Search

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 s...

We report the first in situ variable temperature neutron powder diffraction (NPD) study of the solid-state ion conductor Li3PS4 which delineates the transitions between its three polymorphs (γ → β → α) and determines their crystalline Li-sublattices. They are compared to those of nanoporous β-Li3PS4, and β′-type Si-substituted Li3.25Si0.25P0.75S4. Importantly, Rietveld refinement of the NPD data elucidates the crystal structure of the high-temperature α-Li3PS4 polymorph for the first time and shows that it crystallizes in the Cmcm space group. The lithium diffusion pathways of both the bulk β and α polymorphs are evaluated using the maximum entropy method and bond valence site energy calculations, revealing that the structure of α-Li3PS4 favors facile 3D conduction, supported mainly by its bcc S-sublattice. The variable temperature in situ NPD study of nanoporous β-Li3PS4 identifies the presence of an amorphous organic component that is only completely removed upon transformation to the α-phase at elevated temperature, and which may play a role in stabilizing this material at room temperature. Si-substitution of Li3PS4 not only stabilizes the β′-Li3.25Si0.25P0.75S4 phase at room temperature but also prevents phase transformation to the α polymorph upon heating owing to its larger lattice volume.

Solid electrolytes are the foundation of all-solid-state batteries (ASSB) and have the potential to provide improved safety and higher energy density than existing liquid battery systems. In the important search for new lithium ion conductors with fast ionic conductivity and good mechanical properties, thiophosphates are a particularly promising class of materials. The first experimental elucidation of new lithium ion conductors in the Li1+2xZn1-xPS4 (LZPS) solid solution whose existence had been predicted by theory is reported herein. Using neutron and synchrotron x-ray powder diffraction together with electrical impedance, and Raman studies, we resolve their crystalline nature and correlate this with ionic conductivity upon increasing the lithium/zinc ratio. We show that the materials exhibit high experimental ionic conductivities - up to 8 × 10-4 S·cm-1 - and reveal the nature of likely pathways for lithium ion conduction.

We report a new sodium superionic conductor, Na11Sn2PS12, that crystallizes in an unprecedented three-dimensional structure type and exhibits an ionic conductivity of 1.4 mS cm−1, with a very low activation energy barrier for Na-ion mobility of 0.25 eV. A combination of structural elucidation via single crystal X-ray diffraction and ab initio molecular dynamics simulations show that Na+-ion conduction pathways flow through equi-energetic sodium–sulfur octahedra interconnected by partial vacancy cross-over sites in all crystallographic dimensions, providing an understanding of the underlying isotropic 3D fast-ion conduction in this material.

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.