Solid electrolyte for calcium ion secondary battery, and calcium ion secondary battery

Abstract

To provide a solid electrolyte for a calcium ion secondary battery, the solid electrolyte being capable of exhibiting performance for stabilizing a negative electrode even when charging and discharging are repeated, while high energy capacity is maintained, in a calcium ion secondary battery.SOLUTION: A solid electrolyte for a calcium ion secondary battery includes a calcium salt, a polymer compound, and a polymer solvent. The calcium salt is a calcium complex hydride salt and / or a calcium closo-complex hydride salt, the polymer compound is an ether-based polymer, and the polymer solvent is at least one selected from the group consisting of THF (tetrahydrofuran), G1(1,2-dimethoxyethane; glyme), G2 (diglyme), G3 (triglyme), and Me-THF (methyltetrahydrofuran).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a calcium ion secondary battery that uses divalent calcium ions as an ion conductive medium, and a solid electrolyte for the calcium ion secondary battery that is applied to the calcium ion secondary battery. [Background technology]

[0002] There is a strong demand for the development of high-capacity, high-output secondary batteries as power sources for hybrid electric vehicles, plug-in hybrid electric vehicles, electric vehicles, etc. In addition, with the widespread use of portable electronic devices such as smartphones, tablet terminals, digital cameras, and notebook computers, there is a strong demand for the development of small, lightweight secondary batteries with high energy density.

[0003] Lithium ion secondary batteries are a type of secondary battery that meets these requirements. Lithium ion secondary batteries are composed of a negative electrode, a positive electrode, and a non-aqueous electrolyte or solid electrolyte, and the active materials used as the materials for the negative electrode and positive electrode are materials that can extract and insert lithium. The non-aqueous electrolyte includes a non-aqueous electrolyte solution obtained by dissolving a lithium salt, which is a supporting salt, in an organic solvent, and the solid electrolyte includes an inorganic or organic solid electrolyte that is non-flammable and has lithium ion conductivity.

[0004] When comparing the energy capacity per unit mass, the energy capacity of lithium (Li) alone is larger and superior to that of other elements. However, lithium secondary batteries have safety issues, lithium is a limited resource, is expensive, and there are problems with its long-term stable supply. For this reason, development of secondary batteries using elements that replace lithium is underway.

[0005] As an example of a secondary battery using an alternative element, there is a calcium ion secondary battery that uses divalent calcium ions as an ion conductive medium.

[0006] Calcium (Ca) is an abundant resource and is much cheaper than lithium. Although calcium has a larger atomic weight than lithium, it is divalent, and therefore, in principle, it is possible to provide a suitable secondary battery material with a higher energy density. Moreover, calcium ions do not ignite in abnormal situations, unlike lithium ions, and therefore calcium is highly safe when used as a secondary battery material. Therefore, calcium may be suitably applied to secondary batteries for automobile power sources, which require characteristics such as high capacity, high output, and high speed charging and discharging, as well as high safety.

[0007] For example, Japanese Patent Application Laid-Open No. 08-312305 discloses a calcium ion secondary battery having a positive electrode in which CaSi2 and / or CaGe2 is used as a positive electrode active material, a negative electrode in which a carbon material is used as a calcium ion storage material, and a non-aqueous electrolyte consisting of a non-aqueous electrolyte solution containing a calcium salt and a solvent.

[0008] On the other hand, from the viewpoint of improving the safety of secondary batteries, the development of all-solid-state secondary batteries using solid electrolytes that are free from the risk of liquid leakage, etc. All-solid-state secondary batteries are composed of a three-layer structure consisting of a positive electrode layer containing a positive electrode active material and a solid electrolyte, a negative electrode layer containing a negative electrode active material (or a negative electrode active material and a solid electrolyte), and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer.

[0009] For example, JP 2020-064864 A discloses an all-solid-state secondary battery comprising a positive electrode layer containing a sulfur-based positive electrode active material and a complex hydride solid electrolyte, a negative electrode layer, and a solid electrolyte layer having lithium ion conductivity and made of a complex hydride solid electrolyte, which is disposed between the positive electrode layer and the negative electrode layer.

