Negative electrode material, preparation method thereof and battery

Abstract

The invention provides a negative electrode material, a preparation method thereof and a battery, and relates to the field of materials for batteries. The negative electrode material comprises graphite, a lithium carbonate layer coated on the surface of the graphite, and a lithium azide layer coated on the surface of the lithium carbonate layer. The lithium carbonate layer can effectively inhibit thermal stress damage of the material and remarkably enhance the crystallization quality and purity of the material, and the lithium azide layer can further optimize a lithium ion transmission channel and reduce interface resistance; the graphite, the lithium carbonate layer and the lithium azide layer cooperate with one another, so that the negative electrode material has relatively high rate capability and excellent cycling stability, and the problem that the rate capability and the cycling performance of an existing graphite negative electrode material are relatively poor is solved.

Description

Technical Field

[0001] The present application relates to the field of battery materials, and in particular to a negative electrode material and a preparation method thereof, and a battery. Background Art

[0002] Driven by the urgent need for global energy transformation and environmental protection, the innovation of energy storage technology has become the forefront of scientific and technological development. As the leader of future energy storage technology, solid-state batteries are gradually leading the profound changes in the field of energy storage with their excellent energy density, inherent safety characteristics and potential long-term cycle capacity. However, the comprehensive promotion and application of solid-state battery technology still faces many challenges. Among them, the performance optimization of electrode materials, especially negative electrode materials, has become a key factor restricting the overall performance improvement of solid-state batteries.

[0003] Most of the mainstream lithium-ion batteries on the market currently use graphite as the negative electrode material. Although graphite exhibits good performance under normal conditions, its structure is prone to deterioration during high-power charging and discharging, resulting in a sharp decline in capacity and a significant reduction in cycle stability. In addition, the flammability and potential explosion risks of traditional liquid electrolyte systems further limit the expansion of lithium-ion batteries in high-energy-efficiency applications. To solve these problems, researchers have conducted a large number of modification studies on graphite negative electrodes, such as coating with metal oxides or carbon-based materials. However, these methods generally have problems such as uneven coating, high process complexity, and high cost. It is difficult to ensure long-term cycle stability while improving high-rate performance, which has become a significant obstacle for solid-state battery technology to overcome. Summary of the invention

[0004] The purpose of this application is to provide a negative electrode material and a preparation method thereof and a battery to solve the above problems.

[0005] To achieve the above objectives, this application adopts the following technical solutions:

[0006] A negative electrode material comprises: graphite, a lithium carbonate layer coated on the surface of the graphite, and a lithium azide layer coated on the surface of the lithium carbonate layer.

[0007] According to an embodiment of the present application, the mass ratio of the graphite to the lithium carbonate layer and the lithium azide layer is 1:(0.01-0.1):(0.01-0.08).

[0008] The present application also provides a method for preparing the negative electrode material as described above, comprising:

[0009] Forming a lithium carbonate layer on the surface of graphite to obtain a graphite-lithium carbonate material;

[0010] A lithium azide layer is formed on the surface of the graphite-lithium carbonate material to obtain a negative electrode material.

[0011] According to an embodiment of the present application, preparing the graphite-lithium carbonate material includes:

[0012] Preparation of lithium hydroxide coated graphite material;

[0013] The lithium hydroxide coated graphite material is heat treated to convert the lithium hydroxide into lithium carbonate.

[0014] According to an embodiment of the present application, the material for preparing lithium hydroxide coated graphite includes: mixing graphite, a lithium source, and a solvent to obtain a material for lithium source coated graphite;

[0015] The lithium source coated graphite material is mixed with an alkaline solution and dried to obtain a lithium hydroxide coated graphite material.

[0016] According to an embodiment of the present application, the graphite includes flaky graphite;

[0017] And / or, the lithium source includes at least one of lithium chloride, lithium hydride, lithium bromide, and lithium iodide, and the mass of the lithium source accounts for 2.5% to 10% of the mass of the graphite;

[0018] And / or, during the mixing of the graphite, the lithium source and the solvent, the method further comprises adding a surfactant, wherein the surfactant comprises at least one of tetraethyl orthosilicate, polyethylene glycol, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, polyoxyethylene sorbitan fatty acid ester and a silane coupling agent;

[0019] and / or, the alkaline solution comprises a sodium hydroxide solution;

[0020] And / or, the concentration of the alkaline solution is 0.5-1 mol / L;

[0021] And / or, the amount of the alkaline solution added is controlled so that the pH value of the system after the lithium source-coated graphite material is mixed with the alkaline solution is in the range of 10 to 12.

