Patent Description:
With the development and progress of electrochemical apparatuses (for example, lithium-ion batteries), increasing requirements have been placed on their cycling performance. The patent application <CIT> describes a battery with a negative electrode active material comprising silicon oxide and graphite.

Although the current technologies for improving electrochemical apparatuses can improve the cycling performance of the electrochemical apparatuses to some extent, they are not satisfactory, and further improvements are expected.

An embodiment of this application provides a negative electrode material, including: a silicon-based material, or a silicon-based material and graphite. The silicon-based material includes a silicon oxide material and a carbon layer located on a surface of the silicon oxide material. A particle size distribution of the silicon-based material satisfies <NUM>≤(Dn<NUM>-Dn<NUM>)/Dn<NUM>≤<NUM>, Dn<NUM>≥<NUM>, and Dn<NUM>≥<NUM>, where Dn<NUM>, Dn<NUM>, and Dn<NUM> are particle sizes when the cumulative number of particles calculated in ascending order of particle sizes reaches <NUM>%, <NUM>%, and <NUM>% of the total number of particles, respectively, in a number distribution diagram of particle sizes of the silicon-based material.

In some embodiments, a mass of the silicon-based material accounts for <NUM>% to <NUM>% of a total mass of the silicon-based material and graphite.

In some embodiments, graphite includes at least one of natural graphite, artificial graphite, or meso-carbon microbeads.

In some embodiments, the silicon oxide material includes SiOx, where x satisfies <NUM><x<<NUM>; and SiOx includes at least one of a crystal state or an amorphous state.

In some embodiments, in a Raman spectrum of the carbon layer, a ratio of a peak intensity at <NUM>-<NUM> I<NUM> to a peak intensity at <NUM>-<NUM> I<NUM> satisfies <NUM><I<NUM>/I<NUM><<NUM>. In some embodiments, the carbon layer is of a velvet structure, with a length of <NUM> to <NUM>.

Another embodiment of this application provides an electrochemical apparatus, including: a positive electrode plate, a negative electrode plate, and a separator disposed between the positive electrode plate and the negative electrode plate, where the negative electrode plate includes a negative electrode current collector and a negative electrode active substance layer disposed on the negative electrode current collector, where the negative electrode active substance layer includes the negative electrode material according to any one of the foregoing embodiments.

In some embodiments, the negative electrode active substance layer includes a binder, where the binder includes at least one of polyacrylate, polyimide, polyamide, polyamideimide, polyfluoroethylene, styrene butadiene rubber, sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium hydroxymethyl cellulose, or potassium hydroxymethyl cellulose.

In some embodiments, the electrochemical apparatus further includes an electrolyte, where the electrolyte includes an organic solvent and a lithium salt; the organic solvent includes at least one of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), polypropylene carbonate, or ethyl propionate; and optionally, the organic solvent includes FEC, where a mass percentage of the FEC in the electrolyte is <NUM>% to <NUM>%.

An embodiment of this application further provides an electronic apparatus, including the electrochemical apparatus according to any one of the foregoing embodiments.

The negative electrode material provided in the embodiments of this application includes a silicon-based material, or a silicon-based material and graphite. The silicon-based material includes a silicon oxide material and a carbon layer located on a surface of the silicon oxide material. A particle size distribution of the silicon-based material satisfies <NUM>≤(Dn<NUM>-Dn<NUM>)/Dn<NUM>≤<NUM>, Dn<NUM>≥<NUM>, and Dn<NUM>≥<NUM>. In the negative electrode material provided in the embodiments of this application, a number distribution of particle sizes of the silicon-based material can be controlled so that large and small particles are better matched, thereby improving cycling performance of an electrochemical apparatus.

With reference to the accompanying drawings and the Detailed Description below, the above and other features, advantages, and aspects of the embodiments of this application will become more apparent. Throughout the drawings, the same or similar reference numbers indicate same or similar elements. It should be understood that the drawings are exemplary and that the components and elements are not necessarily drawn to scale.

Embodiments of this application will be described in more detail below with reference to the accompanying drawings. Although some embodiments of this application are illustrated in the accompanying drawings, it should be understood that this application may be implemented in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided for a more thorough and complete understanding of this application. It should be understood that the accompanying drawings and embodiments of this application are for illustrative purposes only, and are not intended to limit the scope of protection of this application.

