Comparative evaluation of MAX, MXene, NanoMAX, and NanoMAX-derived-MXene for microwave absorption and Li ion battery anode applications

Arundhati Sengupta a, B. V. Bhaskara Rao b, Neha Sharma a, Swati Parmar a, Vinila Chavan a, Sachin Kumar Singh a, Sangeeta Kale *b and Satishchandra Ogale *ac
aDepartment of Physics and Centre for Energy Science, Indian Institute of Science Education and Research (IISER) Pune, Maharashtra-411008, India. E-mail: satishogale@iiserpune.ac.in
bDefence Institute of Advanced Technology, Pune, Maharashtra-411025, India. E-mail: sangeetakale2004@gmail.com
cResearch Institute for Sustainable Energy (RISE), TCG Centres for Research and Education in Science and Technology (TCG-CREST), 16th Floor, Omega, BIPL Building, Blocks EP & GP, Sector V, Salt Lake, Kolkata 700091, India. E-mail: satish.ogale@tcgcrest.org

Received 31st December 2019 , Accepted 20th March 2020

First published on 20th March 2020


Abstract

MAX and MXene phases possess unique physical properties, encompassing the realms of both ceramics and metals. Their nanolaminated layered configuration, high anisotropic electrical conductivity, and ability to scatter electromagnetic radiation are beneficial in multiple applications. Herein, detailed applications of MAX and MXene are studied in the fields of microwave absorption and Li ion batteries (LIB). In particular, coatings based on MAX, MXene, ball-milled NanoMAX, and NanoMAX-derived-MXene (MXene-N) and their composites are examined in terms of their comparative efficacy for the aforesaid applications. NanoMAX and MXene-N based composites with graphite exhibit superior performance with specific reflection loss values (representing absorbance when measured with metal-backing) of −21.4 and −19 dB cm3 g−1, respectively, as compared to their bulk counterparts, that too with a low density (0.63 g cm−3) and very small thickness (0.03 mm). These performance improvements in absorbance in only 30 μm coatings can be attributed to reflective losses compounded with multiple internal reflections within the nanocomposite intensified by dielectric losses, arising from high interface density. The pristine samples were also studied for their performance as Li ion battery anodes. Herein, MXene-N exhibits the best performance with a specific capacity of 330 mA h g−1 at 100 mA g−1 and excellent cycling stability tested up to 1000 cycles.


1. Introduction

In recent years, intense research has been done on an interesting class of layered ternary transition-metal carbides/nitrides, referred to as MAX phases (with the general formula Mn+1AXn, where M is an early transition metal, A is an element of group 13 or 14, X is either C or N, and n = 1–3), and the corresponding 2D forms Mn+1XnTx (Tx representing the etchant-induced surface-anchored groups such as F, OH, and O) termed as MXenes; the terminology is derived from graphene. The research in this field has advanced rapidly with several pioneering and innovative contributions by the groups of Gogotsi, Barsoum, and others.1,2 MAX phases concurrently possess ceramic-like and metal-like properties with excellent mechanical strength and chemical stability.3–5 MXenes show high electrical conductivity (of the order of 103 S cm−1) and high surface hydrophilicity, which are useful for forming composites/hybrids/coatings by solution-processing in polar solvents, opening up several application avenues for these rather special-property materials.6 Indeed, their applications and excellent performance in several areas such as electrochemical energy storage, water splitting, pressure sensing, water purification, and electromagnetic (EM) interference (EMI) shielding have already been demonstrated.7–12 In this study, we have focussed on two specific application areas, which by virtue of the peculiar property-needs invite detailed explorations of MAX and MXene phases in suitably engineered forms, namely a microwave absorption material (MAM) and a Li ion battery (LIB) anode material. We have differentiated EM wave absorbance (studied herein and discussed later) with the total EMI shielding effectiveness (SE), which represents the compounded effect of very high metallic reflectivity and absorption, and has already yielded highly impressive result with MXene, as shown in the work by Gogotsi and coworkers.8

The research on EM wave (microwave) absorbing materials (MAMs) has grown significantly in the last decade owing to their major military applications in stealth technology such as hiding targets like aircraft, ships, missiles, and other defence vehicles from radar surveillance systems,13,14 as well as more recent applications as shields against data infringement in the domains of specific electronic and telecommunication devices. MAMs operate via relaxation of dipolar and interfacial polarizations, and ohmic loss.15,16 An ideal MAM is generally in the form of a coating or a paint and should show high reflective loss (high absorption when measured against metal backing) over a broad band-width, while having low density and thickness with flexibility. As a result, a lot of research in this field is focused on using carbon-based materials such as graphite, graphene, carbon nanotubes/nanofibers, carbides such as SiC and TiC, and their composites, which are well-known as light-weight high loss dielectrics.16,17 The use of a composite or a hybrid configuration with properly-tailored microstructure offers interfacial polarization, which can help in improving the microwave absorption capacity of a material.18–20 2D materials such as graphene can provide a large interface density (high specific surface area), aiding the microwave absorption process. As shown by some recent studies, MAX and MXene phases can bring additional benefits in this context.21–23 Further detailed explorations are clearly needed in order to understand the influence of microstructural variations (2D sheets, 0D nanoparticles) and interface density on the microwave absorption properties of these materials, especially when these are prepared to apply practically on suitable surfaces as thin paints. This serves as the basis of our motivation to pursue these studies.

