JP2000331683A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery capable of suppressing lowering of service capacity at a low temperature while lifetime thereof is long. SOLUTION: This lithium secondary battery is provided with a positive electrode 1 containing a complex oxide composed of Li and at least one element selected from Co and Ni, and the complex oxide has a specific surface area S (m2/g) calculated by the BET method is 0.45<=S<=0.8 and an average particle size D (μm) at measuring particle size distribution is 3.5<=D<=7.0, wherein the S and the D satisfy D>=9.0S-1.2 and an average pore diameter P (nm) at a pore distribution measurement is 8<=P<=10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery having an improved positive electrode.

[0002]

2. Description of the Related Art In recent years, with the development of electronic equipment, development of a lithium secondary battery that is small, lightweight, has a high energy density, and is capable of being repeatedly charged and discharged has been advanced.
As a positive electrode active material of this lithium secondary battery, Ti
Lithium-containing oxides such as S ₂ , MoS ₂ , HbSe ₂ , and V ₂ O ₅ , and lithium composite oxides such as lithium cobalt oxide, lithium manganese composite oxide, and lithium nickel oxide are known. .

[0003] Lithium manganese composite oxides and lithium nickel oxides have attracted attention because they are richer in resources and cheaper than lithium cobalt oxides. However, lithium manganese composite oxide is
Inferior in capacity compared to lithium cobalt oxide,
For example, in the operating range of the 4V region (3.0 to 4.3V), lithium cobalt oxide has a theoretical capacity of about 280 mAh / g,
While the actual capacity is 150 to 160 mAh / g, the theoretical capacity of the lithium manganese composite oxide is about 150 mAh / g.
/ G, actually 110-120 mAh / g. Further, although lithium nickel oxide can have a higher capacity than lithium cobalt oxide, thermal stability during overcharge and stability during storage are slightly inferior to lithium cobalt oxide.

[0004]

In a lithium secondary battery provided with a positive electrode containing lithium nickel oxide or lithium cobalt oxide as an active material, the charge / discharge cycle life and discharge characteristics at low temperatures are improved. Is required. As a method of improving the charge / discharge cycle life, it is known to add another element to the active material or to reduce the specific surface area of the active material. However, according to these methods, although the cycle life is improved, there is a problem that the capacity is reduced. Further, the addition of other elements increases the manufacturing cost and complicates the manufacturing process. As a method of improving the low-temperature discharge characteristics, a method of reducing the particle size of the active material can be considered, but the cycle life is reduced. Thus, it has been difficult to achieve both low-temperature discharge characteristics and cycle life in a lithium secondary battery.

An object of the present invention is to provide a long-life lithium secondary battery in which a decrease in discharge capacity at a low temperature is suppressed.

[0006]

A lithium secondary battery according to the present invention is a lithium secondary battery having a positive electrode including a composite oxide of Li and at least one element selected from Co and Ni. The composite oxide has a specific surface area S (m ² / g) determined by the BET method of 0.45 ≦ S ≦ 0.8.
And the average particle size D (μm) in the particle size distribution measurement was 3.5.
≦ D ≦ 7.0, and S and D are D ≧ 9.0S−
1.2, and the average pore diameter P (nm) in the pore distribution measurement is 8 ≦ P ≦ 10.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a positive electrode active material of a lithium secondary battery according to the present invention will be described.

[0008] The positive electrode active material comprises a composite oxide of Li and at least one element selected from Co and Ni. The composite oxide has a specific surface area S by a BET method.
(M ² / g) is 0.45 ≦ S ≦ 0.8, the average particle size D (μm) in the particle size distribution measurement is 3.5 ≦ D ≦ 7.0,
The S and D satisfy D ≧ 9.0S−1.2, and the average pore diameter P (nm) in the pore distribution measurement is 8 ≦
P ≦ 10.

The composite oxide has the general formula Li _x MO
₂ (where M is Co and / or Ni and x is
03 ≦ x ≦ 1.02). Atomic ratio x of lithium in the composite oxide
Becomes smaller when the secondary battery is in a charged state, and becomes larger in a discharged state. The fluctuation range is 0.03 ~
1.02.

[0010] The composite oxide can take a spherical or nearly spherical form.

The specific surface area S is defined in the above range for the following reason. 0.4 specific surface area
If it is less than 5 m ² / g, it becomes difficult to improve the discharge capacity at low temperatures. On the other hand, the specific surface area S is 0.8 m ² / g
Is exceeded, the grain growth of the composite oxide is insufficient.
In the X-ray diffraction measurement, a peak due to the unfired material is shown, and both the discharge capacity at low temperatures and the cycle life are inferior. A more preferable range of the specific surface area S is 0.5 to 0.1.
75 m ² / g.

