JP7594890B2 - Non-aqueous electrolyte secondary battery with excellent capacity retention after reflow soldering - Google Patents

Description

The present invention relates to a non-aqueous electrolyte secondary battery having an excellent capacity retention rate after reflow soldering .

Japanese Patent Application Laid-Open No. 2003-233699 discloses that a small non-aqueous electrolyte secondary battery using silicon oxide (SiO _x ) with a carbon-coated surface can provide excellent initial capacity and cycle characteristics.
Furthermore, in recent years, small non-aqueous electrolyte secondary batteries have been required to be compatible with reflow soldering in order to increase the efficiency of soldering when mounting the batteries on circuit boards.
Conventionally, a small non-aqueous electrolyte secondary battery that combines a lithium manganese oxide positive electrode active material with a lithium aluminum alloy negative electrode active material has been provided with a structure that allows reflow soldering.

JP 2013-101770 A

Incidentally, in order to make a secondary battery using the silicon oxide as the negative electrode, instead of the combination of active materials described above, compatible with reflow soldering, it is considered necessary to use a material with high heat resistance, as well as to suppress unexpected reactions of the electrodes and electrolyte, and to stabilize charging and discharging.
In particular, it is considered that the cell capacity will decrease due to the heat applied during reflow soldering, but it is desirable to have a large capacity retention rate, which is the ratio of the capacity after assembly to the capacity after reflow.

In view of the above problems, an object of the present invention is to provide a small nonaqueous electrolyte secondary battery that is excellent in cycle characteristics and long-term storage stability, and that is compatible with reflow mounting and has an excellent capacity retention rate after reflow soldering .

"1" In order to solve the above-mentioned problems, a nonaqueous electrolyte secondary battery having an excellent capacity retention rate after reflow soldering according to one embodiment of the present invention is a nonaqueous electrolyte secondary battery in which a positive electrode, a negative electrode, an electrolyte solution containing a supporting salt and a solvent, and a separator are contained in a container formed of a positive electrode can and a negative electrode can, the positive electrode contains a spinel-type lithium manganese oxide as an active material, the negative electrode contains carbon-coated SiO _x as an active material, the electrolyte solution contains a mixed solvent containing ethylene carbonate (EC) and vinylene carbonate (VC) in a glyme-based solvent, the separator contains glass fiber, and the value of the ratio of the negative electrode capacity to the positive electrode capacity (negative electrode capacity/positive electrode capacity) is in the range of 1.7 to 2.4.

In this embodiment, a combination of a positive electrode active material containing a spinel-type lithium manganese oxide and a negative electrode active material containing carbon-coated SiOx is combined with an electrolyte solution consisting of a mixed solvent containing ethylene carbonate and vinylene carbonate, and the value of (negative electrode capacity/positive electrode capacity) is set to a range of 1.7 to 2.4. This configuration makes it possible to provide a small nonaqueous electrolyte secondary battery that is compatible with reflow soldering, has excellent capacity retention after heating associated with reflow soldering, and has excellent cycle characteristics and long-term storage properties.

"2" In the nonaqueous electrolyte secondary battery having an excellent capacity retention rate after reflow soldering according to one embodiment of the present invention, the value of (negative electrode capacity/positive electrode capacity) is preferably in the range of 1.9 to 2.4.
"3" In the nonaqueous electrolyte secondary battery having an excellent capacity retention rate after reflow soldering according to one embodiment of the present invention, the value of (negative electrode capacity/positive electrode capacity) is preferably in the range of 1.94 to 2.39.

In this embodiment, by setting the value of (negative electrode capacity/positive electrode capacity) in the range of 1.9 to 2.4, it is possible to obtain an excellent initial capacity and an excellent capacity retention rate.

"3" In the nonaqueous electrolyte secondary battery having an excellent capacity retention rate after reflow soldering according to one embodiment of the present invention, the positive electrode can is cylindrical with a bottom, the negative electrode can is fixed to the inside of an opening of the positive electrode can with a gasket interposed therebetween, the storage container is sealed by providing a crimping portion that crimps the opening of the positive electrode can to the negative electrode can side, and the positive electrode, the negative electrode, a separator, and the electrolyte are stored in the storage container.

In this embodiment, a button-type nonaqueous electrolyte secondary battery with a sealed structure can be provided in which the negative electrode can and the positive electrode can are crimped via a gasket. In addition, this nonaqueous electrolyte secondary battery can provide a button-type secondary battery that has little capacity loss after reflow soldering, excellent capacity retention rate, and excellent initial capacity.

