JP5183680B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery.

  In recent years, as one of new secondary batteries with high output and high energy density, a lithium secondary battery using a non-aqueous electrolyte and charging and discharging by moving lithium ions between a positive electrode and a negative electrode has been developed. It's being used.

  As such a lithium secondary battery, a secondary battery using a material alloyed with lithium as a negative electrode active material has been studied. However, the volume of the active material alloyed with lithium expands / contracts when lithium is occluded / released. For this reason, the active material is pulverized with charge / discharge, or the active material is peeled off from the current collector, so that there is a problem that the current collection property is lowered and the charge / discharge cycle characteristics are deteriorated.

  The present applicant has proposed a negative electrode for a lithium secondary battery in which an active material layer made of a silicon material and a binder is provided on the surface of a current collector made of a conductive metal foil and sintered in a non-oxidizing atmosphere. doing. In this negative electrode, it has been found that the active material layer is in close contact with the current collector, and thus exhibits good charge / discharge cycle characteristics (Japanese Patent Application No. 2000-401501).

  In addition, the present applicant uses an amorphous or microcrystalline silicon thin film formed by a sputtering method or a CVD method on a current collector made of a conductive metal foil as a negative electrode active material. It has been found that cycle characteristics can be obtained (Patent Document 1).

International Publication No. 01/31720 Pamphlet

  However, even in such a negative electrode for a lithium secondary battery, the volume of the active material expands and contracts when the active material absorbs and releases lithium, so that the active material layer cracks, and the contact resistance in the active material layer As a result, there is a problem that the current collecting property in the electrode is lowered, and the charge / discharge cycle characteristics are lowered.

  The objective of this invention is providing the lithium secondary battery which can improve the current collection property in an electrode, and can improve charging / discharging cycling characteristics, and its manufacturing method.

  The present invention is a lithium secondary battery comprising a negative electrode having an active material layer containing an active material that electrochemically occludes / releases lithium on a current collector, a positive electrode, and a non-aqueous electrolyte. It is characterized in that a solid electrolyte is disposed inside a crack formed in the active material layer by occlusion / release.

  According to the present invention, by arranging the solid electrolyte inside the crack formed in the active material layer, the current collecting property in the electrode can be enhanced, and the charge / discharge cycle characteristics can be improved. Moreover, since it can suppress that an active material layer peels from a collector with a solid electrolyte, a charge / discharge cycle characteristic can be improved also from such a point.

  In the present invention, the whole nonaqueous electrolyte may be a solid electrolyte. Therefore, the solid electrolyte disposed inside the crack of the active material layer may be a part of the entire solid electrolyte. In the present invention, a part of the nonaqueous electrolyte may be a solid electrolyte. For example, the solid electrolyte may be disposed only inside the crack of the active material layer, and the other part may be a liquid electrolyte.

  Examples of the solid electrolyte in the present invention include a solid electrolyte formed by combining a polymer and an electrolytic solution containing a lithium salt into a gel. That is, a gel polymer solid electrolyte in which an electrolyte containing a lithium salt is held by a polymer can be used. Such a solid polymer electrolyte is excellent in adhesion to the active material, and the charge / discharge cycle characteristics are particularly excellent because the adhesion to the active material is not lowered even by charge / discharge. Examples of the polymer that holds the electrolytic solution include a polyether solid polymer, a polycarbonate solid polymer, and a polyacrylonitrile solid polymer. Moreover, the copolymer which consists of 2 or more types of these polymer monomers, and the polymer which bridge | crosslinked 2 or more types of these polymers are mentioned.

  Other polymers that hold the electrolyte include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymers, and fluorine-based polymers such as polytetrafluoroethylene, polyamide-based polymers, polyimide-based polymers, Examples include imidazole polymers, polyoxazole polymers, polymelamine formaldehyde polymers, polypropylene polymers, and polysiloxane polymers.

  Further, the solid electrolyte of the present invention may be a completely solid electrolyte having Li ion conductivity. Examples of such a completely solid electrolyte include those composed of a lithium salt and a polymer. Examples of the polymer constituting such a complete solid electrolyte include polyether polymers, polysiloxane polymers, and polyphosphazene polymers.

  In the present invention, the surface roughness Ra of the surface of the current collector provided with the active material layer is preferably 0.2 μm or more. By using the current collector having such a surface roughness Ra, the contact area between the active material layer and the current collector is increased, so that the adhesion between the active material layer and the current collector is improved. In addition, when the binder is included in the active material layer, since the binder enters the uneven portions on the surface of the current collector, an anchor effect is exhibited between the binder and the current collector, so that even better adhesion is obtained. . For this reason, peeling of the active material from the current collector due to the charge / discharge reaction can be suppressed. When the active material layers are provided on both sides of the current collector, the surface roughness Ra on both sides of the current collector is preferably 0.2 μm or more.

  Moreover, it is preferable that the average space | interval S of the local peak on the surface of a collector has the relationship of said surface roughness Ra and 100Ra> = S. The surface roughness Ra and the average interval S between the local peaks are defined in Japanese Industrial Standards (JIS B 0601-1994), and can be measured by, for example, a surface roughness meter.

