Method for modifying sodium battery positive electrode active material

Abstract

The application relates to the field of battery materials and discloses a modification method of a sodium battery positive electrode active material, which comprises the following steps: proportionally weighing sodium sources, mixed iron sources, phosphorus sources, lithium sources, fluorine sources, boron sources, organic carbon sources and deionized water to prepare mixed slurry; wet sand grinding the mixed slurry and controlling the solid phase particle size; spray drying and granulating to obtain spherical powder; and placing the spherical powder in a protective atmosphere to implement stepwise heat treatment. The application utilizes the decomposition characteristics of the mixed iron sources in the stepwise heat treatment to induce oxygen vacancies on the material surface to in-situ anchor doped atoms, realizes uniform solid solution of lithium, fluorine and boron elements at lattice points, solves the segregation and escape problems of modified components in a high-temperature environment, cooperatively improves the electronic and ionic transmission efficiency of the material, and utilizes the in-situ generated conductive carbon network to maintain the interface stability and structural integrity.

Description

Technical Field

[0001] This invention relates to a method for modifying the positive electrode active material of sodium batteries, belonging to the field of battery material technology. Background Technology

[0002] Sodium iron phosphate pyrophosphate, as a cathode material for polyanionic sodium-ion batteries, possesses characteristics such as stable three-dimensional framework structure, high safety performance, and moderate voltage plateau. Industrial preparation often employs solid-state reaction methods, supplemented by carbon source coating to overcome its inherently low electronic conductivity. To improve sodium-ion transport rate and rate capacity, lithium doping at sodium sites, fluorine doping at oxygen sites, and boron doping at phosphorus sites have become modification pathways. However, a physical conflict exists between the high-temperature crystallization kinetics of the polyanionic system and the thermal stability of the dopant components. To ensure… The main phase has high crystallinity and a complete framework structure. The sintering temperature usually needs to be higher than 550℃. At this temperature, active components such as fluorine and boron are prone to volatilization due to gas phase partial pressure, or component segregation occurs at the grain nucleation interface, thereby generating an electrically insulating enriched layer on the surface of the primary particles.

[0003] Besides optimizing the component system ratio, there are shortcomings in the micro-dynamic control of heat treatment. For example, Chinese invention patent CN116344773B discloses a composite sodium iron phosphate pyrophosphate cathode material and its preparation method. By introducing multiple dopant salts and carbon sources for composite modification and using segmented heating process for heat treatment, the specific capacity and rate performance of the material are improved. However, under actual working conditions, no specific lattice trapping mechanism is constructed for volatile or segregated components such as fluorine and boron. After entering the high-temperature region required for the crystallization of the main phase, the doped atoms are in a state of dynamic runaway. They are very likely to migrate along the grain boundary to the surface to form a heterogeneous phase layer, or directly cause the stoichiometric ratio to deviate due to vaporization. The lack of atomic-level in-situ pinning ability in sintering logic limits the modification effect and makes it difficult to fundamentally solve the bottleneck of the dramatic increase in interfacial charge transfer impedance under high-rate conditions.

[0004] Therefore, the technical problem to be solved by this invention is how to achieve stable solid solution of doped components in the main phase framework through raw material system design and thermal field evolution control, and simultaneously construct a low-impedance interface transport network. Summary of the Invention

[0005] To address the problems in the background art, the technical solution of the present invention is as follows: A method for modifying the positive electrode active material of a sodium battery, the modification method comprising the following steps:

[0006] Step S101: Weigh out sodium source, mixed iron source system, phosphorus source, lithium source, fluorine source and boron source according to stoichiometric ratio, add organic carbon source and deionized water to prepare mixed slurry; wherein, the mixed iron source system is composed of ferric phosphate dihydrate and ferrous oxalate dihydrate. Step S102: The mixed slurry is fed into a sand mill for wet grinding to obtain the modified precursor slurry; Step S103: The modified precursor slurry is dried to obtain spherical precursor powder. Step S104 involves performing a step-by-step heat treatment on the spherical precursor powder under a protective atmosphere, including the following sub-steps: Step S1041 involves performing a first-stage heat treatment within the temperature range where ferrous oxalate undergoes thermal decomposition, using carbon monoxide generated from the thermal decomposition of ferrous oxalate to reduce the surface of the precursor powder, thereby forming oxygen vacancy defects on the powder surface; Step S1042 involves increasing the temperature for a second-stage heat treatment, causing the precursor powder to undergo a solid-state reaction, using the lattice attraction generated by the oxygen vacancy defects to bind the doped atoms released from the fluorine and boron sources, resulting in an active material of sodium iron pyrophosphate with doped elements distributed at lattice sites and coated with a conductive carbon layer on the surface.

[0007] Preferably, in step S101, based on the total molar amount of the mixed iron source system, the molar proportion of ferrous oxalate dihydrate is 15% to 45%; the solid content of the mixed slurry is 35% to 50%; in step S102, the particle size D50 of the solid particles in the mixed slurry is controlled to be 0.25 μm to 0.45 μm to obtain the modified precursor slurry; in step S103, the drying treatment is spray drying to obtain spherical precursor powder; in step S1041, the temperature of the first stage heat treatment is 300℃ to 480℃ and the holding time is 1h to 4h; in step S1042, the temperature of the second stage heat treatment is 550℃ to 700℃ and the holding time is 6h to 12h.

