CN117976829A

CN117976829A - Protection method suitable for lithium metal negative electrode

Info

Publication number: CN117976829A
Application number: CN202410036788.5A
Authority: CN
Inventors: 张进
Original assignee: Individual
Current assignee: Individual
Priority date: 2024-01-10
Filing date: 2024-01-10
Publication date: 2024-05-03

Abstract

The invention discloses a protection method suitable for a lithium metal anode, which comprises the following steps of: adding a molecular sieve into fluorosilane to remove water in the fluorosilane; coating the fluorosilane after removing the water on the surface of the lithium metal anode, and reacting for preset time; after cleaning and drying, forming a protective layer on the surface of the lithium metal negative electrode; the protective layer is formed by a double-layer structure, the inner side of the protective layer is an inorganic layer, the protective layer has high mechanical strength, can effectively isolate side reactions of lithium metal and electrolyte, has high ionic conductivity and low electronic conductivity, and can enable lithium ions to quickly and uniformly pass through the protective layer to be deposited on a lithium cathode so as to avoid dendrite generation; the outside is an organic layer, so that side reactions of lithium and electrolyte caused by serious volume change and crushing of the lithium cathode in the charge and discharge process of the inorganic protective layer can be effectively avoided, and the consumption of extra lithium is reduced. The invention can effectively improve the coulomb efficiency and the cycle stability of the lithium metal battery.

Description

Protection method suitable for lithium metal negative electrode

Technical Field

The invention relates to the technical field of lithium metal batteries, and in particular relates to a protection method suitable for a lithium metal negative electrode.

Background

Along with the popularization of new energy automobiles, the requirements of people on the endurance mileage are continuously improved, and the energy density of the lithium ion battery is required to be further improved. However, since the lithium ion battery adopts graphite as the negative electrode material, the theoretical specific capacity is low and only 372mAh g-1 is available, so that the energy density of the lithium ion battery reaches the limit and the requirements of people cannot be met. However, metallic lithium has an ultra-high theoretical specific capacity of 3860mAh g-1, which is far in excess of the theoretical specific capacity of current graphite cathodes. When the metal lithium is matched with commercial lithium iron phosphate, lithium nickel cobalt manganese oxide and other high-voltage anodes, the energy density of the lithium metal battery is expected to reach 500Wh Kg-1, and the lithium metal battery is favorable for further popularization of new energy automobiles.

Lithium metal has higher theoretical specific capacity (3860 mAh g-1) and lower reduction potential (-3.04V relative to a standard hydrogen electrode), is a key candidate material for breaking through the energy density of a new energy battery by 500Wh Kg-1, and is regarded as a 'holy cup' cathode of a next-generation high-energy-density battery. However, in the practical application process of the lithium metal anode, a series of problems such as poor cycle stability, low coulombic efficiency and the like exist. The root cause of the above problems is: lithium itself has high reduction activity, it is apt to take place the side reaction to produce the unstable solid electrolyte layer in the component (such as polar solvent) in the electrolyte, in the course of charging and discharging of the battery, because there is large volume change of the negative pole, the solid electrolyte layer can break in this course, then regenerate again, repeat this course continuously, cause the uneven solid electrolyte to produce and make lithium ion deposit in the negative pole unevenly, this uneven deposit can produce the area of charge enrichment again, cause the formation of lithium dendrite, dendrite puncture the diaphragm to cause the battery to short out, make the battery circulate unstably; in addition, the continuous cracking and formation of the solid electrolyte layer also continues to consume lithium metal, resulting in a continuous decrease in the available lithium source in the battery, resulting in low coulombic efficiency.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a protection method suitable for a lithium metal anode, wherein a protection layer is formed on the surface of the lithium metal anode, and the method comprises the following steps:

adding a molecular sieve into fluorosilane to remove water in the fluorosilane;

coating the fluorosilane after removing the water on the surface of the lithium metal anode, and reacting for preset time;

After cleaning and drying, forming a protective layer on the surface of the lithium metal negative electrode; wherein, the protective layer is formed by a double-layer structure, the inner side is an inorganic layer, and the outer side is an organic layer.

