A radical anion based functionalization of the basal plane of hexagonal boron nitride (h-BN) and other two-dimensional materials is proposed in the present study. The resulting materials can reversibly be oxidized without the detachment of the functional groups from the basal plane and can thus serve as surface-intercalation type cathode electroactive species and fast solid ion conductors at the same time. The functionalization of h-BN with [·OBX3]- radical anions (X=F, Cl) in the presence of Li, Na or Mg cations provides one example of such systems. This material can be realized in a proposed simple, two step synthesis. In the first step, a symmetric Lewis adduct of the corresponding Li, Na or Mg peroxides is formed with BX3. In the second step, the anion of the Lewis adduct is thermally split into two identical [·OBX3]- radical anions that covalently functionalize the B atoms of h-BN. In the maximum density surface packing functionalization, the product of the synthesis is A n [(BN)2OBX3] (A = Li, Na with n = 1 or A = Mg with n = 0.5). Its ionic conductivity is predicted to be in the order of 0.01-0.1 S cm-1 at room temperature, on the basis of Grotthus-like (or paddle-wheel) ion transport. In the highly oxidized states (0 ≤ n ≤ 1 for Li and Na and 0 ≤ n ≤ 0.5 for Mg), the electronic conductivity of this material is in the order of 1 S cm-1, similar to carbon black. In the fully reduced states (n = 2 for Li and Na and n = 1 for Mg), the material becomes an insulator, like h-BN. The tunability of the electronic properties of A n [(BN)2OBX3] via the cation concentration (n) allows for its application as multifunctional material in energy storage devices, simultaneously serving as cathode active species, solid electrolyte, electroconductive additive, separator, heat conductor and coating for metal anodes that enables dendrite-free plating. This multifunctionality reduces the number of phases needed in an all-solid-state battery or supercapacitor and thus reduces the interfacial impedance making energy storage devices more efficient. For example, Li[(BN)2OBF3] is predicted to have 5.6 V open circuit voltage versus Li metal anode, capacity of 191 mAh g- 1, specific energy of 1067 Wh kg- 1 and can store energy at a (materials only) cost of 24 USD kWh- 1.