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Ultrahigh Energy Density Li-ion Batteries Based on Cathodes of 1D Metals with -Li-N-B-N- Repeating Units in alpha-LixBN2 (1<x<3)

Ultrahigh energy density batteries based on alpha-Li_xBN2 (1<x<3) positive electrode materials are predicted using density functional theory calculations. The utilization of the reversible LiBN2 + 2 Li+ + 2 e- -> Li3BN2 electrochemical cell reaction leads to a voltage of 3.62 V (vs Li/Li$^{+}$), theoretical energy densities of 3251 Wh/kg and 5927 Wh/L, with capacities of 899 mAh/g and 1638 mAh/cm3, while the cell volume of alpha-Li3BN2 changes only 2.8% per two-electron transfer. These values are far superior to the best existing or theoretically designed intercalation or conversion-based positive electrode materials. For comparison, the theoretical energy density of a Li-O2/peroxide battery is 3450 Wh/kg (including the weight of O2), that of a Li-S battery is 2600 Wh/kg, that of Li3Cr(BO3)(PO4) (one of the best designerintercalation materials) is 1700 Wh/kg, while already commercialized LiCoO2 allows for 568 Wh/kg. alpha-Li3BN2 is also known as a good Li-ion conductor with experimentally observed 3 mS/cm ionic conductivity and 78 kJ/mol (~ 0.8 eV) activation energy of conduction. The attractive features of alpha-LixBN2 (1<x<3) are based on a crystal lattice of 1D conjugated polymers with -Li-N-B-N- repeating units. When some of the Li is deintercalated from alpha-Li3BN2 the crystal becomes a metallic electron conductor, based on the underlying 1D conjugated pi-electron system. Thus alpha-LixBN2 (1<x<3) represents a new type of 1D conjugated polymers with great potential for energy storage and other applications.