Zhengwu Fang

NaSICON (Na Super Ionic Conductor) ceramics with formula Na _{1+ x} Zr ₂ Si _x P _{3− x} O ₁₂ (NZSP) are leading solid electrolyte candidates for sodium‐based energy storage. However, the mechanical properties governing membrane reliability, namely elastic modulus, hardness, fracture toughness, and flexural strength, have not been systematically characterized across the full composition range, owing to confounding effects from porosity, glassy grain‐boundary phases, and ZrO ₂ transformation‐induced and grain‐boundary microcracking in conventionally sintered materials. Here, we report the first comprehensive mechanical characterization of high‐purity (total secondary‐phase content < 10%), fully dense NZSP across x = 2.0–2.8, prepared by rapid induction hot‐pressing of powders prepared by solution‐assisted solid‐state reaction. Hot‐pressed samples achieve relative densities exceeding 99%, nanosized grains (150–200 nm), nanosized ZrO ₂ particles, and glass‐free grain boundaries across most compositions. Elastic modulus increases from 109 GPa ( x = 2.0) to 124 GPa ( x = 2.8), attributed to progressive replacement of compliant phosphate units by stiffer silicate groups and the corresponding increase in the stiffer rhombohedral phase. Hardness follows the same trend, rising from 7.20 to 9.61 GPa. Fracture toughness falls between 1.0 and 1.5 MPa·m ^1/2 . Biaxial flexure strength reaches a maximum of 195 MPa at x = 2.4. The critical grain size for thermal expansion anisotropy‐driven cracking falls from 9 µm ( x = 2) to 0.5 µm ( x = 3), for both phases above the hot‐pressed grain size at every composition.

Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) garnet material is a promising inorganic electrolyte for all-solid-state lithium metal batteries because of its safety, broad electrochemical stability and low air sensitivity. However, LLZTO faces critical challenges related to sintering and grain boundaries that adversely affect its mechanical and electrochemical properties. Here we propose oversaturation doping of yttrium into Li_6.72La_3.00Y_0.22Zr_1.28Ta_0.50O₁₂ (LLYZTO) using 1.0 wt% of (Y₂O₃)_0.08(ZrO₂)_0.92 (YSZ). During annealing, a La–Y–O (LYO) nanoscale interphase forms at the grain boundaries. This material nanoengineering approach improves indentation fracture toughness, increases the critical current density and enhances bulk ionic conductivity while maintaining low electronic conductivity. The LLYZTO with 1.0 wt% of YSZ (LLYZTO–1.0 wt% YSZ) is more lithiophobic than its undoped counterpart, and accommodates the volume changes of LiCoO₂ during battery operation, enabling a 2.2 mAh cm⁻² all-solid-state Li||LiCoO₂ coin cell to deliver 2.0 mAh cm⁻² at 0.66 mA cm⁻² for over 100 cycles at 30 °C and 1 MPa of stack pressure. The fracture toughness of LLYZTO–1.0 wt% YSZ also allows the sintering fabrication of a square 5.29-cm² and 150 µm-thick electrolyte membrane, which can be effectively used in lab-scale Li||LiCoO₂ pouch cells. We also demonstrate the extension of the LYO interphase approach to other Li-based inorganic solid electrolytes.

NaSICONs, or sodium superionic conductors, are promising solid electrolyte materials for Na-based all-solid-state and aqueous redox-flow battery applications. Here, the composition Na_3.4Zr₂Si_2.4P_0.6O₁₂ has been prepared through solution-assisted microwave processing and densification through rapid induction hot pressing. The samples display remarkably high total ionic conductivities between 7 and 9 mS cm^–1, competitive with liquid electrolytes. A combination of synchrotron X-ray and neutron diffraction, electrochemical impedance spectroscopy, and variable-temperature solid-state nuclear magnetic resonance studies reveals rapid Na-ion transport through the rigid skeletal framework of this solid ion conductor. Variable-temperature synchrotron X-ray diffraction reveals structural phase separation in this composition on cooling at temperatures close to 430 K with the samples at room temperature displaying a mix of monoclinic C2/c and rhombohedral R3̅c components that appear to collectively contribute to the high conductivity. The observation of very high ionic conductivity in a region of compositional space that is associated with structural instability is proposed as a design principle for superionic conduction.

