The escalating demand for high-energy-density rechargeable batteries to power electronics, electric vehicles, and grid-scale energy storage has spurred extensive research into lithium-based battery technologies. Lithium metal batteries (LMBs), with their potential for significantly higher energy density than current lithium-ion batteries (LIBs), are at the forefront of this effort. Lithium metal (Li⁰) anodes offer a high theoretical specific capacity (3857 mAh g⁻¹) and a low reduction potential (−3.04 V vs standard hydrogen electrode), making them an attractive choice for achieving the desired energy density improvements. However, the practical application of LMBs faces considerable hurdles. The inherent instability of the Li⁰/electrolyte interface leads to the formation of lithium dendrites—needle-like structures that can pierce the separator, causing short circuits and posing safety risks. Furthermore, the uncontrolled growth of dead Li⁰, inactive lithium that doesn't participate in the electrochemical reactions, contributes to low Coulombic efficiency (CE) and poor battery lifespan. Various strategies have been explored to address these challenges, including interfacial engineering, electrolyte modifications, host material design for Li⁰, and separator modifications. While some progress has been made in partially mitigating dendrite growth, a complete resolution requires a deeper understanding of the underlying dendrite formation mechanisms. The formation of dendrites is intricately linked to the inhomogeneous chemical and morphological nature of the solid electrolyte interphase (SEI) formed in situ on the Li⁰ surface. This heterogeneity causes uneven current density distributions, leading to preferential lithium deposition in certain areas, which initiates and accelerates dendrite growth. While theoretical models exist, such as Sand's equation and the diffusion-limited aggregation model, they often oversimplify the complex interplay of factors that govern Li⁰ deposition at the interface, particularly within the context of the SEI. A key factor is the diffusion energy barrier for Li⁺ ions across the SEI. Lowering this barrier and increasing the Li⁺ transference number (which reflects the pure contribution of Li⁺ to the overall ion transport) are crucial for promoting uniform Li⁰ deposition. Current research often focuses on solid-state electrolytes (polymers and ceramics), which, while potentially achieving high transference numbers, suffer from issues like low mechanical strength, poor ionic conductivity (in polymers), and poor interfacial contact (in ceramics), severely limiting their practicality. Therefore, a novel strategy is needed to engineer the Li⁰/electrolyte interface to provide dual protection—both high Li⁺ transference and robust mechanical strength—for stable, high-performance LMBs.

Extensive research has focused on improving the stability of lithium metal anodes. Interfacial engineering approaches, like creating artificial SEIs, have shown promise in mitigating dendrite formation and improving cycling stability. Studies have explored various materials for artificial SEIs, aiming to achieve a balance between high ionic conductivity for fast Li⁺ transport and robust mechanical strength to prevent dendrite penetration. However, a clear understanding of the correlation between Li⁺ transport behavior within the SEI and the mechanical properties of the SEI remains elusive. Many previous works have focused on modifying the electrolyte or using various host materials to improve the stability and reversibility of lithium metal anodes. These studies have shown that electrolyte additives can effectively suppress dendrite growth by modifying the SEI composition and properties or enhancing Li⁺ transference numbers. The use of lithiophilic materials as hosts for lithium has also shown promising results in improving cycling performance and preventing dendrite formation. However, existing methods often face challenges in achieving uniform coatings or maintaining good contact between the solid-state electrolyte and the lithium metal anode. The challenge remains in finding a solution that combines both high Li⁺ transference numbers and high mechanical strength to suppress lithium dendrite formation effectively.

This study introduces a facile and scalable solution-processed approach to construct a phase-pure, single-crystalline Li₃N-rich artificial SEI on lithium metal anodes. The process involves immersing lithium chips in tetramethylethylenediamine (TEMED), which spontaneously reacts with Li⁰ to form α-phase Li₃N. The reaction time was optimized to ensure complete surface coverage by the SEI while maintaining desirable ionic conductivity. The researchers systematically characterized the resulting SEI using a variety of techniques. Contact angle measurements were performed to assess the wettability of the electrolyte on the treated Li⁰ surface, indicating the quality of the interface. X-ray diffraction (XRD) analysis was used to confirm the phase purity and crystalline structure of the Li₃N SEI. Transmission electron microscopy (TEM) provided further structural insights. Electrochemical impedance spectroscopy (EIS) was employed to investigate the interfacial resistance and Li⁺ ion conductivity. The Li⁺ transference number was evaluated using the Bruce-Vincent approach, which involves measuring the steady-state current under a small applied voltage. The activation energy for Li⁺ diffusion was determined using Arrhenius plots from EIS data. Phase-field simulations were integrated with experimental data to elucidate the relationship between the SEI's properties (phase purity and Li⁺ diffusivity) and its effectiveness in suppressing dendrite growth. These simulations provided detailed visualizations of Li⁺ concentration gradients and electric field distributions near the electrode surface under different SEI conditions. The mechanical properties of the SEI were characterized using atomic force microscopy (AFM) to evaluate surface roughness and Young's modulus. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the SEI and confirm the presence of Li₃N. Finally, the electrochemical performance of the TEMED-treated Li⁰ electrodes was evaluated using both symmetric cells and full cells (LFP and NMC cathodes) under various cycling conditions. Symmetric cells were cycled at different current densities and capacities to assess the stability of Li plating/stripping. Full cells were cycled at different rates to evaluate their performance in practical battery configurations. Scanning Electron Microscopy (SEM) analysis was performed before and after cycling to visualize the surface morphology of the lithium electrodes.

