Pole piece, preparation method thereof, secondary battery, and electric device

Abstract

This application provides an electrode sheet and its preparation method, a secondary battery, and an electrical device thereof. The electrode sheet includes a current collector and a safety coating disposed on at least one surface of the current collector. The safety coating, on its surface facing away from the current collector, also has an electrode active material layer. The safety coating includes a binder, a conductive agent, an active material, and halloysite nanotubes, with lithium-containing materials loaded within the lumens of the halloysite nanotubes. This electrode sheet not only improves the safety performance of the battery but also compensates for irreversible lithium-ion loss, thereby increasing the battery's energy density.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to electrode sheets and their preparation methods, secondary batteries and electrical equipment. Background Technology

[0002] As market demands for the safety performance of rechargeable batteries increase, major battery manufacturers are increasingly opting to pre-coat safety surfaces on the electrodes to address safety concerns such as short circuits, thermal runaway, and even fires and explosions that can occur when batteries are punctured or compressed. The safety coating can be placed between the current collector and the active material layer. When a foreign object punctures the active material layer, the safety coating can encapsulate the object, preventing direct contact between the object and the current collector, thus improving battery safety. Understandably, the thicker the safety coating, the better the effect. However, the introduction of a safety coating also leads to a significant reduction in battery energy density. Therefore, there is an urgent need to develop an electrode that can provide sufficient safety performance while also improving battery energy density. Summary of the Invention

[0003] In view of this, this application provides an electrode that can not only improve the safety performance of the battery, but also compensate for irreversible lithium-ion loss in the battery, thereby improving the energy density of the battery.

[0004] The first aspect of this application provides an electrode sheet, including a current collector and a safety coating disposed on at least one side surface of the current collector, wherein the safety coating on the side surface opposite to the current collector is further provided with an electrode active material layer;

[0005] The safety coating includes an adhesive, a conductive agent, an active material, and halloysite nanotubes, wherein the lumen of the halloysite nanotubes is loaded with a lithium-containing material.

[0006] When the aforementioned electrode is applied to a battery, the lithium-containing material loaded in the halloysite nanotubes gradually decomposes during battery charging to generate lithium ions. These lithium ions migrate to the negative electrode to participate in the formation of the solid electrolyte membrane or are directly deposited on the negative electrode, thereby compensating for irreversible loss of active ions and improving the battery's energy density. Furthermore, after the lithium salt is removed, the basic structure of the safety coating remains unchanged. When the battery is punctured or squeezed by a foreign object during subsequent use, the halloysite nanotubes will slip, and the binder will contact the active material before the foreign object, forming a protective layer that cuts off the short-circuit current and prevents thermal runaway, thus significantly improving the battery's safety performance. If the battery suffers a severe impact and short-circuit heat generation, the halloysite nanotubes, with their heat insulation, flame retardant properties, and low coefficient of thermal expansion, will decompose / peel from the current collector due to the significant difference in their thermal expansion rates. This will significantly increase the internal resistance and prevent thermal runaway, further enhancing the battery's safety performance.

[0007] Therefore, the aforementioned electrode sheets can be used to provide batteries that combine high safety performance and high energy density.

[0008] A second aspect of this application provides a method for preparing an electrode sheet, comprising:

[0009] A safety coating is formed on at least one side of the current collector. After drying, an active material layer is formed on the surface of the safety coating. After drying, an electrode is obtained.

[0010] The safety coating includes an adhesive, a conductive agent, an active material, and halloysite nanotubes, wherein the lumen of the halloysite nanotubes is loaded with a lithium-containing material.

[0011] The above preparation method has simple steps and high process reliability, making it suitable for large-scale industrial production.

[0012] A third aspect of this application provides a secondary battery, including the electrode provided in the first aspect of this application or the electrode prepared by the method provided in the second aspect of this application. Due to the use of the aforementioned electrode, this secondary battery can possess both high safety performance and high energy density.

[0013] The fourth aspect of this application provides an electrical device, including the secondary battery provided in the third aspect of this application. Attached Figure Description

[0014] Figure 1 This is a simplified schematic diagram of a variation of the electrode sheet in an embodiment of this application;

[0015] Figure 2 This is a simplified schematic diagram of another variation of the electrode sheet in the embodiments of this application;

[0016] Figure 3 These are scanning electron microscope (SEM) images of halloysite nanotubes loaded with lithium-containing materials before and after in Example 1 of this application;

[0017] Figure 4 This is the temperature-time curve of the battery in some embodiments of this application during the nail penetration test.

[0018] Labeling explanation: 100-Electrode; 10-Current collector; 20-Safety coating; 30-Electrode active material layer; 21-First sublayer; 22-Second sublayer. Detailed Implementation

[0019] Currently, to address the issue of thermal runaway in batteries caused by foreign object puncture or compression, the industry often incorporates safety coatings into the electrode plates. Commonly used safety coatings typically consist of boehmite, lithium iron phosphate, a conductive agent, and a binder. Lithium iron phosphate conducts active ions, while the conductive agent conducts electrons to maintain normal battery operation. The low conductivity and high-temperature resistance of boehmite contribute to improved battery safety. However, this structure leads to a loss of battery energy density, significantly reducing the market competitiveness of these batteries.

[0020] To address the aforementioned problems, this application provides an electrode 100, which can be found in the embodiments described above. Figures 1-2 The electrode 100 includes a current collector 10 and a safety coating 20 disposed on at least one side surface of the current collector 10. The side surface of the safety coating 20 facing away from the current collector 10 is also provided with an electrode active material layer 30.

