JPH11154512A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery that can improve its reliability during high temperature storage by using a composite oxide, as a positive electrode active material, in which the content of sulfur is not more than 4000 ppm and which includes lithium and manganese. SOLUTION: In a nonaqueous electrolyte secondary battery that includes a negative electrode active material capable of occluding and effusing lithium ions, a lithium ion conductive nonaqueous electrolyte and a positive electrode active material formed from a lithium containing metal oxide capable of occluding and effusing lithium ions, this nonaqueous electrolyte secondary battery is characterized by such features that the lithium containing metal oxide is a composite oxide including spinel group lithium and manganese that is expressed by a general formula, Li[Lix Mn2-x ]O4 , where the value x is in the range of 0<=x<=0.18 and the content of sulfur contained in the composite oxide is not more than 4000 ppm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of storage characteristics of a non-aqueous electrolyte secondary battery using a composite oxide of lithium and manganese as a positive electrode active material.

[0002]

2. Description of the Related Art With the recent development of electronic technology, devices have been reduced in size and weight at a surprising speed. For this reason, mobile devices such as mobile communication devices and portable computers have begun to spread widely, and secondary batteries having a high energy density have been demanded as power sources for these mobile devices. Above all, the non-aqueous electrolyte secondary battery has a higher voltage than conventional nickel-cadmium batteries and nickel-metal hydride batteries, so that the energy density per weight and volume is improved. Therefore, there is a strong demand for a power source that can be expected to further reduce the size and weight of the device. However, in non-aqueous electrolyte secondary batteries using lithium metal and lithium alloy as the negative electrode material, when charge and discharge are repeated, lithium dendrites are formed on the negative electrode, resulting in poor cycle performance and reliability at high temperatures. It was not easily put into practical use due to problems such as the nature of the product.

As means for solving these problems, a carbon material capable of inserting and extracting lithium as a negative electrode active material and a nonaqueous electrolyte using a composite oxide of lithium and a transition metal having a layered structure as a positive electrode active material are used. Battery (Patent No. 19
No. 89293), which has a voltage of 4 V or more in a charged state, and is widely used as a power source for mobile devices. However, current non-aqueous electrolyte secondary batteries are expensive because they contain a large amount of cobalt, and there has been a limit in reducing the price as a power source.
For this reason, attempts to replace cobalt with nickel or manganese are active. In particular, manganese, which is inexpensive among transition metals, is most expected to replace cobalt.

[0004] Lithium manganese oxide having a stoichiometric composition has poor cycle performance, and as a method for improving this, it has been proposed to replace part of manganese with lithium as disclosed in, for example, JP-A-5-205744. Have been. Such a lithium manganese oxide in which a part of manganese is replaced by lithium cannot be obtained unless a Mn raw material and a Li raw material are mixed at a desired ratio and heat-treated at a relatively low temperature of 700 to 750 ° C or lower. This is 800-9
When heat treatment is performed at a high temperature of about 50 ° C., desired Li / Mn
This is because a lithium manganese oxide having a ratio of spinel and a single phase cannot be obtained. The non-aqueous electrolyte secondary battery using the lithium manganese oxide thus obtained is
Since the internal impedance increases during storage at high temperatures, the capacity greatly decreases after storage, and the container swells, it lacks reliability particularly during storage at high temperatures, and is insufficient as a power source for portable devices.

[0005]

Lithium manganese oxide in which part of manganese has been replaced by lithium has a certain effect in preventing an increase in internal impedance near room temperature, but is not yet sufficient. Furthermore, in a more severe situation where the temperature during storage is high, there is a problem that the internal impedance increases and the capacity of the battery decreases, and the battery swells during storage. An object of the present invention is to provide a non-aqueous electrolyte secondary battery having improved reliability during high-temperature storage by using a composite oxide containing lithium and manganese manufactured by a specific method.

[0006]

Means for Solving the Problems The heat treatment conditions of the spinel-based composite oxide of lithium and manganese, the method of substituting a part of manganese with lithium, and the cause of lowering the reliability during high-temperature storage were investigated. As a result, they have found that a composite oxide containing lithium and manganese with a reduced sulfur content is particularly suitable as a positive electrode material for a non-aqueous electrolyte secondary battery, and have reached the present invention.

