Power storage element and method for using the same

Abstract

To provide a power storage element that includes a wound electrode body and improves characteristics, and to provide a method of using the same.SOLUTION: A power storage element comprises: a wound electrode body that contains lithium manganate in a positive electrode active material as a main component; and a container for housing the wound electrode body. The wound electrode body has: a mixture layer formation part formed with a mixture layer; and a mixture layer non-formation part located at least at one end in a first direction parallel to a winding axis. For the wound electrode body, a ratio of a dimension in the first direction to a dimension in a second direction orthogonal to the first direction in a plan view is equal to or more than 1.45.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an energy storage element and a method for using the same. [Background technology]

[0002] In recent years, energy storage elements such as lithium-ion secondary batteries have been used in a wide range of fields, including as power sources for portable terminals such as notebook personal computers and smartphones, renewable energy storage systems, and power sources for IoT devices. They are also being actively developed as power sources for next-generation clean energy automobiles, such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] Lithium-ion secondary batteries use lithium transition metal composite oxides such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide as positive electrode active materials (see, for example, Patent Document 1).

[0004] In order to reduce the dead space in the container and improve the energy density of the electricity storage element, various structures have been proposed (for example, see Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2003-157844 A [Patent Document 2] JP 2019-003880 A Summary of the Invention [Problem to be solved by the invention]

[0006] Conventionally, lithium ion batteries often use a so-called wound type electrode assembly in which a long positive electrode plate and a long negative electrode plate are stacked with a long separator interposed therebetween and the stack is wound. The wound type electrode assembly is easy to manufacture at low cost. However, wound-type electrode bodies are generally considered unsuitable for improving energy density because the current collecting parts (such as the current collector, which is a metal plate part) occupy a relatively large space inside the container.

[0007] An object of the present disclosure is to provide an energy storage element having improved characteristics using a wound electrode body, and a method for using the same. [Means for solving the problem]

[0008] An energy storage element according to one embodiment of the present disclosure includes a wound electrode body containing lithium manganate as a main component in a positive electrode active material, and a container that contains the wound electrode body. The wound electrode body has a composite layer forming portion in which a composite layer is formed, and a composite layer non-forming portion located at least at one end in a first direction parallel to a winding axis. The ratio of the dimension of the wound electrode body in the first direction to the dimension in a second direction perpendicular to the first direction in a plan view is 1.45 or more. Effect of the Invention

[0009] According to the present disclosure, it is possible to provide an energy storage element having improved characteristics using a wound electrode body, and a method of using the same. [Brief description of the drawings]

[0010] [Figure 1] 1 is a perspective view showing an energy storage device including an energy storage element according to an embodiment. [Diagram 2] FIG. 2 is an exploded perspective view showing a configuration example of an energy storage element. [Diagram 3] FIG. 2 is a schematic diagram showing a configuration example of an electrode body. [Figure 4] FIG. 2 is an explanatory diagram illustrating a configuration example of an electrode body. [Diagram 5] 1 is a charge / discharge curve showing the relationship between SOC and voltage for an LMO battery. [Figure 6] 1 is a charge / discharge curve showing the relationship between SOC and voltage for an LFP battery. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The energy storage element includes a wound electrode body containing lithium manganate as a main component in a positive electrode active material, and a container that contains the wound electrode body. The wound electrode body has a composite layer forming portion in which a composite layer is formed, and a composite layer non-forming portion located at least at one end in a first direction parallel to a winding axis. In a plan view, the wound electrode body has a ratio of a dimension in the first direction to a dimension in a second direction perpendicular to the first direction of 1.45 or more. The ratio of the dimension in the first direction to the dimension in the second direction may be 1.82 or more.

[0012] In this specification, the "winding axis" may be a virtual axis as the center of winding, or a physical axis such as a winding core. From the viewpoint of improving the energy density, the winding axis is preferably a virtual linear axis. In this specification, "planar view" refers to the case where the wound electrode body, which is contained in a container and cannot be seen, is viewed from a third direction perpendicular to the first and second directions described above after being removed from the container or before being contained in the container. The composite layer non-forming portion may be provided at only one end of the wound electrode body in the first direction, or at both ends of the wound electrode body in the first direction. In the latter case, the composite layer forming portion is provided between the composite layer non-forming portions. As an example, the dimensional ratio is 1.45 when the first direction dimension of the wound electrode body in a planar view is 196.65 mm and the second direction dimension is 135.60 mm, and as another example, the dimensional ratio is 1.82 when the first direction dimension of the wound electrode body is 246.65 mm and the second direction dimension is 135.60 mm.

