Sodium battery positive electrode material, preparation method and application thereof

Abstract

The invention discloses a sodium battery positive electrode material as well as a preparation method and application thereof. The preparation method comprises the following steps: regulating and controlling in-situ growth of polyanions and an oxide composite crystal phase; under the condition of the stoichiometric ratio of polyanions, a sodium source and a transition metal source are further preferably and excessively added, and the first-stage heat treatment process is carried out under the condition of oxygen or air or inert gas, so that the material formed after the first-stage heat treatment is in an oxygen-enriched state; in the second-stage heat treatment process, under the combined action of reaction thermodynamics and kinetics, when a polyanion crystal structure grows, excessive sodium sources and transition metal sources are combined with oxygen atoms to generate an oxide positive electrode structure, and the oxide positive electrode structure and polyanions further construct a composite crystal phase; in the first-stage heat treatment process, transition metal may be oxidized into a high valence state, and a valence state in a target product is generated along with carbon thermal reduction in the subsequent second-temperature-stage heat treatment process, so that the transition metal can be used as an electrode material to exert electrochemical performance in a battery through an oxidation-reduction reaction.

Description

Technical Field

[0001] This invention relates to the field of alkali metal ion batteries, to electrode materials for alkali metal ion batteries, and particularly to polyanionic and oxide composite crystal phases, preparation methods, and positive and / or negative electrode materials for alkali metal ion batteries. Background Technology

[0002] Promoting the development of new energy storage batteries such as alkali metal ion batteries is a key measure to strengthen national energy reserves and promote the transformation of the national energy structure. It is also one of the key technologies to achieve carbon peaking and carbon neutrality. Breaking through the existing technological barriers of alkali metal ion batteries and accelerating their application in the energy storage field has become a key research and development direction in the energy storage field at this stage.

[0003] In alkali metal batteries, lithium-ion and sodium-ion batteries are widely studied. Sodium and lithium are in the same group of the periodic table and have similar chemical properties. Compared with lithium-ion batteries, sodium-ion batteries have advantages in resource abundance, raw material cost, and safety performance. In addition, lithium-ion batteries form aluminum-lithium alloys under low voltage conditions at the negative electrode, while sodium-ion batteries do not have this alloying effect. Therefore, aluminum current collectors can be used instead of copper current collectors, thereby further reducing the cost of sodium-ion batteries and increasing their energy density. Based on this, sodium-ion batteries will have greater resource and price advantages in the future energy development field, and it is particularly important to conduct in-depth research and solve the current problems in the development of sodium-ion battery technology. However, compared with lithium-ion batteries, the cost of sodium-ion batteries is still relatively high. The main reason is that sodium-ion batteries cannot overcome the low specific energy, and the key limiting factor is the low available specific capacity of sodium-ion battery cathode materials.

[0004] Currently reported sodium-ion battery cathode materials mainly fall into three categories: layered oxides, Prussian blue compounds, and vanadium-based polyanionic compounds. Sodium oxide cathodes, primarily composed of nickel, copper, iron, and manganese, possess excellent conductivity and sodium diffusion capabilities, resulting in superior specific capacity and rate performance. However, during cycling, structural phase transitions and lattice oxygen release lead to rapid performance degradation and safety concerns related to byproducts. Prussian blue analogues, with their relatively open sodium diffusion channels, and iron-manganese-based electrode materials, exhibit excellent specific capacity and rate performance. However, the presence of structural water and coordinated cyanide ions causes rapid performance degradation and potential safety hazards. Polyanionic compounds, such as sodium vanadium phosphate, sodium fluorophosphate, and composite sodium iron phosphate, possess unique advantages due to their structural stability, rapid sodium diffusion, and high safety. However, rigid polyanionic compounds (PO4)... 3- SO4 2- As inert groups, they do not participate in electrochemical reactions and have low electronic conductivity, resulting in low specific capacity and increased cell cost. Summary of the Invention

[0005] To address the limitations of sodium-ion battery cathode materials in simultaneously achieving high specific capacity, high rate capability, and stable performance, this invention innovatively provides a polyanion-oxide composite crystalline phase, its preparation method, and its application. During high-temperature heat treatment, the polyanion-oxide composite crystalline phase is grown in situ through precise control of reaction conditions, dominant crystalline phase, and elemental stoichiometry. Under the condition of a stoichiometric ratio of polyanions, an excess of sodium source and transition metal source is further preferred. The first stage of heat treatment is carried out under oxygen, air, or inert gas conditions, ensuring that the material formed after the first stage is in an oxygen-rich state. During the second stage of heat treatment, under the combined effects of reaction thermodynamics and kinetics, while the polyanion crystal structure grows, the excess sodium source and transition metal source combine with oxygen atoms to generate an oxide cathode structure, further constructing a composite crystalline phase with the polyanion. During the first stage of heat treatment, the transition metal may be oxidized to a high valence state, which is then converted to the valence state of the target product through carbothermic reduction during the subsequent second-stage heat treatment. This product serves as an electrode material in the battery, thereby exerting electrochemical performance through redox reactions.

[0006] A method for preparing a polyanionic and oxide composite crystalline phase, wherein the composite crystalline phase is formed by mixing a polyanionic crystalline phase and an oxide crystalline phase:

[0007] The chemical formula of the polyanionic crystal phase is A. x M m+ y (XO b ) o- n Z p- w The mass ratio of the polyanionic crystal phase to the polyanionic and oxide composite crystal phase is 1.

[0008] The chemical formula of the oxide crystal phase is A. z M m+ Q r- q The mass ratio of the oxide crystalline phase to the polyanionic and oxide composite crystalline phase is f;

[0009] 0.25≤l≤0.85, 0.15≤f≤0.75, l+f=1;

[0010] In the aforementioned polyanionic crystal phase A x M m+ y (XO b ) o- n Z p- w and oxide phase A z M m+ Qr- q The elements of A and M in it have the same composition;

[0011] A is selected from one or more of Li, Na, and K, 2 ≤ x ≤ 5 (preferably 2.5 ≤ x ≤ 4.0, where x represents the molar stoichiometry of element A in the polyanion crystal phase); 0.5 ≤ z < 2.0 (preferably 0.7 ≤ z ≤ 1.35, where z represents the molar stoichiometry of element A in the oxide crystal phase);

[0012] M is selected from one or more of V, Fe, Mn, Si, Zr, Co, Ni, Cu, Ti, Cr, Zn, Al, Mg, Sc, Mo, Ga, Sn, and Pb, 0.7 ≤ y ≤ 3.2 (preferably 1.0 ≤ y ≤ 3.0, where y represents the molar stoichiometry of the corresponding element M);

[0013] X is selected from one or more of P, As, Sb, Si, Ge, C, B, and S, 2.0 ≤ b ≤ 4 (where b represents the molar stoichiometry of the corresponding element O);

[0014] 1.0 ≤ n ≤ 4, preferably 2 ≤ n ≤ 3; (n represents the molar stoichiometry of the corresponding XO b of

[0015] Z is selected from one or more of halogens N (N = one or more of F, Cl, Br, and I), or a mixture of O and halogens N (N = one or more of F, Cl, Br, and I); when Z is selected as halogens N (N = one or more of F, Cl, Br, and I), 0 ≤ w ≤ 4 (preferably 1 ≤ w ≤ 3, where w represents the molar stoichiometry of the corresponding element Z); when Z is selected as a mixture of O and halogens N (N = one or more of F, Cl, Br, and I), the said Z p- w is denoted as O 2- w1 N - w2 , 0 < w1 ≤ 1, 1 ≤ w2 < 3, (w, w1, and w2 respectively represent the molar stoichiometries of the corresponding elements O and halogens N in Z, 2w1 + w2 = p*w, 1 ≤ p ≤ 2);

[0016] Q is selected from one or more of O or halogens N (N = F, Cl, Br, I), and the said Q r- q is denoted as O 2- q1 N - q2, 0≤q1≤0.5, preferably 0≤q1≤0.2, 3≤q2≤4, preferably 3≤q2≤3.5; or 1.5≤q1≤2, preferably 1.8≤q1≤2, 0≤q2≤1, preferably 0≤q2≤0.4 (q1 and q2 represent the molar quantities of the corresponding element O and halogen N source in Q, respectively, 2q1+q2=q*r, 1≤r≤2);

[0017] The above values ​​take values ​​within the range that satisfy A. x M m+ y (XO b ) o- n Z p- w and A z M m+ Q r- q All are electrically neutral; m+, o-, p-, and r- are M, XO, and O, respectively. b The valence states of Z and Q;

[0018] A x M m+ y (XO b ) o- n Z p- w The following relationship exists: x + y*m = n*o + w*p;

[0019] A z M m+ Q r- q The following relationship exists: z + m = r * q;

[0020] When the polyanionic and oxide composite crystal phase has the structure shown, the A source, M source, and XO source are... b The molar ratio of the sources is (lx+fz):(ly+f):ln, where A source, M source, and XO are present. b The molar ratios of the sources are expressed in terms of the molar amounts of A, M, and X, respectively; or when the polyanionic and oxide composite crystal phase has the structure shown, the molar ratios of source A, source M, and source XO are... b The molar ratio of source A and source N is (lx+fz):(ly+f):ln:(lw2+fw4), and the molar ratio of source A, source M, and XO is (lx+fz):(ly+f):ln:(lw2+fw4). b The molar ratio of source A and source N is expressed in terms of the molar amounts of A, M, X, and N, respectively.

