JP2020129432A - Secondary battery - Google Patents

Abstract

To provide a secondary battery capable of improving safety against overcharge.SOLUTION: A cylindrical secondary battery 1 including a positive electrode 174, a negative electrode 175 and a current interruption mechanism 181 is characterized in that a positive electrode mixture contains lithium carbonate of 1 wt.% or more and 4 wt.% or less, an NP ratio which is a ratio of the capacity of the positive electrode to the capacity of the negative electrode is 1.4 or more and 2.0 or less, and the coating amount of the positive electrode mixture is 110 g/mor more and 140 g/mor less.SELECTED DRAWING: Figure 8

Description

The present invention relates to a secondary battery used as a power source for electric vehicles and hybrid vehicles, for example.

In recent years, lithium-ion secondary batteries have been used as a power source for electric vehicles and hybrid vehicles. A lithium-ion secondary battery for automobiles is required to have high output, high energy density and high safety.

Further, in the lithium-ion secondary battery, it is important not to impair the safety of the battery system even when overcharged due to an erroneous operation. In order to improve the safety during overcharge, it is required to appropriately control the current cutoff mechanism and heat generation of the secondary battery.

In Patent Document 1, lithium carbonate is used as a compound that generates a decomposed gas at the time of overcharging and activates a current cutoff mechanism, and the content of lithium carbonate in a specific positive electrode mixture is numerically limited.

JP, 2013-138014, A

However, according to the method of Patent Document 1, when the content of lithium carbonate cannot be increased, the configuration contents of the positive electrode and the negative electrode of the secondary battery are changed to improve safety against overcharge. I can't let you do it.

The present invention aims to solve the above problems. Specifically, it is to provide a secondary battery capable of improving safety against overcharge.

In order to solve the above problems, the secondary battery according to the present invention includes a positive electrode coated with a positive electrode mixture, a negative electrode coated with a negative electrode mixture, and a battery containing the positive electrode and the negative electrode. A secondary battery comprising a container and a current interrupting mechanism for interrupting an electric current when the internal pressure of the battery container reaches a predetermined operating pressure, wherein the positive electrode mixture contains 1% by weight or more and 4% by weight or less. Of lithium carbonate, the NP ratio, which is the ratio of the capacity of the positive electrode to the capacity of the negative electrode, is 1.4 or more and 2.0 or less, and the coating amount of the positive electrode mixture is 110 g/m ² or more. It is characterized by being 140 g/m ² or less.

Further, it is preferable that the NP ratio is 1.4 or more and 1.6 or less, and the coating amount of the positive electrode mixture is 120 g/m ² or more and 140 g/m ² or less. Further, it is preferable that the NP ratio is 1.5 and the coating amount of the positive electrode mixture is 130 g/m ² or more and 140 g/m ² or less. Further, the coating amount of the positive electrode mixture is preferably 130 g/m ² .

According to the present invention, it is possible to provide a secondary battery having excellent safety against overcharge. Further features related to the present invention will be apparent from the description of the present specification and the accompanying drawings. Further, problems, configurations and effects other than those described above will be clarified by the following description of the embodiments.

1 is a cross-sectional view of a cylindrical secondary battery as an embodiment of the present invention. FIG. 2 is an exploded perspective view of the cylindrical secondary battery shown in FIG. 1. FIG. 2 is a perspective view showing a state in which a part of the electrode group of FIG. 1 is cut to show the details. 1 is an external perspective view of a prismatic secondary battery according to an embodiment of the present invention. FIG. 5 is an exploded perspective view of the prismatic secondary battery shown in FIG. 4. The perspective view which shows a winding electrode group. The table|surface which shows the structure of positive electrode mixture coating amount and NP ratio. The table which shows the test result of an overcharge test.

FIG. 1 is a vertical sectional view showing an embodiment of a cylindrical secondary battery of the present invention. The cylindrical secondary battery 1 has, for example, an outer diameter of 40 mmφ and a height of 100 mm. This cylindrical secondary battery 1 is configured by accommodating a power generation unit 20 inside a bottomed cylindrical battery can 60 whose opening is sealed by a sealing lid 50. First, the battery can 60 and the power generation unit 20 will be described, and then the sealing lid 50 will be described.

