Ternary carbonate molten salt electrolyte with low melting point and high ionic conductivity and its preparation and application

Abstract

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, and relates to the field of high-temperature electrochemical materials. The electrolyte is prepared from lithium carbonate, sodium carbonate and potassium carbonate through the steps of optimizing the eutectic ratio, vacuum drying, and mixed melting under an inert atmosphere. The melting point of the obtained molten salt is not higher than 396°C, and the ionic conductivity can reach above 1.50 S / cm at 700°C. This method is also combined with molecular dynamics simulation to analyze its microstructure and migration mechanism, thereby improving the interface stability and conductive performance. The electrolyte of the present invention has the characteristics of controllable cost, uniform structure, and suitability for high-temperature fuel cells. It solves the problems of high melting point, low conductivity and poor electrode interface stability 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 in particular to a ternary molten carbonate electrolyte with a low melting point and high ionic conductivity, specifically including its preparation method and application in molten carbonate fuel cells (MCFCs), and belongs to the design and preparation technology direction of key electrolyte materials for molten carbonate fuel cells. Background Art

[0002] As the global energy structure shifts toward a low-carbon future, fuel cells, as a new, efficient, and low-emission energy conversion device, are attracting increasing attention. Molten carbonate fuel cells (MCFCs), in particular, have become a key development direction for high-temperature fuel cell technology due to their ability to directly utilize a variety of hydrocarbon fuels, high electrochemical reaction efficiency, and suitability for combined heat and power systems.

[0003] The typical operating temperature range of MCFC is 600℃~700℃, and it is necessary to use a heat transfer medium that can conduct CO3 at high temperature. 2- The molten salt system of ions is used as the electrolyte. Currently, the mainstream electrolyte mostly uses the Li2CO3-K2CO3 binary eutectic system, which has high ionic conductivity and a relatively mature engineering application basis. Its typical molar ratio is 62:38, corresponding to a melting point of about 500°C.

[0004] However, as MCFCs develop towards higher power density, longer life, and lower cost, traditional electrolyte systems face several key challenges, including:

[0005] 1. High melting point: The melting point of traditional binary systems is generally above 500°C, resulting in a large overall heat load on the system, increasing the difficulty of thermal management of the battery and the aging rate of the material, which is not conducive to long-term stable operation;

[0006] 2. Limited ion migration performance: In some medium-temperature operating ranges, the CO3 2- Insufficient migration capacity restricts further improvement of fuel cell power density;

[0007] 3. Unstable costs: As the main component, the raw material cost of Li2CO3 is subject to significant fluctuations in the lithium resource market, making it difficult to control the overall cost of the electrolyte system;

[0008] 4. Weak interface research: Existing research mostly focuses on the physical parameters of the molten salt system itself, and lacks systematic analysis of the interface structure between it and the electrode, ion transmission path, etc., which poses a potential obstacle to improving the overall battery efficiency.

[0009] To address these issues, researchers have attempted to introduce Na2CO3 to create a ternary eutectic molten salt system: Li2CO3-Na2CO3-K2CO3. By adjusting the ratio of the three carbonates, they achieved a synergistic optimization of melting point reduction and ion migration performance. Preliminary studies have shown that this ternary molten salt system can further lower the melting point while maintaining structural stability and enhancing the material's conductivity.

[0010] However, the currently available research on ternary molten salt still has the following deficiencies:

[0011] There is a lack of systematic exploration of the eutectic ratio, especially in the evaluation of structural stability and conductivity in different thermodynamic regions;

[0012] The melting environment and drying process are not strictly controlled, resulting in low purity of the obtained samples and large fluctuations in performance;

[0013] The lack of theoretical support for the microscopic mechanisms of electrolyte / electrode interface structure, migration channels and ion activation mechanisms limits the depth of material optimization design.

