JP2013041742A - Negative electrode and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the cycle characteristics of a secondary battery.SOLUTION: A negative electrode 15 comprises a sheet-like negative electrode collector 18, a first negative electrode active material layer 19 formed by coating the external surface 181 side (the side where a convex surface is formed by winding) of the negative electrode collector 18 with a negative electrode active material, and a second negative electrode active material layer 20 formed by coating the internal surface 182 side (the side where a concave surface is formed by winding) of the negative electrode collector 18 with a negative electrode active material. The first negative electrode active material layer 19 contains graphite and SiOx as the negative electrode active material capable of occluding and discharging lithium ions. The wt.% of graphite to the composite material in the second negative electrode active material layer 20 is larger than the wt.% of graphite to the SiOx in the first negative electrode active material layer 19.

Description

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

  A secondary battery in which a material capable of inserting and extracting lithium ions (negative electrode active material) is applied to a plate-shaped negative electrode current collector (see, for example, Patent Documents 1 and 2) has high energy density and excellent cycle characteristics. Battery. A negative electrode active material is applied to both surfaces of the plate-like negative electrode current collector to form a negative electrode active material layer, and the negative electrode active material layer occludes and releases lithium ions during charge and discharge. The negative electrode active material layer increases in volume when lithium ions are occluded and decreases in volume when lithium ions are released.

JP-A-8-17472 JP 2009-16339 A

  A configuration in which a thin negative electrode is wound as disclosed in Patent Documents 1 and 2 is preferable in increasing the reaction area of the negative electrode. When the negative electrode is wound (bent), stress is generated in the direction in which the concave surface of the bent negative electrode is compressed, but when the negative electrode active material layer on the concave surface side of the negative electrode occludes lithium ions, the negative electrode active material layer on the concave surface side of the negative electrode Generates a stress that tends to extend in the opposite direction of the stress. This stress has no escape because there is the compressive stress, stress concentrates on the negative electrode active material layer on the concave side of the negative electrode, cracks occur in the negative electrode active material layer on the concave side of the negative electrode, and the negative electrode active material on the concave side Layers can peel or fall off. When the negative electrode active material layer on the concave surface side of the negative electrode is peeled or dropped, the cycle characteristics are deteriorated.

  An object of this invention is to provide the negative electrode in the secondary battery which can improve cycling characteristics.

  The inventions of claims 1 to 4 are directed to a sheet-like negative electrode including a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector. The negative electrode has a bent portion that is bent, and a first negative electrode active material layer is formed on a convex surface of the curved portion that is at least part of the outer surface side of the negative electrode, and at least one of the inner surface side of the negative electrode. A second negative electrode active material layer is formed on the concave surface of the curved portion, which is a portion, and the first negative electrode active material layer is formed by absorption or release of lithium from the first active material and the first active material. A second active material having a small volume change rate, wherein the second negative electrode active material layer includes the second active material, and the weight percent of the second active material in the second negative electrode active material layer is the second active material. It is larger than the weight% of the second active material in one negative electrode active material layer.

  The volume change rate here is a volume change with respect to insertion and extraction of lithium ions. Since the second negative electrode active material layer on the concave surface side of the negative electrode has a small volume change rate with respect to insertion and extraction of lithium ions, stress concentration in the second negative electrode active material layer is relaxed. As a result, the peeling or dropping of the second negative electrode active material layer on the concave surface side is suppressed, and the cycle characteristics are improved.

In a preferred example, the first active material is a silicon oxide represented by SiOx (0.3 ≦ x ≦ 1.6), and the second active material is a carbon-based material.
In a preferred example, the first active material is a silicon oxide represented by SiOx (0.3 ≦ x ≦ 1.6), and the second active material is graphite.

  In a preferred example, the second negative electrode active material layer includes the first active material, and the weight percent of the second active material with respect to the first active material in the second negative electrode active material layer is the first negative electrode. It is larger than the weight% of the second active material with respect to the first active material in the active material layer.

In addition, the comparison of the weight% of a 1st negative electrode active material layer and a 2nd negative electrode active material layer is performed with respect to per unit layer thickness of a 1st negative electrode active material layer and a 2nd negative electrode active material layer.
As the negative electrode active material, use of silicon oxide (SiOx: x is about 0.5 ≦ x ≦ 1.5) has been studied. It is known that SiOx decomposes into Si and SiO2 when heat-treated. This is called a disproportionation reaction, and if it is a homogeneous solid silicon monoxide SiO having a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO 2 phase by solid internal reaction. The Si phase obtained by separation is very fine. That is, the silicon oxide represented by SiOx (0.3 ≦ x ≦ 1.6) refers to a composite material of Si and SiO2 that decomposes into Si and SiO2 when heat-treated.

