Positive electrode material, preparation method thereof, positive electrode sheet and sodium ion battery

Abstract

This application provides a positive electrode material and its preparation method, a positive electrode sheet, a sodium-ion battery, and an electrical device, belonging to the field of batteries. The positive electrode material includes a core and a carbon-containing coating layer covering at least a portion of the surface of the core. The core contains sodium vanadium fluorophosphate. A coin cell is fabricated using the positive electrode material. The coin cell undergoes an Nth charge-discharge test. In the discharge curve of the Nth charge-discharge test, the discharge specific capacity is C0, and the total discharge specific capacity of the discharge plateau corresponding to a voltage of 3.3V to 3.4V is C1, where C1 / C0 < 6.7%, and N is an integer greater than or equal to 1. The conditions for the Nth charge-discharge test are: charging the coin cell to 4.3V at a constant current rate of 0.2C at 20℃ to 30℃, and then discharging it to 2V at a constant current rate of 0.2C. The positive electrode material provided in this application exhibits high discharge specific capacity and electronic conductivity.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a positive electrode material and its preparation method, a positive electrode sheet, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries (SIBs) are another promising type of rechargeable battery after lithium-ion batteries (for example, in new energy vehicles). Their electrochemical performance primarily depends on the performance of their cathode. Among various cathode materials for sodium-ion batteries, sodium vanadium fluorophosphate (Na3V2(PO4)2F3, NVPF) has a theoretical operating voltage of approximately 3.85V and a theoretical specific capacity of 128.3mAh / g, making it a highly promising cathode material. However, currently available NVPF materials still suffer from poor capacity, resulting in low energy density in sodium-ion batteries. Furthermore, while NVPF materials possess good ionic conductivity, their electronic conductivity is relatively poor. Summary of the Invention

[0003] In view of the technical problems existing in the background art, this application provides a positive electrode material and its preparation method, a positive electrode sheet and a sodium-ion battery, aiming to solve the technical problems of poor capacity and poor electronic conductivity of the currently prepared NVPF materials.

[0004] In a first aspect, embodiments of this application provide a cathode material, including a core and a carbon-containing coating layer covering at least a portion of the surface of the core, wherein the core comprises sodium vanadium fluorophosphate (Na3V2(PO4)2F3);

[0005] A coin cell is fabricated using the positive electrode material. The coin cell is subjected to the Nth charge-discharge test. In the discharge curve of the Nth charge-discharge test, the discharge specific capacity is C0, the discharge specific capacity of the discharge plateau corresponding to the voltage of 3.3V to 3.4V is C1, and C1 / C0 < 6.7%, where N is an integer greater than or equal to 1.

[0006] The conditions for the Nth charge-discharge test are as follows: at 20℃~30℃, the coin cell battery is charged to 4.3V at a constant current rate of 0.2C, and then discharged to 2V at a constant current rate of 0.2C.

[0007] In the technical solution of this application embodiment, controlling C1 / C0 < 6.7% indicates that the core of the cathode material has high purity of Na3V2(PO4)2F3, while the mass proportion of impurity phases such as Na3V2(PO4)3 is low, effectively improving the capacity of the cathode material and the energy density of the sodium-ion battery. Simultaneously, the carbon-containing coating layer in the cathode material can effectively improve the electronic conductivity of the NVPF, further enhancing the material's capacity.

[0008] In some implementations, at least one of the following conditions is met:

[0009] (1) C1 / C0 ≤ 3.5%;

[0010] (2) The carbon-containing coating layer covers the surface of the core with a coating rate of 89% to 100%, preferably 95% to 100%;

[0011] (3) The porosity of the positive electrode material is 10% to 69%, preferably 15% to 30%.

[0012] In the technical solutions of this application embodiment, the porosity being within the aforementioned range is beneficial for ensuring the entry and exit of sodium ions while maintaining a high density of the carbon-containing coating layer, thereby improving the compaction density of the cathode material. The Cl / C0 ratio being within the aforementioned range allows for higher purity of the NVPF phase in the cathode material, further effectively improving the capacity of the cathode material and the energy density of the sodium-ion battery. The carbon-containing coating layer covering the surface of the core being within the aforementioned range is beneficial for improving the capacity and corrosion resistance of the cathode material, and also enables the cathode material to possess good electronic conductivity.

[0013] In some embodiments, the mass fraction of carbon in the cathode material is 1.25% to 6%, preferably 1.5% to 3.5%.

[0014] In the technical solution of this application embodiment, the mass fraction of carbon element in the cathode material is within the above range, which enables the cathode material to have high compaction density and electronic conductivity; the coating rate of the carbon coating layer covering the surface of the core is within the above range, which is beneficial to improving the capacity and corrosion resistance of the cathode material, and enables the cathode material to have good electronic conductivity.

[0015] In some embodiments, the compaction density of the positive electrode material is 1.4 g / cm³. 3 ~2g / cm 3 The preferred value is 1.53 g / cm³. 3 ~1.99g / cm 3 .

[0016] In the technical solution of this application embodiment, the positive electrode material has a high compaction density, which is beneficial to enable the sodium-ion battery to have a high energy density.

[0017] Secondly, embodiments of this application provide a method for preparing a cathode material, including:

[0018] A mixed slurry is provided, the mixed slurry containing a sodium source, a fluorine source, a vanadium source, a phosphorus source and a carbon source;

[0019] The mixed slurry is dried to obtain a precursor;

[0020] The precursor is calcined to obtain the cathode material;

[0021] The positive electrode material comprises a core and a carbon-containing coating layer covering at least a portion of the surface of the core. The core contains sodium vanadium fluorophosphate (Na3V2(PO4)2F3). A coin cell is fabricated using the positive electrode material, and the coin cell is subjected to an Nth charge-discharge test. In the discharge curve of the Nth charge-discharge test, the discharge specific capacity is C0, and the discharge specific capacity of the discharge plateau corresponding to the voltage between 3.3V and 3.4V is C1, where C1 / C0 < 6.7%, and N is an integer greater than or equal to 1. The conditions for the Nth charge-discharge test are as follows: at 20℃ to 30℃, the coin cell is charged at a constant current rate of 0.2C to 4.3V, and then discharged at a constant current rate of 0.2C to 2V.

[0022] The cathode material obtained using the preparation method described in this application has a high purity of Na3V2(PO4)2F3 in its core, while the mass proportion of impurity phases such as Na3V2(PO4)3 is low, thereby effectively improving the capacity of the cathode material and the energy density of the sodium-ion battery. Simultaneously, the carbon-containing coating layer in the cathode material obtained using the above preparation method can effectively improve the electronic conductivity of the NVPF, further enhancing the material's capacity.