[0008] Preferably, in step S101, the sodium source is selected from sodium carbonate, sodium acetate, or sodium pyrophosphate; the phosphorus source is selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or sodium pyrophosphate; the lithium source is selected from lithium carbonate, lithium hydroxide, lithium phosphate, or lithium fluoride; the fluorine source is selected from sodium fluoride or ammonium fluoride; the boron source is selected from boric acid, boron oxide, sodium borate, or lithium borate; the organic carbon source is selected from one or more of glucose, sucrose, citric acid, starch, polyvinyl alcohol, or polyethylene glycol; and according to the theoretical chemical formula of the generated sodium iron phosphate pyrophosphate active material, the lithium doping amount in the lithium source is 0.01 mol to 0.08 mol, the fluorine doping amount in the fluorine source is 0.02 mol to 0.20 mol, and the boron doping amount in the boron source is 0.01 mol to 0.03 mol.

[0009] Preferably, in step S101, based on the total molar amount of the mixed iron source system, the molar percentage of ferrous oxalate dihydrate is preferably 25% to 35%; in step S102, the wet grinding speed is 2000 rpm to 3500 rpm, the grinding time is 3 h to 6 h, and the grinding media used is zirconia beads with a diameter of 0.2 mm to 0.6 mm.

[0010] Preferably, in step S103, the inlet air temperature of the spray drying process is 240°C to 280°C, and the outlet air temperature is 100°C to 120°C; the average particle size of the obtained spherical precursor powder is 5μm to 20μm.

[0011] Preferably, in step S104, the heating rate in step S1041 is 3°C / min to 8°C / min, and the heating rate in step S1042 is 2°C / min to 5°C / min; the protective atmosphere includes nitrogen or argon, and the flow rate of the protective atmosphere is 0.5L / min to 2L / min.

[0012] Preferably, the amount of organic carbon source added is such that the mass content of carbon element in the generated sodium iron phosphate pyrophosphate active material is 1.0% to 2.0% by weight percentage; the sodium iron phosphate pyrophosphate active material is formed by the agglomeration of primary single crystal particles into secondary spherical particles, and the particle size of the primary single crystal particles is 50 nm to 300 nm.

[0013] Preferably, in step S1041, carbon monoxide and carbon dioxide generated by the thermal decomposition of ferrous oxalate dihydrate are used to form a local reducing atmosphere. The carbon monoxide combines with oxygen atoms on the surface of the spherical precursor powder and detaches in the form of carbon dioxide, thereby inducing the generation of oxygen vacancy defects.

[0014] Preferably, in step S1042, the pinning effect of oxygen vacancy defects on fluoride ions and boron atoms is used to reduce the diffusion activation energy of the elements, so that fluoride is dissolved in the oxygen sites of the crystal lattice and boron is dissolved in the phosphorus sites of the crystal lattice, and the vaporization of the fluoride source and the boron source is suppressed when the temperature is higher than 550°C.

[0015] Preferably, the prepared sodium iron phosphate pyrophosphate active material has a three-dimensional framework structure, wherein lithium occupies sodium lattice sites, fluorine occupies oxygen lattice sites, and boron occupies phosphorus lattice sites.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. In the modification of positive electrode active materials for sodium batteries, a composite modification path of lithium doping at sodium sites, fluorine doping at oxygen sites, and boron doping at phosphorus sites is selected. Elements with different radii and electronegativity generate synergistic gains on the sodium iron phosphate lattice of pyrophosphate. The introduction of lithium ions with smaller radii at sodium sites can expand the sodium ion diffusion channel and reduce the ion migration barrier. The doping of fluorine elements with stronger electronegativity at oxygen sites enhances the ionicity of the chemical bond between the metal and the anion, thereby broadening the energy band and improving the intrinsic electronic conductivity of the material. The substitution of boron groups at phosphorus sites induces lattice distortion and forms defect sites that facilitate rapid ion diffusion. This multi-site synergistic modification mechanism eliminates the lattice zoning restrictions caused by single doping, enabling the material to exhibit high-throughput ion transport characteristics and structural stability during power conversion, and avoiding the collapse of the crystal framework under high-voltage desodium removal conditions.

[0017] 2. A mixed iron source system consisting of ferrous oxalate dihydrate and ferric phosphate dihydrate, combined with a two-stage step sintering process, is used to achieve kinetic control of the solid solution behavior of dopant elements at the microscale. During the low-temperature sintering process, the thermal decomposition of ferrous oxalate in a specific molar ratio generates a locally high-concentration reducing micro-atmosphere. This atmosphere washes over the precursor surface and induces a high density of transient oxygen vacancies. These vacancies act as chemical anchors in the subsequent high-temperature crystallization stage, pinning volatile components such as fluorine and boron to lattice sites in situ through electrostatic attraction and spatial concession mechanisms, thus blocking the segregation of components at grain boundaries. This solid solution path driven by the decomposition kinetics of the iron source ensures that the dopant elements are atomically uniformly distributed in the main phase framework, resolving the thermodynamic contradiction between component volatilization and interfacial impurity enrichment in traditional solid-phase reactions.

