High-strength bipolar pole piece and preparation method thereof

Abstract

The invention provides a high-strength bipolar pole piece and a preparation method thereof. The bipolar pole piece comprises a current collector layer, a positive electrode layer and a lithium-carbon composite material layer, the positive electrode layer and the lithium-carbon composite material layer are arranged on the two sides of the current collector layer, and the lithium-carbon composite material layer comprises metal lithium and carbon nanotubes with the mass fraction of 9%-65% and can further comprise a second carbon source with the mass fraction smaller than 10%. The lithium-carbon composite material layer is used as a negative electrode layer, and the carbon nanotube is used as a host skeleton of metal lithium, so that the local current density can be reduced, the growth of lithium dendrites can be inhibited, the strength of the lithium-carbon composite material can be regulated and controlled, and the bipolar pole piece can meet the high-voltage condition of all-solid-state battery assembly and use; and after the metal lithium in the negative electrode layer is pulled out to form the hole, the exposed carbon nano tube can continuously maintain the interface contact between the negative electrode layer and the solid electrolyte layer, so that the stable and long-term operation of the all-solid-state battery is ensured, the limitation condition that the all-solid-state battery must be used under relatively high external pressure is eliminated, and the practical progress of the all-solid-state battery is greatly promoted.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and particularly relates to a high-strength bipolar electrode for all-solid-state batteries and a preparation method thereof. Background Art

[0002] Due to its high safety, high energy density and other characteristics, all-solid-state batteries have always been a research hotspot in the academic and industrial fields. Especially when the all-solid-state battery adopts a unique internal series design, that is, a bipolar electrode, the all-solid-state battery will have a smaller internal resistance and a higher output voltage. However, when preparing all-solid-state batteries, in order to ensure sufficient contact between the interfaces of battery materials, a powder press or an isostatic pressing device is generally used to obtain a large assembly pressure, which not only increases the assembly difficulty and cost of all-solid-state batteries, but also puts higher requirements on the material strength.

[0003] To improve the energy density of all-solid-state batteries, metallic lithium with a large specific capacity and a low electrode potential is usually used as the negative electrode. During the rolling process of the bipolar electrode, in order to ensure a large compaction density on the positive electrode side or ensure a tight fit between the metallic lithium negative electrode and the current collector, a large rolling pressure is required, and the soft metallic lithium negative electrode is easily deformed under a large pressure, resulting in poor thickness uniformity of the bipolar electrode. During the assembly process of all-solid-state batteries, the bipolar electrode with poor thickness uniformity will lead to a loose fit between the metallic lithium negative electrode and the solid electrolyte and an increase in the internal resistance of the battery cell. In addition, during the use of all-solid-state batteries, the insertion and extraction of metallic lithium occur on the surface of the negative electrode. The bipolar electrode with poor thickness uniformity will lead to uneven lithium extraction and insertion of metallic lithium, accelerating the formation of holes on the surface of the negative electrode, reducing the contact between the metallic lithium negative electrode and the solid electrolyte, and ultimately resulting in the deterioration or even failure of the performance of all-solid-state batteries. However, if a large pressure is used to ensure the interface contact between the metallic lithium negative electrode and the solid electrolyte, it is easy to cause deformation of the metallic lithium negative electrode, and even cause battery short circuit or battery structure damage. Obviously, the bipolar electrode prepared with a soft metallic lithium negative electrode is not compatible with the large assembly and use pressures, which greatly hinders the industrialization process of all-solid-state batteries.

[0004] Simply reducing the assembly pressure will result in insufficient compaction of the positive electrode layer in the bipolar electrode, while reducing the use pressure cannot maintain the tight contact between the metallic lithium negative electrode and the solid electrolyte. To improve the thickness uniformity of the bipolar electrode, some researchers have proposed using a lithium alloy with higher strength. However, during the cycling process of all-solid-state batteries, holes will still be formed at the interface between the lithium alloy and the solid electrolyte due to lithium extraction, and using a lithium alloy cannot solve the problem of reduced contact between the metallic lithium negative electrode and the solid electrolyte.

