Positive electrode sheet and electrochemical device

Abstract

Disclosed in the present application are a positive electrode sheet and an electrochemical device. The positive electrode sheet comprises a positive electrode material, the positive electrode material comprises a sodium element, and based on the mass of the positive electrode sheet, the mass proportion of the sodium element is 0.1% to 3%; and within a range of 2 nm to 20 nm from the surface of the positive electrode material, the positive electrode material further comprises a boron element, and based on 100% of the atoms of all elements within a range of 2 nm to 20 nm from the surface of the positive electrode material, the atomic proportion of the boron element is X%, where X is 0.1 to 4, the atomic proportion of the sodium element is Y%, where Y is 0.021 to 5, and X / Y is 0.08 to 185.71. The electrochemical device of the present application can improve the problem of gas production while achieving good high-temperature cycling performance.

Description

Positive electrode plate and electrochemical device Technical Field

[0001] This application relates to the field of electrochemistry, and more particularly to a positive electrode and an electrochemical device. Background Technology

[0002] In the research and development of lithium-ion batteries, the performance of the cathode material directly affects the overall performance of the battery. Ternary materials, such as nickel-cobalt-manganese oxide (NCM) series, have become one of the most popular cathode materials on the market today due to their advantages of high energy density and good cycle performance.

[0003] To improve the electrochemical performance of ternary materials, bulk element doping methods such as doping with Mg or Al are commonly used. However, this method often leads to severe capacity degradation. Alternatively, a single substance can be coated onto the cathode material surface, such as an oxide coating, but uniform coating is difficult to achieve. Currently, methods combining synergistic coating with multiple substances, controlling the content of coating elements in the coating layer, and surface doping are relatively rare. Summary of the Invention

[0004] In view of this, this application provides a positive electrode and an electrochemical device.

[0005] The first aspect of this application provides a positive electrode sheet, which includes a positive electrode material, the positive electrode material including sodium element, and the mass percentage of sodium element is 0.1% to 3% based on the mass of the positive electrode sheet; the positive electrode material also includes boron element within a range of 2nm to 20nm from the surface of the positive electrode material, and the atomic percentage of boron element is X%, X is 0.1 to 4, the atomic percentage of sodium element is Y%, Y is 0.021 to 5, and X / Y is 0.08 to 185.71, based on the atomic percentage of all elements within the range of 2nm to 20nm from the surface of the positive electrode material being 100%.

[0006] This application adds boron and sodium to the cathode material, and when their contents satisfy the above-mentioned relationship, the added boron makes the cathode material surface more compact, reduces the specific surface area of ​​the cathode material, reduces electrolyte penetration, and improves the side reactions on the cathode material surface. Furthermore, due to the inductive effect of sodium and boron, the density of the solid electrolyte interphase (SEI) film on the cathode material surface is improved, making the SEI film thinner. This not only improves the gas generation problem at high temperatures (85°C and above) in electrochemical devices, but also improves their high-temperature (45°C and above) cycle performance.

[0007] Based on the first aspect, in some possible implementations, based on the mass of the positive electrode sheet, the mass percentage of sodium is 1.6% to 3%; taking the atoms of all elements in the range of 2nm to 20nm on the surface of the positive electrode material as 100%, the atomic percentage of boron is X%, X is 1.2 to 1.8, the atomic percentage of sodium is Y%, Y is 0.8 to 1.5, and X / Y is 0.8 to 2.25.

[0008] This application further improves the high-temperature (85°C and above) gas generation problem and high-temperature (45°C and above) cycle performance of electrochemical devices by further controlling the mass ratio of sodium in the positive electrode sheet and the atomic ratio of sodium and boron in the range of 2nm to 20nm on the surface of the positive electrode material to meet the above range.

[0009] Based on the first aspect, in some possible implementations, within a range of 2nm to 20nm from the surface of the positive electrode material, the positive electrode material also includes nitrogen element, and the atomic percentage of nitrogen element is 0.3% to 1.8% based on the atomic composition of all elements within a range of 2nm to 20nm from the surface of the positive electrode material as 100%.

[0010] This application introduces nitrogen elements within a range of 2nm to 20nm from the surface of the cathode material. Due to the interaction between nitrogen and the electrolyte, the products of electrolyte decomposition on the cathode material surface form a thinner and more uniform SEI film. This suppresses the direct contact between the cathode material surface and the electrolyte, as well as the dissolution of transition metals in the cathode material at high temperatures. This further improves the high-temperature (85°C and above) gas generation problem and the high-temperature (45°C and above) cycling performance of the electrochemical device.

