WO2022061531A1 - 负极复合材料、负极、电子化学装置和电子装置 - Google Patents

负极复合材料、负极、电子化学装置和电子装置 Download PDF

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WO2022061531A1
WO2022061531A1 PCT/CN2020/116872 CN2020116872W WO2022061531A1 WO 2022061531 A1 WO2022061531 A1 WO 2022061531A1 CN 2020116872 W CN2020116872 W CN 2020116872W WO 2022061531 A1 WO2022061531 A1 WO 2022061531A1
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negative electrode
composite material
electrode composite
lithium titanate
carbon nanotubes
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PCT/CN2020/116872
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English (en)
French (fr)
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邓攀
谢远森
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宁德新能源科技有限公司
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Priority to PCT/CN2020/116872 priority Critical patent/WO2022061531A1/zh
Priority to CN202080011389.3A priority patent/CN113454815A/zh
Publication of WO2022061531A1 publication Critical patent/WO2022061531A1/zh

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/05—Preparation or purification of carbon not covered by groups C01B32/15, C01B32/20, C01B32/25, C01B32/30
    • D—TEXTILES; PAPER
    • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
    • D01D—MECHANICAL METHODS OR APPARATUS IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS
    • D01D5/00—Formation of filaments, threads, or the like
    • D01D5/0007—Electro-spinning
    • D01D5/0061—Electro-spinning characterised by the electro-spinning apparatus
    • D01D5/0069—Electro-spinning characterised by the electro-spinning apparatus characterised by the spinning section, e.g. capillary tube, protrusion or pin
    • D—TEXTILES; PAPER
    • D04—BRAIDING; LACE-MAKING; KNITTING; TRIMMINGS; NON-WOVEN FABRICS
    • D04H—MAKING TEXTILE FABRICS, e.g. FROM FIBRES OR FILAMENTARY MATERIAL; FABRICS MADE BY SUCH PROCESSES OR APPARATUS, e.g. FELTS, NON-WOVEN FABRICS; COTTON-WOOL; WADDING ; NON-WOVEN FABRICS FROM STAPLE FIBRES, FILAMENTS OR YARNS, BONDED WITH AT LEAST ONE WEB-LIKE MATERIAL DURING THEIR CONSOLIDATION
    • D04H1/00—Non-woven fabrics formed wholly or mainly of staple fibres or like relatively short fibres
    • D04H1/70—Non-woven fabrics formed wholly or mainly of staple fibres or like relatively short fibres characterised by the method of forming fleeces or layers, e.g. reorientation of fibres
    • D04H1/72—Non-woven fabrics formed wholly or mainly of staple fibres or like relatively short fibres characterised by the method of forming fleeces or layers, e.g. reorientation of fibres the fibres being randomly arranged
    • D04H1/728—Non-woven fabrics formed wholly or mainly of staple fibres or like relatively short fibres characterised by the method of forming fleeces or layers, e.g. reorientation of fibres the fibres being randomly arranged by electro-spinning
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/054—Accumulators with insertion or intercalation of metals other than lithium, e.g. with magnesium or aluminium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present application belongs to the technical field of secondary batteries, and in particular relates to a flexible self-supporting negative electrode composite material that can be directly used as a negative electrode, as well as negative electrodes, electrochemical devices and electronic devices comprising the negative electrode composite material.
  • Lithium titanate has been used as a negative electrode material for lithium-ion batteries for a long time, and its ultra-high safety performance (lithium deintercalation platform ⁇ 1.5V, no lithium precipitation), long cycle life (> 1000 cycles) and charge-discharge
  • the ultra-small volume change in the process (no SEI film formation, basically no volume change in the unit cell) has significant advantages, and is called "zero-strain" material.
  • the electronic conductivity of lithium titanate is very poor, and its electronic conductivity is about 10 -13 S/cm at room temperature, which is almost an insulator, which greatly limits the electrochemical performance of lithium titanate as an electrode material for lithium ion batteries.
  • the method of coating the surface of the material with a carbon layer can simply and effectively improve the electrical conductivity of the material, and it is also the most commonly used method for the modification of lithium titanate.
  • the existing methods only carry out simple carbon coating.
  • the uniformity of the coating layer presents great challenges.
  • the coating layer itself is mostly hard carbon or soft carbon. For use in secondary batteries, it is necessary to develop a new negative electrode material.
  • the present application provides a self-supporting flexible negative electrode composite material and a preparation method thereof.
  • a first aspect of the present application provides a negative electrode composite material for a secondary battery, the negative electrode composite material comprising fibrous self-supporting amorphous carbon having titanium on the fibrous self-supporting amorphous carbon Lithium oxide and carbon nanotubes.
  • the meaning of having lithium titanate and carbon nanotubes on the fibrous self-supporting amorphous carbon means that the fibrous self-supporting amorphous carbon is distributed between and/or at least partially surrounded by the amorphous carbon.
  • Lithium titanate and carbon nanotubes contained in the amorphous carbon More specifically, lithium titanate and carbon nanotubes are mainly contained in amorphous carbon, but it is not excluded that there are dispersed lithium titanate and carbon nanotubes between amorphous carbons, that is, lithium titanate and carbon nanotubes. Although there is contact with amorphous carbon, they are not embedded in each other.
  • self-supporting amorphous carbon refers to carbon materials that have both good electrical conductivity and supporting properties, and can be directly used as negative electrodes of secondary batteries without additional current collectors and binders.
  • amorphous carbon is fibrous and has good shape retention properties, and will not collapse due to external force.
  • the negative electrode composite material composed of self-supporting amorphous carbon, lithium titanate and carbon nanotubes has good pressure bearing capacity and good flexibility, making the negative electrode composite material very suitable for direct use as the negative electrode of secondary batteries.
  • the fibrous self-supporting amorphous carbon has a diameter of 200 nm to 600 nm.
  • fibrous self-supporting amorphous carbon there are many methods for forming fibrous self-supporting amorphous carbon, such as gel method or freeze-drying method.
  • the fibrous self-supporting amorphous carbon is obtained by calcining the polymer under vacuum or an inert atmosphere.
  • Self-supporting amorphous carbon can be obtained by calcining polymers (eg, polyvinylpyrrolidone, polyacrylonitrile, polyvinyl alcohol) under high temperature conditions.
  • the polymer includes at least one of polyvinylpyrrolidone (PVP), polyacrylonitrile (PAN), and polyvinyl alcohol (PVA).
  • PVP polyvinylpyrrolidone
  • PAN polyacrylonitrile
  • PVA polyvinyl alcohol
  • PVP polyvinylpyrrolidone
  • PAN polyacrylonitrile
  • PVA polyvinyl alcohol
  • One-dimensional amorphous carbon layer composites lithium titanate and carbon nanotubes to obtain one-dimensional carbon nanofibers.
  • Carbon nanotubes and one-dimensional carbon nanofibers form a conductive network all over the negative electrode composite material, which greatly improves the conductivity of the negative electrode composite material, and the negative electrode composite material exhibits flexible self-supporting properties as a whole, which can be directly used as a secondary negative electrode.
  • the obtained amorphous carbon encapsulates the lithium titanate and carbon nanotubes inside the amorphous carbon to form a free form of carbon. Shape carbon nanofibers, and finally form a sheet-like composite material with complete structure and good flexibility. After drying, cold pressing and cutting, the composite material can be directly used as a negative electrode plate of a secondary battery, so the negative electrode composite material of the present application is called a self-supporting flexible negative electrode composite material.
  • the self-supporting flexible negative electrode composite material of the present application can be directly used as a negative electrode due to its own self-supporting properties, without the need to use additional current collectors and negative electrode active materials like traditional negative electrodes.
  • the negative electrode composite material of the present application shows a great improvement in electrical conductivity and electrochemical performance, and the self-supporting and flexible properties have great potential in the application of flexible batteries.
  • the secondary battery using the negative electrode composite material of the present application can realize high rate discharge and ensure long cycle.
  • the negative electrode composite material of the present application includes lithium titanate, carbon nanotubes, and an amorphous carbon coating layer.
  • the lithium titanate can be commercial lithium titanate or lithium titanate prepared according to the method of the present application.
  • the lithium titanate prepared by the method of the present application is formed by the reaction of lithium chloride monohydrate (LiCl ⁇ H 2 O) and tetrabutyl titanate (C 16 H 36 O 4 Ti) during the preparation process, and the obtained titanate
  • the mixing of lithium with carbon nanotubes and amorphous carbon is more uniform, and the final anode composite has more uniform properties.
  • the mass percentages of lithium titanate, carbon nanotubes, and coated amorphous carbon are 40% to 65%: 3% to 10%: 30% to 50%. At this ratio, lithium titanate and carbon nanotubes can be dispersed evenly.
  • carbon nanotubes are used as conductive "wires", and the outer layer is coated with a layer of one-dimensional carbon nanofibers. , the lithium titanate and the conductive mesh wire can be wrapped as a whole to form a complete three-dimensional conductive network.
  • the lithium titanate may be commercial lithium titanate and lithium titanate synthesized in-situ in the present application.
  • the lithium titanate is a spinel-structured lithium titanate, and the lithium titanate has a crystallinity of 75% to 100%. It has been observed that the crystallinity of lithium titanate can affect the capacity of the cell. The higher the crystallinity of lithium titanate, the higher the capacity of the cell. When the crystallinity of lithium titanate basically reaches more than 80%, the influence on the battery cell is limited. When selecting the raw material of lithium titanate, it is appropriate to select lithium titanate with a crystallinity of 75% to 100%.