[0021] The safety coating 20 comprises an adhesive, a conductive agent, an active material, and halloysite nanotubes, with lithium-containing materials loaded within the lumens of the halloysite nanotubes. The outer surface of the halloysite tube has a relatively smooth silicon-oxygen tetrahedral structure, while the inner wall, both ends of the nanotube, and defect sites contain a certain number of aluminum or silanol groups. Therefore, the inner wall of its lumen exhibits strong polarity. Combined with its large cavity structure, this allows for the fixation of decomposable lithium salts within the cavity. Furthermore, halloysite nanotubes possess high flame retardancy, a low coefficient of thermal expansion (1%-5%), and are widely available. It is understood that in this application, a very small amount of the aforementioned self-decomposing lithium salt may also be dispersed on the surface of the halloysite nanotubes.

[0022] After the electrode 100 is assembled into the battery, during the charging process, the lithium-containing material gradually decomposes to generate lithium ions. These lithium ions migrate to the negative electrode to participate in the formation of the SEI film or are directly deposited in the pores of the negative electrode as "pre-stored lithium," thereby compensating for the irreversible loss of active ions and improving the battery's energy density. After the lithium-containing material is removed, the halloysite nanotubes maintain their original structure due to their rigidity and structural stability. Therefore, the structure of the safety coating 20 remains essentially unchanged. When the battery is punctured or squeezed, the foreign object passes through the electrode active material layer 30 and comes into contact with the safety coating 20. The halloysite nanotubes then slip, causing the binder and active material to encapsulate the foreign object, increasing the resistance to cut off the short-circuit current and thus preventing thermal runaway or even fire and explosion. If the battery suffers a severe impact and short-circuit heat generation, halloysite nanotubes have certain heat insulation and flame retardant properties that can slow down the heat generation process and suppress heat overflow. In addition, the thermal expansion coefficient is small, and its volume will not change much. However, the binder expands rapidly when heated, causing the safety coating 20 to delaminate / peel from the current collector 10, thereby drastically increasing the internal resistance and preventing thermal runaway. Therefore, the safety performance of the battery can be further improved.

[0023] In some embodiments of this application, the lithium-containing materials mentioned above include, but are not limited to, lithium salts and lithium metal. The lithium salts include, but are not limited to, at least one of lithium sulfide, lithium nitride, lithium fluoride, lithium oxide, lithium oxalate, and lithium peroxide. These substances have suitable decomposition voltages and can be completely decomposed into lithium ions during battery charging. Other elements in the lithium salt can escape as gases or participate in the formation of the SEI film along with the lithium ions. In some cases, lithium sulfide can be selected. Lithium sulfide has a delithiation potential of 3.5V, which is more universal; in other cases, when adapted to high-voltage batteries, for example, 4.48V, 4.5V (vs Li... +The lithium oxide, lithium fluoride, lithium oxide, lithium oxalate, and lithium peroxide can be selected from lithium nitride, lithium fluoride, lithium oxide, lithium oxalate, and lithium peroxide. In some embodiments of this application, the safety coating 20 has a first sub-layer and a second sub-layer stacked together. The first sub-layer is disposed close to the current collector 10. The active material includes a first active material and a second active material; the binder includes a first binder and a second binder; and the conductive agent includes a first conductive agent and a second conductive agent. The first sub-layer 21 includes halloysite nanotubes loaded with lithium-containing material, the first active material, the first binder, and the first conductive agent. The second sub-layer 22 includes a binder, a second active material, a second binder, and a second conductive agent. The first active material and the second active material can be the same; the first binder and the second binder can be completely identical or different. The first conductive agent and the second conductive agent can be completely identical or different. Thus, halloysite nanotubes are concentrated near the current collector 10, allowing foreign objects to penetrate the electrode active material layer 30 of the electrode 100 and contact the second sub-layer 22 before reaching it. The second conductive agent and second binder in the second sub-layer 22 can react more rapidly, encapsulating the foreign object and buffering direct contact between the foreign object and the first sub-layer, thus reducing / interrupting the short-circuit current. Furthermore, the first sub-layer 21 has higher density and better mechanical properties, providing more mechanical support to the current collector 10 while maintaining the same mass percentage of the safety coating 20. In addition, in some cases, lithium-containing materials may be attached to the outer surface of the halloysite nanotubes. The mass percentage of this lithium-containing material is very small, and its decomposition may create a few vacancies in the safety coating 20. The binder in the second sub-layer can fill these vacancies, further improving the structural stability of the safety coating 20 during charging and discharging, thereby enhancing the overall performance of the battery.

[0024] In some embodiments of this application, the aforementioned active materials include, but are not limited to, at least one of lithium nickel oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, and lithium manganese iron phosphate. These active materials possess suitable ionic conductivity and high safety performance, which is beneficial to the battery's cycle performance, rate performance, and safety performance.

[0025] In some embodiments of this application, the adhesive in the safety coating 20 may be selected from materials well known to those skilled in the art. Exemplarily, the adhesive includes, but is not limited to, at least one of polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyvinyl alcohol, and polyurethane.

[0026] In some embodiments of this application, a portion of the binder is in situ coated onto the halloysite nanotubes. Specifically, the first binder is in situ coated onto the halloysite nanotubes. The first binder can be coated on its surface, or it can be partially attached to its outer wall and partially located in its inner cavity. In this way, the binder is more uniformly dispersed in the safety coating 20, which is more conducive to the timely and thorough peeling of the safety coating 20 from the current collector 10 by the difference in thermal expansion rates between the binder and the halloysite nanotubes when the battery generates heat, thereby preventing thermal runaway more promptly.

