Atomistic Insights into FeF3 Nanosheet: An Ultrahigh-Rate and Long

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Atomistic insights into FeF3 nanosheet: An ultrahighrate and long-life cathode material for Li-ion batteries Zhen-Hua Yang, Shu Zhao, Yanjun Pan, Xianyou Wang, Hanghui Liu, Qun Wang, Zhijuan Zhang, Bei Deng, Chunsheng Guo, and Xingqiang Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17127 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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ACS Applied Materials & Interfaces

Atomistic insights into FeF3 nanosheet: An ultrahigh-rate and long-life cathode material for Li-ion batteries Zhenhua Yang,*,†,§ Shu Zhao,†,§ Yanjun Pan,†,§ Xianyou Wang,┴ Hanghui Liu,†,§ Qun Wang,†,§ Zhijuan Zhang,†,§ Bei Deng,# Chunsheng Guo,▽ and Xingqiang Shi*,# Key Laboratory of Materials Design and Preparation Technology of Hunan Province,

†

School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, Hunan, China Key Laboratory of Low Dimensional Materials & Application Technology (Ministry

§

of Education), School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, Hunan, China Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry

┴

of Education, Hunan Province Key Laboratory of Electrochemical Energy Storage and Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China Department of Physics, Southern University of Science and Technology, Shenzhen

#

518055, China Superconductivity and New Energy R & D Center, Southwest Jiaotong University,

▽

Mail Stop 165#, Chengdu 610031, China

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ABSTRACT: Iron fluoride with high operating voltage and theoretical energy density has been proposed as a high-performance cathode material for Li-ion batteries. However, the inertness of pristine bulk FeF3 results in poor Li kinetics and cycling life. Developing nanosheet-based electrode materials is a feasible strategy to solve these problems. Herein, based on first-principles calculations, first the stability of FeF3 (012) nanosheet with different atomic terminations under different environmental conditions was systematically studied, then the Li-ion adsorption and diffusion kinetics were thoroughly probed, and finally the voltage for different Li concentrations were given. We found that F-terminated nanosheet is energetically favorable in a wide range of chemical potential, which provide a vehicle for lithium ion diffusion. Our Li-ion adsorption and diffusion kinetics study revealed that: (1) the formation of Li dimer is the most preferred; (2) The Li diffusion energy barrier of Li dimer is lower than isolated Li atom (0.17 eV for Li dimer vs 0.22eV for Li atom); (3) The diffusion coefficient of Li is 1.06×10-6cm2·s-1, which is orders of magnitude greater than that of Li diffusion in bulk FeF3 (10-13~10-11 cm2·s-1). Thus FeF3 nanosheet can be acted as an ultrahigh-rate cathode material for Li-ion batteries. More importantly, the calculated voltage and specific capacity of Li on the FeF3 (012) nanosheet demonstrate that it has a much more stable voltage profile than bulk FeF3 for a wide range of Li concentration. So, few layers FeF3 nanosheet is prior in providing the desired long-life energy density in Li-ion batteries. These above findings in the current study shed new lights on the design of ultrahigh-rate and long-life FeF3 cathode material for Li-ion batteries.

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KEYWORDS: nanosheet, diffusion barrier, diffusion coefficient, iron fluoride, atomistic insights 1. INTRODUCTION Li-ion batteries (LIBs) are now the dominating rechargeable energy-supply systems for portable electronic devices and electrical vehicles.1-3 To satisfy the increasing demand of reliable power sources, developing LIBs with high-voltage, high-capacity and high-rate performance is the most urgent challenges. Most importantly, design of new cathode materials based on reversible conversion reaction is critical. Recently, transition metal fluorides,4-16 acting as cathode materials, have attracted great interest owing to large theoretical capacities and high discharge voltages. The high ionicity bonds between transition metal and fluoride results in their high reaction potentials. Moreover, metal fluorides usually have open structures for transportation of Li-ion.17 And all the oxidation states of the metal can be utilized in the reversible conversion reaction,18 which contributes to large theoretical energy density. Among transition metal fluorides, great attentions have been focused on iron fluoride due to their excellent properties, such as high theoretical specific capacity (712 mAh/g), low cost and non-toxicity.19-28 Especially, it is found that FeF3 exhibits superior thermal stability at elevated temperatures as a cathode for LIBs.29-30 For all these reasons, many investigators have developed themselves to exploring the intrinsic physical properties of bulk FeF3 and its reaction mechanism with Li, obtaining the atomistic mechanism about electrochemical reaction of bulk FeF3. It is confirmed that FeF3 belongs to antiferromagnetic system and its (024) vacant plane between FeF3 (012)

