CN113130857A

CN113130857A - Nano silicon material and preparation method thereof

Info

Publication number: CN113130857A
Application number: CN201911396836.7A
Authority: CN
Inventors: 赵明才; 张娟; 汪炜
Original assignee: Jiangsu Zaichi Technology Co ltd
Current assignee: Jiangsu Zaichi Technology Co ltd
Priority date: 2019-12-30
Filing date: 2019-12-30
Publication date: 2021-07-16
Anticipated expiration: 2039-12-30
Also published as: CN113130857B

Abstract

The invention relates to a nano silicon material with an amorphous state/nano crystal composite structure. In the application of a lithium/sodium ion battery, after crystalline silicon materials are firstly intercalated with lithium/sodium, a composite structure of an amorphous region (silicon-lithium alloy) and a crystalline region (non-intercalated with lithium) is formed, and volume expansion and structural change are generated; after delithiation/sodium, the volume shrinkage leads to structural collapse, i.e. cracking and pulverization of the silicon particles. In order to provide enough lithium/sodium insertion space in advance and inhibit the volume change of the first lithium/sodium insertion/removal, the invention provides an amorphous/nanocrystalline composite structure and a controllable preparation method thereof, namely, a spark discharge and high-energy ball milling combined process is adopted to prepare a nano silicon material with the amorphous/nanocrystalline composite structure, wherein an amorphous area surrounds nanocrystals, the crystal face orientation of the nanocrystals is randomly distributed, the proportion range of the amorphous area is controllable, and the amorphous/nanocrystalline composite structure belongs to an isotropic material. In lithium and sodium ion battery applications, the structure can effectively relieve the problems of material cracking and pulverization caused by expansion/contraction of silicon materials due to lithium/sodium insertion/removal, thereby improving the cycle performance of the silicon negative electrode.

Description

Nano silicon material and preparation method thereof

Technical Field

The invention relates to an amorphous/nanocrystalline composite structure nano silicon material, belonging to the technical field of lithium battery cathode materials.

Background

In recent years, with the rapid development of new energy pure electric vehicles, plug-in hybrid electric vehicles and electric tools, higher requirements are put on the energy density, safety and cycle stability of lithium ion batteries. Graphite as a commercial negative electrode material of a lithium ion battery (with a theoretical specific capacity of 372 mAh g^-1) The market demand for high energy density and large cells has not been met. The silicon-based negative electrode material has higher theoretical specific capacity of 4200 mAh g^-1The lithium ion battery cathode material has the advantages of low charge and discharge platform (close to the potential platform of graphite), environmental friendliness, high safety and the like, and is considered to be the most lithium-embedded lithium ion battery cathode material. Different from the principle that a graphite laminated structure provides a lithium ion embedding space, crystalline silicon is a covalent tetrahedral structure, an alloy compound can be formed with lithium ions in the lithium embedding process, the crystalline silicon is gradually converted into amorphous silicon lithium alloy along with the increase of lithium embedding amount and the severe expansion (up to 300%) of the volume, in the lithium removing process, the amorphous silicon lithium alloy is gradually converted into amorphous silicon particles from outside to inside, the volume can be severely contracted, the severe volume change (more than 300%) caused by the repeated lithium removing/embedding process causes the mechanical damage/pulverization of the particles, a solid electrolyte interface film (SEI film) is always in the dynamic change of destruction-reconstruction, the electrolyte is continuously consumed, further the decay of the conductive ring environment outside the material, the aggravation of electrode polarization, the reduction of specific capacity, the reduction of coulombic efficiency and the like are caused, and the exertion of the cycle performance and the multiplying power performance of the material is seriously influenced, reducing battery life.

In order to solve the problem of silicon volume expansion, the volume expansion of silicon is mainly inhibited by reducing the size of silicon particles, such as preparing silicon nanoparticles, silicon nanowires, silicon nanotubes and porous nano-silicon, so that the electrochemical cycling stability of the silicon-based negative electrode material can be improved. However, the synthesis method of the material mainly comprises a chemical vapor deposition method, a laser ablation method, an electron beam evaporation method, a magnetron sputtering method and a metal-assisted chemical etching method, but the preparation methods have extremely high requirements on equipment and harsh process conditions, mainly use silane or silicon tetrachloride as a silicon source, have high cost and are difficult to produce in a large scale. The nano material prepared by the method is mostly crystalline nano silicon, volume expansion has anisotropy in charge and discharge, generated mechanical stress is unevenly distributed, the material is easy to crack and crush, and long-term cycle performance is difficult to maintain.

Disclosure of Invention

The invention provides a nano silicon material with an amorphous/nano crystal composite structure, which can provide enough lithium embedding space in advance and inhibit the volume change of first lithium removal/lithium embedding. The invention adopts spark discharge combined high-energy ball milling technology to prepare the nano silicon material with an amorphous/nano crystal composite structure, namely, an amorphous area surrounds a nano crystal area.

