A method for preparing a hard carbon negative electrode material, a hard carbon negative electrode material and its application

Abstract

The present invention discloses a preparation method of a hard carbon negative electrode material, a hard carbon negative electrode material and an application thereof. The preparation method comprises the following steps: step S1, performing multiple step-rate over-discharge treatments on a carbon fluoride battery after discharge to obtain a carbon fluoride battery to be disassembled; wherein the lower limit of the discharge voltage of the multiple step-rate over-discharge treatments is less than or equal to 1.5V; step S2, disassembling the carbon fluoride battery to be disassembled, removing the carbon fluoride positive electrode, cleaning it, and then collecting the carbon fluoride material; step S3, performing high-temperature sintering treatment on the carbon fluoride material under an inert atmosphere and / or treating it with an acidic solution to obtain a hard carbon negative electrode material; the hard carbon negative electrode material contains 2wt% to 35wt% of in-situ generated LiF; the preparation method has the advantages of stable raw materials, low cost, good product consistency and simple manufacturing process. The prepared hard carbon negative electrode material is applied to secondary batteries, has excellent electrochemical properties, and is conducive to commercial use.

Description

Technical Field

[0001] The present invention relates to the field of new energy technology, and in particular to a preparation method of a hard carbon negative electrode material, the hard carbon negative electrode material and applications thereof. Background Art

[0002] At present, the most widely used battery negative electrode materials are still mainly carbon-based. Taking lithium-ion batteries as an example, the negative electrodes of commercial lithium-ion batteries are usually graphite or mesophase carbon microbeads. However, the lamellar structures of both have poor compatibility with electrolytes. During the charge and discharge process, the repeated deintercalation and insertion of solvated ions will cause structural damage. Their cycle stability and coulombic efficiency are less than ideal, especially in the field of power batteries that require high current charging and discharging. In addition, due to the small spacing between the carbon layers of layered graphite materials, sodium-ion batteries cannot embed them to form stable compounds, which greatly limits their application.

[0003] Hard carbon refers to carbon that is difficult to graphitize. It is a carbonaceous material obtained by thermal decomposition of high molecular polymers. The hard carbon negative electrode material has the structural characteristics of isotropy, short-range order and long-range disorder, and a large interlayer spacing. Studies have shown that the charge and discharge reaction mechanism of hard carbon negative electrode materials is an adsorption-desorption mechanism rather than the embedding-desorption mechanism of layered graphite electrodes. Therefore, hard carbon negative electrode materials theoretically have a very high theoretical specific capacity (100mAh / g-500mAh / g) and excellent rate performance. In addition, due to the large interlayer spacing of hard carbon negative electrode materials, this makes its application in sodium ion batteries more obvious, and it has received increasing attention in the field of sodium ion batteries. Therefore, hard carbon negative electrode materials have good application prospects, but there are currently many problems in the preparation process of hard carbon materials. There are certain technical difficulties and barriers in their production and manufacturing, which are mainly reflected in the process control and technology accumulation in the processes of raw material selection, cross-linking treatment, carbonization, purification, and modification.

[0004] According to the different sources of hard carbon material precursors, they can be divided into resin-based, asphalt-based, biomass-based and anthracite-based. Judging from the current development of hard carbon preparation technology, the production process of biomass-based hard carbon is less difficult and is expected to be applied first, but it is difficult to ensure the supply stability, low cost and consistency of raw materials in the mass production stage. The current production process of asphalt-based / resin-based hard carbon is difficult and the performance is also poor. In general, the current hard carbon preparation process generally has problems such as poor raw material supply stability, high cost, poor consistency of prepared products, or difficulty in production, and poor electrochemical performance. How to obtain hard carbon negative electrode materials with stable raw materials, low cost, good product consistency, simple preparation process and excellent electrochemical performance is an urgent problem to be solved in this field.

[0005] On the other hand, the lithium batteries that were first put into use have reached the end of their lifespans, resulting in a large number of waste batteries. How to use them more reasonably, efficiently and greenly has become an issue that society urgently needs to consider.

[0006] Usually, metal oxide positive electrodes can be recycled through metallurgy or by adding certain components. However, the structure of the layered carbon material is greatly damaged after long cycles, the recycling cost is high, and the residual utilization value is not large. Carbon fluoride is the product of carbon materials after fluorination treatment. It has excellent properties such as high specific capacity, electrochemical stability, and a smooth discharge platform, and is widely used as a primary lithium battery positive electrode. It is the primary battery positive electrode material with the highest specific energy. Carbon fluoride lithium batteries are widely used in certain special application fields such as pacemakers, aerospace, and other scenarios. With the diversification of social needs, the demand for it is getting higher and higher, but as a primary battery, large-scale use will make the recycling and treatment of waste electrodes after the end of the battery life a difficult problem.

[0007] Currently, the most common method for disposing of spent carbon fluoride batteries is centralized incineration, which exacerbates environmental pollution and may pose safety risks. Therefore, exploring the use of carbon fluoride as an active material in secondary batteries and finding secondary uses for spent carbon fluoride have become current technical challenges.

[0008] When used as a negative electrode material for secondary batteries, it is difficult to achieve in a short period of time due to the limitations of the current practical electrolyte voltage window. Although there are reports that it can be used in room-temperature sodium secondary batteries (the decomposition potential of NaF is lower than that of LiF, and reversible charge and discharge can be performed within the voltage window of existing electrolytes), the discharge platform is low and the polarization is too large, which is still a long way from practical application. Considering that the spent carbon lithium fluoride primary battery has only been discharged once, its material still retains its original structure, so recycling and reusing the spent electrodes is a more feasible solution.

[0009] At present, some existing technologies, such as Chinese invention patent CN113526488A (publication date is October 22, 2021), disclose a method for recovering and modifying the positive electrode material in a waste carbon fluoride battery. The specific steps include dismantling the discharged lithium / carbon fluoride original battery, obtaining the waste carbon fluoride positive electrode, washing the waste carbon fluoride with a first solvent and then putting it into a second solvent for ultrasonic stripping, and collecting the carbon fluoride to obtain a carbon material. The preparation process of the carbon material in this patent application is simple, and the raw materials are derived from waste carbon fluoride primary lithium batteries. The source of raw materials is rich and stable. At the same time, since it is a recovery of the positive electrode material of a primary lithium battery, its cost is relatively low. This method requires the removal of the F element (including LiF, etc.) in the carbon fluoride as much as possible, thereby obtaining a carbon material with higher purity. Although this method can effectively collect carbon fluoride, the electrochemical performance of the carbon material finally obtained is poor. According to Example 1, after 100 cycles, the modified carbon material obtained by the above method has a stable capacity between 170mAh / g-210mAh / g, and an initial coulombic efficiency of 30%-40%. After concentrated acid immersion and heat treatment (the purpose of which is to completely remove LiF), the highest specific capacity is only 280mAh / g, and the initial coulombic efficiency is about 65%. According to the accompanying drawings in the specification, it can be seen that as the number of cycles increases, its specific capacity is lower and the cycle performance is poor. The carbon material obtained by the method disclosed in this invention patent has a very low specific capacity, poor initial coulombic efficiency and poor cycle performance. It is not suitable as a negative electrode material for secondary batteries such as lithium-ion batteries and sodium-ion batteries, and is difficult to apply to high-energy-density battery systems.

[0010] In summary, the electrochemical performance of hard carbon negative electrode materials or carbon-based materials prepared in the current existing technology still has major defects. Their processing process is complicated, the electrochemical performance such as specific capacity and first coulombic efficiency is low, and the cycle performance needs to be improved. The overall properties of existing hard carbon negative electrode materials are still far from commercial applications and have no practical utilization value. Summary of the Invention

[0011] The present invention provides a method for preparing a hard carbon negative electrode material, a hard carbon negative electrode material, and its application. In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is how to obtain a hard carbon negative electrode material with stable raw materials, low cost, good product consistency, simple preparation process, and excellent electrochemical properties (such as discharge specific capacity, cycle performance, and first coulombic efficiency). The present invention achieves the complete conversion of F in the waste fluorinated carbon material into LiF by improving the recycling process of waste fluorinated carbon electrodes, and regulates the LiF content in the hard carbon negative electrode material to an optimal range through high-temperature sintering and / or acidic solution treatment, thereby preparing a hard carbon negative electrode material with excellent performance for commercial use.

[0012] To this end, in a first aspect, an embodiment of the present invention provides a method for preparing a hard carbon negative electrode material, the preparation method comprising the following steps:

[0013] Step S1, performing multiple step-rate over-discharge treatments on the carbon fluoride battery after discharge to obtain a carbon fluoride battery to be disassembled; wherein the lower limit of the discharge voltage of the multiple step-rate over-discharge treatments is less than or equal to 1.5V;

[0014] Step S2, disassembling the carbon fluoride battery to be disassembled, taking out the carbon fluoride positive electrode, cleaning it, and collecting the carbon fluoride material;

[0015] Step S3, sintering the fluorinated carbon material at high temperature under an inert atmosphere and / or treating it with an acidic solution to obtain a hard carbon negative electrode material;

[0016] The hard carbon negative electrode material contains 2 wt% to 35 wt% of in-situ generated LiF;

[0017] The shape of the hard carbon negative electrode material includes one or more of a layered structure, a block structure, an amorphous structure or a tubular structure.

[0018] Preferably, the lower limit of the discharge voltage of the multiple step-rate over-discharge treatment is between 0.3V and 1.5V;

[0019] The specific process of the multiple step-rate over-discharge treatment is as follows: the carbon fluoride battery after discharge is discharged at a rate of 0.1C to 0.2C until the voltage is less than or equal to 1.5V, then discharged at a rate of 0.05C to less than 0.1C until the voltage is less than or equal to 1.0V, and then discharged at a rate of 0.02C to less than 0.05C until the voltage is between 0.3V and 0.5V;

[0020] Preferably, during the process of the multiple step-rate over-discharge treatments from high to low rates, the lower limit of the discharge voltage is gradually reduced.

[0021] Preferably, the fluorinated carbon material includes one or more of: fluorinated graphite, fluorinated graphite polymer, fluorinated carbon nanotube, fluorinated carbon fiber, fluorinated graphene, fluorinated hard carbon, fluorinated coke, and fluorinated carbon black;

[0022] The content of LiF accounts for 5 wt % to 15 wt % of the hard carbon negative electrode material.

[0023] Preferably, the high temperature sintering process includes one-stage sintering or segmented sintering;

[0024] The specific conditions of the one-stage sintering are: setting the heating rate to 1°C / min to 10°C / min, the sintering temperature to be greater than or equal to 800°C, and the holding time to be 0.5 hours to 24 hours;

[0025] The staged sintering is specifically as follows: placing the carbon fluoride material in a heating device under an inert gas atmosphere, heating it to a first temperature at a heating rate of 1°C / min to 10°C / min, and keeping it at that temperature for 0.5 to 24 hours, heating it to a second temperature at a heating rate of 1°C / min to 10°C / min, and keeping it at that temperature for 0.5 to 24 hours, and then heating it to a third temperature at a heating rate of 1°C / min to 10°C / min, and keeping it at that temperature for 0.5 to 8 hours;

[0026] The first temperature is greater than or equal to 300° C. and less than 650° C.;

[0027] The second temperature is greater than or equal to 650° C. and less than 800° C.;

[0028] The third temperature is greater than or equal to 800°C.

[0029] Preferably, the acidic solution treatment method is specifically: using an acidic solution to immerse, rinse, spray or spray the carbon fluoride material or the carbon fluoride material that has undergone the high-temperature sintering treatment to obtain a hard carbon negative electrode material with a controlled LiF content;

[0030] After the acidic solution treatment, the hard carbon negative electrode material with the adjusted LiF content is cleaned with an alkaline solution and / or a neutral reagent;

[0031] The acidic solution includes one or more of an inorganic acid or an organic acid;

[0032] The alkaline solution includes one or more of an inorganic base, an organic base or an alkaline salt solution.

[0033] Further preferably, the acidic solution specifically includes: one or more of sulfuric acid, nitric acid, phosphoric acid, sulfurous acid, carbonic acid, boric acid, oxalic acid, acetic acid, carbonic acid, and sulfinic acid;

[0034] The alkaline solution specifically includes: one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, ammonia water, ferric hydroxide, aluminum hydroxide, magnesium hydroxide, sodium acetate, sodium silicate, and sodium phosphate.

