JP2010009948A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a nonaqueous electrolyte secondary battery equipped with superior input and output characteristics by using an easily graphitizable carbon in which the surface is covered with an amorphous carbon layer as a negative electrode active material of the nonaqueous electrolyte secondary battery, and by optimization of a range of thickness of the amorphous carbon layer and a range of filling density of the negative electrode mixture layer. <P>SOLUTION: In the nonaqueous electrolyte secondary battery equipped with a positive electrode, a negative electrode containing the negative electrode active material and the nonaqueous electrolyte, the negative electrode active material is made of the easily graphitizable carbon in which the surface is covered with the amorphous carbon layer, and the thickness of the amorphous carbon layer is 0.005 to 1.5 μm. Moreover, it is preferably arranged that the average particle diameter of this negative electrode active material is 4 to 40 μm, and the filling density of the negative active mixture layer is 1.0 to 1.3 g/cm<SP>3</SP>. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery using easily graphitizable carbon whose surface is covered with an amorphous carbon layer as a negative electrode active material.

Non-aqueous electrolyte secondary batteries include a transition metal composite oxide such as lithium cobaltate (LiCoO ₂ ) and spinel-type manganese dioxide (LiMn ₂ O ₄ ) as a positive electrode active material, various carbon materials, silicon, silicon oxide as a negative electrode active material A material that can occlude and release lithium, such as a product, an organic electrolyte solution in which a lithium salt such as LiPF ₆ is dissolved in an organic solvent as an electrolyte, and a wound power generation element in which a strip electrode is wound through a separator or a flat plate A laminated power generation element in which electrodes are laminated via a separator is housed in a battery case, hermetically sealed with a battery lid, and a positive electrode terminal and a negative electrode terminal are attached to the outside of the battery.

  This non-aqueous electrolyte secondary battery is widely used as a power source for various portable electronic devices such as a mobile phone, a personal computer, and a video camera, taking advantage of its small size, light weight, and high energy density.

  As the negative electrode active material for non-aqueous electrolyte secondary batteries, graphite having excellent charge / discharge cycle characteristics has been mainly used. However, there is a problem that graphite has a small capacity per unit weight. In order to solve this problem, it has been proposed to use amorphous carbon as the negative electrode active material instead of graphite. However, amorphous carbon has a large capacity per unit weight, but is inferior in charge / discharge cycle characteristics. There was a problem.

  Therefore, for example, Patent Document 1 discloses a non-aqueous electrolyte having a high capacity and excellent charge / discharge cycle characteristics by using a composite carbon material in which an amorphous carbon thin film is formed on the surface of graphite powder particles as a negative electrode active material. It is described that a secondary battery can be obtained. Patent Document 2 discloses a non-aqueous electrolyte that has excellent charge / discharge cycle characteristics by using, as a negative electrode active material, a composite carbon material in which the surface of graphitizable carbon as a base material is coated with amorphous carbon. It is described that a secondary battery can be obtained.

  Recently, there has been an increasing trend to apply non-aqueous electrolyte secondary batteries to power sources for electric vehicles and industrial large electric devices. Non-aqueous electrolyte secondary batteries used for power supplies of these large devices are required to have high energy density and excellent input / output characteristics.

  When graphitizable carbon with a relatively developed crystal structure is used as the negative electrode active material of the non-aqueous electrolyte secondary battery, excellent characteristics such as high energy density can be obtained, but the crystal structure is not developed. Compared with the material, the reactivity with the electrolytic solution was high, and excellent input / output characteristics could not be obtained after long-term use.

  Regarding the average particle diameter of graphite used for the negative electrode active material of the non-aqueous electrolyte secondary battery, for example, in Patent Document 2, a battery having excellent high rate charge / discharge characteristics can be obtained by setting the average particle diameter in the range of 1 to 30 μm. Are listed. However, regarding the average particle diameter of graphitizable carbon, Patent Document 3 describes that a powder of 50 μm or less is used when mixed with the positive electrode, but describes the optimum range when used for the negative electrode active material. There was no literature.

