Method for forming secondary battery, secondary battery, electric device

Abstract

The application discloses a formation method of a secondary battery, the secondary battery and a power utilization device. The formation method of the secondary battery comprises: sequentially performing first formation treatment, second formation treatment and third formation treatment on the secondary battery, wherein the first formation treatment comprises: charging for 60-120 min at a current I1 and a cutoff voltage U1; the second formation treatment comprises: charging for 60-120 min at a current I2 and a cutoff voltage U2; and the third formation treatment comprises: charging for 60-120 min at a current I3 and a cutoff voltage U3; wherein I1 is 0.02C-0.05C, I2 is 0.05C-0.1C, I3 is 0.1C-0.5C, and I1I2I3. Thus, the secondary battery with small internal polarization and low interface impedance can be obtained, and the cycle performance of the secondary battery is improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a method for forming a secondary battery, a secondary battery, and an electrical device. Background Technology

[0002] In recent years, rechargeable batteries have made great strides and can be widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in electric vehicles, power tools, military equipment, and aerospace. With the gradual increase in market demand, further improving the cycle performance of rechargeable batteries has become a key research and development challenge.

[0003] It should be noted that the above statements are only used to provide background information related to this application and do not necessarily constitute prior art. Summary of the Invention

[0004] In a first aspect, this application proposes a method for forming a secondary battery, comprising: sequentially performing a first formation treatment, a second formation treatment, and a third formation treatment on the secondary battery, wherein the first formation treatment includes: charging with current I1 for 60 min to 120 min, with a cutoff voltage of U1; the second formation treatment includes: charging with current I2 for 60 min to 120 min, with a cutoff voltage of U2; and the third formation treatment includes: charging with current I3 for 60 min to 120 min, with a cutoff voltage of U3; wherein I1 is 0.02C to 0.05C, I2 is 0.05C to 0.1C, and I3 is 0.1C to 0.5C, and I1 < I2 < I3. Therefore, by employing a continuous three-stage step formation with a small current, the SEI film is continuously grown in one step using a continuous charging process. The resulting SEI film structure is dense and not easily cracked after long-term cyclic charging and discharging, thus obtaining a secondary battery with low internal polarization and low interfacial impedance, which helps to improve the cycle performance of the secondary battery.

[0005] In some embodiments, the first formation process sequentially includes: using current I 1A Charge for 30-60 minutes, with current I 1B Charging for 30-60 minutes, with a cutoff voltage of U1; where I 1A <I 1B As a result, the SEI film formed is more uniform and dense, which helps to obtain a secondary battery with less internal polarization and lower interfacial impedance, further improving the cycle performance of the secondary battery.

[0006] In some embodiments, the second formation process sequentially includes: using current I 2A Charge for 30-60 minutes, with current I 2B Charging for 30-60 minutes, the cutoff voltage is U2; where I2A <I 2B As a result, the SEI film formed is more uniform and dense, which helps to obtain a secondary battery with less internal polarization and lower interfacial impedance, further improving the cycle performance of the secondary battery.

[0007] In some embodiments, the third formation process sequentially includes: using current I 3A Charge for 30-60 minutes, with current I 3B Charging for 30-60 minutes, the cutoff voltage is U3; where I 3A <I 3B As a result, the SEI film formed is more uniform and dense, which helps to obtain a secondary battery with less internal polarization and lower interfacial impedance, further improving the cycle performance of the secondary battery.

[0008] In some embodiments, the cutoff voltage U1 is 2.8V~3.4V, the cutoff voltage U2 is 3.4V~3.8V, and the cutoff voltage U3 is 3.8V~4.1V. Therefore, charging with current I1 for a period of time to the cutoff voltage U1 facilitates the slow and uniform reduction of the electrolyte, forming a dense and continuous SEI film substrate on the surface of the negative electrode. Then, charging with current I2 for a period of time to the cutoff voltage U2 allows a small number of metal active ions to begin embedding into the negative electrode active material, causing the SEI film to thicken slowly, releasing micro-stress on the electrode, and reducing the problem of SEI film cracking due to excessive internal stress. Finally, charging with current I3 for a period of time to the cutoff voltage U3 allows a larger number of metal active ions to stably embed into the negative electrode active material, fully activating the active sites inside the negative electrode active material particles, which helps to fully utilize the capacity of the secondary battery.

