A method for predicting Li + / Mg 2+ Method of isolating the performance of confined mass transfer graphene oxide membranes

Abstract

This invention relates to a method for predicting Li based on molecular dynamics simulations. + / Mg 2+ A method for separating the properties of confined mass transfer graphene oxide films includes the following steps: 1) constructing a membrane containing water molecules and Li + Mg 2+ 1) Obtain the initial structural data file of the graphene oxide confined mass transfer model; 2) Optimize the structure of the model; 3) Perform molecular dynamics calculations on the optimized model; 4) Analyze the kinetic model and statistically analyze the water and Li molecules. + / Mg 2+ The separation ratio and water molecule flux are calculated by counting the number of channels, and the diffusion coefficient is obtained through further mean square displacement analysis. This invention, by modeling and simulating graphene oxide channels with confined mass transfer effects during experiments, can quickly determine the separation effect of confined mass transfer graphene oxide membranes, effectively shortening the experimental cycle and saving experimental costs.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of computer simulation of film materials, and particularly relates to a method for predicting a graphene oxide film for Li + / Mg 2+ separation based on molecular dynamics simulation. BACKGROUND

[0002] Batteries are indispensable in modern life, and lithium is widely used as a battery raw material. With the increasing demand for lithium ion batteries, it is of great significance to extract lithium ions. About 70% of lithium is derived from salt lake brine. The biggest challenge in extracting lithium from salt lakes is the interference of many other ions, such as Na + , Ca 2+ and Mg 2+ . Since Li + (0.38 nm) has a similar hydrated ion radius as Mg 2+ (0.43 nm), it is very difficult to separate Li + and Mg 2+ . Membrane separation technology is a green and environmentally friendly process that can be used to extract lithium. However, the current commercial separation membrane has low efficiency, and the hydrated radius and interlayer limited mass transfer effect cannot be fully considered, so it cannot well separate Li + / Mg 2+ .

[0003] Graphene oxide (GO) has a sub-nanometer interlayer spacing and a surface-adjustable functional group, and exhibits excellent ion transport characteristics, which can achieve good screening of ions and has attracted extensive attention from researchers. When the interlayer channel of graphene oxide is reduced to nanoscale, the fluid flux in the channel increases, and the flow speed is accelerated, resulting in a limited mass transfer effect. The limited mass transfer effect and the ion hydrated radius will affect the separation effect of Li + / Mg 2+ . CN111013398A discloses a Janus nanochannel dominant nanofiltration membrane for selectively removing charged drugs and a preparation method thereof. The separation of the entire membrane is dominated by introducing a charge-modified MOF in the middle layer of the nanofiltration membrane. Due to the limited mass transfer effect of the MOF itself, the flux of the membrane is effectively improved, and the retention rates of positively charged drugs and negatively charged drugs are also improved. However, there are many disadvantages in testing the separation performance from the experimental aspect, such as wasting a lot of time and money in synthesizing and characterizing the membrane without obtaining correct results. A more serious problem is that the existing characterization method can only characterize the structure of the membrane, and the dynamic behavior of the fluid in the channel is difficult to characterize.

[0004] CN112802558A discloses a method for predicting the performance of a covalent organic framework for lithium extraction from salt lakes based on molecular simulation, comprising: setting a water box containing water molecules, Li + and Mg 2+ , combining the water box with a COF separation membrane, and constructing a COF salt water model box, optimizing the structure of the COF salt water model box, and setting the density of the COF salt water model box according to the actual salt water density; molecular dynamics simulation is performed on the set COF salt water model box, and the transmission speed of magnesium ions and lithium ions in the pore channel of the COF separation membrane is counted after the simulation is completed, and the separation factor is calculated according to the transmission speed of magnesium ions and lithium ions. The present application adopts a non-equilibrium molecular dynamics simulation method, constructs a COF model, and indirectly calculates the separation efficiency by counting the transmission speed of ions.

