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5068978 
Journal Article 
V-insertion in Li(Fe,Mn)FePO4 
Wu, T; Liu, J; Sun, L; Cong, L; Xie, H; Abdel-Ghany, A; Mauger, A; Julien, CM 
2018 
Yes 
Journal of Power Sources
ISSN: 0378-7753 
383 
133-143 
WOS:000430904200017 
Insertion of 3% vanadium in LiMn1-yFeyPO4 has been investigated, with y = 0.2 corresponding to the highest manganese concentration before the stress/strain field degrades the electrochemical performance. V substitutes for Fe2+ in the trivalent state V3+. This substitution is accompanied with the formation of Fe vacancies while Mn remains in the Mn2+ valence state, leading to a composition LiMn0.8Fe0.2-0.045 square(0.03)square 0.015PO4 where square is a Fe vacancy. The comparison between electrochemical properties of a pristine sample and a sample with 3 mol.% vanadium made of particles with the same morphology (spherical particles with the same dispersion 100-150 nm in size) and same carbon coating (same conductivity of the carbon layer) is reported. Although the vanadium is in the V3+ state at open circuit voltage (2.6 V) before cycling, a reversible V3+/V2+ is observed when the potential of the half-cell is lowered below the redox potential of 1.8 V vs Li+/Li, due to Li-vacancies. The V insertion improves the electrochemical properties, due to a synergetic effect of an increase of the lithium diffusion coefficient by a factor two and an increase of the electric conductivity at any Li-concentration during the cycling process, in contradiction with prior claims that attributed the increase of conductivity to V-based impurities. 
Olivine; Vanadium doping; Cathode materials; Lithium-ion batteries