New Mechanistic Insights on Na-Ion Storage in Nongraphitizable Carbon

Nongraphitizable carbon, also known as hard carbon, is considered one of the most promising anodes for the emerging Na-ion batteries. The current mechanistic understanding of Na-ion storage in hard carbon is based on the “card-house” model first raised in the early 2000s. This model describes that Na-ion insertion occurs first through intercalation between graphene sheets in turbostratic nanodomains, followed by Na filling of the pores in the carbon structure. We tried to test this model by tuning the sizes of turbostratic nanodomains but revealed a correlation between the structural defects and Na-ion storage. Based on our experimental data, we propose an alternative perspective for sodiation of hard carbon that consists of Na-ion storage at defect sites, by intercalation and last via pore-filling

Synthesis and Systematic Trends in Structure and Electrical Properties of [(SnSe)_1.15]_<i>m</i>(VSe₂)₁, <i>m</i> = 1, 2, 3, and 4

Four compounds [(SnSe)_1.15]_<i>m</i>(VSe₂)₁, where <i>m</i> = 1–4, were synthesized to explore the effect of increasing the distance between Se–V–Se dichalcogenide layers on electrical transport properties. These kinetically stable compounds were prepared using designed precursors that contained a repeating pattern of elemental layers with the nanoarchitecture of the desired product. XRD and STEM data revealed that the precursors self-assembled into the desired compounds containing a Se–V–Se dichalcogenide layer precisely separated by a SnSe layer. The 00<i>l</i> diffraction data are used to determine the position of the Sn, Se, and V planes along the <i>c</i>-axis, confirming that the average structure is similar to that observed in the STEM images, and the resulting data agrees well with results obtained from calculations based on density functional theory and a semiempirical description of van der Waals interactions. The in-plane diffraction data contains reflections that can be indexed as <i>hk</i>0 reflections coming from the two independent constituents. The SnSe layers diffract independently from one another and are distorted from the bulk structure to lower the surface free energy. All of the samples showed metallic-like behavior in temperature-dependent resistivity between room temperature and about 150 K. The electrical resistivity systematically increases as <i>m</i> increases. Below 150 K the transport data strongly indicates a charge density wave transition whose onset temperature systematically increases as <i>m</i> increases. This suggests increasing quasi-two-dimensional behavior as increasingly thick layers of SnSe separate the Se–V–Se layers. This is supported by electronic structure calculations

High Capacity of Hard Carbon Anode in Na-Ion Batteries Unlocked by PO_<i>x</i> Doping

The capacity of hard carbon anodes in Na-ion batteries rarely reaches values beyond 300 mAh/g. We report that doping PO_<i>x</i> into local structures of hard carbon increases its reversible capacity from 283 to 359 mAh/g. We confirm that the doped PO_<i>x</i> is redox inactive by X-ray adsorption near edge structure measurements, thus not contributing to the higher capacity. We observe two significant changes of hard carbon’s local structures caused by doping. First, the (002) <i>d</i>-spacing inside the turbostratic nanodomains is increased, revealed by both laboratory and synchrotron X-ray diffraction. Second, doping turns turbostratic nanodomains more defective along <i>ab</i> planes, indicated by neutron total scattering and the associated pair distribution function studies. The local structural changes of hard carbon are correlated to the higher capacity, where both the plateau and slope regions in the potential profiles are enhanced. Our study demonstrates that Na-ion storage in hard carbon heavily depends on carbon local structures, where such structures, despite being disordered, can be tuned toward unusually high capacities