JPH10241687A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery in which cycle performances are superior even at the temperature of a room temperature or more, since keeping high capacity having such a structure as lithium- containing metal oxide is spinel lithium manganese oxide, an X-ray diffraction peak exists in specified ranges, and the half-value width of the X-ray diffraction peak exists in a specified range. SOLUTION: In a nonaqueous electrolyte secondary battery, a lithium- containing metal oxide is spinel lithium manganese oxide which is expressed by a general formula Li[Lix Mn2-x ]O4 (where 0.05<=x<=0.18), no X-ray diffraction peak exists in ranges of 4.26±0.02Å, 4.08±0.02Å, 2.75±0.02Å, at least the X-ray diffraction peaks exist in ranges of 4.74±0.02Å, 2.47±0.02Å, 2.05±0.02Å, and the half-value widths of the X-ray diffraction peak exist in a range of 0.1±0.05 respectively. Therein, the initial capacity is high, no occurrence of Mn elusion occurs even at the temperature of a room temperature or more, and good cycle performances are maintained. Thereby, using the low-cost lithium manganese oxide, a high performance nonaqueous electrolyte secondary battery, which is in no way inferior to the one using high-cost lithium cobalt oxide, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of cycle performance at high temperature of a non-aqueous electrolyte secondary battery using lithium manganese oxide as a positive electrode active material.

[0002]

2. Description of the Related Art In recent years, the development of electronic technology has been remarkable, and devices have been reduced in size and weight. For this reason, mobile devices such as mobile communication devices and portable computers have become widespread, and high energy density secondary batteries have been demanded as power sources for these mobile devices.
In particular, since a non-aqueous electrolyte secondary battery can be expected to have a high voltage, it is desired 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 non-aqueous electrolyte secondary battery using a carbon material capable of inserting and extracting lithium as a negative electrode active material and a composite oxide of lithium and cobalt as a positive electrode active material ( Patent No. 1989293
Has been developed and has a voltage of 4 V or more in a charged state, and thus has become 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 other transition metals are active.
Among the transition metals, manganese, which is inexpensive, is most expected to replace cobalt. However, a lithium manganese oxide having a stoichiometric composition has poor cycle performance.
It has been proposed to replace part of manganese with lithium, as disclosed in US Pat.

[0004] A conventional method for synthesizing a lithium manganese oxide in which a part of manganese is replaced with lithium is as follows.
It was obtained by mixing the n raw material and the Li raw material at a desired ratio and performing a heat treatment at a relatively low temperature of 700 ° C. or less. In particular, heat treatment at a temperature of 500 ° C. or less by using lithium hydroxide having a low melting point (anhydrous and having a melting point of 445 ° C.) or lithium nitrate (a melting point of 255 ° C.) instead of lithium carbonate has been proposed. . However, in any case, since the heat treatment is performed at a low temperature, the crystallinity does not increase, and as a result, there is a problem that the reversible capacity around about 4 V with respect to the oxidation-reduction potential of lithium metal decreases. Furthermore, when lithium hydroxide is used as the Li raw material, if it remains unreacted, the paste is gelled due to the alkali component when the paste is prepared by wet mixing with a binder to form a battery. There was a problem that it became impossible to use it. In addition, when heat treatment is performed at a high temperature of 700 ° C. or more, a sub-phase such as Li ₂ MnO ₃ is easily generated, and manganese is not sufficiently replaced with lithium. Was eluted, and there was a problem in the cycle performance at high temperatures.

[0005]

Lithium manganese oxide in which part of manganese has been replaced by lithium has been able to improve the cycle performance near room temperature, but still has a high temperature and in more severe conditions. There is a problem that manganese is eluted and the cycle performance is reduced. Further, there is a problem that the crystallinity is low and the capacity is greatly reduced. An object of the present invention is to replace a part of manganese with lithium, and by using a lithium manganese oxide having good crystallinity, while maintaining high capacity, particularly good cycle performance even at room temperature or higher. To provide a nonaqueous electrolyte secondary battery.

