Negative electrode sheet, secondary battery, and electrical device

Abstract

The present application provides a negative electrode sheet, a secondary battery, and an electrical device. The negative electrode sheet comprises a current collector and a negative electrode film layer. The negative electrode film layer comprises: a first film layer, provided on one side or both sides of the current collector; and a second film layer, provided on the side of the first film layer distant from the current collector. The negative electrode active material of the first film layer and the negative electrode active material of the second film layer each independently comprise a silicon-based negative electrode material, the silicon-based negative electrode material of the second film layer comprises a porous silicon negative electrode material, and the content of the silicon-based negative electrode material of the first film layer is less than that of the silicon-based negative electrode material of the second film layer. The charging capacity and the cycle performance of the secondary battery are improved.

Description

Negative electrode sheet, secondary battery and power-consuming device

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application 202310798399.1, filed on June 30, 2023, entitled “A negative electrode plate, secondary battery and electrical device,” the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application relates to the field of battery technology, and in particular to a negative electrode plate, a secondary battery, and an electrical device. Background Art

[0004] Lithium-ion batteries have advantages such as high energy density, high power density, and long cycle life, making them widely used in portable electronic devices such as laptops, mobile phones, digital cameras, and other electronic products. In recent years, with the rapid development of new energy and clean energy vehicles, higher requirements have been placed on the performance and safety of new power batteries and energy storage batteries, and the demand for batteries to withstand various extreme operating conditions is also growing.

[0005] Currently, to improve the performance of lithium-ion batteries, much research is focused on the development and improvement of electrode materials and electrolytes. For example, silicon materials have a high theoretical specific capacity (>4000mAh / g), far higher (approximately 10 times) than graphite, which is already nearing its capacity limit, and have a low voltage to lithium, making them a promising candidate for high-energy-density batteries. Although using silicon anodes to replace some traditional graphite anodes can increase the specific capacity of the anode active material, silicon undergoes a dramatic volume expansion during the lithium insertion / extraction process, causing the silicon material to rapidly pulverize and fall off the electrode during cycling, thereby losing contact with the current collector and preventing the silicon material from fully exerting its activity.

[0006] Summary of the Invention

[0007] The present application provides a negative electrode plate, a secondary battery and an electrical device to improve the charging capacity and cycle performance of the secondary battery.

[0008] In a first aspect of the present application, a negative electrode plate is provided, comprising a current collector and a negative electrode film layer, the negative electrode film layer comprising: a first film layer, arranged on one side or both sides of the current collector; a second film layer, arranged on a side of the first film layer away from the current collector; the negative electrode active material in the first film layer and the negative electrode active material in the second film layer each independently comprise a silicon-based negative electrode material, and the silicon-based negative electrode material in the second film layer comprises a porous silicon negative electrode material, and the content of the silicon-based negative electrode material in the first film layer is less than the content of the silicon-based negative electrode material in the second film layer.

[0009] In the negative electrode sheet of the present application, the lower first film layer also contains silicon-based negative electrode material, thereby improving the energy density of the negative electrode sheet. At the same time, the silicon-based negative electrode content in the lower first film layer is relatively small, which effectively alleviates the problem of separation of the silicon-based negative electrode material from the current collector due to pulverization during the cycle. The second film layer located on the surface has a porous silicon negative electrode material, thereby effectively avoiding the problem of reduced battery capacity due to the use of pore-forming agents such as porous carbon; and the porous silicon negative electrode material can increase the surface pores, improving the defect that lithium ions cannot be embedded in the negative electrode of the electrode sheet due to damage to the surface pores caused by rolling; and the content of silicon-based negative electrode material in the upper second film layer is greater than that in the lower first film layer, especially the upper layer has a porous silicon negative electrode material, so the upper layer preferentially embeds lithium, the lithium embedding path is shorter, the kinetics are better, thereby improving the charging capacity of the negative electrode sheet, and effectively alleviating lithium plating, thereby improving the cycle performance of the battery.

[0010] In any embodiment of the first aspect, the surface density M1 of the first film layer is in the range of 4.5 mg / cm 2 ~20mg / cm 2 The surface density M2 of the second film layer is in the range of 4.5 mg / cm 2 ~20mg / cm 2 .

[0011] In any embodiment of the first aspect, the weight ratio of the first film layer to the second film layer is 95:5 to 30:70, and can further be 90:10 to 40:60, thereby improving the synergistic effect of the two film layers.

[0012] In any embodiment of the first aspect, the porosity of the first membrane layer ranges from 20% to 50%, the porosity of the second membrane layer ranges from 20% to 70%, and the porosity of the negative electrode membrane layer ranges from 20% to 70%, which can not only improve the lithium ion embedding ability, but also increase the gram capacity of the negative electrode sheet.

[0013] In any embodiment of the first aspect, the thickness of the second film layer is 10 μm to 20 μm, which fully solves the problem of pores on the surface of the negative electrode sheet being crushed due to rolling.

[0014] In any embodiment of the first aspect, the mass content of the silicon-based negative electrode material in the first film layer is 0.5% to 50%; and / or the mass content of the porous silicon negative electrode material in the second film layer is 0.5% to 70%. Optionally, the mass content of the porous silicon negative electrode material in the negative electrode film layer is 1% to 60%. This allows the silicon-based negative electrode materials in both film layers to be used to maximize the gram capacity of the negative electrode sheet.

[0015] In any embodiment of the first aspect, the second film layer comprises a porous silicon negative electrode material, a graphite negative electrode material, a binder, a dispersant, and a conductive agent, wherein the conductive agent comprises one or more of conductive carbon and carbon nanotubes; optionally, the mass percentage of the porous silicon negative electrode material in the second film layer is 0.5% to 70%; optionally, the mass percentage of the graphite negative electrode material in the second film layer is 25% to 90%; optionally, the mass percentage of the binder in the second film layer is 1% to 8%; optionally, the mass percentage of the dispersant in the second film layer is 0.5% to 2%; optionally, the mass percentage of the conductive carbon in the second film layer is 0.5% to 5%; optionally, the mass percentage of the carbon nanotubes in the second film layer is 0.05% to 2%. The porous silicon negative electrode material and the graphite negative electrode material are combined, and the structural stability of the graphite negative electrode material is utilized to improve the volume stability of the second film layer during the cycle, and the porous silicon negative electrode material is used to improve the gram capacity and lithium ion transmission capacity of the second film layer.

[0016] In any embodiment of the first aspect, the first film layer comprises a silicon-based negative electrode material, a graphite negative electrode material, a binder, a dispersant, and a conductive agent, wherein the conductive agent comprises one or more of conductive carbon and carbon nanotubes; optionally, the mass percentage of the silicon-based negative electrode material in the first film layer is 0.5% to 50%; optionally, the mass percentage of the graphite negative electrode material in the first film layer is 45% to 97.5%; optionally, the mass percentage of the binder in the first film layer is 1% to 3%; optionally, the mass percentage of the dispersant in the first film layer is 0.3% to 1.5%; optionally, the mass percentage of the conductive carbon in the first film layer is 0 to 3%; optionally, the mass percentage of the carbon nanotubes in the first film layer is 0 to 0.5%. The combination of the silicon-based negative electrode material and the graphite negative electrode material improves the adhesion stability between the first film layer and the current collector, and can also increase the gram capacity of the first film layer to a certain extent.

[0017] In any embodiment of the first aspect, the silicon-based negative electrode material of the first film layer includes any one or more of the group consisting of a silicon material, a silicon oxide material, and a porous silicon negative electrode material, and the content of the porous silicon negative electrode material in the first film layer is less than the content of the porous silicon negative electrode material in the second film layer, so as to improve the uniformity of the overall pores of the negative electrode film layer.

[0018] In any embodiment of the first aspect, the Dv of the porous silicon negative electrode material 50 Between 3μm and 20μm, optionally between 4μm and 15μm; and / or the BET specific surface area of ​​the porous silicon negative electrode material is 1m 2 / g~30m 2 / g; optionally between 6m 2 / g~20m 2 / g. To further improve the volume energy density of the negative electrode.

[0019] In any embodiment of the first aspect, the porous silicon anode material comprises: a core comprising porous silicon, the porous silicon comprising elemental silicon and a silicon compound, the silicon compound comprising silicon oxide; and a coating layer coated on the surface of the core. The porous silicon anode material employing a core-shell structure improves the energy density of the anode electrode while mitigating the volume expansion of the silicon-based anode material, thereby improving the cycling stability of high-energy-density batteries.

[0020] In any embodiment of the first aspect, the coating layer comprises any one or more of a metal compound of silicon, lithium silicate, amorphous carbon, and carbon nanotubes. By coating the porous silicon anode material with the one or more materials, the mechanical stability or conductivity of the porous silicon anode material is improved.

[0021] In any embodiment of the first aspect, the coating layer includes: a first coating layer, the first coating layer is coated on the surface of the core, the first coating layer includes lithium silicate; a second coating layer, the second coating layer is coated on the surface of the first coating layer, the second coating layer includes a metal compound of silicon, optionally the metal element of the metal compound of silicon includes Ti, Mg and / or Al; a third coating layer, the third coating layer is coated on the surface of the second coating layer, the third coating layer includes amorphous carbon, optionally the material of the amorphous carbon includes hard carbon and / or soft carbon. The first coating layer includes lithium silicate, which can generate lithium salts and inert phases (lithium oxide) during the lithium insertion process. The generation of lithium salts can reduce the consumption of lithium ions in the positive electrode active material during the first lithium insertion process, so that the battery has a higher first coulombic efficiency. The generation of inert phases can effectively alleviate volume expansion and improve the cycle performance of the battery. The second coating layer provided between the first coating layer and the third coating layer includes a metal compound of silicon, which has strong mechanical strength and can buffer the volume expansion of the internal silicon material, thereby improving the cycle performance. On the one hand, the third coating layer can buffer the volume expansion of the first coating layer and improve the cycle performance; on the other hand, it can improve the protection of the first coating layer, avoid direct contact between the first coating layer and water, and prevent the dissolution and gas production of lithium silicate in the first coating layer.

