Transition metal doped perovskite type oxide and preparation method and application thereof

Abstract

The invention relates to the field of perovskite type oxide materials, in particular to a transition metal doped perovskite type oxide and a preparation method and application thereof. The invention relates to a transition metal doped perovskite type oxide, the chemical general formula of which is LaFe1-xMxO3, M is at least one of Al, Co, Ni and Zn, and 0 lt; < = 0.3; the transition metal doped perovskite type oxide has a cubic perovskite structure, and a 3d orbit of B-site doped metal and a 2p orbit of O form covalent interaction through hybridization. According to the invention, the conductivity and electrochemical performance of the perovskite type oxide are improved, the problem of material agglomeration is solved by optimizing the preparation process, and a new solution is provided for the application of high-performance battery materials such as nickel-metal hydride batteries.

Description

Technical Field

[0001] This application relates to the field of perovskite oxide materials, and particularly to a transition metal-doped perovskite oxide, a preparation method thereof, and an application thereof. Background Art

[0002] With the worsening of energy and environmental problems, it is crucial to develop new green and environmentally friendly energy and energy-saving technologies. Developing green and environmentally friendly electric vehicles, energy-saving and emission-reducing hybrid vehicles, and energy storage devices is an appropriate way to solve these problems, and batteries play a key role as power conversion technologies and storage devices. Several types of rechargeable batteries, such as lead (Pb) acid batteries, nickel-cadmium (Ni-Cd) batteries, nickel-metal hydride (Ni / MH) batteries, and lithium (Li) ion batteries, have been developed and practically applied. Although lithium ion batteries have a relatively high energy density, as an energy source device for electric vehicles, there are still some potential safety hazards. Ni / MH batteries have high power density, overcharge / discharge resistance, environmental friendliness, and safety. Although their energy density is lower than that of lithium ion batteries, they are more advantageous for portable power tools and electric vehicles. In addition, nickel-metal hydride (Ni / MH) also has the advantage that the electrolyte is not consumed during the cycling process. The design of nickel-metal hydride rechargeable batteries is similar to the nickel-cadmium system (Ni / Cd). The main difference is that the former uses hydrogen adsorbed in a metal alloy as the active negative electrode material, while the latter uses cadmium.

[0003] Therefore, metal hydrides are considered to be the best choice for the negative electrode materials of nickel / metal hydride (Ni / MH) batteries. The most important electrochemical properties of the hydrogen storage compounds used in these batteries include capacity, cycle life, exchange current density, and equilibrium potential. The type of intermetallic compound, microstructure, properties and amounts of each element in the intermetallic compound affect its electrochemical performance, so the composition of the hydrogen storage alloy can be designed to provide the best electrochemical performance. In nickel-metal hydride batteries, the most important is the negative electrode material of the battery, which affects the physical and chemical properties of the battery. Currently, AB5-type and lanthanum-magnesium-nickel-based AB3 hydrogen storage alloys are severely corroded by the electrolyte at high temperatures, resulting in a reduced cycle life. In addition, the cost of hydrogen storage alloys is relatively high, and there is an urgent need to develop new negative electrode materials. As a new type of negative electrode material for nickel-metal hydride batteries, perovskite oxides have the advantages of high energy density, high temperature resistance, low cost, and no pollution. In particular, its high temperature resistance performance is relatively superior, and it can be applied to some equipment with special working temperatures. Although the current application of perovskite oxides in nickel-metal hydride batteries is in the research and development stage, certain research results have been achieved, especially its low cost and high temperature resistance performance have received great attention. At present, there are still some defects that limit its application: (1) Poor self-conductivity. (2) Low hydrogen diffusion coefficient in nickel-metal hydride batteries. (3) Severe agglomeration phenomenon and small specific surface area during the synthesis of materials. Therefore, improving these defects to make it better applied to nickel-metal hydride batteries is of great significance. In recent years, in order to develop new negative electrode materials for nickel-metal hydride batteries, researchers have found that perovskite oxides have great application potential in the fields of energy conversion and storage due to their special crystal structure. In particular, perovskite oxides ABO3 have been widely used in photocatalysis, supercapacitors, photovoltaic cells, and electrode materials for solid oxide fuel cells (SOFC), gas sensors, electronic and magnetic materials, etc., becoming a popular research material in recent years.

