JP4746272B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery and a negative electrode for a non-aqueous electrolyte secondary battery.

  In recent years, electronic devices such as mobile communication devices, notebook computers, palmtop computers, integrated video cameras, portable CD (MD) players, cordless telephones, etc. have been reduced in size and weight. As a power source, a battery having a small size and a large capacity is particularly demanded.

  Examples of batteries that are widely used as power sources for these electronic devices include primary batteries such as alkaline manganese batteries, and secondary batteries such as nickel cadmium batteries and nickel metal hydride batteries. Among them, non-aqueous electrolyte secondary batteries that use lithium composite oxide for the positive electrode and carbonaceous materials that can occlude and release lithium ions for the negative electrode are small and light, have a high unit cell voltage, and a high energy density. It is attracting attention because it is obtained.

  In this non-aqueous electrolyte secondary battery, since the negative electrode density increases as the capacity increases, the impregnation of the non-aqueous electrolyte in the negative electrode is problematic.

  As a method for improving the impregnation rate of the nonaqueous electrolyte solution of the electrode, Patent Document 1 discloses that a power generation element having a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution is sealed in an exterior member and then pressurized in a pressure vessel. It has been proposed to process. Moreover, in this patent document 1, after the pressurization process in a pressure vessel, the electrostatic capacitance between positive and negative electrodes is measured and the impregnation state of a non-aqueous electrolyte is determined.

However, according to the method of Patent Document 1, even a secondary battery determined to be fully impregnated may have a short charge / discharge cycle life. That is, even if the electrolytic solution is to be uniformly diffused by pressure treatment as in Patent Document 1, the distribution of the electrolytic solution is biased toward the separator having the highest wettability, so the impregnation state of the positive electrode and the negative electrode is insufficient. And impregnation spots occur. When the impregnation state of the electrolyte is determined by the capacitance between the positive and negative electrodes, the capacitance between the positive and negative electrodes is a comprehensive evaluation as a parallel plate capacitor, and the impregnation state of each electrode is evaluated. Therefore, even if there is a slight problem in the impregnation state of the electrode, it is possible to obtain a favorable evaluation. As a result, the initial charge performed after the pressurization process is mottled, and further repeated charge / discharge cycles cause electrodeposition of Li deposition in the negative electrode, thus shortening the charge / discharge cycle life.
JP 2002-110252 A

  An object of the present invention is to provide a nonaqueous electrolyte secondary battery having a long charge / discharge cycle life.

  Moreover, an object of this invention is to provide the negative electrode for nonaqueous electrolyte secondary batteries which can improve the charging / discharging cycle life of a nonaqueous electrolyte secondary battery.

A non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode,
A negative electrode comprising a current collector and an active material-containing layer carried by the current collector, and satisfying the following formula (1):
A non-aqueous electrolyte having a viscosity (20 ° C.) in the range of 3 to 50 (cP) and a non-aqueous electrolyte having a conductivity (20 ° C.) in the range of 2 to 10 (mS / cm). It is a feature.

5 ≦ Cs / Cs _Cu ≦ 15 (1)
Where Cs is the capacitance (nF) of the negative electrode, and Cs _Cu is the capacitance (nF) of the current collector.

  According to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery having an improved charge / discharge cycle life.

  Moreover, according to this invention, the negative electrode for nonaqueous electrolyte secondary batteries which can improve the charging / discharging cycle life of a nonaqueous electrolyte secondary battery can be provided.

  First, the positive electrode, negative electrode, and nonaqueous electrolyte of the nonaqueous electrolyte secondary battery according to the present invention will be described.

1) Positive electrode The positive electrode includes a current collector and an active material-containing layer supported on one or both surfaces of the current collector.

  The active material-containing layer may contain a binder or a conductive agent together with the positive electrode active material.

