Patent ID: 12224406

DETAILED DESCRIPTION

Hereinafter, one embodiment of the secondary battery disclosed herein is described with reference to the drawings. Matters other than those specifically mentioned in the present description but necessary for implementation may be recognized as design matters for a person skilled in the art based on conventional techniques in the art. The present invention can be implemented based on the content disclosed in the present description and a common general technical knowledge in the art. In the drawings below, members and parts exhibiting the same effect are assigned with the same numerals or symbols.

The term “battery” used herein refers to a power storage device in general from which electrical energy can be taken out and is a concept including a primary battery and a secondary battery. The term “secondary battery” refers to a power storage device in general that can be charged and discharged repeatedly and includes a so-called storage battery such as a lithium secondary battery, a nickel-hydrogen battery, and a nickel-cadmium battery. Hereinafter, the secondary battery disclosed herein will be described in detail while illustrating a lithium-ion secondary battery, one of the secondary batteries. However, the secondary battery disclosed herein is not limited to the embodiments described herein.

FIG.1is a sectional view schematically illustrating the inner structure of a secondary battery100according to the present embodiment. In the present embodiment, the secondary battery100is placed in a three-dimensional space indicated by a first direction D1, a second direction D2, and a third direction D3intersecting (orthogonal to, in this embodiment) one another. The symbols “F”, “Rr”, “L”, “R”, “U”, and “D” in the drawings represent “front”, “rear”, “left”, “right”, “up”, and “down”, respectively. InFIGS.1,3, and4, the first direction D1indicates a front-rear direction, the second direction D2indicates a left-right direction, and the third direction D3indicates a up-down direction. The first direction D1is also a direction in which a winding axis W (seeFIG.2) extends.

As illustrated inFIG.1, the secondary battery100according to the present embodiment is a sealed lithium-ion secondary battery provided with a battery case30, a wound electrode body20, and a non-aqueous electrolyte10.

The battery case30houses the wound electrode body20and the non-aqueous electrolyte10thereinside in a sealed manner. In the present embodiment, the battery case30has a rectangular parallelepiped and flat rectangular shape. The battery case30includes a main body31and a lid32. The main body31is a rectangular hollow member having an opening (not illustrated) at one end (for example, the upper end). The lid32has a plate shape and seals the opening of the main body31. The lid32is attached to the main body31.

The lid32is provided with a positive electrode terminal42and a negative electrode terminal44for external connection and a safety valve36. The safety valve36releases an inner pressure of the battery case30when the inner pressure increases to a predetermined pressure or higher. The battery case30is provided with an inlet port (not illustrated) for injecting the non-aqueous electrolyte10into the main body31. The material of the battery case30is not particularly limited, but, for example, a metallic material which is light and has high thermal conductivity, such as aluminum, may be used as the material of the battery case30.

FIG.2is a schematic view illustrating a constitution of the wound electrode body20of the secondary battery100according to the present embodiment. As illustrated inFIG.2, the wound electrode body20includes an elongated positive electrode sheet50, an elongated negative electrode sheet60, and an elongated separator70. The separator70is interposed between the positive electrode sheet50and the negative electrode sheet60. In the present embodiment, the separator70includes a first separator71and a second separator72and is constituted of two separators. In the wound electrode body20, the positive electrode sheet50, the negative electrode sheet60, and the separator70are superimposed and wound around the winding axis W. In the present embodiment, the positive electrode sheet50, the first separator71, the negative electrode sheet60, and the second separator72are superimposed in this order so that the directions thereof are aligned in the longitudinal direction of the developed positive electrode sheet50, and wound around the winding axis W.

In the positive electrode sheet50, elongated positive electrode active material layers54containing a positive electrode active material are formed on one surface or both surfaces (both surfaces in the present embodiment) of a sheet-shaped positive electrode current collector52along the longitudinal direction thereof. An unformed part52ain which the positive electrode active material layer54is not formed is disposed at an end on one end side (left end side inFIG.2) in a direction (the first direction D1in this embodiment) in which the winding axis W in the positive electrode current collector52extends. The first direction D1is a direction along a short direction of the positive electrode sheet50in a developed state. The unformed part52aof the positive electrode sheet50is a part in which the positive electrode current collector52is exposed. As illustrated inFIG.1, a positive electrode collector plate42ais joined to the unformed part52aof the positive electrode sheet50. A positive electrode terminal42is electrically connected to the positive electrode collector plate42a.

In the present embodiment, a positive electrode current collector for secondary batteries of this kind can be used as the positive electrode current collector52without any particular limitation. As the positive electrode current collector52, a metallic positive electrode current collector having good conductivity may preferably be used. Metallic materials such as aluminum, nickel, titanium, and stainless steel can be adopted as the positive electrode current collector52. In particular, aluminum (for example, aluminum foils) is preferably used as the positive electrode current collector52.

Examples of the positive electrode active material contained in the positive electrode active material layer54include lithium composite metal oxides having, for example, layered structures, spinel structures, or the like (for example, LiNi1/3Co1/3Mn1/3O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi0.5Mn1.5O4, LiCrMnO4, LiFePO4or the like). The positive electrode active material layer54may be formed by dispersing the positive electrode active material and an optional material (for example, a conductive material, a binder, or the like) in a suitable solvent (for example, N-methyl-2-pyrrolidone: NMP), preparing a paste (or slurry) composition, adding a suitable amount of the composition on a surface of the positive electrode current collector52, and drying the composition.

