Ternary carbonate molten salt electrolyte with low melting point and high ionic conductivity as well as preparation and application of ternary carbonate molten salt electrolyte

Abstract

The invention provides a ternary carbonate molten salt electrolyte with low melting point and high ionic conductivity, and preparation and application thereof, and relates to the field of high-temperature electrochemical materials. The electrolyte is prepared by using lithium carbonate, sodium carbonate and potassium carbonate as raw materials through the steps of eutectic proportion optimization, vacuum drying, mixing and melting in an inert atmosphere and the like, the melting point of the obtained molten salt is not higher than 396 DEG C, and the ionic conductivity at 700 DEG C can reach 1.50 S / cm or above. The method is also combined with molecular dynamics simulation to analyze the microstructure and migration mechanism, and the interface stability and conductivity are improved. The electrolyte provided by the invention has the characteristics of controllable cost, uniform structure and suitability for high-temperature fuel cells, solves the problems of high melting point, low conductivity, poor electrode interface stability and the like of the existing molten salt system, and has good industrial application value.

Description

Technical Field

[0001] The present invention relates to the field of high-temperature electrochemical materials, and particularly to a ternary carbonate molten salt electrolyte with low melting point and high ionic conductivity, specifically including its preparation method and application in molten carbonate fuel cells (MCFCs), belonging to the technical direction of the design and preparation of key electrolyte materials for molten carbonate fuel cells. Background Art

[0002] With the global energy structure transforming towards low-carbon, fuel cells, as new energy conversion devices with high efficiency and low emissions, are receiving increasing attention. Among them, molten carbonate fuel cells (MCFCs), with the characteristics of directly using various hydrocarbon fuels, having high electrochemical reaction efficiency, and being suitable for combined heat and power systems, have become an important development direction of high-temperature fuel cell technology.

[0003] The typical operating temperature range of MCFCs is 600°C to 700°C, and a molten salt system capable of conducting CO 3 2- ions at high temperatures is required as the electrolyte. Currently, the mainstream electrolytes mostly adopt the Li 2 CO 3 -K 2 CO 3 binary eutectic system, which has high ionic conductivity and a relatively mature engineering application foundation. Its typical molar ratio is 62:38, and the corresponding melting point is about 500°C.

[0004] However, with the development of MCFCs towards higher power density, longer lifespan, and lower cost, the traditional electrolyte system faces several key challenges, mainly including: 1. High melting point: The melting points of traditional binary systems are generally above 500°C, resulting in a large overall heat load of the system, increasing the difficulty of battery thermal management and the material aging rate, which is not conducive to long-term stable operation; 2. Limited ion migration performance: In some intermediate temperature operating ranges, the CO 3 2- migration ability of the existing molten salt system is insufficient, restricting the further improvement of the power density of fuel cells; 3. Unstable cost: As the main component, the raw material cost of Li 2 CO 3 is significantly affected by the fluctuations in the lithium resource market, making it difficult to control the overall cost of the electrolyte system; 4. Weak interface research: Existing research mostly focuses on the physical property parameters of the molten salt system itself, lacking a systematic analysis of the interface structure and ion transport path between it and the electrode, which poses a potential obstacle to improving the overall battery efficiency.

[0005] To solve the above problems, researchers have tried to introduce Na 2 CO 3 to construct a ternary eutectic molten salt system of Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 By regulating the proportion of the three carbonates, the synergistic optimization of the melting point reduction and ion migration performance is achieved. Preliminary studies have shown that the ternary molten salt system can further lower the melting point while maintaining structural stability, enhancing the conductivity of the material.

[0006] However, the current publicly available research on ternary molten salts still has the following deficiencies: There is a lack of systematic exploration of the eutectic ratio, especially in the evaluation of the structural stability and conductivity in different thermodynamic regions; The control of the melting environment and drying process is not strict, resulting in low purity and large performance fluctuations of the obtained samples; There is a lack of theoretical support for the microscopic mechanisms of the electrolyte / electrode interface structure, migration channels, and ion activation mechanisms, limiting the depth of material optimization design.

