LiZnSO4F made in an ionic liquid: a ceramic electrolyte composite for solid-state lithium batteries.

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 LiZnSO4F fluorosulfate phase having a sillimanite LiTiOPO4-type structure, which on simply pressed samples shows a room-temperature ionic conductivity of 10 – 10 7 Scm 1 together with a 0–5 V electrochemical stability window range, while ionic-liquid-free LiZnSO4F shows an ionic conductivity four orders of magnitude lower (10 11 Scm ). 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 today s energy-conscious society, hence placing rechargeable batteries as one of the major technological sciences in this new century. 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. This is the reason why LiFePO4, 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, LiFePO4/C composite overcomes transport limitations and is capable of reversibly and rapidly intercalating 0.9 Li (ca. 160 mA hg ) 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. Further exploring the chemistry of polyanionic-based insertion electrodes, we recently synthesized, by an ionothermal process, a novel 3.6 V LiFeSO4F electrode showing a reversible capacity nearing 140 mAh g 1 (theoretical capacity = 151 mAh g ), good rate capability, and cycling stability. This fluorosulfate was found to crystallize in a tavorite structure (space group P 1) with three-dimensional channels for Li diffusion as opposed to the one-dimensional channels in LiFePO4. Most likely, from the 3D versus 1D change in the conduction path, the use of LiFeSO4F 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 AMSO4F (A = Li, Na and M = Co, Ni, Mn, etc.) homologues. This new family of materials, practically unknown a year ago, counts no less than 20 members showing related structures with either promising electrochemical or attractive ionic properties. Among them, the sodium-based 3d-metal fluorosulfates, which crystallize in a titanite structure (derived from the tavorite structure, space group P21/c) and have localized positions for the Na ions, were found to show a four-fold increase in ionic conductivity as compared to their Li-based counterparts on cold-pressed powders (10 7 S cm 1 for Na vs. 10 11 Scm 1 for Li at room temperature). While far from the hallmark solid-state electrolytes for future Li batteries such as Li1.5Al0. 5Ge1.5(PO4)3 (LAG), Li1.3Al0. 3Ti1.7(PO4)3 (LAT), and Li3+xPO4 xNx (LIPON), which have room-temperature conductivities of 2.8 10 4 Scm , 10 3 S cm , and 10 6 S cm , respectively, such a finding was an impetus to look for further fluorosulfate members as part of the effort to develop new ceramic electrolyte materials with increased conductivity, thus allowing a switch from thin-film to bulk technology in all solid-state batteries. Besides high ionic conductivity, a pivotal figure of merit for solid-state electrolytes is the width of their electrochemical stability window. This window is limited for ionic conducting ceramics containing 3d-metal elements, such as Li1.3Al0. 3Ti1.7(PO4)3, owing to the reduction of Ti 4+ in Ti at approximately 2.4 V. So our strategy was to search for other members of the fluorosulfate AMSO4F family containing divalent metals that cannot be easily reduced or oxidized. Besides LiMgSO4F, the first reported fluorosulfate, [7] other attractive candidates could enlist lead, tin, or zinc to prepare AMSO4F phases. Mindful of the previously reported struc[*] Dr. P. Barpanda, Dr. J.-N. Chotard, Dr. C. Delacourt, M. Reynaud, Prof. M. Armand, Prof. J.-M. Tarascon Laboratoire de R activit et Chimie des Solides Universit de Picardie Jules Verne, CNRS UMR 6007 33, rue Saint Leu, 80039 Amiens (France) E-mail: jean-marie.tarascon@sc.u-picardie.fr Homepage: http://jmtarascon.tech.officelive.com