Positive electrode sheet, battery and preparation method therefor, battery pack, and electric device

Abstract

Provided in the present application are a positive electrode sheet, a battery and a preparation method therefor, a battery pack, and an electric device. The positive electrode sheet comprises a positive electrode active material. A CEI layer is present on the surface of the positive electrode active material, wherein the atomic percentage of fluorine in the CEI layer is less than or equal to 12 at.%, and the atomic percentage of oxygen in the CEI layer is less than or equal to 23at.%. The present application can reduce the resistivity of the positive electrode sheet and improve the discharge energy of the battery.

Description

A positive electrode, a battery, a method for preparing the same, a battery pack, and an electrical device thereof.

[0001] This application claims priority to Chinese Patent Application No. 202510080981.3, filed on January 16, 2025, entitled "A positive electrode sheet, a battery and a method for preparing the same, a battery pack and an electrical device thereof", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of batteries, specifically to a positive electrode, a battery and its preparation method, a battery pack and an electrical device. Background Technology

[0003] In recent years, lithium-ion batteries have been widely used in the field of portable power supplies. However, with technological advancements, the requirements for the energy density and other performance characteristics of portable power supplies are becoming increasingly stringent. For example, the actual specific capacity of lithium iron phosphate batteries is approximately 155 mAh / g, which is still far from the theoretical specific capacity (170 mAh / g), necessitating a technological breakthrough. The positive electrode is a crucial component of lithium-ion batteries, affecting their capacity and other performance characteristics. However, current technologies generally suffer from defects such as high resistivity of the positive electrode and low battery discharge capacity, which urgently need to be addressed. Summary of the Invention

[0004] This application provides a positive electrode sheet, a battery and a method for preparing the same, a battery pack and an electrical device, to at least solve the problems of high resistivity of the positive electrode sheet and low discharge capacity of the battery in the prior art.

[0005] In one aspect, this application provides a positive electrode sheet comprising a positive electrode active material, wherein a CEI layer is present on the surface of the positive electrode active material, wherein the atomic percentage of fluorine in the CEI layer is less than or equal to 12 at.%, and the atomic percentage of oxygen in the CEI layer is less than or equal to 23 at.%.

[0006] According to one embodiment of this application, the atomic percentage of fluorine in the CEI layer is 3 at.% to 12 at.%.

[0007] According to one embodiment of this application, the atomic percentage of oxygen in the CEI layer is 20 at.% to 23 at.%.

[0008] According to one embodiment of this application, the CEI layer further contains carbon, and the atomic percentage of carbon in the CEI layer is greater than or equal to 25 at.%.

[0009] According to one embodiment of this application, the atomic percentage of carbon in the CEI layer is 25 at.% to 36 at.%.

[0010] According to one embodiment of this application, the thickness of the CEI layer is 0.5 nm to 9 nm.

[0011] According to one embodiment of this application, the positive electrode active material includes lithium iron phosphate and / or carbon-coated lithium iron phosphate.

[0012] According to one embodiment of this application, the resistivity of the positive electrode is 20 to 38 Ω·cm.

[0013] According to one embodiment of this application, the resistivity of the positive electrode is 24 to 32 Ω·cm.

[0014] Another aspect of this application provides a battery including the aforementioned positive electrode.

[0015] Another aspect of this application provides a method for preparing the above-mentioned battery, comprising the following steps: placing a cell precursor in a casing and adding an electrolyte therein to obtain a battery precursor; wherein the cell precursor includes a positive electrode precursor, and the positive electrode precursor includes a positive electrode active material; performing a formation treatment on the battery precursor to obtain the battery; wherein the formation treatment process includes: charging the battery precursor to a state of charge of 1% to 30% and then performing a heat preservation treatment, followed by an aging treatment on the battery precursor after the heat preservation treatment to obtain the battery; wherein the temperature of the heat preservation treatment is 25°C to 70°C, and the heat preservation treatment time is 1h to 96h.

[0016] According to one embodiment of this application, during the formation process, the battery precursor after the heat preservation treatment is charged to a full charge state and then subjected to the aging treatment to obtain the battery.

[0017] According to one embodiment of this application, the heat preservation treatment time is 6h to 72h.

[0018] Another aspect of this application provides a battery pack, including the battery described above or a battery prepared according to the method described above.

[0019] Another aspect of this application provides an electrical device, including the battery described above, or a battery manufactured according to the method for manufacturing the battery described above, or the battery pack described above.

[0020] The positive electrode sheet, battery, preparation method thereof, battery pack and electrical equipment provided in this application have a CEI layer on the surface of the positive electrode active layer in the positive electrode sheet. The mass percentage of fluorine in the CEI layer is less than or equal to 12 at.%, and the atomic percentage of oxygen in the CEI layer is less than or equal to 23 at.%. Under such a CEI layer composition system, the resistivity of the positive electrode sheet can be reduced, the discharge energy of the battery can be improved, and the defects of the prior art can be effectively overcome. Detailed Implementation

[0021] To enable those skilled in the art to better understand the solutions of this application, a further detailed description of this application is provided below. The specific embodiments listed below are merely descriptions of the principles and features of this application; the examples are only for explaining this application and are not intended to limit its scope. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0022] Existing technologies generally suffer from defects such as high resistivity of the positive electrode and low battery discharge capacity, which urgently need to be addressed.

[0023] Specifically, based on the inventors' long-term research, the presence of the cathode / electrolyte interface film (CEI) layer is generally beneficial to improving battery performance. For example, a CEI layer formed on the surface of the positive electrode active material can act as a protective layer to protect the positive electrode active material and reduce problems such as corrosion by electrolyte components. However, the CEI layer has poor electronic conductivity but good ionic conductivity. Currently, whether the CEI layer is automatically grown during the battery manufacturing process or is modified or constructed through doping (e.g., doping inorganic elements into the positive electrode active material to modify the CEI layer or constructing a CEI layer on the surface of the positive electrode active material), it generally suffers from poor electronic conductivity, which affects the conductivity of the positive electrode active material and thus affects the specific capacity of the positive electrode active material.

[0024] In view of this, embodiments of this application provide a positive electrode sheet, including a positive electrode active material, wherein a CEI layer is present on the surface of the positive electrode active material, the mass percentage of fluorine in the CEI layer is less than or equal to 12 at.%, and the mass percentage of oxygen in the CEI layer is less than or equal to 23 at.%.

[0025] According to the inventors' research, in the above-mentioned positive electrode composition system, the fluorine content in the CEI layer is less than or equal to 12 at.%, and the oxygen content is less than or equal to 23 at.%. This is beneficial for improving the conductivity of the positive electrode while maintaining the protective function of the CEI layer. The reason for this is that the fluorine (F) and oxygen (O) content in the CEI layer is positively correlated with the content of inorganic components in the CEI layer. F and O mainly form inorganic substances such as fluorides and oxides in the CEI layer (fluorides such as lithium fluoride (LiF), and oxides such as lithium oxide (Li2O)). F and O in the CE... The lower the content of the CEI layer, the less inorganic components there are in the CEI layer, and the more organic components there are. The increase in organic components is conducive to the formation of a dense CEI layer, which plays a protective role and prevents the electrolyte from penetrating into the positive electrode active material. At the same time, during the battery manufacturing process, the CEI layer with more organic components is also conducive to inhibiting the contact reaction between the electrolyte and the positive electrode active material, as well as the resulting further thickening of the CEI layer. This also helps to maintain a thinner CEI layer, improve the conductivity of the positive electrode, ensure the capacity of the positive electrode, and thus improve the battery's discharge energy and other performance characteristics.