[0022] In any embodiment of the first aspect, the silicon compound further comprises a metal compound of silicon, wherein the metal compound of silicon is coated on the surface of the silicon element; optionally, the metal element of the metal compound of silicon includes Ti, Mg, and / or Al. Coating the metal compound of silicon on the surface of the silicon element can buffer the volume expansion of the silicon element and improve cycle performance.

[0023] In any embodiment of the first aspect, the coating layer includes: a first coating layer, the first coating layer is coated on the surface of the inner core, and the first coating layer includes lithium silicate; a third coating layer, the third coating layer is coated on the surface of the first coating layer, and the third coating layer includes amorphous carbon. Optionally, the material of the amorphous carbon includes hard carbon and / or soft carbon. The first coating layer includes lithium silicate, which can generate lithium salts and inert phases (lithium oxide) during the lithium insertion process. The generation of lithium salts can reduce the consumption of lithium ions in the positive electrode active material during the first lithium insertion process, so that the battery has a higher first coulombic efficiency. The generation of inert phases can effectively alleviate volume expansion and improve the cycle performance of the battery. On the one hand, the third coating layer can buffer the volume expansion of the first coating layer and improve the cycle performance; on the other hand, it can improve the protection of the first coating layer, avoid direct contact between the first coating layer and water, and prevent the dissolution and gas production of lithium silicate in the first coating layer.

[0024] In any embodiment of the first aspect, the above-mentioned third coating layer also includes carbon nanotubes, which improve the conductivity of the porous silicon negative electrode material, and utilize carbon nanotubes to fully bind the porous silicon negative electrode material, alleviate the volume expansion during the charge and discharge process, control the polarization inside the negative electrode cell, and improve the cycle performance of the cell.

[0025] In any embodiment of the first aspect, the mass of the porous silicon negative electrode material in the first film layer and the second film layer is m1 and m2 respectively; the mass percentage of the carbon nanotubes in the porous silicon negative electrode material in the first film layer and the second film layer is C1 and C2 respectively; excluding the carbon nanotubes in the porous silicon negative electrode material, the mass of the remaining carbon nanotubes in the first film layer and the second film layer is S1 and S2 respectively, the first film layer and the second film layer are the first film layer per unit area and the second film layer per unit area respectively, and the following relationship is satisfied: (m2×C2+S2) / M2=(m1×C1+S1) / M1, m2≥m1, S1≥S2, S2=m1×m2×(C2-C1) / (m2-m1). By utilizing the restrictions of the above relationship and making full use of the carbon nanotubes, the binding capacity of the silicon-based negative electrode is further improved, the rebound of the negative electrode plate is effectively suppressed, and the polarization of the negative electrode plate is minimized.

[0026] In any embodiment of the first aspect, the Dv50 of the silicon element is in the range of 0 < Dv50 ≤ 10 nm; optionally, the Dv50 of the silicon element is in the range of 2 nm < Dv50 ≤ 8 nm. Using nanometer-sized silicon element facilitates mixing with silicon oxide, thereby increasing the energy density of the battery, reducing the difficulty of coating the core structure, and improving the cycling stability of the battery.

[0027] In any embodiment of the first aspect, the chemical formula of the silicon oxide is SiOx, where 0<x≤2, and the Dv50 range of the silicon oxide is 2μm≤Dv50≤13μm, which is conducive to mixing with silicon and improving the energy density of the battery.

[0028] In any embodiment of the first aspect, the lithium silicate comprises one or more of Li2SiO3, Li2Si2O5, Li4SiO4, Li2Si3O7, Li8SiO6, Li6Si2O7, Li4Si2O7, Li2Si4O7, and LiSiO3; optionally, the lithium silicate comprises at least Li2SiO3, and further optionally, the mass of Li2SiO3 is at least 50% of the mass of the first coating layer, and more optionally, the mass of Li2SiO3 is at least 70% of the mass of the first coating layer. This allows the first coating layer to effectively reduce the consumption of lithium ions in the positive electrode active material during the initial lithium insertion process, resulting in a higher initial coulombic efficiency for the battery.

[0029] In any embodiment of the first aspect, the thickness of the first coating layer is 10% to 80% of the particle radius of the porous silicon anode material. Optionally, the thickness of the second coating layer is 1 nm to 50 nm. Optionally, the thickness of the third coating layer is 1 nm to 1000 nm, and optionally 5 nm to 300 nm. Within these thickness ranges, the particles effectively reduce volume expansion of the anode material while increasing battery energy density, thereby improving the cycling stability of high-energy-density batteries.

[0030] The second aspect of the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the negative electrode sheet is any of the negative electrode sheets described above. The secondary battery of the present application has good charging capacity, high energy density and good cycle performance.

[0031] The third aspect of the present application provides an electrical device, comprising a secondary battery, wherein the secondary battery is selected from the secondary batteries mentioned above in the present application. The electrical device of the present application has better safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

[0033] FIG1 is a schematic structural diagram of a negative electrode sheet provided in one embodiment of the present application.

[0034] FIG2 is a schematic structural diagram of a porous silicon negative electrode material provided in one embodiment of the present application.

[0035] FIG3 is a schematic structural diagram of a porous silicon negative electrode material provided in another embodiment of the present application.

[0036] FIG4 is a schematic structural diagram of a porous silicon negative electrode material provided in yet another embodiment of the present application.

[0037] FIG5 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0038] FIG. 6 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 5 .

[0039] FIG7 is a schematic diagram of a battery module according to an embodiment of the present application.

[0040] FIG8 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0041] FIG. 9 is an exploded view of the battery pack shown in FIG. 8 according to an embodiment of the present application.

[0042] FIG. 10 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0043] In the drawings, the drawings are not drawn to scale.

[0044] Explanation of the accompanying drawings: 01 current collector; 02 first film layer; 03 second film layer; porous silicon negative electrode material 10; 11 core; 12 first coating layer; 13 second coating layer; 14 third coating layer; 111 silicon element; 112 silicon oxide; 113 metal compound of silicon; 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0045] The following detailed description of the embodiments of the present application is provided in conjunction with the accompanying drawings and examples. The following detailed description of the embodiments and the accompanying drawings are used to illustrate the principles of the present application, but are not intended to limit the scope of the present application, that is, the present application is not limited to the described embodiments.

[0046] Below, the embodiments of the negative electrode sheet, secondary battery, and electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0047] The "ranges" disclosed herein are defined in terms of lower and upper limits, where a given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise indicated, the numerical range "a to b" is a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0048] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0049] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0050] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0051] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0052] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0053] [Secondary battery]

[0054] Secondary batteries, also known as rechargeable batteries or storage batteries, are batteries that can be recharged to activate the active materials after discharge and continue to be used.

[0055] Typically, a secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the battery's charge and discharge process, active ions (such as lithium ions) are embedded and released back and forth between the positive and negative electrode sheets. The separator is set between the positive and negative electrode sheets, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing active ions to pass through. The electrolyte between the positive and negative electrode sheets mainly plays the role of conducting active ions.

[0056] [Negative electrode]

[0057] In the process of improving the performance of lithium-ion batteries, many studies have gradually discovered that reasonable electrode structure design is very important for the ion and electron transport pathways throughout the electrode. By optimizing the electrode structure, the electrode's conductivity and electrolyte wettability can be improved, and the electron and ion transport rate within the electrode can be increased, thereby improving the battery's energy density and rate performance. However, obtaining thick electrodes that have both good electron / ion transport properties and high active material loading is a major challenge.

[0058] For example, even with thick electrode plates with multiple layers of coating, there is still the problem of surface pores about 10 to 20 μm deep on the surface of the plate being squeezed during the rolling process, which prevents lithium ions in the electrolyte from effectively diffusing into the interior of the plate, resulting in a significant reduction in charging capacity. To solve this problem, there is a technology that increases the surface porosity by adding pore-forming agents to the surface film layer of the negative electrode plate, such as using porous carbon. However, the use of porous carbon leads to a loss in the energy density of the plate, which in turn reduces the specific capacity of the battery. In order to solve the above problems, a typical embodiment of the present application provides a negative electrode plate, as shown in Figure 1, the negative electrode plate includes a current collector 01 and a negative electrode film layer, the negative electrode film layer includes a first film layer 02 and a second film layer 033, the first film layer 02 is arranged on one side or both sides of the current collector 01; the second film layer 03 is arranged on the side of the first film layer 02 away from the current collector 01; the negative electrode active material of the first film layer 02 and the negative electrode active material in the second film layer 03 each independently include a silicon-based negative electrode material, and the silicon-based negative electrode material in the second film layer 03 includes a porous silicon negative electrode material 10, and the content of the silicon-based negative electrode material in the first film layer 02 is less than the content of the silicon-based negative electrode material in the second film layer 03.

