CN110854377A - Porous silica composite material and preparation and application thereof - Google Patents

Porous silica composite material and preparation and application thereof Download PDF

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CN110854377A
CN110854377A CN201911236729.8A CN201911236729A CN110854377A CN 110854377 A CN110854377 A CN 110854377A CN 201911236729 A CN201911236729 A CN 201911236729A CN 110854377 A CN110854377 A CN 110854377A
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silicon
porous silica
silicon oxide
silica composite
particles
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CN110854377B (en
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杨娟
唐晶晶
周向阳
刘晓剑
周昊宸
王鹏
侯林
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Hunan Chenyu Fuji New Energy Technology Co ltd
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Hunan Chenyu Fuji New Energy Technology Co Ltd
Central South University
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Abstract

The invention belongs to the field of lithium ion battery materials, and particularly discloses a porous silicon oxide composite material which comprises an inner core, an intermediate layer compounded on the surface of the inner core, and an outer layer compounded on the surface of the intermediate layer; wherein the inner core is silicon; the middle layer is silicon oxide and silicate of metal M dispersed in the silicon oxide; the outer layer is a carbon coating layer; the metal M is a metal element capable of reducing silicon oxide. The invention also provides a preparation method of the composite material and application of the composite material in serving as a negative electrode active material of a lithium ion secondary battery. The research of the invention finds that the composite material has the characteristics of long cycle life, high first-time efficiency and the like.

Description

Porous silica composite material and preparation and application thereof
Technical Field
The invention belongs to the technical field of lithium battery electrode materials, and particularly relates to a porous silicon monoxide composite powder material and a preparation method thereof.
Background
With the continuous update of consumer electronic products and the increasing development of electric automobiles, the demand for new generation of high specific energy lithium ion batteries is increasing, and the development of cathode materials of the batteries from embedded carbon-based materials (with a theoretical capacity of 372mAh/g) to lithiatable metal materials, metal compounds and composite materials with high theoretical capacity is promoted. Wherein, silicon has the advantages of high theoretical capacity (4211mAh/g), good safety performance, wide source and the like, and becomes the mainstream of the research of novel cathode materials at home and abroad at present. However, silicon expands greatly in volume and has low intrinsic conductivity during charge and discharge, resulting in poor cycle and rate performance of the battery. And among the compounds of silicon, silicon monoxide (SiO)xX is more than 0 and less than 2, and the theoretical specific capacity is higher>2000mAh/g), although lower than elemental silicon, because it can react with lithium ions during the first charge and dischargeReaction to form electrochemically inert Li2O and Li2SiO4The problem of volume expansion of the active material is effectively relieved, and the cycle performance of the battery is well improved. However, the large consumption of lithium ions also makes the first coulombic efficiency of silica low. In the field of silicon-based cathode materials, technical key points and difficulties are focused on how to maintain the cycling stability of a silicon-based cathode and improve the first coulombic efficiency.
At present, the conventional preparation process of the silicon oxide is to fully mix silicon powder and silicon dioxide powder according to equal molar ratio and carry out disproportionation reaction under the conditions of vacuum and high temperature of more than 1000 ℃ to generate the silicon oxide. The formed silica overflows in the form of vapor and is then condensed into a silica bulk. In the prior art, the raw materials of the high-purity silicon powder and the high-purity silicon dioxide powder have high cost, and the uniform mixing is difficult to realize in the material preparation process; the requirements of high temperature and vacuum production conditions are strict, and the control of the technological process is difficult, so that the equipment cost and the energy consumption cost are high; the silicon monoxide product is produced in a block form, the quality uniformity is poor, a large amount of energy is consumed to further crush the silicon monoxide product to a powder form, and the market demand is difficult to meet. Therefore, the search for low-cost raw materials, the development of new preparation processes and composite technologies is an important solution for obtaining low-cost high-quality silicon monoxide negative electrode materials.
Disclosure of Invention
Aiming at overcoming the defects of the prior art and solving the problems of low initial efficiency and complex preparation process of the silicon monoxide negative electrode material, the invention provides a brand-new negative electrode active material (porous silicon monoxide composite material) with a special structure and special components, and aims to improve the electrical properties such as initial efficiency, cycling stability and the like.
The second objective of the invention is to provide a preparation method for preparing the porous silica composite material by a selective reduction mechanism, aiming at obtaining a negative active material with excellent first efficiency and cycling stability by a brand new preparation idea.
The third purpose of the invention is to provide the application of the porous silicon oxide composite material in the lithium secondary battery.
It is a fourth object of the present invention to provide a lithium secondary battery comprising the porous silica composite.
