A solid-state electrolyte covered with a zn i2 thin film, and a preparation method and application thereof

Abstract

The application discloses a solid-state electrolyte covered with a ZnI2 film and a preparation method and application thereof. After a compact LLZTO sheet is placed in a ZnI2 solution for a period of time and taken out, the surface of the LLZTO sheet is attached with ZnI2 crystal particles; when molten metal lithium is poured onto the surface of the LLZTO sheet with ZnI2, a reaction occurs to generate LiI and Li-Zn alloy, which is helpful to the infiltration of the metal lithium on the surface of the LLZTO sheet, thereby improving the interface problem between the solid-state electrolyte LLZTO and the lithium metal, greatly reducing the interface impedance, effectively inhibiting the generation of lithium dendrites, and improving the current density, surface capacity and cycle life of the solid-state battery.

Description

Technical Field

[0001] This invention relates to the field of electrochemical technology, specifically to a solid electrolyte covered with a ZnI2 thin film, its preparation method, and its application. Background Technology

[0002] With the booming development of the global economy, people around the world are increasingly aware of the importance of ecological and environmental protection. Coal, oil, and natural gas, widely used globally, not only cause environmental pollution and climate change but are also non-renewable. Therefore, new, clean, and renewable energy sources such as solar, wind, and ocean energy are being gradually developed. The continuous improvement of the energy structure has also driven research into safer and more reliable high-energy, high-power energy storage devices. Lithium-ion batteries, since their advent, have become the main energy storage solution for portable electronic products and electric vehicles. This is mainly due to their high operating voltage, high specific energy, long cycle life, and stable cycle performance. However, traditional lithium-ion batteries mostly use organic electrolytes, which may lead to serious safety accidents such as flammability, explosion, leakage, corrosion, and short circuits causing fires. Research has shown that replacing traditional organic liquid electrolytes with solid electrolytes is an effective way to solve these problems. In addition, solid electrolytes have better compatibility with lithium metal, which can further improve the energy density of lithium batteries. Currently, research on solid electrolytes mainly includes two aspects: one is organic-inorganic composite electrolyte membranes, which involves uniformly dispersing inorganic electrolyte powder in polymers to prepare flexible composite electrolyte membranes; the other is pure solid electrolytes, which involve sintering electrolyte powder into high-density, regularly shaped ceramic electrolyte sheets at high temperatures.

[0003] However, in the research of solid electrolytes, the poor wettability between pure solid electrolytes and lithium metal leads to a large interfacial impedance at the interface, hindering further development. For flexible composite electrolyte membranes, the interfacial problem is mainly due to the interaction between the inorganic electrolyte and the organic polymer within the composite membrane. The slow migration rate of lithium ions within the polymer results in low ionic conductivity at room temperature. For pure solid electrolytes, due to their dense internal structure, the grain boundary blocking effect is greatly reduced, and the interfacial problem mainly stems from poor contact between the electrolyte itself and the electrode, thus affecting battery performance. Currently, interface treatment typically involves two methods. One is modifying the lithium metal anode to improve its wettability with the solid electrolyte, thereby improving the interface. For example, the paper "Graphitic carbon nitride (g-C3N4): An interface enabler for solid-state lithium metal batteries" (Angew. Chem. Int. Ed. 59, 2020, 3699-3704) describes how adding g-C3N4 to lithium metal changes the interface between lithium metal and the solid electrolyte LLZTO from point contact to a close surface contact, significantly improving the suppression of lithium dendrite formation and reducing the interfacial impedance at 0.3 mA / cm². 2 It cycled stably for 300 hours at a current density. Another approach is to improve the surface of the solid electrolyte by adding a coating to allow for close contact between the lithium metal and the electrolyte. For example, the paper "Building a better li-garnet solid electrolyte / metallic li interface with antimony" (Adv. Energy Mater. 11, 2021, 2102086) introduces an Sb coating on the solid electrolyte surface, presenting a remarkable interface layer that leads to a reduction in the Li / LLZTO electrolyte interface resistance at 0.2 mA / cm². 2 The cells cycled for 200 hours at a given current density. While these approaches have yielded improvements at the lithium metal-solid electrolyte interface, further improvements are needed in current density and cycle life. Therefore, achieving better performance in solid-state batteries remains a significant challenge. Summary of the Invention

