NONAQUEOUS ELECTROLYTE SECONDARY BATTERY

In a prismatic nonaqueous electrolyte secondary battery, a flat winding electrode assembly and a nonaqueous electrolyte are housed in a prismatic outer body. A positive electrode includes a positive electrode substrate exposed portion formed along the longitudinal direction. A negative electrode includes a negative electrode substrate exposed portion formed along the longitudinal direction. The nonaqueous electrolyte contains a lithium salt having an oxalate complex as an anion at the time of making the nonaqueous electrolyte secondary battery. The area of the negative electrode substrate exposed portion is 700 cm2 or more. The area of the positive electrode substrate exposed portion is 500 cm2 or more. The area of the negative electrode substrate exposed portion is larger than the area of the positive electrode substrate exposed portion. The prismatic nonaqueous electrolyte secondary battery above can provide a nonaqueous electrolyte secondary battery that has excellent cycling characteristics and excellent reliability.

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 are 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 flattened 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 shown 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 shown 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 aluminum alloy having a thickness of about from 10 to 20 μm, while the negative electrode substrate be formed using foil of copper or 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, a flattened 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, and2A, 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 an 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 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 number of winding 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 made of 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 made of resin material and holds 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 sputters during resistance welding, thereby preventing a trouble such as an internal short in the winding electrode assembly14due to the sputters. 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.

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, copper alloy, aluminum, 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, copper alloy, aluminum or aluminum alloy.

A plurality of, for example, here two pieces of positive electrode conductive members29are integrally held by the positive electrode intermediate member30made of 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. The positive electrode collector17has a rib17aformed on its main body part where the resistance welding is performed with 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. The negative electrode collector19also has a rib19aformed on its main body part where the resistance welding is performed with 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 sputters generated 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.

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.

InFIG. 3A, when the length of the positive electrode substrate, the width of the positive electrode substrate, the width of the positive electrode active material mixture layer11a, and the width of the positive electrode substrate exposed portion15are Lp, Wp, Wap, and Wcp, respectively, here the following equation holds: Wap=Wp−Wcp. Thus, the area of the positive electrode substrate exposed portions15formed onto both sides of the positive electrode11is as follows: 2×Wcp×Lp, and 2×Wcp−Lp≧500 cm2.

In addition, the following equation holds:

In other words, the area (both sides) of the positive electrode substrate exposed portions15is 5 to 20% of the area of the positive electrode active material mixture layers11aformed onto both sides of the positive electrode11.

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.

InFIG. 3B, when the length of the negative electrode substrate, the width of the negative electrode substrate, the width of the negative electrode active material mixture layer12a, and the width of the negative electrode substrate exposed portion16are Ln, Wn, Wan, and Wcn, respectively, here the following equation holds: Wan=Wn−Wcn. Thus, the area of the negative electrode substrate exposed portion16formed onto both sides of the negative electrode12is as follows: 2×Wcn×Ln, and 2×Wcn×Ln≧700 cm2.

In addition, the following equation holds:

In other words, the area (both sides) of the negative electrode substrate exposed portions16is 5 to 30% of the area of the negative electrode active material mixture layers12aformed onto both sides of the negative electrode12.

Moreover, in the prismatic nonaqueous electrolyte secondary battery10of the embodiment, the area of the negative electrode substrate exposed portion16is larger than the area of the positive electrode substrate exposed portion15. In other words, the following equation holds:

Preparation of Nonaqueous Electrolyte

The nonaqueous electrolyte to be used 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 is reacted 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. The area (both sides) of the negative electrode substrate exposed portions16of the flat winding electrode assembly14is 700 cm2, and the area (both sides) of the positive electrode substrate exposed portions15is 500 cm2. This flat winding electrode assembly14was used to produce a prismatic nonaqueous electrolyte secondary battery without the nonaqueous electrolyte poured. 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 pre-charge be performed after pouring the nonaqueous electrolyte and before sealing the electrolyte pour hole26.

