Lithium metal battery negative electrode and preparation method therefor, and lithium secondary battery

Abstract

The present invention belongs to the technical field of lithium batteries, and provides a lithium metal battery negative electrode and a preparation method therefor, and a lithium secondary battery. The lithium metal battery negative electrode comprises: a current collector and a lithium metal layer provided on the surface of the current collector, and a composite film layer is provided on the side, away from the current collector, of the lithium metal layer. The composite film layer comprises a two-dimensional black phosphorus nanosheet and a polymer piezoelectric material, and the mass ratio of the two-dimensional black phosphorus nanosheet to the polymer piezoelectric material is 1:1-2. The growth of lithium dendrites and the decomposition of an electrolyte solution can be inhibited, and the attenuation of battery performance can be effectively mitigated.

Description

Lithium metal battery negative electrode, preparation method thereof and lithium secondary battery

[0001] Cross-reference to Related Applications

[0002] The present application claims priority to the Chinese patent application No. 2024107316757, filed on June 6, 2024, and entitled "A lithium metal battery negative electrode, preparation method thereof and lithium secondary battery", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present application relates to the technical field of lithium batteries, and particularly relates to a lithium metal battery negative electrode, a preparation method thereof and a lithium secondary battery. BACKGROUND

[0004] Lithium secondary batteries are used as power sources for electric vehicles due to their excellent electrochemical performance, and as the demand for the cruising range of electric vehicles increases, the energy density of the battery needs to be continuously improved. Lithium metal is considered to have application prospects in high specific energy lithium secondary batteries due to its high specific capacity (3860 mAh / g).

[0005] However, uncontrolled lithium dendrite growth and severe electrolyte decomposition in the current lithium secondary battery can cause battery capacity loss or short circuit, hindering the further practical application of lithium metal batteries.

[0006] SUMMARY

[0007] In view of the deficiencies of the prior art, the purpose of the embodiments of the present application includes providing a lithium metal battery negative electrode, a preparation method thereof and a lithium secondary battery. The growth of lithium dendrites and the decomposition of electrolyte can be inhibited, and the performance degradation of the battery can be effectively slowed down.

[0008] In a first aspect, the embodiments of the present application provide a lithium metal battery negative electrode, comprising: a current collector and a lithium metal layer arranged on the surface of the current collector, the lithium metal layer being provided with a composite film layer on the side away from the current collector; the composite film layer comprises two-dimensional black phosphorus nanosheets and a polymer piezoelectric material, and the mass ratio of the two-dimensional black phosphorus nanosheets and the polymer piezoelectric material is 1:1-2.

[0009] The two-dimensional black phosphorus nanosheet and the polymer piezoelectric material both have piezoelectricity, and the polymer piezoelectric material has good flexibility, so that when the two-dimensional black phosphorus nanosheet is matched with the polymer piezoelectric material at a suitable mass ratio, a film layer with good piezoelectricity, a certain thickness and toughness can be formed on the surface of the lithium metal layer. When lithium nucleation deposition in the lithium metal layer forms a lithium dendrite protrusion, it will cause local deformation of the composite film layer, thereby triggering the piezoelectric effect of the composite film layer to form a local piezoelectric reverse overpotential, which can inhibit the growth of lithium dendrites. Moreover, the film layer has a certain thickness and toughness, and can also correspondingly inhibit the growth of lithium dendrites to penetrate the film layer. At the same time, further black phosphorus can react with the lithium dendrite tip to form electrically insulating lithium phosphide, to further inhibit the growth of lithium dendrites. Moreover, black phosphorus can also reduce high-oxidizing free radicals in the electrolyte, protect the electrolyte from decomposition caused by free radicals, thereby effectively slowing down the performance degradation of the battery.

[0010] In some embodiments of the present application, the polymer piezoelectric material includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-hexafluoropropylene copolymer, polylactic acid, polyacrylonitrile, and poly-beta-hydroxybutyrate.

[0011] The at least one polymer piezoelectric material used in the present application has a certain piezoelectric property and excellent flexibility, and can be matched with the two-dimensional black phosphorus nanosheet to form a composite film layer with high piezoelectric property, a certain thickness and toughness.

[0012] In some embodiments of the present application, the thickness of the composite film layer is 3-10 μm.

[0013] The present application uses a composite film layer with a suitable thickness arranged on the side of the lithium metal layer away from the current collector, which can not only ensure a certain thickness to protect the lithium metal negative electrode and effectively inhibit the growth of lithium dendrites, but also avoid excessive thickness to avoid affecting the transmission of lithium ions.

[0014] In some embodiments of the present application, the thickness of the lithium metal layer is 500-700 μm.

[0015] In a second aspect, the embodiments of the present application provide a preparation method of the lithium metal battery negative electrode provided in the first aspect, which comprises:

[0016] (1) Ball milling a phosphorus raw material into black phosphorus fine powder under an inert gas atmosphere, dissolving the black phosphorus fine powder in a first solvent, centrifuging after ultrasonic exfoliation in an ice water bath to obtain two-dimensional black phosphorus nanosheets dissolved in the first solvent;

[0017] (2) Dissolving the two-dimensional black phosphorus nanosheets in a second solvent after centrifugal cleaning, adding a polymer piezoelectric material according to a mass ratio, and adding a lithium ion solution to obtain a composite film layer precursor solution;

[0018] (3) fixing the lithium metal layer on the surface of the current collector, coating the composite film layer precursor solution to the side of the lithium metal layer away from the current collector, and drying to obtain a lithium metal battery negative electrode.

