Bulk and grain-boundary ionic conductivity in sodium zirconophosphosilicate Na3Zr2(SiO4)2PO4 (NASICON)

Interface Ionic/Electronic Redistribution Driven by Conversion-Alloy Reaction for High-Performance Solid-State Sodium Batteries

NASICON-type Na+ conductors show a great potential to realize high performance and safety for solid-state sodium metal batteries (SSSMBs) owing to their superior ionic conductivity, high chemical stability, and low cost. However, the interfacial incompatibility and sodium dendrite hazards still hinder its applications. Herein, a conversion-alloy reaction-induced interface ionic/electronic redistribution strategy, constructing a gradient sodiophilic and electron-blocking interphase consisting of sodium-tin (Na-Sn) alloy and sodium fluoride (NaF) between NASICON ceramic electrolyte and Na anode is proposed. The NaxSny alloy-rich layer near the side of the sodium electrode acts as a superior conductor to enhance the anodic sodium-ion transport dynamics while the NaF-rich layer near the side of the ceramic electrolyte serves as an electron insulator to confine the interfacial electron turning ability, achieving uniform and dendrite-free Na deposition during the cycling. Profiting from the synergistic effect of the gradient interphase, the critical current density (CCD) of the assembled Na symmetric cell is significantly increased to 1.7 mA cm-2 and the cycling stability of that is as high as 1200 h at 0.5 mA cm-2. Moreover, quasi-solid-state sodium batteries with both Na3V2(PO4)3 and NaNi1/3Fe1/3Mn1/3O2 cathode display outstanding electrochemical performance.

Challenges and Breakthroughs in Enhancing Temperature Tolerance of Sodium-Ion Batteries

Lithium-based batteries (LBBs) have been highly researched and recognized as a mature electrochemical energy storage (EES) system in recent years. However, their stability and effectiveness are primarily confined to room temperature conditions. At temperatures significantly below 0 °C or above 60 °C, LBBs experience substantial performance degradation. Under such challenging extreme contexts, sodium-ion batteries (SIBs) emerge as a promising complementary technology, distinguished by their fast dynamics at low-temperature regions and superior safety under elevated temperatures. Notably, developing SIBs suitable for wide-temperature usage still presents significant challenges, particularly for specific applications such as electric vehicles, renewable energy storage, and deep-space/polar explorations, which requires a thorough understanding of how SIBs perform under different temperature conditions. By reviewing the development of wide-temperature SIBs, the influence of temperature on the parameters related to battery performance, such as reaction constant, charge transfer resistance, etc., is systematically and comprehensively analyzed. The review emphasizes challenges encountered by SIBs in both low and high temperatures while exploring recent advancements in SIB materials, specifically focusing on strategies to enhance battery performance across diverse temperature ranges. Overall, insights gained from these studies will drive the development of SIBs that can handle the challenges posed by diverse and harsh climates.

Rechargeable Solid-State Na-Metal Battery Operating at -20 °C

Achieving satisfactory performance for a solid-state Na-metal battery (SSNMB) with an inorganic solid electrolyte (SE), especially under freezing temperatures, poses a challenge for stabilizing a Na-metal anode. Herein, this challenge is addressed by utilizing a Natrium super ionic conductor (NASICON) NASICON-type solid electrolyte, enabling the operation of a rechargeable SSNMB over a wide temperature range from -20 to 45 °C. The interfacial resistance at the Na metal/SE interface is only 0.4 Ω cm2 at 45 °C and remains below 110 Ω cm2 even at -20 °C. Remarkably, long-term Na-metal plating/stripping cycles lasting over 2000 h at -20 °C are achieved with minimal polarization voltages at 0.1 mA cm-2 . Further analysis reveals the formation of a uniform Na3- x Cax PO4 interphase layer at the interface, which significantly contributes to the exceptional interfacial performance observed. By employing a Na3 V1.5 Al0.5 (PO4 )3 cathode, the full battery system demonstrates excellent adaptability to low temperatures, exhibiting a capacity of 80 mA h g-1 at -20 °C over 50 cycles and retaining a capacity of 108 mAh g-1 (88.5% of the capacity at 45 °C) at 0 °C over 275 cycles. This research significantly reduces the temperature threshold for SSNMB operation and paves the way toward solid-state batteries suitable for all-season applications.

