Method for selecting electrolyte for secondary battery

Abstract

To provide a method for selecting an electrolyte for a secondary battery which enables selection of an ionic liquid capable of improving the Coulombic efficiency of the secondary battery without going through any actual experiment.SOLUTION: A method for selecting an electrolyte for a secondary battery has: a calculation step of calculating a maximum value of a partial charge of a cation part of an ionic liquid by performing a first structural optimization calculation using either or both of methods (1a) and (1b) and then performing a second structural optimization calculation using a method (2); and a selection step of selecting an ionic liquid having the calculated maximum value of the partial charge of +0.040 or more. (1a) After assigning a charge to each atom by a charge equilibrium method, structural optimization calculation is performed using a classical molecular dynamics method. (1b) Structural optimization calculation and vibration calculation are performed using a density functional method. (2) Structural optimization calculation, vibration calculation, and energy calculation are performed using a molecular orbital method, and mulliken charges of constituent atoms of a locally stable structure are calculated.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a method for selecting an electrolyte for a secondary battery. [Background technology]

[0002] 2. Description of the Related Art Ionic liquids have conventionally been used as electrolytes in secondary batteries such as lithium ion secondary batteries.

[0003] For example, Patent Document 1 discloses an electrolyte for an air battery, which contains an ionic liquid containing an ether group in the cation structure. Furthermore, Patent Document 2 discloses an electrolyte solution containing N,N-diethyl-N-methyl-N-(2-methoxyethoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide (DEME2TFSA), N,N-diethyl-N-methyl-N-(2-methoxydiethoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide (DEME3TFSA), N,N-diethyl-N-methyl-N-(2-methoxytriethoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide (DEME4TFSA), or a mixture thereof. Furthermore, Patent Document 3 discloses a non-aqueous electrolyte containing an ionic liquid represented by a specific general formula and which is liquid at 50° C. or lower, and an ion-conductive polymer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2013-084430 A [Patent Document 2] JP 2013-084431 A [Patent Document 3] JP 2004-111294 A Summary of the Invention [Problem to be solved by the invention]

[0005] Ionic liquids have been used as electrolytes in secondary batteries, but they can sometimes reduce the Coulombic efficiency (charge / discharge efficiency) of secondary batteries, and a method for selecting ionic liquids that contribute to improving the Coulombic efficiency has been desired.Whether an ionic liquid contributes to improving the Coulombic efficiency can be confirmed by actually fabricating a prototype battery and conducting experiments, but such experiments are time-consuming, so a method for selecting ionic liquids that can improve the Coulombic efficiency without conducting experiments has been desired.

[0006] The present disclosure has been made in consideration of the above circumstances, and aims to provide a method for selecting an electrolyte for a secondary battery, which is capable of selecting an ionic liquid that can improve the Coulombic efficiency in a secondary battery without going through actual experiments. [Means for solving the problem]

[0007] Means for solving the above problems include the following aspects. <1> a calculation step of performing a first structural optimization calculation by one or both of the following methods (1a) and (1b), and then performing a second structural optimization calculation by the following method (2) to calculate the maximum value of the partial charge of the cation portion of the ionic liquid; a selection step of selecting an ionic liquid having a calculated maximum partial charge value of +0.040 or more as an electrolyte for a secondary battery; A method for selecting an electrolyte for a secondary battery. (1a) For the cation portion of the ionic liquid, charges are assigned to each atom by the charge equilibration method, and then a structural optimization calculation is performed using classical molecular dynamics. (1b) Perform geometry optimization and vibration calculations using density functional theory. (2) Using molecular orbital methods, structure optimization calculations, vibration calculations, and energy calculations are performed to calculate the Mulliken charges of the constituent atoms of the locally stable structure. <2> The selection step is a step of selecting an ionic liquid having a calculated maximum partial charge value of +0.370 or more as an electrolyte for a secondary battery. <1> 2. A method for selecting an electrolyte for a secondary battery according to claim 1. Effect of the Invention

[0008] According to the present disclosure, there is provided a method for selecting an electrolyte solution for a secondary battery, which is capable of selecting an ionic liquid capable of improving the Coulombic efficiency in a secondary battery without conducting actual experiments. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 shows the results of calculating the maximum partial charge of the cation moiety for ionic liquid P13. [Diagram 2] FIG. 1 shows the results of calculating the maximum partial charge of the cation moiety for ionic liquid P14. [Diagram 3] FIG. 1 shows the results of calculating the maximum partial charge of the cation moiety for the PP13 ionic liquid. [Figure 4] FIG. 1 shows the results of calculating the maximum partial charge of the cation moiety for the DEME ionic liquid. [Diagram 5] FIG. 1 shows the results of calculating the maximum partial charge of the cation moiety for the EMI ionic liquid. [Figure 6] 1 is a graph summarizing the correlation between the average Coulombic efficiency and the maximum partial charge for each ionic liquid measured in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, a method for selecting an electrolyte for a secondary battery according to an embodiment of the present disclosure will be described in detail.

