Cationically Substituted Bi0.7Fe0.3OCl ... - ACS Publications

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Cationically Substituted Bi0.7Fe0.3OCl Nanosheets as Li Ion Battery Anodes Yoon Myung,†,#,∇ Jaewon Choi,‡,∥,∇ Fei Wu,† Sriya Banerjee,† Eric H. Majzoub,§ Jaewon Jin,∥ Seung Uk Son,∥ Paul V. Braun,‡ and Parag Banerjee*,†,⊥ †

Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, Missouri 63130, United States ‡ Department of Materials Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States § Center for Nanoscience, Department of Physics and Astronomy, University of Missouri, St. Louis, Missouri 63121, United States ∥ Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea ⊥ Institute of Materials Science and Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States # Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul, 05006, Korea S Supporting Information *

ABSTRACT: Cation substitution of Bi3+ with Fe3+ in BiOCl leads to the formation of ionically layered Bi0.7Fe0.3OCl nanosheets. The synthesis follows a hydrolysis route using bismuth(III) nitrate and iron(III) chloride, followed by postannealing at 500 °C. Room temperature electrical conductivity improves from 6.11 × 10−8 S/m for BiOCl to 6.80 × 10−7 S/m for Bi0.7Fe0.3OCl. Correspondingly, the activation energy for electrical conduction reduces from 862 meV for pure BiOCl to 310 meV for Bi0.7Fe0.3OCl. These data suggest improved charge mobility in Bi0.7Fe0.3OCl nanosheets. Density functional theory calculations confirm this behavior by predicting a high density of states near the Fermi level for Bi0.7Fe0.3OCl. The improvement in electrical conductivity is exploited in the electrochemical performance of Bi0.7Fe0.3OCl nanosheets. The insertion capacity of Li+ ions shows an increase of 2.5×, from 215 mAh·.g−1 for undoped BiOCl to 542 mAh·g−1 for Bi0.7Fe0.3OCl after 50 cycles at a current density of 50 mA·g−1. Thus, the direct substitution of Bi3+ sites with Fe3+ in BiOCl results in nanosheets of an ionically layered ternary semiconductor compound which is attractive for Li ion battery anode applications. KEYWORDS: metal oxychloride, hydrolysis, doping, electrochemistry, Li ion batteries stability due to large volume changes during cycling,9,10 which has limited its use. Recently metal oxyhalides, due to their unique layered structure consisting of anionic sheets interleaved with cationic layers, have attracted interest as LIB electrodes.11,12 BiOCl is one such candidate material that has a layered tetragonal crystal structure that consists of alternatively stacked layers of

1. INTRODUCTION Two dimensional nanosheets are promising candidates for lithium ion battery (LIB) electrodes.1−3 In a conventional LIB, the anode is based on graphite. However, graphite has a low theoretical capacity (372 mAh·g−1 for LiC6).4−6 Alternative 2D materials for LIB anodes are metal chalcogenides such as MoS2 nanosheets. MoS2 is capable of a four-electron transfer reaction with Li+ during the charge/discharge process leading to a theoretical lithium storage capacity7,8 of 669 mAh·g−1. Despite this advantage, MoS2 exhibits low conductivity and poor © 2017 American Chemical Society

Received: December 30, 2016 Accepted: April 7, 2017 Published: April 7, 2017 14187

DOI: 10.1021/acsami.6b16822 ACS Appl. Mater. Interfaces 2017, 9, 14187−14196

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ACS Applied Materials & Interfaces [Bi2O2]2+ slabs between two sheets of Cl− ions along the c-axis. This structure can provide fast diffusion pathways along and between the ionic layers.13 Further, the layered structural characteristics can provide large volume expansion for Li atom intercalation and deintercalation.14,15 The electrochemical reaction of BiOCl with lithium is a two-step process and has been proposed as follows.16 BiOCl + 3Li+ + 3e− → Li3OCl + Bi

(1)

Bi + 3Li+ + 3e− → Li3Bi

(2)

Scheme 1. Comparison of BiOCl (Left) and FeOCl (Right) Crystal Structures Showing the 7.3% Difference in the BiOCl (001) vs FeOCl (010) Planes

