The liquid ammoniate of sodium iodide as an alternative electrolyte for sodium ion batteries: The case of titanium dioxide nanotube electrodes

Abstract

Efforts for the development of sodium ion batteries (NIB) are focusing on insertion electrode materials rather than on novel electrolytes. In a radically new approach, sodium insertion into amorphous TiO2 nanotubes, chosen as a typical electrode material for NIBs, is studied for the NaI liquid ammoniate (NaI·3.3NH3) and compared with the behavior in typical organic electrolytes (for instance 1 M NaClO4 in propylene carbonate, PC). The liquid ammoniate leads to significantly larger electrode capacities (between 0.5 and 2.6 V vs. Na+/Na): 145 mA h g−1 in NaI·3.3NH3 versus 105 mA h g−1 in 1M NaClO4/PC at 1 mA cm−2 (14C for the ammoniate). This is linked to the outstanding conductivity and sodium concentration of NaI·3.3NH3, together with the much smaller charge transfer resistance observed for this electrolyte. In more general vein, the prospects for using the NaI·3.3NH3 liquid ammoniate in NIBs are finally discussed.