Patent Application
20150050565
The present disclosure relates to primary and secondary electrochemical cells that include magnesium electrode materials and electrolytes for the same.
There has been a lot of interest in rechargeable electrochemical cells as a way of storing energy for use in portable or mobile devices such as, for example, cell phones, personal digital assistants, plug-in hybrid vehicles, or electric vehicles. Much of the recent work has been to develop lithium-ion electrochemical cells that have high storage capacities and that can be operated safely. Magnesium has been suggested as a potential anode material for rechargeable nonaqueous electrochemical cells due to its abundance, its high charge density, and its ability to transfer two electrons upon ionization.
Hideyuku et al. (JP 2004-265765) have designed a secondary battery that has a sulfur positive electrode and a negative electrode that contains at least one of magnesium metal, magnesium alloy, magnesium oxide, silicon, carbon, and transition metal sulfide as an active material. The battery has a non-aqueous electrolyte containing a magnesium salt such as magnesium [bis(trifluoromethanesulfonyl)imide]2.
NuLi et al. (Electrochemical and Solid-state Letters, 8, (11) C166-C169 (2005)) and Shimamura et al. (Journal of Power Sources, 196, 1586-1588 (2011)) have both reported the deposition and dissolution of magnesium from an ionic liquid. NuLi has reported electrochemical magnesium deposition and dissolution on a silver substrate in the ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonylimide) containing 1M Mg[(CF3SO2)2N]2. Shimamura has reported electrochemical reduction and oxidation of magnesium cation in ionic liquids containing simple magnesium salts. The ionic liquid that they used was N,N-diethyl-N-methyl-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)imide.
One of the challenges for the development of magnesium electrochemical cells and batteries is that of finding useful electrolytes. Magnesium analogues of common electrolyte salts (e.g. Mg(PF6)2, Mg(ClO4)2, Mg(SO3CF3)2) generally have low solubility in electrolyte solvents. Furthermore, common electrolyte solvents are generally believed to form a blocking solid electrolyte interface (SEI) layer at low voltages that can increase internal impedance, reduce the charge rates, and impede electrochemical reactions essential to the efficient operation of magnesium electrochemical cells.
Nonaqueous magnesium electrolytes in which magnesium electrochemistry can be conducted at low overpotentials are typically solutions that contain Grignard reagents. Such solutions typically are composed of a solution of magnesium alkyl halide in tetrahydrofuran. Such solutions are toxic and react spontaneously with oxygen in the air, making them not compatible with dry room environments. They also are oxidatively unstable, limiting the voltage of magnesium batteries to below about 2.5 V.
Provided are concentrated magnesium electrolyte solutions that can be made with magnesium [bis(trifluoromethanesulfonyl)imide]2, or Mg(TFSI)2 salts in common oxidatively stable organic solvents (acetonitrile, carbonates, pyridine). In such solutions magnesium metal can be stripped at low overpotentials. Efficient magnesium intercalation at voltages of 0.5 V or greater can be achieved only when acetonitrile or adiponitrile is used as the solvent. Since acetonitrile and adiponitrile are much more oxidatively stable than THF or the Grignard reagents currently used, electrolytes made from Mg(TFSI)2 and acetonitrile or adiponitrile may be highly useful as electrolytes for high voltage magnesium batteries. The utility of this electrolyte in primary cells has been demonstrated using a Mg metal anode and a Mo6S8 cathode. Electrochemical reversibility of Mg(TFSI)2 electrolytes has also been observed, demonstrating usefulness in high voltage magnesium secondary cells.
In one aspect, an electrochemical cell is provided that includes at least one electrode comprising a magnesium intercalation compound, and an electrolyte. The electrolyte includes a fluorinated imide salt or a fluorinated methide salt substantially dissolved in an oxidatively stable solvent. The at least one electrode can include a magnesium intercalation compound selected from transition metal sulfides, transition metal oxides, magnesium transition metal sulfides, magnesium transition metal oxides, and carbon fluorides. The provided electrochemical cell can include a negative electrode comprising magnesium. The provided electrochemical cell can be a primary (or non-rechargeable) electrochemical cell or a secondary (or rechargeable) electrochemical cell. In some embodiments, the provided electrochemical cell can be operated at temperatures greater than about 40° C. and/or at voltages at or above 3.0V vs. Li/Li+.
