Solid electrolyte, secondary battery and capacitor

Abstract

To provide a solid electrolyte which is excellent in ion conductivity.SOLUTION: A solid electrolyte contains a molecular crystal represented by the general formula (1) [LiaXb(NCCH2CH2CN)c]n...(1), (in the general formula (1), a represents an integer of 1 or greater, b represents an integer of 1 or greater, c represents an integer of 1 or greater, and n represents an integer of 1 or greater. However, in the general formula (1) a=b holds. X has a distance of closest approach between Li and Li in the molecular crystal of 2.00Å or more and 6.00Å or less, and is N(SO2F)2-.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte, a secondary battery, and a capacitor. [Background technology]

[0002] In recent years, attempts have been made to apply solid electrolytes to secondary batteries, etc., rather than liquid electrolytes, in order to reduce the weight of batteries and simplify their structure. The leading candidates for solid electrolytes are ceramics, glass, and polymers, but solid electrolytes containing molecular crystals, which are crystalline organic materials, are also being considered.

[0003] As a solid electrolyte containing a molecular crystal, for example, an ion-conductive solid electrolyte containing a crystal containing at least one of (A) an electron-donating organic compound containing 1 to 3 atoms selected from the group consisting of nitrogen, oxygen, phosphorus, and sulfur atoms, or (B) an anion having a cyclic structure containing at least one atom selected from the group consisting of nitrogen, phosphorus, and sulfur atoms, and at least one of a specific lithium salt and a specific sodium salt has been studied (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6150424 Summary of the Invention [Problem to be solved by the invention]

[0005] The ion-conductive solid electrolyte described in Patent Document 1 does not have sufficient ion conductivity and there is room for improvement.

[0006] The present disclosure has been made in view of the above, and has an object to provide a solid electrolyte having excellent ion conductivity, and a secondary battery and capacitor including the same. [Means for solving the problem]

[0007] The means for solving the above problems include the following aspects. <1> A solid electrolyte comprising a molecular crystal represented by the following general formula (1): [Li a X b (NCCH2CH2CN) c ] n ···(1) (In general formula (1), a represents an integer of 1 or greater, b represents an integer of 1 or greater, c represents an integer of 1 or greater, and n represents an integer of 1 or greater, provided that in general formula (1), a=b is satisfied. X represents a monovalent anion species such that the Li-Li nearest neighbor distance in the molecular crystal is 6.00 Å or less.) <2> In the molecular crystal, the nearest neighbor distance between the Li-Li atoms is 4.00 Å or more. <1> The solid electrolyte according to claim 1. <3> In the general formula (1), each X is independently N(SO2F)2 - , N(SO2CF3)2 - or SCN - is <1> or <2> The solid electrolyte according to claim 1. <4> [LiN(SO2F)2(NCCH2CH2CN)2] n , [Li2{N(SO2CF3)2}2(NCCH2CH2CN)3] n , and [Li2(SCN)2(NCCH2CH2CN)3] n A solid electrolyte comprising at least one molecular crystal selected from the group consisting of: <5> Used as a solid electrolyte in secondary batteries or capacitors <1> ~ <4> 10. The solid electrolyte according to claim 9, wherein the first and second electrodes are electrically connected to each other.

[0008] <6> <1> ~ <5> A secondary battery comprising the solid electrolyte according to any one of the above items. <7> <1> ~ <5> A capacitor comprising the solid electrolyte according to any one of the above items. [Effects of the Invention]

[0009] According to one embodiment of the present invention, it is possible to provide a solid electrolyte having excellent ion conductivity, and a secondary battery and a capacitor including the same. [Brief explanation of the drawings]

