Solid-state electrolyte and cell using the same

Abstract

This invention provides a solid electrolyte for lithium-ion batteries, comprising a garnet-type solid electrolyte and a polymer filler, wherein the polymer filler is dispersed in the garnet-type solid electrolyte. The garnet-type solid electrolyte is lithium carbonate-free, and the content of the garnet-type solid electrolyte is 60% by weight or more based on the total weight of the garnet-type solid electrolyte and the polymer filler.

Description

[Technical Field]

[0001] This invention relates to a solid electrolyte for lithium-ion batteries, and more specifically, to a solid electrolyte comprising a garnet-type solid electrolyte and a polymer filler in a specific ratio. This invention also relates to a lithium-ion battery comprising the solid electrolyte. [Previous Technology]

[0002] To address global warming and climate change, energy conservation and carbon reduction are the primary goals, and improving energy storage efficiency is the most important issue in energy conservation. Lithium-ion batteries are one of the most widely used energy storage systems. Traditional lithium-ion batteries use liquid electrolytes; however, liquid electrolytes have safety issues such as leakage and the separator being susceptible to lithium dendrite puncture, leading to short circuits or even explosions. Therefore, recent research on lithium-ion batteries has focused on the development of solid-state electrolytes. Garnet-type solid-state electrolytes are considered one of the most promising solid-state electrolytes due to their high mechanical strength, high ionic conductivity (approximately 10⁻³ S / cm (Siemens / cm) to 10⁻⁴ S / cm), and high chemical stability.

[0003] However, all-solid-state garnet-type ceramic electrolyte sheets are brittle and difficult to thin, and are prone to breakage due to extrusion during processing, which greatly limits their application. In addition, garnet-type solid electrolytes readily react with carbon dioxide (CO2) and water (H2O) present in the air to form a lithium carbonate (Li2CO3) passivation layer on their surface, resulting in an increase in the interfacial impedance between the all-solid-state garnet-type ceramic electrolyte sheet and lithium metal. [Summary of the Invention]

[0004] In view of the above-mentioned technical problems, the present invention provides a solid electrolyte that combines good flexibility and mechanical strength with high ionic conductivity by combining a lithium carbonate-free garnet-type solid electrolyte with polymer fillers. The solid electrolyte is particularly suitable for use in lithium-ion batteries, and the provided lithium-ion battery can combine good mechanical properties, high ionic conductivity, high electrochemical stability, and high cycle stability.

[0005] Therefore, one object of the present invention is to provide a solid electrolyte for lithium-ion batteries, comprising a garnet-type solid electrolyte and a polymer filler, wherein the polymer filler is dispersed in the garnet-type solid electrolyte, and the garnet-type solid electrolyte is free of lithium carbonate, and wherein the content of the garnet-type solid electrolyte is 60% by weight or more based on the total weight of the garnet-type solid electrolyte and the polymer filler.

[0006] In one embodiment of the present invention, the garnet-type solid electrolyte system is selected from the group consisting of: Li 6.4La 3Zr 1.4Ta 0.6O 12 (LLZTO), Li 6.4La 3Zr 2Al 0.2O 12 (LLZAO), Li 6.4La 3Zr 2Ga 0.2O 12 (LLZGO), Li 6.25Al 0.20La 3Zr 1.85Nb 0.15O 12 (LALZNO), and combinations thereof.

[0007] In one embodiment of the present invention, the garnet-type solid electrolyte is provided by lithium carbonate-free garnet-type solid electrolyte ceramic powder. The average particle size of the ceramic powder is preferably less than 100 micrometers.

[0008] In one embodiment of the present invention, the polymer filler material is selected from the group consisting of polytetrafluoroethene (PTFE), poly(ethylene oxide, PEO), polyacrylonitrile (PAN), poly(methyl methacrylate, PMMA), and combinations thereof.

[0009] In one embodiment of the present invention, the content of the polymer filler is 0.1% by weight to less than 40% by weight, more preferably 0.1% by weight to less than 10% by weight, and even more preferably 0.1% by weight to less than 5% by weight, based on the total weight of the garnet-type solid electrolyte and the polymer filler.

[0010] Another object of the present invention is to provide a lithium-ion battery comprising a positive electrode, a negative electrode and a solid electrolyte as described above.

