JP2019164964A - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery having excellent initial charge / discharge efficiency and charge / discharge cycle characteristics are provided. A negative electrode for a lithium ion secondary battery is a negative electrode having a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material. A thiol compound, wherein at least a part of the hydrogen of the thiol group is replaced by lithium. The lithium ion secondary battery has the negative electrode of the above embodiment, a positive electrode, and a non-aqueous electrolyte. [Selection diagram] Fig. 1

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery.

  In recent years, lithium ion secondary batteries have been widely used as power sources for portable electronic devices, automobiles, and power storage. The lithium ion secondary battery is mainly composed of a positive electrode, a negative electrode, a separator that insulates the positive electrode from the negative electrode, and a non-aqueous electrolyte that enables ions to move between the positive electrode and the negative electrode. Since lithium ion secondary batteries have high energy density, they have been put into practical use as power sources for electronic portable devices such as mobile phones and laptop computers, and are widely spread. In recent years, with the remarkable development of portable electronic devices, communication devices, and the like, there is a strong demand for lithium ion secondary batteries with higher energy density from the viewpoints of downsizing and weight reduction of devices. Further, there is a strong demand for extending the life of automobile batteries.

By the way, negative electrode active materials such as semimetals or metal oxides such as silicon (Si) and silicon oxide (SiO _x ) have a theoretical capacity much higher than the theoretical capacity of 372 mAh / g of graphite currently in practical use. Therefore, it is the most promising material for increasing the energy density of batteries. However, for example, in the case of a negative electrode using a negative electrode active material containing silicon, it is known that the behavior of expansion and contraction of the negative electrode accompanying the insertion and desorption of lithium ions due to charge / discharge is larger than that of graphite. Therefore, in the lithium ion secondary battery using the negative electrode, excellent charge / discharge cycle characteristics can be obtained by pulverizing the negative electrode active material particles by peeling and dropping from the current collector by repeated charging and discharging. There was a problem that it was difficult. In response to such a problem, Patent Document 1 discloses using silicon-containing particles in which the surface of silicon particles is coated with a thermosetting resin as a negative electrode active material. In Patent Document 1, it is described that polyimide and polyamideimide are preferable as the thermosetting resin.

JP 2014-192064 A

  In a lithium ion secondary battery, charge / discharge cycle characteristics may be deteriorated by irreversibly trapping lithium ions in the positive electrode active material layer or the negative electrode active material layer during charge / discharge. In particular, in the polyimide and polyamideimide described in Patent Document 1, the imide group easily captures lithium ions, and in the lithium ion secondary battery containing these, the initial charge / discharge efficiency is greatly reduced, and the subsequent charge / discharge cycle The deterioration of the characteristics sometimes became large.

  The objective of this invention is made | formed in view of the said situation, and is providing the negative electrode for lithium ion secondary batteries and the lithium ion secondary battery which were excellent in first time charge / discharge efficiency and charging / discharging cycling characteristics.

As a result of intensive studies, the inventor has shown that the initial charge / discharge efficiency and the charge / discharge cycle are improved by adding a thiol compound in which the hydrogen of the thiol group is replaced by lithium to the negative electrode active material layer of the lithium ion secondary battery. As a result, the present invention has been completed.
That is, in order to solve the above problems, the following means are provided.

(1) A negative electrode for a lithium ion secondary battery according to a first aspect is a negative electrode having a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, wherein the negative electrode active material The layer includes a negative electrode active material and a thiol compound, and in the thiol compound, hydrogen of at least a part of the thiol group is replaced by lithium.

(2) In the negative electrode for a lithium ion secondary battery according to the above aspect, the thiol compound may be attached to the surface of the negative electrode active material layer.

(3) In the negative electrode for a lithium ion secondary battery according to the above aspect, the ratio of thiol groups in which hydrogen is replaced by lithium with respect to the total amount of thiol groups of the thiol compound is 10 mol% or more and 100 mol% or less. It may be within the range.

(4) In the negative electrode for a lithium ion secondary battery according to the above aspect, the content of the thiol compound in the negative electrode active material layer is 0.1% by mass or more and 2.0% by mass or less with respect to the negative electrode active material. It may be within the range.

(5) In the negative electrode for a lithium ion secondary battery according to the above aspect, the negative electrode active material layer may include a binder, and the binder may be either polyamideimide or polyimide.

