Soft and hard composite coated sodium ion battery metal-based polyanion positive electrode material and preparation method thereof

Abstract

The invention discloses a soft and hard composite coated sodium ion battery metal-based polyanion positive electrode material and a preparation method thereof.The positive electrode material comprises an active material inner core of metal-based phosphate and derivatives thereof, and the active material inner core is sequentially coated with a hard coating layer and a soft coating layer; the active material core and the hard coating layer are molded through gradient coating, and the hard coating layer and the soft coating layer are molded through dynamic bonding. The soft and hard composite coated sodium ion battery metal-based polyanion positive electrode material has the characteristics of high first cycle discharge capacity, good cycle retention rate, small interface impedance and low crack rate.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-ion batteries, and specifically refers to a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material and a preparation method thereof. Background Art

[0002] In the context of the global active promotion of the "dual carbon" goal, energy storage technology has become a key link in the efficient utilization of renewable energy. Sodium-ion batteries, with their significant advantages such as abundant sodium resource reserves, low cost, and high safety, show great application potential in the field of large-scale energy storage and have received extensive attention from researchers and the industrial community.

[0003] As an important branch of cathode materials for sodium-ion batteries, polyanion compounds have outstanding characteristics such as stable structure, long cycle life, and good thermal stability. However, currently, polyanion sodium-ion cathode materials still face many challenges in practical applications.

[0004] Such as volume expansion and structural pulverization: The insertion and extraction of sodium ions in polyanion materials will cause lattice distortion. For example, the volume change of Na3MnTi(PO4)3 is about 8%. With the increase in the number of cycles, stress concentration occurs inside the particles, resulting in cracking and pulverization, seriously affecting the structural stability and cycle life of the material. Although nanoscale design (such as preparing nanoparticles and porous structures) can relieve stress, it will increase the specific surface area of the material, exacerbating the side reaction between the electrolyte and the material; Optimizing the binder (such as using the aqueous binder CMC) improves the binding performance to a certain extent, but the adhesion is still insufficient.

[0005] Such as poor interfacial stability: Side reactions will occur between the electrolyte and the material surface, such as the dissolution of Fe²⁺ and the hydrolysis of PO4³⁻, resulting in an increase in electrode impedance and a decrease in battery performance. Surface coating (such as Al2O3, LiNbO3) can inhibit side reactions to a certain extent, but the coating layer is prone to cracking during charge and discharge, losing its protective effect; The regulation of the solid electrolyte interface (SEI) has achieved certain results in lithium-ion batteries, but the research in the field of sodium-ion batteries is still in its infancy, and the related technologies are not yet mature.

[0006] To address the above problems, researchers have been exploring new synthesis processes and material structure designs. Carbon coating process route: The in-situ growth of graphene coating technology using CVD method. Although this technology can effectively improve the material properties, it has high process energy consumption, is difficult to achieve large-scale production, and cannot solve the problems of volume expansion and dynamic self-healing of the material; doping modification route: The ion diffusion performance is improved by co-doping of Mg²⁺ / F⁻, but the combination of doping elements is single and fails to comprehensively solve key problems such as volume expansion and interface stability; binder optimization route: The use of polyacrylate aqueous binder, although it improves the bonding performance to a certain extent, lacks self-healing function; and the structure design route: The core-shell structure (NVP@C) is proposed. However, the shell modulus is single and the stress buffering ability is insufficient, unable to effectively cope with the complex stress changes during the charge and discharge process of the battery. Summary of the Invention

[0007] The object of the present invention is to provide a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material and its preparation method, which have the characteristics of high first-cycle discharge capacity, good cycle retention rate, small interfacial impedance and low crack rate.

[0008] The present invention can be realized by the following technical solutions:

[0009] The present invention relates to a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material, which includes an active material core of metal-based phosphate and its derivatives. The active material core is successively coated with a hard coating layer and a soft coating layer. The active material core and the hard coating layer are formed by gradient coating, and the hard coating layer and the soft coating layer are formed by dynamic bond formation.

[0010] Furthermore, the hard coating layer is a composite material of a highly cross-linked polymer and a low cross-linked polymer containing amino groups and phenolic hydroxyl groups.

[0011] Specifically, the highly cross-linked polymer is one or more of tannic acid (TA), polydopamine (PDA), chitosan (CS), polyaniline (PANI), which has strong adhesion, surface film-forming property (uniformly coating the active material core), and chemical active sites.

[0012] Specifically, the low cross-linked polymer is one or more of polyurethane (PU), acrylate (PA), polyacrylamide (PAM), epoxy resin (EP). Its core role is to solve the problems of stress concentration and interface debonding of the material during the charge and discharge process through gradient design.

[0013] Furthermore, the soft coating layer is a prepolymer containing disulfide bonds (-S-S-), and the prepolymer is polyimide (PI-SS) and / or polythiourethane (PTU). Its operating mechanism is: R-S-S-R → (heating) → R-S* + R-S* → (recombination) → R-S-S-R. The broken sulfur radicals (S*) recombine under the drive of thermal motion, and the self-healing ability of the electrode is realized through the reversible breakage and recombination of dynamic covalent bonds, solving the performance decay caused by the accumulation of microcracks during the cycling process.

[0014] Furthermore, the metal-based in the metal-based phosphate and its derivatives is one or more of vanadium-based, iron-based, titanium-based, chromium-based, zirconium-based, and manganese-based; the iron-based phosphate and its derivatives are Na4Fe3(PO4)2P2O7, NaFePO4, and their modified materials; the manganese-based phosphate and its derivatives are Na3MnTi(PO4)3, Na3MnZr(PO4)3, Na3MnV(PO4)3, Na3MnCr(PO4)3, and their modified materials.

[0015] Another aspect of the present invention is to protect a method for preparing the above-mentioned soft-hard composite-coated sodium-ion battery metal-based polyanionic cathode material, including the following steps:

[0016] S1. Preparation of the precursor sol: Dissolve the sodium source, phosphorus source, transition metal M source, and citric acid in a wet process under temperature control to obtain the precursor sol;

[0017] S2. Sintering of the active core material: Sinter the sol at a high temperature to obtain the active core material;

[0018] S3. Surface activation of the active core material: Acid-treat the active core material, wash and dry it to obtain the dried powder of the activated active core material; Acid treatment can activate the surface of the active core material, remove surface impurities and oxides, and increase surface active sites, which is beneficial to the adsorption and polymerization of polydopamine in the follow-up. After soaking, rinse the core material repeatedly with deionized water until the washing liquid is neutral, and then place it in an oven for drying. The drying process should avoid excessive drying causing cracking or agglomeration on the material surface;

[0019] S4. Preparation of the hard coating layer: The surface-activated active core material after drying is placed in a Tris-HCl buffer solution dissolved with amino- and phenol hydroxyl-containing polymers and PEI, and the temperature is controlled for reaction to coat the hard coating layer on the active core material; in the reaction system of the buffer solution, by oscillating the reaction in a constant-temperature shaker, for example, in this process, the amino- and phenol hydroxyl-containing polymers undergo oxidative polymerization under the action of PEI and the Tris-HCl buffer solution to form a uniform hard coating layer on the surface of the core material. As a promoter, PEI can accelerate the polymerization reaction of the amino- and phenol hydroxyl-containing polymers and at the same time enhance the adhesion between the hard region and the surface of the core material;

[0020] S5. Treatment of the gradient structure: A glycol solution dissolved with a low-crosslinked polymer is coated on the surface of the hard coating layer to form an outer soft region, and pulsed UV irradiation is used to form a gradient structure from the hard region to the soft region of the hard coating layer; pulsed UV irradiation is used for crosslinking (the pulsed UV crosslinking method can avoid local overheating, enabling the gradient crosslinked polymer to form a crosslinked network on the surface of the polydopamine coating layer to achieve the construction of a gradient structure from the hard region to the soft region;

[0021] S6. Preparation of the soft coating layer: A prepolymer solution containing disulfide bonds is coated on the surface of the hard coating layer, and then dried to obtain a gradient-coated and dynamically bonded soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material. In this process, the prepolymer undergoes covalent bonding with the amino and phenol hydroxyl groups in the hard region of the hard coating layer, and at the same time, the molecular chains are wound in the porous structure of the hard coating layer to achieve firm chemical grafting and physical anchoring.

