Structural Change and Morphological Surface Degradation upon Electrochemical Li Extraction from a Single Crystal of Spinel-type LiNi0.5Mn1.5O4

The space-group change and surface degradation of spinel-type LiNi0.5Mn1.5O4 upon the Li extraction reaction at 4.9 V were investigated using single-crystal samples. The stoichiometric LiNi0.5Mn1.5O4 single crystals having an octahedral shape with 0.07 × 0.07 × 0.07 mm3 of the maximum size were synthesized via a self-flux method utilizing high-temperature molten chlorides, i.e., MnCl2 and NiCl2, at 750 °C. Single-crystal X-ray diffraction (XRD) structure analysis of LiNi0.5Mn1.5O4 revealed the cubic P4332 space group, cell parameter of a = 8.1723(2) Å, and preferred occupation of the Ni atoms in two types of octahedral sites with occupancy values of 77.4% and 7.5% for the 4b and 12d sites, respectively. Moreover, electrochemical lithium extraction experiments were carried out using single-crystal LiNi0.5Mn1.5O4 samples, and fully delithiated single-crystal samples were successfully obtained following charging at 4.9 V. Single-crystal XRD analysis of delithiated Ni0.5Mn1.5O4 revealed the structural change from the P4332 space group to Fd3̅m, together with the cell parameter change to a = 8.0095(5) Å. The valence changes of the Ni atoms from +2 to +4 resulted in the apparent atomic disordering of Mn and Ni in the Ni0.5Mn1.5O4 structure. Surface morphology changes of the single crystals were observed following charging at 4.9 V, suggesting the micrometer scale degradation originated from the lattice volume change upon the Li extraction reaction.