Enhanced low-temperature Li-ion storage in MXene titanium carbide by surface oxygen termination

MAX phase Ti₃AlC₂ was prepared by ball-milling a mixture of Ti₂AlC powder (>90 wt%, Forsmon) and TiC powder (99%, Johnson Matthey Electronic, NY) with a molar ratio of 1:0.8 for 18 h. The mixture was sintered at 1350 °C for 2.5 h under Ar. Then the material was ball milled for 6 h to smash the bulk particles.

To synthesize Ti₃C₂T_x(F/OH) MXene nanosheet, Ti₃AlC₂ MAX phase was treated with 40% concentrated HF (American Chemical Society grade, BDH) for 6 h at 30 °C to etching the Al out. The as-obtained powder was then washed by using deionized water for several times until the pH  >  6.5, followed by drying in a vacuum oven at 60 °C for 12 h. The MXene-Ti₃C₂T_x(O) with more O-terminated surfaces was obtained by annealing Ti₃C₂T_x(F/OH) powder at 300 °C under ambient air. And the Ti₃C₂O_x/TiO₂ were obtained by annealing Ti₃C₂T_x(F/OH) powder in pure O₂ at 400 °C for 1h.

Powder x-ray diffraction (XRD) data were collected on a Miniflex 600 by employing a Cu Ka (λ  =  0.154 18 nm) radiation source. The morphology was observed by scanning electron microscopy (SEM, Verios 460L Field Electron and Ion Co.) and HRTEM with field-emission gun (HRTEM, Talos F200 X, FEI). The surface elemental information was measured by x-ray photoelectron spectrometer (Escalab 250Xi, ThermoFisher Scientific) using monochromatized Al Kα x-rays as the excitation source. Raman spectrum were measured with a Horiba Scientific at excitation laser wavelength of 532 nm.

Electrochemical performance was carried out in CR2032 coin type cells. The working electrodes were prepared by coating the slurry of the active materials, Ketjen black, and sodium carboxymethyl cellulose at a weight ratio of 7:2:1 onto copper foil and dried in a vacuum at 60 °C. The electrolyte was composed of 1.0 M LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) with a volume ratio of 1:1. Celgard film was employed as the separator. The coin cells were assembled in an argon-filled glove box (Mikrouna) with the concentrations of O₂ and H₂O  <  0.1 ppm.

The galvanostatic discharge–charge measurements were performed in the potential range of 0.01–3.0 V versus Li⁺/Li on a Land battery test system (Land CT2001A). And the temperature was controlled by a high and LT environmental test oven. Cyclic voltammetry (CV) and impedance data were collected by using an electrochemical workstation (CHI 760E).

DFT based first-principles calculations were performed using the Vienna ab initio simulation package (VASP) [33, 34] within the projector augmented-wave approach [35]. Generalized gradient approximation (GGA) in the parameterization of Perdew, Burke, and Ernzerhof (PBE) [36] pseudopotential was used to describe the exchange–correlation potential. The Li-ion diffusion barrier was evaluated with the nudged elastic band (NEB) method [37]. This approach duplicates a series of images between the starting point and the end point of diffusing ion to simulate the intermediate states, with the positions of the starting point and the end point fixed. The supercell comprises 4  ×  4  ×  1 conventional unit cells, corresponding to 32 formula units of Ti₃C₂O₂/Ti₃C₂O₂. For the large supercell adopted in NEB calculations, only the Γ point is adopted for k-point sampling to reduce the computational cost. The plane-wave cut off was set to 520 eV. Geometry optimizations were performed by using a conjugate gradient minimization until all the forces acting on ions were less than 0.01 eV Å⁻¹ per atom.

The preparation and surface engineering of MXene Ti₃C₂T_x is schematically shown in figure 1. Firstly, MXene Ti₃C₂T_x(F/OH) is obtained through etching Al out by HF from Ti₃AlC₂. The –F or –OH group is formed as terminals lying on the aqueous solution condition. Exposed to different atmospheres at 300 °C, the O replace F on the surface of MXene gradually. The partial O substituted Ti₃C₂T_x(O) and totally oxidized Ti₃C₂O_x/TiO₂ can be obtained in ambient air and pure O₂, respectively.