[0018] 3. By controlling the ratio of mixed iron sources and the synergistic effect of organic carbon sources, a three-dimensional interconnected conductive network and porous microstructure are constructed in situ during the material synthesis process. The gaseous products released by the decomposition of ferrous oxalate generate abundant pore-forming effects inside the particles, increasing the wetting area of ​​active materials and electrolytes. At the same time, the decomposition products promote the atomic-level dispersion of organic matter and form chemical bonds with the crystal surface after multi-element solid solution. This in-situ self-generated conductive carbon network not only improves electron transport efficiency but also effectively buffers the changes in cell volume during charging and discharging. This synergistic mechanism enables the material to have low polarization loss characteristics under high current impact, ensuring the capacity performance and cycle performance of the battery under high rate conditions. Attached Figure Description

[0019] Figure 1 This is a process flow diagram of the multi-site synergistic modification process of the sodium battery cathode material of the present invention; Figure 2 This is a structural diagram of the closed-loop regulation system for preparing the positive electrode active material of the present invention.

[0020] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0022] A method for modifying a positive electrode active material for sodium batteries, the method comprising the following steps: Step S101: Weigh out sodium source, mixed iron source system, phosphorus source, lithium source, fluorine source and boron source according to stoichiometric ratio, add organic carbon source and deionized water to prepare mixed slurry; wherein, the mixed iron source system is composed of ferric phosphate dihydrate and ferrous oxalate dihydrate. Step S102: The mixed slurry is fed into a sand mill for wet grinding to obtain the modified precursor slurry; Step S103: The modified precursor slurry is dried to obtain spherical precursor powder. Step S104 involves performing a step-by-step heat treatment on the spherical precursor powder under a protective atmosphere, including the following sub-steps: Step S1041 involves performing a first-stage heat treatment within the temperature range where ferrous oxalate undergoes thermal decomposition, using carbon monoxide generated from the thermal decomposition of ferrous oxalate to reduce the surface of the precursor powder, thereby forming oxygen vacancy defects on the powder surface; Step S1042 involves increasing the temperature for a second-stage heat treatment, causing the precursor powder to undergo a solid-state reaction, using the lattice attraction generated by the oxygen vacancy defects to bind the doped atoms released from the fluorine and boron sources, resulting in an active material of sodium iron pyrophosphate with doped elements distributed at lattice sites and coated with a conductive carbon layer on the surface.

[0023] Preferably, in step S101, based on the total molar amount of the mixed iron source system, the molar proportion of ferrous oxalate dihydrate is 15% to 45%; the solid content of the mixed slurry is 35% to 50%; in step S102, the particle size D50 of the solid particles in the mixed slurry is controlled to be 0.25 μm to 0.45 μm to obtain the modified precursor slurry; in step S103, the drying treatment is spray drying to obtain spherical precursor powder; in step S1041, the temperature of the first stage heat treatment is 300℃ to 480℃ and the holding time is 1h to 4h; in step S1042, the temperature of the second stage heat treatment is 550℃ to 700℃ and the holding time is 6h to 12h.

[0024] Preferably, in step S101, the sodium source is selected from sodium carbonate, sodium acetate, or sodium pyrophosphate; the phosphorus source is selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or sodium pyrophosphate; the lithium source is selected from lithium carbonate, lithium hydroxide, lithium phosphate, or lithium fluoride; the fluorine source is selected from sodium fluoride or ammonium fluoride; the boron source is selected from boric acid, boron oxide, sodium borate, or lithium borate; the organic carbon source is selected from one or more of glucose, sucrose, citric acid, starch, polyvinyl alcohol, or polyethylene glycol; and according to the theoretical chemical formula of the generated sodium iron phosphate pyrophosphate active material, the lithium doping amount in the lithium source is 0.01 mol to 0.08 mol, the fluorine doping amount in the fluorine source is 0.02 mol to 0.20 mol, and the boron doping amount in the boron source is 0.01 mol to 0.03 mol.

[0025] Preferably, in step S101, based on the total molar amount of the mixed iron source system, the molar percentage of ferrous oxalate dihydrate is preferably 25% to 35%; in step S102, the wet grinding speed is 2000 rpm to 3500 rpm, the grinding time is 3 h to 6 h, and the grinding media used is zirconia beads with a diameter of 0.2 mm to 0.6 mm.

[0026] Preferably, in step S103, the inlet air temperature of the spray drying process is 240°C to 280°C, and the outlet air temperature is 100°C to 120°C; the average particle size of the obtained spherical precursor powder is 5μm to 20μm.

[0027] Preferably, in step S104, the heating rate in step S1041 is 3°C / min to 8°C / min, and the heating rate in step S1042 is 2°C / min to 5°C / min; the protective atmosphere includes nitrogen or argon, and the flow rate of the protective atmosphere is 0.5L / min to 2L / min.

[0028] Preferably, the amount of organic carbon source added is such that the mass content of carbon element in the generated sodium iron phosphate pyrophosphate active material is 1.0% to 2.0% by weight percentage; the sodium iron phosphate pyrophosphate active material is formed by the agglomeration of primary single crystal particles into secondary spherical particles, and the particle size of the primary single crystal particles is 50 nm to 300 nm.

[0029] Preferably, in step S1041, carbon monoxide and carbon dioxide generated by the thermal decomposition of ferrous oxalate dihydrate are used to form a local reducing atmosphere. The carbon monoxide combines with oxygen atoms on the surface of the spherical precursor powder and detaches in the form of carbon dioxide, thereby inducing the generation of oxygen vacancy defects.

