A lithium manganese iron phosphate material and a preparation method thereof

Abstract

The application provides a lithium manganese iron phosphate material and a preparation method thereof. The material is a core-shell structure, wherein the core is lithium manganese iron phosphate nanocrystalline grains, and the shell is a fluorocarbon layer coated on the surface of the nanocrystalline grains. The method comprises the following steps: mixing waste lithium iron phosphate black powder which is not subjected to acid immersion, alkali washing or high-temperature calcination treatment, a lithium source, a manganese source and an additional transient conductive agent to press into a green body, and regulating the resistivity to 0.1-10 Ω·cm; applying a high-voltage pulse current by using a multi-stage variable-frequency pulse strategy, so that the temperature of the green body is increased to above 1200 DEG C; and cooling at a rate greater than 1000 DEG C / s. The application applies the transient Joule heat technology to waste lithium iron phosphate recovery for the first time, realizes lattice reconstruction upgrading to lithium manganese iron phosphate within a millisecond level time, simultaneously realizes in-situ carbon coating and self-purification, and has low energy consumption, a short process and excellent electrochemical performance of the product.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery recycling technology, specifically to a lithium manganese iron phosphate material and its preparation method. Background Technology

[0002] With the widespread use of lithium iron phosphate (LiFePO4) batteries and their eventual retirement, the recycling and reuse of these end-of-life batteries has become an urgent issue. Lithium iron phosphate (LFP) batteries have captured a significant market share due to their high safety and long cycle life. However, as early-used LFP batteries gradually enter their retirement phase, how to efficiently and economically recycle and dispose of these used batteries has become a major challenge for the industry.

[0003] Existing LFP recycling technologies mainly suffer from the following pain points: Poor economic efficiency. LFP cathode materials do not contain precious metals such as cobalt and nickel. The economic value of recovering lithium carbonate and iron phosphate using traditional hydrometallurgical (acid leaching-extraction) processes is often insufficient to cover the costs of chemical reagent consumption and wastewater treatment, resulting in a lack of economic incentive for recycling companies.

[0004] Limitations of remediation. Current direct regeneration methods (such as physical remediation and solid-phase lithium replenishment remediation) can only remediate waste LFP into LFP materials with performance close to that of virgin materials. They cannot improve the energy density and voltage platform of the materials, resulting in low added value and making it difficult to achieve the upgrade and utilization of "turning waste into treasure".

[0005] Energy consumption and pollution issues. Traditional pyrometallurgical or solid-state sintering repair processes require long-term (10-20 hours) high-temperature roasting, resulting in huge energy consumption; and the binders (such as polyvinylidene fluoride, PVDF) remaining in the waste electrode sheets may produce toxic fluoride gases during traditional incineration, posing environmental risks.

[0006] Therefore, how to develop a new recycling technology that can increase the added value of recycled products, directly upgrade from low-value LFP to high-energy-density cathode materials, and also has the characteristics of low energy consumption, short process and environmental friendliness is a technical problem that urgently needs to be solved in the field of waste battery recycling. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a lithium manganese iron phosphate material and its preparation method.

[0008] The specific technical solution of the present invention is as follows: In a first aspect, the present invention provides a lithium manganese iron phosphate material, wherein the material has a core-shell structure, wherein the core is lithium manganese iron phosphate nanocrystals and the shell is a fluorinated carbon layer covering the surface of the nanocrystals.

[0009] Furthermore, the core has a grain size of 100–200 nm, and the shell has a thickness of 2–5 nm.

[0010] Furthermore, the X-ray diffraction peaks of the material are shifted by 0.15° to 0.25° towards lower angles.

[0011] The lithium manganese iron phosphate (LMFP) material provided by this invention possesses unique microstructural characteristics. The 100–200 nm primary nanocrystal size originates from the freezing of high-temperature grains by ultrafast cooling (>1000℃ / s). The 2–5 nm continuous carbon layer contains fluorine, which is derived from the in-situ carbonization and fluorine doping of residual PVDF in waste black powder. This fluorinated carbon layer effectively enhances the electrochemical performance of the material. The XRD pattern of this material exhibits unique fingerprint characteristics, distinguishing it from conventional LMFP materials. The peak position shifts to a lower angle by 0.15°–0.25°, due to the larger ionic radius of Mn. 2+ (0.83 Å) entered the lattice and replaced part of the Fe. 2+ (0.78Å) increased the cell parameters, and the offset corresponds precisely to the Mn content, proving that Mn was successfully dissolved.

[0012] Furthermore, the lithium manganese iron phosphate nanocrystals have an olivine-type crystal structure and their chemical formula is LiMn. x Fe 1-x PO4, where 0.2 ≤ x ≤ 0.8.

[0013] Furthermore, in the chemical formula of the lithium manganese iron phosphate nanocrystals, x is 0.3 to 0.4.

[0014] When x is between 0.3 and 0.4, the material exhibits both high energy density and good structural stability. If the manganese content is too low (x < 0.2), the contribution of the 4.1V high-voltage plateau is insufficient, resulting in limited improvement in energy density and a lack of economic value. If the manganese content is too high (x > 0.5), the lattice distortion energy exceeds the limit, the Jahn-Teller effect intensifies, structural stability decreases, electronic conductivity deteriorates, and the electrochemical performance of the product deteriorates. Therefore, x = 0.3 to 0.4 is the optimal ratio range. In Example 1, when x = 0.33, the initial discharge capacity reaches 158 mAh / g, and the 4.1V plateau accounts for approximately 33%, demonstrating the best overall performance.

[0015] Furthermore, the X-ray diffraction pattern of the material does not contain any impurity phase diffraction peaks of Fe2O3, Mn2O3, or MnO.

