New P2-Type Honeycomb-Layered Sodium-Ion Conductor: Na2Mg2TeO6

Overview of Inorganic Electrolytes for All-Solid-State Sodium Batteries

All-solid-state sodium batteries (AS3B) emerged as a strong contender in the global electrochemical energy storage market as a replacement for current lithium-ion batteries (LIB) owing to their high abundance, low cost, high safety, high energy density, and long calendar life. Inorganic electrolytes (IEs) are highly preferred over the conventional liquid and solid polymer electrolytes for sodium-ion batteries (SIBs) due to their high ionic conductivity (∼10-2-10-4 S cm-1), wide potential window (∼5 V), and overall better battery performances. This review discusses the bird's eye view of the recent progress in inorganic electrolytes such as Na-β"-alumina, NASICON, sulfides, antipervoskites, borohydride-type electrolytes, etc. for AS3Bs. Current state-of-the-art inorganic electrolytes in correlation with their ionic conduction mechanism present challenges and interfacial characteristics that have been critically reviewed in this review. The current challenges associated with the present battery configuration are overlooked, and also the chemical and electrochemical stabilities are emphasized. The substantial solution based on ongoing electrolyte development and promising modification strategies are also suggested.

Characterization of co-fired sodium-ion conductive Na2Ni2TeO6 and Na2Zn2TeO6 with honeycomb layer structure

We investigated the reactivity of P2-type honeycomb layered oxides Na2Ni2TeO6 (NNTO) and Na2Zn2TeO6 (NZTO) co-fired at the temperature from 500 °C to 800 °C. From X-ray diffraction measurements, it was found that the reaction between NNTO and NZTO is unremarkable at the temperature below 700 °C. However, when annealed at 800 °C, they formed the solid-solution phase without any secondary phases. The NNTO and NZTO composite pellets co-fired at 800 °C showed sodium-ion conductivity well above 10-4 S cm-1 at room temperature, indicating that the solid-solution phase of NNTO and NZTO has good ionic conductivity. A maximum room temperature conductivity of 7.4 × 10-4 S cm-1 was confirmed at the mixing ratio NNTO: NZTO = 0.5 : 1.5. These results can be applied to the fabrication of all-solid-state batteries using NNTO as the cathode active material and NZTO as the solid electrolyte via a simple co-sintering process.

Chlorine-Rich Na6-xPS5-xCl1+x: A Promising Sodium Solid Electrolyte for All-Solid-State Sodium Batteries

Developing argyrodite-type, chlorine-rich, sodium-ion, solid-state electrolytes with high conductivity is a long-term challenge that is crucial for the advancement of all-solid-state batteries (ASSBs). In this study, chlorine-rich, argyrodite-type Na6-xPS5-xCl1+x solid solutions were successfully developed with a solid solution formation range of 0 ≤ x ≤ 0.5. Na5.5PS4.5Cl1.5 (x = 0.5), displaying a highest ionic conductivity of 1.2 × 10-3 S/cm at 25 °C, which is more than a hundred times higher than that of Na6PS5Cl. Cyclic voltammetry and electrochemical impedance spectroscopy results demonstrated that the rich chlorine significantly enhanced the ionic conductivity and electrochemical stability, in addition to causing a reduction in activation energy. The Na5.5PS4.5Cl1.5 composite also showed the characteristics of a pure ionic conductor without electronic conductivity. Finally, the viability of Na5.5PS4.5Cl1.5 as a sodium electrolyte for all-solid-state sodium batteries was checked in a lab-scale ASSB, showing stable battery performance. This study not only demonstrates new composites of sodium-ionic, solid-state electrolytes with relatively high conductivity but also provides an anion-modulation strategy to enhance the ionic conductivity of argyrodite-type sodium solid-state ionic conductors.

