ARTICLE pubs.acs.org/JPCC
L-Serine-Assisted
Synthesis of Superparamagnetic Fe3O4 Nanocubes for Lithuium Ion Batteries
Huaqiang Cao,*,† Renlong Liang,†,‡ Dong Qian,‡ Jin Shao,§ and Meizhen Qu§ †
Department of Chemistry, Tsinghua University, Beijing 100084, People's Republic of China School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, People's Republic of China § Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041 People's Republic of China ‡
bS Supporting Information ABSTRACT: Monodisperse superparamagnetic Fe3O4 nanocubes with the edge length up to approximately 16 nm were synthesized by using a biomolecule-assisted solvothermal approach. The Fe3O4 nanocubes are characterized by X-ray powder diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. The magnetic properties of the Fe3O4 nanocubes are studied by zerofield cooling (ZFC) and field cooling (FC) procedures, i.e., MT and MH curves. The electrochemical behavior of the Fe3O4 nanocubes was tested in lithium ion batteries, which presents high specific capacitance of 695.1 mA 3 h/g at a current rate of 0.2 C, higher than that of commonly used graphite electrode (372 mA 3 h/g), as well as excellent Coulombic efficiency of above 95% after the 11th cycle in the subsequent cycles.
1. INTRODUCTION Magnetite (Fe3O4), as the oldest magnetic material and one of the most important magnetic materials, has aroused great interest for various applications such as low-field magnetic separation,1 lithium ion batteries,2 mimetic enzymes,3 a dual imaging probe for caner,4 and two-photon fluorescence indicator.5 Different morphologies and structures of Fe3O4 nanostructures have been successfully synthesized, such as nanoparticles,6 binary nanoparticle superlattices,7 hollow nanospheres,8 nanoprisms,9 nanowires,10 nanotubes,11 nanoflowers,12 and nanocubes.13 Uniform size and shape of nanostructures make it possible to distinguish the properties inherent to the material from other effects. So it is still great challenge to develop novel synthesis methods for generating monodisperse Fe3O4 nanocrystals. Recently, it has aroused our great interest to develop a new synthesis method to prepare inorganic nanostructures,14 such as SnO2 and ZrO2 nancrystals with photocatalytic activities,14a,b Mg(OH)2 complex nanostructures with superhydrophobicity and flame retardant effects,14c hydroxyapatite nanocrystals with inhabiting the proliferation of human HeLa cells,14d In(OH)3 nanocubes with bifunctions, i.e, superhydrophobicity and photocatalytic activity,14e β-Ni(OH)2 complex nanostructures with enhanced electrochemical activity and superhydrophobicity,14f Ag2Se complex nanostructures with photocatalytic activity and superhydrophobicity,14g Bi@Bi2O3 microspheres with photocatalytic activity,14h PbSe complex nanostructures with superhydrophobicity,14i and MoO3 nanowires with excellent pseudocapacitor r 2011 American Chemical Society
behavior,14j etc. It is known that high-quality materials will lead to new physicochemical properties and potential applications. Herein, we report a simple synthesis of Fe3O4 nanocubes by an amino acid assisted solvothermal process.
2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. FeCl3 3 6H2O (Analytical pure, AR),
L-serine
[CH2(OH)CH(NH2)COOH]) (purity > 99%, Beijing Kebio Biotechnology Co., Ltd.), glycol (AR, Beijing Chemical Works), and hexamethylenetetramine (HMT, also called urotropine, C6H12N4, (CH2)6N4, AR, Sinopharm Chemical Reagent Co., Ltd.) were used without further purification. In a typical synthesis, 1 mmol FeCl3 3 6H2O was dissolved in 15 mL of glycol with stirring for 10 min and forming solution A. 1.5 mmol hexamethylenetetramine and 1 mmol L-serine acid were dissolved in 25 mL of H2O with stirring for 10 min and forming solution B. Solution B was added into stirred solution A in 30 min at room temperature. The resulting mixture was transferred to and sealed in a Teflon-lined autoclave, heated to 200 °C, and maintained at this temperature for 10 h. After the autoclave was cooled down to room temperature naturally, the products were collected via a centrifugal method washing with deionized water Received: October 7, 2011 Revised: November 10, 2011 Published: November 14, 2011 24688
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Figure 2. Raman spectrum of Fe3O4 nanocubes. Figure 1. XRD pattern of Fe3O4 nanocubes.
