Fundamental Principles toward Designing High Na-Containing P2-Structured “Layered” Na-Transition Metal Oxides as High-Performance Cathode Materials for Na-Ion Batteries

The present work proposes and establishes a universal strategy toward facilitating the development of desired structural types (viz., P-type vs O-type) of "layered" Na- transition metal (TM) oxides, with the desired Na-content and properties. In this regard, the structure type, allowable Na-content, and Na-layer/"inter-slab" spacing have been found to depend on the "charge:size" ratio of the TM-ions, concomitant electronegativity and covalency of TM–O bonds, and the charge neutrality aspect. Overall, increases in the average "charge:size" ratio of the cation combination in the TM-layer and concomitant TM–O bond covalency result in a lower effective negative charge on the O-ions. This renders the prismatic coordination of O-ions around the Na-ions more favorable even at a higher Na-content, but with the latter needing some compromise over the charge neutrality aspect. Accordingly, by careful selection of the combination of non-TM/TM-ions in the TM-layer, a high Na-containing (viz., ∼0.84 per formula unit) P2-type Na0.84([]0.06Li0.04Mg0.02Ni0.22Mn0.66)O2 has been successfully developed here, which, as a cathode material for Na-ion batteries, exhibits a high desodiation capacity of ∼178 mAh/g (@ C/5; within 2–4 V vs Na/Na+), exceptional cyclic stability pertaining to a ∼98% capacity retention after 500 galvanostatic desodiation/sodiation cycles at a high current density (2.5C), and also stability upon exposure to air/water. The suitable combination of a high Na-content and "charge:size" ratio in the TM-layer of the as-developed P2-type Na-TM-oxide is again the factor responsible for the above properties/performances. Furthermore, going with the proposed scientific basis, mere replacement of Mn4+, having a higher "charge:size" ratio (∼7.5 Å–1), with Ti4+, having a lower "charge:size" ratio (∼6.5 Å–1), keeping everything else the same, has been found to yield the O3-type structure.