Method for predicting constant-current discharge performance of magnesium anode by using first principle

Abstract

The invention belongs to the field of electrochemical behaviors of seawater battery anodes, and particularly relates to a method for predicting constant-current discharge performance of a magnesium anode by using a first principle. Comprising the following steps: (1) establishing a matrix model according to atomic configurations and arrangement modes of different anodes; cutting the sample surface, and arranging a vacuum layer and a solvent layer; (2) carrying out geometric optimization on the structure formed in the step (1); (3) in combination with a joint density functional theory, putting the geometrically optimized structure into a solvent environment for solvation, and calculating the total energy, interface electron density, charge density and electrode potential of the structure; and (4) predicting a potential-time relationship of anode constant current discharge within the set acting time by using a calculation result in the step (3) and combining a discharge dynamic law of a chronopotentiometry. According to the method, the magnesium anode surface electric double-layer model in seawater can be obtained, and the electrochemical properties of the magnesium anode surface electric double-layer model can be quickly and effectively predicted based on the joint density functional theory.

Description

Technical Field

[0001] The invention belongs to the field of electrochemical behavior of seawater battery anodes, and specifically relates to a method for predicting the constant current discharge performance of magnesium anodes by using first principles. Background Art

[0002] The magnesium anode is considered to be the most promising anode for the next generation of energy storage devices due to its high theoretical capacity and suitable operating voltage, as well as its low cost, abundant reserves, no pollution from corrosion products, and easy replacement. However, its further commercialization is hindered by the potential fluctuations of its different alloys during the discharge process, the negative difference effect that increases with the degree of polarization, and the block effect that leads to increased magnesium alloy loss. First-principles modeling is considered to be an effective strategy for simulating electrode processes. In first-principles calculations, the current effective evaluation of magnesium anode performance is mainly based on work function and state density, but a widely applicable evaluation method is still needed for the actual anode performance evaluation. Summary of the invention

[0003] The purpose of the present invention is to provide a method for predicting the constant current discharge performance of a magnesium anode using first principles, which can obtain a double electric layer model of the magnesium anode surface in seawater, and based on the joint density functional theory, can quickly and effectively predict its electrochemical properties.

[0004] The technical solution to achieve the purpose of the present invention is: a method for predicting the constant current discharge performance of a magnesium anode using first principles, comprising the following steps:

[0005] Step (1): Using Material Studio to build a matrix model based on the atomic configuration and arrangement of different anodes; cutting the sample surface, setting the vacuum layer and solvent layer;

[0006] Step (2): geometrically optimizing the structure formed in step (1);

[0007] Step (3): Combined with joint density functional theory, the geometrically optimized structure is placed in a solvent environment for solvation, and the total energy of the structure and the interface electron density, charge density, and electrode potential are calculated;

[0008] Step (4): Using the calculation results of step (3) and the discharge dynamics of chronopotentiometry, the potential-time relationship of the anode constant current discharge within the set working time is predicted.

[0009] Furthermore, in step (1), a pure Mg substrate is selected as the anode material.

[0010] Furthermore, step (1) is specifically as follows: calculating the lattice parameters of the magnesium unit cell, establishing a magnesium electrode containing nine metal atomic layers, and establishing a 4*4*1 super unit cell; cutting along the unit cell plane (1 0 0); translating the unit based on the unit cell model, translating along the c direction at the cutting plane to increase a vacuum layer with a thickness of 40 angstroms, and forming an initial crystal plane model;

[0011] Two identical molecular models were replicated, and four water molecules and one chloride ion were adsorbed on the top position of the magnesium anode plate surface respectively.

[0012] Furthermore, in step (2), VASP is used to perform geometry optimization in vacuum, and the bottom 6 metal atomic layers are fixed, allowing the remaining surface layer atoms and a layer of water molecules to move freely.

[0013] Furthermore, step (2) is specifically as follows: using the augmented plane wave method in the first-principles calculation method of density functional theory, using the Perdew-Burke-Ernzerhof function based on the generalized gradient approximation as the exchange-correlation function, setting the plane wave cutoff energy, the atomic force convergence criterion, the energy convergence criterion of the self-consistent field, the electronic configuration of magnesium, the K value of the Brillouin zone and the van der Waals force correction.

[0014] Furthermore, the solvation parameters in step (3) are as follows: the fluid mode is selected as LinearPCM, the pcm variant is CANDLE, and the solvent is water with a set concentration of NaCl;

[0015] The exchange-correlation functional is GGA-PBEsol;

[0016] Allow Coulomb interaction and set the Coulomb correction to the center symmetric position of the metal layer;

[0017] K-point is set to Gamma mode with a value of 5-5-1;

[0018] Set the maximum number of iterations, convergence threshold and cutoff energy for the electronic step.

