From left to right: (A) a Zn anode (1cm in diameter), (B) an EMIHSO4 – PVA separator (laying on a syringe needle to illustrate thickness and transparency), and (C) a PbO2 – carbon cathode. |
Scientists
at the NRL Materials Science and Technology Division are providing
solid evidence that there is a new route towards developing novel,
lightweight energy storage devices. By moving away from centuries of
caustic, hazardous aqueous-based battery cells and instead using
non-volatile, thermally-stable ionic liquids, scientists predict
multiple new types of batteries. Rather than depend on highly acidic
electrolytes, ionic liquids are used to create a solid polymer
electrolyte composed of an ionic liquid and polyvinyl alcohol,
developing novel types of solid state batteries with discharge voltages
ranging up to 1.8 volts.
The
unique properties of ionic liquids have fostered this explosive
interest in battery applications. Ionic liquids are room temperature
molten salts that possess many important characteristics, such as nearly
no vapor pressure, non- flammability and lack of reactivity in various
electrochemical or industrial applications. “It is the high thermal and
electrochemical stability of the ionic liquids which has fostered the
growing interest in ionic liquids for use in various electrochemical
processes,” said Dr. Thomas Sutto. “These new types of solid-state cells
mimic standard alkaline cells, but without the need for caustic
electrolytes.”
Limits
imposed by using corrosive electrolytes often result in severe
restrictions to standard battery geometry and the need for special
corrosive-resistant battery containers. The use of reactive ionic
liquids in non-aqueous cells replaces the more hazardous highly alkaline
electrolytes such as manganese oxide (MgO) and zinc (Zn) found in
traditional batteries.
The
root of this work began during standard corrosion studies of different
metals in ionic liquids. While working with ionic liquids based on
mineral acids, such as hydrogen sulphates, it was observed that Zn metal
would react to form zinc sulphate. Since this is similar to that
observed for the zinc anode in a standard alkaline cell, a series of
experiments were then performed to determine how different metal oxides
reacted in these types of ionic liquids.
Electrochemical
experiments demonstrate that not only can these reactive ionic liquids
act as the electrolyte/separator in both solid state and liquid
batteries, but they can also act as a reactive species in the cell’s
electrochemical makeup. Using a non-aqueous approach to primary and
secondary power sources, batteries are designed using standard cathode
and anode materials such as magnesium dioxide (MgO2), lead dioxide
(PbO2) and silver oxide (AgO). The ionic liquid that is the main focus
of this work is 1-ethyl-3-methylimidazolium hydrogen sulphate (EMIHSO4),
however, other ionic liquids such as those based on the nitrate and
dihydrogen phosphate anions (negatively charged ions) have also been
found to work well in this type of a battery design.
The
use of these electrolytes suggests the potential for new types of
rechargeable systems, such as replacement electrolytes in nickel-metal
hydride (NiMH) batteries, or even the standard lead-acid battery.
Experimental work is currently underway to develop such a rechargeable
ionic liquid power source. The ability to create solid separators also
allows for the formation of many new types of batteries via a number of
fabrication techniques.