Rechargeable batteries have gained importance over the last three decades as energy storage devices due to their ability of delivering high energy and power density with long cycle life, which is useful for energy applications such as grid level storage of renewable energy and in the rapidly evolving field of electric and hybrid electric transport vehicles. In this field, extensive research is in progress on different aspects of battery materials such as anode, cathode, electrolyte, and electrode–electrolyte interfaces.24,25 A great deal of research focus has been laid on the anode material and its morphology, which serves as a pivotal component in tuning their performance efficiency. Nanostructured anode is known to be advantageous because it provides a large surface area, short path length for diffusion of alkali ions, and high rate of electron transport.26–29 In spite of some successes achieved with some material systems, the search for new materials continues owing to the need to address persisting problems of low capacity, electronic conductivity, or coulombic efficiency, and/or safety issues, large volume change (structural stability issues), or large potential hysteresis.30,31 After the discovery of MXenes in 2011, these materials have acquired attention as anode materials for LIBs.32–35 Although several reports have shown enhanced battery performance using MXene by making their composites with other state-of-the-art materials,36–38 there still exist ample possibilities to tune the microstructure and/or surface chemistry/defects of bare MXene to improve the battery performance further, which provides motivation for further studies while keeping the methods simple and low-cost.

In the present work, we have examined microwave absorption properties in the X-band (8.2–12.4 GHz) in detail for the case of paints based on Ti3AlC2 (MAX), Ti3C2Tx (MXene), ball-milled Ti3AlC2 nanoparticles (NanoMAX), and NanoMAX-derived-MXene (MXene-nanoparticles or MXene-N) and their composites with graphite. NanoMAX and MXene-N with graphite cases are seen to exhibit superior microwave absorption performance with reflection loss (RL) values of −13.8 dB and −12 dB, respectively (which correspond to specific reflection loss (SRL) values of −21.4 and −19 dB cm3 g−1, respectively; 94–96% absorption), over the RL in other cases at the resonance peak of 8.9 GHz of the X-band with only a low thickness of the material coat of about 0.03 mm (30 μm). The RL for other cases are: −2 dB (MAX), −3 dB (MXene), −2 dB (NanoMAX), −3.5 dB (MXene-N), −11.5 dB (MAX-graphite), −7 dB (MXene-graphite), and −4 dB (graphite). We attribute these high reflection losses (RL) to the intensified dielectric polarizations and related dielectric losses due to high interface density and multiple internal reflections.37,38 The four cases of pristine samples were explored for LIB anode application as well. The evaluation of these materials as LIB anodes shows an interesting trend in their performance. MXene-N stands out among all the samples. It delivers a specific capacity of 330 mA h g−1 at 100 mA g−1 with an excellent cycling stability up to 1000 cycles. This performance of MXene-N is attributed to the high density of surface functional groups introduced during the etching of the NanoMAX sample, which has abundant surface defects introduced during the ball-milling of MAX as well as enhanced density of edges, thus providing effective adsorption sites. The results obtained are unique and consolidated comparative studies on the above materials in 2D and 0D forms as low-cost thin paints or coats for microwave absorption and LIB have not been reported to the best of our knowledge.

2. Experimental section

2.1. Materials synthesis and device fabrication

The details regarding the synthesis and characterization of bulk Ti3AlC2 (MAX), the bulk-derived Ti3C2Tx (MXene), nanostructured Ti3AlC2 (NanoMAX) obtained by ball-milling, and NanoMAX-derived Ti3C2Tx (MXene-N) are given in the ESI.

2.2. Microwave absorption measurements

Microwave absorber paint samples were prepared by first dissolving 0.1 g of ethyl cellulose in ∼1.5 mL isopropyl alcohol and then adding 0.2 g microwave absorber material, which is a mixture of graphite with MAX, MXene, NanoMAX, or MXene-N, i.e., MAX-graphite, MXene-graphite, NanoMAX-graphite, or MXene-N-graphite (1[thin space (1/6-em)]:[thin space (1/6-em)]1 wt ratio). The paint mixture was allowed to homogenize using a sonicator for ∼5 min with each solid additive. The homogenized mixture was drop-cast on the desired area (2.28 cm × 1.01 cm) of a polyethylene terephthalate (PET) sheet (0.3 mm), allowed to air-dry, and then laminated for microwave absorption measurements by a Vector Network Analyzer (VNA). The individual components of the above absorber materials and the uncoated PET sheet were studied as the control samples. The details regarding the microwave absorption property characterization, the corresponding procedures, theoretical considerations, and the formulae to obtain the parameters of interest are provided in the ESI.

