title: Anomalously high Na+and low Li+mobility in intercalated Na2Ti6O13.
abstract: We report an anomalous diffusion behavior in intercalated Na2Ti6O13. Using first-principles calculations, the direct migration of inserted Na+along the tunnel direction is predicted to have a barrier of 0.24-0.44 eV, while the migration of inserted Li+along the tunnel direction has a barrier of 0.86-1.15 eV. Although Li+can also diffuse along a zig-zag path in the tunnel, the barrier of 0.86-0.99 eV is still much higher than that for Na+. Our results surprisingly lead to the conclusion that the diffusion of larger Na+is 4-8 orders of magnitude faster than Li+in the same host lattice, and explain the experimentally observed exceptional rate capability of Na2Ti6O13as the Na-ion battery anode. The anomalous diffusion behavior is attributed to the geometric features of Na2Ti6O13. For migration of Li+it is necessary to weaken Li-O bonds and to overcome the repulsion between Li and host Na ions simultaneously, while for Na+diffusion the improved Na-O bonding at the transition state partially compensates for the energy penalty from the repulsion of host Na ions.
authors.affiliation: Toyota Research Institute of North America, 1555 Woodridge Ave, Ann Arbor, Michigan, USA. chen.ling@toyota.com.
authors.forename: Chen
authors.initials: C
authors.lastname: Ling
authors.forename: Ruigang
authors.initials: R
authors.lastname: Zhang