High-Voltage Lithium-Ion Batteries for a Sustainable Transport

The development of lithium-ion batteries (LIBs) with specific energy >200 Wh/kg plays a key role to boost the progress of sustainable transport and to promote the large-scale diffusion of electric vehicles with long electric-driving-range. The battery energy density can be increased by using high-voltage and/or high-capacity cathode materials. LiNi0.4Mn1.6O4 and LiNi0.5Mn1.5O4 are among the most promising cathode materials for the high theoretical specific capacity and high nominal operating voltage. However, the major concern is about their reactivity towards conventional electrolytes that decompose at high potentials leading to thick surface layers on the cathode resulting in capacity loss. This PhD work deals with the development of high energy and power LIBs featuring LiNi0.4Mn1.6O4 and LiNi0.5Mn1.5O4 cathodes, mainly for hybrid electric vehicle (HEV) applications. Starting from the challenging study of cell components such as electrolyte, separator, electrode conductive additive and binder, full cells with graphite anodes were assembled and tested according to the US Dept. of Energy (DOE) protocols for the use in power-assist and plug-in HEV applications. The use of a carbonate-based electrolyte with a fluorinated non-conventional lithium salt and of the reinforced polyvinylidene fluoride (PVdF) macroporous membrane as separator significantly improve the electrochemical performance of graphite//LiNi0.4Mn1.6O4 cells with respect to those with the conventional electrolyte and commercial separator. The study of different conductive additives on the cycling performance of LiNi0.5Mn1.5O4 electrodes demonstrated that reduced graphene oxides improve the electrode/electrolyte interface by acting as a protective barrier that hinders the formation of a thick passivation layer on the cathode surface. The study of water-soluble binder proved that carboxymethyl cellulose remarkably improve the cycling performance of LiNi0.5Mn1.5O4 electrodes compared to those with conventional PVdF binder. Characterization tests according DOE protocols on graphite//LiNi0.4Mn1.6O4 and graphite//LiNi0.5Mn1.5O4 confirmed that these cells meet the DOE targets of energy and power for power-assist and plug-in HEVs.