Patent Description:
The commercially used lithium ion batteries existing mainly adopt the pure graphite, or graphite/silicon-carbon mixture doped with a small amount of silicon as the negative electrode material, however, given that the theoretical specific capacity of the graphite is only 372mAh/g, the specific capacity of the graphite/silicon-carbon mixture is generally below 500mAh/g, the further improvement of the specific energy of the lithium-ion batteries is limited, and it cannot meet the requirements imposed by the development of new energy industries such as existing electric automobiles. The silicon negative electrode based on alloying reactions has a theoretical lithium storage capacity of up to <NUM>,200mAh/g, which is the ideal choice for the next generation of lithium-ion battery negative electrode material. However, the silicon has a large volume expansion (><NUM>%) during the alloying reaction with lithium, which leads to pulverization and deactivation of the particles, so that the cycling stability of the silicon negative electrode is poor, especially the silicon-carbon negative electrode with a high specific capacity.

Although the porous structure can accommodate the expansion of silicon during the lithium deintercalation process, the particle conductivity is poor due to the absence of conductive particles between the pores, thereby causing the performance decline of the multiplying power. In addition, the pore structure is also prone to collapse and crack, so that the silicon is in direct contact with the electrolyte, and the cycle performance of the battery is degraded.

<CIT> discloses a silicon-carbon composite negative electrode material, a preparation method thereof, and a lithium-ion battery, wherein the silicon-carbon composite negative electrode material is a material with a core-shell structure, wherein a core part comprises nano-silicon, amorphous carbon, graphene and carbon nanotubes, the surface of the nano-silicon is coated with the amorphous carbon, the nano-silicon coated with the amorphous carbon is distributed on the surfaces of the graphene and the carbon nanotubes, the carbon nanotubes form a three-dimensional cross-linked network, and the graphene is uniformly distributed in the three-dimensional cross-linked network, and a shell part is a carbon layer. However, the graphene of <CIT> is not the in-situ generated graphene, but the externally added graphene. The externally added structure is only in physical contact with other raw materials in the particles, the association is not sufficiently close, such that the structure merely plays a common conductive role.

<CIT> discloses a negative active material includes: an active material core; and a composite coating layer located on a surface of the active material core, wherein the composite coating layer includes a lithium-containing oxide having an orthorhombic crystal structure, and a first carbonaceous material, the lithium-containing oxide.

<CIT> provides a core-shell type silicon-carbon composite material which comprises silicon-carbon composite particles, the silicon-carbon composite particles comprise graphite and silicon nanoparticles adsorbed on the surface of the graphite, and the core-shell type silicon-carbon composite material further comprises a first carbon coating layer and at least one second carbon coating layer which coat the surfaces of the silicon nanoparticles, after the first carbon coating layer and the second carbon coating layer are mixed with the silicon nanoparticles by adopting a mixed carbon source treated by a surface modifier, the polarity of the first carbon coating layer and the second carbon coating layer is changed by the surface modifier, and the first carbon coating layer and the second carbon coating layer repel each other; and the first carbon coating layer has adsorbability on the silicon nanoparticles. The core-shell type silicon-carbon composite material provided by the invention has relatively low volume expansion rate and relatively good cycle performance. The invention also provides a preparation method of the core-shell type silicon-carbon composite material and application of the core-shell type silicon-carbon composite material.

<CIT> discloses a spherical silicon-oxygen-carbon negative electrode composite material, which is of a three-layer structure comprising an inner layer, an intermediate layer and an outer layer, wherein the inner layer is an SiOx/graphite substrate; the intermediate layer is an amorphous carbon coating layer; the outer layer is a carbon nanotube coating layer; the mass of the inner layer SiOx/graphite substrate accounts for <NUM>%-<NUM>% of total mass of the spherical silicon-oxygen-carbon negative electrode composite material; the mass of the intermediate layer amorphous carbon accounts for <NUM>%-<NUM>% of total mass of the spherical silicon-oxygen-carbon negative electrode composite material; and the outer layer carbon nanotube accounts for <NUM>%-<NUM>% of total mass of the spherical silicon-oxygen-carbon negative electrode composite material. The grain diameter of the adopted SiOx substrate is smaller than <NUM> microns; the grain diameter is relatively small; intercalation and deintercalation of active substances are facilitated; higher specific capacity can be obtained; meanwhile, a dispersing agent is added when an SiOx sample is ground; and condition that the SiOx with a relatively small grain diameter is agglomerated in quantity to affect the performance is prevented.

