JP5157222B2 - Electrode and electrochemical device - Google Patents

Description

  The present invention relates to an electrode and an electrochemical device.

As an electrode of an electrochemical device such as a lithium secondary battery, an electrode having an active material-containing layer provided on a current collector is known. Such an electrode is manufactured by applying a paste containing active material particles, a binder, a conductive additive and a solvent on a current collector, drying the solvent, and then pressing the coating film. One purpose of this press is to increase the volumetric energy density of the electrode.
JP 05-307957 A Japanese Patent Laid-Open No. 05-307958 Japanese Patent Laid-Open No. 07-37618 Japanese Patent Application Laid-Open No. 08-287952 JP 09-213372 A JP-A-10-92414 JP 2004-178819 A JP 2004-127913 A JP 2000-133267 A Japanese Patent Laid-Open No. 2004-210634 JP 2003-323896 A JP 2002-8655 A

  Incidentally, in recent years, further improvement in volume energy and reduction in charging time have been demanded. In order to increase the density of the active material of the electrode, a method of using particles having a high degree of graphitization such as natural graphite and pressing with a roll press at a sufficiently high pressure can be considered. However, since natural graphite is soft, pores are likely to be clogged, and the density of the active material of the electrode is increased by such a method.

  On the other hand, as described in Patent Document 12, it is conceivable to increase the filling rate by mixing small particles and large particles of graphite. However, in this case, there is a problem that cycle characteristics are deteriorated and rate characteristics cannot be sufficiently improved due to an increase in the surface area of graphite.

  The present invention has been made in view of the above problems, and an object thereof is to provide an electrode capable of rapid charging and having excellent cycle characteristics while sufficiently increasing the density of the electrode, and an electrochemical device using the same. .

The electrode according to the present invention includes a current collector and an active material-containing layer provided on the current collector. The active material-containing layer includes first carbon particles and second carbon particles having an average particle size smaller than that of the first carbon particles. Moreover, the average particle diameter of the first carbon particles is 7 to 25 μm, and the average particle diameter of the second carbon particles is 0.08 to 0.3 μm. Furthermore, when the ratio (P101 / P100) between the peak intensity P101 of the (101) plane and the peak intensity P100 of the (100) plane in the X-ray diffraction pattern of each carbon particle is defined as the graphitization degree of each carbon particle, The graphitization degree of one carbon particle is 1.5 to 3.1, and the graphitization degree of the second carbon particle is 0.5 to 0.8.

The packing density of the active material of the electrode can be increased by mixing carbon particles having different particle diameters. When the average particle diameter of the first carbon particles is smaller than 7 μm, the electrolytic solution tends to be decomposed on the surface of the first carbon particles. When the average particle diameter of the first carbon particles is larger than 25 μm, the voids of the electrodes become large and the electrode density tends to decrease. Moreover, when the average particle diameter of the second carbon particles is smaller than 0.08 μm, the electrolytic solution tends to be decomposed on the surface of the second carbon particles. When the average particle diameter of the second carbon particles is larger than 0.3 μm, the gap between the electrodes becomes large and the electrode density tends to decrease. In addition, the particles having a low graphitization degree are hard, or even when pressed, fine pores suitable for the penetration of the electrolytic solution are formed between the second carbon particles, so that the electrolytic solution is likely to penetrate. Furthermore, when a carbon material having a small average particle diameter is used, the surface area becomes large and decomposition of the electrolytic solution becomes remarkable, but since particles having a low graphitization degree are used as carbon particles having a small particle diameter, It is difficult to cause decomposition. Further, since the graphitization degree of the second carbon particles is not 0, the second carbon particles can also perform sufficient occlusion / release of electrolyte ions. In particular, by using two types of carbon particles having a graphitization degree in the above range, the density of the electrode can be reduced without reducing the lithium ion insertion / desorption reaction and without causing the decomposition reaction of the electrolytic solution. It is possible to obtain an electrode that can be rapidly charged and has excellent cycle characteristics while sufficiently increasing the resistance.

