CN114421093A

CN114421093A - Flexible boron nitride nanotube inorganic diaphragm, preparation thereof and application thereof in lithium secondary battery

Info

Publication number: CN114421093A
Application number: CN202011091490.2A
Authority: CN
Inventors: 段曦东; 申晓华
Original assignee: Hunan University
Current assignee: Hunan University
Priority date: 2020-10-13
Filing date: 2020-10-13
Publication date: 2022-04-29
Anticipated expiration: 2040-10-13
Also published as: CN114421093B

Abstract

The invention relates to the technical field of lithium secondary battery diaphragms, in particular to a flexible boron nitride nanotube (BN) inorganic diaphragm which is an inorganic diaphragm material formed by interlacing boron nitride nanotubes. The invention also provides a tube-in-tube preparation method of the BN inorganic diaphragm and an application method of the diaphragm in a lithium secondary battery. The all-inorganic BN diaphragm provided by the invention has excellent flexibility and electrochemical performance under a limit condition.

Description

Flexible boron nitride nanotube inorganic diaphragm, preparation thereof and application thereof in lithium secondary battery

Technical Field

The invention belongs to the technical field of lithium secondary battery diaphragm materials, and particularly relates to a flexible boron nitride nanotube inorganic diaphragm.

Technical Field

With the progress of the times, the automatic devices such as power trolleybuses, mobile phones, household electric devices, medical instruments and the like are popularized and applied in various aspects such as life, machinery, chemical engineering, medical use, aviation, military industry and the like, so that the lithium battery with high safety and high endurance time is more and more emphasized¹. However, the commonly used lithium battery separator is a polypropylene separator, mainly comprising PP and PE, both of which have very low melting points (PP, melting point 165 ℃ C.; PE, 135 ℃ C.)². The lithium metal has higher theoretical capacity (3860mAh g^-1) And a relatively negative chemical potential (-3.04V), so that the lithium metal battery becomes the optimal choice for large-scale power energy storage. The fresh lithium sheet has more or less certain defects on the surface, when the lithium metal battery is charged and discharged, the defects of the lithium sheet have larger electric field than the plane, so that lithium ions are unevenly distributed and preferentially deposited at the bulges, serious lithium dendrites can be generated in the continuous circulation process, joule heat can be generated in the process, the temperature can reach 200 ℃ instantly, and the diaphragm is seriously shrunk to cause the contact of the positive electrode and the negative electrode to be short-circuited or even explode³。

Commonly used modifications include 1) the use of polymers with good heat resistance, but most are difficult to achieve at 500 ℃. 2) The surface of PP or PE is coated with one or more of aluminum trioxide, silicon dioxide, titanium dioxide and zirconium dioxide⁴. The method has large surface energy, and the charge and discharge are accompanied with the falling of the nano particles, so the mechanical property is limited. The development of commercial spray technology has greatly improved the coating process, but the tortuous and convoluted channels between particles can reduce ionic conduction. 3) Constructing an artificial SEI film improves the mechanical strength of the separator and mitigates the formation of dendrites, but other physical properties of the separator are also limited. It is difficult to prepare a lithium battery separator capable of balancing all the parameters.

BN has good insulativity, electrochemical stability and thermal conductivity because the lone pair electrons are deflected to the N atom by the extremely strong electronegativity of the N atom and a certain ionic bond is presented⁵. There have been some reports in the literature on the work including composite coating of BN as an electrolyte additive, and composite coating of BN nanosheets and a binder on a PP separator or 3D printing of the separator. Due to the limitation of the addition amount and the use of the binder, the prepared separator has limited ionic conductivity and mechanical properties, and the dendrite inhibition and heat resistance are greatly influenced.

Reference to the literature

1.(a)Zhang,T.W.；Chen,J.L.；Tian,T.；Shen,B.；Peng,Y.D.；Song,Y.H.；Jiang,B.；Lu,L.L.；Yao,H.B.；Yu,S.H.,Sustainable Separators for High-Performance Lithium Ion Batteries Enabled by Chemical Modifications.Advanced Functional Materials 2019,29(28),1902023；(b)Zhang,C.；Shen,L.；Shen,J.；Liu,F.；Chen,G.；Tao,R.；Ma,S.；Peng,Y.；Lu,Y.,Anion-Sorbent Composite Separators for High-Rate Lithium-Ion Batteries.Adv Mater 2019,31(21),e1808338.

2.Yan,J.；Zhao,Y.；Wang,X.；Xia,S.；Zhang,Y.；Han,Y.；Yu,J.；Ding,B.,Polymer Template Synthesis of Soft,Light,and Robust Oxide Ceramic Films.iScience 2019,15,185-195.

3.(a)Waqas,M.；Ali,S.；Feng,C.；Chen,D.；Han,J.；He,W.,Recent Development in Separators for High-Temperature Lithium-Ion Batteries.Small2019,1901689；(b)Wan,J.；Xie,J.；Kong,X.；Liu,Z.；Liu,K.；Shi,F.；Pei,A.；Chen,H.；Chen,W.；Chen,J.；Zhang,X.；Zong,L.；Wang,J.；Chen,L.-Q.；Qin,J.；Cui,Y.,Ultrathin,flexible,solid polymer composite electrolyte enabled with aligned nanoporous host for lithium batteries.Nature Nanotechnology 2019,14(7),705-711；(c)Song,Q.；Li,A.；Shi,L.；Qian,C.；Feric,T.G.；Fu,Y.；Zhang,H.；Li,Z.；Wang,P.；Li,Z.；Zhai,H.；Wang,X.；Dontigny,M.；Zaghib,K.；Park,A.-H.；Myers,K.；Chuan,X.；Yang,Y.,Thermally stable,nano-porous and eco-friendly sodium alginate/attapulgite separator for lithium-ion batteries.Energy Storage Materials 2019；(d)Shin,M.；Song,W.J.；Han,J.G.；Hwang,C.；Lee,S.；Yoo,S.；Park,S.；Song,H.K.；Yoo,S.；Choi,N.S.；Park,S.,Metamorphosis of Seaweeds into Multitalented Materials for Energy Storage Applications.Advanced Energy Materials 2019,9(19),1900570.

