Activating interfaces for faster lithium transport in solid-state batteries

Solid-state lithium-metal batteries are considered one of the most promising technologies for next-generation energy storage because they can potentially deliver high energy density while improving battery safety. Yet their practical development is still limited by a fundamental challenge: lithium ions must move efficiently across solid-solid interfaces, where contact, chemistry, and transport are often poorly matched.

This challenge is particularly important in polymer-ceramic composite solid electrolytes. Garnet-type LLZTO is known for its high lithium-ion conductivity and good chemical stability, while PVDF-based polymers provide flexibility and processability. However, combining the two materials does not automatically create a fast ion-transport network. LLZTO surfaces are easily covered by lithium carbonate and lithium hydroxide after air exposure, and these surface species hinder lithium-ion movement. Meanwhile, semicrystalline PVDF itself has limited lithium-ion conductivity at room temperature. These issues make the ceramic-polymer interface a major bottleneck for solid-state battery performance.

To address this problem, a research team led by Professor Renjie Chen at Beijing Institute of Technology developed an interphase activator strategy for PVDF-LLZTO composite solid electrolytes. Instead of treating ceramic surface modification and polymer ion transport as two separate problems, the strategy couples both processes through one additive.

In this study, SbF₃ was used as a representative interphase activator. It performs two connected functions. First, it converts the native lithium carbonate and lithium hydroxide species on the LLZTO surface into a reconstructed Sb₂O₃/LiF-rich interphase. This reconstructed interphase provides a more favorable local environment for lithium-ion transfer at the garnet surface. Second, together with ion-conducting cellulose, SbF₃ regulates the coordination environment in the PVDF-based polymer phase, reduces polymer chain ordering, and promotes faster lithium-ion coordination and decoordination.

Through this dual-function design, the optimized electrolyte, named FPIT-11, forms a continuous lithium-ion percolation pathway across the LLZTO bulk, the reconstructed interphase, and the polymer coating. Experimental characterizations and theoretical calculations show that the Sb₂O₃/LiF heterogeneous interface lowers the lithium-ion migration barrier, while the polymer phase becomes more disordered and more favorable for lithium-ion transport.

The optimized FPIT-11 electrolyte delivers an ionic conductivity of 6.7 × 10⁻⁴ S cm⁻¹, a lithium-ion transfer number of 0.84, and a lithium-ion conductivity of 5.6 × 10⁻⁴ S cm⁻¹ at 30 ℃. It also shows a wide electrochemical stability window of 5.25 V versus Li/Li⁺, indicating potential compatibility with high-voltage cathodes.

The electrolyte also demonstrates strong stability against lithium metal. Symmetric lithium cells using FPIT-11 cycle stably for nearly 2400 hours at 0.2 mA cm⁻² and reach a critical current density of 3 mA cm⁻² under 3 MPa pressure. Further interfacial analysis reveals the formation of a lithium nitride-rich multiphase inorganic interphase on the lithium-metal surface, which helps support stable lithium plating and stripping.

Full-cell tests further demonstrate the practical potential of this strategy. LiFePO₄, NCM523, and lithium-rich manganese-based cathode cells using FPIT-11 show stable cycling performance at 30 ℃. In particular, NCM523-based solid-state cells maintain stable cycling for 770 cycles, while pouch-cell tests also show good operational stability and safety.

This work provides a new design principle for garnet-polymer composite solid electrolytes. By integrating ceramic surface reconstruction with polymer-phase transport activation, the interphase activator strategy offers a practical path toward solid-state lithium-metal batteries with improved ion transport, interfacial stability, and cycling performance.

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