[0005] In summary, it is indeed necessary to provide a bipolar electrode with high strength and good contact between the metallic lithium negative electrode and the solid electrolyte. Summary of the Invention

[0006] The present invention provides a high-strength bipolar electrode. This bipolar electrode uses a lithium-carbon composite material as the negative electrode. By changing the content of carbon nanotubes in the lithium-carbon composite material, the strength of the negative electrode material and the bipolar electrode is regulated, overcoming the defect of the softness of the metallic lithium negative electrode, so that the bipolar electrode containing the lithium-carbon composite material meets the assembly and use conditions under high pressure. In addition, the lithium-carbon composite material may further contain a second carbon source. Besides improving the interface between the carbon material and metallic lithium to increase the lithium-ion transport channels, the second carbon source will remain at the interface between the negative electrode and the solid electrolyte and redistribute on this interface as the interface changes. When dendrites are generated on the negative electrode, the second carbon source can physically hinder the spread and growth of the dendrites. The second carbon source can also serve as the active sites of metallic lithium. During the battery cycling process, the second carbon source filled in the carbon nanotube framework can ensure that the local current densities inside and outside the carbon nanotube framework are consistent, promoting the more uniform extraction and deposition of metallic lithium. The exposed carbon nanotubes and the second carbon source maintain the contact between the negative electrode layer and the solid electrolyte interface, enabling the bipolar electrode to get rid of the limitation that all-solid-state batteries must be used under a relatively high external pressure, greatly promoting the practical process of all-solid-state batteries.

[0007] To achieve the above technical effects, according to one aspect of the present invention, there is provided a bipolar electrode, characterized in that it includes a current collector layer, and a positive electrode layer and a lithium-carbon composite material layer located on both sides of the current collector layer, wherein the lithium-carbon composite material layer contains metallic lithium and carbon nanotubes with a mass fraction of 9% to 65%, and optionally, further contains a second carbon source with a mass fraction of less than 10%.

[0008] In some embodiments, the mass fraction of carbon nanotubes in the lithium-carbon composite material layer is 8% to 50%.

[0009] In some embodiments, the thickness of the high-strength bipolar electrode is 2 microns to 500 microns, wherein the thickness of the current collector layer is 20 nanometers to 20 microns.

[0010] In some embodiments, the active material in the positive electrode layer is at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium cobalt oxide, lithium iron phosphate, lithium titanate, lithium manganate, lithium nickelate, elemental sulfur, oxygen positive electrode material, sulfonated polyacrylonitrile, iron disulfide.

[0011] In some embodiments, the thickness of the lithium-carbon composite material layer is 1 micron to 100 microns.

[0012] In some embodiments, the second carbon source in the lithium-carbon composite material layer is at least one of carbon black, graphene, whisker carbon nanotubes, graphite, and mesophase carbon microspheres.

[0013] In some embodiments, the secondary particle size of the carbon material in the lithium-carbon composite material layer is less than 30 microns.

[0014] In some embodiments, the material of the current collector layer includes at least one of copper foil, tin foil, nickel foil, stainless steel foil, conductive carbon paper with a metal coating on the surface, and conductive plastic with a metal coating on the surface.

[0015] In some embodiments, the thickness deformation of the bipolar electrode under a pressure of 100 Mpa is less than 15% of the initial thickness, or the thickness deformation under a pressure of 300 Mpa is less than 25% of the initial thickness.

[0016] According to another aspect of the present invention, there is provided a method for preparing the above bipolar electrode, which is characterized by comprising the following steps:

[0017] S1. Construct a unipolar electrode

[0018] Coat the positive active material on the current collector layer to prepare a unipolar positive electrode, or roll the lithium-carbon composite material onto the current collector layer to prepare a unipolar negative electrode;

[0019] S2. Construct a bipolar electrode

[0020] Roll the lithium-carbon composite material layer on the blank side of the unipolar positive electrode constructed in S1 or coat the positive active material on the blank side of the unipolar negative electrode, and roll it after drying to form a high-strength bipolar electrode.

[0021] The present invention has at least the following advantages:

[0022] 1. In the lithium-carbon composite material, carbon nanotubes serve as the host framework for metallic lithium, which can reduce the local current density and inhibit the growth of lithium dendrites.

[0023] 2. By changing the content of carbon nanotubes, the strength of the lithium-carbon composite material is regulated, overcoming the defect of the softness of metallic lithium, so that the bipolar electrode containing the lithium-carbon composite material meets the assembly and use conditions under high pressure.

[0024] 3. Inside the carbon nanotubes and at the interface between the carbon material and metallic lithium can serve as a dual transport channel for lithium ions. Especially with the cooperation of the second carbon source, it promotes the simultaneous insertion and extraction of lithium ions inside and on the surface of the lithium-carbon composite material, overcoming the defect that metallic lithium only inserts and extracts lithium ions on the surface, so that the bipolar electrode has better charge and discharge performance.

[0025] 4. When metallic lithium is removed, even under a small pressure or even without pressure, the exposed carbon nanotubes can still maintain the interface contact between the negative electrode layer and the solid electrolyte, ensuring the stable long-term operation of the all-solid-state battery.

