CN104617259B - The protection processing of cathode of lithium in lithium secondary battery - Google Patents

Abstract

The invention discloses a method for in-situ protection treatment of a lithium negative electrode. The metallic lithium treated with in-situ protection of the lithium negative electrode can be used for high-performance lithium secondary batteries. The method for the in-situ protection treatment of the lithium negative electrode provided by the present invention includes the method of in-situ generating silicon dioxide on the surface of the lithium negative electrode. At a certain temperature, the treatment solution reacts with the passivation layer on the surface of the lithium metal to obtain a protective layer containing silicon dioxide. The preparation method for lithium in-situ protection provided by the invention is simple, easy to control and has a high degree of practicality. The use of in-situ protected lithium metal for lithium secondary batteries can greatly improve the energy density and cycle performance of current batteries, and has high practical value.

Description

Protection treatment of lithium negative electrode in lithium secondary battery

technical field

The invention relates to a method for protecting and treating lithium negative electrodes in lithium secondary batteries.

Background technique

Lithium secondary batteries and lithium-ion batteries play an important role in energy storage devices. Lithium is the metal with the smallest mass and the lowest potential among all simple substances, and its theoretical specific capacity reaches 3860mA h/g. Lithium batteries composed of lithium have the characteristics of high operating voltage, high mass specific capacity and large specific energy. However, lithium batteries using lithium as the anode will generate lithium dendrites and safety issues. In the early 1990s, Sony Corporation of Japan used graphite that can deintercalate lithium as the negative electrode, and successfully prepared a lithium-ion battery with better safety performance. After 30 years of development, lithium-ion batteries have achieved great commercial success. However, the specific capacity of graphite is only 372mAh/g, and the specific capacity of positive electrode materials that can deintercalate lithium is difficult to exceed 200mAh/g. Therefore, it is difficult for the current commercial lithium-ion battery to exceed 200W h/Kg, and it is difficult to meet the requirements of the development of new energy technology for high-performance secondary batteries. Therefore, lithium secondary batteries with high specific energy are attracting more and more attention, and lithium-sulfur batteries and lithium-air batteries are the research hotspots in the field of secondary batteries today.

The biggest obstacle to the commercialization of lithium secondary batteries is still lithium dendrites and the safety of lithium. Due to the fact that "lithium dendrites" may form on the surface of the metal lithium negative electrode during the cycle of the secondary lithium metal battery, as the number of cycles increases, the "lithium dendrites" grow rapidly and break through the electrolyte to contact the positive electrode, resulting in a short circuit inside the battery and the battery eventually At the same time, because the "lithium dendrites" on the surface of lithium metal are easily dissolved in the electrolyte to form "dead lithium", they lose contact with electrons and cannot perform electrochemical reactions. On the one hand, the generation of "dead lithium" reduces the cycle efficiency of metallic lithium, and on the other hand, highly active "dead lithium" stays in the electrolyte and is prone to some side reactions with the electrolyte, posing a threat to the safety of the battery.

PolyPlus, in conjunction with Eveready Bettery and Sheldahl, has developed a relatively mature lithium-sulfur battery by using a method of coating to protect the lithium negative electrode. PolyPlus company's method is: the negative electrode of the battery is based on copper or polymer, and metal lithium is plated on the substrate by vapor deposition method, and then wrapped with a phosphate protective film that can conduct lithium ions. This method of PolyPlus Company has relatively high preparation cost and is not suitable for large-scale production. Zhang et al. (Naturecommunication 2014, 5:3015) used a carbon layer to protect lithium metal sheets to improve the performance of lithium-sulfur batteries. They mixed graphite, conductive carbon, and binder PVDF into a slurry, then coated it on a stainless steel mesh, and then pressed the stainless steel mesh with graphite on the metal lithium sheet. The research results showed that this method effectively suppressed lithium branches. crystal growth. However, this method does not grow the cladding layer in situ on the lithium sheet, which has a certain discount on the effect of inhibiting lithium dendrites, and the preparation method is relatively complicated.

The present invention creatively adopts the method of growing a silicon dioxide in-situ protective layer on metal lithium to inhibit the growth of lithium dendrites, and effectively controls the thickness of the protective layer by controlling the reaction time and the amount of reactants. The in-situ grown protective layer can effectively inhibit the growth of lithium dendrites, thereby improving the cycle performance of lithium secondary batteries. This method of growing the protective layer in situ can be widely used in lithium secondary batteries (lithium-sulfur batteries, lithium-air batteries, etc.).

