JP2001273903A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a low-temperature property in a lithium secondary battery that is constituted from a positive electrode in which lithium containing complex oxide is used as an active material, a negative electrode which is composed of carbonaceous materials wherein occlusion and emission of lithium ion is electrochemically possible, a separator, and a nonaqueous electrolytic solution. SOLUTION: By adding Co3O4 which has a surface area of specific range, an electro-conductivity of positive electrode can be enhanced and a lithium secondary battery which can be superior in the low-temperature property can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a positive electrode using a lithium-containing composite oxide as an active material, a negative electrode made of a carbon material such as graphite capable of electrochemically occluding and releasing lithium ions, a separator, and a non-aqueous electrolyte. The present invention relates to a lithium secondary battery using a liquid, and more particularly to a lithium secondary battery having excellent low-temperature characteristics.

[0002]

2. Description of the Related Art A lithium secondary battery using a non-aqueous electrolyte, a carbon material such as graphite as a negative electrode material, and a lithium-containing composite oxide as a positive electrode active material has a higher voltage and voltage than an aqueous secondary battery. It is expected to be used as a main power source for consumer electronic devices, which have a high energy density and are becoming portable and cordless in recent years. Also, since lithium metal is not used for the negative electrode, cycle stability and safety are excellent. Furthermore, a polymer that absorbs and retains the non-aqueous electrolyte is mixed with the active material layer,
A lithium polymer secondary battery using a film made of the polymer is also being developed as a thin and lightweight battery.

[0003] However, with the widespread use of lithium ion secondary batteries characterized by light weight and high energy density, and lithium polymer secondary batteries characterized by thinning, the usage environment of these secondary batteries has become more severe. It has become something. When mounted on a portable device, it is expected that the device will be used outdoors under low temperatures, and it is required to exhibit higher battery performance even in such an environment.

However, since these lithium secondary batteries use an organic solvent-based electrolyte having a higher viscosity and a lower electric conductivity than an aqueous electrolyte, there is a problem in low-temperature characteristics.

[0005]

To solve the above problem, for example, Japanese Patent Application Laid-Open No. Hei 10-334916 proposes to heat-treat a graphite surface in a non-oxidizing atmosphere to modify the surface. High production costs cannot be avoided. Japanese Patent Application Laid-Open Nos. Hei 5-217602 and Hei 10-31825 propose the use of γ-butyrolactone as a solvent for an electrolytic solution. However, there is a problem of gas generation due to decomposition of the solvent during high-temperature storage. The properties are not always satisfactory.

For the purpose of improving the deterioration of battery characteristics during high-temperature storage, Japanese Patent Application Laid-Open No. 9-330719 discloses FeOb, CoOc, MnOd (0 <b) in addition to a lithium-transition metal composite oxide as an active material. , C, d <
It has been proposed to add a transition metal oxide such as 1.35), but the surface area is not described.

It is an object of the present invention to provide a lithium secondary battery using a lithium-containing composite oxide as a positive electrode active material and a carbon material capable of electrochemically occluding and releasing lithium ions for a negative electrode. Without loss, improvement of discharge capacity decrease due to resistance rise inside battery at low temperature,
For example, the discharge capacity at 20 ° C is 50 at -20 ° C.
% To maintain the capacity.

[0008]

In order to solve this problem, the present invention uses a lithium-containing composite oxide as a positive electrode active material and a carbon material capable of electrochemically storing and releasing lithium ions as a negative electrode. In the lithium secondary battery, by including Co ₃ O ₄ having a specific range of surface area in the positive electrode, there is no gas generation due to oxidative decomposition of the electrolyte during high-temperature storage, thereby improving the conductivity of the positive electrode, Based on the finding that the reduction in discharge capacity can be improved.

According to a first aspect of the present invention, there is provided a positive electrode using a lithium-containing composite oxide as an active material, a negative electrode made of a carbon material such as graphite capable of electrochemically storing and releasing lithium ions, and impregnating them. In a lithium secondary battery comprising a non-aqueous electrolyte, a surface area of the positive electrode ranges from 15 m ² / g to
By containing 65 m ² / g of Co ₃ O ₄ , the discharge characteristics at low temperatures are improved and the occurrence of gas during high-temperature storage is reduced.

According to a second aspect of the present invention, there is provided an active material mixture layer containing an active material comprising a lithium-containing composite oxide and a polymer capable of absorbing and retaining a non-aqueous electrolyte, and a current collector supporting the active material mixture layer. A positive electrode, a negative electrode comprising a mixture layer containing a carbon material such as graphite capable of electrochemically occluding and releasing lithium ions and a polymer absorbing and retaining a non-aqueous electrolyte, and a current collector supporting the mixture layer; a porous film comprising a polymer that absorbs retain aqueous electrolyte, a lithium secondary battery comprising these absorption retained non-aqueous electrolyte, a surface area in the positive electrode is 10m ² / g~25m ² / g
By containing Co ₃ O ₄ , the discharge characteristics at low temperatures are improved, and gas generation during high-temperature storage is reduced.

