Voltage curves are linearly related to the slope of the free energy of the electrode material. If the electrode host material forms a solid solution with Li as panel (a), it will exhibit a smooth sloping voltage curve as illustrated in panel (b). If, however, Li insertion is accompanied by a first-order phase transformation from a Li poor phase,α, to Li rich phase,β, it will have a free energy curve as illustrated in panel (c), and resulting in a plateau in the voltage curve as illustrated in panel (d). Steps in the voltage profile appear if an intermediate phase is stable as in panels (e) and (f).^[14]