Structural defects and lattice strain are intrinsic to many crystalline materials, yet their roles in controlling chemical reaction mechanisms and directing crystallization pathways remain poorly understood. Here, we revealed the three-dimensional evolution of strain and dislocation defects at the nanoscale during the growth of heterogeneously nucleated barite (BaSO₄) and calcite (CaCO₃) crystals by using coherent X-ray scattering, electron microscopy, and molecular simulations. Unlike barite, which formed with minimal internal strain, calcite developed dislocation defects and exhibited spatially varying strain that increased during growth. During growth in Sr-rich solutions, calcite likely incorporates Sr²⁺ into the defects, which further modulates the local lattice structure and increases both the compressive and tensile strain. These findings suggest that calcite crystallization was likely dominated by attachment of precursor phases, which gave rise to defect-enriched domain structures not predicted by classical growth models. By linking defect formation to ion incorporation and growth dynamics, this work provides fundamental insight into how lattice-level strain heterogeneity governs the chemical reactivity of ionic crystals.

While the development of new solid electrolytes (SEs) is crucial for advancing energy storage technologies, revisiting existing materials with significantly improved knowledge of their physical properties and synthesis control offers significant opportunities for breakthroughs. Na1+xZr2SixP3−xO12 (NaSICON) SEs have recently regained attention for applications in both solid-state and aqueous redox flow batteries due to their improved electrochemical and mechanical properties, along with their inherent electrochemical stability, air robustness, and low manufacturing cost. Recent improvements in NaSICON have primarily targeted macroscopic property enhancements and synthesis techniques. To enable further breakthroughs in the performance of NaSICON SEs, future efforts should focus on understanding how modified synthesis conditions influence atomic and microscopic-scale features, such as conduction channels, electronic structures, phase distributions, and grain boundaries. These features ultimately control ion conductivity, mechanical properties, and electrochemical stability of NaSICON and its interfaces. Here, we review the current understanding of the structure-chemistry-property relationships of NaSICON SEs, focusing on atomic and microscopic levels. First, we introduce the proposed ionic conduction mechanisms in NaSICON crystallites. Then, we explore experimental investigations at phase and grain boundaries to assess ionic conduction and interfacial stability. We also examine strategies to address interfacial challenges such as high resistance and chemical reactions between SEs and electrodes, highlighting the difficulties in analyzing interfaces at the nano/atomic scale. Finally, we provide an outlook on advancing microscopy and spectroscopy techniques to enhance insights into NaSICON SEs ionic conduction and interfacial stability, supporting the development of improved long-duration energy storage devices.

Grain boundary (GB) structural change is commonly observed during and after stress-driven GB migration in nanocrystalline materials, but its exact atomic scale transformation has not been explored experimentally. Here, using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we observed the dynamic GB structural transformation stemming from reversible facet transformation and GB dissociation during the shear-mediated migration of faceted GBs in gold nanocrystals. A reversible transformation was found to occur between (002)/(111) and Σ11(113) GB facets, accomplished by the coalescence and detachment of ( 1 ¯ 1 ¯ 1 ) / ( 002 ) -type GB steps or disconnections that mediated the GB migration. In comparison, the dissociation of (002)/(111) GB into Σ11(113) and Σ3(111) GBs occurred via the reaction of ( 111 ) / ( 11 1 ¯ ) -type steps that involved the emission of partial dislocations. Furthermore, these transformations were loading dependent and could be accommodated by GB junctions. This work provides atomistic insights into the dynamic structural transformation during GB migration.