Preparation method of aluminum fluoride coated nano silicon material

Abstract

The aluminum fluoride coated nano silicon material is prepared from nano silicon powder, aluminum nitrate nonahydrate, ammonium fluoride, absolute ethyl alcohol and deionized water, and an aluminum fluoride coating layer and a nano silicon negative electrode material are compounded to form an aluminum fluoride coated nano silicon material structure. The aluminum fluoride coating layer on the surface can be converted into a fast ion conductor in the battery circulation process, so that the lithium ion diffusion rate is increased, and migration of lithium ions between the nanometer silicon negative electrode and the electrolyte is promoted. Under the coating protection effect of the coating layer, the volume expansion of the nano-silicon material is improved, the voltage attenuation speed is slowed down, the cycling stability is greatly improved, the cycling performance of the nano-silicon negative electrode material is improved, and silicon particle breakage, material pulverization and falling off from a pole piece are avoided. The coating amount of the aluminum fluoride coating layer is compared, and the optimal coating amount is given. The method has the advantages of low cost and high controllability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a preparation method of a coated nano-silicon negative electrode material. BACKGROUND

[0002] Compared with other rechargeable batteries such as nickel-cadmium batteries and nickel-hydrogen batteries, lithium ion batteries have the advantages of high energy density, wide working voltage range, good circuit performance, small environmental pollution and the like, and thus have been widely applied, especially in the field of electric vehicle power, and have achieved rapid development. With the upgrading and replacement of automobiles and electronic products, higher requirements are put forward for the charging speed, endurance time and battery life. In addition, the emergence of hybrid electric vehicles and electric vehicles puts forward higher requirements for the energy density of power supply equipment, and thus improving the performance of lithium batteries is a far-reaching thing. With the improvement of the energy density of lithium ion batteries, it is urgent to improve the process method for large-scale production of high-capacity new lithium ion battery negative electrode materials.

[0003] At present, silicon (Si) has been considered as a promising alternative anode material due to its high theoretical capacity 4200 (mAhg -1 However, in the practical application of lithium ion batteries, the silicon (Si) material has the problem of huge volume expansion during the process of lithium extraction and insertion, which leads to the collapse of the overall material structure due to the continuous expansion and contraction during charging and discharging, and thus a stable solid electrolyte interface layer (SEI) cannot be formed, resulting in rapid capacity decay during charging and discharging. Some researchers disperse negatively charged mesoporous carbon spheres and sucrose in a solvent and stir to mix and carbonize to prepare, but this method has the problem of easy oxidation of raw materials, which makes it difficult to ensure the yield of the product, and the long cycle stability of the silicon-carbon material is not obviously improved. Another method is to in-situ grow phenolic resin on the surface of silicon particles, carbonize and then etch with hydrofluoric acid (HF), but this method will cause irreversible expansion and contraction of the silicon particles, causing the surface SEI film to continuously break and generate, thereby causing the electrolyte interface layer (SEI) film to thicken, the battery internal resistance to increase and the polarization to increase, and thus mass production is also difficult. There is also a method of coating silicon dioxide with aluminum sulfate and then carbon-coating, and the coating layer structure can inhibit the volume expansion of the silicon-carbon negative electrode during the cycle process, but this method leads to poor weather resistance and poor chemical corrosion resistance. In order to overcome the above defects, surface coating is adopted to improve the cycle stability of the silicon negative electrode material. However, the thickness of the coating layer of the coating material will affect the transmission of lithium ions to the negative electrode, thereby reducing the rate performance of the material, and it is difficult to accurately control the thickness of the coating layer in large-scale production.

