A negative electrode composite material, a preparation method therefor, and use thereof

Abstract

The application provides a negative electrode composite material and a preparation method and application thereof. The negative electrode composite material comprises ultra-micro N-type silicon crystal material and conductive carbon material, the ultra-micro N-type silicon crystal material is dispersed in the conductive carbon material, the outer surface of the negative electrode composite material particle is not exposed to the ultra-micro N-type silicon crystal material, and the D50 particle size of the ultra-micro N-type silicon crystal material is less than or equal to 5 nm. The negative electrode composite material has good structural stability, high electrical conductivity, good rate performance, good expansion characteristics and cycle performance, and can be used to provide an electrochemical device with good rate performance and cycle performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative electrode composite material and a preparation method and application thereof. BACKGROUND

[0002] Crystalline silicon material is often used as a negative electrode active material of a battery due to its high theoretical specific capacity. However, the crystalline silicon material has poor anti-expansion performance, and the volume changes greatly during charging and discharging, and each volume increase and decrease will form cracks, affecting the normal performance of the battery. In addition, elemental silicon is a semiconductor material, and has poor electronic conductivity, which greatly limits its application as an electrode active material in batteries. At present, the industry often forms a coating layer on the surface of silicon material particles to improve the anti-expansion performance of the silicon material, but the effect of relieving the expansion effect is limited, and the silicon material particles are still prone to breakage, which deteriorates the cycle performance of the battery. SUMMARY

[0003] In view of this, the present application provides a negative electrode composite material. The negative electrode composite material has good structural stability, and has high electrical conductivity, good rate performance, good expansion characteristics and cycle performance, and can be used to provide an electrochemical device with good rate performance and cycle performance.

[0004] The first aspect of the present application provides a negative electrode composite material, which comprises ultra-micro N-type silicon crystal material and conductive carbon material, the ultra-micro N-type silicon crystal material is dispersed in the conductive carbon material; the outer surface of the negative electrode composite material particles is not exposed to the ultra-micro N-type silicon crystal material, and the D50 particle size of the ultra-micro N-type silicon crystal material is less than or equal to 5 nm.

[0005] The ultra-micro N-type silicon crystal material has a large number of free electrons inside, has strong conductivity, and has a very small size, so that it can be dispersed in the conductive carbon. Therefore, the negative electrode composite material has good uniformity and strong conductivity, so that the negative electrode composite material 100 as a whole has high rate performance. In addition, each ultra-micro N-type silicon crystal material is wrapped by dense conductive carbon material, the risk of rupture of the ultra-micro N-type silicon crystal material is significantly reduced, thereby the expansion characteristics and long cycle characteristics of the negative electrode composite material can be significantly improved, and the stability of the negative electrode composite material is improved.

[0006] Optionally, the conductive carbon material comprises amorphous carbon.

[0007] Optionally, the negative electrode composite material is in a granular form, and the mass content of the ultra-micro N-type silicon crystal material in a single negative electrode composite material gradually decreases from the center to the outer surface of the negative electrode composite material.

[0008] Optionally, in the single negative electrode composite, the minimum mass concentration of silicon element in the region where the ultrafine N-type silicon crystal material is distributed is A, and the maximum mass concentration of silicon element is B, wherein A is greater than or equal to 0.9B.

[0009] Optionally, the constituent elements of the ultrafine N-type silicon crystal material include Si element and doping element; the doping element includes but is not limited to phosphorus element.

[0010] Optionally, the atomic volume concentration of the doping element in the ultrafine N-type silicon crystal material is less than or equal to 5×10 21 atoms / cm 3 . Preferably, the atomic volume concentration of the doping element in the ultrafine N-type silicon crystal material is less than or equal to 1×10 21 atoms / cm 3 . Further preferably, the atomic volume concentration of the doping element in the ultrafine N-type silicon crystal material is less than or equal to 2×10 20 atoms / cm 3 . Still further preferably, the atomic volume concentration of the doping element in the ultrafine N-type silicon crystal material is less than or equal to 1×10 20 atoms / cm 3 .

[0011] Optionally, the mass percentage of silicon element in the negative electrode composite is 25%-75%. Preferably, the mass percentage of silicon element in the negative electrode composite is 40%-70%.

[0012] Preferably, the D50 particle size of the ultrafine N-type silicon crystal material is less than or equal to 0.5 nm. Further preferably, the D50 particle size of the ultrafine N-type silicon crystal material is less than or equal to 0.2 nm.

[0013] Optionally, the grain size of the ultrafine N-type silicon crystal material is less than or equal to 3 nm. Preferably, the grain size of the ultrafine N-type silicon crystal material is in the range of 0.1 nm-1 nm.

[0014] Optionally, the D50 particle size of the negative electrode composite is less than or equal to 30 μm. Preferably, the D50 particle size of the negative electrode composite is less than or equal to 15 μm. Further preferably, the D50 particle size of the negative electrode composite is in the range of 3 μm-15 μm.

[0015] Optionally, the powder resistivity of the negative electrode composite is less than or equal to 2.5 mΩ·cm.