[0030] Preferably, in step S1042, the pinning effect of oxygen vacancy defects on fluoride ions and boron atoms is used to reduce the diffusion activation energy of the elements, so that fluoride is dissolved in the oxygen sites of the crystal lattice and boron is dissolved in the phosphorus sites of the crystal lattice, and the vaporization of the fluoride source and the boron source is suppressed when the temperature is higher than 550°C.

[0031] Preferably, the prepared sodium iron phosphate pyrophosphate active material has a three-dimensional framework structure, wherein lithium occupies sodium lattice sites, fluorine occupies oxygen lattice sites, and boron occupies phosphorus lattice sites. In the sodium iron phosphate pyrophosphate active material, Fe is an iron-deficient phase and its molar amount ranges from 2.8 to 3.0.

[0032] Example 1: In the preparation of sodium iron phosphate pyrophosphate cathode active materials for high-rate energy storage, the method provided by this invention reduces interfacial segregation of dopant components by controlling the evolution path of the chemical potential in the precursor thermal field, and prepares materials with the general formula... The active ingredients, sodium carbonate, ferric phosphate dihydrate, ferrous oxalate dihydrate, ammonium dihydrogen phosphate, lithium carbonate, and sodium fluoride, were weighed according to stoichiometric ratios. Based on the total molar amount of the mixed iron source system, the molar percentage of ferrous oxalate dihydrate was set at 15%. The raw materials and glucose (as an organic carbon source) were added to deionized water to prepare a mixed slurry with a solid content of 40%. This slurry was then ground using a sand mill with 0.4 mm diameter zirconia beads until the median particle size of the solid particles in the slurry reached the specified value. Reaching 280 nm, the resulting slurry, after spray drying and granulation, produces spherical precursor powder with an average particle size of 12 μm, wherein... The median particle size of the sample particles.

[0033] Spherical precursor powder was subjected to step-by-step heat treatment under a protective atmosphere. In the first stage, the temperature was increased to 400℃ at a heating rate of 5℃ / min and held for 2 hours. Within this temperature range, ferrous oxalate dihydrate underwent thermal decomposition to produce carbon monoxide gas. Carbon monoxide induced oxygen vacancy defects on the surface of the spherical precursor powder. Based on the thermodynamic principle of solid surface defects, carbon monoxide preferentially stripped non-bridging oxygen atoms from the precursor powder surface within this specific temperature range, preventing the reduction of bridging oxygen in the internal polyanionic framework, maintaining the integrity of the polyanionic framework, and blocking the reduction and precipitation of ferrous iron crystal phases. Within the controlled temperature range of 300℃ to 480℃, the thermal activation energy provided by the system was only sufficient to overcome the bonding energy barrier of terminal oxygens with low coordination numbers on the surface. Simultaneously, due to the limitation of mass transfer depth in the gas-solid interface reaction, carbon monoxide molecules in this temperature range... The kinetic energy between the particles is insufficient to penetrate into the dense polyanionic bulk lattice, and the POP and PO-Fe bridging oxygen within the framework possess extremely high covalent bond dissociation energies. This confines the reduction reaction to a few nanometers of the powder's surface layer in a physicochemical mechanism. Thus, without disrupting the bulk framework structure or inducing elemental iron precipitation, the controllable induction of transient surface oxygen vacancies is achieved. In the second stage, the temperature is increased to 650℃ at a rate of 3℃ / min and held for 10 hours. During the phase reconstruction process induced by the temperature rise, surface oxygen vacancies diffuse into the lattice along grain boundaries and concentration gradients, transforming into a bulk vacancy network that penetrates the crystal framework. The oxygen vacancies generated in the first stage create lattice attraction to bind fluorine atoms, reducing their vaporization loss at high temperatures. Furthermore, lithium elements enter sodium sites, resulting in a uniform distribution of dopant elements. The active material within the crystal lattice; by utilizing the in-situ decomposition of ferrous oxalate dihydrate to regulate the defect state through solid solution, the contradiction between the crystallization process of sodium iron phosphate pyrophosphate and the thermal stability of the doping components was balanced. The resulting active material showed an initial discharge specific capacity of 115 mAh / g and an initial coulombic efficiency of 94.5% under 0.1C discharge conditions, and exhibited a low polarization potential at a 10C charge-discharge rate, reflecting that the solid solution of fluorine atoms at oxygen sites enhanced the interfacial ion migration rate of the cathode material.

