JP2005310621A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion battery having an excellent capacity recovery factor after it is over-discharged, and enhanced in the stability and reliability of itself when it is over-charged and over-discharged, wherein the thermal stability of a negative electrode is enhanced when it is over-charged, high safety and reliability is maintained when it is over-charged, and a negative electrode collector is restrained from being dissolved even when it is over-discharged. <P>SOLUTION: In this nonaqueous electrolyte secondary battery, the nonaqueous electrolyte has γ butyrolactone as a main solvent and LiBF<SB>4</SB>as a main electrolyte, and copper or a copper alloy the surface of which is coated with Cu<SB>2</SB>O is used as its negative electrode collector. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery excellent in overcharge characteristics and overdischarge characteristics.

  Lithium ion batteries that use non-aqueous electrolytes, lithium-containing composite oxides as the positive electrode active material, and carbonaceous materials that can occlude and release lithium ions, such as graphite, are negative electrode active materials compared to aqueous secondary batteries. High-voltage, high energy density, recently expanded its application range, and the number is rapidly increasing, while a cheaper lithium ion battery is desired.

  It is indispensable to install a safety protection circuit for this lithium ion battery, and this safety protection circuit protects the lithium ion battery from overcharge and overdischarge. The share of this safety protection circuit is large.

  Moreover, it is normal that copper foil is used for the collector for negative electrodes of this lithium ion battery. This is because copper is relatively stable in the battery, can be industrially thinned to about 4 μm to 20 μm, is inexpensive, and has excellent versatility. However, among these, the chemical stability of copper is superior to other materials, so it cannot be said that it is sufficient as a current collector for a battery.

Therefore, a proposal to form a non-conductive film by plating with a nickel film or chromium film of 1 μm or more on a copper foil (see, for example, Patent Document 1) or a composite film containing copper hydroxide and copper oxide is formed. The proposal (for example, refer patent document 2) to disclose is disclosed.
JP-A-8-306390 JP-A-10-94764

  However, recent portable devices are required to be small, light, and driven for a long time, and improvement of energy density is essential. In order to cope with this, it is essential to reduce the thickness of the current collector. However, in the negative electrode plate manufactured using the negative electrode current collector disclosed in Patent Document 1, a non-conductive film is required to be 1 μm or more, and the proportion of the film when the current collector is thinned is It cannot be ignored. In addition, since the non-conductive coating becomes thicker, the adhesion between the current collector and the active material is reduced. Therefore, when the charge / discharge cycle is repeated, there is a problem that the active material is peeled off from the current collector. is there.

  Furthermore, in order to provide cheaper lithium-ion batteries in response to growing demand for lithium-ion batteries, it is effective to delete safety protection circuits that account for a large percentage of lithium-ion battery material costs. It is necessary to improve the safety and reliability of the lithium ion battery itself against charging and over-discharging.

  For overcharging, which is one of the functions of the safety protection circuit, a configuration using a separator shutdown function, which is a function specific to battery-mounted separators, is widely used. Normally, the separator plays a role in preventing the short circuit between the positive electrode and the negative electrode can, but the separator using porous polyolefin or the like is a battery due to an external short circuit or an overcharge due to a failure in the voltage control function of the charger or the phone body. When the temperature of the battery rises significantly, the porous separator softens, so that it becomes substantially non-porous and has a so-called shutdown function that prevents current from flowing. By this shutdown, the temperature rise of the battery is around 150 ° C. Stop. By the time of shutdown, there is metal lithium deposited on the surface of the negative electrode without being fully occluded in the carbonaceous material used as the negative electrode active material, but the safety protection circuit is removed and the safety of overcharging of the lithium ion battery itself, In order to improve the reliability, it is necessary that the thermal stability of the negative electrode on which the metal lithium is deposited has a sufficient margin with respect to the battery temperature stopped by the shutdown.