[0005] Molecular dynamics is a method of simulating the microcosmic behavior of fluid through the membrane by using computer technology. There is no report on the performance of graphene oxide membrane for separating Li + / Mg 2+ under the condition of nanometer confinement from the perspective of molecular dynamics, so we study the performance of membrane material for separating Li + / Mg 2+ under the condition of nanometer confinement from this perspective. The present application can quickly judge the separation effect of the confined mass transfer graphene oxide membrane. At the same time, it can promote the application of the confined mass transfer graphene oxide membrane for separating Li + / Mg 2+ . Greatly reduce the application cost, save experimental resources, and reduce the time from research to industrialization. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a method for predicting the performance of confined mass transfer graphene oxide membrane for separating Li + / Mg 2+ based on molecular dynamics simulation, which simulates the membrane separation process by computer simulation and predicts the performance of confined mass transfer graphene oxide membrane for separating Li + / Mg 2+ from the perspective of molecular dynamics.

[0007] Specifically, the technical scheme of the present application is as follows:

[0008] Step one: setting a water box containing water molecules and Li + and Mg 2+ according to the actual salt water density, assembling the water box after annealing and graphene oxide confined mass transfer model.

[0009] Step two: structure optimization of graphene oxide salt water box.

[0010] Step three: molecular dynamics simulation is performed on the structure-optimized graphene oxide brine box, and the number of magnesium ions and lithium ions distributed on the discharge side of the graphene oxide separation membrane is counted after the simulation, and subsequent analysis including Li + / Mg 2+ separation ratio, mean square displacement, diffusion coefficient, etc.

[0011] Further, in the method for predicting the confined mass transfer performance of graphene oxide for Li + / Mg 2+ separation based on molecular dynamics simulation, the method comprises the following steps:

[0012] In step one, the density of the brine box can be any density within 0.3-0.7 mol / L, such as 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L or 0.7 mol / L, but not limited thereto, and other unlisted numbers within the range are also applicable. The interlayer spacing of the graphene oxide confined mass transfer channel is set to 0.8-1.2 nm. For example, it can be 0.8 nm, 0.9 nm, 1.0 nm, 1.1 nm or 1.2 nm, but not limited thereto, and other unlisted numbers within the range are also applicable. The oxidation degree of the graphene oxide confined mass transfer membrane is set to 10%-40%. For example, it can be 10%, 20%, 30% or 40%, but not limited thereto, and other unlisted numbers within the range are also applicable.

[0013] In step one, the annealing process for the water box is as follows: under NVT ensemble, the initial temperature is 300 K, the final temperature is 500 K, the cycle number is 5, the structure is optimized each time, and the Smart algorithm is selected.

[0014] In step two, the force field type is Universal force field. When optimizing the structure, the algorithm is selected as Smart; the force field distribution method is automatic distribution; the charge distribution method is Use current; and the maximum iteration number is 500.

[0015] In step three, during the molecular dynamics simulation of the graphene oxide brine box, the graphene oxide skeleton is kept fixed. The ensemble is selected as NVT; the initial velocity is selected as Random; and the temperature is 300 K. The molecular dynamics simulation process includes two parts, preliminary simulation and simulation for analysis. The pressure of all simulations is applied in the direction parallel to the graphene oxide channel, and the size is 13.89 MPa / nm 2 . The preliminary simulation is simulated at a step length of 1 fs for 2000 ps, and the simulation for analysis is performed at a step length of 1 fs for 2000 ps.

[0016] As a preferred scheme of the present application, the Li+ Li 2+ The separation ratio is calculated according to formula (1):

[0017]

[0018] wherein N(Li + ) represents the number of lithium ions on the discharge side after passing through the channel. N(Mg 2+ ) represents the number of magnesium ions on the discharge side after passing through the channel.

[0019] As a preferred scheme of the present application, the Li + / Mg 2+ The mean square displacement curve is calculated according to formula (2), the Li + / Mg 2+ The diffusion coefficient is calculated by formula (3).

