Intensive research effort is currently focused on the development of efficient, reliable, and environmentally safe electrochemical energy storage systems due to the ever-increasing global energy storage demand. Li ion battery systems have been used as the primary energy storage device over the last three decades. However, low abundance and uneven distribution of lithium and cobalt in the earth crust and the associated cost of these materials, have resulted in a concerted effort to develop beyond lithium electrochemical storage systems. In the case of non-Li ion rechargeable systems, the development of electrode materials is a significant challenge, considering the larger ionic size of the metal-ions and slower kinetics. Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides, MXenes and phosphorene, have garnered significant attention recently due to their multi-faceted advantageous properties: large surface areas, high electrical and thermal conductivity, mechanical strength, etc. Consequently, the study of 2D materials as negative electrodes is of notable importance as emerging non-Li battery systems continue to generate increasing attention. Among these interesting materials, graphene has already been extensively studied and reviewed, hence this report focuses on 2D materials beyond graphene for emerging non-Li systems. We provide a comparative analysis of 2D material chemistry, structure, and performance parameters as anode materials in rechargeable batteries and supercapacitors.

Growing concerns regarding the safety, flammability and hazards posed by Li-ion systems have led to research on alternative rechargeable metal-ion electrochemical storage technologies. Among the most notable of these are Na-ion supercapacitors and batteries, motivated, in part, by the similar electrochemistry of Li and Na ions. However, sodium ion batteries (SIBs) come with their own set of issues, especially the large size of the Na ⁺ ion, its relatively sluggish kinetics and low energy densities. This makes the development of novel materials and appropriate electrode architecture of absolute significance. Transition metal dichalcogenides (TMDs) have attracted a lot of attention in this regard due to their relative ease of exfoliation, diverse morphologies and architectures with superior electronic properties. Here, we study the electrochemical performance of Mo-based two-dimensional (2D) layered TMDs (e.g. MoS ₂ , MoSe ₂ and MoTe ₂ ), exfoliated in a superacid, for battery and supercapacitor applications. The exfoliated TMD flakes were interfaced with reduced graphene oxide (rGO) to be used as composite electrodes. Electron microscopy, elemental mapping and Raman spectra were used to analyse the exfoliated material and confirm the formation of 2D TMD/rGO layer morphology. For supercapacitor applications in aqueous electrolyte, the sulfide-based TMD (MoS ₂ ) exhibited the best performance, providing an areal capacitance of 60.25 mF cm ⁻² . For SIB applications, TMD electrodes exhibited significantly higher charge capacities than the neat rGO electrode. The initial desodiation capacities for the composite electrodes are 468.84 mAh g ⁻¹ (1687.82 C g ⁻¹ ), 399.10 mAh g ⁻¹ (1436.76 C g ⁻¹ ) and 387.36 mAh g ⁻¹ (1394.49 C g ⁻¹ ) for MoS ₂ , MoSe ₂ and MoTe ₂ , respectively. Also, the MoS ₂ and MoSe ₂ composite electrodes provided a coulombic efficiency of near 100 % after a few initial cycles.

Two-dimensional (2D) materials have elicited considerable interest in the past decade due to a diverse array of novel properties ranging from high surface to mass ratios, a wide range of band gaps (insulating boron nitride (BN) to semiconducting transition metal dichalcogenides), high mechanical strength and chemical stability. Given the superior chemo-thermo-mechanical properties, 2D materials may provide transformative solution to a familiar yet persistent problem of significant socio-economic burden: the corrosion of stainless steel (SS). With this broader perspective, we investigate corrosion resistance properties of SS-coated with 2D nanomaterials; molybdenum disulfide (MoS₂), BN, bulk graphite in 3.5 wt% aqueous NaCl solution. The nanosheets were prepared by a novel liquid phase exfoliation technique and the coatings were made by a paint brush to achieve uniformity. Open circuit potential (OCP) and potentiodynamic plots indicate the best corrosion resistance is provided by the MoS₂coatings. Superior performance of the coating is attributed to low electronic conductivity, large flake size and uniform coverage of SS substrate, which probably impeded the corrosive ions from the solution from diffusing through the coating.

