A lithium negative electrode and a method for preparing the same

Abstract

The application discloses a lithium negative electrode and a preparation method thereof, and belongs to the technical field of batteries. The lithium negative electrode comprises a modified diaphragm and a metal lithium negative electrode sheet, wherein the modified diaphragm is located on one side of the metal lithium negative electrode sheet and closely adheres to the metal lithium negative electrode sheet, and after heat pressing treatment, the lithium negative electrode is formed in an integrated manner, so that the corrosion of the environment to the lithium negative electrode can be effectively reduced, and the storage and processability of the lithium negative electrode are increased. The preparation method of the lithium negative electrode has the characteristics of simple process, low cost, safety and reliability, and the preparation process does not need to modify the lithium negative electrode or the lithium metal interface, and is convenient for large-scale mass production. The integrated lithium negative electrode obtained by the application is applied to a lithium metal battery, and the prepared lithium battery has a longer cycle life and a higher coulomb efficiency.

Description

TECHNICAL FIELD

[0001] The application discloses a lithium negative electrode and a preparation method thereof, and belongs to the technical field of batteries. BACKGROUND

[0002] In recent years, lithium ion secondary batteries have been widely used in consumer electronics and communication fields. With the rapid development of electric vehicles, smart grids and large-scale energy storage fields, it is necessary to further improve the performance of lithium ion batteries, such as higher energy density and safety, and longer cycle life. However, due to the limited theoretical capacity, the current development and utilization has reached the limit.

[0003] Metal lithium has ultra-high theoretical specific capacity and the lowest redox potential, and is an ideal negative electrode material for constructing next-generation high-specific-energy secondary battery systems such as lithium-oxygen, lithium-sulfur and solid-state batteries. However, metal lithium also has strong activity, and is easy to react with organic electrolyte during charging and discharging to generate an anisotropic and unstable SEI (Solid Electrolyte Interface) film. At the same time, the non-uniformity of metal lithium during deposition is prone to grow into lithium dendrites, which may pierce the separator and cause serious safety hazards. In addition, during the deposition of metal lithium, there will be a huge volume expansion, which will cause the continuous rupture of the SEI film, so that the deposited lithium dendrites will fall off from the substrate into the electrolyte, exposing fresh lithium, which will continuously consume the electrolyte, and so on, resulting in rapid decline of battery performance.

[0004] In order to solve the above technical problems, various solutions have been proposed by those skilled in the art, such as the design of new three-dimensional current collectors, electrolyte additives, solid-state electrolytes or the construction of new artificial modification layers. The three-dimensional current collector can reduce the current density on the electrode surface and reduce the generation of lithium dendrites, but it will reduce the utilization rate of lithium and the overall energy density of the battery. The electrolyte additive can participate in the in-situ generation of the SEI film and guide the uniform distribution of lithium ions, but the electrolyte additive will be continuously consumed during the electrochemical cycle until it is completely consumed, losing its original protective effect. Solid-state electrolytes are highly expected in the safety of lithium negative electrodes, but there are still problems such as large interface impedance and low ionic conductivity. The construction of an artificial modification layer on the surface of the lithium negative electrode can effectively inhibit the growth of lithium dendrites and reduce the side reaction between lithium metal and electrolyte, but the preparation process is often complex and the preparation environment is also harsh, which is not conducive to large-scale production. Therefore, those skilled in the art urgently need to develop a simple and feasible method for protecting metal lithium negative electrode, thereby improving the cycle life and rate performance of lithium metal battery and promoting the commercial application of lithium metal negative electrode. SUMMARY

[0005] The application aims to solve the problems of poor storage and processability of lithium negative electrode, and provides a lithium negative electrode and a preparation method thereof.

[0006] The application solves the above problems by adopting the technical scheme that the lithium negative electrode comprises a modified separator and a metal lithium negative electrode sheet, the modified separator is tightly attached to one side of the metal lithium negative electrode sheet to form an integrated lithium negative electrode.

[0007] The modified separator comprises a separator and a functional coating layer, the thickness of the separator is 5-25 microns, and the thickness of the functional coating layer is 0.5-20 microns.

[0008] The separator is one or more of PE separator, PP separator, PP / PE / PP three-layer separator, thin mu stone coated separator and aramid coated separator.

[0009] The preparation method of the lithium negative electrode comprises the following steps:

[0010] Step one, the functional coating layer is pre-coated on the separator to obtain a modified separator;

[0011] Step two, the modified separator is tightly attached to one side of the metal lithium negative electrode, and then a lithium negative electrode is formed by hot pressing, wherein the hot pressing temperature of the double-plate hot press machine is adjusted to 30-80 DEG C, the hot pressing time is 0.5-6 hours, and the hot pressing pressure of the double-plate hot press machine is adjusted to 1-10 Mpa.

[0012] The raw material weight parts of the functional coating layer are as follows:

[0013] Lithium-containing compound 4-8 parts

[0014] Organic polymer 1-4 parts

[0015] Binder 1 part

[0016] The lithium-containing compound is one or more of nano-morphology lithium aluminum silicate, petalite, lepidolite, lithium magnesium silicate, hectorite, lithium lanthanum zirconium oxide, lithium lanthanum zirconium tantalum oxide, lithium germanium phosphorus sulfur, lithium phosphorus sulfur chloride; or the lithium-containing compound is one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium dihydroxyborate, lithium difluoro hydroxyborate, lithium difluorophosphate, lithium bisfluorosulfonylimide, lithium bis(trifluoromethylsulfonyl)imide, lithium perchlorate, lithium nitrate, lithium nitride, lithium phosphide, lithium oxide, lithium sulfide, lithium selenide, lithium fluoride, lithium chloride, lithium bromide and lithium iodide.

[0017] The organic polymer is one or more of polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyacrylamide, polyacrylic acid, polyacrylonitrile, polymethyl methacrylate, polyvinylidene carbonate, polyurethane, polydimethylsiloxane, perfluorosulfonic acid resin, and lithiumated perfluorosulfonic acid resin.

[0018] The binder is one or more of gelatin, gum arabic, chitosan, starch, cyclodextrin, polyvinylpyrrolidone, carboxymethyl cellulose, sodium carboxymethyl cellulose, sodium alginate, butadiene-styrene rubber, LA133, polyvinylidene fluoride, polyethylene oxide, polyamide, polyethyleneimine, and polyimide.

[0019] The preparation method of the functional coating comprises the following steps:

[0020] After the lithium-containing compound, the organic polymer, and the binder are selected according to the weight ratio, they are mixed and dispersed in a solvent by ball milling, and then the functional coating slurry is obtained after uniform dispersion.

[0021] The functional coating slurry is uniformly coated on one side of the separator, and then the modified separator containing the functional coating is obtained after drying at a temperature of 40-80℃ for 12-48h.

[0022] The solvent is one or more of water, ethanol, isopropanol, acetone, ethylene glycol, tetrahydrofuran, dimethyl sulfoxide, N-methyl pyrrolidone, and N,N-dimethylformamide.

[0023] The lithium negative electrode comprises a modified separator and a metal lithium negative electrode sheet, wherein the modified separator is located on one side of the metal lithium negative electrode sheet and closely adheres to the metal lithium negative electrode sheet, and the lithium negative electrode is formed after heat pressing treatment, so that the corrosion of the lithium negative electrode by the environment can be effectively reduced, thereby increasing the storage and processability of the lithium negative electrode. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 Flowchart of the preparation process of the integrated lithium negative electrode

[0025] Figure 2 The voltage-time curve of the tested lithium-lithium symmetric battery under a current density of 1mA cm -2 and a capacity of 1mA h cm -2

[0026] In the figure, a is the curve of the ordinary lithium negative electrode, and b is the curve of the integrated lithium negative electrode.