Silicon-based materials has much a higher specific capacity than carbon materials. Therefore, more and more electrochemical apparatuses are using silicon-based materials as their negative electrode materials. However, Silicon-based materials as negative electrode materials are still subject to certain defects. During cycling, a significant volume change of the silicon-based material causes the silicon-based material to be pulverized and separated from a negative electrode current collector, thereby degrading cycling performance of the electrochemical apparatus.

An embodiment of this application provides a negative electrode material. The negative electrode material includes a silicon-based material, or includes a silicon-based material and graphite. The negative electrode material includes the silicon-based material, thereby helping increase a specific capacity of the negative electrode material. The silicon-based material includes a silicon oxide material and a carbon layer located on a surface of the silicon oxide material. The carbon layer on at least part of the surface of the silicon oxide material can increase conductivity of the silicon-based material and reduce swelling of the silicon-based material during cycling.

A particle size distribution of the silicon-based material satisfies <NUM>≤(Dn<NUM>-Dn<NUM>)/Dn<NUM>≤<NUM>, Dn<NUM>≥<NUM>, and Dn<NUM>≥<NUM>. Dn<NUM>, Dn<NUM> and Dn<NUM> are particle sizes when the cumulative number of particles calculated in ascending order of particle sizes reaches <NUM>%, <NUM>%, and <NUM>% of the total number of particles, respectively, in a number distribution diagram of particle sizes of the silicon-based material. In some embodiments, in a number distribution diagram of particle sizes of the silicon-based material, integral areas are calculated in ascending order of particle sizes, and the integral areas that are <NUM>%, <NUM>% and <NUM>% of the total area correspond to particle sizes Dn<NUM>, Dn<NUM> and Dn<NUM>, respectively.

In some embodiments, when particles of the silicon-based material satisfy Dn<NUM><<NUM> or Dn<NUM><<NUM>, due to the excessively small particle size and excessively large surface area of the silicon-based material, too much electrolyte is consumed to form an SEI (solid electrolyte interphase) film, which causes more side reactions, more byproducts generated, and more gas produced by an electrochemical apparatus during cycling. In addition, a continuous increase in the SEI film degrades cycling performance of the electrochemical apparatus. In some embodiments, when (Dn<NUM>-Dn<NUM>)/Dn<NUM><<NUM>, the silicon-based material has a small range of particle size distribution, which may be inconducive to filling voids of the silicon-based material, thereby resulting in an inadequate compacted density; and when (Dn<NUM>-Dn<NUM>)/Dn<NUM>><NUM>, more gas may be produced by an electrochemical apparatus during cycling. In this embodiment of this application, particle sizes of the silicon-based material can be controlled so that a number distribution of the particle sizes of the silicon-based material satisfies <NUM>≤(Dn<NUM>-Dn<NUM>)/Dn<NUM>≤<NUM>, Dn<NUM>≥<NUM>, and Dn<NUM>≥<NUM>. In this way, a compacted density of a negative electrode can be increased, and side reactions of an electrochemical apparatus during cycling can be decreased, thereby increasing an energy density, improving the cycling performance of the electrochemical apparatus and reducing production of gas during cycling.

In some embodiments, a mass of the silicon-based material accounts for <NUM>% to <NUM>% of a total mass of the silicon-based material and the graphite. When a ratio of the mass of the silicon-based material to the total mass of the silicon-based material and the graphite is less than <NUM>%, an energy density may be increased insignificantly due to a too low percentage of the silicon-based material. In some embodiments, the graphite includes at least one of natural graphite, artificial graphite, or meso-carbon microbeads.

In some embodiments, the silicon oxide material includes SiOx, where x satisfies <NUM><x<<NUM>; and SiOx includes at least one of a crystal state or an amorphous state. In some embodiments, when x is too large, SiOx generates more Li<NUM>O and Li<NUM>SiO<NUM> of irreversible phases during cycling, resulting in decreased specific energy and first-cycle coulombic efficiency; when x is too small, cycling performance of SiOx may be degraded.