Examples of the positive electrode active material include various oxides such as manganese dioxide, lithium-containing composite oxides, chalcogen compounds such as titanium disulfide and molybdenum disulfide. Examples of the lithium-containing composite oxide include lithium manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel cobalt oxide, lithium-containing iron oxide, and vanadium oxide containing lithium. be able to. Among them, when a lithium-containing cobalt oxide (for example, LiCoO ₂ ), a lithium-containing nickel cobalt oxide (for example, LiNi _0.8 Co _0.2 O ₂ ), or a lithium manganese composite oxide (for example, LiMn ₂ O ₄ , LiMnO ₂ ) is used. This is preferable because a high voltage can be obtained. As the positive electrode active material, one kind of oxide may be used alone, or two or more kinds of oxides may be mixed and used.

  Examples of the conductive agent include acetylene black, carbon black, and graphite.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyether sulfone, ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). Can be used.

  As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, aluminum, stainless steel, or nickel.

  The positive electrode is produced, for example, by suspending a conductive agent and a binder in an appropriate solvent in a positive electrode active material, applying the suspension to a current collector, and drying to form a thin plate.

2) Negative electrode The negative electrode includes a current collector and an active material-containing layer supported on one or both surfaces of the current collector, and satisfies the following formula (1).

5 ≦ Cs / Cs _Cu ≦ 15 (1)
Where Cs is the capacitance (nF) of the negative electrode, and Cs _Cu is the capacitance (nF) of the current collector.

  The active material-containing layer may contain a binder together with a negative electrode active material that occludes / releases lithium.

  Examples of the negative electrode active material include carbon materials that occlude and release lithium. Examples of the carbon material include graphite, coke, carbon fiber, spherical carbon, pyrolytic gas-phase carbonaceous material, resin fired body, and other graphite materials or carbonaceous materials; thermosetting resin, isotropic pitch, mesophase pitch. Graphite material obtained by performing heat treatment at 500 to 3000 ° C. on carbon-based carbon, mesophase pitch-based carbon fiber, mesophase microspheres, etc. (especially, mesophase pitch-based carbon fiber is preferable because of high capacity and charge / discharge cycle characteristics) or It is preferable to use two or more kinds of materials having different shapes from carbonaceous materials.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like. Can be used.

  As the current collector, it is desirable to use a copper plate such as a copper foil, but it may have a porous structure.

The reason for defining the capacitance ratio (Cs / Cs _Cu ) within the above-described range will be described. The current collector is hardly impregnated with a non-aqueous electrolyte, and the current collector has a substantially constant capacitance (Cs _Cu ). Therefore, the current collector is activated depending on the value of the capacitance ratio (Cs / Cs _Cu ). The non-aqueous electrolyte impregnation property of the substance-containing layer can be shown. If the capacitance ratio (Cs / Cs _Cu ) is less than 5, the non-uniform distribution of the non-aqueous electrolyte in the negative electrode increases, so that the initial charge is mottled. Occurs and the charge / discharge cycle life is shortened. On the other hand, when the capacitance ratio (Cs / Cs _Cu ) exceeds 15, the negative electrode absorbs too much non-aqueous electrolyte, and the non-aqueous electrolyte is unevenly distributed on the negative electrode side. Due to depletion, the amount of the nonaqueous electrolyte held by the peripheral edge of the positive electrode and the separator is insufficient, charging / discharging is not successful, and charging / discharging cycle characteristics deteriorate. A more preferable range of the capacitance ratio (Cs / Cs _Cu ) is 8-12.

The negative electrode is prepared by, for example, kneading a negative electrode active material and a binder in the presence of a solvent, applying the obtained suspension to a current collector, drying it, and then pressing it once at a desired pressure or 2 It is prepared by multi-stage pressing up to 5 times. At this time, the capacitance ratio (Cs) is adjusted by adjusting the types of the active material and the binder and the addition amount thereof, the roller diameter, the pressing pressure and the number of times of pressing. / Cs _Cu ) can be set within the range of equation (1) described above. The reason why the capacitance ratio (Cs / Cs _Cu ) can be set within the range of the above-described formula (1) by adjusting these manufacturing conditions is not clear, but is related to the fact that the homogeneity of the negative electrode can be improved. Presumed to be.

  A separator can be interposed between the positive electrode and the negative electrode.