  The lithium ion secondary battery according to the second aspect includes the negative electrode of the above aspect, a positive electrode, and a non-aqueous electrolyte.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the negative electrode for lithium ion secondary batteries and the lithium ion secondary battery which were excellent in first time charge / discharge efficiency and charging / discharging cycling characteristics.

It is a cross-sectional schematic diagram 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 according to this embodiment. A lithium ion secondary battery 100 shown in FIG. 1 mainly includes a laminated body 40, a case 50 that accommodates the laminated body 40 in a sealed state, and a pair of leads 60 and 62 connected to the laminated body 40. Although not shown, the nonaqueous electrolyte is housed in the case 50 together with the laminate 40.

  The stacked body 40 is configured such that the positive electrode 20 and the negative electrode 30 are disposed to face each other with the separator 10 interposed therebetween. The positive electrode 20 is obtained by providing a positive electrode active material layer 24 on a plate-like (film-like) positive electrode current collector 22. The negative electrode 30 is obtained by providing a negative electrode active material layer 34 on a plate-like (film-like) negative electrode current collector 32.

  The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10. Leads 62 and 60 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the ends of the leads 60 and 62 extend to the outside of the case 50. In FIG. 1, the case 50 has one laminated body 40 in the case 50, but a plurality of laminated bodies 40 may be laminated.

"Negative electrode"
The negative electrode 30 includes a negative electrode current collector 32 and a negative electrode active material layer 34 provided on the negative electrode current collector 32.

(Negative electrode current collector)
The negative electrode current collector 32 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used. The negative electrode current collector 32 is preferably not alloyed with lithium, and copper is particularly preferable. The thickness of the negative electrode current collector 32 is preferably 6 to 30 μm.

(Negative electrode active material layer)
The negative electrode active material layer 34 includes a negative electrode active material and a thiol compound. Moreover, the negative electrode active material layer 34 has a negative electrode binder and a negative electrode electrically conductive material as needed.

(Negative electrode active material)
As a negative electrode active material, the well-known negative electrode active material for lithium ion secondary batteries which can occlude / release lithium ion can be used. Specifically, for example, metallic lithium, a carbon material, a metal combined with lithium, a compound mainly composed of an oxide, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like can be given.
Examples of the carbon material used as the negative electrode active material include graphite (natural graphite, artificial graphite), carbon nanotube, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, and the like. Examples of the metal that combines with lithium include aluminum, silicon, and tin. Examples of the compound mainly composed of oxide include silicon oxide (SiO _x (0 <x <2)) and tin dioxide.
Among these, it is particularly preferable to use silicon or silicon oxide as the negative electrode active material. Since silicon and silicon oxide are materials having a large theoretical capacity, they contribute to increasing the capacity of the lithium ion secondary battery 100.

(Thiol compound)
The thiol compound is a lithium-substituted thiol group (—SLi) in which at least a part of the thiol groups (—SH) is substituted with lithium during the production of the lithium ion secondary battery 100 and in a charged state. That is, the thiol compound may contain an unsubstituted thiol compound and a lithium-substituted thiol compound substituted with lithium, or may contain a lithium-substituted thiol compound alone. The presence of the lithium-substituted thiol compound in the negative electrode active material layer 34 improves the initial charge / discharge efficiency and charge / discharge cycle characteristics of the lithium ion secondary battery 100. Although the details of this reason are not clear, it can be considered as follows.

  First, the lithium-substituted thiol compound present in the negative electrode active material layer at the time of manufacturing the lithium ion secondary battery 100 functions as a solid electrolyte membrane (SEI), thereby suppressing side reactions for generating SEI. It is conceivable that consumption of the nonaqueous electrolyte is suppressed. Second, the lithium-substituted thiol compound present in the negative electrode active material layer at the time of producing the lithium ion secondary battery 100 releases a lithium ion and has a disulfide bond during the discharge of the first charge / discharge cycle. It is conceivable that the amount of lithium ions that can be reversibly used in the lithium ion secondary battery 100 increases by generating the compound. Thirdly, in the second and subsequent charge / discharge cycles, the disulfide bond of the compound having a disulfide bond generated at the time of discharge absorbs a part of lithium ions during charging to generate a lithium-substituted thiol compound. It is conceivable that the amount of lithium ions occluded in the negative electrode active material can be reduced without changing the lithium ion occlusion amount of the entire negative electrode active material layer, thereby suppressing deterioration of the negative electrode active material. For example, when silicon is used as the negative electrode active material, lithium ions occluded in silicon may break Si—Si bonds in silicon to form SiLi alloys, but some of the lithium ions are disulfide bonds. By being occluded by the compound having, SiLi alloy is hardly generated, and deterioration of silicon itself is suppressed.