In some embodiments, in a Raman spectrum of the carbon layer, a ratio of a peak intensity at <NUM>-<NUM> I<NUM> to a peak intensity at <NUM>-<NUM> I<NUM> satisfies <NUM><I<NUM>/I<NUM><<NUM>. In some embodiments, I<NUM>/I<NUM> of the carbon layer reflects a defect density in the carbon layer. The defect density is proportional to I<NUM>/I<NUM>. When I<NUM>/I<NUM> is too small, conduction of electrons is affected, and direct current impedance of an electrochemical apparatus may be increased. When I<NUM>/I<NUM> is too large, inhibition of swelling of a silicon oxide material by the carbon layer may be less effective, and increased defects may cause more side reactions with an electrolyte.

In some embodiments, the carbon layer is a velvet structure with a length of <NUM> to <NUM>. In some embodiments, a transmission electron microscopy image may be taken for the carbon layer, and a length of the velvet structure may be calculated from the transmission electron microscopy image. The carbon layer of a velvet structure has a long-range conduction effect, which is conducive to electrical contact during cycling, thereby helping increase a cycling capacity retention rate.

An embodiment of this application further provides an electrochemical apparatus. As shown in <FIG>, the electrochemical apparatus includes: a positive electrode plate <NUM>, a negative electrode plate <NUM>, and a separator <NUM> disposed between the positive electrode plate <NUM> and the negative electrode plate <NUM>, where the negative electrode plate <NUM> includes a negative electrode current collector and a negative electrode active substance layer disposed on the negative electrode current collector, and the negative electrode active substance layer includes the negative electrode material described in any one of the above embodiments.

In some embodiments of this application, the electrochemical apparatus further includes an electrolyte; the electrolyte includes an organic solvent and a lithium salt; and the organic solvent includes at least one of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), polypropylene carbonate, or ethyl propionate.

In some embodiments of this application, the organic solvent includes FEC, and a mass percentage of FEC in the electrolyte is <NUM>% to <NUM>%. In some embodiments, FEC can improve mechanical properties of an SEI film, thereby reducing a cycling swelling rate and increasing a cycling capacity retention rate of the electrochemical apparatus. With an excessively small amount of FEC, improvement may be limited. With an excessively large amount of FEC, the SEI film may be continuously formed, thereby increasing an impedance of the electrochemical apparatus and resulting in a decreased cycling capacity retention rate. In some embodiments, addition of FEC having a mass percentage of <NUM>% to <NUM>% can significantly increase the cycling capacity retention rate and reduce the cycling swelling rate of the electrochemical apparatus using a silicon-based material.

In some embodiments of this application, the lithium salt includes: at least one of lithium hexafluorophosphate (LiPF<NUM>), lithium tetrafluoroborate (LiBF<NUM>), lithium difluorophosphate (LiPO<NUM>F<NUM>), lithium bis(trifluoromethanesulfonimide) LiN(CF<NUM>SO<NUM>)<NUM>(LiTFSI), Lithium bis(fluorosulfonyl)imide Li(N(SO<NUM>F)<NUM>)(LiFSI), lithium bis(oxalate)borate LiB(C<NUM>O<NUM>)<NUM>(LiBOB), or lithium difluoro(oxalato)borate LiBF<NUM>(C<NUM>O<NUM>)(LiDFOB).

In some embodiments, the negative electrode active substance layer may further include a conductive agent. The conductive agent in the negative electrode active substance layer may include at least one of carbon black, acetylene black, Ketjen black, lamellar graphite, graphene, carbon nanotubes, carbon fibers, or carbon nanowires. It should be understood that the materials disclosed here are merely examples, and any other suitable material may be adopted for the negative electrode active material layer. In some embodiments, a mass ratio of the negative electrode material, conductive agent, and binder in the negative electrode active substance layer may be (<NUM> to <NUM>):(<NUM> to <NUM>):(<NUM> to <NUM>). It should be understood that this is merely an example and not intended to limit this application.

