ABSTRACT
The storage of energy by means of reversible intercalation of bivalent magnesium ions represents, nowadays, the shortest route for doubling the energy density of conventional lithium ion batteries. Contrary to the intercalation of monovalent lithium ions, the intercalation of Mg2+ is a kinetically limited process. Herein we demonstrate a new approach for improving Mg2+ intercalation, which is based on dual intercalation of Li+ and Mg2+ ions with a synergic effect. The concept is proved on the basis of eco-compatible oxides such as magnesium manganate spinels, MgMn2O4, and lithium titanates Li2TiO3 and Li4Ti5O12 with a monoclinic and spinel structure. These two types of oxides are selected since they exhibit high and low potentials of ion intercalation due to the redox couples Mn2,3+/Mn3,4+ and Ti3+/Ti4+, respectively. Through a newly developed method of synthesis, we succeeded in the preparation of well-crystallized nanosized spinels with a specific cationic distribution. The intercalation properties of MgMn2O4, Li2TiO3 and Li4Ti5O12 are first examined in model cells versus the metallic Li anode. The Li+ and Mg2+ intercalation is directed by the kind of the used electrolyte: a lithium electrolyte consisting of 1 M LiPF6 solution in EC : DMC and a magnesium electrolyte consisting of 0.5 Mg(TFSI)2 solution in diglyme. The mechanism of Li+ and Mg2+ co-intercalation is assessed by ex situ X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM). Finally, a new type of hybrid Li-Mg ion cell combining MgMn2O4 and Li4Ti5O12 oxides as electrodes is constructed. The cell configuration allows reaching an operating voltage of around 1.7 V by using an electrolyte containing 0.5 M LiTFSI in diglyme.
