First-principles modelling of lithium iron oxides as battery cathode materials

Starting from published charge/discharge curves and X-ray data on Pmmn-LiFeO(2) and LiFe(5)O(8) as cathode materials vs. Li anode, a scheme of electrochemical reactions is proposed to explain the unclear electrode functionality of the 'corrugated layer' LiFeO(2) phase. The scheme was validated by quantum-mechanical calculations (CRYSTAL09 code, hybrid B3LYP Hamiltonian) on a number of structural models for Li(1-x)FeO(2), LiFe(5)O(8), and Li(3)Fe(5)O(8). Magnetic interactions were taken into account, finding antiferromagnetic (Li(1-x)FeO(2)) and ferrimagnetic (LiFe(5)O(8) and Li(3)Fe(5)O(8)) orderings as stable states. At variance with spinel-like LiFe(5)O(8), Li(3)Fe(5)O(8) displays a rocksalt-type superstructure. The computed energies for reactions (I) 4LiFeO(2) --> 4Li(0.75)FeO(2) + Li, (II) 4Li(0.75) FeO(2) + Li --> 4/5LiFe(5)O(8) + 8/5Li(2)O, and (III) 1/2LiFe(5)O(8) + Li <-> 1/2Li(3)Fe(5)O(8) are 4.44, -3.62, and -2.10 eV, respectively. Such values compare satisfactorily with the average charge/discharge voltages observed for positive electrodes made up of Pmmn-LiFeO(2) and of LiFe(5)O(8). (C) 2010 Elsevier B.V. All rights reserved.