Contribution of Anode and Cathode Potentials during Charging to the Performance of Aluminium-Ion Battery with Triethylamine-AlCl3 Electrolyte

Abstract

Aluminium ion batteries (AIBs) are a promising emerging battery technology, thanks to the high theoretical gravimetric and volumetric energy density of Aluminium as well as its very large abundancy and recycling capability.[1] In aprotic graphite-Al cell, however, practical energy density is limited by both electrochemical stability window of the electrolyte (2.4-2.6V) and specific capacity of graphite for long-term reversible storage of large AlCl4 - anions (radius 530 pm vs. 68 pm for Al3+). This works aims at studying the influence of cut-off voltage value during charging step in 1:1.5 triethylamine hydrochloride / aluminium tetrachloride (TEA:AlCl3) electrolyte on Al and graphite electrode potential under half-cell and full-cell conditions. In Aluminium symmetric cell, potentials during Al deposition on an Al foil at 1 mA cm-2 were 60-70 mV higher in TEA-AlCl3 electrolytecompared to the cell measured with EMIMCl-AlCl3 electrolyte as reference system. Because of higher electrochemical stability of TEA-AlCl3 solution than EMIMCl-AlCl3 electrolyte, cut-off voltage of charging step in AIB full-cell was set to 2.45 V compared to 2.40 V for the reference cell. During long-term cycling at 1 A g-1, the specific capacity of the AIB cell employing TEA-AlCl3 increased from 60to 94 mAh g-1 (57%) after 1000 cycles while that of cell with EMIMCl-AlCl3 was quite constant at about 60 mAh g-1. This behaviour was explained by continuous decreasing in Al electrode potential at End-of-Charge state (EOC) during deposition reaction (charging step) in TEA-AlCl3 allowing an increase in graphite electrode potential and consequently in AIB cell capacity over the whole experiment. This increase in capacity was also accompanied by a raise of defect sites in graphite material which was revealed using Raman spectroscopy.