[0014] 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 interface transport mechanism to meet the multiple performance requirements of the new generation of molten carbonate fuel cells for electrolytes in terms of operating temperature range, conductive efficiency and system compatibility. Summary of the Invention

[0015] To address the problems of existing molten carbonate electrolytes, such as high melting point, limited ionic conductivity, poor interface stability, and difficulty in controlling the preparation environment, 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 Li2CO3, Na2CO3, and K2CO3, strictly controlling the drying and melting processes, and combining molecular dynamics simulation, the melting point is lowered to below 396°C while the ionic conductivity in the high temperature range (700°C) is increased to above 1.50 S / cm, significantly enhancing the thermal stability and interface conductivity of the material, and providing a new electrolyte solution for molten carbonate fuel cell systems that is efficient, stable, and cost-effective.

[0016] In one possible embodiment, a method for preparing a ternary carbonate molten salt electrolyte with a low melting point and high ionic conductivity is provided, comprising the following steps:

[0017] 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%;

[0018] 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 melted at a static constant temperature of 620°C to 630°C for 8 to 10 hours (without stirring), and cooled after forming a homogeneous melt;

[0019] S3. Establish a molecular dynamics model of the interface between the molten salt electrolyte and the electrode, and simulate and analyze the ion migration path.

[0020] In one possible embodiment, the eutectic molar ratio of Li2CO3, Na2CO3 and K2CO3 is 59.5±1:22.5±1:18±1.

[0021] In one possible embodiment, the melting process is carried out in a closed container into which 99.999% high-purity argon gas is continuously introduced.

[0022] In one possible embodiment, the water and oxygen contents in the inert atmosphere glove box are both ≤0.1 ppm.

[0023] In one possible embodiment, 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 has a composition of: Li2CO3 55% to 65%, Na2CO3 20% to 25%, and K2CO3 15% to 20%, and satisfies: melting point ≤ 396°C; ionic conductivity ≥ 1.50 S / cm at 700°C.

[0024] In one possible embodiment, the molar ratio of Li2CO3, Na2CO3 and K2CO3 is 59.5±1:22.5±1:18±1.

[0025] In one possible embodiment, a molten carbonate fuel cell is provided, comprising: a porous nickel anode; a lithiated nickel oxide cathode; and the above-mentioned ternary carbonate molten salt electrolyte layer; wherein the operating temperature of the fuel cell is 580° C. to 680° C.

[0026] In a possible embodiment, the conductivity of the electrolyte layer decreases by ≤5% after continuous operation at 650° C. for 500 hours.

[0027] Based on the above technical solution, the present invention's method for preparing a low-melting-point, high-ionic-conductivity ternary carbonate molten salt electrolyte utilizes a comprehensive approach of thermodynamic optimization, refined raw material processing, and controlled melting to effectively create a novel molten salt material with a uniform structure, stable composition, and excellent electrical conductivity. By introducing Na₂CO₃ and K₂CO₃ to adjust the Li₂CO₃ content, this material lowers its overall melting point to below 396°C. At the same time, it achieves an ionic conductivity exceeding 1.50 S / cm at 700°C, balancing both low-temperature startup performance and high-temperature operating efficiency.

[0028] The process employed in this invention involves transferring and grinding raw materials within an inert atmosphere glove box, significantly reducing the risk of interference from water vapor and oxygen, ensuring electrolyte purity and structural consistency. Furthermore, the strategy of rapidly cooling to form a glassy structure facilitates the formation of continuous, low-barrier ion migration channels, providing structural support for improving electrolyte performance.

[0029] In addition, molecular dynamics simulation was used to theoretically model and verify the ion distribution and migration behavior, and it was clear that high Li + The intrinsic mechanism of denser ion structure and higher diffusion rate under lower content provides an important reference for the subsequent design of electrolyte materials.

