MXene/graphene oxide heterojunction as a high performance anode material for lithium ion batteries

Abstract

MXene/graphene oxide composites with strong interfacial interactions were constructed by ball milling in vacuum. Graphene oxide (GO) acted as a bridge between Ti₃C₂T_x nanosheets in the composite material, which could buffer the mechanical shear force during the ball milling process, avoid the structural damage of nanosheets and improve the structural stability of the composite material during the lithium process. Partial oxidation of Ti₃C₂T_x nanosheets is caused by high temperatures during ball milling, which is beneficial to improve the intercalation of lithium ions in the material, reduce the stress and electrostatic repulsion between adjacent layers, and cause the composite to have better lithium storage performance. Under the high current density of 2.5 A g⁻¹, the reversible capacity of the Ti₃C₂T_x/GO composite material after 2000 cycles was 116.5 mA h g⁻¹, and the capacity retention was as high as 116.6%.

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1. Introduction

With the development of portable electronic devices and electric vehicles, lithium-ion batteries became a research hotspot because of their high energy density, lack of memory effect and wide operating voltage window. Ti₃C₂T_x has the advantages of adjustable layer spacing, abundant surface functional groups and multiple active sites, which had unique advantages in the field of lithium-ion battery electrode materials. However, the layer spacing decreased due to the van der Waals attraction between the layers, which seriously hindered the penetration of electrolyte, reducing the rapid migration of lithium ions, and affecting the electrochemical performance.^1,2

In order to further improve the lithium storage performance of Ti₃C₂T_x, one of the most promising strategies was to combine Ti₃C₂T_x nanosheets with other nanomaterials to form heterogeneous structures.^3,4 At present, many materials such as transition metal oxides (TMOs),^5–7 transition metal sulphides (TMDCs),^8,9 phosphorus,^10,11 silicon,^12,13 polymers,^14,15 and carbon materials^16–19 are loaded between Ti₃C₂T_x nanosheets, which could increase the ion transport rate and buffer the volume expansion of the nanoparticles, and ensure the fast charge transport in the heterojunction materials. The existence of a Ti₃C₂T_x nano-sheet layer could avoid the agglomeration of nano-particles, which is beneficial to the effective utilization of surface active sites, thus improving the electrochemical properties of heterojunction composites.

In this paper, the Ti₃C₂T_x/GO heterojunction composites were prepared by high-speed ball milling in vacuum. Graphene oxide acted as a spacer between Ti₃C₂T_x nano-layers in order to enlarge the space between them, which increased the charge storage capacity of the composite and promoted the diffusion of electrolyte in the electrode reaction. Moreover, graphene oxide exhibited high conductivity, which contributed to interlayer charge transfer and improved the multiples properties of the composites.

2. Experimental

2.1 Preparation of Ti₃C₂T_x and GO

Graphene oxide (GO) was prepared by the oxidation reaction of graphite powder (99.9% purity, Alfa Aesar, USA) using a modified Hummers' method [18]. H₂SO₄ (12 mL), K₂S₂O₈ (2.5 g) and P₂O₅ (2.5 g) were mixed and heated in a flask, and graphite powder (3 g, 325 mesh) was added at 80 °C and kept at this temperature for 4.5 h. After heating, dilute it with 500 mL deionized water and cool it to room temperature naturally. Let it stand for 10 h before removing it. The seed water is drained and filtered to remove residual acid and dried. Subsequently, the product was placed in concentrated sulfuric acid (120 mL) under ice bath conditions and KMnO₄ (15 g) was slowly added. After stirring for 2 h, dilute with 250 mL deionized water and continue stirring for 2 h. After that, 700 mL of deionized water was added to dilute the solution, and 20 mL of 30%H₂O₂ was used to neutralize the solution until the color of the mixture turned bright yellow with bubbling. The mixture is then filtered and washed with 10% HCl solution to remove metal ions, and then washed repeatedly with deionized water until the pH value of the filtrate is ∼6. Finally, the precipitate is dried in a freezer to obtain graphene oxide (GO). It was then ball milled at 500 rpm for 12 h to be used as backup for reserve.

