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 P-type silicon crystal material and conductive carbon material, the ultra-micro P-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 P-type silicon crystal material, and the D50 particle size of the ultra-micro P-type silicon crystal material is less than or equal to 5 nm. The negative electrode composite material has high electrical conductivity, good rate performance, good expansion characteristics and cycle performance, and can maintain structural stability in the charging and discharging cycle process, thereby being used for providing an electrochemical device with good rate performance and long cycle performance.

Description

TECHNICAL FIELD

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

[0002] Crystalline silicon material is often used as the 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 change during charging and discharging is extremely large, which forms cracks during each volume increase and decrease, affecting the normal performance of the battery. In addition, elemental silicon is a semiconductor material, and its electronic conductivity is poor, 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 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 high electrical conductivity, good rate performance, good expansion characteristics and cycle performance, and can maintain structural stability during the charging and discharging cycle, thereby being used to provide an electrochemical device with good rate performance and long cycle performance.

[0004] The first aspect of the present application provides a negative electrode composite material, which comprises ultra-micro P-type silicon crystal material and conductive carbon material, the ultra-micro P-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 P-type silicon crystal material, and the D50 particle size of the ultra-micro P-type silicon crystal material is less than or equal to 5 nm.

[0005] The ultra-micro P-type silicon crystal material has a large number of electron holes inside, and has strong conductivity, which can sufficiently improve the electrical conductivity of the negative electrode composite material. In addition, the ultra-micro P-type silicon crystal material is dispersed in the excellent conductor, i.e., the conductive carbon material, and the composite material has good uniformity, so that the negative electrode composite material has high rate performance. In addition, each ultra-micro crystal P-type silicon material has a small size and is buried in the dense conductive carbon material, so that the risk of breakage of the ultra-micro P-type silicon crystal material is significantly reduced, thereby significantly improving the expansion characteristics and cycle characteristics of the negative electrode composite material and improving the stability of the negative electrode composite material.

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

[0007] Optionally, the negative electrode composite material is in the form of particles, and the mass content of the ultra-micro P-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 P-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 P-type silicon crystal material include Si element and doping element; the doping element includes but is not limited to boron element and / or beryllium element.

[0010] Optionally, the atomic volume concentration of the doping element in the ultrafine P-type silicon crystal material is less than or equal to 5×10 20 atoms / cm 3 . Preferably, the atomic volume concentration of the doping element in the ultrafine P-type silicon crystal material is less than or equal to 2×10 20 atoms / cm 3 . Further preferably, the atomic volume concentration of the doping element in the ultrafine P-type silicon crystal material is less than or equal to 1×10 20 atoms / cm 3 . Still further preferably, the atomic volume concentration of the doping element in the ultrafine P-type silicon crystal material is less than or equal to 5.5×10 19 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 P-type silicon crystal material is less than or equal to 0.5 nm. Further preferably, the D50 particle size of the ultrafine P-type silicon crystal material is less than or equal to 0.2 nm.

[0013] Optionally, the grain size of the ultrafine P-type silicon crystal material is less than or equal to 3 nm. Preferably, the grain size of the ultrafine P-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 ultrafine P-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 ultrafine P-type silicon crystal material and the electrically conductive carbon material, the ultrafine P-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 ultrafine P-type silicon crystal material, and the D50 particle size of the ultrafine P-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 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, during the heat treatment, the amount of the silicon source introduced into the heating device gradually decreases.

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

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

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

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

[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 present application provides a negative electrode tab, which includes the negative electrode composite material provided in the first aspect of the present application or prepared by the preparation method provided in the second aspect of the present 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 provided in an embodiment of the present application is shown in FIG. 1.

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

[0037] Figure 2B A corresponding relationship between the mass concentration of the ultra-fine P-type silicon crystal material and the gray scale is shown in FIG. 3.

[0038] Figure 3 A transmission electron microscope (TEM) photo of the negative electrode composite provided in Embodiment 2 of the present application is shown in FIG. 4.

[0039] The reference signs are explained as follows: 100-negative electrode composite; 10-ultra-fine P-type silicon crystal material; 20-conductive carbon material. DETAILED DESCRIPTION

[0040] Specifically, the negative electrode composite provided in the present application can be prepared by the following steps. Figure 1 The present application provides a negative electrode composite 100, which comprises an ultra-fine P-type silicon crystal material 10 and a conductive carbon material 20, the ultra-fine P-type silicon crystal material 10 is dispersed in the conductive carbon material 20; the outer surface of the particle of the negative electrode composite 100 is not exposed to the ultra-fine P-type silicon crystal material 10, and the D50 particle size of the ultra-fine P-type silicon crystal material 10 is less than or equal to 5 nm.

