Electrochemical Properties of Birnessite As a Positive Material for Aqueous Calcium-Ion Batteries

Abstract

Recently, calcium-ion batteries (CIBs) have received attention as a candidate for next generation batteries due to slight difference of standard reduction potential about 0.17 V above that of lithium and relative abundance of calcium compared to lithium.1 It is known that the birnessite-type manganese oxide, characterized by a layered structure consisting of edge-shared MnO6 octahedral with interlayer distances of ~7 Å as shown in Fig. 1a, allows high mobility of the interlayer cations of lithium or sodium with fast kinetics and little structural rearrangement.2 In this study, the electrochemical properties of the birnessite have been investigated as a cathode candidate material for rechargeable aqueous CIBs. The birnessite was synthesized by thermal decomposition of KMnO4 according to the previous report.3 The working electrodes consisted of the active material (birnesstie), conducting carbon (super P), and binder (polyacrylonitrile) with the weight ratio of 80:10:10. Activated carbon and Ag/AgCl were served as the counter and the reference electrode, respectively. The galvanostatic discharge/charge experiments were carried out at C/10 in the voltage range of -0.4­ – 0.8 V vs. Ag/AgCl in 1.0 M Ca(NO3)2 aqueous electrolyte. Reversible calcium-ion intercalation was observed in the electrolyte system as shown in Fig. 1b, c. The initial discharge/charge capacities were 182 mAh g-1 and 151 mAh g-1, respectively. The average discharge voltage was estimated to be around 2.8 V vs. Ca2+/Ca. The capacities were faded rapidly from the following cycles, probably due to the instability of the host material accompanying manganese dissolution. Detailed analyses with ICP-OES, EDX, XRD, and cyclic voltammetry (CV) with different scan rates indicated that both the intercalation and pseudo-capacitance are responsible for the reactions. As the research on calcium-ion batteries at an initial stage, this work demonstrates an example suggesting a possibility of discovering other new electrode materials for the calcium-ion batteries.