[0035] Preferably, the preparation method further comprises: when the hard carbon negative electrode material is in a layered structure, treating the hard carbon negative electrode material by one or more of a physical method, a chemical method or an electrochemical method to increase the reactive area of ​​the hard carbon negative electrode material;

[0036] The physical method includes: one or more methods of solvent thermal intercalation, ultrasonic stripping, tape stripping or high-energy ball milling;

[0037] The chemical method is to cause the hard carbon negative electrode material to undergo an oxidation-reduction reaction, specifically comprising treating the hard carbon negative electrode material with an inorganic acid, followed by dilution and oxidation with a strong oxidant;

[0038] The electrochemical method specifically comprises: using the hard carbon negative electrode material to prepare a pole piece and assembling a half-cell, and then over-discharging at a rate greater than or equal to 3C to peel off the carbon layer through excessive ion intercalation.

[0039] In a second aspect, an embodiment of the present invention provides a hard carbon negative electrode material prepared by the preparation method described in the first aspect, wherein the hard carbon negative electrode material comprises a fluorinated carbon material and in-situ generated LiF;

[0040] The mass of the LiF accounts for 2% to 35% of the total mass of the hard carbon negative electrode material;

[0041] The hard carbon negative electrode material includes one or more of a layered structure, a block structure, an amorphous structure or a tubular structure.

[0042] Preferably, the fluorinated carbon material includes one or more of: fluorinated graphite, fluorinated graphite polymer, fluorinated carbon nanotube, fluorinated carbon fiber, fluorinated graphene, fluorinated hard carbon, fluorinated coke, and fluorinated carbon black;

[0043] The mass of the LiF accounts for 5% to 15% of the total mass of the hard carbon negative electrode material;

[0044] The specific surface area of ​​the hard carbon negative electrode material is greater than or equal to 100m 2 / g.

[0045] In a third aspect, an embodiment of the present invention provides a secondary battery, comprising the hard carbon negative electrode material obtained by any method of the first aspect or the hard carbon negative electrode material described in any of the second aspects; preferably, the secondary battery is a lithium ion battery or a sodium ion battery.

[0046] The present invention provides a method for preparing a hard carbon negative electrode material, a hard carbon negative electrode material, and its application. The present invention has the following technical effects:

[0047] (1) The hard carbon negative electrode material prepared by the preparation method of the hard carbon negative electrode material provided by the present invention can improve the electrochemical performance of the hard carbon negative electrode material by in-situ generation of LiF during the preparation process, avoiding the presence of other forms of fluorine as much as possible, and controlling the LiF content within the range of 2 wt% to 35 wt%. The reasons are as follows:

[0048] Since the F element has the strongest electronegativity, excessive F element in battery materials will reduce the conductivity of the material, causing the battery polarization to increase and the capacity to be unable to function normally. Therefore, previous studies have all hoped to remove the F element from the battery materials as much as possible; however, the inventors of the present application have discovered that by controlling the LiF content in the hard carbon negative electrode material and minimizing the presence of other fluorides in the hard carbon negative electrode material, it helps to improve the performance of the negative electrode material. This is because LiF is the main component of the SEI film on the surface of the secondary battery negative electrode material and LiF is beneficial to improving the microstructure of the hard carbon negative electrode material.

[0049] To this end, the present invention provides a method for preparing a hard carbon negative electrode material. The method uses waste carbon fluoride primary batteries as raw materials and uses an electrochemical method of multiple step-rate overdischarge treatments to convert the fluoride in the carbon fluoride material into LiF as much as possible, thereby reducing the presence of unfavorable fluorides. In addition, the method combines high-temperature sintering treatment and / or acidic solution treatment to control the LiF content in the hard carbon negative electrode material, thereby improving the electrochemical performance of the negative electrode material.

[0050] Specifically, the present invention uses an electrochemical method, i.e., a step-by-step rate method, to repeatedly over-discharge waste fluorinated carbon to a lower voltage range to convert the fluoride in the waste fluorinated carbon into LiF as much as possible, i.e., reduce the presence of unfavorable fluorides, thereby improving the conductivity of the material and preventing the occurrence of battery polarization, and promote and regulate the generation of LiF, i.e., control the conversion of fluorine element into LiF by the magnitude of the discharge current, the discharge voltage, and the discharge time, so as to achieve the highest possible LiF content in the hard carbon negative electrode material, up to 60wt% or more, and almost no other fluorides are present; then, through subsequent high-temperature sintering treatment and / or acidic solution treatment process, the LiF content is further controlled to 2wt%- 35wt%, preferably in the range of 5wt%-15wt%, so that the negative electrode material has excellent specific capacity and cycle performance. For lithium batteries, when the LiF content is between 2wt%-35wt%, the discharge specific capacity of the prepared hard carbon negative electrode material for the first time or after cycle stabilization is ≥300mAh / g; when the LiF content is between 5wt%-15wt%, the discharge specific capacity of the prepared hard carbon negative electrode material for the first time or after cycle stabilization is ≥320mAh / g; specifically, in order to adjust the LiF content, the subsequent treatment process can be carried out by heating under high temperature inert conditions, which can not only effectively remove impurity atoms on the surface of the hard carbon negative electrode material, but also remove residual CF in the hard carbon negative electrode material. XHeating and volatilization can reduce the presence of undesirable fluorides, and the content of LiF can also be effectively controlled by heating treatment. In addition, in order to adjust the content of LiF, the content of LiF can also be adjusted and controlled by acidic solution treatment. Through the preparation process of the present invention, it is possible to achieve mixing of LiF and hard carbon negative electrode matrix material, and part of the LiF will enter the body of the negative electrode material, which is different from the simple physical mixing method when preparing the existing electrode sheet, so that the role of LiF in the negative electrode can be fully exerted.

[0051] (2) The hard carbon negative electrode material prepared by the present invention has a structure including one or more of a layered structure, a block structure, an amorphous structure or a tubular structure. Regardless of the structure, the prepared hard carbon negative electrode material mainly adopts the adsorption-desorption principle of lithium ions and / or sodium ions during the charge and discharge process. Therefore, it has a high specific capacity, significantly faster reaction kinetics, and excellent rate performance. The reasons are as follows.

[0052] The hard carbon negative electrode material obtained by the preparation process of the present invention, regardless of the structure of the waste fluorinated carbon raw material, when it is used as the negative electrode of the secondary battery, lithium, sodium and other ions will not be embedded in the carbon layer during the charge and discharge process, but will be stored in an adsorption-desorption manner, that is, the active ions will not be stored in a de-embedding manner. This is different from the working method of the existing commercial negative electrode; the reason is that a large amount of LiF generated during the battery discharge process and the decomposition products of the electrolyte solvent will enter the CF x The structure of the body, thereby hindering the reaction of lithium / sodium ion insertion; and some CF2 in the carbon fluoride itself does not participate in the reaction, forming an inactive area; in addition, during the fluorination process, the fluorine atoms embedded in the carbon layer will break the spacing between carbon atoms and the interlayer spacing, which causes the carbon layer structure to transform from the original planar structure to a tortuous boat-type or chair-type structure. Therefore, regardless of the structure of the carbon fluoride used as the raw material, that is, whether the carbon fluoride is layered, blocky, amorphous, or tubular, the electrochemical reaction mechanism during the charge and discharge process of the hard carbon negative electrode prepared by the present invention is mainly based on the active ion adsorption-desorption mechanism, and only a very small part of the capacity in this process is provided by the active ion insertion-desorption mechanism.

[0053] (3) The preparation method provided by the present invention also provides a stable channel for obtaining raw materials of hard carbon negative electrode materials, which greatly alleviates the environmental burden brought by waste carbon fluoride batteries and effectively improves their utilization value. Through the simple process treatment of the present invention, not only high-quality hard carbon negative electrode materials can be obtained, but also the problem of waste batteries is solved, which has very high commercial value.

[0054] Therefore, the hard carbon negative electrode material prepared by the present invention has a special chemical composition, that is, the hard carbon raw material contains LiF, a component that helps to form a stable solid electrolyte film on the negative electrode surface and improve the material microstructure; the hard carbon negative electrode material of the present invention has a high discharge capacity and first coulombic efficiency. By controlling the content of LiF within a preferred range and improving the distribution characteristics of LiF in the hard carbon negative electrode material matrix, a hard carbon negative electrode material based on the active ion adsorption-desorption mechanism is formed, thereby enabling the hard carbon negative electrode material prepared by the present invention to have a specific capacity of more than 300 mAh / g, which is effective for lithium ion. The battery has an initial coulombic efficiency of more than 73%, even as high as more than 76%, and the specific capacity does not decay after 100 charge and discharge cycles, showing excellent cycle performance; even when applied to sodium batteries, the initial coulombic efficiency exceeds 70%, and the specific capacity decays less after 100 charge and discharge cycles, showing good cycle performance; the hard carbon negative electrode material of the present invention is applied to secondary batteries, which can have excellent electrochemical properties (such as discharge specific capacity, cycle performance and initial coulombic efficiency), and the hard carbon negative electrode material of the present invention can be applied not only to lithium-ion batteries, but also has good application prospects in sodium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The technical solutions of the embodiments of the present invention are further described in detail below through the accompanying drawings and examples.

[0056] Figure 1 A flow chart of a method for preparing a hard carbon negative electrode material provided in an embodiment of the present invention;

[0057] Figure 2 (a) is a scanning electron microscope (SEM) image of the hard carbon negative electrode material provided in Example 1 of the present invention without surface treatment;

[0058] Figure 2 (b) is a scanning electron microscope (SEM) image of the hard carbon negative electrode material after surface carbonization treatment provided in Example 1 of the present invention;

[0059] Figure 3 (a) is a discharge curve of a CR2032 button-type lithium battery assembled with the hard carbon negative electrode material provided in Example 1 of the present invention;

[0060] Figure 3 (b) A discharge curve of a CR2032 button-type sodium battery assembled with the hard carbon negative electrode material provided in Example 1 of the present invention;

[0061] Figure 4 (a) Cycling performance and Coulombic efficiency curve of a CR2032 button-type lithium battery assembled with the hard carbon negative electrode material provided in Example 1 of the present invention;

[0062] Figure 4(b) Cycling performance and coulombic efficiency curve of a CR2032 button-type sodium battery assembled with the hard carbon negative electrode material provided in Example 1 of the present invention;

[0063] Figure 5 A discharge capacity curve of a lithium metal button cell assembled with a hard carbon negative electrode material having an acidic solution treatment to adjust the LiF content provided in Example 2 of the present invention;

[0064] Figure 6 (a) shows the adsorption-desorption isotherm and pore size distribution of nitrogen for the hard carbon negative electrode material before physical modification provided in Example 3 of the present invention;

[0065] Figure 6 (b) Nitrogen adsorption-desorption isotherm and pore size distribution of the hard carbon negative electrode material modified by physical methods provided in Example 3 of the present invention;

[0066] Figure 7 A comparison chart of the cycle performance and coulombic efficiency of a CR2032 button-type lithium battery assembled with the hard carbon negative electrode material before and after physical modification provided in Example 3 of the present invention;

[0067] Figure 8 This is an SEM image of the hard carbon negative electrode material provided in Example 4 of the present invention;

[0068] Figure 9 A graph showing the cycle performance and coulombic efficiency of a CR2032 button-type lithium battery assembled with the hard carbon negative electrode material provided in Example 4 of the present invention;

[0069] Figure 10 A comparison chart of the cycling performance and coulombic efficiency of CR2032 button-type lithium batteries assembled with the hard carbon negative electrode material of Example 1 of the present invention and the hard carbon material without LiF of Comparative Example 1;

[0070] Figure 11 A comparison of the cycling performance and coulombic efficiency of CR2032 button-type lithium batteries assembled with the hard carbon negative electrode material of Example 1 of the present invention and the fluorinated carbon material of Comparative Example 2;

[0071] Figure 12 A comparison chart of the cycling performance and coulombic efficiency of CR2032 button-type lithium batteries assembled with the hard carbon negative electrode material of Example 1 of the present invention and the hard carbon negative electrode material of Comparative Example 3 that has not been subjected to multiple step over-discharges;

[0072] Figure 13 This is a comparison chart of the cycle performance and coulombic efficiency of CR2032 button-type lithium batteries assembled with the hard carbon negative electrode material of Example 1 of the present invention and the hard carbon material of Comparative Example 4 with unregulated LiF content.

[0073] Figure 14This is a comparison chart of the second discharge curves of CR2032 button-type lithium batteries assembled with the hard carbon negative electrode material of Example 1 of the present invention and the hard carbon material obtained by pre-treatment in different step-by-step over-discharge steps of Example 5.

[0074] Figure 15 This is a comparison chart of the second discharge curves of CR2032 button-type lithium batteries assembled with the hard carbon negative electrode material of Example 1 of the present invention and the hard carbon material obtained by different high-temperature surface treatment methods of Example 6. DETAILED DESCRIPTION

[0075] The present invention is further described in detail below through the accompanying drawings and specific embodiments, but it should be understood that these embodiments are for a better understanding of the present invention, are not limited to the described best mode of implementation, and do not limit the content and scope of protection of the present invention. Any product that is identical or similar to the present invention and is obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the scope of protection of the present invention.