[0041] The ultrafine P-type silicon crystal material 10 has more electron holes inside, and has strong conductivity, which can improve the conductivity of the negative electrode composite material 100. The ultrafine P-type silicon crystal material 10 is dispersed in the excellent conductor, the conductive carbon material 20. The dispersion specifically refers to that the size of the ultrafine P-type silicon crystal material 10 is small enough, and when dispersed in the negative electrode composite material 100, the particle feeling is weak, and the conductive carbon material 20 is almost integrated, so that the negative electrode composite material has good uniformity, and the negative electrode composite material 100 has high rate performance. In addition, the ultrafine P-type silicon crystal material 10 is buried in the dense conductive carbon material 20. The smaller the size of the silicon crystal material, the greater the Gibbs free energy of the increased rupture under the condition of equal molar quantity (that is, the greater the energy barrier required for particle rupture), and the energy barrier required for silicon crystal material rupture can effectively inhibit the rupture and pulverization of the silicon crystal material. In addition, the smaller the size of the ultrafine P-type silicon crystal material 10, the larger the specific surface area, and the more active sites on the material surface, but since it is coated in the conductive carbon material 20, the problem of excessive active sites leading to increased side reactions can be effectively solved. 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, so it is difficult for the silicon crystal material to break the wrapping of the conductive carbon material 20 and expose it, so the conductive carbon material 20 can also play a role in inhibiting the expansion of the ultrafine P-type silicon crystal material 10, thereby improving the expansion and cycle characteristics of the negative electrode composite material 100, and improving the stability of the negative electrode composite material 100.

[0042] In this application, the D50 particle size of the ultrafine P-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 ultrafine P-type silicon crystal material 10 is too large, it will cause the particle feeling of the negative electrode composite material 100 to increase, the mixing effect of the ultrafine P-type silicon crystal material 10 and 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 and long cycle characteristics of the negative electrode composite material 100.

[0043] In some embodiments of the present application, the D50 particle size of the ultrafine P-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 P-type silicon crystal material 10 is less than or equal to 0.2 nm. Understandably, the ultrafine P-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 the growth rate of the crystal is similar under the atmospheric condition, so the size of the ultrafine P-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 P-type silicon crystal particles in the cross-section sample of the negative electrode composite material 100 observed under a transmission electron microscope (TEM) is considered as the D50 particle size of the ultrafine P-type silicon crystal particles defined in the present application.

[0044] In some embodiments of the present application, the grain size of the ultrafine P-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 P-type silicon crystal material 10 is in the range of 0.1 nm-1 nm.

[0045] 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 1 μm, 2 μm, 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, so that the electrochemical properties of the negative electrode active material (i.e., the battery) can be fully exerted.

[0046] In some embodiments of the present application, the negative electrode composite material 100 is in a granular form, please refer to Figures 2A-2BThe mass content of the ultra-micro P-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 P-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 P-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.

[0047] 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.

[0048] 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 P-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 P-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 P-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.

[0049] In some embodiments of the present application, the constituent elements of the ultrafine P-type silicon crystal material 10 include Si element and doping elements; the doping elements include boron element and / or beryllium element. In some cases, the doping elements can also include other trivalent elements, for example, one or more of gallium element, indium element, aluminum element, etc.

[0050] In some embodiments of the present application, the atomic volume concentration of the doping elements is less than or equal to 5×10 20 atoms / cm 3 . Preferably, in some specific embodiments, the atomic volume concentration of the doping elements in the ultrafine P-type silicon crystal material 10 is less than or equal to 2×10 20 atoms / cm 3 . Further preferably, in some specific embodiments, the atomic volume concentration of the doping elements in the ultrafine P-type silicon crystal material 10 is less than or equal to 1×10 20 atoms / cm 3 . Still further preferably, in some specific embodiments, the atomic volume concentration of the doping elements in the ultrafine P-type silicon crystal material 10 is less than or equal to 5.5×10 19 atoms / cm 3 . The greater the atomic volume concentration of the doping elements, the better the conductivity of the ultrafine P-type silicon crystal material 10, but the appropriate amount of doping elements can ensure that the ultrafine P-type silicon crystal material 10 has good conductivity while also ensuring that it has good lithium ion intercalation capability. For example, the atomic volume concentration of the doping elements in the ultrafine P-type silicon crystal material 10 can be 5×10 20 atoms / cm 3 , 3.5×10 20 atoms / cm 3 , 2×10 20 atoms / cm 3 , 1×10 20 atoms / cm 3 , 5.5×10 19 atoms / cm 3 , 5×10 19 atoms / cm 3 , etc.

