The Li-rich spinel Li4Mn5O12 (Li(Mn5/3Li1/3)O4) historically only shows a reversible cation-redox reaction, with a theoretical capacity of 135.5 mA h g-1. However, we found that a simple 400 °C solid-state synthesis method gives a Li4Mn5O12-like nanoparticulate cathode that yields significant reversible hybrid cation- and anion-redox capacities. A high specific capacity of 212 mA h g-1 was achieved. The reversible anion-redox contribution is attributed to the tiny particle size (<10 nm), which facilitates electron tunneling, and a possible random solid-solution in the Li(Mn5/3Li1/3)O4 lattice due to the low synthesis temperature.