Although covalent organic frameworks (COFs) are promising organic cathodes for lithium-ion batteries, their compact layered bulk structure limits ion/electron transport, which results in low active site utilization and poor rate capability. Here, we present an in situ composite strategy: polymerizing electroactive monomers within COF pores. Bulk COFs are converted into few-layer nanosheets with polymer penetrating COFs (COF@Polymer) by taking advantage of polymerization stress and polymer- COF interactions to reduce interlayer bonding. Compared to traditional exfoliation, this method delivers a higher yield, wider applicability, and better nanosheet quality. The COF@Polymer cathodes exhibit triple advantages: the nanosheet morphology enhances active site accessibility and shortens mass transfer distances; the penetrated polymers establish efficient and additional carrier transport pathways; and polymer-COF interactions boost structural stability. Thus, COF@Polymer cathodes perform better than pristine COFs, conventionally exfoliated nanosheets, and COF-carbon composites in terms of active site utilization (95% for DACOF@FS-5, 98% for BTCOF@FS-20 and 92% for PTCOF@FS-10), rate performance ( ～ 70% capacity retention at 10 A g? 1 comparing to the capacity at 0.2 A g? 1 for BTCOF@FS-20 and PTCOF@FS-10), cycle stability, energy density, and power density (30.5 kW kg? 1 with an energy density up to 336 Wh kg? 1 for BTCOF@FS-20).