[0009] In a second aspect, this application proposes a secondary battery prepared by the aforementioned formation method for a secondary battery. Therefore, the secondary battery of this application exhibits excellent cycle performance.

[0010] In some embodiments, the secondary battery includes a positive electrode sheet, which includes a positive current collector and a positive electrode film layer located on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes one or more of Prussian blue compounds, layered metal oxides, and polyanionic compounds. Therefore, the positive electrode active material exhibits high structural stability, which helps to further improve the cycle performance of the secondary battery.

[0011] In some embodiments, the secondary battery includes a negative electrode sheet, which includes a negative current collector and a negative electrode film layer located on at least one side of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes one or more of graphite, hard carbon, and soft carbon. This helps to improve the energy density and cycle performance of the secondary battery.

[0012] In some embodiments, the secondary battery includes an electrolyte, which includes a solvent, and the solvent includes a carbonate. Therefore, the electrolyte of this application has a wide electrochemical stability window, which can be adapted to a wide operating voltage range of the positive electrode active material, and also helps to preferentially reduce and form a dense and stable SEI film on the negative electrode surface, thereby improving the cycle performance of the secondary battery.

[0013] In a third aspect, this application proposes an electrical device comprising the aforementioned secondary battery. Therefore, the electrical device of this application possesses all the features and advantages of the aforementioned secondary battery, which will not be repeated here. Detailed Implementation

[0014] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit this application; unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0015] The terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are open-ended expressions, meaning they include what is specified in this application but do not exclude other aspects.

[0016] In the description of this application, all figures disclosed herein, whether or not the words "approximately" or "about" are used, are approximate values. Each figure may vary by less than 10% or by a difference that is considered reasonable by one of the art, such as 1%, 2%, 3%, 4%, or 5%.

[0017] 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 expected that ranges of 60-110 and 80-120 are also included. 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 "ab" 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.

[0018] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. "First feature" and "second feature" may include one or more of the indicated feature.

[0019] In the description of this application, "multiple" means two or more.

[0020] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0021] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0022] The formation method for secondary batteries is generally to charge them directly at a constant current of 0.2C to 0.5C to the rated upper limit voltage of the battery. This formation method is simple and quick, but the current is too large in the early stage of charging, and a loose and uneven solid electrolyte interphase (SEI) film is quickly formed on the surface of the negative electrode. In addition, the decomposition of the electrolyte produces a large amount of gas, making the secondary battery prone to bulging. Lithium plating is also prone to occur on the negative electrode, resulting in a large irreversible capacity loss of the secondary battery. The final secondary battery has excessive internal polarization and high interfacial impedance, which seriously affects the cycle performance of the secondary battery.

[0023] In this application, during the first formation stage, the cell is first charged to a low voltage plateau with a very small constant current. The small current can significantly reduce cell polarization and reduce the violent interfacial reactions caused by large current. The low voltage environment reduces the side reactions of the electrolyte in the positive electrode region, promoting the in-situ formation of a uniform and dense initial SEI film at the positive and negative electrode interfaces, improving interfacial ion transport efficiency, and reducing interfacial impedance. Then, in the second formation stage, the charging current is appropriately increased, and constant current charging continues to the intermediate voltage. A small number of active ions begin to embed into the negative electrode, the SEI film slowly thickens, the micro-stress of the electrode is released, and the SEI film cracking caused by excessive internal stress is reduced, further stabilizing the interface. In the third formation stage, the standardized formation current is used to charge to the target activation voltage, completing the entire formation process. Therefore, by adopting a three-stage continuous formation with low current and controlling the charging time of the three stages over a longer period of time, the SEI film can be grown continuously in one go by utilizing the continuous charging process and the long charging time, allowing the SEI to have sufficient growth time. The resulting SEI film structure is more compact and less prone to cracking after long-term charge-discharge cycles. This results in a secondary battery with low internal polarization and low interface impedance, which helps to improve the cycle performance of the secondary battery.

