Nano-silver / graphene composite modified lithium metal negative electrode as well as preparation method and application thereof

Abstract

The invention relates to a nano-silver / graphene composite modified lithium metal negative electrode and a preparation method and application thereof.The method comprises the steps that nano-silver particles and graphene oxide powder are added into an organic solvent according to the mass ratio of 1: 1-5: 1 to be subjected to ultrasonic dispersion, a uniform and stable mixed spraying solution is formed, and the nano-silver / graphene composite modified lithium metal negative electrode is prepared through the spraying, rolling and low-temperature reduction combined technology. And constructing a compact composite protective layer on the surface of the lithium metal. In the composite structure, the nano-silver particles provide a large number of uniformly distributed lithium-philic nucleation sites, so that uniform deposition of lithium ions is effectively induced, a graphene sheet layer forms a three-dimensional skeleton structure, the mechanical stability of an interface is enhanced, and dendritic crystal growth is inhibited. The synergistic effect of the two can significantly reduce side reactions, improve interface stability, and improve cycle life and rate capability. The preparation method is simple in process and mild in condition, and the prepared negative electrode has high safety, excellent electrochemical performance and good industrialization prospect and is suitable for practical application of a high-energy-density lithium metal battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrochemical energy storage, in particular to a nano-silver / graphene composite modified lithium metal negative electrode, a preparation method and application thereof. BACKGROUND

[0002] The power transportation, portable electronics and large-scale energy storage scenarios continue to pursue higher energy density, and the traditional graphite negative electrode lithium ion system has approached the theoretical limit. Even combined with high-nickel ternary positive electrode and Si-C composite negative electrode, the specific energy of a single body still fluctuates around 300Wh·kg -1 , and it is difficult to reach the medium and long-term goal of 500Wh·kg -1 . Metal lithium is generally considered as a key negative electrode candidate to break through the bottleneck due to its theoretical capacity of 3860mAh·g -1 , the lowest negative electrode potential of-3.04V(vs SHE) and the low density of 0.534g·cm -3 .

[0003] However, the surface of metal lithium has very high chemical activity, and in the deposition / detachment process of common carbonate or ether electrolyte, the solvent is easily consumed and the SEI with mechanical weakness and chemical instability is formed. The rupture of SEI exposes fresh lithium repeatedly, and the side reaction is amplified. The concentration gradient in the electrolyte and the electric field tip effect are superimposed, which drives the lithium to segregate and grow dendrites in the high-curvature area, further inducing failure modes such as "dead lithium", separator puncture and capacity drop.

[0004] In the existing modification ideas, a single metal or alloy layer can reduce the nucleation overpotential due to its lithium affinity, but it is mostly applied in the form of bulk material or rolled sheet, and the macroscopic grain and continuous size are difficult to coordinate >300% volume change under high surface capacity and high rate. Local stress concentration still leads to interface tearing and polarization increase; a single carbon material (such as a graphene thin layer) has high conductivity and certain shielding effect, but its chemical affinity with lithium is limited, and it is prone to interlayer peeling or crack propagation during the cycle process, making it difficult to maintain a smooth and uniform deposition interface for a long time. Only relying on two-dimensional micron-level pressing or dense coating, the transmission path of electric charge and ions in the thick electrode is still long, and the polarization is significantly increased with the increase of surface capacity, and the improvement of service life is limited.

[0005] Therefore, a new lithium metal negative electrode is needed to achieve uniform nucleation on a microscale and provide sufficient toughness and adaptability on a macroscale to meet the dual requirements of stability and service life under high surface capacity and high-rate charging and discharging. SUMMARY

[0006] The present application provides a nano-silver / graphene composite modified lithium metal negative electrode, a preparation method and application thereof, which aims to solve the technical problems of unstable lithium metal negative electrode interface, dendrite growth and limited cycle life in the prior art.

[0007] To achieve the above object, the technical scheme of the present application is as follows:

[0008] In a first aspect, the present application provides a preparation method of a nano-silver / graphene composite modified lithium metal negative electrode, comprising the following steps:

[0009] a) Preparation and purification of nano-silver particles: mix silver nitrate aqueous solution with polyvinylpyrrolidone (PVP) dispersant, slowly add sodium borohydride aqueous solution under the condition of heating and stirring to carry out reduction reaction, centrifuge the obtained nano-silver sol after the reaction is completed, and wash the precipitate with deionized water and anhydrous ethanol alternately, then vacuum dry and grind to obtain pure nano-silver particles;

[0010] b) Preparation of composite spraying liquid: add the nano-silver particles obtained in step a) and graphene oxide powder in a mass ratio of 1:1-5:1 into an organic solvent, and form a uniform and stable nano-silver / graphene oxide mixed spraying liquid after ultrasonic treatment;

[0011] c) Spraying and forming of lithium negative electrode: under the protection of inert atmosphere, uniformly spray the nano-silver / graphene oxide mixed spraying liquid obtained in step b) on the surface of a lithium metal sheet, roll after drying, and form a nano-silver / graphene oxide composite modified layer on the surface of the lithium metal sheet.

[0012] Further, in step a):

[0013] The concentration of the silver nitrate aqueous solution is 0.01-0.5 mol / L;

[0014] The molar ratio of polyvinylpyrrolidone (PVP) to silver nitrate is 1:1-1:100;

[0015] The molar ratio of sodium borohydride to silver nitrate is 1:1-5:1;

[0016] The reaction temperature of the reduction reaction is 30-60℃;

[0017] The centrifugal speed of the centrifugal separation is 3000-10000 rpm;

[0018] The temperature of the vacuum drying is 50-80℃.

