GeO2-DOPED SOLID ELECTROLYTE, METHOD FOR PRODUCING THE SAME, AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME

Abstract

To provide an LATP solid electrolyte that exhibits high ion conductivity, a method for producing the same, and a lithium secondary battery containing the same.SOLUTION: A method for producing a solid electrolyte is provided, wherein the solid electrolyte contains Li1+xAlxTi2-x(PO4)3 (LATP) doped with GeO2, where x is 0.3-0.5, and the production method comprises a step of forming a powder mixture by mixing a precursor powder of Li1+xAlxTi2-x(PO4)3 with GeO2 powder, and a step of sintering the powder mixture.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a GeO2-doped solid electrolyte, a method for producing the same, and a lithium secondary battery containing the same. [Problem information] [National research and development project that supported this invention] [Project unique number] 1711188320 [Project Number] 2022R1A2C1010617 [Ministry name] Ministry of Science, ICT and Communication [Name of issue management (specialized) organization] Korea Research Foundation [Research project name] Individual basic research (Ministry of Science and Technology Information) [Research title] Development of thin-film solid electrolytes using spontaneous interfacial reactions containing metal atoms and powder deposition processes [Project Implementation Organization Name] Kwangwoon University Industry-Academia Cooperation Group [Research period] 2023.03.01~2024.02.29 [National research and development project that supported this invention] [Project unique number] 1711197946 [Project number] 00222124 [Ministry name] Ministry of Science, ICT and Communication [Name of issue management (specialized) organization] Korea Research Foundation [Research project name] Group research support [Research title] Production of carbonate ceramic materials for carbon dioxide storage and their use as energy elements [Project Implementation Organization Name] Kwangwoon University Industry-Academia Cooperation Group [Research period] 2023.06.01~2024.02.29 [Background technology]

[0002] As the global energy crisis caused by global warming worsens, the demand for high-performance energy storage devices is exponentially increasing. Among these, lithium-ion batteries are widely used in electric vehicles, energy storage systems, mobile electronic devices, and more. However, lithium-ion batteries, which use liquid electrolytes, have limited energy density and are constantly facing stability issues, such as explosions. In contrast, solid electrolytes have attracted attention because they offer inherent non-flammability, high energy density, a wide operating temperature range, and a wide electrochemical window, thereby overcoming the shortcomings of conventional liquid electrolytes.

[0003] Among solid electrolytes, NASICON-type electrolytes have emerged as a promising electrolyte candidate for next-generation lithium-ion batteries. Among them, Li-ion batteries are composed of TiO6 octahedra and PO4 tetrahedra arranged alternately, with the TiO6 octahedra and PO4 tetrahedra sharing oxygen atoms at their corners. 1+x Al x Ti 2-x (PO4)3 (where 0.3≦x≦0.5) compound (LATP) -4 ~10 -3 S cm -1 LATP has recently attracted attention as a highly electrochemically stable material with high intrinsic ionic conductivity. However, even in the case of LATP, the ionic conductivity is limited by grain-boundary scattering, making its widespread use difficult. Therefore, in order to commercially use LATP in all-solid-state lithium-ion batteries, the ionic conductivity of LATP needs to be further improved.

[0004] Li 1+x Al x Ti 2-xMany efforts have been made to improve the ionic conductivity of (PO4)3 compounds. In LATP, when the cation is replaced with a cation having a different ionic radius and valence, the ionic conductivity is improved due to lattice distortion. For example, Dharmesh et al. 3+ The larger trivalent cation (Ga 3+ ,Sc. 3+ , Y 3+ ) was doped, but the size difference of the ions caused elastic strain, and Li + (Kothari DH, Kanchan D. Effect of doping of trivalent cations Ga 3+ ,Sc. 3+ , Y 3+ in Li 1.3 Al 0.3 Ti 1.7 (PO4)3(LATP)system on Li+ ion conductivity.Physica B:Condensed Matter.2016;501:90-4 and Ma F, Zhao E, Zhu S, Yan W, Sun D, ​​Jin Y, et al.Preparation and evaluation of high lithium ion conductivity Li1.3Al 0.3 Ti 1.7 (PO4)3 solid electrolyte obtained using a new solution method. Solid State Ionics. 2016;295:7-12 (Reference). However, Sc 3+ and Y 3+ Large cations such as Li 1+x Al x Ti 2-x It is difficult to incorporate into the (PO4)3 lattice and tends to segregate at grain boundaries, which negatively affects ionic conductivity.

[0005] Other studies have shown that Ti 4+ Position Nb 5+When replaced with, the ionic conductivity is 0.75 × 10 -3 S cm -1 It has been confirmed that the tetrahedral P 5+ Position Si 4+ When replaced with LiTiOPO4 の The formation of ionic conductivity is enhanced by 10 -3 S cm -1 (Zhu J, Xiang Y, Zhao J, Wang H, Li Y, Zheng B, et al.Insights into the local structure, microstructure and ionic conductivity of silicon doped NASICON-type solid electrolyte Li 1.3 Al 0.3 Ti 1.7 P3O 12 .Energy storage materials. 2022;44:190-6).

