Ti3C2Tx MXene as Janus separators for redox-enhanced electrochemical capacitors with reduced self-discharge

Redox-enhanced electrochemical capacitors (redox ECs) are promising energy-storage devices with the potential to deliver both high power and energy performances, although overcoming the severe self-discharge remains a great challenge due to the shuttle effect of redox species such as I − /I 3 − ions introduced in the electrolytes for gaining additional capacities. Herein, we propose a cation-initiated self-assembly process of Ti 3 C 2 T x MXene nanosheets onto glass fiber membranes to form Janus separators that can mitigate the shuttling of I − /I 3 − ions in iodine-based redox ECs. The interlayer spacing of the MXene films can be controlled by selecting different cations such as imidazolium (C 4 mim + ), Mg 2+ , and Al 3+ , therefore the resulting Janus separators exhibit tailored nanochannels for ion sieving. Both molecular dynamic (MD) simulations and electrochemical tests indicate that C 4 mim + -intercalated MXene Janus separators (C 4 mim + -MXene) can block the diffusion of I 3 − ions while allowing the pass of other electrolyte ions. As a result, self-discharge caused by the shuttle effect can be much reduced in C 4 mim + -MXene-based redox ECs, which deliver voltage and energy retentions substantially higher than that of the cells using glass fiber membrane separators (voltage retention: 70% vs. 10%, energy retention: 51% vs. 6.5%) after 24-h open-circuit test.