The average particle diameter D means a particle diameter D50 having a volume accumulation frequency of 50% in the particle size distribution. The reason for defining the average particle diameter D in the above range is as follows. When the average particle diameter D is less than 3.5 μm, the grain growth of the composite oxide is insufficient, so that a peak due to the unfired material is shown in the X-ray diffraction measurement, and both the discharge capacity and the cycle life at low temperature are inferior. It will be. On the other hand, the average particle diameter D
Exceeds 7.0 μm, it becomes difficult to improve the discharge capacity at low temperatures. A more preferable range of the average particle diameter D is
It is 3.6 to 6.5 μm.

In the composite oxide, when the specific surface area S is kept constant and the average particle diameter D is changed, the cycle life fluctuates. When the average particle diameter D is kept constant and the specific surface area S is changed, the low-temperature discharge capacity fluctuates. Tend to. When the average particle size D is (9.
If the value is smaller than the value represented by (0S-1.2), one of the low-temperature discharge capacity and the cycle life decreases.

The reason why the average pore diameter P is defined in the above range is as follows. Average pore diameter P
Is less than 8 nm, the degree of permeation of the active material into the non-aqueous electrolyte decreases, so that it becomes difficult to improve the discharge capacity at low temperatures. On the other hand, when the average pore diameter P exceeds 10 nm, the reaction area of the active material is large, so that a high discharge capacity can be obtained at a low temperature, but the charge / discharge cycle life decreases.
A more preferable range of the average pore diameter P is 8.5 to 9.3.
nm.

The positive electrode active material is produced, for example, by the method described below. After mixing the oxides, hydroxides, carbonates, nitrates, etc. of the respective metals of Li, Ni and Co, they are calcined at 600 to 900 ° C. (more preferably 680 to 900 ° C.) in air or an oxygen atmosphere. I do. The composition of the calcined powder is made uniform by mixing the calcined powder with a solvent. If the calcined powder composition is uneven, it becomes difficult to form fine pores on the surface of the active material. After drying the mixed powder, graphite is further added and mixed. Next, when calcined at 600 to 900 ° C. (more preferably 680 to 900 ° C.) for 12 to 48 hours in air or an oxygen atmosphere, graphite is evaporated after the mixed powder is melted, and fine pores are formed on the particle surface. Thus, the composite oxide described above is obtained. The temperature, time, and atmosphere in the calcination and firing, the mixing ratio of the mixed powder and graphite, and the particle size of graphite are set so that the composite oxide satisfies the above-described conditions. In addition, according to such a method, a composite oxide generally having the composition represented by the general formula described above and having an atomic ratio x of lithium in the range of 0.95 to 1.05 is obtained. In addition, the secondary battery including the positive electrode including the composite oxide having the composition represented by the general formula described above is in a discharged state when the atomic ratio x of lithium is within the above range.

Hereinafter, an example in which the lithium secondary battery according to the present invention is applied to a polymer lithium secondary battery will be described.

As shown in FIG. 1, a polymer lithium secondary battery mainly comprises a positive electrode 1, a negative electrode 2, and an electrolyte layer 3 disposed between the positive electrode 1 and the negative electrode 2. Power generating element. The positive electrode 1 includes a porous current collector 4 and a positive electrode layer 5 bonded to both surfaces of the current collector 4. On the other hand, the negative electrode 2 includes a porous current collector 6 and a negative electrode layer 7 bonded to both surfaces of the current collector 6. The band-shaped positive electrode terminal 8 is obtained by extending the current collector 4 of each of the positive electrodes 1 in a band shape. On the other hand, the strip-shaped negative electrode terminal 9 is obtained by extending the current collector 6 of the negative electrode 2 in a strip shape. For example, a positive electrode lead 10 made of a strip-shaped aluminum plate is connected to the two positive electrode terminals 8. For example, a negative electrode lead (not shown) made of a strip-shaped copper plate is connected to the negative electrode terminal 9. The power generation element having such a configuration is hermetically sealed in a state in which the positive electrode lead 10 and the negative electrode lead extend from the exterior material 11 in an exterior material 11 having a barrier function against moisture, air, and the like.

As the positive electrode, the negative electrode, the electrolyte layer, and the package of the polymer lithium secondary battery, for example, those described below can be used.

(1) Positive Electrode 1 The positive electrode 1 includes a porous current collector 4, a positive electrode active material, a non-aqueous electrolyte, and a polymer holding the electrolyte, which are adhered to both surfaces of the current collector 4. And a positive electrode layer 5.

As the positive electrode active material, a composite oxide of at least one element selected from Co and Ni and Li is used. The composite oxide has a specific surface area S (m ² / g) determined by the BET method of 0.45 ≦ S ≦ 0.8 and an average particle diameter D (μm) in the particle size distribution measurement of 3.5 ≦ D ≦ 7. 0
Wherein the S and D satisfy D ≧ 9.0S-1.2, and the average pore diameter P (nm) in the pore distribution measurement.
Is 8 ≦ P ≦ 10.

The non-aqueous electrolyte is prepared by dissolving an electrolyte in a non-aqueous solvent.

Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and dimethyl carbonate (D
MC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone (γ-
BL), sulfolane, acetonitrile, 1,2-dimethoxyethane, 1,3-dimethoxypropane, dimethyl ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran and the like. The non-aqueous solvents may be used alone or as a mixture of two or more.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ) and lithium hexafluorophosphate (L
iPF _6), boric tetrafluoride lithium (LiBF _4), lithium hexafluoroarsenate (LiAsF _6), lithium salts such as lithium trifluoromethane sulfonate (LiCF ₃ SO ₃₎ may be mentioned.

The amount of the electrolyte dissolved in the non-aqueous solvent is desirably 0.2 mol / l to 2 mol / l.

The polymer desirably has a binding function in addition to the function of holding the non-aqueous electrolyte. Examples of such a polymer include a polyethylene oxide derivative, a polypropylene oxide derivative, a polymer containing the derivative, a polytetrafluoropropylene, a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP), and a polyvinylidene fluoride ( PVdF) or the like can be used. Among them, VdF
—HFP copolymers are preferred.

The positive electrode may include a conductive material from the viewpoint of improving conductivity. Examples of the conductive material include artificial graphite, carbon black (eg, acetylene black), nickel powder, and the like.

As the porous current collector, for example, a mesh made of aluminum or an aluminum alloy, an expanded metal, a punched metal, or the like can be used.

(2) Negative Electrode 2 The negative electrode 2 has a porous current collector 6 and a negative electrode which is laminated on both surfaces of the current collector 6 and contains an active material, a non-aqueous electrolyte and a polymer holding the electrolyte. And a layer 7.

Examples of the negative electrode active material include carbonaceous materials that occlude and release lithium ions. Such carbonaceous materials include, for example, those obtained by firing organic polymer compounds (eg, phenolic resin, polyacrylonitrile, cellulose, etc.), those obtained by firing coke and mesophase pitch, and those made by artificial graphite. And carbonaceous materials represented by natural graphite and the like. Among them, it is preferable to use a carbonaceous material obtained by firing the mesophase pitch at a temperature of 500 ° C. to 3000 ° C. in an inert gas atmosphere such as an argon gas or a nitrogen gas under normal pressure or reduced pressure.

As the non-aqueous electrolyte, those similar to those described above for the positive electrode are used.

The polymer desirably has a binding function in addition to the function of holding the non-aqueous electrolyte. As such a polymer, the same type of polymer as that described for the above-described positive electrode can be used.
HFP copolymers are preferred.

As the porous current collector, for example, a mesh made of copper or a copper alloy, an expanded metal, a punched metal, or the like can be used.

(3) Electrolyte Layer 3 The electrolyte layer 3 contains a non-aqueous electrolyte and a polymer holding the electrolyte.

From the viewpoint of further improving the strength, the electrolyte layer may contain an organic filler or an inorganic filler such as silicon oxide powder.

The polymer desirably has a binding function in addition to the function of holding the non-aqueous electrolyte. As such a polymer, the same type of polymer as that described for the above-described positive electrode can be used.
HFP copolymers are preferred.

(4) Exterior material 11 The exterior material 11 may be formed of a laminated film in which a heat-fusible resin is disposed on the sealing surface and a thin metal film such as aluminum (Al) is interposed therebetween. preferable. Specifically, a laminated film of acid-modified polypropylene (PP) / polyethylene terephthalate (PET) / Al foil / PET laminated from the sealing surface side to the outer surface; a laminated film of acid-modified PE / nylon / Al foil / PET; Laminated film of ionomer / Ni foil / PE / PET; ethylene vinyl acetate (EV
A) A laminated film of / PE / Al foil / PET; an ionomer / PET / Al foil / PET laminated film or the like can be used. Here, the film other than the acid-modified PE, the acid-modified PP, the ionomer, and the EVA on the sealing surface has moisture resistance, air resistance, and chemical resistance.

In FIG. 1 described above, an example in which the current collector of the negative electrode has a porous structure is described, but a metal thin film made of copper or a copper alloy may be used.

Further, in FIG. 1 described above, an example in which a power generating element having a five-layer structure in which a positive electrode, an electrolyte layer, a negative electrode, an electrolyte layer, and a positive electrode are stacked in this order has been described. Alternatively, a power generating element having a three-layer structure including a positive electrode, an electrolyte layer, and a negative electrode may be used. The power generating element having a three-layer structure includes a positive electrode having a structure in which a positive electrode layer is supported on both surfaces of a porous current collector, a negative electrode having a structure in which a negative electrode layer is supported on both surfaces of the porous current collector, and a positive electrode and a negative electrode. A positive electrode having a structure consisting of an electrolyte layer adhered between them, or a structure in which a positive electrode layer is supported on one side of a metal thin film (made of aluminum or aluminum alloy), and one side of a metal thin film made of copper or copper alloy A negative electrode having a structure in which a negative electrode layer is supported thereon, and an electrolyte layer bonded between the positive and negative electrode layers.

