JP5512355B2 - Negative electrode active material for non-aqueous electrolyte secondary battery, non-aqueous electrolyte secondary battery using the same, and production method thereof - Google Patents

Description

  The present invention relates to an improvement in the negative electrode active material of a non-aqueous electrolyte secondary battery.

  Nonaqueous electrolyte secondary batteries have high energy density and high capacity, and are therefore widely used as drive power sources for portable devices.

  Conventionally, graphite has been used as a negative electrode active material used in non-aqueous electrolyte secondary batteries. Graphite includes natural graphite and artificial graphite. Artificial graphite has the advantage of being superior in cycle characteristics and safety compared to natural graphite, but has the disadvantage of being inferior in low temperature characteristics to natural graphite. In particular, when a negative electrode using scaly artificial graphite is compressed at a high pressure to increase the capacity, the basal surface of graphite, which is hard to absorb and desorb lithium ions, is oriented parallel to the current collector (core body). As a result, the occlusion / desorption of lithium ions was hindered, and there was a problem that the discharge characteristics at low temperatures and high load discharges were further deteriorated.

  Under such circumstances, techniques for improving discharge characteristics by coating graphite particles with a carbon material have been proposed (see Patent Documents 1 to 6).

JP 2004-127723 A Japanese Unexamined Patent Publication No. 4-368778 JP-A-10-294111 JP 2000-90925 A Japanese Patent Laid-Open No. 11-54123 JP 2007-317551 A

  Patent Document 1 discloses a negative electrode material for a lithium ion secondary battery in which granular graphite is used as a core material, and scaly graphite is attached to all or part of the surface of the core material. According to this technology, it is said that characteristics such as initial charge / discharge efficiency, cycle characteristics, safety, rapid charge / discharge characteristics, and conductivity can be improved.

  Patent Document 2 discloses a technique in which a surface in contact with an electrolytic solution of carbon serving as an active material is covered with amorphous carbon. According to this technique, it is said that the charging efficiency can be reduced and the carbon material can be prevented from being destroyed due to the decomposition of the electrolytic solution.

  Patent Document 3 discloses that a carbon or graphite powder having a ratio a / b of a major axis a to a minor axis b of 1 ≦ a / b ≦ 3 and an average particle size of 1 to 30 μm is coated with a coal-based or petroleum-based pitch. In addition, a technique is disclosed in which the pitch of the surface is infusible, pulverized (lightly pulverized), carbonized, and graphitized. According to this technique, the charge / discharge capacity can be increased and the cycle characteristics can be improved.

  Patent Document 4 discloses a carbon material for a negative electrode made of a fired product of a mixture of at least one of artificial graphite and natural graphite and a carbon material containing a volatile component. According to this technique, a carbon material having a small specific surface area and low production cost can be provided without impairing the stability of the electrolytic solution.

In Patent Document 5, (1) the (002) plane spacing (d002) by wide-angle X-ray diffraction is less than 3.37 mm, and the crystallite size (Lc) in the C-axis direction is at least 1000 mm or more (2 ) FWHM of R values and 1580 cm ^-1 peak is 0.3 or less, which is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum is 24cm ^-1 or less, (3) average particle The average value of the thickness of the thinnest part having a diameter of 10 to 30 μm is at least 3 μm to an average particle size, and (4) a specific surface area by the BET method is 3.5 m ² / g to 10.0 m ² / g, (5) Graphite powder having a tapping density of 0.5 g / cc or more and 1.0 g / cc or less, (6) X-ray diffraction peak intensity ratio of (110) / (004) by wide-angle X-ray diffraction method is 0.015 or more The Disclosed is a carbonaceous powder having a multi-layer structure in which a core is coated with a carbon precursor on the surface of the core and then fired in a temperature range of 700 to 2800 ° C. in an inert gas atmosphere to form a carbonaceous surface layer. ing. According to this technique, high temperature storage characteristics and low temperature discharge characteristics can be improved.

  Patent Document 6 discloses a technique in which mesocarbon microbeads whose surface is at least partially coated with natural graphite are used for the negative electrode. According to this technique, a secondary battery having a large capacity and excellent cycle characteristics can be provided.

  However, even with these techniques, the low temperature characteristics and load characteristics have not been improved sufficiently.

