A team of Chinese and Korean researchers has developed a
lithium-iodine (Li-I2) battery system which has
double the energy density of conventional lithium-ion (Li-ion)
Research published in the journal
Nature Communications by the scientists from
Japans RIKEN centre showed that the performance of
lithium-based batteries could be improved by using an aqueous
system, in which the organic electrolyte in the cathode of
conventional Li-ion electrochemical cells is replaced with
|The higher energy density of
could increase the mileage of EVs without
adding to the weight of battery packs (image: dj
The RIKEN team is involved in alternative energy research
specifically, improving the performance of
lithium-based battery technologies, for which increased
commercial application requires advances in energy density,
lifespan and rechargeability.
battery system, the researchers investigated the performance of
an aqueous cathode, which accelerates reduction and oxidation
(redox) reactions to improve battery performance.
This aqueous Li-I2 battery can solve some
disadvantages associated with the commercial, solid-state and
nonaqueous electrolyte-based batteries, Dr Yu Zhao and Dr
Hye Ryung Byon, who led the research, told IM.
Finding suitable reagents for the aqueous cathode proved to be
a tricky proposition, however. According to Yu and Byon, water
solubility is the most important criterion for screening new
materials, since this determines the batterys energy
Furthermore, the redox reaction has to take place in a
restricted voltage range in order to avoid water electrolysis
the decomposition of water into oxygen and hydrogen,
which can cause explosions due to the increased pressure inside
After extensive research,
in which they tested various materials, the researchers settled
on using iodine,
an element with high water solubility and a pair of ions, known
as the triiodide/iodide redox couple.
The presence of this pair of ions made iodine an ideal material
for the aqueous cathode, the researchers explained, because its
solubility in alkali iodide means that it readily undergoes
aqueous electrochemical reactions,
and it is more stable and less toxic compared with other heavy
Energy density for EVs
Li-ion batteries are commonly used in devices such as mobile
phones and laptop computers, but efforts to use them to power
electric vehicles (EVs)
have been hampered their relatively low energy
This has meant that EV engineers have been faced with the
challenge of packing enough batteries into a vehicle to provide
the desired power without introducing storage and weight issues
something that has so far proved difficult.
According to Yu and Byon, aqueous lithium-iodine batteries
offer a number of major advantages over conventional Li-ion
batteries, the first of which is higher energy
Iodines solubility corresponds to its high energy
density. By using a tri-iodide/iodide redox, the RIKEN team
achieved an energy density of approximately 0.33
, compared to an average of 150-170 Wh/kg for conventional
Yu and Byons team have also managed to raise
Li-I2 energy densities even further by using a
flowing-cathode configuration that stores aqueous
fuel in an external reservoir.
This modification should make Li-I2 batteries more
amenable to EV specifications, because with a
cathode-flow-through-mode the energy density of the battery can
be increased tens or even hundreds of times. Because the
aqueous solution can be stored externally, this will limit the
weight of the battery pack.
Other aqueous advantages
Testing by Yu
and Byons team found that the conductivity of aqueous
batteries outperforms that of solid-state batteries because the
aqueous chemical reaction does not create any solid
precipitation, allowing the lithium ions to travel unobstructed
through the water.
liquid cathode also affords greater flexibility of battery
design. Because the lithium ions can travel far more quickly
through liquid than solids, the target energy capacity can be
controlled by the volume of liquid in the aqueous electrolyte
Li-I2 batteries are easier to make than solid-state
batteries as they do not need additional materials such as
conducting additives and binders. They also require only a low
level of maintenance.
solid-state li-ion batteries, the aqueous Li-I2
battery electrodes do not undergo structural deformation when
generating power. Solid batteries experience a number of
unintended side-reactions, in addition to the main
electrochemical reaction, which take place at the interfaces
between the numerous components (electrolyte, carbon additive,
binder, etc.), thereby reducing their lifespan.