[0004] Perovskite oxide LaFeO3 is considered a promising negative electrode material for nickel-metal hydride batteries due to its low cost, environmental friendliness, and high temperature resistance. In 2004, the Japanese scholar Esaka's group first reported a peroxide SrCe 0.95 Yb 0.05 O 3-δ , which has the ability to perform electrochemical repeated charging and discharging with protons as charge carriers in alkaline solutions. Although researchers have used different methods to improve the conductivity of perovskite oxides to enhance their electrochemical performance, there has been little research on LaFeO3 as a negative electrode material for nickel-metal hydride batteries, especially on the influence of suitable doping elements and doping amounts on electrochemical performance during the doping process. After doping other elements at the B-site of perovskite oxides, active centers can be introduced. If the B-site is doped with other valence state elements, the valence state of the B-site cations will also change. The change in the valence state of the B-site cations is beneficial to the formation of B-OH, thus increasing the conductivity and the mobility of O 2- ions. In addition, the doping of B-site elements can produce a synergistic effect between different valence states of B-site cations, resulting in an electronic structure at the B-site that is conducive to the reaction. By exploring the research idea of doping different transition metal elements at the B-site, it is expected to solve the problems of poor self-conductivity, material agglomeration, and low hydrogen diffusion coefficient of perovskite oxides when used as negative electrode materials for nickel-metal hydride batteries. Summary of the Invention

[0005] In view of the problems of poor electrical conductivity of perovskite oxides themselves, low hydrogen diffusion coefficient in nickel-metal hydride batteries, and severe agglomeration during the synthesis of materials, etc., this application provides a transition metal-doped perovskite oxide, its preparation method and application. This application proposes a strategy of doping transition metal elements to improve the electrical conductivity and electrochemical performance of the material by adjusting the chemical state of the B-site cations in the perovskite oxide. At the same time, for the transition metal-doped perovskite oxide material prepared in this application, after doping the B-site with Al, Co, Ni, and Zn elements, the covalency of the Al-O, Co-O, Ni-O, and Zn-O bonds is stronger than that of the Fe-O bond, which is beneficial to enhancing the covalent bond interaction between the 3d orbitals of the B-site transition metals (Al, Co, Ni, Zn) and the O 2p orbital, making electron migration easier. At the same time, in LaFeO3, since -Fe-O- is the electron transport channel, after doping the Fe site, the distance of -Fe-O- is further shortened, thereby greatly increasing the electrical conductivity. And the prepared transition metal-doped perovskite oxide is used as the negative electrode material of the nickel-metal hydride battery. The prepared nickel-metal hydride battery, through testing, shows that it can greatly improve the high-temperature electrochemical performance of the nickel-metal hydride battery.

[0006] In the first aspect, this application provides a transition metal-doped perovskite oxide, and the following technical solution is adopted: A transition metal-doped perovskite oxide, whose chemical general formula is LaFe 1-x M x O3, where M is at least one of Al, Co, Ni, and Zn, and 0 < x ≤ 0.3; the transition metal-doped perovskite oxide has a cubic perovskite structure, and the 3d orbitals of the B-site doped metal and the 2p orbitals of O form a covalent interaction through hybridization.

[0007] By adopting the above technical solution, by doping the B-site with Al / Co / Ni / Zn elements, their 3d orbitals and the 2p orbitals of O form a covalent bond stronger than the Fe-O bond, promoting the migration of electrons in the lattice. At the same time, the doping shortens the transmission channel distance of the Fe-O bond, significantly improving the electrical conductivity and solving the problem of poor inherent electrical conductivity of the perovskite material. The stability of the cubic perovskite structure is enhanced, making the diffusion path of hydrogen atoms in the lattice smoother.

[0008] Preferably, its chemical general formula is LaFe 0.8 M 0.2 O3, where M is at least one of Al, Co, Ni, and Zn; the transition metal-doped perovskite oxide has a cubic perovskite structure, and the 3d orbitals of the B-site doped metal and the 2p orbitals of O form a covalent interaction through hybridization.

[0009] In a second aspect, the present application provides a method for preparing a transition metal-doped perovskite-type oxide, adopting the following technical solution: As a general technical concept, the present application also provides the method for preparing the above-mentioned transition metal-doped perovskite-type oxide, including the following steps: S31. According to the stoichiometry of the chemical formula, La2O3, Fe2O3 and the doped metal oxide are respectively added into a ball mill, and ball milling and mixing are carried out with the mass ratio of the ball milling balls to the raw materials being 10:0.8 - 1.2 to obtain a mixture; S32. Under an argon atmosphere, the mixture is subjected to high-temperature calcination and cooled to room temperature to obtain the transition metal-doped perovskite-type oxide.

