Facile synthesis of rutile TiO₂/carbon nanosheet composite from MAX phase for lithium storage

1. Introduction

Titanium oxide (TiO₂) has been extensively researched in lithium-ion battery (LIB) application due to its excellent cycling stability, high environmentally friendliness, and low cost [1,2]. Different from the silicon with alloying reaction mechanism [3,4] and other transition metal oxides (such as Co₃O₄, Fe₃O₄, Cu₂O et al.) with the conversion reaction mechanism [[5], [6], [7], [8]], TiO₂ exhibits low volume expansion (<4%) and good cycle reversibility due to its phase transition mechanism by transforming to Li_xTiO₂ (0 < x < 1) [1,9]. However, intrinsic limitations, such as low electrical conductivity (10^-12-10^-7 S cm^-1) and poor rate capability, still inhibit its practical application [10]. Nanosize design is an efficient way to improve the electrochemical properties of TiO₂. Compared to the bulk TiO₂, nanosize structures, such as TiO₂ quantum dots [11], 1D TiO₂ nanomaterials [12], 2D TiO₂ nanosheets [13,14] and porous TiO₂ [15,16], usually demonstrate large specific surface area, tunable electronic properties, attractive chemical activity and probably extraordinary mechanical strength, leading to better dynamic properties [17,18]. Combining TiO₂ with other conductive materials, especially with carbonaceous materials, also proves beneficial for enhancing the electrochemical performances. The carbonaceous materials in TiO₂-based composites can favor electron transport, buffer the volume change, and act as a passivation layer to prevent the aggregation of active materials during the charge/discharge process [[19], [20], [21], [22], [23], [24]]. However, it is a challenge to obtain a well-dispersed composite material with an ex-situ synthesis method.

Recently, a new family of 2D materials, i.e., transition metal carbides, nitrides, and carbonitrides known as MXenes, was proposed to have a wide potential in energy storage systems due to their abundant surface functional groups and ultra-high electrical conductivity [25,26]. Various metal oxide [27,28], sulfides [29] and alloys [30,31] have been assembled on the surface of 2D MXene sheets, resulting in enhanced conductivity and structure stability and thus improved cycle and rate performances. For example, Liu et al. [32] assembled TiO₂ nanorods on MXene nanosheets via the Van der Waals interaction and the material has excellent performance in LIB. More interestingly, TiO₂-based materials, produced by directly oxidizing Ti-based MXenes (Ti₃C₂T_x or Ti₂CT_x) with H₂O₂ [33] or treated at high temperature [34,35], also perform magnificently as LIB anode materials. However, MXenes are usually synthesized by selective etching the “A” layers of MAX phase (a group of layer compounds with a general formula of M_n+1AX_n, where M is an early transition metal, A is generally group IIIA and IVA elements, X is C and/or N, and n = 1, 2 or 3) with corrosive hydrogen fluorine or the mixed solution of LiF and HCl [25,36]. The process is complicated, time-consuming, and not scale-up.

Herein, we adopted a facile strategy to synthesize rutile TiO₂/carbon nanosheet composite by directly etching MAX (Ti₃AlC₂) with Na₂CO₃ at high temperature. The lamellar carbon atoms in the MAX phase are converted to 2D carbon nanosheets, on which urchin-like TiO₂ assembled from 1D rutile TiO₂ nanorods anchored. The urchin-like TiO₂ decreases the diffusion distance of lithium ions and the 2D carbon nanosheets increase the overall electrical conductivity of the material. This unique architecture promises the composite with excellent lithium storage performances as anode for LIBs.

2. Experimental

2.1. Synthesis of TiO₂/C composites

MAX (Ti₃AlC₂) and Na₂CO₃ were mixed uniformly at a mass ratio of 1:10. The mixture was calcinated at 880 °C for 2 h under flowing nitrogen at a ramping rate of 10 °C min^-1 afterwards. The calcinated product was slowly thrown into 3 M HCl solution to remove the redundant Na₂CO₃, and then the solution was heated at 100 °C for 2 h in the water bath under the continuous stirring. Finally the sample was obtained by washed with distilled water and dried, donated as TiO₂/C-100. Samples obtained under water-bath temperatures of 60 and 80 °C are donated as TiO₂/C-60 and TiO₂/C-80.

