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Building an Electronic Bridge via Ag-Decoration to Enhance Kinetics of Iron Fluoride Cathode in Lithium Ion Batteries Yu Li, Xingzhen Zhou, Ying Bai, Guanghai Chen, Zhaohua Wang, Hui Li, Feng Wu, and Chuan Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on April 30, 2017

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ACS Applied Materials & Interfaces

Building an Electronic Bridge via Ag-Decoration to Enhance Kinetics of Iron Fluoride Cathode in Lithium Ion Batteries Yu Li,† Xingzhen Zhou,†,‡ Ying Bai,*,† Guanghai Chen,† Zhaohua Wang,† Hui Li,† Feng Wu, †,§ Chuan Wu*,†,§ †

Beijing Key Laboratory of Environmental Science and Engineering, School of

Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡

National Active Distribution Network Technology Research Center, Beijing Jiaotong

University, Beijing 100044, PR China §

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, PR

China

KEY WORDS: lithium-ion battery, FeF3·0.33H2O cathode, Ag-decoration, electronic bridge, kinetics ABSTRACT: As a typical multi-electron cathode material for lithium ion batteries, iron fluoride (FeF3) and its analogues suffer from poor electronic conductivity and low actual specific capacity. Herein, we introduce Ag nanoparticles by silver mirror reaction into the FeF3·0.33H2O cathode to build the electronic bridge between the solid (active materials) and liquid (electrolyte) interface. The crystal structures of as-prepared samples are characterized by XRD and Rietveld refinement. Moreover, the density of states (DOS) of FeF3·0.33H2O and FeF3·0.33H2O/Ag (Ag-decorated FeF3·0.33H2O) samples are calculated using the first principle density functional theory (DFT). The FeF3·0.33H2O/Ag cathodes exhibit significant enhancements on the electrochemical performance in terms of the cycle performance and rate capability, especially for the Ag-decorated amount of 5%. It achieves an initial capacity of 168.2 mAh g-1 and retains a discharge capacity of 128.4 mAh g-1 after 50 cycles in the voltage range of 2.0–4.5 V. It demonstrates that Ag-decoration can reduce the band gap, improve electronic conductivity and elevate intercalation/deintercalation kinetics. 1

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INTRODUCTION Lithium ion batteries (LIBs) as an electrochemical energy storage system have become one of the most attractive research hotspots due to the environmental deterioration and energy crisis.1-3 Thereinto, cathode materials are considered as a primary determinant for improving energy densities of cell system because their capacities are relatively lower than those of anode materials.4-7 In the past few years, some traditional cathodes (e.g., LiCoO2, LiFeO4) have been commercialized. However, the capacities of these cathodes are achieved by shuttling Li+ to trigger just one unit valence change per transition metal cation, which limits their capacities to some extent.8-9 Exploring a possible multi-electron reaction in electrochemical conversion process as a new mechanism to replace Li+ insertion-deinsertion reaction, seems to be an effective approach to break capacities bottleneck.10-11 For this reason, the transition metal compounds with high-valence oxidation states, such as MxZy (M=Fe, Co, Ni…; Z=F, O, S…), have jumped into our view.12-15 Among them, iron fluoride (FeF3), a representative and typical iron-based fluoride, has drawn increasing attention due to its high theoretical specific capacity of 712 mAh g-1 with 3e- transfer at an average voltage of 2.7 V.15-17 In fact, its practical application in LIBs is undesirably impeded since a poor electronic conductivity is originated from the large band-gap of Fe-F bonds, leading to the actual specific capacity of only 80 mAh g-1 far below the theoretical value.18-20 To overcome this issue, multitudinous efforts have been made to improve FeF3 cathodes electrochemical performances, such as partially substituting Fe3+ with other transition ions (Co2+ or Ti4+) to reduce band gap,8, 21 designing the size of materials to shorten both electronic and ionic pathways,19,

22-24

and forming compounds with

conductive materials (graphene, carbon nanotubes or conductive carbon black) to enhance electronic conductivity.25-28 Nevertheless, the conductive carbon-based materials will reduce the energy density because it is electrochemically inactive as a part of cathode in LIBs.29 It is generally known that Ag is a superexcellent conductor of electricity and can be easily prepared. Some researchers have adopted Ag to modify electrode materials, 2


