Room temperature lithium superionic conductivity in high entropy oxides

Abstract

Impedance spectroscopy measurements evidence superionic Li + mobility (>10 À3 S cm À1 ) at room temperature and fast ionic mobility for Na + (5 Â 10 À6 S cm À1 ) in high entropy oxides, a new family of oxide-based materials with the general formula (MgCoNiCuZn) 1ÀxÀy Ga y A x O (with A ¼ Li, Na, K). Structural investigations indicate that the conduction path probably involves oxygen vacancies.

Charge compensation and electrostatic transferability in three entropy-stabilized oxides: Results from density functional theory calculations

“…16 Noting that the crystal ionic radius of Li is larger than the average radii for the J14 elements, they attribute the decrease in lattice constant with the oxidation of Co þ2 to Co þ3 , and to the formation of O defects. 16,17 The former is supported by X-ray Photoelectron Spectroscopy (XPS) data, which indicate oxidation of some of Co, although not enough to fully compensate the charge state of all of the added Li. For the compositions containing Li, the DFT calculations (see Table I) yield lattice constants for the binary, all of the ternary compositions, and the J14 þ Li random structure that are all smaller than those for the J14 elements alone.…”

“…Similarly, the effective ionic radius for Li þ1 fits within the radius range but would require oxidation of one or more of the J14 elements. As measured by Berardan et al, 16,17 oxidation of Co þ2 to Co þ3 is a viable mechanism, but based on Figure 1 the radius of Co þ3 ions is below the radius range defined by the horizontal dashed line. We note also that the radii for Ni þ3 and Cu þ3 are below this regime.…”

“…17 The Maria research group has also shown that the entropic rocksalt structure is maintained for the addition of other oxideforming elements, including Sc, Cr, Sb, Ge, Sn, and Ca. 18 Details of the experiments with Sc (a cation commonly associated with a þ3 charge formal state) are discussed below.…”

“…16,17 The former is commonly associated with a þ1 formal charge state, and so the authors suggest charge compensation involving oxidation of Co þ2 to Co þ3 and/or creation of O vacancies. They report that the addition of Ga without Li resulted in formation of a CuGa 2 O 4 phase, which they suggest may be due, in part, to the system being unable to accommodate the Ga þ3 formal charge state.…”

“…There are clearly tremendous possibilities for designing materials of this type with potentially interesting and unique properties and applications. [15][16][17] Zhang et al 3 demonstrated that the stability of nonoxide MHEAs can be reasonably predicted based on two empirical parameters that can be derived from the component elements, a measure of the differences in atomic radii and the mixing enthalpy for a solid solution. In addition, to enhance the free energy of the MHEAs the entropy of mixing should be maximized, which occurs for equi-molar concentrations.…”