2.3. Electrochemical measurements

The electrodes for all the samples were prepared by mixing the sample, conducting carbon, and polyvinylidene difluoride (PVDF) binder in the weight ratio of 80[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]10 in N-methyl-2-pyrrolidone (NMP) as the solvent. The slurry was coated onto a Cu foil and kept for drying at 60 °C under vacuum for 5 h. The foil was then punched into 1 cm2 circular discs. The coated anodes were used as the working electrodes and Li metal was used as the reference electrode in the fabrication of CR-2032 coin cells. The separator used was Whatmann and the electrolyte used was LiPF6 in EC[thin space (1/6-em)]:[thin space (1/6-em)]DMC (1[thin space (1/6-em)]:[thin space (1/6-em)]1 vol. ratio). Galvanostatic charge discharge measurements were performed with a BTS-Neware (China) 5 V-10 mA battery tester. The impedance and cyclic voltammetry were performed with a VMP3 biologic system equipped with a potentiostat and galvanostat channels.

3. Results and discussion

3.1. Material characterization

The XRD patterns of bulk Ti3AlC2 (MAX) and nanostructured (ball-milled) Ti3AlC2 (NanoMAX) are shown in Fig. 1a. Their XRD patterns match well with the JCPDS pattern 052-0875 of Ti3AlC2 (hexagonal lattice, P63/mmc space group). The XRD peak broadening of NanoMAX with respect to that of MAX reflects the nano-sizing achieved upon ball-milling of MAX. The XRD patterns of MXene and MXene-N (NanoMAX-derived-MXene) obtained from Al-etching of MAX and NanoMAX (see experimental details in ESI), respectively, are given in Fig. 1b. These match well with those of Ti3C2Tx synthesized by different routes.39–41 MXene and MXene-N typically show increased interplanar spacing (d002 = 1.484 nm and d002 = 1.315 nm, respectively) for the (002) plane as compared to these planes of MAX and NanoMAX (d002 = 0.904 nm for both) phases, confirming Al-removal. The MXene sample exhibits a significantly intense and sharp (002) peak, which is known to appear when Li+Cl (here, from LiF + HCl) is present during its synthesis (the etching process) from the MAX phase.39 This is because of Li+ intercalation, leading to greater structural ordering of the 2D sheets of MXene. Other peaks are suppressed in comparison to the (002) peak in this sample. For MXene-N, which is essentially nanoparticles (0D material, refer microstructure analysis below), the surface energy is high along all the directions and preferential orientation along (002) is no longer possible. Thus, other planes show up almost as prominently as the (002) plane, along with the expected peak-broadening of the 0D system. The XRD patterns of the paints made with graphite added to the above four samples and coated on PET sheets are given in Fig. S1.
image file: c9nr10980c-f1.tif
Fig. 1 The XRD patterns of (a) bulk Ti3AlC2 (MAX) and nanostructured Ti3AlC2 (NanoMAX) and the derived (b) MXene (from MAX) and MXene-N (from NanoMAX).

The FESEM (Field Emission Scanning Electron Microscopy) images of the micron-sized stacked layers of MAX are shown in Fig. 2a. The MXene derived from it by etching-out of Al (Fig. 2b) clearly shows increased spacing between the layers, thus supporting the XRD data. The FESEM images of NanoMAX and the NanoMAX-derived-MXene or MXene-N are shown in Fig. 2c and d, respectively. The HRTEM (High Resolution Transmission Electron Microscopy) images of MXene-N in Fig. 2e shows particles of average 10 nm size.


image file: c9nr10980c-f2.tif
Fig. 2 FESEM images of (a) Ti3AlC2 (MAX), (b) Ti3C2Tx (MXene), (c) nanostructured Ti3AlC2 (NanoMAX), and (d) the NanoMAX-derived-Ti3C2Tx (MXene-N); (e) HRTEM and (f) lattice images of MXene-N.

The magnified image of the region marked with a ‘square’ (Fig. 2f) shows the crystallite corresponding to the (008) plane with interplanar spacing d008 = 0.309 nm, which is characteristic of the hexagonal Ti3C2Tx structure. Atomic Force Microscopic (AFM) studies were also performed on exfoliated MXene (Fig. S2). These show a sheet size of ∼2 μm and thickness ∼1 nm.