A cylindrical canned battery can 60 has a caulked part 61 formed at the can open end (upper side in the figure). By caulking and fixing the sealing lid 50 to the battery can 60 with the caulking portion 61, the opening is hermetically sealed, and the sealing performance of the sealed cylindrical secondary battery 1 using the non-aqueous electrolyte is secured. The caulking portion 61 includes an annular protrusion, a bent portion 62 formed by bending an opening end portion of the can inward, and a grooving portion 63 protruding inward at a position apart from the bottom surface of the battery by a predetermined distance. As will be described later, the sealing lid 50 is caulked and fixed with the gasket 43 interposed between the bent portion 62 and the grooving portion 63 to seal the battery.

The power generation unit 20 is configured by integrally uniting the electrode group 10, the positive electrode current collecting member 31, and the negative electrode current collecting member 21 as described below. The electrode group 10 has a shaft core 15 at the center, and a positive electrode, a negative electrode, and a separator are wound around the shaft core 15. The hollow cylindrical shaft core 15 has a large-diameter recess 15a formed on the inner surface of the upper end in the axial direction (vertical direction in the drawing), and the positive electrode current collecting member 31 is press-fitted into the recess 15a. The positive electrode current collecting member 31 is made of, for example, aluminum, and has a disk-shaped base portion 31a, and a lower cylindrical portion 31b that protrudes toward the shaft core 15 side at the inner peripheral portion of the base portion 31a and is press-fitted into the inner surface of the shaft core 15. , And an upper cylindrical portion 31c protruding toward the sealing lid 50 at the outer peripheral edge. An opening 31d for discharging gas generated inside the battery is formed in the base 31a of the positive electrode current collecting member 31.

All the positive electrode leads 16 of the positive electrode sheet 11a are welded to the upper tubular portion 31c of the positive electrode current collecting member 31. In this case, the positive electrode lead 16 is overlapped and joined on the upper cylindrical portion 31c of the positive electrode current collecting member 31. Since each positive electrode lead 16 is very thin, a single one cannot take out a large current. For this reason, a large number of positive electrode leads 16 are formed at predetermined intervals over the entire length from the beginning to the end of winding around the shaft core 15.

The positive electrode lead 16 of the positive electrode sheet 11a and the ring-shaped pressing member 32 are welded to the outer periphery of the upper cylindrical portion 31c of the positive electrode current collecting member 31. The large number of positive electrode leads 16 are brought into close contact with the outer periphery of the upper tubular portion 31c of the positive electrode current collecting member 31, the pressing member 32 is wound around the outer periphery of the positive electrode lead 16 and temporarily fixed, and then welded in this state.

Since the positive electrode current collecting member 31 is oxidized by the electrolytic solution, reliability can be improved by forming it from aluminum. As soon as the surface of aluminum is exposed by some processing, an aluminum oxide film is formed on the surface, and the aluminum oxide film can prevent oxidation by the electrolytic solution. Further, by forming the positive electrode current collecting member 31 from aluminum, the positive electrode lead 16 of the positive electrode sheet 11a can be welded by ultrasonic welding or spot welding.

A step portion 15b having a small outer diameter is formed on the outer periphery of the lower end portion of the shaft core 15, and the negative electrode current collecting member 21 is press-fitted and fixed to the step portion 15b. The negative electrode current collecting member 21 is formed of, for example, copper, has an opening 21b formed in a disc-shaped base portion 21a and press-fitted into the step portion 15b of the shaft core 15, and has an outer peripheral edge facing the bottom portion side of the battery container 60. An outer peripheral cylindrical portion 21c that protrudes is formed.

All the negative electrode leads 17 of the negative electrode sheet 12a are welded to the outer peripheral cylindrical portion 21c of the negative electrode current collecting member 21 by ultrasonic welding or the like. Since each negative electrode lead 17 is very thin, a large number of negative electrode leads 17 are formed at predetermined intervals over the entire length from the beginning to the end of winding around the shaft core 15 in order to extract a large current.

The negative electrode lead 17 of the negative electrode sheet 12a and the ring-shaped pressing member 22 are welded to the outer periphery of the outer peripheral cylindrical portion 21c of the negative electrode current collecting member 21. The large number of negative electrode leads 17 are brought into close contact with the outer circumference of the outer peripheral cylindrical portion 21c of the negative electrode current collecting member 21, and the holding member 22 is wound around the outer circumference of the negative electrode lead 17 and temporarily fixed, and welded in this state.

A negative electrode current-carrying lead 23 made of copper is welded to the lower surface of the negative electrode current collecting member 21. The negative electrode conducting lead 23 is welded to the battery can 60 at the bottom of the battery can 60. The battery can 60 is made of, for example, carbon steel having a thickness of 0.5 mm, and the surface thereof is nickel-plated. By using such a material, the negative electrode conducting lead 23 can be welded to the battery can 60 by resistance welding or the like.