and enthanol for three time cycling, followed by drying at 80 °C for 8 h. 2.2. Materials Characterization. The X-ray powder diffraction measurement was carried out on a X-ray diffractometer (Druker D8 Advance) with Cu Kα radiation (λ = 1.54056 Å) in a 2θ range from 10 to 80°. Transmission electron microscopy (TEM) and high resolution electron microscopy (HRTEM) measurement was carried out on a JEOL JEM-2010 electron microscope, operating at 200 kV. Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet 560 FT-IR spectrophotometer. Raman spectra (Renishaw, RM 1000) were measured with excitation from the 514-nm line of an Ar ion laser with a power of about 5 mW. 2.3. Magnetic Behavior Measurement. Magnetic properties of the sample were measured using a Physical Property Measurement System (PPMS-9T) with temperature capabilities of 5300 K and magnetic field up to 500 Oe for measuring the magnetization (M). 2.4. Electrochemical Measurement. Electrochemical experiments were performed using CR 2032 type coin cells assembled in an argonfilled glovebox (MBRAUN). The working electrode was prepared by mixing the Fe3O4 nanocubes and Carboxymethyl Cellulose Sodium (CMC, 3%) at a weight ratio of 90:10, followed by pasting on pure Cu foil (15 μm). Celgard 2400 was used as a separator. Lithium foil was used as the counter electrode. The electrolyte consisted of a solution of LiPF6 (1 M) containing vinylene carbonate (2%) in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1, volume ratio). A galvanostatic cycling test of the assembled cells was carried out on a BS9300K system in the voltage range of 0.0013.0 V (vs Li+/Li) at current density of 0.2 (200 mA/g), 0.5, 1.0, 2.0, and 5.0 C, respectively. The weight of Fe3O4 nanocubes in the working electrode was used to estimate the specific discharge capacity of the LIBs, which was expressed in mA 3 h/g of Fe3O4 nanocubes.
3. RESULTS AND DISCUSSION The X-ray diffraction (XRD) pattern of the as-synthesized sample presents eight characteristic peaks at 18.31 (111), 30.36 (220), 35.77 (311), 43.35 (400), 53.76 (422), 57.16 (511), 62.98 (440), and 74.47° (533) (Figure 1), which correspond very well with the standard XRD data card of Fe3O4 (JCPDS card. No. 851436). The Fe3O4 nanocubes approximately 14 nm in edge length of nanocubes according to the Scherrer equation:15 The strongest diffraction peak is on (311) lattice plane. D(311) = kλ/b cos θ, where D, k, λ, b, and θ represent the size of Fe3O4
Figure 3. FT-IR spectrum of Fe3O4 nanocubes.
nanocubes vertical to the analyzed lattice plane with the Miller indices (hkl), the constant factor (0.9), the wavelength of the X-ray applied in the experiment, the peak width at half intensity (in a 2θ intensity plot) of the (311) diffraction peak, and the Bragg angle corresponding to the (311) diffraction peak, respectively. At ambient conditions, the Raman spectrum of the assynthesized sample shows four peaks at 215, 280, and 500 cm1 and a broad and main peak at 680 cm1 (Figure 2). The main peak at 680 cm1 is the characteristic peak for magnetite, attributed to the A1g mode, i.e., symmetric stretch of oxygen atoms along FeO bonds.16 The weak peaks at 215 and 500 cm1 are attributed to T2g(1) mode (translator movement of the whole FeO4 unit) and T2g(2) mode (asymmetric stretch of Fe and O), respectively.17 And the weak peak at 280 cm1 is attributed to Eg mode (symmetric bends of oxygen with respect to Fe).18 The Raman data further demonstrate that the as-synthesized sample is pure Fe3O4 phase. Figure 3 presents the Fourier transform infrared (FT-IR) spectrum of the as-synthesized Fe3O4 nanocrystals which demonstrates that the Fe3O4 nanocrystals are free of organic contaminants, such as L-serine and urotropine. The peaks at 582, 1381, 1631, and 3404 cm1 are attributed to Fe3O4, CO2, and H2O, respectively.19 The peak at 582 cm1 corresponds to the FeO bond of Fe3O4 phase.20 Figure 4 shows the crystal structure of Fe3O4. The relatively large O2 ions form a face-centered cubic lattice and Fe cations occupying the interstitial tetrahedral position.21 Fe3O4 (FeO 3 Fe2O3, [Fe3+]tet[Fe2+,Fe3+]octO4) belongs to the inverse spinel structure ([B]tet[A,B]octO4) (A, octahedral sites; B, tetrahedral and octahedral sites) where Fe3+ (d5) has no crystal field stabilization energies, CFSE) in either the octahedral or tetrahedral sites, Fe2+ (d6) has a preference for octahedral sites.22 The Fe2+ and Fe3+ at octahedral sites are close together due to edge-sharing octahedra; thus positive holes can migrate easily from Fe2+ to Fe3+ ions. 24689
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Figure 4. Crystal structure of the Fe3O4.