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

Sodium-ion batteries (SIBs) are experiencing a large-scale renaissance to supplement or replace expensive lithium-ion batteries (LIBs) and low energy density lead-acid batteries in electrical energy storage systems and other applications. In this case, layered oxide materials have become one of the most popular cathode candidates for SIBs because of their low cost and comparatively facile synthesis method. However, the intrinsic shortcomings of layered oxide cathodes, which severely limit their commercialization process, urgently need to be addressed. In this review, inherent challenges associated with layered oxide cathodes for SIBs, such as their irreversible multiphase transition, poor air stability, and low energy density, are systematically summarized and discussed, together with strategies to overcome these dilemmas through bulk phase modulation, surface/interface modification, functional structure manipulation, and cationic and anionic redox optimization. Emphasis is placed on investigating variations in the chemical composition and structural configuration of layered oxide cathodes and how they affect the electrochemical behavior of the cathodes to illustrate how these issues can be addressed. The summary of failure mechanisms and corresponding modification strategies of layered oxide cathodes presented herein provides a valuable reference for scientific and practical issues related to the development of SIBs.

Unraveling Sodium-Ion Dynamics in Honeycomb-Layered Na2MgxZn2-xTeO6 Solid Electrolytes with Solid-State NMR

All-solid-state sodium-ion batteries (SIBs) have the potential to offer large-scale, safe, cost-effective, and sustainable energy storage solutions by supplementing the industry-leading lithium-ion batteries. However, for the enhanced bulk properties of SIB components (e.g., solid electrolytes), a comprehensive understanding of their atomic-scale structure and the dynamic behavior of sodium (Na) ions is essential. Here, we utilize a robust multinuclear (23Na, 125Te, 25Mg, and 67Zn) magnetic resonance approach to explore a novel Mg/Zn homogeneously mixed-cation honeycomb-layered oxide Na2MgxZn2-xTeO6 solid solution series. These new intermediate compounds exhibit tailorable bulk Na-ion conductivity (σ) with the highest σ = 0.14 × 10-4 S cm-1 for Na2MgZnTeO6 at room temperature suitable for SIB solid electrolyte applications as observed by powder electrochemical impedance spectroscopy (EIS). A combination of powder X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, and field emission scanning electron microscopy (FESEM) reveals highly crystalline phase-pure compounds in the P6322 space group. We show that the Mg/Zn disorder is random within the honeycomb layers using 125Te nuclear magnetic resonance (NMR) and resolve multiple Na sites using two-dimensional (triple-quantum magic-angle spinning (3QMAS)) 23Na NMR. The medium-range disorder in the honeycomb layer is revealed through the combination of 25Mg and 67Zn NMR, complemented by electronic structure calculations using density functional theory (DFT). Furthermore, we expose very fast local Na-ion hopping processes (hopping rate, 1/τNMR = 0.83 × 109 Hz) by using a laser to achieve variable high-temperature (∼860 K) 23Na NMR, which are sensitive to different Mg/Zn ratios. The Na2MgZnTeO6 with maximum Mg/Zn disorder displays the highest short-range Na-ion dynamics among all of the solid solution members.

Recent Progress on Honeycomb Layered Oxides as a Durable Cathode Material for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are becoming promising candidates for energy storage devices due to the low cost, abundant reserves, and excellent electrochemical performance. As the most important unit, layered cathodes attract much attention, where honeycomb-layered-oxides (HLOs) manifest outstanding structural stability, high redox potential, and long-life electrochemistry. Here, recent progress on HLOs as well as Na₃ Ni₂ SbO₆ and Na₃ Ni₂ BiO₆ as two representative materials are introduced, and the crystal and electronic structure, electrochemical performance, and modification strategies are summarized. The advanced high nickel HLOs are highlighted toward development of state-of-the-art sodium-ion batteries. This review would deepen the understanding of superstructure in layered oxides, as well as structure-property relationship, and inspire more interest in high output voltage, long lifespan sodium-ion batteries.