[0013] By using a wound electrode body that is long in the first direction and has a ratio of the dimension in the first direction to the dimension in the second direction of 1.45 or more, and housing the wound electrode body in a container that is as long as the wound electrode body, the ratio of the space occupied by the current collecting components in the container to the volume of the container can be reduced. This improves the ratio of the volume of the wound electrode body to the volume of the container (so-called volume occupancy), and provides an energy storage element with improved energy density. However, when a wound electrode body long in the first direction is used and charging and / or discharging is performed through a composite layer non-forming portion located at one end of the electrode body, a reaction is unlikely to occur in a part of the wound electrode body far from the composite layer non-forming portion due to the influence of electrical resistance up to that part. For example, in the case of a lithium ion battery, the insertion and removal reaction of lithium ions is unlikely to occur. In other words, when a long wound electrode body is used, reaction variations are likely to occur within the electrode body (lithium ions are likely to be unevenly distributed).

[0014] When an electrode body containing lithium manganese oxide as a main component in the positive electrode active material as in the above configuration is used, when the energy storage element is in a non-energized state (for example, when the energy storage element is left unused), reaction variations within the electrode body are naturally eliminated.

[0015] The reason for this will be explained by comparing an electric storage element (e.g., an LMO battery) having an electrode body containing lithium manganate as a main component in the positive electrode active material with an electric storage element (e.g., an LFP battery) having an electrode body containing lithium iron phosphate as a main component in the positive electrode active material. FIG. 5 is a charge / discharge curve showing the relationship between SOC (State of Charge) and voltage for an LMO battery. FIG. 6 is a charge / discharge curve showing the relationship between SOC and voltage for an LFP battery. The horizontal axis is SOC (%), and the vertical axis is voltage (V). In the figure, a solid line indicates the charge curve, and a dashed line indicates the discharge curve. Although not shown, a curve showing OCV (Open Circuit Voltage) is located almost halfway between the charge curve and the discharge curve. OCV is the voltage of a battery when the voltage of the battery is not affected by polarization or is negligibly small, such as when no charge / discharge current flows continuously. The negative electrodes of LMO batteries and LFP batteries are graphite.

[0016] As shown in Figure 5, the charge / discharge curve of an LMO battery has a gradient over a wide range of SOC (there is a voltage difference according to the change in SOC). Therefore, even if reaction variations occur within the electrode body, electricity flows within the electrode body when no current is applied from the part where the charging reaction has progressed and the voltage has increased to the part where the charging reaction has not progressed and the voltage remains low, eliminating the reaction variations.

[0017] For comparison, in the LFP battery shown in FIG. 6, the voltage hardly changes over a wide range of the SOC (a plateau region with an extremely small gradient). Therefore, there is almost no voltage difference between the part where the charging reaction has progressed and the part where the charging reaction has not progressed, so that the reaction variation in the electrode body is difficult to eliminate even in a non-energized state. If charging and discharging are resumed in a state where the reaction variation has not been eliminated, the reaction variation in the electrode body is further promoted. This tendency is particularly noticeable in a low-temperature environment. Therefore, by using a wound electrode body containing a lithium transition metal, which has a gradient in the charge / discharge curve over a wide range of SOC as the main component, such as lithium manganate, in the positive electrode active material, the reaction variation in the wound electrode body that is long in the first direction can be reduced.

[0018] The energy storage element may have an open circuit voltage (OCV) of 3.6 V or more over 95% or more of the charge / discharge range in which it is used.

[0019] When a wound electrode body containing lithium manganate as a main component in the positive electrode active material is used for the storage element, the OCV is maintained at 3.6V or higher even when the SOC is low (for example, when the SOC is 5%). Furthermore, the OCV is maintained at 3.6V or higher in almost the entire charge / discharge range in which the storage element is used (for example, SOC 5% to SOC 100%). Therefore, overdischarge is unlikely to occur. For example, even when the storage element is discharged at a high rate (for example, 1C discharge) in a low-temperature environment such as -30°C, there is a margin until the discharge end voltage (discharge cut-off voltage), and overdischarge is unlikely to occur. The discharge end voltage is, for example, 3.0V.