  The present invention has been made in view of the above, and an object thereof is to provide a negative electrode active material for a non-aqueous electrolyte secondary battery excellent in low-temperature characteristics and load characteristics, and a non-aqueous electrolyte secondary battery using the same. And

The first aspect of the present invention for solving the above problems is configured as follows.
At least a part of the surface of the scaly artificial graphite having an average particle diameter of 25 to 35 μm is covered with a coating layer composed of amorphous carbon and natural graphite having an average particle diameter of 0.1 to 3 μm. The mass ratio of the crystalline carbon to the natural graphite is 95: 5 to 50:50, and the mass ratio of the scaly artificial graphite to the total mass of the amorphous carbon and the natural graphite is 99: 1 to 97. : The negative electrode active material for nonaqueous electrolyte secondary batteries which is 3.

  The amorphous carbon contained in the coating layer acts to improve the lithium ion acceptability of the scaly artificial graphite at low temperatures and high loads. Further, the natural graphite of 0.1 to 3 μm contained in the coating layer also acts to improve the lithium ion acceptability at low temperatures and high loads. In addition, when the scaly artificial graphite is completely covered with amorphous carbon, there is a problem that the initial charge / discharge efficiency decreases (the irreversible capacity increases). In addition, since natural graphite is contained, the scaly artificial graphite is not completely covered with amorphous carbon, and a reduction in initial charge / discharge efficiency can be prevented. In the configuration of the first aspect of the present invention, these effects act synergistically, and the low temperature characteristics and load characteristics can be improved without causing a decrease in the initial charge / discharge efficiency.

  Here, when the average particle size of natural graphite is too small or too large, the load characteristics are deteriorated. Therefore, the average particle size of natural graphite is 0.1 to 3 μm.

  Moreover, when the average particle diameter of the scale-like artificial graphite used as a nucleus is too small, an irreversible capacity | capacitance will become large and initial stage charge / discharge efficiency will fall. On the other hand, if the average particle size of the scaly artificial graphite is too large, it is difficult to form a good quality coating layer, and the effect of the coating layer cannot be sufficiently obtained. For this reason, the average particle diameter of scale-like artificial graphite shall be 25-35 micrometers.

  Further, if the mass of the coating layer (natural graphite + amorphous carbon) is small relative to the scale-like artificial graphite mass serving as the nucleus, the coating layer becomes too thin, and the effect of the coating layer cannot be sufficiently obtained. Moreover, since the coating layer becomes too thick when the mass ratio of the coating layer is large, the irreversible capacity increases and the initial charge / discharge efficiency decreases. For this reason, mass ratio of the scale artificial graphite used as a nucleus and a coating layer (amorphous carbon + natural graphite) shall be 99: 1-97: 3.

  Further, if the amount of natural graphite contained in the coating layer is too small, the low temperature characteristics cannot be sufficiently improved. Further, if the amount of natural graphite contained in the coating layer is too large, the low temperature characteristics and load characteristics cannot be sufficiently improved. For this reason, the mass ratio of natural graphite to amorphous carbon in the coating layer is set to 5:95 to 50:50.

  A second aspect of the present invention for solving the above problems is a non-aqueous electrolyte secondary battery using the negative electrode active material for a non-aqueous electrolyte secondary battery.

The third aspect of the present invention for solving the above problems is configured as follows.
A scaly artificial graphite having an average particle diameter of 25 to 35 μm, a pitch as an amorphous carbon source, and natural graphite having an average particle diameter of 0.1 to 3 μm are mixed and fired at 850 to 1000 ° C. A method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery, comprising a coating step of coating at least a part of the surface of the scaly artificial graphite with amorphous carbon made of carbides of the pitch and the natural graphite.

  According to the above configuration, the scaly artificial graphite, pitch, and natural graphite are mixed and fired to form a coating layer composed of amorphous carbon and natural graphite on the surface of the scaly artificial graphite. The same effect as the present invention can be obtained.

  A fourth aspect of the present invention for solving the above-described problems is a method for producing a non-aqueous electrolyte secondary battery using the above-described method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery.

  As described above, according to the present invention, a nonaqueous electrolyte secondary battery excellent in initial charge / discharge efficiency, low temperature characteristics, and load characteristics can be realized.

  EMBODIMENT OF THE INVENTION The form for implementing this invention is demonstrated in detail through a following example. In addition, this invention is not limited to the following form, In the range which does not change the summary, it can change suitably and can implement.

[Example 1]
[Production of positive electrode]
90 parts by mass of lithium cobaltate, 5 parts by mass of graphite powder as a conductive agent, 5 parts by mass of polyvinylidene fluoride as a binder, and N-methyl-2-pyrrolidone as a dispersion medium are mixed. An active material slurry was obtained. This positive electrode active material slurry was applied to both surfaces of an aluminum current collector having a thickness of 15 μm by a doctor blade method, and NMP was volatilized and removed, followed by rolling with a roll press machine and cutting to obtain a positive electrode.