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planes is available for Li insertion.31 Furthermore, according to detailed experiment32 and density functional theory (DFT) studies,33 the entire reversible electrode reactions between bulk FeF3 and Li can be involved as the following: FeF3  Li   e   LiFeF3 LiFeF3  2Li   2e   Fe  3LiF

(1) (2)

However, pristine bulk FeF3 still suffers from poor Li kinetics and cycling life due to its inertness. To alleviate these problems, various techniques, such as synthesis novel nanocomposites materials34-41 and ionic doping,42-43 have been developed to improve its electronic conductivity, trying to enhance Li kinetics. And the electrical conductivity of FeF3 has been improved to some extent. But FeF3 still suffers from poor ionic conductivity, which lead to its Li kinetics cannot be significantly improved. Moreover, specific capacity fading and stability degradation of FeF3 tend to follow, which poses significant technological bottleneck. Nowadays, breakthrough in terms of enhanced Li kinetics and cycling life is urgently needed by the rapid development of other technologies for FeF3. Excitingly, the development of 2D material provides more promising choices. In the last decade, nanosheet materials, such as carbon,44-45 ZrS2,46 TiO2,47 Ti3C2,48 LiCoO249 and Nb2O5,50 have shown extraordinary performances in LIBs because of their unique electronic, mechanical, and electrochemical properties. It is generally understood that the nanosheet performs three functions: (1) its micrometer-scale dimensions can buffer volume change during Li-ion insertion/deinsertion, thus improving stability;51 (2) its ultrathin thickness can accelerate Li-ion and electron transport, building high-power LIBs;52 (3) its high

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surface area can provide favorable conditions for contact between the active material and the electrolyte, promoting the electrochemical activation.47 Therefore, nanosheet may be one effective method to improve Li kinetics and cycling life of FeF3. Subsequently, FeF3 nanosheet with exposed (012) plane has been successfully prepared in experiment.53 And it has been proposed that the formation of sheet-structure contributes to the large reversible energy-storage capacity during insertion/deinsertion process as well as in conversion reaction.53 All these research established foundation to reveal atomistic mechanism of interaction between FeF3 (012) nanosheet and Li ion. Usually, it is difficult to experimentally justify the reaction mechanism between Li-ion and FeF3 (012) nanosheet at the atomic scale. To obtain detailed atomic insights of the basic principles and potential applications of FeF3 (012) nanosheet in LIBs, we carried out comprehensive computational modeling of the behaviors of Li intercalation and diffusion on the FeF3 (012) nanosheet, including atomic-level analysis of surface structures, and elucidated the Li-FeF3 interaction. Accordingly, an unambiguous atomistic reaction model of FeF3 (012) nanosheet with lithium has been revealed. 2. COMPUTATIONAL DETAILS 2.1 Computational method First principles calculations within the framework of density functional theory (DFT) were implemented using the VASP code.54-55 The spin-polarized generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) method was used to treat the exchange-correlation energy. The projector augmented wave (PAW)

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method55 was used to describe the core electrons. The valence electronic states of 3s23p63d6, 2s22p5 and 2s1 are taken into account as the valence states for Fe, F and Li, respectively. The plane wave energy cut-off was set to 500 eV. The 4×4×2 and 4 × 4 × 1 grids of k-points were selected for bulk and surface calculations, respectively. All atoms are fully relaxed to their equilibrium positions until the maximum residual forces on the constituent atoms are less than 0.03eV/Å. GGA+U method was employed to treat localized Fe 3d orbits. The effective single parameters U-J of 5.0 eV was set.31 Spin-polarized calculations were performed for all calculations. For the determination of the activation barriers and paths of the Li-ion diffusion across the FeF3 (012) nanosheet, the climbing image nudged elastic band method (CI-NEB)56-58 were carried out to compute transition-state structures. During CI-NEB calculations, the norm of the forces to the path is less than 0.03 eV/Å. Correspondingly, the diffusion coefficients can be calculated according to the following equation:

D  d 2 0 exp(

Ea ) k BT

(3)

where d is the hopping distance of Li, Ea is the energy barrier, k B is the Boltzmann constant, T is the temperature, and  0 is the attempt frequency (which is assumed to be 1013 Hz59). 2.2 Surface models A unit cell of FeF3 possesses hexagonal crystal structure with R-3C space group.60 Before starting the surface calculation, bulk structure of the hexagonal FeF3 phase is 6


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fully optimized and the calculated lattice constants are: a=5.247 Å and c=13.427 Å, which agrees well with the experimental values of a=5.198 Å and c=13.330 Å.60 Based on the optimized bulk geometries, the FeF3 (012) slabs are constructed. Two types of terminations, FeF termination and F termination, are taken into account (details see Figure S1 in Supporting Information). And they are labeled as FeF3 (012)-FeF and FeF3 (012)-F, respectively. To obtain the equilibrium structure of those slabs, all layers of the slabs are allowed to be optimized by the total energy and atomic force calculations. Seven alternating FeF and F atomic layers were used in the slab models. A vacuum region with a thickness of 10 Å is included to avoid the interactions between repeated slabs. 3. RESULTS AND DISCUSSION 3.1 Surface structure and surface energy of FeF3 (012) nanosheet Firstly, we analyzed and compared the surface structures of FeF3 (012) nanosheet with FeF and F termination. It is found that the F-terminated surface is much more stable: strong surface reconstruction occurs in the FeF-termination, but this has not happened in the F-termination (details see in Supporting Information). Therefore, Figure 1(a) only gives the optimized slab models of the F-terminated FeF3 (012). Surface energy is one physical quantity for the study of the relative stability of different surfaces. If the surface energy is smaller, the surface will be more stable. Under F-rich conditions (see Figure 1(b)), F-terminated surface is the energetically favorable structure. Therefore, given enough fluoride pressure, the FeF3 (012) nanosheet can be experimentally fabricated. In order to compare the relative stability

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of FeF-terminated (FeF3 (012)-FeF) and F-terminated (FeF3 (012)-F) surfaces, we calculated the surface energy (Esur) according to following equation.

Esur 

1 ( Etot  N Fe Fe  N F F ) 2A

Here, A is the area of the corresponding slab. The factor of

(4)

1 appears, which means 2

the cell contains two surfaces. E tot is the total energy of the relaxed slab.  Fe and

F are the chemical potential of per Fe atom and per F atom, respectively. N Fe and N F represent the numbers of Fe and F atoms in the slab, respectively. Note that  Fe and F are not independent. Their relationship can be expressed as bulk EFeF  Fe  3F 3

(5)

bulk Where EFeF is the energy per formula unit of FeF3. Therefore, surface energy (Esur) 3

can be defined as

Esur 

1 bulk [ Eslab  N Fe EFeF  (3N Fe  N F ) F ] 3 2A

(6)

Besides, the formation energy ( H f ) of FeF3 can be written as follow. bulk 0 H f  EFeF  Fe  3F0 3

(7)

 Fe0 is the total energy per atom of bulk Fe.  F0 is one half of total energy of F2 molecule. According to Eq. (5) and Eq. (7), the variation range of F

can be

obtained as following:

1 F0  H f  F  F0 3

(8)

According to Eq. (6), we calculate the Esur for both FeF3 (012)-FeF and FeF3 (012)-F slabs as a

function of F . The calculated results are plotted in Figure 1(b), 8


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which shows that the F-terminated surface is much more energetically favorable in a wide range of chemical potential.