  Therefore, as the negative electrode active material, graphitizable carbon is used as the core material, and the surface of the graphitizable carbon is coated with amorphous carbon having low crystallinity and low reactivity with the electrolyte. It is conceivable to use materials. However, when such a composite carbon material is used, a non-aqueous electrolyte secondary battery excellent in input / output characteristics cannot be obtained depending on conditions.

  In other words, the amorphous carbon present on the surface of the negative electrode active material is brittle compared to the graphitizable carbon used as the core material, and if the amorphous carbon layer becomes too thick, the charge / discharge reaction of the battery When the negative electrode active material repeatedly expands and contracts, there is a problem that brittle fracture occurs, graphitizable carbon is exposed, and reacts with the electrolytic solution. However, in Patent Document 4, since the thickness of the amorphous carbon that is the coating layer is not studied, the relationship between the thickness of the amorphous carbon and the input / output characteristics of the battery is unknown.

  Thus, it is conceivable to optimize the packing density of the negative electrode mixture layer. The packing density of the negative electrode mixture layer is described in, for example, Patent Document 5, and is a composite carbon in which graphite is used as the negative electrode active material and the surface of graphitizable carbon is coated with amorphous carbon. In this case, since the optimum value of the packing density of the negative electrode mixture layer was unknown, the relationship between the packing density of the negative electrode mixture layer and the input / output characteristics was not known.

  However, when the packing density of the negative electrode mixture layer is too high, the surface amorphous carbon layer is destroyed because it is necessary to apply an excessive pressing pressure in the pressing step, and graphitizable carbon is exposed, There was a problem of reacting with the electrolyte.

Accordingly, an object of the present invention is to use graphitizable carbon whose surface is covered with an amorphous carbon layer as a negative electrode active material of a nonaqueous electrolyte secondary battery, and to optimize the thickness range of the amorphous carbon layer. Accordingly, it is an object of the present invention to obtain a non-aqueous electrolyte secondary battery that can maintain excellent input / output characteristics even after long-term use.
WO96 / 14173 Publication JP 11-045715 A WO04 / 034491 Japanese Patent Laid-Open No. 11-147832 JP-A-11-214042

  The invention according to claim 1 is a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte, wherein the negative electrode active material is easily graphite whose surface is coated with an amorphous carbon layer. Carbon is characterized in that the amorphous carbon layer has a thickness of 0.005 to 1.5 μm.

  The invention of claim 2 is characterized in that, in the non-aqueous electrolyte secondary battery, the particle diameter of the negative electrode active material is in the range of 4 to 40 μm.

Furthermore, the invention of claim 3 is characterized in that, in the non-aqueous electrolyte secondary battery, the filling density of the negative electrode mixture layer containing the negative electrode active material is 1.0 to 1.3 g / cm ³ .

  According to the invention of claim 1, a non-aqueous electrolyte secondary battery having high energy density and excellent input / output characteristics is obtained by optimizing the thickness of the amorphous carbon layer on the surface of the negative electrode active material. Can do.

  According to the invention of claim 2, a nonaqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics can be obtained.

  According to the invention of claim 3, it is possible to obtain a non-aqueous electrolyte secondary battery further excellent in input / output characteristics.

  Hereinafter, the best embodiment of the present invention will be described.

In the non-aqueous electrolyte secondary battery of the present invention, graphitizable carbon whose surface is covered with an amorphous carbon layer is used as the negative electrode active material. And the range of the thickness of the surface amorphous carbon layer shall be 0.005-1.5 micrometers. Moreover, the range of the average particle diameter of this negative electrode active material shall be 4-40 micrometers, and also the range of the packing density of the negative mix layer containing this negative electrode active material shall be 1.0-1.3 g / cm < ³ >. preferable.