[0010] It is conceivable to provide a secondary battery having the characteristics of both a calcium ion secondary battery and an all-solid-state secondary battery, and a calcium-based polymer solid electrolyte has been proposed as a solid electrolyte applicable to such secondary batteries.

[0011] As calcium-based polymer solid electrolytes, solid electrolytes in which a calcium salt is combined with a polymer compound have been reported to date as follows. (1) CaCl2 + PVA (polyvinyl alcohol) and PVP (polyvinylpyrrolidone) mixed solvent or copolymer, (2) Ca(NO3)2 + PEGDA (polyethylene glycol diacrylate), (3) Ca(NO3)2 + PEGDA and epoxy resin mixed solvent or copolymer, (4) Ca(BF4)2 or Ca(TFSI)2 or Ca(ClO4)2 + PEGDA, (5) Ca(TFSI)2 + PVI (polyvinylimidazole).

[0012] These calcium-based polymer solid electrolytes are synthesized by mixing with a monomer, oligomer, or polymer or dissolving them in a solvent, and then carrying out a polymerization reaction.

[0013] These calcium-based polymer solid electrolytes have a thermal conductivity of 1×10 at room temperature. -6 S / cm or more 10 -3 It exhibits ionic conductivity of less than S / cm. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Japanese Patent Application Publication No. 08-312305 [Patent Document 2] JP 2020-064864 A Summary of the Invention [Problem to be solved by the invention]

[0015] For the negative electrode of a calcium ion secondary battery, metallic calcium, calcium alloys, or carbon materials as calcium ion storage materials have been proposed. Among these, metallic calcium is preferably used as the negative electrode from the viewpoint of obtaining a high energy capacity by utilizing the characteristics of metallic calcium, which has a reduction potential and capacity per volume equivalent to those of metallic lithium.

[0016] However, because metallic calcium has a high reducing power, calcium salts and polymer compounds are easily reductively decomposed to form a passive layer at the interface between the negative electrode and the solid electrolyte, and for this reason, a calcium ion secondary battery using a combination of a conventional calcium-based polymer solid electrolyte and a negative electrode made of metallic calcium has not yet been realized.

[0017] In addition, there have been reports of constant current dissolution and deposition tests using metallic calcium as the anode for solid electrolytes consisting of a combination of Ca(BF4)2 or Ca(TFSI)2 or Ca(ClO4)2 + PEGDA, and a combination of Ca(TFSI)2 + PVI. However, the overvoltage is very large at around 2 V, and the solid electrolytes are not suitable for battery operation.

[0018] In this way, when a calcium-based polymer solid electrolyte is used, metallic calcium cannot be used as the negative electrode active material, and therefore charge / discharge tests that require negative and positive electrode active materials compatible with the solid electrolyte cannot be performed. Therefore, the reality is that a combination of a calcium-based polymer solid electrolyte, a negative electrode active material made of metallic calcium or a calcium alloy, and a positive electrode active material that constitutes a secondary battery with a cell voltage of 1 V or more has not yet been realized.

[0019] Therefore, an object of the present disclosure is to provide a solid electrolyte for calcium ion secondary batteries, which can exhibit performance that stabilizes the negative electrode even after repeated charging and discharging while maintaining a high energy capacity, particularly in calcium ion secondary batteries that use metallic calcium or a calcium alloy as the negative electrode active material, and a calcium ion secondary battery using the same. [Means for solving the problem]

[0020] A solid electrolyte for a calcium ion secondary battery according to one embodiment of the present disclosure includes a calcium salt, a polymer compound, and a polymer solvent, The calcium salt is a calcium complex hydride salt and / or a calcium closo-based complex hydride salt, The polymer compound is an ether-based polymer, and The polymer solvent is at least one selected from the group consisting of THF (tetrahydrofuran), G1 (1,2-dimethoxyethane; glyme), G2 (diglyme), G3 (triglyme), and Me-THF (methyltetrahydrofuran); It is characterized by:

[0021] The calcium complex hydride salt is preferably Ca(BH4)2 (calcium borohydride).

