Porous Ti3C2Tx MXene for Ultrahigh-Rate Sodium-Ion Storage with

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Porous Ti3C2Tx MXene for Ultrahigh-Rate Sodium-Ion Storage with Long Cycle Life Xiuqiang Xie, katja kretschmer, Babak Anasori, Bing Sun, Guoxiu Wang, and Yury Gogotsi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00045 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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ACS Applied Nano Materials

Porous Ti3C2Tx MXene for Ultrahigh-Rate Sodium-Ion Storage with Long Cycle Life Xiuqiang Xie,1,2 Katja Kretschmer,2 Babak Anasori,1 Bing Sun,2 Guoxiu Wang,2,* Yury Gogotsi1,3* 1

Dr. X. Xie, Dr. B. Anasori, Prof. Y. Gogotsi A. J. Drexel Nanomaterials Institute, and Materials Science and Engineering Department Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA E-mail: [email protected] 2

Dr. X. Xie, K. Kretschmer, Dr. B. Sun, Prof. G. Wang Centre for Clean Energy Technology, School of Mathematical and Physical Sciences University of Technology Sydney, Broadway, Sydney, NSW 2007, Australia E-mail: [email protected] 3

Prof. Y. Gogotsi Key Laboratory of Physics and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun 130012, China

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ABSTRACT The development of anode materials remains a challenge to satisfy the requirements of sodium-ion storage for large-scale energy storage applications, which is ascribed to the low kinetics of ionic/electron transfer of electrode materials. Here we show that controlled anisotropic assembly of highly conductive Ti3C2Tx (MXene) nanosheets to form a porous structure can enhance the sodium-ion storage kinetics. At high current densities of 1 and 10 A g-1, the porous Ti3C2Tx electrode delivered capacities of 166 and 124 mA h g-1, respectively. Even at an extremely high current density of 100 A g-1, a capacity of 24 mA h g-1 can be achieved. The porous Ti3C2Tx electrode also exhibited long cycle life that can be extended to 1000 cycles with no capacity decay at a current density of 1 A g-1. This work demonstrates the successful control of Ti3C2Tx architecture to push electrochemical sodium-ion storage closer to large-scale applications and is expected to shed light on the rational utilization of the outstanding properties of MXenes by controlling their microscopic assembling. Keywords: porous structure; MXene; high rate; long cycle life; sodium-ion storage


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Electrochemical sodium-ion storage is emerging as a promising cost-effective alternative for grid-scale electrical energy storage (EES) because of the advantages of the low cost and natural abundance of sodium precursor materials. However, sodium-ion storage systems are still facing several challenges, such as low-energy/power densities and short cycle life. To satisfy criteria for smart grid management, it requires sodium-ion storage systems to shift electrical energy from peak to off-peak periods.1 In this context, it is important to develop electrode materials that can handle high-rate charging/discharging, which concurrently requires high electronic and ionic conductivities. Currently, many anode materials have been synthesized and evaluated, such as carbon materials,2-6 materials based on different processes such as, intercalation (Ti-oxides),7-12 conversion reaction (metal oxides and sulfides) 13-15 and alloying (Sn, Sb, P).16-23 However, they are still in an early stage and further improvements are required. The major concern is that most of these anode materials normally suffer from the well-recognized trade-off between the power and energy densities because of the sluggish sodium-ion storage kinetics, which limits their high-rate applications. Another central obstacle to expanding the use of these anode materials is the limited cycle life originating from pulverization issues. Anode materials with long cycle life remain being intensively pursued. Two-dimensional (2D) transition metal carbides/nitrides MXenes are emerging as very appealing candidates for sodium-ion storage due to the metallic electronic conductivity as high as 9880 S cm-1 and a low Na+ diffusion barrier (0.1-0.2 eV),24-27 which are the necessary prerequisites to afford high-rate sodium-ion storage. MXenes have been reported as electrode materials for sodium-ion storage.28-31 Yamada and coworkers reported on a multilayered Ti2CTx MXene negative electrode that delivered a reversible capacity of ~175 mA h g−1 with good cycle stability (12 and 19% losses of the second capacity after 50 and 100 cycles, respectively).

