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| Discharge curves of the Li–O2/CO2 batteries with various ratio of CO2 in O2/CO2 mixed gas at 25 °C (current density: 0.2 mA cm-2). Inset shows CO2 ratio dependence on relative capacities as compared with the Li–O2 battery (CO2 0%). Credit: RSC, Takechi et al. Click to enlarge. |
Researchers at the Advanced Battery Lab, Toyota Central R&D Laboratories, Inc., have developed a new advanced gas-utilizing battery using a mixture of O2 and CO2 and featuring a very high discharge capacity of up to almost three times that of a non-aqueous Li–air (O2) battery. A paper on their work was published in the Royal Society of Chemistry journal Chemical Communications.
Although the new battery has to be a primary battery (i.e., non-rechargeable) due to the difficulty of electrochemical decomposition of
Li2CO3 in the cathode, its very high discharge capacity offers the potential for an alternative energy source with the use of
CO2-rich gas such as exhaust gas from vehicles or factories, the researchers conclude. Additionally, the basic mechanism of this battery can in
principle be extended to non-lithium systems.
Because of its potential very high energy density, Li–air (O2) chemistry is one of the promising candidates looked to for providing the significant improvement in batteries required to meet the demands of future vehicles, electronics and other applications. (Earlier post.) The oxygen reduction at the cathode (air electrode) is the most important process in the battery. However, the team of Kensuke Takechi, Tohru Shiga and Takahiko Asaoka explain, the reduced oxygen forms superoxide anion
radical species (O2•-) in non-aqueous electrolyte. A subsequent reaction between O2 and Li+ leads to final discharge products: Li2O2, Li2O, and/or the decomposed compounds of solvent in the electrolyte.
Because these discharge products are expected to be stored in the cathode, many researchers have selected porous carbon material due
to its large void volume. However, Takechi et al. note, since the usual Li–air battery only consumes oxygen, an intermittent radical species,
highly reactive LiO2•, is generated during the discharge reaction.
This reactive species limits the discharge capacity
of the battery due to the rapid precipitation of the discharge
products and their poor electron conducting property, which
causes the incomplete filling of the void volume in the cathode.
In this study, we developed a new strategy in enhancing the
discharge capacity of Li–air battery by introducing a mixed
gas of O2 and CO2 (Li–O2/CO2 battery). It is well known that O2•- can be captured by CO2, and the reaction has been
applied for CO2 sensors or molten-carbonate fuel cells
(MCFC). We noticed that the series of the reactions between
O2•- and CO2 was expected to slow down the
precipitation speed of the discharged products in the cathode
and have demonstrated the performance of the Li–O2/CO2
battery.—Takechi et al.
In their experiments, the basic structure of the cell and electrodes were identical to those of the Li–O2 battery. The team introduced a mixed gas of O2 and CO2 in different concentrations into the cell directly with a pressure
of 2 kgf cm-2 (28 psi) at 25 °C.
Among their findings were that the discharge capacity of the Li–O2/CO2 battery (CO2 ratio: 50%) was 5860 mAh g-1, or 289% as compared
with that of the Li–O2 battery (CO2 ratio: 0%). Only 10% of CO2 in the mixed gas boosted the discharge
capacity of the battery twice as much as the standard Li–O2
cell, and 30% CO2 enhanced the performance almost three
times as much. The appropriate CO2 ratio to obtain the
maximum capacity was from 30 to 70%. When the CO2 ratio was higher than 80%, the capacity was dramatically reduced
because of the low O2 concentration.
The unique point of the battery, the researchers say, is the rapid consumption of the superoxide anion radical by CO2
as well as the slow filling property of the Li2CO3 in the cathode.
Resources
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Kensuke Takechi, Tohru Shiga and Takahiko Asaoka (2011) A Li–O2/CO2 battery. Chem. Commun., doi: 10.1039/C0CC05176D