On the Origin of the Non-Arrhenius Na-ion Conductivity in Na3OBr

The sodium-rich antiperovskites (NaRAPs) with composition Na3OB (B=Br, Cl, I, BH4, etc.) are a family of materials that has recently attracted great interest for application as solid electrolytes in sodium metal batteries. Non-Arrhenius ionic conductivities have been reported for these materials, the origin of which is poorly understood. In this work, we combined temperature-resolved bulk and local characterisation methods to gain an insight into the origin of this unusual behaviour using Na3OBr as a model system. We first excluded crystallographic disorder on the anion sites as the cause of the change in activation energy; then identified the presence of a poorly crystalline impurities, not detectable by XRD, and elucidated their effect on ionic conductivity. These findings improve understanding of the processing-structure-properties relationships pertaining to NaRAPs and highlight the need to determine these relationships in other materials systems, which will accelerate the development of high-performance solid electrolytes.

On the Origin of the Non-Arrhenius Na-ion Conductivity in Na3 OBr

The sodium-rich antiperovskites (NaRAPs) with composition Na3 OB (B=Br, Cl, I, BH4 , etc.) are a family of materials that has recently attracted great interest for application as solid electrolytes in sodium metal batteries. Non-Arrhenius ionic conductivities have been reported for these materials, the origin of which is poorly understood. In this work, we combined temperature-resolved bulk and local characterisation methods to gain an insight into the origin of this unusual behaviour using Na3 OBr as a model system. We first excluded crystallographic disorder on the anion sites as the cause of the change in activation energy; then identified the presence of a poorly crystalline impurities, not detectable by XRD, and elucidated their effect on ionic conductivity. These findings improve understanding of the processing-structure-properties relationships pertaining to NaRAPs and highlight the need to determine these relationships in other materials systems, which will accelerate the development of high-performance solid electrolytes.

Investigation of the sodium-ion transport mechanism and elastic properties of double anti-perovskite Na3S0.5O0.5I

Sodium-rich anti-perovskites have unique advantages in terms of composition tuning and electrochemical stability when used as solid-state electrolytes in sodium-ion batteries. However, their Na+ transport mechanism is not clear and Na+ conductivity needs to be improved. In this paper, we investigate the stability, elastic properties and Na+ transport mechanisms of both the double anti-perovskite Na3S0.5O0.5I and anti-perovskite Na3OI. The results indicate that the NaI Schottky defect is the most favorable intrinsic defect for Na+ transport and due to the substitution of S2- for O2-, Na3S0.5O0.5I has stronger ductility and higher Na+ conductivity compared to Na3OI, despite the electrochemical window being slightly narrower. Divalent alkaline earth metal dopants can increase the Na+ vacancy concentration, while impeding Na+ migration. Among the dopants, Sr2+ and Ca2+ are the optimal dopants for Na3S0.5O0.5I and Na3OI, respectively. Notably, the Na+ conductivity of the non-stoichiometric Na3S0.5O0.5I at room temperature is 1.2 × 10-3 S cm-1, indicating its great potential as a solid-state electrolyte. Moreover, strain effect calculations show that biaxial tensile strain is beneficial for Na+ transport. Our work reveals the sodium-ion transport mechanism and elastic properties of double anti-perovskites, which is of great significance for the development of solid-state electrolytes.

Opportunities of Flexible and Portable Electrochemical Devices for Energy Storage: Expanding the Spotlight onto Semi-solid/Solid Electrolytes

