Vacancy Associates Evoked Hematite Mesocubes with Enhanced

Oct 1, 2018 - ... Hematite Mesocubes with Enhanced Efficiency in Li−Storage Behaviors ... By comparing the Li storage performance between two hemati...
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Cite This: J. Phys. Chem. C 2018, 122, 23377−23384

Vacancy Associates Evoked Hematite Mesocubes with Enhanced Efficiency in Li Storage Behaviors Sheng-qi Guo,†,‡ Juan Wang,‡ Yan-tao Chen,‡ Bing-chuan Gu,§ and Boxiong Shen*,† †

School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China Tianjin Key Lab for Photoelectric Materials & Devices, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China § State Key Laboratory of Particle Detection and Electronics, University of Science & Technology of China, Hefei, Anhui 230026, China Downloaded via UNIV OF SUNDERLAND on October 29, 2018 at 09:06:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Vacancies can significantly determine the physical and chemical properties of inorganic materials, which play crucial roles in many areas of applications, such as electronics, catalysis, and energy storage devices. However, little is known about the relationships between vacancy feature and lithium (Li) storage behaviors of inorganic materials. Here, we discovered that the types and abundance of atom vacancies on anode materials strongly affect their Li storage performance. By comparing the Li storage performance between two hematite (α-Fe2O3) cubes with similar morphology but different predominant vacancies and applying the density functional theory computations on the Li-ion diffusion rate and substitution energy of Fe ions by Li ions, we show that the vacancies, in particular the large size vacancies on the surface of anode materials, could effectively reduce the barriers of Li-ion diffusion and decrease the substitution energy of Fe ions. This new finding may advance the design and development of novel anode materials with excellent performance for Li-ion batteries.

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million host atoms.17 Crystalline semiconductors (e.g., metal oxides and sulfides) with different types and abundance of vacancies possess different electronic structures and may exhibit significantly varied reactivity as well as spatial storage performance. For example, the sodium storage capacity of TiO2(B) with oxygen vacancies was twice as much as that of TiO2(B) without oxygen vacancies.18 It is therefore reasonable to expect that the types and abundance of the vacancies of inorganic materials can also greatly affect theirLi storage performance. To date, a few articles have considered the vacancydependent performance of Li storage of anode materials. For example, Kung group introduced a high density of in-plane, nanometer-size carbon vacancies in graphene sheets, which greatly enhanced the capacity of batteries.19 In addition, Zhou et al. revealed that the oxygen-vacancy-rich MnO2 anode possessed higher capacity and rate capability than any nonvacancy reference samples.20 However, no previous publications have compared the vacancy type-dependent performance of Li storage of crystals with similar morphology, which is important to assess whether vacancy engineering is an

INTRODUCTION The rapid development of various applications from portable electronic devices to electrical vehicles stimulates the booming demands for Li-ion batteries (LIBs) with higher power/energy density.1,2 Presently, the commercial anode of LIBs is dominated by graphite because of its satisfying safety, low voltage plateau, and environmental compatibility. Yet, its low theoretical capacity (∼372 mA h g−1) and unsatisfactory rate capability cannot meet the requirements of the dramatically increasing market.3−5 In the past decade, great efforts have been devoted on exploiting advanced substitute materials, and a large number of alternatives based on modification of inorganic materials (including morphological manipulation, doping, and combining with carbon materials) has been reported.6−13 Although extraordinary progress has been made, the low capacity, poor ionic/electronic conductivity, and large volume fluctuation of these alternatives seriously hinder their practical applications. It is still crucial and challenging to seek better alternatives and especially to explore novel and effective strategies to enhance the capacity of Li storage materials. Recent research studies have shown that the type and abundance of atom vacancies in semiconductors strongly affect their chemical and physical properties.14−16 The electronic structures of semiconductors can be significantly changed at vacancy concentrations as low as one vacancy per one hundred © 2018 American Chemical Society

Received: August 25, 2018 Revised: September 27, 2018 Published: October 1, 2018 23377

