A lithium ion-doped sodium iron silicate composite material coated with biochar, a preparation method and application thereof

Abstract

The application provides a biochar-coated lithium ion-doped sodium silicate composite material and a preparation method and application thereof, and belongs to the technical field of sodium ion batteries. The sodium silicate is modified by a lithium ion doping method, so that the discharge specific capacity of the sodium silicate can be effectively improved. The lithium ion-doped sodium silicate material is carbon-coated by biochar, so that the cycle performance of the sodium silicate can be effectively improved. The lithium ion doping and carbon coating method can make the electrode material have the characteristics of high conductivity, high specific capacity and excellent cycle stability. Results show that the biochar-coated lithium ion-doped sodium silicate composite material prepared by the application has a capacity retention of 82% under a 0.05C current density in a 1.5-4.5V voltage window after 50 cycles, and the cycle performance is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a biochar-coated lithium-ion-doped sodium iron silicate composite material, its preparation method, and its application. Background Technology

[0002] With the increasing consumption of fossil fuels, a series of intractable environmental problems have emerged. Therefore, replacing fossil fuels with clean energy has become a crucial issue. Clean energy is mostly intermittent, and efficient energy storage devices are directly related to the storage and utilization of these energy sources. Among them, rechargeable batteries, as a feasible energy storage technology, can integrate renewable energy into the power grid.

[0003] Lithium-ion batteries have attracted widespread attention due to their portability and high energy density. However, the limited and uneven distribution of lithium reserves in the Earth's crust has led to a continuous rise in lithium salt prices. Sodium, with its similar electrochemical properties to lithium and abundant, low-cost sodium salt reserves, has made sodium-ion batteries increasingly popular. Among these components, the cathode material is a key factor determining the battery's energy density. Therefore, researching high-energy-density cathode materials is crucial for sodium-ion batteries.

[0004] Sodium iron silicate has attracted much attention in sodium-ion batteries due to its high theoretical specific capacity, but its poor conductivity seriously affects its rate performance and actual specific capacity, limiting its promotion in practical applications. Summary of the Invention

[0005] The purpose of this invention is to provide a biochar-coated lithium-ion-doped sodium iron silicate composite material, its preparation method and application, wherein the biochar-coated lithium-ion-doped sodium iron silicate composite material has excellent rate performance and high specific capacity.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing a biochar-coated lithium-ion-doped sodium iron silicate composite material, comprising the following steps:

[0008] Biomass raw materials are mixed with gas-generating aids and subjected to a first grinding process. The resulting ground material is then subjected to a first calcination process in a first protective gas and followed by acid washing to obtain biochar.

[0009] Sodium source, iron source, lithium source, acid accelerator, silicon source and organic solvent are mixed and sol-gelled to obtain the precursor;

[0010] The biochar is mixed with the precursor and then subjected to a second grinding. The resulting mixture is then subjected to a second calcination in a second protective gas to obtain a biochar-coated lithium-ion-doped sodium iron silicate composite material.

[0011] Preferably, the biomass raw material includes coffee grounds, leaves, cow dung, straw, or wood chips; the gas-generating aid includes calcium carbonate, magnesium carbonate, sodium carbonate, sodium bicarbonate, zinc carbonate, or copper carbonate; the mass ratio of the biomass raw material to the gas-generating aid is 1:0.5 to 1.5; and the first grinding time is 0.5 to 1.5 hours.

[0012] Preferably, the first protective gas includes argon or nitrogen; the first calcination temperature is 750–950°C, and the time is 30–90 min.

[0013] Preferably, the sodium source includes at least one of anhydrous sodium acetate, sodium hydroxide, sodium carbonate, sodium oxalate, sodium nitrite, sodium dihydrogen phosphate, sodium bicarbonate, sodium bisulfate, sodium hypophosphite, sodium citrate, anhydrous sodium sulfate, potassium sodium tartrate tetrahydrate, sodium alginate, disodium hydrogen phosphate dodecahydrate, sodium acetate trihydrate, sodium bisulfite, and sodium nitrate.

[0014] The iron source includes at least one of ferrous oxalate and ferrous nitrate;

[0015] The silicon source includes at least one of silicon dioxide, tetraethyl orthosilicate, and methyl silicate.

