Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li6+xP1–xGexS5I as Probed by Neutron Diffraction and NMR

Tuning Ion Mobility in Lithium Argyrodite Solid Electrolytes via Entropy Engineering

The development of improved solid electrolytes (SEs) plays a crucial role in the advancement of bulk-type solid-state battery (SSB) technologies. In recent years, multicomponent or high-entropy SEs are gaining increased attention for their advantageous charge-transport and (electro)chemical properties. However, a comprehensive understanding of how configurational entropy affects ionic conductivity is largely lacking. Herein we investigate a series of multication-substituted lithium argyrodites with the general formula Li6+x[M1aM2bM3cM4d]S5I, with M being P, Si, Ge, and Sb. Structure-property relationships related to ion mobility are probed using a combination of diffraction techniques, solid-state nuclear magnetic resonance spectroscopy, and charge-transport measurements. We present, to the best of our knowledge, the first experimental evidence of a direct correlation between occupational disorder in the cationic host lattice and lithium transport. By controlling the configurational entropy through compositional design, high bulk ionic conductivities up to 18 mS cm-1 at room temperature are achieved for optimized lithium argyrodites. Our results indicate the possibility of improving ionic conductivity in ceramic ion conductors via entropy engineering, overcoming compositional limitations for the design of advanced electrolytes and opening up new avenues in the field.

Exploring Layered Disorder in Lithium-Ion-Conducting Li3Y1-xInxCl6

Li3Y1-xInxCl6 undergoes a phase transition from trigonal to monoclinic via an intermediate orthorhombic phase. Although the trigonal yttrium containing the end member phase, Li3YCl6, synthesized by a mechanochemical route, is known to exhibit stacking fault disorder, not much is known about the monoclinic phases of the serial composition Li3Y1-xInxCl6. This work aims to shed light on the influence of the indium substitution on the phase evolution, along with the evolution of stacking fault disorder using X-ray and neutron powder diffraction together with solid-state nuclear magnetic resonance spectroscopy, studying the lithium-ion diffusion. Although Li3Y1-xInxCl6 with x ≤ 0.1 exhibits an ordered trigonal structure like Li3YCl6, a large degree of stacking fault disorder is observed in the monoclinic phases for the x ≥ 0.3 compositions. The stacking fault disorder materializes as a crystallographic intergrowth of faultless domains with staggered layers stacked in a uniform layer stacking, along with faulted domains with randomized staggered layer stacking. This work shows how structurally complex even the "simple" series of solid solutions can be in this class of halide-based lithium-ion conductors, as apparent from difficulties in finding a consistent structural descriptor for the ionic transport.

Understanding the role of aliovalent cation substitution on the li-ion diffusion mechanism in Li6+xP1-xSixS5Br argyrodites

Due to their high ionic conductivity, lithium-ion conducting argyrodites show promise as solid electrolytes for solid-state batteries. Aliovalent substitution is an effective technique to enhance the transport properties of Li6PS5Br, where aliovalent Si substitution triples ionic conductivity. However, the origin of this experimentally observed increase is not fully understood. Our density functional theory (DFT) study reveals that Si4+ substitution increases Li diffusion by activating Li occupancy in the T4 sites. Redistribution of Li-ions within the lattice results in a more uniform distribution of Li around the T4 and neighboring T5 sites, flattening the energy landscape for diffusion. Since the T4 site is positioned in the intercage jump pathway, an increase in the intercage jump rate is found, which is directly related to the macroscopic diffusion and bulk conductivity. Analysis of neutron diffraction experiments confirms partial T4 site occupancy, in agreement with the computational findings. Understanding the aliovalent substitution effect on interstitials is crucial for improving solid electrolyte ionic conductivity and advancing solid-state battery performance.

Solid-State Electrolytes and Electrode/Electrolyte Interfaces in Rechargeable Batteries

Solid-state batteries (SSBs) are considered to be one of the most promising candidates for next-generation energy storage systems due to the high safety, high energy density and wide operating temperature range of solid-state electrolytes (SSEs) they use. Unfortunately, the practical application of SSEs has rarely been successful, which is largely attributed to the low chemical stability and ionic conductivity, ineluctable solid-solid interface issues including limited ion transport channels, high energy barriers, and poor interface contact. A comprehensive understanding of ion transport mechanisms of various SSEs, interactions between fillers and polymer matrixes and the role of the interface in SSBs are indispensable for rational design and performance optimization of novel electrolytes. The categories, research advances and ion transport mechanism of inorganic glass/ceramic electrolytes, polymer-based electrolytes and corresponding composite electrolytes are detailly summarized and discussed. Moreover, interface contact and compatibility between electrolyte and cathode/anode are also briefly discussed. Furthermore, the electrochemical characterization methods of SSEs used in different types of SSBs are also introduced. On this basis, the principles and prospects of novel SSEs and interface design are curtly proposed according to the development requirements of SSBs. Moreover, the advanced characterizations for real-time monitoring of interface changes are also brought forward to promote the development of SSBs.

