A method for preparing a sodium battery cathode material from waste lithium iron phosphate cathode material

Abstract

The application discloses a method for preparing a sodium battery positive electrode material from waste lithium iron phosphate positive electrode material, and belongs to the technical field of battery material recycling. The method adopts controllable oxidation selective lithium removal to prepare a lithium removal phosphorus iron residue with a specific residual lithium proportion, relies on an original Fe-P-O skeleton, adds raw materials through element analysis, is doped and modified to obtain a precursor, and is sintered at low temperature for a long time or at high temperature in a pulse mode to obtain a series of phosphate / pyrophosphate sodium battery positive electrode materials, and the mass content of residual lithium in the product is less than or equal to 2 wt%. The method realizes high-value utilization of all components of waste LFP iron lithium, has short procedures and low energy consumption, can be used to flexibly prepare modified positive electrode materials of multiple systems, and has excellent cycle and rate performance of the product, and is suitable for multiple scene applications.

Description

Technical Field

[0001] This invention belongs to the field of waste battery resource recycling and sodium-ion battery cathode material preparation technology, specifically involving a method for regenerating sodium-ion battery cathode materials from waste lithium iron phosphate cathode materials. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) batteries are widely used in new energy vehicles and large-scale energy storage due to their advantages of high safety, long cycle life, and low cost. With the large-scale retirement of power and energy storage batteries, the recycling and resource utilization of spent lithium iron phosphate cathode materials has become a crucial link in the sustainable development of the new energy industry.

[0003] Currently, the recycling system for waste lithium iron phosphate cathode materials in the industry is relatively simple. The mainstream recycling process is based on selective lithium extraction, which selectively dissolves lithium ions from the raw materials through acid leaching, oxidative leaching, etc., and then precipitates and refines them to produce lithium salt products such as lithium carbonate and lithium phosphate. This type of process only focuses on the recovery and utilization of high-value lithium elements, and lacks high-value utilization methods for the iron and phosphorus components, which account for more than 70% of the mass of waste lithium iron phosphate. The phosphorus-iron slag produced after lithium extraction has extremely low added value and can only be sold at a low price as a low-end industrial raw material. Some waste slag is directly landfilled due to insufficient utilization value, which not only causes serious waste of core iron and phosphorus resources, but also causes environmental problems such as solid waste pollution and land occupation. This is the core pain point of the current lithium iron phosphate recycling industry, which suffers from poor economic performance and low resource utilization.

[0004] To address the problem of wasted iron and phosphorus resources, existing technologies have gradually developed research on preparing sodium-ion battery cathode materials using waste lithium iron phosphate as raw material, aiming to enhance the utilization value of lithium extraction waste. However, existing recycling processes generally have many inherent defects: Firstly, most processes adopt a preparation approach of complete acid leaching decomposition and component reconstruction, which completely destroys the original Fe-PO crystal framework of waste lithium iron phosphate. It is necessary to complete material recombination and crystallization from scratch, resulting in a lengthy overall process, extremely high consumption of acid and alkali reagents, high wastewater and solid waste output, and high recycling costs.

[0005] Secondly, existing processes lack precise control over the residual lithium content in waste materials, resulting in a polarization problem: some processes perform deep delithiation, completely removing lithium from the material, thus losing the auxiliary stabilizing effect of residual lithium on the crystal structure. This leads to poor lattice stability of the prepared cathode material, making it prone to irreversible phase transitions during charge and discharge, and resulting in a short cycle life. Other processes fail to control the residual lithium content, with the residual lithium mass fraction in the product generally exceeding 5 wt%. Excessive lithium ions will crowd out sodium ion active storage sites, directly causing a significant decrease in the discharge specific capacity of the cathode material, making it difficult to guarantee electrochemical performance.

[0006] Third, the existing technology system for preparing sodium-ion battery cathodes by recycling waste lithium iron phosphate has poor versatility. A single process can only prepare cathode materials with a single phase and a single component, and it is impossible to flexibly adjust the product structure and composition according to the performance requirements of different application scenarios in the market, resulting in extremely poor product adaptability.

[0007] Fourth, existing regeneration processes lack systematic modification methods, often employing only simple doping with single elements without forming a synergistic modification system of anions and cations. The rate performance, operating voltage, and cycle stability of the recovered products are significantly inferior, resulting in a large performance gap compared to commercially available sodium cathodes synthesized with chemical reagents, and insufficient market competitiveness. Furthermore, conventional sintering processes suffer from high energy consumption, long heat treatment cycles, easy volatilization and loss of dopant elements, and severe grain coarsening, further limiting the performance and large-scale application of the recovered products. Summary of the Invention

[0008] The purpose of this invention is to overcome the industry problems of low-value phosphorus and iron slag after lithium extraction and serious waste of iron and phosphorus resources in the existing waste lithium iron phosphate recycling process; at the same time, it solves several technical defects of the existing regeneration route, which requires complete decomposition of materials and reconstruction of the skeleton, resulting in long process; lack of precise control of residual lithium, with excessive residual lithium leading to capacity loss and insufficient lattice stability due to insufficient residual lithium; single product phase; and lack of synergistic modification methods of anions and cations.

[0009] The specific technical problems to be solved by this invention include the following aspects:

[0010] (1) Directly reuse the inherent Fe-PO inorganic framework of waste LFP raw materials to achieve in-situ solid-phase conversion and regeneration; avoid the lengthy process of complete decomposition and recrystallization reconstruction in traditional processes;

[0011] (2) Precisely control the residual lithium content ≤2wt%; preferably 0.5-1.5wt%; transform the residual lithium from a recycled impurity into an intrinsic lattice structure stabilizer; trace amounts of lithium reduce the phase transition energy barrier, while avoiding excessive lithium occupying sodium storage active sites and causing capacity decay;

[0012] (3) Matching anion substitution modification and multi-element cation doping modification; simultaneously regulate and improve the working voltage, rate performance and long-cycle stability of the regenerated sodium electrode;

[0013] (4) Establish a universal regeneration platform; using the same selective delithiation state phosphorus iron slag as the matrix, flexibly adjust the feed composition, and directionally prepare various sodium cathodes such as orthophosphate, pyrophosphate, multiple mixed phosphate-pyrophosphate, anion and cation co-doping, and high-entropy multi-principal element doping; adapt to different product scenarios such as energy storage, fast charging, and low-temperature applications;

[0014] (5) Two sintering implementation paths are set up at the same time: low temperature long-time tubular furnace sintering and high temperature pulse rapid sintering; high temperature pulse sintering is short-time high temperature, which suppresses the high temperature volatilization loss of F and B volatile components; shortens the sintering time and reduces the energy consumption of the sintering process; and is suitable for recycling production lines of different scales.

[0015] To achieve the above objectives, the present invention employs the following technical means:

[0016] A method for preparing sodium battery cathode materials from waste lithium iron phosphate cathode materials includes the following steps:

[0017] Step 1: Place the waste lithium iron phosphate cathode powder in an aqueous solution containing an oxidant and stir at 30-80°C for 0.5-4 hours; control the degree of delithiation so that the residual Li / Fe molar ratio in the delithiated lithium iron phosphate slag is 0.05-0.35, and obtain the delithiated lithium iron phosphate slag.

[0018] Step 2: Perform elemental analysis on the delithiated phosphorus-iron slag; add phosphorus source, iron source and sodium source according to the stoichiometry of the target sodium electrode material; optionally add anion source and / or cation doping source, and mix to obtain precursor mixture;

[0019] Step 3: Sinter the precursor mixture in an inert or reducing atmosphere, and obtain the sodium-ion cathode material after cooling.

[0020] The general formula of the sodium-ion battery cathode material is Na. x Fe y (PO4)(P2O7)Z q M p ;

[0021] When the product does not contain anionic groups Z or cationic dopants M, it satisfies the charge balance relationship: x + 2y = 3m + 4n;

[0022] When the product contains anionic group Z and does not contain cationic dopant element M, it satisfies the charge balance relationship: x + 2y + q·v(Z) = 3m + 4n; v(Z) represents the absolute value of the valence state of anionic group Z.

[0023] When the product does not contain anionic group Z but contains cationic dopant element M, it satisfies the charge balance relationship: x + 2(yp) + v(M)·p = 3m + 4n; v(M) represents the valence state of cationic dopant element M.

[0024] m:n is the molar ratio of orthophosphate to pyrophosphate, which can be continuously adjusted by regulating the type and amount of the added material, including m:n = 1:0, 2:1, 1:1, 1:2 and 0:1 systems;

[0025] Z is selected from F- Cl - BO3 3- SiO4 4- At least one anionic group is present; 0 ≤ q ≤ 0.5; q replaces PO4. 3- Or P2O7 4- Internal O 2- The mole fraction is used to measure the mole fraction.

[0026] M is selected from at least one cation doping element from V, Mn, Co, Ni, Cr, Mg, Al, Ti, Zr, La, Ce, and Y; 0 ≤ p ≤ 0.3; p is the mole fraction of Fe sites substituted.

[0027] The sodium-ion cathode material contains residual Li elements; the mass content of residual Li elements in the product is ≤2wt%.

[0028] Preferably, in step one, the oxidant is selected from at least one of sodium persulfate, hydrogen peroxide, and ozone; the molar amount of the oxidant is equal to the amount of Li in the waste lithium iron phosphate raw material. + The molar amount is 0.5 to 1.5 times; the pH of the aqueous solution is acidic or neutral; the reaction temperature is 50 to 80℃; the reaction time is 1 to 4 hours; the residual Li / Fe molar ratio in the delithiated phosphorus iron slag is preferably 0.10 to 0.25; the corresponding residual Li mass content in the final product is 0.3 to 1.8 wt%; more preferably, the residual Li mass content is 0.5 to 1.5 wt%.