[0022] The calcium closo-based complex hydride salt is Ca(CB 11 H 12 )2 is preferred.

[0023] The positive electrode active material for a calcium ion secondary battery preferably further contains LiBH4 (lithium borohydride).

[0024] A calcium ion secondary battery according to one embodiment of the present disclosure is configured with a three-layer structure including a positive electrode layer including a positive electrode active material capable of receiving and releasing calcium ions, a negative electrode layer including a negative electrode active material capable of receiving and releasing calcium ions, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, The present invention is characterized in that the solid electrolyte for a calcium ion secondary battery according to one aspect of the present disclosure described above is used as a solid electrolyte constituting the solid electrolyte layer.

[0025] The negative electrode active material preferably contains metallic calcium and / or a calcium alloy.

[0026] The negative electrode active material is preferably made of metallic calcium. Effect of the Invention

[0027] The solid electrolyte for calcium ion secondary batteries according to one embodiment of the present disclosure is not reductively decomposed even when calcium metal with high reducing power is used as the negative electrode active material, and therefore it is possible to use calcium metal as the negative electrode of a calcium ion secondary battery.

[0028] Therefore, by applying the solid electrolyte for calcium ion secondary batteries according to one embodiment of the present disclosure, it is possible to provide a calcium ion secondary battery that can exhibit battery performance in which the negative electrode is stabilized even after repeated charging and discharging while maintaining a high energy capacity. [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a diagram showing the crystal structure of a calcium complex hydride salt. [Diagram 2] FIG. 2 is a schematic explanatory diagram of an example of impedance evaluation measurement and an equivalent circuit used in the analysis. [Diagram 3] FIG. 3 is a diagram showing a schematic structure of a calcium ion secondary battery. [Figure 4]FIG. 4 is a graph showing the results of the constant current voltage test in Example 1. [Diagram 5] FIG. 5 is a graph showing the results of a constant current voltage test in the comparative example. [Figure 6] FIG. 6 is a graph showing the results of the constant current voltage test in Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The inventors of the present disclosure have conducted extensive research into combinations of positive electrode active materials, negative electrode active materials, and solid electrolytes that can maintain a high energy capacity and stabilize the negative electrode even after repeated charging and discharging in a calcium ion secondary battery using metallic calcium in the negative electrode.

[0031] As a result, it was discovered that the stability against metallic calcium can be greatly improved by applying a solid electrolyte obtained by synthesizing a calcium complex hydride salt and / or a closo-type complex hydride salt as the electrolyte salt (calcium salt), an ether-based polymer, in particular pTHF (polytetrahydrofuran), as the polymer compound, and at least one selected from the group consisting of THF (tetrahydrofuran), G1 (1,2-dimethoxyethane; also known as "glyme" or "monoglyme"), G2 (diglyme), G3 (triglyme), and Me-THF (methyltetrahydrofuran).

[0032] Furthermore, it has been found that by further adding a lithium salt to the solid electrolyte, the electrochemical characteristics of the calcium ion secondary battery can be improved, and the cycle characteristics can be improved.

[0033] The present disclosure has been completed based on these findings. Hereinafter, an example of an embodiment of the present disclosure will be described in detail.

[0034] 1. Solid electrolyte for calcium ion secondary batteries A solid electrolyte for a calcium ion secondary battery according to an example embodiment of the present disclosure comprises a calcium salt, a polymer compound, and a polymer solvent, the calcium salt being a calcium complex hydride salt and / or a calcium closo-based complex hydride salt, the polymer compound being an ether-based polymer such as pTHF, and the polymer solvent being THF or the like.

[0035] More specifically, the solid electrolyte for calcium ion secondary batteries is composed of a gel-like solid electrolyte obtained by synthesizing a calcium salt consisting of a calcium complex hydride salt and / or a calcium closo-based complex hydride salt, the polymer compound, and the polymer solvent.