The micron and submicron silicon material with amorphous state/nanocrystalline composite structure can be prepared by utilizing a spark discharge processing technology, the bulk silicon material is used as a workpiece electrode, pulse current is applied between the workpiece electrode and a tool electrode, the bulk silicon material can be melted or gasified in a local tiny area, the melted or gasified material is rapidly condensed under the action of cooling liquid, the internal structure is reconstructed into the micron and submicron silicon material with amorphous state/nanocrystalline composite structure, and the silicon particles obtained by condensation have a very loose structure and are particularly suitable for ball milling, the cold welding effect generated among the particles in the ball milling process can be avoided, on one hand, the ball milling efficiency can be greatly improved, on the other hand, the nano material with amorphous state/nanocrystalline composite structure can be obtained, and the high-energy ball milling technology is independently adopted to prepare the nano silicon particles, the structure is not provided, and micron and submicron-scale particles formed by the aggregation of nano silicon are obtained due to the cold welding effect generated among particles in the ball milling process.

In a first aspect of the present invention, there is provided:

a nanometer silicon material containing amorphous/nanometer crystal structure is prepared by mixing amorphous region and crystalline region in single silicon nanometer particle of nanometer silicon material, and crystal face orientation of crystalline region is distributed randomly.

In one embodiment, the individual silicon nanoparticles have an average size in the range of 3 to 100 nm.

In one embodiment, the average size of the crystalline domains is between 1 and 20 nm.

In one embodiment, the single silicon nanoparticle may be an intrinsic material, or may be doped silicon doped with one or two of the elements phosphorus, nitrogen, arsenic, boron, indium, and aluminum.

In one embodiment, the crystalline regions have an area fraction of 10-98%; more preferably 15 to 40%.

In a second aspect of the present invention, there is provided:

the preparation method of the nano silicon material containing the amorphous/nano crystal structure comprises the following steps:

step 1, using a blocky silicon material as a workpiece electrode, respectively connecting the workpiece electrode and a tool electrode to two poles of a pulse power supply, forming continuous pulse discharge between the tool electrode and the workpiece electrode in working liquid, melting or gasifying the workpiece electrode in a local extremely micro area by using high temperature generated by spark discharge, and rapidly condensing the melted or gasified material under the action of cooling liquid to obtain micron and submicron silicon particles;

and 2, transferring the micron and submicron silicon material with the amorphous/nanocrystalline composite structure obtained by the spark discharge into high-energy ball milling equipment, carrying out ball milling treatment, and finally drying to obtain the nano silicon material with the amorphous/nanocrystalline composite structure.

In one embodiment, the bulk silicon material used in step 1 may be intrinsic silicon, or a doped material doped with one or two of phosphorus, nitrogen, arsenic, boron, indium, and aluminum.

In one embodiment, in step 1, the tool electrode may be copper, copper-based alloy, copper-based composite material, graphite, or diamond.

In one embodiment, in the step 1, the cooling liquid is deionized water or aviation kerosene.

In one embodiment, the pulse width of the electrical pulses generated by the pulse power source is between 50ns and 500 μ s.

In one embodiment, the pulse width of the electrical pulses generated by the pulsed power supply is preferably 50-300 ns.

In one embodiment, the high speed flushing pressure is preferably in the range of 1MPa to 20 MPa.

In one embodiment, in the step 2, a dispersion medium is added during the ball milling process to adjust the mass fraction of the slurry to be 1-30%, and the mass ratio of the grinding medium to the silicon powder is 1-3: 1, grinding media adopts 0.03-0.3mm grinding balls, the rotating speed of the equipment is adjusted to 500-.

In the step 2, the added dispersion medium is one or more of deionized water, acetone, butanone, toluene, ethanol, ethylene glycol, isopropanol, butanol cyclohexane or cyclohexanone.

In the step 2, the grinding ball is made of zirconia, alumina or stainless steel.

In the step 2, the drying adopts a spray dryer, a suction filter or a freeze dryer.

In a third aspect of the present invention, there is provided:

the electric spark discharge machining equipment is applied to the preparation of the nano silicon material.

In a fourth aspect of the present invention, there is provided:

the nano silicon material is applied to preparing a lithium ion battery cathode material.

In a fifth aspect of the present invention, there is provided:

a method of reducing the area fraction of crystalline silicon in a single silicon nanoparticle comprising the steps of:

in the process of electric spark discharge machining, the pulse width of a smaller electric pulse is used; the pulse width is preferably 50-300 ns.

Advantageous effects

The volume change of the silicon is fundamentally inhibited by regulating and controlling the internal structure of the silicon particles by adopting a method of a spark discharge (spark discharge) and bead-milling (beads-milling) combined process. The single crystal silicon ingot is prepared into micro-nano silicon particles by spark discharge, and meanwhile, crystal grains are refined to be in a nano size, the crystal orientation is disordered, and the difference caused by the anisotropy of the crystal is reduced; the bead milling can further refine the crystal grains to a plurality of nanometers and generate more amorphous structures, finally a composite structure of amorphous surrounding nanocrystalline is formed, the structure has isotropy and good mechanical property, and the mechanical stress generated by the expansion of silicon particles can be reduced in the lithium embedding process, so that the cracking and pulverization of the particles are effectively relieved, and the cycle performance of the particles is improved. By adjusting the parameters of spark discharge and bead milling, the nano silicon particles with controllable particle size, controllable nano crystal size and controllable amorphous proportion can be obtained. Under the condition of a spark discharge parameter with small pulse width, 0.1mm of zirconia beads and 12hd of grinding, nano silicon particles with the average particle diameter of 91.8nm, the average grain size of about 4.23nm and the average crystal area ratio (the average crystal area ratio) of about 15.45 percent are obtained, and the reversible specific capacity of the modified material applied to the negative electrode of the lithium ion battery is 1804.7 mAh g^-1The first effect is 62.97%, and after 100 cycles, the capacity retention rate is still as high as 60.64%.