Fe3O4 is a good conductor which is usually attributed to a halfmetal with the highest known Curie temperature (858 K, 523 °C).23 Fe3+ ions are in a state with spin S = 5/2 and zero orbital moment, while Fe2+ ions have a spin of 2, so Fe3+ and Fe2+ should contribute 5 and 4 μB, respectively. Magnetic order at the A and B sites is antiparallel, leading to ferromagnetic order with an excess magnetic moment of about 4 μB per formula and it has a high Curie temperature of 858 K.24 The morphology and microstructure of the as-synthesized Fe3O4 was studied by transmission electron microscopy (TEM) and high resolution TEM (HRTEM) (Figure 5). The TEM image shows the Fe3O4 is cube-shaped structures. The edge length of cube-shaped Fe3O4 nanocrystals is up approximately 16 nm (Figure 5a), which is close to the size of D(311) calculated based on Scherrer formula. More information about the crystal can be derived from the HRTEM images taken on the Fe3O4 nanocubes (parts b and c of Figure 5). The lattice fringes (d = 0.30 nm) observed in the HRTEM image agree well with the separation between the (220) planes of Fe3O4. To understand the crystal growth process, a series of experiments were performed. The first study was the effect of the reaction time. Figures S1a, 2a, 3a (of the Supporting Information) and 5 shows TEM images of the products synthesized in identical concentrations (Fe3+ concentration = 3 mmol/ 40 mL, L-serine concentration = 18 mmol/40 mL, HMT concentration = 1.5 mmol/40 mL) and the identical reaction temperature of 200 °C but for different reaction times, that is, 30 min, 1 h, 5 h, and 10 h, with these being denoted as Fe3O4-1, Fe3O4-2, Fe3O4-3, and Fe3O4-4 (i.e., the typical called Fe3O4 characterized by Figures. 1,3 and 58 in the text), correspondingly. All these samples belong to Fe3O4 based on XRD analyses (Figures S1b, 2b, 3b (of the Supporting Information) and 1). However, the morphology and size are different. The samples Fe3O4-1 (Figure S1a of the Supporting Information) and Fe3O42 (Figure S2a of the Supporting Information) are nanoparticles with sizes of about ∼5 nm and ∼7.5 in diameter. Both Fe3O4-3 (Figure S3a of the Supporting Information) and Fe3O4-4 (Figure 5) are nanocubes with ∼10 nm and ∼16 nm in edge length, respectively. The phenomenon of enlarged sizes accompanying with prolonging the reaction time is attributed to the Ostwald ripening process, in which larger particles are energetically favorable to growing with the expense of smaller, less stable particles.25 When we carried out a similar experimental parameter but without L-serine, we just obtained red product of rice-shaped
Figure 5. (a) TEM image and (b and c) HRTEM images of the assynthesized Fe3O4 nanocrystals.
