This study presents the preparation and electrochemical performance of cobalt-doped Li2FeSiO4/C (Co-doped LFS/C) composite by using a solid-state reaction assisted with refluxing process. The Co-doped LFS/C composite delivers a higher discharge capacity of 142.5 mAh g−1 at 3.0 C even after 100 cycles compared with pristine LFS/C. A clear feature of cobalt-doping in Li2FeSiO4 has been specified by X-ray powder diffraction, X-ray photoelectron spectroscopy coupled with Ar-ion sputtering, X-ray absorption spectroscopy, and positron annihilation lifetime spectroscopy. It is confirmed that Co has been successfully doped into the lattice of LFS, and Co-doping does not change the monoclinic structure of LFS and the oxidation state of Fe. Positron annihilation lifetime spectroscopy analysis further demonstrates that Co-doping increases the defect concentration and the electronic conductivity of LFS. The results clarify the nature of cobalt-doping in LFS and are helpful for understanding the enhancement mechanism of the electrochemical performance of LFS and other cathode materials.