  Here, the degree of graphitization is defined by the ratio (P101 / P100) of the peak intensity P101 of the (101) plane and the peak intensity P100 of the (100) plane in the X-ray diffraction pattern, and this (P101 / P100 ) Means that the degree of graphitization is higher.

  Further, when the average particle diameter of the first carbon particles is 1, it is preferable that the average particle diameter of the second carbon particles is 0.002 to 0.05. When the average particle diameter of the second carbon particles is smaller than 0.002, the surface area of the second carbon particles increases, so that the decomposition reaction of the electrolytic solution tends to occur. Moreover, when larger than 0.05, the space | gap of an electrode becomes large and there exists a tendency for an electrode density to fall.

  Moreover, it is preferable that a 1st carbon particle contains scaly graphite. By using scaly graphite, the charge / discharge efficiency of the first carbon particles can be improved.

  The shape of the second carbon particles is preferably spherical. By making the shape of the second carbon particles spherical, the second carbon particles can easily enter the gaps between the first carbon particles, so that the electrode density can be improved.

  Moreover, it is preferable that 5-200 weight part of 2nd carbon particles are included when a 1st particle is 100 weight part. When the amount of the second carbon particles is less than 5 parts by weight, the filling of the voids formed by the first carbon particles becomes insufficient, so that the voids of the electrodes become large and the electrode density tends to decrease. When the amount of the second carbon particles is more than 200 parts by weight, there is a tendency that the electrolytic solution is easily decomposed because the factor of the electrolytic solution decomposition increases.

  Moreover, the electrochemical device which concerns on this invention is an electrochemical device provided with the above-mentioned electrode.

  According to the present invention, it is possible to provide an electrode capable of rapid charging and having excellent cycle characteristics while sufficiently increasing the density of the electrode and an electrochemical device using the electrode.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratio in each drawing does not necessarily match the actual dimensional ratio.

(electrode)
First, the electrode according to the present embodiment will be described with reference to FIG. The electrode 10 is a product in which an active material-containing layer 14 is provided on a current collector 12.

  As the current collector 12, for example, a copper foil, a nickel foil, or the like can be used.

  The active material-containing layer 14 is a layer containing active material particles, a binder (not shown), and a conductive additive (not shown) blended as necessary.

  In the present embodiment, the active material particles include the first carbon particles 5 and the second carbon particles 6 having an average particle size smaller than that of the first carbon particles 5. The first carbon particles 5 have a graphitization degree of 1.5 to 3.1, and the second carbon particles 6 have a graphitization degree of 0.5 to 0.8.

  Here, the degree of graphitization is defined by the ratio (P101 / P100) between the peak intensity P101 of the (101) plane and the peak intensity P100 of the (100) plane in the X-ray diffraction pattern of each carbon particle. The larger this (P101 / P100), the higher the degree of graphitization.

  An average particle diameter can be defined as D50 which is a 50% diameter in a volume reference | standard particle size distribution, for example. The particle size distribution can be easily obtained by a laser diffraction scattering method or the like.

  Specific examples of the first carbon particles 5 include artificial graphite particles and natural graphite particles. In particular, natural graphite particles having a scaly structure are preferred. The average particle diameter of the first carbon particles is preferably 7 to 25 μm.

As the second carbon particles 6, it is preferable to use carbon powder called non-graphitized carbon powder. The average particle diameter (D50) of the second carbon particles 6 is preferably 0.08 to 0.3 μm. In particular, the second carbon particles 6 have an average particle diameter D50 of 0.3 μm or less, Lc <30 nm, La <50 nm, 3.365 <d ₀₀₂ <3.395, and a BET area of 20 m ² / g or less. preferable. Such carbon powder can be obtained, for example, by thermally decomposing hydrocarbon gas such as toluene together with hydrogen gas.

