CN113644235A

CN113644235A - Method for constructing LiF protective layer on three-dimensional lithium-carbon composite material and application of LiF protective layer

Info

Publication number: CN113644235A
Application number: CN202110928971.2A
Authority: CN
Inventors: 刘振源; 周旭峰; 刘兆平
Original assignee: Ningbo Institute of Material Technology and Engineering of CAS
Current assignee: Ningbo Institute of Material Technology and Engineering of CAS
Priority date: 2021-08-13
Filing date: 2021-08-13
Publication date: 2021-11-12

Abstract

The invention provides a method for constructing a LiF protective layer on a three-dimensional lithium-carbon composite material, which comprises the following steps: mixing graphene, fluorine-containing organic binder solution and metal lithium powder to obtain metal lithium/graphene composite material slurry; coating the metal lithium/graphene composite material slurry on a current collector to obtain an electrode; and carrying out thermal lithiation compounding on the electrode to obtain the LiF protective layer on the three-dimensional size. Different from other methods of introducing a fluorine-containing protective layer on the surface of a blocky lithium foil lithium sheet through an additional reaction step in the prior art, the method provided by the invention synthesizes the metal lithium powder/graphene three-dimensional composite anode material with LiF protection through a one-step method of homogenate coating and heating compounding, does not need to use complex equipment, has simple and time-saving preparation process, and can be used for large-area production. The invention also provides an application of the LiF protective layer on the three-dimensional lithium-carbon composite material.

Description

Method for constructing LiF protective layer on three-dimensional lithium-carbon composite material and application of LiF protective layer

Technical Field

The invention belongs to the technical field of electrodes, and particularly relates to a method for constructing a fluorine-rich protective layer on the surface of lithium metal powder and application thereof.

Background

The Solid Electrolyte Interface (SEI) formed by the reaction of the traditional lithium metal negative electrode and the electrolyte is fragile, and is easy to crack due to volume expansion caused by lithium deposition in the repeated charge-discharge cycle process, and the exposed new lithium continuously reacts with the electrolyte, so that the continuous irreversible loss of active substances and the electrolyte is caused, dead lithium without electrochemical activity is generated on the surface of the lithium negative electrode, the internal resistance of the battery is increased, and the capacity of the battery is reduced.

One common means for improving the lithium metal negative electrode is to protect the surface of the lithium metal, and artificially construct a protective layer on the surface of the body lithium with extremely high activity, so as to relieve the reaction between the lithium and the electrolyte, regulate and control the deposition behavior of the lithium in the circulation process, and inhibit the growth of lithium dendrites.

The fermi level of lithium metal is generally higher than the lowest unoccupied electron Level (LUMO) of most organic solvents used as electrolyte, so that lithium spontaneously reacts with the electrolyte, and reaction products are accumulated on the surface of lithium to form a solid electrolyte membrane (SEI film) to block the two from subsequent side reactions, and thus, the SEI film plays a crucial role in the performance of the lithium battery. However, in the process of charging the battery, the lithium dendrite caused by the uneven nucleation growth of the lithium at the interface of the negative electrode can break the SEI, so that the SEI loses the passivation effect, and the continuous side reaction between the lithium negative electrode and the electrolyte can not be prevented, so that the loss is caused continuously. Therefore, the surface protection of the metal lithium negative electrode is a very common modification means, and the artificially introduced protective layer can induce the uniform deposition of lithium by regulating and controlling the distribution of lithium ion current, provide high Young modulus rigidity to inhibit the growth of lithium dendrites, and protect the metal lithium negative electrode from the view points of multiple functions such as a compact interface barrier electrode, an electrolyte and the like. LiF is the optimization component of lithium metal battery SEI membrane acknowledged in the academic world at present, and LiF's fast lithium ion conductor characteristic can guide the smooth deposit of lithium ion, alleviates the dendritic crystal growth and the volume expansion problem of lithium negative pole, stabilizes the SEI structure to reach the purpose that improves battery cycling stability. Therefore, many scientific researches and process researches are aimed at constructing a lithium metal cathode surface protection layer by using fluorine-containing substances, particularly LiF, so as to obtain a safer and more efficient lithium metal battery.

In the prior art, fluorine-containing substances are introduced to modify a lithium metal cathode as a target, so that the preparation process is complicated, large-scale preparation is difficult, and the like, and the application of the technologies in an actual battery system is limited. For example, some methods require special heating, melting, liquefying and spraying devices, hot pressing devices and the like, and require inert atmosphere protection, strict equipment requirements make the preparation process complicated and have extremely high safety requirements, the obtained protective layer has poor uniformity and a general passivation effect, and the cost of manpower and material resources is high. Some technologies need to prepare passivation layers through solid-liquid reaction and are limited by objective reaction conditions, large-scale batch preparation cannot be achieved through the technologies, the quality of a protective layer depends on chemical reaction raw materials and reaction time, the lithium sheet needs to be polished before reaction to remove surface impurities, the reaction is facilitated, residual solution needs to be removed through repeated cleaning after the reaction, and the operation is complex, time-consuming and labor-consuming.