The study found that the TEMED-derived Li₃N-rich SEI exhibits superior performance compared to conventionally produced Li₃N SEIs. The phase-pure, single-crystalline nature of the TEMED-derived SEI resulted in a significantly lower diffusion energy barrier for Li⁺ ions, increasing Li⁺ mobility, and leading to a more uniform Li⁰ deposition. Contact angle measurements showed improved electrolyte wettability on the TEMED-treated Li⁰, facilitating homogeneous ion distribution. XRD and TEM analysis confirmed the formation of a phase-pure α-Li₃N SEI. EIS measurements revealed that the TEMED-treated Li⁰ exhibited lower impedance, indicating improved Li⁺ ion conductivity. The Li⁺ transference number was considerably higher in the TEMED-treated cells (*t*⁺ = 0.668) than in untreated cells (*t*⁺ = 0.37). Arrhenius plots showed a significantly lower activation energy for Li⁺ diffusion in the TEMED-treated Li⁰ (0.48 eV) compared to untreated (0.703 eV) and conventionally N2 treated Li (0.613 eV). Phase-field simulations corroborated these findings, demonstrating that the higher Li⁺ diffusivity in the TEMED-derived SEI resulted in significantly reduced dendrite growth and a more uniform Li⁰ deposition profile. The Li₃N SEI also exhibited high mechanical strength (Young's modulus of 6.85 GPa), significantly higher than untreated Li (0.32 GPa) and the critical value for dendrite suppression (6.0 GPa). This high modulus contributes to the stability of the interface and prevents dendrite propagation. XPS analysis confirmed the absence of other organic components, confirming that the improved performance is solely due to the Li₃N SEI. Electrochemical testing revealed significantly enhanced performance of the TEMED-treated Li⁰ in both symmetric and full cells. Symmetric cells displayed extended cycle life, exceeding 3500 h at 0.5 mA cm⁻². Full cells using LFP and NMC cathodes showed remarkable stability and capacity retention over numerous cycles, far exceeding untreated Li⁰ cells. SEM analysis confirmed the absence of dendrites in the TEMED-treated Li⁰ after cycling, showcasing uniform Li⁰ deposition.

The results clearly demonstrate that the facile TEMED treatment method offers a highly effective strategy for enhancing the stability and performance of lithium metal anodes. The formation of the phase-pure, single-crystalline Li₃N SEI is crucial for achieving the observed improvements. The lower Li⁺ diffusion energy barrier and higher Li⁺ transference number in this SEI lead to a more uniform Li⁺ flux distribution at the electrode/electrolyte interface, effectively suppressing dendrite formation and improving cycling stability. The high mechanical strength of the Li₃N SEI further contributes to the stability by preventing mechanical failure and dendrite penetration. The observed improvements in both symmetric and full cell performance highlight the practical relevance of this approach for developing high-energy-density and long-life LMBs. The agreement between experimental observations and phase-field simulations supports the proposed mechanism of dendrite suppression through manipulating the Li⁺ diffusion barrier and transference number. This work significantly advances the field of LMB research by offering a scalable and highly effective method for constructing stable and durable artificial SEIs.

This research presents a facile and effective solution-processed method to create a phase-pure Li₃N-rich artificial SEI on lithium metal anodes using TEMED treatment. This SEI dramatically improves the cycling stability and capacity retention of both symmetric and full lithium-metal batteries. The significant enhancements stem from the low Li⁺ diffusion energy barrier, high Li⁺ transference number, and high mechanical strength of the single-crystalline Li₃N SEI. Future work could focus on exploring other suitable precursors for SEI formation to further optimize performance and investigate the long-term stability and scalability of this approach for various battery chemistries and applications. Exploring different electrolyte compositions and optimizing the SEI thickness could also lead to further performance improvements.

While this study demonstrates significant improvements in lithium metal battery performance, some limitations exist. The long-term stability of the TEMED-derived SEI under extreme conditions (high temperature, high current density) requires further investigation. The scalability of the TEMED treatment process for large-scale battery manufacturing needs to be assessed. A comprehensive cost-benefit analysis comparing this method to other SEI formation techniques would also be valuable. The study primarily focused on LFP and NMC cathodes. Investigating the compatibility of this SEI with other cathode materials would broaden the applicability of the findings.