Bi-Stacked Titanium Carbide (MXene) Anodes for Hybrid Sodium Ion

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Bi-Stacked Titanium Carbide (MXene) Anodes for Hybrid Sodium Ion Capacitors Narendra Kurra, Mohamed Alhabeb, Kathleen Maleski, Chueh-Han Wang, Husam N. Alshareef, and Yury Gogotsi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01062 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Energy Letters

Bi-Stacked Titanium Carbide (MXene) Anodes for Hybrid Sodium Ion Capacitors Narendra Kurraa, Mohamed Alhabeba, Kathleen Maleskia, Chueh-Han Wanga, Husam N. Alshareefb and Yury Gogotsia* a A. J. Drexel Nanomaterials Institute, and Department of Materials Science and Engineering, Drexel University, Philadelphia, PA19104, USA b

Materials Science and Engineering, King Abdullah University of Science and (KAUST), Thuwal 23955-6900, Saudi Arabia

Technology

Corresponding Author E-mail: [email protected]

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ABSTRACT

Two-dimensional transition metal carbides (MXenes) have shown great promise as electrode materials for high-rate pseudocapacitive energy storage. In this study, we report on the fabrication of bi-stacked 2D titanium carbide electrodes which are free of binder, conductive additives and current-collector. This MXene electrode is capable of reversible electrochemical storage of sodium ions with good cycling stability and rate capability. A prototype hybrid Na-ion capacitor was assembled by combining the bi-stacked MXene anode with an activated carbon cathode, which showed an energy density of 39 Wh/kg (including the total weight of bi-stacked MXene and activated carbon, 6 mg/cm2) at 1C rate and maintained up to 60% of its performance at a 60 C rate, in the operating voltage window of 3.4 V. This study opens new avenues for developing selfstanding binder and additive-free MXene electrodes for metal-ion batteries/capacitors.

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Electrochemical energy storage is a rapidly growing field of research due to the increasing demands for portable electronics, electric/hybrid vehicles, and smart grids.1-3 Currently, state-ofthe-art lithium-ion batteries (LIBs) – with their high energy density and reliability – have become dominant in these applications. 4, 5 On the contrary, electrical double layer capacitors (EDLCs) rely on electrostatic adsorption/desorption of ions at the electrode/electrolyte interface, delivering high power at the expense of energy density.6, 7 This trade-off between the power and energy densities of EDLCs and batteries depends on the fundamental charge storage mechanism associated with the corresponding electrode materials and electrolytes.8 Recently, pseudocapacitive and hybrid energy storage has become attractive as it bridges the gap between batteries and supercapacitors by offering higher storage capacity compared to EDLC at higher charge-discharge rates than batteries.9, 10 A variety of transition metal oxides, such as RuO2,11 MnO2,12 MoO3,13 and Nb2O514, which exhibit rapid surface or intercalation redox reactions with charge transfer events, have been explored for pseudocapacitive energy storage. In addition, transition metal oxides must be used in the form of nanoparticles or porous structures to offer a high specific area, however, their low electronic conductivity (except expensive RuO2) limits rate performance. Two-dimensional transition metal carbides (MXenes) have shown great promise in pseudocapacitive energy storage.15,

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MXenes are electronically conductive, permitting fast

electron transfer, and have transition metal oxide-like surfaces, enabling fast surface redox reactions.17-19 MXenes are typically synthesized from ternary layered ceramic materials, such as MAX phases, which can be represented by the general formula Mn+1AXn, where M is a transition metal (e.g., Ti, Nb, Mo, V, Cr, Ta), A is an element from group 13 or 14 in the periodic table (e.g.,