On the first discharge, lithiation leads to the decomposition of BiOCl into metallic Bi and Li3OCl. During further charge/ discharge cycles, Bi alloying/dealloying up to the theoretical limit of Li3Bi can occur. Thus, BiOCl theoretical capacity is estimated to be 385 mAh·g−1, which while being high is not as attractive as compared to MoS2. An attractive strategy to further boost capacity is to susbtitutionally dope BiOCl nanosheets. Doping is considered to be an effective strategy to improve electrode performance in LIBs.17,18 For example, Bi1−xMxOCl in which Mn+ has replaced Bi3+ site could greatly increase capacity without compromising the advantages of the ionically layered structure of BiOCl. However, to date, such a performance enhancement in the BiOCl system has not been realized. Therefore, in this paper we demonstrate Fe3+ doped BiOCl anodes with a capacity of 542 mAh·g−1 after 50 charge− discharge cycles at a current density of 50 mA.g−1. This value is 1.4× the theoretical capacity of Li3Bi (385 mAh·g−1) which is formed during Li alloying/dealloying of undoped BiOCl. We choose the candidate dopant as Fe3+ for three reasons. First, the smaller Fe3+ ionic radius (0.645 Å) compared to Bi3+ (1.17 Å) can lead to creation of open volume within the ionic layers for faster diffusion during Li insertion/extraction.19 Second, Fe has a lower atomic mass as compared to Bi, and thus, a higher gravimetric capacity can be realized. These two reasons may lead one to conclude that FeOCl, rather than BiOCl, may be a better anode material. However, FeOCl shows a larger intercalation associated volumetric strain11 compared to BiOCl. Thus, a mixed BiOCl−FeOCl phase can help mitigate this volumetric strain, aid better stress management, and simultaneously improve the kinetics of charge intercalation/ deintercalation. These motivations have led us to explore the LIB performance of Fe3+ doped BiOCl as an anode material. We note that Fe3+ decorated BiOCl sheets have been demonstrated in the past.20 However, our report successfully incorporates Fe3+ ions inside the BiOCl lattice, substituting the Bi3+ sites. Achieving such substitutional doping is a challenge since the reacting Bi and Fe salts result in the formation of the more thermodynamically stable BiFeO3 phase.21,22 Therefore, to incorporate Fe3+ in the BiOCl lattice, we use the following strategy as given below. Scheme 1 compares the crystal structure of BiOCl and FeOCl. BiOCl has a lattice parameter of 7.34 Å along [001] direction and FeOCl has a lattice parameter of 7.95 Å along [010] direction.23 The lattice parameter difference between BiOCl (001) and FeOCl (010) is only 7.3%, thus raising the possibility of successful Fe3+ substitution into the BiOCl lattice. Careful Fe3+ incorporation in BiOCl lattice can be conducted by control of pH via a facile room temperature hydrolysis reaction under acidic conditions. The substitutional doping of Fe3+ at the Bi3+ sites is then completed via a thermal annealing step at 500 °C which prevents the formation of the BiFeO3

phase or phase segregation into separate BiOCl and FeOCl phases. The resulting homogeneously doped BiOCl nanosheet, i.e., Bi0.7Fe0.3OCl, is stable and leads to a marked improvement in the electrical conductivity of the material and Li ion capacity and stability for LIB anodes.

2. EXPERIMENTAL SECTION All reagents were purchased from Sigma-Aldrich (ACS quality) and were used as received without further purification. Pure BiOCl nanosheets were prepared by the hydrolysis reactions described in our previous work.24 Bi0.7Fe0.3OCl nanosheets were prepared using 1:1 molar ratio of Bi(NO3)3·5H2O and FeCl3·6H2O. Bi(NO3)3·5H2O (1 mmol) was dissolved in 100 mL of DI water and stirred continuously. Subsequently, 100 mL of 10 mM FeCl3·6H2O solution was added to Bi(NO3)3·5H2O solution. The pH was maintained at 3 during hydrolysis step. After 60 min under vigorous magnetic stirring, an orange precipitate was formed at room temperature. This nanosheet suspended solution was rinsed with distilled water and isolated by centrifugation at 8000 rpm, 10 min, and repeated 3 times. This process yields 200 mg of sample powder per run. Post-thermal annealing was performed in a horizontal tube furnace in the temperature range of 100−500 °C under air. The nanosheet morphology was characterized by field-emission transmission electron microscopy (FE-TEM, JEOL-2100F). X-ray diffraction (XRD) was performed on a Rigaku Geigerflex D-MAX/A diffractometer using Cu Kα radiation. Raman spectra of the samples were obtained using a Renishaw inVia Raman instrument, with a wavelength of 514 nm. X-ray photoelectron spectroscopy (XPS) was measured using the PHI Versa Probe II spectrometer (Physical Electronics, Inc.) with a photon energy of 1486.6 eV (Al Kα). Elemental analysis was performed by an inductively coupled plasma optical emission spectrometer (PerkinElmer Optima 7300DV) to confirm atomic ratio of Bi and Fe. Photolithography was used to pattern a two-electrode structure consisting of thermally evaporated metallic Al on a SiO2 substrate. The gap distance between the two electrodes was 20 μm. A drop of 20 mg of BiOCl or Bi0.7Fe0.3OCl nanosheets dispersed in 1 mL of isopropanol solution was placed on the Al patterned electrode, and the solvent was evaporated to form a uniform film covering the electrode. All electronic transport property measurements were carried out in a commercial probe station (Janis ST500-1-2CX) with Cu−Be probe tips, 50 μm in tip diameter. Temperature dependent current−voltage (I−V) tests were done with temperature varying from 300 to 425 K, with a step of 5 K. The pressure in the chamber was maintained at atmosphere (1000 mbar). I−V measurements were made using a Keithley 2400 source measure unit coupled to a Labview control program. The applied voltage was increased from 0 to 30 V at a rate of 0.05 V per step. Li ion batteries were fabricated by the following process. First, the anode material (2 g), carbon black (0.2 g), and polyvinylidene fluoride (PVDF) binder (0.4 g) were mixed in N-methylpyrrolidone (NMP, 1 g). After coating the copper foil with this slurry, the electrode was 14188