DOI: 10.1021/acs.jpcc.8b08283 J. Phys. Chem. C 2018, 122, 23377−23384

Article

The Journal of Physical Chemistry C

Figure 1. SEM and TEM images showing that two Fe2O3 samples have similar morphology and exposed facets. SEM images of (a,c) Fe2O3-450 and (b,d) Fe2O3-150. TEM images of (e) Fe2O3-450 and (f) Fe2O3-450. The HR-TEM images of (g) Fe2O3-450 and (h) Fe2O3-150 indicate that these two samples had dominantly exposed {012} facets.

measurements were carried out in the aqueous solution (pH ≈ 7), tested by the Nano-ZS zetasizer (Malvern Instruments). Room-temperature electron spin resonance (ESR) spectra were measured on an ESR spectrometer (JEOL JES-FA200) at 300 K and 9.062 GHz. Positron Annihilation Measurements. The positron lifetime experiments were carried out using a fast−fast coincidence ORTEC system (Oak Ridge Technology & Engineering Cooperation) with a time resolution of ∼240 ps full width at half-maximum. A 30 μCi source of Na was sandwiched between two identical samples, and the total count was 1 million. Positron lifetime calculations were carried out using the ATSUP method,21 in which the electron density and the positron crystalline Coulomb potential were constructed by the non-self-consistent superposition of free-atom electron density and Coulomb potential in the absence of the positron. We used 3 × 3 × 3 supercells for positron lifetime calculations of unrelaxed structure monovacancy defects and vacancy associates in Fe2O3. To obtain the electron density and the Coulomb potential due to the nuclei and the electron density, several self-consistent calculations for electronic structures were performed with the fit-QMCGGA (QMC: quantum Monte Carlo; GGA: generalized gradient approximation) approach or electron−electron exchange correlations. To obtain the positron lifetimes, the GGA form of the enhancement factor proposed by Barbiellini was chosen.22 Computational Details. First-principles calculations were performed using plane wave ultrasoft pseudopotential based upon the density functional theory in the Materials Studio (Accelrys Inc., San Diego. CA). These computations utilized PBE (Perdew−Burke−Ernzerhof)23 in GGA (the general gradient approximation) to describe the exchange−correlation interaction. A cutoff energy of 300 eV was employed for all computations. The TPSD24 optimization method was used, and total energy convergence, max ionic force, max ionic displacement, and max stress component tolerance were 2.0 × 10−6 eV/atom, 0.5 × 10−1 eV/Å, 0.2 × 10−2 Å, and 0.1 GPa, respectively. We used the complete LST/QST method to perform the Li-ion diffusion barrier calculations with the module CASTEP (Cambridge Serial Total Package).25 Electrochemical Measurements. For electrochemical tests, the working electrodes were composed of active materials, acetylene black, and polytetrafluoroethylene at a weight ratio of 80:10:10. The average weight of the electrodes was ∼1.5 mg. In the test cells, lithium metal was used as the

effective strategy to enhance the performance of Li storage of materials. In this work, we investigate the vacancy-dependent Li storage performance of two hematite (α-Fe2O3) cubes with similar morphology but different types and abundance of Febased vacancies: α-Fe2O3 cubes with abundant isolated Fe vacancies versus α-Fe2O3 cubes with abundant larger size Fe3+−oxygen vacancy associates. By comparing differences in Li storage performance between the two samples and conducting theoretical computations for the Li-ion diffusion barriers and the substitution energy of Fe ions by Li ions, we demonstrate that the types and abundance of vacancies are vital for the Li storage performance. We believe that this new finding will advance the design of high-performance Li storage materials.

