Pyrazine polymer PHATNs up cathode in sodium-ion batteries

• Scientists at the University of Maryland have developed an organic polymer electrode which they claim demonstrates stable function for a sodium-ion battery over 50,000 cycles and also offers encouraging performance in magnesium-ion and aluminum-ion storage devices.

Scientists from the U.S. and China have demonstrated a high performance cathode material they say could aid the development of low cost, durable sodium-ion batteries.

Replacing the lithium in batteries with a safer, more durable and lower cost material is a key concern for scientists working on energy storage technologies, alongside boosting energy density and cycle lifetime.

Sodium-ion batteries offer one possibility and easily fulfill the requirement of cheap, abundant, low fire risk materials. While such batteries have seen limited commercial application – including powering a sewage pumping station in Australia – the technology has struggled to match rivals in terms of energy density and long term stability.

Scientists led by Chunsheng Wang, of the University of Maryland’s Department of Chemical and Biomolecular Engineering, however, believe a key reason for this is a lack of suitable electrode materials, particularly ones free of expensive heavy metals.

Pyrazine compound

The researchers identified a pyrazine-derived organic compound called hexaazatrinaphthalene (HATN), which has already achieved impressive performance in lithium-ion batteries, as a suitable material to work with. Initially, as with many organic electrode materials, the HATN dissolved into the electrolyte, rendering the battery unstable during cycling.

The next stage of the work described in the paper A Pyrazine-Based Polymer for Fast Charging Batteries – published in Angewandte Chemie – saw the scientists attempt to stabilize the structure by linking individual molecules. They created polymeric HATN (PHATN), which they proceeded to test in a variety of battery chemistries.

Encouraging results

In a sodium battery, the PHATN cathode demonstrated reversible capacity of 220 milliamp hours per gram (mAh/g-1), corresponding to energy density of 440 watt-hours per kilogram. The battery retained a capacity of more than 100 mAh/g-1 after 50,000 cycles and the scientists reported it could be operated at a maximum 3.5 V. The paper described that result as “among the best performances in sodium-ion batteries”.

Encouraging results were also observed after combining the PHATN cathode with less well developed battery technologies. That maintained reversible capacities of 110 mAh g−1 after 200 cycles in a magnesium battery and 92 mAh g−1 after 100 cycles in an aluminum device.

The researchers hope to see their pyrazine-based cathodes employed in “environmentally benign, high-energy-density, fast and ultra-stable, next generation rechargeable batteries,” according to an announcement by Wiley, the publishing house which produces Angewandte Chemie.