The Raman spectra of the samples are compared in Fig. 3a. The MAX sample shows Raman bands at about 127 cm−1 (split into 116, 134 cm−1, E2g symmetry), 260 cm−1 (E1g, E2g), and 402 cm−1 (A1g) corresponding to the shear and longitudinal Ti–Al vibrational modes.1,42 The bands in the range of 500–750 cm−1 (560, 608, 650, 700 cm−1) are associated with the vibrations of C atoms of Ti–C (E1g, E2g, and A1g symmetries, marked in Fig. 3a). NanoMAX exhibits the basic vibrational modes observed in MAX but does not reflect the splitting of Eg modes of Ti–Al and Ti–C vibrations owing to the localized crystalline ordering upon nano-sizing. A slight strengthening of the longitudinal Ti–Al A1g vibrational mode is observed in the nano-dimension. In general, the treatment of MAX with Al-etchant is known to cause deterioration of the hexagonal crystal structure/symmetry (lowering of crystalline order) in MXene.1,43 However, the presence of Li+ (in LiF-HCl etchant) aids the structural ordering in MXene by intercalation (preserving some of the hexagonal symmetry elements of the parent MAX),39 as also described in the XRD section. Thus, the basic Raman modes of MAX in the range of 200–650 cm−1 are seen to be preserved in our MXene sample (also justified by the rather sharp (002) and (004) XRD peaks of MXene in Fig. 1b) but is significantly downshifted in MXene-N, in accordance with the literature.1,43 Furthermore, the bands show broadening and positional shifts as per the influencing surface terminating group (marked with a star ‘*’ in Fig. 3a). With the removal of Al in MXene/MXene-N, the Raman bands at about 127 cm−1 of the MAX/NanoMAX samples disappear and new bands appear. These and the evidence of surface terminations in the overall MXene/MXene-N spectra are described as follows: the MXene and MXene-N samples exhibit a band at 153 cm−1 corresponding to the in-plane Ti–C vibrations of Ti3C2 with Eg symmetry.42 This vibrational mode is shifted upon terminations with O, F, and/or OH in Ti3C2Tx and shows up at about 111 cm−1 and 107 cm−1 in MXene and MXene-N, respectively. A band at ∼250 cm−1 in the MXene can be ascribed to the mixed contributions from the out-of-plane Ti–C vibrations of Ti3C2 (A1g symmetry) and the vibrations of H atoms of Ti3C2(OH)2.44,45 A second band corresponding to the out-of-plane Ti–C vibrations appears around 608 cm−1. In MXene-N, the first band (of out-of-plane Ti–C vibrations) is shifted to about 207 cm−1 and the second band is shifted to about 627 cm−1. This indicates a predominance of surface terminations in MXene-N (greater surface energy of 0D NanoMAX than that of bulk MAX causes greater number of surface (and edge) functional groups to be attached in the derived MXene-N than in MXene during the etching process). This causes the weakening of the out-of-plane vibrations of the surface Ti atoms while strengthening the out-of-plane vibrations of C atoms in MXene-N.44 The band at ∼418 cm−1 in MXene corresponds to the in-plane vibrations of O-atoms (Eg symmetry) of OH-terminated MXene Ti3C2(OH)2. This band is shifted to ∼400 cm−1 in MXene-N. A weak band at about 720 cm−1 corresponding to the C–C vibrations of Ti3C2O2[thin space (1/6-em)]45 is observed in both the MXene and MXene-N samples.


image file: c9nr10980c-f3.tif
Fig. 3 (a) Raman spectra of MAX, NanoMAX, MXene, and MXene-N. (b) FTIR spectra of MAX, NanoMAX, MXene, and MXene-N.

The FTIR spectra of MAX, NanoMAX, MXene, and MXene-N samples, given in Fig. 3b, exhibit Ti–C stretching vibrations in the range of 500–650 cm−1.46 The surface terminating groups O, OH, and/or F in MXene and MXene-N samples were confirmed from the C–O, C–F stretching and the O–H bending bands, characteristic of Ti3C2Tx, occurs at about 920–1160 cm−1, 1050–1360 cm−1, and 1400–1650 cm−1, respectively, in both the samples, which are in accordance with the reported literature.47–50 The Ti–O and Ti–OH vibrations appear in the ranges of 650–880 cm−1 and 1400 cm−1, respectively. The adsorbed CO2 vibrations show up at about 1365 cm−1, in the ranges of 2270–2430 cm−1 and 1990–2180 cm−1.51 Clearly, the surface terminating groups are more pronounced in MXene-N as compared to MXene owing to the more available surface comprising the terminations. MAX and NanoMAX samples can be differentiated from the MXene and MXene-N samples by means of the absence of C–F stretching bands in the former, although they show evidence of a small amount of adsorbed CO2/H2O and surface O–H (negligible in MAX but somewhat pronounced in NanoMAX owing to the higher surface energy in the nano-dimension). The Ti 2p and C 1s XPS spectra of the samples are compared in Fig. 4a and b, respectively.


image file: c9nr10980c-f4.tif
Fig. 4 XPS spectra of (a) Ti 2p levels, (b) C 1s levels of MAX, NanoMAX, MXene, and MXene-N, and (c) Al 2p levels of MAX and NanoMAX.

