K x Mn[Mn(CN)6] as a High Capacity Positive Electrode Material for Potassium Batteries

Abstract

Recently, Na-ion and K-ion batteries have attracted attention as next-generation secondary batteries.1 Prussian blue analogues (PBAs) are promising cathode materials for these batteries. In 2014, it was reported that Na x Mn[Mn(CN)6] exhibited higher capacity in Na cells than the theoretical capacity based on Mn2+/3+ redox, which could be attributed to the reduction of Mn in [Mn(CN)6] to monovalence.2 Thus, PBA with Mn at the C-coordination site exhibits unique properties, but there is no report of high capacity in K cells. In this study, K x Mn[Mn(CN)6] (KMnHCM) was synthesized, and K+ ion extraction/insertion properties were evaluated. KMnHCM was synthesized by the solution precipitation method.3 The solution was sufficiently deoxygenated by bubbling nitrogen gas, and synthesis was performed under N2 atmosphere. The obtained samples were washed three times with deionized water under N2 atmosphere, and then dried at 150 °C under vacuum. The active material was mixed with conductive carbon (KB) and binder (PVdF) at a weight ratio of 7:2:1 in a glove box under an Ar atmosphere. Galvanostatic charge-discharge tests were conducted in a two-compartment three-electrode cell using a composite electrode coated on an Al foil as the working electrode, an activated carbon counter electrode, and an Ag+/Ag reference electrode. Figure 1(a) shows the X-ray diffraction (SXRD) patterns and Rietveld refinement results of the synthesized sample. The diffraction patterns of KMnHCM can be attributed to the monoclinic structure with space group P21/n and are almost identical to that previously reported. SXRD patterns were successfully fitted with the satisfied R values, and the composition of KMnHCM was estimated to be K1.87Mn[Mn(CN)6]0.96·0.29H2O. Figure 1(b) shows the charge-discharge curves of KMnHCM. The electrode showed an initial charge capacity of 163 mAh g-1 and an initial discharge capacity of 220 mAh g-1, corresponding to respective ~2.0 and 2.76 potassium extraction and insertion, respectively, from/into K x Mn[Mn(CN)6]. The discharge capacity exceeds the theoretical capacity of 160 mAh g-1 based on the Mn2+/3+ redox of K1.87Mn[Mn(CN)6]0.96·0.29H2O. Furthermore, the additional plateau was observed at the low voltage of around 1.5 and 1.7 V in the discharge and charge curves, respectively. The extremely high capacity and the third plateau at low voltage are similar to Na x Mn[Mn(CN)6] in Na cells.2 Thus, the third plateau would be attributed to the reversible redox of [MnII(CN)6]4-⇄[MnI(CN)6]5-. In the talk, we will further discuss the charge-discharge mechanisms, including redox behavior and structural changes of KMnHCM during charge/discharge.