  On the other hand, when overdischarge protection, which is one of the functions of the safety protection circuit, is deleted, the carbonaceous material used as the negative electrode active material has a potential range associated with lithium doping / dedoping under normal charge / discharge conditions. Although it is in the range of 0.1 V to 1.0 V on the basis of lithium, the negative electrode potential exceeds 1 V when an overdischarge state occurs due to voltage control abnormality of the telephone body. A copper current collector is very stable when the potential of the negative electrode active material is in the range of 0.1 to 1 V, but the potential of the negative electrode active material exceeds 1 V and further reaches the redox potential of copper. Then it begins to dissolve. As a result, the current collecting performance of the current collector is deteriorated, or dissolved copper is deposited on the surface of the positive electrode, adversely affecting the performance, and the battery capacity is impaired. As described above, the performance of the electrode once deteriorated at the time of overdischarge hardly recovers even after charging again, and the capacity remains greatly deteriorated.

  When this overdischarged state occurs and the negative electrode potential exceeds 1 V, the copper water disclosed in Patent Document 2 is formed on the copper foil even if the composite film containing the copper hydroxide and the copper oxide is formed. Even when an oxide or copper oxide is CuO, it dissolves to reach the oxidation-reduction potential, and the overdischarge characteristics deteriorate.

In order to solve the above-described problems, the present invention provides a non-aqueous electrolyte solution using γ-butyrolactone (GBL) which is a cyclic carbonate as a main solvent as a non-aqueous solvent, and LiBF ₄ as a main electrolyte as an electrolyte. It is characterized by using. In the vicinity of 150, which is the battery temperature stopped by the above-described shutdown during overcharge, heat is generated during Li ₂ CO ₃ coating generation due to the reaction between the electrolyte and Li. γ-butyrolactone (GBL) has a flash point higher than that of conventional electrolytes, and reduces the reactivity of metallic lithium by forming a film on metallic lithium by polymerizing without reducing it to gas even when reduced. On the other hand, LiBF ₄ also has an effect of suppressing the exothermic reaction between the deposited metallic lithium and the electrolyte by forming an inert thick LiF layer, so that the thermal stability of the negative electrode on which metallic lithium is deposited is up to about 180 ° C. The safety and reliability of overcharging are improved.

Further, as a negative electrode collector made of copper or a copper alloy, an anode current collector, characterized in that the surface is coated with Cu ₂ O film, the Cu ₂ O film, the oxygen concentration is 15 vol. %, And the film thickness is preferably 10 nm or more and 500 nm or less in terms of the oxide film thickness by the cathode reduction method.

  Using such a non-aqueous electrolyte and a negative electrode current collector, the current collector includes a negative electrode plate having a negative electrode active material layer having a carbon material as a main component of the negative electrode active material on at least one surface and a lithium transition metal composite oxide. A non-aqueous electrolyte secondary battery in which an electrode plate group, which is insulated from a positive electrode plate having a main component of a positive electrode active material via a separator, is accommodated in a battery case and a non-aqueous electrolyte is injected, Maintains high safety and reliability during charging, suppresses dissolution of the thinned negative electrode current collector even in an overdischarged state, and excels in capacity recovery with respect to the initial capacity even after overdischarged In addition, a non-aqueous electrolyte secondary battery can be provided.

  As described above, according to the present invention, the heat resistance of the negative electrode on which metallic lithium is deposited during overcharge is improved, high safety and reliability during overcharge are maintained, and even when an overdischarge state occurs. Thus, dissolution of the thinned negative electrode current collector can be suppressed, and a battery excellent in capacity recovery rate relative to the initial capacity can be obtained even after overdischarge.

  In addition, the adhesion between the current collector and the negative electrode active material layer is improved, and a battery with little capacity deterioration can be obtained even when charging and discharging are repeated.

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

  FIG. 1 is a cross-sectional view of a cylindrical lithium secondary battery according to an embodiment of the present invention.