Alternatively to this, the method may comprise, the fluorosilane is trimethoxy (pentafluorophenyl) silane, triethoxy [5,5,6,6,7,7,7-heptafluoro-4, 4-bis (trifluoromethyl) heptyl ] silane, trimethoxy (1H, 2H-nonafluorohexyl) silane triethoxy (1H, 2H-nonafluorohexyl) silane one of trimethoxy (1H, 2H-tridecafluoron-octyl) silane or trimethoxy (1H, 2H-heptadecafluorodecyl) silane.

Optionally, the volume of the fluorosilane is 0.2-1mL; the diameter of the lithium metal is 8-16mm, and the thickness is 0.025-0.5mm.

Alternatively, the volume of fluorosilane, a (mL), has the following mathematical relationship with the total surface area of lithium metal, b (cm 2):

Optionally, the main component of the inorganic layer is LiF or LiF and Li3N, and the main component of the organic layer is ROSiOxLiy (R is an organic group).

Optionally, in the surface reaction process of the fluorosilane and the lithium metal negative electrode, the lithium metal is put into an oven at 40-80 ℃ to be baked for 0.5-12 h.

Alternatively, the reaction time of the fluorosilane with the surface of the lithium metal anode is 0.5h-12h.

According to the technical scheme, after removing the moisture in the fluorosilane, the fluorosilane is coated on the surface of the lithium metal negative electrode, and the reaction is carried out for a preset time, so that the lithium metal negative electrode and the fluorosilane are formed on the surface of the lithium metal negative electrode, and two layers of protection layers are constructed, wherein one layer is an inorganic layer and is positioned on the inner side of the protection layer, the protection layer has high mechanical strength, can effectively isolate side reactions of lithium metal and electrolyte, has high ionic conductivity and low electronic conductivity, and can enable lithium ions to quickly and uniformly pass through the protection layer to be deposited on the lithium negative electrode, so that dendrite generation is avoided; the other layer is an organic layer and is positioned at the outer side of the protective layer, so that side reactions of lithium and electrolyte caused by serious volume change and crushing of the lithium cathode in the charge and discharge process of the inorganic protective layer can be effectively avoided, and the consumption of extra lithium is reduced. The coulomb efficiency and the cycle stability of the lithium metal battery can be effectively improved by constructing the inner protective layer and the outer protective layer.

Drawings

Exemplary embodiments of the present invention may be more completely understood in consideration of the following drawings:

Fig. 1 is a voltage-time graph of a lithium-ion battery prepared in example 7 and a lithium-ion battery prepared in comparative example 1, in which (a) is a battery assembled with untreated lithium sheets and (b) is a battery assembled with treated lithium sheets;

FIG. 2 is a schematic view showing the surface and cross section of the lithium sheet obtained in example 7, wherein (a) is the surface of the treated lithium sheet and (b) is the cross section of the treated lithium sheet;

Fig. 3 is a graph showing charge discharge of lithium-oxygen batteries assembled by lithium sheets prepared in example 8 at 1 st, 10 th, 20 th, 30 th, 40 th and 50 th turns.

Detailed Description

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention, as illustrated in the accompanying drawings.

The invention provides a novel protection method suitable for a lithium metal negative electrode, which comprises the steps of coating fluorosilane on the surface of the lithium metal negative electrode after removing moisture in fluorosilane, reacting for a preset time to enable the lithium metal negative electrode and the fluorosilane to construct two layers of protection layers on the surface of the lithium metal negative electrode, wherein one layer is an inorganic layer and is positioned on the inner side of the protection layer, and the protection layer has high mechanical strength and can effectively isolate side reactions of lithium metal and electrolyte, has high ion conductivity and low electron conductivity, can enable lithium ions to quickly and uniformly pass through the protection layer to be deposited on the lithium negative electrode, and avoids dendrite generation; the other layer is an organic layer and is positioned at the outer side of the protective layer, so that side reactions of lithium and electrolyte caused by serious volume change and crushing of the lithium cathode in the charge and discharge process of the inorganic protective layer can be effectively avoided, and the consumption of extra lithium is reduced.

