A method for preparing a lithium metal lithium-free negative electrode

Abstract

The patent relates to a preparation method of a lithium metal lithium-free negative electrode, the porous aluminum foil has excellent electrolyte wettability and high specific surface area, which helps to improve the transmission efficiency of lithium ions and electrons, thereby improving the lithium storage activity. Meanwhile, the patent also provides an electrochemical reconstruction method of an aluminum negative electrode, a special lithium-aluminum alloy layer is formed on the surface of the aluminum negative electrode through alloying reaction; by adjusting the electrochemical reaction conditions, the morphology and microstructure of the lithium-aluminum alloy are optimized, so that the uneven deposition of lithium metal when the lithium-aluminum alloy is used as a lithium metal battery lithium-free negative electrode is inhibited. The results show that the lithium aluminum negative electrode of the lithium metal battery described in the patent is significantly better than the conventional aluminum foil, the commercial porous aluminum foil and the self-made porous aluminum foil without activation treatment in terms of cycle stability and safety. The present application has important application value in the field of lithium metal battery technology, and can provide an effective solution for the development and commercialization of high-energy-density lithium metal batteries.

Description

TECHNICAL FIELD

[0001] The present application relates to the preparation of lithium metal lithium-free negative electrode, in particular to the technical field of porous aluminum foil material preparation, for lithium metal lithium-free negative electrode, belongs to the technical field of lithium metal battery manufacturing. BACKGROUND

[0002] Lithium metal battery uses lithium as the negative electrode, compared with lithium ion battery, has higher theoretical specific capacity and output voltage (redox potential-3.040 V), which embodies the broad development prospect and competitive advantage. However, in the cycle process, the metal lithium negative electrode still has the following problems to be solved. First, in the process of charging and discharging, the uncontrollable growth of lithium dendrite formed by the uneven deposition-dissolution of lithium metal and the interface instability; second, lithium metal has high reaction activity with electrolyte, which is easy to cause side reaction, thus leading to the decline of cycle performance. In order to solve the above problems, researchers optimize the lithium metal battery negative electrode material from the composition and structure design of the current collector, surface protection and other aspects, in order to improve the electrochemical cycle stability and safety of lithium metal battery.

[0003] From the selection of lithium metal battery negative electrode material, electrochemically inert metals such as copper are widely studied. On the one hand, copper has good electrical conductivity and electrochemical stability, which is conducive to the uniform deposition / peeling of lithium metal, and has a certain stability. However, the deposition process often shows a large polarization, indicating that the reaction kinetics is not good, and the cycle performance under large current is not ideal. On the other hand, the processing technology of copper foil is mature, which is conducive to its large-scale application and industrialization, and significantly reduces the cost. At the same time, by modifying the active metal elements that can form alloy with lithium on the surface of copper, the lithium deposition overpotential can be significantly reduced, and the deposition / peeling reaction kinetics can be further improved, thereby significantly improving its performance. Reasonable use of the stability of electrochemically inert metals and the rapid lithiation reaction kinetics of electrochemically active metal elements can be expected to develop high-performance lithium metal lithium-free negative electrode materials.

[0004] Aluminum is a traditional metal material, which is rich in reserves, low in cost and mature in processing technology. It is not only widely used in traditional fields such as architecture, decoration, vehicles, military, aerospace, etc., but also shows great application prospect in emerging new energy battery field. As the positive electrode of lithium ion battery, the positive and negative electrode current collector of sodium ion battery, it shows more obvious cost advantage than copper. However, aluminum itself is also a high-capacity lithium ion battery negative electrode material, which can alloy with lithium, and as a lithium metal battery negative electrode, it will cause structure pulverization due to reaction with lithium, thus causing serious performance degradation. Therefore, so far, there are few reports on high-performance aluminum negative electrode of lithium metal battery. This patent aims to make full use of the cost advantage of aluminum, overcome the deficiency of structure damage caused by reaction with lithium through electrochemical means, and develop a kind of high-performance aluminum negative electrode of lithium metal battery.