[0019] Further, in step b):

[0020] The organic solvent is selected from one or more of n-hexane, cyclohexane or toluene;

[0021] In the nano-silver / graphene oxide mixed spraying liquid, the total concentration of nano-silver and graphene oxide is 1-10 mg / mL;

[0022] The ultrasonic treatment power is 200-600 W, and the time is 30-120 min.

[0023] Further, step c) further comprises a reduction and standing step:

[0024] The inert atmosphere is argon or nitrogen, wherein the content of water and oxygen is less than 0.1 ppm;

[0025] During the spraying process, the distance between the spray gun nozzle and the surface of the lithium metal sheet is 5-20 cm.

[0026] The spraying rate is 0.5-2 mL / min.

[0027] Further, step c) further comprises a reduction and standing step:

[0028] After rolling and standing drying, the lithium metal negative electrode is placed at 10-50℃ in a glove box for 1-12 hours, so that the graphene oxide part of the composite modified layer is reduced to reduced graphene oxide, and the adhesion between the composite modified layer and the lithium metal sheet is enhanced.

[0029] In a second aspect, the application provides a nano-silver / graphene composite modified lithium metal negative electrode obtained by the above preparation method.

[0030] Further, the nano-silver / graphene oxide composite modified layer is densely and firmly covered on the surface of the lithium metal sheet, the thickness of the composite modified layer is 30 nm-5 μm, the reduced graphene oxide sheet layer in the composite modified layer forms a three-dimensional skeleton structure, and the nano-silver particles are uniformly embedded in the surface and interstices of the three-dimensional skeleton structure.

[0031] In a third aspect, the application provides an application of a nano-silver / graphene composite modified lithium metal negative electrode in a copper foil half-cell, wherein the copper foil half-cell comprises:

[0032] a) using the lithium metal sheet after modification by spraying as a working electrode;

[0033] b) using a copper foil as a counter electrode and a current collector;

[0034] c) using Celgard 2400 polypropylene porous membrane as a separator, the electrolyte is a solution of 1 mol / L lithium bis(trifluoromethylsulfonyl)imide in a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane with a volume ratio of 1:1, and containing 2% lithium nitrate;

[0035] d) assembling a CR2032 type button cell in an argon glove box with the content of water and oxygen being less than 0.1 ppm, and packaging under a pressure of 350-500 psi.

[0036] In a fourth aspect, the present application provides a use of the nano-silver / graphene composite modified lithium metal negative electrode in a symmetrical battery, wherein the symmetrical battery comprises:

[0037] a) using the sprayed modified lithium metal sheet as the positive electrode and the negative electrode, respectively;

[0038] b) using Celgard 2400 polypropylene porous membrane as the separator, and the electrolyte is a solution of 1 mol / L lithium bis(trifluoromethylsulfonyl) imide in a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane with a volume ratio of 1:1, and containing 2% lithium nitrate;

[0039] c) assembling the CR2032 button cell in a xenon glove box with water and oxygen less than 0.1 ppm;

[0040] d) packaging under a pressure of 350-500 psi.

[0041] In a fifth aspect, the present application provides a use of the nano-silver / graphene composite modified lithium metal negative electrode in a lithium iron phosphate full battery, wherein the lithium iron phosphate full battery comprises:

[0042] a) using the sprayed modified lithium metal sheet as the negative electrode, and the positive electrode is a composite electrode with an active material surface density of 3-4 mg / cm 2 and a mass ratio of LiFePO4, polyvinylidene fluoride (PVDF) and conductive carbon black (Super P) of 8:1:1;

[0043] b) using Celgard 2400 polypropylene porous membrane as the separator, and the electrolyte is a solution of 1 mol / L lithium bis(trifluoromethylsulfonyl) imide in a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane with a volume ratio of 1:1, and containing 2% lithium nitrate;

[0044] c) assembling the CR2032 button cell in a xenon glove box with water and oxygen less than 0.1 ppm;

[0045] d) packaging under a pressure of 350-500 psi.

[0046] Compared with the prior art, the present application has the following beneficial effects:

[0047] The nano-silver / graphene composite modified lithium metal negative electrode prepared by the method has significant advantages in structure and performance; the nano-silver particles in the composite layer are uniformly dispersed and closely combined in the reduced graphene oxide sheet layer, can provide a large number of uniformly distributed lithiumophilic nucleation sites, induce lithium to uniformly deposit on the interface, and significantly inhibit the generation of dendrites; at the same time, the graphene sheet layer constructs a solid three-dimensional skeleton structure, provides mechanical support for the electrode and disperses stress, helps to maintain the interface integrity and conductive continuity; the composite modified layer formed thereby is overall dense and stable, can effectively reduce the side reaction of active lithium and electrolyte, significantly prolong the cycle life of the battery, improve the coulomb efficiency, and maintain a lower polarization voltage and a higher capacity retention rate under high rate conditions.

[0048] The preparation method provided by the application has mild process conditions and simple steps, and is suitable for industrial application. By controlling the mass ratio of nano-silver and graphene oxide, the ultrasonic dispersion conditions and the spraying parameters, the composition and thickness of the composite layer can be accurately controlled to ensure the uniformity and stability of the composite structure; the spraying combined with rolling and the subsequent low-temperature standing reduction process can convert part of the graphene oxide into reduced graphene oxide, thereby enhancing the bonding force with the lithium substrate and further improving the overall stability and durability of the composite layer; compared with the traditional single metal coating or carbon-based coating method, the method can not only significantly improve the cycle stability and safety of the lithium metal negative electrode, but also has the advantages of easy process control, strong adaptability and scalability.

