Secondary battery, method for manufacturing the same, and electric device

Abstract

A secondary battery includes a positive electrode tab, a negative electrode tab, and a separator disposed between the positive electrode tab and the negative electrode tab; the positive electrode tab includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer including modified graphite, the modified graphite including graphite and anions located between layers of the graphite.

Description

Technical Field

[0001] This application relates to the field of secondary batteries, and more specifically to secondary batteries, their preparation methods, and electrical devices. Background Technology

[0002] The statements herein are provided only as background information in connection with this application and do not necessarily constitute prior art.

[0003] During the first charge and discharge of a lithium-ion battery, the electrode material and the electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material. This passivation layer is an interface layer with the characteristics of a solid electrolyte. It is an electronic insulator but an excellent conductor of lithium ions. Lithium ions can freely insert and extract through this passivation layer. Therefore, this passivation film is called a "solid electrolyte interface" (SEI film) for short.

[0004] The formation of the SEI film consumes some active lithium ions, increasing the irreversible capacity during the first charge-discharge cycle and reducing the battery's cycle performance. Therefore, to compensate for the performance loss caused by SEI film formation, lithium replenishment is often necessary. However, traditional lithium replenishment technologies often require the introduction of lithium replenishing agents that negatively impact battery energy density or stability, or complex processes. Therefore, how to further simplify lithium replenishment for rechargeable batteries and avoid excessive negative impacts on other battery performance aspects remains a pressing issue in the rechargeable battery field. Summary of the Invention

[0005] According to various embodiments of this application, a secondary battery, a method for preparing the same, and an electrical device thereof are provided.

[0006] A first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and a separator, wherein the separator is disposed between the positive electrode and the negative electrode;

[0007] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes modified graphite, which includes graphite and anions located between the layers of graphite.

[0008] By embedding anions into the interlayer of graphite used as a positive electrode additive, these anions are less likely to escape during cycling. To maintain charge balance, lithium ions in the corresponding electrolyte are easily embedded into the negative electrode. This allows for simple and efficient lithium replenishment without significantly altering existing battery manufacturing processes, effectively improving battery cycle performance. Furthermore, the modified graphite with embedded anions also enhances conductivity, further improving the battery's electrical performance.

[0009] In some embodiments, the anion is a monovalent anion; optionally, the monovalent anion includes PF6. - ClO4 - TFSI - FSI - and NO3 - One or more of these. Suitable types of anions can be better embedded between graphite layers without affecting battery performance, and have a lower extraction rate during cycling, which is more conducive to improving lithium replenishment efficiency.

[0010] In some embodiments, the X-ray diffraction pattern of the modified graphite exhibits characteristic peaks in the range of 10° to 20°. Irreversible phase changes occur after the anions that are difficult to extract during the interlayer embedding cycle of graphite, thus enabling the detection of characteristic X-ray diffraction peaks in the 10° to 20° range in the obtained modified graphite.

[0011] In some embodiments, the positive electrode active material layer uses one or more of lithium iron phosphate and ternary positive electrode materials; optionally, the ternary positive electrode material includes Li a Ni x Co y M z O2, wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe, Mo, and B, 0.95≤a≤1.2, x>0, y>0, z>0, and x+y+z=1. The working voltage of a suitable type of positive electrode active material is better matched to the voltage required for anion embedding between graphite layers, avoiding failure of anion embedding between graphite layers due to voltage mismatch during preparation, or causing electrolyte decomposition.

[0012] In some embodiments, the graphite content in the positive electrode active material layer is 1% to 8% by mass; optionally, the graphite content in the positive electrode active material layer is 2% to 4% by mass. By controlling the graphite content in the positive electrode active material layer within a suitable range, the lithium replenishment requirement is met without excessively encroaching on the space of the positive electrode active material, thus avoiding a decrease in battery capacity and achieving a balance between lithium replenishment and battery capacity.