A porous silica composite material comprises an inner core, a middle layer compounded on the surface of the inner core and an outer layer compounded on the surface of the middle layer; wherein the inner core is silicon; the middle layer is silicon oxide and silicate of metal M dispersed in the silicon oxide; the outer layer is a carbon coating layer;
the metal M is a metal element capable of reducing silicon oxide.
The porous silicon oxide composite powder material is a three-layer composite structure with an inner core, an intermediate layer and an outer layer which are sequentially compounded, wherein the inner core is silicon, the intermediate layer is silicon oxide and silicate of metal M dispersed in the silicon oxide, and the outer layer is a carbon coating layer. The research of the invention finds that the composite material has the characteristics of long cycle life, high first-time efficiency and the like.
Preferably, the silicate of the metal M is dispersed in the intermediate layer.
Preferably, the metal M is at least one of magnesium, aluminum, sodium and potassium.
Preferably, the porous silica composite has a metal M content of 1 to 5 wt%.
Preferably, the porous silica composite has a molar ratio of silicon to oxygen of, for example, 1 (0.1 to 1).
Preferably, the porous silica composite material contains carbon in an amount of 5 to 20 wt%.
Preferably, the particle size of the porous silica composite material is micron or submicron; i.e. 50nm-30 μm. The particles have porous structure and specific surface area of 10-200m2(ii)/g, the average pore diameter is 2-50 nm.
The second purpose of the invention is to provide a preparation method of the porous silica composite material; the method comprises the following steps:
step (1): surface oxidation is carried out on the silicon particles to obtain silicon particles (Si @ SiO) wrapped by silicon oxide in situ2)；
Step (2): mixing silicon particles coated by silicon oxide, metal M, compound molten salt and a carbonaceous adhesive for granulation to obtain a composite precursor; the compound molten salt comprises water-soluble salts of two or more alkali metals and/or alkaline earth metals; the eutectic temperature of the compound molten salt is 400-800 ℃;
and (3): and carrying out sintering reaction on the composite precursor in an inert atmosphere, and then washing with water to obtain the porous silica material.
The existing idea of obtaining the silicon monoxide material is to carry out disproportionation reaction of SiO2 and Si, the method is complex to operate and high in raw material cost, and the first-turn capacity and the cycle performance of the obtained material need to be improved. In the prior art, the idea and means for directly reducing silicon dioxide to prepare a silicon protoxide material do not exist, and the main reason is that the reduction degree of the silicon dioxide is difficult to control, the silicon dioxide is easily and directly reduced to a silicon simple substance, and the silicon protoxide material cannot be obtained. To this end, the present invention provides the above preparation method, which innovatively etches the silicon surface into silicon oxide through the selective surface oxidation in step (1), and innovatively performs the selective reduction of the metal M under the synergistic effect of the compound molten salt and the carbonaceous binder, thus unexpectedly controlling the degree of reduction, and realizing the uniform distribution of the silicate interlayer of the metal M and the control of the morphology. The preparation method fills the technical blank that the silicon oxide material is not directly reduced, and can unexpectedly obtain the cathode active material with excellent first-turn efficiency and optimal cycling stability.
In the present invention, the silicon particles may be of industrial grade. Preferably, the silicon content of the silicon particles is greater than 99 wt%. The granularity is 5-50 mm.
The research of the invention discovers that the innovative material surface oxidation method converts the surface of silicon into silicon oxide in situ, and is further matched with the selective reduction mechanism in the step (2), thereby being beneficial to further improving the first-turn capacity and the cycling stability of the prepared material.
Preferably, in the step (1), the ball milling is carried out in an oxygen-containing atmosphere to realize the silicon particlesThe surface is oxidized. It was found that surface oxidation of silicon particles by in-situ surface oxidation (Si @ SiO) can be obtained directly by surface oxidation of silicon particles with an oxygen-containing atmosphere with the mechanical assistance of ball milling2)。
The oxygen-containing atmosphere is, for example, an atmosphere containing 5 to 40% of oxygen; preferably air.
Preferably, the surface oxidation step of step (1) is:
preferably, the silicon particles are put into a tumbling ball mill, and positive pressure air having a certain temperature and humidity is blown to perform a ball milling reaction (surface oxidation) to produce silicon fine particles coated with silicon oxide.
Preferably, the pressure (absolute pressure-standard atmospheric pressure) for blowing air to form positive pressure is 700 to 800 Pa. The air temperature is 100-150 ℃. The relative humidity is 50-70%.
Preferably, the rotating speed of the ball mill is 20-100 r/min; the filling coefficient is 10-35%.
In the invention, after the ball milling partial oxidation is finished, silicon particles (silicon oxide-coated silicon fine particles) with oxidized surfaces can be transferred out of the reaction device through negative pressure. The pressure of the negative pressure (standard atmospheric pressure-absolute pressure) is, for example, 550 to 650 pa.
