Designing amino functionalized titanium‑organic framework on separators toward sieving and redistribution of polysulfides in lithium‑sulfur batteries

Lithium–sulfur batteries (LSBs) are poised to reshape energy storage with their ultra-high theoretical capacity (1 675 mAh g^-1) and energy density (2 800 Wh kg^-1). Yet, the notorious “polysulfide shuttle” continues to erode cycle life and slash sulfur utilization. Now, researchers from the State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals at Lanzhou University, led by Professor Fen Ran, unveil a precision separator coating that turns the old problem into a new advantage. Their work, reported in Nano-Micro Letters, delivers a sub-nanometer ionic gate that not only blocks polysulfides but also re-uses them.

Why This MOF Coating Matters

• Sub-nanometer precision gate: The –NH₂ functional group shrinks Ti–MOF pore size from 0.95–1.10 nm to a uniform 0.83 nm—exactly the “sweet spot” that sieves out long-chain polysulfides while letting solvated Li⁺ slip through unhindered.
• Electrostatic recycling station: Positively charged –NH₂ moieties act as a dynamic depot, adsorbing escaped polysulfides via electrostatic attraction, then releasing them back to the cathode during charge—boosting active-material utilization and extending cycle life.
• Lewis acid–base fast lanes: Nitrogen lone-pairs form transient Li⁺–N coordination bonds, lowering desolvation energy and guiding directional Li⁺ transport, cutting polarization to 0.15 V at 0.1 C.

Engineering the Functional Separator

• One-step solvothermal synthesis of NH₂–Ti–MOF nanocakes (~200 nm) followed by low-temperature activation (250 °C, N₂) to open micropores without collapse.
• Doctor-blade coating onto commercial Celgard® 2500 yields a dense 9-μm layer (0.37–0.50 mg cm^-2) that withstands 180 °C thermal abuse and retains structural integrity even after 1 003 cycles.
• Coin-cell validation with Ketjenblack/S cathodes (0.84–1.0 mg cm^-2 S) shows an ultralow capacity fade of 0.045 % per cycle over 1 000 cycles at 1 C—outperforming bare PP (0.073 %) and Ti–MOF-only (0.056 %) controls.

Characterizing the Nano-Gate

• Visual shuttle test: In an H-cell, a 0.83 nm NH₂–Ti–MOF membrane keeps Li₂S₆ solution colorless for 24 h, while PP and Ti–MOF separators show visible yellowing—direct evidence of suppressed crossover.
• In-situ Raman mapping: After discharge to 1.7 V, separator-facing Li anode surfaces reveal zero polysulfide signatures for NH₂–Ti–MOF, versus strong S₆^2- peaks for PP.
• DFT & MD calculations: Adsorption energies for Li₂S₄, Li₂S₆, and Li₂S₈ on –NH₂ sites jump by 0.3–0.5 eV compared to pristine Ti–MOF, while MD shows a 150.8 kJ mol^-1 diffusion barrier for S₆^2- through 0.83 nm pores.

Future Outlook

• Scalable roll-to-roll coating: The cake-like morphology and benign aqueous/methanol processing route promise compatibility with industry-scale separator production lines.
• High-loading cathodes: Preliminary tests at 3.0 mg cm⁻² sulfur still deliver 403.7 mAh g^-1 after 153 cycles, indicating robustness under practical areal capacities.
• Next-gen chemistries: The same gating concept can be extended to Na–S, K–S, or even solid-state systems by tuning pore size and surface functionality.

With a single, elegantly engineered layer, the Lanzhou team transforms the separator from passive barrier to active polysulfide gatekeeper—ushering lithium–sulfur batteries toward real-world, long-lifetime deployment.