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-MnO2-A2SO4 (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-MnO2-Li2SO4 due to excellent mobility (migration barrier 0.168 eV) of lithium ions. Better cycling stability appears in δ-MnO2-K2SO4, which is attributed to larger adsorption energy (−0.655 eV) between potassium ions and δ-MnO2. Moreover, compared with a pure Li2SO4 electrolyte, our calculations suggest that incorporation of moderate Na2SO4 or K2SO4 into the Li2SO4 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-MnO2 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-MnO2 electrochemical energy storage devices.