Promoting fast Na-storage and fabricating a stable layered cathode for sodium-ion batteries via site-selective substitution in triple crystallographic sites

Abstract

Herein, to design a high-performance layered oxide cathode, Na_0.66Ca_0.05[Ni_0.25Li_0.11Mn_0.64]O_1.95F_0.05 (doped-NNM) was designed and investigated via a site-selective substitution strategy, in which Ca²⁺, Li⁺ and F⁻ were introduced into the Na, transition-metal and O²⁻ sites, respectively. Neutron diffraction not only revealed a larger super cell (P6₃ space group) to suitably account for all diffraction peaks instead of the unit cell (P6₃/mmc space group) via X-ray diffraction, but also confirmed the different distributions of Ca, Li and F ions in triple crystallographic sites. Different roles of Ca, Li and F ions in doped-NNM cathodes were revealed, which are responsible for the excellent high rate-capability (76 mA h g⁻¹ at 10C and 64 mA h g⁻¹ at 15C) and long cycling-life (80.8% capacity retention after 500 cycles). Theoretical calculation not only confirmed an easy Na-ion motion within 2a/2b sites compared with that within 2a/2b sites to 6c sites in the super cell model for the first time, but also demonstrated improved electronic conductivity by reducing the energy band gap after doping, which resulted in fast Na-storage and high-rate performance of doped-NNM. Moreover, a reversible P2-Z phase transition in doped-NNM demonstrated by in situ X-ray diffraction measurement is favorable to enhance structural integrity and increase the cycling life. This study offers a fundamental understanding of the design and optimization of the material structure and electrochemical performance of P2-type sodium-based layered cathode materials.