Characterizing Grain Boundary Effects on Mg2+ Conduction in Metal-Organic Frameworks

Next-generation materials for fast ion conduction have the potential to revolutionize battery technology. Metal-organic frameworks (MOFs) are promising candidates for achieving this goal. Given their structural diversity, the design of efficient MOF-based conductors can be accelerated by a detailed understanding and accurate prediction of ion conductivity. However, the polycrystalline nature of solid-state materials requires consideration of grain boundary effects, which is complicated by challenges in characterizing grain boundary structures and simulating ensemble transport processes. To address this, we have developed an approach for modeling ion transport at grain boundaries and predicting their contribution to conductivity. Mg²⁺ conduction in the Mg-MOF-74 thin film was studied as a representative system. Using computational techniques and guided by experiments, we investigated the structural details of MOF grain boundary interfaces to determine accessible Mg²⁺ transport pathways. Computed transport kinetics were input into a simplified MOF nanocrystal model, which combines ion transport in the bulk structure and at grain boundaries. The model predicts Mg²⁺ conductivity in the MOF-74 film within chemical accuracy (<1 kcal/mol activation energy difference), validating our approach. Physically, Mg²⁺ conduction in MOF-74 is inhibited by strong Mg²⁺ binding at grain boundaries, such that only a small fraction of grain boundary alignments allow for fast Mg²⁺ transport. This results in a 2-3 order-of-magnitude reduction in conductivity, illustrating the critical impact of the grain boundary contribution. Overall, our work provides a computation-aided platform for molecular-level understanding of grain boundary effects and quantitative prediction of ion conductivity. Combined with experimental measurements, it can serve as a synergistic tool for characterizing the grain boundary composition of MOF-based conductors.

Nanoscale interface engineering of inorganic Solid-State electrolytes for High-Performance alkali metal batteries

All-solid-state metal batteries (ASSMBs) have been regarded as the ideal candidate for the next-generation high-energy storage system due to their ultrahigh specific capacity and the lowest redox potential. However, the uncontrollable chemical reactivity during cycling which directly determines the growth behaviour of metal dendrites, the low coulombic efficiency and the safety concerns severely limit their real-world applications.. Crystallographic optimization based on solid-state electrolytes (SSEs) provides an atomic-scale and fundamental solution for the inhibition of dendrite growth in metal anodes, which has attracted widespread attentions. From this perspective, we summarize the recent advance of the crystallographic optimization for various classes of solid-state electrolytes. We highlight the recent experimental findings of crystallographic optimization for a new generation of all-solid-state batteries, including lithium-ion batteries, sodium-ion batteries, magnesium-ion batteries, with the aim of providing a deeper understanding of the crystallographic reactions in ASSMBs. The challenges and prospects for the future design and engineering of crystallographic optimization of SSEs are discussed, providing ideas for further research into crystallographic optimization to improve the performance of rechargeable batteries.

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

Solid electrolytes are at the heart of future energy storage systems. Li-bearing argyrodites are frontrunners in terms of Li⁺ ion conductivity. Although many studies have investigated the effect of elemental substitution on ionic conductivity, we still do not fully understand the various origins leading to improved ion dynamics. Here, Li_6+xP_1–xGe_xS₅I served as an application-oriented model system to study the effect of cation substitution (P⁵⁺ vs Ge⁴⁺) on Li⁺ ion dynamics. While Li₆PS₅I is a rather poor ionic conductor (10^–6 S cm^–1, 298 K), the Ge-containing samples show specific conductivities on the order of 10^–2 S cm^–1 (330 K). Replacing P⁵⁺ with Ge⁴⁺ not only causes S^2–/I^– anion site disorder but also reveals via neutron diffraction that the Li⁺ ions do occupy several originally empty sites between the Li rich cages in the argyrodite framework. Here, we used ⁷Li and ³¹P NMR to show that this Li⁺ site disorder has a tremendous effect on both local ion dynamics and long-range Li⁺ transport. For the Ge-rich samples, NMR revealed several new Li⁺ exchange processes, which are to be characterized by rather low activation barriers (0.1–0.3 eV). Consequently, in samples with high Ge-contents, the Li⁺ ions have access to an interconnected network of pathways allowing for rapid exchange processes between the Li cages. By (i) relating the changes of the crystal structure and (ii) measuring the dynamic features as a function of length scale, we were able to rationalize the microscopic origins of fast, long-range ion transport in this class of electrolytes.