It is believed that the decrease in discharge capacity at low temperature is mainly caused by an increase in the resistance inside the battery, more specifically, the positive electrode. According to the present invention, however, Co ₃ O ₄ contained in the positive electrode is reduced.
It is presumed that by having a high oxidation state in the charged state and having conductivity, it has a function of suppressing an increase in the resistance of the positive electrode at low temperatures.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view of a cylindrical lithium ion secondary battery, 1 is a battery case, 2 is a polyethylene insulating plate, 3 is a polypropylene gasket, 4 is a sealing plate incorporating a safety member, and 5 is a sealing plate. A positive electrode lead made of aluminum, 6 is a positive electrode plate using lithium cobalt composite oxide or the like as an active material, 7 is a separator made of polyethylene, 8 is a negative electrode plate using spherical graphite or the like as a negative electrode material, 9 is a negative electrode lead made of copper It is. The positive electrode plate 6 is formed by applying a paste obtained by mixing and dispersing an active material, a conductive agent composed of a carbon material, and a binder such as a fluororesin in an aqueous solution of carboxymethyl cellulose (CMC) to an aluminum foil as a positive electrode current collector, It is obtained by cutting the dried and rolled product into predetermined dimensions. The negative electrode plate 8 is obtained by applying a paste obtained by mixing and dispersing spheroidal graphite powder in an aqueous solution of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders onto a copper foil as a negative electrode current collector, followed by drying and rolling. Is obtained by cutting into a predetermined size.

FIG. 2 is a cross-sectional view of a flat type lithium polymer secondary battery, in which 11 is a battery case made of a resin laminate containing aluminum foil, 12 is a welding seal of a lead portion, and 1
5 is a positive electrode lead made of an aluminum sheet, 16 is a positive electrode plate using a lithium cobalt composite oxide or the like as an active material, 17 is a vinylidene fluoride-6-propylene copolymer P (VDF + HFP), and a non-aqueous electrolyte Is a film that absorbs and retains and forms a gel state, 18 is a negative electrode plate using spherical graphite or the like capable of electrochemically storing and releasing lithium ions, and 19 is a negative electrode lead made of a copper sheet. The positive electrode plate 16 absorbs and holds a lithium cobalt composite oxide as an active material, carbon black as a conductive agent, and a non-aqueous electrolyte.
Vinylidene fluoride-6-fluoropropylene copolymer as a polymer that also acts as a binder, N-methyl-2-pyrrolidone, cyclohexanone as a polymer solvent,
A paste obtained by adding and mixing and dispersing dibutyl phthalate as a pore-forming material to an aluminum lath material serving as a positive electrode current collector, drying and cutting into a predetermined size. The negative electrode plate 18 is a mixture of spherical graphite, vinylidene fluoride-6-propylene copolymer as a polymer that absorbs and holds a non-aqueous electrolyte and also acts as a binder, and N-methyl as a solvent. A paste obtained by adding and mixing and dispersing 2-pyrrolidone, acetone and dibutyl phthalate into a copper lath material as a negative electrode current collector, drying and cutting into a predetermined size.

The film 17 is formed by pasting the vinylidene fluoride-6-propylene copolymer, the solvent, dibutyl phthalate and the fine particles of silicon oxide as a structural material in the same manner as described above. It is obtained. The positive electrode plate 16, the film 17, and the negative electrode plate 18 were heated and integrated with a heat roller under pressure, and then immersed in xylene to elute dibutyl phthalate, whereby a porous polymer electrode group was obtained. After this electrode group was placed in the battery case 11, an electrolytic solution was injected and heated to form a gel polymer electrolyte having a polymer portion in a gel state. Finally, the resin laminate at the opening was heat-welded to obtain a flat lithium polymer secondary battery. In the above description, the cylindrical battery and the flat battery are shown. However, the present invention is effective regardless of the shape of the battery.
It is also possible to use it for a coin-shaped battery or the like.

By the way, the preferable range of the surface area of Co ₃ O ₄ is 15 m ² / g to lithium ion secondary battery.
65 m ² / g. If the surface area is less than 20 m ² / g, conductivity which can ensure low-temperature characteristics cannot be obtained.
If it exceeds m ² / g, conductivity that can ensure low-temperature characteristics can be obtained, but gas generation during high-temperature storage due to oxidative decomposition of the electrolytic solution on the Co ₃ O ₄ surface cannot be suppressed.