The present invention provides (1) a negative electrode active material capable of inserting and extracting lithium ions, a non-aqueous electrolyte having lithium ion conductivity, and a lithium-containing metal oxide capable of inserting and extracting lithium ions. In a non-aqueous electrolyte secondary battery provided with a positive electrode active material made of a material, the lithium-containing metal oxide has a general formula Li [Li _x Mn _2-x ] O ₄ (where O ≦ x ≦ 0.18 A) is a spinel-based composite oxide containing lithium and manganese,
A non-aqueous electrolyte secondary battery characterized in that sulfur contained in the composite oxide is 4000 ppm or less; (2) a general formula Li [Li _x Mn _2-x ] O ₄ (where O ≦ x ≦ 0. 1
8) in the range of O ≦ x ≦ 0.18.
The nonaqueous electrolyte secondary battery according to the above (1), wherein a cleaning treatment is performed after the heat treatment described above, and then a second heat treatment is performed.

Hereinafter, the present invention will be described specifically.
The manganese raw material of the lithium manganese oxide used in the present invention is EMD (Electrolytic Manga).
(nice Dioxide) is preferred. Lithium raw materials include, for example, Li ₂ CO ₃ , LiOH, LiCl, Li
NO ₃ , Li ₂ SO ₄ and CH ₃ COOLi can be mentioned, but Li ₂ CO ₃ is preferable. When a part of Mn is substituted with a transition metal, an oxide of the transition metal to be substituted can be mixed with EMD so as to have a desired Mn / M ratio.

[0009] The composite oxide containing lithium and manganese used in the present invention is prepared as follows. EMD and Li ₂ pulverized to an average particle size of 5 to 25 μm
CO ₃ is preliminarily prepared in a desired Li / Mn ratio, preferably in a state where Li is less than desired, more preferably Li / M
The first heat treatment is performed by mixing so that the n ratio becomes 0.5 to 0.55. The first heat treatment temperature is preferably 800
950 ° C., more preferably 850-900 ° C.
It is. If the heat treatment temperature is lower than 800 ° C., the sulfate groups contained in EMD remain undesirably even in the subsequent cleaning. On the other hand, when the temperature is 950 ° C. or higher, the amount of sub-phases other than spinel increases, which is not preferable. Li
/ Mn ratio is also high because of heat treatment at high temperature.
When the / Mn ratio is large, a sub-phase such as Li ₂ MnO ₃ is likely to appear, and therefore, it is preferable to perform the heat treatment in a state where Li is smaller than a desired Li / Mn ratio. Also, Li / Mn
If the ratio is less than 0.5, sub-phases such as Mn ₂ O ₃ and Mn ₃ O ₄ appear, which is also not preferable.

[0010] The obtained composite oxide containing lithium and manganese is washed after the first heat treatment. The sulfate group contained in EMD by the first heat treatment is converted to water-soluble Li
Changes to ₂ SO ₄ . Therefore, Li ₂
SO ₄ can be removed, and the amount of sulfur contained in the composite oxide can be reduced. The washed composite oxide and a Li material such as Li ₂ CO ₃ are mixed so as to have a final desired Li / Mn ratio, and a second heat treatment is performed. The second heat treatment is performed at 550 to 750 ° C.
It is preferable to carry out at a relatively low temperature. If the temperature is 550 ° C. or lower and 750 ° C. or higher, the finally obtained composite oxide containing lithium and manganese is not a single spinel phase, which is not preferable. Further, when the composite oxide containing lithium and manganese after washing already has a final desired amount of Li / Mn ratio, the second raw material such as Li ₂ CO ₃ is mixed without mixing the Li raw material such as Li ₂ CO ₃ . Is performed.