[0016] The second aspect of the present application provides a preparation method of a negative electrode composite, comprising the following steps:

[0017] heating a certain amount of a silicon source, a doping source and a carbon source in a heating device, the carbon source forms an electrically conductive carbon material, and the electrically conductive carbon material is dispersed with ultra-micro N-type silicon crystal material formed in situ by the silicon source and the doping source, to obtain a negative electrode composite material;

[0018] The negative electrode composite material includes the ultra-micro N-type silicon crystal material and the electrically conductive carbon material, the ultra-micro N-type silicon crystal material is dispersed in the electrically conductive carbon material, the outer surface of the negative electrode composite material particle is not exposed to the ultra-micro N-type silicon crystal material, and the D50 particle size of the ultra-micro N-type silicon crystal material is less than or equal to 5 nm.

[0019] The preparation method is simple in operation, strong in controllability and high in production efficiency, and is suitable for large-scale industrialized preparation.

[0020] Optionally, the heating treatment of the certain amount of the silicon source, the doping source and the carbon source in the heating device at least includes the following steps:

[0021] (1) preheating treatment: introducing the carbon source into the heating device;

[0022] (2) heat treatment: replacing the carbon source with the silicon source and the doping source;

[0023] (3) post-treatment: replacing the silicon source with the carbon source.

[0024] Optionally, in the heat treatment, the amount of the silicon source introduced into the heating device decreases.

[0025] Optionally, the silicon source includes but is not limited to SiH4.

[0026] Optionally, the doping source includes but is not limited to phosphine.

[0027] Optionally, the carbon source includes but is not limited to acetylene.

[0028] Optionally, the heating treatment is performed at 400-2000°C for 0.25-4h.

[0029] Optionally, an electrically conductive carbon skeleton material is pre-set in the heating device.

[0030] Optionally, the electrically conductive carbon skeleton material includes but is not limited to activated carbon.

[0031] The third aspect of the application provides a negative electrode tab, which includes the negative electrode composite material provided in the first aspect of the application or prepared by the preparation method provided in the second aspect of the application.

[0032] The electrode tab has good rate performance, cycle performance and safety performance.

[0033] The fourth aspect of the present application provides an electrochemical device, which comprises the negative electrode provided in the third aspect of the present application.

[0034] The electrochemical device comprises a secondary battery, and the secondary battery has high rate performance, good cycle performance and high safety. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 A structural schematic diagram of the negative electrode composite material provided in an embodiment of the present application is shown in the figure.

[0036] Figure 2A A schematic diagram of the distribution of the ultra-fine N-type silicon crystal material in the negative electrode composite material provided in an embodiment of the present application is shown in the figure.

[0037] Figure 2B The correspondence between the mass concentration of the ultra-fine N-type silicon crystal material and the gray scale.

[0038] Legend: 100-negative electrode composite material; 10-ultra-fine N-type silicon crystal material; 20-conductive carbon material. DETAILED DESCRIPTION

[0039] Specifically, refer to Figure 1 The embodiment of the present application provides a negative electrode composite material 100, which comprises an ultra-fine N-type silicon crystal material 10 and a conductive carbon material 20, the ultra-fine N-type silicon crystal material 10 is dispersed in the conductive carbon material 20; the outer surface of the particle of the negative electrode composite material 100 is not exposed to the ultra-fine N-type silicon crystal material 10, and the D50 particle size of the ultra-fine N-type silicon crystal material 10 is less than or equal to 5nm.

[0040] The ultra-micro N-type silicon crystal material 10 has a large number of free electrons and strong conductivity. The D50 particle size of the ultra-micro N-type silicon crystal material 10 is less than 5 nm, and the size is extremely small. When the ultra-micro N-type silicon crystal material 10 is dispersed in the conductive carbon material, the particle feeling is extremely weak, and the two can be highly mixed, so that the negative electrode composite material has good uniformity and strong conductivity, and the overall negative electrode composite material 100 has high rate performance. In addition, each ultra-micro N-type silicon crystal material 10 is wrapped by the dense conductive carbon material 20. Under the condition of equal molar quantity, the smaller the size of the silicon crystal material, the greater the Gibbs free energy of the increased rupture (that is, the greater the energy barrier that the particle needs to cross to break). Small size can effectively inhibit the rupture and pulverization of the silicon crystal material. On the other hand, the presence of the conductive carbon material 20 can also protect the silicon crystal material, and effectively solve the problem of increased side reactions between the negative electrode material and the electrolyte caused by the large specific surface area of the silicon crystal material. In addition, the absolute thermal expansion volume of the small-size silicon crystal material is small, and the impact force on the conductive carbon material 20 formed when the volume expansion occurs is small. The silicon crystal material is difficult to break the wrapping of the conductive carbon material 20 and expose it, so that the conductive carbon material 20 can persistently inhibit the expansion of the ultra-micro N-type silicon crystal material 10 during the charge and discharge cycle, and further improve the expansion characteristics and long cycle characteristics of the negative electrode composite material 100.

