Effects of Nanoscale Surface Lithium Depletion on the Optical Properties and Electronic Band Structures of Lithium Transition-Metal Phosphates

The optical absorption properties of lithium transition-metal phosphates (LiTMPO4, TM = Mn, Fe, Co, Ni) with nanoscale particle sizes (300-500 nm in diameter) have been measured. The measured edges have been compared with band gaps determined from their corresponding electronic band structure calculations. Various functionals for density functional theory calculations have been compared and validated against experimental results. Gradual increases and intermediate peaks in optical absorption spectra before the main absorption edges have been attributed to intervalence charge transfer between TM2+ and TM3+ due to surface Li depletion. The functional, screened-exchange local density approximation (sX-LDA), which includes screened Hartree-Fock exchange, shows the highest overall accuracy for electronic band structure prediction with acceptable computational cost. The voltage plateaus determined from calculated enthalpies also display the best match with calculations using sX-LDA.