A method for identifying ion transport modes in gel polymer electrolytes based on molecular simulation

Abstract

The present invention provides an algorithm for identifying the ion transport mode of gel polymer electrolytes based on molecular simulation, which realizes the identification of ion transport mode by extracting the numbers of atoms and molecules around cations and using Prs values ​​for quantitative description and dynamic change analysis. The implementation of the present invention helps to reveal the conductive mechanism of gel polymer electrolytes at the molecular level, which can effectively complement the current experimental theoretical research and provide a clear direction for the regulation of the conductivity of gel polymer electrolytes.

Description

Technical Field

[0001] The invention relates to the technical field of molecular simulation, and in particular to an algorithm for identifying ion transport modes of gel polymer electrolytes based on molecular simulation. Background Art

[0002] With the rise of new energy vehicles, the market demand for high-performance batteries is gradually increasing. As a key component affecting battery performance, electrolyte plays an important role in the development of batteries. Due to the volatility and leakage of solvents, traditional liquid electrolytes have the risk of being flammable and explosive. On the contrary, thanks to the affinity of polymer molecules to solvent molecules, gel polymer electrolytes have the characteristics of being difficult to leak. Therefore, gel polymer electrolytes are expected to replace traditional liquid electrolytes to become the next generation of green electrolytes and make important contributions to the development of green batteries.

[0003] However, due to the limited understanding of the conductive mechanism of gel polymers, the current technical direction for improving conductivity is relatively vague, and the conductivity of gel polymer electrolytes is still at a low level, which limits the application and development of gel polymer electrolytes. At the microscopic level, the conductivity of the electrolyte is determined by the ion transport mode. Different ion transport modes face different environmental resistances, so different transport modes themselves have different movement speeds. For example, ions usually move faster in a solution environment than in a solid environment. This is because ions usually carry the surrounding polar molecules with them, while the solid environment itself has poor mobility and stronger structural resistance. In general, the study of ion transport modes helps to deeply understand the conductive mechanism of gel polymer electrolytes and provide a clear direction for regulating the conductivity of gel polymer electrolytes.

[0004] At present, the ion transport mode in gel polymer electrolytes is still unclear. Experimental data speculate that due to the presence of solvent molecules, the ion transport mode in gel polymer electrolytes may be similar to that in traditional liquid electrolytes. Experimental data also speculate that due to the presence of polymer molecules, the ion transport mode in gel polymer electrolytes may be similar to that in all-solid polymer electrolytes. Limited by the research scale of macroscopic experiments, neither of these two viewpoints has been widely and directly verified and studied. Although experimental techniques have been continuously developed and improved in recent years, providing an unprecedented perspective for the microstructural characterization of gel polymer electrolytes, the dynamic ion transport process cannot be directly studied, and a new technical method is urgently needed. Summary of the invention

[0005] In view of this, the present invention aims to propose an algorithm for identifying the ion transport mode of gel polymer electrolytes based on molecular simulation. Molecular dynamics is first used to simulate the structural characteristics and ion dynamic evolution process of gel polymer electrolytes, and then the dynamic change process of particles is described by calculating the interaction between molecular atoms. The molecular dynamics trajectory is read and analyzed to study various transport modes.

[0006] An algorithm for identifying ion transport modes in gel polymer electrolytes based on molecular simulation, the algorithm comprising the following steps:

[0007] 1) Model construction

[0008] The gel polymer electrolyte model was constructed using GROMACS software, which included solvent molecules, polymers and conductive salts.

[0009] 2) Output of trajectory file

[0010] Place the well-balanced gel polymer electrolyte model in an external electric field environment for non-equilibrium simulation, and output a structure file that records the position information of each atom;

[0011] 3) Identification of transmission method

[0012] First, all atoms and molecules are numbered consecutively, and then the total serial number of atoms and molecules in the range of 0.3-0.4nm around the cation in each frame is analyzed and obtained. The changes of atoms and molecules around the cation in the two frames before and after are compared, including the polymer state around the cation, the molecular serial number around the cation, and the atomic serial number around the cation, and then the ion transmission mode is determined.

