Oxide solid electrolyte and co-sintering method for layered solid electrolyte

Abstract

An oxide solid electrolyte and a method for co-sintering a laminated solid electrolyte are disclosed. The oxide solid electrolyte is a NASICON (Na Super Ionic Conductor) based oxide solid electrolyte and comprises a laminated structure including a first NASICON-based electrolyte layer and a second NASICON-based electrolyte layer joined together, wherein the first NASICON-based electrolyte layer has a relatively higher ionic conductivity than the second NASICON-based electrolyte layer and the second NASICON-based electrolyte layer has a relatively higher chemical stability than the first NASICON-based electrolyte layer.

Description

Technology Field

[0001] The present invention relates to an oxide solid electrolyte and a method for co-sintering a solid electrolyte, and can be applied to the field of solid electrolyte technology for electrochemical energy storage devices including solid batteries, all-solid-state secondary batteries, and lithium metal batteries. Background Technology

[0002] Sodium superionic conductors (Na Super Ionic Conductors, NASICON) with the general chemical formula AM2(XO4)3 are a group of materials discovered in the 1970s, Na + It features a three-dimensional skeletal structure with interconnected channels that enable rapid ion movement. Due to these structural characteristics, NASICON materials possess both low electronic conductivity and high ionic conductivity, making them widely studied as solid-state electrolytes (SSEs). A key feature that distinguishes NASICON-based electrolytes from other types of SSEs is their excellent stability in the atmosphere, specifically that they do not react with moisture in the air.

[0003] However, despite this excellent atmospheric stability, the chemical stability of NASICON-based electrolytes for lithium metal depends on the element at the M site. If Al is substituted at the M site of LiTi2(PO4)3(LTP), Li 1+x Al x Ti 2-x (PO4)3(LATP) is formed, which is 10 -4 - 10 -3 S cm -1 It is evaluated as a promising solid electrolyte due to its high ionic conductivity within the range. However, despite its high conductivity, Ti 4+ is 2.4V (vs. Li +Li is irreversibly reduced at low potentials, leading to the formation of harmful secondary phases during electrochemical cycling. These secondary phases generated by reduction cause volume changes at the Li|SSE interface, resulting in mechanical stress, increased interfacial resistance, and reduced cycle stability of the material. This interfacial instability is the biggest obstacle to the commercialization of LATP. Therefore, developing strategies to protect the LATP surface without degrading ionic conductivity remains an important research task.

[0004] One of the most widely studied approaches to mitigate the electrochemical instability of LATP with respect to lithium metal is the insertion of a stabilizing interlayer between the lithium cathode and the LATP electrolyte, commonly referred to as the "sandwich strategy." The selection of the interlayer material and the deposition method have a decisive influence on interfacial stability and ion transport through the layer. Organic polymer coatings are preferred because their flexibility and excellent wettability enable close contact with lithium and provide low interfacial resistance. However, polymer coatings have limitations such as complex processes, flammability, and a narrow electrochemical window. On the other hand, inorganic materials are attracting attention as an alternative for interfacial protection due to their relatively simple synthesis processes and high thermal stability. Compared to polymer coatings, inorganic coatings such as metal oxides offer enhanced electrochemical stability but have the disadvantage of high initial resistance due to low ionic conductivity.

[0005] To address the problem of the ion-nonconductive layer, some studies have introduced NASICON-based ceramics as an interlayer. Zhao et al. [researched] Li, which has a crystal structure similar to LATP and exhibits similar ion conductivity. 1.3 Al 0.3 Ge 1.7A (PO4)3(LAGP) intermediate layer was used. As a result, the LAGP layer improved interfacial stability while maintaining high ionic conductivity. However, since LAGP also reacts with lithium metal, it is not a fundamental solution to the LATP reduction problem. The case of LAGP suggests that to fundamentally solve the LATP reduction problem, an intermediate layer that is structurally compatible and chemically inert to lithium metal is required.

[0006] Among strategies for introducing various inorganic layers, the co-sintering process is considered the most ideal approach in terms of simplicity and cost-effectiveness. This process minimizes interfacial resistance by simultaneously sintering two different ceramic layers in a single heat treatment step to form strong physical and chemical bonds at the interface. However, the process of integrating different ceramics into a single monolithic structure entails fundamental technical challenges due to the shrinkage inevitably associated with the sintering process. Mismatches in interlayer shrinkage behavior can generate internal stress, leading to mechanical failures such as warping or cracking. This issue stems from the densification process, which is governed by atomic diffusion, as the diffusion and shrinkage behaviors between ceramics differ depending on the difference in atomic mass. In fact, the initial green density significantly influences shrinkage, while denser green bodies undergo relatively smaller volume changes. Methods such as tape casting, hot pressing, and magnetron sputtering have been applied, but these processes are not suitable for mass production.

[0007] Accordingly, a simple yet robust LATP-based ceramic bilayer structure was fabricated through cold pressing and co-sintering following the existing powder sintering process without additional equipment. This scalable process is inherently efficient and highly reliable, and the remaining challenge is to precisely control the green density of each layer to suppress failure due to internal stress.

[0008] To address the interfacial instability between the lithium metal and LATP, an ion-conductive LiZr2(PO4)3 (LZP) layer that does not react with lithium was introduced onto the LATP electrolyte via a cold-press co-sintering process. This suppressed dendrite formation and ensured interfacial stability. This scalable approach forms a structure consisting of a porous LZP top layer and a dense LATP bottom layer, simultaneously achieving strong interfacial bonding and structural stability. The designed bilayer structure suppresses Ti reduction and Li dendrite formation, resulting in 1.7 × 10⁻⁶ -4 S cm -1 High ionic conductivity of 4.3 mA cm -2 It exhibits a critical current density, which is a performance improvement of more than 14 times compared to a single LATP.

