Electrochemical Characterization of LiMnFeF₆for Use as Positive Electrode in Lithium-Ion Batteries

Abstract

To replace common commercial lithium metal oxides or polyanionic cathode materials, lithium transition metal fluorides are of particular interest, as they potentially provide a high energy density, safety, and cyclability. The quaternary fluoride LiMnFeF 6 that crystallizes in the Na 2 SiF 6 -type structure was synthesized without toxic chemicals by a sol-gel process with trifluoroacetic acid as fluorine source. After structural and morphological characterization, the as-synthesized material was ball-milled with carbon. The novel composite cathode material produced in this way was cycled versus lithium. The electrochemical potential range, cycling ability, and rate performance were investigated and the electrochemically active redox couple Fe 3+ /Fe 2+ was confirmed by Mössbauer spectroscopy. Structural changes of the LiMnFeF 6 host structure after Li insertion were investigated by X-ray powder diffraction.Currently, lithium transition metal oxides and phosphates are the most preferred cathode materials for portable and stationary applications. Compared to LiMn 2 O 4 , LiCoO 2 , or LiNiO 2 , positive electrode materials with at least two different transition metal cations like LiNi 0.5 Mn 1.5 O 4 or LiNi 1-x-y Mn x Co y O 2 possess better electrochemical properties like a higher redox potential, increased specific capacity or enhanced cycling performance. 1-3 In general, substitution of oxygen in the cathode host structure by the more electronegative fluorine results in a higher redox potential, leading to an increased energy density of the positive electrode. 4,5 Since the report of Abraham 6 and Arai et al. 7 on metal fluoride-containing positive electrodes, lithium insertion and conversion during cycling of the host material (e.g. FeF 3 , CuF 2 ) has been investigated in great detail. [8][9][10][11] The first lithium transition metal fluoride with an applicable electrochemical performance reported in literature was monoclinic Li 3 FeF 6 . 12 Specific discharge capacities of monoclinic and orthorhombic Li 3 FeF 6 , 13-16 monoclinic Li 3 VF 6 , 17 Li 2 TiF 6 , 18 and LiFeFeF 6 19,20 were reported to be at least 75 mAh/g. Lithium transition metal fluorides were investigated for their Li insertion and extraction potential, Li diffusion properties, and influence of divalent cation doping 21-24 using several theoretical methods. Several patents cover the use of Li 3 FeF 6 , Li 3 VF 6 , Li 2 TiF 6 , Li 2 NiF 4 , and LiFeFeF 6 as positive electrode materials for Li-ion batteries. [25][26][27][28][29] In the last four years, this class of materials attracted increasing attention. Still, lithium transition metal fluorides are not yet competitive with established electrode materials for use in lithium-ion batteries.However, electrochemical properties of ternary Li y MF x or quaternary LiM (1) M (2) F 6 (with M (1) = M (2) , e.g. M = Cr, Mn, Fe, Co, Ni) 3d lithium transition metal fluorides have not yet been studied due to their challenging synthesis process. It usually requires the usage of hazardous chemicals like hydro...

Electrochemical characterization of monoclinic and orthorhombic Li3CrF6 as positive electrodes in lithium-ion batteries synthesized by a sol–gel process with environmentally benign chemicals

“…Such behavior was also observed for the different Li 2 MnSiO 4 polymorphs (P21/n, Pn, Pmnb, and Pmn21) [47]. Similar discharge potentials for the electrochemical redox couple Fe III /Fe II were recently reported for quaternary lithium transition metal fluorides LiNiFeF 6 [10], LiMgFeF 6 (trirutile-type) [9] and LiMnFeF 6 (Na 2 SiF 6 -type) [8]. Based on quantum chemical calculations, Koyama et al [48] predicted similar intercalation voltages for the quaternary lithium transition metal fluoride materials.…”

“…This was also reported for the redox couple Fe III /Fe II in different structure types (e.g. monoclinic and orthorhombic Li 3 FeF 6 , tetragonal LiNiFeF 6 , and trigonal LiMnFeF 6 ), where the different crystal structures have no significant influence on the electrochemical potential curves [6,8,10].…”

“…In general, these syntheses are based on toxic chemicals like HF, LiF or F 2 . Recently, a solegel process with trifluoroacetic acid as fluorine source was established for synthesis of nano-scale Li 3 FeF 6 [6], Li 2 NiF 4 [7], and quaternary compounds like LiMnFeF 6 [8], LiMgFeF 6 [9], and LiNiFeF 6 [10].…”

LiCaFeF 6 : A zero-strain cathode material for use in Li-ion batteries

“…The material presented here belongs to a novel group of compounds, that are quaternary lithium transition metal fluorides with formula AM II M III F 6 [15][16][17][18]. A close packed arrangement of fluorine anions forms the basis of their crystalline structure.…”

“…According to the ratio of ionic radii of the different valent metal cations A, M II , and M III various cationic arrangements in an octahedral coordination are possible resulting in the three structure types, the trirutile-type ( ), the Na 2 SiF 6 -type ( ), and the colquiriite type ( ) structure [19]. The electrochemical properties of trirutile-type (LiNiFeF 6 [15,16,18,20], LiMgFeF 6 [20,21] , LiCoFeF 6 [20]) and Na 2 SiF 6 -type compounds (LiMnFeF 6 [17]) were studied recently. Furthermore, the lithium insertion mechanisms of the trirutile-type host structures of LiFe 2 F 6 [15] and LiNiFeF 6 [18] were investigated by in situ X-ray diffraction.…”