  As the separator, a microporous film, a woven fabric, a non-woven fabric, a laminate of the same material or different materials among these can be used. In particular, the microporous membrane causes a so-called shutdown phenomenon in which the resin constituting the separator plastically deforms and closes the fine pores when the temperature of the electrode group rises abnormally due to heat generated by overcharging, etc., and the flow of lithium ions Is prevented, and further overheating is prevented, and the overcharge state can be safely terminated, which is preferable. Examples of the material for forming the separator include polyethylene, polypropylene, ethylene-propylene copolymer, and ethylene-butene copolymer. As a material for forming the separator, one type or two or more types selected from the types described above can be used.

  An electrode group is formed by combining the positive electrode, the negative electrode, and the separator. For example, (i) the positive electrode and the negative electrode are wound in a flat shape or a spiral shape with a separator interposed therebetween, or (ii) the positive electrode and the negative electrode are wound in a spiral shape with a separator interposed therebetween. Rotating and then compressing in the radial direction, (iii) bending the positive electrode and the negative electrode one or more times with a separator interposed therebetween, or (iv) laminating the positive electrode and the negative electrode with a separator interposed therebetween It is produced by.

  The electrode group need not be pressed, but may be pressed to increase the integrated strength of the positive electrode, the negative electrode, and the separator. It is also possible to heat at the time of pressing.

  In the present invention, as long as the positive electrode and the negative electrode are impregnated with the nonaqueous electrolytic solution, a solid or gel electrolyte layer can be used instead of the separator.

4) Non-aqueous electrolyte As the non-aqueous electrolyte, a liquid one can be used. Examples of the liquid non-aqueous electrolyte include a so-called non-aqueous electrolyte containing a non-aqueous solvent and an electrolyte (for example, lithium salt) dissolved in the non-aqueous solvent.

  Examples of the non-aqueous solvent include γ-butyrolactone (GBL), cyclic carbonate (eg, ethylene carbonate (EC), propylene carbonate (PC), etc.), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), phenylethylene carbonate. (PhEC), chain carbonate (eg, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc.), γ-valerolactone (VL), methyl propionate (MP), ethyl propionate ( EP), 2-methylfuran (2Me-F), furan (F), thiophene (TIOP), catechol carbonate (CATC), ethylene sulfite (ES), 12-crown-4 (Crown), tetraethyleneglycol Examples include rudimethyl ether (Ether) and trioctyl phosphate (TOP). The kind of the non-aqueous solvent to be used can be one kind or two or more kinds.

  Among them, preferred are those containing cyclic carbonate and GBL, those containing cyclic carbonate and chain carbonate, and those containing EC and PC. Particularly suitable is a non-aqueous solvent containing cyclic carbonate and GBL. This non-aqueous solvent will be described in detail.

a. γ-butyrolactone (GBL)
Since GBL has low reactivity with the positive electrode in a charged state, that is, in a state where the potential of the positive electrode is high, the decomposition reaction and exothermic reaction of the nonaqueous solvent in the charged state can be suppressed. The GBL ratio is desirably in the range of 40 to 80% by weight.

b. Cyclic carbonates Cyclic carbonates react with GBL and lithium ions occluded in the negative electrode active material without losing the advantages of GBL, ie, the low freezing point, high lithium ion conductivity, and excellent safety. Can be suppressed. Among the cyclic carbonates, ethylene carbonate (EC) and propylene carbonate (PC) are preferable. In particular, EC is preferable because it has a large effect of suppressing the reaction between lithium ions and GBL. In addition, the kind of cyclic carbonate may be one, or two or more kinds.

  The ratio of the cyclic carbonate to the total weight of the non-aqueous solvent is preferably in the range of 20 to 50% by weight.

c. Subcomponent Other solvents other than GBL and cyclic carbonate can be contained in the non-aqueous solvent. Preferred as an auxiliary component is one containing vinylene carbonate (VC). Subcomponents containing VC can improve the denseness of the protective film on the negative electrode surface, and thus can improve long-term high-temperature storage characteristics in a charged state. It is desirable that the weight ratio of the minor component in the non-aqueous solvent is within the range of 10% by weight or less.

