Effect of additives in the electrodepositon of aluminum from ionic liquids for application to the development of secondary rechargeable batteries

Aluminum (Al) anodes can provide a secondary battery for high energy density applications, such as stationary energy storage or transportation needs. Al cannot be electrodeposited from aqueous solutions due to breakdown of water. It has been shown that Al electrodeposited from mixture of ionic liquid based on imidazolium chloride and aluminum chloride (AlCl ₃) can be used as rechargeable battery electrode. In this work, cycle life was limited due to the compositional changes taking place in the electrolyte solution upon cycling which was proved with NMR spectroscopy. The intial 33% Depth of Dicharge (DOD) for 15 cycles showed the improvement of cycle life of deposited Al over 100% DOD cycling for 70 cycles due to the formation of base layer of Al. The effect of addition of co-solvent such dry benzene improves the conductivity of electrolyte and reduces the kinematic viscosity. This helps in improved current distribution and dense and compact Al deposition. As the concentration of benzene increased the Al was deposited with equiaxed grains and no entrained salt was observed in the deposit. In our previous work, cycle life of deposited Al was limited to ∼100 cycles due to the formation of undesireable surface products which were rich in Cl and Al. To improve the cycle life and minimize the compositional changes taking place in the electrolyte, organic solvents such as acetonitrile and dry benzene were added to the IL electrolyte. The cycle life and surface morphology of deposited Al from this mixture has been studied as a function of IL: solvent concentration. At various concentrations of the solvent (20-80 wt %), the galvanostatic cycling (charging and discharging) was studied with three-electrode electrochemical setup. It has been found that more than 100 cycles were obtained with 100% depth of discharge (DOD) at 20% dry benzene concetration. The addition of solvent helped in improving mobility of electroactive species in the electrolyte. The NMR spectroscopic measurements have been carried out with various concentrations of organic additives to the mixture of IL and AlCl ₃. Also the variation of the morphology of the electrodeposited Al was studied with SEM and EDAX with varying the concentrations of the organic additives. Though the addition of organic additives improve the battery performance, it will also increase the risk of developing unsafe batteries due to the flammability or high vapor pressure associated with these organic additives. The flammability and vapor pressure was monitored with varying concentration of the electrolyte and organics, using a bomb calorimeter setup. Optimization between battery safety and performance is the desired goal.