[0019] The phosphorus raw material is ball milled into black phosphorus fine powder, and then the black phosphorus fine powder is subjected to liquid phase ultrasonic stripping in a water bath, so that two-dimensional black phosphorus nanosheets with good piezoelectric properties can be efficiently obtained in a simple and low-cost manner; the two-dimensional black phosphorus nanosheets, the polymer piezoelectric material, the lithium ion solution and the solvent are mixed to obtain a composite film layer precursor solution, and then the composite film layer precursor solution is coated to the side of the lithium metal layer away from the current collector to form a composite film layer, so as to protect the lithium metal negative electrode, effectively inhibit the growth of lithium dendrites and the decomposition of electrolyte, and effectively slow down the performance degradation of the battery.

[0020] In some embodiments of the present application, the mass ratio of the two-dimensional black phosphorus nanosheets, the polymer piezoelectric material, the lithium ion solution and the second solvent is 1:1-2:1:7-9.

[0021] The two-dimensional black phosphorus nanosheets, the polymer piezoelectric material, the lithium ion solution and the second solvent are mixed in a suitable mass ratio to form a composite film layer precursor solution, so that a composite film layer with high piezoelectric properties and certain thickness and toughness can be formed.

[0022] In some embodiments of the present application, the phosphorus raw material includes any one of red phosphorus, black phosphorus bulk and black phosphorus crystal.

[0023] Any one of the above-mentioned phosphorus raw materials can be ball milled to obtain black phosphorus fine powder, so as to obtain two-dimensional black phosphorus nanosheets in subsequent preparation.

[0024] In some embodiments of the present application, the first solvent is a polar aprotic solvent containing an alkaline salt.

[0025] The polar aprotic solvent containing the alkaline salt is used to dissolve the black phosphorus fine powder, so that the black phosphorus fine powder can be ultrasonically stripped to obtain two-dimensional black phosphorus nanosheets efficiently.

[0026] In some embodiments of the present application, the second solvent is a polar aprotic solvent.

[0027] The polar aprotic solvent is used to dissolve the two-dimensional black phosphorus nanosheets, the polymer piezoelectric material and the lithium ion solution, so that a uniform and stable composite film layer precursor solution can be obtained for subsequent preparation of a stable composite film layer.

[0028] In some embodiments of the present application, the alkaline salt includes at least one of sodium hydroxide, potassium hydroxide and lithium hydroxide.

[0029] In some embodiments of the present application, the polar aprotic solvent comprises at least one of N-methylpyrrolidone, N,N-dimethylformamide, 1,3-dimethyl-2-imidazolidinone, 1,2-dimethoxyethane.

[0030] In some embodiments of the present application, the ball milling is at 200-2000 rpm for 1-48 h.

[0031] The phosphorus raw material can be ball milled into black phosphorus fine powder with a suitable particle size by the above-mentioned ball milling at a suitable speed for a suitable time, so as to be efficiently ultrasonically exfoliated into two-dimensional black phosphorus nanosheets with higher piezoelectricity.

[0032] In some embodiments of the present application, the mass ratio of the phosphorus raw material to the ball milling steel ball is 1:10-50.

[0033] The phosphorus raw material can be ball milled into black phosphorus fine powder with a suitable particle size by mixing the phosphorus raw material with the ball milling steel ball at a suitable mass ratio.

[0034] In some embodiments of the present application, the concentration of the black phosphorus fine powder dissolved in the first solvent is 0.5-5 mg / mL.

[0035] The black phosphorus fine powder is dissolved in the first solvent to form a black phosphorus solution with a suitable concentration, so as to be efficiently ultrasonically exfoliated in the subsequent step, and two-dimensional black phosphorus nanosheets with higher piezoelectricity are efficiently obtained.

[0036] In some embodiments of the present application, the frequency of the ultrasonic wave is 20-30 kHz, the power is 600-1500 W, and the ultrasonic exfoliation is performed for 1-10 h.

[0037] The black phosphorus is ultrasonically exfoliated for a suitable time by using the ultrasonic wave with a suitable frequency and power, so as to be efficiently exfoliated to obtain two-dimensional black phosphorus nanosheets with higher piezoelectricity.

[0038] In some embodiments of the present application, the centrifugation in step (1) is performed at 2000-8000 rpm for 5-10 min.

[0039] The solution after the ultrasonic exfoliation is centrifuged for a suitable time by using a suitable speed, so as to improve the yield of the two-dimensional black phosphorus nanosheets.

[0040] In some embodiments of the present application, the centrifugation in step (2) is performed at 10000-13333 rpm for 5-10 min.

[0041] The two-dimensional black phosphorus nanosheets are centrifuged for a suitable time by using a suitable speed, so as to be efficiently cleaned, and the two-dimensional black phosphorus nanosheets are beneficial to the preparation of a composite film layer precursor solution in the subsequent step.

[0042] In some embodiments of the present application, the mixing in step (2) is performed for 6-48 h under an argon atmosphere with an oxygen content of less than 0.1 ppm.

