Lithium Transport Properties in LiMn_1−αFe_αPO₄ Olivine Cathodes

Abstract

We report a comparative study of the electrochemical lithium diffusion properties within the olivine structure of LiMn 0.5 Fe 0.5 PO 4 , LiFePO 4 and LiMnPO 4 materials prepared by solvothermal pathway.The study includes careful analysis performed by potentiodynamic cycling with galvanostatic acceleration (PCGA), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT), carried out in order to investigate the features of the Fe 3+ /Fe 2+ and Mn 3+ /Mn 2+ redox processes and the lithium ion transport within the olivine structure. The electrochemical investigation reveals a shift of the redox potential of Fe 3+ /Fe 2+ and Mn 3+ /Mn 2+ couples toward higher and lower values, respectively, in LiMn 0.5 Fe 0.5 PO 4 in respect to the bare materials. Interestingly, the study reveals lithium diffusion coefficients depending on the state of the charge of the materials as well as on the adopted technique. Accordingly, CV leads to lithium diffusion coefficient of the order of 10 −12 cm 2 s −1 for LiMnPO 4 , 10 −9 cm 2 s −1 for LiFePO 4 and 10 −11 cm 2 s −1 for LiMn 0.5 Fe 0.5 PO 4 . EIS mainly indicates lower value of lithium diffusion coefficient, i.e. 10 −13 cm 2 s −1 for LiMnPO 4 , 10 −12 for LiFePO 4 and 10 −13 cm 2 s −1 for LiMn 0.5 Fe 0.5 PO 4 . GITT provides a wide range of Li + diffusion coefficient, depending on the Li 1-x MePO 4 stoichiometry, that is, 10 −14 -10 −10 cm 2 s −1 for LiMnPO 4 and LiFePO 4 , and 10 −13 -10 −10 cm 2 s −1 for LiMn 0.5 Fe 0.5 PO 4 . The wide diffusion coefficient window obtained by changing the state of charge and the adopted technique, sheds light on the complex trend of the lithium diffusion in olivines and indicates that the adopted technique may actually influence the materials evaluation.

Density functional theory study of lithium diffusion at the interface between olivine-type LiFePO4and LiMnPO4

“…Currently, the relatively low conductivity and poor Li transport properties can be improved by reducing crystal size and changing the microstructure/morphology, using surface modification and forming composite structures. Using these various methods, the electronic conductivity and Li diffusion coefficients can be increased up to 10 -1 S/cm and 10 -9 cm 2 /s, respectively [7][8][9][10][11][12][13][14][15][16]. Another drawback of the LiFePO4 is its relatively small voltage (3.4 V vs.…”

“…LiMnPO4 has a voltage of 4.1 eV, which makes it also attractive, because this voltage value is considered to be the maximum limit for most liquid electrolytes and larger than that of the LiFePO4. However, compared to the LiFePO4, electrochemical performance of the LiMnPO4 as the cathode material for the LIBs is poorer, therefore, various methods have been applied to solve this problem, such as doping modification or reducing the crystal size down to hundreds or even tens of nanometers [10,[17][18][19]. However, even after applying these methods, it still cannot meet the requirements for fast charging/discharging rate.…”

A High Voltage Olivine Cathode for Application in Lithium‐Ion Batteries

“…Table reports the lattice parameters obtained by Rietveld refinement of the patterns. These values are consistent with the expected lattice parameters obtained using Vegard's law considering cell volumes of LiFePO 4 , LiMnPO 4 , and LiCoPO 4 (synthesized following a similar solvothermal pathway) with molar ratio Fe/Mn/Co=0.25:0.50:0.25. The inset of Figure shows thermogravimetric analysis (TGA) results for C‐LiFe 0.25 Mn 0.5 Co 0.25 PO 4 .…”

“…The GITT discharge reveals favored Li + diffusion within the vacant one‐dimensional channels of the olivine lattice at x ≈1 ( D GITT of about 10 −9 cm 2 s −1 ), and subsequent D GITT decrease at about 10 −11 –10 −10 cm 2 s −1 by Mn 3+ /Mn 2+ and Fe 3+ /Fe 2+ reductions. Briefly, GITT provides Li + diffusion values within the range 10 −11 –10 −10 cm 2 s −1 for 0.2< x <0.8 (i.e., values in agreement with those revealed by CV as well as with D reported for LiMn 0.5 Fe 0.5 PO 4 olivine materials prepared by similar solvothermal pathway) . A large part of the literature, mainly focused on LiFePO 4 , indicates a lithium exchange reaction through two‐phase formation .…”

“…LiFe 0.25 Mn 0.5 Co 0.25 PO 4 olivine material was synthesized by a solvothermal recipe optimized in our laboratories . Two aqueous solutions, one containing lithium hydroxide monohydrate (LiOH⋅H 2 O) and another one containing lithium di‐hydrogen phosphate (LiH 2 PO 4 ), MnSO 4 ⋅H 2 O, FeSO 4 ⋅7 H 2 O, CoSO 4 ⋅7 H 2 O, and sucrose were added to ethylene glycol (EG) under vigorous stirring (final ratio EG/H 2 O 2:1 v / v ).…”