Patent Application
20230411696
The embodiments of the present disclosure relate to a composition for a calcium battery electrolyte, an electrolyte containing the composition, and a calcium battery containing the electrolyte.
Lithium-ion batteries, which have high energy density, have been applied to storage for portable electronic devices and grid devices. In expectation of forthcoming widespread of electric vehicles and smart grids, demands have arisen for large-scale power storage system. However, current Li-ion battery performance are approaching their theoretical limits. Furthermore, lithium material is unevenly distributed and exhaustion of natural abundance and rise of cost of lithium material have been concerned. For the above, development of batteries using alternative material to lithium has been expected.
In recent years, calcium batteries have been attracted for next-generation secondary batteries. Calcium batteries have high energy densities comparable with those of lithium batteries. Further, rich abundance of calcium allows remarkable cost reduction.
For practical use of calcium batteries, development of a calcium battery electrolyte has been on its way. The performance of a calcium battery is determined in terms of three factors of Ca ion conductivity, plating/stripping stability of Ca ion reaction, and an electrochemical potential window of Ca ion.
Non-patent Document 1 discloses an electrolyte prepared by dissolving a calcium salt Ca(BF4)2 in a solvent of a mixture of ethylene carbonate and propylene carbonate (EC+PC). However, a battery containing this electrolyte, which battery functions at 150° C. or higher and also which has poor plating/stripping stability of Ca ion, has a low Coulombic efficiency.
Non-patent Document 2 discloses an electrolyte prepared by dissolving Ca(BF4)2 in a solvent of tetrahydrofuran (THF), which exhibits a high Coulombic efficiency on an Au electrode. The electrolyte is compatible with a Ca metal, but has anodic stability as low as 2.4 V vs. Ca2+/Ca, which leads to a narrow electrochemical potential window and difficulty in achieving a high voltage.
Non-patent Documents 3 and 4 each disclose an electrolyte prepared by dissolving Ca(4DME)[B(hfip)4]2 in a solvent of 1,2-dimethoxyethane (DME). The electrolyte obtains a preferable anode stability (>4.0 V), but generates calcium fluoride CaF2 on electrodes in the course of charging and discharging because of containing fluorine, resulting in becoming unable to conduct Ca ions.
[Non-Patent Document 1] A. Ponrouch et al., Nature Materials 2016, 15, 169-172
[Non-Patent Document 2] D. Wang et al., Nature Materials 2018, 17, 16-20
[Non-Patent Document 3] Z. Y. Li et al., Energy and Environmental Science 2019, 12, 3496-3501
[Non-Patent Document 4] A. Shyamsunder et al., ACS Energy Lett. 2019, 4, 2271-2276
With the foregoing problems in view, at least one of the embodiments of the present disclosure can provide a composition for a calcium battery electrolyte, a calcium battery electrolyte, and a calcium battery free from halogens such as fluorine, high in Ca ion conductivity, stable in Ca ion plating/stripping reaction, and wide in electrochemical potential window.
To Solve the above problem, Inventors synthesized a novel calcium battery electrolyte composition, generated a novel electrolyte containing this composition, and generated a calcium battery including this electrolyte.
According to an aspect of the embodiment, a composition for a calcium battery electrolyte includes: a calcium salt containing at least a calcium atom, a boron atom, and a hydrogen atom and having a cage structure.
According to another aspect of the embodiment, a calcium battery electrolyte includes: a medium for the calcium battery electrolyte; and a calcium salt containing at least a calcium atom, a boron atom, and a hydrogen atom and having a cage structure.
According to an additional aspect of the embodiment, a calcium battery comprising: a positive electrode; a negative electrode; an medium for an electrolyte; and a calcium battery electrolyte a calcium salt containing at least a calcium atom, a boron atom, and a hydrogen atom and having a cage structure.
Hereinafter, description will now be made in relation to one embodiment of the present disclosure. The following description shows a generic or specific example. The one embodiment is merely an example and the scope of the present disclosure should by no means be limited to the following embodiment.
As described above, the performance of a calcium battery is determined in terms of three factors of Ca ion conductivity, plating/stripping reaction stability of Ca ion, and an electrochemical potential window of Ca ion. Specifically, the Ca ion conductivity and the electrochemical potential window of Ca ion are affected by selection of an electrolyte, and a plating/stripping reaction stability of Ca ion is affected by the combination of a positive electrode and a negative electrode. Here, the electrolyte will now be described, and the combination of the positive and negative electrodes will be described below.
