Nonaqueous electrolyte secondary battery

A nonaqueous electrolyte secondary battery includes a flat winding electrode assembly including a positive electrode substrate exposed portion on one end and a negative electrode substrate exposed portion on the other end. The winding numbers of the positive and the negative electrode substrate exposed portions are each 30 or more. The positive and negative electrode substrate exposed portions each have an outermost surface welded and connected with a positive and a negative electrode collectors, respectively. A nonaqueous electrolyte used to fabricate the battery contains a lithium salt having an oxalate complex as an anion. At the welded connection portions, all of the layers of the positive electrode substrate exposed portion are melted to be welded and connected to the positive electrode collector, and all of the layers of the negative electrode substrate exposed portion are melted to be welded and connected to the negative electrode collector.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte secondary battery that has excellent cycling characteristics.

BACKGROUND ART

Alkaline secondary batteries typified by nickel-hydrogen batteries and nonaqueous electrolyte secondary batteries typified by lithium ion batteries have been widely used as a power supply for driving portable electronic equipment, such as cell phones including smartphones, portable personal computers, PDAs, and portable music players. In addition, alkaline secondary batteries and nonaqueous electrolyte secondary batteries have been widely used as a power supply for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs and PHEVs), and in stationary storage battery systems for suppressing output fluctuation of solar power generation and wind power generation, for example, and for a peak shift of system power that utilizes the power during the daytime while saving the power during the nighttime.

The use of EVs, HEVs, and PHEVs or the stationary storage battery system especially requires high capacity and high output characteristics. The size of each battery is therefore increased, and a plurality of batteries are connected in series or in parallel for use. Therefore, nonaqueous electrolyte secondary batteries have been generally used for these purposes in view of space efficiency. When physical strength is needed, a metal prismatic outer body with one side open, and a metal sealing plate for sealing this opening are generally adopted as an outer body of a battery.

Increasing longevity is essential in nonaqueous electrolyte secondary batteries used for the above-mentioned purposes. Therefore, various additives are added to a nonaqueous electrolyte in order to prevent degradation. For example, JP-A-2009-129541 discloses that, in a nonaqueous electrolyte secondary battery, a cyclic phosphazene compound and various salts having an oxalate complex as an anion are added to a nonaqueous electrolyte. JP-T-2010-531856 and JP-A-2010-108624 describe the addition of lithium bis(oxalato)borate (Li[B(C2O4)2], hereinafter referred to as “LiBOB”), which is a lithium salt having an oxalate complex as an anion, as represented by the following structural formula (I).

For example, Japanese Patent No. 3439085 discloses the invention of a nonaqueous electrolyte secondary battery in which lithium difluorophosphate (LiPF2O2) is added to a nonaqueous electrolyte in order to prevent self-discharge at charge storage and improve storage characteristics after charging.

When a cyclic phosphazene compound and various salts having an oxalate complex as an anion disclosed in JP-A-2009-129541 are added to the nonaqueous electrolyte, fire resistance of the nonaqueous electrolyte is improved, which can provide a nonaqueous electrolyte secondary battery having excellent battery characteristics and high safety. When LiBOB disclosed in JP-T-2010-531856 and JP-A-2010-108624 is added to a nonaqueous electrolyte, a protective layer including a lithium ion conductive layer that is thin and extremely stable is formed on the surface of a carbon negative electrode active material of the nonaqueous electrolyte secondary battery. This protective layer is stable even in a high temperature, consequently preventing the carbon negative electrode active material from decomposing the nonaqueous electrolyte. This leads to an advantage of providing excellent cycling characteristics and improving the safety of a battery.

In the nonaqueous electrolyte secondary battery disclosed in Japanese Patent No. 3439085, LiPF2O2and lithium react with each other to form a high-quality protective covering onto an interface of a positive electrode active material and a negative electrode active material. This protective covering prevents direct contact between an active material in a state of charge and an organic solvent, thereby preventing decomposition of the nonaqueous electrolyte due to contact between the active material and the nonaqueous electrolyte. Consequently, an advantageous function effect of improving charge storage characteristics can be attained.

A nonaqueous electrolyte secondary battery having high capacity and high output characteristics uses not only a positive electrode and a negative electrode with a large width, and a separator, but also a flat winding electrode assembly produced by winding these many times.

A problem has been found in that a reaction inside an electrode is likely to occur in a non-uniform manner in a winding electrode assembly including a positive electrode with a large width and a negative electrode with a large width. In the nonaqueous electrolyte secondary battery, a lithium salt having an oxalate complex as an anion is added to a nonaqueous electrolyte in order to improve cycling characteristics. However, when the reaction inside the electrode occurs in a non-uniform manner, a protective covering derived from the lithium salt having the oxalate complex as an anion is less likely to be formed uniformly on the surface of the negative electrode, and the cycling characteristics are less likely to be improved.

SUMMARY

An advantage of some aspects of the invention is to provide a nonaqueous electrolyte secondary battery that has excellent cycling characteristics even if a winding electrode assembly including a positive electrode with a large width and a negative electrode with a large width is used.

A nonaqueous electrolyte secondary battery of an aspect of the invention includes: a flat winding electrode assembly formed by winding an elongated positive electrode and an elongated negative electrode with an elongated separator interposed therebetween; and an outer body storing the flat winding electrode assembly and a nonaqueous electrolyte. The flat winding electrode assembly includes a positive electrode substrate exposed portion wound on one end and a negative electrode substrate exposed portion wound on the other end. The winding number of the wound positive electrode substrate exposed portion is 30 or more, and the winding number of the wound negative electrode substrate exposed portion is 30 or more. The wound positive electrode substrate exposed portion has an outermost surface welded and connected with a positive electrode collector, and the wound negative electrode substrate exposed portion has an outermost surface welded and connected with a negative electrode collector. The nonaqueous electrolyte contains a lithium salt having an oxalate complex as an anion at the time of making the nonaqueous secondary battery. All of the layers of the positive electrode substrate exposed portion are melted at the welded connection portion between the positive electrode substrate exposed portion and the positive electrode collector to be welded and connected to the positive electrode collector. All of the layers of the negative electrode substrate exposed portion are melted at the welded connection portion between the negative electrode substrate exposed portion and the negative electrode collector to be welded and connected to the negative electrode collector.