  • the particle diameter of lithium titanate needs to be within a suitable range. If the particle diameter of lithium titanate is too large, it exceeds the size range of nanoscale lithium titanate. In addition, due to cost considerations, the particle diameter of lithium titanate should not be too small, and the particle diameter of lithium titanate is too small, which increases the difficulty of production and increases the cost. In the present application, the particle diameter of lithium titanate is 10 nm to 200 nm.
  • the carbon nanotubes can be multi-walled carbon nanotubes, single-walled carbon nanotubes, or a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes.
  • Multi-walled carbon nanotubes are widely available and low in cost, and are very suitable for use as raw materials for carbon nanotubes in this application.
  • single-walled carbon nanotubes are employed.
  • Single-walled carbon nanotubes have a better effect on improving the performance of secondary batteries because of their more excellent electrical conductivity and physicochemical properties.
  • the diameter and length of carbon nanotubes will affect the mixing of carbon nanotubes and lithium titanate.
  • the carbon nanotubes have a diameter of 10 nm to 70 nm and a length of 100 nm to 200 nm.
  • one-dimensional amorphous carbon nanofibers are obtained by carbonization of polymer PVP or PAN under high temperature vacuum or high temperature inert atmosphere.
  • the carbon nanofibers obtained after high temperature calcination are stable in structure and can be used as excellent coating layers.
  • the stability of the overall structure of the negative electrode composite can be maintained during the charging and discharging process.
  • the present application also provides a negative electrode, the negative electrode is composed of the negative electrode composite material as described above, and the expansion ratio of the negative electrode is 3% to 5%.
  • a second aspect of the present application provides a method for preparing the above-mentioned negative electrode composite material, the method specifically comprises the following steps:
  • solution A wherein, the polymer is one of polyvinylpyrrolidone, polyacrylonitrile, and polyvinyl alcohol.
  • the organic solvent is selected from ethanol, N,N-dimethylformamide (DMF), or a mixed solvent composed of ethanol and N,N-dimethylformamide;
  • the present application adopts the method of electrospinning to prepare the precursor solution of the negative electrode composite material (ie, mixed solution C), and the mixed solution C is injected into a syringe connected with a blunt metal needle for electrospinning.
  • the nanofiber membrane collected on the receiving device was placed in a vacuum drying oven to dry completely. Then, the dried nanofiber membrane was pre-oxidized in a muffle furnace for a certain period of time, and finally carbonized at a high temperature for a certain period of time under vacuum or inert atmosphere in a tube furnace to obtain a fibrous self-supporting flexible negative electrode composite.
  • step (2) the amount of lithium titanate added is 4 times to 25 times the mass of the carbon nanotubes.
  • step (2) the amount of glacial acetic acid used is 75L to 85L per mole of tetrabutyl titanate.
  • the stirring time of the mixed solution C at room temperature is 8 hours to 20 hours.
  • a stirring time of less than 8 hours may result in uneven dispersion of the material, which has a negative impact on the uniformity of the negative electrode composite material obtained subsequently. It is not necessary for the stirring time to exceed 20 hours, and it will also increase the production cost.
  • the pre-oxidation treatment is performed at a temperature of 200° C. to 280° C.; the pre-oxidation treatment time may be 1 hour to 3 hours.
  • Pre-oxidation is carried out under aerobic conditions and is a process of converting easily oxidizable components in polymers into oxides.
  • the operation of pre-oxidation may include: placing the spun yarn obtained in the previous step in a muffle furnace, and performing pre-oxidation treatment at a lower temperature in an atmosphere of air or oxygen.
  • the temperature increase rate is 1°C/min to 10°C/min. The heating rate should not be too fast to prevent damage to the structure of the carbon nanofibers formed in the subsequent carbonization process.
  • the lithium titanate is commercial lithium titanate.
  • the added amount of carbon nanotubes is 3.6% to 40% by mass of the lithium titanate.
  • the high-temperature carbonization is performed in a vacuum or an inert atmosphere; the high-temperature carbonization temperature is 650° C. to 900° C., and the holding time is 5 hours to 15 hours; the heating rate is 1°C/min to 10°C/min.
  • the inert atmosphere is an argon atmosphere, a nitrogen atmosphere, a mixed inert gas atmosphere of a combination of argon and nitrogen.
  • the temperature of high temperature carbonization is between 650°C and 900°C. Too low temperature cannot achieve carbonization of nanofiber membranes, and too high temperature not only leads to unnecessary cost increase, but more importantly, too high temperature will lead to increased brittleness of the negative electrode composite obtained after carbonization, which is not conducive to its use as negative electrode material.
  • the lithium titanate can be commercialized lithium titanate, or can be synthesized by the method of the present application.
  • the method of the present application includes the following steps:
  • the organic solvent includes at least one of polyvinylpyrrolidone, polyacrylonitrile, and polyvinyl alcohol; the organic solvent is selected from ethanol and/or N,N-dimethylformamide;
  • step (1) the mass fraction of the polymer in solution A is 30% to 50%; the mass fraction of lithium chloride monohydrate in solution A is 3% to 6%.
  • the mass of the carbon nanotubes is 7% to 46% of the mass of the lithium chloride monohydrate.
  • the molar ratio of tetrabutyl titanate to lithium chloride monohydrate is from 1:1 to 10:9.
  • the solubility of lithium chloride and tetrabutyl titanate in the solvent can be improved In a suitable range, it is avoided that the solubility of these materials is too high and is not conducive to the dispersion of the materials.
  • the negative electrode composite material directly constitutes the negative electrode of the secondary battery, and the expansion ratio of the negative electrode is 3% to 5%.
  • a third aspect of the present application provides an electrochemical device comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is composed of the negative electrode composite material described in the present application.
  • the electrochemical device may be a lithium-ion battery, a sodium-ion battery, or a magnesium-ion battery.
  • the lithium ion battery includes: a positive electrode, a negative electrode, and an electrolyte.
  • the positive electrode material can be a lithium-containing compound such as lithium cobaltate, lithium manganate, lithium iron phosphate, and the like.
  • the negative electrode material is the negative electrode composite material described in the first aspect of the application, or the negative electrode material is the negative electrode composite material prepared according to the method described in the second aspect of the present application.
  • the electrolyte may be ethylene carbonate, fluorinated ethylene carbonate, propylene carbonate, dimethyl carbonate, or the like.
  • the negative electrode composite material provided by the present application has good conductivity, excellent rate performance, and self-supporting properties, and can be directly used as a negative electrode of a lithium ion battery, thereby getting rid of the dependence on the negative electrode current collector.
  • 1 is a schematic diagram of an electrospinning device
  • Example 2 shows a schematic diagram of a circular sheet made of the negative electrode composite material obtained in Example 1;
  • Example 3 shows a schematic diagram of a circular sheet made of the negative electrode composite material obtained in Example 1 after being bent;
  • Example 4 is a part of a scanning electron microscope (SEM) image of a circular sheet made of the negative electrode composite material obtained in Example 1; in the figure, lithium titanate particles are located on the surface of a hollow or non-hollow amorphous carbon fiber tube to form spurs or Inside the amorphous carbon fiber tube;
  • SEM scanning electron microscope
  • Example 5 is a part of a scanning electron microscope image of a circular sheet made of the negative electrode composite material obtained in Example 1; in the figure, carbon nanotubes are partially contained in the fibrous amorphous carbon, and a part is exposed in the amorphous carbon carbon external;
  • Fig. 6 is the X-ray diffraction (XRD) pattern of the negative electrode composite material obtained by Example 1, Comparative Examples 1, 2;
  • Example 7 is a comparison diagram of the rate performance of the negative electrode composite materials obtained from Example 1 and Comparative Examples 1 and 2 as a negative electrode of a lithium ion battery;
  • FIG. 8 is a cycle performance comparison diagram of the negative electrode composite materials obtained from Example 1 and Comparative Examples 1 and 2 used as negative electrodes of lithium ion batteries.
  • the relative crystallinity is represented by the ratio of the peak intensity of (111) crystal plane in the XRD pattern of the materials in each example or comparative example to the peak intensity of (111) crystal plane in the standard XRD pattern of lithium titanate (Hereinafter, crystallinity is synonymous with relative crystallinity).
  • the negative electrode composite material obtained in this application is dried, cold pressed, and cut into a negative electrode piece, and the metal lithium piece is used as the counter electrode.
  • 1M LiPF 6 is dissolved in ethylene carbonate (EC) and dimethyl carbonate. (DMC) and other solvents, as electrolyte, assembled into 2032 button battery.
  • the voltage range of the cycle test is 1.0V to 2.5V, and the cycle test temperature is 25°C.
  • the rate performance is that the battery is charged and discharged 10 times at the current density of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C, respectively, and the battery capacity obtained is tested.
  • Step 1 Add 0.5g polyacrylonitrile (PAN), 0.2g polyvinylpyrrolidone (PVP) and 15mg multi-wall carbon nanotubes to 10mL DMF and stir until the polymer is fully dissolved.
  • PAN polyacrylonitrile
  • PVP polyvinylpyrrolidone
  • Step 2 Add 2.25 mmol of LiCl ⁇ H 2 O to the solution described in Step 1 and continue to stir until dissolved to obtain solution A.
  • Step 3 Mix 2.5 mmol of tetrabutyl titanate (C 16 H 36 O 4 Ti) and 200 ⁇ L of glacial acetic acid, and stir until a clear solution B is obtained.