[0043] In the present application, the two-dimensional black phosphorus nanosheet, the polymer piezoelectric material, the lithium ion solution and the second solvent are mixed under suitable conditions for a suitable time to obtain a uniform and stable composite film layer precursor solution.

[0044] In some embodiments of the present application, the spin coating in step (3) is performed at 500-1500 rpm for 30-60 s.

[0045] In the present application, the composite film layer is formed on the side of the lithium metal layer away from the current collector by spin coating at a suitable speed for a suitable time to form a composite film layer with a suitable thickness.

[0046] In some embodiments of the present application, the drying is performed at 25-100°C for 6-24 h.

[0047] In the present application, the composite film layer is dried at a suitable temperature for a suitable time to form a stable and tough composite film layer.

[0048] In a third aspect, the embodiments of the present application provide a lithium secondary battery comprising the lithium metal battery negative electrode provided in the first aspect.

[0049] In the present application, the lithium metal battery negative electrode with a composite film layer on the side of the lithium metal layer away from the current collector is used to prepare a lithium secondary battery, and the performance of the lithium secondary battery is good. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. If the specific conditions are not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are used. If the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be purchased on the market.

[0051] In order to alleviate the problem that uncontrolled lithium dendrite growth and serious electrolyte decomposition in the lithium secondary battery can cause capacity loss or short circuit, the embodiments of the present application provide a lithium metal battery negative electrode, which comprises: a current collector and a lithium metal layer arranged on the surface of the current collector, and a composite film layer arranged on the side of the lithium metal layer away from the current collector; the composite film layer comprises two-dimensional black phosphorus nanosheets and a polymer piezoelectric material, and the mass ratio of the two-dimensional black phosphorus nanosheets to the polymer piezoelectric material is 1:1-2.

[0052] The two-dimensional black phosphorus nanosheet and the polymer piezoelectric material both have piezoelectricity, and the polymer piezoelectric material has good flexibility, so that the two-dimensional black phosphorus nanosheet and the polymer piezoelectric material can be mixed in a suitable mass ratio to form a film layer with good piezoelectricity, a certain thickness and toughness on the surface of the lithium metal layer. In a first aspect, when lithium nucleates and deposits to form lithium dendrite protrusions in the lithium metal layer, local deformation of the composite film layer is caused, thereby triggering the piezoelectric effect of the composite film layer to form a local piezoelectric reverse overpotential in the composite film layer. The reverse overpotential can weaken the diffusion and deposition driving force of lithium ions to the nucleation and deposition site of the lithium dendrite protrusion tip, thereby inhibiting the formation of lithium dendrites in the local area. In addition, the composite film layer has a certain thickness and toughness, which can also inhibit the growth of lithium dendrites through the film layer, thereby making the surface of the lithium metal layer flat. In a second aspect, if the lithium dendrites continue to grow and the lithium dendrite tip penetrates into the composite film layer, the black phosphorus has electrochemical activity and reacts with the lithium dendrite tip to form electrically insulating lithium phosphide (Li3P), thereby inhibiting the continuous deposition of lithium ions to the lithium dendrite tip and avoiding the further growth of lithium dendrites, thereby reducing the risk of short circuit caused by lithium dendrites piercing the separator during the cycle process and improving the safety of the battery. In a third aspect, the two-dimensional black phosphorus nanosheet also has a certain reducing property and can selectively reduce high-oxidizing free radicals in the electrolyte to protect the electrolyte from decomposition caused by free radicals, thereby effectively slowing down the performance degradation of the battery.

[0053] As an example, the polymer piezoelectric material can include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polylactic acid (PLA), polyacrylonitrile (PAN), and poly-beta-hydroxybutyrate (PHB). The above-mentioned polymer piezoelectric material has a certain piezoelectric property and excellent flexibility, and can be mixed with the two-dimensional black phosphorus nanosheet to form a composite film layer with high piezoelectric property, a certain thickness and toughness. Further, the weight average molecular weight of the polymer piezoelectric material is 100000-1000000, preferably 400000-800000.

[0054] In some embodiments of the present application, the thickness of the composite film layer is 3-10 μm. Further, the thickness of the composite film layer is 5-10 μm. As an example, the thickness of the composite film layer can be, but is not limited to, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm. The composite film layer with a suitable thickness is arranged on the side of the lithium metal layer away from the current collector, which can not only ensure a certain thickness to protect the lithium metal negative electrode and effectively inhibit the growth of lithium dendrites, but also avoid the influence of too thick thickness on the transmission of lithium ions.

[0055] In some embodiments of the present application, the thickness of the lithium metal layer is 500-700 μm. By way of example, the thickness of the lithium metal layer can be, but is not limited to, 500 μm, 550 μm, 600 μm, 650 μm, 700 μm.

