Ion-Pairing Limits Crystal Growth in Metal-Oxygen Batteries

Aprotic alkali metal-oxygen batteries are widely considered to be promising high specific energy alternatives to Li-ion batteries. The growth and dissolution of alkali metal oxides such as Li2O2 in Li-O2 batteries and NaO2 and KO2 in Na- and K-O2 batteries, respectively, is central to the discharge and charge processes in these batteries. However, crystal growth and dissolution of the discharge products, especially in aprotic electrolytes, is poorly understood. In this work, we chose the growth of NaO2 in Na-O2 batteries as a model system and show that there is a strong correlation between the electrolyte salt concentration and the NaO2 crystal size. With a combination of experiments and theory, we argue that the correlation is a direct manifestation of the strong cation-anion interactions leading to decreased crystal growth rate at high salt concentrations. Further, we propose and experimentally demonstrate that cation-coordinating crown molecules are suitable electrochemically stable electrolyte additives that weaken ion-pairing and enhance discharge capacities in metal-oxygen batteries while not negatively affecting their rechargeability.Comment: 13 Pages, 4 Figures, 14 pages of Supplementary Informatio

Efficient separation of rare earths recovered by a supported ionic liquid from bauxite residue leachate

Bauxite residue (BR) contains substantial concentrations of rare-earth elements (REEs), but their recovery is a challenge. Acidic BR leachates typically comprise much higher concentrations of base elements (g L−1) than those of the REEs (ppm). Thus, adsorbents that are highly selective for the REEs over the base elements are required for the separation. The novel supported ionic liquid phase (SILP) betainium sulfonyl(trifluoromethanesulfonylimide) poly(styrene-co-divinylbenzene) [Hbet-STFSI-PS-DVB] was evaluated for the uptake of REEs (Sc, Y, Nd, Dy) in the presence of base elements (Ca, Al, Fe) from BR leachates. Breakthrough curves from acidic nitrate and sulfate media were investigated, as both HNO3 and H2SO4 are commonly used for leaching of BR. The SILP exhibited a superior affinity for REEs in both media, except in the case of Sc(III) from the sulfate feed. The recovery rates of the trace amounts of REEs from the real nitrate feed were remarkably high (71.7–100%) via a simple chromatography separation, without requiring complexing agents or a pretreatment for the removal of interfering elements. The REEs were purified from the base elements and separated into three sub-groups (scandium, light REEs and heavy REEs) by an optimized elution profile with H3PO4 and HNO3 in a single chromatographic separation step

Selection criteria of diluents of tri-n-butyl phosphate for recovering neodymium(III) from nitrate solutions

The selection of a proper diluent should be based on several criteria such as the distribution ratio, phase disengagement time, cost, safety and environmental impact of the process. The effect of different diluents on the solvent extraction of Nd(III) by the neutral extractant tri-n-butylphosphate (TBP) from nitrate feed solutions was studied. The nature of the diluent had little effect on the extraction kinetics of Nd(III) by TBP above 2.5 min. In general, phase disengagement times were relatively shorter for aromatic diluents compared to aliphatic diluents. Conversely, extraction efficiencies were the highest for aliphatic diluents, slightly lower for mixed aliphatic-aromatic diluents and much lower for aromatic diluents. The poorer extraction efficiencies of aromatic diluents maybe due to the lower concentration of free extractant as a result of the stronger interactions of the diluent with water and/or of the diluent with the extractant. Under the experimental conditions, the differences in extraction between aliphatic and aromatic diluents decreased with increasing the salting-out effect of nitrate ions in the feed. At nitrate concentrations of 4.5 mol L−1 or more, the different diluents had a limited influence on the metal extraction with 1 mol L−1 TBP from feed solutions of 1 g L−1 Nd(III). Thus, under these conditions, the selection of the diluent can be preferably based on its cost, safety and biodegradability rather than on its physico-chemical properties

Non-aqueous solvent extraction of rare-earth ions

Poster presentation + flash presentation by N.K. Batchustatus: publishe

Sustainable process for the separation of dysprosium and neodymium: a solvometallurgical approach