[0021] Step (1): Mix source A, source M, and XO according to the required stoichiometric ratio. b The source, Z source (with or without N source), and carbon source are mixed evenly in a solvent to obtain a precursor slurry, which is then dried to obtain precursor powder P1.

[0022] Step (2): Heat-treat the precursor powder P1 obtained in step (1). After the heat treatment, a product P2 with a structure of a composite crystal phase of polyanion and oxide can be obtained.

[0023] The heat treatment includes a process of treating at a first temperature T1 sequentially and a process of treating at a second temperature T2 after the treatment at the first temperature T1.

[0024] The first temperature T1 is 200°C ≤ T1 ≤ 400°C (preferably 280°C - 380°C), and the treatment time is 0.5 - 6 h (preferably 2 h - 4 h); the atmosphere for the heat treatment at the first temperature is under the condition of oxygen or air atmosphere or inert atmosphere.

[0025] The second temperature T2 is 450°C < T1 ≤ 950°C (550°C < T1 ≤ 900°C), and the treatment time is 3 h - 20 h (preferably 5 h - 10 h); the heat treatment at the second temperature is carried out in an inert atmosphere or an inert atmosphere containing H2; in the atmosphere for the heat treatment at the second temperature, the molar content of H2 is 0 - 50%, preferably 10% - 20%.

[0026] [[ID=,14]]In step (1), the methods of uniformly mixing the A source, M source, XO b source, Z source (with or without adding the N source), and carbon source include one or more of ball milling, tank milling, and sand milling; the solid content of the precursor slurry is 15 wt% - 60 wt%, preferably 25 wt% - 45 wt%.

[0027] The solvent is water, ethanol, or a mixed solvent of water and ethanol, and the proportion of water in the mixed solvent in the total mass of the mixed solvent is (30 - 80) wt%, preferably (45 - 55) wt%. s

[0028] The drying method is spray drying. Among them, the feeding flow rate is controlled at 20 mL / min - 60 mL / min, the inlet air temperature is controlled at 140°C - 225°C, the outlet air temperature is 70°C - 120°C, and the compressed air pressure is 0.2 Mpa - 0.7 Mpa.

[0029] ]The composite crystal phase is composed of a mixture at the crystal cell level of a polyanion crystal phase and an oxide crystal phase.

[0030] The addition amount of the carbon source makes the carbon content in the finally generated product P2 with a structure of a composite crystal phase of polyanion and oxide be 1 wt% - 20 wt% (preferably 2 wt% - 15 wt%, more preferably 7 wt% - 12 wt%).

[0031] The inert atmosphere gas is one or more of nitrogen and argon.

[0032] The particle size of the product P2 is 5-10 micrometers.

[0033] The A and M sources are respectively selected from one or more of the oxides, hydroxides, acetates, nitrates, carbonates, oxalates, phosphates, pyrophosphates, chlorides, citrates, and sulfates corresponding to their elements;

[0034] XO b Represents oxyacid anions containing one or more of the elements P, As, Sb, Si, Ge, C, B, and S, derived from XO. b One or more compounds corresponding to oxyacid anions;

[0035] The N source is selected from one or more of the elements and compounds corresponding to them;

[0036] The carbon source is one or more of the following: polyethylene glycol, cellulose, starch, polyvinyl alcohol, formic acid, acetic acid, citric acid, malic acid, lactic acid, oxalic acid, tartaric acid, ascorbic acid, cyclohexanediol, salicylic acid, caffeic acid, ethylene acrylate copolymer, maleic acid copolymer, polyacrylic acid, polyvinyl acid, polyamic acid, polybutenoic acid, glucose, sucrose, starch, fructose, and carbon nanotubes.

[0037] The polyanionic and oxide composite crystal phase prepared by the method described above.

[0038] The polyanionic and oxide composite crystal phase prepared by the preparation method described herein, or the application of the polyanionic and oxide composite crystal phase described herein, can be used as a positive electrode active material or a negative electrode active material in the positive or negative electrode of a sodium-ion battery.

[0039] A positive or negative electrode for a sodium-ion battery, wherein the positive or negative electrode contains a polyanionic and oxide composite crystalline phase prepared by a preparation method or contains the polyanionic and oxide composite crystalline phase;

[0040] Preferably, in the positive electrode, the content of the polyanionic and oxide composite crystalline phase is 40-96 wt%; preferably, in the negative electrode, the content of the polyanionic and oxide composite crystalline phase is 50-98 wt%.

[0041] Preferably, the positive electrode material further contains a conductive agent and a binder, and the mass ratio of the polyanionic and oxide composite crystal phase, the conductive agent, and the binder is (40-96):(2-30):(2-30).

[0042] Preferably, the negative electrode material further contains a conductive agent and a binder, and the mass ratio of the polyanionic and oxide composite crystal phase, the conductive agent, and the binder is (50-98):(1-25):(1-25).

[0043] Beneficial effects

[0044] The synthesis method of this invention is simple, directly growing a composite crystal phase of polyanions and oxides in situ. The oxides can improve the electrical conductivity of the composite crystal phase, and as the charging process proceeds, MO formed when sodium ions are extracted from the oxide lattice... x The compound can further improve the conductivity of the composite phase structure, and the polyanion can enhance the structural stability of the composite crystal phase during the charge and discharge process. In addition, the three-dimensional framework structure of the polyanion can further enhance the transport of sodium ions in the composite crystal phase, while buffering the volume expansion of the composite crystal phase under high-rate charge and discharge conditions. Furthermore, the formed grain boundaries also contribute to the improvement of electrochemical performance. By further controlling the reaction conditions to optimize the ratio of the two phases, the obtained material has high rate performance and cycle stability. Therefore, this high-performance sodium-ion battery cathode material and preparation method have great application prospects in sodium-ion batteries. Detailed Implementation

[0045] Example 1

[0046] Weigh 13.64g of vanadium pentoxide (V2O5), 23.40g of sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), 4.31g of sodium acetate (CH3COONa), and 53.59g of glucose and grind them in a ball mill jar with a ball-to-material ratio of 2:1. After grinding, transfer the mixture to a quartz crucible and sinter it at 350℃ in air for 3 hours. After cooling, remove the crucible, grind it further, and then place it in a tube furnace at 850℃ in argon atmosphere for 5 hours. This is recorded as sample 1#.

[0047] TG, XRD, and XRF tests were performed on sample 1#. XRD showed that sample 1# was a mixture of 80% sodium vanadium phosphate (Na3V2(PO4)3) and 20% sodium vanadate (NaVO2) by mass. TG test showed that the carbon content of sample 1# was 8.5wt%. XRF test showed that the molar ratio of Na / V / P elements in sample 1# was 1.332:0.997:0.973. HR-TEM and refined XRD showed that sodium vanadate (NaVO2) (O3 type, layered structure, vacancy) was present in the crystal lattice. The diffraction spots of a crystal (interstitial group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, sodium ions located in the inter-layer octahedral interstices) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework), thus indicating that the two are mixed at the unit cell level; simultaneously, the conductivity of sample 1# was measured to be 3.9*10 using the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529).-2 S / cm.