[0010] [Figure 1] 1 shows the results of measuring the ionic conductivity of the solid electrolytes of Examples 1 and 2. [Figure 2] 1 shows the results of measuring the ionic conductivity of the solid electrolyte of Example 3. [Figure 3] 1 shows the results of measuring the ionic conductivity of the solid electrolytes of Example 3, Comparative Example 1, and Comparative Example 2. [Figure 4] 1 shows the results of measuring ionic conductivity in the solid electrolyte of Comparative Example 3. [Figure 5] 1 shows the results of measuring the ionic conductivity of the solid electrolyte of Comparative Example 4. [Figure 6] 10 shows the results of measuring the ionic conductivity of the solid electrolyte of Comparative Example 5. [Figure 7] 1 shows the results of DSC for the solid electrolytes of Examples 1 and 2. [Figure 8] 1 shows the results of DSC for the solid electrolyte of Example 3. [Figure 9] 1 is a graph showing the results of measuring the transport number of the solid electrolyte of Example 1. [Figure 10] 1 is a graph showing the results of measuring the transport number of the solid electrolyte of Example 2. [Figure 11] 1 shows the measurement results of the potential window of the solid electrolyte of Example 1. [Figure 12] 1 shows the measurement results of the potential window of the solid electrolyte of Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present disclosure will be described in detail below. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. In the present disclosure, each component may contain multiple substances corresponding to the component. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified.

[0012] First Embodiment [Solid electrolyte] The solid electrolyte according to the first embodiment of the present disclosure includes a molecular crystal represented by the following general formula (1). [Li a X b (NCCH2CH2CN) c ] n ···(1) (In general formula (1), a represents an integer of 1 or greater, b represents an integer of 1 or greater, c represents an integer of 1 or greater, and n represents an integer of 1 or greater, provided that in general formula (1), a=b is satisfied. X represents a monovalent anion species such that the Li-Li nearest neighbor distance in the molecular crystal is 6.00 Å or less.) In the present disclosure, the Li-Li nearest neighbor distance in a molecular crystal is a value measured by crystal structure analysis under conditions of -50°C or -100°C.

[0013] The solid electrolyte of this embodiment has excellent ionic conductivity. The reasons for this can be explained, for example, as follows. First, according to the HSAB (Hard and Soft Acids and Bases) rule, by forming a molecular crystal that combines lithium ions, which are hard acids, with succinonitrile (NCCH2CH2CN), which is a soft base and contains a nitrile group, the interaction acting around the lithium ions is reduced. It is believed that reducing this interaction promotes ion diffusion, thereby increasing the conductivity of lithium ions. Furthermore, it is presumed that applying an anion species that results in a relatively small Li-Li nearest neighbor distance in the molecular crystal in the combination of lithium ions and succinonitrile reduces the distance between the hopping site and Li, thereby increasing the conductivity of lithium ions. Furthermore, since succinonitrile has a relatively high boiling point of 265°C, volatilization of the ligand under heating conditions can be suppressed. Therefore, the solid electrolyte of the present disclosure is expected to have a wider range of applications than conventional solid electrolytes that use molecular crystals.

[0014] The solid electrolyte of this embodiment may be used as a solid electrolyte in, for example, a secondary battery, a capacitor, or an air battery, and may be preferably used as a solid electrolyte in a secondary battery or a capacitor.

[0015] (molecular crystal) The solid electrolyte of this embodiment contains a molecular crystal represented by the general formula (1). a X b (NCCH2CH2CN) c ] are crystals formed by intermolecular interactions.

[0016] In the molecular crystal represented by the general formula (1), the nearest neighbor distance between Li-Li is preferably 2.00 Å or more, more preferably 3.00 Å or more, and even more preferably 4.00 Å or more.

[0017] In the molecular crystal represented by the general formula (1), the nearest neighbor distance between Li and Li is preferably 5.80 Å or less, more preferably 5.50 Å or less, from the viewpoint of ionic conductivity.

[0018] In the general formula (1), each X is independently N(SO2F)2 - (FSA), N(SO2CF3)2 - (TFSA) or SCN - From the viewpoint of the transference number of lithium ions, it is preferable that N(SO2F)2 - or N(SO2CF3)2 - is more preferable, and from the viewpoint of ionic conductivity, N(SO2F)2 - It is more preferable that:

[0019] In general formula (1), a is preferably 1 or 2, and b is preferably 1 or 2. In general formula (1), c is preferably 1 to 3, and more preferably 1 or 2 from the viewpoint of the ionic conductivity of the solid electrolyte.