[0041] In this application, the D50 particle size of the ultra-micro N-type silicon crystal material 10 can be 0.1 nm, 0.15 nm, 0.2 nm, 0.25 nm, 0.3 nm, 0.35 nm, 0.4 nm, 0.45 nm, 0.5 nm, 0.55 nm, 0.6 nm, 0.65 nm, 0.7 nm, 0.75 nm, 0.8 nm, 0.85 nm, 0.9 nm, 0.95 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, etc. If the D50 particle size of the ultra-micro N-type silicon crystal material 10 is too large, the particle feeling of the negative electrode composite material will be enhanced, the dispersion effect of the ultra-micro N-type silicon crystal material 10 in the conductive carbon material 20 will be poor, and the Gibbs free energy required for the rupture will be reduced, which will damage the expansion characteristics and long cycle characteristics of the negative electrode composite material 100.

[0042] In some embodiments of the present application, the D50 particle size of the ultrafine N-type silicon crystal material 10 is less than or equal to 0.5 nm. Further preferably, in some specific embodiments, the D50 particle size of the ultrafine N-type silicon crystal material 10 is less than or equal to 0.2 nm. Understandably, the ultrafine N-type silicon crystal material in the negative electrode composite material 100 provided by the embodiments of the present application is obtained by vapor phase growth, and under the atmospheric condition, the growth speed of the crystal is similar, so the size of the ultrafine N-type silicon crystal material in the same negative electrode composite material 100 is also similar. In the present application, the size of a certain number of ultrafine N-type silicon crystal particles in the cross-section sample of the negative electrode composite material 100 observed under the transmission electron microscope (TEM) in different regions is considered as the D50 particle size of the ultrafine N-type silicon crystal particles defined in the present application.

[0043] In some embodiments of the present application, the grain size of the ultrafine N-type silicon crystal material 10 (calculated by the Debye-Scherrer formula after X-ray diffraction test) is less than or equal to 3 nm. Preferably, in some specific embodiments, the grain size of the ultrafine N-type silicon crystal material 10 is in the range of 0.1 nm-1 nm. Exemplarily, the grain size of the ultrafine N-type silicon crystal material 10 can be 0.1 nm, 0.2 nm, 0.3 nm, 0.4 nm, 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, 1.0 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, etc.

[0044] In some embodiments of the present application, the D50 particle size (average particle size, measured by dynamic light scattering method) of the negative electrode composite material 100 is less than or equal to 30 μm. In some specific embodiments, the D50 particle size of the negative electrode composite material 100 is in the range of 3 μm-15 μm. Exemplarily, the D50 particle size of the negative electrode composite material 100 can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 20 μm, 25 μm, 30 μm, etc. Controlling the D50 particle size of the negative electrode composite material 100 in the above range is beneficial to ensure that the specific surface area of the negative electrode composite material 100 is in a suitable range, so as to ensure that the electrolyte in the subsequent battery can be well infiltrated into the negative electrode active material, and also can control the de-intercalation path of the active metal ion to be short, so that the electrochemical property of the negative electrode active material (i.e., the battery) can be fully played.

[0045] In some embodiments of the present application, the negative electrode composite material 100 is in a granular shape, please refer to Figures 2A-2BThe mass content of the ultra-micro N-type silicon crystal material 10 in the single negative electrode composite material 100 gradually decreases from the center to the surface of the negative electrode composite material 100. Understandably, the central part of the negative electrode composite material 100 has more distribution of the ultra-micro N-type silicon crystal material, and the mass content of the conductive carbon material is higher along the extension direction of the diameter, and the connection between the conductive carbons is more compact, so that the structure of the entire negative electrode composite material is more stable. From a macroscopic point of view, it is relatively difficult to break the outer layer material from the center of the sphere or sphere-like structure. Such a structural design is beneficial to further reducing the risk of rupture and pulverization of the negative electrode composite material due to the volume expansion of the silicon crystal material. In addition, the conductive carbon material 20 has stronger conductivity and ion conductivity than the ultra-micro N-type silicon crystal material 10 (equivalent to a semiconductor material). Such a structural design is also beneficial to constructing an efficient conductive and ion-conductive network inside the single negative electrode composite material, thereby further improving the rate performance of the negative electrode composite material.

[0046] In some embodiments of the present application, the mass percentage of silicon in the negative electrode composite material 100 is 25%-75%. In some specific embodiments, the mass percentage of silicon in the above-mentioned negative electrode composite material is 40%-70%. Exemplarily, the mass percentage of silicon in the negative electrode composite material 100 can be 25%, 30%, 35%, 40%, 42.5%, 45%, 47.5%, 50%, 52.5%, 55%, 60%, 62.5%, 65%, 67.5%, 70%, 75%, etc. Controlling the mass percentage of silicon within a certain range can not only ensure a high content of negative electrode active material, but also ensure that there is sufficient conductive carbon material 20 in the negative electrode composite material to form an efficient conductive and ion-conductive network, and the structural stability of the negative electrode composite material 100.