The commercial Ti₃AlC₂ powder was purchased from 11 Technology Co., Ltd (Jilin, China). The layered Ti₃AlC₂ was chemically etched to remove the Al layer by wet etching as follows: after 1 g LiF and 20 mL 9 M HCl solution were magnetically stirred in a Teflon beaker for 30 min, 1 g Ti₃AlC₂ powder was slowly added into the above solution, and the etching reaction was carried out for 36 h at 35 °C under magnetic stirring. Subsequently, the precipitation was obtained by centrifugation at 6000 rpm and washed with deionized water several times until the pH value reached ∼6. Then, the sediment was placed in 200 mL 0.25 M LiOH solution, ultrasonic treatment and stirred for 24 h. The alkalized mixture is repeatedly filtered and washed with deionized water until the pH of the solution reaches ∼8. Finally, the precipitate was dried in a freeze-dryer to obtain multilayer Ti₃C₂T_x MXene (multi-Ti3C2T_x). Subsequently, Ti₃C₂T_x was obtained by ball milling at 500 rpm for 12 h in a vacuum environment for backup.

2.2 Preparation of Ti₃C₂T_x/GO composite

The multi-Ti₃C₂T_x and GO nanosheets were taken in a mass ratio of 9 : 1 and placed in a vacuum environment with mixed ball milling at a speed of 500 rpm for 12 h. After that, by calcination at 300 °C for 3 h under inert atmosphere with a heating rate of 2 °C min⁻¹. The as-prepared product was designated as Ti₃C₂T_x/GO. Control the GO content but different composite materials was prepared.

2.3 Electrochemical measurements

As for the lithium-ion battery tests, all of the CR2032-type coin cells were assembled in an Ar-filled glove box. The working electrode was prepared by mixing Ti₃C₂T_x or S–Ti₃C₂T_x, acetylene black, and polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidinone (NMP) with a weight ratio of 6 : 2 : 2. Lithium foil was used as counter electrode, Celgard polypropylene as separators and 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) a volumetric ratio of 1 : 1 as the electrolyte. All electrochemical performance tests were conducted under 25 °C. Cyclic voltammetry experiments were performed by LAND CT2001A battery testing system at a scan rate of 0.1 mV s⁻¹ between 0.05 and 3 V. Electrochemical impedance spectroscopy (EIS) measurements were performed using an AC amplitude of 10 mV and a frequency range of 200 kHz to 0.01 Hz.

2.4 Characterization

The morphology and structural information were conducted by SEM (SU-8020, Hitachi) and TEM (JEOL JEM-2100F). The XRD patterns were collected on a Bruker D8 diffractometer (Cu Kα radiation, λ = 1.5406 Å). The element surface analysis was carried out by XPS (Axis Ultra DLD, Kratos Analytical) with Al Kα radiation. The specific surface area of the sample was determined by N₂ absorption at 77 K with a BeiShiDe 3H-2000PM1 instrument. The Raman spectra were collected on a Raman spectrometer (Labram-010) using 532 nm laser.

3. Results and discussion

As shown in Fig. 1a, Ti₃C₂T_x presented a typical layered structure after chemical etching, but there was still a serious self-stacking phenomenon. GO had very large horizontal and vertical dimensions, with internal three-dimensional holes connected to each other (Fig. 1b).²⁰ When the two mixtures were milled and treated at high temperature, the size of the particles was significantly reduced, but the flake graphene was like a ribbon floating between the Ti₃C₂T_x particles (Fig. 1c and d).

Fig. 1 SEM images of (a) Ti₃C₂T_x; (b) GO; (c and d) Ti₃C₂T_x/GO composites.

