Thermally reduced graphene/MXene film for enhanced Li-ion storage Shuaikai Xu,[a] Yohan Dall’Agnese,[a] Junzhi Li,[a] Yury Gogotsi,*[a, b] and Wei Han*[a, c] Abstract: Two-dimensional transition metal carbides called MXenes Two-dimensional (2D) materials such as graphene,[7] are emerging electrode materials for energy storage due to their sulfides,[8,9] nitrides,[10,11] and oxides[12,13] have attracted metallic electrical conductivity and low ion diffusion barrier. In this considerable interest because of their unique and beneficial work, we combined Ti2CTx MXene with graphene oxide (GO) followed physical and chemical properties when used as LIB anode by a thermal treatment to fabricate flexible rGO/Ti2CTr film, where materials. However, limited electronic conductivity is an issue for electrochemically active rGO and Ti CT nanosheets impede the 2 r many 2D materials. The largest family of highly conductive 2D stacking of layers and synergistically interact producing ionically and materials is transition metal carbide/carbonitride labeled MXenes, electronically conducting electrodes. The effect of the thermal which were discovered recently.[14] MXene are synthesized by treatment on the electrochemical performance of Ti2CTx is evaluated. selectively etching the A (Al, and other) layers from Mn+1AXn As anode for Li-ion storage, the thermally treated Ti2CTr possesses a phases (where M is an early transition metal, X is carbon or higher capacity in comparison to as-prepared Ti2CTx. The nitrogen and n=1-3). When the A-layers are etched out, they are freestanding hybrid rGO/Ti2CTr films exhibit excellent reversible capacity (700 mAh g-1 at 0.1 A g-1), cycling stability and rate replaced by a combination of surface terminations such as OH, O and F, therefore the correct chemical designation for MXenes is performance. Additionally, flexible rGO/Ti3C2Tr films are made using the same method and also present improved capacity. Therefore, this Mn+1XnTx (Tx refers to surface functional groups). MXenes are study provides a simple, yet effective, approach to combine rGO with emerging as a promising anode material for Li-ion batteries [15] different MXenes, which can enhance their electrochemical properties due to their metallic conductivity and 2D structure. Although [16] for Li-ion batteries. Ti3C2Tx and Ti2CTx have good electrical conductivity and a low Li+ diffusion barrier,[17] their capacity as anodes is not high enough compared to Sn, Si or other advanced nanomaterials. As- Introduction synthetized Ti3C2Tx and Ti2CTx have capacities around 100 mAh/g at 1C rate,[18-20] which limits their application as electrode Development of high-performance electrochemical energy materials. To improve the capacity of Ti3C2Tx anode, Ti3C2Tx storage devices has attracted increasing attention with the ‘paper’ was fabricated by filtering delaminated few-layer Ti3C2Tx growing use of renewable energy sources,[1-3] and penetration of [20] colloidal solution, or producing hybrid Ti3C2Tx/carbon wearable devices into daily life. The Li-ion battery (LIB) is nanotubes electrodes.[21,22] More recently, Sn(+IV)-complexed regarded as the most suitable candidate to satisfy energy storage ions decorating and pillaring highly conductive Ti3C2Tx electrodes needs due to its advanced development stage.[4-5] Many research were used to produce anodes for advanced LIBs with outstanding efforts aim to develop advanced materials with higher capacities capacities.[17,23] Additionally, it is predicted by DFT calculation that and lifetimes than current graphite or lithium titanate anodes.[6] M2C (M = Sc, Ti, V, and Cr) MXenes have gravimetric capacities over 400 mA h g-1, which is higher than the gravimetric capacity of graphite, and can be doubled by forming Li metal bilayers [a] Dr. S. Xu, Dr. Y. Dall’Agnese, Dr. J. Li, Prof. Y. Gogotsi, Prof. W. Han between MXene layers.[24] Besides, the capacities of MXenes Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education) were predicted to significantly depend on the nature of surface Jilin University, Changchun 130012, (P.R. China) terminal groups (-F, -O or -OH).[24-26] As the thinnest and lightest E-mail: [email protected] [b] Prof. Y. Gogotsi MXene, Ti2CTx, which is the closest to Ti3C2Tx in composition, Department of Materials Science and Engineering, and A. J. Drexel Nanomaterials Institute has a higher theoretical capacity than Ti3C2Tx but lower Drexel University, Philadelphia, Pennsylvania 19104, USA E-mail: [email protected] conductivity. [c] Prof. W. Han International Center of Future Science Directly after synthesis, the electronic contact between the Jilin University, Changchun 130012, (P.R. China) Ti2CTx MXene blocks is poor due to the large size of the particles, resulting in isolated MXene blocks and limiting the electrochemical performance. This problem can be solved by introducing conductive “bridges” to connect Ti2CTx particles. Graphene, another important 2D material, has been widely used graphene oxide (GO) were removed at the same time, resulting as an electrode for energy storage due to its good electronic in improved electrical conductivity and bridging between Ti2CTr 2 - conductivity, large theoretical specific surface area (~2630 m g and rGO nanosheets. In addition, the Ti2CTx particles acted as 1), and superior mechanical properties.