Beyond intercalation-based supercapacitors: The electrochemical oxidation from Mn₃O₄ to Li₄Mn₅O₁₂ in Li₂SO₄ electrolyte

The difficulty of enlarging the voltage window for aqueous asymmetric supercapacitors seriously impedes the enhancement in energy density, thus affecting their practical applications. Herein, lithium-rich Li₄Mn₅O₁₂ nanoflakes are firstly in situ synthesized on carbon cloth through the electrochemical oxidation of prefabricated Mn₃O₄ nanowalls in a traditional three-electrode cell using Li₂SO₄ as electrolyte. It is intriguingly found that the potential windows for the Mn₃O₄ and Li₄Mn₅O₁₂ electrodes can be enlarged to 0–1.2 V (vs Hg/Hg₂Cl₂) with high specific capacitances of 527 and 627 F g⁻¹ at 1 mA cm⁻², respectively. The CV kinetic analysis reveals different charge storage mechanisms for the Mn₃O₄ and Li₄Mn₅O₁₂ electrodes, major capacitances of which are identified as the surface capacitive contribution and diffusion-controlled contribution, respectively. Making the best of separate potential window of the Li₄Mn₅O₁₂ cathode and activated carbon anode, the as-assembled aqueous asymmetric supercapacitor device exhibits a wide voltage window of 2.2 V with a large energy density of up to 78 Wh kg⁻¹ at 295 W kg⁻¹ as well as ideal cycle stability, significantly outstripping previously reported Mn-based supercapacitors. Therefore, the excellent energy storage property together with low cost enables the assembled aqueous asymmetric supercapacitors to become a highly potential candidate for more possible future applications.