[0056] The present application provides a preparation method of the above-mentioned lithium metal battery negative electrode, comprising:

[0057] (1) In an inert gas atmosphere, mix the phosphorus raw material with the ball-milled steel balls at a mass ratio of 1:10-50, ball mill at 200-2000 rpm for 1-48 h to obtain black phosphorus fine powder, dissolve the black phosphorus fine powder in a polar aprotic solvent containing an alkaline salt to form a black phosphorus solution with a concentration of 0.5-5 mg / mL, ultrasonically exfoliate in an ice water bath at a frequency of 20-30 kHz and a power of 600-1500 W for 1-10 h to obtain a brown-black suspension; centrifuge the brown-black suspension at 2000-8000 rpm for 5-10 min to obtain two-dimensional black phosphorus nanosheets dissolved in a polar aprotic solvent containing an alkaline salt;

[0058] (2) Centrifuge the two-dimensional black phosphorus nanosheets at 10000-13333 rpm for 5-10 min to remove the polar aprotic solvent containing the alkaline salt, then repeatedly wash the centrifuged two-dimensional black phosphorus nanosheet precipitate with the polar aprotic solvent several times, and then mix the two-dimensional black phosphorus nanosheets, the polymer piezoelectric material, the lithium ion solution, and the polar aprotic solvent at a mass ratio of 1:1-2:1:7-9 in an argon atmosphere with a water and oxygen content of less than 0.1 ppm for 6-48 h to obtain a composite film layer precursor solution;

[0059] (3) Fix the lithium metal layer on the surface of the current collector, spin coat the composite film layer precursor solution on the side of the lithium metal layer away from the current collector at 500-1500 rpm for 30-60 s, and dry at 25-100 °C for 6-24 h to obtain a lithium metal battery negative electrode with a 3-10 μm composite film layer on the side of the lithium metal layer away from the current collector.

[0060] The phosphorus raw material is ball-milled into black phosphorus fine powder, and then the black phosphorus fine powder is subjected to liquid-phase ultrasonic exfoliation in a water bath, which is a simple and low-cost method that can efficiently obtain two-dimensional black phosphorus nanosheets with good piezoelectric properties. Then, the two-dimensional black phosphorus nanosheets are mixed with a polymer piezoelectric material, a lithium ion solution, and a solvent to obtain a composite film layer precursor solution, and the composite film layer precursor solution is spin-coated onto the side of the lithium metal layer away from the current collector to form a composite film layer, thereby protecting the lithium metal negative electrode, effectively inhibiting the growth of lithium dendrites and the decomposition of the electrolyte, and effectively slowing down the performance degradation of the battery.

[0061] By way of example, the inert gas can include, but is not limited to, any one of argon, nitrogen, and helium.

[0062] Further, the mass ratio of the phosphorus raw material to the ball-milled steel ball is 1:20-30. As an exemplary, the mass ratio of the phosphorus raw material to the ball-milled steel ball can be but not limited to 1:10, 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45, 1:50. The phosphorus raw material and the ball-milled steel ball with a suitable mass ratio are mixed and ball-milled, and the phosphorus raw material can be efficiently ball-milled into black phosphorus fine powder with a suitable particle size.

[0063] Further, the phosphorus raw material is ball-milled at 500-2000 rpm for 24-48 h to form black phosphorus fine powder. As an exemplary, the phosphorus raw material can be but not limited to ball-milled at 200 rpm, 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, 1500 rpm, 1600 rpm, 1700 rpm, 1800 rpm, 1900 rpm, 2000 rpm. It can be but not limited to ball-milled for 1 h, 6 h, 12 h, 16 h, 18 h, 20 h, 22 h, 24 h, 28 h, 30 h, 36 h, 40 h, 42 h, 46 h, 48 h. The phosphorus raw material can be efficiently ball-milled into black phosphorus fine powder with a suitable particle size at a suitable speed for a suitable time, so as to be efficiently ultrasonically exfoliated into two-dimensional black phosphorus nanosheets with higher piezoelectricity. As an exemplary, the particle size of the black phosphorus fine powder is 300-500 nm.

[0064] Further, the concentration of the black phosphorus solution is 0.5-2 mg / mL. As an exemplary, the concentration of the black phosphorus solution can be but not limited to 0.5 mg / mL, 1 mg / mL, 1.5 mg / mL, 2 mg / mL, 2.5 mg / mL, 3 mg / mL, 3.5 mg / mL, 4 mg / mL, 4.5 mg / mL, 5 mg / mL. The black phosphorus fine powder is dissolved in a solvent to form a black phosphorus solution with a suitable concentration, so as to be efficiently ultrasonically exfoliated for obtaining two-dimensional black phosphorus nanosheets with higher piezoelectricity.

[0065] Further, the solution after ultrasonic exfoliation is centrifuged at 4000-8000 rpm for 5-8 min. As an example, the solution after ultrasonic exfoliation can be but not limited to centrifuged at 2000 rpm, 3000 rpm, 4000 rpm, 5000 rpm, 6000 rpm, 7000 rpm, 8000 rpm. By centrifuging the solution after ultrasonic exfoliation at an appropriate speed for an appropriate time, it is beneficial to improve the yield of two-dimensional black phosphorus nanosheets.

[0066] Further, the solution after ultrasonic exfoliation is centrifuged at 4000-8000 rpm for 5-8 min. As an example, the solution after ultrasonic exfoliation can be but not limited to centrifuged at 2000 rpm, 3000 rpm, 4000 rpm, 5000 rpm, 6000 rpm, 7000 rpm, 8000 rpm. By centrifuging the solution after ultrasonic exfoliation at an appropriate speed for an appropriate time, it is beneficial to improve the yield of two-dimensional black phosphorus nanosheets.

[0067] As an example, the particle size of the two-dimensional black phosphorus nanosheet is 80-200 nm, and the thickness is 5 nm.