The ever-increasing demand for flexible and portable electronics has stimulated research and development in building advanced electrochemical energy devices which are lightweight, ultrathin, small in size, bendable, foldable, knittable, wearable, and/or stretchable. In such flexible and portable devices, semi-solid/solid electrolytes besides anodes and cathodes are the necessary components determining the energy/power performances. By serving as the ion transport channels, such semi-solid/solid electrolytes may be beneficial to resolving the issues of leakage, electrode corrosion, and metal electrode dendrite growth. In this paper, the fundamentals of semi-solid/solid electrolytes (e.g., chemical composition, ionic conductivity, electrochemical window, mechanical strength, thermal stability, and other attractive features), the electrode-electrolyte interfacial properties, and their relationships with the performance of various energy devices (e.g., supercapacitors, secondary ion batteries, metal-sulfur batteries, and metal-air batteries) are comprehensively reviewed in terms of materials synthesis and/or characterization, functional mechanisms, and device assembling for performance validation. The most recent advancements in improving the performance of electrochemical energy devices are summarized with focuses on analyzing the existing technical challenges (e.g., solid electrolyte interphase formation, metal electrode dendrite growth, polysulfide shuttle issue, electrolyte instability in half-open battery structure) and the strategies for overcoming these challenges through modification of semi-solid/solid electrolyte materials. Several possible directions for future research and development are proposed for going beyond existing technological bottlenecks and achieving desirable flexible and portable electrochemical energy devices to fulfill their practical applications. It is expected that this review may provide the readers with a comprehensive cross-technology understanding of the semi-solid/solid electrolytes for facilitating their current and future researches on the flexible and portable electrochemical energy devices.

Bilayer Halide Electrolytes for All-Inorganic Solid-State Lithium-Metal Batteries with Excellent Interfacial Compatibility

Inorganic solid-state electrolytes (ISSEs) have been extensively researched as the critical component in all-solid-state lithium-metal batteries (ASSLMBs). Many ISSEs exhibit high ionic conductivities up to 10^-3 S cm^-1. However, most of them suffer from poor interfacial compatibility with electrodes, especially lithium-metal anodes, limiting their application in high-performance ASSLMBs. To achieve good interfacial compatibility with a high-voltage cathode and a lithium-metal anode simultaneously, we propose Li₃InCl₆/Li₂OHCl bilayer halide ISSEs with complementary advantages. In addition to the improved interfacial compatibility, the Li₃InCl₆/Li₂OHCl bilayer halide ISSEs exhibit good thermal stability up to 160 °C. The Li-symmetric cells with sandwich electrolytes Li₂OHCl/Li₃InCl₆/Li₂OHCl exhibit long cycling life of over 300 h and a high critical current density of over 0.6 mA cm^-2 at 80 °C. Moreover, the all-inorganic solid-state lithium-metal batteries (AISSLMBs) LiFePO₄-Li₃InCl₆/Li₃InCl₆/Li₂OHCl/Li fabricated by a facile cold-press method exhibit good rate performance and long-term cycling stability that stably cycle for about 3000 h at 80 °C. This work presents a facile and cost-effective method to construct bilayer halide ISSEs, enabling the development of high-performance AISSLMBs with good interfacial compatibility and thermal stability.

Unravelling the alkali transport properties in nanocrystalline A3OX (A = Li, Na, X = Cl, Br) solid state electrolytes. A theoretical prediction

Transport properties of the halogeno-alkali oxides A3OX (A = Li, Na, X = Cl, Br) nanocrystalline samples with the presence of ∑3(111) grain boundaries were computed using large-scale molecular dynamic simulations. Results on the diffusion/conduction process show that these nanocrystalline samples are characterized with higher activation energies as compared to previous theoretical studies, but closer to experiment. Such a performance can be attributed to the larger atomic density at the ∑3(111) grain boundary regions within the nanocrystals. Despite a minor deterioration of transport properties of the mixed cation Li2NaOX and Na2LiOX samples, these halogeno-alkali oxides can also be considered as good inorganic solid electrolytes in both Li- and Na-ion batteries.

