Surface-modified solid electrolytes, processes for preparing them, and their use in electrochemical cells

Abstract

The present technology relates to a process for producing a coated solid electrolyte comprising a metal-based coating layer deposited on at least a part of the surface of a solid electrolyte, the process comprising (i) depositing a precursor powder of a metal-based coating material on at least a part of the surface of the solid electrolyte; (ii) subjecting the precursor powder of the metal-based coating material to a rapid heating method to produce a molten metal-based coating material; and (iii) solidifying the molten metal-based coating material to produce the coated solid electrolyte. Also described are the coated solid electrolyte obtained by the process, as well as an electrochemical cell and a battery comprising the coated solid electrolyte. For example, the battery can be a lithium battery or a lithium-ion battery.

Description

Technical Field

[0001] Related Applications This application claims priority under the applicable law to U.S. Provisional Patent Application No. 63 / 368,165, filed on July 12, 2022, the content of which is hereby incorporated by reference in its entirety for all purposes.

[0002] Technical Field This application relates to the field of solid electrolytes and their use in electrochemical applications. More specifically, this application relates to solid electrolytes having at least one modified surface, their manufacturing processes, and their use in electrochemical cells and batteries, particularly all-solid-state batteries.

Background Art

[0003] Background As the demand for renewable energy is increasing, the development of high-performance energy storage devices is required. Lithium-ion batteries (LIBs) are the main energy storage devices for portable electronic devices and have dominated the electric vehicle market. However, current LIBs with liquid electrolytes and graphite anodes have reached the limit of their theoretical energy density (Choi, Jang Wook, and Doron Aurbach. "Promise and reality of post-lithium-ion batteries with high energy densities." Nature Reviews Materials 1, no. 4 (2016): 1-16). One of the most promising strategies to further improve the energy density of LIBs is to replace the graphite anode with lithium metal, which is widely recognized as the "holy grail" of battery research and can increase the capacity of the anode by a factor of 10 through the host-free lithium storage mechanism of the lithium metal anode.

[0004] The main problem currently faced by lithium metal anodes is the unstable interface with liquid electrolytes, which causes dendritic lithium growth and ultimately leads to battery short - circuit (Lin, Dingchang, et al. "Reviving the lithium metal anode for high - energy batteries." Nature nanotechnology 12, no. 3 (2017): 194 - 206). Using solid electrolytes is widely recognized as one of the most promising solutions to suppress dendritic growth because they have higher mechanical strength compared to conventional liquid electrolytes. All - solid - state lithium - metal batteries (ASSLMBs) with solid electrolytes and lithium - metal anodes potentially have much higher energy than conventional LIBs (Tikekar, Mukul D., et al. "Design principles for electrolytes and interfaces for stable lithium - metal batteries." Nature Energy 1, no. 9 (2016): 1 - 7).

[0005] However, the interface between the solid electrolyte and the lithium metal anode is very different from that of the liquid electrolyte (Liu, Bin, et al. "Advancing lithium metal batteries." Joule 2.5 (2018): 833-845). For example, many solid electrolytes, including sulfide-based electrolytes (Lau, Jonathan, et al. "Sulfide solid-state electrolytes for lithium battery applications." Advanced Energy Materials 8.27 (2018): 1800933), argyrodite-based electrolytes (Yu, Chuang, et al. "Recent development of lithium argyrodite solid-state electrolytes for solid-state batteries: synthesis, structure, stability and dynamics." Nano Energy 83 (2021): 105858), and halide-based electrolytes (Li, Xiaona, et al. "Progress and perspectives on halide lithium conductors for all-solid-state lithium batteries." Energy & Environmental Science 13.5 (2020): 1429-1461), have been shown to be electrochemically unstable against lithium reduction. When the solid electrolyte comes into contact with the lithium metal anode, a solid electrolyte interface (SEI) layer is formed, which increases the interfacial resistance and the likelihood of dendrite formation.Among all solid electrolytes, garnet-type solid electrolytes have a wide electrochemical stability window and are among the few that are stable at the electrochemical potential of lithium metal, making them excellent candidates for use in ASSLMBs (Thangadurai, Venkataraman et al. "Garnet-type solid-state fast Li ion conductors for Li batteries: critical review." Chemical Society Reviews 43.13 (2014): 4714-4727).

[0006] Despite the theoretical stability of garnet-type electrolytes paired with lithium metal anodes, the interfacial contact between these two components is actually extremely poor due to Li2CO3 surface contamination and the inherent lithiophobic properties of the garnet-type electrolyte. When a garnet-type solid electrolyte is combined with a lithium metal anode, the interfacial resistance is 1000 Ω cm -2It has been widely reported that it may exceed (Wang, Chengwei, et al. "Garnet-type solid-state electrolytes: materials, interfaces, and batteries." Chemical reviews 120.10 (2020): 4257-4300; Zhao, Ning, et al. "Solid garnet batteries." Joule 3.5 (2019): 1190-1199; and Krauskopf, Thorben, et al. "Lithium-metal growth kinetics on LLZO garnet-type solid-state electrolytes." Joule 3.8 (2019): 2030-2049). Large interfacial resistance causes severe voltage polarization during lithium plating / stripping cycles, which leads to the formation of dendritic lithium. Eventually, dendritic lithium penetrates the grain boundaries of the electrolyte, causing internal short circuits and battery failure (Porz, Lukas, et al. "Mechanism of lithium metal penetration through inorganic solid-state electrolytes." Advanced Energy Materials 7.20 (2017): 1701003; and Ning, Ziyang, et al. "Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells." Nature Materials 20.8 (2021): 1121-1129). To address the issue of interfacial contact, various interfacial coating layers have been developed in the past few years, but it remains a challenge to completely eliminate such interfacial resistance with a simple, cost-effective, and scalable technique. For example, an interfacial coating layer made of germanium reduces the interfacial resistance to 115 Ω cm -2can be reduced to (Luo, W., et al. "Reducing Interfacial Resistance between Garnet-Structured Solid-State Electrolyte and Li-Metal Anode by a Germanium Layer." Adv. Mater 29 (2017): 1606042). In the chemical treatment method using ammonium fluoride, surface contamination is removed, a LiF coating layer is formed, and the interfacial resistance is reduced to 38.7 Ω cm -2 (Duan, Hui, et al. "Building an air stable and lithium deposition regulable garnet interface from moderate-temperature conversion chemistry." Angewandte Chemie 132, no. 29 (2020): 12167-12173). The graphite-based interfacial coating layer has also been shown to reduce the interfacial resistance to 105 Ω cm -2 (Shao, Yuanjun, et al. "Drawing a soft interface: an effective interfacial modification strategy for garnet-type solid-state Li batteries." ACS Energy Letters 3.6 (2018): 1212-1218).

[0007] The conversion reaction between the intermediate layer MoS2 and lithium metal promotes the wetting of the interface, and the interfacial resistance of the garnet electrolyte containing lithium is 14 Ω cm -2has been shown to be reduced (Fu, Jiamin, et al. "In situ formation of a bifunctional interlayer enabled by a conversion reaction to initiatively prevent lithium dendrites in a garnet solid-state electrolyte." Energy & Environmental Science 12.4 (2019): 1404-1412). To date, the best-performing interfacial coating with an interfacial resistance of 1 Ω cm -2 has been achieved by a thin Al2O3 layer deposited on the surface of the garnet electrolyte by atomic layer deposition (Han, Xiaogang, et al. "Negating interfacial impedance in garnet-based solid-state Li metal batteries." Nature materials 16.5 (2017): 572-579). However, there is still a need to develop new coating materials to protect the interface between the solid electrolyte and the negative electrode, especially those that are superior to conventional coating materials.

Prior Art Documents

Non-Patent Documents

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Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 8

Non-Patent Document 9

Non-Patent Document 10

Non-Patent Document 11

Non-Patent Document 12

Non-Patent Document 18

Summary of the Invention

Means for Solving the Problems

[0009] Overview According to one aspect, the present technology is a process for producing a coated solid electrolyte including a metal-based coating layer deposited on at least a part of the surface of a solid electrolyte, the process comprising the following steps: (i) depositing a precursor powder of a metal-based coating material on at least a part of the surface of the solid electrolyte; (ii) subjecting the precursor powder of the metal-based coating material to a rapid heating method to produce a molten metal-based coating material; and (iii) solidifying the molten metal-based coating material to produce a coated solid electrolyte relates to a process.

[0010] In one embodiment, step (i) is carried out by a mechanical coating process or a chemical coating process. In an interesting embodiment, step (i) is carried out by a powder deposition technique. In a preferred embodiment, the powder deposition technique is a powder spraying technique, a powder rubbing technique, or a powder dipping technique.

[0011] In another embodiment, the process further includes a step of removing an excess amount of the precursor powder of the metal-based coating material before step (ii).

[0012] In another embodiment, the rapid heating method is selected from a Joule heating method, a microwave radiation method, a discharge plasma sintering method, an induction heating method, a laser sintering method, an infrared radiation method, and an electric pulse solidification method. In an interesting embodiment, the rapid heating method is a Joule heating method.

[0013] In another embodiment, the rapid heating method is carried out for a period of less than about 90 seconds, or less than about 80 seconds, or less than about 70 seconds, or less than about 60 seconds, or less than about 50 seconds, or less than about 40 seconds, or less than about 30 seconds, or less than about 25 seconds, or less than about 20 seconds, or less than about 15 seconds, or less than about 10 seconds.

[0014] In another embodiment, the rapid heating method is carried out for a period in the range of about 1 second to about 90 seconds, or about 1 second to about 80 seconds, or about 1 second to about 70 seconds, or about 1 second to about 60 seconds, or about 1 second to about 50 seconds, or about 1 second to about 40 seconds, or about 1 second to about 30 seconds, or about 1 second to about 25 seconds, or about 1 second to about 20 seconds, or about 1 second to about 15 seconds, or about 1 second to about 10 seconds, or about 2 seconds to about 10 seconds, or about 3 seconds to about 10 seconds.

[0015] In another embodiment, the rapid heating method is carried out at a temperature in the range of about 550 °C to about 1400 °C, or about 600 °C to about 1350 °C, or about 650 °C to about 1300 °C, or about 700 °C to about 1250 °C, or about 700 °C to about 1200 °C.

[0016] In another embodiment, the rapid heating method is about 5×10 2°C / min -1 ~ about 1.44×10 4 °C / min -1 is carried out at a heating temperature gradient rate within the range of. In an interesting embodiment, the rapid heating method is about 3×10 3 °C / min -1 of the heating temperature gradient rate.

[0017] In another embodiment, step (iii) is about 5×10 2 °C / min -1 ~ about 4.8×10 3 °C / min -1 of the cooling temperature gradient rate. In an interesting embodiment, step (iii) is about 3×10 3 °C / min -1 of the cooling temperature gradient rate.

[0018] In another embodiment, the above process further includes a step of preparing a solid electrolyte.

[0019] In another embodiment, the above process further includes a step of densifying the solid electrolyte. In an interesting embodiment, the densification step is carried out by a rapid heating method. In a preferred embodiment, the rapid heating method is selected from the group consisting of Joule heating method, microwave radiation method, discharge plasma sintering method, induction heating method, laser sintering method, infrared radiation method, and electric pulse solidification method. In a more preferred embodiment, the rapid heating method is the Joule heating method.

[0020] According to another aspect, the present technology relates to a coated solid electrolyte obtained by the process defined herein.

[0021] In one embodiment, the metal-based coating layer is uniformly deposited on the surface of the solid electrolyte. In an alternative embodiment, the metal-based coating layer is heterogeneously dispersed on the surface of the solid electrolyte.

[0022] In another embodiment, the metal-based coating material is selected from the group consisting of metal elements, metal alloys, metal oxides, fluorinated metals, and combinations of at least two of these.

[0023] In another embodiment, the metal-based coating material is a metal element. In an interesting embodiment, the metal element is selected from the group consisting of Al, Cu, Ag, Sn, Sb, and Bi. In a preferred embodiment, the metal element is Cu, Ag, or Sn.

[0024] In another embodiment, the metal-based coating material is a metal alloy. For example, the metal alloy includes a first metal component selected from the metal elements of Group 14 and Group 15 of the periodic table and a second metal component, and the second metal component is different from the first metal component. In an interesting embodiment, the first metal component is selected from Sn, Sb, and Bi. In another interesting embodiment, the second metal component is an alkali metal, an alkaline earth metal, a transition metal, a post-transition metal, a metalloid, or a lanthanide. For example, the second metal component is selected from the group consisting of Al, Mn, Co, Ni, Cu, Ag, Sn, Sb, La, Tb, and Bi. In a preferred embodiment, the metal alloy is an Sn-Mn, Sn-Co, Sn-Ni, Sn-Cu, Sn-Cu-Tb, Sn-Ag, Sn-La, Sn-Bi-Ag, Sb-Cu, Sb-Ag, or Bi-Ag based alloy. In another preferred embodiment, the metal alloy is Cu3Sn or Cu6Sn5. In another preferred embodiment, the metal alloy is AgSn x Bi 1-x where x is 0 ≦ x ≦ 1. In a more preferred embodiment, the metal alloy is AgSn, AgSn 0.8 Bi 0.2 , AgSn 0.6 Bi 0.4 , AgSn 0.4 Bi 0.6 and is selected from the group consisting of AgBi.

[0025] In another embodiment, the metal-based coating material is a fluorinated metal. In an interesting embodiment, the fluorinated metal is selected from the group consisting of SnF2, SnF4, ZnF2, InF3, GaF3, SbF3, TlF, PbF2, CuF2, BiF3, AlF3, AgF, and LiF.

[0026] In another embodiment, the metal-based coating material is a metal oxide. In an interesting embodiment, the metal oxide is selected from the group consisting of SnO, SnO2, CuO, Cu2O, Bi2O3, Al2O3, and Ag2O.

[0027] In another embodiment, the solid electrolyte is a ceramic solid electrolyte. In an interesting embodiment, the ceramic solid electrolyte is a garnet-type solid electrolyte. In a preferred embodiment, the garnet-type solid electrolyte is Li7La3Zr2O 12 (LLZO), Li 6.25 Al 0.25 La3Zr2O 12 (Al-LLZO), Li 6.5 La3Zr 1.5 Ta 0.5 O 12 (LLZTO), Li 6.35 Al 0.05 La3Zr2Ta 0.5 O 12 (Al-LLZTO), Li 6.25 Nd3Zr 1.5 Ta 0.5 O 12 (LNZTO), Li 6.25 Sm3Zr 1.5 Ta 0.5 O 12 (LSZTO), and Li 6.25 (Sm 0.5 La 0.5 )3Zr 1.5 Ta 0.5 O 12 (LSZTO) selected from the group consisting of. In a more preferred embodiment, the garnet-type solid electrolyte is Li7La3Zr2O 12 (LLZO), Li 6.25 Al 0.25 La3Zr2O 12 (Al-LLZO), Li 6.5 La3Zr 1.5 Ta 0.5 O 12 (LLZTO), and Li 6.35 Al 0.05 La3Zr2Ta 0.5 O 12It is selected from the group consisting of (Al-LLZTO).