[0034] Example 2: In the experiment verifying the interfacial kinetic stability of sodium iron phosphate pyrophosphate cathode active material under high-rate charge and discharge conditions, an electrochemical workstation was used to target the general formula... Constant current charge-discharge tests were conducted on active material samples and multiple control group samples. The current sampling accuracy of the electrochemical workstation was set to 10 pA and the potential resolution to be no less than 0.1 μV. During the experiment, a 50 Hz power frequency interference signal with an amplitude of 2 mV was introduced into the test circuit to simulate the electromagnetic noise in the energy storage system's operating environment. The molar ratio of ferrous oxalate dihydrate in the mixed iron source system was investigated in detail on its effect on charge transfer resistance. The molar ratio of ferrous oxalate dihydrate was selected as the core influencing parameter. Its value was determined based on the balance between the concentration of in-situ induced oxygen vacancies and the thickness of the surface-coated carbon layer. When the ratio increased from 15% to 45%, the oxygen vacancy concentration on the surface of the active material particles was observed to increase monotonically using paramagnetic resonance spectroscopy. This reflects that the increase in carbon monoxide partial pressure in the first stage of heat treatment enhanced the ability to capture oxygen atoms from the lattice. The discharge specific capacity of the active material obtained by experimental group A with a 15% ratio was 98.4 mAh / g at a rate of 10C. The control sample group F used a pure iron phosphate dihydrate system without ferrous oxalate dihydrate. Under the same test conditions, due to the lack of oxygen vacancies anchoring fluorine atoms, the actual composition deviated from the stoichiometry. The measured discharge capacity was only 72.5 mAh / g, and the initial coulombic efficiency decreased from 94.2% in experimental group A to 86.8%. This proves the contribution of the reducing micro-atmosphere generated by the decomposition of mixed iron source in constructing efficient ion transport channels.

[0035] Performance tests were conducted within the boundary range of the modified parameters. Experimental group B used a ferrous oxalate dihydrate content limited to the upper limit of 45%, achieving a capacity retention of 82.5% at a 20C rate. However, when the content exceeded the boundary and increased to 55%, due to the overlapping deposition of organic decomposition products and oxalate decomposition products, an amorphous carbon layer exceeding 15.6 nm in thickness was observed on the particle surface. This caused a sharp increase in the diffusion resistance of sodium ions during charge and discharge, resulting in a non-linear drop in the discharge capacity at a 20C rate to 81.3 mAh / g. 52.6% of the experimental group B The mutation level rose to 138.4. This data difference establishes a working window of 15% to 45% as the range for achieving a synergistic effect of high electronic conductivity and low ion diffusion resistance. The above experimental results for different component ratios and heat treatment conditions show that, by utilizing the kinetic characteristics of ferrous oxalate dihydrate in the initial stage of step heat treatment, lattice defects that suppress the vaporization loss of modified components can be effectively induced. The resulting positive electrode active material maintains the physical integrity of the electrochemical interface during high-rate continuous cycling, effectively reducing the polarization internal resistance loss of the electrode in the process of directly converting chemical energy into electrical energy.

[0036] Example 3: This example combines Figures 1 to 2 The modification method for the positive electrode active material of sodium batteries is explained, such as... Figure 1As shown, the modification method for the positive electrode active material of a sodium battery includes steps S101, S102, S103, and S104. In step S101, sodium source, a mixed iron source system consisting of ferric phosphate dihydrate and ferrous oxalate dihydrate, phosphorus source, lithium source, fluorine source, and boron source are weighed according to stoichiometric ratio, and organic carbon source and deionized water are added to prepare a mixed slurry. Then, in step S102, the mixed slurry is fed into a sand mill for wet grinding to obtain a modified precursor slurry. In step S103, the modified precursor slurry is dried to obtain... The spherical precursor powder is then subjected to a step-by-step heat treatment in a protective atmosphere in step S104. First, the spherical precursor powder is subjected to a first-stage heat treatment within the temperature range where ferrous oxalate undergoes thermal decomposition. The carbon monoxide generated by the thermal decomposition of ferrous oxalate dihydrate is used to reduce the powder surface and form oxygen vacancy defects. Then, the temperature is increased to carry out a second-stage heat treatment to induce a solid-state reaction. The lattice attraction generated by the oxygen vacancy defects is used to bind the doped atoms, ultimately obtaining sodium iron pyrophosphate active material with doped elements distributed at lattice sites and a conductive carbon layer on the surface.

[0037] like Figure 2 As shown, the system includes a sand mill for wet milling a mixed slurry to produce a modified precursor slurry, and a spray dryer for receiving the slurry through a feeding process. The spray dryer is equipped with a humidity sensor to collect real-time ambient relative humidity and optimize drying conditions accordingly to produce spherical precursor powder. The obtained spherical precursor powder is placed in a tubular atmosphere furnace and subjected to step heat treatment under a protective atmosphere to generate sodium iron phosphate pyrophosphate active material. During this process, the system uses a detection terminal including an online gas chromatograph to monitor the concentration of carbon monoxide in the furnace exhaust gas in real time, thereby accurately locking the defect-induced intensity during the heat treatment process through a decomposition kinetic feedback mechanism.

[0038] Example 4: In response to In large-scale industrial preparation of active materials, to address the engineering challenges of decreased component thermal stability and uneven interfacial crystallization caused by high-concentration fluorine doping, this invention provides a method that precisely locks the defect-induced intensity by establishing a decomposition kinetic feedback mechanism. Based on the atomic ratio of the product formula, sodium carbonate, ferric phosphate dihydrate, ferrous oxalate dihydrate, ammonium dihydrogen phosphate, lithium carbonate, and sodium fluoride are calculated and weighed as raw materials. Using the total molar amount of the mixed iron source system as a benchmark, the molar proportion of ferrous oxalate dihydrate is determined to be 30%. The above raw materials are then added together with glucose to deionized water to prepare a mixed slurry with a solid content of 45%. This slurry is then continuously ground using a sand mill with 0.3 mm diameter zirconia beads until the median particle size of the solid particles in the slurry reaches the specified value. Reaching 310 nm, the resulting slurry was spray-dried to produce spherical precursor powder with an average particle size of 15 μm, wherein... The median particle size of the sample particles.

