  Lithium-air batteries have been considered as ultimate solutions for the
power source of long-range electrified transportation, but state-of-the-art
prototypes still suffer from short cycle life, low efficiency and poor power
output. Here, a lithium-bromine rechargeable fuel cell using highly
concentrated bromine catholytes is demonstrated with comparable specific
energy, improved power density, and higher efficiency. The cell is similar in
structure to a hybrid-electrolyte Li-air battery, where a lithium metal anode
in nonaqueous electrolyte is separated from aqueous bromine catholytes by a
lithium-ion conducting ceramic plate. The cell with a flat graphite electrode
can discharge at a peak power density around 9mW cm-2 and in principle could
provide a specific energy of 791.8 Wh kg-1, superior to most existing cathode
materials and catholytes. It can also run in regenerative mode to recover the
lithium metal anode and free bromine with 80-90% voltage efficiency, without
any catalysts. Degradation of the solid electrolyte and the evaporation of
bromine during deep charging are challenges that should be addressed in
improved designs to fully exploit the high specific energy of liquid bromine.
The proposed system offers a potential power source for long-range electric
vehicles, beyond current Li-ion batteries yet close to envisioned Li-air
batteries.