  The graphitizable carbon used for the core material of the negative electrode active material of the present invention is that the average plane spacing of the lattice plane (d002) by X-ray diffraction is 0.339 to 0.355 nm, and the crystallite size is c-axis direction. Non-graphitic carbon that can be converted to graphite by a high-temperature treatment represented by 1.6 to 3.0 nm in (Lc), such as carbon materials, coke, and mesophase microspheres obtained by heat-treating pitches and tars.

  The amorphous carbon of the surface coating layer is obtained by heat treatment using an easily graphitizable resin or a hardly graphitizable resin as a precursor. These are carbons that have lower crystallinity than the graphitizable carbon, also called amorphous carbon, and are low-temperature treated products of graphitizable carbon that can be converted to graphite by high-temperature treatment, and are not converted to graphite even by high-temperature treatment. It contains non-graphitizable carbon.

  In the present invention, the method for producing graphitizable carbon whose surface is coated with an amorphous carbon layer is not particularly limited, but in the solution of the graphitizable resin or the hardly graphitizable resin dissolved in a solvent, A method of mixing and dispersing graphitizable carbon, removing the solvent and drying, and carbonizing this easily obtains graphitizable carbon whose surface is uniformly coated with an amorphous carbon layer. Is preferable.

  The solvent used in preparing the solution of the graphitizable resin or the non-graphitizable resin is not particularly limited as long as it can dissolve the resin used, and quinoline, pyridine, toluene, benzene, tetrahydrofuran, creosote Oil etc. can be used.

  After preparing a composite of graphitizable carbon and resin, the resin is carbonized to obtain composite carbon particles of graphitizable carbon and amorphous carbon. There is a method in which a carbon mixture is heated to heat a resin. As the atmosphere used for carbonization, an inert atmosphere, a vacuum atmosphere, or a low oxygen-containing atmosphere can be employed. The maximum temperature during carbonization is preferably 800 to 2000 ° C. Moreover, the obtained composite_body | complex is crushed and classified as needed. A crusher such as a cutter mill or a pin mill can be used for crushing, and a wind-type or mechanical classifier can be used for classification.

  Further, in the negative electrode active material of the present invention, since the surface amorphous carbon layer is more brittle than the graphitizable carbon used for the core material, if the surface layer is too thick, the active material particles during the charge / discharge cycle There is a problem that brittle fracture occurs due to expansion and contraction of the material, and if the thickness of the surface layer is too small, the reaction between the graphitizable carbon used for the core material and the electrolytic solution cannot be prevented. Therefore, the thickness of the amorphous carbon layer in the graphitizable carbon whose surface is coated with the amorphous carbon layer of the present invention is set to 0.005 to 1.5 μm.

  The thickness of the amorphous carbon layer on the surface is such that the graphitizable carbon used for the core material and the graphitizable resin or non-graphitizable resin used as the raw material for the amorphous carbon layer covering the surface It can be controlled by the mixing ratio (weight ratio) of the two when producing the composite.

  In the non-aqueous electrolyte secondary battery of the present invention, if the particles of the negative electrode active material are too large, there are few active materials and the bonding surface between the active material and the current collector. Is liable to be cut, and if it is too small, the growth of the negative electrode surface coating layer is promoted with an increase in the specific surface area. Therefore, the range of the average particle diameter of the negative electrode active material is 4 to 40 μm.

  In the present invention, the “average particle diameter” of the negative electrode active material means a particle diameter at a point that intersects the axis of 50% integrated amount in an integrated curve of relative particle amounts in the particle size distribution, and is also referred to as median diameter. It is a called value. The average particle size of the negative electrode active material particles was measured by a laser diffraction / scattering method using Nikkiso Microtrac UPA and HRA.

  In the present invention, the graphitizable carbon having the target average particle diameter can be obtained by sieving after finely pulverizing with a ball mill.

  The negative electrode mixture layer of the nonaqueous electrolyte secondary battery is usually composed of a carbon material as an active material and a polymer material as a binder. In some cases, acetylene black or the like as a conductive additive may be included.

