Hard carbon negative electrode material and preparation method thereof, negative electrode sheet and application thereof

Abstract

The application relates to a hard carbon negative electrode material and a preparation method thereof, a negative electrode sheet and application thereof. The preparation method is as follows: oxidized nanocellulose and a dispersing agent are mixed, ultrasonic dispersion and stirring are carried out, a sodium source and a dopant are added, stirring treatment is carried out, and a cellulose mixture mixed with the dopant is obtained; hard carbon particles are added into the cellulose mixture mixed with the dopant, mixing and dispersion are carried out, a first polymer is added, the hard carbon particles are coated, and first polymer-coated hard carbon particles are obtained; the first polymer-coated hard carbon particles are placed in a reaction cavity, heating and carbonization are carried out in an inert atmosphere, a polymer carbonization layer is obtained; carbon-containing mixed gas is introduced, surface heating and deoxidization flushing treatment are carried out, and a gaseous carbon deposition layer coated on the surface of the polymer carbonization layer, that is, the hard carbon negative electrode material, is obtained. The obtained hard carbon negative electrode material has a relatively high initial coulomb efficiency.

Description

Technical Field

[0001] This invention relates to the field of secondary battery negative electrode active materials technology, and in particular to a hard carbon negative electrode material and its preparation method, negative electrode sheet and its application. Background Technology

[0002] Between 1980 and 1999, research on sodium-ion batteries and lithium-ion batteries was at roughly the same level. However, the emergence of carbon materials, represented by graphite, directly accelerated the commercialization of lithium-ion batteries. Before 2000, the lack of suitable anode materials to match the cathodes of sodium-ion batteries was one of the reasons why sodium-ion battery research lagged behind lithium-ion battery development. Therefore, for sodium-ion batteries to achieve a breakthrough in industrialization, it is necessary to find a suitable, practical, inexpensive, and efficient anode material.

[0003] Current research has found that amorphous carbon, including hard carbon and soft carbon, is the most promising anode material for sodium-ion batteries. Its simple structure, high specific capacity, low cost, and environmental friendliness have attracted considerable attention. However, it also has several drawbacks. For example, the relatively low first-cycle coulombic efficiency of hard carbon anode materials limits their practical application in sodium-ion batteries. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a hard carbon anode material, its preparation method, anode sheet, and its applications.

[0005] The first objective of this invention is to provide a method for preparing a hard carbon anode material, comprising the following steps:

[0006] (1) Oxidized nanocellulose and dispersant are mixed and ultrasonically dispersed and stirred, then sodium source and dopant are added and stirred to obtain a cellulose mixture with dopant.

[0007] (2) Add hard carbon particles to the cellulose mixture containing dopants, mix and disperse, add polymer, stir evenly, and obtain hard carbon particles coated with the first polymer.

[0008] (3) The hard carbon particles coated with the first polymer are placed in the reaction chamber, heated and carbonized under an inert atmosphere, cooled and then ball-milled and sieved to obtain hard carbon particles coated with a polymer carbonization layer containing carbon nanofibers.

[0009] (4) Introduce a carbon-containing mixed gas into the reaction chamber and heat it to obtain a gaseous carbon deposition layer coated on the surface of the polymer carbonization layer, thereby obtaining the hard carbon anode material.

[0010] In one embodiment of the present invention, in step (1), the oxidized nanocellulose is prepared by the following method: nanocellulose is mixed with an oxidant solution, heated to react, washed and dried to obtain oxidized nanocellulose.

[0011] In one embodiment of the present invention, the nanocellulose is selected from one or more of cellulose nanofibers, cellulose nanowhiskers, cellulose nanofibers, microfibrillated cellulose, bacterial nanocellulose, and electrospun fibers; the length of the nanocellulose is 50-500 nm and the diameter is 10-300 nm.

[0012] In one embodiment of the present invention, at least one or more of the following conditions are satisfied:

[0013] The oxidant in the oxidant solution is selected from one or more of nitric acid, sulfuric acid, ammonium persulfate, potassium persulfate, and sodium persulfate;

[0014] The concentration of the oxidant solution is 0.2–38 wt%.

[0015] The mass-to-volume ratio of the nanocellulose to the oxidant solution is 10–200 g / L;

[0016] The heating reaction conditions are 40–90°C with stirring for 30 min–8 h.

[0017] In one embodiment of the present invention, in step (1), at least one of the following conditions is satisfied:

[0018] The dispersant is selected from one or more of dimethylformamide, sodium pyrophosphate, sodium tripolyphosphate, sodium hexametaphosphate, and sodium alkyl sulfonate.

[0019] The sodium source is selected from at least one of sodium acetate, sodium carbonate, sodium formate, sodium phenolate, sodium phenylate, sodium p-phenolate, sodium benzenesulfonate, and sodium powder.

[0020] The dopant is one or more of ammonium bicarbonate, ammonium carbonate, sodium hypophosphite, ammonium hypophosphite, ammonium phosphate, ammonium tripolyphosphate, and ammonium polyphosphate.

[0021] In one embodiment of the present invention, in step (2), the hard carbon particles are prepared by the following method: the undersize product discarded during sieving after the second polymer is treated at a high temperature of 1000-2000°C, with a particle size of less than or equal to 3.0 μm.

[0022] In one embodiment of the present invention, the first polymer or the second polymer is independently selected from at least one of polyacrylonitrile, polybutadiene, polystyrene, polyethylene, polyethersulfone, polyetherimide, polyimide, phenolic resin, and epoxy resin.

[0023] In one embodiment of the present invention, in step (3), the inert atmosphere is one or more of nitrogen, helium, neon, and argon.

[0024] In one embodiment of the present invention, in step (4), the carbon-containing mixed gas includes an inert gas and an organic gas that does not contain oxygen; the gas flow ratio of the inert gas to the organic gas that does not contain oxygen is 10:0.2 to 8; and the heating temperature is 400 to 1000°C.

