JPH11283624A - Lithium secondary battery and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery having a voltage gradient from the initial discharging to the completion in such a manner as to facilitate detection of a residual capacity by a discharging voltage. SOLUTION: A lithium compound such as lithium hydroxide is mixed with lithium oxide, followed by heat treatment, thus preparing a ramsdellite type crystal structure expressed by a general formula: Li2 Ti3 O7 . A material including mainly lithium titanium oxide having the resultant crystal structure is used as a negative electrode, thus constituting a lithium secondary battery capable of exhibiting variable voltage characteristics with progression of charging/ discharging without deteriorating excellent charging/discharging cycle characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a portable device such as a mobile communication terminal, a chargeable / dischargeable lithium secondary battery used as a backup power source for a memory, and more particularly to a general formula Li
_{The present} invention relates to a lithium secondary battery using a negative electrode mainly composed of lithium titanium oxide represented by ₂ Ti ₃ O ₇ and a method for producing the same.

[0002]

2. Description of the Related Art Due to the rapid development of technologies in the field of electronics in recent years, electronic devices have been reduced in size, and there has been an increasing demand for small and lightweight batteries having a high energy density as power supplies for these devices. Against this background, lithium secondary batteries using lithium for the negative electrode have attracted particular attention, and are being actively developed.

However, in a secondary battery using lithium for the negative electrode, repetition of charging and discharging causes dendritic lithium called dendrites to grow on the surface of the negative electrode using lithium, which breaks through the separator and causes an internal short circuit. Due to the phenomena that occur, the charge / discharge cycle life is significantly impaired.

As a solution, metal oxides such as niobium pentoxide and lithium titanium oxide having a spinel type crystal structure can be used as a material capable of inserting and extracting lithium in the negative electrode.
Alternatively, a lithium secondary battery using carbon such as natural graphite, artificial graphite or carbon fiber, and a lithium alloy such as lithium-aluminum or lithium-zinc has been proposed.

[0005] Among such materials capable of occluding and releasing lithium, lithium titanium oxide having a spinel type crystal structure disclosed in JP-A-6-275263 has a ratio of Li / Li ⁺ to 1. It has a very flat charge / discharge curve near 5V. Furthermore, lithium titanium oxide having a spinel-type crystal structure is excellent in long-term charge-discharge cycle characteristics because lithium ions are easily absorbed and released, and there is no generation of dendrites due to repeated charge and discharge. A lithium secondary battery using lithium titanium oxide for a negative electrode has been put to practical use.

[0006]

In recent years, in an electronic device using a small secondary battery as a power supply, a function of detecting the remaining capacity of the battery has become an indispensable configuration. More recently, limited battery capacity must be used effectively to meet the conflicting demands of higher battery capacity due to increased power usage on the device side and weight reduction for the purpose of reducing device weight. There is. As a method of detecting the discharge capacity of a battery, various methods have been proposed in which the charging / discharging current and voltage are measured, and the battery capacity is detected and displayed. However, this method causes an increase in the weight of the electronic device, and is not suitable for a device that emphasizes portability. Therefore, a configuration is adopted in which the relationship between the battery voltage and the remaining capacity is input to an IC in advance, and the remaining capacity is displayed based on the level of the battery voltage.

The lithium titanium oxide having a spinel-type crystal structure described above has characteristics such that it shows a flat potential at the time of charge and discharge, and shows a sudden change in potential at the end of each phase. Therefore, a lithium secondary battery in which lithium titanium oxide is used for the negative electrode shows voltage characteristics in which a region where voltage is displaced flatly and a region where voltage is rapidly displaced are present. For this reason, there is a problem that the accuracy of the remaining capacity detected based on the voltage level is reduced.

The present invention provides a lithium secondary battery that has a voltage gradient from the beginning to the end of discharge so that the remaining capacity can be easily detected by voltage, and makes it easy to detect the remaining capacity by the discharge voltage. .

[0009]

In order to solve the above-mentioned problems, a lithium secondary battery of the present invention comprises a positive electrode, a negative electrode capable of doping / dedoping lithium, and an electrolyte using a lithium salt as a supporting salt. In the lithium secondary battery, a lithium titanium oxide represented by a general formula Li ₂ Ti ₃ O ₇ is used as a main component of the negative electrode.

According to this configuration, it is possible to provide a lithium secondary battery having a so-called voltage gradient in which the voltage changes as charging and discharging progress, without impairing excellent charge and discharge cycle characteristics. When the equipment using this battery is in use, the discharge capacity decreases as the use time elapses, and the discharge voltage of the battery also decreases.Therefore, a voltage check is performed based on a predetermined relationship between the discharge capacity and the voltage. The remaining capacity can be easily detected.