  The negative electrode plate is made by mixing a negative electrode active material and a binder (in some cases with a conductive auxiliary agent), adding an organic solvent to the mixture to form a negative electrode mixture paste, and using this negative electrode mixture paste as a strip-shaped current collector A negative electrode mixture layer containing a negative electrode active material, a binder, and (in some cases, a conductive auxiliary agent) is attached to the surface of the belt-shaped current collector.

  In the present invention, the filling density of the negative electrode mixture layer is controlled by the coating mass of the negative electrode mixture layer and the thickness of the mixture layer after pressing. The roll press is performed by heating the roll in the range of 100 to 170 ° C. so that a high packing density can be obtained with a low linear pressure.

  In the present invention, the method for calculating the packing density on the negative electrode mixture layer is to cut out a negative electrode having a mixture layer applied on both sides into a size of 10 cm × 10 cm, measure the mass and thickness of the electrode, and calculate from the following formula: did.

(Electrode mass−current collector mass) / [(electrode thickness−current collector thickness) × area]
In the negative electrode active material of the present invention, if the packing density of the negative electrode mixture layer is too high, there is a problem that the amorphous carbon layer on the surface is destroyed because it is necessary to apply an excessive pressing pressure in the pressing step. If the packing density is too low, the contact surfaces between the negative electrode active materials are reduced, so that the internal resistance increases due to the cutting of the conductive path accompanying the charge / discharge cycle, and the input / output characteristics are degraded. Therefore, the range of the packing density of the negative electrode mixture layer of the present invention is set to 1.0 to 1.3 g / cm ³ .

In the non-aqueous electrolyte secondary battery of the present invention, as the positive electrode active material, the composition formula Li _x MO ₂ , Li _y M ₂ O ₄ , the composition formula Na _x MO ₂ (where M is one or more transition metals, 0 A composite oxide represented by ≦ x ≦ 1, 0 ≦ y ≦ 2), a metal chalcogenide or a metal oxide having a tunnel structure or a layered structure can be used. Specific examples thereof include LiCoO ₂ , LiCo _x Ni _1-x O ₂ , LiMn ₂ O ₄ , Li ₂ Mn ₂ O ₄ , MnO ₂ , FeO ₂ , V ₂ O ₅ , V ₆ O _{13 and the} like. Moreover, you may use these various active materials in mixture.

  As a solvent for the non-aqueous electrolyte, ethylene carbonate, propylene carbonate, butylene carbonate, trifluoropropylene carbonate, γ-butyrolactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, Non-aqueous solvents such as 3-methyl-1,3-dioxolane, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl propyl carbonate, dibutyl carbonate, etc. These solvents can be used alone or in combination.

The nonaqueous electrolyte is used by dissolving the supporting salt in these nonaqueous solvents. Examples of the supporting salt include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ CO ₂ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiCF ₃ CF ₂ CF ₂ SO ₃ , LiN (SO ₂ CF ₃ ). ₂ , salts such as 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 mixtures thereof may be used. it can.

  Moreover, as a separator, a woven fabric, a nonwoven fabric, a synthetic resin microporous film, etc. can be used, and especially a synthetic resin microporous film can be used suitably. Among these, polyolefin microporous membranes such as polyethylene and polypropylene microporous membranes, or microporous membranes composed of these are preferably used in terms of thickness, membrane strength, membrane resistance, and the like.

  The shape of the battery and the shape of the battery are not particularly limited, and the present invention can be applied to non-aqueous electrolyte secondary batteries having various shapes such as a square, an oval, a coin, a button, and a sheet. is there.

[Examples 1 to 5 and Comparative Examples 1 to 3]
[Example 1]
A nonaqueous electrolyte secondary battery in which a wound power generation element was housed in a rectangular battery case was produced. Lithium cobaltate as the positive electrode active material, graphitizable carbon whose surface is coated with an amorphous carbon layer as the negative electrode active material, LiPF as a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate in a volume ratio of 1: 1 as the electrolyte solution An organic electrolyte solution in which ₆ was dissolved to a concentration of 1 mol / L was used.

