Improving stability using a mixed ion/hybrid electrolyte strategy in a sodium ion capacitor

In Sodium-ion capacitors (SICs), employing high-capacity anodes based on alloying/conversion reactions can potentially alleviate the performance issues but are susceptible to rapid capacity fading due to consistent dissolution of the electrode surface by the electrolyte. Herein, we extend the “hybrid mixed electrolyte” strategy to ion capacitors for overcoming the rapid cycle decay issues in high-energy SICs: molybdenum disulphide (MoS₂, MS) nanocrystals grown on carbon nitride (C₃N₄, CN), denoted as MS/CN, as anode exhibited remarkable performance in both sodium electrolyte and replaced with a hybrid mixed electrolyte comprising larger Na⁺ ions and smaller Li⁺ ions. Thus, constructed SICs exhibited a high energy density of 124 Wh Kg⁻¹ with a mixed ion electrolyte, and low energy loss (71% retention after 40000 cycles) with a hybrid electrolyte (lower than conventional sodium-ion-based electrolyte: 37% retention after 15000 cycles). Systematic investigations, including X-ray photoelectron spectroscopy and electron probe microanalysis, revealed an advantageous formation of a stable interface for electrode and electrolyte.