JP2013062029A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wound lithium ion secondary battery with small performance degradation.SOLUTION: The lithium ion secondary battery comprises a wound electrode group having a positive electrode, a negative electrode, and a separator (1a), a shaft core around which the wound electrode group is wound, and a battery can (3) housing the wound electrode group and the shaft core (2). The lithium ion secondary battery is cylindrical, and since the diameter at the central part of the shaft core in the longitudinal direction is smaller than the diameters at both ends of the shaft core in the longitudinal direction, performance degradation of the secondary battery can be minimized when the electrode expands or contract.

Description

  The present invention relates to a lithium ion secondary battery.

  Generally, the charge / discharge part of a lithium ion secondary battery has a structure in which a positive electrode, a separator, a negative electrode, and a separator are stacked as one layer. A cylindrical lithium ion secondary battery has a structure in which a laminated unit in which a positive electrode, a separator, a negative electrode, and a separator are stacked is wound around a cylindrical shaft core to form a wound electrode group and housed in a cylindrical battery can It is. A wound prismatic lithium ion secondary battery has a structure in which the one laminated unit is wound around a flat plate to form a rectangular wound body and housed in a rectangular battery can. In addition, a single stacked unit that is simply stacked in layers and accommodated in a rectangular battery can is covered with a flexible thin film instead of a stacked rectangular lithium ion secondary battery or battery can. And what the circumference | surroundings welded and sealed is called a laminate-type lithium ion secondary battery.

  A general structure of a cylindrical lithium ion secondary battery is shown in FIG. A wound electrode group 1 is formed by winding one laminated unit composed of a positive electrode, a separator, a negative electrode, and a separator around the shaft core 2, and this is housed in a battery can 3. A positive electrode tab 4 protrudes from one end of the wound electrode group, and a negative electrode tab 7 protrudes from the other end. The positive electrode tab 4 is connected to the positive electrode current collector 5 by a method such as ultrasonic welding, and this is connected to the battery positive electrode 6 to connect to an external circuit. Similarly, the negative electrode tab 7 is welded to the negative electrode current collector 8 by a method such as ultrasonic welding, and this is connected to the battery negative electrode 9 to be connected to an external circuit. The battery can 3 is normally manufactured by deep drawing, and a battery positive electrode is attached to the positive electrode side via a seal portion 10 and sealed.

The detailed structure of the wound electrode group is shown in the enlarged portion in FIG. The positive electrode is obtained by applying a positive electrode active material layer 1c on both sides of an aluminum foil 1b, and the negative electrode is obtained by applying a negative electrode active material layer 1e on both sides of a copper foil 1d. The positive electrode (1c + 1b + 1c) and the negative electrode (1e + 1d + 1e) are alternately stacked with the separator 1a interposed therebetween. The positive electrode active material layer 1c, and a negative electrode active material layer 1e together porous material, the pores are filled with electrolyte solution carrying lithium ions Li ^+, the lithium ion Li ⁺ adsorption between the active material and the electrolyte solution -Desorption occurs. In the case of discharging, electrons are desorbed from lithium in the negative electrode active material layer 1e, become lithium ion Li ⁺ , dissolve in the electrolyte solution, and move into the positive electrode active material layer 1c. In the case of charging, on the contrary, lithium in the positive electrode active material layer 1c moves into the negative electrode active material layer 1e.

  In this way, lithium comes and goes into the positive and negative active materials as it is charged and discharged, but when it comes in, the volume of the active material itself is expanded, and when it comes out, the volume of the active material itself is reduced. Conventionally, an active material has been used in which the amount of expansion and contraction falls within an amount that does not cause a problem. Typical examples are the amount of manganese-based material for the positive electrode and amorphous carbon for the negative electrode. However, in order to increase the battery capacity, ordinary carbon, that is, graphite has been used for the negative electrode active material, and in this case, expansion and contraction of about 10% occur.

When the volume of the installation space does not change when the active material repeats expansion and contraction, the internal stress increases during expansion and the pores collapse, and the active material is cut out during contraction, increasing the number of parts that are partially isolated, The conductive network of electrons and lithium ion Li ⁺ is destroyed, and the performance deteriorates.

  The prior art for preventing this will be described.