The positive electrode plate is prepared by mixing 88 wt% of LiMn ₂ O ₄ as a positive electrode active material, 4 wt% of acetylene black (AB) as a conductive additive, and 8 wt% of polyvinylidene fluoride (PVdF) as a binder. N-methylpyrrolidone (NMP) is added to the mixture to form a positive electrode mixture paste, this positive electrode mixture paste is applied to both sides of a 20 μm-thick belt-shaped aluminum current collector, dried, and then compression molded by a roll press. is there. The size of the strip-like positive electrode plate was 493 mm in length, 30 mm in width, 72 mm in width of the positive electrode mixture layer uncoated portion, and the total thickness of the aluminum current collector and the positive electrode mixture layers on both sides was 220 μm.

Pitch coke was used as the negative electrode active material, and the surface of this pitch coke was coated with an amorphous carbon layer obtained by carbonizing an easily graphitizable resin having a thickness of 0.10 μm. The average particle diameter of the obtained negative electrode active material was 20 μm. The negative electrode plate is prepared by mixing 95 wt% of the negative electrode active material and 5 wt% of PVdF as a binder, adding NMP to the mixture to form a negative electrode mixture paste, and forming the negative electrode mixture paste with a 10 μm-thick strip-shaped copper It is applied to both sides of the current collector, dried and then compression molded with a roll press. The packing density of the obtained negative electrode mixture layer was 1.2 g / cm ³ . The size of the strip-shaped negative electrode plate was 444 mm in length, 31 mm in width, 17 mm in width of the negative electrode mixture layer uncoated portion, and the total thickness of the copper current collector and the negative electrode mixture layers on both sides was 146 μm.

  As the separator, a microporous polyethylene film having a width of 34 mm and a thickness of 25 μm was used. The wound power generation element is obtained by winding a belt-like positive electrode plate and a belt-like negative electrode plate around a winding core in a long cylindrical shape via a belt-like separator. And the nonaqueous electrolyte secondary battery A of Example 1 which produced the winding type electric power generation element in the aluminum battery case was produced.

  The design capacity of the obtained nonaqueous electrolyte secondary battery A was 400 mAh. The “design capacity” means an initial discharge capacity between 4.1 and 2.75 V at 25 ° C.

  FIG. 1 shows the appearance of the obtained nonaqueous electrolyte secondary battery. In FIG. 1, 1 is a nonaqueous electrolyte secondary battery, 2 is a battery case, 3 is a battery can, 4 is a battery lid, A positive electrode terminal, 6 is a negative electrode terminal, 7 is an insulator, 8 is an electrolyte injection port, and 9 is a safety valve.

[Example 2]
As a negative electrode active material, the surface of pitch coke was coated with an amorphous carbon layer obtained by carbonizing an easily graphitizable resin, and the thickness of the amorphous carbon layer was 0.005 μm. Thus, a nonaqueous electrolyte secondary battery B of Example 2 was produced.

[Example 3]
As a negative electrode active material, the surface of pitch coke was coated with an amorphous carbon layer obtained by carbonizing a graphitizable resin, and the thickness of the amorphous carbon layer was set to 0.01 μm. Thus, a non-aqueous electrolyte secondary battery C of Example 3 was produced.

[Example 4]
As the negative electrode active material, the surface of pitch coke was coated with an amorphous carbon layer obtained by carbonizing a graphitizable resin, and the thickness of the amorphous carbon layer was 1.0 μm. Thus, a non-aqueous electrolyte secondary battery D of Example 4 was produced.

[Example 5]
As the negative electrode active material, the surface of pitch coke was coated with an amorphous carbon layer obtained by carbonizing a graphitizable resin, and the thickness of the amorphous carbon layer was 1.5 μm. Thus, a nonaqueous electrolyte secondary battery E of Example 5 was produced.