[0013] Furthermore, the ion transport modes include solid polymer electrolyte properties, liquid electrolyte properties and transition properties.

[0014] Further, the specific judgment method of step 3) is:

[0015] First, the transport properties are determined based on the state of the polymer around the cation. If there is polymer around the cation in both frames, it is a solid polymer electrolyte property. If there is no polymer around the cation in both frames, it is a liquid electrolyte property. If there is polymer around only one of the two frames, it is a transition property.

[0016] Then determine the changes in atoms and molecules, and by comparing the molecular serial numbers around the cation and the atomic serial numbers around the cation, further determine whether the surrounding polymer molecules, polymer atoms, and solvent molecules have changed, thereby determining the specific transmission method.

[0017] Furthermore, the specific method for determining the transmission attribute is:

[0018] When there are polymer molecules around the cation, let P = 1, otherwise P = 0;

[0019] when , indicating that there are polymers in both the front and back frames, and the cation transport mode shows the properties of solid polymer electrolyte;

[0020] when , indicating that there is no polymer in the two frames before and after, and the cation transport mode shows the properties of liquid electrolyte;

[0021] when , indicating that there is polymer in one of the two frames, and the cation transport mode shows transition properties.

[0022] Furthermore, under the properties of solid polymer electrolyte, the molecular number of the polymer around the cation is recorded as P_resid. If the molecular number of the polymer around the cation is the same before and after, then ,on the contrary, ; The atomic serial number of the polymer around the cation is recorded as P_serial. If the atomic serial number of the polymer around the cation is the same before and after, then ,on the contrary, ;

[0023] Under the liquid electrolyte properties, the molecular number of the solvent around the cation is recorded as S_resid. If the molecular number of the solvent around the cation is the same before and after, then ,on the contrary, ; There are solvent molecules around the cation, then ,on the contrary, ; S_resid is used to determine whether the cation carries the surrounding solvent molecules with it.

[0024] Further, when hour:

[0025] , , is the chain transmission mode; in this mode, the cation moves along the polymer chain, so the cation is near the polymer molecule at all times, P2=P1=1, P_resid is the same at all times, but P_serial is constantly changing;

[0026] , , which is the cooperative transport mode; in this mode, the cation moves with the polymer chain, P2=P1=1, P_resid is the same before and after, and P_serial is also unchanged;

[0027] , , which is the bridging transport mode; in this mode, the cation moves by adsorbing additional polymer chains or desorbing part of the polymer chains, P2=P1=1, the P_resid before and after is different, but there is an overlap in P_resid;

[0028] , , which is a jumping transport mode; in this mode, the cation jumps from one polymer chain segment to another, P2=P1=1, the P_resid before and after are different, and there is no overlapping part in P_resid.

[0029] Further, when hour:

[0030] , , which is the structural transfer mode; in this mode, the cation does not carry any surrounding solvent molecules to move together, so there is no polymer chain around the cation, P2=P1=0, although there are solvent molecules around the cation, the S_resid is different and there is no overlapping part;

[0031] , , which is the medium transport mode; in this mode, the cation carries the surrounding solvent molecules to move together, so there is no polymer chain around the cation, P2=P1=0, there are solvent molecules around the cation and the solvent molecule number remains unchanged;

[0032] , , which is a mixed transport mode. In this mode, the cation only carries part of the surrounding solvent molecules to move together, so there is no polymer chain around the cation, P2=P1=0, there are solvent molecules around the cation but only part of the solvent molecule number changes.

[0033] Further, when , where P2=1, P1=0, or P2=0, P1=1, is the cross-boundary transport mode; in this mode, the cation jumps between the polymer region and the solution region, so the cation is located near the polymer molecule at one and only one of the previous and subsequent moments.

[0034] Furthermore, the polymer is polyethylene oxide PEO, the degree of polymerization is 50, and the conductive salt is lithium salt LiTFSI;

[0035] In step 2), the external electric field strength is 3 V / nm and the non-equilibrium simulation lasts for 10 ns.

