Batteries: Shuttling chloride ions

Angew. Chem. Int. Ed. 55, 4285–4290 (2016)

Chloride-ion batteries are one of the latest additions to the rechargeable battery family. In contrast to lithium-ion batteries in which lithium ions move between electrodes, chloride-ion batteries rely on chloride ion (Cl⁻) transport, offering high theoretical energy capacity that rivals their lithium counterparts. However, research into Cl⁻ batteries is still in its infancy, and the current challenge lies in finding suitable electrodes and electrolytes that can offer high cyclability. Maximilian Fichtner and colleagues in Germany have now reported a Cl⁻ battery consisting of a vanadium oxychloride cathode (VOCl), a lithium anode and a solution of 1-butyl-1-methylpiperidinium chloride (PP₁₄Cl) in propylene carbonate as an electrolyte, which displays high stability over 100 cycles.

The researchers revealed that upon discharge, the PP₁₄⁺ ions diffuse into the VOCl cathode, expanding the VOCl interlayers, while the vanadium ions are partially reduced to V²⁺; meanwhile, the Cl⁻ ions diffuse into the lithium anode, which is then oxidized into LiCl. While charging, V²⁺ can be oxidized back into V³⁺; importantly, the enlarged VOCl interlayers due to the intercalated PP₁₄⁺ ions allow the return of Cl⁻ into the cathode. These multistep reactions ensure good reversibility during the discharge/charge process in the battery.