[0013] In some embodiments, the Dv50 particle size of the graphite is 10 μm to 20 μm; optionally, the Dv50 particle size of the graphite is 16 μm to 18 μm. Controlling the particle size of the graphite within a suitable range can make the positive electrode slurry more uniform during slurry preparation, the electrode compaction density more suitable, and the ion diffusion path moderate, thereby further improving the lithium replenishment and conductivity.

[0014] A second aspect of this application provides a method for preparing a secondary battery according to one or more of the foregoing embodiments, comprising the following steps:

[0015] The graphite, other raw materials for preparing the positive electrode active material layer, and solvent are mixed to prepare a positive electrode slurry;

[0016] The positive electrode slurry is coated onto at least one surface of the positive electrode current collector, and then dried and pressed to obtain a positive electrode sheet.

[0017] The positive electrode, negative electrode, and separator are assembled, injected with electrolyte, and formed at a voltage of 4.5V to 4.8V.

[0018] The voltage of the formation process is controlled at 4.5V to 4.8V, which allows anions to be smoothly inserted into the interlayer of graphite without easily detaching, and does not cause electrolyte decomposition, thus affecting battery performance.

[0019] In some embodiments, the solute in the electrolyte includes one or more of LiPF6, LiClO4, LiTFSI, LiFSI, and LiNO3. A suitable electrolyte solute provides a suitable anion species that can better intercalate between graphite layers during formation and is less prone to extraction during cycling, thereby achieving better lithium replenishment.

[0020] In some embodiments, the lithium ion concentration in the electrolyte is 1 mol / L to 1.5 mol / L; optionally, the lithium ion concentration in the electrolyte is 1.2 mol / L to 1.3 mol / L. In this application, the electrolyte not only provides basic electron and ion conduction functions but also provides a source of lithium ions after formation, thus playing a role in lithium replenishment. Therefore, a suitable lithium ion concentration in the electrolyte is an important prerequisite for maintaining the basic performance of the battery while achieving lithium replenishment.

[0021] In some embodiments, the formation current is less than 0.1C. Controlling the formation current within a suitable range allows anions to be more uniformly embedded in the interlayer of graphite, reducing anion release during cycling and thus improving lithium replenishment efficiency.

[0022] In some embodiments, when the formation process is performed at a voltage of 4.5V to 4.8V, the specific capacity of the anions embedded in the graphite ranges from 100mAh / g to 200mAh / g.

[0023] In some embodiments, when the formation process is performed at a voltage of 4.5V to 4.8V, the specific capacity of the anions extracted from the graphite ranges from 20mAh / g to 100mAh / g.

[0024] By employing appropriate formation conditions, the specific capacity of anions embedded in graphite can be made greater than that of anions released, thereby achieving lithium replenishment.

[0025] A third aspect of this application provides an electrical device comprising a secondary battery as described in one or more of the foregoing embodiments.

[0026] Details of one or more embodiments of this application are set forth in the following drawings and description. Other features, objects, and advantages of this application will become apparent from the specification, drawings, and claims. Attached Figure Description

[0027] To better describe and illustrate embodiments or examples of the inventions disclosed herein, reference may be made to one or more accompanying drawings. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed inventions, the embodiments or examples currently described, or the best mode of these inventions as currently understood.

[0028] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0029] Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of this application is shown.

[0030] Figure 3 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0031] Explanation of reference numerals in the attached figures:

[0032] 1: Secondary battery; 11: Casing; 12: Electrode assembly; 13: Cover plate; 2: Electrical device.

[0033] Figure 4 These are X-ray diffraction patterns of the graphite raw materials used in the cathodes of the various embodiments and comparative examples of this application.

[0034] Figure 5 This is the X-ray diffraction pattern (Test 1) of the graphite in the positive electrode of the battery prepared in Example 1 of this application.

[0035] Figure 6 This is the X-ray diffraction pattern (Test 2) of the graphite in the positive electrode of the battery prepared in Comparative Example 2 of this application.

