Electrochemical, Magnetic, and Structural Investigation of the Li_x(Mn_yFe_1-y)PO₄ Olivine Phases

Abstract

A series of synthetic heterosite-purpurite, (Mn y Fe 1-y )PO 4 (y < 0.8), with negligible disorder and impurities, was obtained by chemical oxidation of the well-crystallized isotypic tryphillite-lithiophilite series, Li(Mn y Fe 1-y )PO 4 (ordered olivine structure, space group Pnma). Comparative magnetic and X-ray/ neutron powder diffraction investigations of the two solid-solution lines were performed as a function of Mn content to increase understanding of the electrochemical activity loss of Mn 3+ /Mn 2+ in the Li x (Mn y Fe 1-y )PO 4 electrode system. Introducing Mn ions into the 4c site did not cause significant change in the local geometry of M 2+ O 6 and PO 4 polyhedra, while the M 3+ O 6 octahedra became severely distorted with an increase in the number of Jahn-Teller active Mn 3+ ions. The edge-sharing geometry of M 3+ O 6 and PO 4 polyhedra fixed the shared O3′-O3′ interatomic distance, causing selective strong elongation of the M 3+ -O3′ distance with small shrinkage of other M 3+ -O1, M 3+ -O2, and M 3+ -O3 bond lengths. The overall distortion of the MO 6 octahedra with M ) Mn 3+ was much larger than the corresponding change in the unit-cell orthorombicity and significantly increased asymmetry in the M-O-M superexchange interaction. All samples exhibited antiferromagnetism; however, the trivalent series had more than a sevenfold larger decrease in Neel temperature T N (from ca. 130 K at y ) 0 to ca. 50 K at y ) 0.8) compared to the divalent series (from ca. 52 K at y ) 0 to ca. 35 K at y )1) as a function of the Mn content y.

LiMn0.8Fe0.2PO4: An Advanced Cathode Material for Rechargeable Lithium Batteries

“…Previous reports on Li(MnFe)PO 4 compounds suggest that such mixed-transition-metal olivine compounds may have a higher rate capability than LiMnPO 4 ; however, it is not clear what the optimal ratio of Mn and Fe in these compounds may be or to what extent these compounds are stable as cathodes for lithium batteries. [1,9,10] Since the redox potential of the Fe ions in the olivine compounds is lower by 600-700 mV than that of Mn ions, it is important to maximize the Mn/Fe ratio in Li(MnFe)PO 4 to ensure that most of the capacity is available in the high-voltage domain.…”

Synthesis and characterization of olivine LiNiPO4 for aqueous rechargeable battery

“…The PO 4 unit tends to reduce the covalency of the M -O bond, modifying the redox potential for the M 2+ / 3+ couple and thus producing a practical voltage while lithium insertion and re-insertion occurs [7][8]. Among the LiMPO 4 reported in the literature [7][8][9][10], lithium iron phosphate (LiFePO 4; [7]) and lithium manganese phosphate (LiMnPO 4 ; [10]) have been recognised as promising cathode material for rechargeable battery applications. This is mainly because a large reversible capacity of 164 mAh/g was achieved over a wide composition range of Li 1-y Mn y PO 4 and Li 1-y Fe y PO 4 , even at Mn & Fe contents as high as y = 0.75 [11].…”

Phase diagram and electrochemical properties of mixed olivines from first-principles calculations

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Specifically, the prospect of improved energy density over LiFePO 4 due to the higher redox potential of Mn 3+ / Mn 2+ compared to Fe 3+ / Fe 2+ ͓ϳ4.1 V compared to ϳ3.4 V vs Li/ Li + ͑Ref. 16͔͒ has fuelled research into Li͑Fe 1−y Mn y ͒PO 4 .…”