  In recent years, impressive advances in harvesting renewable energy have led
to pressing demand for the complimentary energy storage technology. Here, a
high coulombic efficiency (~ 99.7%) Al battery is developed using
earth-abundant aluminum as the anode, graphite as the cathode, and a cheap
ionic liquid analogue electrolyte made from a mixture of AlCl3 and urea in 1.3
: 1 molar ratio. The battery displays discharge voltage plateaus around 1.9 V
and 1.5 V (average discharge = 1.73 V) and yielded a specific cathode capacity
of ~73 mAh g-1 at a current density of 100 mA g-1 (~ 1.4 C). High coulombic
efficiency over a range of charge-discharge rates and stability over ~150-200
cycles was easily demonstrated. In-situ Raman spectroscopy clearly showed
chloroaluminate anion intercalation/deintercalation of graphite in the cathode
side during charge/discharge and suggested the formation of a stage 2 graphite
intercalation compound when fully charged. Raman spectroscopy and nuclear
magnetic resonance suggested the existence of AlCl4-, Al2Cl7- anions, and
[AlCl2. (urea)n]+ cations in the urea/AlCl3 electrolyte when an excess of AlCl3
was present. Aluminum deposition therefore proceeded through two pathways, one
involving Al2Cl7- anions and the other involving [AlCl2.(urea)n]+ cations. This
battery is a promising prospect for a future high performance, low cost energy
storage device.