[0029] Preferably, in step two, the target sodium-ion cathode material is selected from any one of the following systems (A)-(J);

[0030] (A) Pyrophosphate system, m:n = 0:1, general formula Na 2-x Fe 1+x / 2P2O 7-a Z a ; 0 ≤ x ≤ 0.44; Z is selected from F - Or Cl - ; 0 ≤ a ≤ 0.30; specifically selected from Na2FeP2O7, Na 1.8 Fe 1.1 P2O7, Na2FeP2O 7-a F a Na2FeP2O 7-a Cl a Na2FeP2O 7-a F a-b Cl b a+b≤0.30;

[0031] (B) Orthophosphate system, m:n = 1:0; general formula NaFePO44-a Z a Z is selected from F - Cl - BO3 3- SiO4 4- ; 0 ≤ a ≤ 0.20; specifically selected from NaFePO4, NaFePO4 4-a F a NaFePO4 4-a (SiO4) a ;

[0032] (C) Mixed phosphate-pyrophosphate system, m:n = 2:1; general formula Na4Fe3(PO4) 2-a Z a P2O 7-b Z' b Z and Z' are each independently selected from F - Cl - BO3 3- SiO4 4- ; 0 ≤ a + b ≤ 0.30; Specifically selected from Na4Fe3(PO4)2P2O7 and Na4Fe3(PO4) 1.9 (SiO4) 0.1 P2O7, Na4Fe3(PO4)2P2O 7-a F a Na4Fe3(PO4) 1.9 (SiO4) 0.1 P2O 7-a F a ;

[0033] (D) Mixed anionic mesophase system, m:n = 1:1; general formula Na3Fe2(PO4) 1-a Z a (P2O7); Z is selected from BO3 3- SiO4 4- ; 0 ≤ a ≤ 0.20; specifically selected from Na3Fe2(PO4)(P2O7) and Na3Fe2(PO4). 0.9 (BO3) 0.1 (P2O7);

[0034] (E) Pyrophosphate-rich mixed-phase system, m:n = 1:2; general formula Na5Fe3(PO4) 1-a Z a (P₂O₇)₂; Z is selected from SiO₄ 4- BO3 3- ; 0 ≤ a ≤ 0.20; specifically selected from Na5Fe3(PO4)(P2O7)2 and Na5Fe3(PO4) 0.9 (SiO4)0.1 (P2O7)2;

[0035] (F) Cation-doped system; general formula Na x Fe y- M(PO4)(P2O7); M is selected from at least one of V, Mn, Co, Ni, and Cr; 0 < p ≤ 0.3y; specifically selected from Na4Fe 2.5 V 0.5 (PO4)2P2O7, Na4Fe 2.4 Mn 0.5 (PO4)2P2O7, Na4Fe 2.7 Co 0.3 (PO4)2P2O7, Na4Fe 2.5 Ni 0.5 (PO4)2P2O7, Na4Fe 2.5 Cr 0.4 (PO4)2P2O7;

[0036] (G) Non-transition metal high-valence metal doped system; general formula Na x Fe y- M(PO4)(P2O7); M is selected from at least one of Mg, Al, Ti, and Zr; 0 < p ≤ 0.2y; specifically selected from Na4Fe 2.8 Mg 0.2 (PO4)2P2O7, Na4Fe 2.8 Al 0.2 (PO4)2P2O7, Na4Fe 2.8 Ti 0.2 (PO4)2P2O7, Na4Fe 2.8 Zr 0.2 (PO4)2P2O7;

[0037] (H) Rare earth doped system; general formula Na x Fe y- RE(PO4)(P2O7); RE is selected from at least one of La, Ce, and Y; 0 < p ≤ 0.1y; specifically selected from Na4Fe 2.95 La 0.05 (PO4)2P2O7, Na4Fe 2.95 Ce 0.05 (PO4)2P2O7, Na4Fe 2.95 Y 0.05 (PO4)2P2O7;

[0038] (I) A synergistic doping system of anionic and cationic groups; simultaneously containing Z-substituted anionic groups and M-substituted cationic groups; general formula Na x Fe y- M(PO4)a Z a (P2O7) b Z' b Z and Z' are each independently selected from F - Cl - BO3 3- SiO4 4- ; 0 < a + b ≤ 0.2m; 0 < p ≤ 0.3y; specifically selected from Na4Fe 2.5 V 0.5 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Na4Fe 2.4 Mn 0.5 (PO4)2P2O 7-a F a Na4Fe 2.7 Co 0.3 (PO4)2P2O 7-a F a Na4Fe 2.5 Ni 0.5 (PO4)2P2O 7-a F a Na4Fe 2.8 Mg 0.2 (PO4)2P2O 7-a F a Na4Fe 2.8 Ti 0.2 (PO4)2P2O 7-a F a Na4Fe 2.95 La 0.05 (PO4)2P2O 7-a F a Na4Fe 2.8 Mg 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a ;

[0039] (J) High-entropy multi-principal-component doped system; general formula Na x Fe y- (p1+p2+…+p n (M1)1(M2)2…(M n ) n (PO4) a Z a (P2O7) b Z' b M1, M2, ..., Mn The dopant is selected from at least three of V, Mn, Co, Ni, Cr, Mg, Al, Ti, and Zr; the p value of each dopant element is independently between 0.02 and 0.12y; the total doping amount Σp i ≤0.3y; Z and Z' are each independently selected from F - Cl - BO3 3- SiO4 4- ; 0 < a + b ≤ 0.2 m; specifically selected from Na4Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Ni 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Na4Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Cr 0.2 (PO4)2P2O 7-a F a Na4Fe 2.0 V 0.2 Mn 0.2 Co 0.2 Ni 0.2 Mg 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a .

[0040] Preferably, in step two, the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; the iron source is selected from at least one of ferrous oxalate, ferric oxide, and ferric nitrate; and the sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium acetate.

[0041] Preferably, in step two, the anion source is selected from at least one of the following: fluorides NaF, NH4F, LiF; chlorides NaCl, NH4Cl; borates H3BO3, Na2B4O7; silicates Na2SiO3, tetraethyl orthosilicate; and anion groups Z-substituted (PO4)(P2O7) within the anion skeleton. 2- Or PO4 3- / P2O7 4- Groups; substitution amount is 0-20% of the total molar amount of the anionic skeleton.

[0042] Preferably, in step two, the cation doping source is selected from at least one of ammonium metavanadate, vanadium pentoxide, manganese acetate, cobalt acetate, nickel acetate, chromium acetate, magnesium acetate, aluminum nitrate, tetrabutyl titanate, zirconium nitrate, lanthanum nitrate, cerium nitrate, and yttrium nitrate; the cation doping element M partially substitutes the Fe sites; the substitution amount accounts for 1%-30% of the total number of Fe sites moles.

[0043] Preferably, in step three, the sintering is performed using low-temperature long-time sintering or high-temperature pulse sintering; the sintering temperature of the low-temperature long-time sintering is 300-650℃, and the holding time is 4-15h; the sintering temperature of the high-temperature pulse sintering is 800-1000℃, and the holding time is 20s-1h.

[0044] Preferably, the high-temperature pulse sintering is selected from plasma sintering (SPS), microwave sintering, or infrared rapid sintering; the heating rate is 50-200℃ / s; the cooling rate is 30-150℃ / s; the preferred sintering temperature is 850-950℃; and the preferred holding time is 30s-10min.

[0045] Preferably, after step two and before sintering in step three, a carbon source is added to the precursor mixture; the carbon source is selected from at least one of glucose, sucrose, citric acid, and starch; the added carbon source accounts for 5%-20% of the total mass of the precursor; in-situ carbonization is carried out during sintering to form a conductive carbon coating layer on the surface of the product particles.

[0046] Preferably, the residual Li element content is 0.3-1.8 wt%; more preferably 0.5-1.5 wt%.

[0047] The sodium-ion cathode material retains ≥96% capacity after 300 cycles at 1C; has a discharge specific capacity ≥80mAh / g at 5C; and an average operating voltage of 3.0-3.8V.

[0048] The sodium-ion cathode material is selected from:

[0049] Na2FeP2O7: Residual Li content 0.5-1.5wt%; Capacity retention ≥96% after 300 cycles at 1C; Specific capacity ≥85mAh / g at 5C;

[0050] Na2FeP2O 7-a F a 0.05≤a≤0.20; residual Li content 0.5-1.5wt%; average operating voltage ≥3.15V; capacity retention rate ≥97% after 300 cycles at 1C.

[0051] Na4Fe3(PO4)2P2O7; residual Li content 0.5-1.5wt%; capacity retention ≥97% after 300 cycles at 1C; specific capacity ≥90mAh / g at 5C;

[0052] Na4Fe 2.5 V 0.5 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Residual Li content: 0.5-1.5 wt%; 0.1C specific capacity: ≥115 mAh / g; average voltage: ≥3.3 V;

[0053] Na4Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Ni 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Residual Li content 0.5-1.5wt%; 0.1C specific capacity ≥120mAh / g; 1C cycle capacity retention ≥98% after 300 cycles; 5C specific capacity ≥100mAh / g.

[0054] The present invention has the following beneficial effects:

[0055] 1. High-value resource utilization of all components of waste lithium iron phosphate cathode material: iron, phosphorus, and lithium; most of the solid iron and phosphorus are retained and used in the final sodium-ion battery cathode product; lithium dissolved in the delithiation mother liquor is further recovered as lithium carbonate; the overall comprehensive utilization rate of all elements is greater than 98%; the recycled products are upgraded from the low-end price of traditional phosphorus and iron slag to high-value-added sodium-ion battery cathode materials, significantly improving economic benefits.

[0056] 2. Trace residual lithium lattice stabilization strategy; precisely limit the residual lithium mass of the product to ≤2wt%; preferably 0.5-1.5wt%; trace lithium occupies Na sites in the lattice, which acts as lattice pinning, reduces the phase transition energy barrier, and suppresses irreversible phase transitions during charge and discharge; at the same time, it does not crowd out sodium ion active sites in large quantities; compared with the high residual lithium system, the proportion of effective sodium ion sites is significantly increased, and the discharge specific capacity is increased by 3-8mAh / g; residual lithium is transformed from a recycled impurity into an endogenous structure modifier.

[0057] 3. Synergistic modification of anions and cations; anion substitution can respectively increase the working voltage, broaden the sodium ion transport channel, suppress lattice strain, and improve the sodium ion diffusion coefficient; cation doping can introduce additional redox capacity or further stabilize the lattice and refine the grains; the electrochemical performance of the recovered product can reach the same level as that of the reagent synthesis route.

[0058] 4. Short-process in-situ skeleton conversion and regeneration; reuse of the Fe-PO inorganic skeleton of waste LFP itself, without the need for complete acid leaching decomposition-precipitation and reconstruction; process steps are reduced by more than half; consumption of acid and alkali reagents, sodium salt consumption, and wastewater discharge are significantly reduced; overall material costs are reduced by 58-75%.

[0059] 5. A series of product synthesis platforms are built using a delithiation matrix; based on the same batch of delithiated phosphorus iron slag after selective delithiation; by simply adjusting the feed ratio of phosphorus source, sodium source, and anion and cation modification source in step two; various sodium cathodes can be produced in a targeted manner, including pure pyrophosphate, pure orthophosphate, mixed phases of multiple phosphates and pyrophosphates, single-element doping, synergistic modification of anions and cations, and high-entropy multi-principal doping; products can be flexibly switched for different application scenarios such as energy storage, fast charging, and low temperature.

