JP2019169346A - Lithium ion secondary battery - Google Patents

Abstract

To provide a lithium ion secondary battery having excellent cycle characteristics.SOLUTION: The lithium ion secondary battery includes a power generation element including: a positive electrode having a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector; a negative electrode having a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector, facing the positive electrode; and a separator sandwiched between the positive electrode and the negative electrode. The density of a first negative electrode active material layer located on a surface on the side not facing the positive electrode at the negative electrode located at the outermost of the power generation element is greater than the density of a second negative electrode active material layer located inner side of the power generating element with respect to the first negative electrode active material layer.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a lithium ion secondary battery.

  In recent years, electronic devices such as mobile phones and personal computers have been rapidly reduced in size and cordless, and the demand for secondary batteries having a small size, light weight, and high energy density as driving power sources has been increasing.

  One of the characteristics required for a lithium ion secondary battery is cycle characteristics. If the cycle characteristics deteriorate, stable charge / discharge cannot be performed for a long time. The expansion and contraction of the negative electrode during charge and discharge, the depletion of the electrolyte, and the like are factors that degrade the cycle characteristics.

  Patent Document 1 describes a secondary battery in which the density of the negative electrode in the outermost layer of a plurality of stacked negative electrodes is reduced. A low-density negative electrode among the plurality of negative electrodes serves as a cushion, and the stress caused by the expansion and contraction of the secondary battery is relieved.

  Patent Document 2 describes a sealed secondary battery in which a porous body in which an electrolytic solution is absorbed is disposed at a position adjacent to an electrode plate group in a battery case. By supplying the electrolytic solution from the porous body, depletion of the electrolytic solution is suppressed.

JP 2010-233201 A JP 2003-17115 A

  However, the secondary battery described in Patent Document 1 cannot be said to have sufficient cycle characteristics. The low density negative electrode located in the outermost layer can alleviate the rapid expansion and contraction of the internal negative electrode, but does not suppress the expansion and contraction of the internal negative electrode itself. That is, cracks and peeling may occur in the internal negative electrode due to expansion and contraction, and the cycle characteristics were not sufficient.

  In addition, since the sealed secondary battery described in Patent Document 2 has a porous body provided outside, the overall size becomes large. That is, the energy density of the lithium ion secondary battery is lowered. Further, the portion of the electrode plate group in the battery case that is not adjacent to the porous body is depleted of the electrolytic solution, and the cycle characteristics are deteriorated.

  The present invention has been made in view of the above problems, and an object thereof is to provide a lithium ion secondary battery having excellent cycle characteristics.

  As a result of intensive studies, the present inventors have found that the cycle characteristics are improved by increasing the density of the negative electrode active material layer in the outermost layer as compared with the density of the negative electrode active material layer in other portions. That is, this invention provides the following means in order to solve the said subject.

(1) A lithium ion secondary battery according to a first aspect includes a positive electrode having a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector, a negative electrode current collector, and the negative electrode current collector. A negative electrode active material layer located on at least one surface of the electric body, and a negative electrode facing the positive electrode, and a separator sandwiched between the positive electrode and the negative electrode, and a power generation element, The density of the first negative electrode active material layer located on the surface of the negative electrode located on the outermost side of the power generation element that does not face the positive electrode is such that the second negative electrode located on the inner side of the power generation element than the first negative electrode active material layer It is higher than the density of the active material layer.

(2) In the lithium ion secondary battery according to the above aspect, the density D ₁ of the first negative electrode active material layer and the density D ₂ of the second negative electrode active material layer are 1 <D ₁ / D ₂ <1. 7 relationship may be satisfied.

(3) In the lithium ion secondary battery according to the above aspect, the density D ₂ of the second negative electrode active material layer may be 1.0 g / cm ³ or more.

  The lithium ion secondary battery according to the above aspect is excellent in cycle characteristics.

