A mischphase lamellar cathode material, its preparation and use

Abstract

The application provides a mixed-phase layered positive electrode material, a preparation method and application thereof, the crystal structure of the mixed-phase layered positive electrode material comprises P-phase layers and O-phase layers which are stacked in sequence; the composition of the P-phase layer is sodium lithium cobalt manganese oxide, and the composition of the O-phase layer is lithium-rich lithium manganese oxide. The mixed-phase layered positive electrode material provided by the application, in which the P-phase and the O-phase are alternately stacked, effectively inhibits the phase change of the positive electrode material in the charging and discharging process of the sodium ion battery through the synergistic effect between different crystal phases, improves the structural stability of the layered positive electrode material, so that the sodium ion battery prepared has higher capacity performance, excellent rate performance and cycle performance. In addition, the mixed-phase layered positive electrode material provided by the application has a higher sodium content, which expands the possibility of practical application of the layered positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, and particularly relates to a mixed-phase layered cathode material, its preparation method and application. Background Technology

[0002] Sodium is not only abundant in resources but also widely distributed. Compared to lithium-ion batteries, sodium-ion batteries have a cost advantage and greater potential for sustainable development. However, the capacity of existing sodium-ion battery cathode materials is far lower than that of the anode, making it crucial to improve the performance of cathode materials to increase the energy density of sodium-ion batteries. Layered cathode materials are a commonly used type of sodium battery cathode material, and based on their different structural types, they can be divided into O-type and P-type layered materials; when Na... + When Na occupies the interstices of octahedrons composed of transition metal elements, this structure is called an O (octahedral) type material. + When it occupies the gaps in a triangular prism composed of transition metal elements, this structure is called a P (prismatic) type material.

[0003] Currently, commonly used layered cathode materials are generally single-phase structures. During charging and discharging, due to the insertion and extraction of sodium ions, single-phase layered materials undergo significant volume expansion and contraction, and the transition metal layers experience interlayer slippage, leading to phase transitions. Therefore, single-phase layered cathode materials exhibit poor structural stability during battery charging and discharging, resulting in poor electrochemical performance of sodium-ion batteries.

[0004] CN113292113A discloses an O3-phase layered oxide cathode material for sodium-ion batteries and its preparation method. The preparation method includes: dissolving a transition metal salt and a soluble sodium salt in deionized water and mixing them to obtain a mixed salt solution; subjecting the mixed salt solution to spray pyrolysis to obtain a sodium-containing oxide precursor; pressing the sodium-containing oxide precursor into sheets and then performing high-temperature solid-state sintering to obtain the O3-phase layered oxide cathode material. The layered cathode material obtained using this preparation method still has a single-phase structure, which cannot solve the problem of easy interlayer slippage and poor structural stability of the cathode material during battery charging and discharging.

[0005] CN113135602A discloses a method for inducing the structural transformation of sodium-ion layered oxide cathode materials to the P2 phase. This method involves controlling the type and content of doped metal elements to reduce the average radius of the transition metal, thereby regulating the structure of the sodium-ion layered oxide cathode material and promoting its transformation to the P2 phase. While this method addresses the problem of synthesized layered cathode materials often accompanied by a series of impurities by reducing the average radius of the transition metal M, and improves the single-phase purity of the layered cathode material, it still cannot solve the technical problem of structural instability of single-phase layered cathode materials during charge and discharge, leading to low performance of sodium-ion batteries.

[0006] CN113140724A discloses a method for synthesizing sodium manganate, a sodium-ion battery cathode material with a tunneled layered symbiotic phase. The method employs a solid-state sintering process to synthesize sodium-ion cathode material Na with a tunneled phase and a layered phase symbiotic structure in one step. 0.5 MnO 2-x F x In this cathode material, the tunnel phase particles are prismatic, while the layered phase particles are irregular spherical, which is equivalent to a physical mixture of two different morphologies. Although this improves the electrochemical performance of sodium-ion batteries to some extent, it still suffers from the instability of single-phase layered cathode materials.

[0007] Therefore, developing mixed-phase layered cathode materials with two or more crystal structures can combine the advantages of different materials and the synergistic effect between different crystal phases, which is crucial for improving the structural stability of cathode materials during charge and discharge processes and the electrochemical performance of sodium-ion batteries. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a mixed-phase layered cathode material, its preparation method, and its applications. The mixed-phase layered cathode material prepared by this invention, with alternating P and O phases, effectively suppresses phase transitions in the cathode material during the charging and discharging process of sodium-ion batteries, improving the structural stability of the layered cathode material. This results in sodium-ion batteries with high capacity performance, excellent rate performance, and good cycle performance. Furthermore, the mixed-phase layered cathode material provided by this invention has a high sodium content, expanding the possibilities for practical applications of layered cathode materials.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a mixed-phase layered cathode material, wherein the crystal structure of the mixed-phase layered cathode material comprises sequentially stacked P-phase layers and O-phase layers.

[0011] The P phase layer is composed of sodium lithium cobalt manganese oxide, and the O phase layer is composed of lithium-rich manganese oxide.