  Next, the electrolyte will be described.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenide (LiAsF ₆ ), and trifluorometa. Lithium salts such as lithium sulfonate (LiCF ₃ SO ₃ ), bistrifluoromethylsulfonylimide lithium (LiN (CF ₃ SO ₂ ) ₂ ), bispentafluoroethylsulfonylimide lithium (LiN (C ₂ F ₅ SO ₂ ) ₂ ) Can be mentioned. The type of electrolyte used can be one type or two or more types.

Among them, LiBF ₄ is preferable because it has a low reactivity with the positive electrode when the temperature of the secondary battery rises, and thus can quickly terminate the decomposition reaction of the nonaqueous electrolyte after the current interruption. Further, (a lithium salt of at least one of LiN (CF ₃ SO _{2) 2} and _{LiN (C 2 F 5 SO 2} ) 2, or a mixed salt containing a lithium salt comprising LiBF _4, or LiBF ₄ and When a mixed salt containing LiPF ₆ is used, the cycle life at a high temperature can be further improved.

  The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2.5 mol / L.

  The amount of the non-aqueous electrolyte is preferably 0.2 to 0.6 g per 100 mAh of battery unit capacity.

The viscosity (20 ° C.) of the non-aqueous electrolyte is in the range of 3 to 50 (cP), and the conductivity (20 ° C.) of the non-aqueous electrolyte is in the range of 2 to 10 (mS / cm). preferable. This is due to the reason explained below. The capacitance ratio (Cs / Cs _Cu ) can vary depending on the viscosity or conductivity of the non-aqueous electrolyte, so even if the negative electrode impregnation is poor, the viscosity or conductivity of the non-aqueous electrolyte is changed. As a result, the capacitance ratio (Cs / Cs _Cu ) can be kept within the range of the above-described equation (1). However, in such a case, not only the impregnation property of the negative electrode is low, but also the viscosity or the electrical conductivity may be out of the range suitable for charging / discharging. By setting the viscosity and conductivity of the non-aqueous electrolyte within the above-described ranges, it is possible to obtain a negative electrode having excellent impregnation properties while ensuring the characteristics required for the non-aqueous electrolyte.

  There is no restriction | limiting in particular in the container in which the positive electrode mentioned above, a negative electrode, and a nonaqueous electrolyte are accommodated, For example, it can form from a metal plate, metal foil, a resin film, a laminate film, etc.

  The nonaqueous electrolyte secondary battery according to the present invention can be applied to various forms such as a rectangular shape, a cylindrical shape, and a thin shape. A thin nonaqueous electrolyte secondary battery as an example will be described in detail with reference to FIGS.

  As shown in FIG.1 and FIG.2, the electrode group 2 is accommodated in the container main body 1 which makes | forms a rectangular cup shape. The electrode group 2 has a structure in which a laminate including a positive electrode 3, a negative electrode 4, and a separator 5 disposed between the positive electrode 3 and the negative electrode 4 is wound into a flat shape. The nonaqueous electrolyte is held in the electrode group 2. A part of the edge of the container body 1 is wide and functions as the lid plate 6. The container body 1 and the cover plate 6 are each composed of a laminate film. The laminate film includes an external protective layer 7, an internal protective layer 8 containing a thermoplastic resin, and a metal layer 9 disposed between the external protective layer 7 and the internal protective layer 8. A lid 6 is fixed to the container body 1 by heat sealing using a thermoplastic resin of the inner protective layer 8, whereby the electrode group 2 is sealed in the container. A positive electrode tab 10 is connected to the positive electrode 3, and a negative electrode tab 11 is connected to the negative electrode 4, and each is pulled out of the container and serves as a positive electrode terminal and a negative electrode terminal.