[0004] Therefore, although the development of nano-silicon negative electrode material has been more than ten years, but there is still no low-cost and high-controllable coating modification process can make silicon negative electrode material meet the needs of commercial production. SUMMARY

[0005] In view of the defects in the prior art, the purpose of the present application is to provide a low-cost and high-controllable preparation method of aluminum fluoride coated nano-silicon material. The process method uses an aluminum fluoride coating layer to composite with a nano-silicon negative electrode material to form an aluminum fluoride coated nano-silicon material structure. The surface aluminum fluoride coating layer will be converted into a fast ion conductor during the battery cycle process to improve the lithium ion diffusion rate, thereby promoting the migration of lithium ions between the nano-silicon negative electrode and the electrolyte. Under the protection of this coating layer, the volume expansion of the nano-silicon material is improved, the voltage decay rate is slowed down, and the cycle stability is greatly improved. However, a high coating amount of the coating layer will block the conduction of lithium ions, etc. The present application compares the coating amount of the aluminum fluoride coating layer and gives the optimal coating amount. Thanks to this special structure, the aluminum fluoride coated on the negative electrode surface can inhibit the volume expansion of the nano-silicon negative electrode during the cycle process, avoiding the rupture of silicon particles, the pulverization of the material, and the falling off from the pole piece. During the initial cycle, the AlF3 coating layer can be lithiated into a fast lithium ion conductive LiF and Li-Al-F layer, which not only can act as an intermediate separation layer between the active material and the electrolyte, reducing the occurrence of side reactions, and being more friendly to the electrolyte. Moreover, it also promotes the interface ion diffusion, thereby ensuring the smooth intercalation and deintercalation of lithium ions, and predicting the improved electrochemical performance, including cycle stability and rate performance.

[0006] The present application provides a preparation method of aluminum fluoride coated nano-silicon material. The aluminum fluoride coated nano-silicon material of the method is prepared from nano-silicon powder, aluminum nitrate nonahydrate, ammonium fluoride, anhydrous ethanol and deionized water; the method comprises the following specific steps:

[0007] S1: 5g of nano-silicon powder with a diameter of 100nm is dispersed in a beaker containing 50ml of anhydrous ethanol, and then placed in an ultrasonic cleaner for ultrasonic treatment for 30 minutes to make the nano-silicon material uniformly dispersed.

[0008] S2: 0.11g-1.1g of aluminum nitrate nonahydrate is taken to prepare an aluminum nitrate solution A with 400ml of deionized water, and reserved.

[0009] S3: 0.033g-0.33g of ammonium fluoride is taken to prepare an ammonium fluoride solution B with 50ml of deionized water, and reserved.

[0010] S4: the dispersed nano-silicon material of S1 is mixed with the solution A of S2 according to a mass ratio of 1:80, stirred for 30 minutes, and then placed in a 80℃ water bath environment for continuous stirring for 3 hours to obtain a mixed solution.

[0011] S5: S3 solution B with a mass ratio of 1:8 to S2 solution A is added dropwise to the mixed solution of S4, the rate of adding solution B is 2-4 ml / min, and the adding is controlled to be completed within 30 minutes.

[0012] S6: The mixed solution of S5 is placed in a water bath at 80℃ and continues to stir for three hours.

[0013] S7: The mixed solution after stirring of S6 is centrifuged by a centrifuge, the rotation speed is controlled at 8000r / min, and centrifugation is carried out for 1 minute, the supernatant after centrifugation is removed; the remaining solid powder is washed with deionized water, and then centrifuged by a centrifuge, the rotation speed is controlled at 12000r / min and centrifuged for 3 minutes, the supernatant after centrifugation is removed; the remaining solid powder is washed with anhydrous ethanol, and continues to be centrifuged by a centrifuge, the rotation speed is controlled at 12000r / min and centrifuged for 1 minute, and the supernatant is removed to obtain a solid powder; finally, the solid powder is placed in a blast drying oven for drying for 2 hours, and then placed in a vacuum drying oven for drying for 4 hours.

[0014] S8: The solid powder obtained after drying of S7 is poured into a grinder and ground for 10 minutes.