[0047] In some embodiments of the present application, in the single negative electrode composite material 100, the minimum mass concentration of silicon in the region where the ultra-micro N-type silicon crystal material 10 is distributed is A, and the maximum mass concentration is B, wherein A≥0.9B. Exemplarily, the value of A can be 0.9B, 0.91B, 0.92B, 0.93B, 0.94B, 0.95B, 0.96B, 0.97B, 0.98B, 0.99B, etc. The mass concentration distribution difference of silicon represents the mass concentration distribution difference of the ultra-micro N-type silicon crystal material 10 inside the negative electrode composite material 100. Controlling the concentration distribution difference of silicon within the above-mentioned range can not only form an efficient conductive and ion-conductive network inside the negative electrode composite material 100, but also avoid the concentrated effect of the ultra-micro N-type silicon crystal material due to excessive concentration in the center of the negative electrode composite material 100, thereby further improving the structural stability of the negative electrode composite material 100.

[0048] In some embodiments of the present application, the constituent elements of the ultrafine N-type silicon crystal material 10 include Si elements and doping elements. The doping elements include, but are not limited to, phosphorus elements and / or arsenic elements. In some cases, the doping elements can also include other pentavalent elements, such as nitrogen elements, etc.

[0049] In some embodiments of the present application, the atomic volume concentration of the doping elements is less than or equal to 5×10 21 atoms / cm 3 . Preferably, in some specific embodiments, the atomic volume concentration of the doping elements in the ultrafine N-type silicon crystal material 10 is less than or equal to 1×10 21 atoms / cm 3 . Further preferably, in some specific embodiments, the atomic volume concentration of the doping elements in the ultrafine N-type silicon crystal material 10 is less than or equal to 2×10 20 atoms / cm 3 . Still further preferably, in some specific embodiments, the atomic volume concentration of the doping elements in the ultrafine N-type silicon crystal material 10 is less than or equal to 1×10 20 atoms / cm 3 . The greater the atomic volume concentration of the doping elements, the better the conductivity of the ultrafine N-type silicon crystal material 10. However, an appropriate amount of doping elements can ensure that the ultrafine N-type silicon crystal material 10 has good conductivity and good intercalation capacity for active metal ions.

[0050] In some embodiments of the present application, the powder resistivity of the negative electrode composite material 100 is less than or equal to 2.5 mΩ·cm. For example, the powder resistivity of the negative electrode composite material 100 can be 1.4 mΩ·cm, 1.5 mΩ·cm, 1.6 mΩ·cm, 1.7 mΩ·cm, 1.8 mΩ·cm, 1.9 mΩ·cm, 2.0 mΩ·cm, 2.1 mΩ·cm, 2.2 mΩ·cm, 2.3 mΩ·cm, 2.4 mΩ·cm, 2.5 mΩ·cm, etc.

[0051] The present application also provides a preparation method of a negative electrode composite material, which is suitable for preparing the negative electrode composite material 100 described above. The preparation method comprises the following steps:

[0052] A certain amount of silicon source, doping source and carbon source are introduced into a heating device for heating. The carbon source forms a conductive carbon material, and the conductive carbon material is dispersed with the ultrafine N-type silicon crystal material formed in situ by the reaction of the silicon source and the doping source, to obtain a negative electrode composite material.

[0053] The negative electrode composite material includes an ultrafine N-type silicon crystal material and a conductive carbon material, wherein the ultrafine N-type silicon crystal material is dispersed in the conductive carbon material; the outer surface of the negative electrode composite material particles does not expose the ultrafine N-type silicon crystal material, and the D50 particle size of the ultrafine N-type silicon crystal material is less than or equal to 5 nm.

[0054] A silicon source, a doping source, and a carbon source are introduced into a fluidized bed furnace, so that the decomposed silicon source and the decomposed doping source co-deposit to form an ultrafine N-type silicon crystalline material, and the ultrafine N-type silicon crystalline material is suspended in the fluidized bed furnace. The carbon source also decomposes and deposits on the surface of the ultrafine N-type silicon crystalline material. As the conductive carbon material continuously deposits on the surface of the material unit, the ultrafine silicon crystal particles with the conductive carbon material deposited layer continuously fuse in the fluidized bed furnace, ultimately forming the negative electrode composite material provided in the embodiments of the present application.

[0055] The preparation method is simple to operate, has strong process controllability, and high production efficiency, and is suitable for large-scale industrial preparation.

[0056] In the present application, the silicon source and doping source are first vaporized under high temperature, or gaseous silicon source and gaseous doping source are directly added to cause a gas phase reaction between the silicon source, doping source and carbon source.

[0057] In some embodiments of the present application, the process of introducing a certain amount of silicon source, dopant source, and carbon source into a heating device for heating includes at least the following steps: (1) preheating: introducing a carbon source into the heating device; (2) heat treatment: replacing the carbon source with the silicon source and the dopant source; and (3) post-treatment: replacing the silicon source with the carbon source. That is, during the preheating process, raw materials other than the silicon source and the dopant source are introduced; during the heat treatment process, raw materials other than the carbon source are introduced; and during the post-treatment process, raw materials other than the silicon source and the dopant source are introduced. Preparing the negative electrode composite material according to the above steps can better disperse the ultrafine N-type silicon crystals in the conductive carbon material, further optimizing the electrochemical performance of the negative electrode composite material.

