Order-Disorder Transition driven Superionic Conduction in the New Plastic Polymorph of Na4P2S6

Sodium thiophophates are promising materials for large-scale energy storage applications benefiting from high ionic conductivities and the-political abundance of the elements. A representative of this class is Na4P2S6, which currently shows two known polymorphs–α and β. This work describes a third polymorph of Na4P2S6, γ, that forms above 580◦C, exhibits fast ion conduction with low activation energy, and is mechanically soft. Based on high-temperature diffraction, pair distribution function analysis, thermal analysis, impedance spectroscopy, and ab initio molecular dynamic calculations, γ-Na4P2S6 is identified to be a plastic crystal, characterized by dynamic orientational disorder of the P2S64– anions on a translationally fixed body centered cubic lattice. The prospect of stabilizing plastic crystals at operating temperatures of solid-state batteries and benefiting from their high ionic conductivities as well as mechanical properties could have a strong impact in the field of solid-state battery chemistry.

Ionic and Electronic Conductivities of Lithium Argyrodite Li6PS5Cl Electrolytes Prepared via Wet Milling and Post-Annealing

Lithium argyrodite Li6PS5Cl powders are synthesized from Li2S, P2S5, and LiCl via wet milling and post-annealing at 500°C for 4 h. Organic solvents such as hexane, heptane, toluene, and xylene are used during the wet milling process. The phase evolution, powder morphology, and electrochemical properties of the wet-milled Li6PS5Cl powders and electrolytes are studied. Compared to dry milling, the processing time is significantly reduced via wet milling. The nature of the solvent does not affect the ionic conductivity significantly; however, the electronic conductivity changes noticeably. The study indicates that xylene and toluene can be used for the wet milling to synthesize Li6PS5Cl electrolyte powder with low electronic and comparable ionic conductivities. The all-solid-state cell with the xylene-processed Li6PS5Cl electrolyte exhibits the highest discharge capacity of 192.4 mAh·g−1 and a Coulombic efficiency of 81.3% for the first discharge cycle.

Microstructural impacts on ionic conductivity of oxide solid electrolytes from a combined atomistic-mesoscale approach

Although multiple oxide-based solid electrolyte materials with intrinsically high ionic conductivities have emerged, practical processing and synthesis routes introduce grain boundaries and other interfaces that can perturb primary conduction channels. To directly probe these effects, we demonstrate an efficient and general mesoscopic computational method capable of predicting effective ionic conductivity through a complex polycrystalline oxide-based solid electrolyte microstructure without relying on simplified equivalent circuit description. We parameterize the framework for Li7-xLa3Zr2O12 (LLZO) garnet solid electrolyte by combining synthetic microstructures from phase-field simulations with diffusivities from molecular dynamics simulations of ordered and disordered systems. Systematically designed simulations reveal an interdependence between atomistic and mesoscopic microstructural impacts on the effective ionic conductivity of polycrystalline LLZO, quantified by newly defined metrics that characterize the complex ionic transport mechanism. Our results provide fundamental understanding of the physical origins of the reported variability in ionic conductivities based on an extensive analysis of literature data, while simultaneously outlining practical design guidance for achieving desired ionic transport properties based on conditions for which sensitivity to microstructural features is highest. Additional implications of our results are discussed, including a possible connection between ion conduction behavior and dendrite formation.

Theoretical and Experimental Studies of ion Transport in Mixed Polyanion Solid Electrolytes

Lithium and sodium (Na) mixed polyanion solid electrolytes for all-solid-state batteries display some of the highest ionic conductivities reported to date. However, the effect of polyanion mixing on ion transport properties is still debated. Here, we focus on Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3) NASICON electrolyte to elucidate the role of polyanion mixing on Na-transport properties. Although there is a large body of data available on this NASICON system, transport properties extracted from experiments or theory vary by orders of magnitude, signifying the need to bridge the gap between different studies. Here, more than 2,000 distinct ab initio-based kinetic Monte Carlo simulations have been used to map the statistically vast compositional space of NASICON over an unprecedented time range and spatial resolution and across a range of temperatures. We performed impedance spectroscopy of samples with varying Na compositions revealing that the highest ionic conductivity (~ 0.1 S cm–1) is achieved in Na3.4Zr2Si2.4P0.6O12, in line with our predictions (~0.2 S cm–1). Our predictions indicate that suitably doped NASICON compositions, especially with high silicon content, can achieve high Na+ mobilities. Our findings are relevant for the optimization of mixed polyanion solid electrolytes and electrodes, including sulfide-based polyanion frameworks, which are known for their superior ionic conductivities.

Can substitutions affect the oxidative stability of lithium argyrodite solid electrolytes?