According to the present embodiment, it is possible to provide a small-sized nonaqueous electrolyte secondary battery that is compatible with reflow soldering, has excellent cycle characteristics and long-term storage stability, and exhibits an excellent capacity retention rate after reflow soldering .

1 is a cross-sectional view showing a nonaqueous electrolyte secondary battery according to a first embodiment. 1 is a graph showing the relationship between the positive and negative electrode capacity balance and the capacity retention rate when a heat treatment equivalent to reflow soldering is applied to a plurality of nonaqueous electrolyte secondary batteries produced in the examples. FIG. 1 is a graph showing the relationship between the positive and negative electrode capacity balance and the capacity after subjecting a plurality of nonaqueous electrolyte secondary batteries produced in the examples to a heat treatment equivalent to reflow soldering, exposing them to a high temperature and high humidity environment of 60° C. and 90% RH, and leaving them for 20 days.

Below, an example of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention is given, and its configuration is described in detail with reference to FIG. 1. The nonaqueous electrolyte secondary battery described in the present invention is a secondary battery in which an active material used as a positive electrode or a negative electrode and a separator are housed in a container. In addition, in the drawings used in the following description, the scale of each component is appropriately changed to make each component recognizable.

[First embodiment of non-aqueous electrolyte secondary battery]
1 is a so-called coin (button) type battery. The nonaqueous electrolyte secondary battery 1 includes a cylindrical metal positive electrode can 12 with a bottom, a cylindrical metal negative electrode can 22 with a lid that closes the opening of the positive electrode can 12, and a gasket 40 provided along the inner circumferential surface of the positive electrode can 12.
This nonaqueous electrolyte secondary battery 1 is provided with a thin (flat) storage container 2 configured by placing a positive electrode can 12 on the outside of a negative electrode can 22 with a gasket 40 between them, and crimping the inner periphery of the opening of the positive electrode can 12 inward. Within the storage container 2, a storage space surrounded by the positive electrode can 12 and the negative electrode can 22 is formed, and in this storage space, a positive electrode 10 and a negative electrode 20 are disposed facing each other with a separator 30 between them, and the storage space is further filled with an electrolyte 50.
The material of the positive electrode can 12 is a conventionally known material, for example, stainless steel such as SUS316L or SUS329JL.
The material of the negative electrode can 22 is, like the material of the positive electrode can 12, a conventionally known stainless steel, such as SUS316L, SUS329JL, or SUS304-BA. The negative electrode can may also be made of a clad material made by crimping copper, nickel, or the like onto stainless steel. The outer diameter of the storage container 2 is, for example, about 4 to 12 mm.

(Positive electrode)
In this embodiment, the positive electrode 10 is electrically connected to the inner surface of the positive electrode can 12 (the upper surface of the bottom wall of the storage container 2 in FIG. 1 ) via the positive electrode current collector 14, and the negative electrode 20 is electrically connected to the inner surface of the negative electrode can 22 (the lower surface of the ceiling wall of the storage container 2 in FIG. 1 ) via the negative electrode current collector 24. Note that the positive electrode current collector 14 and the negative electrode current collector 24 may be omitted, and the positive electrode 10 may be directly connected to the positive electrode can 12 so that the positive electrode can 12 has the function of a current collector, or the negative electrode 20 may be directly connected to the negative electrode can 22 so that the negative electrode can 22 has the function of a current collector.
The gasket 40 is connected to the outer periphery of the separator 30 inside the storage container 2, and the gasket 40 holds the separator 30. The positive electrode 10, the negative electrode 20, and the separator 30 are impregnated with an electrolyte solution 50 filled in the storage container 2.

In the positive electrode 10, the type of positive electrode active material is not particularly limited, but it is preferable to use, for example, a positive electrode active material containing a spinel-type lithium manganese oxide.
The content of the positive electrode active material in the positive electrode 10 is determined taking into consideration the discharge capacity required for the nonaqueous electrolyte secondary battery 1 and can be in the range of 50 to 95 mass %. When the content of the positive electrode active material is equal to or more than the lower limit of the above preferred range, a sufficient discharge capacity is easily obtained, and when it is equal to or less than the preferred upper limit, the positive electrode 10 is easily formed.

