Anand I. Bhatt

Stabilizing lithium metal using ionic liquids for long-lived batteries.

Suppressing dendrite formation at lithium metal anodes during cycling is critical for the implementation of future lithium metal-based battery technology. Here we report that it can be achieved via the facile process of immersing the electrodes in ionic liquid electrolytes for a period of time before battery assembly. This creates a durable and lithium ion-permeable solid–electrolyte interphase that allows safe charge–discharge cycling of commercially applicable Li|electrolyte|LiFePO4 batteries for 1,000 cycles with Coulombic efficiencies >99.5%. The tailored solid–electrolyte interphase is prepared using a variety of electrolytes based on the N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide room temperature ionic liquid containing lithium salts. The formation is both time- and lithium salt-dependant, showing dynamic morphology changes, which when optimized prevent dendrite formation and consumption of electrolyte during cycling. This work illustrates that a simple, effective and industrially applicable lithium metal pretreatment process results in a commercially viable cycle life for a lithium metal battery.

Lithium–sulfur batteries—the solution is in the electrolyte, but is the electrolyte a solution?

At first glance, the combination of the lightest, most electropositive metal (lithium) with a safe, abundant (and reasonably light) non-metal (sulfur) makes good sense as a prospective battery. However, while the lithium–sulfur battery offers a very high theoretical specific energy (∼2600 W h kg−1) the actual performance delivered is proving to be severely limited—in many cases, this is directly related to the role of the electrolyte. The fundamental issue is that the reduction of sulfur proceeds through a series of polysulfide species, which are for the most part soluble in common organic solvents, including those employed in battery electrolyte solutions. So, despite the fact that the ultimate product (Li2S) is essentially insoluble, the intermediate stages of discharge see a migration of redox-active species out of the cathode, from where they can react with the lithium anode, which sets in train a series of equilibria that cause both a loss of charging efficiency and a gradual loss of discharge capacity. In the last decade, a major stream of the research to overcome this complex situation has focused on minimizing the solubility of polysulfides. From this we now have a range of media in which the lithium–sulfur system can operate with much improved charge–discharge characteristics: ionic liquids (and blends with organic media); super-saturated salt-solvent mixtures; polymer-gelled organic media; solid polymers; solid inorganic glasses. Underlining the multi-faceted nature of interactions within the lithium–sulfur cell, though, none of these improved electrolytes has been able to bring the performance of this system up to the levels of reliability and capacity maintenance (without sacrificing high specific energy) that are benchmarks in energy storage applications. Our survey indicates that only by combining particular electrolytes with cathode materials that are designed to actively retain sulfur and its reduction products, have a relatively few studies been able to obtain the desired levels of performance. Ultimately the successful development of the lithium–sulfur battery requires careful coordination of the choice of modified electrolyte with the specific nature of the cathode material, underpinned by the assumption that the resulting electrolyte composition will meet established criteria for compatibility with the lithium anode.

Ionic Liquids with the Bis(fluorosulfonyl)imide Anion: Electrochemical Properties and Applications in Battery Technology

Room temperature ionic liquids (RTILs) with the bis(fluorosulfonyl)imide (FSI) anion exhibit higher conductivities than the corresponding bis(trifluoromethanesulfonyl)imide (TFSI) compounds, thereby generating interest as novel electrolytes for lithium batteries. The electrochemical properties of a series of FSI RTILs, at inert metal and lithium electrodes, have been investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy. Addition of LiBF 4 , LiPF 6 , or LiTFSI extends cathodic limits to significantly more negative values and allows reversible lithium electrodeposition. Variable-current cycling of symmetrical Li | Li coin cells reveals significant changes in electrode-electrolyte interphasial impedance, which depends on the identity of the lithium salt anion, the concentration of the salt, and the RTIL cation. For most cells, voltage-time curves become unsteady early in duty, which is consistent with the formation of dendrites on the lithium surface. A stable voltage behavior returns within around 20 cycles, at notably a lower current density presumably because detachment/reattachment of dendrites eventually re-establishes a contiguous lithium electrode with a higher surface area. Importantly, the combination of the kinetics of lithium deposition and morphology of the deposit in FSI anion-based RTIL media does not result in lithium penetration of the separator. Therefore, FSI-based electrolytes can play a key role in the development of a viable lithium-metal battery technology.