[0011] <How to select electrolyte for secondary batteries> A method for selecting an electrolyte for a secondary battery according to an embodiment of the present disclosure includes a calculation step of calculating a maximum value of a partial charge of a cation portion of an ionic liquid, and a selection step of selecting an ionic liquid having a calculated maximum value of partial charge equal to or greater than a predetermined value as an electrolyte for a secondary battery. In the calculation step, a first structural optimization calculation is performed by either or both of the following methods (1a) and (1b), and then a second structural optimization calculation is performed by the following method (2) to calculate the maximum value of the partial charge of the cation portion of the ionic liquid. (1a) For the cation portion of the ionic liquid, charges are assigned to each atom by the charge equilibration method, and then a structural optimization calculation is performed using classical molecular dynamics. (1b) Perform geometry optimization and vibration calculations using density functional theory. (2) Using molecular orbital methods, structure optimization calculations, vibration calculations, and energy calculations are performed to calculate the Mulliken charges of the constituent atoms of the locally stable structure. Moreover, the predetermined value in the selection process is +0.040.

[0012] The method for selecting an electrolyte for a secondary battery according to an embodiment of the present disclosure can select an ionic liquid that can improve the Coulombic efficiency of a secondary battery by going through the above steps without conducting actual experiments.

[0013] Conventionally, ionic liquids have been used as electrolytes in secondary batteries such as lithium-ion secondary batteries. However, depending on the ionic liquid used, the Coulombic efficiency (charge / discharge efficiency) of the secondary battery may be low, and a method for selecting an ionic liquid that contributes to improving the Coulombic efficiency has been desired. In addition, by actually manufacturing a prototype battery using the ionic liquid and conducting experiments to measure the Coulombic efficiency, it is possible to confirm whether the ionic liquid contributes to improving the Coulombic efficiency. However, it is very time-consuming to conduct such experiments for all ionic liquids prior to the selection of the ionic liquid, and therefore a method for selecting an ionic liquid that can improve the Coulombic efficiency of a secondary battery without going through actual experiments has been desired.

[0014] Therefore, the present inventors have investigated a method for selecting an ionic liquid that can contribute to improving the Coulombic efficiency, and have found that there is a high correlation between the maximum partial charge of the cation moiety in the ionic liquid and the average Coulombic efficiency of a secondary battery using the ionic liquid as an electrolyte.The inventors have also found that by selecting an ionic liquid whose maximum partial charge calculated by the above-mentioned method is +0.040 or more and using it as an electrolyte in a secondary battery, a high Coulombic efficiency (for example, average Coulombic efficiency: 96% or more) can be achieved.

[0015] (Calculation process: How to calculate the maximum partial charge) Next, a method for calculating the maximum value of the partial charge of the cation moiety in the ionic liquid, which is performed in the calculation step, will be described.

[0016] -First structural optimization calculation- In the calculation process, first, a rough structural optimization calculation is performed by either or both of the following methods (1a) and (1b). (1a) For the cation portion of the ionic liquid, charges are assigned to each atom by the charge equilibration method, and then a structural optimization calculation is performed using classical molecular dynamics. (1b) Perform geometry optimization and vibration calculations using density functional theory.

[0017] In the structural optimization calculation shown in (1a), charges are assigned to each atom of the cation part of the ionic liquid by the charge equilibration method (Charge(Q) Equilibration(Eq): QEq method). For calculations using the charge equilibration method, the charge assignment function of the molecular modeling simulation software Visualizer is used.

[0018] Next, in the structural optimization calculation of (1a), a structural optimization calculation is performed using the classical molecular dynamics method. For the classical molecular dynamics calculation, a molecular dynamics calculation tool Forcite (Quality: ultra-Fine, Force field: COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies), Universal, Dreiding, etc.) is used to perform structural optimization (search for a locally stable structure close to the most stable structure).

[0019] In the structure optimization calculation shown in (1b), the cation part of the ionic liquid is subjected to structure optimization calculation and vibration calculation using density functional theory (DFT). For the density functional calculation, the electronic structure program Gaussain 03 (B3L YP / 6-31G(d)) or the quantum mechanics program Dmol3 (GGA / PW91 DNP / Fine) is used to perform structure optimization calculation and vibration calculation (locally stable structure if there is no imaginary vibration).

[0020] As described above, a rough structural optimization calculation is performed by either or both of methods (1a) and (1b).

