A negative electrode active material for lithium ion batteries, a preparation method therefor, and use thereof

Abstract

The application provides a negative electrode material for a lithium ion battery and a preparation method and application thereof. The negative electrode active material for the lithium ion battery comprises a nano-porous silicon material, a first carbon coating layer, a second carbon coating layer and a third carbon coating layer which are sequentially coated on the nano-porous silicon material, and a protective layer. The first carbon coating layer is formed by nitrogen-doped and / or sulfur-doped conductive carbon material, and the bonding force between the first carbon coating layer and the nano-porous silicon material at the interface is 15-25 MPa. The second carbon coating layer is formed by boron-doped graphitized carbon material, and the graphitization degree coefficient of the graphitized carbon material is 0.6-0.9. The third carbon coating layer comprises 30-50 wt% of amorphous carbon and 50-70 wt% of crystalline carbon. The lithium ion battery formed by using the negative electrode material has excellent initial coulomb efficiency, capacity retention rate and rate performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy, and particularly relates to a negative electrode active material for a lithium ion battery and a preparation method and application thereof. BACKGROUND

[0002] Silicon-carbon negative electrode materials are considered as ideal candidate materials for the next generation of lithium ion batteries due to their high specific capacity and good cycle performance. However, silicon in the silicon-carbon negative electrode material will undergo a huge volume change during charging and discharging, which can reach 300%, resulting in problems such as electrode structure damage, active material peeling, and continuous decomposition of electrolyte, thereby causing rapid capacity decay and short cycle life of the battery.

[0003] To solve this problem, researchers have proposed various carbon coating strategies to improve the performance of silicon-based negative electrode materials. Traditional carbon coating methods mainly include liquid phase carbon coating and gas phase carbon coating. However, forming a single carbon coating on the surface of the silicon-based negative electrode often fails to meet the comprehensive requirements of conductivity, stability, and mechanical strength of the negative electrode material. For example, the carbon layer formed in the prior art is usually not uniform and dense, which cannot effectively alleviate the volume expansion of the silicon material. Or the carbon layer is unstable in quality and prone to pores and cracks, which cannot effectively protect the silicon material. In addition, the silicon-carbon negative electrode material is prone to side reactions with the electrolyte during long-term cycling, forming an unstable solid electrolyte interface (SEI) film, which affects its long-term stability.

[0004] Therefore, it is urgent to develop a high-performance lithium ion battery negative electrode material to meet the increasing performance requirements of negative electrode materials for the next generation of high-energy-density lithium ion batteries. SUMMARY

[0005] To solve all or part of the above technical problems, the application provides the following technical solutions:

[0006] The application provides a negative electrode active material for a lithium ion battery, comprising:

[0007] a nano-porous silicon material;

[0008] a first carbon coating layer formed by a nitrogen and / or sulfur doped conductive carbon material and coated on the nano-porous silicon material, and at least part of N atoms and / or S atoms in the first carbon coating layer are chemically bonded with at least part of Si atoms in the nano-porous silicon material at the interface between the first carbon coating layer and the nano-porous silicon material; and the bonding force between the first carbon coating layer and the nano-porous silicon material at the interface is 15-25 MPa according to the nanoindentation test method.

[0009] a second carbon coating layer formed of boron-doped graphitized carbon material and coated on the first carbon coating layer, wherein the graphitized carbon material has a graphitization degree coefficient of 0.6-0.9 according to a Raman spectroscopy method;

[0010] a third carbon coating layer containing 30-50 wt% amorphous carbon and 50-70 wt% crystalline carbon and coated on the second carbon coating layer, wherein the amorphous carbon and the crystalline carbon are uniformly mixed and distributed;

[0011] a protective layer including one or a combination of inorganic oxide, inorganic fluoride or LiPON (lithium phosphorous oxynitride) and coated on the third carbon coating layer.

[0012] In the method for calculating the graphitization degree coefficient according to the Raman spectroscopy method, the graphitization degree coefficient R = IG / (IG+ID), wherein IG and ID are the integral intensities of G peak and D peak in the Raman spectrum, respectively, the G peak (about 1580 cm -1 ) represents a graphite structure, and the D peak (about 1350 cm -1 ) represents a defect or disordered structure. The higher the graphitization degree, the closer the R value to 1.