[0020] MSD(Δt) = <[r(t-Δt)-r(t)] 2 > formula (2)

[0021]

[0022] The standard for judging effective separation is: if the final model channel layer spacing is less than 2 nm, the Li + / Mg 2+ The separation ratio is greater than 2. The maximum value of the mean square displacement curve of Li + inside the channel is greater than that of Mg 2+ , and the diffusion coefficient of Mg 2+ is smaller than that of Li + . At the same time, the number of water molecules and ions passing through the channel is greater than that in the ordinary channel. It is proved that this model is based on the effect of limited mass transfer and the separation of Li + / Mg 2+ has excellent performance. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 It is a flowchart of the research method of the graphene oxide film mass transfer with limited mass transfer effect of the present application;

[0024] Figure 2 It is the initial molecular structure model of the graphene oxide channel with limited mass transfer effect of the present application;

[0025] Figure 3 It is the molecular structure model of the graphene oxide channel structure after optimization treatment with limited mass transfer effect of the present application;

[0026] Figure 4 It is the molecular structure model of the graphene oxide channel after kinetic simulation with limited mass transfer effect of the present application. DETAILED DESCRIPTION

[0027] The application will be further described in details below with specific examples. It should be understood that the specific examples are only used to explain and introduce the application, and cannot limit the application scope of the application. Any modification and change made to the application without departing from the purpose and scope of the application shall fall within the protection scope of the application.

[0028] Example 1

[0029] A method for predicting Li + / Mg 2+ The method for separating the performance of the confined mass transfer graphene oxide film specifically comprises the following steps:

[0030] (1) Building an initial model of graphene oxide and solution;

[0031] Building a graphene oxide structure, water molecule and ion structure model. The water molecule and ion structure model contains H2O, MgCl2 and LiCl, and the concentration is set to 0.3 mol / L. After the water box is assembled, annealing is performed. The degree of oxidation of graphene oxide is set to 10%, and the spacing between the two graphene oxide layers is set to 0.9 nm. The graphene is placed on both sides of the graphene oxide box. The annealed water box is placed on the left side of the graphene oxide box.

[0032] (2) Structure optimization of the graphene oxide salt water box;

[0033] Setting the parameters for structure optimization, when optimizing the structure model of the graphene oxide salt water box, the Geometry optimization task in the Forcite module is adopted, the Smart algorithm is adopted, Energy=0.001, Force=0.5, The calculation step is 500 steps, the force field is Universal force field, the charge is Forcefieldassigned, the charge group is automatically assigned by the force field, the electrostatic term is PPPM summation method, the accuracy is 0.001, the van der Waals term is Atom-based summation method, the truncation method is Cubic spline, and the cutoff radius is The bond width is Long range correction is used during calculation. After the calculation is completed, the graphene oxide salt water box structure data file after energy minimization processing is obtained.

[0034] (3) Dynamic calculation of the optimized molecular structure;

[0035] The parameters for setting up the dynamics calculation are optimized when the saltwater box model of graphene oxide is set up. The Dynamics task in the Forcite module is used. First, the canonical ensemble is used, the temperature is set to 300 K, the Nose temperature control method is used, the time step is 0.5 fs, the time length is 100 ps, the force field is the Universal force field, the charge is Forcefield assigned, the charge group is automatically assigned by the force field, the electrostatic term is the PPPM summation method, the accuracy is 0.001, the van der Waals term is the Atom-based summation method, the truncation method is Cubic spline, and the cutoff radius is The bond width is Long range correction is used during the calculation; then further dynamics calculation is performed under a pressure of 13.89 MPa / nm 2 , using a time step of 1 fs and a time length of 2000 ps, and other settings are the same. Finally, dynamics calculation is performed using a time step of 1 fs and a time length of 2000 ps for analysis.

[0036] (4) The model after dynamics processing is subjected to dynamics analysis.

[0037] The Analysis in the Forcite module of the Materials Studio software is used to extract the dynamics index parameters, and the concentration distribution, density, radial distribution function, mean square displacement, diffusion coefficient, number of hydrogen bonds, and state parameter information of each output frame can be obtained.

[0038] By counting the distribution of ions on both sides and the distribution of water molecules, it can be concluded that the ion separation efficiency and water flux are improved. By analyzing the mean square displacement and diffusion coefficient, it can be concluded that the diffusion speed of ions is greatly accelerated. The properties of the channel have a great influence on the passage of fluid in the channel.

