CN113130857B

CN113130857B - Nano silicon material and preparation method thereof

Info

Publication number: CN113130857B
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: 2023-09-12
Anticipated expiration: 2039-12-30
Also published as: CN113130857A

Abstract

The invention relates to a nano silicon material with an amorphous/nano crystal composite structure. In lithium/sodium ion battery applications, after the crystalline silicon material is first intercalated with lithium/sodium, a composite structure of an amorphous region (silicon lithium alloy) and a crystalline region (without lithium intercalation) is formed, and volume expansion and structural change are generated; after delithiation/sodium, the volume shrinkage leads to structural collapse, i.e. cracking and chalking of the silicon particles. In order to provide enough lithium/sodium intercalation space in advance and inhibit the volume change of first lithium/sodium intercalation/deintercalation, 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 an amorphous/nanocrystalline composite structure, wherein amorphous areas surround nanocrystals, crystal face orientations of the nanocrystals are randomly distributed, and the amorphous areas have controllable occupation ratio range, thus the nano silicon material belongs to isotropic materials. In lithium and sodium ion battery applications, the structure can effectively alleviate the problems of material cracking and pulverization caused by expansion/shrinkage of the silicon material due to lithium/sodium intercalation/deintercalation, 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 rapid development of new energy pure electric vehicles, plug-in hybrid electric vehicles and electric tools, higher requirements are being put on the energy density, safety and cycle stability of lithium ion batteries. Graphite as lithium ion battery commercial anode material (theoretical specific capacity is 372 mAh g) ^-1 ) The market demand for high energy density large batteries has not been met. The silicon-based anode material has higher theoretical specific capacity 4200 mAh g ^-1 Lower charge-discharge plateau (with graphitePotential platform approach), green environmental protection, high safety and the like, is considered as the lithium ion battery cathode material with the most lithium intercalation. Unlike the principle that a layered structure of graphite provides lithium ion intercalation space, crystalline silicon is a covalent tetrahedral structure, an alloy compound is formed with lithium ions in the lithium intercalation process, along with the increase of the lithium intercalation amount, crystalline silicon is gradually converted into amorphous silicon lithium alloy and is accompanied by the severe expansion of the volume (up to 300%), in the lithium removal process, the amorphous silicon lithium alloy is gradually converted into amorphous silicon particles from outside to inside, the volume is severely contracted, the volume is severely changed (more than 300 percent) caused by the repeated lithium removal/intercalation process, the mechanical damage/pulverization of the particles is caused, the solid electrolyte interface film (SEI film) is always in the dynamic change of destruction-reconstruction, electrolyte is continuously consumed, further, the external conductive environment of the material decays, the electrode polarization is aggravated, the specific capacity is reduced, the coulomb efficiency is reduced and the like, the circulation performance and the multiplying power performance of the amorphous silicon lithium alloy are seriously influenced, and the service life of a battery is reduced.

Aiming at the problem of silicon volume expansion, the silicon volume expansion is mainly restrained by reducing the size of silicon particles, such as preparing silicon nano particles, silicon nano wires, silicon nano tubes and porous nano silicon, so that the electrochemical cycling stability of the silicon-based anode material can be improved. However, the above material synthesis methods are mainly chemical vapor deposition, laser ablation, electron beam evaporation, magnetron sputtering and metal assisted chemical etching, but these preparation methods have extremely high requirements on equipment, severe process conditions, mainly use silane or silicon tetrachloride as silicon source, high cost and difficult mass production. Most of the nano materials prepared by the method are crystalline nano silicon, have anisotropy in volume expansion in charge and discharge, produce uneven distribution of mechanical stress, easily cause cracking and crushing of the materials, and are difficult to maintain long-term cycle performance.