[0016] The lithium source includes at least one of lithium carbonate, lithium acetate dihydrate, and anhydrous lithium chloride.

[0017] Preferably, the molar ratio of the sodium source, iron source, lithium source and silicon source is 2:(0.9~0.99):(0.01~0.1):1;

[0018] The acid accelerator includes citric acid, lactic acid, malic acid, tartaric acid, or ascorbic acid; the molar ratio of the acid anion in the acid accelerator to the iron element in the iron source is 1:0.9 to 1.1.

[0019] Preferably, the sol-gelation temperature is 80–100°C and the time is 24–36 h.

[0020] Preferably, the mass ratio of the precursor to biochar is 1:0.01 to 0.1; and the second grinding time is 15 to 45 minutes.

[0021] Preferably, the second protective gas includes argon or nitrogen; the second calcination temperature is 500–700°C, and the time is 5–8 hours.

[0022] The present invention provides a biochar-coated lithium-ion-doped sodium iron silicate composite material prepared by the preparation method described in the above technical solution.

[0023] This invention provides the application of the biochar-coated lithium-ion-doped sodium iron silicate composite material described above in sodium-ion batteries.

[0024] This invention provides a method for preparing a biochar-coated lithium-ion-doped sodium iron silicate composite material. Modifying sodium iron silicate through lithium-ion doping effectively improves its discharge specific capacity. The lithium-ion-doped sodium iron silicate material is carbon-coated using biochar raw materials. The N and S elements generated during the biochar calcination process are utilized for doping. The S and N co-doped carbon has a high binding energy, and this strong bond interaction is beneficial to stabilizing the composite material, thereby effectively improving the cycling performance of sodium iron silicate. Therefore, this invention, by simultaneously employing lithium-ion doping and biochar coating, enables the electrode material to possess characteristics such as high conductivity, high voltage (discharge voltage around 4.0V), high specific capacity, and excellent cycling stability. Results show that the biochar-coated lithium-ion-doped sodium iron silicate composite material prepared by this invention retains 82% of its capacity after 50 cycles within a voltage window of 1.5–4.5V and a current density of 0.05C, demonstrating a significant improvement in cycling performance.

[0025] This invention utilizes coffee grounds to prepare biochar. The method is safe, pollution-free, green, economical, simple, highly controllable, and low-cost, making it suitable for large-scale promotion. Attached Figure Description

[0026] Figure 1 The rate performance of the electrode materials prepared in Comparative Example 1 and Example 1;

[0027] Figure 2 For long-cycle testing of the electrode materials prepared in Comparative Example 1 and Example 1;

[0028] Figure 3 Impedance diagrams of the electrode materials prepared for Comparative Example 1 and Example 1. Detailed Implementation

[0029] In this invention, unless otherwise specified, the raw materials or reagents required for preparation are all commercially available products well known to those skilled in the art.

[0030] This invention provides a method for preparing a biochar-coated lithium-ion-doped sodium iron silicate composite material, comprising the following steps:

[0031] Biomass raw materials are mixed with gas-generating aids and subjected to a first grinding process. The resulting ground material is then subjected to a first calcination process in a first protective gas and followed by acid washing to obtain biochar.

[0032] Sodium source, iron source, lithium source, acid accelerator, silicon source and organic solvent are mixed and sol-gelled to obtain the precursor;

[0033] The biochar is mixed with the precursor and then subjected to a second grinding. The resulting mixture is then subjected to a second calcination in a second protective gas to obtain a biochar-coated lithium-ion-doped sodium iron silicate composite material.

[0034] This invention involves mixing biomass raw materials with a gas-generating aid, performing a first grinding, calcining the resulting ground material in a first protective gas, and then acid washing to obtain biochar.

[0035] In this invention, the biomass raw materials preferably include coffee grounds, leaves, cow dung, straw, or wood chips.

[0036] The present invention does not have any particular limitation on the specific type and source of the biomass raw material, and any corresponding biomass raw material of a type known in the art can be used; in the embodiments of the present invention, it is specifically coffee grounds from IIAC gold medal beans.