[0051] 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. Illustratively, 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.

[0052] The present application also provides a preparation method of a negative electrode composite material, which is suitable for preparing the above-mentioned negative electrode composite material 100. The preparation method comprises the following steps:

[0053] A certain amount of a silicon source, a doping source and a 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 ultra-micro P-type silicon crystal material formed in situ by the reaction of the silicon source and the doping source, so as to obtain a negative electrode composite material.

[0054] In the negative electrode composite material, the ultra-micro P-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 P-type silicon crystal material, and the D50 particle size of the ultra-micro P-type silicon crystal material is less than or equal to 5 nm.

[0055] The silicon source, the doping source and the carbon source are introduced into a boiling furnace, so that the decomposed silicon source and the decomposed doping source can be co-deposited to form ultra-micro P-type silicon crystal material, and the above-mentioned ultra-micro P-type silicon crystal material is suspended in the boiling furnace. At this time, the carbon source will also be decomposed and deposited on the surface of the ultra-micro P-type silicon crystal material. With the continuous deposition of the conductive carbon material on the surface of the material unit, the ultra-micro silicon crystal particles each having a deposition layer of the conductive carbon material continuously fuse in the boiling furnace, and finally form the negative electrode composite material provided in the present application.

[0056] The above-mentioned preparation method is simple in operation, strong in controllability and high in production efficiency, and is suitable for large-scale industrialized preparation.

[0057] In the present application, the above-mentioned silicon source and doping source will be gasified under the action of high temperature, or gaseous silicon source and gaseous doping source are directly added, so that the silicon source, the doping source and the carbon source can react in the gas phase.

[0058] In some embodiments of the present application, the silicon source can be a material known to those skilled in the art. The silicon source includes but is not limited to SiH4.

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

[0060] 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.

[0061] In some embodiments of the present application, the heating condition is that the treatment is carried out at 400-2000℃ for 0.25-4h.

[0062] In some embodiments of the present application, the heating of the amount of silicon source, doping source and carbon source into the heating device at least includes the following steps: (1) preheating treatment: introducing the carbon source into the heating device; (2) heat treatment: replacing the carbon source with the silicon source and the doping source; (3) post-treatment: replacing the silicon source with the carbon source. That is, in the preheating treatment process, raw materials other than the silicon source and the doping source are introduced; in the heat treatment process, raw materials other than the carbon source are introduced; in the post-treatment process, raw materials other than the silicon source and the doping source are introduced. According to the above steps, the ultra-micro P-type silicon crystals can be better dispersed in the conductive carbon material, and the electrochemical performance of the negative electrode composite material is further optimized.

[0063] In some embodiments of the present application, the amount of silicon source introduced into the heating device gradually decreases during the heat treatment process. At this time, it is beneficial to ensure that the mass content of the ultra-micro P-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. In some specific embodiments, the decrease is carried out at a rate of 50wt.% per hour. At this time, it is more beneficial to adjust the mass content of the ultra-micro P-type silicon crystal material in the negative electrode composite material to gradually decrease from the center to the outer surface of the negative electrode composite material.

[0064] In some embodiments of the present application, a conductive carbon skeleton material is also pre-placed in the heating device. That is, before introducing other raw materials 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 P-type silicon crystal material, and the subsequently introduced carbon source can also deposit in the skeleton carbon material to fill the gaps, so that the ultra-micro P-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.

[0065] The present application also provides a negative electrode sheet with the negative electrode composite material 100 provided by the present application.

[0066] The electrode sheet has good rate performance, cycle performance and safety performance.

[0067] In some embodiments of the present application, the negative electrode composite 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 can be coated on the negative electrode current collector, and then dried, rolled and cut to obtain the negative electrode sheet.

[0068] The present application provides an electrochemical device with the negative electrode sheet provided by the embodiments of the present application.

