Next generation (Gen.5) Li-ion batteries: investigation of confinement effects on reversibility of conversion cathode materials

The introduction of novel materials and improvements on the internal cell architecture have led to a gradual increase in the energy density of secondary Li-ion batteries. The pathway for further evolutionary improvements is expected to reach its limit by 2030 with gen. 4c batteries. New material systems and cell types will continue to offer improved energy density beyond this point, reaching energy densities up to 2000 Wh/L. Such next generation batteries, typically defined as Gen. 5 in the Strategic Energy Technology Plan (SET Plan) proposed by the European Commission, are expected to accelerate the transition to electrified road transport and possibly pave the way for an application in the aviation industry.

Transition metal oxides, commonly used as positive electrode materials in Li-ion batteries are intercalation compounds, in which Li-ions can reside in empty lattice sites, without drastically impacting the lattice structure. As such the host material retains its mechanical integrity during long term operation. Lithium-ion batteries based on the conversion reaction mechanism may break the energy density barrier imposed by conventional systems exploiting this well-known intercalation chemistry. Despite potentially offering unparalleled energy, the conversion chemistry is currently not fully controlled and shows intrinsic limitations which hinder its widespread implementation in practical systems. In fact, conversion cathode materials exhibit large voltage hysteresis, microstructural instability, and poor reversibility, which prevent an efficient use in rechargeable batteries ensuring long cycle life.

Electrode nanostructuring and physical confinement at the nanoscale may affect the reaction pathways in the electrochemical cell, thereby improving the reversibility of the Li-ion exchange. However, these confinement effects on the conversion mechanism are currently not fully understood. Nonetheless, this methodology has the potential to turn materials which are currently only suitable for primary (non-rechargeable) batteries reversible. This PhD project will shed light on the reaction mechanism in conversion cathode materials for high-energy lithium-ion batteries and investigate on technological approaches to enhance the charge/discharge reversibility of Gen. 5 cells. An interdisciplinary approach involving material engineering at the nanoscale, fundamental electrochemistry, and material integration in battery systems aim to redefine the boundaries of the lithium-ion technology. Meticulous analysis of the conversion reactions will be conducted, comprising both physico-chemical characterization of the reaction products and electrochemical characterization of the nano-engineered conversion electrodes. The learnings will be exploited to engineer the conversion electrode and enable reversible operation.