[0011]

Embodiments of the present invention will be described below.

The invention according to claim 1 provides a lithium secondary battery comprising a positive electrode, a negative electrode capable of doping / dedoping lithium, and an electrolyte using a lithium salt as a supporting salt, wherein the lithium secondary battery has a general formula of Li ₂ Ti ₃ O A negative electrode mainly composed of lithium titanium oxide represented by ₇ is used.

The lithium titanium oxide represented by the general formula Li ₂ Ti ₃ O ₇ has a ramsdellite type crystal structure. Unlike the known spinel type crystal structure, this lithium titanium oxide has a reduction potential of 2 with respect to Li / Li ^+.
Starting from around V, the potential proceeds with a gentle potential gradient to around 0.5 V. In the oxidation reaction, the potential slightly increases and proceeds in the opposite direction. The reason is that in the case of the spinel type crystal structure, the oxidation-reduction proceeds at a certain potential due to the heterogeneous reaction (reaction in which the reactant and the product coexist), and the ramsdellite type In the case of a crystal structure, it is presumed that a gentle S-shaped potential gradient is generated due to a homogeneous reaction (a reaction in which a product changes uniformly).

Therefore, when the lithium titanium oxide is used as the active material for the negative electrode, even when the positive electrode is combined with an active material having a flat charge / discharge potential, the formed battery has a gentle S-shaped voltage gradient. Will be shown.

The lithium titanium oxide is Cu
When the X-ray diffraction is performed using as a target, the plane spacing is at least 4.45 °, 2.69 °, 2.24 °, 1.7.
7 ° (each ± 0.02 °). Further, C
The peak intensity ratio between the peak value at 4.45 ° (± 0.02 °) and the peak value at 2.69 ° (± 0.02 °) in the X-ray diffraction pattern obtained from the X-ray diffraction method targeting u , 100: 20-100: 60
Is in the range. A negative electrode using a lithium titanium oxide having a ramsdellite-type crystal structure having such physical properties also has a feature that an energy density can be increased as compared with a spinel-type crystal structure.

According to a sixth aspect of the present invention, there is provided a lithium secondary battery comprising a positive electrode, a negative electrode containing lithium titanium oxide capable of doping / dedoping lithium, and an electrolyte using a lithium salt as a supporting salt. Wherein a lithium compound and a titanium oxide are mixed and then heat-treated to produce a lithium titanium oxide. The lithium titanium oxide produced by this production method has a ramsdellite type crystal structure represented by the general formula Li ₂ Ti ₃ O ₇ , and a lithium secondary battery using this as a negative electrode is charged and discharged. Shows a characteristic in which the voltage changes as the state progresses. This has the effect of facilitating detection of the remaining capacity by voltage check.

In the above production method, the lithium compound is selected from lithium hydroxide, lithium oxide, lithium nitrate and lithium carbonate. After mixing one or more selected lithium compounds and titanium oxide,
Heat treatment is performed. At this time, the mixing ratio of the lithium compound and titanium oxide affects the crystal structure of the generated lithium titanium oxide.

Further, in the step of performing the heat treatment, the crystal structure of the generated lithium titanium oxide also differs depending on the temperature conditions at which the heat treatment is performed. The lithium titanium oxide having a ramsdellite-type crystal structure represented by the general formula Li ₂ Ti ₃ O ₇ can be obtained by performing a heat treatment at a temperature of 950 ° C. or higher.

The temperature condition of the heat treatment will be described in detail. When the heat treatment is performed at 800 ° C., lithium titanium oxide having a spinel type crystal structure is generated. 950 ° C
In the case of heat treatment, formation of a lithium titanium oxide having a ramsdellite type crystal structure is observed. Therefore,
Under this temperature condition, the X-ray diffraction peak of lithium titanium oxide having a ramsdellite type crystal structure is weak. When the heat treatment is performed at 1000 ° C. or higher, the intensity of the X-ray diffraction peak of the ramsdellite type crystal structure is increased, and the spinel type crystal structure and the ramsdellite type crystal structure of lithium titanium oxide coexist.

The upper limit of the heat treatment temperature is a temperature close to the temperature at which the lithium titanium oxide melts, that is, 1400 ° C.
It is possible to near. However, considering the efficiency of the heat treatment, the heat treatment temperature is preferably lower, and the heat treatment temperature of 1000 ° C. to 1300 ° C. at which a lithium titanate having a ramsdellite-type crystal structure is obtained is considered industrially preferable. Can be At this time, as the heat treatment temperature is higher, the peak of lithium titanium oxide having a spinel crystal structure is weakened, and the peak of lithium titanium oxide having a ramsdellite crystal structure is dominant. Of course, the method for producing Li ₂ Ti ₃ O ₇ of the present invention may be another method.

