Lithium-sulfur (Li-S) batteries are highly regarded as the next-generation high-energy-density secondary batteries due to their high capacity and large theoretical energy density. However, the practical application of these batteries is hindered mainly by the polysulfide shuttle issue. Herein, we designed and synthesized a new lithium sulfonylimide covalent organic framework (COF) material (COF-LiSTFSI, LiSTFSI = lithium (4-styrenesulfonyl) (trifluoromethanesulfonyl)imide), and further used it to modify the common polypropylene (PP) separator of Li-S batteries. The COF-LiSTFSI with sulfonylimide anion groups features stronger electronegativity, thus can effectively facilitate the lithium ion conduction while significantly suppress the diffusion of polysulfides via the electrostatic interaction. Compared with the unmodified PP separator, the COF-LiSTFSI modified separator results in a high ionic conductivity (1.50 mS·cm⁻¹) and Li⁺ transference number (0.68). Consequently, the Li-S battery using the COF-LiSTFSI modified separator achieves a high capacity of 1229.7 mAh·g⁻¹ at 0.2 C and a low decay rate of only 0.042% per cycle after 1000 cycles at 1 C, compared with those of 941.5 mAh·g⁻¹ and 0.061% using the unmodified PP separator, respectively. These results indicate that by choosing suitable functional groups, an effective strategy for COF-modified separators could be developed for high-performance Li-S batteries.

Co-free Li-rich Mn-based layered oxides are promising candidates for next-generation lithium-ion batteries (LIBs) due to their high specific capacity, high voltage, and low cost. However, their commercialization is hindered by limited cycle life and poor rate performance. Herein, an in-situ simple and low-cost strategy with a nanoscale double-layer architecture of lithium polyphosphate (LiPP) and spinel phase covered on top of the bulk layered phase, is developed for Li_1.2Mn_0.6Ni_0.2O₂ (LMNO) using Li⁺-conductor LiPP (denoted as LMNO@S-LiPP). With such a double-layer covered architecture, the half-cell of LMNO@S-LiPP delivers an extremely high capacity of 202.5 mAh·g⁻¹ at 1 A·g⁻¹ and retains 85.3% of the initial capacity after 300 cycles, so far, the best high-rate electrochemical performance of all the previously reported LMNOs. The energy density of the full-cell assembled with commercial graphite reaches 620.9 Wh·kg⁻¹ (based on total weight of active materials in cathode and anode). Mechanism studies indicate that the superior electrochemical performance of LMNO@S-LiPP is originated from such a nanoscale double-layer covered architecture, which accelerates Li-ion diffusion, restrains oxygen release, inhibits interfacial side reactions, and suppresses structural degradation during cycling. Moreover, this strategy is applicable for other high-energy-density cathodes, such as LiNi_0.8Co_0.1Mn_0.1O₂, Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, and LiCoO₂. Hence, this work presents a simple, cost-effective, and scalable strategy for the development of high-performance cathode materials.