4.Ryou,M.-H.；Lee,D.J.；Lee,J.-N.；Lee,Y.M.；Park,J.-K.；Choi,J.W.,Excellent Cycle Life of Lithium-Metal Anodes in Lithium-Ion Batteries with Mussel-Inspired Polydopamine-Coated Separators.AdvancedEnergy Materials 2012,2(6),645-650.

5.Jimin Shim,a.H.J.K.,a Byoung Gak Kim,bc Yong Seok Kim,bc Dong-Gyun；Kim,b.a.J.-C.L.,2D boron nitride nanoflakes as a multifunctional additive in gel polymer electrolyte for safe,long cycle life and high rate lithium metal batteries.2017.

Disclosure of Invention

In order to solve the technical problems of poor heat resistance, poor heat dissipation, serious lithium dendrite formation, limited mechanical property and the like of the prepared diaphragm in the existing preparation method, the invention aims to provide a flexible boron nitride nanotube inorganic diaphragm, and aims to obtain a boron nitride nanotube all-inorganic diaphragm material which has flexibility, excellent high temperature resistance and excellent electrochemical property under the limit condition.

The second purpose of the invention is to provide a preparation method of the flexible boron nitride nanotube inorganic membrane.

The third purpose of the invention is to provide an application method of the flexible boron nitride nanotube inorganic diaphragm in a lithium secondary battery.

A fourth object of the present invention is to provide a lithium secondary battery comprising the flexible boron nitride nanotube inorganic separator.

The BN nano material has better performance, the BN nano material is compounded in a diaphragm polymer for enhancing the performance of the diaphragm in the prior art, although a certain effect can be achieved, the problems of incompatibility of the BN nano material and the diaphragm polymer interface, BN compounding stability and the like are solved, and the technical advantages of the technical means, particularly the advantages under the high-temperature condition, are not obvious. For this reason, the present invention has for the first time attempted to provide a BN all-inorganic separator material. Researches find that the all-inorganic diaphragm has strong material brittleness and poor flexibility and can not meet the use requirements of the lithium secondary battery diaphragm; in order to solve the problems of poor flexibility, non-ideal performance and the like of the inorganic diaphragm, the invention provides the following technical scheme through intensive research:

a flexible boron nitride nanotube inorganic diaphragm is an inorganic diaphragm material formed by interlacing (weaving) boron nitride nanotubes.

The invention provides an all-inorganic diaphragm material formed by interlacing and weaving boron nitride hollow nanotubes, and researches show that the material has good flexibility unexpectedly, can meet the use requirement of a lithium secondary battery diaphragm, and has excellent multiplying power and cycling stability, particularly excellent high-temperature resistance, and can remarkably improve the electrochemical stability of the lithium secondary battery under extreme conditions (such as high temperature, high voltage and the like).

The research of the invention finds that the BN nanotube structure and the diaphragm formed by interweaving the BN nanotubes are the key points for realizing good flexibility, improving heat resistance, and improving multiplying power and electrochemical stability under high-temperature conditions.

In the invention, the BN nano tube can be understood as a tubular fiber which has a hollow structure along the length direction, the tube diameter is nano-scale, and the length is more than micro-scale.

In the invention, the pipe diameter (outer diameter) of the boron nitride nanotube is 20-600 nm; the length is 5 to 100 μm;

preferably, the inorganic diaphragm material has a porosity of 60-90%; the thickness is 10 to 30 μm.

The invention also provides a preparation method of the flexible boron nitride nanotube inorganic diaphragm, which comprises the following steps:

step (1): carbon fiber membrane template preparation

Obtaining a carbon fiber membrane (template) by adopting an electrostatic spinning method;

step (2): boron nitride preparation

Placing a raw material powder containing B powder and an auxiliary agent and the carbon fiber film in a small tube; the tube is then placed in a tube furnace and a chemical vapor deposition reaction (also referred to herein as a deposition reaction) is carried out in a carrier gas containing a source of N,

wherein, one end of the small tube is closed, and the other end is open; the small tube is arranged along the direction of the carrier gas flow, the open end of the small tube is positioned at the upstream of the carrier gas flow, and the closed end of the small tube is positioned at the downstream of the carrier gas flow;

in the small tube, the carbon fiber film is placed on the upper part of the raw material powder;

the temperature in the deposition reaction process is greater than or equal to 1200 ℃;

and (3): stripper plate

And (3) removing the carbon fiber membrane template from the material prepared in the step (2) to obtain the flexible boron nitride nanotube inorganic diaphragm.

In order to solve the problems that the BN inorganic material has poor flexibility and is difficult to be successfully used in a lithium secondary battery, and the performances of heat resistance, multiplying power, high temperature resistance, stability and the like in the lithium secondary battery are not ideal, the invention innovatively provides a boron nitride nanotube inorganic diaphragm which takes an electrostatic spinning carbon fiber film as a template and innovatively utilizes a tube-in-tube deposition means, thus being capable of unexpectedly successfully obtaining flexibility and having excellent multiplying power, high temperature resistance and electrochemical performance in the lithium secondary battery.

The carbon fiber membrane template can be prepared by adopting the existing electrostatic spinning means.

Preferably, the step (1) comprises the following steps: and (3) carrying out electrostatic spinning on the spinning solution containing the carbon source to obtain a film precursor, and then carrying out preheating and carbonization treatment to obtain the carbon fiber film.

Preferably, the carbon source is a polymer carbon source, and more preferably at least one of Polyacrylonitrile (PAN), polyvinyl alcohol (PVA), and polyvinylpyrrolidone (PVP).

In the electrostatic spinning process, the concentration of the carbon source in the spinning solution is preferably 0.01-1 g/mL; preferably 0.08-0.2 g/mL.

Preferably, the flow rate of the spinning solution is 0.1-1 mL h^-1(ii) a Preferably 0.5 to 0.9mL h^-1. The voltage of the spinning process is preferably 15-25 KV. The distance between the spray nozzle and the receiving plate in the spinning process is preferably 10-20 cm.

Preferably, the temperature of the preheating process is 250-350 ℃. The preheating treatment is performed in an oxygen-containing atmosphere such as an air atmosphere.

Preferably, the temperature of the carbonization process is 600-700 ℃. The carbonization process is carried out under a protective atmosphere, wherein the protective atmosphere is nitrogen or inert gas.