[0025] In one embodiment of the present invention, the oxygen-free organic gas is CH4 (methane), C2H4 (ethylene), C2H6 (ethane), C3H8 (propane), n-C4H8 (n-butene), i-C4H8 (isobutene), 1.2-C4H6 (1,2-butadiene), 1.3-C4H6 (1,3-butadiene), Cis-2-C4H6 (cis-dibutene), Tans-2-C4H6 (trans-dibutene), n-C4H 10 (n-Butane), i-C4H 10 (Isobutane), i-C4H 10 One or more of the following: (isobutane), C3H6 (propylene), C3H6 (cyclopropane), methylhexane, n-heptane, cyclohexane, dodecane, isododecane, nonane, isopentane, hexadecane, n-butane, methylcyclohexane, neohexane, n-octane, 1-nonyne, 1-octane, 1-butyne, 2-butyne, 2-pentyne, acetylene, 1-pentyne, n-nonyne, cyclohexyethyne, octane, and vinylacetylene; wherein the inert gas is one or more of nitrogen, helium, neon, and argon.

[0026] The second objective of this invention is to provide a hard carbon anode material, wherein hard carbon particles are used as the core, and a polymer carbonization layer is coated on the surface of the hard carbon particles; and a gaseous carbon deposition layer is coated on the surface of the polymer carbonization layer.

[0027] The polymer carbonization layer comprises carbon nanofibers;

[0028] The hard carbon particles and carbon nanofibers are doped with N and / or P.

[0029] In one embodiment of the present invention, one or more of the following conditions are satisfied:

[0030] The hard carbon particles have a size of 0.02–10 μm;

[0031] The thickness of the polymerized carbonized layer of the hard carbon anode material is 0.2–30 μm;

[0032] The thickness of the gaseous carbon deposition layer of the hard carbon anode material is 0.002–0.3 μm;

[0033] The median particle size D50 of the hard carbon anode material is 2–40 μm;

[0034] The carbon content of the hard carbon anode material is >85wt%;

[0035] The porosity of the hard carbon anode material is 0.05–35%.

[0036] A third objective of the present invention is to provide a hard carbon anode sheet comprising the hard carbon anode material.

[0037] The preparation method of the above-mentioned hard carbon negative electrode includes the following steps:

[0038] (1) One-time mixing: Dry mix the hard carbon anode material and the conductive material at a speed of 200-3000 r / min for 20-200 min to obtain a mixture;

[0039] (2) Secondary mixing: Add binder and water, stir at 100-3000 r / min for 5 min-10 h, until the content of solid matter is 40-60% (preferably 45-55%), adjust the viscosity to 1.0-6 Pa·s (preferably 2.5-4 Pa·s) to obtain mixed hard carbon slurry, coat the mixed hard carbon slurry on the negative electrode current collector, dry at 80-105℃, roll and cut to obtain the hard carbon negative electrode sheet.

[0040] A fourth objective of this invention is to provide a sodium-ion secondary battery, including the aforementioned hard carbon negative electrode. The sodium-ion secondary battery is prepared by the following method:

[0041] The positive electrode, separator, and hard carbon negative electrode are stacked and wound in sequence to obtain a bare cell. The tabs are ultrasonically welded, the bare cell is placed in a battery case, dried at 150-180°C to remove moisture, electrolyte is injected into the battery case, and then it is packaged to obtain a sodium-ion secondary battery.

[0042] The technical solution of the present invention has the following advantages compared with the prior art:

[0043] (1) The present invention increases the interlayer spacing by atomic doping of phosphorus and nitrogen elements inside the polymer carbonized layer; at the same time, the interlayer spacing can be widened by inserting carbonized nanocellulose inside the polymer carbonized layer, reducing sodium insertion / extraction, improving sodium ion diffusion kinetics, and increasing sodium ion transport rate.

[0044] (2) This invention utilizes the decomposition of oxygen-free organic gas at high temperature into highly reactive small carbon species (e.g., carbon free radicals). These free radicals react with oxygen-functional groups (such as hydroxyl, carbonyl, and carboxyl groups) on the surface of some polymer carbonized layers to produce H2O and CO2, releasing oxygen and forming surface defects. The oxygen-free organic gas is then carbonized at high temperature to form a carbonized deposition layer. Combined with N and P doping, this repairs excessive defect sites. The carbonized deposition layer tightens the porosity on the hard carbon surface caused by excessive defects, forming more micropores (~0.8 nm), resulting in a gaseous carbon deposition layer with an amorphous structure and fewer residual oxygen atoms and defects. The amorphous carbon layer can reduce Na... + Diffusion resistance on the surface of hard carbon anode material; the gaseous carbon deposition layer synthesizes ultrapores on the surface of the material and hard carbon material rich in sodium adsorption active sites. The ultrapores can prevent the entry of electrolyte and excessive consumption of sodium ions on the surface, which can improve the first coulombic efficiency of hard carbon anode material.

[0045] (3) Hard carbon particles are waste products discarded during sieving after polymers are treated at high temperatures of 1000-2000℃; while nanocellulose is abundant and widely sourced, and is mostly a by-product of industrial production, thus realizing the reuse of waste. Attached Figure Description

[0046] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein...

[0047] Figure 1 This is a schematic diagram of the hard carbon anode material structure of the present invention.

[0048] Figure 2 These are the XRD patterns of the hard carbon anode materials obtained in Example 1 and Comparative Example 2 of this invention.

[0049] Figure 3 This is a TEM image of the hard carbon anode material obtained in Example 1 of the present invention.

[0050] Explanation of reference numerals in the accompanying drawings: 1. Polymer carbonization layer; 2. Carbon nanofibers; 3. Hard carbon particles; 4. Gaseous carbon deposition layer. Detailed Implementation

[0051] To address the relatively low coulombic efficiency of hard carbon anode materials in the first cycle, this invention proposes the following technical solution:

[0052] The first objective of this invention is to provide a method for preparing a hard carbon anode material, comprising the following steps:

[0053] (1) Oxidized nanocellulose and dispersant are mixed and ultrasonically dispersed and stirred, then sodium source and dopant are added and stirred to obtain a cellulose mixture with dopant.