When the nonaqueous electrolyte contains a lithium salt having an oxalate complex as an anion at the time of making the nonaqueous electrolyte second battery, a protective covering that is stable even in a high temperature is formed on the surface of the negative electrode due to a reaction between the lithium salt having an oxalate complex as an anion and a negative electrode active material. This prevents the negative electrode active material from decomposing the nonaqueous electrolyte, thereby providing excellent cycling characteristics and improving the safety of the battery. In the nonaqueous electrolyte secondary battery of the invention, the winding numbers of the positive electrode and the negative electrode in the flat winding electrode assembly are each 30 or more. In other words, the numbers of stacked layers of the positive electrode and the negative electrode are each 60 or more. This provides a nonaqueous electrolyte secondary battery that can easily achieve high capacity and high output characteristics.

In the nonaqueous electrolyte secondary battery of the invention, the wound positive electrode substrate exposed portion has an outermost surface welded and connected with a positive electrode collector, and the wound negative electrode substrate exposed portion has an outermost surface welded and connected with a negative electrode collector. Furthermore, all of the layers in the stacked direction of the positive electrode substrate exposed portion are melted at the welded connection portion between the positive electrode substrate exposed portion and the positive electrode collector to be welded and connected to the positive electrode collector, and all of the layers in the stacked direction of the negative electrode substrate exposed portion are melted at the welded connection portion between the negative electrode substrate exposed portion and the negative electrode collector to be welded and connected to the negative electrode collector. In other words, all of the stacked layers of the positive electrode substrate exposed portion and the negative electrode substrate exposed portion are each melted and integrated at the welded connection portion. Even in the nonaqueous electrolyte secondary battery including the flat winding electrode assembly that has a large winding number, this structure allows a reaction in the electrode to proceed uniformly on the winding start side and on the winding end side of the winding electrode assembly. Thus, a protective covering derived from the lithium salt having the oxalate complex as an anion is formed uniformly on the surface of the negative electrode, and the nonaqueous electrolyte secondary battery can be obtained that has more excellent cycling characteristics. In addition, even if the nonaqueous electrolyte penetrates the positive electrode substrate exposed portion or the negative electrode substrate exposed portion, resistance in the positive electrode substrate exposed portion or the negative electrode substrate exposed portion at the welded connection portion is less likely to change. Whether all of the stacked layers of the positive electrode substrate exposed portion or the negative electrode substrate exposed portion are melted at the welded connection portion can be confirmed by breaking down the welded connection portion after resistance-welding and examining a weld mark formed at the time of resistance-welding.

A compound capable of reversibly absorbing and desorbing lithium ions may be selected to be used as appropriate as the positive electrode active material that can be used in the nonaqueous electrolyte secondary battery of the invention. Such electrode active materials include lithium transition-metal composite oxides that are represented by LiMO2(M is at least one of Co, Ni, and Mn) and are capable of reversibly absorbing and desorbing lithium ions, namely, LiCoO2, LiNiO2, LiNiyCo1-yO2(y=0.01 to 0.99), LiMnO2, LiCoxMnyNizO2(x+y+z=1), LiMn2O4, or LiFePo4. Such lithium transition-metal composite oxides may be used alone, or two or more of them may be mixed to be used. Furthermore, lithium cobalt composite oxides with different metal element such as zirconium, magnesium, and aluminum added thereto may be used as well.

The following shows examples of a nonaqueous solvent that can be used for the nonaqueous electrolyte in the nonaqueous electrolyte secondary battery of the invention: a cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); a fluorinated cyclic carbonate; a cyclic carboxylic ester such as γ-butyrolactone (γ-BL) and γ-valerolactone (γ-VL); a chain carbonate such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methylpropyl carbonate (MPC), and dibutyl carbonate (DBC); fluorinated chain carbonate: a chain carboxylic ester such as methyl pivalate, ethyl pivalate, methyl isobutyrate, and methyl propionate; an amide compound such as N,N′-dimethylformamide and N-methyl oxazolidinone; and a sulfur compound such as sulfolane. It is desirable that two or more of them be mixed to be used.

In the nonaqueous electrolyte secondary battery of the invention, the lithium salt that is commonly used as an electrolyte salt for an nonaqueous electrolyte secondary battery may be used as the electrolyte salt dissolved in the nonaqueous solvent. Examples of such a lithium salt are as follows: LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, LiAsF6, LiClO4, Li2B10Cl10, Li2B12Cl12, and mixtures of these substances. In particular, among them, it is preferable that LiPF6(lithium hexafluorophosphate) be used. The amount of dissolution of the electrolyte salt with respect to the nonaqueous solvent is preferably from 0.8 to 1.5 mol/L.

