Composite hard carbon negative electrode material, preparation method and application thereof

Abstract

The application discloses a composite hard carbon negative electrode material and a preparation method and application thereof. The composite hard carbon negative electrode material comprises hard carbon and nano copper, the content of the nano copper is 0.09% to 0.45% based on the total mass of the composite hard carbon negative electrode material, the hard carbon comprises closed micropores and open mesopores, the pore volume of the closed micropores is >0.03 cm 3 / g obtained through carbon dioxide adsorption test.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, specifically relating to a composite hard carbon anode material, its preparation method, and its application. Background Technology

[0002] Hard carbon is a key anode material for sodium-ion batteries. However, current hard carbon technologies suffer from drawbacks such as low reversible specific capacity and low initial coulombic efficiency. Furthermore, because hard carbon is a carbon material that is difficult to graphitize at high temperatures, its carbon atoms are arranged irregularly, resulting in poor electrical conductivity. Existing technologies generally improve its conductivity by increasing the carbonization temperature of hard carbon, but excessively high carbonization temperatures can lead to glass formation, rendering the hard carbon unusable. Moreover, excessively high carbonization temperatures can also reduce the interlayer spacing of the d002 layers in hard carbon, leading to poor rate performance and failing to meet the requirements for long-cycle operation. Summary of the Invention

[0003] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, the objective of this invention is to propose a composite hard carbon anode material, its preparation method, and its applications. This invention improves the conductivity of hard carbon materials while also increasing their reversible specific capacity, ensuring a high initial coulombic efficiency for the composite hard carbon anode material, and further enhancing its adsorption capacity for electrolytes.

[0004] In one aspect of the present invention, a composite hard carbon anode material is provided. According to an embodiment of the present invention, the composite hard carbon anode material comprises:

[0005] The composite hard carbon anode material contains hard carbon and nano-copper. Based on the total mass of the composite hard carbon anode material, the content of nano-copper is 0.09%-0.45%. The hard carbon comprises closed micropores and open mesopores, and the pore volume of the closed micropores, as determined by carbon dioxide adsorption testing, is >0.03 cm³. 3 / g.

[0006] The composite hard carbon anode material according to embodiments of the present invention improves the conductivity of the hard carbon material, enhances its reversible specific capacity, ensures a high initial coulombic efficiency, and strengthens its adsorption capacity for electrolyte.

[0007] In addition, the composite hard carbon anode material according to the above embodiments of the present invention may also have the following additional technical features:

[0008] In some embodiments of the present invention, the pore volume of the open mesopores, as determined by nitrogen adsorption testing, is ≥0.01 cm³. 3 / g.

[0009] In some embodiments of the present invention, the pore size of the closed micropore is 0.4 nm-1.0 nm; and / or, the pore size of the open mesopore is 2 nm-50 nm.

[0010] In some embodiments of the present invention, in a battery using sodium metal as the counter electrode, the reversible specific capacity of the composite hard carbon anode material is >310 mAh / g; and / or, in a battery using sodium metal as the counter electrode, the initial coulombic efficiency of the composite hard carbon anode material is >88%.

[0011] In some embodiments of the present invention, the composite hard carbon anode material has a conductivity of >12 S / cm at 5 MPa.

[0012] In some embodiments of the present invention, the particle size of the nano-copper is 20nm-80nm.

[0013] In a second aspect, the present invention provides a method for preparing the composite hard carbon anode material of the above embodiments. According to an embodiment of the present invention, the method includes:

[0014] (1) Mix hard carbon precursor, nano copper oxide and oxalic acid or oxalate to obtain a mixture, and place it in a sealed reaction vessel;

[0015] (2) The mixture is heated to a first temperature and held at that temperature to decompose the oxalic acid or the oxalate;

[0016] (3) Heat the mixture obtained in step (2) to a second temperature and keep it warm so that the hard carbon precursor undergoes an activation reaction and the nano copper oxide is reduced;

[0017] (4) Transfer the reactants from step (3) to a protective atmosphere device, heat them to a third temperature and keep them warm to obtain composite hard carbon anode material.

[0018] The method for preparing composite hard carbon anode materials according to embodiments of the present invention achieves the simultaneous introduction of nano-copper into hard carbon materials and increases the pore volume of closed micropores in hard carbon materials through the decomposition of oxalic acid, the reduction of nano-copper oxide, the activation of hard carbon precursors, and the closure of open micropores. That is, a single process can simultaneously improve the conductivity and reversible specific capacity of hard carbon materials, eliminating the need for separate processes to enhance these two properties, reducing process complexity, simplifying the process, and saving significant manpower and resources. It also fully utilizes the CO and H2 generated during the activation of the hard carbon precursor, avoiding waste. Furthermore, compared to directly using nano-copper, nano-copper oxide is relatively inexpensive, reducing costs.