Therefore, the method can effectively improve the coulombic efficiency and the cycle stability of the lithium metal battery by constructing the inner protective layer and the outer protective layer.

The technical scheme, the implementation process, the principle and the like are further explained as follows.

The invention relates to a protection method suitable for a lithium metal anode, which comprises the following steps:

adding a molecular sieve into fluorosilane to remove water in the fluorosilane;

The fluorosilane of the present invention is trimethoxy (pentafluorophenyl) silane, triethoxy [5,5,6,6,7,7,7-heptafluoro-4, 4-bis (trifluoromethyl) heptyl ] silane, trimethoxy (1H, 2H-nonafluorohexyl) silane triethoxy (1H, 2H-nonafluorohexyl) silane one of trimethoxy (1H, 2H-tridecafluoron-octyl) silane or trimethoxy (1H, 2H-heptadecafluorodecyl) silane.

The volume of the fluorosilane is 0.2-1mL; the diameter of the lithium metal is 8-16mm, and the thickness is 0.025-0.5mm.

The volume a (mL) of the fluorosilane of the present invention has the following mathematical relationship with the total surface area b (cm 2) of the lithium metal:

The main component of the inorganic layer is LiF or LiF and Li3N, and the main component of the organic layer is ROSiOxLiy (R is an organic group).

In the surface reaction process of fluorosilane and lithium metal cathode, lithium metal is baked in a baking oven at 40-80 deg.c for 0.5-12 hr.

The reaction time of the fluorosilane and the surface of the lithium metal negative electrode is 0.5h-12h.

The technical solution of the present invention will be described in further detail below with reference to a number of preferred embodiments and accompanying drawings, and it is apparent that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention. The test methods in the following examples, in which specific conditions are not noted, generally follow conventional conditions.

Example 1

The fluorosilane of this example was commercially available triethoxy (1H, 2H-nonafluorohexyl) silane, and the lithium metal was commercially available as lithium flakes having a diameter of 8mm and a thickness of 0.025mm.

Molecular sieves were first added to triethoxy (1H, 2H-nonafluorohexyl) silane to remove moisture from the triethoxy (1H, 2H-nonafluorohexyl) silane. Then placing lithium metal with the diameter of 8mm and the thickness of 0.025mm into a 10mL glass bottle with a cover, adding 0.2mL triethoxy (1H, 2H-nonafluorohexyl) silane solution into the bottle, soaking for 0.5h to enable the solution to fully react with the surface of the lithium metal cathode, and in the process, placing the solution and the lithium metal cathode into an oven to be heated at the temperature of 40 ℃ to improve the reaction speed between the solution and the lithium metal cathode. And then the treated lithium metal is washed by DME (ethylene glycol dimethyl ether), and then is put into an oven for drying, so that the button cell can be assembled.

Example 2

The fluorosilane of this example was commercially available triethoxy (1H, 2H-nonafluorohexyl) silane, and the lithium metal was a commercially available lithium sheet having a diameter of 16mm and a thickness of 0.5mm.

Molecular sieves were first added to triethoxy (1H, 2H-nonafluorohexyl) silane to remove moisture from the triethoxy (1H, 2H-nonafluorohexyl) silane. Then putting lithium metal with the diameter of 16mm and the thickness of 0.5mm into a 10mL glass bottle with a cover, adding 1mL triethoxy (1H, 2H-nonafluorohexyl) silane solution into the bottle, soaking for 12h to enable the solution to fully react with the surface of a lithium metal anode, and in the process, putting the lithium metal anode and the lithium metal anode into an oven to heat at 80 ℃ to improve the reaction speed between the lithium metal anode and the lithium metal anode. And then the treated lithium metal is washed by DME (ethylene glycol dimethyl ether), and then is put into an oven for drying, so that the button cell can be assembled.

Example 3

This embodiment is substantially the same as embodiment 1 described above. In this example, a previously prepared solution of triethoxy (1H, 2H-nonafluorohexyl) silane dissolved in 0-0.2M LiNO3 (lithium nitrate) was added to the bottle, and the other preparation was the same as in example 1.