In the nonaqueous electrolyte secondary battery of the invention, the lithium salt having the oxalate complex as an anion is preferably contained in the nonaqueous electrolyte in an amount of 0.01 to 2.0 mol/L, more preferably from 0.05 to 0.2 mol/L at the time of making the nonaqueous electrolyte secondary battery. In the nonaqueous electrolyte secondary battery of the invention, the additive amount of the lithium salt having the oxalate complex as an anion in the nonaqueous electrolyte may be added as the electrolyte salt whose principal element is the lithium salt having the oxalate complex as an anion. However, a large additive amount of the lithium salt having the oxalate complex as an anion in the nonaqueous electrolyte increases the viscosity of the nonaqueous electrolyte. Therefore, various electrolyte salts as above may be used as principal elements, and the lithium salt having the oxalate complex as an anion may be added as an additive substance in a small amount, for example, about 0.1 mol/L. When the lithium salt having the oxalate complex as an anion is added as the additive substance, depending on the additive amount thereof, all of the lithium salt having the oxalate complex as an anion may be consumed for forming the protective covering at the initial charge, resulting in a case in which no lithium salt having the oxalate complex as an anion remains substantially in the nonaqueous electrolyte. The invention also includes this case. Thus, the invention includes any case in which the nonaqueous electrolyte of the nonaqueous electrolyte secondary battery before the initial charge contains the lithium salt having the oxalate complex as an anion.

In the nonaqueous electrolyte secondary battery of the invention, it is preferable that the welded connection portion between the positive electrode substrate exposed portion and the positive electrode collector be provided at least at one point on each of both sides across the winding central position of the flat winding electrode assembly, and that the welded connection portion between the negative electrode substrate exposed portion and the negative electrode collector be provided at least at one point on each of both sides across the winding central position of the flat winding electrode assembly.

Such a structure has the welded connection portion formed on both sides (on a sealing plate side and the bottom portion side of the outer body) across the winding central position of the flat winding electrode assembly. Therefore, the electrode reaction is likely to proceed uniformly over the whole flat winding electrode assembly, and the above-mentioned effect can be successfully attained.

In the nonaqueous electrolyte secondary battery of the invention, it is preferable that the positive electrode substrate exposed portion have a melting area of 2.7 mm2or more in a plane passing through the central axis of the welded connection portion and being perpendicular to the winding axis of the winding electrode assembly within the welded connection portion with the positive electrode collector.

Furthermore, it is preferable that the negative electrode substrate exposed portion have a melting area of 1.8 mm2or more in a plane passing through the central axis of the welded connection portion and being perpendicular to the winding axis of the winding electrode assembly within the welded connection portion with the negative electrode collector.

In the nonaqueous electrolyte secondary battery, the positive electrode collector and the positive electrode substrate are usually formed using aluminum or an aluminum alloy, and the negative electrode collector and the negative electrode substrate are usually formed using copper or a copper alloy. Copper or a copper alloy has a higher melting point, higher hardness, and higher strength than aluminum or an aluminum alloy. Thus, the melting area of the positive electrode substrate exposed portion is made larger than that of the negative electrode substrate exposed portion to allow mechanical strength of the welded connection portion of the positive electrode to be almost equal to that of the welded connection portion of the negative electrode.

In the nonaqueous electrolyte secondary battery of the invention, it is preferable that the wound positive electrode substrate exposed portion and the wound negative electrode substrate exposed portion be each divided into two segments, that the two segments of the wound positive electrode substrate exposed portion be disposed so that a positive electrode conductive member is arranged therebetween, and that the two segments of the wound negative electrode substrate exposed portion be disposed so that a negative electrode conductive member is arranged therebetween. In such a case, it is preferable that a plurality of positive electrode conductive members be held in one positive electrode intermediate member with electric insulating properties, and that a plurality of negative electrode conductive members be held in one negative electrode intermediate member with electric insulating properties.

Such a structure enables the two segments of the substrate exposed portion, the conductive member, and a collector to be connected at a time by the series resistance welding method, and allows the resistance welding to be performed so as to form a weld mark passing through each stacked portion of the two segments of the substrate exposed portion. Therefore, less current is needed for the resistance welding, compared to the case where the resistance welding is performed so as to form a weld mark passing through the whole stacked portion of the non-divided positive electrode substrate exposed portion or negative electrode substrate exposed portion. In addition, a plurality of conductive members on the positive electrode side and the negative electrode side are each held in one intermediate member with electric insulating properties. This allows the conductive members to be stably positioned and disposed between the two segments of the substrate exposed portion, and improves the quality of the resistance welded portion to achieve low resistance.

In the nonaqueous electrolyte secondary battery of the invention, the battery preferably has a capacity of 20 Ah or more.

When the battery capacity is 20 Ah or more, the winding numbers of the positive electrode and the negative electrode are large. Therefore, the above-mentioned effect of the invention can be seen better.

In the nonaqueous electrolyte secondary battery of the invention, it is preferable that the lithium salt having the oxalate complex as an anion be lithium bis(oxalato)borate (Li[B(C2O4)2], hereinafter referred to as “LiBOB”).

Using LiBOB as the lithium salt having the oxalate complex as an anion provides the nonaqueous electrolyte secondary battery capable of attaining further preferable cycling characteristics.

In the nonaqueous electrolyte secondary battery of the invention, it is preferable that the nonaqueous electrolyte used to produce the nonaqueous electrolyte secondary battery of the invention contain LiPF2O2.

When LiPF2O2is contained in the nonaqueous electrolyte as an additive, it reacts with lithium at the charge and discharge to form a high-quality protective covering onto the interface of the positive electrode active material and the negative electrode active material. This protective covering prevents direct reaction between an active material in a state of charge and an organic solvent, thereby preventing decomposition of the nonaqueous electrolyte and providing a nonaqueous electrolyte secondary battery that has excellent charge storage characteristics. Depending on the added amount of LiPF2O2, all of the LiPF2O2may be consumed for forming a protective covering at the initial charge and discharge, resulting in a case in which no LiPF2O2remains substantially in the nonaqueous electrolyte. The invention also includes this case. Thus, the invention includes any case in which the nonaqueous electrolyte of the nonaqueous electrolyte secondary battery before the initial charge contains LiPF2O2. LiPF2O2is preferably contained in an amount of from 0.01 to 2.0 mol/L, more preferably from 0.01 to 0.1 mol/L at the time of making the nonaqueous electrolyte secondary battery.

