JPH04329268A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To prevent the heat generation with quick temperature rise and the relatively quick damage even if over-charging is performed with high current by forming a part, which is coated with lithium carbonate, in the surface of a positive electrode mainly composed of lithium compound oxide. CONSTITUTION:A positive electrode 2, which are mainly composed of LixMO2, and a negative electrode 2, which can be doped and undoped with lithium, are wound through a eparators 3, and the positive electrode 1 and the negative electrode 2 are housed in a battery can 5 under the condition that insulating plates 4 are placed in the upper and the lower of the winding body to form a nonaqueous electrolyte secondary battery. M means transition metal, and at least one kind of Co and Ni is desirable, and 0.05<=x<=1.10. Furthermore, a current cutting thin plate 8 to be pushed up by the pressure rise inside of a battery is comprised. The surface of the positive electrode 2 is coated with lithium carbonate, and even if over-charging is performed with high current abnormal reaction inside of the battery with over-charging is prevented.

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 that uses a lithium composite oxide as a positive electrode and is equipped with current interrupting means that operates in response to an increase in battery internal pressure.

0002

BACKGROUND OF THE INVENTION In recent years, advances in electronic technology have led to higher performance, smaller size, and more portable electronic equipment, and there has been an increasing demand for high energy density secondary batteries used in these electronic equipment. Conventionally, nickel-cadmium batteries and lead batteries have been known as secondary batteries used in these electronic devices, but these batteries have a low discharge potential and are difficult to obtain in terms of obtaining batteries with high energy density. It is still insufficient. In recent years, secondary nonaqueous electrolytes have been developed that use materials that can be doped and dedoped with lithium ions, such as lithium, lithium alloys, or carbon materials, as negative electrodes, and lithium composite oxides such as lithium cobalt composite oxides as positive electrodes. Research and development of batteries is underway. This non-aqueous electrolyte secondary battery has high battery voltage and high energy density.
It has little self-discharge and excellent cycle characteristics.

By the way, in general, if a battery has a sealed structure, if the internal pressure of the battery increases for some reason, the battery will rapidly break, causing the battery to lose its function or even damage peripheral equipment. Sometimes it happens. In particular, when a non-aqueous electrolyte secondary battery as described above is manufactured with a sealed structure, if for some reason a current of more than a predetermined amount of electricity flows during charging, resulting in an overcharged state, the battery voltage will increase.
The electrolyte, etc. decomposes and generates gas, increasing the internal pressure of the battery. If such an overcharged state continues, abnormal reactions such as rapid decomposition of the electrolyte and active material may occur, resulting in a rapid rise in battery temperature.

As a countermeasure to this problem, a non-aqueous electrolyte secondary battery has been proposed, which is equipped with a current interrupting means that operates in response to an increase in battery internal pressure and whose positive electrode contains lithium carbonate. In this secondary battery, for example, when the overcharge state progresses, lithium carbonate in the positive electrode is electrochemically decomposed and carbon dioxide gas is generated. When the internal pressure of the battery begins to rise due to this gas generation, the current interrupting means is activated due to this increase in internal pressure, and the charging current is interrupted. This stops the progress of abnormal reactions inside the battery, and prevents a rapid rise in battery temperature and battery internal pressure.

[0005]

[Problems to be Solved by the Invention] However, in recent years, with the rapid charging of batteries, safety at higher charging currents has been required. For this reason, in a non-aqueous electrolyte secondary battery that is equipped with the above-mentioned current interrupting means and contains lithium carbonate in the positive electrode, safety can be ensured in the event of overcharging with the conventional charging current, but even higher current If the battery is overcharged, it may exhibit damage conditions such as heat generation accompanied by a rapid temperature rise and relatively rapid damage. Therefore, the present invention provides a highly safe non-aqueous electrolyte secondary battery that prevents heat generation accompanied by rapid temperature rise and relatively rapid damage even if it is overcharged with a higher current. The purpose is to provide a water electrolyte secondary battery.

[0006]

[Means for Solving the Problems] As a result of repeated studies to achieve the above-mentioned object, the present inventors have developed a method in which the surface of a positive electrode mainly composed of lithium composite oxide is covered with lithium carbonate. The inventors have discovered that, by doing so, heat generation accompanied by a rapid temperature rise and relatively rapid damage can be prevented, leading to the present invention. That is, the non-aqueous electrolyte secondary battery of the present invention has a positive electrode mainly composed of Lix MO2 (where M represents at least one transition metal and satisfies 0.05≦X≦1.10), The battery comprises a negative electrode capable of doping and dedoping, a nonaqueous electrolyte, and a current interrupting means that operates in response to an increase in battery internal pressure, and the positive electrode has a surface covered with lithium carbonate. That is.