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Figure 1. (a) TEM image of Bi0.7Fe0.3OCl nanosheets: the lattice resolved image for the crystalline shown in (b) as-synthesized and (c) after thermal annealed Bi0.7Fe0.3OCl nanosheets.

Figure 2. (a) XRD patterns of different temperature annealed nanosheets. Magnified scaled of the (b) (001) and (c) (110) and (102) peaks. dried under vacuum at 120 °C for 3 h. The diameter of the circular copper electrode was 14 mm. The loading level of electrode materials was 10 mg/cm2. Cell tests were conducted using coin type half cells (CR 2032) with lithium metal as the counter electrode and 1.3 M LiPF6 in ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate (EC/EMC/DMC, 3:4:3, volume ratio) as the electrolyte. Discharge/charge cycle tests were performed using WBCS3000 automatic battery cycler system and Biologic VMP3 potentiostats. CV curves of the coin half cell were measured in the potential range of 0.05−2.5 V (vs Li+ /Li) at a scan rate of 0.2 mV·s−1. Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP).25 A plane wave basis set was used for the electronic wave functions with an energy cutoff of 600 eV. Monkhorst−Pack grids were used for Brillouin zone integration with a minimum k-point density of 3000/Å−3. Projector augmented wave (PAW)26,27 pseudopotentials with the generalized gradient approximation (GGA) were used with the exchange correlation of Perdew and Wang (PW91).28 Computationally tractable supercells were constructed from P4/nmm BiOCl to examine Fesubstituted materials. The composition of the 1 × 2 × 1 supercell containing four formula units of BiOCl was Bi3Fe1O4Cl4, corresponding closely to the experimentally prepared Bi0.7Fe0.3OCl materials. Structural relaxation of the P4/nmm BiOCl structure and supercells with Fe substitution on Bi sites were performed using the conjugate gradient algorithm in VASP until the forces on the atoms were less than 0.01 eV/Å. Band structures were calculated using either the standard PAW GGA pseudopotentials or the hybrid B3LYP functional where indicated.

confirmed by FE-TEM image in Figure 1a and via SEM in Figure S1 in Supporting Information. The FE-TEM images show the presence of sheets of size ∼200 nm. Figure 1b shows the lattice resolved image of an as-synthesized Bi0.7Fe0.3OCl nanosheet. The d-spacing of tetragonal BiOCl (003) planes is measured and found to be 0.25 nm which is consistent with that of JCPDS No. 85-0861 (a = 3.890 Å, c = 7.370 Å). The corresponding FFT-ED pattern displays (003), (114), and (111) planes. Figure 1c shows the lattice resolved image of an annealed Bi0.7Fe0.3OCl nanosheet. The nanosheet exhibits good crystallinity and clear lattice fringes with a lattice spacing of 0.28 and 0.26 nm in the orthogonal directions. This matches well with the (110) and (111) planes of the tetragonal BiOCl crystal system. XRD data of Bi0.7Fe0.3OCl nanosheets are shown in Figure 2a. All peaks can be indexed relative to the tetragonal BiOCl phase (JCPDS No. 85-0861; a = 3.890 Å, c = 7.370 Å). Figure 2b shows the magnified BiOCl (001) peak for the various samples studied. The (001) peak shifts to lower angle Δ(2θ) by 0.16°. This result implies that the lattice parameter expands slightly upon Fe3+ doping. One might suppose that the substitution of Bi3+ ions (ionic radius, r(Bi3+) = 1.17 Å) with the smaller radius Fe3+ ions (r(Fe3+) = 0.645 Å) would lead to a contraction in the lattice constant of BiOCl. To explain our observed results, we compare lattice parameters of orthorhombic FeOCl (JCPDS No. 74-1369; a = 3.750 Å, b = 7.950 Å, c = 3.300 Å) with the BiOCl crystal structure. As mentioned in Scheme 1 previously, the lattice parameters of FeOCl (010) and BiOCl (001) are 7.95 and 7.37 Å, respectively; the FeOCl