  In order to make the surface roughness Ra of the current collector surface 0.2 μm or more, a surface roughening treatment may be performed. Examples of such roughening treatment include a plating method, a vapor phase growth method, an etching method, and a polishing method. The plating method and the vapor phase growth method are methods for roughening the surface by forming a thin film layer having irregularities on the surface on a current collector. Examples of the plating method include an electrolytic plating method and an electroless plating method. Examples of the vapor phase growth method include a sputtering method, a CVD method, and a vapor deposition method. Examples of the etching method include a physical etching method and a chemical etching method. Examples of the polishing method include sandpaper polishing and blasting.

  The current collector used in the present invention is preferably a conductive metal foil. Examples of such metal foil include metals such as copper, nickel, iron, titanium, cobalt, and alloys made of combinations thereof. In particular, those containing a metal element that easily diffuses into the active material are preferable. When the active material is silicon or the like, the copper element is easily diffused, and therefore a metal foil containing the copper element is preferably used. Therefore, copper foil and copper alloy foil are preferably used.

  For the purpose of improving the adhesion between the current collector and the active material layer, a metal foil having a layer containing a copper element on the surface of the current collector in contact with the active material layer may be used as the current collector. As such a thing, what formed the copper or copper alloy layer on the surface of the metal foil which consists of metal elements other than copper is mentioned. Examples of the metal foil in which the surface roughness Ra of the copper layer or the copper alloy layer is 0.2 μm or more include a copper or copper alloy plating film formed on the metal foil by an electrolytic plating method. Specific examples include an electrolytic copper foil in which a copper plating film is formed on a copper foil by an electrolytic plating method, and a copper plating film formed on the surface of a nickel foil.

  In the present invention, the thickness of the current collector is not particularly limited, but is preferably in the range of 10 to 100 μm.

  Further, the upper limit of the surface roughness Ra of the current collector is not particularly limited, but since the thickness of the current collector is preferably in the range of 10 to 100 μm, the surface roughness Ra is substantially reduced. The upper limit is preferably 10 μm or less.

  In the present invention, the thickness X of the active material layer preferably has a relationship of 5Y ≧ X and 250Ra ≧ X with the thickness Y and surface roughness Ra of the current collector. When the thickness X of the active material layer is larger than 5Y or 250Ra, the volume of the active material layer during the charge / discharge reaction is greatly expanded and contracted, and therefore the adhesion between the active material layer and the current collector due to the unevenness of the current collector surface. May not be maintained, and the active material layer may peel from the current collector.

  The thickness X of the active material layer is preferably 100 μm or less, and more preferably in the range of 10 to 100 μm.

  The active material layer in the present invention may be an active material layer provided on a current collector by binding active material particles with a binder, or the active material is deposited on the current collector in the form of a thin film. It may be an active material layer formed as described above.

  When the active material layer is a layer containing active material particles and a binder, a slurry containing active material particles and a binder is applied to the current collector and then sintered together with the current collector in a non-oxidizing atmosphere. It is preferable that they are formed. In such a case, it is preferable that the binder remains without being completely decomposed even after the heat treatment for sintering. Since the binder remains without being decomposed even after the heat treatment, in addition to the improvement of the adhesion due to the sintering between the active material particles and the current collector and between the active material particles, the binding force by the binder is also added, and these adhesions are added. The sex can be further enhanced. In addition, when a current collector having a surface roughness Ra of 0.2 μm or more is used, as described above, the binder enters an uneven portion on the surface of the current collector, thereby anchoring between the binder and the current collector. The effect is exhibited and the adhesion can be further improved. For this reason, peeling of the active material from the current collector due to expansion and contraction of the volume of the active material at the time of occlusion and release of lithium can be suppressed, and good charge / discharge cycle characteristics can be obtained.

  The binder in the present invention preferably contains polyimide. Polyimide can also be obtained by heat-treating polyamic acid. The polyamic acid is dehydrated and condensed by heat treatment to produce polyimide. In the present invention, a polyimide having an imidization ratio of 80% or more is preferable. When the imidation ratio of the polyimide is less than 80%, the adhesion between the active material particles and the current collector may be deteriorated. Here, the imidization rate is the mol% of the produced polyimide with respect to the polyimide precursor. Those having an imidization ratio of 80% or more can be obtained, for example, by heat-treating a polyamic acid N-methyl-2-pyrrolidone (NMP) solution at a temperature of 100 to 400 ° C. for 1 hour or more. When heat treatment is performed at 350 ° C., the imidization rate is 80% after about 1 hour, and the imidation rate is 100% after about 3 hours. In the present invention, it is preferable that the binder remains without being completely decomposed even after the heat treatment for sintering. Therefore, when polyimide is used as the binder, the sintering is performed at 600 ° C. or less at which the polyimide is not completely decomposed. It is preferable.

  The amount of the binder is preferably 5% by weight or more of the total weight of the active material layer, and the volume occupied by the binder is preferably 5% or more of the total volume of the active material layer. When the amount of the binder is less than these ranges, the amount of the binder with respect to the active material particles is too small, and the adhesion in the electrode by the binder may be insufficient. In addition, when the amount of the binder is excessively increased, the resistance in the electrode increases, so that initial charging may be difficult. Therefore, the binder amount is preferably 50% by weight or less of the total weight of the active material layer, and the volume occupied by the binder is preferably 50% or less of the total volume of the active material layer.