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for instance, LiFePO430, LiCoO231, LiMn2O432, Li4Ti5O1233 and Si34. It is proved that Ag-coating can greatly elevate the rate property and cycling stability of the electrodes by reinforcing the electronic conductivity between electrode particles and reducing the polarization of cathode. In this work, Ag-decorated FeF3· 0.33H2O cathodes are synthesized by a masterly silver mirror reaction process. The effects of surface modification on the electrochemical behaviors of FeF3· 0.33H2O electrodes are investigated. In order to understand

the

reason

why

Ag-decoration

can

significantly

enhance

intercalation/deintercalation kinetics of FeF3· 0.33H2O cathode, XRD refinement, DOS and reaction mechanism hypothesis of Ag-decorated FeF3·0.33H2O are studied and discussed in details. 1. EXPERIMENTAL SECTION 2.1. Synthesis. The synthetic process is schematically exhibited in Scheme 1. Typically, Fe(NO3)3·9H2O and NH4HF2 were utilized as raw materials and the polyethylene glycol (PEG 20000) was selected as a dispersant. 15 g PEG was dissolved in a co-solvent mixture of 20 ml deionized water and 150 ml ethanol in duplicate as solution A and B. Solution A was transferred to a Teflon beaker, and a required amount of NH4HF2 was dissolved in it to form solution C. And 1 M AgNO3 ammonia distilled water solution and methanal solution were added slowly into the solution B to produce silver mirror reaction and quickly transferred to 0.25 M Fe(NO3)3 ethanol solution to form solution D. With peristaltic pump, the solution C was added dropwise to solution D accompanying by agitated stirring. After 12 h aged time, the product was washed by deionized water and ethanol alternately for three times and then dried at 120 oC for 12 h in vacuum atmosphere. The gradient calcinations were conducted on all precursors. Precursors are pre-sintered at 120 oC in vacuum oven to remove the absorbed water, then transferred into tube furnace to be calcinated at 200 oC for 3 h and further heated at 300 oC for 3 h to obtain FeF3·0.33H2O. In order to further enhance the conductivity of the calcined iron fluoride sample, a conductive agent of Super P (iron fluoride: SP=85:15 by weight) was mixed by high energy ball-milling at 3



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300 rpm for 3 h. At last, the final product were converted into an iron fluoride/silver/carbon nanocomposite, hereinafter the compounds are referred to as Ag 0%, Ag 1%, Ag 3% and Ag 5%, respectively. 2.2. Material Characterization. Crystal structure of the electrode materials was analyzed between 10° and 80° 2θ by Rigaku2400 powder X-ray diffraction (XRD) with Cu Kα radiation at a scan rate of 8° 2θ min-1. The surface morphologies were observed by using a FEI Quanta 250 field-emission scanning electron microscope (FE-SEM) and a Tecnai G2 F20 high-resolution

transmission

electron

microscope

(HR-TEM).

Thermogravimetric-differential thermal analysis (TG-DTA) was detected on a Seiko 6300 instrument at a heating rate of 10 oC min-1 from 25 oC to 700 oC under argon atmosphere. The content of silver element was tested by using a 5300DV inductively coupled plasma optical emission spectroscopy (ICP-OES). 2.3. Electrochemical Tests. Each of the as-prepared composites was formed into a cathode slurry. The nano-composite (80 wt.%), super P (SP, 10 wt.%) and polyvinylidene fluoride (PVDF, 10 wt.%) in N-methyl-2-pyrrolidinone (NMP) solvent were mixed to give a uniform slurry, and then coated on Al foil. The coated electrodes were put into a vacuum drying oven at 120 oC for 12 h. The electrodes were cut into discs and the mass loading density of each electrode slice was about 1.0 mg cm-2. Two-electrode coin cells (CR2025) employing lithium foil as counter electrodes and Celgard 2400 as separators were assembled in a glove box filled by argon. The electrolyte consisted of 1 M solution of LiPF6 dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylene methyl carbonate (EMC) solvents (1:1:1 v/v/v). The charge/discharge measurements were tested on a LAND-CT2001A instrument at selected current rates, the voltage range was 2.0-4.5 V vs Li+/Li. In this voltage range, only one electron occurs transfer in FeF3, namely the theoretical specific capacity of FeF3 is 237 mAh g-1, so we define 1 C = 237 mA g-1. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments were all conducted with a CHI660e electrochemical workstation. CV tests were recorded at a scanning 4