[0012] The mixed-phase layered cathode material with alternating P and O phases provided by this invention effectively suppresses phase transitions in the cathode material during the charge and discharge process of sodium-ion batteries through the synergistic effect between different crystalline phases, thereby improving the structural stability of the layered cathode material. This results in sodium-ion batteries with high capacity performance, excellent rate performance, and good cycle performance. Furthermore, the mixed-phase layered cathode material provided by this invention has a high sodium content (sodium atomic percentage reaches 0.8%), expanding the possibilities for practical applications of layered cathode materials.

[0013] Furthermore, the chemical formula of lithium manganese oxide rich in lithium is Li₂MnO₃, and the overall chemical formula of the mixed-phase layered cathode material is Na. x Li M Co y Mn z O2, where x ≥ 0.67, 0 < y < 1, 0 < z < 1. Through the interaction between the transition metal element and the doped lithium element, a mixed-phase structure of the layered cathode material is formed. Simultaneously, the doping of lithium element further improves the electrochemical cycling performance of the cathode material. In this invention, the mixed-phase cathode material Na... x Li M Co y Mn z The range of M in O2 is 0.15 to 0.25, preferably 0.2.

[0014] As a preferred embodiment of the present invention, the P phase layer is of the P2 crystal form.

[0015] Preferably, the O phase layer is of the O3 crystal form.

[0016] As a preferred embodiment of the present invention, the mixed-phase layered cathode material is in the form of bulk particles.

[0017] Preferably, the particle size of the mixed-phase layered cathode material is 0.2 to 5 μm, for example, it can be 0.2 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0018] In a second aspect, the present invention provides a method for preparing the mixed-phase layered cathode material described in the first aspect, the method comprising:

[0019] A precursor is obtained by reacting a transition metal salt solution with a coprecipitant solution. The precursor, lithium salt, and sodium salt are then mixed and subjected to a first sintering and a second sintering to obtain the mixed-phase layered cathode material.

[0020] In this invention, the precursor is first sintered in a muffle furnace, followed by a second sintering in a tube furnace, during which a change in crystal structure occurs. Under high-temperature conditions, the precursor first decomposes to generate corresponding metal oxides. These metal oxides then react with sodium and lithium salts during high-temperature sintering, subsequently leading to nucleation and growth of crystal particles.

[0021] As a preferred embodiment of the present invention, the method for preparing the precursor includes:

[0022] A transition metal salt is dissolved in a first solvent to form a transition metal salt solution, and a coprecipitant is dissolved in a second solvent to form a coprecipitant solution. Subsequently, the transition metal salt solution and the coprecipitant solution are added dropwise to a third solvent to coprecipitate, and the precursor is obtained after drying.

[0023] This invention employs a coprecipitation method to prepare precursors. The coprecipitation method is simple to operate, consumes less energy, and can obtain relatively pure cathode materials. Furthermore, by controlling the stirring rate and time during the precursor preparation process, the metal precipitate salt can achieve an atomic-level degree of mixing. At the same time, the particle size and morphology of the precursor prepared by the coprecipitation method are controllable, and the uniformity of the target product particles can be effectively guaranteed.

[0024] Preferably, the transition metal salt includes cobalt salt and manganese salt.

[0025] Preferably, the cobalt salt includes any one of cobalt sulfate, cobalt nitrate, or cobalt acetate.

[0026] Preferably, the manganese salt includes any one of manganese sulfate, manganese nitrate, or manganese acetate.

[0027] Preferably, the coprecipitant comprises sodium oxalate or sodium hydroxide.

[0028] In this invention, the types of cobalt salts, manganese salts, and coprecipitants can be freely combined, as long as the coprecipitant can form a precipitate with the transition metal salts (cobalt salts and manganese salts).

[0029] Preferably, the first solvent, the second solvent, and the third solvent are all deionized water, and more preferably ultrapure water.

[0030] In this invention, the amount of the first solvent and the second solvent added is 500 mL, and the amount of the third solvent added is 50 mL.

[0031] Preferably, the coprecipitation is carried out under stirring.

[0032] Preferably, the stirring rate is 450 to 550 rpm / min, for example, it can be 450 rpm / min, 480 rpm / min, 500 rpm / min, 520 rpm / min or 550 rpm / min, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] Preferably, the dropping rate of both the transition metal salt solution and the coprecipitant solution into the third solvent is 4.5–5.5 mL / min, for example, it can be 4.5 mL / min, 4.6 mL / min, 4.7 mL / min, 4.8 mL / min, 4.9 mL / min, 5 mL / min, 5.1 mL / min, 5.2 mL / min, 5.3 mL / min, 5.4 mL / min or 5.5 mL / min, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0034] Preferably, the coprecipitation includes a first stage and a second stage.

