JP2003051309A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

(57) Abstract: The present invention relates to a non-aqueous electrolyte secondary battery, which suppresses expansion and contraction of a negative electrode active material due to charge / discharge and precipitation of lithium due to overvoltage, and has a cycle life characteristic and a high rate. An object is to provide a nonaqueous electrolyte secondary battery having excellent charge / discharge characteristics. SOLUTION: In a non-aqueous electrolyte secondary battery using a positive electrode using a transition metal composite oxide containing lithium as a positive electrode active material and a negative electrode using a carbon material capable of occluding and releasing lithium as a negative electrode active material, The substance is 0.37 nm or more,
A low-crystalline carbon material having a lattice spacing (d002) of 0.40 nm or less, characterized in that the potential at the time of occlusion of lithium is 30 mV or more based on lithium metal.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art In recent years, a lithium secondary battery has been rapidly growing as a battery system leading a small battery as a power source for driving a portable electronic device such as a mobile phone, a notebook type personal computer and a video camcorder. Also, environmental issues,
From the standpoint of energy issues, large-scale batteries for electric vehicles or night-time power storage are being actively developed, with higher capacity, higher energy density, and excellent charge / discharge cycle characteristics.
Moreover, there is a strong demand for realizing a lithium secondary battery with excellent economical efficiency. The lithium secondary battery is superior to other battery systems in that it has a high operating voltage and a high energy density, but since it uses a non-aqueous electrolyte, it compares with an aqueous electrolyte,
It has a drawback that the ionic conductivity is low and it is difficult to take out an electric current. As an improvement to this, LiCoO ₂ , LiNiO ₂ or Li was formed on a thin aluminum foil core.
A positive electrode having a thin mixture layer made of a lithium-containing composite oxide such as Mn ₂ O ₄ and a negative electrode having a thin mixture layer made of a carbon material on a thin copper foil core are wound with a separator interposed therebetween or Batteries that have a large area and are easy to take out current by stacking have been put to practical use. However, as a high power density battery that can discharge a large current by a pulse of several seconds to several tens of seconds as required for power tool applications and hybrid electric vehicle applications that are practically used for Ni-Cd batteries and Ni-MH batteries. However, there are still insufficient points, and it has hardly been put to practical use.

On the other hand, when a lithium secondary battery is charged and discharged, a positive electrode made of a lithium-containing composite oxide releases and stores lithium, and conversely, a negative electrode stores and releases lithium.
At that time, in both the positive and negative electrodes, the crystal structure expands or contracts due to the insertion and desorption of lithium into the crystal structure, which causes the mixture layer to expand and contract. Therefore, by repeating charge and discharge, the conductivity between the mixture layer itself, that is, the active material is reduced, the adhesion between the current collector and the mixture layer is reduced, and the porosity of the electrode plate is reduced. The electrolyte is pushed out of the electrode plate, causing local depletion of the electrolyte, resulting in an increase in internal resistance and an overvoltage. And when the negative electrode potential becomes less than the redox potential of lithium, it has a problem that cycle life characteristics and high rate charge / discharge characteristics deteriorate due to the deposition of lithium having poor charge / discharge reversibility on the negative electrode surface. Was there.

[0004]

When graphite is used as the carbon material as the negative electrode active material, the lattice spacing (d002) is 0.3354 nm when lithium is not occluded, but it does not occlude lithium up to C ₆ Li. When it is carried out, the carbon layers are spread to 0.37 nm, and the degree of expansion is large. JP 2000-2
In Japanese Patent Laid-Open No. 00624, the lattice spacing (d002) is 0.3.
Although it is disclosed that non-graphitizable carbon of 4 nm or more is used, 0.34 nm cannot avoid expansion due to lithium occlusion.

Further, as shown in FIG. 1, changes in the occlusion and release potentials of lithium in amorphous carbon, that is, the charge and discharge potentials, are not flat like graphite, but have a slope. The negative electrode potential in a state where the occlusion amount is small is much higher than the redox potential of lithium, and approaches the redox potential of lithium as the occlusion amount of lithium increases. Charging in the vicinity of the redox potential of lithium may cause lithium to be deposited when it becomes overvoltage, so that it is used for electric tools and hybrid electric vehicles that require high input / output characteristics.
Further, there is a problem that cycle life characteristics and high rate charge / discharge characteristics are deteriorated for electric vehicle applications requiring long life characteristics.

The present invention is intended to solve such conventional problems, and an object thereof is to provide a non-aqueous electrolyte secondary battery having high cycle life characteristics and high rate charge / discharge characteristics.