The Ti 2p spectrum of MAX exhibits an intense peak at 454.5 eV, which is characteristic of Ti 2p3/2 of Ti–C (of Ti3AlC2), with the 2p1/2 component at 460.5 eV.1,52 The contributions from Ti–Al (Ti 2p3/2 and 2p1/2) appear at lower binding energy (BE) values 453.9 and 459.9 eV.53,54 The contributions from the surface Ti–O (Ti 2p3/2 and 2p1/2) for various oxidation states from Ti2+ to Ti4+ appear at higher BE values in the ranges of 455–459 eV and 461–465 eV (positions marked in Fig. 4a), respectively. For NanoMAX, the Ti–C 2p3/2 peak is lowered in intensity with respect to the peak at about 459 eV (from surface Ti–O contribution) and is shifted towards higher BE (possibly due to O–Ti–C surface bonding), depicting the increased surface contribution in the nano-dimension, consistent with the FTIR data. For MXene, the main peak positions are shifted towards further higher BE values (than in MAX/NanoMAX) and are significantly broadened, consistent with the contributions from the increased number of surface functional groups (O, OH, and/or F, all leading to a negative surface charge), arising from the Al-etching process. The contributions from Ti–F appear at higher BE values than Ti–O.53,55 The intensity of the Ti–C 2p3/2 peak with respect to the peak due to surface Ti–O/F at 459.5 eV is reasonably preserved in the micron-sized sheets of MXene. However, in MXene-N, the surface Ti–O/F contribution predominates in the nano-dimension, thus suppressing the intensity of the Ti–C 2p3/2 peak.

The C 1s spectrum of MAX shows a characteristic peak at 281.7 eV corresponding to Ti–C,52,53 which is shifted to higher BE value (281.9 eV) in NanoMAX, MXene, and MXene-N owing to the surface contributions described earlier. The second broad peak in the range of 284 to 288 eV has contributions from C–C, C–H (from surface adventitious carbon and/or graphitic carbide-derived carbon), C–O (from surface O), and –COO (from adsorbed CO2, see FTIR analysis).51,55 In the 2D microsheets of MXene, the Ti–C peak intensity is higher than the second broad peak comprising of surface contributions, which is consistent with the literature.53 The Al 2p spectra of MAX and NanoMAX in Fig. 4c show the presence of Al–Ti together with the surface Al–O and interfacial Al–Ti–O. Such surface contributions are commonly observed in these carbide materials.52,54 The O 1s spectra of the samples (Fig. S3a) support the presence of –O, –OH, and adsorbed H2O (refer to FTIR).

The F 1s spectra (Fig. S3b) of MXene and MXene-N show the presence of C–Ti–F together with contributions from Li–F (reported BE for LiF is 685.5 eV)56 and/or Al–F/Al–O–F (from AlFx/Al(OF)x formed during Al-etching process; BE for AlF3 is 686.7 eV (ref. 55 and 57)), and/or C–F (reported 689.2 eV).56

3.2. Applications

3.2.1. Microwave absorption application. Microwave absorption properties of various cases of interest are shown in Fig. 5. In this case, we have made the measurements using 1 port Vector Network Analyzer (VNA) method, in which reflection loss (RL) measurements were performed with metal backing, which essentially and directly represent microwave absorption in the X-band (8.2–12.4 GHz) wave guide of Port 1. The experimental setup is made so that only the reflection parameter S11 is measured in the instrument. The metal backing is made up of aluminium material, which is of the same kind as that used in aircrafts. Our 1-port measurement is to be contrasted with traditional 2 port VNA measurements used in the evaluations of EMI shielding effectiveness or SE (e.g., in ref. 8 used by Gogotsi and coworkers) wherein two S-parameters (S11, S12) were considered, which represent the fractional reflected and transmitted power (R = |S11|2, T = |S12|2, and A = 1 − RT). In our case of reflection loss (RL), only S11 is relevant due to the metal backing and 1 port VNA measurement.
image file: c9nr10980c-f5.tif
Fig. 5 (a) Microwave absorption (RL), (b) real permittivity (ε′), and (c) tangent loss (tan[thin space (1/6-em)]δ) of MAX-graphite, MXene-graphite, NanoMAX-graphite, and MXene-N-graphite, and (d) a model mechanism of microwave absorption in the MXene-N-graphite sample.