[0036] Figure 7 This is the X-ray diffraction pattern (Test 3) of the graphite in the positive electrode of the battery prepared in Comparative Example 3 of this application.

[0037] Figure 8This is the X-ray diffraction pattern (test 4) of the graphite of the positive electrode of the battery prepared in Example 1 of this application after voltage formation treatment at 2.5V. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0040] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is understood that ranges of 60–110 and 80–120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "a–b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0041] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0042] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0043] Unless otherwise specified, all steps in this 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 it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0044] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0045] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: 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).

[0046] As market demand for longer battery life gradually increases, effectively improving battery cycle performance has become a research hotspot in the field of rechargeable batteries. The formation of the SEI film is a significant cause of impaired battery cycle performance; therefore, additional lithium replenishment has become an effective measure to improve battery cycle performance. Current lithium replenishment methods can be broadly categorized into positive electrode lithium replenishment and negative electrode lithium replenishment. Positive electrode lithium replenishment often involves introducing a certain amount of lithium salt replenishing agent into the positive electrode active material layer. However, these lithium salt replenishing agents often cause the collapse of the corresponding positions in the active material layer after lithium replenishment, thus requiring corresponding reinforcement design of the positive electrode active material layer or the entire electrode structure. Furthermore, this process occupies the position of the positive electrode active material and has no other function besides lithium replenishment, significantly reducing battery capacity. Negative electrode lithium replenishment often uses elemental lithium as the replenishing agent. Elemental lithium is highly reactive and readily reacts with water and oxygen, thus requiring special electrode structure design or improved manufacturing processes to achieve, and also posing certain safety risks.

[0047] Based on the above background, the first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and a separator, wherein the separator is disposed between the positive electrode and the negative electrode.

[0048] The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes modified graphite, which includes graphite and anions located between the layers of graphite.

[0049] By embedding anions into the interlayer of graphite used as a positive electrode additive, these anions are less likely to escape during cycling. To maintain charge balance, lithium ions in the corresponding electrolyte are easily embedded into the negative electrode. This allows for simple and efficient lithium replenishment without significantly altering existing battery manufacturing processes, effectively improving battery cycle performance. Furthermore, the modified graphite with embedded anions also enhances conductivity, further improving the battery's electrical performance.

[0050] In some embodiments, the anion is a monovalent anion; optionally, the monovalent anion includes PF6. - (hexafluorophosphate ion), ClO4 - (perchlorate ion), TFSI - (bis(trifluoromethanesulfonylimide) ion), FSI - (Difluorosulfonyl imide ion) and NO3 - One or more of the following (nitrate ions): Suitable types of anions can be better embedded between graphite layers without affecting battery performance, and have a lower extraction rate during cycling, which is more conducive to improving lithium replenishment efficiency.

[0051] In some embodiments, the X-ray diffraction pattern of the modified graphite exhibits characteristic peaks in the range of 10° to 20°. Optionally, the X-ray diffraction pattern of the modified graphite exhibits characteristic peaks at any of the following values ​​or within any combination of two of these values: 11°, 12°, 13°, 14°, 15°, 16°, 17°, 18°, or 19°. Irreversible phase changes occur after the anions that are difficult to extract during the interlayer intercalation cycle of graphite, thus enabling the detection of characteristic X-ray diffraction peaks in the 10° to 20° range in the obtained modified graphite.

[0052] In some embodiments, the positive electrode active material layer uses one or more of lithium iron phosphate and ternary positive electrode materials; optionally, the ternary positive electrode material includes Li a Ni x Co y M z O2, wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe, Mo, and B, 0.95 ≤ a ≤ 1.2, x > 0, y > 0, z > 0, and x + y + z = 1. Further optionally, M is Mn, LiNi x Co y Mn z O2 materials include NCM523 NCM 622 and NCM 811 One or more of the following. The working voltage of the appropriate type of positive electrode active material is more matched with the voltage required to insert anions into the graphite interlayer, so as to avoid failure of anion insertion into the graphite interlayer due to voltage mismatch during preparation, or electrolyte decomposition.