Under Pressure: Mechanochemical Effects on Structure and Ion Conduction in the Sodium-Ion Solid Electrolyte Na3PS4

Fast-ion conductors are critical to the development of solid-state batteries. The effects of mechanochemical synthesis that lead to increased ionic conductivity in an archetypical sodium-ion conductor Na₃PS₄ are not fully understood. We present here a comprehensive analysis based on diffraction (Bragg and pair distribution function), spectroscopy (impedance, Raman, NMR and INS), and ab initio simulations aimed at elucidating the synthesis-property relationships in Na₃PS₄. We consolidate previously reported interpretations regarding the local structure of ball-milled samples, underlining the sodium disorder and showing that a local tetragonal framework more accurately describes the structure than the originally proposed cubic one. Through variable-pressure impedance spectroscopy measurements, we report for the first time the activation volume for Na⁺ migration in Na₃PS₄, which is ∼30% higher for the ball-milled samples. Moreover, we show that the effect of ball-milling on increasing the ionic conductivity of Na₃PS₄ to ∼10^-4 S/cm can be reproduced by applying external pressure on a sample from conventional high-temperature ceramic synthesis. We conclude that the key effects of mechanochemical synthesis on the properties of solid electrolytes can be analyzed and understood in terms of pressure, strain, and activation volume.

Fluoride-Ion Batteries: On the Electrochemical Stability of Nanocrystalline La0.9Ba0.1F2.9 against Metal Electrodes

Over the past years, ceramic fluorine ion conductors with high ionic conductivity have stepped into the limelight of materials research, as they may act as solid-state electrolytes in fluorine-ion batteries (FIBs). A factor of utmost importance, which has been left aside so far, is the electrochemical stability of these conductors with respect to both the voltage window and the active materials used. The compatibility with different current collector materials is important as well. In the course of this study, tysonite-type La0.9Ba0.1F2.9, which is one of the most important electrolyte in first-generation FIBs, was chosen as model substance to study its electrochemical stability against a series of metal electrodes viz. Pt, Au, Ni, Cu and Ag. To test anodic or cathodic degradation processes we carried out cyclic voltammetry (CV) measurements using a two-electrode set-up. We covered a voltage window ranging from −1 to 4 V, which is typical for FIBs, and investigated the change of the response of the CVs as a function of scan rate (2 mV/s to 0.1 V/s). It turned out that Cu is unstable in combination with La0.9Ba0.1F2.9, even before voltage was applied. The cells with Au and Pt electrodes show reactions during the CV scans; in the case of Au the irreversible changes seen in CV are accompanied by a change in color of the electrode as investigated by light microscopy. Ag and Ni electrodes seem to suffer from contact issues which, most likely, also originate from side reactions with the electrode material. The experiments show that the choice of current collectors in future FIBs will become an important topic if we are to develop long-lasting FIBs. Most likely, protecting layers between the composite electrode material and the metal current collector have to be developed to prevent any interdiffusion or electrochemical degradation processes.

Lithium ion dynamics in LiZr2(PO4)3 and Li1.4Ca0.2Zr1.8(PO4)3

High ionic conductivity, electrochemical stability and small interfacial resistances against Li metal anodes are the main requirements to be fulfilled in powerful, next-generation all-solid-state batteries. Understanding ion transport in materials with sufficiently high chemical and electrochemical stability, such as rhombohedral LiZr2(PO4)3, is important to further improve their properties with respect to translational Li ion dynamics. Here, we used broadband impedance spectroscopy to analyze the electrical responses of LiZr2(PO4)3 and Ca-stabilized Li1.4Ca0.2Zr1.8(PO4)3 that were prepared following a solid-state synthesis route. We investigated the influence of the starting materials, either ZrO2 and Zr(CH3COO)4, on the final properties of the products and studied Li ion dynamics in the crystalline grains and across grain boundary (g.b.) regions. The Ca2+ content has only little effect on bulk properties (4.2 × 10-5 S cm-1 at 298 K, 0.41 eV), but, fortunately, the g.b. resistance decreased by 2 orders of magnitude. Whereas, 7Li spin-alignment echo nuclear magnetic resonance (NMR) confirmed long-range ion transport as seen by conductivity spectroscopy, 7Li NMR spin-lattice relaxation revealed much smaller activation energies (0.18 eV) and points to rapid localized Li jump processes. The diffusion-induced rate peak, appearing at T = 282 K, shows Li+ exchange processes with rates of ca. 109 s-1 corresponding, formally, to ionic conductivities in the order of 10-3 S cm-1 to 10-2 S cm-1.

Spatial confinement – rapid 2D F− diffusion in micro- and nanocrystalline RbSn2F5

NMR and conductivity spectroscopy reveal 2D diffusion in both microcrystalline and nanocrystalline RbSn₂F₅.

Substitutional disorder: structure and ion dynamics of the argyrodites Li6PS5Cl, Li6PS5Br and Li6PS5I

Li NMR spectroscopy reveals rapid Li ion dynamics in the poor Li ion conductor Li₆PS₅I; long-range motion is, however, only possible for Li₆PS₅Br and Li₆PS₅Cl with anion site disorder.