Zoom In Zoom Out Reset image size

The characterizations of as-prepared materials are shown in figure 2. We can understand its structural characteristics more intuitively as displayed in figure 2(a). The 2D sheets are obtained by means of selectively etching Ti₃AlC₂ [11, 14]. The presence of phases and structures has been identified by XRD experiments as illustrated in figure 2(b). The pattern of Ti₃AlC₂ is consistent with the standard card (JCPDS card No. 52-0875) basically, in which a small amount of TiC served as impurities. As for the Ti₃C₂T_x(F/OH), the (0 0 2) peak shifts from 2θ  =  9.5° to 2θ  =  8.7° and the corresponding c axis lattice parameter of ex-Ti₃C₂ changes from 18.5 to 20.3 Å [21, 38]. The layer distance increased as the Al etching out. The crystallinity decreased after O partially substituting while the layered MXene structures still maintains. After deeply oxidation in pure O₂, the main phase can be indexed as TiO₂ (JCPDS card No. 21-1272) as displayed in figure S1 (stacks.iop.org/TDM/6/045025/mmedia).

Zoom In Zoom Out Reset image size

Figure S2 shows the Raman spectra of the Ti₃C₂T_x(F/OH), Ti₃C₂T_x(O), and Ti₃C₂O_x/TiO₂ samples. Compared to the Ti₃C₂T_x(F/OH) sample, Ti₃C₂T_x(O) sample shows a major peak centered at 154 cm⁻¹ corresponding to anatase TiO₂ and the intensity of which has slight changed with Ti₃C₂T_x(F/OH) sample. This proved that a small amount of TiO₂ appears on the surface of the material [32, 39]. The other three broad Raman peaks centered nearly around 254, 410, and 610 cm⁻¹ representing the vibration modes can be assigned to nonstoichiometric titanium carbide [40]. Compared to the Ti₃C₂T_x(F/OH) sample, the band centered around 610 cm⁻¹ shifts to higher energy for all annealed samples.

It has been reported that for layered materials, as the layer thickness decreases, the band position shifts to higher energy representing a slight hardening of the bonds as the layer thickness decreases [39, 41]. Raman spectra indicate a slight thinning of layers in the annealed samples, leading to larger interplanar distance.

SEM was carried out to study the morphology evolution. As shown in figures 2(c) and (d), an accordion-like multilayer structured MXene Ti₃C₂T_x(F/OH) is observed after etching out Al from Ti₃AlC₂, which confirms successful exfoliation of the pristine Ti₃AlC₂. Regular nanoparticles formed on the sheet surface after heat treatment in the air (figure 2(e)). And the sheets are completely covered by nanoparticles with a size of 20–30 nm after deeply oxidation as revealed in figure 2(f). The nanoparticle is confirmed to be TiO₂ combined with the XRD results. What's more, it still maintains layered structure, which is consistent with the results of SEM.