  As shown in FIG. 1, an electrode plate group in which a positive electrode plate 5 and a negative electrode plate 6 are wound in a spiral shape with a separator 7 interposed therebetween is housed in a bottomed cylindrical battery case 8, and the negative electrode plate 6 is connected to the case 8 via the lower insulating plate 10, and the positive electrode lead 3 connected to the positive plate 5 is connected to the internal terminal of the sealing plate 1 via the upper insulating plate 4. A non-aqueous electrolyte (not shown) is injected, and the sealing plate 1 and the battery case 8 are caulked and sealed via the insulating gasket 2.

  This positive electrode plate 5 is obtained by kneading and dispersing a positive electrode active material and a binder, and if necessary, a conductive agent in a solvent on one or both sides of a current collector made of an aluminum foil, a lathed or etched foil. The paste can be applied, dried and rolled. And the thickness of a positive electrode plate needs to be wound as faithfully as possible in the shape using a winding core, and it is preferable that it is 120 micrometers-210 micrometers, and has flexibility.

As the positive electrode active material, for example, a lithium-containing transition metal compound that can accept lithium ions as a guest is used. For example, a composite metal oxide of at least one metal selected from cobalt, manganese, nickel, chromium, iron, and vanadium and lithium, LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiCo _x Ni _(1-x) O ₂ ( 0 <x <1), LiCrO ₂ , αLiFeO ₂ , LiVO _{2 and the} like are preferable.

  Examples of the binder include a fluororesin material that maintains adhesion between active materials, a polymer material having a polyalkylene oxide skeleton, and a styrene-butadiene copolymer. As the fluorine resin material, polyvinylidene fluoride (PVDF), copolymer P (VDF-HFP) of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) is preferable.

  As the conductive agent added as necessary, carbon-based conductive materials such as acetylene black, graphite, and carbon fiber are preferable.

  As the dispersion medium, a dispersion medium in which the binder can be dissolved is suitable. In the case of the organic binder, an organic solvent such as acetone, cyclohexanone, N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone (MEK), or the like is used. Single or a mixed dispersion medium in which these are mixed is preferable. In the case of an aqueous binder, water is preferable.

  The negative electrode plate 6 is prepared by applying, drying, and rolling a paste obtained by kneading and dispersing a negative electrode active material and a binder, and if necessary, a conductive agent in a dispersion medium on one side or both sides of a current collector. Can do. The thickness of the negative electrode plate needs to be wound as faithfully as possible using a winding core, and is preferably 130 μm to 210 μm thick and flexible like the positive electrode plate.

The copper or copper alloy used as the negative electrode current collector is not particularly limited as long as a Cu ₂ O film is formed, and examples thereof include a rolled foil and an electrolytic foil. It may be a perforated foil, an expanded material, a lath material, or the like.

The Cu ₂ O film formed on the surface of the negative electrode current collector can be easily produced by heat treatment in an atmosphere having an oxygen concentration of 15% by volume or less, and the film thickness is an oxide film formed by a cathode reduction method. The thickness value is preferably 10 nm or more and 500 nm or less.

When the Cu ₂ O film is less than 10 nm, it is not preferable because a uniform film without defects cannot be formed. When the Cu ₂ O film exceeds 500 nm, the adhesion between the negative electrode active material and the negative electrode current collector decreases, and charge / discharge is caused. Repeating the cycle is not preferable because the negative electrode active material peels off from the negative electrode current collector.

  Further, the copper thickness is preferably as the tensile strength is strong. However, when the thickness is increased, the void volume inside the battery is reduced and the energy density is lowered.

  By using such a negative electrode current collector, the adhesion with the negative electrode active material layer is improved, a battery with little capacity deterioration can be obtained even after repeated charge and discharge, and even when overdischarged, Dissolution of the negative electrode current collector can be suppressed, and a battery having an excellent capacity recovery rate after overdischarge can be obtained.