Disclosure of Invention

In view of the above, the present invention provides a method for constructing a LiF protective layer on a three-dimensional lithium-carbon composite material and an application thereof, and the method provided by the present invention does not need to use complex equipment, is simple and time-saving in preparation process, and can be used for large-area production.

The invention provides a method for constructing a LiF protective layer on a three-dimensional lithium-carbon composite material, which comprises the following steps:

mixing graphene, fluorine-containing organic binder solution and metal lithium powder to obtain metal lithium/graphene composite material slurry;

coating the metal lithium/graphene composite material slurry on a current collector to obtain an electrode;

and carrying out thermal lithiation compounding on the electrode to obtain the LiF protective layer.

Preferably, the mass ratio of the graphene to the solute in the fluorine-containing organic binder solution is (2-4): (3-5);

the mass ratio of the metal lithium to the graphene is (6-10): 1.

Preferably, the mass fraction of the fluorine-containing organic binder solution is 5-20%.

Preferably, the fluorine-containing organic binder is one or more selected from polyvinylidene fluoride, hexafluoropropylene, polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene copolymer and ethylene-tetrafluoroethylene copolymer.

Preferably, the thickness of the coating is 200 to 500 micrometers.

Preferably, the temperature of the hot lithiation compounding is 200-400 ℃.

Preferably, the time for the thermal lithiation recombination is within 1 minute.

Preferably, the thermal lithiation further comprises, after the compounding:

and mechanically rolling the product after the hot lithiation compounding to obtain the LiF protective layer.

Preferably, the rolling thickness is 1/3-1/2 of the coating thickness.

The present invention provides a battery comprising: the LiF protective layer is prepared by the method in the technical scheme.

Directly and mechanically homogenizing and mixing lithium metal powder, graphene powder and a fluorine-containing organic binder in a polar solvent N-methyl pyrrolidone (NMP), coating the mixture on a copper foil in a freely adjustable thickness, and drying the mixture in vacuum to form a lithium metal/graphene composite foil; and then, heating on a glove box heating table for a short time to carry out a thermal lithiation compounding process, pyrolyzing the fluorine-containing organic binder by utilizing heat released by graphene lithiation reaction, and forming a graphene-coated and LiF-uniformly-dotted protective layer on the surface of lithium particles by a one-step method. The technology combined with slurry coating and heating can adopt simple preparation technology and equipment to carry out large-scale material production. More importantly, the invention does not need to additionally add a third phase substance while introducing the fluorine-containing protective layer, the binder is simultaneously a fluorine source of LiF and exists as the binder during homogenate compounding, LiF is generated by reaction with Li in the heating and lithiation process after the foil is formed, and the decomposition product H₂The reaction process can be autocatalyzed, the compounding process is rapid, additional heating is not needed once the reaction is started, and energy can be saved. The whole reaction process is finished by a one-step method, other substances are not required to be added or pretreatment and post-treatment are not required, the operation is simple and convenient, and the application prospect is wide.

The invention provides a method for constructing an artificial LiF protective layer by an in-situ reaction of an ingenious hydrogen autocatalytic pyrolysis organic binder on the surface of a lithium metal powder/graphene powder composite cathode. After a short time of heating induction, the lithium is coated on the surfaceThe graphene of (a) reacts to form a lithium-philic lithium carbon compound (LiC)₆) And releasing heat, the fluorine-containing binder is thermally decomposed and reacts with lithium to generate a large amount of LiF, an artificial protective layer with graphene anchoring coating and LiF decoration is formed on the surface of the metal lithium powder particles, and H generated by the pyrolysis reaction₂And the in-situ reaction can be self-catalyzed, and finally a protective layer rich in LiF is constructed on the surface of the lithium metal powder/graphene composite cathode by a one-step method, the protective layer can effectively standardize the deposition and dissolution behavior of lithium ions, relieve the side reaction of lithium metal and electrolyte, stabilize SEI, and solve the problems of dendritic crystal growth, dead lithium accumulation, low cycle life and the like of the traditional lithium metal battery.

Different from other methods of introducing an additional reaction step to introduce a fluorine-containing protective layer on the surface of a blocky lithium foil lithium sheet in the prior art, the method provided by the invention synthesizes the metal lithium powder/graphene composite anode material with LiF surface protection by a one-step method of homogenate coating and heating compounding, does not need to use complex equipment, has simple and time-saving preparation process, and can be used for large-area production.

