Synthesis, structural and electrochemical characterizations of the sol–gel birnessite MnO 1.84·0.6H 2O

Abstract

The electrochemical features of the sol–gel birnessite MnO 1.84·0.6H 2O are reported here in terms of OCV data, cycling behavior kinetics of Li transport in the oxide. We have confirmed that the sol–gel birnessite remains one of the best manganese oxides as rechargeable cathodic materials with an initial specific capacity of 270 mAh g −1 due to the high amount of Mn(IV). However, from a practical viewpoint, this hydrated compound has been proved to exhibit an important and constant capacity fading in the potential window 4.2–2 V associated with a strong disordering process. The ac impedance measurements have proved this behavior was mainly due to an increase of the cathode impedance and a strong decrease in kinetics of Li transport in the oxide. Such a phenomenon can be overcome when the Co-doped form Co 0.15Mn 0.85O 1.84·0.6H 2O is used. In that case, a constant capacity as high as 170 mAh g −1 can be maintained over more than 50 cycles with no change in the electrochemical behavior of the Co-doped sample. This clearly shows that stable capacity can be expected even from a hydrated oxide network.