Lithium-based-montmorillonite-coated lifepo 4 core-shell material, preparation method therefor, and use thereof

Abstract

The present disclosure belongs to the technical field of metallurgy and relates to a lithium-based-montmorillonite-coated LiFePO4 core-shell material, a preparation method therefor, and a use thereof. The lithium-based-montmorillonite-coated LiFePO4 core-shell material comprises LiFePO4 as a core layer and lithium-based montmorillonite as a shell layer, wherein the lithium-based montmorillonite coats the surface of LiFePO4. The lithium-based-montmorillonite-coated LiFePO4 core-shell material prepared according to the present disclosure has a relatively high ion conduction capability, can compensate for the deficiency of large Li+ conduction resistance at a LiFePO4-water interface, and can facilitate the conduction of Li+ between a water phase and an LiFePO4 crystal grain, that is, the overall resistivity of a paste is reduced, and the working voltage is lower, so that the energy consumption of the electrochemical extraction of lithium from a salt lake can be further reduced. The lithium-based-montmorillonite-coated LiFePO4 core-shell material prepared according to the present disclosure can be widely applied to the extraction of lithium from salt lake brine.

Description

A lithium-based montmorillonite-coated LiFePO4 core-shell material and its preparation method and application Technical Field

[0001] The present disclosure relates to the field of metallurgy technology, and in particular to a lithium-based montmorillonite-coated LiFePO4 core-shell material and a preparation method and application thereof. Background Art

[0002] Lithium resources are widely used in lithium-ion batteries and other related fields. With the rapid development of the lithium-ion battery industry, the demand for lithium resources in industrial production has increased at an increasingly rapid rate. In recent years, there has even been a shortage of lithium raw materials. How to efficiently and quickly extract lithium from salt lakes has always been a scientific problem to be solved. Salt lakes have a low lithium content and a high content of other metals such as magnesium. Due to the similar properties of magnesium and lithium, it is difficult to selectively extract lithium resources from salt lakes. Compared with traditional methods such as precipitation, calcination and leaching, carbonization, nanofiltration membrane, solvent extraction and adsorption, the electrochemical method of extracting lithium from salt lake brine can achieve the advantages of cleaner extraction, lower energy consumption and higher extraction efficiency.

[0003] The electrochemical method of extracting lithium from salt lake brine is based on the fact that lithium ion sieve can realize redox reaction at different voltages and complete Li + The principle of the embedding / extraction process. Existing lithium ion sieve substrates include: spinel-type LiMn2O4, layered oxide-type LiMO2 (M is a transition metal ion, i.e. Ni, Co, Mn, etc.), and polyanion salt LiFePO4. However, the first two are oxides and usually involve metal ion dissolution, resulting in a poor substrate life. Therefore, LiFePO4 is most suitable for electrochemical work under aqueous conditions. + The insertion / extraction process will cause Li + There is polarization phenomenon in the insertion / extraction voltage platform (the charging and discharging voltage is higher).

[0004] In view of this, the present disclosure is proposed.

[0005] Summary of the Invention

[0006] The purpose of the present disclosure is to provide a lithium-based montmorillonite-coated LiFePO4 core-shell material and its preparation method and application.

[0007] In order to achieve at least one of the above-mentioned objectives of the present disclosure, the following technical solutions may be adopted:

[0008] In a first aspect, the present disclosure provides a lithium-montmorillonite-coated LiFePO4 core-shell material, which includes LiFePO4 as a core layer and lithium-montmorillonite as a shell layer, and the lithium-montmorillonite is coated on the surface of the LiFePO4.

[0009] In some embodiments of the present disclosure, the mass ratio of the LiFePO4 to the lithium-based montmorillonite is 1:0.1% to 1%.

[0010] In a second aspect, the present disclosure provides a method for preparing a lithium-montmorillonite-coated LiFePO4 core-shell material, which comprises mixing LiFePO4 and lithium-montmorillonite, adding an alcohol solvent, performing a gel reaction, and drying and grinding the obtained gel.

[0011] In some embodiments of the present disclosure, the mass ratio of the LiFePO4 to the lithium-based montmorillonite is 1:0.1% to 1%.

[0012] In some embodiments of the present disclosure, the amount of the alcohol solvent added accounts for 30% to 300% of the mass of the LiFePO4.

[0013] In some embodiments of the present disclosure, the alcohol solvent includes any one or a combination of at least two of ethanol, ethylene glycol, polyethylene glycol, propanol or methanol.

[0014] In some embodiments of the present disclosure, the temperature of the gel reaction is 70-100°C.

[0015] In some embodiments of the present disclosure, the gel is dried at a temperature of 80 to 120° C. and a drying time of 1 to 18 hours.

[0016] In some embodiments of the present disclosure, the method for preparing the lithium-based montmorillonite includes: mixing and infiltrating the montmorillonite with a first lithium solution, then filtering and collecting the solid powder, and drying the solid powder.

[0017] In some embodiments of the present disclosure, the lithium ion concentration in the first lithium solution is greater than 1 g / L.

[0018] In some embodiments of the present disclosure, the soaking temperature is 70-100° C., and the soaking time is greater than 12 hours.

[0019] In some embodiments of the present disclosure, the solid powder is dried at a temperature of 80 to 120° C. and for a drying time of 3 to 12 hours.

[0020] In a third aspect, the present disclosure provides the use of the lithium-montmorillonite-coated LiFePO4 core-shell material as described in any of the aforementioned embodiments or the lithium-montmorillonite-coated LiFePO4 core-shell material prepared by the preparation method of the lithium-montmorillonite-coated LiFePO4 core-shell material as described in any of the aforementioned embodiments in lithium extraction from salt lake brine.

[0021] In a fourth aspect, the present disclosure provides a method for extracting lithium from salt lake brine, comprising: using the lithium-based montmorillonite-coated LiFePO4 core-shell material as described in any one of the aforementioned embodiments or the lithium-based montmorillonite-coated LiFePO4 core-shell material prepared by the preparation method of the lithium-based montmorillonite-coated LiFePO4 core-shell material as described in any one of the aforementioned embodiments as a lithium ion sieve, recorded as M-LiFePO4, and delithiating a portion of the lithium-based montmorillonite-coated LiFePO4 core-shell material to prepare a lithium-deficient ion sieve, recorded as M-FePO4;

[0022] The M-LiFePO4 and the M-FePO4 are respectively added into the anode chamber and the cathode chamber to perform electrochemical lithium extraction;

[0023] or,

[0024] The M-LiFePO4 and the M-FePO4 are coated on an anode plate and a cathode plate respectively to perform electrochemical lithium extraction.