[0006] Thus, grain boundary engineering is important for improving electrolyte performance. There is a continuing need for innovative methods that can further improve ionic conductivity and easily control the structure of grain boundaries. Against this background, the present inventors have conducted extensive research to develop LATP solid electrolytes with high ionic conductivity, and have completed the present disclosure. [Prior art documents] [Non-patent literature]

[0007] Kothari DH, Kanchan D.Effect of doping of trivalent cations Ga 3+,Sc. 3+ , Y 3+ in Li 1.3 Al 0.3 Ti 1.7 (PO4)3(LATP)system on Li+ ion conductivity. Physica B:Condensed Matter.2016;501:90-4 Summary of the Invention [Problem to be solved by the invention]

[0008] A primary object of the present invention is to provide a LATP solid electrolyte having high ionic conductivity, a method for producing the same, and a lithium secondary battery containing the same. [Means for solving the problem]

[0009] The present disclosure provides a method for preparing a solid electrolyte, wherein the solid electrolyte is a GeO2-doped Li 1+x AlxTi 2-x (PO4)3(LATP), where x is 0.3 to 0.5, and the manufacturing method includes Li 1+x AlxTi 2-x The method includes mixing a precursor powder of (PO4)3 and a GeO2 powder to form a powder mixture, and sintering the powder mixture.

[0010] The present disclosure also provides a solid electrolyte produced by the above-described method, wherein the solid electrolyte includes a Ge-enriched amorphous phase at grain boundaries, and a lithium-ion battery including a positive electrode, a negative electrode, and the solid electrolyte. [Brief explanation of the drawings]

[0011] [Figure 1] As for the GeO2-LATP XRD patterns depending on the GeO2 concentration, Figure 1a shows the XRD patterns over the entire 2-theta range, and Figures 1b to 1d show the XRD patterns over the 24-28°, 28-31°, and 36-38° ranges, respectively. [Figure 2]The SEM image (Fig. 2a), Ge energy dispersive X-ray spectroscopy (EDS) mapping image (Fig. 2b), and bright-field TEM image (Fig. 2c) and high-resolution TEM image (Fig. 2d) of the 4 wt% GeO2-LATP sample are shown. [Figure 3] SEM images and elemental analysis results by EDS for 2 wt% GeO2-LATP (Fig. 3a) and 8 wt% GeO2-LATP (Fig. 3b) are shown. [Figure 4] SEM images of GeO2-LATP samples with different GeO2 concentrations are shown below: 0 wt% (Fig. 4a), 1 wt% (Fig. 4b), 2 wt% (Fig. 4c), 4 wt% (Fig. 4d), 8 wt% (Fig. 4e), and 10 wt% (Fig. 4f). [Figure 5] The grain size distribution of the GeO2-LATP sample according to the GeO2 concentration corresponds to 0 wt% (Fig. 5a), 1 wt% (Fig. 5b), 2 wt% (Fig. 5c), 4 wt% (Fig. 5d), 8 wt% (Fig. 5e), and 10 wt% (Fig. 5f), respectively. [Figure 6] XPS analysis results for the 4 wt% GeO2-LATP sample show a survey scan (Fig. 6a), a Ti 2p scan (Fig. 6b), and a Ge 3d scan (Fig. 6c). [Figure 7] 1 is a Nyquist plot of GeO2-LATP samples according to GeO2 content. [Figure 8] The Nyquist plots of the GeO2-LATP samples according to GeO2 content are shown for 0 wt% (Fig. 8a), 1 wt% (Fig. 8b), 2 wt% (Fig. 8c), 4 wt% (Fig. 8d), 8 wt% (Fig. 8e), and 10 wt% (Fig. 8f). [Figure 9] A schematic mechanism of improved lithium ion transport due to the presence of amorphous phases at grain boundaries is shown for Li-ion conduction in pure LATP (left) and Li-ion conduction in GeO2-LATP (right). [Figure 10] The relative density and ionic conductivity are shown as a function of GeO2 content. [Figure 11]Arrhenius plots for pure LATP and 4 wt % GeO2-LATP samples are shown. [Figure 12] Nyquist plots as a function of temperature are shown for pure LATP (Fig. 12a) and 4 wt% GeO2-LATP (Fig. 12b). [Figure 13] The performance of symmetric Li / electrolyte / Li cells using pure LATP or 4 wt% GeO2-LATP as the electrolyte is shown, showing the cycling performance (Fig. 13a), the voltage at the initial cycle (Fig. 13b), and the voltage after 400 hours of operation (Fig. 13c). [Figure 14] EIS graphs before cycling (FIG. 14a) and after 400 hours of operation (FIG. 14b) are shown for symmetric Li / electrolyte / Li cells using pure LATP or 4 wt. % GeO2-LATP as the electrolyte. [Figure 15] Figure 1 shows the rate performance in a Li / 4 wt% GeO2-LATP / Li symmetric cell. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. However, various modifications may be made to the embodiments, and the scope of the patent application is not limited or restricted by such embodiments. It should be understood that all modifications, equivalents, and alternatives to the embodiments are included in the scope of the patent.

[0013] The terms used in the embodiments are merely used for the purpose of explanation and are not to be construed as limiting. A singular expression includes a plural expression unless the context clearly indicates otherwise. In this specification, the terms "comprise" or "have" indicate the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, and should be understood as not precluding the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0014] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Commonly used predefined terms should be interpreted as having a meaning consistent with the meaning they have in the context of the relevant art, and should not be interpreted as having an ideal or overly formal meaning unless expressly defined herein.