Drawings

FIG. 1 is a surface topography of a modified lithium carbon composite prepared in example 1;

FIG. 2 is a cross-sectional profile of a modified lithium-carbon composite prepared in example 1;

FIG. 3 is a surface topography of a modified lithium carbon composite prepared in example 2;

FIG. 4 is a cross-sectional profile of a modified lithium-carbon composite prepared in example 2;

FIG. 5 is a surface topography of a modified lithium carbon composite prepared in example 3;

FIG. 6 is a cross-sectional profile of a modified lithium-carbon composite prepared in example 3;

FIG. 7 shows the cycling performance of the symmetrical batteries prepared in examples 1-3;

FIG. 8 is a topographical view of a lithium-carbon composite material prepared in example 4;

FIG. 9 is the electrochemical performance of the symmetric cell prepared in example 4;

FIG. 10 is a topographical view of a lithium-carbon composite material prepared in example 5;

FIG. 11 is the electrochemical performance of the symmetric cell prepared in example 5;

FIG. 12 is a topographical view of a lithium-carbon composite material prepared in example 6;

FIG. 13 is the electrochemical performance of the symmetric cell prepared in example 6;

FIG. 14 is a topographical view of a lithium-carbon composite material prepared in example 7;

FIG. 15 is the electrochemical performance of the symmetric cell prepared in example 7;

FIG. 16 is a topographical view of a lithium-carbon composite material prepared in example 8;

fig. 17 is the electrochemical performance of the symmetric cell prepared in example 8.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other examples, which may be modified or appreciated by those of ordinary skill in the art based on the examples given herein, are intended to be within the scope of the present invention. It should be understood that the embodiments of the present invention are only for illustrating the technical effects of the present invention, and are not intended to limit the scope of the present invention. In the examples, the methods used were all conventional methods unless otherwise specified.

In the invention, other carbon materials or other two-dimensional materials can be used instead for coating and compounding the lithium particles and constructing the three-dimensional conductive framework.

In the invention, the graphene is preferably graphene powder; the graphene is preferably dried before use.

In the present invention, the drying is preferably carried out in an oven; the drying temperature is preferably 75-85 ℃, more preferably 78-82 ℃ and most preferably 80 ℃; the drying time is preferably 24 hours or more, and more preferably 48 hours or 72 hours, so as to ensure that the graphene powder is dried without absorbing water.

In the invention, the mass ratio of the graphene to the solute in the fluorine-containing organic binder solution is preferably (2-4): (3-5), more preferably (2.5-3.5): (3.5 to 4.5), most preferably 3: 4.

in the present invention, the solvent in the fluorine-containing organic binder solution is preferably a polar solvent, and more preferably N-methylpyrrolidone (NMP).

In the present invention, the solvent is preferably subjected to molecular sieve water removal before use.

In the present invention, the molecular sieve is preferably subjected to an activation treatment before use.

In the present invention, the method of activation treatment preferably includes:

and (3) cleaning the molecular sieve with ethanol, then cleaning with water, and then drying and heating to complete activation.

In the present invention, the ethanol is preferably anhydrous ethanol; the water is preferably deionized water; the washing is preferably carried out for several times until the water is clear.

In the present invention, the drying is preferably performed in an oven; the drying temperature is preferably 75-85 ℃, more preferably 78-82 ℃ and most preferably 80 ℃.

In the present invention, the heating is preferably performed in a muffle furnace; the heating method is preferably as follows:

and sequentially carrying out first temperature heat preservation for the first time, second temperature heat preservation for the second time, third temperature heat preservation for the third time and fourth temperature heat preservation for the fourth time.

In the invention, the first temperature is preferably 80-120 ℃, more preferably 90-110 ℃, most preferably 100 ℃, and the first time is preferably 1-3 hours, more preferably 1.5-2.5 hours, most preferably 2 hours; the second temperature is preferably 230-270 ℃, more preferably 240-260 ℃, and most preferably 250 ℃; the second time is preferably 1 to 3 hours, more preferably 1.5 to 2.5 hours, and most preferably 2 hours; the third temperature is preferably 330-370 ℃, more preferably 340-360 ℃ and most preferably 350 ℃; the third time is preferably 1 to 3 hours, more preferably 1.5 to 2.5 hours, and most preferably 2 hours; the fourth temperature is preferably 70-90 ℃, more preferably 75-85 ℃, and most preferably 80 ℃; the fourth time is preferably 8 to 12 hours, more preferably 9 to 11 hours, and most preferably 10 hours. In the invention, the heating speed in the heating process is preferably 0.5-1.5 ℃/min, more preferably 0.8-1.2 ℃/min, and most preferably 1 ℃/min.