Fig. 5a shows the reflection loss (RL) of four different cases of samples achieved in the X-band with wave guide dimensions of 2.28 × 1.01 cm2. The RL value of −10 dB means 90% the of input microwave radiation is absorbed. The PET sheet without the sample coat has the RL value of about −1 dB.

Amongst the four cases, the NanoMAX-graphite sample (with a low sample coat thickness of only 0.03 mm or 30 μm) is seen to exhibit the maximum RL dip of ∼−13.8 dB at 8.9 GHz, that is, ∼96% of the input power is getting absorbed. For the same sample, another RL dip (−9.2 dB) is noted at about 10.2 GHz. Other samples, namely, the MAX-graphite, MXene-graphite, and MXene-N-graphite show RL values of about −11.5 dB, −7 dB, and −12 dB, respectively, at about 8.9 GHz (for the same sample coat thickness of 30 μm). The individual components, namely, graphite, MAX, MXene, NanoMAX, MXene-N, and PET show RL values of less than −5 dB (see ESI, Fig. S4a). This loss is due to absorption by the dipoles (interfacial polarisation) and ohmic loss, destructive interference effects, and multiple internal reflections. All these phenomena are coupled together to render the final reflection loss.

Further analysis of the material properties such as real permittivity (ε′) and tangent loss (tan[thin space (1/6-em)]δ) were measured using the material measurement software, namely, Flexera (ID-9-3F5236FE) equipped with a vector network analyser (VNA) through scattering parameters set by the Nicolson–Ross–Weir (NRW) method.58,59 The real part of permittivity shows the ability of the material to pass the electric field and to store as the electrical potential energy. The imaginary part of permittivity represents the dielectric loss, which is a function of conductivity (σ = ε0ωε′′). Tangent loss is the ratio of the imaginary and real parts of permittivity (tan[thin space (1/6-em)]δ = ε′′/ε′). A summary of the microwave absorption data is presented in Table 1. The comparative RL data of NanoMAX-graphite and MXene-N-graphite cases with respect to other reports on MXene and composites are presented in Table 2.16,21,22

Table 1 Summary of the microwave absorption data of different samples
Sample Average ε Average tan[thin space (1/6-em)]δ RL [dB] at ∼8.9 GHz
PET 1 0.1 −1
Graphite 3 10 −4
MAX 1 0.2 −2
MXene 1 0.2 −3
NanoMAX 1.2 0.2 −2
MXene-N 2.2 2.1 −3.5
MAX-graphite 2 1 −11.5
MXene-graphite 3 2.5 −7
NanoMAX-graphite 1.8 1.5 −13.8
MXene-N-graphite 1.5 1.8 −12


Table 2 Comparison of the microwave absorption data of the present work against other reports on MXene and composites
Sample Thickness [mm] Frequency [GHz] Reflection loss [dB] Specific reflection loss [dB cm3 g−1] Ref.
Ti3C2Tx (MXene) 1.7 11.6 (8.2–12.4) −48.4 Low due to high density 21
Ti3C2 nanosheet/paraffin 3 7.8 (2–18) −40 Low due to high density 22
Graphite/TiC/Ti3C2 2.1 10.5 (8.2–12.4) −63 Low due to high density 16
NanoMAX-graphite 0.03 8.9 (8.2–12.4) −13.8 −21.4 Present work
MXene-N-graphite 0.03 8.9 (8.2–12.4) −12 −19 Present work


Fig. 5b and c show the frequency dependent real permittivity (ε′) and tangent loss, respectively, of various samples. As the frequency increases, the real permittivity and loss tangent values decrease, as expected.

From the above, we observe that the average ε′-values are lower for NanoMAX and MXene-N based composites as compared to the other composites. The tangent loss values of graphite, MXene-N, and MXene-graphite cases (Table 1 and Fig. S4c) are quite high (>2). This is due to factors such as high electrical conductivity and/or surface functionalities/surface charge (as per the material), which results in high reflection (high impedance mismatch) and this is in accordance with their low RL-values, as observed and stated above. The tangent loss values of MAX-graphite, NanoMAX-graphite, and MXene-N-graphite are about 1, 1.5, and 1.8. The values are ≤0.2 for individual components except for graphite and MXene-N.

The trends in the real permittivity/tangent loss values, the reflection loss increments, and the band broadening/shifts can be ascribed to the overall morphology (layered nature of MAX and MXene, nano-size effects in NanoMAX and MXene-N, leading to high interface density), physicochemical, and/or surface properties of the materials and their composites. Fig. 5d shows the model mechanism of microwave absorption (RL) with a metal backing 1 port mode of VNA in a typical NanoMAX-graphite system, which is rich in interfaces. It shows that when microwave radiation falls on the material medium, the wave undergoes partial reflection from the surface as well as maximum wave attenuation by several dipoles (interfacial polarisation), ohmic loses, multiple internal reflections, and destructive interference of the input and reflected power from the metal backing.