[0013] In the negative electrode active material provided by the application, the first carbon coating layer forms strong interaction with the silicon surface through the abundant functional groups on the conductive carbon material, providing excellent interface bonding force, effectively preventing the carbon coating layer from falling off during the cycle process, and maintaining the structural integrity of the material; the graphitized structure of the second carbon coating layer provides good mechanical strength, and the boron doping introduces additional electron carriers, improving the electrical conductivity of the carbon layer, the second carbon coating layer can enhance the conductive network of the entire electrode, accelerate electron transmission, and at the same time provide mechanical support to inhibit the volume expansion of the silicon material; the amorphous carbon in the third carbon coating layer provides flexibility and porosity, and the crystalline carbon provides high electrical conductivity, forming a "soft and hard" structure that has good buffering capacity to absorb the stress generated by the volume change of the silicon material and prevent the material from cracking, and also can maintain high electrical conductivity to ensure fast electron transmission.

[0014] Due to the synergistic effect of the strong interfacial bonding force of the first carbon coating layer and the high conductivity of the second carbon coating layer, the negative electrode active material can maintain a stable electron transport network in long-term cycles; and the negative electrode active material also has both flexible buffering and rigid support, and the amorphous carbon in the third carbon coating layer cooperates with the graphite structure of the second coating layer to provide a buffering effect while maintaining the stability of the overall structure; the negative electrode active material also has a multi-scale pore structure, and the three layers of carbon coating together construct a multi-scale pore network from micro to macro, which is conducive to the penetration of electrolyte and the rapid transmission of lithium ions. The present invention found that the three-layer carbon coating of the above structure also helps to form a more stable and uniform SEI film, reducing the irreversible capacity loss of the battery; and improving the thermal conductivity of the material, which helps to better manage heat during high-rate charge and discharge.

[0015] In some embodiments, the specific surface area of ​​the nanoporous silicon material is 800 to 1200 m 2 / g.

[0016] In some embodiments, the nanoporous silicon material has a three-dimensional interconnected pore structure, and the pores contained in the three-dimensional interconnected pore structure have a pore diameter of 10-30 nm.

[0017] In some embodiments, the thickness ratio of the first carbon coating layer, the second carbon coating layer, and the third carbon coating layer is (2-3):(5-10):(5-8).

[0018] In some preferred embodiments, the thickness of the first carbon coating layer is 2-3 nm, the thickness of the second carbon coating layer is 5-10 nm, and the thickness of the third carbon coating layer is 5-8 nm.

[0019] In some embodiments, the thickness of the protective layer is 1-5 nm.

[0020] In some embodiments, the median particle size D50 of the negative electrode active material for lithium-ion batteries is 1 to 10 μm.

[0021] In some embodiments, the conductive carbon material includes one or more of dopamine, polypyrrole, or polythiophene.

[0022] In some embodiments, the boron doping amount in the second carbon coating layer is 0.2-2 wt %.

[0023] In some embodiments, the third carbon coating layer is formed by carbonizing a mixture of soft pitch and hard pitch at 600-1000°C.

[0024] In some embodiments, the inorganic oxide includes one or more of Al2O3, TiO2, and ZrO2, and the inorganic fluoride includes AlF3.

[0025] The second object of the present application is to provide a preparation method of the negative electrode active material for lithium ion batteries, comprising:

[0026] performing liquid-phase carbon coating on the nanoporous silicon material using a nitrogen-doped and / or sulfur-doped conductive carbon material to form a first carbon coating layer on the surface of the nanoporous silicon material, thereby obtaining a first intermediate material;

[0027] performing chemical vapor deposition on the first intermediate material using a gaseous carbon source, and introducing a boron source during the chemical vapor deposition to form a boron-doped graphitized carbon material on the surface of the first carbon coating layer, thereby forming a second carbon coating layer, thereby obtaining a second intermediate material;

[0028] adsorbing a carbon precursor on the surface of the second intermediate material, and performing carbonization treatment on the second intermediate material with the adsorbed carbon precursor to convert the carbon precursor into amorphous carbon and crystalline carbon, thereby forming a third carbon coating layer, thereby obtaining a third intermediate material;

[0029] depositing one or more of inorganic oxides, inorganic fluorides or LiPON on the surface of the third intermediate material by at least atomic layer deposition technology, thereby forming a protective layer, thereby obtaining a negative electrode material for lithium ion batteries.