[0037] In this invention, the biomass raw material is preferably washed, dried in an oven, and then transferred to a ball mill with a gas-generating aid for grinding. The drying temperature of the biomass raw material is preferably 120-140°C, more preferably 130°C, and the drying time is preferably 8-12 hours.

[0038] In this invention, the gas-generating aid preferably includes calcium carbonate, magnesium carbonate, sodium carbonate, sodium bicarbonate, zinc carbonate, or copper carbonate.

[0039] In this invention, the mass ratio of the biomass raw material to the gas-generating agent is preferably 1:0.5 to 1.5, more preferably 1:0.6 to 1.2, and even more preferably 1:1; this invention utilizes the gas generated by the high-temperature decomposition of the gas-generating agent to give the biochar a larger specific surface area.

[0040] In this invention, the first grinding time is preferably 0.5 to 1.5 hours, more preferably 1 hour.

[0041] In this invention, the first protective gas preferably includes argon or nitrogen; the first calcination temperature is preferably 750-950°C, more preferably 850-900°C, and the time is preferably 30-90 min, more preferably 60-80 min.

[0042] After the first calcination is completed, the present invention preferably acid washes the obtained material until the pH value is neutral, and then dries it to obtain biochar; the acid used for acid washing is preferably dilute hydrochloric acid, and the concentration of the dilute hydrochloric acid is preferably 1-3 mol / L, more preferably 2 mol / L; the drying temperature is preferably 130℃.

[0043] This invention involves mixing sodium source, iron source, lithium source, acid promoter, silicon source and organic solvent, and then performing sol-gelation to obtain a precursor.

[0044] In this invention, the sodium source preferably includes at least one of anhydrous sodium acetate, sodium hydroxide, sodium carbonate, sodium oxalate, sodium nitrite, sodium dihydrogen phosphate, sodium bicarbonate, sodium bisulfate, sodium hypophosphite, sodium citrate, anhydrous sodium sulfate, sodium potassium tartrate tetrahydrate, sodium alginate, disodium hydrogen phosphate dodecahydrate, sodium acetate trihydrate, sodium bisulfite, and sodium nitrate; more preferably, it is at least one of anhydrous sodium acetate, sodium acetate trihydrate, sodium hydroxide, sodium carbonate, and sodium bicarbonate; when the sodium source is two or more of the above, this invention does not have a special limitation on the ratio of different types of sodium sources, and any ratio is acceptable.

[0045] In this invention, the iron source preferably includes at least one of ferrous oxalate and ferrous nitrate; when the iron source is two or more of the above, this invention does not have a special limitation on the ratio of different types of iron sources, and any ratio is acceptable.

[0046] In this invention, the silicon source preferably includes at least one of silicon dioxide, tetraethyl orthosilicate, and methyl silicate; when the silicon source is two or more of the above, this invention does not have a special limitation on the ratio of different types of silicon sources, and any ratio is acceptable.

[0047] In this invention, the lithium source preferably includes at least one of lithium carbonate, lithium acetate dihydrate, and anhydrous lithium chloride; when the lithium source is two or more of the above, this invention does not have a special limitation on the ratio of different types of lithium sources, and any ratio is acceptable.

[0048] In this invention, the molar ratio of the sodium source, iron source, lithium source and silicon source is preferably 2:(0.9~0.99):(0.01~0.1):1, more preferably 2:(0.9~0.96):(0.02~0.06):1.

[0049] In this invention, the acid accelerator preferably includes citric acid, lactic acid, malic acid, tartaric acid, or ascorbic acid, more preferably citric acid; the molar ratio of the acid anion in the acid accelerator to the iron element in the iron source is 1:0.9–1.1, more preferably 1:0.96–1.0, and even more preferably 1:1. This invention utilizes the acid accelerator to accelerate the hydrolysis of the iron source, thereby accelerating the reaction rate.

[0050] In this invention, the organic solvent is preferably ethanol; the amount of the organic solvent used is not specifically limited, and can be adjusted according to actual needs to ensure uniform mixing of materials.