[0068] As an example, the mass ratio of the two-dimensional black phosphorus nanosheet, the polymer piezoelectric material, the lithium ion solution, and the polar aprotic solvent can be but not limited to 1:1:1:7, 1:1:1:8, 1:1:1:9, 1:2:1:7, 1:2:1:8, 1:2:1:9. The two-dimensional black phosphorus nanosheet, the polymer piezoelectric material, the lithium ion solution, and the polar aprotic solvent are mixed to form a composite film layer precursor solution by an appropriate mass ratio, which is beneficial to subsequently form a composite film layer with high piezoelectric performance and a certain thickness and toughness.

[0069] As an example, the phosphorus raw material can be but not limited to any one of red phosphorus, black phosphorus bulk, and black phosphorus crystal. Ball milling can obtain black phosphorus fine powder for subsequent preparation of two-dimensional black phosphorus nanosheets. As an example, the particle size of the red phosphorus powder is 1-50 μm.

[0070] As an example, the alkaline salt can be but not limited to at least one of sodium hydroxide (NaOH), potassium hydroxide (KOH), and lithium hydroxide (LiOH).

[0071] As an example, the polar aprotic solvent can include, but is not limited to, at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), 1,3-dimethyl-2-imidazolidinone (DMI), 1,2-dimethoxyethane (DME).

[0072] As an example, the lithium ion solution can include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium bis-trifluoromethylsulfonylimide (LiTFSI).

[0073] As an example, the lithium ion solution can further be an electrolyte including at least one of the above lithium ion solutions, and a solvent in the electrolyte can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC).

[0074] Further, in step (3), spin coating is performed at 500-1000 rpm for 30-60 s. As an example, the spin coating speed can include, but is not limited to, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, 1500 rpm. By spin coating at a suitable speed for a suitable time, a composite film layer with a suitable thickness is formed on the side of the lithium metal layer away from the current collector.

[0075] Further, in step (3), drying is performed at 25-60℃ for 6-12 h. As an example, the drying temperature can include, but is not limited to, 25℃, 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, 100℃. The drying time can include, but is not limited to, 6 h, 8 h, 10 h, 12 h, 16 h, 18 h, 20 h, 22 h, 24 h. By drying the composite film layer at a suitable temperature for a suitable time, a stable and tough composite film layer is formed.

[0076] The embodiment of the present application provides a lithium secondary battery including the above lithium metal battery negative electrode. The lithium metal battery negative electrode has a composite film layer on the side of the lithium metal layer away from the current collector, which can inhibit the growth of lithium dendrites and the decomposition of electrolyte, and the lithium secondary battery formed by the preparation has good performance.

[0077] The features and performance of the present application are further described in detail below in conjunction with the embodiments.

[0078] Embodiment 1

[0079] The embodiment provides a lithium metal battery negative electrode, and a preparation method thereof includes:

[0080] (1) Take 2 g of red phosphorus powder into a high-energy ball milling jar, add 60 g of steel balls, vacuumize the ball milling jar and fill it with argon, ball mill at 500 rpm for 24 h to obtain black phosphorus fine powder; dissolve the black phosphorus fine powder in a saturated NMP solvent containing NaOH, keep the concentration of the black phosphorus fine powder solution at 2 mg / mL; use an ultrasonic cell disruptor to perform ultrasonic exfoliation on the black phosphorus in the above solution in an ice water bath, the ultrasonic frequency is 20 kHz, the power is 1500 W, and the ultrasonic exfoliation time is 5 h to obtain a brown-black suspension; centrifuge the brown-black suspension at 8000 rpm for 5 min, take the supernatant and store it at -20℃ to obtain two-dimensional black phosphorus nanosheets dissolved in a saturated NMP solvent containing NaOH;

[0081] (2) Centrifuge the two-dimensional black phosphorus nanosheet solution dissolved in a saturated NMP solvent containing NaOH at 10000 rpm for 10 min, remove the supernatant, and then repeatedly wash the centrifuged two-dimensional black phosphorus nanosheet precipitate with DME solvent for 3 times; then transfer it into an argon atmosphere glove box with low humidity and low oxygen (both water and oxygen values are less than 0.1 ppm), mix according to the mass ratio of two-dimensional black phosphorus nanosheets: PVDF (Mw = 400000): 1M LiClO4 EC / PC (volume ratio 1:1) electrolyte: DME = 1:1:1:7, and stir mix for 48 h to obtain a composite film layer precursor solution;

[0082] (3) Fix a 700 μm lithium metal foil on the surface of a current collector, use a spin coater to spin coat the composite film layer precursor solution on the side of the lithium metal foil away from the current collector at 500 rpm for 30 s, and then place it in an oven and dry at 25℃ for 6 h to obtain a lithium metal anode with a 10 μm composite film layer on the side of the lithium metal foil away from the current collector.

[0083] Example 2

[0084] This example provides a lithium metal battery anode, which is different from Example 1 only in that in step (2), the mixing is carried out according to the mass ratio of two-dimensional black phosphorus nanosheets: PVDF (Mw = 400000): 1M LiClO4 EC / PC (volume ratio 1:1) electrolyte: DME = 1:2:1:7.