[0015] In addition, in the description with reference to the accompanying drawings, the same components are denoted by the same reference numerals regardless of the reference numerals, and redundant description thereof will be omitted. In the description of the embodiments, if a detailed description of related known technology is determined to unnecessarily obscure the gist of the embodiments, the detailed description thereof will be omitted.

[0016] Furthermore, in describing components of the embodiments, terms such as first, second, A, B, (a), (b), etc. are used to distinguish the component from other components, and do not limit the essence, order, or sequence of the component.

[0017] Components that have the same functions as components included in one embodiment will be described using the same names in other embodiments. Unless otherwise specified, the description of one embodiment may also be applied to other embodiments, and detailed descriptions will be omitted to the extent that they overlap.

[0018] "%" used throughout this specification to indicate the concentration of a particular substance is (wt / wt)% for solid / solid, (wt / vol)% for solid / liquid, and (vol / vol)% for liquid / liquid, unless otherwise specified.

[0019] The term "about" is understood to indicate a range of numbers that one of skill in the art would consider equivalent to the recited value in a manner that achieves the same function or result.

[0020] Every numerical range given throughout this specification includes its upper and lower limits, and every narrower numerical range subsumed within that range, and all such narrower numerical ranges are intended to be expressly and specifically written herein.

[0021] Amorphous films at grain boundaries are observed in many polycrystalline systems, and these amorphous films penetrate along the grain boundaries into the liquid phase during sintering, playing an important role in the dynamics of microstructural evolution, such as grain growth. For example, amorphous films affect the mechanical and electrical properties of materials. Koo et al. demonstrated that the dielectric properties of Nb-doped SrTiO3 can be controlled by adjusting the liquid film migration (Koo SY, Lee GG, Kang SJL, Nowotny J, Sorrell C. Suppression of Liquid Film Migration and Improvement of Dielectric Properties in Niobium-Doped Strontium Titanate. Journal of the American Ceramic Society. 2004;878:1483-7). Similarly, Choi et al. demonstrated that by adjusting such films, the dielectric properties of Ti -reported that the grain growth behavior can be controlled with barium-rich BaTiO3 (Choi SY, Yoon DY, Kang S-JL. Kinetic formation and thickening of intergranular amorphous films at grain boundaries in barium titanate. Acta materialia. 2004;5212:3721-6). This means that the formation of a liquid film during sintering can improve both the sintering behavior and ionic conductivity.

[0022] The present inventors have disclosed a new method for preparing solid electrolytes by incorporating GeO2 during the electrolyte synthesis process. The incorporated GeO2 significantly enhanced the ionic conductivity of the solid electrolyte by forming a liquid phase film at the grain boundaries of LATP during sintering.

[0023] On the other hand, Fan Bai et al. 1.4 Al 0.4 Ti 1.6 GeO2 was introduced into LATP by adding GeO2 to (PO4)3 (Fan Bai et al. Lithium-ion conduction of Li 1.4 Al 0.4 Ti 1.6(PO4)3-GeO2 composite solid electrolyte. Solid State Ionics. 2019;329;40-45). Here, the added GeO2 forms a solid crystalline secondary phase. Whether or not GeO2 is a solid crystalline secondary phase can be confirmed by observing that the majority of the added GeO2 is an impurity phase as measured by XRD. In contrast, the present disclosure differs from the GeO2-LATP disclosed by Fan Bai et al. in that GeO2 is introduced during the electrolyte synthesis step, resulting in the formation of a Ge-rich amorphous phase at the grain boundaries. Furthermore, the solid electrolyte of the present disclosure exhibits grain boundary conductivity up to 14 times higher than LATP without GeO2 addition, while the GeO2-LATP disclosed by Fan Bai et al. exhibits grain boundary conductivity up to approximately 3 times higher than LATP without GeO2 addition. This indicates that the grain boundary conductivities differ depending on whether the added Ge exists mainly in the form of an amorphous secondary phase or in the form of a crystalline secondary phase.

[0024] The GeO2-doped LATP electrolyte of the present disclosure has high ionic conductivity and can operate stably for more than 500 hours in a Li / electrolyte / Li symmetric cell system. Furthermore, the method for producing the solid electrolyte of the present disclosure can be easily applied to conventional solid-phase synthesis methods. In other words, the solid electrolyte of the present disclosure, its production method, and lithium secondary batteries containing the same have the advantages of high ionic conductivity and stability, as well as favorable commercialization.

[0025] In one aspect, the present disclosure provides a method for producing a solid electrolyte, wherein the solid electrolyte is a GeO2-doped Li 1+x Al x Ti 2-x (PO4)3(LATP), where x is 0.3 to 0.5, and the manufacturing method includes 1+x Al x Ti 2-xThe method includes the steps of mixing a (PO4)3 precursor powder and a GeO2 powder to form a powder mixture, and sintering the powder mixture. According to one embodiment, x is 0.4 to 0.5. Preferably, x is 0.5.

[0026] According to one aspect, Li 1+x Al x Ti 2-x The precursor powders for (PO4)3 include Li2CO3, Al2O3, TiO2, and (NH4)2HPO4.