Poster presentation by N.K. Batchustatus: publishe

Solvometallurgical separation of rare-earth ions by extraction with two mutually immiscible organic phases

Poster presented by Nagaphani Kumar Batchustatus: publishe

Effect of the diluent on the solvent extraction of neodymium(III) by bis(2-ethylhexyl)phosphoric acid (D2EHPA)

The effect of different types of diluents on the extraction of neodymium(III) from a chloride aqueous feed solution by the acidic extractant bis(2-ethylhexylphosphoric) acid (D2EHPA) was investigated. A total of 11 aliphatic, mixed aliphatic-aromatic and aromatic diluents were considered. D2EHPA was very well miscible with all diluents, even at 90% (v/v) of extractant. Aliphatic diluents gave the highest extraction efficiencies, while aromatic diluents did suppress the formation of emulsions or gels. Although a good separation of Dy(III) over Nd(III) was observed at 0.5 mol/L D2EHPA in all diluents, aromatic diluents were found to yield the highest separation factors. This study shows that the effect of the diluent on the extraction behaviour of Nd(III) is primarily determined by the aliphatic/aromatic content of the diluent. Diluents with the same concentration of aromatics show very similar behaviour. The choice of a diluent can be based on factors other than the extraction behaviour, for instance price and volatility.status: publishe

Separation of yttrium and europium by non-aqueous solvent extraction

Poster presentation by N.K. Batchustatus: publishe

Separation of rare-earth ions from ethylene glycol (+LiCl) solutions by non-aqueous solvent extraction with Cyanex 923

The separation of a mixture of rare earths by non-aqueous solvent extraction with two immiscible organic phases has been studied. The more polar organic phase was ethylene glycol with dissolved lithium chloride and the less polar organic phase was the extractant diluted in n-dodecane. Cyanex 923 was found to be the most performant extractant amongst the investigated acidic, basic and solvating extractants: Cyanex 272, Cyphos IL 101, Aliquat 336, bis(2-ethylhexyl)amine, trioctylphosphine oxide (TOPO) and Cyanex 923. The replacement of the aqueous chloride feed solutions by non-aqueous ethylene glycol feed solutions had a profound effect on the distribution ratios and separation factors. The separation factors for extraction of pairs of rare earths from aqueous chloride solutions by Cyanex 923 are too low to be of practical use. On the contrary, a mixture of rare earths can be separated conveniently in four different groups by extraction with Cyanex 923 from ethylene glycol (+LiCl) solutions. The influence of several parameters such as the chloride concentration, the type of chloride salt, the addition of other polar solvents to the ethylene glycol phase, the addition of second extractant to the less polar organic phase, and the addition of complexing agents to the ethylene glycol phase has been studied. The extraction mechanism for extraction of ytterbium(III) was studied by slope analysis experiments. The ytterbium(III) species in the ethylene glycol phase and the extracted species in the n-dodecane phase were determined by EXAFS. Furthermore, a conceptual flow sheet for the fractionation of rare earths from an ethylene glycol (+LiCl) feed solution into different groups by extraction with Cyanex 923 has been proposed. The new extraction system is useful for extraction of scandium and for separation of scandium from the other REEs.status: publishe

Non-aqueous solvent extraction of rare-earth nitrates from ethylene glycol to n-dodecane by Cyanex 923

A solvent extraction process comprising two immiscible organic phases has been developed for the extraction of rare earths. The more polar organic phase was ethylene glycol with dissolved rare-earth nitrate salts and lithium nitrate, while the less polar phase was a solution of the neutral extractant Cyanex® 923 dissolved in n-dodecane. The solvent extraction mechanism was determined by slope analysis and the main species in the organic phase was identified by Extended X-ray Absorption Fine Structure (EXAFS) studies. The extraction from the ethylene glycol solution was compared with extraction from an aqueous feed solution. When compared to aqueous feed solutions, the light rare-earth elements (LREEs) are less efficiently extracted and the heavy rare-earth elements (HREEs) more efficiently extracted from an ethylene glycol feed solution, resulting into the easy separation of HREEs from LREEs. The separation factors between neighboring elements are higher for this non-aqueous solvent extraction process than for extraction from an aqueous feed solution.status: publishe