High-entropy doped manganese / iron-based layered material, preparation method thereof, pole piece and battery

Abstract

The invention discloses a high-entropy doped manganese / iron-based layered material and a preparation method thereof, a pole piece and a battery, the material contains six different metal cations of manganese, iron, copper, titanium, lithium and magnesium in a transition metal layer, the material contains two different metal cations of sodium and calcium in a sodium layer, the chemical formula of the layered material is Na < 0.62 > Ca < 0.03 > Mn < 0.58 > Fe < 0.23 > Cu < 0.085 > Mg < 0.01 > Ti < 0.015 > Li < 0.08 > O < 2 >, the problem that the specific capacity is reduced and unstable in the prior art is solved.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and in particular to a high-entropy doped manganese / iron-based matrix material and a preparation method thereof, a pole piece and a battery. Background Art

[0002] In recent years, due to the widespread application of lithium-ion batteries and people's concerns about the shortage of lithium resources, sodium-ion batteries, which have similar mechanisms to lithium-ion batteries, have attracted global attention and have been widely studied. Valuable research work is mainly focused on the development of advanced positive electrode materials, negative electrode materials, and separators, which have shown good performance in the application of sodium-ion batteries. The development of sodium-ion batteries has been hindered by the lack of low-cost and high-performance electrode materials. Among all common positive electrode materials, layered Na x TMO 2 Due to its high theoretical capacity, high output voltage and simple preparation process, it has aroused great interest and high expectations among researchers. x TMO 2 It is mainly divided into P2 type and O3 type. Due to the structural characteristics of P2 type positive electrode materials and wide sodium ion diffusion channels, it exhibits excellent rate performance and long cycle stability at low voltage. However, at high voltage (>4.2V), the P2 type will undergo an irreversible P2-O2 phase transition, which leads to a sharp drop in capacity, greatly limiting its application. O3 type positive electrode materials have a high theoretical specific capacity due to their high sodium content. However, the O3 type structure requires sodium ions to diffuse through tetrahedral sites surrounded by adjacent octahedral sites during the cycle, which limits its electrochemical kinetics. P2-type manganese-based or manganese / iron-based oxides are considered to be the most promising sodium-ion battery positive electrode materials due to their high capacity and extremely abundant iron and manganese resources. Layered manganese-based materials (Na x MnO 2 ) as an example, manganese is mainly Mn 3+ and Mn 4+ There are two valence states. Mn 3+ It has a high spin electronic configuration with a single electron occupying the eg* orbital, which causes the octahedron to be distorted, the so-called Jahn-Teller distortion. 3+The local distortion caused by ions can lead to irreversible structural changes, resulting in rapid capacity decay. So far, researchers have made a lot of efforts to solve the above problems, such as doping with elements such as lithium, magnesium, aluminum, titanium, copper, zirconium, tin, etc. to improve structural stability, surface modification to avoid metal dissolution, or developing P2-O3 composite structures to take advantage of the advantages of both phases. Despite the encouraging progress, manganese / iron-based cathode materials still cannot meet industrial needs. Building a strong structure is the basis for layered cathode materials to withstand repeated sodium ion insertion / extraction. Therefore, there is an urgent need for an effective method to significantly enhance the structure of layered cathode materials.

[0003] High entropy materials are a new type of material. Due to their high structural stability, mechanical properties, electrical properties and catalytic properties, they have been widely used in electrochemical energy storage, catalysis and environmental protection. High entropy materials are usually a mixture of five or more metal cations with an atomic percentage between 5% and 35%. When the concept of high entropy is introduced into the positive electrode material of sodium ion batteries, the sodium ions and metal cations in the transition metal layer of the high entropy material are disordered, making it difficult to form a sodium ion / vacancy ordered structure, which can delay or inhibit the formation of phase transitions in the positive electrode material. At the same time, various metal cations are randomly distributed on the same site, which increases the configurational entropy and stabilizes the crystal structure, making the electrode material have high energy density and good cycle stability. The main redox metal centers in the positive electrode materials of sodium ion batteries are only iron, cobalt, nickel and manganese. Traditional high entropy materials usually introduce a large amount of inert elements that do not undergo redox reactions to achieve solid solution of five or more metal elements. In addition, in order to achieve the maximum configurational entropy, the ratio of all doping elements is usually equimolar. Due to the stability brought by high entropy and the synergistic effect of multiple components, high entropy layered oxide cathode materials usually exhibit high ionic conductivity and stable sodium ion extraction behavior. However, the influence of high entropy structure and different doping elements on the phase transition of manganese / iron-based oxides during the cycle process has not been fully explored. A large amount of inert elements and equimolar properties will lead to the loss of theoretical specific capacity of cathode materials, affecting the practical application of high entropy materials. Summary of the invention

[0004] The present invention provides a high-entropy doped manganese / iron-based matrix material and a preparation method thereof, a pole piece and a battery, which solve the problems of reduced specific capacity and instability in the prior art.

[0005] In order to solve this technical problem, this solution provides the following technical solutions:

[0006] A high entropy doped manganese / iron-based layered material, wherein the material comprises six different metal cations of manganese, iron, copper, titanium, lithium and magnesium in the transition metal layer, and two different metal cations of sodium and calcium in the sodium layer, wherein the chemical formula of the layered material is Na0.62 Ca 0.03 Mn 0.58 Fe 0.23 Cu 0.085 Mg 0.01 Ti 0.015 Li 0.08 O 2 .

[0007] Preferably, the high entropy doped manganese / iron based layered material is of P2 type.

[0008] High entropy doped Mn / Fe based hierarchical materials, electrochemically active Fe 3+ and Mn 4+ Provides charge compensation, which is the main contribution to specific capacity; low-valent Mg 2+ and Cu 2+ It helps to increase the valence of Mn and alleviate the Jahn-Teller distortion; at the same time, Cu 2+ The presence of can effectively reduce the structural distortion of local OO and enhance the redox reversibility of lattice O. Doping Li in the TM layer plays the role of a structural stabilizer because the monovalent Li + It is beneficial to retain more Na in the deep desodium structure + , maintaining electrostatic balance, thereby inhibiting the O2 phase transition. 4+ and Mn 4+ The significant difference in Fermi level between the two 4+ The inhibition of cation and charge ordering in the TM layer and Na + / vacancy ordering. Ca doping in the Na layer 2+ Can play the role of "pillar", because Ca 2+ and O 2- The strong interaction between them is beneficial to improve the structural and cyclic stability of the material. In addition to the various functions brought by different metal cations, the structural stability driven by high entropy doping also suppresses the P2-P'2 phase transition and particle cracking, which is beneficial to reduce the Na + Therefore, the functional integration of different metal cations, combined with the improvement of structural stability driven by high entropy doping, synergistically brings about the high reversible capacity, high rate performance and long cycle stability of NFM-HEO.

