Theoretical calculation method for screening solvated ionic liquids matching solid electrolytes

Abstract

The method for theoretical calculation screening of solvated ionic liquids matching solid electrolytes belongs to the field of solid-state lithium battery technology, and specifically includes the following steps: Step 1, using theoretical calculation software to establish the initial structures of solvated ionic liquid organic ligands and lithium salts respectively; Step 2, constructing an amorphous unit cell, relaxing the unit cell, performing molecular dynamics simulation, and linear fitting Li + The mean square displacement of Li + Diffusion coefficients of Li2O3 solvated ionic liquids were calculated. + Conductivity, screening out Li + Several kinds of solvated ionic liquids with high conductivity; Step 3, for high Li + Several solvated ionic liquids with different conductivity and a reference material, LiPF6, are used to establish the initial structures of the reactants and products of their thermal decomposition reactions. In step 4, the single point energy of each substance is calculated, and the heat of reaction of the thermal decomposition reaction is calculated to determine the thermal stability of the several solvated ionic liquids. This method improves the efficiency of SIL screening and saves manpower, financial resources, and time costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of solid-state lithium batteries, and in particular relates to a method for screening a solvated ionic liquid matching a solid electrolyte through theoretical calculation. Background Art

[0002] For lithium batteries, the safety performance and energy density of traditional liquid organic electrolyte batteries need to be further improved. Due to the decomposition of LiPF6, organic liquid electrolytes begin to show significant mass loss at about 70°C. Solid electrolytes have the advantages of being non-flammable, non-volatile, and leak-free, which significantly improves the safety performance of lithium batteries; its electrochemical stability window is larger and can match high-voltage positive electrode materials. In addition, metallic lithium can be used as the negative electrode, thereby increasing the energy density of the battery. Among solid electrolytes, oxide electrolytes Li 1.5 Al 0.5 Ge 1.5 (PO4)3(LAGP) has Li + The advantages of high conductivity and good chemical stability. However, compared with the solid-liquid interface of electrode / liquid electrolyte, the solid-solid interface of electrode / LAGP electrolyte formed in solid-state lithium batteries has poor contact performance, which cannot fully exert the excellent performance of oxide electrolytes. This is manifested in an increase in the electrochemical impedance of the battery, and the capacity and cycle life are significantly lower than those of liquid electrolyte lithium batteries.

[0003] Solvate ionic liquid (SIL) is a compound obtained by mixing lithium salt and organic solvent in a ratio greater than or equal to equimolar ratio. It is liquid at room temperature and has similar properties to ionic liquids. Compared with ionic liquids, it has the advantages of simple preparation process, Li + High conductivity, Li + If a small amount of SIL is added to the LAGP electrolyte, the wettability of the solid electrolyte can be appropriately improved while maintaining the solid form of the electrolyte. In this way, the Li + At the same time, it increases the contact area between the electrode and the electrolyte, promotes the battery's active reaction, and improves the battery's capacity and cycle life.

[0004] SIL is a complex formed by lithium salt and ether solvent through chelation, and its combination is diverse. The SIL that can better match LAGP should have Li + In order to find such a SIL with high conductivity and strong thermal stability, a large number of experimental tests are required according to traditional thinking, which brings a huge workload to construct a SIL-LAGP with excellent performance, and increases manpower, financial resources and time costs. Summary of the Invention

[0005] The present invention calculates the Li + Conductivity, calculate the thermal decomposition reaction heat according to the structure of the substances involved in the thermal decomposition reaction of the electrolyte, and then screen out Li + This theoretical calculation method has good reliability and significantly improves the efficiency of SIL screening, avoids drug waste, and saves experimental time.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for theoretical calculation and screening of a solvated ionic liquid matching a solid electrolyte, characterized by comprising the following steps:

[0008] Step 1: Using theoretical calculation software, establish the initial structures of the organic ligands and lithium salts for a series of solvated ionic liquids, and perform geometry optimization;

[0009] Step 2: Construct an amorphous unit cell, perform relaxation and molecular dynamics simulation on the unit cell, and linearly fit Li + The mean square displacement of Li + The diffusion coefficient of Li in a series of solvated ionic liquids was calculated using the Nernst-Einstein equation. + Conductivity, screening out Li + Several solvated ionic liquids with high electrical conductivity;

[0010] Step 3: For high Li + Several solvated ionic liquids with high conductivity and the reference material LiPF6 were used to establish the initial structures of the reactants and products of their thermal decomposition reactions and perform geometric optimization.

