JP2012009152A - Lithium-ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve cycle characteristics of a lithium-ion secondary battery equipped with a negative electrode in which a plurality of columnar alloy-based active materials are supported on the surface of a negative electrode collector.SOLUTION: A lithium-lion secondary battery 1 comprises a positive electrode 3 having a positive electrode active material layer capable of absorbing and discharging lithium ions, a negative electrode 4 with a plurality of columnar alloy-based active materials being supported on the surface of a negative electrode collector, a separator 5 intervening between the positive electrodes 3 and the negative electrode 4, and a nonaqueous electrolytic solution. On the surface of the positive electrode active material layer of the positive electrode 3, there is provided a swellable film 6 made of easily swellable resin having a swelling degree of 10% or more to the nonaqueous electrolytic solution.

Description

  The present invention relates to a lithium ion secondary battery. More specifically, the present invention relates to an improvement in non-uniform distribution of a nonaqueous electrolyte solution in a lithium ion secondary battery including a negative electrode in which a columnar alloy-based active material is supported on the surface of a negative electrode current collector.

  A lithium ion secondary battery using an alloy active material as a negative electrode active material (hereinafter sometimes referred to as an “alloy secondary battery”) is more than a conventional lithium ion secondary battery using graphite as a negative electrode active material. It is known to have a high capacity and energy density. Therefore, the alloy-based secondary battery is expected not only as a power source for electronic devices but also as a main power source or auxiliary power source for transportation equipment, machine tools, and the like. Known alloy-based active materials include silicon-based active materials such as silicon and silicon oxide, and tin-based active materials such as tin and tin oxide.

  However, the alloy-based active material undergoes significant expansion of its particles during charging and generates internal stress. As a result, the negative electrode active material layer may be detached from the negative electrode current collector, or the negative electrode may be deformed.

  In order to reduce internal stress generated when the alloy-based active material expands, a negative electrode made of an alloy-based active material and having a plurality of micron-sized columnar bodies formed on the surface of the negative electrode current collector is known (patent) Reference 1). In such a negative electrode, a gap is formed between a pair of adjacent columnar bodies. Such voids alleviate the generation of internal stress when the alloy-based active material expands. That is, even if the alloy-based active material expands significantly during charging, the generation of stress is suppressed by the voids formed between adjacent columnar bodies. As a result, the alloy active material is prevented from dropping from the negative electrode current collector, or the negative electrode is deformed.

  By the way, Patent Document 2 describes a positive electrode including a positive electrode active material layer including a positive electrode active material and a binder, a negative electrode including a negative electrode active material layer including a negative electrode active material and a binder, and a positive electrode and a negative electrode. A separator interposed therebetween, and a polymer support interposed between the positive electrode active material layer and / or the negative electrode active material layer and the separator, the solvent swelling degree of the polymer constituting the polymer support is Disclosed is a non-aqueous electrolyte secondary battery characterized by having a degree of solvent swelling of a binder contained in a positive electrode active material layer and / or a negative electrode active material layer in contact with a polymer support.

  The non-aqueous electrolyte secondary battery disclosed in Patent Document 2 aims to eliminate leakage of the non-aqueous electrolyte and the like with the above-described configuration.

International Publication WO2008 / 026595 JP 2008-047402 A

  In the alloy-based secondary battery including the negative electrode having a plurality of columnar bodies described above, the cycle characteristics may be deteriorated as the number of charge / discharge cycles is increased. As a result of repeated studies on this cause, the present inventors have obtained the following knowledge.

  In the negative electrode having a plurality of columnar bodies, the volume of voids between the columnar bodies decreases due to the columnar bodies occluding lithium ions and remarkably expanding during charging. On the other hand, at the time of discharge, the columnar body releases lithium ions and contracts remarkably, so that the void volume increases remarkably.

  Therefore, when the volume of the gap of the negative electrode increases due to the discharge, the nonaqueous electrolyte stored in the alloy-based secondary battery is absorbed into the gap by capillary action. As a result, the mobility of the non-aqueous electrolyte in the battery may be poor. In such a case, the distribution of the non-aqueous electrolyte in the electrode surface becomes non-uniform at the end of discharge. As a result, variations in the depth of discharge occur. In addition, since the negative electrode having a plurality of columnar bodies has higher wettability with respect to the non-aqueous electrolyte than the positive electrode, there is a possibility that the non-aqueous electrolyte is unevenly distributed in the negative electrode while the positive electrode is in a dry state. is there.

  Thus, when non-uniform distribution of the non-aqueous electrolyte occurs, the charge / discharge reaction does not proceed in the region where the non-aqueous electrolyte is depleted, and the positive and negative electrodes are maintained in the region where the non-aqueous electrolyte is retained. The amount of charge / discharge reaction of becomes excessively large. As described above, when the load on the positive and negative electrodes increases, the cycle characteristics may be deteriorated.

  An object of the present invention is to provide a lithium ion secondary battery having a negative electrode including an alloy-based active material as a negative electrode active material and having excellent cycle characteristics.

  The lithium ion secondary battery of the present invention includes a positive electrode including a positive electrode active material layer capable of occluding and releasing lithium, a negative electrode in which a plurality of columnar alloy-based active materials are supported on the surface of a negative electrode current collector, A coating comprising a separator interposed between a positive electrode and the negative electrode and a non-aqueous electrolyte, and comprising a readily swellable resin having a degree of swelling with respect to the non-aqueous electrolyte of 10% or more on the surface of the positive electrode active material layer It is characterized by having.

  Thus, by forming a resin film having a high non-aqueous electrolyte absorption capacity on the surface of the positive electrode, it is possible to suppress the dry-up of the non-aqueous electrolyte of the positive electrode during discharge. As a result, in the initial stage of charging, lithium ions can be smoothly released from the positive electrode, and deterioration of the positive electrode can be suppressed.

  According to the present invention, a lithium ion secondary battery having high capacity and high energy density and excellent cycle characteristics is provided.