the aqueous batteries only experience one simple interface
reaction between the lithium ions and the aqueous cathode for
the necessary transfer of electrons to generate
Additionally, Yu and Byons team found that the aqueous
battery could be successfully recharged numerous times,
demonstrating 200 cycles without any loss of
Most commercial Li-ion batteries can be charged and discharged
for around 300 cycles with capacity retention of approximately
60%. However, their capacity keeps fading with each cycle due
to the side reactions between the active materials,
electrolyte, carbon additives and binders.
the researchers, the Li-I2 batteries could represent
an economical alternative to conventional Li-ion batteries.
price of cathode material in the Li-ion battery, for example
lithium cobalt oxide (LiCoO2, 99.8% purity,
Sigma-Aldrich) is around $115/100g. This is nearly 40% higher
than that of the price of iodine (>99.8% purity,
Sigma-Aldrich), which is around $70.4/100g, they
explained, adding that it takes around 1.4 kg iodine to
generate 1 KWh of electricity.
addition, the organic electrolyte used in a Li-ion battery
accounts for over 20% of the total cost, whereas the water in
the Li-I2 battery is almost free. The assembly
process for solid-state Li-ion batteries is also more expensive
than that for the aqueous Li-I2 battery, and because
the iodine is easily recycled we can save the cost of
Yu and Byon
note that there is still the high cost of the Li-ion ceramic
separator component, which splits the electrolyte between the
anode and cathode sections of the battery, to be considered,
but she believes that this can be developed further to reduce
costs and improve ionic conductivity.
No eye for the
RIKEN team has demonstrated that aqueous Li-I2
batteries offer significant advantages over conventional Li-ion
technology, Yu and Byon do not believe that the aqueous
chemistry will take market share away from the Li-ion battery
than competing with or replacing Li-ion batteries, we think
that the Li-I2 battery can be employed in its own
target field, they said.
batteries, such as Li-ion, lead-acid, nickel metal hydride
(Ni-MH) and sodium sulphur (Na-S) batteries, have been used for
different markets because of their unique chemistries and
characteristics. We expect that Li-I2 batteries with
an aqueous electrolyte reservoir can be applied for large-scale
grid storage, and, perhaps, for future EVs, if the solid
electrolyte separator can be further developed.
researchers also noted that the technology is still in the
early stages of development, and that a number of areas need
perfecting before Li-I2 batteries can become
The first one
of these is the anode: We demonstrated the
Li-I2 battery using metallic lithium for the anode.
This has a very high energy capacity but also presents safety
issues [because metallic lithium is flammable when it comes
into direct contact with moisture], Yu and Byon said,
but we can probably replace the metallic lithium with
graphite, or we could develop a liquid anode with an
electrolyte reservoir to overcome the safety risks without
issue, according to the researchers, is the performance of the
solid electrolyte as the separator. The ionic conductivity of
the commercially available ceramic solid electrolyte is
relatively low, which is a bottleneck to improving the power
density of the battery.
and Byons team expects that concentrated research in this
area will be able to enhance the ionic conductivity of solid
electrolyte next 5-10 years.
A third major
issue is the energy density of Li-I2 batteries.
While they significantly outperform existing Li-ion chemistry,
they fall a long way short of their full potential.
iodine delivers an energy density of 0.74 KWh/kg. So far, the
maximum energy density we have obtained is 0.33 KWh/kg,
the researchers explained.
Increasing the weight fraction of iodine in the aqueous
solution is challenging. Gels may be able to provide a
solution, but this needs to further investigation, they
Yu and Byon
are ambitious, however, and hope to reduce the development
timescale for aqueous Li-I2 batteries to make them a
realistic near-term prospect.
back into the history of rechargeable batteries, lead-acid
batteries, Ni-MH batteries and Li-ion batteries have taken more
than 20 years from the proof-of-concept stage to final
commercialisation. We want to see commercial Li-I2
batteries available in the 2020s, if possible, they