Spectroscopic Evidence of Surface Li-Depletion of Lithium Transition-Metal Phosphates

The surface chemistries of carefully synthesized, phase pure lithium transition-metal phosphates LiTMPO4 (TM = Fe, Mn, Co, Ni) have been determined using a combination of surface spectroscopic techniques. Results obtained with X-ray photoelectron spectroscopy, Raman spectroscopy, and soft X-ray absorption spectroscopy indicate the presence of surface Li-depletion and mixed chemical states of transition-metal (TM) ions across the family. The extent of surface TM mixed chemical states has been found responsible for changes in color and optical absorption edge positions, which are often used to validate electronic band structure calculations. These characteristics are common in many complex metal oxide ceramics and may not be unique to lithium metal phosphate battery materials.