Activating Magnesium Electrolytes through Chemical Generation of Free Chloride and Removal of Trace Water

Mg batteries are attractive next-generation energy storage systems due to their high natural abundance, inexpensive cost, and high theoretical capacity compared to conventional Li-ion based systems. The high energy density is achieved by electrodeposition and stripping of a Mg metal anode and requires the development of effective electrolytes enabled by a mechanistic understanding of the charge-transfer mechanism. The magnesium aluminum chloride complex (MACC) electrolyte is a good model system to study the mechanism as the solution phase speciation is known. Previously, we reported that minor addition of Mg(HMDS)₂ to the MACC electrolyte causes significant improvement in the Mg deposition and stripping voltammetry resulting in good Coulombic efficiency on cycle one and, therefore, negating the need for electrochemical conditioning. To determine the cause of the improved electrochemistry, here we probe the speciation of the electrolyte after Mg(HMDS)₂ addition using Raman spectroscopy, ²⁷Al nuclear magnetic resonance spectroscopy, and ¹H–²⁹Si heteronuclear multiple bond correlation spectroscopy on MACC + Mg(HMDS)₂ at various Mg(HMDS)₂ concentrations. Mg(HMDS)₂ scavenges trace H₂O, but it also reacts with MACC complexes, namely, AlCl₄⁻, to form free Cl⁻. We suggest that although both the removal of H₂O and the formation of free Cl⁻ improve electrochemistry by altering the speciation at the interface, the latter has a profound effect on electrodeposition and stripping of Mg

Conditioning-Free Mg Electrolyte by the Minor Addition of Mg(HMDS)₂

Mg-based batteries are an attractive next-generation energy storage chemistry due to the high natural abundance and inexpensive cost of Mg, along with the high theoretical energy density compared to that of conventional Li-ion chemistry. The greater energy density is predicated on a Mg metal anode, and pathways to achieving reversible Mg electrodeposition and stripping are reliant on the development of Mg electrolytes. Although Mg electrolyte chemistry has advanced significantly from the reactive Grignards of the 1920s to the carboranes of this decade, there remains significant challenges in correlating the Mg metal anode electrochemistry with the composition of the electrolyte salts as a result of the complicated interface of Mg metal and the electrolyte. To probe the effect of the interface on Mg electrodeposition, we turn to an electrolyte with a known solution-phase composition: the magnesium aluminum chloride complex (MACC) electrolyte. The MACC electrolyte requires electrolytic conditioning to support reversible Mg electrodeposition and stripping. Here, we show that a small concentration (2–5 mM) of Mg(HMDS)₂ with respect to the MACC electrolyte salts suppresses Al³⁺ deposition and promotes reversible Mg electrodeposition and stripping in the first cycle. The significant effect of a small concentration of additive is attributed to changes to the electrode interface. The impact of the Mg interface on the observed electrochemical performance is discussed

Continued investigation of LDEF's structural frame and thermal blankets by the Meteoroid and Debris Special Investigation Group

This report focuses on the data acquired by detailed examination of LDEF intercostals, 68 of which are now in possession of the Meteoroid and Debris Special Investigation Group (M&D SIG) at JSC. In addition, limited data will be presented for several small sections from the A0178 thermal control blankets that were examined/counted prior to being shipped to Principal Investigators (PI's) for scientific study. The data presented here are limited to measurements of crater and penetration-hole diameters and their frequency of occurrence which permits, yet also constrains, more model-dependent, interpretative efforts. Such efforts will focus on the conversion of crater and penetration-hole sizes to projectile diameters (and masses), on absolute particle fluxes, and on the distribution of particle-encounter velocities. These are all complex issues that presently cannot be pursued without making various assumptions which relate, in part, to crater-scaling relationships, and to assumed trajectories of natural and man-made particle populations in LEO that control the initial impact conditions

Reversible Capacity of Conductive Carbon Additives at Low Potentials: Caveats for Testing Alternative Anode Materials for Li-Ion Batteries

The electrochemical performance of alternative anode materials for Li-ion batteries is often measured using composite electrodes consisting of active material and conductive carbon additives. Cycling of these composite electrodes at low voltages demonstrates charge storage at the operating potentials of viable anodes, however, the conductive carbon additive is also able to store charge in the low potential regime. The contribution of the conductive carbon additives to the observed capacity is often neglected when interpreting the electrochemical performance of electrodes. To provide a reference for the contribution of the carbons to the observed capacity, we report the charge storage behavior of two common conductive carbon additives Super P and Ketjenblack as a function of voltage, rate, and electrolyte composition. Both carbons exhibit substantial capacities after 100 cycles, up to 150 mAh g^(−1), when cycled to 10 mV. The capacity is dependent on the discharge cutoff voltage and cycling rate with some dependence on electrolyte composition. The first few cycles are dominated by the formation of the SEI followed by a fade to a steady, reversible capacity thereafter. Neglecting the capacity of the carbon additive can lead to significant errors in the estimation of charge storage capabilities of the active material

Selective formation of pyridinic-type nitrogen-doped graphene and its application in lithium-ion battery anodes