[0010] By adopting the above technical solution, ball milling and mixing: Ball milling is a physical mixing method, in which the raw materials are impacted and ground by high-speed rotating balls to achieve uniform mixing of the raw materials. This process can reduce the particle size of the raw materials, increase the specific surface area, and contribute to subsequent chemical reactions. At the same time, the ball milling process may also affect the crystal structure of the raw materials. Ball milling and mixing ensure the uniform distribution and contact of the raw materials, contributing to improving the uniformity and efficiency of the reaction. At the same time, the lattice distortion that may be generated during the ball milling process also contributes to subsequent chemical reactions and the improvement of the material properties. High-temperature calcination is a heat treatment process. By heating the mixture at a high temperature, chemical reactions can be promoted, such as the formation of oxides and the growth of crystals. At the same time, high-temperature calcination can remove the moisture and organic impurities in the raw materials, improving the purity and stability of the material. High-temperature calcination not only promotes the formation of chemical bonds and the growth of crystals, but also helps to improve the structural stability and electrochemical properties of the material. At the same time, the lattice distortion and phase transformation generated during the high-temperature calcination process may also enhance the conductivity and electrochemical properties of the material. Cooling to room temperature: The cooling process helps to stabilize and solidify the material structure. Rapid cooling after high-temperature calcination can avoid secondary crystallization or phase separation of the material during the cooling process, maintaining the uniformity and properties of the material. By rapid cooling, the stability of the crystal structure and chemical bonds formed during the high-temperature calcination process can be maintained, while reducing the structural changes of the material during the cooling process, ensuring that the properties of the material will not be reduced due to the cooling process. In summary, each step plays a key role in the entire preparation process. Through the method of combining physical mixing and heat treatment, not only the uniform mixing of the raw materials and the smooth progress of chemical reactions are ensured, but also the structure and properties of the material are optimized by controlling parameters such as temperature and time. The synergistic effect of these steps jointly promotes the preparation of the transition metal-doped perovskite-type oxide, improves the conductivity and electrochemical properties of the material, making it a good choice for the negative electrode material of nickel-metal hydride batteries.

[0011] Preferably, in step S31, the doped metal oxide is selected from at least one of Al2O3, Co3O4, Ni2O3, and ZnO.

[0012] Preferably, in step S31, the ball mill is a planetary ball mill; the grinding balls are zirconia balls with a diameter of 3-5 mm. In step S31, the conditions for ball milling mixture include: the rotation speed is 400-500 rpm, and the ball milling time is 3-5 h.

[0013] By adopting the above technical solution, by optimizing the ball milling parameters (such as rotation speed, time) and using zirconia balls, the agglomerates are effectively broken, and a uniformly dispersed precursor is obtained.

[0014] Preferably, in step S32, the conditions for high-temperature calcination are: the heating rate is 3-5 °C / min below 600 °C, the heating rate is 0.8-1.5 °C / min from 600 °C to 1200 °C, and finally keep the temperature at 1200 °C for 4-6 hours.

[0015] By adopting the above technical solution, in the low-temperature stage (below 600 °C, 3-5 °C / min): the relatively fast heating rate can quickly pass through the organic matter decomposition stage, avoid the residue of intermediate products, and at the same time prevent the premature sintering of the precursor due to staying in the low-temperature zone for too long, ensuring the uniformity of subsequent reactions. In the high-temperature stage (600-1200 °C, 0.8-1.5 °C / min): slowing down the heating rate enables more sufficient lattice reconstruction, promotes the ion substitution of B-site doped metals (Al / Co / Ni / Zn) and Fe 3+ . The slow atomic migration is beneficial to the formation of a uniform solid solution structure, avoiding lattice distortion caused by rapid heating. Keeping the temperature at 1200 °C for 4-6 hours allows the grains to grow sufficiently and eliminates grain boundary defects. The improvement of the cubic perovskite structure can enhance the octahedral sharing of vertices and shorten the Fe-O bond length; the long-time high temperature promotes the 3d-2p orbital hybridization, making the 3d orbitals of the doped metals form covalent bonds with the 2p orbitals of O. The combination of stepwise heating and heat preservation enables the material to obtain dual optimization: quickly passing through the decomposition stage reduces the impurity phase, and the slow high-temperature section realizes atomic-level doping.

[0016] In the third aspect, the present application provides an application of a transition metal-doped perovskite-type oxide, adopting the following technical solution: As a general technical concept, the present application also provides the above transition metal-doped perovskite-type oxide as a negative electrode material for use in a nickel-metal hydride battery.