[0059] In the negative electrode sheet of the present application, the lower first film layer 02 also contains a silicon-based negative electrode material, thereby improving the energy density of the negative electrode sheet. At the same time, the silicon-based negative electrode content in the lower first film layer 02 is relatively low, which effectively alleviates the problem of the silicon-based negative electrode material peeling off from the current collector 01 due to pulverization during the cycle. The second film layer 03 located on the surface has a porous silicon negative electrode material 10, thereby effectively avoiding the problem of reduced battery specific capacity caused by the use of pore-forming agents such as porous carbon; and the porous silicon negative electrode material 10 can increase the surface pores, improving the defect that lithium ions cannot be embedded in the negative electrode of the electrode sheet due to damage to the surface pores caused by rolling; and the content of silicon-based negative electrode material in the upper second film layer 03 is greater than that in the lower first film layer 02. In particular, the upper layer has the porous silicon negative electrode material 10, so the upper layer preferentially embeds lithium, the lithium embedding path is shorter, and the kinetics are better, thereby improving the charging capacity of the negative electrode sheet and effectively alleviating lithium plating, thereby improving the cycle performance of the battery.

[0060] The above-mentioned first film layer 02 can be directly provided on the current collector 01, or a primer layer can be provided between the first film layer 02 and the current collector 01; the above-mentioned second film layer 03 can be directly provided on the first film layer 02, or other negative electrode film layers can be provided between the two, as long as the second film layer 03 is located on the surface of the negative electrode sheet.

[0061] The content of the above-mentioned silicon-based negative electrode material can be determined by referring to the following method: the first step is to prepare a plurality of negative electrode sheet samples with different mass ratios, which are obtained by mixing silicon-based negative electrode material and graphite material (if there are other negative electrode materials in the negative electrode sheet to be measured, the negative electrode sheet samples can be prepared with reference to the material type of the negative electrode sheet to be measured); the second step is to take samples respectively, place them in a scanning electron microscope for energy spectrum testing, and obtain the relative mass percentage of silicon element in each negative electrode sheet sample; the third step is to use the relative mass percentage of silicon element in each negative electrode sheet sample to obtain the relative mass percentage of silicon element in each negative electrode sheet sample. The standard curve is drawn and fitted with the mass percentage of the silicon-based negative electrode material in each negative electrode sample as the horizontal axis and the mass percentage of the silicon-based negative electrode material in each negative electrode sample as the vertical axis to obtain a quantitative relationship. In the fourth step, for the negative electrode sheet to be tested, the thickness of the upper and lower film layers is confirmed by CP (ion beam) section, and then the powder scraping method is used to scrape out the powder of the upper second film layer, and then the powder of the lower first film layer is scraped out, and the relative mass percentage of the silicon element in each is measured, and then the mass percentage of the silicon-based negative electrode material in the first and second film layers to be tested is calculated according to the quantitative relationship.

[0062] In some embodiments, the surface density M1 of the first film layer 02 is in the range of 4.5 mg / cm 2 ~20mg / cm 2 The surface density M2 of the second film layer 03 is in the range of 0.2 mg / cm 2 ~20mg / cm 2 The above weight range ensures the effective thickness of the two film layers. In some embodiments, the surface density M1 of the first film layer 02 can be 4.5 mg / cm 2 , 5mg / cm 22 , 7mg / cm 2 、10mg / cm 2 、15mg / cm 2 and 20 mg / cm 2 , preferably 5 mg / cm 2 ~7mg / cm 2 In some embodiments, the surface density M2 of the second film layer 03 may be 0.2 mg / cm 2 , 0.3mg / cm 2 , 0.4mg / cm 2 , 1mg / cm 2 , 2mg / cm 2 , 3mg / cm 2 , 5mg / cm 2 、10mg / cm 2 、17mg / cm 2 and 20 mg / cm 2 , preferably 0.3 mg / cm 2 ~17mg / cm2 .

[0063] The main function of the second film layer 03 of the present application is to utilize the porous silicon negative electrode material 10 therein to increase the surface porosity. In some embodiments, the weight ratio of the above-mentioned first film layer 02 and the second film layer 03 is 95:5~30:70, and can be further optionally 90:10~40:60, such as 90:10, 80:20, 70:30, 60:40, 50:50 or 40:60. In the above embodiments, especially when the first film layer 02 is utilized as the main body of the negative electrode film layer, the main function of the negative electrode plate is exerted, and the second film layer 03 is utilized to compensate for the defects of the first film layer 02, thereby improving the synergistic effect of the two film layers.

[0064] In some embodiments, the porosity of the first film layer 02 is in the range of 20% to 50%, the porosity of the second film layer 03 is in the range of 20% to 70%, and the porosity of the negative electrode film layer is in the range of 20% to 70%, for example, 20%, 30%, 40%, 50%, 60% or 70%. The negative electrode film layer with the above porosity can not only utilize the pores to improve the lithium ion embedding ability, but also utilize the negative electrode active material to improve the gram capacity of the negative electrode sheet.

[0065] In some embodiments, the thickness of the second film layer 03 is 10 μm to 20 μm, such as 10 μm, 12 μm, 15 μm, 18 μm or 20 μm. This thickness fully solves the problem of the pores on the surface of the negative electrode sheet being crushed due to rolling.

[0066] The method for determining the weight of the above two film layers can refer to the following process: confirm the thickness of the upper and lower film layers through the CP section, and then use the powder scraping method to first scrape out the powder of the upper second film layer and weigh it; then scrape out the powder of the lower first film layer and weigh it, and you can get the weight and ratio of the first film layer and the second film layer respectively.

[0067] The above porosity determination method can be referred to as follows: take a picture of the CP section, scan it through imaging software, calculate the porosity of the corresponding section and the first film layer 02 and the second film layer 03, then scan and take pictures of multiple CP sections, and calculate through software to obtain the porosity of the first film layer 02 and the second film layer 03 in the electrode.

[0068] In order to improve the energy density of the negative electrode plate as much as possible, in some embodiments, the mass content of the silicon-based negative electrode material in the first film layer 02 is 0.5% to 50%, which can be optionally 1% to 45%, 1% to 15%, 2% to 10% or 5% to 10%, such as 1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50%; and / or the mass content of the porous silicon negative electrode material 10 in the second film layer 03 is 0.5% to 70%, which can be optionally 5% to 65%, 10% to 60%, 10% to 15%, 2% to 10%, 10% to 15%, 20% to 25%, 30% to 35%, 40% to 45%, 50% to 50%; 5% to 55% or 20% to 50%, such as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65% or 70%; Optionally, the mass content of the porous silicon negative electrode material 10 in the negative electrode film layer is 1% to 60%, optionally 5% to 55%, 5% to 25%, 5% to 20% or 10% to 20%, such as 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or 60%. In order to utilize the silicon-based negative electrode material in the two film layers to maximize the gram capacity of the negative electrode sheet.

[0069] In some embodiments, in addition to silicon-based negative electrode materials, the negative electrode active material may also use negative electrode active materials for batteries known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, tin-based materials, and lithium titanate. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0070] In addition to containing negative electrode active materials, the first film layer and the second film layer of the present application also contain binders, dispersants and conductive agents contained in conventional negative electrode film layers. In some embodiments, the above-mentioned second film layer includes porous silicon negative electrode material, graphite negative electrode material, binder, dispersant and conductive agent, and the conductive agent includes one or more of conductive carbon and carbon nanotubes; optionally, the mass percentage of porous silicon negative electrode material in the second film layer is 0.5% to 70%; optionally, the mass percentage of graphite negative electrode material in the second film layer is 25% to 90%; optionally, the mass percentage of binder in the second film layer is 1% to 8%; optionally, the mass percentage of dispersant in the second film layer is 0.5% to 2%; optionally, the mass percentage of conductive carbon in the second film layer is 0.5% to 5%; optionally, the mass percentage of carbon nanotubes in the second film layer is 0.05% to 2%. A combination of porous silicon negative electrode material and graphite negative electrode material is used. The structural stability of the graphite negative electrode material is used to improve the volume stability of the second membrane layer during the cycle, and the porous silicon negative electrode material is used to improve the gram capacity and lithium ion transmission capacity of the second membrane layer.

[0071] In some embodiments, as an example, the binder includes at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0072] In some embodiments, as examples, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0073] In some embodiments, as examples, the dispersant includes at least one of sodium carboxymethylcellulose (CMC), polyvinylpyrrolidone (PVP), and polyacrylamide (PAM).

[0074] In some embodiments, the first film layer comprises a silicon-based negative electrode material, a graphite negative electrode material, a binder, a dispersant, and a conductive agent, wherein the conductive agent comprises one or more of conductive carbon and carbon nanotubes. Optionally, the mass percentage of the silicon-based negative electrode material in the first film layer is 0.5% to 50%; alternatively, the mass percentage of the graphite negative electrode material in the first film layer is 45% to 97.5%; alternatively, the mass percentage of the binder in the first film layer is 1% to 3%; alternatively, the mass percentage of the dispersant in the first film layer is 0.3% to 1.5%; alternatively, the mass percentage of the conductive carbon in the first film layer is 0 to 3%; alternatively, the mass percentage of the carbon nanotubes in the first film layer is 0 to 0.5%. The content of the graphite negative electrode material in the first film layer is higher than that of the graphite negative electrode material in the second film layer, thereby improving the structural stability of the first film layer and the adhesion stability of the current collector. Moreover, the addition of the silicon-based negative electrode material can increase the gram capacity of the first film layer to a certain extent.