The positive electrode 10 may contain a conductive assistant (hereinafter, the conductive assistant used in the positive electrode 10 may be referred to as a "positive electrode conductive assistant").
Examples of the positive electrode conductive assistant include carbonaceous materials such as furnace black, ketjen black, acetylene black, and graphite.
The positive electrode conductive assistant may be used alone or in combination of two or more of the above.
The content of the positive electrode conductive assistant in the positive electrode 10 is preferably 2 to 20 mass %, more preferably 4 to 15 mass %. If the content of the positive electrode conductive assistant is equal to or greater than the lower limit of the above-mentioned preferred range, sufficient conductivity is easily obtained. Furthermore, the electrode is easily molded into a pellet. On the other hand, if the content of the positive electrode conductive assistant in the positive electrode 10 is equal to or less than the upper limit of the above-mentioned preferred range, sufficient discharge capacity is easily obtained by the positive electrode 10.

The positive electrode 10 may contain a binder (hereinafter, the binder used in the positive electrode 10 may be referred to as the "positive electrode binder").

As the positive electrode binder, a conventionally known substance can be used. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polyacrylic acid (PA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), etc. can be selected, and a binder formed by combining a plurality of these can be used.
The positive electrode binder may be one of the above-mentioned materials alone or two or more of them may be combined and used as the positive electrode binder. The content of the positive electrode binder in the positive electrode 10 may be, for example, 1 to 20 mass %.
In this specification, when a numerical range is indicated by using "to" to indicate an upper and lower limit, the range includes the upper and lower limits unless otherwise specified. Thus, for example, 1 to 20% by mass means 1% by mass or more and 20% by mass or less.
The positive electrode current collector 14 may be a conventionally known material, and an example of such a material is a conductive resin adhesive containing carbon as a conductive filler.

In addition, in this embodiment, the positive electrode active material may contain other positive electrode active materials in addition to the lithium manganese oxide described above, and may contain, for example, one or more of other oxides such as molybdenum oxide, lithium iron phosphate compound, lithium cobalt oxide, lithium nickel oxide, vanadium oxide, etc.

(Negative electrode)
In the negative electrode 20, the type of the negative electrode active material is not particularly limited, but it is preferable that the negative electrode active material contains, for example, silicon oxide.
In the negative electrode 20, the negative electrode active material is preferably made of carbon-coated SiO _x , for example, silicon oxide represented by SiO _x (0≦x<2) coated with carbon.

Furthermore, the negative electrode 20 may contain, in addition to the above-mentioned SiO _x (0≦x<2) as the negative electrode active material, other negative electrode active materials, such as Si, C, etc.
When granular SiO _x (0≦x<2) is used as the negative electrode active material, the particle diameter (D50) thereof is not particularly limited, but may be selected from the range of, for example, 0.1 to 30 μm, and more preferably from the range of 1 to 10 μm. If the particle diameter (D50) of SiO _x is less than the lower limit of the above range, for example, when the nonaqueous electrolyte secondary battery 1 is stored or used in a harsh high-temperature and high-humidity environment, or reactivity due to a reflow soldering process is increased, and the battery characteristics may be impaired, and if the particle diameter (D50) exceeds the upper limit, the discharge rate may be reduced.

The content of the negative electrode active material in the negative electrode 20, i.e., SiO _x (0≦x<2), is determined taking into consideration the discharge capacity required for the nonaqueous electrolyte secondary battery 1, and can be selected from the range of 50 mass % or more, and preferably from the range of 60 to 80 mass %.
In the negative electrode 20, when the content of the negative electrode active material composed of the above elements is equal to or more than the lower limit of the above range, a sufficient discharge capacity is easily obtained, and when it is equal to or less than the upper limit, the negative electrode 20 is easily formed.

The negative electrode 20 may contain a conductive assistant (hereinafter, the conductive assistant used in the negative electrode 20 may be referred to as a "negative electrode conductive assistant"). The negative electrode conductive assistant is similar to the positive electrode conductive assistant.
The content of the negative electrode conductive assistant in the negative electrode 20 is, for example, 1 to 45 mass %.

The negative electrode 20 may contain a binder (hereinafter, the binder used in the negative electrode 20 may be referred to as the "negative electrode binder").
As the negative electrode binder, polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polyacrylic acid (PA), carboxymethyl cellulose (CMC), polyimide (PI), polyamideimide (PAI), or the like can be selected.