[0020] The solid electrolyte of this embodiment may contain a molecular crystal other than the molecular crystal represented by general formula (1). From the viewpoint of ion conductivity, the content of the molecular crystal represented by general formula (1) in the solid electrolyte of this embodiment is preferably 50% by mass to 100% by mass, more preferably 70% by mass to 100% by mass, and even more preferably 90% by mass to 100% by mass, based on the total amount of the molecular crystal.

[0021] The solid electrolyte of this embodiment may contain components other than the molecular crystals, including, but not limited to, a gel electrolyte, a polymer electrolyte such as polyethylene oxide, an organic electrolyte salt such as lithium bis(trifluoromethanesulfonyl)imide, an inorganic electrolyte salt such as lithium hexafluorophosphate, a sulfide-based inorganic solid electrolyte, an oxide-based inorganic solid electrolyte, a binder, a conductive additive, a positive electrode active material, and a negative electrode active material. Preferred examples of the sulfide-based inorganic solid electrolyte and the oxide-based inorganic solid electrolyte include the lithium ion conductive sulfide-based inorganic solid electrolyte and the oxide-based inorganic solid electrolyte described in WO 2017 / 111131.

[0022] In the solid electrolyte of this embodiment, the content of the molecular crystal is, for example, preferably 15% by mass to 99% by mass, more preferably 70% by mass to 95% by mass, and even more preferably 80% by mass to 95% by mass. When the content of the molecular crystal is 15% by mass or more, the flexibility of the solid electrolyte tends to be excellent.

[0023] Second Embodiment [Solid electrolyte] The solid electrolyte of the second embodiment of the present disclosure is [LiN(SO2F)2(NCCH2CH2CN)2] n , [Li2{N(SO2CF3)2}2(NCCH2CH2CN)3] n , and [Li2(SCN)2(NCCH2CH2CN)3] n The solid electrolyte of the present embodiment contains at least one molecular crystal (hereinafter also referred to as a "specific molecular crystal") selected from the group consisting of: The solid electrolyte of the present embodiment contains the specific molecular crystal, and therefore has excellent ionic conductivity.

[0024] The secondary battery of the present disclosure includes the solid electrolyte of the present disclosure. The secondary battery of the present disclosure tends to have excellent output characteristics, low-temperature operating characteristics, etc., due to the excellent flexibility and ionic conductivity of the solid electrolyte.

[0025] The secondary battery of the present disclosure preferably includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and the above-described solid electrolyte provided between the positive electrode and the negative electrode.

[0026] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material. The positive electrode can be produced, for example, by forming a positive electrode mixture layer on a positive electrode current collector using a composition containing the positive electrode active material. The composition containing the positive electrode active material may be a mixture of the positive electrode active material and an organic binder, a solvent, a conductive additive, etc. Conventionally known materials used in producing positive electrodes can be used as the positive electrode current collector, the positive electrode active material, the organic binder, the solvent, the conductive additive, etc.

[0027] The negative electrode includes a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material. The negative electrode can be produced, for example, by forming a negative electrode mixture layer on the negative electrode current collector using a composition containing the negative electrode active material. The composition containing the negative electrode active material may be a mixture of the negative electrode active material and an organic binder, a solvent, a conductive additive, etc. Conventionally known materials used in producing negative electrodes can be used as the negative electrode current collector, the negative electrode active material, the organic binder, the solvent, the conductive additive, etc.

[0028] The capacitor of the present disclosure includes the solid electrolyte of the present disclosure. The capacitor of the present disclosure tends to have excellent output characteristics, low-temperature operating characteristics, etc., due to the excellent flexibility and ionic conductivity of the solid electrolyte.