[0011] Step 4: Calculate the single point energy of each substance, and then calculate the reaction heat of the thermal decomposition reaction to determine the thermal stability of the several solvated ionic liquids;

[0012] Step 5: Based on the above theoretical calculation results, select Li + Solvated ionic liquids with high conductivity and strong thermal stability.

[0013] Furthermore, the specific process of step 1 is: using theoretical calculation software Materials Studio, the organic ligand, lithium salt anion and lithium salt cation of each solvated ionic liquid are modeled separately, wherein the organic ligand includes glycol dimethyl ether (CH3O(CH2CH2O) nCH3, i.e. one of the glyme) and crown ether organic solvents, the lithium salt includes one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), LiBF4, lithium trifluoromethanesulfonate (LiOTf), LiClO4, lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), lithium trifluoroacetate (LiTFA), and LiNO3. The Forcite module is used to perform geometry optimization of the organic ligand, lithium salt anion, and lithium salt cation, respectively.

[0014] Furthermore, the specific process of step 2 is as follows: using the AmorphousCell module, an amorphous cell composed of organic ligands, lithium salt anions and lithium salt cations is constructed, wherein the number of organic ligands, lithium salt anions and lithium salt cations is the same, which is 10 to 20; using the Forcite module, the cell is relaxed by minimizing the cell energy, annealing and 50 to 80 ps short-time molecular dynamics simulation, and a 400 to 800 ps long-time molecular dynamics simulation is performed in the NVE ensemble at a temperature of 293 to 308 K; by linear fitting Li + The mean square displacement of Li + The diffusion coefficient of Li is calculated by the Nernst-Einstein equation. + Conductivity, comparison of Li2O3 solvated ionic liquids + Conductivity, screen out several solvated ionic liquids with higher conductivity.

[0015] Furthermore, the specific process of step three is as follows: the first thermal decomposition reaction in the solvated ionic liquid is the evaporation of the organic ligand, and the first thermal decomposition reaction in the organic electrolyte is the decomposition of LiPF6. The solvated ionic liquid matching the solid electrolyte should have stronger thermal stability than the organic electrolyte, so in this step, the LiPF6 in the organic electrolyte is used as a reference for thermal stability calculation. + The reactants and products involved in thermal decomposition were modeled within the range of several solvated ionic liquids with high conductivity and the reference material LiPF6. 3 The module performs geometric optimization on these structures, where the maximum number of iterations is 300-600, the maximum number of self-consistent field (SCF) cycles is 300-600, and the tail value is 0.005-0.01Ha.

[0016] Furthermore, the specific process of step 4 is: using DMol 3Module, calculates the single point energy of each substance after geometric optimization, where the maximum self-consistent field (SCF) cycle number is 300-600 and the tail value is 0.005-0.01Ha; the reaction heat is the sum of the single point energies of each substance after thermal decomposition minus the sum of the single point energies of each substance before thermal decomposition. The reaction heat of the thermal decomposition reaction can be used to judge the thermal stability of the substance: the greater the reaction heat, the stronger the thermal stability of the reactant; the smaller the reaction heat, the weaker the thermal stability of the reactant. The reaction heats of several solvated ionic liquids and LiPF6 are compared to screen out solvated ionic liquids with higher thermal stability than LiPF6.