  Carbon-based materials refer to carbon-based materials. Specifically, in addition to graphite, pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), glassy carbon Organic polymer compound fired body (furan resin etc. fired at an appropriate temperature and carbonized), carbon fiber, activated carbon and the like.

  The volume change rate of graphite is smaller than the volume change rate of the composite material of Si and SiO 2, and the volume change rate of the negative electrode active material layer can be changed by changing the weight percent of graphite and the composite material. That is, a mixture of graphite and a composite material is a negative electrode active material suitable for obtaining a desired volume change rate.

  The invention of claim 4 is directed to a secondary battery in which a positive electrode, a negative electrode, and a non-aqueous electrolyte in which the positive electrode and the negative electrode are immersed are contained in a container, and the negative electrode is described in claims 1 to 3. The negative electrode described in any one of the items was used.

  The present invention has an excellent effect that the cycle characteristics of the secondary battery can be improved.

Sectional drawing which shows 1st Embodiment. AA sectional view taken on the line AA of FIG. A 2nd embodiment is shown and (a) is a sectional view. (B) is the BB sectional drawing of Fig.3 (a).

Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 1 and 2.
As shown in FIG. 1, the container 11 constituting the secondary battery 10 includes a vessel part 12 that opens upward and a lid 13 that closes the upper opening of the vessel part 12.

  As shown in FIG. 2, the wound electrode 14 and the non-aqueous electrolyte are accommodated in the vessel portion 12. The wound electrode 14 is immersed in a nonaqueous electrolytic solution. As the nonaqueous electrolytic solution, a lithium salt as an electrolyte and dissolved in an organic solvent is used.

  The wound electrode 14 includes a sheet-like negative electrode 15, a sheet-like positive electrode 16, and a separator 17 interposed between the negative electrode 15 and the positive electrode 16. The positive electrode 16 and the negative electrode 15 are formed in a shape that is wound in a state where the separator 17 is sandwiched between the positive electrode 16 and the negative electrode 15.

  As shown in FIG. 1, a negative electrode lead 24 is electrically connected to the negative electrode 15, and the negative electrode lead 24 is electrically connected to the vessel portion 12. A positive electrode lead 25 is electrically connected to the positive electrode 16, and the positive electrode lead 25 is electrically connected to a positive electrode pin 26 fixed to the lid 13.

  As shown in FIG. 2, the negative electrode 15 includes a sheet-like negative electrode current collector 18 and a negative electrode on the outer surface 181 side of the negative electrode current collector 18 (the outer surface side of the negative electrode 15 that is wound to produce a convex surface). The negative electrode active material is applied to the first negative electrode active material layer 19 formed by applying the active material and the inner surface 182 side of the negative electrode current collector 18 (the side on the inner surface side of the negative electrode 15 that is wound to form a concave surface). The second negative electrode active material layer 20 is formed by coating. In the present embodiment, the thickness of the first negative electrode active material layer 19 and the thickness of the second negative electrode active material layer 20 are substantially the same. In FIG. 2, the curved portion 141 (curved portion of the negative electrode 15) of the wound electrode 14 is enlarged and illustrated, and the convex surface 181 </ b> C of the outer surface 181 and the concave surface 182 </ b> C of the inner surface 182 are enlarged and illustrated.

  The positive electrode 16 has positive electrode actives on the sheet-like positive electrode current collector 21, and on the outer surface 211 side (the side that is wound to produce a convex surface) and the inner surface 212 side (the side that is wound to produce a concave surface). It is comprised from the positive electrode active material layers 22 and 23 formed by apply | coating a substance.

  The positive electrode current collector 21 is made of, for example, aluminum, and the positive electrode active material layers 22 and 23 are, for example, LixMO2 that is a composite oxide of lithium and a transition metal (where M represents one or more transition metals, and x Contains, for example, 0.05 to 1.10) as the positive electrode active material. Graphite is added as a conductive agent to the lithium composite oxide, and for example, polyvinylidene fluoride is used as the binder.