Antiperovskite Electrolytes for Solid-State Batteries

Solid-state batteries have fascinated the research community over the past decade, largely due to their improved safety properties and potential for high-energy density. Searching for fast ion conductors with sufficient electrochemical and chemical stabilities is at the heart of solid-state battery research and applications. Recently, significant progress has been made in solid-state electrolyte development. Sulfide-, oxide-, and halide-based electrolytes have been able to achieve high ionic conductivities of more than 10^-3 S/cm at room temperature, which are comparable to liquid-based electrolytes. However, their stability toward Li metal anodes poses significant challenges for these electrolytes. The existence of non-Li cations that can be reduced by Li metal in these electrolytes hinders the application of Li anode and therefore poses an obstacle toward achieving high-energy density. The finding of antiperovskites as ionic conductors in recent years has demonstrated a new and exciting solution. These materials, mainly constructed from Li (or Na), O, and Cl (or Br), are lightweight and electrochemically stable toward metallic Li and possess promising ionic conductivity. Because of the structural flexibility and tunability, antiperovskite electrolytes are excellent candidates for solid-state battery applications, and researchers are still exploring the relationship between their structure and ion diffusion behavior. Herein, the recent progress of antiperovskites for solid-state batteries is reviewed, and the strategies to tune the ionic conductivity by structural manipulation are summarized. Major challenges and future directions are discussed to facilitate the development of antiperovskite-based solid-state batteries.

Antiperovskite Superionic Conductors: A Critical Review

Antiperovskites of composition M3AB (M = Li, Na, K; A = O; B = Cl, Br, I, NO2, etc.) have recently been investigated as solid-state electrolytes for all-solid-state batteries. Inspired by the impressive ionic conductivities of Li3OCl0.5Br0.5 and Na3OBH4 as high as 10-3 S/cm at room temperature, many variants of antiperovskite-based Li-ion and Na-ion conductors have been reported, and K-ion antiperovskites are emerging. These materials exhibit low melting points and thus have the advantages of easy processing into films and intimate contacts with electrodes. However, there are also issues in interpreting the stellar materials and reproducing their high ionic conductivities. Therefore, we think a critical review can be useful for summarizing the current results, pointing out the potential issues, and discussing best practices for future research. In this critical review, we first overview the reported compositions, structural stabilities, and ionic conductivities of antiperovskites. We then discuss the different conduction mechanisms that have been proposed, including the partial melting of cations and the paddlewheel mechanism for cluster anions. We close by reviewing the use of antiperovskites in batteries and suggest some practices for the community to consider.

Polymorph of LiAlP2O7: Combined Computational, Synthetic, Crystallographic, and Ionic Conductivity Study

We report a new polymorph of lithium aluminum pyrophosphate, LiAlP₂O₇, discovered through a computationally guided synthetic exploration of the Li-Mg-Al-P-O phase field. The new polymorph formed at 973 K, and the crystal structure, solved by single-crystal X-ray diffraction, adopts the orthorhombic space group Cmcm with a = 5.1140(9) Å, b = 8.2042(13) Å, c = 11.565(3) Å, and V = 485.22(17) ų. It has a three-dimensional framework structure that is different from that found in other LiM^IIIP₂O₇ materials. It transforms to the known monoclinic form (space group P2₁) above ∼1023 K. Density functional theory (DFT) calculations show that the new polymorph is the most stable low-temperature structure for this composition among the seven known structure types in the A^IM^IIIP₂O₇ (A = alkali metal) families. Although the bulk Li-ion conductivity is low, as determined from alternating-current impedance spectroscopy and variable-temperature static ⁷Li NMR spectra, a detailed analysis of the topologies of all seven structure types through bond-valence-sum mapping suggests a potential avenue for enhancing the conductivity. The new polymorph exhibits long (>4 Å) Li-Li distances, no Li vacancies, and an absence of Li pathways in the c direction, features that could contribute to the observed low Li-ion conductivity. In contrast, we found favorable Li-site topologies that could support long-range Li migration for two structure types with modest DFT total energies relative to the new polymorph. These promising structure types could possibly be accessed from innovative doping of the new polymorph.