[0036] Furthermore, the specific steps of step 1) are: construct the system in proportion, set the simulation parameters, including force field parameters, intermolecular interaction parameters, temperature parameters, pressure parameters, long-range force parameters and simulation time, and then perform energy minimization and equilibrium simulation in sequence to obtain a stable model.

[0037] Furthermore, the specific steps of step 1) are as follows: the force field parameters of the polymer, solvent and salt molecules are set by selecting the GAFF force field, the cutoff radius of the long-range force parameter is 1.2 nm, the temperature is set to 300K, and the pressure is controlled at about 1 bar to simulate the temperature and pressure conditions when synthesizing the gel polymer electrolyte in the experiment; after the initial gel polymer model is processed by energy minimization and NPT pre-equilibrium simulation, it is then cycled three times for 30 ns of simulated annealing to accelerate molecular mixing and 10 ns of NPT simulation to release the pressure within the configuration, followed by a 50 ns equilibrium simulation under the NPT ensemble to finally obtain a stable gel polymer electrolyte configuration.

[0038] The algorithm was finally implemented using the programming language C++ and converted into a computer executable program.

[0039] Compared with the prior art, the algorithm for identifying the ion transport mode of gel polymer electrolyte based on molecular simulation described in the present invention has the following advantages:

[0040] The algorithm for identifying the ion transport mode of gel polymer electrolytes based on molecular simulation described in the present invention combines molecular dynamics simulation with algorithm analysis, converting the coordinate data output by molecular dynamics simulation, which is not conducive to direct observation of the transport process by naked eyes, into data that can be read and analyzed, providing an effective means for further exploring the types and performance of ion transport modes in gel polymer electrolytes. The implementation of this algorithm helps to reveal the conductive mechanism of gel polymer electrolytes at the molecular level, which can effectively complement the current experimental theoretical research and provide a clear direction for the regulation of the conductivity of gel polymer electrolytes. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0042] Figure 1 for the ion transport modes in different electrolyte systems;

[0043] Figure 2Solvent molecules selected for the embodiments of the present invention and different gel polymer electrolyte models constructed; a represents different solvent molecules, b represents a gel polymer electrolyte model solvated by DMC, c represents a gel polymer electrolyte model solvated by CYH, d represents a gel polymer electrolyte model solvated by DMA, e represents a gel polymer electrolyte model solvated by PC, f represents a gel polymer electrolyte model solvated by EC, and g represents a gel polymer electrolyte model solvated by VC;

[0044] Figure 3 To identify the ion transport mode in gel polymer electrolytes;

[0045] Figure 4 is the conversion algorithm of ion transport mode in gel polymer electrolyte; where P_resid refers to the polymer molecular serial number around the cation, P_serial refers to the polymer atomic serial number around the cation, and S_resid refers to the solvent molecular serial number around the cation;

[0046] Figure 5 This is the current contribution diagram of each ion transport mode in different gel polymer electrolytes. DETAILED DESCRIPTION

[0047] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0048] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] All molecular dynamics simulations involved in the present invention were completed using the open source simulation software GROMACS 2019.6. The simulation construction was completed using the GROMACS built-in commands, and the simulation was visualized and analyzed using VMD 1.9.3 software.

[0050] Gel polymer electrolytes (GPEs) are mixtures of polymers, solvent molecules, and salts. They can be considered as electrolytes composed of solid polymer electrolytes (SPEs) and liquid lithium ion electrolytes (LIBs). Due to the similarity of the components, the ion transport mode in GPEs may have the same ion transport mode in these two electrolytes. In LIBs, there are three transport modes for cations (e.g. Figure 1As shown in a), they are medium transmission, structural transmission, and mixed transmission; medium transmission means that cations move with all the surrounding solvent molecules; structural transmission means that cations move by detaching from the surrounding old solvent molecules and combining with new solvent molecules at the next moment; and mixed transmission means that cations move with some of the surrounding old solvent molecules, which is a combination of medium transmission and structural transmission. In SPEs, cations also have three transmission modes (such as Figure 1 (b) are chain transport, cooperative transport, and hopping transport; chain transport means that the cations move along a polymer chain; cooperative transport means that the cations are adsorbed on the polymer chain and move with the polymer chain segments without relative motion; and hopping transport means that the cations move by jumping from one polymer chain segment to another.