[0048] Example 2

[0049] The conditions and procedures are the same as in Example 1, except that 11.70g of sodium dihydrogen phosphate dihydrate NaH2PO4·2H2O is weighed out to replace 23.40g of sodium dihydrogen phosphate dihydrate NaH2PO4·2H2O, and 8.61g of sodium acetate is weighed out to replace 4.31g of sodium acetate. This is recorded as Sample 2#.

[0050] Sample 2# was subjected to TG, XRD, and XRF tests. XRD analysis showed that sample 2# was a mixture of 50% sodium vanadium phosphate (Na3V2(PO4)3) and 50% sodium vanadate (NaVO2). TG analysis showed that sample 2# had a carbon content of 10.3 wt%. XRF analysis showed that the molar ratio of Na / V / P in sample 2# was 1.161:1.002:0.476. HR-TEM and refined XRD revealed the presence of sodium vanadate (NaVO2) in the crystal lattice. 2) (O3 type, layered structure, space group R-3m, VO6 octahedra share edges to form a two-dimensional layered framework, sodium ions are located in the inter-layer octahedral gaps) and sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra share vertices to form a three-dimensional framework, sodium ions are distributed in the three-dimensional framework) diffraction spots alternate, indicating that the two are a mixture at the unit cell level; at the same time, the conductivity of sample 2# was tested by the four-probe method (GB / T 39978-2021, GB / T30835-2014, ASTM F1529) to be 4.2*10 -2 S / cm.

[0051] Example 3

[0052] The conditions and procedures are the same as in Example 1, except that 5.85g of sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O) and 10.46g of sodium acetate are weighed and designated as sample 3#.

[0053] Sample 3# was subjected to TG, XRD, and XRF tests. XRD showed that sample 3# was a mixture of 30% sodium vanadium phosphate (Na3V2(PO4)3) and 70% sodium vanadate (NaVO2). TG test showed that the carbon content of sample 3# was 12.7 wt%. XRF test showed that the mass ratio of Na / V / P elements in sample 3# was 1.088:1.005:0.250. HR-TEM and refined XRD showed that vanadium phosphate (Na3V2(PO4)3) in the crystal lattice was present. )3) (Rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra share vertices to form a three-dimensional framework, sodium ions are distributed in the three-dimensional framework) The diffraction spots of sodium crystal are uniformly embedded in the diffraction spots of sodium vanadate (NaVO2) (O3 type, layered structure, space group R-3m, VO6 octahedra share edges to form a two-dimensional layered framework, sodium ions are located in the inter-layer octahedral gaps), thus showing that the two are mixed at the unit cell level; at the same time, the conductivity of sample 3# was tested by the four-probe method (GB / T 39978-2021, GB / T 30835-2014, ASTM F1529) to be 4.6*10 -2 S / cm.

[0054] Example 4

[0055] The conditions and process are the same as in Example 1, except that the first step of heat treatment is sintering at 350°C in an argon atmosphere for 3 hours, and this is designated as sample 4#.

[0056] Sample 4# was subjected to TG, XRD, and XRF tests. XRD showed that sample 4# was a mixture of sodium vanadium phosphate (Na3V2(PO4)3) and sodium vanadate (NaVO2) with a mass content of 83%. TG test showed that the carbon content of sample 4# was 8.9 wt%. XRF test showed that the mass ratio of Na / V / P elements in sample 4# was 1.353:1.004:1.044. Sodium vanadate (Na3V2(PO4)3) in the crystal lattice could be seen from HR-TEM and refined XRD. The diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (a rhombohedral crystal system, NASICON-type three-dimensional structure, space group R-3c, where VO6 octahedra share vertices to form a two-dimensional layered framework, and sodium ions are located in the inter-layer octahedral gaps) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (a rhombohedral crystal system, NASICON-type three-dimensional structure, space group R-3c, where VO6 octahedra and PO4 tetrahedra share vertices to form a three-dimensional framework, and sodium ions are distributed within the three-dimensional framework). This indicates that the two are mixed at the unit cell level. Simultaneously, the conductivity of sample #4, measured using the four-probe method (GB / T 39978-2021, GB / T 30835-2014, ASTM F1529), is 3.7*10⁻⁶. -2 S / cm.

[0057] Example 5

[0058] The conditions and process are the same as in Example 1, except that the second heat treatment is sintered at 750°C in an argon atmosphere for 5 hours, and is designated as sample 5#.

[0059] Sample 5# was subjected to TG, XRD, and XRF tests. XRD showed that sample 5# was a mixture of sodium vanadium phosphate (Na3V2(PO4)3) and sodium vanadate (NaVO2) with a mass content of 81% and 19% respectively. TG test showed that the carbon content of sample 5# was 9.1 wt%. XRF test showed that the mass ratio of Na / V / P elements in sample 5# was 1.359:1.025:1.023. HR-TEM and refined XRD showed that sodium vanadate (NaVO2) (O3 type, layered structure, vacancy) was present in the crystal lattice. The diffraction spots of a crystal (interstitial group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, sodium ions located in the inter-layer octahedral interstices) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework), thus indicating that the two are mixed at the unit cell level; simultaneously, the conductivity of sample 5# was measured to be 3.1*10⁻⁶ using the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529). -2 S / cm.

[0060] Example 6

[0061] The conditions and process are the same as in Example 1, except that the second heat treatment is sintered at 900°C in an argon atmosphere for 5 hours, and is designated as sample 6#.

[0062] Sample 6# was subjected to TG, XRD, and XRF tests. XRD showed that sample 6# was a mixture of 85% sodium vanadium phosphate (Na3V2(PO4)3) and 15% sodium vanadate (NaVO2) by mass. TG test showed that the carbon content of sample 6# was 7.3wt%. XRF test showed that the mass ratio of Na / V / P elements in sample 6# was 1.345:0.985:1.073. HR-TEM and refined XRD showed that sodium vanadate (NaVO2) (O3 type, layered structure, vacancy) was present in the crystal lattice. The diffraction spots of a crystal (interstitial group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, sodium ions located in the inter-layer octahedral interstices) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework), thus indicating that the two are mixed at the unit cell level; simultaneously, the conductivity of sample 6# was measured to be 2.8*10 using the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529). -2 S / cm.

[0063] Example 7

[0064] Weigh 60.60g of ferric nitrate nonahydrate Fe(NO3)3·9H2O, 24.57g of sodium dihydrogen phosphate dihydrate NaH2PO4·2H2O, 5.54g of sodium acetate, and 63.04g of glucose and grind them in a ball mill jar with a ball-to-material ratio of 2:1. After grinding, transfer the mixture to a quartz crucible and sinter it at 350℃ in air atmosphere for 3h. After cooling, remove the mixture, grind it further, and then place it in a tube furnace at 550℃ in argon atmosphere for 5h. This mixture is recorded as sample 7#.

[0065] Sample 7# was subjected to TG, XRD, and XRF tests. XRD showed that sample 7# was a composite sodium iron phosphate (Na4Fe3(PO4)2P2O7) with a mass content of 85% (orthorhombic crystal system, space group Pnma). The PO4 and adjacent FeO6 share edges or angles, extending along the bc plane. The bc plane is bridged by P2O7 along the a-axis to form a three-dimensional structure, with sodium ions distributed within the three-dimensional framework. (Composite sodium iron phosphate (Na4Fe3(PO4)2P2O7) prepared separately using this method showed a powder resistivity of 1.9 × 10⁻⁶ using a four-probe test.) -3 (S / cm) and 15% sodium ferrite (Na2FeO2) (O3 type, layered structure, space group R-3m, FeO6 octahedra share edges to form a two-dimensional layered framework, sodium ions are located in the inter-layer octahedral interstices) (Sodium ferrite (Na2FeO2) was prepared alone according to this method, and the powder resistivity was 2.1*10 by four-probe testing). -2The mixture (S / cm) showed that the carbon content of sample 7# was 6.5wt% according to TG analysis, and the mass ratio of Na / Fe / P in sample 7# was 1.475:0.980:1.044 according to XRF analysis. HR-TEM and refined XRD showed that the diffraction spots of sodium ferrite crystals were uniformly embedded in the diffraction spots of the composite sodium iron phosphate, indicating that the two are mixed at the unit cell level. Meanwhile, the conductivity of sample 7# was measured to be 3.4*10⁻⁶ using the four-probe method (GB / T 39978-2021, GB / T 30835-2014, ASTM F1529). -2 S / cm.