[0085] Example 3

[0086] This example provides a lithium metal battery anode, which is different from Example 1 only in that the polymer piezoelectric material PVDF is replaced by PVDF-TrFE.

[0087] Example 4

[0088] This example provides a lithium metal battery anode, which differs from Example 1 only in that the spin-coating speed in step (3) is increased to 1000 rpm, resulting in a lithium metal anode with a 5 pm composite film layer on the side of the lithium metal layer away from the current collector.

[0089] Example 5

[0090] This example provides a lithium metal battery anode, which differs from Example 1 only in that the ball-milling speed in step (1) is increased to 2000 rpm.

[0091] Example 6

[0092] This example provides a lithium metal battery anode, which differs from Example 1 only in that the ball-milling time in step (1) is extended to 48 h.

[0093] Example 7

[0094] This example provides a lithium metal battery anode, which differs from Example 1 only in that the concentration of the black phosphorus fine powder solution during ultrasonic exfoliation in step (1) is reduced to 0.5 mg / mL.

[0095] Example 8

[0096] This example provides a lithium metal battery anode, which differs from Example 1 only in that the centrifugal speed of the brown-black suspension obtained after ultrasonic exfoliation in step (1) is reduced to 4000 rpm.

[0097] Example 9

[0098] This example provides a lithium metal battery anode, which differs from Example 1 only in that the mixing ratio in step (2) is 2D black phosphorus nanosheet: PVDF (Mw = 400000): 1M LiClO4 EC / PC (volume ratio 1:1) electrolyte: DME = 1:1:1:9.

[0099] Example 10

[0100] This example provides a lithium metal battery anode, which differs from Example 1 in that black phosphorus crystal powder is used instead of red phosphorus powder as the raw material for ball-milling in step (1).

[0101] Example 11

[0102] This example provides a lithium metal battery anode, which differs from Example 1 only in that the stirring and mixing time of the composite film layer precursor solution in step (2) is shortened to 24 h.

[0103] Example 12

[0104] This example provides a lithium metal battery negative electrode, which is only different from example 1 in that the drying temperature in step (3) is increased to 60°C.

[0105] Example 13

[0106] This example provides a lithium metal battery negative electrode, which is different from example 1 in that: directly use commercial black phosphorus nanosheets, the preparation method comprises:

[0107] (1) After washing the black phosphorus nanosheets 3 times with DME solvent, transfer them into a low-humidity and low-oxygen (both water and oxygen values are less than 0.1 ppm) argon atmosphere glove box, mix according to the mass ratio of black phosphorus nanosheets: PVDF (Mw=400000): 1M LiClO4 EC / PC (volume ratio 1:1) electrolyte: DME = 1:1:1:7, put in a stirrer to stir and mix for 48h to obtain a composite film layer precursor solution;

[0108] (2) Fix a 700μm lithium metal foil on the surface of the current collector, use a spin coater to spin coat the composite film layer precursor solution on the side of the lithium metal foil away from the current collector at 500rpm for 30s, put it into an oven and dry at 25°C for 6h to obtain a lithium metal negative electrode with a 10μm composite film layer on the side of the lithium metal foil away from the current collector.

[0109] Comparative Example 1

[0110] This comparative example provides a lithium metal battery negative electrode, which is different from example 1 in that: use non-piezoelectric red phosphorus nanoparticles (particle size 50-300nm) instead of two-dimensional black phosphorus nanosheets, the preparation method comprises:

[0111] (1) After washing the red phosphorus nanoparticles 3 times with DME, transfer them into a low-humidity and low-oxygen (both water and oxygen values are less than 0.1 ppm) argon atmosphere glove box, mix according to the mass ratio of red phosphorus nanoparticles: PVDF (Mw=400000): 1M LiClO4 EC / PC (volume ratio 1:1) electrolyte: DME = 1:1:1:7, put in a stirrer to stir and mix for 48h to obtain a composite film layer precursor solution;

[0112] (2) Fix a 700μm lithium metal foil on the surface of the current collector, use a spin coater to spin coat the composite film layer precursor solution on the side of the lithium metal foil away from the current collector at 500rpm for 30s, put it into an oven and dry at 25°C for 6h to obtain a lithium metal negative electrode with a 10μm composite film layer on the side of the lithium metal foil away from the current collector.

[0113] Comparative Example 2

[0114] This comparative example provides a lithium metal battery negative electrode, which is different from example 1 in that: use non-piezoelectric polyethylene oxide (PEO) instead of PVDF.

[0115] Comparative Example 3

[0116] This comparative example provides a lithium metal battery negative electrode, which is different from Example 1 in that barium titanate (BaTiO3) nanoparticles with piezoelectricity (particle size 50-200 nm) are used instead of two-dimensional black phosphorus nanosheets, and the preparation method comprises:

[0117] (1) After the BaTiO3nanoparticles are washed with DME for 3 times, they are transferred into an argon atmosphere glove box with low humidity and low oxygen (both water and oxygen values are less than 0.1 ppm), and mixed according to the mass ratio of BaTiO3nanoparticles: PVDF (Mw=400000): 1M LiClO4EC / PC (volume ratio 1:1) electrolyte: DME = 1:1:1:7, and put into a stirring rod for stirring and mixing for 48h to obtain a composite film layer precursor solution;

[0118] (2) A 700μm lithium metal foil is fixed on the surface of the current collector, and the composite film layer precursor solution is spin-coated on the side of the lithium metal foil away from the current collector at 500rpm for 30s using a spin coater, and is placed in an oven and dried at 25℃ for 6h to obtain a lithium metal negative electrode with a 10μm composite film layer on the side of the lithium metal foil away from the current collector.