[0011] In one embodiment of the present invention, the material of the positive electrode is selected from the group consisting of: lithium iron phosphate (LiFePO4, LFP), lithium cobalt oxide (LiCoO2, LCO), lithium nickel cobalt aluminum oxide (NCA), and lithium nickel cobalt manganese oxide (NCM).

[0012] In one embodiment of the present invention, the material of the negative electrode is selected from the group consisting of lithium metal, lithium indium alloy, lithium aluminum alloy, lithium silicon alloy and combinations thereof.

[0013] In one embodiment of the present invention, an ionic liquid containing lithium salt is present between the positive electrode and the solid electrolyte and between the negative electrode and the solid electrolyte.

[0014] In one embodiment of the present invention, the lithium salt is selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, and combinations thereof.

[0015] In order to make the above-mentioned objectives, technical features and advantages of the present invention more apparent and understandable, the following is a detailed description of some specific embodiments.

Implementation Method

[0017] The following will specifically describe some specific embodiments of the present invention; however, the present invention may be practiced in many different forms and the scope of protection of the present invention should not be limited to the specific embodiments described.

[0018] Unless otherwise stated, the terms “a”, “the” and similar terms used in this specification and the claims shall be understood to include both singular and plural forms.

[0019] Unless otherwise stated, in this specification and the scope of the claims, "positive electrode" refers to the cathode in the battery discharge state, and "negative electrode" refers to the anode in the battery discharge state.

[0020] Unless otherwise stated, in this specification and the claims, the unit of weight average molecular weight (Mw) is "Dalton".

[0021] In this article, the term "ionic liquid" refers to ionic compounds in liquid state, especially ionic compounds that have the function of enhancing lithium ion mobility with the properties of soluble lithium salts.

[0022] In this document, the term "polymer-in-ceramic (PIC)" refers to a composite material of polymer and ceramic, wherein the ceramic content is greater than 50% by weight based on the total weight of the composite material, that is, the polymer content is less than 50% by weight.

[0023] The advantage of this invention compared to the prior art lies in adding a relatively small amount of polymer filler to a garnet-type solid electrolyte to form a polymer-in-ceramic solid-state electrolyte, thereby providing a solid electrolyte that combines good flexibility and mechanical strength with high ionic conductivity. This solid electrolyte can be applied to lithium-ion batteries, and the resulting lithium-ion batteries exhibit good mechanical properties, high ionic conductivity, high electrochemical stability, and high cycle stability. The following provides a detailed description of the solid electrolyte of this invention and its related applications.

[0024] 1. Solid electrolyte

[0025] The solid electrolyte of the present invention comprises a garnet-type solid electrolyte and a polymer filler, wherein the polymer filler is dispersed in the garnet-type solid electrolyte.

[0026] 1.1. Garnet-type solid electrolyte

[0027] Garnet-type solid electrolytes are a type of oxide electrolyte, represented by Li₇La₃Zr₂O₁₂ (LLZO). Generally, garnet-type solid electrolytes exist in a high-temperature metastable cubic phase and a low-temperature / room-temperature stable tetragonal phase. In the tetragonal phase, lithium sites are 100% occupied, while lithium vacancies exist in the cubic phase. This results in the ionic conductivity of tetragonal garnet being two orders of magnitude lower than that of cubic garnet. One method to stabilize cubic garnet is to directly dope and substitute Li⁺ to generate lithium vacancies. Another method is to use high-valence ions to substitute Zr⁴⁺, which can stabilize the cubic phase and achieve a high ionic conductivity close to 10⁻³ S / cm. In one embodiment of the present invention, the garnet-type solid electrolyte is selected from LLZTO, LLZAO, LLZGO, and LALZNO. The aforementioned garnet-type solid electrolytes can be used individually or in combination. In the following embodiments of the present invention, LLZTO is used.

[0028] Garnet-type solid electrolytes readily react with carbon dioxide and water in the air to form lithium carbonate (Li₂CO₃) on their surface. Studies have found that lithium carbonate causes high interfacial resistance between the garnet-type solid electrolyte and lithium metal, leading to a decrease in ionic conductivity and consequently negatively impacting the performance of lithium-ion batteries. Therefore, one technical feature of this invention is that the garnet-type solid electrolyte does not contain lithium carbonate.

