Metal lithium secondary battery negative electrode containing alloy protective layer prepared through physical deposition and preparation method

Abstract

The invention relates to a metal lithium secondary battery negative electrode containing an alloy protection layer prepared through physical deposition and a preparation method. The method comprises the following steps: cutting a lithium strip used as a negative electrode of the lithium metal secondary battery to prepare a sample, and physically depositing a layer of lithium tin alloy film with the thickness of 30-50nm through magnetron sputtering, so that a film interface protection layer is formed on the surface of the negative electrode, and the lithium battery containing the interface protection layer realizes uniform dissolution and deposition of lithium metal in the cycle process; furthermore, the negative electrode with the lithium-tin alloy protection layer can be used in the metal lithium secondary battery, and the lithium-tin alloy protection layer can improve the interface stability of the battery negative electrode, so that the metal lithium secondary battery has better stability, and the battery cycle interface stability and long cycle life are realized.

Description

Technical Field

[0001] The invention belongs to the technical field of chemical power sources, and in particular relates to a negative electrode of a metal lithium secondary battery containing an alloy protective layer prepared by physical deposition and a preparation method thereof. Background Art

[0002] In recent years, with the development of diversified energy demand, higher requirements have been put forward for high energy density and long cycle life energy storage systems. With the progress of research, the actual energy density of commercial batteries has only increased six times from the first generation of lead-acid batteries (~40Wh / kg) to the current lithium-ion batteries (~240Wh / kg and 640Wh / L), and the energy density growth rate of lithium-ion batteries is only 7%-8% per year. Lithium-ion batteries were once considered to be suitable candidates for energy storage, but their energy density has been close to the theoretical value. Lithium metal has an ultra-high capacity (3860mAh / g) and the lowest negative electrochemical potential (-3.040V relative to the standard hydrogen electrode). It is considered to be the "holy grail" electrode and has received extensive research attention. Compared with existing lithium-ion battery anode materials, metallic lithium anode has great potential in meeting the demand for high energy density. Lithium metal secondary batteries with lithium metal anode systems can achieve energy density of 450-600Wh / kg. Although lithium metal anodes have excellent theoretical capacity and energy density, due to the instability of the lithium metal / liquid electrolyte interface caused by dendrite growth during lithium deposition / stripping, the short circuit caused by dendritic lithium crystal growth leads to thermal runaway and the risk of lithium metal secondary battery explosion. Various safety issues have made lithium metal secondary batteries lose the opportunity to enter the commercial market. Therefore, in the past 40 years, lithium battery researchers have been studying strategies to inhibit dendrite growth. Summary of the invention

[0003] In order to solve the problem of cycle stability in the application of new battery systems, the present invention proposes an innovative method for preparing an alloy protective layer by physical deposition, a metal lithium secondary battery negative electrode containing an alloy protective layer prepared by physical deposition and a preparation method.

[0004] The technical solution adopted by the present invention is: a method for preparing a negative electrode of a metal lithium secondary battery containing an alloy protective layer prepared by physical deposition, wherein a lithium-tin alloy thin film layer is deposited on the surface of a cut lithium strip by a DC magnetron sputtering physical deposition method to obtain a battery negative electrode containing a lithium-tin alloy protective layer.

[0005] Preferably, the thickness of the lithium-tin alloy protective layer is 30 nm to 50 nm, and the thickness of the lithium-tin alloy protective layer is controlled by controlling the magnetron sputtering deposition time.

[0006] Preferably, the lithium ribbon is cut, the copper tab is pressed onto the lithium ribbon, and the ribbon is placed on a magnetron sputtering sample stage, and the vacuum degree of the chamber is set to 10 -4Pa, turn on the DC power supply, and deposit a lithium-tin alloy thin film layer with a thickness of 30nm to 50nm on the surface of the lithium negative electrode by controlling the magnetron sputtering deposition time.

