JP2013016353A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery whose service life can be extended and in which an increase in electric resistance at an anode can be suppressed.SOLUTION: A battery 1 (nonaqueous electrolyte secondary battery) includes an anode 4 containing: an anode active material made of graphite whose surface is coated with amorphous carbon and which has an average particle diameter of 3 to 10 μm inclusive; and carbon having an average particle diameter smaller than that of the graphite.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery including a negative electrode including a negative electrode active material made of graphite.

  In nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries, graphite is used as the negative electrode active material. However, when graphite is used as the negative electrode active material, the graphite easily reacts with the non-aqueous electrolyte, so that the life of the non-aqueous electrolyte secondary battery is shortened. Therefore, conventionally, it has been proposed to coat the surface of graphite particles with amorphous carbon, which is less reactive with non-aqueous electrolyte than graphite (see, for example, Patent Document 1).

  Patent Document 1 proposes a lithium ion battery using a negative electrode active material made of graphite having a surface coated with amorphous carbon such as non-graphitizable carbon or easily graphitizable carbon. In this proposed lithium ion battery, a negative electrode active material made of graphite whose surface is coated with amorphous carbon having a substantially uniform thickness, a binder made of polyvinylidene fluoride, and a solvent made of N-methylpyrrolidone. The negative electrode plate is formed by applying the mixture mixed in the form of a slurry to a copper current collector. Note that the graphite particles as the negative electrode active material contained in the negative electrode plate have an average particle diameter of 6 μm or more and 50 μm or less.

JP-A-11-329436

  Here, as a result of various studies, the inventor of the present application has determined that the average particle diameter of the graphite particles disclosed in Patent Document 1 above is negative electrode using graphite (negative electrode active material) whose surface is coated with amorphous carbon. (6 μm or more and 50 μm or less), when the average particle size of the graphite particles of the negative electrode active material is large, even if the surface of the graphite particles is coated with amorphous carbon having low reactivity with the nonaqueous electrolyte, It has been found that there is a problem that the life of the water electrolyte secondary battery is shortened. On the other hand, the present inventor has also found that there is a problem in that the electrical resistance at the negative electrode increases due to charge and discharge when the average particle size of the graphite particles of the negative electrode active material is small.

  The present invention has been made to solve the above-described problems, and one object of the present invention is to extend the life of the non-aqueous electrolyte secondary battery and to provide an electric A non-aqueous electrolyte secondary battery capable of suppressing an increase in resistance is provided.

Means for Solving the Problems and Effects of the Invention

  As a result of extensive studies by the inventor of the present application in order to achieve the above object, the following configurations have been found in order to solve the above problems. That is, the nonaqueous electrolyte secondary battery according to one aspect of the present invention includes a negative electrode active material comprising graphite having a surface coated with amorphous carbon and an average particle size of 3 μm or more and 10 μm or less, and an average particle size of graphite. And a negative electrode including carbon having an average particle diameter equal to or smaller than the diameter.

  In the non-aqueous electrolyte secondary battery according to one aspect of the present invention, as described above, the surface of the non-aqueous electrolyte secondary battery is coated with amorphous carbon and includes a negative electrode active material made of graphite having an average particle size of 3 μm or more and 10 μm or less. By configuring the negative electrode, an increase in irreversible capacity due to charging / discharging can be suppressed, so that the life of the lithium ion battery can be extended.

  In the nonaqueous electrolyte secondary battery according to one aspect of the present invention, the negative electrode has a negative electrode active material composed of graphite whose surface is coated with amorphous carbon, and an average particle size equal to or less than the average particle size of graphite. By containing carbon, fine carbon having an average particle diameter equal to or smaller than the average particle diameter of graphite can be effectively introduced between the graphite particles. Thereby, since it can suppress that the electrical resistance between the particles of graphite increases, it can suppress that the electrical resistance in a negative electrode increases.

  In the non-aqueous electrolyte secondary battery according to the above aspect, the carbon is preferably acetylene black having an average particle size equal to or less than the average particle size of graphite. If comprised in this way, the fine acetylene black which has an average particle diameter below the average particle diameter of graphite can be easily entrapped between the graphite particles. Thereby, the electrical contact between the graphite particles can be reliably maintained.