[0029] X-ray photoelectron spectroscopy (XPS), X-ray fluorescence spectrometry (XRF), and Raman spectroscopy are commonly used for elemental analysis of ceramic materials. Raman spectroscopy can detect the elemental composition of a sample down to a depth of approximately 0.1 micrometers and has a low analytical limit, down to 0.1 ppm. Therefore, in this paper, Raman spectroscopy was used to confirm that the garnet-type solid electrolyte does not contain lithium carbonate. A 532-nanometer laser source and a laser power of 10 milliwatts (mW) were used to analyze the surface of the garnet-type solid electrolyte, confirming that no lithium carbonate signal was present in the obtained Raman spectrum. Since lithium carbonate is formed by the reaction of carbon dioxide and water on the surface of the garnet-type solid electrolyte, the absence of lithium carbonate on the surface of the garnet-type solid electrolyte confirms that it does not contain lithium carbonate. Therefore, Raman spectroscopy confirms that the lithium carbonate content of the garnet-type solid electrolyte used in this invention is 0 or less than 0.1 ppm. In the appended embodiments, the Raman spectrometer used is a DXR Raman Microscope purchased from Thermo Fisher Scientific, USA.

[0030] A lithium carbonate-free garnet-type solid electrolyte can be obtained by acid washing of the garnet-type solid electrolyte. First, garnet-type solid electrolyte ceramic powder is added to an organic solvent, followed by a strong acid aqueous solution to obtain an initial mixed solution. The initial mixed solution is ultrasonically vibrated, and an appropriate amount of ultrapure water is added to prevent excessive strong acid from corroding the surface of the garnet-type solid electrolyte ceramic powder. The ultrasonically vibrated initial mixed solution is then centrifuged. The precipitate obtained after centrifugation is the lithium carbonate-free garnet-type solid electrolyte ceramic powder. Examples of the organic solvent include, but are not limited to, ethanol, isopropanol, and acetone. Each of the aforementioned organic solvents can be used alone or in combination. Examples of the strong acid aqueous solution include, but are not limited to, hydrochloric acid aqueous solution, sulfuric acid aqueous solution, and nitric acid aqueous solution. Each of the aforementioned strong acid aqueous solutions can be used alone or in combination. The concentration of the strong acid aqueous solution can be from 0.1 M to 2.0 M, for example 0.1 M, 0.2 M, 0.3 M, 0.4 M, 0.5 M, 0.6 M, 0.7 M, 0.8 M, 0.9 M, 1.0 M, 1.1 M, 1.2 M, 1.3 M, 1.4 M, 1.5 M, 1.6 M, 1.7 M, 1.8 M, 1.9 M, or 2.0 M, or within a range consisting of any two of the above values.

[0031] In one embodiment of the present invention, the content of garnet-type solid electrolyte is 60% by weight or more, more specifically 60% to 99% by weight, based on the total weight of garnet-type solid electrolyte and polymer filler. For example, based on the total weight of the garnet-type solid electrolyte and the polymer filler, the content of the garnet-type solid electrolyte can be 60 wt%, 61 wt%, 62 wt%, 63 wt%, 64 wt%, 65 wt%, 66 wt%, 67 wt%, 68 wt%, 69 wt%, 70 wt%, 71 wt%, 72 wt%, 73 wt%, 74 wt%, 75 wt%, 76 wt%, 77 wt%, 78 wt%, 79 wt%, 80 wt%, 81 wt%, 82 wt%, 83 wt%, 84 wt%, 85 wt%, 86 wt%, 87 wt%, 88 wt%, 89 wt%, 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, 97 wt%, 98 wt%, or 99 wt%, or a range consisting of any two of the above values. In order to obtain better mechanical properties, ionic conductivity, electrochemical stability and cycle stability of lithium-ion batteries, the content of garnet-type solid electrolyte is preferably 90% to 99% by weight, more preferably 95% to 99% by weight, for example 95% by weight, 95.5% by weight, 96% by weight, 96.5% by weight, 97% by weight, or 97.5% by weight, or within the range of any two of the above values.

[0032] 2. Polymer filler

[0033] The solid electrolyte of the present invention can improve the flexibility and mechanical strength of the solid electrolyte by adding polymer filler, thereby improving the processability of the solid electrolyte.

[0034] Examples of the polymer filler materials include, but are not limited to, PTFE, PEO, PAN, and PMMA. The aforementioned polymer filler materials can be used individually or in combination. In the examples attached below, PTFE filler is used.