[0009] Cross-sectional analysis after charge-discharge confirmed that Li was uniformly deposited along the pore surface of LZP, demonstrating that it effectively mitigates dendrite growth. In the Li|SSE|LiFePO4 battery configuration, the double-layer SSE exhibited a lower interfacial resistance of 72.3 Ω compared to 157.7 Ω for LATP. This improvement in interfacial stability led to an improvement in cycle life, with the double-layer SSE achieving a capacity retention rate of 81% after 250 charge-discharge cycles under 0.5 C conditions, which is significantly higher than the 56% of LATP.

[0010] The interfacial engineering approach presented in this invention significantly enhances the commercialization potential of LATP. Furthermore, the proposed cold-pressure co-sintering process provides a scalable framework applicable to other solid electrolytes requiring co-sintering. This approach offers a solution that goes beyond the conventional "sandwich strategy" by utilizing the different sintering characteristics of LZP and LATP to construct a robust double-layer interface. Overall, this invention advances the development of practical all-solid-state batteries by resolving the inherent interfacial instability issues of LATP while presenting a universal co-sintering process applicable to NASICON-based solid electrolytes. The problem to be solved

[0011] Oxide solid electrolytes possess excellent chemical stability and mechanical strength, making them highly promising for application in all-solid-state secondary batteries; however, a single material has limitations in simultaneously satisfying both ionic conductivity and chemical stability. Consequently, attempts have been made to stack oxide solid electrolyte layers with different properties; however, when these stacked solid electrolytes are manufactured by simultaneous sintering, problems arise such as mechanical deformation occurring during the sintering process, including warping, cracking, or delamination.

[0012] The present invention aims to provide a new laminated solid electrolyte and a method for manufacturing the same, which can suppress mechanical deformation by effectively controlling interlayer shrinkage behavior during the co-sintering process of the laminated solid electrolyte, in order to solve the problems of the conventional technology described above. means of solving the problem

[0013] In one aspect, the present invention provides an oxide solid electrolyte comprising a NASICON (Na Super Ionic Conductor)-based oxide solid electrolyte, wherein the oxide solid electrolyte comprises a laminated structure including a first NASICON-based electrolyte layer and a second NASICON-based electrolyte layer joined together, wherein the first NASICON-based electrolyte layer has a relatively higher ionic conductivity than the second NASICON-based electrolyte layer and the second NASICON-based electrolyte layer has a relatively higher chemical stability than the first NASICON-based electrolyte layer. Accordingly, the laminated structure can simultaneously achieve high ionic conductivity and high chemical stability, which are difficult to achieve simultaneously with a single electrolyte.

[0014] The present invention relates to an oxide solid electrolyte having a NASICON (Na Super Ionic Conductor) structure, and more specifically, to a technology that aims to simultaneously satisfy ionic conductivity and chemical stability by realizing a single solid electrolyte structure through the stacking of NASICON-based electrolytes having different physical properties.

[0015] In this specification, “NASICON-based electrolyte” refers to an oxide-based ion conductor having a crystal framework of AM2(XO4)3 and encompasses an electrolyte comprising a structure capable of transporting lithium ions or sodium ions. Additionally, “laminated structure” refers to a structure in which different electrolyte layers are arranged in the thickness direction and joined through an interface.

[0016] In this specification, the term “green layer” refers to an electrolyte layer in a state prior to sintering, in which electrolyte powder is pressure-molded to have a certain shape and density. The term “green density” refers to the relative density of the green layer prior to sintering.

[0017] In order to realize a solid electrolyte with a stacked structure as described above, a first electrolyte powder and a second electrolyte powder were stacked and then sintered to form a single structure. However, it was discovered that when electrolyte powders having different compositions and physical properties are simply stacked and sintered simultaneously under the same pressure conditions, mechanical defects such as warping, cracking, or delamination occur in the structure due to differences in shrinkage behavior of each electrolyte layer during the sintering process. These problems not only reduce the mechanical reliability of the stacked solid electrolyte but can also lead to a decrease in electrolyte performance by causing interfacial defects.

[0018] To solve the above problem, experiments were conducted by varying the lamination process and molding conditions. For example, various attempts were made, such as applying the two electrolyte powders to the same pressure, evenly dispersing the upper electrolyte powder over the lower electrolyte layer, using a flat plate, or applying different pressures to the upper and lower electrolyte layers. As a result, it was confirmed that when the first electrolyte powder is pressed to a relatively high pressure to form the first green layer, and the second electrolyte powder placed on top is pressed to a relatively low pressure to form the second green layer, and then sintered simultaneously, the mechanical deformation occurring during the sintering process is significantly suppressed. This is understood to be because the shrinkage behavior of the two electrolyte layers acts complementarily during the co-sintering process, thereby suppressing deformation of the entire laminated structure and achieving stable interfacial bonding.

[0019] In a stacked solid electrolyte structure formed by such process control, the first NASICON-based electrolyte layer can form a more dense microstructure near the interface compared to a single-layer structure, and the second NASICON-based electrolyte layer can be formed with a structure containing residual pores. These structural features contribute to maintaining or improving the efficiency of ion conduction through the first electrolyte layer, while simultaneously ensuring chemical stability and interfacial stability through the second electrolyte layer.

[0020] Accordingly, the oxide solid electrolyte according to the present invention can effectively combine the advantages of each electrolyte layer through a stacked structure and simultaneously provide excellent structural stability and electrochemical performance by suppressing mechanical defects during the co-sintering process. Due to these effects, the oxide solid electrolyte of the present invention can be advantageously applied to various electrochemical energy storage devices, such as all-solid-state secondary batteries and lithium metal batteries.

