Ca and Ti co-doped O3 phase layered oxide positive electrode material and preparation method and application thereof

Abstract

The invention relates to the field of sodium ion batteries, and discloses a Ca and Ti co-doped O3 phase layered oxide positive electrode material and a preparation method and application thereof, the crystal structure of the positive electrode material is an O3 phase layered structure, and the chemical general formula is Na < 1-x > Ca < x > Ni < 0.33 > Fe < 0.33 > Mn < 0.33-y > Ti < y > O < 2, 0 lt >, x is less than or equal to 0.1, 0lt; and y < = 0.1. According to the O3-phase layered oxide positive electrode material provided by the invention, the ion radius of the doped Ca element is larger than that of sodium ions, so that the distance of an alkali metal layer can be increased, a diffusion channel of Na < + > can be improved, the Ca element can also broaden the voltage range of converting the O3-phase material into a P3 phase in the charging process, and the cycling stability of the material in the charging and discharging process is improved. In the charging process, Ca < + > always exists in the alkali metal layer and is not separated from the layered transition metal oxide structure into the electrolyte like Na < + >. Ti < 4 + > replaces Mn < 3 + >, so that the influence of a Jahn-Teller effect can be reduced, common irreversible phase change of a high-voltage region in an O3 phase can be inhibited, the structural stability is improved, and better cycle performance is obtained.

Description

Technical Field

[0001] The present invention relates to the field of sodium ion batteries, and specifically to a Ca, Ti co-doped O3-phase layered oxide cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous increase in social demand for energy and the continuous pursuit of energy conversion technologies, it has become a recognized difficult problem to study renewable energy storage systems with low cost, high efficiency, long life, and good safety. Due to the rich content, wide distribution, and low cost of sodium resources in the earth's crust and oceans, sodium ion batteries show broad application potential in large-scale electrical energy storage. The key characteristics of the battery (such as specific capacity, cycle performance, and working voltage) are mainly determined by the inherent electrochemical characteristics of the electrode material. Therefore, the main problem in the sodium ion battery system is to find suitable electrode materials, especially cathode materials, which largely determine the energy density and safety performance of the battery. Currently, several different types of sodium ion battery cathode materials have been discovered, including metal oxides, polyanion compounds, Prussian blue analogs, and organic compounds. Polyanion compounds have a stable anion framework and exhibit good cycle stability and high safety. However, this material usually has poor conductivity and low specific capacity. Similarly, Prussian blue analogs have a multi-channel structure and high theoretical capacity but still have the disadvantages of poor cycle stability and rate performance. In contrast, although the layered sodium transition metal oxide cathode material Na x MO2 (M = Fe, Mn, Ni, Co, Cr and their combinations) (as Figure 1 shown) has great application potential due to its high energy density, simple structure, and easy synthesis, there are still bottleneck problems such as interlayer slip and complex phase transformation. Due to the great application value of layered metal oxides, in view of the above bottleneck problems, new thinking is needed in theory and methods to inhibit the irreversible phase transformation caused by interlayer slip of layered metal oxides. Therefore, developing cathode materials with high energy density, high power density, and excellent cycle stability is the difficulty and focus of the current development of sodium ion batteries.

[0003] In response to the above problems, researchers have proposed some solutions. Huang et al. used Al doping to completely inhibit the harmful P2-P2' phase transformation by suppressing the Jahn-Teller effect of Mn 3+ . Wang et al. effectively suppressed the intermediate phase transformation through high-entropy design, realized the reversible O3-P3 phase evolution, and thus stabilized the layered structure. Wu et al. introduced Zn and Ti into the P2-type layered Na 0.66 Ni 0.33 Mn 0.67In the transition metal layer of O2, the P2-O2 phase transition under high voltage can be effectively inhibited, thereby improving the cycle durability. After forming a full cell with hard carbon, the capacity retention rate after 100 cycles within the voltage range of 1.0V - 4.2V is 75.6%. Kim et al. proposed the synergistic effect of Li-Cu cations, and through Li substitution and phase transition inhibition in the P3-type layered oxide cathode, the potential of oxygen redox is fully exerted. The P3-type Na 0.7 [Li 0.1 Cu 0.2 Mn 0.7 O2 cathode has a large specific capacity of 212 mAh / g at 15 mA g –1 and the discharge capacity still remains at 90% of the initial capacity after 100 cycles. Although some progress has been made through element substitution, the selection of substitute elements not only needs to consider the impact on the redox voltage, but also the impact on the structural stability. It can be seen that it is difficult to completely inhibit the phase transition of the material during cycling by single element substitution. Therefore, exploring the research idea of multiple element substitution to solve the problems of irreversible phase transition, poor air stability, and low specific capacity is still the focus of current research. Summary of the Invention