[0004] In view of the above-mentioned prior art, the purpose of this invention is to provide a solid electrolyte coated with a ZnI2 thin film, its preparation method, and its application. This invention uses a simple immersion method in solution to attach a layer of ZnI2 to the surface of an LLZTO solid electrolyte. The reaction between ZnI2 and lithium metal ensures close contact between the LLZTO solid electrolyte and the lithium metal.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A first aspect of the present invention provides the application of ZnI2 in reducing the interfacial impedance between a solid electrolyte and lithium metal, characterized in that the interfacial impedance between the solid electrolyte and lithium metal is improved by casting molten lithium metal onto the surface of a solid electrolyte covered with ZnI2.

[0007] Preferably, the solid electrolyte covered with a ZnI2 thin film is prepared by the following method:

[0008] After polishing the solid electrolyte, the solid electrolyte sheet is immersed in ZnI2 solution to obtain a solid electrolyte covered with a ZnI2 film.

[0009] Preferably, the ZnI2 solution is obtained by dissolving ZnI2 powder in a solvent; the solvent is anhydrous ethanol or deionized water.

[0010] Preferably, the concentration of the ZnI2 solution is 0.2-5 mol / L; and the soaking time is 0.1-120 min.

[0011] Preferably, the solid electrolyte is an LLZTO electrolyte; the thickness of the solid electrolyte is 0.01-1 mm.

[0012] A second aspect of the present invention provides the application of the solid electrolyte covered with the ZnI2 thin film in the preparation of solid-state lithium-ion batteries.

[0013] A third aspect of the present invention provides a solid-state lithium-ion battery, characterized in that the solid-state lithium-ion battery uses a solid electrolyte covered with a ZnI2 thin film as the electrolyte, and molten lithium metal is cast onto the surface to form a solid electrolyte-reaction layer-lithium metal interface.

[0014] Preferably, the temperature of the molten lithium is 200-500°C.

[0015] Preferably, the interfacial impedance value of the solid electrolyte-reaction layer-lithium metal interface is 1-200 Ωcm. -2 .

[0016] The beneficial effects of this invention are:

[0017] (1) The present invention uses a simple immersion solution method to treat the surface of solid electrolyte LLZTO. The preparation process is simple and does not damage the solid electrolyte itself. No further chemical treatment is required, making it more environmentally friendly.

[0018] (2) When applied to lithium iron phosphate all-solid-state batteries, the performance is significantly improved. Moreover, the method adopted in this invention can be well applied to the surface treatment of other solid electrolytes, and greatly improves the energy density and cycle stability of the battery.

[0019] (3) By covering the surface of the LLZTO electrolyte with a ZnI2 film, the interfacial impedance is greatly reduced and the formation of lithium dendrites is effectively suppressed, thereby improving the current density, areal capacity and cycle life of the solid-state battery. Attached Figure Description

[0020] Figure 1 XRD patterns of LLZTO and ZnI2@LLZTO at different concentrations in Example 1.

[0021] Figure 2 SEM images of LLZTO and ZnI2@LLZTO at different concentrations in Example 1.

[0022] Figure 3 SEM image of the interface between LLZTO and ZnI2@LLZTO and molten lithium metal in Example 1.

[0023] Figure 4 Schematic diagram of symmetrical battery assembly.

[0024] Figure 5 Example 1: 0.1mA cm -2 Polarization voltage curves of ZnI2@LLZTO symmetric cells at current density.

[0025] Figure 6 Example 1: 0.1mA cm -2 Polarization voltage curves of bare LLZTO symmetrical cells at current density.

[0026] Figure 7 Impedance spectrum of ZnI2@LLZTO symmetrical cell in Example 1.

[0027] Figure 8 Impedance spectrum of bare LLZTO symmetrical cell in Example 1.

[0028] Figure 9 In Example 2, 0.1 mA cm -2 Polarization voltage curves of ZnI2@LLZTO symmetric cells at current density.

[0029] Figure 10 In Example 3, 0.1mA cm -2 Polarization voltage curves of MgF2@LLZTO symmetric cells at current density.

[0030] Figure 11In Example 4, 0.2mA cm -2 Polarization voltage curves of ZnI2@LLZTO symmetric cells at current density.