In FIG. 1 described above, an example in which the present invention is applied to a polymer lithium secondary battery has been described. Can also be applied.

As described in detail above, according to the lithium secondary battery of the present invention, the specific surface area, the average particle diameter and the average pore diameter of the positive electrode active material are regulated so that the charge / discharge cycle life near room temperature can be attained. Without lowering the discharge capacity at low temperatures.

[0041]

Embodiments of the present invention will be described below in detail.

Example 1 Li ₂ CO ₃ and CoCO ₃
Was weighed so that the molar ratio of lithium to cobalt was 1.02: 1.0, and mixed well in an alumina mortar.
Calcination was performed at 0 ° C. for 12 hours. This calcined powder was mixed with ethanol in a ball mill for 10 hours. Next, after the mixed powder was dried, it was weighed so that the weight ratio of the mixed powder to graphite was 1: 0.15, and mixed in a ball mill for 6 hours. 24 hours at 900 ° C in air
Lithium cobalt oxide was synthesized by firing for an hour.

The obtained lithium cobalt oxide is Li
_x CoO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.53 m
² / g, and the average particle size D in the particle size distribution measurement is 3.6 μm.
m, the average pore diameter P in the pore distribution measurement is 9.29
nm. The value of (9.0S-1.2) was calculated from S, and found to be 3.57. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

Next, a test cell was assembled to measure the cycle characteristics at room temperature and the discharge capacity at low temperature of the obtained lithium cobalt oxide.

First, 80 parts by weight of the lithium cobalt oxide (positive electrode active material), 17 parts by weight of acetylene black as a conductive material, and 3 parts by weight of polytetrafluoroethylene powder as a binder were weighed. After mixing the positive electrode active material and acetylene black using an automatic mortar, polytetrafluoroethylene powder was added and mixed until the polytetrafluoroethylene was sufficiently fiberized. Further, the mixture was rolled with a roll press to form a mixture of 0.25 to 0.27 mm.
, And pressure-bonded to a stainless steel net as a current collector. Thereafter, a positive electrode was prepared by welding a titanium lead.

Subsequently, a vitreous separator was placed between a negative electrode obtained by pressing a lithium metal foil on a nickel mesh body to which a nickel lead was welded and a positive electrode obtained, and these were stored together with a reference electrode in a nonaqueous electrolyte. The test cell was immersed in a glass container. The reference electrode had a structure in which a lithium metal foil was pressure-bonded to a stainless steel mesh, and was immersed in the container in proximity to the positive electrode. As the non-aqueous electrolyte, lithium hexafluorophosphate (L
iPF ₆ ) in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (mixing volume ratio 1: 1)
A solution having a composition of mol / l dissolved was used.

Example 2 Li ₂ CO ₃ and CoCO ₃
Was weighed so that the molar ratio of lithium to cobalt was 0.96: 1.0, and mixed well in an alumina mortar.
Calcination was performed at 0 ° C. for 12 hours. This calcined powder was mixed with ethanol in a ball mill for 10 hours. Next, after the mixed powder was dried, it was weighed so that the weight ratio of the mixed powder and graphite became 1: 0.10, and mixed in a ball mill for 6 hours. 24 hours at 870 ° C in air
Lithium cobalt oxide was synthesized by firing for an hour.

The obtained lithium cobalt oxide is Li
_x CoO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.79 m
² / g, and the average particle size D in the particle size distribution measurement is 6.1 μm.
m, the average pore diameter P in the pore distribution measurement is 8.53.
nm. The value of (9.0S-1.2) was calculated from S, and found to be 5.91. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium cobalt oxide was used.

Example 3 Li ₂ CO ₃ and CoCO ₃
Was weighed so that the molar ratio of lithium to cobalt was 0.99: 1.0, and mixed well in an alumina mortar.
Calcination was performed at 0 ° C. for 12 hours. This calcined powder was mixed with ethanol in a ball mill for 10 hours. Next, after the mixed powder was dried, it was weighed so that the weight ratio of the mixed powder and graphite became 1: 0.08, and mixed with a ball mill for 4 hours. 850 ° C in an oxygen atmosphere
For 24 hours to synthesize lithium cobalt oxide.

The obtained lithium cobalt oxide is Li
_x CoO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.66 m
² / g, and the average particle size D in the particle size distribution measurement is 7.0 μm.
m, the average pore diameter P in the pore distribution measurement is 8.09
nm. The value of (9.0S-1.2) was calculated from S, and found to be 4.74. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium cobalt oxide was used.

Comparative Example 1 Li ₂ CO ₃ and CoCO ₃
Was weighed so that the molar ratio of lithium to cobalt was 0.96: 1.0, and mixed well in an alumina mortar.
Calcination was performed at 0 ° C. for 12 hours. This calcined powder was fired in air at 850 ° C. for 24 hours to synthesize lithium cobalt oxide.

