(Keynote) Lithiated Fluorinated “Core-Shell” Nanoparticles As Single-Ion-Conducting Electrolytes for Lithium Batteries

Abstract

The development of high-performing functional materials for application in secondary batteries is nowadays one of the most active fields in the international scientific research panorama [1]. The most used electrolytes in a lithium-ion battery are typically obtained by mixing organic carbonates (e.g., ethylene carbonate, EC, and/or dimethyl carbonate, DMC), with a lithium salt (e.g., lithium hexafluorophosphate, LiPF6) [2]. Despite the high conductivity of these electrolytes (>10-3 S·cm-1 at room temperature, RT) the main drawbacks are associated to: (i) the high volatility and flammability of liquid solvents; and (ii) the chemical instability of LiPF6 salt, which instantly hydrolyzes in the presence of water traces [3]. Recently, a ceramic material based on lithiated fluorinated-TiO2 (LiFT) was proposed acting both as a solid-state electrolyte and as a nanofiller to obtain nanocomposite electrolytes [4-6]. LiFT consists of fluorinated anatase nanoparticles, whose surface is functionalized with Li+ ions through an innovative one-step process involving metallic lithium [7]. The Li+ conductivity in LiFT is due to the hopping of lithium ions between coordination sites present at the grain boundaries between nanoparticles. When used as a nanofiller, LiFT nanoparticles act both as the source of Li+ and as a plasticizing agent of the solvent (e.g., polymer or ionic liquid). Accordingly, LiFT allowed us to devise innovative composite electrolytes for application in lithium batteries with a high thermal stability and conductivity. In this work, we present our recent studies on three different applications of LiFT nanoparticles in lithium secondary batteries: (i) pristine ceramic solid-state electrolyte; (ii) nanofiller and source of Li+ in a family of single-ion conducting polymer electrolytes (Sn-CPEs); and (iii) nanofiller and source of Li+ in a family of ionic liquid (IL)-based electrolytes for lithium conduction. In particular, polymer-based electrolytes are obtained by dispersing different amounts of LiFT in a polyethylene glycol 400 (PEG400) matrix. Instead, in IL-based electrolytes LiFT is dispersed in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI) ILs. The content of LiFT nanofiller in the electrolytes varies from 0 to 86.3 wt% in PEG400-based materials, and it is equal to ca. 70 wt% in the IL-based systems. For all the samples, the atomic composition is obtained from ICP-AES and CHNS elemental analyses. The correlation between structure, thermal properties and conductivity mechanism of the resulting electrolytes is elucidated by a variety of techniques: (i) FT-MIR and -FIR vibrational spectroscopies; (ii) differential scanning calorimetry (DSC); (iii) thermogravimetric analysis (TGA); and (iv) broadband electrical spectroscopy (BES). All these techniques reveal how the different solvents (i.e., PEG400 and EMImX, where X = BF4 - and TFSI-) are able to dissociate and coordinate the Li+ species present on the surface of the LiFT nanoparticles. The highest RT conductivities achieved by the electrolytes presented in this work are 1.1∙10-5, 2.8∙10-4, and 4.1∙10-2 S∙cm-1 for the PEG-based, pristine ceramic solid-state LiFT, and IL-based systems, respectively. The differences and the analogies between the three different families of electrolytes are extensively elucidated. Finally, the most promising electrolytes go through electrochemical studies (e.g., by means of cyclic voltammetry, CV) and are adopted in the fabrication of battery prototypes, that undergo charge/discharge cycles in operative conditions at different C rates (see Figure 1). Figure 1