Structural and electrochemical properties of Na0.72CoO2 as cathode material for sodium-ion batteries

In this work, results of structural studies, performed as a function of temperature (13–1073 K) of Na0.72CoO2 cathode material, are presented, together with microstructural measurements, including detailed transmission microscopy examination, as well as electrochemical studies, showing current rate-dependent charge/discharge characteristics and also voltammetry tests. It was found that thermal expansion of P2 type (P63/mmc) Na0.72CoO2 differs along a- and c-axes. Microstructural studies showed platelet-like grains of the synthesized sodium cobaltate, while detailed transmission microscopy measurements indicated coexistence of well-defined layered regions with regions having somewhat disturbed atomic arrangement. For higher current densities, characteristic plateaus observed on Na/Na+/NaxCoO2 charge/discharge curves diminish, and the voltage dependence on the sodium concentration is smoothed out. Results of the electrochemical tests, performed on cathodes without any carbon additives, indicated that in fact, such additives influence electrochemical behavior during first charge considerably. Additional voltammetry investigations confirmed presence of nine characteristic anodic and cathodic peaks, corresponding well with plateaus observed during charge/discharge of the cells.