On the upper surface of the base portion 31a of the positive electrode current collecting member 31, a flexible positive electrode conductive lead 33 formed by laminating a plurality of aluminum foils is joined by welding one end portion thereof. The positive electrode conductive lead 33 is capable of passing a large current and is provided with flexibility by laminating and integrating a plurality of aluminum foils. That is, it is necessary to increase the thickness of the connection member in order to flow a large current, but if it is formed from a single metal plate, the rigidity is increased and the flexibility is impaired. Therefore, a large number of thin aluminum foils are laminated to give flexibility. The positive electrode conductive lead 33 has a thickness of, for example, about 0.5 mm, and is formed by stacking five aluminum foils having a thickness of 0.1 mm.

FIG. 2 is an exploded perspective view of the cylindrical secondary battery shown in FIG.
The electrode group 10 has a shaft core 15 at the center, and a positive electrode, a negative electrode, and a separator are wound around the shaft core 15. Then, the outermost first separator 13 is fixed by the adhesive tape 19.

The sealing lid 50 includes a cap 3 having an exhaust port 3c, a cap case 37 attached to the cap 3 and having a cleavage groove 37a, a positive electrode connecting plate 35 spot-welded to the back surface of the central portion of the cap case 37, and a positive electrode connecting plate. An insulating ring 41 sandwiched between the upper surface of the peripheral edge of 35 and the back surface of the cap case 37 is provided, and is preassembled as a subassembly.

The cap 3 is formed by plating iron such as carbon steel with nickel. The cap 3 has a disk-shaped peripheral portion 3a and a headless and bottomless cylindrical portion 3b protruding upward from the peripheral portion 3a, and has a hat shape as a whole. An exhaust port 3c is formed in the center of the tubular portion 3b. The cylindrical portion 3b functions as a positive electrode external terminal and is connected with a bus bar or the like.

The peripheral portion of the cap 3 is integrated with a folding flange 37b of a cap case 37 made of an aluminum alloy. That is, the periphery of the cap case 37 is folded back along the upper surface of the cap 3, and the cap 3 is caulked and fixed. An annular ring that is folded back on the upper surface of the cap 3, that is, the flange 37b and the cap 3 are friction-welded to each other. That is, the cap case 37 and the cap 3 are integrated by caulking and fixing by the flange 37b and welding. As described above, the sealing lid 50 includes the flange 37b in which the cap case 37 and the cap 3 are integrated.

In the central circular area of the cap case 37, a circular cleavage groove 37a and a cleavage groove 37a extending radially from the cleavage groove 37a in all directions are formed. The cleaving groove 37a is formed by crushing the upper surface side of the cap case 37 into a V shape by pressing and leaving the remaining portion thin. The cleaving groove 37a is cleaved when the internal pressure in the battery container 60 rises above a predetermined value, and releases the gas inside.

The sealing lid 50 constitutes an explosion-proof mechanism. When the internal pressure exceeds the reference value due to the gas generated inside the battery can 60, a crack is generated in the cap case 37 in the cleavage groove, and the gas inside is discharged from the exhaust port 3c of the cap 3 to prevent the gas inside the battery can 60 from being exhausted. Pressure is reduced. Moreover, the cap case 37 bulges outward from the container due to the internal pressure of the battery can 60, and the electrical connection with the positive electrode connecting plate 35 is cut off, thereby suppressing overcurrent. The cylindrical secondary battery 1 is in an overcharged state, the positive electrode gas generating compound such as lithium carbonate contained in the positive electrode mixture is decomposed by the positive electrode potential at that time to generate gas, and the battery can (battery container) When the internal pressure of 60 rises and reaches a predetermined operating pressure, for example, a pressure in the range of 0.4 MPa or more and 1.0 MPa or less, the cap case 37 bulges outward from the container to interrupt the current (current Blocking mechanism).

The sealing lid 50 is mounted on the upper cylindrical portion 31c of the positive electrode current collecting member 31 in an insulated state. That is, the cap case 37 in which the cap 3 is integrated is placed on the upper end surface of the positive electrode current collecting member 31 in an insulated state via the insulating ring 41. However, the cap case 37 is electrically connected to the positive electrode current collecting member 31 by the positive electrode conductive lead 33, and the cap 3 of the sealing lid 50 serves as the positive electrode of the cylindrical secondary battery 1. Here, the insulating ring 41 has an opening 41a and a side portion 41b protruding downward. The connection plate 35 is fitted in the opening 41 a of the insulating ring 41.

