JP7400580B2 - Positive electrode active material layer, positive electrode and secondary battery using the same - Google Patents

Description

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

A lithium ion secondary battery is a secondary battery that has a high charge/discharge capacity and can achieve high output. Lithium ion secondary batteries are currently used mainly as a power source for portable electronic devices, and are also expected to be used as a power source for electric vehicles, which are expected to become popular in the future. However, when used in these applications, particularly when used as a power source for automobiles, cost reduction and space saving are required. Further, for use in portable electronic devices, which are currently the main application, there is a demand for further reductions in size and weight.

Lithium ion secondary batteries currently in use mainly use lithium metal oxides such as cobalt and nickel as positive electrode active materials. On the other hand, attempts have also been made to use sulfur as a positive electrode active material. Sulfur is a resource-rich and inexpensive material, and theoretically has the highest electric capacity (1675 mAh/g) among known positive electrode active materials. It is said to have about six times the electrical capacity of the currently mainstream lithium cobalt oxide cathode 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 charging and discharging process is soluble in the non-aqueous electrolyte for lithium ion secondary batteries. Therefore, when sulfur is used as a positive electrode active material, there is a problem that the above compound is eluted into the electrolytic solution, causing the positive electrode to gradually deteriorate and the battery capacity to decrease.

Sulfur-based polymer positive electrodes using tetrathionaphthalene polymers or sulfur-modified polyacrylonitrile as active materials have been proposed as an attempt to suppress the above-mentioned elution, although the capacitance is slightly inferior to that of sulfur (Patent Document 1, (see 2).

Generally, the above-mentioned positive electrode is made by applying a slurry containing a positive electrode active material, a conductive additive, and a binder to improve the binding properties between the current collector and the active material and between the active materials onto the current collector, and then drying the slurry. It is created by Due to the chemical potential of the sulfur element, the electrode potential is low and the cell voltage remains at around 2V (current lithium ion batteries are around 3.6 to 3.8V), so 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 effective means for achieving this is to increase the thickness of the positive electrode active material layer, in other words, to improve the positive electrode capacity areal density. However, increasing the thickness causes cracks to occur and separation from the positive electrode current collector. In this case, although there is room for improvement by increasing the amount of binder, the concentration of the positive electrode active material in the entire positive electrode decreases, and as a result, high capacity cannot be achieved at present.

Japanese Patent Application Publication No. 2008-66125

WO2011/129103 publication

In the positive electrode active material layers described in Patent Documents 1 and 2, it has been difficult to improve the positive electrode capacity areal density by increasing the film thickness. The present disclosure has been made in view of the above problems, and aims to provide a positive electrode active material layer that has an excellent positive electrode capacity areal 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 aspect of the present disclosure, the positive electrode active material includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder, and 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 bonded to the skeleton, and the atomic weight ratio (number of fluorine atoms/number of oxygen atoms) in the binder is 5 to 60. layers are provided.

According to this viewpoint, the extremely high interaction between the positive electrode binder and the carbon-sulfur bonds on the surface of the positive electrode active material reduces the probability of aggregation of the positive electrode active materials, leading to the occurrence of cracks due to thicker films and the occurrence of positive electrode aggregation. Peeling from the electric body can be suppressed, and a positive electrode active material layer with excellent capacitance areal density can be realized.

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

According to this viewpoint, the positive electrode capacity areal density is further improved.

A positive electrode active material layer is provided, wherein the thickness of the positive electrode active material layer is 151 to 300 μm.

According to this viewpoint, the positive electrode capacity areal density is further improved.

A positive electrode active material layer is provided, wherein the positive electrode active material is sulfur-modified polyacrylonitrile.

According to this viewpoint, it becomes possible to realize a positive electrode active material layer with excellent positive electrode capacity areal density and rate characteristic ratio.

A positive electrode is provided that includes any of the positive electrode active material layers described above and a positive electrode current collector.

According to this viewpoint, a positive electrode with excellent positive electrode capacity areal density can be realized.

A secondary battery is provided that includes the positive electrode, negative electrode, separator, and electrolyte described above.

According to this viewpoint, a secondary battery with excellent positive electrode capacity areal density can be realized.

As explained above, according to the present disclosure, the positive electrode capacity areal density of a positive electrode active material layer, a positive electrode using the same, and a secondary battery are significantly improved.

FIG. 1 is a cross-sectional view showing a secondary battery according to an embodiment of the present disclosure.

Preferred embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Note that, in this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals and redundant explanation will be omitted.