Application of the N-propyl-N-methyl-pyrrolidinium Bis(fluorosulfonyl)imide RTIL Containing Lithium Bis(fluorosulfonyl)imide in Ionic Liquid Based Lithium Batteries

In seeking to develop ionic liquid based electrolytes for use in lithium metal batteries, we present an investigation of the electrochemical properties of N-propyl-N-methyl-pyrrolidinium bis(fluorosulfonyl)imide and lithium bis(fluorosulfonyl)imide at Ni, Pt, and Li electrodes by cyclic voltammetry, chronoamperometry, and impedance spectroscopy. While lithium electrodeposition and stripping are chemically reversible, the magnitude of peak currents during successive cycles is strongly dependent on the substrate. Severe decreases are observed at Ni, only moderate falls at Pt, while Li electrodes support modest increases in current, consistent with roughening of the electrode with each deposition cycle. We discuss this behavior on the basis of competition between (i) formation of a solid electrolyte interphase at the deposited lithium surface and (ii) strength of interaction between deposited lithium and substrate. Chronoamperometric data indicate that lithium deposition proceeds via instantaneous nucleation and growth, which favors smooth rather than nodular deposit morphology. Symmetrical (Li|electrolyte|Li) cells display excellent cycling behavior (>470 cycles), at current densities up to 10 mA cm ―2 , with only transient evidence of dendrite formation. Initially high impedance is reduced by increasing the concentration (∼0.5 mol kg ―1 ) of lithium salt, although all cells eventually reach relatively low values of < 10 Ω cm 2 . The properties of this electrolyte system make it a strong candidate for future application in lithium metal batteries.

Emerging electrochemical energy conversion and storage technologies

Electrochemical cells and systems play a key role in a wide range of industry sectors. These devices are critical enabling technologies for renewable energy; energy management, conservation, and storage; pollution control/monitoring; and greenhouse gas reduction. A large number of electrochemical energy technologies have been developed in the past. These systems continue to be optimized in terms of cost, life time, and performance, leading to their continued expansion into existing and emerging market sectors. The more established technologies such as deep-cycle batteries and sensors are being joined by emerging technologies such as fuel cells, large format lithium-ion batteries, electrochemical reactors; ion transport membranes and supercapacitors. This growing demand (multi billion dollars) for electrochemical energy systems along with the increasing maturity of a number of technologies is having a significant effect on the global research and development effort which is increasing in both in size and depth. A number of new technologies, which will have substantial impact on the environment and the way we produce and utilize energy, are under development. This paper presents an overview of several emerging electrochemical energy technologies along with a discussion some of the key technical challenges.

Synthesis of Ag and Au nanostructures in an ionic liquid: thermodynamic and kinetic effects underlying nanoparticle, cluster and nanowire formation

The in situ reduction of dissolved Ag+ or Au3+ to the zero valent state in the distillable ionic liquid DIMCARB is reported. The reduction process leads to both 1D and 3D nanostructure formation, i.e. nanoparticles and nanowires and clusters composed of nanoparticles. The nanostructures formed have been characterised using UV/vis spectroscopy, powder X-ray diffraction and atomic force microscopic (AFM) topographic and phase imaging. Ag nanostructure growth occurs as a solution based crystallisation process. However, real time AFM imaging of Ag nanostructures formed when poly(vinyl pyrrolidone) is present shows the growth of nanowires on a mica surface via a surface confined process involving Ag atom diffusion. The growth of Au nanowires also occurs via the surface diffusion process. Based on the experimental results, a nanostructure growth mechanism in DIMCARB is proposed and 1D or 3D nanostructure growth is related to either a thermodynamic or a kinetic pathway.

