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.

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Hard carbon is the leading candidate anode for commercialization of Na-ion batteries. Hard carbon has a unique local atomic structure, which is composed of nanodomains of layered rumpled sheets that have short-range local order resembling graphene within each layer but complete disorder along the caxis between layers. A primary challenge holding back the development of Na-ion batteries is that a complete understanding of the structure-capacity correlations of Na-ion storage in hard carbon has remained elusive. This article presents two key discoveries: first that characteristics of hard carbon's structure can be modified systematically by heteroatom doping, and second, that these structural changes greatly affect Na-ion storage properties, which reveals the mechanisms for Na storage in hard carbon. Specifically, via P or S doping, the interlayer spacing is dilated, which extends the low-voltage plateau capacity, while increasing the defect concentrations with P or B doping leads to higher sloping sodiation capacity. Our combined experimental studies and first principles calculations reveal that it is the Na-ion-defect binding that corresponds to the sloping capacity, while the Na intercalation between graphenic layers causes the low-potential plateau capacity. The new understanding provides a new set of guiding principles to optimize hard carbon for Na-ion battery applications.

This study reveals the transport behavior of lattice water during proton (de)insertion in the structure of the hexagonal WO·0.6HO electrode. By monitoring the mass evolution of this electrode material via electrochemical quartz crystal microbalance, we discovered (1) WO·0.6HO incorporates additional lattice water when immersing in the electrolyte at open circuit voltage and during initial cycling; (2) The reductive proton insertion in the WO hydrate is a three-tier process, where in the first stage 0.25 H is inserted per formula unit of WO while simultaneously 0.25 lattice water is expelled; then in the second stage 0.30 naked H is inserted, followed by the third stage with 0.17 HO inserted per formula unit. Ex situ XRD reveals that protonation of the WO hydrate causes consecutive anisotropic structural changes: it first contracts along the c-axis but later expands along the ab planes. Furthermore, WO·0.6HO exhibits impressive cycle life over 20 000 cycles, together with appreciable capacity and promising rate performance.

The capacity of hard carbon anodes in Na-ion batteries rarely reaches values beyond 300 mAh/g. We report that doping PO x into local structures of hard carbon increases its reversible capacity from 283 to 359 mAh/g. We confirm that the doped PO x 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) d-spacing inside the turbostratic nanodomains is increased, revealed by both laboratory and synchrotron X-ray diffraction. Second, doping turns turbostratic nanodomains more defective along ab 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.