  As a result of intensive research, the inventors of the present invention have a reason that the charge / discharge cycle life is reduced due to the electrolyte impregnated state of the negative electrode. By using the negative electrode satisfying the above-described formula (1), the charge / discharge cycle life is reduced. I found it to be improved. This is because the negative electrode satisfying the above-described formula (1) maintains the electrolyte solution balance between the positive electrode and the negative electrode within an appropriate range (the negative electrode does not absorb the electrolyte too much), and the non-aqueous electrolyte distribution of the negative electrode is uniform. Although it is assumed that it is possible to enhance the nature, details are not clear.

  Subsequently, the manufacturing method of the nonaqueous electrolyte secondary battery which concerns on this invention is demonstrated.

The method for producing a nonaqueous electrolyte secondary battery according to the present invention includes a current collector and an active material-containing layer supported on the current collector, and the capacitance ratio (Cs / Cs _Cu ) is the above (1). Assembling a non-initial charge secondary battery comprising a negative electrode satisfying the formula, a positive electrode, and a non-aqueous electrolyte held by the positive electrode and the negative electrode;
The method includes a step of pressurizing the uncharged secondary battery in a pressure vessel and a step of initially charging the uncharged secondary battery.

(First step)
A negative electrode comprising a current collector and an active material-containing layer carried by the current collector, and having a capacitance ratio (Cs / Cs _Cu ) satisfying the formula (1), a positive electrode, the positive electrode, and the negative electrode A non-initial charge secondary battery having a non-aqueous electrolyte held in the battery is assembled.

  The uncharged secondary battery is prepared by, for example, preparing an electrode group including a positive electrode and a negative electrode, then housing the electrode group in a container, injecting a nonaqueous electrolyte, and subjecting the container to a sealing process. can get. An aging process etc. can be given to the obtained secondary battery of the first charge.

(Second step)
The uncharged secondary battery is pressurized in a pressure vessel.

  For the pressurization, a method of supplying a pressurizing medium into the pressure vessel can be adopted. Examples of the pressurizing medium include carbon dioxide gas, dry air, an inert gas such as argon gas and nitrogen, and water. The type of the pressure medium to be used can be one type or two or more types. As the pressurizing medium, it is desirable to use a gas, particularly a gas not containing moisture.

The applied pressure is preferably in the range of 0.3 MPa to 1 MPa as a gauge pressure. This is due to the reason explained below. If the applied pressure is less than 0.3 MPa in terms of gauge pressure, it may be difficult to make the distribution of the non-aqueous electrolyte in the negative electrode uniform. On the other hand, when the applied pressure exceeds 1 MPa as the gauge pressure, the electrode group is excessively compressed, so that the negative electrode expands greatly during the initial charge due to the reaction, and the electrode group is likely to be distorted. When the electrode group is distorted, variation in the distance between the positive electrode and the negative electrode is increased, so that charging / discharging spots are likely to occur, and a long charge / discharge cycle life may not be obtained. A more preferable range is 0.5 to 0.9 MPa (third step).
First charge the uncharged secondary battery.

In the method for manufacturing a non-aqueous electrolyte secondary battery according to the present invention described above, a pressure vessel is applied to an uninitialized secondary battery provided with a negative electrode having a capacitance ratio (Cs / Cs _Cu ) satisfying the above formula (1). Since the pressure treatment is performed in the battery, the charge / discharge cycle life of the secondary battery can be further improved.

[Example]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Example 1
<Production of negative electrode>
Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) are added as binders to mesophase pitch-based carbon fibers (MCF) as a carbon material in the mixing ratio shown in Table 1 below, and these are added in the presence of water. The slurry was kneaded to prepare a slurry. The slurry is applied to both surfaces of a current collector made of a copper foil having a thickness of 12 μm, dried, and pressed with a pair of metal rolls having a roll diameter of 160 mmφ at the linear pressure shown in Table 1 below, thereby containing an active material. A negative electrode having a structure in which the layer was supported on both sides of the current collector was produced.

  The capacitance (Cs) of the obtained negative electrode was measured using the evaluation cell shown in FIG. This evaluation cell is a three-electrode cell using a negative electrode as a working electrode and Li metal as a counter electrode and a reference electrode.