[0030] Experiments have demonstrated that the molten salt electrolyte of this invention can operate stably at temperatures between 600°C and 700°C. Compared to the traditional Li2CO3-K2CO3 system, it can lower the operating temperature while maintaining high conductivity. Furthermore, the conductivity degradation after 100 thermal cycles is kept to less than 5%. Overall, this electrolyte system boasts abundant material resources, a controllable preparation process, and excellent performance. It has promising industrial feasibility and application prospects, providing key material support for the development of a new generation of high-efficiency MCFC systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the AC impedance spectrum of the Li2CO3-Na2CO3-K2CO3 (59.5±1:22.5±1:18±1 mol%) ternary molten salt prepared in Example 1 at 700°C, with a test frequency range of 0.01 Hz–1 MHz;

[0032] Figure 2 It is a comparison of the impedance spectra of Li2CO3-Na2CO3-K2CO3 (43.5±1:31.5±1:25±1 mol%) molten salt under the same conditions;

[0033] Figure 3 is Li obtained from molecular dynamics simulation + -CO3 2-Radial distribution function (RDF) curve, the horizontal axis is the atomic distance (A), and the vertical axis is the probability density. DETAILED DESCRIPTION

[0034] To make the technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings. The accompanying drawings are used to further explain the technical features and experimental verification results of the present invention, among which:

[0035] Figure 1 The AC impedance spectrum of the ternary molten salt with the optimal ratio of Li2CO3-Na2CO3-K2CO3 (59.5±1:22.5±1:18±1 mol%) at 700°C is shown. The low interfacial resistance characteristics are intuitively reflected by the capacitive arc radius of the Nyquist plot.

[0036] Figure 2 To compare the impedance spectrum of the molten salt with the ratio of Li2CO3-Na2CO3-K2CO3 (43.5±1:31.5±1:25±1mol%), Figure 1 The comparison proves that the ratio of the present invention has a significant advantage in reducing charge transfer resistance;

[0037] Figure 3 The radial distribution function (RDF) curve of molecular dynamics simulation reveals that Li + The difference in coordination structure with carbonate ions provides a microscopic mechanism explanation for the high ionic conductivity.

[0038] Example 1: Preparation process of ternary carbonate molten salt electrolyte

[0039] This example uses high-purity lithium carbonate (Li2CO3, ≥99.99%), sodium carbonate (Na2CO3, ≥99.99%) and potassium carbonate (K2CO3, ≥99.95%) as raw materials. -3 After vacuum drying for 24 h under Pa conditions, the samples were transferred to an inert atmosphere glove box with a water and oxygen content of ≤0.1 ppm for storage and operation.

[0040] The raw materials were precisely weighed to a molar ratio of Li₂CO₃-Na₂CO₃-K₂CO₃ (59.5±1:22.5±1:18±1) and ground in a glove box for 30 minutes, with a particle size of ≤5 μm. The mixed powder was placed in a 99.6% alumina crucible and placed in a sealed tube furnace filled with 99.999% high-purity argon. The temperature was raised at 5°C / min to 625±5°C and maintained for 10 hours. After cooling, a pale yellow glassy solid electrolyte was obtained.

[0041] As a comparative experiment, another sample of Li2CO3-Na2CO3-K2CO3 (43.5±1:31.5±1:25±1 mol%) was prepared with the same process conditions for subsequent performance evaluation.

[0042] Example 2: High-temperature conductivity performance test of molten salt electrolyte

[0043] In order to systematically evaluate the high-temperature electrical conductivity of the ternary carbonate molten salt electrolyte of the present invention, electrochemical impedance spectroscopy (EIS) tests were performed on it at different temperatures.

[0044] 1. Sample Preparation and Pretreatment

[0045] The electrolyte sample prepared in Example 1 was crushed and passed through a 400-mesh sieve to ensure uniform particle size. 1.000 ± 0.001 g of the sample was weighed and placed in a corundum crucible (10 mm inner diameter) to prevent chemical reactions between the electrolyte and the container.

[0046] The sample pretreatment process includes:

[0047] Place in a vacuum drying oven at 110°C for 6 hours to eliminate adsorbed water;

[0048] After cooling to room temperature, the samples were immediately packaged and transferred to an argon glove box (O2 and H2O < 0.1 ppm) for testing.

[0049] 2. Construction of conductivity test platform

[0050] Conductivity measurements were performed using a CHI660E electrochemical workstation. A symmetrical three-electrode system (Pt | molten salt | Pt) was employed, with the following structure:

[0051] 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 three times with deionized water;

[0052] Reference electrode: Ag / AgCl electrode is used to calibrate the system;

[0053] Electrode fixation: Clamp the Pt electrode with a ceramic fixture, maintaining a spacing of 5.00±0.01 mm to ensure good contact and uniform heating;

[0054] Electrolytic cell constant (Ccell) calibration: calibrated with 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 .