The negative electrode binder may be one of the above alone or a combination of two or more of them. When polyacrylic acid is used as the negative electrode binder, the polyacrylic acid may be adjusted in advance to a pH of about 3 to 10. In this case, for example, an alkali metal hydroxide such as lithium hydroxide or an alkaline earth metal hydroxide such as magnesium hydroxide can be used to adjust the pH.
The content of the negative electrode binder in the negative electrode 20 is, for example, in the range of 1 to 20 mass %.

In this embodiment, the size and thickness of the negative electrode 20 can be similar to those of the positive electrode 10 .
In addition, in the nonaqueous electrolyte secondary battery 1 shown in FIG. 1 , a configuration can be employed in which a lithium body 60 such as lithium foil is provided on the surface of the negative electrode 20, i.e., between the negative electrode 20 and a separator 30 described below, although this is not shown in the figure.

"Electrolyte"
The electrolyte 50 is usually a liquid in which a supporting salt is dissolved in a non-aqueous solvent.
In the nonaqueous electrolyte secondary battery 1 of this embodiment, a mixed solvent containing tetraglyme (TEG) as a main solvent, diethoxyethane (DEE) as a sub-solvent, and further containing ethylene carbonate (EC) and vinylene carbonate (VC) as additives can be used as the nonaqueous solvent constituting the electrolyte solution 50. The nonaqueous solvent is usually determined taking into consideration the heat resistance, viscosity, etc. required for the electrolyte solution 50, but in this embodiment, a solvent consisting of the above-mentioned solvents is used.
As the main solvent for constituting the glyme-based solvent, tetraglyme, triglyme, pentagrayme, diglyme, etc. can be used.

In this embodiment, an electrolyte solution 50 using a non-aqueous solvent containing tetraglyme (TEG), diethoxyethane (DEE), and ethylene carbonate (EC) can be used. By adopting such a configuration, DEE and TEG are solvated in the Li ions that form the supporting salt.
At this time, since DEE has a higher donor number than TEG, DEE selectively solvates with Li ions. In this way, DEE and TEG solvate the Li ions that form the supporting salt, protecting the Li ions. As a result, even if moisture enters the inside of the nonaqueous electrolyte secondary battery under a high-temperature and high-humidity environment, it is possible to prevent moisture from reacting with Li, thereby suppressing a decrease in discharge capacity and improving storage stability.

The ratio of each of the above solvents in the nonaqueous solvent in the electrolyte solution 50 is not particularly limited, but can be selected from the following ranges (total 100%): TEG: 30% by mass or more and 48.5% by mass or less (30 to 48.5%), DEE: 30% by mass or more and 48.5% by mass or less (30 to 48.5%), EC: 0.5% by mass or more and 10% by mass or less (0.5 to 10%), VC: 2% by mass or more and 13% by mass or less (2 to 13%).
When the ratio of TEG, DEE and EC contained in the non-aqueous solvent is within the above range, the above-mentioned effect of protecting the Li ions by DEE solvating them can be obtained.
Even within the above range, the VC content is preferably in the range of 2.5% by mass or more and 10% by mass (2.5 to 10%), and more preferably in the range of 5.0% by mass or more and 7.5% by mass (5.0 to 7.5%). The upper limits of the TEG and DEE contents are preferably 48.25% by mass or less, and more preferably 48% by mass or less.
When the VC content is in the range of 2% by mass or more and 13% by mass or less, the change in thickness occurring in the storage container 2 consisting of the positive electrode can 12 and the negative electrode can 22 is small even when it is heated during reflow soldering, and the increase in internal resistance can also be reduced. Furthermore, when the VC content is in the range of 2.5% by mass or more and 10.0% by mass or less, the change in thickness occurring in the storage container 2 can be made smaller even when it is heated during reflow soldering, and the increase in internal resistance can also be made smaller. Even within these ranges, the VC content is most preferably in the range of 5.0% by mass or more and 7.5% by mass or less.

The supporting salt may be a known Li compound used as a supporting salt in the electrolyte of a non- _aqueous electrolyte secondary battery, and examples of the supporting salt include organic acid _lithium salts such as _LiCH3SO3 , _LiCF3SO3 , LiN( _CF3SO2 ) _{2 ,} LiN( _C2F5SO2 ) _{2, LiC(CF3SO2)3, LiN(CF3SO3)2 , and LiN ( FSO2 ) 2 ; and} inorganic acid lithium salts such as _LiPF6 , _LiBF4 , _LiB ( _C6H5 ) _{4 ,} LiCl, and LiBr. Among these, lithium salts that are compounds having lithium ion conductivity are preferred, with LiN( _{CF3SO2 ) 2} , LiN( _FSO2 ) ₂ , and _LiBF4 being more preferred, and LiN( _CF3SO2 ) ₂ being particularly preferred from the viewpoints of heat resistance and _low reactivity with moisture, and of being able to fully exhibit storage characteristics.
The supporting salt may be used alone or in combination of two or more of the above.