  The negative electrode current collector 18 is made of, for example, copper. The first negative electrode active material layer 19 formed on the outer surface 181 side of the negative electrode current collector 18 is made of SiOx (0.3 ≦ x ≦ 1.6) as a first active material capable of inserting and extracting lithium ions. And a carbon-based material (graphite in this embodiment) as a second active material capable of occluding and releasing lithium ions. The first negative electrode active material layer 19 contains acetylene black or ketjen black as a conductive agent and polyamideimide as a binder.

  The weight percent of graphite relative to SiOx in the second negative electrode active material layer 20 formed on the inner surface 182 side of the negative electrode current collector 18 is greater than the weight percent of graphite relative to SiOx in the first negative electrode active material layer 19. Assuming that the total weight percent of graphite and SiOx is 100 weight percent, the weight percent of graphite and the weight percent of SiOx in the first negative electrode active material layer 19 are 30% by weight of graphite: SiOx: 70% by weight to 40% by weight. %: 60% by weight. On the other hand, the weight percentage of graphite and SiOx in the second negative electrode active material layer 20 is 70 wt%: 30 wt% to 60 wt%: 40 wt% of graphite: SiO x.

  That is, the second negative electrode active material layer 20 includes SiOx as the first active material and graphite (carbon-based material) as the second active material, and the graphite (second material) in the second negative electrode active material layer 20. The weight percent of the active material) is greater than the weight percent of graphite (second active material) in the first negative electrode active material layer 19.

Next, the operation of the first embodiment will be described.
When the positive electrode 16 and the negative electrode 15 are wound through the separator 17 to form the wound electrode 14, a larger stress is generated on the inner surface side of the negative electrode 15 than on the outer surface side. Further, when lithium ions are absorbed or released, the volume of graphite and SiOx in the negative electrode 15 changes.

  Si alone has a large volume change rate, but SiOx is one of the preferable materials for suppressing deterioration of cycle characteristics due to volume change. The volume change rate of graphite is smaller than the volume change rate of SiOx. Therefore, the volume change rate of the second negative electrode active material layer 20 in which the graphite weight percent is greater than the SiOx weight percent is the volume change rate of the first negative electrode active material layer 19 in which the SiOx weight percent is greater than the graphite weight percent. Smaller than. As a result, the volume change rate of the second negative electrode active material layer 20 is smaller than the volume change rate of the first negative electrode active material layer 19.

  The reduction in the volume change rate of the second negative electrode active material layer 20 alleviates stress concentration in the second negative electrode active material layer 20 on the concave surface 182C side of the curved portion 141 of the negative electrode 15 where stress concentration tends to occur. As a result, the generation of cracks in the second negative electrode active material layer 20 and, consequently, the peeling and dropping of the second negative electrode active material layer 20 are suppressed.

In the first embodiment, the following effects can be obtained.
(1) In the second negative electrode active material layer 20 on the concave surface side of the negative electrode 15, a negative electrode active material having a small volume change rate with respect to insertion and extraction of lithium ions is selected. Therefore, the stress concentration in the second negative electrode active material layer 20 is relaxed. As a result, the peeling or dropping of the second negative electrode active material layer 20 on the concave surface 182C side of the negative electrode 15 where stress is easily concentrated is suppressed, and the cycle characteristics are improved.

  (2) The volume change rate in graphite is smaller than the volume change rate of the composite material of Si and SiO 2 (SiOx), and the volume of the negative electrode active material layers 19 and 20 can be changed by changing the weight percentage of graphite and SiOx. The rate of change can be changed. That is, a mixture of graphite and SiOx is a negative electrode active material suitable for obtaining a desirable volume change rate.

Next, the second embodiment shown in FIGS. 3A and 3B will be described.
As shown in FIG. 3A, the container 29 that constitutes the secondary battery 28 includes a container part 30 that opens upward and a lid 31 that closes the upper opening of the container part 30.

  As shown in FIG. 3 (b), the container unit 30 accommodates a stacked cell 34 in which a plurality of sheet-like negative electrodes 32 and a plurality of sheet-like positive electrodes 33 are alternately stacked. The laminated cell 34 is formed in a shape bent into a U shape. An insulating separator 35 is interposed between the adjacent negative electrode 32 and positive electrode 33. The separator 35 prevents electrical contact between the negative electrode 32 and the positive electrode 33.

  A current collecting terminal 36 is electrically connected to each negative electrode 32, and a negative electrode lead terminal 37 is electrically connected to the current collecting terminal 36. The negative electrode lead terminal 37 penetrates the lid 31 and protrudes out of the container 29. A current collecting terminal 38 is electrically connected to each positive electrode 33, and a positive electrode lead terminal 39 is electrically connected to the current collecting terminal 38. The positive electrode lead terminal 39 penetrates the lid 31 and protrudes out of the container 29.

