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

Starting from published charge/discharge curves and X-ray data on Pmmn-LiFeO2 and LiFe5O8 as cathode materials vs. Li anode, a scheme of electrochemical reactions is proposed to explain the unclear electrode functionality of the ‘corrugated layer’ LiFeO2 phase. The scheme was validated by quantum-mechanical calculations (CRYSTAL09 code, hybrid B3LYP Hamiltonian) on a number of structural models for Li1−xFeO2, LiFe5O8, and Li3Fe5O8. Magnetic interactions were taken into account, finding antiferromagnetic (Li1−xFeO2) and ferrimagnetic (LiFe5O8 and Li3Fe5O8) orderings as stable states. At variance with spinel-like LiFe5O8, Li3Fe5O8 displays a rocksalt-type superstructure. The computed energies for reactions (I) 4LiFeO2 → 4Li0.75FeO2 + Li, (II) 4Li0.75FeO2 + Li → 4/5LiFe5O8 + 8/5Li2O, and (III) 1/2LiFe5O8 + Li ↔ 1/2Li3Fe5O8 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-LiFeO2 and of LiFe5O8.

Figure optionsDownload as PowerPoint slideResearch highlights▶ Electrochemical reaction energies are computed quantum-mechanically. ▶ ‘Corrugated layer’ LiFeO2 cathode deintercalates Li to Li1−xFeO2 in the first charge. ▶ Li1−xFeO2 transforms to ferrimagnetic spinel-like LiFe5O8 in the first discharge. ▶ LiFe5O8 ↔ rocksalt-type Li3Fe5O8 in subsequent discharge/charge cycles. ▶ Magnetic ordering stabilizes the compounds involved to different extent.