The content of the supporting salt in the electrolyte 50 can be determined taking into consideration the type of supporting salt, and is preferably 0.1 to 3.5 mol/L, more preferably 0.5 to 3 mol/L, and particularly preferably 1 to 2.5 mol/L. If the supporting salt concentration in the electrolyte 50 is too high or too low, the electrical conductivity may decrease, which may adversely affect the battery characteristics.

"(Negative electrode capacity/positive electrode capacity) value"
In the nonaqueous electrolyte secondary battery 1 of this embodiment, it is preferable that the value of "(negative electrode capacity/positive electrode capacity)=positive/negative electrode capacity balance" is in the range of 1.7 to 2.4.
If the value of (negative electrode capacity/positive electrode capacity) is less than 1.7, in other words, if the positive electrode capacity is larger, a sufficient capacity can be obtained after assembly, but the capacity retention rate after reflow heating becomes small, and a sufficient cell capacity cannot be obtained after reflow heating.
On the other hand, when the value of (negative electrode capacity/positive electrode capacity) exceeds 2.4, in other words when the negative electrode capacity is larger, the capacity retention rate is sufficiently high, but the amount of the positive electrode becomes relatively small, and sufficient cell capacity cannot be obtained both after assembly and after reflow.
For this reason, it is preferable that the value of (negative electrode capacity/positive electrode capacity) is in the range of 1.7 to 2.4. If the value of (negative electrode capacity/positive electrode capacity) is in the range of 1.7 to 2.4, a nonaqueous electrolyte secondary battery 1 having a large capacity retention rate after reflow heating and a large initial capacity can be provided.
It is more preferable that the above-mentioned (negative electrode capacity/positive electrode capacity) value is in the range of 1.9 to 2.4. If it is in this range, a sufficiently high capacity can be obtained after assembly, and the capacity retention rate after reflow heating can be increased, resulting in a higher cell capacity after reflow heating.
Even within the above range, in order to reliably secure the initial capacity and to keep the capacity retention rate after reflow heating in a high range, it is most preferable to set the range of 1.94 to 2.39.

The negative electrode capacity shown here means the capacity obtained by multiplying the capacity density of the active material in the negative electrode by the amount of the active material used. The positive electrode capacity means the capacity obtained by multiplying the capacity density of the active material in the positive electrode by the amount of the active material used. As the capacity density of the active material, lithium manganate was used at 137 mAh/g, and silicon monoxide was used at 1775 mAh/g.
The capacity density of the active material is a value estimated from the cell capacity obtained by actually charging and discharging an electrochemical cell in which a positive electrode or a negative electrode faces metallic Li. The theoretical capacity is known to be 148 mAh/g for lithium manganate and 2007 mAh/g for silicon monoxide, but the above-mentioned values are used in this application.

(Negative electrode capacity/positive electrode capacity): A calculation example of the capacity balance is shown below.
The positive electrode mixture ratio was lithium manganese oxide: graphite: polyacrylic acid = 95:4:1 (mass ratio), and 16.4 mg of the positive electrode mixture (15.6 mg of lithium manganate) was used, resulting in a positive electrode capacity of 2.14 mAh.
The negative electrode mixture ratio was silicon monoxide:graphite:polyacrylic acid=75:20:5 (mass ratio), and 3.1 mg of the negative electrode mixture (2.3 mg of silicon monoxide) was used, resulting in a negative electrode capacity of 4.08 mAh.
The amount of Li is adjusted according to the amount of silicon monoxide (designed so that the ratio of the number of Li atoms: the number of SiO molecules is approximately 4:1).
A calculation example of the capacity balance (negative electrode capacity/positive electrode capacity) under the above-described conditions and a calculation example of the designed Li amount are shown in Tables 1 and 2 below.