It is a longitudinal cross-sectional view which shows typically the structure of the lithium ion secondary battery which is 1st Embodiment of this invention. It is a longitudinal cross-sectional view which shows typically the structure of the negative electrode with which the lithium ion secondary battery shown in FIG. 1 is equipped. It is a longitudinal cross-sectional view which shows typically the structure of the columnar body with which the negative electrode shown in FIG. 2 is equipped. It is side surface perspective drawing which shows the structure of a vacuum evaporation system typically.

An embodiment of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a longitudinal sectional view schematically showing a configuration of a lithium ion secondary battery 1 according to the first embodiment of the present invention. In the positive electrode 3 and the negative electrode 4 illustrated in FIG. 1, illustration of the active material layer and the current collector is omitted. FIG. 2 is a longitudinal sectional view schematically showing the configuration of the negative electrode 4 provided in the lithium ion secondary battery 1 shown in FIG.

  A lithium ion secondary battery 1 is a wound electrode group obtained by interposing a separator 5 between a positive electrode 3 having a swellable coating 6 on the surface of a positive electrode active material layer and a negative electrode 4 and winding them. 2 (hereinafter simply referred to as “electrode group 2”), a battery case 14 having a bottomed cylindrical shape, containing the electrode group 2, a non-aqueous electrolyte (not shown), and the like, and functioning as a negative electrode terminal; 14, sealing plate 15 functioning as a positive electrode terminal, gasket 16 disposed so as to be interposed between battery case 14 and sealing plate 15, and positive electrode current collector and sealing plate of positive electrode 3. 15, a negative electrode lead 11 that conducts the negative electrode current collector 20 of the negative electrode 4 and the battery case 14, and an upper insulating plate 12 and a lower insulation that are respectively attached to both ends of the electrode group 2 in the longitudinal direction. Board 1 It is equipped with a.

  First, the configuration of the electrode group 2 will be described. As shown in FIG. 1, the electrode group 2 has a layer structure in which a positive electrode 3 having a swellable coating 6 on the surface of a positive electrode active material layer, a negative electrode 4, and a separator 5 interposed therebetween are laminated. . The swellable film 6 is a resin film made of an easily swellable resin that has a high degree of swelling with respect to the non-aqueous electrolyte. As shown in FIG. 2, the negative electrode 4 includes a negative electrode current collector 20 having a plurality of convex portions 21 and a plurality of columnar alloy-based active materials 23 (hereinafter “columnar bodies 23”) supported by the convex portions 21. And).

  The swellable film 6 is formed on the surface of the positive electrode active material layer of the positive electrode 3 and is made of an easily swellable resin having a degree of swelling with respect to the non-aqueous electrolyte of 10% or more, and has lithium ion conductivity. Such a swellable coating 6 is prepared by, for example, preparing a coating liquid by dissolving the aforementioned easily swellable resin in an organic solvent, applying the coating liquid to the surface of the positive electrode active material layer, and applying the obtained coating film to the coating liquid. It can be formed by drying. The coating method is not particularly limited, and examples thereof include screen coating, die coating, comma coating, roller coating, bar coating, gravure coating, curtain coating, spray coating, air knife coating, reverse coating, dip coating, and dip squeeze coating. It is done. Moreover, the swellable film 6 having a desired thickness can be formed by selecting the concentration of the easily swellable resin in the coating liquid, the type of the organic solvent used in the coating liquid, the liquid temperature of the coating liquid, and the like.

In this specification, the degree of swelling with respect to the non-aqueous electrolyte is measured as follows. First, a resin is dissolved in an organic solvent to prepare a resin solution, this resin solution is applied to a flat glass surface, and the obtained coating film is dried to produce a sheet having a thickness of 50 μm to 100 μm. This sheet is cut into 30 mm × 30 mm and used as a sample. The sample obtained above is immersed in a non-aqueous electrolyte used in the lithium ion secondary battery 1 at 25 ° C. for 24 hours in a sealed container. And the degree of swelling is calculated | required according to a following formula as an increase rate of the mass (Y) of the sample after being immersed in a non-aqueous electrolyte with respect to the mass (X) of the sample before being immersed in a non-aqueous electrolyte.
Swelling degree (%) = {(Y−X) / X} × 100

  In the lithium ion secondary battery 1, by forming a swellable coating 6 made of a readily swellable resin having a degree of swelling of 10% or more with respect to the non-aqueous electrolyte on the surface of the positive electrode active material layer of the positive electrode 3, The balance around the liquid nonaqueous electrolyte in the secondary battery 1 can be kept good. In particular, even when the ability of the negative electrode 4 to hold the non-aqueous electrolyte is increased by discharge, the non-aqueous electrolyte is distributed to the positive electrode 3 without the non-aqueous electrolyte being unevenly distributed on the negative electrode 4. Thereby, lithium ions are smoothly released from the positive electrode 3 at the end of discharge and at the beginning of charging, and the disorder of the structure of the positive electrode active material layer and the deterioration of the positive electrode 3 are suppressed. As a result, the cycle characteristics of the lithium ion secondary battery 1 are further improved.

  The swelling degree of the easily swellable resin of the present embodiment with respect to the non-aqueous electrolyte (hereinafter simply referred to as “swelling degree of the easily swellable resin”) is preferably in the range of 20% to 200%, and is preferably 50% to 160%. % Is more preferable.

  When the swelling degree of the easily swellable resin is less than 10%, the ability of the non-aqueous electrolyte to be retained by the swellable coating 6 decreases, and the non-aqueous electrolyte is unevenly distributed in the negative electrode 4 at the end of discharge and at the beginning of charge. There is a possibility that the positive electrode 3 cannot be sufficiently suppressed and the liquid is dried up. On the other hand, when the degree of swelling of the easily swellable resin becomes too large, the swellable coating 6 may gel, and the separator 5 may be clogged. In this case, the nonaqueous electrolyte solution around the positive electrode 3 becomes insufficient, and the cycle characteristics of the lithium ion secondary battery 1 may be deteriorated.