[0016] The XRD pattern of the material of this invention exhibits unique fingerprint characteristics, distinguishing it from conventional LMFP materials. No impurity diffraction peaks are observed, particularly in the 2θ = 33°–35° range, where no Fe₂O₃, Mn₂O₃, or MnO impurity peaks are present, indicating high product purity and a complete crystal lattice. In Comparative Example 1, due to slow cooling leading to Mn / Fe phase separation, obvious impurity peaks appeared in this range, verifying the high purity characteristics of the material of this invention.

[0017] Secondly, the present invention provides a method for preparing the above-mentioned lithium manganese iron phosphate material, comprising the following steps: S1. Black powder is obtained by dismantling waste lithium iron phosphate batteries; the black powder contains binder and carbon black, and has not been treated by acid leaching, alkali washing or high-temperature calcination. S2. Mix the black powder obtained in S1 with a lithium source, a manganese source and an added transient conductive agent to obtain a precursor powder; S3. Press the precursor powder into a green body; S4. Apply a high-voltage pulse current to the green blank to raise the temperature of the green blank to above 1200°C, thereby realizing the solid solution reaction between manganese and lithium iron phosphate lattice. S5. After the pulse ends, the powder is cooled to obtain lithium manganese iron phosphate material.

[0018] This invention applies transient Joule heating technology to the recycling of waste LFP for the first time, directly using black powder containing binders and impurities as raw material. Ultra-rapid heating is achieved through high-voltage pulsed current, completing lattice reconstruction within milliseconds. Compared to the traditional tubular furnace calcination method (Comparative Example 1, taking 10-18 hours), this invention reduces reaction time by tens of thousands of times and significantly lowers energy consumption. Furthermore, it eliminates the need for complex pretreatment processes such as acid leaching and alkali washing, avoiding the use of acid and alkali reagents and waste liquid treatment issues, thus greatly simplifying the process. The LMFP material prepared by this method achieves an initial discharge capacity of 158 mAh / g, with a 4.1V manganese plateau accounting for approximately 33%, realizing the upgrade and regeneration from low-value LFP to high-value LMFP.

[0019] Furthermore, in step S2, based on the mass of the waste cathode black powder as 100%, the amount of manganese source added is 10-60%, the amount of lithium source added is 5-30%, and the amount of external transient conductive agent added is 5-25%.

[0020] The addition of manganese source at 10-60% corresponds to a final product x=0.12-0.45, covering the practical composition range of LMFP. The addition of lithium source at 5-30% ensures that the lithium sites in the lattice are full without producing excessive residual alkali. The addition of transient conductive agent at 5-25% is designed based on percolation theory: when it is below 5%, the total carbon content of the system (including about 2% residual carbon from the black powder itself) is insufficient to break through the percolation threshold (8%-15%), the conductive network is distributed in an island-like manner, the resistivity cannot be reduced to below 10Ω·cm, the pulse current is severely suppressed, the current tunneling effect occurs, and the product is in a "mixed raw and cooked" state (see Comparative Example 2); when it is above 25%, the proportion of active material is severely diluted, the theoretical capacity per unit mass of material decreases, and the resistivity is too low (<0.1Ω·cm), which easily causes current overshoot, resulting in arc erosion of the contact surface or local temperature exceeding 3000℃, causing the active material to decompose.

[0021] Further, in step S2, the manganese source is at least one of manganese dioxide, manganese tetroxide, manganese carbonate, and manganese oxalate; the lithium source is at least one of LiOH, Li2CO3, Li3PO4, and lithium acetate; the added transient conductive agent is at least one of carbon black, graphene, and carbon nanotubes; the mixing is performed using high-energy ball milling at a speed of 200–500 rpm for 2–4 hours.

[0022] Both the aforementioned manganese and lithium sources are commonly used raw materials for battery production, readily available, and cost-effective. Manganese carbonate (MnCO3) undergoes thermal decomposition at 500℃-650℃, transforming into highly active nano-MnO microcrystals that uniformly adhere to the surface of LFP particles, creating favorable conditions for the subsequent rapid diffusion of manganese atoms into the crystal lattice. High-energy ball milling achieves uniform mixing and preliminary mechanical activation of the raw materials, ensuring a uniform distribution of components at the microscale, laying the foundation for subsequent uniform reactions.

[0023] Furthermore, in step S3, the resistivity of the green blank is adjusted to be between 0.1 Ω·cm and 10 Ω·cm. Adjusting the resistivity of the green blank to 0.1-10 Ω·cm is a necessary condition for achieving a stable and uniform Joule thermal field, ensuring the smooth progress of the subsequent high-temperature solid solution reaction.

[0024] Furthermore, in step S4, the high-voltage pulse current adopts a multi-level frequency conversion pulse strategy, which includes: first applying a low-voltage long pulse with a pulse width of 100-300ms; then applying a high-voltage short pulse with a pulse width of 10-50ms.

[0025] The "multi-stage frequency conversion pulse strategy" of this invention is a "three-stage progressive cascade pretreatment" strategy designed based on the complex thermochemical characteristics of waste impurity-containing black powder system. The first stage of low voltage long pulse (50-100V / 100-500ms) realizes three irreversible cascade processes: ① The PVDF binder is defluorinated and carbonized in an orderly manner at 300℃-400℃, and the fluorine-containing gas is released smoothly, avoiding the risk of micro-explosion at ultra-high temperature; ② The newly formed carbon and the added conductive agent undergo carbon-carbon bridging, triggering the "positive and negative self-reinforcing effect" of the conductive network, which reduces the real-time resistivity of the green blank by about an order of magnitude; ③ Manganese sources such as manganese carbonate are pre-decomposed into highly active nano MnO microcrystals at 500℃-650℃, which are uniformly attached to the surface of LFP particles. In the second stage, a high-voltage short pulse (200-400V / 10-50ms) pushes the temperature above 2000℃ under conditions where the conductive network is fully activated and the manganese source has been pre-decomposed and positioned, allowing active manganese atoms to efficiently enter the LFP lattice via the shortest diffusion path to complete solid solution. If the first stage is skipped and a high-voltage pulse is applied directly (Example 3), it will lead to explosive vaporization of PVDF, violent release of CO2, and insufficient activation of the conductive network. As a result, the initial discharge capacity of the final product drops to 145mAh / g, and the cycle retention rate drops to 80.5%.