3. RESULTS AND DISCUSSION 3.1. Material and Structural Characterization. The shape and morphology of as-synthesized nanosheets were 14189


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ACS Applied Materials & Interfaces (010) lattice parameter is 7.3% greater than BiOCl (001). Therefore, the peak shifting to lower angle in annealed Bi0.7Fe0.3OCl is a result of increasing FeOCl character of the crystal lattice as compared to undoped BiOCl. This result is consistent with the magnified regions of (110) and (102) peaks in Figure 2c which show shifts to lower values in the peak positions as well, indicating an increase in the associated lattice constants. Simulated powder X-ray diffraction for a random substitution of 25% Fe3+ on Bi3+ sites is shown in Figure 3a. The simulation

Figure 4. Raman spectra of nanosheets annealed at different temperatures.

vibration of Bi−O, respectively.29,30 For Bi0.7Fe0.3OCl, all samples show peak broadening at A1g band position. In addition, a red shift is observed at A1g band from BiOCl (144 cm−1) to Bi0.7Fe0.3OCl (142 cm−1). This broadening and frequency shift of A1g mode suggests either (1) formation of complex defects or (2) production of lower charged Bi+3‑δ ion by Fe3+ incorporation in BiOCl lattice.31−34 With increasing annealing temperature, the 220 cm−1, 286 cm−1, and 400 cm−1 bands become stronger (red arrows in Figure 4). These peaks have been observed in FeOOH and Fe2O3 materials and suggest Fe3+ ion is well ordered in Bi0.7Fe0.3OCl lattice.35,36 The substitution mechanism observed is in line with the XRD results discussed above. In addition, no extra peaks of BiFeO3 are detected.22,37,38 Survey scanned XPS spectra of the pure BiOCl and assynthesized and 500 °C thermal annealed samples are shown in Figure S3, Supporting Information. The survey spectra show all of the samples were composed of elements Bi, Fe, O, Cl, and C without any residual component. The atomic percentages of Bi and Fe were calculated by XPS. The Bi to Fe ratio was found to be 0.7:0.3 for both as-synthesized and 500 °C annealed sample. Figure 5a shows Bi 4f peaks of pure BiOCl and as-synthesized and 500 °C thermal annealed Bi0.7Fe0.3OCl samples. The peak binding energies of 158.9 and 164.2 eV are assigned to Bi 4f7/2 and Bi 4f5/2 of the Bi3+ in the pure BiOCl.39,40 The assynthesized Bi0.7Fe0.3OCl Bi 4f7/2 spectrum possesses not only 158.9 eV peak but also a shoulder peak around 160.5 eV, after Fe3+ substitution into BiOCl lattice. The 500 °C thermal annealed Bi0.7Fe0.3OCl does not show a significant peak splitting while the peak binding energy shifts to lower energy (by ∼0.3 eV) as compared to pure BiOCl. This peak shift is probably due to a lower charged Bi3+ due to its higher electronegativity than Fe3+. Another possibility could be the presence of point defects such as those occurring in “black” BiOCl. There, the appearance of oxygen vacancies induces a lower binding energy peak shift as well.41 Furthermore, this result is consistent with our Raman spectra shown above. Figure 5b shows the fine spectrum for Fe 2p3/2 peaks of assynthesized and 500 °C thermal annealed Bi0.7Fe0.3OCl. The characteristic peak appears at 711.8 eV for as-synthesized Bi0.7Fe0.3OCl and 710.5 eV for thermal annealed Bi0.7Fe0.3OCl, respectively. The peak at 711.8 eV in as-synthesized Bi0.7Fe0.3OCl suggests Fe3+ ions are potentially present in an ionic state similar to FeOOH. On the other hand, the binding energy peak at 710.5 eV in 500 °C thermal annealed

Figure 3. Simulated XRD of 25% Fe substitution in BiOCl forming a solid solution on the Bi/Fe sublattice (a). XRD of pure BiOCl and 500 °C annealed Bi0.7Fe0.3OCl (b).

