Entropy Tuning Stabilizing P2-Type Layered Cathodes for Sodium-Ion Batteries

The P2-type layered transition metal oxide cathodes confront formidable challenges, including irreversible deleterious phase transitions, transition metals migration, and sluggish Na+ diffusion kinetics, which hamper their rapid commercial application in sodium ion batteries (SIB). In this work, an entropy tuning with dual-site substitution strategy is proposed to address the aforementioned issues. In the tailored [Na0.67Zn0.05]Ni0.22Cu0.06Mn0.66Ti0.01O2 (NZNCMTO) cathodes, the strategic incorporation of Zn ions serves to occupy Na sites, intentionally disrupting the Na/vacancy ordering and establishing a reinforcing "pillar" effect within the layered framework. Furthermore, the substitution of Cu and Ti for Ni and Mn bolsters covalent bonding with the lattice oxygen, thereby impeding the migration of the transition metal ions and leading to a near-zero strain structural evolution during charge and discharge process. Density functional theory calculations confirmed that entropy-tuned NZNCMTO substantially lowered the migration energy barrier for Na+ ions diffusion and improved electronic conductivity. Consequently, the NZNCMTO cathode exhibits an impressive high practical capacity of 91.54 mAh g-1 at a high discharge rate of 10 C, along with outstanding cycling stability, maintaining near 100% capacity retention over 500 cycles at a current density of 10 C. This work presents an innovative blueprint for designing high-performance sodium-ion battery cathode materials. This research introduces an entropy-tuned and dual-site substitution strategy to overcome challenges in P2-type Na0.67Ni0.33Mn0.67O2 cathodes for sodium-ion battery. The tailored [Na0.67Zn0.05]Ni0.22Cu0.06Mn0.66Ti0.01O2 cathodes, strategically incorporating Zn ions, disrupt Na/vacancy ordering, fortifying the layered structure. More importantly, combining a series of in/ex situ experimental measurements and theoretical calculations, the energy storage mechanism and the reason for the excellent performance are unveiled.image