Example 4

The fluorosilane of this example was commercially available trimethoxy (1H, 2H-tridecyl-fluoro-n-octyl) silane, the lithium metal was commercially available as lithium flakes having a diameter of 8-16mm and a thickness of 0.025-0.5mm.

Molecular sieves were first added to trimethoxy (1H, 2H-tridecyl-fluoro-n-octyl) silane to remove moisture from the trimethoxy (1H, 2H-tridecyl-fluoro-n-octyl) silane. Then a glass fiber diaphragm with the diameter of 10-18mm is fully covered on the lithium sheet, then 20-150 mu L of trimethoxy (1H, 2H-tridecyl fluoride) silane is dripped on the diaphragm, the lithium sheet (covered with the glass fiber diaphragm) is soaked for 0.5-12h, then the lithium sheet is put into an oven, and baked for 0.5-12h (the generated protective layer is between 5 and 30 mu m) at the temperature of 40-80 ℃ to enable the solution and the lithium sheet to fully react, and a protective film is generated on the surface of the lithium sheet. And then cleaning the treated lithium sheet by DME, and then putting the lithium sheet into an oven for drying to assemble the button cell.

Example 5

This embodiment is substantially the same as embodiment 4 described above. In this example, trimethoxy (1H, 2H-tridecafluoron-octyl) silane dissolved with 0-0.2M LiNO3 was added dropwise to the separator, and the procedure was the same as in example 4.

Example 6

The fluorosilane of this example was commercially available trimethoxy (1H, 2H-nonafluorohexyl) silane, and the lithium metal was a commercially available ultrathin lithium tape with a thickness of 0.02-0.1mm.

Molecular sieves were first added to trimethoxy (1H, 2H-tridecyl-fluoro-n-octyl) silane to remove moisture from the trimethoxy (1H, 2H-tridecyl-fluoro-n-octyl) silane. Then spreading the purchased ultrathin lithium belt (with the thickness of 0.02-0.1 mm) on a film coating machine, adjusting the thickness of a film coating to be 0.02-0.2mm, uniformly coating trimethoxy (1H, 2H-tridecafluoron-octyl) silane on the lithium belt through the work of the film coating machine, transferring the lithium belt into a baking oven at the temperature of 40-80 ℃ and baking for 0.5-12h, so as to accelerate the reaction speed. It should be noted that the process of using the applicator is performed in an argon glove box or is completed quickly in a drying room; the method is beneficial to industrial production conversion, and can be used for producing the lithium belt with the protection layers on both sides for assembling the soft package battery.

Example 7

The two lithium metals prepared in example 1 were used as a positive electrode and a negative electrode, respectively, and then assembled in the order of negative electrode case-leaf spring-gasket-lithium sheet-separator-lithium sheet-positive electrode case in a glove box filled with argon gas (H2O content <0.1ppm, O2 content <0.1ppm in the glove box) using a CR2032 type button cell case. The diaphragm is a glass fiber diaphragm, the electrolyte is DMSO (dimethyl sulfoxide) solution containing 1M LiTFSI (lithium bistrifluoromethane sulfonyl imide), and the electrolyte is required to be dripped on the diaphragm. And standing the battery for 12 hours outside after the battery is assembled, so that the electrolyte and the anode and the cathode are fully soaked.

Comparative example 1

Comparative example 1 differs from example 7 in that: the lithium metal negative electrode was not treated and had no protective layer.

The battery prepared in example 7 and the battery prepared in comparative example 1 were separately mounted on a battery test system of martial arts LANHE for testing. The test parameters were as follows: the current is 0.1mAh cm < -2 >, the capacity is 0.1mAh cm < -2 >, and the charge and discharge cycles of the battery are continuously carried out, so that the voltage-time curve of the battery is obtained. Fig. 1 (a) is a voltage-time graph of the battery prepared in comparative example 1, and fig. 1 (b) is a voltage-time graph of the Li-Li symmetric battery prepared in example 7.

It can be seen from fig. 1 that the voltage of the assembled symmetric cell using the treated lithium sheet is significantly lower than the voltage of the assembled cell using the untreated lithium sheet, which illustrates that lithium ions can pass through the surface solid electrolyte layer with less resistance and faster speed, which illustrates that the protective layer we have excellent ion conducting ability by pre-treatment.