[Configuration of secondary battery]
First, the configuration of a secondary battery 100 according to this embodiment will be described based on FIG. 1.

Secondary battery 100 includes a positive electrode 20, a negative electrode 30, and a separator layer 10. The form of secondary battery 100 is not particularly limited. That is, the secondary battery 100 may be cylindrical, prismatic, laminated, button-shaped, or the like.

The positive electrode 20 includes 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 made of aluminum, copper, nickel, stainless steel, or an alloy of these materials 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 that can reversibly absorb and release alkali metal ions such as lithium, sodium, and potassium can be used.

For example, poly(carbon disulfide) of (CSx)n (in the formula, x is a value of 1.2 to 2.3, and n is a value of 2 or more) described in JP-A-7-29599, Examples include the sulfur-atom crosslinked polymer of tetrathionaphthalene described in JP-A-2008-66125, and the sulfur-modified polyacrylonitrile described in WO2011/129103. Among these, sulfur-modified polyacrylonitrile is preferred from the viewpoint of rate characteristic ratio.

Further, the positive electrode active material layer 24 may include a positive electrode conductive additive. Examples of positive electrode conductive additives include carbon powder such as carbon black, carbon nanotubes, carbon materials, metal fine powder such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powder, and conductive oxides such as ITO. can be mentioned. If sufficient conductivity can be ensured with the positive electrode active material alone, the positive electrode active material layer 24 does not need to contain a positive electrode conductive aid.

Further, the positive electrode active material layer 24 includes 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 to the positive electrode current collector 22 . The positive electrode binder has a fluoro copolymer skeleton and a pendant chain bonded to the skeleton.

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

The present disclosers estimate the mechanism of this embodiment as follows. Cathode active materials having carbon-sulfur bonds are extremely likely to aggregate due to surface interactions between the cathode active materials. Therefore, in the process of forming the positive electrode active material layer, the aggregation progresses as the solvent evaporates, and eventually cracks occur due to volumetric contraction, and eventually the positive electrode active material peels off from the positive electrode current collector. This becomes more noticeable as the probability of aggregation of positive electrode active materials increases due to thickening of the film.

The above problem can be solved by setting the atomic weight ratio (number of fluorine atoms/number of oxygen atoms) in the positive electrode binder to 5 to 60, which causes a very high interaction with the carbon-sulfur bond on the surface of the positive electrode active material, which causes the positive electrode active materials to interact with each other. Aggregation can be suppressed. As a result, generation of cracks and peeling from the positive electrode current collector due to thickening of the film can be effectively prevented. If the atomic weight ratio (number of fluorine atoms/number of oxygen atoms) is less than 5, the positive electrode binders will aggregate due to interaction between oxygen atoms, and the above effect will not be achieved. Moreover, when it exceeds 60, interaction with the positive electrode active material is lost due to the high surface energy of fluorine atoms, and the above effects are not achieved.

Note that even if the ratio is 5 to 60, it is a simple blend of a polymer consisting of a fluoro-based copolymer skeleton having no pendant chains and a polymer corresponding to the pendant chains (for example, a polyoxyalkylene polymer). In this case, phase separation occurs between the two, and the above effect cannot be obtained.

The fluoro-based copolymer skeleton of this embodiment includes a copolymer of a fluorinated monomer and a (meth)acrylic monomer.

Examples of fluorinated monomers include C3 to C8 perfluoroolefins such as tetrafluoroethylene and hexafluoropropene, and C2 to C8 hydrogen-containing fluorocarbons such as vinylidene fluoride, vinyl fluoride, 1,2-difluoroethylene and trifluoroethylene. _Olefins , chloro- _and / _or bromo- and/or iodo-C2-C6 fluoroolefins, or bromo- and/or iodo-C2-C6 fluoroolefins of the formula CF ₂ =CFOR _b , where R _b is a C1-C6 fluoro- or perfluoroalkyl, such as CF ₃ , C ₂ F ₅ , C _{( per} )fluoro-oxyalkyl vinyl ethers represented by the formula CF ₂ =CFOR _c (wherein R _c is C1-C12 alkyl, or C1- C12 oxyalkyl or C1-C12 (per)fluorooxyalkyl having one or more ether groups, such as perfluoro-2-propoxy-propyl), or of the formula CF ₂ =CFOCF ₂ OR _d (formula wherein R _d represents 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 A (per)fluoroalkyl vinyl ether represented by a C1-C6 (per)fluorooxyalkyl group having an ether group, or a (per)fluoroalkyl vinyl ether of the formula CF ₂ =CFOR _e (wherein R _e is a C1-C12 alkyl or (per) fluoroalkyl, or C1-C12 oxyalkyl, or C1-C12 (per)fluorooxyalkyl having one or more ether groups, and R _e is a carboxylic acid in the form of its acid, acid halide or salt. Examples include functional (per)fluoro-oxyalkyl vinyl ethers represented by (or containing sulfonic acid), fluorodioxole, particularly perfluorodioxole, and the like.