Disclosure of Invention

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

The method is characterized in that micron and submicron silicon materials with amorphous/nanocrystalline composite structures can be prepared by utilizing a spark discharge machining process, bulk silicon materials are used as workpiece electrodes, pulse current is applied between the workpiece electrodes and tool electrodes, the bulk silicon materials can be melted or gasified in local very small areas, the melted or gasified materials are rapidly condensed under the action of cooling liquid, the internal structures are reconstructed into micron and submicron silicon materials with amorphous/nanocrystalline composite structures, the condensed silicon particles have extremely loose structures, the method is particularly suitable for ball milling, cold welding effect generated among particles in the ball milling process can be avoided, on one hand, the ball milling efficiency can be greatly improved, on the other hand, nano materials with amorphous/nanocrystalline composite structures can be obtained, and the nano silicon particles are prepared by independently adopting a high-energy ball milling process, do not have the structure, and micron and submicron particles agglomerated by nano silicon are obtained due to cold welding effect generated among particles in the ball milling process.

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

a nano silicon material containing amorphous/nano crystal structure is characterized in that in single silicon nano particles of the nano silicon material, amorphous areas wrap crystalline areas and are mixed with each other, and crystal plane orientations of the crystalline areas are randomly distributed.

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

In one embodiment, the crystalline regions have an average size of 1-20nm.

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 region has an area ratio of 10-98%; more preferably 15-40%.

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

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

step 1, taking a bulk silicon material as a workpiece electrode, respectively connecting the workpiece electrode and a tool electrode at two poles of a pulse power supply, forming continuous pulse discharge between the tool electrode and the workpiece electrode in a working solution, and utilizing high temperature generated by spark discharge to melt or gasify a local very small area of the workpiece electrode, wherein the melted or gasified material is rapidly condensed under the action of a cooling solution to obtain micron and submicron silicon particles;

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

In one embodiment, in the step 1, the bulk silicon material may be intrinsic silicon, or may be 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, graphite, or diamond.

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

In one embodiment, the pulse power source generates electrical pulses having a pulse width of 50ns-500 μs.

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

In one embodiment, the high-velocity hydraulic pressure is preferably 1Mpa to 20Mpa.

In one embodiment, in the step 2, a dispersion medium is added in 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 grinding balls with the diameter of 0.03-0.3mm, the rotating speed of equipment is regulated to 500-2500r/min, and the ball milling time is 5-20h.

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

In the step 2, the grinding balls are made of zirconia, alumina or stainless steel.

In the step 2, a spray dryer, a suction filter or a freeze dryer is adopted for drying.

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

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

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

a method of reducing the area ratio of crystalline silicon to individual silicon nanoparticles comprising the steps of:

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

Advantageous effects

The method of adopting a spark discharge (spark discharge) and bead-milling (beads-milling) combined process can radically inhibit the volume change of silicon by regulating the internal structure of silicon particles. The single crystal silicon ingot is prepared into micro-nano silicon particles by spark discharge, and meanwhile, crystal grains are thinned to nano size and the crystal orientation is disordered, so that the difference caused by crystal anisotropism is reduced; the bead mill can further refine the crystal grains to a plurality of nanometers and generate more amorphous structures, and finally, the amorphous composite structure surrounding the nanocrystalline is formed, and has isotropy and good mechanical properties, and the mechanical stress generated by the expansion of silicon particles can be reduced in the lithium intercalation process, so that the cracking and pulverization of the particles are effectively relieved, and the cycle performance of the particle is improved. The particle size can be controlled and the nanocrystalline size can be obtained through spark discharge and the adjustment of the bead grinding parametersAnd nano silicon particles with controllable amorphous proportion. Under the conditions of small pulse width spark discharge parameter, 0.1mm zirconia beads and grinding for 12hd, 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 1804.7 mAh g of the material applied to the negative electrode of the lithium ion battery is improved ^-1 The initial effect is 62.97%, and after 100 circles of circulation, the capacity retention rate is still up to 60.64%.

Drawings

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

FIG. 2 is a graph showing the 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 in 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) image of the silicon particulate material in example 1.

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

fig.7 is a graph showing the first charge and discharge performance of the silicon particulate material of example 1 in preparing a lithium battery;

fig. 8 is a Scanning Electron Microscope (SEM) picture (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 the silicon particulate material of comparative example 1.