  As shown in FIG. 3, a measurement negative electrode (working electrode) 24 in which an active material-containing layer 23 is formed only on one side of a current collector 22 made of copper foil is accommodated in the outer case 21. On the active material-containing layer 23 of the working electrode 24, a measurement separator 25 made of a glass filter is laminated. On the measurement separator 25, a counter electrode obtained by attaching a lithium foil 27 to a nickel net 26 is laminated. On the other hand, a nickel plate 28 is laminated on the current collector 22 of the working electrode 24 in order to collect current from the working electrode 24. The electrode laminate having the above-described configuration is sandwiched between two electrode pressing plates 29a and 29b made of, for example, an acrylic plate. One electrode pressing plate 29 a is stacked on the nickel net 26 and the other electrode pressing plate 29 b is stacked on the nickel plate 28. The electrode pressing plates 29a and 29b are fixed by bolts 30 and nuts 31, and can be fixed in a state where the counter electrode, the separator, and the working electrode are in close contact with each other.

The nonaqueous electrolytic solution 32 for measurement is accommodated in the outer case 21. The composition of the nonaqueous electrolytic solution 32 for measurement is desirably combined with the same composition as that used in an actual cell. In the case of Example 1, lithium tetrafluoroborate (LiBF ₄ ) was added to a non-aqueous solvent in which ethylene carbonate (EC) and γ-butyrolactone (GBL) were mixed at a weight ratio (EC: GBL) of 35:65. A solution dissolved at 5 mol / L is used as the nonaqueous electrolyte solution 32 for measurement. Further, it is desirable that the amount of the nonaqueous electrolytic solution 32 for measurement is such that the entire laminate including the electrode pressing plate is sufficiently immersed.

  A reference electrode 33 made of lithium foil is immersed in the nonaqueous electrolyte solution 32 for measurement. The nickel net 26, the nickel plate 28, and the reference electrode 33 are electrically connected to a current / voltage detector (not shown) by leads.

  Next, the measurement method will be specifically described.

  First, the obtained negative electrode was cut into a 2 × 2 cm square, and one side of the active material-containing layer formed on both sides of the current collector was peeled off to prepare a test negative electrode (working electrode) 24. A Ni metal plate 28 cut into a 2 × 2 cm square and provided with a lead was pressed against the working electrode 24 to collect current on the working electrode. When the capacitance is measured with the active material-containing layer supported on both sides of the current collector, the active material-containing layer on the back side (the active material-containing layer not facing the counter electrode) is also involved in the charge / discharge reaction. Therefore, it becomes impossible to accurately evaluate the characteristics of the active material-containing layer facing the counter electrode. In addition, if the two surfaces are used, the influence of the thickness of the active material-containing layer cannot be ignored. Furthermore, the contact resistance between the working electrode 24 and the Ni metal plate 28 increases. Therefore, in order to accurately measure the electrolyte impregnation property of the active material-containing layer per one side of the current collector, it is desirable to perform the measurement with one active material-containing layer peeled off.

  The capacitance was measured by using an LCR meter 4284A manufactured by Hewlett packard and applying a voltage of 0.1 V at a frequency of 42.0 Hz.

Further, the capacitance (Cs _Cu ) of the current collector was measured in the same manner as described above except that a 2 × 2 cm square copper foil was used as the working electrode 24 instead of the test negative electrode.

The obtained capacitance ratio (Cs / Cs _Cu ) is shown in Table 1 below.

(Examples 2-11 and Comparative Examples 1-7)
The capacitance ratio (Cs / Cs _Cu ) is changed as shown in Table 1 by setting the type of carbon material, CMC addition amount, SBR addition amount, and negative electrode press linear pressure as shown in Table 1 below. Except for the above, a negative electrode was produced in the same manner as described in Example 1 above. In addition, MCMB in Table 1 indicates mesophase pitch-based carbon microbeads.

  Next, using the obtained negative electrode, the thin non-aqueous electrolyte secondary battery having the structure shown in FIGS. 1 and 2 was assembled, and the charge / discharge cycle life was measured.