[0076] An embodiment of the present invention provides a hard carbon negative electrode material, comprising a fluorinated carbon material and in-situ generated LiF; wherein the mass percentage of LiF to the mass percentage of the hard carbon negative electrode material is 2% to 35%, preferably 5% to 15%.

[0077] The fluorinated carbon material includes one or more of fluorinated graphite, fluorinated graphite polymer, fluorinated carbon nanotube, fluorinated carbon fiber, fluorinated graphene, fluorinated hard carbon, fluorinated coke, and fluorinated carbon black.

[0078] The hard carbon negative electrode material includes one or more of a layered structure, a block structure, an amorphous structure or a tubular structure. The specific surface area of ​​the hard carbon negative electrode material provided by the embodiment of the present invention is greater than or equal to 100m 2 / g.

[0079] The method for preparing the hard carbon negative electrode material provided by the embodiment of the present invention is as follows: Figure 1 As shown, it specifically includes the following three steps:

[0080] Step S1, performing multiple step-rate over-discharge treatments on the carbon fluoride battery after discharge to obtain a carbon fluoride battery to be disassembled;

[0081] Among them, the lower limit of the discharge voltage of the multiple step rate over-discharge treatment is less than or equal to 1.5V, preferably between 0.3V and 1.5V; the specific process of the multiple step rate over-discharge treatment is: the carbon fluoride battery after discharge is discharged at a rate of 0.1C to 0.2C to a voltage less than or equal to 1.5V, then discharged at a rate of 0.05C to less than 0.1C to a voltage less than or equal to 1.0V, and then discharged at a rate of 0.02C to less than 0.05C to a voltage between 0.3V and 0.5V;

[0082] Preferably, in this step, during the process of multiple step-rate over-discharge treatments from high to low rates, the lower limit of the discharge voltage is gradually reduced.

[0083] Step S2, disassembling the carbon fluoride battery to be disassembled, taking out the carbon fluoride positive electrode, cleaning it, and collecting the carbon fluoride material;

[0084] Among them, the methods for cleaning the carbon fluoride positive electrode and collecting the carbon fluoride material in this step are conventional methods in the prior art;

[0085] The method for cleaning the carbon fluoride positive electrode is specifically as follows: a first solvent can be used to clean the surface of the carbon fluoride positive electrode to remove residual electrolyte; the first solvent used is any one of ethylene glycol dimethyl ether (DME), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), dimethylformamide (DMF), acetonitrile, and tetrahydrofuran;

[0086] The method for collecting fluorinated carbon materials specifically comprises: placing the waste fluorinated carbon positive electrode disassembled from the fluorinated carbon battery into a second solvent and assisting with ultrasonic stripping, or using a mechanical stripping method, or using a chemical method to collect, so that the fluorinated carbon material is separated from the current collector and collected; the second solvent is dichloromethane, acetone, ethyl acetate, ethanol, 2-butanone, cyclohexane, isopropyl alcohol (IPA), n-heptane, water, butanol, acetic acid, octane, N, N-dimethylformamide (DMF), cyclohexanone, cycloethanol, N, N-dimethylacetamide, 1,2-propylene glycol, ethylene glycol (EG), N-methylpyrrolidone (NMP), acetamide, diethylene glycol, glycerol Any one; among them, the mechanical stripping method for collecting carbon fluoride materials includes: scraping the carbon fluoride directly from the current collector with a tool such as a scalpel, or peeling the carbon fluoride material from the current collector with a tool such as tweezers, or crushing the electrode with a grinder and then screening out the aluminum current collector, or using cotton cloth dipped in water, ethanol or NMP, and separating the carbon fluoride from the current collector by repeated wiping; the chemical method for collecting carbon fluoride materials, specifically, according to the different chemical properties of carbon fluoride and the current collector aluminum foil, using a strong alkaline solution to directly dissolve the aluminum foil without destroying the carbon fluoride material, the alkaline solution is sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, strontium hydroxide, barium hydroxide, etc.

[0087] In the present invention, the carbon fluoride positive electrode cleaning process does not use strong acid (such as sulfuric acid, nitric acid, perchloric acid, selenic acid, hydrobromic acid, hydroiodic acid, chloric acid) as the second solvent to soak the spent carbon fluoride positive electrode. Improper control of strong acid soaking will result in too little lithium fluoride or even close to zero, thereby affecting the performance of the material.

[0088] The fluorinated carbon material collected in step S2 includes one or more of: fluorinated graphite, fluorinated graphite polymer, fluorinated carbon nanotube, fluorinated carbon fiber, fluorinated graphene, fluorinated hard carbon, fluorinated coke, and fluorinated carbon black.

[0089] Step S3, sintering the fluorinated carbon material at high temperature under an inert atmosphere and / or treating it with an acidic solution to obtain a hard carbon negative electrode material;

[0090] The content of LiF in the obtained hard carbon negative electrode material is 2wt% to 35wt%, preferably 5wt% to 15wt%; its shape includes one or more of a layered structure, a block structure, an amorphous structure or a tubular structure;

[0091] In this step, the purpose of the high-temperature sintering and / or acidic solution treatment of the carbon fluoride material obtained in step S2 is to control the LiF content in the carbon fluoride material. In addition, during the high-temperature sintering process, the heteroatom groups generated on the surface of the waste carbon fluoride due to the reaction with the electrolyte during the discharge process can also be carbonized, thereby reducing the occurrence of side reactions in the subsequent cycle process.

[0092] Specifically, the high temperature sintering process includes one-stage sintering or segmented sintering;

[0093] The specific conditions for one-stage sintering are: setting the heating rate to 1℃ / min~10℃ / min, the sintering temperature to be greater than or equal to 800℃, and the holding time to be ≥1 hour;

[0094] The staged sintering is specifically as follows: in an inert gas atmosphere, the carbon fluoride material is placed in a heating device, heated to a first temperature at a heating rate of 1°C / min to 10°C / min, and kept at this temperature for 0.5 to 24 hours, heated to a second temperature at a heating rate of 1°C / min to 10°C / min, and kept at this temperature for 0.5 to 24 hours, and then heated to a third temperature at a heating rate of 1°C / min to 10°C / min, and kept at this temperature for 0.5 to 8 hours; the first temperature is greater than or equal to 300°C and less than 650°C; the second temperature is greater than or equal to 650°C and less than 800°C; and the third temperature is greater than or equal to 800°C;

[0095] The acidic solution treatment method comprises: impregnating, rinsing, spraying or atomizing the fluorinated carbon material or the fluorinated carbon material that has been subjected to high-temperature sintering treatment with an acidic solution to obtain a hard carbon negative electrode material with a controlled LiF content; then, washing the hard carbon negative electrode material with the controlled LiF content with an alkaline solution and / or a neutral reagent;

[0096] The specific operation of the acid solution treatment may include: first determining the total content of LiF in the collected fluorinated carbon material by chemical titration, and then preparing a dilute acid solution with an appropriate molar number to dissolve and control the LiF content contained in the material. For the same batch of materials, a control test method can be used. For example, for the material obtained in step S2, a certain amount of control sample is weighed, and the LiF content is determined by chemical titration. After calculation, different molar numbers of acid solutions are prepared to treat the hard carbon negative electrode material. That is, different molar numbers of acid are used for treatment according to different target LiF content. The specific chemical reaction equation is: LiF + H + →Li + +HF↑;

[0097] The acidic solution is one or more inorganic or organic acids, specifically including one or more of sulfuric acid, nitric acid, phosphoric acid, sulfurous acid, carbonic acid, boric acid, oxalic acid, acetic acid, carbonic acid, and sulfinic acid;

[0098] The alkaline solution adopts one or more of inorganic base, organic base or alkaline salt solution, specifically including one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, ammonia water, ferric hydroxide, aluminum hydroxide, magnesium hydroxide, sodium acetate, sodium silicate and sodium phosphate.

[0099] The application of the hard carbon negative electrode material obtained by the above-mentioned preparation method in the embodiment of the present invention can be used in secondary batteries, and can make the secondary batteries have good electrochemical properties such as discharge specific capacity, cycle performance and first coulombic efficiency, wherein the secondary battery is preferably a lithium-ion battery or a sodium-ion battery; the above-mentioned hard carbon negative electrode material of the present invention can also be used in lithium battery modules or sodium battery modules, and when used in terminal products, it also has high energy and excellent cycle characteristics.

[0100] In this application, regarding the selection of raw materials, the types of waste fluorinated carbons collected include fluorinated graphite, fluorinated graphite polymers, fluorinated carbon nanotubes, fluorinated carbon fibers, fluorinated graphene, fluorinated hard carbon, fluorinated coke, fluorinated carbon black, etc. Compared with waste graphite negative electrodes or mesophase carbon microspheres, the structures of these two types of materials have been destroyed after long cycles, and their performance after recycling is significantly lower than that of the original products. However, the waste fluorinated carbon primary battery materials used in this invention have only undergone a single discharge process, so the material structure remains relatively intact. The use of waste fluorinated carbon after discharge as raw material to prepare hard carbon has a complete structure that can provide more reactive sites, thereby having a larger discharge capacity.

[0101] In particular, due to the unique reaction mechanism of carbon fluoride, the waste carbon fluoride after discharge generates a rich content of LiF in situ, without the need for subsequent separate LiF replenishment. When existing negative electrode materials are supplemented with LiF separately, on the one hand, due to the stable properties of LiF and the extremely small content in the negative electrode SEI film, it is difficult to control the specific addition amount, thereby affecting the electrochemical performance of the battery. In addition, the separately added LiF cannot be guaranteed to be evenly distributed on the surface of the negative electrode material. If the distribution is uneven, it will deteriorate the material performance. The hard carbon negative electrode prepared by the present invention has a unique composition during the preparation process, and also solves the problems caused by the subsequent separate LiF replenishment. Moreover, based on the reaction mechanism and the inherent properties of LiF, the LiF content can be controlled, further improving the optimization effect of LiF on the interface stability, thereby significantly improving the cycle life and capacity of the hard carbon negative electrode prepared by the hard carbon negative electrode material of the present invention.

[0102] In the present application, regarding the influence of the LiF content in the hard carbon negative electrode material, the hard carbon negative electrode material obtained by the above-mentioned preparation method, wherein the LiF content needs to be controlled between 2wt% and 35wt%, and can be any value within the above-mentioned range, such as 2wt%, 10wt%, 20wt%, 25wt%, 35wt%, preferably any value within the range of 5wt% to 15wt%. This is because the applicant found during the research process that when the LiF content in the hard carbon negative electrode material of the present invention is controlled within the above-mentioned range, the electrochemical performance of the hard carbon negative electrode material can be improved; the application of the hard carbon negative electrode material of the present invention in a secondary battery can effectively improve the cycle performance and rate performance of the secondary battery. For example, for a lithium battery, when the LiF content is between 2wt% and 35wt%, the discharge specific capacity is ≥300mAh / g. Optimally, when the LiF content is between 5wt% and 15wt%, the discharge specific capacity is ≥320mAh / g.

[0103] The inventors found that when the LiF content is lower than 2wt% or higher than 35wt%, the discharge specific capacity of the hard carbon negative electrode material is less than or equal to 300mAh / g, but still greater than the discharge specific capacity of 240mAh / g of hard carbon without LiF on the market.

[0104] In the present application, in order to make all the F in the waste fluorinated hard carbon material react in situ to generate LiF, a method of performing multiple step-rate discharges on the discharged waste fluorinated carbon battery is adopted, which can generate as much LiF as possible in the waste fluorinated carbon material and reduce other forms of fluoride.

[0105] The reasons why the present invention adopts a method of multiple step-rate over-discharge treatment to generate LiF in situ, as well as the specific process, are described in detail below.

[0106] During the research process, the applicant found that theoretically, the content of LiF in the carbon fluoride material in the waste carbon fluoride battery is between 0-68wt%. However, depending on the use of the waste carbon fluoride battery, such as the carbon fluoride battery is fully discharged, partially discharged, or scrapped without being discharged, the content of LiF in the waste carbon fluoride material varies greatly. In addition, the existence of fluorine also includes CF x And many other situations.