  In the non-aqueous electrolyte secondary battery in which the carbon is acetylene black having an average particle size equal to or less than the average particle size of graphite, the negative electrode preferably includes carboxymethyl cellulose. If comprised in this way, a fine acetylene black will be disperse | distributed uniformly with carboxymethylcellulose, and acetylene black can be effectively entrapped between graphite particles.

It is sectional drawing which showed the whole structure of the battery by one Embodiment of this invention.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below with reference to the drawings.

  First, with reference to FIG. 1, the structure of the battery 1 by one Embodiment of this invention is demonstrated. The battery 1 is an example of the “nonaqueous electrolyte secondary battery” in the present invention.

  A battery 1 according to an embodiment of the present invention includes a power generation element 2 as shown in FIG. The power generation element 2 is formed by winding a positive electrode 3, a negative electrode 4, and a separator 5. Moreover, in the positive electrode 3, the positive electrode mixture containing a positive electrode active material is apply | coated to both surfaces of the positive electrode electrical power collector which consists of aluminum foil or aluminum alloy foil. Moreover, in the negative electrode 4, the negative mix containing a negative electrode active material is apply | coated to both surfaces of the negative electrode collector which consists of copper foils.

  In addition, the power generation element 2 is housed inside the battery case 6, and the battery cover 7 is connected to the battery so as to close the opening of the battery case 6 in a state where the power generation element 2 is housed inside the battery case 6. The case 6 is attached by laser welding. Further, the positive electrode 3 of the power generation element 2 is connected to the battery lid 7, and the negative electrode 4 of the power generation element 2 is connected to the negative electrode terminal 8 of the battery lid 7. The battery case 6 has a liquid injection port (not shown). The battery 1 is formed by sealing the liquid injection port after injecting a non-aqueous electrolyte into the battery case 6 through the liquid injection port.

  Here, in this embodiment, the negative electrode mixture of the negative electrode 4 contains a negative electrode active material, a negative electrode plate conductive additive, and a thickener. The negative electrode active material is made of graphite particles whose surface is coated with amorphous carbon. The graphite particles are made of at least one of natural graphite and artificial graphite, and are adjusted so that the average particle diameter is about 3 μm or more and about 10 μm or less. The average particle size of the graphite particles is preferably about 3 μm or more and about 5 μm or less. Furthermore, the average particle size of the graphite particles is more preferably about 5 μm.

  Amorphous carbon that coats the graphite particles is non-graphitizable carbon (HC) in which carbon crystallites are hardly arranged in an ordered arrangement, or graphitizable characteristics in which carbon crystallites are arranged in a somewhat ordered arrangement. It consists of carbon (SC). Non-graphitizable carbon is amorphous carbon that cannot be converted to graphite even when heated to an ultra-high temperature of around 3300K under normal pressure or reduced pressure. Easy-graphite carbon is converted to graphite by high-temperature treatment at around 3300K. Amorphous carbon that can be converted. As the amorphous carbon coating the graphite particles, amorphous carbon having d002 of 3.45 mm (0.345 nm) or more is preferable, and further, non-graphitizable carbon having d002 of 3.70 mm (0.370 nm) or more. Is more preferable. As the raw material for amorphous carbon, phenol resin, furan resin, furfuryl alcohol, and the like can be used.

  Here, the thickness of the amorphous carbon covering the graphite particles is very small with respect to the average particle diameter of the graphite particles. For this reason, the average particle diameter of the graphite particles after the surface is coated with amorphous carbon is almost the same as the average particle diameter of the graphite particles before the surface is coated with amorphous carbon.

  Moreover, in this embodiment, the negative electrode plate conductive support agent contained in the negative electrode mixture is made of carbon having an average particle size equal to or less than the average particle size (about 3 μm or more and about 10 μm or less) of the graphite particles of the negative electrode active material. The carbon of the negative electrode plate conductive assistant has conductivity and has a function of improving the electronic conductivity of the negative electrode 4. The negative electrode conductive additive preferably has an average particle size of about 1/3 or less of the average particle size of the graphite particles of the negative electrode active material.

  Specifically, as the carbon of the negative electrode conductive additive, acetylene black (AB) having an average particle diameter of about 1.5 μm to about 2.0 μm, or about 1.2 μm to about 3.5 μm. It is possible to use graphite having an average particle diameter. When graphite having an average particle size of about 3.0 μm or more and about 3.5 μm or less is used as the negative electrode plate conductive additive, the average particle size of the negative electrode plate conductive additive is the average of the graphite particles of the negative electrode active material. It must be smaller than the particle size.