[0049] Of course, implementing each technical solution of the application does not necessarily need to achieve all the advantages described above. BRIEF DESCRIPTION OF DRAWINGS

[0050] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only some embodiments of the application, and for those skilled in the art, other drawings of other embodiments can also be obtained without creative labor on the basis of these drawings.

[0051] Figure 1 SEM image of the nano-silver particles in Example 1 of the application;

[0052] Figure 2 Photo of the nano-silver sprayed on the lithium metal sheet in Example 1 of the application;

[0053] Figure 3 SEM images of the nano-silver / graphene mixture in Example 1 of the application under different magnifications;

[0054] Figure 4Battery cycle plot for Li@Ag nanoparticle||Li@Ag nanoparticle symmetric battery in 1 mA / 1 mAh, 5 mA / 5 mAh test condition in Example 1 of the present application;

[0055] Figure 5 Battery cycle plot for Li@Ag nanoparticle||Cu asymmetric battery in 1.5 mA / 4 mAh, 5 mA / 5 mAh test condition in Example 1 of the present application;

[0056] Figure 6 Battery cycle plot for Li@Ag nanoparticle||LEP full battery in 1C charge test condition in Example 1 of the present application;

[0057] Figure 7 Battery cycle plot for Li@Ag nanoparticle||Li symmetric battery in 1 mA / 1 mAh, 5 mA / 5 mAh test condition in Example 2 of the present application;

[0058] Figure 8 Battery cycle plot for Li@Ag nanoparticle||Cu asymmetric battery in 1.5 mA / 4 mAh, 5 mA / 5 mAh test condition in Example 2 of the present application;

[0059] Figure 9 Battery cycle plot for Li@Ag nanoparticle||Li symmetric battery in 1 mA / 1 mAh, 5 mA / 5 mAh test condition in Example 3 of the present application;

[0060] Figure 10 Battery cycle plot for Li@Ag nanoparticle||Cu asymmetric battery in 5 mA / 5 mAh, 10 mA / 10 mAh test condition in Example 3 of the present application. DETAILED DESCRIPTION

[0061] The application will be further described in details by specific embodiments and drawings. In different embodiments, similar elements are associated with similar element reference numbers. In the following embodiments, many details are described in order to make the application better understood. However, those skilled in the art can easily recognize that some features can be omitted in different cases, or can be replaced by other elements, materials, methods. In some cases, some operations related to the application are not shown or described in the specification, in order to avoid the core part of the application being overwhelmed by too much description, and it is not necessary to describe these related operations in detail for those skilled in the art according to the description in the specification and general technical knowledge in the art.

[0062] In addition, features described in the specification, operations or characteristics can be combined in any appropriate manner to form various embodiments. Meanwhile, the steps or actions in the method description can also be sequentially changed or adjusted in a manner obvious to those skilled in the art. Therefore, the various sequences in the specification and the drawings are only for the purpose of clearly describing a certain embodiment, and do not mean that the sequence is necessary, unless otherwise stated that a certain sequence must be followed.

[0063] Embodiment 1:

[0064] The embodiment provides a preparation method of a nano-silver / graphene composite modified lithium metal negative electrode, comprising the following steps:

[0065] Step a), preparation and purification of nano-silver particles

[0066] Into 40ml 0.1mo / L silver nitrate (AgNO3) solution, 70ml deionized water and 2g polyvinylpyrrolidone (PVP) are added, and a magnetic stirrer is added in a beaker for stirring for 1h under water bath heating at 40℃. Then, 0.2016g sodium borohydride (NaBH4) is dissolved in 50ml deionized water, and a rubber bulb dropper is used to slowly drop the solution into the above solution. During the reaction, the solution color rapidly changes to bright yellow to brown yellow, indicating that nano-silver particles are generated. After the dropping is completed, heating and stirring are continued for 5h to ensure that the reaction is complete. Then, a centrifugal tube is used for separation, and a high-speed centrifuge (3000-5000rpm) is used for centrifugal collection of the obtained nano-silver solution, and deionized water and anhydrous ethanol are used for cleaning at least 3 times to completely remove residual nitrate ions, borate ions and sodium ions. Subsequently, the centrifugal tube is placed in an oven for vacuum drying at 60℃, and finally, grinding is performed in a mortar to obtain fine powder, and the final nano-silver particles are obtained, as shown in Figure 1 It can be seen from the scanning electron microscope image that the prepared nano-silver particles are in a uniform dispersed state, and the particle morphology is regular.

[0067] Step b), preparation of nano-silver / graphene composite spraying solution

[0068] 80mg graphene oxide powder and 240mg finally cleaned nano-silver particles are added into a sample bottle, 10ml n-hexane is added, and the mixture is ultrasonically treated for 30-60min to ensure that the nano-silver particles are in full contact with the graphene oxide powder layers and are uniformly mixed, so that a stable nano-silver / graphene composite spraying solution is formed, as shown in Figure 3 The nano-silver particles are uniformly distributed on the surface and wrinkle area of the graphene oxide layer and are tightly combined with the graphene oxide layer. In the embodiment, the mass ratio of graphene oxide to nano-silver is 1:3.