The obtained lithium cobalt oxide is Li
_x CoO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02) and the specific surface area S by the BET method is 0.40 m
² / g, the average particle size D in the particle size distribution measurement is 7.3 μm.
m, the average pore diameter P in the pore distribution measurement is 7.56
nm. The value of (9.0S-1.2) calculated from S was 2.4. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium cobalt oxide was used.

Comparative Example 2 Li ₂ CO ₃ and CoCO ₃
Was weighed so that the molar ratio of lithium to cobalt was 1.01: 1.0 and mixed well in an alumina mortar.
Calcination was performed at 0 ° C. for 3 hours. This calcined powder was fired at 780 ° C. for 12 hours in an oxygen atmosphere to synthesize lithium cobalt oxide.

The obtained lithium cobalt oxide is Li
_x CoO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.30 m
² / g, and the average particle size D in the particle size distribution measurement is 7.8 μm.
m, the average pore diameter P in the pore distribution measurement is 6.24.
nm. The value of (9.0S-1.2) was calculated from S and found to be 1.5. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium cobalt oxide was used.

Comparative Example 3 Li ₂ CO ₃ and CoCO ₃
Was weighed so that the molar ratio of lithium to cobalt was 1.05: 1.0, and mixed well in an alumina mortar.
Calcination was performed at 0 ° C. for 6 hours. The calcined powder was mixed with ethanol in a ball mill for 6 hours. Next, after the mixed powder was dried, it was weighed so that the weight ratio of the mixed powder to graphite was 1: 0.2, and mixed by a ball mill for 6 hours. Further, at 870 ° C. in an oxygen atmosphere, 12
Lithium cobalt oxide was synthesized by firing for an hour.

The obtained lithium cobalt oxide is Li
_x CoO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.60 m
² / g, and the average particle size D in the particle size distribution measurement is 8.0 μm.
m, the average pore diameter P in the pore distribution measurement is 9.67
nm. The value of (9.0S-1.2) calculated from S was 4.2. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium cobalt oxide was used.

Comparative Example 4 Li ₂ CO ₃ and CoCO ₃
Was weighed so that the molar ratio of lithium to cobalt was 0.99: 1.0, and mixed well in an alumina mortar.
Calcination was performed at 0 ° C. for 3 hours. This calcined powder was fired in an oxygen atmosphere at 900 ° C. for 12 hours to synthesize lithium cobalt oxide.

The obtained lithium cobalt oxide is Li
_x CoO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.47 m
² / g, and the average particle size D in the particle size distribution measurement is 6.6 μm.
m, the average pore diameter P in the pore distribution measurement is 10.1
4 nm. When the value of (9.0S-1.2) was calculated from S, it was 3.03. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium cobalt oxide was used.

(Cycle test at room temperature) The test cells obtained in Examples 1 to 3 and Comparative Examples 1 to 4 were charged at a constant current density of 1.0 mA / cm ^{2 in} a constant temperature room maintained at 20 ° C. When the voltage reaches 4.3 V, charging is terminated and
After a pause of 1 minute, discharge was performed at a constant current density of 1.0 mA / cm ² , and when the voltage reached 3.0 V, the discharge was terminated.
A cycle test was performed in which a 10-minute pause was performed before switching to the next charge. Figure 2 shows the results of such a cycle test.
Shown in However, in FIG. 2, the horizontal axis represents the number of cycles, and the vertical axis represents the capacity retention ratio (%) with respect to the discharge capacity in the first cycle.
Is shown.

(Capacity Measurement at Low Temperature) The test cells of Examples 1 to 3 and Comparative Examples 1 to 4 were charged at a constant current density of 1.0 mA / cm ^{2 in} a constant temperature room maintained at 20 ° C. 4.
When the voltage reaches 3 V, constant voltage charging is performed, and the current is 10 μm
The charge was terminated when the charge / discharge rate reached A / cm ² or less. Next, in a thermostat kept at −10 ° C., 1.0 mA / cm ²
Was discharged at a constant current density of 3.0 V. When the voltage reached 3.0 V, the discharge was terminated, and the ratio of the discharge capacity to the charge capacity at room temperature was determined. The results are shown in Table 1 below.

[0067]

[Table 1]

As apparent from FIG. 2, the specific surface area S (m ² / g) by the BET method is 0.45 ≦ S ≦ 0.8, and the average particle diameter D (μm) in the particle size distribution measurement is 3.5. ≦ D ≦
At 7.0, the S and the D satisfy D ≧ 9.0S-1.2, and the average pore diameter P (n
m) The secondary batteries of Examples 1 to 3 provided with a positive electrode including a lithium cobalt oxide in which 8 ≦ P ≦ 10, wherein any one of the specific surface area S, the average particle diameter D, and the average pore diameter P is as described above. It can be seen that the cycle life at room temperature is longer than those of the secondary batteries of Comparative Examples 1 to 4 including the composite oxides that do not satisfy the above conditions.