[0025] In some embodiments of the present disclosure, when the M-LiFePO4 and the M-FePO4 are added to the anode chamber and the cathode chamber respectively for electrochemical lithium extraction, the following operations are included:

[0026] The M-LiFePO4 is mixed with the second lithium solution to prepare a slurry and the slurry is placed in the anode chamber; the M-FePO4 is mixed with brine to prepare a slurry and the slurry is placed in the cathode chamber;

[0027] An anode plate is inserted into the anode chamber, and a cathode plate is inserted into the cathode chamber. The anode chamber and the cathode chamber are separated by an anion membrane. Voltage is applied to the anode plate and the cathode plate to perform electrochemical lithium extraction. The operation stops when a first cut-off voltage is reached.

[0028] In some embodiments of the present disclosure, the solid-liquid ratio of the M-LiFePO4 and the second lithium solution in slurry preparation is 1:1-30; the solid-liquid ratio of the M-FePO4 and the brine in slurry preparation is 1:1-30.

[0029] In some embodiments of the present disclosure, the anode chamber and the cathode chamber are kept stirred during the lithium extraction process.

[0030] In some embodiments of the present disclosure, the stirring speed is 100 to 1000 r / min.

[0031] In some embodiments of the present disclosure, the main component of the second lithium solution is LiCl, wherein the lithium concentration is <15 g / L.

[0032] In some embodiments of the present disclosure, when the M-LiFePO4 and the M-FePO4 are respectively coated on the anode plate and the cathode plate for electrochemical lithium extraction, the coating thickness of the M-LiFePO4 is 0.01 to 1 cm; the coating thickness of the M-FePO4 is 0.01 to 1 cm.

[0033] In some embodiments of the present disclosure, the anode plate includes one of a graphite electrode, a platinum metal sheet, or a carbon fiber electrode.

[0034] In some embodiments of the present disclosure, the anode plate is a graphite electrode.

[0035] In some embodiments of the present disclosure, the cathode plate includes one of a graphite electrode, a platinum metal sheet, or a carbon fiber electrode.

[0036] In some embodiments of the present disclosure, the cathode plate is a graphite electrode.

[0037] In some embodiments of the present disclosure, the method for preparing the M-FePO4 includes: mixing the M-LiFePO4 with a lithium-containing liquid to prepare a slurry and placing it in an anode chamber while stirring, filling brine into a cathode chamber, inserting an anode plate and a cathode plate into the chamber respectively, and applying power in a constant current manner until the cut-off voltage is reached, taking out the slurry from the anode chamber, filtering and collecting the solid, and thus obtaining the M-FePO4.

[0038] In some embodiments of the present disclosure, the lithium-containing liquid includes a lithium solution or brine.

[0039] In some embodiments of the present disclosure, the current density during power-on is 10 to 100 A / m 2 .

[0040] In some embodiments of the present disclosure, the first cut-off voltage and the second cut-off voltage are both 0.1-0.6V.

[0041] Compared with the prior art, the advantages of the present invention include:

[0042] The lithium-based montmorillonite coated LiFePO4 core-shell material provided by the present invention is obtained by coating the lithium-based montmorillonite on the surface of the LiFePO4 lithium ion sieve, and the product is a montmorillonite core-shell structure. The resistivity of the aqueous slurry prepared by the synthesized core-shell structure material is significantly reduced. The lithium-based montmorillonite coated LiFePO4 core-shell material disclosed in the present invention is based on the principle that the interlayer ions in the montmorillonite layered structure are very easy to undergo mass-to-mass substitution. After the lithium-based montmorillonite is coated, it can produce a strong Li + Conductivity and excellent hydrophilicity significantly reduce the solid-liquid interface Li +Therefore, the lithium-based montmorillonite coated LiFePO4 core-shell material prepared by the present invention has a higher ion conductivity (reduced overall resistance) and can make up for the LiFePO4-water interface Li + Defects with large conduction resistance can promote Li + The conduction between the aqueous phase and the LiFePO4 grains, i.e., the overall resistivity of the slurry, is reduced, and the operating voltage is lowered, thereby further reducing the energy consumption of electrochemical lithium extraction from salt lakes. The lithium-based montmorillonite-coated LiFePO4 core-shell material prepared in this disclosure can be widely used in lithium extraction from salt lake brines. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0044] FIG1 is a schematic diagram of an electrochemical lithium extraction device according to the present disclosure;

[0045] FIG2 is an SEM image of a lithium-based montmorillonite-coated LiFePO4 core-shell material provided in Example 1 of the present disclosure;

[0046] FIG3 is a voltage-specific capacity curve diagram of the electrochemical devices assembled in Example 1 and Comparative Example 1 of the present disclosure. DETAILED DESCRIPTION

[0047] The embodiments of the present disclosure will be described in detail below with reference to the examples. However, those skilled in the art will appreciate that the following examples are intended only to illustrate the present disclosure and should not be construed as limiting the scope of the present disclosure. Where specific conditions are not specified in the examples, the experiments were performed under conventional conditions or the conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, all are commercially available conventional products.

[0048] The endpoints of the ranges and any values ​​disclosed in this disclosure are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0049] The present invention provides a lithium-montmorillonite-coated LiFePO4 core-shell material, which includes LiFePO4 as a core layer and lithium-montmorillonite as a shell layer, wherein the lithium-montmorillonite is coated on the surface of the LiFePO4, wherein the mass ratio of LiFePO4 to lithium-montmorillonite is 1:0.1% to 1%.

[0050] The present disclosure provides a method for preparing a lithium-based montmorillonite-coated LiFePO4 core-shell material, which comprises the following steps:

[0051] S1. Prepare lithium montmorillonite.

[0052] The montmorillonite is mixed with the first lithium solution and infiltrated at a temperature of 70-100° C. for more than 12 hours. The solid powder is then collected by filtration and dried at a temperature of 80-120° C. for 3-12 hours to obtain the product.

