Meaning
Structural phase transformations involve the rearrangement of atoms within layered oxide cathodes during extended cycling. The rhombohedral to spinel transition marks the change from a high capacity layered structure to a more stable but lower voltage spinel phase. This shift occurs primarily in lithium rich or manganese rich materials when lithium ions are extracted and transition metals migrate.
Understanding this path helps in designing more durable cathode chemistries.
Lattice Rearrangement
Oxygen atoms stay relatively fixed while the metal ions move into new sites. In the rhombohedral to spinel transition, the cubic close packed oxygen framework is preserved but the cation ordering changes. This physical movement creates local strain and disrupts the pathways for lithium diffusion.
Voltage Decay
Discharge potentials drop as the electronic environment of the transition metals changes. Because the spinel phase operates at a lower average potential, the rhombohedral to spinel transition results in a loss of energy density. This gradual decline is a major challenge for the commercialization of next generation cathode materials.
Structural Stabilization
Adding elements like aluminum or magnesium inhibits the movement of atoms. Preventing the rhombohedral to spinel transition requires reinforcing the layers to keep the transition metals in their original positions. Successful stabilization ensures the battery maintains a constant voltage profile over its operating life.