[0051] Preferably, in the present invention, a sodium source, an iron source, and a lithium source are mixed in an organic solvent, and the obtained mixed solution is subjected to first heating and stirring, and then a silicon source is added and second heating and stirring is continued; the obtained mixed solution is solubilized until a sol appears, and then transferred to an oven for drying to obtain a precursor; the temperature of the first heating and stirring is 40-60°C, more preferably 50-55°C, and the time is preferably 1-3 h, more preferably 2 h; the temperature of the second heating and stirring is preferably 40-60°C, more preferably 50-55°C, and the time is preferably 3-5 h, more preferably 3-4 h.

[0052] In the present invention, the temperature of the solubilization is preferably 80-100°C, more preferably 85-95°C, and further preferably 90°C; the time is preferably 24-36 h, more preferably 30 h.

[0053] In the present invention, the temperature of the drying is preferably 60°C.

[0054] After obtaining the biochar and the precursor, in the present invention, the biochar and the precursor are mixed and subjected to second grinding, and the obtained mixture is subjected to second calcination in a second protective gas to obtain a biochar-coated sodium iron lithium silicate composite.

[0055] In the present invention, the mass ratio of the precursor to the biochar is preferably 1:0.01-0.1, more preferably 1:0.01-0.07, and further preferably 1:0.03-0.05; the time of the second grinding is preferably 15-45 min, more preferably 30 min.

[0056] In the present invention, the second protective gas preferably includes argon or nitrogen; the temperature of the second calcination is preferably 500-700°C, more preferably 600-650°C, and the time is preferably 5-8 h, more preferably 6-7 h.

[0057] The present invention provides a biochar-coated sodium iron lithium silicate composite prepared by the preparation method described in the above technical solution.

[0058] The biochar-coated sodium iron lithium silicate composite prepared in the present invention is denoted as Na2Fe 1-x Li x SiO4@SC-y (0 < x < 0.1, 1 ≤ y ≤ 10), where x represents the molar number of Li element, and y is the numerical value corresponding to the mass percentage (%) of the biochar in Na2Fe 1-x Li x SiO4.

[0059] The present invention provides an application of the biochar-coated sodium iron lithium silicate composite described in the above technical solution in a sodium ion battery.

[0060] In this invention, the preferred method for applying the biochar-coated lithium-ion-doped sodium iron silicate composite material in a sodium-ion battery is to assemble the biochar-coated lithium-ion-doped sodium iron silicate composite material into a half-cell.

[0061] The biochar-coated lithium-ion-doped sodium iron silicate composite material, conductive agent, and binder were mixed, and N-methylpyrrolidone (NMP) was added and ground until no obvious particles were visible. The resulting slurry was uniformly coated on aluminum foil, transferred to a vacuum oven for drying, and then sliced.

[0062] Using biochar-coated lithium-ion-doped sodium iron silicate composite material as the positive electrode and sodium sheet as the negative electrode, commercial sodium salt electrolyte, glass fiber diaphragm, and CR2032 button battery casing, half-cell button battery assembly was carried out in a glove box.

[0063] In this invention, the conductive agent is preferably SuperP, acetylene black, graphene, and carbon nanotubes, with SuperP being the most preferred; the binder is preferably one or more of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR) emulsion, and carboxymethyl cellulose (CMC), with PVDF being the most preferred; the mass ratio of the biochar-coated lithium-ion-doped sodium iron silicate composite material, the conductive agent, and the binder is preferably 9-ab:0.5+a:0.5+b (0≤a≤2.5, 0≤b≤0.5), and more preferably 7:2:1.

[0064] In this invention, the sodium salt electrolyte is preferably NaPF6 (1.0 mol / L, solvent is EC and DMC in a volume ratio of 1:1, with an additional 5.0% FEC), NaPF6 (1.0 mol / L, solvent is EC and DEC in a volume ratio of 1:1), or NaClO4 (1.0 mol / L, solvent is EC and DEC in a volume ratio of 1:1, with an additional 5.0% FEC), and more preferably NaPF6 (1.0 mol / L, solvent is EC and DEC in a volume ratio of 1:1).

[0065] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0066] The coffee grounds used in the following examples are those from IIAC Gold Medal beans.