[0026] 5. As the lithium extraction and insertion processes occur at the negative electrode, the second carbon source will redistribute at the interface between the negative electrode and the solid electrolyte layer. At the end of the cycle, the second carbon source will accumulate at the interface between the negative electrode and the solid electrolyte, which can not only promote the uniform extraction and deposition of lithium ions at the interface, but also physically prevent the growth of lithium dendrites.

[0027] 6. The second carbon source can also serve as an active site for metallic lithium. During the battery cycling process, the second carbon source filled in the carbon nanotube framework can ensure that the local current densities inside and outside the carbon nanotube framework are consistent, promoting more uniform extraction and deposition of metallic lithium.

[0028] 7. The role of the bare carbon nanotubes in maintaining the contact between the negative electrode layer and the solid electrolyte interface enables the bipolar electrode to get rid of the limitation that all-solid-state batteries must be used under relatively high external pressure, greatly promoting the practical application process of all-solid-state batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic structural diagram of the high-strength bipolar electrode prepared by the present invention.

[0030] Figure 2 It is a diagram showing the structural change of the interface between the high-strength bipolar electrode and the solid electrolyte after lithium extraction.

[0031] Figure 3 It is a diagram showing the structural change of the interface between the ordinary bipolar electrode and the solid electrolyte after lithium extraction.

[0032] Figure 4 It is a diagram showing the thickness change of the bipolar electrodes of each embodiment with the change of external pressure.

[0033] Figure 5 It is a schematic structural diagram of the all-solid-state battery assembled when testing the performance of the bipolar electrode of the present invention.

[0034] Figure 6 It is a diagram showing the correspondence between the capacity and the number of cycles of the all-solid-state batteries assembled using the bipolar electrodes of each embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] In order to make the objectives, technical solutions and advantages of the embodiments of the present disclosure clearer, the following further describes the embodiments of the present disclosure in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present disclosure, and are not used to limit the embodiments of the present disclosure.

[0036] Figure 1Schematic structural diagram of the high-strength bipolar electrode prepared according to the present invention. The high-strength bipolar electrode of the present invention includes a current collector layer 20, and a lithium-carbon composite material layer 10 and a positive electrode layer 30 respectively disposed on both sides of the current collector layer. Among them, in the lithium-carbon composite material layer 10, in addition to metallic lithium, carbon nanotubes 101 and a second carbon source 102 dispersed in the metallic lithium are also included. Figure 2 Schematic diagram of the interfacial structure change between the high-strength bipolar electrode and the solid electrolyte after lithium extraction. Figure 2 In it, the lithium-carbon composite material layer 10 of the high-strength bipolar electrode is in contact with the solid electrolyte 40. It can be seen from the figure that when holes appear at the interface between the lithium-carbon composite material layer 10 and the solid electrolyte 40, the carbon nanotubes in the lithium-carbon composite material can still maintain the interface contact between the negative electrode and the solid electrolyte. Moreover, the interior of the carbon nanotubes is hollow and can serve as a channel for lithium ion transmission, ensuring that metallic lithium can still be extracted or embedded at the hole position. However, the bipolar electrode prepared with pure metallic lithium cannot achieve this effect. Figure 3 Schematic diagram of the interfacial structure change between the ordinary bipolar electrode and the solid electrolyte after lithium extraction. Figure 3 And Figure 2 The difference from

[0037] Example 1

[0038] The LiNi8CoMnO2 positive electrode material, polyvinylidene fluoride and a conductive agent are homogenously coated on a 10-micron-thick stainless steel foil according to a mass fraction ratio of 95:2.5:2.5, and a unipolar positive electrode sheet is prepared by drying and rolling.

[0039] The carbon nanotubes are mixed with molten lithium to prepare a lithium-carbon composite material with a carbon nanotube content of 30%, and a lithium-carbon composite tape with a thickness of 20 microns is prepared by rolling. Through the rolling method, the lithium-carbon composite tape is adhered to the blank side of the above unipolar positive electrode sheet to prepare a bipolar electrode with a thickness of 50 microns.

[0040] Or first prepare a unipolar negative electrode sheet, and then coat the positive electrode active material on the blank side of the unipolar negative electrode sheet, and roll it after drying, and a 50-micron-thick bipolar electrode can also be prepared.

[0041] Example 2

[0042] Other steps are as described in Example 1, only changing the thickness of the lithium-carbon composite material to 5 microns.

[0043] Example 3

[0044] Other steps are the same as those in Example 1, except that the content of carbon nanotubes in the lithium-carbon composite material is changed to 8%.

[0045] Example 4

[0046] Other steps are the same as those in Example 1, except that the content of carbon nanotubes in the lithium-carbon composite material is changed to 48%.

