(Invited) Understanding the Dynamics of Confined Species in Electrochemical Energy Storage Materials

Abstract

Layered structures have played an important role in the development of electrochemical energy storage, for the relative ease of ion motion through two-dimensional diffusion channels, and for the stability of the host structure during battery operation. There have been several recent examples in which the performance of ion conducting materials has been improved through the presence of confined species between the layers, potentially through increasing the stability of the layers or enhancing the diffusion of ions. This may be observed for both electrode and electrolyte materials and is exemplified by the recent alternative to ionic liquid (IL) electrolytes, called conductive “solid-like” electrolytes, or the quasi-solid liquid electrolyte (QSLE). In the QSLE based upon boron nitride, ionic liquid electrolyte is confined within the BN layers. The electrolyte is intended to take the higher conductivity advantage of liquid electrolytes and combine this with the superior mechanical stability of BN based solid electrolytes. This hybrid solid electrolyte is derived from few layer nanoporous BN powders; the powders exhibit high surface area (> 800 m2/g) with layers and micropores (< 2 nm) for the infiltration of IL. X-ray diffraction indicates that the broad peak associated with the inter-layer separation shifts to longer d-spacing with absorption and confinement of IL. These infiltrated powders are then pressed to form a mechanically stable electrolyte, with high ionic conductivity. Such electrolytes can show robust stability against dendrite formation during battery cycling, which is a key issue in battery safety. While the properties look promising, there has been little fundamental effort to understand the nature of conduction in these solid-like electrolytes. The dynamics of the IL are expected to play a role in the diffusive motion of Na+/Li+ cations dissolved within the hybrid electrolyte. Prior work on confinement of ionic liquids using quasi-elastic neutron scattering (QENS), such as in the pores of a mesoporous carbon, have shown that the dynamics of the IL may not follow expected trends. For example, the translational motion of the IL may increase with confinement, and then slow down as the temperature is increased. Here we will present the effect of confinement on the dynamics of ionic liquid in 11BN layers using the BASIS spectrometer, and discuss this in relation to other confined electrochemical energy storage systems.