[0036] (1) Calcium salts Calcium complex hydride salt is an inorganic compound consisting of a metal cation and a molecular complex ion containing hydrogen, and has the general formula: Ca x [M' y H z ] and has a structure as shown in Figure 1. Examples of calcium complex hydride salts include, but are not limited to, Ca(BH4)2 and a complex of Ca(BH4)2 and X (wherein X is one or more compounds selected from CaCl2, CaBr2, CaI2, Ca(NH2)2, and P2S5). The calcium complex hydride salt can be composed of one or more of these.

[0037] The calcium closo-based complex hydride salt is a hydride salt having a cage-shaped cluster-type complex ion with a closo structure. The closo-based hydride salt is a hydride salt having a cation, which is a metal ion, and a compound represented by the general formula: (C m M n-m H n ) (2-m)- (wherein 0≦m<2, 5≦n≦12, and m and n are integers).

[0038] Calcium closo-based complex hydride salts include, but are not limited to, Ca(CB 11 H 12)2, Ca[CB9H 10 ]2, CaB 12 H 12 , CaB 10 H 10 Closo-based complex hydride salts are hydrated in the atmosphere, and therefore have better stability in the atmosphere than other complex hydride salts. In addition, Closo-based complex hydride salts can have a higher ionic conductivity than other complex hydride salts.

[0039] In addition, as a closo-based complex compound salt, Ca(CB 11 H 12 )2 and Ca[CB9H 10 ]2. Such a solid solution can further increase the ionic conductivity.

[0040] In the solid electrolyte for calcium ion secondary batteries of this embodiment, it is preferable to use Ca(BH4)2 as the complex hydride salt constituting the calcium salt, and Ca(CB 11 H 12 It is preferable to use Ca(BH4)2 as the calcium salt. It is most preferable to use Ca(BH4)2 as the calcium salt. This is because Ca(BH4)2 can have two functions as a crosslinking agent and an electrolyte salt, and is more stable in terms of dissolution and deposition of calcium metal than other calcium salts.

[0041] The solid electrolyte for calcium ion secondary batteries of this embodiment may further contain metal ions (Li, K, etc.) having a lower standard potential than calcium metal, such as other lithium ion conductors. Examples of lithium ion conductors include lithium complex hydride salts and / or closo-based complex hydride salts. The solid electrolyte for calcium ion secondary batteries of this embodiment may also contain small amounts of metal ions (Mg, Na, etc.) having a higher standard electrode potential than calcium metal.

[0042] Examples of complex hydride salts include LiBH4, and a complex of LiBH4 and X (wherein X is one or more compounds selected from LiCl, LiBr, LiI, LiNH2, and P2S5). Examples of closo-based complex hydride salts include Li2B 12 H 12 , Li2B 10 H 10 LiCB 11 H 12 , LiCB9H 10 Examples include:

[0043] Among these, it is preferable to use LiBH4 as the lithium ion conductor. By adding a different cation such as LiBH4 or a monovalent cation, it is possible to further improve electrochemical properties such as suppression of overvoltage increase and increase in Coulombic efficiency in calcium ion secondary batteries.

[0044] In the solid electrolyte for calcium ion secondary batteries of this example, calcium complex hydride salt and / or calcium closo-based complex hydride salt is used as the calcium salt, thereby preventing the reductive decomposition of the solid electrolyte by the metallic calcium constituting the negative electrode. This prevents the formation of a passive layer at the interface between the negative electrode and the active material, and provides high stability at the interface between the electrode and the solid electrolyte, making it possible to stabilize the negative electrode even when charging and discharging are repeated while maintaining a high energy capacity.

[0045] Furthermore, in the solid electrolyte for calcium ion secondary batteries of this embodiment, the addition of a lithium complex hydride salt and / or a closo-based complex hydride salt can further improve the stability against metallic calcium.

[0046] (2) Polymer compounds The solid electrolyte for calcium ion secondary batteries of this embodiment is characterized in that an ether-based polymer is used as the polymer compound to be combined with a calcium salt.

