Pyridinic N‐Dominated Hard Carbon with Accessible Carbonyl Groups Enabling 98% Initial Coulombic Efficiency for Sodium‐Ion Batteries

Abstract

Hard carbon (HC) has been widely regarded as the most promising anode material for sodium‐ion batteries (SIBs) due to its decent capacity and low cost. However, the poor initial Coulombic efficiency (ICE) of HC seriously hinders its practical application in SIBs. Herein, pyridinic N‐doped hard carbon polyhedra with easily accessible carbonyl groups and in situ coupled carbon nanotubes are rationally synthesized via a facile pretreated zeolitic imidazolate framework (ZIFs)‐carbonization strategy. The comprehensive ex/in situ techniques combined with theoretical calculations reveal that the synergy of pyridinic‐N and carbonyl groups promoted by the pretreatment and carbonization process would not only optimize the Na+ adsorption energy but also accelerate the desorption of Na+, significantly suppressing the irreversible capacity loss. As a result, the as‐synthesized hard carbon polyhedra as an anode can deliver an unprecedented high ICE of 98% with a large reversible capacity of 389.4 mAh g−1 at 0.03 A g−1. This work may provide an effective strategy for the structural design of HC with high ICE.