Drawings

FIG. 1 is a circuit diagram of the present invention;

FIG. 2 is a graph showing a particle size distribution of the silicon particulate material in example 1;

FIG.3 is a Scanning Electron Microscope (SEM) picture of the silicon particulate material of example 1.

Fig. 4 is an X-ray diffraction (XRD) pattern of the silicon particulate material in example 1.

FIG. 5 is a high resolution Transmission Electron Microscope (TEM) picture of the silicon particulate material of example 1.

FIG. 6 is the cyclic charge and discharge performance of the silicon particulate material in a lithium battery of example 1;

FIG.7 shows the first charge and discharge performance of the silicon particulate material in example 1 in the preparation of a lithium battery;

fig. 8 is Scanning Electron Microscope (SEM) images (a) (b) of silicon particles prepared in comparative example 1 and comparative example 2, respectively.

Fig. 9 is an X-ray diffraction (XRD) pattern of the silicon particulate materials of comparative example 1 and comparative example 2.

FIG. 10 is a high resolution Transmission Electron Microscope (TEM) image of a silicon particulate material of comparative example 1.

FIG. 11 is a high resolution Transmission Electron Microscope (TEM) image of a silicon particulate material of comparative example 2.

Fig. 12 shows the first cycle charge/discharge performance of comparative example 1 and comparative example 2.

Fig. 13 shows the first charge/discharge properties of comparative example 1 and comparative example 2.

Detailed Description

The invention obtains the nano silicon material with an amorphous/nano crystal composite structure, namely, an amorphous area surrounds a nano crystal area, and the crystal face orientation of the nano crystal area is randomly distributed, thereby belonging to isotropic materials. The structure can effectively alleviate the problems of material cracking and powdering caused by the expansion/contraction of the silicon material due to lithium intercalation/deintercalation. Meanwhile, the generation of Li15Si4 crystal phase can be inhibited in the process of charging and discharging, thereby improving the cycle performance of the silicon electrode. The nano silicon material obtained by the invention has small units of amorphous regions and nano crystalline regions, is randomly mixed and distributed, has good uniformity, dispersibility and crystal face orientation randomness, and has good consistency among batches.

In the electric spark discharge machining process, the system comprises a pulse power supply, a servo control system, a workpiece electrode, a tool electrode, working liquid, a pump and a filter. The workpiece electrode and the tool electrode are respectively connected with the positive electrode and the negative electrode of the pulse power supply, the feeding of the tool electrode is controlled by the servo system, meanwhile, working liquid can be introduced into the tool electrode, and the working liquid is recycled by the pump after being filtered by the filter. The formation process of the silicon particles: an electric field is formed immediately after a voltage is applied between positive and negative electrode materials, because the gap between the positive and negative electrodes is small, and the microscopic surface of the electrode materials is uneven, the electric field is strongest in the area where the two electrodes are closest to each other, when the electric field strength reaches the critical value that the inter-electrode insulating medium is broken down, the inter-electrode medium is ionized and broken down to form a discharge channel, sparks are generated and along with the explosion sound, the electrode materials in the processing area are melted and gasified by the high temperature generated in the process, and nano-scale particles formed by the aggregation of gasified silicon atom clusters and micron-scale particles obtained by melting are formed. The melted and gasified silicon particles can be rapidly cooled after meeting low-temperature working solution, and are agglomerated again in the process of releasing energy, and finally formed micro-nano particles can be taken out by the working solution in the through holes.

Silicon is a semiconductor material, electrodes are required to have certain conductivity for spark discharge, boron-doped P-type monocrystalline silicon material (resistivity of 0.01 omega cm) is adopted as a workpiece electrode and a tool electrode in the following embodiment, the workpiece electrode is a cuboid with the thickness of 20mm, the tool electrode is a cuboid with the cross section of 5 mm-5 mm square and a through hole with the diameter of 2mm in the middle, and the working solution is insulating deionized water (resistivity of 10M omega cm).

In the bead milling process, the system comprises two parts, wherein one part is external circulating cold water, and condensed water with the temperature lower than 10 ℃ is provided for cooling a milling area and a slurry tank. The other part is material crushing, and comprises a main motor, a separation motor, a separator, a stirring motor, a zirconia bar pin, zirconia beads and a pump. Wherein, one part of the zirconia bar pin is driven by a main motor to drive the grinding materials (beads) to move at high speed, and the grinding materials moving at high speed are used for impacting and crushing materials; the other part of the zirconia bar pins are fixed on the wall of the inner cavity, and the collision probability is improved by changing the flow field in the grinding cavity, so that the grinding effect is improved. The beads have large mass and large centrifugal force, the separating electrode drives the separator to rotate at high speed to throw the grinding materials out, and the grinding aid dispersed with silicon particles can smoothly pass through the separator to be circularly ground with the help of the pump.

