a mass ratio of 94: 4: 2, and adding N-methyl-2-pyrrolidone and kneading. ..

Next, the above positive positive slurry is applied to one surface of the current collector having a length of 222 mm and a width of 29 mm on a current collector made of an aluminum foil having a thickness of 12 μm, a length of 238 mm and a width of 29 mm, and faces one surface of the current collector. The other surface was coated with a length of 172 mm and a width of 29 mm. The current collector after applying the positive electrode slurry was dried and then rolled to prepare a positive electrode plate. At this time, 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 . ..

Then, a current collecting tab made 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 electrode plate to which the positive electrode mixture was not applied.

Artificial graphite and silicon-containing carbon were used as the negative electrode active material, and carboxymethyl cellulose and styrene-butadiene rubber were used as the binder. Artificial graphite, silicon-containing carbon, carboxymethyl cellulose, and styrene-butadiene rubber are mixed so as to have a mass ratio of 92.2: 5.3: 1.0: 1.5, and water is added and kneaded to obtain a negative electrode. The slurry (negative electrode mixture) was adjusted.

Next, the above negative electrode slurry was applied to one surface of the current collector having a length of 235 mm and a width of 30 mm on a current collector made of an aluminum foil having a thickness of 8 μm, a length of 271 mm and a width of 30 mm, and opposed to one surface of the current collector. The other surface was coated with a length of 178 mm and a width of 30 mm. The current collector after applying the negative electrode slurry was dried and then rolled to prepare a negative electrode electrode plate. At this time, 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} . By performing the rolling in multiple stages, it was possible to suppress an increase in the orientation value represented by C (004) / C (110) of the negative electrode. Further, a current collecting tab made 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 the portion of the negative electrode electrode plate to which the negative electrode mixture was not applied.

A non-aqueous solvent was prepared by mixing ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate in a volume ratio of 20:40:40. _{LiPF 6} as an electrolyte salt was dissolved in the prepared non-aqueous solvent at a concentration of 1.15 mol / L to prepare an electrolytic solution. Further, vinylene carbonate was externally added to the prepared electrolytic solution in an amount of 1.5% by mass based on the total mass of the electrolytic solution, and 1,3,2-dioxathiolane 2,2-dioxide (DTD) was added to the electrolytic solution. It was externally added at 1.0% by mass with respect to the total mass.

A lithium secondary battery was manufactured using the positive electrode, the negative electrode, and the electrolytic solution prepared above. The positive electrode and the negative electrode were arranged so as to face each other via a separator, and these were folded back at a predetermined position, wound, and then pressed to prepare a flat electrode assembly. As the separator, two separators made of a polyethylene porous body having a length of 350 mm and a width of 32 mm were used.

The prepared electrode assembly was housed in a battery container made of aluminum laminate, and an electrolytic solution was injected into the battery container. At this time, the current collecting tabs of the positive electrode and the negative electrode were arranged so that they could be taken out. The design capacity of the manufactured lithium ion secondary battery is 480 mAh.

(Evaluation of lithium-ion secondary battery)
Subsequently, the lithium ion secondary battery produced above was initially charged and discharged. Specifically, in an environment of 25 ° C., the lithium ion secondary battery is charged with a constant current of 48 mA until it reaches 4.3 V, and then charged with a constant voltage of 4.3 V until the current value reaches 24 mA. , A constant current of 48 mA was discharged until it reached 2.8 V. The discharge capacity at this time was set to the initial capacity Q1.

Next, the lithium secondary battery that has been initially charged and discharged as described above is charged in an environment of 25 ° C. with a constant current of 240 mA until it reaches 4.3 V, and further, the current value is a constant voltage of 4.3 V. Was charged until the voltage reached 24 mA. Then, the charged lithium ion secondary battery was disassembled, and the negative electrode plate was taken out. The wide-angle X-ray diffraction profile of the removed negative electrode plate was measured. The X-ray source was CuKα, and the output was 45 kV and 200 mA. Regarding the obtained X-ray diffraction profile, each of the graphite (004) plane in which 2θ is observed in the vicinity of 48 ° to 56 ° and the graphite (110) plane in which 2θ is observed in the vicinity of 76 ° to 79 °. The peak intensity was measured. Then, the ratio C (004) / C (110) of the peak intensity of the (004) plane of graphite and the peak strength of the (110) plane of graphite was calculated. The results are shown in Table 1 below. The higher the ratio C (004) / C (110), the more parallel the graphite layer is to the measurement surface.