[0066] Example 8

[0067] The conditions and procedures were the same as in Example 1, except that the amount of glucose added was 40.71g, designated as sample 8#.

[0068] Sample 8# was subjected to TG, XRD, and XRF tests. XRD showed that sample 8# was a mixture of 76% sodium vanadium phosphate (Na3V2(PO4)3) and 24% sodium vanadate (NaVO2). TG test showed that the carbon content of sample 8# was 2.9wt%. XRF test showed that the mass ratio of Na / V / P elements in sample 8# was 1.34:1.034:0.925. HR-TEM and refined XRD showed that sodium vanadate (NaVO2)(O) was present in the cathode structure lattice. Type 3, layered structure, space group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, sodium ions located in the inter-layer octahedral interstices. The diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (a three-dimensional structure of rhombohedral crystal system, NASICON type, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework). This indicates that the two are mixed at the unit cell level. Simultaneously, the conductivity of sample #8, measured using the four-probe method (GB / T 39978-2021, GB / T 30835-2014, ASTM F1529), is 2.7*10⁻⁶. -2 S / cm.

[0069] Example 9

[0070] The conditions and procedures were the same as in Example 1, except that the amount of glucose added was 75.41g, designated as sample 9#.

[0071] Sample 9# was subjected to TG, XRD, and XRF tests. XRD showed that sample 9# was a mixture of 85% sodium vanadium phosphate (Na3V2(PO4)3) and 15% sodium vanadate (NaVO2). TG test showed that the carbon content of sample 9# was 19.5 wt%. XRF test showed that the mass ratio of Na / V / P elements in sample 9# was 1.342:0.985:1.072. HR-TEM and refined XRD showed that sodium vanadate (NaVO2) (O3 type) was present in the crystal lattice. The diffraction spots of a crystal (layered structure, space group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, sodium ions located in the inter-layer octahedral interstices) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework). This indicates that the two are a mixture at the unit cell level. Simultaneously, the conductivity of sample #9, measured using the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529), is 4.0*10⁻⁶. -2 S / cm.

[0072] Example 10

[0073] Weigh 14.73g vanadium pentoxide (V₂O₅), 7.56g sodium fluoride (NaF), 13.80g ammonium dihydrogen phosphate (NH₄H₂PO₄), 3.45g sodium acetate (CH₃COONa), and 53.59g glucose and grind them in a ball mill jar with a ball-to-material ratio of 2:1. After grinding, transfer the mixture to a quartz crucible and sinter it at 350℃ in air for 3 hours. After cooling, remove the mixture, grind it further, and then place it in a tube furnace at 700℃ in an argon atmosphere for 5 hours. This mixture is recorded as sample 10#.

[0074] TG, XRD, and XRF tests were performed on sample 10#. XRD analysis showed that sample 10# was a mixture of 85% sodium vanadium trifluorophosphate (Na3V2(PO4)2F3) and 15% sodium vanadate (NaVO2). TG analysis showed that the carbon content of sample 10# was 7.8 wt%. XRF analysis showed that the molar ratio of Na / V / P / F elements in sample 10# was 1.477:1.078:0.80:1.20. HR-TEM and refined XRD revealed that the sodium vanadate (NaVO2) crystal lattice (O3 type, layered structure)... The diffraction spots of a crystal (space group R-3m, VO6 octahedra forming a two-dimensional layered framework, sodium ions located in the interstices of the octahedra) are uniformly embedded in the diffraction spots of sodium vanadium trifluorophosphate (Na3V2(PO4)2F3) (tetragonal crystal system, NASICON type three-dimensional structure, space group P42 / mnm, [V2O8F3] dioctahedrons bridged by fluorine atoms, while oxygen atoms are all interconnected through [PO4] units, and sodium ions are distributed in the interstices of the three-dimensional framework). This indicates that the two are mixed at the unit cell level. Simultaneously, the conductivity of sample 10# was measured to be 4.1*10 using the four-probe method (GB / T 39978-2021, GB / T30835-2014, ASTM F1529). -2 S / cm.

[0075] Example 11

[0076] Weigh 48.48g of ferric nitrate nonahydrate, 9.84g of sodium acetate (CH3COONa), 13.80g of ammonium dihydrogen phosphate dihydrate, 4.50g of ferrous fluoride tetrahydrate (FeF2·4H2O), and 53.59g of glucose and grind them in a ball mill jar with a ball-to-material ratio of 2:1. The ball mill jar is filled with an argon atmosphere. After grinding, the ball mill jar is transferred to a glove box. The sample is taken out from the glove box and transferred to a quartz crucible for sealing. After being taken out from the glove box, it is immediately sintered at 350℃ in an argon atmosphere for 3 hours after the seal is broken. After cooling, it is taken out and further ground, and then placed in a tube furnace at 650℃ in an argon atmosphere for 5 hours. This sample is recorded as sample 11#.

[0077] Sample 11# was subjected to TG, refined XRD, and XRF tests. Refined XRD indicated that sample 11# was a mixture of 80% sodium ferric fluorophosphate (Na2FePO4F) and 20% sodium ferric fluoride (NaFeF3). TG testing showed that sample 11# had a carbon content of 7.4 wt%. XRF testing showed that the molar ratio of Na / Fe / P / F in sample 11# was 1.918:1.117:0.80:1.753. HR-TEM and refined XRF analyses also confirmed the presence of these components. As shown in D, the diffraction spots of sodium iron fluoride (NaFeF3) (trigonal crystal system, space group P3121, Na / M layers forming a conjugated structure, FeF3 octahedrons forming a three-dimensional framework structure) are uniformly embedded in the diffraction spots of sodium fluorophosphate (Na2FePO4F) (orthorhombic crystal system, space group Pbcn, layered structure, PO4 tetrahedra and FeO4F2 forming layered units), indicating that the two are mixed at the unit cell level. Simultaneously, the conductivity of sample 11# was measured to be 5.7*10⁻⁶ using the four-probe method (GB / T 39978-2021, GB / T30835-2014, ASTM F1529). -2 S / cm.

[0078] Example 12

[0079] Weigh 13.91g vanadium pentoxide (V2O5), 23.40g sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), 1.23g sodium acetate (CH3COONa), 1.50g lithium carbonate (Li2CO3), and 53.59g glucose and grind them in a ball mill jar with a ball-to-material ratio of 2:1. After grinding, transfer the mixture to a quartz crucible and sinter it at 350℃ in air for 3 hours. After cooling, remove the crucible, grind it further, and then place it in a tube furnace at 880℃ in an argon atmosphere for 5 hours. This sample is designated as sample 12#.

[0080] Sample 12# was subjected to TG, refined XRD, and XRF tests. The refined XRD showed that sample 12# was sodium lithium vanadium phosphate (Na₂O₃) with a mass content of 80%. 2.4 Li 0.6 V2(PO4)3) and 20% sodium lithium vanadate (Na) 0.8 Li 0.2 The mixture of Na and Li (V / P) was analyzed by TG, which showed that sample 12# contained 7.5 wt% carbon. XRF analysis indicated that the molar ratio of Na / Li / V / P in sample 12# was 1.082:0.27:1.023:0.994. HR-TEM and refined XRD revealed the presence of sodium lithium vanadate (Na₂O₂) in the crystal lattice. 0.8 Li 0.2The crystal diffraction spots of VO2 (O3 type, layered structure, space group R-3m, VO6 octahedra share edges to form a two-dimensional layered framework, sodium ions are located in the inter-layer octahedral interstices, thus it can be seen that lithium ions do not change the crystal structure) are uniformly embedded in sodium vanadium phosphate (Na). 2.4 Li 0.6 The diffraction spots of V2(PO4)3 (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedrons and PO4 tetrahedrons share vertices to form a three-dimensional framework, lithium ions and sodium ions are distributed in the three-dimensional framework, thus showing that lithium ions do not change the crystal structure) indicate that the two are mixed at the unit cell level; at the same time, the conductivity of sample 12# was tested by the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529) and found to be 5.6*10 -2 S / cm.

[0081] Example 13

[0082] Weigh out 10.23g vanadium pentoxide (V2O5), 14.24g ferric nitrate nonahydrate (Fe(NO3)3·9H2O), 22.93g sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), 7.38g sodium acetate (CH3COONa), and 53.59g glucose. Grind them in a ball mill jar with a ball-to-material ratio of 2:1. After grinding, transfer the mixture to a quartz crucible and sinter it at 350℃ in air for 3 hours. After cooling, remove the mixture, grind it further, and then place it in a tube furnace at 700℃ in an argon atmosphere for 5 hours. This mixture is recorded as sample 13#.

