Covalent organic frameworks (COFs) represent an emerging class of porous crystalline materials and have recently shown interesting applications in energy storage. Herein, we report the construction of a cycle-stable sulfur electrode by embedding sulfur into a 2D COF. The designed porphyrin-based COF (Por-COF), featuring a relatively large pore volume and narrow pore size distribution, has been employed as a host material for sulfur storage in Li–S batteries. With a 55% sulfur loading in the composite, the thus-prepared cathode delivers a capacity of 633 mA h g⁻¹ after 200 cycles at 0.5C charge/discharge rates. Therefore, embedding sulfur in the nanopores of the Por-COF significantly improves the performance of the sulfur cathode. Considering the flexible design of COFs, we believe that it is possible to synthesize a 2D COF host with a suitable pore environment to produce more stable Li–S batteries, which may help in exploration of the structure–property relationship between the host material and cell performance.

It is demonstrated that a novel eutectic solution including 1,3,5‐trioxane (TXE) and succinonitrile (SN) can be converted into solid‐state polymer electrolyte (SPE) via in situ polymerization triggered by lithium difluoro(oxalato)borate (LiDFOB). It is worth noting that all the precursors (LiDFOB, TXE, and SN) of this novel SPE are totally solid and nonvolatile at room temperature, where, LiDFOB works as a lithium salt and an initiator simultaneously to avoid the introduction of impurity. It is noted that such SPE presents a considerable ionic conductivity of 1.14 × 10 ⁻⁴ S cm ⁻¹ and a sufficiently wide electrochemical window of 4.5 V, which is significant for supporting the high‐energy lithium batteries. In addition, this dedicatedly designed in situ polymerization is powerful to build kinetically favorable polymer‐based protective layers on LiCoO ₂ cathode and Li metal anode simultaneously, guaranteeing outstanding cycling stability (capacity retention of 88% after 200 cycles) of 4.3 V LiCoO ₂ /lithium metal batteries at room temperature. More intriguingly, soft packed LiCoO ₂ /SPE/Li metal batteries can still light a blue light emitting diode (LED) under the harsh conditions of being bent, cut, and stroked by a hammer, demonstrating excellent safety characteristics.