[0035] Preferably, the temperature of the first stage is 65 to 75°C, for example, it can be 65°C, 66°C, 67°C, 68°C, 69°C, 70°C, 71°C, 72°C, 73°C, 74°C or 75°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0036] Preferably, the duration of the first stage is 0.5 to 1.5 hours, for example, it can be 0.5 hours, 0.6 hours, 0.7 hours, 0.8 hours, 0.9 hours, 1 hour, 1.1 hours, 1.2 hours, 1.3 hours, 1.4 hours or 1.5 hours, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0037] Preferably, the duration of the second stage is 2.5 to 3.5 hours, for example, it can be 2.5 hours, 2.6 hours, 2.7 hours, 2.8 hours, 2.9 hours, 3 hours, 3.1 hours, 3.2 hours, 3.3 hours, 3.4 hours or 3.5 hours, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0038] In this invention, the second stage of co-precipitation is carried out at room temperature. This is because the solubility of sodium oxalate is low at room temperature, and the concentration difference between the two solutions is large when co-precipitating at room temperature. Therefore, the two-stage reaction of heating first and then naturally cooling to room temperature can effectively avoid the problem of uneven precipitation caused by the difference in solubility.

[0039] Preferably, a separation process is further included between the co-precipitation and the drying.

[0040] Preferably, the separation process includes filtration or centrifugation.

[0041] As a preferred embodiment of the present invention, the lithium salt includes any one of lithium hydroxide, lithium oxide, or lithium carbonate.

[0042] Preferably, the sodium salt is sodium carbonate.

[0043] Preferably, the precursor, the lithium salt, and the sodium salt are mixed by any one of wet ball milling, sand milling, or grinding.

[0044] Preferably, the mixing time is 1 to 4 hours, for example, it can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0045] Preferably, the solvent added during the mixing process includes ethanol or deionized water.

[0046] Preferably, the rotational speeds of the wet ball milling, the sand milling, and the grinding are all ≥400 rpm / min, for example, 400 rpm / min, 380 rpm / min, 360 rpm / min, 340 rpm / min, 320 rpm / min, 300 rpm / min, 250 rpm / min, or 200 rpm / min, but are not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0047] Preferably, the mixture is dried and pressed once, followed by sintering once and sintering twice to obtain the mixed-phase layered cathode material.

[0048] Preferably, the pressure of the primary tablet compression is 5 to 10 MPa, for example, it can be 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa or 10 MPa, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0049] As a preferred technical solution of the present invention, the temperature of the first sintering is ≤500℃, for example, it can be 500℃, 450℃, 400℃, 350℃, 300℃, 250℃ or 200℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable; preferably, it is 300~500℃.

[0050] Preferably, the sintering time is 4.5 to 5.5 hours, for example, 4.5 hours, 4.6 hours, 4.7 hours, 4.8 hours, 4.9 hours, 5 hours, 5.1 hours, 5.2 hours, 5.3 hours, 5.4 hours or 5.5 hours, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0051] Preferably, after the first sintering, an intermediate is obtained, and the intermediate is subjected to cooling and secondary pressing in sequence before the second sintering is performed.

[0052] In this invention, the heating rate of a single sintering can be 2℃ / min, and the cooling process of a single sintering is natural cooling to room temperature.

[0053] Preferably, the pressure of the secondary tablet compression is 5 to 10 MPa, for example, it can be 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa or 10 MPa, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0054] As a preferred technical solution of the present invention, the heating rate of the secondary sintering is ≤5℃ / min, for example, it can be 5℃ / min, 4.5℃ / min, 4℃ / min, 3.5℃ / min, 3℃ / min, 2.5℃ / min, 2℃ / min or 1.5℃ / min, but is not limited to the listed values, other unlisted values ​​within this range are also applicable; preferably 2 to 4℃ / min.

[0055] This invention limits the heating rate of the secondary sintering to ≤5℃ / min. When the heating rate of the secondary sintering is greater than 5℃ / min, it will cause changes in the particle size and morphology of the cathode material. This is because changing the heating rate will affect the nucleation and growth of crystal particles. An excessively fast heating rate will lead to uneven heating, affecting the consistency of the cathode material obtained after sintering, and thus affecting the electrochemical performance of sodium-ion batteries.

[0056] Preferably, the final temperature of the secondary sintering is ≥700℃, for example, it can be 700℃, 720℃, 750℃, 780℃, 800℃, 820℃, 850℃, 880℃, 900℃, 920℃ or 950℃, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable; preferably 700~900℃.

[0057] This invention limits the final temperature of the secondary sintering to ≥700℃. When the final temperature of the secondary sintering is below 700℃, the performance of the cathode material will be poor. This is because the reaction and sintering of the precursor with lithium / sodium salt is a solid-state sintering. The ion diffusion rate is slow in the solid-state process, and a longer sintering time at a higher temperature is required.

[0058] At different secondary sintering final temperatures, this invention yielded mixed-phase layered cathode materials. During the secondary sintering process, the mixed-phase layered cathode materials underwent a crystal phase change from P3 / O3 to P2 / P3 / O3 to P2 / O3, ultimately forming a P2 / O3 mixed-phase layered cathode material. Furthermore, with increasing secondary sintering temperature, the particle size of the prepared mixed-phase cathode material gradually increased.

[0059] Preferably, the holding time for the secondary sintering is 11 to 13 hours, for example, 11 hours, 11.5 hours, 12 hours, 12.5 hours or 13 hours, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0060] Preferably, the secondary sintering is carried out in an oxygen atmosphere.