  Further, the thickener contained in the negative electrode mixture has a function of uniformly dispersing the negative electrode active material, the negative electrode plate conductive aid, and the like when the negative electrode 4 is manufactured. As this thickener, carboxymethyl cellulose (CMC), polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, polyacrylonitrile, or the like can be used alone or in combination. Here, when acetylene black (AB) is used as the negative electrode conductive additive, CMC is preferably used as the thickener.

  Moreover, you may make a negative electrode mixture contain a negative electrode binder etc. as needed. As the negative electrode binder, particles of styrene-butadiene copolymer rubber or the like can be used.

Further, the positive electrode mixture of the positive electrode 3 contains a positive electrode active material. The positive electrode active material is not particularly limited, and various positive electrode active materials can be used. For example, a composite oxide of lithium and a transition metal can be used as the positive electrode active material. Specifically, the general formula Li _x M1 _p O _(2-δ) (wherein M1 is at least one metal composed of Co, Ni, or Mn, and x is 0.4 ≦ x ≦ 1.2). In addition, p is 0.8 ≦ p ≦ 1.2 and δ is 0 ≦ δ ≦ 0.5, or a general formula Li _x M2 _q O _(4-δ) (wherein M2 Is at least one metal made of Co, Ni or Mn, x is 0.4 ≦ x ≦ 1.2, q is 1.5 ≦ q ≦ 2.2, and δ is 0 ≦ 0. It is possible to use a positive electrode active material represented by (δ ≦ 0.5). It is also possible to use a compound containing at least one element of the group consisting of Al, Fe, Cr, Ti, Zn, P and B for these composite oxides.

Furthermore, the general formula Li _x M3 _u PO ₄ (wherein M3 is a 3d transition metal having a vacant orbit in the 3d orbital. X is 0 ≦ x ≦ 2, and u is 0.8 ≦ u ≦ It is possible to use a phosphoric acid compound having an olivine structure represented by 1.2) as the positive electrode active material. In order to ensure conductivity, it is possible to use a phosphoric acid compound coated with amorphous carbon. Moreover, you may comprise so that two or more types of above-mentioned complex oxide and phosphoric acid compound may be contained as a positive electrode active material.

  Moreover, you may make a positive electrode mixture contain a positive electrode plate conductive support agent, a binder, a viscosity modifier, etc. as needed. As the positive electrode plate conduction aid, acetylene black (AB), carbon black, graphite, or the like can be used. Further, as the binder, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene copolymer rubber, polyacrylonitrile, or the like can be used alone or in combination. As the viscosity modifier, N-methylpyrrolidone (NMP) or the like can be used.

  As the separator 5 of the power generation element 2, a nonwoven fabric, a synthetic resin microporous film, or the like can be used. In addition, it is preferable to use a synthetic resin microporous film from the viewpoint of ease of processing and improvement of durability. In particular, it is more preferable to use a polyolefin microporous film made of polyethylene or polypropylene. Further, the heat resistance of the separator 5 may be improved by forming an aramid layer or the like on the surface of the polyolefin microporous film.

Moreover, what dissolved electrolyte salt in the nonaqueous solvent is used as a nonaqueous electrolyte arrange | positioned inside the battery case 6 with the electric power generation element 2. FIG. Examples of the electrolyte salt of the nonaqueous electrolyte include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ CO ₂ , LiC (CF ₃ ) ₃ , LiC (C ₂ F ₅ ) ₃ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiCF ₃ CF ₂ CF ₂ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ , LiN (COCF ₂ CF ₃ ) ₂ and LiPF ₃ (CF ₂ CF ₃ ) ₃ or the like can be used. Further, the above electrolyte salts may be used alone or in combination of two or more. From the viewpoint of improving conductivity, it is preferable to use LiPF ₆ as the electrolyte salt. Further, as the electrolyte salt, a mixture containing LiPF ₆ as a main component and the above electrolyte salt other than LiPF ₆ such as LiBF ₄ may be used.

  Nonaqueous electrolytes include non-aqueous solvents such as ethylene carbonate (EC), propylene carbonate, butylene carbonate, trifluoropropylene carbonate, γ-butyrolactone, sulfolane, 1,2-dimethoxyethane, tetrahydrofuran, methyl acetate, ethyl acetate, propylene acid. Methyl, ethyl propylene acid, dimethyl sulfoxide, dimethyl carbonate (DMC), diethyl carbonate, ethyl methyl carbonate (EMC), dipropyl carbonate, methylpropyl carbonate, dibutyl carbonate, and the like can be used. The nonaqueous solvent is preferably used by mixing a plurality of nonaqueous solvents described above from the viewpoint of adjusting the conductivity and viscosity of the nonaqueous electrolyte.

