Positive electrode material, preparation method thereof and sodium ion battery

Abstract

The invention provides a positive electrode material, a preparation method thereof and a sodium ion battery. The positive electrode material comprises a positive electrode material core and a metal oxide coating layer coating the surface of the positive electrode material core, the positive electrode material core comprises a metal ion doped positive electrode active material, the chemical general formula of the positive electrode active material is Na [alpha] MnxNiyCozO2, 0.4 < = x < = 0.7, 0.1 < = y < = 0.5, 0.1 < = z < = 0.5, x + y + z = 1, and 0.67 < = alpha < = 1.2; the mass ratio of the metal ions to the positive electrode active material is (0.1-0.5): 100; the mass ratio of the metal oxide coating layer to the positive electrode active material is (0.5-3): 100; the metal ions are transition metal ions and / or alkaline earth metal ions; the metal oxide in the metal oxide coating layer is a transition metal oxide and / or an alkaline earth metal oxide. The positive electrode material is high in structural stability and low in dissolution of doped metal.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a positive electrode material and a preparation method thereof, and a sodium ion battery. Background Art

[0002] Lithium-ion batteries (LIBs) are widely used in portable electronic devices and new energy vehicles due to their excellent electrochemical performance. However, lithium is relatively abundant and unevenly distributed, resulting in high costs. Sodium-ion batteries (SIBs) operate on a similar principle to lithium-ion batteries. In contrast, sodium is more abundant and more widely distributed in the Earth's crust. In addition, aluminum foil can replace expensive copper foil as the current collector for the negative electrode of sodium-ion batteries. Therefore, compared with LIBs, SIBs have the advantage of low cost.

[0003] Among sodium-ion battery cathode materials, layered oxides offer high theoretical capacity and good electrical conductivity. However, during the sodium ion embedding and de-embedding process, these cathode materials suffer from issues such as the easy dissolution of doped metals and the side reactions of residual alkali and active substances on the surface with the electrolyte. This further accelerates the structural collapse of the cathode material, resulting in a reduced cycle life for sodium-ion batteries. Summary of the Invention

[0004] The main purpose of the present invention is to provide a positive electrode material and a preparation method thereof, and a sodium ion battery, so as to solve the problems in the prior art of positive electrode materials such as the dissolution of doped metals and the side reactions of surfactants with electrolytes, which lead to structural instability of the positive electrode materials.

[0005] In order to achieve the above object, according to one aspect of the present invention, a positive electrode material is provided, which comprises a positive electrode material core and a metal oxide coating layer coated on the surface of the positive electrode material core; the positive electrode material core comprises a positive electrode active material doped with metal ions, and the chemical formula of the positive electrode active material is Na α Mn x Ni y Co z O2, wherein 0.4≤x≤0.7, 0.1≤y≤0.5, 0.1≤z≤0.5, x+y+z=1, 0.67≤α≤1.2; the mass ratio of the metal ion to the positive electrode active material is 0.1-0.5:100; the mass ratio of the metal oxide coating layer to the positive electrode active material is 0.5-3:100; the metal ion is a transition metal ion and / or an alkaline earth metal ion; the transition metal ion is selected from Zr 4+ 、Ce 4+ and W 6+ Any one or more of; Alkaline earth metal ion is Al 3+; The metal oxide in the metal oxide coating layer is a transition metal oxide and / or an alkaline earth metal oxide; the transition metal oxide is selected from any one or more of ZrO2, CeO2 and WO3; the alkaline earth metal oxide is Al2O3.

[0006] Furthermore, the transition metal ion is Zr 4+ and / or W 6+ ; and / or, the mass ratio of metal ions to positive electrode active materials is 0.2 to 0.5:100.

[0007] Furthermore, the transition metal oxide is ZrO2 and / or WO3; and / or the mass ratio of the metal oxide coating layer to the positive electrode active material is 1 to 3:100, and / or the thickness of the metal oxide coating layer is 4 to 9 nm.

[0008] According to another aspect of the present invention, a method for preparing the above-mentioned positive electrode material is provided, the preparation method comprising: step S1, providing a positive electrode active material, the chemical formula of the positive electrode active material is Na α Mn x Ni y Co z O2; and step S2, performing a first mixing and a first sintering in sequence on a first raw material including a positive electrode active material and a metal oxide to obtain a positive electrode material; wherein the metal oxide is a transition metal oxide and / or an alkaline earth metal oxide; the transition metal oxide is selected from any one or more of ZrO2, CeO2 and WO3; and the alkaline earth metal oxide is Al2O3.

[0009] Furthermore, in the above step S2, the first raw material is first mixed using a first ball mill; wherein the rotation speed of the first ball mill is 100 to 2000 rpm, and / or the first mixing time is 1 to 24 hours, and / or the mass ratio of the first raw material to the grinding balls in the first ball mill is 1:20 to 70; and / or the mass ratio of the positive electrode active material to the metal oxide is 85 to 100:0.2 to 3.

[0010] Furthermore, in the above step S2, the heating rate of the first sintering is 1-5°C / min, and / or the temperature of the first sintering is 600-950°C, and / or the time of the first sintering is 6-12 hours.