[0007] A battery negative electrode containing a lithium-tin alloy protective layer is prepared by a method for preparing a battery negative electrode containing an alloy protective layer prepared by physical deposition.

[0008] A metal lithium secondary battery comprises a metal lithium secondary battery negative electrode containing a lithium-tin alloy protective layer.

[0009] Preferably, it is a lithium-lithium symmetrical button cell or a metal lithium secondary soft pack cell.

[0010] Preferably, the specific preparation method is as follows:

[0011] Step 1: The positive electrode active material, the conductive agent, and the adhesive are uniformly dispersed in an appropriate amount of NMP solvent by a homogenizer; the uniformly dispersed slurry is coated on an aluminum foil current collector to prepare a positive electrode, and the positive electrode sheet is obtained after rolling, slitting, and die-cutting;

[0012] Step 2: Cut and fix the lithium ribbon, press the copper tab onto the lithium sheet using a hydraulic press, and then deposit a lithium-tin alloy thin film layer on the surface of the cut lithium ribbon using a DC magnetron sputtering physical deposition method to obtain a negative electrode sheet containing a lithium-tin alloy protective layer;

[0013] Step 3: Prepare a lithium-lithium symmetrical button cell: place a button cell spring sheet, a button cell gasket, a lithium negative electrode containing a lithium-tin alloy protective layer, a separator, an electrolyte, and a lithium sheet into a button cell shell in the order of preparing a metal lithium secondary button cell;

[0014] Alternatively, a soft-pack battery is prepared by repeatedly stacking a lithium metal secondary battery electrode group in the order of a separator, a lithium negative electrode containing a lithium-tin alloy protective layer, a separator, and a positive electrode, and injecting an electrolyte to form a lithium metal secondary soft-pack battery after a formation and exhaust process.

[0015] Preferably, the positive electrode active material includes 97wt% high nickel ternary, the conductive agent is 1wt% conductive carbon black and 0.5wt% conductive carbon nanotubes, and the binder is 1.5wt% PVDF, wherein the proportion of PVDF in the NMP solvent is 5wt%.

[0016] The advantages and positive effects of the present invention are: a lithium-tin alloy thin film interface protective layer is physically deposited by magnetron sputtering, the protective layer improves the uneven structure of the electrode surface, has high ionic conductivity, and reduces the diffusion resistance of lithium ions through the film; the protective layer has excellent mechanical properties and adapts to the impact and uneven volume changes during repeated lithium deposition / stripping processes;

[0017] Lithium batteries containing this interface protection layer achieve uniform dissolution and deposition of metallic lithium during the cycle process, thereby achieving battery cycle interface stability and long cycle life. The physical deposition preparation method of the lithium alloy protective layer has high uniformity and excellent performance, and has broad application space in the field of metallic lithium secondary batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a comparison diagram of the cycle overpotential of the lithium-lithium symmetric button cell of Example 1 and the comparative example;

[0019] Figure 2 It is a comparison chart of the cycle capacity retention rate of the metal lithium secondary soft-pack battery of Example 1 and the comparative example. DETAILED DESCRIPTION

[0020] The embodiments of the present invention are described below with reference to the accompanying drawings.

[0021] The present invention relates to a metal lithium secondary battery negative electrode containing an alloy protective layer prepared by physical deposition and a preparation method thereof. A lithium ribbon used as a battery negative electrode is cut, and then a layer of lithium tin alloy is physically deposited by magnetron sputtering, thereby forming a thin film interface protective layer on the surface of the negative electrode. Furthermore, it can be used in a metal lithium battery system with a higher energy density, such as using a negative electrode with a lithium tin alloy protective layer in a metal lithium secondary battery. The lithium tin alloy protective layer can improve the interface stability of the battery negative electrode, so that the metal lithium secondary battery has better stability.

