Composite negative electrode sheet, preparation method thereof and lithium battery

Abstract

The application discloses a composite negative electrode sheet, a preparation method thereof and a lithium battery. The composite negative electrode sheet comprises a current collector and an active material layer, a first insulating and heat-conducting coating layer and a second insulating and heat-insulating coating layer which are sequentially stacked on the current collector, wherein the first insulating and heat-conducting coating layer comprises an insulating and heat-conducting material, the second insulating and heat-insulating coating layer comprises an insulating and heat-insulating material, the particle size D50 of the insulating and heat-conducting material is D v1 , and the particle size D50 of the insulating and heat-insulating material is D v2 . D v1 <D v2 The composite negative electrode sheet can effectively improve the safety performance of the lithium battery.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and relates to a composite negative electrode sheet, its preparation method, and a lithium battery. Background Technology

[0002] Lithium-ion batteries are widely used in electric vehicles due to their high energy density, but thermal runaway caused by internal short circuits remains a significant safety hazard. Overcharging, lithium dendrite growth, mechanical impact, and overheating can all lead to internal short circuits. The short-circuit current causes a rapid accumulation of Joule heat. If this heat cannot dissipate quickly, an exothermic chain reaction occurs, triggering exothermic side reactions in materials such as the negative electrode, positive electrode, electrolyte, and separator, ultimately leading to thermal runaway in the lithium battery.

[0003] Compared to the positive electrode, the initial exothermic reaction of thermal runaway in lithium batteries mostly occurs on the negative electrode side. For example, exothermic processes such as SEI film decomposition and reaction between the lithium-intercalated negative electrode and the electrolyte are prone to occur on the negative electrode side. In addition, the negative electrode has good conductivity, and the heat generated is greater than the heat dissipation, which makes it very easy to accumulate a large amount of heat locally, thereby triggering thermal runaway of the battery.

[0004] CN118522853A discloses a negative electrode heat dissipation layer and a lithium-ion battery. The negative electrode heat dissipation layer includes a negative electrode sheet and a heat dissipation layer located between the negative electrode sheet and a battery separator. The heat dissipation layer is a heat dissipation coating or a heat dissipation separator. The heat dissipation coating is applied to the surface of the negative electrode sheet or to a first surface of the battery separator, the first surface being adjacent to the negative electrode sheet. The heat dissipation separator is disposed between the negative electrode sheet and the battery separator. Both the heat dissipation coating and the heat dissipation separator are made using thermally conductive materials, including at least one of graphite-based nanomaterials, ceramic-based nanomaterials, and metal-based nanofibers. Compared to traditional lithium-ion batteries, the thermal conductivity is almost doubled, enabling the battery to be charged at a higher rate under limited temperature rise conditions.

[0005] CN116666571A discloses a lithium-ion battery negative electrode sheet with high heat dissipation performance and its preparation method. The preparation method includes the following steps: S1, preparing a negative electrode active material layer; S2, preparing a phase change heat dissipation material layer; S3, preparing a lithium-ion battery negative electrode sheet: coating one surface of the negative electrode current collector with the negative electrode active material layer prepared in step S1, coating the other surface of the negative electrode current collector with the phase change heat dissipation material layer prepared in step S2, and drying to obtain the lithium-ion battery negative electrode sheet with high heat dissipation performance. Compared with the prior art, the preparation method proposed in this invention can effectively reduce the operating temperature and heat of the lithium-ion battery, improve the heat dissipation performance and safety performance of the battery, and also improve the cycle performance and rate performance of the battery.

[0006] However, the safety performance of lithium batteries using the above methods needs further improvement. Summary of the Invention

[0007] In view of the above-mentioned technical problems existing in the prior art, the purpose of this invention is to provide a composite negative electrode sheet, its preparation method and a lithium battery.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a composite negative electrode sheet, the composite negative electrode sheet comprising a current collector and an active material layer, a first insulating and thermally conductive coating, and a second insulating and thermally insulating coating sequentially stacked on the current collector, wherein the first insulating and thermally conductive coating comprises an insulating and thermally insulating material, and the second insulating and thermally insulating coating comprises an insulating and thermally insulating material, wherein the particle size D50 of the insulating and thermally conductive material is D v1 The particle size D50 of the insulating and heat-insulating material is D v2 D v1 <D v2 .

