JP2019175653A - Lithium ion secondary battery - Google Patents

Abstract

To provide a lithium ion secondary battery suppressed in metal lithium deposition in a curved surface portion during repeated charge/discharge.SOLUTION: A lithium ion secondary battery includes a wound body including a positive electrode and a negative electrode wound with a separator interposed therebetween and an exterior body for receiving the wound body. The wound body includes a plurality of planar portions and a curved surface portion connecting the planar portions, and, when Rrepresents the charge transfer resistance of the negative electrode in the curved surface portion and Rrepresents the charge transfer resistance in the planar portions, a relationship of R/R≤0.8 is satisfied.SELECTED DRAWING: Figure 1

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 there is an increasing demand for secondary batteries having a small size, light weight, and high energy density as power sources for driving these devices. Under such circumstances, a lithium ion secondary battery having a large charge / discharge capacity and a high energy density has attracted attention.

  The lithium ion secondary battery generally includes an electrode body and a non-aqueous electrolyte, and can be roughly classified into two types, a stacked battery and a wound battery, depending on the shape of the electrode body. Among these, the electrode body of the wound battery is manufactured by winding a long sheet-shaped electrode and a long sheet-shaped separator into a flat shape as a whole. Can be manufactured easily, and has the advantage of excellent productivity.

  However, it is also known that due to the structure of the wound battery, the electrode body has a flat surface portion and a curved surface portion, and a specific problem due to the curved surface portion occurs. For example, regarding the problem of causing a short-circuit in the manufacturing process due to the non-uniformity of stress in the curved surface portion, in Patent Document 1, by providing a surplus separator region in the outermost peripheral portion of the wound body, stress relaxation in the curved surface portion and charge A method for improving the balance of the carrier and solving the above problems is disclosed.

JP 2017-10878

  However, various properties are not yet satisfied by the method of the prior art, and suppression of metallic lithium precipitation at the time of repeated charging / discharging is particularly demanded on the curved surface portion.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide a lithium ion secondary battery in which the deposition of metallic lithium during repeated charging / discharging in the curved surface portion is suppressed.

In order to solve the above problems, a lithium ion secondary battery according to the present invention includes a wound body in which a positive electrode and a negative electrode are wound through a separator, and an exterior body that houses the wound body, The winding body includes a plurality of flat surface portions and a curved surface portion connecting the flat surface portions. The negative electrode charge transfer resistance in the curved surface portion is R ₁ , and the negative charge transfer resistance in the flat surface portion is R _2. In this case, the relationship of R ₁ / R ₂ ≦ 0.8 is satisfied.

According to this, even in a curved surface portion where the density and SEI film formation are likely to be in a non-uniform state, the negative electrode satisfying the relationship of R ₁ / R ₂ ≦ 0.8 is used, so that solvated lithium ions are removed from the curved surface portion. Since solvation is performed quickly and lithium ion insertion proceeds uniformly on the entire negative electrode surface, metal lithium deposition during repeated charging and discharging in the curved surface portion is suppressed.

Lithium-ion secondary battery according to the present invention further charge transfer resistance R ₁ of the negative electrode in the curved portion is preferably 20mΩ or less 1Ah cell conversion.

According to this, it is suitable as a value of R ₁ , and metal lithium deposition during repeated charging / discharging in the curved surface portion is further suppressed.

In the lithium ion secondary battery according to the present invention, it is further preferable that the density of the negative electrode mixture layer in the curved portion is 1.30 g / cm ³ or less.

According to this, even if the density of the negative electrode mixture layer in the curved surface portion is a low density of 1.30 g / cm ³ or less, the lithium ion secondary battery according to the present invention can be repeatedly charged and discharged in the curved surface portion. The metal lithium deposition is further suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery by which metal lithium precipitation at the time of repeated charging / discharging in a curved surface part was suppressed is provided.

It is a schematic cross section of the lithium ion secondary battery according to the present embodiment. It is the figure which expand | deployed the winding body in the lithium ion secondary battery which concerns on this embodiment. It is the schematic cross section which expanded the edge part of the winding body in the lithium ion secondary battery which concerns on 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 according to this embodiment. As shown in FIG. 1, a lithium ion secondary battery 100 according to this embodiment includes a power generation element 1 and an exterior body 2. The power generating element 1 includes a positive electrode 10, a negative electrode 20, and a separator 30. The power generating element 1 shown in FIG. 1 is a wound body in which a positive electrode 10 and a negative electrode 20 are disposed so as to face each other with a separator 30 in between. Each of the positive electrode 10 and the negative electrode 20 is provided with terminals 15 and 25 for electrical connection with the outside.