[0009] This solution also provides a method for preparing the above-mentioned high entropy doped manganese / iron-based matrix material, comprising the following steps:

[0010] S1, the stoichiometric ratio of Fe(NO 3 ) 3 9H 2 O、Mn(NO 3 ) 2·4H 2 O and Cu(NO 3 ) 2 ·3H 2 O is fully dissolved in deionized water and mixed with transition metal salt solution;

[0011] S2, then adding the sodium hydroxide aqueous solution dropwise to the mixed transition metal salt solution; adjusting the pH value of the system with ammonia water after the dropwise addition for 1 hour to obtain a precipitate; filtering the generated precipitate, washing it with deionized water and ethanol, and then drying it to obtain a precursor;

[0012] S3. Then, the dried precursor is mixed with a stoichiometric ratio of TiO 2 、CaCO 3 , Li 2 CO 3 and MgCO 3 , and a 5% excess of stoichiometric Na 2 CO 3 The mixture is ball-milled, and then the mixture is calcined and taken out for grinding;

[0013] S4. Finally, the mixture is calcined at 900°C in an oxygen atmosphere at a heating rate of 5°C / min to form a high entropy doped manganese / iron based matrix material NFM-HEO.

[0014] Preferably, the total concentration of the transition metal salt solution in step S1 is 2.0 mol L -1 .

[0015] Preferably, the concentration of the sodium hydroxide aqueous solution in step S2 is 4.0 mol / L -1 The concentration of ammonia water is 1.0 molL -1 ; Adjust the pH value of the system to 11.

[0016] Preferably, the drying temperature in step S2 is 100° C. and the drying time is 6 hours.

[0017] Preferably, the ball milling speed in step S3 is 600 rpm, and the ball milling time is 6 hours; the calcination temperature is 500° C., and the calcination time is 6 hours.

[0018] Preferably, the calcination time in step S4 is 12 hours.

[0019] The present solution also provides a pole piece, which includes the above-mentioned high-entropy doped manganese / iron-based layered material or the high-entropy doped manganese / iron-based layered material prepared by the preparation method.

[0020] The present solution also provides a battery, which includes the above-mentioned pole piece, counter electrode and diaphragm, and the diaphragm is arranged between the pole piece and the counter electrode.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] The material contains six different metal cations in the transition metal layer (manganese, iron, copper, titanium, lithium, magnesium) and two different metal cations in the sodium layer (sodium, calcium). These different cations have different electrochemical activities, valence states, electronegativity, or ability to bond with oxygen, and play different roles in the high entropy material. 3+ and Mn 4+ Provides charge compensation, which is the main contribution to specific capacity. 2+ and Cu 2+ It helps to increase the valence of Mn and alleviate the Jahn-Teller distortion. 2+ The presence of can effectively reduce the structural distortion of local OO and enhance the redox reversibility of lattice O. Doping Li in the TM layer plays the role of a structural stabilizer because the monovalent Li + It is beneficial to retain more Na in the deep desodium structure + , maintaining electrostatic balance, thereby inhibiting the O2 phase transition. 4+ and Mn 4+ The significant difference in Fermi level between the two 4+ The inhibition of cation and charge ordering in the TM layer and Na + / vacancy ordering. Ca doping in the Na layer 2+ Can play the role of "pillar", because Ca 2+ and O 2- The strong interaction between them is beneficial to improve the structure and cycle stability of the material. Therefore, the functional integration of different metal cations, combined with the improvement of structural stability driven by high entropy doping, synergistically brings about the high reversible capacity, high rate performance and long cycle stability of NFM-HEO. Electrochemical measurements as well as morphological and phase characterizations show that the high entropy structure hinders the P2-P'2 phase transition, stabilizes the P2-OP4 phase transition and anion redox reaction, promotes the sodium ion migration kinetics, and at the same time, due to the use of the reversible electrochemical redox reaction of oxygen at high voltage, the voltage decay is suppressed and the high-rate reversible capacity is improved. The prepared high-entropy layered oxide cathode material exhibits a smooth charge and discharge curve, with a reversible specific capacity of 134 mAh g at a rate of 0.1C. -1 After 500 cycles at 1C rate, the capacity retention rate reaches 85.4%, and it has good rate performance, with a reversible capacity of 70 mAh g at 5C rate. -1. Phase characterization shows that its phase transition at high voltage is well suppressed and it also exhibits reversible redox reaction. The performance improvement of high-entropy doped manganese / iron-based oxide cathode materials may be largely due to the high-entropy driven structural stability and the different functions of various elements. High-entropy P2-type cathode materials have great practical application potential in the field of sodium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0024] Figure 1 (a) X-ray diffraction patterns of high entropy oxide (NFM-HEO) and NFM;

[0025] Figure 1 (b) is a scanning electron microscope image of high entropy oxide (NFM-HEO);

[0026] Figure 1 (c) is a scanning electron microscope image of NFM;

[0027] Figure 1 (d) is a high-resolution transmission electron microscopy image of a high entropy oxide (NFM-HEO), and the inset is the corresponding fast Fourier transform (FFT) pattern and the display of the interlayer spacing;

[0028] Figure 1 (e) Energy dispersive X-ray spectroscopy (EDS) scan of the surface of selected particles of high entropy oxide (NFM-HEO) material;

[0029] Figure 1 (f) is the selected area electron diffraction (SAED) image of high entropy oxide (NFM-HEO);

[0030] Figure 1 (g) is the energy dispersive X-ray spectroscopy (EDS) surface scan of the selected particle surface of the NFM material;

[0031] Figure 2 (a) is the X-ray diffraction refinement pattern of NFM;

