JP2021153029A - Positive electrode active material layer, positive electrode using the same, and secondary battery - Google Patents

Abstract

To provide a positive electrode active material layer which is excellent in positive electrode capacity area density, a positive electrode using the same, and a secondary battery.SOLUTION: There is provided a positive electrode active material layer 24 which includes a positive electrode active material, a positive electrode conductive aid, and a positive electrode binder. The positive electrode active material includes an active material having a carbon-sulfur bond. The positive electrode binder includes a fluoro-based copolymer skeleton and a pendant chain coupled to the skeleton, the atomic weight ratio (number of fluorine atom/number of oxygen atom) in the positive electrode binder being 5-60.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a positive electrode active material layer, a positive electrode using the same, and a secondary battery.

A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of increasing output. Lithium-ion secondary batteries are currently mainly used as a power source for portable electronic devices, and are expected to be a power source for electric vehicles, which are expected to become widespread in the future. However, when used for these purposes, especially when used as a power source for automobiles, cost reduction and space saving are required. Further, for portable electronic devices, which are the main applications at present, further shortening, smallness, and thinning are required.

Most of the lithium ion secondary batteries currently in use use lithium metal oxides such as cobalt and nickel as the positive electrode active material. On the other hand, attempts have been made to use sulfur as a positive electrode active material. Sulfur is a resource-rich and inexpensive material, and theoretically has the largest electric capacity (1675 mAh / g) among known positive electrode active materials. It is said that about 6 times the electric capacity can be obtained as compared with the current mainstream lithium cobalt oxide positive electrode material. Therefore, it is desired to put sulfur into practical use as a positive electrode active material.

However, the compound of sulfur and lithium produced in the positive electrode during the charge / discharge process is soluble in a non-aqueous electrolyte solution for a lithium ion secondary battery. Therefore, when sulfur is used as the positive electrode active material, there is a problem that the positive electrode is gradually deteriorated due to the elution of the above compound into the electrolytic solution, and the battery capacity is lowered.

Although the electric capacity is slightly inferior to that of sulfur, a sulfur-based polymer positive electrode using a tetrathionaphthalene polymer or sulfur-modified polyacrylonitrile as an active material has been proposed as an attempt to suppress the above elution (Patent Document 1, Patent Document 1, 2).

Generally, the positive electrode is coated with a slurry containing a positive electrode active material, a conductive auxiliary agent, and a binder for improving the binding property between the current collector and the active material or between the active materials, and dried. It is produced by doing. The electrode potential is low due to the chemical potential of the sulfur element, and the cell voltage remains at about 2V (current lithium-ion battery is about 3.6 to 3.8V). Therefore, in order to increase the energy density (cell voltage x capacity), It is necessary to increase the ratio of the positive electrode active material layer to the entire battery component. One of the effective means is to thicken the positive electrode active material layer, in other words, to improve the positive electrode capacitance surface density. However, the thickening of the film causes cracks and peeling from the positive electrode current collector. In that case, although there is room for improvement by increasing the amount of the binder, the concentration of the positive electrode active material in the entire positive electrode decreases, and as a result, the capacity cannot be increased.

Japanese Unexamined Patent Publication No. 2008-66125

WO2011 / 129103 Gazette

In the positive electrode active material layer described in Patent Documents 1 and 2, it was difficult to improve the positive electrode capacitance surface density by thickening the film. The present disclosure has been made in view of the above problems, and an object of the present invention is to provide a positive electrode active material layer having an excellent positive electrode capacitance surface density due to a thick film, and a positive electrode and a secondary battery using the same.

In order to solve the above problems, according to a certain viewpoint of the present disclosure, a positive electrode active material, a positive electrode conductive auxiliary agent, and a positive electrode binder are included, and the positive electrode active material includes an active material having a carbon-sulfur bond. The positive electrode binder includes a fluorocopolymer skeleton and a pendant chain bonded to the skeleton, and the atomic weight ratio (number of fluorine atoms / oxygen atoms) in the binder is 5 to 60. Layers are provided.

From this point of view, the very high interaction between the positive electrode binder and the carbon-sulfur bond on the surface of the positive electrode active material reduces the agglomeration probability between the positive electrode active materials, causing cracks due to the thickening of the film and collecting the positive electrodes. Peeling from the electric body can be suppressed, and a positive electrode active material layer having an excellent capacitance surface density can be realized.

A positive electrode active material layer having a binder ratio of 3 to 15% by mass in the positive electrode active material layer is provided.

From this point of view, the positive electrode capacitance surface density is further improved.

A positive electrode active material layer having a thickness of 151 to 300 μm is provided.

From this point of view, the positive electrode capacitance surface density is further improved.

A positive electrode active material layer is provided in which the positive electrode active material is sulfur-modified polyacrylonitrile.

From this point of view, it is possible to realize a positive electrode active material layer having excellent positive electrode capacitance surface density and rate characteristic ratio.

A positive electrode including the positive electrode active material layer according to any one of the above and a positive electrode current collector is provided.