The connection plate 35 is formed of an aluminum alloy, has a substantially dish-like shape in which almost the entire portion except the central portion is uniform and the central portion is bent to a slightly lower position. The thickness of the connection plate 35 is, for example, about 1 mm. A thin, dome-shaped projection 35a is formed at the center of the connection plate 35, and a plurality of openings 35b are formed around the projection 35a. The opening 35b has a function of releasing gas generated inside the battery. The protrusion 35a of the connection plate 35 is joined to the bottom surface of the central portion of the cap case 37 by resistance welding or friction diffusion joining.

Then, the electrode group 10 is housed in the battery can 60, and the hermetically-sealed lid 50, which is manufactured in advance as a partial assembly, is electrically connected to the positive electrode current collecting member 31 and the positive electrode conductive lead 33 and placed on the upper part of the cylinder. Then, the outer peripheral wall portion 43b of the gasket 43 is bent by a press or the like, and the base 43a and the outer peripheral wall portion 43b are caulked so that the sealing lid 50 is pressed in the axial direction. As a result, the sealing lid 50 is fixed to the battery can 60 via the gasket 43.

Initially, the gasket 43 has an outer peripheral wall portion 43b which is formed on the peripheral side edge of the ring-shaped base portion 43a so as to stand substantially vertically toward the upper side, and an inner peripheral side portion which extends downward from the base portion 43a. It has a shape having a cylindrical portion 43c formed by vertically hanging. By caulking the battery can 60, the sealing lid 50 is sandwiched by the battery can 60 via the outer peripheral wall portion 43b.
A predetermined amount of non-aqueous electrolytic solution is injected into the battery can 60. As an example of the non-aqueous electrolyte, it is preferable to use a solution in which a lithium salt is dissolved in a carbonate-based solvent. Examples of the lithium salt include lithium fluorophosphate (LiPF ₆ ), lithium fluoroborate (LiBF ₆ ), and the like. Further, as an example of the carbonate-based solvent, ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), methyl ethyl carbonate (MEC), or a mixture of solvents selected from one or more of the above solvents, Is mentioned.

FIG. 3 is a diagram for explaining the power generation unit, showing the details of the structure of the electrode group 10 and a perspective view of a state in which a part is cut. As shown in FIG. 3, the electrode group 10 has a configuration in which a positive electrode 11, a negative electrode 12, and first and second separators 13 and 14 are wound around an outer periphery of a shaft core 15.

In this electrode group 10, a first separator 13 is wound around the innermost circumference that is in contact with the outer circumference of the shaft core 15, and a negative electrode 12, a second separator 14 and a positive electrode 11 are laminated in this order on the outer side thereof. It has been wound up. A first separator 13 and a second separator 14 are wound several times inside the innermost negative electrode 12. The outermost circumference is the negative electrode 12 and the first separator 13 wound around the negative electrode 12.

The positive electrode 11 is formed of aluminum foil and has a strip shape, and has a positive electrode sheet 11a and a positive electrode treatment portion (a positive electrode mixture layer) in which the positive electrode mixture 11b is applied to both surfaces of the positive electrode sheet 11a. An upper side edge along the longitudinal direction of the positive electrode sheet 11a is a positive electrode mixture untreated portion 11c where the positive electrode mixture 11b is not applied and the aluminum foil is exposed. The positive electrode mixture untreated portion 11c is integrally formed with a large number of positive electrode leads 16 protruding upward in parallel with the shaft core 15 at equal intervals.

The positive electrode mixture 11b includes a positive electrode active material, a positive electrode conductive material, a positive electrode binder, and a positive electrode gas generating compound. The positive electrode active material is preferably lithium oxide. Examples thereof include lithium cobalt oxide, lithium manganate, lithium nickel oxide, lithium composite oxide (lithium oxide containing two or more kinds selected from cobalt, nickel and manganese).

There is no limitation on the positive electrode conductive material as long as it can assist the transfer of the electrons generated by the lithium storage/release reaction in the positive electrode mixture to the positive electrode. Examples of the positive electrode conductive material include graphite and acetylene black.

The positive electrode binder is capable of binding the positive electrode active material and the positive electrode conductive material, and also binding the positive electrode mixture and the positive electrode current collector, as long as it does not significantly deteriorate due to contact with the non-aqueous electrolyte. There is no particular limitation. Examples of the positive electrode binder include polyvinylidene fluoride (PVDF) and fluororubber.