[0027] In some embodiments of this application, the conductive agent in the safety coating 20 may be selected from materials well known to those skilled in the art. Exemplarily, the conductive agent may include, but is not limited to, at least one of acetylene black, Ketjen black, graphene, and carbon nanotubes. In some specific embodiments of this application, the conductive agent includes carbon nanotubes. When carbon nanotubes are selected as the conductive agent, in some cases, some carbon nanotubes may be inserted approximately vertically into the gaps between the laid halloysite nanotubes. The aforementioned approximately vertical insertion means that "the angle between the carbon nanotube and the thickness direction of the electrode sheet is less than or equal to 15°," thereby further enhancing the electronic conductivity of the safety coating 20.

[0028] In some embodiments of this application, the surface of halloysite nanotubes is further modified with a silane coupling agent. In some specific embodiments, the safety coating 20 has a first sublayer 21 and a second sublayer 22 stacked together. The first sublayer 21 includes halloysite nanotubes loaded with lithium-containing material, a first binder, a first conductive agent, and a first active material, and the surface of the halloysite nanotubes is further modified with a silane coupling agent. This facilitates the dispersion of the binder, and some of the binder can form chemical crosslinks with the halloysite nanotubes. When the battery generates heat, it is more conducive to the delamination / peeling of the safety coating and the current collector, thereby further improving the safety performance of the battery.

[0029] In some embodiments of this application, halloysite nanotubes exist in the form of an aerogel in the safety coating 20. For example, the safety coating has a polyvinyl alcohol aerogel doped with halloysite nanotubes, and the lumen of the halloysite nanotubes is loaded with a lithium-containing material.

[0030] In some embodiments of this application, the angle between the halloysite nanotubes and the current collector 10 along a first direction of the electrode 100 is less than or equal to 15°; wherein, the first direction is perpendicular to the thickness direction of the electrode 100. In this case, the halloysite nanotubes are almost "laid out" on the current collector 10. The rigid halloysite nanotubes can form one or more rigid network structures, which can further enhance the mechanical properties of the electrode 100 and greatly alleviate the problem of edge curling of the electrode sheet (e.g., the positive electrode sheet) caused by the impact and shrinkage of the separator in the battery. Therefore, it can effectively prevent safety problems caused by direct reaction between the positive electrode sheet and the negative electrode active material. Furthermore, when the battery is subjected to lateral foreign object compression (the foreign object is compressed in a direction parallel to the current collector 10), the rigid network structure will directly contact the laterally placed foreign object, thereby further reducing the risk of direct contact between the foreign object and the current collector 10, and greatly reducing the risk of a large-area instantaneous short circuit in the battery. For example, the angle between the halloysite nanotubes and the current collector 10 can be 0, 0.2°, 0.5°, 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9°, 10°, 11°, 12°, 13°, 14°, etc. In this application, the electrode can be sliced ​​to expose its cross-section (the cross-section is perpendicular to the first direction mentioned above), and the angle between the halloysite nanotubes and the current collector in the safety coating can be observed under a scanning electron microscope. In some specific embodiments of this application, at least some of the halloysite nanotubes are parallel to the first direction of the current collector 10 (that is, at least some of the halloysite nanotubes have an angle of 0° with the current collector in the first direction). In this case, the effect of the rigid network structure mentioned above is more obvious, and the safety performance of the battery can be further optimized.

[0031] In some embodiments of this application, the thickness of the first sublayer is in the range of 2μm-6μm; the thickness of the second sublayer is in the range of 1μm-4μm. Exemplarily, the thickness of the first sublayer can be 2μm, 2.2μm, 2.5μm, 2.8μm, 3μm, 3.2μm, 3.5μm, 3.8μm, 4μm, 4.2μm, 4.5μm, 4.8μm, 5μm, 5.2μm, 5.5μm, etc. Exemplarily, the thickness of the second sublayer can be 1μm, 1.2μm, 1.5μm, 1.8μm, 2μm, 2.2μm, 2.5μm, 2.8μm, 3μm, 3.5μm, 3.8μm, etc. In some embodiments of this application, the thickness of the safety coating 20 is 3μm-10μm. For example, the thickness of the safety coating 20 can be 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, etc. This allows the final battery to have high safety performance while maximizing its energy density.

[0032] In some embodiments of this application, the thickness ratio of the first sublayer to the second sublayer is 1:(0.3-0.6). Exemplarily, the thickness ratio of the first sublayer to the second sublayer can be 1:0.3, 1:0.35, 1:0.4, 1:0.45, 1:0.5, 1:0.55, 1:0.6, etc. In this way, the rigidity of the first sublayer can be further improved without changing the total thickness and overall composition of the safety coating 20, thus facilitating further improvement of battery safety performance.

[0033] In some embodiments of this application, the mass percentage of lithium-containing material is 1%-10%, based on the total mass of halloysite nanotubes loaded with lithium-containing material. Exemplarily, the mass percentage of lithium-containing material can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc. This ensures sufficient lithium salt to compensate for the irreversible loss of active ions during battery formation, while also preventing large amounts of lithium salt from causing structural deformation within the halloysite nanotubes and affecting the structure of the safety coating 20 after lithium salt decomposition.

[0034] In some embodiments of this application, in the safety coating 20, the mass ratio of halloysite nanotubes loaded with lithium-containing material, active material, binder, and conductive agent is 1:(0.2-4):(0.02-1.5):(0.02-1.5). Exemplarily, the mass ratio of halloysite nanotubes loaded with lithium-containing material, active material, binder, and conductive agent can be 1:(1-4):(0.1-1.5):(0.1-1.5), 1:(1.5-3.5):(0.5-1.2):(0.5-1.2), 1:(2-3):(0.8-1.2):(0.8-1.2), etc. Thus, when the electrode 100 comes into contact with a foreign object, there is sufficient active material and binder to encapsulate the foreign object, and the electrode 100 also has sufficient halloysite nanotubes to enhance the mechanical strength of the electrode 100, further preventing thermal runaway, and benefiting the cycle performance and rate performance of the battery.