2.2. Material characterizations

The morphology of the obtained TiO₂/C composite was characterized by using a Hitachi S4800 scanning electron microscope (SEM). The powder X-ray diffraction (XRD) was performed on a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation (λ =0.154 nm). A Renishaw 1000 Raman spectrometer (514 nm) was used to record the Raman spectra. Nitrogen adsorption/desorption measurements were performed on Micromeritics ASAP 2460. The surface area was calculated by the conventional Brunauer-Emmett-Teller (BET) method, while the pore-size distribution was determined by the density function theory (DFT) method using a carbon slit pore equilibrium model. Thermo-gravimetry (TG) analysis was performed on a Netzsch STA449C instrument. The curve was recorded in air at a heating rate of 5 °C min^-1 up to 1000 °C. X-ray photoelectron spectroscopy (XPS) was performed on a Sigma Probe spectrometer with a high-performance Al monochromatic source operated at 15 kV. The XPS spectra were calibrated using the C_1s spectrum of neutral carbon peak at 284.8 eV. The electrical conductivity was characterized using a Kethley RST-8 four-point probe resistivity meter.

2.3. Electrochemical testing

The active material (80 wt%), super-P (10 wt%), and poly(vinylidene fluoride) (10 wt%) were mixed uniformly in N-methyl-2-pyrrolidone to form a slurry. Electrode was prepared by spreading the slurry onto a Cu foil and then getting dried. With the as-prepared electrode as the working electrode, lithium foil as the counter electrode, Celgard 3500 microporous membranes as the separator, and 1 M LiPF₆ in ethylene carbonate/diethyl carbonate (V/V = 1:1) as the electrolyte, 2025 coin-type half cells were assembled in an argon-filled glove box (with O₂ and H₂O level below 0.1 ppm). The half cells were tested at different current densities within a voltage range of 1.0-3.0 V vs. Li⁺/Li on a Land BT2000 battery test system (Wuhan, China). The capacities are calculated based on the mass of the composites. The cyclic voltammetry (CV) measurements were conducted at a scan rate of 0.1 mV s^-1 between 1.0 and 3.0 V on a CHI600E electrochemical workstation (Chenhua, China). Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz to 0.01 Hz. The loading mass of the active material was 1.0-1.1 mg cm^-2.

3. Results and discussion

Fig. 1 demonstrates the typical morphology of TiO₂/C-100. After the calcination and water-bath process, 2D carbon nanosheets with large surface area and ultrathin thickness were obtained as shown in Fig. 1(b). This structure is derived from the lamellar carbon atoms in MAX phase after Na₂CO₃ etching (Fig. 1(a)). Urchin-like TiO₂ consisted of 1D TiO₂ nanorods are anchored uniformly on the surface of 2D carbon nanosheets. These in situ generated 1D TiO₂ nanorods with a diameter of 10-20 nm (Fig. 1(c)) decrease the diffusion distance of the Li ions and benefit the charge transfer. The 2D carbon nanosheets can favor the electrolyte infiltration, create a conductive network between the urchin-like TiO₂ and increase the overall electrical conductivity of the material. Thus the whole structure will be beneficial for enhancing the dynamic performance of the material.

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Fig. 1.   Schematic diagram for the transformation evolution (a), typical morphology of TiO₂/C-100 (b) and high resolution image of the urchin-like TiO₂ (c).

To further understand the detailed mechanism of the transformation reaction, XRD patterns of the raw MAX, the calcinated product and the final TiO₂/C are recorded and the results are presented in Fig. 2. The initial peaks of the Ti₃AlC₂-type MAX almost disappear after the calcination process. Instead, strong peaks corresponding to Na₂CO₃ and Na₄TiO₄ appear. When the calcinated product was dissolved in water and HCl was added into the upper supernatant dropwise, a white precipitation appears at first and gradually disappears with the increasing HCl amount, implying the formation of Na₃AlO₃. After water-bathing with HCl solution, Na₃AlO₃ and Na₄TiO₄ transform to AlCl₃ and TiCl₄, and TiCl₄ further water-bathing hydrolyzes to yield TiO₂. The final sample exhibits no other peaks except for those of rutile TiO₂ (JCPDS Card No. 01-077-0442) and carbon (the weak peak at 25.3°). Thus, the transformation evolution can be inferred as follows:

2Ti₃AlC₂ + 15Na₂CO₃ (l) → 6Na₄TiO₄ + 2Na₃AlO₃+ 4C + 15CO (g)

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ (g)

NaAlO₂ + 4HCl → AlCl₃ + NaCl + 2H₂O

Na₄TiO₄ + 4HCl → TiCl₄ + 4NaCl + 4H₂O

TiCl₄ + 2H₂O → TiO₂ + 4HCl

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Fig. 2.   XRD patterns of the raw MAX, the calcinated product and the final TiO₂/C-100.