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rate of 0.1 mV s-1 between 2.0 and 4.5 V. The alternating current (AC) amplitude was 5 mV and the frequency range was from 106 Hz to 10-1 Hz. 2.4. First Principal Calculations. The density of states (DOS) of pristine FeF3·0.33H2O and Ag-decorated FeF3·0.33H2O/Ag samples were calculated using the first-principles density functional theory (DFT) through Vienna Ab Simulation Package (VASP).35-36 The exchange-correlation Perdew-Burke-Ernzonhof

functional (PBE)

energy function

was within

processed the

generalized

by

the

gradient

approximation (GGA).37-38 The cut-off energy used for the plane wave expansion of the wave function was 500 eV. Considering the strong correlation effect in the transition metal Fe, electronic structure calculations and structural relaxations were performed by adopting a spin-depedent GGA plus Hubbard U (GGA+U) approach.39-40 The U values for Fe and Ag were set as 5 and 2 eV according to previous works, respectively.8, 41 The Brillouin zone was sampled with 3×2×3 mesh for the electronic structure calculations and structural optimizations. The convergence accuracies for the total energy and force were set to 10-4 and 0.05 eV, respectively. To avoid the influence of other atoms, 10 Å vacuum was added for structural calculations. 2. RESULTS AND DISCUSSION The XRD patterns of precursors FeF3·3H2O/Ag (namely, FeF3·3H2O with different Ag-decoration amounts) are shown in Figure 1a. The major reflections are the characteristics of the tetragonal structure with space group P4/n (PDF NO. 32-0624) except for the weak peaks observed around 13-14o. These weak peaks are ascribed to NH4FeF4, which is produced by excess NH4HF2 reacting with Fe(NO3)3. It is indicated that Ag-decoration may not change the structure of precursors FeF3·3H2O fundamentally. The peaks belonged to Ag gradually show up at 2θ = 38.1o for samples Ag 3 and 5% with the increases of Ag amount. However, it is difficult to clearly make out each peak of silver in the XRD patterns of FeF3·3H2O/Ag. This phenomenon is mainly because of small amount of Ag additives, and most peaks of Ag overlap with the peaks of FeF3·3H2O. In Figure 1b, from the morphology of precursor with 5% 5



Ag-decoration, the sample is formed interpenetration twin, which consists of interlaced nanoplates with thickness of 500-800 nm. In order to confirm the dehydration time and proportion of precursors FeF3·3H2O/Ag, the TG-DTA results tested from 20 to 700 oC are shown in Figure 2. All the TG-DTA curves can be divided into four stages as follows. a) In the process of 20-120 oC, there is almost no change from the TG curves except for a little weight loss of 2-3% observed around 120 oC, corresponding to a weak exothermic peak due to phase transformations of crystals. b) In the process of 120-200 o

C, the TG curves have a rapid decline meanwhile the DTA curves display an obvious

endothermic peak around 150 oC, indicating about 28% hydration water loss in the range. The possible chemical reaction is: · 3 ⟶ · 0.33 + 2.67 . c) In the process of 200-450 oC, the rate of weight change slows down and the weight loss around 300 oC is about 4%, which is equal to the weight percentage of H2O in FeF3·0.33H2O. The chemical reaction of this stage is: · 0.33 ⟶ + 0.33 . d) In the process of 450-700 oC, the weight loss is negligible, which means the composition is thermodynamically stable. According to these results, the gradient calcinations are conducted on all precursors. At first, the precursors are pre-sintered at 120 oC in vacuum oven to remove the absorbed water, then transferred into tube furnace to be calcinated at 200 oC for 3 h and further heated at 300 oC for 3 h to obtain FeF3·0.33H2O, which is relatively stable intermediate phase between FeF3·3H2O and FeF3.22, 42 After ball-milling with SP, the ICP-OES tests for as-prepared FeF3·0.33H2O/Ag/SP samples were carried out. The Ag contents in samples Ag 1%, Ag 3% and Ag 5% are 0.07, 0.25 and 0.44 wt.%, respectively. To explore the crystal structure, the XRD and corresponding Rietveld refinements were conducted on FeF3·0.33H2O/Ag/SP samples. As shown in Figure 3a, all the XRD patterns are indexed to FeF3·0.33H2O (PDF NO. 76-1262), indicating that as-prepared samples are pure phase and crystallized in an orthorhombic structure with CmCm space group.24 The structural parameters of samples determined from the Rietveld refinement are illustrated in Table 1. As is indicated from the results, the cell volume of FeF3·0.33H2O increase after 6