[0022] Furthermore, in step S2, the high-temperature sintering method is two-stage sintering.

[0023] Specifically, the sintering temperature in the first stage is 300 - 400 °C, the sintering atmosphere is one or more of nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen, the sintering time is 2 - 6 h, and the heating rate is 1 - 4 °C / min; the sintering protection can prevent the material from being oxidized in the low-temperature stage, and at the same time remove the organic matter and moisture in the sol, enabling the raw materials to undergo preliminary reactions to form a certain crystal structure;

[0024] Specifically, the sintering temperature in the second stage is 500 - 700 °C, the sintering atmosphere is an argon / hydrogen composite gas with a volume ratio of (85 - 97):(15 - 3), the sintering time is 6 - 12 h, and the heating rate is controlled at 1 - 4 °C / min. The precise regulation of the argon / hydrogen mixed gas plays a key role in the valence state of the M-based. As a reducing agent, H2 can reduce the metal-based transition elements to the appropriate valence state to ensure that the proportion of the metal-based reaches the requirements, thereby obtaining a high-purity active material.

[0025] Further, in step S4, the reaction temperature is 30 - 60°C, and the reaction time is 10 - 20 h. The control of the reaction temperature and time has an important impact on the thickness and quality of the inner hard region. If the temperature is too high or the time is too long, it may cause the inner layer to be too thick or agglomeration to occur.

[0026] Further, in the pulsed UV irradiation of step S5, the irradiation mode is 1 - 10 s on / 1 - 10 s off, the intensity is 10 - 30 mW / cm², and the total time is 10 - 30 min. The selection of the irradiation intensity, time, and mode has a crucial impact on the shell gradient and crosslinking degree, and precise control is required to obtain an ideal gradient structure.

[0027] Further, in step S1, the temperature-controlled reaction is a constant-temperature water bath, the constant-temperature water bath temperature is 50 - 100°C, and the sol pH is 1 - 6.

[0028] Further, in step S3, the soaking time for acid treatment is 1 - 30 min, the drying temperature is 50 - 100°C, and the drying time is 1 - 2 h.

[0029] Further, in step S6, the drying temperature is 100 - 150°C, and the drying time is 1 - 2 h.

[0030] Further, the transition metal M source is one or more of a vanadium source, an iron source, a titanium source, a chromium source, a zirconium source, and a manganese source. The vanadium source is one or more of NH4VO3, VO, V2O3, VO2, V2O5. The iron source is one or more of Fe(OH)3, Fe(OH)2, Fe2O3, FeC2O4, FePO4, Fe(HCOO)2, Fe(HCOO)3, Fe(CH3COO)2, Fe(CH3COO)3, Fe(NO3) 3* 9H2O, Fe(NO3)2. The titanium source is one or more of TiO2, Ti2O3. The chromium source is one or more of Cr2O3, CrCl3. The zirconium source is one or more of ZrO2, ZrCl4. The manganese source is one or more of MnCO3, MnCl2, Mn(NO3)2, MnO2, Mn2O3, Mn3O4. The sodium source is one or more of Na3C6H5O7·2H2O, NaNO3, Na2CO3, Na3PO4, Na4P2O7, Na2HPO4, Na2C2O4, NaH2PO4, CH3COONa, Na2H2P2O7, HCOONa. The phosphorus source is one or more of sodium phosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, ammonium phosphate, pyrophosphoric acid, ammonium dihydrogen phosphate, sodium pyrophosphate, sodium dihydrogen pyrophosphate.

[0031] Further, in S5 and S6, the coating process is one or more of electrostatic spraying, ultrasonic spraying, atomic layer spraying, and supercritical fluid spraying.

[0032] The present invention relates to a soft-hard composite-coated sodium-ion battery metal-based polyanionic cathode material and a preparation method thereof, having the following beneficial effects:

[0033] First, the first-week discharge capacity is high. At a charge-discharge rate of 0.1 and a voltage range of 2.0 - 4.2V, the first-week discharge capacity of the traditional Na4Fe3(PO4)2P2O7 material is only 99 mAh / g, while the material of this patent reaches 109.2 mAh / g, approaching the theoretical capacity. This is because the gradient coating and dynamic bonding design of this patent optimize the structure and interfacial properties of the material, enabling more active substances to participate in the electrochemical reaction, thus significantly improving the first-week discharge capacity.

[0034] Second, the cycle retention rate is good. Under the environmental conditions of a charge-discharge rate of 1C and 25°C, after 1000 cycles, the capacity retention rate of the traditional material is 90.2%, while the material of this patent is increased to 98.2%. The modulus gradient structure of the gradient coating layer effectively buffers the stress during the sodium-ion insertion / extraction process, reduces particle cracking and pulverization; repairs the microcracks inside the material and realizes self-repair of the dynamic bonding layer.

[0035] Fourth, the interfacial impedance is small. Using EIS to test the interfacial impedance (Rct), the traditional material is 215 Ω, while the material of this patent is reduced to 52 Ω. For example, the PI - SS dynamic bonding layer is covalently bonded to the PDA layer through imide bonds, forming a stable interface, reducing the interfacial impedance, and improving the charge-discharge efficiency and cycle stability of the battery.

[0036] Fourth, the crack rate is low. After cycling, the crack rate is statistically analyzed by SEM. The crack area ratio of the traditional material is 28%, while the material of this patent is only 4%. The gradient design of the gradient coating layer from the soft region to the hard region can effectively disperse the stress generated by sodium-ion insertion / extraction and reduce crack generation; the dynamic bonding layer repairs the internal microcracks of the material through the reversible fracture and recombination of disulfide bonds at the battery operating temperature. The double insurance mechanism greatly improves the structural stability of the material. Description of the Drawings

[0037] Figure 1 SEM of the Na4Fe3(PO4)2P2O7@PI-SS*PDA-PA material in Application Example 1;

[0038] Figure 2 SEM of the Na4Fe3(PO4)2P2O7 / C material in Comparative Example 1. Detailed Embodiments

[0039] To enable those skilled in the art to better understand the technical solution of the present invention, the products of the present invention will be further described in detail below in conjunction with embodiments.

[0040] The present invention relates to a soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material, which includes an active material core of metal-based phosphate and its derivatives. The active material core is sequentially coated with a hard coating layer and a soft coating layer. The active material core and the hard coating layer are formed by gradient coating, and the hard coating layer and the soft coating layer are formed by dynamic bond synthesis.

[0041] Further, the hard coating layer is a composite material of a highly cross-linked polymer and a low cross-linked polymer containing amino groups and phenolic hydroxyl groups.