[0069] The electrochemical device includes a secondary battery, which has high rate performance, good cycle performance and high safety.

[0070] 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, 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 using a semi-solid electrolyte or a solid electrolyte, the positive electrode sheet and the negative electrode sheet can also contain semi-solid electrolyte materials or solid electrolyte materials.

[0071] In the present application, the lithium battery can be assembled by the following method:

[0072] S01, in a glove box, the positive electrode sheet, the separator and the negative electrode sheet are sequentially stacked to form an electric core;

[0073] S02, the electric core is packaged with an aluminum plastic film shell and injected with an electrolyte to obtain a battery with a capacity of 3-5 AH. The battery can be tested for electrochemical performance after formation.

[0074] Alternatively, the lithium battery can be assembled by the following method:

[0075] 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;

[0076] S02, the electric core is packaged to obtain a solid or semi-solid battery. The battery can be tested for electrochemical performance after formation.

[0077] 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), and then drying and pressing the slurry. The positive electrode active material is a material known to those skilled in the art, and can be at least one of lithium iron borate, lithium titanate, lithium cobaltate, nickel-manganese-cobalt ternary material, nickel-cobalt-aluminum ternary material, and lithium-rich manganese-based material. The conductive agent is a material 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 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.

[0078] Generally, the aforementioned electrolyte contains an organic solvent, a lithium salt, and an additive; the organic solvent, the lithium salt, and the additive can all be materials known to those skilled in the art. Exemplarily, 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 hexafluoroborate, 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.

[0079] In some embodiments of the present application, the separator is a material known to those skilled in the art, and exemplarily, the separator can be a polyethylene film, a polypropylene film, a polyethylene / polypropylene double-layer film, a polyethylene / polypropylene / polypropylene three-layer film, or the like.

[0080] In some embodiments of the present application, the solid or semi-solid electrolyte is a material known to those skilled in the art.

[0081] The technical solutions of the present application are further illustrated in the following examples.

[0082] Example 1

[0083] N2, SiH4, PH3, and C2H2 were introduced into a heating device-boiling furnace for reaction, and the specific process conditions are shown in Table 1. The prepared negative electrode composite material is denoted as S1. It is determined that the D50 particle size of S1 is 9 μm, and the D50 particle size of the ultrafine P-type silicon crystal material is 5 nm.

[0084] Example 2

[0085] Active carbon powder, N2, SiH4, PH3, and C2H2 were introduced into a heating device-boiling furnace for reaction, and the specific process conditions are shown in Table 1. The prepared negative electrode composite material is denoted as S2. It is determined that the D50 particle size of S2 is 12 μm, and the D50 particle size of the ultrafine P-type silicon crystal material is 2 nm.

[0086] Example 3

[0087] The porous carbon powder, N2, SiH4, PH3, C2H4 were introduced into the heating device-boiling furnace for reaction, and the specific process conditions are shown in Table 1. The prepared negative electrode composite material is recorded as S3. It is determined that the D50 particle size of S3 is 15 μm, and the D50 particle size of the ultrafine P-type silicon crystal material is 5 nm.

[0088] Example 4

[0089] The acetylene black powder, N2, SiH4, PH3, toluene were introduced into the heating device-boiling furnace for reaction, and the specific process conditions are shown in Table 1. The prepared negative electrode composite material is recorded as S4. It is determined that the D50 particle size of S4 is 10 μm, and the D50 particle size of the ultrafine P-type silicon crystal material is 0.5 nm.

[0090] Example 5

[0091] The porous carbon powder, CO2, SiH4, PH3, CH4 were introduced into the heating device-boiling furnace for reaction, and the specific process conditions are shown in Table 1. The prepared negative electrode composite material is recorded as S5. It is determined that the D50 particle size of S5 is 11 μm, and the D50 particle size of the ultrafine P-type silicon crystal material is 0.2 nm.

[0092] Example 6

[0093] The porous carbon powder, N2, ethyl silicate (TEOS), PH3, BBr3, CH4 were introduced into the heating device-vacuum rotary furnace for reaction, and the specific process conditions are shown in Table 1. The prepared negative electrode composite material is recorded as S6. It is determined that the D50 particle size of S5 is 13 μm, and the D50 particle size of the ultrafine P-type silicon crystal material is 5 nm.

