CN111029533B

CN111029533B - Metallic lithium surface protection method, negative electrode and metallic lithium secondary battery

Info

Publication number: CN111029533B
Application number: CN201911049470.6A
Authority: CN
Inventors: 王骞; 吴恺; 申兰耀; 刘�文; 周恒辉
Original assignee: Pulead Technology Industry Co ltd; Beijing Taifeng Xianxing New Energy Technology Co ltd
Current assignee: Pulead Technology Industry Co ltd; Beijing Taifeng Xianxing New Energy Technology Co ltd
Priority date: 2019-10-31
Filing date: 2019-10-31
Publication date: 2021-09-07
Anticipated expiration: 2039-10-31
Also published as: CN111029533A

Abstract

The invention provides a method for protecting the surface of lithium metal, a cathode and a lithium metal secondary battery, wherein the surface of the lithium metal is treated by adopting a fluoride solution of rare earth element cerium, and an interface alloyed protective film containing lithium cerium alloy on the surface is obtained by an in-situ method, wherein the main components of the protective film are LiF and Li₃N, and Li-Ce alloys. Of these components LiF, Li₃N has high mechanical strength and rapid ion transport ability, thereby facilitating Li⁺The transfer at the interface can effectively inhibit the generation of lithium dendrites, and the distribution of the Li-Ce alloy on the interface can also enable electrons to be rapidly transferred at the interface, thereby being beneficial to realizing rapid charge and discharge.

Description

Metallic lithium surface protection method, negative electrode and metallic lithium secondary battery

Technical Field

The invention belongs to the technical field of batteries, and particularly relates to a metal lithium surface protection process, a metal lithium cathode using the protection process and a secondary lithium ion battery.

Background

Lithium ion batteries have been widely used in smart phones, notebook computers, electric vehicles, and large-scale energy storage systems due to their advantages of high energy density, small size, and small self-discharge effect. However, the existing lithium ion battery technology based on graphite as the negative electrode and lithium-containing transition metal oxide as the positive electrode cannot meet the increasing energy density requirement of people. Under such a large background, new secondary lithium ion battery technologies using metallic lithium as a negative electrode, such as lithium sulfur battery (2600Wh/kg), lithium oxygen battery (3580Wh/kg), etc., are considered to be the most ideal substitutes for the current lithium ion battery systems due to their high theoretical energy density.

However, since metallic lithium has very high chemical and electrochemical reactivity, it is easily corroded by the electrolyte, resulting in a decrease in the utilization rate of lithium. Meanwhile, in the process of charging and discharging, the surface of lithium metal is subjected to electrodeposition continuously, uneven lithium deposition is caused due to concentration polarization on the surface of an electrode, the uneven lithium deposition can be amplified gradually in subsequent long circulation, a dendritic lithium dendritic structure is generated, and finally, a diaphragm can be punctured to cause short circuit of a battery, so that safety problems such as thermal runaway, fire explosion and the like are caused.

To solve the above-mentioned problem of dendrites caused by the use of metallic lithium negative electrodes, the most approved strategy at present is to adopt a three-dimensional current collector and a surface protection method. The method is characterized in that a three-dimensional current collector is adopted, namely, the lithium metal is poured into a three-dimensional frame material, such as a foamed nickel three-dimensional current collector (adv. funct. mater.2017, 1700348). The large specific surface area of the three-dimensional current collector is utilized to reduce local current density and inhibit the generation of lithium dendrites. The surface protection is to artificially modify a protective layer on the surface of the metal lithium, such as a polysiloxane protective layer to inhibit the growth of lithium dendrites (adv.mater.2017,29,1603755), and then to utilize the decomposition of the additive in the charge and discharge process based on the strategy of electrolyte additive decomposition to form LiF and Li on the surface of the cathode₃N and the like faster transporting Li⁺Thereby improving the electrochemical performance.

