Reactive Interactions between the Ionic Liquid BMP‐TFSI and a Na Surface

In order to obtain atomistic insights into the initial stages of the formation of the solid electrolyte interphase (SEI) in Na ion or Na metal batteries, we employ surface chemistry experiments and DFT calculations to study the interactions and reactions between a Na surface and the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI), a candidate to be used as electrolyte in batteries. Oxygen-free Na thin films, which were grown on Ru(0001) and characterized by X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), can be understood as model of a Na-rich electrode. After deposition of submonolayer to multilayer BMP-TFSI films on the Na thin films at room temperature, XPS measurements revealed partial decomposition and the formation of a ‘contact layer’ at the Na surface, consisting of mainly TFSI-based decomposition products

Influence of Complexing Additives on the Reversible Deposition/Dissolution of Magnesium in an Ionic Liquid

Aiming at a fundamental understanding of the synergistic effects of different additives on the electrochemical Mg deposition/dissolution in an ionic liquid, we have systematically investigated these processes in a combined electrochemical and theoretical study, using 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMP‐TFSI) as the solvent and a cyclic ether (18‐crown‐6) and magnesium borohydride as additives. Both crown ether and BH4− improve Mg deposition, its reversibility, and cycling stability. The combined presence of both additives and their concentration relative to that of Mg$^{2+}$ are decisive for more facile and reversible Mg deposition/dissolution. These results and those of quantum chemical calculations indicate that 18‐crown‐6 can partly displace TFSI− from its direct coordination to Mg$^{2+}$. Furthermore, the interaction between Mg$^{2+}$ and directly coordinated TFSI− is weakened by coordination with 18‐crown‐6, preventing its Mg$^{+}$‐induced decomposition. Finally, Mg deposition is improved by the weaker overall coordination upon Mg$^{2+}$ reduction to Mg$^{+}$

Solvent-Dictated Sodium Sulfur Redox Reactions: Investigation of Carbonate and Ether Electrolytes

Sulfur-based cathode chemistries are essential for the development of high energy density alkali-ion batteries. Here, we elucidate the redox kinetics of sulfur confined on carbon nanotubes, comparing its performance in ether-based and carbonate-based electrolytes at room temperature. The solvent is found to play a key role for the electrochemical reactivity of the sulfur cathode in sodium–sulfur (Na–S) batteries. Ether-based electrolytes contribute to a more complete reduction of sulfur and enable a higher electrochemical reversibility. On the other hand, an irreversible solution-phase reaction is observed in carbonate solvents. This study clearly reveals the solvent-dependent Na–S reaction pathways in room temperature Na–S batteries and provides an insight into realizing their high energy potential, via electrolyte formulation design

Zinc‐Ion Hybrid Supercapacitors Employing Acetate‐Based Water‐in‐Salt Electrolytes

Halide-free, water-in-salt electrolytes (WiSEs) composed of potassium acetate (KAc) and zinc acetate (ZnAc$_2$) are investigated as electrolytes in zinc-ion hybrid supercapacitors (ZHSs). Molecular dynamics simulations demonstrate that water molecules are mostly non-interacting with each other in the highly concentrated WiSEs, while “bulk-like water” regions are present in the dilute electrolyte. Among the various concentrated electrolytes investigated, the 30 m KAc and 1 m ZnAc$_2$ electrolyte (30K1Zn) grants the best performance in terms of reversibility and stability of Zn plating/stripping while the less concentrated electrolyte cannot suppress corrosion of Zn and hydrogen evolution. The ZHSs utilizing 30K1Zn, in combination with a commercial activated carbon (AC) positive electrode and Zn as the negative electrode, deliver a capacity of 65 mAh g$^{−1}$ (based on the AC weight) at a current density of 5 A g$^{−1}$. They also offer an excellent capacity retention over 10 000 cycles and an impressive coulombic efficiency (≈100%)

Fast kinetics of multivalent intercalation chemistry enabled by solvated magnesium-ions into self-established metallic layered materials

Rechargeable magnesium batteries are one of the most promising candidates for next-generation battery technologies. Despite recent significant progress in the development of efficient electrolytes, an on-going challenge for realization of rechargeable magnesium batteries remains to overcome the sluggish kinetics caused by the strong interaction between double charged magnesium-ions and the intercalation host. Herein, we report that a magnesium battery chemistry with fast intercalation kinetics in the layered molybdenum disulfide structures can be enabled by using solvated magnesium-ions ([Mg(DME)x]2+). Our study demonstrates that the high charge density of magnesium-ion may be mitigated through dimethoxyethane solvation, which avoids the sluggish desolvation process at the cathode-electrolyte interfaces and reduces the trapping force of the cathode lattice to the cations, facilitating magnesium-ion diffusion. The concept of using solvation effect could be a general and effective route to tackle the sluggish intercalation kinetics of magnesium-ions, which can potentially be extended to other host structures

Embedding Heterostructured α‐MnS/MnO Nanoparticles in S‐Doped Carbonaceous Porous Framework as High‐Performance Anode for Lithium‐Ion Batteries

In this work, the synthesis of α-MnS/MnO/S-doped C micro-rod composites via a simple sulfidation process is demonstrated, starting from a Mn-based metal-organic framework. The resulting heterostructured α-MnS/MnO nanoparticles (8±2 nm) are uniformly embedded into the S-doped carbonaceous porous framework with hierarchical micro-/meso-porosity. The combination of structural and compositional characteristics results in the promising electrochemical performance of the as-obtained composites as anode materials for lithium-ion batteries, coupled with high reversible capacity (940 mAh $g^{−1}$ at 0.1 A $g^{−1}$), excellent rate capability as well as long cycling lifespan at high rate of 2.0 A $g^{−1}$ for 2000 cycles with the eventual capacity of ∼300 mAh $g^{−1}$. Importantly, in situ X-ray diffraction studies clearly reveal mechanistic details of the lithium storage mechanism, involving multistep conversion processes upon initial lithiation

Model Studies on Solid Electrolyte Interphase Formation on Graphite Electrodes in Ethylene Carbonate and Dimethyl Carbonate II: Graphite Powder Electrodes

As part of a systematic study on the formation and composition of the solid electrolyte interphase (SEI) in lithium‐ion batteries (LIBs), going stepwise from highly idealized electrodes such as highly oriented pyrolytic graphite and conditions such as ultrahigh vacuum conditions to more realistic materials and reaction conditions, we investigated the decomposition of simplified electrolytes (ethylene carbonate (EC)+1 M LiPF$_{6}$ and dimethyl carbonate (DMC)+1 M LiPF$_{6}$) at binder‐free graphite powder model electrodes. The results obtained from cyclic voltammetry and ex situ X‐ray photoelectron spectroscopy half‐cell measurements – in particular on the effect of cycling rate, solvent and electrode – are explained in terms of a mechanistic model where electrolyte decomposition occurs at the SEI | electrode interface and where transport of solvent and salt species through the growing SEI plays an important role for explaining the observed change from preferential salt decomposition to solvent decomposition with increasing cycling rate