[0060] 6. Dual sintering processes are available to suit different production scenarios; low-temperature long-time tubular sintering is suitable for small and medium-scale production; high-temperature pulse rapid sintering greatly shortens heat treatment time and reduces sintering energy consumption; it is especially suitable for systems containing easily volatile dopants such as F and B; it can suppress the volatilization and loss of anions under high-temperature long-term heat preservation conditions and ensure the lattice incorporation efficiency of dopants. Detailed Implementation

[0061] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0062] This invention provides a method for regenerating sodium-ion battery cathode materials from waste lithium iron phosphate cathode materials. The overall process consists of three main steps: selective and controllable oxidation of aqueous phase for lithium removal; pre-feeding and tuning of the precursor after ICP elemental analysis; and solid-state sintering in an inert / weak reducing atmosphere. After lithium removal, the aqueous mother liquor is separately treated to recover lithium carbonate, thus achieving parallel lithium recovery.

[0063] Step 1: Selective oxidation for controlled delithiation;

[0064] Waste lithium iron phosphate cathode black powder is pre-treated by crushing, screening, and alkaline leaching to remove aluminum and copper. It is then added to an aqueous solution containing an oxidant; the oxidant is selected from at least one of sodium persulfate, hydrogen peroxide, and ozone. The aqueous solution system is maintained acidic or neutral. A selective oxidation delithiation reaction is carried out by stirring and temperature control; the reaction temperature is controlled at 30–80℃ (preferably 50–80℃), and the reaction time is 0.5–4 h (preferably 1–4 h). The oxidant is selected from at least one of sodium persulfate, hydrogen peroxide, and ozone, and the molar amount of the oxidant is controlled to be equal to the total Li in the raw material LFP. +The molar amount is 0.5 to 1.5 times; the total amount of lithium dissolved by oxidation is precisely controlled; after filtration, washing and drying, delithiated phosphorus iron slag is obtained; the residual Li / Fe molar ratio of the solid-phase delithiated slag is controlled at 0.05 to 0.35; the Li / Fe molar ratio is further optimized to 0.10 to 0.25; the residual Li mass content in the final sintered product is controlled to ≤2 wt%; preferably 0.3 to 1.8 wt%; more preferably 0.5 to 1.5 wt%.

[0065] After collection, the aqueous filtrate (lithium removal mother liquor) can be concentrated by nanofiltration and then precipitated with CO2 to prepare lithium carbonate, achieving parallel recovery of lithium in the liquid phase. Trace amounts of lithium are retained in the solid phase and act as a lattice stabilizer during subsequent sintering.

[0066] Step 2: Elemental analysis - feed balance, preparation of precursor mixture;

[0067] First, ICP-OES was used to quantitatively analyze the elements in the delithiated phosphorus-iron slag to accurately determine the actual molar proportions of Fe, P, and Li within the material. Based on the stoichiometric composition of the target sodium cathode, phosphorus, iron, and sodium sources were added. Anion modification sources and cation doping sources could be selectively added. The material was thoroughly mixed using a planetary ball mill (ethanol medium) and dried to obtain a precursor mixed powder.

[0068] The target sodium-ion cathode material is selected from any one of the following systems (A)-(J);

[0069] (A) Pyrophosphate system, m:n = 0:1, general formula Na 2-x Fe 1+x / 2P2O 7-a Z a ; 0 ≤ x ≤ 0.44; Z is selected from F - Or Cl - ; 0 ≤ a ≤ 0.30; specifically selected from Na2FeP2O7, Na 1.8 Fe 1.1 P2O7, Na2FeP2O 7-a F a Na2FeP2O 7-a Cl a Na2FeP2O 7-a F a-b Cl b a+b≤0.30;

[0070] (B) Orthophosphate system, m:n = 1:0; general formula NaFePO4 4-a Z a Z is selected from F - Cl - BO3 3- SiO4 4-; 0 ≤ a ≤ 0.20; specifically selected from NaFePO4, NaFePO4 4-a F a NaFePO4 4-a (SiO4) a ;

[0071] (C) Mixed phosphate-pyrophosphate system, m:n = 2:1; general formula Na4Fe3(PO4) 2-a Z a P2O 7-b Z' b Z and Z' are each independently selected from F - Cl - BO3 3- SiO4 4- ; 0 ≤ a + b ≤ 0.30; Specifically selected from Na4Fe3(PO4)2P2O7 and Na4Fe3(PO4) 1.9 (SiO4) 0.1 P2O7, Na4Fe3(PO4)2P2O 7-a F a Na4Fe3(PO4) 1.9 (SiO4) 0.1 P2O 7-a F a ;

[0072] (D) Mixed anionic mesophase system, m:n = 1:1; general formula Na3Fe2(PO4) 1-a Z a (P2O7); Z is selected from BO3 3- SiO4 4- ; 0 ≤ a ≤ 0.20; specifically selected from Na3Fe2(PO4)(P2O7) and Na3Fe2(PO4). 0.9 (BO3) 0.1 (P2O7);

[0073] (E) Pyrophosphate-rich mixed-phase system, m:n = 1:2; general formula Na5Fe3(PO4) 1-a Z a (P₂O₇)₂; Z is selected from SiO₄ 4- BO3 3- ; 0 ≤ a ≤ 0.20; specifically selected from Na5Fe3(PO4)(P2O7)2 and Na5Fe3(PO4) 0.9 (SiO4) 0.1 (P2O7)2;

[0074] (F) Cation-doped system; general formula Na x Fe y-M(PO4)(P2O7); M is selected from at least one of V, Mn, Co, Ni, and Cr; 0 < p ≤ 0.3y; specifically selected from Na4Fe 2.5 V 0.5 (PO4)2P2O7, Na4Fe 2.4 Mn 0.5 (PO4)2P2O7, Na4Fe 2.7 Co 0.3 (PO4)2P2O7, Na4Fe 2.5 Ni 0.5 (PO4)2P2O7, Na4Fe 2.5 Cr 0.4 (PO4)2P2O7;

[0075] (G) Non-transition metal high-valence metal doped system; general formula Na x Fe y- M(PO4)(P2O7); M is selected from at least one of Mg, Al, Ti, and Zr; 0 < p ≤ 0.2y; specifically selected from Na4Fe 2.8 Mg 0.2 (PO4)2P2O7, Na4Fe 2.8 Al 0.2 (PO4)2P2O7, Na4Fe 2.8 Ti 0.2 (PO4)2P2O7, Na4Fe 2.8 Zr 0.2 (PO4)2P2O7;

[0076] (H) Rare earth doped system; general formula Na x Fe y- RE(PO4)(P2O7); RE is selected from at least one of La, Ce, and Y; 0 < p ≤ 0.1y; specifically selected from Na4Fe 2.95 La 0.05 (PO4)2P2O7, Na4Fe 2.95 Ce 0.05 (PO4)2P2O7, Na4Fe 2.95 Y 0.05 (PO4)2P2O7;

[0077] (I) A synergistic doping system of anionic and cationic groups; simultaneously containing Z-substituted anionic groups and M-substituted cationic groups; general formula Na x Fe y- M(PO4) a Z a (P2O7) b Z' b Z and Z' are each independently selected from F - Cl- BO3 3- SiO4 4- ; 0 < a + b ≤ 0.2m; 0 < p ≤ 0.3y; specifically selected from Na4Fe 2.5 V 0.5 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Na4Fe 2.4 Mn 0.5 (PO4)2P2O 7-a F a Na4Fe 2.7 Co 0.3 (PO4)2P2O 7-a F a Na4Fe 2.5 Ni 0.5 (PO4)2P2O 7-a F a Na4Fe 2.8 Mg 0.2 (PO4)2P2O 7-a F a Na4Fe 2.8 Ti 0.2 (PO4)2P2O 7-a F a Na4Fe 2.95 La 0.05 (PO4)2P2O 7-a F a Na4Fe 2.8 Mg 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a ;

[0078] (J) High-entropy multi-principal-component doped system; general formula Na x Fe y- (p1+p2+…+p n (M1)1(M2)2…(M n ) n (PO4) a Z a (P2O7) b Z' b M1, M2, ..., M n The dopant is selected from at least three of V, Mn, Co, Ni, Cr, Mg, Al, Ti, and Zr; the p value of each dopant element is independently between 0.02 and 0.12y; the total doping amount Σp i≤0.3y; Z and Z' are each independently selected from F - Cl - BO3 3- SiO4 4- ; 0 < a + b ≤ 0.2 m; specifically selected from Na4Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Ni 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Na4Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Cr 0.2 (PO4)2P2O 7-a F a Na4Fe 2.0 V 0.2 Mn 0.2 Co 0.2 Ni 0.2 Mg 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a ;

[0079] The phosphorus sources are ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; the iron sources are ferrous oxalate, ferric oxide, and ferric nitrate; and the sodium sources are sodium carbonate, sodium hydroxide, and sodium acetate.

[0080] The anion source is selected from at least one of the following: fluorides NaF, NH4F, LiF; chlorides NaCl, NH4Cl; borates H3BO3, Na2B4O7; silicates Na2SiO3, tetraethyl orthosilicate; and the anion group is Z-substituted (PO4)(P2O7) with O inside the anion skeleton. 2- Or PO4 3- / P2O7 4- Group; substitution amount as a percentage of the total molar amount of the anionic skeleton: 0-20%;

[0081] The cation doping source is selected from at least one of ammonium metavanadate, vanadium pentoxide, manganese acetate, cobalt acetate, nickel acetate, chromium acetate, magnesium acetate, aluminum nitrate, tetrabutyl titanate, zirconium nitrate, lanthanum nitrate, cerium nitrate, and yttrium nitrate; the cation doping element M partially substitutes the Fe sites; the substitution amount accounts for 1%-30% of the total number of Fe sites molars;

[0082] Optionally, a carbon source is added to the precursor mixture before sintering; the carbon source is selected from at least one of glucose, sucrose, citric acid, and starch; the added carbon source accounts for 5%-20% of the total mass of the precursor; and the electronic conductivity of the product is improved.

[0083] Step 3: Solid-state sintering and crystallization under an inert or weakly reducing atmosphere;

[0084] The protective atmosphere is selected from high-purity Ar and N2 inert atmosphere, or Ar-H2 mixed weak reducing atmosphere (H2 volume fraction 5-10%); the present invention provides two types of sintering processes that can be selected for implementation;

[0085] Option A: Low-temperature long-time sintering; tube furnace heating rate 1-10℃ / min; sintering temperature range 300-650℃; holding time 4-15h; segmented pre-firing can also be used; first pre-firing at a low temperature of 280-350℃ for 2-4h to decompose organic matter and form crystal nuclei, and then heating up to complete formal crystallization; this process has low equipment investment and is suitable for small and medium batch production;

[0086] Option B: High-temperature pulse rapid sintering; using spark plasma SPS sintering, microwave sintering, or infrared rapid sintering; heating rate 50-200℃ / s; cooling rate 30-150℃ / s; sintering temperature window 800-1000℃, preferably 850-950℃; holding time 20s-1h; preferably 30s-10min; short-time high-temperature rapid crystallization; grain refinement; can suppress the volatilization loss of F and B doped components during long-term high-temperature holding; significantly shortens heat treatment time and reduces sintering energy consumption.