[0007] In the present invention, Lix MO2 is used as the positive electrode.
(However, M represents at least one type of transition metal, preferably represents at least one type of Co or Ni. Also, X satisfies 0.05≦X≦1.10.) be done. Such active materials include LiCoO2
, LiNiO2 , LiNiy Co(1-y) O
2 (however, 0.05≦X≦1.10, 0<y<1). The above composite oxide is
For example, it can be obtained by using carbonates of lithium, cobalt, and nickel as starting materials, mixing these carbonates according to their compositions, and firing the mixture at a temperature range of 600° C. to 1000° C. in an atmosphere containing oxygen. Furthermore, the starting materials are not limited to carbonates, and can be similarly synthesized from hydroxides and oxides. In the present invention, the surface of the positive electrode is provided with a portion coated with lithium carbonate. Examples of methods for providing a portion covered with lithium carbonate on the surface of the positive electrode include a method in which lithium carbonate remains during synthesis of the active material, or a method in which lithium carbonate is mixed with the obtained active material after synthesizing the active material. A method of remelting lithium carbonate at a predetermined temperature may be considered, but is not limited to this method. The amount of lithium carbonate introduced is preferably 0.5 to 15% by weight in the positive electrode active material. As a method for confirming that a portion coated with lithium carbonate is formed on the surface of the positive electrode, for example, energy dispersive X-ray spectroscopy (E
There is a method of analyzing elements present on the surface of a positive electrode active material using DX).

On the other hand, in the present invention, for example, a carbon material is used for the negative electrode, but this carbon material may be any material as long as it can be doped and dedoped with lithium, such as pyrolytic carbons,
Cokes (pitch coke, needle coke, petroleum coke, etc.), graphite, glassy carbons, fired organic polymer compounds (carbonized by firing phenol resin, furan resin, etc. at an appropriate temperature), carbon fiber , activated carbon, etc. Alternatively, in addition to carbon materials, metallic lithium, lithium alloys (for example, lithium-aluminum alloys), and polymers such as polyacetylene and polypyrrole can also be used.

[0009] As the electrolytic solution, for example, an electrolytic solution in which a lithium salt is used as an electrolyte and is dissolved in an organic solvent is used. Examples of organic solvents include, but are not limited to, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, γ-butyl lactone,
Tetrahydrofuran, 2-methyltetrahydrofuran,
1,3-dioxolane, sulfolane, acetonitrile,
Diethyl carbonate, dipropyl carbonate, and the like can be used alone or as a mixed solvent of two or more.

[0010] As the electrolyte, LiClO4, LiA
sF6, LiPF6, FiBF4, LiB(C6
H5)4, LiCl, LiBr, CH3 SO3
Li, CF3 SO3 Li, etc. can be used. Further, as the current interrupting means, any current interrupting means normally provided in this type of battery can be employed, and any current interrupting means may be used as long as it can interrupt the current according to the internal pressure of the battery.

[0011]

[Operation] By using LiCoO2 whose surface is coated with lithium carbonate as the positive electrode, overcharging at a higher current can prevent the battery from generating heat accompanied by a rapid temperature rise and from relatively rapid damage. Although the reason for this is not clear, it is thought to be as follows. That is, lithium carbonate is electrochemically decomposed during overcharging to generate carbon dioxide gas, thereby ensuring safety during overcharging. Here, if lithium carbonate is simply mixed with the positive electrode active material in the positive electrode, the reaction area of lithium carbonate is small. Therefore, when overcharging is performed with a higher charging current, the decomposition reaction of lithium carbonate may not be completed in time, and an abnormal reaction may proceed within the battery. However, by partially providing a layer of lithium carbonate on the surface of the active material to cover the surface of the active material, the surface area of lithium carbonate, that is, the reaction area, increases. As a result, even if overcharged with a higher charging current, lithium carbonate decomposes quickly, promoting the generation of carbon dioxide gas, suppressing the progress of abnormal reactions within the battery, and preventing a rapid temperature rise in the battery. This seems to have made it possible to prevent the accompanying heat generation and relatively rapid damage.