  • Step 4 Slowly add solution B to solution A and stir at room temperature for 12 hours to obtain mixed solution C, which is used as an electrospinning precursor solution.
  • Step 5 inject the above mixed solution C into a syringe connected with a blunt metal needle to conduct an electrospinning experiment.
  • the schematic diagram of the electrospinning experiment device is shown in FIG. 1 . After electrospinning, the nanofiber membranes were collected and vacuum dried.
  • Step 6 The dried nanofiber membrane is placed in a muffle furnace for pre-oxidation at 230° C. for 2 hours.
  • Step 7 carbonizing the pre-oxidized nanofiber film at 800° C. under vacuum conditions, and maintaining the temperature for 10 hours to obtain one-dimensional nanofibers (LTO/CNT-V 0 ) of lithium titanate composite carbon nanotubes.
  • FIG. 2 is a schematic diagram of a circular sheet of the negative electrode composite LTO/CNT-V 0 prepared in this example.
  • Figure 3 is a schematic view of the circular sheet after being bent. It can be seen from Figure 3 that the circular sheet made of negative electrode composite material has a good self-supporting flexible structure, high toughness, and is not easily broken when bent, and can be directly used as an electrode. The self-supporting flexible structure is free from current collectors. At the same time, the removal of the pole piece manufacturing process is conducive to increasing the specific energy density of the battery and shortening the battery manufacturing cycle.
  • FIG. 4 is a portion of a scanning electron microscope image of the negative electrode composite material of the present application. It can be seen from Figure 4 that the fibrous amorphous carbon has lighter colored dots. These lighter colored dots are lithium titanate particles. Lithium titanate particles are located on the surface of the hollow or non-hollow amorphous carbon fiber tube to form spurs or inside the amorphous carbon fiber tube, and appear as scattered light-colored granular objects under the scanning electron microscope.
  • FIG. 5 is another part of the electron microscope image of the negative electrode composite material of the present application. As can be seen from FIG. 5 , the carbon nanotubes in the figure are partially contained in the fibrous amorphous carbon, and a part is exposed outside the amorphous carbon.
  • the top is the XRD pattern of the negative electrode composite material in the application, which can be seen to be consistent with the standard pattern of lithium titanate, indicating that the main material of the negative electrode composite material is lithium titanate.
  • the top is the rate performance diagram of the LTO/CNT-V 0 negative electrode composite material directly used as a lithium ion battery pole piece. It can be seen that the LTO/CNT-V 0 negative electrode composite material is directly used as a pole piece. The effect is excellent And the rate performance has been greatly improved, and there is still a reversible capacity of 78mAh/g at 10C ultra-high rate.
  • the top is the cycle diagram of the LTO/CNT-V 0 negative electrode composite material directly used as a lithium-ion battery pole piece in this example.
  • the battery still maintains ⁇ 140mAh after 500 cycles at 1C current. /g of reversible capacity.
  • the mass ratio of lithium titanate, carbon nanotubes, and amorphous carbon is 52:8:40, wherein the particle size of lithium titanate is about 50 nm (the particle size here refers to the average particle size of the particles) , measured by a conventional particle distribution tester), the smaller particle size is conducive to the transmission of ions, the carbon nanotubes play the role of conductive "network lines", and the main role of the amorphous carbon after high temperature carbonization is to build a conductive network, Stabilize the overall structure of the anode composite.
  • the negative electrode composite material in this example has greater electronic conductivity and better rate performance.
  • the carbonization temperature was raised to 900°C on the basis that other conditions were kept the same as those in Example 1.
  • the amorphous carbon will be further carbonized, the proportion of amorphous carbon will decrease slightly (from 40% to 39%), the conductivity will further increase, and the thickness will decrease slightly; at the same time, high temperature will promote the growth of lithium titanate crystals (about 50nm). to about 55 nm), which is not conducive to maintaining the overall stability of the negative electrode composite material and the transport of lithium ions.
  • the carbonization temperature is increased to 1000°C on the basis of keeping other conditions consistent with Example 1.
  • the conductivity of the amorphous carbon increases as the temperature increases, but the quality does not change, and the lithium titanate crystal will be caused by excessive temperature. If it grows up, the amorphous carbon may not be able to coat it well, the overall stability of the negative electrode composite material will become worse, and the process cost will increase significantly.
  • Example 2 on the basis of keeping other conditions consistent with those in Example 1, the high temperature carbonization conditions were changed from vacuum to argon atmosphere, and the carbonization yield of the polymer was slightly reduced under the argon atmosphere, accompanied by the flow of pores in the atmosphere. There is also a small increase in porosity (about 30% to about 32%), and the increased porosity consumes more electrolyte than under vacuum conditions.
  • the carbonization temperature was reduced to 650°C.
  • the crystal form of lithium titanate is not fully formed, the degree of crystallinity is low (41%), the structure is unstable, and the particle size is also small. .
  • the degree of carbonization of the polymer is low, and the electrical conductivity cannot be effectively improved.
  • the turbostratic structure refers to the structure after carbonization of the polymer at low temperature
  • the usage amounts of lithium chloride monohydrate and tetrabutyl titanate are increased.
  • Increasing the amount of lithium salt and titanium salt increases the mass percentage of lithium titanate in the negative electrode composite material in this embodiment, and improves the loading of active material lithium titanate.
  • Appropriately increasing the lithium titanate loading carbon nanotubes or amorphous carbon within the specified range will not cause a large loss in the conductivity of the overall negative electrode composite, while also maintaining flexibility for direct use as a negative electrode, the battery's The rate and cycle performance did not deteriorate significantly.
  • the increase of lithium titanate content the gram capacity of the negative electrode composite at small current increases slightly, and the energy density increases, but the power density will decrease.
  • the amount of carbon nanotubes used is reduced (from 15 mg to 10 mg). Reducing the amount of carbon nanotubes reduces the effective contact between lithium titanate and carbon nanotubes, and electron transfer in some areas needs to rely on lithium titanate itself (the conductivity of lithium titanate itself is only 10 -13 S/cm), and the electronic conductivity The rate performance of the battery will be slightly worse, but it will not be greatly affected. Due to the increased mass percentage of amorphous carbon, the overall stability of the negative electrode composite material will be improved, and the battery cycle stability will be enhanced.
  • the amount of polymer used is reduced to 60% of that in Example 1.
  • the amorphous carbon content of the negative electrode composite material is greatly reduced after high-temperature carbonization, resulting in that the amorphous carbon cannot effectively coat lithium titanate and carbon nanotubes, and the negative electrode composite material cannot be maintained.
  • Stable Due to its toughness, it cannot be directly used as an electrode material for lithium-ion batteries. It requires conventional stirring, coating and other processes to make pole pieces, which not only increases the technological process, but also uses current collectors, binders, etc. to reduce the energy density of the battery. .
  • the length and diameter of carbon nanotubes are reduced on the basis of keeping other conditions consistent with those in Example 1.
  • the length and diameter of carbon nanotubes are reduced (see Table 1 and Table 2 for specific values), and its size is closer to the size of lithium titanate in this example, which is conducive to better dispersion and contact between the two;
  • the small length and diameter are conducive to the uniform coating of the amorphous carbon, so that the coating thickness is more uniform, and the overall capacity and rate performance of the negative electrode composite material are improved to a certain extent.
  • multi-walled carbon nanotubes are replaced with single-walled carbon nanotubes.
  • Single-walled carbon nanotubes have better electrical conductivity than multi-walled carbon nanotubes, so the overall electronic conductivity of the negative electrode composite material in this example is better than that of Example 1, and the rate performance will be improved.
  • the high cost of walled carbon nanotubes is not conducive to high-volume applications.
  • Step 1 Add 0.5 g of polyacrylonitrile (PAN), 0.2 g of polyvinylpyrrolidone (PVP) and 15 mg of multi-walled carbon nanotubes to 10 mL of DMF and stir until the polymer is fully dissolved.
  • PAN polyacrylonitrile
  • PVP polyvinylpyrrolidone
  • Step 2 Add 2.5 mmol of spinel lithium titanate (particle size about 50 nm) to step 1, stir and disperse to obtain a suspension of lithium titanate, and stir at room temperature for 12 hours to obtain mixed solution C.
  • Mixed solution C As an electrospinning precursor suspension.
  • Step 3 inject the above mixed solution into a 10mL syringe connected with a blunt metal needle, apply a DC high voltage of 8kV between the syringe needle and the receiving device through a high-voltage DC power supply, and the solution is advanced by a syringe pump at a speed of 0.6mL/h, and the needle The distance between it and the receiving device is 15 cm, and the schematic diagram of the experimental process is shown in Figure 1. After the end, the nanofiber membranes were collected and dried under vacuum.
  • Step 4 The dried nanofiber membrane is placed in a muffle furnace for pre-oxidation at 230° C. for 2 hours.
  • Step 5 carbonizing the pre-oxidized nanofiber membrane at 800° C. under vacuum conditions, and maintaining the temperature for 10 hours to obtain one-dimensional carbon nanofibers composited with lithium titanate and carbon nanotubes.
  • the multi-walled carbon nanotubes are replaced by single-walled carbon nanotubes under the condition that other conditions in Example 11 remain unchanged.
  • the introduction of single-walled carbon nanotubes slightly enhanced the rate capability of the battery.
  • Example 13-15 the dosages of PAN and PVP were adjusted on the basis of keeping other conditions consistent with those in Example 1.
