Liquid Metal Mediated Heterostructure Fluoride Solid Electrolytes of High Conductivity and Air Stability for Sustainable Na Metal Batteries

Fluoride-based solid electrolytes (SEs) have emerged as a promising component for high-energy-density rechargeable solid-state batteries (SSBs) in view of their wide electrochemical window, high air stability, and interface compatibility, but they still face the challenge of low ion conductivity and the lack of a desired structure for sodium metal SSBs. Here, we report a sodium-rich heterostructure fluoride SE, Na3GaF6-Ga2O3-NaCl (NGFOC-G), synthesized via in situ oxidation of liquid metal gallium and in situ chlorination using low-melting GaCl3. The distinctive features of NGFOC-G include single-crystal Na3GaF6 domains within an open-framework structure, composite interface decoration of Ga2O3 and NaCl with a concentration gradient, exceptional air stability, and high electrochemical oxidation stability. By leveraging the penetration of gallium at NaF grain boundaries and the in situ self-oxidation to form Ga2O3 nanodomains, the solid-phase reaction kinetics of NaF and GaF3 is activated for facilitating the synthesis of main component Na3GaF6. The introduction of a small amount of a chlorine source during synthesis further softens and modifies the boundaries of Na3GaF6 along with Ga2O3. Benefiting from the enhanced interface ion transport, the optimized NGFOC-G exhibits an ionic conductivity up to 10-4 S/cm at 40 °C, which is the highest level reported among fluoride-based sodium-ion SEs. This SE demonstrates a "self-protection" mechanism, where the formation of a high Young's modulus transition layer rich in NaF and Na2O under electrochemical driving prevents the dendrite growth of sodium metal. The corresponding Na/Na symmetric cells show minimal voltage hysteresis and stable cycling performance for at least 1000 h. The Na/NGFOC-G/Na3V2(PO4)3 cell demonstrates stable capacity release around 100 mAh/g at room temperature. The Na/NGFOC-G/FeF3 cell delivers a high capacity of 461 mAh/g with an excellent stability of conversion reaction cycling.

Ultrahigh Nitrogen Content Carbon Nanosheets for High Stable Sodium Metal Anodes

Sodium metal, with a high theoretical specific capacity of 1165 mAh g^-1 , is the ultimate anode for sodium batteries, yet how to deal with the inhomogeneous and dendritic sodium deposition and the infinite relative dimension change of sodium metal anodes during sodium depositing/stripping is still challenging. Here, a facile fabricated sodiuphilic 2D N-doped carbon nanosheets (N-CSs) are proposed as sodium host material for sodium metal batteries (SMBs) to prevent dendrite formation and eliminate volume change during cycling. Revealing from combined in situ characterization analyses and theoretical simulations, the high nitrogen content and porous nanoscale interlayer gaps of the 2D N-CSs can not only concede dendrite-free sodium stripping/depositing but also accommodate the infinite relative dimension change. Furthermore, N-CSs can be easily process into N-CSs/Cu electrode via traditional commercial battery electrode coating equipment that pave the way for large-scale industrial applications. On account of the abundant nucleation sites and sufficient deposition space, N-CSs/Cu electrodes demonstrate a superior cycle stability of more than 1500 h at a current density of 2 mA cm^-2 with a high coulomb efficiency of more than 99.9% and ultralow nucleation overpotential, which enable reversible and dendrites-free SMBs and shed light on further development of SMBs with even higher performance.

Regulating Na Electrodeposition by Sodiophilic Grafting onto Porosity-Gradient Gel Polymer Electrolytes for Dendrite-Free Sodium Metal Batteries

Sodium metal batteries have been emerging as promising candidates for post-Li battery systems owing to the natural abundance, low costs, and high energy density of Na metal. However, exploiting an Na metal anode is accompanied by uncontrolled Na electrodeposition, particularly concerning dendrite growth, hampering practical Na metal battery applications. Herein, we propose sodiophilic gel polymer electrolytes with a porosity-gradient Janus structure to alleviate Na dendrite growth. Tethering only 1.1 mol % sodiophilic poly(ethylene glycol) to poly(vinylidene fluoride-co-hexafluoropropylene) suppresses Na dendrites by regulating homogeneous Na⁺ distribution, which relies on molecular-level coordination between Na⁺ and the sodiophilic functional groups. By exploiting the porosity-gradient Janus structure, we have demonstrated that regular porosity and well-defined morphology of polymer electrolytes, particularly at the Na/electrolyte interface, significantly impact dendrite growth. This study provides new insights into the rational design of Na dendrite-suppressing polymer electrolytes, primarily focusing on the ion-regulating ability achieved by surface engineering.