However, in the above-mentioned method for suppressing the generation of lithium dendrites, the use of a three-dimensional current collector may reduce the energy density of the battery due to the high weight ratio of the current collector itself, and thus is not considered in the commercial application. The scheme of polymer protection on the surface is limited by low mechanical strength and low lithium ion transmission rate, and quick charge and discharge under high current density are difficult to realize. LiF and Li can be formed on the surface of the negative electrode despite the strategy of additive decomposition₃N and the like faster transporting Li⁺But these components have only ionic conductivity and no electronic conductivity, and the electron transfer from the electrode surfaceThe output is suppressed and it is also difficult to achieve rapid charge and discharge at high current density. Therefore, how to realize simultaneous transmission of ions and electrons on the surface of the electrode, thereby realizing rapid charge and discharge and long cycle under high current density is still one of the problems to be solved at present.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to provide a metal lithium surface protection process, which realizes the simultaneous and rapid transmission of electrons and ions of a metal lithium cathode surface protection film. The method comprises the following steps: the surface of lithium metal is treated by adopting a fluoride solution of rare earth element cerium, and an interface alloyed protective film containing lithium cerium alloy on the surface is obtained by an in-situ method, so that the fast ion transmission rate is ensured, the electron transmission is realized, the growth of lithium dendrite is inhibited, and the fast charge and discharge under high current density are realized.

One aspect of the present invention is to provide a method for protecting a surface of lithium metal, comprising:

dripping the metal lithium treatment solution on the surface of a metal lithium sheet, standing for reaction for 20-40min, and then cleaning and airing the surface of the lithium sheet by using an organic solvent;

wherein the solvent of the lithium metal treatment solution is an organic solvent which does not react with lithium metal, the solute is fluoride containing cerium, and the concentration of the solute is 0.1-20mg/mL, preferably 1-10mg/mL, and more preferably 1-3 mg/mL.

Further, the above method wherein the lithium metal treatment solution solvent comprises one or more of N' N-Dimethylformamide (DMF), Tetrahydrofuran (THF),1, 3-Dioxolane (DOL), ethylene glycol dimethyl ether (DME), Ethylene Carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), Propylene Carbonate (PC); preferably, the solvent is one or more of N' N-Dimethylformamide (DMF), Tetrahydrofuran (THF),1, 3-Dioxolane (DOL) and ethylene glycol dimethyl ether (DME); more preferably N' N-Dimethylformyl (DMF).

Further, in the method, the solute of the lithium metal treatment solution is cerium trifluoromethanesulfonate (CeTFSI) or cerium trifluoride (CeF)₃) Or cerium tetrafluoride (CeF)₄) One or more ofA plurality of types; preferably, the solute is cerium triflate (CeTFSI) or cerium tetrafluoride (CeF)₄) One or more of; more preferably cerium triflate (CeTFSI).

Further, the metallic lithium treatment solution was dropped on the surface of a metallic lithium wafer having a diameter of 8mm and a thickness of 500 μm, and the amount of the treatment solution was controlled. Preferably 20 to 75. mu.l, more preferably 50. mu.l. When the dripped treatment liquid is less than 20 microlitres, the treatment liquid can not completely cover the surface of the metal lithium, so that no protective film is generated on partial surface of the lithium; when the dropwise added treatment liquid is higher than 75 microliters, the treatment liquid overflows on the surface of the metal lithium, more treatment liquid is driven to flow out due to the liquidity of the liquid, the waste of the treatment liquid is caused, the residual amount of the surface treatment liquid is small, and the texture of the generated protective film is poor.

Further, the organic solvent used for cleaning the lithium sheet is a common volatile reagent which does not react with lithium metal, and comprises ethylene glycol dimethyl ether, tetrahydrofuran, 1, 3-dioxolane and the like.

Further, the reaction time of the treatment liquid with the metallic lithium needs to be controlled within a suitable range. Preferably 20-40min, more preferably 30 min. When the reaction time is less than 20min, the reaction degree is low, the reaction is incomplete, and a protective film cannot be generated on a part of the surface of the metal lithium or the generated protective film has poor quality; when the reaction time is longer than 40min, the reaction is already complete, the time is not meaningful to be continuously increased, and meanwhile, the treatment solution solvent is volatilized, so that more impurities remain on the surface of the metal lithium, and the subsequent cleaning is inconvenient.

Another aspect of the present invention is to provide a lithium metal anode, the surface of which is treated by the above method.

Another aspect of the present invention is a lithium metal secondary battery including the negative electrode, an electrolytic solution, a positive electrode, and a separator.