Evidence of Long-Range and Short-Range Magnetic Ordering in the Honeycomb Na3Mn2SbO6 Oxide

We present a comprehensive study of the synthesis, structure, and magnetic properties of the honeycomb oxide Na₃Mn₂SbO₆ supported by neutron diffraction, heat capacity, and magnetization measurements. The refinements of the neutron diffraction patterns (150, 50, and 45 K) using the Rietveld method confirm the monoclinic (S. G. C2/m) structure. Temperature-dependent magnetic susceptibilities measured at varying fields along with the heat capacity measurements demonstrate the coexistence of long-range ordering (∼42 K) and short-range ordering (∼65 K). The field-dependent isothermal magnetization measurements at 5 K indicate a spin-flop transition around 5 T. Rietveld refinements of the low-temperature (below 45 K) neutron diffraction data further confirm the long-range magnetic ordering. In addition, the temperature variation of the lattice parameters obtained from the neutron powder diffraction analysis exhibited a distinct anomaly near the antiferromagnetic transition temperature. The appearance of the concomitant broadened backgrounds in the neutron powder diffraction data collected at 80, 50, and 45 K supports the short-range ordering. The resultant magnetic structure consists of spins that are aligned antiparallel with the nearest neighbors and also with the spins of the adjacent honeycomb layers. The occurrence of a fully ordered magnetic ground state (Neel antiferromagnetic (AFM)) in Na₃Mn₂SbO₆ consolidates the significance of fabricating new honeycomb oxides.

Honeycomb-Layered Oxides With Silver Atom Bilayers and Emergence of Non-Abelian SU(2) Interactions

Honeycomb-layered oxides with monovalent or divalent, monolayered cationic lattices generally exhibit myriad crystalline features encompassing rich electrochemistry, geometries, and disorders, which particularly places them as attractive material candidates for next-generation energy storage applications. Herein, global honeycomb-layered oxide compositions, Ag2 M2 TeO6 (M = Ni , Mg , etc $M = \rm Ni, Mg, etc$ .) exhibiting Ag $\rm Ag$ atom bilayers with sub-valent states within Ag-rich crystalline domains of Ag6 M2 TeO6 and Ag $\rm Ag$ -deficient domains ofAg 2 - x Ni 2 TeO 6 ${\rm Ag}_{2 - x}\rm Ni_2TeO_6$ (0 < x < 2 $0 < x < 2$ ). The Ag $\rm Ag$ -rich material characterized by aberration-corrected transmission electron microscopy reveals local atomic structural disorders characterized by aperiodic stacking and incoherency in the bilayer arrangement of Ag $\rm Ag$ atoms. Meanwhile, the global material not only displays high ionic conductivity but also manifests oxygen-hole electrochemistry during silver-ion extraction. Within the Ag $\rm Ag$ -rich domains, the bilayered structure, argentophilic interactions therein and the expected Ag $\rm Ag$ sub-valent states (1 / 2 + , 2 / 3 + $1/2+, 2/3+$ , etc.) are theoretically understood via spontaneous symmetry breaking of SU(2)× U(1) gauge symmetry interactions amongst 3 degenerate mass-less chiral fermion states, justified by electron occupancy of silver4 d z 2 $4d_{z^2}$ and 5s orbitals on a bifurcated honeycomb lattice. This implies that bilayered frameworks have research applications that go beyond the confines of energy storage.

Ion Hopping: Design Principles for Strategies to Improve Ionic Conductivity for Inorganic Solid Electrolytes

Solid electrolytes are considered as an ideal substitution of liquid electrolytes, avoiding the potential hazards of volatilization, flammability, and explosion for liquid electrolyte-based rechargeable batteries. However, there are significant performance gaps to be bridged between solid electrolytes and liquid electrolytes; one with a particular importance is the ionic conductivity which is highly dependent on the material types and structures. In this review, the general physical image of ion hopping in the crystalline structure is revisited, by highlighting two main kernels that impact ion migration: ion hopping pathways and skeletons interaction. The universal strategies to effectively improve ionic conductivity of inorganic solid electrolytes are then systematically summarized: constructing rapid diffusion pathways for mobile ions; and reducing resistance of the surrounding potential field. The scoped strategies offer an exclusive view on the working principle of ion movement regardless of the ion species, thus providing a comprehensive guidance for the future exploitation of solid electrolytes.