[0004] Aiming at the problems of poor air stability, irreversible phase transition during charge and discharge, and poor cycle stability faced by sodium-ion layered cathode materials, the present invention provides a Ca, Ti co-doped O3-phase layered oxide cathode material.

[0005] To achieve the above object, the present invention provides a Ca, Ti co-doped O3-phase layered oxide cathode material, the crystal structure of the O3-phase layered oxide cathode material is in an O3-phase layered structure, and the chemical general formula is: Na 1- x Ca x Ni 0.33 Fe 0.33 Mn 0.33-y Ti y O2, where 0 < x ≤ 0.1 and 0 < y ≤ 0.1.

[0006] As a further preferred technical solution of the present invention, 0.05 ≤ x ≤ 0.1 and 0.05 ≤ y ≤ 0.1.

[0007] According to another aspect of the present invention, the present invention also provides a preparation method of a Ca, Ti co-doped O3-phase layered oxide cathode material, which includes the following steps:

[0008] The raw materials Na2CO3, CaCO3, NiO, Fe2O3, Mn2O3, and TiO2 are ball-milled and mixed to obtain a precursor powder; then the precursor powder is pressed and sintered to obtain a Ca, Ti co-doped O3-phase layered oxide cathode material.

[0009] As a further preferred technical solution of the present invention, the ball-milling speed is 300 - 600 r.p.m, and the ball-milling time is 1 - 12 h.

[0010] As a further preferred technical solution of the present invention, the sintering temperature is 800 - 1000 °C, and the sintering time is 12 - 24 h.

[0011] As a further preferred technical solution of the present invention, in the precursor powder, the ratio of each raw material component is Na2CO3:CaCO3:NiO:Fe2O3:Mn2O3:TiO2 = 1 - x:x:0.33:0.33:0.33 - y:y (0.05 ≤ x ≤ 0.1, 0.05 ≤ y ≤ 0.1).

[0012] According to another aspect of the present invention, the present invention also provides an application of a Ca, Ti co-doped O3-phase layered oxide cathode material as a cathode material in a sodium-ion battery.

[0013] As a further preferred technical solution of the present invention, the Ca, Ti co-doped O3-phase layered oxide cathode material, a conductive agent Super-P, and a binder polyvinylidene fluoride are added to an N-methylpyrrolidone solvent and ball-milled to form a slurry; then the slurry is coated on a metal aluminum foil and vacuum-dried to obtain a cathode electrode sheet for a sodium-ion battery.

[0014] The Ca, Ti co-doped O3-phase layered oxide cathode material proposed by the present invention, because the ionic radius of the incorporated Ca element is larger than that of sodium ions, can not only increase the spacing of the alkali metal layer and improve the diffusion channel of Na + , but also the Ca element can broaden the voltage range for the O3-phase material to transform into the P3 phase during charging, increasing the cycle stability of the material during charge and discharge. During charging, Ca + always exists in the alkali metal layer and does not escape into the electrolyte from the layered transition metal oxide structure like Na + . Ti 4+ substituting Mn 3+ can not only reduce the influence of its Jahn-Teller effect, but also inhibit the irreversible phase change in the common high-voltage region in the O3 phase, improve the structural stability, and thus obtain better cycle performance.