[0022] When preparing the negative electrode with a lithium-tin alloy protective layer, the lithium ribbon is cut and fixed on the magnetron sputtering sample stage through a tooling, and the magnetron sputtering vacuum chamber door is closed and the chamber is evacuated until the vacuum degree of the chamber drops to 10 -4 Pa, turn on the DC power supply, and deposit a lithium-tin alloy film layer with a thickness of 30nm to 50nm on the surface of the lithium negative electrode by controlling the magnetron sputtering deposition time to 0.5h, in which the tin content of the lithium-tin alloy is 1% to 5%; then use a die-cutting tool to cut the negative electrode containing the alloy protective layer into the required size.

[0023] The lithium-tin alloy protective layer improves the uneven structure of the electrode surface and has high ionic conductivity to reduce the diffusion resistance of lithium ions through the membrane; the protective layer has excellent mechanical properties and can adapt to the impact and uneven volume changes during repeated lithium deposition / stripping; the protective layer has extraordinary stability in morphology, structure and chemical properties in long-term cycles, improves the stability of the battery negative electrode interface, thereby achieving more uniform lithium dissolution / deposition during the cycle, and has a certain inhibitory effect on the growth of dendritic / granular lithium dendrites.

[0024] The negative electrode of the battery containing a lithium-tin alloy protective layer can be prepared to form a metal lithium secondary battery. The positive electrode active material of the battery can use a high-nickel ternary (NCM) material. The electrolyte is composed of a lithium bis(fluorosulfonyl)imide electrolyte and ester and ether organic solvents. The diaphragm is a ceramic-coated composite diaphragm. After preparing the various components of the battery, assembling them according to conventional methods can obtain a metal lithium secondary battery including a negative electrode containing a lithium-tin alloy protective layer. Specifically, it can be a lithium-lithium symmetrical button cell or a metal lithium secondary soft-pack cell. During the cycle of the lithium-lithium symmetrical button cell and the metal lithium secondary soft-pack cell, the lithium-tin alloy protective layer realizes uniform dissolution and deposition of metal lithium on the negative electrode surface, thereby achieving the cycle interface stability and long cycle life of the metal lithium secondary battery.

[0025] The scheme of the present invention is described below in conjunction with the accompanying drawings, wherein the experimental methods without specific operating steps are all carried out in accordance with the corresponding product instructions, and the instruments, reagents, and consumables used in the examples can all be purchased from commercial companies unless otherwise specified.

[0026] Embodiment 1:

[0027] Positive electrode preparation: 97wt% of the positive electrode active material high nickel ternary (NCM), 1wt% of conductive carbon black and 0.5wt% of the conductive agent composed of carbon nanotubes, and 1.5wt% of the adhesive PVDF are evenly dispersed in an appropriate amount of NMP solvent through a homogenizer, wherein the proportion of PVDF in the NMP solvent is 5wt%; the evenly dispersed slurry is coated on an aluminum foil current collector to prepare a positive electrode, and the positive electrode sheet is obtained after rolling, slitting and die-cutting.

[0028] Preparation of lithium negative electrode: Cut the 0.06mm thick lithium ribbon into suitable size and press the copper tab onto the lithium sheet at a fixed position using a hydraulic press. Prepare a lithium alloy negative electrode protective layer on the surface of the lithium negative electrode. Cut the lithium ribbon and fix it on the magnetron sputtering sample stage with a tool. Close the magnetron sputtering vacuum chamber door and evacuate the chamber until the vacuum degree of the chamber drops to 10 - 4 Pa, turn on the DC power supply, and deposit a lithium-tin alloy film layer with a thickness of 30nm to 50nm on the surface of the lithium negative electrode by controlling the magnetron sputtering deposition time to 0.5h, and then cut the negative electrode containing the alloy protective layer into the required size by a die-cutting tool.

