Abstract
Improving the accessibility of ions in the electrodes of electrochemical energy storage devices is vital for charge storage and rate performance. In particular, the kinetics of ion transport in organic electrolytes is slow, especially at low operating temperatures. Herein, we report a new type of MXene-carbon nanotube (CNT) composite electrode that maximizes ion accessibility resulting in exceptional rate performance at low temperatures. The improved ion transport at low temperatures is made possible by breaking the conventional horizontal alignment of the two-dimensional layers of the MXene Ti3C2 by using specially designed knotted CNTs. The large, knot-like structures in the knotted CNTs prevent the usual restacking of the Ti3C2 flakes and create fast ion transport pathways. The MXene-knotted CNT composite electrodes achieve high capacitance (up to 130 F g−1 (276 F cm−3)) in organic electrolytes with high capacitance retention over a wide scan rate range of 10 mV s−1 to 10 V s−1. This study is also the first report utilizing MXene-based supercapacitors at low temperatures (down to −60 °C).
Highlights
Improving the accessibility of ions in the electrodes of electrochemical energy storage devices is vital for charge storage and rate performance
As discussed in the previous section, the mixing of nanomaterials with MXenes has already been used to modify the interlayer spacing of MXene; the restacking of the MXene flakes still leads to a planar, paper-like electrode architecture with an in-plane orientation of the 2D sheets, severely limiting the accessibility of the electrolyte (Supplementary Fig. 1), which hinders the pseudocapacitive performance of MXenes at high scan rates in aqueous electrolyte and results in poor rate performance in organic electrolytes[18,19,20,21]
We have demonstrated how the architecture of an electrode can lead to enhanced rate performances in organic electrolytes
Summary
Improving the accessibility of ions in the electrodes of electrochemical energy storage devices is vital for charge storage and rate performance. We report a new type of MXene-carbon nanotube (CNT) composite electrode that maximizes ion accessibility resulting in exceptional rate performance at low temperatures. The solvation shells of ions in organic electrolytes have a larger size than those in aqueous electrolyte These properties hinder charge storage and slow the ion transport kinetics of electrode materials, limiting high-rate performances, especially at low temperatures[4,5,6]. Efforts to improve ion accessibility for high-rate performance in organic electrolytes have been addressed using two approaches: electrolyte formulation and modification of electrode structure. Another study on tuning electrolyte composition showed that matching between the size of the electrode pores of carbon electrodes with the solvated ions of the electrolyte played an important role in achieving exceptional low-temperature performance[9].
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