  As the easily swellable resin, any resin can be used without particular limitation as long as it has a degree of swelling of 10% or more and can exhibit lithium ion conductivity by contact with a non-aqueous electrolyte, but vinylidene fluoride and hexafluoro A copolymer with propylene, a copolymer of tetrafluoroethylene and hexafluoropropylene, and the like are preferable. In these copolymers, by setting the content ratio of the hexafluoropropylene unit to 2% by mass to 50% by mass of the total amount of the copolymer, a copolymer which is an easily swellable resin having a degree of swelling of 10% or more is obtained. Obtainable.

  If the content ratio of the hexafluoropropylene unit is too small, the ability of the non-aqueous electrolyte to be retained by the swellable coating 6 is reduced, and the non-aqueous electrolyte can be sufficiently prevented from being unevenly distributed in the negative electrode 4 at the initial stage of discharge and the initial stage of charge. Therefore, there is a possibility that the positive electrode 3 is in a liquid-drying state. On the other hand, when the content ratio of the hexafluoropropylene unit is too large, the swellable coating 6 is gelled, so that the state in which the swellable coating 6 is reliably attached to the surface of the positive electrode active material layer cannot be obtained. Clogging occurs. In this case, the nonaqueous electrolyte solution around the positive electrode 3 becomes insufficient, and the cycle characteristics of the lithium ion secondary battery 1 may be deteriorated.

  The content ratio of the hexafluoropropylene unit is more preferably 5% by mass to 35% by mass with respect to the total amount of the copolymer. Thereby, the difference between the holding capacity of the non-aqueous electrolyte by the laminate of the positive electrode 3 and the swellable coating 6 and the holding capacity of the negative electrode 4 by the non-aqueous electrolyte becomes very small. As a result, the amount of the non-aqueous electrolyte that reaches the positive electrode 3 increases, the liquidity of the non-aqueous electrolyte in the lithium ion secondary battery 1 is further improved, and the cycle characteristics of the lithium ion secondary battery 1 are significantly improved. improves.

  Such an easily swellable resin is preferably selected from the range of 10,000 to 2,000,000, and more preferably selected from the range of 100,000 to 1,000,000. Thereby, the adhesiveness between the swellable coating 6 and the surface of the positive electrode active material layer is improved, and even when charging / discharging is repeated, peeling of the swellable coating 6 from the surface of the positive electrode active material layer is suppressed.

  If the number average molecular weight is too small, the ability of the easily swellable resin to hold the non-aqueous electrolyte is reduced, and the effect of using the easily swellable resin may not be sufficiently exhibited. If the number average molecular weight is too large, the lithium ion conductivity of the swellable coating 6 is lowered, and the load characteristics of the lithium ion secondary battery 1 may be lowered.

  The thickness of the swellable film 6 is not particularly limited, but is preferably 0.1 μm to 20 μm, more preferably 1 μm to 10 μm. As a result, a swellable coating 6 having a good balance between the retention of the nonaqueous electrolyte and the lithium ion conductivity is obtained. If the thickness of the swellable coating 6 is too small, the ability to hold the non-aqueous electrolyte by the laminate of the positive electrode 3 and the swellable coating 6 may be reduced. Further, the mechanical strength of the swellable coating 6 is lowered, and there is a possibility that the swellable coating 6 may be peeled off from the surface of the positive electrode active material layer. If the thickness of the swellable coating 6 is too large, the lithium ion conductivity of the swellable coating 6 may be reduced, and the load characteristics of the battery 1 may be reduced.

The positive electrode 3 includes a positive electrode current collector (not shown) and a positive electrode active material layer formed on both surfaces of the positive electrode current collector, and has the swellable coating 6 on the surface of each positive electrode active material layer.
As the positive electrode current collector, a metal foil made of a metal material such as aluminum, an aluminum alloy, stainless steel, or titanium can be used. Among the metal materials, aluminum and aluminum alloys are preferable. The thickness of the positive electrode current collector is not particularly limited, but is preferably 10 μm to 30 μm. In the present embodiment, the positive electrode current collector has a strip shape.

  In the present embodiment, the positive electrode active material layer is provided on both surfaces of the positive electrode current collector, but may be provided on one surface. The positive electrode active material layer contains a positive electrode active material, a binder, and a conductive agent. The positive electrode active material layer can be formed, for example, by applying a positive electrode mixture slurry to the surface of the positive electrode current collector, and drying and rolling the obtained coating film. The positive electrode mixture slurry can be prepared, for example, by mixing a positive electrode active material, a binder, a conductive agent, and a solvent.

  As the positive electrode active material, a positive electrode active material for a lithium ion secondary battery can be used. Among them, lithium-containing composite oxides and olivine type lithium phosphate are preferable.

  The lithium-containing composite oxide is a metal oxide containing lithium and a transition metal element, or a metal oxide in which a part of the transition metal element in the metal oxide is substituted with a different element. Examples of the transition metal element include Sc, Y, Mn, Fe, Co, Ni, Cu, and Cr. Among transition metal elements, Mn, Co, Ni and the like are preferable. Examples of the different elements include Na, Mg, Zn, Al, Pb, Sb, and B. Among the different elements, Mg, Al and the like are preferable. Each of the transition metal element and the different element can be used alone or in combination of two or more.

Specific examples of the lithium-containing composite oxide, for _{example, Li Z CoO 2, Li Z} NiO 2, Li Z MnO 2, Li Z Co m Ni 1-m O 2, Li Z Co m M 1-m O n, _{Li Z Ni 1-m M m} O n, Li Z Mn 2 O 4, Li Z Mn 2-m MnO 4 ( in each of the formulas above, M is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, It represents at least one element selected from the group consisting of Cu, Zn, Al, Cr, Pb, Sb and B. 0 <Z ≦ 1.2, 0 ≦ m ≦ 0.9, 2.0 ≦ n ≦ 2. .3) and the like. Among _{these, Li Z Co m M 1-} m O n is preferred.

Specific examples of the olivine-type lithium phosphate include, for example, LiXPO ₄ , Li ₂ XPO ₄ F (wherein X represents at least one element selected from the group consisting of Co, Ni, Mn, and Fe). Etc.

