Adam S. Best

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.

Lithium electrochemistry and cycling behaviour of ionic liquids using cyano based anions

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.

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.

Transport Properties and Phase Behaviour in Binary and Ternary Ionic Liquid Electrolyte Systems of Interest in Lithium Batteries

A binary ionic liquid (IL) system based on a common cation, N-methyl-N-propylpyrrolidinium (C(3) mpyr(+)), and either bis(trifluoromethanesulfonyl)imide (NTf(2) (-)) or bis(fluorosulfonyl) imide (FSI(-) as the anion is explored over its entire composition range. Phase behavior, determined by DSC, shows the presence of a eutectic temperature at 247 K and composition around an anion ratio of 2:1 (FSI(-) :NTf(2)(-)) with the phase diagram for this system proposed (under the thermal conditions used). Importantly for electrochemical devices, the single phase melting transition at the eutectic is well below ambient temperatures (247 K). To investigate the effect of such anion mixing on the lithium ion speciation, conductivity and PFG-NMR diffusion measurements were performed in both the binary IL system as well as the Li-NTf(2) -containing ternary system. The addition of the lithium salt to the mixed IL system resulted in a decrease in conductivity, as is commonly observed in the single-component IL systems. For a fixed lithium salt composition, both conductivity and ion diffusion have linear behaviour as a function of the anion ratio, however, the rate of change of the diffusion coefficient seems greater in the presence of lithium. From the application point of view, the addition of the FSI(-) to the NTf(2)(-) IL results in a considerable increase in lithium ion diffusivity at room temperature and no evidence of additional complex ion behaviour.

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.

Structure of aluminum fluoride coated Li[Li1/9Ni1/3Mn5/9]O2 cathodes for secondary lithium-ion batteries

The structural properties of layered Li[Li1/9Ni1/3Mn5/9]O2 positive electrodes nominally coated with aluminum fluoride are studied. Coatings were prepared by using aqueous solutions with various concentrations of aluminum and fluorine and are compared with samples treated under similar conditions but with aqueous HCl solutions. Samples were investigated following heat treatment at 120 °C and 400 °C with powder X-ray diffraction, transmission electron microscopy including energy dispersive X-ray spectroscopy (TEM/EDS), elemental analysis via inductively coupled plasma-optical emission spectroscopy (ICP-EA), and both 6Li and 27Al magic angle spinning NMR spectroscopy. The TEM/EDS and 27Al NMR data provide support for an aluminum-rich amorphous coating that, following drying at 120 °C, comprises six coordinated, partially hydrated aluminum environments. Heat treatment at 400 °C results in a phase that resembles partially fluorinated γ- or γ′-Al2O3, at least locally. An Al : F ratio of 2 : 1 is obtained in stark contrast to the ratio used in the original solution (1 : 3). No AlF3 is detected by PXRD and instead some evidence for a protonated phase (formed by ion exchanging protons for lithium) is detected along with Li[Li1/9Ni1/3Mn5/9]O2 after drying. This phase disappears on heating to 400 °C, suggesting some reorganization of bulk Li[Li1/9Ni1/3Mn5/9]O2 and possibly some incorporation of Al into the structure. This is in agreement with the 6Li NMR spectra, which indicate that the local environments that are found in the Ni-free end member of the series Li[Li(1/3−2x/3)NixMn(2/3−x/3)]O2 (i.e. Li2MnO3) are enhanced on sintering.

Towards elucidating microscopic structural changes in Li-ion conductors Li1+yTi2–yAly[PO4 ]3 and Li1+yTi2–yAly[PO4 ]3–x [MO4 ]x(M=V and Nb): X-ray and27Al and31P NMR studies

A combination of X-ray powder diffraction (XRD) and nuclear magnetic resonance (NMR) studies has demonstrated that attempted substitutions of Al, V and Nb into the framework of LiTi2(PO4)3 yield several impurity phases in addition to direct substitutions of Al into Ti and V, Nb into P sites. Direct substitutions were confirmed by changes in the unit cell dimensions as indicated by the peak shifts observed in the X-ray diffractographs and by analyses of the 27Al and 31P magic angle spinning (MAS) spectra. A major impurity phase was identified as AlPO4 (found in at least two polymorphs) and the amount present increases with increasing Al additions. The formation of AlPO4 appeared to be enhanced by further V but suppressed by Nb substitution. These results suggest that the presence of AlPO4 , together with the non-stoichiometric modified LTP, may be the cause for the observed densification of this material upon sintering and the increased ionic conductivity.

Physical properties of high Li-ion content N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide based ionic liquid electrolytes

Electrolytes based on bis(fluorosulfonyl)imide (FSI) with a range of LiFSI salt concentrations were characterized using physical property measurements, as well as NMR, FT-IR and Raman spectroscopy. Different from the behavior at lower concentrations, the FSI electrolyte containing 1 : 1 salt to IL mole ratio showed less deviation from the KCl line in the Walden plot, suggesting greater ionic dissociation. Diffusion measurements show higher mobility of lithium ions compared to the other ions, which suggests that the partial conductivity of Li(+) is higher at this higher composition. Changes in the FT-IR and Raman peaks indicate that the cis-FSI conformation is preferred with increasing Li salt concentration.