[0005] To this end, the present patent develops a preparation process of porous aluminum foil. On the one hand, compared with traditional aluminum powder materials, the exposed surface area of the porous aluminum foil is reduced, which can minimize the surface oxide layer and improve the material reactivity. On the other hand, the porous structure can effectively enhance the contact between the material and the electrolyte and improve the reaction kinetics, while effectively enhancing the structural stability of the material in the electrochemical reaction. On this basis, by optimizing the electrochemical reaction parameters, a special structure of lithium-aluminum alloy protective layer is generated on the surface of the active aluminum, which maintains the stable lithium metal deposition / stripping reaction while providing good protection for the aluminum matrix. The new type of lithium metal battery negative electrode prepared by the porous aluminum foil after electrochemical reconstruction has excellent electrochemical performance, which has great research value and application potential. SUMMARY

[0006] The present application provides a preparation method of a new type of lithium metal battery negative electrode. The prepared porous aluminum foil is used to assemble a battery, and a surface lithium-aluminum alloy layer is formed by lithiation / delithiation electrochemical activation to protect the aluminum matrix, thereby improving the cycle stability of the electrode as a lithium metal battery negative electrode. Through electrochemical activation, the morphology and microstructure of the lithium-aluminum alloy surface layer can be effectively controlled, further activating the active sites on the surface, thereby realizing more uniform lithium deposition / stripping reaction. At the same time, the activation process can also promote the formation and stability of the SEI film, reduce the irreversible consumption of lithium and capacity decay in the subsequent charge and discharge process, thereby effectively improving the coulombic efficiency and cycle life of the lithium metal battery. The design method of the aluminum negative electrode provided by the present application is expected to provide a new idea and solution for the commercial application of lithium metal batteries.

[0007] Technical scheme

[0008] The technical scheme of the present application comprises the following steps:

[0009] (1) First, a certain amount of polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) is added to N,N-dimethylformamide (DMF) and stirred for a period of time (50℃ water bath heating for 5h), to obtain a colorless transparent solution as A solution;

[0010] (2) A certain amount of ethylene glycol is directly taken as B solution;

[0011] (3) The A solution is prepared into nanofibers by electrospinning, and the B solution is sprayed by electrostatic spraying to form a carbon nanofiber membrane and deposit on the aluminum foil to obtain a precursor carbon nanofiber membrane / aluminum foil composite;

[0012] (4) The (3) is dried in a blast drying oven to remove the solvent;

[0013] (5) sintering (4) for a period of time under air atmosphere, and then heat treating it under Ar / H2 gas protection, so as to decompose or carbonize the polymer, and at the same time, melt the aluminum foil and flow along the surface of the carbon material to form a porous aluminum structure.

[0014] (6) using the porous aluminum foil material as the negative electrode, and the lithium sheet as the positive electrode to assemble a button cell, and performing electrochemical alloying to obtain a Li-Al alloy, which is the lithium metal without lithium negative electrode.

[0015] Preferably, the mass ratio of polyacrylonitrile (PAN) to polyvinylpyrrolidone (PVP) in step (1) is 0.3-0.6 g: 0.1-0.2 g;

[0016] Preferably, the stirring time in step (1) is 30-48 h;

[0017] Preferably, the ethylene glycol in step (2) is 2-5 mL;

[0018] Preferably, the electrospinning combined with electrostatic spraying conditions in step (3) are as follows: the temperature is 40-60℃, the humidity is 20-30%, the voltage is 15-20 kV, and the receiving distance is 15-25 cm, and the electrospinning / spraying time is 8-15 hours;

[0019] Preferably, the drying temperature in step (4) is 50℃-80℃, and the drying time is 18-24 h;

[0020] Preferably, the sintering temperature under air atmosphere in step (5) is 200-300℃, and the sintering time is 2-5 h;

[0021] Preferably, the heat treatment temperature under Ar / H2 gas protection in step (5) is 650-750℃, and the time is 2-5 h;

[0022] Preferably, in the battery test process, the first three circles are activated by controlling the current density and the battery capacity, so that lithium ions can effectively embed into the structure of the porous aluminum foil to form a Li-Al alloy, further improving the electrochemical performance of the battery;

[0023] Preferably, in the activation process, the charge and discharge current density is 0.01-0.05 A g -1 , and the discharge capacity each time is 50-200 mAh g -1 , so as to ensure that lithium ions can form a surface alloy layer with the porous aluminum in an appropriate amount and maintain the structural integrity of the porous aluminum.

