LLZO solid electrolyte doped with single element, method for preparing same, and all-solid-state lithium secondary battery including same

Abstract

Disclosed is a method for preparing a solid electrolyte that has a cubic crystal structure with high structural stability by doping the Zr sites of LLZO with a single element (Sb, In, or Cd) in order to solve the problems of structural instability and low ionic conductivity when preparing LLZO by co-precipitation, wherein the method can improve ionic conductivity by increasing lithium ion transport pathways.

Description

Single element-doped LLZO solid electrolyte, method for manufacturing the same, and all-solid-state lithium secondary battery including the same

[0001] Cross-citation with related application(s)

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2024-0164518 filed November 18, 2024, and all contents disclosed in the literature of said Korean patent applications are incorporated herein as part of this specification.

[0003] The present invention relates to a single-element doped LLZO solid electrolyte, a method for manufacturing the same, and an all-solid-state lithium secondary battery containing the same.

[0004] Lithium-ion batteries possess large electrochemical capacity, high operating potential, and excellent charge-discharge cycle characteristics. Demand for these lithium-ion batteries is increasing for applications such as portable information terminals, portable electronic devices, small home power storage devices, motorcycles, electric vehicles, and hybrid electric vehicles. With the expansion of these applications, there is a demand for improved safety and high performance of lithium-ion batteries.

[0005] Conventional lithium-ion batteries use liquid electrolytes, which cause them to ignite easily when exposed to water in the air, raising constant safety concerns. This safety issue is becoming an even greater concern as electric vehicles become a reality.

[0006] Accordingly, research on all-solid-state secondary batteries utilizing solid electrolytes made of non-combustible inorganic materials has recently been actively conducted to enhance safety. All-solid-state secondary batteries are attracting attention as next-generation secondary batteries due to their safety, high energy density, high power output, long lifespan, simplified manufacturing processes, larger and more compact battery sizes, and reduced costs.

[0007] All-solid-state secondary batteries consist of a positive electrode, a solid electrolyte layer, and a negative electrode; among these, the solid electrolyte in the solid electrolyte layer requires high ionic conductivity and low electronic conductivity.

[0008] Solid electrolytes that satisfy the requirements of the solid electrolyte layer of all-solid-state secondary batteries include sulfide-based and oxide-based types. Among these, sulfide-based solid electrolytes have problems such as the generation of resistive components due to interfacial reactions with positive or negative active materials, strong hygroscopicity, and the generation of hydrogen sulfide (H2S) gas, which is a toxic gas.

[0009] Japanese Registered Patent Publication No. 4,779,988 discloses an all-solid-state lithium secondary battery having a stacked structure of a positive electrode, a solid electrolyte layer, and a negative electrode, and composed of a sulfide-based solid electrolyte layer.

[0010] Oxide-based solid electrolytes include LLTO (Li3xLa 2 / (3-x) TiO3) system, LLZO(Li7La3Zr2O 12 ) systems are widely known, and among them, LLZO, which is known to have relatively higher grain boundary resistance compared to LLTO but excellent dislocation window characteristics, is attracting attention as a promising material.

[0011] Despite the advantages of the above-mentioned LLZO, such as high ionic conductivity, low reactivity with electrode materials, and a wide potential window (0-6V), it is difficult to establish process conditions due to the volatilization of lithium (Li) during the sintering process, and its manufacturing process is complex and demanding due to its difficult-to-sinter nature, making it difficult to apply in practice. Furthermore, since there are significant differences in ionic conductivity depending on the crystal structure, it is necessary to develop a technology to control the crystal structure of LLZO by adjusting the composition of the starting material, calcination, and sintering conditions.

[0012] (Patent Document 1) Japanese Registered Patent No. 4779988

[0013] LLZO presents problems of structural instability and low ionic conductivity when prepared by a coprecipitation method based on the Taylor reaction. In this invention, the Zr of LLZO 4+ By doping a single element at a site, it has a crystal structure that is a single-phase cubic structure with high structural stability, and the movement path of lithium ions is increased by the doping element within the crystal structure, thereby improving ion conductivity.

[0014] The present invention aims to provide a single-element doped LLZO solid electrolyte, a method for manufacturing the same, and an all-solid-state lithium secondary battery containing the same.

[0015] The present invention provides a lithium lanthanum zirconium oxide (LLZO) solid electrolyte represented by the following chemical formula 1 and doped with a single element (M=Sb, In, or Cd).

[0016] [Chemical Formula 1]

[0017] Li x La y Zr z M p O 12

[0018] (In the above formula, x, y, z, and p follow as described in the specification)

[0019] In addition, the present invention comprises the step of (a) preparing a solid electrolyte precursor slurry by co-precipitation reaction in a Taylor vortex state with a mixed solution comprising an aqueous metal precursor solution containing a lanthanum salt, a zirconium salt, and a single element (M=Sb, In, or Cd) salt, a complexing agent, and a pH adjuster;

[0020] (b) a step of preparing a solid electrolyte precursor by washing and drying the solid electrolyte precursor slurry;

[0021] (c) a step of reacting the above solid electrolyte precursor with a lithium source;

[0022] (d) a step of performing a primary heat treatment for calcination of the obtained reactant; and

[0023] (e) a step of performing a secondary heat treatment for sintering the obtained calcined mixture; the present invention provides a method for preparing a lithium lanthanum zirconium oxide (LLZO) solid electrolyte of Formula 1.