[0022] According to an embodiment of the present application, heat treating the lithium hydroxide-coated graphite material includes: heating the lithium hydroxide-coated graphite material to a first temperature at a first heating rate, then heating it to a second temperature at a second heating rate, and keeping it warm at the second temperature; wherein the first heating rate is greater than the second heating rate, the heating to the first temperature and the heating to the second temperature are carried out under inert gas protection, and the keeping warm at the second temperature is carried out in an atmosphere containing carbon dioxide.

[0023] According to an embodiment of the present application, the first heating rate is 10-20°C / min, and the second heating rate is 1-5°C / min;

[0024] And / or, the first temperature is 80-250° C., the second temperature is 350-650° C., and the insulation time is 1-2 hours.

[0025] According to an embodiment of the present application, forming a lithium azide layer on the surface of the graphite-lithium carbonate material includes: depositing an organic lithium compound and an azide source on the surface of the graphite-lithium carbonate material by atomic layer deposition.

[0026] According to an embodiment of the present application, the deposition temperature is 130-160°C;

[0027] And / or, the deposition pressure is 0.08-0.12 Pa;

[0028] And / or, the organic lithium compound includes at least one of butyl lithium, phenyl lithium, and a lithium amide compound, and the lithium amide compound includes lithium hexamethyldisilazide;

[0029] And / or, the azide source includes at least one of sodium azide, potassium azide, and ammonia;

[0030] and / or, the pulse time ratio of the organolithium compound to the azide source is 1:20 to 1:50;

[0031] And / or, the number of cycles of the atomic layer deposition method is 100-1000 times;

[0032] And / or, after the deposition is completed, the method further comprises: annealing the deposited material to obtain a negative electrode material; wherein the annealing temperature is 100-200° C., and the annealing time is 1-2 hours.

[0033] The present application also provides a battery, comprising the negative electrode material as described above or the negative electrode material prepared by the preparation method as described above.

[0034] Compared with the prior art, the beneficial effects of this application include:

[0035] The negative electrode material of the present application includes graphite, and a lithium carbonate layer and a lithium azide layer coated on the surface of the graphite. The lithium carbonate layer can effectively inhibit the thermal stress damage of the material and significantly enhance the crystal quality and purity of the material. The lithium azide layer can further optimize the lithium ion transmission channel and reduce the interface resistance. The graphite, lithium carbonate layer, and lithium azide layer cooperate with each other to make the negative electrode material have a higher rate performance and excellent cycle stability, which improves the problem of poor rate performance and cycle performance of existing graphite negative electrode materials.

[0036] The present application forms a lithium carbonate layer on the surface of graphite, which can effectively block the erosion of unfavorable substances in the electrolyte, reduce the occurrence of side reactions, and also enhance the connection strength between graphite particles by chemical bonding, preventing the fragmentation and collapse of the structure during the cycle, fundamentally improving the cycle durability of the negative electrode material, and ensuring that the battery can still maintain a high capacity retention rate after multiple charge and discharge cycles. The present application also forms a lithium azide layer on the surface of the lithium carbonate layer, which can reduce the mass transfer resistance of lithium ions at the interface, promote the rapid diffusion of lithium ions, and reduce the energy loss in the charge transfer process. In addition, the lithium azide layer can also effectively regulate the formation of the SEI film, avoid the generation of large particles, thereby maintaining a good electrochemical response during high-rate charge and discharge, and improving the instantaneous power output of the battery. The negative electrode material prepared by the present application method has a high rate performance and excellent cycle stability. Moreover, the present application method does not need to use harmful solvents, has the advantages of environmental friendliness, and in addition, the present application method also has the advantages of low production cost, which is conducive to large-scale promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope of the present application.

[0038] Figure 1 This is a SEM image of the negative electrode material prepared in Example 1 of the present application;

[0039] Figure 2 This is a comparison chart of the cycle performance of the negative electrode materials prepared in Example 1 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0040] As used herein:

[0041] "Prepared from" is synonymous with "comprising." As used herein, the terms "comprising," "including," "having," "containing," or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0042] The conjunction "consisting of excludes any unspecified element, step, or component. If used in a claim, this phrase renders the claim closed-ended so that it does not include materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim rather than immediately following the subject matter, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0043] When an amount, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper preferred values ​​and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pairing of any range upper limit or preferred value with any range lower limit or preferred value, regardless of whether the range is disclosed separately. For example, when a range of "1 to 5" is disclosed, the described range should be interpreted as including ranges "1 to 4", "1 to 3", "1 to 2", "1 to 2 and 4 to 5", "1 to 3 and 5", etc. When a numerical range is described in this article, unless otherwise stated, the range is intended to include its end values ​​and all integers and fractions within the range.

[0044] In these examples, parts and percentages are by mass unless otherwise indicated.