Figure 1. (a)The front view of FeF3 (012) nanosheet with F-termination after geometric optimization. (b) The surface energies of FeF3 (012)-FeF and FeF3 (012)-F surfaces as a function of  F . The vertical dashed lines show the low and high limits of the chemical potential. 3.2 Li adsorption behavior Now let us move to further explore the Li adsorption behavior on FeF3 (012)-F surface. First, consider the isolated Li adsorption cases. Three possible Li initial adsorption sites with high symmetry were considered, as indicated in Figure 2: (1) the site on top of a F atom in the outer layer (abbreviated as top site); (2) the centre of two neighboring F atom in the outer layer (bridge site); (3) the centre of three neighboring F atom in the outer layer (hollow site). The adsorption energy is a criterion to determine the energetically stability of the adsorption. And the adsorption energy( Eads )

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of Li can be expressed as Eads  Etotal  Esub  ELi

(9)

where Etotal , Esub and ELi are total energies of Li/FeF3 (012) full system, the FeF3 (012) and isolated Li, respectively. After geometric optimization, Li atom in all these three initial adsorption structures are all very prone to move to the bridge site. The calculated adsorption energy of Li is -3.77 eV, which is enough to guarantee a rapid loading process of the Li ion on the FeF3 (012) nanosheet. As the Li atom is adsorbed at bridge site, Li atom almost loses all valence electrons and the charges transferred from Li atom to the FeF3 (012) nanosheet is 0.89 |e| according to Bader charge analysis.61-64 The Li-F bond length is 1.733 Å, which indicates that strong ionic bond occurs between Li and F atoms. Next, we considered the cases of multiple Li atoms adsorption (details see in Supporting Information). It is found that Li atoms prefer to form Li dimer with a Li-Li separation of 2.554 Å. Moreover, we found that the stability of lithium adsorption descend with increasing distance of Li dimmers (details see Figure S5 and Figure S6 in Supporting Information).In order to further reveal the reason of the formation of Li dimer on the FeF3 (012) nanosheet, we calculate its charge density deference (  ), which is defined as    total   Li   FeF3

(10)

where  total ,  Li and  FeF3 are the charge densities of Li/FeF3, Li and FeF3, respectively. The Li-induced charge redistribution is presented in Figure 2(b).

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Figure 2. (a) The adsorption model of isolate Li atom on the FeF3 (012) nanosheet. Bader charge of Li and its neighboring atoms, together with Li-F bond length, are presented. (b) Differential charge density of Li dimer on the FeF3 (012) nanosheet. The yellow/blue region represents charge aggregation/depletion, and the isosurface is 0.003 e/Å3. (c) Li-F bond length and Bader charge for atoms neighboring the Li dimer. For comparison, the local configurations in pure FeF3 (012) is also shown.

As shown in Figure 2(b), it is demonstrated that the electron states of two Li atoms are both polarized. The Li dimer induces electron redistribution and electron depletion regions appear between F atom layers and their adjacent Fe atom layers, which indicates that the Li foreigner weakens the interaction between F layer and Fe layer. Moreover, Li atoms almost lose their valence electrons and obvious Li-F ionic bond occurs. Figure 2(c) shows the local structure neighboring Li dimer. For each Li

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atom, neighboring F atoms located at different layers all obtain valence electrons from one Li atom. The charge of Li is +0.90 |e|, while the calculated charges of fluoride located different layers are −0.81 |e|, −0.72 |e| and −0.76 |e|, respectively. Compared with initial FeF3 (012) nanosheet, it is obvious that the charge of Li dimer is transferred to the adjacent F atoms located at top, second and third layers. Most of all, the strongest interaction between Li atom and F atoms located at top layer are predominantly ionic due to most valence electrons obtained by top-layer F atom. The Li-F bond length is 1.806 Å, which is close to ideal ionic bond (2.090 Å). For second-layer and third-layer F atoms, the corresponding Li-F bond lengths are 2.099 Å and 2.560 Å, respectively. Therefore, two Li atoms are connected together due to the strong interaction of Li-F ionic bond. 3.3 Ionic conductivity In connection with the rechargeable battery performance, the rate of Li-ion diffusion directly affects ionic conductivity during charging and discharging. Easier Li diffusion means higher charge-discharge rates of LIBs. Therefore, we employ the well-established climbing image nudged-elastic-band (CI-NEB) technique to explore Li-ion diffusion behavior on the FeF3 (012) nanosheet. The type of isolate lithium diffusion dominates at low Li loading. At higher Li loading, Li dimer is prone to form during multiple adsorptions of lithium atoms. Consequently, examination of Li dimer mobility and diffusion pathways on the FeF3 (012) nanosheet is of crucial importance. The diffusion of isolated Li and Li dimer are both systematically investigated in the following.