[0067] Example 1

[0068] A 3% by mass biochar-coated lithium-ion-doped sodium iron silicate composite material (Na2Fe) was used. 0.96 Li 0.04 Preparation of SiO4@SC-3):

[0069] S1. Mix 2.7216 g (0.02 mol) sodium acetate trihydrate, 1.7269 g (0.0096 mol) ferrous oxalate dihydrate, 0.0408 g (0.0004 mol) lithium acetate dihydrate and 0.6404 g (0.0033 mol) citric acid in ethanol, and stir the resulting mixture at 50 °C for 2 h.

[0070] S2. Add 2.0833 g (0.01 mol) of tetraethyl orthosilicate to the mixture and heat and stir at 50 °C for 3 h.

[0071] S3. Heat and stir the above mixture at 90°C for 30 hours until a sol appears, then transfer it to an oven and dry at 60°C to obtain the precursor.

[0072] S4. Wash the coffee grounds and place them in an oven at 130°C overnight.

[0073] S5. After drying the coffee grounds, weigh 5g of the dried material and 5g of calcium carbonate and transfer them to a ball mill. Mill for 1 hour.

[0074] S6. Transfer the ball-milled material to a tube furnace and calcine it at 850°C for 1 hour under argon protection.

[0075] S7. Wash the calcined material with 20 mL of 2M dilute hydrochloric acid until the pH value is neutral, then transfer it to an oven and dry at 130°C to obtain biochar.

[0076] S8. Take 3g of precursor and 0.09g of biochar and transfer them to a ball mill. Mill for 0.5h.

[0077] S9. The ball-milled precursor was transferred to a tube furnace and calcined at 600℃ for 6 hours under argon protection to obtain a biochar-coated lithium-ion-doped sodium iron silicate composite material, denoted as Na2Fe. 0.96 Li 0.04 SiO4@SC-3.

[0078] Example 2

[0079] The difference from Example 1 is that: 1% by weight of biochar is used to coat the electrode material.

[0080] S1 to S7 are the same as in Example 1;

[0081] S8. Take 3g of precursor and 0.03g of biochar and transfer them to a ball mill. Mill for 0.5h.

[0082] S9. The ball-milled precursor was transferred to a tube furnace and calcined at 600℃ for 6 hours under argon protection to obtain a biochar-coated lithium-ion-doped sodium iron silicate composite material, denoted as Na2Fe. 0.96Li 0.04 SiO4@SC-1.

[0083] Example 3

[0084] The difference from Example 1 is that: 5% by weight of biochar is used to coat the electrode material.

[0085] S1 to S7 are the same as in Example 1;

[0086] S8. Take 3g of precursor and 0.15g of biochar and transfer them to a ball mill. Mill for 0.5h.

[0087] S9. The ball-milled precursor was transferred to a tube furnace and calcined at 600°C for 6 hours under inert gas protection to obtain a biochar-coated lithium-ion-doped sodium iron silicate electrode material, denoted as Na2Fe. 0.96 Li 0.04 SiO4@SC-5.

[0088] Example 4

[0089] The difference from Example 1 is that 7% by weight of biochar is used to coat the electrode material.

[0090] S1 to S7 are the same as in Example 1;

[0091] S8. Take 3g of precursor and 0.21g of biochar and transfer them to a ball mill. The ball milling time is 0.5h.

[0092] S9. The ball-milled precursor was transferred to a tube furnace and calcined at 600℃ for 6 hours under inert gas protection to obtain a biochar-coated lithium-ion-doped sodium iron silicate electrode material, denoted as Na2Fe. 0.96 Li 0.04 SiO4@SC-7.

[0093] Example 5

[0094] The difference from Example 1 is that: for Na2Fe 0.98 Li 0.02 SiO4 for biochar coating:

[0095] S1. Mix 2.7216 g (0.02 mol) sodium acetate trihydrate, 1.7629 g (0.0098 mol) ferrous oxalate dihydrate, 0.0204 g (0.0002 mol) lithium acetate dihydrate and 0.6404 g (0.0033 mol) citric acid in ethanol, and stir the resulting mixture at 50 °C for 2 h.

[0096] S2. Add 2.0833g of tetraethyl orthosilicate to the mixture and heat and stir at 50°C for 3 hours.

