LiZnSO₄F Made in an Ionic Liquid: A Ceramic Electrolyte Composite for Solid‐State Lithium Batteries

Abstract

The search for good solid electrolytes constitutes a major goal towards the development of safer lithium batteries. A few candidates do exist, but they suffer either from narrow electrochemical window stability or too low ionic conductivity. Herein we report the ionic-liquid-assisted synthesis of a novel LiZnSO 4 F fluorosulfate phase having a sillimanite LiTiOPO 4 -type structure, which on simply pressed samples shows a room-temperature ionic conductivity of 10 À5 -10 À7 S cm À1 together with a 0-5 V electrochemical stability window range, while ionic-liquid-free LiZnSO 4 F shows an ionic conductivity four orders of magnitude lower (10 À11 S cm À1 ). While robustly reproducible but not yet fully understood, this finding offers new opportunities to tailor inorganic composites with higher ionic conductivity. The origin of such results is demonstrated to be rooted in a surface effect associated with the grafting of a lithium-containing ionic liquid layer. This finding opens up new opportunities for the design of ceramic composites with higher ionic conductivity and should serve as an impetus for further exploiting the chemistry of ionic liquid grafting on oxides.Renewable energy sources and electric automotive transportation are popular topics in todays energy-conscious society, hence placing rechargeable batteries as one of the major technological sciences in this new century.[1] Advances in energy storage are a tribute to chemists abilities to design new and better materials. In the hunt for novel electrode materials, notions of sustainability must be considered.[2] This is the reason why LiFePO 4 , which is made of inexpensive and abundant chemical elements, has attracted the attention of the research community despite its poor conducting properties. By particle downsizing and carbon nanocoating, LiFePO 4 /C composite overcomes transport limitations and is capable of reversibly and rapidly intercalating 0.9 Li (ca. 160 mA h g À1 ) at a redox voltage of 3.43 V versus Li. Thus, it has become one of the most praised electrode materials for the next generation of rechargeable batteries for high-volume applications. [3] Further exploring the chemistry of polyanionic-based insertion electrodes, we recently synthesized, by an ionothermal process, a novel 3.6 V LiFeSO 4 F electrode showing a reversible capacity nearing 140 mA h g À1 (theoretical capacity = 151 mA h g À1 ), good rate capability, and cycling stability.[4] This fluorosulfate was found to crystallize in a tavorite structure (space group P " 1 1) with three-dimensional channels for Li diffusion as opposed to the one-dimensional channels in LiFePO 4 . Most likely, from the 3D versus 1D change in the conduction path, the use of LiFeSO 4 F powders will obviate the need for nanosizing or carbon coating, while the same cost and environmental advantages are maintained.Since our early report, we have considerably enlarged the fluorosulfate family with the discovery of AMSO 4 F (A = Li, Na and M = Co, Ni, Mn, etc.) homologues. [5] This new family of materials, pract...

Enabling the Li-ion conductivity of Li-metal fluorosulphates by ionic liquid grafting

“…A comparison between the high resolution 1 H (e) and 19 F (f) NMR spectra of LiZnSO 4 F-IL and LiZnSO 4 F-C clearly shows the presence of EMI and TFSI molecules in the samples (e, f), together with some residual water, either in LiZnSO 4 F-C ( ≈ 6.5 ppm) or in the grafted ionic liquid layer for LiZnSO 4 F-IL ( ≈ 1.8 ppm) (e). Some LiF is also detected by 19 F NMR (b, f) lower ionic conductivity for the sillimanite LiZnSO 4 F than the tavorite LiMSO 4 F phases, the opposite of what we observed [7,8,11,16]. So, solely from structural perspective, the high value of conductivity is difficult to explain.…”

“…However, the high conductivity value in the case of LiZnSO 4 F-IL was a surprise, which can arise either due to structure or surface modification. LiZnSO 4 F stabilizes into a different structure (orthorhombic, s.g. Pnma [11]) than other LiMSO 4 F compounds (triclinic, s.g. P-1), which leads to a one-dimensional diffusion of Li for LiZnSO 4 F as compared to a three-dimensional one for LiMSO 4 F phases. It leads to the expectation of having a /cooling (a, c), seemingly indicating no change in the bulk LiZnSO 4 F lithium mobility.…”

“…We zeroed in to synthesize LiZnSO 4 F as its precursor ZnSO 4 ·H 2 O is isostructural to other MSO 4 ·H 2 O precursors and Zn is electrochemically inactive (hence, stable). Using a two-step low temperature synthesis method (ionic liquid or solid-state synthesis) [8], we successfully produced sillimanite-structured LiZnSO 4 F. Performing conductivity measurement, we accidentally discovered that the involvement of ionic liquids during synthesis led to six orders of improvement in effective ionic conductivity of LiZnSO 4 F [11]. Combining several techniques such as solid-state Nuclear Magnetic Resonance (SS-NMR), X-ray Photoelectron Spectroscopy (XPS) and AC impedance spectroscopy, here we report a detailed spectroscopy analysis of ionic liquid-prepared LiZnSO 4 F samples.…”

Lithium metal borate (LiMBO3) family of insertion materials for Li-ion batteries: a sneak peak

“…The polyanionic systems form a treasure box for chemist to unravel new materials with rich structural diversity and formidable electrochemical properties. The role of 3d transition metals is striking in forming diversified structures and polymorphs depending upon different synthesis conditions [12][13][14]. Add to this, the polyanionic frameworks have been shown to deliver the highest redox potential for Fe 3+ /Fe 2+ (ca.…”

Comparative Study and Electrochemical Properties of LiFePO4F Synthesized by Different Routes

“…Therefore, this type of materials becomes a candidate of the new-generation cathode material for Li + batteries. [1][2][3] Recently, it was reported that LiZnSO 4 F with a 3D structure framework was adequate for a ceramic electrolyte, 4 and the ferrous homologue LiFeSO 4 F and NaFeSO 4 F were reported as intriguing electrode materials 2,5 with obvious advantages such as low toxicity and high Li + diffusivity. The analogues based on PO 4 F 4− polyanion (e.g.…”