[0045] "Parts by mass" refers to the basic unit of measurement for expressing the mass ratio of multiple components. 1 part can represent any unit mass, such as 1g or 2.689g. If we say that the mass of component A is a parts and the mass of component B is b parts, it means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, it means that the mass of component A is aK and the mass of component B is bK (K is an arbitrary number, indicating a multiple factor). It should not be misunderstood that, unlike the mass parts, the sum of the mass of all components is not limited to 100 parts.

[0046] "And / or" is used to indicate that one or both of the stated situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0047] In order to better explain the technical solution provided by this application, before the embodiments, the technical solution is first described as a whole, as follows:

[0048] A negative electrode material comprises: graphite, a lithium carbonate layer coated on the surface of the graphite, and a lithium azide layer coated on the surface of the lithium carbonate layer.

[0049] The lithium carbonate layer in the negative electrode material of the present application can effectively inhibit thermal stress damage of the material and significantly enhance the crystallization quality and purity of the material. The lithium azide layer can further optimize the lithium ion transmission channel and reduce the interface resistance. The graphite, lithium carbonate layer, and lithium azide layer cooperate with each other to enable the negative electrode material to have a higher rate performance and excellent cycle stability, which is beneficial to improving the electrochemical performance and overall safety of the negative electrode material.

[0050] Specifically, the lithium carbonate layer builds a chemically stable and highly adherent protective barrier on the graphite surface, which can not only effectively block the erosion of adverse substances in the electrolyte and reduce the occurrence of side reactions, but also enhance the connection strength between graphite particles by chemical bonding, prevent the fragmentation and collapse of the structure during the cycle, and fundamentally improve the cycle durability of the negative electrode material, ensuring that the battery can still maintain a high capacity retention rate after multiple charge and discharge cycles. The lithium azide layer realizes the precise regulation of the ion transmission path. The introduction of the lithium azide layer significantly reduces the mass transfer resistance of lithium ions at the interface, and its highly uniform and dense structure promotes the rapid diffusion of lithium ions and reduces the energy loss during charge transfer. In addition, the lithium azide layer can also effectively regulate the formation of the SEI film and avoid the formation of large particles, thereby maintaining a good electrochemical response during high-rate charge and discharge, and improving the instantaneous power output of the battery. The lithium carbonate layer and the lithium azide layer work together to improve the safety of the battery from the intrinsic material level and the interface level. The lithium carbonate layer has good thermal stability and can maintain structural integrity in high temperature environments, reducing the risk of thermal runaway. At the same time, the lithium azide layer can also inhibit the side reactions between the electrolyte and the electrode, reduce the generation of gas and the possibility of internal short circuits, thereby enhancing the overall safety performance of the battery.

[0051] According to the embodiment of the present application, the mass ratio of graphite to lithium carbonate layer and lithium azide layer is 1:(0.01-0.1):(0.01-0.08). When the contents of the three are within the above range, the negative electrode material can have higher rate performance and excellent cycle stability.

[0052] For example, the mass ratio of graphite to lithium carbonate layer and lithium azide layer can be 1:0.01:0.01, 1:0.015:0.045, 1:0.055:0.019, 1:0.055:0.045, 1:0.055:0.075, 1:0.075:0.045, 1:0.1:0.08 or any value between 1:(0.01-0.1):(0.01-0.08).

[0053] The present application also provides a method for preparing the negative electrode material as described above, comprising:

[0054] Forming a lithium carbonate layer on the surface of graphite to obtain a graphite-lithium carbonate material;

[0055] A lithium azide layer is formed on the surface of the graphite-lithium carbonate material to obtain a negative electrode material.

[0056] According to an embodiment of the present application, preparing the graphite-lithium carbonate material includes:

[0057] Preparation of lithium hydroxide coated graphite material;

[0058] The lithium hydroxide coated graphite material is heat treated to convert the lithium hydroxide into lithium carbonate.

[0059] According to an embodiment of the present application, the material for preparing lithium hydroxide coated graphite includes: mixing graphite, a lithium source, and a solvent to obtain a material for lithium source coated graphite;

[0060] The lithium source coated graphite material is mixed with an alkaline solution and dried to obtain a lithium hydroxide coated graphite material.

[0061] According to an embodiment of the present application, the graphite includes flaky graphite;

[0062] And / or, the lithium source includes at least one of lithium chloride, lithium hydride, lithium bromide, and lithium iodide, and the mass of the lithium source accounts for 2.5% to 10% of the mass of the graphite; for example, the mass of the lithium source accounts for 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any value between 2.5% and 10% of the mass of the graphite.

