Study on MnO2/MXene–Ti3C2 composite materials as cathode materials for magnesium batteries

In this paper, MnO2/MXene–Ti3C2 composites with different molar ratios were successfully prepared by a one-step hydro-thermal method, and the optimum proportion was confirmed by XRD and SEM comparative analysis. The optimum proportion of MnO2/MXene–Ti3C2 composites and MnO2 was used as a cathode material for magnesium batteries to carry out the electrochemical performance test. The results showed that the charge–discharge capacity of the MnO2/MXene–Ti3C2 composite was up to 105 mA h g−1, much higher than that of MnO2 (64 mA h g−1), and meanwhile it had good rate performance. At the same time, this also opened up the application of MXene–Ti3C2, a new two-dimensional material, in the field of battery electrode materials.


Introduction
With the rapid development of social economy, increasing consumption of fossil energy and escalating environmental pollution, it is extremely urgent to nd clean, environmentally friendly, efficient and sustainable energy sources. 1-3 Therefore, some new environmentally friendly secondary batteries with high-capacity have been favored by domestic and overseas researchers. Among them, lithium-ion batteries are widely used because of their high energy density and mature technology, but they have the problems of high cost and poor safety. 4,5 Magnesium and lithium are located in a diagonal position in the periodic table, and have similar ionic radii and chemical properties. Compared with lithium metal (180.5 C), the melting point of magnesium (648.8 C) is much higher, and its activity is poorer. So, it is easy to process and also safer. Moreover, magnesium ions can carry two charges. Although the mass specic capacity (2205 mA h g À1 ) is not as high as lithium metal (3862 mA h g À1 ), it is still considerable. [6][7][8][9] At the same time, China is rich in magnesium resources, ranking rst in the world, and the price is low and it's easy to obtain. [10][11][12] So, magnesium ion batteries have attracted much attention from researchers around the world.
Based on advanced theoretical algorithms, modeling, simulation, and computer technology, the reasonable design of materials, batteries, devices and batteries in the eld of lithium ion batteries is gradually being realized. 13,14 Some researchers have also used the rst principle to do research about the stability on electrode materials for magnesium batteries. 15 The calculation and experiment of large shared database connection can greatly help to narrow some of the current experimental and technical gaps, as well as predict the path-independent characteristics, and help to fundamentally understand the pathindependent performance in multiple space-time scales.
Magnesium ion battery is mainly composed of three parts: compound cathode which can reversibly imbed-detach magnesium ion, metal magnesium and its alloy's negative electrode, and electrolyte. [16][17][18] When discharging, the metal magnesium of the negative electrode dissolves out and forms the magnesium ion. It reaches the positive electrode through a series of processes such as dissolution, adsorption and transfer of electrolyte, and then it is embedded in the cathode materials. When charging, the magnesium ions embedded in the cathode material are detached and reach the negative electrode under the action of external electric eld, and then they are deposited again to form the metal magnesium. [19][20][21][22] At present, the studies on magnesium ion batteries at home and abroad mainly focus on cathode materials and electrolytes, which are all still in the preliminary stage. Cathode material is the key factor which restricts the development of magnesium ion battery, and some domestic and overseas scholars are trying to nd and develop new cathode materials. 23-25 MnO 2 is a kind of electrode material with high theoretical specic capacity, low cost and easy availability, which has been widely used as battery electrode material, but its poor conductivity and stability limit the application of MnO 2 as electrode material. [26][27][28] Therefore, many researchers have improved the electrochemical performance of MnO 2 by improving MnO 2 's nanostructure or combining MnO 2 with some conductive materials. [29][30][31][32] MXene is a new type of transition metal carbide, carbonitride and nitride (which are collectively called MXene) with twodimensional graphene-like structure. The general formula of MXene is M n+1 X n T x (n ¼ 1, 2, 3), of which is an early transition metal, X is carbon or nitrogen, and T x represents O 2À , OH À , F À and other surfaces functional group, MXene has been widely used in various elds. [33][34][35] MXene is mainly obtained by selectively etching a xed atomic layer in the MAX phase of the precursor. MAX phase is the general term of a class of ternary layered compounds, which have a uniform chemical formula M n+1 AX n (A is III, IV main group elements). Common chemical etchants are hydrouoric acid or a mixture of hydrochloric acid and lithium uoride. [36][37][38] MXene is a new type of twodimensional layered material, which has the following advantages as electrode material: (1) good electrical conductivity, which is conducive to the electron transmission; (2) large specic surface area, which can provide more storage sites; (3) layered structure, which is conducive to the rapid diffusion of electrolyte ions between layers, providing excellent rate performance; (4) high density of magnesium metal, which makes the volume specic capacity high; (5) adjustable surface chemical structure, which means different MXene can provide different working potential windows. 39-43 So far Ti 3 C 2 T x is the most studied MXene. It has been successfully used as electrode material for supercapacitors, sodium-ion batteries and lithiumion batteries. [44][45][46] Among them, Ti 3 C 2 T x as electrode material for lithium ion battery is the most widely studied. Some researchers have also prepared some Ti 3 C 2 T x matrix composites for lithium batteries and shows excellent capacity and rate performance. 47,48 Moreover, MXene has excellent optical, mechanical and thermal stability properties, and has a wide range of applications in the elds of electromagnetic shielding, water treatment, gas, biosensor and photo electrochemistry catalyze. 49,50 According to the advantages of MXene-a new type of material, we deposited MnO 2 nanoparticles on the surface of MXene-Ti 3 C 2 material with high specic surface areas and high electrical conductivity by hydro-thermal method in this article, and its electrochemical performance was tested as the cathode for magnesium batteries.

