Energy density. Rechargeable batteries are mainly evaluated in terms of energy density, power density, and cycling stability. The energy density (Ed) is directly related to the specific capacity (Q) and output voltage (E) (Eq. 1), so both parameters should be increased to improve battery performance through molecular design, electrode composition, electrolyte optimisation, etc. Ed = E x Q (Eq.1) Both the theoretical specific capacity and voltage can be tuned by selecting the appropriate electroactive organic building blocks and by structural design (see below). However, the practical capacities can be influenced by several factors such as the electrode composition, electronic conductivity, ion diffusion, binder, particle size of the active material, or the electrolyte used. The maximum specific capacities and average discharge voltages reported for some representative COF-based cathodes for lithium batteries are summarised in Fig. . Capacity. According to Eq. 1, high specific capacity is required to achieve high energy density. The specific capacity is defined as the electric charge that can be stored in a cell per mass of active material. The theoretical specific capacity (Qtheo) of an electrode material can be calculated from the following equation: