A Fluorine‐Free, Low Ion Pair Electrolyte for Transition Metal‐Free and Halogen‐Free Sodium Batteries

ABSTRACT

Fluorinated solvents, salts, and binders are widely used in modern rechargeable batteries. High durability and superior high‐rate ability are often attributed to fluorine‐rich interphases or fluorine‐tuned solvation structures. To date, fluorine‐free battery development is still nascent, with the interphases and structure‐property‐performance relationship thereby remaining poorly understood. Here, we investigate the impact of anion substituents in Na‐ion salts on electrolyte ion transport properties and interphase compositions on Na‐ion anodes. A fluorine‐free sodium tetraphenylborate‐diglyme electrolyte can cycle hard carbon anode over 2000 cycles at 1C with a capacity retention exceeding 98%. Sodium metal anode exhibits stripping‐plating efficiencies of 99.93% over 1000 cycles at 0.5 mA/cm ² and 0.5 mAh/cm ² . The steric effect of the bulky phenyl substituent leads to negligible ion pairing and good ionic conductivity, supporting high‐rate performance of Na‐ion electrodes. We demonstrate that a transition‐metal(TM)‐free and fluorine‐free battery as proof of concept, in a perylene‐3,4,9,10‐tetracarboxylic acid diimide (PTCDI)||hard carbon full cell configuration, delivers stable cycling with a ∼85% capacity retention at 1C after 500 cycles and good fast‐charging capability reaching 80% specific energy in less than 4 min. These results show that it is possible to design TM‐free and fluorine‐free sodium ion batteries without an anion‐rich solvation structure or fluorine‐based interphases.