In some embodiments, the positive electrode plate <NUM> includes a positive electrode current collector and a positive electrode active substance layer disposed on the positive electrode current collector. The positive electrode active substance layer may be located on one or two sides of the positive electrode current collector. In some embodiments, the positive electrode current collector may be made of an aluminum foil. Certainly, other positive electrode current collectors commonly used in the prior art may also be used. In some embodiments, a thickness of the positive electrode current collector may be <NUM> to <NUM>. In some embodiments, the positive electrode active substance layer may be applied on part of the positive electrode current collector. In some embodiments, a thickness of the positive electrode active substance layer may be <NUM> to <NUM>. It should be understood that this is merely an example, and any other suitable thickness may be adopted.

In some embodiments, the positive electrode active substance layer includes a positive electrode material. In some embodiments, the positive electrode material may include at least one of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium iron manganese phosphate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, or lithium nickel manganate, and the positive electrode material may undergo doping and/or coating processing. In some embodiments, the positive electrode active substance layer further includes a binder and a conductive agent. In some embodiments, the binder in the positive electrode active substance layer may include at least one of polyvinylidene fluoride, a vinylidene fluoride-hexafluoropropylene copolymer, a styrene-acrylate copolymer, a styrene-butadiene copolymer, polyamide, polyacrylonitrile, a polyacrylic ester, polyacrylic acid, a polyacrylic salt, carboxyl methyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinylether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. In some embodiments, the conductive agent in the positive electrode active substance layer may include at least one of conductive carbon black, acetylene black, Ketjen black, lamellar graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, a mass ratio of the positive electrode material, conductive agent, and binder in the positive electrode active substance layer may be (<NUM> to <NUM>):(<NUM> to <NUM>):(<NUM> to <NUM>). It should be understood that the descriptions above are merely examples, and any other suitable materials, thicknesses, and mass ratios may be adopted for the positive electrode active substance layer.

In some embodiments, the separator <NUM> includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. In particular, polyethylene and polypropylene can well prevent short circuit, and can enhance stability of a battery through a turn-off effect. In some embodiments, a thickness of the separator is in a range from about <NUM> to <NUM>. In some embodiments, a surface of the separator may further include a porous layer. The porous layer is disposed on at least one surface of the separator and includes at least one of inorganic particles or a binder. The inorganic particles are selected from at least one of aluminum oxide (Al<NUM>O<NUM>), silicon oxide (SiO<NUM>), magnesium oxide (MgO), titanium oxide (TiO<NUM>), hafnium dioxide (HfO<NUM>), tin oxide (SnO<NUM>), cerium dioxide (CeO<NUM>), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO<NUM>), yttrium oxide (Y<NUM>O<NUM>), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, pores of the separator have a diameter of about <NUM> to <NUM>. The binder in the porous layer is selected from at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylic ester, polyacrylic acid, polyacrylic salt, carboxyl methyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The porous layer on the surface of the separator can improve heat resistance, oxidation resistance, and electrolyte infiltration performance of the separator, enhancing binding between the separator and an electrode plate.

In some embodiments of this application, an electrode assembly of the electrochemical apparatus is a wound electrode assembly or a stacked electrode assembly. In some embodiments, the electrochemical apparatus is a lithium-ion battery, but this application is not limited thereto.

In some embodiments of this application, taking a lithium-ion battery as an example, a positive electrode plate, a separator, and a negative electrode plate are wound or stacked in sequence to form an electrode assembly which is then loaded into, for example, an aluminum-plastic filmed housing for packaging, an electrolyte is injected, and the lithium-ion battery is obtained after formation and packaging. Then, performance testing is performed on the prepared lithium-ion battery.

Those skilled in the art will understand that a preparation method of the electrochemical apparatus (for example, a lithium-ion battery) described above is merely an example. Without departing from the content disclosed in this application, other methods commonly used in the art may be adopted.

An embodiment of this application further provides an electronic apparatus including the electrochemical apparatus described above. The electronic apparatus according to this embodiment of this application is not particularly limited, and may be any known electronic apparatus used in the prior art. In some embodiments, the electrochemical apparatus may include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable fax machine, a portable copier, a portable printer, a stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notebook, a calculator, a memory card, a portable recorder, a radio, a standby power source, a motor, an automobile, a motorcycle, a motor bicycle, a bicycle, a lighting appliance, a toy, a game console, a clock, an electric tool, a flash lamp, a camera, a large household battery, a lithium-ion capacitor, or the like.