Contents of the invention

The object of the present invention is to provide a preparation method and application of a metal lithium in-situ protective layer of a negative electrode material of a lithium secondary battery.

The preparation method of the in-situ protective layer of the metal lithium sheet provided by the present invention comprises the following steps: placing the lithium sheet in the treatment solution for a period of time or coating the treatment solution on the surface of the lithium sheet, and after reacting for a certain period of time, taking out the lithium sheet, wiping off the surface excess treatment solution, to obtain lithium flakes containing a _SiO2 protective layer.

In the above preparation method, the treatment liquid includes one or more of various esters and silanes that react with lithium hydroxide to form silica. Esters include ethyl orthosilicate, methyl orthosilicate, etc. Silanes include trimethylchlorosilane, dimethylchlorosilane, methyltrichlorosilane, triisopropylchlorosilane, methyldiphenyl chloride Silane, tert-butyldiphenylchlorosilane, dodecyldimethylchlorosilane, and siloxane, etc.

In the above preparation method, the state of the lithium sheet in the treatment liquid includes partial immersion of the lithium sheet in the treatment liquid and complete immersion in the treatment liquid.

In the above preparation method, the soaking time of the lithium sheet in the treatment solution is 10s-10h, preferably 30s-90s.

In the above preparation method, the reaction temperature is -20-50°C, preferably 5-35°C.

The application provided by the invention is the application of metal lithium with a protective layer as the negative electrode material of the lithium secondary battery, especially as the application of the lithium-sulfur battery.

Compared with the prior art, the preparation method of metal lithium protection treatment provided by the present invention has the advantage that metal lithium generates a protective layer through in-situ reaction, and the in-situ protective layer is regulated by adjusting the type and reaction time of metal lithium and reaction liquid, etc. composition and thickness. The treated metallic lithium can effectively control the growth of lithium dendrites, and can greatly improve its cycle performance when used in lithium secondary batteries, especially lithium-sulfur secondary batteries. The preparation method of the metal lithium in-situ protection layer is simple, the raw material is easily obtained, and is suitable for large-scale production.

In the present invention, when the metal lithium protective layer is used for lithium secondary batteries, the positive electrode material adopts carbon/sulfur composite material, wherein the carbon material is microporous carbon, activated carbon, mesoporous carbon, macroporous carbon, mesoporous microporous carbon, macroporous One or more of porous microporous carbon, macroporous mesoporous carbon, macroporous mesoporous microporous carbon. In addition, the positive electrode material can also be the positive electrode material commonly used in lithium ion batteries at present, such as lithium cobalt oxide, lithium nickel oxide, lithium manganate, lithium iron phosphate, lithium cobalt nickel manganese oxide, and the like.

In the present invention, when the metallic lithium protective layer is used in lithium secondary batteries, one or more of expanded graphite, thermally exfoliated graphene, carbon nanotubes, Ketjen black, carbon black and Super P are used as conductive additives.

When the metal lithium protective layer is used for lithium secondary batteries in the present invention, the binding agent adopts polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), sodium hydroxymethyl cellulose (CMC ), one or more of styrene-butadiene rubber (SBR), gelatin and cyclodextrin.

In the present invention, when the metal lithium protective layer is used in lithium secondary batteries, the electrolyte is ester electrolyte or ether electrolyte, and the concentration of lithium salt is 0.1-5M, preferably 0.5-2.5M.

In the described ester electrolyte, the solvent is ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and propylene carbonate (PC). One or more, the lithium salt is lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium bisoxalate borate (LiBOB), lithium tetrafluoroborate (LiBF ₄ ), bis(trifluoromethanesulfonyl) One or more of lithium imide (LiTFSI).

In the ether electrolyte, the solvent is one or more of triethylene glycol dimethyl ether (TEGDME), 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME). The lithium salt is selected as lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium bisoxalate borate (LiBOB), lithium tetrafluoroborate (LiBF ₄ ) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) one or more of them.

In the present invention, the separator for lithium secondary battery is one of PE film, PP film, PP/PE film, PP/PE/PP film and glass cellulose film (Whatman).

Description of drawings

Fig. 1 is the SEM picture and the corresponding Mapping picture of the SiO ₂ protective layer in embodiment 1.