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Al, Si, Ga, In), and X is carbon (C) and/or nitrogen (N), with n = 1, 2, or 3.15, 20, 21 For instance, the most studied MXene, titanium carbide (Ti3C2) has shown metal-like conductivity (~ 8700 S/cm)20 with proton-induced pseudocapacitance and high packing density (3-4 g/cm3) giving rise to high rate capability and unprecedented volumetric performance (volumetric capacitance ~ 1.5 kF/cm3) in aqueous acidic electrolytes.17-18, 22 Many studies have been devoted to understanding the intercalation of cations into MXenes in both aqueous and non-aqueous electrolytes.18, 22-26 Notably, Wang et al. demonstrated the pseudocapacitance of Ti2C electrodes in non-aqueous Na electrolyte with a sodiation capacity up to 175 mAh/g.25 Furthermore, the Na ion intercalation mechanism between Ti3C2Tx layers was investigated using solid state nuclear magnetic resonance (NMR) spectroscopy, showing a reversible capacity up to 100 mAh/g at a current density of 20 mA/g.26 Given the promising pseudocapacitive characteristics of MXenes, it is clear that they may be suitable electrode materials for high-rate hybrid energy storage devices.27 Recently, researchers started exploring the feasibility of using MXene electrodes for Liand Na-ion hybrid capacitors.28, 29 Wang et al. fabricated Na-ion capacitor prototypes using a negative Ti2C electrode with a Na2Fe2(SO4)3 cathode.25 Xie et al. demonstrated a Na-ion capacitor based on self-standing porous Ti3C2/CNT negative electrode with Na0.44MnO2 as positive electrode.30 V2C MXene was employed as cathode material for fabricating a Na-ion capacitor with hard carbon anodes.29 However, from the device perspective, metal current collectors, such as copper (Cu) and aluminum (Al), add to the total weight of the device, yet do not contribute to capacity, thus reducing the energy density of the device by at least a factor of two.31 We demonstrate the possibility of using a bi-stacked structure by employing a highly conductive MXene film as a current collecting layer which provides mechanical support for the multi-layered MXene particles. One of the challenges with 2D material electrodes is a limited electrode thickness


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ACS Energy Letters

(usually does not exceed tens of microns) due to diffusion limitations in stacked 2D flakes.32 3D designs can produce thicker electrodes, but at the expense of volumetric performance.27 However, if current-collector free electrode films can be used, the thickness is less of problem as long as ionic transport can occur with sufficiently high rate.33 In this study, we demonstrate high-rate Naion hybrid capacitors by employing a novel, free-standing, bi-stacked MXene electrode design as a negative electrode, while using conventional activated carbon as cathode. The bi-stacked Ti3C2 MXene negative electrode architecture was obtained by vacuum filtration, avoiding the use of metal current collectors, binders, or additives. We have not only matched capacity performance, but also achieved kinetic balance of two electrodes in designing the high-rate hybrid Na-ion capacitors. Ti3C2 MXene was synthesized by selective etching of Al layers out of the parent MAX phase, Ti3AlC2, with HF-H2SO4 aqueous etching solution (see experimental details in Supporting Information). As illustrated in Figure 1a, the etching process uses 5 wt.% HF mixed with 25 wt.% of H2SO4 (remaining 70 wt.% water) and proceeds for 15 hours at 40 ºC. The resulting multilayered samples are designated as Ti3C2Tx, where Tx denotes surface functional groups such as – OH, -O and –F. Presumably, the supporting acid (in this case H2SO4) helps in dissolving etching products such as AlFx and maintaining low pH, however, the surface chemistry variations were not studied here and will be reported elsewhere. The (002) peak of Ti3C2Tx shifts to a lower angle of 9.2° compared to Ti3AlC2 (2ϴ = 9.5°), indicating that the interlayer distance expands from 9.42 to 9.73 Å (Figure S1, Supporting information).20, 25 It is evident that even 5 wt.% HF is sufficient to etch Al layers.20 A slightly higher d-spacing of Ti3C2Tx samples with respect to Ti3AlC2 is apparent. As samples were dried in vacuum at 200 °C, the possibility of intercalated water layers in affecting the d-spacing of Ti3C2Tx MXenes can be eliminated. Since only 5 wt.% HF etchants