[0030] In some embodiments, the preparation method specifically comprises: immersing the nanoporous silicon material in a solution containing 0.1-20 mg / mL of nitrogen-doped and / or sulfur-doped conductive carbon material, and reacting at a temperature of 60-80°C for 4-8 h, thereby obtaining the first intermediate material.

[0031] In some embodiments, the nitrogen-doped and / or sulfur-doped conductive carbon material comprises one or more of dopamine, polypyrrole or polythiophene.

[0032] In some embodiments, the temperature of the chemical vapor deposition is 600-1000°C, and the time is 2-30 h.

[0033] In some embodiments, the gaseous carbon source comprises acetylene and / or methane.

[0034] In some preferred embodiments, the gaseous carbon source comprises acetylene and methane in a volume ratio of 2-4:1-2.

[0035] In some embodiments, the boron source comprises triethylboron.

[0036] In some embodiments, the amount of the boron source introduced is 1-5 wt% of the gaseous carbon source.

[0037] In some embodiments, the preparation method specifically includes: dispersing the carbon precursor in a solvent to obtain a solution containing 3 to 30 wt% of the carbon precursor, and immersing the second intermediate material in the solution for 12 to 48 hours so that the carbon precursor is adsorbed on the surface of the second intermediate material; and carbonizing at a temperature of 600 to 1000°C for 2 to 4 hours to obtain the third intermediate material.

[0038] In some embodiments, the carbon precursor includes one or more combinations of soft pitch, glucose, sucrose, starch, hard pitch, phenolic resin, and furan resin.

[0039] In some preferred embodiments, the carbon precursor includes soft pitch and hard pitch.

[0040] The method of depositing one or more of inorganic oxides, inorganic fluorides or LiPON on the surface of the third intermediate material using the atomic layer deposition technology to form a protective layer can adopt any existing method, and the present invention does not impose any particular limitation on this.

[0041] A third object of the present invention is to provide a negative electrode for a lithium ion battery, comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer is any one of the negative electrode materials for lithium ion batteries described above.

[0042] A fourth object of the present invention is to provide a lithium-ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the negative electrode is the negative electrode of the lithium-ion battery.

[0043] Compared with the prior art, the present invention has at least the following beneficial effects: the porous silicon material in the negative electrode active material provided by the present invention can effectively alleviate the volume expansion during the charge and discharge process and provide abundant ion channels; the multi-layer carbon coating structure can improve the conductivity of the material, enhance the interfacial bonding force, and optimize the carbon structure; the outer protective layer can provide uniform and dense surface protection for the material; through the synergistic effect between the layers, the multi-layer synergy achieves a comprehensive improvement in the conductivity, structural stability, ion transmission capacity and surface protection performance of the negative electrode active material; compared with a single carbon-coated negative electrode material, the negative electrode material provided by the present invention can increase the first coulombic efficiency of the lithium-ion battery by 10 to 15%, the rate performance by more than 50%, and the performance attenuation in a high-temperature cycle of 60°C by 40%; the capacity retention rate of the negative electrode material after 1,000 cycles reaches more than 92%, and the capacity still maintains more than 80% at a high current of 10C. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in this application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0045] Figure 1 is a SEM image of the negative electrode active material prepared in one embodiment of the present invention;

[0046] Figure 2 is a multi-layer cross-sectional view of a negative electrode active material prepared in one embodiment of the present invention;

[0047] Figure 3 1 is an XRD pattern of the negative electrode active material prepared in one embodiment of the present invention. DETAILED DESCRIPTION

[0048] The technical solutions of the present invention are described in detail below in conjunction with specific embodiments so that those skilled in the art can better understand and implement the technical solutions of the present invention. The specific functional details disclosed herein should not be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching those skilled in the art to employ the present invention in various ways in virtually any appropriate detailed embodiment.

[0049] Unless otherwise specified, in the specific embodiments of the present invention, various raw materials, reagents, reaction equipment, and testing equipment used can be obtained through commercial purchase and other channels, and the testing methods are conventional methods in the field.