Preferably, the silica-encapsulated silicon particles have a particle size in the micrometer or submicrometer range, i.e. 100nm to 100 μm, and an oxygen content of the particles of 20 to 80 wt%.
Preferred step (1): the silicon particles are put into a roller ball mill, positive pressure air with certain temperature and humidity is blown in, ball milling reaction is carried out at certain rotating speed, silicon fine particles wrapped by silicon oxide are generated, and the fine particles are taken out of the roller ball mill through negative pressure airflow and enter a silicon powder collecting device. In the step (1), the silicon content in the silicon particles used as the raw material is more than 99 wt%, and the particle size is 5-50 mm; blowing air to form positive pressure of 700-800Pa, and forming negative pressure of 550-650Pa between the ball mill and the collecting device; the air temperature is 100 ℃ and 150 ℃, and the relative humidity is 50-70%; the rotating speed of the roller is 20-100 r/min, and the filling coefficient is 10-35%; the silicon coated by the silicon oxide has the particle size of micron or submicron grade, namely 100nm-100 mu m, and the oxygen content of the particles is 20-80 wt%.
In the invention, the surface oxidized silicon dioxide is selectively reduced by the metal M innovatively by adopting the compound molten salt with the eutectic temperature and the carbonaceous adhesive.
The compound molten salt is soluble alkali metal or alkaline earth metal salt capable of forming eutectic point molten salt, and can be selected from at least two of fluoride salt, chloride salt, nitrate salt and sulfate salt of lithium, sodium, potassium, magnesium and calcium.
Preferably, the compound molten salt is two or more of lithium chloride, sodium chloride, potassium chloride, magnesium chloride and sodium fluoride;
further preferably, the compound molten salt is prepared from the following components in a molar ratio of 5-8: 2-5 of a mixture of lithium chloride and sodium chloride; or the molar ratio is 3-5: 5-7 of a mixture of magnesium chloride and potassium chloride; or the molar ratio is 2-4: 2-4: 2-6 of a mixture of sodium chloride, potassium chloride and sodium fluoride.
The carbonaceous binder is selected from one or more of coal tar with high carbon content, coal pitch, petroleum pitch, phenolic resin, epoxy resin, sucrose and glucose.
Preferably, the mass ratio of the silicon oxide-coated silicon particles, the metal M, the compound molten salt and the carbonaceous binder is 1 (0.1-0.5) to (5-15) to (0.5-2).
The mixing granulation may be a known granulation method, including drum granulation, spray granulation, extrusion granulation, and disc granulation.
The inert atmosphere (protective atmosphere) refers to a nitrogen or argon atmosphere.
The sintering reaction is carried out at a speed of 2-10 ℃/min.
Preferably, the sintering temperature is greater than or equal to the eutectic temperature of the compound molten salt, and further preferably is 600-1000 ℃.
Preferably, the time of the sintering reaction is 1 to 12 hours.
And after sintering, washing with deionized water, and drying (drying) and crushing (scattering) to obtain the material.
The drying temperature is 80-105 ℃, and the drying time is 12-24 hours.
The breaking equipment can be selected from common crushing equipment, including a jaw crusher, a cone crusher, a hammer crusher, a roller crusher, an air flow crusher and a planetary ball mill, and the particle size of the broken material is in a submicron level.
The preparation method of the preferred porous silica composite powder material takes industrial-grade silicon particles as raw materials, and comprises the following steps:
the first step is as follows: putting silicon particles into a roller ball mill, blowing positive pressure air with certain temperature and humidity, performing ball milling reaction at a certain rotating speed to generate silicon fine particles wrapped by silicon oxide, and taking the fine particles out of the roller ball mill through negative pressure airflow to enter a silicon powder collecting device;
the second step is that: mixing and granulating the silicon particles coated by the silicon oxide collected in the first step, metal magnesium powder, mixed salt particles (compound molten salt) and a carbonaceous adhesive according to a certain proportion to obtain a composite precursor;
the third step: and (3) placing the composite precursor obtained in the second step into a reaction vessel, placing the reaction vessel into a high-temperature furnace, carrying out sintering reaction in an inert atmosphere, naturally cooling, washing out water-soluble salts in the product by using deionized water, drying and scattering to obtain the porous silica powder material.