Further, the lithium secondary battery that has been initially charged and discharged as described above is charged in an environment of 25 ° C. with a constant current of 240 mA until it reaches 4.3 V, and further, the current value changes at a constant voltage of 4.3 V. It was charged until it reached 24 mA. Then, the charged lithium ion secondary battery was disassembled, and the negative electrode plate was taken out. A cross section of the negative electrode mixture was prepared from the taken-out negative electrode electrode plate by an Ar ion milling process (cross section polisher), and an SEM image of the prepared cross section was obtained. With respect to the acquired SEM image, the particles of the graphite active material and the voids in the particles were discriminated by image processing, and the cross-sectional area ratio of each was calculated to calculate the void ratio in the particles of the graphite active material. The results are shown in Table 1 below.

Further, the lithium secondary battery that has been initially charged and discharged as described above is charged in an environment of 25 ° C. with a constant current of 240 mA until it reaches 4.3 V, and further, the current value changes at a constant voltage of 4.3 V. It was charged until it reached 24 mA. Then, the charged lithium ion secondary battery was disassembled, and the negative electrode plate was taken out. The SEI film formed on the surface of the negative electrode plate taken out was analyzed by X-ray photoelectron spectroscopy. The X-ray source was monochromatic AlKα (1486.6 eV), and the analysis area of the negative electrode plate was 700 μm × 300 μm. From the spectrum obtained by X-ray photoelectron spectroscopy, the composition ratio of sulfur was calculated in atomic%. The results are shown in Table 1 below.

Furthermore, the state of the sulfur atom contained in the SEI film was analyzed from the spectrum obtained by X-ray photoelectron spectroscopy. The F (1s) peak and the S (2p) peak were used for the sulfur state analysis.

For example, according to Non-Patent Document (Journal of the Electrochemical society, 154 (12) A1088-A1099 (2007)), the peak of F (1s) is observed 685.1EV, the peak of S (2p) is SO ₄ And SO ₃ are observed at 168.4 eV to 171.1 eV, and 165.5 eV to 167.5 eV, 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, the low B. of F (1s) measured by X-ray photoelectron spectroscopy. E. The value of the peak top on the side is assumed to be LiF, the observed position of each peak of S (2p) is assumed to be near the peak top, and the sulfur state is determined by the peak shift amount based on the peak of F (1s). did.

That is, in the negative electrode plate according to Example 1, the LiF (F (1s)) peak was 681.7 eV and the S (2p) peak was 166.1 eV. Therefore, the difference between the observed positions of the F (1s) peak and the S (2p) peak was 515.6 eV. This, according to Non-Patent Document, the peak of F (1s), for corresponding to a difference between the peak of SO ₄ S (2p), is included in the SEI film negative electrode plate according to Example 1 state of the sulfur atom, it was confirmed that the SO _4.

Subsequently, the lithium secondary battery that was initially charged and discharged as described above was charged in an environment of 25 ° C. with a constant current of 240 mA until it reached 4.3 V, and then the current value was a constant voltage of 4.3 V. Was charged to 24 mA, and then discharged to 2.8 V with a current of 240 mA. With this as one cycle, 100 cycles of charging and discharging were repeated. Then, from the initial capacity Q1 and the discharge capacity Q [0.5C] at the 100th cycle, the capacity retention rate in the 0.5C cycle was determined using the following formula.
Capacity retention rate (%) in 0.5C cycle = (Q [0.5C] / Q1) × 100

Further, the lithium secondary battery that has been initially charged and discharged as described above is charged in an environment of 25 ° C. with a constant current of 960 mA until it reaches 4.3 V, and then the current value changes at a constant voltage of 4.3 V. The battery was charged to 24 mA and then discharged to 2.8 V with a current of 240 mA. With this as one cycle, 99 cycles of charging and discharging were repeated. Then, in the 100th cycle, the battery is charged with a constant current of 240 mV until it reaches 4.3 V, charged with a constant voltage of 4.3 V until the current value reaches 24 mV, and then discharged with a constant current of 240 mA until it reaches 2.8 V. Was done. From the initial capacity Q1 and the discharge capacity Q [2C] at the 100th cycle, the capacity retention rate in the 2.0C cycle was determined using the following formula.
Capacity retention rate (%) in 2.0C cycle = (Q [2C] / Q1) × 100

The evaluation results of each of the examples and comparative examples are shown in Table 1 below.