[0021]

An embodiment of the present invention will be described below with reference to the drawings. It should be noted that the following embodiments are only examples embodying the present invention, and do not limit the technical scope of the present invention.

One mole of titanium oxide is mixed with 2/3 mole of lithium hydroxide, and is mixed at 800 ° C. and 95 ° C. in an oxygen atmosphere.
0 ° C, 1000 ° C, 1100 ° C, 1300 ° C, 1400
Each was heat-treated at 8 ° C. for 8 hours to prepare lithium titanium oxide. X-ray diffraction was performed on these samples using Cu as a target. Table 1 shows the main peak positions and peak intensity ratios in the X-ray diffraction pattern. Further, a ramsdellite type crystal structure (JCPDS No. 34-039)
3) and a spinel type crystal structure (JCPDS No. 2)
Table 2 shows the peak positions and peak intensity ratios of the X-ray diffraction of the lithium titanium oxide of 6-1198).

[0023]

[Table 1]

[0024]

[Table 2]

As can be seen by comparing Tables 1 and 2, 80
At 0 ° C., lithium titanium oxide has a spinel-type crystal structure, and at 950 ° C. or higher, lithium titanium oxide having a ramsdellite-type crystal structure is formed and is a main component. Almost the same peak was observed even when the heat treatment temperature was set at 1000 ° C. or higher, and lithium titanium oxide was obtained stably. From these facts, the X-ray diffraction pattern characterizing the lithium titanium oxide of the present invention has peak positions of at least 4.45 °, 2.69 °, 2.24 °, and 1.77.
(Each ± 0.02 °), and the peak intensity ratio between the peak value at 4.45 ° and the peak value at 2.69 ° is in the range of 100: 20 to 100: 60,
You can check.

These samples were kneaded with 90 wt%, 5 wt% of acetylene black as a conductive material and 5 wt% of a fluororesin as a binder so as to have a weight ratio of 5 wt%.
mm, formed into a pellet having a thickness of 0.6 mm, and then dehydrated by vacuum drying at 250 ° C.
Lithium doped in an electrolytic solution was used as a negative electrode. Here, the filling amount of the lithium titanium oxide is 100 m
g. Further, 3 against Li / Li ⁺ as a positive electrode.
A lithium manganese oxide having a potential of 0 V was used.

FIG. 1 shows the configuration of a coin-type lithium secondary battery of the present invention. 1 is a case that also serves as a positive electrode terminal, 2 is a sealing plate that also serves as a negative electrode terminal, 3 is a polypropylene gasket that insulates the case 1 from the sealing plate 2, 4 is a positive electrode, 90% by weight of lithium manganese oxide, and acetylene is a conductive material. 5 wt% black and 5 wt% fluororesin as binder
Kneaded so as to have a weight ratio of 15 mm and a thickness of 0.1 mm.
The pellets were formed into a size of 6 mm and dehydrated by vacuum drying at 250 ° C. Reference numeral 5 denotes a negative electrode, which is the lithium titanium oxide of the present invention. Reference numeral 6 denotes a polypropylene nonwoven fabric having a thickness of 0.3 mm. The electrolytic solution is a mixture of propylene carbonate (PC), ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) in an equal volume of 1 mol / l of LiPF _6.
Is dissolved in the ratio of Battery size is outer diameter 2
0 mm and height 2.0 mm. These batteries were heated at a heat treatment temperature of 950 ° C. for lithium titanium oxide at 100 ° C.
For each of 0 ° C, 1100 ° C, 1300 ° C, and 1400 ° C, battery A-1, battery A-2, battery A-3, and battery A
-4 and battery A-5. These batteries A-1 to A-
Reference numeral 5 denotes a negative electrode active material of the present invention in which lithium titanium oxide having a ramsdellite type crystal structure is used as a main component. The heat treatment temperature of lithium titanium oxide is 800 ° C.
Was designated as Comparative Battery B. Comparative battery B uses lithium titanium oxide having a spinel crystal structure as a main component as a negative electrode active material.

In the discharge test, the battery was discharged at a constant current of 1 mA at room temperature, and the discharge sustaining voltage up to 0.5 V was shown in FIG. 2, and the energy density was shown in FIG.