[0054] (2) Add hard carbon particles to the cellulose mixture containing dopants, mix and disperse, add the first polymer, stir evenly, and obtain polymer-coated hard carbon particles.

[0055] (3) The polymer-coated hard carbon particles are placed in the reaction chamber, heated and carbonized under an inert atmosphere, cooled and then ball-milled and sieved to obtain hard carbon particles coated with a polymer carbonization layer containing carbon nanofibers.

[0056] (4) Introduce a carbon-containing mixed gas into the reaction chamber and heat it to obtain a gaseous carbon deposition layer coated on the surface of the polymer carbonization layer, thereby obtaining the hard carbon anode material.

[0057] In a specific embodiment, in step (1), the oxidized nanocellulose is prepared by the following method: nanocellulose is mixed with an oxidant solution, heated to react, washed and dried to obtain oxidized nanocellulose. Oxidizing nanocellulose can increase the content of surface peroxide bonds, carbon-oxygen bonds, hydroxyl groups, and other structures, thereby increasing surface activity and surface contact.

[0058] In a specific embodiment, the nanocellulose is selected from one or more of cellulose nanofibers, cellulose nanowhiskers, cellulose nanofibers, microfibrillated cellulose, bacterial nanocellulose, and electrospun fibers; the length of the nanocellulose is 50-500 nm and the diameter is 10-300 nm.

[0059] In a specific embodiment, at least one or more of the following conditions are met:

[0060] The oxidant in the oxidant solution is selected from one or more of nitric acid, sulfuric acid, ammonium persulfate, potassium persulfate, and sodium persulfate;

[0061] The concentration of the oxidant solution is 0.2–38 wt%.

[0062] The mass-to-volume ratio of the nanocellulose to the oxidant solution is 10–200 g / L;

[0063] The heating reaction conditions are 40–90°C with stirring for 30 min–8 h.

[0064] In a specific embodiment, in step (1), at least one of the following conditions is satisfied:

[0065] The dispersant is selected from one or more of dimethylformamide, sodium pyrophosphate, sodium tripolyphosphate, sodium hexametaphosphate, and sodium alkyl sulfonate.

[0066] The sodium source is selected from at least one of sodium acetate, sodium carbonate, sodium formate, sodium phenolate, sodium phenylate, sodium p-phenolate, sodium benzenesulfonate, and sodium powder; furthermore, the sodium powder needs to be passivated, which is a conventional step in the art, such as surface carbon coating; by adding a sodium source, the initial coulombic efficiency of the hard carbon anode is improved, and the sodium ion loss caused by the formation of the SEI film is compensated, thereby significantly improving the reversible capacity of the sodium-ion battery and improving the electrochemical performance of the hard carbon.

[0067] The dopant is at least one of ammonium bicarbonate, ammonium carbonate, sodium hypophosphite, ammonium hypophosphite, ammonium phosphate, ammonium tripolyphosphate, and ammonium polyphosphate. The dopant provides a source of N and P elements, which can optimize the carbon interlayer spacing, reduce sodium insertion / extraction, improve sodium ion diffusion kinetics, and increase the sodium ion transport rate.

[0068] In a specific embodiment, in step (2), the hard carbon particles are prepared by the following method: the undersize product discarded during sieving after the second polymer is treated at a high temperature of 1000-2000℃, with a particle size of less than or equal to 3.0 μm. Alternatively, the undersize particles from step (3) can be used. This achieves material recycling.

[0069] In a specific embodiment, the first polymer or the second polymer is independently selected from at least one of polyacrylonitrile, polybutadiene, polystyrene, polyethylene, polyethersulfone, polyetherimide, polyimide, phenolic resin, and epoxy resin. Further, the first polymer or the second polymer is preferably the same material, which ensures that the physical and electrochemical properties of the internal hard carbon particles and the external carbonized polymer layer remain consistent.

[0070] In a specific embodiment, in step (3), the gas in the inert atmosphere is one or more of nitrogen, helium, neon, and argon.

[0071] In a specific embodiment, in step (3), the sieving is to select hard carbon particles coated with a polymer carbonization layer containing carbon nanofibers that meet the particle size requirements. Particles that do not meet the particle size requirements can be reused as raw materials for hard carbon particles in step (2).

[0072] In a specific embodiment, in step (3), the carbon nanofibers are obtained by heating and carbonizing nanocellulose.

[0073] In a specific embodiment, in step (4), the carbon-containing mixed gas includes an inert gas and an oxygen-free organic gas; the gas flow ratio of the inert gas to the oxygen-free organic gas is 10:0.2-8; and the heating temperature is 400-1000℃. During the heating process, surface scouring and deoxidation are simultaneously performed using the gas flow. Surface scouring and deoxidation creates defects on the surface of the polymer carbonized layer. Since some oxygen-free organic compounds are liquid at room temperature, they mix with the inert gas after heating to a gaseous state, forming a carbon-containing mixed gas.

[0074] In a specific embodiment, the oxygen-free organic gas is one or more of methylhexane, n-heptane, cyclohexane, dodecane, isododecane, nonane, isopentane, hexadecane, methane, n-butane, propane, ethane, methylcyclohexane, neohexane, n-octane, 1-nonyne, 1-octane, 1-butyne, 2-butyne, 2-pentyne, acetylene, 1-pentyne, n-nonyne, cyclohexyethyne, octane, and vinylacetylene; the inert gas is one or more of nitrogen, helium, neon, and argon.

[0075] The second objective of this invention is to provide a hard carbon anode material, wherein the hard carbon anode material has a core-shell structure, with hard carbon particles as the core, and a polymer carbonization layer coated on the surface of the hard carbon particles; and a gaseous carbon deposition layer coated on the surface of the polymer carbonization layer.