Examples of the positive electrode gas generating compound include a carbonate inorganic compound, an oxalic acid inorganic compound, and a nitric acid inorganic compound that generate gas at a positive electrode potential in an overcharged state. Among them, inorganic carbonate compounds are preferable, and examples thereof include lithium carbonate. The content of lithium carbonate in the positive electrode mixture is preferably 1% by weight or more and 4% by weight or less, but in Example 1, it was set to 1.8% by weight.

The method for forming the positive electrode mixture layer is not limited as long as the positive electrode mixture is formed on the positive electrode. An example of a method of forming the positive electrode mixture 11b is a method of applying a dispersion solution of constituent materials of the positive electrode mixture 11b on the positive electrode sheet 11a.

Examples of the method of applying the positive electrode mixture 11b to the positive electrode sheet 11a include a roll coating method and a slit die coating method. After adding N-methylpyrrolidone (NMP) or water as a solvent example of the dispersion solution to the positive electrode mixture 11b and kneading the slurry, the slurry was uniformly applied to both sides of an aluminum foil having a thickness of 20 μm and dried. , Press and cut. An example of the coating thickness of the positive electrode mixture 11b is about 40 μm on each side. When cutting the positive electrode sheet 11a, the positive electrode lead 16 is integrally formed. In Example 1, the coating amount of the positive electrode mixture on one surface of the positive electrode was 140 g/m ² .

The negative electrode 12 is formed of a copper foil and has a strip shape, and has a negative electrode sheet 12a and a negative electrode treatment portion (negative electrode mixture layer) in which the negative electrode mixture 12b is applied to both surfaces of the negative electrode sheet 12a. The lower side edge along the longitudinal direction of the negative electrode sheet 12a is a negative electrode mixture untreated portion 12c where the negative electrode mixture 12b is not applied and the copper foil is exposed. In the negative electrode mixture untreated portion 12c, a large number of negative electrode leads 17 extending in the direction opposite to the positive electrode lead 16 are integrally formed at equal intervals.

The negative electrode mixture 12b includes a negative electrode active material, a negative electrode binder, and a thickener. The negative electrode mixture 12b may include a negative electrode conductive material such as acetylene black. Graphite carbon is preferably used as the negative electrode active material. By using graphite carbon, a lithium ion secondary battery for a plug-in hybrid vehicle or an electric vehicle that requires a large capacity can be manufactured. The method of forming the negative electrode mixture 12b is not limited as long as it is a method of forming the negative electrode mixture 12b on the negative electrode sheet 12a. An example of a method of applying the negative electrode mixture 12b to the negative electrode sheet 12a is a method of applying a dispersion solution of the constituent material of the negative electrode mixture 12b onto the negative electrode sheet 12a. Examples of the coating method include a roll coating method and a slit die coating method.

As an example of a method of applying the negative electrode mixture 12b to the negative electrode sheet 12a, N-methyl-2-pyrrolidone or water as a dispersion solvent was added to the negative electrode mixture 12b, and the kneaded slurry was rolled into a rolled copper foil having a thickness of 10 μm. Both sides are evenly coated, dried and then pressed to cut. An example of the coating thickness of the negative electrode mixture 12b is about 40 μm on each side. When cutting the negative electrode sheet 12a, the negative electrode lead 17 is integrally formed.

The width of the first separator 13 and the second separator 14 is W _S , the width of the negative electrode mixture 12b formed on the negative electrode sheet 12a is W _C , and the width of the positive electrode mixture 11b formed on the positive electrode sheet 11a is W _A. In such a case, it is formed so as to satisfy the following formula.
W _S >W _C >W _A

That is, the width W _{C of the} negative electrode mixture 12b is always larger than the width W _{A of the} positive electrode mixture 11b. This is because in the case of a lithium ion secondary battery, lithium, which is a positive electrode active material, is ionized and permeates the separator, but when the negative electrode active material is not formed on the negative electrode side and the negative electrode sheet 12a is exposed, the negative electrode sheet is exposed. This is because lithium is deposited on 12a and causes an internal short circuit.

The negative electrode capacity/positive electrode capacity, which is the ratio of the capacities of the formed positive electrode and negative electrode, was defined as the NP ratio. The NP ratio is preferably 1.1 or more, but in Example 1, it was 1.4.

(Example 1)
As described above, in the secondary battery of Example 1, the content of lithium carbonate in the positive electrode mixture was 1.8% by weight, the NP ratio was 1.4, and the coating amount of the positive electrode mixture on one side of the positive electrode was It was 140 g/m ² .

(Example 2)
A secondary battery of Example 2 was produced in the same manner as in Example 1 except that the amount of the positive electrode mixture applied was 130 g/m ² .