[0021] The second NASICON-based electrolyte layer of the present invention is characterized by comprising a porous structure. In the present invention, the fact that the second NASICON-based electrolyte layer comprises a “porous structure” means a structure in which a plurality of pores are formed within the electrolyte layer, so that empty spaces coexist with a continuous solid phase. Such a porous structure can be formed as the second NASICON-based electrolyte layer is molded with a relatively low green density during the co-sintering process.

[0022] The porous structure of the present invention is characterized by suppressing lithium dendrite growth at the interface with lithium metal. The porous structure of the second NASICON-based electrolyte layer plays an important role at the interface in contact with lithium metal. That is, the pores provide a pathway for the movement of lithium ions while simultaneously mitigating the local concentration of lithium metal precipitation, thereby suppressing the localization of current density. Accordingly, the phenomenon of lithium metal growing in a dendritic form can be suppressed. Consequently, the second NASICON-based electrolyte layer having the porous structure improves interface stability with lithium metal and provides the effect of preventing short circuits or battery performance degradation caused by dendrite growth.

[0023] The first NASICON-based electrolyte layer of the present invention is characterized by being a NASICON-based electrolyte comprising Al and Ti, and the first NASICON-based electrolyte layer is Li 1+x Al x Ti 2-x It may include (PO4)3(0 < x ≤ 0.5).

[0024] The second NASICON-based electrolyte layer of the present invention is characterized as being a NASICON-based electrolyte containing Zr, wherein the second NASICON-based electrolyte layer may include LiZr2(PO4)3, and the first NASICON-based electrolyte layer is Li 1+x Al x Ti 2-x It may include (PO4)3(0 < x ≤ 0.5). As a non-limiting example, LiZr2(PO4)3 / Li 1.3 Al 0.3 Ti 1.7 When designed with a (PO4)3(LZP / LATP) bilayer structure, it improves interfacial stability with lithium metal and effectively suppresses Ti reduction and the subsequent formation of secondary phases, and 1.7 x 10 -4 S cm -1 High ionic conductivity of 4.3 mA cm-2 It exhibits excellent critical current density characteristics.

[0025] The laminated structure of the present invention is Li 1+x Al x Ti 2-x The invention is characterized by being manufactured by applying a first pressure to a powder containing (PO4)3 (0 < x ≤ 0.5) to form a first green layer; applying a powder containing LiZr2(PO4)3 to the first green layer and applying a second pressure lower than the first pressure to form a second green layer; and then co-sintering a laminate containing the first green layer and the second green layer.

[0026] As a non-limiting example, Li 1.3 Al 0.3 Ti 1.7 The first pressure applied to the powder containing (PO4)3 may be in the range of 20 to 30 MPa, and preferably may be applied at a condition of about 25 MPa. Since LZP has a relatively low intrinsic ionic conductivity, it is desirable to form a solid electrolyte bonded to LATP with a relatively thin thickness so as to exhibit a chemical protective function while minimizing the reduction in ionic conductivity. The powder containing LiZr2(PO4)3 may be laminated to a thickness of 35 to 45 μm, and at this time, the second pressure may be applied in the range of 5 to 10 MPa, and preferably may be applied at a condition of about 5 MPa.

[0027] In another aspect, the present invention provides a method for co-sintering a laminated solid electrolyte, comprising: (a) a step of forming a first green layer by pressing a first electrolyte powder with a first pressure; (b) a step of forming a second green layer by stacking a second electrolyte powder on the first green layer and pressing it with a second pressure lower than the first pressure; and (c) a step of co-sintering a laminate comprising the first green layer and the second green layer.

[0028] The first electrolyte powder of the present invention is Li 1+x Al x Ti 2-x It may include (PO4)3(0 < x ≤ 0.5).

[0029] The second electrolyte powder of the present invention may include LiZr2(PO4)3. Since LZP has a relatively low intrinsic ionic conductivity, it is desirable to form a solid electrolyte bonded to LATP with a relatively thin thickness so as to minimize the reduction in ionic conductivity while still providing a chemical protective function. The second electrolyte powder containing LiZr2(PO4)3 may be laminated to a thickness of 35 to 45 μm, wherein the second pressure is characterized by being in the range of 5 to 10 MPa, and preferably pressurized to a condition of about 5 MPa.

[0030] The first pressure of the present invention is characterized by being in the range of 20 to 30 MPa, and preferably can be pressurized to about 25 MPa.

[0031] Li, a NASICON-based solid electrolyte (SSE) 1.3 Al 0.3 Ti 1.7 (PO4)3(LATP) provides high ionic conductivity but is chemically unstable with respect to lithium metal. Upon direct contact with lithium metal, Ti 4+Ions are reduced, leading to the formation of harmful secondary phases. These secondary phases increase interfacial resistance and promote dendrite growth, thereby hindering the commercialization of LATP. In this invention, a chemically stable LiZr2(PO4)3 (LZP) protective layer is formed on LATP by co-sintering while maintaining high ionic conductivity. In particular, a novel co-sintering process is proposed that strategically utilizes the differences in shrinkage behavior between different materials. The co-sintered LZP / LATP forms a functional double layer consisting of a porous LZP layer that performs a chemical protective function and a mechanically robust and dense LATP layer. Electrochemical and ex-situ analysis results show that the LZP layer is Ti 4+ It was confirmed that it improves the chemical stability of LATP by inhibiting the reduction of and preventing the formation of a secondary phase. As a result, LZP / LATP is 1.7 × 10 -4 S cm -1 High ionic conductivity of 4.3 mA cm -2 It exhibited an excellent critical current density (CCD), which is 14.3 times better than single LATP, enabling high-power operation. This high CCD is attributed to the porous LZP structure that inhibits dendrite growth. In full-cell evaluations, LZP / LATP reduced the Li|SSE interfacial resistance to 46% of that of LATP and demonstrated superior cycle stability, showing a capacity retention rate of 81% even after 250 cycles under 0.5 C conditions, compared to 56% for LATP. In conclusion, the present invention presents an interfacial design strategy that simultaneously achieves chemical and physical stability by engineering inherent material property differences. This approach provides a scalable path to accelerate the commercialization of all-solid-state batteries with safe and high-power characteristics. Effects of the invention