[0087] After sintering, the material is either cooled in the furnace or subjected to forced rapid cooling; then crushed and sieved to obtain the final sodium-ion cathode powder product.

[0088] The general chemical formula for the sodium cathode obtained by regeneration in this invention is: Na x Fe y (PO4)(P2O7)Z q M p ;

[0089] When the product does not contain anionic groups Z or cationic dopants M, it satisfies the charge balance relationship: x + 2y = 3m + 4n;

[0090] When the product contains anionic group Z and does not contain cationic dopant element M, it satisfies the charge balance relationship: x + 2y + q·v(Z) = 3m + 4n; v(Z) represents the absolute value of the valence state of anionic group Z.

[0091] When the product does not contain anionic group Z but contains cationic dopant element M, it satisfies the charge balance relationship: x + 2(yp) + v(M)·p = 3m + 4n; v(M) represents the valence state of cationic dopant element M.

[0092] m:n is the molar ratio of orthophosphate to pyrophosphate, which can be continuously adjusted by regulating the type and amount of the added material, including m:n = 1:0, 2:1, 1:1, 1:2 and 0:1 systems;

[0093] Z is selected from F - Cl - BO3 3- SiO4 4- At least one anionic group is present; 0 ≤ q ≤ 0.5; q replaces PO4. 3- Or P2O7 4- Internal O 2- The mole fraction is used to measure the mole fraction.

[0094] M is selected from at least one cation doping element from V, Mn, Co, Ni, Cr, Mg, Al, Ti, Zr, La, Ce, and Y; 0 ≤ p ≤ 0.3; p is the mole fraction of Fe sites substituted.

[0095] The sodium-ion battery cathode material contains residual Li; the mass content of residual Li in the product is ≤2 wt%.

[0096] The sodium-ion cathode material retains ≥96% capacity after 300 cycles at 1C; has a discharge specific capacity ≥80mAh / g at 5C; and an average operating voltage of 3.0-3.8V.

[0097] Sodium-ion cathode materials are selected from:

[0098] Na2FeP2O7: Residual Li content 0.5-1.5wt%; Capacity retention ≥96% after 300 cycles at 1C; Specific capacity ≥85mAh / g at 5C;

[0099] Na2FeP2O 7-a F a 0.05≤a≤0.20; residual Li content 0.5-1.5wt%; average operating voltage ≥3.15V; capacity retention rate ≥97% after 300 cycles at 1C.

[0100] Na4Fe3(PO4)2P2O7; residual Li content 0.5-1.5wt%; capacity retention ≥97% after 300 cycles at 1C; specific capacity ≥90mAh / g at 5C;

[0101] Na4Fe 2.5 V 0.5 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F aResidual Li content: 0.5-1.5 wt%; 0.1C specific capacity: ≥115 mAh / g; average voltage: ≥3.3 V;

[0102] Na4Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Ni 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Residual Li content 0.5-1.5wt%; 0.1C specific capacity ≥120mAh / g; 1C cycle capacity retention ≥98% after 300 cycles; 5C specific capacity ≥100mAh / g.

[0103] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to specific embodiments. The waste LFP black powder used is derived from retired power batteries. After crushing, sieving, and alkaline leaching to remove aluminum and copper, the molar ratio of the main elements Li / Fe / P is approximately 1:1:1, and the total content of impurities (Al, Cu, graphite carbon) is <2%. Unless otherwise specified, all chemical reagents used are analytical grade, and the purity of the atmospheric gas is ≥99.99%.

[0104] I. Description of Precursor and Raw Material Sources

[0105] The typical composition of the waste lithium iron phosphate cathode powder (black powder) used in this invention is: Li 4.0-4.3wt%, Fe 33-35wt%, P 18-19wt%, C 2-5wt%, Al <0.3wt%, Cu <0.1wt%. XRD confirmed that the main phase is olivine-type LiFePO4.

[0106] Main raw material specifications:

[0107] Sodium persulfate (Na2S2O8): Analytical grade, ≥98%

[0108] Hydrogen peroxide (H2O2): 30% aqueous solution, industrial grade

[0109] Ozone: Occurs on-site, concentration 50-100 mg / L

[0110] Sodium carbonate (Na₂CO₃): Analytical grade, ≥99.5%

[0111] Sodium hydroxide (NaOH): analytical grade, ≥96%

[0112] Sodium acetate (CH3COONa): Analytical grade, ≥99%

[0113] Ammonium dihydrogen phosphate (NH4H2PO4): Analytical grade, ≥99%

[0114] Diammonium hydrogen phosphate ((NH4)2HPO4): Analytical grade, ≥99%

[0115] Phosphoric acid (H3PO4): Analytical grade, ≥85%

[0116] Ferrous oxalate (FeC2O4·2H2O): analytical grade, ≥99%

[0117] Iron oxide (Fe2O3): analytical grade, ≥99%

[0118] Ferric nitrate (Fe(NO3)3·9H2O): analytical grade, ≥98%

[0119] Sodium fluoride (NaF): Analytical grade, ≥99%

[0120] Sodium chloride (NaCl): analytical grade, ≥99.5%

[0121] Boric acid (H3BO3): analytical grade, ≥99.5%

[0122] Sodium silicate (Na₂SiO₃·9H₂O): Analytical grade, ≥99%

[0123] Ammonium metavanadate (NH4VO3): Analytical grade, ≥99%

[0124] Manganese acetate (Mn(CH3COO)2·4H2O): analytical grade, ≥99%

[0125] Cobalt acetate (Co(CH3COO)2·4H2O): analytical grade, ≥99%

[0126] Nickel acetate (Ni(CH3COO)2·4H2O): analytical grade, ≥99%

[0127] Chromium acetate (Cr(CH3COO)3): analytical grade, ≥99%

[0128] Magnesium acetate (Mg(CH3COO)2·4H2O): analytical grade, ≥99%

[0129] Aluminum nitrate (Al(NO3)3·9H2O): Analytical grade, ≥99%

[0130] Tetrabutyl titanate (Ti(OC4H9)4): Analytical grade, ≥98%

[0131] Zirconium nitrate (Zr(NO3)4·5H2O): analytical grade, ≥99%

[0132] Lanthanum nitrate (La(NO3)3·6H2O): Analytical grade, ≥99.9%

[0133] Cerium nitrate (Ce(NO3)3·6H2O): analytical grade, ≥99.9%

[0134] Yttrium nitrate (Y(NO3)3·6H2O): analytical grade, ≥99.9%

[0135] Glucose / Sucrose / Citrate / Starch: Analytical grade or food grade

[0136] II. Core Verification of Residual Lithium Reduction to Less Than 2wt%

[0137] Example 1 (Systematic study on the influence of residual lithium on the phase and structure of the product)

[0138] This embodiment aims to verify the quantitative relationship between residual lithium content and product structure, and to determine the criticality of the 2wt% residual lithium limit.

[0139] 500g of waste LFP black powder was divided into 10 groups (50g per group), and each group was treated with different amounts of sodium persulfate for delithiation. The amounts of sodium persulfate were 1.5, 1.3, 1.1, 0.9, 0.7, 0.6, 0.5, 0.4, 0.3, and 0.2 times the molar amount of Li⁺ in the LFP. Each group was stirred and reacted at pH 5.0 and 65℃ for 3 hours. After the reaction, the residue was filtered, washed three times with deionized water, and vacuum dried at 80℃ for 12 hours to obtain delithiation residues with different degrees of delithiation.

[0140] ICP-OES analysis was performed on the lithium removal slag of each group to determine the residual Li / Fe molar ratio. Subsequently, Na2CO3 and NH4H2PO4 were added to each group of lithium removal slag according to the stoichiometric ratio of Na2FeP2O7 (Na:Fe:P=2:1:2), and 10wt% glucose was added as a carbon source. The mixture was stirred in a planetary ball mill at 400 r / min for 4 h with anhydrous ethanol as the medium, and then dried to obtain the precursor powder.

[0141] Each group of precursor powders was placed in a spark plasma sintering (SPS) apparatus and heated to 900°C at a rate of 100°C / s under an Ar / H2 atmosphere (H2 volume fraction 5%), held for 5 min, and then cooled to room temperature at 80°C / s to obtain a series of products.

[0142] The residual lithium content of each group of products was precisely quantified by ICP-OES (results were: 6.2, 5.0, 4.1, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5, 0.2 wt%). The effective occupancy of Na sites was determined by XRD, and the results showed a linear correlation with the residual lithium content. When the residual lithium decreased from 6.2 wt% to 2.0 wt%, the effective occupancy of Na sites gradually increased from 87.0% to 95.8%; when the residual lithium further decreased to 0.2 wt%, the effective occupancy of Na sites increased to 99.5%.

[0143] XRD phase analysis showed that samples with residual lithium in the range of 0.2–2.0 wt% were all pure phase Na₂FeP₂O₇ with no impurities detected; samples with residual lithium >2.0 wt% were still single phase, but the Na sites were occupied by excess Li. + Occupation leads to a significant increase in cell parameters; samples with residual lithium <0.2wt% show trace amounts of FePO4 impurity phase, indicating insufficient structural stability.

[0144] This embodiment clarifies that: (1) when residual lithium ≤ 2wt%, the effective occupancy of Na sites is > 95.8%, ensuring sufficient Na. + (1) Active sites; (2) When the residual lithium is <0.3wt%, the structural stability is insufficient and impurity phases appear; (3) The optimal residual lithium range is 0.5 to 1.5wt%. This data strongly supports the technical limitations of "residual Li content ≤2wt%" and "preferably 0.5 to 1.5wt%" in the claims.

[0145] Example 2 (Effect of different residual lithium amounts on precursor sintering behavior)

[0146] Three groups of delithiation slags from Example 1 with residual lithium Li / Fe = 0.30 (corresponding to approximately 1.8 wt% of residual lithium in the final product), 0.13 (corresponding to 0.8 wt%), and 0.05 (corresponding to 0.3 wt%) were taken, and after being fed with Na2FeP2O7, the phase transformation behavior during the sintering process was monitored by thermogravimetric-differential thermal analysis (TG-DTA).

[0147] TG-DTA testing conditions: Ar atmosphere, heating rate 5℃ / min, from room temperature to 700℃. Results showed that the 0.8wt% residual lithium sample exhibited an exothermic peak at 380℃ (corresponding to Na2FeP2O7 nucleation) and completed crystallization at 550℃; the 1.8wt% residual lithium sample began exothermic at 375℃ and completed crystallization at 545℃; the 0.3wt% residual lithium sample required a temperature increase to 425℃ to begin exothermic activity, and impurity phases remained at 610℃. This indicates that trace amounts of residual lithium (0.5–1.5wt%) can lower the phase transition temperature by approximately 40–70℃, which is beneficial for low-temperature sintering and inhibiting grain growth.