[0028] In another embodiment, the coated solid electrolyte further comprises at least one additional component. In an interesting embodiment, the additional component is selected from the group consisting of ion conductors, inorganic particles, glass or ceramic particles, nanoceramics, salts, and other similar additives.

[0029] In another embodiment, the coated solid electrolyte further comprises a second coating material deposited on at least a portion of the surface opposite to the solid electrolyte. In an interesting embodiment, the second coating material is a succinonitrile-based coating material. For example, the succinonitrile-based coating material contains a lithium salt.

[0030] According to another aspect, the present technology relates to an electrochemical cell comprising a negative electrode, a positive electrode, and a coated solid electrolyte as defined herein.

[0031] In one embodiment, the metal-based coating layer of the coated solid electrolyte faces the negative electrode.

[0032] In another embodiment, when present, the second coating material of the coated solid electrolyte faces the positive electrode.

[0033] In another embodiment, the negative electrode comprises an electrochemical active material comprising an alkali metal, an alkaline earth metal, an alloy comprising at least one alkali metal or alkaline earth metal, a non-alkali and non-alkaline earth metal, or an alloy or intermetallic compound. In an interesting embodiment, the electrochemical active material of the negative electrode comprises lithium metal or an alloy thereof.

[0034] In another embodiment, the positive electrode contains an electrochemically active material. In an interesting embodiment, the electrochemically active material of the positive electrode is selected from the group consisting of metal oxides, lithium metal oxides, metal phosphates, lithium metal phosphates, titanates, lithium titanates, metal fluorophosphates, lithium metal fluorophosphates, metal oxyfluorophosphates, lithium metal oxyfluorophosphates, metal sulfates, lithium metal sulfates, metal halides (e.g., fluorides), lithium metal halides (e.g., fluorides), sulfur, lithium sulfur, selenium, lithium selenium, and combinations of at least two of these, or includes them. For example, the metal of the electrochemically active material is selected from the group consisting of titanium (Ti), iron (Fe), magnesium (Mg), manganese (Mn), vanadium (V), nickel (Ni), cobalt (Co), aluminum (Al), zirconium (Zr), zinc (Zn), niobium (Nb), and combinations of at least two of these.

[0035] In another embodiment, the positive electrode further contains at least one electron conductive material. In an interesting embodiment, the electron conductive material is selected from the group consisting of carbon black, acetylene black, graphite, graphene, carbon fiber, carbon nanofiber, carbon nanotube, and combinations of at least two of these.

[0036] In another embodiment, the positive electrode further contains at least one binder. In an interesting embodiment, the binder is selected from the group consisting of polyether type polymer binders, fluorinated polymers, and water-soluble binders.

[0037] In another embodiment, the positive electrode further contains at least one additional component. In an interesting embodiment, the additional component is selected from the group consisting of ion conductors, inorganic particles, glass or ceramic particles, nanoceramics, salts, and other similar additives.

[0038] According to another aspect, the present technology relates to a battery including at least one electrochemical cell as defined herein.

[0039] In one embodiment, the battery is selected from the group consisting of a lithium battery, a lithium ion battery, a sodium battery, a sodium ion battery, a potassium battery, a potassium ion battery, a magnesium battery, and a magnesium ion battery. In an interesting embodiment, the battery is selected from the group consisting of a lithium battery or a lithium ion battery.

Brief Description of the Drawings

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DETAILED DESCRIPTION OF THE INVENTION

[0083] Detailed Description The following detailed description and examples are illustrative and should not be construed as further limiting the scope of the invention. Rather, it is intended to cover all alternatives, modifications, and equivalents that can be included as defined in this description. The objects, advantages, and other features of this solid electrolyte, system, method, and their uses will become more apparent and better understood by reading the following non-limiting description and referring to the accompanying drawings.

[0084] All technical and scientific terms and expressions used in this specification have the same definition as those commonly understood by those skilled in the art when related to this technology. Nevertheless, for clarity, the definitions of some terms and expressions used in this specification are provided below.

[0085] When the term "about" is used in this specification, it means approximately, within or around that range. When the term "about" is used in relation to a numerical value, it changes the numerical value, for example, within a variation range of plus or minus 10% of its nominal value. This term can also take into account the rounding of numerical values or the possibility of random errors in experimental measurement values due to, for example, equipment limitations.

[0086] When a range of values is recited in this specification, unless otherwise specified, the lower and upper limits of that range are always included in the definition. When a range of values is recited in this application, all intermediate ranges and sub-ranges, as well as the individual values included in that range, are intended to be included.

[0087] Throughout the following description, when the article "a" is used to introduce an element, it is worth noting that it does not mean "only one", but rather "one or more". When the specification describes that a step, component, feature, or property "may", "might", "can", or "could" be included, it should be understood that the particular component, feature, or property need not be included in all options.

[0088] This application describes solid electrolytes, their production methods and systems, and their use in electrochemical cells and batteries, such as all-solid-state metal batteries.

[0089] This technology relates to a process for producing a coated solid electrolyte comprising a metal-based coating layer deposited on at least a part of the surface of a solid electrolyte. More specifically, this process comprises the following steps: (i) Depositing a precursor powder of a metal-based coating material on at least a part of the surface of the solid electrolyte; (ii) Subjecting the precursor powder of the metal-based coating material to a rapid heating method to produce a molten metal-based coating material; and (iii) Solidifying the molten metal-based coating material to produce a coated solid electrolyte.

[0090] It should be understood that the processes described herein rely on heat treatment techniques, rapid sintering techniques, or melt quenching techniques. The term "rapid heating method" should be understood to refer to the entire heat treatment process that can include, for example, heating, dwell, and cooling steps.

[0091] According to one example, the step of depositing a precursor powder of a metal-based coating material on at least a part of the surface of a solid electrolyte can be carried out by any suitable method. The deposition step can be carried out by a mechanical coating process or a chemical coating process.

[0092] For example, the deposition step can be carried out by a powder deposition technique. The powder deposition technique can be, for example, a powder spraying technique, a powder rubbing technique, or a powder dipping technique. However, various other methods can be used to apply the precursor powder of the metal-based coating material to the surface of the solid electrolyte.

[0093] According to another example, the precursor powder of the metal-based coating material can adhere to the surface of the solid electrolyte by an attractive force such as van der Waals force.

[0094] According to another example, the above process may further include, if necessary, a step of removing an excess amount of the precursor powder of the metal-based coating material before the step of subjecting the precursor powder of the metal-based coating material to a rapid heating method. The step of removing the excess amount of the precursor powder of the metal-based coating material can be carried out by any suitable method. For example, a compressed gas can be used to simply blow away the excess precursor powder of the metal-based coating material.

[0095] According to another example, the rapid heating method can be carried out by any suitable method. For example, the rapid heating method can be selected from the Joule heating method, the microwave radiation method, the discharge plasma sintering method, the induction heating method, the laser sintering method, the infrared radiation method, and the electric pulse solidification method. According to an interesting example, the rapid heating method is the Joule heating method (also known as the resistive heating method, the resistance heating method, or the ohmic heating method).

[0096] It should be understood that the precursor powder of the metal-based coating material is subjected to the rapid heating method at a temperature, temperature gradient rate, and for a period sufficient to melt at least one component of the precursor powder of the metal-based coating material.

[0097] According to another example, the rapid heating method can be carried out for a period of less than about 90 seconds. For example, the rapid heating method can be carried out for a period of less than about 80 seconds, or less than about 70 seconds, or less than about 60 seconds, or less than about 50 seconds, or less than about 40 seconds, or less than about 30 seconds, or less than about 25 seconds, or less than about 20 seconds, or less than about 15 seconds, or less than about 10 seconds. Alternatively, the rapid heating method can be carried out for a period in the range of about 1 second to about 90 seconds, including the limit value. For example, the rapid heating method can be carried out for a period in the range of about 1 second to about 80 seconds, or about 1 second to about 70 seconds, or about 1 second to about 60 seconds, or about 1 second to about 50 seconds, or about 1 second to about 40 seconds, or about 1 second to about 30 seconds, or about 1 second to about 25 seconds, or about 1 second to about 20 seconds, or about 1 second to about 15 seconds, or about 1 second to about 10 seconds, or about 2 seconds to about 10 seconds, or about 3 seconds to about 10 seconds, including the limit value. According to an interesting example, the rapid heating method can be carried out for a period of about 3 seconds.

[0098] According to another example, the rapid heating method can be carried out at a temperature in the range of about 550°C to about 1400°C, including the limit value. For example, the rapid heating method can be carried out at a temperature in the range of about 600°C to about 1350°C, or about 650°C to about 1300°C, or about 700°C to about 1250°C, or about 700°C to about 1200°C, including the limit value.

[0099] According to another example, the rapid heating method can be carried out at a heating temperature gradient rate of about 5×10 2 °C / min -1 ~ about 1.44×10 4 °C / min -1 For example, the rapid heating method can be carried out at a heating temperature gradient rate of about 3×10 3 °C / min -1 Alternatively, the rapid heating method can be isothermal and can be carried out at a constant heating temperature. Alternatively, the rapid heating method can have a substantially short initial heating gradient, for example, it can be heated from the ambient temperature to the final temperature in a short time of about 0 seconds. Alternatively, the rapid heating method can include at least one heating temperature gradient and at least one isothermal heating cycle.

[0100] According to another example, the solidification step can be carried out by any suitable method. According to an interesting example, the solidification step can be an ultra-rapid cooling step or a rapid cooling step for forming a substantially uniform metal-based coating layer on the surface of the solid electrolyte. For example, the solidification step can be carried out at a cooling temperature gradient rate in the range of about 5×10 2 °C / min -1 ~ about 4.8×10 3 °C / min -1 For example, the solidification step can be carried out at a cooling temperature gradient rate of about 3×10 3 °C / min -1 Alternatively, the solidification step can be isothermal and can be carried out at a constant cooling temperature. Alternatively, the solidification step can have a substantially short initial cooling temperature gradient, for example, it can be cooled from the first temperature to the ambient temperature in a short time of about 0 seconds. Alternatively, the solidification step can include at least one cooling temperature gradient and at least one isothermal cooling cycle.

[0101] According to another example, the above process further optionally includes a step of preparing a solid electrolyte before step (i). Any suitable method for preparing the solid electrolyte is contemplated. In some examples, the solid electrolyte is a garnet-type solid electrolyte and can be obtained by conventional solid synthesis or rapid sintering techniques.

[0102] For example, a solid electrolyte powder precursor can be weighed to obtain the desired solid electrolyte. Next, the raw material powder can be ball-milled, for example, at a speed of about 300 rpm for about 10 hours to be substantially uniformly mixed. After mixing, the raw material powder can be compressed into pellets and annealed, for example, by a rapid heating method or at a temperature of about 900 °C in a muffle furnace for about 12 hours.

[0103] According to another example, the above process further optionally includes a step of densifying the solid electrolyte. The densification step can be carried out by any suitable method. For example, the densification step can be carried out by a heat treatment technique that substantially improves the final pellet density. For example, the densification step can be carried out by rapid heating methods such as Joule heating, microwave radiation, spark plasma sintering, induction heating, laser sintering, infrared radiation, and electric pulse consolidation. For example, the rapid heating method can be carried out under the above-mentioned conditions. According to an interesting example, the densification step can be carried out, for example, by Joule heating for about 10 seconds.

[0104] To understand the present technology in more detail, first refer to FIG. 1, which provides in (a) a schematic diagram of a process for producing a pure solid electrolyte and in (b) a schematic diagram of a process for producing a coated solid electrolyte according to a possible embodiment.

[0105] As shown in FIG. 1(b), the deposition step can be carried out by a powder coating process that includes spreading a metal-based coating material precursor powder over at least a portion of the surface of the solid electrolyte. Next, the solid electrolyte can be transferred to a rapid melting and quenching device with the surface coated with the metal-based coating material precursor powder facing upward. Next, the metal-based coating material precursor powder can be subjected to a rapid temperature increase to substantially melt at least one component of the precursor powder of the metal-based coating material and form a molten metal-based coating material. The molten metal-based coating material can spread substantially or completely over the entire surface of the solid electrolyte. Next, the molten metal-based coating material can be subjected to a rapid temperature decrease, thereby solidifying the molten metal-based coating material and producing a coated solid electrolyte.

[0106] According to another example, the process further optionally includes the step of depositing a second coating material on at least a portion of the surface opposite to the surface of the solid electrolyte to form a second coating layer. It should be understood that the second coating layer is deposited on the surface opposite to the surface of the solid electrolyte on which the metal-based coating layer is deposited.

[0107] Referring now to FIG. 2(b), this figure provides a digital photograph of a rapid heating device for producing a pure solid electrolyte and / or a coated solid electrolyte according to a possible embodiment. The device for producing a pure solid electrolyte and / or a coated solid electrolyte by the rapid heating method needs to be able to provide a substantially high-temperature environment with a substantially high heating and cooling rate. Any suitable device is contemplated.

[0108] We modified the previously reported ultra-high-speed high-temperature sintering device (Wang, Chengwei, et al. "A general method to synthesize and sinter bulk ceramics in seconds." Science 368.6490 (2020): 521-526) to construct a laboratory-scale rapid heating device. All heat treatments of the present disclosure were carried out using the laboratory-scale rapid heating device. As shown in Fig. 2(b), the rapid heating device can include a pyrometer (1), a heating chamber (2) with a rubber seal O-ring, an electrical connection (3), a gas inlet / outlet (4), a heating element (5), a transducer (6), and a power supply (7).

[0109] Continuing to refer to Fig. 2(b), any suitable heating element (5) through which current passes and the temperature rises is contemplated. For example, a graphite sheet can be an effective heating element. To prevent oxidation of the graphite, the heating element can be placed in an airtight chamber filled with an inert gas such as argon or nitrogen. The current can be supplied by a programmable power supply, and the temperature of the heating element can be controlled by changing the amplitude of the current. A pyrometer can be attached to the top of the chamber to measure the temperature, and the infrared light radiated from the heating element can be captured by the pyrometer and converted into temperature. The temperature of the heating element is determined by Equation 1, where Q is the Joule heat, Q loss represents the amount of heat transfer from the heating element to the surroundings, I and R represent the current and resistance of the graphite heating element respectively, A is the surface area of the graphite heating element, ε and σ are the emissivity and Stefan-Boltzmann constant respectively, h is the heat transfer rate, T is the actual temperature of the heating element, and T0 is the ambient temperature. Q = Q loss = I 2 R = hA(T - T0) + εσA(T 4 - T0 4 ) (1)

[0110] However, if the heating element operates at a substantially high temperature, the effect of thermal radiation becomes much larger than that of heat conduction and convection. As a result, the temperature of the heating element can be approximated by Equation 2, where the actual temperature of the heating element is proportional to the square root of the current and the measured temperature agrees with the theoretical prediction (Figure 2(c)).