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EXPERIMENTAL SECTION Synthesis of Fe2O3-150 and Fe2O3-450. All reagents were of analytical grade and used as received without further purification. To synthesize Fe2O3-150, 1.0 mmol (0.40 g) Fe2(SO4)3, 6.0 mmol (0.24 g) NaOH, and 3.0 mmol (1.05 g) sodium dodecyl benzene sulfonate were dissolved in 30 mL of water by vigorously stirring for 10 min. Then, the mixture was sealed in a 50 mL Teflon-lined stainless-steel autoclave and heated at 150 °C for 12 h. Next, the mixture was gradually cooled to room temperature. A reddish brown precipitate was collected and washed with deionized water and absolute ethanol, and the product was kept in absolute ethanol for future use. The Fe2O3-450 sample is prepared by calcination of the Fe2O3-150 sample at 450 °C for 2 h in a muffle furnace, which is denoted as Fe2O3-450. Material Characterization. The morphology of the Fe2O3 samples were characterized with scanning electron microscopy (SEM) (Nova Nano SEM 230, FEI) and transmission electron microscopy (TEM) (Tecnai G2 F20, FEI). The X-ray diffraction (XRD) patterns of the materials were recorded using an X-ray diffractometer (Rigaku D/max 2500) with Cu Kα radiation (λ = 1.54056 Å). The X-ray photoelectron spectroscopy (XPS) analysis was done using a Kratos Axis Ultra DLD XPS. The Fourier transform infrared (FT-IR) spectra were obtained on a NICOLET FT-IR spectrometer. The Brunauer−Emmett−Teller (BET) specific surface area of the samples was analyzed by nitrogen adsorption using a Tristar 3000 nitrogen adsorption apparatus. The zeta potential 23378

DOI: 10.1021/acs.jpcc.8b08283 J. Phys. Chem. C 2018, 122, 23377−23384

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The Journal of Physical Chemistry C counter and reference electrodes. The electrolyte was 1 M LiPF6 dissolved in a 1:1:1 mixture of ethylene carbonate, ethylene methyl carbonate, and dimethyl carbonate. The cells were assembled in a glove box filled with high-purity argon. Discharge/charge tests of the cells were performed between 0.01 and 3.0 V (vs Li/Li+) under a LAND-CT2001A instrument at room temperature. Cyclic voltammetry (CV) tests were performed at different scanning rates from 0.1 to 5.0 mV s−1 between 0.01 and 3.0 V (vs Li/Li+) after the initial four discharge/charge cycles. Electrochemical impedance spectroscopy (EIS) was recorded by the ZAHNER ZENNIUM CIMPS-1 electrochemical workstation with the frequency range from 0.1 to 106 Hz and an ac signal of 5 mV in amplitude as the perturbation. Note: we calculated the specific capacity of electrodes according to the corresponding total weight of active materials in each electrode.

Figure S5), respectively. The longest lifetime (τ3) was assigned to the large defect clusters and the interface present in a material, whereas the shortest lifetime (τ1) was attributed to the monovacancies. Here, the measured τ1 for Fe2O3-150 and Fe2O3-450 was 180.3 and 179.6 ps, respectively, both of which ‴ ), based on the corresponded to the isolated Fe vacancy (V Fe calculated positron lifetime values of different types of vacancies of Fe2O3 in Table 2. τ2 for two samples (around

RESULTS AND DISCUSSION A sample of α-Fe2O3 with abundant isolated Fe vacancies was prepared via a facile hydrothermal process at 150 °C (denoted as Fe2O3-150). Another sample of α-Fe2O3 with abundant Fe3+-oxygen vacancy associates was fabricated by calcination of Fe2O3-150 at 450 °C for 2 h (named Fe2O3-450). The XRD patterns (Figure S1) indicated that both Fe2O3 samples were pure hexagonal α-Fe2O3 in the space group R3̅c (JCPDS no. 33-0664). The XPS analysis only identified the presence of Fe (711.2 eV) and O (531.5 eV) in the two samples (Figure S2), indicating that both the samples were pure. The FT-IR demonstrated the absence of organic molecules on the surface of two Fe2O3 samples, implying the formation of a pure phase of α-Fe2O3 (Figure S3). The morphology of Fe2O3-150 and Fe2O3-450 was characterized by SEM and TEM. As shown in Figure 1a−f, both Fe2O3 samples consisted entirely of equalsized and uniform mesocube structures without impure particles or aggregates. Obviously, there was no change in the morphological and crystal size of Fe2O3-150 through calcination treatment at 450 °C. High-resolution TEM (HRTEM) analysis (Figure 1g,h) confirmed their highly crystalline nature and revealed that the two samples had dominantly exposed {012} facets. Furthermore, both Fe2O3-150 and Fe2O3-450 were mesoporous materialsthe average pore sizes are 30.1 nm for Fe2O3-150 and 22.4 nm for Fe2O3-450 (Figure S4), and the specific surface area of Fe2O3-150 and Fe2O3-450 was determined by the BET method as 33.6 and 27.7 m2 g−1, respectively. We monitored the vacancies of the two Fe2O3 samples using positron annihilation spectrometry. As an accurate method to characterize defects in solids, positron annihilation spectrometry can provide information on the type and relative concentration of defects/vacancies by measuring the lifetime of positrons, even at the parts-per-million level.26,27 Both Fe2O3-150 and Fe2O3-450 yielded three lifetime components, τ1, τ2, and τ3, with relative intensities I1, I2, and I3 (Table 1 and