[0047] As with the conventional combination of electrolyte salt and polymer compound, the solid electrolyte for calcium ion secondary batteries of this example can be obtained in the form of a gel by dissolving a mixture of calcium salt and ether-based polymer in a polymer solvent and polymerizing them. An example of the ether-based polymer is polytetramethylene ether glycol such as pTHF.

[0048] In particular, in the solid electrolyte for calcium ion secondary batteries of this example, calcium complex hydride salt and / or calcium closo-based complex hydride salt are used as calcium salts, and these calcium salts can be easily dissolved in water or organic solvents. Therefore, by combining these calcium salts with a polymer compound made of an ether-based polymer and a polymer solvent made of THF or the like, it is possible to easily produce a gel-like solid electrolyte.

[0049] By using an ether-based polymer as the polymer compound, it is possible to ensure stability against metallic calcium and high ionic conductivity even when a calcium complex hydride salt and / or a calcium closo-based complex hydride salt is used as the calcium salt.

[0050] (3) Polymer solvent The solid electrolyte for calcium ion secondary batteries of this example is characterized in that at least one selected from the group consisting of THF, G1, G2, G3, and Me-THF is used as a polymer solvent for obtaining a gel-like solid electrolyte.

[0051] By using these polymer solvents, it is possible to ensure stability against metallic calcium and high ionic conductivity when calcium complex hydride salts and / or calcium closo-based complex hydride salts are combined with polymer compounds such as pTHF.

[0052] (4) Ionic conductivity The solid electrolyte for calcium ion secondary batteries in this example has an ionic conductivity of 1.0×10 -6 S / cm or more 1.0×10 -4 S / cm or less, preferably 1.0×10 -5 S / cm or more, preferably 2.0×10 -5 S / cm or more. Ion conductivity is 1.0×10 -6 By setting the capacitance to be 1.0 S / cm or more, it is possible to obtain an all-solid-state secondary battery that stably exhibits high energy capacity.

[0053] The ionic conductivity of the solid electrolyte for calcium ion secondary batteries is determined as follows. First, a battery is constructed by combining a positive electrode and a negative electrode made of metal molybdenum (Mo) with a solid electrolyte for calcium ion secondary batteries, and a Nyquist plot as shown in Figure 2 is obtained using an impedance measurement method. Based on this Nyquist plot, an equivalent circuit as shown in Figure 2 is set, and the resistance of the solid electrolyte is calculated by fitting and analyzing the equivalent circuit and the Nyquist plot. The ionic conductivity is calculated from the following formula using the calculated resistance, the thickness of the solid electrolyte, and the electrode area. Ionic conductivity (S / cm) = thickness of solid electrolyte (cm) / (resistance of solid electrolyte (Ω) × electrode area (cm 2 ))

[0054] In this example, when assembling the cell for impedance measurement, the glass fiber (separator) is impregnated with the mixed solution before solidification, so the thickness of the glass fiber is used in place of the thickness of the solid electrolyte in the above formula.

[0055] 2. Calcium-ion secondary battery As shown in FIG. 3 , a calcium ion secondary battery (hereinafter also referred to as a “secondary battery”) according to an example of an embodiment of the present disclosure is configured with a three-layer structure including a positive electrode layer including a positive electrode active material, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, and includes the above-mentioned solid electrolyte for calcium ion secondary batteries according to an example of an embodiment of the present disclosure as the solid electrolyte.

[0056] It should be noted that the following description is merely illustrative, and the secondary battery can be embodied in various forms, including the forms described below, and in which various modifications and improvements have been made based on the knowledge of those skilled in the art. In addition, the use of the secondary battery is not particularly limited.

[0057] (1) Positive electrode layer The positive electrode layer includes a positive electrode active material. Examples of the positive electrode active material include oxide active materials. Examples of the oxide active material include LiCoO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, etc., rock salt layered active materials, LiMn2O4, Li4Ti5O 12 and olivine type active materials such as LiFePO4.

[0058] In the calcium ion secondary battery of this example, among these, Li4Ti5O 12 When a calcium complex hydride salt and / or a calcium closo-based complex hydride salt is used as the calcium salt, LTO, which has a low average operating voltage of 1.55 V (Li / LTO), can be preferably used since these electrolyte salts have poor stability on the oxidation side.