  Here, the change of the thiol compound at the time of charging / discharging is demonstrated taking the case where the thiol compound is cysteine lithium which has a lithium substituted thiol group as an example. At the time of manufacturing the lithium ion secondary battery 100 and in a charged state, at least a part of cysteine exists as cysteine lithium (left side). At the time of discharge (oxidation of the negative electrode), cysteine lithium releases lithium ions, and the cysteine from which the lithium ions have been released is bonded via disulfide to generate cystine (right side). On the other hand, during charging (reduction of the negative electrode), cystine absorbs lithium ions and the disulfide bond is cleaved to generate cysteine lithium.

It can be confirmed by the infrared absorption spectrum and the X-ray photoelectron spectrum that cysteine lithium is generated in the negative electrode active material layer 34 after charging. For example, the presence of cysteine, -CH ₂ - ^{2960 cm} -1 due to the group, ^{2550 cm} -1 due to -SH group (thiol group), ^{1587 cm} -1 attributable to -COOH groups, due to the -NH ₂ group This can be confirmed by the presence or absence of each spectrum at 1545 cm ⁻¹ . Moreover, it can confirm that the thiol group is substituted by lithium by the presence or absence of the X-ray photoelectron spectroscopy spectrum which belongs to lithium. The infrared absorption spectrum can be measured by FT-IR (Fourier transform infrared spectroscopy). Further, the X-ray photoelectron spectrum can be measured by XPS (X-ray photoelectron spectroscopy).

  Moreover, it can confirm that the cysteine lithium is producing | generating in the negative electrode active material layer 34 after charge also by the mass spectrum measured using TOF-SIMS (time-of-flight type secondary ion mass spectrometry). it can. For example, the presence of cysteine lithium can be confirmed from the presence or absence of cysteine fragments and the mass spectra of -SLi and -Li.

On the other hand, the formation of a compound having a disulfide bond in the negative electrode active material layer 34 after discharge can be confirmed by an infrared absorption spectrum. For example, the presence of cystine can be confirmed by the presence or absence of each spectrum at 500 cm ⁻¹ due to an —SS—bond (disulfide bond).

  The thiol compound may be a monothiol compound having one thiol group in the molecule or a polyvalent thiol compound having two or more thiol groups in the molecule. The polyvalent thiol compound is preferably a dithiol compound or a trithiol compound. The thiol compound is preferably solid at room temperature.

  Examples of monothiol compounds include cysteine, glutathione, and 4-aminobenzenethiol. Examples of dithiol compounds include 1,2-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenedithiol, 1,5-naphthalenedithiol, 4,4′-biphenyldithiol, 4,4′-thiobis Examples include benzenethiol, toluene-3,4-dithiol, 3,6-dioxa-1,8-octanediol, bismuthiol, and 1,4-benzenedimethanethiol. Examples of the trithiol compound include 1,3,5-benzenetrithiol and 1,3,5-triazine-2,4,6-trithiol (thiocyanuric acid).

  The thiol compound is preferably attached to the surface of the negative electrode active material layer 34. The surface of the negative electrode active material layer 34 means a surface opposite to the negative electrode current collector 32 side of the negative electrode active material layer 34. Since the thiol compound is attached to the surface of the negative electrode active material layer, the function as SEI can be further exerted, whereby the consumption of the non-aqueous electrolyte is further suppressed, and the first time of the lithium ion secondary battery 100. Charge / discharge efficiency and charge / discharge cycle characteristics are further improved. The amount (coverage) of the thiol compound adhering to the surface of the negative electrode active material layer 34 can be measured, for example, as follows. First, element mapping of the surface of the negative electrode active material layer 34 is performed using a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS). The element mapping image is binarized by image processing with the portion where C, S, and Li are detected in the obtained element mapping image as a portion covered with cysteine lithium. Then, the area of the part covered with cysteine lithium and the area of the part not covered are respectively converted into the number of pixels, and the coverage is calculated from the ratio of the number of pixels.