Lithium ion conducting argyrodites are among the most studied solid electrolytes due to their high ionic conductivities. A major concern in a solid-state battery is the solid electrolyte stability. Here we present a systematic study on the influence of cationic and anionic substitution on the electrochemical stability of Li6PS5X, using step-wise cyclic voltammetry, optical band gap measurements, hard X-ray photoelectron spectroscopy along with first-principles calculations. We observe that going from Li6PS5Cl to Li6+xP1-xMxS5I (M = Si4+, Ge4+), the oxidative degradation does not change. Considering the chemical bonding shows that the valence band edges are mostly populated by non-bonding orbitals of the PS43- units or unbound sulfide anions and that simple substitutions in these sulfide-based solid electrolytes cannot improve oxidative stabilities. This work provides insights on the role of chemical bonding on the stability of superionic conductors and shows that alternative strategies are needed for long-term stable solid-state batteries.

Effects on Microstructure and Ionic Conductivity of the Co-Doping with Strontium and Samarium of Ceria with Constant Oxygen Vacancy Concentration

Partially substituted cerias are attractive materials for use as electrolytes in intermediate-temperature solid oxide fuel cells (SOFCs). Ceria doped with Sm or Gd has been found to have high ionic conductivities. However, there is interest in whether doping with multiple elements could lead to materials with higher ionic conductivities. The present study looks at the effects of co-doping Sr and Sm in ceria. A compositional series, Ce0.8+xSm0.2−2xSrxO2−δ (with x = 0–0.08), designed to have a constant oxygen vacancy concentration, was successfully prepared using the citrate–nitrate complexation method. A solubility limit of ~5 cation% Sr was found to impact material structure and conductivity. For phase-pure materials, with increasing Sr content, sinterability increased slightly and intrinsic conductivity decreased roughly linearly. The grain boundaries of phase-pure materials showed only a very small blocking effect, linked to the high-purity synthesis method employed, while at high %Sr, they became more blocking due to the presence of a SrCeO3 impurity. Grain capacitances were found to be 50–60 pF and grain boundary capacitances, 5–50 nF. The variation in the bulk capacitance with Sr content was small, and the variation in grain boundary capacitance could be explained by the variation in grain size. Slight deviations at high %Sr were attributed to the SrCeO3 impurity. In summary, in the absence of deleterious effects due to poor microstructure or impurities, such as Si, there is no improvement in conductivity on co-doping with Sr and Sm.

Understanding fast-ion conduction in solid electrolytes

The ability of some solid materials to exhibit exceptionally high ionic conductivities has been known since the observations of Michael Faraday in the nineteenth century (Faraday M. 1838 Phil. Trans. R. Soc. 90 ), yet a detailed understanding of the atomic-scale physics that gives rise to this behaviour remains an open scientific question. This theme issue collects articles from researchers working on this question of understanding fast-ion conduction in solid electrolytes. The issue opens with two perspectives, both of which discuss concepts that have been proposed as schema for understanding fast-ion conduction. The first perspective presents an overview of a series of experimental NMR studies, and uses this to frame discussion of the roles of ion–ion interactions, crystallographic disorder, low-dimensionality of crystal structures, and fast interfacial diffusion in nanocomposite materials. The second perspective reviews computational studies of halides, oxides, sulfides and hydroborates, focussing on the concept of frustration and how this can manifest in different forms in various fast-ion conductors. The issue also includes five primary research articles, each of which presents a detailed analysis of the factors that affect microscopic ion-diffusion in specific fast-ion conducting solid electrolytes, including oxide-ion conductors Gd 2 Zr 2 O 7 and Bi 4 V 2 O 11 , lithium-ion conductors Li 6 PS 5 Br and Li 3 OCl , and the prototypical fluoride-ion conductor β - PbF 2 . This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

Diffuson-mediated thermal and ionic transport in superionic conductors

Ultra-low lattice thermal conductivity as often found in superionic compounds is greatly beneficial for thermoelectric performance, however, a high ionic conductivity can lead to device degradation. Conversely, high ionic conductivities are searched for materials in solid-state battery applications. It is commonly thought that ionic transport induces low thermal conductivity and that ion and thermal transport are not completely independent properties of a material. However, no direct comparison or underlying physical relationship has been shown between the two. Here we establish that ionic transport can be varied independent of thermal transport in Ag+ superionic conductors, even though both phenomena arise from atomic vibrations. Thermal conductivity measurements, in conjunction with two-channel lattice dynamics modeling, reveals that the vast majority of Ag+ vibrations have non-propagating diffuson-like character, which provides a rational for how these two transport properties can be independent. Our results provide conceptually novel lattice dynamical insights to ionic transport and confirm that ion transport is not a requirement for ultra-low thermal conductivity. Consequently, this work bridges the fields of solid state ionics and thermal transport, thus providing design strategies for functional ionic conducting materials from a vibrational perspective.