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Abstract LiCoO2 is a historic lithium-ion battery cathode that continues to be used today because of its high energy density. However, the practical capacity of LiCoO2 is limited owing to the harmful phase transition at high voltages, which prevents the realization of its theoretical capacity. Here, we treat LiCoO2 particles with a molten salt of MgF2–LiF as a reaction accelerator to facilitate the diffusion and doping of magnesium into bulk LiCoO2 and to form a stable coating layer on the particle surface. Ex situ X-ray diffraction analysis confirms the inhibition of the harmful phase transition and the emergence of a different phase as the modified LiCoO2 was charged up to 4.7 V. The modified LiCoO2 shows high electrochemical performance during high-voltage operation. This technology provides a guideline for the suppressing fundamental degradation associated with phase transition and achieving ultra-high energy density LiCoO2 cathodes.