No abstract

Lithium metal has the highest volumetric and gravimetric energy density of all negative-electrode materials when used as an electrode material in a lithium rechargeable battery. However, the formation of lithium dendrites and/or 'moss' on the metal electrode surface can lead to short circuits following several electrochemical charge-discharge cycles, particularly at high rates, rendering this class of batteries potentially unsafe and unusable owing to the risk of fire and explosion. Many recent investigations have focused on the development of methods to prevent moss/dendrite formation. In parallel, it is important to quantify Li-moss formation, to identify the conditions under which it forms. Although optical and electron microscopy can visually monitor the morphology of the lithium-electrode surface and hence the moss formation, such methods are not well suited for quantitative studies. Here we report the use of in situ NMR spectroscopy, to provide time-resolved, quantitative information about the nature of the metallic lithium deposited on lithium-metal electrodes.

Because of their potentially superior safety characteristics, room temperature ionic liquids (RTILs or ILs) have been vigorously researched as a potential replacement for current commercial lithium battery electrolytes, which are based on volatile and flammable organic carbonates. However, relatively poor battery performance, which is a consequence of the higher viscosity and lower conductivity of these materials, has prevented them becoming mainstream electrolytes for commercial lithium batteries. Amongst various RTILs, those containing the bis(fluorosulfonyl)imide (FSI) anion exhibit high conductivities and diffusivities, making them interesting potential electrolytes for lithium metal batteries. Here, we evaluate the electrochemical stability, lithium electrochemistry, and Li + transference numbers of FSI-based ionic liquid electrolytes intended for use in rechargeable Li metal batteries. We show that ILs containing high concentrations of lithium, up to 3.2 mol.kg −1 in C 3 mpyr FSI, have excellent rate capability (higher than that of standard battery electrolytes) with both the lithium metal electrode and LiCoO 2 cathode, in spite of their significantly higher viscosities and lower conductivities. This unusual behavior is ascribed to the concomitant increase in transference number with increasing Li-salt concentration.

Whether lithium–sulfur batteries succeed hinges largely on taming sulfur's complex electrochemistry, over which, choice of electrolyte exerts an equally complex influence.

Chemical reaction studies of N-methyl-N-propyl-pyrrolidinium-bis(fluorosulfonyl)imide-based ionic liquid with the lithium metal surface were performed using ab initio molecular dynamics (aMD) simulations and X-ray Photoelectron Spectroscopy (XPS). The molecular dynamics simulations showed rapid and spontaneous decomposition of the ionic liquid anion, with subsequent formation of long-lived species such as lithium fluoride. The simulations also revealed the cation to retain its structure by generally moving away from the lithium surface. The XPS experiments showed evidence of decomposition of the anion, consistent with the aMD simulations and also of cation decomposition and it is envisaged that this is due to the longer time scale for the XPS experiment compared to the time scale of the aMD simulation. Overall experimental results confirm the majority of species suggested by the simulation. The rapid chemical decomposition of the ionic liquid was shown to form a solid electrolyte interphase composed of the breakdown products of the ionic liquid components in the absence of an applied voltage.

Ionic liquids (ILs) are widely studied as a safer alternative electrolyte for lithium‐ion batteries. The properties of IL electrolytes compared to conventional electrolytes make them more thermally stable, but they also have poor wetting with commercial separators. In a lithium‐ion battery, the electrolyte should completely wet out the separator and electrodes to reduce the cell internal resistance. Investigations of cell materials with IL electrolytes have shown that the wetting issues in IL–electrolyte cells are most likely due to poor separator compatibility, not electrode compatibility. A compatible separator must be developed before IL electrolytes can be used in commercial lithium‐ion batteries. Herein, separators for IL electrolytes, including commercial and novel separators, are reviewed. Separators with different processing methods, polymers, additives, and different IL electrolytes are considered. Collated, the separator studies show a strong correlation between ionic conductivity and membrane porosity, even more than the electrolyte type. The challenge of a suitable separator for IL electrolytes is not solved yet. Herein, it is revealed that a separator for IL electrolytes will most likely require a combination of high thermal and mechanical stability polymer, ceramic additives, and an optimized manufacturing process.

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.

Room temperature ionic liquids ͑RTILs͒ with the bis͑fluorosulfonyl͒imide ͑FSI͒ anion exhibit higher conductivities than the corresponding bis͑trifluoromethanesulfonyl͒imide ͑TFSI͒ 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 ͑CV͒ 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.Increasing consumer demand for power and energy density is driving the development of the next generation of lithium-ion batteries. 1 To date, however, these developments have been incremental, with the active materials employed in today's batteries being little different from those in Sony's first release back in 1991. It can be argued that this lack of progress is largely due to a failure to develop improved electrolytes. New "high voltage" cathode materials have been described regularly over the years; yet any benefits are seldom realized due to the limited electrochemical stability of organic carbonate-based electrolytes. 2 These systems are also compromised by poor thermal stability, appreciable volatility ͑and flammability͒, and significant toxicity.Certain members of the vast family of compounds known as room temperature ionic liquids ͑RTILs͒ have properties that address many of the concerns with classic organic electrolytes. 3 Holzapfel et al. were one of the first groups to report sustained cycling of a lithium-ion battery with an ionic-liquid electrolyte medium. 4 They used 1-ethyl-3-methyl-imidazolium bis͑trifluoromethanesulfo-nyl͒imide ͑C 2 mimTFSI͒ and LiTFSI doped with vinylidene carbonate ͑VC͒ in an effort to form a stable solid electrolyte interphase ͑SEI͒ at the graphite anode. 4 While these cells displayed a reasonably constant discharge capacity, VC is progressively consumed during long-term charge-discharge cycling, 5 with the result that intercalation of the imidazolium cation into the gra...