Exposing {010} Active Facets by Multiple-Layer Oriented Stacking Nanosheets for High-Performance Capacitive Sodium-Ion Oxide Cathode.

As one of the most promising cathodes for rechargeable sodium-ion batteries (SIBs), O3-type layered transition metal oxides commonly suffer from inevitably complicated phase transitions and sluggish kinetics. Here, a Na[Li Ni Mn Cu Mg ]O cathode material with the exposed {010} active facets by multiple-layer oriented stacking nanosheets is presented. Owing to reasonable geometrical structure design and chemical substitution, the electrode delivers outstanding rate performance (71.8 mAh g and 16.9 kW kg at 50C), remarkable cycling stability (91.9% capacity retention after 600 cycles at 5C), and excellent compatibility with hard carbon anode. Based on the combined analyses of cyclic voltammograms, ex situ X-ray absorption spectroscopy, and operando X-ray diffraction, the reaction mechanisms behind the superior electrochemical performance are clearly articulated. Surprisingly, Ni /Ni and Cu /Cu redox couples are simultaneously involved in the charge compensation with a highly reversible O3-P3 phase transition during charge/discharge process and the Na storage is governed by a capacitive mechanism via quantitative kinetics analysis. This optimal bifunctional regulation strategy may offer new insights into the rational design of high-performance cathode materials for SIBs.