In the nonaqueous electrolyte secondary battery of the invention, it is preferable that the positive electrode collector have a rib formed in a region facing the positive electrode substrate exposed portion and on an end on the center side of the flat winding electrode assembly, and that the negative electrode collector have a rib formed in a region facing the negative electrode substrate exposed portion and on an end on the center side of the flat winding electrode assembly.

Such a structure allows the rib to prevent scatter of spatters generated when the collector is resistance-welded to the outermost surface of a stack of a plurality of layers of the positive electrode substrate exposed portion or a stack of a plurality layers of the negative electrode substrate exposed portion. Therefore, these spatters are less likely to enter the inside of the flat winding electrode assembly, whereby a nonaqueous electrolyte secondary battery can be obtained that has excellent reliability. In addition, high heat is generated when the positive electrode collector or the negative electrode collector is welded to the positive electrode substrate exposed portion or the negative electrode substrate exposed portion. However, the rib serves as a heat releasing fin, which prevents a portion other than the welded portion of the positive electrode collector and the negative electrode collector to be melted.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

An embodiment of the invention will be described below in detail with reference to the accompanying drawings. However, the embodiment described below is merely an illustrative example for understanding the technical spirit of the invention and is not intended to limit the invention to the embodiment. The invention may be equally applied to various modifications without departing from the technical spirit described in the claims.

Embodiment

First, a prismatic nonaqueous electrolyte secondary battery in accordance with an embodiment will be described with reference toFIGS. 1 to 4. As shown inFIG. 4, this nonaqueous electrolyte secondary battery10includes a flat winding electrode assembly14. In the electrode assembly14, a positive electrode11and a negative electrode12are wound while being insulated from each other with a separator13interposed therebetween. The winding electrode assembly14has its outermost side covered with the separator13and has the negative electrode12disposed on a further outer side than the positive electrode11.

As illustrated inFIG. 3A, a positive electrode11is produced by the following process: a positive electrode active material mixture is applied onto both sides of a positive electrode substrate of aluminum foil; the resultant object is dried and extended by applying pressure; and the positive electrode11is slit so as to expose the aluminum foil in a strip along the end of one side in the wide direction. The part of the aluminum foil exposed in a strip is a positive electrode substrate exposed portion15. As illustrate inFIG. 3B, a negative electrode12is produced by the following process: a negative electrode active material mixture is applied onto both sides of a negative electrode substrate of copper foil; the resultant object is dried and extended by applying pressure; and the negative electrode12is slit so as to expose the copper foil in a strip along the end of one side in the wide direction. The part of the copper foil exposed in a strip is a negative electrode substrate exposed portion16.

The width and length of a negative electrode active material mixture layer12aof the negative electrode12are larger than those of a positive electrode active material mixture layer11a. It is preferable that the positive electrode substrate be formed using foil of aluminum or an aluminum alloy having a thickness of about from 10 to 20 μm, while the negative electrode substrate be formed using foil of copper or a copper alloy having a thickness of about from 5 to 15 μm. A specific composition of the positive electrode active material mixture layer11aand the negative electrode active material mixture layer12awill be described later.

As shown inFIGS. 2A and 2B, the flat winding electrode assembly14having a plurality of stacked layers of the positive electrode substrate exposed portion15on one end and a plurality of stacked layers of the negative electrode substrate exposed portion16on the other end is produced by the following process: the positive electrode11and the negative electrode12produced as above are displaced so that the aluminum foil exposed portion of the positive electrode11and the copper foil exposed portion of the negative electrode12are not overlapped by the active material mixture layers of the opposing electrodes; and the positive electrode11and the negative electrode12are wound while being insulated from each other with a separator13interposed therebetween. A microporous polyolefin membrane is preferably used as the separator13.

The stacked layers of the positive electrode substrate exposed portion15are electrically connected to a positive electrode terminal18of aluminum material with a positive electrode collector17of aluminum material interposed therebetween. Likewise, the stacked layers of the negative electrode substrate exposed portion16are electrically connected to a negative electrode terminal20of copper material with a negative electrode collector19of copper material interposed therebetween. As shown inFIGS. 1A, 1B, and 2A, the positive electrode terminal18and the negative electrode terminal20are fixed to a sealing plate23of aluminum material or other material with insulating members21and22, respectively, interposed therebetween. Where appropriate, the positive electrode terminal18and the negative electrode terminal20are connected to an external positive electrode terminal and an external negative electrode terminal (neither shown in the drawings), respectively.

The flat winding electrode assembly14produced as above is inserted into a prismatic outer body25of aluminum material or other material with one side thereof open with an insulating resin sheet24interposed in the periphery except for the sealing plate23side. The sealing plate23is then fitted to a opening portion of the prismatic outer body25, and a fitting portion between the sealing plate23and the prismatic outer body25is laser-welded. Moreover, a nonaqueous electrolyte is poured through an electrolyte pour hole26, and then the electrolyte pour hole26is sealed. Consequently, the nonaqueous electrolyte secondary battery10of the embodiment 1 is produced. In the prismatic nonaqueous electrolyte secondary battery10of the embodiment, as shown inFIG. 4, starting from the prismatic outer body25, the resin sheet24, the separator13, the negative electrode12, the separator13, the positive electrode11, the separator13, the negative electrode12, . . . are disposed.