[0097] S3. Heat and stir the above mixture at 90°C for 30 hours until a sol appears, then transfer it to an oven and dry at 60°C to obtain the precursor.

[0098] S4. Wash the coffee grounds and place them in an oven at 130°C overnight.

[0099] S5. After drying the coffee grounds, weigh 5g of the dried material and 5g of calcium carbonate and transfer them to a ball mill. Mill for 1 hour.

[0100] S6. Transfer the ball-milled material to a tube furnace and calcine it at 850°C for 1 hour under argon protection.

[0101] S7. Wash the calcined material with 20 mL of 2M dilute hydrochloric acid until the pH value is neutral, then transfer it to an oven and dry at 130°C to obtain biochar.

[0102] S8. Take 3g of precursor and 0.21g of biochar and transfer them to a ball mill. The ball milling time is 0.5h.

[0103] S9. The ball-milled precursor was transferred to a tube furnace and calcined at 600℃ for 6 hours under inert gas protection to obtain a biochar-coated lithium-ion-doped sodium iron silicate electrode material, denoted as Na2Fe. 0.98 Li 0.02 SiO4@SC- 7 .

[0104] Example 6

[0105] The difference from Example 1 is that: for Na2Fe 0.94 Li 0.06 SiO4 for biochar coating:

[0106] S1. Mix 2.7216 g (0.02 mol) sodium acetate trihydrate, 1.691 g (0.0094 mol) ferrous oxalate dihydrate, 0.0612 g (0.0006 mol) lithium acetate dihydrate and 0.6404 g (0.0033 mol) citric acid in ethanol, and stir the resulting mixture at 50 °C for 2 h.

[0107] S2. Add 2.0833g of tetraethyl orthosilicate to the mixture and heat and stir at 50°C for 3 hours.

[0108] S3. Heat and stir the above mixture at 90°C for 30 hours until a sol appears, then transfer it to an oven and dry at 60°C to obtain the precursor.

[0109] S4. Wash the coffee grounds and place them in an oven at 130°C overnight.

[0110] S5. After drying the coffee grounds, weigh 5g of the dried material and 5g of calcium carbonate and transfer them to a ball mill. Mill for 1 hour.

[0111] S6. Transfer the ball-milled material to a tube furnace and calcine it at 850°C for 1 hour under argon protection.

[0112] S7. Wash the calcined material with 20 mL of 2M dilute hydrochloric acid until the pH value is neutral, then transfer it to an oven and dry at 130°C to obtain biochar.

[0113] S8. Take 3g of precursor and 0.21g of biochar and transfer them to a ball mill. The ball milling time is 0.5h.

[0114] S9. The ball-milled precursor was transferred to a tube furnace and calcined at 600℃ for 6 hours under inert gas protection to obtain a biochar-coated lithium-ion-doped sodium iron silicate electrode material, denoted as Na2Fe. 0.94 Li 0.06 SiO4@SC-7.

[0115] Comparative Example 1

[0116] The difference from Example 1 is that the electrode material was not coated with biochar.

[0117] Lithium-ion doped sodium iron silicate electrode material Na2Fe 0.96 Li 0.04 Preparation of SiO4

[0118] S1. 2.7216 g (0.02 mol) sodium acetate trihydrate, 1.7269 g (0.0096 mol) ferrous oxalate dihydrate, 0.0408 g (0.0004 mol) lithium acetate dihydrate and 0.6404 g (0.0033 mol) citric acid were mixed in ethanol to obtain a mixture, which was then stirred at 50 °C for 2 h.

[0119] S2. Add 2.0833g of tetraethyl orthosilicate to the mixture and continue heating and stirring at 50°C for 3 hours.

[0120] S3. Heat and stir the above mixture at 90°C for 30 hours until a sol appears, then transfer it to an oven and dry at 60°C to obtain the precursor.

[0121] S4. The precursor is transferred to a tube furnace and calcined at 600℃ for 6 hours under inert gas protection to obtain lithium-ion doped sodium iron silicate electrode material Na2Fe. 0.96 Li 0.04 SiO4.