From this point of view, it is possible to realize a positive electrode having an excellent positive electrode capacitance surface density.

A secondary battery including the positive electrode, the negative electrode, the separator, and the electrolytic solution described above is provided.

From this point of view, a secondary battery having an excellent positive electrode capacitance surface density can be realized.

As described above, according to the present disclosure, the positive electrode active material layer and the positive electrode capacitance surface density of the positive electrode and the secondary battery using the same are significantly improved.

It is sectional drawing which shows the secondary battery which concerns on embodiment of this disclosure.

Preferred embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. In the present specification and the drawings, components having substantially the same functional configuration are designated by the same reference numerals, so that duplicate description will be omitted.

[Secondary battery configuration]
First, the configuration of the secondary battery 100 according to the present embodiment will be described with reference to FIG.

The secondary battery 100 includes a positive electrode 20, a negative electrode 30, and a separator layer 10. The form of the secondary battery 100 is not particularly limited. That is, the secondary battery 100 may be any of a cylindrical type, a square type, a laminated type, a button type, and the like.

The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24 provided on one surface thereof. The positive electrode current collector 22 may be made of a conductive material, and for example, a thin metal plate such as aluminum, copper, nickel, stainless steel, or an alloy-based material thereof can be used.

As the positive electrode active material used for the positive electrode active material layer 24, an active material having a carbon-sulfur bond capable of reversibly advancing the occlusion and release of alkali metal ions such as lithium, sodium and potassium can be used.

For example, poly (carbon denaturide) of (CSx) n (x is a value of 1.2 to 2.3 and n is a value of 2 or more) described in JP-A-7-295999. Examples thereof include the sulfur atomic cross-linked polymer of tetrathionaphthalene described in JP-A-2008-66125, and the sulfur-modified polyacrylonitrile described in WO2011 / 129103. Of these, sulfur-modified polyacrylonitrile is preferable from the viewpoint of rate characteristic ratio.

Further, the positive electrode active material layer 24 may have a positive electrode conductive auxiliary agent. Examples of the positive electrode conductive auxiliary agent include carbon powder such as carbon black, carbon nanotubes, carbon material, metal fine powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and metal fine powder, and conductive oxide such as ITO. Can be mentioned. When sufficient conductivity can be ensured only by the positive electrode active material, the positive electrode active material layer 24 does not have to contain the positive electrode conductive auxiliary agent.

Further, the positive electrode active material layer 24 contains a positive electrode binder. The positive electrode binder binds the positive electrode active materials to each other and also binds the positive electrode active material and the positive electrode current collector 22. The positive electrode binder has a fluoropolymer skeleton and a pendant chain bonded to the skeleton.

The atomic weight ratio (number of fluorine atoms / number of oxygen atoms) between the number of fluorine atoms and the number of oxygen atoms in the positive electrode binder is preferably 5 to 60. Within the above range, the generation of cracks due to the thickening of the film and the peeling from the positive electrode current collector can be effectively suppressed.

The Disclosers presume the mechanism of this embodiment as follows. The positive electrode active material having a carbon-sulfur bond is very likely to aggregate due to the surface interaction between the positive electrode active materials. Therefore, in the process of forming the positive electrode active material layer, the aggregation proceeds as the solvent volatilizes, and finally, cracks occur due to volume shrinkage, and eventually the positive electrode active material is peeled off from the positive electrode current collector. This becomes more remarkable as the probability of aggregation between the positive electrode active materials increases due to the thickening of the film.

The problem is that by setting the atomic weight ratio (number of fluorine atoms / oxygen atoms) in the positive electrode binder to 5 to 60, a very high interaction with the carbon-sulfur bond on the surface of the positive electrode active material occurs, and the positive electrode active materials have a very high interaction with each other. Aggregation can be suppressed. As a result, it is possible to effectively prevent the generation of cracks due to the thickening of the film and the peeling from the positive electrode current collector. When the atomic weight ratio (number of fluorine atoms / number of oxygen atoms) is less than 5, the positive electrode binders agglomerate due to the interaction between oxygen atoms, and the above effect is not exhibited. If it exceeds 60, the interaction with the positive electrode active material is lost due to the high surface energy of the fluorine atom, and the above effect is not exhibited.

Even when the ratio in the previous term is 5 to 60, a simple blend of a polymer composed of a fluoropolymer skeleton having no pendant chain and a polymer corresponding to the pendant chain (for example, a polyoxyalkylene polymer). Then, both cause phase separation, and the above effect cannot be obtained.

Examples of the fluoropolymer skeleton of the present embodiment include a copolymer of a fluorinated monomer and a (meth) acrylic monomer.