[0107] The preparation method of the novel hard carbon negative electrode provided by the present invention adopts the electrochemical method of multiple step-rate over-discharge treatment described above, and the obtained hard carbon negative electrode material contains LiF, a key component that helps form a more stable solid electrolyte film on the negative electrode surface. On the one hand, it is well known that the main components of the SEI film on the negative electrode surface include LiF, etc. The presence of LiF can form a more stable electrode / electrolyte film, greatly improving the cycle performance of the material, especially carbonaceous materials. Currently, there is a large amount of research on how to introduce LiF on the surface of the negative electrode material. However, the present invention does not require the additional introduction of LiF into the negative electrode material, and the presence of an appropriate amount of LiF in the negative electrode material can be achieved only through the preparation process, thereby significantly improving the discharge capacity and cycle performance of the material. In addition, the element F has the strongest electronegativity. Excessive F or fluorine compounds present in other forms will reduce the conductivity of the material, causing the battery to polarize and the capacity to be unable to function normally. Therefore, it is necessary to control the F content in the negative electrode material. The main fluoride present in waste carbon fluoride is carbon fluoride, which is hydrophobic in nature. The most effective method is to remove the F element by electrochemical method. Therefore, the present invention subjects used waste carbon fluoride batteries to multiple over-discharges and strictly controls the discharge voltage and discharge time to fully allow the F element contained to react with Li to convert into the beneficial LiF. This can effectively remove the F element in the raw material that causes adverse factors and effectively control the resulting LiF content.

[0108] For the above different situations of waste carbon fluoride batteries, multiple step rate over-discharges can be used to remove the electrochemically active F in the material and at the same time remove the CF x The present invention sets a wider voltage range and a smaller discharge current to remove the electrochemically active F element in the original waste fluorinated carbon material as much as possible, and can also make more metal lithium and CF x reaction, and the active F element can be gradually removed by multiple over-discharges at step rates, ultimately achieving the goal of almost completely removing the active F element and promoting the in-situ generation of LiF in the fastest way to the greatest extent.

[0109] The present invention uses the above-mentioned multiple step-rate overdischarge process to generate LiF from the F element in situ. The step rate and voltage are set specifically according to the LiF content in the waste fluorinated carbon material: if the LiF content in the fluorinated carbon material is low, the discharged fluorinated carbon battery can be overdischarged to a lower voltage using a small rate current multiple times. This can promote the generation of more LiF and reduce other fluorides in the material system. The main reaction is to convert other fluorides into LiF. If the LiF content in the fluorinated carbon material is high, the multiple step-rate overdischarge process of the present invention can also minimize the content of other fluorides, thereby avoiding performance deterioration of the obtained hard carbon negative electrode material when used in a secondary battery.

[0110] When the above-mentioned multiple step-rate over-discharge process is used to electrochemically treat the spent carbon fluoride battery, in the over-discharge treatment stage, the voltage is set to be less than the discharge voltage of the carbon fluoride material and greater than the voltage at which the metallic lithium and the aluminum current collector form an alloy. The specific lower limit of the discharge voltage is less than or equal to 1.5V, preferably between 0.3V and 1.5V, and the rate is set to a small rate current in a step-like mode to promote the formation of LiF.

[0111] The specific process of the multiple step-rate over-discharge treatment of the present invention is to over-discharge the carbon fluoride battery after the discharge is completed through the following three stages:

[0112] The first stage of overdischarge is to discharge at a rate of 0.1C to 0.2C until the voltage is less than or equal to 1.5V, where the rate and voltage values ​​can be any values ​​within the above range, such as the rate of 0.1C, 0.12C, 0.14C, 0.15C, 0.16C, 0.18C, 0.2C, etc., and the lower limit of the discharge voltage can be 1.5V, 1.4V, 1.3V, 1.2V, 1.1V, 1.0V, 0.8V, 0.5V, 0.4V, 0.3V, etc., but is not limited to the values ​​listed above; the lower limit of the discharge voltage in this stage is set to below 1.5V to promote the formation of LiF;

[0113] The second stage of overdischarge is to discharge at a rate of 0.05C to less than 0.1C until the voltage is less than or equal to 1.0V. It can be any value within the above range, such as the rate of 0.05C, 0.06C, 0.07C, 0.08C, 0.09C, and the lower limit of the discharge voltage can be 1.0V, 0.9V, 0.8V, 0.7V, 0.6V, 0.5V, 0.4V, 0.3V, etc., but is not limited to the listed values. In this stage, overdischarge at a lower rate is used to better promote the formation of LiF, and the lower overdischarge voltage can further promote the conversion of more fluorine elements into LiF.

[0114] The third stage of overdischarge is to discharge at a rate of 0.02C to less than 0.05C until the voltage is between 0.3V and 0.5V. It can be any value within the above range, such as 0.02C, 0.03C, 0.04C, and the lower limit of the discharge voltage can be 0.3V, 0.35V, 0.4V, 0.45V, 0.5V, etc., but is not limited to the listed values. In this stage, further overdischarge at a lower rate can be used to maximize the generation of LiF.

[0115] Preferably, during the multiple step-rate over-discharge treatments from high to low, the lower limit of the discharge voltage is gradually reduced. By gradually reducing the over-discharge voltage, this step-by-step voltage transition can also promote the conversion of fluorine element into LiF to the greatest extent.

[0116] It should be further explained that, in the process of multiple step-rate over-discharge treatment from high to low, the lower limit of the discharge voltage is gradually reduced, which is a preferred solution. The lower limit value of the discharge voltage in the process of multiple step-rate over-discharge can also be the same or irregular. For example, the lower limit values ​​of the discharge voltage are 0.3V, 0.5V, and 0.4V, respectively, which can meet the requirement of promoting the conversion of fluorine element into LiF.

[0117] In the present application, regarding how to control the LiF content, after the LiF-containing carbon fluoride material is collected after the above-mentioned multiple step-rate over-discharge treatments, it is necessary to further perform high-temperature sintering treatment and / or acid solution treatment to adjust the LiF content to between 2wt% and 35wt%.

[0118] The principle and specific process of controlling the content of in-situ generated LiF by high-temperature sintering treatment in the present invention are described in detail below.

[0119] During the research process, the inventors found that when the temperature is around 800°C, some LiF has been separated from the main carbon material under the blowing of inert gas. By heating the collected waste fluorocarbons under high temperature inert conditions, for example, heating at a temperature above the melting point of LiF under inert gas conditions, the LiF content can be controlled within a suitable range by controlling the heating temperature and holding time. For example, by carrying out the above-mentioned high-temperature calcination in a tubular furnace, the LiF is melted and then carried away by the inert gas flow, such as holding at 900°C for 2 hours, the LiF content is generally reduced by about 4wt%. However, when different batches of waste fluorocarbons and the types of waste fluorocarbons are used for treatment vary, the reduction in LiF content brought about by the above-mentioned high-temperature treatment also fluctuates within a certain range.

[0120] In this application, the high-temperature sintering process includes two parallel schemes: one-stage sintering and segmented sintering.

[0121] Among them, the specific conditions of one-stage sintering are: under an inert gas atmosphere, place the carbon fluoride material in a heating device, set the heating rate to 1℃ / min~10℃ / min, the sintering temperature is greater than or equal to 800℃, and the holding time is ≥1 hour; the one-stage sintering is simple to operate and low in cost.

[0122] The staged sintering is to place the carbon fluoride material in a heating device under an inert gas atmosphere, and then go through three sintering stages, namely:

[0123] The first sintering stage involves heating the material to a temperature between 300°C and 650°C at a rate of 1°C / min to 10°C / min, and then maintaining the temperature for 0.5 to 24 hours. During this sintering stage, the heteroatom groups formed on the surface of the waste fluorinated carbon material due to reaction with the electrolyte during the discharge process are carbonized and removed, thereby reducing side reactions in the subsequent material recycling process.

[0124] The second sintering stage is to heat the temperature to 650℃ or higher and less than 800℃ at a heating rate of 1℃ / min to 10℃ / min and keep the temperature for 0.5 hours to 24 hours. In this sintering stage, the inactive solid CF in the recovered waste fluorinated carbon material is x It will be converted into gas and volatilized, which can further remove the unnecessary inactive F elements in the material;

[0125] The third sintering stage involves heating the material to 800°C or higher at a rate of 1°C / min to 10°C / min and holding the temperature for 0.5 to 8 hours. During this sintering stage, the LiF gradually melts, and further increasing the temperature causes the liquefied LiF to volatilize. By controlling the heating temperature and holding time, the LiF content can be controlled within an appropriate range.

[0126] During the discharge process of carbon fluoride batteries, a large amount of inorganic salts and organic impurity functional groups will be formed on the electrode surface. These generated impurities will not be removed during the raw material collection process, but will subsequently have a great impact on the specific capacity and coulomb efficiency of the material. The present invention adopts an inert gas-protected high-temperature pyrolysis method to remove impurities and salts on the surface of the waste carbon fluoride raw materials obtained, thereby improving the capacity and coulomb efficiency of the material. Usually, the carbonization temperature of organic impurities is 300°C. Above this temperature, the organic matter will be carbonized and converted into electrochemically active beneficial substances, while the salts will undergo thermal decomposition and can be removed by subsequent washing with pure water or ethanol. In order to ensure that the carbon material of the waste carbon fluoride itself is not oxidized during the pyrolysis process, the pyrolysis process is carried out under inert gas conditions. Preferably, the heating rate of the pyrolysis process is 1°C / min-10°C / min, and the pyrolysis time is 0.5 hours-24 hours. In particular, the heating temperature can be heated at a lower temperature for a period of time, and then further kept at a sintering temperature ≥650°C for a certain period of time, and the inactive solid CF in the recovered waste carbon fluoride is recovered.x It will be converted into gaseous state and volatilized, which can further remove the F element in the material. When the temperature is ≥800℃, heating at this temperature for a certain time, which is close to or exceeds the melting point of LiF, can further control the content of LiF in the negative electrode material by adjusting the heating temperature and time and keeping it at a certain temperature for a period of time. That is, when the content of LiF is excessive, part of LiF can be removed within the above temperature range, thereby achieving the purpose of controlling the content of LiF.

[0127] The hard carbon negative electrode material obtained by the above-mentioned preparation method of the present invention has many morphologies. Depending on the type of the collected fluorinated carbon raw material, it can be a layered structure, a block structure, an amorphous structure or a tubular structure. Therefore, the present invention can obtain hard carbon materials of corresponding morphologies simply by selecting the morphology of the raw material. The processing technology is simple and has diverse morphological characteristics, which is significantly different from the hard carbon materials currently used commercially. The hard carbon currently circulating on the market is limited by the preparation technology, and most of them are irregular blocks, and a few are spherical or flaky. The hard carbon obtained by the present invention has multiple morphological characteristics, which is a unique effect of the preparation technology of the present invention.

[0128] Furthermore, the electrochemical reaction mechanism of the hard carbon negative electrode material prepared by the present invention differs from that of currently commercialized carbon materials. For example, in carbonaceous graphite negative electrodes, the interlayer spacing of graphite's carbon atoms is 0.142 nm, held together by covalent bonds, and the interlayer spacing is 0.335 nm, held together by van der Waals forces. Therefore, other substances or elements can easily be embedded between the graphite layers. During the charge and discharge process of lithium / sodium batteries, the electrochemical reaction mechanism is an insertion-extraction mechanism, exhibiting a typical charge-discharge plateau. However, hard carbon negative electrodes lack a regular layered structure, and their electrochemical reaction mechanism for storing active lithium / sodium ions is an absorption-extraction mechanism, resulting in a steep charge-discharge curve. For layered negative electrode materials like graphite, the electrochemical reaction mechanism for storing active lithium / sodium ions during charge and discharge is an extraction-extraction mechanism. However, unlike commercially used graphite materials, the negative electrode material prepared by the present invention, even if it is a hard carbon material with a layered structure prepared from graphite fluoride as a raw material, the lithium / sodium ions will not be embedded in the interlayer of the negative electrode material during the charge and discharge process, that is, the electrochemical reaction will not be carried out in the form of deintercalation, but the active ions will be stored in the form of adsorption-desorption. For the hard carbon negative electrode material prepared by the present invention, regardless of its structure, its reaction mechanism is the adsorption-desorption mechanism. This is because a large amount of LiF and electrolyte decomposition products generated in the waste fluorinated carbon enter the CF xIn the matrix structure, part of the CF2 does not participate in the reaction and is an inactive area, which hinders the lithium / sodium insertion reaction. In addition, during the fluorination process, the fluorine atoms embedded in the carbon layer will break the distance between carbon atoms and the interlayer distance. The carbon layer structure is transformed from the original planar structure to a tortuous boat-type or chair-type structure. This results in the raw material being a layered structure, a block structure, an amorphous structure or a tubular structure. After the hard carbon negative electrode is prepared by the present invention, the electrochemical reaction mechanism is mainly an adsorption-desorption mechanism rather than an embedding-desorption mechanism. Only a very small part of the capacity is provided by the embedding-desorption mechanism. This is due to the structural characteristics of the hard carbon negative electrode product of the present invention that are significantly different from the prior art. Moreover, when the raw material is fluorinated graphite or other fluorinated carbon materials containing lamellar morphology, the hard carbon negative electrode material prepared by the present invention can also retain the original layered structure, which greatly improves the reaction active area.