[0069] Step c), spraying and forming of the lithium negative electrode

[0070] In a glove box filled with high-purity argon, a piece of substrate paper was fixed on the surface of the workbench, and a lithium metal sheet with a thickness of 150-300 pm was adhered to one side using double-sided tape to keep the lithium metal sheet stable in an inert atmosphere and facilitate subsequent spraying operations; the mixed spraying liquid was loaded into the liquid tank of the ultrasonic spray gun, the distance between the nozzle and the surface of the lithium metal sheet was set to 10 cm, the spraying rate was 1 mL / min, and the surface of the lithium metal sheet was sprayed back and forth until all the composite spraying liquid was sprayed, see Figure 2 , the surface of the lithium metal sheet after spraying treatment was uniformly covered with a composite modified layer, and the whole presented a dense and continuous film-like structure; the sprayed lithium metal sheet was rolled by a rolling machine and left to dry, and the composite layer was more closely attached to the lithium metal substrate by moderate mechanical pressure, eliminating the pores or microcracks that may be formed during the spraying process, and ensuring the density and uniformity of the modified layer. Then, in a glove box at 10-50 °C, the mixed spraying liquid was reduced and left to stand for 3 h, so that part of the graphene oxide in the mixed spraying liquid was reduced to reduced graphene oxide (rGO), further improving the conductivity and stability of the composite layer, and promoting its combination with the surface of the lithium substrate, thereby obtaining a nano-silver / graphene composite modified lithium metal negative electrode with a dense structure, a stable interface, and excellent electrical conductivity.

[0071] The performance test of the lithium metal negative electrode prepared in this example is as follows:

[0072] Assembled lithium symmetric button cell: a battery shell with a type of 2025 was selected, and a polyethylene separator was used. The negative electrode and the positive electrode both used the sprayed lithium metal sheet. The polyethylene separator was placed between the positive and negative electrodes and 75 pL of electrolyte 1M LiTFSI-DME:DOL 1:1+2wt% LINO3 was added on both sides. Finally, the lithium button cell was packaged. The assembled battery was tested for Li||Li symmetric battery cycle performance on a new Wei tester, and the test conditions were 1 mA / 1 mAh and 5 mA / 5 mAh. The results of Li@Ag nanoparticle rGo-3:1 are shown in Figure 4 , which can be stably cycled for 1000 h and 500 h at 1 mA / 1 mAh and 5 mA / 5 mAh, respectively, and the symmetric battery shows good cycle stability.

[0073] Assembled lithium asymmetric button cell: a battery shell with a type of 2025 was selected, and a polyethylene separator was used. The negative electrode used the sprayed lithium metal sheet, and the positive electrode was a copper foil. The polyethylene separator was placed between the positive and negative electrodes and 75 pL of electrolyte was added on both sides. Finally, the lithium button cell was packaged. The assembled battery was tested for Li||Cu asymmetric battery cycle performance on a new Wei tester, and the test conditions were 1.5 mA / 4 mAh and 5 mA / 5 mAh. The results of Li@Ag nanoparticle rGo-3:1 are shown in Figure 5As shown, the capacity retention rate is 95% and the coulombic efficiency is above 99% after 250 cycles at 1C rate.

[0074] Assemble the button lithium full battery: select the battery shell with the type of 2025 and the polyethylene diaphragm, the negative electrode uses the sprayed lithium metal sheet, the positive electrode is the LEP pole piece, the polyethylene diaphragm is placed in the positive and negative electrodes and 75 μL of electrolyte is added on both sides, and finally the button lithium battery is packaged. The assembled battery is tested on the new Wei tester for Li||LEP cycle performance test, and the test condition is 1C discharge. The results of Li@Ag nanoparticle rGo-3:1 are as follows Figure 6 As shown, the coulombic efficiency is above 99% and the capacity retention rate is 95% after 250 cycles under the test condition; the performance of the LEP full battery is good.

[0075] Example 2:

[0076] The embodiment provides a preparation method of a nano-silver / graphene composite modified lithium metal negative electrode, comprising the following steps:

[0077] Step a), preparation and purification of nano-silver particles

[0078] Into 40 ml of 0.1 mo / L silver nitrate (AgNO3) solution, 70 ml of deionized water and 2 g of polyvinylpyrrolidone (PVP) are added, and a magnetic stirrer is added in a beaker for stirring for 1 h under water bath heating at 40℃. Then, 0.2016 g of sodium borohydride (NaBH4) is dissolved in 50 ml of deionized water, and a rubber bulb dropper is used to slowly drop the solution into the above solution. During the reaction process, the solution color rapidly changes from bright yellow to brown yellow, indicating the formation of nano-silver particles. After the dropping is completed, the heating and stirring are continued for 5 h to ensure the completion of the reaction. Then, a centrifugal tube is used for separation, and a high-speed centrifuge (3000-5000 rpm) is used for centrifugal collection of the obtained nano-silver solution, and deionized water and anhydrous ethanol are used for cleaning at least 3 times to completely remove residual nitrate ions, borate ions and sodium ions. Subsequently, the centrifugal tube is placed in an oven for vacuum drying at 60℃, and finally, the obtained nano-silver particles are ground into fine powder in a mortar.

[0079] Step b), preparation of nano-silver / graphene composite spraying solution

[0080] 80 mg of graphene oxide powder and 160 mg of finally cleaned nano-silver particles are added into a sample bottle, 10 ml of n-hexane is added, and the mixed solution is ultrasonically treated for 30-60 min to ensure that the nano-silver particles and the graphene oxide powder layers are fully contacted and uniformly mixed to form a stable nano-silver / graphene composite spraying solution. In this embodiment, the mass ratio of graphene oxide to nano-silver is 1:2.

