A sodium supplement and a preparation method thereof, a sodium supplement positive electrode and a preparation method thereof, and a sodium ion battery

Abstract

The application relates to a sodium supplement, a preparation method of the sodium supplement, a sodium supplement positive electrode and a preparation method of the sodium supplement positive electrode and a sodium ion battery. x MO4, wherein x is 3-4.5, the M comprises a first element, the first element is any one or a combination of at least two of Co, Fe, Mn, Cr, Ti, V, Zr, Mo or W; and the sodium supplement is a triclinic crystal structure. The sodium supplement comprises a triclinic crystal structure of Na x MO4, the crystal structure of the sodium supplement is unstable, is easy to decompose at a low voltage, can be matched with most positive electrode materials, the decomposition product is an oxide, has no negative effect on the positive electrode material and has a high theoretical specific capacity, therefore, the sodium supplement is a high-efficiency sodium supplement, has good compatibility with a sodium ion positive electrode active material, and can improve the capacity and cycle stability of the sodium ion battery. x MO4 can be used as a high-efficiency sodium supplement and has good compatibility with a sodium ion positive electrode active material, thereby improving the capacity and cycle stability of the sodium ion battery.

Description

Technical Field

[0001] This invention relates to the field of sodium replenishment technology for sodium-ion batteries, and particularly to a sodium replenishing agent and its preparation method, a sodium-replenishing positive electrode and its preparation method, and a sodium-ion battery. Background Technology

[0002] Since its commercialization, the lithium-ion battery (LIB) field has been developing rapidly. However, the scarcity and uneven distribution of lithium resources, coupled with the emergence and widespread adoption of electric vehicles and stationary applications in recent years, have placed unprecedented pressure on the lithium-ion battery value chain, leading to a demand for alternative energy storage chemicals. Sodium-ion batteries, with similar characteristics to lithium-ion batteries and abundant sodium resources, have become a promising alternative to commercial lithium-ion batteries. However, during the charge and discharge process of sodium-ion batteries, the formation of the SEI (Sediment Ion) at the negative electrode consumes a large amount of active sodium ions, directly affecting the battery's energy density and cycle performance. Currently, pre-sodiumization is one of the more effective and easier-to-implement methods to address this issue.

[0003] Pre-sodiuming can be broadly categorized into four methods: direct contact, chemical pre-sodiuming, electrochemical pre-sodiuming, and positive electrode sodium replenishment. Direct contact involves coating the electrode surface with sodium metal under pressure to achieve uniform pre-sodiuming. Because it directly involves the use of sodium metal, large-scale implementation is challenging. Chemical pre-sodiuming is achieved by immersing the electrode in a strongly reducing chemical reagent. This method is simple to operate and can be implemented on a large scale, but it suffers from reagent residue issues. Electrochemical pre-sodiuming involves first assembling and cycling the negative electrode with metallic sodium to pre-generate an SEI layer on the negative electrode side. The battery is then disassembled, and the pre-sodiumed negative electrode is reassembled with a new battery to construct a full cell. This method allows for good control of negative electrode pre-sodiuming and modulation of the SEI layer, but it requires additional assembly and disassembly processes, increasing process steps and costs. Positive electrode sodium replenishment involves adding a small amount of sodium replenishing agent to the positive electrode. During the first charge cycle, an irreversible sodium source is released for sodium replenishment. This method has attracted considerable attention due to its ease of operation and wide applicability.

[0004] An ideal sodium replenisher needs to meet the following conditions: (1) high irreversible capacity; (2) low decomposition voltage, allowing it to decompose within the battery's operating window; (3) decomposition products that have no negative impact on the battery; and (4) good air stability. Currently, sodium replenishers studied include Na2O2, NaN3, Na3P, Na2CO3, Na2C2O4, and EDTA-4Na. Among these, Na3P has a high capacity but also the highest toxicity; Na2CO3 has a small actual sodium replenishment capacity; and Na2C2O4 is inexpensive and easy to prepare, but its sodium removal voltage (approximately 4.3V) is higher than the operating voltage of sodium-ion batteries (approximately 4.0V), which has always been a problem limiting its industrialization.

[0005] CN117878452A discloses a composite sodium supplement agent, comprising sodium oxalate, conductive porous carbon, and a polymer dispersant. This agent uses a conductive carbon substrate to support nano-sodium oxalate as a positive electrode sodium supplement additive for sodium-ion batteries to reduce the decomposition voltage of sodium oxalate, resulting in excellent sodium supplementation. However, the decomposition voltage remains high, making it difficult to adapt to sodium-ion battery positive electrodes and hindering industrial production. Currently, no sodium supplement agent meets all requirements in all aspects; they all suffer from problems such as toxicity, low capacity utilization, or poor compatibility with the positive electrode. Most importantly, their decomposition voltages are generally high, exceeding the cutoff voltage of sodium-ion batteries, posing significant challenges to industrialization.