[0053] In some embodiments, the mass percentage of graphite in the positive electrode active material layer is 1% to 8%; alternatively, the mass percentage of graphite in the positive electrode active material layer may be, for example, 3%, 5%, 6%, or 7%, or even 2% to 4%. By controlling the mass percentage of graphite in the positive electrode active material layer within a suitable range, the lithium replenishment requirement is met without excessively encroaching on the space of the positive electrode active material, thus avoiding a decrease in battery capacity and achieving a balance between lithium replenishment and battery capacity.

[0054] In some embodiments, the Dv50 particle size of the graphite is 10 μm to 20 μm; alternatively, the Dv50 particle size of the graphite can be, for example, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, or 19 μm, or even 16 μm to 18 μm. Controlling the particle size of the graphite within a suitable range can make the positive electrode slurry more uniform during slurry preparation, the electrode compaction density more suitable, and the ion diffusion path moderate, thereby further improving the lithium replenishment and conductivity.

[0055] In this application, Dv50 refers to the particle size at which the cumulative volume distribution number of particles reaches 50% in the particle size cumulative distribution curve. Physically, it means that the volume percentage of particles smaller than (or larger than) this particle size value is 50%. As an example, Dv50 can be conveniently determined using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, according to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0056] A second aspect of this application provides a method for preparing a secondary battery according to one or more of the foregoing embodiments, comprising the following steps:

[0057] A positive electrode slurry is prepared by mixing graphite, other raw materials for the positive electrode active material layer, and solvent.

[0058] The positive electrode slurry is coated onto at least one surface of the positive electrode current collector, and then dried and pressed to obtain the positive electrode sheet.

[0059] The positive electrode, negative electrode, and separator are assembled, and an electrolyte is injected. Formation is then performed at a voltage of 4.5V to 4.8V. The formation voltage can also be, for example, 4.55V, 4.6V, 4.65V, 4.7V, or 4.75V.

[0060] The voltage of the formation process is controlled at 4.5V to 4.8V, which allows anions to be smoothly inserted into the interlayer of graphite without easily detaching, and does not cause electrolyte decomposition, thus affecting battery performance.

[0061] In some embodiments, the solute in the electrolyte includes one or more of LiPF6, LiClO4, LiTFSI, LiFSI, and LiNO3. A suitable type of electrolyte solute provides a suitable type of anion that can be better embedded in the interlayer of graphite during formation and is less prone to extraction during cycling, thereby achieving better lithium replenishment.

[0062] In some embodiments, the lithium ion concentration in the electrolyte is 1 mol / L to 1.5 mol / L; alternatively, the lithium ion concentration in the electrolyte may also be 1.1 mol / L or 1.4 mol / L, or even 1.2 mol / L to 1.3 mol / L. In this application, the electrolyte not only provides basic electron and ion conduction functions, but also provides a source of lithium ions after formation, thus playing a role in lithium replenishment. Therefore, having a suitable lithium ion concentration in the electrolyte is an important prerequisite for maintaining the basic performance of the battery while achieving lithium replenishment.

[0063] In some embodiments, the formation current is less than 0.1C. Optionally, the formation current may be, for example, 0.09C, 0.08C, 0.07C, 0.06C, 0.05C, 0.04C, 0.03C, 0.02C, or 0.01C. Controlling the formation current within a suitable range allows anions to be more uniformly embedded in the graphite interlayers, reducing anion release during cycling and thus improving lithium replenishment efficiency.