[0020] For comparison, an energy storage device (such as an LFP battery) having an electrode body containing lithium iron phosphate as a main component in the positive electrode active material has an OCV lower than 3.6 V over the entire charge / discharge range in which the energy storage device is used. In a storage element (e.g., NMC111 battery) having an electrode body containing the three components nickel, cobalt, and manganese in the positive electrode active material, the OCV is 3.6 V or higher only in the region where the SOC is 50% or higher. In the region where the SOC is less than 50%, the OCV is less than 3.6 V, and the lower the SOC, the lower the OCV.

[0021] The method of using the electricity storage element is such that when the open circuit voltage (OCV) of the above electricity storage element is 3.6V or higher, discharging is started.

[0022] For an energy storage element using a wound electrode body containing lithium manganate as a main component in the positive electrode active material, discharging is started when the open circuit voltage (OCV) is 3.6V or more, thereby preventing overdischarge and enabling the element to be used favorably. In general, overdischarge is controlled by controlling the discharge current using a control device or the like, but there is a risk of instantaneous overdischarge occurring due to delays in control response or the like. By using a wound electrode body containing lithium manganate as a main component in the positive electrode active material, discharging can be started almost constantly from a high voltage of 3.6V or more, so that a sufficient margin can be secured up to the discharge end voltage and overdischarge can be prevented.

[0023] In the above-mentioned method of use, the discharge of the storage element may be started in a temperature range of -30°C or lower. In a low-temperature environment, the internal resistance of the storage element increases, and the voltage drop of the storage element due to discharge increases. By using a wound electrode body containing lithium manganate as a main component in the positive electrode active material, it is possible to ensure a margin to the discharge end voltage and prevent overdischarge even in a low-temperature environment.

[0024] Hereinafter, the present invention will be described in detail with reference to the drawings showing embodiments thereof. In the following description and drawings, a first direction is a direction parallel to the winding axis of the wound electrode body in the energy storage element and is the width direction (lateral direction) of the wound electrode body. A second direction is a direction perpendicular to the winding axis of the electrode body and is the height direction (vertical direction) of the electrode body. A third direction is a direction perpendicular to the winding axis of the electrode body and is the thickness direction of the electrode body.

[0025] <Electricity storage element> FIG. 1 is a perspective view showing an energy storage device 100 including an energy storage element 1 according to an embodiment. FIG. 1 shows an example of an energy storage device 100 in which energy storage units each including a plurality of electrically connected energy storage elements 1 are further assembled. The energy storage elements 1 have a rectangular parallelepiped shape, and a positive electrode terminal 11 and a negative electrode terminal 12 are provided at the center of both end faces. Adjacent positive electrode terminals 11 and negative electrode terminals 12 of adjacent energy storage elements 1 are connected by a bus bar (not shown) or the like, and the energy storage elements 1 are connected in series. The energy storage device 100 may include a battery management unit (BMU) and / or a cell monitoring unit (CMU) that monitors the state of the energy storage elements 1.

[0026] The energy storage element 1 is a battery cell such as a lithium ion secondary battery. The energy storage element 1, in the form of an energy storage unit or an energy storage device 100 (battery pack) in which a plurality of energy storage elements 1 are electrically connected, is applied to power sources for automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power sources for electronic devices, power sources for power storage, and the like.

[0027] 2 is an exploded perspective view showing a configuration example of the energy storage element 1. The energy storage element 1 is configured by housing a flat-shaped wound electrode body (hereinafter also simply referred to as an electrode body) 13 and an electrolyte (not shown) in a hollow rectangular parallelepiped container 14. The container 14 is made of a metal material such as aluminum or stainless steel.

[0028] Fig. 3 is a schematic diagram showing an example of the configuration of the electrode body 13. Fig. 3 shows a partially developed view of the wound state of the electrode body 13. The electrode body 13 includes a positive electrode 15, a negative electrode 16, and two sheet-like separators 17. The electrode body 13 is formed by overlapping the positive electrode 15 and the negative electrode 16 with the separator 17 interposed therebetween and winding the electrode body 13 around a winding axis X.

[0029] The positive electrode 15 is an electrode plate in which a positive electrode active material layer 152 is formed on the surface of a sheet-like positive electrode substrate 151 made of aluminum, an aluminum alloy, or the like. The positive electrode 15 has an uncoated positive electrode portion 153 where the positive electrode active material layer 152 is not formed at one end in the first direction. The negative electrode 16 is an electrode plate in which a negative electrode active material layer 162 is formed on the surface of a sheet-like negative electrode substrate 161 made of copper, a copper alloy, or the like. The negative electrode 16 has an uncoated negative electrode portion 163 where the negative electrode active material layer 162 is not formed at the other end in the first direction.

