JPH05325971A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To provide a nonaqueous electrolyte secondary battery of long life by using compound oxide, composed of lithium, boron and cobalt, as a positive electrode active material so as to improve a charge/discharge cycle characteristic. CONSTITUTION:As a positive electrode active material of a nonaqueous electrolyte secondary battery in which a positive electrode 1, negative electrode 2 and a separator 3 are arranged in a battery case 4, compound oxide composed of lithium, boron and cobalt, represented by LiBxCo(1-x)O2, is used. In the positive electrode active material composed of the lithium boron cobalt compound oxide, generation of gas in the battery can be suppressed with a long charge/ discharge cycle life, and the nonaqueous electrolyte secondary battery of long life can be obtained. Further, an alphabet (x) in the general formula LiBxCo(1-x) O2 is preferable in a relation where 0.001<=x<=0.25.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, which provides a non-aqueous electrolyte secondary battery having a long charge / discharge cycle life by using a stable positive electrode active material. It is a thing.

[0002]

2. Description of the Related Art In recent years, with the miniaturization of various electronic devices, there has been a demand for secondary batteries having higher energy density. A secondary battery using a non-aqueous electrolytic solution has an energy density several times higher than that of a battery using a conventional aqueous electrolytic solution, and thus its practical application is awaited. The non-aqueous electrolyte is
It is a solution of a metal salt serving as an electrolyte in an aprotic organic solvent. For example, for lithium salts, LiC
propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, γ-butyrolactone, dioxolane, 2-O ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO _3, etc.
A single solvent such as methyltetrahydrofuran, diethylcarbonate, dimethylcarbonate, sulfolane or the like, or a mixture of these dissolved in a solvent is used. These non-aqueous electrolytes are used by being injected into a battery container, but impregnated into a porous separator, added high molecular weight resin to make it highly viscous, or gelled to lose fluidity. It is sometimes used in the state. Further, a polymer electrolyte represented by polyethylene oxide is also being studied as an electrolyte for a non-aqueous electrolyte secondary battery.

Various materials have been studied as a negative electrode active material for a non-aqueous electrolyte battery, but a lithium-based negative electrode is most suitable because a high energy density is expected. Particularly, as a negative electrode of a non-aqueous electrolyte secondary battery, lithium metal, lithium alloy, carbon having lithium ions retained, and the like are being studied.

As a positive electrode active material for a non-aqueous electrolyte secondary battery,
As shown in JP-A-55-136131, research on lithium cobalt composite oxide (LiCoO ₂ ) has been actively conducted in recent years. Batteries using this active material have an average operating voltage of 3.
Since it has a voltage of about 6V, which is about three times higher than 1.2V of nickel-cadmium batteries, it is possible to further increase energy density and downsize batteries.

The present invention relates to an improvement of such a non-aqueous electrolyte secondary battery.

[0006]

Lithium-cobalt composite oxide LiCoO ₂ is a stable substance as it is,
When used as a positive electrode active material of a non-aqueous electrolyte battery, there was a drawback that the capacity was greatly deteriorated with charge / discharge cycles. LiCoO ₂ becomes unstable in crystal structure when Li is removed by charging, and oxygen is separated to gradually separate Co ₂ O ₃
It has the property of changing to. Co ₂ O ₃ does not return to the original LiCoO ₂ even when discharged. When charging and discharging are repeated, there is a drawback that the capacity decreases and the charging and discharging cycle is short. In addition, an increase in battery internal pressure due to gas generation is also recognized.

[0007]

The present invention solves such a problem by adding a small amount of boron B to LiCoO ₂ so that the capacity of the non-aqueous electrolyte solution is less deteriorated as the cycle progresses. The next battery is available. That is, the present invention has a general formula as a positive electrode active material of a non-aqueous electrolyte secondary battery.
It is characterized by using a lithium boron cobalt complex oxide represented by LiB _x Co _(1-x) O ₂ (0.001 ≤ x ≤ 0.25).