[0021] -Second structural optimization calculation- After the first structural optimization calculation, finer structural optimization calculations are performed by the following method (2). (2) Using molecular orbital methods, structure optimization calculations, vibration calculations, and energy calculations are performed to calculate the Mulliken charges of the constituent atoms of the locally stable structure.

[0022] In the structure optimization calculation shown in (2), structure optimization calculation, vibration calculation, and energy calculation are performed using the molecular orbital method (MO method). For calculation by the molecular orbital method, the electronic structure program Gaussain 03 (HG / 6-31G(d) and HF / 6-311G(d,p), MP2 / 6-311G(d,p) for energy calculation only) is used to calculate the Mulliken charge of the constituent atoms of the locally stable structure close to the most stable structure.

[0023] As described above, the maximum partial charge of the cation part of the ionic liquid is obtained by performing detailed structural optimization calculations using method (2). Note that the above calculation does not take into account the solvent effect. The maximum partial charge obtained by the above calculation is the maximum value among the atoms excluding the hydrogen atoms in the cation.

[0024] (Selection process) In the method for selecting an electrolyte for a secondary battery according to an embodiment of the present disclosure, an ionic liquid is selected depending on whether or not the maximum value of the partial charge determined in the calculation step is equal to or greater than a predetermined value. The predetermined value is +0.040. When an ionic liquid having a maximum partial charge value of +0.040 or more is used as an electrolyte in a secondary battery, it can achieve high Coulombic efficiency (for example, average Coulombic efficiency: 96% or more).

[0025] From the viewpoint of achieving a higher coulombic efficiency (for example, average coulombic efficiency: 97% or more), the predetermined value is preferably +0.370.

[0026] (Ionic Liquid) Examples of ionic liquids include N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(fluorosulfonyl)imide (DEME), 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI), and 1-ethyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide (DEMI-FSI).

[0027] <Secondary battery> Next, a secondary battery using, as an electrolyte solution, an ionic liquid selected by the method for selecting an electrolyte solution for a secondary battery according to an embodiment of the present disclosure will be described.

[0028] A secondary battery (non-aqueous secondary battery) includes, for example, a negative electrode, a positive electrode, a separator, and an electrolyte.

[0029] (electrolyte) ·solvent The electrolytic solution contains a solvent (non-aqueous solvent) and an electrolyte. As the solvent, an ionic liquid selected by the method for selecting an electrolyte for a secondary battery according to an embodiment of the present disclosure is used.

[0030] ·Electrolyte The electrolyte in the electrolytic solution may be, for example, a Li salt, such as lithium bis(fluorosulfonyl)imide (LiFSI), LiPF6 (lithium hexafluorophosphate), lithium tetrafluoroborate (LiBF4), or Li[N(CF3SO2)2]. The amount of the electrolyte may be, for example, 1.0 to 2.0 mol / L, and is preferably 1.0 to 1.5 mol / L.

[0031] The electrolytic solution may contain various additives, such as a thickener, a film-forming agent, a gas generating agent, etc., in addition to the solvent and the electrolyte. The electrolyte is typically a non-aqueous electrolytic solution that is liquid at room temperature (e.g., 25±10°C). The electrolytic solution is typically liquid in the environment in which the battery is used (e.g., a temperature environment of -20 to +60°C).

[0032] (Negative electrode) The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector. The negative electrode current collector is preferably a conductive member made of a metal (for example, copper) having good electrical conductivity. The negative electrode active material layer contains a negative electrode active material. Examples of the negative electrode active material include graphite-based carbon such as natural graphite, artificial graphite, and amorphous coated graphite. The graphite-based carbon has a graphite content of approximately 50% by mass or more, preferably 80% by mass or more. The negative electrode active material layer may be composed of only the negative electrode active material, or may contain components other than the negative electrode active material, such as a thickener or a binder, as necessary. Examples of the thickener include celluloses such as carboxymethyl cellulose (CMC). Examples of the binder include rubbers such as styrene butadiene copolymer (SBR) and vinyl halide resins such as polyvinylidene fluoride (PVdF).

[0033] (positive electrode) The positive electrode includes, for example, a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector. The positive electrode current collector is preferably a conductive member made of a metal (e.g., aluminum) having good electrical conductivity. The positive electrode active material layer includes at least a positive electrode active material and a conductive material. The positive electrode active material layer may be composed of a positive electrode active material and a conductive material, or may include other components such as a binder and various additives. Examples of the binder include vinyl halide resins such as polyvinylidene fluoride (PVdF).