According to the technical scheme, silicon particles are put into a roller ball mill, air with certain temperature and humidity is blown into the roller ball mill, particles on the surfaces of the silicon particles are displaced under continuous mutual friction and cylinder wall friction, under the action of high-temperature air with certain humidity, edges of crystal faces, displaced on the surfaces of the silicon particles, are partially oxidized with oxygen in the air, fall off from the surfaces of the silicon particles under friction impact, and are further ground to generate silicon particles coated with silicon oxide. The thickness of the silicon oxide layer and the oxygen content of the particles can be controlled by controlling the amount of the silicon particles and the blast amount as well as the temperature and the humidity of the air. According to the oxygen content of the granules, magnesium powder as a reducing agent and a carbonaceous binder are quantitatively added for granulation and sintering, under the action of eutectic point mixed molten salt, a system maintains a specific reaction temperature, stable and uniform reduction reaction is carried out, silicon oxide is reduced into low-oxygen-content silicon monoxide, uniformly dispersed magnesium silicate is generated in the granules, and a carbon coating layer is generated on the surfaces of the granules. After the mixed salt in the reaction product is washed away, the porous structure of the silicon monoxide composite powder is formed.
The invention also provides an application of the porous silicon monoxide composite material as a negative electrode active material of a lithium secondary battery.
The composite material is preferably used as a negative active material and is used for being compounded with a conductive agent and a binder to prepare a negative material. The conductive agent and the binder are all materials known in the industry.
In a further preferable application, the negative electrode material is compounded on the surface of a negative electrode current collector to prepare a negative electrode. The negative electrode material of the present invention may be formed by combining the negative electrode material on a current collector by a conventional method, for example, by a coating method. The current collector is any material known in the industry.
In a further preferred application, the negative electrode, the positive electrode, the separator and the electrolyte are assembled into a lithium secondary battery.
The invention also provides a lithium secondary battery which comprises the porous silicon oxide composite material.
Preferably, a negative electrode of the lithium secondary battery comprises the porous silica composite.
Further preferred is a lithium secondary battery, wherein the negative electrode material in the negative electrode sheet of the lithium secondary battery comprises the porous silica composite material.
The technical scheme of the invention has the beneficial effects that:
(1) the surface oxidation in-situ formation of partial silicon oxide composite particles is adopted, and a drum-type ball mill winnowing method is preferably adopted to prepare a silicon/silicon oxide coating structure, so that the raw material pretreatment process is simplified, the combination degree of silicon and silicon oxide is enhanced, the silicon oxide content is controlled, and the next reduction reaction is facilitated;
(2) eutectic mixed molten salt is adopted to stabilize the reduction temperature, local overheating is avoided, and the reduction degree of silicon oxide can be accurately controlled, so that the direct preparation of the silicon oxide powder with low oxidation degree is realized;
(3) magnesium thermal reaction generates a byproduct magnesium silicate with electrochemical inertia and an adhesive is carbonized to form a carbon coating layer, which is beneficial to improving the first coulombic efficiency and improving the circulation stability;
(4) the porous structure formed after the water-soluble salt in the product is dissolved further relieves the volume expansion of the silicon monoxide in the charging and discharging processes, and maintains a stable electrode structure;
(5) the method has the advantages of wide sources of main raw materials, low cost, simple and convenient processes such as ball milling, granulation and sintering, strong controllability, easy realization of large-scale production and good practical prospect.
Drawings
FIG. 1: scanning electron micrograph of the silica composite powder Material in example 1
FIG. 2: x-ray diffraction pattern of the silica composite powder material in example 1
Detailed Description
The specific procedures of the present invention are illustrated below by way of examples, it being understood that these examples are intended to illustrate the invention and are not intended to limit the scope of the invention in any way. Various procedures and methods not described in detail herein are conventional methods well known in the art.
Example 1
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon oxide-coated silicon particles, the collected metal magnesium powder, the collected mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, and the eutectic point is 570 ℃) and the collected medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
Observing the appearance of the sample by using a scanning electron microscope, wherein the average particle size of the particles is 8 mu m, as shown in figure 1; detecting sample phase by X-ray diffraction, as shown in figure 2, wherein the main components of the powder comprise simple substance silicon and silicon monoxide; the pore structure distribution is tested by nitrogen adsorption and desorption, the average size of pore channels is 25nm, and the specific surface area is 36m2(ii)/g; the carbon content in the powder is 9.2% by adopting a thermogravimetric differential calorimeter test, and the magnesium content in the powder is 3.8% by adopting an ICP-AES test. The silicon monoxide negative plate is assembled into a CR2032 type lithium ion button cell to carry out electrochemical performance detection in a voltage range of 0.01-1.2V at room temperature, and the current density of charge and discharge test is 200mA/g (in the following cases, the determination method is the same as the case except for special statement). The first reversible capacity was recorded at 1610mAh/g, coulombic efficiency 85%, capacity retention 93% after 100 cycles.