In another aspect, a method of making an electrochemical cell is provided that includes dissolving a fluorinated imide or fluorinated methide salt in an oxidatively stable solvent to form an electrolyte, immersing at least one electrode that includes a magnesium intercalation compound into the electrolyte, and immersing a second electrode comprising magnesium into the electrolyte. The at least one electrode can be selected from transition metal sulfides, transition metal oxides, magnesium transition metal sulfides, magnesium transition metal oxides, and carbon fluorides.
In yet another aspect a magnesium electrochemical cell is provided that includes a liquid organic electrolyte, wherein the electrochemical cell is operated at a temperature greater than 40° C.
In this disclosure:
The provided electrochemical cells and methods of making the same provide an electrolyte salt that has high solubility in nitrile-containing solvents. Furthermore, the provided electrochemical cells and methods resist the formation of a blocking solid electrolyte interphase layer that can impede electrochemical reactions essential to the efficient operation of magnesium electrochemical cells.
The above summary is not intended to describe each disclosed embodiment of every implementation of the present invention. The brief description of the drawings and the detailed description which follows more particularly exemplify illustrative embodiments.
In the following description, reference is made to the accompanying set of drawings that form a part of the description hereof and in which are shown by way of illustration several specific embodiments. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense.
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.
Electrochemical cells are provided that include at least one electrode that includes a magnesium intercalation compound and an electrolyte that includes a fluorinated imide salt or a fluorinated methide salt that is substantially dissolved in an oxidatively stable solvent. A number of materials are known to intercalate magnesium. Exemplary materials include TiS2, V6O13, V2O5, WO3, MoO3, MnO2, InSe, and some sulfides of molybdenum. These materials are discussed, for example, in P. G. Bruce et al., “Chemical Intercalation of Magnesium into Solid Hosts”, J. Mater. Chem., 1(4), 705-706 (1991) and Z. D. Kovalyuk et al, “Electrical Properties of Magnesium-Intercalated InSe”, Inorganic Materials, 45(8), 846-850 (2009). Another set of materials that can intercalate magnesium are the chevrel phase materials of molybdenum sulfide, selenide, and telluride. For example, MgxMo3S4 intercalation cathodes have been disclosed in an article by D. Aurbach et al., Nature, 407, 724 (2000) and in U.S. Pat. Appl. Publ. No. 2004/0137324 (Itaya et al.). Magnesium chevrel phases can have various stoichiometries such as MgMo3S4 or Mg2Mo6S8. Since magnesium intercalates into the chevrel materials, the amount of magnesium can vary depending upon the amount of intercalation. Other intercalation electrode materials that may be useful are described in International PCT Pat. App. Publ. No. WO2011/150093 (Doe et al.).
The provided electrochemical cells may include a current collector comprising one or more elements selected from the group consisting of carbon, Al, Cu, Ti, Ni, stainless steel, and alloys thereof, as described in U.S. Pat. App. Publ. No. 2011/0159381 (Doe et al.).
The provided electrochemical cells include an electrolyte that includes a fluorinated imide salt or a fluorinated methide salt substantially dissolved in an oxidatively stable solvent. The fluorinated imide or fluorinated methide salts include a bis(trifluoromethylsulfonyl)imide anion or a bis(trifluoromethylsulfonyl)methide anion having the formulae:
wherein each Rf group is, independently, F or a fluoroalkyl group having 1-4 carbon atoms, which may optionally contain catenary oxygen or nitrogen atoms within the carbon chain, and wherein any two adjacent Rf groups may optionally be liked to form a 5-7 membered ring. The salts can have cations selected from ammonium, imidazolium, pyrazolium, triazolium, thiazolium, oxazolium, pyridinium, pyridazinium, pyrimidonium, and pyrazinium. In some embodiments, the cations can be metals such as sodium, lithium, potassium, or magnesium. Generally, at least some of the cations are magnesium cations. Typically, magnesium [bis(trifluoromethanesulfonyl)imide]2 or magnesium [tris(trifluoromethanesulfonyl)methide]2 are employed in the provided electrolytes. Magnesium [bis(trifluoromethanesulfonyl)imide]2 can be produced by reacting magnesium carbonate or magnesium hydroxide or magnesium metal with bis-(trifluoromethanesulfonyl)imide acid. Magnesium [tris(trifluoromethanesulfonyl)methide]2 can be produced in an analogous manner from tris-(trifluoromethanesulfonyl)methide acid.
