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RIKEN team develops high-performance lithium-iodine battery system with higher energy density than conventional Li-ion
8 July 2013
|The working concept of I3–/I– redox reaction in the aqueous Li-I2 battery. Zhao et al. Click to enlarge.|
A team from Japan’s RIKEN, led by Hye Ryung Byon, has developed a lithium-iodine (Li-I2) battery system with a significantly higher energy density than conventional lithium-ion batteries. RIKEN is Japan’s largest research organization, with institutes and centers in locations throughout Japan.
In a paper published in Nature Communications, they report that aqueous lithium-iodine batteries based on the triiodide/iodide redox reaction show high battery performance. The high solubility of triiodide/iodide redox couples results in an energy density of ~ 0.33 kWh kg−1. The reversible redox reaction without the formation of resistive solid products promotes rechargeability, demonstrating 100 cycles with negligible capacity fading.
They suggest that a low cost, non-flammable and heavy-metal-free aqueous cathode can contribute to the feasibility of scale-up of lithium-iodine batteries for practical energy storage.
According to the Battery Roadmap 2010 announced by the New Energy and Industrial Technology Development Organization in Japan, the main target for rechargeable batteries is an improvement of the energy density up to 0.5 kWh kg-1cell (1.0 kWh l-1cell) by 2030 to enable electric vehicles to extend the driving range to a comparable level with gasoline-powered internal combustion engine vehicles (ca. 500 km) [311 miles]. However, achieving more than three times the energy density raised from the currently employed battery systems (<0.2 kWh kg-1cell) is an exceptional challenge, because the current battery technology has almost reached its performance limitation. Accordingly, new battery systems using new chemistries and system configurations are needed, which are capable of achieving higher energy density than the current ones.
...The alternative to make a less problematic storage system [than lithium-sulfur or lithium-O2] is aqueous lithium batteries. The aqueous solution has a high ionic conductivity in the presence of completely ionized substances, which leads to rapid redox reactions in electrochemical cells. The idea has been to employ these aqueous electrolytes in a cathode, referred to as the aqueous cathode using redox couple reactions that do not deteriorate the aqueous cathode and electrically conductive current collector, as well as not leave over any solid product and precipitation residue at the aqueous cathode/current collector interface. The aqueous cathode can, therefore, offer negligible polarization and volume expansion.
A promising aqueous cathode can be determined from the redox couples possessing high solubility and a suitable redox potential avoiding the electrolysis of water. The solubility is proportional to the energy density. In the diagram of redox couple solubility with respect to the standard reduction potential, the triiodide/iodide (I3–/I–) redox couple reaction shows a favorable solubility...The redox potential of the I3–/I– couples (0.536 V versus standard hydrogen electrode (SHE)) is also suitable to avoid water electrolysis....in this work for the first time, we present the aqueous cathode operated by the I3–/I– redox couples and apply this for a lithium-iodine (Li-I2) battery.
The aqueous Li-I2 battery we demonstrate is noticeably different from either the conventional all-solid-state or non-aqueous electrolyte-based Li-I2 batteries, which have performed at extremely low discharge current rate or shown low Coulombic efficiency with the formation of a lithium iodide (LiI) layer. The I3–/I– redox reaction in aqueous cathode is rapid and performed up 12 mA cm-2 of discharge current rate without serious potential drop. The aqueous Li-I2 battery attains superior storage capacity (~98% of the theoretical capacity), Coulombic efficiency (>99.5%), and cyclic performance (>99.5% capacity retention for 100 cycles) which is, to the best of our knowledge, the best result among previous reports using new chemistries and system configurations.—Zhao et al.
|Schematic illustration of the aqueous Li-I2 battery. Zhao et al. Click to enlarge.|
The aqueous Li-I2 batteries consist of a lithium anode (Cu mesh/Li metal/organic electrolyte/buffer layer), ceramic separator, the aqueous cathode, and current collector (Super P carbon/Ti foil). Li metal with 1 M of LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte was used for the anode. The Super P carbon-coated Ti foil was employed as the current collector in the aqueous cathode.
The Li-I2 batteries showed high energy density and excellent recharge ability.
Specific capacity was ~207 mAh g-1 at a current rate of 2.5 mA cm-2, which approaches ~98% of the theoretical capacity.
Energy density was ~0.35 kWh kg-1 calculated from the mass of aqueous cathode containing saturated I2 and KI and Li metal in anode and ~0.33 kWh kg-1 estimated from the experimental result—several times higher than previously reported for alkaline aqueous cathodes.
No significant capacity fading was observed during cycles; ~99.6% capacity retention and 99.5-100% Coulombic efficiency for 100 cycles is superior to Li-S and Li-O2 batteries, as well as other aqueous Li batteries using a solid Ni(OH)2 cathode.
- The open circuit potential was ~3.8 V versus Li+/Li and the discharge and charge potentials were stable at 3.50 and 3.70 V versus Li+/Li on cycling, which resulted in overpotentials of 0.04 and 0.16 V, respectively. The overpotential value at such a high current rate is still comparable to Li-ion batteries, and superior to Li-S and Li-O2 batteries at current rates even an order of magnitude lower, demonstrating ~90% overall energy efficiency on cycling,
Byon and colleagues now plan to develop a three-dimensional, micro-structured current collector that could enhance the diffusion-controlled triiodide/iodide process and accelerate charge and discharge. They are also seeking to raise energy densities even further by using a flowing-electrode configuration using an external reservoir.
In summary, the I3–/I– redox reaction-operated aqueous Li-I2 battery exhibited excellent cyclic performance with considerable energy and power densities. Ideal Coulombic efficiency and capacity retention with no degradation of cathode establish the aqueous Li-I2 battery as one of the outstanding post-Li batteries. The specific energy density (~0.33 kWh kg-1) is fairly promising in comparison to the conventional lead-acid, nickel-metal hydride and Li-ion batteries, and can be further improved to be ~0.4 kWh kg-1 using NaI or LiI instead of aqueous KI...The aqueous cathode system can be also extendable to aqueous Na-I2 and K-I2 batteries. We believe that this energy storage system can be scaled up with a reasonably increased energy density by combining a flow-through-mode system.—Zhao et al.
Zhao, Y., Wang, L. & Byon, H. R. (2013) High-performance rechargeable lithium-iodine batteries using triiodide/iodide redox couples in an aqueous cathode. Nature Communications 4, 1896 doi: 10.1038/ncomms2907
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