A carbon nanotube (CNT) mat interspersed with metallic iron nanoparticles is investigated as a structural supercapacitor electrode material that could be incorporated into fiber-reinforced composites. Both ionic liquid and aqueous potassium hydroxide electrolytes are trialed to examine pseudocapacitive mechanisms and the long-term electrochemical stability of the CNT mat in symmetrical supercapacitors. High-resolution transmission electron microscopy showed the high level of interconnection of the CNTs and evenly distributed metallic iron nanoparticles, which enhances the structural and electrical performance of the electrode. Raman and X-ray photoelectron spectroscopies combined with thermogravimetric and surface area analyses are used to characterize the physico-chemical properties of the CNT mat and identify the different electrochemical mechanisms contributing to the supercapacitive behavior. Three electrode experiments demonstrated the relative contributions of the cathode and anode processes to the total capacitance. Symmetrical supercapacitor coin-cell trials used a structural glass-fiber separator and showed the ionic liquid electrolyte facilitated stable pseudocapacitance with the available iron nanoparticles, leading to specific energy and power as high as 18.3 Wh.kg(-1) at 0.15 kW kg(-1) and 5.6 Wh.kg(-1) at 2.4 kW kg(-1) after 5000 cycles. It is envisioned these materials can readily be incorporated into composite materials for structural energy storage technology. Crown Copyright (c) 2019 Published by Elsevier Ltd. All rights reserved.

Lithium based battery technologies are increasingly being considered for large-scale energy storage applications such as grid storage associated with wind and solar power installations. Safety and cost are very significant factors in these large scale devices. Ionic liquid (IL) electrolytes that are inherently non-volatile and non-flammable offer a safer alternative to mainstream lithium battery electrolytes, which are typically based on volatile and flammable organic carbonates. Hence, in recent years there have been many investigations of ionic liquid electrolytes in lithium batteries with some highly promising results to date, however in most cases cost of the anion remains a significant impediment to widespread application. Amongst the various possible combinations the dicyanamide (DCA) anion based ionic liquids offer exceptionally low viscosities and high conductivities – highly desirable characteristics for Li electrolyte solvents. DCA ILs can be manufactured relatively inexpensively because DCA is already a commodity anion, containing only carbon and nitrogen, which is produced in large amounts for the pharmaceutical industry. In this study we use the non-fluorinated ionic liquid N-methyl-N-butylpyrrolidinium dicyanamide to form non-volatile lithium battery electrolytes. We demonstrate good capacity retention for lithium metal and LiFePO4 in such electrolytes and discharge capacities above 130 mAh.g−1 at 50 °C. We show that it is important to control moisture contents in this electrolyte system in order to reduce capacity fade and rationalise this observation using cyclic voltammetry and lithium symmetrical cell cycling. Having approximately 200 ppm of moisture content produces the optimum cycling ability. We also describe plastic crystal solid state electrolytes based on the DCA anion in the lithium metal–LiFePO4 battery configuration and demonstrate over 150 mAh.g−1 discharge capacity without any significant capacity fading at 80 °C.

Organic ionic plastic crystal (OIPC) electrolytes are among the key enabling materials for solid-state and higher than ambient temperature lithium batteries. This work overviews some of the parameter studies on the Li|OIPC interface using lithium symmetrical cells as well as the optimisation and performance of Li|OIPC|LiFePO4 cells. The effects of temperature and electrolyte thickness on the cycle performance of the lithium symmetrical cell, particularly with respect to the interfacial and bulk resistances, are demonstrated. Whilst temperature change substantially alters both the interfacial and bulk resistance, changing the electrolyte thickness predominantly changes the bulk resistance only. In addition, an upper limit of the current density is demonstrated, above which irreversible processes related to electrolyte decomposition take place. Here, we demonstrate an excellent discharge capacity attained on LiFePO4|10 mol% LiNTf2-doped [C2mpyr][NTf2]|Li cell, reaching 126 mAh g-1 at 50 °C (when the electrolyte is in its solid form) and 153 mAh g-1 at 80 °C (when the electrolyte is in its liquid form). Most remarkably, at high temperature operation, the capacity retention at long cycles and high current is excellent with only a slight (3%) drop in discharge capacity upon increasing the current from 0.2 C to 0.5 C. These results highlight the real prospects for developing a lithium battery with high temperature performance that easily surpasses that achievable with even the best contemporary lithium-ion technology.

Replacement of volatile and combustible electrolytes in conventional lithium batteries is desirable for two reasons: safety concerns and increase in specific energy. In this work we consider the use of an ionic organic plastic crystal material (IOPC), N-ethyl-N-methylpyrrolidinium tetrafluoroborate, [C2mpyr][BF(4)], as a solid-state electrolyte for lithium battery applications. The effect of inclusion of 1 to 33 mol% lithium tetrafluoroborate, LiBF(4), into [C2mpyr][BF(4)] has been investigated over a wide temperature range by differential scanning calorimetry (DSC), impedance spectroscopy, cyclic voltammetry and cycling of full Li|LiFePO(4) batteries. The increases in ionic conductivity by orders of magnitude observed at higher temperature are most likely associated with an increase in Li ion mobility in the highest plastic phase. At concentrations >5 mol% LiBF(4) the ionic conductivity of these solid-state composites is comparable to the ionic conductivity of room temperature ionic liquids. Galvanostatic cycling of Li|Li symmetrical cells showed that the reversibility of the lithium metal redox reaction at the interface of this plastic crystal electrolyte is sufficient for lithium battery applications. For the first time we demonstrate an all solid state lithium battery incorporating solid electrolytes based on IOPC as opposed to conventional flammable organic solvents.