  In each of the above formulas showing the lithium-containing composite oxide and the olivine-type lithium phosphate, the number of moles of lithium is a value immediately after the production thereof, and is increased or decreased by charging and discharging. A positive electrode active material can be used individually by 1 type, or can be used in combination of 2 or more type.

  Examples of the binder include resin materials such as polytetrafluoroethylene and polyvinylidene fluoride, styrene butadiene rubber (trade name: BM-500B, manufactured by Nippon Zeon Co., Ltd.), and styrene butadiene rubber (trade name). : BM-400B, manufactured by Nippon Zeon Co., Ltd.) and the like. Examples of the conductive agent include carbon blacks such as acetylene black and ketjen black, and graphites such as natural graphite and artificial graphite. The content ratio of the binder and the conductive agent in the positive electrode active material layer can be appropriately changed according to, for example, the design of the positive electrode 3 and the lithium ion secondary battery 1.

  As a solvent to be mixed with the positive electrode active material, the binder, and the conductive agent, for example, an organic solvent such as N-methyl-2-pyrrolidone, tetrahydrofuran, dimethylformamide, water, or the like can be used.

  Next, the negative electrode 4 will be described with reference to FIG. The negative electrode 4 includes a negative electrode current collector 20 having a plurality of convex portions 21 on both surfaces, and a negative electrode active material layer 22 including a plurality of columnar bodies 23 supported on the surfaces of the convex portions 21. In the present embodiment, one columnar body 23 is formed on one convex portion 21.

  The columnar body 23 is supported on the surface of the convex portion 21 and extends from the surface of the convex portion 21 to the outside of the negative electrode current collector 20. A gap 24 exists between a pair of columnar bodies 23 adjacent to each other. The voids 24 relieve the stress generated with the volume change of the alloy-based active material. As a result, separation of the columnar body 23 from the convex portion 21, deformation of the negative electrode current collector 20 and the negative electrode 4, and the like are suppressed. Further, in such a negative electrode 4, the columnar body 23 contracts at the time of discharge, so that the volume of the void 24 existing between a pair of adjacent columnar bodies 23 becomes remarkably large and absorbs the nonaqueous electrolytic solution.

  The negative electrode current collector 20 is a metal foil made of a metal material such as copper, copper alloy, stainless steel, or nickel, and has a plurality of convex portions 21 on both surfaces. The thickness of the portion of the negative electrode current collector 20 where the convex portions 21 are not formed is preferably 5 μm to 50 μm. In addition, although the negative electrode collector 20 of this embodiment has the convex part 21 on both surfaces, you may have the convex part 21 only on one surface. Moreover, in this embodiment, the negative electrode collector 20 is strip | belt shape.

  The alloy-based active material constituting the columnar body 23 is a substance that occludes lithium by alloying with lithium and reversibly occludes and releases lithium ions under a negative electrode potential. The alloy-based active material is preferably amorphous or low crystalline. As the alloy-based active material, an alloy-based active material for a lithium ion secondary battery can be used, but a silicon-based active material and a tin-based active material are preferable. An alloy type active material can be used individually by 1 type, or can be used in combination of 2 or more type.

Examples of the silicon-based active material include silicon, silicon compounds, and partially substituted products.
Examples of the silicon compound include silicon oxide represented by the formula SiO _a (0.05 <a <1.95), silicon carbide represented by the formula SiC _b (0 <b <1), and formula SiN _c (0 < a silicon nitride represented by c <4/3), an alloy of silicon and a different element (A), and the like. Examples of the different element (A) include Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Sn, and Ti. In addition, the partially substituted body is a compound in which a part of silicon atoms contained in silicon and a silicon compound is substituted with a different element (B). Specific examples of the different element (B) include B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, Ta, V, W, Zn, C, N, and Sn. Can be mentioned. Of these, silicon and silicon oxide are preferred.

Examples of tin-based active materials include tin, tin compounds, tin oxides represented by the formula SnO _d (0 <d ≦ 2), tin nitride, Ni—Sn alloys, Mg—Sn alloys, Fe—Sn alloys, Cu -Sn alloy, tin alloy, such as Ti-Sn _{alloy, SnSiO 3, Ni 2 Sn 4} , Mg 2 Sn and tin compounds such like. Among the tin-based active materials, tin oxide, tin alloy, tin compound, and the like are preferable.

  The lithium ion secondary battery 1 of the present embodiment has a swelling degree of 2% or more and less than 50% on the surface of the negative electrode active material layer 22 as required, and an easily swellable resin constituting the swellable coating 6. It may have a film (hereinafter referred to as “hardly swellable film”) made of a hardly swellable resin (hereinafter simply referred to as “hardly swellable resin”) having a smaller degree of swelling.

  By laminating a hardly swellable coating on the surface of the negative electrode active material layer 22, the nonaqueous electrolyte holding capacity of the laminate of the positive electrode 3 and the swellable coating 6 and the laminate of the negative electrode 4 and the hardly swellable coating are obtained. The difference between the holding capacity of the non-aqueous electrolyte is reduced. As a result, the fluidity of the non-aqueous electrolyte in the lithium ion secondary battery 1 is further improved, and even in the initial stage of charging, the non-aqueous electrolyte is sufficiently distributed in the positive electrode 3 and the deterioration of the positive electrode 3 is further suppressed. The Thereby, the cycle characteristics of the lithium ion secondary battery 1 can be remarkably improved.

  The hardly swellable film is formed on the surface of the negative electrode active material layer 22. The hardly swellable film can be formed in the same manner as the swellable film 6 except that a hardly swellable resin is used instead of the easily swellable resin. As a hardly swellable resin, the degree of swelling is in the range of 2% to less than 50%, the degree of swelling is smaller than that of the easily swellable resin, and lithium ion conductivity is exhibited by contact with a non-aqueous electrolyte. Although it will not be specifically limited if it becomes resin to become, The copolymer of hexafluoropropylene and vinylidene fluoride, the copolymer of hexafluoropropylene, and tetrafluoroethylene etc. can be used preferably. In these copolymers, the content of hexafluoropropylene units is preferably 10% by mass or less.

  When the content ratio of the hexafluoropropylene unit is too large, the degree of swelling of the obtained copolymer becomes 10% or more, and the effect of forming a hardly swellable film may not be obtained.