[0064] In some embodiments, when the formation treatment is performed at a voltage of 4.5V to 4.8V, the specific capacity of the anions embedded in the graphite ranges from 100mAh / g to 200mAh / g. When the formation treatment is performed at a voltage of 4.5V to 4.8V, the specific capacity of the anions embedded in the graphite can also be, for example, 110mAh / g, 120mAh / g, 130mAh / g, 140mAh / g, 150mAh / g, 160mAh / g, 170mAh / g, 180mAh / g, or 190mAh / g.

[0065] In some embodiments, when the formation treatment is performed at a voltage of 4.5V to 4.8V, the specific capacity of the anions extracted from the graphite ranges from 20mAh / g to 100mAh / g. When the formation treatment is performed at a voltage of 4.5V to 4.8V, the specific capacity of the anions extracted from the graphite can also be, for example, 30mAh / g, 40mAh / g, 50mAh / g, 60mAh / g, 70mAh / g, 80mAh / g, or 90mAh / g.

[0066] In this application, "specific capacity" refers to mass specific capacity, which is the capacity value that a unit mass of graphite can extract or insert anions. For example, a specific capacity of 110 mAh / g for inserting anions in graphite means that the capacity value of inserting anions in 1g of graphite is 110 mAh.

[0067] By employing appropriate formation conditions, the specific capacity of anions inserted into graphite can be made greater than that of anions extracted, thereby achieving lithium replenishment. The specific capacity of anions inserted or extracted into graphite depends on the specific anion type; for example, in some embodiments, when the anion is PF6... - At that time, PF6 is embedded in the positive electrode graphite. - The specific capacity is 140mAh / g, and the PF6 is released. - The specific capacity is 20 mAh / g, and the lithium replenishment capacity is 120 mAh / g; in some embodiments, when the anion is FSI - At that time, FSI is embedded in the positive electrode graphite. - The specific capacity is 130mAh / g, and the FSI is removed. - Its specific capacity is 20mAh / g, and its lithium replenishment capacity is 110mAh / g.

[0068] A third aspect of this application provides an electrical device comprising a secondary battery according to one or more of the foregoing embodiments.

[0069] In addition, the secondary battery and power-consuming device of this application will be described below with appropriate reference to the accompanying drawings.

[0070] In one embodiment of this application, a secondary battery is provided.

[0071] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0072] Positive electrode sheet

[0073] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0074] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0075] In some embodiments, the positive 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 substrate and a metal layer formed on at least one surface of the polymer substrate. 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0076] In some embodiments, the positive electrode active material may be a known battery positive electrode active material. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials 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 include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0077] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0078] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0079] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0080] Negative electrode sheet

[0081] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0082] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0083] 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 substrate and a metal layer formed on at least one surface of the polymer substrate. 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0084] In some embodiments, the negative electrode active material may also include negative electrode active materials known in the art for use in batteries, such as: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional 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.

[0085] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from 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).

[0086] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0087] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0088] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0089] electrolytes

[0090] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0091] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0092] 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 difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0093] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0094] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0095] Separating membrane

[0096] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0097] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven 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.

[0098] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0099] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0100] 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. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0101] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured secondary battery 1.

[0102] In some implementations, refer to Figure 2 The outer packaging may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The shell 11 has an opening communicating with the receiving cavity, and the cover plate 13 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator can be formed into an electrode assembly 12 by a winding process or a stacking process. The electrode assembly 12 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 12. The secondary battery 1 may contain one or more electrode assemblies 12, which can be selected by those skilled in the art according to specific practical needs.

[0103] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Among them, mobile devices may be, for example, mobile phones, laptops, etc.; electric vehicles may be, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited to these.

[0104] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0105] Figure 3 Here is an example of an electrical device 2. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc.

[0106] Another example device could be a mobile phone, tablet, laptop, etc.

[0107] Details of one or more embodiments of this application are set forth in the following drawings and description. Other features, objects, and advantages of this application will become apparent from the specification, drawings, and claims.

