Magnesium hydride has been proposed as innovative anode material for Li ion cells due to its large theoretical capacity and high-energy efficiency compared to other conversion materials. In this work, we report a combined experimental-theoretical study about the origin of voltage hysteresis in the conversion reaction of MgH2 in lithium cells. Experimentally, the extent of the thermodynamic voltage hysteresis in the first galvanostatic discharge-charge cycle has been determined by the GITT technique and decoupled from the kinetic overpotentials. Theoretically, the origin of the thermodynamic voltage hysteresis has been evaluated and studied by means density functional theory calculations within the supercell approach. Different elementary reactions have been modeled upon reduction and oxidation on the surfaces of the active phases (i.e., MgH2, LiH, and Mg), and the associated theoretical voltages have been predicted. Experimental and theoretical results have been compared and discussed to draw a comprehensive description of the elementary surface reactions of the MgH2 conversion in lithium cells. © 2015 American Chemical Society.

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