[0010] When a battery experiences thermal runaway, the exothermic reactions that accumulate heat in the early stages are concentrated on the negative electrode side, including the decomposition of the negative electrode SEI film and the reaction between the lithium-intercalated negative electrode and the electrolyte.

[0011] To address the aforementioned problems, this invention provides a composite negative electrode sheet. A first insulating and thermally conductive coating is applied to the surface of the active material layer using a small-particle-size insulating and thermally conductive material, resulting in a denser packing. This facilitates the diffusion and conduction of heat generated by the negative electrode along the in-plane direction, preventing more severe side reactions caused by localized overheating through directional heat conduction. A second insulating and thermally insulating coating, using a large-particle-size insulating and thermally insulating material, is applied to the surface of the first insulating and thermally conductive coating, creating more gaps and suppressing the diffusion of heat generated by internal side reactions along the interlayer. The combined effect of these factors improves battery safety performance. Simultaneously, the insulating properties of the first and second insulating and thermally conductive coatings increase the resistivity of the electrode sheet, reducing the short-circuit current during internal short circuits and minimizing localized heat release, further enhancing battery safety.

[0012] It should be noted that in this invention, the order of the first insulating thermally conductive coating and the second insulating thermally insulating coating is irreplaceable. This is because: the first insulating thermally conductive coating, which is closer to the active material layer, has better thermal conductivity, which is more conducive to dissipating the heat generated by the side reaction inside the negative electrode along the in-plane of the first insulating thermally conductive coating, preventing local heat accumulation; while the second insulating thermally insulating coating, which is farther away from the active material layer, has better thermal insulation, which can prevent the heat inside the negative electrode from diffusing along the interlayer to the separator and the positive electrode, causing overheating problems in the separator and the positive electrode.

[0013] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0014] Preferably, D v1 The range is from 100nm to 800nm, for example, it can be 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 750nm or 800nm, etc.

[0015] Preferably, D v2 The range is 1000nm to 2000nm, for example, it can be 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1600nm, 1700nm, 1800nm, 1900nm or 2000nm, etc.

[0016] Preferably, the thickness of the first insulating and thermally conductive coating is d1, and the thickness of the second insulating and thermally insulating coating is d2, where d1 < d2. By limiting the first insulating and thermally conductive coating to a relatively thin coating and the second insulating and thermally insulating coating to a relatively thick coating, the present invention achieves better thermal conductivity and interlayer thermal insulation effects.

[0017] Preferably, the thickness d1 of the first insulating and thermally conductive coating is 1μm to 5μm, for example, it can be 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm or 5μm, etc.

[0018] Preferably, the thickness d2 of the second insulating and heat-insulating coating is 5μm to 10μm, for example, it can be 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm or 10μm.

[0019] Preferably, the thermal conductivity of the insulating thermally conductive material is >10 W / (m·K), for example, it can be 10 W / (m·K), 10.5 W / (m·K), 11 W / (m·K), 11.5 W / (m·K), 12 W / (m·K), 12.5 W / (m·K), 13 W / (m·K), 13.5 W / (m·K), 14 W / (m·K), 14.5 W / (m·K), or 15 W / (m·K). Using an insulating thermally conductive material with a high thermal conductivity allows for better diffusion and dissipation of heat generated at the negative electrode along the in-plane direction of the first insulating thermally conductive coating.

[0020] Preferably, the insulating and thermally conductive material includes at least one of hexagonal boron nitride, aluminum nitride, aluminum oxide, silicon carbide, and silicon nitride.

