Lithium metal batteries (LMBs) have emerged as a promising technology for high-energy density applications, exceeding 350 Wh kg⁻¹. The high theoretical specific capacity (3860 mAh g⁻¹) and low redox potential (-3.04 V vs. standard hydrogen electrode) of the lithium metal anode (LMA) are key advantages. However, challenges remain, primarily due to lithium dendrite formation and low Coulombic efficiency (CE). These issues stem from the lack of a stable and uniform solid electrolyte interface (SEI) between the LMA and the electrolyte. An ideal SEI needs fast Li⁺ ion conductivity, negligible electron conductivity, high mechanical strength, and high interfacial energy with the LMA. Therefore, electrolyte engineering is crucial for controlling SEI composition and preventing dendrite growth and side reactions, thereby achieving high CE. LiF is a highly effective SEI component due to its low electronic conductivity and high surface energy. Its small lattice constant allows elastic deformation of the SEI. LiF-rich SEIs have proven effective in suppressing dendrite formation and side reactions. Various strategies to create LiF-rich SEIs have been explored, including fluorinated solvents, electrolyte additives, high-concentration electrolytes (HCEs), and localized HCEs. These approaches have resulted in LMBs with impressive CE values (>99%). However, LiF's poor Li⁺ conductivity (~10⁻¹⁰ S cm⁻¹) creates an inhomogeneous Li⁺ flux across the SEI. This uneven Li⁺ distribution leads to dendritic deposition. Existing models, such as Sand's time (tsand), focus on Li⁺ transfer through the bulk electrolyte but ignore the rate-limiting SEI. Tsand is also impractical for real-world LMBs due to high current density thresholds. Understanding how SEI properties kinetically affect Li deposition is essential for designing advanced electrolytes. This paper addresses these challenges by establishing a mechanistic protocol to decipher the impact of the SEI on Li deposition, validated by assessing the compatibility of successful fluorine-rich electrolytes with the LMA.

Significant research has focused on developing electrolytes that promote the formation of a stable and uniform SEI on the lithium metal anode to mitigate dendrite formation and improve Coulombic efficiency. Various approaches have been investigated, including the use of fluorinated solvents [16-21], electrolyte additives [22-25], high-concentration electrolytes (HCEs) [26-28], and localized HCEs [29-33]. These studies have demonstrated the effectiveness of LiF-rich SEIs in enhancing the reversibility of lithium plating/stripping. However, the inherent low ionic conductivity of LiF remains a significant limitation, leading to inhomogeneous lithium-ion flux and subsequent dendrite growth. Previous models, such as Sand's time, have attempted to describe the onset of dendrite formation, but these models often oversimplify the complex interfacial processes occurring within the SEI [36-41]. The limitations of these existing models highlight the need for a more comprehensive understanding of the interplay between SEI properties and lithium deposition kinetics.

The researchers established a mechanistic model based on the law of Li mass conservation to quantitatively assess the impact of the SEI on Li deposition. The model classifies SEI components into high and low Li⁺ mobility zones based on their diffusion energy barriers. It establishes a linear correlation between capacity loss (Qloss) and current density (j): Qloss = Qir + k*j, where Qir represents irreversible capacity loss, and k is a slope that indicates the homogeneity of Li⁺ flux across the SEI (k → 0 indicates homogeneous diffusion). The intercept Qir represents the maximum achievable CE for a given electrolyte. This model was validated using several state-of-the-art high-fluorine electrolytes, demonstrating a clear linear correlation between Qloss and j. The model parameters k, Qir, and CEmax were evaluated for each electrolyte, revealing that high-concentration electrolytes, despite high CE, exhibited high k values, indicating inhomogeneous Li⁺ flux. An electrolyte with low k and Qir suggests uniform Li⁺ diffusion and high CE. To address this inhomogeneity, a dual-halide electrolyte (1.3 M LiFSI in DME/1,2-dichloroethane (DCE), denoted as 1.3 M LDC) was designed. This electrolyte produces a dual-halide (LiF1-xCx) SEI. Cl doping enhances LiF's conductivity without compromising mechanical stability. This was confirmed through experiments showing improved CE (>99.5%) in Li||Cu cells and extended cycle life (>200 cycles) in full cells. Raman spectroscopy, molecular dynamics (MD) simulations, and X-ray photoelectron spectroscopy (XPS) were employed to investigate the Li⁺ solvation structure and SEI components. MD simulations revealed the dominant AGG solvation structure in the 1.3 M LDC electrolyte and preferential decomposition of FSI anions and DCE molecules to form LiF1-xCx. XPS confirmed the presence of both LiF and LiCl in the SEI. Density functional theory (DFT) calculations were used to study the Li⁺ diffusion energy barriers in LiF and LiF1-xCx, revealing a six-fold reduction in the energy barrier after Cl doping. The electrochemical performance of LMAs was evaluated using Li||Cu cells. The 1.3 M LDC electrolyte demonstrated a higher CE (99.54%) and longer cycle life compared to other electrolytes. Furthermore, full cells (Li||NCM811 and anode-free Cu||NCM523 pouch cells) were tested, demonstrating the effectiveness of the dual-halide electrolyte in achieving long cycle life and high capacity retention. COMSOL simulations further supported the findings by visualizing Li⁺ flux and potential distribution across the electrolyte and SEI, highlighting the superior Li⁺ transport in the dual-halide SEI.