[0019] In addition, the method according to the above embodiments of the present invention may also have the following additional technical features:

[0020] In some embodiments of the present invention, in step (1), the mass ratio of the hard carbon precursor, the nano-copper oxide, and the oxalic acid or oxalate is 100:(0.1-0.5):(15-37.5); and / or, the particle size of the nano-copper oxide is 20nm-80nm.

[0021] In some embodiments of the present invention, in step (2), the first temperature is 190℃-230℃, and the temperature is maintained for 1h-2h.

[0022] In some embodiments of the present invention, in step (3), the second temperature is 900℃-950℃, and the temperature is maintained for 1h-4h.

[0023] In some embodiments of the present invention, in step (4), the third temperature is 1000℃-1050℃, and the temperature is maintained for 8h-24h.

[0024] In a third aspect, the present invention provides a negative electrode sheet. According to embodiments of the present invention, the negative electrode sheet comprises the composite hard carbon negative electrode material described in the above embodiments or the composite hard carbon negative electrode material prepared by the method described in the above embodiments.

[0025] In a fourth aspect, the present invention provides a battery. According to an embodiment of the invention, the battery has the negative electrode sheet described above. This improves both the rate performance and the energy density of the battery.

[0026] In a fifth aspect, the present invention provides an electrical device. According to an embodiment of the invention, the electrical device has the battery described above. Thus, the electrical device possesses all the advantages of a sodium-ion battery, which will not be elaborated further here.

[0027] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0028] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0029] Figure 1 This is a schematic flowchart of a method for preparing composite hard carbon anode material according to an embodiment of the present invention. Detailed Implementation

[0030] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0031] In one aspect of the invention, a composite hard carbon anode material is proposed. According to an embodiment of the invention, the composite hard carbon anode material comprises: hard carbon and nano-copper. Based on the total mass of the composite hard carbon anode material, the content of nano-copper is 0.09%-0.45%. The hard carbon comprises closed micropores and open mesopores, and the pore volume of the closed micropores, obtained by carbon dioxide adsorption testing, is >0.03 cm³. 3 / g. Therefore, this embodiment of the invention improves the conductivity of hard carbon materials, enhances their reversible specific capacity, ensures a high initial coulombic efficiency for the composite hard carbon anode material, and further strengthens the adsorption capacity of the composite hard carbon anode material for electrolyte.

[0032] Existing hard carbon materials suffer from drawbacks such as low reversible specific capacity and low initial coulombic efficiency. Furthermore, they also exhibit poor electrical conductivity. Current methods cannot simultaneously address both of these issues.

[0033] The principle behind the composite hard carbon anode material proposed in this invention achieving the aforementioned beneficial effects will be explained in detail below:

[0034] To address the aforementioned problems, this invention adds nano-copper to hard carbon, significantly improving the conductivity of the hard carbon material. This increases the conductivity of the composite hard carbon anode material to over 12 S / cm at 5 MPa, while under the same conditions, the conductivity of hard carbon material without nano-copper is only around 8 S / cm. The increased conductivity of the composite hard carbon anode material reduces the impedance of the anode sheet, decreases electron transport resistance, and weakens ohmic polarization, thereby improving the rate performance of the battery.

[0035] On the other hand, the hard carbon material of the present invention includes closed micropores and open mesopores. The closed micropores can reversibly store sodium ions. Therefore, increasing the pore volume of the closed micropores in the hard carbon material can effectively increase the reversible specific capacity (i.e., charging capacity) of the hard carbon material, thereby effectively improving the energy density of the battery. Specifically, by increasing the pore volume of the closed micropores in the hard carbon material to >0.03 cm⁻¹ 3 / g, thereby increasing the reversible specific capacity of the composite hard carbon anode material to greater than 310 mAh / g, while ensuring that the initial coulombic efficiency of the composite hard carbon anode material is greater than 88%.

[0036] The aforementioned mesopores are open-type mesopores. Due to capillary action, they can enhance the adsorption of electrolyte by the composite hard carbon anode material, thereby improving the electrolyte retention capacity of the composite hard carbon anode material.

[0037] Meanwhile, based on the total mass of the composite hard carbon anode material, the content of nano-copper can be 0.09%-0.45% (for example, but not limited to 0.09%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, etc.). The inventors found that if the content of nano-copper is too low, the conductivity of the hard carbon material cannot be effectively improved; since nano-copper is an inactive substance, if the content of nano-copper is too high, the specific capacity of the hard carbon material will be reduced. Only by limiting the content of nano-copper to the range of 0.09%-0.45% can the conductivity of the hard carbon material be effectively improved, while avoiding the problem of reduced specific capacity of the hard carbon material due to excessive nano-copper content.

[0038] In summary, the embodiments of the present invention improve the conductivity of hard carbon materials, enhance the reversible specific capacity of hard carbon materials, ensure that the composite hard carbon anode material has a high first coulombic efficiency, and also enhance the adsorption capacity of the composite hard carbon anode material for electrolyte.