[0026] Under normal circumstances, the positive electrode active material is in the form of particles, and the CEI layer exists on the surface of the positive electrode active material particles.

[0027] Specifically, the CEI layer includes inorganic components, which may include inorganic compounds such as fluorides and oxides. For example, in lithium-ion batteries, fluorides may include lithium fluoride, and oxides may include lithium oxide.

[0028] In addition, the CEI layer in this application embodiment also includes organic components (organic compounds). The presence of these organic compounds helps the CEI layer to better perform its protective function, reduces the contact between the positive electrode active material and the electrolyte and the occurrence of side reactions between them, thereby making it more conducive to the capacity utilization of the positive electrode active material and improving the battery's discharge energy and other performance.

[0029] Specifically, the CEI layer also contains carbon (C), which mainly comes from the organic components in the CEI layer and is the main component of the organic components in the CEI layer.

[0030] Generally, the atomic percentage of carbon in the CEI layer can be greater than or equal to 25 at.%, specifically between 25 at.% and 36 at.%, such as 25 at.%, 26 at.%, 27 at.%, 28 at.%, 29 at.%, 30 at.%, 31 at.%, 32 at.%, 33 at.%, 34 at.%, 35 at.%, 36 at.%, or any combination thereof. This range is beneficial for further improving the battery's discharge energy and other performance characteristics. The reason for this is that the CEI layer contains more organic components at this level, which is more conducive to the protective function of the CEI layer. At the same time, it suppresses the problem of the positive electrode active material reacting with the electrolyte during battery manufacturing, which would further lead to the thickening of the CEI layer. This is more conducive to maintaining a lower resistivity of the positive electrode sheet and further improving the battery's discharge energy and other performance characteristics.

[0031] In some embodiments, the mass percentage of fluorine in the CEI layer can be 3 at.% to 12 at.%, for example, a range of 3 at.%, 4 at.%, 5 at.%, 6 at.%, 7 at.%, 8 at.%, 9 at.%, 10 at.%, 11 at.%, 12 at.%, or any two of these ranges, which is beneficial for further improving the battery's discharge energy and other performance characteristics.

[0032] In some embodiments, the mass percentage of oxygen in the CEI layer can be 20 at.% to 23 at.%, for example, 20 at.%, 20.5 at.%, 21 at.%, 21.5 at.%, 22 at.%, 22.5 at.%, 23 at.%, or any combination thereof, which is beneficial for further improving the battery's discharge energy and other performance.

[0033] According to further research by the inventors, as the F content in the CEI layer increases, the resistivity of the positive electrode increases, while the discharge energy of the battery using this positive electrode first increases and then decreases. The reason for this is that, as mentioned above, the F content in the CEI layer is positively correlated with the content of inorganic fluorides in the CEI layer, and also reflects the content of organic components in the CEI layer (for example, the less F content in the CEI layer, the more organic components in the CEI layer). Inorganic fluorides and organic components, as the material composition of the CEI layer, affect the protective function of the CEI layer, as well as its ion transport capacity and electronic conductivity, and thus affect the resistivity and capacity performance of the positive electrode. Therefore, as the F content in the CEI layer increases, the resistivity of the positive electrode increases, while the discharge energy of the battery using this positive electrode first increases and then decreases.

[0034] Based on the above research, when the fluorine content in the CEI layer is 3 at.% to 12 at.%, it is beneficial to improve the battery's discharge capacity and other performance characteristics while ensuring that the positive electrode has a low resistivity.

[0035] Furthermore, the inventors also investigated the impact of oxygen in the CEI layer on the performance of the positive electrode and the battery. The impact on the positive electrode and the battery exhibits the following trend: as the O content in the CEI layer increases, the resistivity of the positive electrode increases, while the discharge energy of the battery using this positive electrode first increases and then decreases. The reason for this is that the O content in the CEI layer is positively correlated with the content of inorganic oxides in the CEI layer, and also reflects the content of organic components in the CEI layer (for example, the less O content in the CEI layer, the more organic components). Inorganic oxides and organic components, as the material composition of the CEI layer, affect the protective function of the CEI layer, as well as its ion transport capacity and electronic conductivity, thus affecting the resistivity and capacity performance of the positive electrode. Therefore, as the O content in the CEI layer increases, the resistivity of the positive electrode increases, while the discharge energy of the battery using this positive electrode first increases and then decreases.

[0036] Based on the above research, when the oxygen content in the CEI layer is 20 at.% to 23 at.%, it is beneficial to further improve the battery's discharge capacity and other performance characteristics while ensuring that the positive electrode has a low resistivity.

[0037] In this embodiment, the CEI layer has a relatively thin thickness, specifically less than or equal to 9 nm, which is beneficial for further improving the conductivity and other properties of the positive electrode and improving the discharge energy and other performance of the battery.

[0038] Specifically, in related technologies, the CEI layer formed on the surface of the positive electrode active material generally suffers from poor electronic conductivity and excessive thickness, affecting the specific capacity of the positive electrode active material. For example, for lithium iron phosphate (LFP), current LFP particles are all nanoscale particles with short ion transport paths, so ionic conductivity is not the decisive factor for LFP specific capacity. LFP itself has poor electronic conductivity, and currently commercially available LFP uses carbon coating to improve its electronic conductivity. However, whether the CEI is naturally grown, doped, or artificially constructed, its electronic conductivity is still poor, and its thickness is generally not less than 10 nm. This leads to a decrease in the electronic conductivity of LFP cathode materials, making electronic conductivity the decisive factor for specific capacity. Therefore, the current characteristics of CEI result in significant polarization in LFP batteries, making it difficult to improve specific capacity. In this embodiment, the thickness of the CEI layer on the surface of the positive electrode active material is lower, specifically less than or equal to 9 nm, which is beneficial to improve the conductivity of positive electrode active materials such as lithium iron phosphate, give full play to the specific capacity of the positive electrode active material, and improve the battery's discharge energy and other performance.

[0039] In some embodiments, the thickness of the CEI layer can be from 0.5 nm to 9 nm, for example, a range of 0.5 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, 7.5 nm, 8 nm, 8.5 nm, 9 nm, or any combination thereof. Thus, a CEI layer thickness of not less than 0.5 nm is more conducive to the protective effect of the CEI layer on the positive electrode active material, further ensuring the capacity of the positive electrode active material. It also helps to reduce the heat preservation time during battery fabrication, thereby reducing the consumption of more active lithium in the negative electrode, further ensuring higher battery capacity, and improving battery fabrication efficiency. Meanwhile, a CEI layer thickness of not more than 9 nm also helps to further improve the conductivity of the positive electrode sheet, reduce battery polarization, and thus further improve the battery's discharge energy and other performance characteristics.

