Lithium (Li) metal batteries are promising next-generation energy storage systems, potentially exceeding the energy density of conventional batteries. However, challenges remain, including the highly reactive nature of Li metal, leading to dendrite growth, inactive Li accumulation, and unstable electrode/electrolyte interfaces. A significant challenge is irreversible Li loss due to incomplete stripping, interface evolution, and corrosion. Corrosion, a common phenomenon in materials science, involves the oxidation of a metal to its ionic species. In Li metal batteries, Li corrosion is a self-discharge process independent of external current or potential polarization, and it's closely tied to the solid-electrolyte-interphase (SEI), a passivation layer formed at the Li metal-electrolyte interface. The SEI undergoes dynamic changes, including swelling, dissolution, breakage, and reformation. This dynamic behavior exposes fresh Li to the electrolyte, causing continuous side reactions and Li depletion. Electrochemical corrosion can be exacerbated by localized galvanic couples. Previous attempts to address corrosion through SEI engineering, such as electrolyte formulation or artificial SEI introduction, have shown limited success, highlighting the need for a deeper understanding of the correlation between Li corrosion and SEI progression.

Extensive research has focused on mitigating Li corrosion primarily by modifying the SEI. This includes formulating electrolytes with fluorinated solvents and additives, and introducing artificial SEIs like nitride films, carbon materials, and self-assembled monolayers. While some progress has been made in stabilizing the SEI and inhibiting Li corrosion, improvements in battery lifespan remain far from ideal. This limitation stems from a lack of understanding of Li corrosion science; corrosion-induced Li loss has not been effectively eliminated. The need for further investigation into the dynamic relationship between Li corrosion and SEI evolution is evident. The current literature lacks a comprehensive understanding of corrosion science within battery systems, necessitating further research.

This study quantitatively monitored Li corrosion behavior through devised electrochemical tools and cryo-electron microscopy. The correlation between Li corrosion, SEI dissolution, and battery failure was investigated. A key aspect of the methodology involved a facile electrochemical protocol to determine Li corrosion by measuring capacity loss during galvanostatic Li plating and stripping with varying rest periods. Another protocol involving repeated galvanostatic charging/discharging with open-circuit pauses assessed SEI dissolution. Cryo-electron microscopy was utilized to directly visualize the morphological, structural, and chemical changes of the SEI before and after electrolyte exposure. The dissolved SEI components were also analyzed to confirm the hypothesis of SEI dissolution leading to Li corrosion. To address the observed Li corrosion and SEI dissolution, the researchers designed an artificial passivation layer comprising a low-solubility polymer (poly(vinylidene fluoride), PVDF) and metal fluoride (MFx@PVDF). This layer was fabricated via a blade-coating method onto Li foil or Cu foil/foam. The selection of metal fluoride was based on low solubility and lithiophilicity. Various characterization techniques were used to analyze the artificial passivation layer, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the artificial passivation layer was evaluated in Li||Cu and Li||Li cells by measuring Coulombic efficiency (CE), impedance, and exchange current density. Cryo-transmission electron microscopy (cryo-TEM) and electron energy loss spectroscopy (EELS) were employed to analyze the microscopic morphology, structure, and elemental distribution of the interface with and without the artificial passivation layer. Finally, the efficacy of the artificial passivation layer was tested in full cells using LiFePO4 and NCM523 cathodes, both in coin-type and pouch cell configurations.

The study established a direct correlation between Li corrosion and SEI dissolution. The continuous corrosion of Li metal is attributed to the dissolution of the native SEI, repeatedly exposing fresh Li to the electrolyte and triggering continuous side reactions. The capacity loss in Li||Cu cells increased with resting time, indicating ongoing Li corrosion. Cryo-TEM confirmed SEI shrinkage and compositional changes (reduced organic components, increased inorganic Li salts) after soaking in electrolyte. The artificial passivation layer, MFx@PVDF (specifically MgF2@PVDF), effectively reduced Li corrosion by 74% after 50 hours of rest, as evidenced by significantly reduced capacity loss. This layer showed high ionic conductivity and facilitated uniform Li deposition. In Li||Cu cells, the MgF2@PVDF-coated Cu electrode achieved a high average Coulombic efficiency (CE) of 96.2% over 500 cycles at 1.0 mA cm², significantly exceeding the performance of the bare Cu electrode. Cryo-TEM images revealed a uniform, double-layered SEI on Li deposits with the MgF2@PVDF layer. The outer amorphous polymer layer acted as a barrier, and the inner layer of Mg- and Li-containing species promoted ion transport. The Li||Li symmetric cells with MgF2@PVDF-coated electrodes exhibited substantially longer lifespans (1700 h) and lower voltage hysteresis compared to bare Cu electrodes. In full cells with LiFePO4 cathodes, the MgF2@PVDF-modified cells demonstrated superior rate capability and long-term cycling stability (maintaining ~80% capacity after 1500 cycles at 1.3 mA cm⁻²). These results were also observed in pouch cells with high-loading LiFePO4 and NCM523 cathodes. The findings demonstrate the ability of the artificial passivation layer to stabilize the Li metal-liquid electrolyte interface, suppress Li corrosion, and improve the lifespan of Li metal batteries.

The findings directly address the research question by demonstrating the effectiveness of an artificial passivation layer in mitigating Li corrosion and enhancing the lifespan of Li metal batteries. The key contribution is the quantitative demonstration of the relationship between Li corrosion and SEI dissolution, a mechanism not fully understood previously. The designed passivation layer, comprising a low-solubility polymer and metal fluoride, successfully suppresses both chemical and electrochemical corrosion. The improved performance of both coin-type and pouch cells validates the practicality and scalability of this approach. The results highlight the importance of considering corrosion science in designing high-performance Li metal batteries. This work expands the fundamental understanding of Li corrosion and provides a practical strategy for developing durable Li metal batteries. The broader impact lies in the potential application of this strategy to other battery systems using reactive metallic negative electrodes.

This research uncovered the dynamic interplay between Li corrosion and SEI dissolution, showing that SEI dissolution is a major contributor to Li loss in Li-metal batteries. A novel artificial passivation layer of PVDF and metal fluorides effectively mitigated Li corrosion, significantly improving battery performance and lifespan in both coin cells and pouch cells. This study provides valuable insights into corrosion science in battery systems and offers a practical strategy for developing more durable and longer-lasting Li metal batteries. Future research could focus on exploring other polymer-fluoride combinations and investigating the long-term stability and performance of this approach under various operating conditions.

The study primarily focused on a specific set of electrolytes and electrode materials. Further investigation is needed to determine the generalizability of the findings to a wider range of battery chemistries. Long-term stability testing under various conditions (temperature, charge/discharge rates) should be conducted to fully assess the durability and reliability of the artificial passivation layer. The scaling up of the artificial passivation layer manufacturing process for commercial applications requires further optimization.