[Example]
Next, an evaluation test of the battery 1 performed for confirming the effect of the present invention will be described. Specifically, as an example corresponding to the above embodiment, a battery 1 according to the following Examples 1 to 5 is manufactured, and as a comparative example, a battery according to the following Comparative Examples 1 to 4 is manufactured, and an evaluation test is performed. Went.

Example 1
(Preparation of positive electrode)
First, a positive electrode mixture was prepared. Specifically, a positive electrode active material made of Li _1.1 Mn _1.8 Al _0.1 O ₄ , a positive electrode plate conductive assistant made of acetylene black (AB), and a binder made of polyvinylidene fluoride. Mixed. At this time, the ratio of the positive electrode active material (Li _1.1 Mn _1.8 Al _0.1 O ₄ ), the positive electrode plate conductive additive (AB) and the binder (polyvinylidene fluoride) was 90% by mass, 5% by mass. % And 5% by mass. Then, a paste-like positive electrode mixture was prepared by adjusting the viscosity by adding an appropriate amount of a viscosity modifier composed of N-methylpyrrolidone (NMP) to this mixture.

  And the produced positive mix was apply | coated on both surfaces of the aluminum foil (positive electrode collector) which has a thickness of 20 micrometers, and was dried. Thereby, the positive electrode 3 to which the positive electrode mixture shown in FIG. 1 was applied was produced.

(Preparation of negative electrode active material)
Moreover, the negative electrode mixture was produced. Specifically, first, the pulverized graphite powder and the raw material of non-graphitizable carbon (HC) which is amorphous carbon were mixed and dry-stirred. At this time, phenol resin, furan resin, furfuryl alcohol, and the like were used as raw materials for non-graphitizable carbon (HC). Thereafter, the dry-stirred mixture was baked at a temperature of 800 ° C. to 3000 ° C. for 1 hour to 5 hours. The fine powder after firing was classified and removed, and a fine powder having an average particle diameter of 3 μm was selected. As a result, graphite particles having an average particle diameter of 3 μm whose surface was coated with HC were obtained as the negative electrode active material.

(Preparation of negative electrode)
Moreover, the negative electrode plate conductive support agent which consists of acetylene black (AB) which has an average particle diameter of 1.8 micrometers was disperse | distributed in the aqueous solution which dissolved the thickener which consists of carboxymethylcellulose (CMC). Thereafter, graphite particles having an average particle diameter of 3 μm whose surface is coated with HC and a negative electrode binder made of styrene-butadiene copolymer rubber particles are mixed in an aqueous solution in which AB is dispersed at a predetermined ratio. A paste-like negative electrode mixture was prepared. At this time, the ratio of the negative electrode active material (graphite particles), the negative electrode plate conductive auxiliary agent (AB), the thickener (CMC), and the negative electrode binder (styrene-butadiene copolymer rubber particles) was 96% by mass and 2% by mass. %, 1% by mass and 1% by mass.

  And the produced negative mix was apply | coated on both surfaces of the copper foil which has a thickness of 15 micrometers, and was dried. This produced the negative electrode 4 with which the negative mix shown in FIG. 1 was apply | coated.

(Preparation of non-injected battery)
Thereafter, as shown in FIG. 1, the positive electrode 3, the negative electrode 4, and the separator 5 were wound with the separator 5 interposed between the positive electrode 3 and the negative electrode 4. Thereby, the power generation element 2 was formed. The positive electrode 3 was connected to the battery lid 7 and the negative electrode 4 was connected to the negative electrode terminal 8 with the power generation element 2 disposed inside the battery case 6. Thereafter, the battery lid 7 was attached to the battery case 6 by laser welding. Thereby, the battery 1 before inject | pouring a nonaqueous electrolyte was produced.