Fig. 2 is the SEM picture after 100 cycles of the lithium sheet with _SiO2 protection layer in embodiment 1.

Fig. 3 is the SEM picture of the lithium sheet of Example 11 after 100 cycles.

Detailed ways

The present invention will be further described below in conjunction with specific examples, but the present invention is not limited to the following examples.

The raw materials used in the following examples can be obtained from commercial sources unless otherwise specified.

Embodiment 1, the preparation of metallic lithium in-situ protective layer

In a high-purity argon atmosphere, immerse the lithium sheet in tetraethyl orthosilicate and react for 90s at a reaction temperature of 35°C. After taking out the lithium sheet and wiping off the excess orthosilicate liquid on the surface, the SiO provided by the invention can be obtained. ₂ protective layers.

The morphology and structure of the SiO ₂ protective layer were observed with a cold field emission scanning electron microscope (SEM), as shown in Figure 1, and the inset is the corresponding Mapping picture of silicon and oxygen elements. It can be seen from the figure that silicon elements and oxygen elements are evenly distributed on the protective layer. In addition, it can be seen from the cross-sectional view of SEM that the thickness of the _SiO2 protective layer is 150 nm. .

The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Embodiment 2, the preparation of metallic lithium in-situ protective layer

In a high-purity helium atmosphere, add tetraethyl orthosilicate liquid dropwise on the surface of the lithium sheet, the reaction temperature is 40°C, react for 300s, wipe off the excess tetraethyl orthosilicate liquid on the surface, and SiO ₂ provided by the invention can be obtained The protective layer.

Compared with Example 1, the inert atmosphere was changed to helium, the amount of tetraethyl orthosilicate was greatly reduced, and it was dropped on the surface of the lithium sheet instead, and the reaction time was changed to 300s.

The morphology and structure of the SiO ₂ protective layer were observed with a cold field emission scanning electron microscope (SEM). The lamellar structure was uniformly grown on the lithium surface, and the elements were silicon and oxygen, and the thickness was 100nm.

The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Embodiment 3, the preparation of metallic lithium in-situ protective layer

Under a high-purity argon atmosphere, immerse the lithium sheet in trimethylchlorosilane and react for 200s at a reaction temperature of 40°C. After taking out the lithium sheet and wiping off the excess trimethylchlorosilane liquid on the surface, the SiO provided by the invention can be obtained. ₂ protective layers.

The morphology and structure of SiO ₂ were observed with a cold field emission scanning electron microscope (SEM), which was composed of a multi-layer sheet structure, and the constituent elements were silicon, oxygen and a small amount of chlorine. In addition, it can be seen from the cross-sectional view of SEM that the thickness of the _SiO2 protective layer is about 120 nm.

The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Embodiment 4, the preparation of metallic lithium in-situ protective layer

In a high-purity argon atmosphere, immerse the lithium sheet in the mixed liquid of ethyl orthosilicate and trimethylchlorosilane for 90 seconds, and the reaction temperature is 35°C. Take out the lithium sheet and wipe off the excess liquid on the surface to obtain this product. The invention provides a SiO ₂ protective layer.

The morphology and structure of SiO ₂ were observed with a cold field emission scanning electron microscope (SEM), which was composed of a multi-layer sheet structure, and the constituent elements were silicon, oxygen and a small amount of chlorine. The protective layer can evenly cover the surface of lithium. In addition, it can be seen from the cross-sectional view of SEM that the thickness of the _SiO2 protective layer is about 120 nm.

The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Embodiment 5, the preparation of metallic lithium in-situ protective layer

In a high-purity argon atmosphere, immerse the lithium sheet in a mixed liquid of methyl orthosilicate, dimethylchlorosilane, and methyltrichlorosilane to react for 120 seconds, take out the lithium sheet and wipe off the excess liquid on the surface, and this product can be obtained The invention provides a SiO ₂ protective layer.

The morphology and structure of SiO ₂ were observed with a cold field emission scanning electron microscope (SEM), which was composed of a multi-layer sheet structure, and the constituent elements were silicon, oxygen and a small amount of chlorine. The protective layer can evenly cover the surface of lithium. In addition, it can be seen from the cross-sectional view of SEM that the thickness of the _SiO2 protective layer is about 50-150nm.

The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur battery are listed in Table 2.