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

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

Fig. 13 shows the first charge/discharge performance 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 region surrounds a nano crystal region, and crystal plane orientations of the nano crystal region are randomly distributed, and the nano silicon material belongs to isotropic materials. The structure can effectively alleviate the problems of material breakage and pulverization caused by expansion/contraction of the silicon material due to lithium intercalation/deintercalation. Meanwhile, the generation of Li15Si4 crystal phase can be restrained in the charging and discharging process, so that the cycle performance of the silicon electrode can be improved. The amorphous area and the nanocrystalline area of the nano silicon material are smaller in units and are randomly mixed, so that the uniformity, the dispersity and the crystal face orientation randomness are good, and the consistency among batches is good.

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 fluid, 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 tool electrode is controlled to feed by a servo system, meanwhile, working fluid is communicated in the tool electrode, and the working fluid is filtered out of silicon particles through a filter and is recycled through a pump. The formation process of the silicon particles: an electric field is formed immediately after a voltage is applied between the positive electrode material and the negative electrode material, as the gap between the positive electrode material and the negative electrode material is very small and the microscopic surface of the electrode material is rugged, the electric field is strongest in the area with the two electrodes being 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 occur and burst sounds are accompanied, and the electrode material in the processing area is melted and gasified by the high temperature generated during the process to form nano-scale particles formed by agglomeration of gasified silicon atom clusters and micron-scale particles obtained by melting. The melted and gasified silicon particles can be rapidly cooled after encountering low-temperature working fluid, and are agglomerated again in the process of releasing energy, and finally the formed micro-nano particles can be carried out by the working fluid in the through holes.

Silicon is a semiconductor material, the spark discharge requires that the electrode has certain conductivity, and in the following examples, boron doped P-type monocrystalline silicon material (resistivity 0.01 Ω. cm) is used as a workpiece electrode and a tool electrode, the workpiece electrode is a rectangular parallelepiped with a thickness of 20mm, the tool electrode is a rectangular parallelepiped with a cross section of 5mm x 5mm square, the middle is a rectangular parallelepiped with a through hole with a diameter of 2mm, and the working fluid is insulating deionized water (resistivity 10M Ω. cm).

During bead milling, the system contains two parts, one part being external circulating cold water, providing condensate below 10 ℃ for cooling of the milling zone and slurry tank. Another part is material crushing, including main motor, separating motor, separator, stirring motor, zirconia rod pin, zirconia beads and pump. Wherein, a part of the zirconia rod pin is driven by a main motor to drive an abrasive (beads) to do high-speed motion, and the abrasive which moves at high speed impacts the broken material; the other part of zirconia rod pin is fixed on the inner cavity wall, 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 separator is driven by the separation electrode to rotate at high speed, the grinding materials are thrown out, and the grinding aid dispersed with silicon particles can smoothly pass through the separator to carry out circulating grinding with the help of the pump.

In the following examples, the grinding aid used was absolute ethyl alcohol, the amounts of micro-nano silicon particles and grinding aid were 200g and 1800g, respectively, in a mass ratio of 1:9, and sedimentation was prevented by continuous stirring by a stirring motor. The effective volume of the bead grinding equipment is 0.7L, 2kg of zirconia abrasive (stacking density is 3.5 kg/L) is placed in the bead grinding equipment, the filling rate of zirconium balls is about 81.6%, the rotating speed of a main motor reaches 2300r/min, and the excircle linear speed of a driven zirconia rod pin reaches 13.5m/s.