[0035] The weight average molecular weight (Mw) of the polymer filler material can be in the range of 300,000 to 1,000,000, for example, it can be 300,000, 310,000, 320,000, 330,000, 340,000, 350,000, 360,000, 370,000, 380,000, 390,000, 400,000, 410,000, 420,000, 4 30,000, 440,000, 450,000, 460,000, 470,000, 480,000, 490,000, 500,000, 510,000, 520,000, 530,000, 540,000, 550,000, 560,000, 570,000, 580,000, 590,000, 600,000, 610,000, 620,000, 63 0,000, 640,000, 650,000, 660,000, 670,000, 680,000, 690,000, 700,000, 710,000, 720,000, 730,000, 740,000, 750,000, 760,000, 770,000, 780,000, 790,000, 800,000, 810,000, 820,000, 830 ,000, 840,000, 850,000, 860,000, 870,000, 880,000, 890,000, 900,000, 910,000, 920,000, 930,000, 940,000, 950,000, 960,000, 970,000, 980,000, 990,000, or 1,000,000, or a range consisting of any two of the above values.

[0036] In the solid electrolyte of the present invention, the content of polymer filler, based on the total weight of garnet-type solid electrolyte and polymer filler, can be from 0.1% by weight to less than 40% by weight, for example 0.1% by weight, 0.5% by weight, 1% by weight, 2% by weight, 3% by weight, 4% by weight, 5% by weight, 6% by weight, 7% by weight, 8% by weight, 9% by weight, 10% by weight, 11% by weight, 12% by weight, 13% by weight, 14% by weight, 15% by weight, 16% by weight, 17% by weight, 18% by weight, 19% by weight, 20% by weight, 21% by weight, 22% by weight, 23% by weight, 24% by weight, 25% by weight, 26% by weight, 27% by weight, 28% by weight, 29% by weight, 30% by weight, 31% by weight, 32% by weight, 33% by weight, 34% by weight, 35% by weight, 36% by weight, 37% by weight, 38% by weight, 39% by weight, or 40% by weight, or within the range of any two of the above values. In the solid electrolyte of the present invention, the preferred content of the polymer filler, based on the total weight of the garnet-type solid electrolyte and the polymer filler, is 1% to less than 10% by weight, more specifically 1% to less than 5% by weight, for example, 2.5% by weight. When the content of the polymer filler falls within the above-mentioned preferred range, the solid electrolyte of the present invention can maintain good ionic conductivity while possessing good flexibility and mechanical strength, and the interfacial contact impedance between the solid electrolyte and the electrodes of the lithium-ion battery can be reduced, thereby effectively improving the mechanical performance and cycle performance of the lithium-ion battery.

[0037] 1.3. Preparation of Solid Electrolytes

[0038] The preparation method of the solid electrolyte of the present invention is not particularly limited. In one embodiment of the present invention, it is prepared by uniformly mixing lithium carbonate-free garnet-type solid electrolyte ceramic powder with polymer filler and then pressing it. The size of the lithium carbonate-free garnet-type solid electrolyte ceramic powder can be adjusted as needed, preferably being lithium carbonate-free garnet-type solid electrolyte ceramic powder with an average particle size of less than 100 micrometers. For example, the average particle size of the lithium carbonate-free garnet-type solid electrolyte ceramic powder can be 95 micrometers, 90 micrometers, 85 micrometers, 80 micrometers, 75 micrometers, 70 micrometers, 65 micrometers, 60 micrometers, 55 micrometers, 50 micrometers, 45 micrometers, 40 micrometers, 35 micrometers, 30 micrometers, 25 micrometers, 20 micrometers, 15 micrometers, 10 micrometers, 5 micrometers, or 1 micrometer, or within a range consisting of any two of the above values.

[0039] 2. Lithium-ion batteries

[0040] The solid electrolyte of the present invention is particularly suitable for lithium-ion batteries. Therefore, the present invention also provides a lithium-ion battery comprising a positive electrode, a negative electrode, and the aforementioned solid electrolyte, wherein the solid electrolyte is disposed between the positive electrode and the negative electrode.

[0041] The negative electrode material of the lithium-ion battery of the present invention can be selected from lithium metal, lithium-indium alloy, lithium-aluminum alloy, and lithium-silicon alloy. The aforementioned negative electrode materials can be used individually or in any combination. In the embodiments attached below, the negative electrode material is lithium.