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies on the heat treatment conditions and the replacement method of a spinel-based lithium manganese oxide in which a part of manganese has been replaced with lithium. Was obtained, and a spinel-based lithium manganese oxide suitable as a positive electrode material of a nonaqueous electrolyte secondary battery was found, and the present invention was reached. That is, the present invention provides: (1) a negative electrode active material capable of inserting and extracting lithium ions, a non-aqueous electrolytic solution having lithium ion conductivity, and a lithium-containing metal oxide capable of inserting and extracting lithium ions. In the non-aqueous electrolyte secondary battery provided with the positive electrode active material, the lithium-containing metal oxide is a spinel-based lithium manganese oxide represented by the following general formula, and has an X-ray diffraction peak of 4.26 ± 0. 02Å, 4.08
± 0.02Å, 2.75 ± 0.02Å, and at least 4.74 ± 0.02Å, 2.47 ± 0.
A non-aqueous electrolyte secondary battery having an X-ray diffraction peak at 0. 02 ± 2.05 ± 0.02 ° and a half-value width of each of the X-ray diffraction peaks of 0.1 ± 0.05. _x Mn _2-x ] O ₄ (provided that 0.05 ≦ x ≦
0.18) (2) General formula Li [Li _x Mn _2-x ] O ₄ (provided that
A nonaqueous electrolyte secondary battery, wherein the lattice constant of the spinel-based lithium manganese oxide represented by the formula: 0.05 ≦ x ≦ 0.18) is not less than 8.20 ° and not more than 8.24 °; General formula Li [Li _x Mn _2-x ] O ₄ (however,
0.05 ≦ x ≦ 0.18) After heat-treating a mixture of electrolytic manganese dioxide and lithium hydroxide or lithium nitrate at a temperature of 700 ° C. or higher in an air atmosphere, the spinel-based lithium manganese oxide represented by the following formula: A non-aqueous electrolyte secondary battery obtained by performing a heat treatment again at a temperature of at least 600 ° C. or lower.

Hereinafter, the present invention will be described specifically.
The manganese raw material of the lithium manganese oxide used in the present invention includes, for example, EMD (Electrolytic).
Manganese Dioxide), CMD (Ch
electronic Manganese Dioxide
e) and γ-MnOOH, but EMD having a high tetravalent Mn content is preferable. Further, as for the lithium raw material, for example, Li ₂ CO ₃ , LiOH, LiCl, L
Examples include iNO ₃ , Li ₂ SO ₄ , and CH ₃ COOLi, with Li ₂ CO ₃ , LiOH or LiNO ₃ being preferred.

[0008] The lithium manganese oxide used in the present invention can be prepared as follows. For example, EM pulverized to have an average particle size of 5 to 25 μm
D and LiOH or LiNO ₃ or Li ₂ CO ₃ are mixed so that the Mn / Li ratio becomes 0.5, heat-treated at 800 to 900 ° C. in the air, cooled to around room temperature, and then cooled to a desired Li. LiOH or LiN
It can be obtained by adding O ₃ , mixing and heat-treating at 300 to 600 ° C., more preferably at 300 to 500 ° C. If the temperature of the second heat treatment is lower than 300 ° C., LiOH may remain in particular, and it is not preferable because a paste for forming an electrode cannot be formed. When the temperature exceeds 600 ° C., a secondary phase such as Li ₂ MnO ₃ is easily synthesized, which is not preferable.

As another example, pulverized EMD and Li
OH or LiNO ₃ may be mixed in advance with a desired Mn / Li ratio and heat-treated. Heat treatment is
First at 800-900 ° C, then at 300-600
C., more preferably at 300 to 500.degree. Also in this case, it is important to perform the second heat treatment. If the second heat treatment is not performed, a phase other than spinel such as Li ₂ MnO ₃ is generated, which is not preferable.

The substitution amount of Mn by Li is 0.05 to 0.1.
The range of 18 atomic% is preferable, and more preferably 0.07 to 0.1
A range of 6 atomic% is preferred. If the substitution amount is less than 0.05 atomic%, the effect of substituting manganese with lithium is small, and the cycle performance at room temperature decreases. Also, 0.
If it exceeds 18 atomic%, the capacity is greatly reduced, which is not preferable.

Next, a method of measuring an X-ray diffraction pattern according to the present invention will be described. The measurement of the X-ray diffraction pattern used RINT2500 manufactured by Rigaku Corporation. C for X-ray source
The measurement was performed using u-Kα1 (wavelength: 1.5405 °) under the following instrument conditions. The tube voltage and current are 50 kV and 160 kV, respectively.
mA, divergence slit 0.5 ゜, scattering slit 0.5 ゜,
The light receiving slit width was 0.15 mm, and a monochromator was used. The measurement was performed at a scanning speed of 2 ° / min.
The scanning was performed under the condition of 2θ / θ at 01 °. Further, the half width was obtained by subtracting the background from the measured value of the diffraction pattern expressed on the 2θ axis, and was defined as the width of the peak at half the height (h / 2) of the diffraction peak intensity (h).