[0007] Therefore, there is an urgent need to develop a ternary carbonate molten salt electrolyte system with a lower melting point, higher ionic conductivity, excellent thermal stability, and a clear interfacial transport mechanism to meet the multiple performance requirements of the electrolyte for the operating temperature range, conductivity efficiency, and system compatibility of the new generation of molten carbonate fuel cells. Summary of the Invention

[0008] To solve the problems of high melting point, limited ionic conductivity, poor interfacial stability, and difficult preparation environment control of existing molten carbonate electrolytes, the present invention provides a ternary carbonate molten salt electrolyte with a low melting point and high ionic conductivity, as well as its preparation and application. By optimizing the eutectic ratio of Li 2 CO 3 , Na 2 CO 3 , and K 2 CO 3 , strictly controlling the drying and melting processes, and combining molecular dynamics simulation methods, while reducing the melting point to below 396 °C, the ionic conductivity in the high-temperature range (700 °C) is increased to above 1.50 S / cm, significantly enhancing the thermal stability and interfacial conduction ability of the material, providing an efficient, stable, and cost-controlled new electrolyte solution for the molten carbonate fuel cell system.

[0009] In one possible implementation, a method for preparing a ternary carbonate molten salt electrolyte with a low melting point and high ionic conductivity is provided, including the following steps: S1. Calculate the eutectic molar ratio of lithium carbonate (Li 2 CO 3 ), sodium carbonate (Na 2 CO 3 ), and potassium carbonate (K 2 CO 3 ), where the molar percentage of Li 2 CO 3 is 55% - 65%, the molar percentage of Na 2 CO 3 is 20% - 25%, and the molar percentage of K 2 CO 3 is 15% - 20%; S2. Weigh the raw materials according to the calculated ratio. After drying Li 2 CO 3 , Na 2 CO 3 , and K 2 CO 3 at 130 °C and 10 -3 Pa for 24 hours, mix and grind them in an inert atmosphere glove box until the particle size ≤ 5 μm, and then melt them at 620 °C - 630 °C under static constant temperature conditions for 8 - 10 hours (without stirring). After forming a homogeneous melt, cool it; S3. Establish a molecular dynamics model of the molten salt electrolyte and electrode interface, and simulate and analyze the ion migration path.

[0010] In a possible implementation, the eutectic molar ratio of the Li 2 CO 3 , Na 2 CO 3 , and K 2 CO 3 is 59.5 ± 1:22.5 ± 1:18 ± 1.

[0011] In a possible implementation, the melting process is carried out in a sealed container continuously filled with 99.999% high-purity argon.

[0012] In a possible implementation, the water and oxygen content in the inert atmosphere glove box are both ≤ 0.1 ppm.

[0013] In a possible implementation, a ternary carbonate molten salt electrolyte with a low melting point and high ionic conductivity is provided, which is prepared by the above method and its composition is: Li 2 CO 3 55% - 65%, Na 2 CO 3 20% - 25%, K 2CO 3 15% to 20%, and satisfying: melting point ≤ 396 °C; ionic conductivity at 700 °C ≥ 1.50 S / cm.

[0014] In a possible implementation, the Li 2 CO 3 , Na 2 CO 3 and K 2 CO 3 have a molar ratio of 59.5 ± 1: 22.5 ± 1: 18 ± 1.

[0015] In a possible implementation, a molten carbonate fuel cell is provided, including: a porous nickel anode; a lithiated nickel oxide cathode; the above ternary carbonate molten salt electrolyte layer; the operating temperature of the fuel cell is 580 °C to 680 °C.

[0016] In a possible implementation, the conductivity decay of the electrolyte layer after continuous operation at 650 °C for 500 hours is ≤ 5%.