[0006] Therefore, given the current problems of poor compatibility between most sodium replenishing agents and the positive electrode, as well as high decomposition voltage, how to develop a high-efficiency sodium ion positive electrode sodium replenishing agent with low decomposition voltage that can effectively achieve the function of sodium replenishment has become an urgent problem to be solved. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a sodium supplement agent and its preparation method, a sodium-supplemented cathode and its preparation method, and a sodium-ion battery. The sodium supplement agent of this invention comprises Na with a triclinic crystal structure. x MO4, Na x MO4 has an unstable crystal structure and easily decomposes under low voltage. It is compatible with most cathode materials, and its decomposition products are oxides, which have no negative impact on the cathode material. Furthermore, it possesses a high theoretical specific capacity. Therefore, Na... x MO4 can be used as a highly efficient sodium supplement and has good compatibility with sodium-ion cathode active materials, thereby improving the capacity and cycle stability of sodium-ion batteries.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a sodium supplement, wherein the chemical formula of the sodium supplement is Na. x MO4, wherein x is 3-4.5, for example x can be 3, 4 or 4.5, etc.; the M includes a first element, which is any one or a combination of at least two of Co, Fe, Mn, Cr, Ti, V, Zr, Mo or W;

[0010] The sodium supplement has a triclinic crystal structure.

[0011] In this invention, the sodium supplement is a triclinic crystal structure Na. x MO4, a triclinic crystal with space group P1, exhibits non-equivalent Na+ sites, each bonded to different O atoms. Furthermore, depending on the value of x, some lattice sites may exhibit vacancies or the presence of other metallic elements. Therefore, Na+... xMO4 has low structural stability and is easily decomposed at low voltages. In addition, it has higher electronic and ionic conductivity than conventional sodium supplements, which can achieve better electrical contact, resulting in less polarization during decomposition and a lower decomposition voltage. Its ease of decomposition at low voltages indicates that it is compatible with most cathode materials and has universality. Moreover, the decomposition products are oxides, which have no negative impact on the cathode material.

[0012] As a preferred embodiment of the present invention, M further includes a second element, which is different from the first element, and the second element is a transition metal element with a highest oxidation state of less than 4.

[0013] Preferably, the sodium supplement includes Na4FeO4 or Na4MnO4.

[0014] In this invention, Na4FeO4 is used as an example. Na4FeO4 has the following advantages: 1) It has a high theoretical specific capacity, up to 505 mAh / g, and also has a high irreversible capacity; 2) It has a low decomposition voltage, about 3.4V. Since Na4FeO4 will decompose and undergo disproportionation reaction in water or humid environments, its chemical instability will also result in a low decomposition voltage, which can ensure that it can play a role in sodium replenishment within the working voltage of sodium batteries; 3) The decomposition products have no negative impact on the positive electrode, making it compatible with most sodium battery positive electrode materials and having high compatibility.

[0015] In a second aspect, the present invention also provides a method for preparing the sodium supplement according to the first aspect, the method comprising the following steps:

[0016] Sodium source and M source were mixed and ball-milled, and then calcined to obtain sodium supplement;

[0017] The sodium supplement has the chemical formula Na. x MO4, where x is 3-4.5, and M in the M source includes a first element, which is any one or a combination of at least two of Co, Fe, Mn, Cr, Ti, V, Zr, Mo or W;

[0018] The sodium supplement has a triclinic crystal structure.

[0019] As a preferred technical solution of the present invention, M in the M source further includes a second element, which is different from the first element, and the second element is a transition metal element with a highest oxidation state of less than 4.

[0020] Preferably, the calcination is carried out in a closed environment.

[0021] It should be noted that the present invention does not impose specific requirements or limitations on the reactor vessel used to support the reaction material in order to form a closed environment. Any type commonly used by those skilled in the art that enables the reaction material to be calcined in a closed environment is applicable to the present invention. For example, it can be a closed crucible or a closed borosilicate glass bottle.

[0022] Preferably, during the calcination process, the oxygen partial pressure in the sealed environment is >21.3 kPa, such as 22 kPa, 24 kPa, 26 kPa, 28 kPa, 30 kPa, 32 kPa, 34 kPa, 36 kPa, 38 kPa or 40 kPa.

[0023] During the calcination process of this invention, oxygen is generated, resulting in an oxygen partial pressure in the sealed environment. This invention controls the oxygen partial pressure to >21.3 kPa by adjusting the relationship between the mass of the reaction material and the volume of the reactor vessel. This ensures that the prepared sodium supplement is a pure phase, free from impurities and unreacted material doping, thus achieving a better sodium supplementation effect. If the oxygen partial pressure is too low, amorphous impurities will be generated, making it impossible to prepare a pure phase.

[0024] Preferably, the calcination is carried out in an argon atmosphere or a nitrogen atmosphere.

[0025] It should be noted that the atmosphere in the sealed environment described in this invention is a calcination atmosphere before calcination. During the calcination process, due to the generation of oxygen, there is a large amount of oxygen atmosphere and a small amount of calcination atmosphere in the sealed environment (the amount of calcination atmosphere is relative to the amount of oxygen atmosphere).