The lithium manganese oxide synthesized by the method described above has at least a (111) plane and a (311) plane.
X-ray diffraction peaks derived from the (222) plane
± 0.02 °, 2.47 ± 0.02 °, 2.05 ± 0.
02 °, and the half width as an index of crystallinity of these diffraction peaks is 0.1 ± 0.05. When the half width is 0.1 ± 0.05 or more, elution of Mn tends to occur, and the capacity is undesirably reduced. Further, a diffraction peak of 2.75 ± 0.02 ° caused by Mn ₂ O ₃ or Mn ₃₄ O4, and a diffraction peak of 4.26 ± 0.02 caused by LiOH and / or Li ₂ MnO ₃ .
02 {4.08 ± 0.02}.

The conventional stoichiometric composition LiMn ₂ O ₄
Is JCPDS (The Joint Committee)
on Power Diffraction Sta
According to the ndards card 35-782, the lattice constant is 8.248 °. The lattice constant of the lithium manganese oxide of the present invention is preferably from 8.20 ° to 8.24 °. The reason why the lattice constant of the lithium manganese oxide of the present invention is small is considered that a part of Mn in the crystal unit cell was replaced with Li having an ionic radius smaller than Mn. When the lattice constant is 8.20 ° or less, the capacity is significantly reduced. This is probably because the average valence of Mn is too close to 4. On the other hand, at 8.24 ° or more, the effect of substituting a part of Mn with Li cannot be obtained, and Mn is eluted to lower the cycle performance.

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. For example, carbon materials such as coke, natural graphite, artificial graphite and non-graphitizable carbon, and metal oxides such as SiSnO Thing, L
Although a metal nitride such as iCoN 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, acetylene black, and the like,
Graphite or a combination of graphite and acetylene black is preferred. The addition amount is not particularly limited, but may be from 1 to 20.
% By weight, and more preferably in the range of 3 to 10% by weight. If the amount is less than 1% by weight, the conductivity will not be uniform, and if it exceeds 20% by weight, the capacity per unit volume will decrease. In addition, the binder is usually polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene terpolymer, carboxymethylcellulose,
Styrene butadiene rubber, fluorine rubber, or the like is used alone or in combination, but is not particularly limited. The amount of these additives is preferably 1 to 20% by weight, more preferably 1 to 10% by weight. If the addition amount is less than 1% by weight, the binding force is weak, and if it exceeds 20% by weight, the movement of Li ions is inhibited, and the performance as a battery is reduced.

As the electrolytic solution, a mixture obtained by dissolving a lithium salt as an electrolyte 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. The organic solvent is not particularly limited, but, for example, propylene carbonate, ethylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate,
A single solvent such as 2-dimethoxyethane and tetrahydrofuran or a mixture of two or more solvents can be used.

[0017]

According to the lithium manganese oxide of the present invention, Mn ₂
Since it does not contain O ₃ and / or Mn ₃ O ₄ , elution of Mn from by-products can be prevented. In addition, since the heat treatment is performed at a low temperature after the heat treatment at a high temperature, Li ₂ Mn ₃ O _{4 as} a by-product disappears, and the substitution between Mn and Li can be accurately substituted in a desired amount. Moreover,
Since the heat treatment is performed once at a high temperature of 700 ° C. or more, the crystallinity can be increased as indicated by the half-width of the peak of the X-ray diffraction pattern. As a result, in the nonaqueous electrolyte secondary battery using the lithium manganese oxide of the present invention, the elution amount of Mn at a high temperature is reduced as compared with the battery using the conventional lithium manganese oxide, and Cycle characteristics can be obtained.

[0018]

EXAMPLES Hereinafter, the present invention will be described in more 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 were mixed at a composition ratio of Li / Mn = 0.5 (atomic ratio), heat-treated in air at 850 ° C. for 20 hours, and then cooled to around room temperature. From the result of comparing the X-ray diffraction pattern of this lithium manganese oxide with the JCPDS card, this lithium manganese oxide was found to be LiMn ₂ O ₄
Was confirmed. Then, the obtained LiMn ₂ O
₄ and LiOH were mixed at a composition ratio of Li / Mn = 0.61 (atomic ratio) and heat-treated in air at 400 ° C. for 10 hours to obtain a lithium manganese oxide of the present invention. As shown in FIG. 1, the X-ray diffraction pattern of this lithium manganese oxide shows the respective diffraction peaks derived from the (111) plane, the (311) plane, and the (222) plane as 4.741Å, 2.476.
{2.03}, and their half-value widths are 0.094, 0.118, and 0.118, respectively, and have high crystallinity, Mn ₂ O ₃ , Mn ₃ O _4, and Li ₂ MnO ₃ , L
Since it did not have a diffraction peak of a secondary phase such as iOH, it was confirmed to be a single cubic spinel.

