JP2008097879A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery high in a high-rate discharge characteristic and a cycle characteristic. <P>SOLUTION: This lithium ion secondary battery includes a positive electrode 10, a negative electrode 20, a separator 30 arranged between the positive electrode 10 and the negative electrode 20, and a nonaqueous electrolyte. The positive electrode 10 includes a positive electrode collector 11, a positive electrode active material layer 12 formed on the positive electrode collector 11, and a conductive layer 13 formed on the positive electrode active material layer 12; and the thickness of the conductive layer 13 is 1-7 μm. The positive electrode active material layer 12 includes a lithium-containing composite oxide and a conductive assistant; the content A of the conductive assistant of the positive electrode active material layer 12 is 1-15 wt.% with respect to the total weight of the positive electrode active material layer 12; the conductive layer 13 contains the conductive assistant; and the content B of the conductive assistant of the conductive layer 13 is 20-80 wt.% with respect to the total weight of the conductive layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery having high charge / discharge characteristics and high cycle characteristics.

  In recent years, with the rapid spread of portable devices such as mobile phones and personal digital assistants (PDAs), the demand for lithium ion secondary batteries having a high energy density as a power source is rapidly expanding. At present, this lithium ion secondary battery has established a position as a standard battery for mobile information devices such as mobile phones and notebook computers, and there is a demand for further improvement in its performance. Under such circumstances, various proposals have recently been made to improve the high rate charge / discharge characteristics and cycle characteristics of lithium ion secondary batteries (see, for example, Patent Document 1).

On the other hand, this lithium ion secondary battery is being put to practical use not only as a power source for mobile information devices but also as a high input / output power source for electric vehicles, hybrid vehicles, electric tools and the like. When a lithium ion secondary battery is used as a power supply for high input / output, higher high rate discharge characteristics and cycle characteristics are required compared to use as a power supply for mobile information equipment. For this reason, conventionally, a lithium ion secondary battery has been used as a high input / output power source by using a lithium ion secondary battery and an electric double layer capacitor in combination. In addition, a proposal has been made to use a lithium ion secondary battery as a high input / output power source without using it together with an electric double layer capacitor (see, for example, Patent Document 2). Furthermore, lithium ion secondary batteries used as power sources for electric vehicles and hybrid vehicles that require particularly high rate charge / discharge characteristics have been addressed by increasing the amount of conductive additive added to the positive and negative electrodes.
JP 2003-249 211 A JP 2002-260634 A

  In Patent Document 1, high-rate discharge characteristics and cycle characteristics are improved by forming a layer containing metal particles or the like on the negative electrode surface of a lithium ion secondary battery. However, no consideration is given to the positive electrode side. Not. Moreover, in the method using a lithium ion secondary battery and an electric double layer capacitor in combination, the energy density of the electric double layer capacitor is low, and there is a problem in practical use. Furthermore, in Patent Document 2, by adding activated carbon to the positive electrode active material as a conductive agent, high energy density, high output density, and low temperature characteristics are improved. However, when activated carbon is added in a large amount to the positive electrode active material. Since the fluidity of the active material paint is lowered and the coating film is easily peeled off from the current collector, there is a limit to the amount of activated carbon added, and the above characteristics cannot be sufficiently improved. Further, when the amount of the conductive additive added to the positive electrode and the negative electrode is increased, there is a problem that the amount of the active material is relatively decreased and the battery capacity is decreased.

  The present invention solves the above problems and provides a lithium ion secondary battery having high rate discharge characteristics and high cycle characteristics without reducing the battery capacity.

  A lithium ion secondary battery of the present invention includes a positive electrode capable of inserting and extracting lithium, a negative electrode capable of inserting and extracting lithium, a separator disposed between the positive electrode and the negative electrode, a non-aqueous electrolyte, A positive electrode current collector, a positive electrode active material layer formed on the positive electrode current collector, and a conductive material formed on the positive electrode active material layer. The conductive layer has a thickness of 1 μm or more and 7 μm or less, the positive electrode active material layer includes a lithium-containing composite oxide and a conductive additive, and the conductive auxiliary of the positive electrode active material layer The content A is 1% by weight or more and 15% by weight or less based on the total weight of the positive electrode active material layer, the conductive layer contains a conductive assistant, and the conductive auxiliary agent content of the conductive layer B is 20 wt% or more and 80 wt% or less with respect to the total weight of the conductive layer. It is characterized by that.