  The ratio of the lithium-substituted thiol group to the total amount of thiol groups of the thiol compound (lithium substitution rate) is preferably in the range of 10 mol% or more and 100 mol% or less. The total amount of thiol groups means the total amount of unsubstituted thiol groups (—SH) and lithium-substituted thiol groups (—SLi) in the thiol compound. For example, when the monothiol compound is 1 mol, the total amount of thiol groups is 1 mol. When the lithium substitution rate is in the above range, the amount of lithium ions that can be used reversibly can be reliably increased, so that the initial charge / discharge efficiency of the lithium ion secondary battery 100 is further improved, and charge / discharge cycle characteristics. Will also improve.

  It is preferable that the content rate of the thiol compound of the negative electrode active material layer 34 exists in the range of 0.1 mass% or more and 2.0 mass% or less with respect to a negative electrode active material. When the content of the thiol compound is in the above range, the amount of lithium ions that can be used reversibly can be increased reliably, so that the initial charge / discharge efficiency of the lithium ion secondary battery 100 is further improved, and charge / discharge is performed. Cycle characteristics are also improved.

(Negative electrode binder)
The negative electrode binder binds the negative electrode active materials and the negative electrode current collector 32 together with the negative electrode active materials or the negative electrode active material and the conductive material. 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) ) And the like.

  In addition to the above, as the binder, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-) TFE fluorine rubber), vinylidene fluoride-pentafluoropropylene fluorine rubber (VDF-PFP fluorine rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluorine rubber (VDF-PFP-TFE fluorine rubber), Vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber The containing rubbers (VDF-CTFE-based fluorine rubber) vinylidene fluoride-based fluorine rubbers such as may be used.

  In addition, as the binder, for example, cellulose, styrene butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, or the like may be used. In the lithium ion secondary battery 100 of the present embodiment, the amount of lithium ions that can be reversibly used by the thiol compound increases, so even when a polyimide resin or polyamideimide resin having an imide group is used, the initial charge / discharge efficiency and charge / discharge efficiency are increased. A decrease in discharge cycle characteristics can be suppressed.

Alternatively, 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. In this case, since the binder also functions as a conductive material, it is not necessary to add a conductive material. As the ion-conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) , Polyphosphazene, etc.) and a lithium salt such as LiClO ₄ , LiBF ₄ , LiPF ₆ , or an alkali metal salt mainly composed of lithium, and the like. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer. The binder content of the negative electrode active material layer 34 is preferably in the range of 5% by mass to 30% by mass.

(Negative electrode conductive material)
As the negative electrode conductive material, for example, a carbon material or a conductive oxide can be used. These may be used individually by 1 type and may be used in combination of 2 or more type. Examples of the carbon material include carbon black such as acetylene black and ethylene black, and carbon nanotubes. Examples of the conductive oxide include ITO (tin-doped indium oxide). Among these, carbon black such as acetylene black and ethylene black is particularly preferable. Note that in the case where sufficient conductivity can be ensured with only the negative electrode active material, the negative electrode active material layer 34 may not contain a conductive material. The content of the negative electrode conductive material in the negative electrode active material layer 34 is preferably in the range of 0% by mass to 30% by mass.

"Production of negative electrode"
The negative electrode 30 can be produced as follows.
A negative electrode active material, a binder, and a solvent are mixed to prepare a paint. A conductive material may be further added to the paint as necessary. As the solvent, for example, water, N-methyl-2-pyrrolidone or the like can be used. The mixing ratio of the negative electrode active material, the binder, and the conductive material is preferably 50 to 90% by mass: 5 to 30% by mass: 0 to 30% by mass. These mass ratios are adjusted to 100% by mass as a whole. The mixing method of these components constituting the paint is not particularly limited, and the mixing order is not particularly limited.

  Next, the coating material is applied to the negative electrode current collector 32. 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.

  Subsequently, the solvent in the paint applied on the negative electrode current collector 32 is removed. The negative electrode active material layer 34 is formed by removing the solvent, and the negative electrode 30 is obtained. The method for removing the solvent is not particularly limited. For example, the negative electrode current collector 32 coated with the paint may be dried at a temperature of 80 to 150 ° C. Here, when a thermosetting resin such as a polyimide resin, a polyamideimide resin, or an acrylic resin is used as the binder, the negative electrode active material layer 34 may be cured by further heating at the curing temperature of the thermosetting resin. .

  Next, the negative electrode active material layer 34 of the negative electrode 30 obtained in this way is pressed to adjust the thickness of the negative electrode active material layer 34. A roll press can be used as the press device.