Embodiment 6, the preparation of metallic lithium in-situ protective layer

Other conditions are the same as in Example 4, except that the reaction temperature is 50°C. Obtain the SiO ₂ protective layer provided by the present invention. The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Embodiment 7, the preparation of metallic lithium in-situ protective layer

Other conditions are the same as in Example 4, except that the reaction temperature is 0°C. Obtain the SiO ₂ protective layer provided by the present invention. The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Example 8, Preparation of metal lithium in-situ protective layer

Other conditions are the same as in Example 4, except that the reaction temperature is 5°C. Obtain the SiO ₂ protective layer provided by the present invention. The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Embodiment 9, the preparation of metallic lithium in-situ protective layer

Under a high-purity argon atmosphere, immerse the lithium sheet in tetraethyl orthosilicate and react for 10s at a reaction temperature of 35°C. After taking out the lithium sheet, wipe off the excess orthosilicate liquid on the surface, and the SiO provided by the invention can be obtained. ₂ protective layers.

The morphology and structure of SiO ₂ were observed with a cold field emission scanning electron microscope (SEM), which was composed of a lamellar structure, and the constituent elements were silicon and oxygen. In addition, it can be seen from the cross-sectional view of SEM that the thickness of the _SiO2 protective layer is about 50 nm.

The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Example 10, Preparation of metal lithium in-situ protective layer

In a high-purity argon atmosphere, immerse the lithium sheet in tetraethyl orthosilicate and react for 600s at a reaction temperature of 35°C. After taking out the lithium sheet, wipe off the excess orthosilicate liquid on the surface to obtain the SiO provided by the invention. ₂ protective layers.

The morphology and structure of SiO ₂ were observed with a cold field emission scanning electron microscope (SEM), which was composed of a lamellar structure, and the constituent elements were silicon and oxygen. In addition, it can be seen from the cross-sectional image of SEM that the thickness of the _SiO2 protective layer is about 2 μm.

The deposition and stripping potential and impedance test results of this lithium flake with _SiO2 protection layer are listed in Table 1. The test results when it is used as the negative electrode of lithium-sulfur batteries are listed in Table 2.

Comparative example 1, the test of common lithium sheet

In order to compare the test performance of lithium-sulfur batteries, the test results when ordinary lithium sheets are used as the negative electrodes of lithium-sulfur batteries are listed in Table 2.

Application example Ordinary lithium sheet is used for lithium-sulfur battery testing

Ordinary lithium sheets are used for lithium-sulfur battery tests, and the test results are listed in Table 2.

In an inert gas glove box (H ₂ O<0.1ppm, O ₂ <0.1ppm), the lithium sheet is used as the negative electrode, Ceglard2325 is used as the diaphragm, the lithium sheet of Example 1-10 is used as the positive electrode, the lithium salt of the electrolyte is LiTFSI, and the solvent is selected The mixture of DOL/DME was assembled into a lithium battery and tested on the LAND2100 battery test system. The test conditions were: constant current charge and discharge at a current density of 0.1 mA, and the charge and discharge capacity was controlled at 1 mAh. The deposition and stripping potentials of lithium ions in Examples 1-10 were measured as shown in Table 1. In addition, Auto Lab is used to test its AC impedance spectrum, and the corresponding resistance values are obtained through fitting, and are listed in Table 1.

In an inert gas glove box (H ₂ O<0.1ppm, O ₂ <0.1ppm), the lithium flakes prepared in Examples 1-10 and common lithium flakes (comparative example 1) are negative electrodes, Ceglard 2325 is a separator, carbon The /sulfur compound is the positive electrode, the conductive carbon is Ketjen black, the binder is PVDF, the electrolyte lithium salt is LiTFSI, the solvent is a mixture of DOL/DME, and the lithium battery is assembled and tested on the LAND2100 battery test system. The test current density is 0.2C (1C=1675mA h/g). The test results are listed in Table 2.

In addition, the morphology of the lithium sheet with a protective layer in Example 1 after 100 cycles is shown in FIG. 2 . The morphology of the ordinary lithium sheet in Comparative Example 1 after 100 cycles is shown in FIG. 3 .

Table 1. Deposition/stripping potential and impedance of lithium ions

Table 2. Capacity and coulombic efficiency of lithium-sulfur batteries in application examples

As can be seen from the protective layer thickness, lithium ion deposition potential, and impedance value comparison of Examples 1-10, the type of reaction solution, reaction temperature, time, etc. have a great influence on the protective layer thickness, and the thicker the reaction layer, The higher the lithium ion deposition potential, the greater the impedance value. Compared with other kinds of reaction solutions, tetraethyl orthosilicate produces the lowest resistance value of the protective layer. The key to the protection of metal lithium anode is to adjust the thickness and composition of the protective layer reasonably by optimizing the parameters.