To further understanding the structural evolution after heat processing, transmission electron microscope (TEM) and the HRTEM was carried out as shown in figure 3. Figure 3(a) and inset display the TEM and HRTEM images of Ti₃C₂T_x(F/OH), after etching process, which displays clear adjacent lattice fringes with d-spacings of 0.255 and 0.244 nm, corresponding to the (1 0 2) and (1 0 3) plane of Ti₃C₂ respectively [42]. Figure 3(b) and inset display the structural information of Ti₃C₂T_x(O). The fringe spacing measured to be 0.265 and 0.223 nm, which can be indexed to the (1 0 0) and (1 0 4) planes of Ti₃C₂, respectively, the crystallinity of the basal planes of the Ti₃AlC₂ phase is preserved well. Therefore, it can be reasonably concluded that the MAX crystal structure (JCPDS card no. 52-0875) of the basal planes is maintained in the MXene. Selected area diffraction (SAED) pattern of the Ti₃C₂T_x(F/OH) sheets (figure S3(a)) demonstrates that the MXene sheets retain the hexagonal symmetry and crystallinity of the basal planes of the parent Ti₃AlC₂ phase. After heat processing, the SAED of Ti₃C₂T_x(O) (figure 3(c)) shows some reflecting peak corresponding to TiO₂, which was produced by the local heat developed at surface modification anneal processing of the Ti₃C₂T_x(F/OH). Furthermore, figure 3(d) displays the dark-field TEM image of the Ti₃C₂T_x(O) and the corresponding Energy Dispersive Spectroscopy (EDS) maps of the Ti₃C₂T_x(O) in figures 3(e)–(h) show a uniform distribution of Ti, C, F, and O. The dark-field TEM image and EDS maps of the Ti₃C₂T_x(F/OH) are shown in figure S3, which also indicate the uniform distribution of elements. The surface elemental analysis was performed on XPS. The spectrum demonstrated in figure 4 clearly reveals that Ti, C and O are presented on the surface of Ti₃C₂T_x(F/OH) and Ti₃C₂T_x(O), respectively. The Ti 2p core level is fitted with three doublets (Ti 2p_3/2  −  Ti 2p_1/2) with a fixed area ratio equaling to 2:1 and the energy difference between Ti 2p_3/2  −  Ti 2p_1/2 peaks is 5.7 eV [26, 43]. As displayed in figure 4(a) of the high-resolution Ti 2p spectrum, the peaks at 461.8eV, 460.1.0 eV, 458.7 eV, 456.1 eV, and 454.8 eV can be assigned to Ti–O, Ti–F and Ti–C of Ti₃C₂T_x(F/OH), respectively. It indicates that a small amount of titanium was oxidized during the etching process in HF aqueous solution. Figure 4(b) show the C 1s spectrum which means the peaks at 282.0, 284.8, 286.2 and 288.8 eV corresponds to Ti–C, C–C, C–O, and C–F, respectively [43, 44]. The peaks for O 1s locate at 529.8, 531.5 and 533.5 eV are associated with Ti–O–H, Ti–O and H₂O respectively [45, 46]. Figures 4(e) and (f) show the XPS spectra of Ti₃C₂T_x(O) which is processed in ambient air at 300 °C. The intensity of Ti 2p_3/2 peaks at 456.1 and 454.8 eV decreases significantly, accompanied with an increase of Ti–O peak centered at 458.7eV, compared with those of the Ti₃C₂T_x(F/OH). This indicates that Ti on the surface has been oxidized after processing. Correspondingly, for the spectrum of C 1s (figure 4(e)), the intensity of Ti–C centered at 281.7 eV decreases substantially. Meanwhile, the intensity of C–O centered at 286.2 eV increases, and the intensity of C–C centered at 284.8 eV remains unchanged. As the high-resolution XPS spectra of O 1s, compared figure 4(c) with 4(f), we can come to realize that the concentration of O on the surface has been greatly improved. These results indicate that the content of O as termination group are increased on the surfaces.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

To further explore the surface engineering effects on Li-storage performance at varied temperatures, the electrochemical performance was investigated carefully by using CR2032 coin cells with lithium metal anode at both room temperature and  −20 °C. Figure 5(a) displays the rate performance of Ti₃C₂T_x(F/OH) and Ti₃C₂T_x(O) at  −20 °C. With the increasing of current densities from 100 to 1000 mA g⁻¹, the discharge capacities decreased correspondingly in which demonstrate a order of Ti₃C₂T_x(O)  >  Ti₃C₂T_x(F/OH)  >  Ti₃C₂O_x/TiO₂. Even at a current density of 1000 mAg⁻¹, Ti₃C₂T_x(O) displays a discharge capacity of 88.4 mAh g⁻¹, far better than 51.2 mAh g⁻¹ for Ti₃C₂T_x(F/OH) and 36.6 mAh g⁻¹ for Ti₃C₂O_x/TiO₂. When returned to a current density of 100 mAh g⁻¹, the discharge capacity also recovered perfectly. This indicates the good cycle stability with prolonged cycles.

Zoom In Zoom Out Reset image size

The commercial graphite was also adopted as anode for the comparing the Li-storage performance at LT. As shown in figure S5, the graphite only shows capacity of 77 mAh g⁻¹ (100 mA g⁻¹) and 15.4 mAh g⁻¹ (1000 mA g⁻¹), which are much lower than that of Ti₃C₂T_x(O). The voltage profiles of Ti₃C₂T_x(O) are shown in figure 5(b). Similar with previous reported MXene-based electrode materials, the present Ti₃C₂T_x(O) electrode displays a capacitor-like behavior upon Li ions insertion/desertion process [27, 29]. Figure 5(c) shows the long cycle performance of up to 1000 cycles at a current density of 100 mA g⁻¹ at  −20 °C. The capacity retention at 1000th cycle is 85.8% (compared with 10th cycle) and corresponding to a capacity lost of 0.014% per cycle, We also carefully compared the current performance with previously reported low-temperature LIBs and a table was given at table S1. It could be found that this work is among one of the best previous results. It is generally considered that there are several reasons account for the performance fading with the decreasing operating temperature. The increased Li ions desolvation energy and electrode polarization at LT directly deteriorate the amounts of Li ions which can inserted into electrode materials [3, 5, 8]. The present enhance of Li ion on storage performance in partial oxidized Ti₃C₂T_x(O) is associated with the well electrode/electrolyte contact and improved Li ions desolvation capability even at LT.