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Ag-decoration, demonstrating an expanded Li+ pathway for Ag-decorated samples. This phenomenon could be due to trace amount of Ag insert into the crystal cell, as atomic radius of Ag is only 1.75 Å. The SEM images of the as-prepared FeF3·0.33H2O/Ag/SP samples are shown in Figure 4, the particle sizes of all samples are 50-100 nm after ball-milling. In Figure 4a, although the particle sizes of sample Ag 0% are uniform, slight aggregation is observed. With increasing Ag-decoration amount, these aggregations are reduced and the sizes of particles become more homogeneous (Figure 4b-d). It is because metallic Ag with high rigidity works as a grinding aid during balling process, which contributes to dispersion of materials and suppresses the aggregation. To get a clear view of the Ag nanoparticles in FeF3·0.33H2O, HR-TEM and selected area electron diffraction (SAED) of sample Ag 5% were performed, and the results are displayed in Figure 5. The relatively dark regions marked by dotted green circle shown in Figure 5a are silver nanoparticles. From the HR-TEM image in Figure 5b, apparent interplanar spacing of center region is 2.3 Å and corresponds to (111) plane of the pure silver phase. And the lattice spacing of (220) and (110) plane of FeF3·0.33H2O can be indexed as 3.2 and 2.6 Å. In addition, amorphous super P is also observed. The in-depth analysis of the SAED patterns in Figure 5c reveals that this is (220) plane for a tunneled FeF3·0.33H2O phase (Cmcm). These results illustrate FeF3·0.33H2O, silver and super P are homogeneously mixed and the introduction of Ag does not destroy the structure of bulk FeF3·0.33H2O. To further identify the element composition and distribution of sample Ag 5%, the energy-dispersive X-ray spectra (EDX) was examined and presented in Figure 5d-i. The elements Ag, F, Fe and C are uniformly distributed in sample Ag 5% and without any phase separation. To understand the impact for Ag-decoration on FeF3·0.33H2O, as shown in Figure 6, the DOS of pristine FeF3·0.33H2O and FeF3·0.33H2O/Ag samples are calculated by first-principles density functional theory (DFT). It is essential to say that, taking no account of the sample concentration, this calculation is only able to illustrate change trend of band structure after Ag-decoration. Figure 6b shows the atomic arrangement of FeF3·0.33H2O, and possible site of Ag in FeF3·0.33H2O is depicted in Figure 6c. 7