[0122] Test case

[0123] 1) The specific surface area of ​​the materials in Examples 1-6 and Comparative Example 1 was tested using the BET method, and the results are shown in Table 1.

[0124] Table 1. Specific surface area data of materials in Examples 1-6 and Comparative Example 1.

[0125] Material <![CDATA[Specific surface area m 2 / g]]> Comparative Example 1 <![CDATA[Na2Fe 0.96 That 0.04 SiO4]]> 2.0475 Example 1 <![CDATA[Na2Fe 0.96 Li 0.04 SiO4@SC-3]]> 4.6304 Example 2 <![CDATA[Na2Fe 0.96 Li 0.04 SiO4@SC-1]]> 4.4416 Example 3 <![CDATA[Na2Fe 0.96 Li 0.04 SiO4@SC-5]]> 4.9856 Example 4 <![CDATA[Na2Fe 0.96 Li 0.04 SiO4@SC-7]]> 5.0717 Example 5 <![CDATA[Na2Fe 0.98 Li 0.02 SiO4@SC-7]]> 4.2834 Example 6 <![CDATA[Na2Fe 0.94 Li 0.06 SiO4@SC-7]]> 4.8155

[0126] As shown in Table 1, the specific surface area of ​​Examples 1 to 6 is greater than that of Comparative Example 1, indicating that biochar coating can effectively increase the specific surface area of ​​lithium-doped sodium iron silicate composite material.

[0127] 2) Assembly of half-cells based on the lithium-ion-doped sodium iron silicate composite materials prepared in Examples 1-6:

[0128] The composite materials prepared in Examples 1 to 6 were used as electrode materials for battery assembly testing. 0.7g of composite material, 0.2g of SuperP and 0.1g of PVDF were mixed and 2500μL of NMP was added for grinding until there were no obvious particles. The mixture was then evenly coated on aluminum foil, transferred to a vacuum oven for drying, and sliced ​​for later use.

[0129] Half-cell button cell assembly was carried out in a glove box, with sodium sheet as the negative electrode, NaPF6 (1.0 mol / L, solvents EC and DMC in a volume ratio of 1:1, with an additional 5.0% FEC) as the electrolyte, glass fiber diaphragm as the diaphragm, and CR2032 button cell casing as the battery case.

[0130] Assembly of a half-cell based on the lithium-ion-doped sodium iron silicate electrode material in Comparative Example 1:

[0131] The electrode material prepared in Comparative Example 1 was subjected to battery assembly test. 0.7g of electrode material, 0.2g of Super P and 0.1g of PVDF were mixed and 2500μL of NMP was added for grinding until there were no obvious particles. The mixture was then evenly coated on aluminum foil, transferred to a vacuum oven for drying, and sliced ​​for later use.

[0132] Half-cell button cell assembly was carried out in a glove box, with sodium sheet as the negative electrode, NaPF6 (1.0 mol / L, solvents EC and DEC, volume ratio of EC to DEC 1:1) as the electrolyte, glass fiber diaphragm as the diaphragm, and CR2032 button cell casing as the battery case.

[0133] The electrochemical performance of the materials in Examples 1-6 and Comparative Example 1 under a voltage window of 1.5–4.5 V and a current density of 0.05 C is shown in Table 2; among them, the rate performance, long-cycle performance, and impedance diagram of the electrode materials prepared in Comparative Example 1 and Example 1 are shown in Table 2. Figures 1-3 .

[0134] Table 2 Electrochemical performance data of materials in Examples 1-6 and Comparative Example 1

[0135] Material Specific capacity mAh / g Impedance Ω Cyclic performance Comparative Example 1 <![CDATA[Na2Fe 0.96 That 0.04 SiO4]]> 102.0 600 43% retained after 50 laps Example 1 <![CDATA[Na2Fe 0.96 Li 0.04 SiO4@SC-3]]> 125.9 350 82% retained after 50 laps Example 2 <![CDATA[Na2Fe 0.96 Li 0.04 SiO4@SC-1]]> 120.5 450 79% retained after 50 laps Example 3 <![CDATA[Na2Fe 0.96 Li 0.04 SiO4@SC-5]]> 119.3 300 75% retained after 50 laps Example 4 <![CDATA[Na2Fe 0.96 Li 0.04 SiO4@SC-7]]> 110.5 200 50 laps, retain 60% Example 5 <![CDATA[Na2Fe 0.98 Li 0.02 SiO4@SC-3]]> 115.3 500 50 laps, retain 80% Example 6 <![CDATA[Na2Fe 0.94 Li 0.06 SiO4@SC-3]]> 105.7 700 75% retained after 50 laps