[0024] In addition, the present invention provides an all-solid-state lithium secondary battery comprising a lithium lanthanum zirconium oxide (LLZO) solid electrolyte of the above chemical formula 1.

[0025] The LLZO solid electrolyte according to the present invention achieves structural stabilization by converting the crystal structure from an unstable tetragonal structure to a single-phase cubic structure through doping of the single element (Sb, In, or Cd), and improves ion conductivity by optimizing the movement path of lithium ions.

[0026] In addition, the all-solid-state lithium battery of the present invention has the effect of improving the performance and characteristics of the battery by using an LLZO solid electrolyte.

[0027] FIG. 1 is a flowchart of a method for manufacturing a single element (Sb, In, or Cd) doped LLZO solid electrolyte according to the present invention.

[0028] Figure 2 is a schematic diagram of the Taylor reactor used in the present invention.

[0029] Figure 3 shows the impedance values ​​of the solid electrolytes prepared in Examples 1 to 3 and Comparative Example 1.

[0030] Figure 4 is the XRD spectrum of the calcined mixture obtained after the first heat treatment when preparing the solid electrolyte in Examples 1 to 3 and Comparative Example 1.

[0031] Figure 5 is the XRD spectrum of the sintered pellet obtained after secondary heat treatment when preparing the solid electrolyte in Examples 1 to 3 and Comparative Example 1.

[0032] FIG. 6 is an XRD Rietveld analysis graph of the calcined mixture after the first heat treatment when preparing the solid electrolyte in Examples 1 to 3 and Comparative Example 1, showing (a) lattice constant and volume, and (b) lattice size.

[0033] FIG. 7 is an XRD Rietveld analysis graph of sintered pellets obtained after secondary heat treatment when preparing solid electrolytes in Examples 1 to 3 and Comparative Example 1, showing (a) lattice constant and volume, and (b) lattice size.

[0034] A solid electrolyte according to one embodiment of the present invention is represented by the following chemical formula 1 and is a lithium lanthanum zirconium oxide (LLZO) doped with a single element (M=Sb, In or Cd).

[0035] [Chemical Formula 1]

[0036] Li x La y Zr z M p O 12

[0037] (In the above formula, M is doped at the Zr site and is one of antimony (Sb), indium (In), or cadmium (Cd), 6≤x≤9, 2≤y≤4, 1≤z+p≤3, and 0 <p≤0.5이다)

[0038] Preferably, the solid electrolyte of Formula 1 is Li 7.1 La3Zr 1.9 Sb 0.1 O 12 , Li 7.1 La3Zr 1.9 In 0.1 O 12 , or Li 7.1 La3Zr 1.9 CD 0.1 O 12 is. In this case, the solid electrolyte of Chemical Formula 1 has an ionic conductivity of 1.6 x 10⁻⁶ -5 S / cm to 5.0x10 -4 It is excellent in S / cm.

[0039] The crystal structure of the solid electrolyte of Chemical Formula 1 above may have a single-phase cubic structure, the lattice constant (a=b=c) ranges from 12.97086 Å to 13.02426 Å, and the volume is 2182.259 Å. 3 to 2209.323Å 3 It has a range, and the lattice size can range from 214.1 Å to 447.0 Å.

[0040] A method for manufacturing a solid electrolyte according to one embodiment of the present invention comprises: (a) preparing a solid electrolyte precursor slurry by co-precipitation reaction in a Taylor vortex state with a mixed solution in which an aqueous solution of a metal precursor containing a lanthanum salt, a zirconium salt, and a single element (M=Sb, In, or Cd) salt is mixed with a complexing agent and a pH adjuster; (b) preparing a solid electrolyte precursor by washing and drying the solid electrolyte precursor slurry; (c) reacting the solid electrolyte precursor with a lithium source; (d) performing a first heat treatment for calcination of the obtained reactant; and (e) performing a second heat treatment for sintering the obtained calcined mixture.

[0041] Preferably, the co-precipitation reaction in step (a) can be carried out in a Taylor reactor. The first heat treatment can be carried out at 700 to 900°C, and the second heat treatment can be carried out at 1,100 to 1,250°C.

[0042] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings so that those skilled in the art can easily implement the present invention.

[0043] However, the following description is not intended to limit the present invention to specific embodiments, and detailed descriptions of related prior art are omitted if it is determined that such detailed descriptions could obscure the essence of the present invention.

[0044] The terms used herein are merely for describing specific embodiments and are not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this application, terms such as "comprising" or "having" are intended to indicate the presence of the features, numbers, steps, actions, components, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, or combinations thereof.

[0045]

[0046] This invention addresses the problems of structural instability and low ionic conductivity when preparing LLZO by a co-precipitation method based on a Taylor reaction, by [addressing] Zr of LLZO. 4+ By doping a single element at the site, the crystal structure has a cubic structure with high structural stability, and the movement path of lithium ions is increased by the doping element within the crystal structure, thereby improving ion conductivity.