[0023] In some embodiments, the carbon source includes at least one of citric acid, polyvinyl alcohol, glucose, sucrose, and oxalic acid, optionally including citric acid and / or polyvinyl alcohol, and further optionally including citric acid and polyvinyl alcohol;

[0024] Optionally, when the carbon source is citric acid and polyvinyl alcohol, the mass ratio of citric acid to polyvinyl alcohol is 2:1 to 1:4;

[0025] Optionally, the polyvinyl alcohol includes at least one of polyvinyl alcohol 2000, polyvinyl alcohol 3000, polyvinyl alcohol 4000 and polyvinyl alcohol 6000.

[0026] In the technical solution of this application embodiment, when the carbon source includes citric acid and polyvinyl alcohol, the compounding of citric acid and polyvinyl alcohol can produce a mutually promoting effect, thereby further improving the compaction density and which is beneficial to improving the energy density of sodium-ion batteries using this cathode material.

[0027] In some implementations, at least one of the following conditions is met:

[0028] (1) The sodium source includes at least one of sodium fluoride, sodium phosphate, sodium bicarbonate and sodium nitrate, and optionally includes sodium fluoride;

[0029] (2) The fluorine source includes at least one of sodium fluoride, ammonium fluoride, potassium fluoride and lithium fluoride, and optionally includes sodium fluoride;

[0030] (3) The vanadium source includes vanadium pentoxide and / or ammonium metavanadate;

[0031] (4) The phosphorus source includes at least one of ammonium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate and triammonium phosphate.

[0032] In the technical solution of this application embodiment, when sodium fluoride is used as the sodium source and / or fluorine source, fluorine can be provided simultaneously with the provision of sodium. Compared with the method of providing sodium and fluorine separately, this is beneficial to reduce the input and waste of raw materials.

[0033] In some embodiments, the method for preparing the mixed slurry includes:

[0034] The sodium source, the fluorine source, the vanadium source, and the phosphorus source are taken according to the stoichiometric ratio of Na3V2(PO4)2F3, and mixed with the solvent and the carbon source to form a first slurry;

[0035] After adjusting the pH of the first slurry to 6-7 using a pH adjuster, the slurry is then ground to obtain the mixed slurry.

[0036] Optionally, the pH adjuster includes at least one of ammonia, sodium hydroxide, sodium carbonate, sodium bicarbonate, and ammonium bicarbonate, and more preferably includes ammonia.

[0037] In the technical solution of this application embodiment, by using a pH adjuster to adjust the pH of the first slurry to 6-7, the F in the mixed slurry can be effectively suppressed. - Hydrolysis inhibits the formation of HF, effectively reduces the loss of F, and reduces the formation of impurities such as sodium vanadium phosphate in the finished product, thereby effectively improving the purity (content) of Na3V2(PO4)2F3 and thus improving the capacity of the prepared cathode material.

[0038] In some embodiments, the mixed slurry also contains a fluoride supplement;

[0039] Optionally, the fluoride supplement includes ammonium fluoride and / or sodium fluoride, and more preferably includes ammonium fluoride;

[0040] Optionally, the molar amount of the fluoride supplement is 1% to 5% of the molar amount of the fluoride source.

[0041] In the technical solution of this application embodiment, when the mixed slurry contains a fluorine replenishing agent, the fluorine replenishing agent can replenish this part of the F loss, effectively reduce the formation of impurity phases such as sodium vanadium phosphate in the finished product, and effectively improve the purity (content) of Na3V2(PO4)2F3, thereby improving the capacity of the prepared cathode material.

[0042] In some embodiments, the calcination temperature is 550℃~800℃, preferably 650℃~800℃; the holding time of the calcination is 3h~10h, preferably 4h~9h; and the calcination is carried out in a protective atmosphere.

[0043] Optionally, the protective atmosphere includes nitrogen and / or argon.

[0044] In the technical solution of this application embodiment, the calcination temperature or holding time is within the aforementioned respective ranges. This improves both the density of the carbon-containing coating layer and the purity of Na3V2(PO4)2F3 in the cathode material. If the calcination temperature is relatively low or the holding time is relatively short, the carbon source forming the carbon-containing coating layer is prone to incomplete carbonization and low graphitization, resulting in poor electronic conductivity and low capacity of the prepared cathode material. Furthermore, when the calcination temperature is relatively low, the formed carbon-containing coating layer is relatively loose, leading to a lower compaction density of the cathode material. If the sintering temperature is relatively high or the holding time is relatively long, it is prone to significant F loss, resulting in lower purity of the Na3V2(PO4)2F3 phase in the cathode material and a reduction in the cathode material's capacity.

[0045] Thirdly, embodiments of this application provide a positive electrode sheet, including the positive electrode material described in the first aspect of this application or the positive electrode material prepared by the preparation method described in the second aspect of this application.

[0046] In this embodiment, the positive electrode sheet contains the aforementioned positive electrode material, thus possessing high capacity and energy density.

[0047] Fourthly, embodiments of this application provide a sodium-ion battery, including the positive electrode sheet of the third aspect of this application.

[0048] In this embodiment, the sodium-ion battery includes the aforementioned positive electrode sheet, thus possessing high capacity and energy density.

[0049] Fifthly, embodiments of this application provide an electrical device, including the sodium-ion battery of the fourth aspect of this application.

[0050] The electrical device of this application includes the sodium-ion battery provided in this application, and therefore has at least the same advantages as the sodium-ion battery.

[0051] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0052] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0053] Figure 1 This is a schematic diagram of the preparation process of the cathode material provided in this application.

[0054] Figure 2 This is a morphology diagram of the cathode material prepared in Example 8 of this application.

[0055] Figure 3 This is a morphology diagram of the cathode material prepared in Example 13 of this application.

[0056] Figure 4 This is a morphology diagram of the cathode material prepared in Example 13 of this application.

[0057] Figure 5 The first charge-discharge curve of the coin cell made of the positive electrode material of Example 6 of this application.

[0058] Figure 6 The first charge-discharge curve of the coin cell made of the positive electrode material of Comparative Example 3 of this application. Detailed Implementation

[0059] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0060] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0061] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0062] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0063] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0064] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0065] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0066] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0067] Currently, the NVPF materials that have been prepared typically exhibit three voltage plateaus in their discharge curves (relative to Na). + / Na, approximately 4.1V, 3.6V, and 3.3V–3.4V), but theoretically, NVPF materials only have two plateaus (relative to Na). + The presence of a low-voltage plateau of 3.3V to 3.4V (approximately 4.1V and 3.6V for Na) reduces the average discharge voltage and discharge specific capacity of the cathode material and the sodium-ion battery using it, thus leading to a decrease in battery energy density.