[0061] In this invention, the secondary sintering is carried out in a tube furnace, wherein the oxygen introduction rate is preferably 20 mL / min.

[0062] Preferably, the cooling process of the secondary sintering includes a primary cooling stage and a secondary cooling stage.

[0063] Preferably, the cooling rate of the first cooling stage is ≤2℃ / min, for example, it can be 2℃ / min, 1.8℃ / min, 1.6℃ / min, 1.4℃ / min, 1.2℃ / min, 1℃ / min, 0.8℃ / min or 0.6℃ / min, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable; preferably 1 to 2℃ / min.

[0064] The present invention limits the cooling rate of a single cooling stage to ≤2℃ / min. When the cooling rate of a single cooling stage is greater than 2℃ / min, it will cause changes in the mixed-phase structure. This is because an excessively fast cooling rate will affect the migration and rearrangement of ions.

[0065] Preferably, the final temperature of the first cooling stage is 75-85°C, for example, it can be 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, 84°C or 85°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0066] Preferably, the secondary cooling stage is natural cooling.

[0067] Preferably, after the secondary sintering, a secondary sintering product is obtained, and the secondary sintering product is successively ground and sieved to obtain the mixed-phase layered cathode material.

[0068] In this invention, the cooling process of secondary sintering is divided into two stages. The first cooling stage is carried out in a tube furnace, where the temperature is lowered to 75-85°C, and then the product is quickly transferred to a drying chamber for natural cooling to room temperature in the second cooling stage. If the temperature is lowered to room temperature in the first cooling stage, the secondary sintering product will absorb moisture severely. Therefore, in order to prevent the obtained secondary sintering product from absorbing moisture, the cooling process of secondary sintering in this invention is divided into two stages.

[0069] As a preferred embodiment of the present invention, the preparation method includes:

[0070] (1) The transition metal salt is dissolved in the first solvent to form a transition metal salt solution, and the coprecipitant is dissolved in the second solvent to form a coprecipitant solution. Then, under a stirring rate of 450-550 rpm / min, the transition metal salt solution and the coprecipitant solution are added dropwise to the third solvent at a rate of 4.5-5.5 mL / min. After keeping the temperature at 65-75℃ for 0.5-1.5 h, the mixture is stirred for 2.5-3.5 h, and then separated and dried to obtain the precursor.

[0071] (2) At a speed of ≥400 rpm / min, the lithium salt, sodium salt, solvent and the precursor obtained in step (1) are mixed for 1 to 4 hours, dried and then compressed once under a pressure of 5 to 10 MPa, followed by sintering at a temperature of ≤500℃ for 4.5 to 5.5 hours, and then naturally cooled to obtain the intermediate.

[0072] (3) Under a pressure of 5-10 MPa, the intermediate obtained in step (2) is subjected to secondary compression. Under an oxygen atmosphere, the temperature is increased to ≥700℃ at a heating rate of ≤5℃ / min for secondary sintering for 11-13h. Then, the temperature is decreased to 75-85℃ at a cooling rate of ≤2℃ / min. After natural cooling, the secondary sintered product is obtained. Then, the secondary sintered product is ground and sieved to obtain the mixed-phase layered cathode material.

[0073] In this invention, Na is used as a mixed-phase layered cathode material x Li M Co y Mn z Weigh each raw material according to the stoichiometric ratio of each element in O2.

[0074] Thirdly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte.

[0075] The positive electrode sheet includes the mixed-phase layered positive electrode material described in the first aspect.

[0076] The present invention provides a method for preparing a positive electrode sheet by mixing a mixed-phase positive electrode material, a conductive agent, and a binder. The positive electrode sheet, a separator, and a negative electrode sheet are then stacked sequentially, and an electrolyte is injected before assembling a Wei-2032 battery. The separator comprises glass cellulose, and the negative electrode sheet comprises metallic sodium or hard carbon.

[0077] Compared with the prior art, the present invention has the following beneficial effects:

[0078] This invention provides a mixed-phase layered cathode material, its preparation method, and its applications. The mixed-phase layered cathode material, with alternating layers of P and O phases, effectively suppresses phase transitions in the cathode material during the charge and discharge process of sodium-ion batteries through the synergistic effect between different crystalline phases, thus improving the structural stability of the layered cathode material. This results in sodium-ion batteries with high capacity performance, excellent rate performance, and good cycle performance. Furthermore, the mixed-phase layered cathode material provided by this invention has a high sodium content, expanding the possibilities for practical applications of layered cathode materials. Attached Figure Description

[0079] Figure 1 This is a scanning electron microscope image of the mixed-phase layered cathode material provided in Embodiment 1 of the present invention.

[0080] Figure 2 This is the lattice stripe pattern of the mixed-phase layered cathode material provided in Embodiment 1 of the present invention.

[0081] Figure 3 This is the elemental distribution diagram of the mixed-phase layered cathode material provided in Embodiment 1 of the present invention.

[0082] Figure 4 This is the XRD pattern of the mixed-phase layered cathode material provided in Embodiment 1 of the present invention.

[0083] Figure 5 This is a scanning electron microscope image of the mixed-phase layered cathode material provided in Embodiment 2 of the present invention.

