  Covalent functionalization is a way to tune the electrochemical properties of
hexagonal boron nitride (h-BN) monolayers. The wide band gap insulator h-BN may
become metallic conductor upon functionalization with strong oxidants, such as
fluorosulfonyl radicals ($\cdot$OSO$_2$F), as known since 1978 [N. Bartlett et
al., J. Chem. Soc. Chem. Comm. {\bf 5}, 200 (1978)], with electrical
conductivity of 1.5 S/cm [C. Shen et al., J. Solid State Chem. {\bf 147}, 74
(1999)] that greatly surpasses commercial cathode material Li$_{x}$CoO$_{2}$
while retaining excellent ionic conductivity. Functionalized boron nitrides
(FBN-s) have great potential for cathode applications in energy storage
devices, for example in solid state batteries. While fluorosulfonyl
functionalization is unlikely to result in rechargeable cathodes, similarly to
graphene fluoride (CF$_x$), some other FBN-s discussed here may do. In the
present work, fluorene, oxygen and combined oxygen and boron trifluoride
functionalizations are studied, on the basis of band structure calculations.
Due to the open surfaces of FBN-s, fast ionic diffusion with Li, Na and Mg ions
is possible, enabling batteries with voltages of 2.1-5.6 V, theoretical energy
densities of 800-1200 Wh/kg and fast charge and discharge.