[0024] In a first aspect, this application proposes a method for forming a secondary battery, comprising: sequentially performing a first formation treatment, a second formation treatment, and a third formation treatment on the secondary battery, wherein the first formation treatment includes: charging with current I1 for 60 min to 120 min, with a cutoff voltage of U1; the second formation treatment includes: charging with current I2 for 60 min to 120 min, with a cutoff voltage of U2; and the third formation treatment includes: charging with current I3 for 60 min to 120 min, with a cutoff voltage of U3; wherein I1 is 0.02C to 0.05C, I2 is 0.05C to 0.1C, and I3 is 0.1C to 0.5C, and I1 < I2 < I3. Therefore, by employing a continuous three-stage step formation with a small current, the SEI film is continuously grown in one step using a continuous charging process. The resulting SEI film structure is dense and not easily cracked after long-term cyclic charging and discharging, thus obtaining a secondary battery with low internal polarization and low interfacial impedance, which helps to improve the cycle performance of the secondary battery.

[0025] In the application, "C" refers to the rated capacity of the secondary battery. For example, if a secondary battery with a rated capacity of 1000mAh is discharged at a current of 0.1C, its discharge current is 100mA.

[0026] In this application, the starting voltage of the first formation treatment is the initial voltage of the secondary battery, the starting voltage of the second formation treatment is U1, and the starting voltage of the third formation treatment is U2.

[0027] In some embodiments, before performing the first formation treatment on the secondary battery, the secondary battery is further subjected to a standing period of 8 to 16 hours. This helps the electrolyte to fully wet the electrode plates.

[0028] In some embodiments, the temperatures of the first formation treatment, the second formation treatment, and the third formation treatment are not particularly limited, as long as the purpose of this application is achieved. For example, the temperatures of the first formation treatment, the second formation treatment, and the third formation treatment can be 20°C to 30°C.

[0029] As an example, I1 can be 0.02C, 0.025C, 0.03C, 0.035C, 0.04C, 0.045C, 0.05C, or any value range between the two.

[0030] As an example, the charging time with current I1 can be 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, 100 min, 105 min, 110 min, 115 min, 120 min, or any value range between the two.

[0031] As an example, I2 can be 0.05C, 0.055C, 0.06C, 0.065C, 0.07C, 0.075C, 0.08C, 0.085C, 0.09C, 0.1C, or any value range between the two.

[0032] As an example, the charging time with current I2 can be 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, 100 min, 105 min, 110 min, 115 min, 120 min, or any value range between the two.

[0033] As an example, I3 can be 0.1C, 0.15C, 0.2C, 0.25C, 0.3C, 0.35C, 0.4C, 0.45C, 0.5C, or any value range between the two.

[0034] As an example, the charging time with current I3 can be 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, 100 min, 105 min, 110 min, 115 min, 120 min, or any value range between the two.