The metallic lithium which is subjected to surface treatment by the method is used as a negative electrode, is suitable for various electrolyte systems such as ethers, esters, ionic liquid, gel electrolyte and the like, and is applied to liquid and condensed lithium secondary batteries and lithium air batteries.

The invention has the following beneficial effects:

in the invention, the fluoride of cerium can effectively react with metallic lithium in solution to form a layer of protective film in situ on the surface of the lithium metal, and the main components of the protective film are LiF and Li₃N, and Li-Ce alloys. Of these components LiF, Li₃N has high mechanical strength and rapid ion transport ability, thereby facilitating Li⁺The transfer at the interface can effectively inhibit the generation of lithium dendrites, and the distribution of the Li-Ce alloy on the interface can also enable electrons to be rapidly transferred at the interface, thereby being beneficial to realizing rapid charge and discharge.

Drawings

Fig. 1A is a Scanning Electron Microscope (SEM) image at a 500 μm scale of a lithium sheet without any treatment.

Fig. 1B is a Scanning Electron Microscope (SEM) image at 100 μm scale of a lithium sheet without any treatment.

Fig. 1C is a Scanning Electron Microscope (SEM) image at a 5 μm scale of a lithium sheet without any treatment.

Fig. 2A is a Scanning Electron Microscope (SEM) image of a surface-treated lithium sheet at a 1mm scale.

Fig. 2B is a Scanning Electron Microscope (SEM) image of the surface-treated lithium sheet at a 500 μm scale.

Fig. 2C is a Scanning Electron Microscope (SEM) image of the surface-treated lithium sheet at a 100 μm scale.

FIG. 3 is a graph showing long cycle performance curves in conventional ether electrolyte solutions for a lithium-lithium symmetric battery at room temperature using lithium without any surface treatment as an electrode (a)) and lithium with surface treatment as an electrode (b)), respectively; the current density for charging and discharging was 10mA cm^-2The amount of lithium metal circulated was controlled to 5mAhcm^-2。

FIG. 4 is a surface substance analysis diagram of a surface-treated lithium sheet in a time-of-flight secondary ion mass spectrometer, a) to f) are elemental analysis diagrams of lithium, cerium, carbon, oxygen, sulfur and nitrogen, respectively, under Ar gun bombardment and high vacuum test.

Detailed Description

1. Solution for surface treatment of metallic lithium

1) Solute

The main component of the lithium metal surface treatment solution can be selected from cerium trifluoromethanesulfonate (CeTFSI) and cerium trifluoride (CeF)₃) And cerium tetrafluoride (CeF)₄) Preferably, it is selected from CeTFSI and CeF₄One or more of (a). Further preferred is CeTFSI. When CeTFSI is used, the compactness and the ion conducting capacity of the protective film on the surface of the lithium metal are obviously superior to those of other cerium salts.

The cerium salt is contained in an amount of 0.1 to 20mg/mL, preferably 1 to 10mg/mL, and more preferably 1 to 3mg/mL in the treatment. When the addition amount of the cerium salt is less than 0.1mg/mL, the numbers of cerium ions and TFSI-in the treatment liquid are reduced, so that the surface reaction effect of the treatment liquid and metal lithium is poor, the surface protection film is poor in compactness, the dendritic crystal inhibition effect is poor, meanwhile, the Li-Ce alloy on the surface is less in component, the electron transmission rate is slow, and the rapid transfer of electrons under high current density is difficult to meet. And its ability to transport electrons and ions is decreased, thereby affecting the electrochemical performance of the lithium metal secondary battery. When the content of the cerium salt is more than 20mg/mL, the solubility of the cerium salt is limited. In addition, when the amount of the cerium salt is too high, the cerium salt reacts with lithium excessively, consuming a large amount of lithium, resulting in an increase in the thickness of a protective film on the surface of lithium, and conversely, causing electrons and ions to enter under the protective film, contact with a lithium metal current collector becomes more difficult, further limiting the transport of ions and electrons. Eventually, it also causes a decrease in electrochemical properties of the lithium metal secondary battery, and particularly, has a great influence on electrochemical properties under rapid charge and discharge.