Fast Ion Transport Mechanism and Electrochemical Stability of Trivalent Metal Iodide-based Na Superionic Conductors Na3XI6 (X = Sc, Y, La, and In)

The exploration of solid-state sodium superionic conductors with high sodium-ion conductivity, structural and electrochemical stability, and grand interface compatibility has become the key to the next-generation energy storage applications with high energy density and long cycling life. Among them, halide-based compounds exhibit great potential with the higher electronegativity of halogens than that of the sulfur element. In this work, combined with first-principles calculation and ab initio molecular dynamic simulation, the investigation of trivalent metal iodide-based Na superionic conductors C2/m-Na₃XI₆ (X = Sc, Y, La, and In) was conducted, including the fast ion transport mechanism, structural stability, and interface electrochemical compatibility with electrode materials. Along with the tetrahedral-center saddle site-predominant three-dimensional octahedral-tetrahedral-octahedral diffusion network, C2/m-Na₃XI₆ possesses the merits of high Na ionic conductivities of 0.36, 0.35, and 0.20 mS cm^-1 for Na₃ScI₆, Na₃YI₆, and Na₃LaI₆, respectively. Benefiting from its structural stabilities, C2/m-Na₃XI₆ exhibits lower interface reaction energy and better electrochemical compatibility in contact with both Na metal and high-voltage poly-anion (fluoro)phosphate cathode materials than those of sulfide-based ones. Our theoretical work provides rational design principles for screening and guiding iodide-based C2/m-Na₃XI₆ (X = Sc, Y, La, and In) as promising Na superionic conductor candidates used in all-solid-state energy storage applications.

Effects of Ga Substitution on the Local Structure of Na2Zn2TeO6

In the work presented here, we prepared Ga-substituted NZTO (Na_2–xZn_2–xGa_xTeO₆, x = 0.00, 0.05, 0.10, 0.15, 0.20) layered materials with a soft chemical, citric acid-based synthesis method and characterized these by means of X-ray diffraction (XRD), ²³Na and ¹²⁵Te NMR, and by density functional theory (DFT) modeling. The influence of randomly distributed Ga cations on the ¹²⁵Te NMR spectra confirms the successful synthesis. With DFT-based linear response computations, we show that the local distribution of Na ions in the two neighboring interlayers influences the ¹²⁵Te chemical shift, consistent with observations. DFT modeling suggests that some of the Na sites are rarely occupied in pure NZTO but become favorable upon Ga substitution. There are clear indications that Ga substitution gives an uneven distribution of Na ions in neighboring interlayers and that the Na structure in one layer affects the adjacent layers.

The influence of randomly distributed Ga cations on the 125Te NMR spectra confirms the successful synthesis.

Prussian Blue Analogues for Sodium-Ion Batteries: Past, Present, and Future

Prussian blue analogues (PBAs) have attracted wide attention for their application in the energy storage and conversion field due to their low cost, facile synthesis, and appreciable electrochemical performance. At the present stage, most research on PBAs is focused on their material-level optimization, whereas their properties in practical battery systems are seldom considered. This review aims to first provide an overview of the history and parameters of PBA materials and analyze the fundamental principles toward rational design of PBAs, and then evaluate the prospects and challenges for PBAs for practical sodium-ion batteries, hoping to bridge the gap between laboratory research and commercial reality.

Mixed alkali-ion transport and storage in atomic-disordered honeycomb layered NaKNi2TeO6

Honeycomb layered oxides constitute an emerging class of materials that show interesting physicochemical and electrochemical properties. However, the development of these materials is still limited. Here, we report the combined use of alkali atoms (Na and K) to produce a mixed-alkali honeycomb layered oxide material, namely, NaKNi2TeO6. Via transmission electron microscopy measurements, we reveal the local atomic structural disorders characterised by aperiodic stacking and incoherency in the alternating arrangement of Na and K atoms. We also investigate the possibility of mixed electrochemical transport and storage of Na+ and K+ ions in NaKNi2TeO6. In particular, we report an average discharge cell voltage of about 4 V and a specific capacity of around 80 mAh g-1 at low specific currents (i.e., < 10 mA g-1) when a NaKNi2TeO6-based positive electrode is combined with a room-temperature NaK liquid alloy negative electrode using an ionic liquid-based electrolyte solution. These results represent a step towards the use of tailored cathode active materials for "dendrite-free" electrochemical energy storage systems exploiting room-temperature liquid alkali metal alloy materials.