28

. Another MXene, V2CTx, was successfully used to pair with hard carbon to

assemble full sodium-ion cells, delivering a maximum cell voltage of 3.5 V and a cell capacity of 50 mA h g−1 29. Multilayered Ti3C2Tx MXene has also been tested as a negative electrode for Na-ion storage and exhibited a capacity around 100 mA h g−1.32-33 However, face-to-face stacking of MXene nanosheets impedes electrode/electrolyte interactions and has so far limited the capacity values and the rate capabilities.34-40 In this context, anisotropically assembling MXene nanosheets to form a porous structure represents a viable strategy to address the issue of MXene stacking, which can afford high-rate sodium-ion storage based on MXene electrode materials but remains a challenge.



Here, we report on the preparation of porous Ti3C2Tx MXene (p-Ti3C2Tx) electrode and its electrochemical performances for sodium-ion storage. Ti3C2Tx nanosheets with high metallic conductivity were assembled into a porous and anisotropic structure by a sulfur loadingremoval strategy (as explained in the following). The porous structure can improve the electrode/electrolyte interactions and shortens the transportation and diffusion of sodium-ions. By the advantageous metallic conductivity and low Na+ diffusion barrier of Ti3C2Tx MXene as well as the porous structure, the p-Ti3C2Tx exhibited a high capacity, good rate capability, and long cycling performance when applied as anode materials for sodium-ion storage. Combining these features makes our porous Ti3C2Tx MXene electrode a good candidate for high-performance electrode materials to push electrochemical sodium-ion storage closer to practical large-scale applications.

Figure 1. Schematic preparation of p-Ti3C2Tx. Starting with Ti3C2Tx colloidal solution (left panel), sulfur-ethlyenediamine (EDA) was dropwisely added, followed by addition of HCl to precipitate sulfur on Ti3C2Tx MXene nanosheets in aqueous solution. The resulting sulfur/Ti3C2Tx composites was collected, washed and heat treated at 400 ºC for 5 h under argon flow. As schematically shown in Figure 1, p-Ti3C2Tx were prepared based on a sulfur loadingremoval strategy. The sulfur-amine complex compound chemistry was used to achieve the mixing of sulfur and Ti3C2Tx in aqueous solution. Briefly, sulfur was dissolved in ethlyenediamine (EDA) to obtain S-EDA, which was added dropwise to the Ti3C2Tx colloidal dispersion. After adding hydrochloric acid (HCl) to precipitate sulfur from its complex with EDA, sulfur/Ti3C2Tx composites were obtained, as evidenced by the X-ray diffraction (XRD) data (Figure S1). The thermogravimetric analysis (TGA, Figure S2) suggests that removal of sulfur can be completed at 350 oC. The out-diffusion of the produced sulfur vapor builds up pressure that balances the attractions between the Ti3C2Tx nanosheets. After thermal evaporation of sulfur at 400 oC under Ar, p-Ti3C2Tx were obtained and no XRD peaks corresponding to sulfur could be detected (Figure S1). The as-prepared p-Ti3C2Tx show a clattice parameter (c-LP) of around 23.9 Å, with the (002) peak located at 7.4°. 4 ACS Paragon Plus Environment