Number

[0111] According to another example, the process defined herein can substantially reduce the sintering time compared to conventional methods for producing pure solid electrolytes and / or coated solid electrolytes. For example, the process defined herein based on rapid heating methods can effectively reduce the sintering time from several hours (about 12 hours in conventional solid synthesis) to several seconds (e.g., less than about 25 seconds), thereby substantially reducing lithium loss and effectively fusing the particles to improve the material quality.

[0112] The present technology also relates to a coated solid electrolyte obtained by the process defined herein. A coated solid electrolyte obtained by the process defined herein is also contemplated.

[0113] According to one example, a metal-based coating material can form a uniform coating layer on the surface of the solid electrolyte. For example, a metal-based coating material can form a substantially uniform metal-based coating layer on the surface of the solid electrolyte. Alternatively, a metal-based coating material can form a coating layer on at least a portion of the surface of the solid electrolyte. For example, a metal-based coating can be heterogeneously dispersed on the surface of the solid electrolyte. According to an interesting example, a metal-based coating material forms a substantially uniform metal-based coating layer on the surface of the solid electrolyte.

[0114] According to another example, the metal-based coating material is selected from the group consisting of metal elements, metal alloys, metal oxides, fluorinated metals, and combinations of at least two of these.

[0115] In some examples, the metallic coating material is a metallic element. The metallic element can be a metal or a metalloid selected from the group consisting of, for example, Al, Cu, Ag, Sn, Sb, and Bi. In some interesting examples, the metallic element is Cu, Ag, or Sn.

[0116] In some other examples, the metallic coating material is a metal alloy, such as a binary, ternary, or quaternary metal alloy. The metal alloy can include a first metal component selected from group 14 and 15 elements and a second metal component different from the first metal component. For example, the second metal component can be an alkali metal, an alkaline earth metal, a transition metal, a post-transition metal, a metalloid, or a lanthanide. In some interesting examples, the first metal component is selected from Sn, Sb, and Bi, and the second metal component is selected from the group consisting of Al, Mn, Co, Ni, Cu, Ag, Sn, Sb, La, Tb, and Bi. Non-limiting examples of the metal alloy include Sn-Mn, Sn-Co, Sn-Ni, Sn-Cu, Sn-Cu-Tb, Sn-Ag, Sn-La, Sn-Bi-Ag, Sb-Cu, Sb-Ag, and Bi-Ag based alloys. In some interesting examples, the metal alloy is Cu3Sn or Cu6Sn5. In some other interesting examples, the metal alloy is AgSn x Bi 1-x where x is 0 ≦ x ≦ 1. For example, x can be 1, 0.8, 0.6, 0.4, or 0, and the metal alloy is AgSn, AgSn 0.8 Bi 0.2 AgSn 0.6 Bi 0.4 AgSn 0.4 Bi 0.6 and can be selected from the group consisting of AgBi.

[0117] In some other examples, the metal-based coating material is a fluorinated metal. For example, the fluorinated metal can be selected from the group consisting of SnF2, SnF4, ZnF2, InF3, GaF3, SbF3, TlF, PbF2, CuF2, BiF3, AlF3, AgF, and LiF.

[0118] In some other examples, the metal-based coating material is a metal oxide. For example, the metal oxide can be selected from the group consisting of SnO, SnO2, CuO, Cu2O, Bi2O3, Al2O3, and Ag2O.

[0119] According to another example, the metal-based coating material can be selected based on its melting point temperature. For example, at least one component of the metal-based coating material precursor is preferably liquid at the temperature at which the rapid heating method is carried out. The metal-based coating material can also be selected based on its ability to chemically react with lithium metal to form a substantially high lithium conductive phase.

[0120] According to another example, the metal-based coating material can further include, if necessary, at least one doping element that can be included in a smaller amount, for example, to modulate or optimize its properties. For example, the metal-based coating material can be doped by partially substituting the metal with other elements. For example, Li + The metal-based coating material can be slightly doped with at least one doping element selected based on its ability to lower the energy barrier of diffusion. For example, the metal-based coating material can be doped with Bi.

[0121] According to another example, the solid electrolyte is in the form of a pellet. For example, the metal-based coating layer can be deposited on at least a part of the surface of the solid electrolyte configured to face the negative electrode of the electrochemical cell.

[0122] According to another example, the solid electrolyte can be a glass or ceramic solid electrolyte, preferably a ceramic solid electrolyte. For example, the solid electrolyte can be a garnet-type solid electrolyte. Non-limiting examples of garnet-type solid electrolytes include Li7La3Zr2O 12 (LLZO), Li 6.25 Al 0.25 La3Zr2O 12 (Al-LLZO), Li 6.5 La3Zr 1.5 Ta 0.5 O 12 (LLZTO), Li 6.35 Al 0.05 La3Zr2Ta 0.5 O 12 (Al-LLZTO), Li 6.25 Nd3Zr 1.5 Ta 0.5 O 12 (LNZTO), Li 6.25 Sm3Zr 1.5 Ta 0.5 O 12 (LSZTO), and Li 6.25 (Sm 0.5 La 0.5 )3Zr 1.5 Ta 0.5 O 12 (LSZTO). For example, the garnet-type electrolyte is selected from the group consisting of Li7La3Zr2O 12 (LLZO), Li 6.25 Al 0.25 La3Zr2O 12 (Al-LLZO), Li 6.5 La3Zr 1.5 Ta 0.5 O 12 (LLZTO), and Li 6.35 Al 0.05 La3Zr2Ta 0.5 O 12 (Al-LLZTO).

[0123] Depending on the final composition of the desired garnet-type solid electrolyte, the oxide precursors used in the preparation of the garnet-type solid electrolyte are single metal oxides such as Li2O, ZrO2, Ta2O5, Al2O3, Nd2O3, Sm2O3, and La2O3, or binary metal oxides such as LiZrO3, LiLaO3, LiNdO2, LiSmO2, LiTaO3, La2Zr2O7, La 0.6 Sm 1.4 O3, and AlLiO2, or ternary metal oxides such as Li7La3Zr2O 12 、Li7Nd3Zr2O 12 、Li5La3Ta2O 12 、LiLa2TaO6、LaZrTa3O 11 、LaNdZr2O7、La 0.25 Sm 0.25 Zr 0.5 O 1.75 、and Li 0.5 La2Al 0.5 O4, or the oxide precursors can be a combination thereof.

[0124] According to another example, the solid electrolyte further optionally includes at least one additional component or additive, such as an ion conductive material, inorganic particles, glass or ceramic particles, such as nanoceramics (e.g., aluminum oxide (Al2O3), titanium dioxide (TiO2), silicon dioxide (SiO2) and other similar compounds). For example, the additional component or additive can be selected from NASICON, LISICON, thio-LISICON, garnet, sulfide, sulfide-halide, phosphate, thiophosphate, and combinations thereof, in crystalline and / or amorphous forms. In one example, the additional component or additive is substantially dispersed within the electrolyte. Alternatively, the additional component or additive may be in a separate layer.

[0125] According to another example, the solid electrolyte may further include a second coating material, which forms a second coating layer. When present, the second coating material can be deposited on at least a part of the surface of the solid electrolyte opposite the side where the metal-based coating layer is deposited. It should be understood that the second coating layer is deposited on at least a part of the surface of the solid electrolyte opposite the surface on which the metal-based coating layer is deposited. More specifically, when present, the second coating layer can be deposited on at least a part of the surface of the solid electrolyte configured to face the positive electrode of the electrochemical cell. For example, the second coating material can be selected based on its ability to improve the interfacial contact between the positive electrode and the solid electrolyte. For example, the second coating material can be a succinonitrile-based coating material and may further include a lithium salt such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) if necessary.

[0126] According to one alternative, the electrolyte can be a polymer-ceramic hybrid solid electrolyte. The polymer-ceramic hybrid solid electrolyte can have a multilayer structure. For example, the polymer-ceramic hybrid solid electrolyte can include a layer of a solid polymer electrolyte containing a salt in a solvated polymer and a layer of a ceramic electrolyte, and the metal-based coating layer is deposited on at least a part of the surface of the ceramic layer. It should be understood that the solid polymer electrolyte layer is deposited on the surface opposite the surface of the ceramic layer on which the metal-based coating layer is deposited.

[0127] In some examples, the ceramic can be the garnet-type solid electrolyte defined above. The solid polymer electrolyte can be selected from any known solid polymer electrolyte that is compatible with the various elements of the electrochemical cell. For example, the solid polymer electrolyte can be selected based on its compatibility with lithium and the positive electrode. The solid polymer electrolyte generally can include one or more solid polar polymers, and a salt, crosslinked as necessary. Polyether-type polymers, such as polyethylene oxide (PEO)-based polyether-type polymers, can be used, although several other compatible polymers, such as polynitrile-type polymers, are also known with respect to the preparation of solid polymer electrolytes. The polymer can be further crosslinked. Examples of such polymers include star or comb-shaped multi-branched polymers, such as those described in PCT application number WO2003 / 063287 (Zaghib et al.).

[0128] For example, the salt can be an ionic salt such as a lithium salt. Non-limiting examples of lithium salts include lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium 2-trifluoromethyl-4,5-dicyanoimidazolate (LiTDI), lithium 4,5-dicyano-1,2,3-triazolate (LiDCTA), lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium nitrate (LiNO3), lithium chloride (LiCl), lithium bromide (LiBr), lithium fluoride (LiF), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium trifluoromethanesulfonate (LiSO3CF3) (LiTf), lithium fluoroalkyl phosphate Li[PF3(CF2CF3)3] (LiFAP), lithium tetrakis(trifluoroacetoxy)borate Li[B(OCOCF3)4] (LiTFAB), lithium bis(1,2-benzenediolato(2-)-O,O’)borate [B(C6O2)2] (LiBBB), and combinations thereof.

[0129] The present technology also relates to an electrochemical cell comprising a negative electrode, a positive electrode, and a coated solid electrolyte as defined herein.

[0130] According to one example, the metal-based coating layer of the coated solid electrolyte faces the negative electrode.

[0131] According to another example, if present, the second coating material of the coated solid electrolyte faces the positive electrode.

[0132] According to another example, if present, the solid polymer electrolyte of the coated solid electrolyte faces the positive electrode.

[0133] According to another example, the negative electrode (counter electrode) contains an electrochemically active material that can be any known material and is selected based on its electrochemical compatibility with various elements of the electrochemical cell as defined herein. Non-limiting examples of the electrochemically active material of the negative electrode include alkali metals, alkaline earth metals, alloys containing at least one alkali metal or alkaline earth metal, non-alkali and non-alkaline earth metals, or alloys or intermetallic compounds. According to an interesting example, the electrochemically active material of the negative electrode can be lithium metal or an alloy thereof.

[0134] According to another example, the positive electrode contains an electrochemically active material that can be any known material and is selected based on its electrochemical compatibility with various elements of the electrochemical cell as defined herein. The electrochemically active material of the positive electrode can be in the form of particles. Non-limiting examples of the electrochemically active material include metal oxides, lithium metal oxides, metal phosphates, lithium metal phosphates, titanates, lithium titanates, metal fluorophosphates, lithium metal fluorophosphates, metal oxyfluorophosphates, lithium metal oxyfluorophosphates, metal sulfates, lithium metal sulfates, metal halides (such as fluorides), lithium metal halides (such as fluorides), sulfur, selenium, and combinations of at least two of these. For example, the electrochemically active material of the positive electrode can be selected from the group consisting of metal oxides, lithium metal oxides, metal phosphates, lithium metal phosphates, and combinations of at least two of these. For example, the metal of the electrochemically active material can be selected from the group consisting of titanium (Ti), iron (Fe), magnesium (Mg), manganese (Mn), vanadium (V), nickel (Ni), cobalt (Co), aluminum (Al), chromium (Cr), copper (Cu), antimony (Sb), zirconium (Zr), zinc (Zn), niobium (Nb), and combinations of at least two of these when applicable. According to some interesting examples, the electrochemically active material of the positive electrode can be lithium iron phosphate (LiFePO4, abbreviated as LFP) or lithium nickel manganese cobalt oxide (LiNiMnCoO2, abbreviated as NMC).

[0135] According to another example, other elements or impurities may be further doped into the electrochemical active material of the positive electrode, and the other elements or impurities may be contained in a smaller amount, for example, to modulate or optimize its electrochemical properties. For example, the electrochemical active material of the positive electrode may be doped by partially substituting a metal with other elements. For example, the electrochemical active material of the positive electrode may be doped with a transition metal (for example, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Y) and / or a non-transition element (for example, Mg, Al, or Sb).

[0136] According to another example, the electrochemical active material of the positive electrode may be in the form of particles (for example, microparticles and / or nanoparticles) that can be newly formed or obtained from a commercially available source, and can further include a coating material. The coating material can be an electronically conductive material. For example, the coating can be a carbon coating.

[0137] According to another example, the positive electrode described herein further includes an electronically conductive material as needed. Non-limiting examples of the electronically conductive material include carbon black (for example, Ketjen TM black and Super P TM ), acetylene black (for example, Shawinigan black and Denka TM black), graphite, graphene, carbon fibers (for example, vapor-grown carbon fibers (VGCF), carbon nanofibers, carbon nanotubes, and combinations of at least two of these). According to some interesting examples, the electronically conductive material can be Super P TM .

[0138] According to another example, the positive electrode described herein further includes a binder as needed. For example, the binder can be selected based on its compatibility with various elements of the electrochemical cell. Any known compatible binder is contemplated. For example, the binder can be a polyether type polymer binder, a fluorinated polymer, or a water-soluble binder. According to one example, the binder is a fluorinated polymer, such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). According to another example, the binder is a water-soluble binder, such as styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), hydrogenated NBR (HNBR), epichlorohydrin rubber (CHR), or acrylate rubber (ACM), and optionally includes a thickening agent such as carboxymethyl cellulose (CMC), or an acidic polymer such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMMA), or a combination thereof. According to another example, the binder is a polymer polyether binder, such as a linear, branched, and / or cross-linked binder based on polyethylene oxide (PEO), poly(propylene oxide) (PPO), or a combination of the two (or EO / PO copolymer), and includes cross-linkable units as needed. According to an interesting example, the binder is PVDF. According to another interesting example, the binder includes succinonitrile (SN), a lithium conductive salt (LiTFSI), and polyacrylonitrile (PAN).

[0139] According to another example, the positive electrode described herein further includes at least one additional component or additive, such as an ion conductor, inorganic particles, glass or ceramic particles, nanoceramics (e.g., aluminum oxide (Al2O3), titanium dioxide (TiO2), silicon dioxide (SiO2) and other similar compounds), salts (e.g., lithium salts) and other similar components, as required. For example, the additional component may be an ion conductor selected from the group consisting of NASICON, LISICON, thio-LiSICON, garnet, sulfide, sulfur halide, phosphate, and thiophosphate, in crystalline and / or amorphous forms, and combinations of at least two of these.