A current interruption mechanism27operated by a gas pressure generated inside the battery is provided between the positive electrode collector17and the positive electrode terminal18. A gas exhaust valve28that is open when a gas pressure higher than the working pressure of the current interruption mechanism27is applied is also provided on the sealing plate23. Therefore, the inside of the nonaqueous electrolyte secondary battery10is sealed. The nonaqueous electrolyte secondary battery10alone may be used, or a plurality of nonaqueous electrolyte secondary batteries10connected in series or in parallel may be used for various purposes. When a plurality of nonaqueous electrolyte secondary batteries10connected in series or in parallel are used, the external positive electrode terminal and the external negative electrode terminal may be provided separately to connect the respective batteries with a bus bar.

The flat winding electrode assembly14used in the prismatic nonaqueous electrolyte secondary battery10of the embodiment is used when high capacity of 20 Ah or more and high output characteristics are required. For example, the winding number of the positive electrode11is 43, in other words, the total number of stacked layers of the positive electrode11is 86. When the winding number is 30 or more, in other words, the total number of stacked layers is 60 or more, the capacity of the battery can be 20 Ah or more without increasing the size of the battery beyond necessity.

When the total number of stacked layers of the positive electrode substrate exposed portion15or the negative electrode substrate exposed portion16is large, a large amount of welding current is needed to form a weld mark15aor16apassing through the whole stacked layer portions of the stacked positive electrode substrate exposed portion15or the negative electrode substrate exposed portion16in resistance-welding the positive electrode collector17and the negative electrode collector19to the positive electrode substrate exposed portion15and the negative electrode substrate exposed portion16, respectively.

As shown inFIGS. 2A to 2C, in the positive electrode11, the stacked positive electrode substrate exposed portion15is divided into two segments, and a positive electrode intermediate member30is interposed therebetween. The positive electrode intermediate member30is formed using a resin material and holds a plurality of positive electrode conductive members29, here, two positive electrode conductive members29. Likewise, in the negative electrode12, the stacked positive electrode substrate exposed portion16is divided into two segments, and a negative electrode intermediate member32is interposed therebetween. The negative electrode intermediate member32is formed using a resin material and holds a plurality of negative electrode conductive members31, here, two negative electrode conductive members31. The positive electrode collector17is disposed on the surfaces of both sides of the outermost side of the two segments of the positive electrode substrate exposed portion15that are disposed on both sides of the positive electrode conductive members29. The negative electrode collector19is disposed on the surfaces of both sides of the outermost side of the two segments of the negative electrode substrate exposed portion16that are disposed on both sides of the negative electrode conductive members31. The positive electrode conductive members29are made of aluminum material as with the positive electrode substrate, and the negative electrode conductive members31are made of copper material as with the negative electrode substrate. The shape of the positive electrode conductive members29and the negative electrode conductive members31may be either the same or different.

When the positive electrode substrate exposed portion15or the negative electrode substrate exposed portion16is divided into two segments, welding current needed to form a weld mark15aor16apassing through the whole stacked layer portion of the stacked positive electrode substrate exposed portion15or the negative electrode substrate exposed portion16is small compared to a case in which there is no division. This prevents sputtering during resistance welding, thereby preventing a trouble such as an internal short in the winding electrode assembly14due to the sputtering. Thus, the resistance welding is performed between the positive electrode collector17and the positive electrode substrate exposed portion15and between the positive electrode substrate exposed portion15and the positive electrode conductive members29. Resistance welding is also performed between the negative electrode collector19and the negative electrode substrate exposed portion16and between the negative electrode substrate exposed portion16and the negative electrode conductive members31.FIG. 2shows two weld marks33formed by resistance-welding in the positive electrode collector17and two weld marks34formed by resistance-welding in the negative electrode collector19.

FIG. 2shows that the positive electrode collector17has a weld mark33formed at two points on both sides across the winding central position CP (the sealing plate23side and the bottom side of the outer body25) by resistance-welding.FIG. 2also shows that the negative electrode collector19has a weld mark34formed at two points on both sides across the winding central position CP. The melting area of the positive electrode substrate exposed portion15or the negative electrode substrate exposed portion16after resistance-welding is represented by the maximum value of the sectional area of a melting portion in the positive electrode substrate exposed portion15or the negative electrode substrate exposed portion16in a plane passing through the central axis of the welded connection portion and being perpendicular to the winding central axis of the winding electrode assembly14, in other words, the maximum value of the sectional area of the weld mark. The most appropriate diameter of these melting areas is determined by the rated current for resistance welding predetermined so as to form the weld marks15aand16a(seeFIG. 2B) where all of stacked layers of the positive electrode substrate exposed portion15and the negative electrode substrate exposed portion16are melted at the welded connection portion. The melting area of the positive electrode substrate exposed portion15is preferably larger than that of the negative electrode exposed portion16.

The resistance-welding methods with the positive electrode intermediate member30including the positive electrode substrate exposed portion15, the positive electrode collector17, and the positive electrode conductive members29, and with the negative electrode intermediate member32including the negative electrode substrate exposed portion16, the negative electrode collector19, and the negative electrode conductive members31in the flat winding electrode assembly14of the embodiment will be described in detail below. In the embodiment, the shapes of the positive electrode conductive members29and the negative electrode conductive members31may be substantially the same, and the shapes of the positive electrode intermediate member30and the negative electrode intermediate member32may be substantially the same. The resistance-welding methods are substantially the same as well. Therefore, the positive electrode11will be described below as an example.

The positive electrode substrate exposed portion15of the flat winding electrode assembly14produced as above is divided into two segments from the winding central part to both sides and is collected centering on a quarter of the thickness of the electrode assembly. Subsequently, the positive electrode collector17is provided on both surfaces on the outermost periphery side of the positive electrode substrate exposed portion15. On the inner periphery side of the positive electrode substrate exposed portion15, the positive electrode intermediate member30including the positive electrode conductive members29is inserted between the two segments of the positive electrode substrate exposed portion15so that respective projections on both sides of the positive electrode conductive members29are brought into contact with the positive electrode substrate exposed portion15. For example, the positive electrode collector17is made of an aluminum plate that has a thickness of 0.8 mm.