[0017] Preferably, the nickel-metal hydride battery includes: Positive electrode: Ni(OH)2 / NiOOH electrode; Negative electrode: the transition metal-doped perovskite-type oxide material as claimed in claim 1 or 2 and nickel carbonyl powder; Electrolyte: 6 mol / L KOH solution; Separator: polypropylene porous membrane.

[0018] In summary, the present application includes at least one of the following beneficial technical effects: 1. Improving conductivity: By doping transition metal elements (Al, Co, Ni, Zn) into perovskite oxides, the valence state of B-site cations is changed, enhancing the conductivity of the material. This is because the covalent bond formed after doping is stronger, making electron migration easier.

[0019] 2. Enhancing electrochemical performance: Especially in the application of nickel-metal hydride batteries, the doped perovskite oxides as anode materials can significantly improve the high-temperature electrochemical performance of the batteries. This means that the batteries can maintain better performance in high-temperature environments, which is particularly important for application scenarios that require high-temperature working environments.

[0020] 3. Reducing agglomeration: During the synthesis of the material, the agglomeration of the material can be effectively reduced by ball milling and high-temperature calcination. This helps to improve the uniformity and stability of the material, thereby enhancing the performance of the final product.

[0021] 4. Optimizing the electron transport channel: After doping at the Fe site, the distance of Fe-O is further shortened, optimizing the electron transport channel, thus greatly increasing the conductivity. This is crucial for improving the overall performance of the battery.

[0022] 5. Flexibility and adjustability: The method provided in the present application allows the performance of the material to be optimized by adjusting the doping ratio and type. This flexibility enables the material to be customized according to specific application requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings used in the embodiments: Figure 1 are the XRD patterns and refined structure diagrams of LaFeO3 and the transition metal-doped perovskite oxides prepared in Examples 1-4; where (a) is the XRD pattern; (b) is the refined structure diagram; Figure 2 are the SEM images of the element distribution diagrams of LaFeO3, the transition metal-doped perovskite oxides prepared in Examples 1-4, and Al, Co, Ni, Zn; where, (a) is LaFeO3; (b) is LaFe 0.8 Al 0.2 O3 prepared in Example 1; (c) is LaFe 0.8 Co 0.2 O3 prepared in Example 2; (d) is LaFe 0.8 Ni 0.2 O3 prepared in Example 3; (e) is LaFe prepared in Example 40.8 Zn 0.2 O3; (f) The figure shows the distribution maps of Al, Co, Ni, and Zn elements; TEM images of transition metal-doped perovskite oxides; among them, (a) shows LaFeO3; (b) shows LaFe prepared in Example 1 0.8 Al 0.2 O3; (c) shows LaFe prepared in Example 2 0.8 Co 0.2 O3; (d) shows LaFe prepared in Example 3 0.8 Ni 0.2 O3; (e) shows LaFe prepared in Example 4 0.8 Zn 0.2 O3; Figure 3 are TEM images of LaFeO3 and transition metal-doped perovskite oxides prepared in Examples 1-4; among them, Figure 3 (a) is the transmission image of LaFeO3; Figure 3 (b) is the transmission image of LaFeO3 doped with Al; Figure 3 (c) is the transmission image of LaFeO3 doped with Co; Figure 3 (d) is the transmission image of LaFeO3 doped with Ni; Figure 3 (e) is the transmission image of LaFeO3 doped with Zn. Figure 4 are the current-voltage curves and conductivity curves of LaFeO3 and transition metal-doped perovskite oxides prepared in Examples 1-4 when made into electrodes; among them, (a) is the current-voltage curve and (b) is the conductivity curve; Figure 5 are the discharge curves of LaFeO3 and transition metal-doped perovskite oxides prepared in Examples 1-4 when made into electrodes; Figure 6 are the HRD diagrams of LaFeO3 and transition metal-doped perovskite oxides prepared in Examples 1-4 when made into electrodes; Figure 7 are the cyclic stability curves of LaFeO3 and transition metal-doped perovskite oxides prepared in Examples 1-4 when made into electrodes. Detailed implementation manners

[0024] The implementation scheme of the present application will be described in detail below in conjunction with embodiments. However, those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be construed as limiting the scope of the present application. For those not specified in the embodiments, the operations are carried out under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used without indicating the manufacturer are all conventional products that can be obtained by purchasing in the market.