[0007]

In order to achieve the above-mentioned object, a non-aqueous electrolyte secondary battery of the present invention comprises a positive electrode using a transition metal composite oxide containing lithium as a positive electrode active material, and storage / release of lithium. In a non-aqueous electrolyte secondary battery using a negative electrode that uses a possible carbon material as a negative electrode active material, the negative electrode active material has a lattice spacing (d) of 0.37 nm or more and 0.40 nm or less.
002), which is a low crystalline carbon material, and has a potential of 30 mV when the lithium storage potential is based on lithium metal.
The above is a feature, and a non-aqueous electrolyte secondary battery having high cycle life characteristics and high rate charge / discharge characteristics can be obtained.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The invention according to claim 1 of the present invention is a positive electrode using a transition metal composite oxide containing lithium as a positive electrode active material, and a carbon material capable of inserting and extracting lithium as a negative electrode active material. In the non-aqueous electrolyte secondary battery including the negative electrode, the negative electrode active material has a thickness of 0.37 nm or more and 0.40 nm.
It is a low crystalline carbon material having the following lattice plane spacing (d002), and the potential when occluding lithium is 30 mV or more (v
s. Li / Li ⁺ ) is a non-aqueous electrolyte secondary battery.

In a carbon material having a layered structure such as graphite, lithium is intercalated between graphite layers by an intercalation reaction, and lithium is stored in an ionic state in a state called a stage structure having extremely large anisotropy. Therefore, the lattice spacing (d002) of carbon is 0.3.
If it is less than 7 nm, the graphite-like property is strengthened rather than the amorphous property, and the crystal structure repeatedly expands and contracts with the occlusion and release of lithium, and the cycle life characteristic is deteriorated.

On the other hand, the lattice spacing (d002) is 0.37n
In the low crystalline carbon material having a crystal structure of m or more, lithium is overwhelmingly stored in the void portion of the carbon crystal structure rather than stored in the interlayer due to the intercalation reaction, and lithium isotropically. Since they are uniformly inserted, lithium ions move quickly into the negative electrode active material. Therefore, the input / output is high even when charging / discharging with a large current. In addition, even if lithium is repeatedly occluded and released according to charge and discharge, the lattice plane spacing (d002) when the lithium is inserted into the graphite is 0.
Since it has a crystal structure of 37 nm or more from the beginning, the crystal structure does not repeatedly expand and contract, and excellent cycle life characteristics can be obtained.

However, when the lattice spacing (d002) of carbon is larger than 0.40 nm, the absorption / desorption reaction does not proceed smoothly because the lithium absorption / desorption site is not secured, and the lithium ion is strongly absorbed in the carbon. As a result of being trapped, cycle life characteristics and high rate charge / discharge characteristics are significantly reduced.

Therefore, 0.37 nm or more, 0.40
The low crystalline carbon material having a lattice spacing (d002) of nm or less exhibits high cycle life characteristics and high rate charge / discharge characteristics.

On the other hand, the change with time of the occlusion and emission potentials of lithium, that is, the charge and discharge potentials, is divided into a sloped portion and a flat portion with a boundary of 30 mV (vs. Li ^/ Li ⁺ ) as shown in FIG. Since the potential of this flat portion is close to the redox potential (0 V) of lithium, charging near this potential may cause lithium to deposit due to overvoltage due to a large current or overvoltage due to an increase in internal resistance accompanying a cycle. There is. Therefore, by occluding lithium at a potential of 30 mV or higher (vs. Li ^/ Li ⁺ ) corresponding to the sloped portion, lithium deposition due to overvoltage is avoided, and deterioration of cycle life characteristics and high rate charge / discharge characteristics is suppressed. be able to.

[0014]

EXAMPLES The present invention will be described in detail below with reference to examples.

Example 1 The positive electrode active material has an average particle size of 20.
μm LiMn ₂ O ₄ was used. Acetylene black as a conductive material and polytetrafluoroethylene as a binder were dissolved in this active material in a solvent of N-methyl-2pyrrolidone, and mixed at a mass ratio of 100: 2.5: 4 to form a paste. . This was applied to an aluminum foil core material on both sides, dried, rolled, and then cut into a predetermined size to obtain a positive electrode plate.