0012

EXAMPLES Preferred examples of the present invention will be described below based on experimental results. First, FIG. 1 shows the structure of a non-aqueous electrolyte secondary battery produced in each of the Examples described below. As shown in FIG. 1, this nonaqueous electrolyte secondary battery includes a negative electrode 1 formed by applying a negative electrode active material to a negative electrode current collector 9, and a positive electrode current collector 1.
0 and a positive electrode 2 coated with a positive electrode active material are wound together with a separator 3 in between, and the wound body is housed in a battery can 5 with insulating plates 4 placed above and below the wound body. . A battery lid 7 is attached to the battery can 5 by tightening it through a sealing gasket 6, and a negative electrode lead 1 is attached to the battery can 5, respectively.
It is electrically connected to a negative electrode 1 or a positive electrode 2 via a positive electrode lead 12 and a negative electrode lead 12, and is configured to function as a negative electrode or a positive electrode of a battery.

In the non-aqueous electrolyte secondary battery of this embodiment, the positive electrode lead 12 is attached to the current interrupting thin plate 8 by welding, and the battery lid 7 is attached via the current interrupting thin plate 8.
An electrical connection is made with the In a battery having such a configuration, when the pressure inside the battery increases,
As shown in FIG. 3, the current interrupting thin plate 8 is pushed up and deformed. Then, the positive electrode lead 12 is connected to the current interrupting thin plate 8.
The welded part is cut and the current is cut off.

Example 1 A positive electrode active material (LiCoO2) was synthesized as follows. First, lithium carbonate and cobalt carbonate are mixed into Li/Co(
Mix so that the molar ratio) = 1.15, and mix in air at 90%
It was baked at 0°C for 5 hours. As a result of performing X-ray diffraction measurements on this material, it was found that it was a mixture of LiCoO2 and lithium carbonate. Further, when the amount of lithium carbonate in this material was quantified, it was found that it contained 6.2% by weight of lithium carbonate. The above amount of lithium carbonate is determined by decomposing a sample with sulfuric acid, introducing Co2 into a barium chloride and sodium hydroxide solution, absorbing it, and then titrating it with a standard hydrochloric acid solution to quantify the Co2. It was calculated from the amount.

[0015] Thereafter, this material was ground using an automatic mortar to obtain LiCoO2. L obtained in this way
Using iCoO2 as a positive electrode active material, a positive electrode mixture was prepared by mixing 91% by weight of this positive electrode active material, 6% by weight of graphite as a conductive material, and 3% by weight of polyvinylidene fluoride as a binder. -Methyl-2-pyrrolidone to form a slurry. Then, this slurry is applied to both sides of a strip-shaped aluminum foil that is the positive electrode current collector 10, and after drying, compression molding is performed using a roller press machine to form the positive electrode 2.
It was created.

Next, the negative electrode active material is prepared by using petroleum pitch as a starting material, introducing 10 to 20% of oxygen-containing functional groups (so-called oxygen crosslinking), and then heating it at 1000° C. in an inert gas.
A non-graphitic carbon material with properties similar to those of glassy carbon was used. As a result of X-ray diffraction measurement of this carbon material, the interplanar spacing of the (002) plane was 3.76 Å.
Also, the true specific gravity was 1.58. This carbon material 90
% by weight, 10% by weight of polyvinylidene fluoride as a binder
A negative electrode mixture was prepared by mixing at a ratio of 1, and this was dispersed in N-methyl-2-pyrrolidone to form a slurry. Then, this slurry was applied to both sides of a strip-shaped copper foil serving as a negative electrode current collector, and after drying, compression molding was performed using a roller press machine to create negative electrode 1.

[0017]Then, these strip-shaped positive electrode 2, negative electrode 1, and separator 3 made of a 25 μm microporous polypropylene film were sequentially laminated and then spirally wound many times to create a wound body. . Next, the insulating plate 4 is inserted into the bottom of the nickel-plated iron battery can 5.
The above-mentioned rolled body was stored. Then, in order to collect current from the negative electrode, one end of the negative electrode lead 11 made of nickel was crimped to the negative electrode 1, and the other end was welded to the battery can 5. In addition, in order to collect current from the positive electrode, one end of the aluminum positive electrode lead 12 was connected to the positive electrode 2.
The other end was welded to a battery lid 7 having a current interrupting device 8 that interrupts current depending on the internal pressure of the battery. This battery can 5
1 m of LiPF6 in a mixed solvent of 50% by volume of propylene carbonate and 50% by volume of diethyl carbonate.
An electrolytic solution in which ol was dissolved was injected. Then, by caulking the battery can 5 through the insulating sealing gasket 6 coated with asphalt, the battery lid 7 was fixed, and a cylindrical battery having a diameter of 14 mm and a height of 50 mm was created.

