Graphene-modified LiFePO4 cathodes for advanced Li-/Na-ion secondary batteries

Nowadays, the ways our modern society faces challenges related to climate change, air pollution and energy production represent key concerns worldwide. The global energy scenario confirms a large growth in the implementation of renewable sources, particularly solar and wind-based technologies, which, nonetheless, require a proper storage system to overcome the difficulties given by their intermittent nature. Lithium-ion batteries (LIBs) are the principal electrochemical energy storage technology for the smart portable electronic market and the choice of reference for powering the newborn electrical automotive sector because they combine high energy and power density with lightweight and design compactness, as well as good cycling stability. Recently, research over materials for electrochemical storage systems opened towards a new path, which combines the striking features of graphene in terms of ionic/electronic conductivity and mechanical stability with the deep knowledge on insertion compounds that can host Li+ ions, responsible for the electrochemical process between the two battery electrodes. So far, much attention has been focused on the materials for the negative electrode and several techniques have been developed to form graphene/insertion compounds hybrids, which exploit the appealing properties of the two materials. However, the cathode is particularly critical in determining the capacity of a Li-ion cell and the enhancement of its characteristics can trigger huge advances in the overall performance of the battery. In this context, this Thesis work focuses on the development and characterization of high performing nanostructured olivine lithium iron phosphate (LiFePO4) cathode materials, having enhanced conductivity, Li+ ion diffusion and rate capability thanks to the specific addition of graphene oxide during electrode preparation. Thorough electrochemical characterisation was performed in terms of cyclic voltammetry and galvanostatic charge/discharge cycling in laboratory-scale lithium cells, with particular attention on the behaviour under high current regimes. In addition, it explores the possibility to use LiFePO4 as pristine material to obtain, through cationic exchange, the sodiated equivalent NaFePO4 that combines the safety and stability of LiFePO4 with the low-cost and environmental advantages of sodium. This part of the experimental work was carried out during an internship carried out in the Solid State Chemistry and Energy lab at the Collège de France, Paris, under the supervision of Professor Jean-Marie Tarascon, leading scientist in the developments of Sodium-ion batteries (NIBs), which represent one of the most promising alternatives to traditional LIBs as medium/large-scale stationary energy storage systems able to meet the flexibility requirements of the electric distribution grids in terms of energy/power density, low cost and environmentally friendliness.