[0039] Spherical precursor powder was subjected to step-by-step heat treatment in a tubular atmosphere furnace. During the first stage of heating, the system was raised from room temperature to 420°C at a rate of 8°C / min. The volume fraction of carbon monoxide in the furnace exhaust gas was monitored in real time using online gas chromatography. When the concentration of carbon monoxide dropped from the peak value to below 10% of that peak value, it was determined that the reducing decomposition reaction of ferrous oxalate dihydrate was approaching its endpoint and the transient oxygen vacancies on the surface were saturated. At this point, the temperature was maintained for 3 hours, and the oxygen vacancy signal factor on the powder surface was measured using electron paramagnetic resonance spectroscopy. Within the temperature range of 2.001 to 2.005, the system seamlessly switches to the second stage, increasing the temperature to 680℃ at a rate of 4℃ / min and holding it for 12 hours. Utilizing the saturated oxygen vacancies generated in the first stage as atomic-level trapping sites, fluorine atoms are bound by lattice attraction to counteract the high-temperature vaporization potential above 550℃, resulting in an active material with isotropically distributed dopant elements within the sodium iron phosphate pyrophosphate lattice. The Landé factor for electron paramagnetic resonance (EPR) spectroscopy is used as the endpoint trigger threshold for this heat treatment control node. This is based on the convergence mapping of data from previous reaction kinetic boundary exploration experiments: in proportional reduction sampling tests, when the dynamic gas chromatograph detects a decrease in carbon monoxide concentration and crosses the critical line of 10% (the highest peak), it indicates a precipitous depletion of the highly reactive non-bridging oxygen atoms at the particle surface. The reaction kinetic model demonstrates that ignoring this signal and continuing to maintain the first-stage reduction heat treatment will cause the bridging oxygen within the bulk phase to be forcibly dissociated due to the reversal of the concentration gradient, leading to irreversible collapse and fracture of the main phase framework. Therefore, the 10% tail gas escape ratio becomes the only engineering equivalent measurement indicator characterizing that the microscopic transient oxygen vacancies have just reached the safe tolerance limit without damaging the unit cell nucleus. The above calibration method, which adjusts the heat treatment time through tail gas component feedback, eliminates localized micro-atmosphere fluctuations caused by differences in feed amount. X-ray photoelectron spectroscopy confirms the presence of oxygen in the product. Elements and The atomic ratio of the elements is 0.050, which deviates from the theoretical stoichiometric ratio of 0.0515 by only 0.0015. The resulting active material retains 93.5% of its capacity after 1000 cycles at 1C, and its discharge specific capacity remains at 88.6 mAh / g under a high current load of 20C. This demonstrates the stability of maintaining the interfacial charge balance under high-rate conditions by utilizing the mechanism of in-situ induced lattice defects by decomposition products.

[0040] Example 5: In a large-scale production scenario where the purity of different batches of ferric phosphate dihydrate and ferrous oxalate dihydrate raw materials fluctuates, the heat treatment temperature switching threshold was calibrated based on the thermogravimetric characteristics of the materials. A 10mg sample of the mixed iron source system was extracted and heated to 800℃ at a rate of 10℃ / min in a nitrogen-protected flow field with a flow rate of 50mL / min. The peak temperature at which the decomposition rate of ferrous oxalate dihydrate was fastest was determined by calculating the first derivative of the thermogravimetric analysis curve. And the endpoint temperature corresponding to the plateau period of quality loss. ,Will As the upper limit of the heat preservation in the first stage of subsequent step heat treatment, among which This is the peak temperature of the thermal decomposition rate. This is the temperature at which thermal decomposition ends.

[0041] When the system faces changes in the furnace volume or adjustments in the protective atmosphere flow rate, the response time of the online gas chromatograph is baseline-calibrated using carbon monoxide standard gas. A tracer gas of known concentration is injected into a protective atmosphere at a preset flow rate, and the delay time required for the detection terminal signal to reach steady state is recorded. According to the delay time The logic decision node for carbon monoxide concentration dropping below 10% of the initial peak value has been corrected, and gas diffusion hysteresis caused by the dead volume of the furnace has been compensated, resulting in active materials with a uniformity deviation of conductive carbon layer thickness on the particle surface within 0.5 nm. The delay time for detecting the signal.

[0042] Example 6: In the application scenario of preparing sodium iron pyrophosphate due to unstable spray drying conditions caused by fluctuations in ambient relative humidity, a humidity sensor is used to collect real-time ambient relative humidity. The system corrects the inlet temperature of the spray dryer based on the offline calibration curve. Specifically, the system calculates the real-time ambient relative humidity. relative humidity relative to reference The deviation is adjusted by regulating the output power of the drying hot air to correct the inlet temperature. Follow logical relationships By dynamically adjusting the inlet temperature, the moisture content of the spherical precursor powder is controlled below 0.5%, thereby reducing the amount of iron oxide impurity phase generated by residual moisture during the subsequent step heat treatment stage. This is the corrected inlet temperature. The reference inlet temperature, This is the humidity compensation coefficient. To measure the relative humidity of the actual environment, Based on relative humidity, this humidity compensation coefficient is used in actual industrial deployments. The effective engineering value range is limited to 0.5℃ / % to 1.5℃ / %, and its specific built-in system value depends on the parameter calibration during the previous water load commissioning phase. The specific derivation and acquisition steps are as follows: Under the control state of locking the feed rate and solid content of the sand mill slurry, the relative humidity of the working workshop is manually adjusted to a step change, and the temperature control module automatically captures the amount of air temperature change required to make the moisture content of the final discharged powder approach the critical index of 0.5%. The edge computing unit extracts the linear fitting slope between the required additional temperature deviation and the real-time humidity increment, and this slope value is directly used as... The static calibration constants are burned into the registers of the hot air compensation controller.