In order to deeply analyze the morphological structure of Ti₃C₂T_x/GO composites, transmission electron microscopy (TEM) was used for further characterization. According to Fig. 2a and b, it can be clearly seen that GO is coated in the outermost layer of the material, where the lattice spacing of 0.22 nm and 0.35 nm corresponds to Ti₃C₂T_x (103) and TiO₂ (110) crystal planes, respectively. In addition, the interface structure of TiO₂/Ti₃C₂T_x appeared in Fig. 2c and d,^21–23 and there is an obvious lattice fringe interruption phenomenon, which indicated that Ti₃C₂T_x is partially oxidized due to mechanical ball milling and large transverse shear force. The XRD results (Fig. 3a) further confirm the presence of a small amount of TiO₂,^22,24 which was consistent with TEM results. Raman spectra (Fig. 3b) further confirmed the successful recombination of Ti₃C₂T_x and GO. The vibrational peak at 141.9 cm⁻¹ in the Ti₃C₂T_x/GO composite represented the A_1g symmetric out-of-plane vibrations of the titanium atoms in the material, while the vibrational peaks at 417.2 and 692.6 cm⁻¹ indicated the in-plane shear vibrations (E_g) of Ti, C and surface oxygen-containing functional groups in the Ti₃C₂T_x material, respectively, where the vibrational peak at 292.3 cm⁻¹ peak then verified the production of TiO₂ in the material.^25,26 Meanwhile, the typical peaks of the carbon material were located at 1346.5 and 1599.9 cm⁻¹, corresponding to the D and G peaks of graphene oxide, respectively, indicating the generation of amorphous carbon during the reaction.^25,27 After graphene oxide composite Ti₃C₂T_x, the intensity ratio (I_D/I_G) of the two peaks increased from 0.92 to 1.08, indicating that most of the oxygen functional groups on its surface were removed and TiO₂ was directly embedded into the graphene surface, increasing the large number of defects generated and exposing more active sites on the composite surface, which is conducive to increasing the contact area of the electrolyte and promoting the ion and charge diffusion.²⁸

Fig. 2 (a and b) TEM images of Ti₃C₂T_x/GO and (c and d) TEM images of TiO₂/Ti₃C₂T_x.

Fig. 3 (a) XRD pattern of GO, Ti₃C₂T_x and Ti₃C₂T_x/GO; (b) Raman spectrum.

In order to analyze the chemical composition and surface electronic states of Ti₃C₂T_x/GO composites, XPS spectroscopy was performed, as shown in Fig. 4. In the XPS full spectrum, the addition of GO material resulted in a significant increase in the intensity of the C 1s characteristic peak of the composites, and the increase in the intensity of the O 1s characteristic peak was attributed to the oxidation of Ti₃C₂T_x nanosheets by high temperature during the ball milling composite. In the XPS high-resolution spectra of Ti 2p orbitals (Fig. 5a and b), there are four distinct characteristic peaks in Ti₃C₂T_x. The characteristic peaks at 455.1 and 460.6 eV correspond to Ti(II) (2p_3/2 and 2p_1/2), 456.6 eV corresponds to Ti(III) (2p_3/2), while the Ti–C peak at 461.3 eV corresponds to the Ti 2p_1/2 orbital, and the characteristic peaks at 458.5 and 464.3 eV correspond to Ti(2p_3/2 and 2p_1/2).^29,30 After the composite graphene oxide, the characteristic peaks on the Ti 2p orbitals show a significant change and the Ti–C intensity in the structure is weak, which is still mainly due to the generation of TiO₂ in the material and is wrapped by the graphene oxide on its surface along with Ti₃C₂T_x. As shown in Fig. 5c and d), the main characteristic peaks present in the C 1s are C–C (284.8 eV), C–O (286.2 eV), C–Ti–O (282.2 eV) and O–C=O (288.8 eV), and the intensity of the C–C peak in the composite was significantly higher than the other characteristic peaks, which then indicated the presence of a large number of C–C bonds in it, which could be attributed to the graphene oxide. Fig. 5e and f) showed the O 1s high-resolution spectra of Ti₃C₂T_x, Ti₃C₂T_x/GO composites with characteristic peaks corresponding to Ti–O (530.1 eV), C–Ti-(OH)_x (531.2 eV) and O=C–OH (533.2 eV), respectively, where O was mainly present in Ti₃C₂T_x materials in the form of Ti–O bonds, followed in the form of oxygen-containing functional groups on the surface of Ti₃C₂T_x, while in the composites, the residual oxygen functional groups of GO caused a slight increase in the peak intensity.