[0076] The polymer carbonization layer comprises carbon nanofibers;

[0077] The hard carbon particles and carbon nanofibers are doped with N and / or P.

[0078] In a specific embodiment, one or more of the following conditions are met:

[0079] The hard carbon particles have a size of 0.02–10 μm;

[0080] The thickness of the polymerized carbonized layer of the hard carbon anode material is 0.2–30 μm;

[0081] The thickness of the gaseous carbon deposition layer of the hard carbon anode material is 0.002–0.3 μm;

[0082] The median particle size D50 of the hard carbon anode material is 2–40 μm;

[0083] The carbon content of the hard carbon anode material is >85wt%;

[0084] The porosity of the hard carbon anode material is 0.05–35%.

[0085] A third objective of the present invention is to provide a hard carbon anode sheet comprising the hard carbon anode material.

[0086] The preparation method of the above-mentioned hard carbon negative electrode includes the following steps:

[0087] (1) One-time mixing: Dry mix the hard carbon anode material and the conductive material at a speed of 200-3000 r / min for 20-200 min to obtain a mixture;

[0088] (2) Secondary mixing: Add binder and water, stir at 100-3000 r / min for 5 min-10 h, until the content of solid matter is 40-60% (preferably 45-55%), adjust the viscosity to 1.0-6 Pa·s (preferably 2.5-4 Pa·s) to obtain mixed hard carbon slurry, coat the mixed hard carbon slurry on the negative electrode current collector, dry at 80-105℃, roll and cut to obtain the hard carbon negative electrode sheet.

[0089] In one embodiment of the present invention, the conductive material in step (1) is selected from at least one of conductive carbon black, acetylene black, graphite, graphene, carbon microwires, carbon nanowires, carbon microtubes, and carbon nanotubes.

[0090] In one embodiment of the present invention, in step (2), the adhesive is one or more of the following: polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, methacrylamide, polyacrylic acid, lithium polyacrylate, polyacrylamide, polyimide, polyacrylate, styrene-butadiene rubber, sodium alginate, polyethylene glycol, and guar gum.

[0091] In one embodiment of the present invention, the mass percentages of the hard carbon anode material, the conductive material, and the binder are 85-99.6 wt%, 0.2-7 wt%, and 0.2-8.0 wt%, respectively.

[0092] In one embodiment of the present invention, the compaction density of the slit hard carbon negative electrode sheet is 0.80–1.80 g / cm³. 3 The thickness is 45–400 μm, preferably 0.90–1.3 g / cm³. 3 The thickness is 90–200 μm.

[0093] In one embodiment of the present invention, the negative electrode current collector in step (2) is one or more of the following: aluminum foil, porous aluminum foil, nickel / aluminum foam foil, zinc-plated aluminum foil, nickel-plated aluminum foil, carbon-coated aluminum foil, nickel foil, and titanium foil. Preferably, aluminum foil, nickel-plated aluminum foil, or carbon-coated aluminum foil is used.

[0094] A fourth objective of this invention is to provide a sodium-ion secondary battery, including the aforementioned hard carbon negative electrode. The sodium-ion secondary battery is prepared by the following method:

[0095] The positive electrode, separator, and hard carbon negative electrode are stacked and wound in sequence to obtain a bare cell. The tabs are ultrasonically welded, the bare cell is placed in a battery case, dried at 150-180°C to remove moisture, electrolyte is injected into the battery case, and then it is packaged to obtain a sodium-ion secondary battery.

[0096] In a specific embodiment, the positive electrode active material in the positive electrode sheet is at least one of sodium nickel manganate, sodium nickel cobalt manganate, sodium nickel cobalt aluminum, sodium vanadium fluorophosphate, sodium iron manganese fluorophosphate, sodium iron manganate, sodium copper manganate, sodium copper manganese ferrate, sodium hexacyanoferrate, sodium hexacyanomanganate, sodium hexacyanonickel, sodium hexacyanoferric nickel, sodium hexacyanomanganate, sodium hexacyanoferric manganate, sodium titanium phosphate, and sodium titanium manganese phosphate.

[0097] In a specific embodiment, the separator is a polymer separator of at least one of polyethylene, polypropylene, polysulfonyl, polyacrylonitrile, polyvinyl alcohol, polyarylethersulfone, polyvinylidene fluoride, and polymalonic acid.

[0098] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0099] Example 1

[0100] This embodiment provides a doped hard carbon anode material, its preparation method, and its application, as detailed below:

[0101] 1. Doped hard carbon anode material:

[0102] (1) Oxidized nanofibrillated cellulose and a solution containing 0.5 wt% dispersant (dimethylformamide) were placed in a reaction vessel at a mass-volume ratio of 2:60 kg / L. The mixture was ultrasonically dispersed and stirred at 65°C to obtain a cellulose dispersion. Then, 3 wt% sodium source (sodium carbonate) and 0.8 wt% dopant (ammonium hypophosphite) were added to the cellulose dispersion and stirred for 45 min to obtain a cellulose mixture.

[0103] (2) Add 200g of hard carbon particles to a reactor containing a cellulose mixture, disperse ultrasonically, stir for 20min, then transfer to a shotcrete machine, add 1500g of polymer (polyacrylonitrile), stir at 110℃ to uniformly coat the hard carbon particles, and obtain a mixed polymer. The volume-to-mass ratio of the cellulose mixture to the hard carbon particles is 1L:200g. The hard carbon particles are prepared by the following method: after the polymer (polyacrylonitrile) is treated at 1400℃, the undersize product discarded during sieving has a particle size of less than or equal to 3.0μm.

[0104] (3) The mixed polymer was then placed in a heating furnace filled with inert gas (argon) and carbonized at 1300℃ for 4 hours. After cooling to room temperature, it was ball-milled and sieved through a 5000-mesh sieve. The material remaining on the sieve was then placed in the heating furnace, and a carbon-containing mixed gas was introduced for surface heating (heating temperature 1000℃) and deoxidation scouring treatment for 50 minutes to obtain the hard carbon anode material. The hard carbon anode material was then characterized, and the results are shown in […]. Figure 2 and Figure 3 .Depend on Figure 3 It can be seen that the surface 10nm layer is a gaseous carbon deposition layer, and the dark stripes inside are carbonized nanocellulose.