Fluorinated imide or methide salts such as, for example, magnesium [bis(trifluoromethylsulfonyl)imide]2 (Mg(TFSI)2) or magnesium [tris(trifluoromethylsulfonyl)methide]2 (Mg(TFSM)2), can be dissolved in oxidatively stable solvents such as organic carbonates, nitriles, and pyridine solvent systems. The disclosed Mg(TFSI)2 or Mg(TFSM)2 electrolyte salts can provide a number of surprising benefits when used in primary or secondary Mg cells. These salts can be both highly soluble and highly dissociated in oxidatively and reductively stable, nonaqueous organic solvents, including but not limited to, organic carbonates, nitriles, and pyridines. In some embodiments, the oxidatively stable organic solvents can include aliphatic nitriles such as acetonitrile, propionitrile, valeronitrile, isobutylnitrile, isopentylnitrile, t-butylnitrile, and dinitriles such as succinonitrile, malononitrile, or adiponitrile.
High concentrations of electrolyte salt possessing high dissociation constants in organic solvents can be highly desirable for achieving high ionic conductivity in the electrolyte and to support high rate charge and discharge performance in Mg cells. With Mg(TFSI)2, electrolyte salt, concentrations up to 1.0 M (molar) or higher are possible, depending on the choice of solvents. For example, Mg(TFSI)2 can be dissolved in acetonitrile to form solutions having a concentration of at least 0.1M, at least 0.5M, or even at least 1.0 M at room temperature. The solubility of Mg(TFSI)2 can be even greater at elevated temperatures such as at temperatures greater than about 40° C. Solvents such as organic carbonates, nitriles, and pyridines provide a wide electrochemical stability window and thus enable production of high voltage Mg batteries. Acetonitrile has been found to be a particularly useful solvent because it is capable of supporting reversible Mg electrochemistry at relatively low overpotentials. Another advantage of Mg(TFSI)2 is that it is chemically stable to air and moisture, unlike certain background art electrolytes, like Grignard reagents, which are extremely difficult to handle due to their air and moisture sensitivity and their pyrophoric nature. Furthermore, whereas Grignard reagents are oxidatively unstable and therefore limit overall cell potentials for Mg batteries, Mg(TFSI)2 is much more stable to oxidation and thereby enables production of high voltage Mg batteries.
The thermally stable nature of electrolytes that include Mg(TFSI)2 or Mg(TFSM)2 and solvents such as organic carbonates, nitriles, and pyridine enables batteries comprising these electrolytes to be operated a elevated temperature. For example, batteries comprising these electrolytes can be operated at temperatures above 30° C., or above 40° C., or above 50° C. or above 60° C., or even higher. The limiting temperature factor can be the boiling point of the most volatile solvent in the electrolyte solution.
Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.