[0063] And / or, during the mixing of the graphite, the lithium source and the solvent, the method further comprises adding a surfactant, wherein the surfactant comprises at least one of tetraethyl orthosilicate, polyethylene glycol, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, polyoxyethylene sorbitan fatty acid ester and a silane coupling agent; the above-mentioned types of surfactants can effectively reduce the surface tension of the dispersion, promote the lithium source to adhere more closely to the graphite surface, and enhance the coating effect.

[0064] In some embodiments, the mass of the surfactant accounts for 1%-3% of the mass of the graphite; for example, the mass of the surfactant may account for 1%, 2%, 3% or any value between 1% and 3% of the mass of the graphite.

[0065] In some embodiments, the concentration of the surfactant is 0.1-15 mol / L; for example, the concentration of the surfactant can be 0.1 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, 10 mol / L, 11 mol / L, 12 mol / L, 13 mol / L, 14 mol / L, 15 mol / L or any value between 0.1-15 mol / L.

[0066] And / or, the alkaline solution includes a sodium hydroxide solution; by adding the alkaline solution, the lithium source Li + With alkaline solution OH - LiOH is generated by in-situ reaction and immediately coated on the surface of graphite particles. This design strengthens the graphite particle structure and significantly improves the migration rate of lithium ions, which is conducive to achieving high-speed charging and discharging of the battery.

[0067] And / or, the concentration of the alkaline solution is 0.5-1 mol / L; for example, the concentration of the alkaline solution can be 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L or any value between 0.5-1 mol / L.

[0068] And / or, the amount of the alkaline solution added is controlled so that the pH value of the system after the lithium source-coated graphite material is mixed with the alkaline solution is in the range of 10 to 12.

[0069] For example, the pH value of the system after the lithium source-coated graphite material is mixed with the alkaline solution can be 10, 11, 12 or any value between 10 and 12.

[0070] According to some embodiments of the present application, after mixing the lithium source-coated graphite material with the alkaline solution, the method further comprises: washing the obtained product with water until it is washed to neutrality, and then drying.

[0071] According to some embodiments of the present application, drying is performed under vacuum conditions, the drying temperature is 50-80° C., and the drying time is 10-14 hours.

[0072] For example, the drying temperature can be 50°C, 60°C, 70°C, 80°C or any value between 50-80°C, and the drying time can be 10 hours, 11 hours, 12 hours, 13 hours, 14 hours or any value between 10-14 hours.

[0073] According to an embodiment of the present application, heat treatment of the lithium hydroxide coated graphite material includes: heating the lithium hydroxide coated graphite material to a first temperature at a first heating rate, then heating it to a second temperature at a second heating rate, and keeping it warm at the second temperature; wherein the first heating rate is greater than the second heating rate, the process of heating to the first temperature and the process of heating to the second temperature are carried out under the protection of an inert gas, and the process of keeping warm at the second temperature is carried out in an atmosphere containing carbon dioxide. The present application adopts a gradient heating strategy, which can reduce thermal stress, prevent material structure damage, and promote a more uniform conversion process. The initial stage of the gradient heating of the present application is carried out in a nitrogen atmosphere, which can avoid the premature start of the generation of lithium carbonate, and the later stage of the gradient heating is carried out in an atmosphere containing carbon dioxide, which helps to improve the purity and crystallinity of lithium carbonate.

[0074] The present application does not specifically limit the content of carbon dioxide in the carbon dioxide-containing atmosphere, as long as it can react to generate lithium carbonate. In some embodiments, the carbon dioxide-containing atmosphere includes an air atmosphere, which can save production costs, reduce operational difficulty, and facilitate the promotion and application of the present application method.

[0075] According to some embodiments of the present application, after the heat treatment is completed, the generated product is treated by a rapid cooling method, and the rapid cooling method includes air cooling or water cooling. This design helps to fix the structure after the transformation, reduce grain growth, improve the cyclic stability of the material, and transform lithium hydroxide into a stable lithium carbonate layer and coat it on the graphite surface.

[0076] According to an embodiment of the present application, the first heating rate is 10-20°C / min, and the second heating rate is 1-5°C / min; for example, the first heating rate may be 10°C / min, 11°C / min, 12°C / min, 13°C / min, 14°C / min, 15°C / min, 16°C / min, 17°C / min, 18°C / min, 19°C / min, 20°C / min or any value between 10-20°C / min, and the second heating rate may be 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min or any value between 1-5°C / min.

[0077] And / or, the first temperature is 80-250° C., the second temperature is 350-650° C., and the insulation time is 1-2 hours.

[0078] For example, the first temperature can be 80℃, 100℃, 120℃, 150℃, 170℃, 200℃, 220℃, 250℃ or any value between 80-250℃, the second temperature can be 350℃, 370℃, 400℃, 420℃, 450℃, 470℃, 500℃, 520℃, 550℃, 570℃, 600℃, 620℃, 650℃ or any value between 350-650℃, and the insulation time can be 1 hour, 1.5 hours, 2 hours or any value between 1-2 hours.