In the following examples, the grinding aid used was absolute ethanol, the amounts of the micro-nano silicon particles and the grinding aid were 200g and 1800g, respectively, in a mass ratio of 1:9, and the grinding aid was continuously stirred by a stirring motor to prevent sedimentation. The effective volume of the bead mill equipment is 0.7L, 2kg of 0.1mm zirconium oxide grinding material (the stacking density is 3.5 kg/L) is placed, the filling rate of the zirconium balls is about 81.6 percent, the rotating speed of the main motor reaches 2300r/min, and the outer circular linear speed of the driven zirconium oxide rod pin reaches 13.5 m/s.

The route of preparing silicon material in the invention is shown in figure 1,

the single crystal silicon ingot is used for preparing small-sized particles through spark discharge, and meanwhile, grains are refined, so that a large amount of grinding time is saved for bead grinding, and the energy consumption is greatly reduced. When the micro-nano silicon particles are prepared by spark discharge, the silicon material can be gasified into atom clusters in the central area of the discharge channel at the temperature exceeding the gasification temperature. Meanwhile, in a region where the temperature is lower but higher than the melting temperature of the silicon material, the silicon material may melt to form submicron and micron particles. After gasification and melting, the silicon particles are in gas state and liquid state, the internal structure is changed into amorphous structure from single crystal structure, the internal particles (silicon atoms) are arranged randomly, the distance between the particles is not equal to the equilibrium distance, and the silicon particles have higher potential energy, and are transformed to the crystal structure with the minimum and most stable internal energy in the condensation process and recrystallized in short time (figure 1). Meanwhile, the silicon particles in the micro-nanometer level have very high surface energy, and need to be agglomerated to further release energy. In the whole process, the maintaining time of the discharge channel has a great influence on the size and the internal structure of the micro-nano silicon particles, because the maintaining time of the discharge channel determines the duration time of high temperature. The long duration of the high temperature leads firstly to enlargement of the melting zone and to a large size of the molten particles formed. Secondly, the cooling speed is increased, the supercooling degree is increased, the nucleation rate can be obviously increased, and the higher the nucleation rate is, the more the number of crystal grains is, the smaller the size of the crystal grains is, and the higher the disordering degree of the crystal orientation is, so that the difference caused by the anisotropy of the crystal is reduced. And the long-time high temperature can cause the temperature reduction speed of the formed particles to be slow, thereby increasing the time of the recrystallization process of the particles, and causing the number of crystal grains in the particles to be small and the size of the crystal grains to be large. Finally, due to agglomerationIn the whole process, the long-time high temperature causes a plurality of formed particles to form agglomeration in a gasification and melting state, covalent bonds are formed among the particles, the agglomeration belongs to hard agglomeration, the bonding among the particles is tight, and the particles are not easy to re-disperse. The agglomerates formed at low temperatures are soft and easily dispersed. The time for maintaining the single discharge channel is related to the pulse width of the power supply, and the smaller the pulse width is, the shorter the current duration is, and the shorter the discharge time is. In the experiment, the comparison parameters are simplified, two pulse widths are selected for comparison, the time of a large pulse width (big pulse duration) is 200 mu s, and the obtained silicon particles correspond to Si_BThe time of small pulse width (small pulse duration) is 200ns, and the obtained silicon particles correspond to Si_S。

Filtering and collecting micro-nano particles obtained by spark discharge through a filter to obtain micro-nano silicon particle slurry, removing an oxide layer by using 1 wt% of hydrofluoric acid solution, cleaning with deionized water, centrifuging, and drying in vacuum at 100 ℃ to obtain micro-nano silicon powder. The dried micro-nano silicon powder can be prepared into nano silicon particles with uniform particle size distribution by further bead grinding. The particle size and the internal structure of the spark discharge are controlled by internal energy, and the particle size and the internal structure of the bead mill are controlled by mechanical energy. The size of the mechanical energy of the zirconium balls directly influences the size of the shearing force and the extrusion force generated during collision, thereby influencing the particle crushing effect. During repeated collisions, the forces experienced by the silicon particles can create stresses within the particles that break the particles into multiple particles when the stresses exceed the limits that the particles can withstand. Meanwhile, when the energy of a certain silicon atom of the crystal grain in the silicon particle is larger than the bond energy, the distance between atoms is increased or decreased, the potential energy is increased, the atomic bond is broken to generate a cavity or lattice offset, the original crystal cell parallelepiped structure is changed, the crystal grain is split into a plurality of smaller crystal grains, and even an amorphous structure with higher potential energy is formed. The larger the number of collisions, the smaller the grain size and the higher the proportion of amorphous structure, and gradually these effects are transmitted from the particle surface to the particle interior, gradually forming amorphous/nanocrystalline (amorphous/nanocrystalline) composite structured nano-silicon particles. Both spark discharge and bead milling will cause the silicon to breakDefects are formed inside the particles and bead milling causes more defects because bead milling does not have a recrystallization process. The repeated collision in the bead milling process enables a large number of point defects (vacancies and the like), line defects (mixed dislocation), surface defects (grain boundaries, twin grain boundaries and the like) and the like to be formed inside the particles, and silicon particles with high amorphous degree have larger volume than crystalline silicon particles due to more defects, so that the pre-expansion effect is achieved, and the expansion degree during lithium ion embedding is weakened. Meanwhile, the amorphous structure is isotropic, the crystal structure is anisotropic, and the amorphous/nanocrystalline composite structure silicon particles with low crystal proportion also have isotropic property, so that the mechanical property of the silicon particles during lithium ion intercalation and deintercalation is improved. The state of the micro-nano silicon particles before ball milling can influence the result after bead milling, so that the micro-nano silicon particles Si obtained by adopting the same bead milling process on different spark discharge parameters_BAnd Si_SPerforming bead grinding for 8h to respectively obtain nano silicon particles Si_B+MAnd Si_S+M. The non-crystallization degree can be directly influenced by the time of the bead mill, so that micro-nano silicon particles Si obtained by the same spark discharge parameter_BBead milling for different time to Si_BBead milling for 12h to obtain Si_S+MM。