[0039] It should be noted that the aforementioned closed micropores refer to micropores that are not connected to the outside (i.e., closed pores) and are capable of mass transfer, for example, capable of reversibly transferring sodium ions.

[0040] According to some specific embodiments of the present invention, the pore volume of the closed micropores (i.e., the HK micropore pore volume) obtained by carbon dioxide adsorption testing is 0.04 cm³. 3 / g-0.08cm 3 / g, thereby significantly improving the reversible specific capacity of the composite hard carbon anode material, while ensuring that the composite hard carbon anode material has a high first coulombic efficiency.

[0041] According to further embodiments of the present invention, in a battery (button cell) using sodium metal as the counter electrode, the reversible specific capacity of the composite hard carbon anode material is 310 mAh / g-340 mAh / g.

[0042] According to some specific embodiments of the present invention, the initial coulombic efficiency of the composite hard carbon anode material is 88%-91% in a battery (e.g., a coin cell) using sodium metal as the counter electrode.

[0043] According to some specific embodiments of the present invention, the conductivity of the composite hard carbon anode material at 5 MPa is increased to 12 S / cm-18 S / cm.

[0044] According to some specific embodiments of the present invention, the pore volume of the mesopores (i.e., the BHJ pore volume) obtained by nitrogen adsorption testing is ≥0.01 cm³. 3 / g, thus further enhancing the adsorption of electrolyte by the composite hard carbon anode material, thereby improving the liquid retention capacity of the composite hard carbon anode material. As a specific example, the pore volume of the mesopores obtained by nitrogen adsorption testing is 0.011 cm³. 3 / g-0.015cm 3 / g.

[0045] In embodiments of the present invention, the pore size of the closed micropore is 0.4-1.0 nm; and / or, the pore size of the open mesopore is in the range of 2 nm-50 nm.

[0046] According to some specific embodiments of the present invention, the particle size of the nano-copper is 20nm-80nm (for example, but not limited to 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, etc.). By limiting the particle size of the nano-copper to the above range, the conductivity of the hard carbon material is further effectively improved.

[0047] In a second aspect, the present invention provides a method for preparing the composite hard carbon anode material of the above embodiments. According to embodiments of the present invention, refer to the appendix... Figure 1 The method includes:

[0048] S100: Mix hard carbon precursor, nano-copper oxide, and oxalic acid or oxalate to obtain a mixture, and place it in a sealed reaction vessel.

[0049] In this step, hard carbon precursor, nano-copper oxide, and oxalic acid or oxalate are mixed to obtain a mixture, which is then placed in a sealed reaction vessel. A vacuum is then created inside the reaction vessel. This sealed reaction vessel can be a high-temperature, high-pressure resistant steel reaction tank. The tank body has a heating zone where the mixture is placed. Water-cooled sealing rings are located on both sides of the tank body. Additionally, the reaction vessel is equipped with a rotary motor to drive its rotation; a circulating water cooling system to cool the water-cooled sealing rings; and waste gas collection pipes and valves.

[0050] In embodiments of the present invention, the pre-carbonized hard carbon raw material is called a hard carbon precursor. The pre-carbonization temperature is preferably 300-800°C; the pre-carbonization time is preferably 1-8 hours; the pre-carbonization needs to be carried out in a protective gas, preferably nitrogen; the pre-carbonization equipment is a box furnace or a rotary kiln.

[0051] Oxalic acid can be replaced with ammonium oxalate. When ammonium oxalate is used, nitrogen will be doped into the composite hard carbon anode material, with a nitrogen content of 0.05%-2.0%, which can be measured using an oxygen-nitrogen-hydrogen element analyzer. Experimental data shows that, compared with oxalic acid, using oxalate as a raw material can effectively improve the conductivity of the composite hard carbon anode material, thereby increasing the material's capacity and initial efficiency.

[0052] According to some specific embodiments of the present invention, the mass ratio of hard carbon precursor, nano copper oxide, and oxalic acid or oxalate can be 100:(0.1-0.5):(15-37.5). By limiting the proportion of each component within the above range, firstly, it is ensured that the nano copper oxide is fully reduced to nano copper, and that the final composite hard carbon anode material contains an appropriate amount of nano copper, thereby ensuring that the final composite hard carbon anode material has a high conductivity; secondly, it is ensured that the final composite hard carbon anode material contains a sufficient amount of closed micropores, thereby effectively increasing the reversible specific capacity (i.e., charging capacity) of the hard carbon material.

[0053] According to some specific embodiments of the present invention, the particle size of the nano-copper oxide can be 20nm-80nm (for example, 20 / 30 / 40 / 50 / 60 / 70 / 80nm), thereby ensuring that the particle size of the reduced nano-copper is in the range of 20nm-80nm, thereby further effectively improving the conductivity of the hard carbon material.