(Preparation and injection of non-aqueous electrolyte)
In addition, the electrolyte salt made of LiPF ₆ is a non-aqueous solution made of a mixed solvent in which the volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) is adjusted to 3: 2: 5. Dissolved in solvent. At this time, as the concentration of LiPF ₆ is 1 mol / L, to prepare a non-aqueous electrolyte by dissolving LiPF ₆ into a mixing solvent of non-aqueous solvents. Furthermore, vinylene carbonate was contained in the nonaqueous electrolyte. This vinylene carbonate was contained so that it might become 1.0 mass% with respect to the total weight of a nonaqueous electrolyte. And the produced non-aqueous electrolyte was inject | poured from the injection port which the battery case 6 does not illustrate. Finally, by sealing the liquid injection port, the battery 1 of Example 1 shown in FIG. 1 was produced.

(Example 2)
A battery 1 according to Example 2 was fabricated in the same manner as in Example 1 except that the average particle diameter of the graphite particles of the negative electrode active material was 5 μm.

(Example 3)
Except that the amorphous carbon covering the surface of the graphite particles of the negative electrode active material is graphitizable carbon (SC) and the average particle diameter of the graphite particles is 5 μm, the same as in Example 1. Thus, a battery 1 according to Example 3 was produced.

Example 4
A battery 1 according to Example 4 was produced in the same manner as in Example 1 except that the average particle diameter of the graphite particles of the negative electrode active material was 8 μm.

(Example 5)
A battery 1 according to Example 5 was produced in the same manner as in Example 1 except that the average particle diameter of the graphite particles of the negative electrode active material was 10 μm.

(Comparative Example 1)
A battery according to Comparative Example 1 was prepared in the same manner as in Example 1 except that the surface of the graphite particles of the negative electrode active material was not covered with amorphous carbon and that the average particle diameter of the graphite particles was 5 μm. Produced.

(Comparative Example 2)
A battery according to Comparative Example 2 was fabricated in the same manner as in Example 1 except that the average particle diameter of the graphite particles of the negative electrode active material was 5 μm and that the negative electrode did not contain the negative electrode plate conductive additive.

(Comparative Example 3)
A battery according to Comparative Example 3 was fabricated in the same manner as Example 1 except that the average particle diameter of the graphite particles of the negative electrode active material was 25 μm.

(Comparative Example 4)
Examples except that the surface of the graphite particles of the negative electrode active material is not coated with amorphous carbon, the average particle diameter of the graphite particles is 25 μm, and the negative electrode does not contain a negative electrode conductive additive. In the same manner as in Example 1, a battery according to Comparative Example 4 was produced.

(Evaluation test)
First, the initial capacity was measured using the batteries 1 of Examples 1 to 5 and the batteries of Comparative Examples 1 to 4. Specifically, until each of the batteries 1 of Examples 1 to 5 and the batteries of Comparative Examples 1 to 4 has a voltage of 4.1 V by flowing a constant current of 450 mA under a temperature condition of 25 ° C. Charged (constant current charge). Thereafter, charging was continued for each battery at a constant voltage of 4.1 V (constant voltage charging). Here, charging was performed such that the total of the elapsed time of constant current charging and the elapsed time of constant voltage charging was 3 hours. Thereafter, each battery was discharged at a constant current of 450 mA until the voltage decreased from 4.1 V as the initial voltage to 2.75 V as the discharge end voltage. And the capacity | capacitance (discharge capacity) discharged until it became discharge final voltage (2.75V) was measured as "initial capacity".

  Then, the internal resistance of the battery was measured using the battery 1 of Examples 1 to 5 and the batteries of Comparative Examples 1 to 4 after measuring the initial capacity. Specifically, until each of the batteries 1 of Examples 1 to 5 and the batteries of Comparative Examples 1 to 4 has a voltage of 3.8 V by flowing a constant current of 450 mA under a temperature condition of 25 ° C. Charged (constant current charge). Thereafter, charging was continued for each battery at a constant voltage of 3.8 V (constant voltage charging). Here, charging was performed such that the total of the elapsed time of constant current charging and the elapsed time of constant voltage charging was 3 hours. As a result, the state of charge (SOC) of the battery was set to 50%. The SOC of the battery means a state of charge of the battery when the SOC at the time of full charge (at the time of full charge) is 100% and the SOC at the time of complete discharge (at the time of empty discharge) is 0%.

  And it was made to hold | maintain for 5 hours on 25 degreeC temperature conditions in the state whose SOC of a battery is 50%. Thereafter, the voltage (E1) when discharged at 90 mA (I1) for 10 seconds and the voltage (E2) when discharged at 225 mA (I2) for 10 seconds were measured for each battery. Then, the resistance value (Rx) of the internal resistance of the battery at 25 ° C. was calculated using the equation Rx = | (E1-E2) / (I1-I2) |. The calculated resistance value of the internal resistance was defined as “resistance value before 45 ° C. cycle life test”.