[0039] Example 2

[0040] A method for predicting the performance of a confined mass transfer graphene oxide membrane for Li + / Mg 2+ separation based on molecular dynamics simulation, specifically comprising the following steps:

[0041] (1) Build an initial model of graphene oxide and solution;

[0042] The structure model of graphene oxide, water molecules and ions was built. The water molecule and ion structure model contains H2O, MgCl2 and LiCl, and the concentration is set to 0.4 mol / L. After the water box is assembled, annealing is performed. The oxidation degree of graphene oxide is set to 30%, and the spacing between the two graphene oxide layers is set to 1.0 nm. The graphene is placed on both sides of the graphene oxide box. The annealed water box is placed on the left side of the graphene oxide box.

[0043] (2) The structure of the graphene oxide salt water box is optimized;

[0044] The parameters of the structure optimization are set. When optimizing the structure model of the graphene oxide salt water box, the Geometry optimization task in the Forcite module is used, the Smart algorithm is used, Energy=0.001, Force=0.5, The calculation step is 500 steps, the force field is Universal force field, the charge is Forcefield assigned, the charge group is automatically assigned by the force field, the electrostatic term is PPPM summation method, the accuracy is 0.001, the van der Waals term is Atom-based summation method, the cutoff method is Cubic spline, and the cutoff radius is The bond width is Long range correction is used during calculation. After the calculation is completed, the structure data file of the graphene oxide salt water box after energy minimization is obtained.

[0045] (3) The optimized molecular structure is calculated by dynamics;

[0046] The parameters of the dynamics calculation are set. When optimizing the graphene oxide salt water box model, the Dynamics task in the Forcite module is used. First, the canonical ensemble is used, the temperature is set to 300K, the Nose temperature control method is used, the time step is 0.5fs, the time length is 100ps, the force field is Universal force field, the charge is Forcefield assigned, the charge group is automatically assigned by the force field, the electrostatic term is PPPM summation method, the accuracy is 0.001, the van der Waals term is Atom-based summation method, the cutoff method is Cubic spline, and the cutoff radius is The bond width is Long range correction is used during calculation. Then, the size of the graphene oxide salt water box is 13.89 MPa / nm 2Further dynamics calculation was carried out under the pressure of 1 fs time step and 2000 ps time length, and other conditions were the same. Finally, dynamics calculation was carried out under the condition of 1 fs time step and 2000 ps time length for analysis.

[0047] The parameters of dynamics were extracted by Analysis in Forcite module of Materials Studio software, and the concentration distribution, density, radial distribution function, mean square displacement, diffusion coefficient, hydrogen bond number and state parameter information of each output frame were obtained.

[0048] Through the statistics of the distribution of ions and water molecules on both sides, the ion separation efficiency and water flux were improved. Through the analysis of mean square displacement and diffusion coefficient, it was found that the diffusion speed of ions was greatly accelerated. The properties of the channel had a great influence on the passage of fluid in the channel.

[0049] Example 3

[0050] A method for predicting Li + / Mg 2+ The method for predicting the performance of a confined mass transfer graphene oxide membrane for Li

[0051] (1) Building an initial model of graphene oxide and solution;

[0052] The structure model of graphene oxide, water molecules and ions was built. The structure model of water molecules and ions contained H2O, MgCl2 and LiCl, and the concentration was set to 0.5 mol / L. After the water box was built, annealing was carried out. The degree of oxidation of graphene oxide was set to 40%, and the spacing between the two graphene oxide layers was set to 1.0 nm. The graphene was placed on both sides of the graphene oxide box. The annealed water box was placed on the left side of the graphene oxide box.

[0053] (2) Structure optimization of graphene oxide salt water box;

[0054] The parameters of structure optimization were set. When optimizing the structure model of graphene oxide salt water box, the Geometry optimization task in Forcite module was adopted, the Smart algorithm was adopted, Energy=0.001, Force=0.5, The calculation step is 500 steps, the force field is Universal force field, the charge is Forcefield assigned, the charge group is automatically assigned by the force field, the electrostatic term is PPPM summation method, the precision is 0.001, the van der Waals term is Atom-based summation method, the truncation method is Cubic spline, and the cutoff radius is The bond width is Long range correction is used in the calculation process. After the calculation is completed, the data file of the graphene oxide salt water box structure after energy minimization processing is obtained.

