The rod-like morphology of HTO is maintained after pillaring with PA, as indicated in the scanning electron micrographs (Fig. ). Atomic force microscopy (AFM) images in tapping mode provide additional information on surface topography of the materials (Fig. ). While pristine HTO exhibits relatively smooth surfaces, step-like surface features can be observed in HTO-PA, which likely result from pillaring. The Na + intercalation properties of HTO and HTO-PA are evaluated in a series of electrochemical experiments in organic 1 M NaPF6 in ethylene carbonate and propylene carbonate (1:1 vol. mixture) electrolyte versus sodium metal electrodes. The first five galvanostatic cycles in a potential range of 2.5 -0.2 V vs. Na + /Na at 50 mA g -1 are shown in Fig. . In the first cycle, a larger irreversibility of HTO can be observed compared to HTO-PA, leading to ICEs of 41 % and 50 %, respectively. Irreversibility in the first cycle arises from solid-electrolyte interphase (SEI) formation, as well as irreversible reactions of the host materials. For pristine HTO, strong irreversibility in the potential region between 1.5 -1.0 V can be observed that has been attributed to H2 formation. This is mostly absent in HTO-PA, leading to an improvement of the ICE. This is a good indication that pillaring as a possible strategy to improve ICE and alleviate the H2 formation issue of HTO host materials.