[0058] In some embodiments of the present application, during the heat treatment, the amount of silicon source introduced into the heating device is gradually reduced. This helps ensure that the mass content of the ultrafine N-type silicon crystals in a single negative electrode composite material gradually decreases from the center of the negative electrode composite material toward the outer surface. In some specific embodiments, the reduction is performed at a rate of 50 wt.% per hour. This is more conducive to regulating the mass content of the ultrafine N-type silicon crystals in the negative electrode composite material to gradually decrease from the center of the negative electrode composite material toward the outer surface.

[0059] In some embodiments of the present application, the silicon source can be a material well known to those skilled in the art. The silicon source includes but is not limited to SiH4.

[0060] In some embodiments of the present application, the doping source can be a material well known to those skilled in the art. The doping source includes but is not limited to phosphine.

[0061] In some embodiments of the present application, the carbon source can be a material well known to those skilled in the art, which can be gaseous or liquid. Specifically, the carbon source includes but is not limited to acetylene.

[0062] In some embodiments of the present application, the heating treatment is performed at 400-2000℃ for 0.25-4h.

[0063] In some embodiments of the present application, a conductive carbon skeleton material is pre-placed in the heating device. That is, before other raw materials are introduced into the heating device, a solid skeleton carbon material is placed in the heating device. The skeleton carbon material itself has a large number of pore structures, which can adsorb silicon elements and doping elements to deposit inside to form ultra-micro N-type silicon crystal material. The subsequently introduced carbon source can also deposit in the skeleton carbon material to fill the gaps, so that the ultra-micro N-type silicon crystal material can be buried in the conductive carbon material. In this way, the production efficiency can be significantly improved. In some embodiments, the conductive carbon skeleton material includes but is not limited to activated carbon.

[0064] The present application also provides a negative electrode sheet, which comprises the negative electrode composite material 100 provided by the present application.

[0065] The electrode sheet has good rate performance, cycle performance and safety performance.

[0066] In some embodiments of the present application, the negative electrode composite material provided by the first aspect of the present application can be formed on a negative electrode current collector (such as a copper foil), and then rolled and cut to obtain a negative electrode sheet. Specifically, a slurry containing the negative electrode composite material can be coated on the negative electrode current collector, and then dried, rolled and cut to obtain the negative electrode sheet.

[0067] The present application also provides an electrochemical device, which comprises the negative electrode sheet provided by the present application.

[0068] The electrochemical device includes a secondary battery, which has high rate performance, good cycle performance and high safety.

[0069] The secondary battery can be a liquid battery using a liquid electrolyte, or a semi-solid or solid battery using a semi-solid electrolyte or a solid electrolyte. In some embodiments, the secondary battery can include a positive electrode sheet, the negative electrode sheet described above, and a separator and an electrolyte disposed between the positive electrode sheet and the negative electrode sheet. In other embodiments, the secondary battery can include a positive electrode sheet, a negative electrode sheet, and a semi-solid electrolyte or a solid electrolyte disposed between the positive electrode sheet and the negative electrode sheet. In addition, when a semi-solid electrolyte or a solid electrolyte is used, the positive electrode sheet and the negative electrode sheet can also contain a semi-solid electrolyte material or a solid electrolyte material.

[0070] In the present application, the lithium battery described above can be assembled by the following method:

[0071] S01, in a glove box, the positive electrode sheet, the separator, and the negative electrode sheet are sequentially stacked to form an electric core;

[0072] S02, the electric core is packaged with an aluminum plastic film shell, and an electrolyte is injected to obtain a battery with a capacity of 3-5 AH. The battery can be tested for electrochemical performance after formation.

[0073] Alternatively, the lithium battery described above can be assembled by the following method:

[0074] S01, in a glove box, the positive electrode sheet with a solid or semi-solid electrolyte layer is aligned with the negative electrode sheet to form an electric core; wherein the solid or semi-solid electrolyte layer is close to the negative electrode sheet;

[0075] S02, the electric core is packaged to obtain a solid or semi-solid battery. The battery can be tested for electrochemical performance after formation.

[0076] In some embodiments of the present application, the positive electrode sheet can be obtained by coating a positive electrode slurry containing a positive electrode active material, a conductive agent, and a binder on a positive electrode current collector (such as an aluminum foil), drying, and pressing. The positive electrode active material is a material well known to those skilled in the art, and can be at least one of lithium iron phosphate, lithium titanate, lithium cobaltate, nickel-manganese-cobalt ternary, nickel-cobalt-aluminum ternary, and lithium-rich manganese-based material. The conductive agent is a material well known to those skilled in the art, and can be at least one of carbon black, conductive graphite, carbon fiber, carbon nanotube, graphene, and mixed conductive slurry. The binder is a material well known to those skilled in the art, and can be at least one of polyvinylidene fluoride, polyamide resin, polyacrylonitrile, sodium carboxymethyl cellulose, and styrene butadiene rubber.

