JP2011150865A - Negative electrode material for lithium secondary battery, manufacturing method therefor, and the lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery having a small irreversible capacity, when it is initially charged and discharged, and having superior charge and discharge cycle characteristics, a negative electrode material capable of constituting the lithium secondary battery, and to provide a manufacturing method therefor. <P>SOLUTION: This negative electrode material for the lithium secondary battery contains graphite of which the R value, that is the peak intensity ratio in 1,360 cm<SP>-1</SP>with respect to a peak intensity in 1,580 cm<SP>-1</SP>in an argon ion laser Raman spectrum, is 0.1-0.5, and of which the plane spacing d<SB>002</SB>of a 002 plane is 0.338 nm or less, and an a natural polysaccharide or its derivative for covering a surface thereof, and this lithium secondary battery includes a negative electrode containing the negative electrode material. The negative electrode material is obtained by the manufacturing method, having a process for dispersing the graphite, in a solution with the natural polysaccharide or its derivative dissolved in water. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery having a small irreversible capacity at the time of first charge / discharge and good charge / discharge cycle characteristics, a negative electrode material capable of constituting the lithium secondary battery, and a method for producing the same.

  Lithium secondary batteries are widely used as power sources for portable devices such as mobile phones and notebook personal computers because of their high energy density. Recently, the performance of portable devices has been remarkably improved, and in accordance with this, for example, a higher capacity is required for lithium secondary batteries used as a power source.

  Various studies have been made to increase the capacity of the lithium secondary battery, and so-called irreversible capacity reduction can be cited as one of the methods.

  In current lithium secondary batteries, a carbon material such as graphite is widely used as a negative electrode material that acts as a negative electrode active material. For example, Patent Document 1 discloses phosphorus-like or flake-like graphite used as a negative electrode material. A technique for reducing the irreversible capacity by coating the surface of the particles with an organic substance such as a starch derivative has been proposed.

JP 2003-168433 A

  However, in the negative electrode material described in Patent Document 1, the graphite particles serving as the base material are in the form of phosphorus or flakes having a large aspect ratio, and therefore accompanying the insertion / extraction of lithium ions during charge / discharge of the battery. There is concern about damage due to expansion and contraction between graphite layers. In other words, by repeating charge and discharge of the battery, the surface of the graphite particles may be exposed due to peeling or cracking in the organic matter covering the surface of the graphite particles, which makes it irreversible for each charge and discharge cycle. There is a possibility that the capacity is generated and the charge / discharge cycle deterioration of the battery is caused.

  For this reason, in addition to reducing the irreversible capacity at the time of initial charge / discharge of the battery, development of a technique capable of suppressing the generation of the irreversible capacity even after repeated charge / discharge is required.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a lithium secondary battery having a small irreversible capacity at the time of initial charge / discharge and good charge / discharge cycle characteristics, and the lithium secondary battery. An object of the present invention is to provide a negative electrode material and a manufacturing method thereof.

Anode material for lithium secondary battery of the present invention were able to achieve the object, R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum from 0.1 to 0. 5 and having a 002 plane spacing d ₀₀₂ of 0.338 nm or less, and a natural polysaccharide or a derivative thereof covering the surface thereof.

The method of the present invention for producing the negative electrode material for a lithium secondary battery of the present invention comprises a peak intensity of 1580 cm ⁻¹ in an argon ion laser Raman spectrum in a solution obtained by dissolving a natural polysaccharide or a derivative thereof in water. And having a R value which is a peak intensity ratio of 1360 cm ⁻¹ to 0.1 to 0.5 and a step of dispersing graphite having a ₀₀₂ plane spacing d ₀₀₂ of 0.338 nm or less. .

  Furthermore, the lithium secondary battery of the present invention has a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, and the negative electrode contains the negative electrode material for a lithium secondary battery of the present invention. Is.

  ADVANTAGE OF THE INVENTION According to this invention, the irreversible capacity | capacitance at the time of first time charging / discharging is small, and the charging / discharging cycling characteristics can provide the lithium secondary battery which can comprise the said lithium secondary battery, and its manufacturing method. .

Anode material for lithium secondary battery of the present invention (hereinafter, simply referred to as "anode material".) Is, R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum is 0. 1 to 0.5, and has a 002 plane spacing d ₀₀₂ of 0.338 nm or less, and a natural polysaccharide or a derivative thereof covering the surface.

  That is, in the negative electrode material of the present invention, a coating layer is formed on the surface of the graphite serving as a base material with a natural polysaccharide or a derivative thereof, whereby a battery using the negative electrode material (the lithium secondary battery of the present invention). Secondary battery), the irreversible capacity during the first charge / discharge can be reduced.

