ZnNi with oxygen vacancy enhanced interface electric field ₂ O ₄ / Fe ₂ O ₃ Heterojunction negative electrode material and preparation method thereof

Abstract

This invention discloses a ZnNi₂O₄ / Fe₂O₃ heterojunction anode material with oxygen vacancy-enhanced interfacial electric field and its preparation method, belonging to the field of lithium-ion battery electrode material technology. This invention utilizes a two-step hydrothermal method combined with a controllable reduction calcination process to construct an oxygen-vacancy-rich ZnNi₂O₄ / Fe₂O₃ heterojunction binder-free anode in situ on nickel foam. Oxygen vacancies enhance directional charge transfer at the heterojunction interface and improve the internal interfacial electric field strength, synergistically optimizing electronic conductivity and lithium-ion transport kinetics. This material achieves an initial discharge capacity of 1326 mAh g⁻¹ at 0.1 A g⁻¹ and a reversible capacity of 540 mAh g⁻¹ at 1.2 Ag⁻¹. The lithium-ion diffusion coefficient is 2.8 times higher than that of pure ZnNi₂O₄, solving the problems of poor conductivity, insufficient rate performance, and rapid cycle decay of traditional transition metal oxide anodes.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrode material technology, specifically relating to a transition metal oxide heterojunction anode material, and more particularly to a binder-free ZnNi2O4 / Fe2O3 heterojunction anode material with oxygen vacancy-enhanced interface built-in electric field, its preparation method, and its application in lithium-ion batteries. Background Technology

[0002] With the rapid development of portable electronic devices and new energy electric vehicles, the market has placed stringent demands on the comprehensive performance of high-energy-density lithium-ion batteries. Commercial graphite anodes, limited by their ultra-low theoretical specific capacity of 372 mAh g⁻¹ and sluggish lithium-ion transport kinetics at high rates, can no longer meet the development needs of next-generation high-energy-density lithium-ion batteries. Transition metal oxides (TMOs), with their multi-electron redox reaction characteristics and ultra-high theoretical specific capacity far exceeding that of graphite, have become the most promising candidate system for breaking through the capacity bottleneck of commercial anodes.

[0003] ZnNi₂O₄, as a typical spinel-type bimetallic oxide, possesses a high theoretical capacity of ~915 mAh g⁻¹, abundant redox active sites due to its multivalent Zn / Ni structure, the zero-strain characteristic of Zn which can alleviate structural collapse, and a spin-polarized electronic structure that facilitates the optimization of intrinsic conductivity, making it a highly promising anode material for lithium-ion batteries. However, its single-phase application is limited by poor intrinsic conductivity, severe volume expansion during repeated lithium insertion / extraction, and slow lithium-ion transport kinetics, leading to rapid capacity decay and poor rate performance.

[0004] Heterojunction engineering is a mature modification strategy for transition metal oxide anodes. It can induce interfacial charge redistribution through the difference in work function between the two phases, forming an internal interfacial electric field, accelerating charge transfer, and enriching lithium-ion storage sites. Fe2O3, with its ultra-high theoretical capacity of ~1007 mAh g⁻¹, abundant crustal reserves, and low cost, is an ideal pairing material for constructing complementary heterojunctions with ZnNi2O4. Oxygen vacancy engineering can improve intrinsic conductivity by narrowing the band gap, provide additional lithium-ion adsorption sites, and enhance interfacial electronic coupling to improve the internal interfacial electric field, thereby achieving synergistic optimization of the electrochemical performance of heterojunctions.

[0005] Currently, various spinel metal oxide / Fe2O3 heterojunctions have been developed for use as lithium-ion battery anodes, but their lithium storage performance still needs further optimization. The synergistic effect of oxygen vacancy engineering and interfacial electric field construction has not been fully explored, and there are no reports on the use of ZnNi2O4 / Fe2O3 heterojunctions as lithium-ion battery anodes. The regulation mechanism of oxygen vacancies on the interfacial electric field of this heterojunction and the optimization law of lithium storage performance have not been systematically elucidated. Summary of the Invention

[0006] 1. Materials Design

[0007] This invention takes an oxygen-vacancy-rich ZnNi2O4 / Fe2O3 heterojunction grown in situ on a nickel foam substrate as its core. By driving the spontaneous charge redistribution at the interface through the difference in work functions between the two phases, an internal interfacial electric field is formed. Oxygen vacancy engineering is used to enhance the directional charge transfer at the interface and improve the interfacial electric field strength, thereby achieving synergistic optimization of electronic conductivity and lithium-ion transport kinetics.

