Stable and high-safety fast-charging lithium metal battery enabled by a polydopamine-functionalized hydroxyapatite/aramid hybrid nanofibers separator

Severe lithium dendrite growth and elevated thermal runaway risks pose significant hurdles for fast-charging lithium metal batteries (LMBs). This study reports a polydopamine-functionalized hydroxyapatite/aramid (PDA@HA) hybrid nanofibers separator to synchronously improve the fast-charging LMB's stability and safety. (1) The separator's surface, enriched with lithiophilic carbonyl and hydroxyl groups, accelerates Li⁺ ion desolvation, while electrophilic imine groups impede anion movement. This dual mechanism optimizes the Li⁺-ion flux distribution on the anode, mitigating dendrite formation. (2) The polar PDA modification layer fosters the development of a Li₃N/LiF-rich solid electrolyte interface, further enhancing Li anode stability. Consequently, Li//Li symmetric cells with PDA@HA separators exhibit extended cycle life in Li plating/stripping tests: 5000 h at 1 mA cm^-2 and 700 h at 20 mA cm^-2, respectively, outperforming PP separators (80 h and 8 h). In LiFePO₄ (LFP, ∼2.1 mg cm^-2)//Li full cell evaluation, the PDA@HA separator enables stable operation for 11,000 cycles at 18.2C with 87% capacity retention, significantly outperforming existing fast-charging LMB counterparts in literature. At a high LFP loading of 15.5 mg cm^-2, the cell maintains 137.6 mAh g^-1 (2.13 mAh cm^-2) over 250 cycles at 3C, achieving 98% capacity retention. Moreover, the PDA@HA separator increases threshold temperature for thermal runaway and reduces the exothermic rate, intensifying the battery's thermal safety. This research underscores the importance of functional separator design in improving Li metal anode reversibility, fast-charging performance, and thermal safety of LMBs.