Aiming at attaining an electrode having high Ca ion conductivity and a wide electrochemical potential window, Inventors of the embodiments of the present disclosure first synthesized the following novel electrolyte composition, and then prepared an electrolyte solution in which the composition is dissolved.
The composition may be in a form of a calcium salt, and specifically may be an inorganic calcium salt and/or an organic calcium salt. The salt is preferably an anhydride. A calcium salt is synthesized by combining monovalent anions to a calcium atom of divalent cation.
Examples of the monovalent anion includes Cl−, Br−, I−, BF4−, PF6−, AsF6−, SbF6−, SiF6−, ClO4−, AlCl4−, FSO3−, CF3SO3−, C4F9SO3−, [N(FSO2)2]−, [N(CF3SO2)2]−, [N(C2F5SO2)2]−, [N(FSO2)(CF3SO2)]−, CF3BF3−, C2F5BF3−, CB11H12−, and the derivatives thereof.
From the viewpoint of electrochemical stability, BF4−, PF6−, ClO4−, AlCl4−, [N(CF3SO2)2]−, [N(C2F5SO2)2]−, or CB11H12− may be selected as the monovalent anion. From the viewpoint of solubility, PF6−, FSO3−, [N(CF3SO2)2]−, [N(C2F5SO2)2]−, or CB11H12− may be selected as the monovalent anion.
In the present embodiment, Inventors mainly focused on anions of closo-type complex hydride or carborane anions. The closo-type complex hydride is one type of group of boron compounds and is expressed by Formula [BnHn]2−. A carborane is also one type of group boron compounds and is a generic name of polyhedral borane having a carbon atom. Each compound has a crystalline structure in the form like a cage (closo-structure; see
Examples of an anion pertaining to a carborane are CB11H12, CB9H10, and CB7H8 among which CB11H12 may be preferably selected. Carborane anions are observed to have common features of superior in reduction stability not to reductively decompose on the surface of a calcium metal electrode and of low coordinating with a calcium cation because weakly interacting with a calcium cation. For the above, if another carborane anion is selected in place of CB11H12, similar fine calcium stripping/plating stability would be obtained.
The composition of the embodiments of the present disclosure is formed of a calcium salt containing at least a calcium atom, a boron atom, and a hydrogen atom and having a cage structure. This calcium salt contains component expressed by a general formula Ca(CBn-1Hn)2 (where, n is an integer of four or more). Specifically, the calcium salt may contain a component selected from a group including Ca(CB11H12)2, and a mixture of Ca(CB11H12)2 and Ca(CB9H10)2. If being a mixed salt, the calcium salt is expressed by a general formula Ca(CB11H12)2-x(CB9H10)x (where, x is an integer of one or more). The following description assumes that the calcium salt is Ca(CB11H12)2. Calcium salts except for Ca(CB11H12)2 can be synthesized in the same method as the method below.
Description will now be made in relation to a method of synthesizing Ca(CB11H12)2. In Step 1, an aqueous solution of the starting material A(CB11H12)x is passed through a cation exchange resin and converted into an aqueous of (H3O) (CB11H12). The symbol A is exemplified by Li, Na, Cs, Me3NH, or Mg, and the illustrated example uses the starting material satisfying A=Cs and x=1 or 2. In Step 2, the obtained aqueous solution of (H3O) (CB11H12) is neutralized in the aqueous solution by adding a calcium compound such as Ca(CO3) or Ca(OH)2 and consequently, hydrate Ca(CB11H12)2 is obtained. In Step S3, the obtained hydrate Ca(CB11H12)2 is heat treated at 160 to 240° C. under vacuum for dehydration and thereby Ca(CB11H12)2 anhydride is obtained.
Material evaluation was demonstrated on hydrate Ca(CB11H12)2 and Ca(CB11H12)2 anhydride obtained in Steps of the above synthesizing method. Sample 1 was Ca(CB11H12)2 (hydrate) before subjected to the heat treatment and Sample 2 was Ca(CB11H12)2 (anhydrate) after subjected to the heat treatment.
The measurement to evaluate the composition and the electrolyte of the present embodiment was conducted under the environment of the temperature of 25° C. (room temperature) and the humidity of 1 ppm or less. In order to avoid contamination of, for example, moisture, the electrolyte was treated in a glovebox filled with inactive gas such as argon.
