Transition Metal Sulfides As Cathode Materials for Rechargeable Aluminum Ion Battery

Rechargeable aluminum battery system is very intriguing due to the following reasons: First of all, aluminum has high capacity due to its trivalency. And aluminum is very cheap since it is the most abundant metal element in earth’s crust. 1 As a result, rechargeable aluminum battery can be very promising in large scale energy storage application. One of the main reasons that hinders the development of rechargeable aluminum battery is the lacking of electrolyte that can enable facile deposition and dissolution of aluminum in the anode side. On the other hand, facile electrochemical deposition and dissolution of aluminum can be achieved in room temperature ionic liquid (molten salt) synthesized by mixing aluminum chloride (AlCl 3 ) with organic salts such as 1-butylpyridinium chloride, 1-ethyl-3-methylimidazolium chloride, etc. at a certain ratio. 2,3 In a previous research, our group proposed Chevrel phase Mo 6 S 8 as the first conventional intercalation type cathode material. 4 The logic of choosing transition metal sulfide instead of transition metal oxide as cathode material for aluminum ion battery is very important. Due to the strong coulombic effect, the energy barrier of multivalent ions transportation in the crystal structure is very high. 5 Thus, a softer anionic framework is needed. Sulfide has a much lower electronegativity than oxide, which makes transition metal sulfide a very promising cathode candidate for rechargeable aluminum ion battery. To further validate this assumption, we synthesized nano sized layered TiS 2 and cubic Ti 2 S 4 as well as investigated their electrochemical properties as cathode materials for ionic liquid electrolyte based rechargeable aluminum ion battery at both room temperature and 50 °C. We further confirmed the aluminum intercalation in the TiS 2 and Ti 2 S 4 crystal structure using ex-situ XRD and XPS. The proposed titanium sulfide cathode materials showed decent reversible capacity and a higher working potential. In order to further understand the mechanism of Al intercalation into Chevrel Phase Mo 6 S 8 , high resolution TEM is used to directly visualize the crystal structure of Al x Mo 6 S 8 from different discharged and charged stages. Powder XRD Rietveld refinement is also undertaken to provide support from a theoretical simulation perspective. 1. Li, Q.; Bjerrum, N. J. J. Power Sources 2002, 110, 1. 2. Endres, F. ChemPhysChem 2002, 3, 144. 3. Jiang, T.; Chollier Brym, M. J.; Dube, G.; Lasia, A.; Brisard, G. M. Surf. Coat. Technol. 2006, 201, 1. 4. Geng, L.; Lv, G.; Xing, X. Guo, J. 2015, 27, 4926−4929. 5. Rong, Z.; Malik, R.; Canepa, P.; Sai Gautam, G.; Liu, M.; Jain, A. Persson, K.; Ceder, G. Chem. Mater., 2015, 27 (17), pp 6016–6021.