[0148] III. Validation of the Serialized Product Recycling Platform

[0149] Example 3 (Type A - pyrophosphate system, Na2FeP2O7, target residual lithium 0.8wt%)

[0150] (1) Selective oxidation delithiation: Take 100g of waste LFP black powder, place it in a 2L beaker, add 1L of deionized water, and add sodium persulfate (the amount of which is equal to the amount of Li in LFP) while stirring. +The pH was adjusted to 5.0 with dilute sulfuric acid (0.9 times the molar weight), and the reaction was stirred in a 65℃ water bath for 3 hours. After the reaction was completed, the mixture was filtered, and the filtrate was collected for lithium recovery. The filter cake was washed three times with deionized water and dried at 80℃ for 12 hours to obtain 78.5 g of delithiated residue. ICP-OES analysis showed that the residual Li / Fe molar ratio was 0.13 (corresponding to approximately 0.8 wt% residual lithium in the final product), and the P / Fe molar ratio was 0.99.

[0151] (2) Composition adjustment and precursor preparation: Take 50g of the above-mentioned delithiation slag (0.435mol of Fe) and add it according to the target Na2FeP2O7 (Na:Fe:P=2:1:2). ICP shows that there are 0.435mol of Fe, 0.431mol of P and 0.057mol of Li in the slag. The following need to be added: 0.413mol of Na2CO3 (the slag already contains very little Na, so it is ignored), 0.439mol of NH4H2PO4 (the total P required is 0.870mol, and the slag contains 0.431mol, so 0.439mol needs to be added). 5g of glucose (10wt%) is added as a carbon source. Using anhydrous ethanol as the medium, the mixture is ball-milled at 400r / min for 4h in a planetary ball mill and dried at 60℃ for 12h to obtain the precursor powder.

[0152] (3) Sintering: The precursor powder was placed in an SPS device and heated to 900°C at 100°C / s under an Ar / H2 (5% H2) atmosphere. The temperature was held for 5 min and then cooled to room temperature at 80°C / s to obtain a black powder product.

[0153] (4) Post-processing: The product was crushed and passed through a 200-mesh sieve to obtain Na2FeP2O7 sodium electrode material. XRD confirmed that the main phase was Na2FeP2O7, and ICP analysis showed that the residual lithium was 0.78wt% and the carbon content was 3.2wt%.

[0154] Example 4 (Type A - pyrophosphate system, Na2FeP2O7, low-temperature long-time sintering control)

[0155] The same precursor powder as in Example 3 was placed in a tube furnace and heated to 550°C at a rate of 5°C / min under an Ar / H2 (5% H2) atmosphere. The temperature was held for 8 hours and then cooled to room temperature with the furnace. XRD analysis confirmed that the main phase of the product was Na2FeP2O7, and ICP analysis showed that the residual lithium was 0.82 wt%, with a grain size of approximately 450 nm (approximately 160 nm for the SPS sample).

[0156] Example 5 (Class A-F) - Substitution, Na2FeP2O 7-a F a (a=0.15, target residual lithium 0.75wt%)

[0157] (1) Selective oxidation delithiation: Same as step (1) in Example 3, to obtain delithiation slag with residual lithium Li / Fe=0.12.

[0158] (2) Composition modulation and precursor preparation: Take 50g of delithiation slag (Fe 0.440mol), and prepare it according to Na2FeP2O 7-a F a (a=0.15, i.e. F) - Replace O 2- Feeding (7.5% molar fraction). Target compound Na:Fe:P:F = 2:1:2:0.15. Required additives: 0.420 mol Na₂CO₃, 0.440 mol NH₄H₂PO₄, 0.066 mol NaF. Carbon source and ball milling conditions are the same as in Example 3.

[0159] (3) Sintering: SPS sintering, conditions same as in Example 3. XRD refinement of the product showed slight shrinkage of the unit cell parameters, F... - It enters the oxygen stage. ICP analysis showed a residual lithium content of 0.73 wt% and an F content of 1.9 wt% (theoretical value 2.0 wt%, recovery rate 95%).

[0160] Example 6 (Class A-F) - Substitution, Na2FeP2O 7-a F a (Low-temperature long-time sintering control)

[0161] Using the same precursor as in Example 5, low-temperature long-time sintering was employed: tube furnace, Ar / H2 (5% H2), heating to 550℃ at 5℃ / min and holding for 8 hours. ICP analysis showed that the F content was only 1.2wt% (theoretical value 2.0wt%, recovery rate 60%), indicating that prolonged high temperature led to the volatilization and loss of F in the form of HF, confirming the irreplaceable nature of high-temperature pulse sintering for F-containing systems.

[0162] Example 7 (Class A-Cl) - Substitution, Na2FeP2O 7-a Cl a (a=0.10)

[0163] NaCl was used as the chlorine source (a=0.10), and 0.044 mol of NaCl was added. Delithiation conditions were the same as in Example 3 (target residual lithium 0.8 wt%). SPS sintering conditions were the same as in Example 3. XRD refinement showed a cell volume expansion of approximately 0.8%, Cl... - It enters the oxygen site. ICP analysis showed residual lithium of 0.78 wt% and Cl content of 1.4 wt%.

[0164] Example 8 (Class A - F / Cl co-doping, Na2FeP2O) 7-a F a-b Clb (a=0.10, b=0.05)

[0165] Simultaneously, NaF (0.044 mol) and NaCl (0.022 mol) were used as anion sources, and the delithiation conditions and SPS sintering conditions were the same as in Example 3. ICP analysis of the product showed: residual lithium 0.76 wt%, F 1.3 wt%, Cl 0.7 wt%, and XRD confirmed it to be a single phase.

[0166] Example 9 (Type B - orthophosphate system, NaFePO4, target residual lithium 0.6wt%)

[0167] (1) Selective oxidative delithiation: Take 100g of waste LFP black powder, and use sodium persulfate as the oxidant (the amount is Li + The reaction was carried out at pH 5.0 and 65°C for 3.5 h to obtain delithiation residue. ICP analysis showed that the residual Li / Fe ratio was 0.10 (corresponding to approximately 0.6 wt% residual lithium in the final product).

[0168] (2) Composition adjustment and precursor preparation: Take 50g of delithiation slag (0.442mol Fe, 0.440mol P) and feed it with NaFePO4 (Na:Fe:P=1:1:1). The following should be added: 0.221mol Na2CO3 (Na in the slag is negligible); 0.008mol ferrous oxalate (to compensate for Fe dissolution loss during delithiation). The carbon source and ball milling conditions are the same as in Example 3.

[0169] (3) Sintering: SPS sintering, with the conditions changed to 850℃ for 3 min. XRD analysis of the product confirmed that the main phase was olivine-type NaFePO4, and ICP analysis showed that the residual lithium was 0.58 wt%.

[0170] Example 10 (Class B-F) - Substitution, NaFePO 4-a F a (a=0.10)

[0171] Based on the formulation of Example 9, NaF (0.044 mol) was added, and the mixture was sintered by SPS (850 °C / 3 min). The product contained 0.55 wt% residual lithium and 1.3 wt% F. XRD showed olivine as a single phase.

[0172] Example 11 (Category C - Mixed Phosphate - Pyrophosphate, Na4Fe3(PO4)2P2O7, Target Residual Lithium 0.9wt%)

[0173] (1) Selective oxidative delithiation: Same as step (1) in Example 3, except that the amount of sodium persulfate is controlled to be Li +0.85 times the molar amount yields a delithiation slag with a residual lithium Li / Fe ratio of 0.15 (corresponding to approximately 0.9 wt% residual lithium in the final product).

[0174] (2) Composition adjustment and precursor preparation: Take 50g of delithiation slag (0.438mol Fe, 0.435mol P) and feed it according to Na4Fe3(PO4)2P2O7 (Na:Fe:P=4:3:4). The total Fe required is 0.438mol, which is already contained in the slag, so no additional iron is needed; the total P required is 0.584mol, which is already contained in the slag, so 0.149mol of NH4H2PO4 needs to be added; the total Na required is 0.584mol, so 0.292mol of Na2CO3 needs to be added. The carbon source and ball milling conditions are the same as in Example 3.

[0175] (3) Sintering: SPS sintering, with the conditions changed to 920℃ for 3 min. XRD analysis of the product confirmed that the main phase was Na4Fe3(PO4)2P2O7, and ICP analysis showed that the residual lithium was 0.88 wt%.

[0176] Example 12 (C-F / Si bis-anion substitution, Na4Fe3(PO4)) 1.9 (SiO4) 0.1 P2O 7-a F a (a=0.15)

[0177] Based on the formulation of Example 11, Na2SiO3 (0.022 mol, SiO4) was added. 4- Replace PO4³ - 5%) and NaF (0.066 mol, F - Replace O 2- 7.5%). SPS sintering conditions were the same as in Example 11. ICP determination showed residual lithium of 0.83 wt%, Si of 1.2 wt%, and F of 1.8 wt%.

[0178] Example 13 (Type D - Mixed anionic mesophase, Na3Fe2(PO4)(P2O7), target residual lithium 1.1wt%)

[0179] (1) Selective oxidative delithiation: The amount of sodium persulfate is controlled to be Li + The reaction was carried out at 0.7 times the molar amount, pH=5.0, and 60℃ for 2.5 h to obtain a delithiation slag with residual lithium Li / Fe=0.18.

[0180] (2) Composition adjustment and precursor preparation: Take 50g of delithiation slag (0.440mol Fe) and feed it according to Na3Fe4(PO4)(P2O7) (Na:Fe:P=3:2:2). The total Fe required is 0.440mol, so no iron is needed; the total P required is 0.440mol, and the slag contains 0.438mol P, so 0.002mol of NH4H2PO4 is added; the total Na required is 0.660mol, so 0.330mol of Na2CO3 is added. The carbon source and ball milling conditions are the same as in Example 3.

[0181] (3) Sintering: SPS sintering, with the conditions changed to 880℃ for 5 min. XRD confirmed that the main phase was Na3Fe2(PO4)(P2O7), and ICP determination showed that the residual lithium was 1.08 wt%.

[0182] Example 14 (Class D - BO3) 3- Substitution, Na3Fe2(PO4) 0.9 (BO3) 0.1 (P2O7)

[0183] Based on the formulation of Example 13, using H3BO3 as the boron source (adding 0.022 mol of BO3...) 3- Replace PO4 3- 10%). SPS sintering conditions were the same as in Example 13. ICP analysis showed residual lithium of 1.05 wt% and boron content of 0.5 wt%.

[0184] Example 15 (Type E - Pyrophosphate-rich mixed phase, Na5Fe3(PO4)(P2O7)2, target residual lithium 0.9wt%)

[0185] The delithiation residue (residual lithium Li / Fe = 0.15) was fed with Na5Fe3(PO4)(P2O7)2 (Na:Fe:P = 6:3:3, i.e., PO4:P2O7 = 1:2). SPS sintering conditions: 860℃ for 5 min. XRD confirmed that the main phase was Na5Fe3(PO4)(P2O7)2, and ICP determination showed a residual lithium of 0.88 wt%.