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 (meth)acrylic monomers include acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, aminoethyl acrylate, 2-isocyanate ethyl acrylate, and glycidyl acrylate. Examples include.

Any method can be used to synthesize the copolymer of a fluorinated monomer and a (meth)acrylic monomer, and specific examples thereof include aqueous suspension polymerization and aqueous emulsion polymerization.

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

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

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. When the positive electrode binder ratio in the positive electrode active material layer is 3 to 15% by mass, a balance between film formability and positive electrode capacity areal density can be particularly ensured. When the amount is less than 3% by mass, the amount of positive electrode binder is too small, resulting in poor film-forming properties, and when it exceeds 15% by mass, the concentration of the positive electrode active material in the entire positive electrode decreases, resulting in a decrease in the positive electrode capacity areal density. Furthermore, if the positive electrode binder covers the positive electrode active material more than necessary, the rate characteristic ratio will deteriorate.

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

Further, the thickness of the positive electrode active material layer is preferably 70 μm or more, more preferably 100 μm or more, and more preferably in the range of 151 to 300 μm. Within this range, a sufficient positive electrode capacity areal density can be ensured. The positive electrode capacity surface density is preferably 5 mAh/cm ² or more. If it exceeds 300 μm, it becomes difficult for the electrolyte to penetrate into the positive electrode active material layer, and the positive electrode capacity areal density and rate characteristic ratio tend to deteriorate.

The negative electrode 30 includes 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, a thin metal plate made of aluminum, copper, nickel, stainless steel, or an alloy of these materials can be used.

The negative electrode active material layer 34 includes a negative electrode active material and, if necessary, a negative electrode binder and a negative electrode conductive aid.

As negative electrode active materials, 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, etc. Alloy based materials such as cobalt-tin can be used.

In addition, when using a material that does not contain an alkali metal as the negative electrode active material, for example, among the above negative electrode active materials, carbon-based materials, silicon-based materials, alloy-based materials, etc., the difference between the positive and negative electrodes due to the generation of dendrites. It is advantageous that short circuits are unlikely to occur.

However, when using a negative electrode active material that does not contain these alkali metals, when used in combination with the positive electrode of the present disclosure, neither the positive electrode nor the negative electrode contains an alkali metal, so the negative electrode must be pre-doped with an alkali metal. processing is required.

For example, lithium pre-doping methods include a method in which a half cell is assembled using metallic lithium as a counter electrode, and lithium is inserted by an electrolytic doping method in which lithium is electrochemically doped; Another method that can be used is a method in which lithium is inserted by a bonding pre-doping method in which the negative electrode is left in an electrolytic solution and doped by utilizing the diffusion of lithium into the negative electrode.

As the negative electrode conductive aid, the same material as the positive electrode conductive aid can be used. If sufficient conductivity can be ensured with the negative electrode active material alone, the negative electrode active material layer 34 does not need to contain the negative electrode conductive aid.

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

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

The negative electrode active material layer 34 is formed by, for example, forming a coating liquid by dispersing a negative electrode active material, a conductive aid, and a binder in a suitable organic solvent (for example, N-methyl-2-pyrrolidone), and using this coating liquid. is applied onto the negative electrode current collector 32, dried, and optionally rolled.

In addition, when using a foil-shaped negative electrode active material, the foil-shaped negative electrode active material with a thickness of 0.01 to 200 μm is pasted on the negative electrode current collector 32 to produce a negative electrode sheet.

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

Further, the above separator material may be coated with inorganic particles or a polymer component. Examples of 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, polymer electrolytes (compounds of polymer materials such as polyethylene glycol and polyethylene carbonate and lithium salt), and ion exchange resins (polydimethyldiallylammonium salt). , polystyrene sulfonate, etc.); others include polyvinyl alcohol, CMC, polybutyral, polyacrylic acid, and the like.