[0032] The laminated solid electrolyte according to the present invention can achieve both high ionic conductivity and excellent chemical stability. In particular, the porous structure of the second electrolyte layer suppresses dendrite growth that may occur at the interface with lithium metal and improves interfacial contact characteristics, thereby providing the effect of improving the stability and lifespan of the battery. Furthermore, according to the present invention, mechanical deformations such as bending, cracking, and delamination, which were pointed out as problems during the co-sintering process of conventional laminated solid electrolytes, are significantly suppressed. Brief explanation of the drawing

[0033] Fig. 1. (a) Results comparing the trade-off relationship between redox potential and ionic conductivity of NASICON-type solid electrolytes (SSE). (b) Warping observed when LATP and LZP powders are compressed at a pressure of 25 MPa in a single step. (c) Cracking observed when a flat plate is used to suppress warping during the sintering process. (d) Layer separation occurring when high pressure is applied to the LZP layer during a multi-stage pressurization process. (e) Non-uniform coating phenomenon occurring when low pressure is applied to the LZP layer. Fig. 2. (a) Schematic diagram showing the defect-free co-sintering process. (b) Cross-sectional SEM image of an LZP / LATP pellet taken in BSE (back-scattered electron) mode. The bright and dark regions represent LZP and LATP, respectively. (c) XRD patterns of LZP / LATP, LATP, and LZP pellets after sintering. Fig. 3. (a) Impedance spectrum of LZP / LATP, (b) LATP, (c) LZP. (d) Equivalent circuit of a Pt|SSE|Pt symmetric cell. (e) 1–6 V (vs. Li + 0.5 mV s in the / Li) voltage range -1 (f) Cyclic voltammetry curves of each pellet measured at the scan rate. (f) Ex-situ XRD patterns of LZP / LATP and LATP pellets after constant current cycling. Fig. 4. (a) Critical current density (CDD) test results of symmetric cells using LZP / LATP, (b) LATP, and (c) LZP electrolytes. (d) Long-term constant current cycling test results under constant current density conditions of symmetric cells using LZP / LATP, (e) LATP, and (f) LZP, showing interfacial stability for more than 1000 hours. Fig. 5. (a) Schematic diagram of the lithium penetration and stabilization mechanism within the porous LZP layer. Highlights the lithium wetting and dendrite suppression effects. (b) Cross-sectional SEM image of an LZP / LATP / LZP pellet, showing the formation of porous LZP layers on both sides of a dense LATP core. In the cross-sectional BSE-SEM image taken after cycling, LZP appears brightest, LATP appears moderately bright, and lithium appears darkest, showing (c) suppressed dendrite growth and (d) improved interfacial stability through the appearance of lithium coated inside the LZP pores. Fig. 6. Results of full-cell evaluation of LZP / LATP, LATP, and LZP batteries using LiFePO4 (LFP) as the anode and lithium metal as the cathode. Nyquist plot of the full cell using the LFP electrode: (a) LZP / LATP, (b) LATP, (c) LZP. Experimental data (symbols) and fitting curves (lines) are shown. (d) Equivalent circuit of the Li|SSE|LFP full cell. (e) specific capacity of each sample, (f) rate performance, and (g) cycle stability. Specific details for implementing the invention

[0034] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. Since the present invention is susceptible to various modifications and may take various forms, specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed forms, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the present invention. Similar reference numerals have been used for similar components in the description of each drawing. In the attached drawings, the dimensions of the structures are shown enlarged compared to the actual dimensions for the clarity of the present invention.

[0035] The terms used in this application are used merely to describe specific embodiments and are not intended to limit the invention.

[0036] In addition, the description of one aspect of the present invention may be applied identically or similarly to the description of other aspects for identical or similar configurations or terms.

[0037] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this application.

[0038] The embodiments of the present invention are described below. However, the embodiments described below are merely partial embodiments of the present invention, and the scope of the present invention is not limited to the following embodiments.

[0039] 실험방법

[0040] 1) Prepare ingredients

[0041] LATP(Li 1.3 Al 0.3 Ti 1.7 (PO4)3) and LZP (LiZr2(PO4)3) powders were synthesized via a standard solid-phase reaction method. Al2O3 (Samchun), TiO2 (Sigma Aldrich), ZrO2 (Sigma Aldrich), and NH4H2PO4 (Alfa Aesar) were prepared in stoichiometric ratios, and Li2CO3 (Junsei) was added in an excess of 10 at.% to compensate for lithium volatilization during the calcination process. The precursor mixtures were placed in alumina crucibles for processing. The LATP precursor was calcined at 700°C for 20 hours, and the LZP precursor at 1200°C for 12 hours, with a heating rate of 5°C / min in both cases. After calcination, the samples were naturally cooled to return to room temperature and then ground by hand milling.

[0042] The anode was prepared by mixing LiFePO4 (LFP, MTI Corporation), carbon black (Super P, MTI Corporation), and polyvinylidene fluoride (PVDF, Sigma Aldrich) in a weight ratio of 8:1:1. This mixture was formed into a slurry using N-methyl-2-pyrrolidone (NMP, Sigma Aldrich) as a solvent, then cast onto aluminum foil and dried.