[0015] S9: The ground powder obtained in S8 is moved to a tube furnace for heat treatment, the tube furnace continuously introduces argon with an analytical purity of 99.9% into the reaction chamber at a rate of 100ml / min, the tube furnace is heated to 300℃ at a heating rate of 5℃ / min and kept for 3 hours, and then naturally cooled to room temperature. The solid powder is collected to obtain a negative electrode active material of lithium ion secondary battery, aluminum fluoride coated nano silicon powder (Si@AlF3) with a mass fraction of 0.5%-5%.

[0016] Compared with the prior art, the present application has the following beneficial effects:

[0017] 1. The aluminum fluoride coated on the surface of the negative electrode can inhibit the volume expansion of the nano silicon negative electrode during the cycle process, avoiding the rupture of the silicon particles, the pulverization of the material, and the falling off from the pole piece;

[0018] 2. The cycle performance of the nano silicon negative electrode material can be improved;

[0019] 3. The "natural" generation of the electrolyte SEI layer during the cycle process is avoided, thereby continuously consuming lithium ions;

[0020] 4. The aluminum fluoride coating layer on the surface will be converted into a fast ion conductor during the battery cycle process, so that the lithium ion can maintain a high passing rate, thereby promoting the migration of lithium ions between the nano silicon negative electrode and the electrolyte;

[0021] 5. Under the sheathing protection of the sheathing layer, the volume expansion of the nanosilicon material is improved, the voltage attenuation rate is slowed down, the cycle stability is greatly improved, and good rate performance is obtained;

[0022] 6. The method has low cost and high controllability;

[0023] 7. The aluminum fluoride sheathing layer on the surface can prevent the electrolyte from corroding the inside and is more friendly to the electrolyte;

[0024] 8. The existence of the sheathing layer reduces the side reaction with the electrolyte and improves the high and low temperature performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0026] Figure 1 It is a structure schematic diagram of nanosilicon composite material;

[0027] Figure 2 It is a cycle stability comparison diagram of negative electrode material with 0.5%, 1%, 3%, 5% aluminum fluoride sheathing layer;

[0028] Figure 3 It is a scanning electron microscope diagram of aluminum fluoride sheathed nanosilicon prepared in Example 1;

[0029] Among them: a is the Si face scan (Mapping) diagram of the scanning electron microscope of aluminum fluoride sheathed nanosilicon;

[0030] b is the F face scan (Mapping) diagram of the scanning electron microscope of aluminum fluoride sheathed nanosilicon;

[0031] c is the scanning electron microscope diagram of aluminum fluoride sheathed nanosilicon;

[0032] d is the Al face scan (Mapping) diagram of the scanning electron microscope of aluminum fluoride sheathed nanosilicon.

[0033] Figure 4 It is a transmission electron microscope diagram of aluminum fluoride sheathed nanosilicon;

[0034] Among them: A is the transmission electron microscope diagram of aluminum fluoride sheathed nanosilicon;

[0035] B is the local magnification diagram of the transmission electron microscope of aluminum fluoride sheathed nanosilicon.

[0036] Figure 5X-ray photoelectron spectroscopy.

[0037] wherein: a is the high resolution F1s X-ray photoelectron spectroscopy of the original sample of nanosilicon;

[0038] b is the high resolution F1s X-ray photoelectron spectroscopy of the nanosilicon coated with aluminum fluoride;

[0039] c is the high resolution Al 2p X-ray photoelectron spectroscopy of the original sample of nanosilicon;

[0040] d is the high resolution Al 2p X-ray photoelectron spectroscopy of the nanosilicon coated with aluminum fluoride. DETAILED DESCRIPTION

[0041] In order to make the person skilled in the art better understand the technical scheme of the present application, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by the person skilled in the art without creative labor should belong to the protection scope of the present application.

[0042] The present application is a preparation method of an aluminum fluoride coated nanosilicon material. The aluminum fluoride coated nanosilicon material in the method is prepared by using nanosilicon powder, aluminum nitrate nonahydrate, ammonium fluoride, anhydrous ethanol and deionized water. The method comprises the following specific steps:

[0043] S1: 5g of nanosilicon powder with a diameter of 100nm is dispersed in a beaker containing 50ml of anhydrous ethanol, and then it is placed in an ultrasonic cleaner for ultrasonic treatment for 30 minutes, so that the nanosilicon material is uniformly dispersed.