In the prismatic nonaqueous electrolyte secondary battery10of the embodiment, the nonaqueous electrolyte containing LiBOB is used, thereby providing the nonaqueous electrolyte secondary battery having excellent cycling characteristics. The area of the negative electrode substrate exposed portion16of the flat winding electrode assembly14is 700 cm2, and the area of the positive electrode substrate exposed portion15is 500 cm2, thereby improving heat release characteristics from the inside of the electrode assembly, preventing an increase in temperature of the negative electrode, and preventing a reaction between the negative electrode where the protective covering derived from LiBOB is formed and the nonaqueous electrolyte. Furthermore, the area of the negative electrode substrate exposed portion is larger than the area of the positive electrode substrate exposed portion, thereby preventing an increase in temperature of the negative electrode and preventing a react ion between the negative electrode where the protective covering is formed and the nonaqueous electrolyte more efficiently.

In the prismatic nonaqueous electrolyte secondary battery10of the above-mentioned embodiment, 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.

In addition, for example, LiPF2O2may be included other than a lithium salt having an oxalate complex as an anion. 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 on the surface of the positive electrode and the negative electrode. This protective covering prevents a direct reaction between an active material in a state of charge and an organic solvent, thereby preventing a decomposition of the nonaqueous electrolyte and obtaining the nonaqueous electrolyte secondary battery that has excellent charge storage characteristics.

First Modification

A negative electrode12A of a first modification has a larger area than the negative electrode12of the embodiment, and has negative electrode substrate exposed portions16and16bformed in a particular width onto both ends in the width direction (lateral direction) as shown inFIG. 5. The negative electrode substrate exposed portion16bis formed on both sides of the negative electrode12. This allows an area of a part where a negative electrode active material mixture layer12aof the negative electrode12A is formed to be the same as that of a part where the negative electrode active material mixture layer12aof the negative electrode12is formed in the embodiment, and also enlarges the area of the negative electrode12for an additionally created negative electrode substrate exposed portion16b. The positive electrode11is used that has the same size and the same structure as the positive electrode11of the embodiment shown inFIG. 3A.

Using the negative electrode12A in such a structure can enlarge the area of the negative electrode substrate exposed portion16b, thereby improving the heat release efficiency of the negative electrode12A. It is preferable that the separators13be interposed on both sides of the additionally created negative electrode substrate exposed portion16bof the negative electrode12A.

Second Modification

In the nonaqueous electrolyte secondary battery10of the above-mentioned embodiment, an example is shown where each of the stacked layers of the positive electrode substrate exposed portion15and the stacked layers of the negative electrode substrate exposed portion16are divided into two segments, and the positive electrode intermediate member30including the positive electrode conductive members29and the negative electrode intermediate member32including the negative electrode conductive member31are interposed therebetween. However, in the present invention, the stacked layers of the positive electrode substrate exposed portion15or the negative electrode substrate exposed portion16may not be divided into two segments.

A prismatic nonaqueous electrolyte secondary battery10A in accordance with a second modification will be described with reference toFIG. 6. In the second modification, neither of the stacked layers of the positive electrode substrate exposed portion15nor the negative electrode substrate exposed portion16is divided into two segments, and no positive electrode conductive member or negative electrode conductive member is used. InFIG. 6, the same numbers are given to the same 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 second modification, a resistance welded portion between the positive electrode substrate exposed portion15and the positive electrode collector17and a resistance welded portion between the negative electrode substrate exposed portion16and the negative electrode collector19have substantially similar structures except for the difference of respective formation materials. Thus,FIG. 6Bshows 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 second modification, the amount of the positive electrode active material mixture layer11aof the positive electrode11and the negative electrode active material mixture layer12aof the negative electrode12per unit area are larger than in the embodiment. In addition, the winding numbers of the positive electrode11and the negative electrode12are 35 and 36, respectively, in other words, the numbers of stacked layers of the positive electrode11and the negative electrode12are 70 and 72, respectively, and the design capacity is 25 Ah. Furthermore, the total numbers of stacked layers of the positive electrode substrate exposed portion15and the negative electrode substrate exposed portion16are 70 and 72, respectively. On the positive electrode11, 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, while 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 so that weld marks (not shown in the drawings) are formed so as to pass through the whole stacked layer portions of the bundle of the positive electrode substrate exposed portion15or the negative electrode substrate exposed portion16.

In the flat winding electrode assembly14used in the prismatic nonaqueous electrolyte secondary battery10A of the second modification, one rib formed across the resistance welding points is used as the rib17aformed onto the positive electrode collector17and the rib19aformed onto the negative electrode collector19.

The prismatic nonaqueous electrolyte secondary battery of the above-mentioned embodiment, the first modification, and the second modification shows 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.