Ultrathin lithium alloy strip for flight power, preparation method of ultrathin lithium alloy strip and lithium metal battery

Abstract

The invention discloses an ultrathin lithium alloy strip for flight power, a preparation method of the ultrathin lithium alloy strip and a lithium metal battery. Li: (Mg + Ag) = (0.3-0.5): (0.7-0.5), and the mass ratio of Mg to Ag is (1: 1)-(1: 2.5). A main body in the alloy belt is a uniformly mixed LiMgAg ternary solid solution alloy phase without a pure lithium phase. Through alloy component design, the machining performance is remarkably improved, the viscosity of the alloy to the roller is reduced, in the preparation process, the roller sticking problem in the machining process is avoided, a release film is abandoned, the machining cost is reduced, and batch stable preparation can be achieved. And a novel alloy system and an adaptive rolling process realize collaborative optimization of solid solution, high strength and ultra thinness. The lithium is uniformly dissolved out and deposited through ternary alloy solid solution, and a three-dimensional porous skeleton structure is formed after the lithium is completely dissolved out, so that the structural integrity of the pole piece is kept, and the damage and pulverization of the pole piece in the battery circulation process are avoided. And when the copper foil with the same thickness is replaced, the alloy mass is only 10% of that of the copper foil with the same thickness, and the cycle performance of the prepared battery is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to an ultra-thin lithium alloy strip for flight propulsion, its preparation method, and a lithium metal battery. Background Technology

[0002] Lithium metal is considered an ideal anode material for next-generation high-energy-density rechargeable batteries due to its extremely high theoretical specific capacity (3860 mAh / g) and extremely low redox potential (-3.04 V vs. RHE). In fact, a lithium metal anode with a thickness of only 15 μm can provide a capacity of about 3 mAh / cm², which is a significant advantage over traditional graphite anodes (which require more than 100 μm to achieve 3.5 mAh / cm²). However, although lithium metal has a good relative elongation (50-70%), its softness, low Young's modulus (5 GPa), and low yield strength (0.5-1.1 MPa) lead to two major technical bottlenecks in its mechanical rolling process: interfacial adhesion (roll force as high as 5-8 N / mm²) and strip fracture (fracture energy <10 J / m² when thickness <50 μm). These problems have led the industry to widely adopt lithium foils thicker than 100μm, resulting in actual battery energy density reaching only 30-40% of the theoretical value. This clearly violates the original goal of achieving higher energy density and causes unnecessary material loss.

[0003] Existing rolling processes produce lithium strips by passing lithium metal billets through one or more pairs of rollers to gradually reduce the strip thickness until the desired thickness is achieved. To prevent problems such as wrinkles, breakage, cracking, and sticking to the rollers during processing, a release film coated with silicone oil (contact angle > 110°) is typically used as a physical isolation layer between the lithium strip and the rollers. However, this technology has three major drawbacks: 1) Uncontrollable surface contamination: Residual silicone oil and other compounds (typically 0.5-1.2 mg / cm²) can cause a surge in interfacial side reactions during battery cycling; 2) High processing costs: Release films are generally only usable once, so a large amount of release film is used in multiple rolling processes, significantly increasing processing costs; 3) Poor process compatibility: The thermal stability limitation of the release film (temperature resistance < 80℃) prevents high-speed rolling (> 5 m / min) and online annealing.

[0004] It is noteworthy that ultrathin lithium strips of appropriate thickness, if possessing sufficient mechanical strength to withstand handling and battery manufacturing processes, can be directly used as the anode in lithium metal batteries without the need for heavier copper current collectors, potentially leading to a significant leap in the mass energy density of lithium metal batteries compared to existing systems. However, while existing alloy designs (such as Li-Al and Li-Si systems) can improve strength, they also result in the formation of high-resistivity interface phases (such as Li9Al4 and Li...). 13The presence of Si4 leads to a polarization voltage increase of >200mV, severely restricting practical applications. Therefore, developing novel alloy systems and adaptable rolling processes to achieve synergistic optimization of "high strength, ultra-thinness, and low interfacial impedance" has become a core challenge that urgently needs to be overcome for the industrialization of lithium metal batteries.