In the present invention, the method for removing water by using the molecular sieve preferably comprises the following steps:

pouring the activated molecular sieve into the solution to be fully contacted, sealing the bottle mouth with a sealing film, and placing the bottle mouth in a drying oven.

In the present invention, the time of said standing in drying is preferably at least 12 hours or more to ensure water removal, more preferably 24 hours or more.

In the invention, after the NMP is used, the bottle mouth is preferably sealed by a sealing film for storage, and the activated molecular sieve is periodically supplemented into the bottle, so that the water absorption failure of the prior molecular sieve is avoided.

In the present invention, the method for preparing the fluorine-containing organic binder solution preferably includes:

mixing fluorine-containing organic binder powder with a solvent to prepare a solution.

In the present invention, the fluorine-containing organic binder is preferably one or more selected from the group consisting of polyvinylidene fluoride (PVDF), Hexafluoropropylene (HFP), polyvinyl fluoride (PVF), Polytetrafluoroethylene (PTFE), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and ethylene-tetrafluoroethylene copolymer (ETFE); in view of the hydrogen content, fluorine content, solubility, adhesiveness, and the like, PVDF-HFP is more preferable.

In the invention, the mass fraction of the organic binder solution is preferably 5-20%.

In the present invention, it is preferable that the fluorine-containing organic binder solution is placed in a jar and the mouth of the jar is sealed with a sealing film, and the jar is placed on a magnetic stirrer to be sufficiently stirred and dissolved.

In the invention, the fluorine-containing organic binder can also be replaced by an organic component containing N and O, and a lithium nitrogen/lithium oxygen/lithium nitrogen oxide compound is introduced into the lithium-carbon composite negative electrode for surface protection; even multiple organic components can be blended in the raw materials to finally prepare a multi-component 'cocktail type' artificial SEI layer, so that the purpose of protecting the synergistic surface of lithium metal is achieved, and various problems of the lithium metal cathode in the battery cycle are solved.

In the present invention, the preparation method of the lithium metal/graphene composite slurry preferably includes:

mixing graphene, a fluorine-containing organic binder solution and a solvent to obtain a mixed solution;

and mixing the metal lithium powder with the mixed solution to obtain the metal lithium/graphene composite material slurry.

In the present invention, the solvent is preferably NMP; according to the invention, preferably, a proper amount of NMP solution is added according to the viscosity of the mixed graphene and fluorine-containing organic binder solution for size mixing to obtain a mixed solution, and the obtained mixed solution is homogenized by a homogenizer.

In the present invention, the total mass of the solid matter (total mass of the graphene and the lithium metal powder) and the mass of the solvent are preferably 1: (3-5), more preferably 1: (3.5 to 4.5), most preferably 1: 4.

in the present invention, it is preferable that the method further comprises, after obtaining the mixed solution:

the resulting mixture was subjected to a first homogenization.

In the present invention, the first homogenization is preferably carried out in a refiner; the method of the first homogenization preferably comprises:

maintaining at a first rotational speed for a first time; then keeping the rotation speed at a second rotation speed for a second time; then maintaining the rotation speed for a third time; then at the fourth speed for a fourth time and finally at the fifth speed for a fifth time.

In the invention, the first rotating speed is preferably 800-1200 rpm, more preferably 900-1100 rpm, and most preferably 1000 rpm; the first time is preferably 25 to 35 seconds, more preferably 28 to 32 seconds, and most preferably 30 seconds. In the invention, the second rotating speed is preferably 1800-2200 rpm, more preferably 1900-2100 rpm, and most preferably 2000 rpm; the second time is preferably 100 to 140s, more preferably 110 to 130s, and most preferably 120 s. In the invention, the third rotating speed is preferably 2300-2700 rpm, more preferably 2400-2600 rpm, and most preferably 2500 rpm; the third time is preferably 100 to 140s, more preferably 110 to 130s, and most preferably 120 s. In the invention, the fourth rotating speed is preferably 800-1200 rpm, more preferably 900-1100 rpm, and most preferably 1000 rpm; the fourth time is preferably 30 to 50s, more preferably 35 to 45s, and most preferably 40 s.

the mixture was transferred into an inert gas glove box, into which metal lithium powder (LMP) was added.

In the invention, the mass ratio of the lithium metal powder to the graphene is preferably (6-10): 1, more preferably (7-9): 1, most preferably 8: 1.

in the present invention, the mass content of the binder in the total mass of the binder, the graphene powder and the lithium metal powder is preferably 6 to 15 wt%, more preferably 8 to 12 wt%, and most preferably 10 wt%.

In the present invention, it is preferable that the lithium metal powder and the mixed solution after mixing further include:

the resulting mixture was subjected to secondary homogenization.