(Separator)
The separator 30 is interposed between the positive electrode 10 and the negative electrode 20, and is made of an insulating film having high ion permeability and mechanical strength.
As the separator 30, any separator conventionally used for non-aqueous electrolyte secondary batteries can be used without any restrictions, and examples thereof include glass such as alkali glass, borosilicate glass, quartz glass, and lead glass, and nonwoven fabrics made of resins such as polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyethylene terephthalate (PET), polyamide-imide (PAI), polyamide, and polyimide (PI). Among these, glass nonwoven fabrics are preferred, and borosilicate glass nonwoven fabrics are more preferred. Glass nonwoven fabrics have excellent mechanical strength and large ion permeability, so that they can reduce internal resistance and improve discharge capacity.
The thickness of the separator 30 is determined taking into consideration the size of the nonaqueous electrolyte secondary battery 1, the material of the separator 30, and the like, and can be, for example, 5 to 300 μm.

(gasket)
The gasket 40 is preferably made of, for example, a resin having a heat deformation temperature of 230° C. or higher. If the resin material used for the gasket 40 has a heat deformation temperature of 230° C. or higher, the gasket can be prevented from being significantly deformed by reflow soldering or heating during use of the nonaqueous electrolyte secondary battery 1, and leakage of the electrolyte 50 can be prevented.
As shown in FIG. 1, the gasket 40 is formed in an annular shape along the inner peripheral surface of the positive electrode can 12, and the outer peripheral end 22a of the negative electrode can 22 is disposed inside the annular groove 41 thereof.
Gasket 40 has a ring-shaped outer edge portion 40A having an outer diameter that allows it to be inserted without gaps into the inner periphery of the opening of positive electrode can 12. Gasket 40 has a ring-shaped inner edge portion 40B having an outer diameter that allows it to be inserted without gaps into the inner periphery of negative electrode can 22. Gasket 40 also has a bottom wall portion 40C that connects the lower ends of outer edge portion 40A and inner edge portion 40B.
Therefore, an annular groove 41 is formed on the upper surface side of the outer periphery of the gasket 40, into which the outer periphery end 22a of the negative electrode can 22 can be inserted.
By crimping the peripheral portion 12b of the opening 12a of the positive electrode can 12 shown in FIG. 1 inward, i.e., toward the negative electrode can 22, the gasket 40 can be sandwiched together with the negative electrode can 22, thereby forming a storage container 2 having a structure in which the storage space is sealed.

Examples of materials for the gasket 40 include polyphenylene sulfide (PPS), polyethylene terephthalate (PET), polyamide, liquid crystal polymer (LCP), tetrafluoroethylene-perfluoroalkylvinylether copolymer resin (PFA), polyetheretherketone resin (PEEK), polyethernitrile resin (PEN), polyetherketone resin (PEK), polyarylate resin, polybutylene terephthalate resin (PBT), polycyclohexanedimethylene terephthalate resin, polyethersulfone resin (PES), polyaminobismaleimide resin, polyetherimide resin, and fluororesin. In addition, glass fiber, mica whisker, ceramic fine powder, etc., added in an amount of 30 mass% or less to these materials can be suitably used. By using such materials, it is possible to prevent the gasket from deforming significantly due to heating and the electrolyte 50 from leaking.

According to the nonaqueous electrolyte secondary battery 1 of this embodiment described above, the nonaqueous solvent is provided with electrolyte solution 50 which contains mainly tetraglyme (TEG) and diethoxyethane (DEE) and contains appropriate amounts of both ethylene carbonate (EC) and the above-mentioned vinylene carbonate (VC). Therefore, the battery has heat resistance sufficient to withstand reflow soldering, and even if it is subjected to the heat associated with reflow soldering, there is little risk of the solvent evaporating and there is little risk of the internal pressure of storage container 2 increasing, making it possible to provide a configuration in which storage container 2 is less likely to deform.
Furthermore, if the solvent is a glyme-based solvent containing tetraglyme and diethoxyethane as main components, the heat resistance of the electrolyte can be increased due to the high boiling points of these solvents.

In addition, according to the nonaqueous electrolyte secondary battery 1 of this embodiment, in addition to the combination of the positive electrode, negative electrode, and electrolyte described above, the value of (negative electrode capacity/positive electrode capacity) is set to a range of 1.7 to 2.4, so that a high capacity after assembly can be ensured and the capacity retention rate after reflow heating can be increased, and sufficient cell capacity can be obtained after reflow heating. Therefore, according to this embodiment, a high-capacity nonaqueous electrolyte secondary battery compatible with reflow soldering can be provided.