[0015] This invention studies the influence of the introduced Ca and Ti on the stability of layered transition metal oxides during charge and discharge processes, proposes the mechanism of co-doping of Ca and Ti to inhibit interlayer slip and irreversible phase transformation, provides theoretical guidance and new ideas for the development of new sodium ion layered transition metal oxide materials, and provides a new approach to meet the major demand for large-scale energy storage.

[0016] This invention can achieve the following beneficial effects:

[0017] Using the prepared Ca and Ti co-doped O3-phase layered oxide cathode material as the cathode material of a half-cell and metallic sodium as the counter electrode, electrochemical performance tests were carried out in the voltage range of 2.0 - 4.0V. Meanwhile, the initial discharge specific capacity of the cathode material before and after doping with Ca and / or Ti was compared. The cathode material co-doped with appropriate amounts of Ca and Ti showed the highest initial reversible discharge capacity and the best capacity retention rate. The discharge specific capacity was as high as 168 mAh / g, much higher than that of the cathode material without doping with Ca and Ti (124 mAh / g). On the one hand, this is because the ionic radius of the incorporated Ca element is larger than that of sodium ions, increasing the interlayer spacing of the alkali metal layer and broadening the diffusion channels of Na + , increasing the cycle stability of the material during charge and discharge. On the other hand, the significant difference in Fermi energy levels between Ti 4+ and Mn 4+ can reduce the repulsive force of the O - O bonds between the layers to avoid phase transformation. Doping with Ti 4+ can also reduce the volume change caused by the influence of the Jahn - Teller effect. The strong Ti - O bonds can inhibit the movement of Mn - O and Fe - O bonds during charge and discharge, stabilizing its structure. After incorporating the Ti element, due to the strong Ti - O bond energy, the transition metal layer of the layered oxide cathode material shrinks, the sodium layer spacing becomes wider, and together with Ca, it makes the interlayer spacing of the alkali metal layer wider, further broadening the diffusion channels of Na + . This synergistic effect between Ca and Ti increases the discharge specific capacity of the material and effectively improves the battery performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The following further elaborates on this invention in detail in conjunction with the drawings and specific embodiments.

[0019] Figure 1 Structural schematic diagrams of O3, P3, P2, and O2 types in the layered Na x MnO2 compound.

[0020] Figure 2 XRD pattern of the Ca and Ti co-doped O3-phase layered cathode material of this invention.

[0021] Figure 3SEM image of the Ca, Ti co-doped O3-phase layered cathode material of the present invention.

[0022] Figure 4 Charge and discharge curve of the O3-phase layered oxide NaNi 0.33 Fe 0.33 Mn 0.33 O2 at a current density of 0.1C (1C = 150 mA / g);

[0023] Figure 5 Charge and discharge curve of the O3-phase layered oxide Na 0.95 Ca 0.05 Ni 0.33 Fe 0.33 Mn 0.33 O2 at a current density of 0.1C (1C = 150 mA / g).

[0024] Figure 6 Charge and discharge curve of the O3-phase layered oxide NaNi 0.33 Fe 0.33 Mn 0.23 Ti 0.1 O2 at a current density of 0.1C (1C = 150 mA / g).

[0025] Figure 7 Charge and discharge curve of the O3-phase layered oxide Na 0.95 Ca 0.05 Ni 0.33 Fe 0.33 Mn 0.23 Ti 0.1 O2 at a current density of 0.1C (1C = 150 mA / g)

[0026] Figure 8 Charge and discharge curve of the O3-phase layered oxide NaNi 0.33 Fe 0.33 Mn 0.33 O2 at a current density of 0.1C (1C = 150 mA / g);

[0027] Figure 9 Charge and discharge curve of the O3-phase layered oxide Na 0.95 Ca 0.05 Ni 0.33 Fe 0.33 Mn 0.33 O2 at a current density of 0.1C (1C = 150 mA / g);

[0028] Figure 10The O3-phase layered oxide NaNi obtained in Example 4 0.33 Fe 0.33 Mn 0.23 Ti 0.1 Cycling performance graph at a current density of 0.1C (1C = 150 mA / g) for O2;

[0029] Figure 11 The O3-phase layered oxide Na obtained in Example 5 0.95 Ca 0.05 Ni 0.33 Fe 0.33 Mn 0.23 Ti 0.1 Cycling performance graph at a current density of 0.1C (1C = 150 mA / g) for O2.