[0008]

[Function] A positive electrode active material composed of lithium boron cobalt composite oxide has a long charge / discharge cycle life, can suppress gas generation in the battery, and enables a long-life non-aqueous electrolyte secondary battery. became.

The lithium-boron-cobalt composite oxide has a hexagonal crystal structure like the conventional lithium-cobalt composite oxide. However, since the diffraction peak of the crystal according to the X-ray diffraction pattern is sharper than that of the conventional product, it is considered to have a crystal structure with a high degree of crystallization and with few lattice defects. Lithium boron cobalt composite oxide
The stability is high in a charged state, that is, a state in which a part of lithium ions is removed from the crystal structure, the capacity does not decrease much even if charging and discharging are repeated, and oxygen gas is less generated by decomposition.

Further, at a relatively low temperature as compared with the conventional LiCoO ₂ ,
It is characterized in that it can be manufactured by heat treatment for a short time. Conventionally, heat treatment at about 900 ° C was required for the synthesis of complex oxides.
The synthesis was possible even at 0 ° C. The reason is not clear,
It seems that a low melting point mixture phase is generated during the heating process, which facilitates crystal growth.

It was confirmed that the effect of boron was not affected by the type of boron raw material. There is no limitation on the raw materials of lithium, cobalt, and boron as long as they can be converted into oxides by heat treatment. The most advantageous raw material can be used in terms of manufacturing process and raw material cost.

[0012]

【Example】

Example 1 First, LiB _x Co _(1-x) O ₂ was synthesized. x is 0, 0.
It is 02, 0.05, 0.1, 0.2, 0.25, 0.3. Note that x = 0
Shows the composition of a conventional product, which is a product with no boron added. The synthesis was performed as follows. Lithium carbonate LiCO ₃
And boric acid H ₃ BO ₃ and basic cobalt carbonate 2CoCO ₃ · 3Co (O
H) ₂ · xH ₂ O (cobalt amount 47.2 w%) was mixed in a mortar at a predetermined molar ratio and dispersed in pure water. Citric acid was added to the dispersion to form a complex citrate salt at room temperature, and the mixture was dried at 120 ° C. This powder was calcined at 500 ° C for 6 hours and then heat-treated at 900 ° C for 24 hours. After the heat treatment, it was crushed with a ball mill. A non-aqueous electrolyte secondary battery was assembled using the obtained lithium boron cobalt composite oxide LiB _x Co _(1-x) O ₂ as a positive electrode active material.

FIG. 1 is a sectional view showing the structure of a battery in an embodiment of the present invention. Reference numeral 1 is a positive electrode, which was manufactured as follows. First, the active material LiB _x Co _(1-x) O ₂
10 parts by weight of graphite, polyvinylidene fluoride
5 parts by weight of N-methyl-2-pyrrolidone was added and mixed to prepare a slurry mixture. It has a thickness of 1.02 mm and a porosity of 95%.
After applying to the foam aluminum of, drying and rolling,
A strip-shaped electrode plate having a thickness of about 0.5 mm, a width of 14 mm and a length of 52 mm was obtained. The amount of electricity discharged from the active material on one positive electrode was adjusted to about 65 mAh. 2 is a negative electrode, thickness 0.15mm, width 14mm, length 52mm
Lithium metal was used. The positive and negative electrodes were composed of four positive electrodes and five negative electrodes.

3 is a separator having a thickness of 0.18 mm and a basis weight of 50
Using g / m ² of polypropylene non-woven fabric, the positive electrode of 1 was covered, and the periphery was heat-sealed and fixed. These electrode groups were inserted into a battery case 4 made of nickel plated with iron. Reference numeral 5 denotes a titanium positive electrode lead, which includes a positive electrode 1 and a positive electrode terminal 8.
And are connected. The positive electrode terminal 8 is fixed to a nickel-plated iron sealing plate 6 via a glass seal 7. Reference numeral 9 denotes a negative electrode lead made of nickel, which is spot-welded to the sealing plate 6.