A nonaqueous electrolyte secondary battery having the configuration shown in FIG. 1 was fabricated and subjected to the evaluation test described below.
A positive electrode mixture was _prepared by _{mixing commercially} available lithium manganese oxide ( _{Li1.14Co0.06Mn1.80O4} ), graphite as a conductive additive, and polyacrylic acid as a binder in a ratio of lithium manganese oxide:graphite:polyacrylic acid = 95:4:1 (mass ratio) to prepare a positive electrode mixture for the positive electrode 10. 16.4 mg of this positive electrode mixture was pressed with a pressure of 2 ton/ ^cm2 and pressure molded into a disk-shaped pellet with a diameter of 2.8 mm.

The obtained pellet (positive electrode) was adhered to the inner surface of a positive electrode can made of stainless steel (SUS316L: t = 0.20 mm) using a carbon-containing conductive resin adhesive, and these were integrated to obtain a positive electrode unit. This positive electrode unit was then dried by heating under reduced pressure at 120°C for 11 hours in the atmosphere. Next, a sealant was applied to the inner surface of the opening of the positive electrode can in the positive electrode unit.

Next, SiO powder with carbon (C) formed on the entire surface was prepared as a negative electrode, and this was used as a negative electrode active material. Then, graphite as a conductive agent and polyacrylic acid as a binder were mixed with this negative electrode active material in a ratio of 75:20:5 (mass ratio) to prepare a negative electrode mixture. 3.1 mg of this negative electrode mixture was pressed with a pressure of 2 ton/ ^cm2 and pressed into a disk-shaped pellet with a diameter of 2.8 mm.

The pellets (negative electrode) were bonded to the inner surface of a stainless steel (SUS316L: t = 0.20 mm) negative electrode can using a conductive resin adhesive containing carbon as a conductive filler, and integrated to obtain a negative electrode unit. This negative electrode unit was then dried under reduced pressure and heating at 160 °C for 11 hours in the atmosphere.
A lithium foil punched to a diameter of 2.8 mm and a thickness of 0.44 mm was further attached by pressure onto this pellet-shaped negative electrode to form a lithium-negative electrode laminate.

As described above, in this example, a nonaqueous electrolyte secondary battery was fabricated by giving the positive electrode can the function of a positive electrode current collector and the negative electrode can the function of a negative electrode current collector without providing the positive electrode current collector and the negative electrode current collector shown in the structure of the embodiment.

Next, the nonwoven fabric made of glass fibers was dried and then punched into a disk shape with a diameter of 3.6 mm to form a separator. This separator was then placed on the lithium foil that was pressed onto the negative electrode, and a gasket made of PEEK resin (polyether ether ketone resin) was placed on the opening of the negative electrode can.

(Preparation of Electrolyte)
The solvents tetraglyme (TEG), diethoxyethane (DEE), ethylene carbonate (EC), and vinylene carbonate (VC) were mixed to obtain a non-aqueous solvent, and LiTFSI (1M) was dissolved in the obtained non-aqueous solvent as a supporting salt to obtain an electrolyte. The mixing ratio of the solvents was TEG:DEE:EC:VC=44.8:42.7:5.0:7.5 by volume.
The positive electrode can and the negative electrode can prepared as described above were filled with the electrolyte of each example prepared by the above procedure in a total amount of 7 μL per battery.

Next, the negative electrode unit was crimped to the positive electrode unit so that the separator was in contact with the positive electrode. The opening of the positive electrode can was fitted to seal the positive electrode can and the negative electrode can, and the can was left to stand at 25° C. for 7 days to obtain a sample.
When preparing the samples, the amount of the positive electrode active material in the positive electrode and the amount of the negative electrode active material in the negative electrode were adjusted in the same manner as in the above-mentioned calculation example of "(negative electrode capacity)/(positive electrode capacity)," and nonaqueous electrolyte secondary batteries of Samples 1 to 9 having different values of (negative electrode capacity/positive electrode capacity) were obtained. All of these sample nonaqueous electrolyte secondary batteries were coin-type secondary batteries with an outer diameter of 4.8 mm and a height of 2.1 mm.
The nonaqueous electrolyte secondary batteries of Samples 1 to 9 are secondary batteries having different values of (negative electrode capacity/positive electrode capacity) as shown in Table 3 below.

