Charging in batteries is often accompanied by a phase transformation of the host material. This process is characterized by distinct peaks in the CV and plateaus in the galvanostatic charge–discharge profiles (Fig. 1d). In contrast, charging of the pseudocapacitor should not be accompanied by a phase transition. Instead, pseudocapacitive materials present a continuous, highly reversible change in the oxidation state during charge/discharge, characterized by CVs with either significantly broadened peaks (intercalation, Fig. 1c) and little separation in peak position on charge/discharge, or almost perfectly rectangular CVs (surface redox, Fig. 1b)211.