[0055] 3. Heating and temperature control

[0056] The tests were conducted in a pit-type resistance furnace, with set temperatures at 600°C, 650°C, and 700°C. The furnace was controlled via a thermocouple-coupled PID control module, achieving a temperature accuracy of ±0.5°C. At each temperature point, the temperature was maintained for at least 45 minutes to ensure complete melting of the molten salt and the achievement of thermodynamic equilibrium.

[0057] The thermocouple and furnace body were calibrated at standard temperature points before and after the test to eliminate errors caused by temperature.

[0058] 4. Impedance spectrum acquisition and fitting method

[0059] The AC impedance test parameters are set as follows:

[0060] Excitation voltage: 10 mV;

[0061] Scan frequency range: 0.01 Hz to 1 MHz;

[0062] Test time: Each point lasts about 8 minutes, and the test is repeated 3 times at each temperature point to take the average value.

[0063] Impedance data were fitted and analyzed using ZsimpWin software, using the R(CR)(QR) equivalent circuit model to extract the bulk resistance, Rb. Conductivity, σ, was calculated using the following formula: σ = (Ccell / Rb).

[0064] The fitting result requires the residual (chi-square) to be less than 10 -4 , ensuring the reliability of the analysis. The total experimental error consists of three parts: electrode spacing deviation, temperature control fluctuation, and fitting error, and is controlled within the range of ±3%.

[0065] 5. Test results analysis

[0066] like Figure 1 As shown in the figure, the Nyquist spectrum of the preferred ratio Li2CO3-Na2CO3-K2CO3 (59.5±1:22.5±1:18±1 mol%) electrolyte at 700℃ is a standard single capacitive arc morphology, indicating that the ion migration resistance is mainly concentrated in the bulk phase 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 of the composition ratio Li2CO3-Na2CO3-K2CO3 (43.5±1:31.5±1:25±1 mol%) shows a larger impedance arc (such as Figure 2 As shown), its Rb reaches 0.42Ω·cm 2 , indicating that the ion migration channel is restricted.

[0067] The specific conductivity data are as follows:

[0068]

[0069] The results show that the optimally proportioned molten salt 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 equipment.

[0070] This test verified the stable electrical conductivity and good electrode contact properties of the electrolyte material under high-temperature conditions. Compared with the control group, the optimized Li2CO3 content can form more continuous and uniform ion channels, reduce the migration energy barrier, and meet the core requirement of high-performance molten carbonate fuel cells (MCFCs) for high-ionic conductivity materials.

[0071] Example 3: Molecular dynamics simulation analysis of molten salt electrolyte and interface

[0072] In order to simulate the ionic behavior of real carbonate molten salt electrolyte in service conditions, a Li + -Na + -K + -CO3 2- The / electrode interface transfer model was simulated using LAMMPS, using the Born-Mayer-Coulmb potential function and the Lennard-Jones potential function, with a cutoff radius of 20 A, and running for 10 ns at 875 K. 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: in the preferred ratio, Li + With CO3 2- The average coordination distance of the Li-ion ligand is shorter (2.05 A), which is significantly better than the comparison ratio (2.12 A). + With CO3 2- The RDF peak intensity is lower. This structural characterization shows that in the preferred system, Li + With CO3 2- The interaction between them is weaker and they can migrate more easily in the electrolyte.

[0073] Example 3 (continued): Correlation analysis between simulation results and conductivity data

[0074] To further elucidate the influence of different ratios on the electrical conductivity of the ternary carbonate molten salt system from a theoretical perspective, this example systematically compares and analyzes the structural parameters obtained by molecular dynamics simulation with the measured high-temperature conductivity results in Example 2, thereby establishing a corresponding relationship between microstructure and macroscopic performance.