[0027] ​The ordinate is the polarization voltage, and the unit is V; the abscissa is the cycle time, and the unit is h.

[0028] Figure 3 SEM images of the surface of the metal lithium after the lithium sulfur battery assembled with the common lithium negative electrode and the integrated lithium negative electrode is cycled for 100 times at 0.5C

[0029] In the figure: a is an integrated lithium negative electrode; b is a common lithium negative electrode.

[0030] Figure 4 SEM images of the surface of the lithium negative electrode after the lithium iron phosphate full battery assembled with the common lithium negative electrode and the integrated lithium negative electrode is cycled for 100 times at 0.5C rate

[0031] In the figure: a is an integrated lithium negative electrode; b is a common lithium negative electrode.

[0032] Figure 5 Comparison chart of cycle performance and coulomb efficiency of the lithium sulfur battery assembled with the common lithium negative electrode and the integrated lithium negative electrode at 0.5C rate

[0033] In the figure: a curve is the cycle performance of the common lithium negative electrode, b curve is the cycle performance of the integrated lithium negative electrode, c curve is the coulomb efficiency of the integrated lithium negative electrode, and d curve is the coulomb efficiency of the common lithium negative electrode. DETAILED DESCRIPTION

[0034] The application will be described in detail below with reference to the embodiments and the accompanying drawings.

[0035] Example 1

[0036] The preparation method of the lithium negative electrode of the application comprises the following steps:

[0037] 1) 1.2g of lithium magnesium silicate, 0.6g of lithiated perfluorosulfonic acid resin and 0.2g of gelatin are weighed in sequence and uniformly dispersed in water by ball milling, the ball milling speed is 600r / min, the ball milling time is 4h, and a uniformly mixed functional coating slurry is obtained;

[0038] 2) the functional coating slurry is coated on one side of a 10μm thick polyethylene (PE) separator by scraping; after drying in a 60℃ vacuum oven for 12h, a modified separator modified by the functional coating is obtained; the functional coating can be uniformly and closely attached to the surface of the PE separator, and the thickness of the functional coating is 2μm;

[0039] 3) the modified separator and the lithium metal negative electrode are placed on a double-plate hot press, and are combined together by hot pressing at 50℃ and 4Mpa pressure for 2h, to form an integrated lithium negative electrode.

[0040] It is tested that the prepared integrated lithium negative electrode can exist for 24h without oxidation on the surface in an air with a relative humidity of 20%.

[0041] The functional coating material can be coated on the separator for lithium battery by one or more of spraying, dropping, spin coating, doctor blading, and rolling, preferably by doctor blading.

[0042] The preparation method of the battery provided in Example 1 specifically comprises:

[0043] Preparation of the positive electrode: lithium metal sheets are used as the positive electrode.

[0044] Preparation of the electrolyte: first, LiTFSI and LiNO3 are added to the DOL / DME solution to prepare the electrolyte, so that the concentration of LiTFSI in the solution is 1 mol / L, and the weight percentage of LiNO3 is 1.0 wt.%;

[0045] Battery assembly: then, the CR2032 lithium-lithium symmetric button cell is assembled using the positive electrode, the electrolyte, and the lithium negative electrode prepared in Example 1.

[0046] Example 2

[0047] 1) 1.2 g of lithium aluminum silicate, 0.4 g of lithiated perfluorosulfonic acid resin, and 0.2 g of polyvinylidene fluoride are weighed in sequence, uniformly dispersed in N-methylpyrrolidone by ball milling, the ball milling speed is 800 r / min, and the ball milling time is 10 h, to obtain a uniformly mixed functional coating slurry.

[0048] 2) The functional coating slurry is coated on one side of a 25 μm thick polypropylene (PP) separator by doctor blading; after drying in a 60°C vacuum oven for 12 h, a modified separator modified with a functional coating is obtained; the functional coating can be uniformly and closely attached to the surface of the PP separator, and the thickness of the functional coating is 10 μm;

[0049] 3) The modified separator and the lithium metal negative electrode are placed on a double-plate hot press, and hot pressed at 60°C and a pressure of 6 MPa for 2 h, to be combined together by hot pressing, and form an integrated lithium negative electrode.

