There is much current interest in materials that are made entirely or mostly of nitrogen atoms. Such materials, polynitrogens, may reveal new aspects of nitrogen chemistry, and are believed to provide a possible basis for novel energetic substances. An interesting family of such materials, in which the N5+ group appears as a cation, was prepared by K. O. Christe and co-workers. Little is known as yet on the microscopic properties of these materials. In this paper, we report theoretical calculations to predict the structure, energetic stability and decomposition dynamics of the polynitrogen molecule N5+B(N3)4-, the building block of a solid prepared by Christe, and of the dimer of this molecule. The structures are computed at the B3LYP-D3 level of DFT. ab initio molecular dynamics simulations are used to explore the thermal stability of the species and the decomposition mechanism. It is found that the N5+B(N3)4- ion-pair decomposes on a picosecond time scale at T = 200 K, with an ultrafast release of four N2 molecules, which is very exothermic. The species B(N3)3 is a product. The dimer is considerably more stable. Sensitivity of the process to temperature and to an applied force is reported. Possible applications of this material are briefly discussed.