A Solid-State Single-Ion Conductor for Lithium Batteries Based on Anionic Nanoparticles of Fluorinated Titania

Nowadays, among the most challenging classes of materials for the development of high-energy density lithium batteries are electrolytes and high-capacity cathode materials [1]. A considerable interest is attracted by metallic lithium anode-based Li-S and Li-air batteries because of their high specific capacity [2]. Nevertheless, the use of a metallic lithium anode compromises the chemical and electrochemical stability of the electrolyte. To address these drawbacks, a significant research effort is recently devoted to the design and preparation of new lithium-conducting electrolytes. Here, a new concept of electrolyte based on a solid-state lithium single-ion conductor is proposed, which opens new perspectives in the development of advanced lithium batteries [3]. The electrolyte is obtained by direct reaction of nanometric fluorinated TiO₂ (FT) with molten metallic lithium, and consists of nanoparticles (NPs) with surface anionic groups neutralized by lithium cations. The resulting nanopowder is labelled LiFT^® [4]. It should be highlighted that after functionalization, the conductivity of fluorinated TiO₂ increases by more than four orders of magnitude. In this work, the structure of LiFT is elucidated by means of several techniques such as: Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES); High-Resolution Transmission Electron Microscopy (HR-TEM), powder X-Ray Diffraction (XRD), Infrared Spectroscopy (FT-MIR and -FIR), electrochemical measurements and solid-state Magic-Angle Spinning-NMR (MAS-NMR) spectroscopy. XRD and HR-TEM measurements show that LiFT has the “core” structure of anatase, with Li cations present only in the external lithium-rich shell of NPs. Thus, no intercalation processes of Li⁺ in “core” LiFT NPs take place. In addition, it is demonstrated that lithium cations can migrate through grain boundaries of electrolyte NPs in a very effective way, achieving: 1) a room-temperature single-ion conductivity of about 3x10^-4 Scm^-1; 2) an excellent electrochemical stability; and 3) a high exchange current density. All these features, together with an easy synthesis process, which is based on precursors obtained from very cheap starting materials, make LiFT a very attractive material for application in several fields such as: a) all-solid-state lithium batteries; b) molten electrode lithium batteries; c) nanocomposite electrolytes and d) lithium-air batteries.

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[2]      P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.-M. Tarascon, Li-O₂ and Li-S batteries with high energy storage., Nat. Mater. 11 (2012) 19–29.

[3]      F. Bertasi, K. Vezzù, E. Negro, S. Greenbaum, V. Di Noto, Single-ion-conducting nanocomposite polymer electrolytes based on PEG400 and anionic nanoparticles: Part 1. Synthesis, structure and properties, Int. J. Hydrogen Energy. (2013).

[4]      V. Di Noto, F. Bertasi, E. Negro, M. Piga, M. Bettiol, F. Bassetto, Solid-state electrolytes based on fluorine-doped oxides, PCT/IB2012/053542, 2013.