Fluorinated monomers include C3-C8 perfluoroolefins such as tetrafluoroethylene and hexafluoropropene, and C2-C8 hydrogenated fluoros such as vinylidene fluoride, vinyl fluoride, 1,2-difluoroethylene and trifluoroethylene. Olefin, perfluoroalkylethylene represented by the formula CH ₂ = CH-R _a (where _Ra is C1-C6 perfluoroalkyl), and chloro- and / or chlorotrifluoroethylene such as chlorotrifluoroethylene. C2-C6 fluoroolefins of bromo- and / or iodo-, and formula CF ₂ = CFOR _b (where R _b is a fluoro- or perfluoroalkyl of C1-C6, such as CF ₃ , C ₂ F ₅ , C. _The (per) fluoroalkyl vinyl ether represented by 3 F ₇ ) or the (per) fluoro-oxyalkyl vinyl _{ether represented by the formula CF 2} = CFOR _{c (in the formula, R c} is C1 to C12 alkyl, or C1 to C1 to C1 to C12 (per) fluorooxyalkyl having one or more ether groups, such as C12 oxyalkyl, or perfluoro-2-propoxy-propyl), or the formula CF ₂ = CFOCF ₂ OR _d (formula). Among them, R _d is one or more C1-C6 fluoro- or perfluoroalkyl groups such as CF ₃ , C ₂ F ₅ , C ₃ F ₇ or -C ₂ F _5- O-CF _3. represented by C1~C6 having an ether group (per) fluorooxyalkyl group) (per) or fluoroalkyl vinyl ethers, wherein _CF 2 = CFOR _e (wherein, _{R e} may, Cl -C 12 alkyl or (per) fluoroalkyl, or Cl -C 12 oxyalkyl, or a Cl -C 12 (per) fluorooxyalkyl having one or more ether groups,, _{R e,} the acid, in the form of an acid halide or salt, a carboxylic acid Alternatively, functional (per) fluoro-oxyalkyl vinyl ether represented by (including sulfonic acid), fluorodioxol, particularly perfluorodioxol, and the like can be mentioned.

The (meth) acrylic monomer has a functional group for forming a pendant chain. Examples of the functional group include a hydroxyl group, a carboxylic acid group, an amino group, an isocyanate group and an epoxy group. Specific examples of the (meth) acrylic monomer include acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, aminoethyl acrylate, 2-isocyanate ethyl acrylate, and glycidyl acrylate. And so on.

As a method for synthesizing a copolymer of a fluoride monomer and a (meth) acrylic monomer, any method can be used, and specific examples thereof include an aqueous suspension polymerization method and an aqueous emulsion polymerization method.

The ratio of the fluorine monomer to the (meth) acrylic monomer may be appropriately determined in consideration of the amount of the pendant chain introduced so that the (fluorine atom number / oxygen atom number) in the polymer component is 5 to 60. Specifically, a range containing 60 mol% to 99.9 mol% of fluorine monomer and 0.1 mol% to 40 mol% (meth) acrylic monomer is preferable.

As the compound for forming the pendant chain, a compound containing a plurality of functional groups for binding to the fluoropolymer skeleton and a plurality of oxygen atoms is preferable. Examples of the functional group include a hydroxyl group, a carboxylic acid group, an amino group, an isocyanate group and an epoxy group. Specifically, a polyoxyalkylene having one or two of the above functional groups and having a molecular weight of about 80 to 10,000 is preferable.

The composition ratio of the positive electrode binder in the positive electrode active material layer 24 is preferably in the range of 1 to 20% by mass, more preferably 3 to 15% by mass in terms of mass ratio. When the ratio of the positive electrode binder in the positive electrode active material layer is 3 to 15% by mass, the balance between the film forming property and the positive electrode capacitance surface density can be particularly ensured. If it is less than 3% by mass, the amount of the positive electrode binder is too small and the film forming property is inferior. Further, when the positive electrode binder coats the positive electrode active material more than necessary, the rate characteristic ratio deteriorates.

The positive electrode active material layer 24 forms a coating liquid by dispersing, for example, the positive electrode active material, the positive electrode conductive auxiliary agent, and the positive electrode binder in an appropriate organic solvent (for example, N-methyl-2-pyrrolidone), and the coating liquid is formed. It is formed by applying a working liquid onto the positive electrode current collector 22, drying it, and rolling it if necessary.

The film thickness of the positive electrode active material layer is preferably 70 μm or more, more preferably 100 μm or more, and more preferably 151 to 300 μm. Within this range, the positive electrode capacitance surface density can be sufficiently secured. The positive electrode capacitance surface density is preferably 5 ^{mAh / cm 2 or more.} If it exceeds 300 μm, the permeability of the electrolytic solution into the positive electrode active material layer becomes difficult, and the positive electrode capacitance surface density and the rate characteristic ratio tend to deteriorate.

The negative electrode 30 has a negative electrode current collector 32 and a negative electrode active material layer 34.

The negative electrode current collector 32 may be any conductive plate material, and for example, aluminum, copper, nickel, stainless steel, or a thin metal plate made of an alloy-based material thereof can be used.

The negative electrode active material layer 34 has a negative electrode active material, and if necessary, a negative electrode binder and a negative electrode conductive auxiliary agent.