[0027] According to one aspect, the precursor powder and the GeO2 powder may be mixed uniformly by wet milling.

[0028] According to one aspect, the method for manufacturing the solid electrolyte further includes milling the mixed powder, drying and calcining the milled electrolyte powder, and pressing the calcined electrolyte powder.

[0029] According to one aspect, the milling step includes ball milling. According to one aspect, the milling step includes ball milling with high-purity ethanol and zirconia balls. Preferably, the milling step is ball milling with high-purity ethanol (99.99%) and zirconia balls for 24 hours.

[0030] According to one embodiment, the drying and calcining step is carried out at 800 to 1000°C for 5 to 7 hours. Preferably, the drying and calcining step may be carried out at 900°C for 6 hours.

[0031] According to one embodiment, the pressurizing step applies a pressure of 50 to 70 MPa. Preferably, the pressurizing step applies a pressure of 60 MPa.

[0032] According to one embodiment, the sintering step is carried out at 900 to 1100° C. for 3 to 7 hours. Preferably, the sintering step may be carried out at 1000° C. for 5 hours.

[0033] In a further aspect of the present disclosure, a solid electrolyte is provided, prepared by the method described in any one of the preceding aspects, wherein the solid electrolyte includes a Ge-enriched amorphous phase at grain boundaries. As used herein, the term "Ge-enriched amorphous phase" refers to the presence of Ge in the amorphous phase at an atomic percentage (at%) of 5 to 40%, 7 to 35%, 9 to 32%, 10 to 30%, or preferably 12 to 27%. Alternatively, the term "Ge-enriched amorphous phase" refers to the presence of about 50 at% to about 99 at% or less, or about 70 at% to about 99 at% of Ge, based on the total added Ge.

[0034] According to one embodiment, the concentration of GeO2 in the solid electrolyte of the present disclosure is 1 wt% to 10 wt% based on LATP. According to one embodiment, the concentration of GeO2 in the solid electrolyte of the present disclosure is 2 wt% to 8 wt% based on LATP. Preferably, the concentration of GeO2 in the solid electrolyte of the present disclosure is 4 wt% to 8 wt% based on LATP. Preferably, the concentration of GeO2 in the solid electrolyte of the present disclosure is 4 wt% based on LATP.

[0035] According to one embodiment, the amorphous phase exists between adjacent crystal grains to a thickness of 5 to 15 nm. According to another embodiment, the amorphous phase exists between adjacent crystal grains to a thickness of 7 to 12 nm. Preferably, the amorphous phase exists between adjacent crystal grains to a thickness of 10 nm.

[0036] According to one embodiment, the solid electrolyte of the present disclosure has an ionic conductivity of 3×10 at 25° C. -4 ~9×10 -4 S cm -1According to one embodiment, the solid electrolyte of the present disclosure has an ionic conductivity of 3.5×10 at 25° C. -4 ~9×10 -4 S cm -1 According to one embodiment, the solid electrolyte of the present disclosure has an ionic conductivity of 4×10 at 25° C. -4 ~9×10 -4 S cm -1 According to one embodiment, the solid electrolyte of the present disclosure has an ionic conductivity of 4.3 × 10 at 25°C. -4 ~9×10 -4 S cm -1 According to one embodiment, the solid electrolyte of the present disclosure has an ionic conductivity of 5×10 at 25° C. -4 ~9×10 -4 S cm -1 According to one embodiment, the solid electrolyte of the present disclosure has an ionic conductivity of 7×10 at 25° C. -4 ~9×10 -4 S cm -1 is.

[0037] In a further aspect of the present disclosure, there is provided a lithium ion battery comprising: a positive electrode; a negative electrode; and a solid electrolyte according to any one of the preceding aspects.

[0038] In the following, specific aspects of the present disclosure will be described by way of embodiments, which are not intended to limit the present disclosure but are merely used for illustration purposes.

[0039] Manufacturing example: Synthesis of LATP powder

[0040] LATP powders with GeO2 contents of 0, 1, 2, 4, 8, and 10 wt% based on LATP were synthesized using Li2CO3 (99.0%, Junsei Chemical), Al2O3 (99.0%, Junsei Chemical), TiO2 (99.0%, Junsei Chemical), (NH4)2HPO4 (99.0%, Junsei Chemical), and GeO2 (99.99%, Kojundo Chemical Laboratory) as starting materials.

[0041] Specifically, the precursor materials of LATP, Li2CO3 (2.785 g), Al2O3 (1.281 g), TiO2 (6.021 g), and (NH4)2HPO4 (19.912 g), were prepared according to the stoichiometric composition, and 3 g of Li2CO3 (approximately 10% of the precursor materials used) and GeO2 were prepared and uniformly mixed by wet milling. GeO2 was mixed to be 1, 2, 4, 8, and 10 wt% based on the total wt% of LATP, respectively, and LATP was mixed to be 1, 2, 4, 8, and 10 wt%. 1.5 Al 0.5 Ti 1.5 The powder was mixed to have the composition (PO4)3. The mixed powder was ball-milled with high-purity ethanol (99.99%) and zirconia balls for 24 hours, then dried and calcined at 900°C for 6 hours. The calcined powder was pressurized with 60 MPa and sintered in air at 1000°C for 5 hours.