Fig. 4 XPS spectra of Ti₃C₂T_x and Ti₃C₂T_x/GO composites.

Fig. 5 High resolution XPS spectra of Ti₃C₂T_x/GO and Ti₃C₂T_x: (a and b) Ti 2p; (c and d) C 1s; (e and f) O 1s.

Ti₃C₂T_x/GO composite was used as working electrode and lithium sheet as paired electrode to investigate its electrochemical properties in lithium-ion battery. In the voltage range of 0.05–3.0 V, the CV curve of the Ti₃C₂T_x/GO composite at sweep velocity of 0.1 mV s⁻¹ is shown in Fig. 6a). At 0.99 V, the irreversible peak appeared on the first discharge curve, which was mainly due to the formation of SEI film on the electrode surface and the irreversible reaction between lithium ion and the functional groups on the material surface. In the subsequent cycle, redox peaks at 0.68 V and 1.7 V correspond to lithium ion intercalation/desorption by the following mechanisms:^26,31

Ti₃C₂T_x + yLi + ye⁻ ↔ Ti₃C₂T_xLi_y (1-1)

Fig. 6 (a and b) CV curve and charge–discharge curve of Ti₃C₂T_x/GO composites; (c and d) Ti₃C₂T_x and Ti₃C₂T_x/GO composites multiplier properties and cycle properties.

The good overlap of the CV curves in the 3rd and 4th circles then indicated that the Ti₃C₂T_x/GO composite has excellent cycling stability in the electrochemical lithium storage process. Fig. 6b shows the charge/discharge curves of the Ti₃C₂T_x/GO composite electrode at a current density of 0.1 A g⁻¹, which could correspond well with the CV curves with initial charge/discharge specific capacities of 506.1 and 814.6 mA h g⁻¹, respectively, and a first Coulomb efficiency of 62.13%, due to the presence of abundant functional groups on the surfaces of GO and Ti₃C₂T_x nanosheets that the hybrid ball milling of GO and Ti₃C₂T_x nanosheets exposed a large number of active sites at the same time as reducing their corresponding sizes, increasing their lithium storage capacity. The subsequent overlap of charge/discharge curves also confirmed the more excellent cycling stability of Ti₃C₂T_x/GO composites.³²

After ball milling, the reduction of material particle size shortened the diffusion path of ions and charges and promoted the penetration of electrolyte, which made Ti₃C₂T_x/GO exhibit a more excellent multiplicative performance than Ti₃C₂T_x, as shown in Fig. 6c. The reversible specific capacities of Ti₃C₂T_x/GO reached 275.6, 210.1, 172.8, 147.2, and 130.8 mA h g⁻¹ at current densities of 0.5, 1.0, 1.5, 2.0, and 2.5 A g⁻¹, respectively, and the specific capacities at each current density were much larger than those of the Ti₃C₂T_x material after ball milling alone. When the current density was reduced to 0.5 A g⁻¹, Ti₃C₂T_x/GO could recover to a reversible specific capacity of 263.6 mA h g⁻¹, showing excellent reversibility. Subsequently, Ti₃C₂T_x/GO as well as Ti₃C₂T_x materials were tested for 1200 cycles of performance, as shown in Fig. 6d. At a current density of 0.5 A g⁻¹, the specific capacity of Ti₃C₂T_x/GO was much higher than that of Ti₃C₂T_x. During the first 500 cycles, the electrolyte continuously penetrated between the active particles and the lithium ions were de-embedded several times, which enabled the exposure of more surface active sites, resulting in a gradual increase in the lithium storage capacity of both. However, the capacity of Ti₃C₂T_x materials decayed afterwards, mainly due to the small size of the particles after ball milling, which prompted the agglomeration of small particles under the continuous embedding of lithium ions and impeded the wetting of the electrolyte, and the irreversible SEI film on the surface of the particles, which seriously affected the rapid ion and charge transport, and the internal lithium storage sites were not fully utilized, resulting in the decay of their capacity.²⁸ However, graphene oxide in Ti₃C₂T_x/GO composites, due to its unique ribbon structure, played a bridging role in it, prompting the activation process of the material to continuously expose the active sites and utilize them effectively, showing extremely excellent cycling stability. Then, the Ti₃C₂T_x/GO composite was further tested for long cycles at a high current density of 2.5 A g⁻¹, as shown in Fig. 7. The specific capacity of Ti₃C₂T_x/GO composite after 2000 cycles was 116.5 mA h g⁻¹, with a capacity retention rate of 116.6%, which fully indicated that the Ti₃C₂T_x/GO composite retained good structural stability and had excellent cycling performance after graphene oxide composite. It has excellent cycling performance.

