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Hard carbon (HC) is the most promising anode material for sodium‐ion batteries (SIBs), nevertheless, the understanding of sodium storage mechanism in HC is very limited. As an important aspect of storage mechanism, the steady state of sodium stored in HC has not been revealed clearly to date. Herein, the formation mechanism of quasi‐metallic sodium and the quasi‐ionic bond between sodium and carbon within the electrochemical reaction on the basis of theoretical calculations are disclosed. The presence of quasi‐metallic sodium is further confirmed with the assistance of a specific reaction between the sodiated HC electrode and ethanol, by analyzing the reaction products with Fourier‐transform infrared spectroscopy, gas chromatography, and nuclear magnetic resonance. Moreover, based on the specific chemical reaction, the composition of fully sodiated HC is estimated to be NaC6.7, and the corresponding capacity of sucrose‐derived HC is calculated to be 333.4 mAh g−1 in SIBs, matching well with the experimental result. This work helps to reveal the steady state of sodium and improve the understanding of sodium storage behavior in HC from the aspect of charge transfer. In addition, the proposed method is also expected to pave the way to investigate sodium storage mechanisms in other electrode systems. [ABSTRACT FROM AUTHOR]

Hard carbon, sodium-ion batteries, sodium storage mechanism, steady state Sodium-Ion Batteries: Probing the Energy Storage Mechanism of Quasi-Metallic Na in Hard Carbon for Sodium-Ion Batteries (Adv. Keywords: hard carbon; sodium-ion batteries; sodium storage mechanism; steady state EN hard carbon sodium-ion batteries sodium storage mechanism steady state 1 1 1 03/22/21 20210318 NES 210318 In article number 2003854, Ying Bai, Hongcai Gao, Chuan Wu and co-workers provide an in-depth understanding of the steady state of sodium in hard carbon. [Extracted from the article]

• Flexible PEGMA-based GPE was synthesized via in-situ thermal-cured technique. • The GPE showed a high ionic conductivity of 0.91 mS cm−1 at room temperature. • Interfacial property of electrolyte/electrode was investigated in depth. • Quasi-solid-state sodium ion battery exhibits good cycling stability. Polymer electrolyte is favored in battery research because of its good flexibility, light weight and preferable interfacial contact. However, the development of which is hindered by the low ionic conductivity at room temperature. Herein, a flexible PPEGMA-based gel polymer electrolyte (PGT32-5%) was prepared via in-situ thermal cured technique, plasticized by nonflammable triethyl phosphate and supported by glass fiber. The optimized flame-retardant electrolyte, PGT32-5%, exhibits a high ionic conductivity (0.91 mS cm−1 at 27 °C and a wide electrochemical window (4.8 V). And an artificial interface between polymer and Na metal anode was built to guarantee the superior cycling stability (capacity retention of 91% after 400 cycles) of quasi-solid-state Na 3 V 2 (PO 4) 3 |PGT32-5%|Na battery. Further, the interfacial property and effect on discharge behavior were analyzed in depth, which paves the way for designing polymer electrolytes with superior comprehensive performances in future. [ABSTRACT FROM AUTHOR]

• Copolyether PMH9 is synthesized via cationic ring-opening polymerization. • Hyperbranched pendants break the ordered arrangement of linear polyether. • Plasticizing effect of TFSI− anion on polyether is revealed. • Composite polymer electrolyte shows a high ionic conductivity at room temperature. • All-solid-state polymer-Na battery exhibits good cycling stability. Polyether, especially, poly (ethylene oxide) has attracted extensive attention in polymer electrolytes since 1973 due to the superior coordination ability with alkali metal ions. However, the ionic conductivity of linear polyether is suppressed by the inherent effortless crystallizability. Here we synthesized hyperbranched polyether for composite polymer electrolytes (named as PMH9-1/1-3 NaTFSI) with an ionic conductivity of 5.7 × 10−4 S cm−1 at room temperature used in all-solid-state sodium ion batteries (ASIBs). The effect of grafting site on segmental motion and the plasticizing effect of TFSI− anion on polyether are revealed by nuclear magnetic resonance and infrared spectrum characterizations. ASIBs using PMH9-1/1-3 NaTFSI exhibit a high capacity retention ratio of 92% at 0.2C after 300 cycles. Studies on the grafting site on polyether backbone and the plasticizing effect of TFSI− anion make sense to fabricating composite electrolytes with high performance for ASIBs. [ABSTRACT FROM AUTHOR]