It is a cross-sectional schematic diagram of the lithium ion secondary battery concerning this embodiment. It is the schematic diagram which expanded the electric power generating element of the lithium ion secondary battery concerning this embodiment.

  Hereinafter, the present embodiment will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, in order to make the characteristics of the present invention easier to understand, there are cases where the characteristic parts are enlarged for the sake of convenience, and the dimensional ratios of the respective components are different from actual ones. is there. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be appropriately modified and implemented without departing from the scope of the invention.

[Lithium ion secondary battery]
FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery 100 according to the present embodiment. As shown in FIG. 1, the lithium ion secondary battery 100 includes a power generation element 40 and an exterior body 50. The power generation element 40 is impregnated with an electrolytic solution. The exterior body 50 prevents the electrolyte from leaking to the outside, and prevents external air and moisture from reaching the power generation element 40.

(Power generation element)
The power generation element 40 includes the positive electrode 20, the negative electrode 30, and the separator 10. In the power generation element 40 shown in FIG. 1, a pair of a positive electrode 20 and a negative electrode 30 are disposed to face each other with the separator 10 interposed therebetween. The number of stacked positive electrodes 20 and negative electrodes 30 is not limited. The power generating element 40 may be a wound body.

"Positive electrode"
The positive electrode 20 includes a positive electrode current collector 22 and a positive electrode active material layer 24 located on at least one surface of the positive electrode current collector 22.

  The positive electrode current collector 22 may be a conductive plate material, and a thin metal plate such as an aluminum foil or a nickel foil that has low reactivity with lithium on the oxidation potential side can be used. In particular, an aluminum foil can be suitably used.

  The positive electrode active material layer 24 includes a positive electrode active material and a binder, and includes a conductive additive as necessary.

The positive electrode active material reversibly advances ion insertion and extraction, ion desorption and insertion (intercalation), or ion and ion counter anion (for example, PF ₆ ⁻ ). The electrode active material which can be used can be used.

For example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium manganese spinel (LiMn ₂ O _4), and the general _{formula: LiNi x Co y Mn z M} a O ₂ (x + y + z + a = 1, 0 ≦ x <1, 0 ≦ y <1, 0 ≦ z <1, 0 ≦ a <1, M is one type selected from Al, Mg, Nb, Ti, Cu, Zn, Cr Complex metal oxides represented by the above elements), lithium vanadium compounds (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr) One or more elements or VO selected from the above, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), LiNi _x Co _y Al _z O ₂ (0.9 <x + y + z < 1.1) and the like, and polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and the like.

  Examples of the conductive assistant include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO. It is done. In the case where sufficient conductivity can be ensured with only the positive electrode active material, the lithium ion secondary battery 100 may not include a conductive additive.

  The positive electrode active material layer includes a binder. A well-known thing can be used for a binder. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoro Fluorine resins such as ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF).

"Negative electrode"
The negative electrode 30 includes a negative electrode current collector 32 and a negative electrode active material layer 34 located on at least one surface of the negative electrode current collector 32.

  The negative electrode current collector 32 may be a conductive plate material, and may be a copper foil, a stainless steel foil, or the like having low reactivity with lithium on the reduction potential side. In particular, a copper foil can be suitably used.

  The negative electrode active material layer 34 includes a negative electrode active material and a binder, and includes a conductive additive as necessary.

A known negative electrode active material can be used as the negative electrode active material. Examples of the negative electrode active material include carbon materials such as metallic lithium, graphite capable of occluding and releasing lithium ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, graphitizable carbon, and low-temperature calcined carbon. Metals that can be combined with lithium such as aluminum, silicon and tin, amorphous compounds mainly composed of oxides such as SiO _x (0 <x <2) and tin dioxide, lithium titanate (Li ₄ Ti ₅ And particles containing O ₁₂ ) and the like.

Among these, when silicon (Si) or silicon oxide (SiO _x ) is used as the negative electrode active material, a high-capacity lithium ion secondary battery can be realized. The capacity of the lithium ion secondary battery mainly depends on the active material of the electrode. The theoretical capacity of silicon (Si) or silicon oxide (SiO _x ) is much larger than that of graphite (372 mAh / g).

