Electrospun Polyacrylonitrile/Polyvinylidene Difluoride Bilayer Promoting the Uniform Lithium Deposition/Stripping in the Zero-Excess Lithium Metal Batteries

The zero-excess lithium metal batteries (ZELMBs) offer a higher energy density and better manufacturing safety compared with the conventional LMBs. However, the practical application of such cells is hindered by the severe dendrite growth originating from the uneven Li⁺ distribution at the copper substrate. Here, we combine a top layer of polyacrylonitrile (PAN) and a bottom layer of poly(vinylidene difluoride) (PVDF), fabricated by electrospinning, to serve as an artificial solid electrolyte interphase (ASEI) promoting membrane, denoted as Cu@PAN + PVDF, for effectively dealing with this challenge. This configuration facilitates the desolvation and uniform flux of Li⁺, leading to form an inorganic-rich SEI layer to favor the uniform lithium deposition. In contrast, reversing the layer order (i.e., PVDF as the top layer and PAN as the bottom layer, denoted as Cu@PVDF + PAN) results in a high nucleation barrier and an uneven lithium deposit. The morphological evolution is further examined using a newly designed half-stripping experiment where lithium is plated and partially stripped at 1 mA cm^-2 for 1 and 0.5 h, respectively. The Cu@PAN + PVDF electrode maintains a dense, uniform surface, whereas Cu@PVDF + PAN exhibits midlayer voids and disconnected "dead Li", indicating the uneven delithiation. The Cu@PAN + PVDF||Li half-cell achieves over 160 cycles with a Coulombic efficiency (CE) exceeding 95%, outperforming the Cu@PVDF + PAN||Li (100 cycles) and bare Cu||Li (110 cycles) cells. This work identifies layer orientation as a governing parameter for the ASEI design and introduces a practical half-stripping methodology for evaluating the interfacial reversibility of the negative electrode in ZELMBs.