[0081] Step c), spraying and shaping of lithium negative electrode

[0082] In a glove box filled with high-purity argon, a piece of substrate paper was fixed on the surface of the workbench, and a lithium metal sheet with a thickness of 150-300 pm was adhered to one side using double-sided tape, so that the lithium metal sheet remained stable in an inert atmosphere and facilitated subsequent spraying operations; the mixed spraying liquid was loaded into the liquid tank of the ultrasonic spray gun, the distance between the nozzle and the surface of the lithium metal sheet was set to 10 cm, the spraying rate was 1 mL / min, and the lithium metal sheet surface was reciprocally sprayed until all the composite spraying liquid was sprayed; the sprayed lithium metal sheet was rolled by a roller press and left to dry, the composite layer was more closely attached to the lithium metal substrate by moderate mechanical pressure, the pores or microcracks that may be formed during the spraying process were eliminated, and the density and uniformity of the modified layer were ensured. Then, the lithium metal sheet was reduced and left to stand for 3 h in a glove box at 10-50 °C, so that part of the graphene oxide in the mixed spraying liquid was reduced to reduced graphene oxide (rGO), further improving the conductivity and stability of the composite layer, and promoting the combination of the composite layer with the surface of the lithium substrate, thereby obtaining a nano-silver / graphene composite modified lithium metal negative electrode with a dense structure, a stable interface, and excellent electrical conductivity.

[0083] The performance of the lithium metal negative electrode prepared in this example was tested as follows:

[0084] Assembled lithium symmetric button cell: a battery shell with a type of 2025 and a polyethylene separator were selected, the negative electrode and the positive electrode both used the sprayed lithium metal sheet, the polyethylene separator was placed between the positive and negative electrodes and 75 pL of electrolyte 1 M LiTFSI-DME:DOL 1:1 + 2 wt% LINO3 was added on both sides, and finally the lithium button cell was packaged. The assembled battery was tested for Li||Li symmetric battery cycle performance on a new Wei tester, and the test conditions were 1 mA / 1 mAh, 5 mA / 5 mAh, and 10 mA / 10 mAh. The results of Li@Ag nanoparticle rGo-2:1 are shown in Figure 7 , which can stably cycle for 670 h and 680 h at 1 mA / 1 mAh and 5 mA / 5 mAh, respectively, showing good cycle stability of the symmetric battery.

[0085] Assembled lithium asymmetric button cell: a battery shell with a type of 2025 and a polyethylene separator were selected, the negative electrode used the sprayed lithium metal sheet, and the positive electrode was a copper foil, the polyethylene separator was placed between the positive and negative electrodes and 75 pL of electrolyte was added on both sides, and finally the lithium button cell was packaged. The assembled battery was tested for Li||Cu asymmetric battery cycle performance on a new Wei tester, and the test conditions were 1.5 mA / 4 mAh and 5 mA / 5 mAh. The results of Li@Ag nanoparticle rGo-2:1 are shown in Figure 8As shown, the stable cycle is 510h and 525h at 1.5mA / 4mAh, 5mA / 5mAh.

[0086] Example 3:

[0087] Step a), preparation and purification of nano-silver particles

[0088] Into 40ml 0.1mo / L silver nitrate (AgNO3) solution, 70ml deionized water and 2g polyvinylpyrrolidone (PVP) were added, and then the mixture was stirred for 1h in a beaker with a magnetic stirrer under water bath heating at 40℃. Subsequently, 0.2016g sodium borohydride (NaBH4) was dissolved in 50ml deionized water, and then the solution was slowly added dropwise into the above solution by using a rubber bulb dropper. During the reaction, the solution color changed rapidly from bright yellow to brown yellow, indicating the formation of nano-silver particles. After the dropwise addition was completed, the heating and stirring were continued for 5h to ensure the completion of the reaction. Then, the resulting nano-silver solution was collected by centrifugation using a centrifuge tube and a high-speed centrifuge (3000-5000rpm), and was washed at least 3 times with deionized water and anhydrous ethanol to completely remove residual nitrate ions, borate ions and sodium ions. Subsequently, the centrifuge tube was placed in an oven for vacuum drying at 60℃, and finally was ground into fine powder in a mortar to obtain the final nano-silver particles.

[0089] Step b), preparation of nano-silver / graphene composite spraying solution

[0090] Into a sample bottle, 80mg graphene oxide powder and 80mg final washed nano-silver particles were added, and then 10ml n-hexane was added. The mixture was ultrasonically treated for 30-60min to ensure that the nano-silver particles were in full contact with the graphene oxide powder layers and were uniformly mixed to form a stable nano-silver / graphene composite spraying solution. In this embodiment, the mass ratio of graphene oxide to nano-silver was 1:1.

[0091] Step c), spraying and forming of lithium negative electrode

[0092] In a glove box filled with high-purity argon, a piece of substrate paper was fixed on the surface of the workbench, and a lithium metal sheet with a thickness of 150-300 pm was adhered to one side using double-sided tape to keep the lithium metal sheet stable in an inert atmosphere and facilitate subsequent spraying operations; the mixed spraying liquid was loaded into the liquid tank of the ultrasonic spray gun, the distance between the nozzle and the surface of the lithium metal sheet was set to 10 cm, the spraying rate was 1 mL / min, and the lithium metal sheet surface was sprayed back and forth until all the composite spraying liquid was sprayed; the sprayed lithium metal sheet was rolled by a roller press and left to dry, the composite layer was more closely attached to the lithium metal substrate through moderate mechanical pressure, and the pores or microcracks that may be formed during the spraying process were eliminated to ensure the density and uniformity of the modified layer. Then, the mixed spraying liquid was reduced and left to stand for 3 h in a glove box at 10-50 °C, so that part of the graphene oxide in the mixed spraying liquid was reduced to reduced graphene oxide (rGO), further improving the conductivity and stability of the composite layer, and promoting its combination with the surface of the lithium substrate, thereby obtaining a nano-silver / graphene composite modified lithium metal negative electrode with a dense structure, a stable interface, and excellent electrical conductivity.