[0029] Preparation and testing of button cells: button cell spring sheet, button cell gasket, lithium negative electrode containing lithium-tin alloy protective layer, diaphragm, electrolyte, and lithium sheet are placed in the button cell shell in the order of the button cell. The button cell is placed in a battery packaging machine and a fixed pressure is applied to seal the battery shell to make a lithium-lithium symmetrical button cell. Use a charge and discharge device at 2.5 mA / cm 2 The current density is 2.5 mAh / cm 2 The capacity density is used for charge and discharge cycles.

[0030] Preparation and testing of soft-pack batteries: The lithium secondary battery electrode group is obtained by stacking the metal lithium negative electrode, lithium battery separator, and positive electrode in the order of stacking sheets, and then encapsulated with aluminum-plastic film for lithium batteries. The lithium secondary soft-pack batteries are prepared through the process of liquid injection, aging, formation, degassing and degassing chamber. The charge and discharge cycle performance test is carried out using a charge and discharge equipment at a rate of 0.2CC / 0.5CD. The charge cut-off voltage is 4.35V and the discharge cut-off voltage is 3.0V.

[0031] Comparative Example:

[0032] The preparation method of the positive electrode in this embodiment is the same as that in embodiment 1, but the negative electrode is not subjected to the protective layer preparation. The battery preparation and testing methods are the same as those in embodiment 1.

[0033] The test results of Example 1 and the comparative example are compared and analyzed. Figure 1 and Figure 2 .

[0034] Figure 1 Comparison of the cycle overpotential of the lithium-lithium symmetric button cell of Example 1 and the comparative example. 2 The current density is 2.5 mAh / cm 2 Under the same charge and discharge regime of capacity density, the button cell Example 1 containing a lithium alloy protective layer has a lower overpotential and better cycle stability than the button cell control example without a protective layer. The battery containing a lithium alloy protective layer has a lower overpotential during the cycle. During 200 lithium deposition / stripping cycles, the battery using a lithium alloy protective layer also achieved a stable voltage curve 400 hours later. The overpotential was stably maintained at 0.15V within 300 hours of battery cycling, and the overpotential was stably maintained at 0.20V within 400 hours of cycling. The battery without a protective layer failed after only 80 hours of cycling. This phenomenon shows that the lithium alloy film protective layer effectively achieves interface deposition consistency during the cycle, inhibits the formation of dendritic lithium during lithium deposition, and promotes the deposition of effective active lithium, thereby improving the cycle stability of the battery.

[0035] Figure 2The performance comparison of the cycle capacity retention rate of the lithium metal secondary battery of Example 1 and the comparative example is shown. It can be seen that under the same charge and discharge system, the lithium metal secondary soft-pack battery Example 1 containing a lithium alloy protective layer has better cycle stability than the soft-pack battery comparative example without a protective layer, and the capacity decay is slower; the capacity retention rate of Example 1 at the 76th week of the cycle is 98.71%, and the capacity retention rate of the comparative example is 81.37%; and the capacity retention rate of Example 1 drops below 80% after 140 weeks of cycling. The above results show that the lithium alloy protective layer (Example 1) has the function of stabilizing the negative electrode interface film, avoiding repeated consumption and regeneration of the negative electrode interface film. The stable negative electrode interface film reduces the reaction between the electrolyte and the lithium metal, effectively weakens the side reaction between the lithium metal negative electrode and the electrolyte, inhibits the growth of lithium dendrites, and improves the cycle stability of the lithium metal negative electrode.

[0036] From the above experimental data, it can be seen that the lithium alloy negative electrode protective layer has a significant effect on improving the stability of the metal lithium negative electrode interface. The capacity retention rate of the metal lithium secondary soft-pack battery containing the lithium alloy protective layer can still reach more than 80% after 139 weeks of stable cycling, while the capacity retention rate of the battery without the protective layer drops to less than 80% after only 76 weeks of cycling. The lithium alloy protective layer is applied to metal lithium secondary batteries. The lithium alloy film has excellent mechanical properties, which reduces the volume expansion of the lithium negative electrode during cycling. In addition, the protective layer has a uniform and densified effect on lithium deposition during the first charging process, thereby improving the subsequent cycle stability of the battery. It provides an engineering solution for the practical application of metal lithium negative electrodes, greatly improves the overall performance of metal lithium secondary batteries, is conducive to improving the market application prospects of batteries, and has important production practice significance.