  According to the present invention, the conductivity of the reaction site of the positive electrode can be increased without reducing the battery capacity, and a lithium ion secondary battery having high rate discharge characteristics and high cycle characteristics can be provided.

  A lithium ion secondary battery according to the present invention includes a positive electrode capable of inserting and extracting lithium, a negative electrode capable of inserting and extracting lithium, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. ing. The positive electrode includes a positive electrode current collector, a positive electrode active material layer formed on the positive electrode current collector, and a conductive layer formed on the positive electrode active material layer. The thickness of the conductive layer is 1 μm or more and 7 μm or less. The positive electrode active material layer includes a lithium-containing composite oxide and a conductive auxiliary agent, and the content A of the conductive auxiliary agent in the positive electrode active material layer is 1 wt% or more and 15 wt% with respect to the total weight of the positive electrode active material layer. It is as follows. The conductive layer contains a conductive additive, and the conductive additive content B of the conductive layer is 20% by weight to 80% by weight with respect to the total weight of the conductive layer.

  As described above, the positive electrode has a two-layer structure of the positive electrode active material layer and the conductive layer, and the content A of the conductive assistant in the positive electrode active material layer and the content B of the conductive assistant in the conductive layer are within the above ranges, respectively. Thus, the conductivity of the reaction site of the positive electrode can be increased without reducing the battery capacity, and the high rate discharge characteristics can be improved. Moreover, since the porosity of the conductive layer is higher than that of the positive electrode active material layer, the permeability and liquid retention of the electrolytic solution are improved, and the cycle characteristics are also improved.

  The conductive assistant is preferably made of a carbon material. This is because the carbon material has high conductivity and is stable with respect to the charge / discharge reaction of the electrode.

  The thickness of the conductive layer is preferably 1 μm or more and 7 μm or less, and more preferably 1 μm or more and 5 μm or less. This is because by reducing the thickness of the conductive layer, more positive electrode active material can be filled in the same volume, so that the battery capacity can be increased.

  The thickness ratio between the conductive layer and the positive electrode active material layer is preferably 1: 4 to 1:30. This is because if the thickness of the conductive layer with respect to the positive electrode active material layer becomes too thick, the battery capacity decreases, and if the thickness of the conductive layer becomes too thin, the reactivity of the portion near the conductive layer of the positive electrode active material layer only increases. This is because the effect of improving the reactivity of the whole positive electrode active material layer is reduced.

  The conductive layer may further include a lithium-containing composite oxide. Thereby, the battery capacity can be further increased.

  Hereinafter, the lithium ion secondary battery of this invention is demonstrated based on drawing. FIG. 1 is a cross-sectional view showing an example of an electrode laminate used in the lithium ion secondary battery of the present invention. In FIG. 1, the electrode laminate 1 includes a positive electrode 10 capable of inserting and extracting lithium, a negative electrode 20 capable of inserting and extracting lithium, and a separator 30 disposed between the positive electrode 10 and the negative electrode 20. Yes.

  The positive electrode 10 includes a positive electrode current collector 11, a positive electrode active material layer 12 formed on the positive electrode current collector 11, and a conductive layer 13 formed on the positive electrode active material layer 12. The positive electrode active material layer 12 includes a lithium-containing composite oxide and a conductive additive made of a carbon material, and the content of the conductive additive in the positive electrode active material layer 12 is based on the total weight of the positive electrode active material layer 12. The thickness ratio between the conductive layer 13 and the positive electrode active material layer 12 is set to 1: 4 to 1:30.

  In addition, the conductive layer 13 includes a conductive additive, and the content of the conductive additive in the conductive layer 13 is 20% by weight to 80% by weight with respect to the total weight of the conductive layer 13, and the thickness of the conductive layer 13 is It is set to 1 μm or more and 7 μm or less.

  The negative electrode 20 includes a negative electrode current collector 21, a negative electrode active material layer 22 formed on the negative electrode current collector 21, and a conductive layer 23 formed on the negative electrode active material layer 22. The negative electrode active material layer 22 includes an active material made of a carbon material and a conductive auxiliary agent made of a carbon material, and the content of the conductive auxiliary agent in the negative electrode active material layer 22 is based on the total weight of the negative electrode active material layer 21. 1 wt% or more and 15 wt% or less.