As illustrated inFIG.2, in the negative electrode sheet60, elongated negative electrode active material layers64containing a negative electrode active material are formed on one surface or both surfaces (both surfaces in the present embodiment) of a sheet-shaped negative electrode current collector62along the longitudinal direction thereof. An unformed part62ain which the negative electrode active material layer64is not formed is disposed at an end on the other end side (right end side inFIG.2) in a direction in which the winding axis W in the negative electrode current collector62extends. The unformed part62aof the negative electrode sheet60is a part in which the negative electrode current collector62is exposed. As illustrated inFIG.1, a negative electrode collector plate44ais joined to the unformed part62aof the negative electrode sheet60. A negative electrode terminal44is electrically connected to the negative electrode collector plate44a.

In the present embodiment, a negative electrode current collector for secondary batteries of this kind can be used as the negative electrode current collector62without any particular limitation. As the negative electrode current collector62, a metallic negative electrode current collector having good conductivity may preferably be used. For example, copper (such as copper foils) or an alloy mainly composed of copper may be used as the negative electrode current collector62.

Examples of the negative electrode active material contained in the negative electrode active material layer64include particulate (or spherical, scaly) carbon materials at least partially including a graphite structure (for example, a layered structure), lithium-transition metal composite oxides (for example, a lithium-titanium composite oxide such as Li4Ti5O12), lithium-transition metal composite nitrides, or the like. The negative electrode active material layer64may be formed by dispersing the negative electrode active material and an optional material (for example, a binder or the like) in a suitable solvent (for example, ion-exchange water), preparing a paste (or slurry) composition, adding a suitable amount of the composition on a surface of the negative electrode current collector62, and drying the composition.

As illustrated inFIG.2, a separator made of a conventionally-known porous sheet can be used without any particular limitation as the separator70(specifically, the first separator71and the second separator72). As the separator70, for example, porous sheets (for example, films, non-woven fabrics, or the like) made of a resin such as polyethylene (PE), polypropylene (PP), polyesters, celluloses, and polyamides can be mentioned. Such porous sheets may have a single-layer structure or may have a multilayer structure including two or more layers (for example, a three-layer structure in which PP layers are laminated on both surfaces of a PE layer). A structure provided with porous heat-resistant layers on one surface or both surfaces of a porous sheet may be acceptable. These heat-resistant layers can be, for example, layers including an inorganic filler and a binder (for example, a filler layer). As the inorganic filler, for example, alumina, boehmite, silica, or the like can be preferably adopted.

As illustrated inFIG.1, the non-aqueous electrolyte10housed, together with the wound electrode body20, in the battery case30contains a supporting electrolyte in a suitable non-aqueous solvent, and conventionally-known non-aqueous electrolytes may be adopted without any particular limitation. Examples of the non-aqueous solvent which may be used include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or the like. Examples of the supporting electrolyte that may be suitably used include lithium salts (for example, LiBOB, LiPF6, or the like). In the present embodiment, LiBOB is adopted as the supporting electrolyte. In this case, the LiBOB content in the non-aqueous electrolyte10is preferably 0.3 wt % to 0.6 wt %.

Next, the shapes of the wound electrode body20and the battery case30according to the present embodiment will be described in detail.FIG.3is a sectional view schematically illustrating the secondary battery100and is a sectional view of the secondary battery100in a cross section ofFIG.1.FIG.3shows the outer peripheral shape of the wound electrode body20and a state where the wound electrode body20is wound is omitted. As illustrated inFIG.3, the outer peripheral shape of the wound electrode body20according to the present embodiment is a rounded rectangle when seen from a direction of the winding axis W (the first direction D1in this embodiment). In the present embodiment, the outer peripheral shape of the central part in the first direction D1of the wound electrode body20is a rounded rectangle. The central part in the first direction D1of the wound electrode body20herein means, for example, a central piece when the wound electrode body20is divided into three pieces in the first direction D1. The following description relating to the wound electrode body20basically describes the entire structure of the wound electrode body20. However, if the description does not specifically mention or describe a specific part of the wound electrode body20, at least the central part in the first direction D1of the wound electrode body20shall be described.

The wound electrode body20has a first flat surface21, a second flat surface22, a third flat surface23, and a fourth flat surface24. The first flat surface21to the fourth flat surface24each constitute a part of the outer peripheral surfaces around the winding axis W of the wound electrode body20and are positioned on the outer peripheral surfaces around the winding axis W. In the present embodiment, the first flat surface21constitutes a surface at one end side in the second direction D2in the outer peripheral surfaces of the wound electrode body20, namely, a left surface in this embodiment. The first flat surface21is a flat surface spread in the first direction D1(in other words, a direction in which the winding axis W extends) and the third direction D3. The first flat surface21is a flat surface that is longer in the first direction D1than in the third direction D3.