Sluggish anion transport provides good kinetic stability to the anhydrous anti-perovskite solid electrolyte Li3OCl

Some lithium oxyhalides have been proposed as low-cost solid electrolytes for having room-temperature Li⁺ conductivity close to commercial liquid electrolytes, but with the advantages of enabling higher energy densities through the use of the Li metal anode and not being flammable. However, the stability of anhydrous anti-perovskite lithium oxyhalides, such as Li₃OCl, is not well understood yet: whereas theoretical calculations show they should decompose into lithium halides and Li₂O (except at high temperatures), there is no experimental evidence of such decomposition. Thus, here we use a combination of analytical calculations and force-field-based atomistic modelling to investigate the role of kinetics in the stability of anhydrous Li₃OCl. The results show that due to sluggish Cl^- and O^2- transport this material has good kinetic stability below ∼400 K under high concentration gradients, below ∼450 K under typical cell voltages, and at all temperatures against local composition fluctuations. Furthermore, the good kinetic stability explains the apparent discrepancy between theoretical thermodynamics calculations and experimental observations and contributes to enlighten the nature and extent of this material's stability. The methods presented here can also be extended to other battery materials that are predicted to decompose, to access the safe temperature range they can undergo without degrading.

Theoretical insights into the diffusion mechanism of alkali ions in Ruddlesden–Popper antiperovskites

The diffusion properties of alkali ions in a series of RP antiperovskites are investigated by density functional theory, which provides a theoretical guide for enhancing the ionic conductivity of solid-state antiperovskite electrolytes.

Lithium and Chlorine-Rich Preparation of Mechanochemically Activated Antiperovskite Composites for Solid-State Batteries

Assembling all-solid-state batteries presents a unique challenge due to chemical and electrochemical complexities of interfaces between a solid electrolyte and electrodes. While the interface stability is dictated by thermodynamics, making use of passivation materials often delays interfacial degradation and extends the cycle life of all-solid cells. In this work, we investigated antiperovskite lithium oxychloride, Li₃OCl, as a promising passivation material that can engineer the properties of solid electrolyte-Li metal interfaces. Our experiment to obtain stoichiometric Li₃OCl focuses on how the starting ratios of lithium and chlorine and mechanochemical activation affect the phase stability. For substantial LiCl excess conditions, the antiperovskite phase was found to form by simple melt-quenching and subsequent high-energy ball-milling. Li₃OCl prepared with 100% excess LiCl exhibits ionic conductivity of 3.2 × 10⁻⁵ S cm⁻¹ at room temperature, as well as cathodic stability against Li metal upon the extended number of cycling. With a conductivity comparable to other passivation layers, and stable interface properties, our Li₃OCl/LiCl composite has the potential to stably passivate the solid-solid interfaces in all-solid-state batteries.

Progress on Lithium Dendrite Suppression Strategies from the Interior to Exterior by Hierarchical Structure Designs

Lithium (Li) metal is promising for high energy density batteries due to its low electrochemical potential (-3.04 V) and high specific capacity (3860 mAh g^-1 ). However, the safety issues impede the commercialization of Li anode batteries. In this work, research of hierarchical structure designs for Li anodes to suppress Li dendrite growth and alleviate volume expansion from the interior (by the 3D current collector and host matrix) to the exterior (by the artificial solid electrolyte interphase (SEI), protective layer, separator, and solid state electrolyte) is concluded. The basic principles for achieving Li dendrite and volume expansion free Li anode are summarized. Following these principles, 3D porous current collector and host matrix are designed to suppress the Li dendrite growth from the interior. Second, artificial SEI, the protective layer, and separator as well as solid-state electrolyte are constructed to regulate the distribution of current and control the Li nucleation and deposition homogeneously for suppressing the Li dendrite growth from exterior of Li anode. Ultimately, this work puts forward that it is significant to combine the Li dendrite suppression strategies from the interior to exterior by 3D hierarchical structure designs and Li metal modification to achieve excellent cycling and safety performance of Li metal batteries.

Revealing the effect of grain boundary segregation on Li ion transport in polycrystalline anti-perovskite Li3ClO: a phase field study

Lithium ion transport in a polycrystalline solid-state electrolyte (SSE) is directly linked to the properties of lithium ion batteries. Grain boundaries (GBs), as essential defects in SSE, have been found to play a significant role in the overall kinetics of lithium ion transport, however, the mechanism is not well understood due to the complex role of GBs. The GBs could affect the overall kinetics of ionic transport in the SSEs in two ways: (i) Li/Na diffusivities inside the GBs could be different from those in the bulk, and (ii) point defect segregation at the GBs. The first aspect is well recognized, whereas the second one has been rarely studied. In this study, a combination of first principles and phase field calculations were performed, in which the interaction between point defects and grain boundaries were considered at different scales, to reveal the role of GBs in the overall ionic conduction of SSE anti-perovskite Li₃ClO. The results show that defect segregation, which varies significantly with the GB orientation, reinforces the negative contribution of GBs on the overall ionic diffusivity by approximately one-order of magnitude. This study could help improve the fundamental understanding of ionic transport in polycrystalline SSEs, and provide guidance for the design of new SSEs with excellent ionic conductivity.