assumes Bragg−Brentano scattering geometry using Cu Kα radiation and includes the Lorentz polarization correction. Experimental diffraction is shown in Figure 3b. It is evident from the simulated pattern that an Fe/Bi solid solution sublattice produces a drop in the peak intensities, as one would expect. The experimental XRD indicates this drop in intensity but has additional features, including (i) a disproportionate drop in some peak intensities, e.g., the (001) and (002); (ii) relative peak intensity changes, e.g., (110) and (102); (iii) new peaks at about 27° and 35−36° 2θ; (iv) significant strain seen in the Lorentzian character of the Fe-substituted sample, along with diffuse scattering seen at low angles. Figure S2a in Supporting Information shows simulated diffraction of an ordered substitution of a 1 × 2 × 1 supercell in symmetry P4/ nmm with composition Bi3Fe1O2Cl2. All Bi sites are equivalent, so substitution of Fe3+ for any Bi3+ suffices. While a random substitution changes only the structure factors (peak intensities), the ordered substitution produces additional peaks. From a comparison to the experimental data shown in Figure S2b in Supporting Information, it is apparent that the annealed Bi0.7Fe0.3OCl has a weak correspondence with the simulated diffraction from the ordered structure (i.e., the presence of additional peaks). This suggests that postannealing at 500 °C results in Fe3+ substitution in Bi3+ sites and at the least is a partially ordered transformation. Structural issues are further discussed in section 3.4 containing the DFT calculations. The Raman spectra of the samples are shown in Figure 4. The pure BiOCl shows a strong band at 144 and 200 cm−1 and weak band at 398 cm−1 which is assigned to A1g internal Bi−Cl stretching mode, Eg external Bi−Cl stretching mode, and the 14190


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Figure 5. Fine scan XPS spectrum of (a) Bi 4f and (b) Fe 2p, respectively.

Bi0.7Fe0.3OCl suggests that the Fe3+ ions possess Fe2O3 like ionic state.42,43 The lower shift could also be related to changing defect states between as-synthesized and 500 °C thermal annealed Bi0.7Fe0.3OCl samples. The collective materials and structural characterization results suggest that the low pH hydrolysis of Bi(NO3)3 and FeCl3 can lead to the direct formation of substitutionally doped Bi0.7Fe0.3Cl nanosheets via intermediate FeOOH followed by FeOCl formation. BiFeO3 formation is avoided due to the low pH during hydrolysis. The thermal decomposition of BiOCl or FeOCl is avoided during annealing in O2 due to the stabilizing effect of O2 on oxychlorides.44 The detailed reaction sequence and justification for Bi/Fe = 0.7:0.3 is provided in Supporting Information section S4. Additionally, as stated previously in Scheme 1, the close lattice matching of FeOCl (010) with BiOCl (001) helps in the substitutional doping of Fe3+ in Bi3+ sites. 3.2. Electrical Characterization. Temperature dependent I−V characteristics of pure BiOCl and as-synthesized and 500 °C thermal annealed Bi0.7Fe0.3OCl nanosheets were measured under ambient conditions (Figure S4, Supporting Information). All I−V measurements show ohmic conduction behavior. The variation of ohmic conductivity (σ) allows us to calculate the activation energy (Ea) of conduction as σ = σ0 e−Ea /(kT )

Figure 6. Arrhenius plot of conductivity versus reciprocal temperature.

in the Bi0.7Fe0.3OCl is much lower than original BiOCl phase. We also note that the conductivity in thermal annealed Bi0.7Fe0.3OCl is higher compared to BiOCl and as-synthesized Bi0.7Fe0.3OCl. 3.3. Electrochemical Performance. The improved conductivity and smaller ionic radius of the Fe3+ (0.645 Å) as compared to Bi3+ (1.17 Å) raise the possibility of improved electrochemical performance of Bi0.7Fe0.3OCl nanosheets for Li intercalation in LIBs. Furthermore, as stated before, Fe has a lower atomic mass as compared to Bi, and thus, a higher gravimetric capacity can be realized. On the other hand, FeOCl has shown a larger intercalation associated volumetric strain11 compared to BiOCl, and therefore, Bi0.7Fe0.3OCl can help with better stress management. These motivations have led us to explore the LIB performance of Bi0.7Fe0.3OCl as an anode material. As stated in the Introduction, lithiation in BiOCl leads to the decomposition of the oxychloride into metallic Bi and Li3OCl. During further charge/discharge cycles, Bi alloying/dealloying up to the theoretical limit of Li3Bi can occur. For BiOCl, this theoretical capacity is estimated to be 385 mAh·g−1. On the other hand, FeOCl has been shown to lithiate as LixFeOCl. The theoretical capacity in this case is estimated to be 794 mAh· g−1.45,46 Figure 7 demonstrates pure BiOCl and as-synthesized and 500 °C thermal annealed Bi0.7Fe0.3OCl samples charge/ discharge capacity as a function of cycle number for up to 50

