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 galvanostatic intermittent titration technique measurements and in situ electrochemical impedance measurements. Increased capacity and cycling stability of spheres over flakes may be related to smaller changes of the unit cell volume of spheres and thus to reduced structural stress. Co-doped spheres combine the advantages of both strategies and exhibit the best cycling stability.