[0026] Furthermore, in step S4, the current density of the high-voltage pulse current is 50–500 A / cm². 2 The pulse duration is 10ms to 500ms, and the discharge voltage is 50V to 400V, which raises the green temperature to 1200℃ to 2800℃.

[0027] The aforementioned pulse parameter range covers the feasible operating range of this invention. By adjusting the voltage, resistance, and pulse duration, the heat generation of the green compact can be precisely controlled. A current density of 50-500 A / cm² ensures sufficient heat power density; the temperature rises to 1200℃-2800℃, where 1200℃ is the minimum temperature for effective solid solution of manganese, and 2800℃ is the upper limit to avoid phosphate decomposition.

[0028] Furthermore, in step S5, the cooling rate is greater than 1000°C / s.

[0029] This invention employs an ultrafast cooling rate greater than 1000℃ / s, with four core objectives: First, to lock the homogeneous Mn / Fe solid solution, preventing thermodynamic phase separation. When the cooling rate is insufficient, within the miscible gap of 700℃-600℃, manganese atoms spontaneously aggregate towards the manganese-rich region, and iron atoms aggregate towards the iron-rich region, forming "manganese-rich islands" and "iron-rich matrices." Excess manganese is squeezed out of the lattice, generating a dead Mn2O3 phase (comparative Example 1 surface Mn content is only 12.5%). Second, to suppress grain coarsening, freezing the primary grain size to 100-200nm (comparative Example 1 grain size reaches 2-5μm). Third, to achieve "glassification" of the carbon coating layer, maintaining the amorphous structure. Fourth, to preserve the metastable surface structure of the PVDF in-situ fluorine doping. Comparative Example 3 used the same pulse heating but cooled slowly at 0.5℃ / min. As a result, the Mn / Fe phase separation was severe, the fluorine content dropped from 0.35% to 0.08%, the surface fluorination protective layer collapsed, and the electrochemical performance deteriorated significantly, which fully verified the necessity of ultra-fast cooling.

[0030] Thirdly, the present invention provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery comprises the lithium manganese iron phosphate material described in any one of the technical solutions of the present invention.

[0031] Because the LMFP material of this invention has nanoscale grain size, fluorinated carbon coating, and a unique non-equilibrium structure, lithium-ion batteries assembled with it as the positive electrode active material have excellent electrochemical performance.

[0032] Compared with the prior art, the beneficial technical effects of this invention are reflected in: First, it enables the direct upgrade of waste lithium iron phosphate into high-value-added materials. This invention applies transient Joule heating technology to the field of waste lithium iron phosphate recycling. By heating the mixture of waste lithium iron phosphate black powder with manganese and lithium sources to an ultra-high temperature state within milliseconds, the lithium vacancies and iron dislocations generated in the crystal lattice of the waste material due to long-term cycling are used as highly active reaction sites to drive the rapid diffusion and embedding of manganese ions into the crystal lattice. This achieves lattice reconstruction and material upgrade from lithium iron phosphate (discharge platform of about 3.2V) to lithium manganese iron phosphate (discharge platform up to 4.1V), significantly improving the energy density and market value of the recycled products.

[0033] Secondly, the reaction time is extremely short, resulting in significantly reduced energy consumption. This invention employs transient Joule heating technology, completing the entire lattice reconstruction and doping reaction within milliseconds (approximately 0.6 seconds). Compared to traditional tubular furnace calcination processes (which typically require more than 10 hours), the reaction time is shortened by several orders of magnitude. Simultaneously, this technology directly heats the material itself rather than the furnace chamber, avoiding the significant energy waste inherent in traditional processes and drastically reducing the energy cost per unit output.

[0034] Third, the process is concise and requires no acid or alkali reagents. The method of this invention uses black powder obtained from the dismantling of waste batteries as raw material, without the need for wet metallurgical processes such as acid leaching and alkali washing, nor the need to remove residual PVDF binders and conductive agents beforehand. The process is greatly simplified, avoiding the use of acid and alkali reagents and waste liquid treatment problems, and has significant environmental friendliness.

[0035] Fourth, it exhibits excellent electrochemical performance. The lithium manganese iron phosphate material prepared by this invention achieves an initial discharge capacity of 158 mAh / g, with a 4.1V manganese plateau accounting for approximately 33%, and a capacity retention rate of 96.5% after 100 cycles at 1C, meeting the application requirements of rechargeable batteries.

[0036] Fifth, the high-temperature self-purification effect improves product purity. The ultra-high temperature generated by transient Joule heating is far higher than the boiling point of impurities such as aluminum and fluorides, which can instantly vaporize and volatilize residual aluminum foil fragments, fluorides, and other impurities in waste materials, achieving in-situ purification of the product. Compared with traditional recycling processes that require a dedicated impurity removal step, this invention removes impurities while simultaneously reconstructing the crystal lattice, further simplifying the process. Detailed Implementation

[0037] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below.

[0038] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0039] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0040] The embodiments of the present invention will be described in detail below. Unless otherwise specified, the materials, reagents, etc. used in the following embodiments and comparative examples are commercially available.

