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| Example of a lithium-water rechargeable battery. Credit: ACS, Lu et al. Click to enlarge. |
Researchers at the University of Texas, including Dr. John Goodenough, are proposing a strategy for high-capacity next-generation alkali (lithium or sodium)-ion batteries using water-soluble redox couples as the cathode.
In a paper published in the Journal of the American Chemical Society, they report on their demonstration of feasibility of a such a battery having a thin, solid lithium- or sodium-ion electrolyte separating a water-soluble redox couple as the cathode and lithium or sodium in a nonaqueous electrolyte as the anode.
The cell operates without a catalyst and has high storage efficiency. The possibility of a flow-through mode for the cathode allows flexibility of the cell design for safe,
large-capacity electrical-energy storage at an acceptable cost, they conclude.
Although intense effort has
been devoted to the development of carbon or carbon-buffered
alloys as anodes for lithium batteries, lithium metal still has the
highest specific capacity (3860 mA h g-1) and sodium metal
would be cheaper; therefore, an important challenge for the third
generation of alkali-ion batteries is how to fully and effectively
utilize the high capacity of an alkali metal and increase the
capacity of the cathode beyond what is possible with an insertion
compound while operating at ambient temperature. The present
sodium-sulfur battery operates above 300 °C.
Oxygen in air, an “inexhaustible resource” on earth, makes it
possible to match the capacity of lithium metal. In theory, the
specific energy of a lithium-oxygen (air) battery is 5200 W h
kg-1. The high energy storage has stimulated a worldwide study
of Li-air batteries. A typical Li-air battery discharges at
2.5-2.7 V and charges at 4.2-4.4 V. The large discrepancy of
1.7 V between the charge and discharge curves represents a low
Coulombic efficiency (CE), even when expensive catalysts have
been used to lower the overpotential of the oxygen reactions.
Moreover, slow oxygen diffusion in the cathode degrades the
performance of the Li-air battery.
In theory, the decomposition potential of water is 4.27 V vs.
Li/Li+ at room temperature. The high theoretical potential
predicts the possibility of obtaining a lithium-water battery
with a high potential through a proper structural design. Recently,
a Li|β-NiOOH (water) battery with an operating potential
of ~3.4 V was developed. The cathode, β-NiOOH,
exhibited a specific capacity of 256 mA h g-1, but its capacity
is still far smaller than that of lithium metal.
Here we present a new strategy for alkali-ion batteries using
water-soluble redox couples as the cathode.—Lu et al.
The electrochemical reactions at the electrodes are:
anode: nA → nA+ + ne–
cathode:Mz+(aq) + ne– → M(z-n)+(aq)
A = lithium or sodium (Li or Na), M represents a metal and 1 ≤ n < z. The overall reaction is written:
nA + Mz+(aq) → nA+ + M(z-n)+(aq)
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| Electrochemical behavior of the proof-of-concept battery. Credit: ACS, Lu et al. Click to enlarge. |
In their paper, Lu et al. note that an aqueous cathode has a low viscosity and can be easily circulated in a flow-through configuration at room temperature.
The aqueous cathode could be individually
stored in tank, reducing the volume of the
battery and increasing the design flexibility of the battery structure. Their battery has a specific energy high enough to challenge the specific energies of fuel cells.
The flow of the aqueous solution brings heat out of the battery system and
keeps the battery working near ambient conditions. Furthermore, the aqueous cathode does not suffer from H2 evolution from the solution, and the battery is
efficiently rechargeable, they note.
The aqueous cathodes must:
- have proper redox potentials;
- no side reactions;
- good stability in water;
- good reversibility;
- reliable safety; and
- low cost.
In their initial proof of concept study, they used aqueous Fe-
(NO3)3/Fe(NO3)2 as the cathode, with a relatively
thick, commercially available lithium superionic conductor
(LISICON).
…we have demonstrated the feasibility of an
alkali-ion battery utilizing an alkali metal as the anode and a redox
couple soluble in aqueous solution as the cathode. However, the
capacity of an aqueous cathode is small…requiring the concept to be extended to a flow-through
mode for the cathode. Also, sodium rather than lithium might be
used as the anode. This new strategy represents a third-generation
alkali-ion battery promising lower cost than the conventional
lithium-ion rechargeable battery, safe operation, and a
Coulombic efficiency and voltage greater than those of a Li-air
battery with, in principle, a comparable capacity. The demonstrated
power output was limited by the commercially available
solid electrolyte separating the organic-liquid or polymer anolyte
and the aqueous cathode; the need for the design of a superior
solid electrolyte has been indicated. The new strategy promises
to be applicable to both the electric-vehicle market and the
problem of electrical energy storage for the grid.—Lu et al.
John Goodenough. In 2009, US Energy Secretary Steven Chu named Dr. John B. Goodenough as a winner of the Enrico Fermi Award “in recognition for his lasting contributions to materials science and technology, especially the science underlying lithium-ion batteries. Dr. Goodenough, a physicist, identified and developed the cathode materials for the lithium-ion rechargeable battery that is ubiquitous in today’s portable electronic devices. This material has proven to be inexpensive, environmentally friendly, safe, sustainable, and capable of thousands of charge cycles with a constant output voltage without a loss of capacity. Batteries incorporating this cathode material are used worldwide for cell phones and other portable wireless devices, power tools, hybrid automobiles, small all-electric vehicles, as well as increasingly for electrical energy storage for alternative energy, such as wind and solar power.”
Goodenough invented lithium cobalt oxide cathode materials while at Oxford University. His technology was used in the first commercial Li-ion battery, launched by SONY in 1991. More recently, at the University of Texas, Austin, Dr. Goodenough patented a new class of iron phosphate materials with potential to replace the more costly cobalt materials. In 2000, he received the prestigious Japan Prize for his discoveries of the materials critical to the development of lightweight rechargeable batteries.
Resources
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Aqueous Cathode for Next-Generation Alkali-Ion Batteries
Yuhao Lu, John B. Goodenough, Youngsik Kim
Journal of the American Chemical Society doi: 10.1021/ja201118f