(Example 3)
A secondary battery of Example 3 was produced in the same manner as in Example 1 except that the amount of the positive electrode mixture applied was 120 g/m ² .

(Example 4)
A secondary battery of Example 4 was made in the same manner as in Example 1 except that the amount of the positive electrode mixture applied was 110 g/m ² .

(Example 5)
A secondary battery of Example 5 was made in the same manner as in Example 1 except that NP was made to be 1.5.

(Comparative Example 1)
A secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that the coating amount of the positive electrode mixture was 110 g/m ² and the NP ratio was 1.1.

(Comparative example 2)
A secondary battery of Comparative Example 2 was produced in the same manner as in Example 1 except that the amount of the positive electrode mixture applied was 110 g/m ² and the NP ratio was 1.2.

(Comparative example 3)
A secondary battery of Comparative Example 2 was produced in the same manner as in Example 1 except that the amount of the positive electrode mixture applied was 90 g/m ² and the NP ratio was 1.2.

(Comparative example 4)
A secondary battery of Comparative Example 2 was produced in the same manner as in Example 1 except that the NP ratio was 1.3.

(Comparative example 5)
A secondary battery of Comparative Example 5 was produced in the same manner as in Example 1 except that the amount of the positive electrode mixture applied was 130 g/m ² and the NP ratio was 1.3.

(Comparative example 6)
A secondary battery of Comparative Example 6 was produced in the same manner as in Example 1 except that the amount of the positive electrode mixture applied was 120 g/m ² and the NP ratio was 1.3.

The present invention is not limited to a cylindrical secondary battery, but can be applied to a prismatic secondary battery. FIG. 4 is an external perspective view of a prismatic secondary battery 100 as one embodiment of a power storage device, and FIG. 5 is an exploded perspective view showing the configuration of the prismatic secondary battery 100.

As shown in FIG. 4, the prismatic secondary battery 100 includes a battery container including a battery can 101 and a battery lid 102. The material of the battery can 101 and the battery lid 102 is aluminum or aluminum alloy. The battery can 101 is formed into a flat rectangular box shape with one end opened by performing deep drawing. The battery can 101 includes a bottom plate 101c having a rectangular flat plate shape, a pair of wide side plates 101a standing up from a pair of long side portions of the bottom plate 101c, and a pair of narrow side plates 101b standing up from a pair of short side portions of the bottom plate 101c. And have.

As shown in FIG. 5, a wound electrode group 170 (see FIG. 6) is housed in the battery can 101. The positive electrode current collector 180 is joined to the positive electrode 174 of the wound electrode group 170 and the negative electrode 175 of the wound electrode group 170 is joined. The negative electrode current collector 190 and the wound electrode group 170 are composed of an insulating sheet 108a that covers the central portion of the wound electrode group, an insulating sheet 108b that covers the uncoated portion of the positive electrode of the wound electrode group, and an uncoated negative electrode of the wound electrode group. It is housed in the battery can 101 in a state of being covered with an insulating sheet 108c that covers the working portion. The material of the insulating sheets 108a, 108b, 108c is a resin having an insulating property such as polypropylene, and the battery can 101 and the wound electrode group 170 are electrically insulated.

As shown in FIGS. 4 and 5, the battery lid 102 has a rectangular flat plate shape and is laser-welded so as to close the opening of the battery can 101. That is, the battery lid 102 seals the opening of the battery can 101. As shown in FIG. 1, the battery lid 102 is provided with a positive electrode external terminal 104 and a negative electrode external terminal 105 electrically connected to the positive electrode 174 and the negative electrode 175 of the wound electrode group 170.

As shown in FIG. 5, the positive electrode external terminal 104 is electrically connected to the positive electrode 174 (see FIG. 3) of the wound electrode group 170 via the current cutoff mechanism 181 and the positive electrode current collector 180, and the negative electrode external terminal 105. Is electrically connected to the negative electrode 175 of the wound electrode group 170 via the negative electrode collector 190. Therefore, electric power is supplied to the external device via the positive electrode external terminal 104 and the negative electrode external terminal 105, or externally generated power is supplied to the wound electrode group 170 via the positive electrode external terminal 104 and the negative electrode external terminal 105. Be charged.

As shown in FIG. 5, the battery lid 102 is provided with a liquid injection hole 106a for injecting an electrolytic solution into the battery container. The injection hole 106a is sealed by an injection plug 106b after the injection of the electrolytic solution. As the electrolytic solution, for example, a non-aqueous electrolytic solution in which a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ) is dissolved in a carbonate ester organic solvent such as ethylene carbonate can be used.