[0037] The embodiments of the present invention are described in detail above, but the contents described are only preferred embodiments of the present invention and cannot be considered to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A method for preparing a negative electrode of a lithium metal secondary battery having an alloy protective layer prepared by physical deposition, characterized in that: A lithium-tin alloy film layer is deposited on the surface of the cut lithium ribbon by a direct current magnetron sputtering physical deposition method to obtain a battery negative electrode containing a lithium-tin alloy protective layer.

2. The method for preparing a negative electrode of a lithium metal secondary battery having an alloy protective layer prepared by physical deposition according to claim 1, characterized in that: The thickness of the lithium-tin alloy protective layer is 30-50 nm, and the thickness of the lithium-tin alloy protective layer is controlled by controlling the magnetron sputtering deposition time.

3. The method for preparing a negative electrode of a lithium metal secondary battery having an alloy protective layer prepared by physical deposition according to claim 1, characterized in that: Cut the lithium ribbon, press the copper tab onto the lithium ribbon, place it on the magnetron sputtering sample stage, and set the chamber vacuum to 10 -4 Pa, turn on the DC power supply, and deposit a lithium-tin alloy thin film layer with a thickness of 30nm to 50nm on the surface of the lithium negative electrode by controlling the magnetron sputtering deposition time.

4. A battery negative electrode containing a lithium-tin alloy protective layer obtained by the method for preparing a metal lithium secondary battery negative electrode containing an alloy protective layer prepared by physical deposition as claimed in any one of claims 1 to 3.

5. A metal lithium secondary battery, characterized in that: A negative electrode for a lithium secondary battery comprising a lithium-tin alloy protective layer as claimed in claim 4.

6. The metal lithium secondary battery according to claim 5, characterized in that: Specifically, it is a lithium-lithium symmetrical button cell or a metal lithium secondary soft-pack cell.

7. The metal lithium secondary battery according to claim 6, characterized in that: The specific preparation method is as follows: Step 1: The positive electrode active material, the conductive agent, and the adhesive are uniformly dispersed in an appropriate amount of NMP solvent by a homogenizer; the uniformly dispersed slurry is coated on an aluminum foil current collector to prepare a positive electrode, and the positive electrode sheet is obtained after rolling, slitting, and die-cutting; Step 2: Cut and fix the lithium ribbon, press the copper tab onto the lithium sheet using a hydraulic press, and then deposit a lithium-tin alloy thin film layer on the surface of the cut lithium ribbon using a DC magnetron sputtering physical deposition method to obtain a negative electrode sheet containing a lithium-tin alloy protective layer; Step 3: preparing a lithium-lithium symmetrical button cell: placing a button cell spring sheet, a button cell gasket, a lithium negative electrode containing a lithium-tin alloy protective layer, a separator, an electrolyte, and a lithium sheet into a button cell shell in the order of preparing a lithium-lithium symmetrical button cell; Alternatively, a soft-pack battery is prepared by repeatedly stacking a lithium metal secondary battery electrode group in the order of a separator, a lithium negative electrode containing a lithium-tin alloy protective layer, a separator, and a positive electrode, and injecting an electrolyte to form a lithium metal secondary soft-pack battery after a formation and exhaust process.

8. The metal lithium secondary battery according to claim 7, characterized in that: The positive electrode active material includes 97wt% high nickel ternary, the conductive agent is 1wt% conductive carbon black and 0.5wt% conductive carbon nanotubes, and the binder is 1.5wt% PVDF, wherein the proportion of PVDF in the NMP solvent is 5wt%.

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