As an electrolytic solution, a mixed solvent of propylene carbonate and ethylene carbonate (volume ratio 1: 1) was used.
LiPF ₆ dissolved at a rate of 1 mol / liter was used. The battery is sealed by laser welding around the sealing plate. The dimensions of the assembled battery are 6mm thick, 14mm wide,
The length is 60 mm.

In the battery thus produced, _{x of} LiB _x Co _(1-x) O ₂ used as the positive electrode active material was 0,
0.02, 0.05, 0.1, 0.2, 0.25, and 0.3 are batteries A, B, C, D, E, F, and G, and the current value is 58.2.
A charge / discharge cycle test was conducted under the conditions of 4 mA, end-of-charge voltage 4.1V, and end-of-discharge voltage 2.75V. The current value at this time is about 1 mA / cm ² when converted to the current density of the positive electrode.

FIG. 2 shows the results of the charge / discharge cycle test. As the content of boron in LiB _x Co _(1-x) O ₂ increases, the deterioration of the discharge capacity with charge and discharge cycles decreases. The lithium-boron-cobalt composite oxide containing boron is considered to have high stability during charge / discharge cycles.

Since it is considered that the decomposition of the complex oxide was suppressed by containing boron, the batteries A, B, C, D, E, F, and G having the same structure as the charge / discharge cycle test were considered.
Was stored for 30 days in a charged state at a temperature of 40 ° C. and a terminal voltage of 4.1 V, and then the amount of gas generated in the battery was measured. The amount of gas at this time was determined by opening a part of the battery in water and collecting the gas. Table 1 shows the amount of gas generated.

[0019]

[Table 1] The larger the amount of boron in the composite oxide, the smaller the amount of gas generated. Also, a clear relationship was observed between the amount of gas generated in the battery and the deterioration of the discharge capacity with charge / discharge cycles. However, when the generated gas was analyzed, a small amount of hydrogen gas was detected together with oxygen gas.
It is considered that this is because the lithium metal used for the negative electrode reacted with a small amount of water contained in the battery to generate hydrogen gas. It is considered that the change in the capacity of the battery in FIG. 2 is due to the synergistic effect of the decrease in discharge capacity due to the decomposition of the positive electrode active material and the poor contact between the electrodes due to the generated gas. When the boron content x in the composite oxide was in the range of 0.02 to 0.30, it showed superior characteristics to the conventional product containing no boron.

Example 2 LiB _x Co _(1-x) O ₂ having various compositions with different boron contents was prepared and the discharge characteristics in a non-aqueous electrolyte were measured. The value of x was changed in the range of 0.001 to 0.3. Lithium carbonate, boric acid, and cobalt oxide CoO 2 were used as raw material powders. A specified amount of each raw material powder was weighed, mixed in an automatic mortar, calcined at 500 ° C for 6 hours, and further heat-treated at 900 ° C for 24 hours. The fired product obtained is crushed with a ball mill and L
A composite oxide having a composition of iB _x Co _{(1-x )} O ₂ was prepared. For comparison, boron-free LiCoO ₂ was prepared under the same process and heat treatment conditions.

85 parts of composite oxides of various compositions, 8 parts of acetylene black as a conductive agent, and 34 parts of an aqueous solution of PTFE dispersion (containing 15% of polytetrafluoroethylene resin) as a binder were kneaded, and a pair of them were mixed. After being formed into a sheet shape by passing between the rolls, a positive electrode substrate having a thickness of about 0.6 mm was prepared by press-bonding to both surfaces of an expanded metal core material made of aluminum. This substrate was punched out to obtain a strip-shaped positive electrode having a width of 14 mm and a length of 52 mm.

The measurement was carried out in a beaker test cell in order to avoid the influence of generated gas.