Antiperovskites with Exceptional Functionalities

ABX₃ perovskites, as the largest family of crystalline materials, have attracted tremendous research interest worldwide due to their versatile multifunctionalities and the intriguing scientific principles underlying them. Their counterparts, antiperovskites (X₃ BA), are actually electronically inverted perovskite derivatives, but they are not an ignorable family of functional materials. In fact, inheriting the flexible structural features of perovskites while being rich in cations at X sites, antiperovskites exhibit a diverse array of unconventional physical and chemical properties. However, rather less attention has been paid to these "inverse" analogs, and therefore, a comprehensive review is urgently needed to arouse general concern. Recent advances in novel antiperovskite materials and their exceptional functionalities are summarized, including superionic conductivity, superconductivity, giant magnetoresistance, negative thermal expansion, luminescence, and electrochemical energy conversion. In particular, considering the feasibility of the perovskite structure, a universal strategy for enhancing the performance of or generating new phenomena in antiperovskites is discussed from the perspective of solid-state chemistry. With more research enthusiasm, antiperovskites are highly anticipated to become a rising star family of functional materials.

Ruddlesden–Popper phases of lithium-hydroxide-halide antiperovskites: two dimensional Li-ion conductors

The n = 2 Ruddlesden–Popper antiperovskite LiBr(Li₂OHBr)₂ was successfully obtained and the two-dimensional Li-ion conduction was discussed.

Building Better Batteries in the Solid State: A Review

Most of the current commercialized lithium batteries employ liquid electrolytes, despite their vulnerability to battery fire hazards, because they avoid the formation of dendrites on the anode side, which is commonly encountered in solid-state batteries. In a review two years ago, we focused on the challenges and issues facing lithium metal for solid-state rechargeable batteries, pointed to the progress made in addressing this drawback, and concluded that a situation could be envisioned where solid-state batteries would again win over liquid batteries for different applications in the near future. However, an additional drawback of solid-state batteries is the lower ionic conductivity of the electrolyte. Therefore, extensive research efforts have been invested in the last few years to overcome this problem, the reward of which has been significant progress. It is the purpose of this review to report these recent works and the state of the art on solid electrolytes. In addition to solid electrolytes stricto sensu, there are other electrolytes that are mainly solids, but with some added liquid. In some cases, the amount of liquid added is only on the microliter scale; the addition of liquid is aimed at only improving the contact between a solid-state electrolyte and an electrode, for instance. In some other cases, the amount of liquid is larger, as in the case of gel polymers. It is also an acceptable solution if the amount of liquid is small enough to maintain the safety of the cell; such cases are also considered in this review. Different chemistries are examined, including not only Li-air, Li-O2, and Li-S, but also sodium-ion batteries, which are also subject to intensive research. The challenges toward commercialization are also considered.

Predicting Wettability and the Electrochemical Window of Lithium-Metal/Solid Electrolyte Interfaces