Polyoxometalate Li3 PW12 O40 and Li3 PMo12 O40 Electrolytes for High-energy All-solid-state Lithium Batteries

Solid-state lithium (Li) batteries promise both high energy density and safety while existing solid-state electrolytes (SSEs) fail to satisfy the rigorous requirements of battery operations. Herein, novel polyoxometalate SSEs, Li3 PW12 O40 and Li3 PMo12 O40 , are synthesized, which exhibit excellent interfacial compatibility with electrodes and chemical stability, overcoming the limitations of conventional SSEs. A high ionic conductivity of 0.89 mS cm-1 and a low activation energy of 0.23 eV are obtained due to the optimized three-dimensional Li+ migration network of Li3 PW12 O40 . Li3 PW12 O40 exhibits a wide window of electrochemical stability that can both accommodate the Li anode and high-voltage cathodes. As a result, all-solid-state Li metal batteries fabricated with Li/Li3 PW12 O40 /LiNi0.5 Co0.2 Mn0.3 O2 display a stable cycling up to 100 cycles with a cutoff voltage of 4.35 V and an areal capacity of more than 4 mAh cm-2 , as well as a cost-competitive SSEs price of $5.68 kg-1 . Moreover, Li3 PMo12 O40 homologous to Li3 PW12 O40 was obtained via isomorphous substitution, which formed a low-resistance interface with Li3 PW12 O40 . Applications of Li3 PW12 O40 and Li3 PMo12 O40 in Li-air batteries further demonstrate that long cycle life (650 cycles) can be achieved. This strategy provides a facile, low-cost strategy to construct efficient and scalable solid polyoxometalate electrolytes for high-energy solid-state Li metal batteries.

Evidence for Complex Dynamics in Glassy Fast Ion Conductors: The Case of Sodium Thiosilicates

Classical molecular dynamics is used to study the dynamics of alkali ions in a promising fast ion conductor glass system, i.e., Na2S-SiS2. Diffusion in such thiosilicates is found to display various salient features of alkali silicates, i.e., channel-like diffusion with typical length scales emerging as the temperature is decreased to the glassy state, and Arrhenius behavior for both Na ion diffusivity and calculated conductivity. The dynamics appears, however, to be largely heterogeneous as manifested by fast and slow Na ion motion at intermediate times, both in the high-temperature liquid and in the glassy state. In the former, a diffusion-limited regime is found due to the increased motion of the network-forming species that limits the Na ion dynamics, whereas at low temperatures, the typical dynamical heterogeneities are recovered as observed close to the glass transition.

Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF4

Introducing compositional or structural disorder within crystalline solid electrolytes is a common strategy for increasing their ionic conductivity. (M,Sn)F2 fluorites have previously been proposed to exhibit two forms of disorder within their cationic host frameworks: occupational disorder from randomly distributed M and Sn cations and orientational disorder from Sn(II) stereoactive lone pairs. Here, we characterize the structure and fluoride-ion dynamics of cubic BaSnF4, using a combination of experimental and computational techniques. Rietveld refinement of the X-ray diffraction (XRD) data confirms an average fluorite structure with {Ba,Sn} cation disorder, and the 119Sn Mössbauer spectrum demonstrates the presence of stereoactive Sn(II) lone pairs. X-ray total-scattering PDF analysis and ab initio molecular dynamics simulations reveal a complex local structure with a high degree of intrinsic fluoride-ion disorder, where 1/3 of fluoride ions occupy octahedral "interstitial" sites: this fluoride-ion disorder is a consequence of repulsion between Sn lone pairs and fluoride ions that destabilizes Sn-coordinated tetrahedral fluoride-ion sites. Variable-temperature 19F NMR experiments and analysis of our molecular dynamics simulations reveal highly inhomogeneous fluoride-ion dynamics, with fluoride ions in Sn-rich local environments significantly more mobile than those in Ba-rich environments. Our simulations also reveal dynamical reorientation of the Sn lone pairs that is biased by the local cation configuration and coupled to the local fluoride-ion dynamics. We end by discussing the effect of host-framework disorder on long-range diffusion pathways in cubic BaSnF4.