In the invention, the tube-in-tube deposition means is the key for successfully constructing the all-inorganic diaphragm which can meet the use requirement of the lithium secondary battery and has good multiplying power and stability under the limiting condition.

In the present invention, to realize the "tube-in-tube" deposition means, the small tube needs to be closed at one end and open at the other end, and the open end is located at the upstream of the carrier gas flow direction and the closed end is located at the downstream of the carrier gas flow. In the deposition process, the synthesized raw material and the template are loaded in the small tube, and the boron nitride is deposited on the surface of the carbon fiber of the template by virtue of the carrier gas and the temperature rise of the tube furnace.

In the present invention, the shape of the small tube is not particularly limited, and may be, for example, a straight tube (which means that the longitudinal direction (axial direction) is a straight line or a nearly straight line); further, a straight pipe (which means that the radial cross section is circular) may be used.

In the present invention, the small tubes are referred to the reaction tube chamber of the tube furnace. The pipe diameter of the small pipe is smaller than that of a reaction pipe (quartz pipe) of the tube furnace; preferably, the pipe diameter of the small pipe is less than or equal to 90% of the pipe diameter of the reaction pipe of the tubular furnace; preferably 30 to 85%.

In the invention, the small tube can be a small quartz tube with a closed end.

The small tube is placed in a reaction chamber of a reaction tube of a tube furnace, and can be in direct contact with the inner wall of the reaction tube or not, for example, the small tube is suspended in the reaction chamber by being assisted by a tool. The included angle between the length direction of the small pipe and the length direction (carrier gas flow direction) of the reaction pipe is-10 degrees to +10 degrees; preferably, the length direction of the small tubes is parallel to the length direction of the reaction tube (i.e. the included angle is 0 °).

Preferably, the small pipe is flatly placed at the bottom of the inner wall of the reaction pipe of the tubular furnace.

In the invention, the raw material powder is flatly paved at the bottom of the tube wall of the small tube. The template is arranged in the small tube and positioned at the upper part of the raw material powder.

Preferably, the auxiliary agent is magnesium oxide and ferric oxide.

Preferably, the ferric oxide is gamma-ferric oxide. Researches find that the gamma-ferric oxide is beneficial to the preparation of the all-inorganic diaphragm.

Further preferably, the molar ratio of the B powder to the magnesium oxide to the ferric oxide is 1: 0.2-0.8: 0.2-0.4; more preferably 1:0.4 to 0.6:0.2 to 0.3.

Preferably, during the deposition process, the temperature is raised to the deposition temperature in a protective atmosphere in advance, and then the carrier gas is changed into the carrier gas containing the N source to perform the chemical deposition reaction. In the invention, the gas change technology (pre-heating in a protective atmosphere and then depositing in an N-containing source) is adopted, so that the preparation of the all-inorganic diaphragm with excellent flexibility and electrochemical performance in lithium batteries is facilitated.

Preferably, the protective gas is nitrogen or inert gas; the inert gas is at least one of helium, argon and the like.

The flow rate of the shielding gas is preferably 50 to 150sccm, and more preferably 80 to 110 sccm.

Preferably, the heating rate is 6-10 ℃/min.

In the present invention, preferably, the N source is ammonia gas. In the present invention, the carrier gas for the deposition process may be a 100% N source atmosphere, or a mixed atmosphere of the N source and a protective atmosphere.

Preferably, the flow rate of the N source is 70-150 sccm, and more preferably 80-110 sccm during the deposition process.

Preferably, the temperature in the deposition process is 1200-1300 ℃; further preferably 1200 to 1250 ℃.

In the invention, after the deposition is finished, the obtained material is subjected to decarburization fiber membrane template treatment, and the flexible diaphragm formed by the BN nano tube can be obtained in situ. In the invention, the carbon fiber membrane template can be removed by adopting the existing method.

Preferably, the material obtained in the step (2) is treated in an oxygen-containing atmosphere to remove the carbon fiber membrane template. The oxygen-containing atmosphere is, for example, air or oxygen.

The invention also provides a tube furnace for preparing the flexible boron nitride nanotube inorganic diaphragm, which comprises a reaction tube with an air inlet and an air outlet, and a heating device for heating a quartz tube, wherein the quartz reaction tube is also provided with a small tube; one end of the small tube is open (called an open end or an open end), the other end of the small tube is closed (called a closed end), and the open end of the small tube is close to the air inlet side of the reaction tube; the closed end is close to the air outlet side of the reaction tube.

In the invention, based on the tube furnace, the BN nanotube inorganic diaphragm can be synthesized by a tube-in-tube method, and the all-inorganic diaphragm material with better electrochemical stability under the limit condition can be obtained by adopting the equipment.

A preparation method of a preferable flexible boron nitride nanotube inorganic diaphragm comprises the following steps: preparing polyacrylonitrile fiber (PAN) by electrostatic spinning, drying in vacuum to obtain a film precursor, and then carrying out pre-oxidation treatment (preheating treatment) in a muffle furnace at 250-300 ℃; and then preserving the heat at 600-700 ℃ (under protective atmosphere) to obtain the carbon template. By adopting the tubular furnace equipment, the B powder, the magnesium oxide and the gamma-ferric oxide are uniformly mixed and then placed in a small tube, and then the cut flexible carbon template is placed above the reactant. Raising the temperature to 1200-1250 ℃ under the protection of argon in advance, and then carrying out chemical vapor deposition reaction under ammonia gas. The prepared BN nanotube film is of a uniform porous structure, has higher lithium ion conductivity and heat resistance compared with commercial PP, cellulose, ceramic and aramid fiber coating diaphragms, can well inhibit the growth of lithium dendrites (polarization is very small), and has higher capacity (positive NCM523(18mg cm) matched with commercial positive and negative electrodes^-2) (ii) a Negative electrode graphite (10mg cm)^-2)). And is mixed with a lithium iron phosphate anode (15mg cm) at 120 DEG C^-2) The matching exhibits a stable cycle.

The invention also provides application of the flexible boron nitride nanotube inorganic diaphragm as a diaphragm of a lithium secondary battery.

