Solid-state batteries (SSBs) are considered the next generation of lithium-ion batteries due to their potential for enhanced energy density and improved safety compared to their liquid electrolyte counterparts. However, a significant challenge limiting the widespread adoption of SSBs is the growth of lithium dendrites at the lithium metal anode/solid-state electrolyte (SSE) interface. Li dendrite formation leads to the development of uneven electric-field distribution, poor interfacial contact, and ultimately short-circuiting of the battery, resulting in failure. The high electronic conductivity of many SSEs exacerbates this issue. The uneven electric-field distribution often arises from poor interfacial contact between the lithium metal anode and the SSE. This results in non-uniform lithium deposition, which can initiate and accelerate dendrite growth. The research described in this paper focuses on addressing these challenges by developing a novel interfacial layer that mitigates electron transport across the interface, while simultaneously improving the interfacial contact between the lithium metal and the solid-state electrolyte. The goal is to create a stable and dendrite-free interface, thereby enhancing the performance and longevity of solid-state batteries.

Previous research has explored various strategies to prevent Li dendrite formation in SSBs. These include modifying the surface of the SSE to enhance its wettability with lithium metal, creating artificial SEI (Solid Electrolyte Interphase) layers, and employing three-dimensional architectures to mitigate stress concentration. Surface modification often focuses on enhancing lithiophilicity, the affinity of the SSE surface for lithium ions, using methods like coating the electrolyte surface with a thin layer of another material that has a higher lithiophilicity than the base electrolyte. The hope is that this will encourage uniform deposition of lithium metal and prevent dendrite formation. Studies using different materials such as oxides and polymers have shown improvements, but challenges remain in achieving both sufficient ionic conductivity and sufficient electron blocking. The creation of artificial SEI layers mirrors the behavior of the naturally occurring SEI layer in liquid-based lithium-ion batteries that promotes stable Li deposition. However, artificial SEI layers have to be specifically designed for SSE interfaces and this approach often faces challenges in achieving sufficient stability and reliability. Three-dimensional architectures try to manipulate the distribution of the electric field and alleviate stress concentration during lithium plating, by creating structured electrodes that are conducive to uniform lithium deposition. These methods, while promising, often add complexities to battery manufacturing. This paper suggests a different approach of developing a flexible electron blocking interface, offering a novel method to solve the issue.

This study introduces a flexible electron-blocking interfacial shield (EBS) created using poly(acrylic acid) (PAA). The EBS is formed *in situ* via a substitution reaction between the PAA film and molten lithium at 250 °C. The reaction mechanism and products were investigated using first-principles density functional theory (DFT) calculations. These calculations indicated that a substitution reaction, where lithium replaces hydrogen in the PAA -COOH group, is more stable than a recombination reaction. Differential electrochemical mass spectrometry (DEMS) confirmed the substitution reaction by detecting the release of hydrogen gas during the process. The resulting LiPAA layer was characterized using various techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The mechanical properties of the PAA-modified interface were assessed using atomic force microscopy (AFM) to measure Young’s modulus, demonstrating the flexibility of the resulting interface. DFT simulations were used to investigate the electron-blocking properties of the LiPAA EBS. Electrostatic potential profiles and density of states (DOS) simulations demonstrated a high electron-tunneling energy barrier, confirming the effectiveness of the EBS in preventing electron penetration into the electrolyte. The wettability between the modified LLZTO and the lithium metal was analyzed using contact angle measurements from which the work of adhesion was calculated using the Young equation. Electrochemical impedance spectroscopy (EIS) was used to evaluate the interfacial resistance of both modified and unmodified cells. Galvanostatic cycling tests, including critical current density (CCD) measurements, were performed to assess the long-term stability of the cells under different current densities and areal capacities. Finally, full solid-state batteries (SSBs) were assembled with LiFePO₄ (LFP) cathodes and Li metal anodes using both the modified and unmodified LLZTO electrolytes to compare their performance. The performance of the EBS is compared against other interfacial layers like a gold layer and a poly(ethylene oxide) (PEO) layer to investigate the benefits of this method over other commonly used techniques.