[0047] The lithium lanthanum zirconium oxide (LLZO) solid electrolyte according to the present invention is Zr 4+ A single element (M=Sb, In, or Cd) is doped at the site, and is represented by the following chemical formula 1:

[0048] [Chemical Formula 1]

[0049] Li x La y Zr z M p O 12

[0050] (In the above formula, M is doped at the Zr site and is one of antimony (Sb), indium (In), or cadmium (Cd), 6≤x≤9, 2≤y≤4, 1≤z+p≤3, and 0 <p≤0.5이다.)

[0051] Hereinafter, the solid electrolyte of Chemical Formula 1 is represented as '(Sb, In or Cd)-LLZO', and LLZO doped with antimony (Sb) is represented as 'Sb-LLZO', LLZO doped with indium (In) is represented as 'In-LLZO', and LLZO doped with cadmium (Cd) is represented as 'Cd-LLZO'.

[0052] The values ​​denoted by x, y, and z represent the molar ratios of the (Sb, In, or Cd)-LLZO solid electrolyte, and when the molar ratio of each element is controlled within these ranges, a cubic crystal structure can be obtained and high ionic conductivity can be achieved.

[0053] In the above chemical formula 1, 6≤x<7 and 2.5≤y≤3.5.

[0054] In the above chemical formula 1, 1≤z+p≤3, where 0.5≤z≤2.9 and 0.1≤p≤0.5.

[0055] In the above chemical formula 1, 1≤z+p≤2.5, where 0.7≤z≤2.9 and 0.1≤p≤0.3.

[0056] In the above chemical formula 1, 1≤z+p≤2, where 0.7≤z≤1.9 and 0.1≤p≤0.3.

[0057] In the above chemical formula 1, 1≤z+p≤2, and 1.8 <z≤1.9이고, 0.1≤p<0.2이다.

[0058] The molar ratio of the doping element denoted by p is antimony (Sb), indium (In), or cadmium (Cd) is Zr 4+ This is a figure showing the extent to which doping can occur in the position.

[0059] Zr when p=0 4+ There are no doping elements at the site, and when synthesized by the coprecipitation method, it has a tetragonal crystal structure.

[0060] Zr when p≠0 4+Doping elements are present at the site, and when synthesized by the coprecipitation method, it has a crystal structure in which cubic (tetragonal) and tetragonal structures are mixed.

[0061] A cubic structure is a Bravais lattice structure where the lengths (a, b, c) of the width, length, and height are a=b=c, while a tetragonal structure is a structure where the lengths (a, b, c) of the width, length, and height are a=b≠c. The cubic structure has a relatively simple arrangement, resulting in excellent structural stability and enabling high lithium ion mobility, which leads to relatively high ionic conductivity. Compared to the cubic structure, the tetragonal structure exhibits deformation or abnormal behavior, resulting in high structural instability and relatively low ionic conductivity.

[0062] The (Sb, In, or Cd)-LLZO solid electrolyte of the present invention has p≠0 and has a single-phase cubic structure in its crystal structure.

[0063] Specifically, in the present invention, Zr 4+ By doping a single element among antimony (Sb), indium (In), or cadmium (Cd) at the site, the transition from an unstable tetragonal structure to a cubic structure is promoted. Due to this structural stabilization, the transport pathway of lithium ions is optimized, thereby improving ionic conductivity. The (Sb, In, or Cd)-LLZO solid electrolyte of Formula 1 of the present invention resolves structural instability through doping, so that cubic and tetragonal structures do not coexist but exist solely in a stable cubic structure.

[0064] The doping molar ratio of a single element is denoted by p, and in Chemical Formula 1, it is 0 <p≤0.5, 0.1≤p≤0.3, 또는 0.1≤p<0.2의 범위를 갖는다. 만약 그 수치가 너무 높을 경우, 과도한 도핑으로 인해 원자 배열에 스트레스를 높여 결정 구조의 불안정성이 높아지고, 리튬 이온 이동 경로가 막히거나 복잡해져 오히려 이온전도도가 감소할 수 있다. 따라서 상기 범위 내에서 적절히 조절하여 사용한다.

[0065] According to one embodiment, the (Sb, In, or Cd)-LLZO solid electrolyte of Formula 1 according to the present invention is Li 7.1 La3Zr 1.9 Sb 0.1 O 12 , Li 7.1 La3Zr 1.9 In 0.1 O 12 , or Li 7.1 La3Zr 1.9 CD 0.1 O 12 It was selected from among them.

[0066] The (Sb, In or Cd)-LLZO solid electrolyte of Formula 1 according to the present invention has a lithium ion conductivity sufficient for the operation of a lithium battery at room temperature, i.e., 22°C, and specifically, 1.6 x 10⁻⁶ -5 S / cm or more, 1.7x10 -5 S / cm or more, 1.8x10 -5 S / cm or higher, 5.0x10 -5 S / cm or higher, 7.0x10 -5 S / cm or higher, 8.0x10 -5 S / cm or higher, 9.0x10 -5 S / cm or higher, 1.0x10 -4 S / cm or more, or 1.8x10 -4 S / cm or higher, 5.0x10 -4 It has an ionic conductivity of S / cm or less. For undoped LLZO solid electrolyte, 1.55 x 10⁻⁶ -5 With S / cm, the effect of increasing ion conductivity through doping relative to this value can be secured.