[0042] Specifically, the highly cross-linked polymer is one or more of tannic acid, polydopamine, chitosan, and polyaniline.

[0043] Specifically, the low cross-linked polymer is one or more of polyurethane, acrylate, polyacrylamide, and epoxy resin.

[0044] Further, the soft coating layer is a prepolymer containing disulfide bonds, and the prepolymer is polyimide and / or polythiourethane.

[0045] Further, the metal-based in the metal-based phosphate and its derivatives is one or more of vanadium-based, iron-based, titanium-based, chromium-based, zirconium-based, and manganese-based; the iron-based phosphate and its derivatives are Na4Fe3(PO4)2P2O7, NaFePO4 and their modified materials; the manganese-based phosphate and its derivatives are Na3MnTi(PO4)3, Na3MnZr(PO4)3, Na3MnV(PO4)3, Na3MnCr(PO4)3 and their modified materials.

[0046] Another aspect of the present invention lies in protecting a preparation method of the above-mentioned soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material, which includes the following steps:

[0047] S1. Preparation of precursor sol: Sodium source, phosphorus source, transition metal M source, and citric acid are added in sequence and dissolved under wet conditions with temperature control to obtain a precursor sol;

[0048] S2. Sintering of active core material: The sol is sintered at high temperature to obtain an active core material;

[0049] S3. Surface activation of active core material: The active core material is acid-treated, washed, and dried to obtain a dried powder of the activated active core material;

[0050] S4. Preparation of the hard coating layer: Put the surface-activated active core material after drying into the Tris-HCl buffer solution dissolved with amino group, phenolic hydroxyl polymer and PEI, control the temperature for reaction, and coat the hard coating layer on the active core material;

[0051] S5. Treatment of the gradient structure: Coat the ethylene glycol solution dissolved with the low-crosslinked polymer on the surface of the hard coating layer to form an outer soft zone, and use pulsed UV irradiation to form a gradient structure from the hard zone to the soft zone of the hard coating layer;

[0052] S6. Preparation of the soft coating layer: Coat the prepolymer solution containing disulfide bonds on the surface of the hard coating layer, and then perform drying treatment to obtain a gradient-coated and dynamically bonded soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material.

[0053] Further, in step S2, the high-temperature sintering method is two-stage sintering.

[0054] Specifically, the sintering temperature in the first stage is 300-400 °C, the sintering atmosphere is one or more of nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen, the sintering time is 2-6 h, and the heating rate is 1-4 °C / min.

[0055] Specifically, the sintering temperature in the second stage is 500-700 °C, the sintering atmosphere is an argon / hydrogen composite gas with a volume ratio of (85-97):(15-3), the sintering time is 6-12 h, and the heating rate is controlled at 1-4 °C / min.

[0056] Further, in step S4, the reaction temperature is 30-60 °C, and the reaction time is 10-20 h.

[0057] Further, in the pulsed UV irradiation in step S5, the irradiation mode is 1-10 s on / 1-10 s off, the intensity is 10-30 mW / cm², and the total time is 10-30 min.

[0058] Further, in step S1, the temperature-controlled reaction is a constant-temperature water bath, the constant-temperature water bath temperature is 50-100 °C, and the pH of the sol is 1-6;

[0059] Further, in step S3, the soaking time for acid treatment is 1-30 min, the drying temperature is 50-100 °C, and the drying time is 1-2 h.

[0060] Further, in step S6, the drying temperature is 100-150 °C, and the drying time is 1-2 h.

[0061] Further, the transition metal M source is one or more of a vanadium source, an iron source, a titanium source, a chromium source, a zirconium source, and a manganese source. The vanadium source is one or more of NH4VO3, VO, V2O3, VO2, and V2O5. The iron source is one or more of Fe(OH)3, Fe(OH)2, Fe2O3, FeC2O4, FePO4, Fe(HCOO)2, Fe(HCOO)3, Fe(CH3COO)2, Fe(CH3COO)3, Fe(NO3) 3* 9H2O, and Fe(NO3)2. The titanium source is one or more of TiO2 and Ti2O3. The chromium source is one or more of Cr2O3 and CrCl3. The zirconium source is one or more of ZrO2 and ZrCl4. The manganese source is one or more of MnCO3, MnCl2, Mn(NO3)2, MnO2, Mn2O3, and Mn3O4. The sodium source is one or more of Na3C6H5O7·2H2O, NaNO3, Na2CO3, Na3PO4, Na4P2O7, Na2HPO4, Na2C2O4, NaH2PO4, CH3COONa, Na2H2P2O7, and HCOONa. The phosphorus source is one or more of sodium phosphate, monosodium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, ammonium phosphate, pyrophosphoric acid, ammonium dihydrogen phosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.

[0062] Further, in S5 and S6, the coating process is one or more of electrostatic spraying, ultrasonic spraying, atomic layer spraying, and supercritical fluid spraying.

[0063] Example 1

[0064] The present invention relates to a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material, which includes an active material core of a metal-based phosphate and its derivatives. The active material core is sequentially coated with a hard coating layer and a soft coating layer. The active material core and the hard coating layer are formed by gradient coating, and the hard coating layer and the soft coating layer are formed by dynamic bond synthesis.

[0065] In this embodiment, the hard coating layer is a composite material of a highly cross-linked polymer and a low cross-linked polymer containing amino groups and phenolic hydroxyl groups. Specifically, the highly cross-linked polymer is tannic acid; the low cross-linked polymer is polyurethane or acrylate.

[0066] In this embodiment, the soft coating layer is a prepolymer containing disulfide bonds, and the prepolymer is polyimide and.

[0067] The preparation method of the soft-hard composite coated sodium-ion battery metal-based polyanion cathode material in this embodiment includes the following steps:

[0068] S1. Preparation of the precursor sol: Dissolve the sodium source, phosphorus source, transition metal M source, and citric acid under temperature control by wet method to obtain the precursor sol. Specifically, the temperature-controlled reaction is a constant-temperature water bath, the temperature of the constant-temperature water bath is 100 °C, and the pH of the sol is 3.

[0069] S2. Sintering of the active core material: Sinter the sol at high temperature to obtain the active core material. The high-temperature sintering method is two-stage sintering; the sintering temperature in the first stage is 400 °C, the sintering atmosphere is nitrogen, the sintering time is 4 h, and the heating rate is 1 °C / min; the sintering temperature in the second stage is 700 °C, the sintering atmosphere is an argon / hydrogen composite gas with a volume ratio of 90:10, the sintering time is 6 h, and the heating rate is controlled at 4 °C / min.

[0070] S3. Surface activation of the active core material: Treat the active core material with acid, wash and dry it to obtain the dried powder of the activated active core material. Specifically, the soaking time for acid treatment is 30 min, the drying temperature is 80 °C, and the drying time is 1 h.

[0071] S4. Preparation of the hard coating layer: Put the dried and surface-activated active core material into a Tris-HCl buffer solution dissolved with amino group, phenolic hydroxyl polymer, and PEI, and carry out a temperature-controlled reaction to coat a hard coating layer on the active core material. Specifically, the reaction temperature is 60 °C, and the reaction time is 15 h.

[0072] S5. Treatment of the gradient structure: Coat the surface of the hard coating layer with an ethylene glycol solution dissolved with a low-crosslinked polymer to form an outer soft zone, and use pulsed UV irradiation to form a gradient structure from the hard zone to the soft zone of the hard coating layer. Specifically, in pulsed UV irradiation, the irradiation mode is 10 s on / 5 s off, the intensity is 10 mW / cm², and the total time is 30 min; the coating process is electrostatic spraying.

