Molecular capping arrests Ostwald ripening of interfacial zinc hydroxide sulfate for reversible zinc electrochemistry

Technical Summary: Molecular Capping Arrests Ostwald Ripening of Interfacial Zinc Hydroxide Sulfate

Problem Statement
In aqueous zinc metal batteries, the electrochemical generation of interphases is a dynamic process that continues to evolve after initial formation. A critical challenge is the post-nucleation ageing of zinc hydroxide sulfate (ZHS, typically Zn₄(OH)₆SO₄·4H₂O). While ZHS initially forms as nanoscale crystallites, it progressively coarsens through a dissolution–reprecipitation mechanism known as Ostwald ripening. Driven by the Gibbs–Thomson relationship, smaller nanocrystallites possess higher chemical potential and solubility than larger counterparts, leading to the dissolution of small domains and their redeposition onto larger ones. This process transforms dispersed, potentially functional nanocrystallites into coarse, electrically insulating aggregates. These aggregates obstruct Zn²⁺ transport, redistribute local electric fields, and promote heterogeneous Zn growth, dendrite formation, and rapid cell failure. Existing strategies have largely focused on controlling ZHS nucleation or constructing preformed barriers, often neglecting the thermodynamic stabilization of nascent crystallites against post-nucleation coarsening.

Methodology
The authors propose a "post-nucleation thermodynamic locking" strategy using N-ethyl-2-pyrrolidone (NEP) as a molecular capping agent. The study employs a multi-scale approach combining experimental characterization with theoretical simulations:

• Molecular Dynamics (MD) and Density Functional Theory (DFT): Simulations were used to analyze the adsorption affinity of NEP on ZHS surfaces, the displacement of interfacial water molecules, and the reconstruction of the Zn²⁺ solvation environment.
• Spectroscopic Analysis: ¹H NMR and FT-IR spectroscopy were utilized to confirm the incorporation of NEP into the primary Zn²⁺ solvation shell and the displacement of water molecules.
• Electrochemical Characterization: Linear sweep voltammetry (LSV), linear polarization, and in situ electrochemical impedance spectroscopy (EIS) with distribution of relaxation times (DRT) analysis were conducted to evaluate hydrogen evolution reaction (HER) suppression, corrosion rates, and interfacial stability.
• Microstructural Imaging: High-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), scanning electron microscopy (SEM) with backscattered electron (BSE) imaging, and energy-dispersive X-ray spectroscopy (EDS) were used to visualize the morphology and composition of the solid electrolyte interphase (SEI) and Zn deposits.
• Depth Profiling: X-ray photoelectron spectroscopy (XPS) with Ar⁺ sputtering was performed to analyze the chemical composition of the interphase, with specific attention paid to distinguishing intrinsic components from ion-beam-induced artifacts (specifically regarding ZnO and ZnS signals).

Key Contributions and Mechanisms
The paper establishes that NEP functions through three interconnected mechanisms to stabilize the zinc interface:

1. Interfacial Thermodynamic Locking: NEP molecules exhibit a strong preferential affinity for nascent ZHS surfaces, forming a passivating molecular adlayer. This layer displaces interfacial water molecules, weakening solvent-assisted lattice hydration and dissolution. By reducing the effective solubility driving force, NEP arrests Ostwald ripening, preventing the transformation of nanoscale ZHS into coarse insulating aggregates.
2. Solvation Environment Reconstruction: In the bulk electrolyte, NEP (possessing a higher donor number than water) partially displaces water molecules in the primary solvation shell of Zn²⁺. This reduces the population of coordinated water, thereby inhibiting water reduction and corrosion at the electrode surface.
3. Hybrid Interphase Formation: The reductive decomposition of NEP generates a nitrogen-containing organic matrix. This matrix spatially accommodates the stabilized ZHS nanodomains, creating a conformal organic–inorganic interphase that remains ionically accessible while providing electronic insulation.

Results

• Morphological Control: The NEP-modified electrolyte reduced the average surface roughness of deposited Zn by approximately sixfold (from 482 nm to 82 nm). XRD analysis confirmed a 41% reduction in ZHS crystallite size (from 44 nm to 26 nm), indicating the suppression of coarsening.
• Electrochemical Performance:
  • Zn||Cu Cells: Achieved an average Coulombic efficiency (CE) of 99.79% over 3,100 cycles at 1 mA cm⁻².
  • Zn||Zn Symmetric Cells: Operated for over 3,000 hours at 1 mA cm⁻². Under extreme conditions (22.6 mA cm⁻² and 33.9 mAh cm⁻², corresponding to 56.5% Zn depth of discharge), stable operation was sustained for over 800 hours.
  • Zn||I₂ Full Cells: Demonstrated exceptional stability, sustaining 7,500 cycles at 10 A g⁻¹. The pouch cell retained 94.5% capacity after 100 cycles at 1C.
• Mechanistic Validation: In situ optical imaging and AFM confirmed the transition from dendritic growth to compact, planar deposition. Depth-resolved XPS analysis, cross-referenced with cryo-electron tomography literature, identified that observed ZnO and ZnS signals during sputtering were artifacts of ion-beam-induced ZHS decomposition rather than intrinsic SEI components, reinforcing the conclusion that the primary SEI is an organic matrix containing stabilized ZHS nanodomains.

Significance
The paper claims a paradigm shift in managing interfacial precipitates in aqueous metal batteries. Rather than attempting to eliminate the formation of ZHS—an unavoidable byproduct of water reduction and corrosion—the authors demonstrate that molecular regulation of post-nucleation evolution can convert this byproduct into a functional interphase constituent. By arresting Ostwald ripening through molecular capping, the strategy transforms an insulating, transport-blocking phase into a structurally persistent, ionically conductive hybrid interface. This work identifies the control of precipitate ageing as a critical determinant of electrochemical stability, offering a design principle for stabilizing aqueous zinc electrochemistry without relying solely on nucleation control or pre-formed barriers.