The second flat surface22is disposed at a position facing the first flat surface21with the winding axis W of the wound electrode body20interposed therebetween. In the present embodiment, the second flat surface22constitutes a surface on the other end side in the second direction D2in the outer peripheral surfaces of the wound electrode body20, namely, a right surface in this embodiment. The second flat surface22is a flat surface spread in the first direction D1and the third direction D3. The second flat surface22is a flat surface that is longer in the first direction D1than in the third direction D3as with the first flat surface21. The second flat surface22is disposed on the right side from the first flat surface21and arranged side-by-side with the first flat surface21in the second direction D2. Namely, the first flat surface21overlaps the second flat surface22when seen from the second direction D2. The first flat surface21and the second flat surface22are in parallel with each other but may be slightly inclined from a parallel arrangement.

In the present embodiment, the first flat surface21has the same size as the second flat surface22. However, the first flat surface21may be larger or may be smaller than the second flat surface22. In the present embodiment, the length of the first flat surface21in the first direction D1is the same as the length of the second flat surface22in the first direction D1. However, the length of the first flat surface21in the first direction D1may be longer or may be shorter than the length of the second flat surface22in the first direction D1. Similarly, the length of the first flat surface21in the third direction D3is the same as the length of the second flat surface22in the third direction D3but may be longer or may be shorter than the length of the second flat surface22in the third direction D3.

The third flat surface23is disposed between the first flat surface21and the second flat surface22. The third flat surface23constitutes a surface at one end side in the third direction D3in the outer peripheral surfaces of the wound electrode body20, namely, a lower surface in this embodiment. In the present embodiment, the third flat surface23is disposed below the first flat surface21and the second flat surface22. The third flat surface23is a flat surface spread in the first direction D1and the second direction D2and is orthogonal to the first flat surface21and the second flat surface22. The third flat surface23is a flat surface that is longer in the first direction D1than in the second direction D2.

The fourth flat surface24is disposed at a position facing the third flat surface23with the winding axis W of the wound electrode body20interposed therebetween. The fourth flat surface24is disposed between the first flat surface21and the second flat surface22. In the present embodiment, the fourth flat surface24constitutes a surface on the other end side in the third direction D3in the outer peripheral surfaces of the wound electrode body20, namely, an upper surface in this embodiment. The fourth flat surface24is disposed above the first flat surface21and the second flat surface22. The fourth flat surface24is a flat surface spread in the first direction D1and the second direction D2and is orthogonal to the first flat surface21and the second flat surface22as with the third flat surface23. The fourth flat surface24is a flat surface that is longer in the first direction D1than in the second direction D2.

In the present embodiment, the fourth flat surface24is disposed above the third flat surface23and arranged side-by-side with the third flat surface23in the third direction D3. The third flat surface23overlaps the fourth flat surface24when seen from the third direction D3. The third flat surface23and the fourth flat surface24are in parallel with each other but may be slightly inclined from a parallel arrangement. In the present embodiment, the third flat surface23has the same size as the fourth flat surface24. However, the third flat surface23may be larger or may be smaller than the fourth flat surface24. In the present embodiment, the length of the third flat surface23in the first direction D1is the same as the length of the fourth flat surface24in the first direction D1but may be longer or may be shorter than the length of the fourth flat surface24in the first direction D1. Similarly, the length of the third flat surface23in the second direction D2is the same as the length of the fourth flat surface24in the second direction D2but may be longer or may be shorter than the length of the fourth flat surface24in the second direction D2.

In the present embodiment, the third flat surface23and the fourth flat surface24each may be smaller or larger than either of the first flat surface21and the second flat surface22. The third flat surface23and the fourth flat surface24each may have the same size as either of the first flat surface21and the second flat surface22. In the present embodiment, the length of each of the third flat surface23and the fourth flat surface24in the second direction D2are shorter than the length of each of the first flat surface21and the second flat surface22in the third direction D3. However, the length of each of the third flat surface23and the fourth flat surface24in the second direction D2may be the same as the length of each of the first flat surface21and the second flat surface22in the third direction D3or may be longer than the length of each of the first flat surface21and the second flat surface22in the third direction D3. The length of each of the third flat surface23and the fourth flat surface24in the first direction D1are the same as the length of each of the first flat surface21and the second flat surface22in the first direction D1. However, the length of each of the third flat surface23and the fourth flat surface24in the first direction D1may be longer or may be shorter than the length of each of the first flat surface21and the second flat surface22in the first direction D1.

In the present embodiment, the wound electrode body20further includes a first rounded part26, a second rounded part27, a third rounded part28, and a fourth rounded part29, as illustrated inFIG.3. The first rounded part26to the fourth rounded part29respectively constitute rounded parts in the outer peripheral surfaces of the wound electrode body20. In this embodiment, the first rounded part26to the fourth rounded part29are respectively disposed on four corners of the wound electrode body20when seen from the direction in which the winding axis W extends (the first direction D1in this case).

The first rounded part26is disposed between the first flat surface21and the third flat surface23. The first rounded part26is continuous with the first flat surface21and the third flat surface23. The first rounded part26is on the lower right position from the first flat surface21and on the upper left position from the third flat surface23. The second rounded part27is disposed between the first flat surface21and the fourth flat surface24. The second rounded part27is continuous with the first flat surface21and the fourth flat surface24. The second rounded part27is on the upper right position from the first flat surface21and on the lower left position from the fourth flat surface24. The second rounded part27is disposed above the first rounded part26. The first rounded part26and the second rounded part27are arranged side-by-side in the third direction D3, and the first rounded part26overlaps the second rounded part27when seen from the third direction D3.

The third rounded part28is disposed between the second flat surface22and the third flat surface23. The third rounded part28is continuous with the second flat surface22and the third flat surface23. The third rounded part28is on the lower left position from the second flat surface22and on the upper right position from the third flat surface23. The third rounded part28is arranged side-by-side with the first rounded part26in the second direction D2. The third rounded part28overlaps the first rounded part26when seen from the second direction D2. The fourth rounded part29is disposed between the second flat surface22and the fourth flat surface24. The fourth rounded part29is continuous with the second flat surface22and the fourth flat surface24. The fourth rounded part29is on the upper left position from the second flat surface22and on the lower right position from the fourth flat surface24. In the present embodiment, the fourth rounded part29is arranged side-by-side with the second rounded part27in the second direction D2. The fourth rounded part29overlaps the second rounded part27when seen from the second direction D2. The fourth rounded part29is arranged side-by-side with the third rounded part28in the third direction D3. The fourth rounded part29overlaps the third rounded part28when seen from the third direction D3.

In the present embodiment, the first rounded part26to the fourth rounded part29are the same in size. Namely, the first rounded part26to the fourth rounded part29are the same in radius of curvature. However, any of the first rounded part26to the fourth rounded part29may differ in size from another rounded part. Namely, any of the rounded parts of the first rounded part26to the fourth rounded part29may differ in radius of curvature from another rounded part.

The battery case30, which houses the non-aqueous electrolyte10(seeFIG.1) and the wound electrode body20, is a rectangular case as described above. Thus, the battery case30has a rectangular inner space thereinside. The non-aqueous electrolyte10and the wound electrode body20are housed in the inner space.

In the present embodiment, the battery case30has a first case flat surface81, a second case flat surface82, a third case flat surface83, and a fourth case flat surface84as illustrated inFIG.3. The first case flat surface81to the fourth case flat surface84constitute the inner peripheral surfaces of the battery case30. In the present embodiment, the first case flat surface81to the third case flat surface83constitute the inner peripheral surfaces of the main body31of the battery case30, and the fourth case flat surface84constitutes an inner peripheral surface of the lid32of the battery case30.

The first case flat surface81constitutes a surface at one end side in the second direction D2in the inner peripheral surfaces of the battery case30, namely, a left surface in this embodiment. The first case flat surface81faces the first flat surface21of the wound electrode body20along the first flat surface21. In this embodiment, the first case flat surface81is disposed on the left side from the first flat surface21. The first case flat surface81overlaps the first flat surface21when seen from the second direction D2. The first case flat surface81is in parallel with the first flat surface21but may be slightly inclined relative to the first flat surface21.

The second case flat surface82constitutes a surface on the other end side in the second direction D2in the inner peripheral surfaces of the battery case30, namely, a right surface in this embodiment. The second case flat surface82is in parallel with the first case flat surface81and arranged side-by-side with the first case flat surface81in the second direction D2. The second case flat surface82overlaps the first case flat surface81when seen from the second direction D2. The second case flat surface82faces the first case flat surface81with the wound electrode body20interposed therebetween. The second case flat surface82faces the second flat surface22of the wound electrode body20along the second flat surface22. In this embodiment, the second case flat surface82is positioned on the right side from the second flat surface22, and the second case flat surface82overlaps the second flat surface22when seen from the second direction D2. The second case flat surface82is in parallel with the second flat surface22but may be slightly inclined relative to the second flat surface22.

The third case flat surface83constitutes a surface at one end side in the third direction D3in the inner peripheral surfaces of the battery case30, namely, a lower surface in this embodiment. The third case flat surface83is disposed between the first case flat surface81and the second case flat surface82. In the present embodiment, the third case flat surface83is continuous with the first case flat surface81and the second case flat surface82. Specifically, the left end of the third case flat surface83is connected to the lower end of the first case flat surface81. The right end of the third case flat surface83is connected to the lower end of the second case flat surface82. The third case flat surface83faces the third flat surface23of the wound electrode body20along the third flat surface23. In this embodiment, the third case flat surface83is disposed below the third flat surface23. The third case flat surface83overlaps the third flat surface23when seen from the third direction D3. The third case flat surface83is in parallel with the third flat surface23but may be slightly inclined relative to the third flat surface23.

The fourth case flat surface84constitutes a surface on the other end side in the third direction D3in the inner peripheral surfaces of the battery case30, namely, an upper surface in this embodiment. The fourth case flat surface84is disposed between the first case flat surface81and the second case flat surface82as with the third case flat surface83. The fourth case flat surface84is detachably continuous with the first case flat surface81and the second case flat surface82. Specifically, when the lid32is attached to the main body31of the battery case30, the left end of the fourth case flat surface84is connected to the upper end of the first case flat surface81and the right end of the fourth case flat surface84is connected to the upper end of the second case flat surface82. The fourth case flat surface84is in parallel with the third case flat surface83and arranged side-by-side with the third case flat surface83in the third direction D3. The fourth case flat surface84overlaps the third case flat surface83when seen from the third direction D3. The fourth case flat surface84faces the third case flat surface83with the wound electrode body20interposed therebetween. The fourth case flat surface84faces the fourth flat surface24of the wound electrode body20along the fourth flat surface24. In this embodiment, the fourth case flat surface84is positioned above the fourth flat surface24. The fourth case flat surface84overlaps the fourth flat surface24when seen from the third direction D3. The fourth case flat surface84is in parallel with the fourth flat surface24but may be slightly inclined relative to the fourth flat surface24.

In the present embodiment, although the illustration is omitted an insulating film is disposed between an inner peripheral surface of the battery case30and the wound electrode body20. This insulating film is formed of, for example, a resin material such as polypropylene (PP) and polyethylene (PE). In a state where the wound electrode body20is housed in the battery case30, at least one of the first flat surface21, the second flat surface22, the third flat surface23, and the fourth flat surface24of the wound electrode body20is in indirect contact with the inner peripheral surfaces of the battery case30via the insulating film. In the present embodiment, the first flat surface21, the second flat surface22, and the third flat surface23of the wound electrode body20are in indirect contact with the inner peripheral surfaces of the battery case30via the insulating film, and the fourth flat surface24is not in contact with any inner peripheral surface of the battery case30and is separated from the inner peripheral surfaces of the battery case30. Specifically, in a state where the wound electrode body20is housed in the battery case30, the first flat surface21is in contact with the first case flat surface81via the insulating film, the second flat surface22is in contact with the second case flat surface82via the insulating film, and the third flat surface23is in contact with the third case flat surface83via the insulating film. Meanwhile, the fourth flat surface24is not in contact with the fourth case flat surface84and is separated downward from the fourth case flat surface84. The fourth flat surface24may be in contact with an inner peripheral surface of the battery case30via the insulating film.

In the present embodiment, in a state where the wound electrode body20is housed in the battery case30, voids (hereinafter referred to as exterior voids91) are formed between the wound electrode body20and the battery case30as illustrated inFIG.3. The exterior voids91are gaps formed between the outer peripheral surfaces of the wound electrode body20and the inner peripheral surfaces of the battery case30. In the present embodiment, the exterior voids91include a first exterior void91a, a second exterior void91b, a third exterior void91c, and a fourth exterior void91d.

The first exterior void91ais a gap formed between the first rounded part26and an inner peripheral surface of the battery case30. In this embodiment, the first exterior void91ais a gap surrounded by the first rounded part26, the first case flat surface81of the battery case30, and the third case flat surface83. The first exterior void91ais an example of “an exterior void formed between the first rounded part and an inner peripheral surface of the battery case” of the present invention. The second exterior void91bis a gap formed between the second rounded part27and an inner peripheral surface of the battery case30. The second exterior void91bis a gap surrounded by the second rounded part27, the first case flat surface81of the battery case30, and the fourth case flat surface84.

The third exterior void91cis a gap formed between the third rounded part28and an inner peripheral surface of the battery case30. The third exterior void91cis a gap surrounded by the third rounded part28, the second case flat surface82of the battery case30, and the third case flat surface83. In this embodiment, the first exterior void91aand the third exterior void91care the same in size. However, the first exterior void91amay be larger or may be smaller than the third exterior void91c.

The fourth exterior void91dis a gap formed between the fourth rounded part29and an inner peripheral surface of the battery case30. In this embodiment, the fourth exterior void91dis a gap surrounded by the fourth rounded part29, the second case flat surface82of the battery case30, and the fourth case flat surface84. In the present embodiment, the size of the fourth exterior void91dis the same as that of the second exterior void91b. However, the fourth exterior void91dmay be larger or may be smaller than the second exterior void91b. Note that, as in the present embodiment, when the fourth case flat surface84and the fourth flat surface24are separated from each other, the second exterior void91band the fourth exterior void91dare continuous. In this case, for example, the left side void piece when the continuous void is divided into two equal pieces in the second direction D2is assumed to be the second exterior void91b, and the right side void piece is assumed to be the fourth exterior void91d.

Inside at least one of the first rounded part26, the second rounded part27, the third rounded part28, and the fourth rounded part29of the wound electrode body20, the inner voids92as illustrated inFIG.4are formed at gaps in a portion where the positive electrode sheet50, the negative electrode sheet60, and the separator70are superimposed. In the present embodiment, the inner voids92are formed at gaps in a portion where the positive electrode sheet50, the negative electrode sheet60, and the separator70are superimposed inside each of the first rounded part26, the second rounded part27, the third rounded part28, and the fourth rounded part29. A plurality of inner voids92are formed respectively with respect to the rounded parts26to29.FIG.4illustrates the inner voids92formed inside the first rounded part26, and the inner voids92formed inside the second rounded part27to the fourth rounded part29each have a similar constitution to the inner voids92of the first rounded part26. Namely, the constitutions of the inner voids92are the same among the first rounded part26to the fourth rounded part29in the present embodiment. Thus, inner voids92inside the first rounded part26will be described below, and the description about the constitutions of the inner voids92each inside the second rounded part27to the fourth rounded part29are omitted as appropriate.