[0035] In some embodiments of this application, the length of the halloysite nanotubes is 50 nm to 3000 nm. Exemplarily, the length of the halloysite nanotubes can be 50 nm, 60 nm, 80 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1200 nm, 1500 nm, 1800 nm, 2000 nm, 2200 nm, 2500 nm, 2800 nm, etc. In some specific embodiments, the length of the halloysite nanotubes is 800-1000 nm. Thus, the halloysite nanotubes have more suitable rigidity, are easier to slide, and are easier to "lay out," further improving the mechanical properties of the electrode 100 and facilitating further resistance to thermal runaway.

[0036] In some embodiments of this application, the halloysite nanotubes have an inner diameter of 10nm-50nm and an outer diameter of 40nm-100nm. In some specific embodiments, the halloysite nanotubes have an inner diameter of 30-40nm and an outer diameter of 40-60nm. Exemplarily, the inner diameter of the halloysite nanotubes can be 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, etc. Exemplarily, the outer diameter of the halloysite nanotubes can be 40nm, 45nm, 50nm, 55nm, 60nm, 65nm, 70nm, 75nm, 80nm, etc. This is more conducive to the loading of lithium-containing materials and to the dispersion of halloysite nanotubes in the safety coating 20.

[0037] In some embodiments of this application, the lithium-containing material is in the form of nanoparticles. This allows for better loading within halloysite nanotubes and facilitates smooth decomposition during battery formation.

[0038] In some embodiments of this application, the Dv50 particle size of the active material is in the range of 50nm-1000nm. In some specific embodiments, the Dv50 particle size of the active material is in the range of 50nm-500nm. In some specific embodiments, the Dv50 particle size of the active material is in the range of 50nm-300nm. Exemplarily, the Dv50 particle size of the active material can be 50nm, 80nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, 600nm, 700nm, 800nm, 900nm, etc. By controlling the Dv50 particle size of the active material within the above range, it can better fill / disperse in the gaps between halloysite nanotubes together with the binder particles, thereby improving the density and mechanical strength of the safety coating 20. Furthermore, when the battery is damaged, it can quickly encapsulate foreign objects together with the binder particles, thereby further improving the battery's safety performance and volumetric energy density.

[0039] In this application, the electrode active material in the electrode active material layer 30 of the electrode sheet 100 can be selected from materials well known to those skilled in the art. The electrode active material can be the same as or different from the active material in the safety coating 20. For example, when the active material in the safety coating 20 is selected from at least one of lithium nickelate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, the electrode active material in the electrode active material layer 30 can be the same as the active material in the safety coating 20.

[0040] Accordingly, embodiments of this application also provide a method for preparing an electrode, comprising:

[0041] S1. A safety coating is formed on at least one side of the current collector. After drying, an electrode active material layer is formed on the surface of the safety coating. After drying, an electrode sheet is obtained.

[0042] The safety coating includes an adhesive, a conductive agent, an electrode active material, and halloysite nanotubes, wherein the lumen of the halloysite nanotubes is loaded with a lithium-containing material.

[0043] The above preparation method has simple steps and high process reliability, making it suitable for large-scale industrial production.

[0044] In some embodiments of this application, in step S1, forming a safety coating on at least one side surface of the current collector includes the following steps:

[0045] S11. A first slurry is coated onto at least one side surface of the current collector; wherein the first slurry comprises a binder, a conductive agent, and the halloysite nanotubes, the lumens of which are loaded with lithium-containing material; further, in some specific embodiments, after coating the first slurry, the current collector coated with the first slurry is further rolled, and the thickness of the first slurry layer after rolling is controlled to be 70%-80% of the thickness before rolling, so as to further improve the flatness of the halloysite nanotubes (reduce their angle with the current collector in the first direction). Even further, before coating the first slurry, the current collector is subjected to corona treatment. In some embodiments, after coating the surface of the current collector with the first slurry, it is further dried. In some specific embodiments of this application, after the above drying, it is further rolled.

[0046] S12. Provide a second slurry, the second slurry comprising a binder, a conductive agent, and the active material; coat the current collector surface with the dried first slurry, such that at least a portion of the active material settles into the first slurry to form a first sublayer together with the first slurry, and forms a second sublayer on the surface of the first sublayer.

[0047] Compared to the slurry obtained by directly mixing all the raw materials of the safety coating, the solid content of the first and second slurries can be adjusted to be relatively small. In particular, when the solid content of the first slurry is reduced, halloysite nanotubes can be spread evenly on the surface of the current collector when it is coated on the current collector surface (for the definition of spreading, please refer to the previous description). When the second slurry is then coated on the upper layer, the binder, conductive agent, and active material will settle to the lower layer and fill the gaps between the halloysite nanotubes, forming a high-density first sublayer together with the first slurry. In this case, the portion of the settled active material in the product is called the "first active material". In some embodiments of this application, some active material will remain in the second slurry and become part of the second sublayer after drying. In this case, the portion of the unsettled active material in the product is called the "second active material". In some specific embodiments of this application, a binder, a conductive agent, and halloysite nanotubes are dispersed in a solvent and stirred for 10-48 hours to ensure the binder is uniformly dispersed on the surface of the halloysite nanotubes, resulting in a first slurry. The solid content of the first slurry is 18%-20%. The binder, conductive agent, and active material are then dispersed in a solvent and stirred for 5-24 hours to obtain a second slurry. The solid content of the second slurry is 32%-50%. This facilitates the formation of the first and second sublayers and allows the halloysite nanotubes to spread evenly on the current collector surface, thereby improving the density and mechanical properties of the first sublayer.