[0005] In addition, the varying dissolution rates at different locations on the ultrathin lithium anode can lead to localized pitting, which rapidly evolves into cracks during subsequent cycles. Without a current collector, when these cracks penetrate the electrode, they damage it and break it into several electrochemically inert parts, resulting in rapid battery failure. Compared to thicker lithium anodes, this failure mode is much more likely to occur on ultrathin lithium anodes, leading to a significant reduction in battery cycle life and a substantial increase in safety risks. Summary of the Invention

[0006] The purpose of this invention is to provide an ultrathin, low-density lithium alloy and its preparation method. For lithium strip preparation using a rolling process, the lithium metal billet passes through one or more pairs of rollers to gradually reduce the strip thickness until the desired thickness is achieved. This invention, through alloy composition design, significantly improves machinability, significantly reduces the alloy's viscosity on the rollers, avoids roller sticking problems during the rolling process, eliminates the need for various types of release films, reduces processing costs, and enables stable batch production. It also promotes uniform lithium dissolution and deposition; after complete lithium dissolution, a three-dimensional porous framework structure is formed, maintaining the integrity of the electrode.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0008] An ultra-thin lithium alloy strip for flight propulsion, wherein the main body of the lithium alloy strip is a uniformly mixed LiMgAg ternary alloy phase, without pure lithium phase.

[0009] Preferably, the lithium alloy strip has a thickness of 5μm-20μm, a tensile strength ≥10MPa, an elongation ≥18%, and a surface roughness Ra≤0.3μm.

[0010] A method for preparing an ultrathin lithium alloy strip for flight propulsion, comprising the following steps:

[0011] S1. Lithium ingots are mixed with alloying elements Mg powder and Ag powder in a mass ratio of Li:(Mg+Ag)= (0.3-0.5):(0.7-0.5), and heated to 300℃-500℃ to obtain a lithium alloy mixture, wherein the mass ratio of Mg to Ag is 1:1-1:2.5.

[0012] S2. The lithium alloy mixture is poured into a mold and cooled to obtain an alloy ingot;

[0013] S3. The alloy ingot is pressed into a lithium alloy strip with a thickness of 300μm by hydraulic extrusion.

[0014] S4. The thickness of the lithium alloy strip is gradually reduced by using a rolling process. The lithium alloy strip is passed through one or more pairs of rollers to gradually reduce the thickness of the strip. For alloy strips with a thickness greater than 100μm, the gap between the two rollers is adjusted to reduce the thickness by no more than 35% of the current thickness.

[0015] S5. For alloy strips with a thickness of 40μm-100μm, adjust the gap between the two rollers to reduce it by no more than 30% of the current thickness;

[0016] S6. For alloy strips with a thickness of 10μm-40μm, adjust the gap between the two rollers to reduce it by no more than 25% of the current thickness;

[0017] S7. For alloy strips with a thickness of less than 10μm, adjust the gap between the two rollers to reduce it by no more than 10% of the current thickness, and finally obtain an ultra-thin lithium alloy strip of the required thickness.

[0018] Preferably, 0.1wt%-0.5wt% of rare earth elements Yb or La can be added during the smelting process in step S1 to refine the grains and improve the oxidation resistance of the lithium alloy strip.

[0019] Preferably, during the rolling process, the roll surface temperature is controlled between 20℃ and 150℃, the temperature of the lithium alloy strip is maintained between 25℃ and 120℃, and selective cooling treatment is performed between adjacent passes.

[0020] Preferably, after rolling the lithium alloy strip to the required thickness, the surface of the strip is treated with N2 and CF4 plasma to form a dense composite passivation layer of 5nm-20nm.

[0021] A lithium metal battery includes a positive electrode, a separator, and an ultrathin lithium alloy strip prepared by the above method as a negative electrode.

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] 1. This solution achieves solid solution treatment, high strength, and ultra-thinness through alloy design, solving the problems of sticking to the roller and brittleness: through ternary alloy design, solid solution strengthening is achieved in a synergistic manner, improving the strength of the lithium strip and reducing the surface adhesion work (from 350mJ / m² of pure lithium to 210mJ / m²).

[0024] 2. This invention achieves release film-free rolling of ultra-thin lithium alloy strips through alloy design, enabling the preparation of lithium alloy strips ranging from 1.0μm to 20.0μm. This avoids problems such as roller sticking and breakage during the processing of ultra-thin lithium strips. In lithium metal batteries, it can directly replace copper foil of the same thickness for battery performance testing, with the alloy mass being only 10% of that of copper foil of the same thickness, and its cycle performance is significantly improved. Replacing pure lithium strip of the same thickness with it for battery performance testing also significantly improves cycle performance.

[0025] 3. The ultra-thin lithium alloy strip prepared by this invention can significantly improve the uniformity of lithium dissolution and deposition. On the one hand, it avoids the problem of electrode breakage during long battery cycles, and on the other hand, it significantly inhibits the growth of lithium dendrites, thereby significantly improving cycle life compared with pure lithium. Attached Figure Description

[0026] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the roller and lithium alloy strip in Embodiment 1 of the present invention;

[0028] Figure 2 This is a schematic diagram of the rollers and lithium belt in Comparative Example 2 of the present invention;

[0029] Figure 3 This is a schematic diagram showing the tensile strength test results of pure lithium strip and lithium alloy strip.