We show that a previously observed low current, pre-conditioning behavior which allows increased lithium flux through a solid state, symmetric lithium cell containing a pyrrolidinium bis(trifluoromethyl)sulfonamide electrolyte is a more general phenomenon for organic ionic plastic crystal electrolytes. Furthermore, the mechanism of the preconditioning behavior has been highlighted via a post-mortem characterisation of the cells. Differential scanning calorimetry (DSC) indicated a decrease in the melting point by 1.9 °C after 35 h of 0.01 mA cm− 2 cycling and scanning electron microscopy (SEM) images revealed that substantial changes to the OIPC microstructure, in particular grain size, had occurred. Both of these phenomena would lead to an enhancement of lithium ion transport near the Li/electrolyte interface. In a preliminary experiment incorporating a LiFePO4 cathode, similar preconditioning behavior was observed. Furthermore, good battery performance can be achieved, between 129 and 110 mAh g− 1 during 50 cycles at 0.2 C, denoting the promise of these plastic crystal electrolytes in highly stable, all solid-state lithium metal batteries.

The addition of nanoparticles to an organic ionic plastic crystal can result in orders of magnitude increases in ionic conductivity, which makes these materials of interest as solid state electrolytes. However, this effect is not universal and depends on both the nature of the organic ionic plastic crystal and on the type of nanoparticle used. The effect of addition of TiO2, Al2O3 and SiO2 nanoparticles to a range of ionic materials with varying plasticity and rotator phase behaviour has been studied by thermal analysis and conductivity and the effect on the different materials is compared.

Addition of silica nanoparticles functionalised with lithium propane sulfonate to the organic ionic plastic crystal N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C2mpyr][NTf2]) results in a significant increase in ionic conductivity. Analysis of these nanocomposites by impedance spectroscopy, NMR, positron annihilation lifetime spectroscopy (PALS) and Raman spectroscopy suggests that this is the result of higher matrix mobility due to an increase in defect size and concentration. The effect of these functionalised nanoparticles is compared to that previously observed for unfunctionalised nanoparticles in the lithium-doped and pure plastic crystal.

Doping lithium bis(trifluoromethanesulfonyl) amide (Li[NTf(2)]) into the N-ethyl,N'-methylpyrrolidinium bis(trifluoromethanesulfonyl) amide ([C(2)mpyr][NTf(2)]) plastic crystal material has previously indicated order of magnitude enhancements in ion transport and conductivity over pure [C(2)mpyr][NTf(2)]. Recently, conductivity enhancements in this ionic plastic crystal induced by SiO(2) nanoparticles have also been reported. In this work the inclusion of SiO(2) nanoparticles in Li ion doped [C(2)mpyr][NTf(2)] has been investigated over a wide temperature range by differential scanning calorimetry (DSC), impedance spectroscopy, positron annihilation lifetime spectroscopy (PALS), Raman spectroscopy, NMR spectroscopy and scanning electron microscopy (SEM). Solid state (1)H NMR indicates that the addition of the nanoparticles increases the mobility of the [C(2)mpyr] cation and positron lifetime spectroscopy (PALS) measurements indicate an increase in mean defect size and defect concentration as a result of nanoparticle inclusion, especially with 10 wt% SiO(2). Thus, the substantial drop in ion conductivity observed for this doped nanocomposite material was surprising. This decrease is most likely due to the decrease in mobility of the [NTf(2)] anion, possibly by its adsorption at the SiO(2)/grain boundary interface and concomitant decrease in mobility of the Li ion.

High conductivity in solid-state electrolytes is a critical requirement for many advanced energy and other electrochemical applications. Plastic crystalline materials have shown promise in this regard, and the inclusion of nanosized inorganic particles in both amorphous and crystalline materials has indicated order of magnitude enhancements in ion transport induced by space charge or other defect enhancement. In this paper we present conductivity enhancements in the plastic crystal N,N'-ethylmethylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C(2)mpyr][NTf2]) induced by nanosized SiO2 particles. The addition of the nanoparticles dramatically increases plasticity and ion mobility. Positron annihilation lifetime spectroscopy ( PALS) measurements indicate an increase in mean defect size and defect concentration as a result of nanoparticle inclusion. The scaling of the conductivity with size suggests that a "trivial space charge" effect is operable, although a strain induced enhancement of defects ( in particular extended defects) is also likely given the observed increase in plasticity.

The addition of nano-sized ceramic particles to the plastic crystal ethyl-methyl pyrrolidinium bis(trifluoromethane sulfonyl)amide (P(12)TFSA) has been investigated by means of DSC and conductivity. The thermal behaviour of the plastic crystal as a function of filler content suggests that the filter particles decrease the onset temperature of the melting slightly at high loadings, however they do not decrease the crystallinity of the material. Furthermore, the IV -> III transition decreases in intensity, indicating that the addition of filler increases the possibility for the crystal to remain in metastable rotator phases also at lower temperatures. The conductivity shows a more than one order of magnitude increase with the addition of filler, with a filler concentration dependence that levels out above similar to 10 wt.% TiO2. (c) 2006 Elsevier B.V. All rights reserved.