Honeycomb layered oxides: structure, energy storage, transport, topology and relevant insights

The advent of nanotechnology has hurtled the discovery and development of nanostructured materials with stellar chemical and physical functionalities in a bid to address issues in energy, environment, telecommunications and healthcare. In this quest, a class of two-dimensional layered materials consisting of alkali or coinage metal atoms sandwiched between slabs exclusively made of transition metal and chalcogen (or pnictogen) atoms arranged in a honeycomb fashion have emerged as materials exhibiting fascinatingly rich crystal chemistry, high-voltage electrochemistry, fast cation diffusion besides playing host to varied exotic electromagnetic and topological phenomena. Currently, with a niche application in energy storage as high-voltage materials, this class of honeycomb layered oxides serves as ideal pedagogical exemplars of the innumerable capabilities of nanomaterials drawing immense interest in multiple fields ranging from materials science, solid-state chemistry, electrochemistry and condensed matter physics. In this review, we delineate the relevant chemistry and physics of honeycomb layered oxides, and discuss their functionalities for tunable electrochemistry, superfast ionic conduction, electromagnetism and topology. Moreover, we elucidate the unexplored albeit vastly promising crystal chemistry space whilst outlining effective ways to identify regions within this compositional space, particularly where interesting electromagnetic and topological properties could be lurking within the aforementioned alkali and coinage-metal honeycomb layered oxide structures. We conclude by pointing towards possible future research directions, particularly the prospective realisation of Kitaev-Heisenberg-Dzyaloshinskii-Moriya interactions with single crystals and Floquet theory in closely-related honeycomb layered oxide materials.

Electrolytes for Lithium- and Sodium-Metal Batteries

High-energy-density batteries have attracted significant attention due to the huge demand in electric transportation in future. Metal-based batteries, especially lithium metal batteries (LMBs) and sodium metal batteries (SMBs), have been hot research topics nowadays. The uncontrolled growth of metal dendrites has retarded the development of LMBs and SMBs. Various electrolytes have been explored to meet the demand of high-performance metal-based batteries, such as additives-contained electrolytes, polymer electrolytes, and solid-state electrolytes. To guide the development of electrolytes in LMBs and SMBs, we organize this roadmap to give out the status of present research and future challenges in this field. We also hope that the readers can get the knowledge and ideas from this roadmap.

An idealised approach of geometry and topology to the diffusion of cations in honeycomb layered oxide frameworks

Honeycomb layered oxides are a novel class of nanostructured materials comprising alkali or coinage metal atoms intercalated into transition metal slabs. The intricate honeycomb architecture and layered framework endows this family of oxides with a tessellation of features such as exquisite electrochemistry, unique topology and fascinating electromagnetic phenomena. Despite having innumerable functionalities, these materials remain highly underutilised as their underlying atomistic mechanisms are vastly unexplored. Therefore, in a bid to provide a more in-depth perspective, we propose an idealised diffusion model of the charged alkali cations (such as lithium, sodium or potassium) in the two-dimensional (2D) honeycomb layers within the multi-layered crystal of honeycomb layered oxide frameworks. This model not only explains the correlation between the excitation of cationic vacancies (by applied electromagnetic fields) and the Gaussian curvature deformation of the 2D surface, but also takes into consideration, the quantum properties of the cations and their inter-layer mixing through quantum tunnelling. Through this work, we offer a novel theoretical framework for the study of multi-layered materials with 2D cationic diffusion currents, as well as providing pedagogical insights into the role of topological phase transitions in these materials in relation to Brownian motion and quantum geometry.