[0030] Figure 12 The O3-phase layered oxide Na obtained in Example 9 0.97 Ca 0.03 Ni 0.33 Fe 0.33 Mn 0.13 Ti 0.2 Charge-discharge curve graph at a current density of 0.1C (1C = 150 mA / g) for O2.

[0031] The realization, functional features, and advantages of the present invention will be further described with reference to the embodiments and the accompanying drawings. Detailed Embodiments

[0032] The following further details the specific embodiments of the present invention with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not intended to limit the present invention.

[0033] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which the present invention pertains. The test reagents used in the following embodiments are all conventional biochemical reagents, unless otherwise specified; the experimental methods used are all conventional methods, unless otherwise specified.

[0034] The chemical general formula of the Ca- and Ti-codoped O3-phase layered oxide cathode material proposed by the present invention is: Na 1- x Ca x Ni 0.33 Fe 0.33 Mn 0.33-y Ti yO2, where 0 < x ≤ 0.1 and 0 < y ≤ 0.1. The following presents a series of examples and comparative examples based on different values of x and y in the general formula. To ensure an adequate sodium source, in the comparative examples and examples of the present invention, the mass of the sodium source (Na2CO3) is in excess by 5%.

[0035] Comparative Example 1

[0036] An O3-phase layered oxide cathode material (x = 0) (y = 0), and its preparation method is as follows:

[0037] Weigh Na2CO3 (1.5025 g), NiO (0.6682 g), Fe2O3 (0.7128 g), and Mn2O3 (0.7038 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0038] When the tube furnace cools down to room temperature, take out the sintered sample to obtain an O3-phase ternary layered oxide cathode material (NaNi 0.33 Fe 0.33 Mn 0.33 O2), and its XRD test results are as Figure 2 shown. Finally, quickly grind the O3-phase ternary layered oxide cathode material into powder, collect it and store it in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0039] Example 1

[0040] An O3-phase layered oxide cathode material (x = 0.05) (y = 0), and its preparation method is as follows:

[0041] Weigh Na2CO3 (1.4163 g), CaCO3 (0.1339 g), NiO (0.6631 g), Fe2O3 (0.7072 g), and Mn2O3 (0.6984 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0042] When the tube furnace cools down to room temperature, take out the sintered sample to obtain an O3-phase ternary layered oxide cathode material (Na 0.95 Ca 0.05 Ni 0.33 Fe 0.33 Mn 0.33 O2), and its XRD test results are asFigure 2 As shown. Finally, the O3-phase ternary layered oxide cathode material was quickly ground into powder, collected and stored in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0043] Example 2

[0044] An O3-phase layered oxide cathode material (x = 0.1) (y = 0), and its preparation method is as follows:

[0045] Weigh Na2CO3 (1.3317 g), CaCO3 (0.2659 g), NiO (0.6581 g), Fe2O3 (0.7019 g), Mn2O3 (0.6932 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0046] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (Na 0.9 Ca 0.1 Ni 0.33 Fe 0.33 Mn 0.33 O2), and its XRD test results are as Figure 2 shown. Finally, the O3-phase ternary layered oxide cathode material was quickly ground into powder, collected and stored in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0047] Example 3

[0048] An O3-phase layered oxide cathode material (x = 0) (y = 0.05), and its preparation method is as follows:

[0049] Weigh Na2CO3 (1.507 g), NiO (0.6702 g), Fe2O3 (0.714 g), Mn2O3 (0.460 g), TiO2 (0.0832 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0050] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (NaNi 0.33 Fe 0.33 Mn 0.28 Ti 0.05O2), and its XRD test results are as Figure 2 shown. Finally, the O3-phase ternary layered oxide cathode material was quickly ground into powder, collected and stored in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0051] Example 4

[0052] An O3-phase layered oxide cathode material (x = 0) (y = 0.1), and its preparation method is as follows:

[0053] Weigh Na2CO3 (1.512 g), NiO (0.6724 g), Fe2O3 (0.7172 g), Mn2O3 (0.4936 g), and TiO2 (0.2173 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0054] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (NaNi 0.33 Fe 0.33 Mn 0.23 Ti 0.1 O2), and its XRD test results are as Figure 2 shown. Finally, the O3-phase ternary layered oxide cathode material was quickly ground into powder, collected and stored in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0055] Example 5

[0056] An O3-phase layered oxide cathode material (x = 0.05) (y = 0.05), and its preparation method is as follows:

[0057] Weigh Na2CO3 (1.4210 g), CaCO3 (0.1344 g), NiO (0.6652 g), Fe2O3 (0.7096 g), Mn2O3 (0.5945 g), and TiO2 (0.1075 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0058] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (Na 0.95 Ca 0.05 Ni 0.33 Fe0.33 Mn 0.28 Ti 0.05 O2), and its XRD test results are as Figure 2 shown. Finally, the O3-phase ternary layered oxide cathode material is quickly ground into powder, collected and stored in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0059] Example 6

[0060] An O3-phase layered oxide cathode material (x = 0.1) (y = 0.05), and its preparation method is as follows:

[0061] Weigh Na2CO3 (1.3367 g), CaCO3 (0.2669 g), NiO (0.6605 g), Fe2O3 (0.7046 g), Mn2O3 (0.5903 g), TiO2 (0.1067 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0062] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (Na 0.9 Ca 0.1 Ni 0.33 Fe 0.33 Mn 0.28 Ti 0.05 O2). Finally, the O3-phase ternary layered oxide cathode material is quickly ground into powder, collected and stored in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0063] Example 7

[0064] An O3-phase layered oxide cathode material (x = 0.1) (y = 0.1), and its preparation method is as follows:

[0065] Weigh Na2CO3 (1.3402 g), CaCO3 (0.2676 g), NiO (0.6623 g), Fe2O3 (0.7064 g), Mn2O3 (0.4862 g), TiO2 (0.2140 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0066] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (Na 0.9 Ca 0.1 Ni 0.33 Fe 0.33 Mn 0.23 Ti 0.1 O2). Finally, quickly grind the O3-phase ternary layered oxide cathode material into powder, collect it and store it in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0067] Example 8

[0068] An O3-phase layered oxide cathode material (x = 0.05) (y = 0.1), and its preparation method is as follows:

[0069] Weigh Na2CO3 (1.4253 g), CaCO3 (0.1348 g), NiO (0.6672 g), Fe2O3 (0.7117 g), Mn2O3 (0.4898 g), TiO2 (0.2156 g) according to the stoichiometric ratio and put them into a ball milling tank. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0070] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (Na 0.9 Ca 0.1 Ni 0.33 Fe 0.33 Mn 0.23 Ti 0.1 O2). Finally, quickly grind the O3-phase ternary layered oxide cathode material into powder, collect it and store it in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0071] Example 9

[0072] An O3-phase layered oxide cathode material (x = 0.15) (y = 0.2), and its preparation method is as follows:

[0073] Weigh Na2CO3 (1.2639 g), CaCO3 (0.4008 g), NiO (0.6613 g), Fe2O3 (0.7054 g), Mn2O3 (0.2744 g), and TiO2 (0.4275 g) according to the stoichiometric ratio, put them into a ball milling jar, and ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat it to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0074] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (Na 0.85 Ca 0.15 Ni 0.33 Fe 0.33 Mn 0.13 Ti 0.2 O2). Finally, quickly grind the O3-phase ternary layered oxide cathode material into powder, collect it and store it in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0075] Example 10