[0079] According to an embodiment of the present application, forming a lithium azide layer on the surface of the graphite-lithium carbonate material includes: depositing an organic lithium compound and an azide source on the surface of the graphite-lithium carbonate material by atomic layer deposition.

[0080] According to an embodiment of the present application, the deposition temperature is 130-160°C; within the above deposition temperature range, the high-quality growth of the lithium azide layer can be guaranteed, which is beneficial to improving the rate performance and cycle stability of the battery. If the deposition temperature is too high, it may cause uneven growth of the lithium azide layer, and even cause microstructural damage to the material, increase interface defects, and ultimately lead to poor electrochemical performance of the battery. If the deposition temperature is too low, the reaction rate between the organic lithium compound and the azide source will be significantly slowed down, which may result in incomplete reaction or reactants adsorbed on the surface of the material but not effectively converted into lithium azide, which will not only affect the formation speed of the lithium azide layer, but may also cause the formed lithium azide layer to be not dense and uniform enough, thereby reducing its effectiveness as an ion transport channel optimizer.

[0081] For example, the deposition temperature may be 130°C, 140°C, 150°C, 160°C, or any value between 130-160°C.

[0082] And / or, the deposition pressure is 0.08-0.12 Pa; within the pressure range of 0.08-0.12 Pa, the lithium azide layer can be ensured to grow uniformly and densely, ion transport can be optimized, and electrochemical performance can be enhanced. If the deposition pressure is too low, the deposition rate will be too slow and the film will not be formed completely; if the deposition pressure is too high, the precursor will be over-adsorbed, affecting the film quality and structural stability.

[0083] For example, the deposition pressure may be 0.08 Pa, 0.09 Pa, 0.1 Pa, 0.11 Pa, 0.12 Pa or any value between 0.08-0.12 Pa.

[0084] And / or, the organic lithium compound includes at least one of butyl lithium, phenyl lithium and lithium amide compounds, and the lithium amide compound includes lithium hexamethyldisilazide (LiN(Si(CH 3 )3 ) 2 ), lithium hexamethyldisilazide has the advantages of good stability and easy operation.

[0085] And / or, the azide source includes at least one of sodium azide, potassium azide, and ammonia;

[0086] And / or, the pulse time ratio of the organic lithium compound to the azide source is 1:20 to 1:50; at a pulse time ratio of 1:20 to 1:50, the lithium azide layer can be uniformly deposited and the lithium ion conduction path can be optimized. If the pulse time ratio is too small, the reaction will be incomplete and the film quality will be poor; if the pulse time ratio is too large, the precursor will be wasted and the film thickness will be uneven, thereby reducing the electrochemical performance.

[0087] For example, the pulse time ratio of the organolithium compound to the azide source can be 1:20, 1:25, 1:30, 1:35, 1:40, 1:50, or any value between 1:20 and 1:50.

[0088] And / or, the number of cycles of the atomic layer deposition method is 100-1000 times; under 100-1000 cycles, the atomic layer deposition can form a uniform and dense lithium azide layer, effectively improving the electrochemical performance. Too few cycles will result in a thin and discontinuous film; too many cycles will increase the process cost and may cause stress problems, affecting material properties.

[0089] For example, the number of cycles of the atomic layer deposition method can be 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 or any value between 100 and 1000. Atomic layer deposition (ALD) technology has the advantages of high precision and high material utilization, significantly reduces waste generation, and can reduce long-term production costs.

[0090] And / or, after the deposition is completed, the method further includes: annealing the deposited material to obtain a negative electrode material; wherein the annealing temperature is 100-200°C and the annealing time is 1-2 hours. Through the annealing treatment, the crystallinity and stability of the lithium azide layer can be enhanced, which is conducive to obtaining a negative electrode material with high rate and long cycle.

[0091] For example, the annealing temperature can be 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C or any value between 100-200°C, and the annealing time can be 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h or any value between 1-2h.

[0092] Through the method of the present application, a highly uniform lithium azide layer can be formed, which is beneficial to optimizing the lithium ion transmission channel, reducing the interface resistance, enhancing the electrochemical stability and overall safety of the material, and is beneficial to improving the performance of the negative electrode material.

[0093] According to some embodiments of the present application, the thickness of the lithium azide layer is 5-10 nm, for example, 5 nm, 10 nm, or any value between 5-10 nm.

[0094] The present application also provides a battery, including the negative electrode material described above or the negative electrode material prepared by the preparation method described above. The above-mentioned battery includes an electrolyte battery, a semi-solid battery and an all-solid battery, that is, the negative electrode material of the present application can be applied to an electrolyte battery, a semi-solid battery and an all-solid battery.

