Silicon‐Doped LiFePO₄ Single Crystals: Growth, Conductivity Behavior, and Diffusivity

Abstract

Single crystals of silicon doped LiFePO4 with a silicon content of 1% are grown successfully by the floating zone technique and characterized by single‐crystal and powder X‐ray diffraction, secondary ion mass spectroscopy, and chemical analysis. Electron paramagnetic resonance demonstrates the presence of only Fe2+; no traces of Fe3+ are found. Impedance spectroscopy as well as step‐function polarization/depolarization (DC) measurements are carried out using the cells Ti/LiFe(Si)PO4/Ti and LiAl/LiI/LiFe(Si)PO4/LiI/LiAl. The electronic and ionic conductivities as well as the Li‐diffusivity of the sample in the major crystallographic directions ([h00], [0k0], and [00l]) are determined. Within experimental error the transport properties along the b‐ and c‐axes are found to be the same but differ significantly from the a‐axis, which exhibits lower values. Compared to undoped LiFePO4, Si‐doping leads to an increase of the ionic conductivity while the electronic conductivity decreases, which is in agreement with a donor effect. The activation energies of conductivities and diffusivities are interpreted in terms of defect chemistry and relevant Brouwer diagrams are given.

Aluminium-doped LiFePO4 single crystals : Part I. Growth, characterization and total conductivity

“…(We have studied lateral as well as depth profiles with areas of 100 Â100 mm.) Unlike Si-doped samples 24 we have observed no accumulation or spots of Al on the surface of a polished single-crystal block.…”

“…Its value of 40-45 is almost constant within the temperature range of 40-240 1C in all directions which is consistent with Si-doped LiFePO 4 (e = 40-50). 24 As already mentioned, the absence of any additional polarization process at low frequencies for all orientations indicates that the conduction along the three orientations is predominantly due to electronic carriers. In order to corroborate this, DC polarization and depolarization measurements were performed on the same cell along different orientations similar to what we reported for pure LiFePO 4 single crystals, 12 and almost the same features were observed, i.e., the voltage increases in a step-function manner to a stationary value during polarization and analogously, decays in a step-function form on switching off the DC current.…”

“…A detailed description of single crystal X-ray diffraction measurements has been given elsewhere. 24 All the single crystal X-ray patterns indicate clearly the single-crystallinity of the olivine phosphate material.…”

Influence of Carbon Concentration as Coating on Electrical Conductivity of LiFeSi_0.03P_0.97O₄/C

“…However, doping at the P-site is rarely performed. Like a previous study in 2009, Amin reported a LiFePO4 single crystal doped with Si on the P-site [8]. Doping with x = 0.03 on LiFeSixP1-xO4/C give the best electrochemical performance [9].…”

Synthesis and Characterization of Vanadium-Doped LiMnPO4-Compounds: LiMn(PO4)x(VO4)1–x (0.8 ≤ x ≤ 1.0)

“…Although we cannot entirely rule out an influence of the doping on the ionic conductivity, one must take into account that vanadate doping only results in an increase of the cell volume and not in the formation of additional vacancies on the lithium site. Such vacancies were reported to be beneficial for the ionic conductivity of silicon doped LiFePO 4 (Li 1– x Fe(PO 4 ) 1– x (SiO 4 ) x ) and can increase the ionic conductivity by approximately 1 order of magnitude . Therefore, we assume that the effect on the ionic conductivity should be low.…”