[0075] In addition, the carbon nanotubes in the conductive agent in the first film layer and the second film layer have better conductivity, and the linear structure of the carbon nanotubes has a certain binding effect on the silicon-based negative electrode material, effectively alleviating the volume expansion of the silicon-based negative electrode material, reducing its pulverization, and thereby improving the cycle performance of the battery.

[0076] In some embodiments, the silicon-based negative electrode material of the first film layer includes any one or more of the group consisting of silicon material, silicon oxide material, and porous silicon negative electrode material, or the silicon-based negative electrode material of the first film layer is a porous silicon negative electrode material, as long as the content of the porous silicon negative electrode material in the first film layer is less than the content of the porous silicon negative electrode material in the second film layer, so as to improve the uniformity of the overall pores of the negative electrode film layer.

[0077] In some embodiments, the Dv of the porous silicon negative electrode material is 50 Between 3 μm and 20 μm, such as 3 μm, 4 μm, 5 μm, 7 μm, 10 μm, 12 μm, 15 μm or 20 μm, optionally between 4 μm and 15 μm; and / or the BET specific surface area of ​​the porous silicon negative electrode material is between 1 and 30 m 2 / g; optionally between 6m 2 / g~20m 2 / g. To further improve the cycle performance and charging capacity of the negative electrode.

[0078] The Dv50 can be measured using a laser particle size analyzer (e.g., Malvern Master Size 3000) in accordance with standard GB / T 19077.1-2016, wherein Dv50 is the particle size at which the cumulative volume distribution percentage of the measured material reaches 50%.

[0079] The BET specific surface area is the specific surface area of ​​a material measured using the low-temperature nitrogen adsorption-desorption method.

[0080] Material porosity: The porosity of silicon-based negative electrode materials can be tested by a true density porosity tester (such as AccuPycⅡ1340).

[0081] The porous silicon negative electrode material of the present application can adopt the porous silicon negative electrode material commonly used in the art, such as a simple porous silicon material or a porous silicon material with a coating. In some embodiments, as shown in any of Figures 2 to 4, the above-mentioned porous silicon negative electrode material includes a core 11 and a coating layer, the core 11 is porous silicon, and the porous silicon includes a silicon element 111 and a silicon compound, and the silicon compound includes a silicon oxide 112; the coating layer is coated on the surface of the core. The porous silicon negative electrode material with a core-shell structure can improve the charging capacity of the negative electrode while alleviating the volume expansion of the silicon-based negative electrode material, thereby improving the cycle stability of high-energy-density batteries. Those skilled in the art can obtain the composition of the porous silicon in the core of the porous silicon material by excluding the substances in the external coating layer through nuclear magnetic resonance NMR technology (such as equipment BRUKER AVANCEⅢHD 500MHz).

[0082] Silicon has a high theoretical lithium insertion capacity (approximately 4200 mAh / g) and a low lithium insertion potential, making it a promising candidate for battery applications. However, silicon has a significant volume expansion effect, resulting in poor battery cycling stability. Silicon oxides, a binary compound composed of silicon and oxygen, have a smaller volume expansion effect than silicon. Compared to silicon, silicon oxides can mitigate this volume expansion effect and improve battery cycling stability.

[0083] In some embodiments, the coating layer comprises any one or more of a metal compound of silicon, lithium silicate, amorphous carbon, and carbon nanotubes. By coating the one or more of the above materials, the mechanical stability or conductivity of the porous silicon negative electrode material is improved. The coating of each of the above materials can be carried out in a variety of ways. Where feasible, the coating can be a single layer, or the materials can be coated in multiple layers.

[0084] "Lithium silicate" refers to a series of compounds produced by the reaction of metallic lithium and silicic acid; "amorphous carbon" refers to a transition state of carbon, which is an allotrope of carbon, and usually refers to carbon elements other than graphite and diamond.

[0085] As shown in FIG2 , in some embodiments, the coating layer includes a first coating layer 12 and a third coating layer 14. The first coating layer 12 is coated on the surface of the core 11 and includes lithium silicate. The third coating layer 14 is coated on the surface of the first coating layer 12 and includes amorphous carbon. Optionally, the amorphous carbon material includes hard carbon and / or soft carbon. The first coating layer includes lithium silicate, which can generate lithium salts and inert phases (lithium oxide) during the lithium insertion process. The generation of lithium salts can reduce the consumption of lithium ions in the positive electrode active material during the initial lithium insertion process, so that the battery has a higher initial coulombic efficiency and charging capacity. The generation of inert phases can effectively alleviate volume expansion and improve the cycle performance of the battery. The third coating layer can buffer the volume expansion of the first coating layer and improve the cycle performance. On the other hand, it can enhance the protection of the first coating layer, prevent the first coating layer from direct contact with water, and prevent the dissolution and gas generation of lithium silicate in the first coating layer.

[0086] The porous silicon negative electrode material shown in FIG2 can be prepared by the following method:

[0087] 1. Silicon grains and silicon dioxide are synthesized by vapor deposition to obtain silicon oxide. The D of silicon oxide is adjusted by adjusting the vapor deposition temperature between 600℃ and 1200℃. V 50. Disperse silicon dioxide particles in water, then add polyvinyl pyrrolidone (molar ratio to silicon dioxide is 5 to 15:1), heat and boil under reflux for 2 hours to 5 hours, cool to room temperature, add inorganic alkali solution (concentration is 0.01 mmol / L to 0.1 mmol / L, such as sodium hydroxide solution), stir and etch for several hours, centrifuge the obtained suspension, wash it with water 2-3 times, dry it at 100℃ to 110℃ for 1 hour to 3 hours, and then calcine it in a muffle furnace at 500℃ to 600℃ for 3 hours to obtain porous silicon dioxide as the core structure. By grinding the porous silicon dioxide, its D V 50 distribution, such as adjusting the grinding time between 0.5h and 24h to make D V 50 changes, and further D can be screened V 50 for further adjustments;

[0088] 2. Amorphous carbon: An amorphous carbon source such as asphalt is mixed with the core structure and then subjected to high temperature treatment for 1 to 10 hours (for example, the amorphous carbon source and the core structure are mixed in a mass ratio of 0.01:1 to 0.1:1), and the temperature is controlled at 700°C to 1500°C to obtain a coating layer containing amorphous carbon;

[0089] 3. Mixing the material obtained in step 2 above with a lithium source (for example, the material obtained in step 2 and the lithium source are mixed in a mass ratio of 1:0.09 to 1:0.2), such as lithium powder, lithium hydride, lithium oxide, lithium carbonate or organic lithium, and subjecting the mixture to a high temperature treatment at 300°C to 1000°C for 1h to 5h. The lithium ions will pass through the amorphous carbon coating layer, forming a first coating layer containing lithium silicate coated on the core structure inside the amorphous carbon.

[0090] 4. Finally, the material obtained in step 3 is mixed with a solution of carbon nanotubes, wherein the mass ratio of the material obtained in step 3 to the carbon nanotubes is 1:0.02 to 1:0.1, and a bonding reaction occurs under high temperature treatment at 100°C to 400°C, and the carbon nanotubes are coated on the surface of the material to form a third coating layer of a mixture of amorphous carbon material and carbon nanotubes.

[0091] As shown in FIG3 , in some embodiments, the coating layer includes a first coating layer 12, a second coating layer 13, and a third coating layer 14. The first coating layer 12 is coated on the surface of the core 11 and includes lithium silicate. The second coating layer 13 is coated on the surface of the first coating layer 12 and includes a metal compound of silicon, optionally containing Ti, Mg, and / or Al. The third coating layer 14 is coated on the surface of the second coating layer 13 and includes amorphous carbon, optionally containing hard carbon and / or soft carbon. The second coating layer further disposed between the first and third coating layers includes a metal compound of silicon, which has strong mechanical strength, can buffer the volume expansion of the internal silicon material, and improve cycle performance.

[0092] The porous silicon negative electrode material shown in FIG3 can be prepared by the following method:

[0093] 1. Silicon grains and silicon dioxide are synthesized by vapor deposition to obtain silicon oxide. The silicon oxide particles are dispersed in water, and then polyvinyl pyrrolidone (with a molar ratio of 5 to 15:1 to silicon dioxide) is added, heated to boiling reflux for 2 hours to 5 hours, and then added with an inorganic base solution (with a concentration of 0.01 mmol / L to 0.1 mmol / L, such as sodium hydroxide solution) after cooling to room temperature. Then, the mixture is stirred and etched for several hours. The resulting suspension is centrifuged, washed with water 2-3 times, dried at 100°C to 110°C for 1 hour to 3 hours, and then calcined in a muffle furnace at 500°C to 600°C for 3 hours to obtain porous silicon oxide as the core structure.

[0094] 2. Amorphous carbon: An amorphous carbon source such as asphalt is mixed with the above core structure and then subjected to high temperature treatment for 1 to 10 hours at a temperature of 700°C to 1500°C to obtain a coating layer containing amorphous carbon;

[0095] 3. The material obtained in step 2 is mixed with a lithium source, such as lithium powder, lithium hydride, lithium oxide, lithium carbonate or organic lithium, and subjected to high temperature treatment at 300°C to 1000°C for 1 hour to 5 hours. The lithium ions will pass through the amorphous carbon coating layer and form a first coating layer containing lithium silicate on the core structure inside the amorphous carbon;

[0096] 4. Preparation of a metal compound coating layer of silicon: The metal coating raw material is mixed with the material obtained in step 3 at a mass ratio of 0.001:1 to 0.02:1. The mixed material is subjected to a high temperature treatment at 600°C to 1000°C for 0.5h to 2h. The treated material is ball milled and stirred for 10min to 60min. The metal ions will pass through the amorphous carbon coating layer and form a second coating layer of silicon-containing metal compounds inside the amorphous carbon and on the surface of the lithium silicate;

[0097] 5. Finally, the material obtained in step 4 is mixed with the carbon nanotube solution, and subjected to high temperature treatment at 100°C to 400°C to cause a bonding reaction, coating the carbon nanotubes on the surface of the material to form a third coating layer of amorphous carbon material and carbon nanotubes.