[0035] In some embodiments, the first formation process sequentially includes: using current I 1A Charge for 30-60 minutes, with current I 1BCharging for 30-60 minutes, with a cutoff voltage of U1; where I 1A <I 1B Therefore, let's start with I. 1A Charging with a smaller current results in lower polarization at the negative electrode interface under low current density. Precursor molecules for SEI film formation (such as additives in the electrolyte and carbonate reduction intermediates) are uniformly adsorbed and slowly and orderly deposited on the surface of the negative electrode active material, forming a continuous, thin, and dense initial SEI film substrate. The low porosity of the initial SEI film substrate helps reduce direct contact between solvent molecules in the electrolyte and the negative electrode active material, suppressing the violent decomposition between the electrolyte and the negative electrode active material under subsequent high current. Then, I... 1B When charged with a slightly larger current, the reduction product molecules are slowly deposited and replenished only on the already formed SEI film substrate. The newly added reduction products grow in an orderly epitaxial manner along the original dense SEI film substrate, and the SEI film layer is uniformly thickened as a whole. The resulting SEI film is more uniform and dense, which helps to obtain a secondary battery with less internal polarization and lower interfacial impedance, and further improves the cycle performance of the secondary battery.

[0036] As an example, let's take current I... 1A The charging time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any value between two of these.

[0037] As an example, let's take current I... 1B The charging time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any value between two of these.

[0038] In some embodiments, the first formation process sequentially includes: using current I 1A Charge for 30-60 minutes, let stand for 1-5 minutes, and then charge with current I. 1B Charging for 30-60 minutes, with a cutoff voltage of U1; where I 1A <I 1B .

[0039] In some embodiments, the second formation process sequentially includes: using current I 2A Charge for 30-60 minutes, with current I 2B Charging for 30-60 minutes, the cutoff voltage is U2; where I 2A <I 2B As a result, the SEI film formed is more uniform and dense, which helps to obtain a secondary battery with less internal polarization and lower interfacial impedance, further improving the cycle performance of the secondary battery.

[0040] As an example, let's take current I...2A The charging time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any value between two of these.

[0041] As an example, let's take current I... 2B The charging time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any value between two of these.

[0042] In some embodiments, the second formation process sequentially includes: using current I 2A Charge for 30-60 minutes, let stand for 1-5 minutes, and then charge with current I. 2B Charging for 30-60 minutes, the cutoff voltage is U2; where I 2A <I 2B .

[0043] In some embodiments, the third formation process sequentially includes: using current I 3A Charge for 30-60 minutes, with current I 3B Charging for 30-60 minutes, the cutoff voltage is U3; where I 3A <I 3B As a result, the SEI film formed is more uniform and dense, which helps to obtain a secondary battery with less internal polarization and lower interfacial impedance, further improving the cycle performance of the secondary battery.

[0044] As an example, let's take current I... 3A The charging time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any value between two of these.

[0045] As an example, let's take current I... 3B The charging time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any value between two of these.

[0046] In some embodiments, the third formation process sequentially includes: using current I 3A Charge for 30-60 minutes, let stand for 1-5 minutes, and then charge with current I. 3B Charging for 30-60 minutes, the cutoff voltage is U3; where I 3A <I 3B .

[0047] In some embodiments, the cutoff voltage U1 is 2.8V~3.4V, the cutoff voltage U2 is 3.4V~3.8V, and the cutoff voltage U3 is 3.8V~4.1V. Therefore, charging with current I1 for a period of time to the cutoff voltage U1 facilitates the slow and uniform reduction of the electrolyte, forming a dense and continuous SEI film substrate on the surface of the negative electrode. Then, charging with current I2 for a period of time to the cutoff voltage U2 allows a small number of active ions to begin embedding into the negative electrode active material, causing the SEI film to thicken slowly, releasing micro-stress on the electrode, and reducing the problem of SEI film cracking due to excessive internal stress. Finally, charging with current I3 for a period of time to the cutoff voltage U3 allows a larger number of active ions to stably embed into the negative electrode active material, fully activating the active sites inside the negative electrode active material particles, which helps to fully utilize the capacity of the secondary battery.

[0048] As an example, the cutoff voltage U1 can be 2.8V, 2.9V, 3V, 3.1V, 3.2V, 3.3V, 3.4V, or any value range between the two.

[0049] As an example, the cutoff voltage U2 can be 3.4V, 3.5V, 3.6V, 3.7V, 3.8V, or any value between the two.