[0030] The positive electrode 15 and the negative electrode 16 are arranged in a state of being shifted in the first direction. The electrode body 13 formed by winding the positive electrode 15 and the negative electrode 16 has a composite layer forming portion 131 in which the positive electrode active material layer 152 or the negative electrode active material layer 162 is formed, and a composite layer non-forming portion 132 other than the composite layer forming portion 131. In the example of FIG. 3, the electrode body 13 has the composite layer forming portion 131 located at the center in the first direction, the negative electrode composite layer non-forming portion 132 located at the left end, and the positive electrode composite layer non-forming portion 132 located at the right end. A negative electrode current collector (not shown) made of a metal such as copper is joined to the negative electrode composite layer non-forming portion 132. The negative electrode 16 is electrically connected to the negative electrode terminal 12 via the negative electrode current collector. A positive electrode current collector (not shown) made of a metal such as aluminum is joined to the positive electrode composite layer non-forming portion 132. The positive electrode 15 is electrically connected to the positive electrode terminal 11 via a positive electrode current collector.

[0031] The positive electrode active material layer 152 includes a positive electrode active material. The positive electrode active material can be a material that can absorb and release lithium ions and has a voltage difference according to the change in SOC over a wide range of SOC. In this embodiment, the positive electrode active material is mainly composed of lithium manganate (Li x Mn y O Z ). In detail, the positive electrode active material contains secondary particles consisting of an aggregate of primary particles of lithium manganate as active material particles. The secondary particles of lithium manganate are obtained, for example, by mixing lithium manganate powder with a carbon raw material and baking the mixture to burn off the additives. An example of the lithium manganate is LiMnO2.

[0032] The positive electrode active material may further contain other lithium transition metal oxides. As the other lithium transition metal oxides, those having a voltage difference according to the change of SOC in a wide range of SOC, similar to lithium manganate, are preferable. As the other lithium transition metal oxides, for example, lithium nickel cobalt manganese composite oxides such as LiNiMnCoO2 (NMC111) are preferable. Two or more kinds of the other lithium transition metal oxides may be mixed and used.

[0033] The positive electrode active material layer 152 may further include a conductive assistant, a binder, a thickener, etc. Examples of the conductive assistant include carbon black such as acetylene black, and carbon materials such as graphite. Examples of the binder include polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), etc. Examples of the thickener include carboxymethyl cellulose (CMC), methyl cellulose, etc.

[0034] The content of lithium manganate is preferably 50% by weight or more when the entire mixture of lithium manganate and other lithium transition metal oxides is taken as 100% by weight. By adding other lithium transition metal oxides to lithium manganate in the above range, the effects of the present invention can be further enhanced, the energy density of the storage element can be improved, and good safety can be provided. The content of lithium manganate is more preferably 70% by weight or more, and even more preferably 100% by weight.

[0035] The negative electrode active material layer 162 includes a negative electrode active material. The negative electrode active material may be a material capable of absorbing and releasing lithium ions. Examples of the negative electrode active material include carbon materials such as graphite, hard carbon, and soft carbon. The negative electrode active material layer may further include a conductive assistant, a binder, a thickener, and the like. The conductive assistant, binder, thickener, and the like may be the same as those of the positive electrode active material layer 152.

[0036] The separator 17 is formed of a porous resin film. As the porous resin film, a porous resin film made of a resin such as polyethylene (PE) or polypropylene (PP) can be used. The separator 17 may be formed of a resin film having a single layer structure, or may be formed of a resin film having a multi-layer structure of two or more layers. The separator 17 may include a heat-resistant layer.

[0037] The electrolyte contained in the container 14 together with the electrode body 13 may be the same as that used in conventional lithium ion batteries. For example, an electrolyte containing a supporting salt in an organic solvent may be used. Examples of the organic solvent include aprotic solvents such as carbonates, esters, and ethers. Examples of the supporting salt include lithium salts such as LiPF6, LiBF4, and LiClO4. The electrolyte may include various additives such as a gas generating agent, a film forming agent, a dispersing agent, and a thickening agent.