Further, in the negative electrode material of the present invention, the graphite as the base material has an R value and d ₀₀₂ satisfying the above values, and has an effect of improving load characteristics and charge characteristics at a low temperature of the lithium secondary battery. ing. Therefore, the lithium secondary battery using the negative electrode material of the present invention (the lithium secondary battery of the present invention) can ensure good load characteristics and low-temperature charging characteristics.

In addition, when the surface of graphite is coated with a coating layer made of an organic material as in the negative electrode material of the present invention, the load characteristics of a battery using this may be reduced due to resistance by the coating layer. Since graphite having a value and d ₀₀₂ satisfying the above values has the effect of enhancing the load characteristics of the battery as described above, the negative electrode material of the present invention constituted using such graphite can be used for the coating layer. The deterioration of the load characteristics of the battery due to the formation can be suppressed well.

In addition, by using graphite whose R value and d ₀₀₂ satisfy the above values as a base material, it is possible to further suppress the occurrence of peeling and cracking of the coating layer due to natural polysaccharides or derivatives thereof associated with charging / discharging of the battery. Thus, a battery having better charge / discharge cycle characteristics can be configured. By coating the surface of the graphite with a low crystalline carbon material, the R value and d ₀₀₂ satisfy the above values. In the case of such graphite, the low crystalline carbon existing on the surface of the graphite is satisfied. The material has a small degree of expansion and contraction that accompanies charging / discharging of the battery. For this reason, peeling or cracking of the coating layer due to natural polysaccharides or derivatives thereof during charging / discharging of the battery is difficult to occur. Therefore, in the lithium secondary battery using the negative electrode material of the present invention (lithium secondary battery of the present invention), even if charging / discharging is repeated, generation of new irreversible capacity associated therewith is suppressed, and natural polysaccharides or derivatives thereof Since the effect of the coating layer due to is sustained, the charge / discharge cycle characteristics are also improved.

In addition, the graphite whose R value and d ₀₀₂ satisfy the above values has a hard particle surface, and when coated as a sheet-like electrode, the electrode surface tends to be roughened. For example, in a battery, The separator in contact with the graphite-containing negative electrode is compressed and the distance between the positive electrode and the negative electrode is reduced even without short-circuiting, resulting in a decrease in capacity during the charge / discharge cycle, impairing reliability, There is a risk of voltage failure. However, since the negative electrode material of the present invention covers the surface of graphite with natural polysaccharides or derivatives thereof, the negative electrode material can be used even when R value and d ₀₀₂ satisfy the above values. Since the surface of the separator is soft, an effect of reducing reliability due to the compression of the separator and an effect of suppressing the occurrence of defective withstand voltage can be expected.

Graphite whose R value and d ₀₀₂ satisfy the above values are, for example, natural graphite or artificial graphite whose d ₀₀₂ is 0.338 nm or less formed into a spherical shape, and the surface is coated with an organic compound. And after baking at 800-1500 degreeC, it can grind | pulverize and it can obtain by sizing through a sieve. The organic compound covering the base material includes aromatic hydrocarbons; tars or pitches obtained by polycondensation of aromatic hydrocarbons under heat and pressure; tars mainly composed of a mixture of aromatic hydrocarbons. , Pitch or asphalt; In order to coat the base material with the organic compound, a method of impregnating and kneading the base material into the organic compound can be employed. Also, the R value and d ₀₀₂ satisfy the above values by a vapor phase method in which hydrocarbon gas such as propane or acetylene is carbonized by pyrolysis and deposited on the surface of graphite having d ₀₀₂ of 0.338 nm or less. Graphite can be produced.

In addition, it is preferable that the graphite whose R value and d ₀₀₂ satisfy the above values have a wettability to water of 0 to 50 ° in terms of a solid-liquid surface contact angle (θ). If the solid-liquid surface contact angle (θ) satisfies the above value, the graphite has good surface wettability and can further improve the adhesion with the coating layer of natural polysaccharide or its derivative. By improving the adhesion between the surface of the graphite and the coating layer, for example, peeling or cracking of the coating layer is less likely to occur during battery charge / discharge.

The wettability of graphite in which the R value and d ₀₀₂ satisfy the above values can be determined by measuring the contact angle of water by the sessile drop method in accordance with JIS R 3257-6. Specifically, for example, there is a method of measuring in an air atmosphere at 25 ° C. using “WET-1200” manufactured by ULVAC-RIKO.