  The negative electrode active material in the present invention is not particularly limited as long as it is an active material that occludes and releases lithium electrochemically, but a material that occludes lithium by alloying lithium is preferably used. Examples of such materials include silicon, germanium, tin, lead, zinc, magnesium, sodium, aluminum, potassium, indium, and alloys of these metals. In particular, silicon, tin, germanium, aluminum, and alloys of these metals are preferably used. Among these, silicon is particularly preferably used because of its large theoretical capacity. Examples of silicon alloys include solid solutions of silicon and one or more other elements, intermetallic compounds of silicon and one or more other elements, and eutectic alloys of silicon and one or more other elements. It is done. Examples of the method for producing the alloy include an arc melting method, a liquid quenching method, a mechanical alloying method, a sputtering method, a chemical vapor deposition method, and a firing method. In particular, examples of the liquid quenching method include a single roll quenching method, a twin roll quenching method, and various atomizing methods such as a gas atomizing method, a water atomizing method, and a disk atomizing method.

  In the present invention, the surface of the active material particles may be coated with other metals. Examples of the coating method include an electroless plating method, an electrolytic plating method, a chemical reduction method, a vapor deposition method, a sputtering method, and a chemical vapor deposition method. The metal covering the particle surface is preferably the same metal as the metal used as the current collector. By covering the same metal as the current collector, the bonding property with the current collector during sintering is greatly improved, and further excellent charge / discharge cycle characteristics can be obtained.

  The average particle diameter of the active material particles is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less in order to produce effective sintering. As the particle size of the active material particles is smaller, better cycle characteristics tend to be obtained.

  In the present invention, conductive powder may be mixed in the active material layer. By mixing the conductive powder with the slurry containing the active material particles and the binder, an active material layer containing the conductive powder can be formed. By mixing the conductive powder, a conductive network of the conductive powder is formed around the active material particles, so that the current collecting property in the electrode can be further improved. As the conductive powder, a material similar to that of the current collector can be preferably used. Specifically, an alloy made of a metal such as copper, nickel, iron, titanium, cobalt, or a combination thereof, or a mixture thereof can be given. In particular, copper powder is preferably used, and conductive carbon powder can also be preferably used.

  Similarly to the average particle diameter of the active material particles, the average particle diameter of the conductive powder is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less. It is.

  Sintering in a non-oxidizing atmosphere in the present invention is performed in, for example, a nitrogen atmosphere or an inert atmosphere such as argon. You may carry out in reducing environment, such as hydrogen atmosphere. The heat treatment temperature at the time of sintering is preferably a temperature not higher than the melting points of the current collector and the active material particles. For example, when using copper foil as the current collector, it is preferably 1083 ° C. or lower, which is the melting point of copper, more preferably in the range of 200 to 500 ° C., and more preferably in the range of 300 to 400 ° C. is there. As a sintering method, a discharge plasma sintering method or a hot press method may be used.

  Moreover, it is preferable to roll the active material layer together with the current collector after forming the active material layer on the current collector and before sintering. By such rolling, the packing density in the active material layer can be increased, and the adhesion between the active material particles in the active material layer and the adhesion between the active material particles and the current collector can be enhanced. Therefore, the charge / discharge cycle characteristics can be further improved by rolling.

  As described above, the active material layer in the present invention may be formed by depositing an active material in the form of a thin film on a current collector. Examples of methods for forming a thin film of active material include sputtering, chemical vapor deposition (CVD), vapor deposition, thermal spraying, electrolytic plating, and electroless plating. As the active material formed as a thin film, silicon, tin, germanium, aluminum and alloys containing these metals are particularly preferable. Among these, silicon is particularly preferably used. When silicon is used, it is preferably deposited in the form of an amorphous silicon thin film and a microcrystalline silicon thin film.

  As described above, the solid electrolyte in the present invention is preferably a gel obtained by combining a polymer and an electrolytic solution containing a lithium salt. The solvent of the electrolytic solution is not particularly limited, but is a mixed solvent of cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, and chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate. Is exemplified. Further, mixed solvents of the above cyclic carbonate and ether solvents such as 1,2-dimethoxyethane and 1,2-diethoxyethane are also exemplified.

The lithium salts include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F _{9 SO 2), LiC (CF 3} SO 2) 3, LiC (C 2 F 5 SO 2) 3, LiAsF 6, LiClO 4, Li 2 B 10 Cl 10, Li 2 B 12 Cl 12 and the like and mixtures thereof examples Is done. In particular, LiXF _y (wherein X is P, As, Sb, B, Bi, Al, Ga, or In, y is 6 when X is P, As, or Sb, and X is Bi, Al, Ga or a y is 4) when the in, lithium perfluoroalkyl sulfonic acid imide _{LiN (C m F 2m + 1} SO 2) (C n F 2n + 1 SO 2) ( wherein, m and n are each, independently an integer of 1 to 4) or lithium perfluoroalkyl sulfonic acid methide _{LiN (C p F 2p + 1} SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) ( In the formula, p, q and r are each independently an integer of 1 to 4, and a mixed solute is preferably used. Among these, a mixed solute of LiPF ₆ and LiN (C ₂ F ₅ SO ₂ ) ₂ is particularly preferably used.

Examples of the positive electrode material of the lithium secondary battery of the present invention include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1 O ₂ and other lithium-containing transition metal oxides. Examples thereof include metal oxides that do not contain lithium, such as MnO ₂ . In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.