[0050] Tests show that the prepared integrated negative electrode can exist for 6 hours without oxidation on the surface in an air with a relative humidity of 40%.

[0051] The preparation method of the battery provided in Example 2 specifically comprises:

[0052] Preparation of the positive electrode: a ketchen black-sulfur composite material, conductive carbon black, and polyvinylidene fluoride in a weight ratio of 41:4:5 are mixed uniformly, coated, dried, and then prepared to obtain a ketchen black-sulfur positive electrode sheet, the loading of the positive electrode sheet is 2 mg cm -2 , and the diameter is 14 mm.

[0053] Preparation of electrolyte: First, add LiTFSI and LiNO3 in DOL / DME solution to prepare electrolyte, ensure the concentration of LiTFSI in the solution is 1 mol / L, and the weight percentage of LiNO3 is 1.0wt%;

[0054] Battery assembly: Then use the positive electrode, electrolyte and lithium negative electrode prepared in Example 2 to assemble CR2032 lithium-sulfur battery.

[0055] Example 3

[0056] The other conditions are the same as Example 1, except that polyacrylic acid is used instead of lithiumized perfluorosulfonic acid resin. Then use lithium metal sheet as the positive electrode, and the integrated lithium negative electrode prepared in Example 3 to assemble CR2032 lithium-lithium symmetric button cell.

[0057] Example 4

[0058] The other conditions are the same as Example 1, except that lithium mica is used instead of lithium magnesium silicate. Then use lithium metal sheet as the positive electrode, and the integrated lithium negative electrode prepared in Example 4 to assemble CR2032 lithium-lithium symmetric button cell.

[0059] Example 5

[0060] The other conditions are the same as Example 1, except that lithium nitrate is used instead of lithium magnesium silicate. Then use lithium metal sheet as the positive electrode, and the integrated lithium negative electrode prepared in Example 5 to assemble CR2032 lithium-lithium symmetric button cell.

[0061] Example 6

[0062] The other conditions are the same as Example 1, except that LA133 is used instead of gelatin. Then use lithium metal sheet as the positive electrode, and the integrated lithium negative electrode prepared in Example 6 to assemble CR2032 lithium-lithium symmetric button cell.

[0063] Example 7

[0064] The other conditions are the same as Example 2, except that polyvinylpyrrolidone is used instead of polyvinylidene fluoride. Then use the ketchen black-sulfur positive electrode prepared in Example 2, and the integrated lithium negative electrode prepared in Example 7 to assemble CR2032 lithium-sulfur battery.

[0065] Example 8

[0066] The other conditions are the same as Example 2, except that polyacrylonitrile is used instead of lithiumized perfluorosulfonic acid resin. Then use the ketchen black-sulfur positive electrode prepared in Example 2, and the integrated lithium negative electrode prepared in Example 8 to assemble CR2032 lithium-sulfur battery.

[0067] Example 9

[0068] Other conditions are the same as example 2, the difference is that the lithium soapstone instead of lithium aluminum silicate. Then use the example 2 prepared ketchen black-sulfur positive electrode, with the integrated lithium negative prepared in example 9, CR2032 lithium-sulfur battery is assembled together.

[0069] Example 10

[0070] Other conditions are the same as example 2, the difference is that the N,N-dimethylformamide instead of N-methyl pyrrolidone. Then use the example 2 prepared ketchen black-sulfur positive electrode, with the integrated lithium negative prepared in example 10, CR2032 lithium-sulfur battery is assembled together.

[0071] Example 11

[0072] 1) 1.0 g of lithium aluminum silicate, 0.6 g of lithiumated perfluorosulfonic acid resin and 0.2 g of polyvinylidene fluoride were weighed in turn and uniformly dispersed in N-methyl pyrrolidone by ball milling, the ball milling speed was 800 r / min, the ball milling time was 6 h, and a uniformly mixed functional coating slurry was obtained.