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Figure 2. (a, b) SEM images, (c, d) TEM images of the as-prepared p-Ti3C2Tx, the inset in (d) shows three 2D layers of Ti3C2Tx. (e) N2 isotherms of p-Ti3C2Tx and restacked Ti3C2Tx. (f) Raman spectra of p-Ti3C2Tx compared with the pristine restacked Ti3C2Tx, showing that no new peaks appeared after the treatment. Scanning electron microscopy (SEM) images (Figure 2a and b) show that the as-prepared p-Ti3C2Tx nanosheets arranged to form a porous structure. This is inherently different from the common observed restacking of Ti3C2Tx nanosheets in a compact and parallel manner, which is usually observed for the direct deposition of Ti3C2Tx MXene nanosheets from a Ti3C2Tx colloidal solution by either vacuum-assisted filtration (restacked Ti3C2Tx, Figure S3), or spray and spin deposition.41 Moreover, out-of-plane deformations of Ti3C2Tx nanosheets (crumpled sheets) can be found in Figure 2b. The deformation in the third dimension is beneficial to maintain the structural stability of electrodes and limit the intersheet contact area, compared to the restacked Ti3C2Tx. For the restacked Ti3C2Tx, the microscopic corrugations tend to be minimized by bulk face to face intersheet attraction, forming compact assemblies to reduce surface energies (Figure S3). The nanoscale corrugations suggest that the protocol successfully enabled us to manipulate the arrangement of the individual MXene nanosheets and effectively maintain the advantages of 2D layers. The lateral size of Ti3C2Tx nanosheets is around 1-2 µm, as evidenced by the transmission electron microscopy (TEM) image shown in Figure 2c. Figure 2d shows a high resolution TEM image of the as-produced p-Ti3C2Tx, in which 3 sheets of Ti3C2Tx are stacked in less parallel order with an interlayer spacing of 1.25 nm. The thickness of the p-Ti3C2Tx flakes measured by atomic force microscopy (AFM) to be 5 ACS Paragon Plus Environment


3.8 nm (Figure S4), which is about the thickness of two Ti3C2Tx sheets. The minimized restacking and increase of the exposed surface area have been further confirmed by the N2 adsorption/desorption measurement (Figure 2e). The specific surface area of the p-Ti3C2Tx sample was measured to be 84.2 m2 g-1, which is 4.3 times that of the restacked Ti3C2Tx MXene (19.6 m2 g-1).42 The pore sizes are in the range of 1-20 nm (Figure S5).The Raman spectrum of the as-prepared p-Ti3C2Tx was compared with that of the vacuum-assisted filtered restacked Ti3C2Tx without thermal treatment (Figure 2f). Specifically, the modes at 198 (ω2) and 717 cm-1 (ω3) are A1g symmetry out-of-plane vibrations of Ti and C atoms, respectively, while the modes at 284 (ω5), 377 (ω5), and 626 cm-1 (ω4) are the Eg group vibrations, including in-plane (shear) modes of Ti, C, and surface functional group atoms.43-44 The Raman spectra are almost identical and no peaks originating from titanium oxide were observed, suggesting that no oxidation was occurred during the thermal treatment. This open morphology of the as-prepared p-Ti3C2Tx offers interconnected ion-storage reservoirs and enhances the electrode/electrolyte interfacial interaction. The electronic conductivity of pTi3C2Tx is measured to be ~220 S cm-1 (Figure S6).

Figure 3. Half-cell tests of p-Ti3C2Tx electrodes. (a) Charge/discharge curves of p-Ti3C2Tx at 100 mA g-1. (b) Cycling performance of the as-prepared p-Ti3C2Tx, multilayered Ti3C2Tx, and restacked Ti3C2Tx at 100 mA g-1. The Coulombic efficiencies of p-Ti3C2Tx are also presented. (c) Rate performance of p-Ti3C2Tx. (d) Comparison of the rate capability of p-Ti3C2Tx and 6 ACS Paragon Plus Environment