[0019] Furthermore, the discharge dynamics law used in step (4) is as follows:

[0020]

[0021] Where η is the overpotential; j is the given increasing current density; is the reactant concentration, D0 is the diffusion coefficient of the reactant; j0 is the exchange current density, c R (0,t) is the concentration at x=0 on the surface, is the bulk concentration away from the compact layer η is the overpotential, is a constant.

[0022] Compared with the prior art, the present invention has the following significant advantages:

[0023] By applying the technical solution of the present invention, the electrochemical performance of the anode of the seawater battery is predicted by performing molecular modeling, first-principles simulation, and calculating electrochemical performance on the studied substrate and coating, using the method of joint density functional theory. Compared with existing experimental methods, the method of the present invention has lower cost and higher efficiency, does not require any experimental parameter input, and provides a method for evaluating the surface double layer parameters, thereby providing better theoretical guidance for the preparation of electrode materials and practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the method flow of the present invention.

[0025] Figure 2 This is a structural diagram of the magnesium anode surface after water molecules are adsorbed in the embodiment.

[0026] Figure 3 It is the electron and charge density distribution diagram during anode polarization in the embodiment.

[0027] Figure 4 1 is a comparison chart of the magnesium anode discharge performance in the embodiment and the results predicted by the first principles; wherein (a) is the predicted constant current discharge curve of the magnesium anode, and (b) is the experimentally measured constant current discharge curve of the magnesium anode. DETAILED DESCRIPTION

[0028] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following is a method for first-principles prediction of the constant-current discharge performance of magnesium anode in seawater battery in combination with the accompanying drawings and specific implementation methods.

[0029] The above and other technical contents, features and effects of the present invention can be clearly presented in the following detailed description of the specific implementation methods in conjunction with the drawings. Through the description of the specific implementation methods, the technical means and effects adopted by the present invention to achieve the predetermined purpose can be more deeply and specifically understood. However, the attached drawings are only for reference and explanation, and are not used to limit the technical solutions of the present invention. The present invention is further described in detail below in conjunction with the drawings.

[0030] The embodiment of the present invention provides a method for first-principles prediction of the constant-current discharge performance of magnesium anode in seawater battery, which mainly applies advanced calculation methods, including JDFT and van der Waals force correction, based on density functional theory calculations to accurately understand the electronic structure properties of magnesium anode, and to explore the influence of seawater environment on the double-layer properties, discharge kinetics and thermodynamic properties of magnesium anode. The method of predicting the constant-current discharge performance of magnesium anode in seawater battery based on joint density functional theory of this embodiment includes:

[0031] Step (1): Based on the atomic configuration and arrangement of different anodes, the model of the substrate is established using Material Studio software. The sample surface is cut and the vacuum layer / solvent layer is set.

[0032] The lattice parameters of the magnesium unit cell are calculated to be a=3.198 and c=5.184. Based on these values, a nine-layer magnesium plate is built, and a 4*4*1 supercell is built. Use the Cleave Surface function of the Build module in the Materials Studio software to cut along the unit cell surface (1 0 0); translate the unit based on the unit cell model, and translate along the c direction at the cut surface to add a 40 angstrom thick vacuum layer to form an initial crystal surface model. Copy two identical molecular models, and adsorb four water molecules and one chloride ion on the top position of the magnesium anode plate surface.

[0033] Step (2): Optimize the structure formed in the previous step.

[0034] The augmented plane wave method of the first-principles calculation method of density functional theory was selected, and the Perdew-Burke-Ernzerhof function based on the generalized gradient approximation was selected as the exchange-correlation function. The VASP software was used to set the plane wave cutoff energy, the atomic force convergence criterion, the energy convergence criterion of the self-consistent field, the electronic configuration of magnesium, the K value of the Brillouin zone and the van der Waals force correction.

[0035] Set the parameters for structural optimization. When optimizing the pure magnesium structure model, use the geometry optimization function of VASP, ENCUT = 450, PREC = Accurate, EDIFF = 1E-05, EDIFFG = 0.001, ISMEAR = 1, IBRION = 1, the maximum number of iterations of the ion step is 200, use the optb88 function for van der Waals force correction, and set K-point to Gamma mode with a value of 5-5-1. After the calculation is completed, the structural data of the pure magnesium plate and the pure magnesium plate containing the adsorbed atomic layer after energy minimization are obtained.

[0036] Step (3) combines joint density functional theory to place the geometrically optimized structure into a solvent environment and calculate the total energy of the structure and the interface electron density, charge density, and electrode potential.

