Reversible Electrochemical Insertion/Extraction of Magnesium Ion into/from Robust NASICON-Type Crystal Lattice in a Mg(BF₄)₂-Based Electrolyte

Reliable electrochemical investigations of electrode materials are indispensable for the development of next-generation energy storage devices. In the case of multivalent cation-based electrochemistry, intense attention should be paid to the cell configuration for obtaining reliable data. In particular, the electrolyte and reference electrode must be appropriately selected considering the potential window of electrolyte and the validity of reference. Here, we demonstrate the detailed electrochemical examination for the Mg2+-storage capability of the NASICON-type framework derived from Na3V2(PO4)3 (NVP). A combination of the Mg(BF4)2-based electrolyte with high anodic stability and the reliable Ag pseudo-reference electrode offers decent electrochemical test results. Despite suffering from the polarization concerning magnesiation, the desodiated NVP electrode can deliver a well-defined discharge plateau at ∼2.7 V (vs Mg2+/Mg) with the reversible capacity of >100 mAh g-1 at room temperature. The impedance analysis results indicate that the increased charge transfer resistance on discharging due to the high energy barrier for desolvation of divalent cations is responsible for the large polarization but not extremely significant, allowing for the room-temperature operation. The findings obtained herein also highlight the importance of the structural robustness of host lattice, which is required to withstand the strong amorphization during Na+ extraction and Mg2+ insertion/extraction.