[0017] Based on the above technical solutions, the preparation method of the ternary carbonate molten salt electrolyte with low melting point and high ionic conductivity of the present invention comprehensively adopts thermodynamic optimization design, fine raw material treatment and controlled melting process, and effectively constructs a new molten salt material with uniform structure, stable components and excellent conductivity. By introducing Na 2 CO 3 and K 2 CO 3 to adjust the Li 2 CO 3 content, the overall melting point is reduced to below 396 °C, and at the same time, an ionic conductivity above 1.50 S / cm is achieved at 700 °C, taking into account both the low-temperature startup performance and high-temperature operation efficiency of the electrolyte.

[0018] The process route adopted by the present invention performs raw material transfer and grinding operations in an inert atmosphere glove box, significantly reducing the interference risk of water vapor and oxygen, and ensuring the electrolyte purity and structural consistency. At the same time, the strategy of rapid cooling to form a glassy structure helps to generate continuous and low-energy-barrier ion migration channels, providing structural support for improving the electrolyte performance.

[0019] In addition, by combining molecular dynamics simulation means to perform theoretical modeling and verification on ion distribution and migration behavior, the internal mechanism of denser ion structure and higher diffusion rate at high Li + content is clarified, providing an important reference basis for subsequent electrolyte material design.

[0020] Experiments prove that the molten salt electrolyte of the present invention can stably operate at high temperatures of 600 °C to 700 °C, compared with traditional Li2 CO 3 -K 2 CO 3 The system can appropriately reduce the working temperature and ensure high conductivity, and the attenuation of the conductivity after 100 thermal cycles is controlled within 5%. Generally speaking, the electrolyte system has rich material sources, controllable preparation processes, excellent performance, good industrial feasibility and application promotion prospects, providing key material support for the development of a new generation of high-efficiency MCFC systems. Description of the Drawings

[0021] Figure 1 is the AC impedance spectrum of the Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 ternary molten salt (59.5±1:22.5±1:18±1 mol%) prepared in Example 1 at 700 °C, with the test frequency range of 0.01 Hz–1 MHz; Figure 2 is the impedance spectrum of the comparative Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 molten salt (43.5±1:31.5±1:25±1 mol%) under the same conditions; Figure 3 is the radial distribution function (RDF) curve of Li + -CO 3 2- obtained by molecular dynamics simulation, with the horizontal axis being the atomic spacing (A) and the vertical axis being the probability density. Detailed Embodiments

[0022] To make the technical solutions and advantages of the present invention clearer, the present invention will be described in detail below with reference to the accompanying drawings. Each drawing is used to further explain the technical features and experimental verification results of the present invention, where: Figure 1 shows the AC impedance spectrum of the ternary molten salt of the preferred ratio Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (59.5±1:22.5±1:18±1 mol%) at 700 °C, and its low interfacial resistance characteristics are intuitively reflected by the radius of the capacitive reactance arc in the Nyquist plot; Figure 2To compare the proportion of Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (43.5±1: 31.5±1: 25±1 mol%) molten salt impedance spectra, and Figure 1 comparison proves the significant advantage of the proportion of the present invention in reducing the charge transfer resistance; Figure 3 Through the radial distribution function (RDF) curve of molecular dynamics simulation, the coordination structure difference between Li + and carbonate ions is revealed, providing a microscopic mechanism explanation for high ionic conductivity.

[0023] Example 1: Preparation process of ternary carbonate molten salt electrolyte In this example, high-purity lithium carbonate (Li 2 CO 3 , ≥99.99%), sodium carbonate (Na 2 CO 3 , ≥99.99%) and potassium carbonate (K 2 CO 3 , ≥99.95%) are used as raw materials. After each raw material is vacuum dried at 130 °C and 10 -3 Pa for 24 hours, it is transferred to an inert atmosphere glove box with a water and oxygen content ≤0.1 ppm for storage and operation.