Some embodiments of this application further provide a preparation method of an electrochemical apparatus, including the following steps.

Step <NUM>: <NUM> of amorphous silicon monoxide powder having a particle size of <NUM> to <NUM> and a chemical formula of SiOx (<NUM>≤x≤<NUM>) is first mechanically crushed and then jet pulverized with a jet pulverizer, and then grading is performed to obtain silicon monoxide powder with a particle size of <NUM> to <NUM>.

Step <NUM>: The silicon monoxide powder obtained in step <NUM> is placed into a CVD (chemical vapor deposition, Chemical Vapor Deposition) furnace that is then heated to <NUM> to <NUM> and filled with a carbon source gas at a flow rate of <NUM>/min, and the carbon source gas is cut off immediately after <NUM> minutes. The power is cooled down to room temperature in an inert atmosphere, and a cooled powder sample is taken out. The inert atmosphere is a gas mixture of one or more of Ar, N<NUM>, and He; and the carbon source gas is a gas mixture of one or more of CH<NUM>, C2H<NUM>, C7H<NUM>, C<NUM>H<NUM>, and C<NUM>H<NUM>.

Step <NUM>: A product obtained in step <NUM> is subjected to demagnetization and powder grading to obtain a carbon-coated silicon monoxide negative electrode material satisfying <NUM>≤(Dn<NUM>-Dn<NUM>)/Dn<NUM>≤<NUM>, Dn<NUM>≥<NUM>, and Dn<NUM>≥<NUM>.

Powder of the prepared negative electrode material, graphite, a conductive agent, and a binder are taken to prepare a negative electrode plate for lithium-ion battery. The conductive agent includes one or more of conductive carbon black, acetylene black, Ketjen black, lamellar graphite, graphene, and the like, and graphite includes at least one of natural graphite, artificial graphite, or meso-carbon microbeads. Specific steps are as follows.

<NUM> of negative electrode material powder, <NUM> of graphite, and <NUM> of conductive agent are added into an MSK-SFM-<NUM> vacuum stirrer, and stirred for <NUM> at a revolution speed of <NUM> r/min to <NUM> r/min.

<NUM> of binder is added into the stirred mixture, stirring is performed for <NUM> for uniform dispersion, <NUM> of deionized water is then added, and stirring is performed for uniform dispersion to obtain a mixed slurry, with a revolution speed of <NUM> r/min to <NUM> r/min and a rotation speed of <NUM> r/min to <NUM> r/min. A slurry viscosity is controlled between <NUM> mPa. s and <NUM> mPa. s, and a solid content is controlled between <NUM>% and <NUM>%.

The slurry is filtered with a <NUM>-mesh double-layer screen to obtain a negative electrode slurry, the negative electrode slurry is then applied on a copper foil with a coating thickness of <NUM> to <NUM>; and the electrode plate is dried and cold pressed to a double-sided compacted density of <NUM>/cm<NUM> to <NUM>/cm<NUM>.

A positive electrode material LiCoO<NUM>, conductive carbon black, and polyvinylidene fluoride (PVDF) are fully stirred and uniformly mixed in an N-methylpyrrolidone solvent at a mass ratio of <NUM>:<NUM>:<NUM>, and a resulting mixture is applied on an Al foil, followed by drying and cold pressing, to obtain a positive electrode plate.

With a polyethylene porous polymeric film as a separator, the positive electrode plate, the separator, and the negative electrode plate are stacked in sequence, so that the separator is placed between the positive and negative electrode plates for isolation, and the stack is wound to obtain a bare cell. The bare cell is placed in an outer package, an electrode (a volume ratio of ethylene carbonate EC, dimethyl carbonate DMC, and diethyl carbonate DEC in the electrolyte is <NUM>:<NUM>:<NUM>, a mass percentage of FEC is <NUM>%, and a concentration of LiPF<NUM> is <NUM> mol/L) is injected, sealing is performed, and an electrochemical apparatus is obtained after processes of formation, degassing, and trimming.

To better illustrate the technical solutions of this application, some specific examples are listed below to better illustrate this application. Herein, a lithium-ion battery is used as an example.