[0008] The material has a crystal structure of a two-phase heterojunction in which face-centered cubic spinel phase ZnNi2O4 and cubic hematite phase Fe2O3 coexist, without the formation of impurity phases. The material has a porous nanosheet framework morphology, with ZnNi2O4 as the cross-linked ultrathin wavy nanosheet substrate, and Fe2O3 nanoparticles uniformly anchored on the nanosheet surface to form a tightly contacted heterojunction interface. The nanosheet thickness is 50~200 nm, the pore size is 20~100 nm, and oxygen vacancies are enriched at the heterojunction interface.

[0009] In the material, Ni is in the Ni³⁺ valence state as the main electron donor in the heterojunction; Fe is in the Fe³⁺ valence state as the main electron donor, accompanied by a small amount of reduced iron species, as the electron acceptor in the heterojunction; the introduction of oxygen vacancies can enhance the hybridization of Ni 3d-Fe 3d and O 2p-metal d orbitals, narrow the band gap of the material, increase the electronic density of states at the Fermi level, and provide additional high-affinity lithium-ion adsorption sites.

[0010] 2. Preparation method

[0011] Nickel foam pretreatment: Immerse 2×2 cm nickel foam in 3 mol·L⁻¹ HCl solution for 30 min to remove surface oxides and impurities, then ultrasonically clean with deionized water and dry for later use.

[0012] Preparation of ZnNi2O4 / NF precursor: 1 mmol Zn(NO3)2·6H2O and 2 mmol Ni(NO3)2·6H2O were dissolved in a 30 mL mixture of deionized water and anhydrous ethanol (volume ratio 1:1) and stirred magnetically for 30 min. 6 mmol NH4F and 12 mmol urea were added sequentially, and stirring was continued for 20 min to obtain a homogeneous precursor solution. The precursor solution and pretreated nickel foam were transferred to a 50 mL polytetrafluoroethylene-lined autoclave and hydrothermally reacted at 160 °C for 8 h. After natural cooling, the product was alternately washed with deionized water and anhydrous ethanol and dried to obtain the ZnNi2O4 precursor. The precursor was calcined in air at 350 °C for 5 h with a heating rate of 1 °C·min⁻¹ to obtain a pure-phase ZnNi2O4 / NF control sample.

[0013] Preparation of ZnNi2O4 / Fe2O3 / NF heterojunction: The prepared ZnNi2O4 / NF precursor was placed in a 50 mL polytetrafluoroethylene-lined autoclave, and 30 mL of an aqueous solution containing 4.0 mmol urea and 2.0 mmol FeCl2·4H2O was added. The mixture was hydrothermally reacted at 160℃ for 8 h. After natural cooling, the sample was alternately washed with deionized water and anhydrous ethanol, vacuum dried at 60℃ for 4 h, and calcined at 350℃ in air for 2 h with a heating rate of 1℃·min⁻¹ to completely convert the FeO(OH) precursor into Fe2O3, thus obtaining an air-calcined ZnNi2O4 / Fe2O3 / NF heterojunction.

[0014] Controlled reduction calcination to introduce oxygen vacancies: The above ZnNi2O4 / Fe2O3 / NF sample was placed in a tube furnace, and 5% H2 / Ar mixed gas was introduced to purge for 30 min to remove the air. Under a 5% H2 / Ar gas flow of 100 sccm, the temperature was increased to 400℃ for 1 h at 2℃・min⁻¹. The reducing atmosphere was maintained throughout the process until the sample cooled to room temperature to avoid the oxygen vacancies being filled by ambient oxygen. Finally, an oxygen-rich vacancy ZnNi2O4 / Fe2O3 heterojunction binder-free anode was obtained.

[0015] 3. Performance advantages

[0016] Capacity and rate performance: At a current density of 0.1 A g⁻¹, the material achieves an initial discharge capacity of 1326 mAh g⁻¹; at a high current density of 1.2 A g⁻¹, the reversible capacity reaches 540 mAh g⁻¹, which is 3.05 times that of the pure phase ZnNi₂O₄ anode.

