STUDY AND DEVELOPMENT OF LAYERED LI-NI-MN-CO OXIDE POSITIVE ELECTRODE MATERIALS FOR LITHIUM ION BATTERIES

Abstract

Layered Li-Ni-Mn-Co oxides (NMC) with low cobalt content are promising positive electrode materials for Li-ion batteries. However, the detailed structural properties of these materials are still debated. This thesis work, in part, focused on a systematic study of layered NMC samples to understand the dependence of electrochemical properties on structure and transition metal composition, as well as the structural evolution of layered NMC materials during lithium intercalation. The calendar and cycle lifetimes of lithium-ion cells are affected by the structural stability of active electrode materials as well as parasitic reactions between the charged electrode materials and electrolyte that occur in lithium-ion batteries. It is necessary to explore the failure mechanisms of layered NMC/graphite cells to guide future improvements. This thesis work, in part, thoroughly studied the failure mechanisms of LiNi0.8Mn0.1Co0.1O2/graphite cells from the perspectives of the bulk structural stability, surface structure reconstruction and electrolyte oxidation. Core-shell (CS) structured positive electrode materials based on layered NMC could be the next generation of positive electrode materials for high energy density lithium-ion batteries. This is because a high energy core material (Ni-rich NMC), with poor stability against the electrolyte, can be protected by a thin layer of a stable and active shell material with lower Ni and higher Mn content. A large part of this thesis focused on the development of CS materials using Li-rich and Mn-rich materials as the protecting shell for voltages above 4.5 V, and on an understanding of inter-diffusion phenomena observed during the synthesis of core-shell materials.