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some other typical anode materials. (e) Long-term cyclability of the p-Ti3C2Tx electrode at a current density of 1 A g-1 after the rate performance test. Sodium-ion storage performances were tested in order to evaluate the promise of p-Ti3C2Tx as high-rate electrode materials. Figure 3a demonstrates the galvanostatic charge/discharge curves of the p-Ti3C2Tx electrode at 100 mA g-1 in the potential window of 0.01-3.0 V (vs. Na+/Na). The initial discharge and charge capacities were 641 and 194 mA h g-1, respectively, corresponding to a Coulombic efficiency of 30.2%. The initial irreversible capacity could originate from the formation of solid electrolyte interface (SEI) and the irreversible reaction of Na with the surface functional groups of MXene (F, OH, and O).45-46 In the following cycles, no obvious charge/discharge plateaus were found. This behavior correlates with the nanoscale characteristics of the p-Ti3C2Tx morphology, i.e., the ultrathin thickness of Ti3C2Tx nanosheets and the open framework of the as-prepared p-Ti3C2Tx, which facilitate Na-ion storage at the surface of Ti3C2Tx flakes. At a current density of 100 mA g-1, the p-Ti3C2Tx electrode achieved a stable capacity of 180 mA h g-1, which is shown in Figure 3b. For comparison, we also tested the restacked Ti3C2Tx filtered film. In contrast to the p-Ti3C2Tx, the restacked Ti3C2Tx exhibited a reversible capacity of 20 mA h g-1, which is even lower than that of multilayered Ti3C2Tx powder (62 mA h g-1) (Figure 3b). The tap density of p-Ti3C2Tx is measured to be 1.2 g cm-3. Although this value is lower than that of the restacked Ti3C2Tx film (3.2 g cm-3), the volumetric capacity of p-Ti3C2Tx (216 mA h cm-3) is higher than that of the restacked Ti3C2Tx film (64 mA h cm-3) at a current density of 100 mA g-1 due to the improved accessibility of Ti3C2Tx by the anisotropic assembly of Ti3C2Tx nanosheets. It is also noted that the Coulombic efficiency (CE) for p-Ti3C2Tx electrode was improved from ~95.0% for restacked Ti3C2Tx and 96.5% for multilayered Ti3C2Tx to the current value of ~99% by thermal annealing. Study focusing on surface coating is ongoing to further improve the CE of Ti3C2Tx for practical applications. Figure 3c shows the rate performance of the pTi3C2Tx. At the high current densities of 1 and 10 A g-1, the p-Ti3C2Tx electrodes maintained capacities of 166 and 124 mA h g-1, respectively. Even at an extremely high current density of 100 A g-1, a capacity of 24 mA h g-1 with a Coulombic efficiency of 99% can be achieved. This high-rate capability outperforms the majority of the reported anode materials (Figure 3d), such as Ti3C2Tx/CNTs,34 carbon,47 MoS2/graphene paper,48 multilayered Ti2CTx powder,28 porous carbon nanofiber film49 and TiO2@C nanorods50. To the best of our knowledge, this is the best rate performance ever reported so far for sodium-ion storage, demonstrating that p-Ti3C2Tx is promising for high-rate charging applications, such as hybrid sodium-ion capacitors.28 This can be ascribed to the porous structure of the electronically 7 ACS Paragon Plus Environment


conductive p-Ti3C2Tx, which simultaneously ensures efficient ionic transport and fast electron transfer, thereby giving the p-Ti3C2Tx electrode a high-rate performance. The result suggests that by rational control of the assembly of their nanosheets, we can harness the potential of Ti3C2Tx MXene nanosheets and enhance the rate capability for sodium-ion storage. The longterm cycling stability of the p-Ti3C2Tx electrode is shown in Figure 3e. After 1000 cycles at a current density of 1 A g-1, a capacity of 189 mA h g-1 can be retained, verifying the good stability of the p-Ti3C2Tx electrode for practical long-term cycling.

Figure 4. (a) Cyclic voltammograms of the as-prepared p-Ti3C2Tx at different scan rates. (b) Relationship between peak current and scan rate at 0.75 V during cathodic processes. (c) Cyclic voltammetry profile collected at 1 mV s-1 with shaded portions showing the contributions of the processes not limited by diffusion. (d) Survey of non-diffusion limited current contributions at different scan rates. Figure 4a displays the cyclic voltammetry (CV) curves at various scan rates from 0.3 to 10 mV s-1. A pair of broad cathodic/anodic peaks located at around 0.7/0.85 V can be observed. The redox peaks correspond to Na+ insertion into/extraction from the p-Ti3C2Tx electrode, during which the transformation of the oxidation state of Ti-ions occurs.33 Further insights into the electrochemistry of the Na-ion cell based on the p-Ti3C2Tx can be obtained by kinetic analysis based on CV curves. Figure 4b shows the relationship between the measured cathodic current (i) at 0.75 V and the scan rate (v). The log(v)-log(i) plots can be linearly fitted and the slope of the fitted line is 0.967, which is close to a value of 1. This suggests that 8 ACS Paragon Plus Environment

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the current is predominantly non-diffusion limited