In order to obtain a composite oxide containing lithium and manganese according to the present invention, it is possible to remove the sulfur originating from Li ₂ SO ₄ by performing a final heat treatment and then washing. In this case, not only Li ₂ SO ₄ but also a small amount of lithium and manganese are eluted from the crystal structure by washing, so that it is difficult to obtain a composite oxide having a desired Li / Mn ratio, Eventually, the battery capacity will vary. Further, since a drying step is required after the cleaning and the manufacturing cost is increased, it is difficult to reduce the price of the power supply even when changing from cobalt to manganese.

Further, when cleaning is performed at the final stage of the heat treatment, B
The ET specific surface area is increased about twice, and storage characteristics are reduced as compared with the case where the ET is washed during the heat treatment. on the other hand,
When the Li ₂ SO ₄ is removed by washing after the first heat treatment, and then a Li salt is added so as to have a desired Li amount and then the second heat treatment is performed, the increase in the BET specific surface area is suppressed. It is possible. Therefore, it is most desirable to perform a cleaning process between heat treatments. The negative electrode material used in the present invention is not particularly limited as long as it can occlude and release lithium in an ion state. Examples thereof include carbon materials such as coke, natural graphite, artificial graphite, and non-graphitizable carbon; metal oxides such as SiSnO; _Although a metal nitride such as ₂ can be used, a carbon material is preferable.

In the present invention, when the active material is formed into an electrode, a conductive agent can be added as necessary, and the active material can be fixed to the current collector with a binder. Examples of the conductive agent include natural graphite, artificial graphite, carbon black, Ketjen black, and acetylene black, and graphite or a combination of graphite and acetylene black is preferred. Although the addition amount is not particularly limited, it is preferably 1 to 20% by weight, more preferably 3 to 10% by weight. 1% by weight
The following conductivity is not uniform, and when it is 20% by weight or more, the capacity per unit volume decreases. As the binder, polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene terpolymer, carboxymethylcellulose, styrene-butadiene rubber, fluororubber and the like are usually used alone or in a mixture, but there is no particular limitation. Not done. The amount of these additives is preferably 1 to 20% by weight, more preferably 1 to 10% by weight.
At 1% by weight or less, the binding strength is weak, and at 20% by weight or more, L
The movement of i-ions is hindered, and the performance as a battery is reduced.

As the electrolytic solution, a mixture in which a lithium salt is used as an electrolyte and dissolved in various organic solvents is used. The electrolyte is not particularly limited.
_{O 4, LiBF 4, LiPF 6} , LiAsF 6, LiC
A single or a mixture such as F ₃ SO ₃ can be used. Also, the organic solvent is not particularly limited,
For example, propylene carbonate, ethylene carbonate, γ-butyrolactone, dimethyl carbonate,
Diethyl carbonate, methyl ethyl carbonate,
A single solvent such as 1,2-dimethoxyethane and tetrahydrofuran or a mixture of two or more solvents can be used.

[0015]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to specific examples, but the present invention is not limited to these examples. Example 1 EMD having an average particle size of 20 μm as a starting material
And Li ₂ CO ₃ with Li / Mn = 0.55 (atomic ratio)
, And heat-treated in air at 900 ° C. for 20 hours, and then cooled to around room temperature. The obtained oxide and purified water were mixed at a weight ratio of 1:50, stirred for 12 hours, and then filtered. During this time, the temperature of the purified water was kept at 90 ° C. Then, the filtered oxide and Li ₂ CO
₃ was mixed so as to have a composition ratio of Li / Mn = 0.61 (atomic ratio), and again heat-treated in air at 650 ° C. for 20 hours to obtain a lithium manganese oxide. As a result of a chemical analysis, the obtained lithium manganese oxide was represented by the general formula Li [Li _0.11 Mn _1.89 ] O ₄ , contained 100 ppm of sulfur, and had a BET specific surface area of 0.4 m ² / g.
Met. The saturated water content was 1200 ppm.