190 ps) corresponded to positron annihilation trapped at triple Fe3+-oxygen vacancy associates (i.e., Fe−O−Fe vacancy ‴ V O••V Fe ‴ ), which implied the presence of associates, V Fe ‴ V O••V Fe ‴ vacancy associates in two samples. The abundant V Fe ‴ V O••V Fe ‴ ‴ and V Fe schematic illustrations of nonvacancy V Fe vacancy associates on Fe2O3 were depicted on the basis of theoretical calculation of the two types of Fe-based vacancies (Figure 2a−c). The relative intensity (I) of the positron lifetime provides information on the concentration of defects.28,29 As shown in ‴ V O••V Fe ‴ vacancy Table 1, the higher relative intensity of V Fe associates in Fe2O3-450 suggested a higher concentration compared to that yielded in Fe2O3-150. Also, Table 1 revealed ‴ V O••V Fe ‴ ‴ was predominant in Fe2O3-150, but V Fe that V Fe vacancy associates were predominant in Fe2O3-450. It is ‴ V O••V Fe ‴ vacancy noteworthy that, the formation of V Fe associates resulted in more dangling O bonds than that of ‴ for accommodating more electrons,30 which could be V Fe further verified by the surface charge revealed by zeta potentials. The zeta potential for Fe2O3-150 and Fe2O3-450 was −27.6 and −33.5 mV, respectively, which demonstrated the increased negative charges with increasing large-size vacancy concentration. Besides positron annihilation spectrometry and zeta potential, ESR analysis was also carried out to characterize different types and abundance of vacancies between two samples.31,32 The similar g values in ESR spectrums revealed that both Fe2O3-150 and Fe2O3-450 presented strong signals of electrons captured by the Fe-based vacancies; however, the different ESR signal intensity in Figure S6 indicated that Fe2O3-450 possessed obviously higher concentration of large size Fe-based vacancies compared to Fe2O3-150. Briefly, two Fe2O 3 samples with similar morphology but different predominant forms of the vacancy were successfully fabricated, which could serve as ideal models to investigate the correlation between the material vacancy feature and Li storage performance. Two Fe2O3 samples were prepared as working electrodes. With the aim of exploring the above correlation, we first investigate the electrode kinetics with respect to the Li-ion diffusion coefficient by CV, which could serve as a good descriptor to verify whether inorganic materials with different types of vacancy can accelerate the Li storage reaction process, as fast Li-ion diffusion facilitates the Li transformation chemistry at the material interface.33−35 Figure 3a,b shows

Table 2. Theoretically Calculated Positron Lifetime (τ) Values of Different Types of Defects in Fe2O3a

a

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Table 1. Positron Lifetime Parameters of Fe2O3-450 and Fe2O3-150a nanostructures

τ1 (ps)

τ2 (ps)

τ3 (ns)

I1 (%)

I2 (%)

I3 (%)

Fe2O3-450 Fe2O3-150

180.3 179.6

190.2 191.1

2.76 2.92

34.7 81.5

65.0 17.7

0.324 0.781

τ: positron lifetime components of different types of defects. I: relative intensity of different types of defects. a

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defect type

bulk

‴ V Fe

‴ V O•• V Fe

‴ V O••V Fe ‴ V Fe

τ (ps)

145

179

183

190

τ: positron lifetime components of different types of defects.

