High capacity materials for next generation Li-ion batteries

Thesis

With the staggering growth in the recent years of the Lithium Ion Batteries market and the large number of different applications, in which these devices are currently employed, a change of paradigm in terms of the active materials composing the electrodes in these batteries is required. To meet the market needs in terms of performance (energy density and power), safety and respect of the environment, non-toxic light materials with high capacities are highly sought after.

The main goal of this work was to provide an insight into the mechanisms which limit the application of the Li-Rich based cathodes and hard carbon and silicon anodes. These high capacity materials would be ideal candidates to combine into a full cell for next generation batteries with enhanced energy densities.

The mechanisms underpinning the electrochemical behaviour of a family of cobalt-free lithium-rich layered oxide cathodes, with general formula Li_(4/3-2/3x)NixMn_(2/3-1/3x)O₂ (0 ≤ x ≤ 0.3), has been unravelled. Initially, Li_1.2Ni_0.2Mn_0.6O₂ was synthesized via a one-pot synthetic route and studied to understand the origin of the high potential plateau appearing over the first charge. Thanks to a careful structural and electrochemical characterization and powerful spectroscopic techniques (XANES, SXAS, RIXS) the presence of reversible anionic redox activity has been demonstrated. Subsequently the focus of the work has moved on to unravelling the role of the nickel substitution in favouring the oxygen redox activity over the oxygen loss phenomenon for this family of materials.

In parallel, a study on the improvement of the cyclability and efficiency of high capacity silicon-based anodes has been carried out using new polymeric binders, some of which, have been shown to guarantee more stable cycling and higher capacities with respect to the established binder sodium carboxymethylcellulose (Na-CMC). Additionally, in the attempt to address the issue concerning the First cycle Irreversible Capacity (FIC) of the silicon anodes, a highly reducing pre-treatment has been performed on the electrodes. As well, the treatment has been tested on hard carbon anodes that, despite being promising materials for Lithium and also Sodium Ion Batteries, are usually plagued by large FIC. An important improvement in terms of coulombic efficiency at the beginning of cycling has been observed in all the tested materials without any sign of detrimental effects on the electrodes overall performance.

Authors: Guerrini, N

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English