[0067] The ionic conductivity of the Sb-LLZO solid electrolyte is 5.0 x 10⁻⁶ -5 S / cm or more, or 9.0x10 -5 It is greater than S / cm, and the ionic conductivity of the In-LLZO solid electrolyte is 1.0 x 10⁻⁶ -4 S / cm or more, or 1.8x10 -4 It is greater than S / cm, and the ionic conductivity of the Cd-LLZO solid electrolyte is 5.0 x 10⁻⁶ -5 S / cm or more, or 8.0x10 -5 It is S / cm or more.

[0068] The above ion conductivity is the ion conductivity immediately after the second heat treatment (sintering), and refers to the ion conductivity of the solid electrolyte before the grinding process is performed after the second heat treatment.

[0069] The above ionic conductivity is the result calculated using the resistance value obtained by the Electrochemical Impedance Spectroscopy (EIS) method for a sintered pellet body prepared by manufacturing pellets using a molding mold for the (Sb, In or Cd)-LLZO solid electrolyte powder of the present invention.

[0070] The (Sb, In or Cd)-LLZO solid electrolyte of Formula 1 according to the present invention has a peak identical to the peak of ICSD 98-026-1302, which is a cubic structure Ref. structure, when measuring the XRD (X-ray Diffraction) spectrum. The measurement conditions for the XRD spectrum were performed by measuring a range of 10° to 80° at a rate of 1.5° per minute at 40kV and 15mA based on Cu-Kα (wavelength 1.54 Å) radiation.

[0071] The (Sb, In, or Cd)-LLZO solid electrolyte of Formula 1 according to the present invention has a lattice constant (a=b=c) in the range of 12.97086 Å to 13.02426 Å and a volume of 2182.259 Å. 3 to 2209.323Å 3It has a range of , and the lattice size has a range of 214.1 Å to 447.0 Å.

[0072] The parameters of the above crystal structure are calculated through XRD Rietveld refinement of the results obtained after measuring the XRD spectrum of the sintered pellet body, which is prepared by molding pellets of (Sb, In, or Cd)-LLZO solid electrolyte powder using a mold.

[0073]

[0074] According to another embodiment, the present invention provides a method for manufacturing a (Sb, In or Cd)-LLZO solid electrolyte of Formula 1.

[0075] FIG. 1 is a flowchart of a method for manufacturing a single element (Sb, In, or Cd) doped LLZO solid electrolyte according to the present invention.

[0076] The (Sb, In, or Cd)-LLZO solid electrolyte of Chemical Formula 1 is,

[0077] (a) a step of preparing a solid electrolyte precursor slurry by co-precipitation reaction in a Taylor vortex state using a mixed solution comprising an aqueous solution of a metal precursor containing a lanthanum salt, a zirconium salt, and a single element (M=Sb, In, or Cd) salt, a complexing agent, and a pH adjuster;

[0078] (b) a step of preparing a solid electrolyte precursor by washing and drying the solid electrolyte precursor slurry;

[0079] (c) a step of reacting the above solid electrolyte precursor with a lithium source;

[0080] (d) a step of performing a primary heat treatment for calcination of the obtained reactant; and

[0081] (e) a step of performing a secondary heat treatment for sintering the obtained calcined mixture; is included in the manufacturing process.

[0082] Hereinafter, a method for manufacturing a solid electrolyte of the present invention will be described in detail with reference to FIG. 1. However, this is presented as an example and the present invention is not limited thereto, and the present invention is defined only by the scope of the claims set forth below.

[0083]

[0084] (Step a)

[0085] First, a solid electrolyte precursor slurry is prepared by co-precipitation reaction in a Taylor vortex state using an aqueous metal precursor solution mixed with a lanthanum salt, a zirconium salt, and a doping element (M) salt, and a mixed solution mixed with a complexing agent and a pH adjuster.

[0086] The lanthanum salt, zirconium salt, and doping element (M) salt may be nitrates, sulfates, hydrochlorides, carbonates, etc., and preferably may be nitrates. Specifically, the precursor may be lanthanum nitrate or zirconium nitrate, and the doping element (M) salt may be antimony hydrochloride (SbCl3), indium nitrate (In(NO3)3·xH2O), or cadmium nitrate (Cd(NO3)2·4H2O).

[0087] Each salt can be varied depending on the molar ratio of the (Sb, In or Cd)-LLZO solid electrolyte produced at the end, and is not specifically limited in the present invention.

[0088] Complexing agents may include ammonium hydroxide (NH4·OH), sodium hydroxide, etc.

[0089] The pH adjuster can adjust the pH of the above mixed solution to 10 to 12 and may include, but is not limited to, sodium hydroxide (NaOH), ammonia, etc. Any other pH adjuster capable of adjusting the pH of the mixed solution without affecting the physical properties of the ion-conducting solid oxide is also possible.

[0090] The co-precipitation reaction of step (a) can be carried out under Taylor vortex conditions, and preferably in a Taylor reactor. The Taylor reactor is known to have a mass transfer rate four times higher and a stirring intensity seven times higher than that of a conventional batch reactor.

[0091] Figure 2 is a schematic diagram of the Taylor reactor used in the present invention.

[0092] A Taylor reactor is a reactor capable of performing a Taylor vortex reaction.