DOI: 10.1021/acs.jpcc.8b08283 J. Phys. Chem. C 2018, 122, 23377−23384

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The Journal of Physical Chemistry C

‴ and (c) Figure 2. (a) The coordination environment of the Fe atoms in α-Fe2O3. Schematic representations of trapped positrons of (b) V Fe ‴ V O••V Fe ‴ vacancy associates based on positron annihilation data. V Fe

Figure 3. Lithium-ion diffusion properties on the surface of Fe2O3-450 and Fe2O3-150 with mechanism analysis. CV curves of the (a) Fe2O3-450 and (b) Fe2O3-150 electrodes at various scan rates. (c) CV peak current for the anodic oxidation process versus the square root of the scan rates. (d) Energy profiles for diffusion processes of Li ions on the different Fe2O3 models.

scan rate. Here, the slope of the curve (Ip/ν0.5) represents the Li-ion diffusion rate, as n, S, and CLi were unchanged. It can be clearly seen from Figure 3c that the Fe2O3-450 electrode with ‴ V O••V Fe ‴ vacancy associates exhibits the higher Liabundant V Fe ion diffusion rate than that of the Fe2O3-150 electrode with ‴ , indicating that the large size vacancies could abundant V Fe effectively promote the adsorption and substitution reaction of Li ions on the surface of Fe2O3. To validate the above-mentioned points, we simulated the diffusion barriers for Li ions on {012} facets of three Fe2O3 models using Materials Studio (MS), including an Fe2O3 cube ‴ , and an Fe2O3 cube without vacancies, an Fe2O3 cube with V Fe

the CV curves of two electrodes measured between 0.01 and 3.00 V at different scanning rates from 0.1 to 5.0 mV s−1. The pair of corresponding peaks existed in all rates for both samples and was assigned to the conversion between Li ions and Li2O. These peaks have a linear relationship with the square root of scanning rates, indicating the diffusion-limited process.36 Thus, the classical Randles−Sevcik equation can be applied to describe the Li-ion diffusion process: Ip = (2.69 × 105) n1.5SDLi ions0.5CLiν0.5, where Ip is the peak current, n is the charge transfer number, S is the geometric area of the active electrode, DLi ions is the Li-ion diffusion coefficient, CLi is the concentration of Li ions in the cathode, and ν is the potential 23380

DOI: 10.1021/acs.jpcc.8b08283 J. Phys. Chem. C 2018, 122, 23377−23384

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The Journal of Physical Chemistry C

Figure 4. Top-view schematic representations of corresponding diffusion pathways for (a,b) Fe2O3 cubes without vacancies, (c,d) Fe2O3 cubes with ‴ , and (e,f) Fe2O3 cubes with V Fe ‴ V O••V Fe ‴ vacancy associates. Here, the yellow, purple, red, and green balls represent lithium, iron, oxygen, and V Fe vacancy atoms, respectively.

‴ V O••V Fe ‴ vacancy associates. The Li-ion diffusion with V Fe pathways on the surface of three models were illustrated in Figure 4. The diffusion follows the arc curves from one stable point to the other, with the saddle point located in the middle of the pathway. Figure 3d revealed the energy profiles along the diffusion coordinate and the diffusion pathways. Obviously, ‴ V O••V Fe ‴ the diffusion barriers for Fe2O3-450 with abundant V Fe vacancy associates were ∼2.1 eV smaller than those of Fe2O3150, which is consistent with our experimental results, showing that Li ions diffuse faster on Fe2O3-450. Commonly, a lower barrier can lead to an increase in the diffusion rate according to the exponential rule, and faster diffusion on the surface of the inorganic materials can promote the reaction between Li ions and the anode. To further validate the Li-ion diffusion ability between two samples, we carried out EIS. The line in the lowfrequency range was assigned to the Li-ion diffusion process. Compared with Fe2O3-150, the Fe2O3-450 electrode exhibited a smaller impedance and superior Li-ion diffusion ability (Figure 5a), which was consistent with the CV results. Figure 6a,b shows the galvanostatic discharge/charge voltage profiles of Fe2O3-150 and Fe2O3-450 electrodes at various current rates from 0.1 C (1 C = 1007 mA g−1) to 2 C in the potential range of 0.01−3.00 V. Compared with Fe2O3-150, ‴ V O••V Fe ‴ vacancy associates Fe2O3-450 with abundant V Fe exhibited much better Li storage performance. The discharge/charge capacity of Fe2O3-450 could reach 742.0/ 716.3 mA h g−1 even at a high current rate of 2 C in the initial cycle, approximately 2.5 times as much as that of Fe2O3-150. Apparently, the high concentration of larger size vacancies strongly improved the Li storage performance at the initial stage.37 Note that, according to the Li storage mechanism for Fe2O3 (Fe2O3 + 6Li+ + 6e → Li2O + 2Fe),38 the electron transfer should take place in the Fe element, that is, Fe3+ were first reduced to Fe0 during discharging and recover to the original state when charging, giving a theoretical capacity of 1007 mA h g−1. However, the presented reversible capacities in the initial cycle at 0.1 and 0.2 C of Fe2O3-450 were much higher. From where does the huge extra capacity originate? We speculate that the extra capacity comes from the abundance of ‴ V O••V Fe ‴ vacancy associates on the Fe2O3-450 surface. The V Fe large size vacancy in the initial cycle could accommodate a

