Lithium-sulphur batteries have generated tremendous research interest due to their high theoretical energy density and potential cost-effectiveness. The commercial realization of Li-S batteries is still hampered by reduced cycle life associated with the formation of electrolyte soluble higher-order polysulphide (Li2Sx, x = 4–8) intermediates, leading to capacity fading, self-discharge, and a multistep voltage profile. Herein, we have realized a practical approach towards a direct transformation of sulphur to Li2S2/Li2S in lithium-sulphur batteries by alteration of the reaction pathway. A coconut shell derived ultramicroporous carbon-sulphur composite cathode has been used as reaction directing template for the sulphur. The lithiation/delithiation and capacity fading mechanism of microporous carbon confined sulphur composite was revealed by analyzing the subsurface using X-ray photoelectron spectroscopy. No higher-order polysulphides were detected in the electrolyte, on the surface, and in the subsurface of the cathode composite. The altered reaction pathway is reflected by a single-step profile in the discharge/charge of a lithium-sulphur cell.
While the technological importance of carbon-based anodes for sodium-ion batteries is undebated, the underlying mechanism for sodium insertion and storage is still strongly disputed. Here, we present a joint experimental and theoretical study that allows us to provide detailed insights into the process of Na insertion in nongraphitizable (hard) carbon. For this purpose, we combine data from in situ Raman scattering of Na insertion in hard carbon with density functional theory-based lattice dynamics and band structure calculations for Na insertion in graphitic model structures used for a local description of graphitic domains in hard carbon. The agreement of experimental results and computational findings yields a clear picture of the Na insertion mechanism, which can be described by four different stages that are dominated by surface morphology, defect concentration, bulk structure, and nanoporosity. On the basis of the resulting model for sodium insertion, we suggest design strategies to maximize the capacity of hard carbon.
Li-S batteries are promising energy storage technology for the future, however there two major problems remained which need to be solved before successful commercialization. Capacity fading due to polysulfide shuttle and corrosion of lithium metal are directly connected with the type and quantity of electrolyte used in the cells. Several recent works show dependence of the electrochemical behavior of Li-S batteries on type of the electrolyte. In this work we compare and discuss a discharge mechanism of sulfur conversion in three different electrolytes based on measurements with sulfur K-edge XAS. The sulfur conversion mechanism in the ether based electrolytes, the most studied type of solvents in the Li-S batteries, which are enabling high solubility of polysulfides are compared with the fluorinated ether based electrolytes with a reduced polysulfide solubility and in carbonate based electrolytes with the sulfur confined into a ultramicroporous carbon. In all three cases the sulfur reduction proceeds through polysulfide intermediate phases with a difference on the type polysulfides detected at different steps of discharge. Electrification of road transport has increased the pressure on materials' scientists to improve performance of the current Li-ion batteries and to develop new advanced high energy battery technologies. 1Among several post lithium-ion technologies, the lithium sulfur (Li-S) batteries are recognized as the most promising for commercialization in the near future. A combination of lithium and sulfur in the electrochemical cell corresponds to the theoretical energy density of 2600 Wh/kg, while the maximum practically accessible energy density is predicted to be close to 600 Wh/kg.2,3 This is much higher compared to Li-ion batteries and besides that, sulfur is inexpensive and naturally abundant. Nevertheless, problems related to the solubility of polysulfides in the electrolyte and the related redox shuttle phenomena cause short cycle life, potential safety problems, poor cycling efficiency, and relative fast self-discharge. Additional problems of Li-S batteries are very low electronic conductivity of the both end members in the discharge and charge process (i.e. sulfur and Li 2 S) and extensive corrosion of the metal lithium anode.Different directions how to improve Li-S battery cycle life have been explored, like synthesis of the optimized porous host matrices with active sites for polysulfide anchoring, 4,5 design and optimization of separators which can effectively suppress polysulfide cross communication between electrodes 6,7 and protection of lithium by artificial SEI 8 or by additives. 9 While most of the research was performed in the binary mixture of solvents using alkyl ethers (glymes) and heterocyclic acetyl (dioxolane), less attention has been paid to the development of new electrolytes for Li-S batteries. 10 Reasons for that are nested in the requirements which should be fulfilled in the development of new formulations. First of all, electrolytes used in the electrochemical cells must...
Here, we provide a deeper insight into the state of sulfur confined in ultramicroporous carbon (UMC) and clarify its electrochemical reaction mechanism with lithium by corroborating the results obtained using various experimental techniques, such as X-ray photoelectron spectroscopy, electron energy loss spectroscopy, in situ Raman spectroscopy, and in situ electrochemical impedance spectroscopy. In combination, these results indicate that sulfur in UMC exists as linear polymeric sulfur rather than smaller allotropes. The electrochemical reactivity of lithium with sulfur confined in UMC (pore size ≤0.7 nm) is different from that of sulfur confined in microporous carbon (≤2 nm, or ultramicroporous carbon containing significant amount of micropores) and mesoporous carbon (>2 nm). The observed quasi-solid-state reaction of lithium with sulfur in UMC with a single voltage plateau during the discharge/charge process is due to the effective separation of solvent molecules from the active material. The size of carbon pores plays a vital role in determining the reaction path of lithium with sulfur confined in UMC.
Sulfur possesses high specific capacity (1672 mA h g−1) and high specific energy (2600 Wh kg−1), which makes it attractive as a cathode material for lithium–sulfur batteries. However, the areal energy density of sulfur electrodes is usually low due to an excess amount of inactive materials, mostly carbon, in the electrode composite. Typically, the electrode layers must be thin to achieve good specific capacity and cyclic stability. This further reduces the areal capacities, and it is challenging to design high areal density sulfur electrodes. Herein, the possibility of achieving high areal energy density by using ultramicroporous carbon–sulfur (UMC‐S) composite electrodes is investigated. For this purpose, the weight of sulfur per area is systematically varied by varying the amount of UMC‐S, and its electrochemical performance with respect to current density, cycling voltage window, electrolyte amount, and effect of temperature on cycling is investigated. A sulfur loading of up to 9.7 mg cm−2 and areal capacities above 4.5 mAh cm−2 are attained. The results indicate that a high areal energy density can be achieved with UMC‐S electrodes, however, at lower current rates. Higher current rates can be achieved by reducing the particle size and by improving the intrinsic electronic conductivity of the UMC host.
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