[0025] Example 1 Preparation of transition metal-doped perovskite-type oxides Weigh 0.5 mol of high-purity La2O3, 0.1 mol of Fe2O3, and 0.1 mol of Al2O3 respectively. Put the weighed raw materials into a planetary ball mill. The mass ratio of zirconia balls with a diameter of 4 mm to the sum of the masses of La2O3, Fe2O3, and Al2O3 is 10:1, and ball-mill and mix at a speed of 450 rpm for 4 hours. Then, calcine the mixture in a tubular furnace in an argon atmosphere at 1200 °C for 5 hours. The heating rate below 600 °C is 4 °C / min, and the heating rate in the range of 600 °C - 1200 °C is 1 °C / min, and the LaFe 0.8 Al 0.2 O3 material can be obtained.

[0026] Example 2 Preparation of transition metal-doped perovskite-type oxides Weigh 0.5 mol of high-purity La2O3, 0.4 mol of Fe2O3, and 0.1 mol of Co3O4 respectively. Put the weighed raw materials into a planetary ball mill. The mass ratio of zirconia balls with a diameter of 4 mm to the sum of the masses of La2O3, Fe2O3, and Co3O4 is 10:1, and ball-mill and mix at a speed of 450 rpm for 4 hours. Then, calcine the mixture in a tubular furnace in an argon atmosphere at 1200 °C for 5 hours. The heating rate below 600 °C is 4 °C / min, and the heating rate in the range of 600 °C - 1200 °C is 1 °C / min, and the LaFe 0.8 Co 0.2 O3 material can be obtained.

[0027] Example 3 Preparation of transition metal-doped perovskite-type oxides Weigh out 0.5 mol of high-purity La2O3, 0.4 mol of Fe2O3, and 0.1 mol of Ni2O3 separately. Put the weighed raw materials into a planetary ball mill, and mix them with zirconia balls with a diameter of 4 mm in a mass ratio of 10:1 to the sum of the masses of La2O3, Fe2O3, and Ni2O3, and carry out ball milling and mixing at a speed of 450 rpm for 4 hours. Then, calcine the mixture in a tubular furnace in an argon atmosphere at 1200 °C for 5 hours, with a heating rate of 4 °C / min below 600 °C and a heating rate of 1 °C / min in the range of 600 °C - 1200 °C, and then LaFe 0.8 Ni 0.2 O3 material can be obtained.

[0028] Example 4 Preparation of transition metal-doped perovskite-type oxides Weigh out 0.5 mol of high-purity La2O3, 0.4 mol of Fe2O3, and 0.1 mol of ZnO separately. Put the weighed raw materials into a planetary ball mill, and mix them with zirconia balls with a diameter of 4 mm in a mass ratio of 10:1 to the sum of the masses of La2O3, Fe2O3, and ZnO, and carry out ball milling and mixing at a speed of 450 rpm for 4 hours. Then, calcine the mixture in a tubular furnace in an argon atmosphere at 1200 °C for 5 hours, with a heating rate of 4 °C / min below 600 °C and a heating rate of 1 °C / min in the range of 600 °C - 1200 °C, and then LaFe 0.8 Zn 0.2 O3 material can be obtained.

[0029] Example 5 Preparation of transition metal-doped perovskite-type oxides Weigh out 0.5 mol of high-purity La2O3, 0.45 mol of Fe2O3, and 0.05 mol of ZnO separately. Put the weighed raw materials into a planetary ball mill, and mix them with zirconia balls with a diameter of 3 mm in a mass ratio of 10:1 to the sum of the masses of La2O3, Fe2O3, and ZnO, and carry out ball milling and mixing at a speed of 400 rpm for 5 hours. Then, calcine the mixture in a tubular furnace in an argon atmosphere at 1200 °C for 6 hours, with a heating rate of 3 °C / min below 600 °C and a heating rate of 0.8 °C / min in the range of 600 °C - 1200 °C, and then LaFe 0.9 Zn 0.1 O3 material can be obtained.

[0030] Example 6 Preparation of transition metal-doped perovskite-type oxides Weigh 0.5 mol of high-purity La2O3, 0.35 mol of Fe2O3, and 0.15 mol of ZnO separately. Put the weighed raw materials into a planetary ball mill, and mix them with zirconia balls with a diameter of 5 mm at a mass ratio of 10:1.2 to the sum of the masses of La2O3, Fe2O3, and ZnO, and perform ball milling at a speed of 500 rpm for 3 hours. Then, calcine the mixture in a tube furnace at 1200 °C in an argon atmosphere for 4 hours, with a heating rate of 5 °C / min below 600 °C and a heating rate of 1.5 °C / min in the range of 600 °C - 1200 °C, and then LaFe 0.7 Zn 0.3 O3 material can be obtained.