[0032] Figure 2 (b) is the X-ray diffraction refinement pattern of high entropy oxide (NFM-HEO);

[0033] Figure 2 (c) Schematic diagram of the crystal structure of NFM and high entropy oxide (NFM-HEO);

[0034] Figure 3(a) Comparison of the constant current charge-discharge curves of the new high entropy oxide (NFM-HEO) and the NFM half-cell at the first cycle at a rate of 0.1C;

[0035] Figure 3 (b) is the cyclic voltammetry curve of high entropy oxide (NFM-HEO) at a scan rate of 0.1 mV / s;

[0036] Figure 3 (c) is the differential capacity (dQ / dV) curve of high entropy oxide (NFM-HEO);

[0037] Figure 3 (d) Comparison of rate performance between high entropy oxide (NFM-HEO) and NFM;

[0038] Figure 3 (e) is the cyclic voltammetry curve of NFM at a scan rate of 0.1 mV / s;

[0039] Figure 3 (f) is the differential capacity (dQ / dV) curve of NFM;

[0040] Figure 3 (g) Long cycle performance of high entropy oxide (NFM-HEO) and NFM at 1C rate;

[0041] Figure 4 (a) Impedance of high entropy oxide (NFM-HEO) after different cycle times;

[0042] Figure 4 (b) Impedance of NFM after different cycle times;

[0043] Figure 4 (c) Distribution of relaxation time (DRT) analysis of high entropy oxide (NFM-HEO) after different cycle times;

[0044] Figure 4 (d) Distribution of relaxation time (DRT) analysis of NFM after different number of cycles;

[0045] Figure 4 (e) Comparison of constant current intermittent titration (GITT) curves of high entropy oxide (NFM-HEO) and NFM;

[0046] Figure 4 (f) Comparison of the calculated diffusion coefficients of high entropy oxide (NFM-HEO) and NFM;

[0047] Figure 5 (a) X-ray photoelectron spectra of high entropy oxide (NFM-HEO) in different charge states: 2p orbital of iron (Fe);

[0048] Figure 5 (b) X-ray photoelectron spectra of high entropy oxide (NFM-HEO) in different charge states: 2p orbital of manganese (Mn);

[0049] Figure 5 (c) X-ray photoelectron spectra of high entropy oxide (NFM-HEO) in different charge states: 1s orbital of oxygen (O);

[0050] Figure 5 (d) X-ray photoelectron spectra of high entropy oxide (NFM-HEO) in different charge states: 2p orbital of copper (Cu);

[0051] Figure 5 (e) X-ray photoelectron spectra of high entropy oxide (NFM-HEO) in different charge states: 2p orbital of titanium (Ti);

[0052] Figure 5 (f) X-ray photoelectron spectra of high entropy oxide (NFM-HEO) in different charge states: 2p orbital of calcium (Ca);

[0053] Figure 6 (a) is the in situ X-ray diffraction (XRD) pattern of high entropy oxide (NFM-HEO);

[0054] Figure 6 (b) is a local enlarged view of the evolution of X-ray diffraction peaks of high entropy oxide (NFM-HEO) corresponding to the (002), (004) and (102) crystal planes;

[0055] Figure 6 (c) is the comparison of X-ray diffraction patterns of high entropy oxide (NFM-HEO) before and after 100 cycles at 1C rate;

[0056] Figure 7 (a) Constant current charge-discharge curves of high entropy oxide (NFM-HEO) after different exposure times;

[0057] Figure 7 (b) is a small-angle partial magnification of the X-ray diffraction pattern of the high entropy oxide (NFM-HEO) after being exposed to humid air for different lengths of time;

[0058] Figure 7 (c) is a large-angle partial magnification of the X-ray diffraction pattern of the high entropy oxide (NFM-HEO) after exposure to humid air for different lengths of time;

[0059] Figure 7 (d) Constant current charge-discharge curves of NFM after different exposure times;

[0060] Figure 7 (e) NFM is a small-angle partial magnification of the X-ray diffraction pattern after exposure to humid air for different lengths of time;

[0061] Figure 7 (f) NFM is a large-angle partial magnification of the X-ray diffraction pattern after exposure to humid air for different lengths of time;

[0062] Figure 7 (g) The change of open circuit voltage (OCV) and corresponding structural changes of high entropy oxide (NFM-HEO) after exposure to humid air for different time periods;

[0063] Figure 7 (h) The change of open circuit voltage (OCV) and corresponding structural changes of NFM after exposure to humid air for different time periods;

[0064] Figure 8 (a1) shows the particle crack evolution of high entropy oxide (NFM-HEO) after 0 cycles;

[0065] Figure 8 (a2) shows the particle crack evolution of high entropy oxide (NFM-HEO) after 50 cycles;

[0066] Figure 8 (a3) shows the particle crack evolution of high entropy oxide (NFM-HEO) after 100 cycles;

[0067] Figure 8 (a4) shows the particle crack evolution of high entropy oxide (NFM-HEO) after 200 cycles;

[0068] Figure 8 (b1) shows the particle crack evolution of NFM after 0 cycles;

[0069] Figure 8 (b2) shows the particle crack evolution of NFM after 50 cycles;

[0070] Figure 8 (b3) shows the particle crack evolution of NFM after 100 cycles;

[0071] Figure 8 (b4) shows the particle crack evolution of NFM after 200 cycles. DETAILED DESCRIPTION

[0072] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. The illustrative embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0073] The sodium carbonate (Na 2 CO 3 ), sodium hydroxide (NaOH), calcium carbonate (CaCO 3 ), ethanol and ammonia were purchased from Chengdu Jinshan Chemical Reagent Co., Ltd.; ferric nitrate nonahydrate (Fe(NO 3 ) 3 9H 2 O) and titanium dioxide (TiO 2 ) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; manganese nitrate tetrahydrate (Mn(NO 3 ) 2 ·4H 2 O), copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O) and magnesium carbonate (MgCO 3 ) was purchased from Shanghai Titan Technology Co., Ltd.; lithium carbonate (Li 2 CO 3 ) and N-methylpyrrolidone (NMP) were purchased from Shanghai MacLean Biochemical Technology Co., Ltd.; metallic sodium was purchased from Shandong Xiya Chemical Co., Ltd.; conductive carbon black (Super P) was purchased from TIMCAL Graphite & Carbon Co., Ltd.; polyvinylidene fluoride (PVDF) was purchased from Solvay Chemicals Co., Ltd.; Whatman GF / D glass fiber filter paper was purchased from Shanghai Jinpan Biotechnology Co., Ltd.; NC-008 electrolyte (1 M sodium perchlorate (NaClO 4 ) was purchased from Suzhou Duoduo Chemical Technology Co., Ltd.