[0093] The performance test of the lithium metal negative electrode prepared in this example is as follows:

[0094] Assembled lithium symmetric battery: a battery shell with a type of 2025 was selected, and a polyethylene separator was used. The negative electrode and the positive electrode both used the sprayed lithium metal sheet. The polyethylene separator was placed between the positive and negative electrodes, and 75 pL of commercial electrolyte 1M LiTFSI-DME:DOL 1:1+2wt% LINO3 was added on both sides. Finally, the lithium battery was packaged. The assembled battery was tested on a new Wei tester for Li||Li symmetric battery cycle performance test, with test conditions of 1 mA / 1 mAh, 5 mA / 5 mAh. The results of Li@Ag nanoparticle rGo-1:1 are shown in Figure 9 , which can be stably cycled for 660 h and 670 h at 5 mA / 5 mAh, and the symmetric battery shows good cycle stability.

[0095] Assembled lithium asymmetric battery: a battery shell with a type of 2025 was selected, and a polyethylene separator was used. The negative electrode used the sprayed lithium metal sheet, and the positive electrode was a copper foil. The polyethylene separator was placed between the positive and negative electrodes, and 75 pL of electrolyte was added on both sides. Finally, the lithium battery was packaged. The assembled battery was tested on a new Wei tester for Li||Cu asymmetric battery cycle performance test, with test conditions of 1.5 mA / 4 mAh, 5 mA / 5 mAh, and 10 mA / 10 mAh. The results of Li@Ag nanoparticle rGo-1:1 are shown in Figure 10 , which can be stably cycled for 660 h and 500 h at 1.5 mA / 4 mAh and 5 mA / 5 mAh, and can be stably cycled for 190 h at 10 mA / 10 mAh.

[0096] Example 4:

[0097] The embodiment provides a preparation method of a lithium metal negative electrode based on a nano-silver / graphene composite layer, and comprises the following steps:

[0098] Step a, preparation and purification of nano-silver particles

[0099] Take 40 mL of silver nitrate solution with a concentration of 0.1 mol / L, add 70 mL of deionized water, and then add 2 g of polyvinylpyrrolidone. Stir magnetically in a beaker for 1 h and heat under the condition of a 40°C water bath. Dissolve 0.504 g of sodium borohydride in 50 mL of deionized water, and slowly add it to the above mixture using a rubber bulb dropper. During the reaction process, the color of the solution gradually changes from bright yellow to brown yellow, indicating the formation of nano-silver particles. After the addition is completed, continue to heat and stir for 5 h. The obtained solution is divided into centrifuge tubes and centrifuged at 5000 rpm. Wash with deionized water and anhydrous ethanol for 3 times respectively to remove residual ions. Then, the precipitate is dried in a vacuum oven at 60°C, and ground into powder in a mortar to obtain pure nano-silver particles.

[0100] Step b, preparation of a nano-silver / graphene composite spraying solution

[0101] Take 80 mg of graphene oxide powder and 400 mg of the finally washed nano-silver particles (the mass ratio of graphene oxide to nano-silver is 1:5), add 10 mL of cyclohexane solvent, and place it in an ultrasonic device for treatment at a power of 400 W for 60 min to obtain a uniform and stable nano-silver / graphene oxide composite spraying solution.

[0102] Step c, spraying and forming of the lithium negative electrode

[0103] In an argon glove box, fix the base paper on the workbench, and adhere a lithium metal sheet with a thickness of 200 μm to one side. Fill the above mixed spraying solution into the ultrasonic spraying gun liquid tank, set the distance between the nozzle and the surface of the lithium sheet to be 15 cm, the spraying rate to be 1.5 mL / min, and spray back and forth until the mixed solution is sprayed completely. The sprayed lithium sheet is rolled by a rolling machine and dried in the glove box for 3 h, and then reduced and placed at 10-50°C for 6 h to obtain a nano-silver / graphene composite modified lithium metal negative electrode with a dense composite layer formed on the surface.

[0104] Example 5:

[0105] The embodiment provides a preparation method of a lithium metal negative electrode based on a nano-silver / graphene composite layer, and comprises the following steps:

[0106] Step a, preparation and purification of nano-silver particles

[0107] Into 40 mL of silver nitrate (AgNO3) solution with a concentration of 0.1 mol / L, 70 mL of deionized water and 2 g of polyvinylpyrrolidone (PVP) were added, and the mixture was stirred magnetically in a beaker for 1 h and heated in a 40 °C water bath. 0.2016 g of sodium borohydride (NaBH4) was dissolved in 50 mL of deionized water, and the mixture was added slowly to the above mixture with a rubber dropper. During the reaction, the solution gradually changed from bright yellow to brown yellow, indicating the formation of silver nanoparticles. After the addition was completed, the mixture was continuously heated and stirred for 5 h. The obtained solution was divided into centrifuge tubes and centrifuged at 5000 rpm. The residue was removed by washing with deionized water and anhydrous ethanol for 3 times, respectively. Then the precipitate was dried in a vacuum oven at 50 °C, and ground into powder in a mortar to obtain pure silver nanoparticles.

[0108] Step b, preparation of silver nanoparticle / graphene composite spraying solution

[0109] 80 mg of graphene oxide powder and 400 mg of silver nanoparticles (the mass ratio of graphene oxide to silver nanoparticles was 1:5) were weighed, 10 mL of cyclohexane was added, and the mixture was treated in an ultrasonic cleaner at a power of 200 W for 30 min to obtain a uniform and stable silver nanoparticle / graphene composite spraying solution.

