Effectively recovering spent lithium-ion batteries can reduce resource waste and environmental pollution. LiFePO4 (LFP) batteries have been widely used in new energy vehicles. The main reason for the performance degradation of LFP cathodes is the loss of Li, oxidation of Fe, and the destruction of crystal structure and surface carbon layer. Here, an in-situ repair strategy of high-temperature calcination, using impurities remaining in cathode blackmass (e.g., LiPF6, LiPO3, Fe2O3, acetylene black, binder, and electrolyte) as raw materials, was proposed. During the calcination process, the residual LiPF6 is converted into Li, acting as a lithium source to fill the vacancies. The residual binder and electrolyte are pyrolyzed into amorphous carbon and carbon nanotubes, which deposit on the LFP particles to repair the missing carbon layer and enhance the conductivity of the electrode material. The pyrolytic carbon and residual acetylene black can provide a reducing atmosphere, converting Fe3+ to Fe2+. Additionally, the residual LiPO3 and Fe2O3 impurities undergo recrystallization forming new LFP. The failed LFP was restored to the same level as commercial LFP by calcining at 750 °C for 3 h. This in-situ repair strategy reduces the input of raw materials, providing a valuable reference for large-scale recycling of lithium batteries.