  Patent Document 1 has a structure in which stress generated in one electrode is reduced by dividing the electrode. Dividing lines are inserted at some positions in the longitudinal direction of the wound electrode group, and a certain amount of gap is provided between the divided electrodes. This gap absorbs the volume change accompanying the expansion and contraction of the active material.

  Patent Document 2 has a structure in which a positive electrode aluminum foil or a negative electrode copper foil is formed into a corrugated shape to relieve stress applied to the active material from the aluminum foil or copper foil. The corrugation fold is a direction along the longitudinal direction of the axis, and the active material is buried in the gaps between the corrugations of the corrugations, and the electrodes, that is, the active material + the foil + the active material have a uniform thickness. With this configuration, the aluminum foil or the copper foil is freely deformed and the stress generated between the active foil and the active material is relaxed.

Patent Document 3 does not prevent the pore structure from collapsing due to the expansion and contraction of the electrode, but suppresses the performance deterioration by preventing the lithium ions Li ⁺ from diffusing out of the wound electrode group. As the method, a structure in which a gel-like solid electrolyte is installed between the wound electrode group and the battery can is taken.

JP-A-11-120990 JP 2009-181831 A JP 2010-140801 A

In the lithium ion secondary battery, the lithium ion Li ⁺ is transferred by charge and discharge, and the active material repeatedly expands and contracts when it is absorbed and desorbed by the active material of the electrode. Thereby, the pore structure of the electrode is destroyed, and the performance deteriorates. When the cylindrical battery after performance deterioration is disassembled, the amount of compression near the center in the direction along the axis is large, and deterioration near the center is particularly large. When the state of the active material in this part was inspected, it was confirmed that a large amount of lithium ion Li ⁺ remained even after the discharge. This is thought to be because the pore structure was destroyed by the expansion and contraction of the active material, and a large number of island-like parts separated from the conductive network were generated. The reason why such a phenomenon is not observed in the vicinity of both ends in the direction along the axis is considered to be because there is a clearance when the active material expands because both ends are open spaces. In Patent Documents 1 to 3 of the prior art, deterioration near the center in the direction along the axis cannot be efficiently suppressed.

  An object of this invention is to suppress the performance degradation of the axial center longitudinal direction vicinity at the time of the electrode of a lithium ion secondary battery expanding and contracting.

The features of the present invention are as follows, for example.
(1) A lithium ion secondary battery having a wound electrode group having a positive electrode, a negative electrode, and a separator, a shaft core around which the wound electrode group is wound, and a battery can housing the wound electrode group and the shaft core. The lithium ion secondary battery is a lithium ion secondary battery having a cylindrical shape, and the diameter of the central portion in the longitudinal direction of the shaft core is smaller than the diameters of both end portions in the longitudinal direction of the shaft core.
(2) In the above, the outer periphery of the shaft core has a curved shape in which the diameter of the central portion in the longitudinal direction of the shaft core is curved so as to be smaller than the diameter of both end portions in the longitudinal direction of the shaft core. Next battery.
(3) In the above, portions having the same diameter by a certain distance from both ends in the longitudinal direction of the shaft core are provided, and the wound electrode group has the same diameter at both ends in the longitudinal direction of the shaft core. A lithium ion secondary battery that is wound around a portion and a gap is formed between a central portion in the longitudinal direction of the shaft core and the wound electrode group.
(4) The lithium ion secondary battery in which the wound electrode group is wound in accordance with the shape of the shaft core and a gap is formed between the battery can and the wound electrode group.
(5) In the above, a step is provided from a portion having the same diameter at both ends in the longitudinal direction of the shaft toward the central portion in the longitudinal direction of the shaft.
A lithium ion secondary battery in which the diameter of the shaft core is the same from the step toward the central portion in the longitudinal direction of the shaft core.
(6) In the above, the distance in the radial direction of the gap formed between the wound electrode group and the battery can or the gap formed between the wound electrode group and the shaft core is a lithium ion secondary. A lithium ion secondary battery in which the diameters of both end portions and the central portion in the longitudinal direction of the shaft core are set so that the expansion and contraction amount of the wound electrode group accompanying charging and discharging of the battery is 1.5 times or less.