[0073] 2) The above functional coating slurry was coated on one side of a 16 μm thick polypropylene PP separator by means of scraping; after drying in a 60°C vacuum oven for 12 h, a modified separator modified by a functional coating was obtained; the functional coating can be uniformly and closely attached to the surface of the PP separator, and the thickness of the functional coating is 5 μm;

[0074] 3) The modified separator and lithium metal negative electrode were placed on a double-plate hot press, and hot pressed at 60°C and 4Mpa pressure for 4h, and combined together by hot pressing to form an integrated lithium negative electrode.

[0075] Tested, the prepared integrated negative electrode can exist in the air with relative humidity of 30% for 10h without oxidation on the surface.

[0076] Comparative example 1

[0077] In comparative example 1, the integrated negative electrode prepared in example 1 is replaced by an unmodified ordinary metal lithium sheet negative electrode, and a CR2032 lithium-lithium symmetric button cell is prepared according to the same battery preparation method as in example 1.

[0078] Comparative example 2

[0079] In comparative example 2, the integrated negative electrode prepared in example 2 is replaced by an unmodified ordinary metal lithium sheet negative electrode, and a CR2032 lithium-lithium symmetric button cell is prepared according to the same battery preparation method as in example 2.

[0080] Comparative experiment 1

[0081] CR2032 lithium-lithium symmetric button cells prepared from Example 1 and Comparative Example 1 were tested under the same environment. Please refer to Figure 2 Figure 1 is the voltage-time curve of the test cells prepared from Example 1 and Comparative Example 1 at a current density of 1 mA cm -2 and a capacity of 1 mAh cm -2 . Figure 2 As can be seen in Figure 1b, the polarization potential of Example 1 is 20 mV and remains stable for more than 600 h, while the polarization potential of Comparative Example 1 Figure 2 (a) is as high as 50 mV and shows a trend of increasing polarization after 100 h. This is because the functional coating material of the integrated anode of Example 1 spontaneously transfers to the surface of the lithium metal anode to form a uniform and dense SEI film with high ionic conductivity, which can effectively promote the transmission of lithium ions, guide the uniform deposition of lithium, and inhibit the generation of lithium dendrites, making the interface structure more stable, so that the polarization potential can remain stable for a long time. The SEI layer of Comparative Example 1 is located on the surface of lithium, and during the cycle process, the repeated decomposition and generation of the SEI layer due to the volume change of lithium will cause the mixture of lithium ion poor conductors and dead lithium to adhere to the surface of the metal lithium, causing non-uniform deposition of lithium and increasing the polarization potential.

[0082] Therefore, from the above Comparative Experiment 1, it can be seen that compared with the unmodified metal lithium sheet anode, the cycle life of the battery assembled by the integrated lithium anode prepared by Example 1 is significantly improved, and the interface impedance is significantly reduced.

[0083] Comparative Experiment 2

[0084] CR2032 lithium-lithium symmetric button cells prepared from Example 2 and Comparative Example 2 were tested under the same environment. The electrochemical performance of the battery was tested in a battery test system. The test temperature was 25°C, the test voltage interval was 1.8-2.5 V, and the cycle test was carried out at 0.5C under the condition, wherein 1C = 1675 mA g -1 . The test results are shown in Figure 5 , Figure 5 Figure 2 is a comparison of the cycle performance of lithium-sulfur batteries assembled from Example 2 and Comparative Example 2 at 0.5C. As can be seen from Figure 5 the comparison, the lithium-sulfur battery using the integrated lithium anode shows higher coulombic efficiency and better cycle stability, and after 200 cycles, the discharge specific capacity is still 620 mAh g -1 , higher than the 450 mAh g -1 of the ordinary lithium anode; the average coulombic efficiency during the 200 cycles is 99.91%, also higher than the 98.1% of the ordinary lithium anode.