[0048] In some embodiments of this application, the total mass of the binder in the first slurry and the second slurry is used as a basis, and the mass percentage of the binder in the first slurry is 2%-4%.

[0049] In some embodiments of this application, the total mass of the conductive agent in the first slurry and the second slurry is used as a basis, and the mass percentage of the conductive agent in the first slurry is 0.5%-0.8%.

[0050] In some embodiments of this application, the preparation of halloysite nanotubes loaded with lithium-containing materials includes:

[0051] (1) Immerse halloysite nanotubes in a lithium-containing solution, sonicate for 1-5 hours using a cell disruptor, and continue stirring for 4-24 hours to obtain a suspension;

[0052] (2) The suspension was pre-dried at 50-90℃ for 2-24h, then kept at 100-150℃ for 2-24h, and then vacuum dried at 150-180℃ for 2h-24h to obtain halloysite nanotubes loaded with lithium-containing materials.

[0053] In step (1) above, the halloysite nanotube contains aluminum hydrogen hydroxyl and silicon hydrogen hydroxyl groups, and these groups are connected to water molecules through hydrogen bonds. This structure can attract highly polar lithium-containing materials into its inner cavity. Understandably, a very small amount of lithium salt will also adhere to the surface of the halloysite nanotube. After the heating and dehydration treatment in step (2), the lithium salt particles grow and transfer, gradually increasing in size, and are finally sealed in the halloysite nanotube.

[0054] In some specific embodiments of this application, before step (1) above, the halloysite nanotubes are further purified. Specifically, the natural halloysite nanotubes may be repeatedly washed with 0.5% hydrochloric acid, rinsed with water, and dried. Alternatively, they may be highly purified, including: adding the natural halloysite nanotubes to a solution of deionized water and 0.05% sodium hexametaphosphate (the mass of 0.05% sodium hexametaphosphate is equivalent to the mass of the halloysite nanotubes), stirring at 60°C for 2 hours, letting stand for 40 minutes, performing solid-liquid separation, taking the upper emulsion, sieving (200 mesh), repeating 3-5 times, and then washing and drying to obtain the halloysite nanotubes. In some specific embodiments of this application, the natural halloysite nanotubes are subjected to the above-mentioned highly purified and simple purified processes in sequence. In this way, the inner diameter of the original natural halloysite nanotubes can be further expanded, which is more conducive to the loading of lithium-containing materials.

[0055] This application also provides a secondary battery, including the electrode provided in this application or the electrode prepared by the aforementioned method. Due to the use of the electrode provided in this application, the secondary battery can possess both high safety performance and high energy density.

[0056] For example, the above-mentioned electrode can be used as a positive electrode.

[0057] In this application, the aforementioned secondary battery can be a liquid battery with an electrolyte, a solid battery with a solid electrolyte, or a semi-solid battery with a semi-solid electrolyte.

[0058] In some embodiments of this application, the aforementioned electrode sheet is used as the positive electrode sheet of a battery. In this case, the positive active material in the positive active material layer of the electrode sheet can be selected from commonly used positive active materials for lithium-ion batteries in the field. Exemplarily, the positive active material includes, but is not limited to, at least one of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, nickel-cobalt-aluminum ternary materials (NCA) and nickel-cobalt-manganese ternary materials (NMC), lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, and lithium manganese silicate.

[0059] In this application, the negative electrode of the secondary battery may be any negative electrode known in the art.

[0060] This application also provides an electrical device, including the secondary battery provided in this application embodiment. Because it uses the aforementioned secondary battery to power the electrical device, the electrical device provided in this application embodiment has strong market competitiveness.

[0061] In some embodiments of this application, the aforementioned electrical equipment includes, but is not limited to, 3C electronic products, powered vehicles, etc.

[0062] The technical solution of this application is further described below with reference to several embodiments.

[0063] Example 1

[0064] (1) Add 150g of natural halloysite nanotube clay to a 2000mL beaker, add 1500mL of deionized water and sodium hexametaphosphate equivalent to 0.05% of the halloysite nanotube dosage, stir continuously at 60℃ for 2 hours, let stand for 40min, pour off the upper white emulsion, and filter with a 200-mesh molecular sieve. Repeat this operation 3-5 times, centrifuge the obtained white emulsion, and place the white solid in a 60℃ forced-air drying oven to dry for 24h to obtain highly purified halloysite nanotubes. Then wash the above highly purified halloysite nanotubes multiple times with 0.5% hydrochloric acid, let stand, take the upper turbid liquid, filter, wash and dry multiple times to obtain halloysite nanotubes for use, with a tube length of 1000nm, a tube inner diameter of 35nm, and a nanotube outer diameter of 50nm.

[0065] (2) Loading of lithium-containing materials: 6g of Li2S was dispersed in 400ml of anhydrous ethanol and stirred for 12h at room temperature and humidity below 5% until Li2S was completely dissolved and a yellow clear solution was formed. The upper clear solution was taken and 60g of the above halloysite nanotubes were added. The solution was sonicated for 1h using a cell disruptor and then stirred for 12h. The solution prepared in the above steps was pre-dried at 80℃, then kept at 120℃ for 24h, and then vacuum dried at 200℃ for 12h to obtain halloysite nanotubes loaded with lithium-containing material Li2S, wherein the proportion of Li2S was 10%.