[0068] Based on the above phenomena, the inventors discovered through research that the reason why the currently prepared NVPF materials have a low voltage plateau of 3.3V to 3.4V is because the solid-state method is usually used in the preparation of NVPF materials. This method is prone to causing a lot of fluorine loss during the preparation process, resulting in the presence of impurities such as sodium vanadium phosphate (Na3V2(PO4)3) in the finished product. This reduces the purity (content) of Na3V2(PO4)2F3 in the material, thus resulting in a low voltage plateau of 3.3V to 3.4V, leading to lower product capacity and lower energy density of sodium-ion batteries.

[0069] To address the technical problems of poor capacity and low electronic conductivity of current NVPF materials, this application provides a positive electrode material and its preparation method, a positive electrode sheet, a sodium-ion battery, and an electrical device. By improving the current NVPF material and its preparation method, the electronic conductivity and capacity can be improved, thereby enhancing the capacity and energy density of the positive electrode sheet, the secondary battery, and the electrical device.

[0070] In a first aspect, this application provides a positive electrode material comprising a core and a carbon-containing coating layer covering at least a portion of the surface of the core, wherein the core comprises sodium vanadium fluorophosphate (Na3V2(PO4)2F3); a coin cell is fabricated using the positive electrode material, and the coin cell is subjected to an Nth charge-discharge test. In the discharge curve of the Nth charge-discharge test, the discharge specific capacity is C0, the total discharge specific capacity of the discharge plateau corresponding to the voltage between 3.3V and 3.4V is C1, and C1 / C0 < 6.7%, where N is an integer greater than or equal to 1; the conditions for the Nth charge-discharge test are: at 20℃ to 30℃, the coin cell is charged at a constant current rate of 0.2C to 4.3V, and then discharged at a constant current rate of 0.2C to 2V.

[0071] In some embodiments, the coin cell is prepared as follows: the positive electrode material, conductive agent, and binder provided in this application are mixed in a certain mass ratio, and an appropriate amount of solvent is added to form a uniform electrode slurry. The electrode slurry is then uniformly coated onto aluminum foil, vacuum dried, and cut into circular electrode sheets of a certain diameter, which are then transferred to a glove box for later use. Using sodium metal as the counter electrode and glass fiber as the separator, electrolyte is added, and the cells are assembled into coin cells. The entire assembly process is carried out in an argon-filled glove box.

[0072] In some embodiments, the conductive agent includes one or more of carbon black, acetylene black, Ketjen black, and carbon nanotubes.

[0073] In some embodiments, the binder includes polyvinylidene fluoride (PVDF).

[0074] In some embodiments, the mass ratio of positive electrode material, conductive agent and binder in the electrode slurry is (80:10:10) to (90:5:5).

[0075] In some embodiments, the solvent in the electrode slurry includes at least one of N-methylpyrrolidone (NMP) and water.

[0076] In some embodiments, the diameter of the circular electrode sheet is 15 mm to 20 mm.

[0077] In some embodiments, the electrolyte comprises an electrolyte, an organic solvent, and optional additives. The electrolyte comprises at least one of sodium perchlorate (NaClO4) and sodium hexafluorophosphate (NaPF6). The organic solvent comprises one or more of propylene carbonate, ethylene carbonate, and dimethyl carbonate. The additives comprise fluoroethylene carbonate.

[0078] In some embodiments, the concentration of the electrolyte in the electrolyte solution is 1 mol / L to 1.5 mol / L. As an example, the preparation process of the coin cell described in this application is as follows: The positive electrode material, conductive carbon black, and polyvinylidene fluoride (PVDF) provided in this application are mixed in a mass ratio of 85:8:7. An appropriate amount of N-methylpyrrolidone (NMP) is added to form a uniform electrode slurry. The electrode slurry is then uniformly coated onto aluminum foil, vacuum dried, and cut into circular electrode sheets with a diameter of 15 mm. These sheets are then transferred to a glove box for later use. Using sodium metal as the counter electrode, glass fiber as the separator, sodium perchlorate as the solute in the electrolyte, and propylene carbonate, ethylene carbonate, and fluoroethylene carbonate (volume ratio 1:1:0.05) as the solvent, with a sodium perchlorate concentration of 1 mol / L, a CR2032 coin cell is assembled. The entire assembly process is carried out in an argon-filled glove box.

[0079] It should be noted that the aforementioned discharge curve is usually plotted with specific capacity (in mAh / g) on ​​the x-axis and voltage (in V) on the y-axis. In this discharge curve, within the voltage range of 3.3V to 3.4V on the y-axis, there exists a discharge plateau that is approximately parallel to the x-axis. Assuming that the specific capacities corresponding to the two endpoints of this discharge plateau are C1' and C2' respectively, then C1 = |C1' - C2'|, which is the absolute value of the difference between C1' and C2'. C0 is the value at the intersection of the end of the discharge curve and the x-axis, that is, the specific capacity corresponding to a voltage of 0.

[0080] In some embodiments, N can be an integer such as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, etc.

[0081] In some embodiments, C1 / C0 can be 0, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 6.7%, or within any of the above values.

[0082] The cathode material provided in this application contains Na3V2(PO4)2F3 and optionally Na3V2(PO4)3. When this cathode material is made into a coin cell, when the cathode material contains Na3V2(PO4)3, a discharge plateau exists in the voltage range of 3.3V to 3.4V in the Nth discharge curve. This discharge plateau corresponds to the discharge process of Na3V2(PO4)3. The ratio of the discharge specific capacity corresponding to this discharge plateau to the Nth discharge specific capacity, C1 / C0, reflects the mass proportion of the Na3V2(PO4)3 impurity phase in the cathode material. The lower the C1 / C0, the lower the mass proportion of the Na3V2(PO4)3 impurity phase in the cathode material.

[0083] This application achieves a C1 / C0 ratio of <6.7%, indicating that the core of the cathode material possesses high purity Na3V2(PO4)2F3, while the mass proportion of impurity phases such as Na3V2(PO4)3 is low. This is beneficial for effectively improving the capacity of the cathode material and the energy density of the sodium-ion battery. Simultaneously, the carbon-containing coating layer in the cathode material can effectively improve the electronic conductivity of the NVPF, further enhancing the material's capacity.

[0084] In some implementations, C1 / C0 ≤ 3.5%. For example, C1 / C0 can be 0, 0.1%, 0.3%, 0.7%, 0.9%, 1.1%, 1.3%, 1.7%, 2.1%, 2.3%, 2.7%, 3.1%, 3.3%, 3.5%, or within any of the above values.

[0085] In the technical solution of this application embodiment, the C1 / C0 ratio is controlled to be within the above range, that is, the mass ratio of Na3V2(PO4)3 impurity phase in the cathode material is controlled to be lower, and correspondingly, Na3V2(PO4)2F3 has a higher mass ratio in the cathode material. That is, the purity of the NVPF phase in the cathode material is higher, which is beneficial to further effectively improving the capacity of the cathode material and the energy density of sodium-ion battery.