[0077] Generally, the aforementioned electrolyte comprises an organic solvent, a lithium salt, and an additive; the aforementioned organic solvent, lithium salt, and additive can all be materials known to those skilled in the art. Illustratively, the organic solvent can be at least one of ethylene carbonate, methyl ethyl carbonate, ethylene glycol carbonate, fluoroethylene carbonate, vinylene carbonate, tetraethylene glycol dimethyl ether, ethylene glycol dimethyl ether, dimethyl ether, and 1,3-dioxolane; the lithium salt can be at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis-trifluoromethylsulfonylimide, and lithium trifluorosulfonylimide; the additive can be at least one of fluoroethylene carbonate, vinylene carbonate, and lithium nitrate.

[0078] In some embodiments of the present application, the separator is a material known to those skilled in the art; illustratively, the separator can be a polyethylene film, a polypropylene film, a polyethylene / polypropylene double-layer film, a polyethylene / polypropylene / polypropylene triple-layer film, etc.

[0079] In some embodiments of the present application, the solid or semi-solid electrolyte is a material known to those skilled in the art.

[0080] The technical solutions of the present application are further described below in multiple specific embodiments.

[0081] Embodiment 1

[0082] N2, SiH4, PH3, and C2H2 were introduced into a heating device-boiling furnace for reaction; the specific process conditions are shown in Table 1. The prepared negative electrode composite material is denoted as S1. It was determined that the D50 particle size of S1 was 9 μm, and the D50 particle size of the ultrafine N-type silicon crystal material was 5 nm.

[0083] Embodiment 2

[0084] Activated carbon powder, N2, SiH4, PH3, and C2H2 were introduced into a heating device-boiling furnace for reaction; the specific process conditions are shown in Table 1. The prepared negative electrode composite material is denoted as S2. It was determined that the D50 particle size of S2 was 12 μm, and the D50 particle size of the ultrafine N-type silicon crystal material was 2 nm.

[0085] Embodiment 3

[0086] Porous carbon powder, N2, SiH4, PH3, and C2H4 were introduced into a heating device-boiling furnace for reaction; the specific process conditions are shown in Table 1. The prepared negative electrode composite material is denoted as S3. It was determined that the D50 particle size of S3 was 15 μm, and the D50 particle size of the ultrafine N-type silicon crystal material was 5 nm.

[0087] Embodiment 4

[0088] The acetylene black powder, N2, SiH4, PH3, toluene were introduced into the heating device-boiling furnace for reaction, and the specific process conditions were shown in Table 1. The prepared negative electrode composite material was recorded as S4. It was determined that the D50 particle size of S4 was 10 μm, and the D50 particle size of the ultra-micro N-type silicon crystal material was 0.5 nm.

[0089] Example 5

[0090] The porous carbon powder, CO2, SiH4, PH3, CH4 were introduced into the heating device-boiling furnace for reaction, and the specific process conditions were shown in Table 1. The prepared negative electrode composite material was recorded as S5. It was determined that the D50 particle size of S5 was 11 μm, and the D50 particle size of the ultra-micro N-type silicon crystal material was 0.2 nm.

[0091] Example 6

[0092] The porous carbon powder, N2, silicon ethyl ester (TEOS), PH3, CH4 were introduced into the heating device-vacuum rotary furnace for reaction, and the specific process conditions were shown in Table 1. The prepared negative electrode composite material was recorded as S6. It was determined that the D50 particle size of S5 was 13 μm, and the D50 particle size of the ultra-micro N-type silicon crystal material was 5 nm.

[0093] Example 7

[0094] The difference from Example 1 was that the silicon source was introduced into the boiling furnace in a way of reducing 50 wt.% SiH4 per hour in the heat treatment process. It was determined that the D50 particle size of S7 was 9 μm, and the D50 particle size of the ultra-micro N-type silicon crystal material was 5 nm.

[0095] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples are set.

[0096] Comparative Example 1

[0097] The N2, SiH4, C2H2 were introduced into the heating device-boiling furnace for reaction, and the specific process conditions were shown in Table 1. The prepared negative electrode composite material was recorded as DS1. It was determined that the D50 particle size of DS1 was 9 μm, and the D50 particle size of the ultra-micro N-type silicon crystal material was 5 nm.

[0098] Comparative Example 2

[0099] The silicon monoxide particles, N2, C2H2 were introduced into the heating device-boiling furnace for reaction, and the specific process conditions were shown in Table 1. The prepared negative electrode composite material was recorded as DS2. It was determined that the D50 particle size of DS2 was 9 μm, and the D50 particle size of the nano silicon crystal particles contained in the material was 7 nm (calculated by Debye-Scherrer formula after XRD test).

[0100] Comparative Example 3

[0101] N-type single crystal silicon particles were ball milled and sand milled to obtain nano silicon powder with a D50 particle size of 100 nm. The nano silicon powder, flaky graphite, and phenolic resin were granulated and heat treated to obtain a negative electrode composite material, which is denoted as DS3. It was determined that the D50 particle size of DS3 was 20 μm, the mass percentage of silicon element in DS3 was 15%, and the volume concentration of phosphorus element was 1.6×10 16 atoms / cm 3 .

[0102] Secondary batteries with the negative electrode composite materials (negative electrode materials) prepared in the above examples and comparative examples were prepared.