[0050] Example 1

[0051] Example 1 provides a negative electrode material for a lithium ion battery and a preparation method thereof, which specifically includes the following steps:

[0052] Preparation of nanoporous silicon material: Using NaCl particles as sacrificial templates, silane gas was pyrolyzed at 550°C to deposit on the surface of the NaCl particles. The NaCl template was then dissolved and removed with water to obtain a nanoporous silicon material with a three-dimensional interconnected pore structure. Detection showed that the pore size of the nanoporous silicon material was about 20nm, and the specific surface area reached 1000m 2 / g;

[0053] The prepared nanoporous silicon material was immersed in a 2 mg / mL dopamine solution and reacted at 60°C for 6 hours at a pH of 8.5 to form a 2.5 nm thick dopamine carbon layer on the surface of the porous silicon material, which was recorded as the first intermediate material. Nanoindentation testing showed that the bonding strength at the interface between the dopamine carbon layer and the nanoporous silicon material was approximately 17 MPa.

[0054] The first intermediate material was placed in a tube furnace at 800° C., and a mixture of acetylene and methane (volume ratio 3:1) was introduced thereinto. At the same time, 0.5 wt % triethylboron was introduced therein for chemical vapor deposition. The reaction was allowed to proceed for 30 minutes to form a 7 nm thick boron-doped graphitized carbon layer on the surface of the first intermediate material, which was recorded as the second intermediate material. According to Raman spectroscopy, the graphitization coefficient of the boron-doped graphitized carbon layer was calculated to be 0.67.

[0055] Soft asphalt and hard asphalt were mixed in a 1:1 mass ratio and dissolved in tetrahydrofuran to obtain a 20 wt% asphalt tetrahydrofuran solution. The second intermediate material was immersed in the solution for 36 hours, then filtered and carbonized at 800°C for 2 hours, thereby forming a 6 nm thick mixed asphalt carbon layer on the second intermediate material, which was recorded as the third intermediate material. XRD fitting peak detection showed that the mixed asphalt layer contained 27 wt% amorphous carbon and 73 wt% crystalline carbon.

[0056] Atomic layer deposition technology is used, with trimethylaluminum and water as precursors, and 20 cycles of deposition are performed at a temperature of 200°C to form a 2nm thick Al2O3 protective layer on the third intermediate material to obtain a negative electrode material.

[0057] Figure 1 is a SEM image of the negative electrode active material prepared in this example, Figure 2 is a multi-layer cross-sectional view of the negative electrode active material prepared in this embodiment, Figure 3 is the XRD pattern of the negative electrode active material prepared in this example.

[0058] Example 2

[0059] Example 2 provides a negative electrode material for a lithium ion battery and a preparation method thereof, which specifically includes the following steps:

[0060] Preparation of nanoporous silicon material: Porous silicon was prepared by magnesium thermal reduction method. SiO2 and magnesium powder were mixed in a molar ratio of 1:2, reacted at 650℃ for 4 hours, and then washed with hydrochloric acid to remove MgO to obtain nanoporous silicon material. The pore size of the nanoporous silicon material was 15nm and the specific surface area was 950m 2 / g;

[0061] The prepared nanoporous silicon material was dispersed in a 0.1 mol / mL pyrrole monomer solution, and an oxidizing agent, ammonium persulfate, was added at a concentration of 0.05 mol / L. The mixture was reacted at room temperature for 8 hours to form a 3 nm thick polypyrrole layer on the surface of the nanoporous silicon material, which was recorded as the first intermediate material. The binding force between the dopamine carbon layer and the nanoporous silicon material at the interface between the two was measured by nanoindentation testing and was found to be approximately 17 MPa.

[0062] The first intermediate material was placed in a tube furnace at 850° C., and a mixture of acetylene and methane (volume ratio 3:1) was introduced thereinto, and 0.6 wt % triethylboron was simultaneously introduced therein for chemical vapor deposition. The reaction time was 25 minutes, so as to form a 10 nm thick boron-doped graphitized carbon layer on the surface of the first intermediate material, which was recorded as the second intermediate material. According to Raman spectroscopy, the graphitization degree coefficient of the boron-doped graphitized carbon layer was 0.79.

[0063] Soft asphalt and hard asphalt were mixed in a 1:1 mass ratio and dissolved in tetrahydrofuran to obtain a 20 wt% asphalt tetrahydrofuran solution. The second intermediate material was immersed in the solution for 48 hours, then filtered, and carbonized at 600°C for 3 hours to form an 8 nm thick mixed asphalt layer on the second intermediate material, which was recorded as the third intermediate material. XRD fitting peak detection showed that the mixed asphalt layer contained 26 wt% amorphous carbon and 74 wt% crystalline carbon.