[0119] Comparative Example 4

[0120] This comparative example provides a lithium metal battery negative electrode, which is different from Example 9 in that the mass ratio of two-dimensional black phosphorus nanosheets and PVDF is 1:0.5.

[0121] Comparative Example 5

[0122] This comparative example provides a lithium metal battery negative electrode, which is different from Example 9 in that the mass ratio of two-dimensional black phosphorus nanosheets and PVDF is 1:3.

[0123] Table 1 provides the preparation method of the lithium metal battery negative electrode provided in Examples 1-13 and Comparative Examples 1-5 Note: " / " in the table means that the step is not included.

[0124] Implementation Group 1

[0125] The lithium metal battery negative electrode provided in Examples 1-13 and Comparative Examples 1-5 is assembled into a CR2032 type button cell according to the following method:

[0126] The glove box is protected by argon gas, and the partial pressure of water and oxygen is less than 0.1 ppm. The positive electrode uses lithium iron phosphate (LiFePO4, LFP) as the active material, Super P carbon black as the conductive agent, and PVDF as the binder. The LFP: Super P carbon black: PVDF = 8:1:1 by mass is dissolved in NMP to obtain a positive electrode slurry. The above positive electrode slurry is coated on an aluminum foil using a doctor blade, and then dried at 100°C. After drying, the positive electrode is transferred to a vacuum oven and dried at 120°C for 8 hours. The negative electrode uses the lithium metal battery negative electrode provided in each example and comparative example (the composite film layer faces inward). The separator is a polyethylene (PE) separator. The electrolyte is 1.3M LiPF6 in EC / DEC (volume ratio 3:7).

[0127] Test Example 1

[0128] Each button cell prepared in Group 1 is subjected to charge-discharge cycling at a cutoff voltage of 2.5-3.8V at a rate of 0.1C for 3 cycles and then at a rate of 0.5C for 100 cycles. The capacity retention rate and coulombic efficiency of the battery are recorded, and the results are shown in Table 2.

[0129] Table 2: Capacity retention rate and coulombic efficiency of button cells after 100 cycles

[0130] Group 2

[0131] The lithium metal battery negative electrode provided in each of Examples 1-13 and Comparative Examples 1-5 is assembled into a symmetric cell according to the following method:

[0132] The glove box is protected by argon gas, and the partial pressure of water and oxygen is less than 0.1 ppm. The lithium metal battery negative electrode obtained in each example and comparative example is used as two identical symmetric electrodes (the composite film layer faces inward). The separator is a polyethylene (PE) separator. The electrolyte is 1.3M LiPF6 in EC / DEC (volume ratio 1:1).

[0133] Test Example 2

[0134] Each symmetric cell prepared in Group 2 is subjected to constant-current discharge and constant-current charge. The charge-discharge cycling is performed at a current density of 0.5 mA / cm2, and each cycle period is 4 hours. During normal cycling, the voltage increases with the number of cycles due to the lithium deposition side reaction on the surface of the metal lithium layer. When lithium dendrites penetrate the solid-state electrolyte membrane and cause short circuit, the battery becomes a pure resistor, and the voltage drops sharply and remains unchanged. The number of cycles at which short circuit occurs is recorded, and the data are shown in Table 3.

[0135] Table 3: Number of cycles at which short circuit occurs in symmetric cells

[0136] From the results of Table 2 and Table 3, it can be seen that, in Comparative Examples 1-13 and Comparative Examples 1-5, even if the electrolyte system is not stable enough for the lithium metal negative electrode, the lithium metal negative electrode provided by the present application, which has a composite film layer on the side of the lithium metal layer away from the current collector, can significantly improve the cycle performance of the battery, because the present application uses two-dimensional black phosphorus nanosheets and a polymer piezoelectric material in a suitable weight ratio to form a composite film layer with good piezoelectricity and a certain thickness and toughness; when the composite film layer is subjected to stress caused by lithium dendrite growth, the piezoelectric effect is triggered to form a local piezoelectric reverse overpotential in the composite film layer, thereby weakening the diffusion and deposition driving force of lithium ions to the nucleation and deposition site of the lithium dendrite protruding tip, stopping the formation of lithium dendrites in the local area, and the certain thickness and toughness of the composite film layer can also correspondingly inhibit the growth of lithium dendrites to penetrate the film layer, thereby making the surface of the lithium metal layer flat; at the same time, black phosphorus has electrochemical activity, and the electrically insulating lithium phosphide generated by the reaction of black phosphorus with the lithium dendrite tip can inhibit the continuous deposition of lithium ions to the lithium dendrite tip and avoid the further growth of lithium dendrites, and black phosphorus has a certain reducing property, which can selectively reduce high-oxidizing free radicals in the electrolyte to protect the electrolyte from decomposition caused by free radicals, thereby slowing down the performance degradation of the battery.