The Raman spectrum of TiO₂/C-100 (Fig. 3(a)) shows two strong peaks at ∼1345 and ∼1590 cm^-1, corresponding to the D- and G-band of the carbon nanosheets. The intensity ratio of the D- to the G-band (I_D/I_G) is 1.18, indicating the amorphous nature of the carbon in the composite [37,38]. The symmetric shape of the 2D-band at 2700 cm^-1 manifests the few-layer order graphitic structure. The typical four peaks at 145 cm^-1 (B_1g), 445 cm^-1 (E_g), 610 cm^-1 (A_1g), and 240 cm^-1 (for second-order effect) indicate that the generated TiO₂ is rulite structure instead of anatase TiO₂ [39]. This agrees with the XRD patterns. TG analysis was employed to probe the TiO₂ content in the TiO₂/C composite and the result is given in Fig. 3(b). It declares that TiO₂/C-100 has an obvious mass loss at about 500 °C, owing to the oxidation of the carbon in air. The remained TiO₂ content is 93.73 wt%, indicating a high TiO₂ loading.

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Fig. 3.   Raman spectrum (a), TG curve (b), nitrogen (77 K) adsorption/desorption isotherms (c) and DFT pore-size distribution (d) of TiO₂/C-100.

The nitrogen adsorption/desorption isotherms of TiO₂/C-100 (Fig. 3(c)) present type IV adsorption/desorption isotherms according to the classification of IUPAC [40]. The obvious hysteresis loop indicates the presence of mesopores, while the sharp increase close to p/p₀ = 1 implies the existence of macropores. As confirmed in Fig. 3(d), the pore size of the TiO₂/C-100 distributes in a wide range of 1-80 nm, covering mesopores (2-50 nm) and macropores (>50 nm). The hierarchical porous structure of TiO₂/C-100 facilities the electrolyte infiltration, shortens the diffusion distance and further improves the electrochemical performance of the material. The BET surface area and the pore volume are 162 cm² g^-1 and 0.22 cm³ g^-1, respectively. The enlarged surface area and the developed porous structure ensure the adequate electrolyte infiltration and facilitate the charge transfer.

The electrochemical properties of TiO₂/C-100 are further evaluated for lithium storage. The CV curves of TiO₂/C-100 (Fig. 4(a)) at a scan rate of 0.1 mV s^-1 in 1.0-3.0 V show a pair of weak redox peaks at 1.7 and 1.9 V, related to the phase transition mechanism of TiO_2. During the Li⁺ insertion/extraction, the rutile TiO₂ reversibly transforms into Li_xTiO₂ [9]. This process leads to little volume change, promising good stability of the TiO₂/C-100 electrode. The strong peak below 1.5 V in the first cycle, disappearing in the subsequent cycles, might come from the side reaction on the carbon surface and the irreversible structure transformation of TiO₂ upon deeper Li⁺ insertion [41].

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Fig. 4.   CV curves at a scan rate of 0.05 mV s^-1 (a), voltage profiles of the first three cycles at 0.1 C (b), cycle performance at 0.1 C (c), rate performance (d) and cycle performance at 5 C (e) of TiO₂/C-100 between 1.0 and 3.0 V.

The voltage profiles of TiO₂/C-100 in Fig. 4(b) were conducted at 0.1 C (1 C = 168 mA g^-1) between 1.0 and 3.0 V. A slope voltage plateau can be observed at ∼1.7 V, and the material exhibits a reversible specific capacity of 247 mA h g^-1 with a coulombic efficiency of 61.4%, consisted with the CV curves. After the irreversible reaction in the first cycle, the coulombic efficiency quickly increases to nearly 100% and the shape of the voltage profile does not have obvious change in subsequent cycles.