Insights into Crystal Structure and Diffusion of Biphasic Na2Zn2TeO6

The layered oxide Na₂Zn₂TeO₆ is a fast Na⁺ ion conductor and a suitable candidate for application as a solid-state electrolyte. We present a detailed study on how synthesis temperature and Na-content affect the crystal structure and thus the Na⁺ ion conductivity of Na₂Zn₂TeO₆. Furthermore, we report for the first time an O'3-type phase for Na₂Zn₂TeO₆. At a synthesis temperature of 900 °C, we obtain a pure P2-type phase, providing peak performance in Na⁺ ion conductivity. Synthesis temperatures lower than 900 °C produce a series of mixed P2 and O'3-type phases. The O'3 structure can only be obtained as a pure phase by substituting Li on the Zn-sites to increase the Na-content. Thorough analysis of synchrotron data combined with computational modeling indicates that Li enters the Zn sites and, consequently, the amount of Na in the structure increases to balance the charge according to the formula Na_2+xZn_2-xLi_xTeO₆ (x = 0.2-0.5). Impedance spectroscopy and computational modeling confirm that reducing the amount of the O'3-type phase enhances the Na⁺ ion mobility.

Minerals to Functional Materials: Characterization of Structural Phase Transitions and Raman Analysis of a Superionic Phase in Na6Co(SO4)4

In search of promising Na⁺ ion conductors, we have detected a superionic phase in a Vantoffite mineral, Na₆Co(SO₄)₄, at 570 °C, thus enhancing the use of minerals to produce futuristic solid state electrolytes. Na₆Co(SO₄)₄ crystallizes concomitantly to produce di- and tetrahydrate forms from an aqueous solution. Both the crystal forms belong to a triclinic system, space group P1. The mineral transforms to a dehydrated phase as established by in situ single crystal X-ray diffraction at 217 °C and is shown to be isostructural with its Mn analogue. Even though thermal analysis indicates a single structural phase transition at 450 °C, the features associated with in situ powder X-ray diffraction as well as in situ Raman spectroscopy signify a second phase transition ≈540 °C and the behavior of ionic conductivity leads to a superionic phase (σ ≈ 10^-2 S/cm at 570 °C). These observations are significant for the development and understanding of mineral based solid electrolytes.

F-Doped NaTi2(PO4)3/C Nanocomposite as a High-Performance Anode for Sodium-Ion Batteries

We are presenting a sol-gel method for building novel nanostructures made of nanosized F-doped Na_{1-2 x}Ti₂(PO₄)_{3- x}F _x (NTP-F _x, x = 0, 0.02, 0.05, and 0.10) particles embedded in three-dimensional (3D) carbon matrices (NTP-F _x/C). This technique combines advantages of both zero-dimensional materials and 3D-carbon networks. Proper fluorine doping stabilizes the NTP structure and greatly enhances ion/electron transportation, leading to superhigh-rate electrochemical performance and ultralong cycle life. The composite electrode delivers high specific capacities of 121, 115, 112.2, 110.1, 107.7, 103.1, 85.8, and 62.5 mA h g^-1 at 0.2, 0.5, 1, 2, 5, 10, 20, and 30 C, respectively. It retains an unbelievable ∼70% capacity after a thousand cycles at a rate as high as 10 C. Electroanalytical results reveal that fluorine doping significantly enhances Na⁺ diffusion kinetics. Meanwhile, density functional theory calculations demonstrate F-doped NTPs' own outstanding electrochemical properties, which is due to the enhanced intrinsic ionic/electronic conductivity. The results show that anion doping is an efficient way to make high-performance NTP anodes for sodium-ion batteries.

Na3Ca4(TeO3)(PO4)3: a new noncentrosymmetric tellurite phosphate with fascinating multimember-ring architectures and intriguing nonlinear optical performance

A new functional compound Na₃Ca₄(TeO₃)(PO₄)₃ exhibits a fascinating multimember-ring configuration and intriguing nonlinear optical performance.