[0050] As an example, the cutoff voltage U3 can be 3.8V, 3.9V, 4V, 4.1V, or any value between the two.

[0051] In a second aspect, this application proposes a secondary battery prepared by the aforementioned formation method for a secondary battery. Therefore, the secondary battery of this application exhibits excellent cycle performance.

[0052] In some embodiments, the secondary battery includes a positive electrode sheet, which includes a positive current collector and a positive electrode film layer located on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes one or more of Prussian blue compounds, layered metal oxides, and polyanionic compounds. Thus, the positive electrode active material has a stable structure, which helps to further improve the cycle performance of the secondary battery.

[0053] In some embodiments, the Prussian blue compounds include Na x M[Fe(CN)6] y ·nH2O, M is a transition metal, 1.5≤x≤2.0, 0.9≤y≤1.0, 0≤n≤2.0; the layered metal oxide includes at least one of P2 phase manganese layered oxide and O3 phase ternary oxide; the polyanionic compound includes at least one of sodium iron phosphate, sodium manganese phosphate, and pyrophosphate.

[0054] In some embodiments, the positive current collector includes a metal foil; as a specific example, this application uses aluminum foil.

[0055] In some embodiments, the positive electrode film layer may also include a positive electrode binder and a positive electrode conductive agent, and additives with specific functions and effects, such as thickeners, flame retardants, high-temperature / low-temperature stabilizers, etc., may also be added as needed.

[0056] As an example, the positive electrode binder in the positive electrode film layer may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0057] As an example, the positive electrode conductive agent in the positive electrode film may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0058] 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, positive conductive agent, positive 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 current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0059] In some embodiments, the secondary battery includes a negative electrode sheet, which includes a negative current collector and a negative electrode film layer located on at least one side of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes one or more of graphite, hard carbon, and soft carbon. This helps to improve the energy density and cycle performance of the secondary battery.

[0060] In some embodiments, the negative current collector includes copper foil or aluminum foil.

[0061] In some embodiments, the negative electrode film layer may further include a negative electrode binder and a negative electrode conductive agent.

[0062] In some embodiments, the negative electrode binder in the negative electrode film layer 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), carboxymethyl chitosan (CMCS), and sodium carboxymethyl cellulose (CMC).

[0063] In some embodiments, the negative electrode conductive agent in the negative electrode film layer includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0064] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, negative electrode conductive agent, negative electrode 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.

[0065] In some embodiments, the secondary battery includes an electrolyte, which includes a solvent, and the solvent includes a carbonate. Therefore, the electrolyte of this application has a wide electrochemical stability window, which can be adapted to a wide operating voltage range of the positive electrode active material; in addition, it also helps to preferentially reduce and form a dense and stable SEI film on the negative electrode surface, improving the cycle performance of the secondary battery.

[0066] In some embodiments, the carbonate includes at least one of linear carbonate and cyclic carbonate, wherein the linear carbonate includes at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), and the cyclic carbonate includes at least one of ethylene carbonate (EC) and propylene carbonate (PC).

[0067] In some embodiments, the electrolyte also includes lithium salts. Furthermore, additives with specific functions, such as film-forming additives, lithium replenishing agents, flame retardants, and thermal stability additives, may be added to the electrolyte as needed. As an example, the electrolyte may include lithium salts, solvents, and additives.

[0068] In some embodiments, the lithium salt may include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate, etc.

[0069] In some embodiments, the secondary battery further 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.

[0070] In some embodiments, the separator film includes polyethylene separator film, polypropylene separator film, polyethylene / polypropylene composite separator film, etc.

[0071] In some embodiments, the secondary battery of this application may include, but is not limited to, lithium-ion batteries, sodium-ion batteries, etc.