2) Non-aqueous solvent

Since metallic lithium has high chemical reactivity, the solvent must be a non-aqueous solvent. As the non-aqueous solvent, one or more selected from N 'N-Dimethylformamide (DMF), Tetrahydrofuran (THF),1, 3-Dioxolane (DOL), ethylene glycol dimethyl ether (DME), Ethylene Carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), Propylene Carbonate (PC) may be used, and preferably, the solvent is N' N-Dimethylformamide (DMF), Tetrahydrofuran (THF),1, 3-Dioxolane (DOL), ethylene glycol dimethyl ether (DME). Further preferred is N' N-Dimethylformamide (DMF). Wherein when N' N-Dimethylformamide (DMF) solvent is used, the solubility of CeTFSI is optimal, and the compactness and the thickness of the surface protective film of the metallic lithium are optimal. Meanwhile, the ion and electron transmission effect is best, the dendritic crystal growth inhibition effect is best, and the electrochemical performance is improved most obviously.

3) Other additives

Other surface treatment additives commonly used in the art, such as lithium fluoride, lithium sulfide, fluoroethylene carbonate, vinylene carbonate, and the like, may be added to the surface treatment liquid for lithium metal of the present invention as needed.

2. Lithium metal secondary battery

1) And (3) positive electrode: the positive electrode is an electrode having a positive electrode active material layer on a positive electrode current collector. As the positive electrode active material used in the positive electrode active material layer, a material capable of storing and releasing lithium ions during charge and discharge, for example, a layered lithium manganese acid salt such as LiMnO, may be used₂Or Li_xMn₂O₄(0<x<2) Spinel type lithium manganese oxide salt, LiCoO₂、LiNiO₂Substances in which a part of the transition metals present in the above compounds is replaced by other metals, olivine compounds such as LiFePO₄And LiMnPO₄、Li₂MSiO₄(M is at least one selected from Mn, Fe and Co), active nonmetal such as S, I₂And various active loading forms thereof, and the like. They may be used alone or in combination of two or more.

2) Electrolyte solution: any conventional electrolyte for lithium ion batteries may be used, such as LiCl, LiF, LiFSI, LiPF₆、LiTFSI、LiClO₄The lithium salt is dissolved in a common organic solvent such as 1, 3-Dioxolane (DOL), ethylene glycol dimethyl ether (DME), Ethylene Carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), or Propylene Carbonate (PC). At the same time, suitable customary additives may also be added to the electrolyte, such as: lithium fluoride, lithium nitrate, fluoroethylene carbonate, and the like.

3) A diaphragm: any separator may be used as long as it suppresses contact of the positive electrode and the negative electrode, allows charge carriers to permeate, and has durability in the electrolytic solution. Specific materials suitable for the separator may include polyolefins such as polypropylene or polyethylene based microporous films, cellulose, polyethylene terephthalate, polyimide, polyvinylidene fluoride, and the like. They may be used, for example, as porous films, woven or non-woven fabrics.

4) Negative electrode: the surface-protected metallic lithium treated by the method is provided.

And (3) charge and discharge test: in order to explore the effect of the surface film after protection on the improvement of the electrochemical performance, the lithium-lithium symmetric battery accepted in the industry is adopted for characterization. After the surface protection is explored, the effect of inhibiting lithium dendrites is judged, and the influence on the cycle life of the battery is judged. The test conditions were: and (3) carrying out long-time charge-discharge cycle test by adopting a blue battery test system until the battery is short-circuited, and recording the cycle time which is carried out when the battery is short-circuited. The current density for charging and discharging was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2The test temperature was controlled at 25 ℃.

And (4) SEM test: the surface morphology of lithium is observed by a Hitachi S-4800 scanning electron microscope produced in Japan, the test voltage is 10kV, the current is 10 microamperes, the morphology such as the compactness, the thickness and the like of a lithium metal surface film is obtained from the observation result and evaluated, and the evaluation standard is as follows: very good: the surface film has good compactness and moderate film thickness; o: the surface film has poor compactness and is thin; and (delta): the surface film has good compactness and high film thickness; x: the film is very dense and very thin.

The invention is further illustrated by the following examples. These examples are only illustrative and not intended to limit the scope of the invention.