[0074] Example 1

[0075] P2-Na 0.62 Ca 0.03 Mn 0.58 Fe 0.23 Cu 0.085 Li 0.08 Ti 0.015 Mg 0.01 O 2 Preparation of (NFM-HEO)

[0076] First, the stoichiometric ratio (0.23:0.58:0.085) of Fe(NO 3 ) 3 9H 2 O、Mn(NO 3 ) 2 ·4H 2 O and Cu(NO 3 ) 2 ·3H 2 O is fully dissolved in deionized water, and the total concentration of the transition metal salt solution is 2.0 mol L -1 , then 4.0 molL -1 The sodium hydroxide aqueous solution was added dropwise to the mixed transition metal (TM) salt solution at a volume ratio of 2:1. After 1 hour of dropwise addition, 1.0 mol L -1 The pH value of the system was adjusted to 11 by adding ammonia water. The resulting precipitate was filtered, washed with deionized water and ethanol, and then dried at 100 °C for 6 h. Subsequently, the dried precipitate was mixed with a stoichiometric ratio of TiO 2 、CaCO 3 , Li 2 CO 3 and MgCO 3 , and a 5% excess of stoichiometric Na 2 CO 3 Mixed (TiO 2 :CaCO 3 :Li 2 CO 3 :MgCO 3 :Na 2 CO 3 =0.015:0.03:0.04:0.01:0.325), ball milled at 600 rpm for 6 h. The mixture was then calcined at 500°C for 6 h, taken out and ground, and finally calcined at 900°C for 12 h in an oxygen atmosphere at a heating rate of 5°C / min to form a NFM-HEO sample.

[0077] Comparative Example 1

[0078] P2-Na 0.65 Mn 0.6 Fe 0.4 O 2 Preparation of (NFM)

[0079] The NFM cathode material was prepared by coprecipitation. First, the stoichiometric ratio of Fe(NO 3 ) 3 9H 2 O and Mn(NO 3 ) 2 ·4H2 O was fully dissolved in deionized water, and then sodium hydroxide aqueous solution was added dropwise. After 1 hour of dropwise addition, the pH value of the system was adjusted to 11 with aqueous ammonia. The resulting precipitate was filtered, washed with deionized water and ethanol, and then dried at 100°C for 6 hours. Subsequently, the dried precipitate was mixed with an excess of 5% stoichiometric Na 2 CO 3 The mixture was mixed and ball-milled at 600 rpm for 6 h. The mixture was then calcined at 500 °C for 6 h, taken out and ground, and finally calcined at 900 °C for 12 h in an oxygen atmosphere at a heating rate of 5 °C / min to form an NFM sample.

[0080] Example 2

[0081] The NFM-HEO prepared in Example 1 and the NFM material prepared in Comparative Example 1 were mixed with polyvinylidene fluoride (PVdF) and conductive carbon black (SuperP) in a mass ratio of 85:8:7, dispersed in N-methylpyrrolidone (NMP) to form a uniform slurry, and then evenly coated on aluminum foil with a scraper and dried at 120°C for 6 hours. The prepared electrode sheet was double-roll pressed and then cut into discs with a diameter of 14 mm. The active material loading was 2 to 3 mg and used as a working electrode. The metallic sodium was rolled and cut into discs with a diameter of 16 mm and used as a counter electrode. In an argon-filled glove box, GF / D glass fiber filter paper was used as a diaphragm, 1M sodium perchlorate (NaClO 4 ) ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1) solution and 5wt% fluoroethylene carbonate (FEC) as electrolyte were used to assemble CR2032 button cells. Constant current charge and discharge (GCD) test and constant current intermittent titration technique (GITT) test were performed using CT-4008T battery test system (Shenzhen Xinwei Electronics Co., Ltd.) with a pulse current of 20mAg -1 , lasting 0.5 hours, and then different batteries were left to stand for 1 hour. Electrochemical impedance spectroscopy (EIS) tests were performed in the frequency range of 100kHz to 10mHz using a PARSTAT4000A electrochemical workstation (Princeton, USA), and cyclic voltammetry (CV) tests were performed in the voltage range of 2.0 to 4.3V.

[0082] The crystal structure of NFM-HEO and NFM materials was confirmed using a D8AdvanceX-ray diffractometer (XRD, Bruker, Germany) with a CuKα ray of 40kV. XRD data were processed and refined using GSAS-II software. In-situ XRD results were also obtained using a D8AdvanceX-ray diffractometer (Bruker, Germany). The element molar ratios of NFM-HEO and NFM materials were determined using an Avio200 inductively coupled plasma emission spectrometer (ICP-OES, PerkinElmer, USA). The morphologies of NFM-HEO and NFM materials were observed using a JSM7800F field emission scanning electron microscope (FESEM, JEOL). The micromorphology, lattice fringes and element distribution of NFM-HEO and NFM materials were observed using a TecnaiF20 transmission electron microscope (TEM, JEOL). The surface elemental composition of NFM-HEO and NFM materials was determined using a K-AlphaX-ray photoelectron spectrometer (XPS, Thermo Fisher, USA) with MgKα radiation.

[0083] P2 type Na 0.65 Mn 0.6 Fe 0.4 O 2 (NFM) and high entropy doped Na 0.62 Ca 0.03 Mn 0.58 Fe 0.23 Cu 0.085 Li 0.08 Ti 0.01 5 Mg 0.01 O 2 The XRD results of (NFM-HEO) are as follows Figure 1 (a) As shown. XRD results show that all diffraction peaks can be well matched with the P2 phase P63 / mmc space group (JCPDS#00-054-0894) with a hexagonal crystal structure. The two oxides show similar layered structures, in which the transition metal (TM) atoms are coordinated with the surrounding 6 oxygen atoms to form TMO 6 Octahedral unit, sodium atoms interact with oxygen atoms to form NaO 6 Octahedral unit. There are no diffraction peaks in the range of 20° to 29° corresponding to superstructures derived from honeycomb order and sodium ion / vacancy order, which indicates that the sodium ion / vacancy arrangement in the sodium layer and the metal cation arrangement in the transition metal layer are disordered in both materials. In addition, no impurities were observed in NFM-HEO, indicating that all metal cations have entered the P2 structure.

