The electronic structure of a cathode material sets its redox chemistry, energy density, transport kinetics and structural stability, yet it is rarely discussed in conjunction with the structural design strategies that are used to improve performance. This perspective links electronic energy levels to the electrochemical properties of cathode materials. We first describe how transition-metal 3d and O 2p states, orbital hybridization, charge distribution and interfacial energy-level alignment control capacity, voltage, rate capability and cycling stability. We then re-examine four common strategies, namely elemental doping, defect engineering, composite-structure design and surface modificati... More >