[0042] The positive electrode material of the lithium-ion battery of the present invention can be LFP, a monolithic lithium cathode material, a binary lithium cathode material, and a ternary lithium cathode material. Examples of monolithic lithium cathode materials include, but are not limited to, LCO, lithium nickel oxide, and lithium manganese oxide. Examples of binary lithium cathode materials include, but are not limited to, lithium nickel cobalt oxide, LNMO, and lithium manganese cobalt oxide. Examples of ternary lithium cathode materials include, but are not limited to, NCA and NCM. In one embodiment of the present invention, the positive electrode material is selected from the following group: LFP, LCO, NCA, and NCM. Specific examples of NCA include, but are not limited to, LiNi 0.8Co 0.15Al 0.05O 2, LiNi 0.8Co 0.18Al 0.02O 2, and LiNi 0.9Co 0.05Al 0.05O 2. Specific examples of NCM include, but are not limited to, LiNi 0.33Co 0.33Mn 0.33O 2 (NCM111), LiNi 0.5Co 0.2Mn 0.3O 2 (NCM523), LiNi 0.6Co 0.2Mn 0.2O 2 (NCM622), and LiNi 0.8Co 0.1Mn 0.1O 2 (NCM811). In the examples below, the cathode material is LFP.

[0043] In the lithium-ion battery of the present invention, lithium-containing ionic liquids may be present between the positive electrode and the solid electrolyte, and between the negative electrode and the solid electrolyte, to further improve the interfacial stability between the positive electrode and the solid electrolyte, and between the negative electrode and the solid electrolyte, and reduce the interfacial impedance. The lithium-containing ionic liquid can provide enhanced lithium-ion mobility, improve ionic conductivity, and thus improve the cycle performance of the battery. In one embodiment of the present invention, examples of lithium salts contained in the ionic liquid include, but are not limited to, LiTFSI, LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3. The aforementioned lithium salts can be used individually or in combination. Examples of organic solvents that can be used to form the ionic liquid include, but are not limited to, triethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, diethylene glycol diethyl ether, dipropylene glycol dimethyl ether, and propylene glycol dimethyl ether. The aforementioned organic solvents can be used individually or in combination. In the accompanying embodiments, the lithium salt-containing ionic liquid system contains a TEGDME solution of LiTFSI. Without theoretical limitations, it is believed that lithium salt-containing ionic liquids can form stable lithium-ion transfer interfaces and reduce interfacial impedance. Furthermore, lithium salt-containing ionic liquids help mitigate harmful side reactions and improve electrode performance, preventing the electrode from losing its function within a short period.

[0044] 3. Example

[0045] 3.1. Preparation and testing of solid electrolytes

[0046] [Preparation of LLZTO powder without lithium carbonate]

[0047] One gram of Li 6.4La 3Zr 1.4Ta 0.6O 12 (LLZTO) powder was prepared and its surface was analyzed using a Raman spectrometer (model: DXR Raman Microscope, purchased from Thermo Fisher Scientific, USA) with a 532 nm laser light source and a laser power of 10 mW. The results are shown in Figure 1 (indicated as "before acid washing"). As shown in Figure 1, the un-acid-washed LLZTO powder showed a lithium carbonate signal in the spectrum after Raman spectroscopy analysis, indicating that the LLZTO powder contains lithium carbonate.

[0048] The LLZTO powder was subjected to acid washing. First, 1 gram of LLZTO powder (i.e., LLZTO powder containing lithium carbonate) was added to a mixed solvent of 1 mL of ethanol and 3 mL of isopropanol and stirred until homogeneous to form a uniform dispersion. Next, the dispersion was mixed with 12 mL of 1 M hydrochloric acid to obtain an initial mixed solution. Then, the initial mixed solution was ultrasonically shaken for 60 seconds in an ultrasonic shaker, and then 10 mL of ultrapure water was added to dilute the hydrochloric acid. Next, the diluted mixed solution was placed in a centrifuge and centrifuged at 8,000 rpm for 10 minutes. The precipitate obtained after centrifugation was the lithium carbonate-free LLZTO powder.

[0049] Surface analysis of the acid-washed LLZTO powder was performed using a DXR Raman Microscope with a 532 nm laser source and a 10 mW laser power. The results are shown in Figure 1 (referred to as "after acid washing"). As shown in Figure 1, the acid-washed LLZTO powder did not show any lithium carbonate signal in the Raman spectrum after analysis, indicating that the acid-washed LLZTO powder does indeed not contain lithium carbonate.