"Evaluation Test"
(Initial capacity: mAh)
After each sample of the nonaqueous electrolyte secondary battery was prepared, the capacity was measured before heating to the reflow soldering temperature. The capacity measured in this case was defined as the initial capacity.
(Capacity retention rate)
Each sample of the nonaqueous electrolyte secondary battery was heated at 260° C. for 10 seconds, and the capacity after heating was measured and compared with the initial capacity to obtain the capacity retention rate (%). The heat treatment of heating at 260° C. for 10 seconds corresponds to the heating conditions associated with reflow soldering.

(Capacity after storage in high temperature and high humidity environment)
Each sample of the non-aqueous electrolyte secondary battery was heated at 260° C. for 10 seconds, and then exposed to a high-temperature and high-humidity environment of 60° C. and 90% RH using a thermo-hygrostat, and the capacity was measured after leaving the battery for 20 days. The capacity measured in this case is the capacity after storage at high temperature and high humidity.

(result)
For the nonaqueous electrolyte secondary batteries of Samples 1 to 9, the value of "(negative electrode capacity/positive electrode capacity)=(positive and negative electrode capacity balance)", the initial capacity, the capacity retention rate after reflow, and the capacity after high-temperature, high-humidity storage are shown in Table 3. In addition, Fig. 2 shows the relationship between the capacity retention rate and the initial capacity due to the positive and negative electrode capacity balance, and Fig. 3 shows the relationship between the capacity after high-temperature, high-humidity storage and the positive and negative electrode capacity balance, both as graphs.

2, it can be seen that a good range of (negative electrode capacity/positive electrode capacity) in which the initial capacity is high and the capacity retention rate after reflow exceeds 70% is selected to be in the range of 1.72 to 2.39. Therefore, it was found that in order to achieve a range in which the initial capacity is high and the capacity retention rate after reflow is also high, it is desirable to select a range of 1.7 to 2.4 as the value of (negative electrode capacity/positive electrode capacity).
It was also found that a more favorable range in which the initial capacity is high, exceeding 1.97, and the capacity retention rate after reflow exceeds 80% is when the value of (negative electrode capacity/positive electrode capacity) is in the range of 1.9 to 2.4.
Furthermore, when the relationship between the capacity after high temperature and high humidity storage and the positive and negative electrode capacity balance shown in Table 3 is referred to as shown in FIG. 3, since the capacity after high temperature and high humidity storage corresponding to long-term storage is high, it can be said that the value of (negative electrode capacity/positive electrode capacity) is 1.7 or more as a good range, and the value of (negative electrode capacity/positive electrode capacity) is in the range of 1.9 to 2.4 as a more good range.

1...non-aqueous electrolyte secondary battery, 2...container, 10...positive electrode, 12...positive electrode can, 12a...opening, 12b...periphery, 13...positive electrode, 14...positive electrode current collector, 20...negative electrode, 22...negative electrode can, 22a...outer periphery, 24...negative electrode current collector, 30...separator, 40...gasket, 41...annular groove, 50...electrolyte.

Claims (4)

A nonaqueous electrolyte secondary battery in which a positive electrode, a negative electrode, an electrolyte solution containing a supporting salt and a solvent, and a separator are contained in a container formed of a positive electrode can and a negative electrode can,
The positive electrode contains a spinel-type lithium manganese oxide as an active material, the negative electrode contains carbon-coated SiO _x as an active material, the electrolyte contains a mixed solvent containing ethylene carbonate (EC) and vinylene carbonate (VC) in a glyme-based solvent, and the separator contains glass fiber;
A non-aqueous electrolyte secondary battery having excellent capacity retention after reflow soldering , characterized in that the ratio of the negative electrode capacity to the positive electrode capacity (negative electrode capacity/positive electrode capacity) is in the range of 1.7 to 2.4. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode capacity/positive electrode capacity ratio is in the range of 1.9 to 2.4. 2. The nonaqueous electrolyte secondary battery having an excellent capacity retention rate after reflow soldering according to claim 1, wherein the value of (negative electrode capacity/positive electrode capacity) is in the range of 1.94 to 2.39. The positive electrode can is cylindrical with a bottom,
The negative electrode can is fixed to the inside of the opening of the positive electrode can with a gasket interposed therebetween,
4. The nonaqueous electrolyte secondary battery having excellent capacity retention rate after reflow soldering according to claim 1, wherein the container is sealed by providing a crimping portion that crimps an opening of the positive electrode can to the negative electrode can, and the positive electrode, the negative electrode, the separator, and the electrolyte are contained in the container.

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