[0011] Furthermore, in the above-mentioned step S1, the preparation method of the positive electrode active material includes: performing a second mixing and a second sintering in sequence on a second raw material including a nickel source, a cobalt source, a manganese source and a sodium salt to obtain a positive electrode active material; wherein, the second raw material is subjected to a second mixing by a second ball mill to obtain a precursor; the mass ratio of the second raw material to the grinding balls in the second ball mill is 1:1~75, and / or the rotation speed of the second ball mill is 300~1500rpm, and / or the second mixing time is 6~24h, and / or the particle size of the precursor is 1~15μm; and / or the heating rate of the second sintering is 1~5℃ / min, and / or the temperature of the second sintering is 600~950℃, and / or the time of the second sintering is 6~12h.

[0012] Furthermore, the nickel source is selected from any one or more of nickel oxide, nickel trioxide, nickel acetate and nickel acetate hydrate; and / or the cobalt source is selected from any one or more of cobalt oxide, cobalt trioxide, cobalt acetate and cobalt acetate hydrate; and / or the manganese source is selected from any one or more of manganese monoxide, manganese dioxide, manganese acetate and manganese acetate hydrate; and / or the sodium salt is selected from any one or more of sodium carbonate, sodium citrate, sodium acetate and sodium acetate hydrate.

[0013] According to another aspect of the present invention, a positive electrode is provided, comprising a current collector and a positive electrode active material layer, wherein the positive electrode active material layer is disposed on the surface of the current collector, and the positive electrode active material layer comprises the above-mentioned positive electrode material.

[0014] According to another aspect of the present invention, a sodium ion battery is provided, comprising a positive electrode, a negative electrode and a separator, wherein the positive electrode is the positive electrode described above.

[0015] Applying the technical solution of the present invention, the present application, on the one hand, can effectively reduce the dissolution of the doped metal by coating the metal oxide coating layer on the surface of the positive electrode material core, reduce the interfacial side reactions between the surfactant and the electrolyte, and thus stabilize the structure of the positive electrode material. On the other hand, the above-mentioned doped metal ions are preferably used to effectively broaden the transmission channel of the sodium ions, thereby improving the transmission kinetics of the sodium ions. In addition, the present application preferably contains the types of metal ions and metal oxides within the above-mentioned ranges, which can further improve the structural stability of the positive electrode material, thereby further improving the cycle life and rate performance of the sodium ion battery. If the mass content of the metal ions and the metal oxide coating layer is too large, in addition to being distributed in the bulk of the positive electrode active material and coated on the surface of the positive electrode active material particles, there will also be a certain degree of segregation, which can easily reduce the overall electrical performance of the positive electrode material, and because the conductivity of the transition metal oxide is not high, it is easy to cause the problem of reduced rate performance. If the mass content of the metal ions and the metal oxide coating layer is too low, it is insufficient to be fully distributed in the bulk of the positive electrode active material and fully covered on the surface of the positive electrode active material particles, thereby insufficient to improve the cycle performance and rate performance of the battery. Therefore, the present application preferably has the mass ratio of metal ions to positive electrode active materials and the mass ratio of metal oxide coating layer to positive electrode active materials in the above range, which helps to further reduce the dissolution of metal in the positive electrode material during the sodium ion deintercalation and intercalation process, reduce the occurrence of the above-mentioned side reactions and Jahn-Teller effect, and broaden the transmission channel of sodium ions, thereby further improving the structural stability of the positive electrode material, thereby improving the cycle life and rate performance of the sodium ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0017] Figure 1 The figure shows the change of the discharge specific capacity of the battery in Example 1 and Comparative Example 1 of the present application with the number of cycles;

[0018] Figure 2 The graph showing the change in discharge specific capacity of the batteries in Example 2 and Comparative Example 1 of the present application with the number of cycles is shown;

[0019] Figure 3 The figure shows the change of the discharge specific capacity of the battery in Example 3 and Comparative Example 1 of the present application with the number of cycles;

[0020] Figure 4 The figure shows the change of the discharge specific capacity of the battery in Example 1 and Comparative Example 1 of the present application with cycling at different rates;

[0021] Figure 5TEM images of the positive electrode materials in Example 1 and Comparative Example 1 of the present application are shown;

[0022] Figure 6 The XRD patterns of the positive electrode materials in Example 1 and Comparative Example 1 of the present application are shown.

[0023] The above drawings include the following reference numerals:

[0024] 1. Metal oxide coating. DETAILED DESCRIPTION

[0025] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0026] As analyzed in the background technology of this application, the positive electrode materials in the prior art have the problems of metal dissolution of doped metals and side reactions between surfactants and electrolytes, resulting in structural instability of the positive electrode materials. In order to solve the above problems, this application provides a positive electrode material, a preparation method thereof, and a sodium ion battery.

[0027] In a typical embodiment of the present application, a positive electrode material is provided, which includes a positive electrode material core and a metal oxide coating layer coated on the surface of the positive electrode material core; the positive electrode material core includes a metal ion-doped positive electrode active material, and the chemical formula of the positive electrode active material is Na α Mn x Ni y Co z O2, wherein 0.4≤x≤0.7, 0.1≤y≤0.5, 0.1≤z≤0.5, x+y+z=1, 0.67≤α≤1.2; the mass ratio of the metal ion to the positive electrode active material is 0.1-0.5:100; the mass ratio of the metal oxide coating layer to the positive electrode active material is 0.5-3:100; the metal ion is a transition metal ion and / or an alkaline earth metal ion; the transition metal ion is selected from Zr 4+ 、Ce 4+ and W 6+ Any one or more of; Alkaline earth metal ion is Al 3+ ; The metal oxide in the metal oxide coating layer is a transition metal oxide and / or an alkaline earth metal oxide; the transition metal oxide is selected from any one or more of ZrO2, CeO2 and WO3; the alkaline earth metal oxide is Al2O3.