[0044] S2: 0.11g-1.1g of aluminum nitrate nonahydrate is taken and dissolved in 400ml of deionized water to prepare an aluminum nitrate solution (solution A), which is ready for use.

[0045] S3: 0.033g-0.33g of ammonium fluoride is taken and dissolved in 50ml of deionized water to prepare an ammonium fluoride solution (solution B), which is ready for use.

[0046] S4: the nanosilicon material dispersed in step S1 is mixed with solution A at a mass ratio of 1:80, and stirred for 30 minutes, and then placed in a water bath environment at 80°C for continuous stirring for 3 hours, to obtain a uniformly mixed solution.

[0047] S5: solution B with a mass ratio of 1:8 to solution A is added dropwise to the mixed solution of step S4, and the addition rate is controlled to be 2-4ml / min, and the addition is completed within 30 minutes.

[0048] S6: The mixed solution of step S5 is placed in a water bath at 80°C, and stirring is continued for three hours in the 80°C water bath environment.

[0049] S7: (1) The stirred solution of step S6 is centrifuged using a centrifuge, with the rotation speed controlled at 8000 r / min, and centrifuged for 1 minute. The supernatant after centrifugation is removed.

[0050] (2) The remaining solid powder is washed with deionized water, and then centrifuged using a centrifuge, with the rotation speed controlled at 12000 r / min and centrifuged for 3 minutes. The supernatant after centrifugation is removed.

[0051] (3) The remaining solid powder is washed with anhydrous ethanol, and then centrifuged using a centrifuge, with the rotation speed controlled at 12000 r / min and centrifuged for 1 minute. The supernatant is removed to obtain the solid powder.

[0052] (4) Finally, the solid powder is placed in a blast drying oven for drying for 2 hours, and then placed in a vacuum drying oven for drying for 4 hours.

[0053] S8: The solid powder obtained after drying in step S7 is poured into a grinder and ground for 10 minutes.

[0054] S9: The solid powder obtained in step S8 is moved into a tube furnace for heat treatment. The tube furnace continuously supplies argon gas (99.9%) into the reaction chamber at a rate of 100 ml / min, while the tube furnace is heated to 300°C at a heating rate of 5°C / min and held for 3 hours, and then naturally cooled to room temperature. The solid powder is collected, and a nano-silicon powder coated with aluminum fluoride (Si@ALF3-0.5%-5%) with a mass fraction of 0.5%-5% is obtained, which can be used as a negative electrode active material in a lithium ion secondary battery.

[0055] The following are specific embodiments of the present application:

[0056] Example 1

[0057] S1: 5 g of nano-silicon powder with a diameter of 100 nm is dispersed in a beaker containing 50 ml of anhydrous ethanol, and then placed in an ultrasonic cleaner for ultrasonic treatment for 30 minutes to uniformly disperse the nano-silicon material.

[0058] S2: 0.11 g of aluminum nitrate nonahydrate is dissolved in 400 ml of deionized water to prepare an aluminum nitrate solution (solution A), which is ready for use.

[0059] S3: 0.033 g of ammonium fluoride is dissolved in 50 ml of deionized water to prepare an ammonium fluoride solution (solution B), which is ready for use.

[0060] S4: The dispersed nanosilicon material in step S1 was mixed with solution A at a mass ratio of 1:80, stirred for 30 minutes, and then placed in an 80°C water bath environment for continued stirring for 3 hours to obtain a uniformly mixed solution.

[0061] S5: Solution B was added dropwise to the mixed solution of step S4 at a mass ratio of 1:8, with an addition rate of 2-4 ml / min, and the addition rate was controlled to be completed within 30 minutes.

[0062] S6: The mixed solution of step S5 was placed in an 80°C water bath and continued to be stirred in the 80°C water bath environment for three hours.