  Then, the 45 degreeC cycle life test was done using the battery 1 of Examples 1-5 after measuring internal resistance, and the battery of Comparative Examples 1-4. Specifically, until each of the batteries 1 of Examples 1 to 5 and the batteries of Comparative Examples 1 to 4 has a voltage of 4.1 V by flowing a constant current of 450 mA under a temperature condition of 45 ° C. Charged (constant current charge). Thereafter, charging was continued for each battery at a constant voltage of 4.1 V (constant voltage charging). Here, charging was performed such that the total of the elapsed time of constant current charging and the elapsed time of constant voltage charging was 3 hours. Each battery was discharged at a constant current of 450 mA until the voltage decreased from 4.1 V as the initial voltage to 2.75 V as the final discharge voltage. In addition, it was made to rest only for 10 minutes under the temperature conditions of 45 degreeC after charge and discharge, respectively. This series of charge / discharge cycles was taken as one cycle, and 1000 cycles were repeated for each battery. Then, after 1000 charge / discharge cycles were performed, charge / discharge was performed again under the same conditions as the measurement of the initial capacity described above, and the discharge capacity at that time was defined as “post-cycle capacity”. Further, by dividing “capacity after cycle” by “initial capacity”, the retention rate of the discharge capacity before and after the 45 ° C. cycle life test was calculated as “capacity retention rate”.

  Thereafter, using the batteries 1 of Examples 1 to 5 and the batteries of Comparative Examples 1 to 4 after the 45 ° C. cycle life test, the same measurement as the above-described measurement of the internal resistance was performed, and the resistance value of the internal resistance of the battery ( Rx) was calculated. The calculated resistance value of the internal resistance was defined as “resistance value after 45 ° C. cycle life test”. Also, by dividing “resistance value after 45 ° C. cycle life test” by “resistance value before 45 ° C. cycle life test”, the increase / decrease in resistivity before and after the 45 ° C. cycle life test is calculated as the resistance value increase rate. did.

  Table 1 shows capacity retention rates and resistance value increase rates of the batteries 1 of Examples 1 to 5 and the batteries of Comparative Examples 1 to 4. From the results of the evaluation tests shown in Table 1, as in Examples 1 to 5, the negative electrode had an average particle size of 3 μm or more and 10 μm or less, and the surface was coated with amorphous carbon (HC or SC). In the case of including a negative electrode active material made of graphite particles and a negative electrode plate conductive assistant made of acetylene black (AB) having an average particle size (1.8 μm) smaller than the average particle size of the graphite particles, While being able to maintain at 82% or more, the resistance value increase rate was able to be suppressed to 125% or less. On the other hand, when the surface of the graphite particles of the negative electrode is not coated with amorphous carbon (Comparative Examples 1 and 4), when the average particle size of the graphite particles of the negative electrode is larger than 10 μm (Comparative Examples 3 and 4), and the negative electrode When no negative electrode plate conductive assistant was included (Comparative Examples 1 and 4), the capacity retention rate was only 70% or less, and the resistance value increase rate exceeded 160%.

  From the results of the evaluation test described above, the negative electrode has an average particle diameter of 3 μm or more and 10 μm or less, and a negative electrode active material comprising graphite particles whose surfaces are coated with amorphous carbon (HC or SC), and graphite particles By including the negative electrode plate conductive assistant made of acetylene black (AB) having an average particle size (1.8 μm) smaller than the average particle size, the battery life can be extended and the negative electrode It has been found that an increase in resistance in the active material can be suppressed.

  Considering in more detail, from the results of Examples 2 and 3 and Comparative Example 1, capacity retention when graphite particles whose surfaces are coated with amorphous carbon (HC or SC) are used as the negative electrode active material. It has been found that the rate (82%, 85%) is higher by 45% or more than the capacity retention rate (37%) when using graphite particles whose surface is not coated with amorphous carbon. This is considered to be due to the following reasons. That is, when the rough surface of the graphite particles is coated with amorphous carbon, the specific surface area of the graphite particles that react with the nonaqueous electrolyte is reduced. As a result, in addition to the effect of reducing the reactivity with non-aqueous electrolyte due to the coating of highly reactive graphite particles with amorphous carbon having low reactivity, the ratio of graphite particles coated with amorphous carbon It is considered that the surface area can be reduced, the region that reacts with the nonaqueous electrolyte can be reduced, and the battery life can be extended.

