Letter pubs.acs.org/NanoLett
High-Capacity Lithium-Ion Battery Conversion Cathodes Based on Iron Fluoride Nanowires and Insights into the Conversion Mechanism Linsen Li, Fei Meng, and Song Jin* Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: The increasing demands from large-scale energy applications call for the development of lithium-ion battery (LIB) electrode materials with high energy density. Earth abundant conversion cathode material iron trifluoride (FeF3) has a high theoretical capacity (712 mAh g−1) and the potential to double the energy density of the current cathode material based on lithium cobalt oxide. Such promise has not been fulfilled due to the nonoptimal material properties and poor kinetics of the electrochemical conversion reactions. Here, we report for the first time a high-capacity LIB cathode that is based on networks of FeF3 nanowires (NWs) made via an inexpensive and scalable synthesis. The FeF3 NW cathode yielded a discharge capacity as high as 543 mAh g−1 at the first cycle and retained a capacity of 223 mAh g−1 after 50 cycles at room temperature under the current of 50 mA g−1. Moreover, highresolution transmission electron microscopy revealed the existence of continuous networks of Fe in the lithiated FeF3 NWs after discharging, which is likely an important factor for the observed improved electrochemical performance. The loss of active material (FeF3) caused by the increasingly ineffective reconversion process during charging was found to be a major factor responsible for the capacity loss upon cycling. With the advantages of low cost, large quantity, and ease of processing, these FeF3 NWs are not only promising battery cathode materials but also provide a convenient platform for fundamental studies and further improving conversion cathodes in general. KEYWORDS: FeF3, nanowire, conversion cathode, high capacity, lithium-ion batteries
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storage can also be realized by utilizing conversion electrode materials as a battery cathode that undergoes electrochemical conversion/deconversion upon cycling.3,9−11 A very promising and representative example is iron trifluoride (FeF3), which can theoretically react with three Li+ and deliver a high capacity of 712 mAh g−1 at an average potential of ∼2.7 V,12 enabling a high theoretical energy density of 1950 Wh kg−1.
ithium-ion batteries (LIB) have revolutionized portable electronics and become the dominant power sources for mobile devices due to their superior energy density in comparison with other rechargeable batteries.1−3 Current LIB technology is built on intercalation chemistry, which involves Li+ topotactic intercalation/deintercalation into/from the host lattice of electrode materials represented by lithium cobalt oxide (LiCoO2) and graphite.3 Intercalation/deintercalation reaction mostly introduces a small volume change to the crystal lattice of electrode materials without causing structural collapse and/or phase segregation, which enables high-performance LIB with good rate capability and long lifetime.3 However, current LIB fall short of the ever-increasing energy demand from largescale application, such as electrical vehicles and grid-level energy storage.1−5 This is largely due to the intrinsic limit of intercalation electrode materials, which typically allows no more than one Li+ per structural unit to be inserted into the host lattice. Going beyond current LIB requires the development of energy storage technology based on new electrode materials and/or new chemistries, among which Li−O2 and Li−S have attracted most of the attention due to their significantly higher theoretical energy density.6 Important advances have been made for Li−O27 and Li−S8 batteries recently, but significant challenges remain to be overcome before they can become a commercial success.6 High-energy © 2012 American Chemical Society
FeF3 + 3Li+ + 3e− ⇄ 3LiF + α ‐Fe (4.5 − 1.5 V vs Li+/Li)
In the discharge reaction, FeF3 (space group R3̅c, ReO3-type structure) is electrochemically reduced with Li+ uptake to form a nanocomposite consisting of α-Fe nanodomains embedded in a LiF matrix (Figure 1a).13−15 The large interface between the two phases likely facilitates subsequent decomposition of the nanocomposite upon a reverse bias on charging9 and enables the deconversion reaction to form the defect-trirutile FeF3 (Figure 1a) .13−15 In addition to its high energy density, FeF3 is also an earth abundant material that is much less expensive than Received: September 28, 2012 Revised: October 20, 2012 Published: October 29, 2012 6030
dx.doi.org/10.1021/nl303630p | Nano Lett. 2012, 12, 6030−6037
Nano Letters
Letter