[0105] The preparation method of oxidized nanofiber cellulose in step (1) is as follows: 100g of nanofiber cellulose is placed in 1L of a solution containing 23wt% ammonium persulfate as an oxidant, and reacted and stirred in a reaction vessel at 80℃ for 2h. Finally, it is thoroughly washed with deionized water and dried to obtain oxidized nanofiber cellulose.

[0106] In step (3), the surface deoxidation and rinsing process is as follows: the carbon-containing mixed gas is obtained by mixing inert gas argon and acetylene at a gas flow ratio of 10:3, and then rinsing with gas for 50 minutes.

[0107] 2. Applications of doped hard carbon anode materials in the manufacture of hard carbon anode sheets and sodium-ion secondary batteries:

[0108] (1) Preparation of hard carbon negative electrode:

[0109] 1) One-time mixing: The doped hard carbon anode material and conductive material (90wt% conductive carbon black and 10wt% carbon nanotubes) are placed in the mixing tank of the mixer and dry-mixed at 1500r / min for 60min to obtain the mixture.

[0110] 2) Secondary mixing: Add binder (33wt% sodium carboxymethyl cellulose, 33wt% lithium polyacrylate, and 33wt% styrene-butadiene rubber) and deionized water to a container (wherein the mass percentages of the doped hard carbon anode material, conductive material, and binder are 95.5wt%, 1.5wt%, and 3.0wt%, respectively). Add water until the solid content in the mixing tank is 52%, and adjust the viscosity to 3 Pa·s to obtain a mixed hard carbon slurry. Coat the mixed hard carbon slurry onto the aluminum foil of the anode current collector, dry at 90℃, roll, and slit to obtain a compacted density of 0.95 g / cm³. 3 A hard carbon anode sheet with a thickness of 125 μm.

[0111] (2) Preparation of secondary battery: The positive electrode sheet (positive electrode active material is sodium nickel iron manganese oxide), polypropylene separator and hard carbon negative electrode sheet are stacked and wound in sequence to obtain bare cell, and the tabs are ultrasonically welded. The bare cell is placed in the battery case, dried at 180°C to remove moisture, electrolyte is injected into the battery case and sealed to obtain sodium ion secondary battery.

[0112] Example 2

[0113] This embodiment provides a doped hard carbon anode material, its preparation method, and its application, as detailed below:

[0114] 1. Doped hard carbon anode material:

[0115] (1) Oxidized nanofiber cellulose and a solution containing 0.5 wt% dispersant (dimethylformamide) were placed in a reaction vessel at a mass-volume ratio of 2:60 kg / L. The mixture was ultrasonically dispersed and stirred at 65°C to obtain a cellulose dispersion. Then, 3 wt% sodium source (sodium carbonate) and 1.0 wt% dopant (ammonium hypophosphite) were added to the cellulose dispersion and stirred for 45 min to obtain a cellulose mixture.

[0116] (2) Add 200g of hard carbon particles to a reactor containing a cellulose mixture, ultrasonically disperse, stir for 30min, then transfer to a shotcrete machine, add 2000g of polymer (polyacrylonitrile), stir at 110℃ to uniformly coat the hard carbon particles, thus obtaining a mixed polymer. The volume-to-mass ratio of the cellulose mixture to the hard carbon particles is 1L:200g. The hard carbon particles are prepared by the following method: after the polymer polyacrylonitrile is treated at 1400℃, the undersize product discarded during sieving has a particle size of less than or equal to 3.0μm.

[0117] (3) The mixed polymer is sent to a heating furnace and filled with inert gas (argon). It is carbonized at 1400℃ for 4 hours. After cooling to room temperature, it is ball-milled and sieved through a 5000-mesh sieve. The material on the sieve is sent to a heating furnace and a carbon-containing mixed gas is introduced. The surface is heated (heating temperature is 650℃) and deoxidized and washed for 50 minutes. After cooling, it is ball-milled and sieved through a 5000-mesh sieve to obtain hard carbon anode material.

[0118] The preparation method of oxidized nanofiber cellulose in step (1) is as follows: 100g of nanofiber cellulose is placed in 1L of a solution containing 23wt% ammonium persulfate as an oxidant, and reacted and stirred in a reaction vessel at 80℃ for 2h. Finally, it is thoroughly washed with deionized water and dried to obtain oxidized nanofiber cellulose.

[0119] In step (3), the surface deoxidation and rinsing process is as follows: the carbon-containing mixed gas is obtained by mixing inert gas argon and acetylene at a gas flow ratio of 10:3, and then the gas flow is used for rinsing for 50 minutes.

[0120] 2. The application of doped hard carbon anode material in the manufacture of hard carbon anode sheets and sodium-ion secondary batteries is the same as in Example 1.

[0121] Example 3

[0122] This embodiment provides a doped hard carbon anode material, its preparation method, and its application, as detailed below:

[0123] 1. Doped hard carbon anode material:

[0124] (1) Oxidized nanofiber cellulose and a solution containing 0.5 wt% dispersant (dimethylformamide) were placed in a reaction vessel at a mass-volume ratio of 2:60 kg / L. The mixture was ultrasonically dispersed and stirred at 65°C to obtain a cellulose dispersion. Then, 3 wt% sodium source (sodium carbonate) and 1.2 wt% dopant (ammonium hypophosphite) were added to the cellulose dispersion and stirred for 45 min to obtain a cellulose mixture.

[0125] (2) Add 200g of hard carbon particles to a reactor containing a cellulose mixture, disperse ultrasonically, stir for 30min, and then send it to a shotcrete machine. Add 3000g of polymer (polyacrylonitrile), stir at 110℃, and uniformly coat the hard carbon particles to obtain a mixed polymer. The volume mass ratio of the cellulose mixture to the hard carbon particles is 1L:200g. The hard carbon particles are prepared by the following method: after the polymer polyacrylonitrile is treated at 1400℃, the waste product under the sieve is ≤3.0μm.

