Unlocking the potential of P3 structure for practical Sodium-ion batteries by fabricating zero strain framework for Na+ intercalation

Layered sodium transition metal oxides (NaTMO2) have gamed considerate attention as candidates for cathode materials in practical Na-ion batteries because of their potentially high energy density and operating voltage. Among the several categories (P2, P3, O3) of layered NaTMO2 materials, P3 type cathodes are least studied due to the fast capacity/voltage fading induced by P3-O3 phase transition, as shown by the very few compositions visited in previous reports. It is highly desirable to stabilize the P3 phase throughout the sodium extraction/insertion process as P polytype allows facile migration of Na+-ions between two adjacent prismatic sites. In this work, we synthesized a phase pure P3-Na0.75Mg0.08Co0.10Ni0.2Mn0.60O2 (P3-MNCM) via the incorporation of Mg, while successfully maintains the P3 structure during the charging/discharging process. This together with good ionic conductivity, endows good structural reversibility, and consequently leads to superior cycling performance and rate performance than undoped composition. In situ X-ray diffraction (XRD) further verifies that P3-MNCM cathode demonstrates a near zero strain feature. Of special importance is that the P3-O3 phase transition is effectively suppressed with the incoporation of Mg into the transition metal layer. These findings demonstrate the potential of P3 structure as a promising choice for Na ion cathode materials, which open a feritile avenue and will hopefully inspire more endeavors in the exploration of high performance sodium ion batteries.