[0053] In the disclosure, the lithium ion concentration in the first lithium solution is greater than 1g / L, otherwise lithium montmorillonite cannot be obtained. The lithium ion concentration in the first lithium solution can be, for example, 1g / L, 3g / L, 4g / L, 6g / L, 9g / L or 12g / L, or any range between two of them. In the disclosure, the slurry should be kept at a temperature of 70-100°C during the stirring and infiltration process, for example, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C, or any range between two of them. If the infiltration temperature in the disclosure is too low, fully lithiated lithium montmorillonite cannot be obtained. If it is too high, the reaction is not easy to proceed and the lithiation enhancement effect is not significant. The infiltration time should be greater than 12h. If the time is insufficient, the montmorillonite cannot be fully lithiated. The infiltration time can be, for example, 12h, 14h, 15h, 16h, 18h or 24h, or any range between two of them.

[0054] In the present disclosure, the drying temperature should be between 80 and 120°C, for example, it can be any one of 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C or 120°C, or a range value between any two of them. If the drying temperature in the present disclosure is too low, sufficient drying cannot be achieved, and if the temperature is too high, the structure of the lithium montmorillonite is easily destroyed. The drying time should be between 3 and 12 hours, for example, it can be any one of 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h or 12h, or a range value between any two of them. If the drying time in the present disclosure is too short, sufficient drying cannot be achieved, and if the time is too long, the structure of the lithium montmorillonite is easily destroyed.

[0055] S2. Mix LiFePO4 and lithium montmorillonite, add an alcohol solvent, perform a gel reaction, and dry and grind the obtained gel.

[0056] In the present disclosure, the mass ratio of LiFePO4 to lithium montmorillonite is 1:0.1% to 1%. In some typical but non-limiting embodiments, the mass ratio of LiFePO4 to lithium montmorillonite can be, for example, 1:0.1%, 1:0.2%, 1:0.3%, 1:0.4%, 1:0.5%, 1:0.6%, 1:0.7%, 1:0.8%, 1:0.9% or 1:1%, or any range between two of them, and the amount of the alcohol solvent added accounts for 30% to 300% of the mass of the LiFePO4.

[0057] Wherein, the alcohol solvent includes any one of ethanol, ethylene glycol, polyethylene glycol, propanol or methanol or a combination of at least two thereof. The reaction temperature of the gel reaction is 70-100°C, for example, it can be any one of 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C or a range value between any two thereof. If the reaction temperature of the gel reaction in the present disclosure is too low, sufficient coating reaction cannot be achieved, and if it is too high, uneven coating layer will be caused. The drying temperature for drying the gel should be between 80-120°C, for example, it can be any one of 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C or 120°C or a range value between any two thereof. The range value between any two, the drying time in the present disclosure is too low to be fully dried, and too high may damage the material structure; the drying time is 1-18h, for example, it can be any one of 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h or 18h or the range value between any two, the drying time in the present disclosure is too short to be fully dried, and too long to easily damage the structure of the lithium-based montmorillonite-coated LiFePO4 core-shell material.

[0058] The present invention discloses a product having a montmorillonite core-shell structure obtained by coating the lithium-based montmorillonite on the surface of the LiFePO4 lithium ion sieve. The resistivity of the aqueous slurry prepared by the synthesized core-shell structure material is significantly reduced.

[0059] The lithium-based montmorillonite coated LiFePO4 core-shell material disclosed in the present invention is based on the principle that the interlayer ions in the montmorillonite layered structure are very easy to undergo mass-to-mass substitution. After coating the lithium-based montmorillonite, it can produce a strong Li + Conductivity and excellent hydrophilicity significantly reduce the solid-liquid interface Li + Insertion and extraction ion conduction resistance, therefore, the lithium-based montmorillonite prepared by the present disclosure coated LiFePO4 core-shell material can promote Li + The conduction between the aqueous phase and the LiFePO4 grains, that is, the overall resistivity of the slurry is reduced, the operating voltage is lower, and lower energy consumption can be achieved for lithium extraction from salt lake brine.

[0060] Since the extraction unit amount of Li + (Q is a constant value), the theoretical operating voltage should be as small as possible, where energy consumption W = UIt = QU = QIR, U represents the applied voltage (V), I represents the current (A), R is the resistance of the electrochemical working system (Ω), t is the working time (h), and Q is the target amount of extraction Li + When the LiFePO4 electrode works in an aqueous solution, under the condition that the resistance of LiFePO4 itself is fixed, the present disclosure can significantly reduce the Li + Insertion and extraction ion conduction resistance, thereby reducing the actual Li + The de- / embedding potential is increased, ultimately achieving the goal of reducing total energy consumption.

[0061] The lithium-based montmorillonite-coated LiFePO4 core-shell material prepared in the present invention can be widely used in lithium extraction from salt lake brine.

[0062] To achieve the effect of this technology, we first built an electrochemical lithium extraction device, as shown in Figure 1, and based on this, we conducted method verification, which specifically included the following steps:

[0063] The lithium-based montmorillonite-coated LiFePO4 core-shell material is used as a lithium ion sieve, recorded as M-LiFePO4, and a part of the lithium-based montmorillonite-coated LiFePO4 core-shell material is delithiated to prepare a lithium-poor ion sieve, recorded as M-FePO4.

[0064] The method for preparing M-FePO4 comprises: mixing M-LiFePO4 with lithium-containing liquid (lithium solution or brine) to prepare a slurry, placing the slurry in an anode chamber, and stirring the mixture; filling the cathode chamber with brine; inserting an anode plate and a cathode plate into the chamber respectively; and applying electricity in a constant current manner, wherein the current density during the application of electricity is 10 to 100 A / m 2 , until the cut-off voltage reaches 0.1~0.6V, the slurry is taken out from the anode chamber and the solid is collected by filtration to obtain M-FePO4.

[0065] In some typical but non-limiting embodiments, the current density when energized is 10 A / m 2 , 20A / m 2 、30A / m 2 , 40A / m 2 , 50A / m 2 , 60A / m 2 , 70A / m 2 , 80A / m 2 , 90A / m 2 or 100A / m 2The cut-off voltage may be, for example, any one of 0.1V, 0.2V, 0.3V, 0.4V, 0.5V or 0.6V or a range value between any two of them.

[0066] Its application in extracting lithium from salt lake brine includes the following two operations:

[0067] (1) M-LiFePO4 and M-FePO4 were added into the anode chamber and cathode chamber respectively for electrochemical lithium extraction.

[0068] (2) M-LiFePO4 and M-FePO4 are coated on the anode plate and cathode plate respectively for electrochemical lithium extraction.

