Lithium lanthanum titanium oxide solid electrolyte material and preparation method thereof

Abstract

The invention discloses a lithium lanthanum titanium oxide solid electrolyte material and a preparation method thereof, and belongs to the field of solid electrolyte materials. The lithium lanthanum titanium oxide solid electrolyte material is of a perovskite structure, and the chemical composition of the lithium lanthanum titanium oxide solid electrolyte material is Li3xLa2 / 3-xTiO3, wherein x is greater than or equal to 0.10 and less than or equal to 0.15; the relative density of a ceramic sample sintered at 900 DEG C is greater than 90%; and the total impedance of the ceramic is less than 95k omega. According to the invention, a new complexing agent (diethylenetriamine pentaacetic acid or a combination of ethylenediamine tetraacetic acid and diethylenetriamine pentaacetic acid) is adopted to replace a conventional complexing agent citric acid, the stability of the new complexing agent and metal ions is higher, the hydrolysis rate of metal cations is slowed down, the obtained powder is better in quality, and the service life of the powder is prolonged. After being sintered into the lithium lanthanum titanium oxide solid electrolyte material, the lithium lanthanum titanium oxide solid electrolyte material not only has higher density, but also reduces the grain impedance and grain boundary impedance of the solid electrolyte material, and improves the application potential of the lithium lanthanum titanium oxide solid electrolyte material in preparation of a solid lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the field of solid electrolyte materials, and in particular to a lithium lanthanum titanium oxide solid electrolyte material and a preparation method thereof. Background Art

[0002] Lithium lanthanum titanium oxide (LLTO) solid electrolyte material is an inorganic solid electrolyte based on lithium lanthanum titanium oxide, which has high ionic conductivity and good chemical stability, and is one of the important components of all-solid-state lithium batteries. At present, the methods for synthesizing perovskite structure lithium lanthanum titanium oxide (LLTO) inorganic solid electrolyte mainly include solid phase method and sol-gel method.

[0003] The solid phase reaction method is the most common synthesis method for solid ceramics. This method is to mix solid powders and sinter them at high temperature to obtain perovskite-type oxides. The solid phase sintering method has the disadvantages of high sintering temperature, long sintering time, coarse and uneven particles, easy agglomeration, easy mixing of impurities, etc., resulting in poor sample performance. In addition, the solid phase reaction method has high process temperature and long heat preservation time, which makes the Li element easy to volatilize and requires the addition of excessive Li source.

[0004] The sol-gel method is a method in which the raw materials of the corresponding elements are dissolved into a sol state, a gel is obtained after aging, and then a nanopowder is formed by heat treatment. Because its reaction is controllable and the prepared powder has uniform particle size, it is widely used in the synthesis of nanopowders. The advantages of this method are: the reaction is carried out in a solution, the uniformity is high, and the uniformity of the multi-component system can reach the molecular or atomic level; the sintering temperature is greatly reduced, and nanopowders can be obtained at a lower sintering temperature. Compared with the solid phase reaction method, the sol-gel method is easier to control the amount of material added and the powder synthesis process, the synthesis time is short, the synthesis temperature is low, and the prepared powder has better consistency and repeatability.

[0005] At present, the sol-gel method for synthesizing LLTO powder usually uses citric acid as its complexing agent / chelating agent. The complexing stability of this complexing agent with metal cations is poor, resulting in excessive hydrolysis of metal cations. The generated electrolyte particles are very easy to agglomerate or overgrow, and it is also easy to cause uneven composition or structure of the material, so the quality of the prepared powder is affected. Summary of the invention

[0006] The purpose of the present invention is to provide a lithium lanthanum titanium oxide solid electrolyte material with higher phase purity and powder quality; another purpose of the present invention is to provide a method for preparing a lithium lanthanum titanium oxide solid electrolyte material with higher phase purity and powder quality.

[0007] The present invention provides a lithium lanthanum titanium oxide solid electrolyte material, which is characterized by a perovskite structure and a chemical composition of: Li 3x La 2 / 3-xTiO3, wherein 0.10≤x≤0.15; the powder grain size is less than 180nm; the relative density of the ceramic sample sintered at 900°C is greater than 90%; the total impedance of the ceramic is less than 95kΩ.