[0108] Example 1

[0109] (1) Preparation of positive electrode sheet

[0110] Lithium iron phosphate, graphite (Dv50 particle size of 16.9μm), conductive agent acetylene black, and binder PVDF (polyvinylidene fluoride) were mixed in a weight ratio of 92:2:4:2. N-methylpyrrolidone solvent was added and the mixture was stirred and mixed thoroughly to obtain a positive electrode slurry. The slurry was then coated on both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0111] (2) Preparation of negative electrode sheet

[0112] Graphite, conductive agent acetylene black, binder SBR (styrene-butadiene rubber latex), and binder CMC (sodium carboxymethyl cellulose) are mixed in a weight ratio of 95:1.5:3.1:0.4. Deionized water is added as a solvent, and the mixture is stirred thoroughly to obtain a negative electrode slurry. This slurry is then coated onto both surfaces of the negative electrode current collector copper foil. After drying and cold pressing, a negative electrode sheet is obtained.

[0113] (3) Preparation of electrolyte

[0114] In an argon atmosphere glove box with a water content of <10ppm, EC (ethylene carbonate), PC (polycarbonate), and DMC (dimethyl carbonate) were mixed in a weight ratio of EC:PC:DMC = 3:3:3. Then, LiPF6, VC, DTD, and PS were added and stirred until homogeneous to obtain an electrolyte. The concentration of lithium ions was 1.2 mol / L, and the mass percentages of VC, DTD, and PS were 3%, 1%, and 1%, respectively.

[0115] (4) Preparation of the separating membrane

[0116] PP / PE / PP composite film is used as the separator.

[0117] (5) Preparation of lithium-ion secondary batteries

[0118] The positive electrode sheet obtained in step (1), the separator in step (4), and the negative electrode sheet obtained in step (2) are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation, thus obtaining a bare battery cell. The bare battery cell is placed in an outer packaging, injected with the electrolyte prepared in step (3), and sealed. Formation is carried out at a voltage of 4.8V and a current of 0.05C. At this time, PF6 is embedded in the positive electrode graphite. - The specific capacity is 140mAh / g, and the PF6 is released. - The specific capacity is 20mAh / g, and a lithium-ion secondary battery with a lithium replenishment capacity of 120mAh / g is obtained.

[0119] Example 2

[0120] It is basically the same as Example 1, except that the weight ratio of lithium iron phosphate, graphite, conductive agent acetylene black and binder PVDF in step (1) is 93:1:4:2.

[0121] Example 3

[0122] It is basically the same as Example 1, except that the weight ratio of lithium iron phosphate, graphite, conductive agent acetylene black and binder PVDF in step (1) is 86:8:4:2.

[0123] Example 4

[0124] It is basically the same as Example 1, except that the Dv50 particle size of the graphite in step (1) is 20 μm.

[0125] Example 5

[0126] It is basically the same as Example 1, except that the Dv50 particle size of the graphite in step (1) is 10 μm.

[0127] Example 6

[0128] It is basically the same as Example 1, except that the lithium ion concentration of the electrolyte obtained in step (3) is 1 mol / L.

[0129] Example 7

[0130] It is basically the same as Example 1, except that the lithium ion concentration of the electrolyte obtained in step (3) is 1.5 mol / L.

[0131] Example 8

[0132] It is basically the same as Example 1, except that the current generated in step (5) is 0.15C.

[0133] Example 9

[0134] It is basically the same as Example 1, except that lithium iron phosphate in step (1) is replaced with NCM523, the solute in the electrolyte in step (3) is replaced with LiFSI, and the formation voltage in step (5) is adjusted to 4.5V.

[0135] During formation, FSI is embedded in the positive electrode graphite. - The specific capacity is 130mAh / g, and the FSI is removed. - The specific capacity is 20mAh / g, and a lithium-ion secondary battery with a lithium replenishment capacity of 110mAh / g is obtained.

[0136] Comparative Example 1

[0137] It is basically the same as Example 1, except that graphite is not included in step (1), and the weight ratio of lithium iron phosphate, conductive agent acetylene black and binder PVDF is 94:4:2.

