A Solid-State Single-Ion Conductor for Lithium Batteries Based on Anionic Nanoparticles of Fluorinated Titania
Authors: Di Noto Vito , Bertasi Federico , Negro Enrico , Vezzù Keti , Greenbaum Steve , Bassetto Fabio , Zeggio Stefano 
Published: February 2020

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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