Safe Li-Ion Batteries Built upon Porous TiO2 Nanostructures and Ionic Liquid Electrolytes

Since Sony commercialized the first Li-ion battery (LIB) in 1992, this technology has steadily grown, and nowadays LIBs are the state of art of commercial rechargeable batteries. In fact, billions of cells are building by year to power portable electronic devices and more recently electric cars (EV and PHEVs). Besides LIBs are very promising candidates to meet the demands of electrochemical storage for renewable energies sources. Widely publicized hazardous incidents and recalls of LIBs have raised legitimate concerns regarding Li-ion battery safety, especially for the large-size LIBs. Most of the commercial LIBs use carbonaceous materials as anodes. Regarding safety, dangerous Li-dendrite grown during overcharge has been reported in carbonaceous-based anodes. Strategies to improve LIB safety are based on the use of anode materials with higher redox potentials than graphite [1] , and replacement of the now used liquid electrolytes with other more safety liquids. Current liquid electrolytes raise safety concerns. These are usually based on flammable alkyl carbonates that can ignite or even explode. Electrolyte chemistry is thus an active area of research in issues related to LIB safety.

TiO₂ with higher redox potential (ca. 1.7V) than graphite, avoids metallic Li-deposition and notably decreases the electrolyte decomposition [2]. Besides, TiO₂ is environmentally friendly, abundant and inexpensive. The main drawback of TiO₂-based anodes is the low rate capability resulting from their poor electrical conductivity (ca. 10^‑10 Scm^-1 for anatase) and slow Li-diffusion. Nanosized TiO₂samples are thus essential to reach better performances [3]. Specifically, porous nanostructured electrode materials can be considered as ideal components of electrochemical devices because they combine nanoscale properties with good accessibility, high number of active sites, short diffusion distances and good processability.

In this work, we pay particular attention in the electrochemical characterization of porous TiO₂ anatase nanostructures based on (i) mesocrystal  and (ii) 3D inverse opal arrangements. By carefully controlling experimental conditions we are able to reach performances that are in the line of the best reported for TiO₂ based-anodes. Furthermore, we use these refined nanostructures as working electrodes in half-cells that use Room Temperature Ionic Liquids (RTILs) as electrolyte. RTILs exhibit negligible vapour pressure and lack the risks of the conventional electrolytes use in LIB batteries. IWe also report the electrochemical results of half-cells built upon the more suitable TiO₂ samples and RTIL-based electrolytes (liquid and polymeric gel).

Acknowledgements

The research has been supported by the projects MAT2011-22969 and MATERYENER P2009/PPQ-1626 (CAM).

References:

[1] A. Mahmoud, J.M. Amarilla et al. Electrochim. Acta, 93 (2013) 163

[2] G-N. Zhu, Y-G. Wang, Y-Y. Xia, Energy Environ. Sci. 5 (2012) 6652

[3] P. Tartaj, J.M. Amarilla, Advanced Materials, 23 (2011) 4904