Example 2 A positive electrode active material (LiCoO2) was synthesized as follows. First, lithium carbonate and cobalt carbonate are mixed into Li/Co(
Mix so that molar ratio) = 1, and heat at 900°C in air.
It was baked for 5 hours. The results of X-ray diffraction measurements of this material showed good agreement with LiCoO2 in the JCPDS card. Thereafter, the above material was crushed using an automatic mortar to obtain LiCoO2. Li obtained in this way
CoO2 93.8% by weight and lithium carbonate 6.2% by weight
After mixing well, the mixture was baked in air at 750°C for 5 hours to obtain a positive electrode active material. The amount of lithium carbonate in this positive electrode active material was determined to be 6.2% by weight. Then, this positive electrode active material was crushed using an automatic mortar, and the resulting positive electrode active material was 91% by weight, and graphite was used as a conductive material in an amount of 6% by weight.
A cylindrical battery was prepared in exactly the same manner as in Example 1 except that a positive electrode was prepared by mixing polyvinylidene fluoride in a proportion of 3% by weight as a binder.

Comparative Example 1 A positive electrode active material (LiCoO2) was synthesized as follows. First, lithium carbonate and cobalt carbonate are mixed into Li/Co(
Mix so that molar ratio) = 1, and heat at 900°C in air.
It was baked for 5 hours. The results of X-ray diffraction measurements of this material showed good agreement with LiCoO2 in the JCPDS card. Furthermore, when the amount of lithium carbonate in this positive electrode active material was quantified, it was hardly detected and was 0%. Thereafter, the above materials were crushed using an automatic mortar to form LiCoO2
I got it. Using the LiCoO2 thus obtained as a positive electrode active material, a positive electrode was prepared by mixing 91% by weight of this positive electrode active material, 6% by weight of graphite as a conductive material, and 3% by weight of polyvinylidene fluoride as a binder. However, a cylindrical battery was produced in exactly the same manner as in Example 1 except for this.

Comparative Example 2 Using LiCoO2 obtained in Comparative Example 1 above, Li
CoO2 93.8% by weight, lithium carbonate 6.2% by weight
A positive electrode was prepared by mixing 91% by weight of the mixture obtained as above, 6% by weight of graphite as a conductive material, and 3% by weight of polyvinylidene fluoride as a binder. A cylindrical battery was created.

Twenty batteries obtained in each of the above-mentioned Examples and Comparative Examples were prepared, and these batteries were heated to a current of 1.5 A.
The rate of occurrence of batteries (damaged products) in which heat generation accompanied by a rapid temperature rise of the battery and rapid damage in the comparative example occurred when the battery was overcharged at 3.7 A was investigated. The results are shown in Table 1.

[Table 1]

As shown in Table 1, during overcharging at a charging current of 1.5 A, no damaged products occurred except for Comparative Example 1 which did not contain lithium carbonate. For overcharging at an even higher charging current of 3.7 A, Example 1 in which lithium carbonate remained during active material synthesis was mixed with lithium carbonate after active material synthesis, and the lithium carbonate was remelted at 750°C. In Example 2, no damaged products were generated, whereas in Comparative Example 2, in which lithium carbonate was simply mixed after active material synthesis,
Damage was observed in most of the batteries as in Comparative Example 1 which did not contain lithium carbonate.

Therefore, X-ray diffraction measurements were performed on the positive electrode active material in which lithium carbonate remained during the active material synthesis of Example 1 and the positive electrode active material in which lithium carbonate was mixed after the active material synthesis in Comparative Example 2. The results are shown in FIGS. 3 and 4. FIG. 3 is an X-ray diffraction diagram of the positive electrode active material in which lithium carbonate was left in the synthesis of the active material of Example 1, and FIG.
It is a line diffraction diagram. As shown in FIGS. 3 and 4, Example 1
There was no significant difference in the X-ray diffraction patterns of the positive electrode active material of Comparative Example 2 and that of the positive electrode active material of Comparative Example 2, indicating that the positive electrode active material was a mixture of LiCoO2 and lithium carbonate.

Next, using an energy dispersive X-ray spectrometer (EDX) manufactured by KEVEX,
Elemental analysis of the surfaces of minute regions of the positive electrode active material of Example 1 and the positive electrode active material of Comparative Example 2 described above was conducted. The results are shown in FIGS. 5 and 6. Figure 5 shows the surface spectrum of the positive electrode active material in which lithium carbonate remained during the active material synthesis of Example 1, and Figure 6 shows the surface spectrum of the positive electrode active material in which lithium carbonate was mixed after the active material synthesis of Comparative Example 2. show.