Examples of the negative electrode active material include alkali metals such as lithium, sodium and potassium, carbon-based materials such as graphite, silicon, silicon-containing compounds such as silicon oxide represented by SiOx (0 <x <2), copper-tin and the like. Alloy-based materials such as cobalt-tin can be used.

When a material containing no alkali metal, for example, a carbon-based material, a silicon-based material, an alloy-based material, or the like is used as the negative electrode active material among the above-mentioned negative electrode active materials, between the positive and negative electrodes due to the generation of dendride. It is advantageous that a short circuit is unlikely to occur.

However, when these alkali metal-free negative electrode active material materials are used, when used in combination with the positive electrode of the present disclosure, neither the positive electrode nor the negative electrode contains alkali metal, so the negative electrode is pre-doped with an alkali metal in advance. Processing is required.

For example, as a lithium predoping method, a semi-battery is assembled using metallic lithium at the counter electrode, and lithium is inserted by an electrolytic doping method in which lithium is electrochemically doped. Metallic lithium is placed on the surface of the negative electrode or in the negative electrode. After that, a method such as a method in which the negative electrode is left in an electrolytic solution and lithium is inserted by a pasting pre-doping method in which lithium is doped by utilizing the diffusion of lithium into the negative electrode can be used.

As the negative electrode conductive auxiliary agent, the same one as that of the positive electrode conductive auxiliary agent can be used. When sufficient conductivity can be ensured only by the negative electrode active material, the negative electrode active material layer 34 may not contain the negative electrode conductive auxiliary agent.

The negative electrode binder binds the negative electrode active materials to each other and also binds the negative electrode active material and the negative electrode current collector 32. Examples of the negative electrode binder include those equivalent to the positive electrode binder, fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), and vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP). Fluororesin), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFPTFE fluororubber) and other vinylidene fluoride fluororubbers, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin , Polypolyimide resin, acrylic resin and the like.

The contents of the negative electrode active material, the negative electrode conductive auxiliary agent, and the negative electrode binder in the negative electrode active material layer 34 are not particularly limited.

The negative electrode active material layer 34 forms a coating liquid by dispersing, for example, the negative electrode active material, the conductive auxiliary agent, and the binder in an appropriate organic solvent (for example, N-methyl-2-pyrrolidone), and the coating liquid is formed. Is coated on the negative electrode current collector 32, dried, and rolled if necessary.

When a foil-shaped negative electrode active material is used, a foil-shaped negative electrode active material having a thickness of 0.01 to 200 μm is attached onto the negative electrode current collector 32 to prepare a negative electrode sheet.

The separator 10 may be formed from an electrically insulating porous structure, for example, a monolayer of a film made of polyethylene, polypropylene or polyolefin, a laminate or a stretched film of a mixture of the above resins, or cellulose, polyester and Examples thereof include fibrous nonwoven fabrics made of at least one constituent material selected from the group consisting of polypropylene.

Further, the separator material may be coated with inorganic particles or a polymer component. Examples of the inorganic particles include oxides such as alumina, silica, zirconium oxide, titanium oxide and magnesium oxide, and dielectric materials such as barium titanate. Polymer components include binders for negative electrodes, binders for positive electrodes, polyelectrolytes (composites of polymer materials such as polyethylene glycol and polyethylene carbonate and lithium salts), and ion exchange resins (polydimethyldiallyl ammonium salts). , Polystyrene sulfonate, etc.), and other examples include polyvinyl alcohol, CMC, polybutyral, and polyacrylic acid.

The non-aqueous electrolyte solution contains an alkali metal salt which is an electrolyte and a solvent. Lithium salts include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiSO ₃ CF ₃ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ F) (SO ₂ CF ₃ ), LiN. (SO ₂ CF ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₂ CF ₃ ) ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiI, LiCl, LiF, LiPF ₅ (SO ₂ CF ₃ ), LiPF ₄ (SO ₂₎ CF ₃ ) ₂ etc. can be mentioned. Examples of the sodium salt include NaN (SO ₂ F) ₂ , NaN (SO ₂ CF ₃ ) _{2, and the} like. Examples of the potassium salt include KN (SO ₂ F) ₂ , KN (SO ₂ CF ₃ ) _{2, and the} like.

The concentration of the alkali metal salt is preferably about 0.8 to 5.0 mol / L.