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

the monocrystalline silicon ingot prepares small-sized particles through spark discharge, and refines grains, 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 atomic clusters in the central area of the discharge channel in the area with the temperature exceeding the gasification temperature. At the same time, in the region of lower temperature but above the melting temperature of the silicon material, the silicon material will melt to form submicron and micron sized particles. After gasification and melting, the silicon particles are in gas and liquid states, the internal structure is changed from a single crystal structure to an amorphous structure, internal particles (silicon atoms) are irregularly arranged, the distance between the particles is not equal to the balance distance, and the silicon particles have higher potential energy and are in the following conditionDuring condensation, the transition to the least stable crystal structure with the smallest internal energy occurs and the recrystallization takes place in a short time (fig. 1). At the same time, micro-nano silicon particles have very high surface energy, and agglomeration is needed to further release energy. In the whole process, the discharge channel maintaining time greatly affects the size and the internal structure of the micro-nano silicon particles, because the discharge channel maintaining time determines the duration of the high temperature. The high temperature is continued for a long period of time, which first causes the molten zone to expand and the size of the molten particles formed to be large. And secondly, the cooling speed is increased, the supercooling degree is increased, the nucleation rate can be obviously increased, the higher the nucleation rate means that the more crystal grains are, the smaller the crystal grain size is, and the higher the degree of disorder of the crystal orientation is, so that the difference caused by the crystal anisotropism is reduced. The high temperature for a long time can lead to slow cooling speed of the formed particles, thereby increasing the time of the recrystallization process, leading to small number of grains in the particles and large grain size. Finally, because agglomeration occurs in the whole process, a plurality of formed particles form agglomeration under gasification and fusion states due to long-time high temperature, covalent bonds are formed among the particles, the agglomeration belongs to hard agglomeration, the particles are tightly combined, and the particles are not easy to redisperse. And the agglomeration formed at low temperature is soft agglomeration and is easy to disperse. The duration of the single discharge channel is related to the pulse width of the power supply, and the smaller the pulse width is, the shorter the duration of the current is, and the shorter the discharge time is. The experiment simplifies the comparison parameters, 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 _B The time of the small pulse width (small pulse duration) is 200ns, and the obtained silicon particles correspond to Si _S 。

And filtering and collecting the micro-nano particles obtained by spark discharge through a filter to obtain micro-nano silicon particle slurry, removing an oxide layer by using a 1 wt% hydrofluoric acid solution, cleaning by using deionized water, centrifuging, and vacuum drying at 100 ℃ to obtain the 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 spark discharge controls the particle size and internal structure by internal energy, and the bead mill controls the particle size by mechanical energyAnd an internal structure. The magnitude of the mechanical energy of the zirconium ball directly influences the magnitude of shearing force and extrusion force generated during collision, thereby influencing the crushing effect of particles. During repeated collisions, the forces experienced by the silicon particles create stresses within the particles, and when the stresses exceed the limits that the particles can withstand, the particles fracture into a plurality of particles. 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 reduced, the potential energy is increased, the atomic bonds are broken to generate holes or lattice offset, the original unit 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 more collisions, the smaller the grain size, the higher the proportion of amorphous structure and the effect is gradually transferred from the particle surface to the particle interior, gradually forming amorphous/nanocrystalline composite structured nano-silicon particles. Both spark discharge and bead grinding cause defects to form inside the silicon particles, and bead grinding causes more defects because the bead grinding does not have a recrystallization process. The repeated collision in the bead grinding process causes 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 in the particles, and the silicon particles with high amorphization degree have larger volume than the crystalline silicon particles due to the large number of defects, so that the effect of pre-expansion is achieved, and the expansion degree during lithium ion intercalation is weakened. Meanwhile, the amorphous structure is isotropic, the crystal structure is anisotropic, and the silicon particles with low crystal proportion and amorphous/nanocrystalline composite structure also have isotropic property, so that the mechanical property of the silicon particles during lithium ion intercalation and deintercalation is improved. The state of micro-nano silicon particles before ball milling can influence the result after ball milling, so that the same ball milling process is adopted for obtaining the micro-nano silicon particles Si with different spark discharge parameters _B And Si (Si) _S Performing bead grinding for 8h to respectively obtain nano silicon particles Si _B+M Si (Si) _S+M . The time of the bead mill can directly influence the amorphization degree, so that the micro-nano silicon particles Si obtained by the same spark discharge parameter _B Bead milling for different times is carried out on Si _B Performing bead grinding for 12h to obtain Si _S+MM 。

Symbol definition:

Si _B : the time of the large pulse width (big pulse duration) was 200 μs, resulting in silicon particles;

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

Si _B+M : micro-nano silicon particles Si _B Performing bead grinding for 8 hours to obtain nano silicon particles;

Si _S+M : micro-nano silicon particles Si _S Performing bead grinding for 8 hours to obtain nano silicon particles;

