A nanostructured C‐Li2S composite nanopowder is prepared via a scalable, high‐throughput solution‐processing method based on the steric separation of freshly nucleated Li2S nanoparticles and their self‐assembling. Each 100–200 nm particle is composed of smaller 5–20 nm Li2S nanoparticles uniformly distributed within a rigid carbon matrix. When used as a cathode material for Li cells, this composite demonstrates high rate performance, near‐theoretical capacity utilization, and excellent cycle stability.

Lithium-metal fl uoride (MF) batteries offer the highest theoretical energy density, exceeding that of the sulfur-lithium cells. However, conversion-type MF cathodes suffer from high resistance, small capacity utilization at room temperature, irreversible structural changes, and rapid capacity fading with cycling. In this study, the successful application of the approach to overcome such limitations and dramatically enhance electrochemical performance of Li-MF cells is reported. By using iron fl uoride (FeF 2 ) as an example, Li-MF cells capable of achieving near-theoretical capacity utilization are shown when MF is infi ltrated into the carbon mesopores. Most importantly, the ability of electrolytes based on the lithium bis(fl uorosulfonyl)imide (LiFSI) salt is presented to successfully prevent the cathode dissolution and leaching via in situ formation of a Li ion permeable protective surface layer. This layer forms as a result of electrolyte reduction/oxidation reactions during the fi rst cycle of the conversion reaction, thus minimizing the capacity losses during cycling. Postmortem analysis shows the absence of Li dendrites, which is important for safer use of Li metal anodes. As a result, Li-FeF 2 cells demonstrate over 1000 stable cycles. Quantum chemistry calculations and postmortem analysis provide insights into the mechanisms of the passivation layer formation and the performance boost.

ion battery (LIB) chemistry. Such LIB cells offer signifi cantly higher energy density than lead-acid or nickel-metal hydride batteries and reasonably good cycle stability. [ 4 ] However there is a plenty of room for improvement in various aspects, such as cost, safety, energy density, power, thermal, and cycling stabilities. [ 5 ] One strategy to improve LIB is to employ higher capacity cathode materials and, in some cases, simultaneously replace graphite anodes with higher capacity Li electrodes. There has been a signifi cant research interest in the development of higher-specifi c-energy cathodes, particularly in identifying alternatives for the lithium cobalt oxide (LCO), NMC and NCA, which commonly offer capacity in the range from 150 to 200 mAh g -1 . The moderate energy density, insuffi cient safety characteristics and relativity high cost of these cathode materials are among the major motivations for investigating new cathode materials with higher energy density and competitive price. The lithium-sulfur (Li-S) cells have demonstrated promising energy density values, exceeding those of the high energy density LIBs. [ 6,7 ] However, the majority of Li-S cells exhibit relatively poor rate and cycle stability, which limit opportunities for their commercialization. One of the challenges is the dissolution of intermediate lithium polysulfi de species formed during cycling, which induces a shuttle phenomenon (low Coulombic effi ciency). Also, low intrinsic electric conductivity of S and largely insoluble low order lithium polysulfi des precipitation (such as Li 2 S, and Li 2 S 2 ) are also not desirable for power capability and cycling stability.The somewhat similar lithium-selenium (Li-Se) system in non-aqueous media was recently found to offer promising performances. [ 8 ] Selenium has high theoretical volumetric capacity of ≈3270 mAh cm -3 (not considering expansion upon lithiation) and moderately high theoretical gravimetric capacity of ≈678 mAh g -1 based on molecular weight of 78.96 g mol -1 and density of 4.82 g cm -3 . [ 9 ] High volumetric capacity is particularly attractive for both EV and electronic device applications considering battery packs shall be installed in the limited space. Challenges with Li-Se systems are fairly analogous to that of Li-S systems, including polyselenide dissolution during the cell operation and limited electronic conductivity (although an order of magnitude higher than in S). Only few research groups have reported characterizations of Li-Se systems, where they utilized porous carbon as a conductive framework to entrap Se. [ 10,11 ] This approach might be able to slightly reduce polyselenide dissolution and improve electrical conductivity of selenium Nanocomposites of selenium (Se) and ordered mesoporous silicon carbidederived carbon (OM-SiC-CDC) are prepared for the fi rst time and studied as cathodes for lithium-selenium (Li-Se) batteries. The higher concentration of Li salt in the electrolytes greatly improves Se utilization and cell cycle stability. Se-CDC shows signifi...

The lithium/selenium (Li/Se) rechargeable battery chemistry offers a higher energy density than traditional Li ion battery cells. However, high solubility of polyselenides in suitable electrolytes causes Se loss during electrochemical cycling, and leads to poor cycle stability. This study presents a simple technique to form a protective, solid electrolyte layer on the cathode surface. This protective layer remains permeable to Li ions, but prevents transport of polyselenides, thus dramatically enhancing cell cycle stability. The greatly reduced reactivity of polyselenides with fluorinated carbonates (such as fluoroethylene carbonates [FEC]) permits using their in situ reduction for low-cost formation of protective coatings on Se cathodes.

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