[0063] S7: (1) The original solution after stirring in step S6 was centrifuged using a centrifuge, with a rotation speed controlled at 8000 r / min, and centrifuged for 1 minute, and the supernatant after centrifugation was removed;

[0064] (2) The remaining solid powder was washed with deionized water and then centrifuged using a centrifuge, with a rotation speed controlled at 12000 r / min and centrifuged for 3 minutes, and the supernatant after centrifugation was removed;

[0065] (3) The remaining solid powder was washed with anhydrous ethanol and then centrifuged using a centrifuge, with a rotation speed controlled at 12000 r / min and centrifuged for 1 minute, and the supernatant was removed to obtain a solid powder;

[0066] (4) Finally, the solid powder was placed in a blast drying oven for drying for 2 hours, and then placed in a vacuum drying oven for drying for 4 hours.

[0067] S8: The solid powder obtained after drying in step S7 was poured into a grinder and ground for 10 minutes.

[0068] S9: The solid powder obtained in step S8 was moved to a tube furnace for heat treatment, the tube furnace continuously introduced argon gas (99.9%) into the reaction chamber at a rate of 100 ml / min, while the tube furnace was heated to 300°C at a heating rate of 5°C / min and held for 3 hours, and then naturally cooled to room temperature. The solid powder was collected to obtain a mass fraction of 0.5% of aluminum fluoride-coated nanosilicon powder (Si@AlF3-0.5%), which can be used as a negative electrode active material in a lithium ion secondary battery.

[0069] After obtaining the aluminum fluoride-coated nanosilicon, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) analysis were performed. The results are shown in Figure 3 and Figure 4 : Figure 3 The scanning electron micrograph of the aluminum fluoride-coated nanosilicon obtained in Example 1 shows that the Al and F elements are uniformly coated on the substrate of the nanosilicon material. Figure 4The high-resolution transmission electron microscope (HR-TEM) of the nanometer silicon material coated with aluminum fluoride obtained in Example 1 shows that the thickness of the aluminum fluoride coating is about 2 nm. Figure 3 The scanning electron microscope (SEM) image of the nanometer silicon material coated with aluminum fluoride obtained in Example 1 and the corresponding energy spectrum analysis image thereof show the distribution of the silicon (Si), aluminum (Al) and fluorine (F) elements on the surface of the nanometer silicon material. Figure 3 It can be seen that the aluminum element is uniformly distributed on the nanometer silicon material coated with aluminum fluoride obtained in Example 1. Figure 5 It can be seen that the uncoated nanometer silicon does not have F1s and Al2p peaks. In contrast, the coated nanometer silicon has F1s and Al2p peaks, indicating that the aluminum fluoride has been successfully coated on the surface of the nanometer silicon.

[0070] Example 2

[0071] S1: 5 g of nanometer silicon powder with a diameter of 100 nm was dispersed in a beaker containing 50 ml of anhydrous ethanol, and then placed in an ultrasonic cleaner for ultrasonic treatment for 30 minutes to uniformly disperse the nanometer silicon material.

[0072] S2: 0.22 g of aluminum nitrate nonahydrate was dissolved in 400 ml of deionized water to prepare an aluminum nitrate solution (solution A), which was used as a stock solution.

[0073] S3: 0.066 g of ammonium fluoride was dissolved in 50 ml of deionized water to prepare an ammonium fluoride solution (solution B), which was used as a stock solution.

[0074] S4: The nanometer silicon material dispersed in step S1 was mixed with solution A at a mass ratio of 1:80, and stirred for 30 minutes. Then, the mixture was placed in a water bath at 80°C and stirred for another 3 hours to obtain a uniformly mixed solution.

[0075] S5: Solution B was added dropwise to the mixed solution of step S4 at a mass ratio of 1:8. The addition rate was controlled at 2-4 ml / min, and the addition was completed within 30 minutes.

[0076] S6: The mixed solution of step S5 was placed in a water bath at 80°C and stirred for another 3 hours in the water bath at 80°C.