[0084] Figure 2 (a) and Figure 2(b) XRD refinement results show that both NFM and NFM-HEO can be classified as P2 type structures.

[0085] NFM-HEO and NFM The difference in lattice parameters indicates that the substitution of doping elements will cause certain lattice changes. Although five metal cations are introduced on the basis of NFM structure, the change in the spacing between sodium layers is only This indicates that the P2 structure of the initial Mn / Fe-based oxide is well preserved in NFM-HEO. Although it is difficult to characterize the local environment of all metal atoms in NFM-HEO, it can be expected that elements such as Cu, Li, Ti, and Mg favor the occupation of TMO. 6 layers, because their ionic radius is much smaller than that of sodium ions, and calcium is more likely to occupy the NaO 6 layer, because the hexacoordinated sodium ion The radius of the hexacoordinated calcium ion The radius of the iron ion (Fe 3+ , when the spin is high Low spin ) and manganese ions (Mn 4+ , ) has a radius much smaller than that of calcium ions. 2+ ), lithium ion (Li + ), titanium ions (Ti 4+ ) and magnesium ions (Mg 2+ ) is located at the transition metal site, while the calcium ion (Ca 2+ ) is located at the sodium site, which confirms our speculation. We also refined the XRD results of NFM-HEO when calcium is placed in the transition metal layer. In this case, the Rwp is 6.61%, which is higher than that when calcium is placed in NaO 6 The result of layer. Figure 2 (c) shows the schematic diagram of the crystal structure of NFM and NFM-HEO. In addition, the results of the metal element composition and ratio of NFM and NFM-HEO measured by inductively coupled plasma optical emission spectrometry (ICP-OES) are shown in Tables 1 and 2, respectively.

[0086] Table 1

[0087] Na Fe Mn 0.648 0.400 0.599

[0088] Table 2

[0089] Na Ca Fe Mn Cu Mg Ti Li 0.621 0.03 0.23 0.58 0.085 0.011 0.015 0.078

[0090] High entropy doping can change the particle morphology of materials. Figure 1 (b) and Figure 1(c) Scanning electron microscope (SEM) images of NFM-HEO and NFM show that NFM presents an irregular shape with many small particles attached to the particle surface, while NFM-HEO has a more regular appearance with a significantly cleaner and smoother particle surface. This difference in morphology suggests that the introduction of multiple doping metal cations can optimize the surface properties and structure of manganese / iron-based oxides. Figure 1 The energy dispersive spectroscopy (EDS) results of (g) show that the distribution of each element is uniform in NFM, while in NFM-HEO, Figure 1 As shown in (e), various doped metal elements and matrix elements also have a uniform distribution. The selected area electron diffraction (SAED) results are shown in Figure 1 (f) shows that the exposed crystal facets of NFM-HEO are the same as those of NFM, indicating that high entropy doping with copper (Cu), calcium (Ca), lithium (Li), titanium (Ti), and magnesium (Mg) does not destroy its original crystal structure. Figure 1 (d) The clear lattice fringes in the high-resolution transmission electron microscopy (HRTEM) image show that the interplanar spacing (d-spacing) is 2.33 Å, corresponding to the (102) plane. No phase separation and lattice distortion were found, indicating that a single phase with an ordered internal structure has been formed.

[0091] Different compositions and structures can lead to differences in electrochemical performance. In order to evaluate the effect of high entropy doping on the electrochemical performance, various electrochemical tests were performed on NFM-HEO and NFM in the voltage range of 2.0 to 4.3 V. Figure 3 As shown in (a), the first cycle constant current charge-discharge (GCD) curve shows that the initial discharge specific capacity of NFM-HEO is 134 mAh / g, which is slightly lower than that of NFM (140 mAh / g). This is because the redox-active manganese (Mn) and iron (Fe) are partially replaced by doping elements that are not redox-active. Although the capacity is slightly reduced, the average discharge potential of NFM-HEO is significantly higher than that of NFM, resulting in higher output energy. The initial charge capacity of NFM material is low mainly because during the initial charging process, the valence state of manganese in the original NFM material is +4, at which time only Fe 3+ / Fe 4+ The redox couple participates in the reaction. In contrast, the initial charge capacity of NFM-HEO is higher than that of NFM, most likely due to the presence of Mn in the original material. 3+ , and the contribution of high-pressure anion redox reactions is greater. Figure 3 (a) The extension of the high potential discharge platform and Figure 3 (b) and Figure 3 (e) The cyclic voltammetry (CV) curve corresponding to Fe 3+ / Fe 4+The increase of the high potential oxidation peak of 4+ / Fe 3+ The redox reaction has a promoting effect. The CV curves of NFM-HEO at different cycle times (such as Figure 3 (a)) and dQ / dV curves ( Figure 3 (c) The overlap at low potential is significantly better than that of NFM CV curve ( Figure 3 (e)) and dQ / dV curves ( Figure 3 (f)), which indicates that Mn 4+ / Mn 3+ The redox reaction of NFM-HEO has higher reversibility. This feature may be due to the structural stability brought by high entropy doping, which helps to suppress lattice distortion and alleviate phase transition. At the same time, the dQ / dV curve also shows that during the cycle process, NFM-HEO has anionic redox reaction (O 2- / O 2 n- ). Combined with the high cycling stability of NFM-HEO, it can be inferred that in high-entropy doped positive electrode materials, anionic redox reactions can remain stable and reversible. It can also be observed from the GCD curve that the platform of the oxygen redox reaction in NFM-HEO provides a higher charge specific capacity than that of NFM. It has been reported that substitution of inert elements, especially lithium, can initiate and promote the redox reaction of lattice oxygen, thereby obtaining a higher capacity. Therefore, high-entropy doped NFM-HEO materials involving lithium substitution can provide more capacity contributed by oxygen redox reactions than undoped NFM materials. In addition, as Figure 3 As shown in (d), NFM-HEO exhibits improved sodium ion diffusion kinetics and has better rate performance compared with NFM. Figure 3 As shown in (g), high entropy doping also greatly improves the cycling stability, with NFM-HEO retaining an encouraging 85.4% of its capacity after 500 cycles at 1C rate, while the capacity retention of NFM is only 12.2%.

