The lithium‐sulfur battery is a compelling energy storage system because its high theoretical energy density exceeds Li‐ion batteries at much lower cost, but applications are thwarted by capacity decay caused by the polysulfide shuttle. Here, proof of concept and the critical metrics of a strategy to entrap polysulfides within the sulfur cathode by their reaction to form a surface‐bound active redox mediator are demonstrated. It is shown through a combination of surface spectroscopy and cyclic voltammetry studies that only materials with redox potentials in a targeted window react with polysulfides to form active surface‐bound polythionate species. These species are directly correlated to superior Li‐S cell performance by electrochemical studies of high surface area oxide cathodes with redox potentials below, above, and within this window. Optimized Li‐S cells yield a very low fade rate of 0.048% per cycle. The insight gained into the fundamental surface mechanism and its correlation to the stability of the electrochemical cell provides a bridge between mechanistic understanding and battery performance essential for the design of high performance Li‐S cells.
All-solid-state
Li-ion batteries (ASSBs), considered to be potential
next-generation energy storage devices, require solid electrolytes
(SEs). Thiophosphate-based materials are popular, but these sulfides
exhibit poor anodic stability and require specialty coatings on lithium
metal oxide cathodes. Moreover, electrode designs aimed at high energy
density are limited by their narrow electrochemical stability window.
Here, we report new mixed-metal halide Li3–x
M1–x
Zr
x
Cl6 (M = Y, Er) SEs with high ionic conductivityup
to 1.4 mS cm–1 at 25 °Cthat are stable
to high voltage. Substitution of M (M = Y, Er) by Zr is accompanied
by a trigonal-to-orthorhombic phase transition, and structure solution
using combined neutron and single-crystal X-ray diffraction methods
reveal a new framework. The employment of >4 V-class cathode materials
without any protective coating is enabled by the high electrochemical
oxidation stability of these halides. An ASSB showcasing their electrolyte
properties exhibits very promising cycling stability up to 4.5 V at
room temperature.
Elucidation of the structure of a new sodium superionic conductor, Na11Sn2PS12via single crystal XRD and AIMD simulations reveal isotropic 3D Na+-ion conduction pathways.
Na super ion conductor (NaSICON), Na 1+n Zr 2 Si n P 3-n O 12 is considered one of the most promising solid electrolytes; however, the underlying mechanism governing ion transport is still not fully understood. Here, the existence of a previously unreported Na5 site in monoclinic Na 3 Zr 2 Si 2 PO 12 is unveiled. It is revealed that Na + -ions tend to migrate in a correlated mechanism, as suggested by a much lower energy barrier compared to the single-ion migration barrier. Furthermore, computational work uncovers the origin of the improved conductivity in the NaSICON structure, that is, the enhanced correlated migration induced by increasing the Na + -ion concentration. Systematic impedance studies on doped NaSICON materials bolster this finding. Significant improvements in both the bulk and total ion conductivity (e.g., σ bulk = 4.0 mS cm −1 , σ total = 2.4 mS cm −1 at 25 °C) are achieved by increasing the Na content from 3.0 to 3.30-3.55 mol formula unit −1 . These improvements stem from the enhanced correlated migration invoked by the increased Coulombic repulsions when more Na + -ions populate the structure rather than solely from the increased mobile ion carrier concentration. The studies also verify a strategy to enhance ion conductivity, namely, pushing the cations into high energy sites to therefore lower the energy barrier for cation migration.
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