As is clear from Table 1, Example 1
It can be seen that the secondary batteries of Nos. 1 to 3 have higher discharge capacities at low temperatures than the secondary batteries of Comparative Examples 1 to 3.

Therefore, it can be seen from FIG. 2 and Table 1 that the low-temperature discharge characteristics of the secondary batteries of Examples 1 to 3 can be improved while maintaining the cycle characteristics at room temperature.

Example 4 LiNO ₃ and NiCO ₃ were weighed so that the molar ratio of lithium to nickel was 1.01: 1 and mixed well in an alumina mortar.
It was calcined for 2 hours. The calcined powder was mixed with ethanol in a ball mill for 6 hours. Next, after the mixed powder was dried, it was weighed so that the weight ratio of the mixed powder and graphite became 1: 0.1, and mixed with a ball mill for 4 hours. Further, by baking at 780 ° C. for 24 hours in an oxygen atmosphere, lithium nickel oxide was synthesized.

The obtained lithium nickel oxide is Li
_x NiO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02) and the specific surface area S by the BET method is 0.79 m
² / g, and the average particle size D in the particle size distribution measurement is 6.8 μm.
m, the average pore diameter P in the pore distribution measurement is 8.50
nm. The value of (9.0S-1.2) was calculated from S, and found to be 5.91. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium nickel oxide was used.

Example 5 Li ₂ CO ₃ and NiCO ₃ were weighed so that the molar ratio of lithium to nickel was 1.01: 1, mixed well in an alumina mortar, and calcined at 600 ° C. for 12 hours. Did. The calcined powder was mixed with ethanol in a ball mill for 6 hours. Next, after the mixed powder was dried, it was weighed so that the weight ratio of the mixed powder and graphite became 1: 0.08, and mixed with a ball mill for 4 hours. Further, by baking at 820 ° C. for 24 hours in an oxygen atmosphere, lithium nickel oxide was synthesized.

The obtained lithium nickel oxide is Li
_x NiO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.68 m
² / g, and the average particle size D in the particle size distribution measurement is 6.5 μm.
m, the average pore diameter P in the pore distribution measurement is 9.23
nm. The value of (9.0S-1.2) calculated from S was 4.92. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium nickel oxide was used.

Example 6 Li ₂ CO ₃ and NiCO ₃ were weighed so that the molar ratio of lithium to nickel was 1.02: 1, mixed well in an alumina mortar, and calcined at 600 ° C. for 12 hours. Did. The calcined powder was mixed with ethanol in a ball mill for 6 hours. Next, after the mixed powder was dried, it was weighed so that the weight ratio of the mixed powder to graphite was 1: 0.10, and mixed in a ball mill for 4 hours. Further, by baking at 800 ° C. for 24 hours in an oxygen atmosphere, lithium nickel oxide was synthesized.

The obtained lithium nickel oxide is composed of Li
_x NiO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.75 m
² / g, and the average particle size D in the particle size distribution measurement is 6.2 μm.
m, the average pore diameter P in the pore distribution measurement is 8.34
nm. When the value of (9.0S-1.2) was calculated from S, it was 5.55. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium nickel oxide was used.

Comparative Example 5 Li ₂ CO ₃ and NiCO ₃ were weighed so that the molar ratio of lithium to nickel was 1: 1 and mixed well in an alumina mortar, and then calcined at 600 ° C. for 12 hours. . This calcined powder was mixed with ethanol in a ball mill for 3 hours. Next, after the mixed powder was dried, the weight ratio of the mixed powder to graphite was 1: 1.
It was weighed so as to be 0.10 and mixed by a ball mill for 4 hours. Further, by baking at 870 ° C. for 24 hours in an oxygen atmosphere, lithium nickel oxide was synthesized.

The obtained lithium nickel oxide is Li
_x NiO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.52 m
² / g, and the average particle size D in the particle size distribution measurement is 14.7.
μm, the average pore diameter P in the pore distribution measurement is 6.3.
It was 2 nm. The value of (9.0S-1.2) was calculated from S and found to be 3.48. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium nickel oxide was used.

Comparative Example 6 LiOH and Ni (OH) ₂
Was weighed so that the molar ratio of lithium to nickel was 1.05: 1, and mixed well in an alumina mortar.
For 6 hours. This calcined powder was mixed with ethanol in a ball mill for 3 hours, and then calcined at 850 ° C. for 12 hours in an oxygen atmosphere to synthesize lithium nickel oxide.

The obtained lithium nickel oxide is composed of Li
_x NiO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02) and the specific surface area S by the BET method is 0.47 m
² / g, the average particle size D in the particle size distribution measurement is 11.2
μm, the average pore diameter P in the pore distribution measurement is 7.8.
It was 5 nm. When the value of (9.0S-1.2) was calculated from S, it was 3.03. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium nickel oxide was used.