[0098] The metal coating raw materials include but are not limited to metal salts and metal oxides.

[0099] As shown in Figure 4, in some embodiments, based on the structure shown in Figure 2, the silicon compound further includes a silicon metal compound 113, which is coated on the surface of the silicon element. "Silicon metal compound" refers to a compound formed by a transition metal and silicon; the metal element of the silicon metal compound optionally includes Ti, Mg, and / or Al. Coating the silicon metal compound on the surface of the silicon element can mitigate the volume expansion of the silicon element, improving cycling performance and charging capacity.

[0100] The porous silicon negative electrode material shown in FIG4 can be prepared by the following method:

[0101] 1. Silicon grains and silicon dioxide are synthesized by vapor deposition to obtain silicon oxide. The silicon oxide particles are dispersed in water, and then polyvinyl pyrrolidone (with a molar ratio of 5 to 15:1 to silicon dioxide) is added, heated to boiling reflux for 2 hours to 5 hours, and then added with an inorganic base solution (with a concentration of 0.01 mmol / L to 0.1 mmol / L, such as sodium hydroxide solution) after cooling to room temperature. Then, the mixture is stirred and etched for several hours. The obtained suspension is centrifuged, washed with water 2-3 times, dried at 100°C to 110°C for 1 hour to 3 hours, and then calcined in a muffle furnace at 500°C to 600°C for 3 hours to obtain porous silicon oxide.

[0102] 2. Preparation of a metal compound coating layer of silicon: A metal coating raw material is mixed with the porous silicon oxide, and the mixed material is subjected to a high-temperature treatment at 800°C to 1300°C for 1 hour to 6 hours. The high-temperature treated material is then ground. The coating directly reacts with the silicon grains through a phase selective reaction to form a metal compound coating layer of silicon on the surface of the silicon element, which serves as a core structure.

[0103] 3. Amorphous carbon: An amorphous carbon source such as asphalt is mixed with the above core structure and then subjected to high temperature treatment for 1 to 10 hours at a temperature of 700°C to 1500°C to obtain a coating layer containing amorphous carbon;

[0104] 4. The material obtained in step 3 is mixed with a lithium source, such as lithium powder, lithium hydride, lithium oxide, lithium carbonate or organic lithium, and subjected to high temperature treatment at 300°C to 1000°C for 1 hour to 5 hours. The lithium ions will pass through the amorphous carbon coating layer and form a first coating layer containing lithium silicate on the core structure inside the amorphous carbon;

[0105] 5. Finally, the material obtained in step 4 is mixed with the carbon nanotube solution, and subjected to high temperature treatment at 100°C to 400°C to cause a bonding reaction, coating the carbon nanotubes on the surface of the material to form a third coating layer of amorphous carbon material and carbon nanotubes.

[0106] The above-mentioned hard carbon refers to carbon that is difficult to graphitize, and is a thermal decomposition product of high molecular polymers. Common hard carbons include resin carbon, organic polymer pyrolysis carbon, and carbon black. Soft carbon generally refers to black carbon, which is a type of highly aromatic substance produced by pyrolysis and carbonization of fossil fuels and biomass under oxygen-deficient conditions. In the above-mentioned embodiments of the present application, amorphous carbon may be only hard carbon, or only soft carbon, or a mixture of hard carbon and soft carbon. Due to the large interlayer spacing and fast lithium insertion rate of amorphous carbon, it is beneficial to improve the overall performance of the battery.

[0107] In some embodiments, the third coating layer further comprises carbon nanotubes. Carbon nanotubes can improve the conductivity of the porous silicon negative electrode material, and the carbon nanotubes can be used to fully bind the porous silicon negative electrode material, alleviate the volume expansion during the charge and discharge process, control the polarization inside the negative electrode cell, and improve the cycle performance of the cell. Definition: The mass of the porous silicon negative electrode material 10 in the first film layer 02 and the second film layer 03 are m1 and m2 respectively; the mass percentage of the carbon nanotubes belonging to the porous silicon negative electrode material 10 in the first film layer 02 and the second film layer 03 are C1 and C2 respectively; excluding the carbon nanotubes in the porous silicon negative electrode material 10, the mass of the remaining carbon nanotubes in the first film layer 02 and the second film layer 03 are S1 and S2 respectively, and the first film layer 02 and the second film layer 03 are the first film layer 02 per unit area and the second film layer 03 per unit area respectively. In order to ensure sufficient binding of the porous silicon negative electrode material 10 and effectively suppress the rebound of the negative electrode pole piece, in some embodiments, the carbon nanotubes in the third coating layer, the carbon nanotubes in the first film layer 02, and the carbon nanotubes in the second film layer 03 are designed to satisfy the following relationship: (m2×C2+S2) / M2=(m1×C1+S1) / M1, m2≥m1, S1≥S2, S2=m1×m2×(C2-C1) / (m2-m1), which further improves the binding ability of the silicon-based negative electrode, effectively suppresses the rebound of the negative electrode pole piece, and minimizes the polarization of the negative electrode pole piece.

[0108] The masses of the porous silicon negative electrode materials in the first film layer and the second film layer are respectively m1 and m2, which are determined by referring to the aforementioned method for determining the content of silicon-based negative electrode materials.

[0109] Determination of the mass percentage C1 and C2 of the carbon nanotubes on the surface of the above-mentioned porous silicon negative electrode material: SEM can be used to count the number and length of carbon nanotubes on the surface of multiple silicon particles, and by simulating the chain length and molecular number of the carbon nanotubes, the mass of the corresponding single carbon nanotube is obtained. The average value is obtained by calculating the number of multiple carbon tubes as the mass of the carbon nanotubes on the surface of the porous silicon negative electrode material, and then the mass percentage C1 and C2 of the carbon nanotubes in the porous silicon negative electrode material are calculated through the volume and true density of the porous silicon negative electrode material.

[0110] The masses of the remaining carbon nanotubes in the first film layer and the second film layer are S1 and S2 respectively: the thickness of the upper and lower film layers is confirmed by CP (ion beam) sectioning, and then the powder scraping method is used to first scrape out the powder of the upper second film layer, and then scrape out the powder of the lower first film layer, and then each is divided into n parts of equal mass to ensure uniform distribution on the SEM sample stage, and then the number of carbon nanotubes is counted through SEM testing, and the mass of the carbon nanotubes in the first film layer and the second film layer is calculated using the calculation method in the above method.

[0111] In some embodiments, the Dv50 of the silicon element is in the range of 0 < Dv50 ≤ 10 nm; alternatively, the Dv50 of the silicon element is in the range of 2 nm < Dv50 ≤ 8 nm. Using nanometer-sized silicon element facilitates mixing with silicon oxide, thereby improving the battery's charge capacity, and easing the difficulty of coating the core structure, thereby enhancing the battery's cycling stability.

[0112] The chemical formula for "silicon oxide" is SiOx, where 0 < x ≤ 2, such as silicon monoxide (SiO) and silicon dioxide (SiO2). In some embodiments, the silicon oxide used has a Dv50 range of 2 μm ≤ Dv50 ≤ 13 μm. This facilitates mixing with silicon and improves the battery's energy density.

[0113] In some embodiments, the lithium silicate comprises one or more of Li2SiO3, Li2Si2O5, Li4SiO4, Li2Si3O7, Li8SiO6, Li6Si2O7, Li4Si2O7, Li2Si4O7, and LiSiO3. Optionally, the lithium silicate comprises at least Li2SiO3, so that the first coating layer effectively reduces the consumption of lithium ions in the positive electrode active material during the initial lithium insertion process, resulting in a higher initial coulombic efficiency of the battery. Further, the mass of Li2SiO3 is at least 50% of the mass of the first coating layer, such as 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90%. More preferably, the mass of Li2SiO3 is at least 70% of the mass of the first coating layer, ensuring higher lithium conductivity of the first coating layer and improving the charging capacity of the overall material.

[0114] Standard samples with different Li2SiO3 contents were prepared based on the type of lithium silicate in the first coating layer. The test contents of different Li2SiO3 contents were determined using TOF-SIMES, and a standard curve was established between the test contents and the actual contents. The test contents of the first coating layer were determined using TOF-SIMES, and the accurate Li2SiO3 content was obtained by comparing the results with the standard curve.

[0115] In any embodiment of the first aspect, the thickness of the first coating layer is 10% to 80% of the particle radius of the porous silicon negative electrode material, optionally, the thickness of the second coating layer is 1nm to 50nm; optionally, the thickness of the third coating layer is 1nm to 1000nm, optionally 5nm to 300nm. Within the above thickness range, the particles effectively improve the volume expansion of the negative electrode material while improving the energy density of the battery, and improve the cycle stability and charging capacity of the high energy density battery. The above thickness is tested from the outside to the inside of the particle by TOF-SIMES, and the thickness of each coating layer is judged by the material at different depths. When preparing the porous silicon negative electrode material, the amount of raw materials corresponding to each coating layer is adjusted to adjust its thickness.