[0030] Figure 4 The images show the SEM and EDS images of Li-0.15Mg-0.35Ag.

[0031] Figure 5 This is a SEM image of lithium after extraction from Li-0.15Mg-0.35Ag.

[0032] Figure 6 The XRD pattern is for Li-0.15Mg-0.35Ag. Detailed Implementation

[0033] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0034] Example 1

[0035] A method for preparing an ultrathin lithium alloy strip for flight propulsion includes the following steps:

[0036] S1. Under an argon atmosphere, a certain mass of lithium ingots, Mg powder, and Ag powder are placed in a stainless steel crucible at a mass ratio of 0.5:0.15:0.35 and heated to 300℃-500℃ (480℃ in this embodiment, stirred for 30 minutes). Heating and stirring allow the above metal raw materials to react completely. Then, the alloy mixture is poured into a stainless steel mold and cooled to obtain the original lithium alloy ingot.

[0037] The lithium ingots are 99.99% pure, the Mg powder is 99.95% pure, and the Ag powder is 99.99% pure.

[0038] S2. A lithium alloy strip with a thickness of 300μm is prepared by extruding the lithium alloy ingot at 50℃ using a four-column hydraulic press (pressure 200 tons) at an extrusion speed of 2mm / s.

[0039] S3. For a 300μm lithium alloy strip, pass it sequentially through stainless steel rollers with gaps of 200μm, 150μm, and 100μm to obtain a lithium alloy strip with a thickness of approximately 100μm.

[0040] S4. For a 100μm lithium alloy strip, pass it sequentially through stainless steel rollers with gaps of 75μm, 60μm, 50μm, and 40μm to obtain a lithium alloy strip with a thickness of approximately 40μm.

[0041] S5. For a 40μm lithium alloy strip, pass it sequentially through stainless steel rollers with gaps of 32μm, 26μm, 21μm, 16μm, 12μm, and 10μm to obtain a lithium alloy strip with a thickness of approximately 10μm.

[0042] S6. A soft-pack battery is assembled by stacking NCM9 as the positive electrode, a porous PP membrane as the separator, and the alloy foil as the negative electrode. The battery is tested for cycle performance at 25°C, with a charging rate of 0.33C, a charging cut-off voltage of 4.6V, a discharging rate of 1.0C, and a discharging cut-off voltage of 3.0V.

[0043] Example 2

[0044] The difference between this embodiment and Embodiment 1 is that the final lithium strip thickness is 20 μm.

[0045] Example 3

[0046] The difference between this embodiment and Embodiment 1 is that the final lithium strip thickness is 6 μm.

[0047] Example 4

[0048] The difference between this embodiment and Embodiment 3 is that the lithium ingot, Mg powder, and Ag powder are in a mass ratio of 0.5:0.25:0.25.

[0049] Example 5

[0050] The difference between this embodiment and Embodiment 1 is that the gradient rolling steps S3, S4, and S5 are different, and temperature control is strictly enforced:

[0051] First stage (300μm→100μm): Roller surface temperature 25℃, single-pass thinning rate 35% (stainless steel roller gaps are: 180μm→126μm→89μm).

[0052] Second stage (100μm→40μm): Adjust the roller temperature to 25℃, single-pass thinning rate 25% (stainless steel roller gaps are: 67μm→51μm→39μm in sequence), nitrogen cooling for 5s between passes;

[0053] The third stage (40μm→10μm): the roll temperature is 25℃, the single-pass thinning rate is 15% (the stainless steel roll gaps are: 34μm→29μm→25μm→21μm→18μm→15μm→13μm→11μm→9.35μm in sequence), and the surface temperature of the strip after the last rolling pass is 38℃, resulting in a lithium alloy strip with a thickness of 9.35μm.

[0054] Example 6

[0055] The difference between this embodiment and embodiment 5 is that after the lithium alloy strip of the required thickness is obtained by gradient rolling, it is treated with radio frequency CF4 plasma (power 300W, gas pressure 50Pa, treatment time 60s).

[0056] Example 7

[0057] The difference between this embodiment and Embodiment 5 is that the alloy is adjusted as follows: Li:Mg:Ag = 0.3:0.2:0.5, and 0.1wt% Yb is added.