[0043] 2) Pellet manufacturing

[0044] The synthesized LATP and LZP powders were placed into a die with a diameter of 10 mm and uniaxially compressed at a pressure of 25 MPa for 90 seconds to produce green pellets. For the LZP / LATP bilayer structure, a small amount of LZP powder was evenly dispersed over the pre-pressurized LATP green pellets, and then additionally pressurized at a pressure of 5 MPa for 10 seconds.

[0045] Symmetric LZP / LATP / LZP pellets were fabricated for lithium symmetric cell testing. For this purpose, LZP powder was applied to both sides of an LATP green pellet, and then pressed at a pressure of 5 MPa. Subsequently, all green pellets were sintered under the same conditions. They were heated to 1100°C for 2 hours at a heating rate of 5°C / min and then naturally cooled.

[0046] The sintered pellets were polished with diamond abrasive and sandpaper, then ultrasonically cleaned using isopropanol. Before battery assembly, the final thickness of each pellet was measured using a vernier caliper.

[0047] 3) Cell Assembly

[0048] To measure ionic conductivity, a Pt symmetric cell was fabricated by sputtering Pt onto both sides of a pellet as ion-blocking electrodes. To analyze the redox activity of a solid-state electrolyte (SSE), a cell was constructed using lithium foil as the reference electrode and stainless steel (SS) as the inactive electrode.

[0049] The lithium symmetric cell was assembled by placing lithium foil on both sides of the pellet, and for the double-layer structure, symmetric LZP / LATP / LZP pellets were used. The full cell was constructed using a lithium metal anode and the previously prepared LFP cathode.

[0050] 5 μL of liquid electrolyte (1 M LiPF6 in 1:1 v / v ethylene carbonate:dimethyl carbonate, EC:DMC, Donghwa Electrolyte) was added to all cells containing lithium metal or LFP electrodes to ensure sufficient wettability of the interface.

[0051] 4) Analysis of physical properties

[0052] Phase analysis of the sintered pellets was performed using X-ray diffraction (XRD; Rigaku, Japan). After grinding the pellets into a fine powder, data were collected in the range 2θ = 10–60° using Cu-Kα radiation (λ = 1.54 Å). For ex-situ XRD analysis, the cell was disassembled after cycling to recover the pellets, which were then ground and subjected to XRD analysis. The cross-sectional morphology of the pellets was observed using a field-emission scanning electron microscope (FE-SEM; JEOL, Japan). The LATP and LZP layers were distinguished using back-scattered electron (BSE) modes sensitive to the atomic mass difference between Ti and Zr, allowing for the precise measurement of the thickness of each layer.

[0053] 5) Analysis of electrochemical characteristics

[0054] All electrochemical measurements were performed at room temperature. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 1 kHz to 10 mHz using a potentiostat (Bio-Logic, France). Ionic conductivity (σ) was calculated according to the following equation (Equation 1) using the resistance value (R) obtained from the Nyquist plot.

[0055] (1)

[0056] Here, L is the thickness of the pellet and A is the electrode area.

[0057] The cyclic voltammetry (CV) of the Li|SSE|SS cell is 1–6 V (vs. Li + 0.5 mV s in the voltage range of / Li) -1It was performed at a scan rate. The critical current density (CCD) of the Li-symmetric cell was 0.1 mA cm⁻¹ every 10 minutes using a battery cycler (WonATech, Korea). -2 Measurements were taken by increasing the current density. Galvanostatic cycling of the symmetric cell is 0.1 mA cm⁻¹ -2 It was performed at a current density while reversing the current direction every hour. The galvanostatic cycling with potential limitation (GCPL) test of the Li|SSE|LFP battery was conducted at 2.4–4.0 V (vs. Li + It was performed in the potential range of / Li), and the 1 C-rate was 175 mA g -1 It corresponds to. The specific capacity was measured at 0.1 C, and long-term cycle stability was evaluated for 250 cycles at 0.5 C to calculate the capacity retention rate relative to the initial capacity.

[0058] 결과 분석

[0059] LATP (Li for lithium metal in battery applications) 1.3 Al 0.3 Ti 1.7 To address the instability of (PO4)3) solid electrolytes (SSE), a NASICON-type protective layer was designed based on structural and chemical compatibility. As a NASICON-type candidate material LAGP(Li 1.3 Al 0.3 Ge 1.7 (BIT 4 ) 3 )LZP (LiZr2(PO4)3) and LZP are attracting attention as materials that exhibit excellent chemical stability with respect to lithium metal. These two materials are known to show an inverse correlation between redox stability and ionic conductivity with respect to lithium (Fig. 1a).

[0060] Al-doped LAGP Although it has been proposed as an interlayer, its practical application is limited due to the possibility of Ge being reduced to lithium and high costs. On the other hand, LZP has low ionic conductivity (10 -6 - 10 -5 S cm -1 LZP has not received much attention until now due to [this]. However, LZP possesses excellent chemical robustness and thermodynamic stability with respect to lithium metal, which is an aspect that has not received sufficient attention until now, despite having great potential for establishing stable interfaces. Taking this into account, the present invention utilizes chemically stable LZP as a counterpart to LATP to realize a co-sintered NASICON bilayer structure. This co-sintered bilayer structure simultaneously integrates the complementary ion conductivity and chemical stability of the two NASICON phases.

[0061] The present invention aimed to apply a cold-pressed co-sintering method suitable for mass production. When LZP powder was laminated on top of LATP powder and co-sintered, severe internal stress occurred due to the difference in thermal shrinkage behavior between the two materials, which often led to mechanical failure. Accordingly, controlling the mechanical deformation occurring during the sintering process was set as a top priority. As a result of pressurizing at the same pressure of 25 MPa and sintering at 1100°C, a single LATP pellet shrank to 95.4% of its initial diameter, while a single LZP pellet maintained 98.6%, indicating that LATP exhibited a greater shrinkage rate. This is interpreted as LATP, which contains Al and Ti—metal elements that are relatively lighter than Zr in LZP—inducing faster densification and greater shrinkage, thereby generating internal stress during co-sintering.