In the case of a lithium polymer secondary battery,
Is a 10m ² / g~25m ² / g. Surface area 10m ² /
If it is less than g is not obtained conductive capable of ensuring low-temperature properties, when exceeding 25 m ² / g Conversely, although the conductivity can be ensured low temperature properties are obtained, in the Co ₃ O ₄ surface Gas generation during high-temperature storage due to oxidative decomposition of the electrolyte cannot be suppressed.

The difference in the preferable range of the surface area of Co ₃ O ₄ between the lithium ion battery and the lithium polymer battery is that the lithium polymer battery has an intermediate metal thin film such as an aluminum foil having lower mechanical strength than the metal case. Since the battery case made of resin laminate used for one layer is used, Co ₃ O ₄
This is because it is necessary to reduce the surface area of the battery to reduce the deformation of the battery due to gas generation during high-temperature storage.

The preferable addition amount of Co ₃ O ₄ is as follows.
1 part by weight or more and 10 parts by weight or less based on 00 parts by weight. When the amount is less than 1 part by weight, the function as a conductive agent is not sufficiently exhibited, and the improvement in low-temperature characteristics is small. Conversely, when the amount is more than 10 parts by weight, the content of the positive electrode active material decreases, and a problem arises that the battery capacity cannot be sufficiently secured.

[0020]

Next, the present invention will be described in detail with reference to examples and comparative examples.

Example 1 <Preparation of Positive Electrode> As a lithium-containing composite oxide, 100 parts by weight of lithium cobaltate (LiCoO ₂ ) and a surface area of 20 were used.
By kneading and dispersing 1 part by weight of m ² / g Co ₃ O ₄ , 2 parts by weight of acetylene black as a conductive agent, and 5 parts by weight of polyfluoroethylene (PTFE) as a binder with water containing CMC. A paste was prepared, applied to a current collector made of aluminum foil, and dried to obtain a positive electrode.

<Preparation of Negative Electrode> A paste is prepared by kneading and dispersing 100 parts by weight of spherical graphite powder as a material capable of electrochemically absorbing and releasing lithium ions and 2 parts by weight of SBR as a binder in water containing CMC. A negative electrode was prepared by applying this to a copper foil current collector and drying.

<Preparation of Battery> The positive and negative electrodes were made of a polyethylene separator, and ethylene carbonate (E) was used.
C) and ethyl methyl carbonate (EMC) at a volume ratio of 1: 3 in a mixed solvent of 3 g, and lithium hexafluorophosphate (LiPF6) as an electrolyte at 1.5 mol · dm −
The cylindrical lithium secondary battery shown in FIG. 1 was produced using the electrolyte solution to which 3 was added. This was designated as the battery of Example 1.

Example 2 A battery constructed in the same manner as in Example 1 except that the addition amount of Co ₃ O ₄ was changed to 5 parts by weight, was used in Example 2.
And

Example 3 A battery constructed in the same manner as in Example 1 except that the amount of Co ₃ O ₄ was changed to 10 parts by weight was used as Example 3.

Example 4 Co ₃ O _{4 has} a surface area of 60 m ²
A battery constructed in the same manner as in Example 1 except that the ratio was / g was designated as Example 4.

Example 5 <Preparation of Positive Electrode> As a lithium-containing composite oxide, 100 parts by weight of lithium cobalt oxide (LiCoO ₂ ) and a surface area of 10 parts were used.
1 part by weight of m ² / g Co ₃ O ₄ , 5 parts by weight of acetylene black as a conductive agent, and PVDF-HFP as a binder
A paste was prepared by kneading 8 parts by weight with acetone, and this was applied to a current collector made of an aluminum lath material,
It dried and was set as the positive electrode.

<Preparation of Negative Electrode> Spheroidal graphite powder 100 is used as a material capable of electrochemically absorbing and releasing lithium ions.
By weight, kneading 10 parts by weight of P (VDF-HFP) as a binder with acetone to prepare a paste,
This was applied to a copper lath current collector and dried to form a negative electrode.

<Preparation of Battery> P (VDF
-HFP) film, and 3 g of a mixed solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 1: 3, 1.5 L of lithium hexafluorophosphate (LiPF6) as an electrolyte was added. Mol dm
The flat lithium polymer secondary battery shown in FIG. 2 was manufactured using the electrolyte solution to which -3 was added. This was designated as a battery of Example 5.

Example 6 The surface area of Co ₃ O ₄ was 20 m ^2.
A battery constituted in the same manner as in Example 5 except that the ratio was / g was designated as Example 6.

Comparative Example 1 A battery constructed in the same manner as in Example 1 except that Co ₃ O ₄ was not added was used as Comparative Example 1.

(Comparative Example 2) A battery constructed in the same manner as in Example 1 except that the addition amount of Co ₃ O ₄ was 0.5 part by weight was set as Comparative Example 2.