[0066] (3) Take 45g of halloysite nanotubes loaded with Li2S, 48g of active material (specifically, lithium iron phosphate with a Dv50 particle size of 480nm), 1g of conductive agent (specifically, carbon nanotubes, CNT) and 6g of binder (specifically, polyvinylidene fluoride, PVDF) and add them to a solvent (specifically, N-methylpyrrolidone, NMP) to disperse them. After stirring evenly, coat them on the surface of the current collector (specifically, aluminum foil), dry and roll.

[0067] (4) The positive electrode active material (specifically lithium nickelate), conductive agent (specifically CNT) and binder (specifically PVDF) were mixed in a mass ratio of 100:0.7:0.6 and dispersed in NMP. After stirring evenly, a positive electrode slurry was obtained. The above positive electrode slurry was coated on the surface of the safety coating, dried and rolled to obtain the electrode sheet of Example 1. After the safety coating was finally made into a battery, it was observed by slicing. The thickness of the safety coating was about 5 μm, and the lithium element in the safety coating accounted for 9% of the total mass of halloysite nanotubes.

[0068] Example 2

[0069] The difference from Example 1 is that, after the battery was finally manufactured, the thickness of the safety coating was approximately 9 μm when observed in cross-section.

[0070] Example 3

[0071] The difference from Example 1 is that, after the battery was finally manufactured, the thickness of the safety coating was approximately 3 μm when observed in cross-section.

[0072] Example 4

[0073] The difference from Example 1 is that the mass ratio of halloysite nanotubes, active materials, binders and conductive agents is 1:4:1.5:1.5.

[0074] Example 5

[0075] The difference from Example 1 is that the halloysite nanotube has a tube length of 50 nm, an inner diameter of 10 nm, and an outer diameter of 40 nm.

[0076] Example 6

[0077] The difference from Example 1 is that the halloysite nanotube has a tube length of 3000 nm, an inner diameter of 50 nm, and an outer diameter of 100 nm.

[0078] Example 7

[0079] The difference from Example 1 is that step (3) uses a two-step coating method, specifically prepared according to the following method:

[0080] (3-1) Take 45g of halloysite nanotubes loaded with Li2S, 0.5g of conductive agent (specifically carbon nanotubes, CNTs) and 4g of binder respectively and add them to a solvent (specifically NMP). After stirring, a first slurry is obtained. The first slurry is coated on the surface of aluminum foil.

[0081] (3-2) Take 48g of active material (specifically, lithium iron phosphate with a D50 particle size of 480nm), 0.5g of conductive agent (specifically CNT) and 2g of binder respectively, add them to solvent (specifically NMP) and disperse them. After stirring evenly, a second slurry is obtained. The second slurry is coated on the surface of the first slurry layer and dried to obtain a safety coating.

[0082] In the final electrode, the safety coating includes a first sub-layer and a second sub-layer stacked together. After the battery is finally manufactured, a cross-section is observed. The thickness of the first sub-layer is about 2.5 μm, the thickness of the second sub-layer is about 2.5 μm, and the total thickness is about 5 μm.

[0083] Example 8

[0084] The only difference from Example 7 is that after the electrode is finally made into a battery, when it is sliced ​​and observed, the thickness of the first sublayer is about 7.7 μm, the thickness of the second sublayer is about 2.3 μm, and the total thickness is about 10 μm.

[0085] Example 9

[0086] The only difference from Example 7 is that after the electrode is finally made into a battery, when it is sliced ​​and observed, the thickness of the first sublayer is about 4 μm, the thickness of the second sublayer is about 2.5 μm, and the total thickness is about 6.5 μm.

[0087] Example 10

[0088] The only difference from Example 7 is that the thickness of the first sublayer is 3 μm and the thickness of the second sublayer is 3.5 μm.

[0089] Example 11

[0090] The only difference from Example 7 is that the lithium-containing material in the negative safety coating accounts for 0.5% of the total mass of halloysite nanotubes.

[0091] Example 12

[0092] The only difference from Example 7 is that the material in the safety coating accounts for 15% of the total mass of halloysite nanotubes.

[0093] Example 13

[0094] The difference from Example 7 is that the lithium-containing material loaded in the halloysite nanotubes is passivated lithium powder.

[0095] To highlight the beneficial effects of the embodiments of this application, the following comparative examples are provided.

[0096] Comparative Example 1

[0097] Boehmite, lithium iron phosphate, conductive agent (CNT), and binder (specifically polyvinylidene fluoride) were dispersed in a solvent (specifically NMP) at a mass ratio of 45:48:1:6. After stirring evenly, a safety coating slurry was obtained. This slurry was then coated onto the surface of a current collector (specifically aluminum foil), rolled, and dried to obtain a current collector with a safety coating. The thickness of the safety coating was 7 μm, and after the battery was fabricated, a cross-section was observed to be approximately 3 μm thick.

[0098] The same positive electrode slurry as in Example 1 was coated onto the surface of the safety coating, and after drying, rolling, and slitting, the electrode of Comparative Example 1 was obtained.

[0099] Comparative Example 2

[0100] The difference from Example 1 is that the safety coating contains no active material and the thickness of the safety coating is 3 μm.

[0101] Comparative Example 3

[0102] The difference from Example 1 is that this electrode does not contain a safety coating.

[0103] Battery performance test

[0104] (1) Preparation of negative electrode sheet: Artificial graphite, Super-P, binder (specifically styrene-butadiene rubber, SBR) and thickener (specifically sodium carboxymethyl cellulose, CMC) are mixed in a mass ratio of 100:0.5:1.2:1:2. Then they are dispersed in a solvent (specifically deionized water) to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on both sides of a copper foil with a thickness of 8μm. After drying at room temperature, it is transferred to a forced-air oven at 120℃ for 2h. Then, after cold pressing and slitting, the negative electrode sheet is obtained.