Comparative example 1

SEM tests were performed on the lithium sheets without any treatment before assembling the cells, and the results are shown in fig. 1A to 1C. Preparing a conventional lithium battery electrolyte, adding lithium bistrifluoromethanesulfonylimide (LiTFSI) into a mixed solvent of 1, 3-Dioxolane (DOL) -ethylene glycol dimethyl ether (DME) (DOL and DME bodiesVolume ratio of 1:1), preparing 1mol/L electrolyte, stirring and dissolving to form uniform solution. The electrolyte is adopted, a lithium sheet which is not subjected to any treatment is adopted to assemble the lithium-lithium symmetrical battery, and the charging and discharging tests are carried out on the lithium-lithium symmetrical battery. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2. The long cycle performance of its lithium symmetric cell is shown in a) of fig. 3.

Comparative example 2

The lithium metal sheets were treated with a solution containing no cerium salt. Preparing a treatment solution, namely adding lithium bistrifluoromethylsulfonyl imide (LiTFSI) into N' N-dimethyl formyl (DMF) to prepare a 1mg/mL metal lithium treatment solution, and stirring and dissolving the treatment solution to form a uniform solution. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. And then, assembling the lithium-lithium symmetrical battery by adopting 2 lithium sheets after surface treatment, and carrying out charge and discharge tests on the lithium-lithium symmetrical battery. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 1

The lithium metal sheet is treated with a cerium salt. Preparing a treating solution, namely adding cerium trifluoromethanesulfonate (CeTFSI) into N' N-Dimethylformamide (DMF) to prepare a 1mg/mL metal lithium treating solution, and stirring and dissolving the treating solution to form a uniform solution. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. SEM tests were then performed and the results are shown in FIGS. 2A-2C. Next, another surface-treated lithium plate was subjected to surface substance analysis under a time-of-flight secondary ion mass spectrometer, and the results are shown in fig. 4. And then, assembling the lithium-lithium symmetrical battery by adopting 2 lithium sheets after surface treatment, and carrying out charge and discharge tests on the lithium-lithium symmetrical battery. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2. The long-cycle performance of the lithium-lithium symmetrical battery is shown in b) of FIG. 3Shown in the figure.

Example 2

The lithium metal sheet is treated with a cerium salt. Preparing a treating solution, namely adding cerium trifluoromethanesulfonate (CeTFSI) into N' N-Dimethylformamide (DMF) to prepare a 0.1mg/mL metal lithium treating solution, and stirring and dissolving the treating solution to form a uniform solution. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Examples 3 to 6

The lithium metal sheet is treated with a cerium salt. A treatment solution was prepared as in example 2 except that the concentrations of cerium trifluoromethanesulfonate (CeTFSI) were controlled to 3mg/mL, 5mg/mL, 10mg/mL and 20mg/mL, respectively. 50. mu.L of the above treatment solution was dropped on the surface of a lithium metal sheet having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and a cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 7

The lithium metal sheet is treated with a cerium salt. A treatment solution was prepared as in example 1 except that cerium triflate (CeTFSI) was changed to cerium tetrafluoride (CeF)₄). 50. mu.L of the above treatment solution was dropped on the surface of a lithium metal sheet having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and a cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 8

The lithium metal sheet is treated with a cerium salt. A treatment liquid was prepared in the same manner as in example 1 except that cerium trifluoromethanesulfonate (CeTFSI) was changed to cerium trifluoride (CeF)₃). 50. mu.L of the above treatment solution was dropped on the surface of a lithium metal sheet having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and a cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 9

The lithium metal sheet is treated with a cerium salt. Preparing a treatment solution, namely adding cerium trifluoromethanesulfonate (CeTFSI) into Tetrahydrofuran (THF) to prepare a 1mg/mL metal lithium treatment solution, and stirring and dissolving the solution until a uniform solution is formed. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 10

The lithium metal sheet is treated with a cerium salt. Preparing a treating solution, namely adding cerium trifluoromethanesulfonate (CeTFSI) into 1, 3-Dioxolane (DOL) to prepare a 1mg/mL metal lithium treating solution, and stirring and dissolving the treating solution to form a uniform solution. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Then, 2 lithium sheets after surface treatment are adopted to assemble lithiumAnd (4) symmetry of the battery, and performing charge and discharge tests on the battery. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 11