α-Fe2O3 nanocrystals (Figure S4 of the Supporting Information) not black Fe3O4. Our previous work demonstrates that α-Fe2O3 is formed in glycol-NH3 3 H2O system at 280 °C under solvothermal treatment.26 Even if glycol is the reducing agent, no Fe3O4 can be observed, because Fe(OH)3 is easily formed via the deposition of free Fe 3+ in NH 3 3 H 2 O environment. Then, Fe(OH)3 is transformed into β-FeOOH, followed by α-Fe2O3 through a phase transformation. Hou and co-workers demonstrate that α-Fe2O3 cubic particles can be generated by a hydrothermal synthesis at 130 °C from a solution of urotropine and ferric chloride. Urotropine slowly decomposed to generate 24690
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Figure 6. (a) Zwitterionic form and (b) anionic form of amino acid.
formaldehyde and OH. The OH ions favor of the deposition and crystallization of Fe3+ to form cubic α-Fe2O3 particles, not Fe3O4 particles.27 This phenomenon can be attributed to free Fe3+ ions are deposited with OH ions (from the decomposed product of urotropine). This experiment suggests that free Fe3+ ions are not protected by L-serine via coordination at room temperature before solvothermal process. However, when we carried out a similar experimental parameter but without using urotropine, we can obtain a small quantity of Fe3O4 with low yield of 16% (Figure S5 of the Supporting Information), which is quiet lower than our glycol-L-sersine-urotropine solvothermal system with yield of over 91%. However, the size of products is multidisperse particles with different shapes include small spherelike particles and big cubelike particles. The experiment demonstrates that glycol can react with Fe3+ and generate Fe3O4. Xi and co-workers demonstrated that ethylene glycol (glycol) functions as both a reductive agent and a solvent and reacts with Fe3+ ions at 200 °C in a solvothermal system, leading to the generation of Fe3O4.28 This experiment suggests glycol can function as a reducing agent, which can reduce Fe3+ to Fe2+, and Fe3+ ions are free under high temperature of 200 °C. When we carried out a similar experimental parameter expect but without using glycol, we obtained Fe3O4 with yield of ∼93% (Figure S6 of the Supporting Information). However, the size of products in multidisperse particle with different shapes include small spherelike particles and big cubelike particles. This experiment suggests urotorpine functions as reductive agent (due to the generation of CH2O) and reacts with Fe3+ ions at 200 °C in a solvothermal system, leading to the generation of Fe3O4. The higher yield than that of the experiment without using urotropine, indicates that urotropine is more stronger reductive agent than glycol. On the basis of the above control experiments, we know that Lserine functions as a coordination agent which can protect Fe3+ ions to be free of reacting with OH at room temperature. However, the coordination complex can release Fe3+ under heat treatment at 200 °C. Both glycol and urotropine function as reductive agents, but the reducing ability of urotropine is stronger than that of glycol. The possible growth mechanism of Fe3O4 in our glycol-L-serine-urotropine solvothermal system is composed of three stages. First, Fe3+ ions cooperate with Lserine (HSer) and generate the Fe(HSer)3+ complex29 through the carboxyl oxygen, not the nitrogen-containing group of the amino acid.30 It is known that all amino acids can exist as zwitterion structure (A) at physiological pH, and the complexforming species with Fe3+ is the anion (B) (Figure 6).30 The zwitterionic structure (A) has less net attraction for Fe3+ than does the anionic structure (B), which lacks this hydrogen bond, while strong intramolecular hydrogen bonds are invariably formed between these ammonium groups (NH3+) and carboxylate ion (COO) in the zwitterionic structure (A). Urotropine decomposes to give ammonia and leads to the generation of OH. So the mixed solvent is weak basic solution (pH = 7.02 measured by using an acidometer PHSJ-3F), being favorable of the formation of anionic structure (B) of L-serine which has