[0040] In this embodiment of the application, the positive electrode sheet may include a positive current collector and a positive active layer located on at least one side surface of the positive current collector. Specifically, the positive active layer may be provided on one side surface of the positive current collector, or the positive active layer may be provided on both sides of the positive current collector in the thickness direction (i.e., the two surfaces of the positive current collector). The positive active material is present in the positive active layer.

[0041] Generally, the positive electrode active layer may include the aforementioned positive electrode active material (positive electrode active substance), conductive agent, and binder, all of which can be conventional materials in the art. For example, the positive electrode active material may include one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary positive electrode materials. The ternary positive electrode material may include nickel-cobalt-manganese ternary materials and / or nickel-cobalt-aluminum ternary materials. The conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber. The binder may include one or more of polyvinylidene fluoride (PVDF), polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, etc.

[0042] Generally, in the positive electrode active layer, the mass percentage of the positive electrode active material (i.e., the ratio of the mass of the positive electrode active material to the total mass of the positive electrode active layer) can be 70% to 99%, for example, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 99%, or any combination thereof; the mass percentage of the conductive agent can be 0.5% to 15%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any combination thereof; and the mass percentage of the binder can be 0.5% to 15%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any combination thereof.

[0043] The embodiments of this application may employ conventional positive current collectors in the art, for example, positive current collectors may include aluminum foil.

[0044] In some embodiments, the positive electrode active material includes lithium iron phosphate and / or carbon-coated lithium iron phosphate. Specifically, the positive electrode sheet can be a lithium iron phosphate positive electrode sheet used in lithium iron phosphate batteries. By making the mass percentage of fluorine in the CEI layer on the surface of lithium iron phosphate less than or equal to 12 at.% and the oxygen content less than or equal to 23 at.%, the conductivity of the positive electrode sheet can be improved, and the discharge energy and other performance of the lithium iron phosphate battery can be improved.

[0045] Specifically, lithium iron phosphate (LFP) batteries use lithium iron phosphate-based positive electrode active materials (i.e., lithium iron phosphate and / or carbon-coated lithium iron phosphate) as the positive electrode active material. These materials are typically nano-sized lithium iron phosphate particles with short ion transport paths. Therefore, the ionic conductivity of lithium iron phosphate-based positive electrode materials is not the decisive factor in their specific capacity. However, the electronic conductivity of lithium iron phosphate-based positive electrode materials is poor. Furthermore, after formation and other processes during battery manufacturing, the composition and thickness of the CEI layer formed on the surface of the lithium iron phosphate-based positive electrode material (for example, the thickness of the CEI layer formed on the surface of lithium iron phosphate is relatively thick, usually not less than 10 nm) limit the conductivity of the lithium iron phosphate-based positive electrode material, severely affecting its capacity utilization and resulting in low discharge energy of lithium iron phosphate batteries.

[0046] In this embodiment, by controlling the mass percentage of fluorine in the CEI layer on the surface of the lithium iron phosphate cathode material to be less than or equal to 12 at.% and the oxygen content to be less than or equal to 23 at.%, the composition of the CEI layer is adjusted. This helps to exert the protective function of the CEI layer, protecting the lithium iron phosphate cathode material from electrolyte corrosion, thereby ensuring the capacity of the lithium iron phosphate cathode material. Meanwhile, according to the inventors' research, the fluorine (F) and oxygen (O) content in the CEI layer is positively correlated with the content of inorganic components in the CEI layer. F and O mainly form inorganic substances such as LiF and Li₂O in the CEI layer. The lower the content in the CEI layer, the less inorganic components there are, and the more organic components there are. The increase in organic components is beneficial to the protective function of the CEI layer, preventing the electrolyte from penetrating into the positive electrode active material. At the same time, during the battery manufacturing process, the CEI layer with more organic components is also beneficial to inhibit the contact reaction between the electrolyte and the lithium iron phosphate positive electrode material, as well as the resulting further thickening of the CEI layer. This also helps to maintain a thinner CEI layer, improve the conductivity of the lithium iron phosphate positive electrode material, ensure the capacity of the lithium iron phosphate positive electrode material, and thus reduce the resistivity of the positive electrode sheet and improve the discharge energy and other performance of the lithium iron phosphate battery.

[0047] Specifically, carbon-coated lithium iron phosphate includes lithium iron phosphate and carbon material coated on the surface of the lithium iron phosphate. Relatively speaking, using carbon-coated lithium iron phosphate is beneficial for further improving the conductivity of the positive electrode active material and improving the battery's discharge energy and other performance characteristics. In some embodiments, the mass percentage of carbon material in carbon-coated lithium iron phosphate is 3% to 5%, for example, 4%.

[0048] The embodiments of this application can employ conventional lithium iron phosphate and carbon-coated lithium iron phosphate, which are commercially available or can be manufactured using conventional methods in the art. For example, carbon-coated lithium iron phosphate can be commercially available, or carbon materials can be coated onto the surface of lithium iron phosphate using conventional methods in the art. The content of carbon materials in carbon-coated lithium iron phosphate can be controlled using conventional methods, and there are no particular limitations on this.

[0049] Generally, the CEI layer on the surface of the positive electrode active material can also contain elements other than C, O, and F, such as lithium and / or phosphorus. For example, when the positive electrode active material includes lithium iron phosphate, the CEI layer can contain C, O, Li, F, and P.

[0050] In this embodiment, the positive electrode has a low resistivity, specifically less than or equal to 38 Ω·cm, which is beneficial for the positive electrode to have higher conductivity, better capacity utilization, and improved battery discharge energy and other performance characteristics.

[0051] For example, the resistivity of the positive electrode can be a range of 20 Ω·cm, 22 Ω·cm, 24 Ω·cm, 26 Ω·cm, 28 Ω·cm, 30 Ω·cm, 32 Ω·cm, 34 Ω·cm, 35 Ω·cm, 36 Ω·cm, 38 Ω·cm, or any combination thereof.

[0052] In some embodiments, the resistivity of the positive electrode is 20–38 Ω·cm, and more specifically 24–32 Ω·cm, which is beneficial for improving the battery's discharge energy and other performance characteristics while ensuring that the positive electrode has a low resistivity.

[0053] This application also provides a battery including the above-described positive electrode sheet, which has advantages corresponding to the above-described positive electrode sheet, and will not be described in detail here.

[0054] Specifically, the aforementioned battery can be a lithium-ion battery, specifically a lithium iron phosphate battery.

[0055] Generally, a battery includes a cell and a casing that encapsulates the cell. Electrolyte is injected into the cell within the casing. The cell includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrodes. The cell can be a stacked cell, meaning it is composed of alternating layers of positive electrode, separator, and negative electrode; or it can be a wound cell, meaning it is composed of positive electrode, separator, and negative electrode layers stacked sequentially and then wound together.

[0056] In this embodiment, the battery cell can be packaged using conventional housing materials in the art, such as flexible packaging materials like aluminum-plastic film, but is not limited to these.