As the negative electrode active material, for example, a material containing graphite having a graphite-type crystal structure capable of inserting and extracting lithium ions, such as natural graphite and artificial graphite, is used. In particular, it is preferable to use a carbon material having a graphite-type crystal structure in which the lattice spacing ( ₀₀₂ ) has an interval (d ₀₀₂ ) of 0.3350 to 0.3400 nm.

  As the binder, the dispersion medium, and the conductive agent and plasticizer that can be added as necessary, the same materials as those for the positive electrode can be used.

  The separator is preferably a microporous polyolefin resin such as polyethylene resin or polypropylene resin.

  The non-aqueous electrolyte is composed of a non-aqueous solvent and an electrolyte. As the non-aqueous solvent, γ-butyrolactone (GBL), which is a cyclic carbonate, may be used alone, and as other components, cyclic carbonate and Two or more chain carbonates may be used in combination with γ-butyrolactone (GBL). The cyclic carbonate is preferably at least one selected from ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). The chain carbonate is preferably at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like, and the content of γ-butyrolactone (GBL) is The volume ratio is preferably 50% or more.

As the electrolyte, LiBF ₄ may be used as the main electrolyte, and may be used alone. Other lithium salts having strong electron-withdrawing properties, such as LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ or the like may be used in combination of two or more in addition to LiBF ₄ . These electrolytes are preferably dissolved at a concentration of 0.5 M / L to 2.0 M / L in the non-aqueous solvent, and the content of LiBF ₄ is preferably 50% or more in terms of the total electrolyte molar concentration ratio. .

  By using such a non-aqueous electrolyte, the thermal stability of the negative electrode on which metallic lithium is deposited increases to around 180 ° C., and the safety and reliability of overcharging are improved.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example and a comparative example, these do not limit this invention at all.

(Example 1)
As the negative electrode current collector, a rolled copper foil having a thickness of 7 μm was heat-treated at 150 ° C. for 3 hours in a mixed gas atmosphere of N ₂ : O ₂ = 98: 2 to produce the copper foil of the present invention. The oxide film was qualitatively measured by XPS and confirmed to be a Cu ₂ O film. Further, the thickness of the film was measured by the cathode reduction method to be 50 nm.

  Next, a method for manufacturing the negative electrode plate 6 will be described. 50 parts by weight of scaly graphite powder as a negative electrode active material, 5 parts by weight of styrene butadiene rubber as a binder, 23 parts by weight of a thickener aqueous solution dissolved in 99 parts by weight of water with respect to 1 part by weight of carboxymethylcellulose as a thickener, Was kneaded and dispersed to obtain a negative electrode paste. The obtained negative electrode paste was coated on the current collector by a doctor blade method to a thickness of 180 μm on both sides and dried, then rolled and cut to a thickness of 140 μm to prepare a sheet-like negative electrode plate 6.

A method for manufacturing the positive electrode plate 5 will be described. First, 50 parts by weight of LiCoO ₂ powder as a positive electrode active material, 0.8 parts by weight of acetylene black as a conductive agent, 6 parts by weight of PVDF as a binder, and 41.5 parts by weight of an NMP solution as a solvent are mixed, kneaded and dispersed. Thus, a positive electrode active material paste was obtained. Next, this positive electrode active material paste was applied to both sides of an aluminum foil having a thickness of 30 μm to a thickness of about 230 μm by a doctor blade method, dried, rolled to a thickness of 180 μm, and cut to obtain a positive electrode plate 5.

  Next, an electrode group in which the positive electrode plate 5 and the negative electrode plate 6 thus produced are wound in a spiral shape in a state where the positive electrode plate 5 and the negative electrode plate 6 are insulated through a separator 7 made of a polypropylene resin microporous film having a thickness of 20 μm. Housed in a battery case.