In the present invention, the secondary homogenization is preferably carried out in a homogenizer; the method of secondary homogenization preferably comprises:

at the fifth speed for a fifth time, then at the sixth speed for a sixth time, and finally at the seventh speed for a seventh time.

In the invention, the fifth rotating speed is preferably 800-1200 rpm, more preferably 900-1100 rpm, and most preferably 1000 rpm; the fifth time is preferably 25 to 35 seconds, more preferably 28 to 32 seconds, and most preferably 30 seconds. In the invention, the sixth rotating speed is preferably 2300-2700 rpm, more preferably 2400-2600 rpm, and most preferably 2500 rpm; the sixth time is preferably 100 to 140s, more preferably 110 to 130s, and most preferably 120 s. In the invention, the seventh rotating speed is preferably 800-1200 rpm, more preferably 900-1100 rpm, and most preferably 1000 rpm; the seventh time is preferably 25 to 35 seconds, more preferably 28 to 32 seconds, and most preferably 30 seconds.

In the present invention, all the transfer processes during the process of constructing the LiF protective layer on the three-dimensional lithium-carbon composite material are preferably sealed with a sealing film to prevent moisture in the air from entering and reacting with lithium.

In the invention, all raw materials used in the process of constructing the LiF protective layer on the three-dimensional lithium-carbon composite material are preferably strictly dehydrated, so that the phenomenon that trace moisture residue reacts with the metal lithium powder to influence the final material performance is avoided.

In the present invention, the coating process is preferably carried out in an ultra-dry time with a dew point < -30 ℃, more preferably < -40 ℃.

In the present invention, the current collector is preferably a copper foil.

In the invention, the thickness of the coating is preferably 250-350 microns, more preferably 280-320 microns, and most preferably 300 microns; the lithium loading per unit area can be freely controlled by the coating thickness.

In the present invention, it is preferable that the coating further comprises:

the coated product is dried to completely evaporate the solvent.

In the present invention, the drying is preferably performed in a vacuum oven; the drying temperature is preferably 55-65 ℃, more preferably 58-62 ℃, and most preferably 60 ℃; the drying time is at least 6 hours to ensure complete evaporation of the solvent, more preferably 8 to 12 hours, and most preferably 10 hours.

In the present invention, it is preferable that the dried product is vacuum-sealed with an aluminum plastic film and then placed in a glove box after the drying is completed.

In the present invention, the thermal lithiation compounding is preferably performed in an inert gas glove box.

In the invention, the temperature of the hot lithiation compounding is preferably 200-400 ℃, more preferably 250-350 ℃, and most preferably 300 ℃; the time for the thermal lithiation compounding is preferably within 1 minute, more preferably 5-45 s, more preferably 10-30 s, and most preferably 20 s.

In the present invention, the method of thermal lithiation recombination preferably includes:

heating a heating table to 300 ℃, placing the coated product (the current collector is copper foil) on the heating table, and observing that dark gold traces slowly appear on a gray-black composite material after a few seconds to cause lithiation reaction of lithium and graphene; after a few seconds, the heat release of the lithiation reaction is accumulated to a certain extent, the binder begins to react with lithium and pyrolyze, and the copper foil is further changed into bright yellow.

In the present invention, the bright yellow reaction rapidly diffuses, and the entire reaction process is completed within one minute without additional operations.

In the present invention, it is preferable that the thermal lithiation after the recombination further includes:

In the invention, the golden composite material foil of the product after hot lithiation compounding is preferably sealed and then transferred to an ultra-dry room with dew point of-30 ℃ for mechanical rolling, and the pole piece is compacted to ensure the surface to be flat, more preferably the dew point is less than-40 ℃.

In the invention, the mechanical rolling thickness is preferably 1/3-1/2 of the coating thickness, and more preferably 0.4.

In the invention, the upper surface and the lower surface of the product (pole piece) after thermal lithiation compounding are preferably wrapped by PET release films in the mechanical rolling process.

In the present invention, the mechanical rolling is preferably performed to a target thickness in two steps, for example, 150 μm in a stepwise manner, first to 170 μm in a first rolling and second to 150 μm in a second rolling.

In the invention, the foil is flat after mechanical rolling and can be directly used as an electrode material.

In the invention, the battery is a button battery or a soft package battery.

In the present invention, the method for preparing the button cell preferably comprises:

and assembling the LiF protective layer, the diaphragm and the electrolyte to obtain a half cell, a counter cell or a full cell.

In the present invention, the assembly is preferably carried out in an inert gas glove box with < 1ppm of both water and oxygen.

In the present invention, the LiF protective layer (lithium metal/graphene composite with LiF protective layer) is preferably cut into a wafer; the diameter of the wafer is preferably 12-16 mm, more preferably 13-15 mm, and most preferably 14 mm.