[0077] S7: (1) The original solution after stirring in step S6 was centrifuged at a speed of 8000 r / min for 1 minute, and the supernatant was removed.

[0078] (2) The remaining solid powder was washed with deionized water, and then centrifuged at a speed of 12000 r / min for 3 minutes, and the supernatant was removed.

[0079] (4) The residual solid powder is washed with anhydrous ethanol, and centrifugation is continued at a speed of 12000 r / min for 1 minute, and the supernatant is removed to obtain the solid powder;

[0080] (4) Finally, the solid powder is placed in a blast drying oven for drying for 2 hours, and then is placed in a vacuum drying oven for drying for 4 hours.

[0081] S8: The solid powder obtained after drying in step S7 is poured into a grinder and ground for 10 minutes.

[0082] S9: The solid powder obtained in step S8 is moved to a tube furnace for heat treatment, argon (99.9%) is continuously introduced into the reaction chamber at a rate of 100 ml / min, and the tube furnace is heated to 300°C at a heating rate of 5°C / min and kept for 3 hours, and then naturally cooled to room temperature. The solid powder is collected to obtain aluminum fluoride-coated nanosilicon (Si@AlF3-1%) with a mass fraction of 1%, which can be used as a negative electrode active material in a lithium ion secondary battery.

[0083] Example 3

[0084] S1: 5g of nanosilicon powder with a diameter of 100nm is dispersed in a beaker containing 50ml of anhydrous ethanol, and then is placed in an ultrasonic cleaner for ultrasonic treatment for 30 minutes to uniformly disperse the nanosilicon material.

[0085] S2: 0.66g of aluminum nitrate nonahydrate is dissolved in 400ml of deionized water to prepare an aluminum nitrate solution (solution A), which is ready for use.

[0086] S3: 0.198g of ammonium fluoride is dissolved in 50ml of deionized water to prepare an ammonium fluoride solution (solution B), which is ready for use.

[0087] S4: The dispersed nanosilicon material in step S1 is mixed with solution A at a mass ratio of 1:80, and stirred for 30 minutes, and then is placed in a water bath at 80°C for continuous stirring for 3 hours to obtain a uniformly mixed solution.

[0088] S5: Solution B with a mass ratio of 1:8 to solution A is added dropwise to the mixed solution in step S4 at an addition rate of 2-4ml / min, and the addition is completed within 30 minutes.

[0089] S6: The mixed solution in step S5 is placed in a water bath at 80°C, and is continuously stirred in the water bath at 80°C for 3 hours.

[0090] S7: (1) The original solution after stirring in step S6 is centrifuged at a speed of 8000 r / min for 1 minute, and the supernatant after centrifugation is removed;

[0091] (2) The residual solid powder is washed with deionized water, and then centrifuged at a speed of 12000 r / min for 3 minutes, and the supernatant is removed;

[0092] (4) The residual solid powder is washed with anhydrous ethanol, and then centrifuged at a speed of 12000 r / min for 1 minute, and the supernatant is removed to obtain the solid powder;

[0093] (5) Finally, the solid powder is dried in a blast drying oven for 2 hours, and then dried in a vacuum drying oven for 4 hours.

[0094] S8: The solid powder obtained after drying in step S7 is poured into a grinder and ground for 10 minutes.

[0095] S9: The solid powder obtained in step S8 is moved into a tube furnace for heat treatment, the tube furnace continuously introduces argon (99.9%) into the reaction chamber at a rate of 100 ml / min, while the tube furnace is heated to 300°C at a heating rate of 5°C / min and kept for 3 hours, and then naturally cooled to room temperature. The solid powder is collected to obtain aluminum fluoride-coated nanosilicon (Si@AlF3-3%) with a mass fraction of 3%, which can be used as a negative electrode active material in a lithium ion secondary battery.

[0096] Example 4

[0097] S1: 5g of nanosilicon powder with a diameter of 100nm is dispersed in a beaker containing 50ml of anhydrous ethanol, and then placed in an ultrasonic cleaner for ultrasonic treatment for 30 minutes to uniformly disperse the nanosilicon material.