[Comparative Example 1]
A nonaqueous electrolyte secondary battery F of Comparative Example 1 was produced in the same manner as in Example 1 except that only pitch coke was used as the negative electrode active material (no amorphous carbon layer on the surface).

[Comparative Example 2]
As the negative electrode active material, the surface of pitch coke was coated with an amorphous carbon layer obtained by carbonizing a graphitizable resin, and the thickness of the amorphous carbon layer was 0.003 μm. Thus, a non-aqueous electrolyte secondary battery G of Comparative Example 2 was produced.

[Comparative Example 3]
As the negative electrode active material, the surface of pitch coke was coated with an amorphous carbon layer obtained by carbonizing a graphitizable resin, and the thickness of the amorphous carbon layer was set to 2.0 μm. Thus, a nonaqueous electrolyte secondary battery H of Comparative Example 3 was produced.

[Initial output and output drop measurement result after cycle]
The non-aqueous electrolyte secondary batteries A to H of Examples 1 to 5 and Comparative Examples 1 to 3 were subjected to a charge / discharge cycle test, and the initial output and the output after 300 cycles of charge / discharge were measured. The initial output of the battery is the lower limit from the relationship between the voltage and current at the 10th second when discharging at 80 mA, 200 mA and 400 mA for 10 seconds at a discharge depth (DOD) of 50% and then discharging at each current value. The current value when the voltage was 2.5 V was extrapolated, and the output was measured from the obtained current value and the lower limit voltage.

  In addition, the measurement of "initial output" here adjusts the battery which carried out 1 cycle charge / discharge on the same conditions as the charge / discharge cycle test described below to 50% of depth of discharge (DOD) after producing a battery. It was done.

  In the charge / discharge cycle test, charging is performed at a constant current / constant voltage of up to 4.2 V at a constant current of 400 mA, and further at a constant voltage of 4.2 V for 3 hours, and discharging is performed at a constant current of 400 mA and a final voltage of 2.8 V. And 300 cycles were performed. Also, the output after 300 cycles of charge / discharge was measured under the same method and conditions as the initial output measurement, and the ratio of the output after 300 cycles of charge / discharge with respect to the initial output was determined. " The results are summarized in Table 1.

  From the results of Table 1, in the case of the batteries of Examples 1 to 5 in which the thickness of the amorphous carbon layer on the negative electrode active material surface is in the range of 0.005 to 1.5 μm, the output reduction rate after 300 cycles of charge and discharge It was found that a non-aqueous electrolyte secondary battery having an A of 20% or less can be obtained.

[Examples 6 to 15]
[Example 6]
A nonaqueous electrolyte secondary battery I of Example 6 was produced in the same manner as in Example 2 except that the average particle diameter of the negative electrode active material was 50 μm.

[Example 7]
A nonaqueous electrolyte secondary battery J of Example 7 was produced in the same manner as in Example 2 except that the average particle diameter of the negative electrode active material was 40 μm.

[Example 8]
A nonaqueous electrolyte secondary battery K of Example 8 was produced in the same manner as in Example 2 except that the average particle diameter of the negative electrode active material was 4 μm.

[Example 9]
A nonaqueous electrolyte secondary battery L of Example 9 was produced in the same manner as in Example 2 except that the average particle diameter of the negative electrode active material was 2 μm.

[Example 10]
A nonaqueous electrolyte secondary battery M of Example 10 was produced in the same manner as in Example 1 except that the average particle diameter of the negative electrode active material was 40 μm.

[Example 11]
A nonaqueous electrolyte secondary battery N of Example 11 was produced in the same manner as in Example 1 except that the average particle diameter of the negative electrode active material was 4 μm.

[Example 12]
A nonaqueous electrolyte secondary battery O of Example 12 was produced in the same manner as in Example 5 except that the average particle size of the negative electrode active material was 50 μm.