[0129] In addition, the above-mentioned multiple step-rate over-discharge process method can not only control the content of LiF within the preferred range, but also improve the distribution of LiF in the hard carbon negative electrode material matrix. The reason is that: on the one hand, the multiple step-rate over-discharge process makes the CF x As much F in the bulk phase participates in the reaction to generate LiF as much as possible, and during the normal discharge process, this part of CF x It will not participate in the reaction; on the other hand, the post-treatment process (acid washing or high temperature) will remove excess LiF, especially in places where LiF is concentrated (such as layered cross-sections), making the overall distribution of LiF more uniform, and the carbon material can better improve its electronic conductivity.

[0130] In an optional solution, when the hard carbon negative electrode material has a layered structure (such as graphite fluoride), the preparation method provided in the embodiment of the present invention may further include: treating the hard carbon negative electrode material using one or more of a physical method, a chemical method, or an electrochemical method to increase the reactive area of ​​the hard carbon negative electrode material, thereby improving the material specific capacity and the first coulombic efficiency;

[0131] Using physical methods to increase the reactive area of ​​the hard carbon negative electrode material, including: one or more methods of solvent thermal intercalation, ultrasonic stripping, tape stripping or high-energy ball milling;

[0132] Among them, the solvent thermal intercalation method in the physical method is to stir the hard carbon negative electrode material with a solvent with a large molecular weight and a low boiling point, and then place it in a high-temperature reactor. The macromolecular solvent is inserted between the carbon layers under high temperature and high pressure, and peeled off by expansion to achieve the purpose of increasing the reaction active area; the solvent used can be dichloromethane, acetone, chloroform, methanol, tetrahydrofuran, 1,1,1-trichloroethane, ethyl acetate, ethanol, butanone, cyclohexane, acetonitrile, isopropanol, ethylene glycol dimethyl ether, n-heptane, water, toluene, pyridine, ethylenediamine, butanol, acetic acid, ethylene glycol monomethyl ether, octane, butyl acetate, N,N-dimethyl The solvent may be selected from the group consisting of methylformamide, cyclohexanone, cycloethanol, N,N-dimethylacetamide, 1,2-propylene glycol, dimethyl sulfoxide, ethylene glycol, p-cresol, N-methylpyrrolidone (NMP), benzyl alcohol, cresol, acetamide, ethylene glycol carbonate, diethylene glycol, succinonitrile, sulfolane, and glycerol. The temperature may be set to be appropriately higher than the boiling point of the selected solvent. The mass ratio of the material to the solvent may be 0.2 to 0.02. The holding time of the hot intercalation layer may be 0.5 to 24 hours. During the hot intercalation process, in order to ensure a more uniform reaction, the reactor may be set to a rotation mode with a rotation speed of 10 to 160 r / min.

[0133] The ultrasonic stripping method in the physical method is to mix the obtained hard carbon negative electrode material evenly in a solvent, and then place it on an ultrasonic device for ultrasonication to achieve the purpose of stripping the layer and thus improving the electrochemical reaction active sites; the solvent used can be one of dichloromethane, acetone, ethyl acetate, ethanol, butanone, cyclohexane, isopropanol, n-heptane, water, butanol, acetic acid, octane, N,N-dimethylformamide, cyclohexanone, cycloethanol, N,N-dimethylacetamide, 1,2-propylene glycol, ethylene glycol, N-methylpyrrolidone (NMP), acetamide, diethylene glycol, and glycerol; the ultrasonic stripping time can be 0.5 hours to 72 hours.

[0134] The physical method of tape stripping involves evenly spreading the collected hard carbon anode material on a sticky tape, and then repeatedly bonding it with the tape to achieve the purpose of stripping the sheet and increasing the active area.

[0135] The high-energy ball milling method in the physical method specifically places the collected hard carbon negative electrode material in a ball mill for ball milling, and uses the shear stress of the ball mill during the rotation process to peel off the layered structure, so as to achieve the purpose of increasing the reaction active sites; the ball-to-material ratio can be set to any value between 5 / 1 and 25 / 1; the ball mill speed can be any value between 100r / min and 700r / min; the ball milling time can be 0.5 hours to 24 hours.

[0136] Chemical methods are used to increase the active area of ​​hard carbon anode materials, which actually involves causing them to undergo a redox reaction. Specifically, the mature and typical Hummers method involves treating the original waste fluorinated carbon material or the resulting hard carbon anode material with an inorganic acid, followed by dilution and oxidation with a strong oxidant, and finally drying with air at no more than 30°C. The product is further exfoliated using solvent thermal intercalation or ultrasonic methods, following the same process as the physical method above.

[0137] Using electrochemical methods to subject hard carbon negative electrode materials to multiple high-rate over-discharge treatments can also expand the interlayer spacing of the carbon layer and thereby increase the chemical reaction sites. Specifically, the electrode sheet is prepared using hard carbon negative electrode material and further assembled with metallic lithium or metallic sodium. After that, the electrode is over-discharged at a rate greater than or equal to 3C, and the carbon layer is peeled off through excessive ion intercalation, thereby achieving the purpose of expanding the surface area and increasing the active sites.

[0138] For the hard carbon negative electrode materials with other morphologies and structures obtained in the embodiments of the present invention, ball milling or other methods can also be used to increase the surface area of ​​the material, thereby increasing the reactive area.

[0139] To better understand the technical solutions provided by the present invention, the preparation method, specific applications and characteristics of the hard carbon negative electrode material of the present invention are further described in detail below with reference to a number of specific embodiments and accompanying drawings.

[0140] All materials used in the following examples and comparative examples were purchased from Daikin Industries, Ltd., Japan. The electrolyte and electrolyte additives were obtained from Suzhou Duoduo Chemical Technology Co., Ltd. For experimental procedures or conditions not specified in the examples, conventional experimental procedures or conditions described in literature in the field were followed. Reagents or instruments used without manufacturer identification were commercially available.

[0141] Example 1

[0142] This embodiment provides a preparation process and performance test of a hard carbon negative electrode material, which specifically includes the following steps.

[0143] Step S1. After the discharge is completed, the carbon fluoride battery is subjected to multiple step-rate over-discharge treatments to obtain a carbon fluoride battery to be disassembled. The specific process is as follows:

[0144] This example uses discharged spent lithium / carbon fluoride soft-pack batteries, whose positive electrode plates contain graphite fluoride as the carbon fluoride material. First, the lithium / carbon fluoride soft-pack batteries are discharged at a 0.2C rate to 1.5V, then at a 0.08C rate to 1.0V, and finally at a 0.05C rate to 0.4V, converting all the remaining F in the spent graphite fluoride to LiF.

[0145] Step S2. Disassemble the carbon fluoride battery to be disassembled, remove the carbon fluoride positive electrode, clean it, and collect the carbon fluoride material. The specific process is as follows:

[0146] The battery that has undergone multiple discharge treatments in step S1 is disassembled in a vacuum glove box or a super clean room, the carbon fluoride electrode is taken out, the electrode is cleaned with DMC three times and dried under vacuum conditions; then the cleaned electrode is soaked in an ethanol solution and assisted by ultrasonic vibration, and then the carbon material is peeled off from the current collector with the help of tweezers and dried in a vacuum drying oven and ground into powder to collect the carbon fluoride material.

[0147] Step S3. The fluorinated carbon material is subjected to high-temperature sintering treatment under an inert atmosphere to obtain a hard carbon negative electrode material. The specific process is as follows:

[0148] Weigh 0.5 g of the fluorinated carbon material collected in step S2, place it in a 100 ml ceramic boat and place it in a high-temperature tube furnace and heat it to 800 ° C at a heating rate of 10 ° C / min. Keep it warm for 2 hours under argon atmosphere protection, then wash it with alcohol, filter it and dry it to obtain the final product, hard carbon negative electrode material.

[0149] The LiF content in the hard carbon negative electrode material was tested to be 32.5wt%. The titration test process was as follows:

[0150] Prepare the standard solution: Take laboratory analytical grade concentrated sulfuric acid, slowly pour it into pure water with a glass rod, and stir continuously, and finally dilute it to a 1 mol / L dilute sulfuric acid solution;

[0151] Titration of the LiF content in the obtained hard carbon: Weigh 10g of the hard carbon negative electrode material of Example 1 and 10ml of deionized water, place them in a beaker with a volume of 100ml and stir evenly, slowly add the above-prepared dilute sulfuric acid solution and continue stirring, the sulfuric acid will react with LiF and be consumed. Continuously take the supernatant in a certain period of time and place it in another beaker, add a drop of purple litmus solution until the purple litmus solution just turns red, at which point excess acid begins to appear, indicating that the LiF reaction is complete and the titration is over. Calculate the acid solution consumed, and the equivalent calculation shows that the LiF content in the hard carbon is 32.5wt%.

[0152] Structural characterization of the hard carbon negative electrode material of this embodiment: SEM image of the hard carbon negative electrode material of Example 1, as shown in FIG. Figure 2 As shown, Figure 2 (a) shows the original carbon material without surface treatment. It can be seen that after multiple step-by-step over-discharges, the electrolyte generates a large amount of lithium salts and aggregated organic impurity groups on the surface of the carbon layer, and the morphology of the carbon material is basically obscured. Figure 2(b) shows the hard carbon material after high-temperature treatment under inert gas protection. The surface organic functional groups are carbonized, and the resulting salts are decomposed, revealing the underlying morphology. The hard carbon anode material prepared in this example retains the layered structure characteristic of graphite, with block sizes around 100 μm and carbonized surface heteroatom groups.

[0153] The hard carbon negative electrode material of this embodiment was used to prepare hard carbon pole pieces and assemble batteries, and then the electrochemical performance characterization test was performed. The specific process was as follows:

[0154] Preparation of hard carbon pole piece: Take 100 mg of polyvinylidene fluoride (PVDF), dissolve it in 3 ml of N-methylpyrrolidone (NMP), stir to form a uniform solution, then add 800 mg of hard carbon negative electrode material and 100 mg of superconducting carbon black conductive agent Super P, stir for 8 hours to obtain hard carbon slurry, evenly coat it on copper foil with a coating thickness of 250 μm, and dry the pole piece in a forced air drying oven at 60°C.

[0155] Button Cell Assembly: The resulting electrode sheets were punched into 12mm Φ circular pieces and dried in a vacuum oven at 80°C for 24 hours. Subsequently, in a vacuum glove box, CR2032 button-type lithium and CR2032 button-type sodium batteries were assembled using metallic lithium and sodium sheets as negative electrodes, respectively, and hard carbon sheets as positive electrodes. Conventional commercial electrolytes were used as the electrolyte.

[0156] Battery performance test: Discharge to 0.005V at a current of 30mA / g, then charge to 2V, and perform charge and discharge cycles in this voltage range and current.

[0157] The charge and discharge curves of CR2032 button lithium battery and sodium battery are as follows: Figure 3 (a) and Figure 3 (b) shows the cycle performance and coulomb efficiency curves of CR2032 button lithium battery and sodium battery, respectively. Figure 4 (a) and Figure 4 The electrochemical performance data are shown in Table 1, where the 100-cycle capacity retention rate is calculated based on the second discharge capacity as the basic value.

[0158] Table 1 summarizes the electrochemical performance data of the CR2032 button-type lithium battery and CR2032 button-type sodium battery assembled with the hard carbon negative electrode material of this embodiment:

[0159]

[0160]

[0161] The test results show that when the negative electrode of the button battery is metallic lithium, its first discharge capacity is 416.3mAh / g, the first coulombic efficiency is 73.4%, the second discharge specific capacity is 305.61mAh / g, and the battery's remaining discharge specific capacity after 100 cycles is 306.10mAh / g, which has a very high specific capacity. Relative to the second discharge specific capacity, the capacity retention rate is 100.16%, indicating that the hard carbon negative electrode material prepared by the present invention has no attenuation in specific capacity after 100 cycles, and the hard carbon negative electrode material prepared by the present invention has excellent cyclic stability. Moreover, as the cycle progresses, the battery capacity is appropriately improved, which also shows that the hard carbon negative electrode material prepared by the present invention has a stable structure. In addition, the button lithium battery assembled with this material has a high first coulombic efficiency of up to 73.4%. From Figure 4 In (a), it can be seen that the coulombic efficiency of the battery during the cycle is also stable at above 99%, indicating that the charge and discharge capacity of the hard carbon negative electrode material prepared by the present invention is almost completely reversible during the charge and discharge cycle, which also shows that it has excellent structural stability and therefore has excellent cycle performance.

