Rational design of cationic ratio tuned high-performance P2/O3 bi-phase layered oxide materials for sodium-ion batteries

Layered oxides are the widely investigated cathodes for sodium-ion batteries (SIBs), especially their pure phases (e.g., P2, O3). In this work, we report a series of Mn-Ni-Fe based cathode materials with P2/O3 bi-phase, which possesses advantages of both P2 and O3 structures. At first, the manuscript shows a synthesis phase diagram of Na0.83MnxNiyFe1-x-yO2 to pre-screen and select the P2/O3 bi-phase material Na0.83Mn1/2Ni1/3Fe1/6O2. Then, it is found that the cationic ratio has a significant impact on the phase formation. The structure of the materials gradually changes with the various of the Mn/Ni ratio. When the Mn/Ni ratio reaches 3/2, P2/O3 bi-phase is formed. Moreover, Fe content will influence the P2/O3 ratio in the bi-phase material. In particular, the synthesized bi-phase Na0.83Mn3/6Ni2/6Fe1/6O2 has a P2/O3 ratio of 71/29 wt%. It delivers a high specific capacity of 136.6 mA h g−1 (at current density 20 mA g−1) between 2.0 and 4.0 V and 94.8% retention (at current density 100 mA g−1) after 100 cycles. In addition, an 85.6 mA h g−1 discharge capacity can be maintained at a superior high current density of 2000 mA g−1. The work offers critical guidance to the rational design of the bi-phase layered cathode materials.