Si and SiO _x are accompanied by a large volume expansion during charging. When Si combines with Li, it expands in volume up to 4 times. In the lithium ion secondary battery 100 according to the present embodiment, the negative electrode active material layer 34 located in the outermost layer of the power generation element 40 suppresses the expansion of the negative electrode active material layer 34 located inside the power generation element 40. Therefore, even when Si or SiO _x is used as the negative electrode active material, peeling between the negative electrode active material layer 34 and the negative electrode current collector 32, cracking of the negative electrode active material layer 34, and the like are suppressed.

  Further, when the deposition and dissolution reaction of metallic lithium is used in the negative electrode 30 (when a metallic lithium negative electrode is used), the negative electrode active material layer 34 may not be in the initial state. This is because lithium ions in the electrolytic solution are deposited as metallic lithium on one surface of the negative electrode current collector 32. In preparation for the shortage of the amount of lithium contributing to charging / discharging, a lithium foil may be provided on one surface of the current collector from the initial state before charging / discharging.

  As the conductive auxiliary agent and the binder, the same materials as those for the positive electrode can be used. The binder used for the negative electrode 30 is, for example, carboxymethylcellulose (CMC), styrene / butadiene rubber (SBR), polyimide (PI), polyamideimide (PAI), polyacrylic acid (PAA), etc. May be.

FIG. 2 is an enlarged schematic diagram of the power generation element of the lithium ion secondary battery according to the present embodiment. Density D ₁ of the first anode active material layer 301 located on the surface of the positive electrode 20 not facing the side in the negative electrode 30A is located outermost of the power generating element 40, the inside of the power generating element 40 from the first negative electrode active material layer 301 It is higher than the density D _{2 of} the second negative electrode active material layer 302 positioned. The second negative electrode active material layer 302 may be any negative electrode active material layer of the second negative electrode 30B located inside the first negative electrode 30A.

  The density of the second negative electrode active material layer 302 means the density of any one of the negative electrode active material layers positioned inside the power generation element 40 from the first negative electrode active material layer 301. That is, it does not mean the average density of all the negative electrode active material layers positioned inside the power generation element 40 from the first negative electrode active material layer 301.

When the density D _{1 of} the first negative electrode active material layer 301 is high, the gap that can expand in the first negative electrode active material layer 301 is reduced. When the first negative electrode active material layer 301 having a small gap that can be expanded is located in the outermost layer of the power generation element 40, the expansion of the second negative electrode active material layer 302 positioned therein is limited.

  One of the causes of the deterioration of the cycle characteristics of the lithium ion secondary battery 100 is the depletion of the electrolytic solution accompanying the decomposition of the electrolytic solution. The electrolyte is depleted by a side reaction with the new surface generated in the negative electrode 30. The new surface is generated due to separation between the negative electrode active material layer 34 and the negative electrode current collector 32 and cracks in the negative electrode active material layer 34. Separation between the negative electrode active material layer 34 and the negative electrode current collector 32 and cracks in the negative electrode active material layer 34 are caused by expansion and contraction of the negative electrode active material layer 34. That is, the cycle characteristics of the lithium ion secondary battery 100 are improved by limiting the expansion and contraction of the second negative electrode active material layer 302 in which the first negative electrode active material layer 301 is located inside the power generation element 40.

And the density _{D 1} of the first anode active material layer 301 and the density _{D 2} of the second anode active material layer _302, 1 <It is preferable to satisfy the relation of _{D 1 / D 2 <1.7,} 1.1 ≦ D ₁ / D ₂ <1.6 is more preferable, and 1.1 ≦ D ₁ / D ₂ <1.4 is more preferable.