[0037] Using implicit solvation joint density functional software JDFTx, JDFTx is a first-principles calculation program developed by Associate Professor Ravishankar Sundararaman of the School of Materials Science and Engineering at Rensselaer Polytechnic Institute. It stands out in its field for its unique ability to combine traditional density functional theory (DFT) calculations with implicit solvent environments. This introduces a new perspective to the evaluation of electrochemical phenomena because it provides a more accurate description of charge transfer processes in solution environments. JDFTx offers a variety of options, and JDFTx supports all standard functions present in any electronic DFT software. It supports a variety of exchange-correlation functions. JDFTx supports norm conservation and ultrasoft pseudopotentials, supports Coulomb truncation, and implements a hierarchy of continuum solvation models, starting from a simple linear solvation model (LinearPCM in the command Fluid), with multiple variants, including the default GLSSA13 (same as VASPSol), SCCS (same as Quantum Espresso) and CANDLE, which predicts very well for strongly charged and polar solutes. JDFTx also includes nonlinear solvation models as well as the non-empirical solvation model Salsa, which excels at capturing the non-local atomic response of fluid responses at the linear level.

[0038] The CONTCAR file optimized by VASP was extracted and converted into a geometric site file recognizable by JDFTx software. In JDFTx, the CANDLE model was selected to simulate the long-range interaction of water molecules, the ultrasoft pseudopotential was selected, the fluid mode was LinearPCM, the pcm variant was CANDLE, and the solvent was simulated seawater with a concentration of 0.6 mol / L NaCl. The exchange correlation functional is GGA-PBEsol. Coulomb interactions are allowed and the Coulomb correction is set to the central symmetric position of the metal layer. The K-point is set to Gamma mode with a value of 5-5-1. The maximum number of iterations of the electron step is 1000, and the convergence threshold is 1e -6 Hartrees, with a cutoff energy of 20 Hartrees; the study of the effect of different overpotentials on the properties of the double layer can be accomplished by setting different elec-initial-charge options. Here, bias electrons of -0.2, -0.4, -0.5, -0.6, -0.8, and -1.0 are selected.

[0039] Step (4): The electron density, charge density, vacancy energy, adsorption energy and other parameters of the magnesium anode plate are obtained from the calculation results described in step (3), and the discharge behavior of the magnesium anode in seawater is analyzed in combination with the anode discharge kinetics.

[0040] The three-dimensional electron charge distribution is summed up to a two-dimensional distribution along the z-axis and the influence of the zero-charge electrode is removed, and the following is obtained: Figure 3 The polarized electrons and charge density distribution on the anode plate surface are shown in the figure; the functional relationship between the overpotential and the double layer thickness and the double layer ion concentration is obtained, and the discharge kinetic parameters of pure magnesium are calculated by the joint density functional theory, as shown in Table 1. In order to combine the constant current discharge kinetics (Formula (1)) with the joint density functional theory (Table 1):

[0041]

[0042] Substituting the parameters in the following table into formula (3), we can obtain the exchange current density j0; Figure 3 The electron density distribution integral of the electrode surface should be the electron density σ, from which the effective diffusion layer thickness δ under a given bias electron is obtained.

[0043] In our case, unlike formula (1), the reduced product is a solid, so its charge transfer rate is a fixed value, which is approximately 1 here, as shown in formula (6).

[0044]

[0045] Regarding the simulation of the diffusion layer concentration, the idea of ​​this embodiment is that, since there is a certain amount of diffusion layer thickness and charge density data within a certain bias electron range, then according to formula (5) to calculate The functional relationship between bias voltage and concentration can be obtained as follows:

[0046]

[0047] Where n is the number of particles reacting on the electrode surface under steady state, Q is the current density after the steady state is established, and σ is the charge density δ is the thickness of the effective diffuse layer (nm), ε i is the dielectric constant of the corresponding solvent A is the anode area, j0 is the current density. δ is the voltage drop across the electron at a given bias.

[0048] Table 1 Kinetic parameters of UHP-Mg constant current discharge

[0049]

[0050] E eq=-2.27V (vs. SHE), which is very close to the experimental value (-2.30V vs. SHE). It should be pointed out that the δ calculated here is the effective diffusion layer thickness. Its physical meaning is that when the electric field formed by the electrode plate drops to a certain level, other ions in the solution begin to influence each other. The layer in which this influence and the action of the electrode plate electric field are balanced is the effective diffusion layer, which essentially represents the effective control area of ​​the electrode in the electrochemical sense.

[0051] The kinetic behavior of the constant current discharge of magnesium anode was finally obtained by combining the Sand equation and the joint density functional theory, such as Figure 4 As shown, the equation is as follows,

[0052]

[0053] In order to compare with the experimental results, in this embodiment, a series of constant current discharge experiments were carried out to verify the accuracy of the prediction method proposed by us. The anode selected in our experiment has a surface area of ​​1cm 2 Ultra-high purity magnesium (UHP-Mg) of MgO was prepared. The following table shows its chemical composition measured by inductively coupled plasma optical emission spectrometry (ICP-OES). 0.6 M NaCl solution was used to simulate seawater.