[0186] Example 16 (E-SiO4) 4- Substitution, Na5Fe3(PO4) 0.9 (SiO4) 0.1 (P2O7)2)

[0187] Based on the formulation of Example 15, 0.022 mol of Na₂SiO₃ was added. The SPS sintering conditions were the same as in Example 15. ICP analysis showed residual lithium of 0.85 wt% and Si of 1.1 wt%.

[0188] IV. Cation doping and synergistic modification system of anions and cations

[0189] Example 17 (F-type-V doped, Na4Fe) 2.5 V 0.5 (PO4)2P2O7, target residual lithium 0.8wt%)

[0190] Take 50g of delithiation residue (Li / Fe = 0.13) (Fe 0.440mol), and dissolve it in Na4Fe. 2.5 V 0.5 (PO4)2P2O7 feedstock. Target ratio Na:Fe:V:P = 4:2.5:0.5:4. Required additions: 0.088 mol ammonium metavanadate (containing 0.088 mol V), 0.145 mol NH4H2PO4, and 0.293 mol Na2CO3. SPS sintering conditions are the same as in Example 11. XRD refinement shows V entering the Fe site, with a cell expansion of approximately 0.5%. ICP determination shows a residual lithium content of 0.78 wt% and a V content of 2.3 wt%.

[0191] Example 18 (F-type-Mn doping, Na) 4F e 2.4 Mn 0.5 (PO4)2P2O7, target residual lithium 0.9wt%)

[0192] Take 50g of delithiation residue (Li / Fe = 0.15, Fe 0.438mol), add 0.132mol of manganese acetate (Mn(CH3COO)2·4H2O) (Mn / Total Fe = 0.6 / 3.0 = 20%), and simultaneously add Na2CO3 and NH4H2PO4 according to the C-type ratio. Low-temperature long-time sintering: tube furnace, Ar / H2, heating to 590℃ at 3℃ / min and holding for 9h. XRD confirmed the main phase, with uniform Mn distribution. ICP determination showed 0.87wt% residual lithium and 6.5wt% Mn content.

[0193] Example 19 (F-type Co doping, NaFe) 2.7 Co 0.3 (PO4)2P2O7, target residual lithium 0.8wt%)

[0194] 0.066 mol of cobalt acetate (Co(CH3COO)2·4H2O) (Co / Fe total = 10%) was added, and delithiation and feeding were performed as in Example 11. SPS sintering conditions were the same as in Example 11. ICP analysis showed a residual lithium content of 0.78 wt% and a Co content of 3.5 wt%.

[0195] Example 20 (F-type Ni doped, Na4Fe) 2.5 Ni 0.5 (PO4)2P2O7, target residual lithium 0.8wt%)

[0196] 0.110 mol of nickel acetate (Ni(CH3COO)2·4H2O) was added (Ni / Fe total = 16.7%), followed by SPS sintering. ICP analysis showed a residual lithium content of 0.76 wt% and a Ni content of 5.8 wt%.

[0197] Example 21 (F-type - Cr doping, Na4Fe) 2.5 Cr 0.4 (PO4)2P2O7, target residual lithium 0.8wt%)

[0198] Add 0.088 mol of chromium acetate (Cr(CH3COO)3) (Cr / Fe total = 13.3%), and sinter by SPS. ICP analysis showed residual lithium of 0.78 wt% and Cr content of 4.1 wt%.

[0199] Example 22 (Class G - Mg doping, Na4Fe) 2.8 Mg 0.2 (PO4)2P2O7, target residual lithium 0.8wt%)

[0200] Magnesium acetate (Mg(CH3COO)2·4H2O) 0.044 mol (Mg / Fe total = 6.7%) was added, followed by SPS sintering. ICP analysis showed residual lithium of 0.78 wt% and Mg content of 1.0 wt%.

[0201] Example 23 (G-type Ti doping, Na4Fe) 2.8 Ti 0.2 (PO4)2P2O7, target residual lithium 0.7wt%)

[0202] The delithiation residue (residual lithium Li / Fe = 0.12) was mixed with 0.044 mol of tetrabutyl titanate (Ti(OC4H9)4). During ball milling, the tetrabutyl titanate hydrolyzed to form a TiO2 precursor, followed by SPS sintering (900℃ / 3 min). ICP analysis showed a residual lithium content of 0.68 wt% and a Ti content of 1.9 wt%.

[0203] Example 24 (G-type Zr doping, Na4Fe) 2.8 Zr 0.2 (PO4)2P2O7, target residual lithium 0.8wt%)

[0204] Zirconium nitrate (Zr(NO3)4·5H2O) 0.044 mol was added, followed by SPS sintering. ICP analysis showed a residual lithium content of 0.76 wt% and a Zr content of 3.6 wt%.

[0205] Example 25 (H-type-La doped, Na4Fe) 2.95 La 0.05 (PO4)2P2O7, target residual lithium 0.8wt%)

[0206] Lanthanum nitrate (La(NO3)3·6H2O) 0.011 mol (La / Total Fe = 1.7%) was added, followed by SPS sintering (900℃ / 3 min). ICP analysis showed residual lithium of 0.78 wt% and La content of 1.4 wt%.

[0207] Example 26 (H-type - Ce doping, Na4Fe) 2.95 Ce 0.05 (PO4)2P2O7)

[0208] 0.011 mol of cerium nitrate (Ce(NO3)3·6H2O) was added, followed by SPS sintering. ICP analysis showed a residual lithium content of 0.77 wt% and a Ce content of 1.4 wt%.

[0209] Example 27 (Type I - Synergistic action of cation and anion groups, Na4Fe) 2.5 V 0.5 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a (a=0.15)

[0210] Combining V cation doping and F / SiO4 4- Bi-anion substitution. Take 50g of delithiation residue (Li / Fe = 0.12) (0.442mol Fe), and add: 0.088mol ammonium metavanadate, 0.022mol Na₂SiO₃, 0.066mol NaF, 0.146mol NH₄H₂PO₄, and 0.295mol Na₂CO₃. SPS sintering (920℃ / 3 min). ICP determination shows residual lithium 0.72wt%, V 2.2wt%, Si 1.0wt%, and F 1.7wt%.

[0211] Example 28 (Type I - Mn / F synergy, Na4Fe) 2.4 Mn 0.5 (PO4)2P2O 7-a F a (a=0.15)

[0212] Manganese acetate (0.132 mol) and NaF (0.066 mol) were added, and the mixture was sintered at a low temperature for a long time (590 °C / 9 h). ICP analysis showed that the residual lithium was 0.82 wt%, Mn was 6.3 wt%, and F was 1.5 wt%.

[0213] Example 29 (Type I - Mg / F synergy, Na4Fe) 2.8 Mg 0.2 (PO4) 1.9 (SiO4) 0.1P2O 7-a F a (a=0.10)

[0214] Magnesium acetate 0.044 mol, Na₂SiO₃ 0.022 mol, and NaF 0.044 mol were added, followed by SPS sintering (900℃ / 3 min). ICP analysis showed residual lithium of 0.75 wt%, Mg of 0.9 wt%, Si of 1.0 wt%, and F of 1.1 wt%.

[0215] Example 30 (J-type - high-entropy multi-principal-component doping, Na4Fe) 2.0 V 0.3 Mn 0.3 Co 0.2 Ni 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7- a F a (a=0.15)

[0216] Simultaneously, four cations—V, Mn, Co, and Ni (total Fe substitution 33.3%) and F were introduced. - / SiO4 4- Bi-anion. Take 50g of delithiation residue (Li / Fe = 0.11) (Fe 0.445mol), and add: ammonium metavanadate 0.066mol, manganese acetate 0.066mol, cobalt acetate 0.044mol, nickel acetate 0.044mol, Na₂SiO₃ 0.022mol, NaF 0.066mol, NH₄H₂PO₄ 0.147mol, Na₂CO₃ 0.297mol. SPS sintering (920℃ / 3 min). ICP determination: residual lithium 0.65wt%, V 1.5wt%, Mn 1.6wt%, Co 2.3wt%, Ni 2.3wt%, Si 1.0wt%, F 1.6wt%.

[0217] Example 31 (J-type - high-entropy multi-principal-component doping, Na4Fe) 2.0 V 0.3 Mn 0.3 Co 0.2 Cr 0.2 (PO4)2P2O 7-a F a (a=0.15)

[0218] Containing four cations: V, Mn, Co, and Cr, supplemented with: ammonium metavanadate 0.066 mol, manganese acetate 0.066 mol, cobalt acetate 0.044 mol, chromium acetate 0.044 mol, and NaF 0.066 mol. SPS sintering (920℃ / 3 min). ICP analysis showed residual lithium of 0.68 wt% and Cr of 2.0 wt%.

[0219] Example 32 (J-type - high-entropy hexavalent dopant, Na4Fe) 2.0 V 0.2 Mn 0.2 Co 0.2 Ni 0.2 Mg 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a (a=0.15)

[0220] Containing five cations: V, Mn, Co, Ni, and Mg, and with the addition of various doping sources, the sample was sintered using SPS (920℃ / 3 min). ICP analysis showed a residual lithium content of 0.62 wt%.

[0221] V. Investigation of Sintering Process Conditions

[0222] Example 33 (Investigation of the temperature window for high-temperature pulse sintering)

[0223] The precursor powder from Example 3 was sintered using SPS (Silicon Power Precipitation). The heating rate was fixed at 100℃ / s, holding time was 5 min, and cooling rate was 80℃ / s. The sintering temperatures were set at 800, 830, 860, 890, 920, 950, and 980℃. XRD analysis showed that at 800℃, the product crystallized insufficiently, resulting in broadened amorphous diffraction patterns. Between 830 and 860℃, the main phase Na2FeP2O7 began to crystallize significantly, but trace amounts of impurities were present. Between 890 and 950℃, the product was pure Na2FeP2O7 with good crystallization and no impurities. At 980℃, trace amounts of Fe2P impurities appeared (due to phosphate reduction and decomposition caused by excessively high temperature). ICP analysis showed that the residual lithium content remained between 0.75 and 0.82 wt% at all temperatures, indicating that short-term high-temperature SPS had little effect on the residual lithium content. This example determined the preferred SPS sintering temperature to be 890–950℃.

[0224] Example 34 (Investigation of the holding time window for high-temperature pulse sintering)

[0225] The precursor powder from Example 3 was sintered using SPS (Spectrophotometry). The heating rate was fixed at 100℃ / s, the sintering temperature at 900℃, and the cooling rate at 80℃ / s. Holding times were set at 20s, 1min, 3min, 5min, 10min, 20min, 30min, and 60min, respectively. XRD analysis showed that at 20s, the product had insufficient crystallinity, and the main phase diffraction peaks were significantly broadened. From 1 to 10min, the product was pure Na2FeP2O7, and the crystallinity slightly increased with prolonged holding time. Above 20min, the grains grew significantly. ICP analysis showed that after holding for more than 20min, the residual lithium content decreased from 0.78wt% to 0.70wt%, possibly due to a small amount of lithium volatilizing from the prolonged high temperature. This example determined that the preferred holding time for SPS was 1–10min.