[0011] Based on the first aspect, in some possible implementations, within a range of 2nm to 20nm from the surface of the positive electrode material, the positive electrode material also includes nitrogen element, and the atomic percentage of nitrogen element is 0.8% to 1.3% based on the atomic composition of all elements within a range of 2nm to 20nm from the surface of the positive electrode material as 100%.

[0012] This application further improves the high-temperature (85°C and above) gas generation problem and high-temperature (45°C and above) cycle performance of electrochemical devices by further controlling the atomic ratio of nitrogen in the range of 2nm to 20nm on the surface of the cathode material to meet the above range.

[0013] Based on the first aspect, in some possible implementations, the positive electrode material also includes aluminum, with the aluminum content ranging from 0.02% to 0.6% by mass, depending on the mass of the positive electrode sheet.

[0014] This application, by adding aluminum to the cathode material and controlling its mass ratio to meet the above-mentioned range, allows the added aluminum to form a protective film on the surface of the cathode material. This film effectively inhibits direct contact between the electrolyte and the cathode material surface, reducing the occurrence of side reactions. The protective layer also prevents the dissolution of transition metal elements in the cathode material during electrochemical cycling, thus improving the chemical stability of the cathode material. Furthermore, the presence of aluminum can suppress lattice changes in the material during the charging and discharging process of the electrochemical device, reducing the volume expansion and contraction of the structure, and further improving the high-temperature (85°C and above) gas generation problem and high-temperature (45°C and above) cycling performance of the electrochemical device.

[0015] Based on the first aspect, in some possible implementations, the positive electrode material has a layered structure, and the positive electrode material further includes nickel and manganese elements, with the mass percentage of nickel being 28.7% to 51.5% and the mass percentage of manganese being 5.3% to 27.6% based on the mass of the positive electrode sheet.

[0016] This application improves the stability of the electrochemical device by adding manganese to the cathode material and controlling the mass ratio of manganese within the aforementioned range. The added manganese maintains a stable +4 valence state and will not react with the electrolyte. This further improves the high-temperature (85°C and above) gas generation problem and enhances the high-temperature (45°C and above) cycle performance of the electrochemical device. Furthermore, this application also improves the high-temperature (85°C and above) gas generation problem and enhances the high-temperature (45°C and above) cycle performance by adding nickel to the cathode material and controlling the mass ratio of nickel within the aforementioned range.

[0017] A second aspect of this application provides an electrochemical device including a positive electrode as provided in the first aspect of this application.

[0018] Based on the second aspect, in some possible embodiments, the electrochemical device further includes a negative electrode sheet, which comprises a negative electrode material, namely graphite. The negative electrode material selected in this application is graphite, which further improves the high-temperature (85°C and above) gas generation problem and enhances the high-temperature (45°C and above) cycling performance of the electrochemical device.

[0019] Based on the second aspect, in some possible embodiments, the electrochemical device further includes an electrolyte comprising at least one selected from 1,3,6-hexanetrionitrile, butadionitrile, and 1,2,3-tris(2-cyanoethoxy)propane. By selecting the aforementioned electrolyte, this application further improves the high-temperature (85°C and above) gas generation problem and enhances the high-temperature (45°C and above) cycling performance of the electrochemical device.

[0020] Based on the second aspect, in some possible implementations, a coin cell consisting of a positive electrode and a lithium sheet is charged and discharged at a rate of 0.04C within the range of 2.8V to 4.5V. The coin cell exhibits a redox peak between 3.6V and 3.9V, with a peak intensity of F1, which is 400mAh / g / V to 900mAh / g / V.

[0021] Based on the second aspect, in some possible implementations, a coin cell consisting of a positive electrode and a lithium sheet is charged and discharged at a rate of 0.04C within the range of 2.8V to 4.5V. The coin cell exhibits a redox peak between 4.2V and 4.4V, with a peak intensity of F2, which ranges from 800mAh / g / V to 5000mAh / g / V.

[0022] Based on the second aspect, in some possible implementations, a coin cell consisting of a positive electrode and a lithium sheet is charged and discharged at a rate of 0.04C within the range of 2.8V to 4.5V. The coin cell exhibits a redox peak between 3.6V and 3.9V with a peak intensity of F1, and a redox peak between 4.2V and 4.4V with a peak intensity of F2, where 1≤F2 / F1≤12.

[0023] Based on the second aspect, in some possible implementations, a coin cell battery composed of a positive electrode and a lithium sheet is charged and discharged at a rate of 0.04C within the range of 2.8V to 4.5V. The discharge capacity of the coin cell battery between 4.1V and 4.4V is Q1, and the discharge capacity of the coin cell battery between 2.8V and 4.5V is Q2, where 0.1≤Q1 / Q2≤0.3.

