This study presents a series of near-infrared-II (NIR-II) phosphors codoped with Cr³⁺ and Ni²⁺, achieving an unprecedented internal quantum efficiency (IQE) of 97.4%. The phosphors are synthesized in the intermediate spinel MgGa₂O₄ structure, where Cr³⁺ clusters and Cr³⁺—Ni²⁺ pairs play a crucial role in energy transfer. Electron paramagnetic resonance (EPR) studies reveal the interaction between Cr³⁺ clusters and Cr³⁺—Ni²⁺ pairs, explaining the high IQE. The sintering temperature tunes the Ni²⁺ emission intensity, enhancing the overall efficiency. The findings provide a promising pathway for developing highly efficient NIR-II light-emitting diodes (pc-LEDs) with outstanding performance.This study presents a series of near-infrared-II (NIR-II) phosphors codoped with Cr³⁺ and Ni²⁺, achieving an unprecedented internal quantum efficiency (IQE) of 97.4%. The phosphors are synthesized in the intermediate spinel MgGa₂O₄ structure, where Cr³⁺ clusters and Cr³⁺—Ni²⁺ pairs play a crucial role in energy transfer. Electron paramagnetic resonance (EPR) studies reveal the interaction between Cr³⁺ clusters and Cr³⁺—Ni²⁺ pairs, explaining the high IQE. The sintering temperature tunes the Ni²⁺ emission intensity, enhancing the overall efficiency. The findings provide a promising pathway for developing highly efficient NIR-II light-emitting diodes (pc-LEDs) with outstanding performance.