[0050] [Example 1] Preparation of the solid electrolyte of the present invention

[0051] The solid electrolyte sheet of Example 1 was prepared as follows. In a glove box where both water and oxygen concentrations were less than 1 ppm, 1 gram (97.5% by weight) of the aforementioned acid-washed, lithium carbonate-free LLZTO powder and 0.026 grams (2.5% by weight) of PTFE powder were uniformly mixed in a mortar using a solvent-free method to obtain a mixture. The mixture was then placed in a mold and hot-pressed into a ceramic sheet using a CrushIR 15-ton Digital Press (model: 181-1100, purchased from PIKE Technologies) via uniaxial pressing. The hot-pressing conditions were as follows: hot-pressing temperature of 100°C, pressure of 200 MPa, and hot-pressing time of 10 minutes. The ceramic sheet was cut into circular sheets with a diameter of 12 mm and a thickness of 200 micrometers to obtain the solid electrolyte sheet of Example 1.

[0052] The aforementioned ceramic sheet was photographed using a camera, and the photograph is shown in Figure 2. A cross-sectional view of the aforementioned ceramic sheet was taken using a scanning electron microscope (model: JSM-6510, purchased from JEOL), and the image is shown in Figure 3.

[0053] [Comparative Example 1] Preparation of a solid electrolyte not of the present invention

[0054] The solid electrolyte sheet of Comparative Example 1 was prepared in the same manner as in Example 1, except that untreated lithium carbonate LLZTO powder was used instead of acid-treated LLZTO powder.

[0055] [Thermogravimetric Analysis Test of Solid Electrolytes]

[0056] The solid electrolyte sheets of Example 1 and Comparative Example 1 were analyzed using a thermogravimetric analyzer (model: STA 8122, purchased from Rigaku), and the results are shown in Figure 4. The test conditions of the thermogravimetric analyzer were as follows: argon gas was introduced at a rate of 10 mL / min to form an argon atmosphere, and the temperature was increased from room temperature to 800°C at a rate of 5°C / min.

[0057] As shown in Figure 4, the results show that after PTFE decomposes at 400°C, the garnet-type ceramic powder content of both Example 1 and Comparative Example 1 is greater than 90% by weight. However, as the temperature rises to 800°C, it can be observed that the weight loss of the solid electrolyte sheet of Comparative Example 1 (expressed as w / Li 2CO 3) is greater than that of the solid electrolyte sheet of Example 1 (expressed as w / o / Li 2CO 3). This is because lithium carbonate begins to decompose at 500 to 800°C.

[0058] [Preparation and Testing of Symmetrical Cells]

[0059] Symmetrical batteries were prepared using the solid electrolyte sheets of Example 1 and Comparative Example 1, respectively. The following materials were prepared in advance: a button cell casing, a lithium metal sheet with a diameter of 10 mm and a thickness of 0.025 mm as an electrode sheet, and an ionic liquid containing lithium salt with a concentration of 1 M. The 1 M ionic liquid system containing lithium salt was obtained by dissolving LiTFSI, which is used as a lithium salt, in TEGDME.

[0060] Symmetrical batteries were assembled in a glove box where both water and oxygen concentrations were less than 1 ppm. First, the base of a button cell was placed, followed by a lithium metal sheet (electrode sheet), and then 5 μL of a 1 M lithium salt-containing ionic liquid was added to the electrode sheet. Next, the solid electrolyte sheet of Example 1 or Comparative Example 1 was placed, and then 5 μL of a 1 M lithium salt-containing ionic liquid was added to the solid electrolyte sheet. Finally, the lithium metal sheet (electrode sheet) and the top cover of the button cell were placed in sequence to obtain a stack. The stack was pressed into a coin cell using a coin cell pressing machine at a pressure of 40 MPa to obtain the symmetric batteries of Example 1 and Comparative Example 1.

