PVDF nanofiber separator with sustained LiNO3 additive release for dendrite-suppressed anode-free lithium metal batteries

Abstract

Anode-free lithium metal batteries (AFLMBs) offer the highest theoretical energy density among rechargeable systems but suffer from rapid capacity fading caused by uncontrolled lithium loss and unstable solid electrolyte interphase (SEI) formation, particularly in carbonate electrolytes. Although lithium nitrate (LiNO₃) is a highly effective additive for stabilizing SEI, its poor solubility in carbonate solvents remains a critical bottleneck. Here, we address this limitation by functionalizing a commercial polypropylene (PP) separator with an electrospun polyvinylidene fluoride (PVDF)/LiNO₃ nanofiber coating, enabling localized and sustained release of LiNO₃ during cycling. This design delivers three synergistic benefits: (i) piezoelectric beta-phase PVDF nanofibers regulate lithium flux distribution and suppress dendrite growth, (ii) the polar and electronegative nanofiber network enhances Li⁺ transport, yielding enhanced ionic conductivity (1.22 mS cm⁻¹) and Li⁺ transference number of 0.50, and (iii) uniform distribution of LiNO₃ within the nanofibers acts as a self-replenishing additive reservoir that gradually dissolves to sustain continuous formation of the robust Li₃N/LiF-rich SEI. As a result, Li symmetric cells exhibit stable cycling for over 500 hours at 3 mA cm⁻², while Li‖NMC622 full cells retain 83% of their initial capacity after 100 cycles at an N/P ratio of 1.8. Most notably, Cu‖NMC622 anode-free cells achieve 54% capacity retention after 50 cycles, compared to only 18% for cells with a conventional PP separator. This work demonstrates separator functionalization as a simple, scalable, and broadly applicable strategy to overcome additive solubility limitations and enable long-lasting, dendrite-suppressed anode-free lithium metal batteries.