In the invention, the flexible boron nitride nanotube inorganic diaphragm can meet the flexible assembly requirement of the lithium secondary battery, and not only can obviously improve the multiplying power of the material and the cycling stability under the limit condition. For example, when applied to conventional large-capacity lithium batteries and lithium metal batteries, the lithium-based lithium battery has higher capacity and lithium dendrite resistance than commercial separatorsIn the pool at 3.5mA cm^-2The stable circulation can be carried out for more than 700 hours at 24 mV.

In the present invention, the lithium secondary battery is a lithium ion battery or a lithium metal battery.

The invention also provides a lithium secondary battery, and the diaphragm is the flexible boron nitride nanotube inorganic diaphragm.

Advantageous effects

1. The invention provides an all-inorganic diaphragm material formed by interleaving BN nanotubes. The material can unexpectedly solve the technical problems of large brittleness and poor flexibility of inorganic materials, has good flexibility, can meet the requirements of bending and folding and maintaining structural integrity, and can resist mechanical collision in the battery assembly process; moreover, the lithium ion battery is formed by the BN nanotubes in a staggered mode, the pore structure is uniformly distributed, the liquid absorption effect of the electrolyte and the ion conduction are excellent, and the lithium dendrite formation can be effectively inhibited. Research shows that the material has excellent multiplying power and high-temperature cycling stability.

The all-inorganic diaphragm is formed by BN nano-tubes, wherein BN is in an ionic bond, lone pair electrons are biased to N atoms by the larger electronegativity of N, and the characteristic enables the BN to have good insulativity, thermal conductivity and electrochemical stability. In addition, the BN nano tube of the all-inorganic diaphragm has the Lewis acid-base characteristic. The lone pair of N can act as a base with acidic Li during lithium deposition⁺Combined with slow solvation of lithium with solvent molecules, more Li⁺And (4) dissociating. B acts as an acid to trap anions in the electrolyte to dissociate the lithium salt and mitigate decomposition of the electrolyte. The dissociation of the lithium salt increases the lithium ion conduction, shows extremely small polarization, and can well inhibit lithium dendrites.

For example, the material of the invention still shows higher capacity at 120 ℃, and the common diaphragm can not be charged and discharged. Compared with the traditional lithium battery diaphragm, the brand-new diaphragm provided by the invention has better electrolyte wettability and higher electrolyte occupancy rate, so that the brand-new diaphragm has higher ionic conductivity of 2mS cm^-1And a larger ion transport number t_Li ⁺0.86. Book (I)The diaphragm has better flexibility and mechanical property, and avoids short circuit caused by collision between steel sheets and electrodes in the processes of battery assembly and charging and discharging. Compared with the traditional diaphragm, the diaphragm provided by the invention has better lithium dendrite resistance, can stably circulate for 700 hours under a larger current density, has small polarization (the charge-discharge curve is flat), and the polarization potential of the traditional diaphragm reaches 300mV and has larger fluctuation (the charge-discharge curve is arc). Compared with the traditional lithium battery diaphragm, the diaphragm of the invention has higher capacity (such as the positive NCM523(18mg cm) compared with the matching of the traditional lithium battery diaphragm and a commercial positive electrode and a commercial negative electrode^-2) (ii) a Negative electrode graphite (10mg cm)^-2)). Compared with the traditional lithium battery diaphragm, the diaphragm provided by the invention can present stable circulation at 120 ℃, and a lithium iron phosphate positive electrode (15mg cm) is used^-2)。

2. The present invention also provides a means of "tube-in-tube" preparation and has been found to unexpectedly improve the performance of the resulting material as a lithium secondary battery separator.

In addition, the preparation process of the invention has no complicated operation steps and other raw materials, the equipment is simple, and the operation is simple and easy.

The invention provides possibility for safe large-current charging and discharging energy storage devices.

Drawings

FIG. 1 is a schematic view of an improved tube furnace according to the present invention;

FIG. 2 is an electron microscope image of BN nanotubes prepared under deposition condition A;

FIG. 3 is an electron microscope image of BN nanotubes prepared under deposition condition B;

FIG. 4 is a physical diagram of BN nanotubes prepared under deposition condition C;

FIG. 5 SEM image and EDS of the BN nanotube separator made in example 1;

FIG. 6 is an XRD and IR plots of the BN nanotube separator made in example 1;

FIG. 7 shows the heat resistance of the BN nanotube separator prepared in example 1;

FIG. 8 is the ionic conductivity of the BN nanotube separator made in example 1;

fig. 9 is a comparison of the lithium ion transport number of the BN nanotube separator prepared in example 1 and a commercial separator (lithium ion transport number and ionic conductivity);

FIG. 10 is a graph of lithium dendrite resistance (cycle and detailed enlargement) of the BN nanotube separator made in example 1;

FIG. 11 is the full cell (NCM/graphite) 1C room temperature cycling performance of the BN nanotube separator made in example 1 with a commercial separator;

FIG. 12 shows the BN nanotube separator prepared in example 1 and the conventional lithium battery separator at 1C (LiFePO) at 120 ℃₄Li battery) cycle performance;

FIG. 13 is the flexibility of the BN nanotube membrane made in example 1;

FIG. 14 is a graph of the rate capability of the BN nanotube separator made in example 1 versus a full cell (NCM/graphite) commercial separator;

FIG. 15 is an optical picture of the film made in example 2;

FIG. 16 is an optical picture of the film made in example 3;

fig. 17 is an SEM image of the BN nanotube separator prepared in comparative example 1;

FIG. 18 is a lithium dendrite comparison of the BN nanotube separator made in example 1 and the separator made in comparative example 1;

fig. 19 is a physical diagram of the BN nanotube separator prepared in comparative example 2;

fig. 20 is a pictorial view of a BN nanotube separator prepared in comparative example 3;

fig. 21 is a pictorial view of a BN nanotube separator prepared in comparative example 4;

fig. 22 is a pictorial view and an SEM image of the BN nanotube separator prepared in comparative example 5;

fig. 23 is a pictorial view of a BN nanotube separator prepared in comparative example 6;

fig. 24 is a pictorial view of a BN nanotube separator prepared in comparative example 7;

fig. 25 is a pictorial view of a BN nanotube separator prepared in comparative example 8;

the specific implementation method comprises the following steps:

the present invention will be further described below by way of examples, but the present invention is not limited to the following.