[0110] Step c, spraying and forming of lithium anode

[0111] In a glove box filled with high-purity argon, an A4-sized base paper was fixed on the surface of the workbench, and a lithium metal sheet with a thickness of 200 μm was fixed on one side with double-sided tape. The composite spraying solution was injected into the liquid tank of the ultrasonic spray gun, the distance between the nozzle and the surface of the lithium metal sheet was set to 5 cm, the spraying rate was 0.5 mL / min, and the spraying was repeated until the spraying solution was used up. The sprayed lithium sheet was rolled by a rolling machine and dried in the glove box for 1 h, and then reduced and placed in an argon glove box at 10 °C for 1 h to obtain a lithium metal anode with a silver nanoparticle / graphene composite modified layer with a thickness of about 30 nm on the surface.

[0112] Example 6:

[0113] The present embodiment provides a method for preparing a lithium metal anode based on a silver nanoparticle / graphene composite layer, comprising the following steps:

[0114] Step a, preparation and purification of silver nanoparticles

[0115] Take 40 mL of silver nitrate (AgNO3) solution with a concentration of 0.5 mol / L, add 70 mL of deionized water, and add 2 g of polyvinylpyrrolidone (PVP), stir magnetically in a beaker for 1 h, and heat in a 60°C water bath. Dissolve 1.008 g of sodium borohydride (NaBH4) in 50 mL of deionized water, and slowly add it to the above mixture with a rubber dropper. During the reaction, the color of the solution gradually changes from bright yellow to brown yellow, indicating the formation of silver nanoparticles. After the addition is complete, continue to heat and stir for 5 h. The resulting solution is divided into centrifuge tubes and centrifuged at 10,000 rpm. Wash with deionized water and anhydrous ethanol three times each, and remove residual ions. Then, place the precipitate in a vacuum oven at 80°C to dry, and grind it into a powder in a mortar to obtain pure silver nanoparticles.

[0116] Step b, preparation of silver nanoparticle / graphene composite spraying solution

[0117] Weigh 200 mg of graphene oxide powder and 200 mg of silver nanoparticles (the mass ratio of graphene oxide to silver nanoparticles is 1:1), add 10 mL of toluene solvent, and treat it in an ultrasonic cleaner at a power of 600 W for 120 min to obtain a uniformly dispersed silver nanoparticle / graphene composite spraying solution.

[0118] Step c, spraying and shaping of lithium anode

[0119] In a glove box filled with high-purity argon, fix an A4-sized base paper on the surface of the workbench, and fix a lithium metal sheet with a thickness of 300 μm on one side with double-sided tape. Pour the above composite spraying solution into the liquid tank of the ultrasonic spray gun, set the distance between the nozzle and the surface of the lithium metal sheet to 20 cm, and the spraying rate to 2 mL / min. Spray back and forth along the surface of the lithium sheet until the mixed spraying solution is sprayed completely. The sprayed lithium sheet is rolled by a rolling machine and dried for 1 h, and then reduced and placed at 50°C for 12 h in an argon glove box, so that the graphene oxide is partially reduced to reduced graphene oxide (rGO), forming a dense three-dimensional skeleton structure composite modified layer with a thickness of about 5 μm, thereby obtaining a silver nanoparticle / graphene composite modified lithium metal anode.

[0120] Example 7:

[0121] The embodiment provides a lithium metal negative electrode based on a nano-silver / graphene composite layer. After steps a) to c) are completed, the sprayed and rolled and initially statically dried lithium metal sheet is continuously reduced and statically placed at 10-50 DEG C for 1-12 hours in a glove box environment filled with high-purity argon, oxygen and water content less than 0.1 ppm, so that the oxidized graphene in the composite sprayed layer is partially reduced to reduced graphene oxide (rGO). After the treatment, the composite layer forms a dense and continuous three-dimensional skeleton structure on the surface of the lithium metal, the nano-silver particles are uniformly distributed on the surface of the reduced graphene oxide sheet and in the interstices thereof, and are closely combined with the lithium metal matrix.

[0122] By scanning electron microscopy (SEM) characterization of the modified composite layer after spraying, it can be observed that the nano-silver particles and the reduced graphene oxide sheet are uniformly combined, the morphology is clear, the interface transition is smooth, and a stable three-dimensional conductive skeleton is formed. This structure not only provides a large number of uniformly distributed lithiumophilic nucleation sites (from nano-silver) to help achieve uniform deposition of lithium ions, but also uses the mechanical strength of reduced graphene oxide to improve the overall structural stability, avoiding interface tearing and dendrite penetration caused by volume expansion during deposition / detachment, thereby effectively improving the cycle life and safety performance of the lithium metal negative electrode.

[0123] In summary, the present application provides a complete technical solution with significant innovation from material design, preparation method to practical application. The technical solution is not simply a physical mixture of nano-silver and graphene, but a combination of chemical guidance of nano-silver and physical constraint of graphene sheet to construct a uniformly dispersed, dense and stable composite modified layer, thereby effectively inhibiting the generation and disorderly growth of lithium dendrites. The composite structure not only improves the uniformity of lithium deposition and the interface stability, significantly reduces the short circuit risk of the battery, but also reduces the side reactions of active lithium and electrolyte during the cycle process, improves the coulombic efficiency and cycle life of the battery, and exhibits excellent rate performance and application prospect.

[0124] The above application of specific examples to the present application is only used to help understand the present application and does not limit the present application. For those skilled in the art to which the present application belongs, according to the idea of the present application, a number of simple deductions, deformations or substitutions can be made.