[0073] S6. Preparation of the soft coating layer: Coat the surface of the hard coating layer with a prepolymer solution containing disulfide bonds, and then carry out a drying treatment to obtain a gradient-coated - dynamically bonded soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material. Specifically, the drying temperature is 150 °C, and the drying time is 1.5 h; the coating process is ultrasonic spraying.

[0074] In this embodiment, the metal matrix in the metal-based phosphate and its derivatives is an iron matrix; the iron-based phosphate and its derivatives are Na4Fe3(PO4)2P2O7; the transition metal M source is an Fe source, and the Fe source is Fe(OH)3, Fe(OH)2, Fe2O3, FeC2O4, FePO4; the sodium source is Na3C6H5O7·2H2O, Na2H2P2O7, HCOONa; the phosphorus source is sodium phosphate, sodium hydrogen phosphate.

[0075] Example 2

[0076] The present invention relates to a soft-hard composite-coated metal-based polyanion cathode material for sodium-ion batteries, which includes an active material core of metal-based phosphate and its derivatives. The active material core is successively coated with a hard coating layer and a soft coating layer. The active material core and the hard coating layer are formed by gradient coating, and the hard coating layer and the soft coating layer are formed by dynamic bond synthesis.

[0077] In this embodiment, the hard coating layer is a composite material of a highly cross-linked polymer and a low cross-linked polymer containing amino groups and phenolic hydroxyl groups. Specifically, the highly cross-linked polymer is polydopamine; the low cross-linked polymer is acrylate or polyacrylamide.

[0078] In this embodiment, the soft coating layer is a prepolymer containing disulfide bonds, and the prepolymer is polythiourethane.

[0079] The preparation method of the soft-hard composite-coated metal-based polyanion cathode material for sodium-ion batteries in this embodiment includes the following steps:

[0080] S1. Preparation of the precursor sol: The sodium source, phosphorus source, transition metal M source, and citric acid are dissolved by controlling the temperature in a wet method to obtain a precursor sol. Specifically, the temperature-controlled reaction is a constant-temperature water bath, the temperature of the constant-temperature water bath is 70 °C, and the pH of the sol is 1.

[0081] S2. Sintering of the active core material: The sol is subjected to high-temperature sintering to obtain the active core material. The high-temperature sintering method is two-stage sintering; the sintering temperature in the first stage is 350 °C, the sintering atmosphere is argon-hydrogen, the sintering time is 2 h, and the heating rate is 4 °C / min; the sintering temperature in the second stage is 600 °C, the sintering atmosphere is an argon / hydrogen composite gas with a volume ratio of 85:15, the sintering time is 12 h, and the heating rate is controlled at 2 °C / min.

[0082] S3. Surface activation of the active core material: The active core material is subjected to acid treatment, washed and dried to obtain a dried powder of the activated active core material. Specifically, the soaking time for acid treatment is 20 min, the drying temperature is 50 °C, and the drying time is 1-2 h.

[0083] S4. Preparation of the hard coating layer: The surface-activated active core material after drying is put into a Tris-HCl buffer solution dissolved with amino and phenolic hydroxyl polymers and PEI, and the temperature-controlled reaction is carried out to coat the hard coating layer on the active core material. Specifically, the reaction temperature is 50 °C, and the reaction time is 10 h.

[0084] S5. Treatment of the gradient structure: Coat the surface of the hard coating layer with an ethylene glycol solution dissolved with a low-crosslinked polymer to form an outer soft zone, and use pulsed UV irradiation to form a gradient structure from the hard zone to the soft zone of the hard coating layer. Specifically, during the pulsed UV irradiation, the irradiation mode is 50 s on / 1 s off, the intensity is 30 mW / cm², and the total time is 20 min; the coating process is ultrasonic spraying.

[0085] S6. Preparation of the soft coating layer: Coat the surface of the hard coating layer with a prepolymer solution containing disulfide bonds, and then perform a drying treatment to obtain a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material with gradient coating - dynamic bonding. Specifically, the drying temperature is 120 °C and the drying time is 1 h; the coating process is supercritical fluid spraying.

[0086] In this embodiment, the metal-based in the metal-based phosphate and its derivatives is iron-based; the iron-based phosphate and its derivatives are NaFePO4; the transition metal M source is an Fe source, and the Fe source is Fe(CH3COO)2, Fe(CH3COO)3, Fe(NO3) 3* 9H2O; the sodium source is Na3C6H5O7·2H2O, NaNO3, Na2CO3; the phosphorus source is phosphoric acid, ammonium phosphate, pyrophosphoric acid, ammonium dihydrogen phosphate, sodium pyrophosphate, sodium dihydrogen pyrophosphate.

[0087] Example 3

[0088] The present invention relates to a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material, which includes an active material core of a metal-based phosphate and its derivatives. The active material core is sequentially coated with a hard coating layer and a soft coating layer. The active material core and the hard coating layer are formed by gradient coating, and the hard coating layer and the soft coating layer are formed by dynamic bonding.

[0089] In this embodiment, the hard coating layer is a composite material of a highly crosslinked polymer and a low-crosslinked polymer containing amino groups and phenolic hydroxyl groups. Specifically, the highly crosslinked polymer is chitosan and polyaniline; the low-crosslinked polymer is polyacrylamide and epoxy resin.

[0090] In this embodiment, the soft coating layer is a prepolymer containing disulfide bonds, and the prepolymer is polyimide and polythiourethane.

[0091] The preparation method of the soft-hard composite coated sodium-ion battery metal-based polyanion cathode material in this embodiment includes the following steps:

[0092] S1. Preparation of the precursor sol: Wet-chemically control the temperature of dissolution of the sodium source, phosphorus source, transition metal M source, and citric acid to obtain a precursor sol. Specifically, the temperature-controlled reaction is a constant-temperature water bath, the constant-temperature water bath temperature is 50 °C, and the pH of the sol is 6.

[0093] S2. Sintering of the active core material: The sol is sintered at high temperature to obtain the active core material. The high-temperature sintering method is two-stage sintering; the sintering temperature in the first stage is 300 °C, the sintering atmosphere is nitrogen and argon, the sintering time is 6 h, and the heating rate is 2 °C / min; the sintering temperature in the second stage is 500 °C, the sintering atmosphere is an argon / hydrogen composite gas with a volume ratio of 97:3, the sintering time is 9 h, and the heating rate is controlled at 1 °C / min.

[0094] S3. Surface activation of the active core material: The active core material is acid-treated, washed, and dried to obtain the dried powder of the activated active core material. Specifically, the soaking time for acid treatment is 2 min, the drying temperature is 00 °C, and the drying time is 1.5 h.

[0095] S4. Preparation of the hard coating layer: The dried surface-activated active core material is placed in a Tris-HCl buffer solution dissolved with amino, phenolic hydroxyl polymer, and PEI, and the temperature is controlled for reaction to coat the hard coating layer on the active core material. Specifically, the reaction temperature is 30 °C and the reaction time is 20 h.

[0096] S5. Treatment of the gradient structure: The ethylene glycol solution dissolved with the low-crosslinked polymer is coated on the surface of the hard coating layer to form an outer soft zone, and pulsed UV irradiation is used to form a gradient structure from the hard zone to the soft zone of the hard coating layer. Specifically, in the pulsed UV irradiation, the irradiation mode is 2 s on / 10 s off, the intensity is 20 mW / cm², and the total time is 10 min; the coating process is atomic layer spraying.