The nonaqueous electrolyte includes an alkali metal salt as 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 _{(SO2CF2CF3)2, LiC(SO2CF2CF3)3, LiC(SO2CF3 ) 3 , LiI , LiCl , LiF , LiPF5 ( SO2CF3 )} , _LiPF4 ( _SO2 Examples include CF ₃ ) ₂ and the like. Examples of the sodium salt include NaN(SO ₂ F) ₂ and NaN(SO ₂ CF ₃ ) ₂ . Examples of potassium salts include KN(SO ₂ F) ₂ and KN(SO ₂ CF ₃ ) ₂ .

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, 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, carbonate esters such as fluoroethylene carbonate, methyl acetate, ethyl acetate, methyl propionate, Carboxylic acid esters such as ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethyl acetate, and ethyl trimethyl acetate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain sulfonic esters such as dimethyl sulfoxide and dimethyl sulfite; Cyclic sulfonic acid esters such as sulfolane and propane sultone, nitrile compounds such as acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile, and succinonitrile, 1,2-dimethoxyethane, dimethyl ether, methyl ethyl ether , chain ethers such as diethyl ether, butyl methyl ether, dipropyl ether, cyclopentyl methyl ether, dibutyl ether, diisopentyl ether, triglyme, tetraglyme, oxetane, tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,4 - cyclic ethers such as dioxane, hydrofluoroethers such as 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, phosphoric acid esters such as triethyl phosphate, trimethyl phosphate, Phosphonate 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-decafluoro-3-methoxy-4-(trifluoromethyl)pentane and other fluorinated ethers Examples include the following. These may be used alone or in combination. From the viewpoint of solubility of the alkali metal salt, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, triglyme, tetraglyme, and acetonitrile are preferred.

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

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

Examples of anion species include 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 ((SO ₂ (CF ₂ )xF) (SO2(CF2)yF)N ^- (However, x and y are each independent and represent an integer from 0 to 5.)), etc. These anion species may be used alone or in combination.

From the viewpoint of 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, PF ₆ ^- , or BF ₄ ^- , and furthermore, ( More preferred are SO ₂ F) ₂ N ^- , (SO ₂ CF ₃ ) ₂ N ^- , and (SO ₂ CF ₃ ) (SO ₂ F) N ^- .

Note that various additives (negative electrode SEI (Solid Electrolyte Interface) forming agent, surfactant, etc.) may be added to the non-aqueous electrolyte. Examples of such additives include vinylene carbonate, vinylethylene carbonate, phenylethylene carbonate, succinic anhydride, lithium bisoxalate, lithium tetrafluoroborate, dinitrile compounds, propane sultone, butane sultone, propene sultone, 3- Examples include sulfolene, fluorinated allyl ether, fluorinated acrylate, and the like.

The exterior body 50 seals the laminate 40 and the electrolyte therein. The exterior body 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside, intrusion of moisture, etc. into the secondary battery 100 from the outside, and the like.

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

The leads 60 and 62 are made of a conductive additive material 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 the sandwiched battery into the exterior body 50 together with an electrolyte and sealing the entrance of the exterior body 50 .

Hereinafter, the present disclosure will be specifically explained with reference to Examples. As Examples 1 to 27, positive electrode active material layers were prepared as described below, and the film quality was evaluated. In addition, a secondary battery was fabricated, and the positive electrode capacity areal density and 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 fluoro-based copolymer skeletons and pendant chain-forming compounds used in Examples 1 to 27 and Comparative Examples 1 to 3 were No. 1 in Table 1, respectively. F-1 to F-2, and No. of Table 2. Shown as M-1 to M-18.

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

Next, the obtained fluoro-based copolymer skeleton (F-1) was reacted with a 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 added as a catalyst. This was reacted at 80°C for 2 hours. After the reaction was completed, the reaction solution was added dropwise to 1 liter of methanol, and the precipitated polymer component was collected. Washed twice more with methanol. The obtained polymer component was dried in an oven at 60°C (yield: 19.8 g).

1H-NMR revealed that -CH ₂ - moieties (approximately 2.5 and 3 ppm) of the fluoro-based copolymer skeleton of the polymer component and approximately 3.5 to 3.6 ppm of oxyalkylene repeating units derived from pendant chain-forming compounds. By observing related signals, it was confirmed that pendant chains were introduced into the fluoro-based copolymer 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. Although any model can be used as the XPS surface analyzer, in this example, ESCALAB-200R manufactured by VG Scientific was used. Note that (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 a positive electrode active material, 5% by mass of Ketjen black as a positive electrode conductive agent, and 5% by mass of the above positive electrode binder were combined with N-methyl-2. - The coating solution was prepared by dispersing it in pyrrolidone. The total solid content concentration in the coating liquid was 35% by mass. Next, the coating solution was applied onto an aluminum foil serving as a positive electrode current collector using 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 using a contact film thickness meter and was found to be 163 μm.