[0026] Preferably, the sodium source includes any one or a combination of at least two of Na2O2, Na2O, CH3COONa, or Na2CO3.

[0027] Preferably, the M source includes an iron source and / or a manganese source.

[0028] Preferably, the iron source includes any one or a combination of at least two of Fe2O3, FeO, or Fe3O4.

[0029] Preferably, the manganese source includes Mn2O3.

[0030] Preferably, the molar ratio of sodium in the sodium source to M in the M source is (3-4.5):1, for example, 3:1, 3.5:1, 4:1 or 4.5:1, etc.

[0031] Preferably, the rotational speed of the ball mill is 200rpm-600rpm, such as 200rpm, 300rpm, 400rpm, 500rpm or 600rpm.

[0032] Preferably, the ball milling time is 4h-8h, for example 4h, 5h, 6h, 7h or 8h.

[0033] Preferably, the heating rate of the calcination is 3℃ / min-10℃ / min, for example, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min.

[0034] Preferably, the calcination temperature is 450℃-850℃, such as 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 800℃ or 850℃, etc., and preferably 450℃-600℃.

[0035] Preferably, the calcination holding time is 12h-24h, such as 12h, 14h, 16h, 18h, 20h, 22h or 24h.

[0036] Thirdly, the present invention also provides a sodium-supplementing positive electrode, which includes a sodium-supplementing agent as described in the first aspect, or a sodium-supplementing agent prepared by the preparation method described in the second aspect, and the sodium-supplementing positive electrode further includes a positive electrode active material, a binder, and a conductive agent.

[0037] As a preferred technical solution of the present invention, based on the mass of the sodium-supplementing positive electrode, the amount of sodium-supplementing agent added is 0wt%-30wt% and not 0, for example, 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, or 30wt%.

[0038] Preferably, the chemical formula of the positive electrode active material includes Na. m M 1a M 2b M 3c O2, wherein 0 < m ≤ 1, for example m can be 0.2, 0.4, 0.6, 0.8 or 1, etc.; 0 ≤ a ≤ 1, for example a can be 0, 0.2, 0.4, 0.6, 0.8 or 1, etc.; 0 ≤ b ≤ 1, for example b can be 0, 0.2, 0.4, 0.6, 0.8 or 1, etc.; 0 ≤ c ≤ 1, for example c can be 0, 0.2, 0.4, 0.6, 0.8 or 1, etc.; and a + b + c = 1, wherein M1, M2 and M3 each independently include any one of Ni, Fe, Mn, Ti, Co, Cu or Cr.

[0039] Preferably, based on the mass of the sodium-supplemented positive electrode, the amount of positive electrode active material added is 70wt%-85wt%, such as 70wt%, 75wt%, 80wt%, or 85wt%.

[0040] Preferably, the adhesive comprises any one or a combination of at least two of polyvinylidene fluoride, carboxymethyl cellulose, or polytetrafluoroethylene.

[0041] Preferably, based on the mass of the sodium-supplemented positive electrode, the amount of binder added is 0wt%-10wt%, and not 0, for example, 1wt%, 2wt%, 4wt%, 6wt%, 8wt%, or 10wt%.

[0042] As a preferred technical solution of the present invention, the conductive agent includes any one or a combination of at least two of acetylene black, conductive carbon black, graphene, Ketjen black, carbon nanotubes or carbon fibers.

[0043] Preferably, based on the mass of the sodium-supplemented positive electrode, the amount of conductive agent added is 0wt%-30wt%, and is not 0, for example, 1wt%, 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, or 30wt%, etc.

[0044] Fourthly, the present invention also provides a method for preparing the sodium-supplemented positive electrode according to the third aspect, the method comprising the following steps:

[0045] A sodium-supplemented positive electrode is obtained by mixing the positive electrode active material, sodium supplement, conductive agent and binder.

[0046] As a preferred technical solution of the present invention, the mixing includes dry mixing or wet mixing, preferably dry mixing.

[0047] Preferably, the dry mixing process includes: first mixing the positive electrode active material, sodium supplement and conductive agent, then adding a binder and mixing to obtain a sheet solid, and repeatedly folding and rolling the sheet solid.

[0048] In this invention, a dry mixing method is used to prepare the sodium-supplemented cathode, which can maximize the utilization of Na. x MO4 improves sodium replenishment efficiency, fully leveraging its advantages as a sodium replenishing agent. In addition, the dry mixing method, with zero solvent addition, can improve battery production efficiency and reduce costs.

[0049] Preferably, the dry mixing method includes manual grinding or mechanical stirring.

[0050] Preferably, the dry mixing time is 30-60 minutes, such as 30 minutes, 40 minutes, 50 minutes or 60 minutes.