Comparative Example 3 A battery constructed in the same manner as in Example 1 except that the amount of Co ₃ O ₄ was changed to 15 parts by weight was used as Comparative Example 3.

(Comparative Example 4) The surface area of Co ₃ O ₄ was 80 m ²
A battery constructed in the same manner as in Example 1 except that the ratio was / g was designated as Comparative Example 4.

The batteries of Examples 1 to 6 and Comparative Examples 1 to 4 were
After charging to 4.2V at 500mA under 20 ° C environment,
The battery was discharged to 3.0 V at 100 mA in a 20 ° C. environment, and the discharge capacity at 20 ° C. of each battery was measured. Then again 20
After charging to 4.2V at 500mA in an environment of
The battery was discharged to 3.0 V in a 0 ° C. environment, and the discharge capacity of each battery at −20 ° C. was measured. Finally, after charging again to 4.2 V at 500 mA in an environment of 20 ° C., each battery was charged at 90 ° C.
For 12 hours, and the gas generated by opening each battery in liquid paraffin was collected and the amount of generated gas was measured. Table 1 shows the results.

[0036]

[Table 1]

As is apparent from Table 1, in the embodiment of the present invention, the discharge capacity at −20 ° C. is 360 mAh or more,
It is significantly improved as compared with Comparative Example 1 where no additive was added, and Co
_By adding ₃ O ₄ , it was possible to suppress an increase in resistance at low temperatures.

However, in Comparative Example 2 in which the addition amount was as small as 0.5 part by weight, no significant effect of suppressing the resistance could be found. Conversely, in Comparative Example 3 in which the addition amount was 15 parts by weight, the filling property of the positive electrode active material was low, and the discharge capacity at 20 ° C. was significantly reduced.

In Comparative Example 4, Co _{3 having a} surface area of 80 m ² / g
Although O ₄ was added, gas generation, which is considered to be caused by oxidative decomposition of the electrolytic solution on the surface of Co ₃ O ₄ , increased sharply as compared with Example 4.

Further, from Examples 5 and 6, the addition of Co ₃ O ₄ was effective in improving the low-temperature characteristics also in the flat type lithium polymer secondary battery. From the above results, it has been clarified that the optimal amount of Co ₃ O ₄ is 1 part by weight or more and 10 parts by weight or less based on 100 parts by weight of the active material.

[0041]

As described above, the present invention provides a non-aqueous electrolyte-based lithium secondary battery using a lithium-containing composite oxide as a positive electrode active material and a graphite material capable of electrochemically storing and releasing lithium ions for a negative electrode. In the secondary battery, the discharge capacity at low temperature is improved by including Co ₃ O ₄ having a specific surface area in the positive electrode.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a cylindrical lithium secondary battery of the present invention.

Sectional view showing a Bian Flat-type lithium polymer secondary battery of the present invention; FIG

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Insulating plate 3 Gasket 4 Sealing plate 5 Positive electrode lead 6 Positive electrode plate 7 Separator 8 Negative electrode plate 9 Negative electrode lead 11 Battery case 12 Welding seal of lead part 15 Positive electrode lead 16 Positive electrode plate 17 Film 18 Negative electrode plate 19 Negative electrode lead

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H029 AJ02 AK03 AL06 AM00 AM03 AM05 AM07 AM16 BJ02 BJ04 BJ12 BJ14 DJ08 EJ05 HJ01 HJ07 5H050 AA06 AA10 CA07 CB07 DA02 DA09 DA10 EA01 HA01 HA07

Claims (3)

[Claims] 1. A positive electrode using a lithium-containing composite oxide as an active material, a negative electrode made of a carbon material capable of electrochemically occluding and releasing lithium ions, a separator, and a non-aqueous electrolyte impregnated therein. in the lithium secondary battery comprising a lithium secondary battery surface area in the positive electrode is characterized in that it comprises a Co ₃ O ₄ of 15m ² / g~65m ² / g. 2. An active material mixture layer containing an active material made of a lithium-containing composite oxide and a polymer absorbing and retaining a non-aqueous electrolyte, a positive electrode made of a current collector supporting the active material mixture layer, An active material mixture layer containing a carbon material capable of occluding and releasing and a polymer that absorbs and retains a nonaqueous electrolyte, a negative electrode composed of a current collector that supports the active material mixture layer, and a polymer that absorbs and retains the nonaqueous electrolyte In a lithium secondary battery comprising a porous film and a non-aqueous electrolyte absorbed and retained therein, a surface area of 10 m ² /
A lithium secondary battery comprising g to 25 m ² / g of Co ₃ O ₄ . 3. The lithium secondary battery according to claim 1, wherein the Co ₃ O ₄ is contained in an amount of 1 to 10 parts by weight based on 100 parts by weight of the positive electrode active material.