Determination of the synthesis diagram of sodium cobalt oxide and electrochemical study

Abstract

A complete and uniform synthesis diagram of NaxCoO₂ has been proposed based on forty-one samples synthesized at various temperatures from 450°C to 750°C by solid-state reactions with initial Na:Co ratio ranging from 0.60 to 1.05. Four monophasic domains of' O3, O3', P3' and P2 and four biphasic regions were revealed based on an XRD analysis. The sodium contents in these phases were determined according to the d00j-x relations obtained by an in situ XRD experiment and it is found O3, O3' and P3' phase almost form with only one stoichiometry, that is x=1.00, 0.83 and 0.67 respectively, by solid-state reaction while P2 phase forms in a slightly larger composition range from 0.68 to 0.76. Galvanostatic charging on O3-Na₁.₀₀CoO₂ battery reveals several plateaus and steep steps on the voltage curve, the corresponding phase transitions and solid solution behaviors were studied by a simultaneous in situ XRD experiment. The composition driven structural evolution in three layer NaXCoO₂ follows the sequence: O3-O3'-P3'-P3-P3', with a generally increased interslab distance d₀₀l.