The densities of the first negative electrode active material layer 301 and the second negative electrode active material layer 302 are controlled by the pressure applied during rolling during production. It is difficult to produce the power generating element 40 in which D ₁ / D ₂ is larger than 1.7. When D ₁ / D ₂ is greater than 1.7, the difference between the internal resistance of the first negative electrode active material layer 301 and the internal resistance of the second negative electrode active material layer 302 becomes large, and the charge / discharge reaction becomes uneven. . The charge / discharge reaction tends to concentrate on the negative electrode which has a low internal resistance and is easy to react, and the cycle characteristics deteriorate.

The density D ₂ of the second negative electrode active material layer 302 is preferably 1.0 g / cm ³ or more, and preferably 1.2 g / cm ³ or more. The density D ₁ of the first negative electrode active material layer 301 is preferably 1.2 g / cm ³ or more, and preferably 1.4 g / cm ³ or more. If the first negative electrode active material layer 301 and the second negative electrode active material layer 302 have the above relationship, it is easy to adjust the density relationship between the respective negative electrode active material layers. The density _{D 2} of the second anode active material layer 302 as long as 1.0 g / cm ³ or more, nor a volume expansion of the negative electrode becomes too large.

The density D ₃ of the third negative electrode active material layer 303 positioned on the positive electrode 20 and the surface facing the negative electrode 30A is located outermost of the power generating element 40, and the density D ₁ of the first anode active material layer 301 and the second anode It is preferably between the density D ₂ of the active material layer 302. The density of the negative electrode active material layer 34 has a correlation with the expansion amount of the negative electrode active material layer 34. Since the density of the negative electrode active material layer 34 changes stepwise from the inside to the outside of the power generation element 40, it is possible to suppress the application of large stress locally as the negative electrode active material layer 34 expands and contracts. .

  The average density of the negative electrode active material layer 34 in the negative electrode 30 </ b> A located on the outermost side of the power generation element 40 is preferably higher than the average density of the negative electrode active material layer 34 in the negative electrode 30 </ b> B located on the inner side of the power generation element 40. Here, the average density of the negative electrode active material layer in the negative electrode means the average density of the negative electrode active material layers 34 located on both sides of the negative electrode current collector 32.

  The average density of the negative electrode active material layer and the average density of each negative electrode active material layer in the negative electrode are determined as follows. First, the negative electrode 30, the positive electrode 20, and the separator 10 are separated from the power generation element 40. Then, the negative electrode 30 is punched with a predetermined size (for example, 3 cm × 5 cm), and the weight and thickness of the punched portion are measured (Measurement 1). Next, the negative electrode active material layer located on one surface of the punched portion of the negative electrode is peeled off. When the negative electrode active material layer on one surface is peeled off, the negative electrode current collector and the negative electrode active material layer formed on one surface thereof remain. The weight and thickness of the remaining negative electrode current collector and active material layer are measured (Measurement 2). Finally, the negative electrode active material layer on the surface opposite to the punched portion is peeled off. Only the negative electrode current collector remains by peeling, and the weight and thickness of the negative electrode current collector are measured (Measurement 3). By calculating backward based on the results of Measurement 1 to Measurement 3, the average density of the negative electrode active material layer and the average density of each negative electrode active material layer in the negative electrode are obtained.

"Separator"
The separator 10 only needs to be formed of an electrically insulating porous structure, for example, a single layer of a film made of polyethylene, polypropylene, or polyolefin, a stretched film of a laminate or a mixture of the above resins, or cellulose, polyester, and Examples thereof include a fiber nonwoven fabric made of at least one constituent material selected from the group consisting of polypropylene.

(Electrolyte)
The electrolytic solution is impregnated in the power generation element 40. As the electrolytic solution, an electrolyte solution containing a lithium salt or the like (electrolyte aqueous solution, non-aqueous electrolyte solution using an organic solvent) can be used.

  In the non-aqueous electrolyte solution, an electrolyte is dissolved in a non-aqueous solvent, and a cyclic carbonate and a chain carbonate may be contained as a non-aqueous solvent.

  As cyclic carbonate, what can solvate electrolyte can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used.