In addition, various surface treatments may be applied to graphite whose R value and d ₀₀₂ satisfy the above values in order to adjust the wettability thereof to the above preferable value. In particular, when the treatment is performed using oxygen or fluorine, the effect of improving the wettability by increasing the polarity of the graphite surface can be expected. Specific graphite surface treatment methods include, for example, gas treatment heated to 500 to 1200 ° C. in a carbon dioxide or water vapor atmosphere; oxidation treatment in an acidic aqueous solution such as nitric acid; fluorine treatment treated with various fluorides And so on.

In the negative electrode material of the present invention, natural polysaccharides or derivatives thereof are used for covering graphite with R value and d ₀₀₂ satisfying the above values. In order to produce the negative electrode material of the present invention, for example, a method of dispersing the graphite in a solution (slurry or the like) in which the material for coating the graphite is dissolved in water, and then taking out the graphite and drying it, A method of spraying the solution onto the graphite and drying it may be employed. At this time, if the viscosity of the solution in which the material for coating the graphite is dissolved is low, the solution adhered to the graphite surface may drip before drying, and thus the graphite surface may not be coated well. . Therefore, it is desirable that the solution in which the material for coating the graphite is dissolved has a certain degree of viscosity. Therefore, if the amount of the material dissolved in the solution is increased, a thin coating layer is formed on the surface of the graphite. For example, the load characteristics of the battery may be reduced. However, a natural polysaccharide or a derivative thereof can be used to prepare a solution having a high viscosity with a relatively small amount of dissolution. Therefore, it is easy to form a thin coating layer on the surface of the graphite. Can be satisfactorily suppressed.

  Natural polysaccharides or derivatives thereof include, for example, natural polysaccharides such as xanthan gum, welan gum, gellan gum, guar gum, carrageenan, dextrin, pregelatinized starch, etc .; cellulose derivatives (carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, etc.) Derivatives of natural polysaccharides such as In the production of the negative electrode material of the present invention, water is preferably used as a solvent for the solution of the natural polysaccharide or derivative thereof used for coating the graphite from the viewpoint of environmental protection. Saccharides or their derivatives are water-soluble, and the viscosity of the solution can be made relatively high by simply dissolving a small amount in water. Especially, the said natural polysaccharide is especially preferable from the fact that the thickening effect to water is higher, and a highly viscous solution can be prepared with a smaller amount. In addition, as a natural polysaccharide or its derivative (s), the thing of the said illustration may be used individually by 1 type, and 2 or more types may be used together.

In the negative electrode material, the coating thickness of the natural polysaccharide or derivative on the graphite surface where the R value and d ₀₀₂ satisfy the above values is preferably 1 nm or more from the viewpoint of better ensuring the effect of the coating, More preferably, it is 3 nm or more. However, if the coating thickness of the natural polysaccharide or its derivative on the surface of the graphite is too large, the load characteristics are improved by using graphite in which the R value and d ₀₀₂ satisfy the above values in a battery using this. There is a possibility that the effect and the effect of suppressing the decrease in load characteristics due to the use of natural polysaccharides or derivatives thereof may be reduced. Therefore, the coating thickness of the natural polysaccharide or derivative on the graphite surface where the R value and d ₀₀₂ satisfy the above values is preferably 20 nm or less, and more preferably 10 nm or less.

For the negative electrode material of the present invention, for example, a solution (slurry or the like) in which natural polysaccharides or derivatives thereof are dissolved in water is prepared, and graphite in which the R value and d ₀₀₂ satisfy the above values is dispersed in this solution. It can manufacture by the method which has a process.

  The solution in which the natural polysaccharide or derivative thereof is dissolved in water preferably has a viscosity at 25 ° C. (viscosity measured using a vibration viscometer) of 5 to 50 mPa · s. The viscosity of the solution can be adjusted by adjusting the concentration of the natural polysaccharide or its derivative to be dissolved.

Moreover, it is preferable that the surface tension in 25 degreeC of the solution which dissolved the natural polysaccharide or its derivative in water is 4-40 mN / m. By using such a solution, the adhesion between the graphite whose R value and d ₀₀₂ satisfy the above values and the natural polysaccharide or derivative thereof covering the surface is improved, and the natural polysaccharide or The sustainability of the above-described effect by the derivative is further improved. The “surface tension of a natural polysaccharide or its derivative at 25 ° C.” here is a value measured by a Wilhelmy method using a commercially available device (for example, “CBVP-Z” manufactured by Kyowa Interface Science Co., Ltd.). It is.