[0051] Based on this, eight cation transport modes in GPEs can be reasonably speculated (e.g. Figure 1 As shown in c), they are ① along-chain transmission, ② cooperative transmission, ③ bridging transmission, ④ jumping transmission, ⑤ structural transmission, ⑥ medium transmission, ⑦ mixed transmission, and ⑧ cross-boundary transmission. Among them, ①, ②, and ④ transmission modes correspond to the ion transmission modes in SPEs, ③ transmission mode means that cations move by adsorbing additional polymer chains or desorbing part of the polymer chains, ⑤-⑦ transmission modes correspond to the transmission modes in LIBs, and ⑧ transmission mode is a new transmission mode, which means that cations jump directly from the polymer region to the liquid phase region or from the liquid phase region to the polymer region.

[0052] Example 1 Construction of gel polymer electrolyte model

[0053] The solvent molecules are selected from a series of common ones in traditional liquid batteries, such as dimethyl carbonate (DMC), ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMA), ethylene carbonate (VC) and hexylene carbonate (CYH). The structural formulas are as follows: Figure 2 As shown in a; the polymer is polyethylene oxide PEO, and the conductive salt is lithium bis(trifluoromethylsulfonyl)imide LiTFSI.

[0054] Six groups of initial models were constructed using six solvent molecules, DMC, CYH, DMA, PC, EC, VC, polymers, and conductive salts. The corresponding model sizes were 6.46×6.46×6.46 nm. 3 、6.71×6.71×6.71 nm 3 、6.58×6.58×6.58 nm 3 、6.49×6.49×6.49 nm 3 、6.30×6.30×6.30 nm3 and 6.26×6.26×6.26 nm 3 , the number of solvent molecules is 800, the degree of polymerization of the polymer chain is 50, the number of polymer molecules is 50, and the number of LITFSI salt molecules is 30.

[0055] The force field parameters of polymers, solvent molecules and salt molecules are set using the GAFF force field. The cutoff radius of the long-range force parameter is 1.2 nm, the temperature is set to 300 K, and the pressure is controlled at about 1 bar to simulate the temperature and pressure conditions when synthesizing gel polymer electrolytes in experiments. After the initial gel polymer model is processed by energy minimization and NPT pre-equilibrium simulation, three cycles of 30ns simulated annealing are performed to accelerate molecular mixing and 10ns NPT simulation to release the pressure in the configuration, followed by a 50ns equilibrium simulation under the NPT ensemble to finally obtain a stable gel polymer electrolyte configuration. The constructed model is as follows Figure 2 b- Figure 2 As shown in g.

[0056] Finally, the balanced gel polymer electrolyte model was placed in an environment with an external electric field strength of 3 V / nm (such as Figure 2 A 10 ns non-equilibrium simulation under the NVT ensemble was performed to simulate the real working environment of the electrolyte, and a 1000-frame structure file recording the position information of each atom was output.

[0057] Example 2 Identification of transmission mode

[0058] In order for computers to understand and recognize these transmission modes, it is necessary to transform the image description into a concrete logical judgment or mathematical formula. The core idea of ​​the recognition algorithm is to correspond the changes of atoms and molecules in the previous and next frames in a certain area around the cation to a specific transmission mode, such as Figure 3 As shown, in this embodiment, the numbers of atoms and molecules within 0.3 nm around the cation are selected for identification. The specific implementation steps are as follows:

[0059] First, in order to allow the computer to intuitively describe the atoms and molecules around the cation, all atoms and molecules are serially numbered. Therefore, the computer can describe the information of the particles around a certain cation by outputting the serial numbers of all particles around the cation. Then, three independent physical indicators are extracted from the initial serial number list to reduce the information dimension and confusion. They are the polymer state P around the cation, the molecular serial number resid around the cation (which can be abbreviated as r, such as Figure 3 As shown in the figure) and the serial number of atoms around the cation (can be abbreviated as s, such as Figure 3As shown). When there are polymer molecules around the cation, P=1, otherwise P=0. Therefore, the computer can describe the particle information around a certain cation in a certain frame equivalently and concisely by comparing the Prs values ​​of every two adjacent frames within 1000 frames. At the same time, the change of Prs completely represents the change of atoms and molecules, which can be converted into the corresponding transmission mode. The specific conversion algorithm is Figure 4 shown.

[0060] ① Along-chain transmission mode

[0061] At this time, the cation moves along a polymer chain, so the cation is always near a polymer molecule, then P2=P1=1, satisfying the following relationship:

[0062] ;

[0063] And the polymer molecular number (polymer resid, denoted as P_resid) is the same before and after, so,

[0064] { ;

[0065] However, the polymer serial number (P_serial) around the cation is constantly changing.

[0066] .

[0067] ②Cooperative transmission mode

[0068] At this time, the cation moves with a polymer chain, then P2=P1=1, satisfying the following relationship:

[0069] ;

[0070] And the molecular numbers of the polymers are the same before and after, so

[0071] { ;

[0072] At the same time, the polymer atomic number around the cation remains unchanged, so

[0073] .

[0074] ③Bridge transmission model

[0075] At this time, the cation moves between multiple polymer chains, moving by adsorbing additional polymer chains or desorbing some polymer chains, then P2=P1=1, satisfying the following relationship:

[0076] ;

[0077] And the molecular numbers of polymers are different before and after, so

[0078] { ;

[0079] .

[0080] ④ Jump transmission mode

[0081] At this time, the cation jumps from one polymer chain segment to another, then P2=P1=1, satisfying the following relationship:

[0082] ;

[0083] The molecular numbers of the polymers before and after are completely different, so

[0084] { ;

[0085] .

[0086] ⑤Structural transmission mode

[0087] At this time, the cation does not carry any solvent molecules around it to move together, so there is no polymer chain around the cation, then P2=P1=0, satisfying the following relationship:

[0088] ;

[0089] Although there are solvent molecules around the cation, the solvent molecule number (solvent resid, denoted as S_resid) is completely different, so

[0090] ;

[0091] .

[0092] ⑥Media transmission mode

[0093] At this time, the cation carries the surrounding solvent molecules to move together, so there is no polymer chain around the cation, then P2=P1=0, satisfying the following relationship:

[0094] ;

[0095] There are solvent molecules around the cation and the sequence of the solvent molecules remains unchanged, so

[0096] ;

[0097] .

[0098] ⑦ Hybrid transmission mode

[0099] At this time, the cation only carries part of the surrounding solvent molecules to move together, so there is no polymer chain around the cation, then P2=P1=0, satisfying the following relationship:

[0100] ;

[0101] There are solvent molecules around the cation, but only part of the solvent molecule number changes, so

[0102] ;

[0103] .

[0104] ⑧Cross-border transmission mode

[0105] At this time, the cation jumps between the polymer region and the solution region. Therefore, there is only one moment before and after the cation that it is near the polymer molecule, then P2=1, P1=0, or P2=0, P1=1, and the following relationship is satisfied:

[0106] .

[0107] The entire recognition algorithm can ultimately be implemented using the programming language C++ and converted into a computer executable program.

[0108] In order to verify the transport performance, this example uses current to simultaneously describe the existence and transport performance of each transport mode in the electrolyte. The larger the current, the stronger the existence of the transport mode and the stronger the transport performance, while the smaller the current, the weaker the existence of the transport mode and the weaker the transport performance.