From FIG. 6, in the positive electrode active material in which lithium carbonate was mixed after the synthesis of the active material of Comparative Example 2, the spectrum of Co clearly appeared. Furthermore, when elemental analysis is performed on each particle, there are parts with a Co spectrum and parts without it, and the parts with a Co spectrum correspond to LiCoO2, as shown in Figure 7. As shown in FIG. 8, the area without the Co spectrum corresponded to lithium carbonate. From this, it was found that in the positive electrode active material of Comparative Example 2, LiCoO2 and lithium carbonate were separated and present in a mixed state. On the other hand, as shown in FIG. 5, for the positive electrode active material in which lithium carbonate remained during the active material synthesis of Example 1, LiCoO
Although the Co spectrum corresponding to Comparative Example 2 does not appear conspicuously and has a weak peak intensity, and the intensity of the Co spectrum changes slightly even if elemental analysis is performed on each particle, the positive electrode active material of Comparative Example 2 There was no significant change in the Co spectrum as there was.

[0026] From this, a positive electrode active material in which lithium carbonate was left during the synthesis of the active material, like a positive electrode active material in which lithium carbonate was mixed after the synthesis of the active material, exists in a state where LiCoO2 and lithium carbonate are separated and mixed. It can be seen that lithium carbonate exists in a state covering the surface of LiCoO2, rather than being completely covered with it. Therefore, LiC
The spectrum of Co corresponding to oO2 does not appear prominently and the peak intensity is weak. In addition, the positive electrode active material obtained by mixing lithium carbonate and remelting the lithium carbonate at 750°C after the synthesis of the active material in Example 2 has an X-ray diffraction diagram and an EDX
The results were the same as in Example 1. From this, the positive electrode active material of Example 2, like the positive electrode active material of Example 1,
It is thought that lithium carbonate exists in a state covering the LiCoO2 surface.

Therefore, the lithium carbonate in the positive electrode is LiCo
By changing the state from simply being mixed with O2 to covering the surface of LiCoO2, it becomes possible to prevent heat generation and relatively rapid damage due to rapid temperature rise of the battery even when overcharging at a higher current. Ta. In this example, LiCoO2 was used as the positive electrode active material, but other positive electrode active materials (for example, LiX Niy Co(1-y) O
2 (however, 0.05≦X≦1.10, 0<y≦1)), similar effects were confirmed.

[0028]

[Effects of the Invention] As is clear from the above description, in the present invention, by using Lix MO2, whose surface is covered with lithium carbonate, as the positive electrode, it can be overcharged at even higher currents. This prevents abnormal reactions inside the battery due to overcharging, and prevents heat generation and relatively rapid damage due to rapid temperature rise of the battery. Therefore, it is possible to provide a nonaqueous electrolyte secondary battery with high energy density, excellent cycle characteristics, and high safety. Moreover, this non-aqueous electrolyte secondary battery has great industrial and commercial value.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view showing a configuration example of a non-aqueous electrolyte secondary battery of the present invention.

FIG. 2 is a schematic cross-sectional view showing the operating state of the current interrupting means.

FIG. 3 is an X-ray diffraction diagram of a positive electrode active material in which lithium carbonate remained during active material synthesis.

FIG. 4 is an X-ray diffraction diagram of a positive electrode active material mixed with lithium carbonate after active material synthesis.

FIG. 5 is a characteristic diagram showing a surface spectrum obtained by energy dispersive X-ray spectroscopy (EDX) of a positive electrode active material in which lithium carbonate remained during active material synthesis.

FIG. 6 is a characteristic diagram showing an EDX surface spectrum of a positive electrode active material mixed with lithium carbonate after active material synthesis.

FIG. 7 is a characteristic diagram showing the EDX surface spectrum of a portion of the Co spectrum in the elemental analysis of each particle of a positive electrode active material mixed with lithium carbonate after active material synthesis.

FIG. 8 is a characteristic diagram showing a surface spectrum by EDX of a portion without a Co spectrum in the elemental analysis of each particle of a positive electrode active material mixed with lithium carbonate after active material synthesis.

[Explanation of symbols]

1...Negative electrode 2...Positive electrode 3...Separator 8...Thin plate for current interruption

Claims (1)

[Claims] [Claim 1] A positive electrode mainly composed of Lix MO2 (where M represents at least one transition metal and satisfies 0.05≦X≦1.10), and a negative electrode that can be doped and dedoped with lithium. A non-aqueous electrolyte secondary battery comprising: a non-aqueous electrolyte; and a current interrupting means that operates in response to an increase in battery internal pressure, the positive electrode having a surface coated with lithium carbonate. .