Nanostructured Fe3O4/SWNT Electrode: Binder-Free and High-Rate Li-Ion Anode

Rechargeable Li-ion batteries are currently being explored for high-power applications such as electric vehicles. However, in order to deploy Li-ion batteries in next-generation vehicles, it is essential to develop electrodes made from durable, nontoxic, and inexpensive materials with a high charge/discharge rate and a high reversible capacity. Transition metal oxides such as Fe3O4, Fe2O3, MoO3, and Co3O4 [1–5] are capable of Liþ insertion/ extraction in excess of 6 Liþ per formula unit, resulting in a significantly larger reversible capacity than commercially employed graphite. In contrast to the intercalation mechanism that occurs for graphite, the transitionmetal oxides are reduced in a conversion reaction to small metal clusters with the oxygen reacting with Liþ to form Li2O. [1,2,6] This usually leads to large volume expansion and destruction of the structure upon electrochemical cycling, especially at high rate. Hence, optimizing particle size and mixing the particles with various carbon additives have been employed to improve the reversible capacity and rate capability of metal oxide electrodes. Among the transition metal oxides, Fe3O4 is both nontoxic and abundant (inexpensive) and is thus considered one of the most promising electrode materials. However, a truly durable high-rate capability and a high capacity for metal oxide based electrodes including Fe3O4 have not yet been achieved. To achieve high-rate capability and high capacity using metal oxide nanoparticles mixed with carbon materials, there are three key issues that must be considered: i) the size of the nanoparticles must be optimized such that rapid Li-ion diffusion and reaction with metal oxide nanoparticles are achieved, ii) an optimized carbon matrix must be developed that ensures both electrical conductivity and good thermal conductivity (to improve heatdissipation), and iii) the conductive additive must maintain a flexible and strong matrix that accommodates large volume changes. In most conventional electrodes, metal oxide nanoparticles are directly mixed with a carbon additive and a binder to help maintain electrical conductivity, and the large volume expansion then results inmechanical degradation of the electrode when cycled at high rate. Here we employ the unique properties of highly crystalline and long single-walled carbon nanotubes (SWNTs) to simultaneously address all of the three key issues with a simple two-step process to synthesize Fe3O4 nanoparticles embedded uniformly in an interconnected ‘‘SWNT net.’’ Furthermore, no polymer binder is required to maintain electrical conductivity. The electrodes contain 95wt% active material with only 5wt% SWNTs as the conductive additive (typical electrodes contain 80wt% active material and 20wt% conductive and binder additives). Most importantly, by using these binder-free electrodes, we have demonstrated a high reversible capacity of 1000mAhg 1 ( 2000mAh cm ) at C rate as well as high-rate capability and stable capacities of 800mAhg 1 at 5C (both for over 100 deep charge/discharge cycles) and 600mAhg 1 at 10C. Raman spectroscopy suggests that this remarkable rate capability is achieved because the Fe3O4 nanoparticles are actually bound to the flexible nanotube net. We also believe that this fabrication method may be employed for other active materials to achieve a binder-free, high-rate, and durable electrode. The FeOOH nanorods, employed as a precursor in the electrode fabrication process, have a width of 50 nm, length of 250 nm, and thickness of 20 nm and are formed with a simple hydrothermal process. X-ray diffraction (XRD) spectra of the as-prepared nanorods and reference a-FeOOH phase (goethite, JCPDS 81-0463) are shown in Figure 1a. All of the reflection peaks can be indexed to the tetragonal a-FeOOH phase. Next we created Fe3O4 nanoparticles embedded in an interconnected SWNTnetwork using FeOOHnanostructures and SWNTs as precursors for a vacuum-filtration and subsequent annealing process. We found that annealing the FeOOH nanorods without SWNTs to 450 8C in an argon atmosphere leads to a mixture of a-Fe2O3 (hematite) and Fe3O4 (magnetite) as indicated by the XRD patterns in Figure 1b. The peaks marked with * are indexed to the Fe3O4 phase (JCPDS 88-0315) and the remainder of the diffraction peaks are indexed to a-Fe2O3, (JCPDS 33-0664). In contrast, annealing FeOOHnanorods mixed with 5wt% SWNTs at 450 8C in an argon atmosphere leads to the complete reduction of FeOOH to Fe3O4, as indicated in Figure 1c. It is therefore evident that the SWNTs actually facilitate the formation of Fe3O4 nanoparticles, enabling excellent Fe3O4 nanoparticle/SWNT electronic and mechanical contact, which is further confirmed by the Raman spectroscopy analysis discussed later. The elegant morphology of the Fe3O4 nanorods embedded uniformly in the SWNT net is clearly depicted in the scanning electronmicroscope (SEM) image of Figure 2a. Figure 2b displays