Rechargeable salt battery

ANTHONY KING

The commercial battery with the highest energy density is the lithium-thionyl chloride (Li-SOCl2) battery. Developed in the 1970s, this non-rechargeable battery is still widely deployed in military, space, utility metering and GPS tracking applications. It uses thionyl chloride as the catholyte, lithium metal as the anode and amorphous carbon as the cathode.

Inspired by this battery, researchers in the US now report a rechargeable version whose energy comes from the formation of sea salt. To recharge, sodium ions flow back, while chlorine gas remains trapped in porous carbon (Nature, doi: 10.1038/s41586-021-03757-z).

The battery consists of sodium metal as the cathode and highly microporous carbon as the anode. The electrolyte is based on aluminium chloride in SOCl2 with fluorine salt additives.

‘We were aware of lithium-thionyl chloride batteries, and we were playing around with it. We discharged it, and then we decided to recharge it again, and to our surprise we observed recharge, to some extent,’ says Hongjie Dai, a battery chemist at Stanford University, California, who led the research.

The Stanford chemists had no clue how the rechargeability worked. It took almost three years to investigate the chemistry and to develop their rechargeable sodium chloride battery. The structure of the anode, amorphous carbon nanospheres, proved crucial.
On the cathode side, sodium metal is oxidised to sodium ions on discharge. These flow to the carbon anode with its micropores of less than 2nm in diameter, depositing as sodium chloride. During recharge, the NaCl in the carbon pores gets re-oxidised to give chlorine gas and sodium ions, with the sodium flowing back to the metallic sodium cathode.

‘The reaction is sodium plus chlorine gives you table salt [on the anode], then going back to sodium plus chlorine,’ says Dai. ‘But the reaction is not easy to make work in a battery, because chlorine is nasty and very reactive.’ The chlorine atoms remain trapped in the tiny carbon pores on the anode.

This battery dispenses with the classical Li-ion batteries materials, replacing the positive electrode usually derived from transition metals, in particular cobalt which is in short supply on the planet
Michel Armand battery chemist at the CIC energiGUNE research centre, Spain

The electrolyte was further refined with fluorine-containing compounds widely used in the battery industry, which facilitated the reversible flow of sodium ions. If the pores inside the carbon anode are too large, the battery does not work so well and is far less reversible.

The Na/Cl2 battery cycles with a Coulombic efficiency greater than 99% at a reversible capacity of about 1,200mAh/g. ‘Depending on the carbon material we could increase that further, because making the carbon material more microporous can allow you to store more sodium chloride,’ Dai explains.

The researchers then developed a lithium chloride battery along similar lines. Lithium is preferable in terms of processability, says Dai, ‘but in terms of abundance there is no question that sodium is a clear winner.’

Dai and his team now plan to boost the battery’s capacity with new carbon anode materials. ‘We want to push the energy density even further and we also want to improve the cycle life,’ says Dai.

‘This battery dispenses with the classical Li-ion batteries materials, replacing the positive electrode usually derived from transition metals, in particular cobalt which is in short supply on the planet,’ notes Michel Armand, a battery chemist at the CIC energiGUNE research centre in Spain. ‘Chlorine and sodium do not pose any problem of sustainability.’

‘I would have imagined the chlorine would be loosely trapped in the porous carbon, resulting in self-discharge,’ adds Armand. ‘This is not the case, and the keeping of the charge with time seems quite impressive.’

‘This work is highly significant as it offers a very high energy density Na-based battery that would be cheaper and without the reliance on lithium,’ notes Max Lu, chemical engineer and vice-chancellor of the University of Surrey. The innovation ‘is the use of a highly microporous carbon as anode [which] has drastically enhanced the storage capacity, with almost a ten-fold increase from the ordinary carbon paper seawater battery.’ He notes, however, that the battery’s stability was only demonstrated for a limited number of
cycles.

‘The most pressing drawback comes the use of a sodium metal electrode, that could react in a thermal runaway mode with the electrolyte within large batteries, such as for electric vehicles or grid storage,’ says Armand. This could happen if the thin protective layer over the metallic sodium gets disrupted and allows contact with the SOCl2 electrolyte.’