In addition, it can be observed that the curve in fig. 1 (b) is very stable, while the voltage of the curve in fig. 1 (a) is continuously increased along with the time, because the protective layer on the surface of the lithium sheet after being treated by us is very stable and can be kept stable in long-time circulation, and the solid electrolyte layer on the surface of the untreated lithium sheet is continuously generated and broken in the circulation process, so that side reaction occurs, the impedance is continuously increased, the voltage is continuously increased in the diagram, and the stability of the double-layer protective layer artificially constructed by us is reflected from the side surface. As is more apparent from the data in table 1 below, the assembled battery using untreated lithium sheets had a voltage of 22.7mV at 250h and 155.1 at 1000h, and the voltage increased 6.83 times, indicating that the surface of the lithium sheets was severely damaged by side reactions, greatly affecting the deposition and stripping ability of lithium ions, which severely deteriorated the coulomb efficiency and cycle stability of the battery; the voltage of the battery assembled by the lithium sheets after treatment is 12.1mV at 250h, the voltage at 1000h is 14.3, and the voltage is only increased by 1.18 times, which shows that the surface structure of the lithium sheets after treatment is stable, the stable deposition and stripping of lithium ions can be maintained in long-time circulation, and the coulomb efficiency and the circulation stability of the battery can be greatly improved in the whole battery.

TABLE 1

In order to clarify the morphology and composition of the protective film, the surface and cross section of the treated lithium sheet were observed using a scanning electron microscope in this example, as shown in fig. 2. It can be seen from fig. 2 (a) that the surface of the treated lithium sheet is smooth and uniform, and that a protective film of about 26 μm is clearly provided over the lithium sheet, which is clearly distinguished from the bulk of the lithium sheet, and which is very uniform for uniform deposition of lithium, as can be seen from the cross-sectional view of fig. 2 (b).

In order to clarify the composition of the protective film, the present example uses the X-ray energy spectrum analysis technique to analyze the composition of the treated lithium sheet, and finds that the outside is mainly an organic compound composed of si—c-Li-O (Si content is 11%), while the inside is mainly a fluoride (F content is 10%), so that the protective layer is successfully constructed on the surface of the lithium sheet by the treatment of nonafluorohexyltriethoxysilane, and the protective layer is composed of the organic silicon oxide of the outside and LiF of the inside, so that the lithium sheet can be effectively protected, and the cycle stability and coulombic efficiency of the battery can be improved.

Example 8

A lithium-oxygen battery was assembled using the lithium metal prepared in example 3 as a negative electrode. Before the battery is assembled, the solvent, lithium salt, separator, battery case, etc. need to be sufficiently dried to avoid the influence of moisture on the battery, and then transferred into a glove box filled with argon. Wherein, the main component of the air electrode is Carbon Nano Tube (CNT), and the specific process for preparing the electrode is as follows: (1) Cutting a stainless steel mesh into pieces with the length of 1cm multiplied by 1cm, washing the pieces with deionized water and absolute ethyl alcohol for several times to remove surface impurities, then placing the pieces in a vacuum atmosphere and drying the pieces at 100 ℃ for 8 hours, and weighing a current collector after drying for later use. (2) Weighing CNT and PVDF (polyvinylidene fluoride) according to a certain mass ratio, uniformly mixing, adding a proper amount of NMP (N-methyl pyrrolidone), fully grinding to form uniform paste, uniformly coating the paste on a cut stainless steel net, and drying at 110 ℃ for 12 hours in a vacuum atmosphere. Weighing the dried electrode slices again, calculating the load capacity, and then placing the electrode slices into a glove box for standby. The specific combination process comprises the following steps: the negative electrode shell, the lithium sheet, the diaphragm (the electrolyte is a DMSO solution containing 1M LiTFSI) and the air electrode, and the positive electrode shell (with a plurality of pore canals with the diameter of 1 mm) are assembled in sequence from bottom to top. The assembled battery is clamped in a specially made glass bottle for storing gas, and then the bottle is filled with oxygen through ventilation of the oxygen bottle.