[0051] In this invention, the positive electrode active material, sodium supplementer, and conductive agent are first mixed evenly to form a homogeneous mixture. Then, a binder is added, and mixing continues. During the mixing process, friction and collision cause the binder to disperse evenly and adhere to the surface of the mixture, resulting in stronger adhesion. Multiple folding and rolling steps further fiberize the binder, increasing the material's viscosity and formability, leading to a superior sodium-supplementing positive electrode performance. Directly mixing the positive electrode active material, sodium supplementer, conductive agent, and binder would result in uneven mixing, reducing the sodium-supplementing positive electrode performance. Furthermore, the increased number of folding and rolling steps required to bond the material into sheets would consume time and reduce efficiency.

[0052] Preferably, the thickness of the sodium-supplemented positive electrode is 50μm-150μm, such as 50μm, 80μm, 100μm, 120μm or 150μm.

[0053] Fifthly, the present invention also provides a sodium-ion battery, the sodium-ion battery comprising a sodium-supplemented positive electrode as described in the third aspect, or a sodium-supplemented positive electrode prepared by the preparation method described in the fourth aspect.

[0054] Compared with the prior art, the present invention has at least the following beneficial effects:

[0055] 1) This invention uses Na with a triclinic crystal structure. x MO4 acts as a sodium supplement, Na x MO4 has an unstable crystal structure and is easily decomposed at low voltage. It can be compatible with most cathode materials. The decomposition products are oxides, which have no negative impact on the cathode material and have a high theoretical specific capacity.

[0056] 2) This invention uses Na with a triclinic crystal structure. x MO4 is a simple sodium replenishing agent to prepare and use. It can be directly mixed and ground with positive electrode active material, conductive agent and binder, avoiding the cumbersome sodium replenishment process and reducing costs. Attached Figure Description

[0057] Figure 1 The sodium supplement Na provided by this invention x Crystal structure diagram of MO4.

[0058] Figure 2 This is the XRD pattern of Na4FeO4 provided in Embodiment 1 of the present invention.

[0059] Figure 3 This is a scanning electron microscope image of Na4FeO4 provided in Embodiment 1 of the present invention.

[0060] Figure 4 This is the first charge-discharge curve of the Na||Na4FeO4 half-cell assembled with Na4FeO4 provided in Embodiment 1 of the present invention.

[0061] Figure 5 This is the XRD pattern of Na4FeO4 provided in Embodiment 5 of the present invention.

[0062] Figure 6 These are the first-cycle charge-discharge curves of the HC||MFN+9.1%Na4FeO4 full cell and the HC||MFN full cell corresponding to the sodium-supplemented positive electrode provided in Application Example 1 and Comparative Application Example 1 of the present invention.

[0063] Figure 7 These are the first-cycle charge-discharge curves of the HC||MFN+4.8%Na4FeO4 full cell and the HC||MFN full cell corresponding to the sodium-supplemented positive electrode provided in Application Example 5 and Comparative Application Example 1 of the present invention.

[0064] Figure 8 These are the first-cycle charge-discharge curves of the HC||MFN+13%Na4FeO4 full cell and the HC||MFN full cell corresponding to the sodium-supplemented positive electrode provided in Application Example 6 and Comparative Application Example 1 of the present invention.

[0065] Figure 9 The graphs show the cycle stability of the HC||MFN+9.1%Na4FeO4 full cells, HC||MFN+4.8%Na4FeO4 full cells, HC||MFN+13%Na4FeO4 full cells and HC||MFN full cells prepared by Application Example 1, Application Example 5, Application Example 6 and Comparative Application Example 1. Detailed Implementation

[0066] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0067] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0068] Figure 1 This invention provides a sodium supplement, Na. x The crystal structure diagram of MO4 shows that the sodium supplement Na... x MO4 has a triclinic crystal structure.

[0069] Example 1

[0070] This embodiment provides a sodium supplement and its preparation method, wherein the sodium supplement is Na4FeO4;

[0071] The preparation method of Na4FeO4 includes the following steps:

[0072] Na2O2 and Fe2O3 were mixed in a molar ratio of 4:1 and ball-milled at 400 rpm for 6 hours using a planetary ball mill. The mixed powder was then pressed into blocks using a briquetting machine at a pressure of 20 MPa, placed in a sealed crucible, and placed in a tube furnace. The furnace was heated to 500°C at a rate of 5°C / min in an argon atmosphere and held at that temperature for 16 hours. During the calcination process, the oxygen partial pressure in the sealed crucible was 30 kPa. The mixture was then naturally cooled to room temperature to obtain Na4FeO4 sodium supplement.

[0073] Figure 2 The XRD pattern of Na4FeO4 provided in Example 1 of the present invention is shown. As can be seen from the figure, the prepared Na4FeO4 and PDF#070-3355 (Na4FeO4 standard card) have the same characteristic peaks, and no other impurity characteristic peaks appear, indicating that Na4FeO4 is a pure phase and no other impurities are generated.

[0074] Figure 3 The image shows a scanning electron microscope (SEM) image of Na4FeO4 provided in Example 1 of the present invention. As can be seen from the image, the microstructure of Na4FeO4 consists of spheres with a narrow particle size distribution. The particle size is approximately 4μm-10μm, and the morphology is relatively uniform.