Therefore, it has an important significance to research and develop a graphitized porous silicon-carbon negative electrode material.

The present disclosure aims to overcome the defects of poor cycle stability of a silicon-based negative electrode and degraded performance of the multiplying power in the existing and provides a graphitized porous silicon-carbon negative electrode material, a preparation method thereof and a lithium-ion battery, wherein the graphitized silicon-carbon negative electrode material can solve the problems that a porous structure is prone to crack and the multiplying power is reduced.

In order to achieve the above purpose, the first aspect of the present disclosure provides a graphitized porous silicon-carbon negative electrode material comprising a core and a carbon coating layer coated on an outer surface of the core, wherein the core sequentially comprises an inner layer and a shallow surface layer from inside to outside, the graphitized porous silicon-carbon negative electrode material is internally provided with pores in gradient distribution, and the porosity is gradiently decreased from inside to outside along the inner layer, the shallow surface layer and the carbon coating layer; and the core comprises a silicon-carbon composite comprising nano-silicon particles, a conductive agent and a graphitized carbon material.

In a second aspect, the present disclosure provides a method for preparing the aforementioned graphitized porous silicon-carbon negative electrode material comprising:.

In a third aspect, the present disclosure provides a lithium ion battery comprising the aforementioned graphitized porous silicon-carbon negative electrode material.

Through the aforesaid technical scheme, the present disclosure has the following favorable effects:.

The terminals and any value of the ranges disclosed herein are not limited to the precise ranges or values, such ranges or values shall be comprehended as comprising the values adjacent to the ranges or values. As for numerical ranges, the endpoint values of the various ranges, the endpoint values and the individual point value of the various ranges, and the individual point values may be combined with one another to produce one or more new numerical ranges, which should be deemed have been specifically disclosed herein.

As mentioned above, the first aspect of the present disclosure provides a graphitized porous silicon-carbon negative electrode material, as shown in <FIG>, the graphitized porous silicon-carbon negative electrode material comprising a core and a carbon coating layer <NUM> coated on an outer surface of the core, wherein the core sequentially comprises an inner layer <NUM> and a shallow surface layer <NUM> from inside to outside, the graphitized porous silicon-carbon negative electrode material is internally provided with pores in gradient distribution, and the porosity is gradiently decreased from inside to outside along the inner layer, the shallow surface layer and the carbon coating layer; and the core comprises a silicon-carbon composite comprising nano-silicon particles <NUM>, a conductive agent and a graphitized carbon material <NUM>.

The present inventors have discovered that a metal catalyst salt is used for catalyzing a carbon source to be highly graphitized, and a carbon nanotube or grapheme is grown; in addition, the one-dimensional carbon material <NUM> of the present disclosure is dispersed in the whole particle, on one hand, the one-dimensional carbon material <NUM> can desirably play a conductive role, and compensate the deficient conductivity of the semiconductor silicon; on the other hand, the graphitized carbon material can strengthen the connection between the one-dimensional carbon material <NUM>, and enhance the connectivity and uniformity between materials in the particles, prevent cracking of the materials from the inside, and the graphitized carbon has improved conductivity relative to the amorphous carbon, thereby improving the multiplying power performance.

Further, the present inventors have discovered that the porosity in the porous silicon-carbon negative electrode material in the existing is not reasonably controlled, and the volume expansion cannot be favorably controlled. The pore structure in the silicon-carbon negative electrode material of the present disclosure is different from that in the existing, the porosity is sequentially reduced from inside to outside to form a stable inner layer and shallow surface layer, and in combination with the dense carbon coating layer <NUM>, so that the expansion of silicon is more inclined to expand inwards, thereby achieving a desirable effect of limiting expansion; further, the conductive layer in the existing is disposed between the core and the outermost layer, it cannot desirably perform the conductive function.

Furthermore, during the whole technological process of the existing, sintering and crushing are directly carried out on the materials after the ball-milling process, so that the structure of the materials is not sufficiently ordered and have various particle sizes, and the particles are not smooth enough, some silicon powder may be directly exposed outside after crushing, which is not beneficial to the subsequent coating process; the present disclosure carries out granulation treatment on the slurry, thereby overcoming the defective problems in the existing.