[0116] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0117] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0118] [Positive electrode]

[0119] The positive electrode sheet generally includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0120] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0121] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0122] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0123] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0124] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0125] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0126] [Electrolytes]

[0127] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0128] In some embodiments, the electrolyte is liquid and includes an electrolyte salt and a solvent.

[0129] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0130] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0131] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0132] [Isolation film]

[0133] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0134] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0135] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0136] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0137] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0138] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG5 shows a secondary battery 5 having a square structure as an example.

[0139] In some embodiments, referring to Figure 6, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0140] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0141] Figure 7 shows an example battery module 4. Referring to Figure 7 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0142] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0143] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0144] Figures 8 and 9 illustrate an example battery pack 1. Referring to Figures 8 and 9 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0145] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0146] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0147] Figure 10 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0148] [Example]

[0149] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0150] 1. Porous silicon anode materials

[0151] Preparation process of porous silicon negative electrode material 1

[0152] 1. Silicon grains and silicon dioxide are prepared by vapor deposition to obtain silicon oxide, wherein the deposition temperature is controlled at 800 ° C, silicon oxide particles are dispersed in water, and then polyvinyl pyrrolidone (with a molar ratio of 10:1 to silicon dioxide) is added, heated to boiling reflux for 3 hours, and then added with a sodium hydroxide solution with a concentration of 0.05 mmol / L after cooling to room temperature, and then stirred and etched for 4 hours. The obtained suspension is centrifuged and washed with water 2-3 times, dried at 105 ° C for 2 hours, and then calcined in a muffle furnace at 550 ° C for 4 hours to obtain porous silicon oxide. The obtained porous silicon oxide is ground and sieved to adjust its D V 50 size;

[0153] 2. Preparation of a metal compound coating layer for silicon: The metal coating raw material TiO2 and the silicon oxide are mixed at a mass ratio of 0.003:1. The mixed material is subjected to a high-temperature treatment at 1000°C for 4 hours, and the high-temperature treated material is ground. The coating directly reacts with the silicon grains through a phase selective reaction to form a silicon TiSi coating layer on the surface of the silicon element, which serves as a core structure;

[0154] 3. Amorphous carbon: The asphalt and the core structure are mixed at a mass ratio of 0.05:1 and then subjected to high temperature treatment for 6 hours at a temperature controlled at 1100°C to obtain a coating layer containing amorphous carbon;

[0155] 4. The material obtained in step 3 above is mixed with lithium carbonate in a mass ratio of 1:0.2, and subjected to high-temperature treatment at 800°C for 3 hours. The lithium ions pass through the amorphous carbon coating layer, forming a first coating layer containing lithium silicate on the core structure inside the amorphous carbon;

[0156] 5. Finally, the material obtained in step 4 is mixed with the carbon nanotube solution, wherein the mass ratio of the material obtained in step 4 to the carbon nanotubes is 1:0.08, and a bonding reaction occurs under high temperature treatment at 250°C, and the carbon nanotubes are coated on the surface of the material to form a third coating layer of amorphous carbon material and carbon nanotubes.

[0157] Based on the preparation process of the porous silicon negative electrode material 1, porous silicon negative electrode materials 2 to 5 are obtained by adjusting the grinding time and the sieving particle size.

[0158] Based on the preparation process of the porous silicon negative electrode material 1 described above, porous silicon negative electrode materials 6 to 10 were obtained by adjusting the temperature of vapor deposition.

[0159] Based on the preparation process of the porous silicon negative electrode material 1 , the mass contents of the porous silicon material and lithium carbonate are adjusted to obtain porous silicon negative electrode materials 11 to 14 with different first coating layer thicknesses.

[0160] Based on the preparation process of the porous silicon negative electrode material 1 described above, the amount of the metal coating raw material TiO 2 is halved to obtain the porous silicon negative electrode material 15 .

[0161] Preparation process of porous silicon negative electrode material 16

[0162] The only difference from the preparation process of porous silicon negative electrode material 1 is that MgCl2 is used instead of TiO2.

[0163] Preparation process of porous silicon negative electrode material 17

[0164] 1. Silicon grains and silicon dioxide are prepared by vapor deposition to obtain silicon oxide, wherein the deposition temperature is controlled at 800 ° C, silicon oxide particles are dispersed in water, and then polyvinyl pyrrolidone (with a molar ratio of 10:1 to silicon dioxide) is added, heated to boiling reflux for 3 hours, cooled to room temperature, and then sodium hydroxide solution is added, and then stirred and etched for several hours. The obtained suspension is centrifuged, washed with water 2-3 times, dried at 105 ° C for 2 hours, and then calcined in a muffle furnace at 550 ° C for 4 hours to obtain porous silicon oxide as a core structure. The obtained porous silicon oxide is ground and sieved to adjust its D V 50 size;

[0165] 2. Amorphous carbon: The asphalt and the core structure are mixed at a mass ratio of 0.05:1 and then subjected to high temperature treatment for 6 hours at a temperature controlled at 1000°C to obtain a coating layer containing amorphous carbon;

[0166] 3. The material obtained in step 2 above is mixed with lithium carbonate in a mass ratio of 1:1.2. After high-temperature treatment at 800°C for 3 hours, lithium ions will pass through the amorphous carbon coating layer and form a first coating layer containing lithium silicate on the core structure inside the amorphous carbon;

[0167] 4. Preparation of a silicon metal compound coating layer: The metal coating raw material and the material obtained in step 3 are mixed at a mass ratio of 0.015:1. The mixed material is subjected to a high-temperature treatment at 1000°C for 1 hour. The treated material is ball-milled and stirred for 30 minutes. The metal ions will pass through the amorphous carbon coating layer and form a second coating layer of silicon-containing metal compounds inside the amorphous carbon and on the surface of the lithium silicate;

[0168] 5. Finally, the material obtained in step 4 is mixed with the carbon nanotube solution, wherein the mass ratio of the material obtained in step 4 to the carbon nanotube is 1:0.08, and a bonding reaction occurs at a high temperature of 250°C, and the carbon nanotubes are coated on the surface of the material to form a third coating layer of amorphous carbon material and carbon nanotubes.

[0169] Based on the above-mentioned preparation process of the porous silicon negative electrode material 17 , porous silicon negative electrode materials 18 to 21 are obtained by adjusting the amount of the metal coating raw material.

[0170] Based on the above-mentioned preparation process of the porous silicon negative electrode material 17 , the porous silicon negative electrode materials 22 and 23 are obtained by adjusting the amount of asphalt.

[0171] Based on the above-mentioned preparation process of the porous silicon negative electrode material 17 , the porous silicon negative electrode material 24 is obtained by adjusting the amount of carbon nanotubes.

[0172] Preparation process of porous silicon negative electrode material 25

[0173] The only difference from the preparation process of the porous silicon negative electrode material 17 is that MgCl2 is used instead of TiO2.

[0174] Preparation process of porous silicon negative electrode material 26

[0175] 1. Silicon grains and silicon dioxide are prepared and synthesized by vapor deposition to obtain silicon oxide, wherein the deposition temperature is controlled at 800°C, silicon oxide particles are dispersed in water, and then polyvinyl pyrrolidone (with a molar ratio of 10:1 to silicon dioxide) is added, heated to boiling reflux for 3 hours, and then added with a sodium hydroxide solution with a concentration of 0.05 mmol / L after cooling to room temperature, and then stirred and etched for several hours. The resulting suspension is centrifuged, washed with water 2-3 times, dried at 105°C for 2 hours, and then calcined in a muffle furnace at 550°C for 4 hours to obtain porous silicon oxide as a core structure;

[0176] 2. Amorphous carbon: The asphalt and the core structure are mixed at a mass ratio of 1.2:1 and then subjected to high temperature treatment for 6 hours at a temperature controlled at 1100°C to obtain a coating layer containing amorphous carbon;

[0177] 3. The material obtained in step 2 above is mixed with lithium carbonate at a mass ratio of 1:1.2. After high-temperature treatment at 800°C for 3 hours, lithium ions will pass through the amorphous carbon coating layer and form a first coating layer containing lithium silicate on the core structure inside the amorphous carbon;

[0178] 4. Finally, the material obtained in step 3 is mixed with the carbon nanotube solution, wherein the mass ratio of the material obtained in step 3 to the carbon nanotube is 1:0.08, and a bonding reaction occurs at a high temperature of 250°C, and the carbon nanotubes are coated on the surface of the material to form a third coating layer of amorphous carbon material and carbon nanotubes.

[0179] Preparation process of silicon negative electrode material 27

[0180] 1. Silicon grains and silicon dioxide are synthesized by vapor deposition to obtain silicon oxide as the core structure;

[0181] 2. Amorphous carbon: The asphalt and the core structure are mixed at a mass ratio of 1.2:1 and then subjected to high temperature treatment for 6 hours at a temperature controlled at 1100°C to obtain a coating layer containing amorphous carbon;

[0182] 3. The material obtained in step 2 above is mixed with lithium carbonate at a mass ratio of 1:1.2. After high-temperature treatment at 800°C for 3 hours, lithium ions will pass through the amorphous carbon coating layer and form a first coating layer containing lithium silicate on the core structure inside the amorphous carbon;

[0183] 4. Finally, the material obtained in step 3 is mixed with the carbon nanotube solution, wherein the mass ratio of the material obtained in step 3 to the carbon nanotubes is 1:0.08, and a bonding reaction occurs under high temperature treatment at 250°C, and the carbon nanotubes are coated on the surface of the material to form a third coating layer of amorphous carbon material and carbon nanotubes.