[0031] Figure 12 Schematic diagram of a Li-ZnI2@LLZTO lithium iron phosphate full cell assembly.

[0032] Figure 13 Long-cycle performance of Li-ZnI2@LLZTO-lithium iron phosphate full cells at 0.5C. Detailed Implementation

[0033] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0034] As described in the background section, solid-state batteries suffer from severe interface problems. These interface problems lead to uneven charge distribution, inducing the formation and growth of lithium dendrites. Furthermore, during battery cycling, poor interfaces hinder lithium-ion transport, resulting in a significant decrease in cycle stability and lifespan.

[0035] Based on this, the purpose of this invention is to provide a method for solving the interface problem between solid electrolytes and lithium metal using zinc iodide. A ZnI2 layer is deposited on the surface of a solid electrolyte LLZTO using a simple immersion solution method. The prepared LLZTO with ZnI2-coated surface is then bonded to lithium metal at high temperature to obtain a tightly contacted LLZTO / Li interface. The thickness of the ZnI2 layer on the solid electrolyte surface can be controlled by adjusting the concentration of the ZnI2 solution. ZnI2 reacts with lithium metal to form LiI and a lithium-zinc alloy. The resulting mixed electronic / ionic conductive layer facilitates the wetting of lithium metal on the LLZTO surface, thereby improving the interface problem between the solid electrolyte LLZTO and lithium metal, significantly reducing the interface impedance, effectively suppressing the formation of lithium dendrites, and improving the current density, areal capacity, and cycle life of the solid-state battery.

[0036] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0037] The test materials used in the embodiments of this invention are all conventional test materials in the art and can be purchased through commercial channels.

[0038] Example:

[0039] (1) Bare solid electrolyte LLZTO sheets were polished and stored in an argon-filled glove box. 1.0 g of ZnI₂ was added to 15 mL of anhydrous ethanol to prepare a 0.2 mol / L ZnI₂ solution. The polished LLZTO sheets were immersed in the ZnI₂ solution for 2 minutes to obtain LLZTO sheets with ZnI₂ adhering to their surface. XRD analysis was performed on these sheets, and the results are as follows: Figure 1 As shown, the XRD diffraction peaks remained unchanged, indicating that liquid-phase deposition did not damage the physical properties of LLZTO. Figure 2 As shown, the surface of LLZTO sheets underwent significant changes after immersion in ZnI2 solutions of different concentrations. Previously porous, the surface became covered with a villous, fibrous material. Figure 3 It can be observed that the scanning images of the interface between the LLZTO sheet with or without ZnI2 and the molten lithium metal show that the interface, which originally had gaps, became tightly connected.

[0040] (2) In a glove box filled with argon gas, using the ZnI2-coated LLZTO sheet prepared in step (1) as the electrolyte, and with lithium metal at both ends, a symmetrical battery was assembled. Figure 4 ). At 0.1mA cm -2 The current density is 0.1 mAh cm⁻¹. -2 A constant current long-cycle test of symmetrical cells was conducted using a Blue Battery testing system at the given areal capacity. For example... Figure 5 As shown, at 0.1 mA cm -2 At the specified current density, the polarization voltage is 30mV, and it can cycle for 500 hours. Meanwhile... Figure 6 In the study, a bare LLZTO-assembled symmetrical cell short-circuited and failed after 50 hours of cycling under the same conditions, with a polarization voltage as high as 380mV.

[0041] (3) Impedance tests were performed on an AUYOLAB machine by assembling Li / LLZTO / Li and Li / ZnI2@LLZTO / Li symmetrical cells. The results showed that the Li / ZnI2@LLZTO / Li symmetrical cell exhibited a smaller semicircle, indicating a lower interface impedance. Subsequently, an equivalent circuit fitting method was used to fit the impedance curve. Considering that there are two interface contacts in a single cell, the result should be divided by 2. The final result is as follows: Figure 7 As shown, the interfacial impedance of the assembled symmetrical battery is 11 Ωcm. -2 The Li / LLZTO / Li symmetric cell exhibits a larger semicircle ( Figure 8 Its interface impedance is relatively large, specifically 1135 Ωcm. -2 .