[0083] Sample 13# was subjected to TG, refined XRD, and XRF tests. The refined XRD showed that sample 13# was sodium vanadium iron phosphate (Na₂O₃) with a mass content of 80%. 3.5 V 1.5 Fe 0.5 (PO4)3) and 20% sodium vanadate (Na) 1.25 V 0.75 Fe 0.25 The mixture of O2) was analyzed by TG, which showed that the carbon content of sample 13# was 7.8 wt%. XRF analysis showed that the molar ratio of Na / V / Fe / P elements in sample 13# was 1.575:0.749:0.978:0.236. HR-TEM and refined XRD revealed that the diffraction spots of sodium vanadium ferrite (NaVO2) (O3 type, layered structure, space group R-3m, V(Fe)O6 octahedra sharing edges to form a two-dimensional layered framework, with sodium ions located in the inter-layer octahedral interstices, indicating that iron ions did not change the crystal structure) were uniformly embedded in the sodium vanadium ferrite (NaVO2) lattice. 3.5 V 1.5 Fe 0.5The diffraction spots of (PO4)3 (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, V(Fe)O6 octahedrons and PO4 tetrahedrons share vertices to form a three-dimensional framework, with sodium ions distributed within the three-dimensional framework, indicating that iron ions do not alter the crystal structure) show that the two are mixed at the unit cell level; simultaneously, the conductivity of sample 13# was measured to be 7.2*10 using the four-probe method (GB / T 39978-2021, GB / T 30835-2014, ASTM F1529). -2 S / cm.

[0084] Comparative Example 1

[0085] Weigh 13.64g of vanadium pentoxide (V2O5), 35.10g of sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), and 47.28g of glucose and grind them in a ball mill jar with a ball-to-material volume ratio of 2:1. After grinding, transfer the mixture to a quartz crucible and sinter it at 350℃ in an argon atmosphere for 3 hours. Then, further heat the mixture to 850℃ and react it in a tube furnace in an argon atmosphere for 5 hours. This sample is designated as sample A#.

[0086] Sample A# was subjected to TG, XRD, and XRF tests. XRD analysis showed that sample A# is sodium vanadium phosphate (Na3V2(PO4)3) with a purity of 99.5%, and also contains 0.5% vanadium trioxide (V2O3). HR-TEM and refined XRD revealed diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) in the crystal lattice, a rhombohedral crystal system, a NASICON-type three-dimensional structure, space group R-3c, with VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, and sodium ions distributed within this framework. TG analysis showed that sample A# has a carbon content of 10.5 wt%. XRF analysis showed that the mass ratio of Na / V / P in sample A# is 2.975:1.985:2.99. Simultaneously, the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM) was also performed. The conductivity of sample A# (F1529) is 2.4*10⁻⁶. -3 S / cm.

[0087] Comparative Example 2

[0088] Weigh 13.64g of vanadium pentoxide (V2O5), 6.15g of sodium acetate (CH3COONa), and 47.28g of glucose and grind them in a ball mill jar with a ball-to-material volume ratio of 2:1. After grinding, transfer the mixture to a quartz crucible and sinter it at 350℃ in air for 3 hours. Then, further heat the mixture to 850℃ and react it in an argon-atmosphere tube furnace for 5 hours. This mixture is recorded as sample B#.

[0089] Sample B# was subjected to TG, XRD, and XRF tests. XRD analysis showed that sample B# was sodium vanadate (NaVO2) with a purity of 98.9%, and vanadium trioxide (V2O3) with a mass content of 1.1%. HR-TEM and refined XRD revealed diffraction spots of sodium vanadate (NaVO2) in the crystal lattice, of type O3, with a layered structure, space group R-3m. VO6 octahedra shared edges formed a two-dimensional layered framework, with sodium ions located in the inter-layer octahedral interstices. TG analysis showed that sample B# had a carbon content of 6.5 wt%, and XRF analysis showed that the Na / V mass ratio in sample B# was 0.95:1.01. Simultaneously, the conductivity of sample B# was measured to be 1.1 × 10⁻⁶ using the four-probe method (GB / T 39978-2021, GB / T30835-2014, ASTM F1529). -2 S / cm.

[0090] Comparative Example 3

[0091] Weigh 14.46g of vanadium pentoxide (V2O5), 30.42g of sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), 2.46g of sodium acetate, and 53.59g of glucose and grind them in a ball mill jar with a ball-to-material ratio of 2:1. After grinding, transfer the mixture to a quartz crucible and sinter it at 350℃ in air for 3 hours. After cooling, remove the crucible, grind it further, and then place it in a tube furnace at 850℃ in an argon atmosphere for 5 hours. This sample is designated as sample C#.

[0092] Sample C# was subjected to TG, XRD, and XRF tests. XRD showed that sample C# was a mixture of 90% sodium vanadium phosphate (Na3V2(PO4)3) and 10% sodium vanadate (NaVO2) by mass. TG test showed that the carbon content of sample C# was 8.5wt%. XRF test showed that the mass ratio of Na / V / P elements in sample C# was 1.481:1.055:1.278. HR-TEM and refined XRD showed that sodium vanadate (NaVO2) (O3 type, layered structure, vacancy) was present in the crystal lattice. The diffraction spots of a crystal (interstitial group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, sodium ions located in the inter-layer octahedral interstices) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework), thus indicating that the two are mixed at the unit cell level; simultaneously, the conductivity of sample C# was measured to be 6.9*10 using the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529). -3 S / cm.

[0093] Comparative Example 4

[0094] The process and conditions are the same as those of Comparative Example 3, except that 2.340g of sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O) and 12.30g of sodium acetate were weighed and denoted as sample D#.

[0095] Sample D# was subjected to TG, XRD, and XRF tests. XRD showed that sample D# was a mixture of 10% sodium vanadium phosphate (Na3V2(PO4)3) and 90% sodium vanadate (NaVO2). TG test showed that the carbon content of sample D# was 8.5wt%. XRF test showed that the mass ratio of Na / V / P elements in sample D# was 1.023:0.999:0.073. HR-TEM and refined XRD showed that sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral) crystals were present in the lattice. The crystal diffraction spots of sodium vanadate (NaVO2) (O3 type, layered structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, with sodium ions distributed within the three-dimensional framework) are uniformly embedded in the diffraction spots of sodium vanadate (NaVO2) (O3 type, layered structure, space group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, with sodium ions located in the inter-layer octahedral gaps). This indicates that the two are mixed at the unit cell level. Simultaneously, the conductivity of sample D# was measured to be 1.5 × 10⁻⁶ using the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529). -2 S / cm.

[0096] Comparative Example 5

[0097] The process and conditions were the same as those of Comparative Example 3, except that 21.06g of sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), 4.92g of sodium acetate, and 35.00g of glucose were weighed and denoted as sample E#.

[0098] Sample E# was subjected to TG, XRD, and XRF tests. XRD showed that sample E# was a mixture of 75% sodium vanadium phosphate (Na3V2(PO4)3) and 25% sodium vanadate (NaVO2) by mass. TG test showed that the carbon content of sample E# was 0.5wt%. XRF test showed that the mass ratio of Na / V / P elements in sample E# was 1.29:0.999:0.873. HR-TEM and refined XRD showed that sodium vanadate (NaVO2) (O3 type, layered structure, empty space) was present in the crystal lattice. The diffraction spots of a crystal (interstitial group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, sodium ions located in the inter-layer octahedral interstices) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework), thus indicating that the two are mixed at the unit cell level; simultaneously, the conductivity of sample E# was measured to be 3.7*10 using the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529). -4 S / cm.

[0099] Comparative Example 6

[0100] The process and conditions were the same as those of Comparative Example 3, except that 23.40g of sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), 4.31g of sodium acetate, and 85.50g of glucose were weighed and denoted as sample F#.