Comparative Example 7 LiOH and Ni (OH) ₂
Was weighed so that the molar ratio of lithium to nickel was 1.02: 1, and mixed well in an alumina mortar.
Was calcined for 3 hours. This calcined powder was mixed with ethanol in a ball mill for 3 hours. Next, after the mixed powder was dried, it was weighed so that the weight ratio of the mixed powder and graphite became 1: 0.15, and mixed with a ball mill for 4 hours. Further, by baking in air at 750 ° C. for 24 hours, lithium nickel oxide was synthesized.

The obtained lithium nickel oxide is composed of Li
_x NiO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 1.3 m ^2.
/ G, the average particle size D in the particle size distribution measurement is 6.5 μm
And the average pore diameter P in the pore distribution measurement is 11.56.
nm. When the value of (9.0S-1.2) was calculated from S, it was 10.5.

A test cell was assembled in the same manner as in Example 1 except that such a lithium nickel oxide was used.

Comparative Example 8 Li ₂ NO ₃ and NiCO ₃ were weighed so that the molar ratio of lithium to nickel was 1: 1 and mixed well in an alumina mortar, and then calcined at 600 ° C. for 6 hours. . This calcined powder is heated at 700 ° C in air.
For 12 hours to synthesize lithium nickel oxide.

The obtained lithium nickel oxide is composed of Li
_x NiO ₂ (x is in the range of 0.03 ≦ x ≦ 1.02), and the specific surface area S by the BET method is 0.88 m
² / g, and the average particle size D in the particle size distribution measurement is 8.6 μm.
m, the average pore diameter P in the pore distribution measurement is 7.5 n
m. The value of (9.0S-1.2) was calculated from the above S and found to be 6.72. Therefore, the oxide satisfies D ≧ 9.0S-1.2.

A test cell was assembled in the same manner as in Example 1 except that such a lithium nickel oxide was used.

The charge / discharge cycle characteristics and low-temperature discharge characteristics of the obtained secondary batteries of Examples 4 to 6 and Comparative Examples 5 to 8 were measured in the same manner as described above, and the results of the cycle test were shown in FIG. Table 2 shows the results of the low-temperature discharge test. However,
In FIG. 3, the horizontal axis indicates the number of cycles, and the vertical axis indicates the capacity retention ratio (%) with respect to the discharge capacity in the first cycle.

[0093]

[Table 2]

As is clear from FIG. 3, the specific surface area S (m ² / g) by the BET method is 0.45 ≦ S ≦ 0.8, and the average particle diameter D (μm) in the particle size distribution measurement is 3.5. ≦ D ≦
At 7.0, the S and the D satisfy D ≧ 9.0S-1.2, and the average pore diameter P (n
m) The secondary batteries of Examples 4 to 6 provided with a positive electrode containing a lithium nickel oxide in which 8 ≦ P ≦ 10 have any one of the specific surface area S, the average particle diameter D, and the average pore diameter P described above. It can be seen that the cycle life at room temperature is longer than those of the secondary batteries of Comparative Examples 5 to 8 provided with composite oxides that do not satisfy the above conditions.

Further, as is apparent from Table 2, Example 4
6 shows that the secondary batteries of Comparative Examples 5 to 7 have higher discharge capacities at low temperatures than the secondary batteries of Comparative Examples 5 to 7.

Accordingly, it can be seen from FIG. 3 and Table 2 that the secondary batteries of Examples 4 to 6 can improve the low-temperature discharge characteristics while securing the cycle characteristics at room temperature.

(Example 7) <Preparation of positive electrode not impregnated with non-aqueous electrolyte> As an active material, 56% by weight of lithium cobalt oxide and 5% by weight of carbon black as described in Example 1 were used. And vinylidene fluoride-hexafluoropropylene (VdF-H) as a polymer having a function of retaining a non-aqueous electrolyte.
A paste was prepared by mixing 17% by weight of the copolymer powder of FP) and 22% by weight of dibutyl phthalate (DBP) in acetone. The obtained paste was applied on a PET film to prepare a positive electrode sheet not impregnated with a non-aqueous electrolyte. The obtained positive electrode sheet was heated and pressed on both surfaces of a current collector made of expanded metal made of aluminum with a hot roll, and then cut to prepare a positive electrode not impregnated with a nonaqueous electrolyte.

<Preparation of Negative Electrode Impregnated with Nonaqueous Electrolyte> Mesophase pitch carbon fiber was pulverized as an active material,
A powder heat-treated at ℃ was prepared. 58% by weight of this powder
And 17% by weight of VdF-HFP copolymer powder and 25% by weight of dibutyl phthalate (DBP) were mixed in acetone to prepare a paste. The obtained paste was applied on a PET film to produce a non-aqueous electrolyte-unimpregnated negative electrode sheet. The obtained negative electrode sheet was heated and pressed on both surfaces of a current collector made of copper expanded metal with a hot roll, and then cut to prepare a non-aqueous electrolyte-unimpregnated negative electrode.