Examples of the solvent include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, di-n-propyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate, methyl isopropyl carbonate, ethyl-n-propyl carbonate and ethyl isopropyl. Carbonate, di-n-propyl carbonate, diisopropyl carbonate, 3-fluoropropylmethyl carbonate, propylene carbonate, ethylene carbonate, butylene carbonate, 4-chloro-1,3-dioxolan-2-one, 4-fluoro-1,3- Dioxolan-2-one, 4-trifluoromethyl-1,3-dioxolan-2-one, vinylene carbonate, dimethylvinylene carbonate, vinylene carbonate, carbonates such as fluoroethylene carbonate, methyl acetate, ethyl acetate, methyl propionate, Carboic acid esters such as ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate and ethyl trimethylacetate, cyclic esters such as γ-butyrolactone and γ-valerolactone, chain sulfonic acid esters such as dimethylsulfoxide and dimethyl sulfite, Cyclic sulfonic acid esters such as sulfolane and propanesarton, nitrile compounds such as acetonitrile, glutaronitrile, adiponitrile, methoxyacetyl, 3-methoxypropionitrile and succinonitrile, 1,2-dimethoxyethane, dimethyl ether and methyl ethyl ether. , Diethyl ether, butyl methyl ether, dipropyl ether, cyclopentyl methyl ether, dibutyl ether, diisopentyl ether, triglime, chain ether such as tetraglyme, oxetane, tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,4 -Cyclic ethers such as dioxane, hydrofluoro ethers such as 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, phosphate esters such as triethyl phosphate and trimethyl phosphate, Phosphonic acid esters such as dimethyl methylphosphonate, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 2,2,2-trifluoroethyl ether, difluoromethyl 2,2 , 3,3-Tetrafluoropropyl ether, 1,1,1,2,2,3,4,5,5,5-decaf Examples thereof include fluorinated ethers such as ruolo-3-methoxy-4- (trifluoromethyl) pentane. These may be used alone or in admixture of a plurality of types. From the viewpoint of the solubility of the alkali metal salt, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, triglime, tetraglime and acetonitrile are preferable.

Further, an ionic liquid may be contained as a solvent. As the ionic liquid, a compound containing a cationic species and an anionic species that are liquid at −30 ° C. to 120 ° C. can be used.

As the cation species, a nitrogen-based cation containing nitrogen, a phosphorus-based cation containing phosphorus, and a sulfur-based cation containing sulfur can be used. These cation components may be used alone or in combination of two or more. Examples of nitrogen-based cations include chain or cyclic ammonium cations such as imidazolium cations, pyrrolidinium cations, piperidinium cations, pyridinium cations, and azoniaspirocations. Examples of phosphorus-based cations include chain or cyclic phosphonium cations. Examples of sulfur-based cations include chain or cyclic sulfonium cations.

Examples of the anionic _{^{species, AlCl 4 -, NO 2 -}} , NO 3 -, I -, BF 4 -, PF 6 -, AsF 6 -, SbF 6 -, NbF 6 -, TaF 6 -, F (HF) 2. ₃ ⁻ , CH ₃ CO ₂ ⁻ , CF ₃ CO ₂ ⁻ , CH ₃ SO ₃ ⁻ , CF ₃ SO ₃ ⁻ , (CF ₃ SO ₂ ) ₃ C ⁻ , C ₃ F ₇ CO ₂ ⁻ , C ₄ F ₉ SO _{^{3 -, (CF 3 SO 2}} ) (CF 3 CO) N -, (CN) 2 N -, imide anion represented by the following formula _{((SO 2 (CF 2)} xF) (SO2 (CF2) yF) N ^- (However, x and y are independent of each other and indicate an integer of 0 to 5.)), Etc. can be mentioned. These anion species may be used alone or in combination of two or more.

From the viewpoint of the solubility of the alkali metal salt, the cation component is preferably a pyrrolidinium cation or a piperidinium cation, and the anion component is preferably an imide anion, a PF ₆ ^{− or} a BF ₄ ⁻ anion, and further, ( SO ₂ F) ₂ N ⁻ , (SO ₂ CF ₃ ) ₂ N ⁻ , and (SO ₂ CF ₃ ) (SO ₂ F) N ⁻ are more preferred.

In addition, various additives (negative electrode SEI (Solid Electrolyte Interface) forming agent, surfactant, etc.) may be added to the non-aqueous electrolyte solution. Examples of such additives include vinylene carbonate, vinyl carbonate ethylene, phenylethylene carbonate, succinic acid anhydride, lithium bisoxalate, lithium tetrafluoroborate, dinitrile compounds, propane sultone, butane sultone, propensultone, 3-. Examples thereof include sulfolene, fluorinated allyl ether, and fluorinated acrylate.

The exterior body 50 seals the laminate 40 and the electrolytic solution inside the exterior body 50. The exterior body 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and invasion of moisture or the like into the inside of the secondary battery 100 from the outside.

For example, as the exterior body 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, for example, polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferable.

The leads 60 and 62 are formed of a conductive additive such as aluminum. Then, the leads 60 and 62 are welded to the positive electrode current collector 22 and the negative electrode current collector 32, respectively, by a known method, and the separator 10 is placed between the positive electrode active material layer 24 of the positive electrode 20 and the protective film 31 of the negative electrode 30. The secondary battery 100 is manufactured by inserting it into the exterior body 50 together with the electrolytic solution in a sandwiched state and sealing the entrance of the exterior body 50.