(3)

Thus, from the Arrhenius equation above, we can obtain the activation energy. In Figure 6, as-synthesized Bi0.7Fe0.3OCl exhibits an Ea of ∼883 meV. This can be compared to a similar Ea for pure BiOCl of ∼862 meV.24 On the basis of our Raman and XPS data, the Fe3+ incorporation into the BiOCl lattice for the as-synthesized sample is not complete and therefore may result in the near similar Ea observed. On the other hand, the 500 °C thermal annealed Bi0.7Fe0.3OCl reveals two Ea values: ∼310 meV for low temperatures and ∼796 meV for high temperature region. The transition point is 370 K. This characteristic bilinear behavior in Ea is typical of doped semiconductors which are extrinsic at low temperatures, revealing the impact of substitutional dopant addition to the parent lattice. At high temperatures, the behavior is intrinsic and related to the properties of the host semiconductor. The decrease in activation energy indicates that the barrier to charge conduction 14191


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Figure 7. (a) Discharge cycling performance dependent on cycle numbers of BiOCl, Bi0.7Fe0.3OCl, 500 °C thermal annealed Bi0.7Fe0.3OCl nanosheets at the 50 mA/g current density and Coulombic efficiencies of 500 °C annealed Bi0.7Fe0.3OCl. (b) Volumetric capacities of 500 °C annealed Bi0.7Fe0.3OCl at the 50 mA/cm3 and 500 mA/cm3. (c) C-rate performance of BiOCl, Bi0.7Fe0.3OCl, 500 °C annealed Bi0.7Fe0.3OCl nanosheets. (d) Charge−discharge curves from 500 °C annealed Bi0.7Fe0.3OCl at the 50 mA/g current density.

98% after 5 cycles. This large initial capacity loss can be attributed to the formation of solid electrolyte interphase (SEI) layers on the electrode surface during the first discharge step and the storage of Li cations, as reported in previous work.16 The annealed Bi0.7Fe0.3OCl reveals resistivity of 5.9 GΩ at 300 K and a room temperature activation energy of 310 meV, which is 552 meV lower than pure BiOCl. The volumetric capacity of 500 °C annealed Bi0.7Fe0.3OCl is shown in Figure 7b. The cross section SEM image is shown in Figure S5, Supporting Information. The results show that this anode has capacities of 954 mAh·cm−3 at 50 mA·cm−3 after 50 cycles and 726 mAh·cm−3 at 500 mA·cm−3 after 200 cycles, respectively. The Coulombic efficiency is maintained >99% for up to 200 cycles. The rate capabilities are shown in Figure 7c. The 500 °C thermal annealed Bi0.7Fe0.3OCl shows higher rate performance of 555 mAh·g−1 and 458 mAh·g−1 at discharge capacities of 50 mA·g−1 and 1 A·g−1, respectively. This can be compared with pure BiOCl with 223 mAh·g−1 and 92 mAh·g−1 discharge capacities at 50 mA·g−1 and 1 A·g−1, respectively. Figure 7d shows the charge−discharge voltage profiles for 1, 2,

cycles using 2032-type coin cells. The coin cells were tested at various C-rates and cycled between 0.001 and 2.5 V at a current density of 50 mA·g−1. The C-rate capability tests were performed in the range from 50 mA·g−1 to 1 A·g−1 and recovered in the range from 1 A·g−1 to 50 mA·g−1. All cells gradually lost their discharge capacity depending on the C-rate. Figure 7a shows the discharging capacity performance at 50 mA·g−1. The first discharge capacities are 1380, 1480, and 1568 mA·h·g−1 in pure BiOCl and as-synthesized and 500 °C thermal annealed Bi0.7Fe0.3OCl, respectively. The pure BiOCl discharge capacities were 571 mA·h·g−1 at second cycle and 215 mA·h·g−1 at 50th cycle due to the large activation energy of 862 meV and a high resistivity of 750 GΩ at 300 K.18,24 Assynthesized Bi0.7Fe0.3OCl shows the value of discharge capacities as 727 mA·h·g−1 at second cycle and 286 mA·h·g−1 at 50th cycle. This low discharge value was possibly due to the high activation energy of 883 meV. In comparison, 500 °C thermal annealed Bi0.7Fe0.3OCl showed a discharge capacity of 842 mA·h·g−1 for second cycle and 542 mA·h·g−1 at 50th cycle. The initial Coulombic efficiency recorded was 66% and reaches 14192


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Figure 8. CV curves of (a) BiOCl and (b) 500 °C annealed Bi0.7Fe0.3OCl at 0.2 mV/s. The first cycle shows 1.6 and 1.62 V contributed to an irreversible chemical change.

