Highly Adaptive Flame‐Retardant Electrolyte Endows Hard‐Carbon Anodes with Stable Interface Structure for High‐Safety Sodium‐Ion Batteries

Hard carbon (HC) is the most promising anode material for sodium‐ion batteries (SIBs). The main obstacle to HC's application is its incompatibility with the phosphate flame‐retardant additive, a common SIB additive. This incompatibility arises from the unstable solid electrolyte interphase (SEI) caused by phosphate molecule decomposition. For the first time, in this work, a new type of highly adaptive flame‐retardant electrolyte is proposed. The electrolyte ensures the stability of the electrode/electrolyte interface by the introduction of pioneering long‐chain nitrile as the SEI‐forming additive and solvated structural regulator. Long‐chain nitriles decompose before phosphate and form a stable SEI containing N element, which prevents the phosphate from inserting into the HC's layers and reduces its sodium‐storage capacity. As a proof of concept, the HC anode demonstrates a higher initial Coulombic efficiency of 80.78% in the as‐designed highly adaptive flame‐retardant electrolyte, which is about 1.8 times than that before regulation. It can cycle stably for 1250 cycles at 3 C with a capacity retention of 60%. Moreover, the electrolyte has good flame retardancy and can extinguish naturally within 1 s. In this work, an innovative method is provided for developing high‐safety SIBs based on HC anodes.