  The negative electrode 32 includes a sheet-like negative electrode current collector 40 and a first negative electrode active material layer 41 formed by applying a negative electrode active material to the outer surface 401 side of the negative electrode current collector 40 (the side that is bent to produce a convex surface). And a second negative electrode active material layer 42 formed by applying a negative electrode active material to the inner surface 402 side of the negative electrode current collector 40 (the side that is bent to produce a concave surface). In the present embodiment, the thickness of the first negative electrode active material layer 41 and the thickness of the second negative electrode active material layer 42 are substantially the same.

  The positive electrode 33 includes a sheet-like positive electrode current collector 43, and positive electrode active material on the outer surface 431 side (the side where the convex surface is bent and the concave surface is generated) and the inner surface 432 side (the side where the concave surface is bent and the concave surface is generated). The positive electrode active material layers 44 and 45 are formed by coating.

  The ratio of the weight percent of graphite and SiOx in the first negative electrode active material layer 41 is 30 wt%: 70 wt% to 40 wt%: 60 wt%. On the other hand, the weight percentage of graphite and SiOx in the second negative electrode active material layer 42 is 70 wt%: 30 wt% to 60 wt%: 40 wt%.

In the second embodiment, the same effect as in the first embodiment can be obtained.
In the present invention, the following embodiments are also possible.
In the first and second embodiments, the second negative electrode active material layer 20 is formed on the entire inner surface side of the negative electrode, but the second negative electrode active material layer 20 is formed only on the concave surface on the inner surface side of the negative electrode. The first negative electrode active material layer 19 may be formed on the remaining inner surface side. That is, the second negative electrode active material layer 20 may be formed on at least a part of the inner surface side of the negative electrode.

  ○ As a negative electrode active material, instead of graphite, pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), glassy carbons, organic polymer compound fired bodies (furan resin, etc.) at an appropriate temperature Carbon-based materials such as those obtained by firing and carbonization), carbon fiber, activated carbon and the like may be used.

In addition to silicon oxide, titanium oxide may be used as the first active material.
The technical idea that can be grasped from the embodiment described above will be described below.
(B) The secondary battery according to claim 4, wherein the positive electrode and the negative electrode are formed in a shape wound in a state of being along each other.

  10, 27 ... Secondary battery. 11, 29 ... container. 14: A wound electrode. 141: curved portion. 15, 32 ... negative electrode. 16, 33 ... positive electrode. 18, 40 ... negative electrode current collector. 181, 401 ... external surface. 181C ... Convex surface. 182 ... The inner surface. 182C ... concave surface. 19, 41 ... 1st negative electrode active material layer. 20, 42 ... second negative electrode active material layer. (SiOx) A composite material as the first active material.

Claims (6)

In a sheet-like negative electrode comprising a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector,
The negative electrode has a bent portion bent,
A first negative electrode active material layer is formed on the convex surface of the curved portion, which is at least part of the outer surface side of the negative electrode,
A second negative electrode active material layer is formed on the concave surface of the curved portion that is at least a part of the inner surface side of the negative electrode,
The first negative electrode active material layer includes a first active material and a second active material having a smaller volume change rate due to absorption or release of lithium than the first active material,
The second negative electrode active material layer includes the second active material,
The negative electrode characterized in that the weight percent of the second active material in the second negative electrode active material layer is greater than the weight percent of the second active material in the first negative electrode active material layer.   The first active material is a silicon oxide represented by SiOx (0.3 ≦ x ≦ 1.6), and the second active material is a carbon-based material. The negative electrode described.   The first active material is a silicon oxide represented by SiOx (0.3 ≦ x ≦ 1.6), and the second active material is graphite. 3. The negative electrode according to any one of 2 above.   The second negative electrode active material layer includes the first active material, and the weight percentage of the second active material with respect to the first active material in the second negative electrode active material layer is equal to the weight percentage of the first negative electrode active material layer. 4. The negative electrode according to claim 1, wherein the negative electrode is larger than the weight percent of the second active material with respect to the first active material. 5.   A secondary battery comprising a sheet-like positive electrode, the negative electrode according to any one of claims 1 to 4, and an electrolytic solution in which the positive electrode and the negative electrode are immersed.   The secondary battery according to claim 5, wherein the positive electrode and the negative electrode are wound electrodes.

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