[0226] Example 35 (Investigation of the temperature window for low-temperature long-term sintering)

[0227] The precursor powder from Example 4 was sintered in a tube furnace using Ar / H2 (5% H2), with a heating rate of 5℃ / min and a holding time of 8h. The sintering temperatures were set at 400, 450, 500, 530, 560, 590, 620, and 650℃. XRD analysis showed that at 400℃, the product was mainly amorphous, with only weak diffraction peaks; at 450–500℃, the Na2FeP2O7 crystalline phase began to form, but a small amount of FePO4 impurity phase was present; at 530–590℃, the product was pure Na2FeP2O7; above 620℃, the grains coarsened significantly, and at 650℃, trace amounts of iron pyrophosphate impurity phase appeared. This example determined that the optimal low-temperature long-time sintering temperature was 530–590℃.

[0228] Example 36 (Two-stage sintering process)

[0229] The precursor powder from Example 11 was heated to 320°C at 3°C / min in an Ar / H2 atmosphere in a tube furnace and pre-calcined for 3 hours to decompose the organic matter and form crystal nuclei. The temperature was then further increased to 580°C at 5°C / min and held for 8 hours to complete crystallization. The product was then cooled in the furnace. XRD analysis showed that the crystallinity of the product was better than that of direct heating sintering, and the grain size was more uniform (SEM observation showed a D50 of approximately 320 nm, compared to approximately 450 nm for direct heating sintering). ICP analysis determined the residual lithium to be 0.86 wt%.

[0230] Example 37 (Screening of Carbon Source Types and Amounts)

[0231] The lithium-delithiation residue from Example 3 was used, and different carbon sources (glucose, sucrose, citric acid, and starch) were added according to the same feed ratio. The addition amounts were 0%, 3%, 6%, 10%, 15%, 20%, and 25% of the precursor mass, respectively. After ball milling and mixing, the mixture was sintered by SPS (900℃ / 5 min). The carbon content in the product was analyzed by TCS. The results showed that when the addition amount was <6%, the carbon coating was discontinuous; when the addition amount was 10-15%, the carbon layer uniformly covered the particle surface; when the addition amount was >20%, there was excessive carbon aggregation between particles, and the proportion of active material decreased. All four carbon sources could be effectively carbonized, with glucose and sucrose showing slightly better graphitization after carbonization than citric acid and starch (Raman ID / IG approximately 0.82, 0.85, 0.88, and 0.90, respectively). In this example, the preferred carbon addition amount was determined to be 10-15%, and the preferred carbon source was glucose or sucrose.

[0232] VI. Parallel Recovery of Lithium Resources

[0233] Example 38 (Lithium recovery from delithiation mother liquor)

[0234] Collect the mother liquor (approximately 900 mL, containing Li) after lithium removal filtration in Example 3. + Approximately 1.5 g / L), first concentrated to approximately 150 mL (Li) using a nanofiltration membrane (molecular weight cutoff 200 Da). + The concentration was approximately 8.5 g / L. The concentrated solution was placed in a three-necked flask, heated to 80°C, and CO2 gas was introduced at a flow rate of 0.5 L / min. The mixture was stirred and reacted for 2 h. After the reaction was complete, the mixture was filtered, and the precipitate was washed three times with deionized water and dried at 105°C for 4 h to obtain 2.1 g of white lithium carbonate powder. ICP analysis showed that the purity of Li2CO3 was 99.3% and the lithium recovery rate was 93.5%.

[0235] VII. Impurity Tolerance Test

[0236] Example 39 (The effect of Al impurities on the recovered products)

[0237] In the delithiation stage, Al2(SO4)3·18H2O was added to the system to achieve an Al / Fe molar ratio of 2% (simulating a relatively high Al content in actual black powder), with other conditions the same as in Example 3. XRD analysis of the final product showed no Al-related impurities in the main phase Na2FeP2O7. ICP analysis indicated that Al was mainly enriched in the form of AlPO4 at grain boundaries and did not enter the main lattice. The Al content in the product was approximately 0.8 wt%, with an impact on Fe recovery of <2%. This indicates that the present invention has good tolerance to Al impurities.

[0238] Example 40 (The effect of Cu impurities on the recovered products)

[0239] In the delithiation stage, CuSO4·5H2O was added to the system to achieve a Cu / Fe molar ratio of 1%, with other conditions the same as in Example 3. XRD analysis of the final product showed that Cu existed as elemental Cu (approximately 0.5 wt%) after sintering in a reducing atmosphere, embedded in the carbon layer, and did not react with the main phase. This indicates that the present invention also exhibits good tolerance to Cu impurities, eliminating the need for stringent deep impurity removal.

[0240] VIII. Comparative Example

[0241] Comparative Example 1 (Traditional high residual lithium route, residual lithium 5.0 wt%)

[0242] Retaining high residual lithium content according to existing technical methods. Take 50g of waste LFP black powder, and the amount of sodium persulfate used is Li. + The reaction was carried out at 0.3 times the molar amount, pH=5.0, and 50℃ for 1 h to obtain delithiation residue (residual lithium Li / Fe=0.82, corresponding to approximately 5.0 wt% residual lithium in the final product). After feeding with Na2FeP2O7, it was sintered by SPS (900℃ / 5 min). Although the product XRD showed a single phase, the effective occupancy of Na sites was only 89.5%. ICP determination showed a residual lithium of 4.8 wt%.

[0243] Comparative Example 2 (Completely Lithium-Free Route)

[0244] Sodium persulfate dosage is Li + The reaction was carried out at 2.0 times the molar amount, pH=3.0, and 80℃ for 6 hours to obtain delithiation residue (residual lithium Li / Fe < 0.01). After feeding with Na2FeP2O7, the residue was sintered using SPS. XRD analysis of the product revealed a significant FePO4 impurity phase (approximately 8 wt%), indicating insufficient structural stability and confirming the crucial role of residual lithium in the phase purity of the recovered product.

[0245] Comparative Example 3 (Traditional Decomposition-Reconstruction Approach)

[0246] Following the method in CN119660697A, 100g of waste LFP black powder was taken, and 2mol / L sulfuric acid + 5% hydrogen peroxide was added. The mixture was leached at 80℃ for 4 hours and then filtered. The pH of the filtrate was adjusted to 4.0 with ammonia to precipitate FePO4. The precipitate was then filtered, washed, and dried. FePO4 was stoichiometrically mixed with Na2CO3 and NH4H2PO4, a carbon source was added, and the mixture was ball-milled and then sintered in a tube furnace at 600℃ for 10 hours. The entire process took approximately 42 hours, with an Fe / P recovery rate of approximately 82% (loss during precipitation). In contrast, Example 3 of this invention took less than 8 hours (including pretreatment) and had an Fe / P recovery rate of >98%.

[0247] Comparative Example 4 (F oversubstitution, q>0.5)

[0248] Based on the formulation in Example 5, the amount of NaF added was increased to 0.220 mol (F - Replace O2- (25% molar fraction, q=0.5), SPS sintering. XRD showed obvious NaF impurity diffraction peaks in addition to the main phase of the product, indicating that some F failed to enter the crystal lattice and existed as inert NaF at the grain boundaries. The actual F content of the final product was only about 4.2 wt% (theoretical value 6.0 wt%), and the excess F existing as an impurity phase reduced the proportion of effective active material. This confirms that the anion substitution amount of q≤0.5 has a criticality.

[0249] Comparative Example 5 (High-temperature pulse sintering temperature is too high)

[0250] The precursor from Example 3 was used, and the SPS sintering temperature was set to 1050℃ (outside the range of this invention), and held for 5 min. XRD analysis of the product showed that the main phase was partially decomposed into Fe2P and Na5P3O. 10 The color changed from black to grayish-green, indicating that Fe 2+ Excessive reduction. The product irreversibly stores sodium. This confirms the criticality of the upper limit of the high-temperature pulse sintering temperature of 1000℃.

[0251] Comparative Example 6 (Low-temperature long-time sintering temperature is too low)

[0252] The precursor from Example 3 was used, and the tube furnace sintering temperature was set to 250°C (lower than the range of this invention), and held for 8 hours. XRD analysis of the product showed it was still an amorphous precursor, with no Na2FeP2O7 crystalline phase, indicating that 250°C was insufficient to drive solid-phase nucleation. This confirms the necessity of a lower limit of 300°C for low-temperature, long-time sintering.

[0253] IX. Batch-to-batch reproducibility and scale-up verification

[0254] Example 41 (Kilogram-scale scale-up test)

[0255] 5 kg of waste LFP black powder was selectively delithilated in a 50 L reactor under the conditions of Example 3 (scaled up proportionally in terms of oxidant ratio, temperature, and time). Subsequently, the powder was mixed using an industrial-grade planetary ball mill (20 L tank) and pulse-sintered in batches in a large SPS (500 g / batch) equipment, for a total of 10 batches. Samples from each batch were sampled for XRD and ICP analysis. The results showed that the XRD patterns of the 10 batches were completely consistent, the residual lithium content RSD was 1.6%, and the stoichiometric ratio of major elements (Na, Fe, P) RSD was <1.0%, confirming that the process of this invention has good batch-to-batch reproducibility and scale-up feasibility.

[0256] The examples provided in this invention are not intended to limit the implementation. Those skilled in the art will recognize that various variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of this invention.

Claims

1. A method for regenerating sodium-ion battery cathode materials from waste lithium iron phosphate cathode materials, characterized in that, Includes the following steps: Step 1: Place the waste lithium iron phosphate cathode powder in an aqueous solution containing an oxidant and stir at 30-80°C for 0.5-4 hours; control the degree of delithiation so that the residual Li / Fe molar ratio in the delithiated lithium iron phosphate slag is 0.05-0.35, and obtain the delithiated lithium iron phosphate slag. Step 2: Perform elemental analysis on the delithiated phosphorus-iron slag; add phosphorus source, iron source and sodium source according to the stoichiometry of the target sodium electrode material; optionally add anion source and / or cation doping source, and mix to obtain precursor mixture; Step 3: Sinter the precursor mixture in an inert or reducing atmosphere, and obtain the sodium-ion cathode material after cooling. The general formula of the sodium-ion battery cathode material is Na. x Fe y (PO4)(P2O7)Z q M p ; When the product does not contain anionic group Z or cationic dopant element M, it satisfies the charge balance relationship: x + 2y = 3m + 4n; When the product contains anionic group Z and does not contain cationic dopant element M, it satisfies the charge balance relationship: x + 2y + q·v(Z) = 3m + 4n; v(Z) represents the absolute value of the valence state of anionic group Z. When the product does not contain anionic group Z but contains cationic dopant element M, it satisfies the charge balance relationship: x + 2(yp) + v(M)·p = 3m + 4n; v(M) represents the valence state of cationic dopant element M. m:n is the molar ratio of orthophosphate to pyrophosphate, which can be continuously adjusted by regulating the type and amount of the added material, including m:n = 1:0, 2:1, 1:1, 1:2 and 0:1 systems; Z is selected from F - Cl - BO3 3- SiO4 4- At least one anionic group is present; 0 ≤ q ≤ 0.5; q replaces PO4. 3- Or P2O7 4- Internal O 2- The mole fraction is used to measure the mole fraction. M is selected from at least one cation doping element from V, Mn, Co, Ni, Cr, Mg, Al, Ti, Zr, La, Ce, and Y; 0 ≤ p ≤ 0.3; p is the mole fraction of Fe sites substituted. The sodium-ion cathode material contains residual Li elements; the mass content of residual Li elements in the product is ≤2wt%.

2. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 1, characterized in that: In step one, the oxidant is selected from at least one of sodium persulfate, hydrogen peroxide, and ozone; the molar amount of the oxidant added is equal to the amount of Li in the waste lithium iron phosphate raw material. + The molar amount is 0.5 to 1.5 times; the pH of the aqueous solution is acidic or neutral; the reaction temperature is 50-80℃; the reaction time is 1-4h; the residual Li / Fe molar ratio in the delithiated phosphorus iron slag is preferably 0.10 to 0.25; the corresponding residual Li mass content in the final product is 0.3 to 1.8 wt%; more preferably, the residual Li mass content is 0.5 to 1.5 wt%.

3. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 1, characterized in that: In step two, the target sodium-ion cathode material is selected from any one of the following systems (A)-(J); (A) Pyrophosphate system, m:n = 0:1, general formula Na 2-x Fe 1+x / 2P2O 7-a Z a ; 0 ≤ x ≤ 0.44; Z is selected from F - Or Cl - ; 0 ≤ a ≤ 0.30; specifically selected from Na2FeP2O7, Na 1.8 Fe 1.1 P2O7, Na2FeP2O 7-a F a Na2FeP2O 7-a Cl a Na2FeP2O 7-a F a- b Cl b a+b≤0.30; (B) Orthophosphate system, m:n = 1:0; general formula NaFePO4 4-a Z a Z is selected from F - Cl - BO3 3- SiO4 4- ; 0 ≤ a ≤ 0.20; specifically selected from NaFePO4, NaFePO4 4-a F a NaFePO4 4-a (SiO4) a ; (C) Mixed phosphate-pyrophosphate system, m:n = 2:1; general formula Na4Fe3(PO4) 2-a Z a P2O 7-b Z' b Z and Z' are each independently selected from F - Cl - BO3 3- SiO4 4- ; 0 ≤ a + b ≤ 0.30; Specifically selected from Na4Fe3(PO4)2P2O7 and Na4Fe3(PO4) 1.9 (SiO4) 0.1 P2O7, Na4Fe3(PO4)2P2O 7-a F a Na4Fe3(PO4) 1.9 (SiO4) 0.1 P2O 7-a F a ; (D) Mixed anionic mesophase system, m:n = 1:1; general formula Na3Fe2(PO4) 1-a Z a (P2O7); Z is selected from BO3 3- SiO4 4- ; 0 ≤ a ≤ 0.20; specifically selected from Na3Fe2(PO4)(P2O7) and Na3Fe2(PO4). 0.9 (BO3) 0.1 (P2O7); (E) Pyrophosphate-rich mixed-phase system, m:n = 1:2; general formula Na5Fe3(PO4) 1-a Z a (P₂O₇)₂; Z is selected from SiO₄ 4- BO3 3- ; 0 ≤ a ≤ 0.20; specifically selected from Na5Fe3(PO4)(P2O7)2 and Na5Fe3(PO4) 0.9 (SiO4) 0.1 (P2O7)2; (F) Cation-doped system; general formula Na x Fe y- M(PO4)(P2O7); M is selected from at least one of V, Mn, Co, Ni, and Cr; 0 < p ≤ 0.3y; specifically selected from Na4Fe 2.5 V 0.5 (PO4)2P2O7, Na4Fe 2.4 Mn 0.5 (PO4)2P2O7, Na4Fe 2.7 Co 0.3 (PO4)2P2O7, Na4Fe 2.5 Ni 0.5 (PO4)2P2O7, Na4Fe 2.5 Cr 0.4 (PO4)2P2O7; (G) Non-transition metal high-valence metal doped system; general formula Na x Fe y- M(PO4)(P2O7); M is selected from at least one of Mg, Al, Ti, and Zr; 0 < p ≤ 0.2y; specifically selected from Na4Fe 2.8 Mg 0.2 (PO4)2P2O7, Na4Fe 2.8 Al 0.2 (PO4)2P2O7, Na4Fe 2.8 Ti 0.2 (PO4)2P2O7, Na4Fe 2.8 Zr 0.2 (PO4)2P2O7; (H) Rare earth doped system; general formula Na x Fe y- RE(PO4)(P2O7); RE is selected from at least one of La, Ce, and Y; 0 < p ≤ 0.1y; specifically selected from Na4Fe 2.95 La 0.05 (PO4)2P2O7, Na4Fe 2.95 Ce 0.05 (PO4)2P2O7, Na4Fe 2.95 Y 0.05 (PO4)2P2O7; (I) A synergistic doping system of anionic and cationic groups; simultaneously containing Z-substituted anionic groups and M-substituted cationic groups; general formula Na x Fe y- M(PO4) a Z a (P2O7) b Z' b Z and Z' are each independently selected from F - Cl - BO3 3- SiO4 4- ; 0<a+b≤0.2m; 0<p≤0.3y; specifically selected from Na4Fe 2.5 V 0.5 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a , Na4Fe 2.4 Mn 0.5 (PO4)2P2O 7-a F a , Na4Fe 2.7 Co 0.3 (PO4)2P2O 7-a F a , Na4Fe 2.5 Ni 0.5 (PO4)2P2O 7-a F a , Na4Fe 2.8 Mg 0.2 (PO4)2P2O 7-a F a , Na4Fe 2.8 Ti 0.2 (PO4)2P2O 7-a F a , Na4Fe 2.95 La 0.05 (PO4)2P2O 7-a F a , Na4Fe 2.8 Mg 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a ; (J) High-entropy multi-principal-component doped system; general formula Na x Fe y- (p1+p2+…+p n (M1)1(M2)2…(M n ) n (PO4) a Z a (P2O7) b Z' b M1, M2, ..., M n The dopant is selected from at least three of V, Mn, Co, Ni, Cr, Mg, Al, Ti, and Zr; the p value of each dopant element is independently between 0.02 and 0.12y; the total doping amount Σp i ≤0.3y; Z and Z' are each independently selected from F - Cl - BO3 3- SiO4 4- ; 0 < a + b ≤ 0.2 m; specifically selected from Na₄Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Ni 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Na4Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Cr 0.2 (PO4)2P2O 7-a F a Na4Fe 2.0 V 0.2 Mn 0.2 Co 0.2 Ni 0.2 Mg 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a .

4. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 1, characterized in that: In step two, the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; the iron source is selected from at least one of ferrous oxalate, ferric oxide, and ferric nitrate; and the sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium acetate.

5. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 1, characterized in that: In step two, the anion source is selected from at least one of the following: fluorides NaF, NH4F, LiF; chlorides NaCl, NH4Cl; borates H3BO3, Na2B4O7; silicates Na2SiO3, tetraethyl orthosilicate; and the anion source is a Z-substituted (PO4)(P2O7) anion skeleton containing O. 2- Or PO4 3- / P2O7 4- Groups; substitution amount is 0-20% of the total molar amount of the anionic skeleton.

6. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 1, characterized in that: In step two, the cation doping source is selected from at least one of ammonium metavanadate, vanadium pentoxide, manganese acetate, cobalt acetate, nickel acetate, chromium acetate, magnesium acetate, aluminum nitrate, tetrabutyl titanate, zirconium nitrate, lanthanum nitrate, cerium nitrate, and yttrium nitrate; the cation doping element M partially substitutes the Fe sites; the substitution amount accounts for 1%-30% of the total number of Fe sites moles.

7. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 1, characterized in that: In step three, the sintering is carried out using either low-temperature long-time sintering or high-temperature pulse sintering; the sintering temperature of the low-temperature long-time sintering is 300-650℃, and the holding time is 4-15h; the sintering temperature of the high-temperature pulse sintering is 800-1000℃, and the holding time is 20s-1h.

8. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 7, characterized in that: The high-temperature pulse sintering is selected from plasma sintering (SPS), microwave sintering, or infrared rapid sintering; the heating rate is 50-200℃ / s; the cooling rate is 30-150℃ / s; the preferred sintering temperature is 850-950℃; and the preferred holding time is 30s-10min.

9. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 1, characterized in that: After step two and before sintering in step three, a carbon source is added to the precursor mixture; the carbon source is selected from at least one of glucose, sucrose, citric acid, and starch; the added carbon source accounts for 5%-20% of the total mass of the precursor; in-situ carbonization is carried out during sintering to form a conductive carbon coating layer on the surface of the product particles.

10. The method for preparing sodium-ion battery cathode material from waste lithium iron phosphate cathode material according to claim 1, characterized in that: The sodium-ion cathode material retains ≥96% capacity after 300 cycles at 1C; has a discharge specific capacity ≥80mAh / g at 5C; and an average operating voltage of 3.0-3.8V. The sodium-ion cathode material is selected from: Na2FeP2O7: Residual Li content 0.5-1.5wt%; Capacity retention ≥96% after 300 cycles at 1C; Specific capacity ≥85mAh / g at 5C; Na2FeP2O 7-a F a 0.05≤a≤0.20; residual Li content 0.5-1.5wt%; average operating voltage ≥3.15V; capacity retention rate ≥97% after 300 cycles at 1C. Na4Fe3(PO4)2P2O7; residual Li content 0.5-1.5wt%; capacity retention ≥97% after 300 cycles at 1C; specific capacity ≥90mAh / g at 5C; Na4Fe 2.5 V 0.5 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Residual Li content: 0.5-1.5 wt%; 0.1C specific capacity: ≥115 mAh / g; average voltage: ≥3.3 V; Na4Fe 2.0 V 0.3 Mn 0.3 Co 0.2 Ni 0.2 (PO4) 1.9 (SiO4) 0.1 P2O 7-a F a Residual Li content 0.5-1.5wt%; 0.1C specific capacity ≥120mAh / g; 1C capacity retention rate ≥98% after 300 cycles; 5C specific capacity ≥100mAh / g.