[0021] Preferably, the thermal conductivity of the insulating material is <1 W / (m·K), for example, it can be 0.9 W / (m·K), 0.8 W / (m·K), 0.7 W / (m·K), 0.6 W / (m·K), 0.5 W / (m·K), 0.4 W / (m·K), 0.3 W / (m·K), 0.2 W / (m·K), or 0.1 W / (m·K). Using an insulating material with low thermal conductivity can better prevent heat from the negative electrode side from diffusing along the interlayer to the separator and positive electrode.

[0022] This allows the heat generated by the negative electrode to diffuse and dissipate more effectively along the in-plane direction of the first insulating and thermally conductive coating.

[0023] Preferably, the insulating and heat-insulating material includes at least one of vermiculite, montmorillonite, perlite, silica, and boron oxide.

[0024] Preferably, the first insulating and thermally conductive coating further includes a first binder, the mass ratio of the first binder to the insulating and thermally conductive material being 20:80 to 2:98, for example, 20:80, 19:81, 18:82, 17:83, 15:85, 13:87, 12:88, 10:90, 9:91, 8:92, 7:93, 6:94, 5:95, 4:96, or 2:98, etc. The introduction of the first binder facilitates the bonding of the insulating and thermally conductive material to the active material layer.

[0025] Preferably, the first adhesive comprises at least one of polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polymethyl methacrylate.

[0026] Preferably, the second insulating and heat-insulating coating further includes a second adhesive, the mass ratio of the second adhesive to the insulating and heat-insulating material being 20:80 to 2:98, for example, 20:80, 19:81, 18:82, 17:83, 15:85, 13:87, 12:88, 10:90, 9:91, 8:92, 7:93, 6:94, 5:95, 4:96, or 2:98, etc. The introduction of the second adhesive facilitates the bonding of the insulating and heat-insulating material to the first insulating and thermally conductive coating.

[0027] Preferably, the second adhesive comprises at least one of polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polymethyl methacrylate.

[0028] Preferably, the active material layer includes negative electrode active material particles, electronically conductive particles, and a negative electrode binder.

[0029] In a second aspect, the present invention provides a method for preparing a composite negative electrode sheet as described in the first aspect, the method comprising the following steps:

[0030] A negative electrode sheet is provided, comprising a current collector and an active material layer disposed on the surface of the current collector;

[0031] Prepare a first slurry containing an insulating and thermally conductive material and a second slurry containing an insulating and thermally insulating material;

[0032] The first slurry is coated on the surface of the negative electrode sheet and dried to obtain a negative electrode sheet with a first insulating and thermally conductive coating.

[0033] The second slurry is coated on the surface of the first insulating and thermally conductive coating, and after drying, the composite negative electrode sheet is obtained.

[0034] Thirdly, the present invention provides a lithium battery, the lithium battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the separator is located between the positive electrode and the negative electrode, and the negative electrode is the composite negative electrode described in the first aspect.

[0035] This invention does not specify the preparation method of the positive electrode and the negative electrode; those skilled in the art can refer to the prior art for preparation.

[0036] This invention does not specifically limit the types of diaphragms and electrolytes; those skilled in the art can select them as needed.

[0037] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, this invention will not exhaustively list the specific point values ​​included in the range.

[0038] Compared with existing technologies, the present invention has the following beneficial effects:

[0039] This invention provides a composite negative electrode sheet. A first insulating and thermally conductive coating with small-particle-size insulating and thermally conductive material is applied to the surface of the active material layer, resulting in a denser packing. This facilitates the diffusion and conduction of heat generated by the negative electrode along the in-plane direction, preventing more severe side reactions caused by localized overheating through directional heat conduction. A second insulating and heat-insulating coating with large-particle-size insulating and heat-insulating material is applied to the surface of the first insulating and thermally conductive coating, creating more gaps and suppressing the diffusion of heat generated by internal side reactions along the interlayer. The combined effect of these factors improves the battery's safety performance. Simultaneously, the insulating properties of the first and second insulating and heat-insulating coatings increase the electrode sheet's resistivity, reduce the short-circuit current during internal short circuits, and decrease localized heat release, further enhancing battery safety. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the structure of a composite negative electrode sheet according to one embodiment of the present invention, wherein 1 is a current collector, 2 is an active material layer, 3 is a first insulating and thermally conductive coating, and 4 is a second insulating and thermally insulating coating. Detailed Implementation

[0041] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0042] The specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0043] Unless otherwise specified, all reagents or instruments used in the examples are commercially available products.