Preparation of manganese dioxide
Firstly, 145.29 mg of potassium permanganate was weighed and placed into the beaker; added 50 mL distilled water, stirred it for 30 minutes to fully dissolve. Then the solution was transferred to the hydro-thermal reactor and reacted at 150 C for 20 h. Aer the reaction is over, take out the hydro-thermal reactor and cool it to room temperature. Aer cooling, the samples were centrifugally washed with alcohol and water alternately for 6-8 times. Aer that the samples were transferred to the beaker and dried in vacuum for 24 h at 60 C, then it became the manganese dioxide.

Preparation of MXene-Ti 3 C 2
Firstly, 2 g of Ti 3 AlC 2 2 g and lithium uoride were respectively accurately weighed; 40 mL of hydrochloric acid (HCl, 9 M) was weighed using a measuring cylinder and was poured slowly into a beaker. Added 2 g of lithium uoride and stirred it for 60 minutes; added 2 g of Ti 3 AlC 2 slowly into the mixed solution; then magnetically stirred it at room temperature (25 C) for 24 h at a speed of 200 rpm; repeated washing it ultrasonically centrifugally with deoxidized water until the pH value reaches 6; dried the sediment in vacuum at 120 C for standby use, and this is the MXene-Ti 3 C 2 .

Preparation of MnO 2 /MXene-Ti 3 C 2 composites
Firstly, 40 mg of Ti 3 C 2 T x was weighed and placed in the beaker; added 50 mL distilled water, ultrasonically stirred it for 30 minutes; 145.29 mg of potassium permanganate with the mole ratio of 1 : 1 was weighed and fully mixed for 30 min. Then poured the stirred solution into the hydrothermal reactor, and set the temperature at 150 C and the time at 20 h. Aer the reaction is over, the reaction still was taken out and cooled to room temperature. Aer cooling, the resulting solution was centrifuged to get the underlayer sediment; the sediment was washed with absolute ethyl alcohol and distilled water alternately for 6-8 times. The washed sample was put into the beaker and dried in a vacuum drying box at 60 C for 24 h to obtain MnO 2 /MXene-Ti 3 C 2 composites.
When preparing MnO 2 /MXene-Ti 3 C 2 composites with molar ratios of 1 : 2, 1 : 1 and 2 : 1, the mass of KMnO 4 was maintained the same, and different mass of MXene-Ti 3 C 2 mixtures were weighed respectively for reaction, which was consistent with the above-mentioned process. planes of (002), (004), (006), (0010), (0012) and (110) respectively, these are the characteristic diffraction peaks of MXene-Ti 3 C 2 , which are consistent with other reports. 54,55 In the XRD spectrums of MnO 2 /MXene-Ti 3 C 2 composites with molar ratio of 1 : 2, the characteristic diffraction peaks of MnO 2 and MXene-Ti 3 C 2 appeared, which indicated that MnO 2 was successfully loaded on the MXene-Ti 3 C 2 , MnO 2 /MXene-Ti 3 C 2 composites are successfully prepared. But there is a main peak at 26.5 which illustrates that MXene-Ti 3 C 2 is easy to be oxidized to form TiO 2 in the hydro-thermal reaction. Moreover, the characteristic diffraction