Fig. 5a show that the RL keeps increasing rapidly as we go from bulk MAX to exfoliated MXene (large sheets) to graphite. The electrical conductivity of MAX is of the order of 104 S cm−1 (ref. 60) while that of graphite is ∼105 S cm−1;61 thus, the higher loss in graphite can be attributed to its higher conductivity. However, very high electrical conductivity can easily reflect the microwave radiation from the surface of the absorber instead of entering into it due to easy impedance mismatch.23 The conductivity of MXene (1836 S cm−1)62 is lower than these two but there are a number of surface groups (Tx) in Ti3C2Tx and higher interface density due to the few layer sheet-like character of exfoliated MXene, which can enhance the dielectric losses. The MXene-graphite case shows a loss that is higher than either of the two individual materials, which compounds the effects of conductivity and surface dielectric functions. The MAX-graphite case has a significantly higher loss than that in the MXene-graphite case, which can be attributed to the much higher conductivity of MAX than MXene along with higher compactness of the interfaces between the MAX surface (without functional groups Tx) and the polymer. The result for MXene-N-graphite case is close to that of MAX-graphite. This reflects the compensating effect of the enhanced density of the surface groups in the MXene case and the higher electrical conductivity of the bulk MAX phase.

The most interesting case is that of NanoMAX-graphite, which exhibits substantially higher loss as compared to all the other cases discussed above. The NanoMAX-graphite case scores over the MAX-graphite case because it affords the efficient internalization of the incoming radiation via refractive and diffractive routes along with internal microwave scattering. The diffractive effects associated with nanoclusters can help in homogenizing and spreading of the internalized microwave throughout the sample so as to allow higher absorption. A better understanding of the interface phenomena between the different interfaces, which is lacking at present, could throw further light on the processes.

The above studies project that these composite paints, especially NanoMAX-graphite and MXene-N-graphite having very small thickness of 0.03 mm (major advantage of being lightweight) and density of about 0.63 g cm−3 with SRL values of −21.4 dB cm3 g−1 and −19 dB cm3 g−1, are the potential candidates for microwave absorption applications, especially in stealth applications and data protection. It is worth pointing out that the case of EM Shielding Efficiency (SE) addressed in different works, including the highly impressive case of very high SE realized in ultrathin MXene–Na-alginate composite by Gogotsi and co-workers,8 is distinctly different from our case. In their impressively ordered arrangement of MXene sheets obtained in the composite via the favourable functionality of Na-alginate, the reflection itself has 25 dB contribution, implying that 99.6% radiation is directly reflected back by the composite due to the high metallicity. The morphology in our case is more powder-like (see ESI Fig. S5) and therefore, the direct reflection component is already quite low; however, more importantly, the radiation that penetrates inside and returns by reflection at the back surface (Al) is highly absorbed by scattering and de-phasing. This happens due to the multiple internal reflections and enhanced dielectric losses, which arise as a result of the enhanced polarizability generated by the interfaces and their high density.22,23 The differing nature of the attendant phenomena is also reflected by the higher frequency dependent variations in our case with resonances caused by interference effects between the incident and reflected components.

2.2.2. Li ion battery anode application. The samples MAX, MXene, NanoMAX, and MXene-N were explored as LIB anodes. The galvanostatic charge discharge curves in Fig. 6a display the first irreversible specific capacity of nearly 900 mA h g−1 at the current density of 100 mA g−1. The first reversible capacity is 360 mA h g−1 and the material possesses adsorptive and diffusive nature for Li ion insertion as no clear plateaus are seen in the charge discharge plots when cycled between 0.01 to 3 V. To draw a performance comparison of all the samples, rate capability tests were conducted on MAX, NanoMAX, MXene, and MXene-N, as shown in Fig. 6b. MXene-N shows the highest specific capacity among all the four at any given rate. Even at a high rate of 3 A g−1, it gives a specific capacity of ∼110 mA h g−1 and regains its original capacity when cycled at the low current density of 100 mA g−1. The long-term cycling stability of all the samples at 100 mA h g−1 is shown in Fig. 7a. The bulk MAX sample stands to be the lowest with a specific capacity of nearly 50 mA h g−1 possibly due to the restricted Li ion diffusion into the stacked sheets of Ti3AlC2. The ball-milled NanoMAX shows better performance than the latter, which could be due to the surface defects induced by the ball milling process. The MXene sample gives a specific capacity higher than the previous ones because of the functional groups introduced during etching. These functional groups provide additional adsorption sites for the Li ions.37 The MXene-N sample is observed to have a high specific capacity of 350 mA h g−1 in the first cycle, which drops during the initial cycling.
image file: c9nr10980c-f6.tif
Fig. 6 (a) Charge–discharge curves at 100 mA g−1 of the MXene-N anode half-cell w.r.t. Li/Li+ and (b) rate performance of the MAX, NanoMAX, MXene, and MXene-N anode half cells w.r.t. Li/Li+.