[0093] The reactor comprises an outer fixed cylinder and an inner rotating cylinder installed at a predetermined distance from each other inside the reactor and rotating. A fluid passage through which a reaction solution flows is formed between the outer fixed cylinder and the inner rotating cylinder.

[0094] A mixed solution comprising a metal precursor aqueous solution, a complexing agent, and a pH regulator is introduced through the solution injection unit (1). At this time, the reactor is discharged through the temperature control solution injection unit (3) and the temperature control solution discharge unit (2), and the temperature inside the reactor is controlled through the reaction solution temperature control unit (7).

[0095] The metal precursor aqueous solution and mixed solution introduced into the reactor through the solution injection section (1) undergo a co-precipitation reaction in the solution reaction section (8). During the reaction, the inner rotating cylinder is rotated by the stirring rod (6), and a flow is generated in the direction of rotation. At this time, due to centrifugal force and Coriolis force, a force is generated in which the fluids present in the inner cylinder tend to move toward the outer cylinder. As the rotation speed increases, the flow becomes increasingly unstable, forming a vortex in the form of ring pairs that rotate regularly in opposite directions along the axial direction. This flow is called Taylor fluid flow or Taylor vortex. The Taylor vortex promotes the co-precipitation reaction, thereby enabling the production of precursors more advantageously than in conventional co-precipitation reactors. The slurry obtained after the co-precipitation reaction is discharged through the reactant (slurry form) discharge section (5), and the reaction solution is discharged through the drain section (4).

[0096] In the theory related to the Taylor vortex mentioned above, the generation of the Taylor vortex can be represented by the Taylor number (Ta). A lower value indicates a linear fluid flow, while a higher value indicates more severe vortex generation. In the present invention, the Taylor number capable of generating a Taylor vortex may be 550 to 1,500, preferably 630 to 800, and more preferably 640 to 700. These values ​​may vary depending on the conditions of the reactor and can be appropriately adjusted by a person skilled in the art.

[0097] The coprecipitation reaction is carried out at a pH of 10 to 12, 10.5 to 11.5, or 11, at a reaction temperature of room temperature, and for 1 to 3 hours.

[0098] At this time, the content ratio of the metal precursor aqueous solution and the complexing agent is carried out in the range of 140 to 200 parts by weight, 150 to 180 parts by weight, and preferably 150 to 170 parts by weight of the complexing agent relative to 100 parts by weight of the metal precursor aqueous solution.

[0099]

[0100] (Step b)

[0101] Next, the solid electrolyte precursor slurry (e.g., La-Sb-Zr-OH, La-In-Zr-OH, La-Cd-Zr-OH) is washed and dried to produce a solid electrolyte precursor.

[0102] The above washing is performed with distilled water, and can be dried using conventional drying methods such as hot air drying, freeze drying, or vacuum drying.

[0103] The solid electrolyte precursor obtained after drying has a pH in the range of 6 to 8, or about 7.

[0104]

[0105] (Step c)

[0106] Next, the above solid electrolyte precursor and the lithium source are mixed.

[0107] Lithium sources can be, for example, LiOH·H2O, LiOH, LiNO3, LiCO3, etc.

[0108] The lithium content of the above lithium source may be added in excess, taking into account the amount of lithium that evaporates when sintered at 700 to 1,200°C, and may be included in the mixture to include 101 to 112 parts by weight, preferably 101 to 108 parts by weight, and more preferably 102 to 107 parts by weight, relative to 100 parts by weight of the lithium content of the solid electrolyte, which is the final product.

[0109] In other words, the mixture may contain an excess amount of lithium, ranging from 1 to 12 weight percent, preferably 1 to 8 weight percent, and more preferably 2 to 7 weight percent, compared to the amount of lithium element in the final LLZO solid electrolyte. If too much lithium is included, it may be difficult to accurately control the lithium composition of the solid electrolyte.

[0110] The above mixing can be performed by a grinding mixing process. Specifically, it can be ground and mixed while applying mechanical energy, such as a bead mill, planetary ball mill, planetary mill, mechano fusion, mortar and pestle, or auto grinder.

[0111] According to one embodiment, when a planetary ball mill is used as the grinding device, the process can be carried out at a rotational speed of 100 rpm to 2000 rpm for 1 hour to 50 hours, but is not limited thereto. Preferably, 10 mm zirconia balls are fed into the planetary ball mill, then ground at a speed of 400 rpm to 700 rpm, and then further ground using a mortar and pestle or an auto grinder.

[0112]

[0113] (Step d)

[0114] Next, a primary heat treatment is performed for the calcination of the obtained reactant.

[0115] The first heat treatment can be performed at 700 to 900°C, preferably at 750 to 850°C, and more preferably at 770 to 820°C.

[0116] The above first heat treatment can be performed for 1 to 12 hours, preferably for 1 to 9 hours, and more preferably for 1 to 7 hours. However, the first heat treatment time is not necessarily limited to this and may vary depending on the first heat treatment temperature.

[0117] Through the above calcination, the crystal structure of the calcined mixture exists in a state where cubic and tetragonal structures are mixed.