  Next, the negative electrode lead 9 connected to the negative electrode plate 6 is electrically connected to the case 8 via the lower insulating plate 10, and the positive electrode lead 3 connected to the positive electrode plate 5 is sealed via the upper insulating plate 4. After electrically connecting to the internal terminals of the plate 1, a non-aqueous electrolyte (not shown) is injected, and the sealing plate 1 and the battery case 8 are caulked and sealed through the insulating gasket 2, and the diameter is 17 mm. A cylindrical lithium ion secondary battery having a size of 50 mm and a battery capacity of 780 mAh was produced.

The electrolyte solution was a predetermined amount of an electrolyte solution in which 1.5 mol of LiBF ₄ was dissolved as an electrolyte in 100% by volume of γ-butyrolactone (GBL). This electrolytic solution is impregnated in the positive electrode active material layer and the negative electrode active material layer, and is responsible for the movement of Li ions between the positive electrode plate 5 and the negative electrode plate 6 through the separator of the microporous membrane in the battery reaction. This electrolytic solution is impregnated in the positive electrode active material layer and the negative electrode active material layer, and is responsible for the movement of Li ions between the positive electrode plate 5 and the negative electrode plate 6 through the micropores of the porous separator in the battery reaction.

(Example 2)
Except that a predetermined amount of an electrolytic solution in which 1.5 mol of LiBF ₄ was dissolved as an electrolyte was injected into a medium having a volume ratio of γ-butyrolactone (GBL) and ethylene carbonate (EC) of 50:50 as an electrolytic solution. In the same manner as in Example 1, a cylindrical lithium ion secondary battery was produced and designated as Battery B.

(Example 3)
As electrolyte, in a medium having a volume ratio of γ-butyrolactone (GBL) and ethylene carbonate (EC) of 50:50, LiBF ₄ and LiPF ₆ as an electrolyte are in a molar ratio of 50:50, and the total electrolyte concentration is 1. A cylindrical lithium ion secondary battery was produced as Battery C in the same manner as in Example 1 except that a predetermined amount of 5 mol dissolved electrolyte was injected.

(Comparative Example 1)
Cylindrical lithium ion 2 was prepared in the same manner as in Example 1 except that a predetermined amount of an electrolytic solution in which 1.5 mol of LiPF ₆ was dissolved as an electrolyte was poured into 100% by volume of γ-butyrolactone (GBL) as an electrolytic solution. A secondary battery was produced and designated as Battery D.

(Comparative Example 2)
As an electrolytic solution, a predetermined amount of an electrolytic solution in which 1.5 mol of LiBF ₄ was dissolved as an electrolyte was injected into a medium having a volume ratio of ethylene carbonate (EC): ethyl methyl carbonate (EMC) of 30:70. A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1, and designated as battery E.

(Comparative Example 3)
As an electrolytic solution, a predetermined amount of an electrolytic solution in which 1.5 mol of LiPF ₆ was dissolved as an electrolyte was poured into a medium having a volume ratio of ethylene carbonate (EC): ethyl methyl carbonate (EMC) of 30:70. A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1, and designated as Battery F.

Example 4
As a negative electrode current collector, a rolled copper foil having a thickness of 10 μm as in Example 1 was subjected to heat treatment at 150 ° C. for 3 hours in a mixed gas atmosphere of N ₂ : O ₂ = 85: 15. A negative electrode plate produced in the same manner as in Example 1 was used as the negative electrode plate G, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode plate was used. And battery G.

At this time, an oxide film made of Cu ₂ O having a thickness of 40 nm was formed on the surface of the negative electrode current collector.

(Example 5)
Example 1 except that a rolled copper foil having a thickness of 6 μm similar to Example 1 was heat-treated at 150 ° C. for 0.5 hour in a mixed gas atmosphere of N ₂ : O ₂ = 98: 2 as the negative electrode current collector. A negative electrode plate produced in the same manner as in Example 1 was used as the negative electrode plate H using the current collector produced in the same manner, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode plate was used. To make a battery H.

At this time, an oxide film made of Cu ₂ O having a thickness of 10 nm was formed on the surface of the negative electrode current collector.