[0086] In some embodiments, the carbon-containing coating layer covers 89% to 100% of the surface of the core, preferably 95% to 100%, which is beneficial for improving the capacity and corrosion resistance of the cathode material and can give the cathode material good electronic conductivity. For example, the coverage rate of the carbon-containing coating layer covering the surface of the core in the cathode material can be 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, or within any range of the above numbers.

[0087] It is understood that the “coverage ratio” mentioned in this application refers to the proportion of the area of ​​the core surface covered by the carbon-containing coating layer to the core surface area.

[0088] In some embodiments, the mass fraction of carbon in the cathode material is 1.25% to 6%, preferably 1.5% to 3.5%. For example, the mass fraction of carbon in the cathode material can be 1.25%, 1.5%, 1.7%, 1.9%, 2.1%, 2.3%, 2.5%, 2.7%, 2.9%, 3.1%, 3.3%, 3.5%, 3.7%, 4%, 4.3%, 4.5%, 4.7%, 5%, 5.3%, 5.5%, 5.7%, 6%, or within any range of the above values.

[0089] It can be understood that the mass fraction of carbon in cathode materials refers to the percentage of carbon by mass in the cathode material.

[0090] In the technical solution of this application embodiment, the mass fraction of carbon in the cathode material is within the above-mentioned range, which is beneficial for balancing compaction density and electronic conductivity. If the mass fraction of carbon in the cathode material is relatively high, it is easy for too many loose and porous carbon layers to form on the surface of the cathode material, resulting in a decrease in compaction density. If the mass fraction of carbon in the cathode material is relatively low, the electronic conductivity is poor and the material capacity is low.

[0091] In some embodiments, the porosity of the cathode material is 10% to 69%, preferably 15% to 30%. For example, the porosity can be 10%, 15%, 17%, 19%, 21%, 23%, 25%, 27%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 69%, or any range thereof. This porosity is advantageous in ensuring the entry and exit of sodium ions while maintaining a high density of the carbon-containing coating, thus improving the compaction density of the cathode material.

[0092] In some embodiments, the compaction density of the positive electrode material is 1.4 g / cm³. 3 ~2g / cm 3 The preferred value is 1.53 g / cm³. 3 ~1.99g / cm 3 For example, the compaction density can be 1.4 g / cm³. 3 1.5g / cm 3 1.6g / cm 3 1.7g / cm 3 1.8g / cm 3 1.9g / cm 3 2g / cm 3 Or it may fall within the range of any of the above values.

[0093] It should be noted that the compaction density of the cathode material refers to the compaction density of the cathode material under a pressure of 30kN.

[0094] In the technical solution of this application embodiment, the positive electrode material has a high compaction density, which enables the sodium-ion battery to have a high energy density.

[0095] Secondly, this application provides a method for preparing a cathode material, which can be used to prepare the cathode material of the first aspect of this application, and may include the following steps S1 to S3:

[0096] S1: Provide a mixed slurry containing a sodium source, a fluorine source, a vanadium source, a phosphorus source, and a carbon source;

[0097] S2: The mixed slurry is dried to obtain a precursor;

[0098] S3: The precursor is calcined to obtain the positive electrode material; wherein, the positive electrode material includes a core and a carbon-containing coating layer covering at least a portion of the surface of the core, the core containing sodium vanadium fluorophosphate (Na3V2(PO4)2F3); a coin cell is made using the positive electrode material, and the coin cell is subjected to the Nth charge-discharge test. In the Nth discharge curve, the discharge specific capacity is C0, the total discharge specific capacity of the discharge plateau corresponding to the voltage of 3.3V to 3.4V is C1, C1 / C0<6.7%, where N is an integer greater than or equal to 1; the conditions for the Nth charge-discharge test are: at 20℃ to 30℃, the coin cell is charged at a constant current rate of 0.2C to 4.3V, and then discharged at a constant current rate of 0.2C to 2V.

[0099] The cathode material obtained using the preparation method described in this application has a high purity of Na3V2(PO4)2F3 in its core and a low mass proportion of impurity phases such as Na3V2(PO4)3, which effectively improves the capacity of the cathode material and the energy density of the sodium-ion battery. Simultaneously, the carbon-containing coating layer in the cathode material obtained using the above preparation method effectively improves the electronic conductivity of the cathode material, further enhancing its capacity.

[0100] In some embodiments, the carbon source includes at least one of citric acid, polyvinyl alcohol, glucose, sucrose, and oxalic acid; optionally, it includes citric acid and / or polyvinyl alcohol (PEG).

[0101] In the technical solution of this application embodiment, when the carbon source includes citric acid, the mixed slurry contains a vanadium source. In this case, citric acid can act as a reducing agent, thereby reducing the valence state of vanadium and complexing it with V. 4+ and / or V 5+ This increases the solubility of vanadium in the mixed slurry, thereby achieving ionic-level mixing of multiple elements. This results in a more uniform elemental distribution in the precursor obtained after drying, facilitating ion migration and fusion during calcination, and thus helping to improve the purity of the Na3V2(PO4)2F3 phase in the product.

[0102] In the technical solution of this application embodiment, when the carbon source includes polyvinyl alcohol, the carbon-containing coating layer formed after polyvinyl alcohol sintering is more dense, which helps to improve the compaction density of the cathode material, thereby improving the energy density of the sodium-ion battery.

[0103] In some embodiments, the carbon source includes citric acid and polyvinyl alcohol.

[0104] In the technical solution of this application embodiment, when the carbon source includes citric acid and polyvinyl alcohol, the compounding of citric acid and polyvinyl alcohol can produce a mutually promoting effect, thereby further improving the compaction density and which is beneficial to improving the energy density of sodium-ion batteries using this cathode material.

[0105] In some embodiments, when the carbon source is citric acid and polyvinyl alcohol, the mass ratio of citric acid to polyvinyl alcohol is 2:1 to 1:4, preferably 2:1 to 1:3. For example, the mass ratio of citric acid to polyvinyl alcohol can be 2:1, 1:1, 1:2, 1:3, 1:4, or within any of the above ranges, which is beneficial for further improving the compaction density of the cathode material.

[0106] In some embodiments, the polyvinyl alcohol includes at least one of polyvinyl alcohol 2000, polyvinyl alcohol 3000, polyvinyl alcohol 4000, and polyvinyl alcohol 6000.

[0107] It should be noted that "polyvinyl alcohol 2000" mentioned in this application refers to polyvinyl alcohol with a weight average molecular weight of 2000; other polyvinyl alcohols are similar, referring to polyvinyl alcohols with a weight average molecular weight of 3000, 4000 or 6000 respectively.