Si _{S+M M} : micro-nano silicon particles Si _B Performing bead grinding for 12 hours to obtain nano silicon particles;

example 1

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

The pulse width of the discharge pulse generated by the pulse power supply is 200 mu s and 200ns respectively, the duty ratio is 1:4, rectangular pulse voltage with open-circuit voltage of 160V is applied between the workpiece electrode and the tool electrode, an ionization and breakdown insulating working medium forms a plasma discharge channel, and the generated high-temperature melting and gasification workpiece electrode is condensed to obtain the micron and submicron silicon material with an amorphous/nanocrystalline composite structure. Filtering by a centrifugal machine to obtain silicon particles, removing an oxide layer by using a 1 wt% hydrofluoric acid solution, cleaning by using deionized water, centrifuging, and vacuum drying at 100 ℃ to obtain the micro-nano silicon powder. The obtained particles are Si respectively _B And Si (Si) _S 。

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

Si is mixed with _B And Si (Si) _S Ball milling for 8h respectively to obtain Si _B+M And Si (Si) _S+M The method comprises the steps of carrying out a first treatment on the surface of the And, si is _S Ball milling for 12h to obtain Si _{S+M M} 。

Comparative example 1

Preparation of nano silicon particles by direct ball milling method

Step 1, coarsely grinding, weighing commercial silicon powder (with the grain size of about 10 um) with a certain mass, pouring the commercial silicon powder into a stirring barrel, adding ethanol to adjust the solid content of slurry to 10%, adjusting the rotating speed to 500r/min, and stirring for 4 hours to uniformly disperse the commercial silicon powder. Then transferring the mixture into a planetary ball mill, wherein the mass ratio of grinding media to silicon powder is 2:1, the grinding medium adopts zirconia grinding balls with the grain diameter of 5mm, the ball milling time is 10 hours, the rotating speed is 800r/min, and ethanol medium is required to be 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.

Step 2, finely grinding, namely transferring the obtained coarse grinding silicon slurry into a horizontal sand mill, wherein the mass ratio of grinding media to silicon powder is 2:1, the grinding media adopts zirconia balls with the grain diameter of 0.1mm, the ball milling time is 10 hours, the rotating speed is 1000r/min, and ethanol media are required to be continuously added in the ball milling process to keep the solid content of the silicon slurry to be 10 percent. Finally, the nano silicon particles with the particle size of 160nm are obtained.

Comparative example 2

Preparation of crystalline nano silicon particles

100sccm (standard cubic centimeter per minute; 1sccm = 1cm3/min of gas, 0 and atmospheric pressure) of SiH 4/hydrogen mixture (mixture 1) and a mixture of argon and hydrogen of 10000sccm (mixture 2) were introduced into a microwave reactor through 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 greater than the reactor. The pressure in this space and in the reactor was adjusted to 200mbar. Separating the powder product from the gas substance in a downstream filtering device to obtain crystalline nanometer silicon powder.

Characterization of particle size and SEM

Fig. 2 shows the particle size distribution of 5 particles prepared in the above example, with logarithmic axes on the abscissa. FIG.3 is an SEM photograph of the 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 _B And Si (Si) _S The particle size distribution curves of (a) can be obtained that the average particle sizes are 4060nm and 390nm, respectively, si _B The particles are larger and the distribution is wider. As can also be seen from FIG.3, si _B The overall grain size of (C) is greater than that of Si _S Is a particle size of (3). Si (Si) _B Has obvious double peaks on the grain size distribution curve, the micron-sized grains are obviously more than submicron-sized grains, and Si _S Mainly submicron particles, which are more obtained by gasifying and agglomerating silicon materials. Indicating that the large pulse width lengthens the duration of high temperature, which leads to the expansion of the melting area in the discharge channel, the particle size of the formed melting particles becomes large and the quantity ratio is high, and the two micro-nano particles are milled for 8 hours under the same bead milling parameters to obtain Si _B+M And Si (Si) _S+M ，Si _S+M Has an average particle diameter of 99nm, which is superior to Si _B+M 108nm, si _S Specific Si _B It is easier to grind to the nanometer scale. This is due to Si _B On the other hand due to the longer duration of Si at elevated temperatures _B More hard agglomerates exist in the process, the raw materials are large in size, the binding force is strong, more energy is needed for refining particles, and more bead grinding time is needed. For Si _S Further prolonging the bead grinding time to 12h to obtain Si _S+MM The average particle diameter is 91nm, and Si _B+M The particle size was reduced somewhat but slowly. It can also be seen from 3 that the spherical silicon particles formed after spark discharge break continuously after repeated collisions during bead milling to form plate-like particles, which are much smaller than 100nm in thickness dimension, possibly only a few to tens of nanometers.