Exploring the Relationship Between Halide Substitution, Structural Disorder, and Lithium Distribution in Lithium Argyrodites (Li6-xPS5-xBr1+x)

Lithium argyrodite superionic conductors have recently gained significant attention as potential solid electrolytes for all-solid-state batteries because of their high ionic conductivity and ease of processing. Promising aspects of these materials are the ability to introduce halides (Li6-xPS5-xHal1+x, Hal = Cl and Br) into the crystal structure, which can greatly impact the lithium distribution over the wide range of accessible sites and the structural disorder between the S2- and Hal- anion on the Wyckoff 4d site, both of which strongly influence the ionic conductivity. However, the complex relationship among halide substitution, structural disorder, and lithium distribution is not fully understood, impeding optimal material design. In this study, we investigate the effect of bromide substitution on lithium argyrodite (Li6-xPS5-xBr1+x, in the range 0.0 ≤ x ≤ 0.5) and engineer structural disorder by changing the synthesis protocol. We reveal the correlation between the lithium substructure and ionic transport using neutron diffraction, solid-state nuclear magnetic resonance (NMR) spectroscopy, and electrochemical impedance spectroscopy. We find that a higher ionic conductivity is correlated with a lower average negative charge on the 4d site, located in the center of the Li+ "cage", as a result of the partial replacement of S2- by Br-. This leads to weaker interactions within the Li+ "cage", promoting Li-ion diffusivity across the unit cell. We also identify an additional T4 Li+ site, which enables an alternative jump route (T5-T4-T5) with a lower migration energy barrier. The resulting expansion of the Li+ cages and increased connections between cages lead to a maximum ionic conductivity of 8.55 mS/cm for quenched Li5.5PS4.5Br1.5 having the highest degree of structural disorder, an 11-fold improvement compared to slow-cooled Li6PS5Br having the lowest degree of structural disorder. Thereby, this work advances the understanding of the structure-transport correlations in lithium argyrodites, specifically how structural disorder and halide substitution impact the lithium substructure and transport properties and how this can be realized effectively through the synthesis method and tuning of the composition.

Liquid-Like Li-Ion Conduction in Oxides Enabling Anomalously Stable Charge Transport across the Li/Electrolyte Interface in All-Solid-State Batteries

The softness of sulfur sublattice and rotational PS4 tetrahedra in thiophosphates result in liquid-like ionic conduction, leading to enhanced ionic conductivities and stable electrode/thiophosphate interfacial ionic transport. However, the existence of liquid-like ionic conduction in rigid oxides remains unclear, and modifications are deemed necessary to achieve stable Li/oxide solid electrolyte interfacial charge transport. In this study, by combining the neutron diffraction survey, geometrical analysis, bond valence site energy analysis, and ab initio molecular dynamics simulation, 1D liquid-like Li-ion conduction is discovered in LiTa2 PO8 and its derivatives, wherein Li-ion migration channels are connected by four- or five-fold oxygen-coordinated interstitial sites. This conduction features a low activation energy (0.2 eV) and short mean residence time (<1 ps) of Li ions on the interstitial sites, originating from the Li-O polyhedral distortion and Li-ion correlation, which are controlled by doping strategies. The liquid-like conduction enables a high ionic conductivity (1.2 mS cm-1 at 30 °C), and a 700 h anomalously stable cycling under 0.2 mA cm-2 for Li/LiTa2 PO8 /Li cells without interfacial modifications. These findings provide principles for the future discovery and design of improved solid electrolytes that do not require modifications to the Li/solid electrolyte interface to achieve stable ionic transport.

Extreme phonon anharmonicity underpins superionic diffusion and ultralow thermal conductivity in argyrodite Ag8SnSe6

Ultralow thermal conductivity and fast ionic diffusion endow superionic materials with excellent performance both as thermoelectric converters and as solid-state electrolytes. Yet the correlation and interdependence between these two features remain unclear owing to a limited understanding of their complex atomic dynamics. Here we investigate ionic diffusion and lattice dynamics in argyrodite Ag8SnSe6 using synchrotron X-ray and neutron scattering techniques along with machine-learned molecular dynamics. We identify a critical interplay of the vibrational dynamics of mobile Ag and a host framework that controls the overdamping of low-energy Ag-dominated phonons into a quasi-elastic response, enabling superionicity. Concomitantly, the persistence of long-wavelength transverse acoustic phonons across the superionic transition challenges a proposed 'liquid-like thermal conduction' picture. Rather, a striking thermal broadening of low-energy phonons, starting even below 50 K, reveals extreme phonon anharmonicity and weak bonding as underlying features of the potential energy surface responsible for the ultralow thermal conductivity (<0.5 W m-1 K-1) and fast diffusion. Our results provide fundamental insights into the complex atomic dynamics in superionic materials for energy conversion and storage.

