P2–NaxCoyMn1–yO2 (y = 0, 0.1) as Cathode Materials in Sodium-Ion Batteries—Effects of Doping and Morphology To Enhance Cycling Stability

Sodium-ion batteries have become a subject of increasing interest and are considered as an alternative to the ubiquitous lithium-ion battery. To compare the effect of two improvement strategies for metal oxide cathodes, specifically Co-doping and morphology optimization, four representatives of the prominent material class of layered NaxMO2 (M = transition metal) have been studied: hexagonal flakes and hollow spheres of P2–NaxMnO2 and P2–NaxCo0.1Mn0.9O2. The better electrochemical performance of the spheres over the flakes and of the Co-doped over the undoped materials are explained on the basis of structural features revealed by operando synchrotron X-ray diffraction. The higher cycling stability of the material doped with ∼10% Co is attributed to three effects: (i) the suppression of a Jahn–Teller-induced structural transition from the initial hexagonal to an orthorhombic phase that is observed in NaxMnO2; (ii) suppression of ordering processes of Na+; and (iii) enhanced Na+ kinetics as revealed by galv...