[0054] S200: Heat the mixture to a first temperature and hold it at that temperature.

[0055] In this step, the circulating water cooling system, heating device and rotary motor of the above reaction tank are turned on to heat the mixture to a first temperature and keep it at that temperature, during which oxalic acid or oxalate decomposes.

[0056] The equation for the decomposition of oxalic acid is: H₂C₂O₄ → CO₂↑ + H₂O↑ + CO↑

[0057] The equation for the decomposition of hydrogen oxalate is: NH4HC2O4→NH3↑+CO2↑+H2O↑+CO↑

[0058] According to some specific embodiments of the present invention, the first temperature can be 190℃-230℃, and the holding time can be 1h-2h, thereby ensuring that oxalic acid or oxalate is fully decomposed.

[0059] The water vapor and CO2 generated in the above steps can be used to activate the hard carbon precursor, and CO can be used to reduce nano-copper oxide. Since the mass ratio of precursor, oxalic acid, and nano-copper oxide is predetermined, the reducing agent CO is present in excess, indicating that nano-copper oxide can be completely reduced to nano-copper and mixed in the hard carbon material as a conductive agent. However, the reaction temperature in step S200 is insufficient for both activation and reduction reactions to occur.

[0060] S300: Heat the mixture to a second temperature and hold it at that temperature.

[0061] In this step, the mixture obtained in step S200 is heated to a second temperature and held at that temperature, during which the activation reaction of the hard carbon precursor and the reduction reaction of nano-copper oxide occur. The activation reactions of the hard carbon precursor are: CO2 + C = 2CO and H2O + C = CO + H2. The H2 and CO generated in the reaction are both reducing agents for nano-copper oxide, further ensuring its reduction. After the reaction is complete, a cooling process is performed. After the reaction vessel cools down, the waste gas is collected first (excess H2 and CO are incinerated for safety reasons), and then the material in the reaction vessel is removed for later use.

[0062] The activation reaction described above actually consumes carbon, i.e., hard carbon precursor, and uses a chemical reaction to create pores on the hard carbon precursor. Water vapor tends to create mesopores, while CO2 tends to create micropores, thus yielding hard carbon with different pore structures. The pores created during the activation stage are open. Open micropores can store sodium, but the stored sodium is essentially irreversible, meaning that in coin cell operation, it exhibits high discharge capacity but low charge capacity, i.e., low reversible capacity, resulting in low initial coulombic efficiency. Therefore, open pores require subsequent closure treatment.

[0063] According to some specific embodiments of the present invention, the second temperature is 900℃-950℃ (for example, but not limited to 900℃, 910℃, 920℃, 930℃, 940℃, 950℃), and the holding time is 1h-4h (for example, but not limited to 1h, 2h, 3h, 4h, etc.), thereby ensuring the full progress of the activation reaction of the hard carbon precursor and the full progress of the reduction reaction of the nano copper oxide.

[0064] The heating rate in this step is not particularly limited, and those skilled in the art can set it according to actual needs. As some specific embodiments, the heating rate in this step can be 1-10℃ / min (e.g., but not limited to 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min).

[0065] In the S300, the reaction vessel rotation speed is 0.1 rpm to 5 rpm.

[0066] S400: Transfer the reactants from step S300 to a protective atmosphere apparatus, heat to the third temperature, and hold at that temperature.

[0067] In this step, the reactants from step S300 are transferred to a protective atmosphere apparatus, heated to a third temperature, and held at that temperature. The high temperature in this step causes the open micropores obtained in step S300 to close, transforming them into closed micropores. These closed micropores can reversibly store sodium ions, thus effectively increasing the reversible specific capacity (i.e., charging capacity) of the hard carbon material. Simultaneously, this ensures the composite hard carbon anode material has a high initial coulombic efficiency, and the conductivity of the hard carbon also increases further. It should be noted that the open mesopores obtained in step S300, due to their relatively large pore size, will not completely close after this high temperature.

[0068] According to some specific embodiments of the present invention, the third temperature can be 1000℃-1050℃ (e.g., but not limited to 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, 1050℃, etc.), and the holding time can be 8h-24h (e.g., but not limited to 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h, etc.). This ensures that the open micropores obtained in step S300 can be fully closed, effectively becoming closed micropores. Considering that hard carbon contains copper, the final carbonization temperature is no greater than 1050℃, which is lower than the melting point of copper (1083℃), but a sufficiently long holding time can still fully close the open micropores, while the conductivity of hard carbon also increases.