[0043] When the system faces pressure fluctuations in the gas supply pipeline or drift in the atmosphere flow rate, the flow field calibration model established by tracer gas injection is used to correct the judgment threshold of carbon monoxide concentration in the thermal decomposition stage. Before the start of the step heat treatment, the response time constant of the detection terminal is recorded using a pulse injection signal. The sampling period of the online gas chromatograph is phase-calibrated using the response time constant, and the judgment point for carbon monoxide concentration dropping below 10% of the initial peak value is corrected based on the flow field resistance deviation. The main board then calculates the corrected concentration judgment threshold. : Among them, the corrected concentration determination threshold As a physical quantity used to trigger the hard-wired logic for temperature zone switching in heat treatment equipment, the unit is volume fraction percentage, and the basic concentration threshold is used for judgment. The preset value is a constant of 10%. The thin-film pressure sensor at the pipeline inlet end collects the difference between the internal absolute pressure of the system and the ambient standard atmospheric pressure, and outputs the flow field resistance deviation. The detection terminal records the signal detection delay time during the gas chromatograph measurement process. Characterizes the physical diffusion hysteresis time of the test gas mixture within a specific furnace volume, and the flow resistance compensation coefficient. The nonlinear effect of constrained flow field geometry on gas impediment is limited to a value between 0.015 and 0.025, and is a dimensionless characteristic constant. It is calculated and locked based on the known concentration of carbon monoxide gas under no-load intake calibration data after replacing furnace pipe fittings. Although α is entered into the underlying algorithm system in dimensionless form at the human-machine interface of the operating terminal, the flow resistance compensation coefficient α has dimensions matching the product of pressure deviation and time delay to ensure the correction term... The dimensionless transformation; this conversion mechanism forcibly normalizes and cancels out the physical units generated by the product of pressure difference and time delay at the moment the operation is triggered, making the combined arithmetic terms (1+α) dimensionless. Before being finally incorporated into the main formula, the physical properties were stripped away, reducing the dimension to a pure scalar weight ratio, thus allowing the basic concentration threshold to have percentage attributes. It can seamlessly perform scalar multiplication amplification or reduction across domains, outputting a final judgment value in percentage form. Based on the aforementioned calculation steps, the system compensates for the impact of gas diffusion hysteresis caused by the furnace dead volume on the determination of the decomposition kinetic endpoint, producing modified positive electrode active materials with a surface oxygen vacancy-induced density deviation of less than 5% and consistent primary particle crystallinity. This is the delay time for signal detection.

[0044] Example 7: In the application scenario of preparing sodium iron phosphate pyrophosphate cathode active material with high energy density and the need to maintain long-term cycling lattice stability, the covalent bond strength of the polyanionic framework is enhanced by introducing a boron source to perform isoelectronic or heterovalent substitution of phosphorus sites. Specifically, boric acid is added as a modifying component during the preparation of the mixed slurry, and the general formula of the resulting product is controlled to be... ,in The value range is set between 0.01 and 0.05. A wet milling process is used to disperse boric acid molecules at the molecular level in the slurry. In the second stage of subsequent step-heat treatment, a high-temperature solid-state reaction is used to allow boron atoms to enter the center of the phosphorus-oxygen tetrahedron. Higher bond energy suppresses lattice distortion of phosphate in the deep desodiumization state, among which This represents the molar replacement ratio of boron atoms with phosphorus atoms.

[0045] The structural stability of the obtained active material was detected using Raman spectroscopy, and representative structures were observed. The characteristic peak of the stretching vibration shifts towards higher wavenumbers, reflecting that the solid solution of boron atoms enhances the internal forces of the anionic clusters. Experimental group D adopted... The boron-doped sample achieved a capacity retention of 88.4% after 2000 cycles at 1C rate under 60℃ conditions, while the control sample G without boron source only achieved a capacity retention of 72.1% under the same high-temperature cycling conditions. Furthermore, transmission electron microscopy revealed obvious crack propagation on the primary particle surface of the control sample G, demonstrating that the lithium, fluorine, and boron ternary components, after occupying sodium, oxygen, and phosphorus sites respectively, produced a structural stability gain effect composed of internal framework reinforcement and surface defect anchoring. This satisfies the compliance requirements of the method of directly converting chemical energy into electrical energy for electrode materials in terms of service life.