[0024] According to the molar ratio of Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (59.5±1: 22.5±1: 18±1), the raw materials are accurately weighed and ground and mixed in the glove box for 30 minutes, and the particle size is controlled to be ≤5 μm. The mixed powder is loaded into a 99.6% alumina crucible and placed in a closed tube furnace filled with 99.999% high-purity argon. It is heated to 625±5 °C at a rate of 5 °C / min and kept molten for 10 hours. After cooling, a pale yellow glassy solid electrolyte is obtained.

[0025] As a comparative experiment, another sample of Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (43.5±1: 31.5±1: 25±1 mol%) is prepared with the same process conditions for subsequent performance evaluation.

[0026] Example 2: High-temperature Conductivity Performance Test of Molten Salt Electrolyte To systematically evaluate the high-temperature conductivity performance of the ternary carbonate molten salt electrolyte of the present invention, electrochemical impedance spectroscopy (EIS) tests were carried out on it at different temperatures.

[0027] 1. Sample Preparation and Pretreatment The electrolyte sample with the preferred ratio prepared in Example 1 was crushed and passed through a 400-mesh sieve to ensure uniform particle size. Weigh 1.000 ± 0.001 g of the sample and put it into a corundum crucible (inner diameter 10 mm) to prevent chemical reactions between the electrolyte and the container.

[0028] The sample pretreatment process includes: Placed in a vacuum drying oven and dried at 110°C for 6 hours to remove adsorbed water; After cooling to room temperature, it was immediately encapsulated and transferred to an argon glove box (O 2 and H 2 O < 0.1 ppm) for testing.

[0029] 2. Construction of Conductivity Test Platform A CHI660E electrochemical workstation was used for conductivity testing. The experiment adopted a symmetric three-electrode system (Pt | molten salt | Pt), and its structure is as follows: Working electrode and counter electrode: Double-sided polished platinum sheet (size 10 × 10 mm), pre-polished with 0.05 μm alumina slurry and ultrasonically cleaned with deionized water 3 times; Reference electrode: Ag / AgCl electrode was used to calibrate the system; Electrode fixation: The Pt electrode was clamped by a ceramic fixture, and the distance was maintained at 5.00 ± 0.01 mm to ensure good contact and uniform heating; Calibration of the cell constant (Ccell): Calibrated with a 0.1 mol / L KCl standard solution (25°C), the test was repeated 3 times, and the average value Ccell = 0.57 ± 0.02 cm -1 .

[0030] 3. Heating and Temperature Control The test was carried out in a vertical resistance furnace, and the set temperature points were 600°C, 650°C and 700°C. The furnace body was linked with a PID control module through a thermocouple to achieve a temperature control accuracy of ±0.5°C. At each temperature point, the temperature was kept constant for more than 45 minutes to ensure that the molten salt was completely melted and reached the thermodynamic equilibrium state.

[0031] Before and after the test, the thermocouple and the furnace body were calibrated at the standard temperature point to eliminate errors caused by temperature.

[0032] 4. Impedance Spectrum Acquisition and Fitting Method The AC impedance test parameters are set as follows: Excitation voltage: 10 mV; Scanning frequency range: 0.01 Hz to 1 MHz; Test time: Each point lasts about 8 minutes, and the test is repeated 3 times at each temperature point and the average value is taken.

[0033] The impedance data is analyzed by fitting through ZsimpWin software, and the R(CR)(QR) equivalent circuit model is selected to extract the bulk resistance Rb. The conductivity σ is calculated by the following formula: σ = (Ccell / Rb).

[0034] The fitting result requires the residual (chi-square) to be less than 10 -4 , ensuring the credibility of the analysis. The total experimental error consists of three parts: electrode spacing deviation, temperature control fluctuation, and fitting error, and is controlled within ±3%.

[0035] 5. Analysis of Test Results As Figure 1 shown, the Nyquist plot of the electrolyte with the preferred ratio Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (59.5±1:22.5±1:18±1 mol%) at 700 °C is in the standard single capacitive reactance arc form, indicating that the ion migration resistance is mainly concentrated in the bulk migration process, and the contribution of the electrode interface resistance can be ignored. The corresponding bulk resistance Rb is 0.38 Ω·cm 2 . The comparison group ratio Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (43.5±1:31.5±1:25±1 mol%) shows a larger impedance arc (as Figure 2 shown), and its Rb reaches 0.42 Ω·cm 2 , indicating that the ion migration channel is restricted.

