Patent Application
20070092801
This invention relates in general to the field of electrolytes for electrochemical devices, and more particularly to electrolytes for lithium and lithium-ion rechargeable batteries and electrochemical capacitors.
Electrochemical devices include cells, batteries, capacitors, timers, coulometers and electrochromic windows. Batteries and capacitors store energy and find use in electric and hybrid vehicles, satellites and portable electronics such as cell phones, computers and music players, to name but a few applications. An ongoing challenge is to develop high gravimetric and volumetric energy density batteries and capacitors capable of delivering high power that are cost effective, rechargeable and safe.
Liquid electrolytes are found in many electrochemical devices including batteries, such as lithium-ion batteries, and electrochemical capacitors. Improvement in ionic conductivity and the cation transport number is desirable. High ionic conductivity is necessary for high-power applications. Liquid electrolytes consist of salt dissolved in one or more solvents. Often the solvents utilized are volatile and flammable. If a battery or electrochemical capacitor is subject to abuse such as heating, crushing, dropping, short-circuiting, puncturing, overcharging and/or overdischarging, the electrolyte can vent from the cell and occasionally ignite, resulting in fire and/or explosion. There exists a need for non-volatile liquid electrolytes with high conductivity and electrochemical devices containing said electrolyte. The electrolytes should perform over a wide temperature range and be cost effective.
Solid electrolytes are non-volatile, but the ionic conductivities are usually several orders of magnitude less than for liquid electrolytes. In addition, solid electrolytes often do not wet electrodes thoroughly. If an electrode changes volume during charge or discharge, contact between a solid electrolyte and electrode may be lost resulting in high cell impedance.
Molten salts, also known as ionic liquids, are non-volatile and operate in a large electrochemical window, yet often have the disadvantage of having high melting points that necessitate high operating temperatures.
A reversible solvent in which a non-ionic liquid (an alcohol and amine base) converts to a molten salt at room temperature upon exposure to carbon dioxide is reported in Nature 436, 11 02 (25 Aug. 2005). The molten salt converts back to the non-ionic liquid when exposed to nitrogen or argon. This reversible solvent is intended for use in organic synthesis and separation where the need to remove and replace solvents between reaction steps is eliminated. The non-ionic liquid is a mixture of 1-hexanol and 1,8-diaza-bicyclo-[5,4,0]-undec-7-ene (DBU). The reaction scheme is as follows.
An object of the present invention is to provide a non-volatile molten salt electrolyte that avoids the above-mentioned problems. A further object of the present invention is to provide electrochemical devices containing such an electrolyte. A further object of the present invention is to provide a method of wetting electrodes and separator with a molten salt electrolyte.
In the present invention, a conductive molten salt electrolyte is obtained by making an organic molten salt and adding additional salt. The additional salt is often necessary because the ions that make-up the molten salt may not be the ions needed for operation of an electrochemical device. For example, lithium and lithium-ion batteries require mobile Li+ in the electrolyte.
The molten salt is made by combining an amine base and an alcohol which convert to a molten salt when exposed to a non-metal oxide. A particularly preferred amine base is 1,8-diaza-bicyclo-[5,4,0]-undec-7-ene (DBU). A particularly preferred alcohol is 1-hexanol. A particularly preferred non-metal oxide is carbon dioxide. When carbon dioxide is bubbled through non-ionic DBU and 1-hexanol, a molten salt, the DBU salt of 1-hexylcarbonate is formed. The molten salt reverts back to the amine base and alcohol when exposed to nitrogen or argon. Dihydric alcohols (diols), trihydric alcohols (triols) such as glycerol and polyols are also suitable.
The cation of the molten salt is a cyclic organic cation selected from the group consisting of the structures below and combinations thereof.
The substituents, R1 to R15, are the same or different and are selected from the group consisting of H, halogens, C1-C15 alkyl and C1-C15 haloalkyl and combinations thereof. R1 to R15 may connect with one another to form bicyclic, tricyclic or multicyclic cations. A preferred example is the following 1,4,5,6-Tetrahydropyrimidine cation shown below.
