Fe-based polyanion materials such as LiFePO₄ or LiFeBO₃ are safe and can achieve high energy density, so they have been considered as cathode materials for large-scale lithium ion batteries. Among these compounds, LiFeSO₄F achieves the highest voltage (3.9 V) among Fe²⁺/Fe³⁺ redox couples and therefore can have higher energy density than LiFePO₄. However, full utilization of 3.9 V LiFeSO₄F is severely limited by its non-scalable synthesis process and poor electrochemical activity. Here, we report a method to synthesize 3.9 V LiFeSO₄F by a scalable solid-state reaction within 1 h by understanding the thermodynamic stability of 3.9 V LiFeSO₄F. The resulting material shows the best electrochemical performance reported to date. The process yields nanosized particles that achieve almost full capacity, 140 mA h g⁻¹, which is 93% of the theoretical capacity and that retain excellent capacity, ∼75 mA h g⁻¹ at 1 C for 150 cycles and high rate capability even at 10 C (6 min). This scalable solid-state reaction for 3.9 V LiFeSO₄F makes it a plausible replacement for LiFePO₄ in the next generation of lithium ion batteries.