Identification of optimal composition with superior electrochemical properties along the zero Mn3+ line in Na0.75(Mn-Al-Ni)O2 pseudo ternary system

Abstract

Biphasic layered oxide cathodes, known for their superior electrochemical performance, are prime candidates for commercializing in Na-ion batteries. Herein, we unveil a series of P3/P2 monophasic and biphasic Al-substituted Na34Mn5-x8Al2x8Ni3-x8O2 layered oxide cathodes that lie along the ‘zero Mn3+ line’ in the Na3/4(Mn-Al-Ni)O2 pseudo-ternary system. The structural analysis showed a larger Na+ conduction bottleneck area in both P3 and P2 structures with a higher Al3+ content, which enhanced their rate performance. In each composition, the P3/P2 biphasic compound with nearly equal fractions of P3 and P2 phases outperformed their monophasic counterparts in almost all electrochemical performance parameters. Operando synchrotron XRD measurements obtained for the monophasic P3 and biphasic P2/P3 samples revealed the absence of the O3 phase during cycling. The high structure stability and faster Na+ transport kinetics in the biphasic samples underpins the enhancement of electrochemical properties in the Al-substituted P3/P2 cathodes. These results highlight fixed oxidation state lines as a novel tool to identify and design layered oxide cathodes for Na-ion batteries in pseudo-ternary diagrams involving Jahn-Teller active cations.