Then, a lithium-substituted thiol compound solution in which a lithium-substituted thiol compound is dissolved in a solvent is applied to the surface of the negative electrode active material layer 34 that has been pressed, and then dried to attach the thiol compound to the surface of the negative-electrode active material layer 34 Let
The lithium-substituted thiol compound solution can be prepared, for example, by adding a lithium salt to an unsubstituted thiol compound solution. Water can be used as a solvent for the lithium-substituted thiol compound solution. As a method of applying the lithium-substituted thiol compound solution to the surface of the negative electrode active material layer 34, an immersion method, a printing method, or a spray method can be used.

"Positive electrode"
The positive electrode 20 includes a positive electrode current collector 22 and a positive electrode active material layer 24 provided on the positive electrode current collector 22.

(Positive electrode current collector)
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum or nickel foil can be used.

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

(Positive electrode active material)
The positive electrode active material used for the positive electrode active material layer 24 includes lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or counter ions (for example, PF ⁶⁻ ) of lithium ions and lithium ions. An electrode active material capable of reversibly proceeding doping and dedoping 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 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 Al, Mg, Nb, Ti, Cu, Zn, A composite metal oxide represented by one or more elements selected from Cr), a lithium vanadium compound (LiV ₂ O ₅ ), an olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb) 1 or more elements selected from Ti, Al, Zr, or VO), lithium titanate (Li ₄ Ti ₅ O ₁₂ ), LiNi _x Co _y Al _z O ₂ ( Examples thereof include composite metal oxides such as 0.9 <x + y + z <1.1), polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene.

(Positive electrode conductive material)
As the positive electrode conductive material, for example, a carbon material or a conductive oxide can be used. As the carbon material and the conductive oxide, those exemplified for the negative electrode conductive material can be used. In addition, when sufficient electroconductivity is securable only with a positive electrode active material, the positive electrode active material layer 24 does not need to contain the electrically conductive material.

(Positive electrode binder)
As the positive electrode binder, for example, a fluororesin, vinylidene fluoride-based fluororubber, or the like can be used. As the fluororesin and vinylidene fluoride-based fluororubber, those exemplified for the negative electrode binder can be used.

  The content of the positive electrode active material in the positive electrode active material layer 24 is preferably in the range of 80% by mass to 96% by mass. Further, the content of the conductive material in the positive electrode active material layer 24 is preferably in the range of 2.0% by mass or more and 10% by mass or less, and the content of the binder in the positive electrode active material layer 24 is 2.0% by mass. It is preferably within the range of 10% by mass or less.

"Production of positive electrode"
The positive electrode 20 can be produced as follows.
A positive electrode active material, a conductive material, a binder and a solvent are mixed to prepare a paint.
As the solvent, for example, water, N-methyl-2-pyrrolidone and the like can be used as in the case of production of the negative electrode. The constituent ratio of the positive electrode active material, the conductive material, and the binder is preferably 80 to 96% by mass: 2.0 to 10% by mass: 2.0 to 10% by mass. These mass ratios are adjusted to 100% by mass 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. The application method is not particularly limited, as in the case of producing a negative electrode, and a method usually employed for producing an electrode can be used.

  Subsequently, the solvent in the paint applied on the positive electrode current collector 22 is removed. The positive electrode active material layer 24 is formed by removing the solvent, and the positive electrode 20 is obtained. The method for removing the solvent is not particularly limited. For example, the negative electrode current collector 32 to which the paint has been applied may be dried in an atmosphere at 80 ° C. to 150 ° C.

  And the positive electrode active material layer 24 of the positive electrode 20 obtained in this way is pressed, and the thickness of the positive electrode active material layer 24 is adjusted. A roll press can be used as the press device.

"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.

"Nonaqueous electrolyte"
The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent. The non-aqueous solvent preferably contains a cyclic carbonate and a chain carbonate.

As cyclic carbonate, what can solvate electrolyte can be used. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or the like can be used. In the cyclic carbonate, part or all of hydrogen may be substituted with fluorine. Examples of the cyclic carbonate substituted with fluorine include fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC).
Since the chain carbonate has a relatively low viscosity compared to the cyclic carbonate, the viscosity of the non-aqueous solvent can be reduced. Examples thereof include diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).
The ratio of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably in the range of 1: 9 to 1: 1 by volume ratio.

  The non-aqueous solvent may contain other organic solvents other than the cyclic carbonate and the chain carbonate. Examples of other organic solvents include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, and 1,2-diethoxyethane.