  The chain carbonate can reduce the viscosity of the cyclic carbonate. Examples thereof include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like may be mixed and used.

  The ratio of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 by volume.

Examples of the electrolyte include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF _{2 SO 2) 2, LiN (} CF 3 SO 2) (C 4 F 9 SO 2), LiN (CF 3 CF 2 CO) 2, lithium salts such as LiBOB can be used. In addition, these lithium salts may be used individually by 1 type, and may use 2 or more types together. In particular, LiPF ₆ is preferably included from the viewpoint of the degree of ionization.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the electrolyte in the non-aqueous electrolyte solution is preferably adjusted to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the lithium ion concentration of the nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. Moreover, by suppressing the electrolyte concentration to within 2.0 mol / L, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It becomes easy.

Even when LiPF ₆ is mixed with another electrolyte, the lithium ion concentration in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

(Exterior body)
The exterior body 50 seals the power generating element 40 and the electrolytic solution therein. The outer package 50 prevents leakage of the electrolytic solution to the outside, penetration of moisture and the like from the outside into the battery, and the like.

  As shown in FIG. 1, the exterior body 50 includes a heat-sealing resin layer 52, a metal layer 54, and a heat-resistant resin layer 56 in order from the power generation element 40. As a material of the heat sealing resin layer 52, polyolefin such as polyethylene and polypropylene can be used. As a material of the metal layer 54, aluminum, stainless steel or the like can be used. As a material of the heat resistant resin layer 56, a polymer having a high melting point, such as polyethylene terephthalate (PET), polyamide (PA), or the like can be used.

(Terminal)
The terminal 60 is made of a conductive material such as aluminum or nickel. One of the terminals is a negative terminal 61 and the other is a positive terminal 62. One end (inner end) of the terminal 60 is connected to the power generation element 40, and the other end (outer end) extends to the outside of the exterior body 50. The two terminals 60 may extend in the same direction or in different directions. The negative electrode terminal 61 is connected to the negative electrode current collector 32, and the positive electrode terminal 62 is connected to the positive electrode current collector 22. The connection method is not particularly limited, and welding, screwing, or the like can be used.

[Method for manufacturing lithium ion secondary battery]
The lithium ion secondary battery 100 according to the present embodiment can be manufactured, for example, by the following method.

  First, the positive electrode 20 and the negative electrode 30 are produced. The positive electrode 20 and the negative electrode 30 differ only in the substance used as an active material, and can be produced by the same manufacturing method.

  A positive electrode active material, a binder, and a solvent are mixed to prepare a paint. You may add a conductive support agent further as needed. As the solvent, for example, water, N-methyl-2-pyrrolidone, N, N-dimethylformamide or the like can be used. The constituent ratio of the positive electrode active material, the conductive additive, and the binder is preferably 80 wt% to 90 wt%: 0.1 wt% to 10 wt%: 0.1 wt% to 10 wt% in terms of weight ratio. These weight ratios are adjusted to 100 wt% as a whole.

  The mixing method of these components constituting the paint is not particularly limited, and the mixing order is not particularly limited. The paint is applied to the positive electrode current collector 22. There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method. Similarly, a coating material is applied to the negative electrode current collector 32 for the negative electrode.

  Subsequently, the solvent in the paint applied on the positive electrode current collector 22 and the negative electrode current collector 32 is removed. The removal method is not particularly limited. For example, the positive electrode current collector 22 and the negative electrode current collector 32 to which the paint is applied are dried in an atmosphere of 80 ° C. to 150 ° C. Finally, rolling is performed as necessary to complete the positive electrode 20 and the negative electrode 30. The difference in the density of the negative electrode active material layer 34 can be changed by changing the pressure applied during the rolling process.

  When the power generating element 40 is a laminate, the positive electrode 20, the negative electrode 30, and the separator 10 are laminated. When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around each other. In any case, the separator 10 is disposed between the positive electrode 20 and the negative electrode 30.