[0028] On the one hand, the present application can effectively reduce the dissolution of the doped metal and reduce the interfacial side reactions between the surfactant and the electrolyte by coating the metal oxide coating layer on the surface of the positive electrode material core, thereby stabilizing the structure of the positive electrode material. On the other hand, the above-mentioned doped metal ions are preferably used to effectively broaden the transmission channel of the sodium ions, thereby improving the transmission kinetics of the sodium ions. In addition, the present application preferably contains the types of metal ions and metal oxides within the above ranges, which can further improve the structural stability of the positive electrode material, thereby further improving the cycle life and rate performance of the sodium ion battery. If the mass content of the metal ions and the metal oxide coating layer is too large, in addition to being distributed in the bulk of the positive electrode active material and coated on the surface of the positive electrode active material particles, there will also be a certain degree of segregation, which can easily reduce the comprehensive electrical properties of the positive electrode material, and because the conductivity of the transition metal oxide is not high, it is easy to cause the problem of reduced rate performance. If the mass content of the metal ions and the metal oxide coating layer is too low, it is not enough to be fully distributed in the bulk of the positive electrode active material and fully cover the surface of the positive electrode active material particles, thereby not enough to improve the cycle performance and rate performance of the battery. Therefore, the present application preferably has the mass ratio of metal ions to positive electrode active materials and the mass ratio of metal oxide coating layer to positive electrode active materials in the above range, which helps to further reduce the dissolution of metal in the positive electrode material during the sodium ion deintercalation and intercalation process, reduce the occurrence of the above-mentioned side reactions and Jahn-Teller effect, and broaden the transmission channel of sodium ions, thereby further improving the structural stability of the positive electrode material, thereby improving the cycle life and rate performance of the sodium ion battery.

[0029] Specifically, the transition metal ion W 6+ The effects of doping and WO3 oxide coating on the cathode material are as follows: 1) The bond energies of Ni-O, Co-O and Mn-O are 392kJ / mol, 368kJ / mol and 402kJ / mol respectively. 6+ When doped into the core phase of the cathode material, the bond energy of WO is 672kJ / mol, which is stronger, thus helping to make the peroxide (O2) in the cathode material n- More stable, and reduce the release of irreversible lattice oxygen, thereby improving the structural stability of the positive electrode material. 6+ The introduction of WO3 coating can broaden the transmission channel of sodium ions and improve the transmission kinetics of sodium ions. 2) WO3 coating helps to strengthen the binding of Mn-O bond, thus leading to the 4+The increase in Mn reduces the Jahn-Teller effect of Mn. The WO3 coating layer also helps to reduce the direct contact between the positive electrode material and the electrolyte, thereby reducing the occurrence of interfacial side reactions. At the same time, the positive electrode material reduces the dissolution of transition metals during the sodium ion deintercalation and intercalation process. 3) Compared with the positive electrode active material, WO3 has a higher Fermi level, which helps to provide electrons for the positive electrode active material to inhibit oxygen release. The effects of the other metal ions and metal oxides on layered oxide positive electrode materials are similar and will not be repeated here.

[0030] In addition, the mass ratio of metal ions to positive electrode active materials can be 0.1:100, 0.15:100, 0.2:100, 0.25:100, 0.3:100, 0.4:100 or 0.5:100. Of course, the mass ratio of metal ions to positive electrode active materials can be any point value within the above range, which will not be repeated here.

[0031] The mass ratio of the metal oxide coating layer to the positive electrode active material can be 0.5:100, 1:100, 1.5:100, 2:100, 2.5:100 or 3:100. Of course, the mass ratio of the metal ion to the positive electrode active material can be any point value within the above range, which will not be repeated here.

[0032] Preferably, α=1. For example, the molecular formula of the positive electrode active material is NaMn 0.6 Ni 0.2 Co 0.2 O2.

[0033] In order to further broaden the transmission channel of sodium ions and improve the structural stability of the positive electrode material, in one embodiment of the present application, the transition metal ion is Zr 4+ and / or W 6+ ; and / or, the mass ratio of transition metal ions to positive electrode active materials is 0.2 to 0.5:100; the transition metal oxide is ZrO2 and / or WO3; and / or, the mass ratio of the metal oxide coating layer to the positive electrode active material is 1 to 3:100, and / or, the thickness of the metal oxide coating layer is 4 to 9 nm.

[0034] The thickness of the metal oxide coating layer is preferably within the above range, which helps to better protect the core of the positive electrode material, thereby reducing the dissolution of metal in the core of the positive electrode material and the side reactions between the surfactant and the electrolyte.

[0035] In another typical embodiment of the present application, a method for preparing the above-mentioned positive electrode material is provided, the preparation method comprising: step S1, providing a positive electrode active material, the chemical formula of the positive electrode active material is Na α Mn x Niy Co z O2; and step S2, a first raw material including a positive electrode active material and a metal oxide is sequentially mixed and sintered for a first time to obtain a positive electrode material; wherein the metal oxide is a transition metal oxide and / or an alkaline earth metal oxide; the transition metal oxide is selected from any one or more of ZrO2, CeO2 and WO3; the alkaline earth metal oxide Al2O3.