Step (<NUM>): <NUM> of amorphous silicon monoxide powder having a particle size of <NUM> to <NUM> and a chemical formula of SiOx (<NUM>≤x≤<NUM>) was mechanically crushed first, and then jet-pulverized by using a jet pulverizer to obtain silicon monoxide powder having a particle size of <NUM> to <NUM>, and grading was performed to obtain SiOx satisfying Dn<NUM>≥<NUM>.

Step (<NUM>): SiOx obtained in step (<NUM>) was placed into a CVD furnace that was then heated to <NUM> and filled with methane at a gas flow of <NUM>/min, and the methane gas was cut off immediately after <NUM>. Cooling to room temperature was performed in an inert atmosphere, and a carbon-coated silicon oxide material was taken out after cooling.

Step (<NUM>): The carbon-coated silicon oxide material obtained in step (<NUM>) was subjected to demagnetization, grading, and <NUM>-mesh screening to obtain a carbon-coated silicon monoxide negative electrode material.

<NUM> of negative electrode material, <NUM> of graphite, and <NUM> of conductive agent were added into an MSK-SFM-<NUM> vacuum stirrer, and stirred for <NUM> at a revolution speed of <NUM> r/min to <NUM> r/min.

<NUM> of binder was added into the stirred mixture, stirring was performed for <NUM> for uniform dispersion, <NUM> of deionized water was then added, and stirring was performed for uniform dispersion to obtain a mixed slurry. During stirring, a revolution speed was <NUM> r/min to <NUM> r/min, and a rotation speed was <NUM> r/min to <NUM> r/min; after stirring, a viscosity of the mixed slurry was controlled between <NUM> mPa. s and <NUM> mPa. s, and a solid content of the mixed slurry was controlled at <NUM>% to <NUM>%.

The mixed slurry was filtered with a <NUM>-mesh double-layer screen to obtain a negative electrode slurry which was then applied on a copper foil at a coating thickness of <NUM>; and the electrode plate was dried and cold pressed to a double-sided compacted density of <NUM>/cm<NUM>.

A positive electrode material LiCoO<NUM>, conductive carbon black, and polyvinylidene fluoride (PVDF) were fully stirred and uniformly mixed in an N-methylpyrrolidone solvent at a mass ratio of <NUM>:<NUM>:<NUM>, and a resulting mixture was applied on an Al foil, followed by drying and cold pressing, to obtain a positive electrode plate.

A porous polyethylene polymeric film was used as a separator. The positive electrode plate, the separator, and the negative electrode plate were stacked in sequence, so that the separator was placed between the positive and negative electrode plates for isolation, and the stack was wound to obtain a bare cell. The bare cell was placed in an outer package, an electrode (a volume ratio of ethylene carbonate EC, dimethyl carbonate DMC, and diethyl carbonate DEC in the electrolyte was <NUM>:<NUM>:<NUM>, a mass percentage of FEC was <NUM>%, and a concentration of LiPF<NUM> was <NUM> mol/L) was injected, sealing was performed, and a lithium-ion battery was obtained after processes of formation, degassing, and trimming.

Example <NUM> is the same as Example <NUM>, except that a <NUM>-mesh screen was used for screening in step (<NUM>) of preparation of a negative electrode material.

Example <NUM> is the same as Example <NUM>, except that an <NUM>-mesh screen was used for screening in step (<NUM>) of preparation of a negative electrode material.

Example <NUM> is the same as Example <NUM>, except that SiOx obtained in step (<NUM>) was placed in a CVD furnace that was then heated to <NUM> in step (<NUM>) of preparation of a negative electrode material.

Example <NUM> is the same as Example <NUM>, except that the preparation of the negative electrode material was different.

Preparation of a negative electrode material in Example <NUM>:.

Example <NUM> is the same as Example <NUM>, except that instead of methane, acetylene was introduced in step (<NUM>) of preparation of a negative electrode material.

Example <NUM> is the same as Example <NUM>, except that an electrolyte used in Example <NUM> contained no FEC.

Example <NUM> is the same as Example <NUM>, except that FEC contained in an electrolyte used in Example <NUM> had a mass percentage of <NUM>%.

The following describes testing methods for various parameters of this application. SEM (scanning electron microscopy) test: Scanning electron microscope characterization was recorded with a PhilipsXL-<NUM> field emission scanning electron microscope. Testing was performed under the conditions of <NUM> kV and <NUM> mA.