[0036] The specific conductivity data is as follows:

[0037] The results show that the molten salt with the preferred ratio has higher ionic conductivity in the entire high-temperature range, especially showing significant advantages at 650 °C and above, and is suitable for medium- and high-temperature electrochemical devices.

[0038] This test verified the stable conductivity and good electrode contact characteristics of the electrolyte material of the present invention under high-temperature conditions. Compared with the control group, the optimized Li 2 CO 3 content can construct a more continuous and uniform ion channel, reduce the migration energy barrier, and meet the core requirements of high-performance molten carbonate fuel cells (MCFCs) for materials with high ionic conductivity.

[0039] Example 3: Molecular dynamics simulation analysis of molten salt electrolyte and interface To simulate the ion behavior under the service conditions of real carbonate molten salt electrolyte, a Li + -Na + -K + -CO 3 2- / electrode interface transfer model containing 6000 ions was constructed. LAMMPS simulation was used, and the Born-Mayer-Coulmb potential function and Lennard-Jones potential function were adopted. The cutoff radius was 20 Å, and it was run at 875 K for 10 ns. In the molecular structure analysis of the two systems of the preferred ratio (59.5±1:22.5±1:18±1 mol%) and the comparative ratio (43.5±1:31.5±1:25±1 mol%), the RDF (radial distribution function) results showed that the average coordination distance between Li + and CO 3 2- in the preferred ratio was shorter (2.05 Å), significantly better than that of the comparative ratio (2.12 Å), and the RDF peak intensity of Li + and CO 3 2- in the preferred ratio was lower. This structural characterization indicates that in the preferred system, the interaction between Li + and CO 3 2- is weaker and it is easier to migrate in the electrolyte.

[0040] Example 3 (continued): Correlation analysis between simulation results and conductivity data To further clarify the influence of different ratios on the conductivity of the ternary carbonate molten salt system from a theoretical level, in this example, the structural parameters obtained from molecular dynamics simulation were systematically compared with the measured high-temperature conductivity results in Example 2, so as to establish the corresponding relationship between the microscopic structure and the macroscopic properties.

[0041] The Li + diffusion coefficient further extracted through the Einstein diffusion formula showed that the Li⁺ diffusion coefficient in the preferred ratio system was 1.2×10 -5 cm² / s, significantly higher than 9.6×10 -6cm² / s. This is highly consistent with the conductivity trend measured in Example 2, where the conductivity of the optimized ratio sample at 700 °C is 1.50 S / cm, while that of the comparative sample is only 1.35 S / cm, indicating that the simulation data effectively reflects the enhanced migration ability of the dominant ions (Li + ) in the material system.

[0042] In the simulation of interfacial ion migration, three representative models with Li 2 CO 3 contents of 45 mol%, 60 mol%, and 70 mol% were constructed respectively, and the intensity of the main peak of RDF in the interfacial region was analyzed. The results show that the main peak of RDF of Li + -CO 3 2- is the lowest at the 60 mol% ratio, indicating the weakest association degree and the largest migration freedom, providing the optimal conditions for the formation of continuous ion channels. This simulation finding is completely corresponding to the phenomenon that the sample with the ratio of Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (59.5 ± 1: 22.5 ± 1: 18 ± 1 mol%) has the optimal conductivity in the actual measurement, theoretically establishing the scientific basis for the ratio optimization strategy of the present invention.

[0043] In summary, in this example, by constructing the molecular dynamics models of the electrolyte bulk phase and interface, the functional relationship between the Li + migration behavior and the coordination structure was clarified; and the structural parameters (bond lengths) obtained from the simulation were quantitatively compared with the conductivity test data to form a complete closed-loop verification system of "microstructure - diffusion rate - conduction efficiency". This research not only reveals the mechanism basis of the molten salt system of the present invention in terms of ratio optimization, but also provides a feasible path and parameter reference for the theoretical dominant design of subsequent electrolyte materials.