[0108] In some embodiments, the sodium source includes at least one of sodium fluoride, sodium phosphate, sodium bicarbonate, and sodium nitrate, and optionally includes sodium fluoride.

[0109] In some embodiments, the fluorine source includes at least one of sodium fluoride, ammonium fluoride, potassium fluoride, and lithium fluoride, and optionally includes sodium fluoride.

[0110] In the technical solution of this application embodiment, when sodium fluoride is used as the sodium source and / or fluorine source, fluorine can be provided simultaneously with the provision of sodium. Compared with the method of providing sodium and fluorine separately, this is beneficial to reduce the input and waste of raw materials.

[0111] In some embodiments, the vanadium source includes vanadium pentoxide and / or ammonium metavanadate.

[0112] In some embodiments, the phosphorus source includes at least one of ammonium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, and triammonium phosphate.

[0113] In some embodiments, the method for preparing the mixed slurry may include the following steps:

[0114] Step S10: Take the sodium source, the fluorine source, the vanadium source, and the phosphorus source according to the stoichiometric ratio of Na3V2(PO4)2F3, and mix them with the solvent and the carbon source to form a first slurry;

[0115] Step S20: After adjusting the pH value of the first slurry to 6-7 using a pH adjuster, the slurry is then ground to obtain the mixed slurry.

[0116] In the technical solution of this application embodiment, grinding the mixed slurry helps to reduce the particle size in the mixed slurry, increase the solubility of various raw materials, thereby facilitating the ionic mixing of multiple elements, making the element distribution in the obtained precursor more uniform, and facilitating the migration and fusion of ions during calcination, thereby helping to improve the purity of the Na3V2(PO4)2F3 phase in the product.

[0117] In some embodiments, the drying process is spray drying.

[0118] During their research, the inventors discovered that the mixed slurry contained a fluorine source, and the F in the fluorine source... - Partial hydrolysis will generate HF, which can easily corrode the spray drying equipment. More importantly, some HF will volatilize during the spray drying process, leading to fluorine loss. This results in the formation of more impurities such as sodium vanadium phosphate in the finished product, reducing the purity (content) of Na3V2(PO4)2F3. Consequently, a low-voltage plateau of 3.3V will appear, resulting in a lower product capacity.

[0119] The preparation method provided in this application, by adjusting the pH value of the mixed slurry to 6-7, can effectively suppress F in the mixed slurry. - Hydrolysis and inhibition of HF formation can effectively reduce F loss and decrease the formation of impurities such as sodium vanadium phosphate in the finished product, thereby effectively improving the purity (content) of Na3V2(PO4)2F3 and thus increasing the capacity of the prepared cathode material.

[0120] It is understood that the "purity of Na3V2(PO4)2F3 (phase)" mentioned in this application refers to the mass percentage content of Na3V2(PO4)2F3 in the cathode material.

[0121] In some embodiments, the pH value of the mixed slurry may be 6, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7 or within any range of the above values.

[0122] In some embodiments, the pH adjuster includes at least one of ammonia, sodium hydroxide, sodium carbonate, sodium bicarbonate, and ammonium bicarbonate, optionally including ammonia.

[0123] In the technical solution of this application embodiment, when ammonia water is used as a pH adjuster, the ammonia water is easy to volatilize by generating ammonia gas during the subsequent calcination process, so it will not introduce other impurity elements, which can improve the purity of Na3V2(PO4)2F3 in the prepared positive electrode material.

[0124] In some embodiments, the type of solvent is not limited, and water, which is commonly used in the art, can be used as the solvent; for example, at least one of pure water, deionized water, or distilled water.

[0125] In some embodiments, the mixed slurry also contains a fluoride supplement.

[0126] During the calcination of the precursor in step S3, some fluorine (F) is typically lost. When the slurry contains a fluorine replenishing agent, the agent can compensate for this F loss, effectively reducing the formation of impurities such as sodium vanadium phosphate in the finished product. This effectively improves the purity (content) of Na3V2(PO4)2F3, thereby increasing the capacity of the prepared cathode material.

[0127] In some embodiments, the fluoride supplement includes ammonium fluoride and / or sodium fluoride, and further includes ammonium fluoride.

[0128] In the technical solution of this application embodiment, when ammonium fluoride is used as a fluoride supplement, it can, on the one hand, make F - On the one hand, ammonium ions can be replenished; on the other hand, during the subsequent calcination process, ammonium ions can be volatilized in the form of ammonia gas, thus avoiding the introduction of other impurity elements, which can improve the purity of Na3V2(PO4)2F3 in the prepared cathode material.

[0129] In some embodiments, the molar amount of the fluoride supplement is 1% to 5% of the molar amount of the fluorine source, preferably 3% to 5%. For example, the molar amount of the fluoride supplement can be 1%, 1.5%, 2%, 2.5%, 3%, 3.1%, 3%, 3.3%, 3.5%, 3.7%, 4%, 4.1%, 4.3%, 4.5%, 4.7%, 5% of the molar amount of the fluorine source, or within any range of the above values, effectively supplementing fluorine. - At the same time, it can also minimize the adverse effects of excessive impurities and poor electrochemical performance in the finished product caused by a relatively large molar amount of fluoride supplement.

[0130] In some embodiments, the calcination temperature is 550°C to 800°C, preferably 650°C to 800°C. For example, the calcination temperature can be 550°C, 600°C, 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 800°C, or within any range of the above values.

[0131] In some embodiments, the holding time for the calcination treatment is 3h to 10h, preferably 4h to 9h. For example, the holding time for the calcination treatment can be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, or within any range of the above values.

[0132] It can be understood that the holding time for calcination treatment refers to the time during which the temperature is raised to the calcination treatment temperature and then held at that temperature.

[0133] In the technical solution of this application embodiment, the calcination temperature or holding time is within the aforementioned respective ranges. This improves both the density of the carbon-containing coating layer and the purity of Na3V2(PO4)2F3 in the cathode material. If the calcination temperature is relatively low or the holding time is relatively short, the carbon source forming the carbon-containing coating layer is prone to incomplete carbonization and low graphitization, resulting in poor electronic conductivity and low capacity of the prepared cathode material. Furthermore, when the calcination temperature is relatively low, the formed carbon-containing coating layer is relatively loose, leading to a lower compaction density of the cathode material. If the sintering temperature is relatively high or the holding time is relatively long, it is prone to significant F loss, resulting in lower purity of the Na3V2(PO4)2F3 phase in the cathode material and a reduction in the cathode material's capacity.

[0134] In some embodiments, the temperature is raised to the calcination temperature, and the corresponding heating process includes the following stages:

[0135] Phase 1: Raise the temperature from room temperature to 120℃~150℃ and keep it at 120℃~150℃ for 2~4 hours to consume the precursor as much as possible and avoid excessive moisture content and consumption of carbon source in the higher temperature range.