The lithium metal sheet is treated with a cerium salt. Preparing a treatment solution, namely adding cerium trifluoromethanesulfonate (CeTFSI) into ethylene glycol dimethyl ether (DME) to prepare a 1mg/mL metal lithium treatment solution, and stirring and dissolving the solution until a uniform solution is formed. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 12

The lithium metal sheet is treated with a cerium salt. Preparing a treatment solution, namely adding cerium trifluoromethanesulfonate (CeTFSI) into Ethylene Carbonate (EC) to prepare a 1mg/mL metal lithium treatment solution, and stirring and dissolving the solution until a uniform solution is formed. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 13

The lithium metal sheet is treated with a cerium salt. Preparing a treatment solution, namely adding cerium trifluoromethanesulfonate (CeTFSI) into diethyl carbonate (DEC) to prepare a 1mg/mL lithium metal treatment solution, and stirring and dissolving the solution until a uniform solution is formed. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. Standing for 30 minutes, and using ethylene glycol to make the surface of the lithium sheetCleaning with dimethyl ether, and air drying. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 14

The lithium metal sheet is treated with a cerium salt. Preparing a treatment solution, namely adding cerium trifluoromethanesulfonate (CeTFSI) into dimethyl carbonate (DMC) to prepare a 1mg/mL metal lithium treatment solution, and stirring and dissolving the solution until a uniform solution is formed. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

Example 15

The lithium metal sheet is treated with a cerium salt. Preparing a treatment solution, adding cerium trifluoromethanesulfonate (CeTFSI) into Propylene Carbonate (PC) to prepare a 1mg/mL metal lithium treatment solution, and stirring and dissolving the treatment solution to form a uniform solution. Then, 50. mu.L of the treatment solution was dropped on the surface of a metal lithium plate having a diameter of 8mm and a thickness of 500 μm to carry out a reaction between the surface lithium metal and the cerium salt. And (5) standing for 30 minutes, cleaning the surface of the lithium sheet by using glycol dimethyl ether, and airing. After which SEM tests were performed. Next, 2 lithium sheets after surface treatment were used to assemble a lithium-lithium symmetric battery, and a charge-discharge test was performed thereon. The test current density was 10mA cm^-2The amount of the circulating lithium metal was controlled to 5mAh cm^-2。

The surface protective film effects and the charge and discharge test results of comparative example 1 and examples 1 to 15 are shown in table 1 below.

Very good: the surface film has good compactness and moderate film thickness; o: the surface film has poor compactness and is thin; and (delta): the surface film has good compactness and high film thickness; x: the film is very dense and very thin.

From the above results, it can be seen that:

1. under the premise of controlling the consistency of other conditions such as electrolyte, diaphragm and the like, when the lithium metal with protected surface is used as the electrode, as in examples 1 to 15, the cycle life of the lithium-lithium symmetric battery is much longer than that of the lithium without any treatment used as the electrode in comparative example 1 under the same conditions. This indicates that the introduction of the surface protective film effectively suppresses the growth of lithium dendrites, improving the cycle life of the battery.

2. On the premise of controlling the consistency of other conditions such as electrolyte, diaphragm and the like, the analysis of comparative example 1 and comparative example 2 shows that when the LiTFSI is used as the surface treatment reagent of the lithium metal, only LiF and Li are formed on the surface₃The protective film composed of N and other components adopts the lithium metal treated by the method as an electrode, and the electrochemical cycle performance of the protective film is obviously improved compared with that of the electrode which is not protected by any metal lithium. This is mainly due to the presence of LiF, Li on the surface₃And when the protective film is composed of N and the like, the protective film has certain mechanical strength, and can inhibit the penetration of lithium dendrites to a certain extent, so that the electrochemical cycle performance of the battery is improved to a certain extent.