[0103] (1) Preparation of negative electrode sheets: the negative electrode composite materials (negative electrode materials), graphite, and binders (specifically SBR and polyacrylic acid), and conductive agents (specifically acetylene black and carbon nanotubes) prepared in the examples and comparative examples were added to a solvent-water in a mass percentage of 5:95:10:5, and after being fully stirred, a negative electrode slurry was obtained. A certain amount of the negative electrode slurry was coated on the surface of a negative electrode current collector-copper foil, and after drying, rolling, and slitting, a negative electrode sheet was obtained.

[0104] (2) Preparation of positive electrode sheets: a positive electrode active material-LiNi 0.8 Co 0.1 Mn 0.1 (NCM811), a binder-PVDF5130, and a conductive agent-super P were dissolved in N-methyl pyrrolidone (NMP) in a mass ratio of 8.7:0.8:0.5, and after being fully stirred, a positive electrode slurry was obtained. The above positive electrode slurry was coated on a positive electrode current collector-aluminum foil, and after drying, rolling, and slitting, a positive electrode sheet was obtained.

[0105] (3) Preparation of secondary batteries: a plurality of the above negative electrode sheets, separators, and positive electrode sheets were alternately stacked to prepare batteries in a stacking manner, wherein the positive and negative electrode sheets were arranged alternately, and the adjacent positive and negative electrode sheets were separated by a separator to obtain a dry battery. The dry battery was placed in an aluminum plastic film outer package, electrolyte was injected, and then vacuum sealed after 48 h of standing at 60°C. After pressure layering, secondary packaging, degassing, and capacity grading at 60°C, a stacking soft-pack full battery with a capacity of 2.2 Ah was obtained. The batteries of the examples are denoted as S1-S7, and the batteries of the comparative examples are denoted as DS1-DS3.

[0106] Related characterization:

[0107] (1) Transmission Electron Microscope (TEM) test was performed on each negative electrode composite material to observe its micro-morphology and determine the D50 particle size of the ultra-micro N-type silicon crystal material in the negative electrode composite material.

[0108] (2) Energy dispersive spectroscopy (EDS) surface scanning was performed on each negative electrode composite material to semi-quantitatively test the distribution of silicon elements inside the ultrafine N-type silicon crystal material.

[0109] (3) The powder resistivity of the negative electrode composite materials prepared in each example and the comparative example was measured: the powder to be measured was placed in a specific container, 50 MPa was applied for 10 s, and the 4-probe method was used for measurement. The results are summarized in Table 3.

[0110] (4) The electrochemical performance of the batteries S1-S7, DS1-DS3 with the negative electrode composite materials (negative electrode materials) of each example and the comparative example was tested. The following steps were included:

[0111] (a) Normal temperature battery cycle test: the battery was subjected to 1C / 1C charge-discharge cycle test at 25°C, the voltage range was 4.2V-3.0V, and the expansion rate and capacity retention rate of the battery after 200 cycles were recorded. The results are summarized in Table 4. The test was continued, and the expansion rate and capacity retention rate of the battery after 400 cycles were recorded. The results are summarized in Table 4.

[0112] (b) High temperature battery cycle test: the battery was subjected to 1C / 1C charge-discharge cycle test at 45°C, the voltage range was 4.2V-3.0V, and the expansion rate and capacity retention rate of the battery after 200 cycles were recorded. The results are summarized in Table 4. The test was continued, and the expansion rate and capacity retention rate of the battery after 400 cycles were recorded. The results are summarized in Table 4.

[0113] (c) Rate performance test: the ratio of discharge capacity of the battery under 1C and 3C conditions was tested.

[0114] (5) The content of silicon elements and boron elements in the negative electrode composite material was determined by inductive coupled plasma emission spectrometer (ICP). The content of carbon elements in the negative electrode composite material was determined by carbon-sulfur analyzer. Among them, the B element in the ICP test result is identified as wt% (C wt ), and the conversion relationship between atoms / cm 3 , can be converted by the empirical formula C wt = k x C nl , wherein the value of k is generally between 2 x 10 -23 -5 x 10 -22 , and the specific value of k needs to be determined according to the true density of the measured material.

[0115] (6) The X-ray diffraction (XRD) test is performed on each negative electrode composite material, and the Scherrer formula is used for peak fitting, so as to semi-quantitatively test each component and content, and verify the test results of the element contents obtained in step (5).

[0116] Table 1: Main process parameters of each example and comparative example

[0117]

[0118] Table 2: Contents of elements in negative electrode composite materials of each example and comparative example

[0119] Sample No. Si (wt%) C (wt%) P (wt%) [P(atoms / cm 3 )]]> O (wt%) Example 1 52.4 41.8 0.7 <![CDATA[7.16×10 19 ]]> 5.1 Example 2 53.0 42.4 0.1 1.02 x 10 19 ]]> 4.5 Example 3 52.2 42.2 0.3 3.07 x 10 19 ]] 5.3 Example 4 52.7 41.9 0.7 7.16 x 10 19 ]] 4.7 Example 5 52.4 41.8 1.2 1.23 x 10 20 ]]> 4.6 Example 6 52.1 42.0 1.0 1.02 x 10 20 ]]> 4.9 Example 7 36.3 55.1 0.8 6.15 x 10 19 ]]> 7.8 Comparative Example 1 52.2 42.4 0.0 0 4.5 Comparative Example 2 39.8 9.6 0.0 0 50.6 Comparative Example 3 47.5 42.4 0.0 0 10.1