[0042] Embodiment 1: GeO 2 -Analysis of the composition and microstructure of LATP electrolyte

[0043] The sintered samples were ground into fine powder and characterized by X-ray diffraction (XRD, D8 Advance A25 Plus / Bruker) using Cu Kα radiation. For microstructural analysis, the sintered samples were cross-sectioned and polished to a cutoff of 1 μm, followed by thermal etching at 800°C for 20 min.

[0044] The microstructure was observed using a field-emission scanning electron microscope (FE-SEM, JSM-7610F / JEOL). The grain size distribution was determined from the SEM images using Matrox Inspector 4.1 (Matrox Electronic Systems). Detailed microstructural observations were performed using a transmission electron microscope (TEM, Titan Themis Z / FEI). Chemical bonding was analyzed using X-ray photoelectron spectroscopy (XPS, K-Alpha / Thermo Scientific (UK)).

[0045] XRD pattern analysis

[0046] Figure 1a shows the XRD patterns for GeO2-LATP prepared according to the above preparation example, with varying GeO2 concentrations. At all concentrations, the XRD patterns exhibit peaks of the main crystalline phase with a rhombohedral NASICON-type structure (R-3c space group). Specifically, the XRD peaks hardly shift with the addition of GeO2, indicating that the lattice parameters of the main crystalline phase are unaffected by GeO2 doping. This suggests that Ge is not incorporated into the LATP lattice, but rather forms a secondary phase at the intergranular or intragranular regions.

[0047] Figures 1b-d show the XRD patterns at 24-28°, 28-31°, and 36-38°, respectively. The pure LATP sample exhibits the XRD pattern of a crystalline secondary phase of LiTiOPO4. However, as the GeO2 content increases (up to 4 wt%), this crystalline secondary phase gradually disappears. Indeed, the XRD results show that the 4 wt% GeO2-LATP sample has a nearly single-phase structure. Furthermore, when the GeO2 content is increased to 8 wt%, a distinct crystalline secondary phase of GeO2 is observed. These XRD results suggest that the added GeO2 reacts with the conventional LiTiOPO4 secondary solid phase during sintering to form an amorphous liquid phase (which is not detected by XRD). If the amount of GeO2 added is small, the secondary phase of LiTiOPO4 crystalline reacts with GeO2 to form an amorphous phase, and the remaining secondary phase of LiTiOPO4 crystalline is detected by XRD. On the other hand, if an excessive amount of GeO2 is added that exceeds the solubility limit, GeO2 cannot completely dissolve in the amorphous liquid phase, and a secondary phase of GeO2 crystalline is formed, which is detected by XRD.

[0048] Analysis of SEM, EDS, and TEM images

[0049] Figure 2 shows the SEM image (Figure 2a), energy dispersive X-ray spectroscopy (EDS) mapping image (Figure 2b), and TEM (Figures 2c and d) images of the 4 wt% GeO2-LATP sample. Figure 2d is an enlarged view of the boxed area in Figure 2c. Figure 2 confirms that the 4 wt% GeO2-LATP sample has an amorphous structure with a high Ge concentration at the grain boundary regions. Specifically, the SEM image does not clearly distinguish the sample's grain structure. This is due to the formation of a liquid phase at the grain boundary regions during sintering. Furthermore, the EDS mapping image reveals that Ge is minimally present within the grains, while significantly present at the grain boundaries. The TEM image confirms that the 4 wt% GeO2-LATP sample has a 10 nm thick phase, which differs from the conventional grain boundary structure. This indicates that Ge is highly concentrated at the grain boundaries, and that this Ge-enriched phase is an amorphous phase.

[0050] Meanwhile, Figures 3a and 3b show SEM and EDS results for sintered pellets of 2 wt% and 8 wt% GeO2-LATP, respectively. Amorphous phases along the grain boundaries are clearly visible in both samples. Additionally, rounded grains are observed within the grains in the 8 wt% GeO2-LATP sample. Compositional analysis reveals that the intragranular Ge concentration is less than 1 at%. The remaining Ge is highly concentrated in the amorphous phase at the grain boundaries, suggesting that only a small amount of Ge is incorporated into the LATP lattice. Furthermore, the rounded grains in the 8 wt% GeO2-LATP sample are confirmed to be GeO2, as predicted by XRD analysis.

[0051] In other words, the added GeO2 interacts with the LiTiOPO4 secondary phase, so that the volume of the LiTiOPO4 phase decreases with increasing GeO2 concentration, and the added GeO2 forms a Ge-rich grain boundary amorphous phase or liquid film. To obtain a structure without crystalline secondary phases (e.g., LiTiOPO4 secondary phase or GeO2 secondary phase), 4 wt% is the most preferred concentration.

