Diffusion kinetics of “layered” Na-TMO2: Regulation of Na+ layer stability by dual ion doping in hybrid electrolyte

Aqueous sodium-ion batteries (ASIBs) are promising for energy storage applications because of their low cost and safe operational properties. Mn-based layered transition metal oxides are favorable positive electrode materials for ASIBs. Still, rapid capacity decay due to the Jahn-Teller effect and Mn dissolution in the aqueous electrolytes during cycling restrict their applicability. To resolve these issues, a dual strategy of using a hybrid electrolyte and doping is used to suppress the hydroxide formation during electrochemical cycling to boost the performance of ASIBs. Cu and Fe dual ion doped Na_0.7MnO₂ (NFCM) is synthesized via the solid-state method for cathode material. The X-ray diffraction spectra confirm good crystallinity and the presence of the P2 phase in pristine and doped samples with minimal impurity. Scanning electron microscopy shows rod-like structures for pristine Na_0.7MnO₂ (NMO), while the doped sample comprises both rod-like and sphere-like structures. The full cell configuration of doped NFCM || activated carbon results best with a hybrid 1 M NaClO₄ electrolyte with a discharge capacity of 141 mAh g⁻¹. The binding of OH⁻ ions with ethanol through hydrogen bonding leads to a decrease in Mn dissolution and a stable cycle life of up to 100 cycles.