In the present embodiment, the inner voids92include a first inner void92a, a second inner void92b, a third inner void92c, and a fourth inner void92das illustrated inFIG.4. The first inner void92ato the fourth inner void92dare some examples of the inner voids of the present invention. The first inner void92ais a gap formed between the positive electrode sheet50and the first separator71positioned inside the first rounded part26. The second inner void92bis a gap formed between the first separator71and the negative electrode sheet60positioned inside the first rounded part26. The third inner void92cis a gap formed between the negative electrode sheet60and the second separator72positioned inside the first rounded part26. The fourth inner void92dis a gap formed between the second separator72and the positive electrode sheet50positioned inside the first rounded part26. In the present embodiment, the first inner void92a, the second inner void92b, the third inner void92c, and the fourth inner void92das described above are formed each inside the second rounded part27, the third rounded part28, and the fourth rounded part29, although specific illustration thereof is omitted.

In the present embodiment, the sizes of the first inner void92ato the fourth inner void92dare the same in size. The expression “the same” herein shall include some error. The first inner void92ato the fourth inner void92dare the same in volume. However, any one of the inner voids of the first inner void92ato the fourth inner void92dmay differ in size from another inner void.

In the present embodiment, the first exterior void91ato the fourth exterior void91d(seeFIG.3) are each larger than the first inner void92a, the second inner void92b, the third inner void92c, and the fourth inner void92d. In other words, each volume of the first exterior void91ato the fourth exterior void91dis larger than the volume of any of the first inner void92ato the fourth inner void92d. For example, each volume of the first exterior void91ato the fourth exterior void91dis 1.01 times to 1.1 times the volume of any of the first inner void92ato the fourth inner void92d. However, each volume of the first exterior void91ato the fourth exterior void91dmay be equal to or larger than 1.1 times the volume of any of the first inner void92ato the fourth inner void92d.

In the present embodiment, the non-aqueous electrolyte10penetrates the exterior voids91and the inner voids92. The exterior voids91and the inner voids92are collectively and simply called “voids95”. In this embodiment, in a state where the wound electrode body20is housed in the battery case30, the ratio of the voids95in the battery case30is 5% to 20%, preferably 5% to 15%, and more preferably 5% to 10%.

As stated above, a constitution of the secondary battery100according to the present embodiment is described. Next, a method for manufacturing the secondary battery100according to the present embodiment is described along the flow chart inFIG.5using the figures inFIGS.6to9. Note thatFIGS.6to9only show the shapes of the outlines of the wound electrode bodies20and20A. The method for manufacturing a secondary battery100includes a winding step S1, an arranging step S2, and a pressing step S3, as illustrated inFIG.5.

First, in the winding step S1inFIG.5, a columnar wound electrode body20A as disclosed inFIG.6is prepared. In the winding step S1, as illustrated inFIG.2, a positive electrode sheet50, a negative electrode sheet60, and a separator70are superimposed and wound around a winding axis W extending in the first direction D1, to prepare a wound electrode body20A (seeFIG.6). In this step, the positive electrode sheet50, a first separator71, the negative electrode sheet60, and a second separator72are superimposed in this order. Then, the positive electrode sheet50, the first separator71, the negative electrode sheet60, and the second separator72are wound around the winding axis W in the superimposed state. Due to this step, the columnar wound electrode body20A which is not yet flattened as disclosed inFIG.6is prepared. In the present embodiment, the wound electrode body20having the first flat surface21to the fourth flat surface24as illustrated inFIG.9by pressing the columnar wound electrode body20A.

As illustrated inFIG.5, the winding step S1is followed by the arranging step S2. In the arranging step S2, the wound electrode body20A, which was wound in the winding step S1, a first position fixed body101, and a second position fixed body102are arranged as illustrated inFIG.7. The first position fixed body101and the second position fixed body102define the length of the wound electrode body20in the third direction D3as illustrated inFIG.3. The positions of the first position fixed body101and the second position fixed body102are fixed.

In the present embodiment, as illustrated inFIG.7, the first position fixed body101has a first fixed surface105, which is contactable with the wound electrode body20A. The first fixed surface105is a surface spread in the first direction D1(in other words, a direction in which the winding axis W extends) and the second direction D2. A third flat surface23is formed on an outer peripheral surface of the wound electrode body20as illustrated inFIG.9by pressing the wound electrode body20A against the first fixed surface105.

As illustrated inFIG.7, the second position fixed body102has a second fixed surface106, which is contactable with the wound electrode body20A. The second fixed surface106is a surface spread in the first direction D1and the second direction D2as with the first fixed surface105. A fourth flat surface24is formed on an outer peripheral surface of the wound electrode body20as illustrated inFIG.9by pressing the wound electrode body20A against the second fixed surface106.

In the arranging step S2inFIG.5, the first position fixed body101and the second position fixed body102are arranged with the wound electrode body20A, which was wound in the winding step S1, interposed therebetween as illustrated inFIG.7. In this step, the first position fixed body101, the wound electrode body20A, and the second position fixed body102are arranged in this order along the third direction D3. At this time, the first fixed surface105and the second fixed surface106face each other with the wound electrode body20A interposed therebetween. In other words, the first fixed surface105and the second fixed surface106each face the wound electrode body20A. In this step, the first position fixed body101is arranged so that the first fixed surface105faces the wound electrode body20A side. The second position fixed body102is arranged so that the second fixed surface106faces the wound electrode body20A side. The distance from the first fixed surface105to the second fixed surface106corresponds to a length of the wound electrode body20in the third direction D3as illustrated inFIG.3, in other words, a length in the third direction D3from the third flat surface23to the fourth flat surface24. As stated above, in the arranging step S2inFIG.5, the first position fixed body101, the wound electrode body20A, and the second position fixed body102are arranged as illustrated inFIG.7.