[0126] (3) The mixed polymer is sent to a heating furnace and filled with inert gas (argon). It is carbonized at 1500℃ for 4 hours. After cooling to room temperature, it is ball-milled and sieved through a 5000-mesh sieve. The material on the sieve is sent to a heating furnace and a carbon-containing mixed gas is introduced. The surface is heated (heating temperature is 650℃) and deoxidized and washed for 50 minutes. After cooling, it is ball-milled and sieved through a 5000-mesh sieve to obtain the doped hard carbon anode material.

[0127] The preparation method of oxidized nanofiber cellulose in step (1) is as follows: 100g of nanofiber cellulose is placed in 1L of a solution containing 23wt% ammonium persulfate as an oxidant, and reacted and stirred in a reaction vessel at 80℃ for 2h. Finally, it is thoroughly washed with deionized water and dried to obtain oxidized nanofiber cellulose.

[0128] In step (3), the surface deoxidation and rinsing treatment is carried out by mixing inert gases nitrogen and acetylene at a gas flow ratio of 10:3, and then rinsing with gas for 50 minutes.

[0129] 2. The application of doped hard carbon anode material in the manufacture of hard carbon anode sheets and sodium-ion secondary batteries is the same as in Example 1.

[0130] Example 4

[0131] This embodiment provides a doped hard carbon anode material, its preparation method, and its application, as detailed below:

[0132] 1. Doped hard carbon anode material:

[0133] (1) Oxidized nanocellulose and a solution containing 0.8 wt% dispersant (dimethylformamide) were placed in a reaction vessel at a mass-volume ratio of 5:60 kg / L. The mixture was ultrasonically dispersed and stirred at 80°C to obtain a cellulose dispersion. Then, 5.5 wt% sodium source (sodium formate) and 1.2 wt% dopant (sodium hypophosphite) were added to the cellulose dispersion and stirred for 60 min to obtain a cellulose mixture.

[0134] (2) Add 100g of hard carbon particles to a reactor containing a cellulose mixture, ultrasonically disperse and stir for 50min, then transfer to a shotcrete machine, add 800g of polymer (phenolic resin), stir at 150℃ to uniformly coat the hard carbon particles, and obtain a mixed polymer. The volume-to-mass ratio of the cellulose mixture to the hard carbon particles is 1L:100g. The hard carbon particles are prepared by the following method: after the polymer phenolic resin is treated at 1400℃, the sieved product is discarded, and the particle size is less than or equal to 3.0μm.

[0135] (3) The mixed polymer is sent to a heating furnace and filled with inert gas (argon). It is carbonized at 1300℃ for 4 hours. After cooling to room temperature, it is ball-milled and sieved through a 5000-mesh sieve. The material on the sieve is sent to a heating furnace and a carbon-containing mixed gas is introduced. The surface is heated (heating temperature is 1100℃) and deoxidized and washed for 30 minutes. After cooling, it is ball-milled and sieved through a 5000-mesh sieve to obtain the doped hard carbon anode material.

[0136] The preparation method of oxidized nanocellulose in step (1) is as follows: 100g of nanofibrillated cellulose is placed in 1L of a solution containing 23wt% ammonium persulfate as an oxidant, and reacted and stirred in a reaction vessel at 80℃ for 2h. Finally, it is thoroughly washed with deionized water and dried to obtain oxidized nanofibrillated cellulose.

[0137] In step (3), the surface deoxidation and scouring treatment is carried out by mixing carbon-containing gas with inert gas argon and methane at a gas flow ratio of 10:5, and then scouring with gas for 30 minutes.

[0138] 2. Applications of doped hard carbon anode materials in the manufacture of hard carbon anode sheets and sodium-ion secondary batteries:

[0139] (1) Preparation of hard carbon negative electrode

[0140] 1) One-time mixing: The doped hard carbon anode material and conductive material (90wt% conductive carbon black and 10wt% carbon nanotubes) are placed in the mixing tank of the mixer and dry-mixed at 1000r / min for 90min to obtain the mixture.

[0141] 2) Secondary mixing: Add binder (50wt% sodium carboxymethyl cellulose and 50wt% lithium polyacrylate) and deionized water to a container (wherein the mass percentages of the doped hard carbon anode material, conductive material, and binder are 94wt%, 2wt%, and 4wt%, respectively). Add water until the solid content in the mixing tank is 49%, and adjust the viscosity to 3.8 Pa·s to obtain a mixed hard carbon slurry. Coat the mixed hard carbon slurry onto the aluminum foil of the anode current collector, dry it at 90℃, roll it, and cut it to obtain a compacted density of 0.98 g / cm³. 3 A hard carbon anode sheet with a thickness of 123 μm.

[0142] (2) Preparation of secondary battery: The positive electrode sheet (positive electrode active material is sodium vanadium fluorophosphate), polypropylene separator and hard carbon negative electrode sheet are stacked and wound in sequence to obtain bare cell, electrode tabs are ultrasonically welded, the bare cell is placed in battery case, dried at 180°C to remove moisture, electrolyte is injected into battery case and sealed to obtain sodium ion secondary battery.

[0143] Example 5

[0144] This embodiment provides a doped hard carbon anode material, its preparation method, and its application, as detailed below:

[0145] 1. Doped hard carbon anode material:

[0146] (1) Oxidized nanofibrillated cellulose and a solution containing 0.8 wt% dispersant (dimethylformamide) were placed in a reaction vessel at a mass-volume ratio of 5:60 kg / L. The mixture was ultrasonically dispersed and stirred at 80°C to obtain a cellulose dispersion. Then, 5.5 wt% sodium source (sodium formate) and 1.2 wt% dopant (sodium hypophosphite) were added to the cellulose dispersion and stirred for 60 min to obtain a cellulose mixture.