[0097] S6. Preparation of the soft coating layer: The prepolymer solution containing disulfide bonds is coated on the surface of the hard coating layer, and then dried to obtain the gradient-coated - dynamically bonded soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material. Specifically, the drying temperature is 100 °C and the drying time is 2 h; the coating process is supercritical fluid spraying.

[0098] In this embodiment, the metal-based in the metal-based phosphate and its derivatives is zirconium-based and manganese-based; the manganese-based phosphate and its derivatives are Na3MnZr(PO4)3; the transition metal M source is zirconium source and manganese source, the zirconium source is ZrO2, ZrCl4, the manganese source is MnCO3, MnCl2; the sodium source is Na3C6H5O7·2H2O, NaNO3, Na2CO3, Na3PO4; the phosphorus source is sodium phosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate.

[0099] Example 4

[0100] The present invention relates to a soft-hard composite coated sodium-ion battery metal-based polyanionic cathode material, which includes an active material core of metal-based phosphate and its derivatives. The active material core is sequentially coated with a hard coating layer and a soft coating layer. The active material core and the hard coating layer are formed by gradient coating, and the hard coating layer and the soft coating layer are formed by dynamic bond synthesis.

[0101] In this embodiment, the hard coating layer is a composite material of a highly cross-linked polymer and a low cross-linked polymer containing amino groups and phenolic hydroxyl groups. Specifically, the highly cross-linked polymer is tannic acid and polydopamine; the low cross-linked polymer is polyurethane and acrylate.

[0102] In this embodiment, the soft coating layer is a prepolymer containing disulfide bonds, and the prepolymer is polyimide and polythiourethane.

[0103] The preparation method of the soft-hard composite coated sodium-ion battery metal-based polyanionic cathode material in this embodiment includes the following steps:

[0104] S1. Preparation of the precursor sol: The sodium source, phosphorus source, transition metal M source, and citric acid are dissolved under temperature control by wet method to obtain the precursor sol. Specifically, the temperature-controlled reaction is a constant temperature water bath, the constant temperature water bath temperature is 60 °C, and the pH of the sol is 3.

[0105] S2. Sintering of the active core material: The sol is subjected to high-temperature sintering to obtain the active core material. The high-temperature sintering method is two-stage sintering; the sintering temperature in the first stage is 330 °C, the sintering atmosphere is nitrogen, the sintering time is 3 h, and the heating rate is 3 °C / min; the sintering temperature in the second stage is 600 °C, the sintering atmosphere is an argon / hydrogen composite gas with a volume ratio of 90:10, the sintering time is 8 h, and the heating rate is controlled at 3 °C / min.

[0106] S3. Surface activation of the active core material: The active core material is subjected to acid treatment, washed and dried to obtain the dried powder of the activated active core material. Specifically, the soaking time of the acid treatment is 20 min, the drying temperature is 80 °C, and the drying time is 1.2 h.

[0107] S4. Preparation of the hard coating layer: The surface-activated active core material after drying is put into a Tris-HCl buffer solution dissolved with amino group and phenolic hydroxyl group polymers and PEI, and the temperature-controlled reaction is carried out to coat the hard coating layer on the active core material. Specifically, the reaction temperature is 40 °C and the reaction time is 15 h.

[0108] S5. Treatment of the gradient structure: Coat the surface of the hard coating layer with an ethylene glycol solution dissolving a low-crosslinking polymer to form an outer soft zone, and use pulsed UV irradiation to form a gradient structure from the hard zone to the soft zone of the hard coating layer. Specifically, in the pulsed UV irradiation, the irradiation mode is 5 s on / 5 s off, the intensity is 17 mW / cm², and the total time is 18 min; the coating process is electrostatic spraying.

[0109] S6. Preparation of the soft coating layer: Coat the surface of the hard coating layer with a prepolymer solution containing disulfide bonds, and then perform a drying treatment to obtain a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material with gradient coating-dynamic bonding. Specifically, the drying temperature is 110 °C and the drying time is 1.3 h; the coating process is supercritical fluid spraying.

[0110] In this embodiment, the metal base in the metal-based phosphate and its derivatives is a titanium base and a manganese base; the manganese-based phosphate and its derivatives are Na3MnTi(PO4)3; the transition metal M source is a titanium source and a manganese source, the titanium source is TiO2, Ti2O3, and the manganese source is Mn(NO3)2, MnO2, Mn2O3, Mn3O4; the sodium source is Na3PO4, Na4P2O7, Na2HPO4, Na2C2O4, NaH2PO4, CH3COONa, Na2H2P2O7, HCOONa; the phosphorus source is sodium phosphate, sodium hydrogen phosphate.

[0111] Example 5

[0112] The present invention relates to a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material, which includes an active material core of a metal-based phosphate and its derivatives. The active material core is sequentially coated with a hard coating layer and a soft coating layer. The active material core and the hard coating layer are formed by gradient coating, and the hard coating layer and the soft coating layer are formed by dynamic bonding.

[0113] In this embodiment, the hard coating layer is a composite material of a highly crosslinked polymer and a low-crosslinked polymer containing amino groups and phenolic hydroxyl groups. Specifically, the highly crosslinked polymer is tannic acid, polyaniline; the low-crosslinked polymer is polyurethane, epoxy resin.

[0114] In this embodiment, the soft coating layer is a prepolymer containing disulfide bonds, and the prepolymer is polyimide and polythiourethane.

[0115] The preparation method of the soft-hard composite coated sodium-ion battery metal-based polyanion cathode material in this embodiment includes the following steps:

[0116] S1. Preparation of precursor sol: The sodium source, phosphorus source, transition metal M source, and citric acid are dissolved under temperature control by wet method to obtain the precursor sol. Specifically, the temperature-controlled reaction is a constant-temperature water bath with a temperature of 70 °C and the pH of the sol is 3.

[0117] S2. Sintering of active core material: The sol is subjected to high-temperature sintering to obtain the active core material. The high-temperature sintering method is two-stage sintering; the sintering temperature in the first stage is 380 °C, the sintering atmosphere is nitrogen and argon, the sintering time is 2 - 6 h, and the heating rate is 2 °C / min; the sintering temperature in the second stage is 600 °C, the sintering atmosphere is an argon / hydrogen composite gas with a volume ratio of 92:8, the sintering time is 7 h, and the heating rate is controlled at 2 °C / min.

[0118] S3. Surface activation of active core material: The active core material is subjected to acid treatment, washed and dried to obtain the dried powder of the activated active core material. Specifically, the soaking time for acid treatment is 20 min, the drying temperature is 90 °C, and the drying time is 1 - 2 h.

[0119] S4. Preparation of hard coating layer: The dried and surface-activated active core material is put into a Tris-HCl buffer solution dissolved with amino, phenolic hydroxyl polymer, and PEI, and the reaction is carried out under temperature control to coat the hard coating layer on the active core material. Specifically, the reaction temperature is 50 °C and the reaction time is 13 h.

[0120] S5. Treatment of gradient structure: The ethylene glycol solution dissolved with low-crosslinked polymer is coated on the surface of the hard coating layer to form an outer soft zone, and a gradient structure from the hard zone to the soft zone of the hard coating layer is formed by pulsed UV irradiation. Specifically, in the pulsed UV irradiation, the irradiation mode is 8 s on / 5 s off, the intensity is 20 mW / cm², and the total time is 20 min; the coating process is electrostatic spraying.