1. Introduction – Sodium batteries are being studied to overcome the drawbacks of Li ion based batteries viz. cost, poor cycle life of Li etc. [1]. Layered compounds are finding applications as anodes and cathodes as these are able to intercalate the large sized Na+ ion better [2]. Prussian blue and layered transition metal oxides are the compounds of choice to facilitate easy intercalation, increase cell performance and provide higher specific capacities [3, 4]. The TiO2 based anodic system is analyzed from different aspects to obtain the optimum system in this case. 2. Fabrication procedures – Fabrication process for the prussian blue cathode involves dissolution of 3 m mol of Na4Fe(CN)6.10H2O in 40 ml of 0.1 M conc. HCl solution followed by vigorous stirring at 500 rpm for 5 minutes. Once this is done, the resulting solution is dried at 80 °C for 20 hours. For the anode, the crystalline TiO2 anatase and the amorphous TiO2 rutile powder are used as obtained. Na2Ti3O7 is obtained by heating 1g of TiO2 and 0.36 g of NaOH at 750 °C for 20 hours, after they were mechanically milled for 4 hours. 3. Experimental steps and results – The various characterization procedures are as follows: (i) Analysis of the phase of the TiO2 and Na2Ti3O7 anode. Figure 1 shows the XRD data of TiO2 (blue, bottom) and Na2Ti3O7 anodes (orange, top). The main peaks are seen and no impurities peaks are noticed, especially in the Na2Ti3O7 sample which indicate phase formation is complete. (ii) Analysis of the Prussian blue cathode before and after the cell cycling to show the degradation of the cathode. The graph in figure 2 represents the XRD data of the pristine Na-PB cathode (bottom, in blue) and the cathode after 10 cycles of operation (top, in orange). The post cycling XRD graph shows the presence of the peaks in the same position indicating very little structural damage. (iii) Cell cycling is performed in a 2 electrode set-up within in an aqueous electrolyte in a voltage range 0 V to 0.7 V. Figure 3 gives the specific capacity with respect to the voltage for the 1st, 2nd, 5th and 10th cycles respectively. The open circuit voltages can be seen to be approximately around slightly higher than 0.55 V and a maximum specific capacity of 76.52 mAh/g. It is also seen that the graphs are very close to one another which indicate very little capacity fading. (vii) Comparison of the stability behavior of different TiO2 anodes. Figure 4 shows the stability characteristic of the different TiO2 based anode systems. As can be seen, the Na2Ti3O7 system is the most stable whereas the amorphous system demonstrates the least stability. 4. Conclusion & future work – Sodium rich prussian blue system has been tried out as a positive electrode versus a transition metal oxide e.g. TiO2 as the negative electrode. The TiO2 system has been tried from different aspects (crystalline, amorphous, heat treated and heat treated) and the imporrtant structural aspects and cell performances have been discussed. Future work would involve analysis if the corrosion mechanism at the electrode-electrolyte interface and study of the Prussian blue cathode in greater detail. References: [1] M.S. Islam, C.A.J. Fisher, Chemical Society Reviews, 43 (2014) 185-204. [2] D. Kim, E. Lee, M. Slater, W. Lu, S. Rood, C.S. Johnson, Electrochemistry Communications, 18 (2012) 66-69. [3] A.A. Karyakin, Electroanalysis, 13 (2001) 813-819. [4] Y. Yue, A.J. Binder, B. Guo, Z. Zhang, Z.-A. Qiao, C. Tian, S. Dai, Angewandte Communications, 53 (2014) 3134-3137. Figure 1

With the existing drawbacks of the Li ion batteries (LIBs) in terms of expenditure and safety, there is an increasing research focus on Na ion batteries (NIBs) as a viable replacement especially for large scale grid storage applications. Considering the rather bulky Na ion and the challenges it would pose for efficient reversibility, choice of optimum electrode materials is of significance. Among the various types of materials being considered for applications as the negative electrode, 2-dimensional (2D) transition metal dichalcogenides (TMDs) have elicited interest due to their layered structure and existing in a range of compositions. Among the various TMDs that have been studied, MoS₂ has received the most focus due to its excellent cyclability as well as stability, however, others in the TMD family have not received as much focus. In this work, we study the sodium recycling properties of exfoliated TMD (MoSe₂, WSe₂ and MoTe₂)- reduced graphene oxide (rGO) composite. Electrochemical analysis was performed by using the TMDs in coin cells with Na foil being the counter electrode in organic electrolyte within a cycling window of 2.25 V – 10 mV. Cell cycling results have indicated that WSe₂ electrode performed the best and provided a reversible capacity more than 250 mAhg^-1 whereas pristine rGO demonstrated only about 80 mAhg^-1. Also, all the TMD/rGO composite electrodes demonstrated a first cycle efficiency greater than 50 %. Similarly, capacity retention results demonstrate a similar pattern, with WSe₂ exhibiting about 80 % capacity retention followed by MoSe₂ (∼60 %) and MoTe₂ (∼45%). Results in aqueous media exhibit similar trends. In conclusion, it can be said that different 2D layered TMDs have shown promise for application as negative electrodes in NIBs in organic and aqueous environment. This provides hope in their extensive application as stable, high reversible capacity negative electrode materials in NIBs.