The negative electrode active material has a lattice spacing (d002)
Carbon having a particle diameter of 0.37 nm and an average particle diameter of 15 μm and polytetrafluoroethylene as a binder were dissolved in a solvent of N-methyl-2pyrrolidone and mixed at a mass ratio of 91: 9 to form a paste. This was applied to both sides of a copper foil core material, dried, rolled, and then cut into a predetermined size to obtain a negative electrode plate. Then, the positive electrode plate and the negative electrode plate were spirally wound with a separator interposed between them, and the insulating plate 6 was placed above and below the separator and housed in a battery case that also serves as a negative electrode terminal.

For the electrolytic solution, a solvent prepared by mixing propylene carbonate and dimethyl carbonate in a volume ratio of 6: 4,
What dissolved 1.0 mol / l lithium hexafluorophosphate as a solute was injected, and the battery was sealed. The upper edge of the battery case is sealed with an insulating packing by a sealing plate also serving as a positive electrode terminal provided with a safety valve.

Example 2 The lattice spacing (d
002) A battery was produced in the same manner as in Example 1 except that carbon having 0.38 nm and an average particle size of 15 μm was used.

(Embodiment 3) The lattice spacing (d
002) A battery similar to that of the example was manufactured except that carbon having 0.40 nm and an average particle diameter of 20 μm was used.

(Comparative Example 1) A battery similar to that of Example 1 was prepared except that the applied mass of the negative electrode was 60% of that of the negative electrode of Example 1. By reducing the coating mass of the negative electrode, the negative electrode potential becomes lower than that of the battery of Example 1.

(Comparative Example 2) A battery similar to that of Example 3 was prepared except that the applied mass of the negative electrode was 50% of that of the negative electrode of Example 3. By reducing the applied mass of the negative electrode, the negative electrode potential becomes lower than that of the battery of Example 3.

Comparative Example 3 The lattice spacing (d
002) A battery was produced in the same manner as in Example 1 except that carbon having 0.36 nm and an average particle size of 10 μm was used.

(Comparative Example 4) The lattice spacing (d
002) A battery was produced in the same manner as in Example 1 except that carbon having 0.41 nm and an average particle size of 15 μm was used.

For these batteries, the charge / discharge current was 0.4.
A (corresponding to 0.2 CmA), the end-of-charge voltage 4.3 V,
Eight cycles of charging and discharging were performed under an environment of 25 ° C. under the condition of the final discharge voltage of 2.5 V to stabilize the battery capacity. Then, after charging to 4.3 V with a charge / discharge current of 2 A (corresponding to 1 CmA), the battery was disassembled, the negative electrode was taken out, and the potential when lithium metal was used as a reference was measured. This means that the negative electrode of each battery uses a potential higher than this measured value during the charging process.

A battery different from the disassembled battery is charged and discharged at a current of 2 A.
(Equivalent to 1 CmA), end-of-charge voltage 4.3 V, end-of-discharge voltage 2.5 V were subjected to a cycle life test under an environment of 25 ° C., and the capacity retention rate after 1000 cycles with respect to the first cycle capacity was determined. Lattice plane spacing of the negative electrode active material (d0
02), the negative electrode potential of the decomposition battery, and the capacity retention rate after 1000 cycles of the cycle life test battery are shown in Table 1.

[0026]

[Table 1]

The batteries of Examples 1, 2 and 3 were charged and discharged,
Lattice plane spacing (d002) 0.37n when lithium is inserted into graphite even after repeated insertion and extraction of lithium
Since it has a crystal structure of m or more from the beginning, expansion and contraction are not repeated, and the potential of the negative electrode is 3 during the charging process.
Since it was set to 0 mV or more (vs. Li ^/ Li ⁺ ), lithium was not deposited on the negative electrode surface, and excellent cycle life characteristics were obtained.

In the batteries of Comparative Examples 1 and 2, since the negative electrode potential during charging is close to the redox potential of lithium, the crystal structure of the negative electrode active material does not expand or contract, but the internal resistance due to charging / discharging does not increase. When the voltage increased due to increase, lithium deposition occurred and the capacity decreased. In the battery of Comparative Example 3, since the lattice spacing is smaller than 0.37 nm, the crystal structure expands and contracts due to the absorption and desorption of lithium, which decreases the conductivity between the active materials of the negative electrode mixture layer and the core of the mixture layer. The cycle life characteristics deteriorated due to desorption from the material. Further, in the battery of Comparative Example 4, the lattice spacing was larger than 0.40 nm and the lithium absorption / desorption sites were not secured, so the absorption / desorption reaction did not proceed smoothly and the cycle life characteristics deteriorated.

Example 4 The positive electrode active material has an average particle size of 15
A battery similar to that of Example 1 was prepared by using LiNiO ₂ of μm and using carbon having a lattice spacing (d002) of 0.39 nm and an average particle size of 15 μm as the negative electrode active material.

