Anomalous Electrochemical Aging Strengthening Behavior of MXene Electrodes for Synergistic Anion‐Cation Storage in Dual‐Ion Batteries

Abstract Electrochemical aging of electrode materials usually leads to capacity decay and voltage drop in conventional rocking‐chair batteries. Here we report an anomalous electrochemical aging strengthening behavior of Ti 3 C 2 T x MXene electrode in a dual‐ion battery, whose capacity/power grows under cycling and eventually reaches an optimal state once the co‐storage of PF 6 − anion and Li + cation is activated by a wide electrochemical aging window. Experimental and theoretical results reveal that there is anion‐cation synergy between PF 6 − and Li + during co‐storage process in Ti 3 C 2 T x electrodes, where small Li + intercalation expands interlayer spacing of Ti 3 C 2 T x at low discharge voltages, and the residual Li + during charging can act as an anchor center to promote storage of large PF 6 − at high charging voltages; meanwhile, the residual PF 6 − during discharge in turn provides additional active sites to coordinate with Li + , raising Li + intercalation voltage and capacity. Thereafter, Ti 3 C 2 T x electrode offers a high capacity of 310 mAh/g, robust cycling stability over 800 cycles, and rate performance up to 1000 mA/g, which is among the best reported results of anion‐cation co‐storage. This counter‐intuitive discovery, resembling aging‐strengthening phenomenon in metallurgy, deepens our understanding of unconventional battery chemistry and provides a new avenue for design of high‐performance electrode materials.