[0042] Example 2

[0043] The difference from Example 1 is that the current density controlled in step (2) of this example is 0.2 mA cm⁻¹. -2 The surface capacity is 0.2mAh cm⁻¹ -2 .like Figure 11 As shown, at 0.2mA cm -2 At current density, the polarization voltage reaches 67mV, and it can be stably cycled for 500 hours. The ZnI2 capping layer enables LLZTO to maintain good performance even at higher current densities.

[0044] Comparative Example 1

[0045] The difference from the example is that the concentration of the ZnI2 solution used in step (1) is 0.1 mol / L. After assembling the symmetric cell, its long-cycle performance is tested, such as... Figure 9 As shown, the polarization voltage reached 87 mV, and the cycle time was 200 hours. Reducing the concentration of the ZnI2 solution may result in a thinner ZnI2 film covering the surface, failing to improve the interface.

[0046] Comparative Example 2

[0047] The difference from Example 1 is that in step (1), the ZnI2 solution was replaced with a MgF2 solution of the same concentration. After testing the symmetric cell under the same conditions, its long-cycle performance is as follows: Figure 10 As shown. After 100 hours of cycling, the battery short-circuited. Experiments have shown that using MgF2 can also provide some improvement at the interface, but the effect is generally limited.

[0048] Comparative Example 3

[0049] The difference from Example 1 is that in step (1), the ZnI2 solution was replaced with a ZnCl2 solution of the same concentration. After testing the symmetric cell under the same conditions, its long-cycle performance was poor: the cell short-circuited after 42 hours of cycling. This indicates that the use of ZnCl2 solution does not significantly improve the interface.

[0050] Application examples

[0051] Using LLZTO prepared in Example 1 as the electrolyte, lithium metal as the negative electrode, and lithium iron phosphate as the positive electrode, a coin cell was assembled. Figure 12 A long-cycle constant-current experiment was conducted on a full-cell battery. (For example...) Figure 13 As shown, due to the presence of the ZnI2 layer, the full cell maintains a capacity of 152.7 mAh g⁻¹ after 200 hours of stable cycling at 0.5C. -1 It also has a high coulomb efficiency of 99.87%.

[0052] This invention addresses the interface problem between solid electrolytes and lithium metal by coating the electrolyte surface with a ZnI2 thin film. The ZnI2 film acts as a buffer layer, and the ZnI2 reacts with lithium metal to generate LiI and a lithium-zinc alloy. The resulting lithium-loving alloy facilitates the wetting of lithium metal on the LLZTO surface, thereby improving the interface problem between the solid electrolyte LLZTO and lithium metal. This significantly reduces the interface impedance and effectively suppresses the formation of lithium dendrites, thus improving the current density, areal capacity, and cycle life of the solid-state battery.

[0053] A lower polarization voltage results in a larger specific capacity and better cycle and rate performance. Compared to the bare LLZTO electrolyte, which has a polarization voltage of 380mV, the polarization voltage of the ZnI2-covered electrolyte is significantly lower.

[0054] The ZnI2 thin film used in this invention overcomes the disadvantage of poor interface contact between solid electrolyte and lithium metal. The solid electrolyte covered with the ZnI2 layer has significantly better performance than the solid electrolyte without any treatment.

[0055] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. The application of ZnI2 in reducing the interfacial impedance between solid electrolytes and lithium metal, characterized in that, The application includes the following steps: (1) Polish the LLZTO solid electrolyte sheet with a thickness of 0.01-1 mm and place it in an argon atmosphere; (2) Dissolve ZnI2 powder in anhydrous ethanol to prepare a ZnI2 solution with a concentration of 0.2 mol / L. Immerse the polished LLZTO solid electrolyte sheet in the ZnI2 solution for 2 min to attach a ZnI2 layer to the surface of the LLZTO solid electrolyte sheet, and obtain an LLZTO solid electrolyte covered with a ZnI2 film. (3) Molten lithium metal at a temperature of 200-500℃ is poured onto the surface of an LLZTO solid electrolyte covered with a ZnI2 film, so that ZnI2 reacts in situ with the molten lithium metal on the surface of the LLZTO solid electrolyte to form a LiI and Li-Zn alloy reaction layer, thus forming an LLZTO solid electrolyte-reaction layer-lithium metal interface. The interfacial impedance value of the interface is 11 Ω·cm. 2 .

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