Symbol definition:

Si_B: a large pulse width (big pulse duration) time of 200. mu.s, resulting in silicon particles;

Si_S: the time of small pulse width (small pulse duration) is 200ns, and silicon particles are obtained;

Si_B+M: micro-nano silicon particle Si_BPerforming bead grinding for 8 hours to obtain nano silicon particles;

Si_S+M: micro-nano silicon particle Si_SPerforming bead grinding for 8 hours to obtain nano silicon particles;

Si_{S+M M}: micro-nano silicon particle Si_BCarrying out bead grinding for 12h to obtain nano silicon particles;

example 1

The boron-doped P-type monocrystalline silicon material (with the resistivity of 0.01 omega cm) is used as a workpiece electrode and a tool electrode, the workpiece electrode is a cuboid with the thickness of 20mm, the tool electrode is a cuboid with the cross section of 5 mm-5 mm square, the middle part is a cuboid with a through hole with the diameter of 2mm, and the working solution is insulating deionized water (with the resistivity of 10M omega cm).

The pulse width of the discharge pulse generated by a pulse power supply is respectively 200 mus and 200ns, the duty ratio is 1:4, and the rectangular pulse voltage of the open-circuit voltage 160V is applied between the workpiece electrode and the tool electrode, the insulating working medium is ionized and punctured to form a plasma discharge channel, and the generated high-temperature melting and gasification workpiece electrode is condensed to obtain the micron and submicron silicon materials with the amorphous/nanocrystalline composite structure. Filtering with a centrifuge to obtain silicon particles, removing an oxide layer by using 1 wt% of hydrofluoric acid solution, cleaning with deionized water, centrifuging, and drying at 100 ℃ in vacuum to obtain the micro-nano silicon powder. The obtained particles are respectively Si_BAnd Si_S。

And (2) further refining the collected micron and submicron silicon materials with amorphous/nanocrystalline composite structures by adopting a high-energy ball milling method, wherein the adopted grinding aid is absolute ethyl alcohol, the amounts of the micron and submicron silicon particles and the grinding aid are respectively 200g and 1800g, the mass ratio is 1:9, and the micron and submicron silicon materials are continuously stirred by a stirring motor to prevent sedimentation. The effective volume of the bead mill equipment is 0.7L, 2kg of 0.1mm zirconium oxide grinding material (the stacking density is 3.5 kg/L) is placed, the filling rate of the zirconium balls is about 81.6 percent, the rotating speed of the main motor reaches 2300r/min, and the outer circular linear speed of the driven zirconium oxide rod pin reaches 13.5 m/s.

Mixing Si_BAnd Si_SBall milling is carried out for 8 hours respectively to obtain Si_B+MAnd Si_S+M(ii) a And, Si is mixed with_SBall milling for 12h to obtain Si_{S+M M}。

Comparative example 1

Preparation of nano silicon particles by direct ball milling method

Step 1, coarse grinding, weighing a certain mass of commercial silicon powder (the particle size is about 10 um) and pouring the commercial silicon powder into a stirring barrel, adding ethanol to adjust the solid content of the slurry to be 10%, adjusting the rotating speed to be 500r/min, and stirring for 4 hours to uniformly disperse the slurry. And then transferring the mixture into a planetary ball mill, wherein the mass ratio of the grinding media to the silicon powder is 2: 1, grinding media adopt zirconia grinding balls with the particle size of 5mm, the ball milling time is 10 hours, the rotating speed is 800r/min, and an ethanol medium is continuously added in the ball milling process to keep the solid content of the silicon slurry unchanged. Coarse ground silicon slurry with a particle size of 1.3um was obtained.

And step 2, fine grinding, namely transferring the obtained rough ground silicon slurry into a horizontal sand mill, wherein the mass ratio of the grinding medium to the silicon powder is 2: 1, grinding media adopt zirconia balls with the particle size of 0.1mm, the ball milling time is 10 hours, the rotating speed is 1000r/min, and an ethanol medium is continuously added in the ball milling process to keep the solid content of the silicon slurry to be 10 percent. Finally obtaining the nano silicon particles with the particle size of 160 nm.

Comparative example 2

Preparation of crystalline silicon nanoparticles

100sccm (standard cubic centimeters per minute; 1sccm =1cm3/min of gas, 0 and atmospheric pressure) SiH 4/hydrogen mixture (mixture 1) and a mixture of both argon and hydrogen of 10000sccm (mixture 2) were introduced into the microwave reactor via a two-flow nozzle. An output of 500W was introduced into the gas mixture from the microwave generator and a plasma was generated therefrom. The plasma spot exiting the reactor through the nozzle expands to a volume of about 20 liters, which is larger than the reactor. The pressure in this space and in the reactor was adjusted to 200 mbar. And separating the powder product from gas substances in a downstream filtering device to obtain crystalline nano silicon powder.

