Magnesium (Mg) metal is a promising anode material for next-generation batteries due to its high theoretical capacity (3833 mAh cm⁻³), low redox potential (−2.37 V vs. standard hydrogen electrode), abundance, and dendrite-resistant nature. However, its use is hampered by severe surface passivation in conventional electrolytes, particularly in the presence of water. Even trace amounts of water (>20 ppm) can significantly impact electrolyte solvation structure and lead to the formation of a passivating layer composed of magnesium oxides (MgO) and hydroxides (Mg(OH)₂), resulting in high overpotentials and rapid battery failure. Previous strategies to mitigate this issue include using water scavengers or constructing artificial solid electrolyte interphases (SEIs). However, creating truly waterproof Mg anodes that function reliably after direct water contact remains a challenge due to a lack of comprehensive understanding of water-induced passivation mechanisms. This study aims to address this gap by investigating the role of MgH₂ in the passivation process and developing a novel approach to create waterproof Mg anodes.

Extensive research has focused on understanding passivation layers in various metal-based batteries. In lithium-metal anodes, the presence and role of lithium hydride (LiH) in the SEI layer have been a subject of debate, with studies confirming its formation and its impact on SEI stability. Similarly, the role of sodium hydride (NaH) in sodium-metal batteries has been investigated. In contrast, research on magnesium hydride (MgH₂) in Mg metal anodes is limited. While a recent study demonstrated reversible MgH₂ formation on a CuSe cathode in an aqueous electrolyte, its role and formation mechanism on Mg anodes remain unclear. This lack of understanding underscores the need for a deeper investigation into the water-induced passivation of Mg anodes and the potential involvement of MgH₂.

This study employed a combination of experimental and theoretical techniques. Experimentally, a pencil-drawn graphite (PDG) interphase was created on Mg foil by simply drawing with a pencil. The structure and composition of the PDG layer and its interaction with water were characterized using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The electrochemical performance of the PDG-Mg anodes, both with and without prior water treatment, was evaluated using galvanostatic cycling tests in symmetric and full cells with different electrolytes (0.3 M Mg(OTf)₂ and 0.2 M MgCl₂ in DME, and 5 M MgCl₂ aqueous electrolyte). Theoretically, first-principles calculations based on density functional theory (DFT) were used to investigate the electronic structure, electrochemical stability window, and adsorption energies of MgH₂, Mg(OH)₂, MgO, and the O-containing graphite in the PDG layer. The Mg diffusion energy barriers were calculated using the climbing image nudged elastic band (CI-NEB) method. Finite element simulations using COMSOL Multiphysics were employed to model ion flux distribution and electric field across the Mg anode surface with and without the PDG interphase.

The key findings of this study are: 1. **Identification of MgH₂ in the Passivation Layer:** In addition to MgO and Mg(OH)₂, MgH₂ was identified as a component of the water-induced passivation layer on Mg anodes. TOF-SIMS and Raman spectroscopy confirmed its presence and spatial distribution within the passivation layer. 2. **Mechanism of Water-Induced Passivation:** The formation of MgH₂, MgO, and Mg(OH)₂ is attributed to the reaction of Mg with water, leading to hydrogen gas evolution that subsequently reacts with Mg to form MgH₂. 3. **Properties of the Passivation Layer:** DFT calculations showed that MgH₂, Mg(OH)₂, and MgO exhibit poor Mg²⁺ adsorption and transport, leading to concentration polarization and high overpotentials. 4. **Development of a Waterproof Anode:** A simple pencil-drawn graphite (PDG) interphase effectively prevents water-induced passivation. The PDG interphase is hydrophobic, exhibits high Mg²⁺ diffusivity (energy barrier of 0.02 eV), and strong magnesiophilicity, facilitating uniform Mg plating/stripping. 5. **Improved Electrochemical Performance:** PDG-Mg anodes exhibit significantly improved stability compared to pristine Mg anodes, both in symmetric cells (over 900 h at 1.0 mA cm⁻², 1.0 mAh cm⁻²) and full cells (over 500 cycles at 0.5 C with a Mo₆S₈ cathode), even after water exposure. COMSOL simulations confirmed the role of the PDG layer in homogenizing ion flux and electric field distribution at the interface, thus minimizing overpotential.

The findings of this study directly address the longstanding challenge of water-induced passivation in Mg anodes. The identification of MgH₂ as a key component of the passivation layer provides a crucial mechanistic insight that has been previously overlooked. The development of the simple, scalable, and effective PDG interphase offers a practical solution to enhance the water resistance of Mg anodes. The combination of experimental and theoretical analyses provides a comprehensive understanding of the interplay between the electrode material, electrolyte, and interphase, paving the way for designing more robust and high-performing Mg metal batteries. The improved electrochemical performance observed in both symmetric and full cells demonstrates the significant potential of this approach for practical applications.

This research successfully identified MgH₂ as a crucial component in water-induced passivation of Mg metal anodes, providing a key mechanistic understanding. Furthermore, a simple pencil-drawn graphite interphase was developed and shown to effectively protect Mg anodes from water, leading to remarkable improvements in cycling stability in both symmetric and full cells. Future research could explore other types of interphases with enhanced properties or investigate the applicability of this method to other water-sensitive battery chemistries.

While this study demonstrates significant progress in creating waterproof Mg anodes, further research is needed to optimize the long-term stability and cyclability under more demanding conditions, including higher current densities and prolonged cycling. The influence of different types of graphite and pencil hardness on the interphase's properties also warrants further investigation. The study primarily focused on a specific electrolyte; exploring the effectiveness of the PDG interphase with a wider range of electrolytes would strengthen the conclusions.