[0118] As shown in the XRD graph presented in Fig. 4, the crystal structure of the calcined mixture varies depending on the type of single element doped in it. Antimony (Sb) exhibits a cubic structure with a major peak, while in the case of indium (In) or cadmium (Cd), cubic and tetragonal structures exist in a mixed state. Therefore, additional heat treatment can be performed to convert the tetragonal structure into a cubic structure.

[0119]

[0120] (Step e)

[0121] Next, a secondary heat treatment is performed to sinter the obtained calcined mixture to produce a (Sb,In or Cd)-LLZO solid electrolyte.

[0122] The above secondary heat treatment can be performed at 1,100 to 1,250°C, and preferably at 1,150 to 1,220°C.

[0123] The above secondary heat treatment can be performed for 3 to 7 hours, and preferably for 4 to 6 hours. However, the secondary heat treatment time is not necessarily limited to this and may vary depending on the secondary heat treatment temperature.

[0124] In the crystal structure of the (Sb, In or Cd)-LLZO solid electrolyte sintered as described above, only a single-phase cubic structure exists.

[0125] Looking at the XRD graph presented in Fig. 5, it can be seen that all solid electrolytes doped with antimony (Sb), indium (In), or cadmium (Cd) in the sintered pellets obtained after the second heat treatment have a cubic crystal structure regardless of the type of doping element.

[0126]

[0127] The (Sb, In or Cd)-LLZO solid electrolyte of Formula 1 of the present invention, manufactured through the above steps, can be applied to the battery field.

[0128] The (Sb, In or Cd)-LLZO solid electrolyte of the present invention has high ionic conductivity and can be applied to lithium secondary batteries, particularly all-solid-state lithium secondary batteries.

[0129] An all-solid-state lithium secondary battery is composed of a positive electrode, a negative electrode, and a solid electrolyte layer interposed between them, and the (Sb, In or Cd)-LLZO solid electrolyte can be used in either the positive electrode or the solid electrolyte layer.

[0130]

[0131] [Example]

[0132] Hereinafter, preferred embodiments of the present invention will be described. However, this is for illustrative purposes only and does not limit the scope of the present invention.

[0133]

[0134] Example 1: Sb-LLZO solid electrolyte (Li 7.1 La3Zr 1.9 Sb 0.1 O 12 )manufacturing

[0135] Lanthanum nitrate (La(NO3)3·xH2O), zirconium hydrochloride (ZrOCl2·xH2O), and antimony hydrochloride (SbCl3) were dissolved in distilled water such that the molar ratio of their metal elements La:Zr:Sb was 3:1.9:0.1 to prepare an aqueous solution of a metal precursor with a starting material concentration of 1 molar. Here, x is any one of integers from 0 to 15.

[0136] Referring to FIG. 2, a solid electrolyte was prepared using a Taylor reactor. The Taylor reactor comprises a solution injection section (1), a temperature control solution discharge section (2), a temperature control solution injection section (3), a reaction solution drain section (4), a reactant (slurry form) discharge section (5), a stirring rod (6), a reaction solution temperature control section (7), and a solution reaction section (8). Through the solution injection section (1) of the Taylor reactor, an appropriate amount of the metal precursor aqueous solution, 160 parts by weight of ammonia water (complexing agent) relative to 100 parts by weight of the metal precursor aqueous solution, and a sodium hydroxide aqueous solution were added to form a mixed solution with a pH adjusted to 11. The reaction temperature was set to 25°C, the reaction time to 2 hours, and the stirring speed of the stirring rod to 1000 rpm, thereby co-precipitating the precursor slurry in the form of a liquid slurry and discharging it to the discharge section (5).

[0137] The above precursor slurry was washed with purified water to adjust the pH to 7, and dried in a forced hot air circulation dryer (Lab Tech, LDO-150F) at 110°C for 24 hours to produce a dried precursor.

[0138] The above-mentioned precursor was ground using a ball mill, and then excess LiOH·H2O was added and mixed using a ball mill to prepare a mixture. The LiOH·H2O content was added such that it was 110 parts by weight (10 wt% excess) relative to the theoretical Li content of 100 parts by weight in the solid electrolyte. The mixture was subjected to a first heat treatment at 900°C for 2 hours.

[0139] The obtained calcined mixture was formed into pellets using a molding mold and then sintered at 1,150°C for 5 hours to produce an Sb-LLZO pellet sintered body.

[0140]

[0141] Example 2: In-LLZO solid electrolyte (Li 7.1 La3Zr 1.9 In 0.1 O 12 ) manufacturing

[0142] The above Example 1 was performed in the same manner, but indium nitrate (In(NO3)3·xH2O) was used instead of antimony hydrochloride to prepare an In-LLZO pellet sintered body.

[0143]

[0144] Example 3: Cd-LLZO solid electrolyte (Li 7.1 La3Zr 1.9 CD 0.1 O 12 ) manufacturing

[0145] The above Example 1 was performed in the same manner, but using indium cadmium nitrate (Cd(NO3)2·4H2O) instead of antimony hydrochloride to prepare a Cd-LLZO pellet sintered body.

[0146]

[0147] Comparative Example 1: LLZO solid electrolyte (Li7La3Zr 2.0 O 12 ) manufacturing

[0148] The above Example 1 was performed in the same manner, but an LLZO pellet sintered body was prepared using an aqueous metal precursor solution in which lanthanum nitrate (La(NO3)3·xH2O) and zirconium hydrochloride (ZrOCl2·xH2O) were dissolved in distilled water such that the molar ratio of their metal elements La:Zr was 3:2.