[0069] Among them, scheme (1) includes the following operations:

[0070] M-LiFePO4 is mixed with a second lithium solution to form a slurry, which is then placed in the anode chamber. M-FePO4 is mixed with brine to form a slurry, which is then placed in the cathode chamber. The second lithium solution is primarily composed of LiCl, with a lithium concentration of <15g / L. The solid-to-liquid ratio of the M-LiFePO4 to the second lithium solution is 1:1-30, while the solid-to-liquid ratio of the M-FePO4 to the brine is 1:1-30. A too low solid-to-liquid ratio prevents sufficient contact between the material and the plate, while a too high ratio results in low current efficiency.

[0071] Insert an anode plate into the anode chamber and a cathode plate into the cathode chamber. The anode chamber and the cathode chamber are separated by an anion membrane. Voltage is applied to the anode plate and the cathode plate to perform electrochemical lithium extraction. The operation stops when the cut-off voltage (0.1-0.6V) is reached.

[0072] The anode chamber and the cathode chamber are stirred during the lithium extraction process. The stirring speed is 100-1000 r / min, for example, any one of 100 r / min, 200 r / min, 300 r / min, 400 r / min, 500 r / min, 600 r / min, 700 r / min, 800 r / min, 900 r / min or 1000 r / min, or a range between any two thereof.

[0073] In solution (2):

[0074] The coating thickness of M-LiFePO4 is 0.01 to 1 cm; the coating thickness of M-FePO4 is 0.01 to 1 cm.

[0075] The anode plate and cathode plate used in solution (1) or solution (2) can be universal. Specifically, the anode plate includes one of a graphite electrode, a platinum metal sheet, or a carbon fiber electrode; preferably, the anode plate is a graphite electrode. The cathode plate includes one of a graphite electrode, a platinum metal sheet, or a carbon fiber electrode; preferably, the cathode plate is a graphite electrode.

[0076] The lithium-based montmorillonite coated LiFePO4 core-shell material provided by the present invention has a high ion conductivity (reducing overall resistance) and can make up for the LiFePO4-water interface Li + The defect of large conduction resistance can significantly reduce the working voltage of electrochemical lithium extraction, thereby further reducing the energy consumption of electrochemical lithium extraction from salt lakes.

[0077] The features and performance of the present disclosure are further described in detail below with reference to the embodiments.

[0078] The concentrations of the main elements in the salt lake brine used in the examples and comparative examples provided in this disclosure are all referred to Table 1:

[0079] Table 1. Concentrations of major elements in salt lake brine

[0080] Example 1

[0081] Step S1, preparation of lithium montmorillonite: commercial montmorillonite is used as raw material, immersed in a lithium solution with a concentration of 3 g / L, kept at 80°C with stirring for 16 hours, and then the resulting solid powder is collected by filtration and dried at 100°C for 6 hours to obtain lithium montmorillonite;

[0082] Step S2, preparing a lithium-montmorillonite-coated LiFePO4 core-shell material: using commercial LiFePO4 as a raw material, the lithium-montmorillonite obtained in step 1 was mixed with it at a mass ratio of 0.2%, and ethanol was added at 30% of the mass of LiFePO4, and a sol-gel reaction was carried out at 100°C. After the above gel was evaporated to dryness, it was dried at 80°C for 18 hours and ground evenly to obtain a lithium-montmorillonite-coated LiFePO4 core-shell material (abbreviated as M-LiFePO4) as shown in Figure 2;

[0083] Step S3, preparation of lithium-deficient ion sieve material: M-LiFePO4 prepared in step S2 and lithium chloride solution with a lithium concentration of 1g / L were mixed in a liquid-solid ratio of 2:1 to prepare a slurry, which was placed in the anode chamber and stirred at 100r / min. Brine was filled in the cathode chamber, and graphite electrodes were inserted into the chamber as anode plates and cathode plates respectively. The mixture was stirred at 20A / m 2 The slurry was removed from the anode chamber and filtered to collect the solid, which was named as lithium-poor ion sieve material (M-FePO4);

[0084] Step S4, electrochemical lithium extraction: M-LiFePO4 in step S2 and lithium chloride solution with a lithium concentration of 5g / L are mixed in a liquid-solid ratio of 5:1 to prepare a slurry and placed in the anode chamber. M-FePO4 in step S3 and brine are mixed in a ratio of 5:1 to prepare a slurry and placed in the cathode chamber. Graphite electrodes and graphite electrodes are inserted into the chamber as anode plates and cathode plates respectively. Both chambers are stirred at 100r / min and separated by an anion membrane. External power is connected to the cathode and anode at 20A / m 2 The electrochemical lithium extraction process stops when the cut-off voltage reaches 0.3V. + It is extracted by M-FePO4 and released in the anode chamber. The anode chamber solution is filtered and collected to obtain lithium-rich solution.

[0085] Example 2

[0086] Step S1, preparation of lithium montmorillonite: commercial montmorillonite is used as raw material, immersed in a lithium solution with a concentration of 6 g / L, kept at 70°C with stirring for 14 hours, and then the resulting solid powder is collected by filtration and dried at 80°C for 12 hours to obtain lithium montmorillonite;

[0087] Step S2, preparing a lithium-montmorillonite-coated LiFePO4 core-shell material: using commercial LiFePO4 as a raw material, the lithium-montmorillonite obtained in step 1 was mixed with it at a mass ratio of 0.1%, and polyethylene glycol was added at 300% of the mass of LiFePO4 to carry out a sol-gel reaction at 80°C. After the above-mentioned gel was evaporated to dryness, it was dried at 120°C for 1h and ground uniformly to obtain a lithium-montmorillonite-coated LiFePO4 core-shell material (referred to as M-LiFePO4);

[0088] Step S3, preparation of lithium-deficient ion sieve material: M-LiFePO4 prepared in step S2 and lithium chloride solution with a lithium concentration of 2g / L were mixed in a liquid-solid ratio of 6:1 to prepare a slurry, which was placed in the anode chamber and stirred at 1000r / min. Brine was filled in the cathode chamber, and platinum metal sheets and platinum metal sheets were inserted into the chamber as anode plates and cathode plates, respectively. The mixture was stirred at 10A / m 2 The slurry was removed from the anode chamber and filtered to collect the solid, which was named as the lithium-poor ion sieve material (M-FePO4);

[0089] Step S4, electrochemical lithium extraction: M-LiFePO4 in step S2 and lithium chloride solution with a lithium concentration of 1g / L are mixed in a liquid-solid ratio of 5:1 to prepare a slurry and placed in the anode chamber. M-FePO4 in step S3 and brine are mixed in a ratio of 5:1 to prepare a slurry and placed in the cathode chamber. Graphite electrodes and platinum metal sheets are inserted into the chamber as anode plates and cathode plates respectively. Both chambers are stirred at 400r / min and separated by an anion membrane. External power is connected to the cathode and anode at a pressure of 10A / m 2 The electrochemical lithium extraction process stops when the cut-off voltage reaches 0.6V. + It is extracted by M-FePO4 and released in the anode chamber. The anode chamber solution is filtered and collected to obtain lithium-rich solution.

