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 -/I3- ions introduced in the electrolytes for gaining additional capacities. Herein, we propose a cation-initiated self-assembly process of Ti3C2Tx 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 (C4mim+), Mg2+, and Al3+, 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 C4mim+-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.

Automated summary

In this research, a cation-initiated self-assembly process of Ti3C2Tx MXene nanosheets onto glass fiber membranes is proposed to form Janus separators that can mitigate the shuttle effect of I-/I3- ions in iodine-based redox ECs. The resulting Janus separators exhibit tailored nanochannels for ion sieving, which can effectively block the diffusion of I-3(-) ions while allowing the pass of other electrolyte ions. Experimental results show that the self-discharge caused by the shuttle effect can be greatly reduced in redox ECs using this Janus separator, resulting in higher voltage and energy retentions compared to cells using glass fiber membrane separators.