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Watered up: aqueous iodine doubles Li-ion battery density

By Laura Syrett
Published: Thursday, 15 August 2013

New Japanese chemistry could mean more mileage for electric vehicles

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) batteries.

 
The higher energy density of Li-I2 batteries
could increase the mileage of EVs without
adding to the weight of battery packs (image: dj venus).
Research published in the journal Nature Communications by the scientists from Japan’s 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 water.

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.

For their Li-I2 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 battery’s energy density.

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 the cathode.

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 metal ions.

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 density.

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 density.

Iodine’s 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 ? kWh / kg -1 , compared to an average of 150-170 Wh/kg for conventional Li-ion batteries.

Yu and Byon’s 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 Byon’s 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.

Having a 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 or cathode.

Aqueous 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.

Unlike 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.

By contrast, 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 electricity.

Additionally, Yu and Byon’s team found that the aqueous battery could be successfully recharged numerous times, demonstrating 200 cycles without any loss of capacity.

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.

Cost effective

According to the researchers, the Li-I2 batteries could represent an economical alternative to conventional Li-ion batteries.

“The 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.

“In 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 environmental penalties.”

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 Li-ion’s share

Although 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 industry.

“Rather 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.

“Many 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.”

The 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 commercially viable.

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 compromising capacity.”

The second 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.

However, Yu and Byon’s 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.

“Pure 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 said.

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.

“Looking 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 said.