[0090] Example 3

[0091] Step S1, preparation of lithium-based montmorillonite: commercial montmorillonite is used as a raw material, immersed in a lithium solution with a concentration of 1 g / L, kept at 100°C with stirring for 24 hours, and then the resulting solid powder is collected by filtration and dried at 80°C for 9 hours to obtain lithium-based montmorillonite;

[0092] Step S2, preparing a lithium-montmorillonite-coated LiFePO4 core-shell material: using commercial LiFePO4 as a raw material, the lithium-montmorillonite obtained in step 1 was mixed with it at a mass ratio of 0.6%, and ethylene glycol was added at 200% of the mass of LiFePO4 to carry out a sol-gel reaction at 70°C. After the above-mentioned gel was evaporated to dryness, it was dried at 100°C for 6 hours and ground uniformly to obtain a lithium-montmorillonite-coated LiFePO4 core-shell material (referred to as M-LiFePO4);

[0093] Step S3, preparation of lithium-poor ion sieve material: M-LiFePO4 prepared in step S2 was mixed with lithium chloride solution with a lithium concentration of 6 g / L at a liquid-solid ratio of 10:1 to prepare a slurry, which was placed in the anode chamber and stirred at 200 r / min. Brine was filled in the cathode chamber, and carbon fiber electrode sheets and carbon fiber electrode sheets were inserted into the chamber as anode plates and cathode plates, respectively. The mixture was stirred at 100 A / m 2 The slurry was removed from the anode chamber and filtered to collect the solid, which was named as lithium-poor ion sieve material (M-FePO4);

[0094] Step S4, electrochemical lithium extraction: M-LiFePO4 in step S2 and lithium chloride solution with a lithium concentration of 3g / L are mixed in a liquid-solid ratio of 20:1 to prepare a slurry and placed in the anode chamber. M-FePO4 in step S3 and brine are mixed in a ratio of 20:1 to prepare a slurry and placed in the cathode chamber. Carbon fiber electrode sheets and carbon fiber electrode sheets are inserted into the chamber as anode plates and cathode plates respectively. Both chambers are stirred at 600r / min and separated by an anion membrane. External power is connected to the cathode and anode at 100A / m 2 The electrochemical lithium extraction process stops when the cut-off voltage reaches 0.1V. + It is extracted by M-FePO4 and released in the anode chamber. The anode chamber solution is filtered and collected to obtain lithium-rich solution.

[0095] Example 4

[0096] Step S1, preparation of lithium montmorillonite: commercial montmorillonite is used as raw material, immersed in a lithium solution with a concentration of 12 g / L, kept at 90°C with stirring for 12 hours, and then the resulting solid powder is collected by filtration and dried at 90°C for 9 hours to obtain lithium montmorillonite;

[0097] Step S2, preparing a lithium-montmorillonite-coated LiFePO4 core-shell material: using commercial LiFePO4 as a raw material, the lithium-montmorillonite obtained in step 1 was mixed with it at a mass ratio of 0.6%, and propanol was added at 150% of the mass of LiFePO4 to carry out a sol-gel reaction at 100°C. After the above gel was evaporated to dryness, it was dried at 90°C for 9 hours and ground evenly to obtain a lithium-montmorillonite-coated LiFePO4 core-shell material (referred to as M-LiFePO4);

[0098] Step S3, preparation of lithium-poor ion sieve material: M-LiFePO4 prepared in step S2 and lithium chloride solution with a lithium concentration of 9g / L were mixed in a liquid-solid ratio of 12:1 to prepare a slurry, which was placed in the anode chamber and stirred at 100r / min. Brine was filled in the cathode chamber, and graphite electrodes were inserted into the chamber as anode plates and cathode plates respectively. The mixture was stirred at 60A / m 2 The slurry was removed from the anode chamber and filtered to collect the solid, which was named as lithium-poor ion sieve material (M-FePO4);

[0099] Step S4, electrochemical lithium extraction: M-LiFePO4 in step S2 and lithium chloride solution with a lithium concentration of 10g / L are mixed in a liquid-solid ratio of 10:1 to prepare a slurry and placed in the anode chamber. M-FePO4 in step S3 and brine are mixed in a ratio of 10:1 to prepare a slurry and placed in the cathode chamber. Graphite electrodes and graphite electrodes are inserted into the chambers as anode plates and cathode plates respectively. Both chambers are stirred at 900r / min and separated by an anion membrane. External power is connected to the cathode and anode at 30A / m2 The electrochemical lithium extraction process stops when the cut-off voltage reaches 0.6V. + It is extracted by M-FePO4 and released in the anode chamber. The anode chamber solution is filtered and collected to obtain lithium-rich solution.

