Chemically Derived K2Mn3O7 As a Potassium Insertion Material:Structure and Electrochemical Study

Abstract

Potassium-ion batteries (KIBs) are fast emerging as economic alternatives for partial replacement of Li-ion batteries in stationary large-scale storage application. With the rising concern over the price and availability of Li-based resources, Na-ion and K-ion batteries have come up to serve applications unrestricted by gravimetric/ volumetric energy density. When compared to Na-ion batteries, KIBs offer several merits including feasible K+ (de)insertion into graphite anode (244 mA h g-1), lower standard voltage (-2.93 V vs. SHE), high diffusion kinetics along with lower solvation radius [1].To realize novel potassium insertion materials, we have proposed application of pristine and chemically modified (e.g. ion-exchange) sodium insertion compounds as cathodes in KIBs. Owing to the similar size of Na and K species, various sodium-based materials can act of host for K+ insertion. As a proof-of-concept study, NaxCoO2 and Na2Mn3O7 were successfully used as KIB cathodes with reversible capacity values of 82 and 132 mAh g-1 respectively [2-3]. Focusing on layered Na2Mn3O7, we have discovered a novel oxide phase by topotactic ion-exchange route using K2SO4 at room temperature. Assuming a layered structure, this new compound exhibits a reversible specific capacity of ~100 mA h g-1 at the current rate of C/20. It involves step-wise voltage profile with a nominal Mn4+/Mn3+ redox potential ~2.7 V (vs. K/K+) (Figure 1). Further, it exhibited robust cycling stability at different current rates, retaining a discharge capacity of 63 mA h g-1 even at 1C rate in the potential window of 1.5 to 3.8 V. The synthesis, structural features and electrochemical charge storage mechanism of this novel potassium insertion compound will be presented. Reference s : [1] H. Kim, J. C. Kim, M. Bianchini, D-H Seo, J. R Garcia, G. Ceder, Adv. Energy Mater. 8 (2017) 1702384.[2] K. Sada, B. Senthilkumar, P. Barpanda, Chem. Commun. 61 (2017) 8588–8591.[3] K. Sada, B. Senthilkumar, P. Barpanda, ACS Appl. Energy Mater. 10 (2018) 5410–5416. Figure 1