[0072] The preparation process of the secondary battery described in this application is well known to those skilled in the art, and this application has no particular limitations. For example, it may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain an electrochemical device; or stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain an electrochemical device. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the packaging bag as needed to prevent pressure rise and overcharging / discharging inside the electrochemical device.

[0073] In a third aspect, this application proposes an electrical device comprising the aforementioned secondary battery. Therefore, the electrical device of this application possesses all the features and advantages of the aforementioned secondary battery, which will not be repeated here.

[0074] In some embodiments, the specific type of electrical device is not particularly limited and can be any device that uses a battery as a power source or energy storage unit. Examples of electrical devices include, but are not limited to, electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), mobile terminals (e.g., mobile phones, laptops, game consoles, wearable devices, etc.), drones, aerospace equipment, satellites, ships, energy storage systems, and so on.

[0075] It is understandable that, in addition to the secondary battery mentioned above, the electrical device also includes other necessary structures and components, all of which can be made with reference to conventional technologies. For example, an electric vehicle may include a body, chassis, tires, navigation system, radar system, steering system, braking system, lubrication system, cooling system, driving system, etc., which will not be described in detail here.

[0076] The following specific embodiments illustrate the solution of this application. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0077] Example 1 <Preparation of Negative Electrode Sheets> A negative electrode slurry was prepared by uniformly mixing hard carbon (anode active material), CMC (anode binder), SBR (anode binder), and superconducting carbon (SuperP) (anode conductive agent) in deionized water at a mass ratio of 94:1:2:3. The solid content of the negative electrode slurry was 42%. The negative electrode slurry was coated on both sides of an aluminum foil and dried, followed by cold pressing to obtain the negative electrode sheet. The single-sided coating weight of the negative electrode film was 6.5 mg / cm². 2 The thickness of the negative electrode film on one side is 45 μm.

[0078] <Preparation of the positive electrode> A positive electrode slurry was prepared by uniformly mixing sodium nickel iron manganese oxide, polyvinylidene fluoride, and superconducting carbon (Super P) in an N-methylpyrrolidone solvent at a mass ratio of 93:4:3. The solid content of the positive electrode slurry was 52%. The positive electrode slurry was coated onto both sides of an aluminum foil and dried, followed by cold pressing to obtain the positive electrode sheet. The single-sided coating weight of the positive electrode film was 12 mg / cm². 2 The thickness of the positive electrode film on one side is 60 μm.

[0079] <Preparation of the separating membrane> A glass fiber with a thickness of 16μm was used as the separator.

[0080] <Preparation of Electrolyte> In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), ethylene carbonate and dimethyl carbonate were mixed at a mass ratio of 3:7 to obtain a base solvent. Then, the electrolyte salt NaPF6 was added to the base solvent to obtain the electrolyte. The electrolyte salt NaPF6 comprised 12% by mass, with the remainder being the base solvent.

[0081] <Preparation of Sodium-ion Batteries> The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and vacuum-baked at 85°C for 48 hours. The electrolyte prepared above is then injected, and the sodium-ion battery is obtained through vacuum sealing, settling, formation, and shaping processes.

[0082] The formation method involves sequentially performing a first formation treatment, a second formation treatment, and a third formation treatment on the secondary battery. The first formation process includes: charging at a current of I10.03C for 120 minutes, with a cutoff voltage of U1 of 3.2V; The second formation process includes: charging at a current of I20.08C for 120 minutes, with a cutoff voltage of U2 of 3.6V; The third formation process includes charging at a current of I30.20C for 120 minutes, with a cutoff voltage of U3 of 4.1V.

[0083] Example 2 Except for adjusting the current I1 of the first formation process to 0.02C and the cutoff voltage U1 to 3.0V; the current I2 of the second formation process to 0.06C and the cutoff voltage U2 to 3.5V; and the current I3 of the third formation process to 0.15C and the cutoff voltage U3 to 3.9V, the rest is the same as in Example 1.