Fig. 4(c-e) exhibits the cycle performance and the rate property of TiO₂/C-100. The composite shows an excellent cycle stability as the specific capacity remains 200 mA h g^-1 after the 200th cycle with a capacity retention of 81.0% at 0.1 C. Fig. 4(d) indicates that TiO₂/C-100 possesses an excellent rate property with the specific capacities of 220, 188, 155, 130, 113, 89, 65, 48 and 43 mA h g^-1 at current densities of 0.1, 0.2, 0.5, 1, 2, 5, 10, 20 and 30 C. Even at a rate of 50 C, the specific capacity of TiO₂/C still maintains 38 mA h g^-1, again proclaiming the outstanding dynamic property of the sample. When the current density returns to 0.1 C, the specific capacity goes back to 218 mA h g^-1 immediately, indicating the outstanding reversiblity. When cycled at 5 C, TiO₂/C-100 exhibits a specific capacity of ∼110 mA h g^-1 after 2000 cycles with a capacity retention of 72.4%, i.e. only 0.014% decay in each cycle, showing an excellent cycling stability. This attractive electrochemical performance of TiO₂/C-100 under high rate can be ascribed to (1) the significantly enhanced electrical conductivity (2.5 × 10^-2 S cm^-1) by the conductive carbon network and (2) the large electrode-electrolyte interface for the charge transfer in the composite improving the dynamic properties.

To further proving the stability of the composite, the cells after 200 cycles were disassembled. SEM image of the cycled electrode (Fig. S1(b) in the Supporting Information) shows the similar particle size to the fresh electrode (Fig. S1(a) in the Supporting Information) with no obvious fissure, implying the structural stability. Moreover, Ti_2p spectra of the electrodes before and after 200 cycles (Fig. S2 in the Supporting Information) both display characteristic peaks of TiO₂ located at ∼458.5 eV with negligible shift, further confirming the stability. EIS spectra, consisting of a depressed semicircle in high frequency region, a semicircle related with the charge transfer resistance (R_ct) at medium frequency region and a straight line corresponding to Li⁺ diffusion resistance in electrode bulk at lower frequency, are fitted with the equivalent circuit (Fig. S3 in the Supporting Information) [42,43]. The charge transfer resistance of the electrode after 5 cycles (56 Ω) is lower than that before cycle (168 Ω) due to the sufficient infiltration of the electrolyte in the hierarchical pores. After 200 cycles, the transfer resistance only slightly increases to 67 Ω, indicating the superiority of the unique architecture of the composite on kinetics and cycle stability.

As the TiO₂ is generated during the water-bath process, the temperature may influence the morphology and performance of the composites. Therefore, samples prepared at water-bath temperatures of 60 and 80 °C were also synthesized, marked as TiO₂/C-60 and TiO₂/C-80, respectively. Fig. S4 in the Supporting Information shows that the morphology of TiO₂/C-60 is mainly composed of the 2D large-size carbon nanosheets and no urchin-like TiO₂ can be observed. Its XRD pattern shows only peaks of carbon and TG indicates that the TiO₂ content is only 35.44 wt% in TiO₂/C-60 (Fig. S5 in the Supporting Information), much lower than 93.73 wt% of TiO₂/C-100. TiO₂/C-80 has similar morphology and XRD pattern to TiO₂/C-100, but the size of the urchin-like TiO₂ is much smaller with a TiO₂ content of 79.75 wt%. All these characterization results declare that the higher water-bath temperature promotes the fully hydrolyzation of TiCl₄ to generate rutile TiO₂. Electrochemical performances of TiO₂/C-60 and TiO₂/C-80 are also shown in Figs. S6 and S7 in the Supporting Information. Compared to TiO₂/C-60 and TiO₂/C-80, TiO₂/C-100 shows much higher capacity and better rate performance due to its high TiO₂ content and the unique architecture. The urchin-like TiO₂ consisted of 1D nanorods with nanometer-level diameter and the 2D ultrathin carbon nanosheets establish a structure with a high surface area, providing more active positions for lithium storage. The special morphology of the composite guarantees the adequate electrolyte infiltration and shortens the diffusion length of ions. The 2D carbon nanosheets link the urchin-like TiO₂, create a conductive network and increase the overall electrical conductivity of the material, thus resulting in an excellent rate performance.

4. Conclusion

In summary, rutile TiO₂/carbon nanosheet composite is synthesized through a facile way by directly calcinating MAX and Na₂CO₃ together and a follow-up water-bath process in an acid environment. The lamellar carbon atoms in the MAX phase are converted to 2D conductive carbon nanosheets with urchin-like TiO₂ anchored on. This unique architecture improves the dynamic properties of the composite in LIB application. A high reversible specific capacity (247 mA h g^-1), excellent rate performance (38 mA h g^-1 at 50 C) and stable cycling performance (0.014% decay in each cycle during 2000 cycles) can be reached. This method might be extended to other MAX phases and offers a new way to synthesize transition metal oxide/carbon composites.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC, No. 51572011), the National Key Research and Development Program of China (No. 2017YFB0102004) and the Fundamental Research Funds for the Central Universities (No. ZY1802).

The authors have declared that no competing interests exist.