51

. This can be further confirmed by

quantifying the contribution of the non-diffusion limited current (Figure 4c). According to i(V) = k1ν + k2ν1/2, where i(V), k1ν, k2ν1/2, and ν are the current at a fixed potential, nondiffusion limited and diffusion-controlled currents, and the scan rate, respectively, the nondiffusion limited current at a certain scan rate can be determined by calculating the value of k1.51-54 Accordingly, the shaded portions in Figure 4c represent the non-diffusion limited component, while the non-shaded portion indicates the diffusion-controlled charge storage. The non-diffusion limited current contributes 62.4% to the overall charge at 1 mV s-1 and becomes more pronounced at higher scan rates as shown in Figure 4d. At 10 mV s-1, 86.8% of the charge originated from the non-diffusion limited portion. The predominate nondiffusion limited charge storage mechanism in combination with the metallic conductivity of Ti3C2Tx MXene can afford ultrafast sodium-ion storage over the as-prepared p-Ti3C2Tx as discussed above, overcoming the trade-off between the power and energy densities. The above results suggest that the favorable porous structure of p-Ti3C2Tx anisotropically assembled from 2D Ti3C2Tx nanosheets can effectively maintain the advantages of 2D Ti3C2Tx for sodium-ion storage, leading to high capacity and ultrafast sodium-ion storage. The mitigative restacking and high surface area of p-Ti3C2Tx facilitates accessibility of MXene nanosheets to the electrolyte, providing more electrochemical active sites than the restacked Ti3C2Tx due to the higher surface-to-volume ratio and leading to high sodium-ion storage capacity. In addition, the porous structure of p-Ti3C2Tx significantly shortens the transportation and diffusion distance of electrolyte, which is beneficial for the rapid transportation and diffusion of sodium-ions. It is also worth to mention that the previous investigations show that the diffusion barriers of Na+ on bare, F-, O-, and OH-functionalized Ti3C2 MXene are 0.02, 0.19, 0.20, and 0.013 eV, respectively, and Ti3C2Tx possesses high metallic conductivities (∼6000−8000 S cm-1).55-57 Consequently, the p-Ti3C2Tx is preferable for ultrafast transport of Na+ and electrons, which is required for sodium-ion storage at ultrahigh rates. In summary, we reported a rational design and preparation of porous Ti3C2Tx electrode materials for sodium-ion storage. Open structure conductive 2D Ti3C2Tx nanosheets were prepared by insertion and heat treatment removal of sulfur. This treatment produced an anisotropic assembly of highly conductive 2D Ti3C2Tx nanosheets into a porous structure, which can concurrently afford high electronic conductivity and fast electrolyte ion transportation and diffusion, leading to the best rate capability among all reported materials so far. At the high current densities of 1 and 10 A g-1, the porous Ti3C2Tx electrodes delivered 9 ACS Paragon Plus Environment


capacities of 166 and 124 mA h g-1, respectively. Even at a very high current density of 100 A g-1, a capacity of 24 mA h g-1 could be achieved. Additionally, the as-prepared porous Ti3C2Tx electrode possesses excellent cycling stability for 1000 cycles. Our results demonstrate the promise of the porous structure of conductive Ti3C2Tx nanosheets to push electrochemical sodium-ion storage systems closer to practical large-scale applications where high-rate charging/discharging is desired. Supporting Information Additional experimental details, XRD patterns of S/Ti3C2Tx composites and the as-prepared p-Ti3C2Tx, TGA and DTG curves of Ti3C2Tx/S, SEM images of a restacked Ti3C2Tx film, AFM image and the acquired height profile of p-Ti3C2Tx on a silicon wafer. Acknowledgements This work was proudly supported by the Commonwealth of Australia through the Automotive Australia 2020 Cooperative Research Centre (AutoCRC). Y.G. acknowledges the national long-term project (WQ20152200273) of “Thousand Talents Plan of Bureau of Foreign Experts Affairs” of People's Republic of China. The authors thank Dr. Meng-Qiang Zhao for TEM characterization, Dr. Taron Makaryan (both, Drexel University) for Raman analysis on MXene and the Core Research Facilities of Drexel University for providing access to XRD, SEM, and TEM instruments. REFERENCES 1.

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