[0041] Example 1 A method for lattice reconstruction and upgrading of waste lithium iron phosphate to lithium manganese iron phosphate includes the following steps: S1. 100g of lithium iron phosphate black powder obtained after discharging, dismantling, crushing, and sieving waste lithium iron phosphate batteries (based on composition analysis, this genuine commercially retired electrode black powder contains approximately 90g of lithium iron phosphate as the main material, approximately 2g of PVDF binder, approximately 2g of conductive carbon black, approximately 2g of trace aluminum foil residue, and the remainder being approximately 4g of trace SEI film residue and electrolyte degradation products) is directly ball-milled with 12.8g of lithium carbonate (Li2CO3, battery grade, purity ≥99.9%, D50≈3μm), 32.5g of manganese carbonate (MnCO3, high-purity battery grade, purity ≥99.95%, D50≈2μm), and 5g of added transient conductive carbon black at a high-energy ball milling speed of 300rpm for 2 hours to obtain a precursor mixed powder.

[0042] S2. Take 5g of the above mixed powder and press it into a cylindrical green body with a diameter of 10mm and a length of 20mm under a pressure of 10MPa. Using a four-probe powder resistivity tester (such as RTS-9 dual electrical four-probe tester), the resistivity of the green body was successfully controlled to 2.5Ω·cm under constant pressure due to the addition of 5g of transient conductive agent.

[0043] S3. The pressed green compact cylinder is placed into a high-temperature resistant quartz insulating tube, clamped at both ends with graphite electrodes, and argon gas is introduced as a protective atmosphere. After connecting the power supply, a multi-stage frequency conversion pulse discharge strategy is adopted: first, the discharge voltage is set to 50V to trigger the discharge, and the current pulse width is 500ms (preheating and carbonizing a very small amount of PVDF and activating the overall conductive network); then, the discharge voltage is set to 300V, and the current pulse width is 50ms. The transient pulse current through the green compact reaches approximately 47 A, corresponding to a current density of approximately 60 A / cm³. 2 The powder temperature was rapidly increased to approximately 2150℃ within tens of milliseconds using a dual-color infrared high-speed thermometer (response time <1ms) for online monitoring and display.

[0044] Under this extreme thermal shock and strong reducing atmosphere, trace aluminum impurities (2g) were vaporized and carried away, while manganese and iron elements underwent deep solid solution.

[0045] S4. After the high-voltage pulse time (50ms) ends, the power is cut off immediately. The system undergoes spontaneous rapid quenching under the convection conduction of cold graphite electrodes and argon gas flow at a flow rate of 0.5-2L / min. The actual average cooling rate recorded by infrared thermography is approximately 1100℃ / s, resulting in carbon-coated lithium manganese iron phosphate composite material LiMn. x F e1 x PO4.

[0046] Product characterization: (1) Chemical composition analysis Weigh 0.1 g of the product powder obtained from S4 above, add aqua regia (concentrated nitric acid:concentrated hydrochloric acid = 1:3 volume ratio) and dissolve completely in a microwave digester. After adjusting the volume, use an inductively coupled plasma optical emission spectrometer (ICP-OES, such as an Agilent 5110 model, RF power 1.2 kW, nebulizer gas flow rate 0.7 L / min) to determine the intensity of characteristic emission lines of Mn and Fe in the solution. The test results show that the molar percentage of Mn in the product (Mn / (Mn+Fe)) is 33.2%, and the molar percentage of Fe is 66.8%. Therefore, the molar ratio of Mn / Fe in the product is determined to be 0.33 / 0.67, and the chemical formula is LiMn. 0.33 Fe 0.67 PO4.

[0047] The fluorine content in the product was determined by ion chromatography (IC), and the result showed that the fluorine content in the product was 0.35 wt%. The total carbon content in the product was determined by a high-frequency infrared carbon-sulfur analyzer (CS-800), and the result showed that the total carbon content was approximately 6.6 wt%.

[0048] (2) Phase structure analysis Phase analysis of the product was performed using an X-ray diffractometer (Bruker D8 Advance). Test conditions: Cu Kα radiation, λ = 1.5406 Å, tube voltage 40 kV, tube current 40 mA, scan range 10°–80°, step size 0.02°, scan speed 2° / min. The results showed that all diffraction peaks of the product were consistent with the standard olivine-type crystal structure with the PnA space group. Compared to the standard lithium iron phosphate JCPDS 83-2092 card, all diffraction peaks of the product were shifted to lower angles by approximately 0.18°. This shift is due to the larger Mn ionic radius. 2+ (0.83 Å) entered the lattice and replaced part of the Fe. 2+ The peak width at half maximum (FWHM) of the product is (0.78 Å). No Fe₂O₃, MnO, or Mn₂O₃ impurity phase diffraction peaks were detected in the 2θ = 33°–35° range, confirming that the product is a pure-phase LMFP solid solution. Furthermore, the product's FWHM is significantly wider than that of conventional sintered LMFP materials, indicating that the product possesses nanocrystal size and lattice microstress.

[0049] (3) Microscopic morphology and structural characterization The morphology of the product was observed using scanning electron microscopy (SEM, Hitachi SU8010 or S-4800) and high-resolution transmission electron microscopy (HRTEM, FEI Talos F200X). SEM results showed that the product consisted of highly uniform, near-spherical nanoparticles with primary grain sizes ranging from 100 to 200 nm. HRTEM results revealed that the nanoparticles were tightly coated with a continuous, dense amorphous carbon layer approximately 2–5 nm thick. Energy dispersive spectroscopy (EDS) analysis showed the presence of fluorine in this carbon layer, indicating that fluorine radicals released during PVDF carbonization were in situ doped into the carbon layer, forming a fluorinated carbon coating.