[0017] Kinetic characteristics: Lithium-ion diffusion coefficient reaches 3.08×10⁻¹ 4 cm²・s⁻¹, relatively pure ZnNi₂O₄ (1.09×10⁻¹) 4 The osmotic pressure (cm²·s⁻¹) was increased by 2.8 times; the charge transfer resistance was reduced to 101.7 Ω, an 84.4% decrease compared to pure phase ZnNi₂O₄ (651.9 Ω);

[0018] Cyclic stability: After 120 cycles at a current density of 0.1 A g⁻¹, the capacity retention rate reached 60.7%, which is 41.8% higher than that of pure phase ZnNi2O4 (42.8%), and the coulombic efficiency can be quickly stabilized to over 93%. Attached Figure Description

[0019] Figure 1: XRD and XPS spectra of the material, used to confirm the phase composition of the material and the presence of oxygen vacancies;

[0020] Figure 2: TEM image, EDS elemental distribution map and HRTEM high-resolution image of the material, used to demonstrate the regulatory role of oxygen vacancies in the formation of ZnNi2O4 / Fe2O3 heterostructure;

[0021] Figure 3: SEM images of the material, used to illustrate the effect of hydrogen reduction annealing on the microstructure of ZnNi2O4 / Fe2O3 heterostructure;

[0022] Figure 4: CV curves, charge-discharge curves, rate performance curves, cycle performance curves, and EIS AC impedance spectra of the materials, used to compare the lithium storage performance of ZnNi2O4 single-phase materials and ZnNi2O4 / Fe2O3 heterojunction materials, while reflecting the charge transfer resistance and lithium-ion diffusion capability of the materials;

[0023] Figure 5: Band structure diagram of the material, used to illustrate the effect of constructing the ZnNi2O4 / Fe2O3 heterojunction on improving the conductivity of the material;

[0024] Figure 6: Electronic density of states diagram of the material, combined with band structure diagram to further reveal the mechanism by which orbital hybridization effect in heterojunction enhances the conductivity of the material;

[0025] Figure 7: Work function distribution and differential charge diagram of the material, used to reveal the enhancement mechanism of the built-in electric field of ZnNi2O4 / Fe2O3 heterostructure by oxygen vacancies;

[0026] Figure 8: Analysis diagram of lithium-ion diffusion barrier and adsorption energy of the material, used to verify the advantages of ZnNi2O4 / Fe2O3 heterojunction material in lithium-ion diffusion kinetics and lithium storage adsorption capacity. Detailed Implementation

[0027] Example 1: Material Preparation

[0028] Nickel foam pretreatment: Take a 2×2 cm piece of nickel foam, immerse it in 3 mol・L⁻¹ HCl solution for 30 min to remove the surface oxide layer and oil stains, then ultrasonically clean it 3 times with deionized water for 5 min each time, and dry it in a vacuum drying oven at 60℃ for later use.

[0029] Preparation of ZnNi2O4 / NF precursor: 1 mmol Zn(NO3)2·6H2O and 2 mmol Ni(NO3)2·6H2O were dissolved in a 1:1 mixture of deionized water and anhydrous ethanol at 30 mL. The mixture was magnetically stirred at room temperature for 30 min until completely dissolved. 6 mmol NH4F and 12 mmol urea were added sequentially, and stirring was continued for 20 min to obtain a homogeneous and transparent precursor solution. The precursor solution and pretreated nickel foam were transferred to a 50 mL stainless steel autoclave lined with polytetrafluoroethylene. After sealing, the autoclave was placed in a forced-air drying oven for hydrothermal reaction at 160 °C for 8 h. After the reaction, the mixture was allowed to cool naturally to room temperature. The sample was then removed and rinsed three times alternately with deionized water and anhydrous ethanol, and vacuum dried at 60 °C for 4 h to obtain the ZnNi2O4 precursor. A portion of the precursor was calcined in a muffle furnace at 350 °C under air atmosphere for 5 minutes. h, heating rate 1℃・min⁻¹, after furnace cooling, a pure phase ZnNi2O4 / NF control sample was obtained;

[0030] Preparation of ZnNi2O4 / Fe2O3 / NF heterojunction: The ZnNi2O4 / NF precursor prepared above was placed in a 50 mL polytetrafluoroethylene-lined autoclave, and 30 mL of an aqueous solution containing 4.0 mmol urea and 2.0 mmol FeCl2·4H2O was added. After sealing, the mixture was hydrothermally reacted at 160℃ for 8 h. After natural cooling, the sample was taken out and rinsed three times alternately with deionized water and anhydrous ethanol. It was then vacuum dried at 60℃ for 4 h and subsequently calcined at 350℃ for 2 h in an air atmosphere in a muffle furnace at a heating rate of 1℃·min⁻¹ to obtain an air-calcined ZnNi2O4 / Fe2O3 / NF heterojunction.

[0031] Controlled reduction calcination to introduce oxygen vacancies: The ZnNi2O4 / Fe2O3 / NF sample after air calcination was placed in a tube furnace and purged with a 5% H2 / Ar mixed gas for 30 min to remove the air from the furnace; the gas flow rate was maintained at 100 sccm, and the temperature was increased to 400℃ at a rate of 2℃・min⁻¹, and calcined at a constant temperature for 1 h; after calcination, the reducing atmosphere was maintained throughout the process until the sample cooled to room temperature, and finally, an oxygen-vacancy-rich ZnNi2O4 / Fe2O3 heterojunction binder-free anode was obtained.

