A SYSTEMATIC EVALUATION OF COBALT-FREE NICKEL-RICH CORE-SHELL STRUCTURED POSITIVE ELECTRODE MATERIALS

Abstract

Layered Co-free Ni-rich (>80%) oxide positive electrode materials have attracted significant attention in recent years due to the high demand for lower cost and higher energy density lithium-ion batteries. The main difficulty in the development of high specific capacity Ni-rich materials primarily lies in obtaining excellent charge-discharge cycle life. Utilizing a core-shell structure with a Ni-rich core and a lower Ni content shell can be an excellent design strategy to balance specific capacity and cycle life. A low Ni content shell of long cycle life may prevent direct electrolyte contact with the high specific capacity Ni-rich core, enabling the core-shell material to have improved cycle life compared to the Ni-rich core alone.

The first part of the thesis explores a systematic series of Co-free Ni-rich core-shell materials with various core/shell compositions, core diameters, shell thickness and lithiation temperatures. A relationship between the cycling performance of core-shell materials and the lithiation temperature during synthesis is proposed which considers the crystallinity of the material, the average percentage of Ni in the Li layer and the presence or absence of the shell phase after the heating step. The “best” core-shell material demonstrates comparable performance with commercial-grade Co-containing materials in terms of capacity retention, powder electrical resistance and cell internal impedance, highlighting the potential of Co-free core-shell materials as promising alternative positive electrode materials.

The second part of the thesis focuses on the evaluation of the “best” core-shell material in commercial-grade pouch cells. Two versions of the core-shell material, washed and unwashed, were obtained from a reputable vendor. The charge-discharge cycling performance of the core-shell pouch cells is worse than commercial-grade single crystal LiNi0.8Mn0.1Co0.1O2 and LiNi0.5Mn0.3Co0.2O2 cells in terms of capacity retention and impedance growth. Cross-section SEM/EDS showed particle microcracking of core-shell materials due to calendering which is detrimental to the cell performance. Washing core-shell materials worsened the mechanical and electrochemical performance of the material.