Furthermore, the inventors of the present disclosure found that in the existing, the graphene of <CIT> is not the in-situ generated graphene, but the externally added grapheme; the externally added structure is only in physical contact with other raw materials in the particles, the association is not sufficiently close, such that the structure merely plays a common conductive role. The present disclosure catalyzes graphitization of the carbon source, which not only has the function of increasing conductivity, but also has the advantage of in-situ generation, namely, the graphitized carbon directly grown from the inside of the particles is more closely connected with other raw materials in the particles, so that the graphitized carbon can play a better role in performing electric conduction and strengthening the connection in the particles.

According to the present disclosure, the whole particle of the graphitized porous silicon-carbon negative electrode material has a graphitization degree within the range from <NUM>% to <NUM>%, preferably within the range from <NUM>% to <NUM>%, more preferably within the range from <NUM>% to <NUM>%.

According to the present disclosure, the whole particle of the graphitized porous silicon-carbon negative electrode material contains silicon, a one-dimensional carbon material, amorphous carbon, and graphitized carbon; the content of the silicon is within the range from 40wt. % to <NUM> wt. %, preferably within the range from 40wt. % to <NUM> wt. %, and the total content of the one-dimensional carbon material, the amorphous carbon, and the graphitized carbon is within the range from 20wt. % to <NUM> wt. %, preferably within the range from 50wt. % to <NUM> wt. %, based on the total weight of the whole particle of the graphitized porous silicon-carbon negative electrode material;.

According to the present disclosure, an average pressure borne by the graphitized porous silicon-carbon negative electrode material is within the range from 500MPa to <NUM>,<NUM> MPa, preferably within the range from 700MPa to <NUM>,000MPa, and more preferably within the range from 781MPa to <NUM> MPa.

According to the present disclosure, the graphitized porous silicon-carbon negative electrode material satisfies the following conditions:
ID:IG=<NUM>-<NUM>; preferably, ID:IG = <NUM>-<NUM>;
wherein ID denotes the defect degree of the carbon atom crystal lattice, and IG represents the in-plane stretching vibration of sp<NUM> hybridization of the carbon atom, namely the graphitization degree; ID: IG denotes the intensity ratio of the two parameters, the larger is the value, it represents the more defects of the carbon atom crystals; the smaller is the value, it represents the higher graphitization degree of the material. I2D/IG = <NUM>-<NUM>; preferably, I2D/IG = <NUM>-<NUM>;
wherein I2D represents the characteristic peak intensity of the graphite slice layer, and the appearance of a 2D peak in a Raman spectrum represents the graphitization of the material.

According to the present disclosure, the graphitized porous silicon-carbon negative electrode material has a conductivity within the range from <NUM>×<NUM><NUM>S/m to <NUM>×<NUM><NUM>S/m, preferably within the range from <NUM>×<NUM><NUM>S/m to <NUM>×<NUM><NUM>S/m, more preferably within the range from <NUM>×<NUM><NUM>S/m to <NUM>×<NUM><NUM>S/m.

According to the present disclosure, the nano-silicon particles have a D<NUM> within the range from <NUM> to <NUM>, preferably within the range from <NUM> to <NUM>; the nano-silicon particles in the present disclosure are silicon powder.

According to the present disclosure, the conductive agent comprises a metallic material and/or a non-metallic material; preferably, the conductive agent includes a one-dimensional carbon material <NUM>; more preferably, the one-dimensional carbon material <NUM> is a conductive carbon material; still further preferably, the conductive carbon material comprises at least one selected from the group consisting of single-walled CNTs (single-walled carbon nano tubes), multi-walled CNTs (multi-walled carbon nano tubes), and VGCF (vapor grown carbon fiber).

According to the present disclosure, the one-dimensional carbon material has an aspect ratio of (<NUM>-<NUM>,<NUM>): <NUM>, preferably (<NUM>-<NUM>): <NUM>.

The conductive agent of the present disclosure is dispersed in the whole particle and can produce a better conductive effect.

According to the present disclosure, the silicon-carbon negative electrode material has pores in gradient distribution therein, and the porosity is gradiently decreased from inside to outside along the inner layer <NUM>, the shallow surface layer <NUM>, and the carbon coating layer <NUM>.