Characterization of particle size and SEM

Fig. 2 shows the particle size distribution of the 5 particles prepared in the above example, with the abscissa using a logarithmic axis. FIG.3 is an SEM photograph of these 5 particles, (a) Si_B, (b)Si_S, (c)Si_B+M, (d)Si_S+M and (e)Si_S+MM. In FIG. 2, Si_BAnd Si_SThe average particle diameters of the particles are 4060nm and 390nm, respectively, and Si is obtained_BThe particles are larger and more widely distributed. As can also be seen in FIG.3, Si_BHas an overall particle diameter larger than that of Si_SThe particle size of (1). Si_BHas obvious double peak on the particle size distribution curve, micron-sized particles are obviously more than submicron-sized particles, and Si_SMainly submicron-sized particles, and the particles are obtained by gasifying and agglomerating silicon materials.The large pulse width can prolong the high-temperature duration, so that the melting area in the discharge channel is enlarged, the particle size of the formed melting particles is enlarged, the number of the formed melting particles is high, and the two micro-nano particles are milled for 8 hours under the same bead milling parameters to obtain Si_B+MAnd Si_S+M，Si_S+MHas an average particle diameter of 99nm, which is superior to that of Si_B+M108nm, Si_SBisi_BIt is easier to grind to nanometer level. This is due to Si on the one hand_BOn the other hand, due to the longer duration of Si at high temperatures_BThe hard agglomerates are more, the raw materials are large in size, the hard agglomerates with strong binding force are more, and more energy is needed to refine particles, namely, more bead milling time is needed. To Si_SFurther prolonging the bead milling time to 12h to obtain Si_S+MMHaving an average particle diameter of 91nm, with Si_B+MThe particle size is reduced, but the reduction is slow. From 3, it can also be seen that the spherical silicon particles formed after spark discharge are broken into plate-like particles after repeated collision in the bead milling process, and the thickness dimension of the plate-like particles is far less than 100nm, and may be only a few to a dozen nanometers.

SEM images of the silicon nanomaterials prepared in comparative example 1 and comparative example 2 are shown in fig. 8 (a) (b), respectively. It can be seen that the number of particles in FIG. 8 (a) of comparative example 1 is more than that between 150 and 200 nm; comparative example 2 larger particles were present in the nanoparticles prepared in fig. 8 (b).

Characterization of XRD

In fig. 4, the XRD pattern of each silicon particle is shown. Wherein, (a) an amorphous silicon ingot, (b) Si_B, (c)Si_S, (d)Si_B+M, (e)Si_S+M and (f)Si_S+MM。Diffraction peaks of Si_B and Si_SThe significant cubic phases of silicon (JCPDS. Card No.01-0787) are observed in the XRD diffraction peaks of (111), (220), (47.3), (311), (56.1), (400), (69), (331), (76.4) and (442) 88.

The XRD patterns of the silicon nanomaterials prepared in comparative examples 1 and 2 above are shown in fig. 9.

TEM characterization

Fig. 5 is a high resolution transmission electron micrograph of individual silicon particles.

Wherein a, b, c, d, e each represent Si_B、Si_S、Si_B+M、Si_S+M、 Si_{S+M M}. The number 1 is the selected area electron diffraction, the number 2 is the transmission electron microscope image, and the numbers 3 and 4 are partial enlarged views in the transmission electron microscope image, which are used for displaying crystalline and amorphous regions. As can be seen from the figure:

Si_Bdiffraction peak intensity ratio of Si_SHigher and narrower peak widths, indicating Si_BHas high crystallinity and larger grain size. From Si_BAnd Si_SCan be seen from the selected area electron diffraction pattern of (A), Si_BThe single crystal characteristics of (a) are more pronounced, also indicating that its grain size is larger. Si is obtained by calculation of the Xiele formula_BAnd Si_SThe average grain sizes of (A) were 68.92nm and 26.47nm, respectively (Table 1). From a comparison of) a3) (a4) with (b3) (b4), it was verified that Si_SGrain size of (2) is larger than that of Si_BSmaller of (a), Si_BThe average crystal area ratio of the two selected fractions (a3) (a4) was 97.25%, slightly higher than that of Si_SThe degree of crystallization was higher than 90% for 94.5% of FIG.7(b3) (b4) (Table 1). As analyzed above, the small pulse width and the short high temperature time accelerate the cooling rate of the formed particles, improve the nucleation rate during recrystallization, and form more crystal grains with smaller size and high degree of disorder of the crystal orientation.