[0069] The method for preparing composite hard carbon anode materials according to embodiments of the present invention achieves the simultaneous introduction of nano-copper into hard carbon materials and increases the pore volume of closed micropores in hard carbon materials through the decomposition of oxalic acid, the reduction of nano-copper oxide, the activation of hard carbon precursors, and the closure of open micropores. That is, a single process can simultaneously improve the conductivity and reversible specific capacity of hard carbon materials, eliminating the need for separate processes to enhance these two properties, reducing process complexity, simplifying the process, and saving significant manpower and resources. It also fully utilizes the CO and H2 generated during the activation of the hard carbon precursor, avoiding waste. Furthermore, compared to directly using nano-copper, nano-copper oxide is relatively inexpensive, reducing costs.

[0070] Meanwhile, on the one hand, this method successfully added nano-copper to hard carbon, significantly improving the conductivity of the hard carbon material. The conductivity of the composite hard carbon anode material at 5 MPa was increased to greater than 12 S / cm, while the conductivity of hard carbon material without nano-copper under the same conditions was only about 8 S / cm. On the other hand, the hard carbon material prepared by this method includes closed micropores and mesopores. The closed micropores can reversibly store sodium ions; therefore, increasing the pore volume of the closed micropores in the hard carbon material can effectively increase the reversible specific capacity (i.e., charging capacity). Specifically, by increasing the pore volume of the closed micropores in the hard carbon material to greater than 0.005 cm⁻¹… 3 / g, thereby increasing the reversible specific capacity of the composite hard carbon anode material to greater than 310 mAh / g, while ensuring that the initial coulombic efficiency of the composite hard carbon anode material is greater than 88%. The above-mentioned mesopores are open mesopores, which, due to capillary action, can enhance the adsorption of electrolyte by the composite hard carbon anode material, thereby improving the electrolyte retention capacity of the composite hard carbon anode material.

[0071] In a third aspect, the present invention provides a negative electrode sheet. According to embodiments of the present invention, the negative electrode sheet comprises the composite hard carbon negative electrode material described in the above embodiments or the composite hard carbon negative electrode material prepared by the method described in the above embodiments. Specifically, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer being disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, a negative electrode binder, and a conductive agent, the negative electrode active material comprising the composite hard carbon negative electrode material as described above.

[0072] In embodiments of the present invention, the specific type of the negative electrode current collector is not particularly limited. For example, metal foil, porous metal plate, or composite current collector can be used. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). As another example, the negative electrode is a negative electrode of a sodium-ion battery, and the negative electrode current collector can be copper foil or aluminum foil.

[0073] In embodiments of the present invention, the specific type of the negative electrode binder is not particularly limited. For example, the negative electrode binder may be selected from at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0074] In some exemplary embodiments, the negative electrode active material layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0075] In embodiments of the present invention, the negative electrode sheet can be prepared according to conventional methods in the art. For example, the above-mentioned composite hard carbon negative electrode material, along with optional other negative electrode active materials, optional conductive agents, binders, and optional thickeners, are dispersed in a solvent, which can be N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is then coated onto a negative electrode current collector, and the negative electrode sheet is obtained through processes such as drying and cold pressing.

[0076] In a fourth aspect, the present invention provides a battery. According to an embodiment of the invention, the battery has the negative electrode sheet described above. This improves both the rate performance and the energy density of the battery.

[0077] According to some specific embodiments of the present invention, the discharge plateau of the battery is less than 0.1V > 40mV, thereby avoiding sodium deposition on the negative electrode of the battery.

[0078] Specifically, the aforementioned battery can be either a sodium-ion battery or a lithium-ion battery. The following explanation uses a sodium-ion battery as an example:

[0079] The sodium-ion battery includes a negative electrode, a positive electrode, and a separator as described in the above embodiments, with the separator disposed between the negative electrode and the positive electrode. The separator includes at least one of PP separator, PE separator, single-sided ceramic separator, double-sided ceramic separator, non-woven fabric separator, and glass fiber separator.

[0080] The positive electrode sheet includes a positive current collector and a positive active material layer formed on the positive current collector. The positive active material layer includes a positive active material, a positive binder, and a positive conductive agent. In embodiments of the present invention, the positive current collector can be made of a material with good conductivity and mechanical strength, preferably aluminum foil.

[0081] In the embodiments of the present invention, the specific types of positive electrode active materials are not particularly limited, and those skilled in the art can select them according to actual needs. As some specific examples, the positive electrode active materials include at least one of sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium iron phosphate, sodium iron pyrophosphate, and compound sodium iron phosphate.

[0082] In the embodiments of the present invention, the specific type of positive electrode conductive agent is not particularly limited, and those skilled in the art can select according to actual needs. As some specific examples, the positive electrode conductive agent includes at least one of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0083] Similarly, the specific type of positive electrode binder is not particularly limited, and those skilled in the art can choose according to actual needs. As some specific examples, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

[0084] The preparation method of the positive electrode sheet includes: mixing the positive electrode active material, positive electrode binder and positive electrode conductive agent evenly according to a preset ratio, adding solvent and stirring evenly to form a positive electrode slurry, then coating it onto the current collector, drying it, and finally cutting it into a specific shape of positive electrode sheet for later use according to the different battery casings.

