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theses

Sodium-ion batteries are an emerging technology aiming to significantly reduce the associated barriers of high performance batteries (high cost, and raw material abundance) in order to enable wide scale utilization of electrochemical energy storage devices. While being similar monovalent ions, Na+ and Li+ demonstrate considerably different characteristics when considering reversible ion-insertion materials, and it is of primary interest to ascertain whether or not analogous sodium chemistries can be derived from the well established lithium-ion compositions reported in literature. A number of high-voltage cathode materials have been identified for Li-ion battery applications, and due to the intrinsic difference in standard redox potentials between Na/Na+ and Li/Li+ (ca. 0.3V), it is desirable to consider high-voltage sodium chemistries to overcome the inevitable lower voltages associated with sodium compositions. LiMn1.5+δNi0.5-δO4 spinel has been previously investigated for application in high performance Li-ion batteries, operating on a reversible redox reaction at ca. 4.7V vs. Li/Li+. Here, the spinel crystal structure provides a 3D-interconnected network of diffusional pathways, promoting facile Li+ diffusion; and is considered for study in an analogous Na-ion battery as the combination of facile diffusional pathways and a high-voltage redox reaction (theoretically at ca. 4.4V vs. Na/Na+) would act as a significant improvement in sodium energy storage cathodes. Herein, LiMn1.5+δNi0.5-δO4 spinel is utilized to form the λ-Mn1-xNixO2 (λ-MNO) structure by electrochemical delithiation. The resulting materials are then investigated as a positive electrode for sodium-ion batteries, as the spinel framework enables facile 3D-interdiffusion via vacant crystallographic sites. Three profound conclusions may be derived from this work: (1) Na+ insertion into the λ-MNO structure is possible, with Na+ demonstrating almost exclusive occupancy of the 8a tetrahedral sites. (2) The Na+ insertion reaction occurs at ca. 3.6V (vs. Na/Na+), enabling direct replacement of existing commercial Li-ion cells. (3) Na+ can be reversibly inserted / deinserted from the spinel host structure, with the possibility of further optimization.

ETD_7047

Ph.D.

Includes bibliographical references

by Joshua Ryan Kim

doi:10.7282/T37P91J5

ETD doctoral

The author owns the copyright to this work.