  FIG. 2 is a developed view of the wound body in the lithium ion secondary battery according to the present embodiment. The positive electrode 10 is a plate-shaped positive electrode current collector 12 provided with a positive electrode active material layer 14. The negative electrode 20 is a plate-shaped negative electrode current collector 22 provided with a negative electrode active material layer 24. An insulating tape 40 is attached to a part of the positive electrode 10 and the negative electrode 20. The insulating tape 40 prevents the terminals 15 and 25 from being short-circuited and suppresses the active material layer from being separated from the current collector.

FIG. 3 is an enlarged schematic cross-sectional view of an end portion of the wound body in the lithium ion secondary battery according to the present embodiment. The wound body end has a plane portion R _P, a curved surface portion R _C that connects the flat portion R _P.

<Positive electrode>
The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 provided on both surfaces of the positive electrode current collector 12.

(Positive electrode current collector)
The positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a positive electrode binder, and a positive electrode conductive additive.

(Positive electrode active material)
As the positive electrode active material, lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of a counter anion (for example, PF ₆ ⁻ ) of the lithium ion are reversibly performed. If it can be made to advance, it will not specifically limit, A well-known positive electrode active material can be used. As the positive electrode active material, for example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium nickel manganese _{oxide, Li (Ni 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, and M is selected from Al, Mg, Nb, Ti, Cu, Zn, Cr Li _a M _b (PO ₄ ) _c (1 ≦ a ≦ 4, 1 ≦ b ≦ 2, 1 ≦ c ≦ 3, and M is Fe, V, Co. , At least one selected from Mn, Ni, and VO), and the like.

(Binder for positive electrode)
The positive electrode binder bonds the positive electrode active materials to each other and bonds the positive electrode active material layer 14 and the positive electrode current collector 12. The binder is not particularly limited as long as the above-described bonding is possible. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) ), Etc., vinylidene fluoride-hexafluoropropylene (VDF-HFP), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene (VDF-HFP-TFE), vinylidene fluoride Vinylidene fluoride fluoropolymers such as id-chlorotrifluoroethylene (VDF-CTFE), styrene / butadiene rubber (SBR), ethylene / propylene rubber (EPR), polyamide (PA), polyimide (PI), polyamideimide (PAI) A non-fluorine resin such as) may be used.

Further, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene, polythiophene, and polyaniline. Examples of the ion conductive conductive polymer include those obtained by combining a polyether polymer compound such as polyethylene oxide and polypropylene oxide and a lithium salt such as LiClO ₄ , LiBF ₄ , and LiPF _6. It is done.

(Conductive aid for positive electrode)
The conductive auxiliary agent for positive electrode is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metals such as copper, nickel, stainless steel, and iron, and conductive oxides such as indium tin oxide (ITO). Moreover, when sufficient electroconductivity is securable only with a positive electrode active material, the positive electrode active material layer 14 does not need to contain the conductive support agent.

<Negative electrode>
The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 provided on both surfaces of the negative electrode current collector 22.

(Negative electrode current collector)
The negative electrode current collector 22 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as copper can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a negative electrode binder, and a negative electrode conductive additive.

(Negative electrode active material)
As the negative electrode active material, lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of a counter anion (for example, PF ₆ ⁻ ) of the lithium ion are reversibly performed. If it can be made to advance, it will not specifically limit, A well-known negative electrode active material can be used. Examples of the negative electrode active material include carbon materials such as graphite, hard carbon, and soft carbon, metals that can form an alloy with lithium such as aluminum, silicon, and tin, and amorphous materials such as silicon oxide and tin oxide. oxide, lithium titanate _{(Li 4 Ti 5 O 12)} , and the like.

(Binder for negative electrode)
There is no limitation in particular as a binder for negative electrodes, The thing similar to the binder for positive electrodes described above can be used.

(Conductive aid for negative electrode)
There is no limitation in particular as a conductive support agent for negative electrodes, The thing similar to the conductive support agent for positive electrodes described above can be used.

  Hereinafter, the negative electrode according to the present embodiment will be described in more detail.

The negative electrode according to the present embodiment, when the charge transfer resistance of the negative electrode in the curved portion _{R C} and _{R 1,} the negative electrode charge transfer resistance of the flat portion _{R P} and _{R 2,} is _R 1 / R ₂ ≦ 0.8 .