TEM (transmission electron microscopy) test: Transmission electron microscope characterization was tested on a JEOL JEM-<NUM> transmission electron microscope, under a working voltage of <NUM> kV.

Powder compacted density test: SUNS UTM7305 was used to add <NUM> of powder into a <NUM> mold, pressure was raised at a rate of <NUM>/min, and the pressure was relieved at a rate of <NUM>/min after the pressure reached <NUM> tons. When the pressure reached <NUM> tons, the pressure was maintained for <NUM>, volume of the powder was measured, and a powder compacted density PD measured in g/cm<NUM> was obtained based on a mass to volume ratio.

Particle size test: About <NUM> of a powder sample was added into a <NUM> clean beaker, about <NUM> of deionized water was added, a few drops of a <NUM>% surfactant were dropwise added to make the powder fully dispersed in the water, the powder was subjected to ultrasound for <NUM> in a <NUM> W ultrasonic cleaning machine, and a particle size distribution was tested with a MasterSizer <NUM>.

Particle size test method for a silicon-based material in an electrode plate: The electrode plate was soaked and cleaned with DMC (dimethyl carbonate) and then dried in a vacuum oven at <NUM> for <NUM> hours, a sample was taken for SEM test, and pictures of five points in a <NUM>×<NUM> range were selected for counting particle sizes of the silicon-based material.

Lithium-ion battery cycling test: At <NUM>, the battery was charged at a constant current of <NUM>. 7C to <NUM> V, then charged at a constant voltage to <NUM>. 025C, left standing for <NUM>, and then discharged at <NUM>. 5C to <NUM> V. A capacity obtained in this cycle was an initial capacity C0. Then, a <NUM>. 7C charge/<NUM>. 5C discharge cycling test was performed. A ratio of a capacity of each cycle to the initial capacity was calculated to obtain a capacity attenuation curve, and a capacity C1 after <NUM> cycles was recorded. Capacity retention rate after <NUM> cycles at <NUM> = C1/C0 × <NUM>%.

Cycle swelling rate test for a lithium-ion battery: A spiral micrometer was used to measure a thickness d0 of the lithium-ion battery at initial <NUM>% SOC. At the <NUM>th cycle, the lithium-ion battery was in a <NUM>% SOC state, and then a thickness d1 of the lithium-ion battery at that time was measured with the spiral micrometer.

Parameters and performance test results of the examples are shown in Table <NUM> (wherein examples <NUM>-<NUM> are not according to the present application).

Performance test data of each Example is shown in Table <NUM>. <FIG> are number distribution diagrams of particle sizes of the negative electrode materials of Example <NUM> and Example <NUM> respectively. From comparison of the test data of Examples <NUM> to <NUM>, it can be learned that Example <NUM> had a higher swelling rate after <NUM> cycles at <NUM> than Examples <NUM> to <NUM>, and a lower compacted density than Examples <NUM> to <NUM>. This was because Dn<NUM> and Dn<NUM> in Example <NUM> were relatively small, both less than <NUM>, and a value of (Dn<NUM>-Dn<NUM>)/Dn<NUM> was as large as <NUM>. In this case, particles with smaller particle sizes in the negative electrode material accounted for a large proportion in number distribution, while particles with larger particle size accounted for a small proportion in number distribution. This led to a decreased bulk density and compacted density of the negative electrode material. In addition, the presence of the particles with smaller particle sizes might cause more violent reactions with an electrolyte during electrochemical reactions, and more byproducts were generated, increasing production of gas during cycling, causing the loss of lithium sources and a continuous increase in the SEI film, and degrading cycling performance.

<FIG> shows the variation of the cycling capacity retention rates in Examples <NUM> and <NUM> with the number of cycles. From comparison of Examples <NUM> and <NUM>, it can be learned that when Dn<NUM>><NUM>, <NUM><(Dn<NUM>-Dn<NUM>)/Dn<NUM><<NUM> and Dn<NUM>><NUM>, with the increase of (Dn<NUM>-Dn<NUM>)/Dn<NUM>, a compacted density gradually increased, the capacity retention rate after <NUM> cycles at <NUM> gradually increased, and the swelling rate after <NUM> cycles at <NUM> gradually decreased. This was because within the specified particle size range, a number distribution of particle sizes of negative electrode materials was even, facilitating the filling of voids and the accumulation of particles, thereby increasing the compacted density, improving the cycling performance and reducing production of gas during cycling.

