Ion storage mechanism of δ-MnO2 as supercapacitor cathode in multi-ion aqueous electrolyte: Experimental and theoretical analysis

The ion storage mechanism and ion concentration play crucial roles in determining the electrochemical energy storage performances of multi-ion-based batteries and/or capacitors. Here, we take δ-MnO-ASO (A = Li, Na, K) as an example system to explore the physical and chemical mechanisms related to electrochemical energy storage using experimental analysis and first-principles calculations. Among the studied systems, superior capacitance performance is found in δ-MnO-LiSO due to excellent mobility (migration barrier 0.168 eV) of lithium ions. Better cycling stability appears in δ-MnO-KSO, which is attributed to larger adsorption energy (−0.655 eV) between potassium ions and δ-MnO. Moreover, compared with a pure LiSO electrolyte, our calculations suggest that incorporation of moderate NaSO or KSO into the LiSO electrolyte could considerably elongate the cycling lifetime. Overdose of Na or K is, however, adverse to the capacitance performance as verified by our experiments. We argue that the dominance role of Na or K ions played in the hybrid electrolyte originates from the larger formation enthalpy and adsorption energy of Na or K when reacting with δ-MnO compared with those of Li. Our findings suggest that understanding of the ion storage mechanism can provide useful clues for searching the proper ion concentration ratio, which takes advantages of individual ions in multi-ion-based δ-MnO electrochemical energy storage devices.