Monovalent manganese could enable new batteries

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Using a range of X-ray techniques, researchers from the US Lawrence Berkeley National Laboratory (LBNL) have confirmed a chemical state of manganese first proposed about 90 years ago. According to the team, its discovery could enable low cost sodium-ion batteries, along with new areas for battery research.

Speculation dating back to 1928 contended that manganese could exist in a monovalent state, in which a manganese atom loses only one electron, rather than the usual two or more electrons. While such a state would enable a voltage range useful for battery anodes, no work had been done to confirm the existence of monovalent manganese.

LBNL’s work, conducted in association with New York University and Natron Energy, was based on a Natron sodium-ion battery whose anode featured a blend of elements – including manganese, carbon and nitrogen.

“Typically, in lithium-ion and sodium-ion batteries, the anode is more often carbon-based,” said Berkeley Lab scientist Wanli Yang. However, the Natron battery features an anode based on transition metals and a cathode with copper, nitrogen, carbon and iron. “The interesting part here is that both electrodes are based on the chemistry of transition metals in the same type of materials,” he added.

The team used soft X-ray absorption spectroscopy at first, which appeared to show mainly the 2+ form of manganese. The researchers then used a new system called in situ resonant inelastic X-ray scattering, or iRIXS. This showed a tell tale contrast in the electrons during the battery’s charge and discharge cycles.

“A very clear contrast immediately showed up with iRIXS,” Yang said. "We later realised that manganese 1+ behaves very closely to the typical 2+ state in other conventional spectroscopy, which is why it had been difficult to detect for so many decades.”

The Natron battery has been shown to deliver up to 90% of its total energy during a five minute discharge and to retain about 95% of its discharge capacity for 1000 cycles.

Yang said the team’s work could inspire other R&D in new types of battery electrodes. “The operation of a battery could drive the emergence of atypical chemical states that do not exist in our conventional thinking. This basic understanding could trigger other novel designs,” he concluded.