The battery lid 102 is provided with a gas exhaust valve 103. The gas discharge valve 103 is formed by partially thinning the battery lid 102 by pressing. The thin film member may be attached to the opening of the battery lid 102 by laser welding or the like, and the thin portion may be used as a gas exhaust valve. The gas discharge valve 103 is opened when the prismatic secondary battery 100 generates heat due to an abnormality such as an internal short circuit and gas is generated, and the pressure inside the battery container rises to reach a predetermined pressure, and the gas is discharged from the inside. The pressure in the battery container is reduced by discharging.

The current cutoff mechanism 181 operates when the prismatic secondary battery 100 is in an overcharged state, the positive electrode potential at that time decomposes the positive electrode gas generating compound, gas is generated, and the internal pressure rises. The current cutoff mechanism 181 has, for example, a diaphragm, and when the inside of the battery container reaches a predetermined operating pressure, for example, a pressure in the range of 0.4 MPa or more and 1.0 MPa or less, the diaphragm deforms to cut off the contact. Have a configuration.

The wound electrode group 170 will be described with reference to FIG. FIG. 6 is a perspective view showing the wound electrode group 170, showing a state where the winding end side of the wound electrode group 170 is expanded. The wound electrode group 170, which is a power generation element, has a laminated structure in which a strip-shaped positive electrode 174 and a negative electrode 175 are wound in a flat shape around the winding central axis W with the separators 173a and 173b interposed therebetween. ..

The positive electrode 174 is formed by forming a positive electrode mixture layer 176 on both surfaces of the positive electrode foil 171. The positive electrode mixture is a mixture of a positive electrode active material with a binder and a positive electrode gas generating compound. Examples of the positive electrode gas generating compound include a carbonate inorganic compound, an oxalic acid inorganic compound, and a nitric acid inorganic compound that generate gas at a positive electrode potential in an overcharged state. Among them, inorganic carbonate compounds are preferable, and examples thereof include lithium carbonate.
The negative electrode 175 has a negative electrode mixture layer 177 formed on both surfaces of the negative electrode foil 172. The negative electrode mixture is a negative electrode active material mixed with a binder.

The positive electrode foil 171 is an aluminum foil having a thickness of about 20 to 30 μm, and the negative electrode foil 172 is a copper foil having a thickness of about 15 to 20 μm. The material of the separators 173a and 173b is a microporous polyethylene resin through which lithium ions can pass. The positive electrode active material is a lithium-containing transition metal composite oxide such as lithium manganate, and the negative electrode active material is a carbon material such as graphite that can reversibly store and release lithium ions.

One end of the width direction of the wound electrode group 170, that is, both ends in the direction of the winding center axis W orthogonal to the winding direction is a laminated part of the positive electrode 174 and the other is a laminated part of the negative electrode 175. There is. The laminated portion of the positive electrode 174 provided at one end is a laminated portion of the positive electrode uncoated portion where the positive electrode mixture layer 176 is not formed, that is, the exposed portion of the positive electrode foil 171. The laminated portion of the negative electrode 175 provided at the other end is a laminated portion of the negative electrode uncoated portion where the negative electrode mixture layer 177 is not formed, that is, the exposed portion of the negative electrode foil 172. The laminated portion of the positive electrode uncoated portion and the laminated portion of the negative electrode uncoated portion are respectively crushed in advance and connected to the positive electrode current collector 180 and the negative electrode current collector 190 by ultrasonic bonding, respectively.

Subsequently, FIG. 7 is a table showing the configurations of the coating amount of the positive electrode mixture and the NP ratio of the cylindrical secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 6. In Examples 1 to 5 and Comparative Examples 1 to 6, the content of lithium carbonate is 1.8% by weight (wt%).

FIG. 8 is a table showing the test results of the overcharge tests of Examples 1-5 and Comparative Examples 1-6.
Before the test, all the batteries were put into a charged state by constant-current constant-voltage charging at 10 A, 4.2 V, and 3 hours. At a temperature of 25° C., a direct current of 8.4 V and 10 A was applied to each battery by using a direct current power source capable of knowing the accumulated amount of electricity supplied. The test was terminated when the current cutoff mechanism was activated and the battery temperature reached around 20°C.

In addition, as a result of each overcharge test, the charging depth of each battery at the end of the test was represented by SOC% based on the accumulated amount of electricity supplied. Note that SOC 100% corresponds to the state before the overcharge test.