[0149]

[0150] Test Example 1: Measurement of Ionic Conductivity

[0151] The results of calculating the ionic conductivity using the resistance values ​​(Fig. 3) measured by the Electrochemical Impedance Spectroscopy (EIS) method for the LLZO pellet sintered bodies prepared in the above examples and comparative examples are shown in Table 1 below.

[0152] Composition Specimen Thickness (cm) Specimen Area (cm) 2 )Resistance (Ω) Resistivity (Ω·cm) Ionic Conductivity (S / cm) Comparative Example 1Li7La3Zr 2.0 O 12 0.0780.187926749.564444.31.55x10-5 Example 1 Li 7.1 La3Zr 1.9 Sb 0.1 O 12 0.090.19744673.1110249.69.76x10 -5 Example 2Li 7.1 La3Zr 1.9 In 0.1 O 12 0.1010.21922468.755358.121.87x10 -4 Example 3Li 7.1 La3Zr 1.9 CD 0.1 O 12 0.0980.18096063.9811196.58.93x10 -5

[0153] Referring to Table 1 and Figure 3, it can be seen that the resistance and ion conductivity of the LLZO pellet sintered bodies of Examples 1 to 3 are improved compared to Comparative Example 1. In particular, compared to the LLZO of Comparative Example 1, the Sb-LLZO of Example 1 showed an improvement of about 6.3 times, the Cd-LLZO about 5.8 times, and the In-LLZO about 12 times.

[0154] These results show that doping Zr with a single element such as Sb, In, or Cd is a way to improve the ionic conductivity of LLZO.

[0155]

[0156] Test Example 2: XRD Analysis

[0157] To compare the XRD results of LLZO according to single-element doping, the XRD of LLZO after the first heat treatment and the second heat treatment was analyzed.

[0158] Figure 4 is the XRD spectrum of the calcined mixture obtained after the first heat treatment when preparing the solid electrolyte in Examples 1 to 3 and Comparative Example 1, and Figure 5 is the XRD spectrum of the sintered pellet obtained after the second heat treatment. For comparison, the peak of ICSD 98-026-1302, which is the Ref. structure of the cubic structure, and ICSD 98-024-6817, which is the Ref. structure of the tetragonal structure, are shown together at the bottom of each spectrum.

[0159] Looking at Fig. 4, the calcined mixture of Comparative Example 1 showed a peak corresponding to a tetragonal crystal structure, while the calcined mixtures of Examples 1 to 3 showed a peak corresponding to a cubic structure. Among these, the calcined mixture of Example 2 (In doping) showed a peak that most closely corresponded to a cubic structure.

[0160] As shown in Fig. 5, after the second sintering, the sintered pellet of Comparative Example 1 still maintained a tetragonal crystal structure, and it was confirmed that all (Sb,In or Cd)-LLZO of Examples 1 to 3 had a cubic structure peak corresponding to the peak of ICSD 98-026-1302.

[0161]

[0162] Test Example 3: XRD Rietveld Analysis

[0163] XRD Rietveld analysis was performed using the results of Test Example 2 above.

[0164] FIG. 6 is an XRD Rietveld analysis graph of the calcined mixture after the first heat treatment when preparing solid electrolytes in Examples 1 to 3 and Comparative Example 1, showing (a) lattice constant and volume, and (b) lattice size, and FIG. 7 is an XRD Rietveld analysis graph of the sintered pellets obtained after the second heat treatment, showing (a) lattice constant and volume, and (b) lattice size. The results of the above graphs are shown in Table 2 below.

[0165] Composition, Crystal System, Lattice Parameter (Å), Volume (Å) 3 ) Lattice size (Crystallite size, Å) Comparative Example 1Li7La3Zr 2.0 O 12 Tetragonal a=b=13.1152c=12.66282178.141362.6 Example 1 Li 7.1 La3Zr 1.9 Sb 0.1 O 12 Cubic a=12.9818(a=b=c)2187.8460.5 Example 2Li 7.1 La3Zr 1.9 In 0.1 O 12 Tetragonal a=b=13.10556c=12.68832179.296556.5 Example 3Li 7.1 La3Zr 1.9 CD 0.1 O 12 Tetragonal a=b=13.114c=12.66352177.8311194.5

[0166] Looking at the results in Figure 6 and Table 2, the Sb-LLZO calcined mixture of Example 1 showed a cubic crystal structure, whereas the calcined mixtures of Comparative Example 1, Example 2, and Example 3 showed a tetragonal crystal structure.

[0167] In particular, when compared to the calcination mixture of Comparative Example 1, it can be seen that the calcination mixture of Example 1 has a larger volume and a significantly reduced lattice size as it has a cubic structure.