  By this invention, the performance fall of a secondary battery when an electrode expand | swells and shrinks can be suppressed. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

Sectional drawing which shows the structure in 1st embodiment of this invention. Sectional drawing which shows the structure in 2nd embodiment of this invention. Sectional drawing which shows the structure in 3rd embodiment of this invention. Sectional drawing which shows the structure of a prior art.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various modifications by those skilled in the art are within the scope of the technical idea disclosed in this specification. Changes and modifications are possible. In all the drawings for explaining the present invention, components having the same function are denoted by the same reference numerals, and repeated description thereof may be omitted.

  The configuration of the first embodiment of the present invention is shown in FIG. FIG. 1 is a diagram showing a cross section taken along the axial center of a cylindrical lithium ion secondary battery, similarly to FIG. 4 showing the prior art. In FIG. 1, the wound electrode group 1 and the shaft core 2 are housed in a battery can 3.

  As shown in FIG. 4, the shape of the shaft core 2 in the prior art is a straight cylindrical shape, that is, a cylindrical shape having a copper diameter at any location along the shaft core 2. On the other hand, in the shaft core 2 in the first embodiment of the present invention, the diameter of the central portion in the longitudinal direction of the shaft core 2 is smaller than the diameter of both end portions in the longitudinal direction of the shaft core 2 as shown in FIG. Yes. Specifically, it has a curved shape so that the diameter decreases from both longitudinal ends of the shaft core 2 toward the center. As the shaft core 2, aluminum, a stainless alloy, or a resin material (PP, PE, etc.) can be used, but a resin material is preferable in that the weight can be reduced.

  When an electrode group in which a positive electrode, a separator, a negative electrode, and a separator are stacked around the shaft core 2 is wound, the outer periphery thereof, that is, the outer periphery of the wound electrode group 1 becomes a shape along the curved shape of the shaft core 2. When this wound electrode group 1 is placed in a straight cylindrical shape, that is, a cylindrical battery can 3 having the same diameter in any longitudinal direction, the outer circumference of the wound electrode group 1 and the inner circumference of the battery can 3 are A gap 11 is generated between them. The shape of the gap 11 is the same as the curved shape of the shaft core 2, and naturally the same as the curved shape of the outer periphery of the wound electrode group 1. Accordingly, the gap 11 is the largest in the direction along the axis 2, that is, in the central portion in the longitudinal direction, and becomes smaller as it approaches both ends, and there is no gap at both ends in the longitudinal direction, that is, the wound electrode. The outer periphery of the group 1 and the inner periphery of the battery can 3 are in contact with each other. From the viewpoint of absorbing the amount of expansion, as shown in FIG. 1, the gap 11 formed on the outer periphery of the wound electrode group 1 has the same radial thickness as compared to the case where it is formed on the inner periphery of the wound electrode group 1. This is preferable because the space volume becomes large.

As the positive electrode active material layer 1c usable in the lithium ion battery, an oxide containing lithium can be considered. Examples of the oxide containing lithium include an oxide having a layered structure such as LiCoO ₂ , LiNiO ₂ , LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiMn _0.4 Ni _0.4 Co _0.2 O ₂ , Lithium-manganese composite oxides having a spinel structure such as LiMn ₂ O ₄ and Li _{1 + x} Mn _2-x O ₄ , or in these oxides, part of Mn is replaced with other elements such as Al and Mg Can be used.

  As the negative electrode active material layer 1e usable in the lithium ion battery, a carbon material capable of occluding and releasing lithium ions can be considered. Examples of the carbon material that can be used include graphite such as natural graphite and artificial graphite, and amorphous carbon. Since graphite has a large expansion amount associated with insertion and extraction of lithium ions, application of the present invention is particularly effective.