TABLE 1

The spark discharge refines the crystal grains to nanometer level in advance, thereby reducing the difficulty of bead grinding and reducing the energy consumption. From 4, the micro-nano silicon particles are subjected to bead grinding, the diffraction peak intensity is weakened, and the peak width is widened, because the stress can generate micro strain in the repeated collision process of the silicon particles in the grinding material, the surface has lattice distortion, the crystal grains are refined, the amorphous amount is increased, and the diffraction peak is widened on an XRD (X-ray diffraction) diagram. To pairAs can be seen from comparison of (d) and (e) in FIG. 4, Si_B+MHas a peak intensity higher than that of Si_S+MBut Si_S+MThe wider the peak width, the smaller the grain size of the bead milled material. Comparing (e) and (f) in FIG. 4, Si is present as the polishing time is prolonged_S+MMThe diffraction peak intensity of the crystal is further weakened, and the peak width is further widened, so that the grains can be further refined and the amorphous amount can be increased by increasing the grinding time. Si is obtained by calculation of the Xiele formula_B+M，Si_S+MAnd Si_S+MMRespectively, of 11.73nm, 6.18nm and 4.23nm (Table 1). From comparison of (c3) (c4), (d3) (d4) and (e3) (e4) in fig. 5, it was verified that Si_B+M，Si_S+MAnd Si_S+MMThe crystal grain size of (a) is gradually reduced and the degree of crystallization is also reduced, and a structure in which nanocrystals are embedded in an amorphous structure is formed inside the silicon particles, in accordance with the structure in fig. 1. Si_B+MThe average crystal area ratio of (c3) (c4) in the two selected portions of FIG. 5 was 63.35%, Si_S+MCorresponding to (d3) (d4) in FIG. 5, it is 37.25%, Si_S+MMCorresponding to (e3) (e4) in fig. 5, is 15.45% (table 1). The selective electron diffraction of (c2), (d2) and (e2) in FIG. 5 can also be derived from Si_B+M，Si_S+MTo Si_S+MMThe degree of amorphization increases in turn. As analyzed by fig.3 and 2.2 Process scheme, repeated collisions of beads gradually increase the stress in the particles, internal defects increase, and the outer layer diffuses into the inner layer, resulting in grain splitting, an increased amorphous structure, and a higher proportion of amorphous particles with longer grinding time.

In addition, transmission electron micrographs of the silicon nanomaterials prepared in comparative example 1 and comparative example 2 are shown in fig. 10 and 11, respectively. It can be seen that the material in comparative example 1 is directly formed by mixing crystalline particles and amorphous particles; whereas the silicon material in comparative example 2 was constituted by a single crystalline region.

Electrochemical performance

The electrochemical test is carried out by a CR2032 type button cell, Si is used as an electrode material, a metal lithium sheet (the diameter is 15 mm) is used as a counter electrode material, a celagrd2500 microporous polypropylene membrane (the diameter is 19 mm) is used as a diaphragm, and LiPF6/EC + DEC (the volume ratio is 1: 1) is used as an electrolyte. The button cell is assembled in a glove box, wherein the glove box is filled with high-purity argon with the purity requirement of more than 99.999 percent, and the water and oxygen content are strictly controlled to be not more than 0.1 ppm.

FIG. 6 is a silicon particulate material (Si)_B, Si_S, Si_B+M, Si_S+M and Si_S+MM) In the cycle charge and discharge performance of the lithium battery, the electrode is firstly charged/discharged for 4 times at 0.05C rate, and then is charged/discharged for 96 times at 0.1C rate, and the voltage is cycled between 0.01 and 1V. FIG.7 shows the first charge-discharge performance of a silicon particulate material in the preparation of a lithium battery; as can be seen from FIG. 6, Si with the largest particle size_BHas the highest first charge-discharge specific capacity of 3918.1 mAh g respectively^-1，3494.1 mAh g^-1The first effect was also highest, reaching 89.18% (table 2). Si_SThe first discharge/charge specific capacity and the first effect are 3698.9 mAh g respectively^-1，3033.0 mAh g^-182.0% (Table 2). Si_B+MThe first discharge/charge specific capacity and the first effect are 3067.7 mAh g respectively^-1，2081.3 mAh g^-167.84% (Table 2)). Si_S+MThe first discharge/charge specific capacity and the first effect are 3023.6 mAh g respectively^-1，1974.2 mAh g^-165.29% (Table 2)). Si_S+MMThe first discharge/charge specific capacity and the first effect are 2866.1 mAh g respectively^-1，1804.7 mAh g^-162.97% (Table 2)). From the above results, it can be seen that, as the particle size of the silicon particles decreases, the first charge-discharge specific capacity and the first effect are both significantly reduced, and the nanoparticles obtained by first spark discharge and then bead milling are most significantly reduced, mainly because the specific surface area increases after the silicon particles are refined, more SEI films are formed during the first cycle, more embedded lithium ions are consumed, and the first effect is reduced. As can be seen from FIG. 6, the micro-nano particles Si_BAnd Si_SAll of which decay rapidly, but Si_SThe cycle performance of the alloy is slightly better than that of Si_BThis is mainly because of the presence of a large number of submicron and micron silicon particles produced by spark discharge, bodies of silicon during the intercalation and deintercalation of lithiumThe product undergoes violent expansion/contraction, so that the silicon particles are separated from the contact with the conductive agent and the current collector, the conductive environment is gradually lost, and a large-area dead volume is formed, so that the capacity is rapidly reduced, and more particles with smaller particle size are formed under a small pulse width, so that the phenomenon is better. Nanoscale particles of Si_B+M, Si_S+M and Si_S+MMThe cycle performance of the catalyst is obviously superior to that of micro-nano Si_BAnd Si_SThe reversible capacity after 100 cycles was 661.7, 890.4, 1094.4 mAh g-1 in this order, and the capacity retention rate was 31.79%, 45.10%, 60.64%, respectively (Table 2)). The silicon grain sizes of the three electrodes were successively decreased, and the average crystal grain size and the average crystal area ratio were also successively decreased (table 1). This shows that the nano silicon particles with small crystal size and high amorphization degree (or amorphous proportion) can effectively relieve the volume expansion of the silicon material in the charging and discharging processes. As analyzed above, the nano-silicon particles having an amorphous/nano-crystalline composite structure have a larger volume than the same-quality crystal particles, and thus have a large number of defects inside, which can provide some space for lithium ion intercalation and alleviate partial volume expansion. A small amount of anisotropic nanocrystals are embedded in the isotropic amorphous silicon particles, so that the whole particles have isotropic properties, and lithium ions are uniformly expanded in all directions during the embedding, so that the mechanical properties of the particles are improved, and meanwhile, the dynamic performance and the diffusion speed of the silicon particles during the embedding of the lithium ions are improved due to the addition of the amorphous region, and the cycle performance of the material is further improved.

