Stable High-Capacity Elemental Sulfur Cathodes with Simple Process for Lithium Sulfur Batteries.

Lithium sulfur batteries are suitable for drones due to their high gravimetric energy density (2600 Wh/kg of sulfur). However, on the cathode side, high specific capacity with high sulfur loading (high areal capacity) is challenging due to the poor conductivity of sulfur. Shuttling of Li-sulfide species between the sulfur cathode and lithium anode also limits specific capacity. Sulfur-carbon composite active materials with encapsulated sulfur address both issues but require expensive processing and have low sulfur content with limited areal capacity. Proper encapsulation of sulfur in carbonaceous structures along with active additives in solution may largely mitigate shuttling, resulting in cells with improved energy density at relatively low cost. Here, composite current collectors, selected binders, and carbonaceous matrices impregnated with an active mass were used to award stable sulfur cathodes with high areal specific capacity. All three components are necessary to reach a high sulfur loading of 3.8 mg/cm2 with a specific/areal capacity of 805 mAh/g/2.2 mAh/cm2. Good adhesion between the carbon-coated Al foil current collectors and the composite sulfur impregnated carbon matrices is mandatory for stable electrodes. Swelling of the binders influenced cycling retention as electroconductivity dominated the cycling performance of the Li-S cells comprising cathodes with high sulfur loading. Composite electrodes based on carbonaceous matrices in which sulfur is impregnated at high specific loading and non-swelling binders that maintain the integrated structure of the composite electrodes are important for strong performance. This basic design can be mass produced and optimized to yield practical devices.

High Energy Density Rechargeable Batteries Based on Li Metal Anodes. The Role of Unique Surface Chemistry Developed in Solutions Containing Fluorinated Organic Co-solvents.

To date, lithium ion batteries are considered as a leading energy storage and conversion technology, ensuring a combination of high energy and power densities and prolonged cycle life. A critical point for elaboration of high energy density secondary Li batteries is the use of high specific capacity positive and negative electrodes. Among anode materials, Li metal anodes are considerably superior due to having the highest theoretical specific capacity (3860 mAh g-1) and lowest negative redox potential (-3.040 V vs a standard hydrogen electrode). Combination of Li metal anodes with Li[NiCoM]O2-layered cathodes with a high stable specific capacity of about 200 up to 250 mAh g-1 is particularly attractive. The development of advanced electrolyte solutions which ensure effective passivation of the electrodes' surfaces is of critical importance. Considerable efforts have been focused on fluorinated organic co-solvents and specifically fluoroethylene carbonate (FEC) due to the formation of thin, flexible Li-ions-conducting surface films with excellent protective properties. However, in the FEC-based solutions, detrimental "cross talk" between the Li anodes and the Li[NiCoM]O2 cathodes leads to worsening of the passivation of Li metal anodes, consumption of the electrolyte solutions, and limited cycle life of full Li|Li[NiCoM]O2 cells cycled with a low amount of the electrolyte solution and practical cycling parameters. The addition of difluoroethylene carbonate (DFEC) co-solvent with lower LUMO energy leads to a significant improvement in the cycling behavior of full cells. Using fluorinated co-solvents possessing synergistic effects is very promising and paves the way for developing rechargeable batteries with the highest energy density.

High-Performance Cells Containing Lithium Metal Anodes, LiNi0.6Co0.2Mn0.2O2 (NCM 622) Cathodes, and Fluoroethylene Carbonate-Based Electrolyte Solution with Practical Loading.

We report on the highly stable lithium metal|LiNi0.6Co0.2Mn0.2O2 (NCM 622) cells with practical electrodes' loading of 3.3 mA h g-1, which can undergo many hundreds of stable cycles, demonstrating high rate capability. A key issue was the use of fluoroethylene carbonate (FEC)-based electrolyte solutions (1 M LiPF6 in FEC/dimethyl carbonate). Li|NCM 622 cells can be cycled at 1.5 mA cm-2 for more than 600 cycles, whereas symmetric Li|Li cells demonstrate stable performance for more than 1000 cycles even at higher areal capacity and current density. We attribute the excellent performance of both Li|NCM and Li|Li cells to the formation of a stable and efficient solid electrolyte interphase (SEI) on the surface of the Li metal electrodes cycled in FEC-based electrolyte solutions. The composition of the SEI on the Li and the NCM electrodes is analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. A drastic capacity fading of Li|NCM cells is observed, followed by spontaneous capacity recovery during prolonged cycling. This phenomenon depends on the current density and the amount of the electrolyte solution and relates to kinetic limitations because of SEI formation on the Li anodes in the FEC-based electrolyte solution.

Fluoroethylene carbonate as an important component in electrolyte solutions for high-voltage lithium batteries: role of surface chemistry on the cathode.

The effect of fluorinated ethylene carbonate (FEC) as a cosolvent in alkyl carbonates/LiPF6 on the cycling performance of high-voltage (5 V) cathodes for Li-ion batteries was investigated using electrochemical tools, X-ray photoelectron spectroscopy (XPS), and high-resolution scanning electron microscopy (HRSEM). An excellent cycling stability of LiCoPO4/Li, LiNi0.5Mn1.5O4/Si, and LiCoPO4/Si cells and a reasonable cycling of LiCoPO4/Si cells was achieved by replacing the commonly used cosolvent ethylene carbonate (EC) by FEC in electrolyte solutions for high-voltage Li-ion batteries. The roles of FEC in the improvement of the cycling performance of high-voltage Li-ion cells and of surface chemistry on the cathode are discussed.

Unusually high stability of a poly(alkylquaterthiophene-alt-oxadiazole) conjugated copolymer in its n and p-doped states.

Incorporation of electron accepting units (oxadiazole) into the 2,5-thienylene conjugated chain leads to a significant improvement in the n-doping-undoping redox stability of the resulting polymer.

Conformational dynamics in nitrogen-fused azabicycles.

Using molecular mechanics (MM3 force field)-based methodology, conformational dynamics have been studied for 1-azabicyclo[2.2.0]hexane, 1-azabicylo[3.3.0]octane, and 1-azabicylo[4.4.0]decane. Obtained conformational schemes describe the flexibity of these parent azabicyles as well as permit us to estimate conformational mobility in related N-fused systems. Quantum mechanics ab initio calculations have been used in order to check the reliability of molecular mechanics-provided estimates of relative energy of conformers. The previous dynamic NMR (DNMR) data have been reinterpreted for some polycyclic alkaloids.