image file: c9nr10980c-f7.tif
Fig. 7 (a) Cycling at 100 mA g−1 of MAX, NanoMAX, MXene, and MXene-N, and (b) long-term cycling stability of the MXene-N anode half-cell w.r.t. Li/Li+.

After structural reformation of the material, the specific capacity increases and after 1000 charge–discharge cycles (Fig. 7b), it attains the value of approximately 330 mA h g−1. Thus, the MXene-N is seen to be better in terms of the specific capacity because of the presence of abundant surface functional groups introduced during the etching of high surface energy NanoMAX sample (having surface defects generated by ball milling). The reversible capacity and stability of MXene-N prepared in this work is compared against those of other differently prepared MXenes and MXene-derivatives in Table S1 (in the ESI).

To explore the charge transfer kinetics of the MXene, electrochemical impedance spectroscopy was employed. The Nyquist plot of MXene-N shown in Fig. 8a reveals a low charge transfer resistance in the high frequency region. This can be attributed to its 0D nature (∼10 nm particle size) with high surface to volume ratio, which makes ion percolation into the electrode facile.35,63 The Warburg impedance for mass transfer in the electrode shows Li+ diffusion in the MXene-N. The cyclic voltammetric curve of MXene-N with Li as the counter as well as the reference electrode is given in Fig. 8b. The first cathodic scan at 0.1 mV s−1 shows three distinctive peaks. These can be attributed to the solid electrolyte interphase (SEI) formation, electrolyte decomposition, and phase transformation of the material. After stabilization and cycling, in the subsequent cycles, two redox couples are observed at 0.8/1.1 V and 1.5/1.75 V. These peaks are indicative of Li+ insertion into Ti3C2Tx by the following mechanism:64,65

Ti3C2Tx + yLi+ + ye → Ti3C2TxLiy.


image file: c9nr10980c-f8.tif
Fig. 8 (a) Nyquist plot of the MXene-N half-cell in the frequency range from 300 kHz to 1 MHz with peak to peak amplitude of 10 mV and (b) cyclic voltammograms of the MXene-N anode w.r.t. Li/Li+ at the scan rate of 0.1 mV s−1 in the half-cell configuration.

The charge discharge behaviour of these samples is examined as seen from Fig. S6–S9 and is discussed in the ESI. The above discussion brings out that amongst the four specific cases, the MXene-N derived from the etched ball-milled NanoMAX clearly stands out in terms of LIB anode performance.

4. Conclusion

Bulk MAX phase, MXene, nanosize MAX (or NanoMAX), and MXene-N (derived by etching of NanoMAX) were comparatively evaluated in terms of their functional properties for applications in microwave absorption and Li ion battery anode applications. Thin (0.03 mm) composite paint coatings made with graphite and NanoMAX or MXene-N were found to exhibit much superior microwave absorption performance with reflection loss (RL) values (basically representing the absorbance when measured with metal backing) of −13.8 dB and −12 dB, respectively (or SRL values of −21.4 and −19 dB cm3 g−1, respectively) as compared to MAX or MXene. This nano-enabled performance enhancement can be ascribed to the increased reflective losses caused by multiple internal reflections along with intensified dielectric polarizations and related dielectric losses due to high interface density. The ease of solution-processability of these materials, arising from the surface functional groups, is promising for the application of these systems as thin paint coats on aircrafts and devices for stealth application. In the case of LIB application, the MXene-N sample is observed to perform the best as the LIB anode material among all the other samples. It shows 330 mA h g−1 specific capacity at 100 mA g−1 current density with a high cycling stability of 1000 cycles. The reduced particle size due to ball-milling and overall defect surface chemistry enhance the battery performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to acknowledge research funding from DST (Clean Energy Research Initiative) and DST Nanomission Thematic unit. S. O. would like to acknowledge the Department of Atomic Energy (BRNS) for the award of the Raja Ramanna Fellowship. A. S. would like to thank SERB-N-PDF (PDF/2017/001957).

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Footnotes

Electronic supplementary information (ESI) available: Experimental details, characterizations like XRD, FESEM, AFM, XPS, reflection loss and comparison table of battery performance. See DOI: 10.1039/c9nr10980c
These authors have contributed equally to the work.

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