[0085] Cell preparation: The positive and negative electrode sheets are added to the separator and wound. After winding, the positive and negative electrode tabs are welded. Then, the bare cell is encapsulated in an aluminum-plastic film. After encapsulation, the cell is vacuum baked for 10-20 hours. Then, after liquid injection, standing, high temperature and high pressure formation, degassing and encapsulation, and capacity testing, the sodium-ion battery is obtained.

[0086] In a fifth aspect, the present invention provides an electrical device. According to an embodiment of the invention, the electrical device has a sodium-ion battery as described above. Thus, the electrical device possesses all the advantages of a sodium-ion battery, which will not be elaborated further here.

[0087] Specifically, the aforementioned electrical equipment can be, but is not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0088] The embodiments of the present invention are described in detail below. It should be noted that the embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. In addition, unless otherwise specified, all reagents used in the following embodiments are commercially available or can be synthesized according to the methods described herein or known to others. For reaction conditions not listed, they are also readily available to those skilled in the art.

[0089] Example 1

[0090] This embodiment provides a composite hard carbon anode material, the preparation method of which includes:

[0091] Step 1: After thoroughly mixing the coconut shell precursor with oxalic acid and copper oxide with a particle size of 40 nm, the mass ratio of each component is shown in Table 1. The mixture is then placed into a reaction vessel, sealed, and vacuumed.

[0092] Step 2: After vacuuming is completed, turn on the heating device of the reaction vessel to 200°C and keep it at that temperature for 2 hours;

[0093] Step 3: Increase the temperature to the second temperature of 920℃ at a rate of 5℃ / min and hold for 2 hours, during which time the rotation speed of the reaction vessel is controlled at 0.5 rpm; after the reaction vessel cools down, remove the waste gas and collect the powder for later use.

[0094] Step 4: Heat the powder obtained in step 3 in a nitrogen atmosphere furnace, raise the temperature to the third temperature of 1000℃ at a rate of 5℃ / min and hold for 12 hours. After cooling, the desired composite hard carbon anode material is obtained.

[0095] Examples 2-4

[0096] The only difference between Examples 2-4 and Example 1 is the amount of oxalic acid used, as shown in Table 1. All other contents are the same as in Example 1.

[0097] Examples 5-6

[0098] The only difference between Examples 5-6 and Example 2 is the amount of nano copper oxide used, as shown in Table 1. All other contents are the same as in Example 2.

[0099] Examples 7-8

[0100] The only difference between Examples 7-8 and Example 2 is the second temperature and the holding time at the second temperature, as shown in Table 1. All other contents are the same as in Example 2.

[0101] Examples 9-10

[0102] The only difference between Examples 9-10 and Example 2 is the third temperature and the holding time at the third temperature, as shown in Table 1. All other contents are the same as in Example 2.

[0103] Example 11

[0104] The only difference between Example 11 and Example 2 is that ammonium oxalate is used instead of oxalic acid; all other contents are the same as in Example 2.

[0105] Comparative Example 1

[0106] The only difference between Comparative Example 1 and Example 2 is that the amount of oxalic acid added is too small, as shown in Table 1. All other contents are the same as in Example 2.

[0107] Comparative Example 2

[0108] The only difference between Comparative Example 2 and Example 2 is that no nano copper oxide was added, as shown in Table 1. All other contents are the same as in Example 2.

[0109] Comparative Example 3

[0110] The only difference between Comparative Example 3 and Example 2 is that the amount of nano copper oxide added is too small, as shown in Table 1. All other contents are the same as in Example 2.

[0111] Comparative Example 4

[0112] The only difference between Comparative Example 4 and Example 2 is that the amount of nano copper oxide added is too much, as shown in Table 1. All other contents are the same as in Example 2.

[0113] Comparative Example 5

[0114] The only difference between Comparative Example 5 and Example 2 is that the second temperature is too low, as shown in Table 1. All other contents are the same as in Example 2.

[0115] Comparative Example 6

[0116] The only difference between Comparative Example 6 and Example 2 is that the heat preservation time at the second temperature is too short, as shown in Table 1. All other contents are the same as in Example 2.

[0117] Comparative Example 7

[0118] The only difference between Comparative Example 7 and Example 2 is that the heat preservation time at the third temperature is too short, as shown in Table 1. All other contents are the same as in Example 2.

[0119] Comparative Example 8

[0120] The only difference between Comparative Example 8 and Example 2 is that the third temperature is too low, as shown in Table 1. All other contents are the same as in Example 2.

[0121] Comparative Example 9

[0122] Comparative Example 9 provides a coconut shell precursor that is the same as that in Example 1, with the differences shown in Table 1.