The key findings of this research demonstrate the effectiveness of the flexible electron-blocking interfacial shield (EBS) in suppressing lithium dendrite growth and enhancing the performance of solid-state lithium metal batteries. DFT calculations showed a high electron-tunneling energy barrier from the lithium metal to the EBS (1.92 eV), confirming its electron-blocking capability. This contrasts with the absence of such a barrier in unmodified garnet electrolytes, which allows for electron transfer into the electrolyte and subsequent dendrite formation. The average Young's modulus for the LLZTO@PAA interface decreased from 20.6 GPa to 3.3 GPa, indicating the flexibility of the interface. This flexibility is crucial in accommodating the volume changes during lithium plating and stripping, maintaining good interfacial contact and preventing dendrite growth. Electrochemical Impedance Spectroscopy (EIS) measurements demonstrated a significant reduction in interfacial resistance, from 1104.3 Ω cm² to 54.5 Ω cm², confirming improved Li⁺ transport at the interface. The critical current density (CCD), a measure of dendrite suppression, was significantly enhanced from 0.2 mA cm⁻² for unmodified cells to 1.2 mA cm⁻² for EBS-protected cells at room temperature, a value reported as the highest ever for garnet electrolytes. Galvanostatic plating/stripping experiments revealed the superior long-term stability of EBS-protected cells. These cells exhibited stable cycling for over 1000 h at 0.2 mA cm⁻² and 400 h at 1 mA cm⁻², while unmodified cells quickly short-circuited. The smooth surface morphology of the EBS-protected LLZTO, observed even after prolonged cycling, in contrast to the rough and defective surface of unmodified LLZTO, visually confirmed the effectiveness of the EBS in preventing dendrite penetration. Full cell tests with LiFePO₄ cathodes showed improved performance for EBS-modified cells, with higher discharge capacity and better cycle life compared to unmodified cells.

The results demonstrate that the proposed strategy of using a flexible electron-blocking interfacial shield (EBS) effectively addresses the critical issue of Li dendrite formation in solid-state batteries. The combination of electron blocking and enhanced interfacial wettability leads to a uniform electric field distribution and suppresses dendrite nucleation and propagation. The flexibility of the LiPAA polymer accommodates Li volume changes during cycling, maintaining good interfacial contact and preventing the formation of voids or gaps at the interface, which can cause stress concentration and dendrite formation. The high CCD value and prolonged cycle life observed in the EBS-protected cells highlight the superiority of this method over other interfacial modifications, particularly electron-conducting materials such as gold. The improved performance of full cells further demonstrates the applicability of this strategy in practical devices. The superior performance of the LiPAA EBS compared to other polymer interfacial layers like PEO is attributed to the improved Li+ transport due to a more favorable substitution reaction with the -COOH group and lower molecular weight of PAA, leading to decreased crystallinity and improved ion mobility. These findings provide valuable insights into the design of efficient and stable interfaces for high-performance solid-state batteries.

This research successfully demonstrated a novel approach to suppress lithium dendrite growth in solid-state batteries by creating a flexible electron-blocking interfacial shield (EBS) using poly(acrylic acid) (PAA). The *in situ* formation of the LiPAA EBS significantly reduces interfacial resistance, improves wettability, and effectively prevents electron penetration into the electrolyte. The resulting cells exhibit a remarkable improvement in critical current density and long-term cycling stability. This strategy offers a promising pathway for developing high-performance, dendrite-free solid-state batteries. Future research could focus on exploring other polymer systems with enhanced properties or investigating the scalability and manufacturability of the EBS for practical applications. Further investigation into the long-term stability under more demanding conditions, such as higher temperatures or wider voltage windows, would also be beneficial.

While this study demonstrates significant improvements in the performance of solid-state batteries, some limitations should be noted. The study focuses on a specific type of garnet electrolyte (LLZTO). The effectiveness of the EBS approach may vary with different electrolytes. The long-term stability of the EBS under extreme operating conditions (very high temperatures or very low temperatures) has not been fully explored, while the long-term stability at high current densities is encouraging. The scalability and cost-effectiveness of the EBS fabrication process for mass production also need further investigation. Further research is needed to fully understand and quantify the long-term stability, degradation mechanisms, and potential side reactions in full cell applications.