[0109] The following formula describes how the current is calculated:

[0110] ;

[0111] in, is the cation current, C N is the total number of cations, is the charge value of the cation, is the displacement along the direction of the electric field, is the length of the model box along the direction of the electric field, Represents a period of time, select = 10ns. Due to the heavy mass of anions, the anion current is not as significant as the cation current. Therefore, in order to simplify the study, the anion current can be ignored. All currents in this embodiment represent the cation current.

[0112] Through the analysis and calculation of ion transport modes in the above six gel polymer electrolyte models, the final results are as follows Figure 5 As shown. The current calculation results show that there are 7 transmission modes in these 6 gel polymer electrolytes, and only the structural transmission mode does not exist. It is worth noting that the currents contributed by these 7 transmission modes are either strong or weak. Among them, there are four transmission modes with generally excellent performance, namely, along-chain transmission, medium transmission, mixed transmission, and cross-boundary transmission, while the currents contributed by the remaining transmission modes are relatively weak and can be ignored.

[0113] The algorithm described in the present invention converts the coordinate data output by molecular dynamics simulation, which is not conducive to direct observation of the transmission process by naked eyes, into data that can be read and analyzed, providing an effective means for further exploring the types and performance of ion transmission modes in gel polymer electrolytes.

[0114] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for identifying ion transport modes of gel polymer electrolytes based on molecular simulation, characterized in that: The method comprises the following steps: 1) Model construction The gel polymer electrolyte model was constructed using GROMACS software, which included solvent molecules, polymers and conductive salts. 2) Output of trajectory file Place the well-balanced gel polymer electrolyte model in an external electric field environment for non-equilibrium simulation, and output a structure file that records the position information of each atom; 3) Identification of transmission method First, all atoms and molecules are numbered consecutively, and then the total serial number of atoms and molecules within 0.3-0.4nm around the cation in each frame is analyzed and obtained. The changes of atoms and molecules around the cation in the previous and next two frames are compared, including the polymer state around the cation, the molecular serial number around the cation, and the atomic serial number around the cation, and then the ion transmission mode is determined; The specific judgment method is: When there are polymer molecules around the cation, let P = 1, otherwise P = 0; when When , it means that there are polymers in both the previous and next frames, then the cation transport mode is characterized by the properties of a solid polymer electrolyte, and the molecular number of the polymer around the cation is recorded as P_resid. If the molecular number of the polymer around the cation is the same at the previous and next moments, then ,on the contrary, ; The atomic serial number of the polymer around the cation is recorded as P_serial. If the atomic serial number of the polymer around the cation is the same before and after, then ,on the contrary, ; , , is the transmission mode along the chain; , , is the cooperative transmission mode; , , is the bridge transmission mode; , , is the jump transmission mode; when When , it means that there is no polymer in the previous and next frames, then the cation transport mode shows the properties of liquid electrolyte, and the number of solvent molecules around the cation is recorded as S_resid. If the number of solvent molecules around the cation is the same before and after, then ,on the contrary, ; There are solvent molecules around the cation, then ,on the contrary, ; Use S_resid to determine whether the cations carry the surrounding solvent molecules with them; , , is the structural transmission mode; , , is the medium transmission mode; , , is a hybrid transmission mode; when , indicating that there is polymer in one of the two frames before and after, then the cation transport mode shows transition properties, where P2=1, P1=0, or P2=0, P1=1, which is a cross-boundary transport mode.

2. The method for identifying the ion transport mode of gel polymer electrolyte based on molecular simulation according to claim 1, characterized in that: The polymer is polyethylene oxide PEO, and the conductive salt is lithium salt LiTFSI; In step 2), the external electric field strength is 3 V / nm and the non-equilibrium simulation lasts for 10 ns.

3. The method for identifying the ion transport mode of gel polymer electrolyte based on molecular simulation according to claim 1, characterized in that: The specific steps of step 1) are: construct the system in proportion, set the simulation parameters, including force field parameters, intermolecular interaction parameters, temperature parameters, pressure parameters, long-range force parameters and simulation time, and then perform energy minimization and equilibrium simulation in sequence to obtain a stable model.

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