The development of solid electrolytes (SEs) is expected to enhance the safety of lithium-ion batteries. Additionally, a viable SE could allow the use of a Li-metal negative electrode, which would increase energy density. Recently, several antiperovskites have been reported to exhibit high ionic conductivities, prompting investigations of their use as an SE. In addition to having a suitable conductivity, phenomena at the interface between an SE and an electrode are also of great importance in determining the viability of an SE. For example, interfacial interactions can change the positions of the band edges of the SE, altering its stability against undesirable oxidation or reduction. Furthermore, the wettability of the SE by the metallic anode is desired to enable low interfacial resistance and uniform metal plating and stripping during cycling. The present study probes several properties of the SE/electrode interface at the atomic scale. Adopting the antiperovskite SE Li₃OCl (LOC)/Li-metal anode interface as a model system, the interfacial energy, work of adhesion, wettability, band edge shifts, and the electrochemical window are predicted computationally. The oxygen-terminated interface was determined to be the most thermodynamically stable. Moreover, the large calculated work of adhesion for this system implies that Li will wet LOC, suggesting the possibility for low interfacial resistance. Nevertheless, these strong interfacial interactions come at a cost to electrochemical stability: strong interfacial bonding lowers the energy of the conduction band minimum (CBM) significantly and narrows the local band gap by 30% in the vicinity of the interface. Despite this interface-induced reduction in electrochemical window, the CBM in LOC remains more negative than the Li/Li⁺ redox potential, implying stability against reduction by the anode. In sum, this study illustrates a comprehensive computational approach to assessing electrode/electrolyte interfacial properties in solid-state batteries.

Chemistry Design Towards a Stable Sulfide-Based Superionic Conductor Li4 Cu8 Ge3 S12

Sulfide-based superionic conductors with high ionic conductivity have been explored as candidates for solid-state Li batteries. However, moisture hypersensitivity has made their manufacture complicated and costly and also impeded applications in batteries. Now, a sulfide-based superionic conductor Li4 Cu8 Ge3 S12 with superior stability was developed based on the hard/soft acid-base theory. The compound is stable in both moist air and aqueous LiOH aqueous solution. The electrochemical stability window was up to 1.5 V. An ionic conductivity of 0.9×10-4  S cm with low activation energy of 0.33 eV was achieved without any optimization. The material features a rigid Cu-Ge-S open framework that increases its stability. Meanwhile, the weak bonding between Li+ and the framework promotes ionic conductivity. This work provides a structural configuration in which weak Li bonding in the rigid framework promotes an environment for highly conductive and stable solid-state electrolytes.

Promises, Challenges, and Recent Progress of Inorganic Solid-State Electrolytes for All-Solid-State Lithium Batteries

All-solid-state lithium batteries (ASSLBs) have the potential to revolutionize battery systems for electric vehicles due to their benefits in safety, energy density, packaging, and operable temperature range. As the key component in ASSLBs, inorganic lithium-ion-based solid-state electrolytes (SSEs) have attracted great interest, and advances in SSEs are vital to deliver the promise of ASSLBs. Herein, a survey of emerging SSEs is presented, and ion-transport mechanisms are briefly discussed. Techniques for increasing the ionic conductivity of SSEs, including substitution and mechanical strain treatment, are highlighted. Recent advances in various classes of SSEs enabled by different preparation methods are described. Then, the issues of chemical stabilities, electrochemical compatibility, and the interfaces between electrodes and SSEs are focused on. A variety of research addressing these issues is outlined accordingly. Given their importance for next-generation battery systems and transportation style, a perspective on the current challenges and opportunities is provided, and suggestions for future research directions for SSEs and ASSLBs are suggested.

Enhanced Li ion conductivity in Ge-doped Li0.33La0.56TiO3 perovskite solid electrolytes for all-solid-state Li-ion batteries

Ge-Doped Li_0.33La_0.56TiO₃ perovskites are synthesized by solid-state reactions and have been demonstrated as Li-ion battery solid electrolytes with excellent ion conductivity and electrochemical stability.

Atomic-Scale Influence of Grain Boundaries on Li-Ion Conduction in Solid Electrolytes for All-Solid-State Batteries

Solid electrolytes are generating considerable interest for all-solid-state Li-ion batteries to address safety and performance issues. Grain boundaries have a significant influence on solid electrolytes and are key hurdles that must be overcome for their successful application. However, grain boundary effects on ionic transport are not fully understood, especially at the atomic scale. The Li-rich anti-perovskite Li₃OCl is a promising solid electrolyte, although there is debate concerning the precise Li-ion migration barriers and conductivity. Using Li₃OCl as a model polycrystalline electrolyte, we apply large-scale molecular dynamics simulations to analyze the ionic transport at stable grain boundaries. Our results predict high concentrations of grain boundaries and clearly show that Li-ion conductivity is severely hindered through the grain boundaries. The activation energies for Li-ion conduction traversing the grain boundaries are consistently higher than that of the bulk crystal, confirming the high grain boundary resistance in this material. Using our results, we propose a polycrystalline model to quantify the impact of grain boundaries on conductivity as a function of grain size. Such insights provide valuable fundamental understanding of the role of grain boundaries and how tailoring the microstructure can lead to the optimization of new high-performance solid electrolytes.