Hereinafter, the present disclosure will be specifically described with reference to Examples. As Examples 1 to 27, a positive electrode active material layer was prepared as described below and the film quality was evaluated. In addition, a secondary battery was manufactured, and the positive electrode capacitance surface density and the rate characteristic ratio were measured. For comparison, Comparative Examples 1 to 3 were prepared and evaluated in the same manner. The results are shown in Table 3. The fluoropolymer skeleton and the pendant chain-forming compound used in Examples 1 to 27 and Comparative Examples 1 to 3 are No. 1 in Table 1, respectively. F-1 to F-2, and No. 1 in Table 2. It is shown by M-1 to M-18.

(Example 1)
[Synthesis of positive electrode binder]
The fluoropolymer skeleton (F-1) was synthesized as follows.
500 g of pure water and 0.2 g of METHOCEL® K100GR anti-precipitation agent were sequentially introduced into a 2 liter reactor equipped with a mechanical stirrer operating at a speed of 1000 rpm. The inside of the reactor was depressurized, the mixture was pressurized to 1 bar with nitrogen, 2 g of a 75% by volume t-amyl perpivalate / isododecane solution was introduced into the reactor, and then 220 g of vinylidene fluoride was introduced. Next, it was gradually heated to 55 ° C. Further, 150 ml of a 10 mass% aqueous solution of methacrylic acid was added, and the reaction was carried out at a pressure of 110 bar for 5 hours. After cooling to room temperature, the mixture was opened to atmospheric pressure and the obtained polymer was recovered. After washing with pure water and methanol in this order, the mixture was dried in an oven at 50 ° C. (yield 208 g). The fluoropolymer skeleton thus obtained contained 98.8 mol% vinylidene fluoride and 1.2 mol% methacrylic acid as measured by NMR measurement.

Next, the obtained fluoropolymer skeleton (F-1) was reacted with the pendant chain forming compound (M-2). 20 g of F-1 and 2.0 g of M-2 were dissolved in 100 ml of N-methyl-2-pyrrolidone, and 0.15 g of triphenylphosphine was further added as a catalyst. This was reacted at 80 ° C. for 2 hours. After completion of the reaction, the reaction solution was added dropwise to 1 liter of methanol to recover the precipitated polymer component. It was washed with methanol twice more. The obtained polymer component was dried in an oven at 60 ° C. (yield 19.8 g).

_{From 1H-NMR, the −CH 2} − portion (about 2.5 and 3 ppm) of the fluoropolymer skeleton of the polymer component and the oxyalkylene repeating unit of about 3.5 to 3.6 ppm derived from the pendant chain forming compound were obtained. By observing the related signals, it was confirmed that the pendant chain was introduced into the fluoropolymer skeleton.

The atomic weight ratio (number of fluorine atoms / number of oxygen atoms) in the positive electrode binder can be measured using an XPS surface analyzer. Any model can be used as the XPS surface analyzer, but in this embodiment, ESCALAB-200R manufactured by VG Scientific Co., Ltd. was used. The (number of fluorine atoms / number of oxygen atoms) was 45.

[Preparation of positive electrode active material layer and positive electrode]
90% by mass of sulfur-modified polyacrylonitrile (trade name: SPAN (registered trademark), manufactured by ADEKA) as the positive electrode active material, 5% by mass of Ketjen Black as the positive electrode conductive aid, and 5% by mass of the above positive electrode binder as N-methyl-2. -The coating solution was adjusted by dispersing in pyrrolidone. The total solid content concentration in the coating liquid was 35% by mass. Next, the coating liquid was applied onto an aluminum foil which is a positive electrode current collector with a doctor blade having a gap of 450 μm, and vacuum dried at 100 ° C. for 2 hours to form a positive electrode active material layer 24. The thickness of the positive electrode active material layer after drying was measured with a contact film thickness meter and found to be 163 μm.

[Membrane quality evaluation]
Visually observe the state of the positive electrode active material layer 24, and if there are no cracks or peeling from the positive electrode current collector, 〇, if some cracks are seen but there is no peeling from the positive electrode current collector, △, cracks or When there was peeling from the positive electrode current collector, it was evaluated as x. The results are shown in Table 3.

[Preparation of negative electrode]
A lithium foil having a thickness of 20 μm was crimped onto a copper foil having a thickness of 10 μm to prepare a φ13 size negative electrode 30.

[Adjustment of electrolyte]
LiN (SO) as a lithium salt in a solvent obtained by mixing dimethyl carbonate and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether as a fluorinated ether in a volume ratio of 80:20. ₂ F) The _{electrolytic solution was adjusted by dissolving 2} to a concentration of 4.2 mol / L.

[Making secondary batteries]
A polypropylene separator having a thickness of 13 μm was prepared, and a separator 10 having a size of φ15 was prepared. The positive electrode 20, the separator 10, and the negative electrode 30 were stacked in this order and placed on the outer body of the coin cell. Further, the above electrolytic solution was added and sealed to prepare a secondary battery 100.