[0057] The electrolyte in this application embodiment can be a conventional electrolyte in the art. For example, the electrolyte is a non-aqueous electrolyte, which may specifically include organic solvents, additives, and electrolyte salts. Organic solvents include one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and propylene carbonate (PC). Additives include, for example, fluoroethylene carbonate (FEC) and / or vinylene carbonate (VC). Electrolyte salts may include lithium salts, such as lithium hexafluorophosphate (LiPF6), but are not limited thereto.

[0058] In this embodiment, the separator is used to separate the positive electrode and the negative electrode to prevent the positive electrode and the negative electrode from short-circuiting due to contact. Conventional separators in the art can be used in this embodiment, and there are no special restrictions.

[0059] In some embodiments, the separator includes a base membrane and a ceramic layer located on at least one side of the base membrane, for example, the ceramic layer is provided on the surface of the base membrane facing the positive electrode and / or the ceramic layer is provided on the side of the base membrane facing the negative electrode.

[0060] Specifically, the base membrane can be a polymer membrane, which can be a conventional membrane material in the art. For example, the base membrane includes a polyethylene (PE) membrane. Exemplarily, the separator can be a PE-coated ceramic membrane (i.e., the separator includes a PE membrane and a ceramic layer located on at least one side of the PE membrane).

[0061] Specifically, the ceramic layer may include a ceramic material and an adhesive. The ceramic material may be a conventional ceramic material in the art, such as alumina. The adhesive may be a conventional adhesive material in the art, such as PVDF.

[0062] Generally, the ceramic material content in the ceramic layer can be 95% to 99% by mass, for example, 97%, and the binder content can be 1% to 5% by mass, for example, 3%.

[0063] In this embodiment, conventional negative electrode sheets in the art can be used, and there are no particular limitations. For example, the negative electrode sheet may include a negative current collector and a negative active layer located on at least one side surface of the negative current collector. Specifically, the negative active layer may be provided on one side surface of the negative current collector, or negative active layers may be provided on both opposite sides of the negative current collector in the thickness direction.

[0064] Specifically, the negative electrode active layer may include a negative electrode active material, a conductive agent, and a binder, all of which can be conventional materials in the art. For example, the negative electrode active material may include graphite, which may include artificial graphite and / or natural graphite; the conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber; the binder may include one or more of sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0065] The embodiments of this application may employ conventional negative electrode current collectors in the art, for example, negative electrode current collectors may include copper foil.

[0066] In this embodiment, the negative electrode sheet can be prepared by conventional methods in the art, such as by coating. Specifically, the components used to form the negative electrode active layer, such as the negative electrode active material, conductive agent, and binder, can be dispersed in a second solvent, such as water, to prepare a negative electrode slurry. This slurry is then coated onto the surface of the negative electrode current collector, and after drying, rolling, and other processes, the negative electrode sheet is obtained. The coating, drying, and rolling processes involved are conventional operations for preparing negative electrode sheets using the coating method, and are not particularly limited thereto.

[0067] This application embodiment also provides a method for preparing the above-mentioned battery, including the following steps: placing a cell precursor in a casing and adding an electrolyte therein to obtain a battery precursor; wherein, the cell precursor includes a positive electrode precursor, and the positive electrode precursor includes a positive electrode active material; performing a formation treatment on the battery precursor to obtain a battery; wherein, the formation treatment process includes: charging the battery precursor to 1% to 30% of its state of charge (SOC) and then performing a heat preservation treatment, followed by aging the heat-preserved battery precursor to obtain a battery; wherein, the heat preservation treatment temperature is 25°C to 70°C, and the heat preservation treatment time is 1 hour to 96 hours.

[0068] For example, the state of charge of the battery precursor can be a range of 1%, 5%, 10%, 15%, 20%, 25%, 30%, or any two of these, before the insulation treatment.

[0069] For example, the temperature of the heat preservation treatment can be a range of 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C or any combination thereof.

[0070] For example, the heat preservation time can be a range of, for example, 1h, 6h, 12h, 18h, 24h, 30h, 36h, 42h, 48h, 54h, 60h, 66h, 72h, 78h, 84h, 90h, 96h, or any combination thereof.

[0071] According to the inventors' research, the above-mentioned preparation process can form a CEI layer with a fluorine content of less than or equal to 12 at.% and an oxygen content of less than or equal to 23 at.% on the surface of the positive electrode active material. Under this positive electrode composition system, the conductivity of the positive electrode in the resulting battery can be improved, thus enhancing the battery's discharge energy and other performance characteristics. The reason for this is that during the battery preparation process, in the initial stage of CEI layer formation on the positive electrode surface, a thinner CEI film with a higher organic content is formed through processes such as heat preservation. The denser organic components of the CEI film inhibit further thickening during subsequent formation processes. This results in a CEI layer with a high organic content and a relatively thin thickness on the surface of the positive electrode active layer in the resulting battery. This improves the conductivity of the positive electrode, making it easier for lithium to be delithiated and intercalated, thereby improving the battery's discharge capacity and discharge energy.

[0072] In the process of heat preservation after charging the battery precursor to 1%–30% of its state of charge, if the battery precursor is charged to an excessively low state of charge (<1% SOC) before heat preservation, or if the heat preservation temperature is too low (<25℃) or the time is too short (<1h), the organic content in the CEI film layer formed on the surface of the positive electrode active material will be too low. During subsequent heat preservation processes, the CEI film layer will have a poor barrier effect on the positive electrode active material and electrolyte, easily forming a CEI layer with insufficient organic content and a large thickness, which is detrimental to the conductivity of the positive electrode sheet. Improving the battery's performance can also affect its discharge energy and other properties. However, if the battery precursor is charged to an excessively high state of charge (>30% SOC) before heat treatment, or if the heat treatment temperature is too high (>96℃) or the heat treatment time is too long (>72h), it will lead to excessive consumption of active ions (such as active lithium), which will also affect the battery's capacity. Furthermore, it will result in excessive organic content in the CEI film layer formed on the surface of the positive electrode active material, affecting the conductivity of the positive electrode and thus hindering its capacity utilization, which is detrimental to improving the battery's discharge energy and other performance characteristics. In addition, excessively long heat treatment times will also affect the battery's manufacturing efficiency.

[0073] Therefore, in the above-mentioned battery preparation process, by charging the battery precursor to 1% to 30% of its state of charge and then performing heat preservation treatment, and by coordinating and controlling the temperature (25℃ to 70℃) and time (1h to 96h) of the heat preservation treatment, a CEI layer with a fluorine content of less than or equal to 12 at.% and an oxygen content of less than or equal to 23 at.% can be formed on the surface of the positive electrode active material. This results in the positive electrode formed by the positive electrode precursor in the prepared battery having higher conductivity and improving the battery's discharge energy and other performance characteristics.

[0074] In some embodiments, the above-mentioned heat preservation treatment time can be 1h to 72h, and more specifically 6h to 72h, which is conducive to the formation of a more suitable CEI layer, while reducing the excessive consumption of active ions (such as active lithium), further improving the battery's discharge energy and other performance, and also improving the battery's manufacturing efficiency.