[0029] The capacitor of the present disclosure preferably includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and the above-described solid electrolyte provided between the positive electrode and the negative electrode. The positive electrode includes a positive electrode current collector and a positive electrode active material layer, and the negative electrode includes a negative electrode current collector and a negative electrode active material layer. Examples of the positive electrode active material and the negative electrode active material in the capacitor include activated carbon.

[0030] The secondary battery or capacitor of the present disclosure may have a series stacked structure in which multiple components each including a positive electrode, a solid electrolyte, and a negative electrode are stacked. When the secondary battery or capacitor of the present disclosure has a series stacked structure, the container and the like can be simplified compared to when secondary batteries or capacitors using an electrolytic solution are arranged in a series structure, and the mass and volume of the system can be reduced.

[0031] Applications of the solid electrolyte of the present disclosure are not particularly limited, and examples thereof include notebook computers, pen-input personal computers, mobile personal computers, e-book players, mobile phones, mobile fax machines, mobile copiers, mobile printers, headphone stereos, video movie machines, liquid crystal televisions, handheld vacuum cleaners, portable CDs, minidiscs, walkie-talkies, electronic organizers, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, aircraft, automobiles, motorcycles, mopeds, bicycles, lighting equipment, toys, game machines, clocks, power tools, strobes, cameras, load-leveling power supplies, natural energy storage power supplies, and medical equipment. [Example]

[0032] The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to these examples.

[0033] [Example 1] Li(FSA) and NCCH2CH2CN(SN) were mixed in a molar ratio of 1:2 (Li(FSA):NCCH2CH2CN) under an argon atmosphere. Then, [Li(FSA)(SN)2] was added to the mixture in an oil bath. n The mixture was heated to a temperature above the melting point of [Li(FSA)(SN)2]. The mixture was then allowed to cool to room temperature, resulting in the formation of the molecular crystal [Li(FSA)(SN)2]. n A solid electrolyte containing 5.03 Å of Li-Li nearest neighbor distance was prepared.

[0034] (Measurement of ionic conductivity) The solid electrolyte obtained in Example 1 was pressure-molded into a disk-shaped measurement sample, and the ionic conductivity was measured by an AC impedance method using gold electrodes in a sealed cell. The ionic conductivity was measured while the measurement sample was heated in a temperature range below the melting point of the molecular crystal. The results are shown in Figure 1. Furthermore, the activation energy for ionic conductivity of Example 1 was determined from the Arrhenius plot to be 32.0 kJ / mol.

[0035] (Differential Scanning Calorimetry) Differential scanning calorimetry (DSC) was performed on the solid electrolyte obtained in Example 1. Specifically, a differential scanning calorimeter (DSC-60, manufactured by Shimadzu Corporation) was used and the measurement was performed at a temperature increase rate of 10°C per minute. The results are shown in Figure 7. As shown in Figure 7, a large endothermic peak was observed at 52°C, which was found to be a peak derived from molecular crystals. The endothermic peak observed at -76°C is presumably due to cracks in the molecular crystals.

[0036] (Transportation number measurement) Using the solid electrolyte obtained in Example 1, the lithium ion transport number was measured as follows. - Coin cell fabrication The obtained electrolyte was crushed in a glove box filled with argon gas, and then pressure-molded into a disk (13φ) using a hydraulic press. The sample was then sandwiched between lithium foils cut into a circle using a punch, and the sample was introduced into a sealed cell to prepare a cell. Electrolyte impedance measurement (before DC polarization) The coin-shaped cell thus prepared was kept in a thermostatic chamber at 40°C for 24 hours, and then impedance measurements were carried out at 40°C from 1 Hz to 1 MHz using an impedance analyzer VMP3 (manufactured by Biologic). DC polarization measurement of electrolytes Using an impedance analyzer VMP3 (manufactured by Biologic), direct current polarization measurements were carried out at 40° C. and 25 mV for 3000 seconds. Electrolyte impedance measurement (after DC polarization) After DC polarization, impedance measurements were carried out at 40°C from 1 Hz to 1 MHz using an impedance analyzer VMP3 (manufactured by Biologic). The transference number was calculated by substituting each measurement result into the following formula. Formula: {Post-polarization current value I s [A] × (applied voltage V (V) - current value before polarization I0 [A] × impedance before polarization (Ω0))} / {current value before polarization I0 [A] × (applied voltage V (V) - current value after polarization I s[A] x Impedance after polarization (Ω s ))}, the lithium ion transport number t Li+ [-] was calculated. The results are shown in Figure 9.