  The surface tension of a solution obtained by dissolving a natural polysaccharide or a derivative thereof in water can be adjusted by using an additive such as a surfactant. Examples of the additive for adjusting the surface tension of the solution include organic solvents such as various alcohols and acetone, and commercially available surface conditioners such as modified silicone and modified polyether (for example, “SN wet series manufactured by San Nopco” ”,“ SN Clean Act Series ”,“ SN Deformer Series ”, etc.). In adjusting the surface tension of the solution, it is preferable to add the exemplified additives so as to have a concentration of about 0.01 to 5% by mass.

After the graphite having the R value and d ₀₀₂ satisfying the above values is dispersed in the solution, the negative electrode material of the present invention can be obtained by taking out the graphite from the solution and drying it.

In addition, the negative electrode material of the present invention is dried by spraying a method other than the above method, for example, a solution in which a natural polysaccharide or a derivative thereof is dissolved in water onto graphite whose R value and d ₀₀₂ satisfy the above values. It can also be produced by a method. In this case as well, a solution in which a natural polysaccharide or a derivative thereof is dissolved in water may have a viscosity or surface tension that is not within the above-mentioned preferred values. It is preferable to use it.

  The lithium secondary battery of the present invention only needs to have a negative electrode containing the negative electrode material of the present invention, and the other configurations and structures are the same as those of conventionally known lithium secondary batteries. Can be applied.

As the negative electrode, for example, a negative electrode mixture layer containing the negative electrode material of the present invention, a binder or the like formed on one side or both sides of a current collector can be used. In the negative electrode, graphite whose R value and d _{002 according} to the negative electrode material of the present invention satisfy the above values acts as the negative electrode active material.

  In addition, another negative electrode active material can also be used together with the negative electrode material of this invention for a negative electrode active material. Examples of the other negative electrode active materials include graphite whose surface is not coated with a polysaccharide or a derivative thereof, pyrolytic carbons, cokes, glassy carbons, a fired body of an organic polymer compound, mesocarbon micro Examples thereof include carbon-based materials capable of occluding and releasing lithium, such as beads (MCMB) and carbon fibers.

  However, when other negative electrode active materials are used in combination with the negative electrode material of the present invention, the negative electrode material of the present invention and other negative electrode active materials are used from the viewpoint of ensuring the effect of the use of the negative electrode material of the present invention. The amount of the negative electrode material of the present invention in the total amount is preferably 70% by mass or more, and more preferably 80% by mass or more.

  The negative electrode is, for example, a slurry-like or paste-like negative electrode mixture in which a negative electrode material of the present invention, other negative electrode active materials, a binder, and a negative electrode mixture containing a conductive auxiliary agent used as necessary are dispersed in a solvent. The agent-containing composition can be produced by applying the pressure-sensitive composition to one side or both sides of the current collector and drying, and then applying a press treatment as necessary to adjust the thickness of the negative electrode mixture layer. In addition, you may produce the negative electrode which concerns on this invention by methods other than the above.

  As the binder for the negative electrode, a fluorine resin such as polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), or the like can be used. Moreover, carbon materials, such as carbon black, etc. can be used for the conductive support agent of a negative electrode.

  The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per one surface of the current collector, for example. The composition of the negative electrode mixture layer is the amount of the negative electrode active material (in the case of using only the negative electrode material of the present invention, the amount thereof. When using another negative electrode active material in combination, the negative electrode material of the present invention And the other negative electrode active material) is preferably 60 to 95% by mass, and the amount of the binder is preferably 1 to 15% by mass. Moreover, when using a conductive support agent, it is preferable that the quantity of the conductive support agent in a negative mix layer is 0.5-5 mass%.

  As the current collector for the negative electrode, a foil made of copper or nickel, a punching metal, a net, an expanded metal, or the like can be used, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is preferably 5 μm.

  The lead portion on the negative electrode side is usually provided by leaving the exposed portion of the current collector without forming the negative electrode mixture layer on a part of the current collector and forming the lead portion at the time of preparing the negative electrode. However, the lead portion is not necessarily integrated with the current collector from the beginning, and may be provided by connecting a copper foil or the like to the current collector later.