Figure 6a displays the minimum conduction band of FeF3·0.33H2O/Ag, whose value of 0.79 eV is lower than that of FeF3·0.33H2O, suggesting a band gap reduction. Thus, we can conclude, according to calculating results, that moderate Ag-decoration may lower the band gap of FeF3·0.33H2O, which makes the electrons much easier to migrate and then improves the conductivity. Cycle performances of FeF3·0.33H2O/Ag/SP samples at a current density of 0.1 C are shown in Figure 7a. The initial discharge capacities of samples Ag 0%, Ag 1%, Ag 3% and Ag 5% are 142.3, 101.7, 121.9 and 168.2 mAh g-1, respectively. Since the irreversible formation of solid electrolyte interphase (SEI) in the first cycle, the specific capacities in the second cycle are consequently regarded as reversible.8, 43-44 After several cycles, we can clearly see the specific capacities of Ag 3% and Ag 5% samples are higher than that of pristine sample. Figure 7b and c are the initial and 50th charge–discharge profiles of FeF3·0.33H2O/Ag/SP, respectively. After 50 cycles, sample Ag 5% has the highest discharge capacities of 128.4 mAh g-1, which is much higher than that of sample Ag 0%. In addition, the sample Ag 5% exhibits a higher discharge plateau and a lower charge plateau, demonstrating weak polarization and excellent reversibility, which agrees with the results based on cyclic voltammograms tested on all samples shown in Figure S1 and Table S1. It is obvious that the cycleability is enhanced considerably when Ag-decorated content is 5%. These results suggest that appropriate Ag-decoration (samples Ag 3% and Ag 5%) may benefit both increasing discharge capacities and improving conductivities of FeF3·0.33H2O/SP, while deficient (samples Ag 1%) or excess Ag-decoration (samples Ag 7%, whose cycle performance is shown in Figure S2) fail to improve electrochemical performance. To figure out the reason why a low amount of Ag-decoration (Ag 1%) has a negative impact on electrochemical property, galvanostatic intermittent titration technique (GITT) is carried out to evaluate the chemical diffusion coefficient of Li+ (DLi+) in samples Ag 0% and Ag 1%. Figure 8(a) shows the GITT curves of these two samples during the initial discharge process between 2.0 and 4.5 V. The DLi+ is calculated according to Eq. (1) derived by Weppner and Huggins as follows45: 8


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= (

!# " ) () ! ' %# $ &

≪ + # )

(1)

where Vm is the molar volume of FeF3·0.33H2O/Ag/SP samples, which is 4.28×10-4 m3 mol-1 calculated from the crystallographic data. m and M are the mass and molecular weight of samples, respectively. A is surface area of the electrode piece in cm2. L is the radius of the electrode particle. Figure 8(b) shows a typical t vs. E profiles for a single titration. If E vs. t-1/2 shows a straight-line behavior over the entire period of current flux, as shown in Figure 8(c), Eq. (2) can be further simplified as45:

= ( ,

- ∆!/ ) (∆! ) 0

(2)

The diffusion coefficients of Li+ at different voltages are obtained based on Eq. (2) and GITT tests shown in Figure 8(c). With the decrease of voltage, the DLi+ of both samples are reduced. At a range of 3.0-2.6 V, DLi+ of Ag 1% sample is lower than that of Ag 0% sample, suggesting Ag-decoration, in some extent, may influence the diffusion of Li+ and even make the electrochemical performance poor, which explains the low discharge capacity of Ag 1% sample. Ag-decorated amount is too low to improve electronic conductivity and accelerate the diffusion of Li+, ultimately causing the poor electrochemical behavior. To highlight the effect of Ag decoration on the rate capability of FeF3·0.33H2O/SP samples, Figure 9a shows the discharge capacities of samples at various current densities from 0.1 to 1 C and finally back to 0.1 C. Sample Ag 5% has superior rate property, exhibiting a high capacity of 169.4 mAh g-1, and achieving reversible capacities of 169.0, 154.9, 132.4, and 116.5 mAh g-1 at 0.1 C, 0.2 C, 0.5 C and 1.0 C, respectively. Once the current density recovers to 0.1 C, the capacity of sample Ag 5% still can reach 165.6 mAh g-1, suggesting the excellent rate performance and wonderful reversibility. Electrochemical impedance spectroscopy (EIS) profiles of Ag 0%, Ag 1%, Ag 3% and Ag 5% cathodes and the corresponding equivalent circuit for fitting are shown in Figure 9b to explore deeper kinetic information. At high frequencies the semicircle is associated with the charge-transfer resistance (Rct), standing for the charge transfer 9



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kinetics. At low frequency, a line corresponds to the Warburg impedance of Li+ diffusion within the cathode. As is well known, the smaller radius of semicircle indicates lower Rct, which demonstrates easier electrons transport between the liquid (electrolyte) and solid (electrode) phases. Among the four samples, with the increase of Ag-decorated amount, the value of Rct becomes lower. Obviously, sample Ag 5% has the lowest Rct, revealing Ag-decoration on FeF3·0.33H2O/SP cathodes may contribute to transport of electrons between the electrolyte and electrode, reduce the charge-transfer resistance and promote the conductivities of materials. To acquire the specific information on electronic conductivity of Ag-decorated FeF3·0.33H2O/SP cathode, electronic conductivity experiment was carried out by a RTS-4 linear four-point probe measurement system and the results are shown in the inset of Figure 9b. An increase in electronic conductivity is observed with the increasing Ag-decorated amounts, which is in line with the results of EIS analysis. In general, the persuasive reaction mechanism of Ag-decorated FeF3·0.33H2O/SP cathodes

can

be

concluded

as

described

below.