(Comparative Example 5) A battery similar to that of Example 2 was prepared except that the applied mass of the negative electrode was 60% of that of the negative electrode of Example 2. Note that the negative electrode potential becomes lower than that of the battery of Example 4 by reducing the applied mass of the negative electrode.

The charge / discharge current of these batteries was 0.4.
A (corresponding to 0.2 CmA), the end-of-charge voltage 4.3 V,
Eight cycles of charging and discharging were performed under an environment of 25 ° C. under the condition of the final discharge voltage of 2.5 V to stabilize the battery capacity. Next, after charging 60% of the battery capacity at this time, the battery was disassembled, the negative electrode was taken out, and the potential when lithium metal was used as a reference was measured. Charge a battery other than the decomposition battery to 60% of the battery capacity, perform pulse charging / discharging for 10 seconds at a charging / discharging current of 8 A (equivalent to 4 CmA) 100000 times, and then charge / discharge current of 0.4 A (equivalent to 0.2 CmA). And the end-of-charge voltage 4.
Two cycles of charging and discharging at 3 V and a discharge end voltage of 2.5 V were performed, and the capacity at the second cycle was used as the sustaining capacity, and the capacity retention ratio to the capacity before the pulse charge / discharge test was calculated. Table 2 shows the negative electrode potential of the decomposition battery and the capacity retention rate after 100000 cycles of the pulse charge / discharge test battery.

[0032]

[Table 2]

In each of the batteries, the negative electrode active material has a lattice spacing (d002) of 0.39 nm, and the crystal structure does not expand or contract due to lithium absorption and desorption. Furthermore, Example 4
Battery has a negative electrode potential of 30 mV or higher (vs. Li / L
i ⁺ ), lithium deposition did not occur even when charged with a large current of 8 A (corresponding to 4 CmA), and a high capacity retention rate was obtained in the pulse charge / discharge test. On the other hand, in the battery of Comparative Example 5, the negative electrode potential was 30 mV or less (vs. Li ^/ Li ⁺ ), so that lithium was deposited due to overvoltage due to a large current of 8 A (corresponding to 4 CmA), and the capacity retention ratio decreased.

LiM used in this example as the positive electrode active material
In addition to n ₂ O ₄ and LiNiO ₂ , similar effects can be obtained when LiCoO ₂ or a material obtained by adding a different element to the above active material is used.

In addition to the electrolytes used in this embodiment, the electrolyte may be other solvent such as cyclic carbonates such as ethylene carbonate and butylene carbonate, chain carbonates such as diethyl carbonate and ethylmethyl carbonate, 1,2- Known compounds such as ethers such as dimethoxyethane and 2-methyltetrahydrofuran can be used alone or as a mixed solvent.

Further, as for solutes, LiBF ₄ , Li
Known materials such as ClO ₄ can be used.

[0037]

As described above, according to the present invention, it is possible to suppress the expansion and contraction of the negative electrode active material due to charging / discharging and the precipitation of lithium due to overvoltage, and to provide a non-aqueous material excellent in cycle life characteristics and high rate charging / discharging characteristics. An electrolyte secondary battery can be obtained.

[Brief description of drawings]

FIG. 1 is a charge / discharge characteristic diagram of amorphous carbon.

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 5H029 AJ02 AJ05 AK03 AL06 AM03                       AM05 AM07 HJ04 HJ13 HJ18                 5H030 AA01 AS11 AS14 BB01                 5H050 AA02 AA07 BA05 CA07 CA08                       CA09 CB07 FA19 HA04 HA13                       HA18

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode using a transition metal composite oxide containing lithium as a positive electrode active material, and a negative electrode using a carbon material capable of occluding / releasing lithium as a negative electrode active material. 0.37 nm or more active material,
A low crystalline carbon material having a lattice spacing (d002) of 0.40 nm or less and a potential of 30 m when occluding lithium.
A non-aqueous electrolyte secondary battery having a voltage of V or more (vs. Li ^/ Li ⁺ ). 2. A non-aqueous electrolyte secondary battery comprising a positive electrode using a transition metal composite oxide containing lithium as a positive electrode active material, and a negative electrode using a carbon material capable of absorbing and desorbing lithium as a negative electrode active material. 0.37 nm or more active material,
A low crystalline carbon material having a lattice spacing (d002) of 0.40 nm or less, wherein the low crystalline carbon material has a charging potential of 30 mV or higher (vs. Li ^/ Li ⁺ ). Charging method for electrolyte secondary battery.