[0050] (4) Analysis of impurity removal effect ICP-OES elemental quantitative analysis was performed on the raw material black powder and the product of Example 1. The results showed that the aluminum content in the raw material black powder was 2.0 wt%, while the aluminum content in the product of Example 1 was reduced to 0.05 wt%, and the aluminum removal rate reached 97.5%. The fluorine content in the raw material black powder was 1.19 wt%, while the fluorine content in the product of Example 1 was 0.35 wt%. About 30% of the fluorine element was retained on the surface of the product to form a fluorinated protective layer. The above data indicate that the transient thermal shock process of the present invention achieves an intelligent self-purification effect of "selective removal of harmful impurities and directional retention of beneficial components".

[0051] Example 2 The only difference between this embodiment and Example 1 is the amount of transient conductive carbon black added in step S1. The specific steps are as follows: S1. 100g of lithium iron phosphate black powder obtained after discharging, dismantling, crushing, and sieving waste lithium iron phosphate batteries (based on composition analysis, this genuine commercially retired electrode black powder contains approximately 90g of lithium iron phosphate as the main material, approximately 2g of PVDF binder, approximately 2g of conductive carbon black, approximately 2g of trace aluminum foil residue, and the remainder being approximately 4g of trace SEI film residue and electrolyte degradation products) is directly ball-milled with 12.8g of lithium carbonate (Li2CO3, battery grade, purity ≥99.9%, D50≈3μm), 32.5g of manganese carbonate (MnCO3, high-purity battery grade, purity ≥99.95%, D50≈2μm), and 10g of added transient conductive carbon black at a high-energy ball milling speed of 300rpm for 2 hours to obtain a precursor mixed powder.

[0052] S2. Take 5g of the above mixed powder and press it into a cylindrical green body with a diameter of 10mm and a length of 20mm under a pressure of 10MPa. The resistivity of the green body was measured to be 1.2Ω·cm using a four-probe powder resistivity tester.

[0053] Steps S3-S4 are the same as in Example 1. The actual average cooling rate recorded by infrared thermography is approximately 1150°C / s, ultimately yielding LiMn. x Fe 1-x PO4 (x=0.33) material.

[0054] Product characterization: Characterization was performed using the same method as in Example 1. The results showed that the primary grain size of the product was 100–200 nm, and the surface was coated with a fluorinated carbon layer with a thickness of approximately 2–5 nm. The tap density was 1.08 g / cm³, and the BET specific surface area was 21.2 m² / g. The fluorine content in the product was 0.32 wt%, and the total carbon content was 8.32 wt%. XRD results showed that the product was a pure-phase LMFP with no impurity peaks.

[0055] Comparative Example 1 (Conventional Tubular Furnace Calcination) This comparative example uses the traditional tubular furnace calcination method, and the specific steps are as follows: S1. 100g of lithium iron phosphate black powder (based on composition analysis, this genuine commercially available retired electrode black powder contains approximately 90g of lithium iron phosphate as the main material, 2g of PVDF binder, 2g of conductive carbon black, approximately 2g of trace aluminum foil residue, and the remainder being approximately 4g of trace SEI film residue and electrolyte degradation products) obtained from the discharge, dismantling, crushing, and sieving of waste lithium iron phosphate batteries was directly mixed with 12.8g of lithium carbonate (Li2CO3, battery grade, purity ≥99.9%, D50≈3μm) and 32.5g of manganese carbonate (MnCO3, high-purity battery grade, purity ≥99.95%, D50≈2μm) in a high-energy ball mill at 300rpm for 2 hours to obtain a precursor mixed powder. (No external transient conductive agent was added).

[0056] S2. The mixed powder was placed in a tube furnace and heated to 700°C at a heating rate of 5°C / min under the protection of an argon-hydrogen mixture (Ar:H2=95:5). The temperature was held for 10 hours and then allowed to cool naturally in the furnace. The total time was about 12 hours, and the product (x<0.15) was obtained.

[0057] Product characterization: (1) Chemical composition analysis 0.1 g of the product powder obtained from S2 was weighed and dissolved completely in aqua regia (concentrated nitric acid:concentrated hydrochloric acid = 1:3 volume ratio) in a microwave digester. After adjusting the volume, the characteristic emission line intensities of Mn and Fe in the solution were measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, such as an Agilent 5110 model, RF power 1.2 kW, nebulizer gas flow rate 0.7 L / min). The test results showed that the molar percentage of Mn in the product (Mn / (Mn+Fe)) was 33.5%, and the molar percentage of Fe was 66.5%. However, point scanning analysis of the particle surface using a high-resolution transmission electron microscope with energy dispersive spectroscopy (HRTEM-EDS) revealed that the actual molar percentage of Mn was only 12.5%, while the percentage of Fe reached 87.5%, indicating that the actual product was LiMn. 0.125 Fe 0.875 PO4 indicates that Mn is extremely unevenly distributed inside and on the surface of the particles, and there is a severe Mn / Fe phase separation phenomenon.

[0058] (2) Phase structure analysis X-ray diffraction (XRD) results showed that although the characteristic peaks of the olivine main phase were present, the peaks were significantly broadened and exhibited splitting asymmetry (especially the (211) peak), indicating the presence of two sublattices: a Mn-rich phase and a Fe-rich phase. At the same time, strong Mn2O3 and Fe2O3 diffraction peaks were detected near 2θ = 33° to 35°.

[0059] (3) Microscopic morphological characterization SEM testing of the product revealed severe sintering and agglomeration of the particles, with the primary grain size growing wildly to 2–5 μm, and the surface carbon layer oxidizing and falling off, thus losing its coating effect.