In the present invention, the carbon template is prepared by the following method except for the specific statement:

PAN (Mw 150000) (1.2g) was dissolved in N, N-dimethylformamide (10mL) and magnetically stirred at 75 ℃ for 12h to form a homogeneous pale yellow solution. The solution was then charged to a 12mL syringe pump at 0.8mLh^-1Flow rate of (d), voltage injection of 18 KV. The fibers were collected from a substrate 15cm from the needle and dried under vacuum at 70 ℃ overnight. Raising the temperature to 280 ℃ in a muffle furnace at a speed of 2 ℃/min, keeping the temperature for 3h (air atmosphere), and pre-oxidizing to keep the shape of the fiber. Then at 5 deg.C for min^-1And heating to 650 ℃ at the heating rate in the Ar atmosphere, and keeping the temperature for 2h to obtain the carbon template (electrostatic spinning membrane template).

In the experimental case, the schematic diagram of the tube-in-tube reaction equipment adopted in the invention is shown in fig. 1, and the tube-in-tube reaction equipment comprises a tube furnace, wherein the tube furnace comprises a reaction tube with an air inlet and an air outlet, and a heating device for heating a quartz tube, and a small tube is also arranged in the quartz reaction tube; one end of the small tube is open (called an open end or an open end), the other end of the small tube is closed (called a closed end), and the open end of the small tube is close to the air inlet side of the reaction tube; the closed end is close to the air outlet side of the reaction tube.

The tube Furnace is a double-temperature tube Furnace (Tianjin Zhonghuan Electric Furnace Co. LTD) (SK-G06123K-2-420), and the tube diameter (external diameter) of the reaction tube of the tube Furnace is 5cm.

The small tube is a round and straight quartz tube, wherein one end of the small tube is closed, and the other end of the small tube is open; the pipe diameter (outer diameter) is 4cm (thickness is 0.3-0.5 cm).

First, screening deposition conditions

Deposition conditions A:

100mg of boron powder with the total mass purity of 99.8 percent, magnesium oxide and alpha-ferric oxide are mixed according to the molar ratio of 1:0.5:0.5 to be used as raw materials, the raw materials are ground for a certain time and then placed in an alumina porcelain boat, the temperature is increased to 1200 ℃ at the speed of 10 ℃/min in a vacuum tube furnace (SK-G06123K-2-420), the flow rate of carrier gas (argon) is set to be 100sccm/min, and ammonia gas is replaced by 100sccm/min at the temperature of 1200 ℃ for 2 hours. After washing the sample for several times with dilute hydrochloric acid, distilled water and ethanol, drying in a vacuum drying oven to obtain white powder with yield of 20% and little yield, only a small amount of raw materials on the surface react to form the nano tube. The electron micrograph is shown in FIG. 2.

And (3) deposition conditions B:

100mg of boron powder with the total mass purity of 99.8 percent, magnesium oxide and gamma-ferric oxide are mixed according to the molar ratio of 1:0.5:0.5 to be used as raw materials, the raw materials are ground for a certain time and then are heated to 1200 ℃ at the speed of 10 ℃/min in a vacuum tube furnace (SK-G06123K-2-420), the flow rate of carrier gas (argon) is set to be 100sccm/min, and ammonia gas is replaced by 100sccm/min at the temperature of 1200 ℃ for 2 hours. The sample was washed several times with dilute hydrochloric acid, distilled water and ethanol and dried in a vacuum oven to obtain white powder with a yield of 60%. The nanotube tips are more particulate due to more catalyst. The electron micrograph is shown in FIG. 3.

Deposition conditions C:

100mg of boron powder with the total mass purity of 99.8 percent, magnesium oxide and gamma-ferric oxide are mixed according to the molar ratio of 1:0.5:0.25 to be used as raw materials, the raw materials are ground for a certain time and then are heated to 1200 ℃ at the speed of 10 ℃/min in a vacuum tube furnace (SK-G06123K-2-420), the flow rate of carrier gas (argon) is set to be 100sccm/min, and ammonia gas is replaced by 100sccm/min at the temperature of 1200 ℃ for 2 hours. The sample was washed several times with dilute hydrochloric acid, distilled water and ethanol and dried in a vacuum oven to obtain white powder with a yield of 60%. The prepared nanotubes are overall better. The electron micrograph is shown in FIG. 4.

Example 1

In the reaction equipment shown in figure 1, the synthesis specifically comprises the following steps:

uniformly mixing the B powder, magnesium oxide and gamma-ferric oxide according to the molar ratio of 1:0.5:0.25, placing the mixture in an alumina porcelain boat, placing the boat in a small quartz tube with one closed end (the length is 5cm, and the width is 4cm, and the width is the outer diameter of the tube), paving the boat, and then placing a carbon fiber template above a reactant. And (2) placing a small quartz tube into the center of a tubular furnace temperature zone, wherein the open end of the small tube is positioned at the upstream of the gas flow, the closed end is positioned at the downstream of the gas flow (as shown in figure 1), heating to 1200 ℃ at 10 ℃/min under argon (100sccm/min), then converting the carrier gas into ammonia gas flow (with the flow rate of 100sccm/min), carrying out heat preservation reaction for 2 hours, and converting the ammonia gas into argon gas to protect and reduce the temperature to room temperature after the reaction is finished. And sintering in air to remove the template to obtain the BN nanotube film. The prepared diaphragm has flexibility and long diameter, is in a cross-linked uniform pore size structure, can have a large electrolyte occupancy rate, and well transmits lithium ions.

The SEM image and EDS of the BN nanotube film prepared by the method are shown in fig. 5, and the BN nanotube film prepared by the method is in a three-dimensional network structure with the mutual connection, is uniform in element distribution and more in pores and is suitable for being infiltrated by electrolyte as can be seen from fig. 5.