The positive electrode conductive members29held by the positive electrode intermediate member30of the embodiment have projections that have, for example, a shape of a circular truncated cone and are formed on two surfaces facing each other on the cylindrical main body. As long as the positive electrode conductive members29are made of metal and blockish, any shape such as a cylinder, a prism, and an elliptic cylinder may be adopted. Materials made of copper, a copper alloy, aluminum, an aluminum alloy, tungsten, molybdenum, etc., may be used as a formation material of the positive electrode conductive members29. Among the materials made of these metals, the following configurations may be adopted: the projection on which nickel plate is applied; and the projection and its base area formed of metal material that facilitates heat generation such as tungsten and molybdenum and, for example, brazed to the main body of the cylindrical positive electrode conductive members29made of copper, a copper alloy, aluminum, or an aluminum alloy.

A plurality of, for example, here two pieces of positive electrode conductive members29are integrally held by the positive electrode intermediate member30formed using a resin material. In such a case, the respective electrode conductive members29are held so as to be in parallel with each other. The positive electrode intermediate member30may have any shape such as a prism and cylinder. However, a landscape prism is desirable in order that the positive electrode intermediate member30is stably positioned and fixed between the two segments of the positive electrode substrate exposed portion15. It is preferable that the corners of the positive electrode intermediate member30be chamfered in order not to hurt or deform the soft positive electrode substrate exposed portion15even if contacting the positive electrode substrate exposed portion15. At least a part to be inserted between the two segments of the positive electrode substrate exposed portion15may be chamfered.

The length of the prismatic positive electrode intermediate member30varies depending on the size of the prismatic nonaqueous electrolyte secondary battery10, but it may be from 20 mm to tens of mm. The width of the prismatic positive electrode intermediate member30may be as much as the height of the positive electrode conductive members29, but at least both ends of the positive electrode conductive members29as welded portions may be exposed. It is preferable that both ends of the positive electrode conductive members29protrude from the surface of the positive electrode intermediate member30, but the positive electrode conductive members29do not necessarily protrude. Such a structure enables the positive electrode conductive members29to be held in the positive electrode intermediate member30, and the positive electrode intermediate member30to be stably positioned and disposed between the two segments of the positive electrode substrate exposed portion15.

Subsequently, the flat winding electrode assembly14, which includes the positive electrode collector17and the positive electrode intermediate member30holding the positive electrode conductive members29disposed therein, is arranged between a pair of resistance welding electrodes (not shown in the drawings). The pair of resistance welding electrodes are each brought into contact with the positive electrode collector17disposed on both surfaces of the outermost periphery side of the positive electrode substrate exposed portion15. An appropriate pressure is then applied between the pair of resistance welding electrodes, thereby performing the resistance welding under predetermined certain conditions. In this resistance welding, the positive electrode intermediate member30is stably positioned and disposed between the two segments of the positive electrode substrate exposed portion15, which improves the dimensional accuracy between the positive electrode conductive members29and the pair of resistance welding electrodes, enables the resistance welding to be performed in an accurate and stable state, and curbs variation in the welding strength.

Next, the detailed structure of the positive electrode collector17and the negative electrode collector19of the embodiment will be described with reference toFIG. 2. As shown inFIGS. 2A and 2B, the positive electrode collector17is electrically connected to a plurality of layers of the positive electrode substrate exposed portion15stacked on one side edge of the flat winding electrode assembly14by the resistance welding method. The positive electrode collector17is electrically connected to the positive electrode terminal18. Likewise, the negative electrode collector19is electrically connected to a plurality of layers of the negative electrode substrate exposed portion16stacked on the other side edge of the flat winding electrode assembly14by the resistance welding method. The negative electrode collector19is electrically connected to the negative electrode terminal20.

The positive electrode collector17is produced, for example, by punching out an aluminum plate in a particular shape and bending it. This positive electrode collector17has a rib17aformed on a main body part where resistance welding is performed to a bundle of the positive electrode substrate exposed portion15. The negative electrode collector19is produced, for example, by punching out a copper plate in a particular shape and bending it. This negative electrode collector19also has a rib19aformed on the main body part where the resistance welding is performed to a bundle of the negative electrode substrate exposed portion16.

The rib17aof the positive electrode collector17and the rib19aof the negative electrode collector19serve as a shield in order to prevent sputtering during the resistance welding from entering the inside of the flat winding electrode assembly14, and as a radiation fin in order to prevent a portion other than the resistance welded portion of the positive electrode collector17and the negative electrode collector19from being melted by heat generated during the resistance welding. The ribs17aand19aare provided at a right angle from the main body of the positive electrode collector17and the negative electrode collector19, respectively, but the angle need not necessarily be vertical. Even a tilt of about ±10° from the right angle brings the same function effect.

In the prismatic nonaqueous electrolyte secondary battery10of the embodiment, the example shows that two ribs are provided corresponding to the resistance welding position along the longitudinal direction as the rib17aof the positive electrode collector17and the rib19aof the negative electrode collector19. However, the configuration is not limited to this case. One rib may be provided, or ribs may be formed on both sides in the width direction. When ribs are formed on both sides in the width direction, their heights may be either the same or different. If their heights are different, it is preferable that the rib around the flat winding electrode assembly14be provided at a higher position than the other.

The positive electrode collector17and the negative electrode collector19are each preferably disposed across the winding central position CP of the positive electrode substrate exposed portion15and the negative electrode exposed portion16. The resistance-welding is preferably performed so that the positive electrode collector17has the weld mark33formed at two points on both sides across the winding central position CP (the sealing plate23side and the bottom side of the outer body25; the same will apply hereinafter), and the negative electrode collector19has the weld mark34formed at two points on both sides across the winding central position CP. Such a structure allows the electrode reaction to easily proceed uniformly through the whole flat winding electrode assembly14.