Hidden chemical order in disordered Ba7Nb4MoO20 revealed by resonant X-ray diffraction and solid-state NMR

The chemical order and disorder of solids have a decisive influence on the material properties. There are numerous materials exhibiting chemical order/disorder of atoms with similar X-ray atomic scattering factors and similar neutron scattering lengths. It is difficult to investigate such order/disorder hidden in the data obtained from conventional diffraction methods. Herein, we quantitatively determined the Mo/Nb order in the high ion conductor Ba₇Nb₄MoO₂₀ by a technique combining resonant X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and first-principle calculations. NMR provided direct evidence that Mo atoms occupy only the M2 site near the intrinsically oxygen-deficient ion-conducting layer. Resonant X-ray diffraction determined the occupancy factors of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively. These findings provide a basis for the development of ion conductors. This combined technique would open a new avenue for in-depth investigation of the hidden chemical order/disorder in materials.

Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH5 (PO4 )3

The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH₅ (PO₄ )₃ (ZP3), which exhibits record-high proton conductivity of 0.5-3.1×10^-2  S cm^-1 in the range 25-110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25-110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer.

Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li6+xP1-xSixO5Cl

Argyrodite is a key structure type for ion-transporting materials. Oxide argyrodites are largely unexplored despite sulfide argyrodites being a leading family of solid-state lithium-ion conductors, in which the control of lithium distribution over a wide range of available sites strongly influences the conductivity. We present a new cubic Li-rich (>6 Li⁺ per formula unit) oxide argyrodite Li₇SiO₅Cl that crystallizes with an ordered cubic (P2₁3) structure at room temperature, undergoing a transition at 473 K to a Li⁺ site disordered F4̅3m structure, consistent with the symmetry adopted by superionic sulfide argyrodites. Four different Li⁺ sites are occupied in Li₇SiO₅Cl (T5, T5a, T3, and T4), the combination of which is previously unreported for Li-containing argyrodites. The disordered F4̅3m structure is stabilized to room temperature via substitution of Si⁴⁺ with P⁵⁺ in Li_6+xP_1-xSi_xO₅Cl (0.3 < x < 0.85) solid solution. The resulting delocalization of Li⁺ sites leads to a maximum ionic conductivity of 1.82(1) × 10^-6 S cm^-1 at x = 0.75, which is 3 orders of magnitude higher than the conductivities reported previously for oxide argyrodites. The variation of ionic conductivity with composition in Li_6+xP_1-xSi_xO₅Cl is directly connected to structural changes occurring within the Li⁺ sublattice. These materials present superior atmospheric stability over analogous sulfide argyrodites and are stable against Li metal. The ability to control the ionic conductivity through structure and composition emphasizes the advances that can be made with further research in the open field of oxide argyrodites.

Deciphering Fast Ion Transport in Glasses: A Case Study of Sodium and Silver Vitreous Sulfides

High-capacity solid-state batteries are promising future products for large-scale energy storage and conversion. Sodium fast ion conductors including glasses and glass ceramics are unparalleled materials for these applications. Rational design and tuning of advanced sodium sulfide electrolytes need a deep insight into the atomic structure and dynamics in relation with ion-transport properties. Using pulsed neutron diffraction and Raman spectroscopy supported by first-principles simulations, we show that preferential diffusion pathways in vitreous sodium and silver sulfides are related to isolated sulfur S_iso, that is, the sulfur species surrounded exclusively by mobile cations with a typical stoichiometry of M/S_iso ≈ 2. The S_iso/S_tot fraction appears to be a reliable descriptor of fast ion transport in glassy sulfide systems over a wide range of ionic conductivities and cation diffusivities. The S_iso fraction increases with mobile cation content x, tetrahedral coordination of the network former and, in case of thiogermanate systems, with germanium disulfide metastability and partial disproportionation, GeS₂ → GeS + S, leading to the formation of additional sulfur, transforming into S_iso. A research strategy enabling to achieve extended and interconnected pathways based on isolated sulfur would lead to glassy electrolytes with superior ionic diffusion.