[0031] Performance detection test 1. Sample the perovskite-type oxides doped with transition metals prepared in Examples 1 - 4 and LaFeO3 with a purity of 99.9%, and test them with a scanning electron microscope (SEM), a transmission electron microscope (TEM), and an X-ray diffractometer (XRD). The test results are as Figure 1 、 Figure 2 and Figure 3 shown.

[0032] 2. Preparation and testing methods of the battery Preparation of the test electrode and the battery (1) After drying the perovskite-type oxides doped with transition metals prepared in Examples 1 - 4 and the purchased LaFeO with a purity of 99.9% separately, accurately weigh 0.15 g of perovskite-type oxide powder and 0.75 g of nickel carbonyl powder on an analytical balance, and mix the weighed perovskite-type oxide powder and nickel carbonyl powder. Note: The perovskite-type oxide powder is a general term for the perovskite-type oxides doped with transition metals prepared in Examples 1 - 4 and the purchased LaFeO with a purity of 99.9%.

[0033] (2) Press the mixed powder on a tablet press into a grayish-black electrode thin sheet with a diameter of 10 mm and a thickness of 1 mm.

[0034] (3) Polish the surface of the electrode sheet to avoid damaging the nickel foam during the charge and discharge process, weigh the polished electrode sheet, and calculate the effective mass.

[0035] (4) Place the weighed electrode sheet in the center of the nickel foam and then put it on the tablet press to apply pressure. Finally, use a spot welder to weld the nickel foam wrapped with the electrode sheet to the nickel sheet to prepare a welded electrode.

[0036] (5) The welded electrode is used as the negative electrode of the nickel-metal hydride battery and assembled with the nickel hydroxide positive electrode into a simple battery. Its electrochemical performance is tested at 60 °C. The test method is as follows: charging at 0.2C for 8 h, with a cut-off voltage of 1.2V; standing for 10 min, and then discharging at 0.2C with a cut-off voltage of 0.7V. The number of cycling: 100.

[0037] The electrolyte of the test device is KOH solution (6molL –1 ), the anode is the LaFeO3 and its composite electrode to be tested, the cathode is the Ni(OH)2 / NiOOH electrode, and the polypropylene porous membrane is used as a separator to prevent the short circuit of the positive and negative electrodes of the battery. The battery is reinforced with nickel wire and soaked in KOH solution for 24 hours, and then relevant electrochemical performance tests are carried out at 60 °C. The test results are as shown in Figure 4 、 Figure 5 、 Figure 6 and Figure 7 .

[0038] Brief Introduction of Electrochemical Test Method (1) Maximum Discharge Capacity Test The maximum discharge capacity is the maximum value that appears during the activation process of the electrode, and the discharge capacity at this time is the maximum discharge capacity of the battery. The charging regime adopted in this application is: the charging current density is 60 mA g –1 , the charging time is 8 hours. After the battery is fully charged, it is left standing for 10 min, and then discharged at a discharge current density of 60 mA g –1 . The cut-off voltage for discharge is 0.7V (vs. Ni(OH)2 / NiOOH). The maximum discharge capacity of the electrode is represented by C max .

[0039] (2) High-rate Discharge Performance Test The activated battery is charged at a current density of 300 mA g –1 . After standing, it is discharged at discharge current densities of 60 mA g –1 , 300 mA g –1 , 600 mA g –1 , 900 mA g –1 , 1200 mA g –1 , 1500 mA g –1 respectively, with a cut-off voltage of 0.7V. The discharge capacities C i at different discharge current densities are marked. The high-rate discharge performance of the electrode can be calculated by the following formula: where C i is the discharge capacity at a discharge current density of i d , and Cmax is the maximum discharge capacity of the electrode.

[0040] (3) Cycling stability performance test After activation, the electrode was charged at a current density of 300 mA / g –1 and discharged at a current density of 60 mA / g with the number of cycles being an integer multiple of 20, denoted by C n ; the discharge capacity when discharging; the capacity retention rate of the electrode at the nth cycle is represented by S –1 and is calculated by the following formula: n

[0041] Figure 1 are the XRD patterns and refined structure diagrams of LaFeO3 and the transition metal-doped perovskite-type oxides prepared in Examples 1-4; as can be seen from Figure 1 , all the characteristic diffraction peaks of the XRD match the standard PDF card (JCPDS 74-2203) of LaFeO3, the space group is Pbnm, and there are no other impurities, indicating that Al, Co, Ni, and Zn are incorporated into the perovskite phase lattice. To study the influence of the incorporated elements on the lattice structure of LaFeO3, we performed Rietveld refinement on all the doped samples using the FullProf software according to the position where the doped atoms occupy Fe. The refined structure diagram is shown in Figure 1 Figure (b) in.