FIG. 3 shows the structure of the beaker test cell.
Reference numeral 10 is a beaker of a glass container, which holds an electrolytic solution 11 therein and whose opening is covered with a polypropylene lid 12. 13 is a positive electrode of the test electrode, and 14 and 14 'are counter electrodes. Reference numeral 15 is a reference electrode, and lithium was used.
Using lithium as a counter electrode, a single electrode charge / discharge cycle test was performed in an equal volume mixture of ethylene carbonate and diethyl carbonate in which 1 molar concentration of LiPF ₆ was dissolved. The test was carried out in a dry box in an argon atmosphere to prevent adverse effects of water and oxygen. After charging to 4.1V against lithium potential with a current of 20mA, it was discharged up to 2.75V against lithium potential with the same current of 20mA, and the change in discharge capacity per unit weight of the composite oxide was obtained.

FIG. 4 shows the boron content x in the composite oxide.
The relationship of the discharge capacity at the first cycle is shown. Compared to the conventional boron-free LiCoO ₂ , the composite oxide containing a small amount of boron at x = 0.001 to 0.05 showed high discharge capacity.
Even if the content of boron is as small as x = 0.001, it is effective, and the discharge capacity becomes the highest at around x = 0.03, and when X = 0.1 or more, the discharge capacity at the first cycle gradually increases as the amount of boron increases. Decreased to.

FIG. 5 shows the change in discharge capacity per unit weight of the composite oxide with charge / discharge cycles. It was found that the boron-containing composite oxide was less deteriorated during charge / discharge cycles. Boron content is X =
Even with a complex oxide as high as 0.25, the discharge capacity is higher at 200 cycles or more than when it is not added. In other words, when performing a charge / discharge cycle of 200 cycles or more, x
The composite oxide having a value of 0.001 to 0.25 has superior properties to conventional LiCoO ₂ .

Some of the powder X-ray diffraction patterns of the composite oxides produced are shown in FIGS. 6, 7, 8, 9, and 10. The diffraction peak of LiB _x Co _(1-x) O ₂ containing x = 0.001 to 0.25 containing boron coincided with that of LiCoO ₂ . However, in all cases, the peak on the d003 plane is sharp, and it is expected that crystallization will be more advanced due to the inclusion of boron. The value of x is 0.
For the composition of 3, some unknown peaks were observed,
The composition could not be identified. It seems that boron compounds are formed as a mixture.

Example 3 Using the strip-shaped electrode produced in Example 2, a battery similar to that of Example 1 was assembled and a charge / discharge test was conducted. The content of boron is the conventional battery with x = 0 and the battery of the present invention with x = 0.03.

In the battery structure diagram of FIG. 1, the method for producing the positive electrode 1 is as follows. 85 parts of the composite oxide of the positive electrode active material, 8 parts of acetylene black as the conductive agent and an aqueous solution of PTFE dispersion as the binder (polytetrafluoroethylene resin 15%
34 parts) were kneaded, passed through a pair of rolls to form a sheet, and then pressure-bonded to both surfaces of an expanded metal core material made of aluminum to produce a positive electrode substrate having a thickness of about 0.6 mm. This substrate was punched out to obtain a strip-shaped positive electrode having a width of 14 mm and a length of 52 mm.