The

unblocked

intercalation/deintercalation reaction in LIBs usually depends on both lithium ions and electrons between active materials and electrolyte.33 In terms of the pristine FeF3·0.33H2O/SP cathode (Figure 9c left), the active particles are surrounded by Li+ derived from electrolyte, making sure the construction of “Li+ bridge” around them. Nevertheless, only partial region of FeF3·0.33H2O surface is connected with electrons through the super P (conductive carbon) | FeF3·0.33H2O interface, leading to small portion of FeF3·0.33H2O surface to be activated for intercalation/deintercalation reaction and poor kinetics. For Ag-decorated FeF3·0.33H2O/SP cathodes (Figure 9c right), the silver nanoparticles disperse uniformly around the FeF3·0.33H2O surface, providing a perfect “e- bridge” to connect solid | liquid interface. Since moderate amount of Ag are not attached with active materials densely, there is still a reliable “Li+ bridge” maintained for Li+ diffusion. These two bridges create more active sites for insertion/extraction process, increasing the electrochemically active surface area of electrode and improving the intercalation/deintercalation kinetics. It is why deficient or excess Ag-decoration on FeF3·0.33H2O (samples Ag 1% and Ag 7%) 10


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cannot significantly enhance the electrochemical performance. In detail, deficient Ag-decoration may not remarkably elevate electronic conductivity, nevertheless excess Ag-decoration may seriously hinder Li+ diffusion. CONCLUSION To summarize, we apply the silver mirror reaction to successfully synthesize a series of Ag-decorated FeF3·0.33H2O/SP cathodes (Ag 0%, Ag 1%, Ag 3% and Ag 5%). XRD refinement results indicate that Ag-decoration can increase the cell volume, returning an expanded Li+ pathway. First principle calculation demonstrates that Ag-decoration can help reducing the band gap of FeF3·0.33H2O and improving its electronic conductivity. The optimized sample Ag 5% delivers the initial discharge capacity of 168.2 mAh g-1 and retains a discharge capacity of 128.4 mAh g-1 after 50 cycles. Enhanced electrochemical property can be assigned to the excellent “e- bridge” built by moderate Ag-decoration, improving electronic conductivity and meanwhile ensuring unblocked Li+ diffusion. Our work provides a new sight for enhancing electronic conductivity of FeF3·0.33H2O, which can be developed on the other low-conductive promising electrodes to improve their electrochemical performance for the next-generation LIBs. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.******. CV curves of FeF3·0.33H2O/SP/Ag samples, the potential differences of four samples, and cycle performance of sample Ag 7% (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Bai); [email protected] (C. Wu). NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS The present work is supported by the National Key R&D Program of China: 11