[0168] Composition, Crystal System, Lattice Parameter (Å), Volume (Å) 3 ) Lattice size (Crystallite size, Å) Comparative Example 1Li7La3Zr 2.0 O 12 Tetragonal a=b=13.0976c=12.69512177.847303.2 Example 1 Li 7.1 La3Zr 1.9 Sb0.1 O 12 Cubic a=13.01718(a=b=c)2205.721214.1 Example 2Li 7.1 La3Zr 1.9 In 0.1 O 12 Cubic a=12.97086(a=b=c)2182.259447.0 Example 3Li 7.1 La3Zr 1.9 CD 0.1 O 12 Cubic a=13.02426(a=b=c)2209.323238.7

[0169] Looking at the results in Figure 7 and Table 3, all (Sb, In, or Cd)-LLZO of Examples 1 to 3 have a cubic structure with lattice constants a=b=c, and both the lattice constant and volume were at nearly the same level. Since the lattice size can vary depending on the size of the doping element, In-LLZO showed the highest MS lattice size value.

[0170] Through these results, it can be seen that the crystal structure of (Sb,In or Cd)-LLZO can be converted from a tetragonal structure to a cubic structure through secondary heat treatment.

[0171]

[0172] The scope of the present invention is defined by the claims set forth below rather than by the detailed description above, and all modifications or variations derived from the meaning and scope of the claims and equivalent concepts thereof should be interpreted as being included within the scope of the present invention.

[0173] (Explanation of symbols)

[0174] 1: Solution injection port

[0175] 2: Temperature control solution discharge section

[0176] 3: Temperature control solution injection port

[0177] 4: Reaction solution drain section

[0178] 5: Reactant (slurry form) discharge section

[0179] 6: Stirring rod

[0180] 7: Reaction solution temperature control unit

[0181] 8: Solution reaction section

[0182] The single-element doped LLZO solid electrolyte according to the present invention can be used in lithium secondary batteries and mounted in various electronic devices driven by electrical energy.

Claims

1. A lithium lanthanum zirconium oxide (LLZO) solid electrolyte represented by the following chemical formula 1 and doped with a single element (M=Sb, In, or Cd): [Chemical Formula 1] Li x La y Zr z M p O 12 (In the above formula, M is doped at the Zr site and is one of antimony (Sb), indium (In), or cadmium (Cd), 6≤x≤9, 2≤y≤4, 1≤z+p≤3, and 0 <p≤0.5이다.) 2. In Paragraph 1, A solid electrolyte in which 6≤x<7, 2.5≤y≤3.5, 1≤z+p≤3, where 0.5≤z≤2.9 and 0.1≤p≤0.

5.

3. In Paragraph 1, The solid electrolyte of Chemical Formula 1 above is Li 7.1 La3Zr 1.9 Sb 0.1 O 12 , Li 7.1 La3Zr 1.9 In 0.1 O 12 , or Li 7.1 La3Zr 1.9 CD 0.1 O 12 Phosphorus, solid electrolyte.

4. In Paragraph 1, The solid electrolyte of Chemical Formula 1 above has an ionic conductivity of 1.6 x 10⁻⁶ -5 S / cm to 5.0x10 -4 Solid electrolyte with S / cm.

5. In Paragraph 1, The crystal structure of the solid electrolyte of the above chemical formula 1 is a solid electrolyte having a single-phase cubic structure.

6. In Paragraph 1, The solid electrolyte of Chemical Formula 1 above has a lattice constant (a=b=c) in the range of 12.97086 Å to 13.02426 Å and a volume of 2182.259 Å. 3 to 2209.323Å 3 A solid electrolyte having a range of and a lattice size in the range of 214.1 Å to 447.0 Å. 7.(a) A step of preparing a solid electrolyte precursor slurry by co-precipitation reaction in a Taylor vortex state using a mixed solution comprising an aqueous solution of a metal precursor containing a lanthanum salt, a zirconium salt, and a single element (M=Sb, In, or Cd) salt, a complexing agent, and a pH adjuster; (b) a step of preparing a solid electrolyte precursor by washing and drying the solid electrolyte precursor slurry; (c) a step of reacting the above solid electrolyte precursor with a lithium source; (d) a step of performing a primary heat treatment for calcination of the obtained reactant; and (e) a step of performing a secondary heat treatment for sintering the obtained calcined mixture; comprising a method for preparing a lithium lanthanum zirconium oxide (LLZO) solid electrolyte of Formula 1 below: [Chemical Formula 1] Li x La y Zr z M p O 12 (In the above formula, M is doped at the Zr site and is one of antimony (Sb), indium (In), or cadmium (Cd), 6≤x≤9, 2≤y≤4, 1≤z+p≤3, and 0 <p≤0.5이다.) 8. In Paragraph 7, A method for producing a solid electrolyte, wherein the co-precipitation reaction in step (a) above is performed in a Taylor reactor.

9. In Paragraph 7, The above lanthanum salt, zirconium salt, or single-element (M=Sb, In, or Cd) salt is, A method for preparing a solid electrolyte comprising one or more salts of nitrate, sulfate, hydrochloride, and carbonate containing lanthanum, zirconium, or a single element (M=Sb, In, or Cd).

10. In Paragraph 7, A method for manufacturing a solid electrolyte, wherein the above first heat treatment is performed at 700 to 900℃.

11. In Paragraph 7, A method for manufacturing a solid electrolyte, wherein the above secondary heat treatment is performed at 1,100 to 1,250℃.

12. An all-solid-state lithium secondary battery comprising the solid electrolyte of claim 1.

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