[0046] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for modifying the positive electrode active material of a sodium battery, characterized in that, The modification method includes the following steps: Step S101: Weigh out sodium source, mixed iron source system, phosphorus source, lithium source, fluorine source and boron source according to stoichiometric ratio, add organic carbon source and deionized water to prepare mixed slurry; wherein, the mixed iron source system is composed of ferric phosphate dihydrate and ferrous oxalate dihydrate. Step S102: The mixed slurry is fed into a sand mill for wet grinding to obtain the modified precursor slurry; Step S103: The modified precursor slurry is dried to obtain spherical precursor powder. Step S104 involves performing a step-by-step heat treatment on the spherical precursor powder under a protective atmosphere, including the following sub-steps: Step S1041 involves performing a first-stage heat treatment within the temperature range where ferrous oxalate undergoes thermal decomposition, using carbon monoxide generated from the thermal decomposition of ferrous oxalate to reduce the surface of the precursor powder, thereby forming oxygen vacancy defects on the powder surface; Step S1042 involves increasing the temperature for a second-stage heat treatment, causing the precursor powder to undergo a solid-state reaction, using the lattice attraction generated by the oxygen vacancy defects to bind the doped atoms released from the fluorine and boron sources, resulting in an active material of sodium iron pyrophosphate with doped elements distributed at lattice sites and coated with a conductive carbon layer on the surface.

2. The method for modifying a sodium battery positive electrode active material according to claim 1, characterized in that, In step S101, based on the total molar amount of the mixed iron source system, the molar proportion of ferrous oxalate dihydrate is 15% to 45%; the solid content of the mixed slurry is 35% to 50%; in step S102, the particle size D50 of the solid particles in the mixed slurry is controlled to be 0.25 μm to 0.45 μm to obtain the modified precursor slurry; in step S103, the drying treatment is spray drying to obtain spherical precursor powder; in step S1041, the temperature of the first stage heat treatment is 300℃ to 480℃ and held for 1h to 4h; in step S1042, the temperature of the second stage heat treatment is 550℃ to 700℃ and held for 6h to 12h.

3. The method for modifying a sodium battery positive electrode active material according to claim 1, characterized in that, In step S101, the sodium source is selected from sodium carbonate, sodium acetate, or sodium pyrophosphate; the phosphorus source is selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or sodium pyrophosphate; the lithium source is selected from lithium carbonate, lithium hydroxide, lithium phosphate, or lithium fluoride; the fluorine source is selected from sodium fluoride or ammonium fluoride; the boron source is selected from boric acid, boron oxide, sodium borate, or lithium borate; the organic carbon source is selected from one or more of glucose, sucrose, citric acid, starch, polyvinyl alcohol, or polyethylene glycol; based on the theoretical chemical formula of the generated sodium iron phosphate pyrophosphate active material, the lithium doping amount in the lithium source is 0.01 mol to 0.08 mol, the fluorine doping amount in the fluorine source is 0.02 mol to 0.20 mol, and the boron doping amount in the boron source is 0.01 mol to 0.03 mol.

4. The method for modifying a sodium battery positive electrode active material according to claim 2, characterized in that, In step S101, based on the total molar amount of the mixed iron source system, the molar percentage of ferrous oxalate dihydrate is preferably 25% to 35%; in step S102, the wet grinding speed is 2000 rpm to 3500 rpm, the grinding time is 3 h to 6 h, and the grinding media used are zirconia beads with a diameter of 0.2 mm to 0.6 mm.

5. The method for modifying a sodium battery positive electrode active material according to claim 2, characterized in that, In step S103, the inlet air temperature of the spray drying process is 240°C to 280°C, and the outlet air temperature is 100°C to 120°C; the average particle size of the obtained spherical precursor powder is 5μm to 20μm.

6. The method for modifying a sodium battery positive electrode active material according to claim 1, characterized in that, In step S104, the heating rate in step S1041 is 3°C / min to 8°C / min, and the heating rate in step S1042 is 2°C / min to 5°C / min; the protective atmosphere includes nitrogen or argon, and the flow rate of the protective atmosphere is 0.5L / min to 2L / min.

7. The method for modifying a sodium battery positive electrode active material according to claim 1, characterized in that, The amount of organic carbon source added is based on a weight percentage, so that the mass content of carbon element in the generated sodium iron phosphate pyrophosphate active material is 1.0% to 2.0%; the sodium iron phosphate pyrophosphate active material is formed by the agglomeration of primary single crystal particles into secondary spherical particles, and the particle size of the primary single crystal particles is 50nm to 300nm.

8. The method for modifying a sodium battery positive electrode active material according to claim 1, characterized in that, In step S1041, carbon monoxide and carbon dioxide generated by the thermal decomposition of ferrous oxalate dihydrate are used to form a local reducing atmosphere. Carbon monoxide combines with oxygen atoms on the surface of the spherical precursor powder and detaches in the form of carbon dioxide, thereby inducing the generation of oxygen vacancy defects.

9. The method for modifying a sodium battery positive electrode active material according to claim 1, characterized in that, In step S1042, the pinning effect of oxygen vacancy defects on fluoride ions and boron atoms is used to reduce the diffusion activation energy of the elements, so that fluoride is dissolved in the oxygen site of the crystal lattice and boron is dissolved in the phosphorus site of the crystal lattice, and the vaporization of fluoride source and boron source is suppressed when the temperature is higher than 550°C.

10. The method for modifying a sodium battery positive electrode active material according to claim 1, characterized in that, The prepared sodium iron phosphate pyrophosphate active material has a three-dimensional framework structure, in which lithium occupies sodium lattice sites, fluorine occupies oxygen lattice sites, and boron occupies phosphorus lattice sites.

Images