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

XRD characterization

In fig. 4, the XRD patterns of the individual silicon particles are shown. Wherein (a) 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 _S The apparent silicon cubic phase (JCPLS. Card No. 01-0787) is seen in XRD diffraction peaks of (1), which are mainly represented by (111) 28.4 °, (220) 47.3 °, (311) 56.1 °, (400) 69.1 °, (331) 76.4 °, (442) 88 °.

The XRD patterns of the silicon nanomaterial prepared in comparative example 1 and comparative example 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 respectively represent Si _B 、Si _S 、Si _B+M 、Si _S+M 、 Si _{S+M M} . The electron diffraction of the selected area is numbered 1, the transmission electron microscope image is numbered 2, and the partial enlarged images in the transmission electron microscope images are numbered 3 and 4, so that crystalline and amorphous areas are displayed. As can be seen from the figures:

Si _B diffraction peak intensity ratio Si of (C) _S Is narrower in peak width, indicating Si _B The crystallinity of (2) is high and the grain size is larger. From Si _B And Si (Si) _S As can be seen from the selected electron diffraction pattern of Si _B The single crystal character of (c) is more pronounced, also indicating that its grain size is larger. Si is calculated by the Shelle formula _B And Si (Si) _S The average grain size of (a) was 68.92nm and 26.47nm, respectively (Table 1). From the comparison of (a 3) (a 4) and (b 3) (b 4), it can be verified that Si _S Grain size of (C) is larger than that of Si _B Smaller, si of _B The average crystal area ratio of the two parts (a 3) (a 4) is 97.25%, which is slightly higher than Si _S 94.5% for FIG.7 (b 3) (b 4), but both had a relatively high degree of crystallization exceeding 90% (Table 1). As analyzed above, the small pulse width and the short high temperature time quicken the cooling speed of the formed particles, improve the nucleation rate during recrystallization, and have more formed grains, smaller size and high degree of disorder of the crystal orientation.

TABLE 1

The spark discharge refines the grains to the nanometer level in advance, thereby reducing the difficulty of bead grinding and reducing energy consumption. The micro-nano silicon particles obtained by the method 4 are subjected to bead grinding, the diffraction peak intensity is weakened, and the peak width is widened, because the silicon particles generate microscopic strain in the repeated collision process of the abrasive, so that lattice distortion occurs on the surface, crystal grains are thinned, the amorphous amount is increased, and the diffraction peak width is shown on an XRD pattern. As can be seen from a comparison of (d) and (e) in FIG. 4, si _B+M The peak intensity is higher than Si _S+M But Si (Si) _S+M The wider the peak width of (c) indicates that the smaller the grain size of the bead mill feedstock, the smaller the grain size of the bead mill feedstock. Comparing (e) with (f) in FIG. 4, si with the increase of the polishing time _S+MM Further decrease in diffraction peak intensity and further increase in peak width, indicating that increasing the grinding time can further refine the crystal grains and increase the amorphous amount. Si is calculated by the Shelle formula _B+M ，Si _S+M And Si (Si) _S+MM The average grain size of (a) was 11.73nm and 6.18nm and 4.23nm (Table 1). From the comparison of (c 3) (c 4), (d 3) (d 4) and (e 3) (e 4) in FIG. 5, si can be verified _B+M ，Si _S+M And Si (Si) _S+MM The crystal grain size of (c) is gradually reduced, and the crystallization degree is also reduced, so that a structure in which nano crystals are embedded in an amorphous structure is formed inside silicon particles, which is consistent with the structure in fig. 1. Si (Si) _B+M The average crystal area ratio in (c 3) (c 4) in FIG. 5 was 63.35% for the two selected portions, si _S+M Corresponding to (d 3) (d 4) in FIG. 5 is 37.25%, si _S+MM Corresponding to (e 3) (e 4) in fig. 5 is 15.45% (table 1). FIG. 5Selected electron diffraction of (c 2), (d 2) and (e 2) in (c 2) can also be derived from Si _B+M ，Si _S+M To Si (Si) _S+MM The degree of amorphization increases in sequence. As analyzed by FIG.3 and 2.2 Process schemes, the stress in the particles increases gradually, internal defects increase continuously, and the outer layer diffuses toward the inner layer, resulting in grain splitting, increased amorphous structure, and higher amorphous ratio for longer grinding times.