[0044] Preparation Example 1

[0045] A positive electrode sheet is provided, and its preparation method is as follows:

[0046] LiNi, a ternary active material for cathodes 0.8 Co 0.1 Mn 0.1 O2, conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed at a weight ratio of 96:2:2. An appropriate amount of N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred until uniformly dispersed to obtain a positive electrode slurry. The positive electrode slurry was coated onto both sides of an aluminum foil and dried at 90°C to obtain an areal density of 20 mg / cm³. 2 The positive electrode sheet.

[0047] Example 1

[0048] A composite negative electrode sheet is provided, comprising a current collector 1, wherein the current collector 1 is a copper foil, and an active material layer 2, a first insulating and thermally conductive coating 3 (thickness d1, d1=3μm) and a second insulating and thermally insulating coating 4 (thickness d2, d2=7μm) are sequentially stacked on the surface of the current collector. The first insulating and thermally conductive coating 3 comprises an insulating and thermally conductive material (crystalline hexagonal boron nitride, particle size D50 is D...). v1 D v1 =500nm) and a first binder (PVDF), wherein the mass ratio of the insulating thermally conductive material to the first binder is 90:10. The second insulating thermal insulation coating 4 includes an insulating thermal insulation material (vermiculite, particle size D50 is D) v2 D v2 =1500nm) and a second binder (PVDF), wherein the mass ratio of the insulating and heat-insulating material to the second binder is 90:10. This embodiment also provides a method for preparing the above-mentioned composite negative electrode sheet, including the following steps:

[0049] S1. Preparation of the negative electrode sheet: Graphite, silicon carbide, conductive carbon black (Super P), hydroxymethyl cellulose binder, and styrene-butadiene rubber latex binder were mixed at a mass ratio of 84.6:9.4:1:1.5:3.5. An appropriate amount of deionized water was added as a solvent, and the mixture was stirred and dispersed evenly to obtain a negative electrode slurry. The negative electrode slurry was coated onto both sides of a copper foil and dried at 70℃ to obtain an areal density of 9.7 mg / cm³. 2 The negative electrode sheet.

[0050] S2. Preparation of the first insulating and thermally conductive coating: Weigh 90g of crystalline hexagonal boron nitride and 10g of PVDF, disperse and mix them evenly in NMP, and coat them on the surface of the negative electrode obtained in S1. After drying, a negative electrode with the first insulating and thermally conductive coating on the surface is obtained.

[0051] S3. Preparation of the second insulating and heat-insulating coating: Weigh 90g vermiculite and 10g PVDF, disperse and mix them evenly in NMP, and coat them on the surface of the first insulating and heat-conducting coating of the negative electrode sheet obtained in S2. After drying, the composite negative electrode sheet is obtained.

[0052] Examples 2-11

[0053] The differences from Example 1 are detailed in Table 1.

[0054]

[0055] Comparative Example 1

[0056] The difference from Example 1 is that the composite negative electrode does not include a second insulating and heat-insulating coating.

[0057] Comparative Example 2

[0058] The difference from Example 1 is that the composite negative electrode does not include the first insulating and thermally conductive coating, and correspondingly, the second insulating and thermally insulating coating is directly disposed on the surface of the active material layer.

[0059] Comparative Example 3

[0060] The difference from Example 1 is that the composite negative electrode does not include the first insulating and thermally conductive coating, nor the second insulating and thermally insulating coating.

[0061] The negative electrode in this comparative example is an uncoated negative electrode sheet.