  There is a certain amount of space around the positive electrode tab 4 or the negative electrode tab 7 in the direction along the axis 2, that is, at both ends in the longitudinal direction, and this space can absorb the expansion and contraction of the electrode. That is, when the electrodes expand, the electrodes at both ends can expand into the space around the positive electrode tab 4 or the negative electrode tab 7. On the other hand, the expansion and contraction of the electrode can be absorbed by the gap 11 in the direction along the axis 2, that is, in the vicinity of the center in the longitudinal direction. That is, when the electrode expands, it can expand into the gap 11. In order to absorb the expansion and contraction of the electrode in the gap 11, the longitudinal length of the shaft core 2 is set so that the distance in the radial direction of the gap 11 is one or more times the expansion and contraction amount of the wound electrode group 1 due to charge / discharge. It is necessary to set the diameters of both ends and the center in the direction. However, if the clearance 11 is excessively large, the volume energy density is reduced and the cost is increased. Therefore, both ends of the axial core 2 in the longitudinal direction are 1.5 times or less of the expansion / contraction amount of the wound electrode group 1. It is desirable to set the diameter of the part and the center part. The size of the gap 11 is the largest in the vicinity of the center in the longitudinal direction along the shape of the axis 2, that is, the outer peripheral shape of the wound electrode group 1, and decreases as it approaches both ends. Since the portion close to both ends of the electrode is allowed to expand and contract in the direction along the axis, that is, in the longitudinal direction, the size of the gap 11 that absorbs the expansion and contraction in the radial direction of this portion is reduced, and in the longitudinal direction. This is because the size of the gap 11 is increased in the vicinity of the center where the expansion and contraction is not allowed.

  If the size of the gap 11 is too large, the curvature of the shaft core 2 is increased, and when the electrode group is wound around the shaft core 2, excessive deformation, that is, partial elongation occurs in the electrode group. The initial performance may be impaired. In order to prevent this, it is necessary to suppress the size of the gap 11 to some extent.

  The gap 11 may be large enough to absorb the expansion of the electrode active material. Particularly problematic is the graphite of the negative electrode active material layer 1e, which expands by about 10% during charging and returns to its original volume during discharging. Table 1 shows the configuration and thickness of parts in the radial direction of a 18650 type as a typical example of a cylindrical lithium ion secondary battery, that is, a battery having a diameter of 18 mm and a length of 65 mm.

  From Table 1, the thickness of the negative electrode active material layer 1e is 2 × 0.56 mm = 1.12 mm. In the case of 10% expansion, the expansion coefficient is 0.112 mm. Therefore, the gap 11 may be such a size, and it is considered that an excessive stress is not applied when the wound electrode group 1 is wound around the shaft core 2.

  In the above description, the size of the gap 11 is obtained assuming that only the negative electrode expands by 10%. However, in the actual battery design, the positive electrode may also expand or contract. At the time of charging, lithium enters the negative electrode active material layer 1e and expands the volume. On the other hand, since lithium is desorbed at the positive electrode, the volume may be slightly reduced. Accordingly, the overall expansion amount is slightly smaller than when only the expansion of the negative electrode is taken into consideration, and the gap 11 can be slightly decreased. Therefore, the amount of deformation when winding the wound electrode group 1 around the shaft core 2 can be reduced to a small extent. The actual expansion amount and contraction amount are determined by the selected positive electrode active material layer 1c and negative electrode active material layer 1e, and the gap 11 is set according to the expansion amount of the selected positive electrode active material layer 1c and negative electrode active material layer 1e. There is a need to.

  The configuration of the second embodiment of the present invention is shown in FIG. FIG. 2 is a view showing a cross section along the axial center of a cylindrical lithium ion secondary battery, similar to FIG. 4 showing the prior art and FIG. 1 showing the first embodiment of the present invention.

  In the first embodiment, the wound electrode group 1 is wound in accordance with the curved shape of the shaft core 2, so that the wound electrode group 1 also has a curved shape similar to that of the shaft core 2. A certain amount of distortion occurs, and there is concern about an adverse effect on initial performance.

  On the other hand, in the second embodiment, the shape of the shaft core 2 is made the same diameter from both ends in the longitudinal direction of the shaft core 2 to a certain distance, and the diameter decreases from there toward the central portion in the longitudinal direction. The shape is curved. That is, it was set as the shape which provides the location which becomes a straight cylinder only for a fixed area in the both ends in the longitudinal direction of the axial center 2. As shown in FIG.

  Furthermore, when the electrode group is wound around the shaft core 2, the electrode core 2 is wound around a portion having the same diameter at both ends in the longitudinal direction, and without following the curved shape near the center of the shaft core 2. Then, it winds up to the outermost circumference while keeping a straight cylindrical shape. In FIG. 2, the inner surface of the wound electrode group 1 does not touch the shaft core 2 except for both ends in the longitudinal direction of the shaft core 2. Therefore, the shape of the wound electrode group 1 is a straight cylindrical shape in which the inner diameter is the same as the outer diameter of both ends in the longitudinal direction of the shaft core 2. The shaft core 2 has a slit for inserting the collar portion of the wound electrode group 1, and the wound electrode group 1 is wound on the shaft core 2 by inserting the wound electrode group 1 into the slit.