[0042] Figure 2 are the SEM images of LaFeO3, the transition metal-doped perovskite-type oxides prepared in Examples 1-4, and the element distribution diagrams of Al, Co, Ni, and Zn; as can be seen from Figure 2 Figure 2 (a) is the SEM image of LaFeO3. The agglomeration of LaFeO3 is relatively serious and the particle size is relatively large. After Al doping, it can be seen that the agglomeration phenomenon is reduced, the particles are partly circular, pores appear between the particles, and at the same time the particle size is reduced ( Figure 2 (b)). Figure 2 (c) is the SEM image after Co doping, and similarly, the agglomeration phenomenon of the particles is also reduced. Figure 2 (d) is the SEM image after Ni doping. It can be seen that after Ni doping, the particle size is greatly reduced, but there is still a certain degree of agglomeration. After Zn doping of LaFeO3, the particle morphology has changed, from the previous irregular and serious agglomeration to a regular polyhedral structure, and at the same time the agglomeration is inhibited. Figure 2 (f), from the element distribution diagram, it can be seen that Al, Ni, Co, and Zn are evenly distributed in the sample.

[0043] Figure 3 TEM images of LaFeO3 and perovskite-type oxides doped with transition metals prepared in Examples 1-4; from Figure 3 it can be seen that Figure 3 (a) is the transmission image of LaFeO3, from which it can be seen that the agglomeration phenomenon is relatively serious. Figure 3 (b) is the transmission image of LaFeO3 doped with Al. It can be seen from the figure that the agglomeration is inhibited to a certain extent after Al doping, and the particles also become smaller. After doping with Co element, the particles change from irregular to quasi-circular structure, and the agglomeration phenomenon is reduced, as Figure 3 (c) shows. From Figure 3 (d), it can be seen that the particle size is significantly reduced after Ni doping, much smaller than that of the undoped LaFeO3 material, and the dispersion degree is also greater than that of the undoped material. Figure 3 (e) is LaFeO3 doped with Zn. It can be seen from the figure that the morphology is similar to regular polygons and has a certain degree of dispersion compared with LaFeO3, which is basically consistent with the SEM results.

[0044] Figure 4 Current-voltage curves and conductivity curves of LaFeO3 and perovskite-type oxides doped with transition metals prepared in Examples 1-4 as electrodes; where (a) is the current-voltage curve and (b) is the conductivity curve. The four-probe method was used to measure the resistance of the electrode sheet, and then the conductivity of the material was calculated. It can be seen from the figure that the conductivity after doping at the Fe site is much higher than that of undoped LaFeO3. The conductivity after doping with Co is 11.08 S / m –1 , nearly twice as high as the conductivity of LaFeO3 (4.308 S / m –1 ), more than doubled compared with LaFe 0.8 Al 0.2 O3 (9.846 S / m –1 ), LaFe 0.8 Ni 0.2 O3 (10.21 S / m –1 ), and LaFe 0.8 Zn 0.2 O3 (9.95 S / m –1 ). This also confirms from the side that the conductivity of LaFeO3 can be improved by doping at the B site.

[0045] Figure 5Discharge curves of electrodes made of LaFeO3 and perovskite-type oxides doped with transition metals prepared in Examples 1-4; First, it can be seen from the capacity that the discharge capacity has increased after doping. From the discharge plateau, the discharge plateau has increased after doping with Co, Ni, and Zn. However, the discharge plateau voltage has decreased after doping with Al, which may be due to the increase in the internal resistance of the battery after doping with Al, resulting in a decrease in the plateau voltage. Figure 4 It can be seen that the conductivity of the material has increased after B-site doping, which is beneficial to the increase of the discharge capacity.

[0046] Figure 6 HRD diagrams of electrodes made of LaFeO3 and perovskite-type oxides doped with transition metals prepared in Examples 1-4; HRD can be calculated by the following formula: HRD = C d / C max ×100%, where C d is the discharge capacity at a current density of 1500 mAg –1 , and C max is the discharge capacity at a current density of 60 mAg –1 . Compared with undoped LaFeO3 (11.2%), the HRD value of LaFe 0.8 Al 0.2 O3 is higher (66.1%). The HRD values after doping with Zn, Co, and Ni are 55.7%, 24.6%, and 42.6% respectively. This is because the agglomeration phenomenon of undoped LaFeO3 powder is relatively serious, which hinders the diffusion of hydrogen. The LaFeO3 material after B-site doping has a larger specific surface area and higher conductivity, making electron migration easier, thereby leading to an increase in high-rate discharge performance.