  As in Examples 1 to 5, when the average particle size of the graphite particles is as small as 3 μm or more and 10 μm or less, the surface of the graphite particles in contact with the non-aqueous electrolyte has many irregularities and the surface has a relatively large portion. Conceivable. For this reason, the rough surface with many unevenness | corrugations of a graphite particle is coat | covered with amorphous carbon, The uneven | corrugated | grooved part of a rough surface is filled with amorphous carbon, and is smoothed. As a result, the surface irregularities are reduced and the specific surface area of the graphite particles that react with the non-aqueous electrolyte is reduced, so that it is possible to suppress an increase in irreversible capacity due to charge and discharge. It is thought that the life could be extended.

  Furthermore, in Examples 1-5, since fine AB having an average particle size smaller than the average particle size of the graphite particles entered between the graphite particles of the negative electrode active material, the graphite particles accompanying the charge / discharge cycle Even if the graphite particles are separated from each other due to the expansion and contraction of the graphite, it is considered that the electrical contact between the graphite particles is maintained by the fine AB that has entered between the graphite particles. Thereby, it can be considered that the electrical resistance between the graphite particles can be prevented from increasing, and as a result, the electrical resistance in the negative electrode can be prevented from increasing.

  As a result of various studies as a thickener to improve the dispersibility of acetylene black (AB), which is a negative electrode conductive aid, and to suppress the aggregation of AB, the inventor of the present application has found that carboxymethylcellulose as a thickener. (CMC) and by adding AB to the CMC aqueous solution, it was found that AB can be uniformly dispersed in the CMC aqueous solution. Also in Examples 1 to 5, in combination with fine AB that easily aggregates CMC was used. Thus, by using CMC as a thickener, it is possible to uniformly disperse fine AB that easily aggregates in the negative electrode, and to effectively enter AB between graphite particles. This also seems to have prevented the increase in electrical resistance between the graphite particles.

  Furthermore, from the results of Example 2 and Example 3, the capacity retention (85%) in the case of using graphite particles whose surfaces are coated with non-graphitizable carbon (HC) as the negative electrode active material is easily graphitizable. It was found that this was higher than the capacity retention rate (82%) in the case of using graphite particles whose surface was coated with carbon (SC). The resistance increase rate (97%) when graphite particles whose surfaces are coated with HC is used as the negative electrode active material is the resistance value increase rate (108%) when graphite particles whose surfaces are coated with SC are used. It was found to be lower than As a result, it has been found that it is preferable to use non-graphitizable carbon (HC) as the amorphous carbon covering the surface of the graphite particles of the negative electrode active material.

  From the results of Examples 1, 2, 4 and 5, the capacity retention (84%, 85%) when graphite particles having an average particle size of 3 μm or more and 5 μm or less are used as the negative electrode active material is It was found that the graphite retention rate (82%) in the case of using graphite particles larger than 5 μm and not larger than 10 μm as the negative electrode active material is higher. The resistance increase rate (97%, 108%) when graphite particles having an average particle size of 3 μm or more and 5 μm or less are used as the negative electrode active material is obtained by using graphite particles having an average particle size larger than 5 μm and 10 μm or less. It was found that the resistance value increase rate (121% to 125%) when used as a substance is low. As a result, it was found that the average particle diameter of the graphite particles of the negative electrode active material is preferably 3 μm or more and 5 μm or less.

  The embodiments and examples disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments and examples but by the scope of claims for patent, and includes all modifications within the meaning and scope equivalent to the scope of claims for patent.

1 battery (non-aqueous electrolyte secondary battery)
4 Negative electrode

Claims (3)

  A negative electrode comprising a negative electrode active material comprising graphite having an average particle size of 3 μm or more and 10 μm or less, the surface of which is coated with amorphous carbon, and a negative electrode comprising carbon having an average particle size equal to or less than the average particle size of the graphite; Non-aqueous electrolyte secondary battery.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon is acetylene black having an average particle size equal to or less than an average particle size of the graphite.   The non-aqueous electrolyte secondary battery according to claim 2, wherein the negative electrode includes carboxymethyl cellulose.

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