An experimental and computational study to understand the lithium storage mechanism in molybdenum disulfide.

The lithium storage mechanism in molybdenum disulfide (MoS(2)) has been comprehensively investigated as the existing conversion-based storage mechanism is unable to explain the reason behind its high practical capacity, high polarization losses, and the change in the discharge profile after the 1(st) charge-discharge cycle. To resolve these issues and to gain a deeper understanding of MoS(2)-based Li-ion batteries, for the first time, we have studied the reaction mechanism of the MoS(2) anode using various experimental techniques such as XRD, Raman spectroscopy, electrochemical impedance spectroscopy, XANES, and EXAFS, as well as ab initio density functional theory based calculations. On the basis of the results presented here, and in line with some experimental findings, we find that the reaction of MoS(2) with Li is not as simple as with usual metal oxide based conversion reactions, but that the pathway of the conversion reaction changes after the first discharge process. In the first discharge process, lithiation is initiated by a limited intercalation process, followed by a conversion reaction that produces molybdenum nanoparticles (Mo) and lithium sulfide (Li(2)S). Whereas, unlike oxide-based conversion materials, MoS(2) does not transverse back during the delithiation process. Indeed, instead of MoS(2) formation, we identified the presence of polysulfur after the complete cycle. In consecutive cycles, polysulfur reacts with lithium and forms Li(2)S/Li(2)S(2), and this Li-S reaction is found to be highly reversible in nature and the only source of the high practical capacity observed in this electrode. To validate our experimental findings, an atomic scale ab initio computational study was also carried out, which likewise suggests that Li first intercalates between the MoS(2) layers but that after a certain concentration, it reacts with MoS(2) to form Li(2)S. The calculations also support the non-reversibility of the conversion reaction, by showing that Mo + Li(2)S formation is energetically more favorable than the re-formation of MoS(2) + Li.

Intercalation anode material for lithium ion battery based on molybdenum dioxide.

MoO2 is one of the most studied anode systems in lithium ion batteries. Previously, the reaction of MoO2 with lithium via conversion reaction has been widely studied. The present study highlights the possible application of MoO2 as an intercalation-based anode material to improve the safety of lithium ion batteries. Nanobelts of MoO2 are prepared by reduction of MoO3 nanobelts under hydrogen atmosphere. The intercalation behavior of MoO2 is specially focused upon by limiting the charge-discharge cycling to narrow potential window of 1.0 to 2.2 V vs Li/Li(+) to avoid conversion reaction. An excellent electrochemical stability over 200 cycles is achieved at a current rate of 100 mAh g(-1). A phase transformation from monoclinic to orthorhombic to monoclinic is observed during the lithiation process, which is reversible during delithiation process and is confirmed by ex-situ XRD and electrochemical impedance spectroscopy. To further demonstrate the viability of MoO2 as a commercial anode material, MoO2 is tested in a full-cell configuration against LiFePO4. The full-cell assembly is cycled for 100 cycles and stable performance is observed. The combination showed an energy density of 70 Wh kg(-1) after 100 cycles.

High-rate and high-energy-density lithium-ion battery anode containing 2D MoS2 nanowall and cellulose binder.

Electrochemically stable molybdenum disulfide (MoS2) with a two-dimensional nanowall structure is successfully prepared by a simple two-step synthesis method followed by thermal annealing at 700 °C in a reducing atmosphere. MoS2 nanowalls provide a better electrochemical performance and stability when cellulose (CMC) binder is used instead of the usual PVDF. The electrodes exhibit a high specific discharge capacity of 880 mA h g⁻¹ at 100 mA g⁻¹ without any capacity fading for over 50 cycles. The electrode also exhibits outstanding rate capability with a reversible capacity as high as 737 mA h g⁻¹ and 676 mA h g⁻¹ at rates of 500 mA g⁻¹ and 1000 mA g⁻¹ at 20 °C, respectively. The excellent electrochemical stability and high specific capacity of the nano structured materials are attributed to the two-dimensional nanowall morphology of MoS2 and the use of cellulose binder. These results are the first of its kind to report a superior stability using bare MoS2 as an active material and CMC as a binder.