Thesis presented December 16, 2022

Abstract:
Energy storage is a major issue of our times. However, electrochemical devices and their current performances, particularly in terms of power, are far from following the growing needs, especially in the field of transportation industry. Ionic liquids (ILs) show remarkable properties: low vapor pressure, high ionic conductivity, high chemical, thermal and electrochemical stability. They meet key criteria for safe energy storage such as lithium batteries. However, the fluctuating nano-segregation observed in bulk ILs acts as transient energy barriers hampering the long-range diffusional processes and hence to the ionic conductivity.
We propose an original battery separator able to boost the transport properties of IL based electrolytes (IL + lithium salt) combining several effects:
- The nanometric confinement of the electrolyte within Carbon NanoTubes (CNTs) to frustrate the formation of the nano-structures observed in bulk.
- A one-dimensional (1D) ionic conduction pathway offered by vertically aligned CNT-based membranes. The interior of the CNTs are the pores (diameter 4 nm) of this polymer composite system.
In order to use this porous membrane as an “all-solid” battery separator, we graft a nanoscale layer of ion-conducting polymers based on IL onto the end of the CNT. This grafted layer electrically insulates the CNT from the electrodes. The CNT are then filled with LI-based electrolytes. We show a drastic increase in the ionic conductivity of these confined (1D) electrolytes in CNT membranes. We report a gain of a magnitude order compared to the bulk analogues. To understand the origin of these phenomena, we carry out a multiscale study of the dynamics of bulk and confined IL. This study is combining PFG-NMR (µm/ms) and neutron scattering (QENS, NSE, ps-ns/ Å-nm). At the molecular scale, the dynamic is activated at lower temperatures in confinement (from 10 to 20°C) than in bulk and is accompanied by a gain of a factor 2 to 3 in the long-range diffusion coefficient.
Results of simulations by molecular dynamics make it possible to attribute the gain in conductivity to an upheaval in the organization of the electrolyte under confinement. The electrolyte is organized in the axis of the CNT according to concentric cylindrical domains. In the area of low density, a preferential path for good conductivity of lithium ions is arranged. A battery separator such as this CNT membrane charged with IL and lithium salts is a promising “all-solid-state” battery.

Keywords:
Nanomaterials, battery, electrochemistry, spectroscopy, polymer

On-line thesis. ​