  The outer shape of the second carbon particles 6 is preferably substantially spherical. Moreover, when the average particle diameter of the first carbon particles 5 is 1, the average particle diameter of the second carbon particles 6 is preferably 0.002 to 0.05. Furthermore, when the first carbon particles 5 are 100 parts by weight, the second carbon particles 6 are preferably included in an amount of 5 to 200 parts by weight.

  As shown in FIG. 1, in the active material containing layer 14, the second carbon particles 6 surround the first carbon particles 5. In the portion filled with the second carbon particles 6, the second carbon particles are not so dense, and fine pores are formed.

  The binder is not particularly limited as long as it can bind the active material particles and the conductive additive to the current collector, and a known binder can be used. For example, fluororesin such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR) and water-soluble polymers (carboxymethylcellulose, polyvinyl alcohol, sodium polyacrylate, dextrin, gluten, etc. ) And the like.

Examples of the conductive assistant include carbon blacks, fine metal powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and fine metal powders, and conductive oxides such as ITO. The conductive assistant is a material added to increase the electronic conductivity of the active material-containing layer 14. As the conductive assistant, acetylene black or carbon black can be suitably used. These have the appearance that carbon aggregates called aggregates or structures are connected in a beaded manner, and have a large specific surface area of 30 m ² / g or more. Since these acetylene blacks or carbon blacks have almost zero graphitization degree, they are clearly distinguished from the second carbon particles referred to in the present invention. Another difference is that acetylene black or carbon black has substantially no charge / discharge characteristics. Therefore, in the present invention, it is difficult to use ordinary carbon black or acetylene black as the second carbon particles.

  Although the thickness of the active material content layer 10 is not specifically limited, It is preferable that it is 30-100 micrometers.

(Method for manufacturing electrode)
Such an electrode can be manufactured as follows. First, a slurry in which an active material particle mixture, a binder, and a necessary amount of a conductive additive are added to a solvent such as N-methyl-2-pyrrolidone or N, N-dimethylformamide is applied to the surface of the current collector 12. And dry. Here, the above-mentioned first carbon particles 5 and second carbon particles 6 are supplied as the active material particle mixture. After drying, the electrode on which the active material-containing layer is formed is pressed with a press such as a roll press. The linear pressure during pressing can be set to 100 to 1000 kgf / cm, for example.
(Action / Effect)

  According to the present embodiment, the active material-containing layer 14 includes the first carbon particles 5 and the second carbon particles 6 having an average particle size smaller than that of the first carbon particles 5 and having a lower graphitization degree. By mixing carbon particles having different particle diameters, the packing density of the active material in the active material-containing layer 14 of the electrode can be increased. In addition, the particles having a low graphitization degree are hard, or even if pressed, fine pores suitable for the penetration of the electrolyte solution are formed between the second carbon particles 6, so that the electrolyte solution easily penetrates. Furthermore, when the second carbon particles having a small average particle diameter are used, the surface area becomes large and the decomposition of the electrolytic solution becomes remarkable. However, since the second carbon particles having a small particle diameter use particles having a low degree of graphitization. It is difficult for the electrolyte to decompose.

(Electrochemical device)
Then, an example of the electrochemical device concerning this invention is demonstrated. FIG. 2 is an example of a lithium ion secondary battery.

  The lithium ion secondary battery 100 mainly includes a laminate 30, a case 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30.

  The laminated body 30 is configured such that a pair of electrodes 10 and 20 are arranged to face each other with the separator 18 interposed therebetween. Each active material-containing layer 14 is in contact with both sides of the separator 18. Leads 60 and 62 are connected to the ends of the current collector 12, and the ends of the leads 60 and 62 extend to the outside of the case 50. One electrode 10 becomes a negative electrode and the other electrode 20 becomes a positive electrode. The negative electrode 10 is the electrode described above.