[0064] Atomic layer deposition technology is used, with titanium tetrachloride and water as precursors, and 25 cycles of deposition are performed at a temperature of 180°C to form a 1nm thick TiO2 protective layer on the third intermediate material to obtain a negative electrode material.

[0065] Example 3

[0066] Example 1 provides a negative electrode material for a lithium ion battery and a preparation method thereof, which specifically includes the following steps:

[0067] Preparation of nanoporous silicon material: Using NaCl particles as sacrificial templates, silane gas was pyrolyzed at 550°C to deposit on the surface of the NaCl particles. The NaCl template was then dissolved and removed with water to obtain a nanoporous silicon material with a three-dimensional interconnected pore structure. Detection showed that the pore size of the nanoporous silicon material was about 20nm, and the specific surface area reached 1000m 2 / g;

[0068] The prepared nanoporous silicon material was immersed in a 20 mg / mL thiophene solution. After uniform dispersion, 10 ml of a 2 mg / mL ferric chloride solution was added dropwise, stirred thoroughly, and reacted at 80°C for 4 hours to form a 2 nm thick polythiophene carbon layer on the surface of the porous silicon material, which was recorded as the first intermediate material. The binding force between the polythiophene carbon layer and the nanoporous silicon material at the interface was measured by nanoindentation testing, and was approximately 18 MPa.

[0069] The first intermediate material was placed in a tube furnace at 1000° C., and a mixture of acetylene and methane (volume ratio 3:1) was introduced thereinto. At the same time, 0.5 wt % triethylboron was introduced therein for chemical vapor deposition. The reaction was allowed to proceed for 30 minutes to form a 5 nm thick boron-doped graphitized carbon layer on the surface of the first intermediate material, which was recorded as the second intermediate material. According to Raman spectroscopy, the graphitization degree coefficient of the boron-doped graphitized carbon layer was calculated to be 0.73.

[0070] Soft asphalt and hard asphalt were mixed in a 1:1 mass ratio and dissolved in tetrahydrofuran to obtain a 20 wt% asphalt tetrahydrofuran solution. The second intermediate material was immersed in the solution for 12 hours, then filtered and carbonized at 600°C for 4 hours, thereby forming a 5 nm thick mixed asphalt carbon layer on the second intermediate material, which was recorded as the third intermediate material. XRD fitting peak detection showed that the mixed asphalt layer contained 21 wt% amorphous carbon and 79 wt% crystalline carbon.

[0071] Atomic layer deposition technology is used, with trimethylaluminum and water as precursors, and 20 cycles of deposition are performed at a temperature of 200°C to form a 5nm thick ZrO2 protective layer on the third intermediate material to obtain a negative electrode material.

[0072] Example 4

[0073] Example 4 is basically the same as Example 1, except that the protective layer in Example 4 is AlF3.

[0074] Example 5

[0075] Example 5 is basically the same as Example 1, except that the protective layer in Example 5 is LiPON.

[0076] Comparative Example 1

[0077] The only difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not form a dopamine carbon layer, that is, a boron-doped graphitized carbon layer, a mixed asphalt carbon layer and an Al2O3 protective layer are directly coated on the surface of the nanoporous silicon material in sequence, and the rest is implemented in the same way as Example 1.

[0078] Comparative Example 2

[0079] The difference between Comparative Example 2 and Example 1 is only that Comparative Example 2 does not form a boron-doped graphitized carbon layer, i.e., a dopamine carbon layer, a mixed pitch carbon layer, and an Al2O3 protective layer are sequentially coated on the surface of the nanoporous silicon material, and the rest is the same as Example 1.

[0080] Comparative Example 3

[0081] The difference between Comparative Example 3 and Example 1 is only that Comparative Example 3 does not form a mixed pitch carbon layer, i.e., a dopamine carbon layer, a boron-doped graphitized carbon layer, and an Al2O3 protective layer are sequentially formed on the surface of the nanoporous silicon material, and the rest is the same as Example 1.

[0082] Comparative Example 4

[0083] The difference between Comparative Example 4 and Example 1 is only that Comparative Example 4 does not form a protective layer, i.e., a dopamine carbon layer, a boron-doped graphitized carbon layer, and a mixed pitch carbon layer are sequentially formed on the surface of the nanoporous silicon material, and the rest is the same as Example 1.

