Ionogels for Safer Energy Storage: The Determining Effect of the Interface

Ionogels represent a route to biphasic materials, for the use of ionic liquids (ILs) for all-solid devices. Confining ILs within host networks enhances their averaged dynamics, resulting in improved charge transport. Fragility, short relaxation times, low viscosity, and good ionic conductivity, all them appear to be related to the IL / host network interface. The presence of ILs at interface neighborhood leads to the breakdown of aggregated, structured regions that are systematically found in bulk ILs. This “destructuration”,[1] as well as segregative interactions at interface,[2,3] coupled with percolation of the bicontinuous solid/liquid interface,[4] make these materials very competitive among the existing solid electrolytes. Such approach could provide (i) a route to lower locally the viscosity of ILs, and (ii) an easier pathway for diffusion of charged species. Several types of ionogels demonstrate this effect, taking into account of fully inorganic, hybrid, polymeric or organic-inorganic host networks . This “all-solid” approach can be applied to several electrochemical energy storage sources, including lithium batteries (Fig 1)[3,5] and supercapacitors (Fig. 2)[6,7]. Strikingly, high performance were shown on these devices, thanks to interfacial effects of confined ILs, with sometimes heightened properties of the chosen ILs as referred to their bulk properties[3,4,7,8]. Such solid electrolytes are particularly well suited for microdevices that we have been or are being developed (Fig. 3)[6,9]. Herein we will emphasize the results of a systematic study of the effect of size of confinement. References [1] “Destructuring ionic liquids in ionogels: enhanced ionic liquid properties for solid devices" A. Guyomard-Lack, P.-E. Delannoy et al., Phys. Chem. Chem. Phys., 16 (2014) 23639-23645. [2] “Ion segregation in an ionic liquid confined within chitosan based chemical ionogels” A. Guyomard-Lack et al., Phys. Chem. Chem. Phys., 17 (2015) 23947-23951 [3] “Enhancement of Lithium Transport by Controlling the Mesoporosity of Silica Monoliths filled by Ionic Liquids “ A. Guyomard-Lack et al., New J. Chem., 40 (2016) 4269-4276. [4] “Biopolymer-silica interpenetrated networks based ionogel: dynamics of confined imidazolium cation in presence of its lithium salt“ C. V. Cerclier et al., Phys. Chem. Chem. Phys., 17 (2015) 29707—29713 [5] “Interfacial stability and electrochemical behavior of Li/LiFePO4 microbatteries using novel soft adhesive photo-ionogels electrolytes“ D. Aidoud et al., J. Power Sources, 330 (2016) 92-103 [6] “Solder-Reflow Resistant Solid-State Micro-Supercapacitor“ M. Brachet et al., J. Mater. Chem. A, 4 (2016) 11835-11843. [7] “Silicon Nanowires and Nanotrees: Elaboration and Optimization of New 3D Architectures for High Performance On-chip Supercapacitors.” D. Gaboriau et al., RSC Adv., 6 (2016) 81017-81027 [8] “Ionic Liquids Confined in Silica Ionogels: Structural, Thermal and Dynamical Behaviors“ S. Mitra et al., Entropy, 19 (2017) 140 DOI: 10.3390/e190401408. [9] "MnO2 Thin Films on 3D Scaffold: Microsupercapacitor Electrodes Competing with “Bulk” Carbon Electrodes", E. Eustache et al., Adv. Energy Mater., 5 (2015) 1500680 Acknowledgments: The authors thank the RS2E for B.A. fellowship. Figure 1