Ultrathin Sheets of MoS2/g-C3N4 composite As a Hosting Material of Sulphur for Lithium-Sulphur Batteries with Long Cycle Life and High Rate Capability

Abstract

High energy density rechargeable batteries are the future of energy storage systems. Lithium ion batteries have brought revolutionary changes in the portable electronic devices for more than thirty years. However, the presently available lithium-ion batteries (LIBs) that follow intercalation-deintercalation mechanism cannot fulfil the energy demand of emerging electric vehicles (EVs), hybrid EVs, and next generation portable electronic devices. Therefore, alternative chemistry of lithium-ion batteries has drawn tremendous attention to overcome this bottleneck of high energy demand. Lithium-sulphur batteries holds enormous potential to meet the future energy demands because of its high theoretical specific capacity (1675mAh/g) and cost effectiveness [1]. The specific energy density of lithium-sulphur battery has a theoretical value of 2567Wh/kg, which is significantly higher than the commercially available lithium-ion batteries (387Wh/kg for LiCoO2 with graphite anode) [1]. In spite of the tremendous potential, lithium-sulphur battery systems have some challenging issues hindering its practical usage. The major challenge is the “shuttle effect” of higher order lithium polysulphides (Li2Sn, 4<n<8) between cathode and anode. This is derived from the dissolution of polysulphides in the organic electrolyte. The shuttle effect results in severe capacity loss, poor rate performance and low coulombic efficiency. Another challenge is the high electrical resistivity of elemental sulphur (1015 Ω-m) that prevents the direct utilization of sulphur as an electrode material. In the present study, MoS2/g-C3N4 nanosheets are utilized to chemically confine the lithium polysulphides. Both MoS2 and graphitic carbon nitride (g-C3N4) will provide anchoring sites for lithium polysulphides leading to superior cycling and rate performance. X-Ray photon spectroscopy (XPS) measurements are performed to investigate the chemical composition and nitrogen content of MoS2/g-C3N4 nanosheets. The N1s spectrum shows three nitrogen components, with the most prominent one being pyridinic-N (398.9 eV) and the other two pyrrolic-N (400.1 eV) and graphitic-N (401.3 eV), respectively [2-4]. The pyridinic-N is the major one accounting for more than half of the overall nitrogen content (42%). The C1s spectra show three deconvoluted peaks that could be assigned to graphite like sp2 carbon (284.8 eV), C-N (285.1 eV) and O-C=O (290.2 eV), respectively [3-4]. The Mo3d spectrum shows the expected Mo3d5/2 (229.8 eV), Mo3d3/2 (232.9 eV) and Mo+6 species (236.1 eV) as commonly observed in partially oxidized MoS2 complexes [5-6]. The S2p spectrum splits into four peaks located at 162.3, 163.4, 163.9, 169.0 eV, which are assigned to S2p3/2, S2p1/2, S-2, and S+4, respectively [5-6]. To our knowledge, this is the first example of utilizing MoS2/g-C3N4 composite as a sulphur host for high performance lithium sulphur batteries. Previously, Ghazi and co-workers[7] had utilized MoS2/Celgard separator as a polysulphide barrier which delivered a reversible capacity of 401mAh/g at 0.5C after 600 cycles with 0.083% capacity decay/cycle and an initial discharge capacity of 550mAh/g at 1C rate. Recently Yu and co-workers [8] reported the utilization of ultrathin mesoporous graphitic carbon frameworks as a sulphur host which showed improved cycling performance with 0.044% capacity decay/ cycle after 400 cycles at 2C and good rate capability by delivering initial discharge capacity of 430mAh/g at 6C. Electrochemical testing of MoS2/g-C3N4 composite showed remarkable cycling and rate performance by delivering specific discharge capacity of around 674, 612 and 510mAh/g at 2, 4 and 8C rate with capacity retention of 80% (0.049% decay/cycle), 77%(0.057% decay/cycle) and 88%(0.03% decay/cycle)respectively, after 400 cycles as shown in the figure. This work demonstrates the development of flexible 2D cathode materials beyond graphene for superior performance of lithium-sulphur batteries.