[0084] Comparative Example 5

[0085] The difference between Comparative Example 5 and Example 1 is only that the thicknesses of the dopamine carbon layer, the boron-doped graphitized carbon layer, and the mixed pitch carbon layer of Comparative Example 5 are 2.5 nm, 7 nm, and 12 nm, respectively, and the rest is the same as Example 1. The present application found that when the thickness of the mixed pitch carbon layer is relatively thick, the comprehensive performance of the battery decreases, which is specifically manifested as a decrease in the initial efficiency of the battery and an acceleration of the surface side reaction at high temperature, which may be due to the existence of active sites in the amorphous carbon in the graphite carbon layer, which promotes the decomposition of the electrolyte to generate a large amount of SEI layer, consumes active lithium ions, and leads to a deepening of the side reaction.

[0086] Comparative Example 6

[0087] The difference between Comparative Example 6 and Example 1 is only that the thicknesses of the dopamine carbon layer, the boron-doped graphitized carbon layer, and the mixed pitch carbon layer of Comparative Example 6 are 2.5 nm, 15 nm, and 6 nm, respectively, and the rest is the same as Example 1. The present application found that when the thickness of the boron-doped graphitized carbon layer is relatively thick, the comprehensive performance of the battery decreases, which is specifically manifested as a decrease in capacity, which is due to the fact that although the boron-doped graphitized carbon layer has good ion conductivity, its capacity is low, and an increase in thickness will lead to a decrease in capacity.

[0088] Comparative Example 7

[0089] The difference between Comparative Example 7 and Example 1 is only that the thicknesses of the dopamine carbon layer, the boron-doped graphitized carbon layer, and the mixed pitch carbon layer of Comparative Example 7 are 6 nm, 7 nm, and 6 nm, respectively, which are the same as those of Example 1. It is found by the present application that when the thickness of the dopamine carbon layer is relatively large, the comprehensive performance of the battery decreases, specifically, the cycle performance and the high-temperature storage performance decrease, which may be due to the fact that the residual N and H elements of the dopamine carbon layer increase the consumption of the electrolyte, especially under high-temperature storage conditions, excessive consumption of the electrolyte leads to loss of active lithium, and increases the irreversible capacity.

[0090] The negative electrode active material in the above examples and comparative examples is used to prepare a lithium ion battery negative electrode, specifically including the following steps:

[0091] (1) Preparation of a positive electrode: the positive electrode is a lithium nickel-cobalt-manganese oxide (NCM523, LiNi 0.5 Co 0.2 Mn 0.3 O2) material. NCM523, a conductive agent (super P), and a binder (PVDF) are mixed in a mass ratio of 92:4:4, an appropriate amount of N-methyl pyrrolidone (NMP) solvent is added, and stirring is performed to form a slurry; the slurry is uniformly coated on an aluminum foil current collector, and after vacuum drying at 80°C for 12 hours, compaction is performed to obtain a positive electrode sheet; the area specific capacity of the positive electrode is 3.5 mAh / cm 2 ;

[0092] (2) Preparation of a negative electrode: the negative electrode material prepared in the above examples or comparative examples, a graphite conductive agent, and a binder CMC-SBR are mixed in a mass ratio of 80:10:10, deionized water is added, and stirring is performed to form a slurry; the slurry is uniformly coated on a copper foil current collector, and after vacuum drying at 60°C for 8 hours, compaction is performed to obtain a negative electrode sheet; the area specific capacity of the negative electrode is 3.0 mAh / cm 2 ;

[0093] (3) The separator is a polyethylene / polypropylene (PE / PP) double-layer composite separator with a thickness of 25 μm and a porosity of 40%;

[0094] (4) The electrolyte is 1M LiPF6 dissolved in a mixed solvent of EC / DMC / EMC (volume ratio 1:1:1), and 2wt% of fluoroethylene carbonate (FEC) is added as an additive;

[0095] (5) Battery assembly: in an argon atmosphere glove box, the positive electrode sheet, the separator, and the negative electrode sheet are sequentially stacked, an appropriate amount of electrolyte is injected, an aluminum plastic film is used for packaging, and a soft pack battery with a capacity of 2000 mAh is prepared;

[0096] (6) Formation and aging: the assembled battery is subjected to first charge and discharge (formation) at 25°C with a current of 0.1C, and then is subjected to aging treatment by storage at 45°C for 24 hours.