When the lattice constant was accurately determined using Si as a standard substance, it was 8.213 °. Furthermore, from the result of elemental analysis, it was confirmed that the _{substance was} Li [Li _0.12 Mn _1.88 ] O ₄ . As a result of observing the surface of the lithium manganese oxide thus prepared with a scanning electron microscope, it was found that the secondary particles were formed by an aggregate of primary particles having a diameter of about 0.3 to 1.0 μm. It could be confirmed. A description will be given of a specific battery preparation 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 100 parts by weight of the lithium manganese oxide, 3 parts by weight based on the total weight are mixed.
The fluororubber was mixed at a ratio of parts by weight, and a mixed solvent of ethyl acetate / ethyl cellosolve, which is a solvent for the fluororubber, was added to obtain a paste by wet mixing. Next, this paste was uniformly applied to both sides of a 20-μm-thick aluminum foil serving as a positive electrode current collector, dried, and then pressure-formed by a roller press to form a belt-shaped positive electrode.
Next, mesocarbon fiber 9 graphitized at 3000 ° C.
To a mixture of 5 parts by weight and 5 parts by weight of scale-like natural graphite, 1 part by weight of carboxymethylcellulose, 2 parts by weight of styrene-butadiene rubber, and purified water as a solvent were added to obtain a paste by wet mixing. Next, this paste was uniformly applied to both surfaces of a copper foil having a thickness of 12 μm serving as a negative electrode current collector,
After drying, a belt-shaped negative electrode was formed by pressure molding with a roller press. 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 cylindrical can, and the wound body was inserted. Next, the negative electrode tab taken out from the wound body was welded to the bottom of the can, and the positive electrode tab was welded to a closing lid made of a gasket, an explosion-proof disk, and a PTC element. An electrolyte in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate was injected into the battery can, and an insulating film was inserted into the upper part of the wound body. A cylindrical non-aqueous electrolyte secondary battery having an outer shape of 17 mm and a height of 500 mm was prepared by inserting the lid and crimping the end of the battery can.

Example 2 A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the method of synthesizing lithium manganese oxide was changed as follows. As a starting material, EMD having an average particle size of 15 μm and LiOH were mixed with Li / Mn =
Mix at a composition ratio of 0.61 (atomic ratio) and 900 ° C in air
And then heat down to 500 ° C for 12 hours.
The lithium manganese oxide of the present invention was obtained by heat treatment for an hour. The X-ray diffraction pattern of the obtained substance shows at least diffraction peaks derived from (111), (311) and (222) planes of 4.751 °, 2.479 °,
2.055 ° and the half-value width is 0.0
94, 0.094, 0.094, high crystallinity,
It was a single cubic spinel with no subphase peak. When the lattice constant was accurately determined using Si as a standard material, it was 8.215 °. Furthermore, from the result of elemental analysis, it was confirmed that the _{substance was} Li [Li _0.11 Mn _1.89 ] O ₄ . As a result of observing the surface of the lithium manganese oxide thus produced with a scanning electron microscope, it was found that the secondary particles were formed by an aggregate of primary particles having a diameter of about 0.5 to 1.5 μm. It could be confirmed.

Comparative Example A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the method of synthesizing lithium manganese oxide was changed as follows. As a starting material, EMD having an average particle diameter of 20 μm and LiOH were mixed with Li / Mn = 0.
6 (atomic ratio) in air at 650 ° C.
After heat treatment for 900 hours, the temperature was raised to 900 ° C. and heat treatment was performed for 12 hours to obtain lithium manganese oxide.
As shown in FIG. 2, the X-ray diffraction pattern of this lithium manganese oxide shows the respective diffraction peaks derived from the (111) plane, the (311) plane, and the (222) plane as 4.767 ° and 2.487.
{2.063}, and their half-value widths are 0.094, 0.118, and 0.141, respectively. Although the crystallinity is high, it is caused by Mn ₂ O ₃ or Mn ₃ O ₄ at 2.767 °. Diffraction peak and Li ₂ MnO ₃
There was a diffraction peak thought to be due to Further, when the lattice constant was accurately determined using Si as a standard substance, it was 8.247 °. From this result, LiMn ₂
O ₄ and Mn ₂ O ₃ and Mn ₃ O ₄ and Li ₂ MnO ₃
And it was not considered to be a uniform cubic spinel single phase.