[0075] Using Na4FeO4 (85wt%) provided in Example 1 of this invention as the positive electrode material and Ketjen black (10wt%) as the conductive carbon black, they were mixed in a mortar and ground thoroughly for 30 minutes to ensure uniform mixing. Then, PTFE (5wt%) was added and grinding continued until the solid powder was fully bonded into a sheet. This sheet was then transferred to a smooth stainless steel plate and folded and rolled back and forth with a stainless steel rod until it was rolled into a smooth sheet with a thickness of approximately 100μm. This sheet was then cut into a circular positive electrode sheet with a diameter of 12mm. Using sodium metal as the negative electrode, a Na||Na4FeO4 half-cell was assembled and subjected to the first charge-discharge test at 25°C, a voltage range of 2-4.5V, and a rate of 0.1C.

[0076] Figure 4 The figure shows the first charge-discharge curve of the Na||Na4FeO4 half-cell assembled by Na4FeO4 according to Embodiment 1 of the present invention. As can be seen from the figure, the specific capacity of the Na||Na4FeO4 half-cell in the first charge cycle is 439mAh / g and the specific capacity in the first discharge cycle is 10mAh / g, which reflects the high irreversible capacity and capacity utilization of Na4FeO4. Moreover, the decomposition voltage is about 3.4V, which is relatively low.

[0077] Example 2

[0078] This embodiment provides a sodium supplement and its preparation method, wherein the sodium supplement is Na4FeO4;

[0079] The preparation method of Na4FeO4 includes the following steps:

[0080] Na2O2 and Fe2O3 were mixed in a molar ratio of 4:1 and ball-milled at 200 rpm for 8 hours using a planetary ball mill. The mixed powder was then pressed into blocks using a briquetting machine at a pressure of 20 MPa. The blocks were placed in a sealed crucible and then placed in a tube furnace. The furnace was heated to 600°C at a rate of 10°C / min in an argon atmosphere and held at that temperature for 12 hours. During the calcination process, the oxygen partial pressure in the sealed crucible was 25 kPa. The mixture was then naturally cooled to room temperature to obtain Na4FeO4 sodium supplement.

[0081] Example 3

[0082] This embodiment provides a sodium supplement and its preparation method, wherein the sodium supplement is Na4FeO4;

[0083] The preparation method of Na4FeO4 includes the following steps:

[0084] Na₂CO₃ and Fe₂O₃ were mixed in a molar ratio of 4:1 and ball-milled at 600 rpm for 4 hours using a planetary ball mill. The mixed powder was then pressed into blocks using a briquetting machine at a pressure of 20 MPa, placed in a sealed crucible, and heated to 450°C at a rate of 3°C / min in a tube furnace under an argon atmosphere and held at that temperature for 24 hours. During the calcination process, the oxygen partial pressure in the sealed crucible was 28 kPa. The mixture was then naturally cooled to room temperature to obtain Na₄FeO₄ sodium supplement.

[0085] Example 4

[0086] This embodiment provides a sodium supplement and its preparation method, wherein the sodium supplement is Na4MnO4;

[0087] The preparation method of Na4MnO4 includes the following steps:

[0088] Na2O2 and Mn2O3 were mixed in a molar ratio of 4:1 and ball-milled at 400 rpm for 6 hours using a planetary ball mill. The mixed powder was then pressed into blocks using a briquetting machine at a pressure of 20 MPa. The blocks were placed in a sealed crucible and then placed in a tube furnace. The furnace was heated to 500°C at a rate of 5°C / min in an argon atmosphere and held at that temperature for 16 hours. During the calcination process, the oxygen partial pressure in the sealed crucible was 30 kPa. The mixture was then naturally cooled to room temperature to obtain Na4MnO4 sodium supplement.

[0089] Example 5

[0090] This embodiment provides a sodium supplement and its preparation method, wherein the sodium supplement is Na4FeO4;

[0091] The difference between the preparation method described herein and Example 1 is that the calcination process is carried out in an air atmosphere, using an ordinary open crucible, and the raw materials are completely exposed to the air, i.e., the oxygen partial pressure is 21.3 kPa. The remaining preparation methods and parameters are consistent with those of Example 1.

[0092] The Na4FeO4 prepared in this embodiment contains significant impurities compared to the Na4FeO4 prepared in Example 1. Figure 5 The XRD pattern of Na4FeO4 provided in Example 5 of the present invention is shown. As can be seen from the figure, compared with PDF#070-3355 (Na4FeO4 standard card), the prepared Na4FeO4 has additional obvious diffraction peaks at 2θ of 16.6°, 20.8° and 34.7°, which indicates that the prepared Na4FeO4 is not a pure phase, but has other impurity phases generated, and has very low crystallinity.

[0093] Example 6

[0094] This embodiment provides a sodium supplement and its preparation method, wherein the sodium supplement is Na4FeO4;

[0095] The difference between the preparation method described herein and Example 1 is that an ordinary open crucible is used to completely expose the raw material to argon gas, i.e., the oxygen partial pressure is 0 kPa. The rest of the preparation method and parameters are the same as in Example 1.