[0024] The third aspect of this application provides an electronic device, including the electrochemical device provided in the second aspect of this application. The electrochemical device has good high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycling performance, which is beneficial to improving the service life of the electronic device. Attached Figure Description

[0025] Figure 1 shows the charge-discharge curves of the lithium-ion button battery in Example 2 during the first week.

[0026] Figure 2 shows the first-week dQ / dV curve of the lithium-ion button cell in Example 2. Detailed Implementation

[0027] The embodiments of this application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. The reagents and materials described in the following embodiments are all commercially available.

[0028] Positive electrode sheet

[0029] One embodiment of this application provides a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on the positive current collector. The positive current collector can be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, including, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The positive electrode material layer includes a positive electrode material, which includes sodium element. Based on the mass of the positive electrode sheet, the mass percentage of sodium element is 0.1% to 3%. Within a range of 2nm to 20nm from the surface of the positive electrode material, the positive electrode material also includes boron element. Based on the atomic percentage of all elements within a range of 2nm to 20nm from the surface of the positive electrode material as 100%, the atomic percentage of boron element is X%, where X is 0.1 to 4, the atomic percentage of sodium element is Y%, where Y is 0.021 to 5, and X / Y is 0.08 to 185.71.

[0030] This application adds boron and sodium to the cathode material, and when their contents satisfy the above-mentioned relationship, the added boron makes the cathode material surface more compact, reduces the specific surface area of ​​the cathode material, reduces electrolyte penetration, and improves the side reactions on the cathode material surface. Furthermore, due to the inductive effect of sodium and boron, the density of the solid electrolyte interphase (SEI) film on the cathode material surface is improved, making the SEI film thinner. This not only improves the gas generation problem at high temperatures (85°C and above) in electrochemical devices, but also improves their high-temperature (45°C and above) cycle performance.

[0031] In some embodiments, based on the mass of the positive electrode sheet, the mass percentage of sodium is 1.6% to 3%; with the atoms of all elements in the 2nm to 20nm range on the surface of the positive electrode material being 100%, the atomic percentage of boron is X%, X is 1.2 to 1.8, the atomic percentage of sodium is Y%, Y is 0.8 to 1.5, and X / Y is 0.8 to 2.25.

[0032] This application further improves the high-temperature (85°C and above) gas generation problem and high-temperature (45°C and above) cycle performance of electrochemical devices by further controlling the mass ratio of sodium in the positive electrode sheet and the atomic ratio of sodium and boron in the range of 2nm to 20nm on the surface of the positive electrode material to meet the above range.

[0033] In some embodiments, within a distance of 2 nm to 20 nm from the surface of the cathode material, the cathode material also includes nitrogen, and the atomic percentage of nitrogen is 0.3% to 1.8% based on the atomic composition of all elements within a distance of 2 nm to 20 nm from the surface of the cathode material as 100%.

[0034] This application introduces nitrogen elements within a range of 2nm to 20nm from the surface of the cathode material. Due to the interaction between nitrogen and the electrolyte, the products of electrolyte decomposition on the cathode material surface form a thinner and more uniform SEI film. This suppresses the direct contact between the cathode material surface and the electrolyte, as well as the dissolution of transition metals in the cathode material at high temperatures. This further improves the high-temperature (85°C and above) gas generation problem and the high-temperature (45°C and above) cycling performance of the electrochemical device.

[0035] In some embodiments, within a distance of 2 nm to 20 nm from the surface of the cathode material, the cathode material also includes nitrogen, and the atomic percentage of nitrogen is 0.8% to 1.3% based on the atomic composition of all elements within a distance of 2 nm to 20 nm from the surface of the cathode material as 100%.

[0036] This application further improves the high-temperature (85°C and above) gas generation problem and high-temperature (45°C and above) cycle performance of electrochemical devices by further controlling the atomic ratio of nitrogen in the range of 2nm to 20nm on the surface of the cathode material to meet the above range.

[0037] In some embodiments, the positive electrode material further includes aluminum, with the aluminum content ranging from 0.02% to 0.6% by mass, based on the mass of the positive electrode sheet.

[0038] This application, by adding aluminum to the cathode material and controlling its mass ratio to meet the above-mentioned range, allows the added aluminum to form a protective film on the surface of the cathode material. This film effectively inhibits direct contact between the electrolyte and the cathode material surface, reducing the occurrence of side reactions. The protective layer also prevents the dissolution of transition metal elements in the cathode material during electrochemical cycling, thus improving the chemical stability of the cathode material. Furthermore, the presence of aluminum can suppress lattice changes in the material during the charging and discharging process of the electrochemical device, reducing the volume expansion and contraction of the structure, and further improving the high-temperature (85°C and above) gas generation problem and high-temperature (45°C and above) cycling performance of the electrochemical device.