[0037] (Measurement of potential window) Using the solid electrolyte obtained in Example 1, the potential window was measured by linear sweep voltammetry (LSV) as follows. A titanium plate as a positive electrode, the solid electrolyte obtained in Example 1, and metallic lithium as a counter electrode were introduced into a bipolar sealed cell in this order. Next, a potential sweep rate of 0.1 mV / s was used to measure the potential. -1 The electrode potential was continuously changed under the conditions of 100°C and 40°C, and the current flowing was measured to obtain a current-voltage curve. The results are shown in Figure 11.

[0038] [Example 2] Li(TFSA) and NCCH2CH2CN were mixed in a molar ratio of 2:3 (Li(TFSA):NCCH2CH2CN) under an argon atmosphere. Then, [Li2(TFSA)2(SN)3] was added to the mixture in an oil bath. n The mixture was heated to a temperature above the melting point of [Li2(TFSA)2(SN)3]. The mixture was then allowed to cool to room temperature, resulting in the molecular crystal [Li2(TFSA)2(SN)3] n A solid electrolyte containing 5.27 Å of Li-Li nearest neighbor distance was prepared. The ionic conductivity of the solid electrolyte was measured, and DSC was performed on the solid electrolyte in the same manner as in Example 1. Furthermore, the transport number and potential window of the solid electrolyte were measured in the same manner as in Example 1. The results are shown in Figures 1, 7, 10, and 12. Furthermore, the activation energy for ionic conductivity of Example 2 was determined from the Arrhenius plot to be 46.0 kJ / mol.

[0039] [Example 3] Under an argon atmosphere, Li(SCN) and NCCH2CH2CN were mixed in a molar ratio of 2:3 (Li(SCN):NCCH2CH2CN). Then, [Li2(SCN)2(SN)3] was added to the oil bath. n The mixture was heated to a temperature above the melting point of [Li2(SCN)2(SN)3]. The mixture was then allowed to cool to room temperature, resulting in the molecular crystal [Li2(SCN)2(SN)3] n A solid electrolyte containing 5.28 Å of Li-Li nearest neighbor distance was prepared. The ionic conductivity of the solid electrolyte was measured, and DSC was performed on the solid electrolyte in the same manner as in Example 1. The results are shown in FIGS. Furthermore, the activation energy for ionic conductivity of Example 3 was determined from the Arrhenius plot to be 46.5 kJ / mol.

[0040] [Comparative Example 1] Li(TFSA) and N,N,N',N'-tetramethyl-1,3-propanediamine (TMPDA) were mixed in equimolar amounts under an argon atmosphere. Then, [Li(TFSA)(TMPDA)] was heated in an oil bath. n The mixture was heated to a temperature above the melting point of [Li(TFSA)(TMPDA)]. The mixture was then allowed to cool to room temperature, yielding the molecular crystal [Li(TFSA)(TMPDA)]. n A solid electrolyte containing 6.65 Å of Li-Li nearest neighbor distance was prepared. The ionic conductivity of the solid electrolyte was measured in the same manner as in Example 1. The results are shown in FIG. Furthermore, the activation energy for ionic conductivity of Comparative Example 1 was determined from the Arrhenius plot to be 73.0 kJ / mol.