[0024] Compared with the prior art, the advantages and beneficial effects of the present application are:

[0025] (1) Cost-effective: Aluminum is a widely used and inexpensive metal material, which offers better cost advantage compared to traditional copper negative electrodes, helping to reduce the production cost of lithium metal batteries.

[0026] (2) Enhanced interface stability: The specific lithium-aluminum alloy layer used as a protective layer for the aluminum negative electrode can effectively improve the interface stability between the aluminum matrix and lithium metal, reduce the structural pulverization phenomenon caused by reaction, and prolong the service life of the battery.

[0027] (3) Excellent electron and ion transmission: The design of porous aluminum foil provides higher specific surface area and good electrolyte wettability, which helps to improve the transmission efficiency of lithium ions and electrons, thus enhancing the lithium storage activity.

[0028] (4) Improved cycle stability: The invention designs a porous structure in the aluminum-based material and combines a special electrochemical reconstruction method to effectively inhibit the growth of lithium dendrites and enhance the cycle stability during charging and discharging.

[0029] (5) Environmentally friendly: Using porous aluminum foil as a negative electrode material has lower environmental impact and better sustainability compared to other heavy metals or negative electrode materials with greater environmental impact, meeting the requirements of modern environmental protection.

[0030] (6) Effective electrochemical protection: The lithium-aluminum alloy layer formed by electrochemical reconstruction not only makes the deposition and stripping reaction of lithium more stable, but also protects the aluminum-based negative electrode material from performance degradation caused by reaction with lithium. BRIEF DESCRIPTION OF DRAWINGS

[0031] The invention will be further described below in conjunction with the accompanying drawings.

[0032] Figure 1 Optical photograph of the sample precursor prepared in Example 1.

[0033] Figure 2 Optical photograph of the sample prepared in Example 1: (a) optical photograph of carbon nanofiber membrane; (b) optical photograph of porous aluminum foil.

[0034] Figure 3 XRD of the porous aluminum foil prepared in Example 1.

[0035] Figure 4 SEM image of the material prepared in Example 1: (a) scanning electron microscope image of carbon nanofiber membrane; (b-c) scanning electron microscope image of porous aluminum foil.

[0036] Figure 5 Contact angle test picture of Examples 1, 3, and 4.

[0037] Figure 6Electrochemical performance plots for Example 1 : (a) first three cycles activated charge-discharge plots; (b) charge-discharge plots for cycles 4-6; (c) cycle efficiency plot.

[0038] Figure 7 Electrochemical performance plots for Example 2: (a) first three cycles charge-discharge plots; (b) charge-discharge plots for cycles 4-6; (c) cycle efficiency plot.

[0039] Figure 8 Electrochemical performance plots for Example 3: (a) first three cycles activated charge-discharge plots; (b) charge-discharge plots for cycles 4-6; (c) cycle efficiency plot.

[0040] Figure 9 Electrochemical performance plots for Example 4: (a) first three cycles activated charge-discharge plots; (b) charge-discharge plots for cycles 4-6; (c) cycle efficiency plot.

[0041] Figure 10 Electrochemical performance plots for Example 5: (a) first three cycles activated charge-discharge plots; (b) charge-discharge plots for cycles 4-6; (c) cycle efficiency plot.

[0042] Figure 11 Electrochemical performance plots for Example 6: (a) first three cycles activated charge-discharge plots; (b) charge-discharge plots for cycles 4-6; (c) cycle efficiency plot.

[0043] Figure 12 Electrochemical performance plots for Example 7: (a) first three cycles activated charge-discharge plots; (b) charge-discharge plots for cycles 4-6; (c) cycle efficiency plot.

[0044] Figure 13 Electrochemical performance plots for Example 8: (a) first three cycles activated charge-discharge plots; (b) charge-discharge plots for cycles 4-6; (c) cycle efficiency plot.

[0045] Figure 14 Time-voltage plot for Example 1. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical scheme and advantages of the present application more clear, the following examples will further illustrate the present application with reference to the drawings. It should be understood that the specific examples described herein are only used to explain the present application and not to limit the present application. On the contrary, the present application covers any alternatives, modifications, equivalent methods and schemes defined by the claims within the spirit and scope of the present application. Further, in order to make the public have a better understanding of the present application, some specific details are described in the following detailed description of the present application. The present application can also be completely understood without the description of these details by those skilled in the art.