[0047] Example 5

[0048] Other steps are the same as those in Example 1, and carbon black with a mass fraction of 8% is added to the lithium-carbon composite material.

[0049] Example 6

[0050] Other steps are the same as those in Example 1. Carbon nanotubes are not used, and the lithium-carbon composite material is prepared only using carbon black with a mass fraction of 8%.

[0051] Comparative Example 1

[0052] Other steps are the same as those in Example 1, and 20-micron pure lithium tape is used as the negative electrode.

[0053] Comparative Example 2

[0054] The LiNi8CoMnO2 cathode material, polyvinylidene fluoride, and conductive agent are homogenized and coated on a 10-micron-thick stainless steel foil in a mass ratio of 95:2.5:2.5, and a unipolar cathode sheet is prepared by drying and rolling.

[0055] The graphite material, binder, and conductive agent are homogenized and coated on the blank side of the above unipolar cathode sheet in a mass ratio of 95:2.5:2.5, and a bipolar electrode sheet with a thickness of 50 microns is prepared by drying and rolling.

[0056] Alternatively, a unipolar anode sheet of graphite anode can be prepared first, and then the positive active material is coated on the blank side of the unipolar anode sheet to prepare a bipolar electrode sheet with a thickness of 50 microns.

[0057] The lithium-carbon composite tape used in Examples 1 to 4 and the pure lithium tape used in Comparative Example 1 are prepared into tensile test specimens. The specimen size is 43 mm wide and 58 mm long, and three specimens are prepared for each material. The tensile test of the specimens is carried out using a tensile tester (manufactured by Jinan Chenxin Testing Machine Co., Ltd.), and the test results are shown in Table 1.

[0058] Table 1. Tensile test results of lithium-carbon composite materials and pure lithium tapes

[0059]

[0060] As can be seen from Table 1, when the thickness is 20 microns in all cases, the average maximum tensile force of the lithium-carbon composite with 30% carbon nanotube content is 2.23 kgf, while that of the pure lithium strip is only 0.62 kgf. This shows that adding carbon nanotubes to the pure lithium strip can significantly enhance the mechanical strength of metallic lithium. As the content of carbon nanotubes added to metallic lithium increases, the maximum tensile force value of the lithium-carbon composite continuously increases, and after adding the second carbon source (such as in Example 5), it is also beneficial to enhance the tensile strength of the lithium-carbon composite. When 48% carbon nanotubes are added to pure lithium, the maximum tensile force value of the 20-micron-thick lithium-carbon composite strip increases to 6.86 kgf, which is 10 times the maximum tensile force value of the pure metallic lithium strip of the same thickness.

[0061] Take the bipolar electrodes prepared in the above examples and comparative examples, punch and prepare circular pieces with a diameter of 14 mm, and use a BER2500 (Yuaneng Technology) deformation tester to conduct pressure tests on the above circular pieces, record the changes in pressure and thickness and plot them, as Figure 4 shown. The deformation amounts of the bipolar electrode thicknesses in each example and comparative example at different pressures are statistically listed in Table 2. At a specific pressure, the deformation amount of the bipolar electrode thickness = (the initial thickness of the bipolar electrode - the thickness of the electrode under the specific pressure) * 100% / the initial thickness of the bipolar electrode.

[0062] Table 2 Deformation amounts of bipolar electrode thicknesses with pressure in each example and comparative example

[0063]

[0064] From Figure 4 and Table 2, it can be seen that as the applied pressure increases, the thicknesses of the bipolar electrodes in each example continuously decrease. When the external pressure increases to 300 Mpa, the deformation amount of the thickness of the bipolar electrode prepared with the lithium-carbon composite in Example 1 is 24%, which is comparable to that of the bipolar electrode prepared with the graphite negative electrode. For the bipolar electrode prepared with pure metallic lithium, its deformation amount of the thickness reaches 38%, and the soft metallic lithium undergoes creep, resulting in a sharp increase in the deformation amount of the bipolar electrode. Although the lithium-carbon composite was used to prepare the bipolar electrode in Example 3, due to the low content of carbon nanotubes, the strength of the prepared bipolar electrode is insufficient, and there is still a large deformation amount of the thickness of the bipolar electrode.