(Production of negative electrode)
(Negative electrode active material preparation process)
Scale-like artificial graphite having an average particle diameter of 27 μm, pitch, and natural graphite having an average particle diameter of 2 μm were mixed at a mass ratio of 99: 0.95: 0.05. Thereafter, firing was performed at 900 ° C., and the pitch was carbonized to obtain a negative electrode active material in which scaly artificial graphite was coated with a coating layer composed of amorphous carbon and natural graphite. In addition, it analyzed with the laser diffraction type | formula particle size analyzer (SALD-2200 by SHIMADZU), and the particle size D50 in the integrated value 50% when integrating from a thing with a small particle size to a thing with a large particle size is average particle size The diameter.

  A negative electrode active material slurry obtained by mixing 98 parts by mass of the negative electrode active material, 1 part by mass of carboxymethyl cellulose as a thickener, 1 part by mass of styrene butadiene rubber as a binder, and water as a dispersion medium. It was. This negative electrode active material slurry was applied to both surfaces of a copper current collector having a thickness of 10 μm by a doctor blade method to volatilize and remove water, and then rolled and cut with a roll press to obtain a negative electrode.

  In addition, the active material application amount of the positive electrode and the negative electrode was set so that the charge capacity ratio (negative electrode charge capacity / positive electrode charge capacity) at a portion where the positive electrode and the negative electrode face each other was 1.1.

(Production of electrode body)
The positive electrode and the negative electrode were wound through a separator made of a polyethylene microporous film, a polypropylene polypropylene tape was attached to the outermost periphery, and then pressed to obtain a flat spiral electrode body.

[Nonaqueous electrolyte adjustment]
Ethylene carbonate, propylene carbonate, and ethyl methyl carbonate are mixed at a volume ratio of 1: 1: 8 (25 ° C., 1 atm), and LiPF ₆ as an electrolyte salt is dissolved to 1.0 M (mol / liter). It became a non-aqueous electrolyte.

[Assembling the battery]
The flat electrode body was inserted into a rectangular outer can. Thereafter, the opening of the outer can is sealed with a sealing body having a liquid injection port, the nonaqueous electrolyte is injected from the liquid injection port, and then the liquid injection port is sealed, so that the nonaqueous solution according to Example 1 is used. An electrolyte secondary battery was produced.

[Example 2]
The nonaqueous electrolyte 2 according to Example 2 is the same as Example 1 except that scaly artificial graphite, pitch, and natural graphite are mixed at a mass ratio of 99: 0.80: 0.20. A secondary battery was produced.

[Example 3]
The nonaqueous electrolyte 2 according to Example 3 is the same as Example 1 except that scaly artificial graphite, pitch, and natural graphite are mixed at a mass ratio of 99: 0.70: 0.30. A secondary battery was produced.

[Example 4]
The nonaqueous electrolyte 2 according to Example 4 is the same as Example 1 except that scaly artificial graphite, pitch, and natural graphite are mixed at a mass ratio of 99: 0.50: 0.50. A secondary battery was produced.

[Example 5]
A nonaqueous electrolyte secondary battery according to Example 5 was produced in the same manner as in Example 3 except that the average particle size of the flaky artificial graphite was 35 μm.

[Example 6]
A nonaqueous electrolyte secondary battery according to Example 6 was produced in the same manner as in Example 3 except that the average particle size of natural graphite was 3 μm.

[Example 7]
A nonaqueous electrolyte secondary battery according to Example 7 was produced in the same manner as in Example 3 except that the average particle size of natural graphite was 0.1 μm.

[Example 8]
The nonaqueous electrolyte 2 according to Example 8 is the same as Example 1 except that scaly artificial graphite, pitch, and natural graphite are mixed at a mass ratio of 97: 2.10: 0.90. A secondary battery was produced.

[Comparative Example 1]
The nonaqueous electrolyte 2 according to Comparative Example 1 is the same as Example 1 except that scaly artificial graphite, pitch, and natural graphite are mixed at a mass ratio of 99: 0.30: 0.70. A secondary battery was produced.

[Comparative Example 2]
A nonaqueous electrolyte secondary battery according to Comparative Example 2 was produced in the same manner as in Example 1 except that scaly artificial graphite and natural graphite were mixed at a mass ratio of 99: 1.

[Comparative Example 3]
A non-aqueous electrolyte secondary battery according to Comparative Example 3 was produced in the same manner as in Example 1 except that flaky artificial graphite and pitch were mixed at a mass ratio of 99: 1.