[0094] Example 7

[0095] The difference from Example 1 is that in the heat treatment stage, the silicon source is introduced into the boiling furnace in a manner of reducing 50 wt.% SiH4 per hour. It is determined that the D50 particle size of S1 is 9 μm, and the D50 particle size of the ultrafine P-type silicon crystal material is 5 nm.

[0096] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples are set.

[0097] Comparative Example 1

[0098] The N2, SiH4, C2H2 were introduced into the heating device-boiling furnace for reaction, and the specific process conditions are shown in Table 1. The prepared negative electrode composite material is recorded as DS1. It is determined that the D50 particle size of DS1 is 9 μm, and the D50 particle size of the ultrafine P-type silicon crystal material is 5 nm.

[0099] Comparative Example 2

[0100] Silicon monoxide particles, N2, 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 DS2. It is determined that the D50 particle size of DS2 is 9 μm, and the D50 particle size of the nanosilicon crystal grains contained in the material is 7 nm (calculated by the Debye-Scherrer formula after XRD testing).

[0101] Comparative Example 3

[0102] After ball milling and sand milling of N-type single crystal silicon particles, nanosilicon powder with a D50 particle size of 100 nm was prepared. Then, the nanosilicon powder, flaky graphite and phenolic resin were granulated and heat treated to prepare a negative electrode composite material, which is denoted as DS3. It is determined that the D50 particle size of DS3 is 20 μm.

[0103] Preparation of secondary batteries with the negative electrode composite materials (negative electrode materials) prepared in the above examples and comparative examples:

[0104] (1) Preparation of negative electrode sheet: 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 into 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.

[0105] (2) Preparation of positive electrode sheet: 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.

[0106] (3) Preparation of secondary battery: a plurality of the above negative electrode sheets, separators, and positive electrode sheets were alternately stacked to prepare a battery by the lamination method, wherein the positive and negative electrode sheets were arranged alternately, and the adjacent positive and negative electrode sheets were separated by the separators to obtain a dry cell. The dry cell was placed in an aluminum plastic film outer package, electrolyte was injected, and then vacuum sealed. After standing at 60°C for 48h, pressure layering, secondary packaging, degassing, and capacity grading were carried out at 60°C to obtain a laminated soft-pack full battery with a capacity of 2.2 Ah. The batteries of each example are denoted as S1-S7, and the batteries of each comparative example are denoted as DS1-DS3.

[0107] Related characterization:

[0108] (1) TEM test was performed on each negative electrode composite material to observe its micro-morphology and determine the D50 particle size of the ultra-fine P-type silicon crystal material. The results of some examples are shown in Table 1. Figure 3 .

[0109] (2) Energy dispersive spectroscopy (EDS) surface scanning test was performed on each negative electrode composite material to observe the distribution of silicon element inside the negative electrode composite material.

[0110] (3) The powder resistivity of the negative electrode composite material prepared in each example and comparative example was measured: the powder to be tested was placed in a specific container, 50 MPa pressure was applied for 10 s, and the measurement was performed by 4-probe method. The results are shown in Table 3.

[0111] (4) Electrochemical performance test was performed on the batteries S1-S7, DS1-DS3 with the negative electrode composite material (negative electrode material) of each example and comparative example. The test included the following steps:

[0112] (a) Normal temperature battery cycle test: the battery was tested at 25°C by 1C / 1C charge-discharge cycle, the voltage range was 4.2V-3.0V, the expansion rate and capacity retention rate of the battery after 200 cycles were recorded, and the results are shown in Table 4. The test was continued, and the expansion rate and capacity retention rate of the battery after 400 cycles were recorded, and the results are shown in Table 4.

[0113] (b) High temperature battery cycle test: the battery was tested at 45°C by 1C / 1C charge-discharge cycle, the voltage range was 4.2V-3.0V, the expansion rate and capacity retention rate of the battery after 200 cycles were recorded, and the results are shown in Table 4. The test was continued, and the expansion rate and capacity retention rate of the battery after 400 cycles were recorded, and the results are shown in Table 4.

[0114] (c) Rate performance test: the ratio of discharge capacity of the battery at 1C and 3C was tested.

[0115] (5) The content of silicon element and boron element in the negative electrode composite material was determined by inductive coupled plasma emission spectrometer (ICP). The content of carbon element in the negative electrode composite material was determined by carbon-sulfur analyzer. Among them, the B element in the ICP test result is identified by wt% (C wt ), and the conversion relationship between it and the unit atoms / cm 3 can be converted by the empirical formula C wt = k x C nl , wherein the value of k is generally 2 x 10-23 -5x10 -22 The specific value of k needs to be determined according to the true density of the material to be measured.