[0065] Assemble all-solid-state batteries using the bipolar electrodes prepared in each example, and use the sulfide solid electrolyte Li7P3S 11 , and assemble in the all-solid-state battery test fixture as Figure 5The all-solid-state battery shown is tested at an external pressure of 200 Mpa. During the actual assembly process, if higher output voltage and energy density are required, multiple bipolar electrodes can be connected in series inside the battery. In order to compare the performance of bipolar electrodes in each embodiment, only one set of bipolar electrodes is provided inside the all-solid-state battery in the present invention. The assembled all-solid-state battery is subjected to a cycle performance test, and the cycle performance is as Figure 6 shown. As can be seen from the figure, the all-solid-state battery assembled with the high-strength bipolar sheet prepared in Example 1 has a capacity decay to 1000 mAh after 150 cycles, while the all-solid-state battery assembled with the pure metal lithium bipolar electrode has a capacity decay to 1000 mAh when the battery is cycled to 52 weeks. The all-solid-state battery prepared with the high-strength bipolar negative electrode has stable cycling and a longer life. And the experiment also proves that using a bipolar electrode prepared with a lithium-carbon composite material with a high carbon nanotube content, such as Example 4, can significantly improve the cycle life of the all-solid-state battery. In addition, it is worth noting that the introduction of the second carbon source can significantly improve the discharge stability of the battery, such as the all-solid-state battery assembled in Example 5. This is because the second carbon source can serve as an active site for metallic lithium. During the battery cycling process, the second carbon source filled in the carbon nanotube framework can ensure that the local current density inside and outside the carbon nanotube framework is consistent, promoting more uniform extraction and deposition of metallic lithium. Therefore, the discharge stability of the battery is significantly improved after the introduction of the second carbon source. However, it can also be seen that the cycle life of the battery with the introduction of the second carbon source has not been effectively improved because the introduction of the second carbon source increases the reaction interface of the electrode and at the same time increases the side reactions at the interface.

[0066] It can be understood that in the embodiments of the present invention, although a high-strength bipolar electrode and its preparation method of the present invention are described in detail in combination with specific embodiments, however, the above is only a description made to meet legal requirements, and the present invention is not limited to the given embodiments. Those skilled in the art can complete the replication of the high-strength bipolar electrode through appropriate operations according to the disclosure and teachings of the specification.

[0067] According to the disclosure and teachings of the above specification, those skilled in the art to which the present invention pertains can make appropriate changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A bipolar pole piece, characterized in that, Comprising: A current collector layer; And A positive electrode layer and a lithium-carbon composite material layer disposed on both sides of the current collector layer, wherein the lithium-carbon composite material layer contains metallic lithium and carbon nanotubes with a mass fraction of 9% to 65%, and optionally, further contains a second carbon source with a mass fraction of less than 10%.

2. The bipolar pole piece according to claim 1, characterized in that, The bipolar pole piece has a thickness of 2 μm to 500 μm, wherein the current collector layer has a thickness of 20 nm to 20 μm.

3. The bipolar electrode plate according to claim 1, characterized in that, The active material in the positive electrode layer is at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium cobalt oxide, lithium iron phosphate, lithium titanate, lithium manganate, lithium nickelate, elemental sulfur, oxygen cathode material, sulfonated polyacrylonitrile, and iron disulfide.

4. The bipolar pole piece according to claim 1, characterized in that, The lithium-carbon composite material layer has a thickness of 1 μm to 100 μm.

5. The bipolar pole piece according to claim 1, wherein The second carbon source in the lithium-carbon composite material layer is at least one of carbon black, graphene, whisker carbon nanotubes, graphite, mesophase carbon microspheres, and metal carbides.

6. The bipolar pole piece according to claim 1, characterized in that, The secondary particle size of the carbon material in the lithium-carbon composite material layer is less than 30 μm.

7. The bipolar electrode plate according to claim 1, wherein The material of the current collector layer includes at least one of copper foil, tin foil, nickel foil, stainless steel foil, conductive carbon paper with a metal coating on the surface, and conductive plastic with a metal coating on the surface.

8. The bipolar pole piece according to claim 1, characterized in that, The bipolar pole piece has a thickness deformation lower than 15% of the initial thickness under a pressure of 100 Mpa, or a thickness deformation lower than 25% of the initial thickness under a pressure of 300 Mpa.

9. A method for preparing a bipolar electrode sheet as described in any one of claims 1 to 8, characterized in that, Including the following steps: S1. Construct a unipolar pole piece Coat the positive electrode active material on the current collector layer to prepare a unipolar positive electrode piece, or roll the lithium-carbon composite material onto the current collector layer to prepare a unipolar negative electrode piece; S2. Construct a bipolar pole piece Roll the lithium-carbon composite material layer on the blank side of the unipolar positive electrode piece constructed in S1 to prepare a bipolar pole piece; or coat the positive electrode active material on the blank side of the unipolar negative electrode piece, and roll it after drying to form a bipolar pole piece.

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