According to the present disclosure, the inner layer <NUM> has a porosity within the range from <NUM>% to <NUM>%, the shallow surface layer <NUM> has a porosity within the range from <NUM>% to <NUM>%, the carbon coating layer (carbon coating layer <NUM>) has a porosity less than <NUM>%, preferably within the range from <NUM>% to <NUM>%; preferably, the inner layer <NUM> has a porosity within the range from <NUM>% to <NUM>%, the shallow surface layer <NUM> has a porosity within the range from <NUM>% to <NUM>%, the carbon coating layer (carbon coating layer <NUM>) has a porosity within the range from <NUM>% to <NUM>%; more preferably, the inner layer <NUM> has a porosity within the range from <NUM>% to <NUM>%, the shallow surface layer <NUM> has a porosity within the range from <NUM>% to <NUM>%, the carbon coating layer (carbon coating layer <NUM>) has a porosity within the range from <NUM>% to <NUM>%. In the present disclosure, by arranging the gradually reduced gradient porosity, the stable inner layer <NUM> and shallow surface layer <NUM> are formed, and the generation of side reaction and outward expansion of silicon are inhibited.

According to the present disclosure, the pore size distribution of the internal pores of the inner layer <NUM> is within the range from <NUM> to <NUM>,<NUM>, the pore size distribution of the internal pores of the shallow surface layer <NUM> is within the range from <NUM> to <NUM>, and the pore size distribution of the internal pores of the carbon coating layer (carbon coating layer <NUM>) is greater than <NUM> and less than or equal to <NUM>, preferably within the range from <NUM> to <NUM>, and more preferably within the range from <NUM> to <NUM>; preferably, the pore size distribution of the internal pores of the inner layer <NUM> is within the range from <NUM> to <NUM>, the pore size distribution of the internal pores of the shallow surface layer <NUM> is within the range from <NUM> to <NUM>, and the pore size distribution of the internal pores of the carbon coating layer (carbon coating layer <NUM>) is within the range from <NUM> to <NUM>. In the present disclosure, the stable inner layer <NUM> and shallow surface layer <NUM> are formed through the design of gradient pores, and in combination with a dense carbon coating layer <NUM>, thereby solving the expansion problem of the silicon-carbon negative electrode.

According to the present disclosure, the silicon-carbon negative electrode material has a D<NUM> within the range from <NUM> to <NUM>; a tap density within the range from <NUM>/cm<NUM> to <NUM>/cm<NUM>; and a specific surface area within the range from <NUM><NUM>/g to <NUM><NUM>/g; preferably, the silicon-carbon negative electrode material has a D<NUM> within the range from <NUM> to <NUM>; a tap density within the range from <NUM>/cm<NUM> to <NUM>/cm<NUM>; and a specific surface area within the range from <NUM><NUM>/g to <NUM><NUM>/g; further preferably, the silicon-carbon negative electrode material has a D<NUM> within the range from <NUM> to <NUM>; a tap density within the range from <NUM>/cm<NUM>-<NUM>/cm<NUM>; and a specific surface area within the range from <NUM><NUM>/g to <NUM><NUM>/g; more further preferably, the silicon-carbon negative electrode material has a D<NUM> within the range from <NUM>-<NUM>; a tap density within the range from <NUM>/cm<NUM>-<NUM>/cm<NUM>; and a specific surface area within the range from <NUM><NUM>/g-<NUM><NUM>/g.

According to the present disclosure, the particle size of the inner layer <NUM> is from <NUM>% to <NUM>%, preferably from <NUM>% to <NUM>%, based on the D<NUM> of the silicon-carbon negative electrode material; that is, the silicon-carbon negative electrode material in the present disclosure has a D50 within the range from <NUM> to <NUM>, and accordingly, the inner layer <NUM> has a particle size within the range from <NUM> to <NUM>, preferably within the range from <NUM> to <NUM>, and more preferably within the range from <NUM> to <NUM>.

According to the present disclosure, the thickness of the shallow surface layer <NUM> is within the range from <NUM> to <NUM>; preferably, the thickness of the shallow surface layer <NUM> is within the range from <NUM> to <NUM>; more preferably, the thickness of the shallow surface layer <NUM> within the range of is from <NUM> to <NUM>; still further preferably, the thickness of the shallow surface layer <NUM> within the range of is from <NUM> to <NUM>.

According to the present disclosure, the thickness of the carbon coating layer (carbon coating layer <NUM>) is within the range from <NUM> to <NUM>; preferably, the thickness of the carbon coating layer (carbon coating layer <NUM>) is within the range from <NUM> to <NUM>; more preferably, the thickness of the carbon coating layer (carbon coating layer <NUM>) is within the range from <NUM> to <NUM>; still further preferably, the thickness of the carbon coating layer (carbon coating layer <NUM>) is within the range from <NUM> to <NUM>.