  In addition, the conductive layer 23 includes a conductive additive, and the content of the conductive additive in the conductive layer 23 is 20 wt% or more and 80 wt% or less with respect to the total weight of the conductive layer 23, and the thickness of the conductive layer 23 is It is set to 1 μm or more and 7 μm or less.

  As described above, in order to improve the high rate charge / discharge characteristics, it is effective to increase the current collecting property of the electrode by increasing the amount of the conductive additive added to the active material. However, when the additive amount of the conductive auxiliary agent is increased, there is a problem in that the capacity is extremely reduced because the number of substances not involved in charging / discharging increases in the electrode, or the electrode density is not increased and the capacity is reduced.

  Therefore, the present inventors have improved the conductivity without reducing the battery capacity by forming a conductive layer having a high content of conductive auxiliary agent on the active material layer, thereby solving the above problem. If the conductive layer is provided at least on the positive electrode side, there is an effect of improving the high rate charge / discharge characteristics, but a similar conductive layer may be provided on the negative electrode side as shown in FIG.

  The conductive layer is formed on the surface of the electrode when charging / discharging at a high rate, because the charge / discharge reaction mainly occurs on the electrode surface facing the counter electrode, so that the conductive layer faces the counter electrode. This is because by providing it on the surface portion of the electrode, the conductivity of the reaction site is improved and the high rate charge / discharge characteristics are improved.

  The content of the conductive assistant in the conductive layers 13 and 23 is related to the thickness of the conductive layers 13 and 23, and is preferably 20% by weight or more and 80% by weight or less. If the amount is less than 20% by weight, sufficient conductivity cannot be obtained, and if it exceeds 80% by weight, the conductive additive inhibits the reaction of the active material.

  Since the porosity of the conductive layers 13 and 23 to which the conductive auxiliary agent is added is higher than the porosity of the positive electrode active material layer 12 and the negative electrode active material layer 22, the permeability and liquid retention of the electrolytic solution are improved, and the cycle Improved characteristics.

Examples of the lithium-containing composite oxide that is the active material of the positive electrode 10 include Li _x CoO ₂ , lithium cobalt composite oxide further containing at least one additive element such as Ge, Ti, and Zr, Li _x NiO _{2, and the} like. Lithium-nickel composite oxide, lithium nickel-cobalt composite oxide, lithium manganese-nickel composite oxide, lithium manganese-nickel-cobalt composite, in which part of nickel in the above lithium-nickel composite oxide is replaced with cobalt or manganese A lithium-containing composite oxide having a layered structure such as an oxide can be used. Examples of the lithium-containing composite oxide include Li _y Mn ₂ O ₄ and a lithium manganese composite oxide containing at least one additional element such as Ge, Zr, and Mg, and Li _4/3 Ti _{5 /} A spinel-structure lithium-containing composite oxide such as a lithium titanium composite oxide such as ₃ O ₄ can also be used. Two or more active materials may be mixed or combined to be used.

  As the conductive aid, carbon materials such as graphite, acetylene black, carbon black, ketjen black, and vapor grown carbon fiber can be used. In particular, when fine particles such as acetylene black, carbon black, ketjen black, and vapor grown carbon fiber are used, the conductive layers 13 and 23 can be easily formed as a thin film layer having a thickness of 1 to 5 μm. These conductive assistants may be used alone or in combination of two or more.

  The positive electrode 10 is formed by, for example, applying a positive electrode mixture obtained by appropriately adding a binder such as polyvinylidene fluoride to the positive electrode active material and the conductive auxiliary agent to a positive electrode current collector 11 made of aluminum foil or the like. After the material layer 12 is formed, the conductive layer 13 is further formed by applying a conductive coating agent, which is obtained by appropriately adding a binder such as polyvinylpyrrolidone to the conductive auxiliary agent, on the positive electrode active material layer 12. As a result, it is formed as a band-shaped molded body. The application of the positive electrode mixture and the conductive coating agent can be performed by a sequential multilayer coating method in which the applied positive electrode mixture is dried and then the conductive coating agent is applied and dried. When the applied positive electrode mixture is dried by a simultaneous multilayer coating method in which a conductive coating agent is applied and dried before drying, the drying process becomes one step and the manufacturing process can be simplified, and the thickness is 1 to 1. The conductive layer 13 having a thickness of 5 μm can be formed uniformly, which is preferable. In the present embodiment, the positive electrode 10 is shown as an example in which the positive electrode active material layer 12 and the conductive layer 13 are formed on both surfaces of the positive electrode current collector 11, but only on one surface of the positive electrode current collector 11 on the separator 30 side. The positive electrode active material layer 12 and the conductive layer 13 may be formed.