[0140] The present technology also relates to a battery including at least one electrochemical cell as defined herein. For example, the battery can be a lithium or lithium-ion battery, a sodium or sodium-ion battery, a magnesium or magnesium-ion battery, or a potassium or potassium-ion battery. According to an interesting example, the battery is a lithium or lithium-ion battery. According to another interesting example, the battery is an all-solid-state battery.

[0141] According to another example, the metal-based coating layer as defined herein can substantially stabilize the interface between the negative electrode and the solid electrolyte. Further, due to the substantially uniform form and lithophilic properties of the metal-based coating layer, the interface resistance can be substantially reduced or even eliminated, thereby enabling dendrite-free lithium plating and stripping on the solid electrolyte interface even at a high current density of 20 mA cm -2 Substantially uniform coating of the metal-based coating material on the surface of the solid electrolyte and easy lithium diffusion through the metal-based coating layer can substantially improve the electrochemical performance. For example, the metal-based coating layer can substantially improve the cyclability.

[0142] To fully exploit the potential of solid electrolytes, the interfacial resistance between the solid electrolyte and the negative electrode should be substantially reduced. There are three prerequisites that need to be met to cancel out the interfacial resistance: (i) Lithium diffusion through the interfacial layer should be much faster than diffusion in the bulk solid electrolyte such that the lithium diffusion resistance in the interfacial layer is substantially negligible; (ii) The interfacial layer should be uniformly coated on the surface of the solid electrolyte so that the local current density is uniformly distributed across the interface during the lithium plating / stripping process; and (iii) The solid electrolyte should be chemically and electrochemically stable with respect to the interfacial layer (Chen, Wan-Ping, et al. "Bridging interparticle Li + conduction in a soft ceramic oxide electrolyte." Journal of the American Chemical Society 143, no. 15 (2021): 5717-5726). For example, many elemental metals or their alloys satisfy the first requirement because they generally have high diffusion rates when alloyed with lithium metal anodes. However, there is a lack of effective and scalable approaches to uniformly coat an ultrathin layer of metal or metal alloy on the surface of solid electrolytes. Previous studies have formed interfacial layers on solid electrolytes using either mechanical coating methods or sputtering methods, but these methods are very difficult to control and often result in a large amount of uncoated area leading to the presence of interfacial resistance despite surface modification. Furthermore, when various metal coating strategies are used, the reported interfacial resistance still shows a large variation of 5 - 150 Ω cm -2 , indicating that completely eliminating such interfacial resistance remains a challenge.

[0143] Using the melt quenching process described herein, a metal-based interfacial layer coating with zero resistance can be applied on the surface of a solid electrolyte. In the melt quenching process described herein, a rapid heating / cooling device is used, and using this, a metal-based material can be formed and coated in-situ on the surface of the solid electrolyte. As candidates for the interfacial layer, a wide range of metals with binary and ternary compositions were examined and their electrochemical performance was comprehensively evaluated. It has been demonstrated that certain metal elements in binary and ternary alloys exhibit a synergistic effect and can uniformly coat an alloy on the surface of the solid electrolyte while reducing the lithium / solid electrolyte interfacial resistance to zero. Some types of metal alloys (the most promising being AgSn 0.6 Bi 0.4 O x among others) have been found to impart a negligible interfacial resistance to the solid electrolyte with a lithium metal anode. To clarify the mechanism of the excellent interfacial stability between the coated solid electrolyte and the lithium metal anode, advanced characterization and theoretical calculations were performed. The results obtained have revealed new mechanistic insights essential for the further development of alloy-based interfacial layers.

Examples

[0144] The following non-limiting examples are illustrative embodiments and should not be construed as further limiting the scope of the present invention. These examples are better understood in combination with the accompanying drawings.

[0145] Example 1 Synthesis of a densified Li 6.5 La3Zr 1.5 Ta 0.5 O 12 (LLZTO) solid electrolyte

[0146] (a) Solid-state synthesis of densified LLZTO solid electrolyte

[0147] The garnet-type solid electrolyte was prepared by solid-state synthesis and densified by a modified rapid heating method (Wang, Chengwei, et al. "A general method to synthesize and sinter bulk ceramics in seconds." Science 368.6490 (2020): 521-526).

[0148] Three different compositions of the garnet-type solid electrolyte, namely Li 6.25 Al 0.25 La3Zr2O 12 (Al-LLZO), Li 6.5 La3Zr 1.5 Ta 0.5 O 12 (LLZTO), and Li 6.35 Al 0.05 La3Zr2Ta 0.5 O 12 (Al-LLZTO) were synthesized. Lithium hydroxide monohydrate (LiOH·H2O), zirconium dioxide (ZrO2), lanthanum oxide (La2O3), tantalum pentoxide (Ta2O5), and aluminum oxide (Al2O3), which are the respective precursors, were weighed so as to obtain the desired stoichiometry. For example, LLZTO was prepared using LiOH·H2O, ZrO2, La2O3 and Ta2O5 precursors in a molar ratio of 7.15:1.5:1.5:0.25. Next, the sample was uniformly mixed in a planetary ball mill at 300 rpm for about 10 hours. Next, the obtained mixture was cold pressed into pellets and subsequently annealed in a muffle furnace at a temperature of about 900 °C for about 12 hours. The as-prepared LLZTO pellets were sandwiched between two graphite heating elements and further densified by rapid heat treatment at a temperature of about 1280 °C for about 10 seconds under an argon atmosphere. Next, the densified LLZTO solid electrolyte was taken out of the rapid heating device and stored in an argon-filled glove box.

[0149] (b) Coating of the metal-based layer on the surface of the LLZTO solid electrolyte prepared in Example 1(a)

[0150] The garnet-type solid electrolyte prepared in Example 1(a) was coated with a layer of a metal-based material containing at least one metal element and at least one second metal element selected from Group 14 and Group 15 elements. For example, the metal elements selected from Group 14 and Group 15 elements can react with lithium to form Li-conductive compounds, and the second metal element can adjust the melting and boiling points of the metal alloy and help maintain the alloy in its liquid form during the heating process. In some examples, the metal alloy is further doped with bismuth.

[0151] The surface of the LLZTO solid electrolyte prepared in Example 1(a) was coated with different metal-based materials. AgSn, CoSn6, LaSn2, MnSn, Ni2Sn, AgSb, CoSb, CuSb, or AgSn x Bi 1-x (where x = 1, 0.8, 0.6, 0.4, and 0) were prepared by weighing the corresponding elemental metal powders and uniformly mixing the powders using either a mortar and pestle or a ball milling method. To coat the garnet-type solid electrolyte surface with the metal-based material by the melt quenching method, the garnet-type solid electrolyte pellet was rubbed against an excess amount of elemental metal powder spread on weighing paper, during which the metal-based particles adhered to the garnet surface by van der Waals forces. Next, the free powder was blown off the garnet surface using a jet of argon gas. Next, the metal-based precursor-treated LLZTO pellet was sandwiched between two graphite heating elements with the coated side facing up, and the temperature was rapidly raised to about 1100 °C to melt the metal precursor and allow the liquid metal to spread completely over the LLZTO surface. The sample was 3 °C / min -1 cooled at a cooling rate of to obtain a uniform metal-based coating.

[0152] The metal precursor powder and the LLZTO powder were mixed at a weight ratio of about 1:40, and then, by performing the melt quenching method of the example, LLZTO powder coated with a metal-based material was also prepared.

[0153] Example 2 Characteristic evaluation of an LLZTO solid electrolyte coated with a layer of the metal-based material prepared in Example 1(b)

[0154] The LLZTO solid electrolyte coated with a layer of the metal-based material prepared in Example 1(b) was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM).

[0155] (a) X-ray diffraction (XRD)

[0156] The crystal phase and purity of the LLZTO solid electrolyte coated with a layer of the metal-based material prepared in Example 1(b) were studied by XRD. As will be described later, the coating layer can also contain metal oxides (for example, SnO / SnO2 and Bi2O3), and therefore, the composition of the entire coating is, hereinafter, AgSn 1-y Bi y O x (0 ≤ y ≤ 1, 0 ≤ x ≤ 3) is formulated. The XRD measurement was carried out using a Rigaku MiniFlex X-ray diffractometer with a 2θ scan from 10° to 60° at a scanning speed of 1.5° per minute -1 of. To confirm the presence of the AgSn 0.6 Bi 0.4 O x coating layer on the LLZTO surface, XRD characteristic evaluations were performed on the densified pure LLZTO solid electrolyte and the AgSn 0.6 Bi 0.4 O x coated LLZTO solid electrolyte, and the spectra obtained as a result are shown in Fig. 3. The diffraction peaks of pure LLZTO substantially coincide with the characteristic peaks of cubic-phase LLZTO. Compared with the XRD spectrum of pure LLZTO, AgSn 0.6 Bi 0.4 Ox Several small peaks corresponding to Ag, Ag3Sn alloy, and Sn - Bi alloy can be observed in the XRD spectrum of the coated LLZTO solid electrolyte, indicating the presence of a composite of different metal - based materials on the surface of LLZTO.

[0157] (b) X - ray photoelectron spectroscopy (XPS)

[0158] The chemical composition of the LLZTO solid electrolyte coated with the metal - based material layer prepared in Example 1(b) was studied by XPS (Kratos Axis Ultra DLD). AgSn 0.6 Bi 0.4 O x To ensure the coated LLZTO solid electrolyte was transferred to the XPS chamber without exposure to air, it was loaded into a sealed capsule filled with argon.

[0159] XPS characterization was performed to study the composition and thickness of the alloy coating. Figure 4(a) shows the AgSn 0.6 Bi 0.4 O x XPS survey spectra of the coated LLZTO solid electrolyte. AgSn 0.6 Bi 0.4 The XPS spectra taken at the outer surface of the Ox - coated LLZTO solid electrolyte show peaks indexed for Ag, Bi, Sn, C, and O. The presence of carbon may be the result of a very thin hydrocarbon layer adsorbed on the sample, while oxygen may arise from the oxidation of the surface of the metal alloy. It should be noted that no distinct peaks for Zr, La, and Ta are observed in the spectra, which means that after surface modification by the melt - quenching method, the LLZTO surface is uniformly covered with the metal alloy. Figure 4(b) shows the AgSn 0.6 Bi 0.4 O xXPS deconvolution composition profiles of Ag, Sn, Bi, La, and Zr at different argon ion sputtering times on the surface of the coated LLZTO solid electrolyte are shown. Figure 4(b) shows AgSn 0.6 Bi 0.4 O x shows the surface of the coated LLZTO solid electrolyte (Region I; Figure 4(b) shows that BiO x has some remaining SnO x is covered with a rich layer; this SnO x rich layer is hereinafter referred to as SnBiO x . The convolution fine XPS spectra of Ag, Sn, and Bi in Regions I and II were recorded to further study the coordination of each element (Figure 4(c)). The Sn and Bi atoms in Region I are partially bonded to oxygen, and as a result, their deconvolution XPS peaks are split into two peaks corresponding to metallic Sn and tin oxide (SnO x ), and metallic Bi and bismuth oxide (BiO x ), respectively. However, the Ag atoms remain substantially in the metallic state, which may be due to their resistance to oxidation compared to Sn and Bi. AgSn 0.6 Bi 0.4 O x / LLZTO / AgSn 0.6 Bi 0.4 O x In the EIS measurement of the sample, it was revealed that the interfacial resistance remained unchanged even after the electrolyte was exposed to the ambient atmosphere for about 60 minutes (Figure 5), which indicates that the presence of a substantially dense SnBiO x layer can be beneficial for protecting LLZTO from reacting with the atmosphere (e.g., H2O and CO2). Both SnBiO x and AgSn 0.6 Bi 0.4 are reactive with lithium metal, which promotes the wetting of lithium metal on the LLZTO surface (Figure 6), and as a result, the interfacial resistance is eliminated. Most of the alloy coating (Region II) consists of Ag, Bi, and Sn, and the composition is AgSn 0.6 Bi 0.4is almost equivalent, which means that AgSn with each element uniformly distributed 0.6 Bi 0.4 layers have been successfully coated on the surface of LLZTO. The deconvolution fine spectra of Sn and Bi in Region II indicate that, compared with the corresponding peaks in Region I, the intensity of the peaks indexed with the metal oxide index has significantly decreased, indicating that the oxygen content in Region II has significantly decreased. After about 200 seconds of argon sputtering, the amount of Zr increases sharply, which 0.6 Bi 0.4 O x shows that the AgSn coating has been completely removed and the LLZTO surface is exposed, which corresponds to an AgSn 0.6 Bi 0.4 O x coating layer with a thickness of approximately 80 nm.

[0160] (c) Scanning electron microscope (SEM)

[0161] The morphology of the LLZTO solid electrolyte and AgSn 0.6 Bi 0.4 O x coated LLZTO solid electrolyte prepared in Example 1(b) was studied by SEM (FEI Quanta 650).

[0162] Measurements were carried out on the pure LLZTO solid electrolyte and AgSn 0.6 Bi 0.4 O x coated LLZTO solid electrolyte. The pure LLZTO solid electrolyte and AgSn 0.6 Bi 0.4 O xThe SEM images of the coated LLZTO solid electrolyte are shown in Figures 7(a)-7(c) and 7(d), respectively. As can be seen in Figures 7(a)-7(c), the conventional solid-state method followed by the rapid heating densification step (Figure 2(b)) results in a densification of LLZTO of about 60%. The pure LLZTO solid electrolyte has a clean and smooth surface consisting of interconnected LLZTO grains with well-defined grain boundaries. As can be seen in Figure 7(d), the surface of LLZTO is coated with AgSn 0.6 Bi 0.4 O x Coating the layer increases the surface roughness, which is due to the fact that the surface is AgSn 0.6 Bi 0.4 O x This may be due to the fact that the surface is covered with a layer.

[0163] This was further substantiated by elemental mapping obtained by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) (Figures 8 and 9). Figures 8 and 9 show the AgSn 0.6 Bi 0.4 O x Elemental mapping images obtained by EDS of the coated LLZTO solid electrolyte, and AgSn 0.6 Bi 0.4 O x A cross-section of the as-coated LLZTO solid electrolyte is shown in Figures 8 and 9, which show uniform Ag, Sn, and Bi signal distributions across the entire scan area.

[0164] Figure 10 shows the AgSn 0.6 Bi 0.4 O x (a) Low magnification and (b) high magnification SEM images showing the interface between the coated LLZTO solid electrolyte and the lithium metal anode.

[0165] (d) Scanning transmission electron microscope (STEM)

[0166] AgSn prepared in Example 1(b) 0.6 Bi 0.4 O xThe coated LLZTO solid electrolyte was further characterized by a scanning transmission electron microscope (STEM, JEOL NEOARM) equipped with an aberration corrector and operating at 80 kV.