  As the solid electrolyte in the present invention, a gel obtained by combining the electrolyte containing the lithium salt and the polymer as described above is preferably used. Preferably, a polymer monomer is added to the electrolytic solution, and then the monomer is polymerized to gel the electrolytic solution.

  In the production method of the present invention, a negative electrode, a positive electrode, and an electrolyte containing a lithium salt are placed in a container to prepare a temporary battery, and the temporary battery is charged and discharged to form a crack in the active material layer. After that, a polymer monomer is added to the electrolytic solution, and the monomer is polymerized to gel the electrolytic solution to obtain a solid electrolyte.

  That is, the production method of the present invention comprises a negative electrode provided with an active material layer containing an active material electrochemically absorbing and releasing lithium on a current collector, a positive electrode, and a non-aqueous electrolyte. Method of manufacturing a lithium secondary battery in which a solid electrolyte formed by combining a polymer and an electrolytic solution containing a lithium salt is placed inside a crack formed in an active material layer by occlusion / release A step of preparing a temporary battery by placing a negative electrode, a positive electrode, and an electrolyte containing a lithium salt in a container; a step of charging and discharging the temporary battery to form a crack in the active material layer; After adding a polymer monomer to the electrolytic solution in the temporary battery after discharging, the monomer is polymerized, and the electrolytic solution is gelled to obtain a solid electrolyte. A step of placing the It is characterized in.

  According to the manufacturing method of the present invention, the active material layer of the negative electrode is cracked by charging / discharging in the temporary battery in which the electrolytic solution in a state before gelation is placed. The inside of the crack formed in the active material layer is in a state in which the electrolytic solution has entered, and after adding a polymer monomer to the electrolytic solution in such a state, the monomer is polymerized, whereby the electrolytic solution Is gelled. Thereby, the electrolyte solution which has penetrated into the inside of the crack of the active material layer is gelled and becomes a solid electrolyte. Therefore, according to the manufacturing method of the present invention, the solid electrolyte can be easily disposed inside the crack formed in the active material layer.

  FIG. 1 is a cross-sectional view schematically showing one embodiment of a negative electrode according to the present invention, and shows an example in which an active material layer is formed of active material particles and a binder. As shown in FIG. 1, unevenness is formed on the surface 1 a of the current collector 1, and the active material layer 2 is provided on the surface 1 a on which the unevenness is formed. The active material layer 2 is formed from the active material particles 2a and the binder 2b, and the active material layer 2 has cracks 4 formed in the thickness direction thereof. The crack 4 is formed when the active material particles 2a of the active material layer 2 occlude / release lithium. The solid electrolyte 3 is arranged in a state of entering the crack 4. Therefore, each of the separated active material layers 2 is covered with the solid electrolyte 3 due to the crack 4.

  In the present invention, since the solid electrolyte 3 having lithium ion conductivity is provided so as to fill the crack 4 portion of the active material layer 2, the current collecting property of the electrode can be improved, and the charge / discharge cycle can be improved. The characteristics can be enhanced.

  Moreover, since the active material layer 2 is held by the mechanical strength of the solid electrolyte 3, peeling of the active material layer 2 from the current collector 1 is suppressed, and the charge / discharge cycle characteristics are also improved in this respect. be able to.

  In another aspect according to the present invention, the active material layer of at least one of the positive electrode and the negative electrode is formed by depositing an active material in the form of a thin film on a current collector, and lithium is occluded / A solid electrolyte is arranged inside the crack formed in the active material layer by being discharged.

  That is, a lithium secondary battery according to another aspect of the present invention includes a positive electrode and a negative electrode on which an active material layer containing an active material that electrochemically absorbs and releases lithium is provided on a current collector, a nonaqueous electrolyte, The active material layer of at least one of the positive electrode and the negative electrode is formed by depositing an active material in the form of a thin film on a current collector, and is activated by inserting and extracting lithium. A solid electrolyte is arranged inside a crack formed in the material layer.

  A method for manufacturing a lithium secondary battery according to another aspect of the present invention includes a positive electrode and a negative electrode on which an active material layer containing an active material that electrochemically stores and releases lithium is provided on a current collector, and a nonaqueous electrolyte The active material layer of at least one of the positive electrode and the negative electrode is formed by depositing an active material in the form of a thin film on a current collector, and lithium is occluded / released. A method of manufacturing a lithium secondary battery in which a solid electrolyte formed by combining a polymer and an electrolyte containing a lithium salt into a gel is disposed inside a crack formed in an active material layer. And a step of producing a temporary battery by putting an electrolytic solution containing a lithium salt in a container, a step of charging and discharging the temporary battery to form a crack in the active material layer, and an electrolysis in the temporary battery after charging and discharging After adding a polymer monomer to the liquid, the monomer Was polymerized by the electrolyte and by gelled solid electrolyte, it is characterized by comprising a step of a state of arranging the solid electrolyte in the interior of the cracking of the active material layer.

  FIG. 5 is a cross-sectional view schematically showing one embodiment of an electrode according to another aspect of the present invention. As shown in FIG. 5, unevenness is formed on the surface 1a of the current collector 1, and a thin film 5 as an active material layer is formed on the surface 1a on which the unevenness is formed. A crack 6 is formed in the thin film 5 in the thickness direction. The crack 6 is formed by the thin film 5 inserting and extracting lithium. The thin film 5 before the crack 6 is formed is a continuous thin film, and is a thin film having unevenness corresponding to the unevenness of the surface 1a of the current collector 1 on the surface. By inserting and extracting lithium, the volume of the thin film 5 expands and contracts. Due to the stress at this time, cracks 6 are formed in the thickness direction starting from the valleys of the irregularities on the surface of the thin film. Therefore, the thin film 5 is formed in a columnar shape by the crack 6.