[0008] The present invention also provides a method for preparing a lithium lanthanum titanium oxide solid electrolyte material, comprising the following steps:

[0009] S1: preparing a precursor solution: weighing a lithium source, a lanthanum source and a titanium source according to a molar ratio of Li:La:Ti=3x: 2 / 3-x:1; dissolving the weighed lithium source and lanthanum source in water to prepare a solution A; dissolving the weighed titanium source and a complexing agent in 60-85°C water to prepare a solution B; mixing the solution A and the solution B evenly, and adjusting the pH of the mixed solution to a weak alkaline state with a pH regulator; heating to 60-85°C, and stirring to make the precursor solution uniform and transparent; the complexing agent is diethylenetriaminepentaacetic acid or a combination of ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid;

[0010] S2: preparing a solid precursor: heating the precursor solution obtained in step S1 until the water evaporates completely to form a solid precursor;

[0011] S3: preparing LLTO electrolyte synthetic powder: heat treating the solid precursor obtained in step S1 to obtain LLTO electrolyte synthetic powder;

[0012] S4 powder granulation: granulating or tableting the LLTO electrolyte synthetic powder obtained in step S3;

[0013] S5 high temperature sintering: the pellets or sheets obtained in step S4 are subjected to high temperature sintering to obtain the lithium lanthanum titanium oxide solid electrolyte material as described above.

[0014] The principle of citric acid complexation reaction is based on the principle of coordination chemistry. Citric acid is 2-hydroxy-1,2,3-hexanetriol, and its structural formula is shown in the following formula (I):

[0015] (I).

[0016] Theoretically, it can ionize 3 protons to form 3 carboxylate -COO- coordination functional groups, plus 1 hydroxyl group, which is a tetradentate ligand. Each coordination site can provide two electron pairs to the metal ion, combining with the empty orbital of the metal ion in the metal salt to form a coordination compound.

[0017] Ethylenediaminetetraacetic acid has the same number of ligands as citric acid, and its structural formula is shown in the following formula (II), which is a tetradentate ligand with 4 carboxylate-COO- coordination functional groups, and can be replaced by a complexing agent with the same number of ligands. In addition, there are complexing agents with more dentate ligands, such as diethylenetriaminepentaacetic acid, with the structural formula shown in the following formula (III), which is a pentadentate ligand (5 carboxylate-COO- coordination functional groups). Ligands with more dentates are beneficial to the chelation effect with metal ions, which can improve the stability of the complex and thus control the hydrolysis rate of metal ions.

[0018] (II); (III).

[0019] Furthermore, in step S1, the molar ratio of the complexing agent to the total metal ions of the lithium source, lanthanum source and titanium source is 0.4-2:1.

[0020] The lithium source is a water-soluble lithium salt, such as lithium nitrate, lithium sulfate, lithium carbonate, etc.; the lanthanum source is a water-soluble lanthanum salt, such as lanthanum nitrate, lanthanum sulfate, lanthanum chloride, lanthanum acetate, etc.; the titanium source is tetrabutyl titanate.

[0021] The pH regulator is aqueous ammonia.

[0022] Furthermore, in step S1, the complexing agent is a combination of ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid.

[0023] The present invention adopts a composition of ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid as a composite complexing agent. By adjusting the ratio of the two, the purpose of regulating the hydrolysis rate of metal cations by adjusting the complexing stability can be achieved, thereby obtaining a highly uniform and high-performance electrolyte powder.

[0024] Furthermore, in step S1, the molar ratio of ethylenediaminetetraacetic acid to diethylenetriaminepentaacetic acid in the complexing agent is 4:1-1:4.

[0025] Furthermore, in step S2, the water in the precursor solution is removed at 200-400°C for 1-4h. During the heating process, as the water evaporates, the precursor solution first turns into a precursor gel, and then the gel volume expands and carbonizes, eventually forming a fluffy and porous solid precursor.

[0026] Furthermore, in step S3, the heat treatment process is to add urea to the solid precursor and grind it evenly, and heat it at 700-850° C. for 2-5 hours.

