Anthony F. Hollenkamp

High Lithium Metal Cycling Efficiency in a Room-Temperature Ionic Liquid

A room-temperature ionic liquid (RTIL) solvent, N-methyl, N-alkyl pyrrolidinium bis(trifluoromethanesulfonyl)amide (P 1 X (Tf) 2 N), has been investigated for use in a lithium metal battery. An average cycling efficiency of>99% is achieved at 1.0 mA cm - 2 (I d e p = I d i s s ) on platinum. At deposition rates up to 1.75 mA cm - 2 optical micrographs indicate that the deposit is uniform and nondendritic. Above 1.75 mA cm - 2 , the deposit becomes dendritic and efficiency decays. High cycling efficiency (>99%) can also be obtained on copper, but at relatively low current density (0.1 mA cm - 2 ). The deposition/cycling history also influences the cycling behavior of the deposit.

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.

Premature capacity loss in lead/acid batteries : a discussion of the antimony-free effect and related phenomena

Abstract Instances of severe capacity loss in apparently healthy lead/acid batteries have been reported over a period of many years, and are still common today. In most cases, these phenomena are linked to the use of antimony-free positive grids and are invoked by repetitive deep-discharge duties. This situation represents probably the greatest barrier to the expansion of markets for lead/acid batteries. To date, research has focused on several possible explanations for capacity loss; notably, degradation of the positive active mass (e.g., relaxable insufficient mass utilization) and the development of electrical barriers around the grid. Although much of the evidence gathered is circumstantial, it does point to the key issues that must be addressed in future work.

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.

Suppressed Polysulfide Crossover in Li–S Batteries through a High-Flux Graphene Oxide Membrane Supported on a Sulfur Cathode

Utilization of permselective membranes holds tremendous promise for retention of the electrode-active material in electrochemical devices that suffer from electrode instability issues. In a rechargeable Li–S battery—a strong contender to outperform the Li-ion technology—migration of lithium polysulfides from the sulfur cathode has been linked to rapid capacity fading and lower Coulombic efficiency. However, the current approaches for configuring Li–S cells with permselective membranes suffer from large ohmic polarization, resulting in low capacity and poor rate capability. To overcome these issues, we report the facile fabrication of a high-flux graphene oxide membrane directly onto the sulfur cathode by shear alignment of discotic nematic liquid crystals of graphene oxide (GO). In conjunction with a carbon-coated separator, the highly ordered structure of the thin (∼0.75 μm) membrane and its inherent surface charge retain a majority of the polysulfides, enabling the cells to deliver very high initial dis...

Compatibility of Li x Ti y Mn1 − y O2 ( y = 0 , 0.11 ) Electrode Materials with Pyrrolidinium-Based Ionic Liquid Electrolyte Systems

The possibility of using electrolyte systems based on room-temperature ionic liquids (RTILs) in lithium-battery configurations is discussed. The nonflammability and wide potential windows of RTIL-based systems are attractive potential advantages, which may ultimately lead to the development of safer, higher energy density devices than those that are currently available. An evaluation of the compatibility of these electrolyte systems with candidate electrodes is critical for further progress. A comparison of the electrochemical behavior of Li/RTIL/Li x MnO 2 and Li x Ti 0.11 Mn 0.89 O 2 cells with those containing conventional carbonate solutions is presented and discussed in terms of the physical properties of two RTIL systems and their interactions with the cathodes. Strategies to improve performance and minimize cathode dissolution are presented.

When is capacity loss in lead/acid batteries ‘premature’?

Abstract Elucidation of the principal mechanism that underlies premature capacity loss (PCL) in lead/acid positive plates has always been hampered by the notion that different forms of PCL are responsible for severe and mild instances of capacity loss. Recently, though, studies focused on the conductivity of the porous mass have provided a clear, universal explanation for all examples of PCL. The evidence required to link the differing views has come from charge/discharge cycling of specially designed plates in which expansion of positive material can be restricted in a controlled fashion. In particular, two findings have bridged the gap between failure at the interface (PCL-1) and failure in the bulk material (PCL-2): (i) plates subjected to extreme conditions of service can cycle at constant capacity for long periods, despite the presence of ‘barrier-layers’; (ii) loss of conductivity in the porous material close to the current-collector can explain severe and rapid capacity loss. On examination, the latter situation is characterized by a localization of lead sulfate in the region close to the current-collector, in line with previous reports of ‘preferential discharge’. The capacity loss for any plate/cell configuration can now be placed on a continuous scale — the rate of loss is determined by the degree to which the configuration, and conditions of service, are able to control the decrease in conductivity of positive material close to the current-collector. Development of positive plates for advanced lead/acid batteries must consider strategies for maintaining conductivity through management of the combined effects of expansion and redistribution of positive material.

The effect of tin on the performance of positive plates in lead/acid batteries

Abstract There are many reports that the use of non- or low-antimonial grids in lead/acid batteries can give rise to the development of a high-impedance ‘passivation’ layer at the grid/active-material interface. It is generally agreed that the layer has a duplex structure that comprises α-PbO deposited directly on the grid surface beneath a compact covering of PbSO 4 ; basic sulfates and α-PbO 2 may also be present. The development of this structure hinders recovery from prolonged deep discharge or self-discharge. Similar phenomena can be observed with dry-charged positive plates. Passivation can also occur during cycling and float operations but, in these duties, the formation of a non-conductive layer of PbSO 4 is thought to be the prime cause of the degradation in plate performance. The incorporation of tin in the positive grid (either in the alloy itself or as a surface layer) is found to reduce the level of α-PbO and greatly alleviate passivation problems relating to charge acceptance. Various mechanisms have been proposed for this tin effect and range from a semiconductor-type doping of α-PbO to changes in the porosity of the PbSO 4 layer and/or the reactivity of α-PbO towards oxidation. The benefits of tin in cycling and float duties are less obvious and it is probable that other features of cell design are more important determinants of battery performance. The action of tin when incorporated in the positive active material requires further exploration.