[0092] The electrochemical impedance of NFM-HEO and NFM after different cycle numbers was then tested, and the electrochemical impedance spectroscopy results were collated and analyzed according to previous reports. Figure 4 (a) and Figure 4 (b) shows that the interface film resistance (RCEI) / charge transfer resistance (R ct ) remained stable during cycling, while the impedance of NFM deteriorated severely after cycling. Figure 4 (c) and Figure 4(d) shows the distribution of relaxation times (DRT) obtained from the EIS data. In 300 cycles, NFM-HEO corresponds to RCEI (s = 10 -4 ~10 -2 ) and Rct(s=10 -2 ~10 -0.5 ) changes little, while those of NFM increase significantly. In addition, the RCEI and R ct It is usually lower than NFM, which indicates that the interface film formed on NFM-HEO is thinner and the charge transfer is faster, which infers that the battery generates less heat during the charging and discharging process. All the above results show that high entropy doping can effectively reduce the RCEI and R ct , and keep it at a low level, thereby improving the electrochemical performance, especially the long-cycle stability. Constant current intermittent titration technique (GITT) tests were also carried out, and the test results and the calculated sodium ion diffusion coefficient are shown in Figure 4 (e) and Figure 4 In (f), it is obvious that the sodium ion diffusion coefficient of NFM-HEO is higher than that of NFM in almost the entire process from 2.0 to 4.3 V, which indicates that the sodium ion diffusion kinetics accompanied by the redox reaction of active metal cations in NFM-HEO is significantly improved, and the redox reaction of oxygen ions under high pressure is also regulated.

[0093] At high voltage, the diffusion coefficient of NFM-HEO is lower than that of NFM. Studies have shown that the diffusion coefficient of high entropy materials at high voltage is about 10 -10 cm 2 / s, while the diffusion coefficient of the original material may be higher. The high entropy structure may increase the electrochemical polarization of the material, resulting in an increase in the migration resistance of sodium ions during the charge and discharge process. This polarization effect may be more obvious in the high voltage region, which will significantly reduce the diffusion coefficient of sodium ions. The degree of electron localization around oxygen in the high entropy material increases, which reduces the electron density between sodium-oxygen (Na-O) bonds. Although this reduces the electrostatic interaction between electrons and interlayer sodium ions, it may also make the diffusion path of sodium ions more tortuous. All of these are the main reasons for the low diffusion coefficient of NFM-HEO at high voltage.

[0094] To explore the possible reasons for the stability of high-entropy doped layered oxide materials, we performed X-ray photoelectron spectroscopy (XPS) tests on NFM-HEO at 0% state of charge (SOC, discharged to 2.0 V) and 100% state of charge (charged to 4.3 V). The results are shown in Figure 2. Figure 5 All elements in the material can be divided into two groups, namely, elements that are electrochemically active during the charge and discharge process and electrochemically inert elements. Figure 5 (d) Figure 5 (e) Figure 5 (f) Comparison of the XPS spectra of Ca, Cu, Ti, Mg, and Li elements at 0% SOC and 100% SOC. There is no obvious change in peak intensity and peak position between the fully charged and fully discharged states, which indicates that they do not participate in the redox reaction during the battery charge and discharge process. Although these components do not participate in providing specific capacity, they play the role of "pillars" in the lattice. At the same time, Figure 5 (a) Figure 5 (b) Figure 5 As shown in (c), for the electrochemically active elements Mn, Fe, and O, the peak areas corresponding to different valence states changed significantly after the sodium ion was deintercalated, revealing their contribution to charge compensation. Among them, the main peak corresponding to Fe2p3 / 2 in the Fe2p spectrum moved significantly toward a higher binding energy direction, as shown in Figure 5 As shown in (a), Fe 4+ The ratio is higher, which means that after charging, Fe 3+ Oxidized to a higher oxidation state (Fe 4+ ). At 100% SOC, the Mn2p spectrum corresponds to Mn 4+ The peak area ratio of is significantly higher than that of 0% SOC state, such as Figure 5 (b), which indicates that Mn 3+ →Mn 4+ The oxidation reaction of is involved in the charge and discharge process. At 100% SOC, the corresponding peroxide species (O 2 n- ) peak, while it disappears at 0% SOC, which indicates that the anion redox reaction (O 2- / O 2 n- ) participates in the battery reaction process, such as Figure 5 (c). All of these are related to Figure 3 (c) is consistent with the dQ / dV results shown. In general, the NFM-HEO structure contains electrochemically active components such as Mn, Fe, O, and electrochemically inert elements such as Ca, Cu, Ti, and Mg, and the active / inert metal atoms are randomly distributed in the bulk phase of the material. When sodium ions are intercalated, local changes in the randomly distributed redox active centers may combine with more diverse local features to help suppress possible phase transitions. From this perspective, the high-entropy doped material NFM-HEO may suppress structural evolution. In addition, the anionic redox reaction (O 2- / O 2 n-) layered oxide materials usually have poor cycling stability. In contrast, NFM-HEO has good cycling stability, which indicates that its anionic redox reaction has good reversibility. To demonstrate this, we performed XPS tests on NFM-HEO after cycling at different charge states and compared the results with those of NFM. After cycling at 100% SOC, the peroxide species (O 2 n- ) peaks still exist, which indicates that the anion redox reaction (O 2- / O 2 n- ) is reversible. In contrast, there is no obvious peak corresponding to the anion redox reaction in NFM after cycling, which indicates that the anion redox reaction involved in NFM has poor reversibility. From this perspective, the high-entropy doped material NFM-HEO may have the effect of stabilizing the anion redox reaction.