Example 2
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 45mm into a roller ball mill, wherein the filling coefficient is 30%; blowing air with the temperature of 120 ℃ and the humidity of 68 percent, keeping the ball mill at the positive pressure of 760Pa, and keeping the collection device at the negative pressure of 580 Pa; performing ball milling reaction at the rotating speed of 50 r/min to generate silicon oxide-coated silicon fine particles, wherein the particle size is 32 mu m, and the oxygen content is 65 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon oxide-coated silicon particles, the collected metal magnesium powder, the collected mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, and the eutectic point is 570 ℃) and the collected medium-temperature asphalt according to the ratio of 1:0.3:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
Sample particle in this example is flatThe average particle diameter is 15 mu m, the average size of the pore channel is 21nm, and the specific surface area is 29m2(ii)/g; the carbon content is 8.5%, and the magnesium content is 4.5%. And assembling the silicon monoxide negative plate into a lithium ion button cell to detect the electrochemical performance, wherein the charge-discharge test current density is 200 mA/g. The first reversible capacity is recorded to be 1560mAh/g, the coulombic efficiency is recorded to be 86%, and the capacity retention rate is recorded to be 91% after 100 times of circulation.
Example 3
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 10mm into a roller ball mill, wherein the filling coefficient is 18%; blowing air with the temperature of 110 ℃ and the humidity of 55 percent, keeping the ball mill at a positive pressure of 730Pa, and keeping the collection device at a negative pressure of 620 Pa; performing ball milling reaction at the rotating speed of 70 r/min to generate silicon oxide-coated silicon fine particles, wherein the particle size is 22 mu m, and the oxygen content is 39 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon oxide-coated silicon particles, the collected metal magnesium powder, the collected mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, and the eutectic point is 570 ℃) and the collected medium-temperature asphalt according to the ratio of 1:0.15:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
In this example, the average particle diameter of the sample particles was 7 μm, the average pore size was 26nm, and the specific surface area was 67m2(ii)/g; the carbon content is 12.4 percent, and the magnesium content is 2.5 percent. And assembling the silicon monoxide negative plate into a lithium ion button cell to detect the electrochemical performance, wherein the charge-discharge test current density is 200 mA/g. The first reversible capacity is recorded to be 1680mAh/g, the coulombic efficiency is 82 percent, and the capacity retention rate is 95 percent after 100 times of circulation.
Example 4
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal magnesium powder, the mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7.5: 2.5, the eutectic point is 550 ℃) and the phenolic resin according to the ratio of 1:0.2:10:2 by adopting a spray granulation method to obtain a composite precursor; placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 800 ℃ at the speed of 5 ℃/min, reacting for 4 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
In this example, the average particle diameter of the sample particles was 10 μm, the average pore size was 20nm, and the specific surface area was 52m2(ii)/g; the carbon content is 10.5%, and the magnesium content is 3.5%. And assembling the silicon monoxide negative plate into a lithium ion button cell to detect the electrochemical performance, wherein the charge-discharge test current density is 200 mA/g. The first reversible capacity was recorded to be 1510mAh/g, the coulombic efficiency was 84%, and the capacity retention rate after 100 cycles was 92%.
Example 5
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal magnesium powder, the mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 5: 5, the eutectic point is 640 ℃) and the phenolic resin according to the ratio of 1:0.2:10:3 by adopting a spray granulation method to obtain a composite precursor; placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 650 ℃ at the speed of 3 ℃/minute, reacting for 7 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
In this example, the average particle diameter of the sample particles was 12 μm, the average pore size was 22nm, and the specific surface area was 42m2(ii)/g; the carbon content is 15.5 percent, and the magnesium content is 2.6 percent. And assembling the silicon monoxide negative plate into a lithium ion button cell to detect the electrochemical performance, wherein the charge-discharge test current density is 200 mA/g. The first reversible capacity is recorded to be 1590mAh/g, the coulombic efficiency is 80%, and the capacity retention rate is 95% after 100 times of circulation.
Example 6
Compared with the embodiment 1, the main difference is that the compound molten salt has a molar ratio of 4: 4: 2 sodium chloride: potassium chloride: sodium fluoride, in particular:
putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal magnesium powder, the mixed salt particles (the molar ratio of sodium chloride to potassium chloride to sodium fluoride is 4: 4: 2, and the eutectic point is 590 ℃) and the medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
In this example, the average particle diameter of the sample particles was 8 μm, the average pore size was 31nm, and the specific surface area was 57m2(ii)/g; the carbon content is 12.2 percent, and the magnesium content is 2.3 percent. Making silicon monoxide negativeThe pole piece is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the charging and discharging test current density is 200 mA/g. The first reversible capacity was recorded as 1650mAh/g, the coulombic efficiency was 83%, and the capacity retention rate after 100 cycles was 92%.