[0017] Further, the screening results in step 2 and step 4 are comprehensively compared and Li + SIL with high conductivity and strong thermal stability.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] Through molecular dynamics simulation and quantum chemical calculation, Li + SIL with high conductivity and strong thermal stability is used to match the solid electrolyte LAGP. + Compared with the conductivity and thermal stability test results, the theoretical calculation method of the present invention has excellent reliability. Moreover, the present invention avoids the disadvantages of traditional screening test methods such as high cost, time and labor consumption, thereby improving the efficiency of SIL screening and significantly saving manpower, financial resources and time costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The first thermal decomposition reactions of [Li(G4)][TFSI], [Li(G4)][FSI], [Li(G3)][TFSI] and LiPF6 and their reaction heats; (a) [Li(G4)][TFSI], (b) [Li(G4)][FSI], (c) [Li(G3)][TFSI], (d) LiPF6;

[0021] Figure 2 These are the theoretical calculated values ​​of the thermal decomposition reaction heat of [Li(G4)][TFSI], [Li(G4)][FSI], [Li(G3)][TFSI] and LiPF6 and the experimental test values ​​of the thermal decomposition temperature. DETAILED DESCRIPTION

[0022] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only part of the embodiments of the invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0023] Example 1

[0024] A method for screening a solvated ionic liquid matching a solid electrolyte by theoretical calculation comprises the following steps:

[0025] 1. Using the theoretical calculation software Materials Studio, tetraethylene glycol dimethyl ether (G4) as the organic ligand and LiTFSI as the lithium salt, the initial structures of these molecules and ions were established, and the Forcite module was used for geometry optimization.

[0026] 2. Using AmorphousCell module, construct a certain number of G4, Li + TFSI - The amorphous unit cell is composed of the Forcite module, and the unit cell is relaxed and simulated by molecular dynamics. After linear fitting Li + The mean square displacement of Li + The diffusion coefficient of Li is calculated by combining the Nernst-Einstein equation. + Electrical conductivity.

[0027] The specific steps are as follows:

[0028] (1) Using theoretical calculation software MaterialsStudio, the G4 molecule and Li + TFSI - The geometry of these molecules and ions were optimized using the Forcite module, and the force field type used in this module was COMPASSIII.

[0029] (2) Using AmorphousCell module to construct G4, Li + TFSI - The number of 15 and the density of 1.40gcm -3The amorphous unit cell is simulated using the Forcite module, which is used to relax the unit cell through unit cell energy minimization, annealing, and short-term molecular dynamics simulation. The annealing temperature range is 300K to 500K, and the number of cycles is 8; the ensemble of molecular dynamics simulation is NVT, the temperature is 303K, and the simulation time is 60ps. Next, a longer molecular dynamics simulation is performed with the ensemble NVE, the temperature is 303K, and the simulation time is 500ps. According to the trajectory file, Li is obtained by ForciteAnalysis. + The mean square displacement is linearly fitted to obtain Li + The diffusion coefficient of [Li(G4)][TFSI] was calculated by combining the Nernst-Einstein equation. + Conductivity is 1.61mScm -1 .

[0030] Example 2

[0031] A method for screening a solvated ionic liquid matching a solid electrolyte by theoretical calculation comprises the following steps:

[0032] 1. Using the theoretical calculation software Materials Studio, triethylene glycol dimethyl ether (G3) as the organic ligand and LiOTf as the lithium salt, the initial structures of these molecules and ions were established, and the Forcite module was used for geometry optimization.

[0033] 2. Using AmorphousCell module, construct a certain number of G3, Li + 、OTf - The amorphous unit cell is composed of the Forcite module, and the unit cell is relaxed and simulated by molecular dynamics. After linear fitting Li + The mean square displacement of Li + The diffusion coefficient of Li is calculated by combining the Nernst-Einstein equation. + Electrical conductivity.

[0034] The specific steps are as follows:

[0035] (1) Using theoretical calculation software MaterialsStudio, the G3 molecule and Li + 、OTf - The geometry of these molecules and ions were optimized using the Forcite module, and the force field type used in this module was COMPASSIII.

