Nature of Alkali Ion Conduction and Reversible Na-Ion Storage in Hybrid Formate Framework Materials

Abstract

The cost advantage of Na-ion batteries has spurred intensive research effort in the last 10 years to develop reversible Na+ storage materials. Although classic host materials—analogous to those in the Li-ion system—are potentially straightforward targets, sluggish Na+ diffusion in many inorganic structures limits options. In this regard, open framework inorganic–organic hybrids such as metal–organic framework materials are considered as viable alternatives. Herein, we introduce heterometallic formate frameworks as potential candidates for reversible Na+ storage. As a first, we present a microwave solvothermal strategy for rapid synthesis of phase-pure microcrystalline Na2Co(HCO2)4 and AB(HCO2)3 (A: Li/Na; B: Co/Mn). By combining in-depth impedance analysis with ab initio molecular dynamics simulation, we reveal that the Li+/Na+ conduction—which follows a “pinball” mechanism—in these materials is extrinsic defect-dominated. Calculation suggests that a librational motion of the formate anions facilitates the diffusion of Na+ compared to Li+, explaining the origin of anomalously higher ionic conductivity for the Na analogue compared to the Li one. Preliminary electrochemical investigation reveals reversible Na+ storage in Na2Co(HCO2)4 and NaMn(HCO2)3 at an average voltage of 2.5–3 V.