In-situ Raman spectral measurement was performed on Samples 1 and 2. This measurement used Raman Spectral instrument (DXR Raman Microscope, product of Thermo SCIENTIFIC INC.).
Nuclear Magnetic Resonance (NMR) measurement was performed on Samples 1 and 2. This measurement used an NMR instrument (AVANCE III 400, product of Bruker Corporation).
Inductivity coupled plasma optical emission spectrometer (ICP-OES) measurement was performed on Sample 1. This measurement used an optical emission spectrometer instrument (iCAP6500, product of Thermo SCIENTIFIC INC.).
Table 1 shows a composition ratio of Ca(CB11H12)2. The continuous ratio of contents calcium, boron, and cesium is logically 1:22:0, and was measured to be 1:21.96:1.5×10−5, which was close to the logical value.
The results of multiple measurements confirmed that Ca(CB11H12)2 was successfully synthesized.
A solvent selected as a medium for the electrolyte may be any liquid that can dissolve a calcium salt, but is preferably a non-aqueous solvent. Example of a non-aqueous solvent are ethers, carbonates, glymes. In particular, an ether solvent is more preferable because an ether oxide having negative polarity against a Ca ion less coordinates and therefore an ether solvent can solve a calcium salt at a high concentration. Examples of a low-coordinating ether solvent is 1,2-dimethoxyethane (hereinafter abbreviated to DME), tetrahydrofuran (hereinafter, abbreviated to THF), triglyme, diglyme, tetraglyme, and propylene carbonate.
Normally, DME or THF is used as a solvent for a calcium electrolyte. Considering the above, the present composition Ca(CB11H12)2 was put into each of DME and THF, but was scarcely dissolved in either solvent. Then, a mixed solution of DME and THF (hereinafter DME/THF mixed solution) was prepared and Ca(CB11H12)2 was put into this mixed solution. Consequently, Ca(CB11H12)2 was well dissolved at a concentration of about 0.5 mol/L or higher. In the present embodiment, the DME/THF mixed solution has a volume ratio of DME to THF of 1:1, which however may be different ratio. When Ca(CB11H12)2 is dissolved in the DME/THF mixed solution, the electrolyte is in a liquid form.
Furthermore, when a mixture (mixed salt) of Ca(CB11H12)2 and Ca(CB9H10)2 was put into the DME/THF mixed solution, the mixed salt was well dissolved at a concentration of about 0.25 mol/L or higher based on each of the salts Ca(CB11H12)2 and Ca(CB9H10)2. When the mixture is dissolved in the DME/THF mixed solution, the electrolyte is in a liquid form.
In Table 2, in regard of the simple solvents DME, THF, Diglyme, and Triglyme, the concentrations of electrolyte solution represent a concentration of electrolyte of saturated solutions; in regard of the mixed solvents of DME/THF1, the concentrations of electrolyte solution represents the concentration of an electrolyte solution of Ca(CB11H12)2 of 0.5 (mol/L); and in regard of the mixed solvents of DME/THF2, the concentrations of electrolyte solution represents the concentration of an electrolyte solution of 0.5 (mol/L) corresponding to the sum of Ca(CB11H12)2 of 0.25 (mol/L) and Ca(CB9H10)2 of 0.25 (mol/L).
In
The method for preparing the electrolyte solution of the present embodiment is accomplished by dissolving the calcium salt Ca(CB11H12)2 in the nonaqueous solvent DME/THF mixed solvent. For example, a solvent DME/THF mixed solution in amount obtaining any concentration of an electrolyte solution is added to calcium salt powder, and then stirred the reaction system. This process was conducted under environment of a temperature of 25° C. (room temperature) and a humidity of 1 ppm or less. Stirring was continued overnight until the solution comes to be completely colorless.
In order to evaluate the characteristic of an electrolyte solution, cyclic voltammetry for stripping/plating Ca ion in the electrolyte was conducted on a three-electrode prototype calcium battery shown in
<Cyclic Voltammogram>
The electrolyte of the present embodiment can be applied to a calcium battery. A calcium battery includes a positive electrode, a negative electrode, and a calcium battery electrolyte. As the electrolyte, the one described in the above “(1) Electrolyte” may be appropriately used. The calcium salt may contain the component Ca(CB11H12)2, and may further contain the component Ca(CB9H10)2.