  • the amount of PAN and PVP used in Example 13 are both 0.4g, wherein PAN has better structural stability after carbonization, and PVP has better conductivity after carbonization. (especially whether it is ultimately flexible) is decisive. Therefore, in Example 14, due to the insufficient amount of the two, the negative electrode composite material cannot be directly used as a flexible material for the pole piece, resulting in a significant reduction in energy density; in Example 15, the amount of PAN used is relatively large, and the carbon content ratio in the product is obvious. Increase, the flexibility of the negative electrode composite material becomes better, but the relative content of the active material lithium carbonate decreases, and the overall energy density of the negative electrode composite material is also significantly lower than that of Example 1.
  • Step 1 Add 0.5g polyacrylonitrile (PAN) and 0.2g polyvinylpyrrolidone (PVP) to 10mL DMF and stir until the polymer is fully dissolved.
  • PAN polyacrylonitrile
  • PVP polyvinylpyrrolidone
  • Step 2 Add 2.25 mmol of LiCl ⁇ H 2 O to the solution described in Step 1 and continue to stir until dissolved to obtain solution A.
  • Step 3 Mix 2.5 mmol of tetrabutyl titanate (C 16 H 36 O 4 Ti) and 200 ⁇ L of glacial acetic acid, and stir until a clear solution B is obtained.
  • Step 4 Slowly add solution B to solution A and stir at room temperature for 12 hours to obtain mixed solution C, which is used as an electrospinning precursor solution.
  • Step 5 inject the above mixed solution C into a 10mL syringe connected with a blunt metal needle, and apply a DC high voltage of 8kV between the syringe needle and the receiving device through a high-voltage DC power supply, and the solution is propelled by a syringe pump at a speed of 0.6mL/h,
  • the distance between the needle and the receiving device is 15 cm, and the schematic diagram of the experimental process is shown in Figure 1. After the end, the nanofiber membranes were collected and dried under vacuum.
  • Step 6 The dried nanofiber membrane is placed in a muffle furnace for pre-oxidation at 230° C. for 2 hours.
  • Step 7 carbonizing the pre-oxidized nanofiber membrane at 800° C. under vacuum conditions, and maintaining the temperature for 10 hours to obtain one-dimensional nanofibers (LTO-V 0 ) of lithium titanate composite carbon nanotubes.
  • the middle curve is the XRD pattern of the LTO-V 0 obtained in this comparative example, and it can be seen that the main material of the negative electrode composite material is also lithium titanate.
  • the middle curve is the rate performance diagram of LTO-V 0 directly used as an electrode, which also shows that the negative electrode composite material has a good effect as a self-supporting flexible electrode, and the rate performance is significantly improved.
  • Step 1 Add 0.5g polyacrylonitrile (PAN) and 0.2g polyvinylpyrrolidone (PVP) to 10mL DMF and stir until the polymer is fully dissolved.
  • PAN polyacrylonitrile
  • PVP polyvinylpyrrolidone
  • Step 2 Add 2.25 mmol of LiCl ⁇ H 2 O to the solution described in Step 1 and continue to stir until dissolved to obtain solution A.
  • Step 3 Mix 2.5 mmol of tetrabutyl titanate (C 16 H 36 O 4 Ti) and 200 ⁇ L of glacial acetic acid, and stir until a clear solution B is obtained.
  • Step 4 Slowly add solution B to solution A and stir at room temperature for 12 hours to obtain mixed solution C, which is used as an electrospinning precursor solution.
  • Step 5 inject the above mixed solution C into a 10mL syringe connected with a blunt metal needle, and apply a DC high voltage of 8kV between the syringe needle and the receiving device through a high-voltage DC power supply, and the solution is propelled by a syringe pump at a speed of 0.6mL/h,
  • the distance between the needle and the receiving device is 15 cm, and the schematic diagram of the experimental process is shown in Figure 1. After the end, the nanofiber membranes were collected and dried under vacuum.
  • Step 6 The dried nanofiber membrane is placed in a muffle furnace for pre-oxidation at 230° C. for 2 hours.
  • Step 7 The pre-oxidized nanofiber membrane is calcined at 800° C. under air conditions, and kept for 10 hours to obtain lithium titanate (LTO-air).
  • the bottom curve is the XRD pattern of the LTO-air obtained in this comparative example, and it can be seen that the phase of the negative electrode composite material is lithium titanate.
  • the bottom curve is the rate performance diagram of the electrode made of LTO-air, indicating that lithium titanate itself has poor rate performance due to its poor conductivity.
  • the first six steps in the preparation of the negative electrode composite materials of Comparative Example 3 and Comparative Example 4 were kept the same as those of Comparative Example 1, except that in Step 7, the carbonization temperature was increased to 900°C and 1000°C, respectively. As the carbonization temperature increases, the conductivity of amorphous carbon becomes better, but the size of lithium titanate also increases. When the temperature is too high, it not only increases the cost, but also has a weak performance improvement effect, and will cause the lithium titanate particles to be too large and negatively affect the performance of the negative electrode composite material.
  • Step 1 Add 0.5g polyacrylonitrile (PAN) and 0.2g polyvinylpyrrolidone (PVP) to 10mL DMF and stir until the polymer is fully dissolved.
  • PAN polyacrylonitrile
  • PVP polyvinylpyrrolidone
  • Step 2 Add 2.25 mmol of commercial nano-lithium titanate into the solution described in Step 1 and continue to stir until the dispersion is uniform to obtain a suspension (solution A).
  • Step 3 200 ⁇ L of glacial acetic acid is solution B.
  • step 4 the solution B is slowly added to the solution A and stirred at room temperature for 12 hours to obtain a mixed solution C, and the mixed solution C is used as the electrospinning precursor solution.
  • Step 5 inject the above mixed solution C into a 10mL syringe connected with a blunt metal needle, and apply a DC high voltage of 8kV between the syringe needle and the receiving device through a high-voltage DC power supply, and the solution is propelled by a syringe pump at a speed of 0.6mL/h,
  • the distance between the needle and the receiving device is 15 cm, and the schematic diagram of the experimental process is shown in Figure 1. After the end, the nanofiber membranes were collected and dried under vacuum.
  • Step 6 The dried nanofiber membrane is placed in a muffle furnace for pre-oxidation at 230° C. for 2 hours.
  • Step 7 calcining the pre-oxidized nanofiber membrane at 800° C. under air conditions, and maintaining the temperature for 10 hours to obtain lithium titanate.
  • Particle size The particle size Dv90 (90% of the particles are smaller than this value) of the particles obtained by the particle distribution tester.
  • the porosity will affect the transport and infiltration of the electrolyte in it, which in turn affects the performance of the cell such as cycling and rate.
  • the test method of conductivity is to measure the resistance of the material under a certain pressure and then calculate the conductivity.
  • Conductivity mainly affects the transport process of electrons in the material, which in turn affects the rate performance.
  • amorphous carbon is also conducive to improving the electrical conductivity of the material and improving the rate performance.
  • the diameter of the amorphous carbon is too small, it cannot play the role of supporting lithium carbonate and carbon nanotubes, and if the diameter of the amorphous carbon is too large, it is not conducive to the performance of the capacity.
  • the porosity of the negative electrode composite is mainly affected by the amorphous carbon content and size, and the porosity further affects the electrolyte infiltration and electrolyte consumption rate. Lithium ion liquid phase transport, too small will affect the electrolyte infiltration, too large will cause deterioration of the first efficiency of the battery.
  • the main function of carbon nanotubes is to improve the electrical conductivity of the negative electrode composite material.
  • increasing the proportion of carbon nanotubes in the negative electrode composite material is beneficial to improve the electronic conductivity and rate performance, but the carbon nanotube itself contributes very little to the capacity, and the overall gram capacity of the negative electrode composite material will decrease. Its proportion is controlled at 3% to 10%.
  • the diameter of lithium titanate particles has a significant impact on its kinetic properties. Smaller-sized nanoparticles are beneficial to the solid-phase transport of lithium ions, but the smaller the particles, the greater the processing cost and difficulty. It is also unfavorable for particle dispersion and embedded in amorphous carbon, so it is better to control its particle size between 10nm and 200nm to exert electrical properties.
  • the crystallinity of lithium titanate mainly affects whether the battery capacity can be fully exerted (as in Example 5). When the calcination temperature exceeds 750°C, the crystallinity of lithium titanate basically reaches more than 80%, and the influence on the performance of the cell is relatively small, so it will not be repeated here.
  • the negative electrode composite material prepared by the method of the present application has self-supporting flexibility, can be used as a negative electrode plate, shows a great improvement in electrical conductivity and electrochemical performance, and has the characteristics of self-supporting flexibility in the application of flexible batteries There is huge potential.
  • the secondary battery using the negative electrode composite material of the present application can realize high-rate discharge and ensure long cycle.