  The thickness of the hardly swellable coating is not particularly limited, but is preferably selected from the range of 0.01 μm to 10 μm. If the thickness of the hardly swellable film is too small, the mechanical strength of the hardly swellable film, the adhesive force to the negative electrode active material layer 22 and the like are reduced, and accordingly the hard swellable film hardly expands as the negative electrode active material layer 22 expands and contracts. There is a possibility that the conductive film peels off from the surface of the negative electrode active material layer 22. As a result, the effect of forming a hardly swellable coating film may not be obtained. On the other hand, when the thickness of the hardly swellable coating is too large, the lithium ion conductivity of the hardly swellable resin is lowered, and the load characteristics of the lithium ion secondary battery 1 may be lowered.

  The negative electrode current collector 20 can be produced by, for example, a roller pressing method. According to the roller pressurization method, two convex rollers having a plurality of concave portions formed on the surface thereof are pressed against each other so that their axes are parallel to form a pressure contact portion, and the metal foil is passed through the pressure contact portion. Thus, by pressing and molding, the convex portion 21 having a shape and a dimension substantially corresponding to the shape and size of the internal space of the concave portion is arranged corresponding to the arrangement of the concave portion on the surface of the convex roller, and the metal foil Formed on both surfaces, the negative electrode current collector 20 is obtained. In order to further increase the bonding strength between the convex portion 21 and the columnar body 23, a metal foil roughened by plating or the like may be passed through the press contact portion. The convex roller used here can be produced, for example, by forming a concave portion by laser processing on the surface of a roller having at least a layer made of forged steel in the surface layer portion.

  The protrusion 21 is a protrusion that extends outward from the surface 20 a of the negative electrode current collector 20. The plurality of convex portions 21 of the present embodiment are staggered on the surface 20a, but are not limited thereto, and may be a close-packed arrangement, a lattice arrangement, a grid arrangement, or the like. Furthermore, it may be arranged irregularly.

  The height and width of the protrusion 21 are the length of the perpendicular dropped from the foremost point of the protrusion 21 to the surface 20a and the maximum length of the protrusion 21 in the direction parallel to the surface 20a in the cross section of the negative electrode 4, respectively. It is. The height of the convex portion 21 is preferably 3 μm to 15 μm. The width of the convex portion 21 is preferably 5 μm to 50 μm. The height and width of the convex portion 21 are obtained by observing the cross section of the negative electrode 4 with a scanning electron microscope and measuring, for example, the height and width of 100 convex portions 21, respectively, and obtaining the average value of the obtained measurement values. be able to.

Examples of the shape of the convex portion 21 in the orthographic projection from above in the vertical direction of the negative electrode current collector 20 include a rhombus, a triangle to an octagon, a parallelogram, a trapezoid, a circle, and an ellipse. .
Furthermore, the convex part 21 has a substantially planar top at the tip part. This top is substantially parallel to the surface 20a.

The number of convex portions 21 is preferably 10,000 pieces / cm ^{2 to} 10 million pieces / cm ² . Moreover, the distance between the axes of the adjacent convex portions 21 is preferably 5 μm to 100 μm. When the shape of the convex portion 21 is a rhombus, polygon, parallelogram, trapezoid, or ellipse, the axis of the convex portion 21 passes through the intersection of diagonal lines or the intersection of the major axis and minor axis and is perpendicular to the surface 20a. It is a straight line. When the shape of the convex portion 21 is circular, the axis of the convex portion 21 is a straight line that passes through the center of the circle and is perpendicular to the surface 20a.

  The plurality of columnar bodies 23 can be simultaneously formed on the surfaces of the plurality of convex portions 21 by a vapor phase method. Examples of the vapor phase method include a vacuum deposition method, a sputtering method, an ion plating method, a laser ablation method, a chemical vapor deposition method, a plasma chemical vapor deposition method, and a thermal spray method. Among these, the vacuum evaporation method is preferable.

  FIG. 3 is a longitudinal sectional view schematically showing the configuration of the columnar body 23. The columnar body 23 is preferably formed as a stacked body of lumps 23a to 23h shown in FIG. 3 by a vacuum deposition method. The number of lumps stacked is not limited to eight, and any number of lumps of two or more can be stacked.

  When forming the columnar body 23 which is a laminate of the chunks 23a to 23h, first, the chunk 24a supported on the surface of the convex portion 21 is formed. Next, the lump 24b is formed so as to be supported by the remaining surface of the convex portion 21 and the surface of the lump 24a. The lump 24c is formed so as to be supported by the remaining surface of the lump 24a and the surface of the lump 24b. Further, the mass 24d is formed so as to be supported by the remaining surface of the mass 24b and the surface of the mass 24c. In the same manner, the columnar body 23 is obtained by alternately stacking the lumps 24e, 24f, 24g, and 24h. A method for forming the columnar body 23 will be described in detail in Examples.

Examples of the three-dimensional shape of the columnar body 23 include a columnar shape, a prismatic shape, and a spindle shape.
The height and width of the columnar body 23 are respectively the length of the perpendicular dropped from the most distal point of the columnar body 23 to the flat top surface of the projection 21 and the columnar shape in the direction parallel to the surface 20a in the cross section of the negative electrode 4. This is the maximum length of the body 23. The height of the columnar body 23 is preferably 3 μm to 30 μm, and the width of the columnar body 23 is preferably 3 μm to 30 μm. The height and width of the columnar body 23 can be obtained by observing the cross section of the negative electrode 4 with a scanning electron microscope in the same manner as the height and width of the convex portion 21.

  As the separator 5 disposed between the positive electrode 3 and the negative electrode 4, a porous sheet having pores, a resin fiber nonwoven fabric, a resin fiber woven fabric, or the like can be used. Among these, a porous sheet is preferable, and a porous sheet having a pore diameter of about 0.05 μm to 0.15 μm is more preferable. Examples of the resin material constituting the porous sheet and the resin fiber include polyolefins such as polyethylene and polypropylene, polyamide, and polyamideimide. The thickness of the separator 5 is preferably 5 μm to 30 μm. The separator 5 of this embodiment is strip-shaped.

