Surface-Enhanced Ni-Rich Cathodes for Reversible Li Intercalation
Authors: Yan Danna , Kim Jae Chul 
Published: December 2024

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. In particular, high-voltage operation for the layered oxide cathodes leads to substantial volume change and structure collapsing, and thereby chemo-mechanical degradation. Cation doping, surface engineering, and/or composition gradient designs are common approaches to stabilizing charged structures at high voltage. They have proven effective in suppressing surface degradation of the layered oxide particles by promoting phase stability or providing extrinsic protection.

In this presentation, we propose a reactive coating method to obtain active cathode particles with protective passivation via transition metal doping, which could primarily address parasitic side reactions occurring at the cathode-electrolyte interface. Our designed Mo (or Ti)-doped layered oxide cathodes contain more than 90% Ni and demonstrate high-voltage stability and good capacity retention under high temperature cycling environment. This results from an electrochemically stable, 10 nm-thick surface phase that was formed in situ during one step calcination process. Electron microscopy combined with elemental analysis indicates that the surface is Mo (or Ti)-rich phase. An electron diffraction pattern from the high-resolution transmission electron microscopy (TEM) reveals that the surface has rocksalt-like structure, while the bulk maintains the layered structure.

This surface-enhanced cathode delivers 216 mAh/g at the second discharge (0.2C, room temperature) with excellent capacity retention of 89% over 50 cycles (1C, 45oC). This outperforms the pristine Ni-rich cathode without doping, in which the retention rate is 78%. Since the capacity fading of these Ni-rich layered cathodes largely originates from the cyclic internal stress caused by the repetitive H2-H3 phase transitions. Here, the differential capacity-voltage analysis and ex situ X-ray diffraction suggest that the surface coating can delay and suppress the unfavorable H2-H3 phase transition at high state of charge for the Ni-rich cathode, extending cycle life. Consequently, the Mo (or Ti) surface disordered rocksalt phase alleviated the structural stress associated with the repetitive phase transition by reducing the abrupt lattice collapse/expansion. The effects of the reduced lattice distortion, Ti/Mo-rich surface phase were reflected by the enhanced cycling stability of the coated cathodes in rate capability analysis. 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 dual modification design represents future research direction towards high-performance cathodes and will inspire waves of efforts to develop multifunctional materials for next-generation batteries.