[0047] The present application adopts electrospinning combined with electrostatic spraying method to prepare porous aluminum foil, which has larger specific surface area and good electrolyte wettability compared with ordinary aluminum foil. The preparation process of the present application is green and environmentally friendly, the reaction process is controllable, the process is simple, the preparation period is short, and the cost is low. Through the first three circles of lithiation / delithiation electrochemical reaction, a surface lithium-aluminum alloy layer is formed to protect the substrate aluminum, thereby improving the cycle stability of the electrode as a negative electrode of lithium metal battery. Through the first three circles of electrochemical activation, the morphology and microstructure of the lithium-aluminum alloy surface layer can be effectively controlled, and the active sites on the surface can be further activated, thereby realizing more uniform lithium deposition / stripping reaction. At the same time, the activation process can also promote the formation and stability of SEI film, reduce the irreversible consumption of lithium and capacity attenuation in the subsequent charge and discharge process, thereby effectively improving the coulombic efficiency and cycle life of lithium metal battery. The design method of aluminum negative electrode provided by the present application is expected to provide new ideas and solutions for the commercial application of lithium metal battery.

[0048] Example 1

[0049] First, 0.45 g of polyacrylonitrile (PAN) and 0.15 g of polyvinylpyrrolidone (PVP) were weighed and added to 4.56 mL of N,N-dimethylformamide (DMF) and stirred for 48 h (with 50°C water bath heating for 5 h), to obtain a colorless transparent solution as A solution. 2.5 mL of ethylene glycol was directly taken as B solution.

[0050] The A solution was moved into a syringe with a nozzle diameter of 0.33 mm and a capacity of 5 mL, and the B solution was moved into a syringe with a nozzle diameter of 0.33 mm and a capacity of 2.5 mL, and the aluminum current collector was used as the receiving end for double-needle spinning. Under the conditions of positive voltage 15 kV, negative voltage 3 kV, temperature 45°C, humidity 30±5%, receiving distance 15 cm, and injection rate 0.42 mL h -1 , electrospinning was carried out for 12 h. The collected product was dried in an oven at 60°C for 20 h to obtain a precursor nanofiber membrane / aluminum foil composite Figure 1 ).

[0051] The precursor nanofiber membrane / aluminum foil composite was then pre-oxidized at 250℃ for 3 h at a heating rate of 1℃ min -1 under air atmosphere, and then annealed at 700℃ for 3 h at a heating rate of 5℃ min -1 under argon atmosphere to obtain carbon nanofiber membrane and porous aluminum foil material. Figure 2 The sintered material was analyzed by XRD pattern, and the obtained diffraction peaks corresponded well to Al (PDF #85-1327). Figure 3 The obtained sample was tested by scanning electron microscope (SEM), as shown in Figure 4 a, the material showed a fibrous morphology. As shown in Figure 4 b, the prepared porous aluminum foil showed a rough and irregular morphology.

[0052] The material was made into a battery as follows: the porous aluminum foil was used as the negative electrode / counter electrode, Celgard 2400 microporous polypropylene was used as the separator, 1M LiTFSI was dissolved in DME+DOL (volume ratio 1:1) solution and 2% LiNO3 additive was used as the electrolyte, and a lithium sheet was used to assemble a CR2025 type button cell in an argon glove box. After all the batteries were assembled, they were left to stand for 8h, and then the electrochemical performance was tested using a LAND CT2001A battery test system.

[0053] First, the current density and battery capacity were controlled for activation (current density was 0.03 A g -1 , capacity was 100 mAh g -1 ) for the first three cycles to enable lithium ions to effectively alloy with the porous aluminum foil to form Li-Al alloy. To ensure that lithium ions can form a surface alloy layer with porous aluminum and maintain the structural integrity of the porous aluminum. Then, the cycle performance test was carried out under the condition of current density of 0.2 A g -1 , capacity of 600 mAh g -1 .

[0054] Example 2

[0055] This example only changes the test method in Example 1, and only the first three cycles of activation are not performed, and the electrochemical performance test is directly carried out under the condition of current density of 0.2 A g -1 , capacity of 600 mAh g -1 .