The positive electrode according to the battery of the present invention is not particularly limited as long as it is a positive electrode used in a conventionally known lithium secondary battery, that is, a positive electrode containing an active material capable of occluding and releasing Li ions. For example, as the active material, Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, etc. Note that the element M is a metal element other than Li and is 10 atoms. A lithium-containing transition metal oxide having a layered structure represented by the following formula: LiMn ₂ O ₄ and a lithium manganese oxide having a spinel structure in which a part of the element is substituted with another element, LiMPO ₄ An olivine type compound represented by (M: Co, Ni, Mn, Fe, etc.) can be used. Specific examples of the lithium-containing transition metal oxide having a layered structure include LiCoO ₂ and LiNi _1-x Co _xy Al _y O ₂ (0.1 ≦ x ≦ 0.3, 0.01 ≦ y ≦ 0. 2) and other oxides containing at least Co, Ni and Mn (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiMn _5/12 Ni _5/12 Co _1/6 O ₂ , LiNi _{3 / 5} Mn _1/5 Co _1/5 O ₂ etc.). In particular, an active material containing 40% or more of Ni is preferable because the battery has a high capacity, and O (oxygen atom) may be substituted with 1 atom% of fluorine or sulfur atom.

  The positive electrode includes, for example, a positive electrode mixture layer made of a positive electrode mixture containing the positive electrode active material, a binder (a fluorine resin such as PVDF) and a conductive additive (a carbon material such as carbon black). It is possible to use one having a structure formed on one or both sides. Such a positive electrode is, for example, coated with a slurry-like or paste-like positive electrode mixture-containing composition in which the positive electrode mixture is dispersed in a solvent, dried on one or both sides of the current collector, and then dried as necessary. Can be manufactured through a step of adjusting the thickness of the positive electrode mixture layer by performing a press treatment. In addition, you may produce the positive electrode which concerns on this invention by methods other than the above.

  The thickness of the positive electrode mixture layer is preferably, for example, 10 to 100 μm per one side of the current collector. Moreover, as a composition of a positive mix layer, it is preferable that the quantity of a positive electrode active material is 60-95 mass%, it is preferable that the quantity of a binder is 1-15 mass%, and the quantity of a conductive support agent is 3. It is preferable that it is -20 mass%.

  As the current collector of the positive electrode, a metal foil such as aluminum, a punching metal, a net, an expanded metal, or the like can be used. Usually, an aluminum foil having a thickness of 10 to 30 μm is preferably used.

  The lead portion on the positive electrode side is normally provided by leaving the exposed portion of the current collector without forming the positive electrode mixture layer on a part of the current collector and forming the lead portion at the time of producing the positive electrode. However, the lead portion is not necessarily integrated with the current collector from the beginning, and may be provided by connecting an aluminum foil or the like to the current collector later.

  The negative electrode and the positive electrode can be used in the form of a laminated electrode body obtained by laminating them via a separator or a wound electrode body obtained by winding the laminated electrode body.

  There is no restriction | limiting in particular about a separator, What is used for the well-known lithium secondary battery is applicable. For example, a microporous polyethylene film having a thickness of 5 to 30 μm and a porosity of 30 to 70%, a microporous polypropylene film, a polyethylene polypropylene composite film, or the like is preferably used. In addition, for the purpose of ensuring higher safety at a high temperature, a heat-resistant separator in which a layer containing a heat-resistant inorganic filler or the like is formed on one side or both sides of the microporous polyethylene film can also be used.

As the nonaqueous electrolytic solution according to the battery of the present invention, a solution in which a lithium salt is dissolved in an organic solvent is used. The lithium salt is not particularly limited as long as it dissociates in a solvent to form Li ⁺ ions and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, inorganic lithium salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ ; LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group]; it can.

  The organic solvent used for the non-aqueous electrolyte is not particularly limited as long as it dissolves the lithium salt and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; Chain ethers such as ethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; nitriles such as acetonitrile, propionitrile and methoxypropionitrile; Sulfites such as ethylene glycol sulfite; and the like, and these may be used alone. , It may be used in combination of two or more thereof. In order to obtain a battery with better characteristics, it is desirable to use a combination that can obtain high conductivity, such as a mixed solvent of ethylene carbonate and chain carbonate. In addition, vinylene carbonates, 1,3-propane sultone, diphenyl disulfide, cyclohexylbenzene, biphenyl, and fluorobenzene are used for the purpose of improving safety, charge / discharge cycleability, and high-temperature storage properties of these non-aqueous electrolytes. , T-butylbenzene, anhydride, sulfurated ester, vinyl ethylene carbonate (VEC) and other additives can be added as appropriate.

  The concentration of the lithium salt in the organic electrolyte is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  Also, instead of the organic solvent, melting at room temperature such as ethyl-methylimidazolium trifluoromethylsulfonium imide, heptyl-trimethylammonium trifluoromethylsulfonium imide, pyridinium trifluoromethylsulfonium imide, guanidinium trifluoromethylsulfonium imide A salt can also be used.