[0084] Example 3 Except for adjusting the charging time of the first formation treatment to 60 minutes and the cutoff voltage U1 to 2.8V, the charging time of the second formation treatment to 60 minutes and the cutoff voltage U2 to 3.4V, and the charging time of the third formation treatment to 60 minutes and the cutoff voltage U3 to 3.8V, the rest is the same as in Example 1.

[0085] Example 4 Except for the formation method being adjusted as follows, everything else is the same as in Example 1.

[0086] The formation method is as follows: the secondary battery is subjected to a first formation treatment, a second formation treatment, and a third formation treatment in sequence. The first formation process includes: using current I 1A Charge at 0.02C for 60 minutes, then charge at current I 1B After charging at 0.03C for 60 minutes, the cutoff voltage U1 is 3.1V. The second formation process includes: using current I 2A Charge at 0.06C for 60 minutes, then charge at current I 2B After charging at 0.08C for 60 minutes, the cutoff voltage U2 is 3.55V. The third formation process includes: using current I 3A Charge at 0.15C for 60 minutes, then charge at current I 3B After charging at 0.20C for 60 minutes, the cutoff voltage U3 is 4.0V.

[0087] Example 5 Except for the formation method being adjusted as follows, everything else is the same as in Example 1.

[0088] The formation method is as follows: the secondary battery is subjected to a first formation treatment, a second formation treatment, and a third formation treatment in sequence. The first formation process includes: using current I 1A Charge at 0.02C for 60 minutes, let stand for 3 minutes, then charge with current I. 1BAfter charging at 0.03C for 60 minutes, the cutoff voltage U1 is 3.1V. The second formation process includes: using current I 2A Charge at 0.06C for 60 minutes, let stand for 3 minutes, then charge with current I. 2B After charging at 0.08C for 60 minutes, the cutoff voltage U2 is 3.55V. The third formation process includes: using current I 3A Charge at 0.15C for 60 minutes, let stand for 3 minutes, then charge with current I. 3B After charging at 0.20C for 60 minutes, the cutoff voltage U3 is 4.0V.

[0089] Comparative Example 1 Except for the formation method being adjusted as follows, everything else is the same as in Example 1.

[0090] The formation method is as follows: charge with a current of 0.02C until the cutoff voltage of 4.1V.

[0091] Comparative Example 2 Except for the formation method being adjusted as follows, everything else is the same as in Example 1.

[0092] The formation method is as follows: charge at a current of 0.5C until the cutoff voltage of 4.1V.

[0093] Comparative Example 3 Except for the formation method being adjusted as follows, everything else is the same as in Example 1.

[0094] The formation method is as follows: charge with a current of 0.05C to the cutoff voltage of 3.4V; then charge with a current of 0.5C to the cutoff voltage of 4.1V.

[0095] Comparative Example 4 Except for the formation method being adjusted as follows, everything else is the same as in Example 1.

[0096] The formation method is as follows: The following charge and discharge processes are performed sequentially: Charge the battery at a current of I10.03C for 120 minutes until the cutoff voltage U1 is 3.2V. Then discharge the battery at a constant current of I1 to 2V. Charge the battery with a current of I20.08C until the cutoff voltage U2 is 3.6V, and then discharge the battery with a constant current of I2 until it reaches 2V; Charge with current I30.20C until the cutoff voltage U3 is 4.1V.

[0097] The negative electrode slurry and sodium-ion battery prepared in the above embodiments and comparative examples were tested as follows, and the test results are shown in Table 1.

[0098] Test methods and equipment Interfacial charge transfer impedance test: The sodium-ion battery was tested using an electrochemical workstation. The test conditions were 25℃, AC disturbance amplitude of 5MV, and test frequency range of 0.01Hz~100000Hz to obtain the interfacial transfer impedance (Rct).

[0099] Testing of charging polarization voltage: At 25°C, the battery is discharged to 2V at a constant current of 1C, and then placed at 0°C and charged to 4.1V at a constant current of 1C. The steady-state polarization voltage during the charging process is recorded to evaluate the degree of electrode polarization.