[0101] Sample F# was subjected to TG, XRD, and XRF tests. XRD showed that sample F# was a mixture of 80% sodium vanadium phosphate (Na3V2(PO4)3) and 20% sodium vanadate (NaVO2) by mass. TG test showed that the carbon content of sample F# was 27wt%. XRF test showed that the mass ratio of Na / V / P elements in sample F# was 1.322:0.998:0.973. HR-TEM and refined XRD showed that sodium vanadate (NaVO2) (O3 type, layered structure, vacancy) was present in the crystal lattice. The diffraction spots of a crystal (interstitial group R-3m, VO6 octahedra sharing edges to form a two-dimensional layered framework, sodium ions located in the inter-layer octahedral interstices) are uniformly embedded in the diffraction spots of sodium vanadium phosphate (Na3V2(PO4)3) (rhombohedral crystal system, NASICON type three-dimensional structure, space group R-3c, VO6 octahedra and PO4 tetrahedra sharing vertices to form a three-dimensional framework, sodium ions distributed within the three-dimensional framework), thus indicating that the two are mixed at the unit cell level; simultaneously, the conductivity of sample F# was measured to be 4.7*10 using the four-probe method (GB / T39978-2021, GB / T 30835-2014, ASTM F1529). -2 S / cm.

[0102] Comparative Example 7

[0103] According to the molar mass, 80% of sample A in Comparative Example 1 and sample 2 in Comparative Example 2 were taken and ball-milled at a ball-to-material ratio of 2:1 for 1 hour. This mixture is denoted as sample G#.

[0104] Sample G# was subjected to TG, XRD, and XRF tests. XRD showed that sample G# was a mixture of 80% sodium vanadium phosphate (Na3V2(PO4)3) and 20% sodium vanadate (NaVO2). TG test showed that the carbon content of sample G# was 7.9 wt%. XRF test showed that the mass ratio of Na / V / P elements in sample G# was 1.324:0.997:0.975. HR-TEM and refined XRD showed that the diffraction spots of sodium vanadium phosphate and sodium vanadate lattices existed separately, indicating that they were only mechanically mixed and there was no mixing at the unit cell level. Meanwhile, the conductivity of sample G# was measured to be 3.78 × 10⁻⁶ using the four-probe method (GB / T 39978-2021, GB / T30835-2014, ASTM F1529). -3 S / cm.

[0105] Implement Test Example 1

[0106] Sample 1# prepared in Example 1 was used as the positive electrode active material for sodium-ion batteries. It was mixed uniformly with conductive agent acetylene black and binder polyvinylidene fluoride at a mass ratio of 8.35:0.82:0.83. N-methylpyrrolidone solvent was added and mixed thoroughly to form a paste. This paste was then applied to an aluminum current collector, dried, and cut into discs with a diameter of 12.8 mm. The areal density of the active material was 8–12 mg / cm³. 2 (Here it is 9.63 mg / cm) 2 This material was used as the positive electrode of the battery. A sodium metal sheet was used as the negative electrode. The electrolyte was a mixture of EC and DEC in 1M NaClO4 (EC / DEC V / V = 1:1). A glass fiber membrane was used as the separator. The assembled battery was subjected to charge-discharge tests. The charging cutoff voltage was 4.5V, and the discharging cutoff voltage was 2.0V. The specific capacity at 0.2C / 5.0C / 10C, ​​the capacity retention rate after 2000 cycles at 10C, the sodium ion diffusion coefficient, and the volume change rate were tested. The test data are recorded in Table 1.

[0107] Sodium ion diffusion coefficient: GITT test. Under the above conditions, the assembled battery is first charged and discharged at 0.05C for one week (2.0V-4.5V). Then, it is charged at 0.05C for 20 minutes, then left to stand for 2 hours, and then charged again until the charging voltage reaches 4.5V. After reaching the 4.5V charging voltage, it is discharged at 0.05C for 20 minutes, left to stand for 2 hours, and then discharged again until the discharge voltage reaches 2.0V. This cycle is repeated for 3-5 weeks. The sodium ion diffusion coefficient is calculated using the following formula.

[0108]

[0109] D(cm 2 / s) is the sodium ion diffusion coefficient, τ is the excitation current time (s); S is the electrode area (cm²). 2 ); ΔEs steady-state voltage change (V), ΔEt transient voltage change (V); V M The molar volume (cm³) of the electrode material 3 / mol); m B Mass (g) of electrode material; M B Molar mass of electrode material (g / mol)

[0110] Volume expansion percentage: The electrodes, electrolyte, and sodium metal obtained above are assembled into an in-situ XRD cell with a charge / discharge range of 2.0V-4.5V, a rate of 0.5C / 0.5C, an XRD angle scan range of 10°-50°, and a scan rate of 5° / min. The cell volume at the end of charging and discharging is calculated by refining the XRD. The volume expansion percentage is calculated as (volume at the end of charging - volume at the end of discharging) / volume at the end of discharging, and recorded as a percentage.

[0111] Implement test case 2-13

[0112] The samples obtained from Test Examples 2#-13# were tested according to the test method of Test Example 1. The specific capacity at 0.2C / 5.0C / 10C, ​​the capacity retention rate after 2000 cycles at 10C, and the sodium ion diffusion coefficient were tested by GITT technology. The test data are recorded in Table 1.

[0113] Comparative Test Case 1

[0114] Sample A#, prepared in Comparative Example 1, was used as the positive electrode active material for sodium-ion batteries. It was mixed uniformly with conductive agent acetylene black and binder polyvinylidene fluoride at a mass ratio of 8.35:0.82:0.83. N-methylpyrrolidone was added as a solvent and mixed thoroughly to form a paste. This paste was then applied to an aluminum current collector, dried, and cut into discs with a diameter of 12.8 mm. The areal density of the active material was 8–12 mg / cm³. 2 (Here it is 9.63 mg / cm)2 This is used as the positive electrode of the battery. The negative electrode is a sodium metal sheet. The electrolyte is a 1M NaClO4 mixture of EC and DEC (EC / DEC V / V = 1:1). The separator is a glass fiber membrane. The assembled battery was subjected to charge-discharge tests. The charging cutoff voltage was 4.5V, and the discharging cutoff voltage was 2.0V. The specific capacity at 0.2C / 5.0C / 10C, ​​the capacity retention rate after 2000 cycles at 10C, the sodium ion diffusion coefficient, and the volume change rate were tested. The test data are recorded in Table 2.

[0115] Sodium ion diffusion coefficient: GITT test. Under the above conditions, the assembled battery is first charged and discharged at 0.05C for one week (2.0V-4.5V). Then, it is charged at 0.05C for 20 minutes, then left to stand for 2 hours, and then charged again until the charging voltage reaches 4.5V. After reaching the 4.5V charging voltage, it is discharged at 0.05C for 20 minutes, left to stand for 2 hours, and then discharged again until the discharge voltage reaches 2.0V. This cycle is repeated for 3-5 weeks. The sodium ion diffusion coefficient is calculated using the following formula.

[0116]

[0117] D(cm 2 / s) is the sodium ion diffusion coefficient, τ is the excitation current time (s); S is the electrode area (cm²). 2 ); ΔEs steady-state voltage change (V), ΔEt transient voltage change (V); V M The molar volume (cm³) of the electrode material 3 / mol); m B Mass (g) of electrode material; M B Molar mass of electrode material (g / mol)

[0118] Volume change rate: The electrodes, electrolyte, and sodium metal obtained above were assembled into an in-situ XRD cell. The charge / discharge range was 2.0V-4.5V, the rate of change was 0.5C / 0.5C, the XRD angle scanning range was 10°-50°, and the scanning speed was 5° / min. The cell volume at the end of charging and discharging was calculated by fine-tuning XRD. The volume expansion percentage was calculated as (volume at the end of charging - volume at the end of discharging) / volume at the end of discharging, and recorded as a percentage.

[0119] Comparative test case 2-7

[0120] The samples B#-G# obtained from comparative examples 2-7 were tested according to the test method of comparative test example 1. The specific capacity at 0.2C / 5.0C / 10C, ​​the capacity retention rate after 2000 cycles at 10C, and the sodium ion diffusion coefficient tested by GITT were recorded in Table 2.

[0121] Table 1 Test data records in the implemented test cases

[0122]

[0123] Table 2 shows the test data records in the comparative test cases.