  Furthermore, a polymer material that contains the non-aqueous electrolyte and gels may be added to make the organic electrolyte into a gel and used for the battery. Polymeric materials for gelling organic electrolytes include PVDF, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyacrylonitrile, polyethylene oxide, polypropylene oxide, ethylene oxide-propylene oxide copolymer And a known host polymer capable of forming a gel electrolyte, such as a crosslinked polymer having an ethylene oxide chain in the main chain or side chain, and a crosslinked poly (meth) acrylate.

  Examples of the form of the battery of the present invention include a cylindrical shape (such as a rectangular tube shape or a cylindrical shape) using a steel can or an aluminum can as an outer can. Moreover, it can also be set as the soft package battery which used the laminated film which vapor-deposited the metal as an exterior body.

  The lithium secondary battery of the present invention can be applied to the same use as a conventionally known lithium secondary battery.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

Example 1
<Production of negative electrode material>
For graphite having an average particle diameter D of _50% of 18 μm, d ₀₀₂ of 0.3361 nm, and a specific surface area of 3.2 m ² / g, the R value was determined by Raman spectroscopy. A target obtained by adhering a very small amount of graphite to frosted glass was used as a target. Each sample folder is placed in a laser Raman measuring device (“U-1000” manufactured by Jovan Yvon), irradiated with an argon ion laser (wavelength 514.5 nm, output 250 mW) focused on 1 μm, and Raman scattering is performed by backscattering. was condensed and integration time 1 sec, thereby integrating three times the range of the 1200～1800Cm ^-1 at a feed step ^{1 cm -1} to obtain a Raman spectrum. The R value of the graphite calculated from the obtained Raman spectrum was 0.18.

  The graphite was placed in an activation furnace and surface-treated at 1000 ° C. for 2 hours under a carbon dioxide gas stream. With respect to the graphite after the surface treatment, the wettability [solid-liquid surface contact angle (θ)] with respect to water was determined by the method described above.

  Xanthan gum which is a natural polysaccharide: 100 g is dissolved in 50 L of water to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After 5 kg of the graphite is added to this solution, the solution is taken out from the solution. It dried and produced the negative electrode material by which the surface of the said graphite was coat | covered with the xanthan gum. In this negative electrode material, the coating thickness of xanthan gum was measured with a transmission electron microscope (TEM).

<Production of negative electrode>
The negative electrode material: 90 parts by mass and PVDF as a binder: 10 parts by mass were mixed in N-methyl-2-pyrrolidone (NMP) in which 0.1 part by mass of oxalic acid was previously dissolved. A negative electrode mixture-containing slurry was prepared by dispersing. This slurry was intermittently applied to both sides of a current collector made of copper foil having a thickness of 10 μm, dried, and then subjected to a calendar treatment to adjust the thickness of the negative electrode mixture layer so that the total thickness became 142 μm. Then, this was cut | disconnected so that it might become 45 mm in width, and the negative electrode was obtained. Further, a tab was welded to the exposed portion of the copper foil of the negative electrode to form a lead portion.

<Preparation of positive electrode>
LiCoO _{2 as a} positive electrode active material: 70 parts by mass, LiNi _0.8 Co _0.2 O ₂ : 15 parts by mass, acetylene black as a conductive auxiliary agent: 10 parts by mass, and PVDF as a binder: 5 parts by mass N-methyl-2-pyrrolidone (NMP) was mixed uniformly as a solvent to prepare a positive electrode mixture-containing paste. This paste is intermittently applied to both sides of an aluminum foil having a thickness of 15 μm to be a current collector, dried, and then subjected to a calendar treatment to adjust the thickness of the positive electrode mixture layer so that the total thickness becomes 150 μm. The positive electrode was produced by cutting to a width of 43 mm. Further, a tab was welded to the exposed portion of the aluminum foil of the positive electrode to form a lead portion.

<Production of model cell>
A model cell was produced using the negative electrode. The model cell uses the negative electrode (the negative electrode mixture layer coated surface cut into 2 × 2 cm square) as the working electrode, Li metal foil as the counter electrode and reference electrode, ethylene carbonate and methyl ethyl carbonate as the non-aqueous electrolyte. Using a solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / L in a mixed solution obtained by mixing 1: 1 at a ratio (volume ratio), and using a polyethylene microporous film (thickness 12 μm) as a separator did.

<Charge / discharge test using model cell>
The model cell was charged with a constant current of 4 mA until a potential of 0.03 V with respect to the reference electrode, and further charged with a constant voltage of 0.03 V until the total charging time was 3 hours. Discharge the model cell after charging to a voltage of 2.5V at a constant current of 4 mA, measure the initial charge capacity and discharge capacity, and charge / discharge efficiency (100 x discharge capacity / charge capacity, unit%) ) Was calculated.