A description will now be given of a specific battery production in the present invention. After mixing 3 parts by weight of acetylene black and 3 parts by weight of scale-like natural graphite as a conductive agent with respect to the lithium manganese oxide 100, polyvinylidene fluoride was mixed at a ratio of 3 parts by weight with respect to the total weight, and N-methylpyrrolidone was added. (NMP) was added to perform wet mixing to obtain a paste. Next, this paste is coated with a 20 μm thick positive electrode current collector.
m was uniformly applied to both sides of an aluminum foil, dried, and then pressure-formed by a roller press to form a belt-shaped positive electrode.

Next, 1 part by weight of carboxymethylcellulose, 2 parts by weight of styrene-butadiene rubber, and purified water as a solvent were added to a mixture of 95% by weight of mesocarbon fiber graphitized at 3000 ° C. and 5% by weight of scale-like natural graphite. To obtain a paste. This paste was uniformly applied to both sides of a copper foil having a thickness of 12 μm as a negative electrode current collector, dried, and then pressed and formed by a roller press to form a strip-shaped negative electrode. Furthermore, a 25-μm-thick polyethylene microporous membrane was sandwiched between the positive electrode and the negative electrode as a separator to form a roll.

An insulating film was inserted into the bottom of a nickel-plated iron square can, and the wound body was crushed and inserted. Next, the negative electrode tab taken out from the wound body was welded to a closing lid having an explosion-proof disc, and the positive electrode tab was welded to the positive electrode pin of the closing lid. Next, the joint between the closing lid and the square can was welded by laser. Through a small hole having a diameter of 1 mm opened in the closing lid, 1 mol / mol of a mixed solvent of ethylene carbonate and diethyl carbonate was introduced into the battery can.
An electrolyte solution in which LiPF ₆ was dissolved at a concentration of 1 liter was injected, and the small hole was closed by welding to obtain an 8.6 mm thick, 34
A square non-aqueous electrolyte secondary battery having a size of 48 mm × 48 mm was prepared.

Example 2 The starting material had an average particle size of 10
μm EMD and Li ₂ CO ₃ with Li / Mn = 0.51
(Atomic ratio), heat-treated in air at 850 ° C. for 20 hours, and cooled down to around room temperature. The obtained oxide and purified water were mixed at a weight ratio of 1:20, stirred for 24 hours, and then filtered. During this time, the temperature of the purified water was kept at room temperature. The powder obtained by filtration was mixed again, and heat-treated at 650 ° C. for 20 hours in the air to obtain a lithium manganese oxide. As a result of chemical analysis, the obtained lithium manganese oxide was represented by the general formula Li [Li _0.07 Mn _1.93 ] O ₄ , and sulfur was 850 p
pm, the BET specific surface area was 0.8 m ² / g, and the saturated water content was 2700 ppm. Using the obtained lithium manganese oxide, a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1.

Examples 3, 4 and 5 As starting materials, EMD having an average particle size of 10 μm and Li ₂ CO ₃ were mixed with Li / Mn =
Mix at a composition ratio of 0.52 (atomic ratio) and 850 ° C in air
At room temperature for 20 hours, and the temperature was lowered to around room temperature. The obtained oxide and purified water were mixed at a weight ratio of 1:20, stirred for 1 hour, 5 hours, and 12 hours, and then each was filtered. During this time, the temperature of the purified water was kept at room temperature. The powder obtained by filtration was mixed again, and heat-treated at 650 ° C. for 20 hours in the air to obtain a lithium manganese oxide. As a result of the chemical analysis, the obtained lithium manganese oxide was represented by the general formula Li [Li _0.07 Mn _1.93 ] O ₄ , and the sulfur content was:
3800 ppm when the stirring time was 1 hour, 2900 ppm when the stirring time was 5 hours, and 1 when the stirring time was 12 hours.
It was 500 ppm. The BET specific surface area is 0.8
m ² / g. Using the obtained lithium manganese oxide, a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1.

(Comparative Example 1) A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 2 except that the lithium manganese oxide was not washed with hot water.