  As shown in FIG. 5, the solid electrolyte 3 is disposed in the crack 6 in a state where it has entered. Accordingly, each of the thin films 5 separated by the cracks 6 is covered with the solid electrolyte 3. Therefore, since the solid electrolyte 3 having lithium ion conductivity is provided so as to fill the crack 6 portion of the thin film 5, the current collecting property of the electrode can be improved, and the charge / discharge cycle characteristics can be improved. it can.

  Moreover, since the thin film 5 is hold | maintained by the mechanical strength which the solid electrolyte 3 has, peeling from the electrical power collector 1 of the thin film 5 is suppressed, and a charge / discharge cycle characteristic can be improved also from this point.

  ADVANTAGE OF THE INVENTION According to this invention, the current collection property in an electrode can be improved and it can be set as the lithium secondary battery excellent in charging / discharging cycling characteristics.

The typical sectional view showing the state of the anode of one example according to the present invention. The top view which shows the lithium secondary battery produced in the Example according to this invention. The scanning electron micrograph which observed the negative electrode in which the crack was formed in the active material layer by charging / discharging from upper direction. The scanning electron micrograph which shows the cross section of the negative electrode by which the active material layer was cracked by charging / discharging. Typical sectional drawing which shows the state of the electrode of the other Example according to this invention. The figure which shows the charging / discharging cycle characteristic in the Example according to this invention. The scanning electron micrograph which shows the cross section of the electrode by which the crack was formed in the thin film by charging / discharging. The figure which shows the charging / discharging cycle characteristic in the Example according to this invention.

  Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range that does not change the gist thereof. It is.

(Experiment 1)
(Production of negative electrode)
8.6 wt% N-methyl-2-pyrrolidone solution containing 81.8 parts by weight of silicon powder (purity 99.9%) having an average particle diameter of 3 μm as an active material and 18.2 parts by weight of polyimide as a binder To make a negative electrode mixture slurry.

  This negative electrode mixture slurry was applied to one surface (rough surface) of an electrolytic copper foil (thickness 35 μm) having a surface roughness Ra of 0.5 μm as a current collector, dried, and then rolled. This was heat-treated at 400 ° C. for 30 hours in an argon atmosphere and sintered. The thickness of the sintered body (including the current collector) was 50 μm. Therefore, the thickness of the active material layer was 15 μm, the thickness of the active material layer / the surface roughness Ra of the current collector was 30, and the thickness of the active material layer / the thickness of the current collector was 0.43.

Moreover, in this negative electrode, the density of the polyimide was 1.1 g / cm ³ , and the volume occupied by the polyimide was 31.8% of the total volume of the active material layer containing polyimide.

[Production of positive electrode]
Using Li ₂ CO ₃ and CoCO ₃ as starting materials, weigh them so that the atomic ratio of Li: Co is 1: 1, mix them in a mortar, press this with a 17 mm diameter mold, pressurize After the molding, it was fired at 800 ° C. for 24 hours in the air to obtain a LiCoO ₂ fired body. This was pulverized in a mortar to prepare an average particle size of 20 μm.

90 parts by weight of the obtained LiCoO ₂ powder and 5 parts by weight of artificial graphite powder as a conductive agent were mixed in a 5% by weight N-methyl-2-pyrrolidone solution containing 5 parts by weight of polyvinylidene fluoride as a binder. Thus, a positive electrode mixture slurry was obtained.

  This positive electrode mixture slurry was applied on an aluminum foil as a current collector, dried, and then rolled to obtain a positive electrode.

(Preparation of electrolyte)
As an electrolytic solution, vinylene carbonate which is 5% by weight of the total weight is added to an electrolytic solution prepared by dissolving 1 mol / liter of LiPF ₆ in a solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 3: 7. It mixed and used as electrolyte solution.

[Preparation of pregel solution]
Tripropylene glycol diacrylate (molecular weight: 300) and the prepared electrolyte solution were mixed at a mass ratio of 1: 7, and 5,000 ppm of t-hexylperoxypivalate as a polymerization initiator was added thereto, and a pregel solution It was.

[Production of battery]
A positive electrode current collector tab and a negative electrode current collector tab are attached to the positive electrode and the negative electrode, respectively, and an electrode group in which a separator made of porous polyethylene is sandwiched between the positive electrode and the negative electrode, and an electrolyte solution in an aluminum laminate exterior body And a temporary battery having an outer dimension of 35 mm wide × 50 mm long × 3.5 mm thick was produced. The temporary battery was charged to 4.2 V at a current value of 50 mA, and then discharged to 2.75 V at a current value of 50 mA. Next, a pregel solution having the same weight as the electrolytic solution in the temporary battery was added to the temporary battery, and these solutions were mixed and then allowed to stand for 4 hours for homogenization. Next, the temporary battery was heated at 60 ° C. for 3 hours to gel the mixed solution of the electrolytic solution and the pregel solution, thereby preparing a battery A1. By heating the mixed solution of the electrolytic solution and the pregel solution, tripropylene glycol diacrylate, which is a polymerizable compound (monomer) in the pregel solution, is polymerized into a polymer, and the electrolytic solution is retained in the polymer network structure. Thus, a so-called gel polymer solid electrolyte is formed.