[0124]

[0125] in conclusion

[0126] The synthesis method of this invention is simple, directly growing a composite crystal phase of polyanions and oxides in situ. Test data from the examples and comparative examples show that the oxides can improve the electrical conductivity of the composite crystal phase, and as the charging process proceeds, the MO formed when sodium ions are extracted from the oxide lattice... x Compounds can further improve the conductivity of the composite phase structure, and polyanions can enhance the structural stability of the composite crystal phase during charge and discharge. In addition, the three-dimensional framework structure of polyanions can further enhance the transport of sodium ions in the composite crystal phase and buffer the volume expansion of the composite crystal phase under high-rate charge and discharge conditions. Furthermore, by controlling the reaction conditions to optimize the ratio of the two phases, a composite crystal phase structure of anions and oxides mixed at the cell level is formed, which helps to improve the electrochemical performance. The resulting material has high rate performance and cycle stability. Therefore, this high-performance sodium-ion battery cathode material and preparation method have great application prospects in sodium-ion batteries.

[0127] As can be seen from Example 1, the mixture prepared by the method of this invention, with a mass content of 80% sodium vanadium phosphate (Na3V2(PO4)3) and 20% sodium vanadate (NaVO2), exhibits excellent electrochemical performance: a gram capacity of 142 mAh / g at 0.2C, 125 mAh / g at 5.0C, and 114 mAh / g at 10C. The capacity retention rate after 2000 cycles at 10C is 92%. Examples 2-3 also show that when sodium vanadium phosphate (Na3V2(PO4)3)... 3) The electrochemical performance varies depending on the mixing ratio of sodium vanadate (NaVO2) with vanadate (0.2C: 142-147 mAh / g, 5.0C: 118-125 mAh / g, 10C: 105-114 mAh / g, 10C 2000-cycle retention: 82%-92%). When the proportion of sodium vanadate is higher, the specific capacity at 5.0C and 10C, as well as the retention after 2000 cycles at 10C, are higher, but the initial capacity at 0.2C is lower. When the proportion of sodium vanadate is higher, the capacity at 0.2C is higher... The specific capacity is relatively high, but other properties are slightly reduced (as in Examples 2-3). As can be seen from Comparative Examples 1-2, sodium vanadium phosphate (Na3V2(PO4)3) (0.2C: 110mAh / g, 5.0C: 108mAh / g, 10C: 105mAh / g, retention rate after 2000 cycles at 10C: 95%) and sodium vanadate (NaVO2) (0.2C: 140mAh / g, 5.0C: 120mAh / g, 10C: 107mAh / g, retention rate after 2000 cycles at 10C: 75%) show better performance. Compared to the single crystal phase, the composite crystal phase exhibits superior cycle stability at 0.2C, 5.0C, 10C, and high rates. The increased specific capacity at 0.2C is attributed to the activation of inert sodium sites in the composite cell structure (especially in sodium vanadium phosphate, where inert sodium accounts for 1 / 3 of the total sodium content). When sodium vanadium phosphate and sodium vanadate are mixed in a cellular manner, the rearrangement of electron cloud density at the grain boundaries provides an electronic transition path for the activation of inert sodium sites. Macroscopically, this is reflected in the increased powder conductivity of the composite crystal phase compared to the single crystal phase (Examples 1-3: powder conductivity 3.9*10). -2 S / cm-4.6*10 - 2 S / cm, Comparative Example 1: 2.4*10 -3 S / cm (sodium vanadium phosphate), Comparative Example 2: 1.1*10 -2The improved electrochemical performance at initial 0.2C and high magnification (5.0C / 10C) is not only due to the increased powder conductivity, but also to the increased sodium ion diffusion coefficient in the structure. The migration path of sodium ions in the mixed crystal phase is different from that in the single crystal phase, and the migration path is more abundant. Especially for the layered structure of sodium vanadate, when it forms a composite crystal phase with sodium vanadium phosphate, the sodium ion diffusion channel changes from two-dimensional to three-dimensional, which is macroscopically reflected in the increase of sodium ion diffusion coefficient (Examples 1-3: sodium ion diffusion coefficient 1.1*10). -10 cm / s-1.9*10 -10 cm / s, Comparative Example 1: 9.5*10 -11 cm / s (sodium vanadium phosphate), Comparative Example 2: 2.7*10 -11 The improved electrical conductivity and sodium ion diffusion coefficient of the powder reduce the polarization of the composite crystal phase during the charge and discharge process. Simultaneously, the volume change of the composite crystal phase during the charge and discharge process is lower than that of the single crystal phase (Examples 1-3: volume change rate: 3.5%-5.1%, Comparative Example 1: 3% (sodium vanadium phosphate), Comparative Example 2: 12% (sodium vanadium vanadium)). Especially compared to the oxide crystal phase sodium vanadium vanadium, the stability is improved in summary. Furthermore, when the ratio of sodium vanadium phosphate to sodium vanadium vanadium in the composite crystal phase is inappropriate, the electrochemical performance is significantly affected by the single crystal phase, and the improvement in electrochemical performance of the composite crystal phase is not obvious, as shown in Comparative Examples 3-4. In addition, the composite crystal phase formed by this invention is different from simple mechanical mixing. When mechanical mixing is used, the electrical conductivity and sodium ion migration coefficient of the powder material formed are basically the statistical average of the single crystal phase, which is different from this invention (Comparative Example 7).

[0128] Similarly, the atmosphere in the first heat treatment stage has a significant impact on the electrochemical performance of the mixed crystal phase. When the first heat treatment stage is in an air atmosphere, the oxygen-rich state may result in higher quality for the subsequent construction of the mixed crystal phase of sodium vanadium phosphate and sodium vanadate. However, if treated in an inert atmosphere, the electrochemical performance is slightly reduced, as in Example 4. Furthermore, the conditions in the second heat treatment stage significantly affect the growth quality of the mixed crystal phase. When the heat treatment temperature of the sodium vanadium phosphate and sodium vanadate composite crystal phase is 750°C and 900°C, the electrochemical performance is slightly lower compared to the heat treatment temperature of 850°C, as in Examples 5-6.

[0129] Meanwhile, the carbon coating structure of the mixed crystal phase also affects the electrochemical performance. When the carbon content is low, the powder conductivity is low and the volume expansion during the cell charging and discharging process is large. When the carbon content is high, the sodium ion diffusion coefficient decreases, which also affects the construction of the oxygen-rich system in the first heat treatment stage, and thus affects the proportion of sodium vanadate and the mixing ratio of the two phases in the composite crystal phase. The suitable carbon loading content is 1wt% to 20wt%, preferably 2wt% to 15wt%, and more preferably 7wt% to 12wt%. The electrochemical performance of the mixed crystal phase is relatively good, as shown in Examples 8-9. When it exceeds this range, the electrochemical performance decreases, as shown in Comparative Examples 5-6.

[0130] The mixed crystal structure and preparation method in this invention patent are also applicable to mixed crystal phases of polyanionic and oxide systems in other systems. For example, the composite crystal phase material of sodium iron phosphate and sodium ferrite prepared in Example 7 has a gram capacity of 141 mAh / g at 0.2C, 123 mAh / g at 5.0C, and 110 mAh / g at 10C, with a capacity retention of 91% after 2000 cycles at 10C. Furthermore, when the anionic portion of the polyanionic crystal phase contains substituent groups (such as halogens or oxygen) or when oxygen atoms in the oxide crystal phase are replaced by halogen atoms, the prepared materials also exhibit excellent electrochemical performance. For example, in Examples 10-11, 0.2C: 140-185 mAh / g, 5.0C: 124-147 mAh / g, 10C: 110-128 mAh / g, and retention rate after 2000 cycles at 10C: 80%-85%; when two transition metal or alkali metal elements are used, the transition metal or alkali metal site composition in the polyanionic crystal phase and the oxide crystal phase is the same, as in Examples 12-13, 0.2C: 149-152 mAh / g, 5.0C: 127-130 mAh / g, 10C: 110-113 mAh / g, and retention rate after 2000 cycles at 10C: 89%-90%.