[0036] (2) Using AmorphousCell module to construct G3, Li + 、OTf - The number of 15 and the density of 1.30gcm-3 The amorphous unit cell is simulated using the Forcite module, which is used to relax the unit cell through unit cell energy minimization, annealing, and short-term molecular dynamics simulation. The annealing temperature range is 300K to 500K, and the number of cycles is 8; the ensemble of molecular dynamics simulation is NVT, the temperature is 303K, and the simulation time is 60ps. Next, a longer molecular dynamics simulation is performed with the ensemble NVE, the temperature is 303K, and the simulation time is 500ps. According to the trajectory file, Li is obtained by ForciteAnalysis. + The mean square displacement is linearly fitted to obtain Li + The diffusion coefficient of [Li(G3)][OTf] was calculated by combining the Nernst-Einstein equation. + Conductivity is 0.28mScm -1 .

[0037] According to the specific steps of Example 1, the Li of [Li(G4)][FSI], [Li(G4)][BF4] and [Li(G3)][TFSI] were calculated respectively. + Conductivity, among which, when the AmorphousCell module is used to construct an amorphous cell, the cell densities corresponding to these three substances are 1.32 g cm -3 、1.22gcm -3 and 1.42gcm -3 , the other calculation parameters are the same as those in Example 1.

[0038] Table 1 shows the Li of some SILs + Comparison of the theoretical calculation and experimental test values ​​of conductivity. It can be seen that the Li + The theoretical calculated values ​​and experimental values ​​of conductivity are in good agreement. + The conductivity method is reliable. By comparison, [Li(G4)][TFSI], [Li(G4)][FSI], and [Li(G3)][TFSI] have higher ionic conductivity. The subsequent theoretical calculations of thermal stability will be carried out on these three substances and the reference material LiPF6.

[0039] Table 1 Li of [Li(G4)][TFSI], [Li(G4)][FSI], [Li(G4)][BF4], [Li(G3)][TFSI] and [Li(G3)][OTf] + Theoretical calculation and experimental test values ​​of electrical conductivity

[0040]

[0041] Example 3

[0042] A method for screening a solvated ionic liquid matching a solid electrolyte by theoretical calculation comprises the following steps: based on Example 1,

[0043] 1. Establish the initial structure of the reactants and products of the thermal decomposition reaction of [Li(G4)][TFSI] and use DMol 3 Module for geometry optimization.

[0044] 2. Through DMol 3 The module calculates the single point energy of each substance involved in the thermal decomposition reaction and obtains the reaction heat of the thermal decomposition reaction of [Li(G4)][TFSI].

[0045] The specific steps are as follows:

[0046] (1) The first thermal decomposition reaction of [Li(G4)][TFSI] is:

[0047] [Li(CH3O(CH2CH2O)4CH3)][CF3SO2NSO2CF3]→Li(CF3SO2NSO2CF3)+CH3O(CH2CH2O)4CH3, the theoretical calculation software MaterialsStudio was used to establish the initial structure of the reactant [Li(G4)][TFSI] and the product Li(CF3SO2NSO2CF3) and G4 in this reaction. 3 The module performs geometry optimization on these materials, where the selected functionals are GGA and BLYP, the basis set is DND, the maximum number of iterations is 500, the maximum number of self-consistent field (SCF) cycles is 500, and the tail value is 0.005Ha.

[0048] (2) Using DMol 3 The module calculates the single-point energies of three substances: [Li(G4)][TFSI], Li(CF3SO2NSO2CF3), and G4. The functionals used are GGA and BLYP, the basis set is DND, the maximum self-consistent field (SCF) cycles are 500, and the tailing value is 0.005Ha. The heat of reaction of the thermal decomposition reaction is the sum of the single-point energies of the substances after decomposition minus the sum of the single-point energies of the substances before decomposition. Based on this, the heat of reaction of the thermal decomposition of [Li(G4)][TFSI] is calculated to be 58.17 kcalmol -1 When a SIL undergoes thermal decomposition, the energy required to break chemical bonds is the heat of reaction. Therefore, the thermal stability of the SIL can be determined based on the magnitude of the heat of reaction. The greater the heat of reaction, the greater the thermal stability of the SIL; the smaller the heat of reaction, the weaker the thermal stability of the SIL.