[0137] Comparative Example 1 and Example 13, the lithium metal negative electrode provided by the present application has higher cycle performance, because the present application ball-mills phosphorus raw material into black phosphorus powder, and then performs liquid-phase ultrasonic exfoliation on the black phosphorus powder in a water bath to efficiently obtain two-dimensional black phosphorus nanosheets with good piezoelectric properties, thereby obtaining a composite film layer with good piezoelectric properties, which can more effectively slow down the performance degradation of the battery.

[0138] Comparative Example 9 and Comparative Examples 4-5, the batteries assembled using the lithium metal negative electrode provided by Comparative Examples 4-5 have slightly poorer cycle performance, because increasing the mass of the polymer piezoelectric material in the composite film layer relatively reduces the proportion of two-dimensional black phosphorus nanosheets in the composite film layer, which makes the piezoelectric properties of the composite film layer decrease to a certain extent, thereby reducing the cycle performance of the battery; reducing the mass of the polymer piezoelectric material in the composite film layer forms a thinner composite film layer that cannot achieve the appropriate thickness uniformity and toughness, which cannot effectively inhibit the growth of lithium dendrites to penetrate the film layer, thereby reducing the cycle performance of the battery.

[0139] The above-described examples are part of the examples of the present application, rather than all the examples. The detailed description of the examples of the present application is not intended to limit the scope of the claimed application, but only represents selected examples of the present application. Based on the examples in the present application, all other examples obtained by a person of ordinary skill in the art without making creative efforts fall within the scope of protection of the present application.

Claims

1. A lithium metal battery anode, characterized in that, The lithium metal battery negative electrode comprises: a current collector and a lithium metal layer arranged on the surface of the current collector, wherein the lithium metal layer is provided with a composite film layer away from one side of the current collector; the composite film layer comprises two-dimensional black phosphorus nanosheets and a polymer piezoelectric material, and the mass ratio of the two-dimensional black phosphorus nanosheets and the polymer piezoelectric material is 1:1-2.

2. The lithium metal battery anode of claim 1, wherein, The polymer piezoelectric material comprises at least one of polyvinylidene fluoride, polyvinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-hexafluoropropylene copolymer, polylactic acid, polyacrylonitrile, and poly-beta-hydroxybutyrate.

3. The lithium metal battery anode of claim 1, wherein, The thickness of the composite film layer is 3-10 μm.

4. The lithium metal battery anode of claim 3, wherein, The thickness of the lithium metal layer is 500-700 μm.

5. A method of producing a lithium metal battery anode as claimed in any one of claims 1-4, characterized in that, The lithium metal battery negative electrode comprises: (1) ball-milling a phosphorus raw material into black phosphorus fine powder under an inert gas atmosphere, dissolving the black phosphorus fine powder in a first solvent, centrifuging after ultrasonic exfoliation in an ice water bath to obtain two-dimensional black phosphorus nanosheets dissolved in the first solvent; (2) dissolving the two-dimensional black phosphorus nanosheets in a second solvent after centrifugal washing, then adding the polymer piezoelectric material according to a mass ratio, and adding a lithium ion solution to obtain a composite film layer precursor solution; (3) fixing the lithium metal layer on the surface of the current collector, coating the composite film layer precursor solution to the side of the lithium metal layer away from the current collector, and drying to obtain a lithium metal battery negative electrode.

6. The production method according to claim 5, wherein The mass ratio of the two-dimensional black phosphorus nanosheets, the polymer piezoelectric material, the lithium ion solution, and the second solvent is 1:1-2:1:7-9.

7. The preparation method according to claim 5, characterized in that, The phosphorus raw material comprises any one of red phosphorus, black phosphorus bulk, and black phosphorus crystal.

8. The preparation method according to claim 5, characterized in that, The first solvent is a polar aprotic solvent containing an alkaline salt; and / or, the second solvent is a polar aprotic solvent; Optionally, the alkaline salt comprises at least one of sodium hydroxide, potassium hydroxide, and lithium hydroxide; Optionally, the polar aprotic solvent comprises at least one of N-methylpyrrolidone, N,N-dimethylformamide, 1,3-dimethyl-2-imidazolidinone, and 1,2-dimethoxyethane.

9. The preparation method according to claim 5, characterized in that, The ball-milling is performed at 200-2000 rpm for 1-48 h; Optionally, the mass ratio of the phosphorus raw material to the steel balls for ball-milling is 1:10-50.

10. The method of claim 5, wherein, The concentration of the black phosphorus fine powder dissolved in the first solvent is 0.5-5 mg / mL; and / or, the frequency of the ultrasonic exfoliation is 20-30 kHz, the power is 600-1500 W, and the ultrasonic exfoliation is performed for 1-10 h; and / or, the centrifugation in step (1) is performed at 2000-8000 rpm for 5-10 min.

11. The method of claim 5, wherein, The centrifugation in step (2) is performed at 10000-13333 rpm for 5-10 min; and / or, the mixing in step (2) is performed under an argon atmosphere with a water and oxygen content of less than 0.1 ppm for 6-48 h.

12. The method of claim 5, wherein, The spin coating in step (3) is performed at 500-1500 rpm for 30-60 s; and / or, the drying is performed at 25-100 °C for 6-24 h.

13. A lithium secondary battery, characterized by comprising: The lithium metal battery negative electrode comprises the lithium metal battery negative electrode according to any one of claims 1-4.

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