[0038] Fig. 4 is an explanatory diagram for explaining a configuration example of the electrode body 13. Using Fig. 4, the ratio of the dimension in a first direction (hereinafter also referred to as the first dimension) of the electrode body 13 in this embodiment to the dimension in a second direction (hereinafter also referred to as the second dimension) will be explained. Fig. 4 is a plan view of the electrode body 13 as viewed from a third direction perpendicular to a plane parallel to the first direction and the second direction. The electrode body 13 has a rectangular shape in a plan view. The first direction corresponds to the width direction of the electrode body 13, and the second direction corresponds to the height direction of the electrode body 13.

[0039] The electrode body 13 has a composite layer forming portion 131 on which a positive electrode active material layer 152 or a negative electrode active material layer 162 is formed, and a negative electrode composite layer non-forming portion 132 and a positive electrode composite layer non-forming portion 132 provided at both ends in a first direction. In this embodiment, the width of the composite layer forming portion 131 corresponds to the width of the negative electrode active material layer 162. The width of the negative electrode composite layer non-forming portion 132 corresponds to the width of the negative electrode uncoated portion 163. The width of the positive electrode composite layer non-forming portion 132 corresponds to the width of the positive electrode uncoated portion 153 minus the overlapping portion of the negative electrode substrate 161. The first dimension is a dimension obtained by summing the widths of the composite layer forming portion 131, the negative electrode composite layer non-forming portion 132, and the positive electrode composite layer non-forming portion 132. The second dimension is a dimension of the electrode body 13 in a second direction.

[0040] The ratio of the first dimension to the second dimension of the electrode body 13 (first dimension / second dimension) is 1.45 or more. By making the first dimension / second dimension 1.45 or more, the energy density can be improved. From the viewpoint of improving the energy density, the first dimension / second dimension is preferably 1.82 or more.

[0041] In the energy storage element 1, when the electrode body 13 is formed by winding the electrode plate around a winding axis, there are a horizontally wound electrode body whose winding axis extends in the up-down direction (vertical direction) and a vertically wound electrode body whose winding axis extends in the left-right direction (horizontal direction). In general, in terms of space efficiency within the container 14, the horizontally wound electrode body has a higher energy density than the vertically wound electrode body. However, when the electrode body 13 becomes longer in the horizontal direction, the space efficiency within the container 14 is reversed between the horizontally wound electrode body and the vertically wound electrode body, and the vertically wound electrode body has a higher energy density than the horizontally wound electrode body.

[0042] Specifically, in the horizontally wound electrode body, even if the electrode body 13 becomes longer in the horizontal direction, the proportion of the composite layer forming portion 131 of the electrode plate in the container 14 does not change much. In contrast, in the vertically wound electrode body, when the electrode body 13 becomes longer in the horizontal direction, the ratio of the composite layer forming portion 131 of the electrode plate to the composite layer non-forming portion 132 increases, so that the proportion of the composite layer forming portion 131 of the electrode plate in the container 14 increases. The composite layer forming portion 131 is a region where the occlusion and desorption reaction of lithium ions occurs. The composite layer non-forming portion 132 is a portion where the substrate is exposed, so that the occlusion and desorption reaction of lithium ions does not occur. By setting the ratio of the first dimension to the second dimension in the vertically wound electrode body 13 to 1.45 or more, the proportion of the composite layer forming portion 131 of the electrode plate in the container 14 can be increased, and the energy density can be improved compared to the horizontal winding type.

[0043] The above describes an example in which the positive electrode uncoated portion 153 and the negative electrode uncoated portion 163 are provided at both ends in the first direction of the electrode body 13. Alternatively, the electrode body 13 may be provided with both the positive electrode uncoated portion 153 and the negative electrode uncoated portion 163 at one end in the first direction, or may be provided with both the positive electrode uncoated portion 153 and the negative electrode uncoated portion 163 at both ends in the first direction.

[0044] <Method of manufacturing energy storage element> An example of a method for manufacturing the energy storage device 1 according to the embodiment of the present invention will be described.

[0045] First, the positive electrode 15 and the negative electrode 16 are prepared. The positive electrode 15 is prepared by applying a positive electrode mixture paste directly or through an intermediate layer to the positive electrode substrate 151 and drying it. At this time, the application position of the positive electrode mixture paste is adjusted so that an uncoated portion 153 of the positive electrode is formed at one end of the positive electrode 15. The positive electrode mixture paste contains each component constituting the positive electrode active material layer 152 such as a positive electrode active material, and a dispersion medium. The positive electrode active material contains the above-mentioned lithium manganate. Similarly, the negative electrode 16 is prepared by applying a negative electrode mixture paste directly or through an intermediate layer to the negative electrode substrate 161 and drying it. At this time, the application position of the negative electrode mixture paste is adjusted so that an uncoated portion 163 of the negative electrode is formed at one end of the negative electrode 16. The negative electrode mixture paste contains each component constituting the negative electrode active material layer 162 such as a negative electrode active material, and a dispersion medium.