[0036] The present application introduces the above-mentioned types of metal oxides in step S2, so that metal ions can be doped into the bulk phase of the positive electrode active material to form a core of the positive electrode material, and the metal oxide forms a coating layer on the outer surface of the core of the positive electrode material, thereby further reducing the dissolution of metal in the positive electrode material during the sodium ion deintercalation and intercalation process, reducing the occurrence of interfacial side reactions and Jahn-Teller effect between the surfactant and the electrolyte, and broadening the transmission channel of sodium ions, thereby further improving the structural stability of the positive electrode material and enhancing the cycle life and rate performance of the sodium ion battery.

[0037] In one embodiment of the present application, in the above step S2, the first raw material is first mixed using a first ball mill; wherein the rotation speed of the first ball mill is 100 to 2000 rpm, and / or the first mixing time is 1 to 24 hours, and / or the mass ratio of the first raw material to the grinding balls in the first ball mill is 1:20 to 70; and / or the mass ratio of the positive electrode active material to the metal oxide is 85 to 100:0.2 to 3.

[0038] The first raw material is preferably first ball-milled in a planetary ball mill. The rotational speed, mixing time, and mass ratio of the first raw material to the grinding balls in the first ball mill are preferably controlled within the aforementioned ranges. This facilitates thorough mixing of the positive electrode active material powder and the transition metal oxide powder, and imparts a certain surface energy to the mixed powder, thereby promoting subsequent solid-phase first sintering. The mass ratio of the positive electrode active material to the metal oxide is preferably controlled within the aforementioned range to facilitate better doping of metal ions into the bulk of the positive electrode active material, forming a core of the positive electrode material, and forming a metal oxide coating on the outer surface of the core of the positive electrode material.

[0039] In one embodiment of the present application, in the above step S2, the heating rate of the first sintering is 1-5°C / min, and / or the temperature of the first sintering is 600-950°C, and / or the time of the first sintering is 6-12h.

[0040] It is preferred to control the heating rate, temperature and time of the first sintering within the above range, which helps to better dope the transition metal ions into the bulk phase of the positive electrode active material to form the core of the positive electrode material, and the transition metal oxide is better coated on the outer surface of the core of the positive electrode material, thereby reducing the dissolution of the transition metal and the occurrence of the above-mentioned side reactions.

[0041] In one embodiment of the present application, in the above-mentioned step S1, the preparation method of the positive electrode active material includes: performing a second mixing and a second sintering on a second raw material including a nickel source, a cobalt source, a manganese source and a sodium salt in sequence to obtain a positive electrode active material; wherein, the second raw material is subjected to a second mixing by a second ball mill to obtain a precursor; wherein, the mass ratio of the second raw material to the grinding balls in the second ball mill is 1 to 75:1, and / or, the rotation speed of the second ball mill is 300 to 1500 rpm, preferably 600 rpm, and / or, the second mixing time is 6 to 24 h, preferably 8 h, and / or, the particle size of the precursor is 1 to 15 μm, preferably 1 to 10 μm; and / or, the heating rate of the second sintering is 1 to 5°C / min, and / or, the temperature of the second sintering is 600 to 950°C, and / or, the time of the second sintering is 6 to 12 h.

[0042] It is preferred to control the mass ratio of the second raw material to the grinding balls in the second ball mill within the above range, which helps to improve the mixing effect of the second mixing and helps to give the second raw material powder sufficient surface energy in the second ball mill, thereby promoting the subsequent solid-phase second sintering reaction.

[0043] The second ball milling speed is preferably controlled within the above range to improve mixing efficiency. The second ball milling time is preferably controlled within the above range to help achieve sufficient mixing and improve production efficiency.

[0044] If the precursor powder's particle size is too coarse, it will be difficult to form the positive electrode active material during the subsequent second sintering process. If the precursor powder's particle size is too fine, it will easily agglomerate, affecting uniformity. Therefore, it is preferable to control the precursor powder's particle size within the above range to facilitate the subsequent second sintering process and better produce the positive electrode active material.

[0045] In addition, in order to further promote the subsequent solid-phase reaction between the positive electrode active material and the transition metal oxide, it is preferred to further crush and sieve after the second sintering to obtain a powder with a target particle size.

[0046] The second sintering is preferably performed in a muffle furnace under an air atmosphere. The heating rate during the second sintering is preferably controlled within the aforementioned range to facilitate the sufficient release of byproducts produced by the thermal decomposition reaction of the sodium salt. The temperature and time during the second sintering are preferably controlled within the aforementioned range to facilitate grain growth and prevent abnormal growth.

[0047] In order to better form a structurally stable positive electrode active material, in one embodiment of the present application, the nickel source is selected from any one or more of nickel oxide, nickel trioxide, nickel acetate and nickel acetate hydrate, preferably nickel oxide; and / or, the cobalt source is selected from any one or more of cobalt oxide, cobalt trioxide, cobalt acetate and cobalt acetate hydrate, preferably cobalt oxide; and / or, the manganese source is selected from any one or more of manganese monoxide, manganese dioxide, manganese acetate and manganese acetate hydrate, preferably manganese monoxide; and / or, the sodium salt is selected from any one or more of sodium carbonate, sodium citrate, sodium acetate and sodium acetate hydrate, preferably sodium carbonate, and more preferably anhydrous sodium carbonate.