[0123] Table 1

[0124]

[0125] The pore volumes of closed micropores in the composite hard carbon anode materials prepared in Examples 1-11, Comparative Examples 1-8, and the hard carbon material in Comparative Example 9 were tested using the carbon dioxide adsorption test method, and the results are shown in Table 2. The pore volumes of open mesopores in the composite hard carbon anode materials prepared in Examples 1-11, Comparative Examples 1-8, and the hard carbon material in Comparative Example 9 were tested using the nitrogen adsorption test method, and the results are shown in Table 2.

[0126] The closed micropore volume mentioned above = the micropore volume after activation (i.e., the third step) - the micropore volume after carbonization (i.e., the fourth step);

[0127] The micropore volume was obtained through carbon dioxide adsorption testing. The specific method is as follows: 1) Test the adsorption capacity of the material at each partial pressure point, with the maximum partial pressure point being 0.03; 2) Establish the correspondence between partial pressure, pore size, and pore volume under the Horvath-Kawazoe analysis model (HK); 3) Obtain the cumulative pore volume within the current pore size range through the pore volume at each partial pressure point.

[0128] The pore volume of the above-mentioned open mesopores is the pore volume of the mesopores after carbonization (i.e., the fourth step). The pore volume of the mesopores is obtained by nitrogen adsorption test. The specific method is as follows: 1) Test the adsorption amount of the material at each partial pressure point, and the maximum partial pressure point is 1.0; 2) Establish the correspondence between partial pressure, pore volume and pore size under the Barrett-Joyner-Halenda analysis model (BJH); 3) Obtain the cumulative pore volume within the current pore size range through the pore volume at each partial pressure point.

[0129] The copper content of the composite hard carbon anode materials prepared in Examples 1-11, Comparative Examples 1-8, and the hard carbon material in Comparative Example 9 was tested using an XRF fluorescence spectrometer, and the results are shown in Table 2. The nitrogen content of the composite hard carbon anode material prepared in Example 11 was tested using an oxygen-nitrogen analyzer, and the results are shown in Table 2.

[0130] The conductivity of the composite hard carbon anode materials prepared in Examples 1-11, Comparative Examples 1-8, and the hard carbon material in Comparative Example 9 at 5 MPa was tested using a powder resistance and compaction density meter. The results are shown in Table 2.

[0131] The composite hard carbon anode materials prepared in Examples 1-11 and Comparative Examples 1-8, as well as the hard carbon material of Comparative Example 9, conductive agent SP, binder CMC, and SBR were mixed and dispersed in water. The mass ratio of anode material, conductive agent, binder, and SBR was 93:3:2:2, forming a uniform anode slurry. The anode slurry was coated onto copper foil as the anode current collector, and the anode sheet was obtained through drying and cold pressing. Then, the anode sheet was used to make a coin cell with a sodium sheet as the counter electrode. The charging capacity and discharging capacity of the composite hard carbon anode materials prepared in Examples 1-11, Comparative Examples 1-8, and Comparative Example 9, as well as the hard carbon material of Comparative Example 9, were tested using coin cells, and the initial coulombic efficiency was calculated. The initial coulombic efficiency = charging capacity / discharging capacity. The results are shown in Table 2.

[0132] The coin cell for the negative electrode material undergoes a process of first discharging and then charging; therefore, the reversible capacity is the same as the charging capacity. A two-stage discharge process is employed: 0.1C discharge to 5mV; rest for 10 minutes; 0.01C discharge to 5mV; rest for 10 minutes. Charging is then performed at 0.1C to 2V.

[0133] Table 2

[0134]

[0135]

[0136]

[0137] As can be seen from Table 2, compared with the hard carbon material of Comparative Example 9, the conductivity of the composite hard carbon anode materials prepared in Examples 1-11 is significantly improved, and the closed micropore volume, mesopore volume, charging capacity and discharging capacity of the composite hard carbon anode materials prepared in Examples 1-11 are all significantly improved.

[0138] As can be seen from Table 2, compared with Example 2, after using ammonium oxalate instead of oxalic acid in Example 11, the composite hard carbon anode material prepared in Example 11 contains 0.08% nitrogen, which improves the conductivity of Example 11 to a certain extent.

[0139] As can be seen from Table 2, compared with Example 2, the closed micropore volume, mesopore volume, charging capacity and discharging capacity of Comparative Example 1 are significantly reduced. This indicates that if the amount of oxalic acid added is too small, the water vapor, CO2 and CO produced by the decomposition of oxalic acid will be less, resulting in the incomplete reduction of nano copper oxide and the inability to carry out the activation reaction fully. This makes it impossible to effectively improve the micropores and mesopores of the hard carbon material, thus leading to a significant reduction in the charging capacity and discharging capacity of the composite hard carbon anode material.

[0140] As can be seen from Table 2, the conductivity of Comparative Examples 2 and 3 is significantly reduced compared to Example 2. This indicates that if no nano-copper oxide is added or the amount of nano-copper oxide added is too small, the conductivity of the composite hard carbon anode material will be significantly reduced.

