Al-Masri D, Yunis R, Hollenkamp AF, Doherty CM, and Pringle JM
Physical chemistry chemical physics : PCCP [Phys Chem Chem Phys] 2020 Aug 24; Vol. 22 (32), pp. 18102-18113.
Ionic liquids and plastic crystals based on pyrrolidinium cations are recognised for their advantageous properties such as high conductivity, low viscosity, and good electrochemical and thermal stability. The pyrrolidinium ring can be substituted with symmetric or asymmetric alkyl chain substituents to form a range of ionic liquids or plastic crystals depending on the anion. However, reports into the use of branched alkyl chains and how this influences the material properties are limited. Here, we report the synthesis of six salts - ionic liquids and organic ionic plastic crystals - where the typically used linear propyl chain substituent is replaced by the branched alternative, isopropyl, to form the cation [C(i3)mpyr]+, in combination with six different anions: dicyanamide, (fluorosulfonyl)(trifluoromethanesulfonyl)imide, bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, tetrafluoroborate and hexafluorophosphate. The thermal and transport properties of these salts are compared to those of the analogous N-propyl-N-methylpyrrolidinium and N,N-diethylpyrrolidinium-based salts. Finally, a high lithium salt content ionic liquid electrolyte based on the bis(fluorosulfonyl)imide salt was developed. This electrolyte showed high coulombic efficiencies of lithium plating/stripping and high lithium ion transference number, making it a strong candidate for use in lithium metal batteries.
Yunis R, Hollenkamp AF, Forsyth C, Doherty CM, Al-Masri D, and Pringle JM
Physical chemistry chemical physics : PCCP [Phys Chem Chem Phys] 2019 Jun 21; Vol. 21 (23), pp. 12288-12300. Date of Electronic Publication: 2019 May 29.
The synthesis and characterisation of new solid-state electrolytes is a key step in advancing the development of safer and more reliable electrochemical energy storage technologies. Organic ionic plastic crystals (OIPCs) are an increasingly promising class of material for application in devices such as lithium or sodium metal batteries as they can support high ionic conductivity, with good electrochemical and thermal stability. However, the choice of OIPC-forming ions is still relatively limited. Furthermore, understanding of the influence of different cations and anions on the thermal, structural and transport properties of these materials is still in its infancy. Here we report the synthesis and in-depth characterisation of a range of new OIPCs utilising the hexamethylguanidinium cation ([HMG]) with five different anions. The thermal, structural, transport properties and free volume in the different salts have been investigated. The free volume within the salts has been investigated by positron annihilation lifetime spectroscopy, and the single crystal and powder X-ray diffraction analysis of [HMG] bis(trifluoromethanesulfonyl)imide ([TFSI]) in phase I and II, [HMG] hexafluorophosphate ([PF 6 ]) and [HMG] tetrafluoroborate ([HMG][BF 4 ]) are reported. The HMG cation can exhibit significant disorder, which is advantageous for plasticity and future use of these materials as high ionic conductivity matrices. The bis(fluorosulfonyl)imide salt, [HMG][FSI], is identified as particularly promising for use as an electrolyte, with good electrochemical stability and soft mechanical properties. The findings introduce a range of new materials to the solid-state electrolyte arena, while the insights into the physico-chemical relationships in these materials will be of importance for the future development and understanding of other ionic electrolytes.
Wei H, Rodriguez EF, Best AS, Hollenkamp AF, Chen D, and Caruso RA
ACS applied materials & interfaces [ACS Appl Mater Interfaces] 2019 Apr 10; Vol. 11 (14), pp. 13194-13204. Date of Electronic Publication: 2019 Mar 26.
The lithium-sulfur battery (LSB) is a promising candidate for future energy storage but faces technological challenges including the low electronic conductivity of sulfur and the solubility of intermediates during cycling. Additionally, current host materials often lack sufficient conductivity and porosity to raise the sulfur loading to over 80 wt %. Here, ordered mesoporous graphitic carbon/iron carbide nanocomposites were prepared via an evaporation-induced self-assembly process using soluble resol, prehydrolyzed tetraethyl orthosilicate (TEOS), and iron(III) chloride as the carbon, silica (SiO 2 ), and iron precursors, respectively. Graphitization and SiO 2 etching were conducted simultaneously via Teflon-assisted, solid-state decomposition at high temperature. A high surface area (∼3100 m 2 g -1 ), large pore volume (∼3.3 cm 3 g -1 ), and graphitized carbon frame were achieved, giving a high sulfur loading (85 wt %) while tolerating volumetric expansion during discharge. Electrochemical testing of a LSB containing the composite/sulfur cathode exhibited a superior reversible capacity exceeding 1300 mAh g -1 at a moderate current (C/10) and a low decay in capacity of 9% after 500 cycles at C/5. The interaction between mesoporous graphitic carbon and sulfur is proposed.