Preparation of Magnesium [bis-(trifluoromethanesulfonyl)imide]2 (Mg(TFSI)2)
High purity Mg chips (99.98% from Aesar, 9.350 g) and 150 g of deionized water (18 MOhm) were charged to a 1.0 L flask equipped with a condenser, nitrogen line, Claisen adapter and an addition funnel A 55.5 weight percent (wt %) solution of H—N(SO2CF3)2 in water (360.36 g, prepared according to PCT Pat. Appl. Publ. No. WO 97/23448, Example #12) was added dropwise with magnetic stirring at room temperature causing a mild exotherm and moderate gas evolution. After all of the imide acid solution was added, the reactor was equipped with a thermocouple probe and a heating mantle and the reaction solution was heated to 90° C. with stirring for two hours. After 2 hours, the pH of the reaction solution was 8.0 (according to pH stick) indicating that the reaction of the imide acid with excess Mg was complete. After the reaction solution was cooled to room temperature, it was filtered by suction through a 0.2 micron filter membrane to remove insoluble magnesium fines to yield 530.2 g of clear colorless aqueous filtrate (pH=8.0). The recovered filtrate was concentrated by short path distillation at atmospheric pressure to a final weight of 321.57 g and the concentrate was filtered again by suction through a 0.2 micron filter membrane to remove a small amount of insoluble solids. The filtered concentrate was transferred to a Pyrex crystallizing dish and evaporated to near dryness at 145° C. in a convection oven and then dried more completely in a vacuum oven at the same temperature. Once dry, the fused white solid was chipped out of the crystallizing dish and transferred to a mortar and pestle where it was ground to a fine powder and immediately transferred to a glass canning jar for further vacuum drying overnight at 145° C. to a final pressure of 45 mTorr. The product was allowed to cool to room temperature in vacuo and then vented with dry nitrogen, immediately capped and transferred to a nitrogen-filled drybox for storage. The isolated yield of anhydrous Mg(TFSI)2 product was 165.2 g (79.5% of theory). Analysis of the product by quantitative 1H and 19F NMR spectroscopy indicated that it was of high purity, containing 99.902% by wt. Mg[(N(SO2CF3)2]2. Water levels measured by Karl-Fischer analysis were 34 ppm and chloride ion levels measured by ion chromatography were <3 ppm. Levels of common metal ion impurities (24 element scan) were determined by ICP-MS and all metallic impurities were found to be present at less than 3 ppm in the product.
Chevrel phase Mo6S8 was prepared by chemical extraction of Cu from Cu2Mo6S8. 5.0839 g of Cu powder (Alfa Aesar, −325 mesh, 10% max +325 mesh, 99% metals basis), 7.7136 g Mo powder (Sigma Aldrich, 1-2 μm, ≧99.9%, trace metals basis) and 25.6209 g of MoS2 powder (Alfa Aesar, −325 mesh, 99% metals basis) were blended together by hand and placed in an alumina boat. The powder mixture was then heated under vacuum at 150° C. for 12 hours, ramped to 985° C. at a rate of 500° C./hr and held at 985° C. for 150 hours. The sample was then cooled to room temperature over 12 hours under vacuum. X-ray diffraction measurement showed that the product was Cu2Mo6S8.
About 10 g of Cu2Mo6S8 were placed in a 100 ml round bottom flask. A 6M HCl was prepared by diluting concentrated HCl (ACP, A.C.S Reagent). About 80 ml of the 6M HCl solution were added to the round bottom flask. The powder/HCl slurry was stirred for 18 hours while oxygen was bubbled through the slurry. The slurry was then filtered through a Buchner funnel and washed with 6M HCl until the filtrate became colorless. The washed powder was dried 120° C. in air for two hours. X-ray diffraction measurements of this powder showed that it was the Chevrel phase of Mo6S8.
3.2 g Mo6S8 powder, 0.4 g of Super P carbon black (MMM Carbon, Belgium), 0.4 g PVDF (Kynar, HSV900) and 10 g N-methylpyrrolidone (Sigma Aldrich, 99.5% anhydrous) were added to a 50 ml hardened steel grinding jar (Retsch) with two 1.25 mm tungsten carbide balls. The slurry was mixed at 120 rpm for one hour using a Retsch PM 200 planetary mill. The slurry was then coated onto aluminum foil using a doctor blade with a 0.008 inch (203.2 μm) gap. The coating was dried at 120° C. for one hour in air prior to use. Electrode disks 12.95 mm in diameter were punched from the foil for use in coin cells. Each disk had approximately 5-6 mg of Mo6S8 active material.