[0039] In some embodiments, the positive electrode material has a layered structure, and the positive electrode material further includes nickel and manganese elements. Based on the mass of the positive electrode sheet, the mass percentage of nickel is 28.7% to 51.5%, and the mass percentage of manganese is 5.3% to 27.6%.

[0040] This application improves the stability of the electrochemical device by adding manganese to the cathode material and controlling the mass ratio of manganese within the aforementioned range. The added manganese maintains a stable +4 valence state and will not react with the electrolyte. This further mitigates the gas generation problem at high temperatures (85°C and above) and enhances the high-temperature (45°C and above) cycle performance of the electrochemical device. Furthermore, this application also improves the high-temperature (85°C and above) gas generation problem and enhances the high-temperature (45°C and above) cycle performance by adding nickel to the cathode material and controlling the mass ratio of nickel within the aforementioned range.

[0041] In some embodiments, a coin cell consisting of a positive electrode and a lithium sheet is charged and discharged at a rate of 0.04C within a range of 2.8V to 4.5V. The coin cell exhibits a redox peak between 3.6V and 3.9V, with a peak intensity of F1, which ranges from 400mAh / g / V to 900mAh / g / V.

[0042] In some embodiments, a coin cell consisting of a positive electrode and a lithium sheet is charged and discharged at a rate of 0.04C within a range of 2.8V to 4.5V. The coin cell exhibits a redox peak between 4.2V and 4.4V, with a peak intensity of F2 ranging from 800mAh / g / V to 5000mAh / g / V.

[0043] In some embodiments, a coin cell consisting of a positive electrode and a lithium sheet is charged and discharged at a rate of 0.04C within the range of 2.8V to 4.5V. The coin cell exhibits a redox peak between 3.6V and 3.9V with a peak intensity of F1, and a redox peak between 4.2V and 4.4V with a peak intensity of F2, where 1≤F2 / F1≤12.

[0044] In some embodiments, a coin cell consisting of a positive electrode and a lithium sheet is charged and discharged at a rate of 0.04C within a voltage range of 2.8V to 4.5V. The discharge capacity of the coin cell between 4.1V and 4.4V is Q1, and the discharge capacity of the coin cell between 2.8V and 4.5V is Q2, where 0.1 ≤ Q1 / Q2 ≤ 0.3.

[0045] electrolyte

[0046] According to some embodiments of this application, the electrolyte includes an organic solvent, a lithium salt, and optional additives.

[0047] In some embodiments, the additive includes at least one selected from 1,3,6-hexanetrionitrile, butadionitrile, and 1,2,3-tris(2-cyanoethoxy)propane. By selecting the above-mentioned electrolyte, this application further improves the high-temperature (85°C and above) gas generation problem and enhances the high-temperature (45°C and above) cycling performance of the electrochemical device.

[0048] In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalateborate)borate LiB(C2O4)2 (LiBOB), or lithium difluorooxalateborate LiBF2(C2O4) (LiDFOB).

[0049] In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate.

[0050] Negative electrode sheet

[0051] The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on the negative current collector. The negative current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or carbon-based current collector, or any composite current collector disclosed in the prior art, including, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The negative electrode material layer includes a negative electrode material, and optionally a conductive agent, a binder, and a thickener.

[0052] The negative electrode material used in this application is graphite, which further improves the high-temperature (85°C and above) gas generation problem of the electrochemical device and enhances the high-temperature (45°C and above) cycle performance.

[0053] The specific type of conductive agent is not limited and can be selected according to requirements. For example, conductive agents include, but are not limited to, conductive graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and at least one of the following:

[0054] The specific type of adhesive is not limited and can be selected according to requirements. As an example, adhesives include, but are not limited to, at least one of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), water-based acrylic resin, and carboxymethyl cellulose.

[0055] The specific type of thickener is not limited and can be selected according to needs. As an example, thickeners include, but are not limited to, sodium carboxymethyl cellulose (CMC).

[0056] Separating membrane

[0057] The material and shape of the separator used in the electrochemical device of this application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator comprises a polymer or inorganic material formed from a material stable to the electrolyte of this application.

[0058] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected.

[0059] A surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixture of polymer and inorganic material. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the polymer material is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0060] Electrochemical device

[0061] One embodiment of this application provides an electrochemical device, including a housing (such as a packaging bag) for accommodating a positive electrode, a separator, a negative electrode, and an electrolyte, as well as other components known in the field of electrochemistry. This application does not limit the scope of these other components. This application does not impose any particular limitation on the housing; it can be a housing known in the art, as long as it achieves the purpose of this application. For example, an aluminum-plastic film packaging bag can be used. This application does not impose any particular limitation on the type of electrochemical device; it can include any device that performs an electrochemical reaction. The electrochemical device of this application exhibits excellent high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycling performance.

