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 Ti₃C₂T_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 PF₆⁻ anion and Li⁺ cation is activated by a wide electrochemical aging window. Experimental and theoretical results reveal that there is anion-cation synergy between PF₆⁻ and Li⁺ during co-storage process in Ti₃C₂T_x electrodes, where small Li⁺ intercalation expands interlayer spacing of Ti₃C₂T_x at low discharge voltages, and the residual Li⁺ during charging can act as an anchor center to promote storage of large PF₆⁻ at high charging voltages; meanwhile, the residual PF₆⁻ during discharge in turn provides additional active sites to coordinate with Li⁺, raising Li⁺ intercalation voltage and capacity. Thereafter, Ti₃C₂T_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.