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were used, thicker, multi-layered Ti3C2Tx MXene flakes are seen with fewer openings of interlayers in Figure S1, Supporting Information. It was reported in the literature that accordionlike morphology of MXenes was more prevalent when the concentration of HF was 30-50%.20 However, in-situ HF generated etchants, such as the LiF/HCl, produce dense MXene particles due to intercalation of Li ions and water molecules.17 Therefore, the morphology of the MXene layers is highly dependent on the concentration and type of the etchant.20 We have fabricated the bistacked MXene film (Figure 1b) through vacuum assisted filtration. Delaminated Ti3C2Tx (dTi3C2Tx) was filtered first followed by filtering multi-layer Ti3C2Tx particles as a second layer. This strategy could be used as a generic method to fabricate a variety of bi-stacked MXene electrodes without employing additives or metal current collectors. Ti

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Figure 1. (a) Schematic illustration for synthesis of multi-layer Ti3C2Tx MXene using HF-H2SO4 etchant. (b) Fabrication of bi-stacked MXene film through vacuum assisted filtration.


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ACS Energy Letters

To investigate electrochemical storage of Na ions, multi-layered Ti3C2Tx powders were slurry casted on Cu foil. Multi-layered Ti3C2Tx was synthesized by employing three different acid mixtures as described in the experimental section (Figure S2, Supporting Information). As shown in Figure 2a, CV scans for the first three cycles are displayed at a scan rate of 0.1 mV/s in the potential range of 0.01 - 3 V (vs. Na+/Na). The first cathodic scan exhibits an irreversible current response with a first cycle Coulombic efficiency of 57%. The initial irreversible capacity is most likely due to the formation of a solid-electrolyte interphase (SEI), and the irreversible reaction of Na-ions with the surface functional groups (-OH, -O and –F) of MXene.25, 26, 29, 30 Additionally, the cathodic and anodic envelops become equal in area starting from the second cycle onward. This result possibly indicates that a stable SEI layer forms during the first cathodic scan, which subsequently controls the diffusion of Na ions across the interphase layer. Further, the CV curves indicate that Ti3C2TX MXene electrodes exhibit a capacitive envelop from 0.02 - 2 V (vs. Na+/Na), which is most likely due to electro-sorption of Na ions between MXene sheets. Interestingly, a pair of reversible redox peaks above 2 V (vs. Na+/Na) were observed, possibly due to a change in redox states of titanium upon insertion/extraction of Na-ions.25 Traditionally, titanium-based electrodes exhibit the redox reaction upon insertion/extraction of Na-ions. The position of each redox peak is highly dependent on the titanium oxidation state and preferential crystallographic sites for Na-ion insertion/extraction.34 As shown in Figure 2b, Ti3C2Tx electrodes show a single pair of redox peaks positioned at 2.34 and 2.11 V (vs. Na+/Na). The reversible nature of redox peaks can be attributed to reversible insertion and extraction of Na+ ions into the MXene structures, causing the reversible change of Ti redox state, which maintains overall charge neutrality.25 It is worth mentioning that Ti3C2TX synthesized using HF and HF/HCl etchants also showed similar redox peaks, but their shape and