[0075] Further extraction of Li by Einstein diffusion formula +Diffusion coefficient shows that the Li⁺ diffusion coefficient in the optimal ratio system is 1.2×10 -5 cm² / s, significantly higher than the 9.6×10 -6 cm² / s. This is highly consistent with the conductivity trend measured in Example 2, where the conductivity of the preferred ratio sample at 700°C is 1.50 S / cm, while the conductivity of the comparative sample is only 1.35 S / cm, indicating that the simulation data effectively reflects the dominant ion (Li + ) Enhanced migration capabilities.

[0076] In the simulation of interfacial ion migration, three representative models with 45 mol%, 60 mol% and 70 mol% Li2CO3 contents were constructed, and the RDF main peak intensity of the interface region was analyzed. The results showed that: at 60 mol% Li + -CO3 2- The RDF peak is lowest, indicating the weakest association and the greatest freedom of migration, providing optimal conditions for the formation of continuous ion channels. This simulation finding fully corresponds to the measured optimal conductivity of the sample with the Li2CO3-Na2CO3-K2CO3 ratio (59.5±1:22.5±1:18±1 mol%), theoretically establishing the scientific basis for the ratio optimization strategy of the present invention.

[0077] In summary, this example clarifies the Li + The functional relationship between migration behavior and coordination structure was investigated; the simulated structural parameters (bond lengths) were quantitatively compared with conductivity test data, forming a complete closed-loop verification system of "microstructure-diffusion rate-conductivity efficiency". This research not only reveals the mechanistic basis for the ratio optimization of the molten salt system of the present invention, but also provides a feasible path and parameter reference for the theoretical-led design of subsequent electrolyte materials.

[0078] Example 4: Application Verification in MCFC Fuel Cell

[0079] The optimal ratio of molten salt 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.

[0080] During 500 hours of continuous operation, the conductivity fluctuation was ≤5%, the OCV and load voltage were stable, and no stratification or structural degradation was observed at the interface, indicating that the molten salt electrolyte has excellent high-temperature stability and interface compatibility, meets the requirements of long-term operation, and has engineering and commercialization potential.

[0081] In practical applications, the technical solutions of the present invention may be reasonably adjusted according to specific implementation conditions, including but not limited to optimization of process parameters, adjustment of raw material ratios, and change of equipment selection. Any improvement based on the core design concept of the present invention shall be deemed to fall within the scope of protection of the present invention as long as its technical features have the same or equivalent technical effects as the technical solutions defined in the claims of the present invention.

Claims

1. A method for preparing a ternary carbonate molten salt electrolyte with a low melting point and high ionic conductivity, characterized in that: The following steps are involved: S1. The eutectic molar ratio of lithium carbonate (Li2CO3), sodium carbonate (Na2CO3) and potassium carbonate (K2CO3) was calculated using the phase diagram software Factsage. The eutectic molar ratio of Li2CO3, Na2CO3 and K2CO3 was 59.5±1:22.5±1:18±1. S2, weigh the raw materials according to the calculated ratio in step S1, and heat Li2CO3, Na2CO3 and K2CO3 at 130℃ and 10 -3 After drying for 24 h under Pa conditions, the samples were transferred to an inert atmosphere glove box with water and oxygen contents ≤ 0.1 ppm and ground to a particle size ≤ 5 μm; S3, heating the ground raw materials to 625±5°C at a rate of 5°C / min, melting them at this temperature for 10 hours, forming a homogeneous molten salt, and then rapidly cooling them to room temperature to obtain a molten salt electrolyte with a glassy structure; S4. Combine molecular dynamics simulation to analyze the ion migration path at the interface between the molten salt electrolyte and the electrode, and optimize the interface stability and conductivity between the electrolyte and the electrode; The melting point of the obtained molten salt electrolyte is ≤396°C, and the ionic conductivity at 700°C is ≥1.50 S / cm.

2. 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.

3. 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 1-2, the molar ratio of Li2CO3, Na2CO3 and K2CO3 is 59.5±1:22.5±1:18±1, and meets the following requirements: melting point ≤396°C; ionic conductivity at 700°C ≥1.50 S / cm.

4. Use of the ternary carbonate molten salt electrolyte as claimed in claim 3 in a molten carbonate fuel cell.

5. The use according to claim 4, 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.

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

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