[0096] The Na4FeO4 prepared in this embodiment contains significant impurities compared to the Na4FeO4 prepared in Example 1, as shown in the corresponding XRD patterns. Figure 5 Similarly, the prepared Na4FeO4 is not a pure phase; other impurity phases are generated, and its crystallinity is very low.

[0097] Application Example 1

[0098] This application example provides a sodium-supplemented cathode and its preparation method. In the sodium-supplemented cathode, Na4FeO4 from Example 1 is used as the sodium-supplementing agent, comprising 77.3 wt% NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (MFN), 9.1 wt% Na4FeO4, 4.5 wt% polytetrafluoroethylene (PTFE) and 9.1 wt% Ketjen black;

[0099] The preparation method includes the following steps:

[0100] Weigh out MFN, Na4FeO4 and Ketjen black according to the above-mentioned amounts, grind them thoroughly for 30 minutes, then add PTFE and continue grinding until the solid powder is fully bonded into a sheet. Place the resulting sheet-like solid on a smooth stainless steel plate and roll it back and forth with a stainless steel rod, fold it in half and roll it again until it is rolled into a sheet with a smooth surface and a thickness of 100μm, thus obtaining the sodium-supplemented positive electrode.

[0101] Application Example 2

[0102] This application example provides a sodium-supplementing cathode and its preparation method. In the sodium-supplementing cathode, Na4FeO4 from Example 2 is used as the sodium-supplementing agent, and the remaining components and contents are consistent with those of Application Example 1.

[0103] The preparation method is consistent with that in Application Example 1.

[0104] Application Example 3

[0105] This application example provides a sodium-supplementing cathode and its preparation method. In the sodium-supplementing cathode, Na4FeO4 from Example 3 is used as the sodium-supplementing agent, and the remaining components and contents are consistent with those of Application Example 1.

[0106] The preparation method is consistent with that in Application Example 1.

[0107] Application Example 4

[0108] This application example provides a sodium-supplementing cathode and its preparation method. In the sodium-supplementing cathode, Na4MnO4 from Example 4 is used as the sodium-supplementing agent, and the remaining components and contents are consistent with those of Application Example 1.

[0109] The preparation method is consistent with that in Application Example 1.

[0110] Application Example 5

[0111] This application example provides a sodium-supplemented cathode and its preparation method. In the sodium-supplemented cathode, Na4FeO4 from Example 1 is used as the sodium-supplementing agent, comprising 81 wt% NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (MFN), 4.8 wt% Na4FeO4, 9.4 wt% polyvinylidene fluoride (PVDF) and 4.8 wt% Ketjen black;

[0112] The difference between the preparation method described above and Application Example 1 is that the quality of the raw materials is adjusted according to the above-mentioned raw material ratio, while the rest of the preparation methods and parameters remain the same as in Application Example 1.

[0113] Application Example 6

[0114] This application example provides a sodium-supplemented cathode and its preparation method. In the sodium-supplemented cathode, Na4FeO4 from Example 1 is used as the sodium-supplementing agent, comprising 74 wt% NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (MFN), 13 wt% Na4FeO4, 4.3 wt% polytetrafluoroethylene (PTFE) and 8.7 wt% Ketjen black;

[0115] The difference between the preparation method described above and Application Example 1 is that the quality of the raw materials is adjusted according to the above-mentioned raw material ratio, while the rest of the preparation methods and parameters remain the same as in Application Example 1.

[0116] Comparative Application Example 1

[0117] This comparative application example provides a sodium-supplemented positive electrode and its preparation method. The difference between this sodium-supplemented positive electrode and Application Example 1 is that the addition of the sodium-supplementing agent is omitted. (NaNi) 1 / 3 Fe 1 / 3 Mn 1 / 3 The content of O2 (MFN) is 85 wt%, polytetrafluoroethylene (PTFE) 5 wt%, and Ketjen Black 10 wt%.

[0118] The difference between the preparation method described above and Application Example 1 is that the quality of the raw materials is adjusted according to the above-mentioned raw material ratio, while the rest of the preparation methods and parameters remain the same as in Application Example 1.

[0119] The sodium-supplemented positive electrode provided in Application Examples 1-6 and Comparative Application Example 1 was cut into a circle with a diameter of 12 mm to obtain the positive electrode sheet of the battery. Hard carbon: conductive carbon black (Super P): binder (PVDF) = 85:8:7 and N-methylpyrrolidone (NMP) was used as a solvent to mix them evenly and coat them on aluminum foil. After vacuum drying at 110°C, they were cut into circles with a diameter of 14 mm as negative electrode sheets. A 19 mm diameter aluminum foil was placed in the positive electrode shell, and 75 μL of 1 mol NaPF6 in PC electrolyte was dropped into the center of the positive electrode shell. Then, a glass fiber separator was placed and 75 μL of electrolyte was dropped in to fully wet the separator. Finally, the hard carbon negative electrode, gasket, spring sheet and negative electrode shell were placed in sequence to obtain a sodium-ion battery.