The results of the overcharge test shown in FIG. 8 are the maximum battery heat generation temperature (° C.) during the test, the SOC (%) of the current cutoff mechanism operation, and the overcharge capacity (Ah).

As shown in the test results of Comparative Examples 1 and 3, when the coating amount of the positive electrode mixture was reduced in order to reduce the overcharge capacity and the battery internal resistance due to the heat generation of the battery, respectively, It was found that the absolute amount of lithium carbonate decreases and the operating SOC (%) of the current interrupting mechanism tends to increase. Furthermore, it was confirmed that the safety against overcharge was lowered and an excessively high temperature state (over temperature) was reached.

Next, as shown in the test results of Comparative Example 1 and Comparative Example 2, and Example 1 and Example 5, the lithium receiving capacity from the positive electrode in the negative electrode increases with the increase of the NP ratio. The charging capacity was reduced and the heat generation of the battery was reduced. It also led to the suppression of lithium dendrite precipitation during overcharging, improving safety against overcharging. However, if the NP ratio is increased too much, it is presumed that the battery internal resistance (DCR) increases, the battery temperature rises, and the safety during overcharge decreases. Further, FIG. 8 shows the range in which the safety at the time of overcharge described in the claims is improved.

The boundary line of the coating amount of the positive electrode mixture of 110 g/m ² in the range of claim 1 is a boundary where the absolute amount of lithium carbonate in the battery is equal to or more than the content of the coating amount of the positive electrode mixture of 110 g/m ² . Further, the boundary line of the NP ratio of 1.4 is a boundary where the overcharge capacity is 14 Ah or less, and the maximum battery heat generation temperature is 120° C. or less, which is highly safe during overcharge. The borderline of the coating amount of the positive electrode mixture of 140 g/m ² is the result that the NP ratio is 1.4 or more, the overcharge capacity is 14 Ah or less, and the maximum battery heat generation temperature is 120° C. or less. It was set because the high boundary was confirmed. The boundary of the NP ratio of 2.0 is set because it is expected that the internal resistance will increase 1.3 times or more as compared with the NP ratio of 1.4, and the heat generation of the battery will become remarkable.

The range of claim 2 is a range in which the safety improvement at the time of overcharging can be expected compared to the range of claim 1.

The range of claim 3 is a range in which safety can be expected to be further improved at the time of overcharging as compared with claim 2.

The point of claim 4 shows the constitution of the coating amount of the positive electrode mixture and the NP ratio, which is expected to improve the safety at the time of overcharge.

Although the embodiments of the present invention have been described above in detail, the present invention is not limited to the above-described embodiments, and various designs can be made without departing from the spirit of the present invention described in the claims. You can make changes. For example, the above-described embodiments have been described in detail in order to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, it is possible to add/delete/replace other configurations with respect to a part of the configurations of the respective embodiments.

DESCRIPTION OF SYMBOLS 1 Cylindrical secondary battery 10 Electrode group 50 Sealing lid 60 Battery can 61 Caulking part 63 Grooving part 100 Square secondary battery 101c Bottom plate 101a Wide side plate 101b Narrow side plate 101 Battery can 102 Battery cover 107 Lid assembly 170 Winding electrode group 174 Positive electrode 175 Negative electrode 176 Positive electrode mixture layer 177 Negative electrode mixture layer 180 Positive electrode current collector 181 Current interruption mechanism

Claims (5)

A positive electrode coated with a positive electrode mixture, a negative electrode coated with a negative electrode mixture, a battery container accommodating the positive electrode and the negative electrode, and the internal pressure of the battery container becomes a predetermined working pressure. A secondary battery having a current interruption mechanism for interrupting current when
The positive electrode mixture contains 1 wt% to 4 wt% of lithium carbonate,
The NP ratio, which is the ratio of the capacity of the positive electrode to the capacity of the negative electrode, is 1.4 or more and 2.0 or less, and the coating amount of the positive electrode mixture is 110 g/m ² or more and 140 g/m ² or less. A secondary battery characterized by being present. The secondary battery according to claim 1, wherein the NP ratio is 1.4 or more and 1.6 or less, and the coating amount of the positive electrode mixture is 120 g/m ² or more and 140 g/m ² or less. .. The secondary battery according to claim 2, wherein the NP ratio is 1.5, and the coating amount of the positive electrode mixture is 130 g/m ² or more and 140 g/m ² or less. The secondary battery according to claim 3, wherein a coating amount of the positive electrode mixture is 130 g/m ² . The secondary battery according to claim 1, wherein the operating pressure of the current interruption mechanism is 0.4 MPa or more and 1.0 MPa or less.

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