3. On the premise of controlling the consistency of other conditions such as electrolyte, diaphragm and the like, the analysis of comparative example 2 and example 1 shows that when CeTFSI is used as a surface treatment reagent of the lithium metal, compared with LiTFSI used as a surface treatment reagent, the electrochemical cycle performance of the finally obtained battery is improved from only 320h of cycle to 1400h of cycle, and is obviously improved. Compared with the method of adopting LiTFSI to only replace Li in the CeTFSI as a surface treatment reagent, the electrochemical cycle of the method is greatly improved. This is mainly because, after replacing Li in Li by Ce, the surface is preservedThe composition of the protective film is formed by the former LiF, Li₃The N and the like become an alloyed surface protective film containing the Li — Ce alloy. Compared with a single protective film, the alloyed surface protective film not only has LiF and Li₃N, etc. can conduct Li⁺The component (A) and the component (B) also have the components which can transmit electrons, such as Li-Ce alloy and the like, and realize the double conduction of ions and electrons. And a protective film which can conduct ions and transmit electrons can effectively realize the obvious improvement of the electrochemical performance under high current density.

4. In examples 1 to 6, when a surface-protected lithium metal is used as an electrode under the condition that other conditions such as an electrolyte and a separator are controlled to be consistent with each other. When the solvent and the solute of the metal lithium treatment solution are controlled to be the same, only the concentration of the solute in the treatment solution is changed, and it can be found that when the concentration of the treatment solution is lower than 1mg/mL, the surface film compactness of the metal lithium is poor, the film is thin, and the electrochemical cycle life is low. The main reason is that when the concentration of the treatment solution is low, the amount of cerium salt is small, the reaction degree with the surface of lithium metal is low, so that the surface film is thin, the compactness is poor, meanwhile, the components capable of transferring electrons such as the Li-Ce alloy on the surface are insufficient, the rapid transmission of electrons cannot be realized, and in addition, the mechanical property of the protective film is insufficient, so that the electrochemical cycle performance is finally reduced. When the concentration of the treatment solution is higher than 3mg/mL, the reaction degree is severe due to overhigh concentration, so that the surface film of the final lithium surface protective film has good compactness and high thickness. This adversely affects the passage of the electron ions through the protective film to the bulk of the lithium metal current collector, and also reduces the electrochemical cycle life of the battery, and also wastes cerium salt.

5. Comparing example 1 with examples 7 and 8, it can be seen that, when the solute in the lithium metal treatment solution is replaced under the condition that the electrolyte, the diaphragm and other conditions are controlled to be consistent, it can be seen that, when CeF is used₃And CeF₄When the lithium metal surface protective film is used as a solute of a treatment solution, the compactness and the thickness of the lithium metal surface protective film are poorer than those of the lithium metal surface protective film when CeTFSI is adopted, and the electrochemical cycle performance of the lithium metal surface protective film is obviously lower than that of the lithium metal surface protective film when CeTFSI is adopted as the solute of the treatment solution. This is mainly due to the fact that CeTFSI is chemically structuredContains F, S, O and other elements, and can effectively form LiF and Li when reacting with the surface of metallic lithium₂S,Li₂O,Li₂CO₃And the inorganic components for fast ion conduction are beneficial to increasing the compactness of the surface protective film, and the components for conducting lithium ions are beneficial to fast transmission of the lithium ions under high current density. Therefore, the growth and puncture of dendritic crystals can be well inhibited, the express ionic conduction can be realized, and the electrochemical cycle life is finally prolonged.

6. Comparing example 1 with examples 9 to 15, it can be seen that, when the solvent in the metallic lithium treatment solution is replaced under the condition of controlling other conditions such as the electrolyte and the diaphragm to be consistent, when the low-boiling point and volatile solvent such as THF, DOL and DME is used, the compactness and thickness of the metallic lithium surface protection film are inferior to those of the high-boiling point and volatile solvent such as EC, DEC, DMC and PC, and the electrochemical cycle performance is also significantly lower than those of the high-boiling point solvent used as the treatment solution. This is mainly because, when a substance having a low boiling point is used as a solvent of the treatment liquid, it volatilizes faster, and the reaction time of lithium and the cerium salt is longer, so that sufficient reaction does not proceed, resulting in poor compactness of the membrane. Ultimately affecting the electrical performance and making the electrochemical cycling performance worse. Therefore, in the present invention, the selection of the solvent for the lithium metal treatment solution is important, and a nonvolatile organic solvent is required.