In Examples 2 and 3 and Comparative Example 1, the orientation of the negative electrode mixture was controlled by reducing the number of multi-step presses during the negative electrode production as compared with Example 1. Further, in Examples 4 and 5 and Comparative Example 3, the abundance ratio of sulfur atoms on the surface of the negative electrode was controlled by reducing the amount of DTD added or by not adding DTD as compared with Example 1. Further, in Comparative Example 2, the bonding state of the sulfur atom on the negative electrode surface was controlled from SO _{4 to SO 3 by adding 1,3-propanesulton (PS) instead of DTD to Example 1.} It is a thing. In each of these Examples and Comparative Examples, the amounts of the positive electrode mixture and the negative electrode mixture on the current collector are the same.

With reference to Table 1, it can be seen that since Examples 1 to 5 have the configuration according to the present embodiment, the capacity retention rate of 2.0 C cycle is improved as compared with Comparative Examples 1 to 3. On the other hand, in Comparative Example 1, since the orientation represented by C (004) / C (110) and the void ratio in the particles are out of the range according to the present embodiment, the capacity retention rate in the 2.0 C cycle is lowered. You can see that. In Comparative Example 2, since the sulfur atom of the surface of the negative electrode not constitute SO ₄ binding unit, it can be seen that the capacity retention rate 2.0C cycle is reduced. Further, in Comparative Example 3, it can be seen that the capacity retention rate in the 2.0 C cycle is lowered because the sulfur atom is not present on the surface of the negative electrode.

Further, in Example 1, the orientation represented by C (004) / C (110) and the void ratio in the particles are included in the preferable ranges according to the present embodiment with respect to Examples 2 and 3. It can be seen that the capacity retention rate of the 0C cycle is higher. Further, in Example 1, since the abundance ratio of sulfur atoms on the surface of the negative electrode is included in the preferable range according to the present embodiment with respect to Examples 4 and 5, the capacity retention rate in the 2.0 C cycle is higher. You can see that.

Here, in the lithium ion secondary batteries according to Examples 1 to 5 and Comparative Examples 1 to 3 above, the degree of battery swelling after 100 cycles was evaluated. Specifically, the rate of change between the thickness of the lithium ion secondary battery after the initial charge and discharge and the thickness of the lithium ion secondary battery after 100 cycles of 2.0 C cycles was measured. The thickness of the lithium ion secondary battery was measured in a fully charged state.

Table 2 below shows the rate of increase in the thickness of the lithium-ion secondary battery after 100 cycles when the thickness of the lithium-ion secondary battery after the initial charge and discharge is used as a reference.

With reference to Table 2, it can be seen that in Examples 1 to 5 having the configuration according to the present embodiment, battery swelling is suppressed as compared with Comparative Examples 1 to 3. That is, it can be seen that the lithium ion secondary battery according to the present embodiment can suppress battery swelling due to charging and discharging at a high rate. Further, in Example 1, either the orientation represented by C (004) / C (110) and the void ratio in the particles, or the abundance ratio of sulfur atoms on the surface of the negative electrode is higher than that of Examples 2 to 5. Since it is included in the preferable range according to the present embodiment, it can be seen that the battery swelling is further suppressed.

Although the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person having ordinary knowledge in the field of technology to which the present invention belongs can come up with various modifications or modifications within the scope of the technical ideas described in the claims. , These are also naturally understood to belong to the technical scope of the present invention.

10 Lithium-ion secondary battery 20 Positive electrode 21 Current collector 22 Positive electrode active material layer 30 Negative electrode 31 Current collector 32 Negative electrode active material layer 40 Separator

Claims (8)

Equipped with a negative electrode containing graphite active material,
The orientation of the negative electrode represented by C (004) / C (110) is 20 or less, and the void ratio in the particles of the graphite active material is 5% or more.
Wherein the negative electrode surface, the presence of sulfur atom constituting the SO ₄ binding, a lithium ion secondary battery. The lithium ion secondary battery according to claim 1, wherein the thickness of the negative electrode mixture containing the graphite active material is 50 μm or more. The lithium ion secondary battery according to claim 2, wherein the thickness of the negative electrode mixture containing the graphite active material is 60 μm or more. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the orientation of the negative electrode represented by C (004) / C (110) is 8 or less. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the graphite active material has an intraparticle void ratio of 8% or more. The lithium ion secondary battery according to any one of claims 1 to 5, wherein the abundance ratio of the sulfur atom on the surface of the negative electrode is 1 atom% or more. The lithium ion secondary battery according to any one of claims 1 to 6, wherein the filling density of the negative electrode mixture containing the graphite active material is 1.5 g / cm ^{3 or more.} The lithium ion secondary battery according to any one of claims 1 to 7, further comprising an electrolytic solution containing 1,3,2-dioxathiolane 2,2-dioxide.

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