From comparisons between Examples <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, it can be learned that with the temperature rise of the CVD furnace, the capacity retention rate after <NUM> cycles at <NUM> first increased and then decreased, and the swelling rate after <NUM> cycles at <NUM> first decreased and then increased. Example <NUM> had the highest capacity retention rate after <NUM> cycles at <NUM> and the lowest swelling rate after <NUM> cycles at <NUM>. This was because when the CVD temperature was less than <NUM>, gas carbonization was incomplete, and the carbon layer was not fully formed; silicon oxide particles had more surface defects, resulting in more byproducts and decreased cycling capacity retention rate. With the temperature increase of the CVD furnace, the particle size of the negative electrode material gradually increased, deficiencies were reduced, and the carbon layer started to form on a surface of a silicon oxide compound. The presence of the carbon layer helped inhibit the swelling of silicon oxide compounds and increase conductivity during cycling. When the temperature of the CVD furnace was too high, the silicon oxide compounds were crystallized significantly, and the ability of the carbon layers to inhibit the silicon oxide compounds was weakened. Therefore, the silicon oxide compounds swell sharply during cycling.

From comparison between Examples <NUM> and <NUM>, it can be learned that when the particles with larger particle sizes in the negative electrode material accounted for a larger portion in number distribution, accumulation of the particles of the negative electrode material had a poor matching, and the powder compacted density was low. In addition, the presence of the large particles increased local swelling, finally resulting in the loss of electrical contact between active substances in the negative electrode plates, and causing the cycling capacity retention rate to reduce and the cycle swelling rate to increase.

<FIG> shows a TEM image of the carbon layer of the negative electrode material in Example <NUM>. <FIG> shows a TEM image of the carbon layer of the negative electrode material in Example <NUM>. It can be learned from <FIG> and <FIG> that the carbon layer in Example <NUM> was of a velvet structure, while the carbon layer in Example <NUM> was of a non-velvet compact structure. From comparison between the data of Examples <NUM> and <NUM>, it can be learned that Example <NUM> with the carbon layer of a velvet structure had a higher capacity retention rate after <NUM> cycles at <NUM> than Example <NUM> with the carbon layer of a non-velvet structure. This was because the carbon layer of a velvet structure had a long-range conduction effect, which was more conducive to electrical contact during cycling and to capacity retention.

From comparison between Examples <NUM>, <NUM> and <NUM>, it can be learned that with the increase of FEC contained in the electrolyte, the capacity retention rate of the lithium-ion battery after <NUM> cycles at <NUM> first increased and then decreased, and the swelling rate after <NUM> cycles at <NUM> first decreased and then increased. When a mass percentage of FEC in the electrolyte for the lithium-ion battery was <NUM>% to <NUM>%, volume swelling during cycling could be significantly reduced and the cycling capacity retention rate could be significantly increased. This was because FEC could improve mechanical properties of SEI films. Therefore, volume swelling during cycling could be effectively reduced. However, too much FEC might cause continuous formation of SEI films, increasing charge transfer impedance at an interface, leading to excessively increased impedance of the lithium-ion battery, and increasing capacity fading at the end of the cycle.

Claim 1:
A negative electrode material, comprising:
a silicon-based material, characterized in that,
the silicon-based material comprises a silicon oxide material and a carbon layer located on a surface of the silicon oxide material; and
a particle size distribution of the silicon-based material satisfies <NUM>≤(Dn<NUM>-Dn<NUM>)/Dn<NUM>≤<NUM>, Dn<NUM>≥<NUM>, and Dn<NUM>≥<NUM>, wherein Dn<NUM>, Dn<NUM> and Dn<NUM> are particle sizes when the cumulative number of particles calculated in ascending order of particle sizes reaches <NUM>%, <NUM>%, and <NUM>% of the total number of particles, respectively, in a number distribution diagram of particle sizes of the silicon-based material.