JP5039933B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  Nonaqueous electrolyte secondary batteries are attracting attention as power sources for electronic devices because of their high energy density and light weight. In particular, button-type nonaqueous electrolyte secondary batteries are widely used in portable electronic devices such as mobile phones because of their small size, and are steadily increasing.

  For mounting this type of button-type non-aqueous electrolyte secondary battery on a printed circuit board, reflow soldering is adopted as the printed circuit board becomes smaller. In reflow soldering, the solder is melted by passing through a furnace having a high temperature atmosphere of 200 ° C. to 260 ° C. above the melting point of the solder, and the non-aqueous electrolyte secondary battery is printed on the printed circuit board with the melted solder. It is to be welded.

By the way, when the reflow soldering is performed, the nonaqueous electrolyte secondary battery is passed through a high temperature atmosphere of 200 ° C. to 260 ° C., so that the electrolyte in the secondary battery is vaporized and decomposed to generate gas. . As a result, due to the generation of this gas, in the secondary battery, the contact inside the battery decreases, and the current collection decreases. Therefore, in Patent Document 1, a concave portion is formed on the bottom surface of a negative electrode can made of a stainless steel layer and an aluminum layer to suppress a decrease in contact due to gas generated by vaporization and decomposition of the electrolytic solution, thereby preventing a decrease in current collection. Proposed.
JP 2004-95399 A

  By the way, in the said recessed part, an electrochemical reaction arises by contacting with electrolyte solution in the state which lithium contact | adhered to the aluminum layer of this recessed part, and a lithium aluminum alloy is made in this recessed part. When the charge / discharge cycle is repeated, insertion and extraction of lithium ions are repeated in the alloy layer, and accordingly, the volume of the alloy layer repeatedly expands and contracts. The stress of the alloy layer accompanying the expansion and contraction is concentrated at the corner of the recess, becomes brittle from the corner, cracks and collapses, and the conduction between the stainless steel layer of the negative electrode can and the lithium-aluminum alloy layer. The area is reduced. As a result, the battery internal resistance increased and the capacity deteriorated during the charge / discharge cycle.

  The present invention has been made in order to solve the above-mentioned problems, and its purpose is to crack or collapse the alloy layer due to expansion and contraction of the volume of the alloy layer accompanying insertion and extraction of lithium ions. It is in providing the nonaqueous electrolyte secondary battery which can suppress.

The invention according to claim 1 is a button type in which a negative electrode using aluminum or an aluminum alloy capable of occluding and releasing lithium ions as an active material is accommodated in an anode can and an electrolyte is injected. In the non-aqueous electrolyte secondary battery, a recess was formed on the bottom surface of the negative electrode can, and the inner surface of the recess was curved.

According to the present invention, even if gas is generated by vaporization and decomposition of the electrolyte, it is possible to prevent the current collection from being lowered because the depression in the battery is suppressed by the recess formed on the bottom surface. Moreover, since the inner surface of the concave portion is curved, there is no continuous portion where the stress is concentrated inside the concave portion, and the stress of the alloy layer when it expands and contracts with the insertion and extraction of lithium ions is dispersed. Therefore, it is difficult for the lithium / aluminum alloy layer to crack or collapse. As a result, the lithium-aluminum alloy layer cracks and collapses early, reducing the conductive area between the stainless steel layer of the negative electrode can and the lithium-aluminum alloy layer, increasing the internal resistance of the battery and degrading the capacity during the charge cycle. Suppress the invitation.

  According to a second aspect of the present invention, in the nonaqueous electrolyte secondary battery, the negative electrode can has a two-layer structure including a stainless steel layer on the outer side and a layer made of aluminum or an aluminum alloy on the inner side. The concave portion was formed in the alloy layer.

  According to this, a layer made of aluminum or an aluminum alloy is made into a lithium-aluminum alloy layer by being brought into contact with the electrolytic solution in a state of being pressed against an adjacent lithium foil.

According to a third aspect of the present invention, in the nonaqueous electrolyte secondary battery, the recess is a hemispherical recess.
According to this, since the inner surface of the concave portion is a hemispherical curved surface, there is no continuous portion where stress is concentrated inside the concave portion, and the alloy layer expands and contracts along with insertion and extraction of lithium ions. Can be dispersed.