The XRD and infrared patterns of the BN nanotube membrane are shown in figure 6;

the following performance measurements were performed on the BN nanotube separator prepared in this example (in each measurement result diagram, the BN refers to the BN nanotube separator prepared in this example):

(1) the heat resistance measuring process comprises the following steps: placing the diaphragm into the center of the temperature zone of the tube furnace, heating at 100 deg.C for 30min, taking out, taking picture, recording, and placing into the tube furnace, heating at 150, 200, 250, 300, 500, and 800 deg.C for 30min, and heating in air at a temperature-rise rate of 10 deg.C/min. The same membrane is used throughout the process. The results are shown in FIG. 7, wherein BN refers to the BN nanotube film produced in this example, and further comparative analyses were performed using a commercial polypropylene film PP (Celgard 2500), a cellulose membrane CF, a ceramic membrane Ce, and an aramid membrane La. From fig. 7, it can be concluded that the commercial separator is severely deformed at a high temperature of 250 ℃ or lower, while the BN separator of the present case is stable even at 800 ℃, which is advantageous for battery operation in a severe high-temperature environment.

(2) The method for measuring the ionic conductivity data comprises the following steps: steel sheets are used as a working electrode and a reference electrode, a diaphragm is arranged between the steel sheets, (the electrolyte is 1mol L)^-1LiPF of₆The solvent is a battery test impedance of a sandwich structure formed by Ethylene Carbonate (EC) and dimethyl carbonate (DMC) (v/v ═ 1:1) + 2% of Vinylene Carbonate (VC)); the measurement results are shown in fig. 8, and it can be understood from fig. 8 that the bulk resistance of the BN diaphragm is the minimum, and is expressed by the formula σ ═ L/(R)_b×A_s) The ionic conductivity of the BN diaphragm is calculated to be 2mS cm^-17.2 times that of the commercial separator PP, 1.5 times that of CF, 3.9 times that of Ce, and 2.1 times that of La, respectively.

(3) The measurement result of the lithium ion migration number of the BN nanotube membrane is shown in FIG. 9, and it can be concluded from FIG. 9 that the BN membrane has excellent electrolyte wettability due to the three-dimensional pore structure, and the Lewis acidity and alkalinity of BN itself can interact with positive and negative ions in the electrolyte to make the BN membrane present the highest lithium ion migration number, which is one of the key factors for inhibiting lithium dendrite.

(4) The lithium dendrite resistance (cycle and detailed enlargement) of the BN nanotube separator is shown in FIG. 10. from FIG. 10, the BN separator can be obtained at a high current density of 3.5mA cm^-2The current can be stably circulated for 700 hours, the current is uniformly distributed, the top end is square, the commercial diaphragm can be stable for only dozens of hours under the current density, then the voltage fluctuates greatly, the current is not uniformly distributed, and the top end is arc-shaped.

(5) Assembling and testing the full battery: LiNi_0.5Co_0.2Mn_0.3O₂(NCM523)(18mg cm^-2) As the positive electrode, commercial graphite (10mg cm)^-2) As a negative electrode, a battery is assembled by a battery case, a positive electrode, a diaphragm, a negative electrode, a steel sheet, a shrapnel and a battery cover in sequence, wherein the electrolyte is a commercial electrolyte (1mol L)^-1LiPF of₆The solvents were Ethylene Carbonate (EC) and dimethyl carbonate (DMC) (v/v ═ 1:1) + 2% Vinylene Carbonate (VC)), and full cell (NCM/graphite) 1C room temperature cycling performance of BN nanotube membranes and commercial membranes was tested on a blue test system. (the assembly method of the full cell and the conditions of the electrochemical test are described) the results are shown in fig. 11. It can be concluded from fig. 11 that the BN membrane exhibits a higher capacity.

(6) High-temperature stability: BN nanotube membrane and traditional lithium battery membrane 1C (LiFePO) at 120 DEG C₄Li battery) cycle performance is shown in fig. 12; it can be concluded from fig. 12 that the BN separator is indeed highly stable at high temperatures, capable of suppressing dendrite generation, and stable cycling is possible for 300 cycles at these temperatures, whereas a cell assembled with a commercial separator is only cycled for a few cycles at these temperatures, and the cell stops operating or the capacity is greatly attenuated, and the separator may shrink and short-circuit.

(7) Flexibility: in this case, the BN membrane is very flexible and can be folded and bent. See fig. 13. However, the existing inorganic ceramics have great brittleness, and are often combined with a binder or other materials to be used as a separator, which greatly reduces the thermal stability of the separator,

(8) rate capability: the multiplying power performance of the BN nanotube membrane obtained in the example is measured, the result is shown in figure 14, and the BN membrane has 97.8mAh g at 5C^-1Capacity of (C), whereas commercial membranes have a capacity fade of substantially 0mAh g at 3C^-1. The BN nano tube film has better charge and discharge performance under high multiplying power.

Example 2

Compared with the example 1, the difference is that the ferric oxide adopts alpha-ferric oxide, specifically:

uniformly mixing the B powder, the magnesium oxide and the alpha-ferric oxide according to the molar ratio of 1:0.5:0.5, placing the mixture in an alumina porcelain boat, placing the boat in a small quartz tube (5 cm in length and 4cm in width) with one closed end, paving the boat with the open end positioned at the upstream of air flow, and then placing a carbon fiber template above a reactant. And (3) placing the small quartz tube into the center of a tubular furnace temperature zone, heating to 1200 ℃ at 10 ℃/min under argon (100sccm/min), reacting for 2 hours at 1200 ℃ under the condition of replacing ammonia gas (the flow rate is 100sccm/min), and then replacing the ammonia gas with argon to protect and reduce to the room temperature. The prepared film is shown in FIG. 15, and BN is not deposited on the surface of the carbon nano tube (BN is white).

Example 3

uniformly mixing the B powder, the magnesium oxide and the gamma-ferric oxide according to the molar ratio of 1:0.5:0.5, placing the mixture in an alumina porcelain boat, placing the boat in a small quartz tube (5 cm in length and 4cm in width) with one closed end, paving the boat with the open end positioned at the upstream of air flow, and then placing a carbon fiber template above a reactant. And (3) placing the small quartz tube into the center of a tubular furnace temperature zone, heating to 1200 ℃ at 10 ℃/min under argon (100sccm/min), reacting for 2 hours at 1200 ℃ under the condition of replacing ammonia gas (the flow rate is 100sccm/min), and then replacing the ammonia gas with argon to protect and reduce to the room temperature. The prepared film is shown in fig. 16, a large amount of residual iron oxide exists on the surface of the diaphragm (the color is red), the diaphragm cannot be removed after air sintering to form a template, and the iron oxide is conductive and can cause short circuit of the battery.

