Microwave-Assisted Synthesis of P2- and O3-Type Na_<i>x</i>CoO₂ Cathode Materials Studied by In Situ Raman Spectroscopy

A microwave heating protocol was developed for the rapid preparation of two polytypes of NaxCoO2 with distinct crystal structures. Two methods using conventional furnace heating with different heating rates were also developed to study the effects of the heating rate and microwave irradiation on the formation pathway and final products. Both O3-type NaCoO2 and P2-type Na0.68–0.76CoO2 were prepared by heating a mixture of Na2O2 and Co3O4 to 850 °C followed by immediate cooling. Adjusting the Na precursor ratio selectively targeted the two phases. In order to compare the formation pathway, intermediate species obtained after the first reaction occurring at around 350 °C were identified by ex situ X-ray diffraction and in situ Raman spectroscopy. A mixture of NaxCoO2 phases was found, showing rapid formation at this temperature, consistent with previous in situ studies. Higher heating rates increased the peak reaction temperature, accelerating reaction completion and increasing the intermediate crystallite size. However, the final product’s composition, structure, and morphology were unaffected by the heating method. Electrochemical performance in coin cells showed that all the O3- and P2-type products had slightly lower initial capacity compared to previous reports but did not vary significantly between heating methods. Capacity retention over 100 cycles was stable for the O3 phase but showed a dramatic reduction for the P2 phase in all heating methods. This study demonstrates a promising rapid synthesis method that could promote the commercialization of Na-containing layered cathode materials.