In the invention, the thickness of the diaphragm is preferably 14-20 micrometers, more preferably 15-18 micrometers, and most preferably 16 micrometers; the membrane is preferably a polypropylene membrane.

In the present invention, it is preferable to use 1mol/L of LiTFSI (lithium bistrifluoromethanesulfonimide) inDOL (1, 3 dioxolane)/DME (ethylene glycol dimethyl ether) ═ 1: 1+ 2% LiNO₃(LiNO₃The mass content of the ether-based electrolyte in the whole solution is 2 percent) to assemble a half cell or a counter cell; 1mol/L of LiPF6 (lithium hexafluorophosphate) in EC (ethylene carbonate)/DMC (dimethyl carbonate) was used as: an ester based electrolyte of 1+ 2% FEC (fluoroethylene carbonate) (FEC content 2% by mass in the total solution) was assembled into a full cell.

In the present invention, the full cell assembly process preferably further includes a positive electrode material, which is preferably a high nickel, lithium iron phosphate or lithium-rich positive electrode material, and a commercial product can be adopted.

In the present invention, the method for preparing the pouch battery preferably includes:

coating a positive electrode material on two sides of the pole piece to obtain a positive pole piece;

placing the membrane in a manual lamination mold;

placing a LiF protective layer (a lithium metal/graphene composite material with the LiF protective layer, a negative plate) on the diaphragm;

welding a reserved level of a tab of the negative plate to a groove on one side of the die, pasting the positive plate on the diaphragm after winding the diaphragm, winding the diaphragm again, placing the negative plate on the diaphragm, and repeating the steps to complete the lamination of the battery core;

respectively welding nickel and aluminum tabs on a negative electrode and a positive electrode by using an ultrasonic welding machine, and taking the aluminum-plastic film for primary packaging after the tabs are welded;

and (3) putting the product subjected to primary packaging into a glove box, injecting electrolyte from the bottom, primarily sealing in the glove box, and taking out for secondary packaging.

In the invention, the cathode material is preferably selected from high nickel, lithium iron phosphate and lithium-rich cathode materials, and can be a commercial product.

In the invention, the single-side capacity in the coating process is preferably 1.5-2.5 mAh/cm²More preferably 2mAh/cm²。

In the present invention, the size of the pole piece may be 43mm × 53 mm.

In the invention, the positions of the positive electrode, the negative electrode and the diaphragm are preferably strictly aligned in the process of preparing the cell lamination.

In the present invention, the method of primary packaging preferably includes:

and arranging the battery cell in the folded aluminum-plastic film, exposing the lug outside the aluminum-plastic film, enabling the hot melt adhesive of the lug to exceed the edge of the aluminum-plastic film by 0.5-2 mm, starting an instrument to finish top sealing and side sealing, and rotating the battery cell and the aluminum-plastic film to finish side sealing.

In the invention, the battery pole lug is preferably pasted with the adhesive tape in the secondary packaging process to prevent the danger caused by the accidental touch and short circuit.

In the invention, after the preparation of the pouch battery is completed, the pouch battery preferably further comprises:

after the battery is kept still for 48 hours, the pressure environment of the battery is adjusted through a pressure die, and then a formation program and a normal circulation program are started: and (3) carrying out soft package full battery circulation test at two different multiplying powers of 0.1C and 0.2C under the voltage window of 2.6-4.3V.

According to the invention, the metal lithium powder, the graphene and the fluorine-containing organic binder are directly coated with slurry, and a thermal lithiation compounding mode is adopted, so that the metal lithium/graphene composite negative electrode foil protected by LiF can be simply and conveniently prepared in a large area. The components in the composite material prepared by the invention, such as lithium loading capacity per unit area, graphene coating degree and fluorine content, can be freely regulated and controlled by the proportion of raw materials and coating thickness. In the process of preparing the LiF protective layer, no additional fluorine-containing substance is needed to be added, the organic binder can be used as a fluorine source to react with lithium to generate LiF, and the decomposition product H₂Has the autocatalysis effect, and the whole reaction process is finished by a one-step method. The product finally obtained by the method provided by the invention is a composite material structure with LiF decorated and graphene anchored and coated on the surface of lithium particles, the surface protective layer is uniform and stable, and LiF and LiC are₆And the protective effect is good due to the synergistic effect of the graphene and the organic silicon compound.

The graphene adopted in the following embodiments of the invention is industrial-grade pure graphene raw powder provided by Ningbo Mexico science and technology Limited, and the copper foil is battery-grade copper foil provided by Shenzhen copper foil manufacturing Limited.