[0060] Comparative Example 2 (without added transient conductive agent) The difference between this comparative example and Example 1 lies in the fact that no external transient conductive carbon black was added in step S1. The specific steps are as follows: S1. 100g of lithium iron phosphate black powder obtained after discharging, dismantling, crushing, and sieving waste lithium iron phosphate batteries (based on composition analysis, this genuine commercial retired electrode black powder contains approximately 90g of lithium iron phosphate as the main material, approximately 2g of PVDF binder, approximately 2g of conductive carbon black, approximately 2g of trace aluminum foil residue, and the remainder being approximately 4g of trace SEI film residue and electrolyte degradation products) is directly ball-milled with 12.8g of lithium carbonate (Li2CO3, battery grade, purity ≥99.9%, D50≈3μm) and 32.5g of manganese carbonate (MnCO3, high-purity battery grade, purity ≥99.95%, D50≈2μm) at a high-energy ball milling speed of 300rpm for 2 hours to obtain a precursor mixed powder.

[0061] S2. Take 5g of the above mixed powder and press it into a cylindrical green body with a diameter of 10mm and a length of 20mm under a pressure of 10MPa. Using a four-probe powder resistivity tester (such as RTS-9 dual electrical four-probe tester), the macroscopic resistivity of the green body was measured to be 185Ω·cm under constant pressure. Moreover, the test results at each point were very different, indicating that the conductive network was distributed in a broken island-like pattern.

[0062] S3. The pressed green compact cylinder is placed into a high-temperature resistant quartz insulating tube, and both ends are clamped with graphite electrodes. Argon gas is introduced as a protective atmosphere. After connecting the power supply, a multi-stage frequency conversion pulse discharge strategy is adopted: first, the discharge voltage is set to 50V to trigger the discharge, and the current pulse width is 500ms (preheating and carbonizing a very small amount of PVDF and activating the overall conductive network); then, the discharge voltage is set to 300V, and the current pulse width is 50ms. At the instant of the 300V high-voltage pulse, due to the overall high impedance of the green compact, the pulse current is limited to about 2A. The infrared high-speed thermometer shows that the macroscopic average temperature of the powder reaches about 650℃, which is far lower than the lattice reconstruction temperature of 1200℃.

[0063] S4. After the pulse ends, the power is cut off and the product is cooled under argon gas to obtain the product. Because the local melting zone forms a dense glassy dead block (losing the porous heat dissipation characteristics), while the particles in most areas remain in the loose and fragmented state of the raw material, the product is in a non-uniform state of "mixed raw and cooked".

[0064] Product characterization: (1) Phase structure analysis X-ray diffraction (XRD) tests on the product showed that the characteristic peaks of the main phase (LFP) hardly shifted to lower angles, indicating that manganese failed to obtain sufficient uniform thermodynamic kinetic energy to enter the crystal lattice. This was accompanied by extremely strong MnO impurity phase peaks.

[0065] (2) Microscopic morphological characterization SEM analysis of the product showed that most of the particles remained loose and fragmented (due to lack of heat and reaction), while the localized current breakdown areas fused and agglomerated into dense, dead blocks of tens of micrometers.

[0066] Comparative Example 3 (Single-stage pulse strategy) The preparation method in this embodiment is basically the same as that in Example 1, except for the pulse discharge strategy in step S3. The specific steps are as follows: S1. 100g of lithium iron phosphate black powder obtained after discharging, dismantling, crushing, and sieving waste lithium iron phosphate batteries (based on composition analysis, this genuine commercially retired electrode black powder contains approximately 90g of lithium iron phosphate as the main material, approximately 2g of PVDF binder, approximately 2g of conductive carbon black, approximately 2g of trace aluminum foil residue, and the remainder being approximately 4g of trace SEI film residue and electrolyte degradation products) is directly ball-milled with 12.8g of lithium carbonate (Li2CO3, battery grade, purity ≥99.9%, D50≈3μm), 32.5g of manganese carbonate (MnCO3, high-purity battery grade, purity ≥99.95%, D50≈2μm), and 5g of added transient conductive carbon black at a high-energy ball milling speed of 300rpm for 2 hours to obtain a precursor mixed powder.

[0067] S2. Take 5g of the above mixed powder and press it into a cylindrical green body with a diameter of 10mm and a length of 20mm under a pressure of 10MPa. Using a four-probe powder resistivity tester (such as RTS-9 dual electrical four-probe tester), the resistivity of the green body was successfully controlled to 2.5Ω·cm under constant pressure due to the addition of 5g of transient conductive agent.

[0068] S3. The pressed green compact cylinder is loaded into a high-temperature resistant quartz insulating tube, clamped at both ends with graphite electrodes, argon gas is introduced, and a power supply is connected. The discharge voltage is directly set to 300V, and the current pulse width is 50ms (single-stage pulse). The transient pulse current through the green compact reaches approximately 47A, corresponding to a current density of approximately 60A / cm². Online monitoring and display are performed using a dual-color infrared high-speed thermometer (response time <1ms). The powder temperature rapidly jumps to approximately 2150℃ within tens of milliseconds.

[0069] S4. After the high-voltage pulse time (50ms) ends, the power is cut off instantly. The system undergoes spontaneous rapid quenching under the convection conduction between the cold graphite electrode and the argon gas flow. The actual average cooling rate recorded by infrared thermography is approximately 1100℃ / s. The high-entropy solid solution structure is instantly frozen, resulting in a carbon-coated lithium manganese iron phosphate composite material LiMn. x Fe1 x PO4 (x=0.33).