[0084] Figure 6 This is the XRD pattern of the mixed-phase layered cathode material provided in Embodiment 2 of the present invention.

[0085] Figure 7 This is a scanning electron microscope image of the mixed-phase layered cathode material provided in Embodiment 3 of the present invention.

[0086] Figure 8 This is the XRD pattern of the mixed-phase layered cathode material provided in Embodiment 3 of the present invention.

[0087] Figure 9 This is a scanning electron microscope image of the mixed-phase layered cathode material provided in Embodiment 4 of the present invention.

[0088] Figure 10This is the XRD pattern of the mixed-phase layered cathode material provided in Embodiment 4 of the present invention.

[0089] Figure 11 This is a scanning electron microscope image of the mixed-phase layered cathode material provided in Embodiment 5 of the present invention.

[0090] Figure 12 This is the lattice stripe pattern of the mixed-phase layered cathode material provided in Embodiment 5 of the present invention.

[0091] Figure 13 This is the XRD pattern of the mixed-phase layered cathode material provided in Embodiment 5 of the present invention.

[0092] Figure 14 This is the elemental distribution diagram of the single-phase layered cathode material provided in Comparative Example 1 of the present invention.

[0093] Figure 15 This is the XRD pattern of the single-phase layered mixed-phase cathode material provided in Comparative Example 1 of the present invention.

[0094] Figure 16 These are rate performance diagrams of the mixed-phase layered cathode materials provided in Examples 1-7 of this invention.

[0095] Figure 17 These are cycle performance diagrams of the mixed-phase layered cathode materials provided in Examples 1-7 of this invention.

[0096] Figure 18 These are cycle performance diagrams of the mixed-phase layered cathode materials provided in Embodiments 1, 8, and 9 of this invention.

[0097] Figure 19 These are cycle performance diagrams of the cathode materials provided in Embodiment 1 and Comparative Example 1 of the present invention.

[0098] Figure 20 These are rate performance diagrams of the cathode materials provided in Embodiment 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0099] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0100] Example 1

[0101] This embodiment provides a method for preparing a mixed-phase layered cathode material, the method comprising:

[0102] (1) 7.025 g of CoSO4·7H2O and 12.675 g of MnSO4·H2O were dissolved in 500 mL of ultrapure water to form a transition metal salt solution, and sodium oxalate was dissolved in 500 mL of ultrapure water to form a coprecipitant solution. Then, under a stirring rate of 500 rpm / min, the transition metal salt solution and the coprecipitant solution were added dropwise to 50 mL of ultrapure water at a rate of 5 mL / min. After being kept at 70 °C for 1 h, the mixture was stirred at room temperature for 3 h. After filtration and drying, the precursor was obtained.

[0103] (2) At a speed of 400 rpm / min, 0.95 g of lithium carbonate, 5.56 g of sodium carbonate, the precursor obtained in step (1) and ethanol were wet ball-milled for 4 h. After drying, the mixture was compressed once under a pressure of 10 MPa. Then, it was sintered once at a heating rate of 2 °C / min to 500 °C for 5 h. After natural cooling, the intermediate was obtained.

[0104] (3) Under a pressure of 10 MPa, the intermediate obtained in step (2) was subjected to secondary tableting. At an oxygen injection rate of 20 mL / min, the temperature was increased to 900 °C at a heating rate of 2 °C / min for secondary sintering for 12 h. Subsequently, the temperature was decreased to 80 °C at a cooling rate of 2 °C / min, and after natural cooling, the secondary sintered product was obtained. The secondary sintered product was then ground and sieved to obtain the chemical formula Na. 0.8 Li 0.2 Co 0.2 Mn 0.6 O2 mixed-phase layered cathode materials, such as Figure 1 , Figure 2 , Figure 3 and Figure 4 As shown.

[0105] Example 2

[0106] The difference between this embodiment and Embodiment 1 is that the final temperature of the secondary sintering in step (3) is 850℃, while the remaining process parameters and operating steps are the same as in Embodiment 1. The scanning electron microscope image and XRD pattern of the mixed-phase cathode material obtained in this embodiment are shown below. Figure 5 and Figure 6 As shown.

[0107] Example 3

[0108] The difference between this embodiment and Embodiment 1 is that the final temperature of the secondary sintering in step (3) is 800℃, while the remaining process parameters and operating steps are the same as in Embodiment 1. The scanning electron microscope image and XRD pattern of the mixed-phase cathode material obtained in this embodiment are shown below. Figure 7 and Figure 8 As shown.

[0109] Example 4

[0110] The difference between this embodiment and Embodiment 1 is that the final temperature of the secondary sintering in step (3) is 750℃, while the remaining process parameters and operating steps are the same as in Embodiment 1. The scanning electron microscope image and XRD pattern of the mixed-phase cathode material obtained in this embodiment are shown below. Figure 9 and Figure 10 As shown.

[0111] Example 5

[0112] The difference between this embodiment and Embodiment 1 is that the final temperature of the secondary sintering in step (3) is 700℃, while the remaining process parameters and operating steps are the same as in Embodiment 1. The scanning electron microscope image and XRD pattern of the mixed-phase cathode material obtained in this embodiment are shown below. Figure 11 , Figure 12 and Figure 13 As shown.