Trackling Key Technology for Development and Industrialization of Power Lithium Ion Battery with High Specific Energy (Grant No. 2016YFB0100508), the Program for New Century Excellent Talents in University (Grant NCET-13-0033), and the Beijing Co-construction Project (20150939014). REFERENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 452, 652-657. (2) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. (3) Li, Y.; Bai, Y.; Bi, X.; Qian, J.; Ma, L.; Tian, J.; Wu, C.; Wu, F.; Lu, J.; Amine, K. An Effectively Activated Hierarchical Nano-/Microspherical Li1.2Ni0.2Mn0.6O2 Cathode for Long-Life and HighRate Lithium-Ion Batteries. ChemSusChem 2016, 9, 728-735. (4) Goodenough, J. B.; Park, K. S. The Li-Ion Rechargeable Battery: A Perspective. J Am. Chem. Soc. 2013, 135, 1167-1176. (5) Whittingham, M. S. Ultimate Limits to Intercalation Reactions for Lithium Batteries. Chem. Rev. 2014, 114, 11414-11443. (6) Li, Y.; Wu, C.; Bai, Y.; Liu, L.; Wang, H.; Wu, F.; Zhang, N.; Zou, Y. Hierarchical Mesoporous Lithium-Rich Li[Li0.2Ni0.2Mn0.6]O2 Cathode Material Synthesized via Ice Templating for Lithium-Ion Battery. ACS Appl. Mater. Interfaces 2016, 8, 18832-18840. (7) Li, Y.; Bai, Y.; Wu, C.; Qian, J.; Chen, G. H.; Liu, L.; Wang, H.; Zhou, X.; Wu, F. Three-Dimensional Fusiform Hierarchical Micro/Nano Li1.2Ni0.2Mn0.6O2 with a Preferred Orientation (110) Plane as a High Energy Cathode Material for Lithium-Ion Batteries. J. Mater. Chem. A 2016, 4, 5942-5951. (8) Bai, Y.; Zhou, X.; Jia, Z.; Wu, C.; Yang, L.; Chen, M.; Zhao, H.; Wu, F.; Liu, G. Understanding the Combined Effects of Microcrystal Growth and Band Gap Reduction for Fe(1−x)TixF3 Nanocomposites as Cathode Materials for Lithium-Ion Batteries. Nano Energy 2015, 17, 140-151. (9) Ko, J. K.; Wiaderek, K. M.; Pereira, N.; Kinnibrugh, T. L.; Kim, J. R.; Chupas, P. 12


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Scheme 1 Schematic diagram of synthetic route for FeF3· 0.33H2O/Ag/SP cathode.

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Figure 1 (a) XRD patterns of precursors FeF3·3H2O/Ag, (b) SEM image of precursor FeF3·3H2O with Ag 5% decoration.

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Figure 2 (a)-(d) TG-DTA curves of precursors FeF3·3H2O/Ag.

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Figure 3 (a) XRD patterns and refinements of cathode FeF3·0.33H2O/Ag/SP; (b) the crystal structure of FeF3·0.33H2O.

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Figure 4 (a)-(d) SEM images of cathode FeF3·0.33H2O/Ag/SP.

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Figure 5 (a) TEM images, (b) HRTEM images, (c) SAED pattern of the green square region in (b), and (d)-(i) EDX elemental distribution mapping and corresponding EDX spectra of sample Ag 5%.

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Figure 6 (a) Calculated DOS of pristine FeF3·0.33H2O and FeF3·0.33H2O/Ag samples; the sketch map of (b) FeF3·0.33H2O and (c) FeF3·0.33H2O/Ag.

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Figure 7 (a) Cycling stability, (b) the initial cycle and (c) the 50th cycle discharge/charge voltage profiles of cathode FeF3·0.33H2O/Ag/SP.

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Figure 8 (a) GITT curves of Ag 0% and Ag 1% samples in the initial discharge process (current flux: 20 mA g-1, time interval: 60 min), (b) linear behaviors of E vs. t-1/2, (c) t vs. E profiles for a single GITT titration, (d) diffusion coefficients of Li+ in Ag 0% and Ag 1% samples at different discharge states.

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Figure 9 (a) Rate capacity profiles of FeF3·0.33H2O/Ag/SP samples, (b) Nyquist plots at low frequencies of FeF3·0.33H2O/Ag/SP samples before cycles, and (c) the sketch for reaction mechanism of FeF3·0.33H2O/Ag/SP samples.

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Table 1 Structural parameters of FeF3·0.33H2O/Ag/SP samples determined from XRD Rietveld refinement. Sample

a/Å

b/Å

c/Å

V/Å3

Rp/%

Rwp/%

Ag 0%

7.4729

12.2782

7.4021

679.174

9.0

12.0

Ag 1%

7.4754

12.7260

7.4986

713.360

10.2

13.7

Ag 3%

7.4232

12.7358

7.5144

710.413

9.3

12.5

Ag 5%

7.5006

12.7685

7.5178

719.989

8.9

11.5

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TOC GRAPHIC

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Building an Electronic Bridge via Ag Decoration To ... - ACS Publications

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