C.L. Li and J. Maier
To realize lithium batteries with satisfactory power and energy densities, exploring new elec-trode materials with optimized microstructure and morphology is the key. While for the anode a variety of options are available, innovations on the cathode still are very rare. Iron trifluo-ride has attracted substantial interest due to its electromotive force (emf) value of 2.7 V (versus Li) and large theoretical capacity of 237 mAh/g [1]. In view of the large bandgap and high ionicity of metal-fluoride bonds in transition metal fluorides, shrinking grain size by nano-decoration technologies provides one of the
few possibilities to improve the electrochemi-cal activity of such insulating fluorides. High-energy mechanical ball-milling combined with conductive carbon additives (>15 wt%) has been demonstrated to be an effective method to create highly reactive carbon-metal-fluoride nanocomposites (CMFNCs). Here, we report briefly on synthesizing a promising iron triflu-oride with a novel structure containing a low amount of hydration water (FeF3⋅0.33 H2O).
The synthesis succeeds by using the ionic liq-uid 1-butyl-3-methylimidazolium tetrafluorob-orate (BmimBF4) close to room temperature
[2]. The ionic liquid serves not only as a non-aqueous solvent and a soft template for nano-structure control, but also as the necessary flu-oride source. Compared to mechanical ball-milling and aqueous chemical approaches, this non-aqueous chemical synthesis exhibits a va-riety of advantages, including low-temperature conditions without the necessity of further cal-cination, precise control of crystallization and ordered microstructure, effective prevention of particle growth and agglomeration. Compared with the poorly conductive ReO3-type FeF3, the FeF3⋅0.33 H2O cathode exhibits an unusual tunnel structure [3], which we hold responsi-ble for the substantial Li storage capacity and perhaps also for the good Li ion transport in view of structural stability. Another advantage of the FeF3⋅0.33 H2O material lies in the self-assembly of a mesoporous morphology, which is beneficial for electrolyte penetration and may be also for the interfacial processes. In addi-tion, the electronic conductivity is good as well.
Therefore, enhanced cycling and rate perfor-mances of FeF3⋅0.33 H2O are expected without in situcarbon coating. Figure 59 shows the pos-sible synthesis mechanism.
A crystallographic view of orthorhombic FeF3⋅0.33 H2O along [001] direction is pre-sented in Fig. 60(a). Two kinds of octahedra (Fe(1)F6 and Fe(2)F6) based on different Fe sites are connected with each other by corner-sharing of coordinating F vertices to generate a roomy hexagonal cavity, with an oxygen atom in the center. In contrast to the ReO3 -type FeF3, a unique one-dimensional tunnel evolves along [001] direction with all the O–F distances being greater than 3 ˚A. Figure 60(b) refers to the microstructure and reveals typ-ical pompon-like clusters with various sizes from 200 nm to 500 nm. These large clusters are made up of many smaller spherical sponges (inset of Fig. 60(b)). Interestingly, the sponge-type aggregate is formed by self-assembly of a stack of needle-shaped nanoparticles, which stretch outwards from the aggregate core. The high-resolution transmission electron micro-scope (HRTEM) image (Fig. 60(c)) from the edge region of mesoporous aggregate reveals well-crystallized nano-needles of about 10 nm in diameter, which grow along [100] direction.
The corresponding diffractogram shown in the inset confirms the distinct crystal symmetry of FeF3⋅0.33 H2O.
Figure 59: Scheme of hydrated iron-based fluoride formation mechanism from BmimBF4ionic liquid and Fe(NO3)3⋅9 H2O precursors.
Figure 60: (a) Projections of orthorhombic FeF3⋅0.33 H2O along [001] direction, with one-dimensional tunnel structure built by hexagonal cavities along [001] direction to accommodate ionic storage and mi-gration. Two kinds of FeF6 octahedra based on different Fe sites are shown in azure. (b) TEM images of FeF3⋅0.33 H2O powders, inset: HRTEM image of a perfect and isolated sponge-type aggregate character-ized by mesoporous morphology. (c) HRTEM image of a needle-shaped nanoparticle located at the edge of sponge shown in (b), inset: The diffractogram of typical crystal structure. (d) Voltage vs. discharge capacity profiles of FeF3⋅0.33 H2O electrode during the second cycle in a rate range of 0.1 – 1C, under a voltage range of 1.6 – 4.5 V at 25∘C, inset: Discharge capacity as a function of cycle number in a rate of 0.1C.
This hierarchy, from fine nano-needles around 10 nm to self-assembled mesoporous sponges or pompons with the sizes of hundreds of nanometer, is considered to be beneficial to electrolyte infiltration and electrochemical re-activity. The soft template of ionic liquid is be-lieved to play a dominant role in the microstruc-ture evolution.
A galvanostatic charge-discharge was per-formed for the carbon-free FeF3⋅0.33 H2O as cathode for lithium cells in a voltage range of 1.6 – 4.5 V at 25∘C, as shown in Fig. 60(d). At
a rate of 0.1C, about 0.66 Li+ per formula is able to intercalate, resulting in a capacity as high as about 150 mAh/g. Accordingly, a Li insertion number of 2/3 is likely to corre-spond to the capacity limit given structural sta-bility during electrochemical cycling, indicat-ing that at maximum two Li+ can be accom-modated in one hexagonal cavity. On the ba-sis of the above discussions, it may be specu-lated that Li ions in FeF3⋅0.33 H2O mainly mi-grate along the characteristic one-dimensional tunnel pathway in [001] direction. This is
fur-ther indicated by the sloped reaction plateaus of 2.7 V, which suggests a solid-solution behav-ior. This single-phase process is totally different from the multiphase transformation mechanism of ReO3-type FeF3. A reversible discharge ca-pacity of 115 mAh/g at 0.1C can be achieved after 50 cycles (inset of Fig. 60(d)). No evi-dent electrochemical degradation was observed at elevated current density, and a reversible capacity of nearly 120 mAh/g is achieved at 1C. The structural and morphological optimiza-tions of the fluoride-based cathodes synthesized by the ionic-liquid-assisted method are signif-icantly beneficial for the accomplishment of high-power lithium batteries.
In summary, a novel iron trifluoride material was detected that is highly suited as a cath-ode for lithium batteries. It was synthesized through an ionic liquid assisted precipitation
method and found to be characterized by both open tunnel structure and mesoporous morphol-ogy. The existence of the small amount of hy-dration water is expected to be crucial for the significant modification of the crystal structure of FeF3 and the generation of favorable Li+ transport and storage sites. This prototype of a hydration-water-induced microstructure may trigger further exploration in this direction for obtaining improved cathode materials for high-power lithium batteries.
References:
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[2] Li, C.L., L. Gu, S. Tsukimoto, P.A. van Aken and J. Maier.Advanced Materials22, 3650–3654 (2010).
[3] Le Blanc, M., G. Ferey, P. Chevallier, Y. Calage and R. De Pape.Journal of Solid State Chemistry47, 53–58 (1983).