[0100] Example 5

[0101] Step S1: taking commercial montmorillonite as a raw material, immersing it in a lithium solution with a concentration of 4 g / L, keeping it at 90°C with stirring for 18 hours, then collecting the resulting solid powder by filtration, and drying it at 120°C for 3 hours to obtain lithium-based montmorillonite;

[0102] Step S2, preparing a lithium-montmorillonite-coated LiFePO4 core-shell material: using commercial LiFePO4 as a raw material, the lithium-montmorillonite obtained in step 1 was mixed with it at a mass ratio of 1%, and methanol was added at 250% of the mass of LiFePO4 to carry out a sol-gel reaction at 90°C. After the above gel was evaporated to dryness, it was dried at 110°C for 3h and ground uniformly to obtain a lithium-montmorillonite-coated LiFePO4 core-shell material (referred to as M-LiFePO4);

[0103] Step S3, preparation of lithium-poor ion sieve material: M-LiFePO4 prepared in step S2 was mixed with lithium chloride solution with a lithium concentration of 12 g / L at a liquid-solid ratio of 16:1 to prepare a slurry, which was placed in the anode chamber and stirred at 300 r / min. Brine was filled in the cathode chamber, and graphite electrodes were inserted into the chamber as anode plates and cathode plates, respectively. The mixture was stirred at 30 A / m. 2 The slurry was removed from the anode chamber and filtered to collect the solid, which was named as the lithium-poor ion sieve material (M-FePO4);

[0104] Step S4, electrochemical lithium extraction: M-LiFePO4 in step S2 and lithium chloride solution with a lithium concentration of 12g / L are mixed in a liquid-solid ratio of 2:1 to prepare a slurry and placed in the anode chamber. M-FePO4 in step S3 is mixed in a slurry of 2:1 with brine and placed in the cathode chamber. Graphite electrodes are inserted into the chambers as anode plates and cathode plates respectively. Both chambers are stirred at 1000r / min and separated by an anion membrane. External power is connected to the cathode and anode at 100A / m 2 The electrochemical lithium extraction process stops when the cut-off voltage reaches 0.6V. + It is extracted by M-FePO4 and released in the anode chamber. The anode chamber solution is filtered and collected to obtain lithium-rich solution.

[0105] Example 6

[0106] Step S1: taking commercial montmorillonite as a raw material, immersing it in a lithium solution with a concentration of 9 g / L, keeping it at 100° C. and stirring it for 15 hours, then collecting the resulting solid powder by filtration, and drying it at 120° C. for 12 hours to obtain lithium-based montmorillonite;

[0107] Step S2, preparing a lithium-montmorillonite-coated LiFePO4 core-shell material: using commercial LiFePO4 as a raw material, the lithium-montmorillonite obtained in step 1 was mixed with it at a mass ratio of 0.3%, and ethanol was added at 180% of the mass of LiFePO4 to carry out a sol-gel reaction at 85°C. After the above gel was evaporated to dryness, it was dried at 90°C for 12 hours and ground evenly to obtain a lithium-montmorillonite-coated LiFePO4 core-shell material (referred to as M-LiFePO4);

[0108] Step S3, preparation of lithium-deficient ion sieve material: M-LiFePO4 prepared in step S2 and lithium chloride solution with a lithium concentration of 15g / L were mixed at a liquid-solid ratio of 20:1 to prepare a slurry, which was placed in the anode chamber and stirred at 500r / min. Brine was filled in the cathode chamber, and platinum metal sheets and platinum metal sheets were inserted into the chamber as anode plates and cathode plates, respectively. The mixture was stirred at 50A / m 2 The slurry was removed from the anode chamber and filtered to collect the solid, which was named as lithium-poor ion sieve material (M-FePO4);

[0109] Step S4, electrochemical lithium extraction: M-LiFePO4 in step S2 and lithium chloride solution with a lithium concentration of 15g / L are mixed in a liquid-solid ratio of 9:1 to prepare a slurry and placed in the anode chamber. M-FePO4 in step S3 and brine are mixed in a ratio of 9:1 to prepare a slurry and placed in the cathode chamber. Platinum metal sheets and platinum metal sheets are inserted into the chamber as anode plates and cathode plates respectively. Both chambers are stirred at 300r / min and separated by an anion membrane. External power is connected to the cathode and anode at 60A / m 2 The electrochemical lithium extraction process stops when the cut-off voltage reaches 0.1V. + It is extracted by M-FePO4 and released in the anode chamber. The anode chamber solution is filtered and collected to obtain lithium-rich solution.

[0110] Example 7

[0111] Steps S1-S3 are the same as those in Example 1.

[0112] Step S4, electrochemical lithium extraction: The M-LiFePO4 in step S2 is coated on the anode plate with a coating thickness of 0.1 cm, and the M-FePO4 in step S3 is coated on the cathode plate with a coating thickness of 0.1 cm. The anode plate coated with M-LiFePO4 and the cathode plate coated with M-FePO4 are inserted into the chamber, and the lithium solution is introduced into the anode chamber and the brine is introduced into the cathode chamber. Both chambers are stirred at 100 r / min, separated by an anion membrane, and the cathode and anode are connected to an external power supply at 20 A / m 2 The electrochemical lithium extraction process stops when the cut-off voltage reaches 0.3V. + It is extracted by M-FePO4 and released in the anode chamber. The anode chamber solution is filtered and collected to obtain lithium-rich solution.

[0113] Comparative Example 1

[0114] The difference from Example 1 is that step S1 and step S2 are not performed, and commercial LiFePO4 material is directly used to prepare the lithium-deficient ion sieve, and then electrochemical lithium extraction is performed.

[0115] Comparative Example 2

[0116] The difference from Example 1 is that in step S1, the commercial montmorillonite material is not subjected to lithiation treatment, and the LiFePO4 material is directly coated with the commercial montmorillonite, and then electrochemical lithium extraction is performed.

[0117] Comparative Example 3

[0118] The difference from Example 1 is that the infiltration temperature in step S1 is too high, and the infiltration is stirred at 120° C. for 16 hours.

[0119] Comparative Example 4

[0120] The difference from Example 1 is that the drying temperature in step S1 is too high, and the drying is performed at 150° C. for 6 h.

[0121] Comparative Example 5

[0122] The difference from Example 1 is that the mass ratio of lithium-based montmorillonite to LiFePO4 material used in step S2 is too small, and they are mixed at 0.01%, and then other steps are performed to electrochemically extract lithium.

[0123] Comparative Example 6

[0124] The difference from Example 1 is that the mass ratio of lithium-based montmorillonite to LiFePO4 material used in step S2 is too large, and they are mixed at 2%, and then other steps are performed to electrochemically extract lithium.

[0125] Comparative Example 7

[0126] The difference from Example 1 is that the ethanol used in step S2 is replaced by pure water, and then other steps are performed to electrochemically extract lithium.

[0127] Comparative Example 8

[0128] The difference from Example 1 is that step S1 is not performed, and the lithium montmorillonite used in step S2 is replaced by lepidolite mineral.

[0129] Comparative Example 9

[0130] The difference from Example 1 is that the LiFePO4 used in step S2 is replaced by LiCoO2.