According to the present disclosure, the coating amount of the carbon coating layer (carbon coating layer <NUM>) is from 1wt. % to <NUM> wt. %, preferably from 10wt. % to <NUM> wt. % of the graphitized porous silicon-carbon negative electrode material.

In a second aspect, the present disclosure provides a method for preparing the aforementioned graphitized porous silicon-carbon negative electrode material comprising the following steps:.

According to the present disclosure, the conditions of the first calcination treatment, the second calcination treatment and the high-temperature carbonization treatment are the same or different, and the respective sintering procedure includes a temperature rise rate within the range from <NUM>/min to <NUM> /min, a final heating temperature within the range from <NUM> to <NUM>,<NUM>, and a heat preservation time within the range from 1hours to <NUM> hours; preferably a temperature rise rate within the range romf <NUM>/min to <NUM>/min, a final heating temperature within the range from <NUM> to <NUM>,<NUM> , and a heat preservation time within the range from 3hours to <NUM> hours.

According to the present disclosure, the sintering is performed under an inert atmosphere; preferably, the inert atmosphere comprises nitrogen gas or argon gas.

According to the present disclosure, the metal catalyst salt comprises one or more selected from the group consisting of iron salt, cobalt salt, nickel salt, and magnesium salt; preferably, the metal catalyst salt comprises one or more selected from the group consisting of nickel acetate, nickel nitrate, nickel sulfate, ferric nitrate, ferric chloride, ferric sulfate, magnesium nitrate, magnesium chloride, magnesium sulfate, cobalt nitrate, and cobalt chloride.

According to the present disclosure, the weight ratio of the used amounts of the nano-silicon particles, the carbon source, the conductive agent, the metal catalyst salt, and the dispersant is (<NUM>-<NUM>): (<NUM>-<NUM>): (<NUM>-<NUM>): (<NUM>-<NUM>): (<NUM>-<NUM>); preferably (<NUM>-<NUM>): (<NUM>-<NUM>): (<NUM>-<NUM>): (<NUM>-<NUM>): (<NUM>-<NUM>).

According to the present disclosure, the carbon source is one or more selected from the group consisting of low-temperature asphalt, medium-temperature asphalt, high-temperature asphalt, water-soluble asphalt, phenolic resin, CMC, glucose and sucrose.

According to the present disclosure, the dispersant comprises one or more selected from the group consisting of PVP (polyvinylpyrrolidone), CTAB (cetyltrimethylammonium bromide), polyethylene glycol, and SDS (sodium dodecyl sulphate).

According to the present disclosure, the slurry in step (<NUM>) has a solid content within the range from 1wt. % to <NUM> wt. %, preferably 2wt. % to <NUM> wt.

According to the present disclosure, the conditions of the heating treatment in step (<NUM>) include: a temperature within the range from <NUM> to <NUM>,<NUM>, preferably within the range from <NUM> to <NUM>,<NUM>; in the present disclosure, under the heating condition, the high-temperature asphalt is completely dissolved, the solvent is slowly evaporated, and the dissolved high-temperature asphalt is infiltrated into the pores of the silicon-carbon composite core <NUM> through the channel established by the solvent, wherein the infiltration amount of the asphalt is within the range from 5wt. % to <NUM> wt.

According to the present disclosure, the solvent in step (<NUM>) comprises one or more selected from the group consisting of tetrahydrofuran, NMP, toluene, and xylene, preferably tetrahydrofuran.

According to the present disclosure, the coating in step (<NUM>) is performed by using at least one of a particle fusion machine, a VCJ machine, and a CVD.

According to the present disclosure, the used amount of the high-temperature asphalt <NUM> is from 5wt. % to <NUM> wt. %, preferably from 20wt. % to <NUM> wt. %, based on the total weight of the silicon-carbon composite core <NUM>.

According to the present disclosure, the used amount of the high-temperature asphalt <NUM> in step (<NUM>) is from 1wt. % to <NUM> wt. %, preferably from 10wt. % to <NUM> wt. %, based on the total weight of the silicon-carbon composite core <NUM>.

According to the present disclosure, the high-temperature asphalt <NUM> and the high-temperature asphalt <NUM> are the same or different, and are respectively one or more selected from the group consisting of high-temperature asphalt with the softening points of <NUM>, <NUM>, <NUM>, and <NUM>.