The capacity of the calcium battery is determined by the amounts of Ca ion stored in the respective material of the positive electrode and the negative electrode and a difference (i.e., voltage) of the reaction potential between the positive electrode and the negative electrode.
The positive electrode 2 may contain a positive electrode active material that is capable of reversibly intercalating and deintercalating Ca ion. Examples of the positive electrode active material include sulfur, titanium sulfide, vanadium oxide, manganese oxide, iron phosphate, vanadium sodium phosphate, calcium molybdate, and prussian blue, among which sulfur is particularly preferable. Sulfur has a large theoretical capacity and is therefore capable of storing five to ten times calcium ion as compared with the above positive electrode active material in an oxide form. Furthermore, sulfur has a relatively low potential to conduct a chemical reaction, which is a demerit that an energy density is not obtained very much but is a merit that a load on an electrolyte solution can be small. For the above, the sulfur positive electrode can obtain a high energy density without decomposing the electrolyte solution.
The negative electrode 3 may contain a negative electrode active material that is capable of reversibly intercalating and deintercalating Ca ion. A preferable example of the negative electrode active material includes metal calcium. Use of Ca ion of metal calcium can obtain a large energy density.
Another example of the negative electrode active material include a calcium alloy which may be exemplified by calcium-tin alloy(CaSnx), silicon-calcium alloy (CaSix), calcium-zinc alloy (CaZnx), calcium-lithium alloy (CaLix), and calcium-sodium alloy (CaNax).
A preferable combination of the positive electrode 2 and the negative electrode 3 is a sulfur (S) positive electrode 2 and a calcium (Ca) negative electrode 3 because of a “theoretical” large capacity ( 1.34 Ah/g) of a calcium metal and the optimal potential of Ca2+/Ca contrast thereof. In addition, since the reaction potential 2.5 V vs. Ca2+/Ca between sulfur and calcium is adequately lower than the oxidization degrading potential (4V vs. Ca2+/Ca or more), prolongation of the lifetime of the calcium battery can be expected.
The positive electrode 2 and the negative electrode 3 of the present example are each preferably a mixture of the corresponding active material ground into grains, conductive material such as acetylene black, carbon black, and graphite, and a binder such as polyvinylidene fluoride or polytetrafluoroethylene. These polymer materials should by no means be limited as far as the materials can bind the active material and the conductive material to obtain the effects of the embodiments of the present disclosure.
The separator 4 insulates the positive electrode 2 from the negative electrode 3. An electrolyte in a liquid form is impregnated in the separator 4. Examples of material of the separator 4 include a porous film and a nonwoven fabric. More specifically, the separator 4 may be formed of a glass fiber, glass ceramic, polyethylene, polypropylene, cellulose, polyvinylidene fluoride, and mixtures containing two or more of the above materials. The separator may contain electrolyte. The calcium battery 1 of the present example may be obtained by, for example, bringing the separator 4 into contact with (e.g., impregnating in) an liquid-form electrolyte.
The electrolyte of the present example was electrically and chemically evaluated.
<Coulombic Efficiency>
Furthermore, the measurement was performed on the following two-electrode prototype calcium battery 1 (see
<charging-discharging profile>
The embodiments of the present disclosure can provide a composition for a calcium battery electrolyte, a calcium battery electrolyte, and a calcium battery that is free from halogens such as fluorine, high in Ca ion conductivity, stable in Ca ion stripping/plating reaction, and wide in electrochemical potential window. The electrolyte composition and the electrolyte of the embodiments of the present disclosure can be used for a calcium battery.
Throughout the specification and the claims, the indefinite article “a” or “an” does not exclude a plurality.
All examples and conditional language recited herein are intended for the pedagogical purposes of aiding the reader in understanding the disclosure and the concepts contributed by the inventor to further the art, and are not to be construed limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the disclosure. Although one or more embodiments of the present disclosures have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-012000 | Jan 2021 | JP | national |
This application is a continuation application of International Application No. PCT/JP 2022/002730, filed on Jan. 25, 2022, and designated the U.S., which claims priority to Japanese Patent application No. 2021-012000, filed on Jan. 28, 2021, the entire contents of each are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2022/002730 | Jan 2022 | US |
| Child | 18226882 | US |