[0047] Figure 7 Cyclic stability curves of electrodes made of LaFeO3 and perovskite-type oxides doped with transition metals prepared in Examples 1-4; It can be seen from Figure 7 that after 100 charge-discharge cycles, the discharge capacities of LaFe 0.8 M 0.2 O3 (M = Al, Co, Ni, Zn) are 223.5 mAhg –1 , 200.5 mAhg –1 , 218.6 mAhg –1 , and 246.8 mAhg –1 respectively, and the discharge capacity of undoped LaFeO3 is only 98.53 mAh g –1 . LaFeO3 and LaFe 0.8 M 0.2The capacity retention rates of the O3 (M = Al, Co, Ni, Zn) electrodes after 100 cycles are 66.0%, 70.6%, 75.2%, 77.5%, and 75.9% respectively. It can be seen that after doping at the B site, the cycle life of the material is greatly improved and the capacity retention rate of the material is increased.

[0048] The above embodiments are only used to explain the technical solutions of the present application rather than to limit it. Although the above embodiments have specifically described the present application, those skilled in the art should understand that the specific implementation manners of the present application can still be modified or equivalently replaced. Any modification and equivalent replacement without departing from the spirit and scope of the present application shall be covered by the protection scope of the present application.

Claims

1. A transition metal-doped perovskite-type oxide, characterized in that, Its chemical general formula is LaFe 1-x M X O3, where M is at least one of Al, Co, Ni, and Zn, and 0 < 𝑥 ≤ 0.3; the transition metal-doped perovskite-type oxide has a cubic perovskite structure, and the 3d orbit of the B-site doped metal forms a covalent interaction with the 2p orbit of O through hybridization.

2. The perovskite-type oxide doped with transition metal according to claim 1, wherein Its chemical general formula is LaFe 0.8 M 0.2 O3, where M is at least one of Al, Co, Ni, and Zn; the transition metal-doped perovskite-type oxide has a cubic perovskite structure, and the 3d orbitals of the B-site doped metal and the 2p orbitals of O form a covalent interaction through hybridization.

3. A method for preparing a transition metal-doped perovskite-type oxide as described in claim 1 or 2, characterized in that, It includes the following steps: S31. According to chemical formula stoichiometry, add La2O3, Fe2O3 and doped metal oxide into a ball mill respectively, and carry out ball milling and mixing with the mass ratio of ball milling balls to raw materials being 10:0.8 - 1.2 to obtain a mixture; S32. Under an argon atmosphere, carry out high-temperature calcination on the mixture, and cool it to room temperature to obtain a transition metal-doped perovskite-type oxide.

4. The preparation method of a transition metal-doped perovskite-type oxide according to claim 3, wherein, In step S31, the doped metal oxide is selected from at least one of Al2O3, Co3O4, Ni2O3, ZnO.

5. The preparation method of a transition metal-doped perovskite-type oxide according to claim 3, characterized in that, In step S31, the ball mill is a planetary ball mill; the ball milling balls are zirconia balls with a diameter of 3 - 5 mm.

6. The preparation method of a transition metal-doped perovskite-type oxide according to claim 3, wherein In step S31, the conditions for the ball milling and mixing include: the rotation speed is 400 - 500 rpm, and the ball milling time is 3 - 5 h.

7. The preparation method of a transition metal-doped perovskite-type oxide according to claim 3, wherein, In step S32, the conditions for the high-temperature calcination are: the heating rate is 3 - 5 °C / min below 600 °C, the heating rate is 0.8 - 1.5 °C / min from 600 °C to 1200 °C, and finally keep the temperature at 1200 °C for 4 - 6 hours.

8. Use of a transition metal-doped perovskite-type oxide as described in claim 1 or 2, characterized in that, The transition metal-doped perovskite-type oxide is used as a negative electrode material in a nickel-metal hydride battery.

9. The application of a transition metal-doped perovskite-type oxide according to claim 8, characterized in that, The nickel-metal hydride battery includes: Positive electrode: Ni(OH)2 / NiOOH electrode; Negative electrode: the transition metal-doped perovskite-type oxide material described in claim 1 or 2 and nickel carbonyl powder; Electrolyte: 6 mol / L KOH solution; Separator: polypropylene porous membrane.

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