High‐Performing All‐Solid‐State Sodium‐Ion Batteries Enabled by the Presodiation of Hard Carbon

All‐solid‐state sodium ion batteries (AS3iBs) are highly sought after for stationary energy storage systems due to their suitable safety and stability over a wide temperature range. Hard carbon (HC), which is low cost, exhibits a low redox potential, and a high capacity, is integral to achieve a practical large‐scale sodium‐ion battery. However, the energy density of the battery utilizing this anode material is hampered by its low initial Coulombic efficiency (ICE). Herein, two strategies, namely i) additional pyrolysis and ii) presodiation by thermal decomposition of NaBH4, are explored to improve the ICE of pristine HC. Raman spectroscopy, X‐ray photoelectron spectroscopy, and electrochemical characterizations elucidate that the thermal treatment increases the Csp2 content in the HC structure, while the presodiation supplies the sodium to occupy the intrinsic irreversible sites. Consequently, presodiated HC exhibits an outstanding ICE (>99%) compared to the thermally treated (90%) or pristine HC (83%) in half‐cell configurations. More importantly, AS3iB using presodiated HC and NaCrO2 as the anode and cathode, respectively, exhibits a high ICE of 92% and an initial discharge energy density of 294Whkgcathode−1$294\ {\rm Wh}\ {\rm{kg}}_{{\rm{cathode}}}^{ - 1}$. The oxygen groups in hard carbon are the intrinsic irreversible sodium storage sites leading to loss of sodium inventory which negatively impacts the energy density of batteries. Hard carbon is presodiated by decomposing sodium‐containing precursors thermally. This supplemental sodium populates the irreversible sites improving the initial Coulombic efficiency and the energy density of all‐solid‐state sodium ion batteries.