[0062] Electronic devices

[0063] The aforementioned electrochemical device is applied to electronic devices to power loads within them. Furthermore, this electrochemical device exhibits excellent high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycling performance, which helps to extend the lifespan of the electronic devices. These electronic devices may include, but are not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0064] Example 1

[0065] <Preparation of cathode materials>

[0066] 1. Prepare a mixed solution containing NiSO4 and MnSO4 according to the elemental molar ratio Ni:Mn = 50:50. Mix this solution with the precipitant NaOH solution and the complexing agent ammonia solution. By controlling the reaction time, ammonia concentration, and pH value, the precursor Ni is obtained. 0.5 Mn 0.5 (OH)2.

[0067] 2. The above-mentioned precursor Ni 0.5 Mn 0.5 (OH)2, lithium hydroxide and sodium hydroxide are ground and mixed evenly in a certain proportion, and calcined at 900℃ in air atmosphere for 48h at a heating rate of 5℃ / min. Then, the mixture is cooled to room temperature in Ar atmosphere at a rate of 5℃ / min. Finally, the mixture is crushed and sieved to obtain positive electrode material 1.

[0068] 3. The above-mentioned sieved positive electrode material 1 is thoroughly mixed with a certain amount of sodium nitrate and boric acid, and then heat-treated at 350°C for 10 hours in a N2 protective atmosphere to obtain the positive electrode material of this application.

[0069] <Preparation of the positive electrode>

[0070] Polyvinylidene fluoride (PVDF) binder, conductive carbon black (Super P) conductive agent, and positive electrode material were mixed uniformly at a weight ratio of 1.5:1.5:97. N-methylpyrrolidone (NMP) was added as a solvent to obtain a positive electrode slurry with a viscosity of 3000 mPas to 6000 mPas. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil current collector and dried at 120°C for 1 hour to obtain a positive electrode sheet with a single-sided coating of a 60 μm thick positive electrode material layer. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of the positive electrode material layer. The sheet was dried under vacuum at 120°C for 1 hour, and then cold-pressed, cut, and had tabs welded to obtain a positive electrode sheet with dimensions of 74 mm × 867 mm.

[0071] <Preparation of Negative Electrode Sheets>

[0072] A negative electrode material (graphite), a thickener (sodium carboxymethyl cellulose, CMC-Na), and a binder (styrene-butadiene rubber, SBR) were mixed in a weight ratio of 96:2:2. Deionized water was added, and the mixture was stirred until homogeneous to obtain a negative electrode slurry with a solid content of 75 wt%. The negative electrode slurry was uniformly coated onto one surface of a 12 μm thick copper foil current collector and dried at 120 °C to obtain a single-sided negative electrode sheet with a coating thickness of 80 μm. The above steps were repeated on the other surface of the copper foil to obtain a double-sided negative electrode sheet. After drying under vacuum at 120 °C for 1 hour, the sheet was cold-pressed, cut, and slit to obtain a negative electrode sheet with dimensions of 78 mm × 875 mm.

[0073] <Preparation of Electrolyte>

[0074] In an argon-atmosphere glove box with a water content of <10 ppm, propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed in a 1:1:1 mass ratio. Thoroughly dried lithium salt LiPF6 was then dissolved in the above non-aqueous solvent, and 2% 1,3-propanesulfonate lactone was added to prepare the electrolyte used in the examples. Based on the mass of the electrolyte, the mass percentage of LiPF6 was 13%.

[0075] <Preparation of the separating membrane>

[0076] A porous polyethylene (PE) film with a thickness of 7 μm was used as the separator.

[0077] <Preparation of Lithium-ion Button Batteries>

[0078] The positive electrode sheet is cut into a circular piece with a diameter of 14 mm as the working electrode, and a lithium metal sheet with a diameter of 18 mm is used as the reference electrode. The two are separated by a separator with a diameter of 20 mm. An appropriate amount of electrolyte is added, and the CR2430 coin cell lithium-ion battery is assembled.

[0079] <Preparation of Lithium-ion Pouch Batteries>

[0080] The positive electrode, separator, and negative electrode prepared above are stacked, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is then wound up. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. Following vacuum sealing, settling, formation (with an upper limit voltage of 4.35V, a formation temperature of 85℃, and a settling time of 2 hours), degassing, and edge trimming, a lithium-ion soft-pack battery is obtained.