[Example 13]
A nonaqueous electrolyte secondary battery P of Example 13 was produced in the same manner as in Example 5 except that the average particle diameter of the negative electrode active material was 40 μm.

[Example 14]
A nonaqueous electrolyte secondary battery Q of Example 14 was produced in the same manner as Example 5 except that the average particle diameter of the negative electrode active material was 4 μm.

[Example 15]
A nonaqueous electrolyte secondary battery R of Example 15 was produced in the same manner as in Example 5 except that the average particle diameter of the negative electrode active material was 2 μm.

[Cycle characteristic measurement results]
The non-aqueous electrolyte secondary batteries I to R of Examples 6 to 15 were subjected to a charge / discharge cycle test. The measurement method and conditions of the battery charge / discharge cycle test were the same as in Example 1. The ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle was defined as the capacity retention rate.

  The results are summarized in Table 2. Table 2 also shows the results of Examples 1, 2, and 5 for comparison.

  From the results of Table 2, when the thickness of the amorphous carbon layer on the negative electrode active material surface is in the range of 0.005 to 1.5 μm and the average particle diameter of the negative electrode active material is in the range of 4 to 40 μm, 300 It was found that excellent charge / discharge cycle characteristics with a capacity retention rate of 80% or more after cycle charge / discharge were obtained.

[Examples 16 to 20]
[Example 16]
A nonaqueous electrolyte secondary battery S of Example 16 was produced in the same manner as in Example 1, except that the packing density of the negative electrode mixture layer was 0.9 g / cm ³ .

[Example 17]
A nonaqueous electrolyte secondary battery T of Example 17 was produced in the same manner as in Example 1, except that the packing density of the negative electrode mixture layer was 1.0 g / cm ³ .

[Example 18]
A nonaqueous electrolyte secondary battery U of Example 18 was produced in the same manner as in Example 1 except that the negative electrode mixture layer had a packing density of 1.1 g / cm ³ .

[Example 19]
A nonaqueous electrolyte secondary battery V of Example 19 was produced in the same manner as in Example 1 except that the packing density of the negative electrode mixture layer was 1.3 g / cm ³ .

[Example 20]
A nonaqueous electrolyte secondary battery W of Example 20 was produced in the same manner as in Example 1 except that the packing density of the negative electrode mixture layer was 1.4 g / cm ³ .

[Initial output and output drop measurement result after cycle]
The non-aqueous electrolyte secondary batteries S to W of Examples 16 to 20 were subjected to a charge / discharge cycle test, and the initial output and the output after 300 cycles of charge / discharge were measured. The measurement method and conditions for the output and charge / discharge cycle tests were the same as in Example 1. The results are summarized in Table 3. Table 3 also shows the results of Example 1 for comparison.

From the results of Table 3, when the filling density of the negative electrode mixture layer is in the range of 1.0 to 1.3 g / cm ³ , the output decrease rate after 300 cycles of charge / discharge is 20% or less. It was found that a secondary battery could be obtained.

The figure which shows the external appearance of a nonaqueous electrolyte secondary electricity.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Nonaqueous electrolyte secondary battery 2 Battery case 3 Battery can 4 Battery cover 5 Positive electrode terminal 6 Negative electrode terminal 7 Insulator 8 Electrolyte injection port 9 Safety valve

Claims (3)

In a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte, the negative electrode active material is graphitizable carbon whose surface is covered with an amorphous carbon layer, A nonaqueous electrolyte secondary battery, wherein the thickness of the crystalline carbon layer is 0.005 to 1.5 μm. The nonaqueous electrolyte secondary battery according to claim 1, wherein the particle diameter of the negative electrode active material is in the range of 4 to 40 μm. ^3. The nonaqueous electrolyte secondary battery according to claim 1, wherein a filling density of the negative electrode mixture layer is 1.0 to 1.3 g / cm ³ .