peaks of MXene-Ti 3 C 2 were obviously weakened aer adding MnO 2 . In the XRD spectrums of MnO 2 /MXene-Ti 3 C 2 composites with molar ratio of 1 : 1 and 2 : 1, the characteristic diffraction peaks of MnO 2 appeared, but the characteristic diffraction peaks of MXene-Ti 3 C 2 gradually weakened and disappeared, showing that MnO 2 nanoparticles can inhibit the recrystallization of two-dimensional MXene-Ti 3 C 2 lamellae in MnO 2 /MXene-Ti 3 C 2 composites, and the larger the proportion, the more obvious the effect. Fig. 2 is the scanning electron microscope images of the samples at different multiples. It can be seen from the Fig. 2a that MXene-Ti 3 C 2 has a layered structure similar to the organ, and the nanosheet of MXene-Ti 3 C 2 are tightly bonded together, so the large specic surface area of MXene-Ti 3 C 2 can not be fully utilized. Fig. 2b is a typical SEM image of layered MnO 2 . It can be seen that MnO 2 is formed by overlapping and agglomeration of nanosheet layers into three-dimensional ower bract shape. The interspace of nanosheet layers is large which forms abundant pore structures. [56][57][58][59] Fig. 2c is a SEM image of MnO 2 / MXene-Ti 3 C 2 nanocomposites with molar ratio of 1 : 2, from which we can see that MnO 2 is evenly distributed between the nanosheet layers and the interspace of MXene-Ti 3 C 2 . At the same time, there are some irregular nanoparticles on the surface and between layers of MXene-Ti 3 C 2 , which consistent with the XRD analysis above. So MXene-Ti 3 C 2 is easy to be oxidized to form TiO 2 in the hydro-thermal reaction. Because MXene-Ti 3 C 2 is obtained by removing A by etching MAX through HF, some surface functional groups will be produced during etching and some defects may occur at the edge of MXene-Ti 3 C 2 lamellae. These defects and active sites provided by surface functional groups create conditions for the uniform distribution of MnO 2 . Fig. 2d1 and d2 is a SEM image of MnO 2 / MXene-Ti 3 C 2 nanocomposites with molar ratio of 1 : 1 at different multiples. From Fig. 2d1, we can see that MnO 2 nanospheres and some agglomerated TiO 2 particles are closely accumulated on MXene-Ti 3 C 2 lamellae, wrapping MXene-Ti 3 C 2 sheet lamellae, and the structure of MXene-Ti 3 C 2 can hardly be seen. As can be seen in Fig. 2d2, MXene-Ti 3 C 2 lamellae surfaces are densely packed with spherical MnO 2 , which seriously hinders the effect of MXene-Ti 3 C 2 . Fig. 2e is a SEM image of MnO 2 /MXene-Ti 3 C 2 nanocomposites with molar ratio of 2 : 1. In the picture we cannot see any MXene-Ti 3 C 2 substance. There were all nano MnO 2 with ower sphere shape. The ower spheres are tightly packed together, forming a honeycomb three-dimensional structure. In conclusion, when the molar ratio of MnO 2 to MXene-Ti 3 C 2 is 1 : 2, they are evenly combined. With the increase of molar ratio, the MXene-Ti 3 C 2 substance decreases and disappears gradually. The increase of MnO 2 inhibits the recrystallization of MXene-Ti 3 C 2 .