[0120] Table 3: Powder resistivity of negative electrode composite materials prepared in each example and comparative example

[0121]

[0122] Table 4: Summary of results of batteries prepared in each example and comparative example after 200 cycles

[0123]

[0124] As can be seen from the data in Tables 1-4, the negative electrode composite material provided in the examples shows superior electronic conductivity performance compared to the comparative material; when the negative electrode composite material provided in the examples is applied to a battery, the rate performance, room temperature and high temperature cycle performance of the battery can be significantly optimized, and at least one of the room temperature battery expansion rate or the high temperature battery expansion rate of the S1-S7 battery is significantly lower than that of the comparative battery, the high temperature cycle performance of the example battery is particularly good, which fully illustrates the superiority of the negative electrode composite material of the examples, and the electrochemical device provided by the negative electrode composite material has good electrochemical performance. When the negative electrode composite material meets the preferred condition of the present application that the mass content of the ultra-fine P-type silicon crystal material in a single negative electrode composite material gradually decreases from the center to the surface of the negative electrode composite material, and other parameters are similar, for example, example 7 and example 4, the comprehensive performance of the battery S7 is obviously better.

[0125] It should be noted that the O element of the example negative electrode composite material comes from the preparation environment and has no significant effect on the electrochemical performance of the material; the high oxygen content of comparative example 2 is caused by the raw material silicon monoxide.

[0126] The above is an exemplary embodiment of the present application. It should be noted that those skilled in the art can make some improvements and refinements without departing from the principles of the present application, and these improvements and refinements are also considered within the scope of protection of the present application.

Claims

1. A negative electrode composite material, characterized in that: The negative electrode composite material includes an ultrafine N-type silicon crystal material and a conductive carbon material, wherein the ultrafine N-type silicon crystal material is dispersed in the conductive carbon material; the ultrafine N-type silicon crystal material is not exposed on the outer surface of the negative electrode composite material particles, and the D50 particle size of the ultrafine N-type silicon crystal material is less than or equal to 5 nm; the negative electrode composite material is in a granular form, and the mass content of the ultrafine N-type silicon crystal material in a single negative electrode composite material gradually decreases from the center of the negative electrode composite material to the outer surface.

2. The negative electrode composite material according to claim 1, characterized in that In a single negative electrode composite material, in the region where the ultrafine N-type silicon crystal material is distributed, the lowest mass concentration of silicon element is A and the highest mass concentration is B, wherein A≥0.9B.

3. The negative electrode composite material according to claim 1, characterized in that The mass percentage of silicon element in the negative electrode composite material is 25%-75%.

4. The negative electrode composite material according to claim 1, characterized in that The constituent elements of the ultrafine N-type silicon crystal material include silicon and doping elements; the doping elements include phosphorus and / or arsenic.

5. The negative electrode composite material according to claim 4, wherein The atomic volume concentration of the doping element is less than or equal to 5×10 21 atoms / cm 3 .

6. The negative electrode composite material according to claim 1, wherein The D50 particle size of the negative electrode composite material is less than or equal to 30 μm.

7. The negative electrode composite material according to any one of claims 1 to 6, characterized in that The powder resistivity of the negative electrode composite material is less than or equal to 2.5 mΩ·cm.

8. A method for preparing a negative electrode composite material, characterized in that: The following steps are involved: A certain amount of silicon source, dopant source, and carbon source are introduced into a heating device for heating, wherein the carbon source forms a conductive carbon material, and ultrafine N-type silicon crystal material formed by in-situ reaction of the silicon source and the dopant source is dispersed in the conductive carbon material, so as to obtain a negative electrode composite material; during the heating process, the amount of silicon source introduced into the heating device is gradually reduced; The negative electrode composite material includes an ultrafine N-type silicon crystal material and a conductive carbon material, wherein the ultrafine N-type silicon crystal material is dispersed in the conductive carbon material; the outer surface of the negative electrode composite material particles does not expose the ultrafine N-type silicon crystal material, and the D50 particle size of the ultrafine N-type silicon crystal material is less than or equal to 5 nm.

9. The preparation method according to claim 8, characterized in that The method also includes pre-setting a conductive carbon skeleton material in the heating device.

10. The preparation method according to claim 9, characterized in that The step of introducing a certain amount of silicon source, doping source, and carbon source into a heating device for heating treatment comprises at least the following steps: (1) Preheating: introducing a carbon source into the heating device; (2) Heat treatment: replacing the carbon source with the silicon source and the doping source; (3) Post-processing: replacing the silicon source with the carbon source.

11. A negative electrode plate, characterized in that: The negative electrode plate comprises the negative electrode composite material according to any one of claims 1 to 7.

12. An electrochemical device, characterized in that The electrochemical device comprises the negative electrode sheet according to claim 11.

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