According to this, even in a curved surface portion where the density and SEI film formation are likely to be in a non-uniform state, the negative electrode satisfying the relationship of R ₁ / R ₂ ≦ 0.8 is used, so that solvated lithium ions are removed from the curved surface portion. Since solvation is performed quickly and lithium ion insertion proceeds uniformly on the entire negative electrode surface, metal lithium deposition during repeated charging and discharging in the curved surface portion is suppressed.

  It is known that the charge transfer resistance is strongly influenced by the surface properties of the active material. In order to control the charge transfer resistance, simply use a mixture of different active materials, or use a mechanical method using a pulverizer or mixer, or a chemical method using a sol-gel method, It is effective to apply a surface coat to the active material. Alternatively, it is also effective to add an appropriate additive at the time of preparing the paint, and to change the drying temperature and atmosphere after application of the active material layer. In addition, it is also effective to change the temperature and the charging speed at the time of the first charge after manufacturing the lithium ion battery.

  In addition, as a method of changing the charge transfer resistance between the curved surface portion and the flat surface portion, there is a method of intermittently applying and drying an active material layer that hits the flat surface portion and then intermittently applying an active material layer that hits the curved surface portion again. After continuously applying the active material layer, a region corresponding to the curved surface portion may be removed using an adhesive tape or the like. It is also effective to apply an appropriate surface coating agent only to either the curved surface portion or the flat surface region after the active material layer is applied. In the embodiments described later, the charge transfer resistance is set to a target value by combining these methods. However, the charge transfer resistance may be controlled by using a method other than the method described here, and such a method may be used. Within the scope of the invention.

  As a method for measuring the charge transfer resistance, a method using a symmetrical cell in which electrodes of the same polarity are opposed to each other is known. For example, the battery to be measured is disassembled in an inert atmosphere such as a glove box, and the electrodes are taken out. With these electrodes separated by a predetermined distance, the active material layers are arranged so that the active material layers face each other. A filled symmetrical cell is made. The charge transfer resistance can be obtained by measuring the symmetric cell using an impedance analyzer and fitting the obtained Cole-Cole plot.

In the negative electrode according to the present embodiment, it is further preferable that the charge transfer resistance R ₁ of the negative electrode in the curved surface portion is 20 mΩ or less in terms of 1 Ah cell.

According to this, it is suitable as a value of R ₁ , and metal lithium deposition during repeated charging / discharging in the curved surface portion is further suppressed.

In the negative electrode according to the present embodiment, it is preferable that the density of the negative electrode mixture layer in the curved surface portion is 1.30 g / cm ³ or less.

According to this, even if the density of the negative electrode mixture layer in the curved surface portion is a low density of 1.30 g / cm ³ or less, the lithium ion secondary battery according to the present invention can be repeatedly charged and discharged in the curved surface portion. The metal lithium deposition is further suppressed.

<Separator>
The separator 30 has an electrically insulating porous structure. Examples of the separator include monolayers or laminates of polyolefins such as polyethylene and polypropylene, and fiber nonwoven fabrics made of cellulose, polyester, polyacrylonitrile, and the like.

  Moreover, the surface of the separator may be coated with an inorganic compound or an organic compound. An insulating metal oxide such as alumina or boehmite can be used as the inorganic compound, and polyvinylidene fluoride (PVDF) or polyethylene oxide (PEO) can be used as the organic compound.

<Electrolyte>
The electrolytic solution according to the present invention is mainly composed of a solvent and an electrolyte.

(solvent)
As said solvent, the solvent generally used for the lithium ion secondary battery can be mixed and used in arbitrary ratios. For example, cyclic carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate, chain carbonate compounds such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), γ-butyrolactone Examples include cyclic ester compounds such as (GBL), and chain ester compounds such as propyl propionate (PrP), ethyl propionate (PrE), and ethyl acetate.

(Electrolytes)
The electrolyte is not particularly limited as long as it is a lithium salt used as an electrolyte of a lithium ion secondary battery. For example, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , lithium bisoxalate borate, LiCF ₃ SO ₃ , An organic acid anion salt such as (CF ₃ SO ₂ ) ₂ NLi, (FSO ₂ ) ₂ NLi, or the like can be used.

