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theses

Minimizing the difference between the theoretical and practically obtained energy densities of layered oxide positive electrode materials is critical to increasing the energy density of Li-ion batteries. Currently, positive electrode materials are incapable of repetitively accessing the high states of charge necessary to approach the theoretical energy density. Primary factors limiting the operating potential of layered oxide positive electrodes include the structural degradation of the positive electrode material, parasitic reactions between the positive electrode and the liquid electrolyte, and the presence of surface impurity species. Parasitic electrode-electrode reactions at high states of charge are investigated through an accelerated aging test developed using a thermally and electrochemically strenuous environment. In conjunction with accelerated aging testing, Li[Ni0.8Co0.15Al0.05]O2 (NCA) is used as a model positive electrode compound to study the chemical and structural stability of highly delithiated layered oxides in the presence of liquid electrolyte. Furthermore, the surface impurity species that develop on NCA upon ambient atmospheric exposure and their impact on the material’s electrochemical performance is thoroughly evaluated. Synthesis and characterization of LixCo1-yAlyO2 and LixNi1-yAlyO2 (1 ≥ x ≥ 0 and 0.2 ≥ y ≥ 0) as well as LixNi0.8Co0.2O¬2 and LixNi0.8Co0.15Al0.05O2 enables the determination of the specific roles that the layered oxide material’s transition metal chemistry has on its structural stability and cycling capabilities at and near full delithiation. The results presented herein emphasize the importance of proper management of layered oxide positive electrode materials and propose handling procedures based on the formation mechanisms of critical surface impurity species. The surface and sub-surface structural decomposition of highly charged positive electrode materials in the presence of electrolyte causes substantial impedance growth and severe transition metal dissolution. Based on the investigation into these intricate electrode-electrolyte reaction mechanisms, strategies to minimize parasitic reactions and avoid degradation of the electrochemical performance are developed. Additionally, elucidated relationships between the material’s transition metal chemistry and stability at high states of charge provide guidelines for designing new positive electrode materials intended to operate near full delithiation.

ETD_8745

Ph.D.

Includes bibliographical references

by Nicholas V. Faenza

doi:10.7282/T3DV1P94

ETD doctoral

The author owns the copyright to this work.