<Production of battery>
The negative electrode and the positive electrode were overlapped with the same separator as that used for the model cell, and wound in a spiral shape to obtain a wound electrode body. The wound electrode body was crushed into a flat shape and loaded into a rectangular outer can. Further, LiPF ₆ was dissolved at a concentration of 1.2 mol / l in a non-aqueous electrolyte (a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 2), and cyclohexylbenzene was added to 1 part by mass of 100 parts by mass of the solvent. 0.02 parts by mass of solution) was poured into the outer can and sealed to prepare a lithium secondary battery. This battery is provided with a cleavage vent for lowering the pressure when the internal pressure rises at the top of the can.

  The following load characteristic test and charge / discharge cycle characteristic test were performed on the lithium secondary battery.

<Load characteristic test>
The battery is charged at a constant current of 1 C and a constant voltage of 4.2 V (total charging time is restricted to 3 hours), and then discharged at a constant current of 0.2 C until the battery voltage reaches 2.75 V. A discharge capacity at 0.2 C was obtained. Next, the battery was charged under the same conditions as described above, and then discharged at a constant current of 2C until the battery voltage reached 2.75V to obtain a discharge capacity at 2C. The ratio of the discharge capacity at 2C to the discharge capacity at 0.2C was expressed as a percentage (%) to evaluate the load characteristics of the battery.

<Charge / discharge cycle characteristics test>
The battery was charged under the same conditions as in the load characteristic test, and then discharged at a constant current of 1 C until the battery voltage reached 2.75V. With this series of operations as one cycle, a 500-cycle charge / discharge test was performed. The ratio of the discharge capacity at the 500th cycle to the discharge capacity at the first cycle was expressed as a percentage (%) to evaluate the charge / discharge cycle characteristics of the battery.

Example 2
The same graphite used for producing the negative electrode material in Example 1 was put in an activation furnace, and surface treatment was performed at 1000 ° C. for 2.5 hours under a carbon dioxide gas stream. With respect to the graphite after the surface treatment, the wettability [solid-liquid surface contact angle (θ)] with respect to water was determined by the method described above.

  Xanthan gum, which is a natural polysaccharide: 70 g is dissolved in 50 L of water, and an appropriate amount of a hydrophobic silica aqueous tension adjuster is added to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After charging 5 kg of the graphite, it was taken out from the solution and dried to prepare a negative electrode material in which the surface of the graphite was coated with xanthan gum. The coating thickness of xanthan gum in this negative electrode material was measured by TEM.

  Except for using the negative electrode material, a model cell was prepared and charged / discharged in the same manner as in Example 1, and a lithium secondary battery was prepared in the same manner as in Example 1 to obtain load characteristics and charge / discharge. Cycle characteristics were evaluated.

Example 3
Xanthan gum, which is a natural polysaccharide: 200 g is dissolved in 50 L of water, and an appropriate amount of a hydrophobic silica aqueous tension adjuster is added to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After charging 5 kg of graphite subjected to the same surface treatment as that used in Example 1, it was taken out from the solution and dried to prepare a negative electrode material in which the surface of the graphite was coated with xanthan gum. The coating thickness of xanthan gum in this negative electrode material was measured by TEM.

  Except for using the negative electrode material, a model cell was prepared and charged / discharged in the same manner as in Example 1, and a lithium secondary battery was prepared in the same manner as in Example 1 to obtain load characteristics and charge / discharge. Cycle characteristics were evaluated.

Example 4
Graphite having an average particle size D of _50% of 20 μm, d ₀₀₂ of 0.3378 nm, and a specific surface area of 1.5 m ² / g (R value obtained in the same manner as in Example 1 is 0.45) depends on carbon dioxide. Without surface treatment, the wettability [solid-liquid surface contact angle (θ)] with respect to water was determined by the above method.

  Xanthan gum, which is a natural polysaccharide: 100 g is dissolved in 50 L of water, and an appropriate amount of a hydrophobic silica aqueous tension adjuster is added to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After charging 5 kg of the graphite, it was taken out from the solution and dried to prepare a negative electrode material in which the surface of the graphite was coated with xanthan gum. The coating thickness of xanthan gum in this negative electrode material was measured by TEM.

  Except for using the negative electrode material, a model cell was prepared and charged / discharged in the same manner as in Example 1, and a lithium secondary battery was prepared in the same manner as in Example 1 to obtain load characteristics and charge / discharge. Cycle characteristics were evaluated.