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Figure 3. Schematic representation of Li-ion diffusion paths on the FeF3 (012) surface. Li-ion migration pathways ①−④ are obtained according to the CI-NEB method. In the top picture, yellow, green and blue spheres represent the first-layer F atom, second-layer F atom and third-layer Fe atom.

Figure 4. Calculated energy profiles for the investigated Li transitions on the FeF3 (012) nanosheet. 13



Firstly, the procedure of isolate Li diffusion was investigated. Considering the stable structure of Li adsorption on the FeF3 (012) nanosheet, four possible diffusion pathways were predesigned between two neighboring bridge sites. And the calculated energy profiles along paths ①−④ are presented in Figure 3. As shown in Figure 3, all Li-ion diffusion paths are arch-like curves due to electrostatic attraction between Li ion and the neighboring F ion. For path ①, the Li ions migrate from one “Li1” site to a “Li1” site across the nearest and nearer “Li1” sites, that is Li1  Li1  Li1  Li1. In path ②, the Li-ions move from one “Li1” site to another “Li1”site across the nearest and nearer “Li2” sites, that is, Li1  Li2  Li2  Li1. For path ③, the Li-ions are allowed to move from “Li1” directly to the nearest neighboring “Li1” sit across the “Li2” site. that is, Li1  Li2  Li1. For path ④, Li ions migrate from one “Li1” site to another “Li1” site across the three neighboring “Li2” sites, that is, Li1  Li2  Li2  Li2  Li1. The energetic barriers for the diffusion of inserted Li+ are plotted in Figure 4. And corresponding energy barrier values of the Li hopping are summarized in Table 1. From our calculations, we have estimated the energy barriers for Li diffusion along path ①, path ②, path ③ and path ④ directions as 0.22 eV, 0.26 eV, 0.26 eV and 0.24 eV, respectively. These results indicate that the most preferable Li diffusion path is along path ① direction. It is noted that the lowest Li-ion barrier energy of 0.22 eV on the FeF3 (012) nanosheet is very close to the reported Li-ion barrier energy for some 2D materials, such as borophene (0.21 eV65) and VS2 (0.22 eV66). Furthermore, it is also smaller than both graphene (0.32eV67) and MoS2 (0.25 eV68). Viewing these results from the perspective of Li-ion barrier, it

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is shown to exhibit very low barrier energy for Li diffusion on the FeF3 (012) nanosheet, which further exhibits that FeF3 can act as a promising cathode material. To clarify the reason for the Li-ion barrier trend and try to reveal the relationship between Li-ion barrier and the crystal structure, we further investigated the crystal structure of Li-ion transport system along path1 and path2 in Figure 5(a). Here, path1 and path2 represent the diffusion paths with the lowest barrier energy and the highest barrier energy, respectively. Figure 5(a) exhibits their schematic ball-polyhedral representation of the Li migration. Where, Li1  Li1 and Li1  Li2 are two different typical transition paths. It is clear that the Li1-F2 interatomic distance turns out to be shorter than the Li1-F1 one by roughly 0.015 Å, thus requiring more energy to break the bond and bringing about a higher energy barrier of Li migration near the F2 atom. To further explain the barrier energy difference between Path1 and Path2, we calculated and compared their Bader charge of transition state before and after Li adsorption at the saddle point. The calculated results are presented in Figure 5(b). It is obvious that the coordination number of Li in Path1 is larger than that of Li in Path2 before and after Li adsorption. Li changes its state from three-coordinated to two-coordinated, leading to lower energy barrier. In Path1, Li-F bond lengths are 1.679 Å, 2.532 Å and 2.552 Å, respectively. In Path2, Li-F bond lengths are 1.702 Å and 2.497 Å, respectively. The larger coordination number of Li is, the more chance of balance of Li-F interaction will occur. The result can contribute to the Li diffusion on the FeF3 (012) nanosheet. And it is another reason that the barrier energy in Path1 is smaller than that in Path2.