Ionic Conductivity of Nanocrystalline and Amorphous Li10GeP2S12: The Detrimental Impact of Local Disorder on Ion Transport

Solids with extraordinarily high Li⁺ dynamics are key for high performance all-solid-state batteries. The thiophosphate Li₁₀GeP₂S₁₂ (LGPS) belongs to the best Li-ion conductors with an ionic conductivity exceeding 10 mS cm^–1 at ambient temperature. Recent molecular dynamics simulations performed by Dawson and Islam predict that the ionic conductivity of LGPS can be further enhanced by a factor of 3 if local disorder is introduced. As yet, no experimental evidence exists supporting this fascinating prediction. Here, we synthesized nanocrystalline LGPS by high-energy ball-milling and probed the Li⁺ ion transport parameters. Broadband conductivity spectroscopy in combination with electric modulus measurements allowed us to precisely follow the changes in Li⁺ dynamics. Surprisingly and against the behavior of other electrolytes, bulk ionic conductivity turned out to decrease with increasing milling time, finally leading to a reduction of σ_20°C by a factor of 10. ³¹P, ⁶Li NMR, and X-ray diffraction showed that ball-milling forms a structurally heterogeneous sample with nm-sized LGPS crystallites and amorphous material. At −135 °C, electrical relaxation in the amorphous regions is by 2 to 3 orders of magnitude slower. Careful separation of the amorphous and (nano)crystalline contributions to overall ion transport revealed that in both regions, Li⁺ ion dynamics is slowed down compared to untreated LGPS. Hence, introducing defects into the LGPS bulk structure via ball-milling has a negative impact on ionic transport. We postulate that such a kind of structural disorder is detrimental to fast ion transport in materials whose transport properties rely on crystallographically well-defined diffusion pathways.

Cation Disorder and Large Tetragonal Supercell Ordering in the Li-Rich Argyrodite Li7Zn0.5SiS6

A tetragonal argyrodite with >7 mobile cations, Li₇Zn_0.5SiS₆, is experimentally realized for the first time through solid state synthesis and exploration of the Li-Zn-Si-S phase diagram. The crystal structure of Li₇Zn_0.5SiS₆ was solved ab initio from high-resolution X-ray and neutron powder diffraction data and supported by solid-state NMR. Li₇Zn_0.5SiS₆ adopts a tetragonal I4 structure at room temperature with ordered Li and Zn positions and undergoes a transition above 411.1 K to a higher symmetry disordered F43m structure more typical of Li-containing argyrodites. Simultaneous occupation of four types of Li site (T5, T5a, T2, T4) at high temperature and five types of Li site (T5, T2, T4, T1, and a new trigonal planar T2a position) at room temperature is observed. This combination of sites forms interconnected Li pathways driven by the incorporation of Zn²⁺ into the Li sublattice and enables a range of possible jump processes. Zn²⁺ occupies the 48h T5 site in the high-temperature F43m structure, and a unique ordering pattern emerges in which only a subset of these T5 sites are occupied at room temperature in I4 Li₇Zn_0.5SiS₆. The ionic conductivity, examined via AC impedance spectroscopy and VT-NMR, is 1.0(2) × 10^-7 S cm^-1 at room temperature and 4.3(4) × 10^-4 S cm^-1 at 503 K. The transition between the ordered I4 and disordered F43m structures is associated with a dramatic decrease in activation energy to 0.34(1) eV above 411 K. The incorporation of a small amount of Zn²⁺ exercises dramatic control of Li order in Li₇Zn_0.5SiS₆ yielding a previously unseen distribution of Li sites, expanding our understanding of structure-property relationships in argyrodite materials.

Lithium ion transport in micro- and nanocrystalline lithium sulphide Li2S

Ion dynamics in binary Li-bearing compounds such as LiF, Li2O and Li2S is rather poor. These compounds do, however, form as decomposition products at the interface between the electrolyte and the electrode materials in lithium-based batteries. They are expected to severely influence the charge transport across this electrode-electrolyte interface and, thus, the overall performance of such systems. Yet, ion dynamics in the nanostructured forms of these binary compounds has scarcely been investigated. Here, we prepared bulk nanostructured Li2S through high-energy ball milling and studied its temperature-dependent ionic conductivity by means of broadband impedance spectroscopy. It turned out that, compared to the unmilled form, Li+ ion conductivity in ball-milled Li2S increased by approximately 3 orders or magnitude. This striking increase is accompanied by a decrease of the average activation energy from ca. 0.9 eV to approximately 0.7 eV. Structural disorder, stress and local distortions are assumed to be responsible for this clear change in macroscopic transport parameters. Fast ion dynamics in or near the interfacial space charge zones might contribute to enhanced dynamics, too. We conclude that Li ion transport in interfacial Li2S, if present in a disordered nanostructured form in lithium-ion batteries, is much faster than originally thought for its ordered counterpart.