[0059] The positive electrode layer can be formed by molding the positive electrode composite material. The positive electrode layer is appropriately processed according to the battery to be used. For example, a pressure compression process using a press or the like can be performed to increase the electrode density. The positive electrode composite material can be formed, for example, by mixing a positive electrode active material and a conductive assistant.

[0060] A gel-like solid electrolyte can be introduced into the positive electrode layer. More specifically, a solution of the solid electrolyte before polymerization is impregnated into the positive electrode mixture. Such a solid electrolyte is added to give the electrode suitable ion conductivity. In the secondary battery of this example, the above-mentioned solid electrolyte for calcium ion secondary batteries can be used as the solid electrolyte.

[0061] In addition to the above, a binder, a conductive material, etc. may be added to the positive electrode mixture. The binder plays a role of binding the positive electrode active material. The binder is not particularly limited, and for example, one or more selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, etc. can be used. The conductive material is added to give the electrode appropriate conductivity. The material of the conductive material is not particularly limited, and for example, graphite such as natural graphite, artificial graphite, and expanded graphite, and carbon black materials such as acetylene black and Ketjen Black (registered trademark) can be used.

[0062] (2) Negative electrode layer The negative electrode layer includes a negative electrode active material. In the calcium ion secondary battery of the present disclosure, metallic calcium (Ca metal) or a calcium alloy can be used as the negative electrode active material. In order to maximize the energy capacity of the negative electrode, it is preferable to use metallic calcium. However, a calcium alloy can also be used to improve battery performance other than the energy capacity, for example, to stabilize the negative electrode against repeated charging and discharging.

[0063] Calcium alloys have the general formula: Ca m M (where M is a metal or semiconductor element, 0≦m≦3).

[0064] The negative electrode layer can be constituted by a foil of calcium metal. In this case, the calcium metal can also act as a current collector. Alternatively, the negative electrode layer can be constituted by a support such as a current collector having a calcium metal coating as the negative electrode active material. This coating can be obtained by depositing calcium metal on the current collector. This coating can be deposited only on a part of the support or on the entire support.

[0065] (3) Solid electrolyte The solid electrolyte is a solid having calcium ion conductivity. In this example, the solid electrolyte for calcium ion secondary batteries according to an example of the embodiment of the present disclosure can be used as the solid electrolyte. The solid electrolyte of this example is preferably applied to a configuration in which the negative electrode layer is made of metallic calcium, but the solid electrolyte of this example can also be applied to negative electrode layers of other configurations.

[0066] (4) Shape and configuration of secondary battery The secondary battery of this example can be made into various shapes, such as a coin shape or a laminated shape. In any shape, the positive electrode layer and the negative electrode layer can be laminated via a solid electrolyte. Then, the positive electrode current collector and the positive electrode terminal leading to the outside, and the negative electrode current collector and the negative electrode terminal leading to the outside can be connected using a current collecting lead or the like, and the secondary battery can be sealed in a battery case. EXAMPLES

[0067] The present disclosure will be described in more detail below using examples and comparative examples, but the present disclosure is not limited to these examples.

[0068] Example 1 A solution of 1.5M Ca(BH4)2 in THF and a solution of pTHF in THF at 1.0 g / ml were prepared, and these were mixed and stirred so that the ratio of calcium salt to polymer compound was 1:1 by volume to cause a polymerization reaction to produce 150 μL of solid electrolyte. Several samples of such solid electrolyte were obtained.

[0069] The infrared absorption spectrum of the obtained sample was obtained using a Fourier transform infrared spectrophotometer (FT-IR, ThermoScientific, Nicolet iN). As a result, no absorption spectrum indicating a hydroxyl group (-OH) was observed in this sample. Therefore, it was confirmed that a solid electrolyte was synthesized by the polymerization reaction of Ca(BH4)2 with THF / pTHF.