<Preparation of positive electrode>
First, 90% by weight of lithium cobalt oxide (Li _x CoO ₂ ; X is 0 <X ≦ 1) powder, 5% by weight of acetylene black, and 5% by weight of polyvinylidene fluoride (PVdF) dimethylformamide (DMF) solution was added and mixed to prepare a slurry. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm, then dried and pressed to produce a positive electrode having a structure in which the positive electrode layer was supported on both sides of the current collector.

<Separator>
A separator made of a microporous polyethylene film having a thickness of 25 μm and a porosity of 45% was prepared.

<Production of electrode group>
After the positive electrode lead made of a strip-shaped aluminum foil (thickness 100 μm) is ultrasonically welded to the positive electrode current collector, and the negative electrode lead made of a strip-shaped nickel foil (thickness 100 μm) is ultrasonically welded to the negative electrode current collector The positive electrode and the negative electrode were spirally wound through the separator between them to prepare an electrode group. The electrode group was formed into a flat shape by applying pressure with a press while heating.

  A laminate film having a thickness of 100 μm in which both surfaces of an aluminum foil were covered with polyethylene was formed into a rectangular cup shape by a press machine, and the electrode group was housed in the obtained container.

  Next, the electrode group in the container was vacuum dried at 80 ° C. for 12 hours to remove moisture contained in the electrode group and the laminate film.

<Preparation of non-aqueous electrolyte>
A non-aqueous solvent was prepared by mixing ethylene carbonate (EC) and γ-butyrolactone (GBL) so that the weight ratio (EC: GBL) was 35:65. Lithium tetrafluoroborate (LiBF ₄ ) was dissolved in the obtained non-aqueous solvent so that its concentration was 1.5 mol / L to prepare a non-aqueous electrolyte. This non-aqueous electrolyte had a viscosity (20 ° C.) of 6.8 (cP) and an electric conductivity (20 ° C.) of 6.3 (mS / cm).

  After injecting the non-aqueous electrolyte into the electrode group in the container so that the amount per battery capacity 1Ah is 4.8 g and sealing by heat sealing, the structure shown in FIGS. An uncharged non-aqueous electrolyte secondary battery having a thickness of 3.6 mm, a width of 35 mm, a height of 62 mm, and a nominal capacity of 0.60 Ah was assembled.

  Subsequently, the secondary battery was placed in an autoclave that was a pressure vessel, and dry air was supplied into the pressure vessel to pressurize the secondary battery at a gauge pressure of 0.5 MPa for 10 minutes.

  The following treatment was applied to the non-aqueous electrolyte secondary battery as an initial charge / discharge process. Constant current / constant voltage charging was performed for 15 hours at room temperature to 4.2 V at 0.2C. Then, it discharged to 3.0V at 0.2C at room temperature, and manufactured the nonaqueous electrolyte secondary battery.

  Here, 1C is a current value necessary for discharging the nominal capacity (Ah) in one hour. Therefore, 0.2 C is a current value necessary for discharging the nominal capacity (Ah) in 5 hours.

(Charge / discharge cycle characteristics)
For each nonaqueous electrolyte secondary battery, a charge / discharge test at a charge / discharge rate of 1 C, a charge end voltage of 4.2 V, and a discharge end voltage of 3.0 V was repeated in an environment at a temperature of 20 ° C., and the discharge capacity was the discharge capacity of the first cycle. The number of cycles when the ratio was reduced to 80% was measured, and the results are shown in Table 1 below.

As is clear from Table 1, the secondary batteries of Examples 1 to 11 having the negative electrode having a capacitance ratio (Cs / Cs _Cu ) in the range of 5 to 15 were discharged after repeated charge and discharge. The number of cycles in which the capacity reaches 80% of the discharge capacity before the cycle exceeds 400, and it is suitable as a power source for portable devices that are used while being repeatedly charged and discharged. This is because the negative electrode is in an appropriate state of impregnation with the electrolyte, and uniform charge / discharge is performed.