[0056] Example 3

[0057] This example only changes the porous aluminum foil used in the assembly of the battery in Example 1 to a commercial porous aluminum foil (Desco Electronics Technology (Kunshan) Co., Ltd.), without further treatment, directly slicing to assemble the battery, and the battery assembly and test methods are exactly the same as in Example 1.

[0058] Example 4

[0059] This example only changes the porous aluminum foil used in the assembly of the battery in Example 1 to a commercial current collector aluminum foil, without further treatment, directly slicing to assemble the battery, and the battery assembly and test methods are exactly the same as in Example 1.

[0060] Example 5

[0061] This example only changes the electrolyte used in the assembly of the battery in Example 1 to an EC+DMC+DEC (volume ratio of 1:1:1) solution with LiPF6 (1 mmol / L) dissolved therein, and the test method is exactly the same as in Example 1.

[0062] Example 6

[0063] This example only changes the porous aluminum foil used in the assembly of the battery in Example 1 to a commercial copper foil, without further treatment, directly slicing to assemble the battery, and the battery assembly and test methods are exactly the same as in Example 1.

[0064] Example 7

[0065] This example only changes the test method in Example 1, only changes the current density during alloying to 0.01 A g -1 , and the capacity to 200 mAh g -1 , and still tests the electrochemical performance under the conditions of a current density of 0.2 A g -1 , and a capacity of 600 mAh g -1 after alloying.

[0066] Example 8

[0067] This example only changes the test method in Example 1, only changes the current density during alloying to 0.05 A g -1 , and the capacity to 200 mAh g -1 , and still tests the electrochemical performance under the conditions of a current density of 0.2 A g -1 , and a capacity of 600 mAh g -1 after alloying.

[0068] Contact angle tests were conducted on Examples 1, 3, and 4. The prepared porous aluminum foil was cleaned, and a certain volume of electrolyte was dropped onto the surface of the foil using a micropipette. After the droplet stabilized, an image of the droplet was captured using a Theta Lite contact angle meter, and the contact angle was automatically measured using software analysis. The analysis results were obtained from... Figure 5 As shown, the porous aluminum foil prepared in Example 1 can reduce the contact angle of the electrolyte from 43.54° to about 25.95° (and is less than the contact angle of 32.37° of commercial porous aluminum foil), indicating that the surface energy of the porous aluminum foil is increased and the surface roughness may also be improved, thereby improving the wettability of the electrolyte.

[0069] Electrochemical performance analysis:

[0070] Electrochemical performance was tested using the LAND CT2001A battery testing system. (See attached image.) Figure 6 a, 8a, 9a, 10a, 11a, 12a, and 13a are charge-discharge curves of the batteries assembled in the examples during the first three activation cycles, where the current density of Examples 1, 3, 4, 5, and 6 is 0.03 A g. -1 The capacity is 100 mAh g -1 The current densities in Examples 7 and 8 were 0.01 A g, respectively. -1 and 0.05A g -1 Both have a capacity of 200 mAh g -1 The horizontal axis represents mass capacity, and the vertical axis represents voltage. The porous aluminum foil prepared in Example 1 shows a significantly improved initial efficiency compared to the commercial porous aluminum foil of Example 3 and the aluminum current collector of Example 4. While the commercial copper foil of Example 6 has a high initial efficiency, its efficiency gradually deteriorates with charging and discharging. This may be because the repeated deposition / stripping process during charging and discharging leads to an uneven crystal structure of lithium metal on the copper foil surface, thus affecting the battery's cycle stability. The alloying process in Example 7 proceeds faster than that in Example 1. The alloying process in Example 8 is essentially the same as that in Example 1. (Appendix) Figure 7 Example a is Example 2, which skips the first three activation cycles and directly operates at a current density of 0.2 A g. -1 The capacity is 600 mAh g -1 The charge-discharge curves under the specified conditions are attached. Figure 6 b-13b is an example at a current density of 0.2 A g -1 The capacity is 600 mAh g -1 The graphs show the 4th, 5th, and 6th charge-discharge curves under the specified conditions. It can be seen from the graph that all discharge curves eventually fall below 0 V, which clearly indicates lithium deposition behavior. (See attached graph.) Figure 6c-13c, the abscissa represents the cycle number, and the ordinate represents the coulombic efficiency. The results show that using porous aluminum foil as the negative material, 1 M LiTFSI dissolved in DME+DOL (volume ratio of 1:1) solution and 2% LiNO3 additive as the electrolyte, and the battery after activation, can effectively realize the alloying reaction between lithium ions and porous aluminum foil, and then form Li-Al alloy. This Li-Al alloy not only can be further used as the negative electrode layer of lithium metal battery, and can also provide good protection for the aluminum layer during the deposition and stripping of lithium, thereby significantly improving the cycle stability and life of the battery. Under the condition of current density of 0.2 A g -1 , the mass capacity is 600 mAh g -1 , it can be stably cycled for more than 300 cycles, and the cycle time is more than 2000 h (as Figure 14 indicated).