[0095] The embodiments of the present application will be described in detail below in conjunction with specific examples, but it will be appreciated by those skilled in the art that the following examples are only used to illustrate the present application and should not be considered as limiting the scope of the present application. If specific conditions are not specified in the examples, they are carried out according to normal conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified in the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0096] Example 1

[0097] Embodiment 1 provides a negative electrode material, and the preparation method thereof comprises:

[0098] Step 1: In situ generation of lithium hydroxide-coated graphite

[0099] Raw material preparation: 10g of flaky graphite is selected as the main material, lithium chloride is selected as the lithium source, and its mass accounts for 5% of the mass of graphite, and 0.1g of polyethylene glycol and 0.1g of ethyl orthosilicate are prepared as surfactants. In addition, 50mL of deionized water and sodium hydroxide solution are prepared, wherein the concentration of sodium hydroxide is 0.75mol / L.

[0100] Preparation process: First, graphite powder is evenly dispersed in deionized water containing lithium source, and ultrasonic dispersion is performed for 30 minutes. During this period, ethyl orthosilicate and polyethylene glycol are added to improve the dispersion effect and promote LiOH coating. Then, sodium hydroxide solution is slowly added to adjust the pH value of the system to 11, and stirring is continued for 2 hours to ensure that LiOH is generated in situ and coated on the surface of graphite particles.

[0101] Post-treatment: After centrifugation, the material was washed with deionized water until neutral and then dried under vacuum at 60 °C for 12 h.

[0102] Step 2: Heat treatment to generate lithium carbonate layer

[0103] The dried coating material was placed in a tube furnace, first heated to 200°C at a rapid heating rate of 15°C / min, then slowly heated to 450°C at 3°C / min, and kept at 450°C for 2 hours, wherein the process of rapid heating to 200°C and slow heating to 450°C was carried out in a nitrogen atmosphere, and the process of keeping at 450°C for 2 hours was carried out in an air atmosphere. Finally, air cooling technology was used to quickly cool down.

[0104] Step 3: ALD coating of lithium azide

[0105] Using ALD technology, lithium hexamethyldisilazide (LiN(Si(CH 3 ) 3 ) 2 ) as an organic lithium compound precursor, NH 3 As an azide source, the deposition temperature was set at 140°C, the pressure in the reaction chamber was maintained at 0.1 Pa, the precursor pulse time ratio was 1:30, 500 cycles of deposition were performed, and the deposition amount was 3% of the mass of graphite to obtain a uniform lithium azide coating layer with a thickness of 5 nm.

[0106] In order to enhance the structural stability of the lithium azide layer, the deposited material was annealed at a temperature of 150° C. for 1.5 hours to obtain a negative electrode material.

[0107] The SEM image of the negative electrode material prepared in Example 1 is as follows: Figure 1 shown.

[0108] In the negative electrode material prepared in Example 1, the mass ratio of graphite to the lithium carbonate layer and the lithium azide layer is 1:0.055:0.045.

[0109] Example 2

[0110] The negative electrode material was prepared by referring to the method of Example 1, except that in Example 2, the mass of lithium chloride accounted for 10% of the mass of graphite, and the vacuum drying temperature in step 1 of Example 2 was increased to 70° C., while other conditions remained unchanged.

[0111] In the negative electrode material prepared in Example 2, the mass ratio of graphite to the lithium carbonate layer and the lithium azide layer is 1:0.075:0.045.

[0112] Example 3

[0113] The negative electrode material was prepared by referring to the method of Example 1, which differs from Example 1 in that when ALD was used to deposit lithium azide in Example 3, the temperature was reduced to 130° C. and the deposition period was reduced to 100 cycles.

[0114] In the negative electrode material prepared in Example 3, the mass ratio of graphite to the lithium carbonate layer and the lithium azide layer is 1:0.055:0.019.

[0115] Example 4

[0116] The negative electrode material was prepared by referring to the method of Example 1, which differs from Example 1 in that the conditions for air heat treatment in step 2 of Example 4 are different from those in Example 1. In Example 1, the air heat treatment is carried out at 450°C for 2 hours, while in Example 4, the air heat treatment is carried out at 450°C for 1 hour.

[0117] In the negative electrode material prepared in Example 4, the mass ratio of graphite to the lithium carbonate layer and the lithium azide layer is 1:0.055:0.045.

[0118] Example 5

[0119] The negative electrode material was prepared by referring to the method of Example 1, which differs from Example 1 in that the lithium azide precursor was changed to butyl lithium and sodium azide.

[0120] In the negative electrode material prepared in Example 5, the mass ratio of graphite to the lithium carbonate layer and the lithium azide layer is 1:0.055:0.045.