[Comparative Example 4]
A nonaqueous electrolyte secondary battery according to Comparative Example 4 was produced in the same manner as in Example 3 except that the average particle size of the flaky artificial graphite was 20 μm.

[Comparative Example 5]
A nonaqueous electrolyte secondary battery according to Comparative Example 5 was produced in the same manner as in Example 3 except that the average particle size of natural graphite was 5 μm.

[Comparative Example 6]
The non-aqueous electrolyte 2 according to Comparative Example 6 is the same as Example 1 except that scaly artificial graphite, pitch, and natural graphite are mixed at a mass ratio of 95: 3.50: 1.50. A secondary battery was produced.

<Initial charge / discharge efficiency test>
Batteries were produced under the same conditions as in Examples 1 to 8 and Comparative Examples 1 to 6, and these batteries were charged with a constant current of 0.2 It (160 mA) until the voltage reached 4.2 V, and then the constant voltage 4 The battery was charged at 0.2 V until the current reached 0.02 It (16 mA). Thereafter, the battery was discharged at a constant current of 0.2 It (160 mA) until the voltage reached 2.75V. The charge capacity and discharge capacity at this time were measured, and the charge / discharge efficiency was calculated by the following equation. In addition, all this charging / discharging was performed on 25 degreeC conditions. The results are shown in Table 1 below.
Initial charge / discharge efficiency (%) = discharge capacity / charge capacity × 100

<Low temperature (0 ℃) characteristic test>
Batteries were produced under the same conditions as in Examples 1 to 4 and Comparative Examples 1 to 3, and these batteries were charged at a constant current of 0.2 It (160 mA) at 25 ° C. until the voltage reached 4.2 V. Thereafter, the battery was charged at a constant voltage of 4.2 V until the current reached 0.02 It (16 mA). Thereafter, the battery was discharged at a constant current of 0.2 It (160 mA) at 25 ° C. until the voltage reached 2.75V. The battery was charged again under the above conditions, and then discharged at 0 ° C. at a constant current of 0.2 It (160 mA) until the voltage reached 2.75V. These discharge capacities were measured, and low temperature characteristics were calculated by the following formula. The results are shown in Table 1 below.
Low temperature characteristics (%) = 0 ° C discharge capacity / 25 ° C discharge capacity × 100

<Load characteristic test>
Batteries were produced under the same conditions as in Examples 1 to 4 and Comparative Examples 1 to 3, and these batteries were charged at a constant current of 0.2 It (160 mA) until the voltage reached 4.2 V, and then the constant voltage 4 The battery was charged at 0.2 V until the current reached 0.02 It (16 mA). Thereafter, the battery was discharged at a constant current of 1 It (800 mA) until the voltage reached 2.75V. The battery was charged again under the above conditions, and then discharged at a constant current of 3 It (2400 mA) until the voltage reached 2.75V. These discharge capacities were measured, and load characteristics were calculated by the following formula. In addition, all this charging / discharging was performed on 25 degreeC conditions. The results are shown in Table 1 below.
Load characteristics (%) = 3 It discharge capacity ÷ 1 It discharge capacity × 100

  From Table 1 above, (1) the average particle size of natural graphite is 0.1 to 3 μm, (2) the average particle size of artificial graphite is larger than 20 μm and not more than 35 μm, and (3) scaly artificial graphite serving as a nucleus, The mass ratio with the coating layer is 99: 1 to 97: 3, and (4) the mass ratio of the natural graphite and amorphous carbon in the coating layer satisfies all the four conditions of 5:95 to 50:50. (Examples 1 to 8), the initial charge / discharge efficiency is 92.6 to 94.7%, the low temperature characteristics are 31.6 to 38.6%, and the load characteristics are 91.8 to 95.4%. I understand that. On the other hand, when at least one of the above four conditions is not satisfied (Comparative Examples 1 to 6), the initial charge / discharge efficiency is at least 92%, the low temperature characteristic is at least 30%, and the load characteristic is at least 90%. It turns out that one is not satisfied.

The reason why such a result was obtained is considered as follows.
The amorphous carbon contained in the coating layer acts to improve the lithium ion acceptability of the scaly artificial graphite at low temperatures and high loads. The natural graphite contained in the coating layer also acts to improve the lithium ion acceptability at low temperatures and high loads. Moreover, when it is completely covered with amorphous carbon, the initial charge / discharge efficiency decreases (the irreversible capacity increases), but the average particle size of the coating layer includes amorphous carbon together with amorphous carbon. The scaly artificial graphite is not completely covered with amorphous carbon, and such a problem does not occur. These effects act synergistically, and the low temperature characteristics and load characteristics can be improved without causing a decrease in the initial charge / discharge efficiency.