[0120] Sodium-ion batteries assembled with sodium-added cathodes provided in Application Examples 1-6 and Comparative Application Example 1 were left to stand for 12 hours, and then subjected to a first charge-discharge test at 25°C, a voltage range of 1.2-4.0V, and a rate of 0.1C. The first discharge specific capacity was obtained, and the specific data are shown in Table 1.

[0121] Sodium-ion batteries assembled with sodium-added cathodes provided in Application Examples 1-6 and Comparative Application Example 1 were left to stand for 12 hours. Then, cycle stability tests were conducted at 25°C, with a voltage range of 1.2-4.0V, a rate of 0.1C for the first three cycles, and a rate of 0.33C thereafter. The discharge specific capacity after 50 cycles was obtained, and the specific data are shown in Table 1.

[0122] Figure 6 The first-cycle charge-discharge curves of the HC||MFN+9.1%Na4FeO4 full cell and the HC||MFN full cell corresponding to the sodium-supplemented cathode provided in Application Example 1 and Comparative Application Example 1 are shown. As can be seen from the figure, the first-cycle discharge specific capacity of the HC||MFN full cell is 108 mAh / g, and the first-cycle discharge specific capacity of the HC||MFN+9.1%Na4FeO4 is 123 mAh / g. This indicates that the sodium-supplementing agent of the Na4FeO4 cathode played a role in offsetting a certain amount of active sodium ions consumed in the formation of the SEI film, thereby improving the first-cycle discharge capacity.

[0123] Figure 7 The first-cycle charge-discharge curves of the HC||MFN+4.8%Na4FeO4 full cell and the HC||MFN full cell corresponding to the sodium-supplemented cathode provided in Application Example 5 and Comparative Application Example 1 are shown. As can be seen from the figure, the first-cycle discharge specific capacity of the HC||MFN full cell is 108 mAh / g, and the first-cycle discharge specific capacity of the HC||MFN+4.8%Na4FeO4 is 113 mAh / g. This indicates that the Na4FeO4 cathode lithium supplementation agent played a role, offsetting a certain amount of active sodium ions consumed in the formation of the SEI film, thereby improving the first-cycle discharge capacity.

[0124] Figure 8 The first-cycle charge-discharge curves of the HC||MFN+13%Na4FeO4 full cell and the HC||MFN full cell corresponding to the sodium-supplemented cathode provided in Application Example 6 and Comparative Application Example 1 are shown. As can be seen from the figure, the first-cycle discharge specific capacity of the HC||MFN full cell is 108 mAh / g, and the first-cycle discharge specific capacity of the HC||MFN+13%Na4FeO4 is 141 mAh / g. This indicates that the Na4FeO4 cathode lithium supplementation agent played a role in offsetting a certain amount of active sodium ions consumed in the formation of the SEI film, thereby improving the first-cycle discharge capacity.

[0125] Figure 9Cyclic stability curves of HC||MFN+9.1%Na4FeO4 full cells, HC||MFN+4.8%Na4FeO4 full cells, HC||MFN+13%Na4FeO4 full cells, and HC||MFN full cells prepared in Application Example 1, Application Example 5, Application Example 6, and Comparative Application Example 1 are shown. As can be seen from the figure, the capacity of the full cell containing Na4FeO4 did not decrease significantly compared with the HC||MFN full cell after multiple charge-discharge cycles. This indicates that Na4FeO4 sodium supplement has a good sodium supplementation effect and can further improve the capacity and stability of the battery.

[0126] Table 1

[0127] project First-cycle discharge specific capacity (mAh / g) Discharge specific capacity (mAh / g) after 50 cycles Application Example 1 123 109 Application Example 2 120 105 Application Example 3 120 106 Application Example 4 121 108 Application Example 5 113 100 Application Example 6 141 129 Comparative Application Example 1 108 88

[0128] The test results show that:

[0129] (1) As can be seen from Examples 1 to 4, by controlling the oxygen partial pressure in the closed environment during the calcination process to be >21.3 kPa, the present invention can ensure that the prepared sodium supplement is a pure phase without impurities or unreacted material doping, thereby achieving a better sodium supplementation effect; if the oxygen partial pressure is too low, impurities will be generated and the preparation of a pure phase cannot be achieved.

[0130] (2) As can be seen from Examples 1 and 5-6, the present invention can ensure the synthesis of Na4FeO4 under certain oxygen partial pressure by calcining the material in a closed environment. If it is completely exposed to air or argon atmosphere, amorphous impurities will be generated and the crystallinity will be very low.

[0131] (3) As can be seen from Application Examples 1 to 6, the sodium supplement prepared by the present invention can achieve better sodium supplementation effect. In hard carbon and MFN based full cells, the discharge specific capacity of the corresponding full cells is significantly improved by adding different proportions of Na4FeO4 sodium supplement. When the positive electrode contains 4.8% Na4FeO4, the discharge specific capacity can be increased to 113 mAh / g. When the positive electrode contains 9.1% Na4FeO4, the discharge specific capacity can be increased to 123 mAh / g. When the positive electrode contains 13% Na4FeO4, the discharge specific capacity can be increased to 141 mAh / g.