[0052] Grain size analysis

[0053] Figure 4 shows SEM images of GeO2-LATP samples with various GeO2 concentrations: 0 wt% (Figure 4a), 1 wt% (Figure 4b), 2 wt% (Figure 4c), 4 wt% (Figure 4d), 8 wt% (Figure 4e), and 10 wt% (Figure 4f). Figure 5 shows the grain size distribution for each concentration. Figures 4 and 5 demonstrate the effect of GeO2 addition on grain growth in the LATP system. Specifically, the grain size distribution in the 0 wt% LATP sample is broad (Figure 5a), indicating rapid and consistent grain growth. However, even with only 1 wt% GeO2, the grain size distribution narrows, demonstrating a bimodal morphology as the GeO2 concentration increases. This abnormal grain growth is due to the formation of a liquid film when GeO2 reacts with LiTiOPO4 during sintering, which then penetrates along the grain boundaries. This LATP grain growth can be explained by the theory of two-dimensional nucleation and grain growth. The addition of GeO2 increases the critical driving force for grain growth by forming a grain boundary liquid film. Grains that exceed the critical driving force exhibit clear growth, while grains that do not meet the critical driving force remain small. As a result, the addition of GeO2 results in a bimodal size distribution. Consequently, the number of small grains increases with increasing GeO2 concentration.

[0054] XPS analysis

[0055] The 4 wt% GeO2-LATP sample was analyzed by XPS (Fig. 6) to analyze its chemical structure. 4+ The grain boundary film is completely oxidized Ge. 4+ The Ti 2p detail spectrum in Fig. 6b shows the Ti 4+The peaks can be easily found, which confirms that the GeO2-doped LATP structure with Ge-rich grain boundary layers has been successfully synthesized.

[0056] Embodiment 2: GeO 2 Electrochemical properties of -LATP electrolyte

[0057] The sintered body was polished to a flat surface to prepare a solid-state LATP electrolyte. Both sides of the fabricated LATP electrolyte were sputter-coated with Au using an SC-701 MkII quick coater. Electrochemical impedance spectroscopy (EIS) of the Au / LATP / Au assembly was performed at room temperature using an impedance phase analyzer (SP-300, Bio-Logic) in the frequency range of 0.1 Hz to 7 MHz with an AC amplitude of 10 mV.

[0058] Figures 7a and 7b show the electrochemical characteristics of LATP samples depending on the GeO2 content. Figure 8 shows the Nyquist plots of Figures 7a and 7b in detail for each GeO2 wt%. The starting point of the semicircle in the EIS plot, where it intersects with the Z axis in the high frequency region, is the contact resistance R s The line behind the semicircle in the low frequency region indicates the Warburg impedance. The semicircle in the high frequency region is again divided into two semicircles as shown in Figure 8, and the diameters of the first and second semicircles are the bulk resistance R b and grain boundary resistance R gb The total resistance of the solid electrolyte was calculated by adding the bulk resistance and the grain boundary resistance (R t =R gb +R b ).

[0059] Referring to Figures 7a and b, when GeO2 was introduced, the R t It can be seen that R clearly decreases due to the formation of the Ge-rich grain boundary phase. gb The mechanism behind this is shown in Figure 9.

[0060] Figure 10 shows the sintered density and ionic conductivity at 25°C depending on the GeO2 content. The ionic conductivity σ is σ = l / R t The sintering time was calculated by the following formula: 1 / A, where 1 and A are the thickness and area of ​​the sintered pellet, respectively. The specific data are shown in Table 1 below.

[0061] [Table 1]

[0062] In relation to ionic conductivity, the ionic conductivity of pure LATP is 1.08 × 10 -4 S cm -1 This agrees with previous results, and it can be seen that the ionic conductivity gradually increases with increasing GeO2 content. In particular, the ionic conductivity of 4 wt% GeO2-LATP is 8.75 × 10 -4 S cm -1 This value is significant, being more than eight times higher than that of pure LATP. Furthermore, as the GeO2 content increases above 4 wt%, the ionic conductivity decreases gradually. This is due to the formation of a secondary phase of GeO2 particles, which is an ion insulator.

[0063] Sintered density (relative density) refers to the value of (measured electrolyte density) / (theoretical density) × 100. Sintered density shows a similar trend to ionic conductivity, but the relative density increases with the addition of GeO2. In particular, the 4 wt% GeO2-LATP sample, which has the highest ionic conductivity, also has the highest relative density at 95.22%.

[0064] In addition, the bulk conductivity remains relatively stable when GeO2 is added (1~2×10 -3 S cm -1 ), the grain boundary conductivity was significantly improved. For example, the grain boundary conductivity of the 4 wt% GeO2-LATP sample was 1.71 × 10 -3 S cm -1Therefore, the grain boundary structure of the Ge-rich film in the GeO2-LATP of the present invention, especially the 4 wt% GeO2-LATP sample, not only improves the sintering density but also promotes lithium ion transport at the grain boundaries, resulting in R gb It can be seen that the amount of lithium in the LATP sample produced in this embodiment is reduced. Meanwhile, the LATP sample produced in this embodiment also contains excess lithium, but recent research has shown that the excess lithium can be separated from the grain boundaries and improve lithium ion transport. In other words, the grain boundary phase of the electrolyte of the present invention, particularly the 4 wt% GeO2-LATP sample, contains a film rich in Ge and lithium, which can significantly improve lithium ion transport.

[0065] Figure 11 shows the Arrhenius plot for pure LATP and 4 wt% GeO2-LATP samples. Figure 12 shows the Nyquist plot for pure LATP and 4 wt% GeO2-LATP samples as a function of temperature. The activation energy for lithium ion conductivity σ can be calculated through the Arrhenius equation:

[0066]

number

[0067] where A is the pre-exponential factor, T is the absolute temperature, and E a is the activation energy, and k is the Boltzmann constant.

