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# Thickness-independent capacitance of vertically aligned liquid-crystalline MXenes

## Abstract

The scalable and sustainable manufacture of thick electrode films with high energy and power densities is critical for the large-scale storage of electrochemical energy for application in transportation and stationary electric grids. Two-dimensional nanomaterials have become the predominant choice of electrode material in the pursuit of high energy and power densities owing to their large surface-area-to-volume ratios and lack of solid-state diffusion1,2. However, traditional electrode fabrication methods often lead to restacking of two-dimensional nanomaterials, which limits ion transport in thick films and results in systems in which the electrochemical performance is highly dependent on the thickness of the film1,2,3,4. Strategies for facilitating ion transport—such as increasing the interlayer spacing by intercalation5,6,7,8 or introducing film porosity by designing nanoarchitectures9,10—result in materials with low volumetric energy storage as well as complex and lengthy ion transport paths that impede performance at high charge–discharge rates. Vertical alignment of two-dimensional flakes enables directional ion transport that can lead to thickness-independent electrochemical performances in thick films11,12,13. However, so far only limited success11,12 has been reported, and the mitigation of performance losses remains a major challenge when working with films of two-dimensional nanomaterials with thicknesses that are near to or exceed the industrial standard of 100 micrometres. Here we demonstrate electrochemical energy storage that is independent of film thickness for vertically aligned two-dimensional titanium carbide (Ti3C2Tx), a material from the MXene family (two-dimensional carbides and nitrides of transition metals (M), where X stands for carbon or nitrogen). The vertical alignment was achieved by mechanical shearing of a discotic lamellar liquid-crystal phase of Ti3C2Tx. The resulting electrode films show excellent performance that is nearly independent of film thickness up to 200 micrometres, which makes them highly attractive for energy storage applications. Furthermore, the self-assembly approach presented here is scalable and can be extended to other systems that involve directional transport, such as catalysis and filtration.

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## Acknowledgements

We thank W.-S. Wei and Z. Davison for providing insights into the assembly of discotic liquid crystals in our experiments. We acknowledge support from the National Science Foundation Materials Science and Engineering Center Grant to the University of Pennsylvania, DMR-1120901 and DMR-1720530 (to S.Y.). Work on MXene synthesis and electrochemical characterization at Drexel University was supported by the Fluid Interface Reactions, Structures & Transport Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (to Y.G.).

## Author information

Authors

### Contributions

Y.X., T.S.M., M.-Q.Z., Y.G. and S.Y. conceived the idea and designed the experiments. Y.X., T.S.M. and M.-Q.Z. performed the experiments. Z.Z., A.D., H.C. and B.A. helped with the experiments. Y.G. and S.Y. supervised the work. Y.X. and T.S.M. drafted the manuscript, and all the authors contributed to the editing of the manuscript.

### Corresponding authors

Correspondence to Yury Gogotsi or Shu Yang.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Size statistics of MXene nanosheets.

a, SEM image of the nanosheets. The flake dimensions used to report the length and width of the sheets are marked. b, Size distribution of MXene nanosheets, measured from SEM over 250 sheets. c, Average sizes of the nanosheets.

### Extended Data Fig. 2 POM images of MXene aqueous solutions.

a, At a concentration of 50 mg ml−1, the MXene solution shows a nearly isotropic phase with low birefringence under crossed polarizers. b, At a concentration of 250 mg ml−1, higher birefringence starts to appear. A nematic phase is formed, as clearly indicated from the Schlieren texture shown in the inset. c, d, POM images of the slow-appearing MXene liquid-crystal phase on top of one-dimensional microchannels at two polarizer angles: 45° (c) and 0° (d). Higher birefringence starts to appear when water evaporates, and the resulting nematic liquid-crystal phase of MXene can be well aligned with microchannels. The microchannels used here have the following dimensions: diameter 2 µm, spacing 2 µm and depth 1.5 µm. The Onsager theory of liquid crystals predicts the formation of the liquid-crystal phase as a function of the volume fraction of molecules in a media: low volume fraction gives an isotropic phase, and high volume fraction gives a liquid-crystal phase. The empirical value of the critical volume fraction of liquid-crystal phase formation can be estimated by $$\varphi \approx \frac{4T}{W}$$, where Φ is the critical volume fraction, W and T are the width and thickness of the nanosheet, respectively. In our MXene system, Φ is estimated to be around 2 vol%, which is equivalent to about 80 mg ml−1 of MXene nanosheets in aqueous solution. However, because the MXene nanosheets are highly polydisperse in terms of size, and the surface charges differ from system to system, the critical value of Φ could vary in experiments. In this work, we demonstrated the isotropic phase of MXene liquid crystal at around 50 mg ml−1, in good agreement with theory, and we showed the nematic phase of MXene at around 250 mg ml−1. This concentration was chosen to be sufficiently high above the critical value of Φ such that a liquid-crystal phase could be ensured.