[0162] When the negative electrode of the button battery is metallic sodium, the first discharge capacity of the button battery is 390.8mAh / g, the first coulombic efficiency is 70.1%, the second discharge capacity is 273.60mAh / g, and the remaining discharge capacity of the battery after 100 cycles is 246.46mAh / g, with a capacity retention rate of 90.08%. The coulombic efficiency of the battery during the cycle is also stable at above 99%. For sodium batteries, due to Na + Radius compared to Li + Due to its large radius and lower thermodynamic activity, its capacity is lower than that of lithium batteries, and its capacity decreases rapidly at the beginning of the cycle, and then stabilizes during the cycle. The capacity decay of the battery mainly occurs in the early stage. For sodium batteries in the current prior art, hard carbon is generally used as the negative electrode material, with a specific capacity between 150mAh / g and 300mAh / g, an initial coulombic efficiency of less than 65%, poor cycle performance, and a low discharge specific capacity retention rate after 100 cycles. However, the hard carbon negative electrode material obtained by the preparation method of the present invention can be used as a hard carbon negative electrode material for sodium batteries to be assembled into sodium battery half-cells due to its sodium storage function. It also has high discharge specific capacity, high initial coulombic efficiency, excellent capacity retention rate, good cycle performance, and excellent battery electrochemical performance.

[0163] Furthermore, the charge-discharge curves of the lithium / sodium battery show that the discharge process (i.e., the lithium / sodium storage process) is sloped, lacking the typical ion intercalation plateau of graphite materials. This clearly demonstrates that the reaction mechanism of the hard carbon anode material of the present invention is a lithium ion absorption-desorption mechanism. Because the presence of LiF, electrolyte solvation products, and active CF2 hinders ion intercalation, the battery's reaction mechanism is an absorption-desorption mechanism, which is typical of hard carbon anode materials. Therefore, the hard carbon anode material prepared by the present invention exhibits excellent electrochemical performance, far exceeding that of existing hard carbon materials.

[0164] Example 2

[0165] In this embodiment, the hard carbon negative electrode material prepared in Example 1 is modified by using an acidic solution treatment to adjust the content of LiF. The specific process is as follows.

[0166] Weigh 10g of the hard carbon anode material from Example 1 and 10ml of deionized water into a 100ml beaker and stir thoroughly. Slowly add the dilute sulfuric acid solution prepared in Example 1 dropwise with continuous stirring. The LiF content was adjusted by calculating the acid content to obtain hard carbon anode materials with LiF contents of 32.5% (untreated), 20%, 10%, 5%, and 0%, respectively.

[0167] CR2032 button-type lithium batteries were prepared using hard carbon negative electrode materials with different LiF contents and electrochemical performance tests were performed. The preparation and testing processes of the button-type batteries were the same as those in Example 1. The discharge capacity curves of batteries assembled with hard carbon negative electrode materials with different LiF contents are shown in FIG. Figure 5 shown.

[0168] Figure 5The second discharge capacity of hard carbon negative electrode materials with 5 different LiF contents is shown. According to the test results, when the LiF content is 0%, its discharge capacity is only 257.5mAh / g, that is, when all LiF is removed, the discharge capacity of the material is low, which is comparable to the level of common biomass hard carbon on the market; when the LiF content is 5%, its discharge capacity is 342.5mAh / g. It can be seen that although it contains a small amount of LiF, the discharge capacity has been greatly improved, far exceeding the capacity of existing commercial hard carbon materials; and when the LiF content is 10%, its discharge capacity is 358mAh / g, which is comparable to the levels of 0% and Compared with the hard carbon negative electrode material with 5% LiF, its discharge specific capacity is further improved, showing the best discharge specific capacity, and its first coulombic efficiency is as high as 76.4%, indicating that by controlling the LiF content in the hard carbon negative electrode material to 10%, it has a very high first coulombic efficiency; when the LiF content is increased to 20%, its discharge specific capacity decreases slightly to 316.4 mAh / g, but still shows a high discharge specific capacity; and when the LiF content is increased to 32.5%, its discharge specific capacity is 305.61 mAh / g, which has a good discharge specific capacity. It can be seen that the second discharge capacity of the hard carbon negative electrode with different LiF contents has a significant difference. When the hard carbon containing 10wt% LiF shows the best electrochemical performance, has the highest discharge specific capacity of 358 mAh / g, and also has a very high first coulombic efficiency. The above LiF content is the optimal value, while when the LiF content is 0%, its specific capacity is very low. Therefore, the electrochemical properties of hard carbon anode materials with different LiF contents are different, which are roughly parabolic in shape. Figure 5 It can be seen that when the LiF content is between 2% and 35%, the hard carbon anode material has a discharge capacity equal to or exceeding 300 mAh / g, showing excellent commercial prospects. The discharge capacity reaches its peak when the LiF content is 10%. Overall, the electrochemical performance of the LiF-containing hard carbon anode material prepared by the preparation process of the present invention is far superior to that of the hard carbon material without LiF.

[0169] Example 3

[0170] This embodiment modifies the hard carbon negative electrode material prepared in Example 1 by physical methods to increase its electrochemical reaction active sites. The specific process is as follows:

[0171] 5 g of the hard carbon negative electrode material of Example 1 was weighed and placed in a high-energy ball mill with a volume of 100 ml. The ball-to-material ratio was set to 20:1, the ball material was zirconia, the ball mill speed was set to 600 r / min, and the ball milling time was 4 hours. The shear force and cutting force of the ball milling process were used to peel off the lamellar hard carbon to expand the active sites of the electrochemical reaction.

[0172] The changes in pores and specific surface area of ​​hard carbon negative electrode materials before and after ball milling are shown in Figure 2. Figure 6 (a) Figure 6 (b), as shown in Table 2; Figure 6 (a) is the adsorption-desorption isotherm curve and pore size distribution of the hard carbon negative electrode material for nitrogen before physical modification. Figure 6 (b) is the adsorption-desorption isotherm curve and pore size distribution diagram of the hard carbon negative electrode material modified by physical methods in this embodiment for nitrogen.

[0173] Depend on Figure 6 As shown in (a) and (b), the curves of the two materials show typical type IV curve characteristics, which indicates that both materials are mesoporous materials before and after ball milling. The specific surface area of ​​the unmilled hard carbon negative electrode material calculated by the BET multi-point method is 125.60 m 2 / g, the total pore volume is 0.095ml / g; and the specific surface area of ​​the hard carbon negative electrode material after ball milling is 220.77m 2 / g, and the total pore volume is 0.21ml / g. After physical ball milling treatment, the specific surface area is increased by nearly two times, and the total pore volume is expanded to 2.2 times the original. The pore size distribution diagram further shows that the pore size of hard carbon before and after ball milling is mainly concentrated between 3nm-20nm, and the peak of the pore size distribution is around 5nm, indicating that ball milling exfoliation greatly increases the specific surface area of ​​hard carbon without destroying the pore structure, which provides a good mass transfer and conductive path for active substances and electron transport.

[0174] Table 2 is a comparison of the changes in pores and specific surface area of ​​hard carbon negative electrode materials before and after modification

[0175] serial number <![CDATA[Specific surface area (m 2 / g)]]> Total pore volume (ml / g) Before physical ball milling modification 125.60 0.095 After physical ball milling modification 220.77 0.210

[0176] CR2032 button-type lithium batteries were prepared using hard carbon anode materials before and after physical modification and their electrochemical performance was characterized:

[0177] The preparation and testing process of CR2032 button lithium battery is the same as that of Example 1. The comparison chart of the cycle performance and coulombic efficiency of the metal lithium button battery assembled with the hard carbon negative electrode material before and after physical modification is shown in the figure. Figure 7 The comparison results of electrochemical performance tests are shown in Table 3.

[0178] Table 3 shows the changes in the performance of lithium button batteries with hard carbon negative electrode materials before and after modification

[0179]

[0180] pass Figure 7The electrochemical test results shown in the figure show that the second discharge capacity of the coin cell assembled with the unmilled hard carbon anode material is 305.61 mAh / g, while the second discharge capacity of the coin cell prepared with the ball-milled hard carbon anode material is increased to 357.50 mAh / g. This shows that physical ball milling can increase the discharge capacity by 17%. This is mainly due to the increase in electrochemical active sites in the material, which significantly improves the battery capacity. After 100 charge and discharge cycles, the residual discharge capacity of the unmilled hard carbon anode is 306.10 mAh / g, while the residual discharge capacity of the ball-milled hard carbon anode is 334.75 mAh / g, with capacity retention rates of 100.16% and 93.64%, respectively. It should be noted that although the capacity retention rate of the ball-milled material is lower than that of the unmodified material after 100 cycles, its specific capacity value is still higher than that of the unmilled hard carbon. The capacity decay of ball-milled hard carbon mainly occurs in the early stage of the cycle. Figure 7 It can be seen that starting from the 10th cycle, the battery's specific capacity has hardly decayed, and the capacity retention rate starting at this time is consistent with the original unmodified capacity retention rate. Overall, the ball milling method can significantly increase the discharge capacity of the material by increasing the electrochemical reaction active sites, which has higher commercial application value.

[0181] Example 4

[0182] The specific preparation process of a hard carbon negative electrode material provided in this embodiment is the same as that in Example 1, except that the collected waste fluorinated carbon positive electrode material is fluorinated carbon nanotubes.

[0183] Structural characterization of hard carbon anode materials prepared from waste fluorinated carbon nanotubes:

[0184] The SEM image of the hard carbon negative electrode material provided in Example 4 of the present invention is as follows: Figure 8 As shown, it can be seen that the material recovered from the fluorinated carbon nanotubes after discharge maintains the tubular structure unchanged, and the tube length is in the micron order.

[0185] The electrochemical performance characterization of the hard carbon negative electrode material, the preparation of the hard carbon negative electrode pole piece, and the assembly of the CR2032 button-type lithium battery prepared in this example are the same as those in Example 1.

[0186] The cycle performance and coulombic efficiency curve of the lithium ion battery assembled with the hard carbon negative electrode material provided in Example 4 are as follows: Figure 9 The electrochemical performance data are shown in Table 4.

[0187] Table 4 is a statistical table of electrochemical performance test data of the hard carbon negative electrode prepared from fluorinated carbon nanotubes collected from waste lithium fluoride batteries in this embodiment.

[0188]

[0189] pass Figure 9 The electrochemical test results in Table 4 show that the hard carbon assembled button battery prepared from waste fluorinated carbon nanotubes as precursor has a second discharge capacity of 297.90 mAh / g; after 100 charge and discharge cycles, the remaining discharge capacity is 304.19 mAh / g, and the capacity retention rate is 102.11%. It can be seen from the cycle performance curve that the battery cycle performance is stable, and as the charge and discharge proceed, the capacity continues to rise slowly, showing excellent cycle performance. This further illustrates that the preparation process of the present invention is suitable for various types of waste fluorinated carbon positive electrodes. Even if the waste fluorinated carbon precursor used is a tubular structure, the hard carbon negative electrode material prepared by the preparation process of the present invention also has excellent electrochemical properties and is suitable for large-scale production and preparation.

[0190] Comparative Example 1

[0191] As described above, the hard carbon negative electrode material obtained by the present invention contains LiF, a component that helps form a stable solid electrolyte film on the negative electrode surface and improves the hard carbon structure. To more clearly illustrate the effect of the presence of LiF on the electrochemical performance of hard carbon, this comparative example compares and analyzes the performance of commercial hard carbon without LiF and the hard carbon negative electrode material prepared using waste fluorinated graphite as raw material in Example 1 of the present invention.

[0192] The preparation of the commercial hard carbon battery pole piece and the assembly method of the CR2032 button-type lithium battery in this comparative example are the same as those in Example 1, and the battery testing procedure is consistent with that in Example 1.

[0193] The comparison chart of the cycle performance and coulombic efficiency of the CR2032 button lithium battery assembled in Example 1 and Comparative Example 1 is as follows: Figure 10 The electrochemical performance data of the two materials are compared in Table 5.

