A hybrid polymer protective layer with uniform Li⁺ flux and self-adaption enabling dendrite-free Li metal anodes

Lithium (Li) metal is considered as an ideal negative electrode material for next-generation secondary batteries; however, the hideous dendrite growth and parasitic reactions hinder the practical applications of Li metal batteries. Herein, a hybrid polymer film composed of polyvinyl alcohol (PVA) and polyacrylic acid (PAA) is adopted as an artificial protective layer to inhibit the dendritic formation and side reactions in Li metal anodes. PVA with large quantities of polar functional groups can induce even distribution of Li ions (Li⁺). Alternatively, PAA can in situ react with Li metal to form highly elastic and ionic conducting lithium polyacrylic acid (LiPAA), thereby enabling tight contact and flexible self-adaption with Li metal anodes. Therefore, such a rationally designed functional composite layer, with good binding ability and relatively high Li⁺ conductivity, as well as excellent capability of homogenizing Li⁺ flow, accordingly enables Li metal anodes to reveal dendrite-free plating/stripping behaviours and minimum volume variation. As a result, the PVA-PAA modified Li metal anode delivered stable cycling for 700 and 250 h, respectively, at current densities of 1 and 3 mA cm^-2 under an areal capacity of 1 mA h cm^-2, in a carbonate ester-based electrolyte without any additive, exhibiting boosted cycling and rate performances. The Li anode with a functional PVA-PAA hybrid interlayer can maintain the dense and smooth texture without dendrite formation after long cycles. The full cell of Li|LiFeO₄ with our modified Li anode and a cathode with a high areal capacity of 2.45 mA h cm^-2 delivers, change to achieved a long-term lifespan of 180 cycles at 1.0 C, with a capacity retention of 96.7%. This work demonstrates a simple and effective strategy of designing multi-functional artificial protective layers, targeting dendrite-free Li anodes.