[0034] The positive electrode active material may be lithium nickel cobalt manganese composite oxide (hereinafter, simply referred to as "LNCM"). The simplest LNCM has the following general formula: LiNi x Co y Mn zIt is represented by O2 (where x, y, and z satisfy 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1). LNCM may contain other additive elements in addition to Li, Ni, Co, and Mn, such as transition metal elements other than Ni, Co, and Mn, and typical metal elements other than Li. LNCM has a layered crystal structure. LNCM may exceed 50% by mass of the entire positive electrode active material, for example, and may account for 80 to 100% by mass. The positive electrode active material may be composed of only LNCM. Examples of other positive electrode active materials include lithium nickel composite oxides, lithium cobalt composite oxides, lithium nickel manganese composite oxides, and the like.

[0035] Examples of the conductive material include graphitized carbon such as non-graphitizable carbon and carbon black, and graphite.

[0036] (Separator) The separator is an electrically insulating porous membrane. The separator electrically isolates the positive electrode and the negative electrode. The separator may have a thickness of, for example, 5 to 30 μm. The separator may be composed of, for example, a porous polyethylene (PE) membrane, a porous polypropylene (PP) membrane, or the like. The separator may have a multilayer structure. For example, the separator may be composed of a porous PP membrane, a porous PE membrane, and a porous PP membrane laminated in this order. The separator may have a heat-resistant layer on its surface. The heat-resistant layer contains a heat-resistant material. Examples of the heat-resistant material include metal oxide particles such as alumina and high melting point resins such as polyimide.

[0037] (Use) Examples of the use of the secondary battery include power sources for hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), battery electric vehicles (BEV), and the like.

Examples

[0038] The following examples are shown to more specifically explain the present disclosure.

[0039] 1. Preparation of Samples The following five types of ionic liquids were prepared. ·P13 FSI: N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ·P14 FSI: N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide PP13 FSI: N-methyl-N-propylpiperidinium bis(fluorosulfonyl)imide DEME FSI: N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(fluorosulfonyl)imide ·EMI FSI: 1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide

[0040] , Lithium bis(fluorosulfonyl)imide (LiFSI) was added as a Li salt to each ionic liquid so that the molar ratio of LiFSI:ionic liquid was 1:3, and five types of sample electrolyte solutions were prepared.

[0041] 2. Dissolution and precipitation test The experimental conditions for the dissolution and precipitation test are as follows. Coin Cell Positive electrode, negative electrode: Cu (9μm, φ14), Li (250μm, φ16) Separator: GC50 (190μm, φ19) is a thick glass separator with a coarse mesh to specialize in impregnation. During precipitation: 5mA cm -2 1h When dissolved: 5mA cm -2 Cutoff at 0.5V Number of cycles: 20 Average Coulombic efficiency: Calculated as the average value for at least 5 cycles, and up to 20 cycles (before short circuit). The results are shown in Table 1.

[0042] [Table 1]

[0043] 3. Calculation of the maximum partial charge of the cation part of the ionic liquid The maximum value of the partial charge of the cation part of the ionic liquid was calculated by the method described above. In the structure optimization calculation of (2), the structure optimization condition was HF / 6-311G(d,p) and the energy calculation condition was MP2 / 6-311G(d,p). The results are shown in Table 2 and Figures 1 to 5.

[0044] [Table 2]

[0045] Furthermore, a graph summarizing the average Coulombic efficiency and maximum partial charge for each ionic liquid is shown in FIG. As shown in Figure 6, there was a very high correlation between the "average Coulombic efficiency" and the "maximum partial charge" for each ionic liquid. Therefore, the magnitude of the maximum partial charge can be an indicator of the Coulombic efficiency in dissolution-precipitation tests when the ionic liquid is used as an electrolyte.

Claims

1. a calculation step of performing a first structural optimization calculation by one or both of the following methods (1a) and (1b), and then performing a second structural optimization calculation by the following method (2) to calculate a maximum value of a partial charge of the cation portion of the ionic liquid; a selection step of selecting an ionic liquid having a calculated maximum partial charge value of +0.040 or more as an electrolyte for a secondary battery; A method for selecting an electrolyte for a secondary battery. (1a) Regarding the cation portion of the ionic liquid, a charge is assigned to each atom by the charge equilibration method, and then a structural optimization calculation is performed using classical molecular dynamics. (1b) A structure optimization calculation and a vibration calculation are performed using a density functional method. (2) Using molecular orbital methods, structure optimization calculations, vibration calculations, and energy calculations are performed to calculate the Mulliken charges of the constituent atoms of the locally stable structure.

2. 2. The method for selecting an electrolyte for a secondary battery according to claim 1, wherein the selection step is a step of selecting an ionic liquid having a calculated maximum value of partial charges of +0.370 or more as the electrolyte for the secondary battery.

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