TABLE 2

From the table, the nano silicon material with the amorphous/nano crystal composite structure prepared by the invention can obviously improve the cycle performance of the silicon cathode. As can be seen from FIGS. 12 and 13, the nano-silicon particles prepared in comparative examples 1 and 2 have poor cycle performance, mainly the prepared nano-particles have crystalline structures, and the volume change is anisotropic during the charge and discharge processesThe anisotropy causes uneven distribution of mechanical stress, easily causes crushing and pulverization of the silicon material, and makes it difficult to maintain long-term cycle stability. In addition, the nano silicon prepared in comparative example 1 can be agglomerated into micron and submicron particles due to the cold welding effect, and the volume change is large in the charging and discharging process, so that the particles are easy to break, and the complete conductive network is difficult to maintain. In the nano silicon material with the amorphous/nano-crystalline composite structure prepared in example 1, the amorphous region surrounds the nano-crystalline region, the crystal plane orientation of the nano-crystalline region is randomly distributed, the expansion direction is basically isotropic in the lithium intercalation process, and the mechanical stress generated by expansion is also dispersed in all directions, so that the problems of material cracking and pulverization caused by expansion/contraction of the silicon material due to lithium intercalation/deintercalation can be effectively alleviated. Meanwhile, the structure can inhibit Li in the charge and discharge process₁₅Si₄The generation of crystal phase can improve the cycle performance of the silicon electrode.

The above embodiments are for illustrating the technical idea of the present invention and do not represent limitations of the present invention.

Claims

1. The nanometer silicon material with amorphous/nanometer crystal structure features that in the nanometer silicon material, the amorphous area and the crystalline area are mixed to form the nanometer silicon material, and the crystal plane orientation of the crystalline area is distributed randomly.

2. The nano-silicon material containing amorphous/nano-crystalline structures as claimed in claim 1, wherein in one embodiment, the average size of the individual silicon nanoparticles is 3-100 nm.

3. The nano-silicon material containing amorphous/nano-crystalline structures as claimed in claim 1, wherein the average size of the crystalline regions is 1-20nm in one embodiment.

4. The nano-silicon material containing amorphous/nano-crystalline structure as claimed in claim 1, wherein the single silicon nano-particle is intrinsic material, or doped silicon doped with one or two of phosphorus, nitrogen, arsenic, boron, indium and aluminum.

5. The nano-silicon material containing amorphous/nano-crystalline structures as claimed in claim 1, wherein the crystalline region has an area ratio of 10-98% in one embodiment; more preferably 15 to 40%.

6. The method of preparing a nano silicon material containing an amorphous/nano crystalline structure as claimed in claim 1, comprising the steps of:

7. The method according to claim 6, wherein the bulk silicon material used in step 1 is intrinsic silicon, or doped material doped with one or two of phosphorus, nitrogen, arsenic, boron, indium and aluminum; in one embodiment, in step 1, the tool electrode may be copper, copper-based alloy, copper-based composite material, graphite, or diamond; in one embodiment, in the step 1, the cooling liquid is deionized water or aviation kerosene; in one embodiment, the pulse width of the electrical pulse generated by the pulse power source is 50ns-500 μ s; in one embodiment, the pulse width of the electrical pulse generated by the pulse power source is preferably 50-300 ns; in one embodiment, the high speed flushing pressure is preferably from 1Mpa to 20 Mpa; in one embodiment, in the step 2, a dispersion medium is added during the ball milling process to adjust the mass fraction of the slurry to be 1-30%, and the mass ratio of the grinding medium to the silicon powder is 1-3: 1, grinding media adopt grinding balls with the diameter of 0.03-0.3mm, the rotating speed of equipment is adjusted to 500-; in the step 2, the added dispersion medium is one or more of deionized water, acetone, butanone, toluene, ethanol, ethylene glycol, isopropanol, butanol cyclohexane or cyclohexanone; in the step 2, the grinding ball is made of zirconia, alumina or stainless steel; in the step 2, the drying adopts a spray dryer, a suction filter or a freeze dryer.

8. The electric spark discharge machining equipment is applied to the preparation of the nano silicon material.

9. The use of the nano silicon material of claim 1 in the preparation of negative electrode materials for lithium ion batteries.

10. A method of reducing the area fraction of crystalline silicon in a single silicon nanoparticle comprising the steps of: in the process of electric spark discharge machining, the pulse width of a smaller electric pulse is used; the pulse width is preferably 50-300 ns.