[0075] Generally, the CEI layer is a solid electrolyte interface film formed on the surface of the positive electrode active material during the formation process of battery manufacturing, based on the reaction between the electrolyte and the positive electrode active material. It has lithium-ion transport capability, that is, it has good ionic conductivity.

[0076] In this embodiment, there is no need to modify the positive electrode active material by elemental doping or constructing a CEI layer. Instead, the CEI layer can be formed based on the formation process during battery manufacturing. A heat preservation process is introduced into the formation process, and the content of elements such as F and O in the CEI layer, as well as the thickness of the CEI layer, are controlled by adjusting the state of charge of the battery precursor before the heat preservation process and the temperature and time of the heat preservation process. This allows for the formation of a CEI layer with preset F and O content on the surface of the positive electrode active material, thereby improving the conductivity of the positive electrode and the discharge energy of the battery. Therefore, this embodiment forms a suitable CEI layer to improve the conductivity of the positive electrode and the discharge capacity of the battery without introducing doping elements into the positive electrode active material. This avoids the impact of doping elements on the performance of the positive electrode active material and the positive electrode. It also has advantages such as simple preparation process of the positive electrode and battery, convenient operation, good compatibility with existing battery production lines, and ease of large-scale production, which is of great significance for practical industrial applications.

[0077] In this embodiment, the aforementioned positive electrode precursor is a positive electrode sheet whose surface has not yet formed a CEI layer (or a positive electrode sheet whose surface has not yet formed a CEI layer). That is, the surface of the positive active material in the positive electrode precursor has not yet formed a CEI layer. Apart from this, the other structures of the positive electrode precursor are basically the same as those of the positive electrode sheet in the battery (the relevant composition and structure of the positive electrode sheet are described above and will not be repeated here).

[0078] Specifically, the positive electrode precursor includes a positive current collector and a positive active layer located on at least one side of the positive current collector, with the positive active material located in the positive active layer. This positive electrode precursor can be prepared using conventional positive electrode preparation methods in the art, such as a coating method. Specifically, the positive active material, conductive agent, binder, and other components used to form the positive active layer are dispersed in a first solvent, such as N-methylpyrrolidone (NMP), to prepare a positive electrode slurry. This slurry is then coated onto the surface of the positive current collector, and after drying, rolling, and other processes, the positive electrode precursor is obtained. The coating, drying, and rolling processes involved are conventional operations in the preparation of positive electrodes using the coating method and are not particularly limited thereto.

[0079] In this embodiment of the application, through the above-described battery preparation process, a CEI layer is formed on the surface of the positive electrode active material, thereby forming a positive electrode precursor. Accordingly, the difference between the cell precursor and the cell in the prepared battery is that the positive electrode in the cell precursor is the aforementioned positive electrode precursor, that is, the cell precursor includes a positive electrode precursor, a separator, and a negative electrode.

[0080] Generally, the battery preparation process may also include: aging the battery precursor, charging the battery precursor to 1% to 30% state of charge, and then performing heat preservation treatment.

[0081] The embodiments of this application can be carried out according to conventional procedures in the art to ensure that the electrolyte fully wets the battery cell. In some embodiments, the aging temperature can be 25°C to 60°C, for example, a range of 25, 30, 35, 40, 45, 50, 55, 60°C or any combination thereof, and the aging time can be 12h to 72h, for example, 12h, 18h, 24h, 30h, 36h, 42h, 48h, 54h, 60h, 66h, 72h or any combination thereof.

[0082] Furthermore, during the battery fabrication process described above, the battery precursor can be charged to a state of charge of 1% to 30% at a first rate before undergoing the aforementioned heat preservation treatment. The first rate can be 0.01C to 0.2C, for example, a range of 0.01C, 0.03C, 0.05C, 0.08C, 0.1C, 0.15C, 0.2C, or any combination thereof. This allows for a more suitable current during battery charging, further avoiding problems such as low organic content, porous, or thick CEI film due to excessive current, while also preventing low battery fabrication efficiency due to insufficient current.

[0083] In this embodiment of the application, after the battery precursor is charged to a preset state of charge, the battery precursor can be placed in a drying device such as a heat preservation box (such as a drying box) and heat preservation treatment is carried out at a preset temperature. Alternatively, the heat preservation treatment can be carried out under ultrasonic vibration, radiation, microwave or other effects.

[0084] In addition, during the preparation of the battery, after the heat preservation treatment is completed, the battery precursor after the heat preservation treatment can be charged to full charge at a second rate. The second rate can be 0.1C to 1C, for example, 0.1C, 0.3C, 0.5C, 0.7C, 0.9C, 1C or any combination thereof, which is beneficial to further balance the organic components of the CEI layer, the thinner thickness and the higher battery preparation efficiency.

[0085] Specifically, the second rate can be greater than the first rate. For example, when the positive electrode active material in the positive electrode active layer is lithium iron phosphate, the first rate can be 0.05C and the second rate can be 0.1C.

[0086] In general, during the manufacturing process of the aforementioned batteries, an aging process can form a solid electrolyte membrane (SEI film) on the surface of the negative electrode sheet. The formation of the SEI film helps protect the negative electrode active material in the negative electrode sheet. In some embodiments, the battery precursor, after heat preservation treatment, can be fully charged before undergoing the aging process.

[0087] In this embodiment, the fully charged state of the battery precursor refers to the state when the battery precursor reaches 100% state of charge (100% SOC), that is, the state when the battery precursor is charged to its upper limit cutoff voltage. In specific implementation, the fully charged state (charging upper limit cutoff voltage) can be determined according to conventional methods in the art based on the positive electrode active material used. For example, for lithium iron phosphate, its charging upper limit cutoff voltage can be 3.8V. That is, when the positive electrode active material is lithium iron phosphate, after the heat preservation treatment is completed, the battery precursor after the heat preservation treatment can be charged to 3.8V (that is, the battery precursor after the heat preservation treatment is charged to the fully charged state) and then the aging treatment is performed.

[0088] In some embodiments, the aging treatment temperature can be 30 to 60°C, for example, a range of 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C or any combination thereof, and the aging treatment time can be 12h to 72h, for example, 12h, 18h, 24h, 30h, 36h, 42h, 48h, 54h, 60h, 66h, 72h or any combination thereof. This can further reduce the consumption of active ions (such as active lithium) during the aging treatment process, further improve the battery's discharge energy and other performance, and further improve the battery's manufacturing efficiency.

[0089] In practice, the positive electrode precursor, separator, and negative electrode are assembled into a battery cell. The battery cell is then placed in a housing, and electrolyte is injected into it through a pre-drilled injection port on the housing. The housing is then sealed (or encapsulated, i.e., the injection port is sealed) to obtain the battery precursor. The assembled battery precursor is then aged to allow the electrolyte to permeate the battery cell. After aging, the battery precursor is charged at a constant current at the first rate to a state of charge of 1% to 30%, and then charging is stopped for heat preservation. After heat preservation, the heat-preserved battery precursor is charged at a constant current at the second rate to a fully charged state, and then charging is stopped for aging. Finally, after degassing and capacity testing, the battery is obtained.

