Electrochemical Intercalation of Magnesium into Graphite Via an Ethylenediamine-Based Electrolyte

Abstract

The formation of dimeric or larger cluster metal intercalates within layered hosts may reveal new chemistries for charge storage electrodes. In previous work, we reported a series of novel ternary stage-1 graphite intercalation compounds (GICs) containing alkaline earth metal cations (M = Mg, Ca, Sr, or Ba) and ethylenediamine (en). Pair distribution function (PDF) analyses indicated the presence M-M bonding within the intercalates in the monolayer galleries.In this work, we present the results of electrochemical intercalation reactions for Mg-en complexes within graphite. Using Mg(SO3CF3)2 in en as the electrolyte, galvanostatic charge and discharge profiles show capacities of 810 and 455 mAh g-1 during the first intercalation and deintercalation steps, respectively. After this formation cycle, a capacity of ~120 mAh g-1 can be retained for over 100 cycles with a columbic efficiency of >93%. Ex-situ X-ray diffraction confirms that stage-1 GICs are produced in the fully reduced state.Cyclic voltammetry (CV) and first-cycle differential capacity measurements (dQ/dV) both show that four redox couples arise in the first charge/discharge process. Very large polarizations (2.2 V) are observed, suggesting that there may be changes in Mg-N bonding in this redox chemistry. Galvanostatic intermittent titration technique (GITT) studies as well as X-ray Raman spectroscopy are currently underway to further examine reaction kinetics and confirm the presence of Mg-Mg bonding. This work may provide a new paradigm toward charge storage in intercalation electrodes.