  Finally, the produced power generation element 40 is sealed in the exterior body 50, and the electrolytic solution is injected into the exterior body 50, whereby the lithium ion secondary battery 100 according to the present embodiment is completed.

  As mentioned above, although this embodiment was explained in full detail with reference to drawings, each composition in each embodiment, those combinations, etc. are examples, and addition, abbreviation, and substitution of composition are within the range which does not deviate from the meaning of the present invention. , And other changes are possible.

"Example 1"
(Preparation of negative electrode)
A copper foil having a thickness of 10 μm was prepared as a negative electrode current collector.

  As the negative electrode active material, silicon powder having an average particle size of 2.5 μm was used. This average particle diameter is an average value of the particle diameters of a plurality of negative electrode active materials, and is the diameter (D50) of a particle having an integrated value of 50% in a distribution curve obtained by particle size distribution measurement. The particle size distribution of the particles was measured by a particle size distribution measuring apparatus using a laser diffraction / scattering method (microtrack method).

  The negative electrode active material described above, acetylene black prepared as a conductive auxiliary agent, and polyamideimide (PAI) prepared as a binder were mixed to obtain a negative electrode mixture. The weight ratio of the negative electrode active material, the conductive additive, and the binder was 80: 5: 15. This negative electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a negative electrode mixture paint. And the negative mix paint was apply | coated by the predetermined application quantity on one surface of 10-micrometer-thick copper foil. The coating amount of the negative electrode mixture paint was adjusted so that the charge capacity ratio was 1.05 to 1.2 times the charge capacity of the positive electrode. And after application | coating, it was made to dry at 100 degreeC, the solvent was removed, and the negative electrode active material layer was formed. Thereafter, the negative electrode active material layer was pressure-formed by a roll press. The density of the negative electrode active material layer in each part was controlled by changing the applied pressure between the negative electrode located inside the power generation element and the negative electrode located in the outermost layer of the power generation element. Finally, in order to bind polyamideimide as a binder more firmly, heat treatment was performed at 350 ° C. for 3 hours under vacuum, and this was used as the negative electrode according to Example 1.

(Preparation of positive electrode)
LiCoO ₂ prepared as a positive electrode active material, acetylene black prepared as a conductive auxiliary agent, and polyvinylidene fluoride (PVDF) prepared as a binder were mixed to obtain a positive electrode mixture. The weight ratio of the positive electrode active material, the conductive additive, and the binder was 90: 5: 5. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode mixture paint. Then, a positive electrode mixture was applied to one surface of an aluminum foil having a thickness of 20 μm with a predetermined application amount. And after application | coating, it was made to dry at 100 degreeC, the solvent was removed, and the positive electrode active material layer was formed. Thereafter, the positive electrode active material layer was pressure-formed by a roll press to produce a positive electrode according to Example 1.

(Production of lithium ion secondary battery for evaluation full cell)
The produced negative electrode and positive electrode were alternately laminated via a polypropylene separator having a thickness of 16 μm, and a laminate was produced by laminating four negative electrodes and three positive electrodes. The density of the negative electrode active material layer (first negative electrode active material layer) located in the outermost layer is 1.1 g / cm ³ , and the negative electrode active material layer (second negative electrode active material) inside the second negative electrode from the outermost layer The density of the layer) was 1.0 g / cm ³ .

  In the negative electrode of the laminate, a negative electrode terminal made of nickel was attached to the protruding end portion of the copper foil not provided with the negative electrode active material layer. Moreover, in the positive electrode of the laminated body, an aluminum positive electrode terminal was attached to the protruding end portion of the aluminum foil not provided with the positive electrode active material layer by an ultrasonic welding machine.

And this closed body was inserted in the exterior body of the laminate film, and heat-sealed except for the surrounding 1 place, and the closed part was formed. The outer package is injected with a solvent in which FEC and DEC are mixed at a volume ratio of 3: 7, and a nonaqueous electrolytic solution to which 1.5M (mol / L) LiPF ₆ is added as a lithium salt. did. And the remaining 1 place was sealed by heat sealing, reducing pressure with a vacuum sealing machine, and the lithium ion secondary battery (full cell) was produced.