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strong affinity to complex with Fe3+. This was demonstrated by an experiment: after forming the mixed solution (i.e., solution A mixed with solution B), we added KOH solution into the above mixed solution, and we did not find any precipitation, suggesting no free Fe3+ in the mixed solution due to the formation of coordination ions of Fe3+ at room temperature. During the second stage, Fe3O4 nuclei are formed. Fe(HSer)3+ will decompose into Fe3+ under 200 °C (eq 1). Both urotropine and glycol function as reductive reagents under solvothermal treatment at 200 °C, which will lead to the formation of Fe2+ from Fe3+ (eqs 2, 4, and 5). After Fe2+/Fe3+ ions react with OH (eq 3), Fe3O4 nuclei will be generated (eqs 6,9).28 The formation of Fe3O4 is therefore believed to proceed via the following steps: Free Fe3+ ions were released from its complex compound: FeðHSerÞ3þ T Fe3þ þ HSer
ð1Þ
HCHO and OH were generated from (CH2)6N4: ðCH2 Þ6 N4 þ 10H2 O f 6HCHO þ 4NH3 3 H2 O
ð2Þ
NH3 3 H2 O T NH4 þ þ OH
ð3Þ
2+
Fe ions were formed via the reaction between reducing agent and Fe3+:28 Fe3þ þ HCHO f Fe2þ þ HCOOH
ð4Þ
Fe3þ þ HOCH2 CH2 OH f Fe2þ þ HOCH2 COOH ð5Þ Thus the codeposition of Fe2+/Fe3+ will occur with OH and generate Fe3O4:31 Fe2þ þ 2OH f FeðOHÞ2
ð6Þ
Fe3þ þ 3OH f FeðOHÞ3
ð7Þ
FeðOHÞ2 þ 2FeðOHÞ3 f Fe3 O4 þ 4H2 O
ð8Þ
The overall reaction is shown as follows: Fe2þ þ 2Fe3þ þ 8OH f Fe3 O4 þ 4H2 O
ð9Þ
Also because glycol is a strong reducing agent with a relatively high point of 246 °C,32 Fe3+ reacts with glycol under the solvothermal system at 200 °C, which can directly generate Fe3O4 nanoparticles.28,33 In the third stage, the newly formed Fe3O4 nuclei tend to grow bigger on the nucleation sites and form Fe3O4 nanocubes. It is known that magnetite is a half-metal with the highest known Curie temperature of 858 K, and its conduction is only attributed to one spin channel, while the other spin channel exhibits a gap at the Fermi level.23 The d electron of solid transition metal compounds is sensitive to both of the crystal structure and the oxidation state of the transition metal. Figure 7 shows the temperature dependent on magnetization at 500 Oe between 10 and 300 K using zero-field cooling (ZFC) and field cooling (FC) procedures, i.e., MT curves. The ZFC and FC curves are usually used to understand the information of the energy barriers. The blocking temperature (TB), estimated based on the peak maximum in ZFC curve,34 is shown at ca. 163 K (for H = 500 Oe). At the TB, the energy of aligned magnetic moments is balanced with the thermal energy kBT of the particles, i.e., the direction of the magnetic moments of the particles begins to 24691
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Figure 7. Zero-field-cooled (ZFC) and field-cooled (FC) curves for the as-synthesized Fe3O4 particles measured with the field of 500 Oe between 10 and 300 K.
Figure 8. Field-dependent magnetization curve at room temperature. Inset: left is the photos of the responsive performance of Fe3O4 nanocubes to an external magnet; right is the magnified section of the hysteresis loop.
wobble about the direction and the moments become disordered.35 Below the blocking temperature TB, given by TB = KV/kB ln(αt/τ0) [where KV is an activation energy to frisk the particle’s magnetization from the direction at θ = 0 to 180° or from the direction at θ = 180 to 0°, K is constant which quantifies the energy density associated with this anisotropy, V is volume of particle, kB is Boltzmann’s constant of 1.3807 1023 J/K, t is the experiment measuring time, τ is relaxation time, and τ0 is typically 109 s, α = 100 when τ is “much longer” than t], each magnetic particle keeps to be locked into one of its two minima (i.e., from θ = 0 to 180° or from 180 to 0°).36 Below TB, the ZFC curve shows an increase as the moments progressively reorient along the applied field at low temperature from 10 to 163 K, while the FC curve shows a decrease at this low temperature. Also we can observe a transition point at ca. 51 K appearing with discontinuous changes in FC and ZFC curves, corresponding to the signature of the Verwey transition temperature (Tv), a designation of chemistry purity in magnetite. For bulk Fe3O4, Tv ≈ 120 K,37 above which fast electron hopping between the Fe2+ and Fe3+ ions on the octahedral B sites occurs. Lower Tv compared with the bulk Fe3O4 have been reported in spherical Fe3O4 nanoparticles with mean particle size of 150 nm (Tv = 98 K) and 50 nm in diameter (Tv ≈ 20 K),38 granular Fe3O4 