Example 1

0.15g of dried graphene raw powder was weighed into a homogenizer cup, 4g of 5 wt% PAN solution (polyacrylonitrile solution) was added thereto, and 2g of NMP was further added dropwise. Cover the bowl cover and adopt the sealing film to seal the rim of a cup, homogenize with the refiner, the rotational speed is as follows: 1000rpm-30s, 2000rpm-120s, 1200rpm-60s, 2500rpm-120s, 1000rpm-40 s; after homogenization, the mixture was transferred into an inert gas glove box, 1.2g of stabilized Lithium Metal Powder (LMP) was added to the glove box and the mouth of the cup was sealed again with a sealing film. The mixture after the lithium powder addition is homogenized for one time by a homogenizer at the following rotation speed: 1000rpm-30s, 2500rpm-120s, 1000rpm-30 s; and homogenizing for two times to obtain the final lithium metal/graphene composite material slurry.

The coating process is carried out in an ultra-dry room with dew point of-30 ℃: coating the prepared composite material slurry on a copper foil with the thickness of 300 microns; transferring the coated film to a vacuum oven at 60 ℃ for drying for 6h to ensure that the solvent is completely volatilized; after drying, vacuum packaging with an aluminum plastic film, and then putting into a glove box.

Carrying out hot lithiation composite reaction in an inert gas glove box: heating a heating table to 300 ℃, placing the dried copper foil coated with the lithium-carbon composite material on the heating table, and observing that dark gold traces slowly appear on the gray-black composite material after a few seconds to cause lithiation reaction of lithium and graphene; after a few seconds, the heat release of the lithiation reaction is accumulated to a certain degree, the binder starts to react with lithium for pyrolysis, and the foil is further changed into bright yellow; the bright yellow reaction diffuses rapidly, the entire reaction process is completed in less than one minute and no additional operation is required.

Sealing the reacted golden composite material foil, taking out the foil from a glove box, transferring the foil to an ultra-dry room with a dew point of-30 ℃ for a mechanical rolling process, compacting the pole piece and ensuring the surface to be smooth; during rolling, the upper part and the lower part of the pole piece are wrapped by PET release films and are gradually rolled to 250 micrometers in two steps (the thickness of two release films is 150 micrometers, namely the composite pole piece is rolled to 100 micrometers, which is 1/3 of coating thickness); the rolled foil is flat and can be directly used as an electrode material after being put into a glove box.

Example 2

The product was prepared according to the method of example 1, differing from example 1 in that a 5% by weight PVDF solution was used as the binder.

Example 3

The product was prepared according to the method of example 1, except that 5 wt% PVDF-HFP solution was used as the binder in example 1.

SEM (Hitachi S4800) detection of the surface morphology and the cross-sectional morphology of the product prepared in the embodiment 1-3 of the invention is carried out, and the detection results are shown in FIGS. 1-6.

A symmetrical cell was prepared as follows:

in both water and oxygen<Symmetric cell assembly was performed in a 1ppm inert gas glove box: cutting the metal lithium/graphene composite material with the LiF protective layer prepared in the embodiment 1-3 into a small wafer with a diameter of 14mm, wherein a polypropylene diaphragm with a thickness of 16 microns is selected as the diaphragm, and 1mol/L of LiTFSI in DOL/DME is selected as the electrolyte as 1:1+2％LiNO₃the ether-based electrolyte is assembled to obtain the CR2032 button-type symmetrical battery.

Constant current charge and discharge tests were performed on the symmetric batteries prepared above using a blue-ray LAND test system of type CT2001A of wuhanjinuo electronics ltd, and the test results are shown in fig. 7.

As can be seen from examples 1 to 3, different organic components are selected, and different hydrogen contents and fluorine contents have great influence on the microstructure and electrochemical performance of the finally obtained surface-protection-treated metal lithium/graphene composite negative electrode material; PVDF-HFP with lower hydrogen content and higher fluorine content, H liberated during the thermal reaction₂Less, the intensity of the composite reaction is low, the integrity of the pole piece structure is easier to maintain, more LiF substances are generated, better protectiveness is achieved, and the optimal electrochemical cycle stability is finally shown. Therefore, when selecting the raw material organic binder, the chemical composition thereof is sufficiently considered, and an organic binder having a low hydrogen content and a high fluorine content is more preferable.

Example 4

The product was prepared according to the method of example 1, except that 6 wt% PVDF-HFP solution was used as the binder in example 1.

A CR2032 button cell battery was prepared according to the procedure of example 1.

Example 5

The product was prepared according to the method of example 1, except that 10 wt% PVDF-HFP solution was used as the binder in example 1.

Example 6

The product was prepared according to the method of example 1, except that 12.5 wt% PVDF-HFP solution was used as the binder in example 1.