  As mentioned above, although preferred embodiment which concerns on this invention was described, this invention is not limited to the said embodiment.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

[Example 1]
(Preparation of positive electrode)
85 parts by mass of Li (Ni _0.80 Co _0.15 Al _0.05 ) O ₂ , 5 parts by mass of acetylene black and 10 parts by mass of PVDF are dispersed in N-methyl-2-pyrrolidone (NMP) to form a positive electrode active material layer The slurry for was prepared. This slurry was applied to one surface of an aluminum metal foil having a thickness of 20 μm so that the applied amount of the positive electrode active material was 9.0 mg / cm ² and dried at 100 ° C. to form a positive electrode active material layer. Then, it pressure-molded with the roller press and produced the positive electrode.

(Preparation of negative electrode)
A slurry for forming a negative electrode active material layer was prepared by dispersing 90 parts by mass of flaky artificial graphite, 5 parts by mass of acetylene black, and 5 parts by mass of PVDF in N-methyl-2-pyrrolidone (NMP). The above slurry is intermittently applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied is 6.0 mg / cm ² , and dried at 100 ° C. A negative electrode active material layer corresponding to the above was formed.

Subsequently, 0.2 wt% of 1,3-propane sultone as an additive was added to the slurry for forming the negative electrode active material layer and dispersed well again. The slurry to which the additive was added was cast on the intermittent part of the negative electrode so that the coating amount of the negative electrode active material was 6.0 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. . Thereafter, the negative electrode mixture layer was pressure-molded by a roller press so that the density of the negative electrode mixture layer was 1.55 g / cm ³ , thereby producing a negative electrode.

(Preparation of wound body)
These were wound around the positive electrode, negative electrode, and polyethylene separator prepared as described above as an axis to prepare a wound body.

(Preparation of electrolyte)
EC volume ratio: DEC = 30: were mixed in a 70, to which LiPF ₆ was dissolved at a concentration of 1.0 mol / L, to prepare an electrolyte solution.

(Production of evaluation lithium-ion secondary battery)
The wound body produced above is housed in an aluminum laminate film exterior body in which a housing space is formed, the electrolytic solution produced above is injected, and vacuum sealing is performed to obtain 2 lithium ion secondary batteries for evaluation. Individually produced.

(Battery)
The lithium ion secondary battery for evaluation produced above was 4.2 V by constant current charging at a charge rate of 0.2 C in a constant temperature bath at 25 ° C. using a charge / discharge test apparatus (manufactured by Hokuto Denko Co., Ltd.). Then, the battery was charged with a constant current discharge at a discharge rate of 0.2 C until the battery voltage became 2.8V. Here, X (C) indicates a current value at which charging is completed in 1 / X time when constant current charging is performed at 25 ° C.

(Measurement of charge transfer resistance)
One of the evaluation lithium ion secondary batteries made into a battery was disassembled in an inert atmosphere in a glove box, and the negative electrode was taken out. About the said negative electrode, after cut | disconnecting the area | region which hits a curved-surface part by 2 mm in width and 50 mm in height, it has arrange | positioned so that it may oppose in the state separated only 5 mm from the prismatic glass cell. Into this glass cell, an electrolytic solution adjusted with EC: DEC = 30: 70 (volume%), 1.0 mol / L LiPF ₆ was poured so that the cut-out negative electrode was immersed in the liquid by a height of 40 mm. A symmetric cell was produced.

The negative electrode not immersed in the liquid of the symmetric cell was connected to the measurement terminal, and the impedance of the negative electrode at 25 ° C. was measured using an impedance analyzer (Bio-Logic). The obtained Cole-Cole plot is an equivalent circuit model in which one resistor (R) and two resistors (R) / capacitors (C) in parallel are connected in series in the range of 10 kHz to 1 Hz. fitting to obtain the charge transfer resistance R ₁ of the negative electrode in the curved portion. Here, the charge transfer resistance is defined as R having the slower time constant τ of the two RC parallel circuits.

For even flat portion of the negative electrode to measure the charge transfer resistance in the same manner as described above to determine the charge transfer resistance R ₂ of the negative electrode in the plane portion.

