Ethylene Glycol Co‐Solvent Enables Stable Aqueous Ammonium‐Ion Batteries with Diluted Electrolyte

Aqueous ammonium-ion batteries (AAIBs) are considered as one of the most promising technologies for large scale energy storage applications, due to their intrinsic safety, environmental friendliness and low cost. Nevertheless, the practical application of AAIBs is impeded by hydrogen evolution reaction (HER) occurring in diluted aqueous electrolyte. Highly concentrated electrolytes can improve electrochemical stability but lead to higher costs and increased hydrolysis of NH4+. In this work ethylene glycol (EG) is proposed as co-solvent, which can act as H-bond modulating agent, to increase the stability of AAIBs with diluted electrolytes. The perturbation of water H-bond network regulated by EG is proved by spectroscopic methods, while the suppressed HER is confirmed by differential electrochemical mass spectrometry (DEMS) results. As a result, full cells with 3,4,9,10-perylenebis(dicarboximide) (PTCDI) anode and FeFe(CN)6 (FeHCF) cathode and EG-added electrolyte exhibit stable cycling performance with capacity retention of 77% after 1950 cycles.