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

Abstract

The optical absorption properties of lithium transition-metal phosphates (LiTMPO 4 , 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 TM 2+ and TM 3+ due to surface Li depletion. The functional, screenedexchange 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.

Validating the Electronic Structure of Vanadium Phosphate Cathode Materials

“…In previous work on Li-ion battery cathodes, this state is generally caused by surface depletion of Li during material synthesis. 69,70 Subsequent transition-metal charge compensation (change from V 3+ → V 4+ -O 2− interactions) and lattice distortion causes a change in the electronic structure and bonding environment, similar to results reported for LiFePO 4 , LiCoPO 4 , and Li 0.6 [Li 0.2 Mn 0.8 ]O 2 . 1,71,721,71,72 Remarkably, the normalized NVP pre-edge shows relatively low integrated intensities compared to LVP and KVP in PEY spectra.…”

“…This change in intensity suggests an increase in the unoccupied states of TM 3d and lengthening of TM–TM bonds at the surface compared to the bulk. In previous work on Li-ion battery cathodes, this state is generally caused by surface depletion of Li during material synthesis. , Subsequent transition-metal charge compensation (change from V 3+ → V 4+ -O 2– interactions) and lattice distortion causes a change in the electronic structure and bonding environment, similar to results reported for LiFePO 4 , LiCoPO 4 , and Li 0.6 [Li 0.2 Mn 0.8 ]­O 2 . ,, ,, …”

“…For example, Li + , Na + , and K + , with substantially different ionic radii, are expected to affect different bonding and coordination response(s) at the solid surface. Furthermore, different surface interactions with residual carbon or reaction during brief atmospheric exposure to CO 2 or H 2 O, forming residual A 2 CO 3 (A = Li, K, Na) or AOH (A = Li, Na) species on the surface, cannot be completely ruled out . Similar phenomena have been widely reported in high-Ni-content NCM cathode materials for Li-ion batteries, where postsynthesis washing is used to remove surface LiOH and Li 2 CO 3 residues. − Nevertheless, surface depletion effects have been detected in the most carefully synthesized and best-performing metal phosphate compounds .…”

Observation of Preferential Cation Doping on the Surface of LiFePO4 Particles and Its Effect on Properties

“…Our prior investigations have shown that the surfaces of lithium metal phosphate particles have differences compared to the cores . These differences can produce significant shifts in optical absorption edges, thus opening to question the validity of prior band structure calculations that did not take surface effects into account …”

Synthesis of Cr doped LiMnPO 4 cathode materials and investigation of their dielectric properties

“…20,21 Moreover, LiMPO 4 has recently attained considerable attention as probable material because of its lower toxicity, ease of availability, and admirable chemical as well as thermal stability. 22,23 In this way, LiMnPO 4 is widely utilized as a basic material in microwave devices as it offers a high voltage of about 4.1 V for operation, which is thought to be the highest limit reachable to most electrolytes. 24 To enhance the device performance, one tactic to improve the AC conductivity and decrease the dielectric loss is through cation doping into LiMPO 4.…”