Microwave-Assisted\nSynthesis of P2- and O3-Type Na<sub><i>x</i></sub>CoO<sub>2</sub> Cathode Materials Studied\nby In Situ Raman Spectroscopy

A microwave heating protocol was developed for the rapid\npreparation\nof two polytypes of Na<sub><i>x</i></sub>CoO<sub>2</sub> with distinct crystal structures. Two methods using conventional\nfurnace heating with different heating rates were also developed to\nstudy the effects of the heating rate and microwave irradiation on\nthe formation pathway and final products. Both O3-type NaCoO<sub>2</sub> and P2-type Na<sub>0.68–0.76</sub>CoO<sub>2</sub> were prepared\nby heating a mixture of Na<sub>2</sub>O<sub>2</sub> and Co<sub>3</sub>O<sub>4</sub> to 850 °C followed by immediate cooling. Adjusting\nthe Na precursor ratio selectively targeted the two phases. In order\nto compare the formation pathway, intermediate species obtained after\nthe first reaction occurring at around 350 °C were identified\nby ex situ X-ray diffraction and in situ Raman spectroscopy. A mixture\nof Na<sub><i>x</i></sub>CoO<sub>2</sub> phases was found,\nshowing rapid formation at this temperature, consistent with previous\nin situ studies. Higher heating rates increased the peak reaction\ntemperature, accelerating reaction completion and increasing the intermediate\ncrystallite size. However, the final product’s composition,\nstructure, and morphology were unaffected by the heating method. Electrochemical\nperformance in coin cells showed that all the O3- and P2-type products\nhad slightly lower initial capacity compared to previous reports but\ndid not vary significantly between heating methods. Capacity retention\nover 100 cycles was stable for the O3 phase but showed a dramatic\nreduction for the P2 phase in all heating methods. This study demonstrates\na promising rapid synthesis method that could promote the commercialization\nof Na-containing layered cathode materials.