  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 characterised by a
range of techniques (SEM, TEM, solid-state NMR, XRD, PDF). Although the
formation of aggregated nanoparticles is favoured 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 Na(1)-Ti3O7 phase, 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 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.