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Figure 5. (a) Schematic ball-polyhedral representation of the Li migration around the “dangling” F1 and F2 atoms of the corresponding FeF3 groups. (b) The local structure and Bader charge of transition state at saddle point in Path1 and Path2. Table 1 Energy barriers and migration path lengths of Li on the FeF3 (012) nanosheet according to CI-NEB Path no 1 2 3 4

path length (Å) 2.17 3.06 2.76 2.24 2.44 2.81 2.92 2.23 2.28

transition type 2.38 Li1 Li1 Li1 Li1 2.34 Li1 Li2 Li2 Li1 Li1 Li2 Li1 3.16 Li1 Li2 Li2 Li2 Li1

16


0.2 0.26 0.20

Ea (eV) 0.22 0.21 0.23 0.26 0.26

0.24

0.23 0.17

0.22

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To shed light on the transformation process of Li dimer, we carefully detected three possible diffusion pathways between two neighboring adsorption sites, namely path1, path2 and path3, respectively, as indicated in Figure 6(a)-(c). And the corresponding barrier energy profiles are shown in Figure 6(d). The calculated energy barriers for Path1, Path2 and Path3 of Li dimer are 0.39 eV, 0.25 eV and 0.17 eV, respectively. To further understand the underlying mechanism of the Li dimer diffusion behavior on FeF3 (012) nanosheet, we presented the local structure of saddle points in Path1, Path2 and Path3, respectively. For Path1, one Li atom is just located at the top of one second-layer F atom, forming strong Li-F ionic bond of 1.948 Å, which play prominent role in preventing Li dimer from diffusion. Accordingly, it results in the largest energy barrier. In the cases of Path2 and Path3, the coordination number of Li in Path3 is larger than that in Path2. Moreover, compared with Path3, Li dimer has better symmetry with surrounding F atoms by analysis of Li-F bond length in Path2, which both leads to the lowest diffusion energy barrier of Li dimer in Path3. Interestingly, this value is even lower than the diffusion barrier of isolate lithium diffusion energy barrier (0.22 eV) on the FeF3 (012) nanosheet, indicating that Li dimer is not trapped at any specific site in spite of Li-Li repulsion. That is to say, it is even easier to diffuse for Li diffusion as the Li concentration increases, which indicates that FeF3 nanosheet has another advantage as a ultrahigh-rate cathode material for LIBs. Based on the above discussion, we further calculated the lithium diffusion coefficient at room-temperature for isolate Li and Li dimer. Table 2 summarizes the

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obtained diffusion coefficient for isolate Li and Li dimer diffusion on the FeF3 (012) nanosheet. From the comparison of the diffusion coefficients, note that a notable difference among them for isolated Li or Li dimer diffusion along different paths, ranging from 10-10 cm2·s-1 to 10-6 cm2·s-1. Our prediction for the Li atom diffusion coefficient along the Path3 direction is the largest value of 1.06×10-6 cm2·s-1, which is orders of magnitude greater than that of Li atom diffusion in bulk FeF3 (10-13~10-11 cm2·s-1).42 Moreover, this ultrahigh mobility implies implies that rate performance of FeF3 (012) nanosheet can be better than other focused high-rate cathode material, such as Li3V2(PO4)3 (10-9~10-10 cm2·s-1),69 Ti2S4 (2×10-8~2×10-9 cm2·s-1)70 and LiNi0.5Mn1.5O4 (2.8×10-7 cm2·s-1).71 As apparent from the discussions so far, FeF3 (012) nanosheet emerges as a potential cathode material for application in LIBs because of its ultrahigh rate. Table 2 Energy barrier, hopping distance and diffusion coefficients at room -temperature for isolate Li and Li dimer diffusion on the FeF3 (012) nanosheet.