[0132] (4) As can be seen from Application Example 1 and Comparative Application Example 1, the addition of Na4FeO4 sodium supplement in this invention can further improve the discharge capacity and cycle stability of sodium-ion batteries.

[0133] In summary, the sodium supplement of the present invention comprises Na with a triclinic crystal structure. x MO4, Na xMO4 has an unstable crystal structure and easily decomposes under low voltage. It is compatible with most cathode materials, and its decomposition products are oxides, which have no negative impact on the cathode material. Furthermore, it possesses a high theoretical specific capacity. Therefore, Na... x MO4 can be used as a highly efficient sodium supplement and has good compatibility with sodium-ion cathode active materials, thereby improving the capacity and cycle stability of sodium-ion batteries.

[0134] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A sodium supplement, characterized in that, The chemical formula of the sodium supplement is Na. x MO4, wherein x is 3-4.5, and M includes a first element, which is any one or a combination of at least two of Co, Fe, Mn, Cr, Ti, V, Zr, Mo or W; The sodium supplement has a triclinic crystal structure.

2. The sodium supplement according to claim 1, characterized in that, The M also includes a second element, which is different from the first element, and the second element is a transition metal element with a highest oxidation state of less than 4; Preferably, the sodium supplement includes Na4FeO4 or Na4MnO4.

3. A method for preparing a sodium supplement according to claim 1 or 2, characterized in that, The preparation method includes the following steps: Sodium source and M source were mixed and ball-milled, and then calcined to obtain sodium supplement; The sodium supplement has the chemical formula Na. x MO4, where x is 3-4.5, and M in the M source includes a first element, which is any one or a combination of at least two of Co, Fe, Mn, Cr, Ti, V, Zr, Mo or W; The sodium supplement has a triclinic crystal structure.

4. The preparation method according to claim 3, characterized in that, The M in the M source also includes a second element, which is different from the first element, and the second element is a transition metal element with a highest oxidation state of less than 4; Preferably, the calcination is carried out in a closed environment; Preferably, during the calcination process, the oxygen partial pressure in the sealed environment is >21.3 kPa; Preferably, the calcination is carried out under an argon atmosphere or a nitrogen atmosphere; Preferably, the sodium source includes any one or a combination of at least two of Na2O2, Na2O, CH3COONa, or Na2CO3; Preferably, the M source includes an iron source and / or a manganese source; Preferably, the molar ratio of sodium in the sodium source to M in the M source is (3-4.5):1; Preferably, the calcination temperature is 450℃-850℃, more preferably 450℃-600℃; Preferably, the calcination holding time is 12h-24h.

5. A sodium-supplementing positive electrode, characterized in that, The sodium-supplementing positive electrode includes the sodium-supplementing agent as described in claim 1 or 2, or the sodium-supplementing agent prepared by the preparation method as described in claim 3 or 4, and the sodium-supplementing positive electrode further includes a positive electrode active material, a binder, and a conductive agent.

6. The sodium-supplementing positive electrode according to claim 5, characterized in that, Based on the mass of the sodium-supplementing positive electrode, the amount of sodium-supplementing agent added is 0wt%-30wt% and not 0; Preferably, the chemical formula of the positive electrode active material includes Na. m M 1a M 2b M 3c O2, wherein 0 < m ≤ 1, 0 ≤ a ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, and a + b + c = 1, and M1, M2 and M3 each independently include any one of Ni, Fe, Mn, Ti, Co, Cu or Cr; Preferably, based on the mass of the sodium-supplemented positive electrode, the amount of the positive electrode active material added is 70wt%-85wt%. Preferably, the adhesive comprises any one or a combination of at least two of polyvinylidene fluoride, carboxymethyl cellulose, or polytetrafluoroethylene; Preferably, based on the mass of the sodium-supplemented positive electrode, the amount of binder added is 0wt%-10wt%, and not 0.

7. The sodium-supplementing positive electrode according to claim 5 or 6, characterized in that, The conductive agent includes any one or a combination of at least two of acetylene black, conductive carbon black, graphene, Ketjen black, carbon nanotubes or carbon fibers. Preferably, based on the mass of the sodium-supplemented positive electrode, the amount of conductive agent added is 0wt%-30wt%, and not 0.

8. A method for preparing a sodium-supplemented positive electrode according to any one of claims 5-7, characterized in that, The preparation method includes the following steps: A sodium-supplemented positive electrode is obtained by mixing the positive electrode active material, sodium supplement, conductive agent and binder.

9. The preparation method according to claim 8, characterized in that, The mixing includes dry mixing or wet mixing, preferably dry mixing; Preferably, the dry mixing process includes: first mixing the positive electrode active material, sodium supplement and conductive agent, then adding a binder and mixing to obtain a sheet solid, and repeatedly folding and rolling the sheet solid.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes a sodium-supplemented positive electrode as described in any one of claims 5-7, or a sodium-supplemented positive electrode prepared by the preparation method described in claim 8 or 9.

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