posted on 2021-07-20, 16:08authored byDanielle Butts, Jürgen Schoiber, Christopher Choi, Günther
J. Redhammer, Nicola Hüsing, Scott Donne, Bruce Dunn
The great interest in Na-ion batteries
has led to the development
of a number of materials based on the analogues of Li-ion battery
materials. However, the larger ion radius and the lower operating
potential of Na compared to those of Li show the limitation of that
approach and underscore the need for new methods of electrode material
design. In this paper, the well-known superionic conductor, Na β″-Al2O3, is converted into a high-rate Na+ negative electrode by substituting Fe for Al and achieving mixed
electron–ion conduction; that is, the presence of Fe2+/Fe3+ in the spinel block of the structure produces electronic
conduction, while Na+ transport is retained in the conduction
plane. The 52 mol % Fe-substituted Na β″-Al2O3 exhibited a high ionic conductivity (∼10–3 S cm–1), near theoretical capacity
values at 1C, and retained ∼70% of theoretical capacity at
20C. Kinetic analysis revealed that a surface-controlled charge-storage
mechanism is responsible for the high-rate capability. Although the
gravimetric capacity of this converted superionic conductor is limited
by the small number of Na-ion storage sites available per transition
metal, the transformation of Na β″-Al2O3 into a mixed electronic–ionic conducting electrode
provides a set of design rules for achieving high-rate redox electrodes
from solid-state electrolytes.