(Example 6)
As a negative electrode current collector, a rolled copper foil having a thickness of 7 μm as in Example 1 was subjected to heat treatment at 180 ° C. for 3 hours in a mixed gas atmosphere of N ₂ : O ₂ = 98: 2 in the same manner as in Example 1. A negative electrode plate produced in the same manner as in Example 1 was used as the negative electrode plate I, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode plate was used. Battery I was obtained.

At this time, an oxide film made of Cu ₂ O having a thickness of 500 nm was formed on the surface of the negative electrode current collector.

(Comparative Example 4)
As the negative electrode current collector, a rolled copper foil having a thickness of 7 μm similar to that in Example 1 was used, but a current collector produced in the same manner as in Example 1 without any treatment was used, and the same as in Example 1. A negative electrode plate produced in this manner was used as a negative electrode plate J, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode plate was used.

Note that an oxide film in which CuO and Cu ₂ O having a thickness of 12 nm were mixed was formed on the surface of the negative electrode current collector at this time.

(Comparative Example 5)
As a negative electrode current collector, a rolled copper foil having a thickness of 7 μm as in Example 1 was subjected to heat treatment at 150 ° C. for 36 hours in a mixed gas atmosphere of N ₂ : O ₂ = 98: 2 as in Example 1, and the same as in Example 1. A negative electrode plate produced in the same manner as in Example 1 was used as the negative electrode plate K, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode plate was used. The battery K was obtained.

At this time, an oxide film made of Cu ₂ O having a thickness of 600 nm was formed on the surface of the negative electrode current collector.

(Comparative Example 6)
As a negative electrode current collector, a rolled copper foil having a thickness of 7 μm as in Example 1 was heat treated at 150 ° C. for 3 hours in a mixed gas atmosphere of N ₂ : O ₂ = 80: 20. A negative electrode plate produced in the same manner as in Example 1 was used as the negative electrode plate L, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode plate was used. The battery L was obtained.

Note that an oxide film containing 52 nm thick CuO and Cu ₂ O was formed on the surface of the negative electrode current collector at this time.

(Comparative Example 7)
A current collector produced in the same manner as in Example 1 was used except that a current collector obtained by subjecting a surface of a rolled copper foil having a thickness of 5 μm to nickel plating of 2 μm as the negative electrode current collector was used. A negative electrode plate produced as described above was used as a negative electrode plate M, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode plate was used.

  Among the batteries obtained in this way, overcharge tests were performed on battery A to battery F at room temperature (20 ° C. to 25 ° C.) at 780 mAh (equivalent to 1 ItA) at an applied voltage of 12 V, and the separator was shut down due to an increase in battery temperature. Immediately after the occurrence of overcurrent, overcharge is stopped, the battery is cooled, the negative electrode is taken out by disassembling the battery, only the negative electrode is accommodated again in the battery case, and installed in a temperature tank with an explosion-proof function. Table 1 shows the results of measuring the ignition temperature of only the negative electrode by heating and comparing the thermal stability of the negative electrode on which metallic lithium is deposited.

  As is clear from Table 1, the ignition temperatures of the negative electrodes of the present inventions A to C are all 180 ° C. or higher, and the temperature of the battery whose rise is stopped by the overcharge separator shutdown is sufficient for the vicinity of 150 ° C. It was found that the battery is highly safe and reliable during overcharge.

  Next, the battery A and the batteries G to L were evaluated for overdischarge characteristics and cycle life characteristics.

  The overdischarge characteristics are constant current-constant voltage charging at 4.2V for 2 hours in an environment of 20 ° C., and constant current charging at 550 mA (0.7 ItA) until the battery voltage reaches 4.2V. Then, after charging until the current value decays to 40 mA (0.05 ItA), charging and discharging are repeated until the discharge end voltage is 3.0 V at a constant current of 780 mA (1 ItA). The initial capacity was used. Then, after connecting a constant resistance of 50Ω from the discharge state and discharging to 0V, the overdischarge state was left for one month, and then the capacity after overdischarge was measured under the above charge / discharge conditions. Table 2 shows the results of the capacity recovery rate for the above.