According to a fourth aspect of the present invention, in the nonaqueous electrolyte secondary battery, the recess is a conical recess.
According to this, since the inner side surface of the concave portion is a conical curved surface, there is no continuous portion where stress is concentrated inside the concave portion, and the alloy layer expands and contracts with insertion and extraction of lithium ions. Can be dispersed.

According to a fifth aspect of the present invention, in the nonaqueous electrolyte secondary battery, the concave portion is a groove having a circular cross section.
According to this, since the concave portion is a groove having a circular cross section, there is no continuous portion where stress is concentrated inside the groove, and the stress of the alloy layer when expanding and contracting with insertion and extraction of lithium ions is eliminated. Can be dispersed.

Hereinafter, an embodiment embodying the present invention will be described with reference to the drawings.
In FIG. 1, a nonaqueous electrolyte secondary battery 10 has a bottomed cylindrical positive electrode can 11, and an opening 11 a of the positive electrode can 11 is circular. The positive electrode can 11 is made of a stainless material. The positive electrode can 11 has a positive electrode pellet 12 disposed on the bottom surface 11 b via a positive electrode current collector 13.

  The positive electrode pellet 12 is prepared by mixing LiMn-containing oxide, graphite as a conductive agent, and polyacrylic acid resin as a binder in a ratio of 90: 7: 3 to form a positive electrode mixture, and pressurizing the positive electrode mixture Molded into a pellet. In this embodiment, 7.5 mg of the positive electrode mixture was processed into pellets having a diameter of 2.2 mm at 2 ton / cm 2.

  The positive electrode pellet 12 is bonded to the bottom surface 11 b of the positive electrode can 11 using a positive electrode current collector 13 made of a conductive resin adhesive containing carbon as a conductive filler. Then, when the positive electrode pellet 12 is bonded to the positive electrode can 11 via the positive electrode current collector 13 and the positive electrode pellet 12 and the positive electrode can 11 are integrated (unitized), the positive electrode unit is dried by heating under reduced pressure. In this embodiment, drying under reduced pressure is performed at 280 ° C. for 8 hours.

A lithium foil 15 is disposed above the positive electrode pellet 12 via a separator 14. The separator 14 separates the positive electrode pellet 12 and the lithium foil 15, and an insulating film having a high ion permeability and a predetermined mechanical strength is used. Further, the separator 14 preferably has heat heat resistance in consideration of reflow soldering. In the present embodiment, a nonwoven fabric made from glass fiber having a thickness of 200 micrometers is dried and then punched into a disk shape of φ3 mm is used.

The lithium foil 15 is punched to a diameter of 2 mm and a thickness of 0.22 mm, and is pressed against the bottom surface 16b of the negative electrode can 16 disposed on the upper side.
The negative electrode can 16 has a cylindrical shape with a lid, and a gasket 17 is attached to the circular opening 16a. Then, the negative electrode can 16 is fitted into the opening 11 a of the positive electrode can 11 from the side of the opening 16 a to which the gasket 17 of the negative electrode can 16 is attached, and the opening 11 a of the positive electrode can 11 is directed toward the gasket 17. The positive electrode can 11 and the negative electrode can 16 are connected and fixed to each other by caulking and sealing. Then, the sealed space S is formed between the positive electrode can 11 and the negative electrode can 16 via the gasket 17 by connecting and fixing the positive electrode can 11 and the negative electrode can 16.

  The sealed space S containing the positive electrode pellet 12, the positive electrode current collector 13, the separator 14, and the lithium foil 15 is filled with an electrolytic solution 18. In this embodiment, the electrolytic solution 18 is a mixed solvent of tetraglyme and diglyme, in which lithium perfluoromethylsulfonylimide is dissolved at a concentration of 1 mol / l in a mixed solvent whose volume percentage is 50:50. The sealed space S was filled with 5 microliters.

  The negative electrode can 16 has a two-layer structure of a stainless steel layer 21 and a hard aluminum layer 22. More specifically, the negative electrode can 16 is obtained by bonding stainless steel and hard aluminum together by rolling, and is formed such that the stainless steel layer 21 is on the outside and the hard aluminum layer 22 is on the inside. And the lithium foil 15 is pressure-bonded to the hard aluminum layer 22 side with respect to the negative electrode can 16 formed in this way. The negative electrode can 16 to which the lithium foil 15 is pressure-bonded is connected and fixed to the positive electrode can 11.