[Test Results] The batteries prepared in Examples 1 and 2 and Comparative Example were all used for the purpose of stabilizing the inside of the batteries.
After the aging period of 4 hours had elapsed, charging was performed in 5 hours with the charging voltage set to 4.2V. Then 5
The battery was discharged at a constant current of 00 mA to 2.7 V, the initial capacity of each battery was measured, and the capacity per unit positive electrode active material in the battery was determined. Next, the battery was placed in a thermostat adjusted to 60 ° C., and the charging voltage was set to 4.2 V to 3
A cycle test was performed in which the battery was charged over time and repeatedly discharged to 2.7 V at a constant current of 1 A, the discharge capacity at the 50th cycle was measured, and the capacity per unit positive electrode active material in the battery was determined. Further, based on these, the deterioration rate of the discharge capacity (Y) at the 50th cycle with respect to the initial capacity (X) was calculated according to the following equation. Deterioration rate (%) = [(XY) / X] × 100

Table 1 shows the discharge amount per unit positive electrode active material calculated from the initial discharge amount and the discharge capacity at the 50th cycle, and the deterioration rate calculated from them. As shown in Table 1, the batteries of Examples 1 and 2 had lower initial capacities than the comparative examples, but had a low deterioration rate after 50 cycles. This is because the heat treatment is performed once at 700 ° C. or higher, so that a uniform spinel having high crystallinity can be formed and the subphase is eliminated, so that the charged lithium substitutes a part of manganese accurately. Conceivable.

[0026]

[Table 1]

[0027]

As described above, the X-ray diffraction peak of the present invention is not present at least at 2.75 ± 0.02 ° but at 4.74 ± 0.02 °, 2.47 ± 0.02 °,
2.05 ± 0.02Å, the half width of each is 0.1
A non-aqueous electrolyte secondary battery using a spinel lithium manganese oxide represented by Li [Li _x Mn _2-x ] O ₄ of ± 0.05 has a high initial capacity and a temperature above room temperature. However, Mn does not elute and good cycle performance is maintained. Further, no gelation of the paste occurs because no LiOH remains. Further, the cost is low because other expensive elements are not used. 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. A high-performance nonaqueous electrolyte secondary battery can be supplied at low cost, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction pattern of a lithium manganese oxide obtained in Example 1.

FIG. 2 shows X of lithium manganese oxide obtained in a comparative example.
It is a line diffraction pattern.

Claims (3)

[Claims] 1. A negative electrode active material capable of inserting and extracting lithium ions, a lithium ion conductive non-aqueous electrolyte,
And a non-aqueous electrolyte secondary battery comprising a positive electrode active material comprising a lithium-containing metal oxide capable of inserting and extracting lithium ions, wherein the lithium-containing metal oxide is represented by the following general formula: A manganese oxide having an X-ray diffraction peak of 4.26 ± 0.02 °,
4.08 ± 0.02Å, not at 2.75 ± 0.02Å, and at least 4.74 ± 0.02Å, 2.47
A non-aqueous electrolyte secondary battery having ± 0.02 ° and 2.05 ± 0.02 °, and a half width of each of the X-ray diffraction peaks is 0.1 ± 0.05. Li [Li _x Mn _2-x ] O ₄ (provided that 0.05 ≦ x ≦
0.18) 2. The lattice constant of a spinel lithium manganese oxide represented by the general formula Li [Li _x Mn _2-x ] O ₄ (where 0.05 ≦ x ≦ 0.18) is at least 8.20 ° 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the angle is not more than .24 °. 3. A spinel-based lithium manganese oxide represented by a general formula Li [Li _x Mn _2-x ] O ₄ (where 0.05 ≦ x ≦ 0.18), comprising electrolytic manganese dioxide and lithium hydroxide or After heat-treating the mixture of lithium nitrate at a temperature of 700 ° C. or more in an air atmosphere,
The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery is obtained by performing heat treatment again at a temperature of 00 ° C. or less.