New Ion Substitution Method to Enhance Electrochemical Reversibility of Co‐Rich Layered Materials for Li‐Ion Batteries

The recent development of high‐energy LiCoO2 (LCO) and progress in the material recycling technology have brought Co‐based materials under the limelight, although their capacity still suffers from structural instability at highly delithiated states. Thus, in this study, a secondary doping ion substitution method is proposed to improve the electrochemical reversibility of LCO materials for Li‐ion batteries. To overcome the instability of LCO at highly delithiated states, Na ions are utilized as functional dopants to exert the pillar effect at the Li sites. In addition, Fe‐ion substitution (secondary dopant) is performed to provide thermodynamically stable surroundings for the Na‐ion doping. Density functional theory calculations reveal that the formation energy for the Na‐doped LCO is significantly reduced in the presence of Fe ions. Na and Fe doping improve the capacity retention as well as the average voltage decay at a cutoff voltage of 4.5 V. Furthermore, structural analysis indicates that the improved cycling stability results from the suppressed irreversible phase transition in the Na‐ and Fe‐doped LCO. This paper highlights the fabrication of high‐energy Co‐rich materials for high voltage operations, via a novel ion substitution method, indicating a new avenue for the manufacturing of layered cathode materials with a long cycle life. Simultaneous doping of Na and Fe ions at LiCoO2 material enhances the structural stability significantly, inhibiting irreversible phase transitions, and thus improving the electrochemical performance. The secondary ion substitution not only made the functional dopant (Na) more readily incorporated into the structure, but also maintained it staying at the Li site, supporting the Li slabs at highly delithiated states.