Electrochemical performance analysis
As shown in Fig. 3a, the capacity of MXene-Ti 3 C 2 as cathode material for magnesium batteries is 18 mA h g À1 at 50 mA g À1 current density, which is low. This is consistent with other literature reports. The rst discharge capacity of MnO 2 at 50 mA g À1 current density is only 64 mA h g À1 , which is reduced to 45 mA h g À1 aer 100 cycles. When 1 : 2 MnO 2 /MXene-Ti 3 C 2 composite is used as cathode material of magnesium battery, the rst discharge capacity is 105 mA h g À1 , which is much higher than that of other manganese dioxide matrix composites. And aer 100 cycles, the discharge capacity is 58 mA h g À1 , the cycling performance is better than that of Mn 2 . Aer adding the MXene-Ti 3 C 2 , the capacity of the composite material is greatly improved, which is due to the addition of MXene-Ti 3 C 2 which has large specic surface area and high electrical conductivity, providing more storage sites for the entry and exit of magnesium ions during the working of magnesium batteries, and shortening the transport path of magnesium ions. Fig. 3b shows the voltage-capacity curves of the two samples in the range of 0.01-2.0 V, and the average voltage is 1.0 V. The discharge curves are similar to those of capacitive storage electrode materials. As shown in Fig. 3b, we can see the MoO 2 / MXene-Ti 3 C 2 displays that most capacity does not come from characteristic behaviors of MoO 2 from the slope between 0.2-1.0 V for the composite. Because we added some carbon black as conductive agent in the process of assembling the electrode sheet, it is also benecial to improve the capacity. Fig. 4 shows the CV curves of the MnO 2 and MnO 2 /MXene-Ti 3 C 2 composites in the voltage range of 0.01-2.0 V. Compared with MnO 2 , MnO 2 /MXene-Ti 3 C 2 composite material has obvious anode and cathode current response, large CV area and obvious capacity. The CV curve shows a sharp peak when it's  This journal is © The Royal Society of Chemistry 2019 RSC Adv., 2019, 9, 33572-33577 | 33575 Paper lower than 0.2 V, which may be due to Mg 2+ embedded at the active site of the electrode with high activation energy. This is consistent with the relatively at discharge curve in the low voltage region of 0.01-0.2 V shown in Fig. 3b. However, aer the battery is stabilized, the curve of charge and discharge voltage between 0.01-2.0 V no longer shows a clear charge and discharge platform, which is similar to the charge and discharge curve of capacitor storage, indicating that the main energy storage mechanism of MnO 2 /MXene-Ti 3 C 2 sample is embedded pseudocapacitance, and the coulombic efficiency of the electrode material is close to 100%. MnO 2 /MXene-Ti 3 C 2 sample has good cycle performance.
MnO 2 /MXene-Ti 3 C 2 composite electrodes also show excellent rate performance. As shown in Fig. 5, at high current densities of 100 mA g À1 , the capacities are 54 mA h g À1 , which are much higher than those of MnO 2 . In the rst ten circles, there is some attenuation of capacity, which is due to the strong electrostatic interaction between Mg 2+ and electrode materials, and Mg 2+ is easy to be captured by some active sites. At a high current density of 100 mA g À1 , the battery still has a stable capacity of 54 mA h g À1 and still has a high capacity at a wide range of current densities, indicating that MnO 2 /MXene-Ti 3 C 2 is a very nice high rate cathode material for magnesium batteries.

Conclusion
MnO 2 and MnO 2 /MXene-Ti 3 C 2 composites with different molar ratios are successfully prepared by one-step hydrothermal method in this paper. When the molar ratio was 1 : 2, MnO 2 evenly adhered on and between MXene-Ti 3 C 2 lamella, and the composite effect was the best, which was conducive to the imbedding and separation of magnesium ions. When 1 : 2 MnO 2 /MXene-Ti 3 C 2 composite is used as cathode material of magnesium battery, the main energy storage mechanism is embedded pseudo capacitance. The rst discharge capacity at 50 mA g À1 current density is 105 mA h g À1 , which is much higher than that of MnO 2 . Even at a high current density of 500 mA g À1 , the MnO 2 /MXene-Ti 3 C 2 composite battery still has a stable capacity of 21 mA h g À1 and good rate performance. At the same time, MXene-Ti 3 C 2 substrate material with high conductivity is very valuable for the study of new electrode materials.

Funding
This work was supported by the Magnesium Industry Collaborative Innovation Center of University of Science and Technology Liaoning (USTLXT201801), Liaoning Natural fund project (2019-ZD-0027) and National Science and Technology Support Plan (2014BAB02B01).

Conflicts of interest
There are no conicts to declare.