Comparative example 1

Compared with the example 1, the difference is that the synthesis is carried out without adopting the tube-in-tube mode, specifically:

uniformly mixing the B powder, magnesium oxide and gamma-ferric oxide according to the molar ratio of 1:0.5:0.25, placing the mixture in an alumina porcelain boat, and then placing a carbon fiber template above a reactant. The porcelain boat is put into the center of a temperature zone of a tubular furnace (SK-G06123K-2-420), the temperature is raised to 1200 ℃ at 10 ℃/min under argon, ammonia gas is exchanged at 1200 ℃ and reacts for 2 hours at the flow rate of 100sccm/min, and the ammonia gas is exchanged into argon gas to be protected and then is reduced to the room temperature after the reaction is finished. And sintering in air to remove the template to obtain the BN nanotube film. The prepared diaphragm has no impurity metal atoms, has a longer diameter, but is brittle. The template is brittle after removal. The SEM image is shown in FIG. 17.

A graph comparing lithium dendrites at the same current density for the BN nanotube separator made in example 1 and the separator made in comparative example 1 is shown in fig. 18; it can be seen from fig. 18 that the separator did not resist lithium dendrite as well as the separator prepared in example 1, and there was a fluctuation in voltage. Showing that the electrochemical performance is greatly different from that of the example 1.

Comparative example 2

Compared with example 1, the difference is only that the reaction temperature is 900 ℃, specifically:

uniformly mixing the B powder, the magnesium oxide and the gamma-ferric oxide according to the molar ratio of 1:0.5:0.25, placing the mixture in an alumina porcelain boat, placing the alumina porcelain boat in a small quartz tube (5 cm in length and 4cm in width) with one closed end, paving the quartz tube, and then placing a carbon fiber template above a reactant. And (3) placing the small quartz tube into the center of a tubular furnace temperature zone, heating to 900 ℃ at 10 ℃/min under argon (100sccm/min), reacting for 2 hours at 900 ℃ under the condition of changing into ammonia gas (the flow rate is 100sccm/min), and reducing the temperature to room temperature under the protection of the argon gas after the reaction is finished. The prepared film is shown in fig. 19, and it can be seen that BN is not deposited on the carbon nanotube film (BN is white), indicating insufficient deposition temperature.

Comparative example 3

Compared with the example 1, the difference is only that the carbon cloth is adopted to replace the carbon film of electrostatic spinning, and the specific steps are as follows:

uniformly mixing the B powder, the magnesium oxide and the gamma-ferric oxide according to the molar ratio of 1:0.5:0.25, placing the mixture in an alumina porcelain boat, placing the alumina porcelain boat in a small quartz tube (5 cm in length and 4cm in width) with one sealed end, paving the quartz tube, and then placing carbon cloth (MA-EN-CU-05021O) above reactants. And (3) placing the small quartz tube into the center of a tubular furnace temperature zone, heating to 1200 ℃ at 10 ℃/min under argon (100sccm/min), reacting for 2 hours at 1200 ℃ under the condition of replacing ammonia gas (the flow rate is 100sccm/min), and then replacing the ammonia gas with argon to protect and reduce to the room temperature. The film produced is shown in fig. 20, and it can be seen that substantially no BN was deposited on the carbon cloth (BN was white).

Comparative example 4

0.7g of boron oxide powder was first dissolved in 10mL of absolute ethanol solvent at 45 ℃ with stirring, and then 0.35g of PVB (Mw 60000) powder was dissolved in 10mL of absolute ethanol solvent, and B was poured into the solution₂O₃Ethanol solvent, the mixture was stirred for 24 hours. Then the output voltage of electrostatic spinning is set to be 16KV, and the flow rate is set to be 0.8 mL/h. Then, the fibers removed are transferred into a tube furnace with 10% O₂/90％NH₃(vol/vol) of the mixed gas (total flux 200sccm/min) was heated to 800 ℃ and after 2 hours of the reaction, the oxygen was turned off and NH was introduced at a rate of 100sccm/min₃Then, the reaction mixture was heated to 1100 ℃ at a heating rate of 5 ℃/min and reacted for 3 hours. Then the ammonia gas is replaced by argon (80sccm/min) for protection and is cooled to the room temperature, and finally the Boron Nitride Nanofiber (BNNF) is obtained. The prepared separator is shown in fig. 21, and the separator is very brittle and can not be used for assembling a battery at all, and the preparation method is the BN fiber separator mentioned in other patents. The need to work with others such as magnesium oxide, ethylene oxide, etc. for the separator is also mentioned.

Comparative example 5

Preparing BN nano-tube by adopting deposition condition C, and adding 30 wt% of H into the BN nano-tube₂O₂Stirring for 12 hours, keeping the temperature in the reaction kettle at 80 ℃ for 24 hours to hydroxylate the surface of the reaction kettle, drying, centrifuging and washing with water and ethanol, ultrasonically dispersing with ethanol, vacuum-filtering to obtain a commercial PP membrane, and observing that the two membranes are combined together and are not firm and can peel and fall off from a material object picture. The topography is shown in FIG. 22. The separator prepared by the method cannot be usedIn electrochemical tests, the electrolyte was dispersed in the electrolyte without being pumped out and assembled in a glove box.

Comparative example 6

Preparing BN nano-tube by adopting deposition condition C, and adding 30 wt% of H into the BN nano-tube₂O₂Stirring for 12 hours, keeping the temperature in the reaction kettle at 80 ℃ for 24 hours to hydroxylate the surface of the BN nano tube, drying, centrifugally washing with water and ethanol, ultrasonically dispersing with water, adding 3 wt% of polyethyleneimine and 2 wt% of polyvinyl alcohol, ultrasonically dripping a plurality of drops on a glass slide, drying and tearing off to obtain the BN nano tube polymeric membrane. The prepared membrane has poor toughness and is easy to crack, and the surface of the membrane can be seen as particles due to poor hydrophobic dispersibility of the BN nano tube. The topography is shown in FIG. 23. The diaphragm prepared by the method has insufficient toughness, and the diaphragm is broken by a pair of tweezers in the battery assembling process and is not easy to assemble into a battery. And the membrane has poor stability because the melting point of the polyethyleneimine is 60 ℃ and the melting point of the polyvinyl alcohol is 260 ℃. Moreover, polyethyleneimine is easy to absorb moisture, and a diaphragm can turn yellow.