(Confirmation of lithium deposition after cycle)
A lithium ion secondary battery for evaluation different from the one in which the charge transfer resistance is measured is charged until the battery voltage becomes 4.2 V by constant current charging at a charging rate of 2.0 C, and then discharged. The battery was discharged at a constant current discharge rate of 1.0 C until the battery voltage reached 2.8V. The charge / discharge pattern was defined as one cycle, and 100 cycles of charge / discharge were performed. Regarding the battery after 100 cycles, the battery was disassembled in an inert atmosphere in the glove box and the negative electrode curved surface portion was confirmed. Slight lithium deposition was confirmed. (About 1% of the total area of the negative electrode curved surface)

[Example 2]
(Preparation of negative electrode)
A slurry for forming a negative electrode active material layer was prepared by dispersing 90 parts by mass of flaky artificial graphite, 5 parts by mass of acetylene black, and 5 parts by mass of PVDF in N-methyl-2-pyrrolidone (NMP). The above slurry is intermittently applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied is 6.0 mg / cm ² , and dried at 100 ° C. A negative electrode active material layer corresponding to the above was formed.

Subsequently, 1.0 wt% of 1,3-propane sultone as an additive was added to the slurry for forming the negative electrode active material layer and dispersed well again. The slurry to which the additive was added was cast on the intermittent part of the negative electrode so that the coating amount of the negative electrode active material was 6.0 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. . Then, it pressure-molded so that the density of the negative mix layer might be set to 1.55 g / cm < ³ > with a roller press, and the negative electrode of Example 2 was produced.

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Example 2 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.
[Example 3]
(Preparation of negative electrode)
A slurry for forming a negative electrode active material layer was prepared by dispersing 90 parts by mass of flaky artificial graphite, 5 parts by mass of acetylene black, and 5 parts by mass of PVDF in N-methyl-2-pyrrolidone (NMP). The above slurry is intermittently applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied is 6.0 mg / cm ² , and dried at 100 ° C. A negative electrode active material layer corresponding to the above was formed.

Subsequently, 1.0 wt% of fluoroethylene carbonate was added as an additive to the slurry for forming the negative electrode active material layer and dispersed well again. The slurry to which the additive was added was cast on the intermittent part of the negative electrode so that the coating amount of the negative electrode active material was 6.0 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. . Then, it pressure-molded so that the density of the negative mix layer might be set to 1.55 g / cm < ³ > with a roller press, and the negative electrode of Example 3 was produced.

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Example 3 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.

[Example 4]
(Preparation of negative electrode)
A slurry for forming a negative electrode active material layer was prepared by dispersing 90 parts by mass of flaky artificial graphite, 5 parts by mass of acetylene black, and 5 parts by mass of PVDF in N-methyl-2-pyrrolidone (NMP). The above slurry is intermittently applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied is 6.0 mg / cm ² , and dried at 100 ° C. A negative electrode active material layer corresponding to the above was formed.

Subsequently, 2.0 wt% of vinylene carbonate as an additive was added to the slurry for forming the negative electrode active material layer and well dispersed again. The slurry to which the additive was added was cast on the intermittent part of the negative electrode so that the coating amount of the negative electrode active material was 6.0 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. . Then, it pressure-molded so that the density of the negative mix layer might be set to 1.55 g / cm < ³ > with a roller press, and the negative electrode of Example 4 was produced.

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Example 4 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.

[Example 5]
(Preparation of negative electrode)
After 90 parts by weight of flaky artificial graphite, 5 parts by weight of acetylene black, and 5 parts by weight of PVDF were dispersed in N-methyl-2-pyrrolidone (NMP), 1 wt% of adiponitrile was added as an additive and dispersed again, and the negative electrode active material A slurry for layer formation was prepared. The above slurry is intermittently applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied is 6.0 mg / cm ² , and dried at 100 ° C. A negative electrode active material layer corresponding to the above was formed.

Subsequently, after 90 parts by weight of scaly artificial graphite, 5 parts by weight of acetylene black, and 5 parts by weight of PVDF were dispersed in N-methyl-2-pyrrolidone (NMP), 2.0 wt% of vinylene carbonate was added as an additive and again. Disperse and prepare another slurry. The slurry was cast on the intermittent portion of the negative electrode so that the amount of the negative electrode active material applied was 6.0 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. Then, it pressure-molded so that the density of the negative mix layer might be set to 1.55 g / cm < ³ > with a roller press, and the negative electrode of Example 5 was produced.

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Example 5 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.

[Example 6]
(Preparation of negative electrode)
A negative electrode of Example 6 was produced in the same manner as in Example 3, except that the negative electrode mixture layer was pressure-molded by a roller press so that the density of the negative electrode mixture layer was 1.35 g / cm ³ .