[0147] (2) Add 100g of hard carbon particles to the reactor containing the cellulose mixture, disperse ultrasonically, stir for 50min, and then send it to the spraying machine. Add 1000g of polymer (phenolic resin), stir at 150℃, and uniformly coat the hard carbon particles to obtain the mixed polymer. The volume mass ratio of the cellulose mixture to the hard carbon particles is 1L:100g. The hard carbon particles are prepared by the following method: after the polymer phenolic resin is treated at 1400℃, the waste product under the sieve is ≤3.0μm.

[0148] (3) The mixed polymer is then sent to a heating furnace, filled with inert gas (argon), and carbonized at a constant temperature of 1400℃ for 4 hours. After cooling to room temperature, it is ball-milled and sieved through a 5000-mesh sieve. The material on the sieve is sent to a heating furnace, and a carbon-containing mixed gas is introduced to perform surface heating (heating temperature is 700℃) and deoxidation rinsing treatment for 30 minutes. After cooling, it is ball-milled and sieved through a 5000-mesh sieve to obtain hard carbon anode material.

[0149] The preparation method of oxidized nanocellulose in step (1) is as follows: 100g of nanofibrillated cellulose is placed in 1L of a solution containing 23wt% ammonium persulfate as an oxidant, and reacted and stirred in a reaction vessel at 80℃ for 2h. Finally, it is thoroughly washed with deionized water and dried to obtain oxidized nanocellulose.

[0150] In step (3), the surface deoxidation and rinsing treatment is carried out by mixing carbon-containing gas with inert gas and methane at a gas flow ratio of 10:5, and then rinsing for 30 minutes.

[0151] 2. The application of doped hard carbon anode material in the manufacture of hard carbon anode sheets and sodium-ion secondary batteries is the same as in Example 4.

[0152] Example 6

[0153] This embodiment provides a doped hard carbon anode material, its preparation method, and its application, as detailed below:

[0154] 1. Doped hard carbon anode material:

[0155] (1) Oxidized nanofibrillated cellulose and a solution containing 0.8 wt% dispersant (dimethylformamide) were placed in a reaction vessel at a mass-volume ratio of 5:60 kg / L. The mixture was ultrasonically dispersed and stirred at 80°C to obtain a cellulose dispersion. Then, 5.5 wt% sodium source (sodium formate) and 1.2 wt% dopant (sodium hypophosphite) were added to the cellulose dispersion and stirred for 60 min to obtain a cellulose mixture.

[0156] (2) Add 100g of hard carbon particles to a reactor containing a cellulose mixture, ultrasonically disperse, stir for 50min, then transfer to a shotcrete machine, add 1500g of polymer (phenolic resin), stir at 150℃ to uniformly coat the hard carbon particles, thus obtaining a mixed polymer. The volume-to-mass ratio of the cellulose mixture to the hard carbon particles is 1L:100g. The hard carbon particles are prepared by the following method: after the polymer phenolic resin is treated at 1400℃, the undersize product discarded during sieving has a particle size of less than or equal to 3.0μm.

[0157] (3) The mixed polymer is sent to a heating furnace and filled with inert gas (argon). It is carbonized at 1500℃ for 4 hours. After cooling to room temperature, it is ball-milled and sieved through a 5000-mesh sieve. The material on the sieve is sent to a heating furnace and a carbon-containing mixed gas is introduced. The surface is heated (heating temperature is 700℃) and deoxidized and rinsed for 30 minutes. After cooling, it is ball-milled and sieved through a 5000-mesh sieve to obtain hard carbon anode material.

[0158] The preparation method of oxidized nanocellulose in step (1) is as follows: 100g of nanofibrillated cellulose is placed in 1L of a solution containing 23wt% ammonium persulfate as an oxidant, and reacted and stirred in a reaction vessel at 80℃ for 2h. Finally, it is thoroughly washed with deionized water and dried to obtain oxidized nanofibrillated cellulose.

[0159] In step (3), the surface deoxidation and rinsing treatment is carried out by mixing carbon-containing gas with inert gas nitrogen and methane at a gas flow ratio of 10:5, and then rinsing for 30 minutes.

[0160] 2. The application of doped hard carbon anode material in the manufacture of hard carbon anode sheets and sodium-ion secondary batteries is the same as in Example 4.

[0161] Comparative Example 1

[0162] The difference from Example 1 is that no dopant was added.

[0163] Comparative Example 2

[0164] The difference from Example 1 is that no oxidized nanofiberized cellulose was added. The resulting hard carbon anode material was structurally characterized, as detailed in [see Example 1]. Figure 2 .

[0165] Comparative Example 3

[0166] The difference from Example 1 is that no surface deoxidation rinsing treatment was performed.

[0167] Performance testing:

[0168] 1. Battery electrical performance testing

[0169] (1) At room temperature of 25°C, the sodium-ion secondary batteries of Examples 1-6 and Comparative Examples 1-3 were first formed and capacitated at the starting and ending voltages of 2.0V and 3.8V, respectively. The charge and discharge capacity of the batteries in the first cycle were recorded, and the coulombic efficiency of the first cycle was calculated (first cycle coulombic efficiency = first cycle discharge capacity / first cycle charge capacity × 100%). The results are shown in Table 1.

[0170] (2) The hard carbon negative electrode sheets of each embodiment and comparative example were cut and punched into 12mm diameter discs, and then assembled into 2032 type button cells in a glove box. The electrolyte solvent was EC and DMC (volume ratio 1:1), and the sodium salt was 1M NaClO4. A polypropylene membrane was used as the separator, and a metallic sodium sheet was used as the counter electrode. The button cells were subjected to discharge / charge tests from 0 to 2.0V. The charge capacity of the button cells was the initial reversible capacity. The results are shown in Table 2.

[0171] Table 1. Coulomb efficiency of the battery in the first cycle.

[0172]

[0173]

[0174] Table 2 Initial reversible capacity of hard carbon anode sheets in the examples and comparative examples.