7. Comparing fig. 1A to 1C, fig. 2A to 2C and the cycle performance of comparative example 1, a) in fig. 3 and the cycle performance of example 1, b) in fig. 3, it was found that pure metallic lithium has a remarkable smoothness without the presence of a protective film on the surface thereof, compared to pure metallic lithium sheets, which causes it to be exposed to an electrolyte during charge and discharge, resulting in its easy corrosion by the electrolyte, thereby causing a decrease in the utilization rate of lithium. As can be seen from fig. 2A-2C, after the surface treatment is performed by the treatment solution, a protective layer with moderate thickness and compactness is generated on the surface of the lithium metal. The compact protective layer can effectively inhibit the growth and puncture of lithium dendrites and improve the electrochemical cycle performance of the battery. This can also be seen in comparison to figure 3, which shows the results of electrochemical long cycle performance tests performed on treated lithium sheets as electrodes and on lithium sheets without any treatment. It can be seen that compared to a) in fig. 3, the electrochemical cycle life of the treated lithium metal used as an electrode is significantly higher than that of the electrode made of a pure lithium sheet without any treatment. The surface protection film is introduced to effectively improve the surface chemical behavior of the metallic lithium negative electrode and inhibit the growth of lithium dendrites, thereby improving the cycle life and the safety performance of the battery.

8. In addition, from the mass spectrometry analysis of the lithium metal surface protection film in fig. 4, it can be seen that the surface protection film mainly contains Li, Ce, C, O, F, S and other components after the surface treatment, which is mainly because the reaction between CeTFSI and Li forms Li-Ce alloy, CeO₂，LiF，Li₂S,Li₂O,Li₂CO₃And the like, and it was found that the substance contents corresponding to most elements were almost unchanged after sputtering for about 100s, indicating that the surface protective film had been completely perforated after sputtering for 100 s. In addition, the protective film has the capability of rapidly conducting lithium ions, and can rapidly realize the transmission of electrons. This can both inhibit the growth and penetration of lithium dendrites and achieve rapid conduction of lithium ions and free electrons. The electrochemical cycle life under the condition of large current rapid charge and discharge is prolonged.

The above-described embodiments are merely preferred embodiments of the present invention, but the present invention is not limited to the above-described embodiments, and corresponding modifications made without departing from the principle of the present invention are also considered to be within the scope of the present invention.

Claims

1. A method of protecting a lithium metal surface, comprising:

dripping 20-75 mu L of metal lithium treatment solution on the surface of a metal lithium sheet with the diameter of 8mm and the thickness of 500 mu m, standing for reaction for 20-40min, and then cleaning and airing the surface of the lithium sheet by using an organic solvent; wherein the content of the first and second substances,

the solvent of the lithium metal treatment solution is an organic solvent which does not react with lithium metal, the solute is fluoride containing cerium, and the concentration of the solute is 0.1-20 mg/mL.

2. The method of claim 1, wherein the solute concentration of the lithium metal treatment solution is 1-10 mg/mL.

3. The method of claim 1, wherein the solvent of the lithium metal treatment solution comprises one or more of N' N-dimethylformamide, tetrahydrofuran, 1, 3-dioxolane, ethylene glycol dimethyl ether, ethylene carbonate, diethyl carbonate, dimethyl carbonate, or propylene carbonate.

4. The method of claim 3, wherein the solvent of the lithium metal treatment solution comprises one or more of N' N-dimethylformamide, tetrahydrofuran, 1, 3-dioxolane, and ethylene glycol dimethyl ether.

5. The method of claim 1, wherein the lithium metal treatment solution solute is one or more of cerium triflate, cerium trifluoride, or cerium tetrafluoride.

6. The method of claim 5, wherein the solute of the lithium metal treatment solution is cerium triflate and/or cerium tetrafluoride.

7. The method for protecting the surface of the lithium metal as claimed in claim 1, wherein the organic solvent used for cleaning the surface of the lithium sheet is a volatile reagent which does not react with the lithium metal and comprises one or more of ethylene glycol dimethyl ether, tetrahydrofuran and 1, 3-dioxolane.

8. A lithium metal negative electrode, characterized in that the surface thereof is treated by the method according to any one of claims 1 to 7.

9. A lithium metal secondary battery comprising the negative electrode according to claim 8, an electrolyte, a positive electrode and a separator.