The inventors of the present disclosure have discovered that the asphalt in the existing is uniformly distributed in the particles by spraying, thereby forming a structure having uniform voids inside and a carbon coating layer on the outside, which is a simple dual structure. However, the asphalt of the present disclosure is distributed on the shallow surface layer of the core under a special process, and after carbonization, the structure with a high core porosity and a low shallow surface layer porosity is formed. In addition, the outermost part of the negative electrode material in the present disclosure is further provided with a more dense carbon coating layer <NUM> having extremely low porosity, thus the negative electrode material in the present disclosure has a triple structure, which sequentially comprises the following components: an inner layer <NUM> with a high porosity, a shallow surface layer <NUM> with a low porosity, and a carbon coating layer <NUM> with extremely low porosity. Due to the structures with gradually decreased porosity from inside to outside, the expansion of silicon can be effectively inhibited, thereby optimizing the cycle performance.

In a third aspect, the present disclosure provides a lithium ion battery comprising the aforementioned graphitized porous silicon-carbon negative electrode material provided with pores in a gradient structure.

In the present disclosure, the silicon-carbon negative electrode material with pores in a gradient structure, a conductive agent, a binder, and deionized water are mixed to form a slurry, which is subjected to coating, drying and cutting, thus a pole piece is obtained; a lithium sheet and a conventional electrolyte are assembled into a button half-cell; wherein the weight ratio of the used amounts of the silicon-carbon negative electrode material with pores in a gradient structure, the conductive agent, and the binder is (<NUM>-<NUM>): (<NUM>-<NUM>): (<NUM>-<NUM>).

The present disclosure will be described in detail below with reference to examples.

In the following Examples and Comparative Examples:.

ID, IG and I2D were parameters obtained by Raman testing.

The hardness parameters were parameters obtained through the testing conducted with a micro compression tester.

The conductivity was the parameter measured by using a conductivity tester.

The porosity was the data derived from the computerized tomography (CT) scanning technique and the algorithm statistics.

The tap density, the specific surface area, and the particle size were the data measured by a tap density instrument, a specific surface area analyzer and a laser particle size analyzer respectively.

The thickness of an inner layer, a shallow surface layer, and a carbon coating layer were the data measured by CP + SEM (ion cutting + scanning electron microscope) and analyzed by the statistical method.

The example served to illustrate the silicon-carbon negative electrode material with pores in a gradient structure prepared in the present disclosure.

The finished product silicon-carbon negative electrode material with pores in a gradient structure was coated with a film and assembled into the CR2032-button cell, wherein the silicon-carbon negative electrode material with pores in a gradient structure, a conductive agent, a binder, and deionized water are mixed according to the weight ratio of <NUM>: <NUM>: <NUM>: <NUM> to form a slurry, which was subjected to coating, drying and cutting to obtain a pole piece; the lithium sheet and the conventional electrolyte were assembled into a button half-cell, and subjected to the charging and discharging test. The charging and discharging cycles were performed with the 1C multiplying power. The battery charging and discharging test was carried out in a multi-channel battery, and the test voltage range of the silicon-carbon negative electrode material was <NUM>. 005V-<NUM>. 8V; the results were shown in Table <NUM>.

In addition, <FIG> was a structural schematic diagram of the silicon-carbon negative electrode material with pores in a gradient structure of the present disclosure; as shown by <FIG>, the silicon-carbon negative electrode material comprised an inner layer <NUM>, a shallow surface layer <NUM> and a carbon coating layer <NUM>; in addition, the inner layer <NUM> had irregular internal pores <NUM>, the porosity was decreased from inside to outside sequentially; the core further included a one-dimensional carbon material <NUM>, nano-silicon particles <NUM> and a graphitized carbon material <NUM>, wherein the one-dimensional carbon material <NUM> can play a role of enwinding and stabilizing the structure; in addition, the graphitized carbon material <NUM> can enhance the connection between the one-dimensional carbon materials <NUM>, and the connectivity and uniformity between materials inside the particles.

<FIG> illustrated an SEM photograph of the silicon-carbon negative electrode material with pores in a gradient structure prepared in Example <NUM> of the present disclosure; as shown by <FIG>, the silicon-carbon negative electrode material was provided with irregular internal pores <NUM>, and the porosity was reduced from inside to outside; moreover, the outermost layer of the silicon-carbon negative electrode material had a dense carbon coating.