Example 7
Compared with the embodiment 1, the main difference is that the compound molten salt is 4: 6, magnesium chloride and potassium chloride, in particular:
putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal magnesium powder, the mixed salt particles (the molar ratio of magnesium chloride to potassium chloride is 4: 6, the eutectic point is 430 ℃) and the medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
In this example, the average particle diameter of the sample particles was 8 μm, the average pore size was 35nm, and the specific surface area was 55m2(ii)/g; the carbon content is 13.4 percent, and the magnesium content is 1.5 percent. And assembling the silicon monoxide negative plate into a lithium ion button cell to detect the electrochemical performance, wherein the charge-discharge test current density is 200 mA/g. The first reversible capacity is recorded to be 1580mAh/g, the coulombic efficiency is 80%, and the capacity retention rate is 94% after 100 times of circulation.
Example 8
Compared with the example 1, the main difference is that the temperature of the selective reduction process is 1000 ℃, and specifically:
putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal magnesium powder, the mixed salt particles (the molar ratio of sodium chloride to potassium chloride to sodium fluoride is 2: 2: 6, the eutectic point is 780 ℃) and the medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 1000 ℃ at the speed of 10 ℃/minute, reacting for 1 hour, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
In this example, the average particle diameter of the sample particles was 7 μm, the average pore size was 25nm, and the specific surface area was 36m2(ii)/g; the carbon content is 12.4%, and the magnesium content is 1%. And assembling the silicon monoxide negative plate into a lithium ion button cell to detect the electrochemical performance, wherein the charge-discharge test current density is 200 mA/g. The first reversible capacity was recorded as 1620mAh/g, the coulombic efficiency was 82%, and the capacity retention after 100 cycles was 93%.
Example 9
Compared with the example 1, the main difference is that the temperature of the selective reduction process is 600 ℃, and specifically:
putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal magnesium powder, the mixed salt particles (the molar ratio of magnesium chloride to potassium chloride is 4: 6, the eutectic point is 430 ℃) and the medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 600 ℃ at the speed of 2 ℃/minute, reacting for 12 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
In this example, the average particle diameter of the sample particles was 9 μm, the average pore size was 18nm, and the specific surface area was 58m2(ii)/g; the carbon content is 14.5%, and the magnesium content is 4.8%. And assembling the silicon monoxide negative plate into a lithium ion button cell to detect the electrochemical performance, wherein the charge-discharge test current density is 200 mA/g. The first reversible capacity was 1530mAh/g, the coulombic efficiency was 85%, and the capacity retention after 100 cycles was 94% was recorded.
Example 10
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal aluminum powder, the mixed salt particles (the molar ratio of sodium chloride to potassium chloride to sodium fluoride is 2: 2: 6, the eutectic point is 780 ℃) and the medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) putting the composite precursor into a burning boat, placing the burning boat in a muffle furnace, carrying out sintering reaction in argon atmosphere, heating to 950 ℃ at the speed of 5 ℃/min, reacting for 4 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
In this example, the average particle size of the sample particles was 10 μm, the average pore size was 23nm, and the ratio tableArea of 78m2(ii)/g; the carbon content was 13.5% and the aluminum content was 2.8%. And assembling the silicon monoxide negative plate into a lithium ion button cell to detect the electrochemical performance, wherein the charge-discharge test current density is 200 mA/g. Recording the first reversible capacity of 1550mAh/g, the coulombic efficiency of 81 percent and the capacity retention rate of 90 percent after 100 times of circulation.
Comparative example 1 Using silicon and silicon oxide Mixed raw Material
Preparing a mixed raw material from simple substance silicon with the purity of 99.9 wt% and the granularity of 25 mu m and silicon oxide according to the mass ratio of 35:65, mixing the mixed raw material with metal magnesium powder, mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, and the eutectic point is 570 ℃) and medium-temperature asphalt according to the ratio of 1:0.2:10:1, and carrying out mixing granulation by adopting an extrusion granulation method to obtain a composite precursor; placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the silica powder material.
The negative silicon oxide plate in the comparative example is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the current density of the charge and discharge test is 200 mA/g. Recording the first reversible capacity of 960mAh/g, the coulombic efficiency of 75 percent and the capacity retention rate of 85 percent after 100 times of circulation.
Comparative example 2 No eutectic mixture salt
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal magnesium powder and the medium-temperature asphalt according to the proportion of 1:0.2:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
The negative silicon oxide plate in the comparative example is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the current density of the charge and discharge test is 200 mA/g. The first reversible capacity was recorded at 1250mAh/g, the coulombic efficiency was 78%, and the capacity retention rate after 100 cycles was 81%.
Comparative example 3 acid washing was used instead of water system to remove magnesium silicate
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon oxide-coated silicon particles, the collected metal magnesium powder, the collected mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, and the eutectic point is 570 ℃) and the collected medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing a product with 1mol/L concentration dilute hydrochloric acid and deionized water, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
The negative silicon oxide plate in the comparative example is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the current density of the charge and discharge test is 200 mA/g. The first reversible capacity was recorded to be 1120mAh/g, the coulombic efficiency was 69%, and the capacity retention after 100 cycles was 86%.