  A plurality of hemispherical recesses 23 are formed on the bottom surface 16b of the negative electrode can 16 (the hard aluminum layer 22 where the lithium foil 15 is pressed). Specifically, as shown in FIG. 2, one recess 23 at the center position of the bottom surface 16b, eight recesses 23 surrounding the recess 23 at the center position at equiangular intervals, and at the same angle on the outside thereof Twelve recesses 23 are formed in total, with twelve recesses 23 surrounding.

  As shown in FIG. 3, each recess 23 formed in the bottom surface 16b is a hemispherical recess having the same shape, and in this embodiment, the recess has a diameter of 0.2 mm and a depth of 0.05 mm. is there. Moreover, the opening edge 23a of each recessed part 23 is formed in circular arc shape so that it may become a curved surface. Accordingly, all the recesses 23 have curved inner surfaces.

  When the positive electrode can 11 and the negative electrode can 16 are connected and fixed, when the electrolytic solution 18 comes into contact with the hard aluminum layer 22 and the lithium foil 15 in pressure contact, the lithium foil is placed in the recess 23 of the hard aluminum layer 22. 15 enters and adheres, and a lithium-aluminum alloy is formed in the recess 23.

Next, the operation of the nonaqueous electrolyte secondary battery 10 configured as described above will be described.
Now, when the hard aluminum layer 22 and the lithium foil 15 are pressure-bonded and the electrolyte solution 18 comes into contact with the lithium foil 15 entering and closely contacting the recess 23 of the hard aluminum layer 22, the electrochemical reaction also occurs in the recess 23. As a result, self-alloying proceeds and a lithium-aluminum alloy layer is formed.

  When the nonaqueous electrolyte secondary battery 10 is repeatedly charged and discharged, lithium ions are repeatedly occluded and released also in the lithium-aluminum alloy layer of the recess 23, and the volume of the alloy layer expands and contracts accordingly. Will be repeated.

  The stress applied to the lithium / aluminum alloy layer due to the expansion and contraction is dispersed without any concentrated points since the recesses 23 are hemispherical, that is, curved. Therefore, since there is no portion where stress is concentrated, the lithium-aluminum alloy layer is hardly cracked or collapsed. As a result, the lithium-aluminum alloy layer cracks and collapses early, reducing the conductive area between the stainless steel layer and the lithium-aluminum alloy layer of the negative electrode can, increasing the internal resistance of the battery, and degrading the capacity during the charge / discharge cycle Is suppressed.

  In the present embodiment, the recess 23 is hemispherical, but it may be a conical recess having a curved surface and no continuous portion where stress is concentrated. Also in this case, it is possible to suppress the occurrence of early cracks and collapse of the lithium / aluminum alloy layer.

Incidentally, the number of charge / discharge cycles was verified for each of the various shapes of the recesses 23 formed on the bottom surface 16b (hard aluminum layer 22).
Therefore, the shape of the plurality of recesses 23 formed on the bottom surface 16b (hard aluminum layer 22) of the present embodiment is shown in FIG. 5 and FIG. 5 as a hemispherical recess 23 (Example 1). Thus, the shape of the plurality of recesses 23 formed on the bottom surface 16b (hard aluminum layer 22) is formed on the bottom surface 16b (hard aluminum layer 22) as shown in FIG. The shape of the plurality of recessed portions 23 is changed to a square pyramid-shaped recessed portion 23c (Comparative Example 1), a plurality of recessed portions 23 formed on the bottom surface 16b (hard aluminum layer 22) as shown in FIG. Each of the secondary batteries having the convex portion 23d (Comparative Example 2) was verified.

  Specifically, the open circuit voltage and the AC internal resistance at 1 kHz were measured for each level of 20 secondary batteries. Thereafter, heat treatment was performed three times in a reflow furnace at a preheating temperature of 180 ° C. for 10 minutes and a maximum heating temperature of 260 ° C. for 20 seconds. After the battery is cooled to room temperature, the open circuit voltage and the AC internal resistance are measured. Under predetermined conditions, charging / discharging is performed 20 times, and the open circuit voltage and the AC internal resistance are measured again. The presence or absence of cracks in the layer portion was confirmed.

