A novel thermal management structure has been developed for Lithium-ion (Li-ion) battery packs operated at extremely high power rates. This structure integrates copper microfibrous media (MFM) with high thermal conductivity (60–65 W m−1K−1) and porosity (70–90 vol%), active cooling structures (i.e. metal cooling tubes), and passive cooling material (i.e. phase change material (PCM)). In this structure the PCM is embedded in the MFM (MFM-PCM). MFM drastically improves interfacial heat transfer and efficiently conducts heat between Li-ion cells, cooling tubes, and PCM. PCM embedded in MFM regulates the cell surface temperature, capping it near the PCM's melting point. It stores the excessive heat generated by the cells under peak use and releases it to the cooling tubes during off-peak use. The MFM-PCM pack enables mid-form Li-ion cells using lithium iron phosphate cathodes to perform at their maximum allowed discharge rates (15C) for individual cells and maintains cell surface temperature below 48 °C, which is much lower than the threshold of 60 °C. As a result, no derating is necessary for safe pack operations. A COMSOL 2D thermal model estimates that the thermal conductivity of MFM-PCM is 58 W m−1K−1; this is close to the measured value. The model predicts that the pack can protect cells from catastrophic cascading failure if one cell experiences thermal runaway and releases up to 130% of its full electric energy (440 kJ) into the pack in 1 s.