Lithium-ion batteries are the most popular with the highest specific energy of all rechargeable batteries. Current materials implement intercalation compounds that have been developed over the past decade. However, intercalation compounds have the problem of one or less than one redox reaction that occur in the electrode host lattice. Therefore, in a bid to increase the specific capacity a new redox mechanism should be developed. Electrochemical conversion reaction uses all the available oxidation state of a high valence metal compound and high capacity is realized.
However, the main issue hampering the use of metal fluorides is poor conductivity of the material. No groups however have focused on testing the aluminum fluoride as an electrode material. It has been identified to coat particular materials to enhance their cyclability and function as an interfacial stabilizer, since it is found to prevent cathode-electrolyte interface degradation.
Nathan Owen and Qi Zhang from Cranfield University in the United Kingdom explored the possibility of implementing aluminum fluoride as an electrode material for high energy density applications. They also indicated the initial charge and discharge profiles of the tested materials. In their study, they also analyzed the redox reaction of aluminum fluoride. Their research work is published in Journal of Applied Electrochemistry.
The authors used aluminum fluoride with nanometer and micron uncoated, coated, and carbon nanocomposite sized particles as high-capacity electrode material for lithium batteries. They indicated the initial discharge capacities to be about 100mAhg-1 for the micrometer size particles and 957 mAhg-1 for other particles.
The charge discharge analyses indicated that the material was reversible but the specific capacity was seen to decrease to less than 5% of the initial discharge capacity after ten cycles. The reversible conversion reaction of aluminum fluoride followed the reduction of aluminum fluoride into lithium fluoride and aluminum in the discharge phase. In the charging phase, aluminum would be oxidized to form aluminum fluoride and lithium again.
It was however noted that not all aluminum was oxidized and this was put down to a combination of a large voltage needed to overcome the large surface/interfacial energy produced through the production of nano lithium fluoride and aluminum particles. The formation of LiAlF6 from lithium fluoride and aluminum fluoride in the short cycle life of the cell indicated that there was unwanted side reactions with the electrolyte. Lack of capacity in the subsequent discharges was then attributed to this.
Owen and Zhang concluded that the material was reversible, and it was important to find the best-engineered electrode and cell electrolyte to enhance cyclability. Synthesizing smaller particles appears to be the best way of improving the material. Reducing the particle size will improve the material kinematics so that higher discharge and charge rates are realized. Another method would entail finding a compatible electrolyte that will not generate unwanted products and would increase charge profile with an aim of improving cycle life and cycling capacities.
Nathan Owen and Qi Zhang. Investigations of aluminum ﬂuoride as a new cathode material for lithium-ion batteries. Journal of Applied Electrochemistry (2017) 47:417–431.Go To Journal of Applied Electrochemistry