All electrolyte and coin cell preparation was performed in an argon glovebox with less than 0.1 ppm moisture and oxygen. Coin cells were constructed from 2325 coin cell hardware. Mg electrodes were prepared by punching 15.60 mm disks from 250 μm Mg foil (99.95%, GalliumSource LLC). Each cell contained a Mg foil electrode, CELGARD 2320 separator, electrolyte, a Mo6S8 disk electrode and a stainless steel spacer.
All electrolyte solvents were dried with molecular sieves (Sigma Aldrich, Type 3 Π, bead 4-8 mesh), unless otherwise stated. Electrolytes were prepared by dissolving Mg(TFSI)2 salt in either acetonitrile (Sigma Aldrich, ≧99.9%, for HPLC), pyridine (Sigma Aldrich, 99.8% anhydrous) and a 1:2 w/w solution of ethylene carbonate (EC)/diethyl carbonate (DEC) (EC/DEC solvent mixture used as received from Novolyte). All coin cells were electrochemically cycled at a rate of C/100 or C/50, based on 122 mAh/g for Mo6S8 using a Maccor Series 4000 Battery Test System. Cycling tests were performed in a thermostatically controlled chamber (±0.5° C.) at either 30° C. or 60° C. Dimethyl carbonate (DMC) was used as received from Novolyte for rinsing electrodes.
A Mo6S8vs. Mg coin cell was constructed using an electrolyte consisting of a 0.5 M solution of Mg(TFSI)2 in acetonitrile. The cell was discharged at a rate of C/100 for 100 hours at 60° C. The voltage curve of the cell is shown in
A Mo6S8 vs Mg coin cell was constructed using an electrolyte consisting of a 0.5 M solution of Mg(TFSI)2 in pyridine. The cell was discharged and charged at a C/50 rate at 60° C. between −0.6 V and 1.2 V and the voltage curve is shown in
A Mo6S8 vs Mg coin cell was constructed as described in Example 1, except that a Mg wire reference electrode was placed between the Mo6S8 and Mg electrodes. The cell was cycled such that the voltage of the Mo6S8 vs Mg wire reference electrode was between 0.5 V and 1.5 V at 60° C. and a C/50 rate. The voltage curve of this cell is shown in
A symmetric Mo6S8 coin cell was constructed as follows. First a Mo6S8 vs Mg coin cell was constructed and discharged as described in Example 1. The cell was then disassembled in an argon-filled glovebox and the discharged Mo6S8 electrode was removed. A new coin cell was then prepared with an electrolyte consisting of a 0.5 M solution of Mg(TFSI)2 in acetonitrile and one electrode being the discharged Mo6S8 electrode and the other electrode being a newly prepared Mo6S8 electrode. The cell was then cycled at a C/40 rate between +/−0.7 V. The voltage curve of this cell is shown in
A Mo6S8 vs Mg coin cell was constructed as described in Example 1, except a solution of 0.5 M Mg(TFSI)2 in adiponitrile was used as the electrolyte. The cell was discharged to −0.8 volts, after which the Mo6S8 electrode reached its full theoretical capacity. The voltage curve of this cell is shown in
A three electrode cell with Mg foil reference and counter electrodes and a 4 mm diameter glassy carbon rod working electrode was constructed in a 20 ml glass vial and covered with a rubber stopper with holes for electrical feedthroughs. Enough electrolyte comprising a 0.5 M solution of Mg(TFSI)2 in acetonitrile was added to the vial to cover the electrode surfaces. Cyclic voltammetry was conducted with this cell at a scan rate of 20 mV/s between 0.5 V and 4 V vs Mg at 25° C. The cyclic voltammagrams obtained are shown in
A Mo6S8vs. Mg coin cell was constructed using an electrolyte consisting of a 0.5 M solution of Mg(TFSI)2 in 1:2 w/w EC:DEC. The cell was discharged at a C/100 rate at 60° C. to zero volts and showed almost no capacity.
Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows. All references cited in this disclosure are herein incorporated by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2013/024805 | 2/6/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61599558 | Feb 2012 | US |