[0032] Example 2: Performance Verification and Phase and Valence State Characterization:

[0033] XRD results show that all characteristic diffraction peaks of the prepared oxygen-vacancy ZnNi2O4 / Fe2O3 heterojunction correspond perfectly to face-centered cubic spinel ZnNi2O4 (JCPDS No. 04-12-2133) and cubic hematite Fe2O3 (JCPDS No. 00-39-1346), confirming the successful construction of the heterojunction. Compared with pure ZnNi2O4, the characteristic diffraction peaks of the heterojunction show a significant shift, confirming that the introduction of oxygen vacancies induces lattice relaxation and changes in interplanar spacing.

[0034] XPS test results showed that the sample contained only four target elements: Zn, Ni, Fe, and O, with no impurities detected. The Zn 2p spectrum corresponded to the characteristic peak of Zn²⁺, the Ni 2p spectrum corresponded to the characteristic peak of Ni³⁺ in spinel ZnNi₂O₄, and the Fe 2p spectrum corresponded to the characteristic peak of Fe³⁺ in Fe₂O₃. Characteristic peaks of reduced iron species were also present. The characteristic peaks of Zn and Fe showed opposite binding energy shifts, confirming that electrons migrated directionally from ZnNi₂O₄ to Fe₂O₃, and the built-in electric field of the interface enhanced by oxygen vacancies was successfully formed. The O 1s spectrum could be divided into peaks of lattice oxygen, oxygen vacancies, and chemisorbed oxygen, confirming that oxygen vacancies were successfully introduced into the heterojunction.

[0035] Morphological and microstructural characterization:

[0036] SEM results show that pure-phase ZnNi2O4 consists of cross-linked ultrathin wavy nanosheets that interweave to form a well-developed porous network. The air-calcined heterojunction retains the nanosheet framework, and Fe2O3 nanoparticles are uniformly anchored on the nanosheet surface. The oxygen-vacancy-rich heterojunction exhibits an oxygen-vacancy-driven porous framework structure with interconnected channels and a defect-rich surface, providing abundant active sites for lithium-ion storage.

[0037] TEM and HRTEM test results show that the sample retains the porous nanosheet morphology, with Zn, Ni, and O elements uniformly distributed on the nanosheet substrate and Fe elements locally enriched at the heterojunction sites. The lattice fringes of the ZnNi2O4 (200) crystal plane and the Fe2O3 (220) crystal plane can be clearly distinguished, forming a clear heterojunction interface. There are obvious lattice distortions and local disordered regions at the interface, which directly confirms the oxygen vacancies enriched at the interface.

[0038] Electrochemical performance testing:

[0039] The prepared sample was used as the working electrode, a lithium metal sheet as the counter electrode, and a glass fiber membrane as the separator to assemble a CR2032 coin cell. Electrochemical performance was tested within a voltage window of 0.01–3.0 V. The results are as follows: At a current density of 0.1 A g⁻¹, the oxygen-vacancy-rich ZnNi₂O₄ / Fe₂O₃ heterojunction achieved an initial discharge capacity of 1326 mAh g⁻¹, an initial charge capacity of 936 mAh g⁻¹, an initial coulombic efficiency of 70.6%, and a reversible capacity of 968 mAh g⁻¹ in the second cycle. After 120 cycles, the capacity retention was 60.7%, significantly better than that of pure ZnNi₂O₄ (42.8%). At current densities of 0.1, 0.4, 0.8, and 1.2 Ag⁻¹, the reversible capacities were 1326, 715, 632, and 540 mAh g⁻¹, respectively, with the current recovering to 0.1 A. After reaching g⁻¹, the capacity rebounded to 680 mAh g⁻¹, and the capacity at 1.2 A g⁻¹ is 3.05 times that of pure-phase ZnNi₂O₄. Electrochemical kinetic tests showed that the charge transfer resistance Rct of the material was 101.7 Ω, and the lithium-ion diffusion coefficient reached 3.08 × 10⁻¹. 4 The concentration of s²·cm²·s⁻¹ was 2.8 times higher than that of pure ZnNi₂O₄, confirming that the interfacial electric field enhanced by oxygen vacancies can significantly accelerate charge transfer and lithium-ion transport.