Stress-Mediated Enhancement of Ionic Conductivity in Fast-Ion Conductors

Finding solid-state electrolytes with high ionic conductivity near room temperature is an important prerequisite for developing all-solid-state electrochemical batteries. Here, we investigate the effects of point defects (vacancies) and biaxial stress on the superionic properties of fast-ion conductors (represented by the archetypal compounds CaF₂, Li-rich antiperovskite Li₃OCl, and AgI) by using classical molecular dynamics and first-principles simulation methods. We find that the critical superionic temperature of all analyzed families of fast-ion conductors can be reduced by several hundreds of degrees through the application of relatively small biaxial stresses (|σ| ≤ 1 GPa) on slightly defective samples (c_v ∼ 1%). In AgI, we show that superionicity can be triggered at room temperature by applying a moderate compressive biaxial stress of ∼1 GPa. In this case, we reveal the existence of a σ-induced order-disorder phase transition involving sizable displacements of all the ions with respect to the equilibrium lattice that occurs prior to the stabilization of the superionic state. In CaF₂ and Li₃OCl, by contrast, we find that tensile biaxial stress (σ < 0) favors ionic conductivity as due to an effective increase of the volume available to interstitial ions, which lowers the formation energy of Frenkel pair defects. Our findings provide valuable microscopic insight into the behavior of fast-ion conductors under mechanical constraints, showing that biaxial stress (or, conversely, epitaxial strain) can be used as an effective means to enhance ionic conductivity.

Mechanocaloric effects in superionic thin films from atomistic simulations

Solid-state cooling is an energy-efficient and scalable refrigeration technology that exploits the adiabatic variation of a crystalline order parameter under an external field (electric, magnetic, or mechanic). The mechanocaloric effect bears one of the greatest cooling potentials in terms of energy efficiency owing to its large available latent heat. Here we show that giant mechanocaloric effects occur in thin films of well-known families of fast-ion conductors, namely Li-rich (Li₃OCl) and type-I (AgI), an abundant class of materials that routinely are employed in electrochemistry cells. Our simulations reveal that at room temperature AgI undergoes an adiabatic temperature shift of 38 K under a biaxial stress of 1 GPa. Likewise, Li₃OCl displays a cooling capacity of 9 K under similar mechanical conditions although at a considerably higher temperature. We also show that ionic vacancies have a detrimental effect on the cooling performance of superionic thin films. Our findings should motivate experimental mechanocaloric searches in a wide variety of already known superionic materials.

Mechanocaloric effects are a promising path towards solid-state cooling. Here the authors perform atomistic simulations on the well-known fast-ion conductor silver iodide and computationally predict a sizeable mechanocaloric effect under biaxial strain.

Li-rich antiperovskite superionic conductors based on cluster ions

Enjoying great safety, high power, and high energy densities, all-solid-state batteries play a key role in the next generation energy storage devices. However, their development is limited by the lack of solid electrolyte materials that can reach the practically useful conductivities of 10^-2 S/cm at room temperature (RT). Here, by exploring a set of lithium-rich antiperovskites composed of cluster ions, we report a lithium superionic conductor, Li₃SBF₄, that has an estimated 3D RT conductivity of 10^-2 S/cm, a low activation energy of 0.210 eV, a giant band gap of 8.5 eV, a small formation energy, a high melting point, and desired mechanical properties. A mixed phase of the material, Li₃S(BF₄)_0.5Cl_0.5, with the same simple crystal structure exhibits an RT conductivity as high as 10^-1 S/cm and a low activation energy of 0.176 eV. The high ionic conductivity of the crystals is enabled by the thermal-excited vibrational modes of the cluster ions and the large channel size created by mixing the large cluster ion with the small elementary ion.