[0131] Experimental Example 1

[0132] The slurry conductivity of the lithium ion sieves provided in Examples 1-7 and Comparative Examples 1-9 was measured using a slurry resistance test system. The test results are shown in Table 2.

[0133] Table 2. Statistical table of slurry conductivity of lithium ion sieves in various examples during slurry preparation

[0134] As can be seen from the above table, in Comparative Example 1, LiFePO4 material is directly used to prepare the lithium-poor ion sieve without coating it, and its slurry conductivity is significantly lower than that of Example 1 and other comparative examples. In Comparative Example 2, montmorillonite is directly used to coat the LiFePO4 material. At this time, since montmorillonite is not coated with lithium, its Li + The conductivity and hydrophilicity are significantly reduced, so although the conductivity of the slurry prepared therefrom is slightly higher than that of Comparative Example 1, it is still significantly lower than that of Example 1. As can be seen from the data of Comparative Examples 3-6, when parameters such as the mass ratio of infiltration, drying, and coating exceed the range of the present disclosure, the conductivity of the slurry will be significantly reduced. As can be seen from the data of Comparative Example 7, when pure water is used instead of ethanol, under the same conditions, pure water is not easy to volatilize, resulting in excessive water content in the gel, which is not conducive to subsequent drying, and thus the conductivity of Comparative Example 7 is significantly lower than that of Example 1. This fully proves that not all solvents can be used to mix LiFePO4 and lithium-based montmorillonite. As can be seen from the data of Comparative Example 8, when other lithium-containing minerals are selected, the conductivity is still significantly lower than that of Example 1, because hectorite has a higher ionic conductivity. As can be seen from the data of Comparative Example 9, when other positive electrode materials are selected as the core, the conductivity of the slurry is poor, mainly due to the high on-start voltage of LiCoO2, which in turn causes its conductivity to be still significantly lower than that of Example 1. Moreover, the dissolution rate of lithium cobalt oxide is high, and it may reach more than 30% after power is applied. 2+ He Li +Therefore, the electrical conductivity of the slurry prepared by the lithium-based montmorillonite-coated LiFePO4 core-shell material provided in the embodiment of the present disclosure is significantly greater than that of the LiFePO4 slurry in other comparative examples.

[0135] Experimental Example 2

[0136] The charge and discharge intermediate voltages and the power consumption required to extract 1 kg of lithium during electrochemical lithium extraction of the lithium ion sieves provided in Examples 1-7 and Comparative Examples 1-9 were tested. The test results are shown in Figure 3 and Table 3:

[0137] Table 3. Statistics of charge and discharge medium voltage, specific capacity, and power consumption required to extract 1kg of lithium for each example

[0138] As can be seen from the above table, to achieve the same level of specific capacity (~80mAh / g), the examples all have a lower charge and discharge medium voltage than the comparative example, wherein the examples are optimized for lithium-based montmorillonite coating, while the comparative example is not coated or coated with other materials. Therefore, the lithium-based montmorillonite-coated LiFePO4 core-shell material has lower energy consumption for extracting lithium per unit mass.

[0139] In summary, the lithium-based montmorillonite coated LiFePO4 core-shell material provided by the present disclosure is a product of montmorillonite core-shell structure obtained by coating the lithium-based montmorillonite on the surface of LiFePO4 lithium ion sieve. The resistivity of the aqueous slurry prepared by the synthesized core-shell structure material is significantly reduced. The lithium-based montmorillonite coated LiFePO4 core-shell material disclosed in the present disclosure is based on the principle that interlayer ions in the montmorillonite layered structure are very easy to undergo mass-to-mass substitution. After the lithium-based montmorillonite is coated, LiFePO4 can produce a strong Li + Conductivity and excellent hydrophilicity significantly reduce the solid-liquid interface Li + Therefore, the lithium-based montmorillonite coated LiFePO4 core-shell material prepared by the present invention has a higher ion conductivity (reduced overall resistance) and can make up for the LiFePO4-water interface Li + Defects with large conduction resistance can promote Li + The conduction between the aqueous phase and the LiFePO4 grains, i.e., the overall resistivity of the slurry, is reduced, and the operating voltage is lowered, thereby further reducing the energy consumption of electrochemical lithium extraction from salt lakes. The lithium-based montmorillonite-coated LiFePO4 core-shell material prepared in this disclosure can be widely used in lithium extraction from salt lake brines.

[0140] The above describes in detail the optional embodiments of the present disclosure, but the present disclosure is not limited thereto. Within the technical concept of the present disclosure, various simple variations of the technical solution of the present disclosure can be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as the contents disclosed by the present disclosure and fall within the scope of protection of the present disclosure. Industrial Applicability

[0141] The lithium-based montmorillonite coated LiFePO4 core-shell material provided by the present invention is obtained by coating the lithium-based montmorillonite on the surface of the LiFePO4 lithium ion sieve, and the product is a montmorillonite core-shell structure. The resistivity of the aqueous slurry prepared by the synthesized core-shell structure material is significantly reduced. The lithium-based montmorillonite coated LiFePO4 core-shell material disclosed in the present invention is based on the principle that the interlayer ions in the montmorillonite layered structure are very easy to undergo mass-to-mass substitution. After the lithium-based montmorillonite is coated, it can produce a strong Li + Conductivity and excellent hydrophilicity significantly reduce the solid-liquid interface Li + Therefore, the lithium-based montmorillonite coated LiFePO4 core-shell material prepared by the present invention has a higher ion conductivity (reduced overall resistance) and can make up for the LiFePO4-water interface Li + Defects with large conduction resistance can promote Li + The conduction between the aqueous phase and the LiFePO4 grains, i.e., the overall resistivity of the slurry, is reduced, and the operating voltage is lowered, thereby further reducing the energy consumption of electrochemical lithium extraction from salt lakes. The lithium-based montmorillonite-coated LiFePO4 core-shell material prepared in this disclosure can be widely used in lithium extraction from salt lake brines.

Claims

1. A lithium-based montmorillonite-coated LiFePO 4 core-shell material, It is characterized in that It includes LiFePO as the core layer 4 and lithium-based montmorillonite as the shell layer, and the lithium-based montmorillonite coats the surface of the LiFePO 4 .

2. The lithium-based montmorillonite-coated LiFePO 4 core-shell material, It is characterized in that The LiFePO 4 and the lithium-based montmorillonite have a mass ratio of 1: 0.1% to 1%.