Alkyl groups where one or more hydrogens are replaced by one or more halogens selected from the group consisting of fluorine, chlorine, bromine and iodine and combinations thereof, are termed haloalkyls.
The anion of the molten salt is selected from the group consisting of organic carbonate anions and organic sulfite anions and combinations thereof. Organic carbonate anions are selected from the group consisting of RCO3−, R(CO3)22−, R(CO3)33− and and R(CO3)nn− and combinations thereof, where R is selected from the group consisting of C1-C15 alkyl and C1-C15 haloalkyl and combinations thereof and n is a positive integer. Organic sulfite anions are selected from the group consisting of RSO3−, R(SO3)22−, R(SO3)33− and R(SO3)nn− and combinations thereof, where R is selected from the group consisting of C1-C15 alkyl and C1-C15 haloalkyl and combinations thereof and n is a positive integer.
Additional salt can be dissolved in the molten salt. The additional salt contains a cation selected from the group consisting of Li+, Na+, K+, Ag+, Rb+, Cs+, Mg2+, Ca2+, Fe2+, Fe3+, Cu+, Cu2+, Zn2+, Al3+ and R4X+ and combinations thereof, where R is selected from the group consisting of H, halogens, C1-C5 alkyl and C1-C5 haloalkyls and combinations thereof and where X is selected from the group consisting of N and P and combinations thereof. The additional salt contains an anion selected from the group consisting of ClO4−, AlCl4−, NO3−, CO32−, SO42−, PF6−, AsF6−, SbF6−, Cl−, Br−, I−, CnR1(2n+1)Q1−, CF3SbF5SO3−, B10Cl102−, B12Cl12−2, B4O72−, R24B−, −N(Q2R3)(Q3R4) and −C(Q4R5)(Q5R6)(Q6R7) and combinations thereof where n=1 to 5 and R1 to R7 are the same or different and are selected from the group consisting of H, halogens, C1-C5 alkyl and C1-C5 haloalkyls, including bridging C1-C5 alkyl and C1-C5 haloalkyls that link Q2 and Q3, Q4 and Q5, Q4 and Q6, Q5 and Q6, and combinations thereof, where Q1 to Q6 are the same or different and are selected from the group consisting of CO and SO2. −N(Q2R3)(Q3R4) and −C(Q4R5)(Q5R6)(Q6R7) are shown below.
For a lithium ion conductive electrolyte, suitable for use in electrochemical capacitors and lithium and lithium-ion batteries, preferred lithium salts include, but are not limited to, LiPF6, LiAsF6, LiSbF6, LiSO3CF3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiCO3 and LiNO3. For electrochemical capacitors, preferred salts include, but are not limited to, (C2H5)4NBF4, (C2H5)4PBF4 and (C2H5)4NClO4.
The electrolyte may contain an anionic polymer lithium salt such as, but not limited to, polyvinyl lithium sulfonate, sufficient to prevent electrolyte crystallization.
The electrolyte may contain high-boiling, high dielectric constant, cyclic solvents selected from the group consisting of cyclic carbonates and cyclic halogenated carbonates and combinations thereof. Cyclic carbonates include, but are not limited to, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate and vinylene carbonate. Cyclic carbonates where one or more hydrogens are replaced by one or more halogens selected from the group consisting of fluorine, chlorine, bromine and iodine and combinations thereof, are termed cyclic halocarbonates. Preferred cyclic halocarbonates include, but are not limited to, chloroethylene carbonate. These high-boiling and high dielectric constant, cyclic solvents assist in forming a passivation layer at electrode/electrolyte interfaces.
Molten salts are sometimes viscous presenting a problem wetting electrodes and separator already folded or rolled into a jellyroll and already in an external can or other packaging. An electrochemical device can be filled and wetted with the molten salt or, preferably, with the amine base and alcohol which are then converted to the molten salt with the addition of a non-metal oxide such as CO2. Additional salt is added at any time.