The electrochemical properties of Ti₃C₂T_x(O) at room temperature were further investigated and shown in figure 6. The CV shown in figure 6(a) were carried out ranging from 0.01 to 3 V (versus Li⁺/Li) at a scan rate of 0.1 mV s⁻¹. In the initial negative scan, four distinct main reduction peaks can be observed at 1.72, 1.09, 0.73 and 0.30 V, respectively. There are two oxidation peaks at 0.83 and 2.0 V appeared at the following positive scan. The irreversible peaks mainly due to the formation of solid electrolyte interphase (SEI) and some unknown side reactions [47, 48]. The enlarged curves inset figure 6(a) exhibit good reprodu-cibility and the oxidation peaks almost overlap, indicating high reversibility. The CV curves of the pristine Ti₃C₂T_x(F/OH) are presented in figure S6. There are only three distinct main reduction peaks observed at 1.63, 0.75 and 0.36 V in the first negative scan process. The shoulder peak at 1.09 V disappeared may be associated with the good Li ions absorption capacity on the surface of Ti₃C₂T_x(O). It is one of the possible reasons for the improved rate performance of partial surface oxidized Ti₃C₂T_x(O). Electrochemical impedance spectra (EIS) were carried out to explore the resistant evolution. Figure 6(b) show the Nyquist plots of the Ti₃C₂T_x(O) and Ti₃C₂T_x(F/OH) electrodes after 50 cycles, the Nyquist plots measured for Ti₃C₂T_x(F/OH) and Ti₃C₂T_x(O) electrodes at different cycle number displayed in figure S7. Insertion figure showed the equivalent circuit model consists of R_s: element accounts for the total ohmic resistance of electrolyte, CPE: the constant phase element that simulates nonideal behavior of the capacitor by combining the surface modification capacitance and the double layer capacitance as a series, R_ct: resistance to charge transfer on the electrode and Z_w: the Warburg impedance of Li⁺ solid phase diffusion. And the resulting R_ct of electrodes after different cycles were concluded in table S2. Disclosing that surface-modified MXene can boost the charge transfer process greatly and thereby resulting in better rate performance.

Zoom In Zoom Out Reset image size

The rate performance of the Ti₃C₂T_x(O) at room temperature is show in figure 6(c). Extraordinary reversible capacities were achieved as 456 (5th cycle, 100 mA g⁻¹), 340 (15th cycle, 200 mA g⁻¹), 310 (25th cycle, 300 mA g⁻¹), and 273 (35th cycle, 500 mA g⁻¹) to 223 mAh g⁻¹ (45th cycle, 1000 mA g⁻¹) at room temperature. Remarkably, when the current density returned to 100 mA g⁻¹, the capacity back to 428 mAh g⁻¹ after 65th cycles, maintaining more than 95% of the initial value. Figure 6(d) presents the voltage profiles at selected cycles at a current density of 100 mA g⁻¹. As can be seen in the profiles, the plateaus on the discharge-charge curves are greatly consistent with the respective peaks on the CV curves. Figure 6(e) shows the cycle performance of Ti₃C₂T_x(O) anode at a current density of 100 mA g⁻¹. Even after 500 cycles, it still delivers a capacity of 404 mAhg⁻¹ and demonstrates ultra-stable cycle performance. The result discloses that the material structure remains stable after long term cycles. Compared to the performance of Ti₃C₂T_x(F/OH) anode in figure S8 and Ti₃C₂O_x/TiO₂ anode in figure S9, the Ti₃C₂T_x(O) anode still exhibits the highest capacity of 404 mAh g⁻¹ and cycle stability. The gradual increase of capacity in Ti₃C₂O_x/TiO₂ anode mainly is associated with the pulverization of TiO₂ and successive decomposition of electrolyte during repeated cycles [49]. We also carefully compared the present performance with previous reported MXene-based anodes and the results are shown in table 1. It could be found that this work is among one of the best.

To understand the electrochemical improvement mechanism after partial O substitution, we employed DFT calculations to evaluate the Li diffusion barrier in MXene with different terminal groups. As shown in figure S10, the Li ion migration barrier has been greatly decreased with O terminal group compared with that of F terminal group. A low migration barrier would facilitate the rapid diffusion of Li ions, which may explain the improved electrochemical performance of the Ti₃C₂T_x(O) electrode in this work.