The researchers developed a mechanistic model that links Li⁺ flux homogeneity in the SEI to Li deposition behavior. This model introduces a key parameter, k, derived from the linear relationship between capacity loss and current density, which quantifies the homogeneity of the Li⁺ flux. A low k-value indicates uniform Li⁺ distribution and is crucial for dendrite-free Li deposition. The model also allows for the prediction of the maximum achievable Coulombic efficiency (CE) for a given electrolyte. The model was successfully validated using several high-performance electrolytes, including high-concentration electrolytes (HCEs). The results show that even highly efficient HCEs exhibit inhomogeneous Li⁺ flux (high k-values) due to the limitations of the LiF-rich SEI. This finding highlights the importance of considering the Li⁺ transport within the SEI, rather than solely focusing on bulk electrolyte properties. To improve Li⁺ transport, a novel dual-halide electrolyte (1.3 M LiFSI in DME/DCE) was formulated. This electrolyte forms a dual-halide (LiF1-xCx) SEI layer on the lithium anode. The addition of chloride ions significantly improves the Li⁺ ionic conductivity of the SEI without compromising its mechanical stability. This leads to a significantly lower k-value, indicating a much more uniform Li⁺ flux. Extensive experimental characterization, including Raman spectroscopy, XPS, and MD simulations, was used to analyze the electrolyte solvation structure and the composition of the resulting SEI. The findings confirm the formation of a dual-halide SEI composed of both LiF and LiCl, with a significantly improved Li⁺ conductivity compared to LiF-only SEIs. The improved performance of the dual-halide electrolyte was demonstrated through extensive electrochemical testing. The Li||Cu cells exhibited an impressive CE of 99.54%, and the full cells (Li||NCM811 and anode-free Cu||NCM523 pouch cells) demonstrated greatly enhanced cycle life and capacity retention, exceeding 200 cycles in some cases and achieving over 125 cycles in practical anode-free pouch cells. The COMSOL simulations of Li⁺ flux and potential distribution support the experimental findings, illustrating the benefits of uniform Li⁺ distribution.

This study significantly advances the understanding of Li⁺ transport in Li metal batteries and its impact on dendrite formation. The developed mechanistic model provides a powerful tool for evaluating the compatibility of electrolytes with Li anodes, going beyond simply measuring CE. The identification of k as an indicator for Li⁺ flux homogeneity offers a new perspective on electrolyte design and opens new avenues for optimizing SEI properties. The successful design and implementation of a dual-halide electrolyte, producing a LiF1-xCx SEI, validates the model's predictions and demonstrate a practical strategy for achieving highly reversible and long-lasting LMBs. The findings highlight the importance of considering SEI properties in addition to bulk electrolyte characteristics for the development of high-performance LMBs. This work provides valuable insights for future electrolyte design, focusing on achieving uniform Li⁺ flux at the SEI to suppress dendrite growth and enable high-energy LMBs for practical applications.

This research successfully established a mechanistic model that links Li⁺ flux homogeneity within the solid electrolyte interphase (SEI) to the success of lithium metal batteries. The model introduces a critical parameter, κ, which effectively predicts electrolyte compatibility with lithium anodes. The results demonstrate that a dual-halide electrolyte, designed based on model predictions, effectively creates a stable SEI with uniform Li⁺ conductivity, leading to significantly improved Coulombic efficiency and extended cycle life in both half-cells and full cells. Future studies could focus on expanding the model to include additional factors influencing SEI formation and Li deposition, such as the influence of different cathode materials and the impact of temperature variations. Further exploration of novel electrolyte compositions based on the principles elucidated in this work is warranted.

While the study provides a significant advancement in understanding Li⁺ flux and its influence on lithium metal batteries, several limitations should be considered. The model simplifies the complex SEI structure, representing it as two distinct zones of high and low Li⁺ mobility. A more detailed, multi-component model might provide an even more accurate description of the SEI behavior. Additionally, the study focuses on specific electrolyte systems, and further investigation may be needed to confirm the generality of the proposed model and the dual-halide approach to a wider range of electrolytes and battery chemistries. The electrochemical tests were performed under specific conditions; the results might differ under different temperature and current density ranges.