  The positive electrode 20 is a product in which an active material containing layer 24 is provided on a current collector 22. The active material-containing layer 24 is a layer containing active material particles, a binder (not shown), and a conductive additive (not shown) blended as necessary.

Examples of the positive electrode active material particles include LiMO ₂ (M represents Co, Ni, or Mn), LiCo _x Ni _1-x O ₂ , LiMn ₂ O ₄ , LiCo _x Ni _y Mn _1-xy O _2. Lithium oxide containing at least one metal selected from the group consisting of Co, Ni and Mn, such as (where x and y are greater than 0 and less than 1), such as LiCo _x Ni _y Mn _1-xy O ₂ is more preferred.

Although the particle size Ra of these active material particles is not particularly limited, for example, it can usually be 0.05 to 20 μm.
As the binder and the conductive additive other than the active material particles, the same materials as those of the negative electrode 10 can be used.

The electrolyte solution is contained in each of the active material-containing layers 14 and 24 and the separator 18. The electrolyte solution is not particularly limited. For example, in the present embodiment, an electrolyte solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, and the withstand voltage during charging is limited to a low level. As the electrolyte solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. Examples of the lithium salt include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ , CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , A salt such as LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB or the like can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Moreover, as an organic solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, etc. are mentioned preferably, for example. These may be used alone or in combination of two or more at any ratio.

  In the present embodiment, the electrolyte solution may be a gel electrolyte obtained by adding a gelling agent in addition to liquid. Further, instead of the electrolyte solution, a solid electrolyte (a solid polymer electrolyte or an electrolyte made of an ion conductive inorganic material) may be contained.

  The separator 18 may be formed of an electrically insulating porous material, for example, a single layer of a film made of polyethylene, polypropylene, or polyolefin, a stretched film of a laminate or a mixture of the above resins, or cellulose, Examples thereof include a fiber nonwoven fabric made of at least one constituent material selected from the group consisting of polyester and polypropylene.

  The case 50 seals the laminated body 30 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside and entry of moisture and the like into the electrochemical device 100 from the outside. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the synthetic resin film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is preferably polyethylene or polypropylene.

  The leads 60 and 62 are made of a conductive material such as aluminum.

  The electrode according to the present invention is not limited to a lithium ion secondary battery, and can be applied as an electrode of an electrochemical capacitor, for example. For example, it can be suitably used as an electrode of an electric double layer capacitor using carbon particles.

Example 1
Amorphous carbon coated natural graphite (average particle size 20 μm, (P101 / P100) = 2.66, 45 parts by weight) as first carbon particles, ungraphitized spherical carbon powder (average particle size D50 = 0.2 μm, (P101 / P100) = 0.63, 45 parts by weight), PVDF as a binder (8 parts by weight), acetylene black (2 parts by weight) as a conductive auxiliary agent, and N− as a solvent Methyl-2-pyrrolidone (NMP) was mixed with a biaxial kneader. Here, the amount of the solvent was adjusted so that a torque of 7 kgm was applied at 60 rpm / min, and the mixture was kneaded for 1 hour. Thereafter, a solvent was further added to adjust the viscosity to form a slurry, which was applied onto a copper foil (thickness: 10 μm) as a negative electrode current collector by a doctor blade method and dried to form an active material-containing layer. The coating amount was adjusted so that the electrode thickness after drying was 37 μm. Then, the positive electrode was manufactured by pressing the negative electrode with a hot roll.

92 parts by weight of LiMn _0.33 Ni _0.33 Co _0.34 O ₂ (wherein the numbers are atomic ratios), 2 parts by weight of acetylene black as a conductive aid, and polyvinylidene fluoride as a binder 3 parts by weight of (PVdF) was mixed into NMP by a planetary mixer to obtain a slurry-like coating solution. The obtained coating solution was applied onto an aluminum foil (15 μm) as a current collector by a doctor blade method and dried. Thereafter, it was roll-pressed so that the amount of the active material supported was 3.2 g / cm ³ .