[0100] Cyclic performance test: At 25℃, the battery is charged at a constant current of 4C to 4.1V, and then discharged at 4C to 2.0V. The resulting capacity is recorded as the initial capacity C0. This is one charge-discharge cycle. The above charge-discharge cycle is repeated for 500 cycles. The discharge capacity after the 500th cycle is recorded as C1. Cyclic capacity retention rate (%) = C1 / C0 × 100%.

[0101] Table 1

[0102] As can be seen from Table 1, the sodium-ion batteries obtained by the three-stage stepped continuous charging formation with low current in the embodiments of this application have lower interfacial charge transfer impedance, lower charging polarization voltage, and higher cycle capacity retention, indicating that the sodium-ion batteries formed by the method of this application have low internal polarization, low interfacial impedance, and better cycle performance. In contrast, Comparative Example 1 uses a lower current value of 0.02C for formation, Comparative Example 2 uses a higher current value of 0.5C for formation, Comparative Example 3 uses only a two-stage formation, and Comparative Example 4 uses a three-stage charging and discharging combination formation method. The performance data from Comparative Examples 1 to 4 show that the sodium-ion batteries prepared in Comparative Examples 1 to 4 have higher interfacial charge transfer impedance, higher charging polarization voltage, and lower cycle capacity retention, indicating that the sodium-ion batteries prepared in the comparative examples have high internal polarization, high interfacial impedance, and poor cycle performance.

[0103] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for forming a secondary battery, characterized in that, include: The secondary battery is subjected to a first formation treatment, a second formation treatment, and a third formation treatment in sequence. The first formation process includes: charging with current I1 for 60 min to 120 min, with a cutoff voltage of U1; The second formation process includes: charging with current I2 for 60 min to 120 min, with a cutoff voltage of U2; The third formation process includes: charging with current I3 for 60 min to 120 min, with a cutoff voltage of U3; Wherein, I1 is 0.02C~0.05C, I2 is 0.05C~0.1C, I3 is 0.1C~0.5C, and I1<I2<I3.

2. The formation method according to claim 1, characterized in that, The first formation process includes, in sequence: With current I 1A Charge for 30-60 minutes, with current I 1B Charge for 30-60 minutes, with the cutoff voltage at U1. Among them, I 1A <I 1B .

3. The formation method according to claim 1, characterized in that, The second formation process includes, in sequence: With current I 2A Charge for 30-60 minutes, with current I 2B Charge for 30-60 minutes, the cutoff voltage is U2; Among them, I 2A <I 2B .

4. The formation method according to claim 1, characterized in that, The third formation process includes, in sequence: With current I 3A Charge for 30-60 minutes, with current I 3B Charge for 30-60 minutes, the cutoff voltage is U3; Among them, I 3A <I 3B .

5. The formation method according to any one of claims 1-4, characterized in that, The cutoff voltage U1 is 2.8V~3.4V, the cutoff voltage U2 is 3.4V~3.8V, and the cutoff voltage U3 is 3.8V~4.1V.

6. A secondary battery, characterized in that, The secondary battery is prepared by the formation method of the secondary battery according to any one of claims 1-5.

7. The secondary battery according to claim 6, characterized in that, The device includes a positive electrode sheet, which includes a positive current collector and a positive electrode film layer located on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes one or more of Prussian blue compounds, layered metal oxides, and polyanionic compounds.

8. The secondary battery according to claim 6, characterized in that, The device includes a negative electrode sheet, which includes a negative current collector and a negative electrode film layer located on at least one side of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes one or more of graphite, hard carbon, and soft carbon.

9. The secondary battery according to any one of claims 6-8, characterized in that, It includes an electrolyte, the electrolyte including a solvent, the solvent including a carbonate.

10. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 6-9.

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