As the negative electrode 2, a carbon negative electrode acting as a host for lithium ions was used instead of lithium.
The negative electrode 2 of Example 3 in FIG. 1 was manufactured as follows. 98 parts of carbon material which is the negative electrode active material, 2 parts of polyvinylidene fluoride as a binder and N-methyl-2-pyrrolidone 30 as a solvent
Part is kneaded into a paste, thickness 1.0mm, porosity 98%
After applying it to the nickel foam, it is dried and rolled,
An electrode substrate with a thickness of 0.5 mm was created. This electrode substrate was punched out to obtain a strip-shaped negative electrode plate having a width of 14 mm and a length of 52 mm. The weight of the active material carbon mixture per one negative electrode was 0.40 g.
The carbon material used here is a carbon fiber produced by a vapor phase epitaxy method, and the physical property values obtained by an X-ray diffraction method are as _{follows: the} distance between crystal layers (d _OO2 ) is 3.36 Å, and the length of the crystallite (Lc) is It is 39 angstrom and has a discharge capacity of 185 mAh / g. As an electrolytic solution, an equal volume mixture of ethylene carbonate and diethyl carbonate in which 1 molar concentration of LiPF ₆ was dissolved was used. The other configurations are the same as in Example 1, but the nominal capacity of the battery is 350 mAh. ..

FIG. 11 shows the results of the charge / discharge cycle test of the inventive battery H of the present invention and the comparative battery I of the comparative example. Unlike Example 1, since lithium was not used for the negative electrode, the influence of the generated gas was reduced, and the capacity of the conventional product was slightly improved, but the battery using the positive electrode active material containing boron was The decrease in discharge capacity was further reduced.

Example 4 LiB _x Co _(1-x) O ₂ was synthesized by lowering the heat treatment temperature.

LiB _x Co of various compositions with different boron contents
_(1-x) O ₂ was prepared and the discharge characteristics in the non-aqueous electrolyte were measured. Lithium carbonate, boric acid, and cobalt oxide were used as raw material powders. The value of x was changed in the range of 0.001 to 0.3. A prescribed amount of each raw material powder was weighed, mixed in an automatic mortar, and then heat-treated at 700 ° C. for 10 hours. The obtained calcined product was crushed with a ball mill to prepare a composite oxide having a LiB _x Co _(1-x) O ₂ composition. Does not contain boron for comparison
LiCoO ₂ corresponding to X = 0 was prepared under the same process and heat treatment conditions.

Using the obtained composite oxide, a strip-shaped positive electrode similar to that in Example 2 was prepared, and a charge / discharge cycle test was conducted under the same conditions as in Example 2 using the beaker test cell shown in FIG. ..

FIG. 12 shows the boron content x and the composite oxide.
The relationship of discharge capacity per unit weight in the first cycle is shown. The discharge capacity of the conventional LiCoO ₂ containing no boron was extremely poor, while the discharge capacity of the boron-containing composite oxide showed almost the same value as in Example 1. The complex oxide containing boron at x = 0.001 to 0.25 showed higher discharge capacity than that without boron at the first cycle. Conventional boron-free LiCoO ₂ is produced at 700 ° C, which is the manufacturing condition here.
The heat treatment for 10 hours seems to be due to insufficient synthesis reaction.

Example 5 LiB _x Co _(1-x) O ₂ was synthesized using various boron raw materials. This limits the oxygen content towards the x = 0.03, boron as raw materials, boric acid H ₃ BO _3, boric oxide-containing B ₂ O _3, lithium borate LiBO ₂ · 2H ₂ O, ammonium borate (N
H ₄₎ using _{2 · 5B 2 O 3 · 8H} 2 O. Lithium carbonate and cobalt oxide were used as raw material powders. Weigh the specified amount of raw material powder, mix in an automatic mortar, and
Heat treated for hours. The obtained fired product was crushed by a ball mill to prepare a composite oxide having a LiB _0.03 Co _0.97 O ₂ composition.

85 parts of the composite oxide, 8 parts of acetylene black as a conductive agent and 34 parts of an aqueous solution of PTFE dispersion (containing 15% of polytetrafluoroethylene resin) as a binder were kneaded.
This was passed between a pair of rolls to form a sheet, and then pressure-bonded to both surfaces of an expanded metal core material made of aluminum to produce a positive electrode substrate having a thickness of about 0.6 mm. This substrate was punched out to obtain a strip-shaped positive electrode having a width of 14 mm and a length of 52 mm. Using the four types of positive electrodes thus manufactured, the unipolar characteristics were measured by the beaker test cell of FIG. 3 in the same manner as in Example 2.
The discharge capacity in the first cycle of all four types of electrodes is 154m.
It was Ah / g. There was no difference in the discharge capacity per unit weight of the composite oxide even if the boron raw materials were different, and the effect of boron was confirmed. It is considered that there is no difference in the complex oxides after the heat treatment because the various boron compounds are easily transformed into oxides by heating in air.