[0105] (2) In an argon glove box with a water content of less than 5 ppm, the above-mentioned electrode sheets were stacked in sequence as positive electrode sheets (the positive electrode sheet of Comparative Example 4 was used directly), separator and negative electrode sheets, wherein the positive and negative electrode sheets were arranged alternately and adjacent positive and negative electrode sheets were separated by the separator to obtain a dry cell. The dry cell was placed in an aluminum-plastic film outer packaging, electrolyte was injected, and then vacuum sealed. After being placed at 60°C for 48 hours, the cells were pressurized at 60°C, repackaged, vented, and capacity tested to obtain the lithium batteries of each embodiment and comparative example.

[0106] (3) Cyclic performance test

[0107] (3-1) Room temperature cycle test: Under the condition of 25±1℃, the battery is charged and discharged at 0.7C and discharged at 0.5CP for a charge-discharge cycle test. The steps are as follows: rest for 10 minutes; charge at 0.5C constant current to 4.4V, charge at constant voltage to 0.05C cutoff; rest for 10 minutes; discharge at 0.5CP constant power to 3.0V, which is one cycle. Repeat this step to test the battery capacity retention rate after 800 cycles.

[0108] (3-2) High-temperature cycling test: Under conditions of 45±1℃, the battery was charged and discharged at 0.7C and then discharged at 0.5CP for a charge-discharge cycle test. The steps are as follows: rest for 10 minutes; charge at 0.7C constant current to 4.4V, then charge at constant voltage to 0.05C cutoff; rest for 10 minutes; discharge at 0.5CP constant power to 3.0V, which is one cycle. Repeat this step and test the battery capacity retention rate after 600 cycles.

[0109] (4) Battery Needle Penetration Safety Test: Ten batteries from each embodiment and comparative example were used for the needle penetration test. Specifically, the batteries were charged to the upper limit voltage at 0.5C and then stopped at 0.05C. After resting for 24 hours, a 3mm steel needle was used to pierce the center point of the battery at a speed of 150mm / s, with five batteries pierced on each side. The test curves of some embodiments and some comparative examples are summarized in […]. Figure 4 middle.

[0110] (5) Battery foreign object compression safety test: Ten batteries were taken for each embodiment and comparative example for foreign object compression test. Specifically, the battery was charged to the upper limit voltage at 0.5C and cut off at 0.05C. After 24 hours, the screw was placed on the surface of the battery and the battery was squeezed with a force of 1000N / s until the pressure reached about 13000N and the pressure was released. Five batteries were squeezed on each side.

[0111] (6) Battery 140°C oven temperature test: 10 batteries were taken from each embodiment and comparative example for 140°C oven temperature test. Specifically, the battery was charged to the upper limit voltage at 0.5C and cut off at 0.05C. Then the battery was placed in the oven and heated to 140°C±2°C at a rate of 5°C±2°C per minute and then kept at 140°C for 30 minutes. After the battery cooled down, the battery was taken out and the appearance of the battery was observed. The temperature rise was tested during the process.

[0112] (7) Test method for battery internal resistance: Test the AC internal resistance of the battery at a frequency of 1kHz.

[0113] (8) Morphology testing of halloysite nanotubes: This also includes observing the aforementioned halloysite nanotubes loaded with Li2S using a scanning electron microscope (SEM). The SEM images of the halloysite nanotubes before and after Li2S loading in Example 1 are shown below. Figure 3 As shown in (a) and (b) in the figure.

[0114] Table 1-1

[0115]

[0116]

[0117] Table 1-2

[0118]

[0119] As can be seen from the parameters in Tables 1-1 and 1-2, for electrodes without a safety coating (Comparative Example 3), or electrodes using a safety coating from the prior art (Comparative Examples 1 and 2), the electrodes provided in this application embodiment can significantly improve the safety performance of the battery. Comparing the electrochemical performance of each embodiment with Comparative Example 3, it is understandable that the battery of Comparative Example 3 has better overall electrochemical performance because it does not have a safety undercoat. However, the safety coating of the electrode in the embodiments helps to reduce the high-temperature expansion rate of the battery, which fully demonstrates the beneficial effect of the electrode in this application embodiment. Secondly, comparing the differences between the embodiments and Comparative Example 1, it is understandable that the thickness of the safety coating will significantly affect the performance of the battery. Therefore, comparing Embodiment 3 and Comparative Example 1 with the same safety coating thickness, the safety performance of Embodiment 3 is significantly better than that of Comparative Example 1, and all electrochemical performance data of Embodiment 3 are better than those of Comparative Example 1, especially energy density, battery internal resistance, and room temperature and high temperature cycling performance.

[0120] Comparing the data between the embodiments, the safety coatings of Embodiments 1 to 3 have the same structure. The greater the thickness of the safety coating, the better it is for improving the safety performance of the battery and for reducing high-temperature expansion. However, within the scope suggested in this application, the smaller the thickness of the safety coating, the better it is for improving the energy density of the battery, the room temperature / high temperature cycle performance, and for reducing the internal resistance of the battery.

[0121] The contents of each substance in the safety coating differ between Example 1 and Example 4. In Example 4, the content of conductive agent is higher and the content of halloysite nanotubes is relatively lower, which will affect the safety performance and high-temperature performance of the battery, but can improve the room temperature cycle performance and energy density of the battery.