<FIG> illustrated a curve schematic diagram of the parameters ID, IG and I2D of the silicon-carbon negative electrode material with pores in a gradient structure prepared in Example <NUM> of the present disclosure; as can be seen from <FIG>, ID: IG = <NUM>; I2D/IG = <NUM>.

The silicon-carbon negative electrode material with pores in a gradient structure was prepared according to the same method as that in Example <NUM>, except that the outermost layer is coated with a CVD coating, the temperature was <NUM>, acetylene gas was used for coating at a gas flow rate of <NUM>/min for 1hour.

As a result, a silicon-carbon negative electrode material having pores in a gradient structure, a dense coating layer at the outside, and a high inner layer porosity and a low shallow surface layer porosity was prepared; the results were shown in Table <NUM>.

A button half-cell was assembled according to the same method as that in Example <NUM>, and subjected to the charging and discharging test; the results were shown in Table <NUM>.

The spray granulation process was performed according to the same method as that in Example <NUM>, except that the asphalt infiltration experiment was not performed, and the fusion coating of the granules was performed directly.

The performance parameters of the prepared silicon-carbon negative electrode material were as shown in Table <NUM>.

The properties of the prepared lithium-ion battery were as shown in Table <NUM>.

The silicon-carbon negative electrode material was prepared according to the same method as that in Example <NUM>, except that the "CNTs" was replaced with "SP", wherein SP was carbon black.

The silicon-carbon negative electrode material was prepared according to the same method as that in Example <NUM>, except that in step (<NUM>), the slurry was not subjected to spray granulation, but subjected to a mixing and evaporating process, in particular, the evenly mixed slurry was stirred and evaporated to dryness at the temperature of <NUM>. The sintering process was then performed, and the asphalt infiltration and mechanical fusion, and coating were further implemented. The performance parameters of the prepared silicon-carbon negative electrode material were as shown in Table <NUM>.

The silicon-carbon negative electrode material with pores in a gradient structure was prepared according to the same method as that in Example <NUM>, except that in step (<NUM>), the metal catalyst salt nickel acetate was not added.

The finished product silicon-carbon negative electrode material with pores in a gradient structure was coated with a film and assembled into a CR2032-button cell according to the same method as that in Example <NUM>, the results were as shown in Table <NUM>.

The silicon-carbon negative electrode material with pores in a gradient structure was prepared according to the same method as that in Example <NUM>, except that the metal catalyst salt nickel acetate was not added, and the "high-temperature asphalt" was replaced with the "graphite with a D<NUM> of <NUM>.

As can be seen from the results in Table <NUM>, the stable inner layer and shallow surface layer are formed by using the silicon-carbon negative electrode materials prepared in Examples <NUM>-<NUM> of the present disclosure, in combination with the dense carbon coating layer, the specific surface area can be reduced and the tap density can be increased; in addition, the porosity is gradiently decreased from inside to outside; and as can be seen, the carbon source may be catalyzed to a high graphitization degree by adding a metal catalyst salt.

As can be seen from the results in Table <NUM>, the silicon-carbon negative electrode materials prepared in Examples <NUM>-<NUM> of the present disclosure can improve the first coulombic efficiency of lithium-ion batteries and can increase the capacity retention rate after <NUM> cycles.

Claim 1:
A graphitized porous silicon-carbon anode material comprising a core and a carbon coating layer coated on an outer surface of the core, wherein the core sequentially comprises an inner layer and a shallow surface layer from inside to outside, the graphitized porous silicon-carbon anode material is internally provided with pores in gradient distribution, and the porosity is gradually decreased from inside to outside along the inner layer, the shallow surface layer and the carbon coating layer, the inner layer has a porosity within the range from <NUM>% to <NUM>%, the shallow surface layer has a porosity within the range from <NUM>% to <NUM>%, the carbon coating layer has a porosity within the range from <NUM>% to <NUM>%; and the core comprises a silicon-carbon composite comprising nano-silicon particles, a conductive agent and a graphitized carbon material, the conductive agent is a one-dimensional carbon material, the one-dimensional carbon material is conductive carbon material, the conductive carbon material comprises at least one selected from the group consisting of single-walled CNTs, multi-walled CNTs, and VGCF; wherein the porosity is obtained by computerized tomography (CT) scanning technique.