科研之友
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摘要
The thermodynamic stability is a very important quantity for the electrode materials, because it is not only related to the electrochemical performances of the materials but also the safety issue of the cells. To evaluate the thermodynamic stability of Li (x) Ni0.5Mn1.5O4 (x = 0, 1), the formation enthalpies from elemental phases and oxides were obtained. The values for LiNi0.5Mn1.5O4 were calculated to be -1341.10 and -141.84 kJ mol(-1), while those for Ni0.5Mn1.5O4 were -949.11 and -49.21 kJ mol(-1). These values are much more negative than those of LiCoO2 and LiNiO2 compounds, indicating that the thermodynamic stability of Li (x) Ni0.5Mn1.5O4 is better than the two classic compounds. To clarify the microscopic origin, the density of states, magnetic moments, and bond orders were systematically investigated. The results showed that the excellent thermodynamic stability of LiNi0.5Mn1.5O4 is attributed to the absence of Jahn-Teller distortions, strong electrostatic interactions of Li-O ionic bond, and strong Ni-O/Mn-O ionic-covalent mixing bonds. After lithium extraction, the disappearance of the pure Li-O bonds leads to an increase of formation enthalpy, indicating a decreasing thermodynamic stability for Ni0.5Mn1.5O4 with respect to LiNi0.5Mn1.5O4.