Preparation of Positive Electrode

The following describes a specific composition of the positive electrode active material mixture layer11aand the negative electrode active material mixture layer12aand a specific composition of the nonaqueous electrolyte used in the prismatic nonaqueous electrolyte secondary battery10of the embodiment. Lithium nickel cobalt manganese composite oxide represented by LiNi0.35Co0.35Mn0.30O2was used as the positive electrode active material. This lithium nickel cobalt manganese composite oxide, carbon powder as a conductive agent, and polyvinylidene fluoride (PVdF) as a binding agent were weighed so that the mass ratio would be 88:9:3, and were mixed with N-methyl-2-pyrrolidone (NMP) as dispersion media to produce a positive electrode active material mixture slurry. This positive electrode active material mixture slurry was applied with a die coater onto both sides of the positive electrode substrate of aluminum foil whose thickness was, for example, 15 μm to form the positive electrode active material mixture layer onto both sides of the positive electrode substrate. Next, the resultant object was dried to remove NMP as an organic solvent, and was pressed with a roll press to have a particular thickness. The electrode thus obtained was slit in a particular width on one end of the electrode in the width direction along the whole longitudinal direction to form the positive electrode substrate exposed portion15that had no positive electrode active material mixture layer formed onto both sides, and whereby the positive electrode11of the structure shown inFIG. 3Awas obtained.

Preparation of Negative Electrode

The negative electrode was produced as follows: 98 parts by mass of graphite powder, 1 part by mass of carboxymethylcellulose (CMC) as a thickening agent, and 1 part by mass of styrene-butadiene-rubber (SBR) as a binding agent were dispersed in water to produce a negative electrode active material mixture slurry. This negative electrode active material mixture slurry was applied with a die coater onto both sides of the negative electrode collector of copper foil whose thickness was 10 μm, and was dried to form the negative electrode active material mixture layer onto both sides of the negative electrode collector. Next, the resultant object was pressed with a press roller to have a particular thickness. The electrode thus obtained was slit in a particular width on one end of the electrode in the width direction along the whole longitudinal direction to form the negative electrode substrate exposed portion16that had no negative electrode active material mixture layer formed onto both sides, and whereby the negative electrode12of the structure shown inFIG. 3Bwas obtained.

Preparation of Nonaqueous Electrolyte

The nonaqueous electrolyte was produced as follows: as a solvent, ethylene carbonate (EC) and methyl ethyl carbonate (MEC) were mixed with a volume ratio (25° C. and 1 atmosphere) of 3:7; LiPF6as an electrolyte salt was added to the mixed solvent so that the concentration would be 1 mol/L; and then LiBOB as a lithium salt having an oxalate complex as an anion was further added so that the concentration would be 0.1 mol/L. The added LiBOB reacts on the surface of the negative electrode at the initial charge to form a protective covering. Therefore, in the prismatic nonaqueous electrolyte secondary battery10of the embodiment, all LiBOB added to the nonaqueous electrolyte is not necessarily present in the form of LiBOB.

Production of Prismatic Nonaqueous Electrolyte Secondary Battery

The negative electrode12and the positive electrode11produced as above were wound while being insulated from each other with the separator13interposed therebetween so as to dispose the negative electrode12onto the outermost periphery side. Subsequently, the resultant object was formed to be flat, and whereby the flat winding electrode assembly14was produced. In the flat winding electrode assembly14, the winding numbers of the positive electrode11and the negative electrode12were 43 and 44, respectively, in other words, the numbers of stacked layers of the positive electrode11and the negative electrode12were 86 and 88, respectively, and the design capacity was 20 Ah. Furthermore, the total numbers of stacked layers of the positive electrode substrate exposed portion15and the negative electrode substrate exposed portion16were 86 and 88, respectively. This flat winding electrode assembly14is used to connect the positive electrode collector17to both sides of the outermost surface of the positive electrode substrate exposed portion15and to connect the negative electrode collector19to both sides of the outermost surface of the negative electrode substrate exposed portion16, by resistance welding with a rated current value determined in advance. The sectional areas of the melting portion formed in the positive electrode substrate exposed portion15were variable, but at least 4.3 mm2. The sectional areas of the melting portion formed in the negative electrode substrate exposed portion16were at least 2.9 mm2. When the stacked layers of the substrate exposed portion are divided into two segments, the conductive member is interposed therebetween, and both ends of the conductive member are each welded and connected to the two segments of the substrate exposed portion, the sum of the sectional areas of the melting portions formed on both sides across the conductive member represents “the melting area of the positive electrode substrate exposed portion in a plane passing through the central axis of the welded connection portion and being perpendicular to the winding axis of the winding electrode assembly within the welding connection portion between the positive electrode substrate exposed portion and the positive electrode collector” and “the melting area of the negative electrode substrate exposed portion in a plane passing through the central axis of the welded connection portion and being perpendicular to the winding axis of the winding electrode assembly within the welding connection portion between the negative electrode substrate exposed portion and the negative electrode collector”.

It is preferable that the positive electrode collector17be attached to the sealing plate23in advance while being electrically connected to a positive electrode terminal18and insulated from the sealing plate23, and the negative electrode collector19be attached to the sealing plate23in advance while being electrically connected to a negative electrode terminal20and insulated from the sealing plate23, before connecting the positive electrode collector17and the negative electrode collector19to the positive electrode substrate exposed portion15and the negative electrode substrate exposed portion16, respectively.