  FIG. 1 is a plan view showing the manufactured lithium secondary battery. The lithium secondary battery is closed by forming a closed portion 12 by heat-sealing the peripheral portion of the aluminum laminate outer package 11. A positive electrode current collecting tab 13 and a negative electrode current collecting tab 14 are taken out above the outer package 11. An electrode group wound with a separator made of porous polyethylene sandwiched between the positive electrode and the negative electrode is inserted into the outer package 11.

[Observation of negative electrode after charging / discharging temporary battery]
3 and 4 are scanning electron micrographs showing the state of the negative electrode after charging and discharging in the temporary battery. FIG. 3 is a photograph observed from above, and FIG. 4 is a cross-sectional view. As apparent from FIGS. 3 and 4, it is understood that cracks in the thickness direction are formed in the active material layer of the negative electrode by charging and discharging in the temporary battery. In this example, after such a crack is formed, a pregel solution is added to form a gel polymer solid electrolyte. Therefore, the gel polymer solid electrolyte is formed in a state where the gel polymer solid electrolyte is embedded in the crack of the active material layer.

(Experiment 2)
In Experiment 1, a battery B1 was manufactured in the same manner as in Experiment 1 except that charging and discharging were not performed after the provisional battery was manufactured.

  In Experiment 1, a battery B2 was produced in the same manner as in Experiment 1, except that an electrolytic solution not containing a monomer and a polymerization initiator was added instead of the pregel solution and heating was not performed.

[Evaluation of charge / discharge cycle characteristics]
Charge / discharge cycle characteristics of the batteries A1, B1, and B2 were evaluated. Each battery was charged to 4.2 V at a current value of 100 mA at 25 ° C., and then discharged to 2.75 V at a current value of 100 mA. The number of cycles to reach 80% of the discharge capacity at the first cycle was measured and defined as the cycle life. The results are shown in Table 1. The cycle life of each battery is an index with the cycle life of the battery A1 as 100.

電池Ａ１は、仮電池において充放電を行い、活物質層に割れを形成した後に、
電解液をゲル化している。従って、活物質層の割れの内部に固体電解質が配置されている。これに対し、電池Ｂ１では、仮電池において充放電を行わずに、電解液をゲル化している。従って、電解液を固体電解質とした後に、活物質層に割れが形成されているので、活物質層の割れの内部には固体電解質が配置されていない。

Battery A1 is charged and discharged in a temporary battery, and after forming a crack in the active material layer,
The electrolyte is gelled. Therefore, the solid electrolyte is disposed inside the crack of the active material layer. On the other hand, in the battery B1, the electrolytic solution is gelled without charging / discharging in the temporary battery. Therefore, since a crack is formed in the active material layer after the electrolytic solution is made a solid electrolyte, the solid electrolyte is not disposed inside the crack of the active material layer.

  In the battery B2, charging and discharging are performed in the temporary battery, but since the pregel solution is not added, the electrolytic solution is not gelled and is in a normal liquid electrolyte state.

  As is clear from the results shown in Table 1, the battery A1 according to the present invention has a longer cycle life than the battery B1. This is presumably because, in the battery A1, the solid electrolyte is disposed inside the crack of the active material layer, so that the current collection in the electrode is improved and the utilization factor of the active material is increased. In addition, since the active material layer is held by the solid electrolyte disposed inside the cracks of the active material layer, peeling of the active material layer from the current collector is suppressed, which also improves the charge / discharge cycle characteristics. It is thought that.

(Experiment 3)
Here, the influence of the surface roughness Ra of the current collector on the charge / discharge cycle characteristics was examined.

  Instead of the electrolytic copper foil having a surface roughness Ra of 0.5 μm, an electrolytic copper foil having a surface roughness Ra of 0.2 μm or an electrolytic copper foil having a surface roughness Ra of 0.17 μm was used. Thus, batteries A2 and A3 were produced.

  For these batteries, the cycle characteristics were evaluated in the same manner as described above. The cycle life is an index with the cycle life of the battery A1 as 100. Table 2 also shows the cycle life of the battery A1.

表２から明らかなように、電池Ａ１及びＡ２は、電池Ａ３に比べ、良好なサイクル特性を示している。このことから、表面粗さＲａが０．２μｍ以上の集電体を用いることにより、サイクル特性が向上することがわかる。これは、表面粗さＲａが大きい集電体を用いることにより、活物質粒子と集電体表面との接触面積が大きくなるため、焼結が効果的に起こり、活物質粒子と集電体との密着性が大きく向上したことによるものと考えられる。さらには、集電体の表面の凹凸部分に、バインダーが入り込み、バインダーと集電体の間にアンカー効果が発現したことにより、さらに密着性が向上し、電極内の集電性が向上したためであると考えられる。

As is clear from Table 2, the batteries A1 and A2 have better cycle characteristics than the battery A3. This shows that the cycle characteristics are improved by using a current collector having a surface roughness Ra of 0.2 μm or more. This is because the use of a current collector with a large surface roughness Ra increases the contact area between the active material particles and the current collector surface, so that sintering occurs effectively, and the active material particles and the current collector This is thought to be due to the significant improvement in the adhesion of the slag. Furthermore, because the binder entered the irregularities on the surface of the current collector and the anchor effect was developed between the binder and the current collector, the adhesion was further improved and the current collection in the electrode was improved. It is believed that there is.