[0068] The activation energies were calculated using the Arrhenius equation from the total conductivity measured in Figure 12, and were found to be 0.187 eV and 0.177 eV for pure LATP and 4 wt% GeO2-LATP, respectively (Figure 11). The addition of GeO2 reduced the activation energy, indicating that lithium ion migration was easier in the 4 wt% GeO2-LATP sample than in the pure LATP sample. In particular, despite the grain size of pure LATP being larger than that of 4 wt% GeO2-LATP, the activation energy for ion conduction in 4 wt% GeO2-LATP is smaller. This unusual phenomenon is due to the excellent ion transport characteristics of the Ge-rich grain boundary phase in 4 wt% GeO2-LATP.

[0069] Embodiment 3: GeO 2 - Cell performance test using LATP solid electrolyte

[0070] To evaluate the cell performance of the synthesized LATP electrolyte, Li / LATP / Li symmetric CR2032 coin cells were fabricated. The coin cells were fabricated in an argon-filled glove box (O2 and HO levels below 0.1 ppm) and tested at 0.01, 0.05, and 0.1 mAcm2 using a multi-channel battery test system (WBCS300Ls21, Won A Tech) in an incubator at 25°C. -2 The cells were tested by galvanostatic cycling at a current density of .

[0071] Figures 13 and 14 show the results for 0.01 mAcm -2This paper shows the performance of symmetric Li / electrolyte / Li cells using pure LATP or 4 wt. % GeO2-LATP as the electrolyte at a current density of 100 s. Here, a liquid electrolyte is not used in the cells. Although a small amount of liquid electrolyte can promote lithium ion transfer between the lithium ion carrier and the solid electrolyte, significantly reducing cell polarization, it can also affect performance measurements due to reasons such as decomposition of the liquid electrolyte. In this embodiment, the use of bare LATP solid electrolyte without a liquid electrolyte allowed for more thorough measurement of the essential properties of the solid electrolyte.

[0072] Specifically, analysis of Figure 13 reveals that the initial polarization voltage for the pure LATP electrolyte was approximately 1.5 V and increased to over 2 V after 400 hours of operation. After 424 hours, the pure LATP cell suddenly shorted out, due to the formation of dendrites within the LATP electrolyte. In contrast, the 4 wt% GeO2-LATP electrolyte, which exhibited the highest ionic conductivity among the samples, had an initial polarization voltage of less than 0.4 V and operated stably for over 500 hours, with an overpotential of less than 0.46 V. The 4 wt% GeO2-LATP electrolyte exhibited large polarization in the initial stages of testing, which is associated with the formation of an interfacial layer between the solid electrolyte and lithium. The 4 wt% GeO2-LATP electrolyte exhibited high stability due to its stable electrolyte composition, and the addition of GeO2 improved ionic conductivity (8.75 × 10 -4 S cm -1 ) has a low polarization voltage. The polarization curve of pure LATP shows a clear arc in the initial stage and after 400 hours of operation, which means that a high concentration of interfacial polarization occurs due to the decomposition of LATP. In contrast, the 4 wt% GeO2-LATP electrolyte significantly improved this polarization. In particular, the potential was easily saturated in the initial stage, and the arc was barely noticeable after 400 hours.

[0073] EIS measurements were taken before and after cycling for pure LATP and 4 wt% GeO2-LATP electrolytes, and are shown in Figure 14. Referring to Figure 14, it can be seen that the 4 wt% GeO2-LATP electrolyte exhibits improved electrolyte performance. For pure LATP electrolyte, the resistance values ​​before and after cycling are greater than 120,000 Ω, while for 4 wt% GeO2-LATP electrolyte, the resistance remains relatively low at less than 20,000 Ω even after cycling.

[0074] The 4 wt% GeO2-LATP sample was then subjected to galvanostatic testing at various current densities to determine its rate performance, which is shown in Figure 15. The lithium symmetric cell with 4 wt% GeO2-LATP exhibited a 0.01 mA cm -2 Polarization voltage below 0.3 V, 0.05 mA cm -2 Polarization voltage of 2.3 V or less and 0.1 mA cm -2 The polarization voltage was below 4.9 V, confirming the stable operation of the 4 wt% GeO2-LATP electrolyte cell. -2 It was confirmed that the cell performance almost retained its initial performance even when the temperature returned to 100°C.

[0075] As mentioned above, considering the excellent ionic conductivity and stable cell performance of the electrolyte of the present disclosure, the GeO2-LATP electrolyte, especially 4 wt% GeO2-LATP, can be used to develop high-performance all-solid-state lithium ion batteries.

[0076] Although the embodiments of the present invention have been described in detail above with reference to the drawings, the present invention is not limited to the above-described embodiments, and those skilled in the art may apply various technical modifications and variations based on the above. For example, the described techniques may be performed in an order different from that described, and / or the components of the described systems, structures, devices, circuits, etc. may be combined or combined in a form different from that described, or may be substituted or replaced by other components or equivalents, and still achieve appropriate results.

[0077] Accordingly, other implementations, other embodiments, and equivalents of the claims are within the scope of the following claims.

[0078] The present disclosure provides the following aspects.

