Tuning the Formation and Structure of the Silicon Electrode/Ionic Liquid Electrolyte Interphase in Superconcentrated Ionic Liquids.

The latest advances in the stabilization of Li/Na metal battery and Li-ion battery cycling have highlighted the importance of electrode/electrolyte interface [solid electrolyte interphase (SEI)] and its direct link to cycling behavior. To understand the structure and properties of the SEI, we used combined experimental and computational studies to unveil how the ionic liquid (IL) cation nature and salt concentration impact the silicon/IL electrolyte interfacial structure and the formed SEI. The nature of the IL cation is found to be important to control the electrolyte reductive decomposition that influences the SEI composition and properties and the reversibility of the Li-Si alloying process. Also, increasing the Li salt concentration changes the interface structure for a favorable and less resistive SEI. The most promising interface for the Si-based battery was found to be in P1222FSI with 3.2 m LiFSI, which leads to an optimal SEI after 100 cycles in which LiF and trapped LiFSI are the only distinguishable lithiated and fluorinated products detected. This study shows a clear link between the nanostructure of the IL electrolyte near the electrode surface, the resulting SEI, and the Si negative electrode cycling performance. More importantly, this work will aid the rational design of Si-based Li-ion batteries using IL electrolytes in an area that has so far been neglected, reinforcing the benefits of superconcentrated electrolyte systems.

Engineering high-energy-density sodium battery anodes for improved cycling with superconcentrated ionic-liquid electrolytes.

Non-uniform metal deposition and dendrite formation in high-density energy storage devices reduces the efficiency, safety and life of batteries with metal anodes. Superconcentrated ionic-liquid electrolytes (for example 1:1 ionic liquid:alkali ion) coupled with anode preconditioning at more negative potentials can completely mitigate these issues, and therefore revolutionize high-density energy storage devices. However, the mechanisms by which very high salt concentration and preconditioning potential enable uniform metal deposition and prevent dendrite formation at the metal anode during cycling are poorly understood, and therefore not optimized. Here, we use atomic force microscopy and molecular dynamics simulations to unravel the influence of these factors on the interface chemistry in a sodium electrolyte, demonstrating how a molten-salt-like structure at the electrode surface results in dendrite-free metal cycling at higher rates. Such a structure will support the formation of a more favourable solid electrolyte interphase, accepted as being a critical factor in stable battery cycling. This new understanding will enable engineering of efficient anode electrodes by tuning the interfacial nanostructure via salt concentration and high-voltage preconditioning.

Tuning Sodium Interfacial Chemistry with Mixed-Anion Ionic Liquid Electrolytes.

The interphase layer that forms on either the anode or the cathode is considered to be one of the critical components of a high performing battery. This solid-electrolyte interphase (SEI) layer determines the stability of the electrode in the presence of a given electrolyte as well as the internal resistance of a battery, and hence the overpotential of a cell. In the case of lithium ion batteries where carbonate based electrolytes are used, additives including hexafluorophosphate (PF6), bis-trifluoromethylsulfonimide (TFSI), (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI), and fluorosulfonimde (FSI) are used to obtain favorable SEI layers. Ionic liquids and salts based on anions containing nitrile groups, including dicyanamide (DCA), offer a less expensive alternative to a fluorinated anion and have also been shown to support stable electrochemistry in lithium and sodium systems. However, longer term cycling leads to the eventual passivation of the electrode, presumed to be due to the instability of the DCA anion. We herein consider the use of a fluorinated anion to control the interfacial electrochemistry and provide a more stable SEI in DCA ILs. We investigate the addition of NaDCA, NaFSI, NaTFSI, and NaFTFSI to the methylpropylpyrrolidinium dicyanamide ([C3mpyr]DCA) ionic liquid. NaFSI was found to generate a more stable SEI layer, as evidenced by extended symmetric cell cycling, while the TFSI and FTFSI salts both lead to thicker, highly passivating surfaces. We use molecular dynamics, infrared spectroscopy and X-ray photoelectron spectroscopy to interrogate and discuss the influence of the anion on the bulk electrolyte, the interfacial electrolyte structure, and the formation of the SEI layer, in order to rationalize the contrasting electrochemical observations.

Homogeneous Metal Nitrate Hydroxide Nanoarrays Grown on Nickel Foam for Efficient Electrocatalytic Oxygen Evolution.

Developing facile routes for fabricating highly efficient oxygen evolution reaction (OER) electrocatalysts is in great demand but remains a great challenge. Herein, a novel molten salt decomposition method to prepare 3D metal nitrate hydroxide (MNH, M = Ni, Co, and Cu) nanoarrays homogenously grown on different conductive substrates, especially on nickel foam (NF) for OER applications, is reported. Compared with the as-prepared CoNH/NF and CuNH/NF, NiNH/NF presents a superior electrocatalytic OER activity and stability in an alkaline solution, with a very low overpotential of only 231 mV versus a reversible hydrogen electrode to deliver a geometrical catalytic current density of 50 mA cm-2 and a low Tafel slope of 81 mV dec-1 , outperforming most reported transition metal compound catalysts. Structural investigation after the OER process reveals the morphology integrity of the nanoarrays but the formation of metal oxyhydroxide (for NiNH and CoNH) or oxide (for CuNH) as the likely real active species. These metal nitrate hydroxide non-noble metal electrocatalysts can be prepared by an economical and simple method, with enhanced intrinsic activity and long-term stability and durability, which might be new candidates for energy conversion and storage applications.

Metal organic framework-derived CoPS/N-doped carbon for efficient electrocatalytic hydrogen evolution.

Electrocatalytic hydrogen evolution has attracted a great deal of attention due to the urgent need for clean energy. Herein, we demonstrate the synthesis of ternary pyrite-type cobalt phosphosulphide (CoPS) nanoparticles supported on a nitrogen-doped carbon matrix, CoPS/N-C, through carbonization and subsequent phosphosulfurization of Co-based zeolitic imidazolate frameworks (ZIF-67), as promising hydrogen evolution reaction (HER) electrocatalysts in both acidic and alkaline solutions. The polyhedral structure of ZIF-67 can be well maintained in the as-prepared CoPS/N-C nanocomposites. In particular, CoPS/N-C provides a geometric catalytic current density of -10 mA cm-2 at overpotentials of -80 and -148 mV vs. a reversible hydrogen electrode (RHE) and a Tafel slope of 68 and 78 mV dec-1 in 0.5 M H2SO4 and 1 M KOH, respectively, which is superior to most of the transition metal phosphosulfide materials. This MOF-derived synthesis of a transition metal phosphosulfide supported heteroatom-doped carbon matrix provides a promising opportunity for the development of highly efficient electrocatalysts for renewable energy devices.

Improving the intrinsic electrocatalytic hydrogen evolution activity of few-layer NiPS3 by cobalt doping.

Here we demonstrate the improvement of the intrinsic electrocatalytic hydrogen evolution activity of NiPS3 by proper cobalt doping. The optimized Ni0.95Co0.05PS3 nanosheets display a geometric catalytic current density of -10 mA cm-2 at an overpotential of 71 mV vs. RHE and a Tafel slope of 77 mV dec-1 in 1.0 M KOH.