low capacity of about 140 mAh g−1 in the voltage window of 4.5−1.5 V even at 70 °C and a small current of 7.58 mA g−1.13 Nanostructuring FeF3 cathodes can potentially alleviate these problems. The decrease in particle size from bulk to nanoscale leads to reduced ion/electrode transport distance and increased surface area,1 which facilitate conversion reaction kinetics and improve the electrochemical cycling performance of FeF3 cathodes.13,16,18−21 Previous FeF3 electrodes are usually made from nanocomposites of FeF3 nanoparticles (NPs) and conductive carbon via an energy-intensive ball-milling process.13,14,16,18,20,22 Even though high capacities have been reported in a few cycles, they were only accessible at elevated temperature and/or small currents13,14 or at the presence of a large amount of carbon (50 wt%).20 Furthermore, the commonly observed capacity fading and voltage hysteresis remain significant challenges for the FeF3 cathode,9 and despite significant experimental and theoretical efforts,13−16,22 complete understanding of the conversion mechanism of FeF3 conversion cathodes is still lacking. One-dimensional (1D) nanostructured electrodes usually exhibit enhanced reversible capacities and longer cycling lives relative to their micrometer-sized/bulk counterparts due to their better capability to withstand stress arising from large structural change upon battery cycling and their better electrical connectivity.23−26 Even though the success has been mostly achieved on anode materials to date, the performance of conversion cathodes could benefit even more from the nanowire (NW) morphology because of their large structural transformation in the electrochemical conversion/deconversion reaction. Moreover, NWs provide a defined 1D nanostructure to perform fundamental studies at the microscopic level27,28 and better understand the challenges facing the FeF 3 conversion cathode. We have previously achieved the inexpensive and scalable solution synthesis of α-FeF3·3H2O NWs29 under low-supersaturation conditions without surfactants and/or catalysts, following the design of dislocation-
Figure 1. (a) Schematics of the crystal structure and microstructure changes that occur in FeF3 cathode material during electrochemical cycling. Blue, green, and red spheres correspond to Fe, Li, and F atoms, respectively. (b) Schematic illustration of the entangled network of FeF3 NWs mixed with carbon black and PVDF binder as LIB cathode. The carbon black is represented by small black dots, and the PVDF binder is not shown for clarity.
the current LiCoO2 cathode material.16 These attributes make FeF3 a very promising cathode material. However, significant challenges have so far prevented these promises from being fulfilled. Because of the insulating character of FeF317 and significant structural transformation upon cycling, FeF3 electrodes made of large FeF3 particles (with crystallite size >150 nm) severely suffered from sluggish kinetics and could only exhibit a
Figure 2. Structural characterization of the FeF3 NW product. (a) SEM images of the precursor α-FeF3·3H2O NWs in a large quantity prepared from a solution synthesis. (b) SEM images of the NWs after dehydration showing well preserved NW morphology. (c) Size distribution of the NWs before and after dehydration. The average diameter of the NWs decreased from 96 to 60 nm after the dehydration. (d) PXRD of the NWs before and after the dehydration in comparison with the reference diffractograms of α-FeF3·3H2O and rhombohedral FeF3, respectively. There is some FeF2 impurity phase, as denoted by the inverted green triangle. (e) A representative TEM image of the dehydrated NWs showing many voids and a rough surface. (f,g) Magnified HRTEM images of the NW in (e), showing the nanodomains of rhombohedral FeF3 and tetragonal FeF2 in the same NW. 6031
dx.doi.org/10.1021/nl303630p | Nano Lett. 2012, 12, 6030−6037
Nano Letters
Letter
Figure 3. Electrochemical and structural characterization of the FeF3 NW cathode. (a) CV of the FeF3 NW electrode scanned between 4.5 and 1.0 V vs Li/Li+ at a scan rate of 0.5 mV s−1. The first four cycles are shown. (b) Voltage profiles for the 1st, 15th, and 30th discharge−charge cycles of the FeF3 NW electrode tested under a current of 50 mA g−1 (1/14.2 C rate) at ∼25 °C. Note that an additional constant-voltage charging step is performed after the constant-current charging step at 4.5 V until the current drops to one-tenth of its original value. (c) Discharge capacity and Coulombic efficiency of the FeF3 NW electrode vs cycle number in comparison with the discharge capacity of the FeF3 cathode made with commercial FeF3 powder (the open red hexagons) and the practical capacity of LiCoO2 (∼140 mAh g−1, the dashed line). Note that all the reported capacity performance in this paper was calculated based on the mass of the FeF3 NWs only. (d) Discharge energy density and energy efficiency of the FeF3 NW electrode vs cycle number shown along with the energy density of LiCoO2 cathode (∼550 Wh kg−1), which is calculated using an average voltage of 3.9 V and a capacity of ∼140 mAh g−1. (e) SEM images before and after the discharge−charge of 50 cycles showing the NW morphology mostly preserved. (f) Size distribution of the NWs before and after the cycling test showing slightly increased NW diameter after 50 cycles.
(Figure 2b) for the first time. The α-FeF3·3H2O NWs underwent complex phase transformations during the dehydration, and mixtures of other phases could be obtained at lower dehydration temperatures (