[0062] Due to this difference in shrinkage rates, when LZP powder was placed on top of LATP powder and sintered under the same pressure of 25 MPa, a warping phenomenon was observed in the LZP / LATP pellets toward the LZP layer (Fig. 1b). This warping hinders uniform contact with the electrode and increases the risk of failure during the cell assembly process. To suppress the warping, a method was applied in which a flat alumina plate was placed on top of the pressure-molded LZP / LATP pellets during the sintering process and weak pressure was applied. As a result, the warping was somewhat alleviated, but electrolyte failure was still observed due to cracking caused by residual internal stress (Fig. 1c).

[0063] To address the issue of shrinkage rate mismatch, a cold pressing process was designed focusing on green pellet density, a key factor in controlling shrinkage. It was hypothesized that lowering the green density of the LZP layer would increase the total shrinkage of the LZP, thereby compensating for the inherently slow shrinkage rate. This approach aimed to relieve internal stress by matching the shrinkage rate of the high-density LATP layer.

[0064] To realize such a density-controlled LZP / LATP double layer, a multi-stage compression process was developed. Specifically, LZP powder was deposited on LATP green pellets pre-compressed at 25 MPa, and then the LZP layers were compressed at different pressures to control the density of each layer differently. When a pressure of 10 MPa or higher was applied to the LZP layers, separation of the layers occurred during the sintering process, while at low pressures of less than 5 MPa, the coating of the LZP layers became non-uniform and the LZP partially delaminated (Figs. 1d and 1e). Accordingly, LZP powder was deposited on pre-pressed LATP green pellets, and an optimal pressure of 5 MPa was applied (Fig. 2a).

[0065] Under these optimal pressure conditions, a green LZP layer with a relatively lower density compared to the LATP layer was formed, which increased the total shrinkage of LZP and effectively relieved internal stress by compensating for the inherently slow shrinkage rate. To confirm this, the co-sintered LZP / LATP pellets exhibited a shrinkage rate of 96.3%, which was greater than that of a single LZP pellet. This new co-sintering process enabled the successful production of high-quality LZP / LATP pellets without mechanical deformation (Fig. 2a).

[0066] Cross-sectional SEM analysis revealed that in the co-sintered double-layer structure, an LZP protective layer approximately 39 μm thick was uniformly formed on an LATP support layer approximately 471 μm thick (Fig. 2b). The LATP layer adjacent to the LZP interface exhibited a very dense structure with minimized interparticle pores compared to the monolithic LATP (Fig. 2b). On the other hand, the LZP protective layer showed higher porosity than the monolithic LZP pellet (Fig. 2b). This unique porous-high-density dual microstructure was engineered by varying the initial green density of each layer.

[0067] When bilayers with different initial green densities are co-sintered, the lower-density layer exhibits a decrease in relative density near the interface, whereas the higher-density layer shows behavior of becoming denser at the interface. Therefore, the lower-density LZP layer becomes porous, while the higher-density LATP layer forms a denser structure at the interface. XRD phase analysis results showed that both single LATP and LZP pellets R 3 It was confirmed that it was sintered into a single phase having a space group (Fig. 2c). In addition, the XRD pattern of the co-sintered LZP / LATP pellet did not show any secondary phases other than the intrinsic peaks of LATP and LZP. This suggests that the two materials form a stable double layer without interfacial reactions even after co-sintering at 1100°C.

[0068] In conclusion, through the co-sintering process, it was possible to successfully manufacture an electrolyte having a strategically designed microstructure consisting of a porous LZP layer and a dense LATP layer.

[0069] Sample Experimental σ(S cm -1 ) LZP / LATP 1.7 x 10 -4 LATP 5.9 x 10 -4 LZP 1.0 x 10 -5

[0070] The co-sintered LZP / LATP bilayer pellet is 1.7 × 10 -4 S cm -1 It represents the ionic conductivity of (Fig. 3a). This value is considered to be 10, which is regarded as a practical standard for solid electrolytes. -4 S cm -1It meets the standard (benchmark). The single LATP and LZP reference pellets used for comparison were 5.9 × 10, respectively. -4 S cm -1 and 1.0 × 10 -5 S cm -1 The ionic conductivity was shown (Figs. 3b and 3c), which is in good agreement with previously reported results.

[0071] Interestingly, the actual measured ionic conductivity of the LZP / LATP pellet is 1.1 × 10⁻⁶, which is the theoretically calculated value. -4 S cm -1 It was found to be higher (Table 1). Ionic conductivity exceeding these theoretical values ​​was consistently reproduced in all manufactured pellets. This result strongly suggests two main effects. First, the interfacial resistance between LZP and LATP is negligibly low, enabling rapid ion transport, and second, the densification of the LZP / LATP bilayer is promoted during the co-sintering process, thereby improving ionic conductivity. Therefore, the introduction of the LZP layer not only provides chemical protection for lithium metal but also promotes the densification of the LATP layer, achieving an overall improvement in ionic conductivity that surpasses theoretical predictions.

[0072] The enhanced stability of the co-sintered LZP / LATP bilayer electrolyte was verified through electrochemical and structural analyses. In the case of LATP pellets, Ti in cyclic voltammetry measurements 4+ Approximately 2.5 V (vs. Li) attributable to the irreversible reduction of + A distinct reaction current was observed near / Li) (Fig. 3e). On the other hand, the LZP pellet was 6 V (vs. Li + It exhibited a very stable current response up to / Li). Crucially, it was confirmed that the reduction current of the LZP / LATP pellet was significantly suppressed, which confirms that the LZP layer effectively extends the electrochemical stability range to 6 V.