[0097] The batteries using the negative active material of the above examples and comparative examples were tested for relevant performance, and the test structure is shown in Table 1.

[0098] Table 1 Performance of lithium ion batteries prepared using the negative material in the examples and comparative examples of the application

[0099]

[0100] From the above examples 1-5, it can be seen that the lithium ion battery assembled using the negative material prepared by the technical scheme of the application has a better first coulomb efficiency and capacity retention rate.

[0101] Comparing comparative example 1 and comparative examples 1-3, the negative material of comparative example 1-3 does not contain a dopamine layer, a boron-doped graphitized carbon layer and a mixed pitch carbon layer respectively, and the first coulomb efficiency and capacity retention rate of the lithium ion battery of comparative example 1-3 are both decreased, indicating that the multi-layer carbon coating structure provided by the application can improve the structural stability and ion transmission capacity of the negative active material through the synergistic effect between the layers.

[0102] Comparing comparative example 1 and comparative example 4, it can be seen that forming a protective layer on the surface of the negative material improves the first coulomb efficiency and capacity retention rate of the lithium ion battery, especially greatly improving the capacity retention rate.

[0103] Comparing comparative example 1 and comparative examples 5-7, it can be seen that the thickness of the dopamine layer, the boron-doped graphitized carbon layer and the mixed pitch carbon layer has an effect on the first coulomb efficiency and capacity retention rate of the lithium ion battery, and within the thickness range provided by the application, the lithium ion battery can maintain a better first coulomb efficiency and capacity retention rate. When the mixed pitch layer of comparative example 5 is thicker, the first efficiency of the battery decreases and the surface side reaction at high temperature increases; when the boron-doped graphitized carbon layer of comparative example 6 is thicker, the capacity of the battery decreases, which may be due to the fact that although the boron-doped graphitized carbon layer has good ion conductivity, its capacity is low, and an increase in thickness will lead to a decrease in capacity; when the dopamine layer of comparative example 7 is thicker, the high-temperature storage performance of the battery decreases, which may be due to the fact that the N and H elements remaining in the dopamine carbon layer will increase the consumption of electrolyte, especially under high-temperature storage conditions, excessive consumption of electrolyte leads to loss of active lithium, resulting in an increase in irreversible capacity.

[0104] Aspects, embodiments, features, and examples of the present application should be considered illustrative in all aspects and are not intended to limit the present application, the scope of which is defined only by the claims. Other embodiments, modifications, and uses will be apparent to those skilled in the art without departing from the spirit and scope of the claimed application.

[0105] In addition, the inventors of the present application have also made tests with other raw materials, process operations, process conditions described in the specification with reference to the foregoing examples, and all ideal results have been obtained.

[0106] While the present application has been described with reference to illustrative embodiments, those with ordinary skill in the art will appreciate that various other alterations, omissions, and / or additions can be made and equivalents can be substituted for elements of the embodiments without departing from the spirit and scope of the present application. Additionally, many modifications can be made to adapt a particular situation or material to the teachings of the present application without departing from the scope of the application. Accordingly, the present application is not intended to be limited to the disclosed embodiments of the application, but rather, the scope of the application is to be defined by the appended claims. Furthermore, unless specifically stated otherwise, any use of the terms first, second, etc., does not indicate any order or importance, but rather the terms first, second, etc., merely indicate distinction between two elements, one element being a first element and the other being a second element.

Claims

1. A negative electrode active material for a lithium ion battery, characterized in that include: Nanoporous silicon materials; a first carbon coating layer formed of a nitrogen- and / or sulfur-doped conductive carbon material and coated on the nanoporous silicon material, wherein at least a portion of the N atoms and / or S atoms in the first carbon coating layer are chemically bonded to at least a portion of the Si atoms in the nanoporous silicon material at an interface between the first carbon coating layer and the nanoporous silicon material; and a binding force between the first carbon coating layer and the nanoporous silicon material at the interface thereof is 15 to 25 MPa according to a nanoindentation test method; a second carbon coating layer, formed of a boron-doped graphitized carbon material and coated on the first carbon coating layer; wherein the graphitization degree coefficient of the graphitized carbon material is 0.6 to 0.9 as calculated by Raman spectroscopy; a third carbon coating layer comprising 30-50 wt% amorphous carbon and 50-70 wt% crystalline carbon, coated on the second carbon coating layer, wherein the amorphous carbon and the crystalline carbon are uniformly mixed and distributed; The protective layer includes one or more of inorganic oxides, inorganic fluorides or LiPON and is coated on the third carbon coating layer.