[0138] Comparative Example 2

[0139] It is basically the same as Example 1, except that the formation voltage in step (5) is 4.0V.

[0140] Comparative Example 3

[0141] It is basically the same as Example 1, except that the formation voltage in step (5) is 4.9V.

[0142] Characterization tests:

[0143] (1) X-ray diffraction (XRD) test:

[0144] XRD tests were performed on the graphite raw material, the graphite in the positive electrode of the battery prepared in Example 1, Comparative Example 2, and Comparative Example 3, respectively, and the results are denoted as Graphite Raw Material, Test 1, Test 2, and Test 3. The obtained spectra are shown in [reference needed]. Figures 4-7 ;

[0145] Repeat steps (1) to (5) in Example 1, then lower the formation voltage to 2.5V, and perform XRD tests on the graphite of the positive electrode in the prepared battery, denoted as Test 4. The obtained spectrum is shown in [reference]. Figure 8 ;

[0146] (2) Cyclic performance test:

[0147] At 25°C, the cell was charged to 3.65V with a constant current of 0.5C, then charged to 0.05C with a constant voltage, and left to stand for 10 minutes. It was then discharged to 2.0V with a constant current of 1C and left to stand for 10 minutes. This test condition was repeated until the cell capacity decayed to 80% SOH. The number of cycles at the time of stopping was recorded. The results are listed in Table 1.

[0148] Table 1

[0149]

[0150]

[0151] Analyzing the data in Table 1, compared to Example 1, the reduced graphite content in Example 2 resulted in a decrease in the capacity available for SEI lithium replenishment, leading to a decline in battery cycle performance. In Example 3, the increased graphite content, while improving the capacity available for lithium replenishment, also encroached on the space of the positive electrode active material, thus contributing to an overall decrease in battery cycle performance. In Example 4, the larger Dv50 particle size of the graphite reduced the electrode compaction density, decreasing the amount of active material that could be accommodated in the same volume, thereby reducing cycle performance. In Example 5, the smaller Dv50 particle size of the graphite, while resulting in high compaction density, increased ion diffusion paths and polarization, necessitating lower... The higher the rate and the longer the time required for lithium replenishment, the less lithium replenishment is needed under the same conditions, resulting in less improvement in cycle performance. In Example 6, the concentration of lithium ions in the electrolyte is low. During the charging and lithium replenishment process, after consuming some lithium salt, the concentration of lithium ions in the electrolyte is lower than 1 mol / L, which reduces the ion diffusion rate between electrode layers and ultimately leads to a decrease in battery cycle performance. In Example 7, the concentration of lithium ions in the electrolyte is high, the electrolyte viscosity is high, wetting is difficult, and the ion diffusion rate between electrode layers is low, which also leads to a decrease in battery cycle performance. In Example 8, the excessive formation current leads to excessive polarization, resulting in incomplete formation reaction and incomplete lithium replenishment reaction, failing to achieve the expected lithium replenishment efficiency.

[0152] In Comparative Example 1, no graphite was added, so lithium replenishment could not be achieved. In Comparative Example 2, the formation voltage was too low, and the anions could not be properly inserted into the graphite, so lithium replenishment could not be achieved either. In Comparative Example 3, the formation voltage was too high, which caused the electrolyte to partially decompose irreversibly, which would seriously degrade the battery performance.

