Single step tranformation of sulphur to Li₂S₂/Li₂S in Li-S batteries

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

The mechanism of cation and oxygen isotope exchange in alkali feldspars under hydrothermal conditions

The mechanism of re-equilibration of albite in a hydrothermal fluid has been investigated experimentally using natural albite crystals in an aqueous KCl solution enriched in 18O at 600°C and 2 kbars pressure. The reaction is pseudomorphic and produces a rim of K-feldspar with a sharp interface on a nanoscale which moves into the parent albite with increasing reaction time. Transmission electron microscopy (TEM) diffraction contrast and X-ray powder diffraction (XRD) show that the K-feldspar has a very high defect concentration and a disordered Al, Si distribution, compared to the parent albite. Raman spectroscopy shows a frequency shift of the Si-O-Si bending vibration from ~476 cm-1 in K-feldspar formed in normal 16O aqueous solution to ~457 cm-1 in the K-feldspar formed in 18O-enriched solution, reflecting a mass-related frequency shift due to a high enrichment of 18O in the K-feldspar silicate framework. Raman mapping of the spatial distribution of the frequency shift, and hence 18O content, compared with major element distribution maps, show a 1:1 correspondence between the reaction rim formed by the replacement of albite by K-feldspar, and the oxygen isotope re-equilibration. The textural and chemical characteristics as well as the kinetics of the replacement of albite by K-feldspar are consistent with an interface-coupled dissolution-reprecipitation mechanism. © Springer-Verlag 2008

The replacement of plagioclase feldspars by albite: Observations from hydrothermal experiments

Oligoclase and labradorite crystals have been experimentally replaced by albite in an aqueous sodium silicate solution at 600°C and 2 kbars. The replacement is pseudomorphic and is characterised by a sharp chemical interface which progresses through the feldspar while preserving the crystallographic orientation. Reaction rims of albite, up to 50 µm thick, can be readily achieved within 14 days. Re-equilibration of plagioclase in an 18O-enriched sodium- and silica-bearing solution results in oxygen isotope redistribution within the feldspar framework structure. The observed characteristics of the reaction products are similar to naturally albitised plagioclase and are indicative of an interface-coupled dissolution- reprecipitation mechanism. Chemical analyses demonstrate that the albitisation is accompanied by the mobilisation of major, minor and trace elements also including elements such as Al and Ti which are commonly regarded as immobile during hydrothermal alteration. The results contribute to developing our understanding of the close association between large-scale albitisation and secondary ore mineralisation which is common in nature. © 2009 Springer-Verlag

Intragranular replacement of chlorapatite by hydroxy-fluor-apatite during metasomatism

The pseudomorphic replacement of chlorapatite by hydroxy-fluor-apatite is associated with metasomatism of the Ødegården metagabbro (Bamble, south Norway). Primary fluor-chlorapatite in the protolith gabbro is non-porous and homogenous in composition with F-content of up to 3.5 wt% and Cl-content up to 2 wt.%. Metasomatism transforms apatite during multi-stage replacement reactions: I) Magmatic apatite was transformed to chlorapatite with Cl-content of up to 6.8 wt.% during scapolitisation. II) A secondary pseudomorphic replacement reaction which transforms chlorapatite to porous hydroxy-fluor-apatite with only minor Cl is related to albitisation. Laser ablation inductively coupled mass spectrometry (LA ICPMS) data from the primary fluor-chlorapatite shows in general a high content of Sr, Y, REE and Th. Chlor- and hydroxy-fluor-apatite show depletion in Co, Ni, Sr, Cd, Pb, Th and U, and enrichment in V and Ba, relative to the primary apatite. The replacement interface of chlorapatite to hydroxy-fluor-apatite is studied by energy-filtered transmission electron microscopy (TEM). The interface is sharp on a nano-scale, and the crystallographic orientation of the apatite is preserved. These observations, together with the porosity development in the hydroxy-fluor-apatite suggest that the replacement mechanism is by interface-coupled dissolution-reprecipitation. © 2009 Elsevier B.V. All rights reserved