[0184] Tests of the prepared porous silicon anode material:

[0185] Dv50: Measured using a laser particle size analyzer (e.g., Malvern Master Size 3000) in accordance with GB / T 19077.1-2016. Dv50 is the particle size at which the cumulative volume distribution percentage of the measured material reaches 50%. The Dv50 of elemental silicon is measured using the aforementioned method when the core structure is formed. The Dv50 of porous silicon anode materials is measured using the aforementioned method after all coating layers are applied.

[0186] BET specific surface area: The specific surface area of ​​porous silicon anode materials was determined using the low-temperature nitrogen adsorption-desorption method.

[0187] Determination of the mass content of Li2SiO3: The corresponding Li2SiO3 content is obtained through TOF-SIMES testing, and the standard curve is tested with standard samples. The accurate Li2SiO3 content is obtained by comparing with the standard curve.

[0188] Determination of the thickness of each coating layer: TOF-SIMES is used to test from the outside to the inside of the particle, and the thickness of each coating layer is determined by the material at different depths.

[0189] Determination of carbon nanotube content: Using SEM, the number and length of carbon nanotubes on the surface of multiple silicon particles can be counted. By simulating the chain length and number of molecules in the carbon nanotubes, the mass of the corresponding single carbon nanotube is calculated. The average value of the calculated number of carbon nanotubes is used as the mass of carbon nanotubes on the surface of the porous silicon anode material. The volume and true density of the porous silicon anode material are then used to calculate the mass percentages of carbon nanotubes C1 and C2 in the two film layers of the negative electrode sheet.

[0190] The test results are recorded in Tables 1 and 2.

[0191] 2. Preparation of negative electrode

[0192] Negative electrode 1:

[0193] Coating weight 10mg / cm 2 The coating weight ratio of the first coating layer and the second coating layer is 70:30; in the first coating layer, the mass contents of the porous silicon negative electrode material 1, graphite, binder, dispersant, conductive agent SP, and conductive agent CNT are 10%, 85.73%, 2%, 1.2%, 1%, and 0.07%, respectively; in the second coating layer, the mass contents of the porous silicon negative electrode material 1, graphite, binder, dispersant, conductive agent SP, and conductive agent CNT are 30%, 65.19%, 2.4%, 1.2%, 1%, and 0.21%, respectively.

[0194] The raw materials of the first coating layer are fully stirred and mixed in an appropriate amount of solvent water according to the above weight ratio to form a uniform negative electrode slurry to obtain a first negative electrode slurry; the second negative electrode slurry can be obtained by the same method; the first negative electrode slurry is coated on the surface of the negative electrode current collector copper foil, and the second negative electrode slurry is coated on the surface of the first negative electrode slurry; after drying and cold pressing, a negative electrode sheet 1 is obtained, and the surface density M1 of the first film layer and the surface density M2 of the second film layer formed in each negative electrode sheet are recorded in Table 2.

[0195] The only difference between the negative electrode sheets 2 to 27 and the negative electrode sheet 1 is that the porous silicon negative electrode material 1 in the negative electrode sheet 1 is replaced by the corresponding porous silicon negative electrode materials 2 to 27 in sequence.

[0196] Negative electrode sheets 27 to 36 all use porous silicon negative electrode material 1, but its usage, surface density, thickness, and film mass percentage in the first film layer and the second film layer are shown in Table 3. Due to the change in usage, the amount of graphite is adjusted so that the total mass fraction of porous silicon negative electrode material 1, graphite, binder, dispersant and conductive agent SP in the first film layer meets 100%, and the amount of graphite is adjusted so that the total mass fraction of porous silicon negative electrode material 1, graphite, binder, dispersant, conductive agent SP, and conductive agent CNT in the second film layer meets 100%.

[0197] The negative electrode sheet 37 uses the silicon negative electrode material 27 to replace the porous silicon negative electrode material 1 in the negative electrode sheet 1 .

[0198] Negative electrode test:

[0199] Determination of the porosity of the negative electrode film layer: take a picture of the CP section, scan it with imaging software, and calculate the porosity of the corresponding section. Then, take pictures of multiple CP sections and calculate them with software to obtain the porosity of the negative electrode film layer.

[0200] Thickness: The thickness was measured by photographing the CP section.

[0201] The weight ratio of the first film layer to the second film layer is recorded as A.

[0202] Calculate the amount of raw materials used and the data in Tables 1 and 2 per 1cm 2 In the first film layer and the second film layer, the mass m1 and m2 of the porous silicon negative electrode material in the first film layer and the second film layer; the mass percentage of the carbon nanotubes belonging to the porous silicon negative electrode material in the first film layer and the second film layer is C1 and C2, excluding the carbon nanotubes in the porous silicon negative electrode material, the mass of the remaining carbon nanotubes in the first film layer and the second film layer are S1 and S2 respectively.

[0203] 3. Secondary batteries

[0204] Example 1

[0205] Negative electrode:

[0206] The above-mentioned negative electrode sheet 1 is used as the negative electrode sheet of the secondary battery of Example 1.

[0207] Positive electrode:

[0208] Coating weight 25.5mg / cm 2The cathode is a single-layer coating, and the mass contents of the ternary materials NCM811, PVDF, conductive agent SP, and conductive agent CNT are 96.5%, 1.5%, 1.5%, and 0.5% respectively;

[0209] The raw materials are fully stirred and mixed in an appropriate amount of solvent NMP according to the above weight ratio to form a uniform positive electrode slurry; the positive electrode slurry is coated on the surface of the positive electrode current collector aluminum foil; after drying and cold pressing, a positive electrode sheet is obtained.

[0210] Electrolyte: The electrolyte includes a solvent, a lithium salt and an additive, wherein the type and content of the solvent are as follows: EC:DMC:DEC=0.3:0.5:0.2, the lithium salt content is 1.0 mol / L, and the film-forming additive includes 8% FEC.

[0211] Diaphragm: PE diaphragm;

[0212] The negative electrode sheets of the secondary batteries of Examples 2 to 37 are negative electrode sheets 2 to 37, respectively, and the positive electrode sheets, electrolytes and separators are the same as those of Example 1.

[0213] The negative electrode sheet of the secondary battery of Comparative Example 1 is negative electrode sheet 38 , and the positive electrode sheet, electrolyte and separator are the same as those of Example 1.

[0214] Gram Capacity Test:

[0215] The negative electrode and lithium sheet were made into a button half-cell for testing, and the charge and discharge test was carried out at 25°C at a rate of 0.04C, with a voltage range of 0.05V-2V. The capacity was tested and the gram capacity of the negative electrode was calculated, gram capacity = test capacity / mass of active material.

[0216] Cyclic performance test:

[0217] At 25°C, the secondary batteries prepared in each example and comparative example were charged at a constant current rate of 1C to a charge cutoff voltage of 4.25V. They were then charged at a constant voltage to a current of ≤0.05C, allowed to rest for 5 minutes, and then discharged at a constant current rate of 1C to a discharge cutoff voltage of 2.8V, allowed to rest for 5 minutes. This constituted one charge-discharge cycle. The batteries were subjected to cyclic charge-discharge testing using this method until the battery capacity decayed to 80%. The number of cycles at this point is the battery's cycle life at 25°C.

[0218] Charging capacity test:

[0219] At 25°C, the batteries of the above embodiments and comparative examples were charged and discharged for the first time at a current of 1C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour). Specifically, the battery was charged at a constant current rate of 1C to a voltage of 4.25V, then charged at a constant voltage to a current of ≤0.05C, allowed to stand for 5 minutes, and then discharged at a constant current rate of 0.33C to a voltage of 2.8V. The actual capacity was recorded as C0.

[0220] Then the battery is charged with constant current at 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, 3.0C0 in sequence to the full battery charge cut-off voltage of 4.25V or the negative electrode cut-off potential of 0V (whichever is reached first). After each charge is completed, it is necessary to discharge at 1C0 to the full battery discharge cut-off voltage of 2.8V. Record the state of charge (SOC) at different charge rates to 10%, 20%, 30%, ..., 80%. The charge rate-negative electrode potential curves at different SOC states were plotted. After linear fitting, the charge rate corresponding to the negative electrode potential of 0V at different SOC states was obtained. This charge rate is the charging window at that SOC state, which is recorded as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC, respectively. The charging time T for charging the battery from 10% SOC to 80% SOC is calculated according to the formula (60 / C20%SOC + 60 / C30%SOC + 60 / C40%SOC + 60 / C50%SOC + 60 / C60%SOC + 60 / C70%SOC + 60 / C80%SOC) × 10%, in minutes. The shorter this time, the better the fast charging performance of the battery.

[0221] The test results are recorded in Table 3.

[0222] Table 3

[0223] According to the comparison of Examples 1 to 5, it can be seen that the D V 50 changes will directly affect the cycle performance and charging capacity of the battery, especially with the increase of the D V As the value of 50 increases, the charging capacity of the battery decreases.

[0224] According to the comparison of Examples 6 to 10, it can be seen that the D V 50 changes will affect the battery's charging capacity and cycle performance. V With an increase of 50, the cycle performance and charging capacity of the battery decrease.

[0225] According to the comparison of Examples 11 to 14, it can be seen that the change in the thickness of the first coating layer containing lithium silicate will affect the charging capacity and cycle performance of the battery. As the thickness of the first coating layer increases, the cycle performance of the battery improves, but the charging capacity improves.