Figure 9. (a) DFT-calculated band structure for BiOCl using the PW91 correlation exchange functional with PAW GGA pseudopotentials. (b) Band structure for BiOCl using the hybrid B3LYP functional. (c) DFT-calculated band structure for Bi3Fe1O4Cl4 using the hybrid B3LYP functional for Fe in the Bi plane and (d) Fe in the oxygen plane. The Fermi level in all cases is indicated by the dotted line.

10, 50 cycles at charge−discharge rate of 50 mA·g−1. The 500 °C thermal annealed Bi0.7Fe0.3OCl shows irreversible discharge curve between 1.8 and 0.7 V (vs Li/Li+) shoulder at first cycle. After this cycle, a consistently reversible 0.7 V (vs Li/Li+) plateau appears from second cycle up to 50 cycles. Thus, outstanding electrochemical performance and stability are demonstrated for 500 °C thermal annealed Bi0.7Fe0.3OCl as a lithium ion battery anode material. The cyclic voltammetry (CV) curves of the BiOCl and 500 °C thermally annealed Bi0.7Fe0.3OCl electrodes are shown in Figure 8a and Figure 8b, respectively. BiOCl response to Li+ intercalation and deintercalation is very similar to results in literature.16 Primarily, after the formation of Li3OCl (1.60 V),

the reduction peaks at 0.7 and 0.56 V correspond to conversion of BiOCl to metallic Bi by formation of Li3Bi alloy. Similarly, 500 °C thermally annealed Bi0.7Fe0.3OCl anode shows peaks at 0.686 and 0.56 V. However, the degradation in these peaks is less severe as compared to BiOCl, suggesting better accommodation of the alloying/dealloying reaction in the Bi0.7Fe0.3OCl lattice. The Fe0/Fe2+ redox peak at 1.60 V and the Fe2+/Fe3+ redox peak at 1.85 V are absent, further confirming that the Fe3+ does not exist as a Fe2O3 phase or as BiFeO3 47,48 but rather as a true substituent ion in the BiOCl lattice. The discharge capacity is calculated to be 494 mAh·g−1 which comprises of 70% of Bi (from a total of 385 mAh·g−1) and 30% of FeOCl (from a total of 794 mAh·g−1). At 50 mA·g−1 current 14193


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bandgap is observed. The latter calculated band structure may suggest that the strain evident in the XRD patterns of our annealed samples is due to the energetic competition of Fe migrating to the O-plane and the resulting lattice strain. Finally, both Fe-substituted structures are predicted to be metallic, with a finite density of states at the Fermi level. These calculations support the electrical data obtained above, which shows a reduced Ea and improved conductivity for 500 °C annealed Bi0.7Fe0.3OCl.