As apparent from FIGS. 2 and 3, 950 ° C.
By performing the above heat treatment, that is, by using lithium titanium oxide having a ramsdellite type crystal structure as a main component, the discharge average discharge voltage is increased by about 0.15 V, and the energy density is increased slightly but to about 3%. . Further, the voltage gradient of the discharge was about 2 mV for the comparative battery B per 10% of the discharge capacity, whereas the battery A-
1 to A-5 are as large as about 60 mV, which makes it easy to detect the remaining capacity by voltage check.

FIG. 3 shows a constant current of 1 mA and 2.5 V
5 shows the change in the discharge capacity of each battery obtained when charging and discharging are repeated between 0.5 and 0.5 V. All of the batteries are stable up to 100 cycles, and the reversibility is not lost even if the spinel crystal structure is changed to a lithium titanium oxide having a ramsdellite crystal structure. This is because the lithium ion doping and undoping reactions of lithium ions during charge / discharge of the lithium titanium oxide crystal having a spinel crystal structure hardly distort the crystal lattice, but from a spinel type to a ramsdellite type. It is considered that the same reason is obtained even if the crystal morphology changes in the structure.

Although lithium hydroxide is used as the lithium compound in the embodiment, lithium oxide, lithium nitrate, and lithium carbonate may be used instead. Further, lithium manganese oxide was used as the positive electrode, but instead of vanadium pentoxide having 3.5 V with respect to Li / Li ⁺ , lithium cobalt oxide having 4.0 V, lithium nickel oxide, spinel type Crystalline lithium manganese oxide may be used. Furthermore, in the electrolyte,
In addition to those used in the examples, for example, LiN (CF ₃ S
O ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiClO ₄ , Li
One or two lithium salts of CF ₃ SO ₃ are
An organic electrolyte dissolved in a single solvent such as diethoxyethane, ethoxymethoxyethane, ethylmethyl carbonate, dimethyl carbonate, γ-butyrolactone, tetrahydrofuran, or a mixture of two or more solvents may be used.

[0032]

As is apparent from the above description, according to the present invention, by using a lithium titanium oxide having a ramsdellite type crystal structure for the negative electrode, the charge-discharge cycle characteristics are excellent and the remaining capacity by the voltage check is reduced. An object of the present invention is to provide a lithium secondary battery having a large voltage gradient so that detection is easy.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lithium secondary battery of the present invention.

FIG. 2 is a discharge characteristic diagram of the present invention and a comparative battery.

FIG. 3 is a diagram showing energy densities of the present invention and a comparative battery.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Case 2 Sealing plate 3 Gasket 4 Positive electrode 5 Negative electrode 6 Separator

Claims (8)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode capable of doping / dedoping lithium, and an electrolyte using a lithium salt as a supporting salt, wherein the negative electrode has a general formula of Li ₂ Ti ₃ O ₇ . A lithium secondary battery comprising a lithium titanium oxide as a main component. 2. The lithium secondary battery according to claim 1, wherein the crystal structure of the lithium titanium oxide is a ramsdellite type crystal structure. 3. The lithium titanium oxide has a plane spacing of 4.45 ° by an X-ray diffraction method using Cu as a target.
2.69Å, 2.24Å, 1.77Å (± 0.02 each)
3. The lithium secondary battery according to claim 1 or 2, wherein 4. A peak intensity ratio between a peak value at 4.45 ° (± 0.02 °) and a peak value at 2.69 ° (± 0.02 °) obtained by an X-ray diffraction method using Cu as a target. The lithium secondary battery according to any one of claims 1 to 3, wherein a lithium titanium oxide in a range of 100: 20 to 100: 60 is used. 5. The negative electrode according to claim 1, wherein a lithium titanium oxide having a ramsdellite type crystal structure and a lithium titanium oxide having a spinel type crystal structure coexist.
Item 7. The lithium secondary battery according to Item 1. 6. A positive electrode, a negative electrode containing lithium titanium oxide represented by the general formula Li ₂ Ti ₃ O ₇ capable of doping / dedoping lithium, and a lithium composed of an electrolyte using a lithium salt as a supporting salt. A method of manufacturing a secondary battery,
A method for producing a lithium secondary battery, wherein the lithium titanium oxide is generated by a step of heat-treating a mixture of a lithium compound and titanium oxide. 7. The method for producing a lithium secondary battery according to claim 6, wherein at least one lithium compound selected from lithium hydroxide, lithium oxide, lithium nitrate, and lithium carbonate is used. 8. A heat treatment temperature in the heat treatment step is 95.
The method for producing a lithium secondary battery according to claim 6, wherein the temperature is in a range of 0 ° C to 1400 ° C.