[0121] Example 6

[0122] The negative electrode material was prepared by referring to the method of Example 1, except that the thickness of the lithium azide layer deposited by ALD was changed to 10 nm.

[0123] In the negative electrode material prepared in Example 6, the mass ratio of graphite to the lithium carbonate layer and the lithium azide layer is 1:0.055:0.075.

[0124] Example 7

[0125] The negative electrode material was prepared by referring to the method of Example 1, except that in Example 7, the mass of lithium chloride accounted for 2.5% of the mass of graphite, and the processing time and temperature remained unchanged.

[0126] In the negative electrode material prepared in Example 7, the mass ratio of graphite to the lithium carbonate layer and the lithium azide layer is 1:0.015:0.045.

[0127] Example 8

[0128] The negative electrode material was prepared by referring to the method of Example 1. The difference from Example 1 is that: on the basis of Example 1, a pretreatment step is added, that is, the graphite is washed with dilute hydrochloric acid to remove impurities, and then the original process is followed to explore the effect of improving the purity on the performance of the final product.

[0129] In the negative electrode material prepared in Example 8, the mass ratio of graphite to the lithium carbonate layer and the lithium azide layer is 1:0.055:0.045.

[0130] Comparative Example 1

[0131] The negative electrode material was prepared by referring to the method of Example 1. The difference from Example 1 is that: in Example 1, no ALD lithium azide layer deposition was performed, and only the lithium hydroxide coating and heat treatment to lithium carbonate were completed to compare the performance of the negative electrode material without the lithium azide layer.

[0132] Comparative Example 2

[0133] The negative electrode material was prepared by referring to the method of Example 1, which differs from Example 1 in that: Comparative Example 2 skipped the lithium hydroxide coating step and directly deposited a lithium azide layer on the surface of natural graphite to investigate the effect of the lack of a lithium carbonate layer on the performance of the negative electrode material.

[0134] Comparative Example 3

[0135] The negative electrode material was prepared by referring to the method of Example 1, which differs from Example 1 in that: in Comparative Example 3, the ALD deposition temperature was increased to 200° C. to observe the negative effects of high temperature on the quality of the lithium azide layer and the battery performance.

[0136] The electrochemical performance of the negative electrode materials prepared in Examples 1-8 and Comparative Examples 1-3 was tested under the same conditions. The test results are shown in Table 1.

[0137] Table 1 Electrochemical test table of Examples 1-8 and Comparative Examples 1-3

[0138]

[0139]

[0140] It can be seen from Table 1 that the specific capacity and long cycle performance of Examples 1-8 at different rates are better than those of Comparative Examples 1-3.

[0141] In Example 8, graphite was purified, and the electrochemical performance of Example 8 was worse than that of Example 1. Although the purification operation can improve the purity of graphite, it may also introduce unfavorable factors. Specifically, this may be due to the dilute hydrochloric acid treatment during the purification process, which changes the surface structure of graphite and affects the dynamics of lithium ion insertion / extraction; the surface properties of the purified graphite change, affecting the quality and uniformity of the coating layer (lithium carbonate and lithium azide); and the formed SEI film is thicker or has a higher resistance, which increases the interface impedance.

[0142] The negative electrode materials of Examples 1-8 include a lithium azide layer. The deposition step of the ALD lithium azide layer is omitted in Comparative Example 1. The negative electrode material of Comparative Example 1 only includes graphite and a lithium carbonate layer located on the surface of the graphite. The negative electrode material of Comparative Example 1 lacks a lithium azide layer. Figure 2 It can be seen that the material of comparative example 1 is inferior to those of embodiments 1-8 in ion conductivity, interface stability and cycle stability. Especially under high rate charge and discharge conditions, the capacity decay of comparative example 1 is faster and the cycle life is shorter.

[0143] The negative electrode materials in Examples 1-8 have a lithium carbonate layer in graphite and lithium azide, which significantly improves the basic structural strength of the material and the diffusion efficiency of lithium ions, and exhibits excellent electrochemical performance in high-rate charge and discharge tests, and has a large number of cycles. Comparative Example 2 deposits lithium azide directly on the graphite surface, and the negative electrode material of Comparative Example 2 does not include a lithium carbonate layer, which results in insufficient structural stability of the material, high interface impedance, affecting the rapid transmission of lithium ions, and thus reducing rate performance and cycle stability.

[0144] Examples 1-8 use a relatively mild ALD deposition temperature to prepare the negative electrode material, which can ensure the high-quality growth of the lithium azide layer, which is beneficial to improving the rate performance and cycle stability of the battery. Comparative Example 3 increases the ALD deposition temperature to 200°C. The electrochemical performance of the negative electrode material prepared in Comparative Example 3 is poor. This may be because the higher ALD deposition temperature will cause uneven growth of the lithium azide layer, and even cause microstructural damage to the material, increase interface defects, and thus lead to poor electrochemical performance of the battery.