Path

Isolated Li

Li dimer

Path length (Å)

Energy barrier

Diffusion coefficient

(eV)

(cm2·s-1)

Path1

3.37

0.22

9.97×10-7

Path2

2.81

0.26

1.27×10-7

Path1

1.80

0.39

2.09×10-10

Path2

2.10

0.25

2.85×10-10

Path3

1.20

0.17

1.06×10-6

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Figure 6. Top views of possible migration paths for Li dimer. IS1 and IS2 represent the start points. While FS1 and FS2 represent final end points. (a) Path1, (b) Path2, (c) Path3, (d) Diffusion energy barriers in the three paths, and (e) the structure of saddle points. 3.4 The Li dimer adsorption form is preferred Considering the low diffusion barriers and high diffusion coefficient of lithium atoms and dimers, other structures for deposited lithium, known as lithium clusters, may form with gradually increasing lithium concentration. As we know, the formation of lithium cluster would seriously reduce the charge/discharge capacity of the LIBs. Besides, the formation of lithium cluster will induce dendrite growth of lithium, 19



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which is harmful for the safety of LIBs. In order to check this, we further calculated cohesive energies of the bulk lithium metal ( Ec ) according to the equation as follows:

1 Ec  ( Ebulk  2Eiso ) 2

(11)

where Ebulk is the total energy of bulk lithium metal and Eiso is the energy of an isolate Li atom. The calculated cohesive energies of the bulk lithium metal is -1.604 eV, which is good agreement with the reported value of -1.630 eV.72 Compared with adsorption energy of Li atom above discussed, the adsorption energy for lithium atom on FeF3 (012) nanosheet is far essentially lower than the corresponding cohesive energies of bulk lithium metal, which suggests that lithium atoms prefer to be uniformly-distributed on the FeF3 (012) nanosheet in the Li dimer form instead of forming lithium metal clusters. 3.5. Band gap, theoretical voltage and storage capacity In order to determine the electronic structure of FeF3 (012) nanosheet at a high Li concentration, we calculated the band gap of FeF3 (012) nanosheet as the increasing of Li concentration in Figure 7(a) (details see Figure S7 in Supporting Information). The band gap decreases gradually with the increasing of x since the larger Li concentration. These observations suggest that the Li atoms mediate the transition and reduce the band gap considerably. In other words, the electron mobility on this material can be increased significantly, which should not limit the adsorption kinetics. As it is well known, the average electrode potential and Li storage capacity are two important parameters which determine the electrochemistrical performance of LIBs. Therefore, we further calculated the theoretical voltage with changes of specific 20


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capacity for FeF3 (012) nanosheet. The Li absorption reaction on the FeF3 (012) nanosheet can be expressed as

Li x1 FeF3  ( x2 - x1 )Li  Li x 2 FeF3

(12)

The theoretical voltage ( Vave ) can be expressed as Vave  

where ELi x

2

FeF3

ELi x

2

FeF3

 E Li

x FeF3 1

 ( x2  x1 ) ELi

( x2  x1 )

(13)

( E Lix FeF3 ) is the total energy of the FeF3 (012) with x2 (x1) Li atoms 1

adsorbed on it. ELi is the total energy per Li atom in bulk phase and its value is -1.898 eV. Here, 2×1×1 FeF3 (012) nanosheet supercell with different number of Li atoms was implemented. The specific capacity (C) of FeF3 (012) for different number of Li atoms is calculated by Faraday's equation

C

xnF 3600M

(14)

where n is the valency of Li, F is Faraday's constant, and M is the molecular weight of LixFeF3. Figure 7(b) shows how the open-circuit voltage change as more Li atoms are added to the FeF3 (012) nanosheet.

Figure 7. (a) The band gap of LixFeF3 as function of Li concentration. (b) Calculated the theoretical voltage for FeF3 (012) as a function of specific capacity.