The nonaqueous electrolytic solution impregnated mainly in the electrode group 2 contains a lithium salt and a nonaqueous solvent. Lithium salts include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiAsF ₆ , LiB ₁₀ Cl ₁₀ , LiCl, LiBr, LiI, LiCO ₂ CF ₃ , LiSO ₃ CF ₃ , Li (SO _{3 CF 3) 2, LiN (SO} 2 CF 3) 2, and lithium imide salt and the like. A lithium salt can be used individually by 1 type, or can be used in combination of 2 or more type. The concentration of the lithium salt in 1 L of the non-aqueous solvent is preferably 0.2 mol to 2 mol, more preferably 0.5 mol to 1.5 mol.

  Non-aqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane. Chain ethers such as γ-butyrolactone, cyclic carboxylic acid esters such as γ-valerolactone, and chain esters such as methyl acetate. A non-aqueous solvent can be used individually by 1 type, or can be used in combination of 2 or more type.

  When the lithium ion secondary battery 1 is manufactured, first, both ends of the positive electrode lead 10 and the negative electrode lead 11 are welded to predetermined positions, respectively. Then, after the upper insulating plate 12 and the lower insulating plate 13 are mounted on the electrode group 2, the electrode group 2 is accommodated in the battery case 14, and further a nonaqueous electrolyte is injected. Next, the gasket 16 and the sealing plate 15 are sequentially attached to the opening of the battery case 14, and the battery case 14 is sealed by caulking the opening end of the battery case 14 toward the sealing plate 15. Thereby, the lithium ion secondary battery 1 is obtained.

  For example, an aluminum lead or the like can be used as the positive electrode lead 10. As the negative electrode lead 11, for example, a nickel lead, a copper lead, or the like can be used. As the battery case 14 and the sealing plate 15, for example, a metal material such as iron or stainless steel formed into a predetermined shape can be used. As the upper insulating plate 12, the lower insulating plate 13, and the gasket 16, for example, an insulating material such as a resin material or a rubber material formed into a predetermined shape can be used.

  Although the lithium ion secondary battery 1 of this embodiment is a cylindrical battery provided with the electrode group 2, it is not limited to it but can take various forms. As the form, for example, a cylindrical battery, an electrode group 2 or a laminated electrode group in which a battery case containing the electrode group 2, a non-aqueous electrolyte, or the like is sealed with a sealing plate made of an insulating material that supports a positive electrode terminal. A rectangular battery housed in a square battery case, a square battery housed in a rectangular battery case by pressing the electrode group 2 into a flat shape by pressurizing the electrode group 2, an electrode group 2 or a flat electrode group, or a laminated type Examples include a laminate film battery in which an electrode group is housed in a battery case made of a laminate film, a coin-type battery in which a laminated electrode group is housed in a coin-type battery case, and the like.

Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
(Example 1)
(A) Preparation of positive electrode 95 parts by mass of a positive electrode active material (LiNi _0.80 Co _0.15 Al _0.05 O ₂ ), 3 parts by mass of graphite powder and 2 parts by mass of polyvinylidene fluoride were mixed with an appropriate amount of N-methyl-2-pyrrolidone, A positive electrode mixture slurry was prepared. The obtained positive electrode mixture slurry was applied to both surfaces of an aluminum foil (positive electrode current collector) having a thickness of 15 μm, and the obtained coating film was dried and rolled to produce a positive electrode having a thickness of 128 μm.

(B) Production of swellable film Copolymer of vinylidene fluoride and hexafluoropropylene (content ratio of hexafluoropropylene unit: 12% by mass, degree of swelling: 115%, number average molecular weight: 400,000, hereinafter "VDF- HFP copolymer (1) ”) was dissolved in dimethyl carbonate (DMC) to prepare a 3% by mass DMC solution of VDF-HFP copolymer (1). After heating this solution to a liquid temperature of 85 ° C., it was applied to the surface of the positive electrode active material layer of the positive electrode obtained above by roller coating and vacuum dried at 100 ° C. for 10 minutes to form a swellable film having a thickness of 4 μm. .

  The laminate of the positive electrode and the swellable coating obtained above was cut into a width that could be inserted into a battery case of a 14400 cylindrical battery (diameter: about 14 mm, height: about 40 mm) to produce a positive electrode plate.

(C) Production of negative electrode (c-1) Production of negative electrode current collector 20 Two forged steel rollers having a plurality of recesses arranged in a staggered pattern on the surface are pressed so that the respective axes are parallel, A nip was formed. The shape of the opening of the concave portion on the roller surface was approximately rhombus.

  Both surfaces of a 26 μm-thick alloy copper foil (trade name: HCL02Z, manufactured by Hitachi Cable Ltd.) were subjected to surface roughening by electrolytic plating to form a plurality of copper particles (particle size: about 1 μm). The surface roughness Rz of the alloy copper foil after the roughening treatment was 1.5 μm. The surface roughness Rz indicates the ten-point average roughness Rz defined in Japanese Industrial Standard (JISB 0601-1994). By passing the copper foil through the nip formed by the two forged steel rollers at a linear pressure of 1000 N / cm, a negative electrode current collector 20 having a plurality of convex portions 21 formed on both surfaces is produced. did.

  The plurality of convex portions 21 had an average height of 8 μm and were arranged in a staggered pattern. Further, the tip portion of the convex portion 21 was a plane substantially parallel to the surface 20 a of the negative electrode current collector 20. Further, in the orthographic projection view from above in the vertical direction of the negative electrode current collector 20, the shape of the convex portion 21 was substantially rhombus. The distance between the axes of the protrusions 21 was 25 μm in the longitudinal direction of the negative electrode current collector and 20 μm in the width direction.

(C-2) Formation of Negative Electrode Active Material Layer FIG. 4 is a side perspective view schematically showing the configuration of a vacuum vapor deposition device 40 (hereinafter referred to as “vapor deposition device 40”). Using the vapor deposition apparatus 40, the columnar body 23 was formed on the surface of the plurality of convex portions 21 of the negative electrode current collector 20 obtained above, and the negative electrode 4 was produced.