2. The negative electrode active material for lithium-ion batteries according to claim 1, wherein: The specific surface area of ​​the nanoporous silicon material is 800-1200 m² / g; And / or, the nanoporous silicon material has a three-dimensional interconnected pore structure, and the pores contained in the three-dimensional interconnected pore structure have a pore diameter of 10 to 30 nm; And / or, the thickness ratio of the first carbon coating layer, the second carbon coating layer, and the third carbon coating layer is (2-3):(5-10):(5-8); And / or, the thickness of the first carbon coating layer is 2-3 nm, the thickness of the second carbon coating layer is 5-10 nm, and the thickness of the third carbon coating layer is 5-8 nm; And / or, the thickness of the protective layer is 1-5 nm; And / or, the median particle size D50 of the negative electrode active material for lithium ion batteries is 1-10 μm.

3. The negative electrode active material for lithium-ion batteries according to claim 1, wherein: The conductive carbon material includes one or more of dopamine, polypyrrole or polythiophene; and / or, the boron doping amount in the second carbon coating layer is 0.2-2 wt %; And / or, the third carbon coating layer is formed by carbonizing a mixture of soft pitch and hard pitch at 600-1000° C.; And / or, the inorganic oxide includes one or more of Al2O3, TiO2, and ZrO2, and the inorganic fluoride includes AlF3.

4. The method for preparing the negative electrode active material for lithium ion batteries according to any one of claims 1 to 3, characterized in that: include: Performing liquid-phase carbon coating on a nanoporous silicon material using a nitrogen- and / or sulfur-doped conductive carbon material to form a first carbon coating layer on the surface of the nanoporous silicon material, thereby obtaining a first intermediate material; Performing chemical vapor deposition on the first intermediate material using a gaseous carbon source, and introducing a boron source during the chemical vapor deposition process to form a boron-doped graphitized carbon material on the surface of the first carbon coating layer, thereby forming a second carbon coating layer, thereby obtaining a second intermediate material; allowing a carbon precursor to adsorb on the surface of the second intermediate material, and performing a carbonization treatment on the second intermediate material adsorbed with the carbon precursor to convert the carbon precursor into amorphous carbon and crystalline carbon to form a third carbon coating layer, thereby obtaining a third intermediate material; Atomic layer deposition technology is used to deposit one or more of inorganic oxides, inorganic fluorides or LiPON on the surface of the third intermediate material to form a protective layer, thereby obtaining a negative electrode material for a lithium-ion battery.

5. The preparation method according to claim 4, characterized in that Specifically include: Immersing the nanoporous silicon material in a solution containing 0.1-20 mg / mL of nitrogen- and / or sulfur-doped conductive carbon material, reacting at a temperature of 60-80° C. for 4-8 hours to obtain the first intermediate material; And / or, the nitrogen and / or sulfur-doped conductive carbon material includes one or more of dopamine, polypyrrole or polythiophene.

6. The preparation method according to claim 4, characterized in that Specifically include: The chemical vapor deposition temperature is 600-1000°C and the time is 2-30 h; and / or, the gaseous carbon source comprises acetylene and / or methane; and / or, the boron source comprises triethylboron; And / or, the amount of the boron source introduced is 1-5 wt % of the gaseous carbon source.

7. The preparation method according to claim 4, characterized in that Specifically include: Dispersing the carbon precursor in a solvent to obtain a solution containing 3-30 wt% of the carbon precursor, and soaking the second intermediate material in the solution for 12-48 hours so that the carbon precursor is adsorbed on the surface of the second intermediate material; The third intermediate material is obtained by carbonizing the obtained product at a temperature of 600-1000° C. for 2-4 hours.

8. The preparation method according to claim 4 or 7, characterized in that: The carbon precursor includes soft pitch and hard pitch.

9. A lithium ion battery negative electrode, characterized in that The invention comprises a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode active material for a lithium ion battery according to any one of claims 1 to 3.

10. A lithium-ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that: The negative electrode is the lithium ion battery negative electrode according to claim 9.

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