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

Abstract

This study investigates a series of cation dopants for LiFePO4 that are reported to be beneficial for rate performance. A solid-state synthesis has been used to be consistent with a common processing method used in prior doping investigations and in commercial processes for this compound. Increased ratios of Fe3+/Fe2+ oxidation on the particle surfaces have been determined using X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (sXAS) for the doped LiFePO4, for all chosen dopants and valences (<, = or >2+), while the cores remain much closer to the characteristics of the pristine (undoped) material. These results indicate that the dopants are predominantly pushed to the particle surfaces during phase formation, even when the dopants are added at the initial fine mixing of precursors. The differential distribution of dopants between the cores and surface layers of the particles results in conductivity improvements and reduction of polarizations in electrochemical impedance spectroscopy measurements of the doped LiFePO4.

Validating the Electronic Structure of Vanadium Phosphate Cathode Materials

“…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 . Based on previous studies of similar transition-metal phosphates, and the robust performance of AVPs at fast charging rates, − this intrinsic surface layer may, in fact, be beneficial to ionic and electronic transport kinetics of wide-band-gap LVP, NVP, and KVP.…”

“…79−81 For example, in prior work, our group has reevaluated the family of olivine phosphate cathode materials, including LiFePO 4 , doped versions, and its derivatives, where evidence of an intrinsic Li-deficient surface layer and segregation of dopants to the surfaces were found ubiquitously amongst the entire family of materials. 42,81 Similarly, during our study of potassium-ion battery material K 3 V 2 (PO 4 ) 3 , a complete XRD phase identification was found to be missing in the literature. 18 This led to our uncovering of the true primary phase and stoichiometry of the material,…”

“…98−100 Nevertheless, surface depletion effects have been detected in the most carefully synthesized and bestperforming metal phosphate compounds. 81 Based on previous studies of similar transition-metal phosphates, and the robust performance of AVPs at fast charging rates, 101−103 this intrinsic surface layer may, in fact, be beneficial to ionic and electronic transport kinetics of wide-band-gap LVP, NVP, and KVP. Surface Layer Junction Effects.…”

“…Correct characterization and validation of the electronic and crystallographic structure of alkali-ion battery materials are fundamental steps toward scientifically directed optimization, and in essence, they are complementary to the goal of accelerating the development and maturation of intercalation-based energy storage technology. These are not easy tasks, and in particular, phosphate-based intercalation materials are notoriously difficult to characterize and model accurately. ,− This is often due to their complex crystallography, ,, indeterminate phase transitions , during intercalation, and changing surface characteristics during battery operation. − For example, in prior work, our group has reevaluated the family of olivine phosphate cathode materials, including LiFePO 4 , doped versions, and its derivatives, where evidence of an intrinsic Li-deficient surface layer and segregation of dopants to the surfaces were found ubiquitously amongst the entire family of materials. , Similarly, during our study of potassium-ion battery material K 3 V 2 (PO 4 ) 3 , a complete XRD phase identification was found to be missing in the literature . This led to our uncovering of the true primary phase and stoichiometry of the material, K 3 V 3 (PO 4 ) 4 ·H 2 O.…”

Effect of Ru Doping on the Properties of LiFePO4/C Cathode Materials for Lithium-Ion Batteries

“…In recent years, lithium iron phosphate (LiFePO 4 ) has become the highly anticipated commercial positive materials for Li-ion batteries because of a reasonable energy value, low price, high safety, and nontoxic elements. − Unfortunately, the inferior electronic conductivity and poor lithium-ion transmission capability strongly impede the commercial development for a high-energy-density battery. − Researchers have carried out various aspects of work to solve these hard problems. − In these modification methods, improving the intrinsic conductivity of LiFePO 4 is still the key problem. The doping of metals is highly effective in improving the intrinsic conductivity of LiFePO 4 . − …”