Example 7

The product was prepared according to the method of example 1, except that 25 wt% PVDF-HFP solution was used as the binder in example 1.

Example 8

The product was prepared according to the method of example 1, except that 33 wt% PVDF-HFP solution was used as the binder in example 1.

The products prepared in examples 4 to 8 were tested according to the method described in the above technical scheme, and the test results are shown in fig. 8 to 17.

It can be seen from examples 4 to 8 that the content of the binder in the composite system has a crucial influence on the properties of the finally obtained Li/G composite protected by LiF. If the content of the binder is too low, a good composite effect cannot be achieved, and the graphene cannot uniformly coat the lithium powder particles and is scattered and accumulated; if the binder content is too high, i.e., if the binder is large and the lithium carbon is small per unit area of the coating, H generated by the thermal lithiation reaction₂Too much, violent reaction, the internal structure of the composite material will be destroyed in the violent reaction, and too much lithium raw material will be consumed by the violent composite reaction, thus reducing the lithium loading in the composite material and affecting the final electrochemical performance.

Therefore, in consideration of the preparation processes of the embodiments with different binder contents and the morphology and electrochemical properties of the obtained composite material, from the process point of view, the binder content is in the range of 3-33 wt%, and the Li/G composite material with LiF protection and a certain performance can be prepared. Below this minimum, the adhesive effect does not support large-area coating preparation of the pole pieces; above this maximum, the binder content is too great and the composite is more destructive than optimal during the thermal reaction. In addition, from the perspective of optimal performance, when the content of the binder is 6-15 wt%, the Li/G composite material with LiF protection and optimal modification effect can be obtained. The homogenate coating process of thick liquids is smooth and easy in this interval, and the thermal reaction composite process is moderate, and graphite alkene is even to be wrapped up and is piled up and form electrically conductive network, and a large amount of LiF exist and form artifical protective layer on the composite surface, and pole piece structure is complete, and electrochemical performance has obvious promotion.

According to the embodiment, lithium and graphene are compounded to form a foil by adopting a method of compounding with slurry coating and thermal lithiation, and then the thermal lithiation exothermic reaction is utilized to pyrolyze the fluorine-containing binder to complete the construction of the fluorinated protective layer of the composite cathode, so that the technology of simply coating and preparing the protective layer in a large area and completing the modification of the protective layer in a one-step method is adopted, on one hand, no additional fluorinating agent is needed to be added, the F element in the binder is directly pyrolyzed and released to react with the lithium to generate the LiF protective layer in situ, the whole reaction process is completed in one step by only heating a small amount, and no additional pretreatment or post-treatment step is needed; on the other hand, the preparation method of the foil coated with the slurry is simple, the preparation method of the protective layer is a technology convenient for large-scale application, and the metal lithium/graphene composite negative foil subjected to lithium affinity modification and surface protection can be obtained without depending on complex equipment and complicated preparation processes.

In addition, from the aspect of modification effect, the metal lithium/graphene composite negative electrode foil prepared by the method not only has the function of uniform local lithium ion flow brought by the LiF fast ion conductor characteristic, but also has additional conductivity provided by the graphene serving as a three-dimensional conductive framework, and golden LiC is generated after lithium reacts and is anchored with the graphene coated on the surface of the graphene₆The modification of the lithium-philic substance is beneficial to the uniform nucleation and growth of lithium on the negative electrode. The simple and rapid composite modification process brings the synergistic effect of various modification effects, and obtains the lithium metal composite modification technology with remarkably improved performance and application prospect at lower cost.

While only the preferred embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

1. A method of constructing a LiF protective layer on a three-dimensional lithium-carbon composite, comprising:

2. The method according to claim 1, wherein the mass ratio of the graphene to the solute in the fluorine-containing organic binder solution is (2-4): (3-5);

the mass ratio of the metal lithium to the graphene is (6-10): 1.

3. The method according to claim 1, wherein the fluorine-containing organic binder solution is present in an amount of 5 to 20% by mass.

4. The method according to claim 1, wherein the fluorine-containing organic binder is one or more selected from the group consisting of polyvinylidene fluoride, hexafluoropropylene, polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene copolymer, and ethylene-tetrafluoroethylene copolymer.

5. The method of claim 1, wherein the coating has a thickness of 200 to 500 micrometers.

6. The method according to claim 1, wherein the temperature of the thermal lithiation compounding is 200-400 ℃.

7. The method of claim 1, wherein the time for the thermal lithiation recombination is within 1 minute.

8. The method of claim 1, further comprising, after the thermal lithiation compounding:

9. The method of claim 8, wherein the rolled thickness is 1/3-1/2 of the coating thickness.

10. A battery, comprising: a LiF protective layer prepared according to the method of claim 1.