[0113] Example 6

[0114] This embodiment provides a method for preparing a mixed-phase layered cathode material, the method comprising:

[0115] (1) 14.55g of Co(NO3)2·6H2O and 8.65g of C4H6MnO4 were dissolved in 500mL of ultrapure water to form a transition metal salt solution, and sodium hydroxide was dissolved in 500mL of ultrapure water to form a coprecipitant solution. Then, under a stirring rate of 450rpm / min, the transition metal salt solution and the coprecipitant solution were added dropwise to 50mL of ultrapure water at a rate of 4.5mL / min. After being kept at 75℃ for 0.5h, the mixture was stirred at room temperature for 3.5h. After filtration and drying, the precursor was obtained.

[0116] (2) At a speed of 450 rpm / min, 0.393 g of lithium oxide, 5.56 g of sodium carbonate, the precursor obtained in step (1) and ethanol were sand-milled for 2 h. After drying, the mixture was tableted once under a pressure of 8 MPa. Then, the temperature was raised to 400 °C at a heating rate of 2 °C / min for 4.5 h and the intermediate was obtained after natural cooling.

[0117] (3) Under a pressure of 8 MPa, the intermediate obtained in step (2) was subjected to secondary tableting. At an oxygen injection rate of 20 mL / min, the temperature was increased to 850 °C at a heating rate of 4 °C / min for secondary sintering for 11 h. Subsequently, the temperature was decreased to 75 °C at a cooling rate of 1.5 °C / min. After natural cooling, the secondary sintered product was obtained. The secondary sintered product was then ground and sieved to obtain the chemical formula Na. 0.8Li 0.2 Co 0.2 Mn 0.6 O2 mixed-phase layered cathode material.

[0118] Example 7

[0119] This embodiment provides a method for preparing a mixed-phase layered cathode material, the method comprising:

[0120] (1) 12.45g of Co(CH3COO)2·4H2O and 12.55g of Mn(NO3)2·4H2O were dissolved in 500mL of deionized water to form a transition metal salt solution, and sodium hydroxide was dissolved in 500mL of deionized water to form a coprecipitant solution. Then, under a stirring rate of 550rpm / min, the transition metal salt solution and the coprecipitant solution were added dropwise to 50mL of deionized water at a rate of 5.5mL / min. After keeping the solution at 65℃ for 1.5h, the solution was stirred at room temperature for 2.5h. The solution was then filtered and dried to obtain the precursor.

[0121] (2) At a speed of 500 rpm / min, 0.95 g of lithium carbonate, 5.56 g of sodium carbonate, the precursor obtained in step (1) and deionized water were ground for 1 h. After drying, the mixture was pressed into tablets at a pressure of 5 MPa. Then, the temperature was raised to 300 °C at a heating rate of 2 °C / min and sintered for 5.5 h. After natural cooling, the intermediate was obtained.

[0122] (3) Under a pressure of 5 MPa, the intermediate obtained in step (2) was subjected to secondary tableting. At an oxygen injection rate of 20 mL / min, the temperature was increased to 800 °C at a heating rate of 5 °C / min for secondary sintering for 13 h. Subsequently, the temperature was decreased to 85 °C at a cooling rate of 1 °C / min. After natural cooling, the secondary sintered product was obtained. The secondary sintered product was then ground and sieved to obtain the chemical formula Na. 0.8 Li 0.2 Co 0.2 Mn 0.6 O2 mixed-phase layered cathode material.

[0123] Example 8

[0124] The difference between this embodiment and Embodiment 1 is that the secondary sintering is omitted, while the remaining process parameters and operating steps are the same as in Embodiment 1.

[0125] Example 9

[0126] The difference between this embodiment and embodiment 1 is that the final temperature of the second sintering in step (3) is 650°C, while the other process parameters and operating steps are the same as in embodiment 1.

[0127] Comparative Example 1

[0128] The difference between this comparative example and Example 1 is that lithium doping was omitted, i.e., lithium hydroxide was not added in step (2), resulting in a product with the chemical formula Na. 0.8 Co 0.2 Mn 0.6 Single-phase (P-phase) layered cathode materials of O2, such as Figure 14 and Figure 15 As shown. The remaining process parameters and operating steps are the same as in Example 1.

[0129] Electrochemical performance tests were performed on the cathode materials provided in Examples 1-9 and Comparative Example 1:

[0130] (1) Preparation of positive electrode sheet: The positive electrode material, conductive carbon black (super p) and polyvinylidene fluoride (PVDF) are dissolved and dispersed in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 and stirred overnight to obtain positive electrode slurry; the positive electrode slurry is coated on aluminum foil, dried under vacuum at 85°C, and then pressed into a positive electrode sheet with a diameter of 12 mm by a certain pressure roller, and the positive electrode sheet is baked in an oven at 85°C for 8 hours.