[0079] Aspect 1: A method for preparing a solid electrolyte, wherein the solid electrolyte is a GeO2-doped Li 1+x AlxTi 2-x (PO4)3(LATP), where x is 0.3 to 0.5, and the manufacturing method includes Li 1+x AlxTi 2-x 1. A method for manufacturing a solid electrolyte, comprising: mixing a precursor powder of (PO4)3 and a GeO2 powder to form a powder mixture; and sintering the powder mixture.

[0080] Aspect 2: The method for producing a solid electrolyte according to Aspect 1, wherein x is 0.5.

[0081] Aspect 3: In any one of the preceding aspects, the Li 1+x AlxTi 2-x The (PO4)3 precursor powder contains Li2CO3, Al2O3, TiO2, and (NH4)2HPO4.

[0082] Aspect 4: In any one of the above aspects, the precursor powder and the GeO2 powder are mixed uniformly by wet milling.

[0083] Aspect 5: In any one of the above aspects, the method for producing a solid electrolyte further includes the steps of milling the mixed powder; drying and calcining the milled electrolyte powder; and pressing the calcined electrolyte powder.

[0084] Aspect 6: The method for producing a solid electrolyte according to any one of the preceding aspects, wherein the milling step includes ball milling.

[0085] Aspect 7: In any one of the above aspects, the drying and calcining step is carried out at 800-1000°C for 5-7 hours.

[0086] Aspect 8: The method for producing a solid electrolyte according to any one of the above aspects, wherein the pressurizing step applies a pressure of 50 to 70 MPa.

[0087] Aspect 9: In any one of the preceding aspects, the sintering step is carried out at 900 to 1100°C for 3 to 7 hours.

[0088] Aspect 10: In any one of the above aspects, the concentration of the GeO2 powder is 1 wt% to 10 wt% based on LATP.

[0089] Aspect 11: The method of any one of the preceding aspects, wherein the solid electrolyte comprises a Ge-enriched amorphous phase at grain boundaries.

[0090] Aspect 12. In any one of the preceding aspects, the solid electrolyte has an ionic conductivity of 3×10 at 25° C. -4 ~9×10 -4 S cm -1 A method for producing a solid electrolyte.

[0091] Aspect 13: A solid electrolyte produced by the method of any one of the preceding aspects, wherein the solid electrolyte comprises a Ge-enriched amorphous phase at grain boundaries.

[0092] Aspect 14: The solid electrolyte of any one of the preceding aspects, wherein the concentration of GeO2 is 1 wt% to 10 wt% based on LATP.

[0093] Aspect 15: The solid electrolyte of any one of the preceding aspects, wherein the amorphous phase exists between adjacent crystal grains and has a thickness of 5 to 15 nm.

[0094] Aspect 16: In any one of the preceding aspects, the ionic conductivity at 25°C is 3 x 10 -4 ~9×10 -4 S cm -1 A solid electrolyte.

[0095] Aspect 17: A lithium ion battery comprising a positive electrode, a negative electrode, and a solid electrolyte according to any one of the preceding aspects.

Claims

1. A method for producing a solid electrolyte, comprising: The solid electrolyte is GeO ２ Doped Li １＋ｘ AlxTi ２－ｘ (P.O. ４ ) ３ (LATP), where x is 0.3 to 0.5; The manufacturing method includes: Li １＋ｘ AlxTi ２－ｘ (P.O. ４ ) ３ Precursor powder and GeO ２ mixing the powders to form a powder mixture; sintering the powder mixture; A method for producing a solid electrolyte, comprising:

2. The Li １＋ｘ AlxTi ２－ｘ (P.O. ４ ) ３ The precursor powder of ２ CO ３ , Al ２ O ３ , TiO ２ , and (NH ４ ) ２ HPO ４ The method for producing the solid electrolyte according to claim 1, comprising:

3. The method for producing the solid electrolyte includes: milling the powder mixture; drying and calcining the milled electrolyte powder; pressing the calcined electrolyte powder; The method for producing a solid electrolyte according to claim 1 , further comprising:

4. The method for producing a solid electrolyte according to claim 1, wherein the sintering step is carried out at 900 to 1100°C for 3 to 7 hours.

5. The Ge ２ 2. The method for producing a solid electrolyte according to claim 1, wherein the concentration of the powder is 1% by weight to 10% by weight based on LATP.

6. The method for producing a solid electrolyte according to claim 1, wherein the solid electrolyte comprises a Ge-enriched amorphous phase at grain boundaries.

7. The solid electrolyte has an ionic conductivity of 3× at 25° C. １０－４ ~9x １０－４ S cm －１ The method for producing a solid electrolyte according to claim 1,

8. A solid electrolyte produced by the method of any one of claims 1 to 7, wherein the solid electrolyte comprises a Ge-enriched amorphous phase at grain boundaries.

9. Ge0 ２ The solid electrolyte according to claim 8, wherein the concentration of is 1% by weight to 10% by weight based on LATP.

10. 9. The solid electrolyte according to claim 8, wherein the amorphous phase is present between adjacent crystal grains with a thickness of 5 to 15 nm.

11. Ionic conductivity is 3x at 25°C １０－４ ~9x １０－４ S cm －１ The solid electrolyte according to claim 8, wherein

12. A positive electrode and a negative electrode; The solid electrolyte according to claim 8 ; Includes a lithium-ion battery.

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