In addition, the number of cycles in which 90% capacity could be maintained when charging / discharging 20 times was examined based on the capacity after reflow.
The verification results are shown in Table 1, Table 2, and Table 3.

表１は、リフロー前後、２０回充放電後の電池水準２０個の直流電圧と交流内部抵抗（１ＫＨｚ）の平均値の結果を示した。

Table 1 shows the results of the average values of the DC voltage and AC internal resistance (1 KHz) of 20 battery levels before and after reflow and after 20 charge / discharge cycles.

Table 2 shows the results of visual check of the lithium / aluminum alloy layer after 20 charge / discharge cycles.
Table 3 shows the results for the number of cycles in which 90% capacity could be maintained when charging / discharging 20 times based on the capacity after reflow.

  As shown in Table 1, in the case of Example 1, although the open circuit voltage is not much different from Example 2 and Comparative Examples 1 and 2, the increase in AC internal resistance was the highest after reflow and after 20 charge / discharge cycles. Small and good results were obtained. In other words, the smaller the internal resistance value, the better the battery.

  Further, as shown in Table 2, Example 1 had the smallest number of visual cracks in the lithium / aluminum alloy layer after 20 charge / discharge cycles, and good results were obtained. Similarly, Example 2 also had a low rate of occurrence compared to Comparative Examples 1 and 2 although cracking occurred.

  As shown in Table 3, when charging / discharging 20 times, the best result was obtained in Example 1 with respect to the number of cycles in which 90% capacity could be maintained. Better results than 2 were obtained.

This is because when the hard aluminum layer 22 is alloyed with the lithium foil 15 and the charge / discharge cycle is repeated, the alloy layer repeats volume expansion / contraction along with insertion / extraction of lithium ions. At that time, in the case of an uneven shape having a corner, stress at the time of volume expansion / contraction of the alloy layer concentrates on the corner, so that it becomes brittle from the corner and easily cracks and collapses.

  Therefore, in Comparative Examples 1 and 2 having many corner portions, cracks and collapse of the alloy layer frequently occurred, and as a result, it was difficult to take a lead with the negative electrode can 16 and an increase in AC internal resistance occurred. Conceivable. On the other hand, in Examples 1 and 2, which do not have corners, cracks and collapse of the alloy layer did not occur as in Comparative Examples 1 and 2, and thus the increase in AC internal resistance is considered to be small.

  In addition, since the lithium-aluminum alloy layer itself is an electrode, Comparative Examples 1 and 2 in which many cracks and collapse of the alloy layer occurred can no longer function as an electrode, and as a result, the number of cycles that can maintain the capacity In contrast, Example 1 and Example 2 are comparative examples in which the uneven shape is a curved surface and there are no corners, so there are few occurrences of cracks and the like, and the number of cycles that can maintain a capacity of 90% or more is a comparative example. 1 and it is considered that the result was better than Comparative Example 2.

Next, effects of the present embodiment configured as described above will be described below.
(1) In this embodiment, the recess 23 is formed on the bottom surface 16 b of the negative electrode can 16. Therefore, even if gas is generated due to vaporization and decomposition of the electrolytic solution 18, it is possible to suppress a decrease in the contact inside the battery by the concave portion 23 formed on the bottom surface and to prevent a decrease in current collection.

  (2) In the present embodiment, the inner surface of the concave portion 23 is formed as a hemispherical curved surface, and the continuous portion where stress is concentrated is eliminated inside the concave portion 23. Therefore, along with insertion and extraction of lithium ions. When expanding and contracting, the stress of the alloy layer can be dispersed.

  Therefore, since there is no portion where stress is concentrated, the lithium-aluminum alloy layer is hardly cracked or collapsed. As a result, the lithium-aluminum alloy layer cracks and collapses early, reducing the conductive area between the stainless steel layer of the negative electrode can and the lithium-aluminum alloy layer, increasing the internal resistance of the battery and degrading the capacity during the charge cycle. Suppress the invitation.

  (3) In the present embodiment, the opening edge 23a of each hemispherical recess 23 is formed in an arc shape so as to be a curved surface. Therefore, since the opening edge 23a of the recess 23 is also a curved surface, the stress of the alloy layer at the time of expansion and contraction can be dispersed even in the portion of the opening edge 23a.