[0027] Furthermore, in the step S3, the mass ratio of the solid precursor to the added urea is 0.5-2:1.

[0028] Adding urea to the mixture and then performing heat treatment can significantly reduce the heat treatment temperature, that is, white LLTO electrolyte synthetic powder can be obtained at a temperature below 900°C. Low temperature is beneficial for synthetic powder, which can reduce the volatility of lithium elements and eliminate the need to add excessive lithium sources, thereby saving energy and reducing production costs.

[0029] Furthermore, in step S4, the powder obtained in step S3 is granulated or tableted once, then ground evenly, and a PVA aqueous solution is added as a granulation binder to perform a second granulation or tableting.

[0030] Furthermore, in step S4, the concentration of the PVA aqueous solution is 5-7%; the ratio of the added mass of the PVA aqueous solution to the mass of the powder is 0.5-13:100; and the mixture is allowed to stand for 1-4 minutes under a pressure of 20-35 MPa for cold pressing.

[0031] Furthermore, the pellets or sheets obtained in step S4 are subjected to high temperature sintering by heat treatment at 900-1200° C. for 3-7 hours at a heating rate of 3-8° C. / min.

[0032] The present invention adopts a new complexing agent (diethylenetriaminepentaacetic acid or a combination of ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid) to replace the conventional complexing agent citric acid. The new complexing agent has higher stability with metal ions, slows down the hydrolysis rate of metal cations, and makes the obtained powder quality better. After sintering into a lithium lanthanum titanium oxide solid electrolyte material, it not only has a higher density, but also reduces the grain impedance and grain boundary impedance of the solid electrolyte material, thereby improving the potential of lithium lanthanum titanium oxide solid electrolyte for use in the preparation of solid-state lithium ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the XRD pattern of the lithium lanthanum titanium oxide solid electrolyte synthesized in Example 1 of the present invention;

[0034] Figure 2 This is a scanning electron microscope image of the LLTO electrolyte synthetic powder synthesized in Example 1 of the present invention;

[0035] Figure 3 is a scanning electron microscope image of the lithium lanthanum titanium oxide solid electrolyte synthesized in Example 1 of the present invention;

[0036] Figure 4 is the electrochemical impedance spectrum of the lithium lanthanum titanium oxide solid electrolyte synthesized in Example 1 of the present invention;

[0037] Figure 5 This is the electrochemical impedance spectrum of the lithium lanthanum titanium oxide solid electrolyte synthesized by citric acid in the prior art. DETAILED DESCRIPTION

[0038] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0039] Example 1

[0040] Preparation of lithium lanthanum titanium oxide solid electrolyte material:

[0041] S1: Prepare the precursor solution: The complexing agent is ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid in a molar ratio of 0.4:0.6, and the ratio (molar ratio) of the complexing agent to the total metal ions of the reactants is 0.6:1; weigh 0.68g of lithium nitrate and 2.35g of lanthanum nitrate and place them in a beaker to prepare solution A, and add 30mL of deionized water. At the same time, weigh 10.21g of tetrabutyl titanate, 3.98g of ethylenediaminetetraacetic acid and 8.03g of diethylenetriaminepentaacetic acid and place them in a beaker, add 100mL of deionized water, and stir in a water bath at 70°C to obtain a clear and transparent solution B. Pour the solution A into the solution B, and add an appropriate amount of deionized water to prepare a 200ml mixed solution and stir in a water bath at 70°C; mix it thoroughly and evenly, adjust the pH value to 8 with ammonia water, and continue stirring for 3h to obtain a precursor solution.

[0042] S2 Preparation of solid precursor: Place the precursor solution in a muffle furnace and heat it at 250° C. for 2 h to obtain a solid precursor.

[0043] S3 Preparation of LLTO electrolyte synthetic powder: Urea is added to the solid precursor for mixing, and the mass ratio of the solid precursor to urea is 1:1. After it is evenly ground, it is placed in a crucible and sent into a muffle furnace for heat treatment, and sintered at 750 ° C for 3 h to obtain LLTO electrolyte synthetic powder.