[0145] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

[0146] In addition, those skilled in the art will appreciate that, although some embodiments herein include certain features included in other embodiments but not other features, the combination of features of different embodiments is meant to be within the scope of the present application and form different embodiments. For example, in the above claims, any one of the claimed embodiments may be used in any combination. The information disclosed in this background technology section is intended only to deepen the understanding of the overall background technology of the present application and should not be regarded as an admission or in any form of implication that the information constitutes prior art known to those skilled in the art.

Claims

1. A negative electrode material, characterized in that: include: Graphite, a lithium carbonate layer coated on the surface of the graphite, and a lithium azide layer coated on the surface of the lithium carbonate layer.

2. The negative electrode material according to claim 1, characterized in that The mass ratio of the graphite to the lithium carbonate layer and the lithium azide layer is 1:(0.01-0.1):(0.01-0.08).

3. A method for preparing a negative electrode material as claimed in claim 1 or 2, characterized in that: include: Forming a lithium carbonate layer on the surface of graphite to obtain a graphite-lithium carbonate material; A lithium azide layer is formed on the surface of the graphite-lithium carbonate material to obtain a negative electrode material.

4. The method for preparing the negative electrode material according to claim 3, characterized in that: The preparation of the graphite-lithium carbonate material comprises: Preparation of lithium hydroxide coated graphite material; The lithium hydroxide coated graphite material is heat treated to convert the lithium hydroxide into lithium carbonate.

5. The method for preparing the negative electrode material according to claim 4, characterized in that: The material for preparing lithium hydroxide coated graphite comprises: mixing graphite, a lithium source, and a solvent to obtain a material for lithium source coated graphite; The lithium source coated graphite material is mixed with an alkaline solution and dried to obtain a lithium hydroxide coated graphite material.

6. The method for preparing the negative electrode material according to claim 5, characterized in that: The graphite includes flake graphite; And / or, the lithium source includes at least one of lithium chloride, lithium hydride, lithium bromide, and lithium iodide, and the mass of the lithium source accounts for 2.5% to 10% of the mass of the graphite; And / or, during the mixing of the graphite, the lithium source and the solvent, the method further comprises adding a surfactant, wherein the surfactant comprises at least one of tetraethyl orthosilicate, polyethylene glycol, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, polyoxyethylene sorbitan fatty acid ester and a silane coupling agent; and / or, the alkaline solution comprises a sodium hydroxide solution; And / or, the concentration of the alkaline solution is 0.5-1 mol / L; And / or, the amount of the alkaline solution added is controlled so that the pH value of the system after the lithium source-coated graphite material is mixed with the alkaline solution is in the range of 10 to 12.

7. The method for preparing the negative electrode material according to claim 4, characterized in that: The heat treatment of the lithium hydroxide coated graphite material comprises: heating the lithium hydroxide coated graphite material to a first temperature at a first heating rate, then heating it to a second temperature at a second heating rate, and keeping it warm at the second temperature; wherein the first heating rate is greater than the second heating rate, the heating to the first temperature and the heating to the second temperature are performed under the protection of an inert gas, and the keeping warm at the second temperature is performed in an atmosphere containing carbon dioxide; The first heating rate is 10-20°C / min, and the second heating rate is 1-5°C / min; The first temperature is 80-250° C., the second temperature is 350-650° C., and the insulation time is 1-2 hours.

8. The method for preparing the negative electrode material according to claim 3, characterized in that: Forming a lithium azide layer on the surface of the graphite-lithium carbonate material includes: depositing an organic lithium compound and an azide source on the surface of the graphite-lithium carbonate material by atomic layer deposition.

9. The method for preparing the negative electrode material according to claim 8, characterized in that: The deposition temperature is 130-160°C; And / or, the deposition pressure is 0.08-0.12 Pa; And / or, the organic lithium compound includes at least one of butyl lithium, phenyl lithium, and a lithium amide compound, and the lithium amide compound includes lithium hexamethyldisilazide; And / or, the azide source includes at least one of sodium azide, potassium azide, and ammonia; and / or, the pulse time ratio of the organolithium compound to the azide source is 1:20 to 1:50; And / or, the number of cycles of the atomic layer deposition method is 100-1000 times; And / or, after the deposition is completed, the method further comprises: annealing the deposited material to obtain a negative electrode material; wherein the annealing temperature is 100-200° C., and the annealing time is 1-2 hours.

10. A battery, characterized in that: The negative electrode material comprises the negative electrode material according to claim 1 or 2, or the negative electrode material prepared by the preparation method according to any one of claims 3 to 9.

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