[0048] In another typical embodiment of the present application, a positive electrode is provided, including a current collector and a positive electrode active material layer, wherein the positive electrode active material layer is arranged on the surface of the current collector, and the positive electrode active material layer includes the above-mentioned positive electrode material.

[0049] The positive electrode comprising the positive electrode material has good electrochemical performance and cycle life.

[0050] Furthermore, the positive electrode active material layer further comprises a conductive agent and a binder, wherein the conductive agent can improve its conductivity, and the binder helps to improve the bonding strength between the positive electrode active material layer, the current collector, and the positive electrode particles.

[0051] Specifically, the conductive agent is selected from any one or more of conductive carbon black, conductive graphite, carbon nanotubes, carbon fibers, and graphene. The binder is selected from any one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), sodium alginate (SA), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR).

[0052] The above-mentioned types of binders are preferred, which can help to further improve the interface contact performance between the positive electrode active material layer and the current collector and the positive electrode active particles, thereby improving the overall performance of the battery.

[0053] Specifically, PVDF has good adhesion and chemical corrosion resistance, can make the motor materials bond into a film under high temperature and high pressure conditions, and ensure good interface contact between the electrode active material and the current collector and between the active particles, thereby improving the performance and cycle life of the battery.

[0054] PTFE has excellent chemical stability, corrosion resistance, and high adhesion. It can efficiently and evenly bond active material particles together to increase the contact area between the active material and the current collector, thereby improving the utilization rate of the active material. At the same time, it can also reduce the interfacial impedance between the current collector and the positive electrode material, thereby improving the electrochemical performance of the battery. In addition, PTFE acts as a conductive network, combining the active material and the electrolyte to promote ion transport.

[0055] SA molecules act as adhesives in the mixed liquid slurry to form a polymer network structure. Through the cross-linking and aggregation of sodium alginate molecules and active materials, the active particles are cross-linked with the active particles and the active particles are cross-linked with SA, which plays a good connecting role, thereby effectively suppressing the phenomenon of electrode powder loss.

[0056] The functions of CMC are as follows: 1) It has a dispersing effect on active materials and conductive agents; 2) It has a thickening, anti-settling and auxiliary bonding effect on the slurry; 3) It stabilizes the processing performance of the electrode, thereby improving the peel strength of the electrode sheet, etc.

[0057] SBR binder can provide bonding force between active material particles and between the active material layer and the current collector, thereby helping to improve the battery's kinetic performance and cycle stability and reduce impedance.

[0058] There are no particular limitations on the method for forming a positive electrode active material layer on the surface of a current collector. For example, the method comprises dispersing 50 to 150 parts by mass of the above-mentioned positive electrode material, 1 to 15 parts by mass of a conductive agent, and 1 to 15 parts by mass of a binder in a solvent to form a positive electrode slurry, and then coating the positive electrode slurry on the surface of the current collector to obtain a positive electrode. The solvent is selected from any one or more of water, methanol, ethanol, and N-methylpyrrolidone.

[0059] In another typical embodiment of the present application, a sodium ion battery is provided, comprising a positive electrode, a negative electrode and a separator, wherein the positive electrode is the positive electrode described above.

[0060] The sodium ion battery including the above-mentioned positive electrode has good cycle life, rate performance and safety performance.

[0061] The beneficial effects of the present application will be further illustrated below with reference to examples.

[0062] Example 1

[0063] Weigh 5.496g of sodium acetate, 1.49g of nickel oxide, 1.50g of cobalt oxide, and 4.26g of manganese oxide to prepare the second raw material for the second mixing. Specifically, place the second raw material in a zirconia ball mill, add zirconia balls, and perform the second ball milling at 600rpm for 8h to obtain a precursor. The mass ratio of the second raw material to the zirconia balls is 1:6. The precursor is placed in a muffle furnace and heated to 900℃ at 3℃ / min in an air atmosphere for 12h for the second sintering, and then cooled to obtain the positive electrode active material Na 0.67 Mn 0.6 Ni 0.2 Co 0.2 O2(MNC).

[0064] The positive electrode active material and WO3 were prepared into the first raw material in a mass ratio of 1:0.022 and mixed for 8 hours in a planetary ball mill at 600 rpm. Ethanol was added as a dispersant. The mass ratio of the first raw material to the grinding balls in the first ball mill was 1:20. The material was then transferred to a muffle furnace and heated to 900°C at 3°C / min in an air atmosphere and kept at this temperature for 12 hours for the first sintering to obtain the positive electrode material WO3@MNC-W. 6+ The mass ratio of the metal oxide (WO3) coating layer to the positive electrode active material MNC is 0.2:100, the mass ratio of the metal oxide (WO3) coating layer to the positive electrode active material MNC is 2:100, and the thickness of the metal oxide (WO3) coating layer is 6 nm.