Mastering the interface for advanced all-solid-state lithium rechargeable batteries

A solid electrolyte with a high Li-ion conductivity and a small interfacial resistance against a Li metal anode is a key component in all-solid-state Li metal batteries, but there is no ceramic oxide electrolyte available for this application except the thin-film Li-P oxynitride electrolyte; ceramic electrolytes are either easily reduced by Li metal or penetrated by Li dendrites in a short time. Here, we introduce a solid electrolyte LiZr₂(PO4)₃ with rhombohedral structure at room temperature that has a bulk Li-ion conductivity σ_Li = 2 × 10^-4 S⋅cm^-1 at 25 °C, a high electrochemical stability up to 5.5 V versus Li⁺/Li, and a small interfacial resistance for Li⁺ transfer. It reacts with a metallic lithium anode to form a Li⁺-conducting passivation layer (solid-electrolyte interphase) containing Li₃P and Li₈ZrO₆ that is wet by the lithium anode and also wets the LiZr₂(PO₄)₃ electrolyte. An all-solid-state Li/LiFePO₄ cell with a polymer catholyte shows good cyclability and a long cycle life.

Li-Ion Conduction and Stability of Perovskite Li3/8Sr7/16Hf1/4Ta3/4O3

A solid Li-ion conductor with a high room temperature Li-ion conductivity and small interfacial resistance is required for its application in next-generation Li-ion batteries. Here, we prepared a cubic perovskite-related oxide with the general formula Li3/8Sr7/16Hf1/4Ta3/4O3 (LSHT) by a conventional solid-state reaction method, which was studied by X-ray diffraction, electrochemical impedance spectroscopy, and (7)Li MAS NMR. Li3/8Sr7/16Hf1/4Ta3/4O3 has a high Li-ion conductivity of 3.8 × 10(-4) S cm(-1) at 25 °C and a low activation energy of 0.36 eV in the temperature range 298-430 K. It exhibits both high stability and small interfacial resistance with commercial organic liquid electrolytes, which makes it promising as a separator in Li-ion batteries.

Antiperovskite Li3OCl Superionic Conductor Films for Solid-State Li-Ion Batteries

Antiperovskite Li₃OCl superionic conductor films are prepared via pulsed laser deposition using a composite target. A significantly enhanced ionic conductivity of 2.0 × 10^-4 S cm^-1 at room temperature is achieved, and this value is more than two orders of magnitude higher than that of its bulk counterpart. The applicability of Li₃OCl as a solid electrolyte for Li-ion batteries is demonstrated.

Antiperovskite Chalco-Halides Ba3(FeS4)Cl, Ba3(FeS4)Br, and Ba3(FeSe4)Br with Spin Super-Super Exchange

Perovskite-related materials have received increasing attention for their broad applications in photovoltaic solar cells and information technology due to their unique electrical and magnetic properties. Here we report three new antiperovskite chalco-halides: Ba₃(FeS₄)Cl, Ba₃(FeS₄)Br, and Ba₃(FeSe₄)Br. All of them were found to be good solar light absorbers. Remarkably, although the shortest Fe-Fe distance exceeds 6 Å, an unexpected anti-ferromagnetic phase transition near 100 K was observed in their magnetic susceptibility measurement. The corresponding complex magnetic structures were resolved by neutron diffraction experiments as well as investigated by first-principles electronic structure calculations. The spin-spin coupling between two neighboring Fe atoms along the b axis, which is realized by the Fe-S···S-Fe super-super exchange mechanism, was found to be responsible for this magnetic phase transition.

MoS2–C/graphite, an electric energy storage device using Na+-based organic electrolytes

Molybdenum disulfide–carbon composite (MoS₂–C) sample has been prepared by a hydrothermal method using Na₂MoO₄, CH₄N₂S and glucose as starting materials and then calcined at 800 °C in an N₂/H₂ mixed atmosphere.