<Preparation of electrolyte layer not impregnated with nonaqueous electrolyte> 33.3 parts by weight of silicon oxide powder, 22.2 parts by weight of VdF-HFP copolymer powder, and dibutyl phthalate (DB)
P) 44.5 parts by weight were mixed in acetone to form a paste. The obtained paste was applied on a PET film, formed into a sheet, and cut to form an electrolyte layer not impregnated with a non-aqueous electrolyte.

<Preparation of Nonaqueous Electrolyte> A nonaqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 2: 1 was mixed with LiPF ₆ as an electrolyte at a concentration of 1 mol / mol. 1 to prepare a non-aqueous electrolyte.

<Battery assembly> A positive electrode not impregnated with a non-aqueous electrolyte was
Sheets, one non-aqueous electrolyte impregnated negative electrode and two non-aqueous electrolyte non-impregnated electrolyte layers were prepared, and these were laminated so that the electrolyte layer was interposed between the positive electrode and the negative electrode. Then, by heating and pressing with a heated rigid roll, a power generating element not impregnated with a non-aqueous electrolyte was produced.

[0102] Such a power generating element was immersed in n-decane, and left with stirring with a magnetic stirrer.
This operation was repeated until the concentration of DBP in methanol by gas chromatography became 20 ppm or less, whereby the plasticizer in the power generating element was removed. After the power generating element was dried, a non-aqueous electrolyte having the above composition was injected into the power generating element, and a polyethylene terephthalate (PET) film, an aluminum foil, and a heat-fusible resin film were laminated in this order from the outermost layer. A polymer lithium secondary battery having the above-described structure shown in FIG. 1 and having a theoretical capacity of 86 mAh was manufactured by sealing with a packaging material made of a laminate film.

Comparative Example 9 A polymer lithium secondary battery was prepared in the same manner as in Example 7 except that the same lithium cobalt oxide as that described in Comparative Example 1 was used as the positive electrode active material. A secondary battery was manufactured.

The obtained secondary batteries of Example 7 and Comparative Example 9 were charged at a constant current of 86 mA to 4.5 V in a constant temperature room maintained at 20 ° C., and then charged at a constant current of 86 mA.
A charge / discharge cycle of discharging to 0 V was performed. 1 at this time
When the capacity retention ratio (relative to the initial capacity) at the 00th cycle was measured, the secondary battery of Example 7 was 85%, while the secondary battery of Comparative Example 9 was 70%.

The secondary batteries of Example 7 and Comparative Example 9 were charged at a constant current of 86 mA to 4.5 V in a constant temperature chamber maintained at 20 ° C., and then charged in a constant temperature chamber maintained at −10 ° C. The battery was discharged to 3.0 V at a constant current of 86 mA, and the ratio of the discharge capacity to the charge capacity at room temperature was determined. The ratio of the secondary battery of Example 7 was 73%, while that of Comparative Example 9 was 73%. The secondary battery was 35%.

In the above-described embodiment, an example in which the present invention is applied to a composite oxide of Co or Ni and Li has been described. However, a similar effect can be obtained with a composite oxide of Co, Ni and Li. It was confirmed.

[0107]

As described above in detail, according to the present invention, it is possible to provide a lithium secondary battery capable of improving low-temperature discharge characteristics while maintaining a long life.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a polymer lithium secondary battery which is an example of a lithium secondary battery according to the present invention.

FIG. 2 is a characteristic diagram showing the relationship between the number of cycles and the capacity retention in the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4.

FIG. 3 is a characteristic diagram showing the relationship between the number of cycles and the capacity retention in the lithium secondary batteries of Examples 4 to 6 and Comparative Examples 5 to 8.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Positive electrode, 2 ... Negative electrode, 3 ... Electrolyte layer, 4 ... Positive electrode collector, 5 ... Positive electrode layer, 6 ... Negative electrode current collector, 7 ... Negative electrode layer, 11 ... Exterior material.

Claims (2)

[Claims] 1. At least one selected from Co and Ni
A lithium secondary battery including a positive electrode including a composite oxide of a kind of element and Li, wherein the composite oxide has a specific surface area S (m ² /
g) is 0.45 ≦ S ≦ 0.8, the average particle diameter D (μm) in the particle size distribution measurement is 3.5 ≦ D ≦ 7.0, and the S and D are D ≧ 9.0S−1. .2 and the average pore diameter P (nm) in the pore distribution measurement is 8 ≦ P ≦ 10
A lithium secondary battery characterized by the following. 2. The composite oxide of the general formula Li _x MO ₂
(Where M is Co and / or Ni and x is 0.0
The lithium secondary battery according to claim 1, wherein the lithium secondary battery has a composition represented by the following formula: 3 ≦ x ≦ 1.02).