Structural stabilities, electronic structures and lithium deintercalation in LixMSiO4 (M=Mn, Fe, Co, Ni): A GGA and GGA+U study

Dilithium-orthosilicate oxides Li2MSiO4 (M denotes transition metals) have been one of the focuses in the field of new cathode materials for Li-ion batteries recently, due to their possible high capacities and probabilities achieving by experiment. Using the density functional theory within both the generalized gradient approximation (GGA) and GGA+U frameworks, the structural stabilities, electronic structures and delithiation process for the dilithium-orthosilicate oxides Li2MSiO4 (M=Mn, Fe, Co, Ni) are systematically investigated. Within the GGA+U approach, LiMSiO4 is shown to be a stable non-stoichiometric structure, while the compound Li1.5MSiO4 are unstable relative to a two-phase form containing Li2MSiO4 and LiMSiO4, which is consistent with the experimental voltage profiles. For Li0.5MSiO4, though the formation energies are negative for Mn-system and Ni-system, the absolute values are so small that they would be likely to also undergo phase separation at room temperature. The average deintercalation voltages calculated by the GGA+U scheme are in good agreement with the available experimental data. Furthermore, the possibility of the exchange of two electrons per M in Li2MSiO4 is also discussed based on the calculated results.