[0141] As can be seen from Table 2, the charging capacity and discharging capacity of Comparative Example 4 decreased compared with Example 2. This indicates that if too much nano-copper oxide is added, it will have a negative impact on the charging capacity and discharging capacity.

[0142] As can be seen from Table 2, compared with Example 2, the conductivity, closed micropore volume, mesopore volume, charging capacity and discharging capacity of Comparative Example 5 are significantly reduced. This indicates that if the second temperature (i.e. the activation temperature) is too low, the activation reaction will not be able to occur, which will result in the inability to effectively improve the micropores and mesopores of the hard carbon material, thereby causing a significant reduction in the charging capacity and discharging capacity of the composite hard carbon anode material.

[0143] As can be seen from Table 2, compared with Example 2, the conductivity, closed micropore volume, mesopore volume, charging capacity and discharging capacity of Comparative Example 6 are all reduced. This indicates that if the holding time at the second temperature (i.e. the activation temperature) is too short, the activation reaction cannot be fully carried out, which will result in the inability to effectively improve the micropores and mesopores of the hard carbon material, thereby reducing the charging capacity and discharging capacity of the composite hard carbon anode material.

[0144] As can be seen from Table 2, compared with Example 2, the closed micropore volume, charging capacity, initial coulombic efficiency and conductivity of Comparative Example 7 are significantly reduced. This indicates that if the holding time at the third temperature is too short, the open micropore cannot close at high temperature, resulting in a reduction in the closed micropore volume, which in turn leads to a significant reduction in charging capacity, initial coulombic efficiency and conductivity.

[0145] As can be seen from Table 2, compared with Example 2, the closed micropore volume, charging capacity, initial coulombic efficiency and conductivity of Comparative Example 8 are significantly reduced. This indicates that if the third temperature is too low, the open micropore cannot close at high temperature, resulting in a reduction in the closed micropore volume, which in turn leads to a significant reduction in charging capacity, initial coulombic efficiency and conductivity.

[0146] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0147] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing composite hard carbon anode materials, characterized in that, include: (1) Mix hard carbon precursor, nano copper oxide and oxalic acid or oxalate to obtain a mixture, and place it in a sealed reaction vessel; (2) The mixture is heated to a first temperature and held at that temperature to decompose the oxalic acid or the oxalate; (3) Heat the mixture obtained in step (2) to a second temperature and keep it warm so that the hard carbon precursor undergoes an activation reaction and the nano copper oxide is reduced; (4) Transfer the reactants from step (3) to a protective atmosphere device, heat them to a third temperature and keep them warm to obtain composite hard carbon anode material; The composite hard carbon anode material comprises hard carbon and nano-copper. Based on the total mass of the composite hard carbon anode material, the content of nano-copper is 0.09%-0.45%. The hard carbon comprises closed micropores and open mesopores, and the pore volume of the closed micropores, as determined by carbon dioxide adsorption testing, is >0.03 cm³. 3 / g.

2. The method according to claim 1, characterized in that, The pore volume of the open mesoporous structure, as determined by nitrogen adsorption testing, is ≥0.01 cm³. 3 / g.

3. The method according to claim 1, characterized in that, The pore size of the closed micropore is 0.4 nm to 1.0 nm; And / or, the pore size of the open mesopore is 2nm-50nm.

4. The method according to claim 1, characterized in that, In a battery using sodium metal as the counter electrode, the reversible specific capacity of the composite hard carbon anode material is >310 mAh / g; And / or, in a battery using sodium metal as the counter electrode, the initial coulombic efficiency of the composite hard carbon anode material is >88%.

5. The method according to claim 1, characterized in that, The composite hard carbon anode material has a conductivity of >12 S / cm at 5 MPa.

6. The method according to claim 1, characterized in that, The particle size of the nano-copper is 20nm-80nm.

7. The method according to claim 1, characterized in that, In step (1), the mass ratio of the hard carbon precursor, the nano copper oxide, and the oxalic acid or oxalate is 100:(0.1-0.5):(15-37.5); And / or, the particle size of the nano-copper oxide is 20nm-80nm.

8. The method according to claim 1, characterized in that, In step (2), the first temperature is 190℃-230℃, and the temperature is maintained for 1h-2h.

9. The method according to claim 1, characterized in that, In step (3), the second temperature is 900℃-950℃, and the temperature is maintained for 1h-4h.

10. The method according to claim 1, characterized in that, In step (4), the third temperature is 1000℃-1050℃, and the temperature is maintained for 8h-24h.

11. A negative electrode sheet, characterized in that, Including composite hard carbon anode materials prepared by the method described in any one of claims 1-10.

12. A battery, characterized in that, Includes the negative electrode sheet as described in claim 11.

13. An electrical appliance, characterized in that, Includes the battery as described in claim 12.

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