where n is the number of electrons transferred per redox reaction, F is the Faraday constant (C mol -1 ) and Mw is the molecular weight (g mol ). Therefore, to increase the specific capacity it is necessary to maximise the number of redox active sites while reducing the molecular weight. An illustrative example of increasing the theoretical capacity by decreasing the molecular weight is the substitution of anthraquinone (AQ) for benzoquinone (BQ) units in β-ketoenamine-linked COFs. As both quinone derivatives undergo two-electron reactions, the calculated theoretical capacities are 155 and 221 mAh g -1 for DAAQ-COF and DABQ-COF, respectively (Fig. ). The theoretical capacity can be further increased to 271 mAh g -1 (PT-COF) by using pyrene-4,5,9,10-tetraone as redox-active moieties, as it involves a 4 e -/Li + process. Therefore, the selection of electroactive building blocks taking into account their molecular weight as well as the number of electrons transferred is crucial to increase the charge storage capacity. Reducing the number or molecular weight of inactive moieties in the framework is also critical to maximise the theoretical capacity of COF electrode materials. An example is the replacement of 1,3,5-triphenylbenzene (Mw = 306 g/mol) by triphenylamine (Mw = 245 g/mol) in PI-COFs, increasing the theoretical capacity of PI-COF-1 up to 142 mAh g -1 (Fig. ). Therefore the connection of linear redox-active building blocks by using electroactive linkers/linkages (quinoxalines, tetraphenyl-p-phenylenediamine, triazines, polyimides, phenazines, 66 etc.) (Fig. ) is an efficient strategy to increase the active sites density. Moreover, the geometry and length of the linkers are determinant for the topology and pore size of the frameworks, which can influence the electrolyte diffusion and thus the practical capacity. However, COFs with small pore size can lead to more rigid structures and be more efficient for charge transport, so the pore diameter needs to be optimised. Other factors affecting the practical capacity, such as the electronic conductivity or particle size, are discussed below. The highest practical capacity among COF-based cathode materials in LIBs reported to date is 502 mAh g -1 (0.05 C) for BQ1-COF (theoretical capacity of 773 mAh g -1 ) (Fig. ). Notably, the estimated energy density for this COF electrode at high current density (20 C) was ~350 Wh kg -1 , being superior to that of commercial transition metal oxide cathodes such as LiCoO2 or LiFePO4 (100-140 Wh kg -1 ). Some key features of this material are its high density of active C=O and C=N groups and its high electrical conductivity, although its structural model remains unclear. In general, the specific capacities refer only to the COF active material, so they can vary significantly when the mass of the whole electrode (conductive additives, binder, etc.) is taken into account. In any case, the specific capacities of some reported redox-active COFs are higher than those of conventional transition metal oxides and among the highest for any type of organic cathode for lithium batteries. Output voltage. Besides capacity, another way to maximise the energy density is to increase the output voltage. The theoretical voltage of a battery can be estimated from the operating potentials of the cathode and anode as shown in Eq. 3. Therefore, in order to achieve a high voltage it is essential to use cathode and anode materials with high and low operating potentials, respectively. E = cathode potential -anode potential (Eq.3) Organic electrode materials can be classified into three different categories according to the charge state change during the redox process: n-type, p-type or bipolar-type. In COFs containing n-type units, the redox-active moieties (e.g., quinones, phenazines, imides, etc.) are reduced to form negatively charged anions, which interact with metal ions (Li + , Na + , etc.) under applied potential. In contrast, ptype building blocks (e.g., phenoxazines, tertiary amines, etc.) can be oxidised to form cations and combine with anions from the electrolyte such as PF6 -, TFSI -or LiClO4 -. In addition, COFs based on bipolar-type building blocks (e.g., organic radicals, porphyrins) can be reduced or oxidised depending on the applied potential.