Abstract

本申请属于二次电池技术领域并提供一种负极复合材料，所述负极复合材料包括纤维状的自支撑无定形碳，所述纤维状的自支撑无定形碳上具有钛酸锂和碳纳米管。本申请提供的负极复合材料导电性好、倍率性能优异，且具有自支撑特性，可以直接用作二次电池(例如锂离子电池)的负极，摆脱对负极集流体的依赖。本申请还提供一种电化学装置和一种电子装置。

Description

负极复合材料、负极、电子化学装置和电子装置 技术领域
本申请属于二次电池技术领域，具体地涉及一种可以直接用作负极的柔性自支撑负极复合材料，以及包含该负极复合材料的负极、电化学装置和电子装置。
背景技术
随着储能市场的发展，二次电池例如锂离子电池作为其中关键的能量存储转化部件受到越来越多研究者及企业的关注，研发循环寿命长(＞10000次循环)、倍率性能好(＞3C)、成本低的新型锂离子电极材料是锂离子电池研发的关键环节。钛酸锂作为锂离子电池负极材料应用已经较久，其超高的安全性能(脱嵌锂平台～1.5V，不会出现析锂)，超长的循环寿命(＞1000次循环)以及充放电过程超小的体积变化(无SEI膜形成，晶胞基本无体积变化)优势显著，被称为“零应变”材料。但是钛酸锂的电子导电性很差，室温下其电子电导率约为10 -13S/cm，几乎为绝缘体，极大地限制了钛酸锂作为锂离子电池电极材料的电化学性能。
通过在材料表面包覆碳层的方法能够简单有效地改善材料的导电性，也是目前对钛酸锂改性用得比较多的方法。现有的方法仅进行简单碳包覆，一方面包覆层均一性存在较大的挑战，另一方面包覆层本身多为硬碳或软碳，导电性改善幅度仍然显得不够，无法有效地应用在二次电池中，因而有必要开发一种新的负极材料。
发明内容
鉴于现有技术中存在的钛酸锂导电性及倍率性能差等问题，本申请提供一种自支撑柔性负极复合材料及其制备方法。
具体地，本申请的第一方面提供一种用于二次电池的负极复合材料，所述负极复合材料包括纤维状的自支撑无定形碳，所述纤维状的自支撑无定形碳上具有钛酸锂和碳纳米管。
在本申请中，纤维状的自支撑无定形碳上具有钛酸锂和碳纳米管的含 义是指纤维状的自支撑无定形碳分布在所述无定形碳之间和/或至少部分地被包含在所述无定形碳中的钛酸锂和碳纳米管。更具体而言，钛酸锂和碳纳米管主要被包含在无定形碳中，但不排除无定形碳之间存在分散的钛酸锂和碳纳米管的情况，即钛酸锂和碳纳米管与无定形碳虽然有接触，但并不相互嵌入彼此之中。
在本申请中，自支撑无定形碳是指同时具有良好导电性和支撑性的碳材料，可以直接用作二次电池的负极而无需额外的集流体和粘结剂。在微观上看，无定形碳呈纤维状且良好的形状保持性质，不会因为外力的作用而导致无定形碳坍塌。由自支撑无定形碳、钛酸锂和碳纳米管构成的负极复合材料具有良好的承压性和良好的柔性，使负极复合材料非常适直接用作二次电池的负极。
在本申请的一些实施例中，所述纤维状的自支撑无定形碳的直径为200nm至600nm。
形成纤维状的自支撑无定形碳的方法可以有很多种，例如凝胶法或者冷冻干燥法等。在本申请中的一种实施方式中，所述纤维状的自支撑无定形碳通过将聚合物在真空或惰性气氛下煅烧而得。
自支撑无定形碳可以由聚合物(例如，聚乙烯吡咯烷酮、聚丙烯腈、聚乙烯醇)在高温条件下煅烧而得。在本申请的一些实施例中，所述聚合物包括聚乙烯吡咯烷酮(PVP)、聚丙烯腈(PAN)、聚乙烯醇(PVA)中的至少一种。聚乙烯吡咯烷酮(PVP)、聚丙烯腈(PAN)和聚乙烯醇(PVA)在煅烧后得到无定形碳层具有有力的支撑性质的结构，不易粉碎，具有较强的韧性。一维无定形碳层将钛酸锂和碳纳米管复合后，得到一维碳纳米纤维。碳纳米管和一维碳纳米纤维形成遍布负极复合材料的导电网络，极大地提升负极复合材料的导电性，且负极复合材料整体表现出柔性自支撑特性，可以直接用作二次的负极。
在本申请的一些实施例中，通过将聚合物、钛酸锂和碳纳米管组成混合物进行高温煅烧，得到的无定形碳将钛酸锂和碳纳米管包覆在无定形碳内部，形成无定形碳纳米纤维，最终形成结构完整且柔韧性良好的片状复合材料。该复合材料经过干燥、冷压、裁切后可以直接用作二次电池的负极极片，因此本申请的负极复合材料被称为自支撑柔性负极复合材料。本申请的自支撑柔性负极复合材料由于其自身的自支撑特性，因此可以直接 用作负极，不需要像传统的负极那样需要使用额外的集流体和负极活性材料。本申请的负极复合材料在导电性以及电化学性能上均表现出极大的提升，且自支撑柔性的特性在柔性电池的应用中有巨大潜力。采用本申请的负极复合材料的二次电池可以实现大倍率放电，并保证长循环。
本申请的负极复合材料包括钛酸锂、碳纳米管以及无定形碳包覆层。钛酸锂可以是商用的钛酸锂，也可以是根据本申请的方法制备得到的钛酸锂。本申请方法制备的钛酸锂由于在制备过程中，通过一水合氯化锂(LiCl·H 2O)和钛酸四丁酯(C 16H 36O 4Ti)反应生成，这样得到的钛酸锂与碳纳米管和无定形碳的混合更加均匀，最终得到负极复合材料具有更均一的特性。
在本申请的负极复合材料的一些实施例中，以钛酸锂、碳纳米管和无定形碳层的总质量计，钛酸锂、碳纳米管、包覆无定形碳的质量百分比为40％至65％∶3％至10％∶30％至50％。在这个比例下，钛酸锂与碳纳米管可以分散均匀，利用碳纳米管优异的导电性，将碳纳米管当作导电“网线”，再加上外层包覆一层一维碳纳米纤维，可以将钛酸锂与导电网线包覆为一个整体，构成完整的立体导电网络。
在本申请的负极复合材料的一些实施例中，还包括钛酸锂可以为商用钛酸锂及本申请中原位合成的钛酸锂。钛酸锂为尖晶石结构的钛酸锂，且所述钛酸锂具有75％至100％的结晶度。经观察发现，钛酸锂的结晶度可以影响电芯的容量，钛酸锂的结晶度越高，电芯的容量越高。当钛酸锂的结晶度基本达到80％以上时，对电芯容易的影响有限。在选择钛酸锂原料时，选择结晶度为75％至100％的钛酸锂为宜。
在本申请的负极复合材料中，钛酸锂的颗粒直径需要在一个适宜的范围内。钛酸锂的颗粒直径太大则超出了纳米级钛酸锂的尺寸范围。另外，出于成本的考虑，钛酸锂的颗粒直径也不宜太小钛酸锂的颗粒直径太小导致生产难度加大，成本上升。在本申请中，钛酸锂的颗粒直径为10nm至200nm。
在本申请的负极复合材料中，碳纳米管可以是多壁碳纳米管，也可以是单壁碳纳米管，或者是多壁碳纳米管和单壁碳纳米管的混合物。多壁碳纳米管来源广泛，成本较低，很适合用作本申请中的碳纳米管原料。在一些情况下，采用单壁碳纳米管。单壁碳纳米管因其更加优异的导电性及 物化性质，对二次电池的性能具有更好的改善作用。另外，碳纳米管直径、长度会影响碳纳米管与钛酸锂的混合，碳纳米管直径、长度太大或太长不易于钛酸锂良好、均匀的混合，会造成无定形碳难以进行包覆，导致负极复合材料的均一性差。在本申请的一些实施例中，碳纳米管的直径为10nm至70nm，长度为100nm至200nm。
在本申请中，一维的无定形碳纳米纤维由聚合物PVP或PAN在高温真空或高温惰性气氛下碳化得到，经过高温煅烧后得到的碳纳米纤维结构稳定，可以作为优异的包覆层，在充电和放电过程可以维持负极复合材料整体结构的稳定性。
本申请还提供一种负极，所述负极由如上所述的负极复合材料构成，且所述负极的膨胀率为3％至5％。
本申请的第二方面提供一种制备如上所述的负极复合材料方法，该方法具体包括以下步骤：
(1)将聚合物、以及碳纳米管加到有机溶剂中，室温下搅拌至聚合完全溶解，得到溶液A；其中，所述聚合物为聚乙烯吡咯烷酮、聚丙烯腈、聚乙烯醇中的一种或多种，所述有机溶剂选自乙醇、N，N-二甲基甲酰胺(DMF)、或乙醇和N，N-二甲基甲酰胺组成的混合溶剂；
(2)将钛酸锂和冰醋酸混合并搅拌至澄清，得到溶液B；
(3)将溶液B加到溶液A中，在常温下搅拌得到的混合溶液C；
(4)通过静电纺丝将所述混合溶液C制备成纳米纤维膜，将得到的纳米纤维膜进行真空干燥，然后进行预氧化处理；预氧化处理结束后，在真空或惰性气氛条件下将预氧化处理后的纳米纤维膜进行碳化，得到所述负极复合材料。
本申请采用静电纺丝的方法制备负极复合材料的前驱体溶液(即，混合溶液C)，将混合溶液C将纺丝前驱体溶液注入接有钝的金属针头的注射器中进行静电纺丝。静电纺丝结束后将接收装置上收集的纳米纤维膜放入真空干燥箱干燥完全。然后，将干燥后的纳米纤维膜放在马弗炉中预氧化一定时间，最后在管式炉的真空或惰性气氛条件下高温碳化一定时长，得到纤维状的自支撑柔性负极复合材料。
在本申请所述方法的一些实施例中，在步骤(2)中，所述钛酸锂的加入量为所述碳纳米管的质量的4倍至25倍。
在本申请所述方法的一些实施例中，在步骤(2)中，冰醋酸的用量为75L至85L每摩尔钛酸四丁酯。
在本申请所述方法的一些实施例中，在步骤(3)中，混合溶液C在常温搅拌时间为8小时至20小时。搅拌时间低于8小时可能导致材料分散不均匀，对后续得到负极复合材料的均一性产生负面影响。搅拌时间超过20小时则没有必要，同时也会造成生产成本的增加。
在本申请所述方法的一些实施例中，在步骤(4)中，所述预氧化处理在200℃至280℃的温度下进行；预氧化处理的时间可以为1小时至3小时。预氧化处理在有氧的条件下进行，是一个将聚合物中的易氧化成分转换为氧化物的过程。预氧化的操作可以包括：将前面的步骤得到的纺丝置于马弗炉中，在空气或氧气氛围下进行较低温度预氧化处理。另外，预氧化处理时，升温速率为1℃/min至10℃/min。升温速率不宜过快，防止对后续的碳化过程中形成的碳纳米纤维的结构造成破坏。
在本申请所述方法的一些实施例中，所述钛酸锂是商用钛酸锂。在这种情形下，碳纳米管的加入量为钛酸锂质量的3.6％至40％。
在本申请所述方法的一些实施例中，在步骤(4)中，高温碳化在真空或惰性气氛中进行；高温碳化温度为650℃至900℃，保温时间为5小时至15小时；升温速率为1℃/min至10℃/min。在高温碳化过程中，惰性气氛为氩气气氛、氮气气氛、氩气和氮气组合的混合惰性气体气氛。高温碳化的温度在650℃至900℃之间。温度过低无法实现对纳米纤维膜的碳化，而温度过高不仅导致不必要的成本增加，而且更重要的是，温度过高会导致碳化后得到的负极复合材料脆性增加，不利于其用作负极材料。
在本申请的方法中，钛酸锂可以是商业化的钛酸锂，也可以通过在本申请的方法来合成。在这种情况下，本申请的方法包括以下步骤：
(1)将聚合物、以及碳纳米管加到有机溶剂中，室温下搅拌至聚合物完全溶解，然后加入一水合氯化锂(LiCl·H 2O)，得到溶液A；其中，所述聚合物包括聚乙烯吡咯烷酮、聚丙烯腈、聚乙烯醇中的至少一种；所述有机溶剂选自乙醇和/或N，N-二甲基甲酰胺；
(2)将钛酸四丁酯(C 16H 36O 4Ti)和冰醋酸混合并搅拌至澄清，得到溶液B；
(3)将溶液B加到溶液A中，得到混合溶液C，在常温下搅拌所述 混合溶液C；
(4)通过静电纺丝将所述混合溶液制C备成纳米纤维膜，将得到的纳米纤维膜进行真空干燥，然后进行预氧化处理；在真空或惰性气氛条件下将预氧化处理后的纳米纤维膜进行碳化，得到所述负极复合材料。
在本申请所述方法的一些实施例中，在步骤(1)中，聚合物在溶液A中的质量分数为30％至50％；一水合氯化锂在溶液A中的质量分数为3％至6％。
在本申请所述方法的一些实施例中，碳纳米管的质量为一水合氯化锂质量的7％至46％。
在本申请所述方法的一些实施例中，钛酸四丁酯与一水合氯化锂的摩尔比为1∶1至10∶9。
通过限定聚合物在溶液A中的质量分数、碳纳米管的质量含量以及钛酸四丁酯与一水合氯化锂的摩尔比，可以使氯化锂以及钛酸四丁酯在溶剂中的溶解度处于一个合适的范围内，避免这些材料溶解度过高而不利于材料的分散。
在本申请中，负极复合材料直接构成二次电池的负极，且负极的膨胀率为3％至5％。
本申请的第三方面提供一种电化学装置，其包括正极、负极、以及电解液，其中所述负极由本申请所述的负极复合材料构成。
具体地，在本申请中，所述电化学装置可以为锂离子电池、钠离子电池、镁离子电池。当电化学装置可以为锂离子电池时，锂离子电池包括：正极、负极、以及电解液。正极电极材料可以为含锂化合物如钴酸锂、锰酸锂、磷酸铁锂等。负极材料为本申请第一方面所述的负极复合材料，或者负极材料为根据本申请第二方面所述的方法制备得到的负极复合材料。电解液可以为碳酸乙烯酯、氟化碳酸乙烯酯、碳酸丙烯酯、碳酸二甲酯等。
本申请提供的负极复合材料导电性好、倍率性能优异，且具有自支撑特性，可以直接用作锂离子电池的负极，从而摆脱对负极集流体的依赖。
附图说明
图1是静电纺丝装置的示意图；
图2示出由实施例1得到的负极复合材料制成的圆形薄片的示意图；
图3示出由实施例1得到的负极复合材料制成的圆形薄片被弯折后的示意图；
图4是由实施例1得到的负极复合材料制成的圆形薄片的扫描电镜图(SEM)的一部分；图中，钛酸锂颗粒位于中空或非中空的无定形碳纤维管的表面形成突刺或位于无定形碳纤维管的内部；
图5是由实施例1得到的负极复合材料制成的圆形薄片的扫描电镜图的一部分；图中，碳纳米管部分地被包含在纤维状的无定形碳中，有一部分露在无定形碳外部；
图6是由实施例1、对比例1、2得到的负极复合材料的X射线衍射(XRD)图谱；
图7是由实施例1、对比例1、2得到的负极复合材料用作锂离子电池负极的倍率性能对比图；
图8是由实施例1、对比例1、2得到的负极复合材料用作锂离子电池负极的循环性能对比图。
具体实施方式
为了使本申请目的、优点更清晰，下面将结合附图对本申请做进一步详细描述。
钛酸锂相对结晶度计算：以各实施例或对比例中材料的XRD图谱中(111)晶面峰强与钛酸锂标准XRD图谱中(111)晶面峰强的比值表示其相对结晶度(在下文中，结晶度与相对结晶度同义)。
电池制备：以本申请中得到的负极复合材料经过干燥、冷压、裁片后为负极极片，金属锂片为对电极，将1M LiPF 6溶于碳酸乙烯酯(EC)、碳酸二甲酯(DMC)等溶剂中，作为电解液，组装成2032扣式电池。
循环及倍率性能测试：循环测试是电池在1C(1C＝175mAh/g)电流密度下进行的充放电循环测试，循环测试的电压范围为1.0V至2.5V，循环测试温度为25℃。倍率性能为电池分别以0.1C、0.2C、0.5C、1C、2C、5C、10C的电流密度进行10次充放电循环，并测试得到的电池容量。
实施例1
步骤一：将0.5g聚丙烯腈(PAN)、0.2g聚乙烯吡咯烷酮(PVP) 以及15mg多壁碳纳米管加到10mL DMF中搅拌至聚合物充分溶解。
步骤二：将2.25mmol的LiCl·H 2O加入步骤一所述溶液中继续搅拌至溶解，得到溶液A。
步骤三：将2.5mmol钛酸四丁酯(C 16H 36O 4Ti)和200μL冰醋酸混合，搅拌至得到澄清溶液B。
步骤四：将溶液B缓慢加到溶液A中常温搅拌12小时，得到混合溶液C，将混合溶液C作为静电纺丝前驱体溶液。
步骤五：将上述混合溶液C注入接有钝金属针头注射器中，进行静电纺丝实验，静电纺丝实验装置示意图如图1所示。静电纺丝结束后收集纳米纤维膜，然后进行真空干燥。
步骤六：将干燥完的纳米纤维膜置于马弗炉中230℃预氧化2小时。
步骤七：将预氧化的纳米纤维膜在真空条件下800℃碳化，保温10小时得到钛酸锂复合碳纳米管一维纳米纤维(LTO/CNT-V 0)。
图2为本实施案例所制备的负极复合材料LTO/CNT-V 0的圆形薄片示意图。图3是圆形薄片被弯折后的示意图。从图3中可以看出，由负极复合材料制作的圆形薄片具有良好的自支撑柔性结构，韧性大，在弯折时不易破碎，可以直接用作电极使用，该自支撑柔性结构摆脱集流体的束缚同时，去除掉极片制作工艺流程，有利于增大电池比能量密度、缩短电池制造周期。
图4是本申请的负极复合材料的扫描电镜图的一部分。从图4中可以看到，纤维状的无定形碳上具有颜色较浅的点状物。这些颜色较浅的点状物即为钛酸锂颗粒。钛酸锂颗粒位于中空或非中空的无定形碳纤维管的表面形成突刺或位于无定形碳纤维管的内部，在扫描电镜下呈现为分散的浅颜色颗粒状物体。
图5是本申请的负极复合材料的电镜图的另一部分。从图5中可以看出，图中的碳纳米管部分地被包含在纤维状的无定形碳中，有一部分露在无定形碳外部。
图6为负极复合材料的XRD图谱，最上方为本申请中的负极复合材料的XRD图谱，可看出与钛酸锂标准图谱保持一致，说明负极复合材料的主体材料为钛酸锂。
在图7中，最上方为LTO/CNT-V 0负极复合材料直接用作锂离子电池 极片的倍率性能图，可看出LTO/CNT-V 0负极复合材料直接用作极片效果极好且倍率性能得到极大改善，在10C超高倍率下仍有78mAh/g的可逆容量。
在图8中，最上方为该实施例中LTO/CNT-V 0负极复合材料直接用作锂离子电池极片的循环图，该实施例中电池在1C电流下循环500圈仍保持有～140mAh/g的可逆容量。
该实施例中钛酸锂、碳纳米管、无定形碳的质量比为52∶8∶40，其中钛酸锂的颗粒粒径为约50nm(此处的颗粒粒径是指颗粒的平均粒径，通过常规的颗粒分布测试仪测得)，较小的粒径有利于离子的传输，碳纳米管起到导电“网线”的作用，高温碳化后的无定形碳的主要作用为构建导电网络，稳定负极复合材料的整体结构。相对于对比例1中的无碳纳米管、对比例2和5中既无碳纳米管也无无定形碳的情形，本实施例中的负极复合材料具有更大的电子电导率及更优的倍率性能。
实施例2
在本实施例中，在其他条件与实施例1保持一致的基础之上，升高碳化温度至900℃。升高温度导致定形碳会进一步碳化，无定形碳比例稍有降低(从40％降低到39％)，导电性进一步增加，厚度略有降低；同时高温会促进钛酸锂晶体长大(约50nm至约55nm)，不利于保持负极复合材料整体稳定及锂离子传输。
实施例3
本实施例在其他条件与实施例1保持一致的基础之上升高碳化温度至1000℃，温度升高无定形碳导电性增大，但质量不再变化且由于温度过高会使钛酸锂晶体长大，有可能无定形碳无法对其进行良好包覆，负极复合材料的整体稳定性将变差，且工艺成本明显上升。
实施例4
在本实施例，在其他条件与实施例1保持一致的基础之上，将高温碳化条件由真空变为氩气气氛，在氩气气氛下聚合物碳化收率略有降低，伴随着气氛流动孔隙率也有小幅上升(约30％至约32％)，升高的孔隙率会 消耗比真空条件下更多的电解液。
实施例5
在本实施例中，在其他条件与实施例1保持一致的基础之上，降低碳化温度为650℃。在此温度下钛酸锂晶型未能完全成型、结晶程度较低(41％)、结构不稳定，颗粒尺寸也较小，钛酸锂脱嵌锂过程难度系数较大，无法实现容量充分发挥。较低的温度聚合物碳化程度低，导电性未能得到有效改善，不但不能起到导电网络的作用，形成的乱层结构(乱层结构是指低温下聚合物碳化后结构)，还可能影响锂离子及电子的传输，进而恶化电池的倍率性能。
实施例6
在本实施例中，在其他条件与实施例1保持一致的基础之上，增加一水合氯化锂及钛酸四丁酯的使用量。增大锂盐及钛盐的用量使得本实施例中的负极复合材料中钛酸锂的质量百分比上升，提高了活性物质钛酸锂的负载。在规定的范围内适当地增加钛酸锂负载、碳纳米管或无定形碳不会对整体负极复合材料的导电性造成较大损失，同时也保持柔性，以便直接用作负极极片，电池的倍率及循环性能无明显恶化。随着钛酸锂含量升高，负极复合材料在小电流下的克容量略微上升，能量密度上升，但功率密度将下降。
实施例7
在本实施例中，在其他条件与实施例1保持一致的基础之上，降低了碳纳米管的使用量(从15mg减少到10mg)。减少碳纳米管用量，钛酸锂与碳纳米管之间的有效接触减少，部分区域电子传递需要依靠钛酸锂自身(钛酸锂自身电导率仅为10 -13S/cm)，电子导电性有所降低，但电池整体的倍率性能会略有变差但并不会受到太大影响。由于无定形碳的质量百分比有所增大，因而负极复合材料整体稳定性将有所提升，电池循环稳定性增强。
实施例8
在本实施例中，在其他条件与实施例1保持一致的基础之上，将聚合物的使用量降低为实施例1中的60％。减少聚合物使用量后，负极复合材料在经过高温碳化后无定形碳含量大为降低，导致无定形碳无法将钛酸锂及碳纳米管进行有效包覆，且无法维持负极复合材料具有稳定的韧性，无法直接用作锂离子电池电极材料，需要进行常规搅料、涂布等流程进行极片制作，不仅增多了工艺流程，也会使用到集流体、粘结剂等，降低电池的能量密度。
实施例9
在本实施例中，在其他条件与实施例1保持一致的基础之上，减小了碳纳米管的长度及直径。碳纳米管长度、直径减小(具体数值见表1、表2)，其大小与本实施例中的钛酸锂的大小更接近，有利于两者之间更好的分散与接触；同时更小的长度、直径有利于无定形碳进行均匀的包覆，从而使包覆厚度更均匀，负极复合材料的整体容量发挥以及倍率性能均有一定提升。
实施例10
在本实施例中，在其他条件与实施例1保持一致的基础之上，将多壁碳纳米管换成单壁碳纳米管。单壁碳纳米管较之多壁碳纳米管有更好的导电性，因而本实施例中的负极复合材料整体的电子导电率较实施例1更好，倍率性能会有所提升，但是由于单壁碳纳米管成本很高，不利于大批量应用。
实施例11
步骤一：将0.5g聚丙烯腈(PAN)、0.2g聚乙烯吡咯烷酮(PVP)和15mg多壁碳纳米管加到10mL DMF中搅拌至聚合物充分溶解。
步骤二：将2.5mmol的尖晶石钛酸锂(粒径约50nm)加到步骤一中搅拌分散得到钛酸锂的悬浊液，并常温搅拌12小时，得到混合溶液C，将混合溶液C作为静电纺丝前驱体悬浊液。
步骤三：将上述混合液注入接有钝的金属针头的10mL注射器中，通过高压直流电源在注射器针头和接收装置之间施加8kV的直流高压，溶 液通过注射泵以0.6mL/h速度推进，针头与接收装置之间的距离为15cm，实验过程示意图如图1所示。结束后收集纳米纤维膜真空干燥。
步骤四：将干燥完的纳米纤维膜置于马弗炉中230℃预氧化2小时。
步骤五：将预氧化的纳米纤维膜在真空条件下800℃碳化，保温10小时得到复合有钛酸锂和碳纳米管的一维碳纳米纤维。
在本实施例中，由于钛酸锂不溶于DMF(N，N-二甲基甲酰胺)中，所以静电纺丝前驱液分散的均匀性较前面十个实施例较差，电池一致性相对较差。
实施例12
本实施例在实施例11其他条件不变的情况下将多壁碳纳米管换为单壁碳纳米管。单壁碳纳米管的引入轻微加强了电池的倍率性能。
实施例13-15
实施例13-15在其他条件保持与实施例1一致的基础上对PAN及PVP的用量进行了调整。实施例13使用PAN及PVP的量均为0.4g，其中PAN碳化后具有更好的结构稳定性，PVP碳化后具有更优异的导电性，两者用量之间的调整对负极复合材料最终导电性(特别是最终是否柔性)具有决定性。因此，在实施例14中由于两者的用量不够，使得负极复合材料不能直接作极片的柔性材料，导致能量密度显著降低；实施例15中使用量比较多的PAN，产物中碳含量比明显增大，负极复合材料的柔性变好，但是活性物质碳酸锂的相对含量下降，负极复合材料整体能量密度较实施例1也明显降低。
实施例16-19
在实施例16-19中，在其他条件与实施例1保持一致的基础上，对碳化温度及碳化保温时间进行了实验设计。温度不变的情况下适当缩短碳化保温时间对负极复合材料的整体影响不会太大。但是，倘若时间太短，钛酸锂结晶度及聚合物碳化过程都尚未完成，负极复合材料性能便会如实施例19中一样恶化严重。但是，碳化时间太长对与负极复合材料性能的提升并无明显作用，且会大大增加时间成本及能源成本(如实施例18)。因 此，需要控制碳化温度及保温时间在合理的范围内，从而达到提升性能及节省成本双重收益。
实施例20
在本实施例中，在对碳纳米管比例及粒径尺寸做了极限研究的同时，用PVA替代部分的PVP，在最低含量3％下使用直径下限10nm，长度下限100nm。由于碳纳米管含量下降，电子导电率将下降，但是其粒径和长度减小有利于与钛酸锂的接触一定程度上减缓了倍率性能恶化。
对比例1
步骤一：将0.5g聚丙烯腈(PAN)、0.2g聚乙烯吡咯烷酮(PVP)加到10mL DMF中搅拌至聚合物充分溶解。
步骤二：将2.25mmol的LiCl·H 2O加入步骤一所述溶液中继续搅拌至溶解，得到溶液A。
步骤三：将2.5mmol钛酸四丁酯(C 16H 36O 4Ti)和200μL冰醋酸混合，搅拌至得到澄清溶液B。
步骤四：将溶液B缓慢加到溶液A中常温搅拌12小时，得到混合溶液C，将混合溶液C作为静电纺丝前驱体溶液。
步骤五：将上述混合溶液C注入接有钝的金属针头的10mL注射器中，通过高压直流电源在注射器针头和接收装置之间施加8kV的直流高压，溶液通过注射泵以0.6mL/h速度推进，针头与接收装置之间的距离为15cm，实验过程示意图如图1所示。结束后收集纳米纤维膜真空干燥。
步骤六：将干燥完的纳米纤维膜置于马弗炉中230℃预氧化2小时。
步骤七：将预氧化的纳米纤维膜在真空条件下800℃碳化，保温10小时得到钛酸锂复合碳纳米管一维纳米纤维(LTO-V 0)。
在图6中，中间的曲线为本对比案例所得LTO-V 0的XRD图，可以看出该负极复合材料的主体材料亦为钛酸锂。
在图7中，的中间的曲线为LTO-V 0直接用作电极的倍率性能图，亦说明该负极复合材料用作自支撑柔性电极的效果良好，倍率性能改善明显。