[0153] Compare Figures 4-7 It can be seen that, relative to the XRD spectrum of graphite raw materials ( Figure 4 In Example 1, the positive electrode graphite obtained contained intercalated anions, forming modified graphite, the spectrum of which (Test 1, ...) is shown in the figure. Figure 5 Characteristic peaks exist in the range of 10° to 20°; in Comparative Example 2, due to the low formation voltage, the anions cannot be properly inserted into the graphite, and modified graphite is not formed, and its XRD spectrum (Test 2, Figure 6 No characteristic peaks were observed in the range of 10° to 20°; in Comparative Example 3, although its XRD spectrum (Test 3, Figure 7 Similar characteristic peaks to those in Example 1 appeared in the range of 10° to 20°, but due to the excessively high formation voltage, the electrolyte underwent partial irreversible decomposition, which severely degraded battery performance; Test 4 ( Figure 8In the process of repeating steps (1) to (5) in Example 1, the formation voltage is then reduced to 2.5V. The XRD spectrum shows characteristic peaks similar to those in Example 1 in the range of 10° to 20°, indicating that the anion insertion into graphite in this application is irreversible. Even if the voltage is reduced in the subsequent use, the anions can still be retained in the graphite, thereby continuously achieving lithium replenishment.

[0154] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0155] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A secondary battery, comprising a positive electrode, a negative electrode, and a separator, wherein the separator is disposed between the positive electrode and the negative electrode; The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes modified graphite, which comprises graphite and anions located between the layers of graphite. The X-ray diffraction pattern of the modified graphite is in the range of 10. o ~20 o Characteristic peaks exist within the range.

2. The secondary battery according to claim 1, characterized in that, The anion is a monovalent anion.

3. The secondary battery according to claim 2, characterized in that, The monovalent anion includes PF6. - ClO4 - TFSI - FSI - and NO3 - One or more of them.

4. The secondary battery according to claim 1, characterized in that, The positive electrode active material layer uses one or more of lithium iron phosphate and ternary positive electrode materials.

5. The secondary battery according to claim 4, characterized in that, The chemical formula of the ternary cathode material is Li. a Ni x Co y M z O2, wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe, Mo, and B, 0.95≤a≤1.2, x>0, y>0, z>0, and x+y+z=1.

6. The secondary battery according to claim 1, characterized in that, In the positive electrode active material layer, the graphite content is 1% to 8% by mass.

7. The secondary battery according to claim 6, characterized in that, In the positive electrode active material layer, the graphite content is 2% to 4% by mass.

8. The secondary battery according to any one of claims 1 to 7, characterized in that, The Dv50 particle size of the graphite is 10 μm ~ 20 μm.

9. The secondary battery according to claim 8, characterized in that, The graphite has a Dv50 particle size of 16 μm to 18 μm.

10. A method for preparing a secondary battery according to any one of claims 1 to 9, comprising the following steps: The graphite, other raw materials for preparing the positive electrode active material layer, and solvent are mixed to prepare a positive electrode slurry; The positive electrode slurry is coated onto at least one surface of the positive electrode current collector, and then dried and pressed to obtain a positive electrode sheet. The positive electrode, negative electrode, and separator are assembled, injected with electrolyte, and formed at a voltage of 4.5 V to 4.8 V.

11. The preparation method according to claim 10, characterized in that, The solute in the electrolyte includes one or more of LiPF6, LiClO4, LiTFSI, LiFSI, and LiNO3.

12. The preparation method according to claim 10, characterized in that, In the electrolyte, the concentration of lithium ions is 1 mol / L to 1.5 mol / L.

13. The preparation method according to claim 12, characterized in that, In the electrolyte, the concentration of lithium ions is 1.2 mol / L ~ 1.3 mol / L.

14. The preparation method according to any one of claims 10 to 13, characterized in that, The current for the formation process is less than 0.1 C.

15. The preparation method according to any one of claims 10 to 13, characterized in that, When the formation process is performed at a voltage of 4.5 V to 4.8 V, the specific capacity of the anions embedded in the graphite ranges from 100 mAh / g to 200 mAh / g.

16. The preparation method according to any one of claims 10 to 13, characterized in that, When the formation process is performed at a voltage of 4.5 V to 4.8 V, the specific capacity of the anions extracted from the graphite ranges from 20 mAh / g to 100 mAh / g.

17. An electrical device comprising a secondary battery as described in any one of claims 1 to 9.

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