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Example 6 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.
[Example 7]
(Preparation of negative electrode)
A negative electrode of Example 7 was produced in the same manner as in Example 3 except that the negative electrode mixture layer was pressure-molded by a roller press so that the density of the negative electrode mixture layer was 1.28 g / cm ³ .

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Example 7 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.

[Comparative Example 1]
(Preparation of negative electrode)
After 90 parts by weight of flaky artificial graphite, 5 parts by weight of acetylene black, and 5 parts by weight of PVDF were dispersed in N-methyl-2-pyrrolidone (NMP), 1 wt% of adiponitrile was added as an additive and dispersed again, and the negative electrode active material A slurry for layer formation was prepared. The slurry was applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied was 6.0 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. Then, it pressure-molded so that the density of the negative mix layer might be set to 1.55 g / cm < ³ > with a roller press, and the negative electrode of the comparative example 1 was produced.

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Comparative Example 1 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.

[Comparative Example 2]
(Preparation of negative electrode)
A negative electrode of Comparative Example 2 was produced in the same manner as Comparative Example 1 except that the negative electrode mixture layer was pressure-molded by a roller press so that the density of the negative electrode mixture layer was 1.35 g / cm ³ .

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Comparative Example 2 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.

[Comparative Example 3]
(Preparation of negative electrode)
A negative electrode of Comparative Example 3 was produced in the same manner as Comparative Example 1 except that the negative electrode mixture layer was pressure-molded by a roller press so that the density of the negative electrode mixture layer was 1.28 g / cm ³ .

(Production of evaluation lithium-ion secondary battery)
A lithium secondary battery for evaluation of Comparative Example 3 was produced in the same manner as in Example 1 except that the negative electrode produced above was used.

(Measurement of charge transfer resistance)
For the evaluation lithium ion secondary batteries produced in Examples 2 to 7 and Comparative Examples 1 to 3, the charge transfer resistance was measured in the same manner as in Example 1. The results are shown in Table 1.

(Confirmation of lithium deposition after cycle)
About the lithium ion secondary battery for evaluation produced in Examples 2-7 and Comparative Examples 1-3, lithium precipitation after cycling progress was confirmed by the method similar to Example 1. FIG. The results are shown in Table 1. In the table, “◎” indicates no lithium deposition, “◯” indicates a slight lithium deposition (1% or less of the total area of the negative electrode curved surface portion), and “Δ” indicates lithium. The state where precipitation was confirmed (5% or less with respect to the total area of the negative electrode curved surface portion), and “x” is a state where a large amount of lithium deposition was confirmed (10% or less with respect to the total area of the negative electrode curved surface portion). Indicates.

  In each of Examples 1 to 7, it was confirmed that lithium deposition in the negative electrode curved surface portion after the cycle was improved as compared with Comparative Examples 1 to 3 in which the charge transfer resistance ratio between the curved surface portion and the flat surface portion was not optimized. .

From the results of Examples 6 and 7, it was confirmed that when the negative electrode mixture layer density was 1.30 g / cm ³ or less, lithium deposition on the negative electrode curved surface portion after the cycle was further improved.

  According to the present invention, there is provided a lithium ion secondary battery in which the deposition of metallic lithium during repeated charging / discharging in the curved surface portion is suppressed.

DESCRIPTION OF SYMBOLS 1 ... Electric power generation element, 2 ... Exterior body, 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 15 ... Terminal, 20 ... Negative electrode, 22 ... Negative electrode collector, 24 ... Negative electrode active material layer, 25 ... Terminal, 30 ... Separator, 40 ... Insulating tape, 100 ... Lithium ion secondary battery, _Rp ... Flat part, _RC ... Curved part

Claims (3)

A wound body in which a positive electrode and a negative electrode are wound through a separator;
An exterior body that houses the wound body,
The wound body includes a plurality of flat surface portions and a curved surface portion connecting the flat surface portions,
When the charge transfer resistance of the negative electrode in the curved surface portion is R ₁ , and the charge transfer resistance of the negative electrode in the flat surface portion is R ₂ , the relationship of R ₁ / R ₂ ≦ 0.8 is satisfied. Next battery. 2. The lithium ion secondary battery according to claim 1, wherein the charge transfer resistance R ₁ of the negative electrode in the curved surface portion is 20 mΩ or less in terms of 1 Ah cell. ^3. The lithium ion secondary battery according to claim 1, wherein a density of the negative electrode mixture layer in the curved surface portion is 1.30 g / cm ³ or less.

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