  The vapor deposition apparatus 40 includes a feed roller 41 around which the negative electrode current collector 20 is wound in advance, transport rollers 42a, 42b, 42c, 42d, 42e, and 42f that guide the negative electrode current collector 20 to the take-up roller 44, and an alloy system. Vapor deposition sources 43a and 43b for storing the raw material of the active material, a winding roller 44 for winding the negative electrode current collector 20 having an alloy-based active material deposited on the surface, and a negative-electrode current collector 20 of the vapor of the alloy-based active material A pair of shielding plates 45a and 45b for regulating the supply area to the surface, oxygen nozzles 46a and 46b for supplying oxygen, a chamber 47 for accommodating these, a vacuum pump 48 for reducing the pressure in the chamber 47, and vapor deposition An electron beam irradiation device (not shown) that irradiates the raw material of the alloy-based active material accommodated in the sources 43a and 43b with an electron beam to generate a vapor of the raw material of the alloy-based active material. There.

  In the vapor deposition apparatus 40, the 1st vapor deposition area | region 50a is formed between the sending-out roller 41 and the conveyance roller 42a, the 2nd vapor deposition area | region 50b is formed between the conveyance roller 42a and the conveyance roller 42b, and it conveys with the conveyance roller 42e. A third vapor deposition region 50 c is formed between the roller 42 f and a fourth vapor deposition region 50 d is formed between the transport roller 42 f and the take-up roller 44.

First, as an alloy-based active material raw material, silicon single crystal (purity 99.9999%, manufactured by Shin-Etsu Chemical Co., Ltd.) was used and accommodated in evaporation sources 43a and 43b. After the chamber 47 was evacuated to 5 × 10 ⁻³ Pa by the vacuum pump 48, oxygen was supplied from the oxygen nozzles 46 a and 46 b into the chamber 47 to create an oxygen atmosphere with a pressure of 3.5 Pa. Next, the scrap silicon accommodated in the evaporation sources 43a and 43b was irradiated with an electron beam (acceleration voltage: 10 kV, emission: 500 mA) to generate silicon vapor. In the middle of the rise of silicon vapor, it mixed with oxygen to generate a mixed gas of silicon vapor and oxygen.

  On the other hand, the negative electrode current collector 20 is fed from the feed roller 41 at a feed rate of 2 cm / min, and silicon vapor and oxygen are formed on the surface of the convex portion 21 formed on one surface of the negative electrode current collector 20 running in the vapor deposition region 50a. And a mixture 23 was deposited to form a mass 23a shown in FIG. Next, a lump 23b was formed on the surface of the convex portion 21 and the surface of the lump 23a of the negative electrode current collector 20 running in the vapor deposition region 50b. Next, on the surface of the convex portion 21 formed on the other surface of the negative electrode current collector 20, a lump 23a was formed in the third vapor deposition region 50c, and a lump 23b was formed in the fourth vapor deposition region 50d. The take-up roller 44 around which the negative electrode current collector 20 was taken up was replaced with the feed roller 41, and in the same manner as described above, the lumps 23c and 23d were stacked on the surface of the lumps 23b on both surfaces of the negative electrode current collector 20. Thereafter, the same operation was repeated twice to form the columnar body 23 that is a laminate of the masses 23a, 23b, 23c, 23d, 23e, 23f, 23g, and 23h on the surface of the convex portion 21 on both surfaces of the negative electrode current collector 20. . In this way, the negative electrode 4 was produced.

The columnar body 23 was supported by the surface of the convex portion 21 and grew to extend outward from the negative electrode current collector 20. One columnar body 23 was formed on one convex portion 21. The columnar body 23 had a substantially cylindrical solid shape. The columnar body 23 had an average height of 16 μm and an average width of 30 μm. Moreover, when the amount of oxygen contained in the columnar body 23 was quantified by a combustion method, the composition of the columnar body was SiO _0.4 .

  Lithium was vapor-deposited on the negative electrode active material layers on both sides of the negative electrode 4 obtained above, and lithium for an irreversible capacity was compensated. The negative electrode 4 after lithium deposition was cut into a width that could be inserted into a battery case of a 14400 cylindrical battery (diameter: about 14 mm, height: about 40 mm) to produce a negative electrode plate.

(D) Preparation of non-aqueous electrolyte Ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1: 8, and LiPF ₆ was dissolved in the obtained mixed solvent at a concentration of 1.2 mol / L. Furthermore, 10 parts by mass of fluoroethylene carbonate was added as an additive to 100 parts by mass of the mixed solvent to prepare a nonaqueous electrolytic solution.

(E) Battery assembly Between the positive electrode plate having a swellable coating on the surface of the positive electrode active material layer and the negative electrode plate, a separator having a thickness of 20 μm (trade name: Hypore, polyethylene porous membrane, manufactured by Asahi Kasei Corporation These were wound with the interposition of a) to produce a wound electrode group. One end of an aluminum lead was connected to the positive electrode plate, and one end of a nickel lead was connected to the negative electrode plate. An upper insulating plate and a lower insulating plate made of polypropylene were respectively attached to both ends in the longitudinal direction of the wound electrode group. Next, the wound electrode group is accommodated in a bottomed cylindrical iron battery case, the other end of the aluminum lead is connected to a stainless steel sealing plate, and the other end of the nickel lead is connected to the inner surface of the bottom of the battery case. Connected to.

  Next, a nonaqueous electrolyte was injected into the battery case by a reduced pressure method. A polypropylene gasket was attached to the periphery of the sealing plate that supported the safety valve, and in this state, the sealing plate was attached to the opening of the battery case. The battery case was hermetically sealed by caulking the open end of the battery case toward the sealing plate. Thus, a cylindrical lithium ion secondary battery having an outer diameter of 14 mm and a height of 40 mm was produced.

(Example 2)
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that the VDF-HFP copolymer (2) was used instead of the VDF-HFP copolymer (1). In addition, VDF-HFP copolymer (2) was HFP content rate: 6 mass%, swelling degree: 70%, and number average molecular weight: 800,000.