[0194] Table 5 Comparison of cycle performance between the hard carbon negative electrode materials of the present invention and commercial hard carbon negative electrode materials

[0195]

[0196] The test results show that the second discharge specific capacities of the hard carbon anode material obtained in Example 1 of the present invention and commercial hard carbon are 305.61 mAh / g and 257.1 mAh / g, respectively. This represents an 18.9% increase in the capacity of the hard carbon anode material of Example 1 compared to the commercial hard carbon. After 100 charge-discharge cycles, the residual discharge specific capacities are 306.10 mAh / g and 239.53 mAh / g, respectively, with corresponding capacity retention rates of 100.16% and 93.17%, respectively. Therefore, it can be seen that the hard carbon anode material obtained in the present invention significantly outperforms commercial hard carbon in terms of both discharge specific capacity and cycle performance. This is because the hard carbon anode material obtained in the present invention contains an appropriate amount of LiF during its preparation, which helps form a more stable solid electrolyte membrane on the anode surface, significantly improving the material's cycle performance. Furthermore, the hard carbon anode material prepared by the present invention has a unique structure and, relative to its specific surface area, more electrochemically active sites, and more ion storage sites, which results in an improved discharge specific capacity.

[0197] Comparative Example 2

[0198] The existing technology, such as Example 3 of the method for recovering and modifying the positive electrode material in a waste carbon fluoride battery disclosed in Chinese invention patent CN113526488A (publication date October 22, 2021), is used as a comparative example.

[0199] The patent requires that the F element in the carbon fluoride be removed as much as possible. This method uses concentrated sulfuric acid with the aid of ultrasonic vibration to remove as much LiF as possible, thereby obtaining a high-purity carbon material. While this method can effectively collect waste carbon fluoride, the resulting carbon material has poor electrochemical performance. According to the specific examples in the patent, button batteries assembled using the carbon material with the F element removed have a maximum capacity of approximately 280 mAh / g and a maximum initial coulombic efficiency of approximately 65%.

[0200] As described above, the hard carbon negative electrode material obtained by the present invention contains LiF, a component that helps form a stable solid electrolyte film on the negative electrode surface and improves the hard carbon structure. To more clearly illustrate the effect of the presence of LiF on the electrochemical performance of hard carbon, the electrochemical performance of the hard carbon negative electrode materials of the aforementioned patent and Example 1 of the present invention was compared and analyzed.

[0201] The preparation of the battery pole piece and the assembly method of the CR2032 button-type lithium battery of this comparative example are the same as those in Example 1, and the battery testing procedure is consistent with that in Example 1.

[0202] Comparison of the cycle performance and coulombic efficiency of the CR2032 button-type lithium battery assembled in Example 1 and Comparative Example 2, as shown in the figure Figure 11The electrochemical performance data of the two materials are compared in Table 6.

[0203] Table 6 Comparison of the cycle performance of the hard carbon negative electrode material of the present invention and the existing patented carbon material

[0204]

[0205]

[0206] According to the test results, the second discharge specific capacity of the hard carbon negative electrode material obtained in Example 1 of the present invention and the existing patented carbon material are 305.61mAh / g and 266.3mAh / g, respectively. The discharge specific capacity of the hard carbon negative electrode material in Example 1 is increased by 14.76% compared with that of Comparative Example 2. After 100 charge and discharge cycles, the remaining discharge specific capacity is 306.10mAh / g and 193.1mAh / g, respectively, and the corresponding capacity retention rates are 100.16% and 72.51%, respectively. Whether from the perspective of discharge capacity or cycle performance, the hard carbon negative electrode material obtained in Example 1 of the present invention is significantly superior to the hard carbon material described in the existing patent in Comparative Example 2. This is due to the LiF contained in the hard carbon negative electrode material obtained by the present invention, which helps to form a more stable solid electrolyte film on the negative electrode surface and improves the structure of the hard carbon negative electrode material, which greatly improves the cycle performance and electrochemical stability of the material.

[0207] Comparative Example 3

[0208] This comparative example provides a preparation process of a hard carbon material, which is different from Example 1 in that step S1 does not perform multiple step-rate over-discharge treatments on the carbon fluoride battery, and the other steps are the same as Example 1.

[0209] In order to illustrate the necessity of multiple step-rate over-discharge treatments of the present invention, the performance of the hard carbon material prepared in Comparative Example 3 was compared with that of the hard carbon negative electrode material in Example 1.

[0210] First, the chemical titration method used in Example 1 was used to measure the content of LiF in the hard carbon material of Comparative Example 3, which was 7.93 wt %.

[0211] The preparation method of the hard carbon battery pole piece and the assembly method of the CR2032 button-type lithium battery in this comparative example 3 are the same as those in Example 1, and the battery testing procedure is also consistent with that in Example 1.

[0212] Comparison of the cycle performance and coulombic efficiency of the CR2032 button-type lithium battery assembled in Example 1 and Comparative Example 3, as shown in the figure Figure 12 The electrochemical performance data of the two materials are compared in Table 7.

[0213] Table 7 Comparison of cycle performance of over-discharged hard carbon and non-over-discharged hard carbon in the present invention

[0214]

[0215] Through Figure 12 As shown, the second discharge specific capacities of the hard carbon negative electrode material of Example 1 after multiple step-rate overdischarge and the unoverdischarged hard carbon material of this comparative example were 305.61 mAh / g and 279.58 mAh / g, respectively. After 100 charge-discharge cycles, the residual discharge specific capacities were 306.10 mAh / g and 218.53 mAh / g, respectively, with corresponding capacity retention rates of 100.16% and 78.16%, respectively. In terms of both discharge capacity and cycling performance, the hard carbon negative electrode material of Example 1 after multiple step-rate overdischarge significantly outperformed the unoverdischarged hard carbon material of this comparative example.

[0216] In addition, compared with the hard carbon negative electrode materials with different LiF contents obtained by multiple step rate over-discharge and acid solution treatment in Example 2 of the present invention, Figure 5 The discharge capacity curves of batteries assembled with hard carbon negative electrode materials having different LiF contents are shown, and the discharge specific capacity value corresponding to the LiF content of 7.93 wt% can be obtained. It can be seen that the discharge capacity of the present invention with a LiF content of 7.93 wt% is greater than 340 mAh / g, which has a very high specific capacity. However, in this comparative example, which did not undergo multiple step-rate over-discharges, the discharge specific capacity of the hard carbon material with a LiF content of 7.93 wt% was only 279.58 mAh / g, which is much smaller than the discharge specific capacity of the hard carbon negative electrode material with the same LiF content in Example 2.

[0217] That is to say, although the hard carbon material of this comparative example has not been subjected to multiple step-rate over-discharges, and its LiF content is within the range that can achieve the optimal hard carbon performance value, since the process of obtaining the hard carbon material in this comparative example has not been subjected to multiple step-rate over-discharges, the F element in the carbon fluoride in its matrix has not been removed, that is, not all of it has been converted into LiF. After the high-temperature treatment, the remaining F becomes electrochemically inert or volatilizes, resulting in increased polarization of the material and poor capacity and cycle performance. This further illustrates the necessity of multiple step-rate over-discharges in the process of preparing the hard carbon negative electrode material in the embodiment of the present invention, as well as the degree of over-discharge for the performance of the material.

[0218] Therefore, as described above, the hard carbon negative electrode material obtained in the embodiment of the present invention contains LiF, a component that helps form a stable solid electrolyte film on the negative electrode surface. After multiple step-rate overdischarges, the LiF content is made as high as possible. Through subsequent modification treatment, the optimal LiF content is between 5wt% and 15wt%. At this time, the corresponding hard carbon negative electrode material has a high discharge specific capacity.

[0219] Since the initial state of the recycled spent carbon fluoride batteries is unknown, that is, if the original carbon fluoride batteries were only discharged at a high rate for a short time as primary batteries, rather than being discharged at multiple step rates, then a large amount of F elements are still not removed from the recovered hard carbon. Even if it contains the LiF content corresponding to the present invention, it is still not enough to improve the electrochemical performance of the material.

[0220] Comparative Example 4

[0221] As mentioned above, the hard carbon negative electrode material prepared in the embodiment of the present invention contains LiF, a component that helps form a stable solid electrolyte film on the negative electrode surface. Preferably, the content of LiF is between 2wt% and 35wt%, and optimally, the content of LiF is between 5wt% and 15wt%.

[0222] The above comparative examples have shown the importance of multiple step-rate over-discharge for LiF formation and F element removal. However, different batches of recycled waste fluorinated carbon have different x Due to the different fluorination degrees, the final LiF content varies greatly. Only after multiple step-rate over-discharges and then regulating the LiF content within the preferred range can the electrochemical performance reach the best.

[0223] This comparative example selects another CF with different degree of fluorination from Example 1 x Waste batteries are used to illustrate the necessity of improving material performance by regulating the LiF content.

[0224] This comparative example uses CF x After the waste battery was subjected to the same preparation steps as in Example 1 to obtain a hard carbon material, the LiF content was determined to be 38.76 wt % using the same chemical titration method as in Example 1.

[0225] The method for preparing battery pole pieces from the hard carbon material obtained in this comparative example 4 and assembling CR2032 button-type lithium batteries is the same as that in Example 1, and the battery testing procedure is also consistent with that in Example 1.

[0226] The comparison chart of the cycle performance and coulombic efficiency of the CR2032 button lithium battery assembled in Example 1 and Comparative Example 4 is as follows: Figure 13 The electrochemical performance data of the two materials are compared in Table 8.

[0227] It should be noted that the LiF content in the hard carbon negative electrode material of Example 1 is 32.5 wt %, which is within the preferred LiF content range; while the LiF content in the hard carbon material of Comparative Example 4 is 38.76 wt %, which is greater than the upper limit of the preferred LiF content range of the present invention.

[0228] Table 8 Comparison of hard carbon cycle performance after overdischarge with and without LiF content control

[0229]

[0230] The test results show that the first coulombic efficiency of the battery containing the hard carbon negative electrode material in Example 1 and the battery containing the hard carbon negative electrode material in Comparative Example 4 were 73.4% and 72.12%, respectively, and the second discharge specific capacity were 305.61 mAh / g and 297.40 mAh / g, respectively. After 100 charge and discharge cycles, the residual capacity was 306.10 mAh / g and 263.9 mAh / g, respectively, and the corresponding capacity retention rates were 100.16% and 88.74%, respectively. This shows that the hard carbon negative electrode material with LiF content in the optimal range prepared in Example 1 of the present invention is superior to the hard carbon material with LiF content not in the optimal range prepared in Comparative Example 4 in terms of first coulombic efficiency, discharge capacity and cycle performance. The hard carbon material obtained in Comparative Example 4 needs to be modified to adjust the LiF content in order to achieve optimal performance.

[0231] From the above comparison, it can be seen that although the waste fluorinated carbon materials (CF x ) are subjected to multiple step-rate over-discharges to generate LiF in situ and remove the F element in the raw materials as much as possible, but according to the original CF x The LiF content in the final material obtained varies with the degree of fluorination. When the LiF content is within the optimal range of the measurement, the electrochemical performance of the material can be effectively improved; but when the LiF content exceeds the upper limit of the optimal range, the electrochemical inertness and insulation of LiF itself will hinder the material capacity, affect the material's cycle performance, and reduce the competitiveness of the material. Therefore, the hard carbon negative electrode material prepared by multiple step-rate over-discharge steps in the embodiment of the present invention needs to be further regulated according to the actual content of LiF to ensure that the final product has the best electrochemical performance.

[0232] Example 5

[0233] This embodiment provides a preparation process and performance test of a hard carbon negative electrode material. The difference from Example 1 lies in the different process conditions for the multiple step-rate over-discharge treatment in step S1. The specific process of the multiple step-rate over-discharge treatment is as follows:

[0234] The discharged waste lithium / carbon fluoride soft-pack batteries used in this embodiment 5 are the same as those used in embodiment 1, and the carbon fluoride material in the positive electrode plate is also graphite fluoride.

[0235] The above-mentioned battery was subjected to multiple step-rate over-discharge treatments. First, the lithium / carbon fluoride soft-pack battery was discharged to 0.5V at a rate of 0.15C, then discharged to a voltage of 0.5V at a rate of 0.05C, and finally discharged to a voltage of 0.5V at a rate of 0.03C, so that the remaining F element in the waste fluorinated graphite was converted into LiF as much as possible.

[0236] The other preparation steps were exactly the same as those in Example 1, and a hard carbon negative electrode material was finally obtained.

[0237] The LiF content of the hard carbon obtained in this example was calibrated using the chemical titration method described in Example 1. The obtained negative electrode material was used to prepare button batteries for electrochemical performance testing. The button battery preparation and testing steps were consistent with those described in Example 1. The second discharge curves of the hard carbon negative electrode obtained by the two different step discharge steps are shown in Figure 2. Figure 14 The performance comparison is shown in Table 9.