[0071] Example 2 is not activated, and after 80 cycles, it is deactivated. Example 3, commercial aluminum foil, starts to decay after about 80 cycles. Example 4, aluminum current collector, starts to decay after about 45 cycles. Example 5, using LiPF6 electrolyte, is deactivated after 130 cycles. Example 6, commercial copper foil, is deactivated after about 103 cycles. Example 7, the current density during alloying is changed to 0.01 A g -1 , and the capacity is changed to 200 mAh g -1 , it starts to decay after about 170 cycles. Example 8, the current density during alloying is changed to 0.05 A g -1 , and the capacity is changed to 200 mAh g -1 , it starts to decay after about 200 cycles. In summary, using porous aluminum foil as the negative material, 1 M LiTFSI dissolved in DME+DOL (volume ratio of 1:1) solution and 2% LiNO3 additive as the electrolyte, and the battery after activation at a current density of 0.03 A g -1 , and the capacity is 100 mAh g -1 , has a stable long cycle life of up to 2000 h.

Claims

1. A method of preparing a lithium metal anode without lithium, characterized in that, An alloying reaction occurs during the charging and discharging process, forming a Li-Al alloy layer, forming a lithium metal negative electrode, and a method for preparing a lithium metal lithium-free negative electrode includes the following steps: (1) First, weigh the polyacrylonitrile PAN and polyvinylpyrrolidone PVP and add them to N,N-dimethylformamide DMF and stir to obtain a colorless transparent solution as A solution; (2) Ethylene glycol as B solution; (3) Prepare nanofibers by electrospinning with A solution, and spray with B solution by electrostatic spraying to form carbon nanofiber membranes and deposit on aluminum foil to obtain a precursor carbon nanofiber membrane / aluminum foil composite; (4) Dry and pre-sinter the precursor carbon nanofiber membrane / aluminum foil composite of (3) and then heat treat to obtain a porous aluminum foil material; (5) using porous aluminum foil material as the negative electrode, lithium metal sheet as the positive electrode to assemble into a button cell, and then performing electrochemical alloying, wherein the charge and discharge current density during the electrochemical alloying treatment is 0.01-0.05 A g -1 , the discharge capacity is 50-200 mAh g -1 , the electrochemical alloying treatment is 3-5 cycles, and a Li-Al alloy, i.e. lithium metal without lithium negative electrode, is obtained.

2. The method of claim 1, wherein the lithium metal anode is prepared without lithium. The mass ratio of polyacrylonitrile to polyvinylpyrrolidone is 3-6:1-2.

3. The method of claim 1, wherein the lithium metal anode is prepared without lithium. The electrospinning combined with electrostatic spraying conditions are: temperature 40-60℃, humidity 20-30%, voltage 15-20 kV, and receiving distance 15-25 cm, electrospinning / spraying 8-15 hours.

4. The method of claim 1, wherein the lithium metal anode is prepared without lithium. The pre-sintering conditions in step (4) are: sintering temperature 200-300℃ in air atmosphere, and sintering time 2-5 h.

5. The method of claim 1, wherein the lithium metal anode is prepared without lithium. The heat treatment conditions in step (4) are: heat treatment temperature 650-750℃ under Ar / H2 gas protection, and time 2-5 h.

6. A lithium ion battery negative electrode material, wherein the lithium metal lithium-free negative electrode is obtained by the method of any one of claims 1-5.

7. A battery material, comprising the lithium metal lithium-free negative electrode obtained by the method of any one of claims 1-5.

Images