Figure 5. Nyquist plots for EIS of (a) Fe2O3-450 and Fe2O3-150 electrodes before cycling and (b) Fe2O3-450 electrode at different states.

large number of Li ions, thereby enhancing the Li storage capacity of the Fe2O3-450 anode. To obtain an in-depth understanding of the correlation between vacancy size and Li storage capacity, we calculated the substitution energies, Es, of one Fe ion substituted by three Li ions in three models mentioned above. A smaller Es value indicated that one Fe ion substituted by three Li ions is energetically more favorable, resulting in a higher likelihood of Fe ions substituted by Li ions on Fe2O3 cubes. The 23381

DOI: 10.1021/acs.jpcc.8b08283 J. Phys. Chem. C 2018, 122, 23377−23384

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The Journal of Physical Chemistry C

Figure 6. Electrochemical performance of the Fe2O3-450 and Fe2O3-150 electrodes. Discharge/charge profiles of (a) Fe2O3-450 and (b) Fe2O3-150 at different current densities in the initial cycle. (c) Cycling performance and Coulombic efficiency of the two Fe2O3 electrodes at 0.1 C for 100 cycles. (d) Cycling performance and Coulombic efficiency of the Fe2O3-450 electrode at 0.2 C for 200 cycles.

can be seen that the effect of high concentration of vacancy associates on the Li storage performance can be maintained during the cycles. More notable was that when the current density was at 0.2 C (Figure 6d), the initial reversible capacity of Fe2O3-450 was 1142.8 mA h g−1 and decreased to 356.8 mA h g−1 after 140 cycles; however, the capacity gradually recovered to 445.8 mA h g−1 in the subsequent 60 cycles, ‴ V O••V Fe ‴ vacancy which implied high concentration of V Fe associates not only stable in the cycle but may also have an activation effect on some solid−electrolyte interface (SEI) components.4 Figure 5b shows EIS of the Fe2O3-450 electrode before and after representative cycles, which proved the activation effect on the SEI. These Nyquist plots consist of typical characteristics of two corresponding impedances in the media-to-high frequency, including SEI resistance (RSEI) and charge-transfer impedance resistance (Rct). We can clearly see that RSEI decreases as the cycle progresses, which confirmed the occurrence of the activation effect. Meanwhile, we found that Rct also significantly decreases during the cycle progresses. This obvious reduction in Rct signifies the enhanced contact between the electrolyte and electrode, which also confirmed that the SEI was gradually activated. To further analyze the structural stability of the vacancies during long-term cycling tests, we first examined the morphological changes of the two samples after 100 cycles with the current density at 0.1 C. From Figure S7, we observed that both Fe2O3-450 and Fe2O3-150 retain the cube structural features, which revealed that the internal structure of two samples can be stably present in the cycling reaction. Compared with Fe2O3-450, more severe deformation was present on the surface of Fe2O3-150. This indicated that the large size vacancies have a mitigating effect on the volume expansion and collapse of the anode materials during the cycling. Subsequently, we detected the vacancy information again of two samples by positron annihilation spectrometry (Table S1). Although the vacancy information of the two samples have changed from before, it was clearly seen that the ‴ V O••V Fe ‴ vacancy associates in two samples still exist and V Fe were the main vacancy types of Fe2O3-450, which demon-

