Key Takeaways & Executive Findings
- •• • The functionally gradient ductile SEI enables Li|PALA|Li cells to cycle for over 4500 h at an ultrahigh current density of 15 mA cm−2 and areal capacity of 15 mA h cm−2, achieving a cumulative capacity of 33750 mA h cm−2—an order of magnitude higher than prior SLMBs (e.g., 11500 mA h cm−2 for Li|porous garnet|Li). • • At −30 °C, the cells stably operate for over 7000 h under practical conditions (5 mA cm−2, 5 mA h cm−2), demonstrating exceptional low-temperature performance critical for cold-climate applications. • • The ductile SEI exhibits a high Li-ion diffusion coefficient of 3.8 × 10−8 cm2 s−1 and low activation energy, facilitating rapid ion transport even at high current densities. • • The SEI's ductility is confirmed by a low generalized stacking fault energy (E_gsf) for AgF/Ag2S interfaces, and it maintains structural integrity at a bending angle of 150°, whereas brittle SEIs fracture—highlighting the importance of mechanical robustness in preventing dendrite growth and contact loss.
Abstract
Solid-state lithium metal batteries (SLMBs) are a promising alternative to conventional lithium-ion batteries due to their potential for higher energy density and improved safety. However, the solid electrolyte interphase (SEI) formed at the lithium metal anode/electrolyte interface is often brittle, leading to poor interfacial contact, high impedance, and dendrite growth, which limits cycle life and rate capability. Here, we report a functionally gradient ductile SEI design that incorporates AgF and Ag2S into the SEI layer, creating a composition gradient with a lithiophilic Ag/Ag–Li alloy at the anode surface. This ductile SEI exhibits a low generalized stacking fault energy, as confirmed by density functional theory calculations, and maintains structural integrity even at a bending angle of 150°, unlike brittle SEIs that fracture. The ductile SEI enables a high Li-ion diffusion coefficient of 3.8 × 10−8 cm2 s−1 and low activation energy. Consequently, Li|PALA|Li symmetric cells demonstrate exceptional cyclability over 4500 h at an ultrahigh current density of 15 mA cm−2 and areal capacity of 15 mA h cm−2, and stable operation for over 7000 h at −30 °C under practical conditions (5 mA cm−2, 5 mA h cm−2). Full cells with LiNi0.8Co0.1Mn0.1O2 cathodes show superior rate performance and capacity retention at both 25 °C and −30 °C. The cumulative capacity reaches 33750 mA h cm−2, an order of magnitude higher than previously reported SLMBs. This work underscores that the mechanical properties of the SEI are as critical as its ionic conductivity and chemical stability, opening a new frontier in interface design for practical, high-energy-density, and safe solid-state batteries.
1. Introduction
Conventional lithium-ion batteries are approaching their theoretical energy density limits, while safety concerns from flammable liquid electrolytes persist. Solid-state lithium metal batteries (SLMBs) offer a path to higher energy density and improved safety by replacing liquid electrolytes with solid counterparts. However, the solid electrolyte interphase (SEI) formed at the lithium metal anode is often brittle, leading to poor interfacial contact, high impedance, and dendrite growth, which severely limits cycle life and rate capability. Existing strategies to engineer the SEI have focused on ionic conductivity and chemical stability, but the mechanical properties—specifically ductility—have been largely overlooked.
This work addresses the interfacial brittleness bottleneck by designing a functionally gradient ductile SEI that incorporates AgF and Ag2S. The composition gradient creates a lithiophilic Ag/Ag–Li alloy at the anode surface, promoting uniform lithium deposition. The ductile nature of the SEI, confirmed by low generalized stacking fault energies, allows it to accommodate mechanical stress without fracturing, even at extreme bending angles. This design enables ultrahigh current densities and areal capacities, as well as stable operation at −30 °C, marking a significant advancement toward practical, high-energy-density, and safe solid-state batteries.
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Wenchao Bi, Guozhong Cao (2026). Functionally Gradient Ductile Solid Electrolyte Interphase for Ultrahigh-Current-Density Solid-State Lithium Metal Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3849-5
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Frequently Asked Questions
What is the failure mechanism of brittle SEIs in solid-state batteries, and how does the ductile SEI mitigate it?
Brittle SEIs crack under mechanical stress from lithium deposition and volume changes, leading to loss of interfacial contact, increased impedance, and dendrite penetration. The ductile SEI, containing AgF/Ag2S, has a low generalized stacking fault energy, allowing it to deform plastically without fracturing. This is evidenced by its intact structure at a bending angle of 150°, whereas brittle SEIs fracture. This mechanical robustness maintains interfacial integrity, enabling stable cycling at ultrahigh current densities.
How does the composition gradient in the SEI influence lithium deposition and ionic transport?
The composition gradient creates a lithiophilic Ag/Ag–Li alloy at the anode surface, which promotes uniform lithium nucleation and deposition, reducing dendrite formation. The ductile SEI also exhibits a high Li-ion diffusion coefficient of 3.8 × 10−8 cm2 s−1 and low activation energy, facilitating rapid ion transport across the interface even at high current densities and low temperatures.
What are the scalability challenges for implementing this ductile SEI in commercial solid-state batteries?
Scalability depends on the manufacturability of the PALA electrolyte and the precise control of the AgF/Ag2S gradient. The synthesis methods must be cost-effective and reproducible at industrial scale. Additionally, the long-term stability of the SEI under practical operating conditions, including high temperatures and mechanical stress, needs to be validated. The reported performance at −30 °C and ultrahigh current densities suggests robustness, but production costs and integration with existing battery manufacturing processes remain key considerations.
How does the cumulative capacity of 33750 mA h cm−2 compare to other state-of-the-art solid-state batteries, and what does this mean for practical applications?
The cumulative capacity of 33750 mA h cm−2 is an order of magnitude higher than previously reported SLMBs, such as Li|porous garnet|Li (11500 mA h cm−2) and Li|Li7N2I-CNT|Li (2400 mA h cm−2). This indicates a significantly extended cycle life, which is crucial for applications requiring long-term reliability, such as electric vehicles and grid storage. The ability to operate at ultrahigh current densities also enables fast charging, a key requirement for consumer adoption.
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