[0044] Example 4: Application verification in MCFC fuel cells The molten salt with the optimized ratio was filled into the experimental MCFC test device. The anode was Ni - Al, the cathode was lithiated NiO, the test temperature was 650 °C, and the initial power density was 0.2 W / cm².

[0045] During the continuous operation for 500 hours, the conductivity fluctuation ≤ 5%, the OCV and the load voltage were stable, and no delamination or structural deterioration was observed at the interface, indicating that the molten salt electrolyte has excellent high - temperature stability and interfacial compatibility, meets the long - term operation requirements, and has the potential for engineering and commercialization.

[0046] The technical solution of the present invention can be reasonably adjusted according to specific implementation conditions in actual applications, including but not limited to the optimization of process parameters, the adjustment of raw material ratios, the change of equipment selection, etc. Any improvement solution based on the core design concept of the present invention, as long as its technical features have the same or equivalent technical effects as the technical solution defined by the claims of the present invention, shall be regarded as falling within the protection scope of the present invention.

Claims

1. A method for preparing a ternary carbonate molten salt electrolyte with low melting point and high ionic conductivity, characterized in that: The following steps are involved: S1. Calculate the eutectic molar ratio of lithium carbonate (Li2CO3), sodium carbonate (Na2CO3) and potassium carbonate (K2CO3) according to the phase diagram software Factsage, where the molar percentage of Li2CO3 is 55% to 65%, the molar percentage of Na2CO3 is 20% to 25%, and the molar percentage of K2CO3 is 15% to 20%; S2. Weigh the raw materials according to the calculated ratio, and heat Li2CO3, Na2CO3 and K2CO3 at 130℃ and 10 -3 After drying for 24 hours under Pa conditions, the mixture was mixed and ground in an inert atmosphere glove box to a particle size of ≤5 μm, and then melted at 620°C to 630°C for 8 to 10 hours using a static constant temperature melting method to form a homogeneous liquid and then cooled; S3. Establish a molecular dynamics interface transfer model for the molten salt electrolyte and electrode interface, and simulate and analyze the migration mechanism of ions under service conditions.

2. The preparation method according to claim 1, characterized in that: The eutectic molar ratios of Li2CO3, Na2CO3 and K2CO3 in step S1 are 59.5±1:22.5±1:18±1 respectively.

3. The preparation method according to claim 1, characterized in that: The melting process in step S2 is carried out in a closed container into which 99.999% high-purity argon gas is continuously introduced.

4. The preparation method according to claim 1, characterized in that The water and oxygen contents in the inert atmosphere glove box in step S2 are both ≤0.1 ppm.

5. A ternary carbonate molten salt electrolyte with low melting point and high ionic conductivity, characterized in that: Prepared by the method according to any one of claims 1 to 4, the composition is: Li2CO3 55%~65%, Na2CO3 20%~25%, K2CO3 15%~20%, and satisfying: melting point ≤396℃; ionic conductivity at 700℃ ≥1.50 S / cm.

6. The ternary carbonate molten salt electrolyte according to claim 5, characterized in that: The molar ratio of Li2CO3, Na2CO3 and K2CO3 is 59.5±1:22.5±1:18±1.

7. Use of the ternary carbonate molten salt electrolyte as claimed in claim 5 or 6 in a molten carbonate fuel cell.

8. The use according to claim 7, characterized in that: The molten carbonate fuel cell comprises: Porous nickel anode; lithiated nickel oxide cathode; Ternary carbonate molten salt electrolyte layer; The operating temperature of the fuel cell is 580°C to 680°C.

9. The use according to claim 8, characterized in that: The conductivity of the electrolyte layer decays by ≤5% after continuous operation at 650° C. for 500 hours.

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