[0040] Theoretical calculation verification:

[0041] Spin polarization density functional theory (DFT) calculations performed using the VASP software package revealed a significant difference in work function between ZnNi2O4 and Fe2O3 (5.275 eV and 4.37 eV, respectively), which is the intrinsic thermodynamic driving force for the formation of the built-in electric field at the interface. The introduction of oxygen vacancies can enhance the directional electron transfer at the interface, increase the interfacial electric field strength, and simultaneously enhance the adsorption capacity of the heterojunction for lithium ions and reduce the lithium ion diffusion barrier, which is in perfect agreement with the experimental results.

Claims

1. A ZnNi2O4 / Fe2O3 heterojunction anode material with oxygen vacancy-enhanced interfacial electric field, characterized in that, The material is a binderless negative electrode grown in situ on a nickel foam matrix. The main body is an oxygen-vacancy-rich ZnNi2O4 / Fe2O3 heterojunction. The heterojunction interface is formed by a close contact between the face-centered cubic spinel phase ZnNi2O4 and the cubic hematite phase Fe2O3, and an internal interfacial electric field is formed at the interface. The material has a porous nanosheet framework morphology, with Fe2O3 nanoparticles uniformly anchored on the surface of ZnNi2O4 nanosheets. The thickness of the nanosheets is 50~200 nm and the pore size is 20~100 nm.

2. The negative electrode material according to claim 1, characterized in that, The oxygen vacancies in the heterojunction are enriched at the heterojunction interface. The introduction of oxygen vacancies can enhance the hybridization of Ni 3d-Fe 3d and O 2p-metal d orbitals, narrow the material band gap, and increase the electronic state density at the Fermi level. In the material, Ni element acts as Ni³⁺ electron donor and Fe element acts as Fe³⁺ electron acceptor. There is a directional migration of electrons from ZnNi₂O₄ to Fe₂O₃ at the heterojunction interface.

3. The negative electrode material according to claim 1, characterized in that, The material exhibits an initial discharge capacity of ≥1300 mAh g⁻¹ at a current density of 0.1 A g⁻¹, a reversible capacity of ≥500 mAh g⁻¹ at a current density of 1.2 A g⁻¹, and a lithium-ion diffusion coefficient that is ≥2.5 times higher than that of pure phase ZnNi₂O₄.

4. A method for preparing a ZnNi2O4 / Fe2O3 heterojunction anode material with oxygen vacancy-enhanced interfacial electric field as described in claim 1, characterized in that, Includes the following steps: (1) Pretreatment of nickel foam: The nickel foam is immersed in HCl solution to remove surface impurities, ultrasonically cleaned with deionized water and then dried for later use. (2) Preparation of ZnNi2O4 / NF precursor: Dissolve zinc source and nickel source in a mixed solution of deionized water and anhydrous ethanol, add NH4F and urea and stir to obtain precursor solution, and perform hydrothermal reaction with pretreated foamed nickel. After washing and drying, the product is ZnNi2O4 precursor; (3) Preparation of ZnNi2O4 / Fe2O3 / NF heterojunction: Place ZnNi2O4 / NF precursor in an aqueous solution of iron source and urea for secondary hydrothermal reaction, and after washing and drying, the product is calcined in air to obtain air-calcined ZnNi2O4 / Fe2O3 / NF heterojunction; (4) Controllable reduction calcination to introduce oxygen vacancies: The heterojunction after air calcination is placed in a reducing atmosphere for controlled calcination, and the reducing atmosphere is maintained throughout the cooling process to obtain oxygen-rich ZnNi2O4 / Fe2O3 heterojunction anode material.

5. The preparation method according to claim 4, characterized in that, In step (1), the concentration of HCl solution is 3 mol·L⁻¹, and the soaking time is 30 min; in step (2), the zinc source is Zn(NO3)2·6H2O, the nickel source is Ni(NO3)2·6H2O, the molar ratio of zinc source to nickel source is 1:2, the hydrothermal reaction temperature is 160℃, the reaction time is 8 h, and the precursor is calcined in air at 350℃ for 5 h with a heating rate of 1℃·min⁻¹.

6. The preparation method according to claim 4, characterized in that, In step (3), the iron source is FeCl2·4H2O, the hydrothermal reaction temperature is 160℃, the reaction time is 8 h, the air calcination temperature is 350℃, the calcination time is 2 h, and the heating rate is 1℃·min⁻¹; in step (4), the reducing atmosphere is a 5% H2 / Ar mixed gas, the calcination temperature is 400℃, the calcination time is 1 h, and the heating rate is 2℃·min⁻¹.

7. The application of the ZnNi2O4 / Fe2O3 heterojunction anode material with oxygen vacancy-enhanced interfacial electric field as described in claim 1 in the anode of a lithium-ion battery.

Images