Ultrafast sulfur redox dynamics enabled by a PPy@N‑TiO2 Z‑scheme heterojunction photoelectrode for photo‑assisted lithium–sulfur batteries

While lithium–sulfur batteries (LSBs) promise 2600 Wh kg⁻¹, the sluggish liquid-solid conversion of polysulfides keeps practical capacities far below theory. Now, researchers at Northwestern Polytechnical University, led by Prof. Yibo He, report a free-standing PPy@N-TiO₂/Carbon-Cloth photocathode that harvests sunlight to co-drive sulfur redox, delivering 1 653 mAh g^-1 (98.7 % of theory) and 333 mAh g^-1 after 5 h of pure photo-charging. Published in Nano-Micro Letters, the work realizes dual-mode energy harvesting in a single cell.

Why Photo-Assisted Strategy Matters

• Polysulfide Bottleneck: Long-chain Li₂S₆/₈ species shuttle and precipitate as electrically insulating Li₂S, wasting active mass.
• External-Field Boost: Optical fields lower activation energy via photocatalysis, yet single-semiconductor electrodes suffer wide band-gaps and fast carrier recombination.
• Sun-in-Sulfur Vision: Integrating solar conversion and storage slashes grid demand during charging, ideal for high-altitude or off-grid devices.

Innovative Heterojunction Design

• Z-Scheme p–n Junction: Vapor-phase polymerized polypyrrole (p-type) coats N-doped TiO₂ nanorods (n-type), building an internal electric field that separates e⁻/h⁺ pairs.
• Band-Engineered Alignment: narrowed gap (2.17 eV) extends visible harvest; CB electrons reduce S₈→Li₂S while VB holes oxidize Li₂S→S₈, closing a catalytic loop.
• Free-standing Architecture: 3-D carbon cloth offers 599 mAh g^-1 at 4 C and 3.3 mAh cm⁻² under 3 mg cm^-2 sulfur, mitigating volume swing and blocking shuttle.

Performance Breakthroughs

• Ultrafast Redox: Tafel slopes drop from 122 → 48 mV dec^-1; Li₂S nucleation time shortens from 3600 → 3 010 s with 17 % higher capacity.
• Dual-Mode Harvesting: 0.33 % solar-to-output efficiency; coin cell powers toy car 288 cm under light vs 212 cm in dark, then 77 cm after 2 h re-charge under ambient sunlight.
• Long-term Stability: 61.7 % retention over 328 cycles at 0.5 C; DRT analysis shows illuminated cell maintains lower R_SEI and R_PS, suppressing parasitic reactions.

Future Outlook
The plug-and-play photocathode design is scalable through roll-to-roll vapor coating, pointing toward solar-assisted EV packs and stratospheric drones where every photon counts.