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 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 non-aqueous electrolyte 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, sufficiently ensure the mobility of lithium ions, and to easily obtain a sufficient capacity during charging and discharging. Become.

Even when LiPF ₆ is mixed with other electrolytes, the lithium ion concentration in the nonaqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol% or more. More preferably it is included.

"Case"
The case 50 seals the laminated body 40 and the nonaqueous electrolyte therein. The case 50 is not particularly limited as long as it can prevent leakage of the nonaqueous electrolyte to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside.

  For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

"Lead"
The leads 60 and 62 are made of a conductive material such as aluminum. Then, the leads 60 and 62 are respectively welded to the positive electrode current collector 22 and the negative electrode current collector 32 by a known method, and a separator is provided between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30. 10 is inserted into the case 50 together with the non-aqueous electrolyte, and the entrance of the case 50 is sealed.

[Method for producing lithium ion secondary battery]
The lithium ion secondary battery 100 can be manufactured as follows.
A positive electrode 20 having a positive electrode active material layer 24, a negative electrode 30 having a negative electrode active material layer 34, a separator 10 interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte are enclosed in a case 50.

  For example, the positive electrode 20, the negative electrode 30, and the separator 10 are stacked, and the positive electrode 20 and the negative electrode 30 are heated and pressed with a press tool from a direction perpendicular to the stacking direction, and the positive electrode 20, the separator 10, and the negative electrode 30. Adhere. For example, the laminated body 40 is put into a bag-like case 50 prepared in advance.

  Finally, a non-aqueous electrolyte is injected into the case 50, whereby the lithium ion secondary battery 100 is manufactured. Instead of injecting the nonaqueous electrolyte into the case, the laminate 40 may be impregnated with the nonaqueous electrolyte.

  In the lithium ion secondary battery 100 of the present embodiment, the negative electrode active material layer 34 includes a thiol compound in which at least some of the thiol group hydrogens are replaced by lithium, so that the initial charge / discharge efficiency and the charge / discharge cycle are excellent. It will be.

Although the embodiments of the present invention have been described in detail with reference to the drawings, the configurations and combinations of the embodiments in the embodiments are examples, and the addition and omission of configurations are within the scope not departing from the gist of the present invention. , Substitutions, and other changes are possible.
For example, in the present embodiment, the thiol compound is preferably attached to the surface of the negative electrode active material layer 34, but the present invention is not limited to this case. For example, a thiol compound may be dispersed in the negative electrode active material layer 34. In this case, the negative electrode 30 can be manufactured by applying a negative electrode mixture paint containing a negative electrode active material and a lithium-substituted thiol compound on the negative electrode current collector 32 and drying it.

[Example 1]
(Preparation of negative electrode)
83 parts by mass of silicon as a negative electrode active material, 2 parts by mass of acetylene black as a conductive material, 15 parts by mass of polyamideimide as a binder, and N-methyl-2-pyrrolidone as a solvent are mixed and dispersed by a hybrid mixer. Thus, a paste-like negative electrode mixture paint was prepared. A negative electrode mixture layer was applied to the surface of the rolled copper foil (thickness: 10 μm) with a predetermined thickness by using this negative electrode mixture paint with a comma roll coater. Next, the solvent in the negative electrode mixture layer was removed by drying in a drying furnace at 100 ° C. to form a negative electrode active material layer. Thereafter, the negative electrode active material layer was applied to the back side of the rolled copper foil in the same procedure. And the negative electrode active material layer formed in both surfaces was crimped | bonded to the rolled copper foil using the roll press machine, and the negative electrode in which the negative electrode active material layer which has a predetermined density was formed was produced. The weight (weight per unit area) of the negative electrode active material contained in the negative electrode active material layer was 1.5 mg · cm ⁻² .

The negative electrode was punched into a 19 mm × 23 mm size electrode with a mold, and then heated to 350 ° C. at a temperature increase rate of 30 ° C./min ^{−1 in} a heat treatment furnace for the purpose of drying and removing the residual solvent for 1 hour After holding, it was rapidly cooled to room temperature. The heat treatment was performed in a vacuum under reduced pressure.

(Lithium-substituted thiol compound coating on negative electrode active material layer)
In 100 g of ion exchange water, 1 g (0.008 mol) of cysteine as a thiol compound was dissolved to prepare an aqueous cysteine solution having a concentration of 1% by mass. Next, 0.00008 mol of lithium hydroxide was added to the aqueous cysteine solution to obtain an aqueous cysteine lithium solution having a thiol group (—SH) substitution rate of 1 mol%.