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position were affected by the etching method as shown in Figure S2, Supporting Information. For instance, Wang et al. demonstrated that the charge storage in Ti2C electrodes in non-aqueous Na+ electrolytes is accompanied by prominent redox peaks above 2 V (vs. Na+/Na).25 Similarly, V2CTx electrodes have shown obvious redox peaks in non-aqueous Na+ electrolyte.29 Therefore, our results are in agreement with the literature data, confirming that careful synthesis and electrode preparation methods are highly important for observing the redox nature of MXene electrodes upon metal ion insertion/extraction in a non-aqueous electrolyte. The low potential peaks at 0.01 V (vs. Na+/Na) can be attributed to the sodiation/desodiation of carbon black, which was used as a conductive additive.35 The sloping charge-discharge profiles are seen in the entire potential range, except above 2V (vs. Na+/Na) where a small plateau, that corresponds to the reversible change of redox state of Ti upon insertion/extraction of Na+ ions is observed.25 The reversible sodiation capacity up to 105 mAh/g was observed at a current density of 25 mA/g. CV curves at various scan rates can provide qualitative estimation of Na-ion kinetics by computing the b-value in the chosen window of scan rates (Figure 2c). Current response is given by the power law relationship as i = avb, where i is current, v is scan rate and a, b are constants.36 As shown in Figure 2d, the anodic peak current at 2.34 V gives a b-value of 0.8 in the scan rate range of 0.1-1 mV/s, which suggests a predominantly surface controlled process (closer to 1 than 0.5). The peak current values are considered for both anodic and cathodic sweeps for estimating the b-value, despite a small polarization in the scan rate range of 0.1 to 1.0 mV/s. In the scan rate range of 0.1 to 1 mV/s, redox peaks at 2 V are evident, indicating a reversible Na+ insertion/extraction accompanied by a change of titanium redox state. However, it is obvious that the low-potential redox peak at 0.01 V diminishes in current amplitude at high scan rates, and is due to diffusion limitations of the intercalation/deintercalation of Na+ ions into carbon black.35 The


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low-potential redox peak completely vanishes at high scan rates, but the Ti redox peaks remain prominent as shown in Figure S2c, Supporting Information.

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Figure 2. (a) Cyclic voltammograms of Ti3C2TX in 1M NaClO4/EC-PC (FEC) electrolyte at 0.1 mV/s for the first three cycles. Arrows represent oxidation and reduction positions. (b) Typical first and third cycle charge-discharge profiles at a current density of 25 mA/g. (c) CVs of Ti3C2Tx /Cu electrode at different scan rates. (d) Linear dependence of log i vs. log ʋ in the scan rate range of 0.1< ʋ < 1.0 mV/s. (e) Rate performance of Ti3C2Tx /Cu electrode at different current densities. (f) Cycling stability of the electrode at a current density of 25 mA/g. Inset shows XRD patterns comparing (002) peak of the electrode after cycling with the pristine sample.


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Rate performance of the Ti3C2Tx electrode was investigated at different current densities from 25 to 1000 mA/g (Figure 2e). Even at a high current density of 500 mA/g, the electrode exhibited a capacity of 58 mAh/g, with 58% retention from 25 mA/g. After testing at 1000 mA/g, the electrode was tested again at a low current density of 25 mA/g, reverting to the same initial capacity values. Since electrodes were sealed in a coin cell in non-aqueous electrolyte, Ti3C2Tx electrodes are stable over many cycles without any phase transformation or degradation of the material. As shown in Figure 2f, cycling stability was tested at a current density of 25 mA/g for 70 cycles in a coin cell. We have conducted ex-situ XRD measurements to investigate the structural changes after the cycling. Pristine Ti3C2Tx cast on Cu foil showed a d-spacing of 9.6 Å which expands to 12.9 Å after the cycling test. Therefore, there is a lattice expansion of 3.3 Å, which is larger than other Na+ intercalation materials (e.g., 1.0 Å for MoS2, 2.8 Å for Ti3C2 and 2.4 Å for Ti2C).25, 26, 37 This kind of expansion was previously observed for Ti2C and Ti3C2 anode materials due to the pillaring effect of Na ions intercalated in the initial cycles of Na-insertion.25, 26 Further, bi-stacked MXene electrodes were fabricated by vacuum filtration to take the advantage of the high electrical conductivity (~ 5000 S/cm) of Ti3C2Tx free-standing films and the performance of multilayered MXenes in Na-ion capacitors. We have employed delaminated Ti3C2Tx (d-Ti3C2Tx) as a current collector38 (see Experimental section for details) and as a mechanical support to attach multi-layer Ti3C2Tx particles deposited by vacuum-assisted filtration. First, d-Ti3C2Tx MXene was vacuum filtered to obtain a free-standing MXene film. ML-Ti3C2Tx was then bath sonicated (sonic frequency ~ 40 kHz) for 2 minutes and filtered on top of d- Ti3C2Tx MXene. Due to high electrical conductivity of Ti3C2Tx free-standing films, metal current collectors were eliminated. Due to hydrophilic nature and functional surface of MXenes, it is possible to