(Experiment 4)
Here, the influence of electrode sintering conditions on cycle characteristics was examined.

  A battery A4 was produced in the same manner as in Experiment 1, except that the electrode was heat-treated at 550 ° C. for 10 hours. Further, in Experiment 1, a battery B3 was manufactured in the same manner except that the electrode was not heat-treated.

  For these batteries, the cycle characteristics were evaluated in the same manner as described above. The cycle life is an index with the cycle life of the battery A1 as 100. Table 3 also shows the cycle life of the battery A1.

表３から明らかなように、熱処理を行った電池Ａ１及び電池Ａ４は、熱処理を行わなかった電池Ｂ３に比べ、良好なサイクル特性を示している。これは、熱処理を行うことにより、活物質粒子と集電体が焼結され、活物質層と集電体との密着性が向上し、電極内の集電性が向上するためであると考えられる。

As is apparent from Table 3, the battery A1 and the battery A4 subjected to the heat treatment show better cycle characteristics than the battery B3 not subjected to the heat treatment. This is considered to be because the active material particles and the current collector are sintered by heat treatment, the adhesion between the active material layer and the current collector is improved, and the current collection in the electrode is improved. It is done.

  Further, the battery A4 subjected to the heat treatment at 550 ° C. for 10 hours has lower cycle characteristics than the battery A1 subjected to the heat treatment at 400 ° C. for 30 hours. This is presumably because the heat treatment at 550 ° C. caused the decomposition of the binder, resulting in a decrease in the adhesion within the electrode due to the binder and a decrease in current collection.

(Experiment 5)
Here, the effect of the conductive powder added to the active material layer on the cycle characteristics was examined.

  A battery A5 was produced in the same manner as in Experiment 1, except that 20% by weight of copper powder having an average particle diameter of 3 μm was added to the silicon powder.

  For this battery, the cycle characteristics were evaluated in the same manner as described above, and the results are shown in Table 4. The cycle life is an index with the cycle life of the battery A1 as 100. Table 4 also shows the cycle life of the battery A1.

表４から明らかなように、導電性粉末を添加した電池Ａ５の方が、導電性粉末を添加していない電池Ａ１に比べ、サイクル特性が向上していることがわかる。これは、導電性粉末を添加することにより、活物質粒子の周りに導電性ネットワークが形成され、活物質層内の集電性が向上したためであると考えられる。

As can be seen from Table 4, the battery A5 to which the conductive powder is added has improved cycle characteristics compared to the battery A1 to which the conductive powder is not added. This is presumably because the addition of the conductive powder formed a conductive network around the active material particles, and improved the current collection in the active material layer.

(Experiment 6)
(Production of negative electrode)
Copper was deposited on the surface of a rolled copper foil having a thickness of 18 μm by an electrolytic method to prepare a roughened copper foil (thickness 26 μm, surface roughness Ra 0.21 μm) having irregularities formed on the surface. An amorphous silicon thin film was deposited by sputtering on the surface of the roughened copper foil on which the irregularities were formed to a thickness of 5 μm. A DC pulse was supplied as power for sputtering. The sputtering conditions were DC pulse frequency: 100 kHz, DC pulse width: 1856 ns, DC pulse power: 2000 W, argon flow rate: 60 scc · BR> incense A gas pressure: 2.0 to 2.5 × 10 ⁻¹ Pa. The formation time was 146 minutes.

  The obtained silicon thin film was cut into a size of 25 mm × 25 mm together with the current collector to obtain a negative electrode.

[Production of positive electrode]
In the same manner as in Experiment 1, a positive electrode mixture slurry was prepared, applied onto an aluminum foil as a current collector, dried, rolled, cut into a size of 20 mm × 20 mm, and used as a positive electrode.

(Preparation of electrolyte)
An electrolyte solution was prepared in the same manner as in Experiment 1.

[Preparation of pregel solution]
A pregel solution was prepared in the same manner as in Experiment 1.

[Production of battery]
A temporary battery was fabricated in the same manner as in Experiment 1 using the above positive electrode and negative electrode.

  The temporary battery was charged to 4.2 V at a current value of 1.3 mA, and then discharged to 2.75 V at a current value of 1.3 mA. Next, a pregel solution having the same weight as the electrolytic solution in the temporary battery was added to the temporary battery, and after these solutions were mixed, they were left for 4 hours for homogenization. Next, the temporary battery was heated at 60 ° C. for 3 hours to gel the mixed solution of the electrolytic solution and the pregel solution, thereby preparing a battery A6. By heating the mixed solution of the electrolytic solution and the pregel solution, tripropylene glycol diacrylate, which is a polymerizable compound (monomer) in the pregel solution, is polymerized into a polymer, and the electrolytic solution is retained in the polymer network structure. Thus, a so-called gel polymer solid electrolyte is formed.

(Experiment 7)
In Experiment 6, a battery B4 was produced in the same manner as in Experiment 6 except that charging and discharging were not performed after the provisional battery was produced.