  With such a shape, there is no gap 11 between the outer periphery of the wound electrode group 1 and the inner periphery of the battery can 3, and there is no gap 11 between the inner periphery of the wound electrode group 1 and the curved portion of the shaft core 2. Can do. The shape of the gap 11 is large in the vicinity of the center in the longitudinal direction along the curved shape of the shaft core 2, and becomes smaller as it is closer to both ends. The expansion of the electrode is absorbed by the gap 11.

  With such a configuration, the same effects as those of the first embodiment can be obtained without giving deformation when the wound electrode group 1 is wound around the shaft core 2.

  The setting of the size of the gap 2 is the same as in the first embodiment.

  The configuration of the third embodiment of the present invention is shown in FIG. In the second embodiment, as shown in FIG. 2, the shaft core 2 has the same diameter from both ends to a certain distance, and is curved so as to become narrower from the center in the longitudinal direction.

  On the other hand, in the third embodiment, as shown in FIG. 3, the shape of the shaft core 2 is the same up to the same diameter from both ends to a certain distance, but a step is formed from there to the center. The diameter of the shaft core 2 is the same diameter that is thinner than both ends. As the shape of the shaft core 2, holes may be provided at both end portions in the longitudinal direction of the shaft core 2, and a center portion in the longitudinal direction of the shaft core 2 may be inserted into the holes provided at both end portions. The effect of forming a gap 11 between the outer periphery of the shaft core 2 and the inner periphery of the wound electrode group 1 is the same as that of the second embodiment, but the shape of the shaft core 2 is simple and the manufacturing cost is reduced. effective. The cross-sectional shape of the shaft core 2 from the step to the center may be a rectangle other than the circle as shown in FIG.

1 wound electrode group 1a separator 1b positive electrode current collector foil (aluminum foil)
1c Positive electrode active material layer 1d Negative electrode current collector foil (copper foil)
1e negative electrode active material layer 2 shaft core 3 battery can 4 positive electrode tab 5 positive electrode current collector 6 battery positive electrode 7 negative electrode tab 8 negative electrode current collector 9 battery negative electrode 10 seal part 11 gap

Claims (6)

A wound electrode group having a positive electrode, a negative electrode and a separator;
An axis around which the wound electrode group is wound;
A battery can that houses the wound electrode group and the shaft core, and a lithium ion secondary battery comprising:
The lithium ion secondary battery is cylindrical.
A lithium ion secondary battery in which a diameter of a central portion in a longitudinal direction of the shaft core is smaller than a diameter of both end portions in a longitudinal direction of the shaft core. In claim 1,
The lithium ion secondary battery in which the outer periphery of the shaft core is curved so that the diameter of the central portion in the longitudinal direction of the shaft core is smaller than the diameter of both end portions in the longitudinal direction of the shaft core. In claim 1 or 2,
Providing a portion having the same diameter by a certain distance from both ends in the longitudinal direction of the shaft;
The wound electrode group is wound around a portion having the same diameter at both ends in the longitudinal direction of the shaft core,
A lithium ion secondary battery in which a gap is formed between a central portion in a longitudinal direction of the shaft core and the wound electrode group. In claim 1 or 2,
The wound electrode group is wound in accordance with the shape of the shaft core,
A lithium ion secondary battery in which a gap is formed between the battery can and the wound electrode group. In claim 3,
A step is provided from a portion having the same diameter at both end portions in the longitudinal direction of the shaft core toward a central portion in the longitudinal direction of the shaft core,
A lithium ion secondary battery in which a diameter of the shaft core is the same from the step toward a central portion in a longitudinal direction of the shaft core. In any one of Claims 1 thru | or 5,
A gap formed between the wound electrode group and the battery can, or
The radial distance of the gap formed between the wound electrode group and the shaft core is not more than 1.5 times the expansion / contraction amount of the wound electrode group associated with charge / discharge of the lithium ion secondary battery. The lithium ion secondary battery in which the diameters of both end portions and the central portion in the longitudinal direction of the shaft core are set so that