[Measurement of positive electrode capacitance surface density]
The positive electrode capacitance surface density was measured by the procedure shown below. At 25 ° C., the battery was discharged to a final voltage of 1.0 V with a constant current corresponding to 0.1 C, and then charged to 3.0 V with a constant current corresponding to 0.1 C. Further, the discharge is performed to a final voltage of 1.0 V with a constant current corresponding to 0.1 C, and the discharge capacity at that time (discharge capacity at 0.1 C) is divided by the positive electrode area to obtain the positive electrode capacitance surface density (mAh / cm). ² ) was calculated. The results are shown in Table 3.
[Rate characteristic ratio measurement]
The rate characteristic ratio was measured according to the procedure shown below. After measuring the positive electrode capacitance surface density, the battery was charged again to 3.0 V with a constant current corresponding to 0.1 C. Further, the battery was discharged to a final voltage of 1.0 V with a constant current corresponding to 1.0 C, and the discharge capacity at that time was defined as the discharge capacity at 1.0 C. The rate characteristic ratio was calculated by dividing the discharge capacity at 1.0 C by the discharge capacity at 0.1 C. As the value, the closer it is to 1, the more preferable. The results are shown in Table 3.

(Example 2)
The procedure was the same as in Example 1 except that the pendant chain forming compound was changed to M-3.

(Example 3)
In the synthesis of the fluoropolymer skeleton, the same procedure as in Example 1 was carried out except that the methacrylic acid was changed to hydroxyethyl acrylate and the pendant chain forming compound was changed to M-6.

(Example 4)
The procedure was the same as in Example 3 except that the pendant chain forming compound was changed to M-7.

(Example 5)
The procedure was the same as in Example 1 except that the pendant chain forming compound was changed to M-4.

(Example 6)
The procedure was the same as in Example 3 except that the pendant chain forming compound was changed to M-8.

(Example 7)
The same procedure as in Example 1 was carried out except that the pendant chain forming compound was changed to M-10 and the catalyst for reacting the fluoropolymer skeleton (F-1) with M-10 was changed to trihexylamine. rice field.

(Example 8)
The procedure was the same as in Example 7 except that the pendant chain forming compound was changed to M-11.

(Example 9)
The procedure was the same as in Example 7 except that the pendant chain forming compound was changed to M-12.

(Example 10)
The procedure was the same as in Example 7 except that the pendant chain forming compound was changed to M-13.

(Example 11)
The procedure was the same as in Example 1 except that the pendant chain forming compound was changed to M-14.

(Example 12)
The procedure was the same as in Example 1 except that the pendant chain forming compound was changed to M-15.

(Example 13)
The procedure was the same as in Example 7 except that the pendant chain forming compound was changed to M-16.

(Example 14)
The procedure was the same as in Example 7 except that the pendant chain forming compound was changed to M-17.

(Example 15)
The procedure was the same as in Example 7 except that the pendant chain forming compound was changed to M-18.

(Example 16)
The same procedure as in Example 1 was carried out except that the positive electrode active material and the positive electrode were prepared by the methods shown below. The results are shown in Table 3.

[Preparation of positive electrode active material layer and positive electrode]
Poly (carbon disulfide) was synthesized as the positive electrode active material as follows. 1.7 g of sodium (50% in paraffin) was washed with hexane. The sodium was added to 50 ml DMSO and stirred until the sodium was dissolved. Next, the same mole of carbon disulfide was added dropwise to DMSO, and the mixture was reflux-heated in a nitrogen atmosphere for 48 hours. When the sodium dicarbon disulfide salt was dissolved in water and acidified with 6N HCl, a dark brown precipitate was formed. The reaction mixture was stirred at room temperature for 24 hours. The solution was then concentrated to 1/4 of its original volume. The precipitate was then separated, washed with water and acetone, and the resulting solid was vacuum dried at 50 ° C. for 24 hours.

Next, 85% by mass of the above positive electrode active material, 5% by mass of Ketjen Black as a positive electrode conductive auxiliary agent, and 10% by mass of the positive electrode binder used in Example 1 were dispersed in N-methyl-2-pyrrolidone. , The coating liquid was adjusted. The total solid content concentration in the coating liquid was 35% by mass. Next, the coating liquid was applied onto an aluminum foil which is a positive electrode current collector with a doctor blade having a gap of 500 μm, and vacuum dried at 100 ° C. for 2 hours to form a positive electrode active material layer 24. From this, a φ12 size positive electrode 20 was produced.

(Example 17)
The same procedure as in Example 16 was carried out except that the positive electrode active material was changed to the tetrathionaphthalene polymer shown below. The results are shown in Table 3.

[Synthesis of positive electrode active material]
A tetrathionaphthalene polymer was synthesized as a positive electrode active material as follows. 15 g of octachloronaphthalene and 6 g of sulfur were placed in a 200 ml flask, and the temperature was raised to 310 ° C. under a nitrogen stream. After holding at 310 ° C. for about 20 minutes, it was allowed to cool. After cooling to room temperature, refluxing and washing with carbon disulfide were repeated twice. After filtration, vacuum drying was performed. 1.1 g of the obtained solid, 0.27 g of sulfur and 0.85 g of sodium carbonate were pulverized and mixed, and placed in a heat-resistant glass tube. The inside of this tube was depressurized using a vacuum pump and then sealed with a burner as it was to prepare a reaction tube. A ribbon heater was wrapped around this sealed tube, the temperature was raised to 320 ° C., and after 12 hours, the mixture was allowed to cool. The black-brown powder in the sealed tube was refluxed and washed with distilled water, acetone, and hot DMF until the filtrate was no longer colored. After filtration, vacuum drying was performed to obtain a tetrathionaphthalene polymer.

