Effects of d0 Ions on δ-formation Mechanism of Mn-rich Disordered Rocksalts

Li-ion cathodes are still a critical bottleneck of rechargeable battery technology. The need for new cathode chemistries that are less expensive, more abundant, and environment friendly than conventional cathode materials, such as LCO and NMC, is crucial to meet society’s ever-growing demands in energy storage and carbon neutrality. Over the past few years, Mn-rich disordered rocksalts (DRX) emerged as low-cost and high-energy cathode candidates for Li-ion batteries. This system faces an unusual phase transformation from a disordered rocksalt to a disordered spinel-like (δ) domains enhancing the rate capability and stability of the DRX compound upon cycling. Although it has been demonstrated that Mn content is the driving force behind the δ transformation little attention has been given to the d0 ions that is usually present is the DRX composition, such as Ti+4, Zr+4, Nb+5 and Mo+6. These high-valent species do not only stabilize DRX local distortions but enhance the Li excess in this system, which is crucial for the Li-ion transport. The relative immobility of the d0 cations could halt the spinel full transformation before becomes detrimental to cycling life, thus being an important component on the δ-formation mechanism. Herein, we propose a systematic study of d0 doping (d0 = Ti+4, Zr+4, Nb+5 and Mo+6) of Mn-rich DRX cathodes to understand the role of d0 ions in the δ transformation. We use a combination of x-ray absorption spectroscopy (XAS) and synchrotron x-ray diffraction (sXRD) to fundamentally understand the δ formation mechanism by linking local structure, atomic speciation and structural rearrangements during DRX cathode cycling.