  The negative electrode was immersed in the cysteine lithium aqueous solution and dried at 60 ° C., thereby supporting cysteine lithium on the negative electrode active material layer. This was repeated a plurality of times to coat 1% by mass of cysteine lithium with respect to the negative electrode active material.

The coating amount (content) of cysteine lithium was calculated by measuring the mass of the electrode after coating, and using the following formula from the mass of the electrode after coating and the mass of the electrode before coating measured in advance.
Covering amount of cysteine lithium = {(mass of electrode after coating−mass of electrode before coating) / mass of electrode before coating} × 100

(Preparation of positive electrode)
90 parts by mass of LiCoO ₂ as a positive electrode active material, 5 parts by mass of acetylene black as a conductive material, and 5 parts by mass of polyvinylidene fluoride (PVDF) as a binder were mixed together to obtain a positive electrode mixture. . Subsequently, the positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a paste-like positive electrode mixture paint. A positive electrode mixture layer was applied to both surfaces of an aluminum foil having a thickness of 15 μm. Next, the solvent in the positive electrode mixture layer was removed by drying in a drying furnace at 100 ° C. to form a positive electrode active material layer. And the positive electrode active material layer formed in both surfaces was crimped | bonded to the aluminum foil using the roll press machine, and the positive electrode in which the positive electrode active material layer which has a predetermined density was formed was produced. The weight (weight per unit area) of the positive electrode active material contained in the positive electrode active material layer was 22 mg · cm ⁻² .

(Production of lithium ion secondary battery)
The above positive electrode and negative electrode were laminated through a separator (porous polyethylene sheet) so that the positive electrode active material layer and the negative electrode active material layer were opposed to each other to obtain a laminate. In the negative electrode of the laminate, a negative electrode lead made of nickel is attached to the protruding end portion of the copper foil not provided with the negative electrode active material layer. On the other hand, in the positive electrode of the laminate, the aluminum foil without the positive electrode active material layer is provided. A positive electrode lead made of aluminum was attached to the end of the protrusion with an ultrasonic welding machine. The laminated body was inserted into the outer body of the aluminum laminate film and heat sealed except for one peripheral portion to form a closed portion. And finally, electrolyte solution was inject | poured in the exterior body, and the remaining one place was sealed with the heat seal, reducing the pressure with the vacuum sealer, and the lithium ion secondary battery was produced. As the electrolytic solution, a nonaqueous electrolyte containing a mixed solvent in which fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7, and lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1 mol / L. The solution was used.

[Examples 2 to 6]
Example 1 except that in the lithium-substituted thiol compound coating of the negative electrode active material, the amount of lithium hydroxide added to the aqueous cysteine solution was changed to adjust the lithium substitution rate to the value shown in Table 1 below. Similarly, a lithium ion secondary battery was produced.

[Comparative Example 1]
A lithium ion secondary battery was produced in the same manner as in Example 1 except that a cysteine aqueous solution having a lithium substitution rate of 0 mol% was used in the lithium-substituted thiol compound coating on the negative electrode active material layer.

[Comparative Example 2]
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode active material layer was not coated with the lithium-substituted thiol compound.

  About the produced lithium ion secondary battery, the state and capacity | capacitance maintenance factor of the thiol compound of a negative electrode active material layer were measured as follows. The capacity retention rate was determined by measuring the initial charge / discharge efficiency and the discharge capacity retention rate at the 50th cycle.

(State of thiol compound)
The lithium ion secondary battery immediately after production (before charging) was disassembled, and the negative electrode was taken out. The negative electrode active material layer of the extracted negative electrode was washed with dimethyl carbonate (DMC) and dried, and then mass spectrum was measured by TOF-SIMS. As a result, for the negative electrode active material layers of Examples 1 to 6, a cysteine fragment and mass spectra of -SLi and -Li were confirmed. On the other hand, for the negative electrode active material layer of Comparative Example 1, a cysteine fragment and a mass spectrum of -SH were confirmed. In the negative electrode active material layer of Comparative Example 2, no cysteine fragment was observed.

[Capacity maintenance rate]
Using a secondary battery charge / discharge tester (made by Hokuto Denko Co., Ltd.), a lithium ion secondary battery is charged at a constant current and a constant voltage up to 4.2 V at a charge rate of 0.5 C, and up to 2.5 V at a discharge rate of 1 C 50 charge / discharge cycles for constant current discharge were performed. Then, the initial charge / discharge efficiency was calculated from the following formula (1), and the discharge capacity retention rate at the 50th cycle was calculated from the following formula (2). The results are shown in Table 1.