[26,27] It has been hybridized conductive spacers and impeded the agglomeration of rGO [28] with Ti3C2Tx MXene for supercapacitor applications. Hence, by nanosheets. The rGO/Ti2CTr films present enhanced combining graphene with Ti2CTx it may be possible to improve the electrochemical performance with high reversible capacity (~700 electrochemical performance of MXene-based anodes for Li-ion mAh g-1 at 0.1 A g-1), high coulombic efficiency, excellent cycling storage.[29,30] stability and rate performance. To demonstrate that this approach In this work, flexible freestanding reduced graphene oxide proposed for rGO/Ti2CTr film anode can be applied to other (rGO)/Ti2CTr films were fabricated by vacuum-assisted filtration MXenes, rGO/Ti3C2Tr films were also fabricated by the same followed by thermal reduction under vacuum. During the thermal method and showed an improved electrochemical performance vacuum annealing, the surface terminal groups of Ti2CTx and for Li-ion storage. Figure 1. a) Schematic illustration of the fabrication process of the flexible rGO/Ti2CTr film and the digital photograph of the flexible rGO/Ti2CTr film. b) Schematic illustration of the surface modification process of Ti2CTx. were fabricated by vacuum-assisted filtration followed by thermal reduction at 573K for 5h under vacuum. During the thermal Results and Discussion treatment process, the interlayer water and -OH terminal groups on the surface of Ti2CTx MXene were concomitantly removed The flexible freestanding rGO/Ti2CTr films were produced (Figure 1b). To achieve the best flexibility and mechanical following the process shown in Figure 1a. The rGO/Ti2CTr films properties of rGO/Ti2CTr films, the weight ratio of rGO:Ti2CTr was optimized and found to be 3:1 (details not shown in this work). open structure than pure rGO (Figure S1c) or GO/Ti2CTx (Figure Therefore, rGO/Ti2CTr films with the weight ratio of 3:1 were 2c) films due to the impeded stacking of rGO nanosheets by the fabricated and characterized hereafter. embedded Ti2CTr particles and the thermal reduction process As shown in Figure 2a, after etching of Al from Ti2AlC for 24h, under vacuum. Figure S1d shows the top view FESEM image of the obtained Ti2CTx has a 2D layered structure, where the Ti2CTx the rGO/Ti2CTr films. Both the wrinkles and folds of the graphene layers were stacked into lamellas, indicating the successful and the embedded Ti2CTr particles were clearly observed. The etching. As shown in Figure S1a, after thermal reduction under results of the corresponding elemental mapping of Ti and C in vacuum, the Ti2CTr maintained 2D layered structure. The few- Figure S1e, f indicated that the Ti2CTr particles were uniformly layer GO nanosheets present flat flake-like morphology (Figure distributed into the hybrid rGO/Ti2CTr films. From Figure 2f, it can S1b), but after thermal reduction under a vacuum, rGO be observed that Ti2CTr particles are surrounded by wrinkled rGO nanosheets present a wrinkled morphology as shown in Figure 2b, nanosheets, improving the conductivity between distant Ti2CTr which is caused by the removal of oxygen-containing functional blocks. The measured BET specific surface areas of the 2 -1 group (-OH) from its surface. The rGO nanosheets possess a GO/Ti2CTx and rGO/Ti2CTr films were 127 and 270 m g , large lateral size and are a few layers thick. As a characteristic respectively (Figure S1g). The increase of the BET-specific feature of rGO nanosheets, the presence of wrinkles and folds surface area is mainly attributed to the pore opening due to the benefit the Li-ion transfer into rGO film. To further enhance the gases released from rGO during reduction under vacuum. The accessibility of the Li-ion to the film electrode and improve its pore width distributions of rGO/Ti2CTr films were evaluated by the electrochemical performance, Ti2CTx powders were introduced as BJH method, and the pore size was determined to be around 3 conductive spacers between the rGO nanoflakes to produce μm (Figure S1h and pores seen in Fig. 2e), which ensured a high flexible rGO/Ti2CTr films, whose typical cross-sectional FESEM ion-accessible surface area and low ion transport resistance. images are shown in Figure 2d, e. rGO/Ti2CTr films possess more Figure 2. FESEM images of a) Ti2CTx and b) rGO nanosheets. c) FESEM cross section image of a hybrid GO/Ti2CTx film. d) Low- and e) high-magnification FESEM cross section images of a hybrid rGO/Ti2CTr film. f) High magnification FESEM image of Ti2CTr particles coated with rGO nanosheets. treatment, indicating that it shifted to a larger d spacing (Δd=0.774 nm). The peak of Ti2AlC at ≈39.2˚ 2θ disappeared, suggesting the complete etching of Al from Ti2AlC.