In order to take full advantage of the secondary resources, in this paper, we reported a template-free process to prepare porous Co microfibers from spent lithium-ion batteries (LIBs). First, the waste LiCoO₂ powders were leached by oxalic acid at a suitable temperature, and rod-like cobalt oxalate powders were obtained. Second, the porous Co microfibers were prepared by using the cobalt oxalate as precursors through a thermal decomposition at 420 °C under nitrogen atmosphere. The prepared Co microfibers possess diameters of 1–2 μm, and each microfiber consists of small particles with size of 100–200 nm. The Co microfibers (25 wt%)/paraffin composite exhibited excellent microwave absorption performance. When the sample thickness is 4.5 mm, the reflection losses reach − 36.14 and − 38.20 dB at 4.16 and 17.60 GHz, respectively, and the effective bandwidth reaches up to 5.52 GHz. This indicates that the Co microfibers can be used as a promising microwave absorber. Therefore, this paper demonstrates a novel process to make a high value-added product through recycling from the spent lithium-ion batteries. In addition, it is advantageous to eliminate the hazard of spent lithium-ion batteries and electromagnetic radiation to environment and human health. Graphical abstract: [Figure not available: see fulltext.].