Eco-compatible oxides enabling energy storage via Li+/Mg2+ co-intercalation

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 Mn^2,3+/Mn^3,4+ 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.