  Examples of the carbon material that is the active material of the negative electrode 20 include graphite, pyrolytic carbons, cokes, glassy carbons, organic polymer compound fired bodies, mesocarbon microbeads, carbon fibers, activated carbon, and the like. . In addition, the same conductive aid used for the positive electrode 10 can be used for the negative electrode 20.

  The negative electrode 20 is formed by, for example, applying a negative electrode mixture obtained by appropriately adding a binder such as polyvinylidene fluoride to the negative electrode active material and the conductive auxiliary agent to a negative electrode current collector 21 made of copper foil or the like. After the material layer 22 is formed, a conductive layer 23 is formed by further applying a conductive coating agent appropriately added with a binder such as polyvinylpyrrolidone to the conductive auxiliary agent on the negative electrode active material layer 22. As a result, it is formed as a band-shaped molded body. Other than these, the negative electrode 20 can be manufactured in the same manner as the positive electrode 10.

  As the separator 30, for example, a microporous separator made of a polyolefin resin such as polyethylene or polypropylene is preferably used. The thickness of the separator 30 may be, for example, 15 to 30 μm.

  In order to manufacture a lithium ion secondary battery using the electrode laminate 1, for example, the surface opposite to the surface where the negative electrode 20 of the electrode laminate 1 is in contact with the separator 30 is further covered with the separator, and then the electrode laminate 1 Is wound to form a positive electrode terminal and a negative electrode terminal. Thereafter, the electrode stack 1 may be inserted into a metal cylindrical battery can, a rectangular battery can, or the like and a lid may be attached, and then an electrolyte may be injected from the injection port. Further, instead of a metal battery can, for example, after the electrode laminate 1 is inserted into a bag-like case formed of an aluminum laminate film or the like, an electrolytic solution is injected, and finally the opening of the bag-like case is sealed. May be.

  As the electrolytic solution, a nonaqueous electrolytic solution in which an electrolyte salt is dissolved in a solvent is used. The solvent used for the non-aqueous electrolyte is preferably an organic solvent having a high dielectric constant. This is because high voltage charging becomes possible. As the organic solvent having a high dielectric constant, ethers, esters, carbonates and the like are preferably used. In particular, it is preferable to use a mixture of esters having a high dielectric constant (dielectric constant of 30 or more). Examples of the ester having a high dielectric constant include sulfur-based esters such as ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, and ethylene glycol sulfite. Cyclic esters are particularly preferable, especially ethylene carbonate. A cyclic carbonate such as

As the electrolyte salt to be dissolved in the solvent of the nonaqueous electrolytic solution, a salt of a fluorine-containing compound such as lithium perchlorate, lithium organic boron, and trifluoromethanesulfonate, or an imide salt is preferably used. . Specific examples of the electrolyte salt, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2, where n is preferably 4 or less), LiN (Rf ₃ OSO ₂ ) ₂ [wherein Rf represents a fluoroalkyl group. ] Etc. are used alone or in admixture of two or more. In particular, LiPF ₆ and LiBF ₄ are desirable because of their good charge / discharge characteristics. This is because these fluorine-containing organic lithium salts have a large anionic property and are easily ion-separated, so that they are easily dissolved in the solvent. The concentration of the electrolyte salt in the electrolytic solution is not particularly limited, but is preferably 0.5 mol / L or more and 1.7 mol / L or less, and more preferably 0.8 mol / L or more and 1.2 mol / L or less.

  Next, based on an Example, this invention is demonstrated more concretely. However, the present invention is not limited to the following examples.

(Example 1)
<Preparation of positive electrode>
86 parts by weight of lithium manganese cobaltate as a positive electrode active material, 9.2 parts by weight of graphite as a conductive additive and 1.8 parts by weight of acetylene black, and 3 parts by weight of polyvinylidene fluoride as a binder, A positive electrode mixture paint was prepared by mixing with N-methyl-2-pyrrolidone. Further, 60 parts by weight of acetylene black as a conductive auxiliary agent and 40 parts by weight of polyvinylpyrrolidone as a binder were mixed with N-methyl-2-pyrrolidone to prepare a conductive coating agent paint.