[0073] The inhibition of Ti reduction was further confirmed through XRD analysis of the pellets after constant current charge / discharge. The LZP / LATP pellets maintained the initial NASICON phase even after cycling (Fig. 3f). However, the LATP pellets Ti 4+ The Li3Ti2(PO4)3 secondary phase was formed due to reduction. The formation of this secondary phase can cause volume changes, leading to mechanical stress and structural deterioration. Consequently, the LZP protective layer significantly improves the stability of the LATP electrolyte by effectively suppressing Ti reduction and the subsequent formation of the secondary phase.

[0074] Sample η ct + η mt at 40 h (V) η ct + η mt at 1000 h (V) Increase rate (%) LZP / LATP 0.019 0.044 132 LATP 0.014 0.090 543 LZP 0.008 0.030 275

[0075] Table 2 shows the overvoltage (η) of lithium symmetric batteries using LZP / LATP, LATP, and LZP. ct + η mt Summary (during 1000 hours of constant current cycling) and values ​​at 40 hours and 1000 hours were presented along with the growth rate compared to 40 hours.

[0076] Critical current density (CCD) was measured using a lithium symmetric cell as an indicator to evaluate the performance of the solid electrolyte (SSE) under high-power conditions. The LZP / LATP bilayer pellet exhibited a significantly higher CCD than the monolayer pellet (Fig. 4a). The monolayer LATP and LZP pellets each measured 0.3 mA cm⁻¹. -2 and 0.7 mA cm -2 CCD values ​​were observed (Figs. 4b and c). In particular, the LZP / LATP pellet showed 4.3 mA cm⁻¹. -2 A very high CCD was achieved, which is about 14.3 times higher than LATP and about 6.1 times higher than LZP.

[0077] Such a groundbreaking improvement in CCDs is due to the chemically stable Li|LZP interface and the high ionic conductivity (1.7 × 10⁻⁶ of the LZP / LATP bilayer -4 S cm -1This stems from the synergistic effect of ). Since there is no interfacial secondary phase at the newly formed Li|LZP interface, interfacial resistance and polarization are minimized; consequently, overpotential is lowered and Li dendrite nucleation is suppressed. In addition, the high ionic conductivity of the LZP / LATP layer results in uniform Li across the cell. + It enables flux to prevent current localization and effectively suppresses dendrite growth.

[0078] Enhanced stability was further confirmed through long-term constant current cycling. Even after 1,000 hours, the LZP / LATP cell maintained a low overvoltage of approximately 0.05 V, which is similar to LZP and significantly lower than LATP (0.085 V), demonstrating excellent long-term interfacial stability (Figs. 4d, e, f). Furthermore, the change in overvoltage excluding the iR drop [indicates] charge transfer (η ct ) and mass transfer (η mt It was analyzed as the sum of the ) components. In the case of LATP, η ct + η mt While the value increased by 543%, the increase for LZP / LATP was only 132%, clearly demonstrating the superior stability of the latter against interfacial degradation (Table 2).

[0079] These results confirm that the inherent chemical stability of LZP / LATP with respect to lithium metal plays a crucial role in ensuring interfacial stability.

[0080] The 14-fold improvement in CCD exhibited by LZP / LATP compared to LATP is attributed not only to its high chemical stability and ionic conductivity but also to the unique morphological characteristics of the co-sintered structure (Fig. 5a). The LZP / LATP / LZP trilayer pellet fabricated for CCD measurement has a structure in which a dense LATP core is sandwiched between two porous LZP layers (Fig. 5b). The dense LATP layer not only enhances the ionic conductivity of the pellet but also provides mechanical rigidity, thereby inhibiting dendrite growth (Fig. 5c).

[0081] The porous LZP layer acts as a buffer layer that suppresses dendrite formation during the charge-discharge process. Li + In the stripping step, lithium dissolves in the peripore region where surface energy is high. Li + In the plating step, the pore surface with low surface energy acts as a preferred deposition site for lithium (Fig. 5d). Instead of growing into dendrites, the lithium deposited on the pore surface increases the contact area between the lithium metal and the solid electrolyte (SSE). This increase in contact area reduces overpotential and contributes to improved overall stability.

[0082] In addition, the porous structure reduces the thickness of the pellet, Li + It serves to shorten the ion migration path. In a long-term constant current cycling test, LZP / LATP exhibits a relatively high overpotential during the first approximately 10 hours because the initial contact area is limited. Subsequently, in the 10-30 hour range, a stabilization phase is observed in which the overpotential decreases as the interfacial contact area gradually increases due to lithium plating (Fig. 4d).

[0083] Sample R b (Oh) R Li|SSE (Oh) LZP / LATP 51.9 72.3 LATP 29.8 157.7 LZP 139.5 19.7

[0084] Table 3 shows the resistance values ​​obtained from EIS, bulk resistance (R b ) and interfacial resistance (R) between lithium and solid electrolyte Li|SSE It represents ).

[0085] To evaluate the impedance of the fabricated LZP / LATP pellets in a real battery environment, a cell was assembled using a LiFePO4 (LFP) cathode and analyzed by electrochemical impedance spectroscopy (EIS) (Figs. 6a, b, c). The obtained impedance spectra were fitted using the equivalent circuit shown in Fig. 6d. The bulk resistance (R) of the cell, determined by ionic conductivity bLATP was the smallest, followed by LZP / LATP and then LZP (Table 3).