[0121] S6. Preparation of soft coating layer: The prepolymer solution containing disulfide bonds is coated on the surface of the hard coating layer, and then dried to obtain the gradient-coated - dynamically bonded soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material. Specifically, the drying temperature is 140 °C and the drying time is 2 h; the coating process is atomic layer spraying.

[0122] In this embodiment, the metal-based group in the metal-based phosphate and its derivatives is an iron-based group, a chromium-based group, and a manganese-based group; the iron-based phosphate and its derivatives are Na4Fe3(PO4)2P2O7; the manganese-based phosphate and its derivatives are Na3MnCr(PO4)3; the transition metal M sources are vanadium source, iron source, titanium source, chromium source, zirconium source, and manganese source, the iron source is Fe(OH)3, Fe(OH)2, Fe2O3, FeC2O4, the chromium source is Cr2O3, CrCl3, and the manganese source is MnCO3, MnCl2; the sodium sources are Na3C6H5O7·2H2O, NaNO3, Na2CO3, Na3PO4, Na4P2O7HCOONa; the phosphorus sources are sodium phosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, ammonium phosphate, and pyrophosphoric acid.

[0123] Application Example 1 Synthesis and Electrochemical Performance of Na4Fe3(PO4)2P2O7@PI-SS*PDA-PA

[0124] This embodiment relates to a soft and hard composite-coated sodium-ion battery metal-based polyanion cathode material Na4Fe3(PO4)2P2O7@PI-SS*PDA-PA, and its preparation method includes the following steps:

[0125] Step 1: Weigh the materials of Fe(NO3) 3* 9H2O, Na2HPO4, and H3PO4 according to the stoichiometric ratio of 3:2:2 in the molecular formula, add them to deionized water, and use citric acid as a complexing agent (the addition amount is 60% of the Mn content), and disperse and dissolve to form a uniform yellowish-red transparent sol. At this time, the pH value of the sol is stable at 3.1;

[0126] Step 2: Transfer the sol to a crucible and place it in a high-temperature furnace. In the first stage, calcine it in an Ar atmosphere at 250 °C for 3 h with a heating rate of 2 °C / min; in the second stage, calcine it in a mixed atmosphere of 500 °C and Ar / H2 (96:4) for 8 h to obtain dry core material powder;

[0127] Step 3: Immerse the core material in a 0.1 mol / L HNO3 solution and soak for 8 min. After soaking, repeatedly rinse the core material with deionized water until the washing liquid is neutral, and then dry it in an oven at 80 °C for 1.5 h to obtain dry powder of the acid-treated core material;

[0128] Step 4: Prepare a reaction solution by dissolving 2 mg / mL of dopamine (PDA) and 1 wt% of PEI (polyethyleneimine) in Tris-HCl buffer solution (pH = 8.5);

[0129] Step 5: Put the dried acid-treated core material into the reaction solution and oscillate and react in a constant-temperature shaker at 45 °C for 10 h to obtain PDA-coated core material;

[0130] Step 6: Mix the solution according to the molar ratio of acrylate (PA) to ethylene glycol of 4:1, and uniformly coat it on the surface of the PDA-coated core material by electrostatic spraying;

[0131] Step 7: Crosslink by pulsed UV irradiation with an irradiation mode of 5 s on / 5 s off, an intensity of 15 mW / cm², and a total duration of 15 min to obtain the PDA-PA coated material;

[0132] Step 8: Uniformly coat the PI-SS prepolymer solution on the surface of the PDA-PA coated core material by atomic layer spraying process, then put it into a vacuum oven and react for 1.5 h under an Ar atmosphere at 120 °C. After natural cooling, the Na4Fe3(PO4)2P2O7@PI-SS*PDA-PA "gradient coating-dynamic bonding" composite cathode material is obtained.

[0133] Mix the Na4Fe3(PO4)2P2O7@PI-SS*PDA-PA composite cathode material and polyvinylidene fluoride in a mass ratio of 7:0.5 to form a homogeneous slurry, then coat the black slurry on the aluminum foil using a 140 μm four-sided coater, and then dry the film in a vacuum drying oven at 110 °C for 1.5 h. Use a punching machine to punch the electrode film into circular wafers with a radius of 0.5 mm, use metallic Na as the counter electrode, 1 mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator to assemble a CR2016 type button cell in a glove box.

[0134] Perform a constant current charge-discharge test on the above button cell with a current density of 0.1 C (1 C = 129 mAh / g). It can be seen from the data in Table 1 that under the condition of 0.1 C rate, this material exhibits a reversible capacity of 109.2 mAh / g. Comparative Example 1 uses a traditional process to prepare the Na4Fe3(PO4)2P2O7 material without gradient coating and dynamic bonding treatment. Under the same test conditions, that is, a charge-discharge rate of 0.1 C and a voltage range of 2.0 - 4.2 V, its first-week discharge capacity is only 99 mAh / g. At a charge-discharge rate of 1 C and an environment of 25 °C, after 1000 cycles, the capacity retention rate is 98.2%, which is much higher than 90.2% of Comparative Example 1. Figure 1SEM image of Na4Fe3(PO4)2P2O7@PI-SS*PDA-PA material after cycling. After statistics, compared with the material in Comparative Example 1 where the crack area accounts for 28%, the crack area of the material in this patent is only 4%. The interfacial impedance (Rct) was tested by EIS and decreased from 215 Ω of the material in Comparative Example 1 to 52 Ω of the material in this patent. This is because during the charge and discharge process, the soft region (PA) is composed of acrylate with low cross-linking degree, which has high elasticity and flexibility, and absorbs the volume expansion caused by the insertion and extraction of sodium ions through elastic deformation; the hard region (PDA) is composed of a highly cross-linked polydopamine layer, which tightly wraps the surface of the cathode material, provides rigid support, inhibits the pulverization of active material particles, and prevents particles from falling off the current collector through strong adhesion. At the same time, the dynamic bonding layer is composed of polyimide (PI-SS) containing disulfide bonds (-S-S-), and its core function is to achieve the self-healing ability of the electrode through the reversible fracture and recombination of dynamic covalent bonds, solving the performance decay caused by the accumulation of microcracks during cycling. Compared with traditional materials, the material in this patent has been greatly improved in terms of electrochemical performance and structural stability.

[0135] Application Example 2 Synthesis and Electrochemical Performance of Na3MnTi(PO4)3@PDA-PA

[0136] This example relates to a soft-hard composite coated sodium-ion battery metal-based polyanion cathode material Na3MnTi(PO4)3@PDA-PA, and its preparation method includes the following steps:

[0137] Step 1: Weigh the materials of Mn2O3, TiO2, and NaH2PO4 according to the stoichiometric ratio of 0.5:1:3 in the molecular formula, add them to deionized water, and use citric acid as a complexing agent (the addition amount is 60% of the Fe content), and disperse and dissolve to form a uniform grayish-brown transparent sol. At this time, the pH value of the sol is stabilized at 3.5;

[0138] Step 2: Transfer the sol to a crucible and place it in a high-temperature furnace. In the first stage, calcine it in an Ar atmosphere at 350 °C for 4 h with a heating rate of 2 °C / min; in the second stage, calcine it in a mixed atmosphere of 650 °C and Ar / H2 (96:4) for 8 h to obtain dry core material powder;