[0175] Example 1 313.3 Example 2 310.5 Example 3 312.6 Example 4 317.2 Example 5 315.5 Example 6 317.6 Comparative Example 1 307.1 Comparative Example 2 308.6 Comparative Example 3 304.6

[0176] As shown in Table 1, this invention increases the interlayer spacing of carbon layers by doping the polymer carbonization layer with phosphorus and nitrogen elements, combined with oxidized nanocellulose, thereby reducing sodium insertion / extraction, improving sodium ion diffusion kinetics, and increasing sodium ion transport rate. Furthermore, the gaseous carbon deposition layer creates ultrapores on the surface of the hard carbon material, preventing electrolyte ingress and excessive sodium ion consumption on the surface, thus significantly improving the material's coulombic efficiency.

[0177] Compared to Comparative Example 1, Example 1 did not include any dopant. As a result, the first-cycle coulombic efficiency of the battery in Example 1 increased significantly, and the corresponding initial reversible capacity also increased. Similarly, compared to Comparative Example 2, Example 1 did not include oxidized nanocellulose, and both the first-cycle coulombic efficiency and initial reversible capacity increased. Likewise, compared to Comparative Example 3, Example 1 did not undergo surface deoxidation scouring treatment, leading to a decrease in the first-cycle coulombic efficiency and initial reversible capacity. In conclusion, the use of dopant and oxidized nanocellulose, combined with surface deoxidation scouring treatment, can synergistically improve the coulombic efficiency of the material.

[0178] Depend on Figure 2 It can be seen that the hard carbon materials of Example 1 and Comparative Example 2 exhibit a broadened diffraction peak between 23 and 24°, corresponding to the characteristic peak of the (002) crystal plane of amorphous carbon. According to Bragg's formula, the interplanar spacing of Example 1 and Comparative Example 2 is calculated to be 0.386 nm and 0.374 nm, respectively. Inserting carbon nanofibers can broaden and increase the interplanar spacing.

[0179] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a hard carbon anode material, characterized in that, Includes the following steps: (1) Oxidized nanocellulose and dispersant are mixed and ultrasonically dispersed and stirred, then sodium source and dopant are added and stirred to obtain a cellulose mixture with dopant; the dopant is one or more of ammonium bicarbonate, ammonium carbonate, sodium hypophosphite, ammonium hypophosphite, ammonium phosphate, ammonium tripolyphosphate and ammonium polyphosphate. (2) Add hard carbon particles to the cellulose mixture containing dopants, mix and disperse, add the first polymer, stir evenly, and obtain hard carbon particles coated with the first polymer. (3) The hard carbon particles coated with the first polymer are placed in the reaction chamber, heated and carbonized under an inert atmosphere, cooled and then ball-milled and sieved to obtain hard carbon particles coated with a polymer carbonization layer containing carbon nanofibers. (4) A carbon-containing mixed gas is introduced into the reaction chamber and heated to obtain a gaseous carbon deposition layer coated on the surface of the polymer carbonization layer, thereby obtaining the hard carbon anode material; the carbon-containing mixed gas includes an inert gas and an organic gas that does not contain oxygen; the gas flow ratio of the inert gas to the organic gas that does not contain oxygen is 10:0.2~8; the heating temperature is 400~1000℃.

2. The preparation method according to claim 1, characterized in that, In step (1), the oxidized nanocellulose is prepared by the following method: nanocellulose is mixed with an oxidant solution, heated to react, washed and dried to obtain oxidized nanocellulose; the oxidant in the oxidant solution is selected from one or more of nitric acid, sulfuric acid, ammonium persulfate, potassium persulfate and sodium persulfate.

3. The preparation method according to claim 1, characterized in that, In step (1), at least one of the following conditions must be met: The dispersant is selected from one or more of dimethylformamide, sodium pyrophosphate, sodium tripolyphosphate, sodium hexametaphosphate, and sodium alkyl sulfonate. The sodium source is selected from one or more of sodium acetate, sodium carbonate, sodium formate, sodium phenolate, sodium phenylate, sodium p-phenolate, sodium benzenesulfonate, and sodium powder.

4. The preparation method according to claim 1, characterized in that, In step (2), the hard carbon particles are prepared by the following method: the undersize product discarded during sieving after the second polymer is treated at a high temperature of 1000~2000℃, and the particle size of the hard carbon particles is less than or equal to 3.0μm.

5. The preparation method according to claim 4, characterized in that, The first polymer or the second polymer is independently selected from at least one of polyacrylonitrile, polybutadiene, polystyrene, polyethylene, polyethersulfone, polyetherimide, polyimide, phenolic resin, and epoxy resin.

6. A hard carbon anode material prepared by the preparation method according to any one of claims 1-5, wherein the hard carbon anode material has a core-shell structure, with hard carbon particles as the core, and a polymer carbonization layer is coated on the surface of the hard carbon particles; and a gaseous carbon deposition layer is coated on the surface of the polymer carbonization layer. The polymer carbonization layer comprises carbon nanofibers; The hard carbon particles and carbon nanofibers are doped with N and / or P elements.

7. The hard carbon anode material according to claim 6, characterized in that, One or more of the following conditions must be met: The size of the hard carbon particles is 0.02~10μm; The thickness of the polymerized carbonized layer of the hard carbon anode material is 0.2~30μm; The thickness of the gaseous carbon deposition layer of the hard carbon anode material is 0.002~0.3μm; The median particle size D50 of the hard carbon anode material is 2~40μm; The carbon content of the hard carbon anode material is >85 wt%; The porosity of the hard carbon anode material is 0.05~35%.

8. A hard carbon negative electrode sheet, characterized in that, This includes the hard carbon anode material obtained by the preparation method according to any one of claims 1-5, and the hard carbon anode material according to claim 6 or 7.

9. A sodium-ion secondary battery, characterized in that, Includes the hard carbon negative electrode sheet as described in claim 8.

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