Comparative Example 2 E of 20 μm as a starting material
MD and Li ₂ CO ₃ were mixed so that Li / Mn = 0.61, and heat-treated at 650 ° C. for 20 hours to obtain a lithium manganese oxide. As a result of the chemical analysis, the obtained lithium manganese oxide was represented by the general formula Li [Li _0.13 Mn
_1.87 ] O ₄ , containing 4500 ppm of sulfur, BE
The T specific surface area is 1.8 m ² / g and the saturated water content is 80
It was 00 ppm. Using this lithium manganese oxide, a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1.

(Test Results) The batteries prepared in the above Examples and Comparative Examples each had a charging voltage of 4.2 V after a 24-hour aging period for the purpose of stabilizing the inside of the batteries.
And charging was performed in 5 hours. Then 400mA
The battery was discharged at a constant current of 3.0 V to 3.0 V. For further stabilization, 10 charge / discharge cycles of 4.2 V charge and 3.0 V discharge were performed, and then a test battery was obtained. The final amount of electricity discharged at this time is calculated as the reference capacity (X) of each test battery.
And

In the charge storage test, the battery was charged in 3 hours with the charging voltage set to 4.2 V, the thickness of the can was measured, and then the battery was placed in a thermostat set at 85 ° C. for 120 hours. Removed after hours. The battery taken out was naturally cooled to around room temperature, the thickness of the can was measured, the battery was discharged to 3.0 V, and then the battery was charged at 4.2 V and discharged at 3.0 V. The latter discharge amount was defined as the recovery discharge capacity (Y) of the test battery. Based on these, the capacity recovery rate of each battery after storage was calculated according to the following equation. (Y / X) × 100 In the discharge preservation test, the thickness of the can was measured after measuring the reference capacity, and was put in a thermostat adjusted to 85 ° C. without charging,
Removed after 120 hours. After the battery was cooled to around room temperature, the thickness of the can was measured. And 4.2V charging,
A 3.0 V discharge was performed. The discharge capacity at this time was defined as the recovery discharge capacity (Z), and the capacity recovery rate was calculated according to the following equation. (Z / X) × 100 Table 1 is a list of test results.

[0025]

[Table 1]

As shown in Table 1, the cleaning treatment was performed,
The recovery rate of the battery with the content of
Improved and the blister of the can is small. Details are unknown, but this
The reason is that sulfur contained in lithium manganese oxide
Decrease, Li_TwoSO_Four/ H_TwoO in battery system
As a result of the reduction of water brought into the tank, the electrolyte Li P
F₆ Is thought to be a controlled result.

[0027]

As described above, a lithium manganese oxide is washed with water between firings, and a spinel lithium manganese oxide having a sulfur content of 4000 ppm or less is used as a positive electrode. In a water electrolyte secondary battery, even when stored at a high temperature of about 80 ° C., the irreversible capacity decrease of the battery is drastically reduced, and the expansion of the container is also reduced. Therefore, a major problem for putting lithium manganese oxide into practical use is solved. As a result, it is possible to provide a non-aqueous electrolyte secondary battery using lithium manganese oxide, which is an inexpensive material, which is comparable to the case using expensive lithium cobalt oxide. High-performance non-aqueous electrolyte secondary batteries can be supplied at low cost, and their industrial value is very large.

Claims (2)

[Claims] 1. A negative electrode active material capable of inserting and extracting lithium ions, a non-aqueous electrolyte having lithium ion conductivity, and a positive electrode active material comprising a lithium-containing metal oxide capable of inserting and extracting lithium ions. In a non-aqueous electrolyte secondary battery provided with a substance, the lithium-containing metal oxide has a general formula Li [Li _x Mn _2-x ] O ₄ (where O ≦ x ≦ 0.1
8) a spinel-based composite oxide containing lithium and manganese represented by the formula: wherein the sulfur contained in the composite oxide is 400
A non-aqueous electrolyte secondary battery characterized by being at most 0 ppm. 2. A spinel-based composite oxide of lithium and manganese represented by the general formula Li [Li _x Mn _2-x ] O ₄ (where O ≦ x ≦ 0.18), wherein O ≦ x ≦
The non-aqueous electrolyte secondary battery according to claim 1, wherein the first heat treatment is performed within a range of 0.18, the cleaning treatment is performed, and then the second heat treatment is performed.