[0081] Examples 2 to 5

[0082] The difference between Examples 2 to 5 and Example 1 is that the amount of sodium hydroxide added in step 2 of the <Preparation of Cathode Material> and the amount of sodium nitrate and boric acid added in step 3 are adjusted. The other conditions / preparation methods are the same as in Example 1. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0083] Examples 6 to 8

[0084] The difference between Examples 6 to 8 and Example 5 is that the amount of sodium nitrate and boric acid added in step 3 of the <Preparation of Cathode Material> is adjusted. The other conditions / preparation methods are the same as in Example 5. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0085] Examples 9 to 11

[0086] The difference between Examples 9 to 11 and Example 7 is that different amounts of nano-alumina are added in step 3 of the <Preparation of Cathode Material>. The other conditions / preparation methods are the same as in Example 7. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0087] Example 12

[0088] The difference between Example 12 and Example 10 is that the content of manganese and nickel in the hydroxide precursor in step 1 of the <Preparation of Cathode Material> is adjusted. The other conditions / preparation methods are the same as in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0089] Example 13

[0090] The difference between Example 13 and Example 10 is that a certain amount of succinate is added in the <Preparation of Electrolyte>. The other conditions / preparation methods are the same as in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Table 1 and Table 2.

[0091] Example 14

[0092] The difference between Example 14 and Example 10 is that a certain amount of 1,3,6-hexanetrionitrile is added in the <Preparation of Electrolyte>. The other conditions / preparation methods are the same as in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0093] Example 15

[0094] The difference between Example 15 and Example 10 is that a certain amount of 1,2,3-tris(2-cyanoethoxy)propane is added in the <Preparation of Electrolyte>. The other conditions / preparation methods are the same as in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0095] Comparative Examples 1 and 2

[0096] The difference between Comparative Examples 1 and 2 and Example 10 is that the amount of sodium hydroxide added in step 2 of the <Preparation of Cathode Material> is adjusted. The other conditions / preparation methods are the same as in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0097] Comparative Examples 3 and 4

[0098] The difference between Comparative Examples 3 and 4 and Example 10 is that the amount of sodium nitrate and boric acid added in step 3 of the <Preparation of Cathode Material> is adjusted. The other conditions / preparation methods are the same as in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0099] Comparative Example 5

[0100] The difference between Comparative Example 5 and Example 10 is that sodium hydroxide is not added in step 2 of the <Preparation of Cathode Material> and sodium nitrate is replaced with lithium nitrate in step 3. The other conditions / preparation methods are the same as those in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0101] Comparative Example 6

[0102] The difference between Comparative Example 6 and Example 10 is that boric acid is not added in step 3 of the <Preparation of Cathode Material>. The other conditions / preparation methods are the same as in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0103] Comparative Example 7

[0104] The difference between Comparative Example 7 and Example 10 is that sodium hydroxide is not added in step 2 of the <Preparation of Cathode Material> and boric acid is not added in step 3, and sodium nitrate is replaced by lithium nitrate. The other conditions / preparation methods are the same as those in Example 10. The specific preparation parameters mentioned above can be adjusted accordingly according to Tables 1 and 2.

[0105] Test methods

[0106] (1) Element content test

[0107] The cathode material was dissolved in aqua regia solution (e.g., 0.4g of cathode material was dissolved in 10ml of aqua regia solution (the volume ratio of aqua regia to deionized water was 1:1, and the volume ratio of concentrated hydrochloric acid to concentrated nitric acid was 3:1). The cathode material was then fully digested in a CEM-Mars5 / Mars6 microwave digester, and the volume was adjusted to 100mL. The mass percentage of elements such as Li, Na, Ni, and Mn in the solution was then tested using an inductively coupled plasma optical emission spectrometer (ICP-OES).

[0108] (2) 45℃ Cyclic Capacity Retention Rate Test (High Temperature)

[0109] The lithium-ion pouch battery was placed in a 45℃ constant temperature chamber and left to stand for 30 minutes to reach a constant temperature. The battery was then charged at a constant current of 1.5C to 4.35V at 45℃, followed by constant voltage charging to 0.02C at 4.35V. After standing for 5 minutes, it was discharged at a constant current of 2C to 2.8V, and then left to stand for 5 minutes. This discharge capacity was recorded as the first cycle discharge capacity. This charge-discharge cycle was repeated 400 times, and the discharge capacity of the battery in the 400th cycle was recorded as the 400th cycle discharge capacity. The capacity retention rate (%) of the lithium-ion pouch battery after 400 cycles at 45℃ = (400th cycle discharge capacity / first cycle discharge capacity) × 100%.