  Next, first, the positive electrode mixture paint is applied to one side of an aluminum foil (current collector) having a thickness of 15 μm, and before the positive electrode mixture paint is dried, the conductive coating agent paint is applied thereon and dried. Thus, a positive electrode active material layer and a conductive layer were formed on one side of the current collector. After that, it is compression-molded by a roll press so that the thickness of the positive electrode active material layer is 30 μm and the thickness of the conductive layer is 5 μm, and then cut, welded with a nickel lead body, and a belt-like positive electrode is formed. Produced. The content A of the conductive assistant relative to the total weight of the positive electrode active material layer is 11% by weight, and the content B of the conductive assistant relative to the total weight of the conductive layer is 60% by weight. Moreover, from the content A and B of the conductive assistant in each layer and the coating weight per unit area of each layer separately measured, the total conductive assistant including the positive electrode active material layer and the conductive layer is combined. The average value of the contents of was found to be 12.8% by weight.

<Production of negative electrode>
N-methyl-2-pyrrolidone was mixed with 88 parts by weight of graphite as a negative electrode active material, 5 parts by weight of acetylene black as a conductive additive, and 7 parts by weight of polyvinylidene fluoride as a binder. An agent paint was prepared. Further, 60 parts by weight of acetylene black as a conductive auxiliary agent and 40 parts by weight of polyvinylpyrrolidone as a binder were mixed with N-methyl-2-pyrrolidone to prepare a conductive coating agent paint.

  Next, first, the negative electrode mixture paint is applied to one side of a copper foil (current collector) having a thickness of 8 μm, and before the negative electrode mixture paint is dried, the conductive coating agent paint is applied thereon and dried. Thus, a negative electrode active material layer and a conductive layer were formed on one side of the current collector. After that, it is compression-molded by a roll press machine so that the thickness of the negative electrode active material layer is 40 μm and the thickness of the conductive layer is 5 μm, and then cut, welded with a nickel lead body, and a strip-shaped negative electrode is formed. Produced. The content of the conductive additive relative to the total weight of the negative electrode active material layer is 5% by weight, and the content of the conductive additive relative to the total weight of the conductive layer is 60% by weight.

<Preparation of non-aqueous electrolyte>
As a nonaqueous electrolytic solution, a solution in which LiPF ₆ was dissolved at 1.0 mol / L in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) in a volume ratio of 1: 2 was prepared.

<Production of battery>
A positive electrode active material layer side of the belt-like positive electrode is overlapped with a negative electrode active material layer side of the belt-like negative electrode through a 25 μm-thick microporous polyethylene separator (porosity: 41%) to form an electrode laminate. After that, it was housed in a bag-like case made of an aluminum laminate film, and the non-aqueous electrolyte was injected. Finally, the opening of the bag-like case was sealed to produce a lithium ion secondary battery.

(Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the positive electrode was produced as follows.

  88 parts by weight of lithium manganese cobaltate as a positive electrode active material, 9.8 parts by weight of graphite as a conductive additive and 0.4 parts by weight of acetylene black, and 1.8 parts by weight of polyvinylidene fluoride as a binder Was mixed with N-methyl-2-pyrrolidone to prepare a positive electrode mixture paint. Further, 55.5 parts by weight of lithium manganese cobaltate as a positive electrode active material, 23.9 parts by weight of acetylene black as a conductive auxiliary agent, and 20.6 parts by weight of polyvinylpyrrolidone as a binder are mixed with N- A conductive coating material paint was prepared by mixing with methyl-2-pyrrolidone.

  Next, first, the positive electrode mixture paint is applied to one side of an aluminum foil (current collector) having a thickness of 15 μm, and before the positive electrode mixture paint is dried, the conductive coating agent paint is applied thereon and dried. Thus, a positive electrode active material layer and a conductive layer were formed on one side of the current collector. After that, it is compression-molded by a roll press machine so that the thickness of the positive electrode active material layer becomes 28 μm and the thickness of the conductive layer becomes 7 μm, then cut, welded with a nickel lead body, Produced. The content A of the conductive assistant relative to the total weight of the positive electrode active material layer is 10.2% by weight, and the content B of the conductive assistant relative to the total weight of the conductive layer is 23.9% by weight. Moreover, from the content A and B of the conductive assistant in each layer and the coating weight per unit area of each layer separately measured, the total conductive assistant including the positive electrode active material layer and the conductive layer is combined. The average value of the contents of was 11% by weight.