[0090] In the embodiments of this application, unless otherwise specified, the positive electrode precursor, separator and negative electrode can be assembled into a battery cell according to conventional procedures in the art, such as by assembling a stacked battery cell using conventional stacking process, or by assembling a wound battery cell using conventional winding process; in the battery manufacturing process, the liquid injection (i.e., injecting electrolyte into the casing), encapsulation, venting, capacity testing and other processes involved are all conventional operations in the art, and there are no special restrictions on them.

[0091] This application also provides a battery pack including the above-described battery, which has advantages corresponding to the above-described positive electrode plate, and will not be described in detail here.

[0092] Generally, a battery pack includes multiple batteries as individual cells, which are connected to form the battery pack. These batteries can be electrically connected using methods conventional in the art, such as series connection, parallel connection, or a combination of these connection methods, without any particular limitation.

[0093] This application also provides an electrical device, including the battery or battery pack described above. This electrical device has advantages corresponding to the positive electrode plate described above, which will not be elaborated further.

[0094] The electrical equipment used in the embodiments of this application can be conventional electrical equipment in the art, such as power equipment (e.g., electric vehicles, electric cars), electronic equipment (e.g., mobile phones, tablets, laptops, digital cameras, etc.), wearable devices (e.g., watches, bracelets, VR glasses, etc.), energy storage power stations, etc., and there are no special limitations on this.

[0095] In this embodiment, the atomic percentages (elemental concentrations) of fluorine, oxygen, and carbon in the CEI layer can be determined by X-ray photoelectron spectroscopy (XPS). Specifically, the atomic percentage of fluorine in the CEI layer refers to the ratio of the number of fluorine atoms to the total number of atoms measured by XPS in the XPS test results of the CEI layer; the atomic percentage of oxygen in the CEI layer refers to the ratio of the number of oxygen atoms to the total number of atoms measured by XPS in the XPS test results of the CEI layer; and the atomic percentage of carbon in the CEI layer refers to the ratio of the number of carbon atoms to the total number of atoms measured by XPS in the XPS test results of the CEI layer. For example, in the XPS test results of the CEI layer, if the XPS detects atoms of F, O, C, and another element A, then the atomic percentage of fluorine in the CEI layer = number of fluorine atoms / (number of fluorine atoms + number of oxygen atoms + number of carbon atoms + total number of A atoms), the atomic percentage of oxygen in the CEI layer = number of oxygen atoms / (number of fluorine atoms + number of oxygen atoms + number of carbon atoms + total number of A atoms), and the atomic percentage of carbon in the CEI layer = number of carbon atoms / (number of fluorine atoms + number of oxygen atoms + number of carbon atoms + total number of A atoms). Here, element A refers to elements in the CEI layer that can be detected by XPS other than F, O, and C. A includes, for example, lithium (Li) and / or phosphorus (P).

[0096] For example, if the XPS test results of the CEI layer show C, O, Li, F, and P, then the atomic percentage of fluorine in the CEI layer = number of fluorine atoms / (number of fluorine atoms + number of oxygen atoms + number of carbon atoms + number of lithium atoms + number of phosphorus atoms), the atomic percentage of oxygen in the CEI layer = number of oxygen atoms / (number of fluorine atoms + number of oxygen atoms + number of carbon atoms + number of lithium atoms + number of phosphorus atoms), and the atomic percentage of carbon in the CEI layer = number of carbon atoms / (number of fluorine atoms + number of oxygen atoms + number of carbon atoms + number of lithium atoms + number of phosphorus atoms).

[0097] Generally, the elemental composition of the CEI layer is significantly different from that of the positive electrode active material. For example, the carbon content in the CEI layer is significantly higher than that in the positive electrode active material (the positive electrode active material used may contain virtually no carbon). Therefore, there is a relatively obvious interface between the CEI layer and the positive electrode active material.

[0098] In practice, XPS etching analysis of the CEI layer can be performed. This involves etching the positive electrode active material particles layer by layer using XPS, and detecting the elemental distribution in each layer during the etching process. When etching reaches the interface between the CEI layer and the positive electrode active material, an inflection point appears in the elemental content (e.g., the atomic percentage of characteristic elements in the CEI layer such as F, O, and C) analyzed by XPS (i.e., a sharp drop or rise in the content of these elements). In other words, the inflection point of the atomic percentage of characteristic elements in the CEI layer is the interface (boundary) between the CEI layer and the positive electrode active material. Specifically, during XPS etching analysis, a curve showing the change in the atomic percentage of the corresponding characteristic elements (e.g., F, O, or C) with the etching depth of the CEI layer can be obtained. The atomic percentage of the element corresponding to the inflection point of this curve (the site where the atomic percentage of the element drops or rises sharply) is essentially the atomic percentage of that element at the interface between the CEI layer and the positive electrode active material. Therefore, the atomic percentage of elements such as F and O, as well as the thickness of the CEI layer, can be measured by XPS.

[0099] Specifically, after obtaining the positive electrode sheet, the atomic percentages of elements such as fluorine, oxygen, and carbon in the CEI layer, as well as the thickness of the CEI layer, can be characterized according to conventional procedures in the art for characterizing film thickness using XPS. For example, as mentioned above, the elemental composition of the CEI layer and the positive electrode active layer are significantly different. The CEI layer can be characterized using XPS etching. For instance, the thickness of the CEI layer can be calculated based on the elemental distribution information of the CEI layer and the XPS sputtering rate. That is, the thickness of the CEI layer can be determined based on the elemental distribution of the CEI layer along the depth direction of the CEI layer (from the surface of the CEI layer away from the positive electrode active material particles to the side of the CEI layer closer to the positive electrode active material particles) measured by XPS etching.

[0100] When testing the thickness of the CEI layer using XPS, the thickness of the CEI layer on the surface of at least 20 positive electrode active material particles can be tested (in the specific embodiment below, the thickness of the CEI layer on the surface of 30 positive electrode active material particles was tested), and then the average value is calculated and used as the test result of the CEI layer thickness.

[0101] In addition, the thickness of the CEI layer on the surface of the positive electrode active material can be directly observed by cryo-electron microscopy. During the test, the thickness of the CEI layer on the surface of at least 20 positive electrode active material particles can be observed by cryo-electron microscopy (the thickness of the CEI layer on the surface of 30 positive electrode active material particles was tested in the following specific embodiment), and then the average value is calculated and used as the test result of the thickness of the CEI layer.

[0102] In this embodiment of the application, the resistivity of the positive electrode can be measured using conventional resistivity instruments and conventional methods in the art. Specifically, when using a resistivity meter to test the resistivity of the positive electrode, the current direction is basically parallel to the thickness direction of the positive electrode. That is, the positive electrode of the resistivity instrument is in contact with one side of the positive electrode in the thickness direction, and the negative electrode of the resistivity instrument is in contact with the other side of the positive electrode in the thickness direction, so as to measure the resistivity of the positive electrode.