Claims

1. Preparation method of polyanion and oxide composite crystal phase, wherein the composite crystal phase is formed by mixing a polyanion crystal phase and an oxide crystal phase: The chemical formula of the polyanionic crystal phase is A. x M m+ y (XO b ) o- n Z p- w The mass ratio of the polyanionic crystal phase to the polyanionic and oxide composite crystal phase is 1. The chemical formula of the oxide crystal phase is A. z M m+ Q r- q The mass ratio of the oxide crystalline phase to the polyanionic and oxide composite crystalline phase is f; 0.25 ≤ l ≤ 0.85, 0.15 ≤ f ≤ 0.75, l + f = 1; In the aforementioned polyanionic crystal phase A x M m+ y (XO b ) o- n Z p- w and oxide phase A z M m+ Q r- q The elements of A and M in the model are the same; A is selected from one or more of Li, Na, K, 2 ≤ x ≤ 5 (preferably 2.5 ≤ x ≤ 4.0, x represents the molar stoichiometry of the corresponding element A in the polyanion crystal phase); 0.5 ≤ z < 2.0 (preferably 0.7 ≤ z ≤ 1.35, z represents the molar stoichiometry of the corresponding element A in the oxide crystal phase); M is selected from one or more of V, Fe, Mn, Si, Zr, Co, Ni, Cu, Ti, Cr, Zn, Al, Mg, Sc, Mo, Ga, Sn, Pb, 0.7 ≤ y ≤ 3.2 (preferably 1.0 ≤ y ≤ 3.0, y represents the molar stoichiometry of the corresponding element M); X is selected from one or more of P, As, Sb, Si, Ge, C, B, S, 2.0 ≤ b ≤ 4 (b represents the molar stoichiometry of the corresponding element O); 1.0≤n≤4, preferably 2≤n≤3; (n represents the corresponding XO) b (molar measurement) Z is selected from halogen N (N is one or more of F, Cl, Br, I), or a mixture of O and halogen N (N is one or more of F, Cl, Br, I); when Z is selected as halogen N (N is one or more of F, Cl, Br, I), 0 ≤ w ≤ 4 (preferably 1 ≤ w ≤ 3, w represents the molar measurement of the corresponding element Z); when Z is selected as a mixture of O and halogen N (N is one or more of F, Cl, Br, I), the said Z p- w is denoted as O 2- w1 N - w2 , 0 < w1 ≤ 1, 1 ≤ w2 < 3, (w1 and w2 respectively represent the molar measurements of the corresponding elements O and halogen N sources in Z, 2w1 + w2 = p * w, 1 ≤ p ≤ 2); Q is selected from one or more of O or halogen N (N = F, Cl, Br, I), wherein Q r- q Notation: O 2- q1 N - q2 , 0≤q1≤0.5, preferably 0≤q1≤0.2, 3≤q2≤4, preferably 3≤q2≤3.5; or 1.5≤q1≤2, preferably 1.8≤q1≤2, 0≤q2≤1, preferably 0≤q2≤0.4 (q1 and q2 represent the molar quantities of the corresponding element O and halogen N source in Q, respectively, 2q1+q2=q*r, 1≤r≤2); The above values ​​take values ​​within the range that satisfy A. x M m+ y (XO b ) o- n Z p- w and A z M m+ Q r- q All are electrically neutral; m+, o-, p-, and r- are M, XO, and O, respectively. b The valence states of Z and Q; A x M m+ y (XO b ) o- n Z p- w The following relationship exists: x + y*m = n*o + w*p; A z M m+ Q r- q The following relationship exists: z + m = r * q; When the polyanionic and oxide composite crystal phase has the structure shown, the A source, M source, and XO source are... b The molar ratio of the sources is (lx+fz):(ly+f):ln, where A source, M source, and XO are present. b The molar ratios of the sources are expressed in terms of the molar amounts of A, M, and X, respectively; or when the polyanionic and oxide composite crystal phase has the structure shown, the molar ratios of source A, source M, and source XO are... b The molar ratio of source A and source N is (lx+fz):(ly+f):ln:(lw2+fw4), and the molar ratio of source A, source M, and XO is (lx+fz):(ly+f):ln:(lw2+fw4). b The molar ratio of source A and source N is expressed in terms of the molar amounts of A, M, X, and N, respectively. Step (1): Mix source A, source M, and XO according to the required stoichiometric ratio. b The source, Z source (with or without N source), and carbon source are mixed evenly in a solvent to obtain a precursor slurry, which is then dried to obtain precursor powder P1. Step (2), subject the precursor powder P1 obtained in step (1) to heat treatment, and after the heat treatment, a product P2 with the structure of the polyanion and oxide composite crystal phase can be obtained; The heat treatment includes a process of treating at the first temperature T1 in sequence and a process of treating at the second temperature T2 after treating at the first temperature T1; The first temperature T1 is 200°C ≤ T1 ≤ 400°C (preferably 280°C - 380°C), the treatment time is 0.5 - 6 h (preferably 2 h - 4 h); the atmosphere for the heat treatment at the first temperature is under the condition of oxygen or air atmosphere or inert atmosphere; The second temperature T2 is 450°C < T1 ≤ 950°C (550°C < T1 ≤ 900°C), the treatment time is 3 h - 20 h (preferably 5 h - 10 h); the atmosphere for the heat treatment at the second temperature is carried out in an inert atmosphere or an inert atmosphere containing H2; in the atmosphere for the heat treatment at the second temperature, the molar content of H2 is 0 - 50%, preferably 10% - 20%.

2. The preparation method according to claim 2, wherein: In step (1), source A, source M, and XO are... b The method for uniformly mixing the source, Z source (with or without N source), and carbon source includes one or more of ball milling, jar milling, and sand milling; the solid content of the precursor slurry is 15wt% to 60wt%, preferably 25wt% to 45wt%. The solvent is water, ethanol, or a mixed solvent of water and ethanol, and the proportion of water in the mixed solvent in the total mass of the mixed solvent is (30 - 80) wt%, preferably (45 - 55) wt%; The drying method is spray drying, wherein the feed flow rate is controlled to be 20 mL / min - 60 mL / min, the inlet air temperature is controlled to be 140°C - 225°C, the outlet air temperature is 70°C - 120°C, and the compressed air pressure is 0.2 Mpa - 0.7 Mpa; The composite crystal phase is composed of a mixture at the crystal cell level of the polyanion crystal phase and the oxide crystal phase.

3. The preparation method according to claim 1, wherein, The addition amount of the carbon source makes the carbon content in the product P2 with the structure of the polyanion and oxide composite crystal phase finally formed be 1 wt% - 20% (preferably 2 wt% - 15 wt%, more preferably 7 wt% - 12 wt%); The inert atmosphere gas is one or more of nitrogen, argon; The particle size of the product P2 is 5-10 micrometers.

4. The preparation method according to claim 2, characterized in that, The A and M sources are respectively selected from one or more of the oxides, hydroxides, acetates, nitrates, carbonates, oxalates, phosphates, pyrophosphates, chlorides, citrates, and sulfates corresponding to their elements; XO b Represents oxyacid anions containing one or more of the elements P, As, Sb, Si, Ge, C, B, and S, derived from XO. b One or more compounds corresponding to oxyacid anions; The N source is selected from one or more of the elements and compounds corresponding to them; The carbon source is one or more of the following: polyethylene glycol, cellulose, starch, polyvinyl alcohol, formic acid, acetic acid, citric acid, malic acid, lactic acid, oxalic acid, tartaric acid, ascorbic acid, cyclohexanediol, salicylic acid, caffeic acid, ethylene acrylate copolymer, maleic acid copolymer, polyacrylic acid, polyvinyl acid, polyamic acid, polybutenoic acid, glucose, sucrose, starch, fructose, and carbon nanotubes.

5. A polyanionic and oxide composite crystal phase prepared by any one of the preparation methods according to claims 1-4.

6. The application of a polyanionic and oxide composite crystal phase prepared by any one of claims 1-4 or the polyanionic and oxide composite crystal phase of claim 5, characterized in that, It is used as a positive or negative electrode active material in the positive or negative electrode of sodium-ion batteries.

7. A positive or negative electrode for a sodium-ion battery, characterized in that, The positive or negative electrode contains a polyanionic and oxide composite crystal phase prepared by any one of the preparation methods of claims 1-4 or contains the polyanionic and oxide composite crystal phase of claim 5; Preferably, in the positive electrode, the content of the polyanionic and oxide composite crystalline phase is 40-96 wt%; preferably, in the negative electrode, the content of the polyanionic and oxide composite crystalline phase is 50-98 wt%. Preferably, the positive electrode material further contains a conductive agent and a binder, and the mass ratio of the polyanionic and oxide composite crystal phase, the conductive agent, and the binder is (40-96):(2-30):(2-30); Preferably, the negative electrode material further contains a conductive agent and a binder, and the mass ratio of the polyanionic and oxide composite crystal phase, the conductive agent, and the binder is (50-98):(1-25):(1-25).

Images