[0122] The difference between Examples 5 and 6 and Example 1 lies in the different morphology and size of halloysite nanotubes, but all are within the range suggested in this application. The performance of the batteries in Examples 1, 5, and 6 is slightly different. Overall, Example 1 performs better. However, considering the different production costs and dispersion difficulties of halloysite nanotubes of different sizes, the electrode sheets of Examples 5 and 6 have a slightly higher cost performance.

[0123] The safety coatings in Examples 7-13 are divided into a first sub-layer and a second sub-layer. This structure results in better overall safety performance for the batteries in Examples 7-13. Furthermore, when the total thickness of the safety coating is the same, the electrochemical performance of the battery in Example 7 is also better than that in Example 1, especially the low high-temperature cycling expansion rate of Example 7, which fully demonstrates the further superiority of this layered structure.

[0124] Comparing the data of Example 9 and Example 10, it can be seen that the total thickness of the safety coating in Example 9 and Example 10 is comparable. However, the ratio of the thickness of the first sublayer to the thickness of the second sublayer in Example 9 is within the range suggested in this application, and the various electrochemical properties of Example 9 are superior.

[0125] Comparing the data from Examples 12 and 7, it can be seen that controlling the content of lithium-containing materials within the range suggested in this application is more conducive to reducing the high-temperature expansion rate of the battery and also to the safety performance of the battery.

[0126] The above description is an exemplary embodiment of this application. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. An electrode sheet, characterized in that, The electrode includes a current collector and a safety coating disposed on at least one side surface of the current collector. The safety coating on the side surface opposite to the current collector is further provided with an electrode active material layer. The safety coating includes a binder, a conductive agent, an active material, and halloysite nanotubes. The halloysite nanotubes have lithium-containing materials loaded in their lumens. The safety coating has a first sublayer and a second sublayer stacked together. The first sublayer is disposed close to the current collector. The active material includes a first active material and a second active material. The binder includes a first binder and a second binder. The conductive agent includes a first conductive agent and a second conductive agent. The first sublayer includes halloysite nanotubes loaded with lithium-containing material, the first active material, the first binder, and the first conductive agent. The second sublayer includes a second binder, a second conductive agent, and a second active material. The thickness ratio of the first sublayer to the second sublayer is 1:(0.3-1).

2. The electrode sheet according to claim 1, characterized in that, Along the first direction of the electrode, the angle between the halloysite nanotube and the current collector is less than or equal to 15°; wherein, the first direction is perpendicular to the thickness direction of the electrode.

3. The electrode sheet according to claim 2, characterized in that, Along the first direction, at least a portion of the halloysite nanotubes are parallel to the current collector.

4. The electrode sheet according to any one of claims 1-3, characterized in that, Based on the total mass of the halloysite nanotubes loaded with the lithium-containing material, the mass percentage of the lithium-containing material is 1%-10%.

5. The electrode sheet according to any one of claims 1-3, characterized in that, In the safety coating, the mass ratio of the halloysite nanotubes loaded with lithium-containing material, the active material, the binder, and the conductive agent is 1:(0.2-4):(0.02-1.5):(0.02-1.5).

6. The electrode sheet according to any one of claims 1-3, characterized in that, The halloysite nanotubes have a length of 50nm-3000nm, an inner diameter of 10nm-50nm, and an outer diameter of 40nm-100nm. The active material has a Dv50 particle size in the range of 50nm-1000nm.

7. The electrode sheet according to any one of claims 1-3, characterized in that, The thickness of the safety coating is 3μm-10μm.

8. The electrode sheet according to any one of claims 1-3, characterized in that, The lithium-containing material includes lithium salts; the lithium salts include at least one of lithium sulfide, lithium nitride, lithium fluoride, lithium oxide, lithium oxalate, and lithium peroxide. The active material includes at least one of lithium nickel oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, and lithium manganese iron phosphate.

9. The electrode sheet according to any one of claims 1-3, characterized in that, The surface of the halloysite nanotubes is also modified with a silane coupling agent.

10. A method for preparing an electrode sheet, characterized in that, include: A safety coating is formed on at least one side of the current collector. After drying, an active material layer is formed on the surface of the safety coating. After drying, an electrode is obtained. The safety coating comprises an adhesive, a conductive agent, an active material, and halloysite nanotubes, wherein the halloysite nanotubes are loaded with lithium-containing material within their lumens. The safety coating has a first sub-layer and a second sub-layer stacked together, the first sub-layer being disposed close to the current collector. The active material comprises a first active material and a second active material. The adhesive comprises a first adhesive and a second adhesive, and the conductive agent comprises a first conductive agent and a second conductive agent. The first sub-layer comprises the halloysite nanotubes loaded with lithium-containing material, the first active material, the first adhesive, and the first conductive agent. The second sub-layer comprises a second adhesive, a second conductive agent, and a second active material. The thickness ratio of the first sub-layer to the second sub-layer is 1:(0.3-1).

11. The preparation method according to claim 10, characterized in that, The formation of a safety coating on at least one side surface of the current collector includes: (1) A first slurry is coated on at least one side surface of the current collector; wherein the first slurry comprises a binder, a conductive agent and the halloysite nanotubes, wherein the halloysite nanotubes are loaded with lithium-containing materials in their lumens; (2) Provide a second slurry, the second slurry comprising a binder, a conductive agent and the active material; coat the surface of the current collector with the first slurry with the second slurry so that at least a portion of the active material settles into the first slurry to form a first sublayer together with the first slurry, and form a second sublayer on the surface of the first sublayer.

12. A secondary battery, characterized in that, The secondary battery includes the electrode sheet as described in any one of claims 1-9 or the electrode sheet prepared by the preparation method as described in claim 10 or 11.

13. An electrical appliance, characterized in that, The electrical equipment includes the secondary battery as described in claim 12.

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