On the other hand, in the secondary batteries of Comparative Examples 1 and 4 to 7 including the negative electrode having a capacitance ratio (Cs / Cs _Cu ) of less than 5, the discharge capacity decreases to 80% after about 300 cycles. This is because the negative electrode with a capacitance ratio (Cs / Cs _Cu ) of less than 5 is poorly impregnated with the electrolyte, and when the initial charge becomes mottled and the cycle is repeated, the deposition of Li occurs. This is because the characteristics deteriorate. As shown in Comparative Examples 5 to 7, when the capacitance ratio (Cs / Cs _Cu ) is less than 5, a long life of 400 cycles or more cannot be obtained even if the pressurization conditions are changed variously. It was.

In addition, the secondary batteries of Comparative Examples 2 and 3 having a negative electrode having a capacitance ratio (Cs / Cs _Cu ) exceeding 15 are also reduced to 80% in about 300 cycles. This is because, according to the negative electrode having a capacitance ratio (Cs / Cs _Cu ) of more than 15, the electrolyte is sucked too much and the liquid is unevenly distributed on the negative electrode side, so the amount of liquid around the separator or the positive electrode is insufficient. This is because charging / discharging is not successful and the cycle characteristics are deteriorated. Therefore, the secondary batteries of Comparative Examples 1 to 7 are inappropriate as a power source for portable devices that are used while being repeatedly charged and discharged.

  Therefore, the nonaqueous electrolyte secondary batteries of Examples 1 to 11 can actually be used as the power source of the portable device based on the results of (capacitance ratio of negative electrode) and (charge / discharge cycle characteristics).

  Next, the relationship between the pressure treatment condition and the capacitance ratio is examined.

  Example 3 in which the secondary batteries of Examples 1, 2, and 4 in which the capacitance ratio is in the range of 8 to 12 have a capacitance ratio of 6 and 14 when the pressure treatment conditions are constant. , 5 had a longer cycle life than the secondary battery. Therefore, it can be seen that the capacitance ratio is desirably 8 to 12 in order to obtain good cycle characteristics.

  Further, when Examples 1, 9, and 10 having the same capacitance ratio of 10 are compared, Examples 1 and 9 having a gauge pressure of 0.5 to 0.9 MPa have an gauge pressure of 1.2 MPa. Compared to Example 10, the cycle life was longer. From this result, it is understood that the gauge pressure is preferably set in the range of 0.5 to 0.9 MPa in order to obtain good cycle characteristics.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The perspective view which shows the thin nonaqueous electrolyte secondary battery which is one Embodiment of the nonaqueous electrolyte secondary battery concerning this invention. FIG. 2 is a partial cross-sectional view of the nonaqueous electrolyte secondary battery in FIG. 1 cut along a short side direction. Configuration diagram showing an outline of a cell for evaluation for measuring the capacitance ratio of the negative electrode (Cs / Cs _Cu) used for the nonaqueous electrolyte secondary battery of Example 1.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Container body, 2 ... Electrode group, 3 ... Positive electrode, 4 ... Negative electrode, 5 ... Separator, 6 ... Cover plate, 7 ... External protective layer, 8 ... Internal protective layer, 9 ... Metal layer, 10 ... Positive electrode tab, 11 DESCRIPTION OF SYMBOLS ... Negative electrode tab, 21 ... Exterior case, 22 ... Current collector, 23 ... Active material containing layer, 24 ... Working electrode, 25 ... Measuring separator, 26 ... Nickel net, 27 ... Lithium foil, 28 ... Nickel plate, 32 ... Nonaqueous electrolyte for measurement, 33... Reference electrode.

Claims (1)

A positive electrode;
A negative electrode comprising a current collector and an active material-containing layer carried by the current collector, and satisfying the following formula (1):
A non-aqueous electrolyte having a viscosity (20 ° C.) in the range of 3 to 50 (cP) and a non-aqueous electrolyte having a conductivity (20 ° C.) in the range of 2 to 10 (mS / cm). Non-aqueous electrolyte secondary battery characterized.
5 ≦ Cs / Cs _Cu ≦ 15 (1)
Where Cs is the capacitance (nF) of the negative electrode, and Cs _Cu is the capacitance (nF) of the current collector.

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