Versatile Battery Insertion Materials: Crystal Structure and Electrochemistry

Abstract

Rechargeable battery technologies became the spotlight of global scientific research in the quest to address growing energy demand. Currently, Li-ion batteries are unanimously occupied consumer electronic sector. These are mostly depending on only two transition elements such as Cobalt (Co) and Nickel (Ni) in popular cathodes LiCoO2 and LiNi1/3Mn1/3Co1/3O2 (NMC). The resource constraints of Co and Ni have paved away for alternate redox chemistry. In this scenario, manganese and vanadium-based layered materials are promising to owe their low-cost, resource-friendly nature with high operational safety. Moreover, the multiple redox states associated with these transition metals are promising to design high energy density cathodes.Here, we worked on the A2M3O7 (M= V, Mn) class of cathode materials. This family of materials exhibits polymorphism and multiple cationic redox properties. Further, bond valence sum energy calculations (BVSE) discovered a very low migration barrier of the alkali-ion (A) site. It inspired us to examine the intercalation properties of different ions including Li, Na, K and Zn. In each case, we observed interesting crystal chemistry with self-ion exchange property. The residual ions after self-ion exchange acted as pillars to lash out structural collapse during (de)intercalation. The ternary layered Na-Mn-O based metal oxide (Na2Mn3O7) was synthesized using different synthetic routes to tune the size and morphology of Na2Mn3O7 particles. The phase pure Na2Mn3O7 crystallizes in a triclinic layered structure (s.g. P-1). It consists of Mn in major +IV and minor +III oxidation states with electrochemically active Mn4+/Mn3+ redox center. The as-synthesized powder material worked as a versatile cathode for the Li, Na, K and Zn-ion batteries1-4. The systematic electrochemical studies were carried in both the aqueous and non-aqueous electrolytes. It delivered a reversible capacity of ~160, ~140, ~134 and ~330 mA h g-1 respectively with Li, Na, K and Zn metal foil as an anode in half-cell architectures. The detailed (de)intercalation mechanism along with diffusional studies of the various alkali-ions (Li/Na/K) along with Zn-ion will be described to present Na2Mn3O7 as a versatile cathode material.