[0062] Comparative Example 4

[0063] The difference from Example 1 is that the stacking order of the first insulating and thermally conductive coating and the second insulating and thermally insulating coating on the surface of the composite negative electrode sheet is adjusted, that is: the second insulating and thermally insulating coating is first set on the surface of the negative electrode sheet, and then the first insulating and thermally conductive coating is set on the second insulating and thermally insulating coating.

[0064] Lithium battery assembly:

[0065] A 12 μm thick polyethylene (PE) separator was selected and stacked in a Z-shape. The positive electrode of Preparation Example 1 and the negative electrode of Examples 1-11 and Comparative Example 1 were placed on both sides of the separator. After stacking, tabs were welded on, and then the separator was placed in an aluminum-plastic film. Top-side sealing, electrolyte injection (solute is 1M LiPF6, solvent is a mixture of EC and DMC in a volume ratio of 3:7), and encapsulation were performed to obtain a lithium battery.

[0066] Performance testing

[0067] (1) Hot box test

[0068] The lithium batteries were fully charged and placed in an oven. The temperature was initially increased from room temperature to 130°C at a rate of 5°C / min and maintained for 30 minutes. The battery was observed for failure. If the battery did not fail, the temperature was increased by 5°C each time and maintained at that temperature for 30 minutes until the battery failed. The failure temperature and holding time were recorded. A higher failure temperature indicates better battery safety, and vice versa. At the same failure temperature, a longer holding time indicates better battery safety, and vice versa.

[0069] (2) Needle prick test

[0070] The lithium batteries obtained in the above embodiments and comparative examples were fully charged. A high-temperature resistant steel needle with a diameter of 5 mm (and a cone angle of 45° at the tip) was then inserted into the geometric center of the battery from a direction perpendicular to the battery electrodes at a speed of 25 mm / s. The steel needle remained in the battery for 1 hour, and the battery was observed to see if it went out of control and whether it emitted smoke, caught fire, or exploded.

[0071] (3) Cyclic performance test

[0072] The lithium battery cycle test was conducted on the Xinwei test system. The battery adopted the constant current-constant potential charging / constant current discharging (CC-CV / DC) mode, with charge and discharge cutoff voltages of 4.2V and 2.8V, respectively. The constant potential cutoff current was 0.05C. The battery was allowed to rest for 5 minutes between each charge and discharge cycle. The battery was cycled at 25℃ with a charge / discharge rate of 0.5 / 1C. The capacity retention rate after 500 cycles was calculated.

[0073] The test results are shown in Table 2.

[0074]

[0075] The test results show that the composite negative electrode sheet provided by the present invention has both an in-plane thermal conductive layer and an in-plane thermal insulating layer, which can significantly improve the safety performance of the battery and has no significant impact on the cycle performance of the battery.

[0076] Comparing Examples 1 and 8, it can be seen that the thickness of the first insulating and thermally conductive coating should not be too thin, otherwise the in-plane thermal conductivity will be poor and the safety performance of the lithium battery will be reduced.

[0077] Comparing Example 1 and Example 9, it can be seen that the thickness of the second insulating and heat-insulating coating should not be too thin, otherwise the surface heat insulation effect will be worse and the safety performance of the lithium battery will be reduced.

[0078] Comparing Example 1 and Example 10, it can be seen that by limiting the thickness of the first insulating and thermally conductive coating to be less than the thickness of the second insulating and thermally insulating coating, it is beneficial to achieve better in-plane thermal conductivity and surface thermal insulation effects, thereby improving the safety performance of lithium batteries.

[0079] Comparing Example 1 and Example 11, it can be seen that by limiting the particle size of the insulating and heat-conducting particles to be smaller than that of the insulating and heat-insulating particles, it is beneficial to achieve better in-plane heat conduction and surface heat insulation effects, thereby improving the safety performance of lithium batteries.

