& Na-Ion Batteries High-Performance Sodium-Ion Batteries and Sodium-Ion Pseudocapacitors Based on MoS 2 /Graphene Composites Yun-Xiao Wang, [a] Shu-Lei Chou,* [a] David Wexler, [b] Hua-Kun Liu, [a] and Shi-Xue Dou [a] Abstract: Sodium-ion energy storage, including sodium-ion batteries (NIBs) and electrochemical capacitive storage (NICs), is considered as a promising alternative to lithium-ion energy storage. It is an intriguing prospect, especially for large-scale applications, owing to its low cost and abun- dance. MoS 2 sodiation/desodiation with Na ions is based on the conversion reaction, which is not only able to deliver higher capacity than the intercalation reaction, but can also be applied in capacitive storage owing to its typically slop- ing charge/discharge curves. Here, NIBs and NICs based on a graphene composite (MoS 2 /G) were constructed. The en- larged d-spacing, a contribution of the graphene matrix, and the unique properties of the MoS 2 /G substantially optimize Na storage behavior, by accommodating large volume changes and facilitating fast ion diffusion. MoS 2 /G exhibits a stable capacity of approximately 350 mAh g 1 over 200 cycles at 0.25 C in half cells, and delivers a capacitance of 50 F g 1 over 2000 cycles at 1.5 C in pseudocapacitors with a wide voltage window of 0.1–2.5 V. Introduction Considering current rechargeable battery systems for renew- able energy storage, lead acid, nickel/cadmium, and vanadium- redox flow batteries all suffer from serious environmental prob- lems and the use of sodium/sulfur batteries is limited because of the high operating temperature (~ 350 8C). Rechargeable lithium-ion batteries (LIBs), the mainstay of energy storage, have been successfully commercialized in portable electronic devices. [1, 2] LIBs, however, are encountering an inevitable chal- lenge with respect to large-scale energy storage owing to re- stricted lithium resources and cost. Thus, it is significant and urgent to develop an essentially new battery system, which needs to be low cost, environmentally friendly, and safe. Sodium, located in the same group in the periodic table as lithium, is one of the most attractive elements to replace lithi- um. It is 4–5 orders of magnitude more abundant than lithium, and the cost of the sodium-based materials is much lower than for lithium-based materials. In addition, the standard po- tential of Na + /Na (2.71 V) is slightly higher than that of Li + /Li (3.05 V). These features point to the great potential of sodium-ion batteries for renewable energy storage and smart grids. [3, 4] To date, various cathode materials, including layered transition metal oxides, [5–8] organic polymers, [3, 9, 10] and polyan- ion fluorophosphates, [11–15] have been reported to reversibly ac- commodate Na ions. Anode research is mainly focused on car- bonaceous material, [16–18] alloyable metals, [19] metal oxides/sul- fides, [20–25] and non-metal materials. [26–28] Furthermore, electro- chemical capacitors are receiving great attention as well, be- cause of their significantly higher power density than batteries with prolonged cycle life, although they also have lower energy density than batteries. [29, 30] To combine the advantages of both batteries and capacitors, Na-based electrochemical ca- pacitors have been proposed and are also a fascinating alter- native for future energy storage. [31, 32] Molybdenum disulfide (MoS 2 ), with an analogous structure to graphite, has attracted extensive interest in terms of energy storage, hydrogen storage, catalysis, and solid lubricants. This layer-structured material possesses S–Mo–S sandwich layers, with the sandwiches stacked together through weak van der Waals interactions. Specifically, atoms within the layers of MoS 2 are bound by strong covalent bonds, whereas the individual layers are bound together by weak van der Waals interactions. Many reports have involved the application of MoS 2 in lithium- ion batteries, as lithium ions can intercalate between the layers. [33–35] Remarkably, expanded MoS 2 , fabricated through a lithiation method with an enlarged c lattice parameter, has achieved higher capacity with a longer cycling span than com- mercial MoS 2 (C-MoS 2 ). The expanded MoS 2 tends to be prone to serious restacking, however, owing to its high surface energy. Moreover, the poor electronic/ionic conductivity be- tween adjacent S–Mo–S sheets further obstructs its application as an electrode material. Thus, many researchers have turned to assembling graphene (G) sheets with expanded MoS 2 to construct three-dimensional (3D) architectures. The obtained MoS 2 /G composites offer high specific surface area, strong me- [a] Y.-X. Wang, Dr. S.-L. Chou, Prof. H.-K. Liu, Prof. S.-X. Dou Institute for Superconducting & Electronic Materials (ISEM) Innovation Campus, University of Wollongong Wollongong, NSW, 2519 (Australia) E-mail: shulei@uow.edu.au [b] Dr. D. Wexler Electron Microscopy Centre (EMC), Innovation campus University of Wollongong, Wollongong, NSW, 2519 (Australia) Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201402563. Chem. Eur. J. 2014, 20, 9607 – 9612  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 9607 Full Paper DOI: 10.1002/chem.201402563