Although it is not clear what action boron has in the complex oxide, observation by an electron microscope shows that all the complex oxides containing boron are crystallized in the heat treatment process. The growth rate was fast, and crystals of 10 μm or more easily grew at a temperature of about 700 ° C. Conventional LiCoO
_In the case of ₂ , a heat treatment condition of at least 900 ° C for 20 hours or more was required. The boron-containing composite oxide can be synthesized by a heat treatment at a lower temperature and a shorter time than ever before.

[0038]

As described above, in the non-aqueous electrolyte secondary battery, LiB _x Co _(1-x) containing boron as the positive electrode active material
By using the composite oxide having the O ₂ composition, the charge / discharge cycle characteristics of the non-aqueous electrolyte secondary battery were improved and it became possible to provide a secondary battery with a long life. The value of x is 0.001
It was effective even with a very small amount. When the value of x exceeds 0.25, the discharge capacity at the beginning of the charging / discharging cycle becomes small, and the generation of foreign matter can be recognized from the results of the X-ray diffraction pattern. The value of x is preferably 0.001 ≤ x ≤ 0.25.

The battery of the present invention has little deterioration during charge and discharge cycles and generates little gas due to decomposition, so that the battery can be hermetically sealed, and by combining with various electrolytes and various negative electrodes, a small button battery can be obtained. Can be applied to large batteries for electric vehicles, and plays a major role in the practical application of non-aqueous electrolyte secondary batteries.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a structure of a battery in an example of the present invention and a comparative example.

FIG. 2 is a graph showing the relationship between the number of cycles and the discharge capacity of an example battery and a comparative example battery of the present invention.

FIG. 3 is a configuration diagram of a beaker test cell.

FIG. 4 is a diagram showing the relationship between the boron content x and the discharge capacity at the first cycle.

FIG. 5 is a diagram showing a change in discharge capacity of a composite oxide with charge / discharge cycles.

FIG. 6 is a powder X-ray diffraction pattern of a composite oxide LiCoO ₂ .

FIG. 7: Powder X of complex oxide LiBxCo (1-x) O ₂ (x = 0.002)
Line diffraction diagram.

FIG. 8: Powder X of complex oxide LiBxCo (1-x) O ₂ (x = 0.01)
Line diffraction diagram.

FIG. 9: Powder X of complex oxide LiBxCo (1-x) O ₂ (x = 0.25)
Line diffraction diagram.

FIG. 10: Powder of complex oxide LiBxCo (1-x) O ₂ (x = 0.30)
X-ray diffraction diagram.

FIG. 11 is a graph showing the relationship between the number of cycles and the discharge capacity of the example battery of the present invention and the comparative example battery.

FIG. 12 is a graph showing the relationship between the boron content x and the discharge capacity at the first cycle.

[Explanation of symbols] 1 positive electrode 2 negative electrode 3 separator 4 battery case 5 positive electrode lead 6 sealing plate 7 glass seal 8 hermetic terminal 9 negative electrode lead 10 beaker 11 non-aqueous electrolyte 12 lid 13 positive electrode 14, 14 'counter electrode 15 reference electrode H book Invention Example Battery I Conventional Battery

Claims (2)

[Claims] 1. A non-aqueous electrolyte solution characterized in that a composite oxide of the general formula LiB _x Co _(1-x) O ₂ composed of lithium, boron and cobalt is used as a positive electrode active material. Next battery. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein x is 0.001 ≤ x ≤ 0.25.