Comparative example 7

Preparing BN nano-tube by adopting deposition condition C, and mixing the BN nano-tube with 30 wt% of H₂O₂And (3) keeping the temperature of the reaction kettle at 80 ℃ for 24 hours after mixing, drying, centrifugally washing water and ethanol, ultrasonically dispersing the mixture by using ethanol, adding 5 wt% of polyvinyl butyral, ultrasonically dripping the mixture on a glass slide, drying, and tearing off the glass slide to obtain the BN nanotube polymeric membrane. The prepared membrane is yellow and has a relatively flat surface, which shows that the BN nano tube is relatively well dispersed in the polyvinyl butyral, but the heat resistance of the membrane is poor due to the limitation of low melting point (90-120 ℃) of the polyvinyl butyral. The topography is shown in FIG. 24. The toughness of the diaphragm is not good, and the tweezers can generate holes and break off when lightly colliding in the battery assembling process.

Comparative example 8

Adding boric acid and melamine into water according to the mass ratio of 3:1, uniformly mixing, heating in a water bath at 90 ℃ for 5 hours until the solution becomes clear to form H₂O-H₃BO₃-C₃N₆H₆The compound was then sonicated at 100W power and the solution cooled to 65 ℃ to ensure that the temperature did not drop. In the course of the cooling process,compound H₂O-H₃BO₃-C₃N₆H₆Precipitation followed by recrystallization into 1D nanowires occurs as follows:

2H₃BO₃+C₃N₆H₆→C₃N₆H₆·2H₃BO₃

then reacted at a temperature of 1100 ℃ and ammonia gas at 200sccm/min for 4 hours to obtain bulk BN. And cutting to obtain the BN film. The membrane prepared by the method has uniform fiber diameter from SEM image, and the appearance is shown in figure 25. However, the foam-like material is similar to foam in a physical diagram, small fragments can fall off, the toughness is poor, the thickness is thick, the foam-like material cannot be cut into micron-sized thickness, the foam-like material cannot be thinned by hot pressing, and slag is easy to fall off, so that the foam-like material cannot be used for assembling batteries.

Claims

1. A flexible boron nitride nanotube inorganic diaphragm is characterized in that the flexible boron nitride nanotube inorganic diaphragm is an inorganic diaphragm material formed by interlacing boron nitride nanotubes.

2. The flexible boron nitride nanotube inorganic membrane of claim 1, wherein the tube diameter of the boron nitride nanotubes is 20-600 nm; the length is 5 to 100 μm.

3. A method for preparing a flexible boron nitride nanotube inorganic membrane according to claim 1 or 2, comprising the steps of:

step (1): carbon fiber membrane template preparation

Obtaining a carbon fiber membrane by adopting an electrostatic spinning method;

step (2): boron nitride preparation

Placing a raw material powder containing B powder and an auxiliary agent and the carbon fiber film in a small tube; then placing the small tube in a tube furnace, and carrying out chemical vapor deposition reaction under the carrier gas containing N source;

and (3): stripper plate

4. The method for preparing a flexible boron nitride nanotube inorganic membrane of claim 3, wherein the step (1) comprises: carrying out electrostatic spinning on spinning solution containing a carbon source to obtain a film precursor, and then carrying out preheating and carbonization treatment to obtain the carbon fiber film;

the carbon source is a polymer carbon source, preferably at least one of Polyacrylonitrile (PAN), polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP);

preferably, in the electrostatic spinning process, the concentration of the carbon source in the spinning solution is 0.01-1 g/mL; preferably 0.08-0.2 g/mL;

preferably, the flow rate of the spinning solution is 0.1-1 mLh^-1(ii) a Preferably 0.5 to 0.9mLh^-1；

The voltage in the spinning process is preferably 15-25 KV;

the distance between the nozzle and the receiving plate in the spinning process is preferably 10-20 cm;

preferably, the temperature of the preheating process is 250-350 ℃;

preferably, the temperature of the carbonization process is 600-700 ℃.

5. The method of claim 3, wherein the auxiliary agent is magnesium oxide or ferric oxide;

preferably, the ferric oxide is gamma-ferric oxide;

further preferably, the molar ratio of the B powder to the magnesium oxide to the ferric oxide is 1: 0.2-0.8: 0.2-0.4.

6. The method of claim 3, wherein the small tubes are straight tubes; preferably a circular straight pipe;

preferably, the pipe diameter of the small pipe is less than or equal to 90% of the pipe diameter of the reaction pipe of the tubular furnace; preferably 30 to 85%.

7. The method of claim 3, wherein the temperature is raised to the deposition temperature in the protective atmosphere, and the carrier gas is converted into the carrier gas containing N source to perform the chemical deposition reaction;

preferably, the shielding gas is nitrogen or inert gas;

the flow rate of the protective gas is preferably 50-150 sccm;

preferably, the N source is ammonia;

the flow rate of the N source is preferably 70-150 sccm;

the heating rate is preferably 6-10 ℃/min;

and (3) treating the material obtained in the step (2) in an oxygen-containing atmosphere to remove the carbon fiber membrane template.

8. A tube furnace for carrying out the method according to any one of claims 3 to 7, comprising a reaction tube having a gas inlet and a gas outlet, and a heating device for heating the quartz tube, wherein the quartz reaction tube further comprises a small tube; one end of the small pipe is open, the other end of the small pipe is closed, and the open end of the small pipe is close to the air inlet side of the reaction pipe; the closed end is close to the air outlet side of the reaction tube.

9. Use of the flexible boron nitride nanotube inorganic membrane according to any one of claims 1 to 2 or the flexible boron nitride nanotube inorganic membrane prepared by the preparation method according to any one of claims 3 to 7 as a membrane for a lithium secondary battery.

10. A lithium secondary battery, characterized in that the diaphragm is the flexible boron nitride nanotube inorganic diaphragm of any claim 1 to 2 or the flexible boron nitride nanotube inorganic diaphragm prepared by the preparation method of any claim 3 to 7.