[0131] (2) Battery assembly: The positive electrode obtained in (1) is used as the positive electrode; a sodium metal sheet is used as the negative electrode; propylene carbonate (PC) is used as the electrolyte, wherein the mass fraction of fluoroethylene carbonate (FEC) is 3wt% and the molar concentration of NaClO4 is 1mol / L; and a glass cellulose membrane is used as the separator to assemble the battery.

[0132] (3) Cyclic performance test: temperature is 30℃, current density is 0.5C, and voltage range is 2~4.5V.

[0133] (4) Rate performance test: The temperature is 30℃, the current density is 0.1C, 0.2C, 0.5C, 1C, 2C, 5C and 10C respectively, and the voltage range is 2~4.5V.

[0134] Analysis of the electrochemical performance diagrams of the cathode materials provided in Examples 1-9 and Comparative Example 1 shows that:

[0135] (1) By Figure 16 and Figure 17It can be seen that the mixed-phase layered cathode materials provided in Examples 1-7 all have high capacity performance, excellent rate performance and cycle performance. This indicates that the mixed-phase layered cathode material with alternating P and O phases provided by the present invention effectively suppresses the phase transition of the cathode material during the charging and discharging process of sodium-ion batteries through the synergistic effect between different crystal phases, and improves the structural stability of the layered cathode material. Thus, the sodium-ion batteries prepared have high capacity performance, excellent rate performance and cycle performance.

[0136] (2) By Figure 18 It can be seen that the electrochemical performance of the mixed-phase layered cathode materials provided in Examples 8 and 9 is lower than that in Example 1. This is because Example 8 omits the secondary sintering, while in the secondary sintering process, the mixed-phase layered cathode material undergoes a crystal phase structure change from P3 / O3 to P2 / P3 / O3 to P2 / O3, ultimately forming a P2 / O3 mixed-phase layered cathode material. The secondary sintering temperature in Example 9 is too low. When the final temperature of the secondary sintering is too low, the ion diffusion rate is slow in the solid-phase process because the precursor reacts and sinters with the lithium / sodium salt, resulting in poor electrochemical performance of the cathode material.

[0137] (3) By Figure 19 and Figure 20 It can be seen that the electrochemical performance of the cathode material provided in Comparative Example 1 is much lower than that in Example 1. This is because Comparative Example 1 omits lithium doping, which... Figure 14 and Figure 15 It can be seen that the cathode material obtained in the comparative example is not a mixed-phase structure, but a single-phase (P-phase) layered cathode material. This indicates that the mixed-phase layered cathode material with alternating P-phase and O-phase layers effectively suppresses the phase transition of the cathode material during the charging and discharging process of sodium-ion batteries through the synergistic effect between different crystal phases, and improves the structural stability of the layered cathode material. As a result, the sodium-ion battery prepared has high capacity performance, excellent rate performance and cycle performance.

[0138] 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 mixed-phase layered cathode material, characterized in that, The crystal structure of the mixed-phase layered cathode material includes sequentially stacked P-phase layers and O-phase layers; The P phase layer is composed of sodium lithium cobalt manganese oxide, and the O phase layer is composed of lithium-rich manganese oxide. The P phase layer is of the P2 crystal form, and the O phase layer is of the O3 crystal form; The mixed-phase layered cathode material is in the form of block particles; the particle size of the mixed-phase layered cathode material is 0.2~5μm; The preparation method of the mixed-phase layered cathode material includes: reacting a transition metal salt solution with a co-precipitant solution to obtain a precursor, mixing the precursor, lithium salt and sodium salt and then performing a first sintering and a second sintering sequentially to obtain the mixed-phase layered cathode material. After mixing, the mixture is dried and compressed once before being sintered once. The pressure of the first pressing is 5~10MPa, the temperature of the first sintering is 300~500℃, and the time of the first sintering is 4.5~5.5h; After the first sintering, an intermediate is obtained. The intermediate is then subjected to cooling and secondary tableting followed by the second sintering. The pressure of the secondary pressing is 5~10MPa, the heating rate of the secondary sintering is ≤5℃ / min, the final temperature of the secondary sintering is 700~900℃, and the holding time of the secondary sintering is 11~13h.

2. A method for preparing the mixed-phase layered cathode material according to claim 1, characterized in that, The preparation method includes: A precursor is obtained by reacting a transition metal salt solution with a coprecipitant solution. The precursor, lithium salt, and sodium salt are mixed and then subjected to a first sintering and a second sintering to obtain the mixed-phase layered cathode material. After mixing, the mixture is dried and compressed once before being sintered once. The pressure of the first pressing is 5~10MPa, the temperature of the first sintering is 300~500℃, and the time of the first sintering is 4.5~5.5h; After the first sintering, an intermediate is obtained. The intermediate is then subjected to cooling and secondary tableting followed by the second sintering. The pressure of the secondary pressing is 5~10MPa, the heating rate of the secondary sintering is ≤5℃ / min, the final temperature of the secondary sintering is 700~900℃, and the holding time of the secondary sintering is 11~13h.

3. The preparation method according to claim 2, characterized in that, The method for preparing the precursor includes: A transition metal salt is dissolved in a first solvent to form a transition metal salt solution, and a coprecipitant is dissolved in a second solvent to form a coprecipitant solution. Subsequently, the transition metal salt solution and the coprecipitant solution are added dropwise to a third solvent to coprecipitate, and the precursor is obtained after drying.