[0054] Table 2 Chemical composition of UHP-Mg used in the control test

[0055]

[0056]

[0057] Electrochemical measurements were performed using a VersaStat 3F electrochemical workstation from Princeton Applied Research, Inc., dedicated for electrochemical studies. UHP-Mg was used as the working electrode in a classical three-electrode system. A saturated calomel electrode (SCE) was used as a reference electrode, and a platinum mesh was placed facing the working electrode as a counter electrode. All these experiments were performed at room temperature between 25 and 30 degrees. Discharge currents of 10, 20, 50, and 100 mA / cm were recorded. 2 The potential-time curve when Figure 3 As shown in the figure on the right, it can be found that the experimental results are very close to the results predicted by the model.

[0058] The data obtained in the embodiments of the present invention can be used to evaluate the discharge kinetics of the metal anode of the water-based electrolyte battery. The measured values ​​and the calculated values ​​are very close, which can fully guide the production practice. The experiment proves that the calculation method designed by the present invention has the advantages of high reliability, flexibility and convenience.

[0059] It should be understood that this specification is only intended to clearly and explicitly explain the calculation process of the constant current discharge behavior of one embodiment. However, to fully evaluate the electrochemical behavior or charge distribution of the material, we need to do further research in cutting different crystal planes, setting different temperatures, or trying different materials. In addition, the technical solutions in each embodiment can be appropriately combined and compared. These supplementary studies and combined analyses will help to further explore the technological development in this field.

[0060] The equipment, operation steps, and calculation methods not involved in the present invention are the same as those in the prior art or can be implemented by using the prior art and will not be described in detail in this application.

[0061] The above specific implementation methods further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific implementation methods of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for predicting the constant current discharge performance of magnesium anode using first principles, characterized in that: The steps include: Step (1): Using Material Studio to build a matrix model based on the atomic configuration and arrangement of different anodes; cutting the sample surface, setting the vacuum layer and solvent layer; Step (2): geometrically optimizing the structure formed in step (1); Step (3): Combined with joint density functional theory, the geometrically optimized structure is placed in a solvent environment for solvation, and the total energy of the structure and the interface electron density, charge density, and electrode potential are calculated; Step (4): Using the calculation results of step (3) and the discharge dynamics of chronopotentiometry, the potential-time relationship of the anode constant current discharge within the set working time is predicted.

2. The method according to claim 1, characterized in that: In step (1), a pure Mg substrate is selected as the anode material.

3. The method according to claim 2, characterized in that Step (1) is specifically as follows: calculating the lattice parameters of the magnesium unit cell, establishing a magnesium electrode containing nine metal atomic layers, and establishing a 4*4*1 supercell; cutting along the unit cell plane (1 0 0); translating the unit based on the unit cell model, translating along the c direction at the cutting surface to increase a vacuum layer with a thickness of 40 angstroms, and forming an initial crystal plane model; Two identical molecular models were replicated, and four water molecules and one chloride ion were adsorbed on the top position of the magnesium anode plate surface respectively.

4. The method according to claim 3, characterized in that In step (2), VASP is used to perform geometry optimization in vacuum, and the bottom 6 metal atomic layers are fixed, allowing the remaining surface layer atoms and a layer of water molecules to move freely.

5. The method according to claim 4, characterized in that Step (2) is specifically as follows: using the augmented plane wave method in the first-principles calculation method of density functional theory, using the Perdew-Burke-Ernzerhof function based on the generalized gradient approximation as the exchange-correlation function, setting the plane wave cutoff energy, the atomic force convergence criterion, the energy convergence criterion of the self-consistent field, the electronic configuration of magnesium, the K value of the Brillouin zone and the van der Waals force correction.

6. The method according to claim 5, characterized in that The solvation parameters in step (3) are as follows: the fluid mode is selected as LinearPCM, the pcm variant is CANDLE, and the solvent is water with a set concentration of NaCl; The exchange-correlation functional is GGA-PBEsol; Allow Coulomb interaction and set the Coulomb correction to the center symmetric position of the metal layer; K-point is set to Gamma mode with a value of 5-5-1; Set the maximum number of iterations, convergence threshold and cutoff energy for the electronic step.

7. The method according to claim 6, characterized in that The discharge dynamics used in step (4) are as follows: Where η is the overpotential; j is the given increasing current density; is the reactant concentration, D0 is the diffusion coefficient of the reactant; j0 is the exchange current density, c R (0,t) is the concentration at x=0 on the surface, is the bulk concentration away from the compact layer η is the overpotential, is a constant.

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