[0139] Step 3: Immerse the core material in a 0.1 mol / L HNO3 solution and soak for 8 min. After soaking, repeatedly rinse the core material with deionized water until the washing liquid is neutral, and then dry it in an oven at 80 °C for 1.5 h to obtain dry acid-treated core material powder;

[0140] Step 4: Prepare a reaction solution by dissolving 2 mg / mL of dopamine (PDA) and 1 wt% of PEI (polyethyleneimine) in Tris-HCl buffer solution (pH = 8.5);

[0141] Step 5: Put the dried acid-treated nuclear material into the reaction solution, and oscillate and react in a constant-temperature shaker at 45 °C for 12 h to obtain the PDA-coated nuclear material;

[0142] Step 6: Mix a solution of acrylate (PA) and ethylene glycol in a molar ratio of 4:1, and evenly coat it on the surface of the PDA-coated nuclear material using electrostatic spraying;

[0143] Step 7: Crosslink by pulsed UV irradiation, with an irradiation mode of 5 s on / 5 s off, an intensity of 15 mW / cm², and a total duration of 15 min to obtain the PDA-PA coated material;

[0144] Step 8: Put the PDA-PA coated material into a vacuum oven, react under an Ar atmosphere at 120 °C for 1.5 h, and naturally cool down to obtain the Na3MnTi(PO4)3@PDA-PA "gradient coating" composite cathode material.

[0145] Mix the Na3MnTi(PO4)3@PDA-PA composite cathode material and polyvinylidene fluoride in a mass ratio of 7:1 to form a homogeneous slurry, then use a 140-μm four-sided preparation device to coat the black slurry on the aluminum foil, and then dry the film in a vacuum drying oven at 110 °C for 1.5 h. Use a punching machine to punch the electrode film into circular pieces with a radius of 0.5 mm, use metallic Na as the counter electrode, 1 mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator to assemble a CR2016 type button battery in a glove box.

[0146] The above coin-type battery was subjected to constant current charge and discharge tests at a current density of 0.1C (1C = 120 mAh / g). From the data in Table 1, it can be seen that under the condition of a 0.1C rate, this material exhibited a reversible capacity of 106.3 mAh / g. In Comparative Example 2, the Na3MnTi(PO4)3 material was prepared by a traditional process without gradient coating treatment. Under the same test conditions, that is, a charge and discharge rate of 0.1C and a voltage range of 2.0 - 4.2V, its first-cycle discharge capacity was only 98 mAh / g. At a charge and discharge rate of 1C and an environment of 25°C, after 1000 cycles, the capacity retention rate was 97.9%, which was much higher than 89.6% of Comparative Example 2. Through SEM observation, the proportion of the crack area of the materials in Example 2 and Comparative Example 2 decreased from 25% to 19%. This is because the soft region (PA) is composed of acrylate with a low cross-linking degree, which has high elasticity and flexibility. It absorbs the volume expansion caused by the insertion and extraction of sodium ions through elastic deformation, preventing the propagation of particle cracks caused by stress concentration. At the same time, the interfacial impedance (Rct) was measured by EIS and decreased from 189Ω of the material in Comparative Example 2 to 45Ω of the material in this patent. This is because during the charge and discharge process, the hard region (PDA) is composed of a highly cross-linked polydopamine layer, which tightly wraps around the surface of the cathode material, providing rigid support, inhibiting the pulverization of active material particles, and preventing particles from falling off the current collector through strong adhesion. Compared with traditional materials, it shows that the material in this patent has improved electrochemical performance and structural stability.

[0147] Synthesis and Electrochemical Performance of Na4Fe3(PO4)2P2O7 / C in Comparative Example 1

[0148] This example relates to a metal-based polyanion cathode material Na4Fe3(PO4)2P2O7 / C for sodium-ion batteries, and its preparation method includes the following steps:

[0149] Step 1: Sucrose was used as a carbon source (the addition amount was 2.9% of the total solid content of the slurry), and Fe(NO3) 3* 9H2O, NaH2PO4, and CH3COONa were weighed according to the stoichiometric ratio in the molecular formula and added to deionized water, dispersed and dissolved to form a uniform precursor solution;

[0150] Step 2: The above precursor solution was spray-dried, with an inlet air temperature of 300°C and an outlet air temperature of 101°C to remove moisture and obtain a dry precursor powder;

[0151] Step 3: In a nitrogen atmosphere, the precursor powder was kept at 600°C for 15h and then naturally cooled to obtain the Na4Fe3(PO4)2P2O7 / C material.

[0152] After mixing Na4Fe3(PO4)2P2O7 / C, acetylene black, and polyvinylidene fluoride into a homogeneous slurry at a mass ratio of 7:2:1, the slurry was evenly coated on aluminum foil using a 140-μm four-sided coater, and then the film was placed in a vacuum drying oven at 110 °C for 1.5 hours. The electrode film was punched into circular discs with a radius of 0.5 mm using a punching machine. Using metallic Na as the counter electrode, 1 mol / L NaClO4 EC + DEC (1:1 vol%) + 5% FEC as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016-type button cell was assembled in a glove box.

[0153] The above button cell was subjected to a constant current charge-discharge test at a current density of 0.1 C (1 C = 129 mAh / g). The results in Table 1 show that the reversible capacity of this material at a 0.1 C rate is 99 mAh / g, which is lower than 109.2 mAh / g in Application Example 1. The reason for the low capacity is related to the structural stability of this material, which will exacerbate the polarization during the charge-discharge process to a certain extent, resulting in abnormal capacity utilization of the material inside some particles, leading to a relatively large internal resistance of this material. The interfacial impedance (Rct) was measured by EIS. The material in Comparative Example 1 was 215 Ω, which is much larger than 52 Ω in Example 1. In addition, after 1000 cycles at a 1 C rate, the capacity retention rate of this material is 90.2%, showing poorer cycle stability compared to Application Example 1, which is related to the lack of "gradient coating - dynamic bonding" on this material. The cycle stability decays severely due to the poor structural stability.

[0154] Synthesis and Electrochemical Performance of Na3MnTi(PO4)3 / C in Comparative Example 2

[0155] This example relates to a metal-based polyanionic cathode material Na3MnTi(PO4)3 / C for sodium-ion batteries, and its preparation method includes the following steps:

[0156] Step 1: Weigh glucose as a carbon source (the addition amount is 3.6% of the total solid content of the slurry), Mn2O3, TiO2, and NaH2PO4 according to the stoichiometric ratio of 0.5:1:3 in the molecular formula, add the materials to deionized water and disperse and dissolve them to form a homogeneous precursor solution;

[0157] Step 2: Spray-dry the above precursor solution with an inlet air temperature of 310 °C and an outlet air temperature of 105 °C to remove moisture and obtain a dry precursor powder;

[0158] Step 3: In a nitrogen atmosphere, keep the precursor powder at 630 °C for 11 h, and then naturally cool it to obtain Na3MnTi(PO4)3 / C.

[0159] Figure 2SEM image of Na3MnTi(PO4)3 / C material, which consists of spherical particles about 3 - 5 μm in size. The larger spherical particles are related to the agglomeration and non-uniformity during the raw material sanding process. Figure 2 XRD of Na3MnTi(PO4)3 material. The diffraction intensity of this material is low and there are many impurity diffraction peaks, indicating poor crystallinity and phase separation of the material, which is related to the poor dispersion uniformity and agglomeration among various elements.