(Example 18)
The same procedure as in Example 16 was carried out except that the positive electrode active material was changed to the sulfur-modified polyacrylonitrile used in Example 1, the ratio of the positive electrode binder was changed to 2% by mass, and the ratio of the positive electrode active material was changed to 93% by mass. .. The results are shown in Table 3.

(Example 19)
The same procedure as in Example 18 was carried out except that the ratio of the positive electrode binder was changed to 3% by mass and the ratio of the positive electrode active material was changed to 92% by mass. The results are shown in Table 3.

(Example 20)
The same procedure as in Example 18 was carried out except that the ratio of the positive electrode binder was changed to 10% by mass and the ratio of the positive electrode active material was changed to 85% by mass. The results are shown in Table 3.

(Example 21)
The same procedure as in Example 18 was carried out except that the ratio of the positive electrode binder was changed to 15% by mass and the ratio of the positive electrode active material was changed to 80% by mass. The results are shown in Table 3.

(Example 22)
The same procedure as in Example 18 was carried out except that the ratio of the positive electrode binder was changed to 18% by mass and the ratio of the positive electrode active material was changed to 77% by mass. The results are shown in Table 3.

(Example 23)
The procedure was the same as in Example 20 except that the gap of the doctor blade when the coating liquid was applied onto the aluminum foil which is the positive electrode current collector was changed to 420 μm. The results are shown in Table 3.

(Example 24)
The same procedure as in Example 23 was carried out except that the gap of the doctor blade when the coating liquid was applied onto the aluminum foil which is the positive electrode current collector was changed to 450 μm. The results are shown in Table 3.

(Example 25)
The same procedure as in Example 23 was carried out except that the gap of the doctor blade when the coating liquid was applied onto the aluminum foil which is the positive electrode current collector was changed to 600 μm. The results are shown in Table 3.

(Example 26)
The same procedure as in Example 23 was carried out except that the gap of the doctor blade when the coating liquid was applied onto the aluminum foil which is the positive electrode current collector was changed to 670 μm. The results are shown in Table 3.

(Example 27)
The same procedure as in Example 23 was carried out except that the gap of the doctor blade when the coating liquid was applied onto the aluminum foil which is the positive electrode current collector was changed to 700 μm. The results are shown in Table 3.

(Comparative Example 1)
The procedure was the same as in Example 1 except that the pendant chain forming compound was changed to M-1. The results are shown in Table 3.

(Comparative Example 2)
The procedure was the same as in Example 3 except that the pendant chain forming compound was changed to M-5. The results are shown in Table 3.

(Comparative Example 3)
The procedure was the same as in Example 3 except that the pendant chain forming compound was changed to M-9. The results are shown in Table 3.

From the results of Examples 1 to 27 and Comparative Examples 1 to 3, the positive electrode binder has a fluoropolymer skeleton and a pendant chain bonded to the skeleton, and has (number of fluorine atoms / number of oxygen atoms). It can be seen that by setting the value to 5 to 60, a good positive electrode active material layer without cracks or peeling from the positive electrode current collector, and a positive electrode capacitance surface density and rate characteristic ratio can be obtained.

10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode current collector, 24 ... Positive electrode active material layer, 30 ... Negative electrode, 31 ... Protective film, 32 ... Negative electrode current collector, 34 ... Negative electrode active material layer, 40 ... Laminated body, 50 ... exterior, 60, 62 ... leads, 100 ... secondary battery

Claims (6)

Positive electrode active material and
Positive electrode conductive auxiliary agent and
Including positive electrode binder,
The positive electrode active material contains an active material containing a carbon-sulfur bond and contains.
The positive electrode binder includes a fluoropolymer skeleton and a pendant chain bonded to the skeleton.
A positive electrode active material layer having an atomic weight ratio (number of fluorine atoms / oxygen atoms) in the positive electrode binder of 5 to 60. The ratio of the positive electrode binder in the positive electrode active material layer is 3 to 15% by mass.
The positive electrode active material layer according to claim 1. The thickness of the positive electrode active material layer is 151 to 300 μm.
The positive electrode active material layer according to any one of claims 1 and 2. The positive electrode active material is sulfur-modified polyacrylonitrile.
The positive electrode active material layer according to any one of claims 1 to 3. The positive electrode active material layer according to any one of claims 1 to 4,
Positive electrode current collector and positive electrode including. The positive electrode according to claim 5 and
With the negative electrode
Separator and
A secondary battery containing an electrolytic solution.

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