[0110] (3) Thickness expansion rate test at 85℃

[0111] A lithium-ion pouch battery was charged at 25°C with a constant current of 0.5C to 4.35V, and then charged at 4.35V with a constant voltage to 0.05C. The thickness of the lithium-ion pouch battery at this point was measured and recorded as H0 using a micrometer. The lithium-ion pouch battery was then stored in an 85°C oven for 24 hours. After removal, the thickness of the lithium-ion pouch battery was measured and recorded using a micrometer, denoted as H1. The thickness expansion rate (%) of the lithium-ion pouch battery after storage at 85°C for 24 hours is calculated as 100% × (H1 - H0) / H0.

[0112] (4) Atomic testing methods for elements in the 2nm to 20nm range on the surface of cathode materials

[0113] X-ray photoelectron spectroscopy (using Thermo Scientific ESCALAB Xi+) was employed to determine the atomic content of each element at different depths of the material's surface coating by controlling the Ar+ etching time.

[0114] (5) Lithium-ion button cell testing and dQ / dV curve

[0115] For coin cell battery testing, the voltage range was 2.8 to 4.5V vs. Li+ / Li, with charging followed by discharging. During charging, the cells were first charged at a constant current of 0.04C (1C = 200mA / g) to 4.5V vs. Li+ / Li, then charged at a constant voltage of 4.5V vs. Li+ / Li until the current was less than 50uA. During discharging, the cells were discharged at a constant current of 0.04C (1C = 200mA / g) to 2.8V vs. Li+ / Li. The dQ / dV curve was obtained by differentiating the capacity-voltage charge-discharge curves.

[0116] Figure 1 is a charge-discharge curve of the lithium-ion button battery of Example 2 in the first week. It can be seen from the figure that the lithium-ion button battery of this application has a clear plateau in the high voltage range and also has a high capacity.

[0117] Figure 2 shows the first-week dQ / dV curve of the lithium-ion coin cell of Example 2. As can be seen from the figure, the lithium-ion coin cell of this application has obvious redox peaks in the voltage range of 4.2V to 4.4V, and the peak intensity is stronger than that of the redox peaks in the low voltage range of 3.6V to 3.9V. This feature indicates that the lithium-ion coin cell of this application has a higher capacity near the plateau in the high voltage range.

[0118] The test data of Examples 1 to 15 and Comparative Examples 1 to 7 are recorded in Table 1.

[0119] Table 1

[0120] Note: " / " in Table 1 indicates that the corresponding substance or parameter does not exist.

[0121] Table 2

[0122] Note: " / " in Table 2 indicates that the corresponding substance or parameter does not exist.

[0123] As can be seen from Tables 1 and 2, compared with Example 1, Examples 2 to 5, by adjusting the amount of sodium hydroxide added in step 2 of the <Preparation of Positive Electrode Material> and the amount of sodium nitrate and boric acid added in step 3, control the sodium content in the positive electrode sheet and the sodium and boron atomic contents in the range of 2nm to 20nm from the surface of the positive electrode material to meet the requirements of this application. Thus, the electrochemical device of this application has good high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycling performance.

[0124] Compared with Example 5, Examples 6 to 8 further improved the high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycle performance of the electrochemical device of this application by further adjusting the amount of sodium nitrate and boric acid added in step 3 of the <Preparation of Cathode Material>, thereby controlling the atomic content of nitrogen element in the range of 2nm to 20nm from the surface of the cathode material to meet the range of this application.

[0125] Compared with Example 7, Examples 9 to 11 further improved the high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycling performance of the electrochemical device of this application by adding nano-alumina in step 3 of <Preparation of cathode material> and controlling the amount added to meet the range of this application.

[0126] Compared with Example 10, Example 12 shows that the content of manganese and nickel in the hydroxide precursor in step 1 of the <Preparation of Cathode Material> can also affect the high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycle performance of the electrochemical device of this application.

[0127] Compared with Example 10, Example 13 further improved the high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycling performance of the electrochemical device of this application by adding a certain amount of succinic acid to the <preparation of electrolyte>.

[0128] Compared with Example 10, Examples 14 and 15 further improved the high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycling performance of the electrochemical device of this application by adding a certain amount of 1,3,6-hexanetrionitrile and 1,2,3-tris(2-cyanoethoxy)propane in the <preparation of electrolyte>.

[0129] By adjusting the amount of sodium hydroxide added in step 2 of the <Preparation of Cathode Material> in Comparative Examples 1 and 2, the sodium content in the cathode electrode of this application and the atomic percentage of sodium in the range of 2nm to 20nm from the surface of the cathode material do not meet the range of this application. It can be seen that compared with Comparative Examples 1 and 2, Example 10 further improves the high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycle performance of the electrochemical device of this application.