3D Sodiophilic Ti3C2 MXene@g-C3N4 Hetero-Interphase Raises the Stability of Sodium Metal Anodes

Owing to several advantages of metallic sodium (Na), such as a relatively high theoretical capacity, low redox potential, wide availability, and low cost, Na metal batteries are being extensively studied, which are expected to play a major role in the fields of electric vehicles and grid-scale energy storage. Although considerable efforts have been devoted to utilizing MXene-based materials for suppressing Na dendrites, achieving a stable cycling of Na metal anodes remains extremely challenging due to, for example, the low Coulombic efficiency (CE) caused by the severe side reactions. Herein, a g-C₃N₄ layer was attached in situ on the Ti₃C₂ MXene surface, inducing a surface state reconstruction and thus forming a stable hetero-interphase with excellent sodiophilicity between the MXene and g-C₃N₄ to inhibit side reactions and guide uniform Na ion flux. The 3D construction can not only lower the local current density to facilitate uniform Na plating/stripping but also mitigate volume change to stabilize the electrolyte/electrode interphase. Thus, the 3D Ti₃C₂ MXene@g-C₃N₄ nanocomposite enables much enhanced average CEs (99.9% at 1 mA h cm^-2, 0.5 mA cm^-2) in asymmetric half cells, long-term stability (up to 700 h) for symmetric cells, and stable cycling (up to 800 cycles at 2 C), together with outstanding rate capability (up to 20 C), of full cells. The present study demonstrates an approach in developing practically high performance for Na metal anodes.

Capacity-Limited Na-M foil Anode: toward Practical Applications of Na Metal Anode

The practical applications of Na metal anodes are severely plagued by unstable Na plating/stripping. Here the fabrication of Na-rich Na-M (M = Au, Sn, and In) alloy anodes is reported as promising alternatives to address this issue. As compared to metallic Na foil anodes, the alloy foils exhibit improved electrolyte/electrode interface and provide abundant sodiophilic sites for efficient Na plating, while the self-evolved porous Na-M structures accommodate volume variation on cycling. Among three alloy foils, the Na-Au system shows the most promising performance. Under practical conditions such as capacity-limited anodes (5 mAh cm^-2 ) and large stripping/plating capacity (1.0 mAh cm^-2 ), the Na_0.9 Au_0.1 anodes demonstrate stable plating/stripping over 350 h. The proof-of-concept full cells assembled with hard carbon or Prussian blue materials and this thin Na_0.9 Au_0.1 anode also have much elongated cycling life than for pristine Na metal anodes. These findings confirm that the rational design of Na-M alloy anodes can be a promising strategy to promote the development of Na metal batteries.

Elastic NaxMoS2-Carbon-BASE Triple Interface Direct Robust Solid-Solid Interface for All-Solid-State Na-S Batteries

The developments of all-solid-state sodium batteries are severely constrained by poor Na-ion transport across incompatible solid-solid interfaces. We demonstrate here a triple Na_xMoS₂-carbon-BASE nanojunction interface strategy to address this challenge using the β″-Al₂O₃ solid electrolyte (BASE). Such an interface was constructed by adhering ternary Na electrodes containing 3 wt % MoS₂ and 3 wt % carbon on BASE and reducing contact angles of molten Na to ∼45°. The ternary Na electrodes exhibited twice improved elasticity for flexible deformation and intimate solid contact, whereas Na_xMoS₂ and carbon synergistically provide durable ionic/electronic diffusion paths, which effectively resist premature interface failure due to loss of contact and improved Na stripping utilization to over 90%. Na metal hosted via triple junctions exhibited much smaller charge-transfer resistance and 200 h of stable cycling. The novel interface architecture enabled 1100 mAh/g cycling of all-solid-state Na-S batteries when using advanced sulfur cathodes with Na-ion conductive PEO₁₀-NaFSI binder and Na_xMo₆S₈ redox catalytic mediator.

Dendrite-Free Sodium Metal Batteries Enabled by the Release of Contact Strain on Flexible and Sodiophilic Matrix

The formation of sodium (Na) dendrites during cycling has impeded the practical application of Na metal anodes. Herein, we developed a flexible graphene-based matrix, e.g., a porous reduced graphene oxide (PRGO) film, to support dendrite-free Na nucleation and plating, contributing to high-performance Na metal batteries. The PRGO film possessed outstanding merits of sodiophilicity and flexibility. The sodiophilic PRGO film enabled uniform Na nucleation in the initial electroplating stage. Furthermore, the flexible PRGO film with a small Young's modulus effectively alleviated the texture deformation of electrodeposited Na, leading to a compact and dendrite-free Na deposition layer. The well-maintained Na metal anodes on the PRGO film exhibited superior cyclability, high Coulombic efficiency, and improved energy density in both half-cell and full-cell testing. This work illustrates the great significance of mechanical properties of the supporting matrix for the Na electroplating, which provides a new strategy to develop high-performance dendrite-free Na metal batteries.