On the Unusually High Fe3+/Fe2+ Redox Potential Exceeding 4V

The potential tunability for Fe³⁺/Fe²⁺ redox couple at unusually high voltage region of 3.5 – 4.1 V will be reviewed based on the new materials discovered in my group over the past 3 years. Overall energetics will be given by decomposing the thermodynamic definition into electronic and structural factors, leading to the guidelines to develop new high voltage cathode materials. In searching new materials, as often missed by the ground state screening, experimental identification of metastable functional phase by entropy gain (including disorder and/or defects) at finite temperature is important and effective. Very promising new cathode materials led by this concept will be introduced as time permits.