[0044] S4 powder granulation: The synthetic powder is put into an agate mortar and ground evenly, then the powder is pre-pressed once for granulation, the pre-pressed powder is ground again, and then a 5% by mass PVA aqueous solution is used as a granulation binder for secondary granulation, and the addition amount is 0.7% of the powder mass. After the PVA aqueous solution and the synthetic powder are fully ground, they are placed in a pressing mold, and at a molding pressure of 30 MPa, they are allowed to stand for 2 min and pressed into a ceramic disc body with a diameter of 19 mm and a thickness of 2 mm.

[0045] S5 high temperature sintering: The obtained ceramic disc body is placed in the crucible mother powder for sintering. In order to avoid the LLTO electrolyte material from contacting the crucible itself at high temperature and reacting or sintering together, and to ensure that the ceramic disc body has the same sintering atmosphere during the heat treatment process, a layer of mother powder is spread under the crucible (the mother powder is the ceramic powder directly sintered from the obtained LLTO electrolyte synthetic powder). Finally, the obtained solid electrolyte sheet is placed in the crucible for burial firing. The burial firing temperature is 900 ℃, the heating rate is 4 ℃ / min, the insulation time is 6 h, and it is naturally cooled to room temperature in the furnace to obtain lithium lanthanum titanium oxide solid electrolyte material.

[0046] The LLTO electrolyte synthetic powder prepared in step S3 and the lithium lanthanum titanium oxide solid electrolyte material prepared in step S4 were observed using a scanning electron microscope. Figure 2-3 As shown, the LLTO electrolyte synthetic powder is a uniform, nearly spherical nanopowder. The LLTO ceramic sintered from the powder has a dense structure, uniform grain growth, and a grain size of ~100nm.

[0047] Example 2

[0048] The difference between this embodiment and embodiment 1 is that the added complex is diethylenetriaminepentaacetic acid, and the added mass is 13.38 g.

[0049] Comparative Example 1

[0050] The performance data of citric acid as the complexing agent, the specific preparation method is detailed in Low temperature synthesis of Li 0.33 La 0.55 TiO3 solid electrolyte with Al 3+ doping by a modified Pechini method, 30 January 2022 Volume 28, pages 1739-1751, (2022).

[0051] Comparative Example 2

[0052] The difference between this embodiment and embodiment 1 is that the added complex is ethylenediaminetetraacetic acid, and the added mass is 9.94 g.

[0053] Performance Test:

[0054] The lithium lanthanum titanium oxide solid electrolyte material prepared in Example 1 was subjected to XRD test. The test results are as follows: Figure 1 As shown in the figure, it can be seen that the lanthanum titanium oxide solid electrolyte material prepared in Example 1 is a single perovskite structure, and no obvious impurity phase is found.

[0055] The lithium lanthanum titanium oxide solid electrolyte materials prepared in Examples 1-2 and Comparative Examples were subjected to SEM scanning, and the particle size distribution was analyzed using Image-Pro Plus 6.0 software. The ceramic density was tested using the Archimedean method, and the ceramic impedance was analyzed using the electrochemical impedance spectroscopy test. The test conditions were: in an open circuit state, a sinusoidal voltage with an amplitude of 0.015 V was used as the disturbance signal, and the frequency range was 10 5 ~10 -2 Hz. The test results are shown in Table 1 below.

[0056] Table 1 Performance test results of examples and comparative examples

[0057] Specific examples Complexing agent type Synthetic powder grain size nm Relative density of ceramic samples sintered at 900℃% Total resistance of ceramics Example 1 EDTA and DTPA Approx. 100 94.8 82 Example 2 DTPA Approx. 170 90.7 92 Comparative Example 1 Citric Acid Approx. 380 81.5 106 Comparative Example 2 EDTA Approx. 310 82.9 101

[0058] like Figure 4-5 As shown, the impedance of the LLTO solid electrolyte synthesized at 900 degrees Celsius in Example 1 is about 82 kΩ, which is significantly better than the impedance of 106 kΩ of the LLTO solid electrolyte at 950°C in Comparative Example 1. As can be seen from Table 1, the relative density of the ceramic samples sintered at 900°C and the total impedance of the ceramics in Examples 1-2 are better than those in Comparative Examples 1-2. At the same time, compared with Example 2 and Comparative Examples 1-2 using a single complexing agent or a traditional citric acid complexing agent, the use of a composite complexing agent in Example 1 is more conducive to controlling the reaction process and the regulation of the powder particle size and uniformity, thereby synthesizing a LLTO solid electrolyte with better performance.