[0167] AgSn 0.6 Bi 0.4 O x Atomic-resolution STEM characterization was performed on the coated LLZTO solid electrolyte. However, due to the limitation of electron beam transmission, AgSn 0.6 Bi 0.4 It was difficult to directly observe the morphology of the coated LLZTO solid electrolyte pellet by STEM. Therefore, the AgSn 0.6 Bi 0.4 O x coated LLZTO nanoparticles were instead characterized by STEM. This is because, even when the pellet is replaced with nanoparticles, despite the change in coating thickness caused by the different surface areas between the two types of samples, AgSn 0.6 Bi 0.4 O x the general morphology of the coating layer remains substantially unchanged. Figure 11(a) shows the low-magnification high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the coated LLZTO nanoparticles. The corresponding elemental mapping image (Figure 11(b)) taken in the same area shows a substantially uniform distribution of Ag, Sn, and Bi on the surface of the LLZTO nanoparticles, indicating that AgSn 0.6 Bi 0.4 O x the alloy is uniformly coated on the surface of the LLZTO nanoparticles. Figure 11(c) shows AgSn 0.6 Bi 0.4 O x the high-resolution HAADF-STEM image of the coated LLZTO nanoparticles. 0.6 Bi 0.4 O xAn atomic-resolution STEM image obtained to show the detailed morphology of the coated LLZTO nanoparticles is shown. The atomic arrangement shows two different types of lattice structures overlapping each other. Fast Fourier transform (FFT) was performed on the image, and the results clearly show that no XRD diffraction pattern can be detected for Sn due to the very thin layer of LLZTO and the surface of LLZTO. 0.95 Bi 0.05 A pattern corresponding to the alloy is shown. LLZTO has

Number

[0168] Example 3 Electrochemical properties of the garnet-type solid electrolyte prepared in Example 1(b)

[0169] (a) Symmetric cell configuration

[0170] Symmetric cells were assembled to evaluate the interfacial stability between lithium metal and the LLZTO solid electrolyte and the coated LLZTO solid electrolyte. Two polished lithium disks with a diameter of 6 mm and a thickness of 200 μm were used as both the working electrode and the counter electrode. The LLZTO solid electrolyte or the coated LLZTO solid electrolyte was sandwiched between the two lithium disks. The average thickness of the LLZTO solid electrolyte or the coated LLZTO solid electrolyte was about 800 μm, and the diameter was about 8 mm. All cells were assembled into 2032-type coin cell cases in an argon-filled glove box with a water and oxygen content of less than about 0.1 ppm.

[0171] (b) Electrochemical cell configuration

[0172] Commercially available LiFePO4 (LFP) or LiNi 0.8 Mn 0.1 Co 0.1 O2 (NMC811) was used as the electrochemically active material for the positive electrode, and a solid full cell was also assembled. First, the NMC811 powder was dried overnight at a temperature of about 200 °C under vacuum. To prepare the composite positive electrode, polyacrylonitrile (PAN, Mw = 150,000, Sigma - Aldrich) blended with succinonitrile (99%, Sigma - Aldrich) plasticizer was used as the lithium - ion conducting component, providing high lithium - ion conductivity and stable operation in the electrochemical cell. Next, the LFP or NMC811 powder was mixed with Super P TM conductive carbon, PAN, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and succinonitrile in a weight ratio of 64.5:10:10:2:13.5 to form a homogeneous slurry. Next, the thus - obtained positive - electrode slurry was cast onto the surface of an aluminum film using a doctor blade and then dried overnight at a temperature of about 60 °C under vacuum to obtain a solid positive - electrode film. Next, from the thus - obtained positive electrode, a disk with a diameter of 6 mm and a standard mass loading of 5 mg cm -2 was cut out and used without further modification. A layer of a metal - based material was coated on one side of the LLZTO electrolyte for the solid - state electrochemical cell, and a polished lithium - metal disk with a diameter of about 6 mm and a thickness of about 200 μm was stacked thereon. To reduce the interfacial resistance at the interface between the solid electrolyte and the LFP or NMC811 positive electrode, the surface of the LLZTO solid electrolyte or the coated LLZTO solid electrolyte facing the NMC811 positive electrode was coated with a thin layer of a polymer composed of 5 wt% LiTFSI / succinonitrile (5 wt% / 95 wt%). All of the above - mentioned components were assembled into a 2032 - type coin - cell case, and the cell was left for about 24 hours before electrochemical measurements were performed.

[0173] (c) Electrochemical Impedance Spectroscopy (EIS)

[0174] The interfacial resistance between the coated LLZTO and lithium metal was measured by EIS using an electrochemical workstation (Biologic VMP-300). An amplitude of 20 mV was used for the EIS measurement. The frequency of the EIS measurement was in the range of 7×10 6 Hz to 1 Hz.

[0175] The metal-based coating material was coated on the surface of LLZTO with a composition corresponding to the most thermodynamically stable phase, and the interfacial resistance between the coated LLZTO and the lithium metal electrode was measured. The interfacial resistance was significantly reduced to less than 5 Ω cm -2 for all types of metal-based coatings (Figure 12(a)), which is in stark contrast to the reported 1000 Ω cm -2 for pure LLZTO solid electrolyte. Although not wishing to be constrained by theory, the lithiophilic property of the metal-based coating material could be one of the most important reasons for the reported low interfacial resistance. In previous studies, by coating a garnet-type solid electrolyte with a lithiophilic metal, the interfacial resistance was reduced to 10 Ω cm -2 ~200 Ω cm -2has been shown to be in the range (Luo, W., et al. "Reducing Interfacial Resistance between Garnet-Structured Solid-State Electrolyte and Li-Metal Anode by a Germanium Layer." Advanced Materials 29 (2017): 1606042; Feng, W., et al. "Building an interfacial framework: Li / garnet interface stabilization through a Cu6Sn5 layer." ACS Energy Letters 4.7 (2019): 1725-1731). The significant reduction in resistance obtained for the coated LLZTO can be the result of a substantially more uniform distribution of the metal-based coating material obtained using the melt quenching process, compared to the conventional sputtering techniques reported in previous studies, which often result in non-uniform coatings.

[0176] As shown in Fig. 12(b), the interfacial resistance of LLZTO coated with a layer of metal-based material having the stoichiometric ratio of MnSn, CoSn6, and AgSn is completely eliminated, while LLZTO coated with a layer of metal-based material having the stoichiometric ratio of Ni2Sn and LaSn2 alloys still has a resistance of about 1 Ω cm -2 ~ about 5 Ω cm -2shows the interfacial resistance. The melting points of the Ni2Sn and LaSn2 alloys are approximately 1100 °C (Okamoto, H. "Supplemental Literature Review of Binary Phase Diagrams: B-Fe, Cr-Zr, Fe-Np, Fe-W, Fe-Zn, Ge-Ni, La-Sn, La-Ti, La-Zr, Li-Sn, Mn-S, and Nb-Re." Journal of Phase Equilibria and Diffusion 37.5 (2016): 621-634; and Nash, P., and A. Nash. "The Ni-Sn (Nickel-Tin) system." Bulletin of alloy phase diagrams 6.4 (1985): 350-359). As a result, incomplete melting and / or wetting of the LLZTO surface by the Ni2Sn and LaSn2 alloy coatings may be the cause of the interfacial resistance. However, it has been confirmed that coating of LLZTO with a metallic coating material by the melt quenching process is effective in reducing the interfacial resistance. To further demonstrate the effectiveness of this coating strategy, LLZTO coated with an antimony-based coating material was prepared using the same method, and according to EIS measurements, the interfacial resistance was also substantially eliminated (Fig. 12(c)).

[0177] (d) Galvanostatic charge-discharge measurements and electrochemical behavior

[0178] The interfacial stability of the coated LLZTO electrolyte was evaluated by galvanostatic charge and discharge tests. The measurements were carried out on the symmetric cells prepared in Example 3(a) at a temperature of about 60 °C with a current density in the range of 0.5 mA cm -2 ~20 mA cm -2 and a fixed capacity of 1, 0.5, and 0.1 mAh cm per cycle -2 .

[0179] The long-term interfacial stability of the coated LLZTO electrolyte was investigated on symmetric cells at 0.5 mA cm -2 and 1.2 mA cm-2 It was measured by cycling at a current density of -2 for 2000 hours.

[0180] The electrochemical performance of the electrochemical solid cell prepared in Example 3(b) was evaluated by performing constant current charge and discharge tests at various C-rates using a battery tester (Neware).

[0181] One of the main goals in developing LLZTO solid electrolytes is to prevent the formation of lithium dendrites during the lithium plating process. Maintaining interfacial contact is essential for smooth lithium plating, but when the current density reaches a limit and the supply of lithium ions from the electrolyte becomes insufficient for lithium plating, lithium dendrites are still formed (Brissot, C., et al. "Dendritic growth mechanisms in lithium / polymer cells." Journal of power sources 81 (1999): 925-929; and Cheng, Xin-Bing, et al. "Toward safe lithium metal anode in rechargeable batteries: a review." Chemical reviews 117.15 (2017): 10403-10473). As a result, despite a significant reduction in the interfacial resistance of the coated LLZTO solid electrolyte, as shown in Figure 13, short circuits are still induced as the current density of the symmetric cell increases. Figure 13 shows the rate performance experimental results of symmetric cells containing LLZTO solid electrolytes coated with metal-based material layers based on Ag-Sn (Ag / Sn: 1 / 1), Co-Sn (Co / Sn: 1 / 6), La-Sn (La / Sn: 1 / 2), Mn-Sn (Mn / Sn: 1 / 1), Ni-Sn (Ni / Sn: 2 / 1), Ag-Sb (Ag / Sb: 1 / 1), and Co-Sb (Co / Sb: 1 / 1) tested at a temperature of 60 °C. With an intermediate layer based on Ag-Sn (Ag / Sn: 1 / 1), 14.6 mA cm -2The highest rate performance is observed at the critical current density of, while the LLZTO electrolytes coated with La2Sn, MnSn, CoSn6, Ni2Sn, AgSb, and CoSb show critical current densities of 7.0, 2.2, 12.8, 13.0, 6.2, and 9.8 mA cm -2 respectively. The different critical current densities resulting from different metal-based compositions can be the result of differences in lithium diffusion rates and lithiophilicity in the metal-based coating layer. Although CoSn6, La2Sn, MnSn, Ni2Sn, AgSb, and CoSb alloys provide excellent interfacial contact at the LLZTO / Li interface, they may have lower lithium diffusion rates, which can lead to insufficient lithium supply during the plating process at high current densities, resulting in the formation of lithium dendrites and potentially short-circuiting of the electrochemical cell. Therefore, it is considered that further modification to improve lithium diffusion can be made to the most functional coating material (i.e., Ag / Sn:1 / 1) to further improve its rate performance.

[0182] Modifications that increase lithium diffusion can be made to the metal-based coatings of this specification to further improve their rate performance. Doping a metal alloy with heteroatoms can create vacancies (Shuai, J., et al. "Manipulating the Ge vacancies and Ge precipitates through Cr doping for realizing the high-performance GeTe thermoelectric material." Small 16.13 (2020): 1906921; and Zhang, X., et al. "Vacancy manipulation for thermoelectric enhancements in GeTe alloys." Journal of the American Chemical Society 140.46 (2018): 15883-15888), which, in turn, has been reported to promote vacancy-mediated lithium diffusion within the alloy (Cui, J., et al. "Rational exploration of conversion-alloying reaction based anodes for high-performance K-ion batteries." ACS Materials Letters 3.4 (2021): 406-413). Bismuth atoms were doped into the Ag-Sn binary alloy to form a ternary alloy. AgSn x Bi 1-x When the materials of the system (x = 1, 0.8, 0.6, 0.4, and 0) were tested, the interfacial resistance remained substantially negligible regardless of the composition change (Figure 14(a)). However, at x = 0.6 and 0.8, it was observed that the critical current density increased substantially with the increase in bismuth content (Figures 14(b) to (d)), which may be due to the increase in the concentration of vacancies that promote the diffusion of lithium in the alloy. However, when the bismuth content further increases, since the tin present in the alloy is less than bismuth, the concentration of vacancies decreases, which, in turn, results in a lower critical current density (Figures 14(e) and (f)).

[0183] Overall, among all the compositions tested, AgSn 0.6 Bi 0.4 O x The coated LLZTO solid electrolyte showed the highest rate performance with a critical current density of 20.0 mA cm -2 at 60 °C. Such a high critical current density not only exceeds the requirements for the stable operation of all-solid-state lithium-metal batteries but is also the highest among all types of batteries with solid electrolytes. AgSn 0.6 Bi 0.4 O x The long-term interfacial stability of the symmetric cell with the coated LLZTO solid electrolyte was also evaluated at room temperature, demonstrating the practical usefulness of the modified electrolyte in all-solid-state batteries. At a current density of 0.5 mA cm -2 and capacities of 1 mAh cm -2 (Figure 15(a)) and 0.1 mAh cm -2 (Figure 15(b)), the symmetric cell showed a low lithium plating and stripping overpotential of approximately 35 mV because the ionic conductivity of LLZTO is high (about 8×10 -4 S cm -1 ) and the interfacial resistance between the coated LLZTO solid electrolyte and lithium metal is negligible. In the symmetric cell, even after operating for over 2000 hours, only a slight increase in overpotential was observed, indicating excellent stability of the coated LLZTO solid electrolyte against lithium metal. AgSn 0.6 Bi 0.4 O x The interfacial stability between the coated LLZTO solid electrolyte and lithium metal was also examined at high current densities of 1.2 mA cm 0.6 and 1 mA cm 0.4 O x with areal capacities of 1 mAh cm 0.6 Bi 0.4 O x The coated LLZTO solid electrolyte showed excellent stability against lithium metal. The interfacial stability between the coated LLZTO solid electrolyte and lithium metal was also evaluated at high current densities of 1.2 mA cm -2 and 1 mA cm -2 with areal capacities of 1 mAh cm -2 (Figure 14(g) and 15(c)) and 0.1 mAh cm -2It was tested in ((Fig. 14(h))). In these symmetric cells, the overpotential initially decreases gradually, which may be due to the activation of the interface at high current density. The overpotential stabilizes at approximately 73 - 78 mV over time at over 700 hours (1 mA cm -2 / 1 mAh cm -2 ) and 2000 hours (1 mA cm -2 / 0.1 mAh cm -2 ). The results suggest that a thin layer of AgSn 0.6 Bi 0.4 O x can fill the inhomogeneity of the LLZTO surface, thereby making the current along the surface substantially more homogeneous and increasing the Li + conductivity at the LLZTO / lithium interface. The alloy formed between the AgSn 0.6 Bi 0.4 O x layer and the lithium metal may limit the formation of dendrites. To observe any morphological changes after long - term cycles of lithium plating and stripping, symmetric cells were disassembled after cycling for approximately 650 hours (340 hours at 0.2 mA cm -2 / 0.1 mAh cm -2 , 310 hours at 1 mA cm -2 / 0.5 mAh cm -2 ), and SEM images were recorded at the cross - section of the lithium and LLZTO / AgSn 0.6 Bi 0.4 O x (Fig. 16(a)). Figures 16(b) - 16(e) are SEM images showing that after 1000 cycles, lithium is evenly plated on LLZTO / AgSn 0.6 Bi 0.4 O x and there is no sign of lithium dendrites penetrating LLZTO. The symmetric cell tests clearly show that an alloy coating on the surface of the LLZTO solid electrolyte can effectively suppress the formation of lithium dendrites and, when combined with a lithium metal electrode, can significantly improve both the rate and cycle stability of the solid electrolyte.