  The cycle life characteristics are constant current-constant voltage charging for 2 hours at 4.2V, constant current charging of 550mA (0.7ItA) until the battery voltage reaches 4.2V, and then the current value decays. Then, after charging to 40 mA (0.05 ItA), a charge / discharge cycle test in which discharge is performed at a constant current of 780 mA (1 ItA) to a discharge end voltage of 3.0 V is repeated in an environment of 20 ° C. Table 2 shows the result of the average value of the capacity retention ratio with respect to the initial capacity when the charge / discharge cycle was repeated 500 times as the initial capacity.

  When the charge / discharge cycle was repeated 500 times, each battery was disassembled and the negative electrode plate A and the negative electrode plate G to negative electrode plate M were taken out and divided into 20 equal parts as shown in FIG. When the negative electrode active material is 50% or more remaining on the negative electrode current collector, one point is counted, and when 50% or more remains on all parts, the negative electrode active material is 20 points. Table 2 shows the results of observing the adhesion between the electrode and the current collector and observing the presence or absence of cracks in the negative electrode current collector.

As is clear from Table 2, the capacity recovery rate with respect to the initial capacity is about 95% for the battery A and the batteries G to L of the present invention in which the Cu ₂ O film is formed on the surface of the negative electrode current collector. It was found that the capacity was hardly deteriorated by overdischarge. In contrast, the batteries J and L (Comparative Example 4 and Comparative Example 6) in which the coating on the surface of the negative electrode current collector is not formed of only Cu ₂ O have a capacity recovery rate regardless of the coating thickness. It was very bad at 24% and 26%, and the capacity deterioration due to overdischarge was remarkable. From this, it was found that using a copper foil coated with a Cu ₂ O coating as a negative electrode current collector is extremely effective in suppressing capacity deterioration due to overdischarge.

  In addition, it was found that the battery of the present invention was superior in cycle characteristics as compared with the battery of the comparative example with little deterioration in capacity even after repeated charge and discharge.

And the negative electrode plate in which the Cu ₂ O film is formed on the surface of the present invention shows high adhesion with the negative electrode active material. However, if the Cu ₂ O film is too thick, the current collector and the Cu ₂ O film or Cu _Since the interface resistance between the ₂ O film and the negative electrode active material layer is large, the characteristics deteriorate. In particular, Comparative Example H has a 2 μm nickel plating on the surface of the current collector, so compared with the Cu ₂ O film. Thus, it is presumed that the adhesion is lowered.

  Further, in the above embodiment, the case where it is applied to a cylindrical battery has been described as an example. However, the present invention has no particular limitation on the battery shape, and has various other shapes such as a flat shape and a square shape. It can be applied to a secondary battery.

Sectional drawing of the lithium secondary battery which shows embodiment of this invention Schematic diagram of adhesion evaluation of negative electrode current collector and active material of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sealing plate 2 Insulating gasket 3 Positive electrode lead 4 Upper insulating plate 5 Positive electrode plate 6 Negative electrode plate 7 Separator 8 Battery case 9 Negative electrode lead 10 Lower insulating plate

Claims (1)

An electrode plate group in which a positive electrode plate using a lithium-containing composite oxide as a positive electrode active material and a negative electrode plate using a carbonaceous material capable of occluding and releasing lithium ions as a negative electrode active material are housed in a case. In the secondary battery obtained by injecting a non-aqueous electrolyte, the non-aqueous electrolyte is a non-aqueous electrolyte containing γ-butyrolactone as a main solvent and LiBF ₄ as a main electrolyte, and the negative electrode collector of the negative electrode plate A non-aqueous electrolyte secondary battery, wherein the electric body is copper or a copper alloy, and the surface thereof is coated with a Cu ₂ O film.

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