Incorporation of Lithium by MgH₂: An Ab Initio Study

The incorporation of lithium by MgH₂ through an electrochemical conversion reaction is a valuable alternative to Li intercalation into graphite for next-generation Li-ion cells. The incorporation of lithium occurs by the reduction of magnesium hydride to magnesium metal nanoparticles surrounded by an amorphous matrix of lithium hydride. In this study we present a computational investigation of the conversion reaction of MgH₂ to give Mg and LiH by first-principles methods. Density functional theory calculations have been performed using plane waves and projector augmented wave (PAW) pseudopotentials within the generalized-gradient approximation by the VASP code. The existence of intermediate phases has been checked by finite temperature ab initio thermodynamic calculations. Also, the occurrence of solid solutions in the first stages of lithium incorporation has been studied by the supercell approach by predicting their thermodynamic stability at 0 K. Five different solid solutions have been mimed by forming 0D defects in the MgH₂ lattice: (a) vacancies in the hydride sites; (b) interstitial lithium insertion; (c) substitution of lithium in hydride sites; (d) substitution of lithium in magnesium sites; (e) substitution of lithium in magnesium sites with parallel formation of vacancies in the hydride sites. Preliminary results about the thermodynamics of the conversion reaction in nanometric MgH₂ clusters are also discussed

Origin of the Voltage Hysteresis of MgH₂ Electrodes in Lithium Batteries

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

Origin of the Voltage Hysteresis of MgH₂ Electrodes in Lithium Batteries

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

Sulfonated Graphene Oxide Platelets in Nafion Nanocomposite Membrane: Advantages for Application in Direct Methanol Fuel Cells

Graphene oxide (GO) is well known as an excellent amphiphilic material due to its oxygen-containing functional groups and its chemical tunability. By intercalation chemistry, organo-modified GO containing sulfonilic terminal groups were prepared and used as nanoadditive in Nafion polymer for the creation of hybrid exfoliated composites. The incorporation of hydrophilic 2D platelike layers in the Nafion membranes is expected to induce advantages in terms of thermal stability and mechanical and barrier properties (limitation of the methanol crossover by increased tortuosity and obstruction effect), although it may negatively affect the proton conductivity. In this work, we show how different preparation methods of the nanocomposites influence morphology, transport properties, and barrier effect to methanol. The hybrid membranes are characterized by powder X-ray diffraction and microscopies (SEM, TEM, and AFM). Water and methanol transport properties inside the nanocomposites are investigated by NMR spectroscopy (diffusivity and relaxation times), unveiling a reduction of the methanol diffusion and, nevertheless, an increase in the proton mobility and water retention at high temperatures. Finally, the electrochemical properties are investigated by direct methanol fuel cell (DMFC) tests, showing a significant reduction of the ohmic losses at high temperatures, extending in this way the operating range of a DMFC