[0080] Comparing Example 1 and Comparative Example 1, it can be seen that the negative electrode sheet of Comparative Example 1 only has a first insulating and thermally conductive coating, which cannot suppress the heat diffusion between layers, resulting in poor safety performance of the lithium battery.

[0081] Comparing Example 1 and Comparative Example 2, it can be seen that the negative electrode sheet of Comparative Example 2 only has a second insulating and heat-insulating coating on its surface, which is not conducive to the in-plane conduction of heat, resulting in poor safety performance of the lithium battery.

[0082] Comparing Example 1 and Comparative Example 3, it can be seen that the negative electrode sheet of Comparative Example 3 has no coating on its surface, resulting in poor safety performance of the lithium battery.

[0083] Comparing Example 1 and Comparative Example 4, it can be seen that the order of the thermal conductive coating and the thermal insulating coating on the surface of the negative electrode sheet in Comparative Example 4 is reversed, making it more difficult for heat to be dissipated, resulting in the worst safety performance of the lithium battery.

[0084] The applicant declares that the detailed method of the present invention is illustrated by the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A composite negative electrode sheet, characterized in that, The composite negative electrode sheet includes a current collector and an active material layer, a first insulating and thermally conductive coating, and a second insulating and thermally insulating coating sequentially stacked on the current collector. The first insulating and thermally conductive coating includes an insulating and thermally conductive material, and the second insulating and thermally insulating coating includes an insulating and thermally insulating material. The particle size D50 of the insulating and thermally conductive material is D. v1 The particle size D50 of the insulating and heat-insulating material is D v2 D v1 <D v2 ; D v1 The range is 100nm~800nm, D v2 The range is 1000nm to 2000nm; The thickness of the first insulating and thermally conductive coating is d1, and the thickness of the second insulating and thermally insulating coating is d2, where d1 < d2; The thickness d1 of the first insulating and thermally conductive coating is 1μm to 5μm, and the thickness d2 of the second insulating and thermally insulating coating is 5μm to 10μm.

2. The composite negative electrode sheet according to claim 1, characterized in that, The thermal conductivity of the insulating and thermally conductive material is >10 W / (m·K); and / or, the insulating and thermally conductive material includes at least one of hexagonal boron nitride, aluminum nitride, aluminum oxide, silicon carbide, and silicon nitride.

3. The composite negative electrode sheet according to claim 1, characterized in that, The thermal conductivity of the insulating material is <1 W / (m·K); and / or the insulating material includes at least one of vermiculite, montmorillonite, perlite, silica and boron oxide.

4. The composite negative electrode sheet according to claim 1, characterized in that, The first insulating and thermally conductive coating further includes a first adhesive, wherein the mass ratio of the first adhesive to the insulating and thermally conductive material is 20:80 to 2:98; and / or, the first adhesive includes at least one of polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polymethyl methacrylate; and / or, the second insulating and thermally insulating coating further includes a second adhesive, wherein the mass ratio of the second adhesive to the insulating and thermally insulating material is 20:80 to 2:98; and / or, the second adhesive includes at least one of polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polymethyl methacrylate.

5. The composite negative electrode sheet according to claim 1, characterized in that, The active material layer includes negative electrode active material particles, electronically conductive particles, and a negative electrode binder.

6. A method for preparing a composite negative electrode sheet as described in any one of claims 1-5, characterized in that, The preparation method includes the following steps: A negative electrode sheet is provided, comprising a current collector and an active material layer disposed on the surface of the current collector; Prepare a first slurry containing an insulating and thermally conductive material and a second slurry containing an insulating and thermally insulating material; The first slurry is coated on the surface of the negative electrode sheet and dried to obtain a negative electrode sheet with a first insulating and thermally conductive coating. The second slurry is coated on the surface of the first insulating and thermally conductive coating, and after drying, the composite negative electrode sheet is obtained.

7. A lithium battery, characterized in that, The lithium battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The separator is located between the positive electrode and the negative electrode. The negative electrode is a composite negative electrode as described in any one of claims 1-5.

Images