[0136] As shown in Table 2, the electrode material of Comparative Example 1 has significantly poor cycle performance. In contrast, the electrode material of Comparative Example 1, which is carbon-coated with biochar prepared from coffee grounds, exhibits excellent electrochemical performance and improved cycle stability.

[0137] Depend on Figures 1-3 It is known that the biochar-coated lithium-doped sodium iron silicate composite material prepared by the present invention has excellent electrochemical performance. Under a voltage window of 1.5 to 4.5 V and a current density of 0.05 C, the capacity retention can reach 82% after 50 cycles.

[0138] In summary, the biochar coating prepared by this invention can effectively improve the electrochemical performance of lithium-ion doped sodium iron silicate electrode materials.

[0139] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a biochar-coated lithium ion-doped sodium iron silicate composite material, characterized in that, The method comprises the following steps: mixing a biomass raw material with a gas production aid, performing first grinding, performing first calcination on the obtained ground material in a first protective gas, performing acid pickling to obtain biochar; mixing a sodium source, an iron source, a lithium source, an acid promoter, a silicon source with an organic solvent to perform sol-gel to obtain a precursor; mixing the biochar with the precursor, performing second grinding, performing second calcination on the obtained mixture in a second protective gas to obtain a biochar-coated lithium ion-doped sodium iron silicate composite material.

2. The production method according to claim 1, characterized by, The biomass raw material comprises coffee grounds, leaves, cow dung, straw or wood chips; the gas production aid comprises calcium carbonate, magnesium carbonate, sodium carbonate, sodium bicarbonate, zinc carbonate or copper carbonate; the mass ratio of the biomass raw material to the gas production aid is 1:0.5-1.5; the first grinding time is 0.5-1.5 h.

3. The production method according to claim 1, characterized by, The first protective gas comprises argon or nitrogen; the first calcination temperature is 750-950 ℃, and the time is 30-90 min.

4. The method of claim 1, wherein, The sodium source comprises at least one of anhydrous sodium acetate, sodium hydroxide, sodium carbonate, sodium oxalate, sodium nitrite, sodium dihydrogen phosphate, sodium bicarbonate, sodium bisulfate, sodium hypophosphite, sodium citrate, anhydrous sodium sulfate, potassium sodium tartrate tetrahydrate, sodium alginate, disodium hydrogen phosphate dodecahydrate, sodium acetate trihydrate, sodium bisulfite and sodium nitrate; The iron source comprises at least one of ferrous oxalate and ferrous nitrate; The silicon source comprises at least one of silicon dioxide, tetraethyl orthosilicate and methyl silicate; The lithium source comprises at least one of lithium carbonate, lithium acetate dihydrate and anhydrous lithium chloride.

5. The preparation method according to claim 4, characterized in that, The molar ratio of the sodium source, the iron source, the lithium source to the silicon source is 2:(0.9-0.99):(0.01-0.1):1; The acid promoter comprises citric acid, lactic acid, malic acid, tartaric acid or ascorbic acid; the molar ratio of the acid radical in the acid promoter to the iron element in the iron source is 1:0.9-1.

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6. The method of claim 1, wherein, The sol-gel temperature is 80-100 ℃, and the time is 24-36 h.

7. The preparation method according to claim 1, characterized in that, The mass ratio of the precursor to the biochar is 1:0.01-0.1; the second grinding time is 15-45 min.

8. The method of claim 1, wherein, The second protective gas comprises argon or nitrogen; the second calcination temperature is 500-700 ℃, and the time is 5-8 h.

9. The biochar-coated lithium ion-doped sodium iron silicate composite material prepared by the preparation method in any one of claims 1-8.

10. The application of the biochar-coated lithium ion-doped sodium iron silicate composite material in claim 9 in a sodium ion battery.

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