(Example 3)
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that a non-swellable film was formed on the surface of the negative electrode active material layer as follows.

[Preparation of hard-to-swell coating]
Polyvinylidene fluoride (swelling degree: 6.5%, number average molecular weight: 400,000) was dissolved in N-methyl-2-pyrrolidone, and a 2 mass% N-methyl-2-pyrrolidone solution of polyvinylidene fluoride (hereinafter referred to as “PVDF”). Solution)) was prepared. The PVDF solution obtained above was applied by roller coating to the surface of the negative electrode active material layer of the negative electrode prepared in the same manner as in Example 1 and further supplemented with lithium for an irreversible capacity, and vacuum dried at 85 ° C. for 10 minutes. Then, a hardly swellable film having a thickness of about 0.1 μm was formed.

(Comparative Example 1)
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that the swellable film was not formed on the surface of the positive electrode active material layer of the positive electrode.

(Comparative Example 2)
In place of the VDF-HFP copolymer (1), polyvinylidene fluoride (swelling degree: 6.5%, number average molecular weight: 400,000) was used in the same manner as in Example 1, except that the cylindrical lithium ion secondary A secondary battery was produced.

[Battery capacity]
The batteries of Examples 1 to 3 and Comparative Examples 1 and 2 are each housed in a constant temperature bath of 20 ° C., and are charged (constant current charging and subsequent constant voltage charging) and discharged (constant current discharging) under the following charging / discharging conditions. The charging / discharging was repeated 3 cycles, and the discharge capacity (0.2 C capacity) for the third time was determined to be the battery capacity (mAh). The results are shown in Table 1.

Constant current charging: charging current 0.7C, end-of-charge voltage 4.2V.
Constant voltage charging: charging voltage 4.2V, charging end current 0.05C, rest time 20 minutes.
Constant current discharge: discharge current 0.2 C, discharge end voltage 2.5 V, rest time 20 minutes.

[Cycle characteristics]
The batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were each housed in a constant temperature bath at 20 ° C., and 1 cycle charge / discharge was performed under the same charge / discharge conditions as in the battery capacity evaluation to obtain the 1 cycle discharge capacity. Then, 198 cycles of charging / discharging were performed on the same charging / discharging conditions as the 1st cycle except changing the electric current value of constant current discharge from 0.2C to 1C. Next, charging / discharging was performed under the same charging / discharging conditions as in the first cycle, and a 200-cycle discharging capacity was obtained. The capacity retention rate (%) was determined as a percentage of the 200 cycle discharge capacity with respect to the 1 cycle discharge capacity. The results are shown in Table 1.

  From Table 1, by forming a swellable coating on the surface of the positive electrode active material layer of the positive electrode, the cycle characteristics of the battery are improved, and even if the number of charge / discharge cycles is increased, the rapid decrease in cycle characteristics is suppressed. I understand. Furthermore, from Example 3 of Table 1, by forming a swellable film on the surface of the positive electrode active material layer of the positive electrode and forming a hardly swellable film on the surface of the negative electrode active material layer of the negative electrode, the cycle characteristics of the battery are further improved. It turns out that it improves.

  The lithium ion secondary battery of the present invention can be used for the same applications as conventional lithium ion secondary batteries, and in particular, as a main power source or auxiliary power source for electronic devices, electrical devices, machine tools, transportation devices, power storage devices, etc. Useful. Electronic devices include personal computers, mobile phones, mobile devices, portable information terminals, portable game devices, and the like. Electrical equipment includes vacuum cleaners and video cameras. Machine tools include electric tools and robots. Transportation equipment includes electric vehicles, hybrid electric vehicles, plug-in HEVs, fuel cell vehicles, and the like. Examples of power storage devices include uninterruptible power supplies.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Winding type electrode group 3 Positive electrode 4 Negative electrode 5 Separator 6 Swellable film 10 Positive electrode lead 11 Negative electrode lead 12 Upper insulating plate 13 Lower insulating plate 14 Battery case 15 Sealing plate 16 Gasket 20 Negative electrode collector 21 Convex part 22 Negative electrode active material layer 23 Columnar body 24 Void 40 Vacuum deposition apparatus

Claims (8)

A positive electrode having a positive electrode active material layer capable of inserting and extracting lithium ions, a negative electrode in which a plurality of columnar alloy-based active materials are supported on the surface of a negative electrode current collector, and interposed between the positive electrode and the negative electrode A lithium ion secondary battery comprising a separator and a non-aqueous electrolyte,
A lithium ion secondary battery comprising a film made of an easily swellable resin having a degree of swelling with respect to the non-aqueous electrolyte of 10% or more on the surface of the positive electrode active material layer.   The lithium ion secondary according to claim 1, wherein the easily swellable resin is a copolymer of hexafluoropropylene and vinylidene fluoride or tetrafluoroethylene containing 2 to 50 mass% of hexafluoropropylene units. battery.   3. The lithium ion secondary battery according to claim 1, wherein a thickness of the coating made of the easily swellable resin is in a range of 0.01 μm to 20 μm.   2. A film made of a hardly-swellable resin having a degree of swelling of 2% or more and less than 50% with respect to the non-aqueous electrolyte on the surface of the negative electrode active material layer and having a degree of swelling smaller than that of the easily-swellable resin. The lithium ion secondary battery of any one of -3.   The lithium ion secondary according to claim 4, wherein the hardly swellable resin is a copolymer of hexafluoropropylene and vinylidene fluoride or tetrafluoroethylene containing a hexafluoropropylene unit in a proportion of 10% by mass or less. battery.   The lithium ion secondary battery according to claim 4 or 5, wherein a thickness of the coating made of the hardly swellable resin is in a range of 0.01 µm to 10 µm.   The lithium ion according to any one of claims 1 to 6, wherein the negative electrode current collector has a plurality of convex portions on a surface thereof, and the columnar alloy-based active material is supported on the surface of the convex portions. Secondary battery.   The lithium ion secondary battery according to claim 1, wherein the alloy-based active material is at least one selected from a silicon-based active material and a tin-based active material.

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