[0065] Preparation of button cells: 0.50 g of positive electrode material, 0.026 g of acetylene black and 0.026 g of PVDF were dissolved in NMP solvent and mixed evenly, coated on an aluminum foil with a thickness of 25 μm, dried at 90 ° C for 1 h, and vacuum dried at 100 ° C for 12 h to make a circular positive electrode sheet with a diameter of 1.2 cm. A metal sodium sheet was used as the negative electrode of the battery. 1.0 mol / L NaPF6, PC and EC with a volume ratio of 1:1 and FEC with a mass concentration of 5.0% electrolyte were added dropwise, and 1.5 g was added. The cells were assembled into 2032 button cells in an argon-filled glove box, which were recorded as WO3@MNC-W batteries.

[0066] Example 2

[0067] The difference from Example 1 is that the positive electrode active material Na 0.67 Mn 0.6 Ni 0.2 Co 0.2 O2(MNC) and ZrO2 were prepared as the first raw material in a mass ratio of 1:0.022 and mixed by the first ball milling in a planetary ball mill, with ethanol added as a dispersant. The raw material was then transferred to a muffle furnace and heated to 900°C at a rate of 3°C / min in an air atmosphere and kept at that temperature for 12 hours for the first sintering to obtain the positive electrode material and ZrO2@MNC-Zr battery. 4+ The mass ratio of the metal oxide (ZrO2) coating layer to the positive electrode active material MNC is 0.2:100, the mass ratio of the metal oxide (ZrO2) coating layer to the positive electrode active material MNC is 2:100, and the thickness of the metal oxide (ZrO2) coating layer is 6 nm.

[0068] Example 3

[0069] The difference from Example 1 is that the positive electrode active material Na 0.67 Mn 0.6 Ni 0.2 Co 0.2O2(MNC) and CeO2 were prepared as the first raw material in a mass ratio of 1:0.022 and mixed by the first ball milling in a planetary ball mill, with ethanol added as a dispersant. The raw material was then transferred to a muffle furnace and heated to 900°C at a rate of 3°C / min in an air atmosphere and kept at that temperature for 12 hours for the first sintering to obtain the positive electrode material and CeO2@MNC-Ce battery. 4+ The mass ratio of the transition metal oxide (CeO2) coating layer to the positive electrode active material MNC is 0.2:100, the mass ratio of the transition metal oxide (CeO2) coating layer to the positive electrode active material MNC is 2:100, and the thickness of the transition metal oxide (CeO2) coating layer is 6 nm.

[0070] Example 4

[0071] The difference from Example 1 is that in the positive electrode material, the transition metal ion W 6+ The mass ratio of the metal oxide (WO3) coating layer to the positive electrode active material MNC is 0.1:100, and the mass ratio of the metal oxide (WO3) coating layer to the positive electrode active material MNC is 1:100. Finally, a WO3@MNC-W battery is obtained, and the thickness of the transition metal oxide (WO3) coating layer is 4 nm.

[0072] Example 5

[0073] The difference from Example 1 is that in the positive electrode material, the transition metal ion W 6+ The mass ratio of the transition metal oxide (WO3) coating layer to the positive electrode active material MNC is 0.5:100, and the mass ratio of the transition metal oxide (WO3) coating layer to the positive electrode active material MNC is 0.5:100. Finally, a WO3@MNC-W battery is obtained, and the thickness of the transition metal oxide (WO3) coating layer is 9 nm.

[0074] Example 6

[0075] The difference from Example 1 is that the heating rate of the first sintering is 5°C / min, the temperature of the first sintering is 600°C, and the time of the first sintering is 12h to obtain the positive electrode material and finally obtain the WO3@MNC-W battery.

[0076] Example 7

[0077] The difference from Example 1 is that the heating rate of the first sintering is 8°C / min, the temperature of the first sintering is 550°C, and the time of the first sintering is 15h to obtain the positive electrode material and finally obtain the WO3@MNC-W battery.

[0078] Example 8

[0079] The difference from Example 1 is that the rotation speed of the first ball mill is 2000 rpm, the mass ratio of the first raw material to the grinding balls is 1:70, and finally the positive electrode material and WO3@MNC-W battery are obtained.

[0080] Example 9

[0081] The difference from Example 1 is that the rotation speed of the first ball mill is 2500 rpm, the mass ratio of the first raw material to the grinding balls is 1:15, and finally the positive electrode material and WO3@MNC-W battery are obtained.

[0082] Example 10

[0083] The difference from Example 1 is that the positive electrode active material Na 0.67 Mn 0.6 Ni 0.2 Co 0.2 O2(MNC) and alkaline earth metal oxide (Al2O3) were prepared as the first raw material in a mass ratio of 1:0.022 and mixed by the first ball milling in a planetary ball mill, with ethanol added as a dispersant. The raw material was then transferred to a muffle furnace and heated to 900°C at a rate of 3°C / min in an air atmosphere and kept at that temperature for 12 hours for the first sintering to obtain the positive electrode material and Al2O3@MNC-Zr battery. 4 + The mass ratio of the metal oxide (Al2O3) coating layer to the positive electrode active material MNC is 0.2:100, the mass ratio of the metal oxide (Al2O3) coating layer to the positive electrode active material MNC is 2:100, and the thickness of the metal oxide (Al2O3) coating layer is 6 nm.

[0084] Comparative Example 1

[0085] The difference from Example 1 is that the positive electrode material is the positive electrode active material Na 0.67 Mn 0.6 Ni 0.2 Co 0.2 O2, the battery is MNC battery.

