Behavior of Li Guest in KNb5O13 Host with One-Dimensional Tunnels and Multiple Interstitial Sites

The demonstration of reversible lithium insertion into oxide hosts1!4 has made the advent of the Liþ-ion battery possible. However, the organic liquid-carbonate electrolytes used have an energy gap (Eg) between their lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) located between 1.1 V and 4.3 V versus Liþ/Li0, which makes the electrochemical potential (the Fermi energy) of lithium poorly matched to the electrolyte LUMO. In an investigation of the position of the Nb(V)/Nb(IV) redox energy in an oxide relative to the LUMO of the carbonate electrolyte, a study of the reversible lithium insertion into KNb5O13 of up to 3.5 Li/formula unit showed an unusual hysteresis of the voltage profile, which indicated that, unlike in a conventional Li-insertion host, the sequence of interstitial sites occupied by the Liþ upon insertion is different from that upon extraction. In this study, only the magnitude of hysteresis was slightly increased upon increasing the current densities from 15 mA/g to 30 mA/g to 60 mA/g; but the voltage profile was not changed. Therefore, theNernst equation coupled with slow diffusion is not the cause of the hysteresis shapeobserved; this shapewould seem to represent a difference in the path sequence on Li insertion and extraction. Since the voltages 1.0 V < V < 2.0 V versus Liþ/Li0 are below the kinetic LUMO of the electrolyte, there is no formation of a passivating SEI layer to complicate the interpretation of this phenomenon. Therefore, we have undertaken modeling and a further experimental study to clarify the insertion versus extraction process. The synthesis and structure of KNb5O13 was first reported by Kwak et al. The orthorhombic (Pbcm) structure with four formula units per unit cell is shown in Figure 1, projected along the a-axis. TheNb atoms occupy three distinguishable octahedral sites, 8e, and 4c symmetry sites. Two-dimensional a!b slabs contain edge-shared Nb2!Nb3!Nb2 triples sharing edges with 180! Nb2!O7!Nb2 bridges connecting the a!b slabs; the triples are bridged by corner-sharing Nb1 octahedra. This peculiar configuration of NbO6 octahedra leaves open a-axis and b-axis channels occupied by the large Kþ ion. Displacement of the Kþ ion in the b-direction from the axis of the a-axis tunnels leaves open 4d1 positions along the a-axis between K þ ions; 4d2 positions between Kþ ions along the b-axis are also available within the a-axis tunnels, as shown in Figure 2. Within the slabs, empty 8e1, 8e2, and 4c1 sites are also available to Li þ.