substitution energy of one Fe ion by three Li ions can be defined as Es = E3Li−Fe + μFe − Eoriginal − 3μLi

where Eoriginal is the total energy of the Fe2O3 cube and E3Li−Fe is the total energy of the Fe2O3 cube with one Fe ion substituted by three Li ions. μFe and μLi are the energy of one Fe and Li atom, respectively, taking into account the spin effect. The calculated Es values were −6.8 eV for Fe2O3 ‴ , and −8.1 eV without vacancy, −6.9 eV for the one with V Fe ‴ V O••V Fe ‴ vacancy associates. The order of Es for the one with V Fe values indicated that Fe ions substituted by Li ions on the surface of the Fe2O3 cube should be energetically more ‴ V O••V Fe ‴ vacancy associates, which is favorable for cubes with V Fe consistent with the experimentally determined vacancy properties of the two samples. On the basis of the computational results, we proposed the following conceptual model to explain why Fe2O3 with ‴ V O••V Fe ‴ vacancy associates effectively facilitates the Li-ion V Fe storage. Along the perimeters of the vacancies, there were many incompletely coordinated Fe and O atoms. These atoms tend to bond with each other, which stretches the bonds between the incompletely coordinated atoms and those immediately adjacent. As the vacancy size becomes larger, this stretching effect increases, resulting in the loose anode material framework, and the Fe ion was more easily substituted. Overall, the presence of vacancy can lower the substitution energy of Fe ions substituted by Li ions. ‴ V O••V Fe ‴ A noteworthy doubt is the deformation of V Fe vacancy associates, that is, it might be gradually corroded by electrolytes and cause serious structural damage especially in long-term and high-rate cycling tests. To investigate this crucial point, both Fe2O3-450 and Fe2O3-150 were tested at a current density of 0.1 C and cycled for 100 cycles (Figure 6c). Fe2O3450 was superior to Fe2O3-150 in terms of Li storage capability and cycle stability; the capacity of Fe2O3-450 decayed from 1390.1 to 622.0 mA h g−1 in the first ten cycles and remains stable in the following 90 cycles; in comparison, the reversible capacity of Fe2O3-150 rapidly decreased to 220.1 mA h g−1. It 23382

DOI: 10.1021/acs.jpcc.8b08283 J. Phys. Chem. C 2018, 122, 23377−23384

Article

The Journal of Physical Chemistry C

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strated that large size vacancies can be stably present in the anode material electrode during the cycling reaction.

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CONCLUSIONS In summary, two α-Fe2O3 cubes with similar morphology but different types and abundance of vacancies have been synthesized and characterized. Considering differences in the Li-ion diffusion ability and storage capacity and based on the density functional theory computations, we determined that the types and abundance of vacancies are critical for the Li storage performance. Vacancies on the surface of materials, in ‴ V O••V Fe ‴ vacancy associates, not only particular the large size V Fe effectively reduce the barriers of Li-ion diffusion but also facilitate Li storage by decreasing the substitution energy of the semiconductor atoms. The new finding in this study further advance the fundamental understanding of nanomaterial structure−activity relationships and may have significant implications for improving semiconductor performance through vacancy engineering by tuning the vacancy type and abundancy.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08283. XRD patterns, XPS spectra, FT-IR spectra, N 2 adsorption/desorption isotherms, positron annihilation spectra, ESR spectra, and SEM images of two α-Fe2O3 cubes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sheng-qi Guo: 0000-0003-1777-4953 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Key Research and Development Project of China (2018YFB0605101), the National Natural Science Foundation of China (grant 21701125, 21171128 and 21271108), Key Project Natural Science Foundation of Tianjin (18JCZDJC39800), and the China Postdoctoral Science Foundation (2015M580208).

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REFERENCES

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DOI: 10.1021/acs.jpcc.8b08283 J. Phys. Chem. C 2018, 122, 23377−23384

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DOI: 10.1021/acs.jpcc.8b08283 J. Phys. Chem. C 2018, 122, 23377−23384