[0136] Stage 2: Increase the temperature from 120℃~150℃ to 400℃~550℃, and keep it at 400℃~550℃ for 1~3 hours. This stage can promote the decomposition of organic matter in the precursor and the reduction of vanadium.

[0137] Phase 3: Increase the temperature from 400℃~550℃ to 550℃~800℃, and maintain the temperature at 550℃~800℃ for 5~9 hours.

[0138] It should be noted that the room temperature mentioned in this application refers to 25℃~35℃.

[0139] In some embodiments, during stage 1, the temperature for heat preservation can be 120°C, 130°C, 140°C, 150°C or within any range of the above values, and the heat preservation time can be 2h, 3h, 4h or within any range of the above values.

[0140] In some embodiments, during stage 2, the temperature for heat preservation can be 400°C, 450°C, 500°C, 550°C or within any range of the above values, and the heat preservation time can be 1 hour, 2 hours, 3 hours or within any range of the above values.

[0141] In some embodiments, in stage 3, the heat preservation temperature can be 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, or a range thereof, and the heat preservation time can be 5h, 6h, 7h, 8h, 9h, or a range thereof.

[0142] In some embodiments, the calcination process is carried out in a protective atmosphere. Optionally, the protective atmosphere includes nitrogen and / or argon.

[0143] Thirdly, embodiments of this application provide a positive electrode sheet, including the positive electrode material of the first aspect of this application or the positive electrode material prepared by the preparation method of the second aspect of this application.

[0144] The positive electrode sheet contains the positive electrode material of the first aspect of this application or the positive electrode material prepared by the preparation method of the second aspect of this application, and thus has high capacity and energy density.

[0145] Fourthly, embodiments of this application provide a sodium-ion battery, including the positive electrode sheet of the third aspect of this application.

[0146] Sodium-ion batteries include a positive electrode sheet according to the third aspect of this application, and therefore have high capacity and energy density.

[0147] Fifthly, embodiments of this application provide an electrical device, including the sodium-ion battery of the fourth aspect of this application.

[0148] The electrical device includes the sodium-ion battery of the fourth aspect of this application, and therefore has at least the same advantages as the sodium-ion battery.

[0149] In some embodiments, the electrical device may be, but is not limited to, a mobile phone, tablet, laptop, electric toy, power tool, electric vehicle, electric car, ship, spacecraft, etc. Electric toys may include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; spacecraft may include airplanes, rockets, space shuttles, and spacecraft.

[0150] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0151] I. Preparation Method

[0152] Example 1

[0153] (1) Sodium fluoride (257.07g), vanadium pentoxide (365.59g) and ammonium dihydrogen phosphate (464.28g) were dispersed in water according to the stoichiometric ratio of Na3V2(PO4)2F3. Citric acid and polyvinyl alcohol (mass ratio of the two is 2:1, weight average molecular weight of polyvinyl alcohol is 2000, where the total mass percentage = total mass of citric acid and polyvinyl alcohol / total mass of sodium fluoride, vanadium pentoxide, ammonium dihydrogen phosphate, citric acid and polyvinyl alcohol) were added as carbon sources to form the first slurry. Ammonia water was added to the first slurry to adjust the pH value to 6. Then it was put into a sand mill for circulating grinding at a speed of 1000 r / min for 3 h to obtain a mixed slurry.

[0154] (2) The mixed slurry is spray-dried to obtain the precursor.

[0155] (3) The precursor is transferred into a sintering kiln and calcined under an argon protective atmosphere. The oxygen content in the sintering kiln is controlled to be ≤10ppm, the moisture content in the highest temperature holding section is ≤10ppm, the CO content is ≤10ppm, and the H2 content is ≤10ppm. The heating process in the sintering kiln is as follows: from room temperature to 140℃, and held at 140℃ for 3 hours; from 140℃ to 500℃, and held at 500℃ for 2 hours; from 500℃ to 705℃, and the calcination temperature is maintained at 705℃ for 6.5 hours to obtain the cathode material.

[0156] Example 2

[0157] The preparation process is similar to that of Example 1, except that in step (1), the pH value is adjusted to 7.

[0158] Examples 3-7

[0159] Similar to the preparation process in Example 1, the main difference is that ammonium fluoride is added to the mixed solution in step (1) as a fluoride supplement, and the molar amount of ammonium fluoride is equivalent to 1%, 2%, 3%, 5%, and 8% of the molar amount of sodium fluoride, respectively.

[0160] Examples 8-11

[0161] The preparation process is similar to that of Example 1, with the main difference being that in step (1), the mass ratio of citric acid to polyvinyl alcohol is 1:1, 1:2, 1:3, and 1:4, respectively.

[0162] Example 12

[0163] The preparation process is similar to that of Example 1, except that in step (1), citric acid of equal mass is used instead of polyvinyl alcohol.

[0164] Example 13

[0165] The preparation process is similar to that of Example 1, except that in step (1), citric acid is replaced with an equal mass of polyvinyl alcohol.

[0166] Example 14

[0167] The preparation process is similar to that of Example 1, except that in step (1), equal amounts of glucose are used instead of polyvinyl alcohol and citric acid.

[0168] Examples 15-21

[0169] Similar to the preparation process in Example 1, the main difference is that in step (1), the total mass ratio of citric acid and polyvinyl alcohol is adjusted so that the mass fraction of carbon element in the prepared cathode material is 1.25%, 1.5%, 2%, 2.3%, 3%, 5%, and 6%, respectively, and the corresponding total mass of citric acid and polyvinyl alcohol is 152.17g, 182.61g, 243.48g, 280g, 365.22g, 608.70g, and 730.43g, respectively.

[0170] Examples 22-27

[0171] Similar to the preparation process of Example 1, the main difference is that in step (3), the calcination temperatures are 500℃, 600℃, 650℃, 750℃, 700℃, and 800℃ respectively.

[0172] Examples 28-35

[0173] Similar to the preparation process in Example 1, the main difference is that in step (3), the calcination treatment holding time is 3h, 4h, 5h, 6h, 7h, 8h, 9h, and 10h respectively.

[0174] Examples 36-37

[0175] The preparation process is similar to that of Example 1, with the main difference being that in step (1), the weight average molecular weights of polyvinyl alcohol are 6000 and 4000, respectively.

[0176] Comparative Examples 1-2

[0177] The preparation process is similar to that of Example 1, with the main difference being that in step (1), the pH is adjusted to 4 and 5 respectively.

[0178] Comparative Example 3

[0179] The preparation process is similar to that of Example 1, except that in step (1), the pH value is adjusted to 8.