Further, transmission electron micrographs of the silicon nanomaterial prepared in comparative example 1 and comparative example 2 are shown in fig. 10 and 11, respectively. It can be seen that the material of comparative example 1 is directly mixed from crystalline particles and amorphous particles; whereas the silicon material in comparative example 2 consisted of a single crystalline region.

Electrochemical Properties

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

Fig. 6 is a silicon particulate material (Si _B , Si _S , Si _B+M , Si _S+M and Si _S+MM ) In the cyclic charge-discharge performance of lithium batteries, the electrode is charged/discharged 4 times at 0.05C rate, then is charged/discharged 96 times at 0.1C rate, and the voltage is cycled between 0.01-1V. Fig.7 is a first charge and discharge performance of a silicon particulate material in preparing a lithium battery; as can be seen from FIG. 6, si with the largest particles _B Has the highest specific capacity of first charge and discharge of 3918.1 mAh g respectively ^-1 ，3494.1 mAh g ^-1 Its first effect was also highest, reaching 89.18% (table 2). Si (Si) _S The first discharge/charge specific capacity and the first effect are 3698.9 mAh g respectively ^-1 ，3033.0 mAh g ^-1 82.0% (Table 2). Si (Si) _B+M The first discharge/charge specific capacity and the first effect are 3067.7 mAh g respectively ^-1 ，2081.3 mAh g ^-1 67.84% (Table 2)). Si (Si) _S+M The first discharge/charge specific capacity and the first effect are 3023.6 mAh g respectively ^-1 ，1974.2 mAh g ^-1 65.29% (Table 2)). Si (Si) _S+MM The first discharge/charge specific capacity and the first effect are 2866.1 mAh g respectively ^-1 ，1804.7 mAh g ^-1 62.97% (Table 2)). From the above results, it can be seen that, as the particle size of the silicon particles is reduced, the specific capacity of charge and discharge for the first time and the first effect are both obviously reduced, and the nanoparticle obtained by spark discharge and bead grinding is most obviously reduced, mainly because the specific surface area of the silicon particles is increased after refinement, more SEI films are formed during the first circulation, more embedded lithium ions are consumed, and the first effect is reduced. As can be seen from FIG. 6, micro-nano particles Si _B And Si (Si) _S The cyclical reversible capacity decay of (c) is very rapid, but Si _S Is slightly better than Si in cycle performance _B This is mainly because there are a large number of submicron and micron silicon particles produced by spark discharge, and the silicon volume expands/contracts drastically during the lithium intercalation/deintercalation process, so that the silicon particles are separated from contact with the conductive agent and the current collector, gradually lose the conductive environment, form a large area of "dead volume", and thus cause rapid capacity decay, and form more particles with smaller particle diameters under small pulse widths, so that the phenomenon will be better. Nanoscale particle Si _B+M , Si _S+M and Si _S+MM Is obviously superior to micro-nano Si in cycle performance _B And Si (Si) _S The reversible capacity after 100 rounds was 661.7, 890.4, 1094.4 mAh g-1, and the capacity retention rate was 31.79%,45.10%,60.64% (Table 2), respectively, which were increased in order. The silicon particle sizes of the three electrodes were sequentially reduced, and the average grain sizes and the average crystal area ratios thereof were sequentially reduced (table 1). This demonstrates that having nano-silicon particles with small grain size and high degree of amorphization (or with a high amorphous ratio) can effectively alleviate the volume expansion of the silicon material during charge and discharge. As analyzed above, the nano silicon particles with amorphous/nano crystal composite structure have larger volume than the mass crystal particles, so that the interior has a large number of defects, can provide some space for lithium ion intercalation, and relieves part of the defectsAnd (5) expanding the volume. A small amount of anisotropic nanocrystals are embedded in the isotropic amorphous material, so that the whole particle has isotropic property, and the particles uniformly expand in all directions during lithium ion embedding, thereby improving the mechanical properties of the particles.