Claims

1. A method for preparing a nano-silver / graphene composite modified lithium metal anode, characterized in that, Includes the following steps: a) Preparation and purification of silver nanoparticles: Silver nitrate aqueous solution was mixed with polyvinylpyrrolidone (PVP) dispersant, and sodium borohydride aqueous solution was slowly added dropwise under heating and stirring conditions to carry out a reduction reaction. After the reaction was completed, the obtained silver nanoparticle sol was centrifuged and the precipitate was washed alternately with deionized water and anhydrous ethanol. After vacuum drying and grinding, pure silver nanoparticles were obtained. b) Preparation of composite spraying liquid: The nano-silver particles and graphene oxide powder obtained in step a) are added to an organic solvent at a mass ratio of 1:1-5:1, and after ultrasonic treatment, a uniform and stable nano-silver / graphene oxide mixed spraying liquid is formed. c) Spraying and forming of lithium anode: Under the protection of an inert atmosphere, the nano-silver / graphene oxide mixed spraying liquid obtained in step b) is uniformly sprayed onto the surface of lithium metal sheet, and after being rolled and allowed to dry, a nano-silver / graphene oxide composite modified layer is formed on the surface of lithium metal sheet.

2. The preparation method of the nano-silver / graphene composite modified lithium metal anode according to claim 1, characterized in that, In step a): The concentration of the silver nitrate aqueous solution is 0.01-0.5 mol / L; The molar ratio of polyvinylpyrrolidone (PVP) to silver nitrate is 1:1 to 1:100; The molar ratio of sodium borohydride to silver nitrate is 1:1 to 5:1; The reaction temperature for the reduction reaction is 30-60℃; The centrifugation speed for the centrifugal separation is 3000-10000 rpm; The vacuum drying temperature is 50-80℃.

3. The method for preparing the nano-silver / graphene composite modified lithium metal anode according to claim 1, characterized in that, In step b): The organic solvent is selected from one or more of n-hexane, cyclohexane, or toluene; In the nano-silver / graphene oxide mixed spray solution, the total concentration of nano-silver and graphene oxide is 1-10 mg / mL; The ultrasonic treatment has a power of 200-600W and a duration of 30-120min.

4. The preparation method of the nano-silver / graphene composite modified lithium metal anode according to claim 1, characterized in that, In step c): The inert atmosphere is argon or nitrogen, wherein the content of water and oxygen is less than 0.1 ppm; During the spraying process, the distance between the spray gun nozzle and the surface of the lithium metal sheet is 5-20cm; The spraying rate is 0.5-2 mL / min.

5. The method for preparing the nano-silver / graphene composite modified lithium metal anode according to claim 4, characterized in that, Step c) is followed by a restoring and settling step: After being rolled and dried, the lithium metal anode is left to stand in a glove box at 10-50°C for 1-12 hours to partially reduce the graphene oxide in the composite modified layer to reduced graphene oxide, thereby enhancing the bonding force between the composite modified layer and the lithium metal sheet.

6. A nano-silver / graphene composite modified lithium metal anode, characterized in that, Obtained by the preparation method according to any one of claims 1-5.

7. The nano-silver / graphene composite modified lithium metal anode according to claim 6, characterized in that, The nano-silver / graphene oxide composite modified layer is densely and firmly covered on the surface of the lithium metal sheet. The thickness of the composite modified layer is 30nm-5μm. The reduced graphene oxide sheets in the composite modified layer form a three-dimensional framework structure, and the nano-silver particles are uniformly embedded in the surface and gaps of the three-dimensional framework structure.

8. The application of the nano-silver / graphene composite modified lithium metal anode according to claim 6 in a copper foil half-cell, characterized in that, The copper foil half-cell includes: a) Using a lithium metal sheet that has been modified by spraying as the working electrode; b) Using copper foil as the counter electrode and current collector; c) Celgard 2400 polypropylene porous membrane is used as the separator, and the electrolyte is a solution of 1 mol / L lithium bis(trifluoromethanesulfonyl)imide in a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane in a volume ratio of 1:1, and contains 2% lithium nitrate. d) Assemble CR2032 button cells in an argon glove box with both water and oxygen content below 0.1 ppm, and encapsulate them under a pressure of 350-500 psi.

9. The application of the nano-silver / graphene composite modified lithium metal anode according to claim 6 in a symmetrical battery, characterized in that, The symmetrical battery includes: a) Using spray-modified lithium metal sheets as the positive and negative electrodes, respectively; b) Celgard 2400 polypropylene porous membrane is used as the separator, and the electrolyte is a solution of 1 mol / L lithium bis(trifluoromethanesulfonyl)imide in a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane in a volume ratio of 1:1, and contains 2% lithium nitrate. c) Assemble CR2032 button cells in an argon glove box where both water and oxygen levels are below 0.1 ppm; d) Encapsulate under a pressure of 350-500 psi.

10. The application of the nano-silver / graphene composite modified lithium metal anode according to claim 6 in lithium iron phosphate full batteries, characterized in that, The lithium iron phosphate full battery includes: a) Using a spray-modified lithium metal sheet as the negative electrode, and the positive electrode having an active material surface density of 3-4 mg / cm³. 2 The composite electrode has a mass ratio of LiFePO4, polyvinylidene fluoride (PVDF), and conductive carbon black (Super P) of 8:1:

1. b) Celgard 2400 polypropylene porous membrane is used as the separator, and the electrolyte is a solution of 1 mol / L lithium bis(trifluoromethanesulfonyl)imide in a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane in a volume ratio of 1:1, and contains 2% lithium nitrate. c) Assemble CR2032 button cells in an argon glove box where both water and oxygen levels are below 0.1 ppm; d) Encapsulate under a pressure of 350-500 psi.

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