[0184] AgSn 0.6 Bi 0.4O x To demonstrate the practical utility of the coated LLZTO solid electrolyte, a solid lithium metal electrochemical cell was assembled using NMC as the electrochemical active material of the positive electrode. The key to the stable operation of the electrochemical cell is AgSn 0.6 Bi 0.4 O x The coated LLZTO solid electrolyte lies in its ability to suppress the formation of dendritic lithium over long cycles. Different from the poor interfacial contact between LLZTO and lithium (Figure 17), AgSn 0.6 Bi 0.4 O x facilitates excellent interfacial contact between LLZTO and the lithium metal negative electrode, as confirmed by cross-sectional SEM images (Figure 10). AgSn 0.6 Bi 0.4 O x Due to the excellent interfacial stability between the coated LLZTO solid electrolyte and the lithium metal negative electrode, the electrochemical cell achieved an excellent reversible lithium storage capacity of about 156 mAh g -1 at 0.1C with a cutoff voltage of 4.1 V (Figure 18(a)). The electrochemical cell maintained a capacity of about 116 mAh g -1 even when tested at a high current density of 1C at room temperature (Figure 18(b)). The excellent rate performance of the electrochemical cell may be mainly due to the reduction of the lithium plating / stripping overpotential, resulting in low polarization in the electrochemical cell (Figure 18(c)). More importantly, the electrochemical cell achieved stable electrochemical performance with an excellent capacity retention rate of about 86% over 1000 cycles at 1C, and no short circuit was observed due to the stable interface between the lithium metal and the solid electrolyte. However, to further increase the energy density by increasing the loading of the positive electrode (e.g., >5 mg cm -2 ), it is necessary to adopt a new polymer cathode liquid system with high Li + conductivity and oxidation potential.

[0185] (e) Theoretical calculation

[0186] All theoretical calculations were based on density functional theory (DFT) using the generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) functional (Perdew, John P., et al. "Generalized gradient approximation made simple." Physical review letters 77.18 (1996): 3865-3868) and the projector augmented wave (PAW) pseudopotential method (Kresse, Georg, et al. "From ultrasoft pseudopotentials to the projector augmented-wave method." Physical review b 59.3 (1999): 1758-1775). DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) (Hafner, Jurgen. "Ab-initio simulations of materials using VASP: Density-functional theory and beyond." Journal of computational chemistry 29.13 (2008): 2044-2078), and the convergence criteria were 10 -6 eV and 10 -2 eV Å -1 for the electronic self-consistent calculations and the Hellmann-Feynman forces, respectively. The kinetic energy cut-off of the plane waves was set to 600 eV. A 3×3×3 dimensional supercell was constructed for the DFT calculations, and the reciprocal lattice space was 0.1 Å -1Sampling was performed using a Monkhorst-Pack mesh with a spacing less than (Pack, James D., et al. "Special points for Brillouin-zone integrations"-a reply." Physical Review B 16.4 (1977): 1748). The diffusion barrier was calculated using the climbed image nudged elastic band (CI-NEB) method that employed a force-based optimizer, and the convergence criterion was set to 10 -2 eVÅ -1 (Henkelman, Graeme, et al. "A climbing image nudged elastic band method for finding saddle points and minimum energy paths." The Journal of chemical physics 113.22 (2000): 9901-9904).

[0187] As described above, the electrochemical performance indicated that doping Bi atoms into the AgSn alloy lattice could significantly improve the overall lithium diffusion rate. Therefore, its improvement mechanism was investigated. Although not bound by theory, the diffusion of lithium ions in the alloy may be mediated by lithium vacancies. The higher the vacancy concentration, the faster the lithium diffusion rate may be, and the vacancy formation energy could be a good indicator of the vacancy concentration, and the former of these can be calculated by DFT according to Equation 3, where E f represents the vacancy formation energy, y is the amount of lithium atoms removed from the unit cell to create vacancies, x is the stoichiometric ratio of Sn in the alloy, and E Li is the DFT energy of the lithium atom.

Number

[0188] Li2AgSn and Li2AgSn 0.6 Bi 0.4The vacancy formation energies are calculated to be 1.07 eV and 0.97 eV, respectively. The fact that the vacancy formation energy of the Bi-doped alloy is lower indicates that the lithium vacancy concentration can be higher in some cases, and as a result, the lithium diffusion rate can be faster. Apart from the vacancy concentration, the diffusion barrier of lithium in the alloy lattice can also affect the lithium diffusion rate. As shown in Fig. 19(a), lithium vacancy diffusion may follow the path of 4a→4c→4a, and the corresponding diffusion energy profile was calculated. Fig. 19(b) shows the comparison of the diffusion barriers between Li2AgSn and Li2AgSn 0.6 Bi 0.4 and shows that when the alloy is doped with Bi atoms, the diffusion barrier significantly decreases from 0.182 eV to 0.124 eV. Therefore, the improvement in rate performance obtained with the Bi-doped alloy may be the result of a combination of a higher concentration of lithium vacancies and a lower diffusion barrier.

[0189] Example 4 Synthesis of a Metal Alloy Coating for Garnet-Type Solid Electrolytes

[0190] (a) Coating of a layer of a metal-based material on the surface of the garnet-type solid electrolyte prepared in Example 1(a)

[0191] The surface of the high-density garnet-type solid electrolyte prepared in Example 1(a) was coated with various metal-based coating materials by the melt quenching method. The metal powder precursors used for the garnet coating included aluminum (Al), tin (Sn), antimony (Sb), bismuth (Bi), and copper (Cu). For the binary metal alloy coating, two types of metal powders in the desired ratio were uniformly mixed in isopropanol for 2 hours at 300 rpm using a planetary ball mill, and then the solvent was evaporated in a vacuum oven for 2 hours. The metal powders were stored in an Ar-filled glove box to avoid oxidation. The high-density garnet-type solid electrolyte pellets were immersed in an excess amount of the metal powder precursor, during which the metal particles adhered to the garnet surface by van der Waals forces. Next, the free powder was blown off the garnet surface using a jet of argon gas. The treated pellets were loaded into a rapid heating system and rapidly heated at a temperature gradient rate of 3×10 3 °C / min -1 for about 3 seconds (temperature range: about 700 °C to about 1200 °C), and then the coated pellets were quickly transferred to an Ar-filled glove box.

[0192] Example 5 Characteristic evaluation of the coated garnet-type solid electrolyte prepared in Example 4(a)

[0193] The coated garnet-type solid electrolyte prepared in Example 4(a) was characterized by XRD, XPS, SEM, EDS, and electrochemical tests. Theoretical calculated values were also obtained.

[0194] (a) X-ray diffraction (XRD)

[0195] The phases of the garnet pellets and the high-density garnet-type solid electrolyte were characterized by XRD (Rigaku Miniflex 600) with and without the metal-based coating.

[0196] (b) X-ray photoelectron spectroscopy (XPS)

[0197] The surface chemistry of the coated garnet-type solid electrolyte was studied by XPS (Kratos Axis Ultra DLD). Using a series of chambers and capsules, air-sensitive samples were transferred from an argon-filled glove box to the XPS chamber to avoid contamination by any ambient atmosphere. Before XPS characterization, a 1-nm-thick hydrocarbon layer was removed by argon ion sputtering.

[0198] (c) Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS)

[0199] The morphology of garnet pellets and high-density garnet-type solid electrolytes was characterized by SEM (FEI Quanta 650) with and without a metal coating. For particle size measurement, the surface of the garnet-type solid electrolyte pellet was polished using 1200-grit sandpaper and then thermally etched at a temperature of 1100 °C to expose the grain boundaries.

[0200] EDS elemental mapping was used to characterize the chemical composition and morphology of the surface coating. The sample was transferred from the argon-filled glove box to the SEM chamber using an air protection transfer protocol that almost completely prevented the sample from being exposed to air.

[0201] (d) Electrochemical tests

[0202] Unless otherwise specified, the thickness of the garnet solid electrolyte pellet in all electrochemical tests was fixed at approximately 800 μm. Li + To measure the conductivity, the solid electrolyte pellet was sandwiched between two blocking electrodes, and the Li conductivity of the electrolyte was measured by EIS (Biologic electrochemical workstation) at an amplitude of 20 mV and a frequency range of 7 MHz to 1 Hz. + The conduction resistance was measured. Li + The conductivity was calculated according to Equation 4, where

Number

Number

[0203] Li + To measure the Li + transport number, a symmetric cell equipped with two lithium disks as the working electrode and the counter electrode was assembled. A 10 mV bias was applied to the cell, and the Nyquist diagrams before and after applying the bias were recorded to measure the change in resistance. The Li

Number

Number

[0204] The interfacial stability between the lithium metal electrode and the garnet-type solid electrolyte is Li +It was evaluated with the same symmetric cell setup as the setup for the measurement of the plating rate. A constant current was applied to the symmetric cell to induce lithium plating / stripping, and the corresponding overvoltage was recorded as a function of time.

[0205] The electrochemical performance of an all-solid-state Li metal battery consisting of a surface-modified garnet-type solid electrolyte, a lithium metal anode, and a composite cathode containing commercially available LiFePO4 or NMC811 as an electrochemical active material was evaluated. To prepare the composite cathode, PAN blended with a succinonitrile plasticizer was used as the Li + conductive component. This has been shown to provide high Li + conductivity and stable operation in electrochemical cells (Lu, Ziheng, et al. "Modulating Nanoinhomogeneity at Electrode-Solid Electrolyte Interfaces for Dendrite-Proof Solid-State Batteries and Long-Life Memristors." Advanced Energy Materials 11.16 (2021): 2003811; and Tran, Hoai Khang, et al. "Composite Polymer Electrolytes Based on PVA / PAN for All-Solid-State Lithium Metal Batteries Operated at Room Temperature." ACS Applied Energy Materials 3.11 (2020): 11024-11035). LFP or NMC powder was mixed with conductive carbon black, PAN, SN, and LiTFSI in a weight ratio of 64.5:10:10:2:13.5. The solid mixture was dispersed in anhydrous NMP to form a homogeneous slurry, which was then cast onto an aluminum foil and dried completely at a temperature of 60 °C under vacuum to obtain a solid cathode film. Next, from the aluminum foil coated with the cathode, a mass loading of about 5 mg cm -2The electrode disk was cut out and used for the assembly of all-solid-state Li-metal batteries. To assemble the electrochemical cell, one side of the garnet-type solid electrolyte pellet was coated with a layer of a metal-based material as described in Example 4(a), and on the other side, a thin solid interface organic layer (5 wt% LiTFSI in SN) was coated to increase the interfacial contact between the cathode and the garnet-type solid electrolyte. Next, the garnet-type solid electrolyte pellet was sandwiched between the cathode and the lithium metal anode and assembled into a 2032-type coin cell case for electrochemical testing. All electrochemical tests were carried out on a Neware battery test system via constant current charging and discharging at different current densities.

[0206] To compare the electrochemical performance, conventional electrochemical cells with liquid electrolytes were also prepared using LFP or NMC811 as the cathode electrochemically active material. The cathodes were prepared by the same slurry casting method, while the composition of the cathodes was changed to 80 wt% LFP or NMC811, 10 wt% conductive carbon black, and 10 wt% polyvinylidene fluoride (PVDF) binder. The cells containing the liquid electrolyte were impregnated with a 1M LiPF6 solution in a non-aqueous solvent mixture of ethylene carbonate / ethyl methyl carbonate (EC / EMC) (1:1 by volume) using a Celgard TM 2400 membrane separator for assembly.

[0207] (e) Simulation

[0208] All simulations were performed using VASP (Hafner, Jurgen. "Ab-initio simulations of materials using VASP: Density-functional theory and beyond." Journal of computational chemistry 29.13 (2008): 2044-2078) based on DFT with the generalized gradient approximation using the PBE function (Perdew, John P., et al. "Generalized gradient approximation made simple." Physical review letters 77.18 (1996): 3865), and the PAW pseudopotential method (Kresse, Georg, et al. "From ultrasoft pseudopotentials to the projector augmented-wave method." Physical review b 59.3 (1999): 1758). In the DFT calculations, the residuals of the electronic self-consistent calculation and the residuals of the Hellmann-Feynman forces were 10 -6 eV and 10 -2 eVÅ -1 For all static calculations, the plane waves were cut off at a kinetic energy of 600 eV and a kinetic energy of 0.1 Å. -1 The reciprocal space was sampled using a Monkhorst-Pack mesh with spacing less than 0.01 mm (Pack, James D., et al. "Special points for Brillouin-zone integrations"-a reply." Physical Review B 16.4 (1977): 1748). To simulate the interaction between the metal atoms and the garnet surface at finite temperatures, first-principles molecular dynamics (AIMD) was performed following the same parameters as the static DFT calculations, except that only the gamma points were sampled in the calculations.

[0209] Example 6 Results and Discussion

[0210] For a garnet-type solid electrolyte to promote uniform lithium plating and stripping and suppress dendrite formation in a Li metal battery, low porosity and high chemical uniformity are essential. Conventional methods such as sintering in a furnace are not only costly and time-consuming, but also inevitable lithium loss at high temperatures makes it impossible to obtain high-quality garnet-type solid electrolytes. On the other hand, rapid densification methods are often preferred because they can minimize the above lithium loss mainly by shortening the sintering time. To synthesize high-quality garnet solid electrolytes and study their interfacial properties later, a rapid heating approach based on high-temperature sintering method has been developed (Wang, Chengwei, et al. "A general method to synthesize and sinter bulk ceramics in seconds." Science 368.6490 (2020): 521-526), which makes it possible to prepare densified garnet solid electrolytes within seconds (Figure 2).

[0211] During the densification step (at a temperature of about 1280 °C for about 10 seconds), the original 10 mm diameter white LLZTO pellet (Figure 20(a)) was rapidly converted into a grayish garnet-type solid electrolyte with a diameter of about 7 mm. Comparing the luminescence images of these two garnet-type solid electrolytes, the densified garnet sample emits intense blue light when irradiated with ultraviolet light of wavelength 254 nm, while the garnet-type sample that has not been densified by rapid heating completely absorbs the 254 nm ultraviolet light (Figure 20(b)), which indicates that the densified garnet-type solid electrolyte has high purity and low porosity (Raukas, M., et al. "Ceramic phosphors for light conversion in LEDs." ECS Journal of Solid State Science and Technology 2.2 (2012): R3168).

[0212] Figures 21(a), 22(a), and 22(b) show the XRD spectra obtained for LLZTO, Al-LLZO, and Al-LLZTO solid electrolytes before and after densification by rapid heating, respectively. All of these show negligible impurities, and all of the characteristic peaks substantially coincide with the standard XRD peaks of the cubic-phase garnet.