The separator is a polyethylene porous sheet, and the electrolyte is propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) in a volume ratio of 2 to 1: 7, and 1.5 mol of LiPF ₆ A lithium ion secondary battery as shown in FIG. 2 was prepared using a nonaqueous electrolyte solution soluted at a ratio of dm ⁻³ and an aluminum laminate film as a case.

(Example 2)
A battery was produced in the same manner as in Example 1 except that the first carbon particles were 60 parts by weight and the second carbon particles were 30 parts by weight.

(Example 3)
An electrode was produced in the same manner as in Example 1 except that 70 parts by weight of the first carbon particles and 20 parts by weight of the second carbon particles were used.

(Comparative Example 1)
An electrode was produced in the same manner as in Example 1 except that the first carbon particle was 90 parts by weight and the second carbon particle was not used.

(Comparative Example 2)
An electrode was produced in the same manner as in Example 1 except that 90 parts by weight of the second carbon particles were used without using the first carbon particles.
(Comparative Examples 3-5)
The electrodes were the same as in Examples 1 to 3, except that the average particle diameter of the first carbon particles was 40 μm, the average particle diameter of the second carbon particles was 0.5 μm, and the degree of graphitization (P101 / P100) was 1.5. Manufactured.

[Measurement of characteristics]
The electrode density was calculated from the electrode thickness and electrode weight measured with a micrometer. Note that the electrode density includes a conductive additive and a binder. The charging time was determined as the time required to charge up to 4.2 V by 30 C constant current constant voltage charging (CCCV) and to charge up to 80% of the rated capacity. The cycle characteristics were measured by repeating 10 C CCCV charge and 10 C CC discharge 500 times and setting the cut-off voltage to 4.2 V-2.5 V. The results are shown in FIG.

  In the example, the high density of the electrode, the shortening of the charging time, and the improvement of the cycle characteristics were realized in a balanced manner.

  FIG. 4 shows an SEM photograph of the negative electrode of Example 1, and FIG. 5 shows an enlarged SEM photograph of the active material-containing layer of FIG.

FIG. 1 is a schematic cross-sectional view of an electrode according to this embodiment. FIG. 2 is a schematic cross-sectional view of the lithium ion secondary battery according to the present embodiment. FIG. 3 is a table showing conditions and results of Examples and Comparative Examples. FIG. 4 is a SEM photograph of the negative electrode of Example 1. FIG. 5 is an enlarged SEM photograph of the active material-containing layer of FIG.

Explanation of symbols

  5 ... 1st carbon particle, 6 ... 2nd carbon particle, 10 ... Electrode (negative electrode), 14 ... Active material content layer, 100 ... Lithium ion secondary battery.

Claims (6)

A current collector,
An active material-containing layer provided on the current collector,
The active material-containing layer includes first carbon particles and second carbon particles having an average particle size smaller than that of the first carbon particles,
The average particle diameter of the first carbon particles is 7 to 25 μm,
The second carbon particles have an average particle size of 0.08 to 0.3 μm;
When the ratio (P101 / P100) between the peak intensity P101 of the (101) plane and the peak intensity P100 of the (100) plane in the X-ray diffraction pattern of each carbon particle is the degree of graphitization of each carbon particle,
The degree of graphitization of the first carbon particles is 1.5 to 3.1,
An electrode in which the second carbon particles have a graphitization degree of 0.5 to 0.8.   2. The electrode according to claim 1, wherein when the average particle diameter of the first carbon particles is 1, the average particle diameter of the second carbon particles is 0.002 to 0.05. The electrode according to claim 1 or 2, wherein the first carbon particles include scaly graphite . The electrode according to claim 1 , wherein the second carbon particles have a spherical shape. The electrode according to any one of claims 1 to 4, comprising 5 to 200 parts by weight of the second carbon particles when the first carbon particles are 100 parts by weight. An electrochemical device comprising the electrode according to claim 1 .