[0095] In order to study the structural evolution of NFM-HEO during the initial charge and discharge process, we further performed in situ X-ray diffraction (XRD) tests. Figure 6 (a) When charging from open circuit potential (OCV) to 4.3 V, some diffraction peaks gradually shift, namely the peaks corresponding to the (002), (004) and (102) crystal planes. Figure 6As shown in (b), during the sodium ion insertion and extraction process, the peaks corresponding to the (002) and (004) crystal planes gradually shift to low angles because the electrostatic repulsion between oxide layers is enhanced, while the peaks corresponding to the (100) and (102) crystal planes shift in the opposite direction because the transition metal-oxygen (TM-O) bond length shortens with increasing oxidation state. After charging to more than 4.2 V, the main peaks corresponding to the (102) and (104) crystal planes become wider and shorter, and a new weak peak appears, which corresponds to the OP4 phase (incomplete transformation of the P2 phase to the O2 phase). According to the XRD results, the electrode material is still dominated by the P2 phase at this time. In the subsequent discharge process, the peak of the OP4 phase gradually disappears, while the (002) peak of the P2 phase gradually strengthens and shifts to a large angle, indicating that the material is completely transformed back to the P2 phase. The presence of the OP4 phase alleviates the drastic change in the interlayer spacing during the transition from the P phase to the O phase, thereby reducing the volume change during the sodium ion insertion / extraction process, which in turn helps to improve the cycling stability of the material. The transition from P2 to OP4 in NFM-HEO materials is consistent with the reported results, indicating that high entropy doping in P2-type manganese / iron-based hierarchical oxides results in a P2-OP4 phase transition rather than a P2-O2 phase transition at high voltages. It is noteworthy that no characteristic peaks of sodium or transition metal ordering were observed during the charge-discharge cycle, and no obvious P'2 phase with monoclinic distortion appeared in the material over the entire sodium ion insertion / deinsertion voltage range.

[0096] In addition, we collected X-ray diffraction (XRD) patterns of NFM-HEO and NFM after cycling and compared them with the results in their pristine states. Figure 6 As shown in (c), there is no obvious difference in the XRD patterns of NFM-HEO before and after the cycle, which indicates that the original P2 structure of NFM-HEO is well preserved after the charge and discharge cycle. The refinement results show that after 100 cycles, the unit cell volume of the high-entropy doped NFM-HEO material changes by only 0.72%. After 100 cycles, the (002) peak corresponding to the P2 phase shifts to a high angle and becomes wider, which indicates that due to the formation of Mn 3+The induced Chiang-Taylor effect leads to the formation of a P'2 phase with strong local distortion. The calculated lattice parameters show that after 100 cycles, the a-axis increases from 2.9068 Å to 2.9228 Å, while the c-axis increases from 11.2619 Å to 11.2721 Å, and the unit cell volume change is 1.19%, which is significantly larger than the 0.72% of NFM-HEO. These results indicate that high-entropy doping can effectively mitigate the volume change during sodium ion insertion / deinsertion and stabilize the framework structure of the material, making it more robust. Despite the P2-OP4 phase transition, the material is still dominated by the P2 phase after 100 cycles at a 1C rate, and the capacity decay rate is low, indicating that it has good long-range structural stability.

[0097] The improved structural stability of high-entropy doped NFM-HEO also enhances its tolerance in humid air. Whether the cathode material can maintain high stability when exposed to humid air is a very important key indicator in practical applications. However, there are few reports on layered cathode materials with high humid air stability in sodium-ion batteries. Here, we studied the stability of NFM-HEO materials when exposed to humid air at 25°C and compared them with NFM. With the increase of exposure time, the XRD peak intensity of both materials gradually decreased, but the position of the peak did not shift significantly. The decrease in peak intensity may be due to the adsorption of air components and moisture on the surface of the material. For NFM, with the increase of exposure time in humid air, additional peaks appeared successively near 12.6° and 25.4°, such as Figure 7 (e) and Figure 7 As shown in (f), these peaks correspond to hydration products. The appearance of these hydrated phase peaks seriously affects the electrochemical performance of NFM, which is manifested as a significant decrease in capacity, such as Figure 7 (d). After exposure to humid air, Figure 7 (b) and Figure 7 As shown in (c), NFM-HEO does not show such a peak, which indicates that NFM-HEO is not easy to react with water, so that its electrochemical performance can be stably maintained, as shown in Figure 7 (a) As shown. It can also be found from the constant current charge and discharge curves that the open circuit voltage (OCV) of both materials increases with the extension of exposure time in humid air, as shown in Figure 7 (g) and Figure 7 (h). As the hydrated phase peak gradually appears in the NFM material, its OCV also gradually increases. The OCV of NFM increased by 10% (from 2.78V to 3.06V), which is greater than the OCV increase of NFM-HEO (increased by 6%, from 2.69V to 2.86V). Both the in-situ XRD test results and the humid air exposure test results show that NFM-HEO has an advantage in structural stability.

[0098] In order to further explore the reasons for the excellent performance of NFM-HEO from the perspective of the mechanical integrity of the material particles, we used a focused ion beam (FIB) to slice the material particles and observed the cross-sectional microstructure of the positive electrode particles before and after cycling. For NFM, after 50 cycles, small cracks appeared on the surface of the particles, while there were fewer cracks inside the particles, indicating that the cracks were initially generated on the surface. As the cycle progressed, the cracks generated on the surface expanded inward and eventually penetrated the entire particle. Figure 8 (b1), Figure 8 (b2), Figure 8 (b3), Figure 8 As shown in (b4), after 200 cycles, a large number of obvious cracks were found on the particles. In sharp contrast, NFM-HEO showed almost no obvious changes before and after the cycle, and no cracks appeared on the surface or inside the particles, as shown in Figure 4. Figure 8 (a1), Figure 8 (a2), Figure 8 (a3), Figure 8 (a4) The rapid increase of cracks in NFM after cycling will destroy its structural stability, leading to more side reactions during the cycling process, which in turn causes the material performance to deteriorate.