[0049] Example 4

[0050] A method for screening a solvated ionic liquid matching a solid electrolyte by theoretical calculation comprises the following steps:

[0051] 1. Establish the initial structure of the reactants and products of the LiPF6 thermal decomposition reaction and use DMol 3 Module for geometry optimization.

[0052] 2. Through DMol 3 The module calculates the single point energy of each substance involved in the thermal decomposition reaction and obtains the reaction heat of the LiPF6 thermal decomposition reaction.

[0053] 3. Comprehensive theoretical calculation results, select Li + SIL with high conductivity and strong thermal stability.

[0054] The specific steps are as follows:

[0055] (1) The first thermal decomposition reaction of LiPF6 is: LiPF6→LiF+PF5. The theoretical calculation software Materials Studio is used to establish the initial structure of the reactant LiPF6 and the products LiF and PF5 in this reaction. 3 The module performs geometry optimization on these materials, where the selected functionals are GGA and BLYP, the basis set is DND, the maximum number of iterations is 500, the maximum number of self-consistent field (SCF) cycles is 500, and the tail value is 0.005Ha.

[0056] (2) Using DMol 3 The module calculates the single point energy of three substances: LiPF6, LiF, and PF5. The functionals used are GGA and BLYP, the basis set is DND, the maximum self-consistent field (SCF) cycle number is 500, and the tail value is 0.005Ha. The calculated reaction heat of thermal decomposition of LiPF6 is 40.20kcalmol -1 .

[0057] (3) Compare the reaction heat of SIL and LiPF6, and select some SILs with higher thermal stability than LiPF6. + The screening results of conductivity and thermal stability show that Li + SIL with high conductivity and strong thermal stability.

[0058] According to the specific steps of Example 3, the thermal decomposition reaction heats of [Li(G4)][FSI] and [Li(G3)][TFSI] were calculated respectively. Among them, the first thermal decomposition reaction of [Li(G4)][FSI] was [Li(CH3O(CH2CH2O)4CH3)][FSO2NSO2F]→Li(FSO2NSO2F)+CH3O(CH2CH2O)4CH3, and the first thermal decomposition reaction of [Li(G3)][TFSI] was [Li(CH3O(CH2CH2O)3CH3)][CF3SO2NSO2CF3]→Li(CF3SO2NSO2CF3)+CH3O(CH2CH2O)3CH3. The other calculation parameters were the same as those in Example 3.

[0059] Figure 1 The thermal decomposition reactions and their heats of reaction are first observed for [Li(G4)][TFSI], [Li(G4)][FSI], [Li(G3)][TFSI], and LiPF6. Based on the magnitude of the heat of reaction, the order of thermal stability of these substances is as follows:

[0060] [Li(G4)][TFSI] > [Li(G4)][FSI] > [Li(G3)][TFSI] > LiPF6. It can be seen that the thermal stability of these three SILs is stronger than LiPF6. In the examples, [Li(G4)][TFSI] has the highest ionic conductivity and the strongest thermal stability. Therefore, in the examples, [Li(G4)][TFSI] is preferred as the best-performing SIL.

[0061] Figure 2 The theoretical calculated values ​​of the thermal decomposition heat of [Li(G4)][TFSI], [Li(G4)][FSI], [Li(G3)][TFSI] and LiPF6 (calculated values ​​of the present invention) and the thermal decomposition temperature (T d ) Experimental test value [1,2] , where the thermal decomposition heat and T d It can be seen that the theoretical calculated values ​​of the thermal decomposition heat of these four substances and T d The order of the experimental test values ​​is consistent, which indicates that the order of the thermal stability under theoretical calculation and experimental test is consistent. Therefore, the theoretical calculation method for the thermal stability of SIL in the present invention is reliable.

[0062] Literature [1]: K.Ueno, K.Yoshida, M.Tsuchiya, N.Tachikawa, K.Dokko, M.Watanabe. Glyme-lithium salt equimolar molten mixtures: Concentrated solutions or solvate ionic liquids? J.Phys.Chem.B,2012,116(36):11323-11331.

[0063] Literature [2]: K. Xu. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem. Rev., 2004, 104(10): 4303-4417.