[0055] (3) The optimized molecular structure is subjected to dynamic calculation;

[0056] The parameters of the dynamic calculation are set, and the Dynamics task in the Forcite module is used to optimize the graphene oxide salt water box model. First, the canonical ensemble is used, the temperature is set to 300K, the Nose temperature control method is used, the time step is 0.5fs, the time length is 100ps, the force field is Universal force field, the charge is Forcefield assigned, the charge group is automatically assigned by the force field, the electrostatic term is PPPM summation method, the precision is 0.001, the van der Waals term is Atom-based summation method, the truncation method is Cubic spline, and the cutoff radius is The bond width is Long range correction is used in the calculation process. Then, under the pressure of 13.89MPa / nm 2 , further dynamic calculation is carried out using time step of 1fs, time length of 2000ps, and other settings being the same. Finally, dynamic calculation is carried out using time step of 1fs, time length of 2000ps for analysis.

[0057] (4) The model after dynamic processing is subjected to dynamic analysis.

[0058] The dynamic index parameters can be obtained by using the Analysis in the Forcite module of the Materials Studio software, including concentration distribution, density, radial distribution function, mean square displacement, diffusion coefficient, number of hydrogen bonds, and output frame state parameter information.

[0059] By counting the distribution of ions and water molecules on both sides, it can be concluded that the ion separation efficiency and water flux are improved. By analyzing the mean square displacement and diffusion coefficient, it can be concluded that the diffusion speed of ions is greatly accelerated. The properties of the channel have a great influence on the passage of fluid in the channel.

Claims

1. A method of predicting Li + / Mg 2+ oxide graphene membranes for Li The method is as follows: S1: Set up the brine box containing water molecules and Li + and Mg 2+ according to the actual density of the brine, assemble the graphene oxide confined mass transfer model after annealing the brine box, place the graphene on both sides of the graphene oxide box, and place the annealed water box on the left feeding side of the graphene oxide box; the density of the brine box is 0.3-0.7 mol / L; the interlayer spacing of the graphene oxide confined mass transfer model is 0.8-1.2 nm, and the oxidation degree is 10%-40%; after selecting the force field for the constructed model, perform structure optimization; S2: Perform molecular dynamics calculation on the structure-optimized graphene oxide brine box; adopt non-equilibrium molecular dynamics simulation, and the specific process is to apply external force to the water box on one side of the graphene oxide channel in the direction parallel to the graphene oxide channel. Under the driving of pressure, the water molecules, magnesium ions and lithium ions in the water box on one side pass through the membrane; after the calculation is completed, the model is analyzed, and the number of water molecules and Li + / Mg 2+ Calculate the separation ratio and water molecule flux through the number of channels.

2. The method of claim 1, wherein, In S1, the water box annealing optimization process comprises: under an NVT ensemble, an initial temperature is T1, a final temperature is T2, a cycle number is N, a structure optimization is performed each cycle, and a Smart algorithm is selected; T1 is set to 200K-400K, T2 is set to 400K-600K; and N is set to 3-10.

3. The method of claim 1, wherein, In S1, the force field type is a Universal force field.

4. The method of claim 1, wherein, In S1, the structure optimization process of the constructed model comprises: the algorithm is selected as Smart; the calculation accuracy is selected as Medium; the force field distribution method is automatic distribution; the charge distribution method is usecurrent; and the maximum iteration number is 500.

5. The method of claim 1, wherein, In S2, the molecular dynamics calculation is simulated using Materials Studio software, and in the simulation process, the graphene skeleton is kept fixed; the ensemble is selected as NVT; the initial velocity is selected as Random; the temperature is selected as 200-400k; and the temperature control method is selected as Nose.

6. The method of claim 1, wherein, The specific process of the molecular dynamics simulation in S2 comprises: A preliminary simulation is performed at a 1fs step for 2000-2500ps, and the system energy and temperature are recorded; The molecular dynamics simulation process is further performed at a 1fs step for 2000-2500ps for subsequent analysis.

7. The method of claim 1, wherein, The Li + / Mg 2+ The separation ratio is calculated according to equation (1): where N(Li + ) represents the number of lithium ions on the discharge side after passing through the channel, and N(Mg 2+ ) represents the number of magnesium ions on the discharge side after passing through the channel.

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