measurement of Bi0.7Fe0.3OCl (RT, i.e., no annealed), the discharge capacity is measured to be 468 mAh·g−1 in 10 cycles, which is close to the capacity calculated above. On the other hand, the discharge capacity of 500 °C annealed Bi0.7Fe0.3OCl after 10 cycles is 644 mAh·g−1. The increase of discharge capacity upon annealing is possibly due to the diffusion of the charge on the surface through the heat treatment and the reaction of the Li+ with the surface. Thus, apart from the reasons cited above, the discharge capacity of Bi0.7Fe0.3OCl annealed at 500 °C is higher than the theoretical capacity partly through the pseudocapacitive lithium storage effect as also shown in other metal oxide−carbon electrodes.49 Finally, post-test analysis was conducted by observing the anode morphology consisting of the embedded Bi0.7Fe0.3OCl nanosheets. This is shown in Figure S6, Supporting Information. Well intact sheets could be observed mixed with carbon black paste and PVDF binder. This suggests that the Bi0.7Fe0.3OCl nanosheets may better handle the stresses of Li ion intercalation and deintercalation thereby, maintaining their morphology after 50 charge/discharge cycles. 3.4. Density Functional Theory Calculations. Density functional theory was used to investigate changes in structure upon Fe-doping and investigate the resulting band structure changes. We note that relaxation of the cell volume of BiOCl in symmetry P4/nmm and the Fe-substituted supercell using standard PAW GGA pseudopotentials results in an unphysical expansion of the c-axis of the crystal to c = 8.78 Å. A relaxation of the 1 × 2 × 1 supercell in composition Bi3Fe1O4Cl4, holding the lattice parameters fixed, indicates that Fe substituted for Bi will relax out of the plane of Bi atoms and into the oxygen plane. This change in structure would also lead to additional diffraction peaks that are not observed in the experimental diffraction spectra (see Figure S7, Supporting Information). The energetics of this relaxation are significant; the relaxed structure in which the Fe is four-coordinated with oxygen is over 2 eV lower in energy, implying a dramatic change in the electronic structure of the two configurations. Indeed, the Fe pstates move to much lower energy when coordinated with oxygen (see Figures S8 and S9, Supporting Information). Yet, this structure is not evident in the experimental XRD spectra. Taken together, the diffraction data and DFT calculations suggest that Fe substitutes for the Bi, resulting in a small lattice parameter change observed in some peaks shifting to lower angle but produces strain and small structural changes in the P4/nmm BiOCl structure as the Fe attempts to relax out of the Bi plane and coordinate with O, as suggested by the Raman and XPS measurements. The band structure of BiOCl in symmetry P4/nmm was calculated by fixing the lattice constants to the experimentally measured values and relaxing only the ionic positions. The predicted indirect band gap is about 2.6 eV using standard PAW GGA pseudopotentials with the PW91 exchange correlation functional (Figure 9a). The band gap is improved slightly by using the hybrid B3LYP functional (Figure 9b) and gives a gap of just over 4 eV, in better agreement with the experimentally measured gap.50 Therefore, we used the B3LYP functional to calculate the Fe-substituted band structures of 1 × 2 × 1 supercells with composition Bi3Fe1O4Cl4. The results are shown in Figure 9c for fixing the Fe position in the Bi layer; here the Fe impurity introduces states near the middle of the gap. Results are shown in Figure 9d for Fe in its structurally relaxed position in the oxygen layer. Specifically, for the Fe in the structurally relaxed position (Figure 9c), a decrease in the

4. CONCLUSIONS Successful substitution of Fe3+ in BiOCl nanosheets has been demonstrated. The close lattice matching of the BiOCl with FeOCl is exploited to synthesize Bi0.7Fe0.3OCl phase. This phase is homogeneous, and no evidence of the thermodynamically stable BiFeO3 is observed. XRD, Raman spectra, and XPS indicate that the Fe3+ incorporation is complete using a 500 °C anneal in ambient air. Room temperature electrical conductivity is improved as is observed in the significant reduction in activation energy, from 862 meV for BiOCl to 310 meV for 500 °C annealed Bi0.7Fe0.3OCl. These measurements are supported by DFT calculations. Finally, anodes made of Bi0.7Fe0.3OCl demonstrate a Li ion capacity of 542 mAh·g−1 after 50 cycles at a current density of 50 mA.g−1 and demonstrates the potential of Bi0.7Fe0.3OCl nanosheets and ionically layered semiconductors, in general, for energy storage applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16822. SEM image of Bi0.7Fe0.3OCl (RT) and Bi0.7Fe0.3OCl (500 °C) nanosheets; simulated and experimental XRD, XPS survey spectra; chemistry of Bi0.7Fe0.3OCl formation; temperature dependendent I−V characteristics of Bi0.7Fe0.3OCl (RT) and Bi0.7Fe0.3OCl (500 °C); cross section SEM image of Li ion battery anode; SEM image of post-test Bi0.7Fe0.3OCl nanosheets mixed in the PVDF and carbon black binder; Simulated XRD and Fe DOS for 1 × 2 × 1 supercell (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yoon Myung: 0000-0002-5774-6183 Paul V. Braun: 0000-0003-4079-8160 Parag Banerjee: 0000-0003-0401-8155 Author Contributions ∇

Y.M. and J.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partial support for this work was provided under the U.S.-India Partnership to Advance Clean Energy-Research (PACE-R) for the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, under Subcontract DE-AC3614194


Research Article


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08GO28308 to the National Renewable Energy Laboratory, Golden, Colorado) and the Government of India, through the Department of Science and Technology under Subcontract IUSSTF/JCERDC-SERIIUS/2012 and is acknowledged. Partial support from U.S. Army RDECOM Acquisition Grant W911NF-15-1-0178, Subgrant RSC15032 is acknowledged. The XPS measurements were made possible through the support from NSF MRI Grant 1337374. The microscopy facility at the Institute of Materials Science and Engineering is acknowledged. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education. (Grants NRF-2016R1A6A3A11930304, NRF-2017R1C1B5018387)

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