Layered Na-Ion Cathodes with Outstanding Performance Resulting from the Synergetic Effect of Mixed P- and O-Type Phases

Abstract

Herein, the synthesis of new quaternary layered Na-based oxides of the type Na x Mn y Ni z Fe 0.1 Mg 0.1 O 2 (0.67≤ x ≤ 1.0; 0.5≤ y ≤ 0.7; 0.1≤ z ≤ 0.3) is described. The synthesis can be tuned to obtain P2- and O3-type as well as mixed P-/O-type phases as demonstrated by structural, morphological, and electrochemical properties characterization. Although all materials show good electrochemical performance, the simultaneous presence of the P- and O-type phases is found to have a synergetic effect resulting in outstanding performance of the mixed phase material as a sodium-ion cathode. The mixed P3/P2/O3-type material, having an average elemental composition of Na 0.76 Mn 0.5 Ni 0.3 Fe 0.1 Mg 0.1 O 2 , overcomes the specific drawbacks associated with the P2- and O3-type materials, allowing the outstanding electrochemical performance. In detail, the mixed phase material is able to deliver specific discharge capacities of up to 155 mAh g -1 (18 mA g -1 ) in the potential range of 2.0-4.3 V. In the narrower potential range of 2.5-4.3 V the material exhibits high average discharge potential (3.4 V versus Na/Na + ), exceptional average coulombic efficiencies (>99.9%), and extraordinary capacity retention (90.2% after 601 cycles). The unexplored class of P-/O-type mixed phases introduces new perspectives for the development of layered positive electrode materials and powerful Na-ion batteries.