[0076] As a control experiment for Example 7, based on the preparation method of Example 7, the only difference is that by changing the doping amount of Ca, the O3-phase ternary layered oxide cathode material Na 0.85 Ca 0.15 Ni 0.33 Fe 0.33 Mn 0.23 Ti 0.1 O2 is obtained. Its preparation method is as follows:

[0077] Weigh Na2CO3 (1.2568 g), CaCO3 (0.3985 g), NiO (0.6576 g), Fe2O3 (0.7014 g), Mn2O3 (0.4827 g), and TiO2 (0.2125 g) according to the stoichiometric ratio, put them into a ball milling jar, and ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat it to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0078] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (Na 0.85 Ca 0.15 Ni 0.33 Fe 0.33 Mn 0.23 Ti 0.1O2). Finally, the O3-phase ternary layered oxide cathode material was quickly ground into powder, collected and stored in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0079] Example 11

[0080] As a control experiment for Example 7, based on the preparation method of Example 7, the only difference is that: by changing the doping amount of Ti, the O3-phase ternary layered oxide cathode material Na 0.9 Ca 0.1 Ni 0.33 Fe 0.33 Mn 0.13 Ti 0.2 O2 was obtained. The preparation method is as follows:

[0081] Weigh Na2CO3 (1.3482 g), CaCO3 (0.2692 g), NiO (0.6662 g), Fe2O3 (0.7106 g), Mn2O3 (0.2764 g), and TiO2 (0.4307 g) according to the stoichiometric ratio and put them into a ball milling jar. Ball mill at a speed of 400 r.p.m. on a ball mill for 5 h. Press the uniformly mixed precursor powder into tablets. The sintering process needs to be carried out under an air atmosphere throughout. Heat up to 900 °C at a heating rate of 2 °C / min and then hold for 16 h.

[0082] When the tube furnace cools down to room temperature, take out the sintered sample to obtain the O3-phase ternary layered oxide cathode material (Na 0.9 Ca 0.1 Ni 0.33 Fe 0.33 Mn 0.13 Ti 0.2 O2). Finally, the O3-phase ternary layered oxide cathode material was quickly ground into powder, collected and stored in a glove box filled with argon to avoid direct contact with moisture and oxygen in the air.

[0083] The O3-phase layered oxide cathode material samples prepared in the above examples and comparative examples were characterized by a series of instruments such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and electrochemical performance.

[0084] The crystal structures of the O3-phase layered oxide cathode materials in Examples 1-5 and Comparative Example 1 above were characterized by an XRD diffractometer, as Figure 2As shown in (a), the main diffraction peaks of the O3-phase layered oxide cathode material are completely consistent with those of the standard card (PDF#25-0581) of the O3-phase layered structure, and the space group belongs to R-3m. No other impurity peaks appear in the XRD pattern, indicating that the layered structure of the material is well maintained. The sharp diffraction peaks suggest good crystallinity of the material. Figure 2 (b) shows the enlarged (003) diffraction peak of the cathode material in the angular range of 16° - 17°. It can be observed from the enlarged view that with the incorporation of Ca and Ti elements, the (003) diffraction peak shifts from a higher angle to a lower angle. According to Bragg's equation 2d sinθ = nλ, when the (003) diffraction peak shifts to the left, the θ value decreases and the d value increases, indicating an increase in the d-spacing of the sodium layer. This is because the radius of Ca + is larger than that of Na + , which enlarges the interlayer spacing of the alkali metal. After the incorporation of Ti element, this may be due to the stronger Ti-O bond energy, which causes the transition metal layer of the layered oxide cathode material to contract and the sodium layer spacing to widen. The widened sodium ion diffusion channels are beneficial to reducing the diffusion energy barrier of sodium ions and promoting the extraction / insertion of sodium ions into the lattice, thereby improving the electrochemical performance of the cathode material.

