Li metal batteries (LMBs) have attracted widespread attention in recent years because of their high energy densities. But traditional LMBs using liquid electrolyte have potential safety hazards, such as: leakage and flammability. Replacing liquid electrolyte with solid polymer electrolyte (SPE) can not only significantly improve the safety, but also improve the energy density of LMBs. However, till now, there is only limited success in improving the various physical and chemical properties of SPE, especially in thickness, posing great obstacles to further promoting its fundamental and applied studies. In this review, the authors mainly focus on evaluating the merits of ultrathin SPE and summarizing its existing challenges as well as fundamental requirements for designing and manufacturing advanced ultrathin SPE in the future. Meanwhile, the authors outline existing cases related to this field as much as possible and summarize them from the perspective of synthetic chemistry, hoping to provide a comprehensive understanding and serve as a strategic guidance for designing and fabricating high‐performance ultrathin SPE. Challenges and opportunities regarding this burgeoning field are also critically evaluated at the end of this review.

Poly(vinylidene fluoride) (PVDF)‐based polymer electrolytes have attracted widespread attention due to their unique Li ⁺ transport mechanism. However, their low ionic conductivity and porous structure, as well as residual solvent limit their application at high current densities. Here, a composite solid electrolyte (CSE) is developed by integrating poly(vinylidene‐co‐trifluoroethylene) [P(VDF‐TrFE)] in its all‐trans conformation with aminofunctionalized metal–organic framework (ZIF‐90‐NH ₂ ). In such a CSE, all F atoms located on one side of the polymer chain, providing fast Li ⁺ transport channels. Concurrently, the functionalized ZIF‐90‐NH ₂ can effectively anchor the residual N, N‐dimethylformamide (DMF) in CSEs while weakening Li ⁺ ‐DMF solvent coordination, inducing the rearrangement of Li ⁺ solvation structure and inhibiting the decomposition of DMF at the interface. Synergistically, ZIF‐90‐NH ₂ can immobilize anions in Li salts, promoting their dissociation. Based on integrating competitive Li ⁺ coordination with immobilized anions, the obtained CSEs exhibit a high Li ⁺ transference number (0.77). The full cells with LiFePO ₄ cathode can run stably over 400 cycles at 5 C, while the Li || LiNi _0.7 Co _0.1 Mn _0.2 O ₂ full cells deliver a high capacity retention (>85%) after 200 cycles at a charge cutoff voltage of 4.5 V. This work opens up a new path for building CSEs with high interfacial stability and fast Li ⁺ transport.