[0180] II. Testing Methods

[0181] 1. The mass fraction of carbon in the cathode material can be tested using the following method: Refer to YS / T 1028.4-2015 Lithium Iron Phosphate Chemical Analysis Methods Part 4 "Determination of Carbon Content", and use the high-frequency combustion infrared absorption method for testing.

[0182] 2. The compaction density of the positive electrode material was tested using a compaction density meter. The test pressure was 3 tons (T) and the pressing time was 30 seconds.

[0183] 3. The porosity test of the cathode material shall refer to TCSTM 00553-2022 Method for determining the porosity of lightweight porous materials.

[0184] 4. The coating efficiency of the carbon-containing coating layer was determined using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX).

[0185] 5. Properties testing of sodium-ion batteries

[0186] (1) Fabrication of button cells:

[0187] Preparation of the positive electrode sheet: The positive electrode material prepared in the above examples or comparative examples, the binder sodium alginate, and the conductive agent acetylene black are dispersed and dissolved in N-methylpyrrolidone (NMP) solvent at a mass ratio of 8:1:1 to obtain a positive electrode slurry. The positive electrode slurry is coated on both sides of an aluminum foil current collector, and after drying, cold pressing, and slitting, a positive electrode sheet is obtained.

[0188] Battery assembly: Using metallic sodium as the counter electrode and glass fiber as the separator, the electrolyte consisted of sodium perchlorate as the solute and propylene carbonate, ethylene carbonate, and fluoroethylene carbonate (volume ratio 1:1:0.05) as the solvent. The concentration of sodium perchlorate in the electrolyte was 1 mol / L. CR2032 button cells were assembled in an argon-filled glove box. After standing for 6 hours, the button cells were used for subsequent electrochemical performance testing.

[0189] (2) Initial charge specific capacity and initial discharge specific capacity test

[0190] The tests were conducted on a Blue Battery Tester (CT2001A). At 20℃~30℃ and normal pressure, the coin cells were charged to 4.3V at a constant rate of 0.2C, and then discharged to 2V at a constant rate of 0.2C. The ambient humidity was below 15%. The cells were cycled for 500 cycles in this charge-discharge mode, and the discharge specific capacity and capacity retention were measured after 500 cycles.

[0191] First-cycle coulombic efficiency = (First discharge specific capacity / First charge specific capacity) × 100%

[0192] Table 1

[0193]

[0194]

[0195] Table 2

[0196]

[0197]

[0198] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0199] Figures 2-4 This indicates that the cathode material prepared in this application has a core-shell morphology and a relatively uniform particle size distribution.

[0200] Figure 5 In the discharge curve, there is almost no voltage plateau in the voltage range of 3.3V to 3.4V, indicating that the content of Na3V2(PO4)3 impurity phase in the cathode material is extremely low. Figure 6 In the discharge curve, the discharge specific capacity of the discharge platform corresponding to 3.3V to 3.4V is (98-89) mAh / g, and the initial discharge specific capacity is 111 mAh / g. Correspondingly, in Comparative Example 3, C1 / C0 = 8.1%, and the content of Na3V2(PO4)3 impurity phase is relatively large.

[0201] As shown in Table 1, the mass fraction of carbon in the cathode materials prepared in each embodiment ranges from 1.25% to 6%, the coating rate of the carbon-containing coating layer ranges from 89% to 100%, the porosity of the cathode materials ranges from 10% to 69%, and the compaction density is 1.4 g / cm³. 3 ~2g / cm 3 The C1 / C0 ratios are all less than 6.7%, and further, the C1 / C0 ratios are all less than or equal to 6.1%; while the cathode materials prepared in Comparative Examples 1 to 3 have a C1 / C0 ratio greater than or equal to 6.7%.

[0202] Furthermore, in each embodiment, by adding ammonia to the first slurry and adjusting the pH to 6-7, the presence of F in the slurry can be effectively suppressed. - Hydrolysis and inhibition of HF generation can effectively reduce F loss and reduce the formation of impurities such as sodium vanadium phosphate in the finished product. Therefore, the mass ratio of Na3V2(PO4)3 impurity phase in the cathode material is relatively low, and the purity of NVPF phase is high. Finally, C1 / C0 are both less than 6.7%. Combined with the first charge specific capacity, first discharge specific capacity, discharge specific capacity after 500 cycles and capacity retention rate after 500 cycles in Table 2, it can be seen that the cathode material provided in this application embodiment has higher capacity and higher capacity retention rate, and higher energy density. In contrast, Comparative Examples 1-3, due to pH values ​​less than 6 or greater than 7, struggled to suppress HF generation and F loss. Consequently, the Na3V2(PO4)3 impurity phase had a relatively high mass ratio in the cathode material, resulting in lower purity of the NVPF phase. Consequently, the C1 / C0 ratio was greater than or equal to 6.7%. Based on the specific capacity of the first charge, the specific capacity of the first discharge, the specific capacity after 500 cycles, and the capacity retention rate after 500 cycles in Table 2, it can be seen that the cathode materials obtained in Comparative Examples 1-3 had lower capacity and capacity retention rates, as well as lower energy density.

[0203] Furthermore, based on the results of Examples 1, 3 to 7 in Tables 1 and 2, it can be seen that, on the basis of Example 1, the addition of an appropriate amount of ammonium fluoride as a fluorine replenishing agent in Examples 3 to 6 replenished the F loss, reduced the formation of impurity phases such as sodium vanadium phosphate in the finished product, and was conducive to improving the purity of Na3V2(PO4)2F3, resulting in a C1 / C0 value comparable to or smaller than that of Example 1, and thus having a discharge specific capacity comparable to or higher than that of Examples 1 and 3 to 6 after 500 cycles. However, in Example 7, due to the relatively large molar amount of ammonium fluoride added, there were too many impurity phases in the finished product, resulting in a lower discharge specific capacity after 500 cycles than that of Examples 1, 3 to 6.

[0204] The results from Examples 1 and 12 in Tables 1 and 2 show that, compared to the use of citric acid as the single carbon source in Example 12, the addition of polyvinyl alcohol in Example 1 makes the carbon-containing coating layer more dense, thus increasing the compaction density. The results from Examples 1 and 13 in Tables 1 and 2 show that, compared to the use of polyvinyl alcohol as the single carbon source in Example 13, the addition of sodium citrate in Example 1 can improve the purity of the NVPF phase to a certain extent, resulting in a lower C1 / C0 ratio and an improved discharge specific capacity after 500 cycles. The results from Examples 1 and 14 in Tables 1 and 2 show that, compared to the use of glucose as the carbon source in Example 14, the combination of citric acid and polyvinyl alcohol in Example 1 increases the compaction density.