Due to the problem of polysulfide ion dissolution in lithium-sulfur batteries, there is a shuttle effect in the charging process, resulting in low Coulombic efficiency and poor cycle performance. The coulombic efficiency of the common lithium sheet as the negative electrode is 95%, and the capacity retention rate after 100 cycles is 63.6% (comparative example 1).

It can be seen from the performance test results of the lithium-sulfur battery in Examples 1-10 and Comparative Example 1 that the lithium sheet with an in-situ protective layer has a great influence on the performance of the lithium-sulfur battery. The type of protective layer has a great influence on the performance of lithium-sulfur batteries. Among them, ethyl orthosilicate, trimethylchlorosilane and their mixed solutions are used as the protective layer prepared by the treatment solution to show better performance for lithium-sulfur batteries ( As in Example 1, Example 3, and Example 4), the lithium-sulfur battery obtained from the protective layer prepared at 5-35° C. is especially treated with a mixed solution of tetraethyl orthosilicate and trimethylchlorosilane Best performance (see Example 4 and Example 8). A too thick protective layer will greatly reduce the capacity and coulombic efficiency of the lithium-sulfur battery, and reduce its cycle performance (as in Example 10); although a too thin protective layer can enable the lithium-sulfur battery to exert a higher specific capacity, it will not work as the cycle time increases. Carrying out, protective layer may be destroyed and cause Coulombic efficiency to reduce (embodiment 9). Therefore, selecting a suitable reaction solution to form a protective layer with a suitable thickness is the key to improving the performance of lithium-sulfur batteries.

In addition, it can be seen from the in-situ SEM image after cycling (Figure 2) that the silicon dioxide protective layer can effectively inhibit the growth of lithium dendrites, thus making the surface very smooth. The lithium sheet without treatment, after 100 cycles in the lithium-sulfur battery, the surface of the lithium is uneven (Figure 3), and the growth of lithium dendrites is very obvious.

In summary, the present invention uses esters, silanes, etc. to react with LiOH on the surface of the lithium metal sheet to generate _SiO2 or a functionalized _SiO2 protective layer on the lithium metal in situ. The prepared in-situ protective layer can effectively prevent the formation of lithium dendrites and improve the cycle stability of lithium secondary batteries. The method has simple preparation, cheap and easy-to-obtain raw materials, is suitable for large-scale production, and has good application prospects.

The above content is only a preferred embodiment of the present invention, and is not intended to limit the implementation of the present invention. Those of ordinary skill in the art can easily make corresponding modifications or modifications according to the main idea and spirit of the present invention. Therefore, the present invention The scope of protection shall be subject to the scope of protection required by the claims.

Claims (4)

1. a kind of preparation method of lithium secondary battery cathode material lithium metal in-situ conservation layer, it is characterised in that protective layer it is main Ingredient is SiO₂, preparation method is as follows：Under an inert atmosphere, lithium metal is immersed in treatment fluid or is coated in treatment fluid On lithium metal, reaction a period of time, the SiO of the functionalization of generation lithium secondary battery cathode material lithium metal₂Protective layer, the place Reason liquid is made of ethyl orthosilicate and trim,ethylchlorosilane, in metallic lithium surface in-situ preparation SiO₂The reaction temperature of protective layer is 5-35 DEG C, in metallic lithium surface in-situ preparation SiO₂The reaction time of protective layer is 30s-90s.

2. according to the method described in claim 1, the inert gas is selected from one or more of argon gas, helium, neon.

3. according to the method described in claim 1, in the generated in-situ functionalization SiO of metallic lithium surface₂The thickness of protective layer is 0.05-1μm。

4. a kind of lithium secondary battery is formed including negative material, diaphragm, electrolyte and positive electrode, positive electrode is selected from cobalt acid Lithium, LiMn2O4, lithium nickelate, LiFePO4, ternary material and rich lithium material, method of the negative material for one of claim 1-3 Obtain the SiO with functionalization₂The lithium piece of protective layer；Diaphragm is selected from PP films, PE films, PP/PE films and PP/PE/PP films；Electrolyte Selected from esters electrolyte, ethers electrolyte and il electrolyte.