[0226] According to the comparison of Examples 17 to 20, it can be seen that the change in the thickness of the metal compound of the second coating layer silicon will affect the charging capacity and cycle performance of the battery. As the thickness of the second coating layer increases, the cycle performance of the battery first increases and then decreases, and the charging capacity gradually decreases. This is because the second coating layer increases the anti-expansion ability of the silicon material and improves the cycle performance. However, the conductivity of the second coating layer is poor, and the increase in coating thickness will deteriorate the dynamics.

[0227] According to the comparison of Examples 1, 17 and 27, it can be seen that different coating methods have a significant impact on the cycle performance and charging capacity of the battery.

[0228] According to the comparison between Example 1, Examples 28 to 37 and Comparative Example 1, it can be seen that the content of porous silicon negative electrode material in the second film layer increases, which helps to improve the charging capacity of the battery; and the gram capacity of Example 28 is more prominent because the content of porous silicon negative electrode in the first film layer is relatively high, and its cycle performance is also reduced because the content of porous silicon negative electrode in the first film layer is relatively high.

[0229] Although the present application has been described with reference to preferred embodiments, various modifications may be made thereto and components may be substituted with equivalents without departing from the scope of the present application. In particular, the various technical features described in the various embodiments may be combined in any manner as long as there are no structural conflicts. The present application is not limited to the specific embodiments disclosed herein, but encompasses all technical solutions within the scope of the claims.

Claims

1. A negative electrode sheet, comprising a current collector (01) and a negative electrode film layer, wherein the negative electrode film layer comprises: A first film layer (02) is arranged on one side or both sides of the current collector (01); A second film layer (03) is arranged on a side of the first film layer (02) away from the current collector (01); The negative electrode active material of the first film layer (02) and the negative electrode active material in the second film layer (03) each independently include a silicon-based negative electrode material, and the silicon-based negative electrode material in the second film layer (03) includes a porous silicon negative electrode material (10), and the content of the silicon-based negative electrode material in the first film layer (02) is less than the content of the silicon-based negative electrode material in the second film layer (03).

2. The negative electrode sheet according to claim 1, wherein: The surface density M1 of the first film layer (02) is in the range of 4.5 mg / cm 2 ~20mg / cm 2 The surface density M2 of the second film layer (03) is in the range of 4.5 mg / cm 2 ~20mg / cm 2 ; Optionally, the weight ratio of the first film layer (02) and the second film layer (03) is 95:5 to 30:70, and can be further optionally 90:10 to 40:60; Optionally, the porosity of the first film layer (02) is in the range of 20% to 50%, the porosity of the second film layer (03) is in the range of 20% to 70%, and the porosity of the negative electrode film layer is in the range of 20% to 70%; Optionally, the thickness of the second film layer (03) is 10μm to 20μm.

3. The negative electrode sheet according to claim 1 or 2, wherein: The mass content of the silicon-based negative electrode material in the first film layer (02) is 0.5% to 50%; and / or the mass content of the porous silicon negative electrode material (10) in the second film layer (03) is 0.5% to 70%; optionally, the mass content of the porous silicon negative electrode material (10) in the negative electrode film layer is 1% to 60%.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein: The second film layer (03) comprises the porous silicon negative electrode material (10), a graphite negative electrode material, a binder, a dispersant and a conductive agent, wherein the conductive agent comprises one or more of conductive carbon and carbon nanotubes; optionally, the mass percentage of the porous silicon negative electrode material (10) in the second film layer (03) is 0.5% to 70%; optionally, the mass percentage of the graphite negative electrode material in the second film layer (03) is 25% to 90%; optionally, the mass percentage of the binder in the second film layer (03) is 1% to 8%; optionally, the mass percentage of the dispersant in the second film layer (03) is 0.5% to 2%; optionally, the mass percentage of the conductive carbon in the second film layer (03) is 0.5% to 5%; optionally, the mass percentage of the carbon nanotubes in the second film layer (03) is 0.05% to 2%.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein: The first film layer (02) comprises the silicon-based negative electrode material, the graphite negative electrode material, the binder, the dispersant and the conductive agent, wherein the conductive agent comprises one or more of conductive carbon and carbon nanotubes; optionally, the mass percentage of the silicon-based negative electrode material in the first film layer (02) is 0.5% to 50%; optionally, the mass percentage of the graphite negative electrode material in the first film layer (02) is 45% to 97.5%; optionally, the mass percentage of the binder in the first film layer (02) is 1% to 3%; optionally, the mass percentage of the dispersant in the first film layer (02) is 1% to 3%. 02) has a mass percentage of 0.3% to 1.5%; optionally, the mass percentage of the conductive carbon in the first film layer (02) is 0 to 3%; optionally, the mass percentage of the carbon nanotubes in the first film layer (02) is 0 to 0.5%; optionally, the silicon-based negative electrode material includes any one or more of the group consisting of silicon material, silicon oxide material and porous silicon negative electrode material (10), and the content of the porous silicon negative electrode material (10) in the first film layer (02) is less than the content of the porous silicon negative electrode material (10) in the second film layer (03).

6. The negative electrode sheet according to any one of claims 1 to 5, wherein: The Dv of the porous silicon negative electrode material (10) 50 The BET specific surface area of ​​the porous silicon negative electrode material (10) is between 1 m 2 / g~30m 2 / g; optionally between 6m 2 / g~20m 2 / g.

7. The negative electrode sheet according to any one of claims 1 to 6, wherein: The porous silicon negative electrode material (10) comprises: A core (11), wherein the core (11) is porous silicon, and the porous silicon includes silicon and silicon compounds, and the silicon compounds include silicon oxides; A coating layer is coated on the surface of the core (11).

8. The negative electrode sheet according to claim 7, wherein: The coating layer includes any one or more of metal compounds of silicon, lithium silicate, amorphous carbon, and carbon nanotubes.

9. The negative electrode sheet according to claim 8, wherein: The coating layer comprises: A first coating layer (12), the first coating layer (12) is coated on the surface of the core (11), and the first coating layer (12) comprises lithium silicate; A second coating layer (13), the second coating layer (13) is coated on the surface of the first coating layer (12), the second coating layer (13) comprises a metal compound of silicon, and optionally the metal element of the metal compound of silicon comprises Ti, Mg and / or Al; A third coating layer (14), the third coating layer (14) is coated on the surface of the second coating layer (13), the third coating layer (14) comprises amorphous carbon, and optionally, the material of the amorphous carbon comprises hard carbon and / or soft carbon.

10. The negative electrode sheet according to claim 8, wherein: The coating layer comprises: A first coating layer (12), the first coating layer (12) is coated on the surface of the core (11), and the first coating layer comprises the lithium silicate; A third coating layer (14), the third coating layer (14) is coated on the surface of the first coating layer (12), the third coating layer (14) comprises the amorphous carbon, and optionally, the material of the amorphous carbon comprises hard carbon and / or soft carbon.

11. The negative electrode sheet according to any one of claims 7 to 8 and 10, wherein: The silicon compound further includes a silicon metal compound, and the silicon metal compound is coated on the surface of the silicon element. Optionally, the metal element of the silicon metal compound includes Ti, Mg and / or Al.

12. The negative electrode sheet according to any one of claims 9 to 11, wherein: The third coating layer also includes the carbon nanotubes.

13. The negative electrode sheet according to claim 12, wherein: The masses of the porous silicon negative electrode material (10) in the first film layer (02) and the second film layer (03) are m1 and m2 respectively; the mass percentages of the carbon nanotubes belonging to the porous silicon negative electrode material (10) in the first film layer (02) and the second film layer (03) are C1 and C2 respectively; Excluding the carbon nanotubes in the porous silicon negative electrode material (10), the masses of the remaining carbon nanotubes in the first film layer (02) and the second film layer (03) are S1 and S2 respectively, the first film layer (02) and the second film layer (03) are the first film layer (02) and the second film layer (03) per unit area respectively, and the following relationship is satisfied: (m2×C2+S2) / M2=(m1×C1+S1) / M1, m2≥m1, S1≥S2, S2=m1×m2×(C2-C1) / (m2-m1).

14. The negative electrode sheet according to any one of claims 7 to 13, wherein: The Dv50 range of the silicon single substance is: 0<Dv50≤10nm; optionally, the Dv50 range of the silicon single substance is: 2nm<Dv50≤8nm.

15. The negative electrode sheet according to any one of claims 7 to 14, wherein: The chemical formula of the silicon oxide is SiOx, wherein 0<x≤2; the Dv50 range of the silicon oxide is: 2μm≤Dv50≤13μm.

16. The negative electrode sheet according to claim 9 or 10, wherein: The lithium silicate includes one or more of Li2SiO3, Li2Si2O5, Li4SiO4, Li2Si3O7, Li8SiO6, Li6Si2O7, Li4Si2O7, Li2Si4O7 and LiSiO3; optionally, the lithium silicate includes at least Li2SiO3, and further optionally, the mass of the Li2SiO3 is at least 50% of the mass of the first coating layer, and more optionally, the mass of the Li2SiO3 is at least 70% of the mass of the first coating layer.

17. The negative electrode sheet according to any one of claims 9 to 16, wherein: The thickness of the first coating layer is 10% to 80% of the particle radius of the porous silicon negative electrode material (10). Optionally, the thickness of the second coating layer is 1nm to 50nm. Optionally, the thickness of the third coating layer is 1nm to 1000nm, and can be 5nm to 300nm.

18. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein: The negative electrode plate is the negative electrode plate according to any one of claims 1 to 17.

19. An electrical device comprising a secondary battery, wherein: The secondary battery is selected from the secondary battery according to claim 18.

Images