Electrolytic Conduction in Transition-Metal-Free Oxides with K2NiF4 Structure

Abstract

La2NiO4 and related materials with the K2NiF4 structure are known to support high electronic conductivity (about 100 S•cm-1) and oxygen ionic conductivity (comparable to or exceeding that of yttria-stabilized zirconia).1, 2 Unlike other well known oxygen ion conductors, these materials conduct via an interstitial rather than a vacancy mechanism. While the K2NiF4 crystal structure is robust across a fairly wide solid solution compositional space, nearly all known compositions exhibit mixed ionic electronic conduction due to one or more transition-metals on the B-site. One exception, LaSrAlO4,3 was previously found to be unstable to the introduction of ionic defects. For these reasons, despite high oxygen ion conductivity, oxides with the K2NiF4 structure have been limited in application as electrode and not electrolyte materials in solid oxide fuel cells (SOFC). In this work, we aimed to develop a new type of SOFC electrolyte material from transition-metal-free oxides with the K2NiF4 structure. The first successful composition, La1.6Sr0.4Al0.4Mg0.6O4,4 was created by solid-state reaction. Oxygen ion defects were able to be created in the lattice by adjusting the La/Sr ratio. We hypothesize that defects are stabilized in this composition but not LaSrAlO4 due to higher charge separation between the rocksalt and perovskite layers in the crystal lattice. Similar compositions substituting Al, Mg or Sr with Ga, Zn or Ca, respectively, were also synthesized and were confirmed to maintain this crystal structure. X-ray diffraction revealed that the crystal structure was consistently more stable to higher concentrations of vacancies relative to interstitial defects. Four-electrode conductivity measurements indicated that the B-site composition had a larger effect on total electrical conductivity than the A-site composition. Specifically, replacing Al or Mg with Ga or Zn, respectively, improved the conductivity by 1 to 2.5 orders of magnitude. Unfortunately, further electrochemical studies suggested significant hole conduction in these samples. Substituting Sr with Ca in the baseline composition had no effect on the total conductivity value, and La1.6+xCa0.4-xAl0.4Mg0.6O4 series samples were all found to be pure ionic conductors. Nevertheless, the oxygen ion conductivity values achieved in the compositions to date remain less than technologically desirable. Results from experiments on newer compositions will show that Li can be placed on the B-site. Such compositions may have utility in both Li-ion and Li-air batteries.