Surface-Modified Ni-Rich Cathodes for Reversible Li Intercalation at High Voltage

Energy-dense nickel (Ni)-rich layered oxide cathodes can enable lithium (Li)-ion batteries to power electric vehicles (EVs) for long distance. Employing Ni-rich cathodes, however, faces challenges in stabilizing cathode-electrolyte interfaces, leading to substantial degradation of the layered structure at the cathode surface during cycling process. Meanwhile, high-voltage operation required to obtain high energy density is particularly critical in accelerating structural degradation, for high Ni cathodes because of their severe lattice strain and irreversible phase transitions. Efforts to stabilize cycle performance at high voltage, such as bulk doping, surface coating, and composition gradient, have proven effective for suppressing degradation of the layered structures.

In this presentation, we propose a reactive coating method to obtain active cathode particles with protective passivation via transition metal coating, which could primarily address parasitic side reactions occurring at the cathode-electrolyte interface. With the coating, our Ni-rich layered oxide cathodes demonstrate high-voltage stability and good capacity retention. This result can be explained bys the formation of an electrochemically stable, surface phase formed in situ during one step calcination process. Electron microscopy combined with elemental analysis indicates that the surface is cation-rich oxide. High-resolution transmission electron microscopy (TEM) reveals that the electron diffraction patterns from the surface are clearly different from that from the layered structure in the bulk.

This surface-enhanced cathode outperforms the pristine Ni-rich cathode, delivering more than 15% improved capacity retentions over 50 cycles. We found that the surface coating can delay or suppress the unfavorable H2-H3 phase transition at high state of charge for the Ni-rich cathode, extending cycle life. We consider that our work demonstrates how doping can stabilize the particle surface of Ni-rich cathodes to promote Li intercalation reversibility and energy density. And we expect that this reactive coating design represents a promising direction towards developing high-performance cathodes.