[0086] On the other hand, the interfacial resistance (R) between the Li cathode and the solid electrolyte (SSE) Li|SSE ) decreased in the order of LZP, LZP / LATP, and LATP. In particular, the LZP cell exhibited a very low resistance of 19.7Ω (Table 3). The LZP / LATP cell had an R of 72.3Ω. Li|SSE It was shown that this value is reduced to about 46% of the 157.7Ω value observed in LATP cells. This reduction in resistance demonstrates that the high chemical stability of LZP contributes to significantly reducing the actual Li|SSE interfacial resistance.

[0087] Meanwhile, R of LZP / LATP cells Li|SSE was larger than that of a pure LZP cell, because the porous LZP layer limits initial contact with the lithium metal cathode. R b and R Li|SSE The sum was lowest in the LZP / LATP cell (Table 3). This suggests that introducing an LZP layer is an important strategy for effectively controlling the Li|SSE interface while minimizing the loss of bulk ion conductivity.

[0088] Galvanostatic cycling with potential limitation (GCPL) measurements performed at a low current density of 0.1 C showed that all three SSEs were approximately 170 mAh g -1It exhibited a similar specific capacity level (Fig. 6e). At these low current densities, the overall reaction rate is determined by the characteristics of the LFP electrode rather than the ionic conductivity of the electrolyte. Conversely, as the current density increases, the ionic conductivity of the SSE acts as the rate-determining step. Consequently, at a current density of 5 C, the rate performance was in the order of ionic conductivity. LATP was the highest at 66%, LZP / LATP at 59%, and LZP was the lowest at 32% (Fig. 6f).

[0089] In a long-term cycle stability test conducted for 250 cycles at a current density of 0.5 C, LZP / LATP showed a capacity retention rate of 81%, which is a significantly improved performance compared to LATP's 56% (Fig. 6g). In addition, LZP / LATP maintained a constant coulombic efficiency (CE) throughout the cycle compared to the LATP cell (Fig. 6g). The stable CE of LZP / LATP suggests that side reactions and electrolyte consumption were mitigated by suppressing Ti reduction.

[0090] Therefore, long-term cycle analysis results demonstrated that the introduction of the LZP layer improves the stability of lithium metal, suppresses dendrite growth, and reduces side reactions, ultimately significantly improving the cycle stability of the battery.

[0091] In conclusion, the present invention introduced a stable LZP layer, a NASICON-based material, onto a chemically unstable LATP solid electrolyte with high ionic conductivity via a co-sintering method. The resulting LZP / LATP electrolyte exhibited excellent stability for lithium metal due to the formation of a porous LZP layer and effectively suppressed dendrite growth. This electrolyte is 1.7 × 10⁻⁶ -4 S cm -1 It showed sufficient ionic conductivity, which reflects the result of the LATP layer being densified during the co-sintering process.

[0092] At the same time, the LZP layer acting as a protective layer is 2.5 V (vs. Li + Irreversible Ti occurring in / Li) 4+ By fundamentally resolving the reduction problem, the formation of undesirable Li3Ti2(PO4)3 secondary phases was effectively prevented. Furthermore, the porous LZP layer formed during the co-sintering process suppressed lithium dendrite formation during charge-discharge cycles and increased the interfacial contact area. This resulted in a performance of 4.3 mA cm⁻¹, a 14.3-fold improvement compared to single LATP. -2 Achieved an excellent critical current density.

[0093] In the Li|SSE|LFP battery configuration, the LZP / LATP electrolyte showed a lower sum of bulk resistance and interfacial resistance than a single LATP or LZP electrolyte, proving that this is a successful strategy that effectively combines the advantages of each material. Accordingly, it demonstrated high cycle stability by recording a capacity retention rate of 81% after 250 cycles under 0.5 C conditions, which is superior performance to the LATP battery, which showed a capacity retention rate of 56%.

[0094] The present invention presents a novel double-layer electrolyte design strategy that enables the control of material properties at the interface through process control. By simultaneously achieving chemical protection and physical stability through a single, scalable cold co-sintering process, it represents a significant advance toward the commercialization of ceramic solid electrolytes.

[0095] Although the present invention has been described above with reference to preferred embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

Claim 1 delete Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 delete Claim 6 delete Claim 7 delete Claim 8 delete Claim 9 delete Claim 10 A method for manufacturing a laminated solid electrolyte by co-sintering comprises: (a) a step of forming a first green layer by pressing a first electrolyte powder with a first pressure; (b) a step of laminating a second electrolyte powder on the first green layer and forming a second green layer by pressing with a second pressure lower than the first pressure; and (c) a step of co-sintering a laminate comprising the first green layer and the second green layer, wherein the first electrolyte powder is Li 1+x Al x Ti 2-x A method for manufacturing a laminated solid electrolyte by co-sintering, comprising (PO4)3 (0 < x ≤ 0.5), wherein the second electrolyte powder comprises LiZr2(PO4)3. Claim 11 In Clause 10, the above Li 1+x Al x Ti 2-x A method for manufacturing a laminated solid electrolyte by co-sintering, characterized by laminating a first electrolyte powder containing (PO4)3 (0 < x ≤ 0.5) to a thickness of 450 to 500 μm. Claim 12 delete Claim 13 A method for manufacturing a laminated solid electrolyte by co-sintering, characterized in that, in claim 10, a second electrolyte powder containing the above LiZr2(PO4)3 is laminated to a thickness of 35 to 45 μm. Claim 14 A method for manufacturing a laminated solid electrolyte by co-sintering, characterized in that, in claim 13, the second pressure is in the range of 5 to 10 MPa. Claim 15 A method for manufacturing a laminated solid electrolyte by co-sintering, characterized in that, in claim 11, the first pressure is in the range of 20 to 30 MPa.

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