We report a high-yield single-step method for synthesizing nitrogen-doped graphene nanostripes (N-GNSPs) with an unprecedentedly high percentage of pyridinic-type doping (>86% of the nitrogen sites), and investigate the performance of the resulting N-GNSPs as a lithium-ion battery (LIB) anode material. The as-grown N-GNSPs are compared with undoped GNSPs using scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), helium ion-beam microscopy (HIM), and electrochemical methods. As an anode material we find that pyridinic-type N-GNSPs perform similarly to undoped GNSPs, suggesting that pyridinic sites alone are not responsible for the enhanced performance of nitrogen-doped graphene observed in previous studies, which contradicts common conjectures. In addition, post-mortem XPS measurements of nitrogen-doped graphene cycled as a lithium-ion battery anode are conducted for the first time, which reveal direct evidence for irreversible chemical changes at the nitrogen sites during cycling. These findings therefore provide new insights into the mechanistic models of doped graphene as LIB anodes, which are important in improving the anode designs for better LIB performance

Elucidating Zn and Mg Electrodeposition Mechanisms in Nonaqueous Electrolytes for Next-Generation Metal Batteries

Cyclic voltammetry and linear sweep voltammetry with an ultramicroelectrode (UME) were employed to study Zn and Mg electrodeposition and the corresponding mechanistic pathways. CVs obtained at a Pt UME for Zn electroreduction from a trifluoromethylsulfonyl imide (TFSI^–) and chloride-containing electrolyte in acetonitrile exhibit current densities that are scan rate independent, as expected for a simple electron transfer at a UME. However, CVs obtained from three different Mg-containing electrolytes in THF exhibit an inverse dependence between scan rate and current density. COMSOL-based simulation suggests that Zn electrodeposition proceeds via a simple one-step, two-electron transfer (E) mechanism. Alternatively, the Mg results are best described by invoking a chemical step prior to electron transfer: a chemical–electrochemical (CE) mechanism. The chemical step exhibits an activation energy of 51 kJ/mol. This chemical step is likely the disproportionation of the chloro-bridged dimer [Mg_2(μ–Cl)_3·6THF]^+ present in active electrodeposition solutions. Our work shows that Mg deposition kinetics can be improved by way of increased temperature

Effect of the Electrolyte Solvent on Redox Processes in Mg–S Batteries

Mg–S batteries are attractive for next-generation energy storage because of their high theoretical capacity and low cost. The foremost challenge in Mg–S batteries is designing electrolytes that support reversible electrochemistry at both electrodes. Here, we target a solution-mediated reduction pathway for the S_8 cathode by tailoring the electrolyte solvent. Varying the solvent in Mg-based systems is complicated because of the active nature of the solvent in solvating Mg^(2+) and the complex dynamics of electrolyte–Mg interfaces. To understand the effect of the solvent on the S_8 reduction processes in the Mg–S cell, the magnesium–aluminum chloride complex (MACC) electrolyte was prepared in different ethereal solvents. Reversible Mg electrodeposition is demonstrated in the MACC electrolyte in several solvent systems. The electrodeposition overpotentials and current densities are found to vary with the solvent, suggesting that the solvent plays a noninnocent role in the electrochemical processes at the Mg interface. Mg–S cells are prepared with the electrolytes to understand how the solvent affects the reduction of S_8. A reductive wave is present in all linear-sweep voltammograms, and the peak potential varies with the solvent. The peak potential is approximately 0.8 V versus Mg/Mg^(2+), lower than the expected reduction potential of 1.7 V. We rule out passivation of the Mg anode as the cause for the low voltage peak potential, making processes at the S8 cathode the likely culprit. The ability to oxidize MgS with the MACC electrolyte is also examined, and we find that the oxidation current can be attributed to side reactions at the C–electrolyte interface

Effect of the Electrolyte Solvent on Redox Processes in Mg–S Batteries

Mg–S batteries are attractive for next-generation energy storage because of their high theoretical capacity and low cost. The foremost challenge in Mg–S batteries is designing electrolytes that support reversible electrochemistry at both electrodes. Here, we target a solution-mediated reduction pathway for the S_8 cathode by tailoring the electrolyte solvent. Varying the solvent in Mg-based systems is complicated because of the active nature of the solvent in solvating Mg^(2+) and the complex dynamics of electrolyte–Mg interfaces. To understand the effect of the solvent on the S_8 reduction processes in the Mg–S cell, the magnesium–aluminum chloride complex (MACC) electrolyte was prepared in different ethereal solvents. Reversible Mg electrodeposition is demonstrated in the MACC electrolyte in several solvent systems. The electrodeposition overpotentials and current densities are found to vary with the solvent, suggesting that the solvent plays a noninnocent role in the electrochemical processes at the Mg interface. Mg–S cells are prepared with the electrolytes to understand how the solvent affects the reduction of S_8. A reductive wave is present in all linear-sweep voltammograms, and the peak potential varies with the solvent. The peak potential is approximately 0.8 V versus Mg/Mg^(2+), lower than the expected reduction potential of 1.7 V. We rule out passivation of the Mg anode as the cause for the low voltage peak potential, making processes at the S8 cathode the likely culprit. The ability to oxidize MgS with the MACC electrolyte is also examined, and we find that the oxidation current can be attributed to side reactions at the C–electrolyte interface