[0086] Comparative Example 2

[0087] The difference from Example 1 is that in the positive electrode material, the transition metal ion W 6+ The mass ratio of WO3 to the positive electrode active material MNC is 1:100, and the WO3@MNC battery is finally obtained.

[0088] Comparative Example 3

[0089] The difference from Example 1 is that the mass ratio of the metal oxide coating layer to the positive electrode active material in the positive electrode material is 5:100, and an MNC-W battery is finally obtained.

[0090] Test method: The constant current and constant voltage method is used for testing, and the voltage range is 1.5~3.9V.

[0091] Battery cycle performance test: First, three cycles at 0.1C, then 100 cycles at 1C. Capacity retention = 1C discharge capacity at that cycle / initial discharge capacity.

[0092] Battery rate performance test: first cycle 0.2C for two cycles, then cycle 1C, 2C, 5C, 8C and 1C for 5 cycles each.

[0093] The batteries of the above examples and comparative examples were subjected to performance tests, and the test results are shown in Table 1.

[0094] Table 1

[0095]

[0096]

[0097] in, Figure 1 The discharge capacity of the battery in Example 1 and Comparative Example 1 changes with the number of cycles. Figure 1 As can be seen, the MNC battery in Comparative Example 1 has a 0.1C first-cycle discharge capacity of 135.8 mAh / g, a 1C first-cycle discharge capacity of 128.9 mAh / g, and a 100-cycle residual discharge capacity of 107.1 mAh / g. Calculated, the 1C discharge capacity retention rate is 83.1%. The WO3@MNC-W battery in Example 1 has a 0.1C first-cycle discharge capacity of 139.6 mAh / g and a 1C first-cycle discharge capacity of 134.8 mAh / g. Calculated, the 1C discharge capacity retention rate is 93.6%.

[0098] Figure 2 The discharge capacity of the battery in Example 2 and Comparative Example 1 changes with the number of cycles. Figure 2 As can be seen from the graph, the ZrO2@MNC-Zr battery of Example 2 has a 0.1C first-cycle discharge capacity of 138.9 mAh / g and a 1C first-cycle discharge capacity of 133.6 mAh / g. Calculation shows that the 1C discharge capacity retention rate is 92.5%.

[0099] Figure 3 The discharge capacity of the battery in Example 3 and Comparative Example 1 varies with the number of cycles. Figure 3 As can be seen from the graph, the 0.1C first-cycle discharge capacity of the CeO2@MNC-Ce battery of Example 3 is 137.6 mAh / g, and the 1C first-cycle discharge capacity is 133.2 mAh / g. Calculation shows that the 1C discharge capacity retention rate is 92.4%.

[0100] It can be seen that in terms of cycle performance, compared with the uncoated MNC positive electrode material, the positive electrode material of the present application shows more excellent cycle performance.

[0101] Figure 4 The graph of the discharge capacity of the battery in Example 1 and Comparative Example 1 as the discharge capacity changes with different cycle rates is shown in FIG. Figure 4 It can be seen that the MNC battery of Comparative Example 1 and the WO3@MNC-W battery of Example 1 have discharge specific capacities of 134.9 mAh / g and 138.2 mAh / g at 0.2C, respectively. As the rate continues to increase, the WO3@MNC-W battery has a higher discharge specific capacity at large current. At an 8C rate, the WO3@MNC-W battery has a discharge specific capacity of 95.2 mAh / g, which is much higher than the 85.4 mAh / g of the MNC battery. Secondly, after a long period of high-rate cycling and returning to a 1C rate, compared with the MNC battery, the WO3@MNC-W battery of Example 1 has better discharge specific capacity reproducibility and reversibility.

[0102] Figure 5 is a TEM image of the positive electrode material in Example 1 and Comparative Example 1, from Figure 5 It can be seen that the positive electrode material of comparative example 1 has no coating layer, and the surface of the core of the positive electrode material of embodiment 1 is evenly coated with a metal oxide coating layer 1, and the thickness of the metal oxide coating layer is 6 nm.

[0103] Figure 6 is the XRD pattern of the positive electrode materials in Example 1 and Comparative Example 1, Figure 6 It can be seen from the figure that the transition metal ions of Example 1 have been doped into the bulk of the positive electrode active material, and the 003 peak shifts to the left, that is, the sodium ion transmission channel becomes wider.

[0104] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0105] On the one hand, the present application can effectively reduce the dissolution of the doped metal and reduce the interfacial side reactions between the surfactant and the electrolyte by coating the metal oxide coating layer on the surface of the positive electrode material core, thereby stabilizing the structure of the positive electrode material. On the other hand, the above-mentioned doped metal ions are preferably used to effectively broaden the transmission channel of the sodium ions, thereby improving the transmission kinetics of the sodium ions. In addition, the present application preferably contains the types of metal ions and metal oxides within the above ranges, which can further improve the structural stability of the positive electrode material, thereby further improving the cycle life and rate performance of the sodium ion battery. If the mass content of the metal ions and the metal oxide coating layer is too large, in addition to being distributed in the bulk of the positive electrode active material and coated on the surface of the positive electrode active material particles, there will also be a certain degree of segregation, which can easily reduce the comprehensive electrical properties of the positive electrode material, and because the conductivity of the transition metal oxide is not high, it is easy to cause the problem of reduced rate performance. If the mass content of the metal ions and the metal oxide coating layer is too low, it is not enough to be fully distributed in the bulk of the positive electrode active material and fully cover the surface of the positive electrode active material particles, thereby not enough to improve the cycle performance and rate performance of the battery. Therefore, the present application preferably has the mass ratio of metal ions to positive electrode active materials and the mass ratio of metal oxide coating layer to positive electrode active materials in the above range, which helps to further reduce the dissolution of metal in the positive electrode material during the sodium ion deintercalation and intercalation process, reduce the occurrence of the above-mentioned side reactions and Jahn-Teller effect, and broaden the transmission channel of sodium ions, thereby further improving the structural stability of the positive electrode material, thereby improving the cycle life and rate performance of the sodium ion battery.