[0238] Table 9 Comparison of physical and chemical data of hard carbon negative electrodes obtained in different step discharge modes of the present invention

[0239]

[0240] According to the test results, the step discharge effect of this embodiment is not much different from that of embodiment 1. Under the discharge mode adopted in embodiment 1, the LiF content of the obtained hard carbon negative electrode is 32.5wt%, and the second discharge specific capacity and the first coulomb efficiency correspond to 305.61mAh / g and 73.4% respectively; while under the discharge mode of this embodiment, the LiF content of the hard carbon negative electrode is measured to be 31.52wt%, and the second discharge specific capacity and the first coulomb efficiency correspond to 303.26mAh / g and 72.86% respectively. Because both embodiments control the lower limit of the step discharge voltage to below 1.5V at a small rate, the F element in the recovered material has been converted into LiF as much as possible, so the electrochemical properties of the material are consistent with the element content. This also shows that the treatment of waste fluorinated carbon batteries within the range of the step discharge mode provided by the present invention has good electrochemical performance.

[0241] Example 6

[0242] This embodiment provides a preparation process and performance test of a hard carbon negative electrode material. Steps S1 and S2 are the same as those in Example 1. The difference from Example 1 is that the high-temperature sintering treatment in step S3 adopts segmented sintering. The specific sintering process is as follows:

[0243] 0.5 g of the fluorinated carbon material collected in step S2 was weighed and placed in a 100 ml ceramic boat. The boat was then placed in a high-temperature tube furnace and heated to 300°C at a heating rate of 5°C / min. The temperature was maintained under argon for 2 hours. The temperature was then increased to 650°C at a heating rate of 10°C / min and maintained for 1 hour. The temperature was then increased to 850°C at a heating rate of 5°C / min and maintained for 2 hours. The material was then washed with alcohol, filtered, and dried to obtain a hard carbon anode material. Testing showed that the LiF content in the hard carbon anode was approximately 26%.

[0244] The obtained negative electrode material was used to prepare button cells for electrochemical performance testing. The button cell preparation and testing steps were the same as those described in Example 1. The second discharge curves of the hard carbon negative electrode obtained by the two different heating methods are shown in Figure 2. Figure 15 The performance comparison is shown in Table 10.

[0245] Table 10 Comparison of electrochemical performance of hard carbon negative electrodes obtained by different heating methods in the present invention

[0246]

[0247] According to the test results, the surface treatment of the recycled hard carbon material using a multi-step heating method will improve the electrochemical performance of the material. Compared with the one-step heating method, the second discharge specific capacity of the multi-step step-by-step heating method increased to 313.50mAh / g, an increase of about 8mAh / g, and the corresponding first coulombic efficiency also increased to 75.93%, an increase of about 2.53%. Under the multi-step heating method, the material has more sufficient reaction time in each temperature range. At 300℃, the organic functional groups on the hard carbon surface are carbonized; at 650℃, the salts such as carbonates generated by the electrolyte side reaction on the hard carbon surface are decomposed, and the inert CFx in the original fluorinated carbon phase that did not participate in the reaction is gradually decomposed, and the F element overflows; at 850℃, the LiF in the hard carbon material begins to melt, and some of it will be carried away from the carbon material surface by the airflow. The multi-step heating treatment reduces impurities on the surface of the carbon material, thereby further reducing side reactions. In addition, the multi-step heating treatment can better control the LiF content, which is 26%, thereby significantly improving the material's first coulombic efficiency and discharge specific capacity.

[0248] Example 7

[0249] This embodiment provides a preparation process and performance test of a hard carbon negative electrode material, which specifically includes the following steps.

[0250] Step S1. After the discharge, the carbon fluoride battery is subjected to multiple step-rate over-discharge treatments to obtain a carbon fluoride battery to be disassembled. The specific process is as follows:

[0251] This example uses discharged spent lithium / carbon fluoride primary batteries, whose positive electrode plates contain carbon black fluoride. First, the batteries are discharged at a rate of 0.15C to 1.5V, then at a rate of 0.1C to 0.8V, and finally at a rate of 0.03C to 0.3V, converting all remaining F in the spent graphite fluoride to LiF.

[0252] Step S2. Disassemble the carbon fluoride battery to be disassembled, remove the carbon fluoride positive electrode, clean it, and collect the carbon fluoride material. The specific process is as follows:

[0253] The battery that has undergone multiple discharge treatments in step S1 is disassembled in a vacuum glove box or a super clean room, the carbon fluoride electrode is taken out, the electrode is cleaned three times with EC and dried under vacuum conditions; then the cleaned electrode is soaked in a dichloromethane solution and peeled with the aid of ultrasonic vibration, and then the carbon material is peeled off from the current collector with the help of tweezers and dried in a vacuum drying oven and ground into powder to collect the carbon fluoride material.

[0254] Step S3. The fluorinated carbon material is subjected to high-temperature sintering treatment under an inert atmosphere to obtain a hard carbon negative electrode material. The specific process is as follows:

[0255] Weigh 0.5 g of the fluorinated carbon material collected in step S2, place it in a 100 ml ceramic boat and place it in a high-temperature tube furnace. Heat it to 845 ° C at a heating rate of 5 ° C / min, keep it warm for 1 hour under argon atmosphere, then wash it with alcohol, filter it and dry it to obtain the final product, hard carbon negative electrode material.

[0256] In summary, the embodiments of the present invention provide a method for preparing a hard carbon negative electrode material, a hard carbon negative electrode material, and its application. By improving the recycling process of waste fluorinated carbon electrodes, all F in the waste fluorinated carbon material is converted into LiF, and the LiF content in the hard carbon negative electrode material is controlled to within an optimal range by high-temperature sintering treatment and / or acidic solution treatment, thereby preparing a hard carbon negative electrode material with excellent performance for commercial use. The hard carbon negative electrode material of the present invention is applied to secondary batteries, which can have excellent electrochemical properties (such as discharge specific capacity, cycle performance, and first coulombic efficiency). In addition, the hard carbon negative electrode material of the present invention can be applied not only to lithium-ion batteries, but also has good application prospects in sodium-ion batteries.

[0257] This invention significantly reduces the environmental burden of waste carbon fluoride electrodes in carbon fluoride batteries, effectively increasing the added value of primary battery electrode materials. Given the significant increase in the use of carbon fluoride batteries, this invention provides a continuous source of precursor raw materials for the hard carbon negative electrode material, laying a solid foundation for the subsequent stabilization and large-scale use of this novel hard carbon.

[0258] The preparation method provided by the embodiment of the present invention has stable raw material structure, low cost, good product consistency and simple preparation process.

[0259] Although the present invention has been described in detail through the above preferred embodiments, it should be understood that the above description is not intended to limit the present invention. After reading the above description, various modifications and substitutions of the present invention will become apparent to those skilled in the art. Therefore, the scope of protection of the present invention should be defined by the appended claims.

[0260] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a hard carbon negative electrode material, characterized in that: The preparation method comprises the following steps: Step S1, performing multiple step-rate over-discharge treatments on the carbon fluoride battery after discharge to obtain a carbon fluoride battery to be disassembled; wherein the lower limit of the discharge voltage of the multiple step-rate over-discharge treatments is less than or equal to 1.5V; Step S2, disassembling the carbon fluoride battery to be disassembled, taking out the carbon fluoride positive electrode, cleaning it, and collecting the carbon fluoride material; Step S3, sintering the fluorinated carbon material at high temperature under an inert atmosphere and / or treating it with an acidic solution to obtain a hard carbon negative electrode material; The hard carbon negative electrode material contains 2 wt% to 35 wt% of in-situ generated LiF; The shape of the hard carbon negative electrode material includes one or more of a layered structure, a block structure, an amorphous structure or a tubular structure; The carbon fluoride battery is a primary battery.

2. The method for preparing a hard carbon negative electrode material according to claim 1, wherein: The lower limit of the discharge voltage of the multiple step-rate over-discharge treatment is between 0.3V and 1.5V; The specific process of the multiple step-rate over-discharge treatment is as follows: the carbon fluoride battery after discharge is discharged at a rate of 0.1C to 0.2C to a voltage less than or equal to 1.5V, then discharged at a rate of 0.05C to less than 0.1C to a voltage less than or equal to 1.0V, and then discharged at a rate of 0.02C to less than 0.05C to a voltage between 0.3V and 0.5V.

3. The method for preparing a hard carbon negative electrode material according to claim 2, wherein: During the multiple step-rate over-discharge processes from high to low rates, the lower limit of the discharge voltage is gradually reduced.

4. The method for preparing a hard carbon negative electrode material according to claim 1, wherein: The fluorinated carbon material includes one or more of fluorinated graphite, fluorinated graphite polymer, fluorinated carbon nanotube, fluorinated carbon fiber, fluorinated graphene, fluorinated hard carbon, fluorinated coke, and fluorinated carbon black; The content of LiF accounts for 5wt% to 15wt% of the hard carbon negative electrode material.

5. The method for preparing a hard carbon negative electrode material according to claim 1, wherein: The high temperature sintering process includes one-stage sintering or segmented sintering; The specific conditions of the one-stage sintering are: setting the heating rate to 1°C / min to 10°C / min, the sintering temperature to be greater than or equal to 800°C, and the holding time to be greater than or equal to 1 hour; The staged sintering is specifically as follows: in an inert gas atmosphere, the carbon fluoride material is placed in a heating device, heated to a first temperature at a heating rate of 1°C / min to 10°C / min, and kept warm for 0.5 hours to 24 hours, heated to a second temperature at a heating rate of 1°C / min to 10°C / min, and kept warm for 0.5 hours to 24 hours, and then heated to a third temperature at a heating rate of 1°C / min to 10°C / min, and kept warm for 0.5 hours to 8 hours; The first temperature is greater than or equal to 300 ℃ to less than 650 ℃; The second temperature is greater than or equal to 650 ℃ to less than 800 ℃; The third temperature is greater than or equal to 800°C.

6. The method for preparing a hard carbon negative electrode material according to claim 1, wherein: The acidic solution treatment method specifically comprises: using an acidic solution to immerse, rinse, spray or spray the carbon fluoride material or the carbon fluoride material that has undergone the high-temperature sintering treatment to obtain a hard carbon negative electrode material with a controlled LiF content; After the acidic solution treatment, the hard carbon negative electrode material with the adjusted LiF content is cleaned with an alkaline solution and / or a neutral reagent; The acidic solution includes one or more of an inorganic acid or an organic acid; The alkaline solution includes one or more of an inorganic base, an organic base or an alkaline salt solution.

7. The method for preparing a hard carbon negative electrode material according to claim 6, wherein: The acidic solution specifically includes: one or more of sulfuric acid, nitric acid, phosphoric acid, sulfurous acid, carbonic acid, boric acid, oxalic acid, acetic acid, and sulfinic acid; The alkaline solution specifically includes: one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, ammonia water, ferric hydroxide, aluminum hydroxide, magnesium hydroxide, sodium acetate, sodium silicate, and sodium phosphate.

8. The method for preparing a hard carbon negative electrode material according to any one of claims 1 to 7, characterized in that: The preparation method further includes: when the hard carbon negative electrode material is in a layered structure, treating the hard carbon negative electrode material using one or more of a physical method, a chemical method, or an electrochemical method to increase the reactive area of ​​the hard carbon negative electrode material; The physical method includes: one or more methods of solvent thermal intercalation, ultrasonic stripping, tape stripping or high-energy ball milling; The chemical method is to cause the hard carbon negative electrode material to undergo an oxidation-reduction reaction, specifically comprising treating the hard carbon negative electrode material with an inorganic acid, followed by dilution and oxidation with a strong oxidant; The electrochemical method specifically comprises: using the hard carbon negative electrode material to prepare a pole piece and assembling a half-cell, and then over-discharging at a rate greater than or equal to 3C to peel off the carbon layer through excessive ion intercalation.

9. A hard carbon negative electrode material prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The hard carbon negative electrode material includes a fluorinated carbon material and in-situ generated LiF; The mass of the LiF accounts for 2% to 35% of the total mass of the hard carbon negative electrode material; The hard carbon negative electrode material includes one or more of a layered structure, a block structure, an amorphous structure or a tubular structure.

10. The hard carbon negative electrode material according to claim 9, characterized in that The fluorinated carbon material includes one or more of fluorinated graphite, fluorinated graphite polymer, fluorinated carbon nanotube, fluorinated carbon fiber, fluorinated graphene, fluorinated hard carbon, fluorinated coke, and fluorinated carbon black; The mass of the LiF accounts for 5% to 15% of the total mass of the hard carbon negative electrode material; The specific surface area of ​​the hard carbon negative electrode material is greater than or equal to 100m 2 / g.

11. A secondary battery, characterized in that: The secondary battery comprises the hard carbon negative electrode material obtained by the method of any one of claims 1 to 8 or the hard carbon negative electrode material according to any one of claims 9 to 10.

12. The secondary battery according to claim 11, wherein The secondary battery is a lithium ion battery or a sodium ion battery.

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