As illustrated inFIG.5, the winding step S1and the arranging step S2are followed by the pressing step S3. In the pressing step S3, as illustrated inFIGS.7,8, and9, the columnar wound electrode body20A, which was wound in the winding step S1, is pressed in the second direction D2and also pressed in the third direction D3to flatten the wound electrode body20A, whereby a flattened wound electrode body20(seeFIG.9) is prepared. In the present embodiment, pressing the wound electrode body20A in the third direction D3is performed by the first position fixed body101and the second position fixed body102.

As illustrated inFIG.7, pressing the wound electrode body20A in the second direction D2is performed by a first pressing body111and a second pressing body112. The first pressing body111presses the wound electrode body20A from one end side in the second direction D2. The first pressing body111has a first pressing surface115. The first pressing surface115is a surface that directly presses the wound electrode body20A and is contactable with the wound electrode body20A. The first pressing surface115is a surface spread in the first direction D1and the third direction D3.

The second pressing body112presses the wound electrode body20A from the other end side in the second direction D2. The second pressing body112has a second pressing surface116. The second pressing surface116is a surface that directly presses the wound electrode body20A and is contactable with the wound electrode body20A as with the first pressing surface115. The second pressing surface116is a surface spread in the first direction D1and the third direction D3.

In the pressing step S3ofFIG.5, as illustrated inFIG.7, the first pressing body111is placed on one end side in the second direction D2of the wound electrode body20A and the second pressing body112is placed on the other end side of in the second direction D2of the wound electrode body20A. At this time, the first pressing surface115and the second pressing surface116both face the wound electrode body20A side.

In the pressing step S3, the wound electrode body20A is pressed by the first pressing body111and the second pressing body112from the second direction D2in the arrow direction illustrated inFIG.7. In this step, the term “press(ing)” refers to applying a pressure on the wound electrode body20A while changing the relative positions of the first pressing body111and the second pressing body112in the second direction D2. By the pressing by the first pressing body111, the first pressing surface115is brought into contact with the wound electrode body20A and the wound electrode body20A is pressed thereafter as illustrated inFIG.8. By this pressing, the first flat surface21(seeFIG.9) is formed on an outer peripheral surface pressed by the first pressing surface115in the wound electrode body20A. Similarly, in the pressing by the second pressing body112, the second pressing surface116is brought into contact with the wound electrode body20A and the wound electrode body20A is pressed thereafter as illustrated inFIG.8. By this pressing, the second flat surface22(seeFIG.9) is formed on an outer peripheral surface pressed by the second pressing surface116in the wound electrode body20A.

In the pressing step S3ofFIG.5, the positions of the first position fixed body101and the second position fixed body102are fixed without moving during a period when the wound electrode body20A is being pressed in the second direction D2. In the pressing step S3, the wound electrode body20A is pressed in the second direction D2until the wound electrode body20A is pressed against the first fixed surface105of the first position fixed body101and the second fixed surface106of the second position fixed body102. By increasing the pressing pressure applied onto the wound electrode body20A, the wound electrode body20A spreads in the third direction D3. Thereafter, the first fixed surface105and the second fixed surface106are brought into contact with the wound electrode body20A and press the wound electrode body20A. Then, the third flat surface23(seeFIG.9) is formed on an outer peripheral surface pressed by the first fixed surface105in the wound electrode body20A. Similarly, the fourth flat surface24(seeFIG.9) is formed on an outer peripheral surface pressed by the second fixed surface106in the wound electrode body20A.

In the present embodiment, the wound electrode body20can be manufactured by performing the pressing step S3. Although the illustration is omitted, the secondary battery100can be manufactured by the step of preparing the wound electrode body20, housing the wound electrode body20in the battery case30, and injecting the non-aqueous electrolyte10into the battery case30.

As described above, in the present embodiment, the wound electrode body20A is pressed in the second direction D2and also pressed in the third direction D3to flatten the wound electrode body20A, as illustrated inFIGS.7to9, in the pressing step S3ofFIG.5. As a result, the first flat surface21and the second flat surface22facing each other with the winding axis W interposed therebetween and the third flat surface23and the fourth flat surface24disposed between the first flat surface21and the second flat surface22and facing each other with the winding axis W interposed therebetween are formed on the outer peripheral surfaces of the wound electrode body20.

In the present embodiment, in the pressing step S3after the arranging step S2, the wound electrode body20A is pressed in the second direction D2until the wound electrode body20A is pressed against the first fixed surface105and the second fixed surface106. By this step, the wound electrode body20A can be pressed not only in the second direction D2but also in the third direction D3simultaneously. Thus, the first flat surface21to the fourth flat surface24can be easily formed in the wound electrode body20by the motion for pressing in the second direction D2.

