Unraveling the Ion-Accumulation-Induced Potential Limitations of MXene-Based Supercapacitors

MXenes are rising star materials for electrochemical energy storage, but their low potential window severely constrains their high-energy-density potential. When subjected to a high potential window, MXenes undergo an irreversible oxidative failure. However, the mechanisms behind this failure are not well understood. Here, we disclose a previously unreported ion-accumulation mechanism that limits the high operating potential of MXenes. Under excessive polarization at high potential, the representative MXene, Ti3C2Tx, shows oxidation behavior but a reversible electrochemical response. Spectroscopic analyses and electrochemical kinetic field simulations disclose the conformational state variation, ion flux distribution, and vertical displacement behavior of MXene electrodes, confirming that electrolyte ions predominantly accumulate at the edges of overly thick stacked MXenes, with only a limited number shuttling freely into the interior. To address this potential-limiting ion-accumulation mechanism, we develop a transferring–engraving method to build free ion-shuttling ultrathin MXenes that guarantees a 100% increase in the potential window and a high volumetric energy density of 45.7 mWh cm–3.