[0046] The positive electrode 15 and the negative electrode 16 are cut to a specified dimension. The positive electrode 15, the negative electrode 16, and the separator 17 are wound to a specified length around the winding axis X to produce an electrode body 13 having a specified first dimension / second dimension. A positive electrode current collector (a tab for a positive electrode terminal) is joined to the composite layer non-forming portion 132 of the positive electrode in the electrode body 13, and a negative electrode current collector (a tab for a negative electrode terminal) is joined to the composite layer non-forming portion 132 of the negative electrode.

[0047] The electrode body 13 and the electrolyte are placed into the container 14 through the opening. The positive electrode current collector is connected to the positive electrode terminal 11, and the negative electrode current collector is connected to the negative electrode terminal 12. The opening of the container 14 is covered and joined by welding or an adhesive or the like. In this way, a battery (energy storage element 1) is obtained.

[0048] <How to use the energy storage element> In the method of use according to the embodiment of the present invention, discharge is started when the open circuit voltage (OCV) of the energy storage device 1 is 3.6 V or higher. In the above-described method of use, discharge may be started in a temperature range of −30° C. or lower.

[0049] By starting discharge from a high voltage state, such as an open circuit voltage (OCV) of 3.6 V or higher, a sufficient margin to the discharge end voltage can be secured, preventing over-discharge. In particular, even in a low-temperature environment such as -30°C, where internal resistance increases, a sufficient margin to the discharge end voltage can be secured. EXAMPLES

[0050] The present invention will now be described in more detail with reference to examples and comparative examples, but it is not intended that the present invention be limited to these examples.

[0051] [Example 1] The storage element of Example 1 was produced by the same steps as in the above-mentioned manufacturing method, using a vertically wound electrode body containing LiMnO2 as the main component in the positive electrode active material, and having the dimensions shown in Table 1 and below. Graphite was used as the main component in the negative electrode active material. The dimensions of the energy storage element of Example 1 other than the first dimension are as follows. Electrode body: Second dimension (height) 135.6 mm, thickness 19.37 mm Container: Width 200 mm, height 145 mm, thickness 22 mm (excluding electrode terminals) Positive substrate: width 180.4mm Positive electrode active material layer: width 166.2mm Uncoated part of positive electrode: 14.2 mm Negative substrate: width 184.5mm Negative electrode active material layer: width 170.3mm Uncoated part of negative electrode: Width 14.2 mm Positive and negative terminal tabs: Width 13.1 mm

[0052] [Table 1]

[0053] [Examples 2 to 8] The first dimension / second dimension of the electrode body was as shown in Table 1, and the width of the composite layer forming portion of the electrode body and the container was changed, and the energy storage elements of Examples 2 to 8 were produced in the same manner as in Example 1. In Examples 2 to 8, the widths of the positive electrode active material layer and the negative electrode active material layer (composite layer forming portion) of the electrode body were increased according to the difference between the first dimension of the electrode body in each Example and the first dimension of the electrode body in Example 1. The second dimension of the electrode body was unified to 135.6 mm, and the thickness was unified to 19.37 mm. Similarly, the width of the container was increased according to the difference between the first dimensions of the electrode bodies. The height of the container was unified to 145 mm, and the thickness was unified to 22 mm.

[0054] [Comparative Examples 1 to 2] The energy storage elements of Comparative Examples 1 and 2 were produced in the same manner as in Example 1, except that the first dimension / second dimension of the electrode body was set as shown in Table 1 and the width of the composite layer forming portion of the electrode body and the container were changed. As in Examples 2 to 8, the width of the composite layer forming portion and the container was reduced according to the difference between the first dimension of the electrode body in each Comparative Example and the first dimension of the electrode body in Example 1.

[0055] [Comparative Examples 3 to 12] The energy storage elements of Comparative Examples 3 to 12 were produced in the same manner as in Example 1, except that LiFePO4 was used as the main component of the positive electrode active material, the first dimension / second dimension of the electrode body were as shown in Table 1, and the width of the composite layer forming portion of the electrode body and the container were changed. As in Examples 2 to 8, the width of the composite layer forming portion and the container was increased or decreased according to the difference between the first dimension of the electrode body in each Comparative Example and the first dimension of the electrode body in Example 1.