[0058] Example 8

[0059] The difference from Example 5 is that,

[0060] In the final rolling stage (20μm→6μm), a single-pass thinning rate of 8% was adopted (gap: 18.4μm→16.9μm→15.5μm→14.3μm→13.1μm→12.1μm→11.1μm→10.2μm→9.4μm→8.6μm→7.9μm→7.3μm→6.7μm→6.2μm); the rolling speed was reduced to 0.8m / min, and the roll temperature was kept constant at 25℃.

[0061] Comparative Example 1

[0062] The difference between this embodiment and Embodiment 1 is that the alloy liquid in step S1 contains only lithium, and the rolling steps in S3-S5 use a silicone oil-based release film as an isolation layer between the rollers and the lithium strip. All other steps remain the same, resulting in a 10μm thick pure lithium strip.

[0063] Comparative Example 2

[0064] The difference between this embodiment and Comparative Example 1 is that the alloy liquid in step S1 contains only lithium, and the rolling steps in S3-S5 do not use silicone oil-based release film. Other steps remain the same. Figure 2 As shown, the lithium strip adheres significantly to the stainless steel roller, making it impossible to form.

[0065] Comparative Example 3

[0066] The difference between this embodiment and Embodiment 1 is that the alloy liquid in step S1 contains only lithium, and the rolling steps in S3-S5 use a silicone oil-based release film as an isolation layer between the rollers and the lithium strip. All other steps remain the same, resulting in a 20μm thick pure lithium strip.

[0067] Comparative Example 4

[0068] In this embodiment, a 6μm copper foil is used as the negative electrode, NCM9 as the positive electrode, and a porous PP membrane as the separator to stack and assemble a soft-pack battery. The battery's cycle performance was tested at 25°C, with a charge rate of 0.33C, a charge cut-off voltage of 4.6V, a discharge rate of 1.0C, and a discharge cut-off voltage of 3.0V.

[0069] like Figure 3 As shown, the tensile strength of pure lithium strip is only around 1.3 MPa, while the tensile strength of the lithium alloy strip in Example 1 exceeds 10 MPa. In lithium metal batteries, when this alloy is used to directly replace copper foil of the same thickness for battery performance testing, the alloy mass is only 10% of that of copper foil of the same thickness, and its cycle performance is significantly improved. Replacing pure lithium strip of the same thickness with this alloy for battery performance testing significantly improves cycle performance.

[0070] Breakthrough in release film-free rolling technology: By solid solution strengthening with a Li-Mg-Ag alloy (Mg / Ag = 0.25 / 0.25 in Example 4), the viscosity of lithium is reduced, enabling silicone oil-free release film rolling and avoiding surface contamination (oxygen content <200ppm, compared to 800ppm in Comparative Example 1). Effective cost reduction: eliminating the release film reduces the processing cost of preparing 10μm lithium strips by 62% (Example 3 vs. Comparative Example 1).

[0071] Significantly improved mechanical properties: The synergistic effect of solid solution strengthening by Mg and dispersion strengthening by Ag enables the 20μm lithium alloy strip to achieve a tensile strength of 15MPa (compared to only 1.2MPa for pure lithium in Example 2), which can withstand winding tension (meeting the requirements for copper-free current collectors). The addition of rare earth elements refines the grain size to 2-5μm (compared to 10-20μm without addition), and increases the elongation to 22%.

[0072] Surface passivation and electrochemical stability: The Li3N passivation layer generated by argon plasma treatment improved the initial coulombic efficiency to 94.5% (compared to 85.2% for copper foil), and after 200 cycles, the lithium deposition / stripping overpotential increased by only 18 mV (compared to 112 mV for untreated). The composite passivation layer suppressed dendrite growth, and after 300 cycles at 4.6V high voltage, the anode thickness expansion rate was ≤5% (compared to 17% in Comparative Example 1).

[0073] Leap in energy density: The 10μm lithium alloy strip has a surface capacity of 4.8mAh / cm², and combined with a copper-free current collector design, the energy density of the entire battery is increased to 550Wh / kg (compared to 320Wh / kg for the traditional lithium copper anode system).