[0085] The O3-phase layered oxide cathode materials in Examples 1, 4, 5 and Comparative Example 1 above were characterized by scanning electron microscopy, as Figure 3 shown. It can be seen from the figure that there is no obvious change in the morphology of the O3-phase layered oxide cathode material without doping Ca and Ti and the materials doped with Ca alone or Ti alone. After co-doping with Ca and Ti, the morphology changes from quasi-hexagonal to approximately elliptical particles.

[0086] The O3-phase layered oxide cathode materials prepared in the above examples and comparative examples were respectively used for electrochemical tests of sodium-ion batteries, as follows:

[0087] 1) Preparation of electrode sheets:

[0088] Aluminum foil was selected as the current collector to prepare the electrode sheet of the sodium-ion battery cathode. The specific method is as follows: The prepared O3-phase layered cathode material, conductive agent Super-P and binder polyvinylidene fluoride (PVDF) were formulated according to a mass ratio of 8:1:1, 400 μL of N-methylpyrrolidone (NMP) solvent was added thereto, and then the weighed slurry was mechanically mixed for 2 hours using a planetary ball mill to obtain a black slurry with appropriate viscosity; the viscous black slurry was transferred onto a suitable-sized, flat and smooth aluminum foil, and a coating operation was carried out using a four-sided coater (100 μm), and then it was transferred to a vacuum drying oven at 110 °C for vacuum drying for 10 - 12 hours; finally, the dried electrode sheet was cut into circular positive electrode sheets with a diameter of 12 mm using a manual button cell slicer.

[0089] 2) Assembly of the battery:

[0090] In the battery test system, metallic sodium was selected as the counter electrode and reference electrode. The electrolyte was 1.0 M NaClO4 dissolved in propylene carbonate (PC). The separator was a glass fiber separator (Whatman, GF / D). A CR2016 type coin cell assembly was selected for assembly. The specific assembly process was as follows: Assemble in the order of the positive electrode case, electrode sheet, separator, sodium sheet, gasket, and negative electrode case. Among them, 100 μL of electrolyte was added in two portions, and a manual coin cell encapsulation machine was used to complete the encapsulation of the coin cell under a certain pressure. The battery assembly process was completed in a glove box filled with argon ([O2] < 0.1 ppm, [H2O] < 0.1 ppm).

[0091] 3) The charge and discharge tests of the battery were carried out on a Neware battery test system. At room temperature, the assembled battery was left standing for 12 h, and then the battery was clamped to the charge and discharge tester. A constant current charge and discharge test was carried out at a current density of 0.1 C (1 C = 150 mA / g) and a voltage range of 2.0 - 4.0 V. The test results are as Figures 4 - 11 .

[0092] Figure 4 is the charge and discharge curve of Comparative Example 1. It can be seen from the figure that the discharge specific capacity of the material in Comparative Example 1 is 115 mAh / g; Figure 5 is the charge and discharge curve of Example 1 (i.e., doping Ca element in the alkali metal layer), and the discharge specific capacity is 133 mAh / g; Figure 6 is the charge and discharge curve of Example 4 (i.e., doping Ti element in the transition metal layer). It can be seen that after doping Ti in the transition metal layer, the discharge specific capacity is 136 mAh / g; Figure 7 is the charge and discharge curve of Example 8 (i.e., doping Ca element in the alkali metal layer and doping Ti element in the transition metal layer). It can be seen that after co-doping with Ca and Ti, the discharge specific capacity of the material is increased to 166 mAh / g, and is much higher than the discharge specific capacity of doping Ca alone or doping Ti alone. This is because after simultaneously doping Ca and Ti elements, the Ca element has a certain strengthening effect on the transition metal layer and can reduce the slip of the transition metal layer. The strong Coulomb repulsion between adjacent transition metal layers sandwiching Ca inhibits the migration of Ni or Fe. Ti doping can reduce the volume change caused by the influence of the Jahn-Teller effect, and the strong Ti-O bond can inhibit the movement of Mn-O and Fe-O bonds during the charge and discharge process. The synergistic effect of Ca and Ti makes the internal structure of the layered oxide more stable.

