Multifunctional materials are powerful tools to support the advancement of energy conversion devices. Materials with prominent electromagnetic and electrochemical properties can realize the conversion of electromagnetic energy and solve the subsequent storage issues. Herein, an electrospinning-thermal reduction method is employed to construct ultrafine nickel nanoparticle modified porous SiO₂/C (Ni-SiO₂/C) hollow nanofibers as promising materials for applications in both electromagnetic wave absorption (EMA) and lithium-ion storage. Impressively, when used as an EMA material, the reflection loss (RL) of Ni-SiO₂/C can reach −47.8 dB at 15.8 GHz with a matching thickness of 2.2 mm. Its excellent microwave absorption performance can be attributed to the enhanced conduction loss, polarization relaxation, synergistic magnetic loss, and preferred impedance matching, which result from multi-component magnetic/dielectric synergy and the unique interconnected multidimensional hollow structure. Furthermore, the electronic conductivity and electrochemical activity of the samples are significantly enhanced due to the uniform distribution of ultrafine Ni nanoparticles in the amorphous SiO₂/C matrix. Meanwhile, the hierarchical hollow porous structure provides sufficient free space for volume change during lithiation/delithiation cycles. Accordingly, the Ni-SiO₂/C nanocomposite exhibits a high reversible capacity of 917.6 mAh·g⁻¹ at 0.1 A·g⁻¹. At a high current density of 2 A·g⁻¹, a capacity of 563.9 mAh·g⁻¹ can be maintained after 300 cycles. An energy conversion-storage device is designed to store waste electromagnetic energy in the form of useful electrical energy. This work inspires the development of high-performance bifunctional materials.