[0103] In practice, the battery can be completely discharged (i.e., discharged to 0% SOC) before disassembly to ensure safety. Specifically, after discharging the battery, the positive electrode is removed and cleaned with organic solvents such as dimethyl carbonate (DMC) to remove electrolyte salts and other electrolyte components from its surface. After drying, the positive electrode is analyzed using XPS, cryo-electron microscopy, and resistivity testing. For example, XPS etching or cryo-electron microscopy measurements can be used to determine the atomic percentage of each element and the thickness of the CEI layer on the surface of the positive electrode active material. For instance, after cleaning the positive electrode with organic solvents and drying it, the positive electrode active layer can be scraped off and prepared as a sample for cryo-electron microscopy characterization. The thickness of the CEI layer on the particle surface can then be observed using cryo-electron microscopy.

[0104] The present application will be further described below through specific embodiments. In the following embodiments and comparative examples, the resistivity of the positive electrode sheet in the prepared lithium-ion battery was measured by a resistivity instrument, and the test method is the same as described above, so it will not be described in detail here; the atomic percentages of oxygen, fluorine, and carbon in the CEI layer on the surface of the positive electrode active material were measured by XPS, and the test method is the same as described above, so it will not be described in detail here.

[0105] In the following examples and comparative examples, the thickness of the CEI layer on the surface of the positive electrode active material in the prepared lithium-ion battery was measured by cryo-electron microscopy. Specifically, the lithium-ion battery was discharged to 2V at a rate of 0.1C (i.e., the lithium-ion battery was completely discharged), and then the positive electrode sheet was removed from the lithium-ion battery. The positive electrode sheet was cleaned with DMC solvent and dried. The positive electrode active layer was scraped off from the positive electrode current collector. The obtained positive electrode powder was then prepared into a cryo-electron microscopy characterization sample and characterized. The thickness of the CEI layer on the particle surface was observed (the thickness of the CEI layer on the surface of 30 positive electrode active material particles was observed by cryo-electron microscopy, and the average value was taken as the final test result).

[0106] In the following examples and comparative examples, the atomic percentages of fluorine, oxygen, and carbon in the CEI layer on the surface of the positive electrode active material in the prepared lithium-ion batteries were measured by XPS. The test method is the same as described above and will not be repeated here.

[0107] Example 1

[0108] 1) Preparation of positive electrode sheet

[0109] Carbon-coated lithium iron phosphate, polyvinylidene fluoride, and acetylene black were mixed at a mass ratio of 100:2.5:2.5, and N-methylpyrrolidone (NMP) was added. The mixture was stirred evenly under vacuum to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated onto both sides of an aluminum foil with a thickness of 13 μm and dried. Then, it was transferred to a vacuum oven and dried at 120°C for 24 hours. After rolling and cutting, the positive electrode sheet was obtained. The carbon-coated lithium iron phosphate comprises lithium iron phosphate and carbon material coated on the surface of the lithium iron phosphate. The mass percentage of carbon material in the carbon-coated lithium iron phosphate is approximately 4%.

[0110] 2) Preparation of negative electrode sheet

[0111] Artificial graphite, acetylene black, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) were mixed in a mass ratio of 94:2:2:2. Deionized water was added, and the mixture was stirred evenly under vacuum to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated on both sides of an 8 μm thick copper foil and dried. Then, it was transferred to a vacuum oven and dried at 120°C for 24 hours. After rolling and cutting, the negative electrode sheet was obtained.

[0112] 3) Preparation of electrolyte

[0113] In a nitrogen-atmospheric glove box with a water content <1 ppm and an oxygen content <1 ppm, ethylene carbonate (EC), dimethyl carbonate (DMC), dimethyl carbonate (DEC), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of EC:DMC:DEC:EMC = 1:1:1:1 to serve as a mixed organic solvent. LiPF6 was then slowly dissolved in the mixed organic solvent, followed by the addition of vinylene carbonate (VC). After thorough mixing, the electrolyte was obtained. Based on the total mass of the electrolyte, the mass percentage of lithium salt LiPF6 was 16%, and the mass percentage of vinylene carbonate (VC) was 3%.

[0114] 4) Separating membrane (diaphragm)

[0115] The total thickness of the separator is 12 μm. The separator consists of a PE membrane, a ceramic layer on the side of the PE membrane facing the positive electrode, and a ceramic layer on the side of the PE membrane facing the negative electrode. Each ceramic layer is composed of alumina and PVDF in a mass ratio of 97:3, and the thickness of each ceramic layer is 1.5 μm.

[0116] 5) Lithium-ion battery manufacturing

[0117] S1. Stack the negative electrode, separator, positive electrode, and separator in sequence, repeat the above sequence to form a stacked battery cell, and fix the stacked battery cell with high-temperature resistant tape (PI material). Then, weld the positive and negative electrode tabs and encapsulate it with aluminum-plastic film (i.e., place the battery cell in the aluminum-plastic film) to obtain the encapsulated battery cell. After vacuum drying the encapsulated battery cell at 105°C for 72 hours, inject the above-mentioned electrolyte into it and seal it to obtain the battery precursor.

[0118] S2. The battery precursor (i.e. the assembled cell) is aged at 45°C for 24 hours, and then the battery precursor is charged at a constant current of 0.05C to a SOC value of 10% (i.e., the state of charge before heat preservation is 10% (i.e., 10% SOC)), and then charging is stopped.

[0119] S3. Then place the above battery precursor in a drying oven at 45°C for 72 hours (i.e., the temperature of the heat treatment is 45°C and the time of the heat treatment is 72 hours).

[0120] S4. Continue to charge the battery precursor after the above heat preservation treatment to 3.8V using a constant current of 0.1C at the second rate, stop charging, and then age it at 45℃ for 48 hours. Then, degas and perform capacity testing to obtain a lithium-ion battery (lithium iron phosphate battery).

[0121] In the prepared lithium-ion battery cathode sheet, the thickness of the CEI layer on the surface of the cathode active material (lithium iron phosphate) is 0.5 nm.

[0122] Examples 2-10 differ from Example 1 in that the SOC value (state of charge before heat preservation treatment) of the battery precursor in step S2, the temperature and time of the heat preservation treatment in step S3 are different (see Table 1 for details). Consequently, the atomic percentages of oxygen (O concentration in the CEI layer of the positive electrode active material in Table 1), fluorine (F concentration in the CEI layer of Table 1), and carbon (C concentration in the CEI layer of Table 1), as well as the thickness of the CEI layer, are different in the resulting lithium-ion batteries (see Table 1 for details). The remaining steps and conditions are the same as in Example 1.

[0123] Comparative Example 1

[0124] The difference between Comparative Example 1 and Example 1 is that step S3 (i.e., heat preservation treatment) is omitted in the battery preparation process; the remaining steps and conditions are the same as in Example 1. The specific preparation process of the battery in Comparative Example 1 is as follows:

[0125] The battery precursor (i.e. the assembled cell) was aged at 45°C for 24 hours, and then the battery precursor was charged at a constant current of 0.05C at the first rate until the SOC value was 10%, and then charging was stopped.

[0126] The battery precursor was then charged to 3.8V using a constant current rate of 0.1C, and charging was stopped. It was then aged at 45°C for 48 hours, followed by venting and capacity testing to obtain a lithium-ion battery (lithium iron phosphate battery). The CEI layer on the surface of the positive electrode active layer in the prepared lithium-ion battery is 10 nm thick.