(Example 3)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that no conductive layer was formed on the negative electrode.

Example 4
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode was produced as described below.

  N-methyl-2-pyrrolidone is mixed with 95 parts by weight of graphite as a negative electrode active material, 1.4 parts by weight of acetylene black as a conductive auxiliary agent, and 3.6 parts by weight of polyvinylidene fluoride as a binder. Thus, a negative electrode mixture paint was prepared. Further, 51 parts by weight of graphite as a negative electrode active material, 23 parts by weight of acetylene black as a conductive auxiliary agent, and 26 parts by weight of polyvinyl pyrrolidone as a binder are mixed with N-methyl-2-pyrrolidone to conduct electricity. An application paint was prepared.

  Next, first, the negative electrode mixture paint is applied to one side of a copper foil (current collector) having a thickness of 8 μm, and before the negative electrode mixture paint is dried, the conductive coating agent paint is applied thereon and dried. Thus, a negative electrode active material layer and a conductive layer were formed on one side of the current collector. After that, after compression molding so that the thickness of the negative electrode active material layer is 36 μm and the thickness of the conductive layer is 9 μm by a roll press machine, cutting is performed, a nickel lead body is welded, and a strip-shaped negative electrode is formed. Produced. The content of the conductive additive relative to the total weight of the negative electrode active material layer is 1.4% by weight, and the content of the conductive additive relative to the total weight of the conductive layer is 23% by weight.

(Comparative Example 1)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that no conductive layer was formed on the positive electrode.

(Comparative Example 2)
A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the thickness of the conductive layer of the positive electrode was 10 μm.

(Comparative Example 3)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the positive electrode was produced as follows.

  86 parts by weight of lithium manganese cobaltate as a positive electrode active material, 9.6 parts by weight of graphite as a conductive additive and 1.7 parts by weight of acetylene black, 2.7 parts by weight of polyvinylidene fluoride as a binder, Was mixed with N-methyl-2-pyrrolidone to prepare a positive electrode mixture paint. Further, 74 parts by weight of lithium manganese cobaltate as a positive electrode active material, 15 parts by weight of acetylene black as a conductive auxiliary agent, and 11 parts by weight of polyvinylpyrrolidone as a binder are added to N-methyl-2-pyrrolidone. A conductive coating material was prepared by mixing.

  Next, first, the positive electrode mixture paint is applied to one side of an aluminum foil (current collector) having a thickness of 15 μm, and before the positive electrode mixture paint is dried, the conductive coating agent paint is applied thereon and dried. Thus, a positive electrode active material layer and a conductive layer were formed on one side of the current collector. After that, it is compression-molded by a roll press machine so that the thickness of the positive electrode active material layer is 30 μm and the thickness of the conductive layer is 7 μm, then cut, welded with a nickel lead body, Produced. The content A of the conductive assistant with respect to the total weight of the positive electrode active material layer is 11.3 wt%, and the content B of the conductive assistant with respect to the total weight of the conductive layer is 15 wt%. This comparative example is an example in which the content B (15% by weight) of the conductive additive in the conductive layer is smaller than the claims of the present invention.

  Table 1 shows the configurations of the positive electrode active material layers and the positive electrode conductive layers of the batteries of Examples 1 to 4 and Comparative Examples 1 to 3.

<Discharge test>
The batteries of Examples 1 to 4 and Comparative Examples 1 to 3 were charged with a constant current and a constant voltage of 1 C and 4.1 V at 20 ° C. for 1.5 hours. Then, the discharge capacity (1 C discharge capacity) when discharged to 2.7 V at 1 C was measured. The results are shown in Table 2.

  The batteries were charged at 20 ° C. with a constant current and a constant voltage of 4.1 V for 1.5 hours. Then, the discharge capacity (20 C discharge capacity) when discharged to 2.5 V at 20 C was measured, and the ratio (discharge rate) of the 20 C discharge capacity to the 1 C discharge capacity was determined. The results are shown in Table 2.