[0056] [Comparative Examples 13 to 14] The energy storage elements of Comparative Examples 13 to 14 were produced in the same manner as in Example 1, except that LiCoO2 was used as the main component of the positive electrode active material, the first dimension / second dimension of the electrode body were as shown in Table 1, and the width of the composite layer forming portion of the electrode body and the container were changed. As in Examples 2 to 8, the width of the composite layer forming portion and the container was increased or decreased according to the difference between the first dimension of the electrode body in each Comparative Example and the first dimension of the electrode body in Example 1.

[0057] <Volumetric energy density> The volumetric energy density was examined for the storage elements of Examples 1 to 8 and Comparative Examples 1 to 14. A charge / discharge test was performed for the storage elements of Examples 1 to 8 and Comparative Examples 1 to 14. In the storage elements of Examples 1 to 8 and Comparative Examples 1 to 2, charging was performed at a constant current and constant voltage at a rate of 0.2C, a voltage of 4.2V, and 7.5 hours, and discharging was performed at a constant current and constant voltage at a rate of 0.2C and a cut voltage of 3.0V. In the storage elements of Comparative Examples 3 to 12, charging was performed at a constant current and constant voltage at a rate of 0.2C, a voltage of 3.5V, and 7.5 hours, and discharging was performed at a constant current and constant voltage at a rate of 0.2C and a cut voltage of 2.5V. In the storage elements of Comparative Examples 13 to 14, charging was performed at a constant current and constant voltage at a rate of 0.2C, a voltage of 4.1V, and 7.5 hours, and discharging was performed at a constant current and constant voltage at a rate of 0.2C and a cut voltage of 3.0V. The discharge capacity (mAh) in this case was calculated. The calculated discharge capacity (mAh) is multiplied by the container size (cm 3 ) to obtain the discharge capacity per volume (mAh / cm 3 The volumetric energy density (Wh / L) was calculated by multiplying the energy density (Wh) by the voltage (V) during discharge. The results are shown in Table 2 below.

[0058] In addition, the volumetric energy density of the energy storage element was calculated when using horizontally wound electrode bodies designed to have the same ratio of the second dimension / first direction using the same materials as in each of Examples 1 to 8 and Comparative Examples 1 to 14. The volumetric energy density is shown in Table 2 below together with the difference between the vertical and horizontal windings.

[0059] [Table 2]

[0060] <Nail penetration test> After the energy storage elements of Examples 1 to 8 and Comparative Examples 1 to 14 were fully charged, a nail penetration test was conducted in which a nail having a diameter of 5 mm was inserted into the energy storage element to a depth of 7 mm. The nail penetration test results were judged to be good or bad based on the presence or absence of smoke or fire. The results are also shown in Table 2. In Table 2, the absence of smoke or fire was indicated as ◯, and the presence of smoke or fire was indicated as ×.

[0061] As is clear from Table 2 above, when the winding direction of the electrode body is vertical winding, the energy density is the same as that of horizontal winding when the first dimension / second dimension is 1.45, and the energy density is higher than that of horizontal winding when the first dimension / second dimension is 1.82 or more. When the first dimension / second dimension is less than 1.45, the energy density is lower than that of horizontal winding. It was confirmed that the energy density of a wound electrode body having a vertical winding structure can be improved when the first dimension / second dimension is 1.45 or more.

[0062] The energy storage elements containing lithium manganate in Examples 1 to 8 had high energy density and good safety. In Examples 1 to 8, the energy density was 316 Wh / L or more. The energy storage elements containing lithium iron phosphate in Comparative Examples 5 to 12 had good safety, but had lower energy density than Examples 1 to 8. The energy storage elements containing lithium cobalt oxide in Comparative Examples 13 and 14 had high energy density, but were insufficient in safety when penetrated by a nail, and white smoke was observed. It was confirmed that by using lithium manganate as the main component of the positive electrode active material, it is possible to provide an energy storage element having high energy density and good safety.

[0063] <Discharge performance characteristics> The discharge performance characteristics were investigated for the energy storage elements produced in Example 2 and Comparative Example 6. A discharge test was carried out at the discharge rates and atmospheric temperatures shown in Table 3 below, and the discharge capacity of the energy storage elements was measured. The discharge cutoff voltage for the energy storage element of Example 2 was 2 V, and the discharge cutoff voltage for the energy storage element of Comparative Example 6 was 2.3 V. The value obtained by dividing the discharge capacity during discharge at each discharge rate and atmospheric temperature by the discharge capacity during discharge at a discharge rate of 0.5 C and a temperature of 25° C. was taken as the discharge capacity (percentage).