[0070] The mixed solution (150 μL) of the sample before polymerization was dropped onto a glass fiber (separator) manufactured by Cytiva (Whatman), and the separator was sandwiched between the positive and negative electrode layers using metal molybdenum (Mo) as the positive and negative electrode layers to obtain an electrode. The obtained electrode was placed in an evaluation cell (manufactured by EC-Frontier), and the AC impedance was measured using an impedance measuring device (manufactured by HIOKI, 3532-80). At this time, the temperature of the evaluation cell was controlled to room temperature (20°C), and a measurement frequency of 4 Hz to 1 MHz was used. Finally, the ionic conductivity was calculated from the surface resistance value of the obtained sample based on the following formula (1). Here, σ is the ionic conductivity (S / cm), d is the thickness of the sample (cm), Rd is the surface resistance value of the sample (Ω), and A is the cross-sectional area of ​​the sample (cm 2 ) σ = d / (Rd A) (1)

[0071] As a result, the ionic conductivity of the solid electrolyte in this example was 1.92 × 10 -5 S / cm, and it was confirmed that the solid electrolyte of Example 1 had calcium ion conductivity.

[0072] The mixed solution (150 μL) of the sample before polymerization was dropped onto a glass fiber (separator) manufactured by Cytiva (Whatman), and the separator was sandwiched between the positive and negative electrode layers using metallic calcium (Ca) polished to a metallic luster as the positive and negative electrode layers to obtain an electrode. The obtained electrode was placed in an evaluation cell, and a charge / discharge test was performed at room temperature (20° C.) using a charge / discharge tester (Solectron Analytical, Cell Test System 1470E) at 0.05 mA / cm. 2 A constant current charge / discharge test was carried out at a current density of 200 cycles.

[0073] As a result, as shown in FIG. 4, the voltage of the evaluation cell did not change with the passage of cycles, and it was confirmed that the solid electrolyte of Example 1 was stable against metallic calcium.

[0074] (Comparative Example) A solution of Ca(BF4)2 dissolved at 1.0M in a mixed solvent of ethylene carbonate and propylene carbonate was mixed with a curing agent containing PEGDA (polyethylene glycol diacrylate), CQ (camphorquinone), and OPPI (p-octyloxyphenylphenyliodonium hexafluoroanitimonate) in a mass ratio of 96:2.5:1.5 in a volume ratio of 1:1. The resulting mixture was stirred in a dark room for 24 hours and irradiated with light to create a gel-like solid electrolyte.

[0075] Metallic calcium (Ca) polished to a metallic luster was used as the positive and negative electrode layers, and the obtained solid electrolyte was sandwiched between the positive and negative electrode layers and pressed to obtain an electrode. The obtained electrode was placed in an evaluation cell, and a charge-discharge test was performed at room temperature (20°C) using a charge-discharge tester at 0.002 mA / cm. 2 A constant current charge / discharge test was performed at a current density of 7 cycles.

[0076] As a result, as shown in FIG. 5, the voltage of the evaluation cell increased with each cycle, and the solid electrolyte of the comparative example was not stable to metallic calcium.

[0077] Example 2 A solution of Ca(BH4)2 at 0.5M and LiBH4 at 1.5M in THF and a solution of pTHF at 1.0g / ml were prepared, and these solutions were mixed and stirred at a volume ratio of 1:1 to cause a polymerization reaction to produce 150μL of a solid electrolyte. Several samples of this solid electrolyte were obtained.

[0078] The mixed solution (150 μL) of the sample before polymerization was dropped onto a glass fiber (separator) manufactured by Cytiva (Whatman), and the separator was sandwiched between the positive and negative electrode layers using metallic calcium (Ca) polished to a metallic luster as the positive and negative electrode layers to obtain an electrode. The obtained electrode was placed in an evaluation cell, and a charge-discharge test was performed at room temperature (20° C.) using a charge-discharge tester at 0.05 mA / cm. 2A constant current charge / discharge test was carried out at a current density of 50 cycles.

[0079] As a result, similarly to Example 1, the voltage of the evaluation cell did not change with the passage of cycles, and it was confirmed that the solid electrolyte of Example 2 was stable against metallic calcium.