Comparative example 4 No use of carbonaceous Binder
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon oxide-coated silicon particles, metal magnesium powder and mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, the eutectic point is 570 ℃) according to the ratio of 1:0.2:10 by adopting an extrusion granulation method to obtain a composite precursor; placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing a product with 1mol/L concentration dilute hydrochloric acid and deionized water, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
The negative silicon oxide plate in the comparative example is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the current density of the charge and discharge test is 200 mA/g. Recording the first reversible capacity as 1310mAh/g, the coulombic efficiency as 76 percent and the capacity retention rate as 78 percent after 100 times of circulation.
Comparative example 5 instead of the in-situ surface oxidation process, the conventional silica hydrolysis coating process was used
Dispersing elemental silicon with the purity of 99.9 wt% and the particle size of 25 mu m into a mixed solution (volume ratio is 1: 10) of ethyl orthosilicate and ethanol, wherein the silicon content is 10%, uniformly stirring for 10min, adding 0.06mol/L hydrochloric acid solution with the relative volume of 0.4, uniformly stirring at 60 ℃ for 3h, and cooling to room temperature; then pouring the mixture into a reactor according to the volume ratio of 1: 1, continuously stirring a mixed solution of ammonia water, deionized water and absolute ethyl alcohol (volume ratio is 1: 2: 3) for 3 hours, and finally repeatedly washing the obtained precipitate with ethanol and water, separating and drying to obtain the ectopic silicon dioxide coated silicon composite raw material; mixing and granulating a silicon composite raw material, metal magnesium powder, mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, the eutectic point is 570 ℃) and medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
The negative silicon oxide plate in the comparative example is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the current density of the charge and discharge test is 200 mA/g. The first reversible capacity is recorded to be 1330mAh/g, the coulombic efficiency is 72 percent, and the capacity retention rate is 75 percent after 100 times of circulation.
Comparative example 5 instead of the Complex molten salt, a Single salt was used
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon particles wrapped by the silicon oxide, the metal magnesium powder, the sodium chloride and the medium-temperature asphalt according to the proportion of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
The negative silicon oxide plate in the comparative example is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the current density of the charge and discharge test is 200 mA/g. The first reversible capacity was recorded at 1430mAh/g, the coulombic efficiency was 69%, and the capacity retention after 100 cycles was 78%.
Comparative example 6 the ball milling control condition did not meet the requirements
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 90 ℃ and the humidity of 40%, keeping the ball mill at the positive pressure of 600Pa, and keeping the collection device at the negative pressure of 500 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles, wherein the particle size is 20 mu m, and the oxygen content is 15 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon oxide-coated silicon particles, the collected metal magnesium powder, the collected mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, and the eutectic point is 570 ℃) and the collected medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (3) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 700 ℃ at the speed of 5 ℃/min, reacting for 6 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
The negative silicon oxide plate in the comparative example is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the current density of the charge and discharge test is 200 mA/g. The first reversible capacity was recorded as 1380mAh/g, the coulombic efficiency was 55%, and the capacity retention after 100 cycles was 67%.
Comparative example 7 sintering temperature not up to the requirement
Putting silicon particles with the silicon content of 99.5 wt% and the particle size of 25mm into a roller ball mill, wherein the filling coefficient is 25%; blowing air with the temperature of 115 ℃ and the humidity of 60%, keeping the positive pressure of the ball mill at 750Pa, and keeping the negative pressure of the collecting device at 600 Pa; performing ball milling reaction at the rotating speed of 60 r/min to generate silicon oxide-coated silicon fine particles with the particle size of 25 mu m and the oxygen content of 45 wt%; the fine particles are carried out of the roller ball mill by negative pressure airflow and enter a silicon powder collecting device; mixing and granulating the collected silicon oxide-coated silicon particles, the collected metal magnesium powder, the collected mixed salt particles (the molar ratio of lithium chloride to sodium chloride is 7: 3, and the eutectic point is 570 ℃) and the collected medium-temperature asphalt according to the ratio of 1:0.2:10:1 by adopting an extrusion granulation method to obtain a composite precursor; and (2) placing the composite precursor into a burning boat and placing the burning boat in a muffle furnace, carrying out sintering reaction under argon atmosphere, heating to 500 ℃ at the speed of 1 ℃/minute, reacting for 15 hours, naturally cooling, washing with deionized water to remove water-soluble composite salt in the product, drying for 24 hours at the temperature of 105 ℃ after solid-liquid separation, and carrying out ball milling, crushing and scattering to obtain the porous silica powder material.