Comparative Example 1
A model cell was prepared and charged / discharged in the same manner as in Example 1 except that the same graphite as that used in Example 1 was used as a negative electrode material without performing surface treatment with carbon dioxide and coating with xanthan gum. While conducting the test, lithium secondary batteries were produced in the same manner as in Example 1, and the load characteristics and charge / discharge cycle characteristics were evaluated.

Comparative Example 2
16μm average particle diameter _{D 50%, d 002} is 0.336 nm, a specific surface area in the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the graphite (argon ion laser Raman spectrum is 3.4 m ² / g With respect to a certain R value of 0.05), the wettability [solid-liquid surface contact angle (θ)] with respect to water was determined by the above method without performing surface treatment with carbon dioxide gas.

  Xanthan gum, which is a natural polysaccharide: 100 g is dissolved in 50 L of water, and an appropriate amount of a hydrophobic silica aqueous tension adjuster is added to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After charging 5 kg of the graphite, it was taken out from the solution and dried to prepare a negative electrode material in which the surface of the graphite was coated with xanthan gum. The coating thickness of xanthan gum in this negative electrode material was measured by TEM.

  Except for using the negative electrode material, a model cell was prepared and charged / discharged in the same manner as in Example 1, and a lithium secondary battery was prepared in the same manner as in Example 1 to obtain load characteristics and charge / discharge. Cycle characteristics were evaluated.

  R value and wettability (solid-liquid surface contact angle) of graphite used for the production of negative electrode materials of Examples 1 to 4 and Comparative Examples 1 and 2, and characteristics of a solution in which xanthan gum was dissolved in water (xanthan gum aqueous solution) (25 Table 1 shows the xanthan gum coating thickness on the graphite surface, and Table 2 shows the evaluation results of the model cells and batteries of Examples 1 to 4 and Comparative Examples 1 and 2.

  As is clear from Table 2, the model cells using the negative electrode materials of Examples 1 to 4 have high initial charge / discharge efficiency, and the irreversible capacity at the first charge / discharge is reduced by using these negative electrode materials. In addition, the lithium secondary batteries of Examples 1 to 4 have a high capacity retention rate at the time of charge / discharge cycle characteristics evaluation, and the use of a specific negative electrode material maintains the effect of reducing the irreversible capacity even when charge / discharge is repeated. Therefore, high charge / discharge cycle characteristics can be secured. Furthermore, the lithium secondary batteries of Examples 1 to 4 have excellent load characteristics.

  On the other hand, in the model cell using the negative electrode material of Comparative Example 1 in which the surface of graphite is not coated with natural polysaccharide, the initial charge / discharge efficiency is low, and in the battery using this negative electrode material, the charge / discharge cycle is The characteristics are inferior. Furthermore, in the battery using the negative electrode material of Comparative Example 2 configured using graphite having an inappropriate R value, the charge / discharge cycle characteristics are inferior, the coating layer on the graphite surface is peeled off, and the effect is sufficient. It is thought that it is not sustained.

Claims (6)

An R value of 0.1 to 0.5 is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum, and the plane spacing _{d 002} of the 002 plane is less than 0.338nm A negative electrode material for a lithium secondary battery, comprising graphite and a natural polysaccharide or a derivative thereof covering the surface thereof.   2. The negative electrode material for a lithium secondary battery according to claim 1, wherein the natural polysaccharide covering the graphite surface or a derivative thereof has a thickness of 1 to 20 nm.   The negative electrode material for a lithium secondary battery according to claim 1 or 2, wherein the wettability of graphite with respect to water is 0 to 50 ° in terms of a solid-liquid surface contact angle (θ). A method for producing a negative electrode material for a lithium secondary battery according to any one of claims 1 to 3,
Natural polysaccharides or derivatives thereof in a solution prepared by dissolving in water, R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum is located at 0.1 to 0.5 And a method for producing a negative electrode material for a lithium secondary battery, comprising the step of dispersing graphite having a surface spacing d _{002 of} 002 planes of 0.338 nm or less.   The lithium secondary solution according to claim 4, wherein a solution obtained by dissolving a natural polysaccharide or a derivative thereof in water has a surface tension of 10 to 40 mN / m and a viscosity of 5 to 50 mPa · s at 25 ° C. A method for producing a negative electrode material for a battery. A lithium secondary battery having a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator,
The said negative electrode contains the negative electrode material for lithium secondary batteries in any one of Claims 1-3, The lithium secondary battery characterized by the above-mentioned.