21



The obtained theoretical voltage varies from 3.37 to 3.91 V. It is worth noting that in our calculation the theoretical voltage is extremely high. This may be attributed to a much stronger adsorption energy of Li ion on the FeF3 (012) nanosheet. Meanwhile, FeF3 (012) nanosheet has a stable voltage profile than bulk FeF3 within the wide range of the Li atom content. Encouragingly, when the FeF3 (012) nanosheet reaches the high Li concentration corresponding to the case of specific capacity arriving from 115.03 mAh/g to 223.35 mAh/g, the theoretical voltage is still as high as 3.37 V. Compared with bulk FeF3, FeF3 (012) nanosheet has a more stable voltage profile than bulk FeF3 within the wide range of the Li atom content. It is therefore important to use few layer FeF3 nanosheet in order to provide the desired long-life energy density in LIBs. 4. CONCLUSIONS In summary, the relaxed FeF3 (012) nanosheet with different terminations and their thermal stability of are clearly presented. The FeF3 (012) nanosheet is proposed as an excellent cathode candidate for LIBs. The thermodynamic surface-stability analysis revealed that FeF3 (012) with F-termination is more stable than FeF3 (012) with FeF-termination in a wide range of chemical potential of fluoride. The most energetically favorable adsorption site for Li ions is predicted at the bridge site of the F atoms. For the adsorption of more Li, Li atoms are expected to form Li dimers and still occupy the bridge sites. Moreover, the Li ions prefer to migrate via neighboring bridge sites. Interestingly, isolate lithium ion diffusion on FeF3 (012) nanosheet with F-termination is extremely easy, with low energy barriers of 0.22 eV, which is much

22


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lower than those of other widely investigated cathod materials. While even lower diffusion energy barrier of around 0.17 eV occurs for Li dimer diffusion, indicating that Li dimers are not only trapped at any specific site, but also Li atoms diffusion will be easy. Furthermore, the diffusion coefficient of Li is 1.06×10-6 cm2·s-1, which is orders of magnitude greater than that of Li diffusion in bulk FeF3(10-13~10-11 cm2·s-1). This contributes FeF3 nanosheet for ultrahigh-rate cathode material in LIBs. Besides, FeF3 (012) nanosheet cathode has high open circuit voltage and much more stable voltage profile than bulk FeF3, which is beneficial for a good cyclability. All of these excellent properties demonstrate that FeF3 (012) nanosheet has a great potential to be applied as the cathode material in LIBs. ASSOCIATED CONTENT Supporting information Details of calculations including slab models of FeF3 (012) nanosheet, optimized structure of FeF3 (012) nanosheet with F-termination and FeF-termination, total energy of the adsorption configuration with two Li atoms on the FeF3 (012) nanosheet, total energies of two Li dimer as function of interatomic distance, optimized structure of Li adsorption with different Li concentration. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Zhenhua Yang) *E-mail: [email protected] (Xingqiang Shi) ORCID Zhenhua Yang: 0000-0002-3967-6249 23



Xingqiang Shi: 0000-0003-2029-1506 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (Grant No. 21573187), Hunan Provincial Natural Science Foundation of China (Grant No.2017JJ2246), A Project supported by Scientific Research Fund of Hunan Provincial Education Department (Grant No.15B228), China Postdoctoral Science Foundation funded project (Grant No.2016M592435), Start-up funds for doctor supported by Xiangtan University (Grant No.12QDZ02), Opening Foundation of Key Laboratory of Materials Design and Preparation Technology of Hunan Province (Grant No.KF20140701), and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501. REFERENCES (1) Tawa, S.; Yamamoto, T.; Matsumoto, K.; Hagiwara, R. Iron(III) fluoride synthesized by a fluorolysis method and its electrochemical properties as a positive electrode material for lithium secondary batteries. J. Fluorine Chem. 2016, 184, 75-81. (2) Reddy, A. L.; Srivastava, A.; Gowda, S. R.; Gullapalli, H.; Dubey, M.; Ajayan, P. M. Synthesis of nitrogen-doped graphene films for lithium battery application. ACS Nano 2010, 4, 6337-6342. (3) Li, C.; Mu, X.; Aken, P. A. V.; Maier, J. A High‐Capacity Cathode for Lithium 24


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Atomistic Insights into FeF3 Nanosheet: An Ultrahigh-Rate and Long

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