### Extended Data Fig. 3 SEM image of nematic MXene liquid crystal after mechanical shear.

Horizontally aligned MXene nanosheets are obtained.

### Extended Data Fig. 4 Schematic of the preparation of MXLLC films.

a, Preparation of the MXLLC slurry. b, Fabrication of the liquid-crystal cell. c, Separation of the MXLLC layer from the liquid-crystal cell. d, The free-standing MXLLC film obtained after supercritical drying.

### Extended Data Fig. 5 POM and SEM images of MXLLC films.

a, b, POM images of MXLLC before (a) and after (b) shear. The inset of b shows the POM image of the MXLLC with the shear direction parallel to the polarizer. ch, Views of the MXLLC film from SEM: random alignment before shear (c), vertical alignment after shear (d), lateral views (e, f), front views (g, h). Dashed red lines indicate the bending directions of the MXene layers.

### Extended Data Fig. 6 AFM images of the spin-coated flat MXene film.

a, Height map. b, Film thickness measured across the boundary of the film and the glass. The thickness of the flat MXene film is estimated to be about 30 nm.

### Extended Data Fig. 7 SEM images of a vacuum-filtered MXene–SWCNT paper with a thickness of around 35 µm.

a, The full cross-section of the freestanding film. b, A higher-magnification image of the top portion of a. Both images show horizontally stacked layers of MXene nanosheets interpenetrated with SWCNTs, a configuration that has been reported to facilitate ion diffusion1.

### Extended Data Fig. 8 Plots of the anodic peak current against scan rate and peak separation against scan rate.

a, Plot of the peak current of the second pair of redox peaks seen at −0.4 V and – 0.6 V versus Hg/HgSO4 in the voltammograms for the MXLCC electrodes in Fig. 3a. b, Plot of the peak separation (ΔEp) for the second pair of redox peaks; the dashed line corresponds to the expected trend for a quasi-electrochemical process2. From a it can be seen that the trend of the current against scan rate of the anodic peak of the second pair of redox peaks is similar (Fig. 3c) to that of the main peaks at – 0.8 V versus Hg/HgSO4 (Fig. 3a), which are characteristic of Ti3C2Tx. The main difference in behaviour for this second pair of peaks compared with previous reports on Ti3C2Tx can be seen in b. Previous characterization of the peak separation against the scan rate for the redox peaks in thin (around 90 nm) Ti3C2Tx electrodes showed a region at low scan rates that corresponds to quasi-equilibrium behaviour3; it is clear from b that this is not the case for the second pair of redox peaks of the MXLLC electrodes. The primary reason for this is probably the large difference in thickness of the MXLLC electrodes (200 μm) compared with previous reports of thin Ti3C2Tx electrodes (90 nm).

### Extended Data Fig. 9 Cyclic voltammograms of pure Ti3C2Tx vacuum-filtered paper and MXLLC film.

a, For the second pair of redox peaks at 0.6 V versus the reference, the current grows slightly during cycling and then stabilizes during continuous cycling. b, The current will fade if the cell is allowed to rest. This pair of peaks is thought to originate from changes occurring in the transition-metal surface of the MXene during cycling and will be the subject of further studies. It is thought that these peaks are more pronounced in the cyclic voltammograms of the MXLLC samples owing to the large amount of active material surface that is exposed to the electrolyte in the MXLLC samples relative to the vacuum-filtered papers. c, d, Cyclic voltammograms of a 35-µm-thick vacuum-filtered MXene paper (c) and a 40-µm-thick MXLLC film (d). For similar film thicknesses, MXLLC films have much better rate-handling ability compared to the vacuum-filtered papers, as only a small decay of the cyclic voltammogram is observed until high scan rates are used.

### Extended Data Fig. 10 Capacitance retention as a function of scan rate for both vacuum-filtered films and MXLLC films.

All data points are normalized to the capacitance value at 10 mV s−1 for each sample curve, respectively. At 2,000 mV s−1, vacuum-filtered MXene papers retain around 14% (35 µm thick) and 30% (6 µm thick) capacitance, whereas MXLLC films maintain more than 75% capacitance over a wide range of film thickness from 40 µm to 200 µm. However, in thicker (320 µm) MXLLC films, the retention curve starts to behave similarly to that of thin vacuum-filtered paper (6 µm). These data again suggest that the optimal thickness of the working-electrode film of MXLLC is 200 µm.

## Supplementary information

### Video 1: Polarized optical microscope (POM) images demonstrating MXene nanosheets aligned by microchannels during water evaporation.

MXene aqueous solution is sandwiched between a patterned epoxy substrate and a flat glass. As water gradually evaporated from the right side of the sample, the MXene LC phase began to form and aligned. Video is accelerated by 4x.

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Xia, Y., Mathis, T.S., Zhao, MQ. et al. Thickness-independent capacitance of vertically aligned liquid-crystalline MXenes. Nature 557, 409–412 (2018). https://doi.org/10.1038/s41586-018-0109-z

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• DOI: https://doi.org/10.1038/s41586-018-0109-z

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