[0205] Based on the results in Tables 1 and 2 for Examples 1, 8-11, it can be seen that when the mass ratio of sodium citrate to polyvinyl alcohol is in the range of 2:1 to 1:3, the corresponding coin cell exhibits better discharge specific capacity after 500 cycles. Based on the results in Tables 1 and 2 for Examples 1, 15-21, it can be seen that when the mass fraction of carbon in the cathode material is between 1.5% and 3.5%, the corresponding coin cell exhibits better discharge specific capacity after 500 cycles. Based on the results in Tables 1 and 2 for Examples 1, 22-27, and 28-35, it can be seen that both the calcination temperature and time affect the purity of NVPF in the cathode material and the compaction density of the cathode material, thus affecting the performance of the corresponding coin cell. Specifically, when the calcination temperature is 650℃-800℃ and the calcination time is 5h-9h, the corresponding coin cell exhibits a relatively higher discharge specific capacity after 500 cycles.

[0206] In summary, by making C1 / C0 < 6.7%, the cathode material and the battery using the cathode material can exhibit better performance in terms of capacity and capacity retention.

[0207] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode material, characterized in that, It includes a core and a carbon-containing coating layer covering at least a portion of the surface of the core, the core comprising sodium vanadium fluorophosphate; A coin cell battery is made using the positive electrode material. The coin cell battery is subjected to the Nth charge-discharge test. In the discharge curve of the Nth charge-discharge test, the discharge specific capacity is C0, the discharge specific capacity of the discharge plateau corresponding to the voltage of 3.3V~3.4V is C1, and C1 / C0 is less than or equal to 0.5%, where N is an integer greater than or equal to 1. The conditions for the Nth charge-discharge test are as follows: at 20°C to 30°C, the coin cell is charged to 4.3V at a constant current rate of 0.2C, and then discharged to 2V at a constant current rate of 0.2C. The carbon-containing coating layer covers 95% to 100% of the surface of the core; the porosity of the cathode material is 17% to 27%; the mass fraction of carbon in the cathode material is 1.5% to 3.5%; and the compaction density of the cathode material is 1.7 g / cm³. 3 ~1.9g / cm 3 .

2. The cathode material according to claim 1, characterized in that, At least one of the following conditions must be met: (1) C1 / C0 ≤ 0.44%; (2) The carbon-containing coating layer covers 96% to 100% of the surface of the core; (3) The porosity of the positive electrode material is 17%~25%; (4) The mass fraction of carbon in the cathode material is 1.7%~3.3%; (5) The compaction density of the positive electrode material is 1.7 g / cm³. 3 ~1.8g / cm 3 .

3. A method for preparing a positive electrode material, characterized in that, include: A mixed slurry is provided, the mixed slurry containing a sodium source, a fluorine source, a vanadium source, a phosphorus source and a carbon source; the carbon source includes polyvinyl alcohol, or polyvinyl alcohol and citric acid; The mixed slurry is dried to obtain a precursor; The precursor is calcined to obtain the cathode material; The positive electrode material comprises a core and a carbon-containing coating layer covering at least a portion of the surface of the core, wherein the core comprises sodium vanadium fluorophosphate; a coin cell is fabricated using the positive electrode material, and the coin cell is subjected to an Nth charge-discharge test; in the discharge curve of the Nth charge-discharge test, the discharge specific capacity is C0, the discharge specific capacity of the discharge plateau corresponding to the voltage of 3.3V~3.4V is C1, and C1 / C0 is less than or equal to 0.5%, wherein N is an integer greater than or equal to 1; the conditions for the Nth charge-discharge test are: at 20°C~30°C, the coin cell is charged at a constant current rate of 0.2C to 4.3V, and then discharged at a constant current rate of 0.2C to 2V; The carbon-containing coating layer covers 95% to 100% of the surface of the core; the porosity of the cathode material is 15% to 30%; the mass fraction of carbon in the cathode material is 1.5% to 3.5%; and the compaction density of the cathode material is 1.53 g / cm³. 3 ~1.99g / cm 3 .

4. The preparation method according to claim 3, characterized in that, One or more of the following conditions must be met: (1) When the carbon source is citric acid and polyvinyl alcohol, the mass ratio of citric acid to polyvinyl alcohol is 2:1 to 1:4; (2) The polyvinyl alcohol includes at least one of polyvinyl alcohol 2000, polyvinyl alcohol 3000, polyvinyl alcohol 4000 and polyvinyl alcohol 6000.

5. The preparation method according to claim 3 or 4, characterized in that, The sodium source includes at least one of sodium fluoride, sodium phosphate, sodium bicarbonate, and sodium nitrate; The fluorine source includes at least one of sodium fluoride, ammonium fluoride, potassium fluoride, and lithium fluoride; The vanadium source includes vanadium pentoxide and / or ammonium metavanadate; The phosphorus source includes at least one of ammonium dihydrogen phosphate, sodium phosphate, ammonium monohydrogen phosphate, and triammonium phosphate.

6. The preparation method according to claim 5, characterized in that, One or more of the following conditions must be met: (1) The sodium source includes sodium fluoride; (2) The fluorine source includes sodium fluoride.

7. The preparation method according to claim 3 or 4, characterized in that, The method for preparing the mixed slurry includes: The sodium source, fluorine source, vanadium source, and phosphorus source are taken according to the stoichiometric ratio of sodium vanadium fluorophosphate, and mixed with the solvent and the carbon source to form a first slurry; After adjusting the pH value of the first slurry to 6-7 using a pH adjuster, the slurry is then ground to obtain the mixed slurry.

8. The preparation method according to claim 7, characterized in that, The pH adjuster includes at least one of ammonia, sodium hydroxide, sodium carbonate, sodium bicarbonate, and ammonium bicarbonate.

9. The preparation method according to claim 8, characterized in that, The pH adjuster includes ammonia.

10. The preparation method according to claim 3 or 4, characterized in that, The mixed slurry also contains a fluoride supplement.

11. The preparation method according to claim 10, characterized in that, One or more of the following conditions must be met: (1) The fluoride supplement includes ammonium fluoride and / or sodium fluoride; (2) The molar amount of the fluoride supplement is 1% to 5% of the molar amount of the fluoride source.

12. The preparation method according to claim 11, characterized in that, Fluoride supplements include ammonium fluoride.

13. The preparation method according to claim 3 or 4, characterized in that, The calcination temperature is 550℃~800℃; the holding time of the calcination is 3h~10h; the calcination is carried out in a protective atmosphere.

14. The preparation method according to claim 13, characterized in that, One or more of the following conditions must be met: (1) The calcination temperature is 650℃~800℃; (2) The holding time for the calcination treatment is 4h~9h; (3) The protective atmosphere includes nitrogen and / or argon.

15. A positive electrode plate, characterized in that, This includes the cathode material as described in claim 1 or 2, or the cathode material prepared by any one of claims 3 to 14.

16. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 15.

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