[0070] Product characterization: Characterization was performed using the same method as in Example 1. The results showed that the product exhibited micro-explosion of particles and carbon layer exfoliation, with uneven grain size distribution and lattice distortion in some areas. XRD results showed slight impurity phase peaks in the product. ICP-OES results showed that the Mn / (Mn+Fe) ratio in the product was 32.8%.

[0071] Comparative Example 4 (Pulse heating + slow cooling) The difference between this comparative example and Example 1 lies in the use of slow cooling instead of rapid cooling in step S4. The specific steps are as follows: S1-S3 are exactly the same as in Example 1; S4. After the pulse ends, instead of rapid cooling, the still high-temperature green billet (along with the quartz tube) is immediately and quickly transferred to a tube furnace preheated to 800°C. Under argon protection, it is cooled from 800°C to room temperature at a rate of 0.5°C / min (i.e. 0.0083°C / s). This slow cooling process takes about 26 hours.

[0072] Product characterization: (1) Phase structure analysis XRD results showed that the product exhibited obvious Mn2O3 impurity phase diffraction peaks in the 2θ = 33°-35° range, while the (211) peak of the olivine main phase showed asymmetric splitting. This is a typical XRD fingerprint characteristic of Mn / Fe undergoing amplitude modulation decomposition to form two sublattices of Mn-rich and Fe-rich phases. The low-angle shift of the main peak decreased from about 0.20° in Example 1 to only about 0.08°, indicating that a large amount of Mn has precipitated from the lattice.

[0073] (2) Chemical composition analysis The fluorine content in the product was determined by ion chromatography (IC), and the results showed that the fluorine content decreased from 0.35 wt% in Example 1 to only 0.08 wt%. During the 26-hour slow cooling, the F atoms that were originally frozen on the surface by quenching gained sufficient thermal energy to gradually desorb or diffuse into the bulk phase, and the surface fluorination protective layer almost completely collapsed.

[0074] Performance testing The tap density of the materials prepared in each example and comparative example was determined using a tap density meter (100mL graduated cylinder, vibrated 1000 times). The specific surface area of ​​the materials prepared in each example and comparative example was determined using the nitrogen adsorption BET method (Micromeritics ASAP 2460). The test data are shown in Table 1.

[0075] Table 1

[0076] In Comparative Example 3, due to the use of a single-stage pulse strategy, the PVDF underwent explosive vaporization, resulting in micro-fracture of the particles, with an extremely wide grain size distribution and irregular fragmentation.

[0077] As shown in Table 1, the primary grain size of the materials prepared in Examples 1-2 of this invention is 100-200 nm, the tap density is 1.08-1.15 g / cm³, and the BET specific surface area is 18.5-21.2 m² / g. Compared with Comparative Example 1 (2-5 μm coarse grains), the materials of this invention exhibit typical nanomaterial characteristics: the lower tap density is due to the surface energy effect of the nanoparticles, and the carbon skeleton acts as a "scaffolding" spacer between the particles, preventing close contact between them; the significantly higher specific surface area is due to the nanosize effect, with the measured specific surface area (18.5 m² / g) being approximately 8 times higher than that of Comparative Example 1 (2.3 m² / g), but lower than the theoretical value (10-50 times), indicating that the carbon coating layer fills some of the nanopores. The above structural characteristics demonstrate that ultrafast cooling (>1000℃ / s) effectively freezes the nanocrystalline structure at high temperatures and retains the spacer support function of the carbon skeleton.

[0078] The materials obtained in the above embodiments and comparative examples were subjected to structural characteristic tests. The test methods and equipment are as follows: (1) Grain size test: The morphology of the material was observed and the grain size was counted using a scanning electron microscope (SEM, model: Hitachi SU8010 or S-4800) and a high-resolution transmission electron microscope (HRTEM, model: FEI Talos F200X).

[0079] (2) Carbon coating layer test: The thickness and morphology of the carbon layer were observed by high resolution transmission electron microscopy (HRTEM); the elemental composition of the carbon layer was determined by energy dispersive spectroscopy (EDS).

[0080] (3) Fluorine content test: The fluorine content in the material was determined by ion chromatography (IC, model: Thermo Scientific Dionex ICS-5000+).

[0081] (4) Phase structure test: Phase analysis was performed using an X-ray diffractometer (XRD, model: Bruker D8 Advance). Test conditions: Cu Kα radiation, λ=1.5406 Å, tube voltage 40kV, tube current 40mA, scanning range 10°-80°, step size 0.02°, scanning speed 2° / min.

[0082] The test results are shown in Table 2.

[0083]

[0084] In Table 2, “—” indicates that the substance was not detected or the content was below the detection limit.

[0085] As can be seen from Table 2, the materials prepared in Examples 1-2 of this invention have unique structural characteristics: grain size of 100-200 nm, surface coated with a continuous fluorinated carbon layer of 2-5 nm, XRD peak shift of 0.18°, and no impurity phase peaks. Comparative Examples 1-4, however, failed to simultaneously possess the above structural characteristics, proving that the technical solution of this invention can prepare LMFP materials with unique microstructures.

[0086] The cathode materials prepared in the above embodiments and comparative examples were subjected to performance tests.

[0087] Electrode preparation and battery assembly: The obtained positive electrode material, conductive agent (acetylene black), and binder (polyvinylidene fluoride, PVDF) were mixed at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was added and ground uniformly to form a slurry. This slurry was coated onto aluminum foil, vacuum dried at 100℃ for 12 hours, and then punched into electrode sheets. Lithium metal sheets were used as the negative electrode. The separator was a Celgard 2400 type single-layer PP membrane (25μm thickness, 41% porosity). The electrolyte was 1.0M LiPF6, EC:DMC:EMC (1:1:1), with 1wt% VC and 1wt% FEC as additives. CR2032 coin cells were assembled in an argon glove box.