films (Tv = 75 K).34b The smaller Tv value of 51 K and more broadened transition Tv can be attributed to the smaller grain size.34b In fact, it is complex to understand the magnetization behavior of Fe3O4 at low temperature, though Verwey proposed that the electron can hop between the Fe2+ and Fe3+ ions on the octahedral B sites in inverse spinel structure in a thermally activated process.37 It has been demonstrated that there is a structural transformation from cubic to triclinic phase at low temperature.39 So we should say the as-synthesized Fe3O4 nanocrystals system undergo a structural transition accompanying with a sudden change of the magnetic and transport properties at Tv. Above the Tv, Fe3O4 has an inverse spinel structure with cubic symmetry, while below the Tv, the cubic symmetry of Fe3O4 crystal is broken by a small lattice distortion.40 The large divergence temperature of ZFC and FC is ca. 300 K which can be attributed to the large amounts of magnetic isotropy energy contributed by the external magnetic field during the cooling procedure. Similar results have been reported in Fe3O4 nanowires34c and nanocrystalline Fe3O4 aerogels.41 The MH curve (Figure 8) measured at room temperature shows nearly no hysteresis loop, which suggests that the assynthesized Fe3O4 nanocrystals possess strong superparamagnetic character, i.e., single-domain particles that orient as large individual magnetic moments in applied magnetic field. The saturation
magnetization (Ms), remnant magnetization (Mr), and coercivity (Hc) for the as-synthesized Fe3O4 nanocrystals are 67 emu/g, 3.5 emu/g (nearly no remanence effect), and 35 Oe, respectively, while the Ms and Hc of bulk Fe3O4 are 85100 emu/g and 115150 Oe, respectively. This difference is attributed to the assynthesized Fe3O4 nanocrystals consisting of small particles.42 The Fe3O4 nanocubes could be removed from solution (Figure 8). The initial rust-colored solution contained Fe3O4 nanocubes homogeneously dispersed in water (1 mg/mL) (inset of Figure 8). Once the solution is placed near a magnet, the solution became clear within 6 min, and Fe3O4 nanocubes keep close to the magnet at the vial wall, where the magnetic field gradient is the largest. This phenomenon suggests that the Fe3O4 nanocubes can find application in low-field magnetic separation.1 The as-synthesized Fe3O4 nanocubes are used as anode material for lithium ion batteries to study the electrochemical properties (Figure 9). Figure 9a shows the potential profiles for the 1st, 2nd, 5th, 28th, 29th, and 30th cycles of the Fe3O4/Li cell. The first specific discharge capacitance is as high as 1200 m 3 A 3 h/g. The phenomenon that the first discharge capacity exceeds the theoretical capacity of Fe3O4 (926 m 3 A 3 h/g, based on the reaction 8 Li+ + Fe3O4 f 3Fe + 4Li2O, assuming the reduction of Fe3+ and Fe2+ to Fe0 during the Li+ intercalation)43 has been observed wildly for transition metal oxide electrodes, such as nanostructured CuO,44 Co3O4 nanowires,45 Co3O4 nanoparticles,46 Co3O4 nanobelts,47 Co3O4 microspheres,48 Fe3O4 nanoparticles,49 etc., which has attributed to the large electrochemical active sites and/or grain boundary area of the nanostructured oxide particles, as well as irreversible reactions (i.e., electrolyte decomposition occurring during the first discharge cycle), or the reversible formation of a Li-bearing solidelectrolyte interface.4549 The discharge capacities of the Fe3O4 electrode in 2nd, 5th, 28th, 29th, and 30th cycles are 838.6, 660.4, 389.4, 376.6, and 366.4 m 3 A 3 h/g, respectively. In the first discharge, there is a steep voltage drop from 3 to ∼0.73 V, which can be attributed to reaction 10 Fe3 O4 þ x Li f Lix Fe3 O4
ð10Þ
An obvious potential plateau at 0.73 V corresponds to the conversion reaction 11 Lix Fe3 O4 þ ð8 xÞLi f 3Fe þ 4Li2 O
ð11Þ
The sloping part of the discharge curve between 0.73 and 0 V can be assigned to the reaction process of Fe and electrolyte to form a gel-like film and inorganic solidelectrolyte interface 24692
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Figure 9. Electrochemical performance of the Fe3O4 nanocubes/Li cells: (a) the dischargecharge profiles in the voltage range 0.013.0 V at the current of 0.2 C rate, (b) the rate performance with the cycling rate of 0.25 C; (c) the cycling performance at 0.2 C rate.