The negative silicon oxide plate in the comparative example is assembled into a lithium ion button cell to carry out electrochemical performance detection, and the current density of the charge and discharge test is 200 mA/g. Recording the first reversible capacity of 920mAh/g, the coulombic efficiency of 75 percent and the capacity retention rate of 69 percent after 100 times of circulation.

Claims (10)

1. The porous silica composite material is characterized by comprising an inner core, a middle layer compounded on the surface of the inner core and an outer layer compounded on the surface of the middle layer; wherein the inner core is silicon; the middle layer is silicon oxide and silicate of metal M dispersed in the silicon oxide; the outer layer is a carbon coating layer;
the metal M is a metal element capable of reducing silicon oxide.
2. The porous silica composite according to claim 1 wherein the silicate of the metal M is dispersed in the intermediate layer;
preferably, the metal M is at least one of magnesium, aluminum, sodium and potassium;
preferably, the porous silica composite has a metal M content of 1 to 5 wt%.
3. The porous silica composite material according to claim 1, wherein the carbon content is 5 to 20 wt%.
4. The porous silica composite according to claim 1, wherein the porous silica composite has a particle size of the order of micrometers or submicrometers; the particles have porous structure and specific surface area of 10-200m2(ii)/g, the average pore diameter is 2-50 nm.
5. A method for preparing the porous silica composite material according to any one of claims 1 to 4, comprising the steps of:
step (1): carrying out surface oxidation on the silicon particles to obtain silicon particles coated by silicon oxide;
step (2): mixing silicon particles coated by silicon oxide, metal M, compound molten salt and a carbonaceous adhesive for granulation to obtain a composite precursor; the compound molten salt comprises water-soluble salts of two or more alkali metals and/or alkaline earth metals; the eutectic temperature of the compound molten salt is 400-800 ℃;
and (3): and carrying out sintering reaction on the composite precursor in an inert atmosphere, and then washing with water to obtain the porous silica material.
6. The method of preparing a porous silica composite according to claim 5, wherein the silicon content in the silicon particles is greater than 99 wt%, and the particle size is 5 to 50 mm;
in the step (1), ball milling is carried out in an oxygen-containing atmosphere to realize the crushing and surface oxidation of silicon particles;
preferably, the crushing and surface oxidation step of step (1) is:
putting silicon particles into a roller ball mill, blowing positive pressure air with certain temperature and humidity, and performing ball milling reaction to generate silicon particles coated by silicon oxide;
preferably, the positive pressure formed by blowing air is 700-800 Pa; the air temperature is 100 ℃ and 150 ℃, and the relative humidity is 50-70%;
preferably, the rotating speed of the ball mill is 20-100 r/min; the filling coefficient is 10-35%;
preferably, the silica-encapsulated silicon particles have a micron or submicron size and the particles have an oxygen content of from 20 to 80 wt%.
7. The method for preparing the porous silica composite material according to claim 5, wherein the compound molten salt is two or more of fluoride salt, chloride salt, nitrate salt and sulfate salt of lithium, sodium, potassium, magnesium and calcium;
preferably, the compound molten salt is two or more of lithium chloride, sodium chloride, potassium chloride, magnesium chloride and sodium fluoride;
further preferably, the compound molten salt is prepared from the following components in a molar ratio of 5-8: 2-5 of a mixture of lithium chloride and sodium chloride; or the molar ratio is 3-5: 5-7 of a mixture of magnesium chloride and potassium chloride; or the molar ratio is 2-4: 2-4: 2-6 of a mixture of sodium chloride, potassium chloride and sodium fluoride.
8. The method of preparing a porous silica composite according to claim 5,
the carbonaceous binder is one or more of coal tar, coal pitch, petroleum pitch, phenolic resin, epoxy resin, sucrose and glucose;
preferably, the mass ratio of the silicon oxide-coated silicon particles, the metal M, the compound molten salt and the carbonaceous binder is 1 (0.1-0.5) to (5-15) to (0.5-2);
preferably, the temperature of sintering is 600-.
9. Use of the porous silica composite material according to any one of claims 1 to 4 or the porous silica composite material obtained by the production method according to any one of claims 5 to 9 as a negative electrode active material for a lithium secondary battery;
preferably, the composite material is used as a negative active material and is used for being compounded with a conductive agent and a binder to prepare a negative material;
preferably, the negative electrode material is compounded on the surface of a negative electrode current collector to prepare a negative electrode;
further preferably, the negative electrode, the positive electrode, the separator and the electrolyte are assembled into a lithium secondary battery.
10. A lithium secondary battery comprising the porous silica composite material according to any one of claims 1 to 4 or the porous silica composite material produced by the production method according to any one of claims 5 to 9;
preferably, the negative electrode of the lithium secondary battery comprises the porous silica composite;
further preferably, the negative electrode material in the negative electrode sheet of the lithium secondary battery comprises the porous silica composite material.
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