[0064] [Table 3]

[0065] As is clear from Table 3 above, the energy storage element of Example 2 exhibited high discharge capacity even in a low-temperature environment. The energy storage element of Example 2 had a discharge capacity of 70% at a discharge rate of 0.5C at a temperature of -30°C, and a discharge capacity of 40% even at a high-rate discharge of a discharge rate of 10C. The energy storage element of Comparative Example 6 had a discharge capacity of 52% at a discharge rate of 0.5C at a temperature of -30°C, and a discharge capacity of 27% at a discharge rate of 10C. It was confirmed that the energy storage element of Example 2 can reduce the decrease in discharge capacity in a low-temperature environment. By using lithium manganate as the main component of the positive electrode active material, it is possible to provide an energy storage element having good discharge performance characteristics over a wide temperature range.

[0066] <Cycle characteristics> The cycle characteristics of the energy storage elements produced in Example 2 and Comparative Example 6 were examined under the two temperature environments shown in Table 4 below. Charge and discharge were repeated under two temperature environments of 25°C and -10°C. Specifically, the energy storage elements were stored at atmospheric temperatures of 25°C and -10°C, and constant current and constant voltage charging was performed at a charge rate of 1C for 1.5 hours, and then the elements were left idle for a predetermined time. The constant voltage charging was 4.2V for the energy storage element of Example 2, and 3.5V for Comparative Example 6. Next, constant current discharging was performed at a discharge rate of 1C, and then the elements were left idle for a predetermined time. The discharge cutoff voltage was 3.0V for the energy storage element of Example 2, and 2.5V for Comparative Example 6. This charge and discharge cycle was repeated, and the discharge capacity of the energy storage element in each cycle was measured.

[0067] The discharge capacity at each cycle was divided by the discharge capacity at the first cycle to obtain the initial capacity ratio (percentage, also called capacity retention rate), and the number of cycles at which the initial capacity ratio reached 80% was determined. The number of cycles at which the initial capacity ratio reached 80% was the number of cycles at which the initial capacity ratio first fell to 80% after repeated charging and discharging. The results are shown in Table 4 below.

[0068] [Table 4]

[0069] As is clear from Table 4 above, the energy storage element of Example 2 maintained an initial capacity ratio of 80% or more up to the 850th cycle in a low-temperature environment of -10°C, and was found to have little deterioration in a low-temperature environment. In contrast, the energy storage element of Comparative Example 6 showed a decrease in the initial capacity ratio to 80% at the 100th cycle in a low-temperature environment of -10°C, and was found to have significant deterioration in a low-temperature environment. It was confirmed that by using lithium manganate as the main component of the positive electrode active material, an energy storage element having good cycle characteristics over a wide temperature range can be provided.

[0070] The embodiments disclosed herein are illustrative in all respects and are not restrictive. The scope of the present invention is defined by the claims, and includes all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0071] 100 Electricity storage device 1 Energy storage element 13 Electrode body (wound electrode body) 131 Composite material layer forming section 132 Part where composite material layer is not formed 14 Container 15 Positive electrode 152 Cathode active material layer 16 negative electrode 162 Negative electrode active material layer

Claims

1. A wound electrode body containing lithium manganate as a main component in a positive electrode active material; A container that accommodates the wound electrode body, The wound electrode body has a composite layer forming portion in which a composite layer is formed, and a composite layer non-forming portion located at least at one end in a first direction parallel to a winding axis, The ratio of a dimension of the wound electrode body in a first direction to a dimension in a second direction perpendicular to the first direction in a plan view is 1.45 or more. Energy storage element.

2. The ratio of the dimension in the first direction to the dimension in the second direction is 4.40 or more. The energy storage element according to claim 1 .

3. The open circuit voltage (OCV) is 3.6 V or higher over 95% or more of the charge / discharge range used. The energy storage element according to claim 1 or 2.

4. The electric storage element according to any one of claims 1 to 3 is caused to start discharging when the open circuit voltage (OCV) is 3.6 V or more. How to use the energy storage element.

5. Discharge starts at temperatures below -30°C A method for using the electric storage element according to claim 4.

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