Initial charge / discharge efficiency = (discharge capacity at the first cycle / charge capacity at the first cycle) × 100 (1)
Discharge capacity maintenance rate at the 50th cycle = (discharge capacity at the 50th cycle / discharge capacity at the first cycle) × 100 (2)

  From the results in Table 1, the lithium ion secondary batteries of Examples 1 to 6 in which the negative electrode active material layer was coated with cysteine lithium were compared with Comparative Example 1 coated with unsubstituted cysteine and Comparative Example 2 not coated with a thiol compound. It can be seen that the initial charge / discharge efficiency and the discharge capacity retention rate at the 50th cycle are improved.

[Examples 7 to 13]
In the lithium-substituted thiol compound coating on the negative electrode active material layer, the negative electrode is immersed in a cysteine lithium aqueous solution, and then the number of times of drying is changed, so that the coating amount of cysteine lithium on the negative electrode active material is as shown in Table 2 below. A lithium ion secondary battery was produced in the same manner as in Example 6 except for the change. About the produced lithium ion secondary battery, the first time charge-and-discharge efficiency and the discharge capacity maintenance factor at the 50th cycle were measured like the above. The results are shown in Table 2 below together with the results of Example 6.

  From the results of Table 2, the lithium ion secondary batteries of Examples 7 to 13 in which the coating amount of cysteine lithium that coats the negative electrode active material layer was changed to the initial charge / discharge efficiency in the same manner as the lithium ion secondary battery of Example 6. Can be seen to improve. In particular, in the lithium ion secondary batteries of Examples 7 to 11 in which the coating amount of cysteine lithium is in the range of 0.1 to 2.0% by mass, the initial charge is similar to the lithium ion secondary battery of Example 6. It can be seen that the discharge efficiency and the discharge capacity retention rate at the 50th cycle are improved. In the lithium ion secondary batteries of Examples 12 and 13 with a large amount of cysteine lithium coating, the discharge capacity retention rate at the 50th cycle was slightly lowered. This is considered to be because a part of cysteine lithium became a resistance film and inhibited lithium ions from being occluded by the negative electrode active material during charging.

[Examples 14 to 25]
A lithium ion secondary battery was fabricated in the same manner as in Example 6 except that in the lithium-substituted thiol compound coating on the negative electrode active material layer, a thiol compound shown in Table 3 below was used instead of cysteine. About the produced lithium ion secondary battery, the first time charge-and-discharge efficiency and the discharge capacity maintenance factor at the 50th cycle were measured like the above. The results are shown in Table 3 below together with the results of Example 6.

  From the results shown in Table 3, the lithium ion secondary batteries of Examples 14 to 25 using the thiol compound shown in Table 3 as the thiol compound have an initial charge / discharge efficiency of 50 as in the lithium ion secondary battery of Example 6. It can be seen that the discharge capacity retention rate at the cycle time is improved.

DESCRIPTION OF SYMBOLS 10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode collector, 24 ... Positive electrode active material layer, 30 ... Negative electrode, 32 ... Negative electrode collector, 34 ... Negative electrode active material layer, 40 ... Laminate, 50 ... Case, 52 ... Metal foil, 54 ... Polymer film, 60, 62 ... Lead, 100 ... Lithium ion secondary battery

Claims (6)

A negative electrode having a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector,
The negative electrode active material layer includes a negative electrode active material and a thiol compound, and the thiol compound is a negative electrode for a lithium ion secondary battery in which hydrogen of at least a part of the thiol group is replaced by lithium.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the thiol compound is attached to a surface of the negative electrode active material layer.   The ratio of thiol groups in which hydrogen is replaced by lithium with respect to the total amount of thiol groups of the thiol compound is in the range of 10 mol% or more and 100 mol% or less. Negative electrode for secondary battery.   The content rate of the said thiol compound of the said negative electrode active material layer exists in the range of 0.1 mass% or more and 2.0 mass% or less with respect to the said negative electrode active material. Negative electrode for lithium ion secondary battery.   The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the negative electrode active material layer includes a binder, and the binder is either polyamideimide or polyimide.   The lithium ion secondary battery which has a negative electrode as described in any one of Claims 1-5, a positive electrode, and a nonaqueous electrolyte.

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