A critical assessment of electrochemistry in a distillable room temperature ionic liquid, DIMCARB

Ionic liquids are frequently advocated as green media for electrochemical studies. However, they are non-volatile and hence difficult to purify or recover. In this paper the electrochemical behaviour of a ‘distillable’ room temperature ionic liquid, DIMCARB, has been investigated. This ionic liquid is unusual because it is readily prepared, in large quantities and at low cost, by mixing of gaseous carbon dioxide with dimethylamine and also easily recovered by decomposition back into its gaseous components followed by reassociation. Almost ideal reversible voltammetry is observed for the Cc+/0 process (Cc = cobalticinium), which therefore is recommended for reference potential calibration. Another IUPAC recommended reference potential process, Fc+/0 (Fc = ferrocene), is only reversible at fast scan rates and occurs near the positive potential limit available. However, decamethylferrocene (DmFc) is reversibly oxidised and behaves ideally as for the reduction of Cc+. The small diffusion coefficients of 1.2 × 10−7 cm2 s−1 (Cc+) and 5.0 × 10−8 cm2 s−1 (DmFc) at 20 °C are attributed to the relatively high viscosity. The potential window of ca. −1.50 V to +0.50 V vs. SHE indicates that DIMCARB is more suitable for electrochemical studies of reductive rather than oxidative processes. Voltammetric studies in DIMCARB reveal a series of reversible reductive processes for the Keggin [α-SiW12O40]4− polyoxometallate. Comparison of reversible potential data reported in other media indicate that the polarity of DIMCARB is intermediate between that of MeCN and the conventional ionic liquid [BMIM][PF6]. The deposition of metallic Pb also has been studied and reveals that Pb(II) is reduced in a single irreversible 2-electron step to the metallic state via a nucleation/growth mechanism. Overall, these studies show that DIMCARB is highly suitable for electrochemical studies, but that it is a potentially reactive medium.

A Combined Scanning Electron Micrograph and Electrochemical Study of the Effect of Chemical Interaction on the Cyclability of Lithium Electrodes in an Ionic Liquid Electrolyte

The effect of storage time on the cyclability of lithium electrodes in an ionic liquid electrolyte, namely 0.5 m LiBF4 in N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide, [C3mpyr+][FSI–], was investigated. A chemical interaction was observed which is time dependent and results in a morphology change of the Li surface due to build up of passivation products over a 12-day period. The formation of this layer significantly impacts on the Li electrode resistance before cycling and the charging/discharging process for symmetrical Li|0.5 m LiBF4 in [C3mpyr+][FSI–]|Li coin cells. Indeed it was found that introducing a rest period between cycling, and thereby allowing the chemical interaction between the Li electrode and electrolyte to take place, also impacted on the charging/discharging process. For all Li surface treatments the electrode resistance decreased after cycling and was due to significant structural rearrangement of the surface layer. These results suggest that careful electrode pretreatment in a real battery system will be required before operation.

Reference Electrodes for Ionic Liquids and Molten Salts

Ionic liquids show promise as electrolytes for a host of electrochemical processes due to their favourable physical and electrochemical properties. However, use of conventional aqueous or non-aqueous reference electrodes with ionic liquids poses problems due to the existence of large junction potentials and possible contamination of the test solution.

Role of H+ in Polypyrrole and Poly(3,4-ethylenedioxythiophene) Formation Using FeCl36H2O in the Room Temperature Ionic Liquid, C4mpyrTFSI

The polymerisation reaction of pyrrole and 3,4-ethylenedioxythiophene using the chemical oxidant FeCl3·6H2O in the room temperature ionic liquid butyl-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (C4mpyrTFSI) has been investigated using cyclic voltammetry, UV/vis and IR spectroscopy. The voltammetric data for the Fe2+/3+ reaction is complicated by the presence of H+ introduced upon dissolution of the iron salt by deprotonation of the coordinated waters. The voltammetric and chemical reaction studies show that H+ itself, introduced to solution as trifluoromethanesulfonic acid (HTFSI), can act as the chemical oxidant for the polymerisation reaction. Voltammetric data also implies that in this system the Fe2+/3+ redox couple may not actually be involved in the polymerisation reaction and that the H+ introduced upon dissolution of the FeCl3·6H2O may be the sole cause of the oxidation reaction.