Nanosize Storage Properties in Spinel Li₄Ti₅O₁₂ Explained by Anisotropic Surface Lithium Insertion

Nanosizing is a frequently applied strategy in recent years to improve storage properties of Li-ion electrodes and facilitate novel storage mechanisms. Due to particle size reduction, surface effects increasingly dominate, which can drastically change the storage properties. Using density functional theory calculations we investigate the impact of the surface environment on the Li-ion insertion properties in defective spinel Li_4+<i>x</i>Ti₅O₁₂, a highly promising negative electrode material. The calculations reveal that the storage properties strongly depend on the surface orientation. The lowest energy (1 1 0) surface is predicted to be energetically favorable for Li-ion insertion into the vacant <i>16c</i> sites. The (1 1 1) surface allows capacities that significantly exceed the bulk capacity Li₇Ti₅O₁₂ at voltages greater than 0 V by occupation of <i>8a</i> sites in addition to the fully occupied <i>16c</i> sites. One of the key findings is that the surface environment extends nanometers into the storage material, leading to a distribution of voltages responsible for the curved voltage profile commonly observed in nanosized insertion electrode materials. Both the calculated surface-specific voltage profiles and the calculated particle size dependent voltage profiles are in good agreement with the experimental voltage profiles reported in literature. These results give a unique insight into the impact of nanostructuring and further possibilities of tailoring the Li-ion voltage profiles and capacities in lithium insertion materials