[0059] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A lithium lanthanum titanium oxide solid electrolyte material, characterized in that: It has a perovskite structure and a chemical composition of: Li 3x La 2 / 3- x TiO3, wherein 0.10≤x≤0.15; the relative density of the ceramic sample sintered at 900°C is greater than 90%; the total impedance of the ceramic is less than 95kΩ.

2. A method for preparing a lithium lanthanum titanium oxide solid electrolyte material, characterized in that: The following steps are involved: S1: preparing a precursor solution: weighing a lithium source, a lanthanum source and a titanium source according to a molar ratio of Li:La:Ti=3x: 2 / 3-x:1; dissolving the weighed lithium source and lanthanum source in water to prepare a solution A; dissolving the weighed titanium source and a complexing agent in 60-85°C water to prepare a solution B; mixing the solution A and the solution B evenly, and adjusting the pH of the mixed solution to a weak alkaline state with a pH regulator; heating to 60-85°C, and stirring to make the precursor solution uniform and transparent; the complexing agent is diethylenetriaminepentaacetic acid or a combination of ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid; S2: preparing a solid precursor: heating the precursor solution obtained in step S1 until the water evaporates completely to form a solid precursor; S3: preparing LLTO electrolyte synthetic powder: heat treating the solid precursor obtained in step S1 to obtain LLTO electrolyte synthetic powder; S4 powder granulation: granulating or tableting the LLTO electrolyte synthetic powder obtained in step S3; S5 high temperature sintering: sintering the pellets or sheets obtained in step S4 at high temperature to obtain the lithium lanthanum titanium oxide solid electrolyte material as claimed in claim 1.

3. The method for preparing a lithium lanthanum titanium oxide solid electrolyte material according to claim 1, characterized in that: In the step S1, the molar ratio of the complexing agent to the total metal ions of the lithium source, the lanthanum source and the titanium source is 0.4-2:

1.

4. The method for preparing a lithium lanthanum titanium oxide solid electrolyte material according to claim 1, characterized in that: In the step S1, the complexing agent is a combination of ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid.

5. The method for preparing a lithium lanthanum titanium oxide solid electrolyte material according to claim 4, characterized in that: In the step S1, the molar ratio of ethylenediaminetetraacetic acid to diethylenetriaminepentaacetic acid in the complexing agent is 4:1-1:

4.

6. The method for preparing a lithium lanthanum titanium oxide solid electrolyte material according to claim 1, characterized in that: In step S3, the heat treatment process is to add urea to the solid precursor and grind it evenly, and heat it at 700-850° C. for 2-5 hours.

7. The method for preparing a lithium lanthanum titanium oxide solid electrolyte material according to claim 6, characterized in that: In the step S3, the mass ratio of the solid precursor to the added urea is 0.5-2:

1.

8. The method for preparing a lithium lanthanum titanium oxide solid electrolyte material according to claim 1, characterized in that: In the step S4, the powder obtained in the step S3 is granulated or tableted once, then ground evenly, and a PVA aqueous solution is added as a granulation binder to perform a second granulation or tableting.

9. The method for preparing a lithium lanthanum titanium oxide solid electrolyte material according to claim 8, characterized in that: In the step S4, the concentration of the PVA aqueous solution is 5-7%; the ratio of the added mass of the PVA aqueous solution to the mass of the powder is 0.5-13:100; and the mixture is allowed to stand for 1-4 minutes under a pressure of 20-35 MPa for cold pressing.

10. The method for preparing a lithium lanthanum titanium oxide solid electrolyte material according to claim 1, characterized in that: In the step S5, the pellets or sheets obtained in the step S4 are subjected to high temperature sintering, which is a heat treatment at 900-1200° C. for 3-7 hours, with a heating rate of 3-8° C. / min.

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