Sol-Gel Based Synthesis of LiNiFeF₆and Its Electrochemical Characterization

Abstract

Lithium transition metal oxides are commonly used as cathode materials in modern mobile and stationary power supplies. Lithium transition metal fluorides are an interesting new class of materials for lithium ion batteries featuring a higher voltage due to substitution of oxygen by the more electronegative fluorine. A sol-gel based process with trifluoroacetic acid as fluorine source was used to synthesize LiNiFeF 6 . Ball-milling with carbon and binder was applied to obtain an electrochemical active LiNiFeF 6 /carbon/binder nano composite. In this study we report on the first electrochemical characterization of a quaternary lithium transition metal fluoride as positive electrode for lithium ion batteries, containing two different transition metals. After 20 cycles of galvanostatic cycling a reversible specific capacity of 88 mAh/g, which is 92% of the initial specific capacity, was retained. In a rate performance test with rates of up to 1C a reversible capacity of 53 mAh/g was obtained. The electrochemically active redox couple Fe 3+ /Fe 2+ was identified by Mössbauer spectroscopy and cyclic voltammetry.The search for alternative cathode materials for lithium batteries to replace common oxide materials has generated considerable research activity to provide reliable battery systems for large-scale applications such as electric vehicles and grid storage. Previous investigations have been performed on a large number of compounds that can be applied as cathode materials for secondary lithium ion batteries such as layered LiMO 2 , silicates Li 2 MSiO 4 and polyanionic olivines LiMPO 4 (M = Fe, Mn, Co). 1,2 Several hundred publications have been published on quaternary lithium metal oxides. 4,5 However, no electrochemical investigations are given about quaternary lithium transition metal fluorides as positive electrode materials. Lithium transition metal fluorides in particular are very promising materials compared to common oxide materials with corresponding electrochemically active cations because the more electronegative fluorine atoms increase the redox potential leading to a higher specific energy. 3 Regarding the theoretic capacity of quaternary lithium transition metal fluorides, they could offer multiple redox couples e.g. M 3+/2+ or M 4+/3+ (e.g. M = V, Cr, Mn, Co or Ni) (eq.

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].…”

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

“…36 In this potential range, LiMnFeF 6 is electrochemically active (Figure 8a) and the electrode polarization increases with increasing cycle number. This indicates the structural degradation of the cathode and the necessity of adjusting the potential range to 2.2 V-4.3 V to significantly reduce the degradation (Figure 8b).…”

“…In this case, no reduction of Fe 3+ to Fe 2+ is observed during milling, contrary to earlier observations for LiNiFeF 6 . 36 After discharging to 2.2 V, the narrow doublets are partly replaced by two broad doublets with isomer shifts in the range from 1.32 to 1.38 mm/s and quadrupole splittings of 2.40 and 1.39 mm/s, respectively (Table III). These values are characteristic of Fe 2+ formed by insertion of Li into LiMnFeF 6 .…”