对比例2
步骤一：将0.5g聚丙烯腈(PAN)、0.2g聚乙烯吡咯烷酮(PVP)加到10mL DMF中搅拌至聚合物充分溶解。
步骤二：将2.25mmol的LiCl·H 2O加入步骤一所述溶液中继续搅拌至溶解，得到溶液A。
步骤三：将2.5mmol钛酸四丁酯(C 16H 36O 4Ti)和200μL冰醋酸混合，搅拌至得到澄清溶液B。
步骤四：将溶液B缓慢加到溶液A中常温搅拌12小时，得到混合溶液C，将混合溶液C作为静电纺丝前驱体溶液。
步骤五：将上述混合溶液C注入接有钝的金属针头的10mL注射器中，通过高压直流电源在注射器针头和接收装置之间施加8kV的直流高压，溶液通过注射泵以0.6mL/h速度推进，针头与接收装置之间的距离为15cm，实验过程示意图如图1所示。结束后收集纳米纤维膜真空干燥。
步骤六：将干燥完的纳米纤维膜置于马弗炉中230℃预氧化2小时。
步骤七：将预氧化的纳米纤维膜在空气条件下800℃煅烧，保温10小时得到钛酸锂(LTO-air)。
在图6中，最下方的曲线为本对比案例所得LTO-air的XRD图，可看出该负极复合材料的的物相为钛酸锂。
在图7中，最下方的曲线为LTO-air做成电极的倍率性能图，说明钛酸锂本身由于导电性很差，倍率性能一般。
对比例3、对比例4
对比例3、对比例4的负极复合材料的制备的前六个步骤保持与对比例1一致，区别在于，在步骤七中，分别将碳化温度升高为900℃、1000℃。随着碳化温度的升高，无定形碳的导电性变好，但是钛酸锂的尺寸也会随之变大。当温度过高不仅增大成本、提升性能效果微弱且会致使钛酸锂颗粒过大负面影响负极复合材料的性能。
对比例5
步骤一：将0.5g聚丙烯腈(PAN)、0.2g聚乙烯吡咯烷酮(PVP)加到10mL DMF中搅拌至聚合物充分溶解。
步骤二：将2.25mmol商用纳米钛酸锂加入步骤一所述溶液中继续搅拌至分散均匀，得到悬浊液(溶液A)。
步骤三：200μL冰醋酸为溶液B。
步骤四，将溶液B缓慢加到溶液A中常温搅拌12小时，得到混合溶液C，将混合溶液C作为静电纺丝前驱体溶液。
步骤五：将上述混合溶液C注入接有钝的金属针头的10mL注射器中，通过高压直流电源在注射器针头和接收装置之间施加8kV的直流高压，溶液通过注射泵以0.6mL/h速度推进，针头与接收装置之间的距离为15cm，实验过程示意图如图1所示。结束后收集纳米纤维膜真空干燥。
步骤六：将干燥完的纳米纤维膜置于马弗炉中230℃预氧化2小时。
步骤七：将预氧化的纳米纤维膜在空气条件下800℃煅烧，保温10小时得到钛酸锂。
表1和表2中列出了各实施例、各对比例制作过程参数及制备成品材料的主要参数。
Figure PCTCN2020116872-appb-000001
Figure PCTCN2020116872-appb-000002
Figure PCTCN2020116872-appb-000003
Figure PCTCN2020116872-appb-000004
表2中的各参数测试方法如下：
颗粒尺寸通过颗粒分布测试仪所得颗粒的粒径大小Dv90(90％的颗粒粒径小于该值)。
钛酸锂结晶度通过测试晶体XRD得到d (101)峰强，再与其标准XRD谱中d (101)标对比，结晶度＝d (101)/d (101)标。
孔隙率通过材料真体积V 0与表观体积V计算而得，孔隙率＝(V 0-V)/V*100％。孔隙率会影响电解液在其中的传输和浸润，进而影响电芯的循环和倍率等性能。
电导率的测试方法为在一定压力下测试材料电阻进而计算电导率。电导率主要影响电子在材料中的传输过程，进而影响倍率性能。
由表2可知，无定形碳的主要作用是支撑钛酸锂及碳纳米管，维持整体结构保持柔性，同时无定形碳也有利于提高材料电导性，提升倍率性能。无定形碳直径越小，负极复合材料整体倍率性能愈佳。但是，无定形碳直径过小，则无法起到负载碳酸锂及碳纳米管的作用，而无定形碳直径太大则不利于容量发挥。
负极复合材料的孔隙率主要受无定形碳含量及尺寸的影响，而孔隙率进一步影响电解液浸润及电解液消耗速率，孔隙率在25％～40％之间电解液能够较好地浸润有利于锂离子液相传输，过小将影响电解液浸润，过大对电池首次效率造成恶化。
类似地，碳纳米管的主要作用为提升负极复合材料的电导性，其粒径一定范围内越小越有利于提升电导，但是过小将无法连接钛酸锂颗粒且影响加工性能，降低比容量；同时增大碳纳米管在负极复合材料中的占比有利于提高电子导电性，提升倍率性能，但碳纳米管本身对容量贡献极小占比增多负极复合材料整体克容量将下降，因而需将其比例控制在3％～10％。
钛酸锂颗粒直径对其动力学性能影响显著，更小尺寸的纳米颗粒有利于锂离子的固相传输，但颗粒越小加工成本及难度也将明显增加，颗粒过大不仅影响锂离子传输，还不利于颗粒分散以及内嵌于无定形碳中，因而需将其粒度控制在10nm～200nm之间更利于电性能发挥。钛酸锂结晶度主要影响电池容量是否能够完全发挥(如实施例5)。当煅烧温度超过750℃，钛酸锂的结晶度基本达到80％以上，对电芯性能的影响相对就比较小了，因而在此不做过多赘述。
通过本申请的方法制备得到的负极复合材料具有自支撑柔性，可以用作负极极片，在导电性以及电化学性能上均表现出极大的提升，且自支撑柔性的特性在柔性电池的应用中有巨大潜力。另外，采用本申请的负极复合材料的二次电池可以实现大倍率放电，并保证长循环。
以上仅为本申请的具体实施案例，但本申请的保护范围并不限于此，任何有关本申请技术范围内轻易能想到的变化或替代都应涵盖在本申请的保护范围之内。

Claims (11)

  1. 一种负极复合材料，其特征在于，所述负极复合材料包括纤维状的自支撑无定形碳，所述纤维状的自支撑无定形碳上具有钛酸锂和碳纳米管。
  2. 根据权利要求1所述的负极复合材料，其特征在于，所述纤维状的自支撑无定形碳的直径为200nm至600nm。
  3. 根据权利要求1所述的负极复合材料，其特征在于，所述纤维状的自支撑无定形碳通过将聚合物在真空或惰性气氛下煅烧而得，其中，所述聚合物包括聚乙烯吡咯烷酮、聚丙烯腈、聚乙烯醇中的至少一种。
  4. 根据权利要求1所述的负极复合材料，其特征在于，在所述负极复合材料中，以钛酸锂、碳纳米管和无定形碳的总质量计，钛酸锂、碳纳米管、纤维状的自支撑无定形碳的质量百分比为40％至65％∶3％至10％∶30至50％。
  5. 根据权利要求1所述的负极复合材料，其特征在于，所述钛酸锂具有如下特征的至少一种：所述钛酸锂具有尖晶石结构；所述钛酸锂具有75％至100％的结晶度；所述钛酸锂的颗粒直径为10nm至200nm。
  6. 根据权利要求1所述的负极复合材料，其特征在于，所述碳纳米管具有如下特征的至少一种：所述碳纳米管包括多壁碳纳米管或单壁碳纳米管中的至少一种；所述碳纳米管的直径为10nm至70nm；所述碳纳米管的长度为100nm至200nm。
  7. 一种制备权利要求1所述的负极复合材料的方法，其特征在于，所述方法包括以下步骤：
    (1)将聚合物、以及碳纳米管加到有机溶剂中，室温下搅拌至聚合物完全溶解，得到溶液A；其中，所述聚合物包括聚乙烯吡咯烷酮、聚丙烯腈、聚乙烯醇中的至少一种，所述有机溶剂选自乙醇和/或N，N-二甲基甲酰胺；
    (2)将钛酸锂和冰醋酸混合并搅拌至澄清，得到溶液B；
    (3)将溶液B加到溶液A中，在常温下搅拌得到的混合溶液C；
    (4)通过静电纺丝将所述混合溶液C制备成纳米纤维膜，将得到的纳米纤维膜进行真空干燥，然后进行预氧化处理；预氧化处理结束后，在 真空或惰性气氛条件下将预氧化处理后的纳米纤维膜进行碳化，得到所述负极复合材料。
  8. 根据权利要求7所述的方法，其特征在于，所述方法还包括：在步骤(1)中，在所述聚合物完全溶解后加入一水合氯化锂，并在步骤(2)中用钛酸四丁酯替代钛酸锂。
  9. 一种负极，其特征在于，所述负极由权利要求1至6任一项所述的负极复合材料构成，且所述负极的膨胀率为3％至5％。
  10. 一种电化学装置，其包括正极、负极、以及电解液，其特征在于，所述负极由权利要求1至6任一项所述的负极复合材料构成。
  11. 一种电子装置，包含权利要求10所述的电化学装置。
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