4. The preparation method according to claim 3, characterized in that, The transition metal salts include cobalt salts and manganese salts.

5. The preparation method according to claim 4, characterized in that, The cobalt salt includes any one of cobalt sulfate, cobalt nitrate, or cobalt acetate.

6. The preparation method according to claim 4, characterized in that, The manganese salt includes any one of manganese sulfate, manganese nitrate, or manganese acetate.

7. The preparation method according to claim 3, characterized in that, The coprecipitant includes sodium oxalate or sodium hydroxide.

8. The preparation method according to claim 3, characterized in that, The first solvent, the second solvent, and the third solvent are all deionized water.

9. The preparation method according to claim 3, characterized in that, The first solvent, the second solvent, and the third solvent are all ultrapure water.

10. The preparation method according to claim 3, characterized in that, The coprecipitation was carried out under stirring.

11. The preparation method according to claim 10, characterized in that, The stirring rate is 450~550 rpm / min.

12. The preparation method according to claim 3, characterized in that, The dropping rate of both the transition metal salt solution and the co-precipitant solution into the third solvent is 4.5~5.5 mL / min.

13. The preparation method according to claim 3, characterized in that, The coprecipitation includes a first stage and a second stage.

14. The preparation method according to claim 13, characterized in that, The temperature in the first stage is 65~75℃.

15. The preparation method according to claim 13, characterized in that, The duration of the first stage is 0.5 to 1.5 hours.

16. The preparation method according to claim 13, characterized in that, The second stage lasts for 2.5 to 3.5 hours.

17. The preparation method according to claim 3, characterized in that, The co-precipitation and drying processes also include a separation process.

18. The preparation method according to claim 17, characterized in that, The separation process includes filtration or centrifugation.

19. The preparation method according to claim 2, characterized in that, The lithium salt includes any one of lithium hydroxide, lithium oxide, or lithium carbonate.

20. The preparation method according to claim 2, characterized in that, The sodium salt is sodium carbonate.

21. The preparation method according to claim 2, characterized in that, The precursor, the lithium salt, and the sodium salt are mixed by any one of wet ball milling, sand milling, or grinding.

22. The preparation method according to claim 2, characterized in that, The mixing time is 1 to 4 hours.

23. The preparation method according to claim 2, characterized in that, The solvents added during the mixing process include ethanol or deionized water.

24. The preparation method according to claim 21, characterized in that, The rotational speeds of the wet ball mill, the sand mill, and the grinding are all ≥400 rpm / min.

25. The preparation method according to claim 2, characterized in that, The heating rate for the secondary sintering is 2~4℃ / min.

26. The preparation method according to claim 2, characterized in that, The secondary sintering is carried out in an oxygen atmosphere.

27. The preparation method according to claim 2, characterized in that, The cooling process of the secondary sintering includes a primary cooling stage and a secondary cooling stage.

28. The preparation method according to claim 27, characterized in that, The cooling rate of the first cooling stage is ≤2℃ / min.

29. The preparation method according to claim 27, characterized in that, The cooling rate during the first cooling stage is 1~2℃ / min.

30. The preparation method according to claim 27, characterized in that, The final temperature of the first cooling stage is 75~85℃.

31. The preparation method according to claim 27, characterized in that, The secondary cooling stage is natural cooling.

32. The preparation method according to any one of claims 2-31, characterized in that, The secondary sintering process yields a secondary sintering product, which is then ground and sieved sequentially to obtain the mixed-phase layered cathode material.

33. The preparation method according to claim 2, characterized in that, The preparation method includes: (1) The transition metal salt is dissolved in the first solvent to form a transition metal salt solution, and the coprecipitant is dissolved in the second solvent to form a coprecipitant solution. Then, the transition metal salt solution and the coprecipitant solution are added dropwise to the third solvent at a stirring rate of 4.5-5.5 mL / min at a stirring rate of 450-550 rpm / min. After keeping the temperature at 65-75℃ for 0.5-1.5 h, the mixture is stirred for 2.5-3.5 h, and then separated and dried to obtain the precursor. (2) At a speed of ≥400 rpm / min, the lithium salt, sodium salt, solvent and the precursor obtained in step (1) are mixed for 1~4 h, dried and then compressed once under a pressure of 5~10 MPa, followed by sintering at a temperature of ≤500℃ for 4.5~5.5 h, and then naturally cooled to obtain the intermediate. (3) Under a pressure of 5~10 MPa, the intermediate obtained in step (2) is subjected to secondary compression. Under an oxygen atmosphere, the temperature is increased to ≥700℃ at a heating rate of ≤5℃ / min for secondary sintering for 11~13h. Then, the temperature is decreased to 75~85℃ at a cooling rate of ≤2℃ / min. After natural cooling, the secondary sintered product is obtained. Then, the secondary sintered product is ground and sieved to obtain mixed-phase layered cathode material.

34. A sodium-ion battery, characterized in that, The sodium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; The positive electrode sheet comprises the mixed-phase layered positive electrode material as described in claim 1.

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