TABLE 2

From the table above, it can be seen that the nano silicon material with amorphous/nano crystal composite structure prepared by the invention can significantly improve the cycle performance of the silicon negative electrode. As can be seen from fig. 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 structure, the volume change is anisotropic in the charge and discharge process, the generated mechanical stress is unevenly distributed, the silicon material is easily crushed and pulverized, and long-term cycle stability is difficult to maintain. And the nano silicon prepared in comparative example 1 is agglomerated into micron and submicron particles due to cold welding, the volume change is large in the charging and discharging process, the particles are easy to break, and the complete conductive network is difficult to maintain. The nano silicon material with amorphous/nano crystal composite structure prepared in example 1, because the amorphous region surrounds the nano crystal region, the crystal plane orientation of the nano crystal region is randomly distributed, the expansion direction is basically isotropic during the lithium intercalation process, and the mechanical stress generated by expansion is dispersed in all directions, the problems of material cracking and pulverization caused by the expansion/contraction of the silicon material due to the lithium intercalation/deintercalation can be effectively alleviated. Meanwhile, the structure can also inhibit Li in the charge and discharge process ₁₅ Si ₄ The generation of the crystal phase can thereby improve the cycle performance of the silicon electrode.

The above embodiments are illustrative of the technical concept of the present invention and are not meant to limit the present invention.

Claims

1. The application of the nano silicon material containing the amorphous nano crystal structure in the preparation of the lithium ion battery is characterized in that the negative electrode material of the lithium ion battery adopts the nano silicon material containing the amorphous nano crystal structure, single silicon nano particles of the nano silicon material are formed by mutually mixing amorphous areas wrapping crystalline areas, and crystal face orientations of the crystalline areas are randomly distributed; the average size of the single silicon nano particles is 3-100nm, and the average size of the crystalline region is 1-20nm;

the area ratio of the crystalline region is 15-40%;

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

step 1, taking a bulk silicon material as a workpiece electrode, respectively connecting the workpiece electrode and a tool electrode at two poles of a pulse power supply, forming continuous pulse discharge between the tool electrode and the workpiece electrode in a working solution, and utilizing high temperature generated by spark discharge to melt or gasify a local very small area of the workpiece electrode, wherein the melted or gasified material is rapidly condensed under the action of the working solution to obtain micron and submicron silicon particles; the pulse width of the electric pulse generated by the pulse power supply is 200-300ns;

step 2, transferring the micron and submicron silicon particles obtained in the step 1 into high-energy ball milling equipment, performing ball milling treatment, and finally drying to obtain the nano silicon material containing the amorphous nano crystal structure;

in the step 2, the mass percentage of the dispersion medium of the adjustment slurry is 1-30% in the ball milling process, and the mass ratio of the grinding medium to the silicon particles in the ball milling process is 1-3:1, grinding media adopts grinding balls with the diameter of 0.03-0.3mm, the rotating speed of equipment is regulated to 500-2500r/min, and the ball milling time is 5-20h;

the lithium ion battery was charged/discharged 4 times at a 0.05C rate, and then after 96 cycles of charging/discharging at a 0.1C rate, the capacity retention rate was 60.64%.

2. The method according to claim 1, wherein in step 1, the bulk silicon material is intrinsic silicon or a doped material doped with one or two of phosphorus, nitrogen, arsenic, boron, indium and aluminum.

3. The use according to claim 1, wherein in step 1 the tool electrode is copper, copper matrix composite, graphite or diamond.

4. The use according to claim 1, wherein in step 1, the working fluid is deionized water or aviation kerosene, and the high-speed flushing pressure of the working fluid is 1MPa-20MPa.

5. The method according to claim 1, wherein the dispersion medium added in step 2 is one or more of deionized water, acetone, butanone, toluene, ethanol, ethylene glycol, isopropanol, butanol, cyclohexane or cyclohexanone.

6. The use according to claim 1, wherein in step 2, the polishing medium is zirconia, alumina or stainless steel; the drying is performed by a spray dryer, a suction filter or a freeze dryer.