[0213] Due to the rapid densification process, the growth of garnet-type solid electrolyte particles is suppressed, and the average particle size of LLZTO is significantly reduced to about 3.7 μm (Figure 21(b)). On the other hand, in the aluminum-doped garnet-type samples, a slight increase in particle size was observed. For example, the particle sizes of Al-LLZO and Al-LLZTO were about 5.2 μm and about 4.0 μm, respectively (Figures 22(c) to (f)). This may be because the doping of aluminum promotes the fusion of particles. However, all of the garnet-type solid electrolytes synthesized by the rapid heating method have substantially high purity and minimal porosity, which regulates the uniform distribution of the local current density, suppresses the formation and growth of lithium dendrites, and is beneficial to the stability of Li metal batteries. LLZTO was thermally etched at a temperature of about 1100 °C for about 12 hours before SEM property evaluation to expose the grain boundaries.

[0214] When the ionic conductivities of LLZTO, Al-LLZO, and Al-LLZTO electrolytes were measured, the conductivity of the LLZTO electrolyte reached 8×10 -4 S cm -1 (Figure 21(c)), which was the highest value among the three electrolyte compositions. It has been widely reported that Ta doping promotes easy Li + conduction, and the high Li + conductivity of the LLZTO electrolyte is consistent with previous reports (Li, Yutao, et al. "Optimizing Li+ conductivity in a garnet framework." Journal of Materials Chemistry 22.30 (2012): 15357-15361).

[0215] The pure garnet electrolyte allows Li to pass through the electrolyte-lithium interface due to poor interfacial contact. + They suffer from transport stagnation, which often results in high interfacial resistance (Figure 23), making them virtually impossible to use directly in Li metal batteries. + Conductive metals and their alloys were screened to find optimal candidates that can eliminate the interfacial resistance during the operation of Li metal batteries. There are four general principles for the screening: (i) the cost of the metal needs to be reasonably low for practical applications; (ii) the metal should form a thermodynamically stable metallic alloy with Li at room temperature and transfer the Li to the electrolyte through an alloying-dealloying process at the electrolyte / lithium interface; + (iii) the metal should be of low toxicity to humans and the environment; (iv) the metal should be in liquid form at the processing temperature (i.e., a temperature of 1100°C) so as to completely wet and spread over the entire garnet surface to form a uniform coating.

[0216] Five candidates (Al, Sn, Sb, Bi, and Cu) were identified, as shown in Figure 21(d). Ultrathin coating layers based on single or binary metals were coated on the surface of LLZTO by melt-quenching method using a rapid heating instrument (Figure 2(a)). EIS spectra of LLZTO solid electrolytes modified with different metal-based coatings are shown in Figure 21(e). The EIS results showed that the Al-, Sn-, Sb-, and Bi-based coatings significantly reduced the interfacial resistance, whereas the Cu-based coatings showed a negligible effect, which may be attributed to the low solubility of Li in Cu. The Sn-based coated LLZTO solid electrolyte showed the lowest interfacial resistance (35 Ω cm) compared to the LLZTO solid electrolytes coated with other metals. -2) This is worthy of note. When Al is doped into the garnet electrolyte, Sn on the grain boundaries rich in Al is difficult to wet, so the effect of Sn on reducing the interfacial resistance is significantly impaired (Fig. 24(b)). As a result, only LLZTO without Al was studied later for property evaluation and electrochemical performance. To further reduce the interfacial resistance, various compositions of Sn-X binary alloys (X: Al, Cu, Bi, and Sb) were tested. However, when the coated garnet-type solid electrolyte is combined with a lithium metal electrode, according to the crystallographic database, the thermodynamically stable Li-Sn-X phase is formed only when X = Cu. As a result, the interfacial resistance was substantially eliminated only when an Sn-Cu (Cu / Sn = 3) -based coating was deposited on the surface of LLZTO (Fig. 21(f)). It is considered that this is the first time that the surface coating of the garnet solid electrolyte has been able to achieve substantial elimination of the interfacial resistance. The elimination of the interfacial resistance is carried out at a heating temperature of 1100 °C with Cu / Sn = 3, but it is assumed that there is a range of temperatures and compositions at which the interfacial resistance can be eliminated. Figs. 21(g) and 24(a) show the EIS spectra of the Cu-Sn-based coating material (Cu / Sn = 3) coated on the LLZTO solid electrolyte prepared at different temperatures but with a fixed composition (the EIS spectrum of the Cu-Sn-based coating material (Cu / Sn = 6 / 5) is shown in Fig. 24(a)). In the case of the Cu-Sn-based coating material (Cu / Sn = 3), the interfacial resistance is reduced to zero only after the heating temperature reaches over 1000 °C, because a relatively high temperature is a prerequisite for forming such a Cu-Sn-based coating material (Furtauer, S., et al. "The Cu-Sn phase diagram, Part I: new experimental results." Intermetallics 34 (2013): 142-147). The influence of the composition of the Cu-Sn-based coating material was also studied, and the interfacial resistance as a function of both temperature and composition is shown in Fig. 21(h).The results show that the temperatures (from about 1050 °C to about 1200 °C) and compositions (corresponding to a Cu mass percentage of 6 / 5 < Cu / Sn < about 39% to about 62%) that can be used to reduce the interfacial resistance to zero are in a wide range, which provides substantially great robustness to this approach in practical applications. As will be described later, the coating layer is covered with an ultrathin layer of thin oxide (SnO / SnO2), and thus the overall composition of the coating is Cu hereinafter. z Sn y O x It is expressed as (6 / 5 < z / y < 3).

[0217] Cu z Sn y O x The morphology of the surface coating was characterized by SEM as shown in Figures 25(a) and 25(b). Cu z Sn y O x The low-magnification morphology of LLZTO with the coating (Figure 25(a)) is almost the same as that of the pure LLZTO surface (Figure 21(b)). It is clear that the surface of the melt-spun LLZTO is covered with a layer of dense nanoplatelets when observed at a higher magnification (Figure 25(b)), which is different from the smooth surface of pure LLZTO (Figure 26(a)). As schematically shown in Figure 25(c), the original micro-sized particles of the Cu-Sn powder (Figure 26(b)) are proposed to melt at high temperature during the melt-spinning process. The Cu-Sn liquid has a strong affinity for LLZTO due to the interaction of Li and O from LLZTO with Sn and Cu respectively, and thus wets and spreads over the entire LLZTO surface. Upon cooling, the Cu-Sn liquid easily converts to solid Cu z Sn y O x and this substantially uniformly covers the LLZTO surface, and the morphology of the nanoplatelets is Cu z Sn y O xDue to crystal growth (Tian, Yanhong, et al. "Relationship between morphologies and orientations of Cu6Sn5grains in Sn 3.0 Ag 0.5 Cu solder joints on different Cu pads." Materials characterization 88 (2014): 58-68). Cu z Sn y O x To further identify the coating composition, XRD and XPS characterization were performed on the coated LLZTO. As shown in Fig. 25(d), in addition to the LLZTO peaks, several characteristic diffraction peaks attributable to Cu6Sn5, β-Sn, Cu, SnO, and SnO2 were identified compared to pure LLZTO. Cu3SnO z Sn y O x The XPS spectrum (Fig. 25(e)) with a lateral resolution of approximately 100 μm (about 730 LLZTO particles) recorded for the coated LLZTO shows only the main characteristic peaks corresponding to Sn, Cu, and O, while the peaks corresponding to LLZTO are not detectable by the surface-sensitive XPS technique, indicating that the LLZTO surface is completely covered by a substantially uniform layer of Cu3SnO x Cu3SnO x Cu3SnO xThe coating layer is then removed by sputtering for about 10 seconds, which corresponds to the removal of a surface coating layer with a thickness of about 5 nm. From the new XPS spectrum, new peaks indexed to La, Zr, and Ta from LLZTO appear (Cheng, Lei, et al. "Garnet electrolyte surface degradation and recovery." ACS Applied Energy Materials 1.12 (2018): 7244-7252; and Sharafi, Asma, et al. "Impact of air exposure and surface chemistry on Li-Li7La3Zr2O 12 interfacial resistance." Journal of Materials Chemistry A 5.26 (2017): 13475-13487), which further indicates the presence of a thin and substantially uniform Cu3SnO x coating on the LLZTO surface. According to the XPS survey spectrum, a peak corresponding to oxygen is detected in the coating layer (Figure 25(e)), and high-resolution spectra of Sn (Figure 25(f)) and Cu (Figure 25(g)) are further recorded to clarify the local coordination of oxygen. Oxygen was found to mainly bond to Sn atoms located on the outer surface of the coating because Sn is inevitably oxidized when in contact with the atmosphere. In this experiment, the cause of the surface oxygen is that Sn is inevitably exposed to oxygen during both the alloy powder preparation and the melt quenching step; the presence of oxygen molecules (i.e., an oxygenated environment) has been shown to improve the wettability of the liquid metal with the garnet substrate and thus improve the uniformity compared to the coating obtained in an oxygen-lean atmosphere. Furthermore, such an oxide layer (SnO / SnO2, hereinafter denoted as SnO x ) may be beneficial as it effectively protects the garnet-type solid electrolyte and the underlying metal-based coating from ambient humidity. As shown in Figure 27, the interfacial resistance of the uncoated LLZTO doubled after 15 minutes of exposure to the ambient, while that of Cu3SnO xThe coated LLZTO remained unchanged, indicating that the LLZTO surface was intact even after exposure to the same ambient conditions. Cu3SnO x The composition profile of the coated LLZTO is schematically shown in Fig. 25(h): a layer containing Cu6Sn5, Sn, and possibly Cu3SnO x substantially uniformly coats the LLZTO surface, while the top of the coating layer is covered with a thin layer of tin oxide.

[0218] To further understand the formation mechanism of the Cu-Sn-based coating, first-principles calculations were performed at both 0 K and finite temperatures. One of the most important factors determining the wetting characteristics of liquid metals on ceramic substrates is the contact angle, which can be derived from Eqs. 4 and 5, where

Eq.

Number

[0219] Figure 28(a) summarizes the calculated adsorption energies of Sn and Cu as a function of the number of layers. The contact angles of Cu and Sn on the LLZTO surface are calculated to be 108° and 47° at 0 K, respectively. If a contact angle smaller than 90° indicates that the liquid can completely wet the substrate, the calculation shows that Sn can easily wet and spread across the surface of LLZTO during melt quenching. The contact angle of Cu is slightly larger than 90° at 0 K, but it has been widely reported that the contact angle may decrease significantly with increasing temperature (Russell, Kenneth C., et al. "Theoretical and experimental studies of ceramic: metal wetting." MRS bulletin 16.4 (1991): 46-52). Therefore, the contact angle of Cu during the melt quenching process becomes smaller than 90°. Thus, to study the wetting behavior at finite temperatures, Cu and Cu x Sn yFirst-principles molecular dynamics simulations were performed for , and the results are shown in Fig. 28(b). It can be observed that Cu atoms interact with the LLZTO surface and tend to gradually form a coating layer over time. This process is accelerated when Cu is mixed with Sn to form an alloy layer on LLZTO, which is consistent with the experiment showing that the Cu-Sn-based coating material (Cu / Sn = 3) is superior to Sn in terms of reducing interfacial resistance (Fig. 21(f)). There are two main factors for the better wetting of the Cu-Sn-based coating material (Cu / Sn = 3): (i) the wetting reaction rate of Cu3Sn is faster than that of Cu, as indicated by a larger mean square displacement (Fig. 29(a)); (ii) it has been revealed that Cu atoms and Sn atoms show a strong synergistic effect when interacting with LLZTO. When Cu3Sn is coated on the surface of LLZTO, Sn atoms tend to strongly bond with Li, and the Gibbs free energy at the interface between Cu3Sn and LLZTO decreases (Fig. 29(b)). As a result, the original Li-O bond on the LLZTO surface weakens. Oxygen atoms have a very strong electron-withdrawing ability and originally attract electrons from lithium on the LLZTO surface. The weakening of the original Li-O bond leads to less strongly coordinated O, which in turn attracts electrons from Cu and strongly adsorbs Cu atoms on the LLZTO surface, substantially improving the wetting of Cu3Sn on the LLZTO surface.

[0220] Similar to the lithium alloying intermediate layers reported so far (Luo, Wei, et al. "Reducing interfacial resistance between garnet-structured solid-state electrolyte and Li-metal anode by a germanium layer." Advanced Materials 29.22 (2017): 1606042; and Krauskopf, Thorben, et al. "Diffusion limitation of lithium metal and Li-Mg alloy anodes on LLZO type solid electrolytes as a function of temperature and pressure." Advanced Energy Materials 9.44 (2019): 1902568), LLZTO / Cu z Sn y O x When contacting the Li metal anode in the battery, Cu z Sn y O x coating forms a ternary system of a Cu-Li-Sn intermediate layer rich in lithium at the interface between the garnet and the lithium metal through a fast alloying process with lithium. Such a lithium-rich ternary intermediate layer alloy (including a trace amount of Li2O) completely eliminates the interfacial resistance, establishes a super-stable electrolyte-electrode interface, and provides high lithium ion conductivity through the alloying-dealloying process at the LLZTO / intermediate layer and Li / intermediate layer interfaces. In fact, LLZTO / Cu z Sn y O x has the same lithium ion conductivity as pure LLZTO (about 8×10 -4 S cm -1 at room temperature, and 7×10 -3 S cm -1 at a temperature of 60 °C), which means that Cu z Sn y O xThis supports that the intermediate layer coating does not sacrifice the high ionic conductivity of the garnet electrolyte. To support the interfacial stability during the operation of the electrochemical cell, the interfaces after extensive lithium plating were characterized by SEM and EDS. Cu3SnO x A lithium symmetric cell was assembled using the coated LLZTO solid electrolyte, and the lithium electrode on one side of the solid electrolyte was completely peeled off and transported to the opposite side at a constant current density of 0.2 mA cm -2 . As confirmed by SEM, no visible voids were formed in the vicinity of the newly formed interface between the plated Li and the Cu3SnO x coated LLZTO solid electrolyte (Figs. 28(c) and 28(d)), which was observed to be in stark contrast to the poor interfacial contact between Li and pure LLZTO (Fig. 31). Despite extensive lithium plating on LLZTO, according to EDS mapping, the Cu3SnO x coating layer remained substantially intact on the LLZTO surface (Fig. 28(e)), while no significant dissolution of LLZTO into lithium was confirmed (Figs. 28(f) - 28(h)), which supports the stability of LLZTO against lithium (Zhu, Yizhou, et al. "Origin of outstanding stability in the lithium solid electrolyte materials: insights from thermodynamic analyses based on first - principles calculations." ACS applied materials & interfaces 7.42 (2015): 23685 - 23693).