[0085] The difference in the microsurfaces of the lithium negative electrodes of Example 2 and Comparative Example 2 after 100 cycles can be seen by Figure 3 and Figure 4 As can be seen, Figure 3 is an SEM image of the surface of the lithium negative electrode of the lithium-sulfur battery assembled in Example 2 of the present application after 100 cycles at 0.5C; Figure 4 is an SEM image of the surface of the lithium negative electrode of the lithium-iron-phosphate full battery assembled in Comparative Example 2 of the present application after 100 cycles at 0.5C. As can be seen, Figure 3 and Figure 4 As can be seen, compared with the ordinary metal lithium sheet negative electrode, the lithium metal surface using the integrated lithium negative electrode is smoother, and no obvious loose and porous lithium dendrite morphology is found. This again shows that the use of the integrated negative electrode can accelerate the lithium ion transmission, effectively realize the uniform deposition of lithium, and thus inhibit the growth of lithium dendrites.

[0086] The above implementation cases show that, compared with the ordinary lithium negative electrode, the functional coating material of the integrated lithium negative electrode can spontaneously transfer to the surface of the lithium metal negative electrode, can effectively promote the lithium ion transmission, guide the uniform deposition of lithium, and inhibit the generation of lithium dendrites, so that the interface structure is more stable, and the polarization potential can be kept stable in long-time cycling. This fully shows that the preparation method is effective, and has important significance for the processing and preparation of the lithium metal negative electrode and the popularization and use of the lithium metal battery.

Claims

1. A lithium negative electrode, characterized by, The lithium negative electrode comprises a modified separator and a metal lithium negative electrode sheet, and the modified separator is tightly attached to one side of the metal lithium negative electrode sheet to form an integrated lithium negative electrode. The modified separator comprises a separator and a functional coating layer, wherein the thickness of the separator is 5-25 μm, and the thickness of the functional coating layer is 0.5-20 μm. The separator is one or more of PE separator, PP separator, PP / PE / PP three-layer separator, boehmite-coated separator and aramid-coated separator. The raw material of the functional coating layer comprises the following components by weight: 4-8 parts of lithium-containing compound 1-4 parts of organic polymer 1 part of binder The lithium-containing compound is nano-morphology magnesium lithium silicate, and the organic polymer is lithiumated perfluorosulfonic acid resin. The preparation method of the functional coating layer comprises the following steps: The lithium-containing compound, the organic polymer and the binder are selected according to the weight ratio, and then mixed and dispersed in a solvent by ball milling, to obtain a functional coating slurry after uniform dispersion; The functional coating slurry is uniformly coated on one side of the separator, and then dried at a temperature of 40-80 ℃ for 12-48 h to obtain a modified separator containing the functional coating layer; The preparation method of the lithium negative electrode comprises the following steps: Step one, the functional coating layer is pre-coated on the separator to obtain a modified separator; Step two, the modified separator is tightly attached to one side of the metal lithium negative electrode, and then a lithium negative electrode is formed by hot pressing, wherein the hot pressing temperature of the double-plate hot press is adjusted to 30-60 ℃, the hot pressing time is 0.5-6 h, and the hot pressing pressure of the double-plate hot press is adjusted to 1-10 Mpa.

2. A lithium negative electrode as claimed in claim 1, wherein, The binder is one or more of gelatin, gum arabic, chitosan, starch, cyclodextrin, polyvinylpyrrolidone, carboxymethyl cellulose, sodium carboxymethyl cellulose, sodium alginate, butadiene rubber, LA133, polyvinylidene fluoride, polyethylene oxide, polyamide, polyethyleneimine and polyimide.

3. The lithium anode of claim 1 wherein the lithium anode is characterized by: The solvent is one or more of water, ethanol, isopropyl alcohol, acetone, ethylene glycol, tetrahydrofuran, dimethyl sulfoxide, N-methyl pyrrolidone and N,N-dimethylformamide.

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