[0093] Figure 8It is the discharge specific capacity graph of the first 50 cycles of Comparative Example 1. It can be seen from the graph that the discharge specific capacity drops to 85 mAh / g after 50 cycles. Figure 9 It is the discharge specific capacity graph of the first 50 cycles of Example 1. It can be seen from the graph that the discharge specific capacity drops to 120 mAh / g after 50 cycles. Figure 10 It is the discharge specific capacity graph of the first 50 cycles of Example 4. It can be seen from the graph that the discharge specific capacity is 105 mAh / g after 50 cycles. Figure 11 It is the discharge specific capacity graph of the first 50 cycles of Example 8. It can be seen from the graph that the discharge specific capacity drops to 140 mAh / g after 50 cycles.

[0094] Figure 12 It is the charge-discharge curve of Example 9 (i.e., the selected x = 0.15, y = 0.2), and the discharge specific capacity is 119 mAh / g.

[0095] The detailed comparison of the above Examples 1-11 and Comparative Example 1 is shown in Table 1:

[0096] Table 1

[0097]

[0098] Based on the comprehensive comparison of the data in Table 1, it can be seen that when the selected ranges of x and y are not within 0 < x ≤ 0.1, 0 < y ≤ 0.1, the discharge specific capacity decreases significantly.

[0099] Although the specific embodiments of the present invention have been described above, those skilled in the art should understand that these are only examples. Various changes or modifications can be made to these embodiments without departing from the principles and essence of the present invention. The protection scope of the present invention is only defined by the appended claims.

Claims

1. A Ca, Ti co-doped O3-phase layered oxide cathode material, characterized in that The crystal structure of this O3-phase layered oxide cathode material is an O3-phase layered structure, and its chemical general formula is: Na 1-x Ca x Ni 0.33 Fe 0.33 Mn 0.33-y Ti y O2, where 0 < x ≤ 0.1 and 0 < y ≤ 0.

1.

2. The Ca, Ti co-doped O3-phase layered oxide cathode material according to claim 1, characterized in that, 0.05 ≤ x ≤ 0.1, 0.05 ≤ y ≤ 0.

1.

3. A method for preparing the Ca, Ti co-doped O3-phase layered oxide cathode material according to claim 1 or 2, characterized in that, It includes the following steps: Ball-mill and mix the raw materials Na2CO3, CaCO3, NiO, Fe2O3, Mn2O3, and TiO2 to obtain a precursor powder; then press and sinter the precursor powder to obtain a Ca, Ti co-doped O3-phase layered oxide cathode material.

4. The preparation method of the Ca, Ti co-doped O3-phase layered oxide cathode material according to claim 3, characterized in that, The ball-milling speed is 300 - 600 r.p.m, and the ball-milling time is 1 - 12 h.

5. The preparation method of the Ca, Ti co-doped O3-phase layered oxide cathode material according to claim 3, characterized in that, The sintering temperature is 800 - 1000 °C, and the sintering time is 12 - 24 h.

6. The preparation method of the Ca, Ti co-doped O3-phase layered oxide cathode material according to claim 3, characterized in that, In the said precursor powder, the molar ratio of each raw material component is Na2CO3:CaCO3:NiO:Fe2O3:Mn2O3:TiO2 = 1 - x:x:0.33:0.33:0.33 - y:y (0.05 ≤ x ≤ 0.1, 0.05 ≤ y ≤ 0.1).

7. Application of the Ca, Ti co-doped O3-phase layered oxide cathode material as claimed in claim 1 or 2 as a cathode material in a sodium-ion battery.

8. The application according to claim 7, wherein Add the Ca, Ti co-doped O3-phase layered oxide cathode material, conductive agent Super-P, and binder polyvinylidene fluoride into N-methylpyrrolidone solvent, ball-mill to form a slurry; then coat the slurry on a metal aluminum foil and dry it under vacuum to obtain a positive electrode sheet for a sodium-ion battery.

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