Next, the flat winding electrode assembly14is inserted into the prismatic outer body25while being covered with the resin sheet24. A fitting portion between the prismatic outer body25and the sealing plate23is then laser-welded to produce a prismatic nonaqueous electrolyte secondary battery that contains no nonaqueous electrolyte. Subsequently, the prismatic outer body25was vacuum-degassed, a particular amount of the nonaqueous electrolyte produced as above was poured through an electrolyte pour hole26provided to the sealing plate23, and the electrolyte pour hole26was then sealed with a blind rivet, thereby preparing the prismatic nonaqueous electrolyte secondary battery10of the embodiment that has the structure shown inFIGS. 1 and 2. It is preferable that a spare charge be performed after pouring the nonaqueous electrolyte and before sealing the electrolyte pour hole26.

The nonaqueous electrolyte secondary battery10of the embodiment can provide excellent cycling characteristics.

In the prismatic nonaqueous electrolyte secondary battery10of the embodiment above, an example of adding LiBOB to the nonaqueous electrolyte as an additive is shown. However, in the present invention, as the lithium salt having an oxalate complex as an anion, lithium difluoro(oxalato)borate, lithium tris(oxalato)phosphate, lithium difluoro(bisoxalato)phosphate, and lithium terafluoro(oxalato)phosphate, for example, may be used.

Modification

The nonaqueous electrolyte secondary battery10of the embodiment shows an example in which the stacked layers of the positive electrode substrate exposed portion15and the stacked layers of the negative electrode substrate exposed portion16are divided into two segments to interpose therebetween the positive electrode intermediate member30including the positive electrode conductive member29and the negative electrode intermediate member32including the negative electrode conductive member31, respectively. However, in the invention, it is not necessary to divide the stacked layers of the positive electrode substrate exposed portion15or the stacked layers of the negative electrode substrate exposed portion16into two segments.

A prismatic nonaqueous electrolyte secondary battery10A in accordance with a modification will be described with reference toFIG. 5, in which neither stacked layers of the positive electrode substrate exposed portion15nor stacked layers of the negative electrode substrate exposed portion16are divided into two segments and neither a positive electrode conductive member nor a negative electrode conductive member is used. InFIG. 5, like numbers are given to like components corresponding to the prismatic nonaqueous electrolyte secondary battery10of the embodiment shown inFIG. 2, and the detailed description thereof is omitted. In the flat winding electrode assembly14of the modification, a resistance welded portion between the positive electrode substrate exposed portion15and a positive electrode collector17and a resistance welded portion between the negative electrode substrate exposed portion16and a negative electrode collector19are different in formation material but are substantially similar in structure. Thus,FIG. 5Bshows a side view of the positive electrode substrate exposed portion15as an example, and a side view of the negative electrode substrate exposed portion16is not shown.

In the flat winding electrode assembly14used in the prismatic nonaqueous electrolyte secondary battery10A of the modification, the amounts per unit area of a positive electrode active material mixture layer11aof the positive electrode11and a negative electrode active material mixture layer12aof the negative electrode12are larger than those in the embodiment. The winding number of the positive electrode11and the negative electrode12are 35 and 36, respectively. In other words, the total numbers of stacking layers of the positive electrode11and the negative electrode12are 70 and 72, respectively. The design capacity is 25 Ah. Furthermore, the total numbers of stacking layers of the positive electrode substrate exposed portion15and the negative electrode substrate exposed portion16are 70 and 72, respectively. On the positive electrode11side, the positive electrode collector17is disposed on the surfaces of both sides of the outermost side of the stacked layers of the positive electrode substrate exposed portion15. On the negative electrode12side, the negative electrode collector19is disposed on the surfaces of both sides of the outermost side of the stacked layers of the negative electrode substrate exposed portion16. The resistance welding is performed at two points on both sides across the winding central position CP (the sealing plate23side and the bottom side of the outer body25) so as to melt the whole stacked layer portion in a plurality of stacked layers of the positive electrode substrate exposed portion15or a plurality of stacked layers of the negative electrode substrate exposed portion16.

In the flat winding electrode assembly14used in the prismatic nonaqueous electrolyte secondary battery10A of the modification, the rib17aformed onto the positive electrode collector17and the rib19aformed onto the negative electrode collector19are formed over one resistance welding point.

In the prismatic nonaqueous electrolyte secondary batteries10and10A of the above-mentioned embodiment and modification, a nonaqueous electrolyte contains a lithium salt having an oxalate complex as an anion such as LiBOB; however, it is preferable that the nonaqueous electrolyte also contain LiPF2O2at the same time. LiPF2O2also reacts with lithium at the initial charge and discharge to form a protective covering on the surface of the positive electrode and the negative electrode. This protective covering prevents direct contact between an active material in a state of charge and an organic solvent, thereby preventing decomposition of the nonaqueous electrolyte due to contact between the active material and the nonaqueous electrolyte, and improving charge storage characteristics.

The nonaqueous electrolyte secondary batteries of the embodiment and the modification each show an example in which the integrated positive electrode collector17or the integrated negative electrode collector19is connected to both of the outermost sides of the positive electrode substrate exposed portion15or both of the outermost side of the negative electrode substrate exposed portion16. However, the positive electrode collector17or the negative electrode collector19may be connected to only one side of the outer utmost sides of the positive electrode substrate exposed portion15or of the outermost sides of the negative electrode substrate exposed portion16, and a mere collector receiving member may be disposed on the other side.

The nonaqueous electrolyte secondary batteries of the embodiment and the modification show an example of connecting between the positive electrode substrate exposed portion15and the positive electrode collector17and between the negative electrode substrate exposed portion16and the negative electrode collector19by resistance-welding, but the connection can be made by ultrasonic welding or irradiation of high-energy rays such as a laser. Furthermore, different connections may be made on the positive electrode side and the negative electrode side.