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

[Preparation of negative electrode]
A lithium foil with a thickness of 20 μm was pressure-bonded onto a copper foil with a thickness of 10 μm, and a negative electrode 30 with a size of φ13 was produced from this.

[Adjustment of electrolyte]
LiN(SO An electrolytic solution was prepared by dissolving _2F ) ₂ to a concentration of 4.2 mol/L.

[Preparation of secondary battery]
A polypropylene separator with a thickness of 13 μm was prepared, and a separator 10 with a diameter of 15 mm was produced. The positive electrode 20, the separator 10, and the negative electrode 30 were stacked in this order and placed on the exterior body of a coin cell. Furthermore, the electrolytic solution described above was added and sealed to produce a secondary battery 100.

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

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

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

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

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

(Example 6)
The same procedure as in Example 3 was carried out 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 fluoro-based copolymer skeleton (F-1) and M-10 was changed to trihexylamine. Ta.

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

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

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

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

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

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

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

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

(Example 16)
Example 1 was carried out in the same manner as in Example 1, except that the production of the positive electrode active material and the positive electrode was changed to the method 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 a 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 of DMSO and stirred until the sodium was dissolved. Next, the same mole of carbon disulfide was added dropwise to DMSO, and the mixture was heated under reflux in a nitrogen atmosphere for 48 hours. The sodium dicarbon disulfide salt was dissolved in water and acidified with 6N HCl, resulting in a dark brown precipitate. 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, by dispersing 85% by mass of the above positive electrode active material, 5% by mass of Ketjenblack as a positive electrode conductive agent, and 10% by mass of the positive electrode binder used in Example 1 in N-methyl-2-pyrrolidone. , the coating solution was prepared. The total solid content concentration in the coating liquid was 35% by mass. Next, the coating solution was applied onto an aluminum foil serving as a positive electrode current collector using 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 positive electrode 20 having a diameter of 12 mm was produced.

(Example 17)
The same procedure as in Example 16 was conducted 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 being held at 310°C for about 20 minutes, it was allowed to cool. After cooling to room temperature, refluxing and washing with carbon disulfide was 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 the mixture was placed in a heat-resistant glass tube. The pressure inside this tube was reduced using a vacuum pump, and the tube was then sealed with a burner to form a reaction tube. A ribbon heater was wrapped around this sealed tube to raise the temperature to 320° C., and after 12 hours, the tube was allowed to cool. The dark brown powder in the sealed tube was repeatedly 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 performed 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 performed 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 performed 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 same procedure as in Example 20 was carried out except that the gap of the doctor blade when coating the coating solution on the aluminum foil serving as 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 coating the coating solution on the aluminum foil serving as 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 coating the coating liquid on the aluminum foil serving as 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 coating the coating liquid onto the aluminum foil serving as 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 coating the coating solution on the aluminum foil serving as the positive electrode current collector was changed to 700 μm. The results are shown in Table 3.

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

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

(Comparative example 3)
The same procedure as in Example 3 was carried out 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 fluoro-based copolymer skeleton and a pendant chain bonded to the skeleton, and has (number of fluorine atoms/number of oxygen atoms). It can be seen that when the ratio is 5 to 60, a good positive electrode active material layer without cracking or peeling from the positive electrode current collector, and a good positive electrode capacity areal density and rate characteristic ratio can be obtained.

DESCRIPTION OF SYMBOLS 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 body, 60, 62... Lead, 100... Secondary battery

Claims (4)

a positive electrode active material;
a positive electrode conductive aid;
a positive electrode binder;
The positive electrode active material includes an active material containing a carbon-sulfur bond,
The positive electrode binder includes a fluoro-based copolymer skeleton and a pendant chain bonded to the skeleton,
the pendant chain includes an oxyalkylene,
The atomic weight ratio (number of fluorine atoms/number of oxygen atoms) in the positive electrode binder is 5 to 60,
The thickness of the positive electrode active material layer is 151 to 300 μm,
The positive electrode binder ratio in the positive electrode active material layer is 3 to 15% by mass,
Cathode active material layer. the positive electrode active material is sulfur-modified polyacrylonitrile;
The positive electrode active material layer according to claim 1 . A positive electrode active material layer according to claim 1 or 2 ,
A positive electrode including a positive electrode current collector. A positive electrode according to claim 3 ;
a negative electrode;
separator and
A secondary battery containing an electrolyte.

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