[0116] (6) The X-ray diffraction (XRD) test is performed on each negative electrode composite material, and the peak fitting is performed by using the Scherrer formula, so as to semi-quantitatively test each component and content, and verify the test result of the element content obtained in step (5).

[0117] Table 1

[0118]

[0119] Table 2: Summary table of element content in negative electrode composite materials of each example and comparative example

[0120]

[0121]

[0122] Table 3: Powder resistivity of negative electrode composite materials of each example and comparative example

[0123] Experiment No. Powder resistivity of the negative electrode composite / mΩ-cm Example 1 2.4 Example 2 2.0 Example 3 2.1 Example 4 1.7 Example 5 2.0 Example 6 2.1 Example 7 1.6 Comparative Example 1 3.1 Comparative Example 2 5.1 Comparative Example 3 3.7

[0124] Table 4: Summary table of electrochemical performance of batteries of each example and comparative example

[0125]

[0126] It can be known from the data in Tables 1-4 that the negative electrode composite material provided in the examples has superior electronic conductivity 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, especially the high temperature expansion rate of S1-S7 is significantly superior to that of DS1-DS3, which fully illustrates the superiority of the negative electrode composite material of the examples, and can be used to provide an electrochemical device with good rate and good cycle. When the negative electrode composite material satisfies the preferred condition of the application "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 1, the comprehensive performance of the battery S7 is obviously better.

[0127] See Figure 3 , Figure 3The TEM picture of the negative electrode composite material of Example 2 is also difficult to identify obvious particles at the magnification of the picture (the scale is 100 nm), which indicates that the size of the ultra-micro P-type silicon crystal particles in the negative electrode composite material provided by the application is small, the particle feeling is weak, and the fusion with the conductive carbon material is high.

[0128] It should be noted that the O element of the negative electrode composite material of the example 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.

[0129] The above is an exemplary embodiment of the application. It should be noted that for those skilled in the art, without departing from the principles of the application, some improvements and refinements can be made, which are also considered within the scope of protection of the application.

Claims

1. A negative electrode composite material, characterized in that, The negative electrode composite material includes ultrafine P-type silicon crystal material and conductive carbon material, wherein the ultrafine P-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 P-type silicon crystal material, and the D50 particle size of the ultrafine P-type silicon crystal material is less than or equal to 5 nm; the negative electrode composite material is granular, and the mass content of ultrafine P-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, The mass percentage of silicon in the negative electrode composite material is 25%-75%.

3. The negative electrode composite material according to claim 1, characterized in that, In a single negative electrode composite material, within the region where the ultrafine P-type silicon crystal material is distributed, the minimum mass concentration of silicon is A and the maximum mass concentration is B, wherein A ≥ 0.9B.

4. The negative electrode composite material according to claim 1, characterized in that, The constituent elements of the ultra-micro P-type silicon crystal material include Si and doping elements; the doping elements include boron and / or beryllium.

5. The negative electrode composite material according to claim 4, characterized in that, The atomic volume concentration of the dopant element is less than or equal to 5 × 10⁻⁶. 20 atoms / cm 3 .

6. The negative electrode composite material according to claim 1, characterized in that, 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 1-6, 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, Includes the following steps: A certain amount of silicon source, dopant source, and carbon source are introduced into a heating device for heating, so that the carbon source forms a conductive carbon material, and the conductive carbon material contains ultrafine P-type silicon crystal material formed by the in-situ reaction of the silicon source and the dopant source, so as to obtain a negative electrode composite material. The heating process includes at least the following steps: (1) Preheating treatment: A carbon source is introduced into the heating device; (2) Heat treatment: The carbon source is replaced with the silicon source and the doping source; and during the heat treatment, the amount of silicon source introduced into the heating device decreases. (3) Post-processing: Replace the silicon source with the carbon source; The negative electrode composite material includes ultrafine P-type silicon crystal material and conductive carbon material, wherein the ultrafine P-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 P-type silicon crystal material, and the D50 particle size of the ultrafine P-type silicon crystal material is less than or equal to 5 nm.

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

10. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the negative electrode composite material as described in any one of claims 1-7.

11. An electrochemical device, characterized in that, The electrochemical device includes the negative electrode as described in claim 10.

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