[0080] Furthermore, using a similar evaluation cell, the current density was changed to 0.010 mA / cm every 10 cycles at room temperature (20°C). 2 , 0.02mA / cm 2 , 0.05mA / cm 2 , 0.10mA / cm 2 , 0.20mA / cm 2 , 0.50mA / cm 2 A constant current charge / discharge test was conducted for 60 cycles with the current being changed to

[0081] As a result, as shown in Fig. 6, the voltage of the evaluation cell at each current density did not change with the passage of cycles, and the test was completed up to 70 cycles. This confirmed that the solid electrolyte of Example 2 was extremely stable against metallic calcium.

[0082] In addition, the mixed solution (150 μL) of the sample before polymerization was dropped onto a glass fiber (separator) manufactured by Cytiva (Whatman), and lithium titanate (Li4Ti5O 12 The separator was sandwiched between the positive and negative electrode layers, and an electrode was obtained by using a positive electrode layer made of a PTFE-coated lithium carbide (LTO) and a negative electrode layer made of metallic calcium (Ca) polished to a metallic luster. The obtained electrode was placed in an evaluation cell, and a charge-discharge cycle test was performed at room temperature (20°C) using a charge-discharge tester. In the charge-discharge cycle test, the C rate was 0.5C, and the cutoff voltage was 1.8V to 0.8V.

[0083] As a result, the initial discharge capacity of the evaluation cell using the solid electrolyte of Example 2 was 154 mAhg-1, and the cycle life was stable for more than 200 times. The 200th discharge capacity was 99 mAhg-1. The coulombic efficiency from the second cycle onwards was 100% or more. Therefore, it was confirmed that the coulombic efficiency and cycle life of the solid electrolyte can be improved by further adding LiBH4 to Ca(BH4)2.

[0084] In this test, the coulombic efficiency of the solid electrolyte of Example 2 exceeded 100%. This is because the BH4 - This is thought to be a side reaction due to the decomposition of .

[0085] Based on the above test results, it can be said that an all-solid-state secondary battery consisting of a solid electrolyte layer made of a solid electrolyte containing LiBH4 in addition to Ca(BH4)2, a positive electrode layer containing a positive electrode active material of a lithium oxide such as LTO, and a negative electrode layer containing a negative electrode active material made of metallic calcium has demonstrated the battery operation at room temperature of a calcium ion secondary battery using metallic calcium in the negative electrode, something that has hardly been reported until now.

Claims

1. The composition includes a calcium salt, a polymer compound, and a polymer solvent, The calcium salt is a calcium complex hydride salt and / or a calcium closo-based complex hydride salt, The polymer compound is an ether-based polymer, and The polymer solvent is at least one selected from the group consisting of THF (tetrahydrofuran), G1 (1,2-dimethoxyethane; glyme), G2 (diglyme), G3 (triglyme), and Me-THF (methyltetrahydrofuran); A solid electrolyte for a calcium ion secondary battery, comprising:

2. The calcium complex hydride salt is Ca(BH ４ ) ２ The solid electrolyte for a calcium ion secondary battery according to claim 1,

3. The calcium closo-based complex hydride salt is Ca(CB １１ H １２ ) ２ The solid electrolyte for a calcium ion secondary battery according to claim 1,

4. LiBH ４ The solid electrolyte for a calcium ion secondary battery according to claim 1 , further comprising:

5. a positive electrode layer including a positive electrode active material capable of accepting and releasing calcium ions; a negative electrode layer including a negative electrode active material capable of accepting and releasing calcium ions; and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer; It is composed of three layers: The solid electrolyte for a calcium ion secondary battery according to any one of claims 1 to 4 is used as a solid electrolyte constituting the solid electrolyte layer. Calcium ion secondary battery.

6. The calcium ion secondary battery according to claim 5 , wherein the negative electrode active material contains metallic calcium and / or a calcium alloy.

7. The calcium ion secondary battery according to claim 5 , wherein the negative electrode active material is made of metallic calcium.

Images