(SEI) layer.50 The Fe3O4 electrodes exhibit high specific capacity in the first cycle, which can deliver 1200 m 3 A 3 h/g in the discharge process and 810 m 3 A 3 h/g in the charge process. Fe3O4 is among a group of metal oxides that abide by the “conversion reaction” mechanism involving the formation and decomposition of Li2O upon subsequent cycling, accompanying the redox of metal nanoparticles. Thus, the electrochemical reaction mechanism of Li with Fe3O4 nanocubes in LIBs can be described as follows: Fe3 O4 þ 8Liþ þ 8e T 3Fe0 þ 4Li2 O
ð12Þ
8Li T 8Liþ þ 8e
ð13Þ
Fe3 O4 þ 8Li T 3Fe0 þ 4Li2 O
ð14Þ
This mechanism of Li reactivity is quite different from the classic Li insertion/deinsertion or Li-alloying mechanisms, the catchwords of the past 30 years.44,51,52 The cycling response at various rates is present in Figure 9b, which shows the discharge of the material at various rates after a slow charge and hold at 3 V to fully charge the material. A rate of n C corresponds to a full discharge in 1/nh.53 At a 0.2 C rate
(corresponding to a time of 5 h to fully discharge the capacity), the Fe3O4 nanocubes discharge to an average 695.1 mA 3 h/g, ∼75.1% of theoretical capacity of Fe3O4, while the Fe3O4 nanocubes reach about 51 mA 3 h/g at the highest rate tested (5 C), corresponding to a time of 720 s to fully discharge the capacity. The capacity decreased stepwise accompanying with the rate increase. When the rate returned back to 0.2 C, the material discharges to 360 mA 3 h/g, ∼38.9% of theoretical capacity of Fe3O4. It is worth pointing out that the average specific capacitance of 695.1 mA 3 h/g at a current rate of 0.2 C is markedly higher specific capacity compared with that of commonly used graphite electrode (372 mA 3 h/g), i.e., about twice that of current carbon-based negative electrode.51 The cycling performance is shown in Figure 9c, which measured the long-cycle characteristics up to 60 cycles at a current rate of 0.2 C. The Fe3O4 nanocube electrode deliver a capacity of 418.4 mA 3 h/g at the 10th cycle with electrode Coulombic efficiency of 94.4%, while 332.5 mA 3 h/g of specific capacity and corresponding electrode Coulombic efficienty of 95.9%, 306.1 mA 3 h/g with Coulombic efficiency of 97.4%, 252.1 mA 3 h/g with Coulombic efficiency of 98.5%, 231.4 mA 3 h/g with Coulombic efficiency of 98.8%, and 221.9 mA 3 h/g with Coulombic efficiency of 99.1%, i.e., approaching 100%, corresponding to the 20th, 30th, 40th, 50th, and 60th cycles (Figure 8c). The Coulombic efficiency of the material is above 95% since 11th cycle in the subsequent cycles, which concludes excellent electrochemical stability. The as-synthesized Fe3O4 nanocubes exhibit a gradual fade in the capacity during cycling, retaining ∼70.4% of its initial capacity after 60 cycles. This suggests that the small nanocubes are initially active but are unstable under the harsh reductionoxidation conditions during the electrochemical cycles. The continuous disintegration of nanocubes is a result of the volume change on cycling. This cracking and crumbling of the nanocubes during cycling may keep on generating new active surfaces that were previously passivated by the stable surface. These new active surfaces consume or trap lithium ions. Thus, the repeated reaction between Fe3O4 and electrolyte leads to fade of the capacity. Similar phenomena have been observed for bare α-Fe2O3 and commercial Fe3O4 fading from 250 and 300 mAh/g to 105 (42% of its initial capacity) and 152 mAh/g (50.7% of its initial capacity), respectively.54 The Sn electrode fading from >1300 mAh/g to ∼50 mAh/g after 30 cycles was attributed to the Sn disintegration.55 Tin nanoparticles encapsulated elastic hollow carbon spheres (TNHCs) exhibited gradual fading from >1625 mAh/g to 550 mAh/g (