In the present embodiment, as illustrated inFIG.3, the wound electrode body20includes the elongated positive electrode sheet50, the elongated negative electrode sheet60, and the separator70interposed between the positive electrode sheet50and the negative electrode sheet60, which are superimposed and wound around the winding axis W. The wound electrode body20has the first flat surface21, the second flat surface22, the third flat surface23, and the fourth flat surface24positioned on the outer peripheral surfaces around the winding axis W. The first flat surface21and the second flat surface22face each other with the winding axis W interposed therebetween. The third flat surface23and the fourth flat surface24are disposed between the first flat surface21and the second flat surface22and face each other with the winding axis W interposed therebetween. Due to this feature, the first flat surface21to the fourth flat surface24are arranged along the inner peripheral surfaces of the rectangular battery case30and, as a result, dead spaces between the battery case30and the wound electrode body20can be made smaller than conventional batteries. Thus, the volume efficiency of the wound electrode body20can be enhanced.

In the present embodiment, the wound electrode body20has the first rounded part26to the fourth rounded part29. Thus, by forming the first flat surface21to the fourth flat surface24and the first rounded part26to the fourth rounded part29in the wound electrode body20, radii of curvature of the rounded parts26to29can be made smaller as compared with conventional electrode body. Thus, dead spaces between the battery case30and the wound electrode body20can be made smaller than conventional batteries and the volume efficiency of the wound electrode body20can be enhanced.

In the present embodiment, the inner voids92as illustrated inFIG.4are formed at gaps in a portion where the positive electrode sheet50, the negative electrode sheet60, and the separator70are superimposed inside at least one of the first rounded part26to the fourth rounded part29(inside all of the first rounded part26to the fourth rounded part29in this embodiment). Specifically, the inner voids92are formed between the positive electrode sheet50and the first separator71, between the first separator71and the negative electrode sheet60, the negative electrode sheet60and the second separator72, and between the second separator72and the positive electrode sheet50inside at least one of the first rounded part26to the fourth rounded part29(inside all of the first rounded part26to the fourth rounded part29in this embodiment). By this feature, the non-aqueous electrolyte10can penetrate the inner voids92formed inside the first rounded part26to the fourth rounded part29.

In the present embodiment, as illustrated inFIG.4, the exterior void91(the first exterior void91ain this embodiment) is formed, for example, between the first rounded part26and an inner peripheral surface of the battery case30. The first exterior void91ais larger than one inner void92(for example, the first inner voids92a) formed inside the first rounded part26. For example, when the inner void92becomes relatively large, the wound electrode body20also becomes relatively large. The wound electrode body20is preferably as small as possible. Thus, in the present embodiment, each inner void92is made smaller than, for example, the first exterior void91a, whereby each inner void92becomes relatively small and therefore, the wound electrode body20can be relatively small.

In the present embodiment, at least one of the first flat surface21to the fourth flat surface24(the first flat surface21to the third flat surface23in this embodiment) is in contact with the inner peripheral surfaces of the battery case30via a so-called insulating film, as illustrated inFIG.3. Due to this feature, gaps between the first flat surface21to the fourth flat surface24and the inner peripheral surfaces of the battery case30can be made small. Thus, dead spaces between the battery case30and the wound electrode body20can be made smaller.

In the present embodiment, the first case flat surface81of the battery case30faces the first flat surface21along the first flat surface21. The second case flat surface82faces the second flat surface22along the second flat surface22. The third case flat surface83faces the third flat surface23along the third flat surface23. The fourth case flat surface84faces the fourth flat surface24along the fourth flat surface24. Due to this feature, dead spaces between the battery case30and the wound electrode body20can be made smaller by housing the wound electrode body20in the battery case30so that the first flat surface21to the fourth flat surface24are arranged along the first case flat surface81to the fourth case flat surface84, respectively.

In the present embodiment, the wound electrode body20includes the first rounded part26to the fourth rounded part29, but the first rounded part26to the fourth rounded part29may be omitted. In this case, the shape of the wound electrode body20is rectangular when seen from the first direction D1. In this case, the upper end of the first flat surface21is connected to the left end of the fourth flat surface24and the lower end of the first flat surface21is connected to the left end of the third flat surface23. The upper end of the second flat surface22is connected to the right end of the fourth flat surface24and the lower end of the second flat surface22is connected to the right end of the third flat surface23.

In the above embodiment, one wound electrode body20is housed in one battery case30. However, the number of the wound electrode bodies20housed in one battery case30is not limited to one, and may be two or more, namely, plural. When a plurality of wound electrode bodies20are housed in the battery case30, the wound electrode bodies20may be housed in the battery case30so that, for example, the plurality of wound electrode bodies20align in the second direction D2, in other words, wide surfaces of wound electrode bodies20(the first flat surface21and the second flat surface22in this case) are arranged mutually superimposedly. Even in this case, dead spaces between the battery case30and each of the wound electrode bodies20can be small by arranging the first flat surface21to the fourth flat surface24of each of the wound electrode bodies20along the inner peripheral surfaces of the rectangular battery case30. Thus, the volume efficiency of the wound electrode body20can be enhanced even when a plurality of the wound electrode bodies20are housed in the battery case30.