"Examples 2 to 5 and Comparative Examples 1 and 2"
The difference from the lithium ion secondary battery according to Example 1 is that the density of the second negative electrode active material layer is fixed to 1.0 g / cm ³ and the density of the first negative electrode active material layer is changed. Other conditions were the same as in Example 1.

“Examples 6 to 10 and Comparative Examples 3 and 4”
The difference from the lithium ion secondary battery according to Example 1 is that the density of the second negative electrode active material layer is changed and fixed to 1.2 g / cm ³ and the density of the first negative electrode active material layer is changed. Other conditions were the same as in Example 1.

"Examples 11 to 13 and Comparative Examples 5 and 6"
The difference from the lithium ion secondary battery according to Example 1 is that the density of the second negative electrode active material layer is changed to 1.4 g / cm ³ and fixed, and the density of the first negative electrode active material layer is changed. Other conditions were the same as in Example 1.

“Examples 14 and 15 and Comparative Examples 7 and 8”
The difference from the lithium ion secondary battery according to Example 1 is that the density of the second negative electrode active material layer is changed to 1.6 g / cm ³ and fixed, and the density of the first negative electrode active material layer is changed. Other conditions were the same as in Example 1.

"Examples 16 to 20 and Comparative Example 9"
It differs from the lithium ion secondary battery according to Example 1 in that the density of the second negative electrode active material layer was changed to 0.95 g / cm ³ and fixed, and the density of the first negative electrode active material layer was changed. Other conditions were the same as in Example 1.

(Capacity maintenance rate measurement test)
About the lithium ion secondary battery produced by the Example and the comparative example, the cycle characteristic was measured in 25 degreeC environment using the secondary battery charging / discharging test apparatus (made by Hokuto Denko Co., Ltd.). 100 cycles of charge / discharge cycles of constant current and constant voltage charge to 4.3V at 0.5C and constant current discharge to 3.0V at 1C, capacity retention rate after 100 cycles is measured, cycle characteristics are represented by cycle retention rate It was evaluated as (unit:%). The evaluation value was an average value of 5 samples. The results are shown in Table 1.

(Cell expansion rate)
About the lithium ion secondary battery produced by the Example and the comparative example, the expansion coefficient of the cell was measured. The cell expansion coefficient is obtained by (“thickness of lithium ion secondary battery after one cycle” − “thickness of lithium ion secondary battery before operation”) / (“thickness of lithium ion secondary battery before operation”). It is done. As for the thickness of the lithium ion secondary battery, the thickness of the lithium ion secondary battery in the stacking direction of the power generation elements is measured from the outside of the outer package.

DESCRIPTION OF SYMBOLS 10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generation element 50 Exterior body 52 Thermal fusion resin layer 54 Metal layer 56 Heat resistant resin layer 60 Terminal 61 Negative electrode terminal 62 Positive terminal 100 Lithium ion secondary battery

Claims (3)

A positive electrode having a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector;
A negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector, and a negative electrode facing the positive electrode;
A power generation element comprising a separator sandwiched between the positive electrode and the negative electrode,
The density of the first negative electrode active material layer located on the surface of the negative electrode located on the outermost side of the power generation element that is not opposed to the positive electrode is a second density located on the inner side of the power generation element than the first negative electrode active material layer. A lithium ion secondary battery having a density higher than that of the negative electrode active material layer. Wherein the first and the density _{D 1} of the negative electrode active material layer and the density _{D 2} of the second anode active material layer _is, 1 <satisfy the relationship _{D 1} / D 2 <1.7, the lithium ion according to claim 1 Secondary battery. ^3. The lithium ion secondary battery according to claim 1, wherein a density D ₂ of the second negative electrode active material layer is 1.0 g / cm ³ or more.

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