Comparative example 2

Comparative example 2 differs from example 8 in that: the lithium metal negative electrode was not treated and had no protective layer.

The battery prepared in example 8 and the battery prepared in comparative example 2 were respectively clamped on a wuhan LANHE battery test system for charge and discharge test, and the test parameters were as follows: and the current density is 200mAg-1, the gram capacity is 500mAh g-1, and the constant-volume constant-current charge-discharge cycle test is carried out. The obtained curves are shown in fig. 3, and the charge and discharge curves of the 1 st, 10 th, 20 th, 30 th, 40 th and 50 th circles are shown in fig. 3, so that the polarization of the battery is gradually increased along with the increase of the number of circles, but the battery is stable all the time, the performance of the treated lithium sheet is very excellent, the lithium sheet can be stably circulated in a lithium-oxygen battery system, and the excellent performance of a protective layer is proved.

Example 9

This embodiment is substantially the same as embodiment 1 described above. In this example, the addition of a compound containing N, cl components to triethoxy (1 h,2 h-nonafluorohexyl) silane was beneficial for forming a protective film, and further enhanced the performance of the protective layer. Other specific preparation procedures were the same as in example 1.

Example 10

This embodiment is substantially the same as embodiment 1 described above. In this example, the lithium sheet was replaced with a lithium tin alloy, and the other specific preparation process was the same as in example 1.

Table 2 below is a comparison of the film thickness of the surface protection film after treatment of lithium metal with different fluorosilanes.

TABLE 2

In summary, in the invention, after removing the moisture in the fluorosilane, the fluorosilane is coated on the surface of the lithium metal negative electrode, and the reaction is performed for a preset time, so that the lithium metal negative electrode and the fluorosilane are reacted, and two protective layers are constructed on the surface of the lithium metal negative electrode. Wherein, after the lithium sheet is treated by fluorosilane, an inorganic protective layer (the main component is LiF/LiF+Li3N) is generated inside, which can effectively avoid side reaction and guide lithium to be deposited uniformly. After the lithium sheet is treated by fluorosilane, an organic protective layer (the main component is ROSiOxLiy (R is an organic group)) is formed outside, so that continuous crushing and formation of a solid electrolyte layer caused by severe volume change of a lithium anode can be effectively inhibited, and consumption of extra lithium is reduced. In addition, the invention uses the fluorosilane solution to treat the lithium sheet, has simple and quick operation and is beneficial to industrial production conversion.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A protection method for a lithium metal anode, characterized in that a protection layer is formed on the surface of the lithium metal anode, comprising the steps of:

adding a molecular sieve into fluorosilane to remove water in the fluorosilane;

2. The method of claim 1, wherein the step of determining the position of the substrate comprises, the fluorosilane is trimethoxy (pentafluorophenyl) silane, triethoxy [5,5,6,6,7,7,7-heptafluoro-4, 4-bis (trifluoromethyl) heptyl ] silane, trimethoxy (1H, 2H-nonafluorohexyl) silane triethoxy (1H, 2H-nonafluorohexyl) silane one of trimethoxy (1H, 2H-tridecafluoron-octyl) silane or trimethoxy (1H, 2H-heptadecafluorodecyl) silane.

3. The method of claim 1, wherein the fluorosilane has a volume of 0.2 to 1mL; the diameter of the lithium metal is 8-16mm, and the thickness is 0.025-0.5mm.

4. The method of claim 1, wherein the volume of fluorosilane, a (mL), is mathematically related to the total surface area of lithium metal, b (cm 2), as follows:

5. The method of claim 1, wherein the inorganic layer has a main component of LiF or LiF and Li3N and the organic layer has a main component of ROSiOxLiy.

6. The method of claim 1, wherein during the surface reaction of the fluorosilane with the negative electrode of lithium metal, the lithium metal is baked in an oven at 40-80 ℃ for 0.5-12 hours.

7. The method of claim 1, wherein the fluorosilane is reacted with the surface of the lithium metal negative electrode for a period of time ranging from 0.5h to 12h.