[0221] Cu3SnO combined with a lithium metal anode xTo evaluate the stability of the coated LLZTO solid electrolyte, symmetric cells were assembled and tested at various current densities and temperatures. The critical current density at which significant dendrite growth and short-circuiting occur (Huang, Xiao, et al. "None-mother-powder method to prepare dense Li-garnet solid electrolytes with high critical current density." ACS Applied Energy Materials 1.10 (2018): 5355-5365; and Song, Yongli, et al. "Revealing the short-circuiting mechanism of garnet-based solid-state electrolyte." Advanced Energy Materials 9.21 (2019): 1900671) was tested for the coated LLZTO solid electrolyte. The assembled symmetric cells could maintain their stability until reaching the critical current density of about 3 mA cm x −2 (Fig. 32(a)) and about 15.2 mA cm -2 −2 (Fig. 32(b)) at room temperature and 60 °C, respectively, as the current density increased. Since the interfacial resistance is completely eliminated, the critical current density is only affected by the ionic conductivity of LLZTO. The ionic conductivity of the electrolyte increases by a factor of 4 at 60 °C compared to room temperature, while the critical current density also increases by approximately 4 times. A critical current density of 3 mA cm -2 −2 is widely recognized as essential for the operation of practical lithium metal batteries with high-energy cathodes (Flatscher, Florian, et al. "The natural critical current density limit for Li7La3Zr2O -2 garnets." Journal of Materials Chemistry A 8.31 (2020): 15782-15788); therefore, Cu3SnO as defined herein 12 ... xThe all-solid-state lithium metal battery based on coated LLZTO not only meets the requirements for practical applications but also possesses a critical current density that is one of the highest values reported for any type of solid electrolyte (Table 1).

[0222]

Table 1

[0223] Li / LLZTO / Cu3SnO x / Li's cycle stability at different current densities and temperatures was also tested, and the results are shown in Figs. 32(c), 32(d), and 33. The Cu / Sn molar ratio of 3:1 (LLZTO / Cu3SnO x ) of Cu3SnO x The symmetric cell composed of the coated LLZTO solid electrolyte can undergo lithium plating / stripping cycles at current densities of 0.2 mA cm -2 and 0.5 mA cm -2 at room temperature for more than 4000 hours, with the change in overvoltage being negligible and no signs of internal short circuit, which demonstrates the exceptional stability of the Cu3SnO x coated LLZTO solid electrolyte in lithium metal batteries. The stable overvoltage of the Cu3SnO x coated LLZTO solid electrolyte is significantly contrasted with the continuously increasing overvoltages of the pure LLZTO solid electrolyte and the Sn-coated LLZTO solid electrolyte (Fig. 34). Furthermore, the cycle performance of the Cu6Sn5O x coated LLZTO solid electrolyte was also tested, and the results are essentially the same as those of the Cu3SnO x coated LLZTO solid electrolyte, both showing similar EIS spectra with negligible interfacial resistance (Fig. 21(h)). The cycle stability of the symmetric cell based on the Cu3SnO x coated LLZTO solid electrolyte was also tested at a temperature of 60 °C and a current density of 4 mA cm -2Evaluated at high current densities and further used operando EIS to characterize the impedance changes over time; the results show that the symmetric cell can operate stably for 1600 hours without an increase in overvoltage (Figure 32(d)). The Nyquist diagrams resulting from operando EIS measurements show that the resistance remains almost constant at 11 Ω cm -2 even when repeating the Li plating / stripping cycles (Figure 32(e)), which is also consistent with the stable overvoltage.

[0224] The results of the symmetric cell tests indicate that the Cu3SnO x coated LLZTO solid electrolyte has a stable interface with the lithium metal anode, and that a lithium metal battery consisting of the Cu3SnO x coated LLZTO solid electrolyte may have excellent stability when combined with a high-energy cathode. To demonstrate the utility of the Cu3SnO x coated LLZTO solid electrolyte in a practical battery, as schematically shown in Figure 35(a), a all-solid-state lithium metal electrochemical cell was assembled using a composite cathode and a lithium metal anode. Figure 35(b) shows the EIS spectra obtained at room temperature for all-solid-state electrochemical cells consisting of LFP and NMC811 cathodes. The semicircle in the high-frequency region reflects the bulk ionic resistance resulting from Li + conduction in the LLZTO solid electrolyte, and the semicircle in the mid-frequency region represents the charge transfer resistance of the cathode (Kim, Sangryun, et al. "A complex hydride lithium superionic conductor for high-energy-density all-solid-state lithium metal batteries." Nature communications 10.1 (2019): 1-9). It is clear that there is no semicircle in the low-frequency region for both the LFP and NMC811 electrochemical cells, which indicates that the interfacial resistance has been substantially completely eliminated, which is consistent with the results of the symmetric cells (Figure 21(g)).

[0225] The cycle performance and rate performance of an electrochemical cell with an LFP cathode were tested at room temperature. According to the results, the electrochemical cell can achieve a reversible capacity of 155 mAh g -1 and was shown to be able to achieve a remarkable initial Coulombic efficiency of 94.6% and an outstanding capacity retention rate of 99% after 100 stable cycles at 0.2C (Figure 36(a)). Furthermore, the electrochemical cell can maintain 80% of its capacity with only a slight increase in charge / discharge overvoltage even when cycled at a high rate of 1C (Figure 36(b)) (Figure 36(c)).

[0226] It is worth noting that composite cathodes containing active materials and polymer electrolytes are being used in electrochemical cells for two main reasons: (i) the polymer electrolyte in the composite cathode promotes better interfacial contact with the LLZTO solid electrolyte, resulting in better rate performance; (ii) in particular, in electrochemical cells equipped with NMC811 cathodes operating at higher voltages, side reactions between the electrolyte and the cathode can be significantly prevented. Due to the absence of side reactions at the cathode / electrolyte interface, all-solid-state electrochemical cells with high-voltage NMC811 cathodes have excellent cycle stability and initial Coulombic efficiency. As shown in Fig. 36(c), the electrochemical cell exhibits an initial Coulombic efficiency of 85.4% and can maintain 74% of its capacity even when cycled at a high rate of 1C. This excellent rate capability of the electrochemical cell is mainly due to the electrolyte / lithium interface without resistance and stable lithium plating / stripping at the anode with low overvoltage, which results in a consistent voltage plateau for the electrochemical cell. The NMC811 cathode is known to have poor cycle stability when operated at high voltages due to side reactions with liquid electrolytes. Compared with the continuous capacity decay of the liquid cell (Fig. 37), the all-solid-state electrochemical cell can achieve 94% of its initial capacity after running for 1000 cycles at 1C for a long time. More importantly, during the stable 1000 cycles of the all-solid-state electrochemical cell, the average Coulombic efficiency exceeds 99.9%, and no internal short circuit due to dendritic lithium growth is observed, indicating excellent safety compared with liquid cells. The electrochemical performance of the present electrochemical cell as defined herein is Cu3SnO x Due to the high interfacial stability at the interface between the coated LLZTO solid electrolyte and the lithium metal anode, it has one of the highest performances among all reported all-solid-state batteries (Table 1).

[0227] In summary, the surface of the garnet electrolyte is Cu z Sn y O xIt is modified by coating a thin layer with (6 / 5 < z / y < 1 / 2). The coating mechanism was systematically studied, and the electrochemical performance of the surface-modified garnet solid electrolyte was comprehensively tested in both symmetric and full electrochemical cells. Four important findings can be summarized as follows: (1) A uniform and dense Cu z Sn y O x coating can be achieved within seconds by the melt quenching approach, and the surface-modified garnet electrolyte shows negligible interfacial resistance when combined with a lithium metal anode. LLZTO / Cu z Sn y O x has lithium ion conductivities of 8.0×10 -4 and 7.0×10 -3 S cm -1 at room temperature and 60 °C, respectively, and electron conductivity and lithium transference number are 7.0×10 -8 S cm -1 and 0.99, respectively. (2) It has been revealed that the synergistic effect between Cu and Sn when Cu and Sn interact with the garnet surface at high temperature is the main reason for forming a uniform coating, and the coating can be formed over a wide range of Cu-Sn compositions and temperatures. (3) Symmetric cells based on the surface-modified garnet electrolyte show critical current densities of 3 mA cm -2 and 15.2 mA cm -2 at room temperature and 60 °C, respectively, and can operate substantially stably for 4000 hours without short circuit. (4) All-solid-state electrochemical cells composed of the surface-modified garnet electrolyte and an NMC811 cathode can achieve 94% of their initial capacity with an average Coulombic efficiency of over 99.9% after 1000 cycles at 1C for a long time.

[0228] Example 7 Densified pure Li 6.5 La3Zr 1.5 Ta0.5 O 12 (LLZTO) Solid Electrolyte Synthesis and Property Evaluation

[0229] Pure garnet-type solid electrolytes can be prepared by the rapid heating method defined herein. As an example, Li 6.5 La3Zr 1.5 Ta 0.5 O 12(LLZTO) was synthesized by the Joule heating method. To obtain a uniform and densified electrolyte, conventional lithium metal hydroxide (LiOH) and lithium metal carbonate precursor (Li2CO3), which are commonly used in the conventional solid synthesis of oxide-based solid electrolytes, were replaced with lithium oxide precursor (Li2O). LiOH and Li2CO3 may release gaseous products during heat treatment, which results in the pulverization of the final solid electrolyte and thus hinders its densification; also, the gaseous products due to the decomposition of LiOH and Li2CO3 precursors may corrode the heating element (e.g., graphite) during Joule heating. In addition, the conventional zirconium dioxide precursor (ZrO2) was also replaced with a lithium zirconium oxide (Li2ZrO3) precursor. The Li2ZrO3 precursor has a much lower melting point (720 °C) compared to ZrO2 (2715 °C), which means that it requires substantially lower sintering temperature and substantially shorter sintering time. Each precursor Li2O, ZrO2 or Li2ZrO3, La2O3, and Ta2O5 was weighed to obtain the desired stoichiometry. For example, LLZTO (Z) was prepared using Li2O, ZrO2, La2O3, and Ta2O5 precursors in a molar ratio of 3.57:1.5:1.5:0.25. In another example, LLZTO (LZ) was prepared using Li2O, LiZrO2, La2O3, and Ta2O5 precursors in a molar ratio of 2.75:1.5:1.5:0.25. Next, the samples were uniformly mixed in a planetary ball mill at 300 rpm for about 10 hours. Next, the obtained mixture was cold pressed into pellets. Next, the prepared precursor pellets were sandwiched between two graphite heating elements and subjected to rapid heat treatment at a temperature of about 1200 °C for about 10 seconds under an argon atmosphere. Next, the densified LLZTO (Z) solid electrolyte and LLZTO (LZ) solid electrolyte were taken out from the rapid heating device and stored in an argon-filled glove box.

[0230] As confirmed by the XRD data shown in FIG. 38, both the LLZTO(Z) solid electrolyte and the LLZTO(LZ) solid electrolyte show substantially high-purity phases without signs of impurities. The SEM images of the LLZTO(Z) solid electrolyte and the LLZTO(LZ) solid electrolyte show that the particle size of LLZTO(LZ) (about 7.5 μm) is larger compared to the particle size of LLZTO(Z) (about 4.1 μm); the larger particle size of LLZTO(LZ) is clearly due to the fact that the sintering temperature of LiZrO2 used in the synthesis of LLZTO(LZ) is lower compared to ZrO2 used in the synthesis of the LLZTO(Z) electrolyte. The EIS measurements of the LLZTO(LZ) solid electrolyte and the LLZTO(Z) solid electrolyte are 5.9×10 -4 S cm -1 and 2.5×10 -4 S cm -1 of Li + conductivity, respectively. The higher Li + conductivity of LLZTO(LZ) is due to its larger particle size and thus fewer grain boundaries, which results in lower grain boundary resistance and can be interpreted as higher Li + conductivity.

[0231] A one-step synthesis process for synthesizing garnet-type solid electrolytes from metal oxide precursors using a Joule heating system was developed. This technique substantially reduced the production cost (i.e., time and energy) compared to the conventional synthesis of synthesizing and sintering garnet-type solid electrolytes using a furnace.

[0232] Example 8 Synthesis and Characterization of Densified Li 6.5 La3Zr 1.5 Ta 0.5 O 12 (LLZTO) Solid Electrolytes

[0233] The densified pure LLZTO (LZ) solid electrolyte prepared in Example 7 was coated with a layer of a metal-based material containing at least one metal element selected from Group 14 and Group 15 elements and at least one metal fluoride having a metal element selected from Group 14 and Group 15 elements. For example, the metal element selected from Group 14 and Group 15 elements can react with lithium to form a Li-conductive compound, and the metal fluoride can react with lithium metal to form lithium fluoride (LiF) and a Li-conductive compound. LiF is an electron insulator and, when formed, later functions as a filler within the coating, promoting Li + conduction and preventing the metal element from dissolving into the lithium metal negative electrode during cycling.

[0234] As an example, a mixture of Sn metal powder and SnF2 precursor corresponding to a weight ratio of 10:0.5, 10:1, 10:2, 10:3, 10:4, 10:5 (Sn:SnF2) was prepared by weighing the corresponding powders and uniformly mixing the powders using either a mortar and pestle or a ball milling method. The coating was applied to the surface of the LLZTO (LZ) pellet prepared in Example 7. To coat the LLZTO (LZ) surface with the metal-based material by the melt quenching method, the LLZTO (LZ) pellet was rubbed against an excess amount of coating precursor powder spread on weighing paper, during which metal particles and metal fluoride particles adhered to the garnet surface by van der Waals forces. Next, the free powder was blown off the garnet surface using a jet of argon gas. Next, the metal-based precursor-treated LLZTO (LZ) pellet was sandwiched between two graphite heating elements with the coated side facing up, the temperature was rapidly raised to about 1100 °C, and maintained at this temperature for about 3 seconds to melt the metal precursor and allow the liquid metal to spread completely over the entire LLZTO (LZ) surface. The sample was superquenched at a cooling rate of about 1 × 10 3 °C / min -1 to obtain a uniform metal-based coating.

[0235] The interfacial resistance of various Sn-SnF2-coated LLZTO (LZ) solid electrolytes prepared in Example 8 was characterized by EIS measurement. The results in Fig. 41 show that the interfacial resistance decreases from Sn:SnF2 10:0.5 (15.6 Ω) to Sn:SnF2 10:3 (<1 Ω), and then increases to 13.1 Ω at Sn:SnF2 10:5, indicating that the coating layer with a composition of Sn:SnF2 10:3 has the lowest interfacial resistance with the lithium metal anode. The critical current density at which significant dendrite growth and short circuit occur was tested for Li / Sn-SnF2-LLZTO (LZ)-Sn-SnF2 / Li with different Sn:SnF2 ratios. The highest rate performance of 5.8 mA cm -2 was observed in the intermediate layer based on Sn:SnF2 (10:3) (Fig. 42); while the LLZTO (LZ) solid electrolytes coated with Sn:SnF2 (10:0.5), Sn:SnF2 (10:1), Sn:SnF2 (10:2), Sn:SnF2 (10:4), and Sn:SnF2 (10:5) showed critical current densities of 1.0, 3.0, 3.4, 3.6, and 2.6 mA cm -2 respectively.

[0236] Without departing from the scope of the present invention, numerous modifications can be made to any of the above embodiments. All references, patents, or scientific literature documents referred to in this application are hereby incorporated by reference in their entirety for all purposes.

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