The potential of potassium-ion batteries for a greener future

Lithium-ion batteries (LIBs) have become the go-to power source for a variety of electronic devices and vehicles over the last two decades. However, the limited availability of lithium, as well as safety and environmental concerns, have prompted scientists to seek out alternative battery technologies, such as aqueous batteries. Potassium-ion batteries (KIBs) are made from abundantly available materials and are safer than lithium-ion batteries. KIBs can also utilise a water-in-salt electrolyte (WISE), which makes them more stable thermally and chemically.

However, the prevention of hydrogen evolution at the negative electrode for its stabilisation is a major challenge in high-voltage aqueous batteries. While solid-electrolyte interphases (SEI) that form between these electrodes and the electrolyte solution help stabilise the electrodes in LIBs, they have been scarcely researched in the context of KIBs. To address this knowledge gap, researchers from the Tokyo University of Science have conducted a study to gain insights into SEI formation and their properties in WISE-based KIBs. Their findings were published in the journal Angewandte Chemie International Edition.

The researchers employed two advanced analytical techniques — scanning electrochemical microscopy (SECM) and operando electrochemical mass spectrometry (OEMS) — to observe how SEI forms and reacts in real time during the operation of a KIB with a 3, 4, 9, 10-perylenetetracarboxylic diimide negative electrode and 55 mol/kg K(FSA) 0.06 (OTf) 0.4∙1H2O, a WISE developed by the team in a previous study.

The experiments revealed that SEI forms a passivating layer in WISE akin to that seen in LIBs, with slow apparent electron transfer rates, helping suppress hydrogen evolution. This ensures stable performance and higher durability of KIBs. However, the researchers observed that the coverage of the SEI layer was incomplete at higher operating voltages, leading to hydrogen evolution. Taken together, the results reveal the need to explore potential avenues to enhance SEI formation in future aqueous batteries. TUS Professor Shinichi Komaba said that while the results reveal interesting details about the properties and stability of SEI found in one particular WISE, the research should also focus on reinforcing the SEI network to achieve improved functionality.

“SEI could perhaps be improved by the development of other electrolytes that produce unique SEIs, but also through the incorporation of electrolyte additives or electrode surface pre-treatment,” Komaba said.

The study also highlighted the power of SECM and OEMS for gaining a solid understanding of electrode–electrolyte interactions in next-generation batteries. “These techniques provide a powerful means for tracking the development, coverage, ion transfer and stability of SEI and can easily be adapted for a variety of electrolytes and electrodes,” Komaba said.

The development of aqueous batteries such as KIBs could be instrumental for future sustainable societies, as they could replace the LIBs currently used in electric vehicles, smart grids and renewable energy systems. By making energy storage more accessible, aqueous batteries will aid the transition toward carbon-neutral energy generation.

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