[0160] After mixing Na3MnTi(PO4)3, acetylene black, and polyvinylidene fluoride in a mass ratio of 7:2:1 to form a homogeneous slurry, the black slurry was coated on aluminum foil using a 140 μm four-sided coater, and then the film was dried in a vacuum drying oven at 110 °C for 1.5 h. The electrode film was punched into circular wafers with a radius of 0.5 mm using a punching machine. Using metallic Na as the counter electrode, 1 mol / L NaClO4 EC + DEC (1:1 vol%) + 5% FEC as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 type button battery was assembled in a glove box.

[0161] The above-mentioned button battery was subjected to constant current charge-discharge testing at a current density of 0.1 C (1 C = 120 mAh / g). The results in Table 1 show that the reversible capacity of this material at a 0.1 C rate is 98 mAh / g, lower than 106.3 mAh / g in Application Example 1. The reason for the lower capacity is related to the structural stability of this material, which will, to a certain extent, exacerbate the polarization during the charge-discharge process, resulting in abnormal capacity utilization of the material inside some particles, leading to a relatively large internal resistance of this material. The interfacial impedance (Rct) was tested by EIS. The material in Comparative Example 1 was 189 Ω, much larger than 45 Ω in Example 1. In addition, after 1000 cycles at a 1 C rate, the capacity retention rate of this material is 89.6%, showing poorer cycle stability compared to Application Example 2. This is related to the lack of "gradient" coating of this material. The cycle stability decays severely due to the poor structural stability, resulting in poor structural stability and powder loss problems during the sodiation / desodiation process.

[0162] Table 1 Comparison table of physical and chemical properties

[0163] Application Example 1 Comparative Example 1 Application Example 2 Comparative Example 2 Discharge specific capacity at 0.1C (mAh / g) 109.2 99 106.3 98 Capacity retention rate after 1000 cycles at 1C (%) 98.2 90.2 97.9 89.6 Crack area (%) (SEM test) 4 28 19 25 Interface impedance (Ω) (EIS test) 52 215 45 189

[0164] The above embodiments are only specific embodiments of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can be made, and these obvious replacement forms all belong to the protection scope of the present invention.

Claims

1. A soft-hard composite coated sodium-ion battery metal-based polyanion cathode material, characterized in that: An active material core including metal-based phosphates and their derivatives, the active material core is sequentially coated with a hard coating layer and a soft coating layer, and the active material core and the hard coating layer are formed by gradient coating, and the hard coating layer and the soft coating layer are formed by dynamic bond formation.

2. The soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material according to claim 1, characterized in that: The hard coating layer is a composite material of a highly cross-linked polymer and a low cross-linked polymer containing amino groups and phenolic hydroxyl groups; The highly cross-linked polymer is one or more of tannic acid, polydopamine, chitosan, and polyaniline; The low cross-linked polymer is one or more of polyurethane, acrylate, polyacrylamide, and epoxy resin.

3. The soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material according to claim 1, wherein: The soft coating layer is a prepolymer containing disulfide bonds, and the prepolymer is polyimide and / or polythiourethane.

4. The soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material according to claim 1, characterized in that: The metal-based in the metal-based phosphates and their derivatives is one or more of vanadium-based, iron-based, titanium-based, chromium-based, zirconium-based, and manganese-based; The iron-based phosphates and their derivatives are Na4Fe3(PO4)2P2O7, NaFePO4, and their modified materials; The manganese-based phosphates and their derivatives are Na3MnTi(PO4)3, Na3MnZr(PO4)3, Na3MnV(PO4)3, Na3MnCr(PO4)3, and their modified materials.

5. A method for preparing a soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material according to any one of claims 1-4, characterized in that It includes the following steps: S1. Preparation of precursor sol: The sodium source, phosphorus source, transition metal M source, and citric acid are dissolved under temperature control by wet method to obtain a precursor sol; S2. Sintering of active core material: The sol is sintered at high temperature to obtain an active core material; S3. Surface activation of active core material: The active core material is acid-treated, washed and dried to obtain a dried powder of the activated active inner material; S4. Preparation of hard coating layer: The surface-activated active core material after drying is put into a Tris-HCl buffer solution dissolved with amino group, phenolic hydroxyl polymer and PEI, and the temperature is controlled for reaction to coat a hard coating layer on the active core material; S5. Treatment of gradient structure: The ethylene glycol solution dissolved with low cross-linked polymer is coated on the surface of the hard coating layer to form an outer soft zone, and pulsed UV irradiation is used to form a gradient structure from the hard zone to the soft zone of the hard coating layer; S6. Preparation of soft coating layer: The prepolymer solution containing disulfide bonds is coated on the surface of the hard coating layer, and then dried to obtain a gradient-coated and dynamically bonded soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material.

6. The preparation method of the soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material according to claim 5, characterized in that: In step S2, the high-temperature sintering method is two-stage sintering; The sintering temperature in the first stage is 300-400 °C, the sintering atmosphere is one or more of nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen, the sintering time is 2-6 h, and the heating rate is 1-4 °C / min; The sintering temperature in the second stage is 500-700 °C, the sintering atmosphere is an argon / hydrogen composite gas with a volume ratio of (85-97):(15-3), the sintering time is 6-12 h, and the heating rate is controlled at 1-4 °C / min.

7. The preparation method of the soft-hard composite coated sodium-ion battery metal-based polyanion cathode material according to claim 5, characterized in that: In step S4, the reaction temperature is 30-60 °C, and the reaction time is 10-20 h.

8. The soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material according to claim 5, characterized in that: In the pulsed UV irradiation in step S5, the irradiation mode is on for 1 - 10 s / off for 1 - 10 s, the intensity is 10 - 30 mW / cm², and the total time is 10 - 30 min.

9. The preparation method of the soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material according to claim 5, wherein: In step S1, the temperature-controlled reaction is a constant-temperature water bath, the temperature of the constant-temperature water bath is 50 - 100 °C, and the pH of the sol is 1 - 6; In step S3, the soaking time for acid treatment is 1 - 30 min, the drying temperature is 50 - 100 °C, and the drying time is 1 - 2 h; In step S6, the drying temperature is 100 - 150 °C, and the drying time is 1 - 2 h.

10. The preparation method of the soft-hard composite-coated sodium-ion battery metal-based polyanion cathode material according to claim 5, wherein: The transition metal M source is one or more of a vanadium source, an iron source, a titanium source, a chromium source, a zirconium source, and a manganese source. The vanadium source is one or more of NH4VO3, VO, V2O3, VO2, and V2O5. The iron source is one or more of Fe(OH)3, Fe(OH)2, Fe2O3, FeC2O4, FePO4, Fe(HCOO)2, Fe(HCOO)3, Fe(CH3COO)2, Fe(CH3COO)3, and Fe(NO3) 3* 9H2O, and Fe(NO3)2. The titanium source is one or more of TiO2 and Ti2O3. The chromium source is one or more of Cr2O3 and CrCl3. The zirconium source is one or more of ZrO2 and ZrCl4. The manganese source is one or more of MnCO3, MnCl2, Mn(NO3)2, MnO2, Mn2O3, and Mn3O4; The sodium source is one or more of Na3C6H5O7·2H2O, NaNO3, Na2CO3, Na3PO, Na4P2O7, Na2HPO4, Na2C2O4, NaH2PO4, CH3COONa, Na2H2P2O7, HCOONa; The phosphorus source is one or more of sodium phosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, ammonium phosphate, pyrophosphoric acid, ammonium dihydrogen phosphate, sodium pyrophosphate, sodium dihydrogen pyrophosphate.

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