[0064] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for theoretical calculation and screening of solvated ionic liquids matching solid electrolytes, characterized in that: The following steps are involved: Step 1: Using theoretical calculation software, establish the initial structures of the organic ligands and lithium salts for a series of solvated ionic liquids, and perform geometry optimization; Step 2: Construct an amorphous unit cell, perform relaxation and molecular dynamics simulation on the unit cell, and linearly fit Li + The mean square displacement of Li + The diffusion coefficient of Li in a series of solvated ionic liquids was calculated using the Nernst-Einstein equation. + Conductivity, screening out Li + Several solvated ionic liquids with high electrical conductivity; Step 3: For high Li + Several solvated ionic liquids with high conductivity and the reference material LiPF6 were used to establish the initial structures of the reactants and products of their thermal decomposition reactions and perform geometric optimization. Step 4: Calculate the single point energy of the thermal decomposition reactants and products after geometric optimization in step 3, and then calculate the reaction heat of the thermal decomposition reaction to determine the thermal stability of the several solvated ionic liquids; Step 5: Based on the above theoretical calculation results, select Li + A solvated ionic liquid with high conductivity, thermal stability higher than that of LiPF6 and strong thermal stability.

2. The method according to claim 1, wherein: The specific process of step one is: using the theoretical calculation software Materials Studio, the organic ligand, lithium salt anion and lithium salt cation of each solvated ionic liquid are modeled separately, wherein the organic ligand includes one of the dimethyl glycol ether and crown ether organic solvents, and the lithium salt includes one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, LiBF4, lithium trifluoromethanesulfonate, LiClO4, lithium bis(pentafluoroethylsulfonyl)imide, lithium trifluoroacetate, and LiNO3. The Forcite module is used to perform geometric optimization of the organic ligand, lithium salt anion and lithium salt cation respectively.

3. The method according to claim 1, wherein: The specific process of step 2 is as follows: using the AmorphousCell module, an amorphous cell composed of organic ligands, lithium salt anions and lithium salt cations is constructed, wherein the number of organic ligands, lithium salt anions and lithium salt cations is the same, which is 10 to 20; using the Forcite module, the cell is relaxed by minimizing the cell energy, annealing and 50 to 80 ps short-time molecular dynamics simulation, and a 400 to 800 ps long-time molecular dynamics simulation is performed in the NVE ensemble at a temperature of 293 to 308 K; by linear fitting Li + The mean square displacement of Li + The diffusion coefficient of Li is calculated by the Nernst-Einstein equation. + Conductivity, comparison of Li2O3 solvated ionic liquids + Conductivity, screen out several solvated ionic liquids with higher conductivity.

4. The method according to claim 1, wherein: The specific process of step three is as follows: the first thermal decomposition reaction in the solvated ionic liquid is the evaporation of the organic ligand, and the first thermal decomposition reaction in the organic electrolyte is the decomposition of LiPF6; the Li + The reactants and products involved in thermal decomposition were modeled within the range of several solvated ionic liquids with high conductivity and the reference material LiPF6. 3 The module performs geometric optimization on these structures, where the maximum number of iterations is 300~600, the maximum number of self-consistent field cycles is 300~600, and the tail value is 0.005~0.01 Ha.

5. The method according to claim 1, wherein: The specific process of step 4 is: using DMol 3 Module, calculates the single point energy of each substance after geometric optimization, where the maximum number of self-consistent field cycles is 300~600 and the tail value is 0.005~0.01 Ha; the reaction heat is the sum of the single point energies of each substance after thermal decomposition minus the sum of the single point energies of each substance before thermal decomposition. The reaction heat of the thermal decomposition reaction can be used to determine the thermal stability of the substance: the greater the reaction heat, the stronger the thermal stability of the reactant; the smaller the reaction heat, the weaker the thermal stability of the reactant. Compare the reaction heats of several solvated ionic liquids and LiPF6, and screen out solvated ionic liquids with higher thermal stability than LiPF6.

6. The method according to claim 1, wherein: Comprehensively compare the screening results in step 2 and step 4, and select Li + SIL with high conductivity and strong thermal stability.

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