Sealed cell improves oxide-peroxide conversion in lithium-ion battery


sealed cell battery-supply-866599918-iStock_MF3d-web-635x357Sealed for success. Oxygen-free cells improve lithium-ion batteries. Credit: battery supply 866599918 iStock MF3d

A new high-energy density and stable lithium-ion battery that works by reversible oxide-peroxide conversion could help in the development of improved “sealed” battery technologies. This is the new result from a team of researchers in Japan and China who have designed an oxygen-free cell in which the Li2O to Li2Oreaction can take place.

Lithium-ion batteries are hitting the headlines this week with news of this year’s Nobel Prize for Chemistry being awarded to John Goodenough, Stanley Whittingham and Akira Yoshino for the development of these devices.

Lithium is the material of choice in these batteries because it has a high specific capacity and low electrochemical potential. In recent years, focus has shifted from the rigid Li-intercalation structures commonly employed in the conventional heavy lithium-transition metal oxide cathodes used in these devices to Li-Obattery technology that exploits oxygen-related redox chemistries that have excellent theoretical gravimetric energy densities. 

Redox reaction between Oand Li2O2

These batteries work thanks to the redox reaction between Oand lithium peroxide, Li2O2. One of the main hurdles hindering their practical application, however, is that they require Ogas as the active species. This needs to be supplied by bulky Ostorage or gas purification devices.

To overcome this problem, and the so-called Ocrossover and electrolyte volatilization in these batteries, researchers led by Haoshen Zhou of the National Institute of Advanced Industrial Science and Technology (AIST) and Nanjing University in China have now designed an O2-free sealed environment for the Li2O to Li2Oreaction.

Li2O/Li2O2 battery system

High-energy density, rechargeable and stable Li-ion battery

Zhou and colleagues did this by embedding Li2O nanoparticles into an iridium-reduced graphene oxide (Ir-rGO) catalytic substrate to successfully control the charging potential within a small region of the device and avoid the unwanted phenomenon of over-polarization.

“The choice of Ir nanoparticles as the catalyst is key, as is the conductive rGO substrate,” explains Zhou. “The Ir can effectively enhance the reaction kinetics and protect the newly formed Li2Ofrom further decomposition (by the formation of the inter-metallic Li2-xO2-Ir compound formed on the particles/substrate interface) while the rGO allows for the remarkable electrical conductivity of the system.”

The researchers also restrained two other serious problems that beset sealed redox systems: the irreversible evolution of Oand the production of superoxide (an aggressive and dangerous product). They did this by controlling the degree of the electrochemical reaction and its cycling depth and thus succeeded in producing a reversible capacity for the device of 400 mAh/g, a value that fares well when compared to other cathode candidates for Li-ion batteries.

The result is a high-energy density (1090 Wh/kg), high energy efficiency (a mere 0.12 V polarization potential), rechargeable Li-ion battery technology that is stable over 2000 cycles with 99.5% coulombic efficiency.

Lithium-ion battery pioneers bag chemistry Nobel prize

 

Although he and his colleagues still need to fully understand the catalysis mechanism at play in the cell, Zhou believes that the sealed Li2O/Li2Obattery system could gradually replace today’s open-cell Li-Obatteries and even become a a “hot” topic for next-generation battery research. “From an applications viewpoint, the very competitive properties of the sealed system could help in the development of cathode materials for commercial Li-ion battery technology,” he tells Physics World.

The researchers, reporting their work in Nature Catalysis 10.1038/s41929-019-0362-zsay they are now looking for more effective catalysts to further boost the reversible capacity region in their device and enhance the reaction kinetics.

Solution coating the easy way


201306047919620Researchers in the US and China have developed the first solution-coating technique capable of producing high-quality, large-area single-crystalline organic semiconductor thin films suitable for high-performance, low power and inexpensive printed electronic circuits. The technique, dubbed FLUENCE (fluid-enhanced crystal engineering) can be used to make thin film organic semiconductors with record charge carrier mobilities.

Fluid flow around micropillars

Solution coating of organic semiconductors is an excellent method for making large-area and flexible electronic materials. However, it is not at all good for making aligned single-crystalline thin films – the ideal form for organic semiconductors and that have the best electronic properties. Aligned crystals are preferred in these materials because charge carrier transport through these structures depends on the crystal orientation.

A team led by Zhenan Bao at Stanford University and Stefan Mannsfeld of the SLAC National Accelerator Laboratory is now reporting on a new solution-coating method that can produce high-quality, millimetre-wide and centimetre-long highly aligned single-crystalline organic semiconductor thin films. The essence of FLUENCE is that we are able to control the flow of liquid in which the organic semiconductor is dissolved, explains team member Ying Diao. During fast printing, this “ink” often distributes itself unevenly – something that leads to defects and other structural imperfections quickly appearing in the semiconducting crystals.

FLUENCE tackles this problem from two angles, she says. First, it works using a microstructured printing blade containing tiny pillars that mixes the ink uniformly. Second, specially designed chemical patterns on the substrate prevent the crystals from aligning randomly or “stochastically” in a direction that would be the opposite to that in which printing is taking place. These two methods combined lead to large-area highly aligned single crystalline films that are much more structurally perfect.

To prove that their technique works, the researchers fabricated an organic semiconductor made from TIPS-pentacene, a routinely used and much studied organic semiconductor material, and found a charge carrier mobility of as high as 11 cm2 V−1 s−1. This is the first time a mobility of greater than 10 cm2 V−1 s−1 has been reported for TIPS-pentacene.

“The concepts we have developed in FLUENCE could easily be scaled up and applied to commercial printing methods,” said Diao. “The significant improvement in structural quality and electrical performance of the thin films printed with our method could allow to make higher performance, lower power, small and inexpensive organic circuitry,” she told nanotechweb.org. “We hope that our work will help advance such a morphology-by-design approach to make organic semiconductors for high-performance, large-area printed electronics.”

The team, which includes researchers from Nanjing University in China, says that it will now look at pattering aligned crystals at length scales suitable for making sub-micron devices.

The present work is reported in Nature Materials.

About the author

Belle Dumé is contributing editor at nanotechweb.org.