[0088] Electrochemical performance testing: Constant current charge-discharge tests were conducted using a battery testing system with a voltage window of 2.5-4.5V. The test results are shown in Table 3.

[0089] Table 3. Comparison of electrochemical performance of materials prepared in each embodiment and comparative example.

[0090] It should be noted that the "total reaction time" mentioned in Table 1 refers to the total discharge time from the start of pulse application to the end of pulse application in the embodiments of the present invention; and to Comparative Example 1, it refers to the entire heat treatment process cycle from the start of heating to cooling to room temperature. In comparison, the method of the present invention can complete lattice reconstruction and doping reaction in milliseconds, significantly shortening the processing time and improving production efficiency.

[0091] As shown in Table 1, Examples 1-2 of this invention utilize transient Joule heating technology to treat waste LFP materials, resulting in an extremely short reaction time (approximately 0.55 seconds), significantly superior to the traditional tubular furnace calcination method (Comparative Example 1, reaction time approximately 12 hours). Regarding electrochemical performance, the material prepared in Example 1 achieved an initial discharge capacity of 158 mAh / g, significantly higher than Comparative Example 1 (130 mAh / g) and Comparative Example 2 (125 mAh / g). This indicates that the LMFP material prepared using waste LFP by the method of this invention possesses excellent electrical properties.

[0092] Comparing Example 1 and Example 2, it can be seen that adding an appropriate amount of transient conductive agent (5-10g) is beneficial to obtaining better electrochemical performance.

[0093] Comparing Example 1 and Comparative Example 3, it can be seen that the multi-stage frequency conversion pulse strategy of "low voltage long pulse + high voltage short pulse" (Example 1) is significantly better than the single-stage pulse strategy (Comparative Example 3), and can obtain a more stable structure and better electrochemical performance. Comparative Example 3 skipped the first stage of preheating, resulting in explosive vaporization of PVDF and micro-fracture of particles, and its performance was significantly degraded, proving the necessity of the multi-stage frequency conversion pulse strategy.

[0094] Comparing Example 1 and Comparative Example 4, it can be seen that even with the same pulse heating conditions, if the cooling rate is too slow (0.0083℃ / s), it will lead to Mn / Fe phase separation, fluorine protective layer disintegration, and significant deterioration of electrochemical performance, proving that ultrafast cooling is one of the key steps to obtain excellent performance.

[0095] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

[0096] It should be particularly noted that the various embodiments listed in this specification are intended to illustrate the technical solutions and advantages of the present invention, and are not intended to limit the scope of protection of the present invention. Without departing from the core ideas and technical effects of the present invention, those skilled in the art can make any form of improvement, substitution, combination, or modification to the structural arrangement, process parameters, material selection, control logic, etc., of the described embodiments; any obvious changes based on the same concept should be considered equivalent solutions of the present invention and should be included within the scope of protection defined by the claims of the present invention. The actual scope of protection of the present invention is determined by the appended claims and should be correctly understood in conjunction with the specification.

Claims

1. A lithium manganese iron phosphate material, characterized in that, The material has a core-shell structure, wherein the core is lithium manganese iron phosphate nanocrystals and the shell is a fluorinated carbon layer covering the surface of the nanocrystals.

2. The lithium manganese iron phosphate material according to claim 1, characterized in that, The core has a grain size of 100–200 nm, and the shell has a thickness of 2–5 nm.

3. The lithium manganese iron phosphate material according to claim 1 or 2, characterized in that, The X-ray diffraction peaks of the material are shifted by 0.15° to 0.25° towards lower angles.

4. A method for preparing the lithium manganese iron phosphate material according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Black powder is obtained by dismantling waste lithium iron phosphate batteries; the black powder contains binder and carbon black; the black powder has not been treated by acid leaching, alkali washing or high-temperature calcination. S2. Mix the black powder obtained in S1 with a lithium source, a manganese source and an added transient conductive agent to obtain a precursor powder; S3. Press the precursor powder into a green body; S4. Apply a high-voltage pulse current to the green billet to raise the green billet temperature to above 1200°C; S5. After the pulse ends, the powder is cooled to obtain lithium manganese iron phosphate material.

5. The method according to claim 4, characterized in that, In step S2, based on the mass of the waste cathode black powder (100%), the amount of manganese source added is 10-60%, the amount of lithium source added is 5-30%, and the amount of added transient conductive agent is 5-25%. In step S2, the manganese source is at least one of manganese dioxide, manganese tetroxide, manganese carbonate, and manganese oxalate; the lithium source is at least one of LiOH, Li2CO3, Li3PO4, and lithium acetate; the added transient conductive agent is at least one of carbon black, graphene, and carbon nanotubes. The mixing is performed using high-energy ball milling at a speed of 200-500 rpm for 2-4 hours.

6. The method according to claim 4, characterized in that, In step S3, the resistivity of the green blank is adjusted to be between 0.1 Ω·cm and 10 Ω·cm.

7. The method according to claim 4, characterized in that, In step S4, the high-voltage pulse current adopts a multi-stage frequency conversion pulse strategy, which includes: first applying a low-voltage long pulse with a pulse width of 100-300ms; then applying a high-voltage short pulse with a pulse width of 10-50ms.

8. The method according to claim 7, characterized in that, In step S4, the current density of the high-voltage pulse current is 50–500 A / cm². 2 The discharge voltage is 50V to 400V, which raises the temperature of the green billet to 1200℃ to 2800℃.

9. The method according to claim 4, characterized in that, In step S5, the cooling rate is greater than 1000°C / s.

10. A lithium-ion battery, characterized in that, The positive electrode of the lithium-ion battery comprises the lithium manganese iron phosphate material as described in any one of claims 1 to 3.

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