  Therefore, cracks and collapse of the lithium / aluminum alloy layer occur and are further difficult. As a result, the lithium-aluminum alloy layer cracks and collapses early, reducing the conductive area between the stainless steel layer of the negative electrode can and the lithium-aluminum alloy layer, increasing the internal resistance of the battery and degrading the capacity during the charge cycle. Further restraining the invitation.

In addition, you may change the said embodiment as follows.
In the above embodiment, the hemispherical recess 23 is formed. However, the present invention is not limited to this, and may be any recess 23 having a curved surface that disperses stress. For example, an oval recess, The cross section may be a striped groove having a semicircular shape or an annular groove.

In the above embodiment, the plurality of recesses 23 are formed at equiangular intervals on the concentric circles. However, they need not be arranged at equiangular intervals.
In the above embodiment, the plurality of recesses 23 formed on the bottom surface 16b of the negative electrode can 16 have the same shape, but may be implemented so that recesses of a plurality of types are mixed. For example, a hemispherical concave portion whose inner diameter and depth become smaller toward the center may be formed, or a hemispherical concave portion and an elliptical concave portion may be mixed.

In the above embodiment, the negative electrode can 16 has a two-layer structure of the stainless steel layer 21 and the hard aluminum layer 22. This may be implemented by replacing the hard aluminum layer 22 with a soft aluminum layer.

In the above embodiment, the recesses 23 are arranged and formed at the same density on the bottom surface 16b. However, for example, the recesses 23 may be arranged and formed so that the center part has a higher density than the outside.
In the above embodiment, the negative electrode can 16 has a two-layer structure of the stainless steel layer 21 and the hard aluminum layer 22, but an aluminum alloy may be used instead of the hard aluminum layer 22. In addition, another metal or the like may be disposed on the outer surface of the stainless steel layer by plating or cladding to form a structure having three or more layers.

Sectional drawing of the nonaqueous electrolyte secondary battery of this embodiment. The top view which looked at the inner side of the negative electrode can for demonstrating the shape of the bottom face of a negative electrode can. The principal part sectional drawing for demonstrating the recessed part formed in the bottom face of a negative electrode can. It is explanatory drawing for demonstrating the recessed part of Example 1, Comprising: (a) is a sectional side view, (b) is a top view. It is explanatory drawing for demonstrating the recessed part of Example 2, Comprising: (a) is a sectional side view, (b) is a top view. It is explanatory drawing for demonstrating the recessed part of the comparative example 1, Comprising: (a) is a sectional side view, (b) is a top view. It is explanatory drawing for demonstrating the convex part of the comparative example 2, Comprising: (a) is a sectional side view, (b) is a top view.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Non-aqueous electrolyte secondary battery, 11 ... Positive electrode can, 12 ... Positive electrode pellet, 13 ... Positive electrode collector, 14 ... Separator, 15 ... Lithium foil, 16 ... Negative electrode can, 16b ... Bottom, 17 ... Gasket, 18 ... electrolyte, 21 ... stainless steel layer, 22 ... hard aluminum layer, 23 ... concave, 23a ... opening edge.

Claims (5)

In the negative electrode can, a negative electrode using aluminum or an aluminum alloy capable of occluding and releasing lithium ions as an active material is housed, and a button-type non-aqueous electrolyte secondary battery into which an electrolyte is injected,
A nonaqueous electrolyte secondary battery, wherein a concave portion is formed on a bottom surface of the negative electrode can and an inner side surface of the concave portion is curved. The nonaqueous electrolyte secondary battery according to claim 1,
The negative electrode can has a two-layer structure of a stainless steel layer on the outer side and a layer made of aluminum or an aluminum alloy on the inner side, and the concave portion is formed in the inner layer made of aluminum or an aluminum alloy. Water electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1 or 2,
The non-aqueous electrolyte secondary battery, wherein the recess is a hemispherical recess. The nonaqueous electrolyte secondary battery according to claim 1 or 2,
The non-aqueous electrolyte secondary battery, wherein the recess is a conical recess. The nonaqueous electrolyte secondary battery according to claim 1 or 2,
The non-aqueous electrolyte secondary battery, wherein the recess is a groove having a circular cross section.

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