3. A preparation method of a lithium-based montmorillonite-coated LiFePO 4 core-shell material It is characterized in that It includes mixing LiFePO 4 with lithium-based montmorillonite, adding an alcohol solvent, carrying out a gel reaction, drying and grinding the obtained gel to obtain the product.

4. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in claim 3 It is characterized in that The LiFePO 4 and the mass ratio of the lithium-based montmorillonite is 1: 0.1% to 1%.

5. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in any one of claims 3-4 It is characterized in that The addition amount of the alcohol solvent accounts for 30%-300% of the mass of the LiFePO 4 4.

6. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in any one of claims 3 to 5 It is characterized in that The alcohol solvent includes any one or a combination of at least two of ethanol, ethylene glycol, polyethylene glycol, propanol or methanol.

7. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in any one of claims 3-6 It is characterized in that The temperature of the gel reaction is 70-100 °C.

8. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in any one of claims 3-7 It is characterized in that The temperature for drying the gel is 80-120 °C, and the drying time is 1-18 h.

9. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in any one of claims 3-8 It is characterized in that The preparation method of the lithium-based montmorillonite includes: mixing and soaking montmorillonite with a first lithium solution, then filtering and collecting the solid powder, and drying to obtain it.

10. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in claim 9 It is characterized in that The lithium ion concentration in the first lithium solution is greater than 1 g / L.

11. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in any one of claims 9-10 It is characterized in that The temperature of the soaking is 70-100 °C, and the soaking time is greater than 12 h.

12. The preparation method of the core-shell material of lithium-based montmorillonite-coated LiFePO 4 as claimed in any one of claims 9-11 It is characterized in that The temperature for drying the solid powder is 80-120 °C, and the drying time is 3-12 h.

13. The lithium-based montmorillonite-coated LiFePO as described in any one of claims 1-2 4 core-shell material or the lithium-based montmorillonite-coated LiFePO as described in any one of claims 3-12 4 core-shell material is prepared to obtain the lithium-based montmorillonite-coated LiFePO 4 Application of the core-shell material in extracting lithium from salt lake brine.

14. A method for extracting lithium from salt lake brine It is characterized in that It includes: The lithium-based montmorillonite-coated LiFePO as described in any one of claims 1-2 4 core-shell material or the lithium-based montmorillonite-coated LiFePO as described in any one of claims 3-12 4 core-shell material is prepared to obtain the lithium-based montmorillonite-coated LiFePO 4 The core-shell material is used as a lithium ion sieve, denoted as M-LiFePO 4 , and a part of the lithium-based montmorillonite-coated LiFePO 4 core-shell material is delithiated to prepare a lithium-deficient ion sieve, denoted as M-FePO 4 ; Add the M-LiFePO 4 and the M-FePO 4 into the anode chamber and the cathode chamber respectively for electrochemical lithium extraction; Or Apply the M-LiFePO 4 and the M-FePO 4 onto the anode plate and the cathode plate respectively for electrochemical lithium extraction.

15. The method for extracting lithium from salt lake brine according to claim 14 It is characterized in that When adding the M-LiFePO 4 and the M-FePO 4 into the anode chamber and the cathode chamber respectively for electrochemical lithium extraction, the following operations are included: Mix the M-LiFePO 4 with a second lithium solution to make a slurry and place it in the anode chamber, and mix the M-FePO 4 with brine to make a slurry and place it in the cathode chamber; Insert an anode plate into the anode chamber, insert a cathode plate into the cathode chamber, separate the anode chamber and the cathode chamber with an anion membrane, apply a voltage to the anode plate and the cathode plate, and perform electrochemical lithium extraction, and stop working when reaching the first cut-off voltage.

16. The method for extracting lithium from salt lake brine according to claim 15 It is characterized in that The M-LiFePO 4 The solid-liquid ratio of the pulp made by combining with the second lithium solution is 1:1 to 30; the M-FePO 4 The solid-liquid ratio of the pulp made by mixing with the brine is 1:1 to 30.

17. The method for extracting lithium from salt lake brine according to any one of claims 15-16 It is characterized in that The anode chamber and the cathode chamber are kept stirred during the lithium extraction process.

18. The method for extracting lithium from salt lake brine according to claim 17 It is characterized in that The stirring speed is 100-1000 r / min.

19. The method for extracting lithium from salt lake brine according to any one of claims 15-18 It is characterized in that The main component of the second lithium solution is LiCl, and the lithium concentration is <15 g / L.

20. The method for extracting lithium from salt lake brine according to claim 14 It is characterized in that When the M-LiFePO 4 and the M-FePO 4 are respectively coated on the anode plate and the cathode plate for electrochemical lithium extraction, the coating thickness of the M-LiFePO 4 is 0.01 - 1 cm; the coating thickness of the M-FePO 4 is 0.01 - 1 cm.

21. The method for extracting lithium from salt lake brine according to any one of claims 14-20 It is characterized in that The anode plate includes one of a graphite electrode, a platinum metal sheet or a carbon fiber electrode.

22. The method for extracting lithium from salt lake brine according to any one of claims 14-21 It is characterized in that The anode plate is a graphite electrode.

23. The method for extracting lithium from salt lake brine according to any one of claims 14-22 It is characterized in that The cathode plate includes one of a graphite electrode, a platinum metal sheet or a carbon fiber electrode.

24. The method for extracting lithium from salt lake brine according to any one of claims 14-23 It is characterized in that The cathode plate is a graphite electrode.

25. The method for extracting lithium from salt lake brine according to any one of claims 15-19 It is characterized in that Method for preparing the M-FePO 4 comprises: mixing the M-LiFePO 4 with a lithium-containing liquid to make a slurry, placing the slurry in an anode chamber, maintaining stirring, filling brine in a cathode chamber, inserting an anode plate and a cathode plate into the chamber respectively, energizing in a constant current manner until a second cut-off voltage is reached, taking out the slurry from the anode chamber, filtering and collecting the solid to obtain the M-FePO 4 .

26. The method for extracting lithium from salt lake brine according to claim 25 It is characterized in that The lithium-containing liquid includes a lithium solution or brine.

27. The method for extracting lithium from salt lake brine according to any one of claims 25-26 characterized in that The current density during power-on is 10 to 100 A / m 2 .

28. The method for extracting lithium from salt lake brine according to any one of claims 25-27, characterized in that both the first cut-off voltage and the second cut-off voltage are 0.1 to 0.6V.

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