Exfoliation of Layered Na-Ion Anode Material Na₂Ti₃O₇ for Enhanced Capacity and Cyclability

We report the exfoliation of layered Na₂Ti₃O₇, a promising anode material for Na-ion batteries, and restacking using HNO₃ and NaOH to form H-[Ti₃O₇] and Na(x)-[Ti₃O₇] compositions, respectively. The materials were characterized by a range of techniques (SEM, TEM, solid-state NMR, XRD, PDF). Although the formation of aggregated nanoparticles is favored under acidic restacking conditions, the use of basic conditions can lead to control over the adherence between the exfoliated layers. Pair distribution function (PDF) analysis confirms that the local TiO₆ connectivity of the pristine material is maintained. The lowest sodium-containing phase Na(1)-[Ti₃O₇], which is the stable product upon Na⁺ leaching after consecutive washing steps, displays the best performance among the compositions studied, affording a stable reversible capacity of about 200 mAh·g^-1 for 20 cycles at a C/20 rate. Washing removes the excess of "free/reactive" Na⁺, which otherwise forms inactive Na₂CO₃ in the insufficiently washed compositions.