  However, when the condition (1) is not satisfied, the following problem occurs. If the average particle size of the natural graphite is too large or too small (Comparative Example 5), the low-temperature characteristics deteriorate (27.8% in Comparative Example 5).

  Further, when the condition (2) is not satisfied, the following problem occurs. If the average particle size of the scale-like artificial graphite serving as the core is too small (Comparative Example 4), the irreversible capacity increases and the initial charge / discharge efficiency decreases (91.9% in Comparative Example 4). On the other hand, if the average particle size of the scaly artificial graphite is too large, it is difficult to form a good quality coating layer, and the effect of the coating layer cannot be sufficiently obtained.

  Further, when the condition (3) is not satisfied, the following problem occurs. If the mass of the coating layer (natural graphite + amorphous carbon) is small relative to the scale-like artificial graphite mass serving as the core, the coating layer becomes too thin, and the effect of the coating layer cannot be sufficiently obtained. Further, when the mass ratio of the coating layer is large (Comparative Example 6), the coating layer becomes too thick, so that the irreversible capacity increases and the initial charge / discharge efficiency decreases (91.8% in Comparative Example 6).

  Further, when the condition (4) is not satisfied, the following problem occurs. If the amount of natural graphite contained in the coating layer is too small (Comparative Example 3), the low temperature characteristics cannot be sufficiently improved (29.8% in Comparative Example 3). If the amount of natural graphite contained in the coating layer is too large (Comparative Examples 1 and 2), the low temperature characteristics and load characteristics cannot be sufficiently improved (Comparative Example 1 has low temperature characteristics of 28.3% and load characteristics of 89.9%). In Comparative Example 2, the low temperature characteristic is 25.3% and the load characteristic is 88.6%).

(Additions)
As the positive electrode active material used in combination with the negative electrode active material of the present invention, it is preferable to use a lithium transition metal composite oxide, a lithium transition metal phosphate compound having an olivine structure, or the like. Examples of lithium transition metal composite oxides include lithium cobalt composite oxide, lithium nickel composite oxide, spinel-type lithium manganese composite oxide, and some transition metal elements contained in these compounds substituted with other metal elements. Compounds are preferred. The lithium transition metal phosphate compound having an olivine structure is preferably lithium iron phosphate. These can be used alone, or can be used in combination of two or more. Moreover, you may add well-known additives, such as lithium carbonate, to a positive electrode.

Furthermore, as the solvent for the nonaqueous electrolyte, cyclic carbonates represented by propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, lactones represented by γ-butyrolactone, γ-valerolactone, diethyl carbonate, dimethyl carbonate, ethylmethyl Chain carbonate typified by carbonate, tetrahydrofuran, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, 1,3-dioxolane, 2-methoxytetrahydrofuran, ether typified by diethyl ether, etc. alone or in combination of two or more. Can be used. As the electrolyte salt in the nonaqueous _electrolyte, it is possible to use _{LiPF 6, LiAsF 6, LiClO 4} , LiBF 4, LiCF 3 SO 3, LiN (CF 3 SO 2) 2 and the like.

  As described above, according to the present invention, it is possible to provide a negative electrode active material for a nonaqueous electrolyte secondary battery excellent in initial charge / discharge efficiency, load characteristics, and low temperature characteristics. Therefore, industrial applicability is great.

Claims (4)

At least a part of the surface of the scaly artificial graphite having an average particle diameter of 25 to 35 μm is covered with a coating layer made of amorphous carbon and natural graphite having an average particle diameter of 0.1 to 3 μm.
The mass ratio of the amorphous carbon to the natural graphite is 95: 5 to 50:50,
A negative electrode active material for a nonaqueous electrolyte secondary battery, wherein a mass ratio of the scaly artificial graphite to the total mass of the amorphous carbon and the natural graphite is 99: 1 to 97: 3.   A non-aqueous electrolyte secondary battery using the negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1.   A scaly artificial graphite having an average particle diameter of 25 to 35 μm, a pitch as an amorphous carbon source, and natural graphite having an average particle diameter of 0.1 to 3 μm are mixed and fired at 850 to 1000 ° C. A method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery, comprising a coating step of coating at least a part of the surface of the scaly artificial graphite with amorphous carbon made of carbides of the pitch and the natural graphite.   The manufacturing method of a nonaqueous electrolyte secondary battery provided with the manufacturing method of the negative electrode active material for nonaqueous electrolyte secondary batteries of Claim 3.