This work reports the facile synthesis of a Sn-P composite combined with nitrogen doped hard carbon (NHC) obtained by ball-milling and its use as electrode material for sodium ion batteries (SIBs). The "Sn 4 P 3 "/NHC electrode (with nominal composition "Sn 4 P 3 ":NHC = 75:25 wt%) when coupled with a diglyme-based electrolyte rather than the most commonly employed carbonatebased systems, exhibits a reversible capacity of 550 mAh g electrode −1 at 50 mA g −1 and 440 mAh g electrode −1 over 500 cycles (83% capacity retention). Morphology and solid electrolyte interphase formation of cycled "Sn 4 P 3 "/NHC electrodes is studied via electron microscopy and X-ray photoelectron spectroscopy. The expansion of the electrode upon sodiation (300 mAh g electrode −1) is only about 12-14% as determined by in situ electrochemical dilatometry, giving a reasonable explanation for the excellent cycle life despite the conversion-type storage mechanism. In situ X-ray diffraction shows that the discharge product is Na 15 Sn 4. The formation of mostly amorphous Na 3 P is derived from the overall (electro)chemical reactions. Upon charge the formation of Sn is observed while amorphous P is derived, which are reversibly alloying with Na in the subsequent cycles. However, the formation of Sn 4 P 3 can be certainly excluded.

Hard carbon (HC) is the negative electrode (anode) material of choice for sodium‐ion batteries (SIBs). Despite its advantages in terms of cost and sustainability, a comprehensive understanding of its microstructure is not complete yet, thus hindering a rational design of high‐performance HC electrodes. In this study, rather than investigating how the precursor and synthesis method influence on the electrochemical properties of HC anodes, we examine the microstructure and surface chemistry of three optimized HC anodes obtained from different precursors by using different synthesis routes. The main goal is to evaluate the influence of the final materials properties (in their optimized state) on the electrochemical reactivity in lithium and sodium cells after a comprehensive structural characterization performed by means of X‐ray photoelectron spectroscopy (XPS), wide‐angle X‐ray scattering (WAXS), Raman spectroscopy, scanning electron microscopy (SEM), and gas sorption measurements. The different electrochemical performance observed in terms of cycling stability and rate capability, and the stability of the solid electrolyte interphase (SEI) formed on the various HCs have been comprehensively investigated. A correlation of the material properties with their electrochemical response upon sodium and lithium uptake and release is clarified. By comparing the Na‐ and Li‐ion storage behavior, a structure‐function relation is identified.

The limited Na‐storage capacity of graphite anodes for sodium‐ion batteries (∼110 mAh g−1) is significantly enhanced by the incorporation of nanosized Sn (17 wt%). The composite (SntGraphite), prepared by simple annealing of graphite with SnCl2, shows a specific capacity of 223 mAh g−1 (at 50 mA g−1) combined with excellent cycle life (i. e., 96 % of capacity retention after 2,200 cycles at 1 A g−1) and initial Coulomb efficiency (90 %). The combined storage of sodium in graphite (by solvent co‐intercalation) and Sn (by alloy formation) is followed by in situ X‐ray diffraction and in situ electrochemical dilatometry (ECD). While the additional tin almost doubles the electrode capacity, its contribution to the electrode expansion (∼3 %) is surprisingly small. The use of SntGraphite as anode for sodium‐ion hybrid capacitors with activated carbon as cathode provides a maximum energy and power density of ∼93 Wh kg−1 and 7.8 kW kg−1, with a capacity retention of ∼80 % after 8,000 cycles.