Fig. 7 Long cycle performance of Ti₃C₂T_x/GO composites at 2.5 A g⁻¹ current density.

In order to further investigate the electrochemical behavior of Ti₃C₂T_x/GO composites, the CV curves of different sweep rates were analyzed accordingly (Fig. 8a). Based on the formula i = av^b, where a is a constant and the value of b can be determined by the slope of the log(v) − log(i) curve. From the fitting curve of Fig. 8b, the b values of reduction peak and oxidation peak are 0.752 and 0.822, respectively. In contrast, diffusion-controlled embedding mechanism and surface pseudocapacitance mechanism coexist in Ti₃C₂T_x/GO electrodes, but the surface-induced pseudo-capacitance process still dominated. In addition, the capacitance contribution ratio of the two reaction mechanisms could be calculated by the following formula:^14,33 i(v) = k₁ν + k₂ν^1/2, where i(v), k₁v, k₂v^1/2 and v are the current, capacitive current, diffusion control current and scanning rate at a given voltage, respectively. According to the CV curve, when the scanning rate is 0.8 mV s⁻¹, the pseudo-capacitance contribution of Ti₃C₂T_x/GO electrode is 52.16%. With the increase in scanning rate, the corresponding pseudo-capacitance contribution also increased (Fig. 8c and d). This showed that in the process of charge and discharge, the mechanical ball milling led to a sharp decrease in the particle size of the material, shortening the ion and charge diffusion path, coupled with the good electrical conductivity of graphene oxide and Ti₃C₂T_x nanosheets, which made the charge transfer process in the surface of Ti₃C₂T_x/GO composites in a dominant position.³⁴

Fig. 8 (a) CV results of Ti₃C₂T_x/GO composites at various scan rates from 0.1 to 3 mV s⁻¹. (b) Relationship between the log(peak current i) and log(scan rate ν). (c) CV curve with corresponding capacitive contribution of the Ti₃C₂T_x/GO composites at a scan rate of 0.8 mV s⁻¹. (d) Contribution ratio of the capacitive and diffusion-controlled charge storage at various scan rates.

4. Conclusion

Ti₃C₂T_x nanosheets and GO were ball milled in vacuum by high energy ball mill self-assembly method. Ti₃C₂T_x/GO composites with strong interfacial interaction could be constructed by forming chemical bonds between the abundant functional groups on the surface of GO and Ti₃C₂T_x, which could effectively promote the transport of ions and electrons at the heterogeneous interface and improve its electrochemical behavior. The specific capacity of Ti₃C₂T_x/GO composite electrode could reach 506.1 mA h g⁻¹ at the current density of 0.1 A g⁻¹, and the reversible capacity of Ti₃C₂T_x/GO composite electrode after 2000 cycles is 116.5 mA h g⁻¹ at the high current density of 2.5 A g⁻¹. The capacity retention rate is as high as 116.6%.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research was financially supported by National Natural Science Foundation of China (21865012, 52202064), Education Department Project Fund of Jiangxi Province (GJJ190427), Ganzhou innovative talent project, the Program for Excellent Young Talents, JXUST, Science and Technology Department of Henan Province (222102230054), New Energy Electric Vehicles High-Voltage Components Inspection and Testing Public Service Platform, Henan Province Education Department of Key Scientific Research Project in Colleges and Universities (21B430012, 23B430012).

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