Studies On Synthesis And Electrochemical Behaviors Of Layered Manganese Dioxides In Aqueous Solution

Manganese dioxide has been widely studied as battery cathode material for many years. It is traditionally employed in aqueous electrolyte manganese batteries and used as the cathode material of lithium primary cells in non-aqueous electrolyte. The alkaline MnO2/Zn system uniquely combines low cost, abundant, and non-toxic reactants with extremely good performance, shelf-life, and temperature characteristics. But numerous attempts to produce a viable secondary battery have failed owing to the irreversible behavior of MnO2 electrodes even under conditions of shallow depth of discharge, and total deterioration of the cell under deep discharge conditions. So a variety of attempts have been done in order to obtaining a sufficient reversible MnO2 by means of physical and chemical modification of the MnO2 raw material.Yao et al., Wroblowa and Dzicciuch et al. have succeeded in improving the cyclability of Birnessite by doping lead and bismuth. Bimessite is a type of manganese dioxide, which has a layer structure with water and alkali ion in the interlayer. Recently Intercaltion of Li+ ions into host layered Birnessite structures has been studied in non-aqueous batteries. There is very limited information on intercalation occurring in aqueous media.The main objective of this work is to investigate whether intercalation of Li+ into the host structure of Birnessite does occur in aqueous solution. Firstly layered manganese dioxides(K-Birnessite) with birnessite structure were synthesized by simple decomposition of KMnO4 at 800C in an oxygen-rich atmosphere and then the electrochemical behavior of K-Birnessite was studied in 2.5M LiOH solution by slow-scanning cyclic voltammetry and galvanostatic discharge/charge. The results indicates the cyclability of the K-Birnessie is poor.Subsequently, a variety of A-Birnessite was prepared by ion-exchange reaction and their cyclic voltammetric behaviors were studied. The results showed the cyclability of Pb-Birnessite and Cr-birnessite was better.So we focused on the electrochemical behaviors of Pb-Birnessite and Cr-Birnessite in 2.5M LiOH aqueous solution. The results indicate the Pb-Birnessite can be rechargeable for 30 cycles and Cr-Birnessite for 70 cycles even underconditions of reactant utilization 100% of its theoretical one-electron capacity and 1C discharge/charge rate in 2.5M LiOH aqueous solution. The enhanced cyclability is ascribed to the suppression of the transition from the layered structure to the spinel structure upon electrochemical cycling.Especially, The Cr in the Cr-Birnessite may be divided into two parties: one party between the interlayer of Cr-Birnessite, one party substation for trivalence manganese in the octahedral site. The Cr between the interlayer acts as pillars and improves its rechargeablity. The Cr substation for trivalence manganese in the octahedral site improves the mean oxidation state of manganese.AAS indicates Li+ can intercalate into and de-intercalate from the Pb-Birnesstie or Cr-Birnessite duing the discharge and charge cycles. The average values of the chemical diffusion coefficient D of Li+ intercalated into Pb-Birnessite and Cr-Bimessite are 8.24 X 10-11cm2s-1 and 1.57 X 10-10 cm2.s-1 respectively.