[0098] S2: 1.1g of aluminum nitrate nonahydrate is dissolved in 400ml of deionized water to prepare an aluminum nitrate solution (solution A), which is ready for use.

[0099] S3: 0.33g of ammonium fluoride is dissolved in 50ml of deionized water to prepare an ammonium fluoride solution (solution B), which is ready for use.

[0100] S4: The dispersed nanosilicon material in step S1 is mixed with solution A at a mass ratio of 1:80, stirred for 30 minutes, and then placed in a 80°C water bath environment for continuous stirring for 3 hours to obtain a uniformly mixed solution.

[0101] S5: Solution B with a mass ratio of 1:8 to solution A is added dropwise to the mixed solution of step S4 at a rate of 2-4ml / min, and the addition rate is controlled to be completed within 30 minutes.

[0102] S6: The mixed solution of step S5 is placed in a 80°C water bath, and continuously stirred in the 80°C water bath environment for three hours.

[0103] S7: (1) Centrifuge the original solution after stirring in step S6 at a speed of 8000 r / min for 1 minute, and remove the supernatant after centrifugation;

[0104] (2) Wash the remaining solid powder with deionized water, centrifuge it again, control the speed at 12000r / min for 3 minutes, and remove the supernatant after centrifugation;

[0105] (3) Wash the remaining solid powder with anhydrous ethanol, continue centrifuging with a centrifuge, control the speed at 12000 r / min and centrifuge for 1 minute, remove the supernatant to obtain a solid powder;

[0106] (4) Finally, the solid powder was placed in a forced air drying oven for 2 hours, and then placed in a vacuum drying oven for 4 hours.

[0107] S8: Pour the solid powder obtained after drying in step S7 into a grinder and grind for 10 minutes.

[0108] S9: The solid powder obtained in step S8 was transferred to a tube furnace for heat treatment. Argon (99.9%) was continuously introduced into the reaction chamber of the tube furnace at a rate of 100 ml / min. The temperature of the tube furnace was simultaneously increased to 300°C at a rate of 5°C / min and maintained at this temperature for 3 hours, followed by natural cooling to room temperature. The solid powder was collected to obtain 5% by mass aluminum fluoride-coated nanosilicon (Si@AlF3-5%), which can be used as a negative electrode active material in lithium-ion secondary batteries.

[0109] The powder solid obtained by the embodiment of the present invention is sieved to obtain qualified aluminum fluoride coated nano silicon powder, which can be further used to manufacture negative electrode sheets. Furthermore, various high-performance batteries can be manufactured using the electrode sheets as negative electrode materials. The performance of the negative electrode of aluminum fluoride coated nano silicon can be further tested by performing electrochemical performance tests such as charging and discharging on the battery. The test results are the first cycle charge and discharge curves of original silicon (Si), Si@AlF3-0.5%, Si@AlF3-1%, Si@AlF3-3%, and Si@AlF3-5% at a current density of 100mA / g. The nano-silicon sample achieved an initial charge capacity of 3512.5 mAh / g and an initial coulombic efficiency of 92.3%. The initial charge capacity and coulombic efficiency of the Si@AlF3-0.5%, Si@AlF3-1%, Si@AlF3-3%, and Si@AlF3-5% samples were 3582.2 mAh / g, 3554.3 mAh / g, 3771.6 mAh / g, and 3512.5 mAh / g, respectively, and were 92.02%, 92.09%, 90.84%, and 91.61%. The initial charge-discharge results showed that the Si@AlF3 sample exhibited a decrease compared to the control sample (see Table 1).