[0127] The 1 / 3C discharge energy of the lithium-ion batteries prepared in each embodiment and comparative example was tested according to the following process: The test was conducted using a Xinwei battery test cabinet. The test process is as follows: At room temperature (25℃), the lithium-ion battery was discharged to 2V at 1 / 3C, and then charged to 3.8V at 1 / 3C constant current and constant voltage with a cutoff current of 0.05C. This charge and discharge process was repeated 3 times. The discharge energy of the 3rd discharge was recorded as the battery discharge energy. The results are shown in Table 1.

[0128] In addition, the SOC value (i.e., the state of charge to which the battery precursor is charged after aging treatment and before heat preservation treatment) in step S2 of each embodiment, the heat preservation treatment temperature and heat preservation treatment time in step S3, and the thickness of the CEI layer on the surface of the positive electrode active material, the atomic percentage of fluorine in the CEI layer (F content in the CEI layer in Table 1), the atomic percentage of oxygen in the CEI layer (O content in the CEI layer in Table 1), and the atomic percentage of carbon in the CEI layer (C content in the CEI layer in Table 1) of the lithium-ion batteries prepared in each embodiment and comparative example are summarized in Table 1.

[0129] Table 1

[0130] As can be seen from Table 1, compared with Comparative Example 1, in Examples 1 to 10, the atomic percentage of fluorine in the CEI layer on the surface of the positive electrode active material is less than or equal to 12 at.% and the atomic percentage of oxygen is less than or equal to 23 at.%, which can significantly reduce the resistivity of the positive electrode and improve the discharge energy and other performance of the lithium-ion battery.

[0131] Furthermore, in Examples 1 to 10, the atomic percentage of carbon in the CEI layer on the surface of the positive electrode active material is greater than or equal to 25 at.%, indicating that the CEI layer contains a relatively large amount of organic components. This is beneficial for forming a dense CEI layer, which can play a protective role and prevent the electrolyte from penetrating into the positive electrode active material. At the same time, during the battery manufacturing process, the CEI layer with a relatively large amount of organic components can also help to suppress the contact reaction between the electrolyte and the positive electrode active material, as well as the resulting further thickening of the CEI layer. This also helps to maintain a relatively thin CEI layer, improve the conductivity of the positive electrode, ensure the capacity of the positive electrode, and thus improve the battery's discharge energy and other performance characteristics.

[0132] Furthermore, as shown in Table 1, the battery discharge energy initially increases and then decreases with increasing CEI layer thickness. Specifically, in Examples 6, 1-3, and Comparative Example 1, the battery discharge energy increases with increasing CEI layer thickness. This is because a longer insulation time for the battery precursor results in a higher organic content in the formed CEI film, which is more conducive to preventing further reaction between the electrolyte and lithium iron phosphate, thus thickening the CEI film. Consequently, a thinner CEI layer is formed on the surface of the positive electrode active material in the resulting battery. However, a thinner CEI layer corresponds to a longer insulation time, leading to greater consumption of active lithium at the negative electrode, which in turn affects the discharge capacity of the resulting lithium-ion battery to some extent. In Examples 3-5 and Comparative Example 1, the battery discharge energy decreases with increasing CEI layer thickness. This is because a thicker CEI layer results in greater battery polarization, leading to a decrease in battery discharge energy.

[0133] Therefore, in lithium-ion batteries, forming a CEI layer on the surface of the positive electrode active material, with the atomic percentage of fluorine in the CEI layer being less than or equal to 12 at.% and the atomic percentage of oxygen being less than or equal to 23 at.%, can reduce the resistivity of the positive electrode and increase the discharge capacity of the lithium-ion battery. Furthermore, as the fluorine and oxygen content in the CEI layer increases, the discharge energy of the lithium-ion battery shows a trend of first increasing and then decreasing. Therefore, by further controlling the atomic percentage of fluorine in the CEI layer within the range of 3 at.% to 12 at.% and the atomic percentage of oxygen within the range of 20 at.% to 23 at.% (e.g., Examples 1 to 5), it is possible to further and more significantly improve the battery's discharge energy while maintaining a low resistivity of the positive electrode.

[0134] Finally, it should be noted that other embodiments of this application will readily conceive of by those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of this application that follow the general principles of this application and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above, and various modifications and alterations may be made without departing from its scope. The scope of this application is limited only by the appended claims.

Claims

1. A positive electrode sheet characterized by comprising: The material includes a positive electrode active material, the surface of which has a CEI layer, wherein the atomic percentage of fluorine in the CEI layer is less than or equal to 12 at.% and the atomic percentage of oxygen in the CEI layer is less than or equal to 23 at.%.

2. The positive electrode sheet according to claim 1, characterized by The atomic percentage of fluorine in the CEI layer is 3 at.% to 12 at.%.

3. The positive electrode sheet according to claim 1, characterized by The atomic percentage of oxygen in the CEI layer is 20 at.% to 23 at.%.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized by, The CEI layer also contains carbon, and the atomic percentage of carbon in the CEI layer is greater than or equal to 25 at.%.

5. The positive electrode sheet according to claim 4, characterized by The atomic percentage of carbon in the CEI layer is 25 at.% to 36 at.%.

6. The positive electrode sheet according to any one of claims 1 to 5, characterized by, The thickness of the CEI layer is 0.5 nm to 9 nm.

7. The positive electrode sheet according to any one of claims 1 to 6, characterized by, The positive electrode active material includes lithium iron phosphate and / or carbon-coated lithium iron phosphate.

8. The positive electrode sheet according to any one of claims 1-7, characterized in that, The resistivity of the positive electrode is 20–38 Ω·cm.

9. The positive electrode sheet according to claim 8, characterized in that, The resistivity of the positive electrode is 24–32 Ω·cm.

10. A battery, characterized in that, Includes the positive electrode sheet as described in any one of claims 1-9.

11. A method for preparing the battery according to claim 10, characterized in that, Includes the following steps: A cell precursor is placed in a housing, and an electrolyte is added thereto to obtain a battery precursor; wherein, the cell precursor includes a positive electrode precursor, and the positive electrode precursor includes a positive electrode active material; The battery precursor is subjected to a formation process to obtain the battery; wherein the formation process includes: charging the battery precursor to a state of charge of 1% to 30% and then performing a heat preservation treatment, followed by aging treatment of the battery precursor after the heat preservation treatment to obtain the battery; wherein the temperature of the heat preservation treatment is 25℃ to 70℃ and the heat preservation treatment time is 1h to 96h.

12. The method for preparing a battery according to claim 11, characterized in that, During the formation process, the battery precursor, after the heat preservation treatment, is charged to a full charge and then subjected to the aging treatment to obtain the battery.

13. The method for preparing a battery according to claim 11 or 12, characterized in that, The heat preservation treatment time is 6h to 72h.

14. A battery pack, characterized in that, This includes the battery as described in claim 10 or the battery prepared according to the method described in any one of claims 11-13.

15. An electrical appliance, characterized in that, This includes the battery as described in claim 10, or the battery prepared according to the method of any one of claims 11-13, or the battery pack as described in claim 14.

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