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achieve adhesion of subsequent MXene layers. We filtered a ML-Ti3C2Tx suspension on top of the wet d-Ti3C2Tx MXene. After complete drying in vacuum oven at 70 °C overnight, this bi-stacked film was pressed by applying a pressure of 500 MPa to ensure good contact between the two MXene layers (Figure S3a, Supporting Information). Furthermore, polymeric binders or

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Figure 3. Electrochemical performance of bi-stacked Ti3C2Tx/d-Ti3C2Tx MXene electrode film. (a) Cross-sectional scanning electron micrograph shows multi-layered Ti3C2Tx/d-Ti3C2Tx; inset shows the 4 cm-diameter bi-stacked MXene film. (b) Cyclic voltammograms (CVs) of the bistacked MXene electrode at different scan rates. (c) Charge and volumetric capacitance variation with scan rate. (d) Galvanostatic charge-discharge profiles at different current densities. (e) Cycling stability at different scan rates of 2, 5 and 10 mV/s. (f) Nyquist spectrum for bi-stacked MXene electrode in Na-electrolyte, inset shows high frequency portion of the spectrum.

As shown in Figure 3a, multi-layered Ti3C2Tx was filtered on d-Ti3C2Tx, in which case the top MXene layer exhibits an open structure with randomly oriented stacks, while the bottom layer of delaminated MXene has a compact morphology. Inset in Figure 3a shows a 4 cm diameter film of bi-stacked Ti3C2Tx/d-Ti3C2Tx. Typical thicknesses of this bi-stacked MXene electrode were ~18


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µm with a density of 1.8 g/cm3 at an areal mass loading of 2.3 mg/cm2. The conductivity of the Ti3C2Tx multi-layered film was found to be 830 S/cm, while the bottom layer of d-Ti3C2Tx film had conductivity up to 5000 S/cm, showing the important differences in the two types of Ti 3C2Tx present in this bi-stacked electrode. When using this architecture as a working electrode in Na-ion half cells, the first cycle Coulombic efficiency was found to be 65% (Figure S3b, Supporting Information). Bi-stacked MXene electrodes showed prominent redox peaks in the scan rate range of 1 to 50 mV/s (Figure 3b). At a scan rate of 1 mV/s, the d-Ti3C2Tx film electrode showed a gravimetric capacitance of 7.2 F/g while bi-stacked MXene electrode has a capacitance of 62 F/g (Figure S3c, Supporting Information). Thus, the d-Ti3C2Tx film electrode exhibited ~11% of capacitance compared to that of the bi-stacked MXene electrode, which is due to compact nature of dense MXene layer which may be hindering the access of Na-ions. Free-standing electrodes (thickness of 18 µm) showed redox peaks even at a scan rate of 50 mV/s. As shown in Figure 3c, charge values varied from 187 to 128 C/g (corresponding areal charge of 430 to 294 mC/cm2) in the scan rate range of 1 to 50 mV/s. Gravimetric capacity values of the bi-stacked MXene electrode are compared with ML- Ti3C2Tx casted on Cu (Figure S3d, Supporting Information). Bi-stacked MXene electrodes showed a volumetric capacitance of 80 F/cm3 (corresponding areal capacitance of 144 mF/cm2) at a scan rate of 50 mV/s. The b-value analysis (i=avb; where i is current; v is scan rate; a and b are constants) for cathodic sweeps (Na-ion insertion) over a wide potentials in the scan rate range of 0.5

Bi-Stacked Titanium Carbide (MXene) Anodes for Hybrid Sodium Ion

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