[0061] The impedance of the symmetrical cells of Example 1 and Comparative Example 1 was measured at room temperature using a modular potentiostat / galvanometer (Autolab PGSTAT302N). The AC frequency range for the measurement was set from 0.1 Hz to 10⁷ Hz, and the results are shown in Figure 5. As can be seen from Figure 5, the impedance of the symmetrical cell of Example 1 (expressed as w / Li₂CO₃) decreased significantly compared to that of the symmetrical cell of Comparative Example 1 (expressed as w / o / Li₂CO₃). This shows that the solid electrolyte without lithium carbonate can effectively reduce the interfacial impedance between the solid electrolyte and the electrode and has a stable interface, that is, the lithium ion distribution inside the symmetrical cell is uniform and the electrode interface is stable.

[0062] Furthermore, the symmetrical cells of Example 1 and Comparative Example 1 were cycled at a current density of 0.05 mAh / cm² (milliampere-hours per square centimeter) at room temperature, and the results are shown in Figure 6. As can be seen from Figure 6, the symmetrical cell of Example 1 (expressed as w / o / Li₂CO₃) maintained stable cycling for over 700 hours at room temperature, while the symmetrical cell of Comparative Example 1 (expressed as w / Li₂CO₃) only maintained stable cycling for approximately 100 hours. This demonstrates that a lithium carbonate-free solid electrolyte can effectively reduce the interfacial impedance between the solid electrolyte and the electrode, thereby improving the cycle life of the lithium-ion battery.

[0063] 3.2. Preparation and Testing of Lithium-ion Batteries

[0064] First, LFP (as the positive electrode active material), KS6 (as the carbon source), and poly(vinylidene fluoride) (PVDF) (as the binder) were placed in a grinding mortar in a weight ratio of 75:20:5 and ground for 20 minutes. Next, 2.0 ml of N-methyl-2-pyrrolidone (NMP) as a solvent was added to the grinding mortar and mixed thoroughly to obtain a slurry. Then, the slurry was coated onto an aluminum foil to form a thickness of approximately 150 micrometers, and the aluminum foil coated with the slurry was placed in a vacuum oven at 100°C and dried for 12 hours to remove the solvent. Next, the sheet formed from the dried slurry was removed from the aluminum foil and cut into circular positive electrode sheets with a diameter of 8 mm and a thickness of 100 micrometers.

[0065] In addition, a lithium metal sheet with a diameter of 8 mm and a thickness of 0.025 mm is prepared as a negative electrode.

[0066] Lithium-ion batteries were prepared using the solid electrolyte sheets of Example 1 and Comparative Example 1. The following materials were prepared in advance: a button cell casing and a 1 M lithium salt-containing ionic liquid, obtained by dissolving LiTFSI (as the lithium salt) in TEGDME. The lithium-ion batteries were assembled in a glove box where both water and oxygen concentrations were less than 1 ppm. First, the base of the button cell was placed, followed by the positive electrode, and then 5 μL of the 1 M lithium salt-containing ionic liquid was added to the positive electrode. Next, the solid electrolyte sheet of Example 1 or Comparative Example 1 was placed, and then 5 μL of the 1 M lithium salt-containing ionic liquid was added to the solid electrolyte sheet. Finally, the negative electrode and the top cover of the button cell were placed sequentially to obtain a stack. The stack was pressed using a button cell pressing machine at a pressure of 40 MPa to obtain the lithium-ion batteries of Example 1 and Comparative Example 1.

[0067] The impedance of the lithium-ion batteries of Example 1 and Comparative Example 1 was measured at room temperature using a modular potentiostat / galvanometer (Autolab PGSTAT302N). The AC frequency range for the measurement was set from 0.1 Hz to 10⁷ Hz, and the results are shown in Figure 7. As can be seen from Figure 7, the sum of the bulk resistance and interfacial resistance of the lithium-ion battery of Comparative Example 1 (expressed as w / Li₂CO₃) is as high as approximately 10,783 Ω, while the sum of the bulk resistance and interfacial resistance of the lithium-ion battery of Example 1 (expressed as w / o / Li₂CO₃) is only about 787 Ω. Furthermore, as shown in the magnified portion of the upper left of Figure 7, the bulk resistance of the lithium-ion battery of Comparative Example 1 is also greater than that of the lithium-ion battery of Example 1 in the high-frequency region. This demonstrates that a lithium carbonate-free solid electrolyte can effectively reduce the interfacial resistance between the solid electrolyte and the electrode.

[0068] Furthermore, the lithium-ion batteries of Example 1 and Comparative Example 1 were subjected to their first charge-discharge cycle at a charge-discharge rate of 0.1 C and at room temperature, and the results are shown in Figure 8. As can be seen from Figure 8, the first charge-discharge cycle of the lithium-ion battery of Comparative Example 1 (expressed as w / Li₂CO₃) could not be completed, while the lithium-ion battery of Example 1 (expressed as w / o / Li₂CO₃) could successfully complete its first charge-discharge cycle, and the first discharge capacity reached 123.25 mAh / g (milliampere-hours / gram). This shows that a lithium carbonate-free solid electrolyte can effectively reduce the interfacial impedance between the solid electrolyte and the electrode and has a stable interface, improving lithium-ion conduction and thus enhancing the electrochemical performance of the lithium-ion battery.

[0069] The above embodiments are merely illustrative of the principles and effects of the present invention and to illustrate the technical features of the present invention, and are not intended to limit the scope of protection of the present invention. Any changes or arrangements that can be easily made by those skilled in the art are within the scope of the present invention. Therefore, the scope of protection of the present invention is as set forth in the appended claims. [Simplified Explanation of the Diagram]

[0016] Figure 1 shows the Raman spectra of the garnet-type solid electrolyte ceramic powders of Example 1 and Comparative Example 1. Figure 2 shows a photograph of the solid electrolyte of Example 1. Figure 3 shows an electron microscope image of the solid electrolyte of Example 1. Figure 4 shows the thermogravimetric analysis (TGA) graphs of the solid electrolytes of Example 1 and Comparative Example 1. Figure 5 shows the impedance analysis test graphs of the symmetrical cells of Example 1 and Comparative Example 1. Figure 6 shows the voltage-time curves of the symmetrical cells of Example 1 and Comparative Example 1. Figure 7 shows the impedance analysis test graphs of the lithium-ion batteries of Example 1 and Comparative Example 1. Figure 8 shows the first charge-discharge curves of the lithium-ion batteries of Example 1 and Comparative Example 1.

Claims

1. A solid electrolyte for lithium-ion batteries, comprising a garnet-type solid electrolyte and a polymer filler, wherein the polymer filler is dispersed in the garnet-type solid electrolyte, and the garnet-type solid electrolyte is free of lithium carbonate, and wherein, based on the total weight of the garnet-type solid electrolyte and the polymer filler, the content of the garnet-type solid electrolyte is 60% by weight or more, and the content of the polymer filler is 1% by weight to less than 10% by weight.

2. The solid electrolyte as claimed in claim 1, wherein the garnet-type solid electrolyte is selected from the group consisting of: Li6.4La3Zr1.4Ta0.6O12 (LLZTO), Li6.4La3Zr2Al0.2O12 (LLZAO), Li6.4La3Zr2Ga0.2O12 (LLZGO), Li6.25Al0.20La3Zr1.85Nb0.15O12 (LALZNO), and combinations thereof.

3. The solid electrolyte as claimed in claim 1, wherein the garnet-type solid electrolyte is provided by garnet-type solid electrolyte ceramic powder that does not contain lithium carbonate.

4. The solid electrolyte as described in claim 3, wherein the average particle size of the lithium carbonate-free garnet-type solid electrolyte ceramic powder is less than 100 micrometers.

5. The solid electrolyte as described in any one of claims 1 to 4, wherein the polymer filler material is selected from the group consisting of polytetrafluoroethylene (PTFE), poly(ethylene oxide) (PEO), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), and combinations thereof.

6. A lithium-ion battery comprising a positive electrode, a negative electrode, and a solid electrolyte as described in any one of claims 1 to 5.

7. The lithium-ion battery as claimed in claim 6, wherein the material of the positive electrode is selected from the group consisting of lithium iron phosphate (LiFePO4, LFP), lithium cobalt oxide (LiCoO2, LCO), lithium nickel cobalt aluminum oxide (NCA), and lithium nickel cobalt manganese oxide (NCM).

8. The lithium-ion battery as claimed in claim 6, wherein the material of the negative electrode is selected from the group consisting of lithium metal, lithium indium alloy, lithium aluminum alloy, lithium silicon alloy, and combinations thereof.

9. The lithium-ion battery as claimed in claim 6, wherein a lithium salt-containing ionic liquid is present between the positive electrode and the solid electrolyte and between the negative electrode and the solid electrolyte.

10. The lithium-ion battery as claimed in claim 9, wherein the lithium salt is selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, and combinations thereof.

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