[0074] Serial Number Thickness / μm Density / gcm-3 Number of cycles Capacity retention rate / % Remark Comparative Example 1 Li 10 0.53 54 93.7 Short circuit Comparative Example 2 Li Unable to form / / / Unable to form Comparative Example 3 Li 20 0.53 97 97.3 Short circuit Comparative Example 4 Cu 6 8.96 6 79.6 Capacity decay Example 1 Li-0.15Mg-0.35Ag 10 0.90±0.02 153 90.1 Capacity decay Example 2 Li-0.15Mg-0.35Ag 20 0.90±0.02 267 90.0 Capacity decay Example 3 Li-0.15Mg-0.35Ag 6 0.90±0.02 98 90.3 Capacity decay Example 4 Li-0.25Mg-0.25Ag 10 0.90±0.02 75 90.2 Capacity decay Example 5 Li-0.15Mg-0.35Ag 9.35 0.87±0.02 167 90.0 Capacity decay Example 6 Li-0.15Mg-0.35Ag 10 0.87±0.02 177 90.1 Capacity decay Example 7 Li-0.2Mg-0.5Ag-0.1wt%Yb 10 1.37±0.02 193 89.9 Capacity decay Example 8 Li-0.15Mg-0.35Ag 6.2 0.87±0.02 163 90.1 Capacity decay

[0075] This application promotes the uniform dissolution and deposition of lithium through alloying. After complete lithium dissolution, a three-dimensional porous framework structure is formed, maintaining the integrity of the electrode. Through alloy composition design, this invention significantly improves machinability and reduces the alloy's viscosity on the rollers, avoiding roller sticking during the rolling process. It also eliminates the need for various types of release films, reducing processing costs and enabling stable batch production. The novel alloy system and compatible rolling process achieve synergistic optimization of "solution treatment - high strength - ultra-thinness". For lithium metal batteries, replacing pure lithium strips of the same thickness in battery performance testing significantly improves cycle performance. Through ternary alloy solution treatment, uniform lithium dissolution and deposition are achieved, maintaining the structural integrity of the electrode and avoiding electrode breakage and pulverization during battery cycling. Replacing copper foil of the same thickness directly in battery performance testing, the alloy mass is only 10% of that of copper foil of the same thickness, and its cycle performance is significantly improved.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An ultra-thin lithium alloy strip for flight propulsion, characterized in that, The lithium alloy strip is mainly composed of a uniformly mixed LiMgAg ternary alloy phase, with no pure lithium phase.

2. The ultra-thin lithium alloy strip for flight propulsion according to claim 1, characterized in that, The lithium alloy strip has a thickness of 5μm-20μm, a tensile strength ≥10MPa, an elongation ≥18%, and a surface roughness Ra≤0.3μm.

3. A method for preparing an ultrathin lithium alloy strip for flight propulsion, characterized in that, The method for preparing the ultrathin lithium alloy strip according to claim 1 or 2 includes the following steps: S1. Lithium ingots are mixed with alloying elements Mg powder and Ag powder in a mass ratio of Li:(Mg+Ag)= (0.3-0.5):(0.7-0.5), and heated to 300℃-500℃ to obtain a lithium alloy mixture, wherein the mass ratio of Mg to Ag is 1:1-1:2.

5. S2. The lithium alloy mixture is poured into a mold and cooled to obtain an alloy ingot; S3. The alloy ingot is pressed into a lithium alloy strip with a thickness of 300μm by hydraulic extrusion. S4. The thickness of the lithium alloy strip is gradually reduced by using a rolling process. The lithium alloy strip is passed through one or more pairs of rollers to gradually reduce the thickness of the strip. For alloy strips with a thickness greater than 100μm, the gap between the two rollers is adjusted to reduce the thickness by no more than 35% of the current thickness. S5. For alloy strips with a thickness of 40μm-100μm, adjust the gap between the two rollers to reduce it by no more than 30% of the current thickness; S6. For alloy strips with a thickness of 10μm-40μm, adjust the gap between the two rollers to reduce it by no more than 25% of the current thickness; S7. For alloy strips with a thickness of less than 10μm, adjust the gap between the two rollers to reduce it by no more than 10% of the current thickness, and finally obtain an ultra-thin lithium alloy strip of the required thickness.

4. The method for preparing an ultrathin lithium alloy strip for flight propulsion according to claim 3, characterized in that, During the smelting process in step S1, 0.1wt%-0.5wt% of rare earth elements Yb or La can be added to refine the grains and improve the oxidation resistance of the lithium alloy strip.

5. The method for preparing an ultrathin lithium alloy strip for flight propulsion according to claim 3, characterized in that, During the rolling process, the roll surface temperature is controlled between 20℃ and 150℃, the temperature of the lithium alloy strip is maintained between 25℃ and 120℃, and selective cooling treatment is performed between adjacent passes.

6. The method for preparing an ultrathin lithium alloy strip for flight propulsion according to claim 3, characterized in that, After rolling to obtain the required thickness of lithium alloy strip, the surface of the strip is treated with N2 and CF4 plasma to form a dense composite passivation layer of 5nm-20nm.

7. A lithium metal battery, characterized in that, The anode includes a positive electrode, a separator, and an ultrathin lithium alloy strip for flight propulsion prepared by the method described in claim 3.

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