[0130] By adjusting the amount of sodium nitrate and boric acid added in step 3 of the <Preparation of Cathode Material> in Comparative Examples 3 and 4, the atomic percentage of boron in the range of 2nm to 20nm from the surface of the cathode material does not meet the range of this application. It can be seen that compared with Comparative Examples 3 and 4, Example 10 further improves the high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycle performance of the electrochemical device of this application.

[0131] Comparative Example 5, by omitting sodium hydroxide in step 2 of the <Preparation of Cathode Material> and replacing sodium nitrate with lithium nitrate in step 3; Comparative Example 6, by omitting boric acid in step 3 of the <Preparation of Cathode Material>; and Comparative Example 7, by omitting sodium hydroxide in step 2 of the <Preparation of Cathode Material> and omitting boric acid in step 3, and replacing sodium nitrate with lithium nitrate, shows that compared to Comparative Examples 5 to 7, Example 10 further improves the high-temperature (85°C and above) gas generation performance and high-temperature (45°C and above) cycle performance of the electrochemical device of this application.

[0132] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A positive electrode sheet, said positive electrode sheet comprising a positive electrode material, characterized in that, The positive electrode material includes sodium, and the mass percentage of sodium is 0.1% to 3% based on the mass of the positive electrode sheet. Within a range of 2nm to 20nm from the surface of the positive electrode material, the positive electrode material also includes boron. Taking the atoms of all elements within the range of 2nm to 20nm from the surface of the positive electrode material as 100%, the atomic percentage of boron is X%, where X is 0.1 to 4, the atomic percentage of sodium is Y%, where Y is 0.021 to 5, and the ratio of X to Y is 0.08 to 185.

71.

2. The positive electrode sheet as described in claim 1, characterized in that, Based on the mass of the positive electrode sheet, the mass percentage of sodium element is 1.6% to 3%; Assuming that all elements within the 2nm to 20nm range on the surface of the cathode material constitute 100%, the atomic percentage of boron is X%, where X is 1.2 to 1.8, the atomic percentage of sodium is Y%, where Y is 0.8 to 1.5, and the ratio of X to Y is 0.8 to 2.

25.

3. The positive electrode sheet as described in claim 1, characterized in that, Within a distance of 2nm to 20nm from the surface of the positive electrode material, the positive electrode material also includes nitrogen element, and based on the atomic composition of all elements within the 2nm to 20nm range of the surface of the positive electrode material being 100%, the atomic percentage of nitrogen element is 0.3% to 1.8%.

4. The positive electrode sheet as described in claim 1, characterized in that, Within a distance of 2nm to 20nm from the surface of the positive electrode material, the positive electrode material also includes nitrogen element, and based on the atomic composition of all elements within the 2nm to 20nm range of the surface of the positive electrode material being 100%, the atomic percentage of nitrogen element is 0.8% to 1.3%.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that, The positive electrode material also includes aluminum, and the mass percentage of aluminum is 0.02% to 0.6% based on the mass of the positive electrode sheet.

6. The positive electrode sheet according to any one of claims 1 to 4, characterized in that, The positive electrode material has a layered structure and further includes nickel, wherein the mass percentage of nickel is between 28.7% and 51.5% based on the mass of the positive electrode sheet; and / or The positive electrode material also includes manganese, and the mass percentage of manganese is between 5.3% and 27.6% based on the mass of the positive electrode sheet.

7. An electrochemical device, characterized in that, Includes the positive electrode sheet as described in any one of claims 1 to 6.

8. The electrochemical device according to claim 7, characterized in that, The electrochemical device further includes a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode material, and the negative electrode material is graphite; The electrolyte comprises at least one of 1,3,6-hexanetrionitrile, succinate, and 1,2,3-tris(2-cyanoethoxy)propane.

9. The electrochemical device according to claim 7 or 8, characterized in that, For coin cells using the aforementioned positive electrode and lithium sheet, a charge-discharge test is conducted at a rate of 0.04C within a voltage range of 2.8V to 4.5V. The coin cell must satisfy at least one of the following characteristics: (1) The coin cell has a redox peak between 3.6V and 3.9V, and the peak intensity of the redox peak is F1, which is 400mAh / g / V to 900mAh / g / V; (2) The coin cell has a redox peak between 4.2V and 4.4V, and the peak intensity of the redox peak is F2, which is 800mAh / g / V to 5000mAh / g / V; (3) 1≤F2 / F1≤12.

10. The electrochemical device according to claim 7 or 8, characterized in that, The button cell battery, composed of the positive electrode and lithium sheet, was charged and discharged at a rate of 0.04C within the range of 2.8V to 4.5V. The discharge capacity of the button cell battery between 4.1V and 4.4V was Q1, and the discharge capacity of the button cell battery between 2.8V and 4.5V was Q2, where 0.1≤Q1 / Q2≤0.3.

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