[0099] In addition, we also studied the dissolution of transition metals (TM) by performing energy dispersive spectroscopy (EDS) characterization on the glass fiber separators disassembled after 200 cycles. NFM-HEO showed almost no obvious dissolution of manganese (Mn) and iron (Fe), while NFM showed severe dissolution of manganese and iron. The remaining peaks in the EDS spectrum are likely to correspond to elements in the glass fiber separator (such as silicon (Si), oxygen (O), sodium (Na), calcium (Ca), magnesium (Mg), titanium (Ti), etc.) and solute components in the electrolyte (such as sodium (Na), carbon (C), nitrogen (N), oxygen (O), fluorine (F), etc.). The dissolution of transition metals into the electrolyte will aggravate the deterioration of the negative electrode solid electrolyte interface (SEI), which also explains the rapid increase in impedance of NFM after cycling.

[0100] In general, high-entropy doping can effectively inhibit the dissolution of transition metals during battery cycling, achieve high structural stability and mechanical integrity, and thus obtain long-term cycling stability.

[0101] The high reversible capacity, high rate performance and long cycle stability of NFM-HEO are mainly due to the specific functions of different metal cations and the structural stability brought by high entropy doping. 3+ and Mn 3+ Provides charge compensation, which is the main contributor to specific capacity. Low-valent Mg 2+ and Cu 2+It helps to increase the valence state of Mn and alleviate the Jahn-Taylor distortion. 2+ The presence of can effectively reduce the structural distortion of local OO bonds and enhance the redox reversibility of lattice oxygen. Doping Li in the transition metal (TM) layer plays the role of a structural stabilizer because the monovalent Li + It is beneficial to retain more Na in the structure of deep desodiumization + , maintaining electrostatic balance, thereby inhibiting the O2 phase transition. 4+ and Mn 4+ There are significant differences in the Fermi levels of Ti 4+ The introduction of will inhibit the orderly arrangement of cations and charges in the transition metal layer, and the Na + / vacancies are arranged in an orderly manner. Ca is doped in the sodium layer 2+ Can play the role of "pillar", and because Ca 2+ With O 2- The strong interaction between them is beneficial to improving the structural stability and cycle stability of the material.

[0102] In addition to the various functions brought by different metal cations, the structural stability driven by high entropy doping also suppresses the P2-P'2 phase transition and particle cracking, and is beneficial to reduce the Na + Therefore, the functional integration of different metal cations, combined with the improvement of structural stability brought by high entropy doping, synergistically endows NFM-HEO with high reversible capacity, high rate performance and long cycle stability.

[0103] In summary, this study has explored the high entropy doping strategy for manganese / iron-based oxide cathode materials for sodium-ion batteries. The electrochemical performance of high-entropy doped layered oxides has been significantly improved, especially the excellent long-cycle stability. This performance improvement depends on the significant improvement in structural stability after high entropy doping. + During the insertion / deinsertion process, in situ XRD test results show that the interlayer spacing of the high-entropy doped NFM-HEO layered oxide cathode material changes smoothly, while the stable contribution of the high-voltage platform and the enhanced cycling performance indicate that high-entropy doping stabilizes the anion redox reaction under high pressure.

[0104] At the same time, the high entropy doping method also improves the stability of layered oxide materials in humid air. After exposure to humid air, the lattice of NFM-HEO does not change significantly, and the electrochemical performance remains relatively stable. In addition, high entropy doping inhibits the generation of particle cracks in layered oxide materials and significantly inhibits the dissolution of transition metals in the material after cycling.

[0105] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A high entropy doped manganese / iron based matrix material, characterized in that: The material contains six different metal cations, namely manganese, iron, copper, titanium, lithium and magnesium, in the transition metal layer and two different metal cations, namely sodium and calcium, in the sodium layer. The chemical formula of the layered material is Na 0.62 Ca 0.03 Mn 0.58 Fe 0.23 Cu 0.085 Mg 0.01 Ti 0.015 Li 0.08 O2.

2. The high entropy doped manganese / iron based matrix material according to claim 1, characterized in that: The high entropy doped manganese / iron based layered material is of P2 type.

3. The method for preparing a high entropy doped manganese / iron based matrix material according to claim 1 or 2, characterized in that: The following steps are involved: S1. Fully dissolve Fe(NO3)3·9H2O, Mn(NO3)2·4H2O and Cu(NO3)2·3H2O in stoichiometric ratio in deionized water and mix the transition metal salt solution; S2, then adding the sodium hydroxide aqueous solution dropwise to the mixed transition metal salt solution; adjusting the pH value of the system with ammonia water after the dropwise addition for 1 hour to obtain a precipitate; filtering the generated precipitate, washing it with deionized water and ethanol, and then drying it to obtain a precursor; S3, then, the dried precursor is mixed with stoichiometric proportions of TiO2, CaCO3, Li2CO3 and MgCO3, and an excess of 5% stoichiometric proportion of Na2CO3, and then the mixture is calcined and taken out for grinding; S4. Finally, the mixture is calcined at 900°C in an oxygen atmosphere at a heating rate of 5°C / min to form a high entropy doped manganese / iron based matrix material NFM-HEO.

4. The method for preparing a high entropy doped manganese / iron based matrix material according to claim 3, characterized in that: The total concentration of the transition metal salt solution in step S1 is 2.0 mol L -1 .

5. The method for preparing a high entropy doped manganese / iron based matrix material according to claim 3, characterized in that: The concentration of the sodium hydroxide aqueous solution in step S2 is 4.0 mol / L -1 , the concentration of ammonia water is 1.0 molL -1 ; Adjust the pH value of the system to 11.

6. The method for preparing a high entropy doped manganese / iron based matrix material according to claim 3, characterized in that: The drying temperature of step S2 is 100° C. and the drying time is 6 hours.

7. The method for preparing a high entropy doped manganese / iron based matrix material according to claim 3, characterized in that: The ball milling speed in step S3 is 600 rpm, and the ball milling time is 6 hours; the calcination temperature is 500° C., and the calcination time is 6 hours.

8. The method for preparing a high entropy doped manganese / iron based matrix material according to claim 3, characterized in that: The calcination time in step S4 is 12 hours.

9. A pole piece, characterized in that: The pole piece comprises the high entropy doped manganese / iron based layered material as described in any one of claims 1-2 or the high entropy doped manganese / iron based layered material prepared by the preparation method as described in any one of claims 3-8.

10. A battery, characterized in that: The battery comprises the pole piece as claimed in claim 9, a counter electrode and a diaphragm, wherein the diaphragm is arranged between the pole piece and the counter electrode.

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