<Cycle test>
Each battery of Examples 1 to 4 and Comparative Examples 1 to 3 was charged at 1C at 20 ° C. to 50% of the theoretical capacity of the positive electrode, and then discharged at 1C, 5C, and 10C, and after 5 seconds from the start of discharge. The measured battery voltage was plotted against the current, and the resistance of the battery (resistance before the cycle test) was measured from the slope of the straight line passing through the plotted points. Next, the above battery was charged and discharged for 100,000 cycles under the environment of 50 ° C. with 10 s charging at 10 C / 10 s discharging at 10 C as one cycle. Thereafter, the resistance after the cycle test was measured in the same manner as described above. From the above results, the resistance increase rate of each battery was calculated by the following formula. The results are shown in Table 2 as relative values with the resistance increase rate of the battery of Comparative Example 1 as 100.

(Equation 1)
Resistance increase rate (%) = (resistance after cycle test−resistance before cycle test) / (resistance before cycle test) × 100

  From Table 2, in Examples 1-4, by providing a conductive layer on the positive electrode active material layer, it is possible to increase the discharge rate without incurring a capacity decrease as compared with Comparative Examples 1-3, and high rate discharge characteristics. The resistance increase rate after the cycle test could be suppressed. The positive electrode of Comparative Example 1 has the same content A of 11% by weight as that of the positive electrode of Example 1, but does not have a conductive layer, and thus has a large difference in characteristics from Example 1. . Further, Comparative Example 2 is different from Example 1 only in the thickness of the conductive layer of the positive electrode, but because the thickness of the conductive layer is too thick, the reaction of the positive electrode active material layer is suppressed on the contrary, As a result, the characteristics deteriorated. Since the content B of the conductive auxiliary agent in the conductive layer was less than 20% by weight, Comparative Example 3 did not function sufficiently as a conductive layer, and the characteristics were inferior to those of Examples 1 to 4. Moreover, in the positive electrode of Example 2, although the average value of the content of the conductive auxiliary agent in the total of the active material layer and the conductive layer was 11% by weight, which was the same as Comparative Example 1, the Comparative Example As is apparent from the fact that the above characteristics were significantly improved from 1, the present invention makes it possible to efficiently function a carbon material as a conductive additive, and a positive electrode having a high reaction efficiency even with a small amount of conductive aid. Can be configured. For this reason, a lithium ion secondary battery excellent in high rate discharge characteristics and cycle characteristics can be obtained.

  As described above, the present invention can provide a lithium ion secondary battery with high rate discharge characteristics and high cycle characteristics without reducing the battery capacity, and only as a power source for mobile information devices such as mobile phones and notebook computers. Instead, it can be widely used as a power source for high input / output of electric vehicles, hybrid vehicles, electric tools and the like.

It is sectional drawing which shows an example of the electrode laminated body of the lithium ion secondary battery of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrode laminated body 10 Positive electrode 11 Positive electrode collector 12 Positive electrode active material layer 13 Conductive layer 20 Negative electrode 21 Negative electrode collector 22 Negative electrode active material layer 23 Conductive layer 30 Separator

Claims (4)

A lithium ion secondary battery comprising: a positive electrode capable of inserting and extracting lithium; a negative electrode capable of inserting and extracting lithium; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte. ,
The positive electrode includes a positive electrode current collector, a positive electrode active material layer formed on the positive electrode current collector, and a conductive layer formed on the positive electrode active material layer,
The conductive layer has a thickness of 1 μm or more and 7 μm or less,
The positive electrode active material layer includes a lithium-containing composite oxide and a conductive additive,
The content A of the conductive additive in the positive electrode active material layer is 1 wt% or more and 15 wt% or less with respect to the total weight of the positive electrode active material layer,
The conductive layer includes a conductive aid,
The lithium ion secondary battery, wherein a content B of the conductive assistant in the conductive layer is 20% by weight to 80% by weight with respect to a total weight of the conductive layer.   The lithium ion secondary battery according to claim 1, wherein the conductive additive is made of a carbon material.   3. The lithium ion secondary battery according to claim 1, wherein a thickness ratio between the conductive layer and the positive electrode active material layer is 1: 4 to 1:30.   The lithium ion secondary battery according to claim 1, wherein the conductive layer further includes a lithium-containing composite oxide.

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