[0041] Comparative Example 2 The molecular crystal [Li(OTf)(TMEDA)] was prepared in the same manner as in Comparative Example 1. n (TMEDA stands for (CH3)2NCH2CH2N(CH3)2. The nearest neighbor distance between Li-Li is 6.89 Å) solid electrolyte was prepared. The ionic conductivity of the solid electrolyte was measured in the same manner as in Example 1. The results are shown in FIG. Furthermore, the activation energy for ionic conductivity of Comparative Example 1 was determined from the Arrhenius plot to be 68.3 kJ / mol.

[0042] Comparative Example 3 The molecular crystal [Li(TFSA)(TMEDA)] was prepared in the same manner as in Comparative Example 1. n A solid electrolyte containing 6.65 Å of Li-Li nearest neighbor distance was prepared. The ionic conductivity of the solid electrolyte was measured in the same manner as in Example 1. The results are shown in FIG.

[0043] Comparative Example 4 The molecular crystal [Li(CPFSA)(TMEDA)] was prepared in the same manner as in Comparative Example 1. n A solid electrolyte containing 7.51 Å of Li-Li nearest neighbor distance was prepared. The ionic conductivity of the solid electrolyte was measured in the same manner as in Example 1. The results are shown in FIG.

[0044] Comparative Example 5 The molecular crystal [Li(NFBSA)(C6H4(OCH3)2)] was prepared in the same manner as in Comparative Example 1. n A solid electrolyte containing Li-Li atoms (estimated to have a nearest neighbor distance of more than 6.00 Å) was prepared. The ionic conductivity of the solid electrolyte was measured in the same manner as in Example 1. The results are shown in FIG.

[0045] As shown in FIGS. 1 to 6, the solid electrolytes of Examples 1 to 3 were superior in ionic conductivity to the solid electrolytes of Comparative Examples 1 to 5. Furthermore, when comparing the activation energies related to ionic conductivity, Examples 1 to 3 exhibited lower values ​​than Comparative Examples 1 and 2, indicating that the interactions acting on lithium ions were reduced in Examples 1 to 3. Furthermore, as shown in FIG. 1, the solid electrolyte of Example 1 exhibited a kinetic energy of approximately 10 s at a low temperature of -30°C. -5 S cm -1 It had high ionic conductivity. Furthermore, the molecular crystal of Example 3 does not contain fluorine atoms. Thiocyanate used to prepare the molecular crystal in Example 3 has the advantages of being less environmentally hazardous and less expensive than fluorine compounds containing fluorine atoms. 9 and 10, high lithium ion transport numbers of 0.9 or more were obtained in both Example 1 and Example 2. This suggests that selective lithium ion hopping is occurring in the molecular crystals of Example 1 and Example 2. The reason that the lithium ion transport numbers were high in Example 1 and Example 2 is speculated to be that anions serve as components of the conduction path, suppressing anion conduction and resulting in selective diffusion of lithium ions. 11 and 12, it was confirmed that Example 1 had a potential window of about 4 V, while Example 2 had a wider potential window of about 5.5 V. The reason for this is presumably that succinonitrile is relatively stable against reduction, and that TFSA is more electrochemically stable than FSA.

Claims

1. It includes a molecular crystal represented by the following general formula (1): A solid electrolyte in which the content of the molecular crystal is 80% by mass to 95% by mass. Li ａ 8 ｂ (83) 2 ２ 38 ２ 38) ｃ ) ｎ ・・・(1) (In the general formula (1), a represents an integer of 1 or more, b is 1, c is 2, and n represents an integer of 1 or more. However, in the general formula (1), a=b is satisfied. X represents a molecular crystal in which the nearest neighbor distance between Li-Li is 2.00 Å or more and 6.00 Å or less, and N(SO ２ F) ２ － It is.)

2. 2. The solid electrolyte according to claim 1, wherein the closest distance between the Li-Li atoms in the molecular crystal is 4.00 Å or more.

3. 3. The solid electrolyte according to claim 1, which is used as a solid electrolyte for a secondary battery or a capacitor.

4. A secondary battery comprising the solid electrolyte according to any one of claims 1 to 3.

5. A capacitor comprising the solid electrolyte according to any one of claims 1 to 3.

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