[Evaluation of charge / discharge cycle characteristics]
The batteries A6 and B4 were evaluated for charge / discharge cycle characteristics.

  Each battery was charged to 4.2 V at a current value of 1.3 mA at 25 ° C., and then discharged to 2.75 V at a current value of 1.3 mA.

  Table 5 shows the initial discharge capacity (discharge capacity at the first cycle) and the capacity retention rate at the 10th cycle. The capacity maintenance rate at the 10th cycle was calculated as follows.

Capacity maintenance ratio (%) at 10th cycle = (discharge capacity at 10th cycle) / (discharge capacity at 1st cycle) × 100
Moreover, the cycle characteristics of the charge / discharge test are shown in FIG.

図６及び表５から明らかなように、本発明に従う電池Ａ６は、比較の電池Ｂ４に比べ、充放電サイクル特性に優れていることがわかる。

As apparent from FIG. 6 and Table 5, it can be seen that the battery A6 according to the present invention is superior in charge / discharge cycle characteristics as compared with the comparative battery B4.

[Observation of negative electrode after charging / discharging temporary battery]
FIG. 7 is a scanning electron micrograph showing the state of the negative electrode after charging and discharging in the temporary battery. As is apparent from FIG. 7, cracks are formed in the thickness direction of the thin film, and the thin film is separated into columns. In this example, after such a crack is formed, a pregel solution is added to form a gel polymer solid electrolyte. Therefore, the gelled polymer solid electrolyte is embedded in the thin film crack.

(Experiment 8)
[Production of positive electrode]
The negative electrode in Experiment 6 was used as the positive electrode.

[Production of battery]
A temporary battery was produced in the same manner as in Experiment 1 except that the above positive electrode and a negative electrode made of lithium metal were used, and a battery A7 was produced from the obtained temporary battery in the same manner as in Experiment 6.

(Experiment 9)
In Experiment 8, a battery B5 was produced in the same manner as in Experiment 8, except that charging and discharging were not performed after the provisional battery was produced.

[Evaluation of charge / discharge characteristics]
The batteries A7 and B5 were evaluated for charge / discharge cycle characteristics.

  Each battery was charged to 0 V at a current value of 4 mA at 25 ° C., and then discharged to 2.0 V at a current value of 4 mA. Table 6 shows the initial discharge capacity and the capacity retention rate at the 10th cycle. FIG. 8 shows cycle characteristics in the charge / discharge test.

図８及び表６から明らかなように、本発明に従う電池Ａ７は、充放電サイクル特性に優れていることがわかる。

As is apparent from FIG. 8 and Table 6, it can be seen that the battery A7 according to the present invention is excellent in charge / discharge cycle characteristics.

DESCRIPTION OF SYMBOLS 1 ... Current collector 1a ... Current collector surface 2 ... Active material layer 2a ... Active material particle 2b ... Binder 3 ... Solid electrolyte 4 ... Crack 5 ... Thin film 6 ... Crack

Claims (12)

In a lithium secondary battery comprising a negative electrode having an active material layer containing an active material that electrochemically occludes / releases lithium on a current collector, a positive electrode, and a non-aqueous electrolyte,
The active material is silicon, tin or germanium, or an alloy containing any of these metals,
The active material layer is formed by applying a slurry containing active material particles and a binder to a current collector and then sintering together with the current collector in a non-oxidizing atmosphere.
A lithium secondary battery, wherein a solid electrolyte is disposed inside a crack formed in the active material layer by insertion and extraction of lithium. In a lithium secondary battery comprising a negative electrode having an active material layer containing an active material that electrochemically occludes / releases lithium on a current collector, a positive electrode, and a non-aqueous electrolyte,
The active material is silicon, tin or germanium, or an alloy containing any of these metals,
The active material layer is formed by depositing an active material in the form of a thin film on the current collector,
A lithium secondary battery, wherein a solid electrolyte is disposed inside a crack formed in the active material layer by insertion and extraction of lithium. The lithium secondary battery according to claim 1 or 2, wherein the whole of the non-aqueous electrolyte is made of a solid electrolyte. The lithium secondary battery according to claim 1, wherein a part of the non-aqueous electrolyte is made of a solid electrolyte. The lithium secondary battery according to any one of claims 1 to 4, wherein the solid electrolyte is a solid electrolyte formed by combining a polymer and an electrolytic solution containing a lithium salt into a gel. The polymer is a polyether solid polymer, a polycarbonate solid polymer, a polyacrylonitrile solid polymer, a copolymer comprising two or more monomers of these polymers, or two of these polymers The lithium secondary battery according to claim 5, wherein the polymer is a polymer obtained by crosslinking the above. The lithium secondary battery according to claim 1, wherein a surface roughness Ra of a surface of the current collector on which the active material layer is provided is 0.2 μm or more. . The lithium secondary battery according to claim 1, wherein the binder is a binder that remains even after heat treatment for sintering. The lithium secondary battery according to claim 1, wherein the binder contains polyimide. 10. The lithium secondary according to claim 1, wherein a conductive powder is mixed in the slurry, and the conductive powder is contained in the active material layer. 11. battery. 11. The active material layer according to any one of claims 1 and 3 to 10, wherein the active material layer is applied with the slurry, dried, rolled with the current collector, and then sintered. The lithium secondary battery as described. The lithium secondary battery according to claim 1, wherein the solid electrolyte is in contact with the current collector.

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