[0106] The above are merely embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A positive electrode material, characterized in that The positive electrode material comprises a positive electrode material core and a metal oxide coating layer coated on the outer surface of the positive electrode material core; The positive electrode material core includes a positive electrode active material doped with metal ions, and the chemical formula of the positive electrode active material is Na α Mn x Ni y Co z O2, where 0.4≤x≤0.7, 0.1≤y≤0.5, 0.1≤z≤0.5, x+y+z=1, 0.67≤α≤1.2; The mass ratio of the metal ion to the positive electrode active material is 0.1 to 0.5:100; the mass ratio of the metal oxide coating layer to the positive electrode active material is 0.5 to 3:100; The metal ions are transition metal ions and / or alkaline earth metal ions; the transition metal ions are selected from Zr 4+ 、Ce 4+ and W 6+ Any one or more of; the alkaline earth metal ion is Al 3+ ; The metal oxide in the metal oxide coating layer is a transition metal oxide and / or an alkaline earth metal oxide; the transition metal oxide is selected from any one or more of ZrO2, CeO2 and WO3; and the alkaline earth metal oxide is Al2O3.

2. The positive electrode material according to claim 1, characterized in that The transition metal ion is Zr 4+ and / or W 6+ ; and / or, the mass ratio of the metal ions to the positive electrode active material is 0.2 to 0.5:

100.

3. The positive electrode material according to claim 1 or 2, characterized in that The transition metal oxide is ZrO2 and / or WO3; and / or the mass ratio of the metal oxide coating layer to the positive electrode active material is 1 to 3:100, and / or the thickness of the metal oxide coating layer is 4 to 9 nm.

4. A method for preparing the positive electrode material according to any one of claims 1 to 3, characterized in that: The preparation method comprises: Step S1, providing a positive electrode active material, the chemical formula of the positive electrode active material is Na α Mn x Ni y Co z O2; and Step S2, sequentially performing a first mixing and a first sintering on a first raw material including the positive electrode active material and the metal oxide to obtain the positive electrode material; Wherein, the metal oxide is a transition metal oxide and / or an alkaline earth metal oxide; the transition metal oxide is selected from any one or more of ZrO2, CeO2 and WO3; and the alkaline earth metal oxide is Al2O3.

5. The preparation method according to claim 4, characterized in that In the step S2, the first raw material is subjected to the first mixing by a first ball mill; wherein the rotation speed of the first ball mill is 100 to 2000 rpm, and / or the first mixing time is 1 to 24 hours, and / or the mass ratio of the first raw material to the grinding balls in the first ball mill is 1:20 to 70; And / or, the mass ratio of the positive electrode active material to the metal oxide is 85-100:0.2-3.

6. The preparation method according to claim 4 or 5, characterized in that In the step S2, the heating rate of the first sintering is 1-5°C / min, and / or the temperature of the first sintering is 600-950°C, and / or the time of the first sintering is 6-12 hours.

7. The preparation method according to any one of claims 4 to 6, characterized in that In step S1, the method for preparing the positive electrode active material includes: sequentially performing a second mixing and a second sintering on a second raw material including a nickel source, a cobalt source, a manganese source and a sodium salt to obtain the positive electrode active material; wherein, the second raw material is subjected to the second mixing by a second ball mill to obtain a precursor; The mass ratio of the second raw material to the grinding balls in the second ball mill is 1:1-75, and / or the rotation speed of the second ball mill is 300-1500 rpm, and / or the second mixing time is 6-24 hours, and / or the particle size of the precursor is 1-15 μm; And / or, the heating rate of the second sintering is 1-5°C / min, and / or, the temperature of the second sintering is 600-950°C, and / or, the time of the second sintering is 6-12 hours.

8. The preparation method according to claim 7, characterized in that The nickel source is selected from any one or more of nickel oxide, nickel trioxide, nickel acetate and nickel acetate hydrate; and / or the cobalt source is selected from any one or more of cobalt oxide, cobalt trioxide, cobalt acetate and cobalt acetate hydrate; and / or the manganese source is selected from any one or more of manganese monoxide, manganese dioxide, manganese acetate and manganese acetate hydrate; and / or the sodium salt is selected from any one or more of sodium carbonate, sodium citrate, sodium acetate and sodium acetate hydrate.

9. A positive electrode comprising a current collector and a positive electrode active material layer, wherein the positive electrode active material layer is disposed on the surface of the current collector, characterized in that: The positive electrode active material layer includes the positive electrode material according to any one of claims 1 to 3.

10. A sodium ion battery comprising a positive electrode, a negative electrode and a separator, characterized in that: The positive electrode is the positive electrode according to claim 9.

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