[0110] Table 1

[0111]

[0112] As Figure 2 shown in the figure, it is the performance curve of original nanosilicon, Si@AlF3-0.5%, Si@AlF3-1%, Si@AlF3-3%, Si@AlF3-5% after 60 cycles of charge-discharge test at 1000 mA / g current density, wherein each battery is pre-activated at a current density of 100 mA / g for three times of charge-discharge cycle. The results show that the discharge specific capacity of original nanosilicon, Si@AlF3-0.5%, Si@AlF3-1%, Si@AlF3-3%, Si@AlF3-5% is 2066.5 mAh / g, 2176.7 mAh / g, 2181 mAh / g, 2370.4 mAh / g, 2185.7 mAh / g respectively after 60 cycles, and the corresponding capacity retention rate is 58.83%, 60.7%, 61.3%, 62.84%, 62.22% respectively, wherein the coated material has higher capacity retention rate, and Si@AlF3-1% exhibits the best cycle stability. The improvement of cycle stability of coated silicon negative electrode is attributed to the fact that the aluminum fluoride coating layer promotes the transmission of lithium ions to the silicon negative electrode during the cycle process, inhibits the decomposition of electrolyte, and helps to form a stable solid electrolyte interphase.

[0113] The above description is only an embodiment of the present application, and does not limit the patent scope of the present application, and any equivalent transformation or direct or indirect application in the related technical field based on the content of the specification and drawings of the present application is also included in the patent protection scope of the present application.

Claims

1. A method for preparing aluminum fluoride-coated nano-silicon material, characterized by: The method for preparing the aluminum fluoride coated nanometer silicon material utilizes nanometer silicon powder, aluminum nitrate nonahydrate, ammonium fluoride, anhydrous ethanol and deionized water; the method comprises the following specific steps: S1: 5g of nanometer silicon powder with a diameter of 100nm is dispersed in a beaker containing 50ml of anhydrous ethanol, and then is placed in an ultrasonic cleaner for ultrasonic treatment for 30 minutes, so that the nanometer silicon material is uniformly dispersed; S2: 0.11g-1.1g of aluminum nitrate nonahydrate is mixed with 400ml of deionized water to prepare an aluminum nitrate solution A, which is reserved for use; S3: 0.033g-0.33g of ammonium fluoride is mixed with 50ml of deionized water to prepare an ammonium fluoride solution B, which is reserved for use; S4: the nanometer silicon material dispersed in S1 is mixed with the solution A in S2 according to a mass ratio of 1:80, and is stirred for 30 minutes, and then is placed in a water bath at 80°C for continuous stirring for 3 hours, so as to obtain a mixed solution; S5: the solution B in S3 is added dropwise into the mixed solution in S4 according to a mass ratio of 1:8 of the solution A, and the rate of dropwise addition of the solution B is 2-4ml / min, and the dropwise addition is completed within 30 minutes; S6: the mixed solution in S5 is placed in a water bath at 80°C, and is continuously stirred for 3 hours; S7: the mixed solution after stirring in S6 is centrifuged by using a centrifuge, the rotating speed is controlled at 8000r / min, and the centrifugal time is 1 minute, and then the supernatant is removed; the remaining solid powder is washed by using deionized water, and then is centrifuged by using a centrifuge, the rotating speed is controlled at 12000r / min, and the centrifugal time is 3 minutes, and then the supernatant is removed; the remaining solid powder is washed by using anhydrous ethanol, and then is centrifuged by using a centrifuge, the rotating speed is controlled at 12000r / min, and the centrifugal time is 1 minute, and then the supernatant is removed to obtain a solid powder; finally, the solid powder is placed in a blast drying oven for drying for 2 hours, and then is placed in a vacuum drying oven for drying for 4 hours; S8: the solid powder obtained after drying in S7 is placed in a grinding machine for grinding for 10 minutes; S9: the ground powder obtained in S8 is moved into a tube furnace for heat treatment, the tube furnace continuously introduces argon with an analytical purity of 99.9% into a reaction chamber at a rate of 100ml / min, the tube furnace is heated to 300°C at a heating rate of 5°C / min and is kept for 3 hours, and then is naturally cooled to room temperature; the solid powder is collected, and an aluminum fluoride coated nanometer silicon powder (Si@AlF3) with a mass fraction of 0.5%-5% as a negative electrode active material in a lithium ion secondary battery is obtained.

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